LINKING INHERITED INDIAN BASEMENT CROSS-STRIKE STRUCTURES UNDER THE HIMALAYAN SYSTEM WITH UPPER CRUSTAL FAULTS IN THE OVER-RIDING ASIAN PLATE
Three northeast-trending palaeotopographic ridges of faulted Precambrian Indian basement underlie the Ganga Basin south of the Himalaya. Analysis of spectrally filtered EGM2008 Bouguer gravity data and edges in its horizontal gradient at different source depths over northern Peninsular India, the Himalaya, and southern Tibet illustrates a crustal-scale fault origin for these ridges and succeeds in determining how far north beneath the Himalayan system they extend and how they ultimately govern the location of upper crustal graben faults in southern Tibet. Our interpretation thus suggests that south Tibet graben are spatially related to deep-seated crustal-scale faults rooted in the underplated Indian crust. These major discontinuities partition the Himalayan range into distinct zones, and ultimately contribute to lateral variability in tectonic evolution along the orogen’s strike.
How the Asian upper plate faults can root into the underplated Indian lower crust, through a zone of low viscosity mid-crust that is potentially plastically flowing eastward, is intriguing. We suggest that the graben-bounding normal faults root in the mid-crustal ductile flow zone, and that the E-W extension in the plateau is linked kinematically to mid-crustal eastward lateral flow. The graben are thus spatially controlled by the stress perturbation around the underlying basement shear zones. In this scenario, reactivation of the basement faults are not directly controlling graben formation, but basement faults act as ʻstress risers’ and serve as catalysts below the mid-crustal ductile channel, localizing stretching strain. Downward-propagating upper crustal normal faults may therefore nucleate above crustal necking zones, or step en échelon and relay above the interpreted basement structures instead of rooting directly into them.