Crustal Evolution and Tectonic Problems

#### **Chapter 1**

## Crustal Evolution of the Himalaya since Paleoproterozoic

*Vikas Adlakha and Kalachand Sain*

#### **Abstract**

Understanding the crustal evolution of any orogen is essential in delineating the nomenclature of litho units, stratigraphic growth, tectonic evolution, and, most importantly, deciphering the paleogeography of the Earth. In this context, the Himalayas, one of the youngest continent-continent collisional orogen on the Earth, has played a key role in understanding the past supercontinent cycles, mountain building activities, and tectonic-climate interactions. This chapter presents the journey of Himalayan rocks through Columbian, Rodinia, and Gondwana supercontinent cycles to the present, as its litho units consist of the record of magmatism and sedimentation since ~2.0 Ga. The making of the Himalayan orogen started with the rifting of India from the Gondwanaland and its subsequent movement toward the Eurasian Plate, which led to the closure of the Neo-Tethyan ocean in the Late-Cretaceous. India collided with Eurasia between ∼59 Ma and ∼40 Ma. Later, the crustal thickening and shortening led to the metamorphism of the Himalayan crust and the development of the north-dipping south verging fold-and-thrust belt. The main phase of Himalayan uplift took place during the Late-Oligocene-Miocene. This chapter also provides insights into the prevailing kinematic models that govern the deep-seated exhumation of Himalayan rocks to the surface through the interplay of tectonics and climate.

**Keywords:** Himalaya, crustal evolution, supercontinent cycles, tectonics

#### **1. Introduction**

*"Present is the key to past,"* the fundamental "*Uniformitarian Principle"* given by Scottish geologist James Hutton [1] holds even today after two centuries when we try to understand the Earth's crust and its evolution in various orogenic belts. Nature has preserved the information about the history of the Earth in various rocks that geologists extract through advanced techniques of geochemistry, geochronology, thermochronology, structural geology, stratigraphy, geophysics, glaciology, climatology, and atmospheric sciences. In this context, Himalayan orogen plays a significant role in understanding the crustal evolution as its rocks provide a vast range of magmatism and sedimentation records from ~2000 to 8 Ma [2–5]. Thus, Himalayan rocks are not only significant in the understanding of past supercontinental cycles but also play an important role to evaluate feedback processes between the lithospheric deformation, atmospheric circulation, tectonic uplift, global climate change, exhumation, and erosion from millennial to decadal scales [6–12]. Such feedback lays the foundations to understand and mitigate natural hazards, such as floods, landslides, and earthquakes, which bear societal relevance. Given

this background, the present chapter focuses on the crustal evolution, deformation, and exhumation of the Himalayan rocks through time since Proterozoic. We evaluate and summarize how the Himalayan rocks have evolved since the Columbian supercontinental cycle to the loftiest and tallest mountain belt of the world in Cenozoic based on geochronological, structural, metamorphic, and thermochronological record. In addition, we also provide insight into the formation of the India-Asia collision zone that resulted from the continent-continent collision between the Indian and Eurasian/Asian Plate through the closure of the Neo-Tethyan Ocean in the Late-Cretaceous-Eocene.

#### **2. Overview of the Himalayan orogen**

The Himalayan orogen is one of the youngest continent-continent collisional mountain belts on the Earth [13]. The orogen is a part of the greater Himalayan-Alpine system, which extends from the Mediterranean Sea in the west to the Sumatra arc of Indonesia in the east over a distance of >7000 km. The composite belt had evolved since the Paleozoic when the Tethyan Ocean closed between two converging landmasses of Eurasia and India. The collision of India and Eurasia took place between ∼59 Ma and <40 Ma [14–23] and it was brought about by rifting of India from Africa and East Antarctica during the Mesozoic. The convergence of the Indian landmass is continuing toward the north relative to stable Eurasian landmass forming an orogenic wedge to the south of the Tibetan Plateau [24].

The Himalayan mountain belt with ~2500 km long arc stretches between the structural syntaxial bends of Nanga Parbat in the west to the Namche Barwa in the east (**Figure 1**). The Gangdese Shan, Karakorum Mountains, also known as Trans-Himalaya [25], and Tibetan Plateau lie in the north of the Himalaya. In the west of the range lies the Hindu Kush Mountains and to the east, the Indo-Burma ranges, also known as the Rongklang range [9, 26]. The Indo-Gangetic plains/depression lies in the south of the raised Himalayan front. The arc can be further divided into western, central, and eastern sectors (**Figure 2**). The width of the Himalayan Mountain

**Figure 1.** *Topographic map of Himalayan orogen (source: www.wikimapia.org).*

*Crustal Evolution of the Himalaya since Paleoproterozoic DOI: http://dx.doi.org/10.5772/intechopen.104259*

**Figure 2.**

*Geological map of the Himalaya showing main tectono-stratigraphic units (after ref. [3]).*

is at its narrowest (100–150 km) in the central sector [9]. The longitudinal river system in the central Himalayas is responsible for the presence of the world's deepest canyons and eight out of the 10 highest mountain peaks on Earth. However, the average relief is significantly less in the western and eastern Himalayas.

#### **3. Geological setting**

The Himalaya has been divided into four tectonostratigraphic domains along the entire arc between Indo-Gangetic Plains in the south and Indus Tsangpo Suture Zone (ITSZ) in the north [3, 10, 27–31]. These are (a) Sub-Himalaya, (b) Lesser Himalayan Sequence (LHS), (c) Greater Himalayan Sequence (GHS)/Higher Himalayan Crystallines (HHC), and (d) Tethyan Sedimentary Zone (TSZ) (**Figure 2**). These domains are characterized as south-vergent fold and thrust belts that have emerged as a result of crustal shortening and thickening.

#### **3.1 Sub-Himalaya**

The Sub-Himalaya represents the southernmost part of the Himalayan orogen. The Main Frontal Thrust (MFT) separates it from the Indo-Gangetic Plains to the south, while the Main Boundary Thrust (MBT) separates it from the LHS to the north (**Figure 2**). It comprises marine sediments of Paleocene-Eocene and sediments from the continental origin of Miocene-Pliocene [32]. The marine sediments comprising shale, sandstone, and limestone are known as Subathu Formation, while the sediments of continental origin are characterized as Dagshai, Kasauli, and Siwalik group of rocks. The sedimentation record of Subathu formation is described to be ~61.5–43.7 Ma from magnetostratigraphic data [32]. The age of Dagshai formation has been estimated to be ~32–25 Ma, followed by overlying Kasauli formation of ~32–22 Ma and Siwalik sediments of ~14 Ma [33–37]. The thickness of this sequence is ~9–10 km, which was deposited by southward flowing river systems of

the Himalaya, forming the erosional history of the orogen [38]. The MFT, bounding the Himalayas to the south is commonly expressed as a zone of folds and blind thrusts [39, 40], which was active during the Pliocene-Holocene [41].

#### **3.2 Lesser Himalayan sequence**

The LHS is bounded by MBT to the south and Main Central Thrust (MCT) zone to the north and consists of three sub-units from south to north [3, 42]. These are: (a) Outer Lesser Himalayan (oLH) belt; (b) Lesser Himalayan Crystalline (LHC) nappe, and (c) Inner Lesser Himalayan (iLH) belt (**Figure 2**). The oLH represents Neoproterozoic-Paleozoic-Mesozoic-Eocene sedimentary sequence between MBT and the Tons Thrust (TT)/North Almora Thrust (NAT). These sequences are locally known as Shimla-Jaunsar (comprising of Mandhali, Chandpur, and Nagthat Formations)-Blaini-Krol-Tal Groups in the NW Himalaya [43]. The detrital zircon U-Pb geochronological ages with the oLH belts are 0.95 Ga in the oldest Mandhali Formation, 0.88 Ga in Chandpur Formation, 0.82 Ga in Nagthat Formation, 0.70 Ga in Balini diamictite and Krol sandstone, and 0.525 Ga in lower Tal trilobite bearing strata [44–47]. These ages provide the maximum timing of their deposition from the source rock and are considered synchronous with the Paleozoic magmatism in the source region.

The LHC nappes are basically the synformal klippe that are thrust over LHS and are equivalent to the GHS rocks, which form their root zone. These are locally named as Jutogh-Ramgarh-Almora-Askot-Chiplakot nappe in the western Himalaya and Kathmandu nappe in the central Nepalese Himalaya. The mylonite orthogneiss of Kulu-Ramgarh Nappe provides the oldest zircon U-Pb age of ~1.85 Ga, which is overlain by the Nathuakhan Formation of ~0.80 Ga [48, 49]. The Almora nappe in the Kumaun region of western Himalaya is characterized by equivalent ~1.85 Ga of mylonite granite-gneiss at its base and younger populations of ~0.85 to 0.58 and 0.55 Ga from garnetiferous-quartzite-schists and intrusive granites, respectively [49, 50].

The iLH belt is the Paleoproterozoic meta-sedimentary sequence between TT/ NAT and MCT and represents the oldest and lowermost sequence of the LHS that was deposited between ~1.90 and 1.80 Ga as constrained by detrital zircon Geochronology [42, 47, 48, 51]. It is noteworthy that the rocks of iLH are characterized by fewer older minor peaks also between ~2.4 and 2.6 Ga [2, 52–59]. These rocks are locally known as Rautgara-Gangolihat-Deoban-Berinag Groups in the western Himalaya, Kushma Group in Central Nepal, and Daling-Shuma Groups in the Eastern Himalayas of Bhutan and Arunachal Pradesh [43, 53, 55, 60]. It is significant to note that there has been a stratigraphic break of nearly ~1 Ga between the timing of deposition of the iLH and oLH [3].

#### **3.3 Greater Himalayan sequence/higher Himalayan Crystallines**

This sequence represents the Himalayan orogen's backbone, which exhibits the most uplifted and most eroded part of the orogen. The sequence is bounded by the South Tibetan Detachment System (STDS) in the north, which separates it from the TSZ. The Munsiari Thrust (MT)/MCT forms the southern boundary of this sequence, where it abuts against the iLH rocks. The ~15 to 20 km thick sequence is divided into two main groups: (a) Munsiari Group/MCT zone and (b) Vaikrita Group. The Munsiari Group overrides the iLH along the MT/MCT 1 or lower MCT (in Nepal Himalaya)/MCT [3, 9, 23, 25, 43, 61–63] and contains mylonitized and imbricated Paleoproterozoic megacryst granite gneiss, fine-grained biotite paragneiss, garnetiferous mica schist, phyllonite and sheared Amphibolite [3]. Based on

#### *Crustal Evolution of the Himalaya since Paleoproterozoic DOI: http://dx.doi.org/10.5772/intechopen.104259*

the geochemistry and geochronological studies, these rocks have been part of the Paleoproterozoic magmatic arc [42], as most of the zircon U-Pb ages of these rocks lie between ~1.97 and 1.75 Ga [55, 59, 60, 64–69], that is, similar to iLH rocks.

The Vaikrita Group consists of amphibolite facies to migmatitic ortho- and para-gneisses rocks and characterizes typical inverted metamorphism [70–80]. The Vaikrita Thrust (VT)/MCT 2 or upper MCT (in Nepal Himalaya) forms the base of this group while these rocks abut against the TSZ rocks along the STDS. This group is different from the Munsiari Group and iLH rocks as the rocks of the Vaikrita Group provide characteristic Neoproterozoic zircon U-Pb ages between ~1.05 and 0.80 Ga with fewer peaks at ~2.50 and ~ 1.80 Ga [3, 4, 60, 69, 81–83].

