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7.2 Plate Tectonics and Passive Margin Evolution

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7.2 Plate Tectonics and Passive Margin Evolution Campanile, Daniel J. (2007) The post-breakup evolution of the western Indian high-elevation passive margin. PhD thesis. http://theses.gla.ac.uk/38/ Copyright and moral rights for this thesis are retained by the author A copy can be downloaded for personal non-commercial research or study, without prior permission or charge This thesis cannot be reproduced or quoted extensively from without first obtaining permission in writing from the Author The content must not be changed in any way or sold commercially in any format or medium without the formal permission of the Author When referring to this work, full bibliographic details including the author, title, awarding institution and date of the thesis must be given Glasgow Theses Service http://theses.gla.ac.uk/ [email protected] The post break-up evolution of the Western Indian high-elevation passive margin Daniel Campanile B.Sc. (Hons.), Glasgow Thesis presented for the degree of Doctor of Philosophy (Ph.D.) University of Glasgow Department of Geographical and Earth Sciences September 2007 Abstract Abstract 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 of Cenozoic 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 to test 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-Kerala basin, 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 evidence for 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 in terms 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 Daniel Campanile i Abstract 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. Daniel Campanile ii Table of contents Table of contents Abstract ................................................................................................................. i Table of contents ................................................................................................ iii List of figures ...................................................................................................... vi List of tables...................................................................................................... viii Acknowledgements ............................................................................................ ix Declaration .......................................................................................................... xi 1 Introduction ................................................................................................. 1 1.1 Introduction ........................................................................................... 1 1.2 Passive margins.................................................................................... 2 1.2.1 Surface uplift mechanisms .........................................................2 1.2.2 Conceptual models of passive margin evolution ........................4 1.3 The Western Indian margin................................................................... 7 1.3.1 Geological background...............................................................7 1.3.2 Margin physiography ................................................................10 1.3.3 Rifting and tectonic history .......................................................12 1.3.4 Margin geomorphology.............................................................15 1.4 The current study ................................................................................ 20 1.5 Predictive forward modelling ............................................................... 21 1.5.1 Introduction...............................................................................21 1.5.2 Methods and model parameters...............................................21 1.5.3 Forward modelling results for the Northern field area...............24 1.5.4 Forward modelling results for the Southern field area ..............26 2 Mass balance analysis.............................................................................. 30 2.1 Introduction ......................................................................................... 30 2.2 The principles of mass balance analysis............................................. 33 2.2.1 The coupled erosional-depositional system .............................33 2.2.2 Mass balance analysis – methods and assumptions................36 2.3 Mass balance analysis of Western India............................................. 37 2.3.1 The offshore area .....................................................................37 2.3.2 The onshore area.....................................................................42 2.4 Methods .............................................................................................. 43 2.4.1 Quantifying sediment in the Konkan-Kerala basin....................43 2.4.1.1 Obtaining compacted sediment volumes ......................43 2.4.1.2 Decompacting sediment volumes .................................46 2.4.2 Calculating the onshore denuded crustal prism........................48 2.5 Results................................................................................................ 50 Daniel Campanile iii Table of contents 2.6 Discussion and implications ................................................................ 52 3 Low temperature thermochronometry..................................................... 55 3.1 Introduction ......................................................................................... 55 3.2 Apatite fission track thermochronometry............................................. 57 3.2.1 Introduction...............................................................................57 3.2.2 AFTT annealing kinetics...........................................................58 3.2.3 Calculating AFTT ages.............................................................61 3.2.4 Fission track confined track length distributions (TLD’s) ..........62 3.2.5 Interpreting AFTT data .............................................................62 3.2.5.1 AFTT age profiles and TLDs .........................................63 3.2.5.2 Forward and inverse-modelling of AFTT data ...............67 3.3 (U-Th)/He thermochronometry ............................................................ 68 3.3.1 Introduction...............................................................................68 3.3.2 (U-Th)/He
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