Most of the seismic activity in Asia occurs in highly deformed orogenic belts and is triggered by active deformations of the crust owing to India-Asia collision zone. However, unusual seismic activity occurs inside the continental Indian Plate (Fig. 1), which is often assumed to be a monolithic unyielding indenter. Although total number of events in the inner parts of the Indian Plate is less than that in the plate boundary zones, such as the Himalayas, the occurrence of strong earthquakes with magnitudes exceeding Mw 6.0 renders these intra-plate regions one of the most hazardous areas in India. For example, the 26 January, 2001 Bhuj earthquake Mw 7.5 occurred in the state of Gujarat in western India far from any tectonic boundaries and killed some 20,000 people. In central India, the May 22, 1997 Jabalpur earthquake Mw 6.0 damaged thousands of houses and killed 39 persons.
Many strong and moderate earthquakes in the inner parts of the Indian Plate occur along the Narmada-Son Lineament (NSL), which is one of the most prominent tectonic elements in continental India. The NSL is a structure with a length of approximately 1000 km and width of 50 km clearly visible in the topography. It is considered as a suture zone between the cratonic blocks that accreted in the geological past. This long E-W trending system appears to be playing a major role in the present day tectonics and crustal shortening in the peninsular India region. The active contractional deformations at NSL were proposed as early as 1971. There are several observations that suggest that the NSL, a failed rift zone under tensional regime in the geological past, is presently reactivating under a compressional regime. Several seismic and magnetotelluric surveys previously performed in different segments of the NSL reveal northward dipping structures that may support the model of underthrusting and shortening in these areas. The focal mechanisms of the main earthquakes along the NSL also indicate compression regime. Finally, the existing GPS observations directly show that the major contractional deformations of continental India are accommodated in the NSL zone. For example, four years measurements at two GPS stations south of Narmada rift at Burwani in the state of Madhya Pradesh (74.90 N 22.04 E) and Sagbara in the state of Maharashtra (73.79 N 21.54 E) indicate deformation rate of more than 3 mm/yr.
The cause of this unusual tectonic activity in intra-continental parts of India is still under debates. To understand the role of collisional processes in the origin of the recent contractional deformations at the NSL, we create a numerical thermo-mechanical model of shortening between the Indian Plate and Asia. We present also a new regional tomography model giving the information about the lithosphere structure of India that is used to define boundary conditions in the numerical modeling.
Collision Zone | Results
The information about the lithospheric structure may help to understand the cause of unusual tectonic activity in the intracontinental parts of India. There have been many studies that estimated the lithosphere thickness of India based on different geophysical investigations; however, many of them appear to be inconsistent, and in some cases contradictory. For example, various receiver functions studies report generally unchanged lithosphere thickness at around 100 km in different parts of continental India. An integral study for the whole Indian continent based on the S-p receiver functions shows significant variations of the lithosphere thickness: relatively thin lithosphere (80–100 km) in the northwestern and southern paths of India and thicker lithosphere (120–140 km) beneath the northeastern part of India. Surface-wave dispersion estimates appear to be not consistent with another regional and global studies based on the surface wave tomography that report thick lithosphere of up to 200 km in the northern India and thinner lithosphere (100–150 km) in the
Southern India. The later models appear to be consistent with body-wave tomography results. These examples show that the problem of the lithosphere thickness determination beneath India is still open and needs further investigations.
Here, we present another seismic model of the upper mantle based on P-wave travel time tomography. The details of the data description, methodology and testing are presented in the Method Section. The results of tomographic inversion are shown as P-wave velocity (Vp) anomalies at 100 km depth (Fig. 2). The other vertical and horizontal sections of this model are given in Supplementary Information. In the tomography results, we clearly observe a high Vp anomaly beneath the northern part of India extending down to 200 km, which can be interpreted as a thicker continental lithosphere. Beneath southern India, however, the high Vp anomaly is less prominent and alternates with lower Vp anomalies. Based on this, as well as previous geophysical studies, we propose that the NSL delimits thicker lithosphere in northern India and thinner lithosphere in southern India. Such a step-shaped structure may be the most plausible place for the initiation of under-thrusting of the thinner plate underneath the thicker one, as shown by previous modeling results. At the western margin of India at the depth of 100 km (Fig. 2), we observe a low-velocity anomaly with the strongest amplitude beneath the Deccan traps. We propose that this anomaly represents a zone of the thinned lithosphere degraded by the Reunion-Deccan hot spot. Previous numerical models have shown that trap-generating plumes may quickly destroy the continental lithosphere owing to the magmatism-induced lithospheric weakening and foundering. In the eastern part of India, we observe another low-velocity anomaly that may be associated with the Rajamahal traps. Weakening of the lithosphere owing to the presence of these two features at both ends of the NSL might be another factor facilitating the origin of a regional-scale fracture zone in the lithosphere.
This paper had been originally published in Scientific Reports. To cite the original work use
Koulakov, T. Gerya, B. K. Rastogi, A. Jakovlev, I. Medved, J. R. Kayal, S. El Khrepy and N. Al-Arifi,
2018, Growth of mountain belts in central Asia triggers a new collision zone in central India,
Scientific Reports, 8:10710 | DOI:10.1038/s41598-018-29105-2.
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