The GHS generally forms a continuous belt along the entire length of the orogen. Still, it also occurs as isolated patches surrounded by low-grade Tethyan strata, such as in the Zanskar and Tso Morari regions of NW India and in the Nanga Parbat massif of northern Pakistan [84–86]. This coeval slip along the MCT and STDS during ~20 to 15 Ma is responsible for the ductile extrusion of the GHS/HHC rocks between these bounding fault zones [30, 87–89]. It is noteworthy that all the northdipping faults in the Himalaya sole into a mid-crustal décollement at depth, the Main Himalayan Thrust [MHT, 90], which lie over the Indian basement.

Apart from the Munsiari and Vaikrita Group of rocks, the presence of Cambro-Ordovician granitoids are unique within the HHC (**Figure 2**). These granitoids belong to Pan-African magmatism and lie to the north of *sensu-stricto* the MCT [9, 91]. These are locally named Central gneiss, Dalhousie, Chauri, Dhauladhar, Palampur, Mandi, Pandoh, and Karsog Granite [27, 92–94]. However, these Paleozoic granites occur sparsely also within the Lesser Himalaya, Tethys Himalaya in the Karakoram and Tibet [95–97]. Geochemical analysis of these granitoids suggests that these rocks were formed in a syn-collision environment and have peraluminous (S-type) and mildly metaluminous (I-type) affinities [98]. Few occurrences of these granitoid bodies exhibit mild alkaline nature that was formed in a post-collision, anorogenic setting [99, 100]. In general, these granitoids belong to early Paleozoic magmatism (ca. 475 Ma) as reported through whole-rock Rb-Sr isochron age U-Pb zircon geochronological data [4, 91, 101–103].

The Proterozoic rocks of GHS have undergone crustal thickening and shortening, metamorphism, and partial melting during Himalayan orogeny, that is, post-India-Asia collision [9]. The leucocratic magmatism in the Himalayas, mainly by muscovite dehydration melting, can be traced along the entire arc of the orogen in the GHS and as well as TSZ [60, 69, 104–106]. In the late stage, the high-grade metamorphic rocks of the GHS exhumed to the surface through the interplay of tectonics and climatic processes [10–12, 107].

#### **3.4 Tethyan sedimentary zone (TSZ)**

Late Precambrian to Eocene siliciclastic and carbonate sedimentary rocks interbedded with Paleozoic and Mesozoic volcanic rocks are exposed to the north of GHS, mainly forming the TSZ [9, 108–119]. It is bounded in the north by Great Counter Thrust where it is juxtaposed with Tso Morari Crystallines and/or ITSZ rocks, and in the south by north-dipping STDS. The STDS is locally named as Zanskar Shear Zone (ZSZ) [79, 120], Rohtang Shear Zone (RSZ) in Himachal Pradesh [121], Trans-Himadri Fault (THF) in Kumaon Himalaya [122], and STDS in Nepal Himalaya [87, 123]. The TSZ has been divided into four subsequences [9]: (a) Early Cambrian to Devonian pre-rift sequence characterized by lithologic units deposited in epicratonal setting, (b) Carboniferous-Lower Jurassic rift and post-rift sequence, (c) Jurassic-Cretaceous passive continental margin sequence, and (d) Cretaceous-Eocene syn-collisional sequence. These rocks form the cover sequence

of the GHS and are also known as Haimanta Formation in the NW Himalaya that yields detrital zircon U-Pb ages between 0.55 and 3.0 Ga [3, 69, 121].

#### **4. Crustal evolution of Himalayan rocks since Paleoproterozoic**

#### **4.1 Paleoproterozoic**

The iLH and MCT zone represent the oldest terrane of the Himalayan rocks that were formed during the Paleoproterozoic. The rocks of both of these terranes have been hypothesized to be a part of an Andean-type arc system that formed during the assembly of the Columbian supercontinent at ~1.9 Ga (**Figure 3a**) [3, 42, 124–128]. The assembly of Columbia involved North America (NA), Eastern Antarctica (EA), North China (NC), and India (I) continents that formed the arc system. The granitoids of the MCT Zone/Munsiari Group were formed due to hydrous partial

#### **Figure 3.**

*Reconstruction of Columbia supercontinent ca. 1.9 Ga showing position of India and the Proterozoic magmatic arc (after ref. [3]). (a) Reconstruction showing continents of NA-North America, EA-eastern Antarctica, NC-North China, I-India. (b) Position of the Proterozoic magmatic arc, its configuration of forearc (blue), backarc (yellow), and the position of India with the (i) Aravalli, (ii) Bundelkhand, and (iii) Singhbhum cratons. Cross-section along XY. (c) Reconstruction showing the evolution of the MCTZ and iLH as the Paleoproterozoic magmatic arc and backarc basin, respectively, during ~2.0–1.8 Ga. Subducted and partially melted oceanic lithosphere caused the emplacement of arc granitoids. The iLH sediments were deposited in rifted back-arc basin and received sediments, from both the arc, that is, MCT zone and Indian craton.*

*Crustal Evolution of the Himalaya since Paleoproterozoic DOI: http://dx.doi.org/10.5772/intechopen.104259*

#### **Figure 4.**

*A possible model for the deposition of iLH and MCT zone rocks in a rift and passive continental margin set up as originated from the mantle plume (after ref. [129]).*

melting of the older crust that involved mafic sources and sediments from the subduction zone (**Figure 3b** and **c**). The volcano-sedimentary sequence of iLH, forming Rautgara-Gangolihat-Berinag, their equivalent formations, was deposited during ~2.0–1.8 Ga in the rifted back-arc basin (**Figure 3c**) [3]. The sediments in this back-arc basin were supplied from both the Proterozoic magmatic and Indian Shield (**Figure 3c**). However, some workers believe that the iLH and MCT zone rocks were part of a rift and passive continental margin set up that was originated from the mantle plume (**Figure 4**) [129, 130]. In this hypothesis, the ~2.0 to 1.8 Ga rocks of the iLH and MCT zone are considered equivalent to the Paleoproterozoic Coronation Supergroup in the Wopmay orogen, northwest Canada [131, 132].

#### **4.2 Neoproterozoic**

The Columbian supercontinent broke up during the Neoproterozoic, resulting in the separation of the Indian craton from Columbia, which thus later became a passive margin along its northern limit. India had reassembled again within a short duration of 1 Ga at ca. 1.1–0.9 Ga with Madagascar, Seychelles, Karakoram Terrane/Pamir, Tarim in the west, South China in the north, Australia in the east, and East Antarctica in the southeast, respectively, forming the Rodinia Supercontinent (**Figure 5**). The Neoproterozoic granitoids have been recognized in the GHS throughout the Himalayan arc and are associated with the Rodinia Supercontinent assembly [60, 66, 81, 134–137]. Apart from the magmatic origin of granitoids within the GHS, the sediments containing detrital zircons of ca. ~1.1 to 0.8 Ga in the Vaikrita Group and the oLH sequences have been sourced from (a) Within-basin magmatic bodies, that is, granites intrusive and orthogneisses, for example, Peshawar, Black mountain of Western Himalaya syntaxis, Chor region of Himachal Himalaya, and Cona, Bhutan and Hapoli regions of Arunachal Himalaya [3], (b) "In-board" Indian Craton, that is, Aravalli Delhi Mobile Belt (ADMB) and Central Indian Tectonic Zone (CITZ), which is collectively known as Great Indian Proterozoic Fold Belt (GIPFOB, **Figure 6**) [3], and (c) external "Outboard" terranes of Nubian-Arabia, Africa, Madagascar, eastern Antarctica, Australia, that is, those belonged to Rodinia Supercontinent assembly [45, 53, 81], through erosion and transportation of sediments by long paleo-river systems.

#### **Figure 5.**

*Reconstruction of Rodinia supercontinent showing the position of India (after refs. [4, 133]).*

#### **Figure 6.**

*Neoproterozoic detrital zircon in the great Himalayan sequence (GHS) and correlatable successions in the lesser Himalaya are sourced from various parts of the Indian craton (after ref. [3]).*

#### **4.3 Late Neoproterozoic to Cambro-Ordovician**

Rodinia supercontinent broke up during 750–600 Ma, which led to the pathway for the formation of Gondwanaland during the Cambrian–Ordovician (**Figure 7**) [e.g., 133, 138, 139]. India was a part of the Gondwanaland that also consisted of South *Crustal Evolution of the Himalaya since Paleoproterozoic DOI: http://dx.doi.org/10.5772/intechopen.104259*

**Figure 7.**

*Position of India during the Gondwana supercontinent assembly in Cambro-Ordovician (after refs. [4, 91]).*

America, Africa, Madagascar, Australia, and Antarctica [140]. Together, these continents formed a subduction system along the northern margin of the Gondwanaland [141–144]. Thus, a thermal event associated with the Pan-African orogeny during the Cambro-Ordovician resulted in the formation of granitoids, such as Dalhousie, Chauri, Dhauladhar, Palampur, Mandi, Pandoh, and Karsog Granite, presently within the GHS and TSZ of the Himalayan arc. Many researchers have proposed the Cambrian-Ordovician event as the pre-Himalayan metamorphic event that resulted in the crustal anataxis of the local Neoproterozoic crustal rocks during syn-to post-collisional crustal thickening, leading to the generation of S-type granitoids [91, 145–153].

#### **4.4 Silurian to cretaceous**

The Gondwanaland was the southern part of the most recent supercontinent Pangea. The Pangea attained its condition of maximum packing at ~250 Ma and started breaking up during ~250 to 230 Ma (**Figure 8**) [140, 154, 155]. The northern part was named Laurasia or Mega Laurasia and contained the northern continents—North America, Greenland, Europe, and northern Asia. The presentday Karakoram Terrane forms the south-western margin of the Tibetan Plateau. It is equivalent to the SE-Pamir terrane and Central Pamir terrane in the west and

**Figure 8.** *Paleogeographic map showing the break-off of the Cimmerian terranes from Pangea (based on refs. [140, 154, 155]).*

Qiangtang terrane in the east, which belonged to Gondwanan ancestry (**Figure 9**). These terranes of Central Pamir, SE-Pamir, Karakoram, and Quiangtang got separated from Gondwana during Permian due to the rifting process that formed the part of the Cimmerian belt [156]. This event resulted in the opening of the Neo-Tethys Ocean (**Figure 10a**). This event was followed by the accretion of these terranes of the Cimmerian belt with the Asian Plate along the Jinsha Suture Zone (JSZ) during the Middle-Cretaceous or maybe earlier [157, 158], resulting in the closure of Paleo-Tethys Ocean (**Figures 9** and **10b**). Initially, the Central Pamir and SE-Pamir were accreted along the Rushan-Pshart suture during Triassic-Jurrasic (**Figure 9**), with slightly later accretion of Southern Pamir and the Karakoram along Tirich Boundary Zone (TBZ) (**Figure 9**) [159].

