Subsidence curves for 27 wells from the western continental margin of India show a characteristic late Oligocene to early Miocene (∼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 ∼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 ≈100 km basin ward 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 fee...
Drilling of the Eratosthenes Seamount south of Cyprus documented incipient collision of the African and Eurasian plates. The oldest sediments recovered, mid?-Cretaceous shallow-water limestones, are overlain by Upper Cretaceous to Lower Oligocene pelagic carbonates, with several hiatuses. Following uplift, a carbonate platform was established in the Miocene; Eratosthenes was then below eustatic sea level during the Messinian desiccation crisis. The platform subsided to bathyal depths during the Lower Pliocene, associated with localized breccia deposition. Further subsidence occurred in Late Pliocene-early Quaternary, coeval with strong surface uplift of southern Cyprus. Subsidence and break-up of Eratosthenes was achieved by a combination of flexural loading and normal faulting. In addition, the Milano and Napoli mud volcanoes were drilled on the northern flank of the Mediterranean Ridge accretionary complex, south of Crete. A mainly extrusive, sedimentary origin is indicated. Multiple debris flows include clasts of sandstone and limestone of at least partly Miocene age. Both mud volcanoes are dated as τ1 Ma old and have been active episodically. Hydrocarbon gas is associated with both mud volcanoes, while methane hydrates (clathrates) exist locally at Milano. The driving force of mud volcanism is overpressuring caused by incipient plate collision. Messinian evaporites may have acted as a localized seal. Material escaped through a zone of backthrusting against rigid Cretan crust to the north.
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