Tertiary palaeosurfaces of the SW Deccan, Western India: implications for passive margin uplift

Two genetically distinct lateritized palaeosurfaces of different ages are recognized in the southwest Deccan Traps region of Western India using a combination of geochemical, topographical, and satellite image data. The Deccan Traps were erupted at the Cretaceous-Tertiary boundary (c. 65 Ma), and comprise a huge area of originally near-horizontal basalt lavas covering much of northwest Peninsular India, and topographically forming the coast-parallel Western Ghats escarpment and elevated Maharashtra plateau to the east. Remnants of the older, palaeosurface currently exist as a series of isolated, laterite-capped plateaux forming the highest elevations along the Western Ghats (15°30'–18°15' N). This surface is of late Cretaceous-early Tertiary age, and originally developed upon flows which lay at, or near to, the top of the lava sequence. This lateritization phase was terminated by a period of uplift and extensive erosion in lower- to mid-Tertiary times during which the low-lying, low-relief coastal (Konkan) plain developed through the eastward recession of the Ghats scarpline. A second phase of lateritisation occurred upon this coastal pediplain during mid- to late Tertiary times. Since the earlier uplift had gently deformed the lava pile prior to the development of the pedimented surface, the low-level Konkian laterite lies with marked angular unconformity upon the lava stratigraphy. Both surfaces have been subject to further large-scale distortion resulting from continuing uplift effects. Development and evolution of these Deccan palaeosurfaces is important since together they provide a record of uplift effects in western India. Moreover, they offer a datum against which the uplift erosional history may be further constrained and demonstrate that uplift effects have acted upon the Indian margin throughout the Tertiary. Since such longevity of uplift is difficult to reconcile with the commonly cited thermal and dynamic post-rift mechanisms known to act upon passive margins, the morphological and structural evolution of the rifted Deccan margin is better described in terms of denudational isostasy