The Neo-Tethyan ocean closed due to the rifting of the Indian Plate from Gondwana and its subsequent journey toward the Asian Plate (**Figure 10b**). The interoceanic Dras volcanic island arc was formed by the initial subduction within the Neo-Tethyan Ocean during Middle Jurassic (**Figure 11a**) [23]. The closure of the Neo-Tethyan ocean resulted in the subduction of the Neo-Tethyan oceanic lithosphere below the southern margin of the Asian Plate along the Shyok Suture Zone (SSZ) in the north-western domain of Karakoram and along the BNS in central Tibet (**Figure 11b** and **c**) [160, 161]. The formation of calc-alkaline continental arc magmatism at ca. ~205–100 Ma due to the subduction of Neo-Tethys oceanic lithosphere along the SSZ produced Karakoram Batholith on the southern margin of the Asian Plate [160–165]. The final collision between the Indian and Asian plates occurred along the ITSZ that was accompanied by the formation of the Kohistan-Ladakh arc (KLA). Subsequently, the formation of ophiolitic and sedimentary sequences took place along the ITSZ (**Figure 11**) [23, 162, 166, 167]. The KLA

*Crustal Evolution of the Himalaya since Paleoproterozoic DOI: http://dx.doi.org/10.5772/intechopen.104259*

#### **Figure 9.**

*Tectonic map of the Himalayan-Tibetan orogenic belt showing a present-day configuration of the Karakoram, SE-Pamir terrane, and central Pamir, which were part of the Cimmerian terrane, rifted from Gondwana and led to the opening of neo-Tethys Ocean (after, ref. [4]).*

#### **Figure 10.**

*(a) Palaeotethys Ocean is getting closed as Cimmerian terrane is approaching toward Eurasia. Please note the position of India, that is, the position of rifting from Gondwana land, (b) Cimmerian terrane that includes the Karakoram as a part of Eurasia. Note the formation of Dras island arc. India is moving toward Eurasia (based on refs. [140, 154, 155]).*

witnessed a major episode of subduction-related magmatism at ∼85 to 40 Ma with small pulses at ∼110 to 100 Ma. The magmatism led to the emplacement of Andeantype plutons during the Late Cretaceous to middle Paleogene [23]. These magmatic rocks are collectively known as the Ladakh Batholith in the western Himalayas, Gangdese Batholith in Tibet, and Lohit Batholith in the Eastern Himalayas [9]. The SSZ closed at ca. 85 Ma through the juxtaposition of the Ladakh arc and Karakoram batholiths [160, 168]. The palaeomagnetic anomalies in the Indian ocean suggest that the convergence of the Indian plate slowed down at ~55 ± 1 Ma [18]. The palaeolatitude evidence suggests that Tethyan succession in the Himalayas overlaps with the Lhasa terrane overlap at 22.8 ± 4.20 N palaeolatitude at 46 ± 8 Ma [169, 170].

#### **Figure 11.**

*Schematic model showing the stages for the collision of Indian and Asian plate (a) Dras-Shyok volcanic formed due to subduction of oceanic lithosphere within the neo-Tethys Ocean arc during middle Jurassic to late cretaceous and formation of Karakoram batholith during early cretaceous due to subduction of Tethyan oceanic lithosphere beneath the southern Asian plate margin along SSZ; (b) closure of SSZ due to collision of Dras volcanic arc and Karakoram terrane during late cretaceous; (c) formation of Ladakh batholith due to subduction of Tethyan oceanic lithosphere below Dras volcanic arc during late cretaceous (after ref. [23]).*

The final closure of the Indian Plate with the Asian Plate took place along the ITSZ between ∼59 Ma to <40 Ma [14–23].

#### **4.5 Cenozoic**

The Cenozoic era represents the main phase of Himalayan orogeny. In the initial stage (~45–23 Ma, **Figure 12a**), after the India-Asia collision, crustal shortening and thickening led to the early prograde regional metamorphism mainly during ~45 to 35 Ma under ~8 to 11 kbar and ~ 600 to 700°C [9, 30]. In this phase, thrusting along the STDS started [9, 171]. This was the time when the GHS/LHS were covered by the TSZ and Gondwana sediments. The initiation of MCT led to the emplacement of the GHS over the LHS during the Early Miocene, that is, at ~23 to 18 Ma (**Figure 12b**) [9, 172–174]. The younger event of metamorphism at ~23 to 15 Ma at ~6 to 8 kbar and ~ 500 to 750°C is considered to be synchronous with the ductile deformation along the MCT zone [9, 30]. The intense ductile shearing caused the formation of the inverted metamorphism sequence across the GHS along with Miocene leucogranites generation [9, 78, 104, 175–181]. The MCT forms the roof of the major thrust duplex within the LHS (**Figure 12c**) [60, 83, 174]. The LHS duplex formation led to erosion of the GHS and the formation of the antiformal stack in the form of LH window zones and synformal nappes in the Himalaya, mainly during ~18 to 14 (**Figure 12c**). Thus, the HHC and LH Window zones became the fastest exhuming bodies, with exhumation rates up to ~3 mm/yr., in the Himalayas since Miocene [10–12, 107, 121, 182]. Later, the activation of the MBT at ~10–12 Ma led to the thrusting of the LHS over the Sub-Himalaya [183–185], while the deposition of Siwalik sediments initiated at ~14 Ma (**Figure 12d**). In the Plio-Quaternary, the initiation of MFT took place that forms the southernmost boundary of the Himalaya and juxtaposes the older rocks of Sub-Himalaya along the modern Indo-Gangetic alluvium (**Figure 12e**). In this phase, the MCT also got reactivated in an out-of-sequence manner that led to the rapid exhumation of the rocks of the MCT zone [186–188].

Apart from the aforementioned general model for the Himalayan evolution, it is noteworthy that the ductile extrusion of the GHS has been explained by three main models. The Channel flow-focused denudation model [8] considers the GHS as a partially molten lower/middle crust that extruded southward from Tibet during Eocene-Oligocene via the formation of the pressure gradient between Tibet and India due to the high elevation of the Tibetan plateau (**Figure 13a**). Wedge-extrusion model states that the MCT and STDS form the tapered core (**Figure 13b**). The gravitational

*Sketch showing the general model for the structural evolution of the Himalayan orogen during the Cenozoic (based on refs. [9, 171]).*

collapse of over thickened continental crust resulted in the development of the STDS [189]. The tectonic wedging model, in which TSZ abut against the LHS (e.g., in the Himachal Pradesh, India) along the MCT due to its termination against the STDS. Thus, the GHS core remains at depth and subsequently forces itself toward the surface (**Figure 13c**) [190]. Thus, fault kinematics, that is, thrusting, folding, gravitational unloading, the geometry of the subsurface in combination with intense orographic precipitation, controlled the Cenozoic development of the Himalayan orogen.

**Figure 13.**

*Tectonic models for the emplacement of the HHC: (a) channel flow model; (b) wedge extrusion model and (c) tectonic wedging (after ref. [69]).*

#### **5. Conclusion**

The Himalayan crust has evolved through multiple stages of supercontinent cycles since ~2.0 Ga. The iLHS and the MCT zone represent the oldest crust that was formed during the Columbian supercontinent assembly. The rocks of the MCT zone have been derived as Andean-type magmatic arc, while the iLHS was evolved as a back-arc basin, the sediments of which were supplied from both the MCT zone and Indian craton. There is no tectonostratigraphic evolutionary record available for ~1 Ga between the timing of deposition of iLHS rocks (~1.85 Ga) and the deposition of the Vaikrita Group of GHS and oLH (~0.85 Ga). Thus, the northern boundary of India was a passive margin before the deposition of GHS and oLH during the Rodinia supercontinent assembly. The Paleozoic granitoids (~0.48 Ga) within the GHS/TSZ represent the record of pre-Himalayan metamorphism during Pan-African orogeny that formed the Gondwanaland as the southern part of Pangea. The Gondwanaland broke up at ca. 230 Ma as the Cimmerian belt consisting of Central Pamir, SE-Pamir, Karakoram/Quiangtang terranes rifted and moved toward the Asian Plate and led to the closure of Paleo-Tethys ocean and opening of Neo-Tethys ocean. India rifted apart from the Gondwanaland at ~230 to 200 Ma and traveled toward Asia, leading to Neo-Tethys ocean's closure. The Neo-Tethys ocean closed along the SSZ at ~85 Ma by forming the junction between the Asian margin and Dras Island Arc/Ladakh Batholith. The northern margin of the Indian continental crust closed along the ITSZ at <40 Ma, the Ladakh Batholith being its northern boundary. The major event of metamorphism and deformation of the Himalayan crust occurred since Eocene-Oligocene, leading to the formation of north-dipping thrust sheets along with MCT, MBT, and MFT. The fault kinematics, that is, thrusting and folding combined with climatic erosion, led to the exhumation of high-grade metamorphic rocks to the surface [9, 12].

#### **Acknowledgements**

We thank the editor Sara Debeuc for the invitation to submit the chapter on Himalayan Crustal Evolution. This work is supported by the CAP Himalaya grant (Activity 7) to V. Adlakha. Prof. A.K. Jain and Shailendra Pundir are thanked for fruitful discussions, informal review, and sharing their figure drafts. Kunal Mukherjee is thanked for extending his help during the final compilation and formatting work. Prof. Nand Lal and R. C. Patel are thanked for their constant

encouragement. K. Sain acknowledges the SERB-DST for awarding him with the J.C. Bose National Fellowship.

### **Conflict of interest**

The authors declare no conflict of interest.

### **Author details**

Vikas Adlakha and Kalachand Sain\* Wadia Institute of Himalayan Geology, Dehradun, India

\*Address all correspondence to: kalachandsain7@gmail.com

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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#### **Chapter 2**

## The Breaking of the Iranian Block during the Cretaceous and the Opening of New Oceanic Basins within the Tethys Ocean: The Case of the Sabzevar-Nain Basin and Its Geodynamic History

*Saidi Abdollah, Khan Nazer Nasser, Hadi Pourjamali Zahra and Farzad Kiana*

#### **Abstract**

The Jurassic subduction of the Neo-Tethys oceanic crust under the western continental margin of the Iranian Block has led to the fragmentation of the Iranian Block in the back-arc basin, leading to the opening of three oceanic basins around it. The ophiolitic belts surrounding central Iran are the indicators of the closure of these basins. The Sabzevar-Nain Basin is one of these basins, which has been created between the micro-block of central Iran in the south and the Alborz Mountain Ranges in the north. This basin opened in the late Jurassic as a rift and then became a trough in the early Cretaceous. Finally, this basin developed into an oceanic basin in the early late Cretaceous. The sedimentation in this basin can be divided into pre-rift, syn-rift and oceanic environments. All of these sediments are strongly folded and faulted. The closure of this basin started during the Paleocene with a subduction under the southern margin of the Alborz Mountain Ranges. The collision event between the northern margin of the micro-block of central Iran and the southern margin of the Alborz Mountain Ranges occurred in the early Eocene. The result of this event was the creation of a wide collision zone, forming a volcanic arc and a back arc basin on the active of the Alborz Mountain Ranges, an ophiolitic belt, and post- collision intrusion masses that appear everywhere in the collision zone. In the point of lithology, these intrusion masses are composed of granite, diorite, and granodiorite. The magmatic activities that started in the Paleocene-early Eocene continued until early Quaternary.

