۱- Introduction

۲- Geology, gravity, seismic network and data

۳- Methodology

۴- Results and discussions

۵- Conclusions

References

Abstract

We herein use the joint inversion of P-receiver functions and fundamental mode group velocity dispersion data (9–۷۰ s) of Rayleigh and Love waves to estimate crustal and lithospheric thicknesses at twenty three-component mobile broadband stations in Kachchh, Gujarat. Modeled Moho depths range from 35 to 43 km, while lithospheric thicknesses vary from 64 to 106 km. The main result of our modelling is the delineation of a marked crustal (∼۲–۴ km) as well as lithospheric (∼۱۰–۲۰ km) thinning and a 2–۶% drop in Vs across the lithosphere-asthenosphere-boundary (LAB), within the Samkhiali graben (associated with gravity high) underlying the central Kachchh rift zone (KRZ), where 95% of the continued aftershock activity took place since 2001. Such a large drop in Vs could be attributed to the presence of carbonatite melts in the upper mantle. Our modeling reveals a 4 km crustal thinning below the central KRZ and a 4 km crustal thickening below the surrounding riftless regions. This kind of crustal structure is inferred to induce large flexural deviatoric stresses (∼۵۰–۱۰۰ MPa) in the upper crust, thereby, these stresses in the presence of regional plate tectonic stresses can bring the Samkhiali graben below the central KRZ near to the critical stress level. While stress-transfer, meteoric water (in the upper crust), and metamorphic fluid as well volatile CO2 flows (in the lower crust) provide the required triggering effect to the critically stressed graben structure (down to 35 km depth) for generating continued aftershock activity in the Kachchh rift zone, since 2001. We also propose that the deeper circulation of volatile CO2 through the inferred conduit (related to the 65 Ma Deccan Plume activity) extending from lower crust down to asthenosphere plays a key role in the generation of uninterrupted occurrence of earthquakes in the Kachchh rift zone, Gujarat, India.

Introduction

The lithosphere underlying the Mesozoic Kachhchh rift zone (KRZ), northwest India, has been significantly affected by two-phase rifting episodes (viz. 184 Ma African and 88 Ma Madagascar) and the 65 Ma Deccan volcanism (Courtillot et al., 1986; Mahony et al., 2002; Sen et al., 2009). The influence of the above tectonic episodes on the crust-mantle structure has been seen through the mapped present-day thinning of crust and lithosphere below the Kachchh rift zone (Mandal, 2016 and 2017). Also, this effect has been reflected in the Bouguer gravity anomaly map of the region, showing alternate regional gravity highs (crustal thinning) and lows (crustal thickening) (Mishra et al., 2005; Khan et al., 2016).This thinning model gets further support from the results of local earthquake tomography, surface wave dispersion and receiver function modelling studies, which have imaged crustal mafic underplating, Moho upwarping and asthenospheric thinning underlying the 2001 Bhuj earthquake region (Mandal and Pujol, 2006; Mandal, 2012). In India, topography and crustal density heterogeneities associated with the Deccan volcanic province (India) have shown to induce intraplate stresses of the order of 100 MPa (Mandal et al., 1996). Thus, the crustal stresses associated with the density heterogeneities might play a key role in generating the intraplate earthquakes in the Kachchh rift zone also. It has been seen that the occurrences of earthquakes in the continental shelf as well as in the continental rift zones are found to be significantly influenced by flexural stresses (Stein et al., 1989; Assumpcao, 1998; Assumpco et al., 2011; Zoback and Richardson, 1996).