Crustal structure along Mehmadabad-Billimora profile in the Cambay basin, India, from deep seismic soundings

Abstract

Deep Seismic Sounding (DSS) investigations were carried out along a 220-km long N-S profile from Mehmadabad (22°50'N 72°46'E) to Billimora (20°46'N 72°58'E) in the Cambay basin, India, for studying the detailed crustal structure in the region. Seismic refraction as well as narrow and wide-angle reflection travel-time data were obtained for various crustal boundaries, starting from the sedimentary section down to the Moho, by continuous profiling. Analysis of reversed reflection and refraction travel-time data has yielded crustal velocity—depth functions for the Mehmadabad—Broach and Broach-Navsari sections along the profile, which are significantly different from each other. The average velocities at depths in the Magmadabad—Broach section have been found to be considerably lower than those in the Broach—Navsari section, by about 25% at 5 km, 17% at 10 km, 7% at 20 km, and 2% at the Moho depth and even deeper down in the upper mantle. The presence of a relatively thicker sedimentary section, especially in the Jambusar—Broach area, as revealed by the reversed refraction data analysis, is responsible for this gross difference in the average velocity functions for the two parts of the profile and the same have therefore been used for obtaining the detailed crustal depth along the profile. The Cambay basin, which is well known to be bounded by step faults on the eastern and western margins, is also dissected into seven major crustal blocks in the N-S direction from Mehmadabad to Billimora. The various crustal blocks identified are, from the north: 1. (I) Mehmadabad block 2. (II) Tarapur block 3. (III) Petlad—Katana block 4. (IV) Jambusar—Broach block 5. (V) Kosamba block 6. (VI) Surat block, and 7. VII) Navsari—Billimora block. These blocks are displaced, up or down, relative to the adjacent blocks due to movements along the faults bounding them. Most of the faults have been found to extend quite deep and to affect the Moho boundary considerably, as is quite evident from the relief of the Moho found along the profile. Further, the seismic activity observed in the region, during historical as well as modern times, suggests that these deep faults may even be currently active. The Jambusar—Broach block, which is well known as a broad regional syncline with the maximum thickness of Cenozoic sediments, has been found to be a major graben revealing a maximum crustal thickness of 38–40 km. The granitic basement in this graben is also the deepest, at 6.0–6.5 km depth. Towards the north and south of this major graben, the Moho as well as the granitic basement are found to be at relatively shallower depths in various crustal blocks. The boundary velocity at the Moho has been found to be 8.1–8.2 km/sec, whereas at the granitic basement level there seems to be a lateral velocity variation from 5.7 to 6.3 km/sec. The crustal depth section obtained in the present study reveals a close relationship between the deep and shallow structural features in this region. The Deccan Traps, which form the basement of the overlying Tertiary sediments over most part of the profile, vary in thickness from 1.0 to 1.8 km. However, the Trap surface rises to very shallow depths on the southern part of the profile. The boundary velocity in the Deccan Traps has been found to vary between 4.8 and 5.2 km/sec. In the Anklesvar-Kosamba area, there is also evidence for the presence of a 1.2 km thick layer of Mesozoic sediments (with a velocity of about 4.0 km/sec) underlying the 1.1 km thick Deccan Traps layer.