**Keywords:** geodynamics, ophiolite, collision, volcanic arc, syn-rift, foreland basin, back-arc basin

#### **1. Introduction**

It is possible to express the evolution of the Iranian crust and basins by looking at their geology and geodynamic history: the consolidation of the Iranian

basement in the Gondwana mega continent; the Precambrian magmatism and metamorphism; deformation and folding in the Precambrian and early Paleozoic rocks; crustal thinning due to an extensional state during the early Paleozoic (Neo-Tethys rifting); rifting in the eastern part of Gondwana in the early Paleozoic [Silurian], (**Figure 1a**); the northward subduction of the paleo-Tethys oceanic crust under the southern margin of the Eurasia supercontinent in the early Paleozoic (**Figure 1b**); the early Cimmerian collision of the Iranian Block with the Turan Block (the southern margin of Eurasia) in the middle Paleozoic (**Figure 1c**); the late Paleozoic-early Jurassic crustal thinning with an instability period in the sedimentary basin after the early Cimmerian orogenic event (**Figure 1d**). The great event on the Iranian Block occurred after the oceanic crust subduction of the Neo- Tethys Ocean under the western margin of the Iranian Block. This subduction led to the global extension and fragmentation of the Iranian crust during the late Jurassic. After this extension and thinning, the crust of the Iranian Block broke in the back-arc basin of the Neo-Tethys Ocean in the early-middle Cretaceous. This event led to the creation of inner oceanic basins such as the Sabzevar-Nain, Nain-Baft, and Sistan-Baluch Basins (**Figure 2**) [1–5]. The ophiolitic belts surrounding central Iran are the indicators of the closure of these basins. They were over thrust on the continental margins accompanied by other materials of the accretionary prism and the continent-continent collision between the adjacent blocks.

In this paper, the authors tried to discuss the geochronology of the opening and closing processes of the Sabzevar-Nain Basin situated between the southern margin of the Alborz Mountain Ranges and the northern margin of

#### **Figure 1.**

*A brief history of the Iranian block evolution from the Precambrian to the late Triassic-early Jurassic; a: The rifting event in the eastern part of Gondwana; b: The subduction of the paleo-Tethys slab under the Turan block (the southern part of Eurasia); c: The early Cimmerian collision of the Iranian block with the Eurasia mega-continent and the development of the neo-Tethys Ocean; d: The crustal thinning and instability in the sedimentary basin after the early Cimmerian collision. GSC: Gondwana supercontinent; PTR: Paleo-Tethys rift; MLC: Mega Lhasa continent; IB: Iranian block; APM: Alborz passive margin; PTOC: Paleo-Tethys oceanic crust; PTT: Paleo-Tethys trench; KMA: Kopeh Dagh magmatic arc; EMC (TB): Eurasia megacontinent (the Turan block); ECCZ: Early Cimmerian collision zone; AP: Arabian plate; ZPM: Zagros passive margin; NTOC: Neo-Tethys oceanic crust; NTT: Neo-Tethys trench; PTS: Paleo-Tethys suture; ECC: Eurasia continental crust.*

#### *The Breaking of the Iranian Block during the Cretaceous and the Opening of New Oceanic… DOI: http://dx.doi.org/10.5772/intechopen.105440*

the micro-block of central Iran (**Figure 3**). The Sabzevar-Nain Basin (SNB) was formed due to an extensional system with crustal thinning accompanied by a north-south rifting which occurred in the back-arc basin of the Neo-Tethys Ocean. The large number of geological structures in this region has motivated many geologists to conduct their researches in it. Several Ph.D. dissertations [including Sadreddini [6], Alavi Tehrani [7], Dehghani [8], and Noghreyan [9]] have been conducted on the oceanic crust remnants (ophiolites) in the Sabzevar region. The first study on the Sabzevar ophiolites was conducted by Sadreddini [6]. This study depended on the petrographic characteristics of the ophiolites in the middle part of the ophiolitic range of Sabzevar. Alavi Tehrani [7] studied the geology and petrology of the ophiolitic rocks in the northwest of Sabzevar. Dehghani [8] described the gravity field and structure of the Iranian crust. Noghreyan [9] stated that the ophiolitic belt of Sabzevar was formed due to an immature arc. These studies were carried out in the frame of the Geological Survey of Iran called the 'Geodynamic Project, [10–12]. The ophiolitic belt was distinguished by rock units including harzburgite, intrusive rocks, a sheeted dyke complex, volcano-sedimentary sequences, an ophiolitic mélange (composed of ophiolitic rocks, intrusive rocks, and oceanic sediments), and metamorphic rocks. The cover rocks were characterized by a Cenozoic sequence and Quaternary rocks. Lensch and Davoudzadeh [13] identified three types of ophiolitic rocks around the micro-block of central Iran including an ophiolitic mélange, ridge type ophiolites, and trench type ophiolites. Baghdadi [14] related the volcanism of the northern part of Sabzevar to a subduction process during

#### **Figure 2.**

*The breaking of the Iranian block and the creation of rifts around the micro-block of Central Iran (a), the locations of the three new basins around the micro-block of Central Iran in the back-arc of the neo-Tethys Ocean (b). AMR: Alborz Mountain ranges; AFB: Afghan block; CIB: Central Iran block; UDMA: Urumieh-Dokhtar magmatic arc; Alborz M.: Alborz Mountains; APM: Alborz passive margin; CIPM: Central Iran passive margin; NBB: Nain-Baft Basin; SNB: Sabzevar-Nain Basin; SBB: Sistan- Baluch Basin; APM: Afghan passive margin; AP: Arabian plate; NTMP: Neo-Tethys passive margin; NTMOR: Neo-Tethys mid-oceanic ridge; NTSZ: Neo-Tethys suture zone.*

#### **Figure 3.**

*The location of the oceanic crust remnants and the suture of the Sabzevar collision zone in the northern part of the micro-block of Central Iran: 1. Cenozoic volcanic rocks; 2. The intrusion of Mesozoic and Cenozoic granites and diorites; 3. The cretaceous-Paleocene ophiolites around the micro-block of Central Iran; 4. The ophiolites of the Zagros suture zone; 5. The ophiolites of the paleo-Tethys continental collision exposed in the north of Iran; 6: The Hormoz formation (pre- Cambrian) in the southeast of the Zagros Mountain ranges; 7: The trace of the paleo-Tethys suture.*

Eocene. Ghassemi and Rezaei-Kahkhaei [15] stated that these rocks were the result of the partial melting of an enriched mantle by an extensional process within the arc. Khalatbari and Etessami [16] worked on the petrology and tectonomagmatic setting of the Eocene volcanic rocks in the Semnan area. They concluded that these volcanic rocks are the result of a subduction event during the Paleocene. Based on her studies on the petrology, petrography, and geochemistry of the intrusive rocks of the Sabzevar-Nain collision zone, Goharshahi [17] concluded that their exposure is due to a subduction event and its continuation after the collision (post-collision intrusion).

During the years 1999–2002, 12 geological maps in the scale of 1:100,000 were prepared in the framework of a project in the Geological Survey of Iran. They contain important data about the petrology, stratigraphy, sedimentology, and structural geology of the geodynamic events.

#### **2. Geological setting**

The geological characteristics of the Sabzevar-Nain collision zone are described as pre- and syn-rift sediments, syn-rift magmatic activities, synsubduction oceanic sediments, syn-subduction magmatism, syn-collision volcanism, post-collision sediments, post-collision magmatism, and postcollision volcanism. The pre-rift sediments are a thick Paleozoic and Mesozoic

#### *The Breaking of the Iranian Block during the Cretaceous and the Opening of New Oceanic… DOI: http://dx.doi.org/10.5772/intechopen.105440*

sequence including the sediments of continental and platform environments. The youngest pre-rift sediments of this basin are late Jurassic-early Cretaceous carbonates which have a wide distribution in the southern part of the Alborz Mountains and the northern part of central Iran (Saidi and Akbarpour [18]; Saidi and Vahdati Daneshmand [19]). Their thickness changes from 450 m in the north of Damghan to 580 m in the south of Sabzevar.

The syn-rift sequence is composed of continental and detrital facies as well as thick-bedded and massive limestone of the platform environment (**Figure 4**). The other syn-rift sediments are thick highly deformed flysch facies consisting of a majority of calcareous shale, some sandstone, and a few limestone lenses. These facies show the high thinning of the continental crust at the time of sedimentation in the Sabzevar-Nain trough. The basalts in the magma chamber under the rift penetrated the broken crust and flowed on the basin floor. In the convergent system between the continental crusts of the Alborz Mountain Ranges and central Iran, these basalts were folded with the deposited sediments in the basin (Salamati and Shafei [20]; Kolivand [21]; Ghaffari Nik [22]) (**Figure 5**).

#### **Figure 4.**

*The two different facies of middle Cretaceous in the middle part of collision zone. The low lands are the synrift sediments derived from the continent (flysch) which are covered by upthrusting the thick bedded, massive limestone of platform environments (high lands).*

#### *Earth's Crust and Its Evolution - From Pangea to the Present Continents*

The pre- and syn-collision late Cretaceous oceanic deposits consist of shale, sandstone, tuffaceous shale, limestone, radiolarian shale, pelagic limestone (Campanian-Maastrichtian in age), tuff, pillow lava, and spilitic basalt. The thickness of these sediments is estimated to be 2000 m (**Figure 6**) [23].

The syn-subduction and syn-collision Paleocene-Eocene volcanic rocks of the volcanic arc in the Sabzevar region are composed of andesite, andesitic tuff, andesitic basalt, olivine alkali breccia, feeder dykes, basaltic andesite, and porphyritic andesite (**Figures 7** and **8**) [24].

The post-collision foreland basin deposits consist of conglomerate, sandy limestone, gypsiferous marl, tuffaceous shale, and sandstone (Eocene and Oligo-Miocene in age). The other sediments of this basin are Miocene sandstone, conglomerate, and gypsiferous marl (**Figures 9** and **10**).

The syn/post-collision intrusive rocks consist of Cenozoic granite, quartz diorite, and diorite (**Figures 11** and **12**).

#### **Figure 6.**

*The late Cretaceous oceanic deposits consist of shale, sandstone, pelagic limestone, radiolarian shale, pillow lava (Campanian- Maastrichtian).*

#### **Figure 7.**

*The morphology characteristics of the Paleocene-Eocene volcanic arc in the western part of Abbasabad (west of Sabzevar).*

*The Breaking of the Iranian Block during the Cretaceous and the Opening of New Oceanic… DOI: http://dx.doi.org/10.5772/intechopen.105440*

#### **Figure 8.**

*The syn-subduction and syn-collision Paleoceane-Eoceane volcanic arc parallel with the ophiolitic belt, north of Sabzevar.*

#### **Figure 9.**

*The brown well-bedded post collision foreland deposits, north west of Sabzevar, composed of conglomerate and sandstone.*

#### **Figure 10.**

*The dark brown thick bedded post collision foreland basin deposits, over the ophiolitic rocks of Sabzevar-Nain Suture (East of Davarzan).*

#### **Figure 11.**

*The great masses of syn/post collision intrusive rocks, consist of Cenozoic granitoid, diorite and granodiorite in Kuh-e-Baharestan (south of Sheshtamad).* 

**Figure 12.** *The granite masses of syn/post collision, in age of Paleocene in the Kuh-e-Mish (south of Sabzevar).* 

The most recent post-collision dacite domes (Miocene and Plio-Quaternary in age) intruded into the ophiolitic belt (**Figure 13**), andesitic lava, tuff, and dacitic lava flow of Sabzevar (**Figure 14**).

#### **3. Geodynamic setting**

The architecture of the present-day Sabzevar-Nain Basin reflects the extensive tectonic regime (**Figure 2a**) which has occurred since the early-middle Cretaceous [2, 4]. This basin has been formed along the east-west direction in the southern part of the Alborz Mountain Ranges and the northern micro-block of central Iran. The tectonostratigraphic sequences of the sediments deposited in the basin between the late Paleozoic and Cenozoic are shown in **Figure 15**. These sediments are composed of shallow water deposits of early Triassic before early Cimmerian orogenic events (the continent-continent collision of the Iranian Block and Eurasia) (**Figure 1c**).

