**2. Seismic investigations of the upper mantle**

the crust and lithospheric mantle architecture with relevant tectonic history of East Antarctica provide evidence of amalgamation and separation of the past supercontinents [6, 7]. The Lützow-Holm Bay (LHB) region, where the Japanese Syowa Station (SYO, 69S, 39E) is located, has been experiencing regional metamorphic events in early Paleozoic [8]. The metamorphic grade increases from amphibolite facies in eastern LHB to granulite facies in the western. During the Pan-African metamorphism, LHB was deformed under compression stress per-

Terre Adélie Craton; G. Cr, Gawler Craton; MR, Miller Range; GSM, Gamburtsev Subglacial Mountains.

**Figure 1.** Gondwana reconstruction at 480 Ma, centered on East Antarctica (modified after [5]) showing the geologic ages of major exposed coastal outcrops [2]. The areas corresponding "undifferentiated Precambrian" terrains belonging to each continental blocks of Gondwanaland (Australia, Africa, South America, and Antarctica) are distinguished by different colors (yellow dot, green dot, brown dot, and light-blue dot), respectively. Abbreviations are as follows: SYO, Syowa Station; LHB, Lützow-Holm Bay; SR, Shackleton range; SPCM, Southern Prince Charles Mountains; LT, Lambert Terrane; EG, Eastern Ghats; PB, Prydz Bay; DG, Denman Glacier; OH, Obruchev Hills; P. Or, Pinjarra Orogeny; TA. Cr,

Enderby Land

**Syowa Station (SYO)** LHB

Seismological evidence with respect to the structure and tectonics of the upper mantle beneath LHB has been derived in the last few decades by both the computer modeling and field observations by the Japanese Antarctic Research Expedition (JARE). Teleseismic data detected at seismic stations in LHB have sufficient signal-to-noise quality for various kinds of analyses so as to clarify local seismicity, heterogeneities of the lithospheric structure, as well as deep interiors of the Earth [10–13]. Several studies had aimed at deriving static structure, tectonics, and dynamics within the crust and mantle depths, associated with geological evolution of the region [14, 15]. In this chapter, by taking into account the tectonic evolution around the Lützow-Holm Bay (LHB) region, passive and active seismic source studies were reviewed in

pendicular to the thermal axis [9].

2 Tectonics - Problems of Regional Settings

Seismological investigations in LHB demonstrated sufficient images of the structure and dynamics in the upper mantle underneath the Antarctic continent. The investigations by using passive seismic sources such as teleseismic events occurring over the globe had demonstrated strong heterogeneity existing in the upper mantle depths. Depth variations of the upper mantle discontinuities (410 and 660 km depths, respectively) were derived from longperiod receiver function analysis (0.2 Hz low-pass filtered), which indicated shallow depths in the 660 km seismic discontinuity beneath continental back azimuths in LHB (**Figure 2**) [16]. The depth distributions of P-S conversion points were also revealed in particular for the 660 km discontinuity. The shallow depths in topography for the 660 km discontinuity were identified beneath the continental azimuths over the ice sheet. These results could provide an evidence of upwelling flow associated with mantle plume in terms of Gondwana breakup

**Figure 2.** Back azimuth distribution of the depth variation in the upper mantle seismic discontinuities by receiver function analyses of broadband seismic data in LHB (modified after [16]). (Left) Location of the strong heterogeneous azimuths in LHB. The area for strong depth variations in upper mantle discontinuities is represented by the light-green open squares, which are almost parallel with the coastal line. Symbolic notation for the P-S conversion points at the mantle discontinuities around 660 km in depth. (Right) Color images represent the smoothed amplitudes of the longperiod receiver functions. Two dashed lines are traced for the maximum amplitudes of both 410 km and 660 km depth discontinuities, respectively. Two back azimuth groups for strong depth variations in the upper mantle discontinuities are circled by the light-green open squares.

process. Moreover, strong heterogeneities were observed in both 410 km and 660 km discontinuities in back azimuths of 20–50° and 200–260°, respectively. These back azimuths are almost parallel to the coast line and are assumed to have a relationship with the breakup of Gondwana supercontinent.

