**4. Transcontinental mega-shear zones (MSZ) and plate dynamics**

Indian Plate is affected by five main ocean-to-continent transcurrent faults as indicated by the extension of important offshore transform/strike-slip faults across the continent. These are, from north to south, the North Kathiawar-Great Boundary fault, SONATA Zone, Alibag, Vengurla, and Tellichery-East Coast-HHL-Naga Hills faults (**Figure 7**) (HHL: Hail-Hakalula lineament). The trans-continental extension of these faults is traced by strong tectonic lineaments matching with mapped fault/ shear zones and important Proterozoic tectonic trends. The matching strikes of North Kathiawar and Great Boundary Fault suggest a continuous trend of crustal shear between Trans Aravalli proto-craton (TAPC) and BPC. These extensive and active fault zones are considered here as mega shear zones (MSZ).

Presently the Indian plate is under compressive stress ([9]; **Figures 1** and **7**). The slab-pull from the Andaman trench is causing the anticlockwise rotation of the plate (**Figure 7**). The Indian plate is divided in the middle by the SONATA TZ which is a mega-shear zone (MSZ) reactivated in the present neotectonic cycle as a transcontinental transform fault (**Figure 7**). This MSZ extends from the Carlsberg Ridge to Upper Assam across the continent along NSG connecting the Dauki fault and Naga

#### **Figure 7.**

*Red lines mark the major ocean-to-continent transcurrent faults (MSZs, numbered) across the Indian shield:*  **(1)** *North Kathiawar – Great Boundary Fault,* **(2)** *Narmada-Son-Dauki-Naga Fault,* **(3)** *Alibag Fault,* **(4)** *Vengurla Fault,* **(5)** *Tellichery – E. coast-HHL-Naga Thrust. Stress/movement directions are shown by black arrows. Yellow arrows show prevailing regional stress directions following plate movement.*

thrust [9]. As a result, the two proto-cratons, BPC & DPC, are rotating with differential motion on either sides of this mid-continental shear zone (**Figure 2**) [3]. The motion of the northern protocraton is constrained by the collision front whereas the southern craton is moving relatively free in response to the anti-clockwise plate motion. The Deccan sub-plate is affected by another mega-shear zone, the Tellichery fault, extending from Carlsberg Ridge in the offshore to Naga frontal thrust along the Naga Hills in AA TZ. This fault extends across the southern part of DPC through the Palghat gap, Kaveri shear zone, along the east coast (bordering Krishna- Godavari rift basin), and across the Bangladesh-Tripura fore-arc prism following Eastern Ghat Precambrian trend (**Figure 7**). This is defined here as Tellichery-Naga-Hills MSZ.

#### *Intra-Plate Dynamics and Active Tectonic Zones of the Indian Plate DOI: http://dx.doi.org/10.5772/intechopen.105647*

Between SONATA MSZ and Tellichery-Naga Hills MSZ, two other offshore faults, Alibag and Vengurla faults, occur. These faults also appear to extend across the shield but the lineaments are obscured by the Deccan Trap cover. The relatively free rotation of the Deccan subplate is creating a tensional stress in the region of the Gulf of Cambay and Narmada (**Figure 2**). This is evident from the occurrence of pull-apart basins in this region [3, 9]. At the same time, in the central and eastern parts of this MSZ, transpressional stress is developed (**Figure 2**). This is evident from the uplift of the Gondwana rifts in the central and eastern parts of this MSZ. South of NSG, the three MSZs across the Deccan sub-plate divide the plate into slices which are slipping left-laterally relative to each other from north to south due to rotation of the plate. This progressive left lateral shift from north to south is apparently responsible for the convex outline of the present coastline.

The Tellichery-Naga MSZ is a resurgent shear zone playing an important role in the present-day plate dynamics. The identification of the mega shear extending from the Carlsberg ridge to the Indo-Burmese plate boundary adds a new dimension in the plate kinematics in the northern Indian Ocean as it appears to be a new or evolving transform plate boundary. Between Eastcoast and AA TZ this MSZ passes through an active zone of seismic activity (**Figure 8**) and it matches with the active TT3 and HHL tectonic lineament of Bangladesh [10] and Tripura-Naga Hills [11] respectively. This transform motion and the stress generated by active convergence of Indian and Burmese plates following oblique collision are responsible for the high degree of seismicity of the Assam-Arakan TZ.

The compressive stress due to continuing north and north-northeastward subduction of the Indian plate is responsible for the seismicity of the Himalayan TZ. The Baluchistan-Karakoram TZ (**Figure 1**) is also highly vulnerable to earthquakes. The recent 2005 Baluchistan earthquake is an example. This is caused by different plate motions along the AA-SD TZ in this northwestern border of the plate. The compression related to the continuing northward subduction of the plate along the Karakoram thrust, the transform motion between the Indian and Afghanistan plates along CT and ONT, and subduction of the Arabian Sea oceanic plate below the Afghan plate along the Makran Fault (MF) in AA-SD TZ, west of the transform boundary are causative forces.

