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Uplift area. The Bouguer gravity anomaly also indicates larger sedimentary thickness in
Wagad as well as Kachchh Mainland Uplift areas due to gravity lows as compared to high
gravity value areas of the Samkhiali basin and Banni area (Singh et al ., 2012 ) .
Figure 6.3c indicates that the south-dipping hidden NWF is a left step-over fault of
the KMF, being separated from it by about 30 km. The step-over zone between Bhachau
and Bharudia is a stressed zone of compressive stress ( Figure 6.1 , inset). The SWF is a
conjugate fault to the NWF and intersects it at mid-crust region. The 2001 mainshock
originated near this intersection. Such step-over faults have been suggested theoretically
by Segall and Pollard ( 1980 ) and Sibson ( 1986 ) , and through an analog sand-box model by
McClay and Bonora ( 2001 ) , and have been mapped in many areas (in Charleston, South
Carolina, by Dura-Gomez and Talwani ( 2009 ) as well as in the New Madrid seismic zone
by Russ ( 1982 ) , Pratt ( 2012 ) and others referred to therein).
6.2.2 Tectono-volcanic events
Tectonic episodes were accompanied by deep crustal magmatic activity (Biswas, 2005 ; Ray
et al ., 2006 ; Paul et al ., 2008 ; Sen et al ., 2009 ) . At least two phases of magmatic activity
are evident. The first activity took place during the extensional stage when ultramafic rocks
intruded into the older Jurassic sediments. Presumably at this time the deep-seatedmagmatic
body was emplaced at the site of the mantle rupture close to the basin center (Biswas, 2005 ) .
The second took place during the Late Cretaceous (65 Ma) post-rift uplift stage when
plume-related alkali and tholeiitic basalts were intruded into the younger Early Cretaceous
sediments and extruded as the Deccan Trap flows. The volcanic activity was associated
with thinning of the sub-Kutch lithosphere. Mandal ( 2010 ) found that the lithospheric
thickness varies from 62 to 63 km in the KR and from 65 to 77 km in the surrounding
region. The rifting started in the Late Triassic-Early Jurassic and continued through the
Deccan volcanism in the Late Cretaceous (Biswas, 1987 ) . The bulk of the Kutch alkalic
lavas came from an Indian Ridge mantle source rather than the Reunion plume. The Kutch
lithosphere was composed of spinel lherzolite that was largely converted to spinel wehrlite
by carbonatite metasomatism. Such carbonatite melts were generated by decompression
melting of the asthenosphere during its rise to shallower levels in response to extension and
thinning of the lithosphere. Mantle xenolith-bearing alkalic magmas were mainly generated
from a mixture dominated by asthenospheric material and the edge of the Deccan/Reunion
plume. These melts ascended, while picking up xenoliths, along pathways created by deep
rift faults. Tholeiites may have come from elsewhere in the south as plumelets, from the
hotter part of the Deccan plume head (Sen et al ., 2009 ) .
6.2.3 Tectonic evolution and existing earthquake generation models of
the Kachchh Rift zone
Rifting was aborted by the trailing edge uplift during the Late Cretaceous pre-collision stage
of the Indian plate, when the leading edge of the plate was slab-pulled towards the Tethyan
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