FLEXURAL AND STRATIGRAPHIC DEVELOPMENT OF THE WEST-INDIAN CONTINENTAL-MARGIN

Subsidence curves for 27 wells from the western continental margin of India show a characteristic late Oligocene to early Miocene (approxima tely 24 +/-5 Ma) rapid increase in subsidence rate superposed on the l ong-lived, slow subsidence typical of the thermal subsidence phase of passive continental margins. By subtracting a best fit negative expone ntial subsidence from the observed subsidence curves, we obtain an est imate of the distribution and magnitude of the ''excess'' subsidence a ffecting the Neogene development of the west Indian margin. The magnit ude of this excess subsidence increases seaward from the coast, rangin g from a few meters to >2000 m near the shelf edge. We examine the fol lowing hypotheses to explain the distribution and timing of this exces s subsidence: (1) modification of basin stratigraphy due to the effect s of lithospheric in-plane compression, (2) creation of accommodation space on the margin by flexural effects associated with Indus fan load ing, and (3) rapid growth of the continental margin and associated fle xural effects. Of the three hypotheses tested, the least important mec hanism to account for the observed excess subsidence is that of variat ions of lithospheric in-plane force, principally because maximum in-pl ane compression within the Indo-Australian plate was only achieved in the late Miocene. Because Indus fan sediment deposition began in the l ate Oligocene to early Miocene, we investigated three-dimensional flex ural effects associated with fan loading as a cause of the excess subs idence beginning at approximately 24 Ma. The distribution and magnitud e of modeled flexural deflection, however, are not consistent with the observed excess subsidence. Interpretation of seismic reflection data indicates that the margin has aggraded and prograded by almost-equal- to 100 km basinward since the Oligocene. Therefore, we evaluate the fl exural effects of this margin growth by estimating the amount of space infilled by margin progradation and aggradation since 24 Ma and compu ting the resulting deflection. This deflection matches the distributio n and magnitude of observed excess subsidence along the margin. In add ition, the distribution of the flexural bulge predicted from the combi ned deflections due to Indus fan and margin loading is spatially coinc ident with the distribution of exposed. marine terraces and drainage d ivides in the Saurastra Peninsula and the regions surrounding the gulf s of Cambay and Kutch, respectively. Available gravity, seismic reflec tion, refraction, and well data are consistent with our prediction of a 4000 to 5000 m thick sediment load developed during the Neogene alon g the outer margin. We propose that flexural deformation due to sedime ntary loading provides a potential tectonic feedback mechanism that af fects coastal and fluvial depositional processes. As regions in close proximity to the load are depressed, regions farther from the load exp erience uplift (i.e., the peripheral bulge), which is sufficient to ca use subaerial exposure of large portions of the shelf and to modify ex isting drainage networks. This feedback represents a mechanism for ind ucing relative sea level changes without invoking glacial eustasy.