School of Earth Sciences - Theses
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ItemThe tectonic evolution of the Songpan Garzê Fold Belt, Southwest China( 2001)Structural observations suggest that all pervasive deformation of the Songpan Garzê Fold Belt occurred continuously and progressively during the Indosinian Orogeny (c.230-200 Ma), and prior to widespread post-tectonic magmatism c.195-180 Ma. Development of the arcuate structural grains and discrete high-grade metamorphic complexes that characterise the southeast Songpan Garzê Fold Belt can be explained entirely in terms of this progressive Indosinian deformation. Likewise, all greenschist-amphibolite facies Barrovian-style metamorphism within the southeast Songpan Garzê Fold Belt is correlated with Indosinian orogenesis. Thermodynamic modelling of micaceous, garnet- and aluminosilicate-bearing metamorphic assemblages, all of which define Indosinian-aged tectonic fabrics, suggests a continuous textural evolution that occurred under the influence of a simple clockwise P-T-t path. There is no structural or metamorphic evidence to substantiate the pervasive Yenshanian (Jurassic-Cretaceous) or Himalayan (Tertiary-Recent) orogenic overprints that previous studies of the terrane have inferred. The distribution of progressive structure and locally variable timing relationship between progressive deformation and peak metamorphism imply that Indosinian deformation occurred diachronously across the southeast Songpan Garzê Fold Belt. Such deformation was characterised by the westward migration of progressive structural ‘fronts’, towards the interior of the terrane, in response to lateral growth of the transpressive interface between the Songpan Garzê Fold Belt and adjacent Yangtze Craton. In contrast, consistent timing relationships between peak metamorphism, basement anatexis and the diapiric exhumation of high-grade metamorphic rocks suggest that the thermal maturation of the terrane was spatially and temporally uniform across the southeast Songpan Garzê Fold Belt. Post-Indosinian modification of the Songpan Garzê Fold Belt was limited to brittle strike-slip dissection, predominantly focussed into the discrete west-northwest-trending Xianshui He and Kunlun fault zones. Sinistral displacement of these two fault zones, from at least the Miocene onward, transported two large tracts of the terrane towards the southeast, effectively intact. This displacement was absorbed by reactivation of the Longmen Mountains Thrust Nappe Belt and transpressive exhumation of the Min Shan Uplift Zone. Modification of the Songpan Garzê Fold Belt did not accommodate any significant vertical distortion that might have contributed to isostatic rebound of the present-day 4500-metre Tibetan Plateau. Nevertheless, uplift of the greater Songpan Garzê Fold Belt occurred from at least the Neogene onward, accommodated by reactivation of the Songpan Garzê/Yangtze interface and present-day boundary of the northeast Tibetan Plateau. New 40Ar/39Ar and Rb/Sr thermochronology from southeast Songpan Garzê Fold Belt identifies two discrete cooling episodes: 1) late-Triassic-Jurassic cooling from peak-metamorphic conditions of >500° C to temperatures of around 350° C; and 2) rapid Tertiary-Recent cooling to near-surface temperatures. Both cooling episodes are interpreted to record erosional exhumation of the terrane. Whilst Mesozoic cooling is correlated with limited post-Indosinian rebound of the thin-skinned Songpan Garzê accretionary wedge, Tertiary-Recent cooling is thought to reflect an isostatic potential associated with Himalayan Orogenesis in central Asia. Surprisingly, this latter event does not seem to have been coupled to the rapid Neogene uplift of the Tibetan Plateau. Tertiary-Recent exhumation, uplift and structural modification of the Songpan Garzê Fold Belt are inferred to have been bolstered by a pervasively thickened viscous lower crust. This thickening is attributed to penetrative east-directed deformation of deeper lithosphere beneath the detached thin-skinned veneer of the Songpan Garzê Fold Belt. Such an interpretation favours penetrative deformation of the mid-lower crust as the dominant mechanism by which India/Eurasia intracontinental convergence was absorbed - not lateral escape or extrusion of eastern Asia - and highlights the role that the Yangtze Craton has played throughout Mesozoic and Tertiary orogenesis: acting as a rigid backstop to penetrative deformation of the more plastic Songpan Garzê Fold Belt.