Abstract The present study focuses on the delineation of the crust–mantle structure underlying the Kachchh rift zone (KRZ), by modelling P-wave receiver functions (P-RFs) and P-wave teleseismic tomography. Our RF study delineates marked crustal and lithospheric thinning below the central KRZ relative to the unrifted surrounding regions. This thinning model receives further support from crust-corrected normalized P-residuals, which suggest dominant negative residuals associated with the central KRZ. Teleseismic tomography using these P-residuals reveals low velocity to a depth of 170 km below the central KRZ, while there are positive residuals associated with the surrounding unrifted zones. Such a complex heterogeneous crust–mantle structure, which could be related to K/T boundary Deccan mantle plume activity and rift-related magmatism, might play a crucial role in seismogenesis of lower crustal earthquakes that have occurred in the KRZ since 2001. Inverted crust–mantle models obtained from P-RFs suggest a low shear velocity zone extending from 70–110 to 170–220 km depth beneath the central KRZ. This receives further support from the presence of low P-wave velocity down to 170 km modelled using teleseismic tomography. This low V p and V s zone in the upper mantle could be explained by the presence of trapped melts related to Deccan volcanism at 65 Ma or older rift-related magmatism.

Abstract Passive source seismic imaging delineates fine crustal and lithospheric structures associated with the Kachchh rift zone (KRZ), Gujarat, India, which suggests a 4-7 km crustal thinning and 6-12 km asthenospheric thinning below the KRZ relative to surrounding regions. The study also depicts a constant lithospheric thickness (~76-78 km) below the median high towards the west of Kachchh rift zone while a marked decrease in lithospheric thickness as well as shear velocity (Vs) is noticed across west to east and south to north of the rift zone, which perhaps suggests an increase in partial melt contents below the central KRZ in comparison to surrounding regions. Further, we notice that the seismogenic zone extends up to 34 km depth below the central KRZ. The hypocenters of the Bhuj 2001 earthquake sequence are found to be mainly concentrated in the mafic-to-ultramafic lower curst (14-34 km depth) below the central KRZ. The coincidence of common area of crustal thinning, asthenospheric upwarping and confined aftershock activity suggests that there is a possible causal relation between the occurrences of continued aftershock activity and a 6-10% drop in Vs (or presence of carbonatite melts) at lithosphere-asthenosphere boundary (LAB) below the central KRZ. This 6-10% drop in Vs can be attributed to a ~1-2% carbonatite melt currently present at the LAB. Thus, we interpret that the presence of aqueous fluids (released during the prograde metamorphic reactions of lower crustal olivine-rich rocks) and volatile CO 2 [emanating from the crystallization of carbonatite melts at shallow upper mantle depths (i.e. 50-70 km)] at the hypocentral depths, might be playing a key role in generating the 2001 Bhuj earthquake sequence covering the entire lower crust.