Seismogenesis of earthquakes occurring in the ancient rift basin of Kachchh, Western India - Intraplate Earthquakes - page 134

trench (Biswas, 2005 ) . The uplift caused structural inversion during the rift-drift transition
stage, when most of the uplifts with drape folding over the edges came into existence by
upthrusting of the basement domino blocks along the master faults. This created first-order
marginal flexures over the foothill uplifts. Lateral motion during the drift stage of the
plate induced horizontal stress and near-vertical normal faults, which were reactivated as
reverse faults during initiation of the inversion cycle, and became strike-slip faults involving
divergent oblique-slip movements (Biswas, 2005 ) . The present structural style evolved by
right-lateral slip, which shifted the uplifts progressively eastward relative to each other
from south to north. This resulted in the present en echelon positioning of the uplifts with
respect to the Kutch Mainland uplift. The strike-slip related structural changes modified
the linear flexures, breaking them into individual folds at the restraining and releasing
bends. Narrow deformation zones complicated by second-order folds and conjugate Riedel
faults formed along the master faults, modifying the initial drape folds (Biswas, 2005 ) .
Syntectonic intrusions further modified the shape and geometry of the individual second-
order structures. Igneous rocks extensively intruded the Mesozoic sediments during rifting
followed by post-rift hotspots related to Deccan volcanism (Sen et al ., 2009 ) . Studies of the
intrusive rocks and seismological data suggest the presence of mafic/ultramafic magmatic
bodies close to the crust-mantle boundary (Mandal and Pujol, 2006 ; Mandal and Chadha,
2008 ; Mandal and Pandey, 2010 ) .
Inversion continued during the post-collision compressive stress regime of the Indian
plate and the KR basin became a shear zone with transpressional strike-slip movements
(with thrusting at depth) along the active sub-parallel rift faults (Biswas, 2005 ) . The same
tectonic phase is continuing, as evident from neotectonic movements along these faults
that are responsible for the present first-order geomorphic features and seismicity. In the
current tectonic cycle, under N-S compressive stresses, the NWF and SWF are the most
active faults, as evident from the concentration of aftershock hypocenters in the overlap
zone (Biswas, 2005 ; Mandal and Horton, 2007 ) . Pulses of movement along these faults are
responsible for generation of new fault fractures within the respective deformation zones.
These new fractures are propagating through the recent piedmont and scarp-fan sediments
in the frontal zones of the thrusts, as seen in the trenches dug close to the KMF and KHF
(Malik et al ., 2008 ; Morino et al. , 2008a, b) and in GPR surveys. The morphotectonic
features also indicate Quaternary uplift along the above-mentioned master faults (Malik
et al ., 2008 ) .
During the present compressive stage, the Radhanpur Arch acts as a stress barrier for
eastward movements along the principal deformation zones, which is creating additional
strain in this part of the basin between the arch and theMedianHigh (Biswas, 2005 ) . Towards
the eastern end of the Mainland uplift, the right lateral KMF becomes the SWF by left-
stepping with an overlap in the region between Samakhiali and Lakadiya (Biswas, 2005 ) .
This overstep zone - formed initially as the Samakhiali-Lakadiya graben - is presently
a convergent transfer zone undergoing transpressional stress in the strained eastern part
of the basin. This is the most strained part of the basin. Expectedly, this is the most
favored site for rupture nucleation. The occurrence of the 2001 Bhuj quake (M w 7.7) in