*The Breaking of the Iranian Block during the Cretaceous and the Opening of New Oceanic… DOI: http://dx.doi.org/10.5772/intechopen.105440*

#### **Figure 13.**

*Youngest post collision dacitic domes penetrated into the ophiolitic rocks of Sabzevar.*

#### **Figure 14.**

*Young post collision dacitic domes penetrated into the foreland basin deposits, north of Mehr (North-West of Sabzevar).*

The early Triassic deposits are unconformably overlain by shale and sandstone detrital facies with an age of late Triassic-early Jurassic and from a new basin in extension after the above-mentioned orogenic events. A discontinuity can be observed in the sedimentary sequences between the late Jurassic and early-middle Cretaceous deposits (Late Cimmerian events). The first stage of rifting occurred in the early-middle Cretaceous in the basins around the central Iranian Block (**Figure 2b**) which were mechanically different from the Nain-Baft and Sistan-Baluch basins. The last two basins were created as pull-apart basins along two transform faults in the western and eastern parts of the micro-block of central Iran. The Sabzevar-Nain Basin was formed in the form of a classical continental rift (**Figure 16a**). This was the time for the micro-block of central Iran to be separated from the Alborz Mountain Ranges and to move toward the south.

During the middle Cretaceous, when a magma chamber was formed below the rift of Sabzevar-Nain, this basin became a large but not very wide trough (**Figure 16b**). After the process of intercontinental extension and the formation

#### **Figure 15.**

*The tectonostratigraphic sequences of the Iranian block before the middle Cretaceous segmentation.*

of the oceanic crust, the Sabzevar-Nain trough changed to an oceanic basin (**Figure 16c**). Simultaneously with the change of the divergent regime to the convergent regime during the Santonian-Campanian (86.3 ± 0.5–72.1 ± 0.2 Ma) [25], the oceanic crust of the Sabzevar-Nain Basin was subducted under the Alborz continental crust (**Figure 16d**). At this time, sedimentation reached its highest rate, whereas the intensity of deposition reached its highest rate in the Maastrichtian. Therefore, in 6.1 Ma, more than 578 m of sediments were deposited in the basin [23].

The process of subduction beneath the Alborz continental crust continued until the beginning of the early Eocene, while the margin of central Iran always remained a passive margin (**Figure 16e**). Since the early Paleocene, there have been highly intense volcanic and intrusive activities which can be observed throughout the collision zone especially in the south of Sabzevar. The oldest age determined for these volcanic and intrusive rocks is Paleocene [7, 9, 13, 14, 17, 26]. However, the age of the volcanic activity in the collision zone and the back-arc of the Sabzevar-Nain Basin varies from Paleocene to early Quaternary [20, 23, 24, 27–31]. The convergence and closure of the Sabzevar-Nain Basin could be due to the northward movement of the micro-block of central Iran. This event took place during the late Eocene-Oligocene (**Figure 16f**). The youngest deposits in the Sabzevar-Nain Basin are early Eocene flysch facies which were upthrust on both continental margins. These sediments are widely spread in the Nain region and the northwest of Bardeskan in the south of Sabzevar.

*The Breaking of the Iranian Block during the Cretaceous and the Opening of New Oceanic… DOI: http://dx.doi.org/10.5772/intechopen.105440*

#### **Figure 16.**

*The geodynamic modeling of the opening (early-middle Cretaceous) and closure (middle-early late Eocene) of the Sabzevar-Nain Basin. CICC: Central Iran continental crust; S N rift: Sabzevar-Nain rift; ACC: Alborz continental crust; CIPM: Central Iran passive margin; APM: Alborz passive margin; SNMOR: Sabzevar-Nain mid oceanic ridge; SNB: Sabzevar-Nain Ocean; SNVA: Sabzevar-Nain volcanic arc; SNCZ: Sabzevar-Nain collision zone; AAM: Alborz active margin; OC: Oceanic crust; E: Eocene; K: Cretaceous.*

#### **4. The structural and petrological characteristics of the Sabzevar-Nain collision zone**

The best and the more complete units of the Sabzevar-Nain collision zone are well appeared around the Sabzevar. In a section from north to south, these units may be described as below:

Back arc basin in which, is deposited the detrital sediments in age of late Eocene, Oligo-Miocene and plio-Quaternary, Volcanic arc mainly composed of Eocene intermediate volcanic rocks and ophiolitic belt (Sabzevar-Nain oceanic crust remnants). Foreland basin in which the shallow water sediments of early- middle Eocene to Plio- Quaternary are accumulated., Syn-post collision intrusive masses of granite, granodiorite, diorite and granitoid, and overthrustihg of ophiolitic units (**Figure 17**) are also observed.


#### **Figure 17.**

*The structural sections of the Sabzevar-Nain collision zone. SNBAB: Sabzevar-Nain back arc basin; SNVA: Sabzevar-Nain volcanic arc; EVSR: Eocene volcano- sedimentary rocks; SNFB: Sabzevar-Nain foreland basin; SNOCR: Sabzevar-Nain oceanic crust remnants (ophiolites); CIPM: Central Iran passive margin; AAM: Alborz active margin; JBB: Joghatay back arc basin; BFB: Bardeskan foreland basin; DF: Daruneh fault; PCDA: Post collision dacites; QVC: Quaternary volcanic Cone; EM: Eocene- Miocene deposits in foreland basin; K2 f : Late Cretaceous oceanic deposits; Ev : Eocene volcanic; Q: Quaternary; M: Miocene deposits; E: Eocene foreland basin deposits; gr: Post collision granites; d: Post collision diorites.*

#### **Figure 18.**

*The syn-rift basaltic flows (K1 v2) folded with the sediments of the Sabzevar-Nain trough. Quaternary: 1. Clay flat, 2. Sand dunes, 3. Cultivated area, 4. Mud flat, 5. Younger alluvium, 6. Older alluvium, 7. High level gravel fan, 8. Low level gravel fan, Cretaceous: 9. Crystal lithic tuff, 10. Andesitic lava (spilite), 11. Calcareous shale and limestone (turbidites), 12. Massive limestone, 13. Shaly limestone, 14. Shale, Paleocene: 15. Conglomerate and sandstone, 16. Shale.*

Middle Cretaceous basalts: These rocks comprise the oldest layer of the Sabzevar-Nain oceanic crust. In fact, these basaltic flows are the highest part of the magma chamber under the Sabzevar-Nain rift. These rocks can be observed within the flysch sediments of the trough and are folded with them during continental convergence (**Figure 18**). These are from alkali basalt series and have a spilitic texture [20]. In the Kharturan area, the syn-rift sediments (flysch facies) and the massive carbonates of the platform environment have been exposed near each other (**Figure 4**).

Late Cretaceous deep-sea sediments: These rocks are widely exposed in the middle part of the collision zone especially in the north and southwest of

#### *The Breaking of the Iranian Block during the Cretaceous and the Opening of New Oceanic… DOI: http://dx.doi.org/10.5772/intechopen.105440*

Kuh-e-Baharestan (Baharestan Mountain) in the south of Sabzevar. These sediments are composed of siliceous sandstone, shale and tuffaceous sandstone with some pillow lava, spilitic vesicular lava, green tuff, radiolarian shale, and pelagic limestone. These deep sediments are strongly folded and faulted. They are a combination of thrusting slices and their boundaries with other rock units are thrust faults.

Oceanic crust remnants: The ophiolitic rocks of the Sabzevar-Nain oceanic crust appear in three areas of the collision zone. The southern exposure is near the northern margin of central Iran among the Eocene flysch and volcanic rocks (**Figure 19**). Here, they are composed of ultramafic rocks, diabase, plagiogranite, and gabbro. The second one is exposed with a fault contact in the middle part of Kuh-e Mish just in the southern flank of the great post-collision exposures of diorite. At their southern limit, these ophiolitic rocks are upthrust on Miocene marls. Lithologically, they are composed of harzburgite, serpentinite, dunite, diabase, and gabbro (**Figure 20**) [7, 13, 23, 32]. The main remnants of the oceanic

#### **Figure 19.**

*Southern exposure of ophiolitic rocks near the northern margin of Central Iran composed of ultramafic rocks, diabase, plagiogranite and gabbro.*

#### **Figure 20.**

*The second ophiolitic rocks are exposed by thrust faults in the middle part of Kuh- e- Mish. In the southern limit, they are upthrust on the Miocene marls.*

crust in the suture zone (**Figure 17**) in the north of Sabzevar are exposed as a mountain range. The length of this mountain range is more than 480 km from east to west and its width is about 20 km. In its southern limits, this ophiolitic range is thrust over the detrital sediments of the foreland basin (**Figure 21**). However, in its northern part, it is limited to the volcanic arc (**Figure 17**). From a lithological point of view, it is composed of harzburgite, lherzolite, dunite, rodingite, serpentinized harzburgite, gabbro, diabase, a complex of sheeted dykes, submarine andesitic basalt, pillow lava, amphibolite, amphibolite schists, serpentinized peridotite, glaucophane schists, hornblende schist, garnet-muscovite schists, epidote-muscovite-chlorite schist, epidote-tremolite-actinolite schists, marble, pegmatite gabbro, monzodiorite, diorite, quartz diorite, and granodiorite [7, 9, 13, 24, 26–28, 33] (**Figure 22**). Obviously, the last intrusions are related to post-collision intrusion events (**Figure 23**) [17].

Volcanic arc: The Sabzevar-Nain volcanic arc is exposed on the Alborz continental margin very close to the suture zone and parallel to the ophiolitic range.

#### **Figure 21.**

*The main remnant of oceanic crust in the suture zone in the north of Sabzevar. These ophiolitic rocks are thrusted over the foreland basin deposits.*

#### **Figure 22.**

*The ophiolitic rocks, situated in the northeast of Sabzevar, composed of harzburgite, lherzolite, dunite, rodengite, serpentinite, gabbro, diabase and pillow basaltic lava.*

*The Breaking of the Iranian Block during the Cretaceous and the Opening of New Oceanic… DOI: http://dx.doi.org/10.5772/intechopen.105440*

#### **Figure 23.**

*The last intrusion related to syn- post collision activities (Kuh- e- Mish) (south of Sabzevar).*

#### **Figure 24.**

*The Sabzevar- Nain volcanic arc exposed on the Alborz continental margin close to the suture zone, mostly composed of intermediate volcanic rocks.*

This can be due to the 30–35-degree angle of the oceanic crust subducted under the Alborz continental crust. Its length is about 660 km from the west of Torbate-Jam in the east of Iran to the north of Semnan in the central southern part of the Alborz Mountain Ranges. Its width in the west of Abbasabad near Miamey is a little more than 10 km. The lithological composition of this arc changes from east to west. In the Sabzevar region, it is reported as red dacite, pyroclastic rocks, massive micrite, and trachyandesite (Oligocene in age). The Eocene volcanic rocks are siliceous tuff interbedded with andesite, porphyritic andesite, basalt, agglomerate andesite basalt to trachyandesite basalt, volcanic breccia, andesite lava, and lithic crystal tuff (**Figure 24**) [23, 27, 28, 30]. In the western part of the arc in the north of Semnan, the volcanic rocks from bottom to top include intermediatebasic lava with a composition of andesite basalt to phitic andesite. In some parts, this sequence is crosscut by andesitic dykes. The eruptions of andesitic-basaltic lava sometimes enter the shallow water environment and produce brecciated hyaloclastites with sandstone, shale, and limestone. In the higher parts of the

sequence, there is some intermediate lava with a phyric andesite composition which is crosscut by quartz feldspathic dykes. Briefly, the Eocene volcanic rocks in the western part of the volcanic arc consist of basalt, basaltic andesite, andesite, dacite, riodacite, riolite, and tuff. Based on their geological, petrographic, and geochemical study (2018), Khalatbari and Etessami concluded that these volcanic rocks are the result of a subduction event during the Paleocene and early-middle Eocene (**Figure 25**).

