Seismogenesis of earthquakes occurring in the ancient rift basin of Kachchh, Western India - Intraplate Earthquakes - page 129

located in the lower crust. The presence of a magmatic intrusive body has been inferred for
KR lower crust, which may accumulate stress and lead to brittle failure. Some low-velocity
patches are found in and around the intrusive, which are speculated to be fluid-filled
asperities.

6.2 Tectonic framework, structure, and tectonic evolution of Kachchh Rift basin

6.2.1 Structure and tectonics

The western margin of the Indian plate is the locale of three failed pericratonic rifts formed
during the breakup of Gondwanaland involving stretching and thinning of the crust (Biswas,
1987 , 2005 ) : the KR formed earliest during the Late Triassic (210 Ma), followed by the
Cambay Rift, which formed during the Early Cretaceous, and the Narmada Rift, which
formed during the Late Cretaceous. The crust is found to be 35-37 km thick in the KR
compared to 38-42 km in the surrounding region (Mandal, 2012a). Rift evolution with syn-
rift sedimentation continued through the Jurassic till the Early Cretaceous as the Indian plate
separated fromAfrica and drifted northward along an anticlockwise path. The rift expanded
from north to south by successive reactivation of primordial faults of the Mid-Proterozoic
Delhi fold belt. The NE-SW strike of the Delhi-Aravalli fold belt swings to E-W in the
Kachchh region (Biswas, 1987 ) . The E-W-trending rift basin has a series of E-W-trending
faults ( Figure 6.2 ) . The KR is bound by the Nagar Parkar uplift on the north and the
Kathiawar uplift (Saurashtra horst) on the south, lying along the sub-vertical Nagar Parkar
and North Kathiawar faults (NPF and NKF). The rift is styled by three main uplifts (from
north to south), Island Belt, Wagad, and Kachchh Mainland, along three intrarift faults,
Island Belt (IBF), Kachchh Mainland (KMF), and South Wagad (SWF), with intervening
grabens and half-grabens. The Island Belt uplift is a narrow south-tilted basement ridge,
which is broken and displaced by tear faults into four separate uplifts described as “islands.”
The uplifts are upthrust basement blocks tilted along sub-vertical faults with initial normal
separation. The structure is characterized by tilted blocks and half-grabens within a south-
tilted asymmetric rift basin. The NKF is the bounding master fault along which the rift
subsided most. All the faults are sub-vertical, dipping 90 towards the adjacent half-

graben or graben (Biswas, 1987 ) . Blanketing sediments over the basement drape over the
tilted edges of the upthrusts as marginal flexures, which are narrow deformation zones along
master faults enclosing complicated folds, locally much faulted and intruded by igneous
rocks. In the western part the step-faulted uplifts are tilted to the south, with flexures draped
over the faulted uplift northern edges (Biswas, 2005 ) . In the eastern part a large uplift,
Wagad, occurs between the Mainland and Island Belt uplifts. It is tilted opposite to the
north with a narrow deformation zone along the faulted southern edge (Biswas, 2005 ) . The
back-slope ends against the Bela horst of the Island Belt uplift. The Mainland and Wagad
uplifts occur in en echelon pattern. The KMF and SWF are parts of a left-stepping dextral
strike-slip fault system (Biswas and Khattri, 2003 ) . The SWF is the eastward continuation
of the KMF after side-stepping with an overlap zone between Bhachau and Adhoi. Another
important tectonic feature in the KR zone is a subsurface basement ridge - Median High -

° to 75 °