The post-breakup evolution of the western Indian high-elevation passive margin

QE Geology;Q Science (General)

Campanile, Daniel J. ; Bishop, Paul

University:University of Glasgow

Department:School of Geographical and Earth Sciences

关键词: Low temperature thermochronometery, Western India, Mass balance analysis, lithospheric flexure, passive margin, fission track, (U-Th)/He;   

Others  :  http://theses.gla.ac.uk/38/1/2007campanilephd.pdf

来源: University of Glasgow

【 摘 要 】

The tectonic development of the Western Indian high-elevation passive margin is complex. At least two major rifting events (India/Madagascar and India/Seychelles) and a major hotspot (The Reunion plume) are believed to have been instrumental in the formation and development of the margin. However, the temporal and spatial extent of these major tectonic events remains poorly constrained. The Western Ghats of India has also been cited as a type example of a downwarped, elevated passive continental margin. However, published low temperature thermochronometry suggests downwearing or parallel escarpment retreat as alternative models of margin evolution. Here are present the results of a sediment mass balance study utilising new data for the offshore portion of the Western Indian margin, new onshore apatite fission track and (U-Th)/He thermochronometry for the onshore portion of the Western Indian margin, and flexural isostatic modelling. The combined methodologies used within this study are used to resolve some of the fundamental questions regarding the tectonic development and subsequent long term landscape evolution of the Western Indian margin.The Konkan-Kerala basin is a major depocentre for sediment from the onshore hinterland of Western India and as such provides a valuable record of the timing and magnitude ofCenozoic denudation along the continental margin. This study presents an analysis of sedimentation in the Konkan-Kerala basin, coupled with a mass balance study, in order totest competing conceptual models for the development of the Western Indian margin. An estimated 109,000 km3 of Cenozoic clastic sediment are present within the Konkan-Keralabasin, a volume difficult to reconcile with the denudation of a downwarped rift flank onshore, and more consistent with denudation of an elevated rift flank. There is evidencefor two major pulses in sedimentation; an early phase in the Palaeocene, and a second beginning in the Pliocene. The Palaeocene increase in sedimentation can be interpreted interms of a denudational response to the rifting between India and The Seychelles, whereas the mechanism responsible for the Pliocene pulse is more enigmatic. Mass balance analysis only provides spatially and temporally averaged estimates for denudation; consequently, this study presents new low temperature thermochronology from onshore regions to constrain the onshore pattern of denudation. Apatite fission track ages increase from c.55 Ma at the coast to <350 Ma at the escarpment with mean confined track length between 11.3 µm and 14.3 µm. Apatite (U-Th)/He ages range from 37 Ma to 123 Ma and zircon (U-Th)/He ages range from 324 Ma to 426 Ma. These data, when modelled, are consistent with accelerated denudation contemporaneous with rifting between India and The Seychelles. Inverse-modelling of the thermochronmetric data suggest denudation of at least 4.5 km at the coast decreasing to more modest amounts of denudation of between 1.5km and 2.5km further inland towards the escarpment. The pattern of denudation inferred from low temperature thermochronometry is consistent with escarpment evolution into an elevated rift flank.The flexural response of the Western Indian margin to sediment loading and denudational unloading can be modelled as a thin elastic beam overlying a fluid substratum. Modelling the isostatic response of the Western Indian lithosphere to sediment loading offshore and denudation onshore infer that flexural isostasy is an important component in the development of the Margin. However, flexural isostasy alone cannot account for the pattern of subsidence offshore or the pattern of uplift onshore and additional mechanisms must be invoked.