#### **Figure 25.**

*Location of the studied samples on the diagrams determination of the tectonomagmatic environment a) Th-Hf/3-Nb/16 diagram [34]. b) Ta/Yb vs. Th/Yb diagram ([35]). (c,d,e) normalized multi- element spider diagrams with N- MORB value [36] for the Ahovan (Semnan) volcanic rock samples. Diagrams to investigate the role of subduction compositions (fluid/melting). f) Ba/Nb diagram vs. Th/Nb [37]. g) Th/Nb diagram vs. Ba/Th [38]. (after [16]).*

In their geological, petrographic, and geochemical studies, Baghdadi [14] and Shahosseini and Ghassemi [39] reached the same conclusion regarding the north and west of Sabzevar, respectively. In their petrochemical and tectonic setting study of the Davarzan-Abassabad volcanic (DAEV) rocks, Ghassemi and Rezaei-Kahkhaei [15] stated that these volcanic rocks are the product of the partial melting of an enriched mantle by an extensional event within the arc (**Figures 26**–**29**).

Post-collision intrusive masses: The post-collision intrusive rocks are cropped out in the form of small and large masses everywhere in the collision zone. The intrusive masses within the ophiolitic belt in the north of Sabzevar are usually small scale. The greatest masses appear in the middle parts of the collision zone in the south and southeast of Sabzevar in the mountains called Borj-Kuh, Kuh-e-Mish, and Kuh-e-Baharestan. The petrological studies on the small intrusive bodies within the ophiolites have shown that they are composed of granite, granophyre, granitoid [29], granite, quartz diorite, diorite, microdiorite [31], micromonzonite,

*The Breaking of the Iranian Block during the Cretaceous and the Opening of New Oceanic… DOI: http://dx.doi.org/10.5772/intechopen.105440*

#### **Figure 26.**

*a) Classification diagrams of volcanic rocks (after [40]). b) Zr vs. Y diagram indicates that the studied area samples belong to transitional to calk alkaline suites. c) K2O vs. Na2O diagram (after [41]), showing that the DAEV samples belong to the Na and K-series. d) TiO2 vs. Mgo diagram (after [42]) indicates that the studied samples have low-Ti contents. (after [15]).*

diorite, granodiorite, granite [28], granite, micromonzonite [27], granite intruded by monzodiorite, quartz diorite, monzodiorite, granite, granodiorite [24], quartz monzonite, and quartz monzodiorite [20]. The age determined for these rocks varies from the Paleocene to early Eocene.

Large intrusive masses have the following characteristics: Granite (post-Paleocene), microgranite to granite (late Cretaceous), diorite, monzodiorite (middle-late Cretaceous) in Kuh-e-Baharestan and Kuh-e-Mish (**Figure 30**) [23] which intrude into the late Cretaceous oceanic sediments (**Figure 31**) as well as granite, granodiorite, diorite, and monzonite which are crosscut by microdioritic and diorite-monzonite dykes [53]. Based on lithological, petrological, and geochemical studies on the intrusive rocks of Kuh-e-Mish, it has been demonstrated that they are a mass of granitoid of which a major part is granodiorite and a minor part is tonalite in terms of composition. This granitoid mass is mostly calc-alkaline. It is meta-alumina to slightly per-alumina in terms of the amount of aluminum and it is a HSS (a true hybrid) in terms of origin. The mentioned magma probably originated from the upper mantle and the base of the crust [17]. Goharshahi also believed that the depth of magma production was about 73 km or a little more and its replacement depth was less than 4 km. This intrusive mass was of the granitoid type and was associated both with the volcanic arc granitoid (VAG) of the continental margin and subduction and its continuation after collision (post-collision events). The estimated time

#### **Figure 27.**

*Chondrite (a) and primitive mantle (b) normalized spider diagrams of DAEV rocks (normalization values are from [36]). (after [15]).*

of collision was probably Eocene and the time of granitoid production was about 37 Ma or more (**Figure 32**) [17].

The youngest post-collision volcanic activities: The age of the youngest volcanic activities in the collision zone varies from Miocene to early Quaternary. They are mainly composed of dacite and most of their outcrops are observed in the ophiolites of northern Sabzevar and also behind the volcanic arc in the Joghatay back-arc basin. Dacites are subvolcanic rocks which mostly appear at the intersections of faults. Young dacites crosscut all the older rocks and sediments in the three forms of dome, plug, and dike. From the petrological point of view, they can be classified into the three groups of biotite bearing dacite, amphibole-bearing dacite, and dacite with a small amount of dark minerals [24]. In the western parts of Sabzevar ophiolites, dacitic domes are more abundant both in number and size (**Figure 33**) [24].

In the northernmost part of the collision zone, dacite rocks show an andesiticdacite composition and appear in the form of domes and lava flows. These lavas cover a large area of the northern margin of the Joghatay back-arc basin. The youngest volcanic cones (Plio-Quaternary in age) can be found in this area ([31]; Radfar [57]) (**Figure 34** and **35**).

*The Breaking of the Iranian Block during the Cretaceous and the Opening of New Oceanic… DOI: http://dx.doi.org/10.5772/intechopen.105440*

#### **Figure 28.**

*Tectonic discriminant diagrams for the DAEV rocks. a) Ta/Yb vs. Th/Yb (after [43]). Vectors show inferred effects of fractional crystallization (FC), assimilation- fractional crystallization (AFC), subduction enrichment and mantle metasomatism. b) Al2O3 vs. TiO2 (after [44]). c) TiO2- MnO-P2O5 (after [45]). d) Nb/Y vs. Rb/Y (after [46]). e) Hf-Th-Ta (after [47]). f) Nb/Yb vs. Th/Yb (after [48]). SHO shoshonite, CA calc-alkaline, TH tholeiite, N-MORB normal MORB, E-MORB enrich MORB, OIT Ocean island tholeiitic basalt, LAT island arc tholeiite, BON boninites OIA Ocean island arc basalts, WPA within-plate alkaline. (after [15]).*

#### **Figure 29.**

*a) Sm/Yb vs. Ce/Sm plot used to identify the mantel source for DAEV rocks (after [49]). b) Zr vs. Y showing the enriched nature of the DAEV rocks (after [50]). c) La/Yb vs. Dy/Yb plot used to determine the degrees of partial melting of the DAEV source rocks (after [51]). d) Ce vs. Ce/Yb diagram used to determine the depths of the melt segregation for the source for DAEC rocks (after [52]).* Ga *garnet (after [15]).*

#### **Figure 30.**

*Low elevation of granite masses (post-Paleocene) and microgranite to granite (late Cretaceous) in vicinity of Estaj village (south of Kouh-e-Baharestan).*

*The Breaking of the Iranian Block during the Cretaceous and the Opening of New Oceanic… DOI: http://dx.doi.org/10.5772/intechopen.105440*

#### **Figure 31.**

*Large intrusive masses of diorite, monzodiorite (late Cretaceous to middle Paleocene) which intruded into the late Cretaceous oceanic sediments north of Anjoman Village (south of Sabzevar).* 

#### **Figure 32.**

*Geochemistry, chemical classification and location of the post collision granitoids (after [17]) a) the location of granitoid samples from Kuh-e-Mish on the a/CNK vs. ANK Maniar and Piccoli [54] diagram, b) the location of the Kuh-e-Mish and Baharestan granitoids on the Batchelor and Bowden [55] diagram, c) Rb-(Y/+Nb), Nb-Y diagrams for the Kuh-e-Mish granitoid samples on the base of Pearce et al. [56], which separate the locations of the Syn-collision from the within plate volcanic arc and oceanic ridge granites.*

**Figure 33.** *The youngest post- collision volcanic activities in the collision zone (Miocene to early quaternary).*

#### **Figure 34.**

*The youngest volcanic cones (Plio-quaternary in age) and the distribution of lava flows in the northernmost part of the Sabzevar-Nain collision zone (after [31]). Quaternary: 1. Younger alluvium, 2. Older alluvium, Plio- quaternary: 3. Andesitic lava, 4. Dacitic lava and dome, Oligo- Miocene: 5. Conglomerate, Eocene: 6. Arc volcanic rocks, 7. Shale and tuff, 8. Shale, 9. Sandstone and conglomerate, 10. Shale, 11. Marl, 12. Sandstone and conglomerate, 13. Marl, 14. Conglomerate, Jurassic: 15. Limestone, 16. Marly limestone, 17. Shale and sandstone, 18. Limestone and sandstone, 19. Silty shale.*

*The Breaking of the Iranian Block during the Cretaceous and the Opening of New Oceanic… DOI: http://dx.doi.org/10.5772/intechopen.105440*

#### **5. Conclusion**

Various processes and tectonic events have been influential on the evolution of the Iranian crust. The major events in this regard can be divided into two stages. One of the events in the second stage is the thinning of the crust during the late Jurassic, leading to a period of instability. After this, the Iranian crust broke and then created interior oceanic basins such as Sabzevar-Nain, Nain-Baft, and Sistan-Baluch Basins [2, 4, 5]. The processes of the opening and closure of the Sabzevar-Nain Basin (SNB) began with crustal thinning in the late Jurassic with an east–west trend between the micro-block of central Iran in the south and the Alborz Mountain Ranges in the north. As a deep trough in the middle Cretaceous, the Sabzavar-Nain rift became an oceanic basin at the beginning of the late Cretaceous. At the end of the late Cretaceous, the Sabzevar-Nain Basin reached its final expansion. With the subduction of the oceanic crust under the continental margin of Alborz, the closure of the Sabzevar-Nain Basin began in the early Paleocene. After the collision of the continental margins of central Iran and the Alborz Mountain Ranges, the basin was closed in the middle Eocene. Based on the geometry of the oceanic crust (the direction of subduction is perpendicular to the trench direction), we propose that this continent-continent collision can be classified as a classical collision. Our reasons based on the structural elements of the collision zone are ophiolitic rocks exposed in the suture, remnants of deep oceanic sediments upthrust on both margins, a volcanic arc parallel to the trend of the suture, a back-arc basin, and finally postcollision intrusive masses.

#### **Acknowledgements**

We thank Mohammad Reza Mirzaie and Mostafa Khoshduni Farahani for accompanying us in the first field trip. We warmly thank Dr. Fodazi for his petrological advice. We appreciate Mr. Jafarian for providing us with data about the Sheshtamad area and Kuh-e-Bahrestan. Our thanks are also extended to the kind people of the region between Nain and Sabzevar for their hospitality, aids, and information. Finally, we thank Miss Shahosseini for drawing the figures.

