Subsidence curves for 27 wells from the western continental margin of India show a characteristic late Oligocene to early Miocene (approximately 24 +/-5 Ma) rapid increase in subsidence rate superposed on the long-lived, slow subsidence typical of the thermal subsidence phase of passive continental margins. By subtracting a best fit negative exponential subsidence from the observed subsidence curves, we obtain an estimate of the distribution and magnitude of the ''excess'' subsidence affecting the Neogene development of the west Indian margin. The magnitude of this excess subsidence increases seaward from the coast, ranging from a few meters to >2000 m near the shelf edge. We examine the following hypotheses to explain the distribution and timing of this excess subsidence: (1) modification of basin stratigraphy due to the effects of lithospheric in-plane compression, (2) creation of accommodation space on the margin by flexural effects associated with Indus fan loading, and (3) rapid growth of the continental margin and associated flexural effects. Of the three hypotheses tested, the least important mechanism to account for the observed excess subsidence is that of variations of lithospheric in-plane force, principally because maximum in-plane compression within the Indo-Australian plate was only achieved in the late Miocene. Because Indus fan sediment deposition began in the late Oligocene to early Miocene, we investigated three-dimensional flexural effects associated with fan loading as a cause of the excess subsidence beginning at approximately 24 Ma. The distribution and magnitude 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 flexural effects of this margin growth by estimating the amount of space infilled by margin progradation and aggradation since 24 Ma and computing the resulting deflection. This deflection matches the distribution and magnitude of observed excess subsidence along the margin. In addition, the distribution of the flexural bulge predicted from the combined deflections due to Indus fan and margin loading is spatially coincident with the distribution of exposed. marine terraces and drainage divides in the Saurastra Peninsula and the regions surrounding the gulfs of Cambay and Kutch, respectively. Available gravity, seismic reflection, refraction, and well data are consistent with our prediction of a 4000 to 5000 m thick sediment load developed during the Neogene along the outer margin. We propose that flexural deformation due to sedimentary loading provides a potential tectonic feedback mechanism that affects coastal and fluvial depositional processes. As regions in close proximity to the load are depressed, regions farther from the load experience uplift (i.e., the peripheral bulge), which is sufficient to cause subaerial exposure of large portions of the shelf and to modify existing drainage networks. This feedback represents a mechanism for inducing relative sea level changes without invoking glacial eustasy.
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