In the SCR zone, the highly rifted Gujarat region is the most active seismic zone in peninsular India. The structural inversion of the rifted structures due to present compressive stress is responsible for the repeated generation of the large earthquakes M > 7.0, particularly in the Kutch rift where the confining stress is enhanced by the local structural framework as discussed below. The SONATA zone is another earthquake-prone linear zone. Several major strong earthquakes M ~ 6.0 occurred around Jabalpur in the past including the recent 1997 earthquake [12]. The focal depth of the 1997 Jabalpur earthquake is estimated at 35 km, at the crust–mantle boundary [13]. The dextral strike-slip motion and related kinematics associated with the parallel faults and their conjugate Riedel faults in the SONATA are the cause of repeated rift basin deep crustal earthquakes within this zone as noted in cases of the 1973 Broach and the 1997 Jabalpur earthquakes M > 6.0.

The Latur and Koyna earthquakes are apparently related to the Koyna-Kurduwadi rift (**Figure 9**) inversion with compressional stress [15]. These rifts are apparently related to Alibag MSZ passing south of the SONATA zone. These events are, however, shallow (depth < 10 km) upper crustal earthquakes.

#### **4.1 The 2001 SCR (BHUJ) earthquake**

January 26, 2001, Republic day EQ earthquake in Bhuj, Gujarat state, is a world example of a recent high magnitude Mw 7.7 earthquake in SCR. Several disastrous

#### **Figure 8.**

*Map showing the focal mechanism of seismic events along Tellichery-Naga Hills MSZ (stippled zone). (Courtsey: Dr. C. Subrhamanyam, NGRI).*

earthquakes occurred in the Kutch rift since ancient times. Strain build-up at the E-W master faults due to intra-plate kinematics is the reason for repeated earthquake generation [16]. The Kutch Mainland Fault (KMF) in the middle of the rift is the main active fault for earthquake generation (**Figure 4**). This fault is currently experiencing dextral transpressional strike-slip movement. Towards the east, the fault tapers off and sidesteps to the left (i.e., shifts to the north) and continues eastward as South Wagad Fault with an approximately 50 km step-over zone (**Figures 4** and **6**).

Intense seismic activity within this step-over zone is indicated by crowding of earthquake epicentres including two major high-intensity earthquakes, the 1956 Anjar, and the 2001Bhuj (**Figure 6**) earthquake. This fault step-over zone is strained by the accumulation of regional compressional stress. Further, the occurrence of massive plutons and geophysical data indicate the presence of a deep-seated igneous body which appears to be syn-rift crustal melt in the deeper crust at 20–40 km depth (**Figure 10**). Seismic tomography study [17] in the Bhuj earthquake epicentre area clearly indicated fluid-filled rock matrix at this depth [18]. The E-W rift ends up against an NW-SE trending basement ridge, the Radhanpur-Barmer arch that separates this rift and the transversely oriented N-S Cambay rift (**Figures 4** and **6**). The easterly horizontal stress along KMF/SWF is constrained by this ridge, which acts as an effective stress barrier. This adds to the strain build-up due to compressive stress within the critical stepover zone. The resistance against the igneous body further

*Intra-Plate Dynamics and Active Tectonic Zones of the Indian Plate DOI: http://dx.doi.org/10.5772/intechopen.105647*

#### **Figure 9.**

*Major tectonic elements south of SONATA zone:* **(1)** *NSG;* **(2)** *Koyna-Kurduwadi rifts,* **(3)** *Godavari rift, and*  **(4)** *west coast fault zone (after [14]).*

#### **Figure 10.**

*Conceptual rift model of Kutch showing causative fault, SWF, extending into the deeper crust causing mantle rupture and lithospheric melt. The igneous body formed by the melt forms the main stress barrier.*

adds to the strain build-up along a rift fault presumably passing over the flank of the igneous mass as shown in the conceptual model (**Figure 10**) drawn on the basis of the available geological and seismotectonic data [19]. The rift fault SWF that extends to the deeper crust is a sub-vertical planar fault bounding the basement domino block in the upper crust. It extends into the deeper crust becoming a low-angle rift fault in the semi-ductile layer of the deeper crust (**Figure 10**). This pattern of the fault along the flank of the igneous mass matches with the pattern of distribution of hypocentres of aftershocks. This indicates that the SWF is the causative fault for repeated earthquake generation [16, 19].