Shear wave splitting analysis by using SKS waves [17], in addition, indicated clear association between "fossil" anisotropies relating to the past tectonics, which appeared to be in present lithospheric mantle structure beneath LHB. A two-layered structure model was assumed for upper mantle anisotropy; upper was supposed to be the "lithosphere," and lower corresponded to the "asthenosphere," respectively. By using the data from local seismic network in LHB, azimuthal variations of the shear wave splitting parameters were obtained (**Figure 3**). The fast polarization directions of the SKS waves were compared with those directions by an absolute plate motion, which are reflecting more recent mantle flow process of the Antarctic Plate (**Figure 3**) [18]. Since the fast polarization directions in lower layer were generally parallel to the directions of the absolute plate motion, the lower layer's anisotropy might reflect the asthenospheric anomalies due to horizontal mantle flow along the plate motion. On the other hand, the fast polarization directions of upper layers did not coincide with the absolute plate motion direction. It was supposed that anisotropic structure could be involved in the past tectonics; the origin of anisotropy was considered as "frozen" within the lithosphere. The Gondwana assembly in early Paleozoic age might be the major aspect in forming the present anisotropy [2].

Furthermore, active seismic source investigations (wide-angle reflection/refraction and near vertical reflection studies) imaged striking lithospheric mantle reflection patterns involving regional tectonics during Pan-African and the next following extension regime at the continental margins of the breaking-upped supercontinent (**Figure 4**) [14, 19]. By these evidences, tectonic evolution model of LHB was estimated in order to explain the heterogeneities in the present upper mantle. For the 2000 active source profile on continental ice sheet in LHB, a single coverage of common depth point (CDP) with only nearer traces was identified in the lower-right panel of **Figure 4**. On the contrary, the CDP stack section with offsets less than 120 km was depicted for the 2002 active source profile. A laminated seismic velocity layer in the lower crustal depths, moreover, was modeled by comparing synthetic receiver functions with those of observed ones in short-period frequencies (0.1–1.0 Hz) [20]. The repetitive crustmantle transition zone derived by 2002 profile suggested an influence of compression stress in NE-SW orientation during the Pan-African, which might occurred at the last stage of formation of a great mobile belt between East and West Gondwana [1]. Successive breakup of the

**Figure 4.** (Left) Map showing the location of deep seismic surveys conducted in LHB (modified after [19]). Solid and open stars indicate the shot locations in 2002 and 2000, respectively. Large and small circles represent the geophone stations on ice sheet for both the active seismic source operations. The size of each shot given is the weight of dynamite used. (Right) Tectonic interpretations of the seismic reflection cross sections. The CDP stack section with offset limited to traces within 120 km for the 2002 profile (upper) and to near traces for the 2000 profile (lower), respectively. Several seismic reflections in the crust and the Moho discontinuity are identified by broken lines and solid arrows. Moreover, reflections from the bottom of the ice sheet are traced by the broken line in the shallow layer of the topmost crust.

Profile 2000 Profile 2002

NE-SW *compression*

5

*At Pan-African ?*

http://dx.doi.org/10.5772/intechopen.71972

Seismological Implication to the Tectonic Evolution of the Lützow-Holm Bay Region (East…

NW-SE *extension*

Profile 2002

Profile 2000

*At Breakup ?*

**Figure 3.** (Left) Upper mantle anisotropy in LHB derived from SKS splitting (modified after [17]). At the stations of AKR, LNG, SKL, SYO, and TOT, the lower layer anisotropy is supposed to be caused by recent asthenospheric flow. For almost all other stations, the direction of anisotropy in the upper layer (corresponds to the "lithosphere") is parallel to NE-SW convergence during the Pan-African age. (Right) Schematic illustration of a two-layered model of seismic anisotropy within the lithosphere and asthenosphere.

Seismological Implication to the Tectonic Evolution of the Lützow-Holm Bay Region (East… http://dx.doi.org/10.5772/intechopen.71972 5

process. Moreover, strong heterogeneities were observed in both 410 km and 660 km discontinuities in back azimuths of 20–50° and 200–260°, respectively. These back azimuths are almost parallel to the coast line and are assumed to have a relationship with the breakup of