#### **Author details**

Saidi Abdollah1 \*, Khan Nazer Nasser1 , Hadi Pourjamali Zahra<sup>2</sup> , Farzad Kiana <sup>2</sup>

1 Geological Survey of Iran, Geological Survey of Iran, Tehran, Iran

2 Rega Zamin Sakht Consulting Engineers, Tehran, Iran

\*Address all correspondence to: abdollahsaidi@yahoo.fr

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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#### **Chapter 3**

Time-Series Analysis of Crustal Deformation on Longstanding Transcurrent Fault: Structural Diversity along Median Tectonic Line, Southwest Japan, and Tectonic Implications

*Yasuto Itoh*

#### **Abstract**

The Median Tectonic Line (MTL) along the longstanding convergent margin of eastern Eurasia has been activated intermittently since ca. 100 Ma. In its incipient phase, propagating strike slips on the MTL generated an elongate pull-apart depression buried by voluminous clastics of the Late Cretaceous Izumi Group. In this study, the complicated deformation processes around this regional arc-bisecting fault are unraveled through a series of quantitative analyses. Our geological survey of the Izumi Group was exclusively conducted in an area of diverse fault morphology, such as jogs and steps. The phase stripping method was introduced to elucidate the time sequence of cumulative tectonic events. After stripping away the initial structure related to basin formation, neotectonic signatures were successfully categorized into discrete clusters originating from progressive wrenching near the active MTL fault system, which has been reactivated by the Quaternary oblique subduction of the Philippine Sea Plate. The method presented here is simple and effective for the detection and evaluation of active crustal failures in mobile belts where records of multiphase architectural buildup coexist.

**Keywords:** oblique subduction, transcurrent fault, pull-apart basin, neotectonics, Median Tectonic Line (MTL)

#### **1. Introduction**

Island arcs exhumed along convergent plate boundaries surrounding the globe are under intensive stress, and hence, they are sites where diverse tectonic forms can be found. Among such features, arc-bisecting faults activated under a regime of oblique subduction [1] have great importance with regard to regional deformation and terrane rearrangement within mobile belts.

The Japanese Islands exhibit the widest range of active landforms since their recent tectonic processes are under the control of four interacting plates around the east Eurasian margin (see **Figure 1** inset). This study focuses on the southwestern part of the arc-trench system, in which transient convergent modes of the Philippine Sea Plate along the Nankai Trough (**Figure 1**) have governed its architectural development.

Except for Kyushu Island in front of a chain of underthrusting bumps on the oceanic plate (Kyushu-Palau Ridge in **Figure 1**), ongoing neotectonic events within southwest Japan can simply be understood by evaluating the obliqueness of plate convergence and related activity levels on a regional arc-bisecting fault, the Median Tectonic Line (MTL).

Here, the research focus is placed on the northwestern part of Shikoku Island (**Figure 1**) because a previous study [3] identified deformation events along with the MTL based on a detailed geologic survey of a Quaternary unit. In the following sections, the tectonics of southwest Japan since the Cretaceous are reviewed, and original results are presented based on an extensive structural analysis making use of the phase stripping method to distinguish recursively overlaid tectonic events, and active crustal damage zones along the MTL are then extracted. This study provides a precedent for further quantitative geologic explorations of mobile belts, where superimposed records of the Earth's evolution remain untouched.

#### **Figure 1.**

*Index maps for the east Eurasian margin and southwest Japan. Offshore geomagnetic anomalies are after [2]. The open arrow shows the present relative motion of the Philippine Sea Plate.*

*Time-Series Analysis of Crustal Deformation on Longstanding Transcurrent Fault: Structural… DOI: http://dx.doi.org/10.5772/intechopen.101329*

#### **2. Geological background**

#### **2.1 Longstanding activities on the MTL**

Most active faults in the Japanese Islands have been vitalized under the complex and unstable regime of plate tectonics through the Quaternary (inset in **Figure 1**). The MTL, however, has an exceptionally long history of activity that dates back to the Cretaceous, when the vigorous northward motion of the Izanagi Plate [4] triggered a breakup of the continental rim and sinistral slips on the proto-MTL. Note that southwest Japan in that period still constituted a part of the east Eurasian margin because the Japan Sea (**Figure 1**) is a back-arc basin that began to open in the mid-Cenozoic [5], although its spreading center is not obviously identified (see the geomagnetic anomalies in **Figure 1** after [2]).

After a period of substantial dormancy during the Paleogene era, the MTL was reactivated under control of the intermittent convergence of the Philippine Sea Plate that had developed during the late Cenozoic around the northwestern Pacific [6]. Southwest Japan at the beginning of the Pliocene was a site of strong inversion. N-S compressive structures that simultaneously built up along the Japan Sea back-arc shelf are suggestive of resumed northward movement of the oceanic plate [7]. Reflecting such a tectonic context, the MTL in the Pliocene stage acted as a lowangle fault, on which watershed mountainous ranges successively emerged [8].

A close observation of the geologic characteristics along the subduction zone [9] found that the Philippine Sea Plate changed its converging azimuth to counterclockwise at approximately 2–1 Ma. This significant fact implies that the westnorthwestward motion of the plate around southwest Japan (arrow in **Figure 1**) inevitably enhanced right-lateral activity on the regional fault. Cumulative lateral separation on the present MTL (e.g., [10]) matches well with this tectonic model. To make an explicit distinction between this and older MTL activity, the Quaternary fault driven by dominantly dextral shear is hereafter referred to as the MTL active fault system (MTLAFS).

#### **2.2 Izumi Group filling a regional pull-apart basin**

As stated above, the MTL was activated as a left-lateral fault along the Eurasian margin during the Late Cretaceous. Around a propagating termination of the regional rupture, enormous trench-parallel pull-apart basins developed and were promptly buried by voluminous marine siliciclastics that are collectively named the Izumi Group [11]. This group is exposed in an area 300 km long by 10–20 km wide along with the MTL, and it topographically coincides with the core of watershed ranges uplifted during the Pliocene.

Ruled by the general development processes of pull-apart basins [12] and contraction in the Pliocene event of N-S inversion, the stereotypical geologic architecture of this Cretaceous unit is an east-plunging syncline. Such a monotonous structural feature is crucial for isolating neotectonic deformation trends by means of a filtering method that is fully explained in Section 3.

#### **2.3 Gunchu Formation: An indicator of active basin formation and exhumation**

#### *2.3.1 Geologic system in the coastal area on the Iyonada Sea*

**Figure 2** depicts the geologic system around the study area in the northwestern part of Shikoku Island [13, 14]. In a broad way, the Cretaceous basin fill of the Izumi Group is in contact with the high-pressure Sanbagawa metamorphic

#### **Figure 2.**

*Geological summary of the northwestern part of Shikoku Island. The geologic system is after [13]. (a) Base map and morphology of the Median Tectonic Line active fault system are after [14–16], respectively. See* **Figure 1** *for the mapped area. (b) Sense of active movements on the Iyo Fault is after [17].*

rock to the south, which ascended rapidly in a subduction zone during the sinistral phase of the MTL (cf. Section 2.1). In contrast, the Izumi Group rests on Cretaceous granitic rock accompanied by low-pressure Ryoke metamorphic rock on its northern border. It is noted that a monoclinal nonmarine Pleistocene formation known as the Gunchu Formation is cropped out on the coast of the inland sea. Its steep structure is a result of Quaternary activity on the MTLAFS that runs through northwest Shikoku and the southern coastal zone of the Iyonada Sea [14–17].

#### *2.3.2 Stratigraphy and structural trend*

The fluvial sequence of the Gunchu Formation was originally divided into three members [18, 19] and redivided by Itoh [3] into two from the viewpoint of the provenance of pre-Neogene clasts. As delineated in **Figure 3**, the Gunchu Formation lies unconformably on an eroded surface of the Izumi Group and has a steep homoclinal structure. Offsets of its stratigraphic boundaries point to the presence of faults crosscutting the unconsolidated strata. Kitabayashi et al. [20] dated zircon grains separated from volcanic ash intercalated in the basal part of the lower member to be 2.2 ± 0.3 Ma (FT age) and 2.13 ± 0.05 Ma (U–Pb age), whereas an ash layer near

*Time-Series Analysis of Crustal Deformation on Longstanding Transcurrent Fault: Structural… DOI: http://dx.doi.org/10.5772/intechopen.101329*

the top of the same member yielded zircon ages of 1.8 ± 0.2 Ma and 1.92 ± 0.05 Ma based on FT and U–Pb methods, respectively [3].

#### *2.3.3 Sedimentology and basin-forming process*

The Gunchu Formation is a stack of channel and bar deposits with minor facies fluctuation that reflects the migration of channels within an alluvial basin. Apart from such phenomena, Itoh [3] recognized drastic changes in gravel compositions probably linked to successive exhumations of hinterlands driven by tectonic uplift.

**Figure 4** exemplifies the compositional variety observed in the fault-bounded Block 1 (**Figure 3**) of the Gunchu Formation. As depicted in the pie charts, the lower member contains a considerable amount of granitic pebbles, the U–Pb ages of which range from 106.5 to 93.8 Ma, as measured for five clusters in four sites [3]. Although these radiometric dates reflect the Cretaceous plutons extensively found in southwest Japan, igneous rocks in that period are enigmatically absent around the present study area. The base of the upper member is defined by an abrupt influx of high-pressure metamorphic clasts. The concentration of schist-originated material is so extreme that some outcrops possess a blue-greenish appearance. The content of metamorphosed

#### **Figure 3.**

*Geologic map of the Pleistocene Gunchu Formation after Itoh [3]. See* **Figure 2b** *for the mapped area. Background topography is a part of the "Kaminada" locality map at 1:25,000, published by the Geospatial Information Authority of Japan.*

#### **Figure 4.**

*Spatiotemporal variation in gravel compositions for Block 1 of the Gunchu Formation after [3]. Relative abundance of rock facies was determined from 100 data points for each observation station.*

gravels tends to decrease upward, and sandstones derived from the nearby Izumi Group become prevalent in the uppermost part of the sedimentary unit.

Itoh [3] attempted to reconstruct the Quaternary basin-forming process based on the above-stated spatiotemporal changes in gravel composition, and additionally, paleocurrent directions that were determined from the imbricated structure of clasts for the same 26 sites in the compositional analysis. **Figure 5** schematically shows the neotectonic evolution of the northwestern part of Shikoku Island. As described previously, granitic pebbles ubiquitously detected in the lower part of the Pleistocene sediments yielded numerical ages concordant with those of the Cretaceous intrusions within the inner zone (north of the MTL; see **Figure 1**). Significant scatter in the U–Pb ages implies that the granites were not derived from a local intrusive body having a uniform cooling history but from asynchronously emplaced plutons, which are widely exposed on the south side of Honshu Island (**Figure 1**). Thus, all the available data point to an assumption that the depocenter at an early stage was located around the area now occupied by the Gunchu Formation (**Figure 5a**).

*Time-Series Analysis of Crustal Deformation on Longstanding Transcurrent Fault: Structural… DOI: http://dx.doi.org/10.5772/intechopen.101329*

**Figure 5.**

*Paleoenvironments during depositional stages of the Gunchu Formation after [3]. Base geologic map is after [13].*

As for the setting of later stages, the massive influx of the Sanbagawa metamorphic rock demonstrates regional uplift in the outer zone (south of the MTL; see **Figure 1**). It is, however, intriguing that the upper member contains a few of the Miocene volcanics that extensively cover the area between the Gunchu basin and the Sanbagawa terrane. Itoh [3] thereby assumed that the region of intensive uplift and erosion progressively expanded northward during the mid-Pleistocene. This resulted in a

sharp increase of upward sandstone clasts coming from the nearby Izumi Group and brought about seaward migration of the depocenter, in which clastics derived from the inner zone are trapped. This is the reason why granite pebbles disappeared from the upper member. This tectono-sedimentary model is summarized in **Figure 5b**. The regional and incremental contraction eventually urged the recent basin fills to build up a steep monoclinal structure along with the coastline.

Our review has thus shown a longstanding history of MTL activity and relevant processes of basin formation and deformation. In the following sections, more extensive and quantitative analyses of fault-related tectonics are discussed based on the results of this study.

#### **3. Analysis**

#### **3.1 Application of phase stripping method**

Previous studies have shown that the structural and sedimentological features of the Pleistocene Gunchu Formation have preserved neotectonic information linked to activity on the MTLAFS. As for advanced research on the fault-bound tectonics, however, confined exposure of the fluvial unit hinders more regional analysis. Therefore, this study focuses on the Cretaceous Izumi Group, which is distributed along the proto-MTL. Although the widespread sediments probably record recent episodes, they also reflect the initial architecture built up during the growth of pullapart basins. In conventional geological mapping, neotectonic signatures related to active deformation are interpreted as secondary features of basin-forming processes or are excluded as noisy data near local faults. Thus, an overlapped event usually ends up as misread or dead information in a one-sided interpretation.

During the course of research on the easternmost part of the MTLAFS, Itoh and Iwata [21] developed a simple method to separate the multiphase deformation. Their "phase stripping method" regards the geologic architecture revealed through field surveys as an integration of an initial tilting and a younger event. The total structure directly measured on an outcrop, *C*, is expressed using a matrix product as follows:

$$\mathbf{C} = BA \tag{1}$$

where *A* is the trend acquired during the initial tectonic phase, a pull-apart basin evolution in this case, and *B* is the pursued phase formed in a recent period. At each observation point, *B* can be determined using the known structural data as follows:

$$B = BAA^{-1} = CA^{-1} \tag{2}$$

In this study, the author applied this simple calculation to field data obtained from 720 outcrops of the Izumi Group.

**Figure 6** represents the concrete procedure of phase stripping. In spite of later disturbance, a general structural trend can be identified based on 280 and 440 points of field data in the northeastern and southwestern blocks of the three-year geologic survey, respectively.

As mentioned before, the typical architecture of the Izumi Group, *A* in Eq. (1), is a monotonous east-plunging syncline developed through the progradational burial of a series of pull-apart basins and is delineated by red contours in the figure. For reference, the Pleistocene Gunchu Formation is located near the north corner of the southwestern block.

*Time-Series Analysis of Crustal Deformation on Longstanding Transcurrent Fault: Structural… DOI: http://dx.doi.org/10.5772/intechopen.101329*

#### **Figure 6.**

*Base map for the phase stripping analysis of the Izumi Group. See magenta enclosure in* **Figure 2a** *for the mapped area. Detailed topographies of the two analyzed blocks are parts of the "Kaminada" and "Gunchu" localities at 1:25,000, published by the Geospatial Information Authority of Japan. There are 280 and 440 data points in the northeastern and southwestern blocks, respectively. Thin red lines and attached dip angles represent the initial east-plunging synclinal structure of the Cretaceous basin fill.*

#### **3.2 Detection of damage zones**

The analytical results of this study are summarized in **Figure 7**. As shown by magenta lines, a previous geomorphological study [17] recognized two branches of the MTLAFS, namely, the minor Kominato Fault and the major Iyo Fault, for which right-lateral offsets were confirmed (**Figure 2b**).

Elaborate field observations led the author to realize that the density of visible fractures in the Izumi Group varies considerably under the possible influence of local deformation. To perform a quantitative evaluation, the fracture density was measured at 18 sites along with the coast. At every station, all the visible cracks residing within a massive sandstone bed were counted on a 10 × 10 grid of graticules ruled in 3-cm divisions, which were drawn on a transparent polyethylene film pasted on the outcrop surface. All the fracture counts at 18 stations are shown in **Figure 7**, and histograms in the figure present selected results of such measurements.

In the same figure, regions of severe deformation are highlighted by red symbols for steep and overturned bedding attitudes of phase *B* in Eq. (2). As for the northeastern part of the analyzed area, a major damage zone lies on or near the Iyo Fault, and minor trends extend along gorges subparallel to the main active fault. In contrast, more diverse structural disturbances characterize the southwestern area. Some data indicating intense motions in phase *B* are aligned near the straight scarp of the Iyo Fault, but others show elongated clusters with different orientations, implying the existence of unknown active failure zones. In both the analyzed areas of **Figure 7**, remarkable phase *B* deformations are found irrelevant to the initial synclinal structure of phase *A*. In the next section, the recent progressive deformation process is examined by integrating geological evidence, and a neotectonic model of the MTLAFS is derived that matches the structural framework.

#### **Figure 7.**

*Extracted neotectonic features based on the phase stripping method. The mapped area is the same as in* **Figure 6***. Previously reported active faults (magenta lines) are after [17]. Refer to the main text for trends A to D (thick gray lines), fracture numbers at 18 stations pasted along the coast, and histograms of the fracture density. Bedding attitudes developed under recent stress are categorized as gentle (yellow, less than 30°), intermediate (orange, between 30° and 60°), and steep (red, 60° or more, including overturned structures).*

#### **4. Discussion**

#### **4.1 Subordinate active faults**

The phase stripping method applied to the Izumi Group distributed around northwestern Shikoku Island successfully delineated recent damage zones in connection with activities on the MTLAFS. As for the wedge between the coastline and the Iyo Fault, four-fault traces were identified based on linear clusters of the strong phase *B* motions (shown by red symbols in **Figure 7**) and are depicted as gray zones A to D in the figure. Trends A and B are concordant with faults cutting the Pleistocene Gunchu Formation (see **Figure 3**). Trends C and D are recognized as deformation zones that branch off from the Iyo Fault and appear to be related with coastal stations showing extreme concentrations (414 and 489 counts for each measurement) of fractures (histograms in **Figure 7**). Such parallel trends aside, the phase *B* data tend to suggest stronger deformation along with the Iyonada Sea coast. Although numerous offshore faults have been found through previous studies (**Figure 2a**), the detailed structure of the shoreline remains ambiguous because research vessels cannot perform sounding surveys in very shallow water. The present results imply an offshore stepped extension of the Iyo Fault that provokes activity on the subordinate faults.

#### **4.2 Actual deformation processes recorded in outcrops**

As mentioned above, the Pleistocene Gunchu Formation possesses a steep homoclinal structure suggestive of strong neotectonic deformation. **Figure 8** represents the unconformity between the Izumi Group and the Gunchu Formation *Time-Series Analysis of Crustal Deformation on Longstanding Transcurrent Fault: Structural… DOI: http://dx.doi.org/10.5772/intechopen.101329*

#### **Figure 8.**

*Sketch of the unconformable relationship between the Cretaceous Izumi Group and the Pleistocene Gunchu Formation after [3]. See* **Figure 7** *for the outcrop location. Bedding attitudes for the Pleistocene unit imply syn-sedimentary structural growth.*

first described by Itoh [3]. Note that the basal part of the Pleistocene gravel strata exhibits overturned bedding, which graduates upward into steep but normal bedding attitudes. Such a change indicates syn-depositional structural buildup. Another point is that the phase *B* data near this exposure (see **Figure 7** for its location) are in the gentle level of recent motions (tilting shallower than 30°). These geologic lines of evidence seem to suggest that a flexure zone developed under a compressive regime during the uplift of the hinterlands, roughly coinciding with the recent unconformable interface.

A sketch of important outcrops is presented in **Figure 9** (see **Figure 7** for the location). Steep tilting in its western part is likely to be affected by lateral motions on the adjacent Iyo Fault. It is notable that the Izumi Group in the central and

#### **Figure 9.**

*Sketch of an outcrop in which accumulation processes of neotectonic deformation are observed. See* **Figure 7** *for the outcrop location. Refer to the main text for the structural interpretation.*

eastern parts of the outcrop shows overturned attitudes. Namely, as we move away from the active fault, the deformation of the strata seems to become stronger. This paradoxical phenomenon implies that an unreported zone of active contraction extends on the eastern side because the intensive signatures of phase *B* are confirmed within broadband around this observation point (see **Figure 7**).

An unconformable surface of the Izumi Group in this exposure is overlain by gravel-rich unconsolidated sediments. This unit seems to be gently inclined eastward, a tendency endorsed by the sedimentary structure of an intercalation of fine volcanic ash (**Figure 9**). Neotectonic activities on the MTLAFS thus resulted in severe deformation and exhumation of the Cretaceous basin fill, and the overlying fluvial deposits are significantly tilted under lingering tectonic stress.

#### **4.3 Integrated neotectonic model**

**Figure 10** depicts an integrated model of neotectonic processes on a part of the MTLAFS, which is modified from the prototype submitted by Itoh [3]. First, an unknown transcurrent fault having a right offset from the Iyo Fault is postulated on the basis of active structures in the Iyonada Sea (**Figure 2**). Such a stepped

#### **Figure 10.**

*Schematic diagrams delineating the probable evolutionary history of the study area adjacent to an active transcurrent fault system. It is modified from Itoh [3] based on the present results.*

#### *Time-Series Analysis of Crustal Deformation on Longstanding Transcurrent Fault: Structural… DOI: http://dx.doi.org/10.5772/intechopen.101329*

morphology accompanied by propagation of fault termination enhanced activity on secondary faults bridging the primary strike-slip features [22]. Compartments divided by the subordinate faults were systematically tilted and rapidly buried by onlapping sediments (**Figure 10a**). Next, a rising contractional regime brought about progressive uplift of the hinterlands and seaward migration of the Pleistocene pull-apart sag. Strong tectonic stress eventually provoked steep tilting of the Gunchu Formation on a narrow flexure zone (see **Figure 10b**) and confined the deformation of the Izumi Group. The latest dextral motions on the MTLAFS may have induced reactivation of the crosscutting faults, which is inferred from the trends of recent intensive deformation (**Figure 7**).

This study has demonstrated that a long and complicated motion history of the MTL governs the architectural development of nearby geologic terranes. As it is a crustal break under the control of subduction modes of oceanic plates, the faultrelated tectonics may have a wider influence over evolutionary processes for the island arc. For example, Itoh et al. [23] performed a volumetric analysis of the Iyonada Sea based on gravity anomalies and found a gigantic buried basin resting against a 4-kmdeep scarp of the MTLAFS. Thus, a regional tectonic model of active margins should be built using multidisciplinary research to shed light on the deep interior of the earth.

### **5. Conclusions**


gation of dextral fault termination at such a structural singularity inevitably formed an active pull-apart basin divided by subordinate faults, which were promptly buried by the Quaternary clastics derived from uplifted hinterlands. A succeeding compressive regime triggered the seaward basin migration and eventual rollover of the recent basin fill. The latest dextral movements on the MTLAFS may have resulted in activity resuming on the basin-dividing faults.

### **Acknowledgements**

The author is grateful to Shigekazu Kusumoto for his thought-provoking discussion during the course of this study. Some of the graphic art was prepared by Rin Itoh.

### **Author details**

Yasuto Itoh Osaka Prefecture University, Osaka, Japan

\*Address all correspondence to: itoh@p.s.osakafu-u.ac.jp

© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Time-Series Analysis of Crustal Deformation on Longstanding Transcurrent Fault: Structural… DOI: http://dx.doi.org/10.5772/intechopen.101329*

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## Section 2