Shear wave splitting analysis by using SKS waves [17], in addition, indicated clear association between "fossil" anisotropies relating to the past tectonics, which appeared to be in present lithospheric mantle structure beneath LHB. A two-layered structure model was assumed for upper mantle anisotropy; upper was supposed to be the "lithosphere," and lower corresponded to the "asthenosphere," respectively. By using the data from local seismic network in LHB, azimuthal variations of the shear wave splitting parameters were obtained (**Figure 3**). The fast polarization directions of the SKS waves were compared with those directions by an absolute plate motion, which are reflecting more recent mantle flow process of the Antarctic Plate (**Figure 3**) [18]. Since the fast polarization directions in lower layer were generally parallel to the directions of the absolute plate motion, the lower layer's anisotropy might reflect the asthenospheric anomalies due to horizontal mantle flow along the plate motion. On the other hand, the fast polarization directions of upper layers did not coincide with the absolute plate motion direction. It was supposed that anisotropic structure could be involved in the past tectonics; the origin of anisotropy was considered as "frozen" within the lithosphere. The Gondwana assembly in early Paleozoic age might be the major aspect in forming the present

> *Absolute Plate Motion*

**Figure 3.** (Left) Upper mantle anisotropy in LHB derived from SKS splitting (modified after [17]). At the stations of AKR, LNG, SKL, SYO, and TOT, the lower layer anisotropy is supposed to be caused by recent asthenospheric flow. For almost all other stations, the direction of anisotropy in the upper layer (corresponds to the "lithosphere") is parallel to NE-SW convergence during the Pan-African age. (Right) Schematic illustration of a two-layered model of seismic anisotropy

Absolute Plate Motion

*SKS fast direction*

Gondwana supercontinent.

4 Tectonics - Problems of Regional Settings

anisotropy [2].

APM; NNR-NUVEL-1 (Argus and Gordon, 1991)

within the lithosphere and asthenosphere.

**Figure 4.** (Left) Map showing the location of deep seismic surveys conducted in LHB (modified after [19]). Solid and open stars indicate the shot locations in 2002 and 2000, respectively. Large and small circles represent the geophone stations on ice sheet for both the active seismic source operations. The size of each shot given is the weight of dynamite used. (Right) Tectonic interpretations of the seismic reflection cross sections. The CDP stack section with offset limited to traces within 120 km for the 2002 profile (upper) and to near traces for the 2000 profile (lower), respectively. Several seismic reflections in the crust and the Moho discontinuity are identified by broken lines and solid arrows. Moreover, reflections from the bottom of the ice sheet are traced by the broken line in the shallow layer of the topmost crust.

Furthermore, active seismic source investigations (wide-angle reflection/refraction and near vertical reflection studies) imaged striking lithospheric mantle reflection patterns involving regional tectonics during Pan-African and the next following extension regime at the continental margins of the breaking-upped supercontinent (**Figure 4**) [14, 19]. By these evidences, tectonic evolution model of LHB was estimated in order to explain the heterogeneities in the present upper mantle. For the 2000 active source profile on continental ice sheet in LHB, a single coverage of common depth point (CDP) with only nearer traces was identified in the lower-right panel of **Figure 4**. On the contrary, the CDP stack section with offsets less than 120 km was depicted for the 2002 active source profile. A laminated seismic velocity layer in the lower crustal depths, moreover, was modeled by comparing synthetic receiver functions with those of observed ones in short-period frequencies (0.1–1.0 Hz) [20]. The repetitive crustmantle transition zone derived by 2002 profile suggested an influence of compression stress in NE-SW orientation during the Pan-African, which might occurred at the last stage of formation of a great mobile belt between East and West Gondwana [1]. Successive breakup of the supercontinent in mid-Mesozoic could explain the formation of stretched reflection structure above the Moho discontinuity as imaged by the 2000 active source profile.

These seismic reflection cross sections were assumed to reflect multi-genetic origins, including igneous intrusions, lithologic/metamorphic layering, mylonite zones, shear zones, seismic anisotropy, and fluid layers [21, 22]. In spite of the multi-genetic origin, metamorphic layering could be principal candidates in the case of LHB. A strong reflectivity in the deeper crust-upper mantle might be expected by layered sequences of mafic and felsic rocks [23]. Moreover, such the reflectivity might be originated when the mafic rocks had been interlayered by a combination of the upper amphibolite and lower granulite facies metapelites [24]. In any continental terrains on the Earth, the primary causes for reflectivity might be enhanced by ductile stretching during a late tectonic extensional process [25]. The reflecting layers near the Moho, moreover, were predominantly identified at the crustal thinning tectonic regimes. In these regards, reflectivity in the lower crust and lithospheric mantle beneath LHB might be enhanced under extensional conditions by the last breakup of Gondwana.
