http://www2.ess.ucla.edu/~yin/05-Publications/papers/091-Yin-2006-ESR.pdf

Cenozoic tectonic evolution of the Himalayan orogen as constrained by along-strike variation of structural geometry, exhumation history, and foreland sedimentation

The Himalaya and Tibet provide an unparalleled opportunity to examine the complex ways in which continents respond to collisional orogenesis. This paper is an attempt to synthesize the known geology of this orogenic system, with special attention paid to the tectonic evolution of the Himalaya and southernmost Tibet since India-Eurasia collision at ca. 50 Ma. Two alternative perspectives are developed. The first is largely historical. It includes brief (and necessarily subjective) reviews of the tectonic stratigraphy, the structural geology, and metamorphic geology of the Himalaya. The second focuses on the processes that dictate the behavior of the orogenic system today. It is argued that these processes have not changed substantially over the Miocene–Holocene interval, which suggests that the orogen has achieved a quasi–steady state. This condition implies a rough balance between plate-tectonic processes that lead to the accumulation of energy in the orogen and many other processes (e.g., erosion of the Himalayan front and the lateral flow of the middle and lower crust of Tibet) that lead to the dissipation of energy. The tectonics of the Himalaya and Tibet are thus intimately related; the Himalaya might have evolved very differently had the Tibetan Plateau never have formed.

Tectonics of the Himalaya and southern Tibet from two perspectives

The timing of India-Asia collision onset – Facts, theories, controversies

Metamorphic chemical geodynamics in continental subduction zones

The channel flow model aims to explain features common to metamorphic hinterlands of some collisional orogens, notably along the Himalaya–Tibet system. Channel flow describes a protracted flow of a weak, viscous crustal layer between relatively rigid yet deformable bounding crustal slabs. Once a critical low viscosity is attained (due to partial melting), the weak layer flows laterally due to a horizontal gradient in lithostatic pressure. In the Himalaya–Tibet system, this lithostatic pressure gradient is created by the high crustal thicknesses beneath the Tibetan Plateau and ‘normal’ crustal thickness in the foreland. Focused denudation can result in exhumation of the channel material within a narrow, nearly symmetric zone. If channel flow is operating at the same time as focused denudation, this can result in extrusion of the mid-crust between an upper normal-sense boundary and a lower thrust-sense boundary. The bounding shear zones of the extruding channel may have opposite shear sense; the sole shear zone is always a thrust, while the roof shear zone may display normal or thrust sense, depending on the relative velocity between the upper crust and the underlying extruding material. This introductory chapter addresses the historical, theoretical, geological and modelling aspects of channel flow, emphasizing its applicability to the Himalaya–Tibet orogen. Critical tests for channel flow in the Himalaya, and possible applications to other orogenic belts, are also presented. The hinterlands of collisional orogens are often characterized by highly strained, high-grade metamorphic rocks that commonly display features consistent with lateral crustal flow and extrusion of material from mid-crustal depths towards the orogenic foreland. A recent model for lateral flow of such weak mid-crustal layers has become widely known as the ‘channel flow’ model. The channel flow model has matured through efforts by several research groups and has also been applied to a variety of geodynamic settings. Thermal-mechanical modelling of collision zones, including the Himalayan–Tibetan system, has brought the concept of channel flow to the forefront of orogenic studies. Original contributors to the concept of channel flow initiated an important paradigm shift (Kuhn 1979), from geodynamic models of continental crust with finite rheological layering to the more encompassing channel flow model. This time-dependent midto lower crustal flow process, which will be reviewed in this chapter, may progress into foreland fold-and-thrust tectonics in the upper crust, thereby providing a spatial and temporal link between the early development of a metamorphic core in the hinterland and the foreland fold-and-thrust belt at shallower structural levels. Outcomes and implications of such a viscous flowing middle to lower crust include a dynamic coupling between mid-crustal and surface processes, and limitations to accurate retro-deformation of orogens (non-restorable orogens, e.g. Jamieson et al. 2006). This Special Publication contains a selection of papers that were presented at the conference ‘Channel flow, extrusion, and exhumation of lower to mid-crust in continental collision zones’ hosted by the Geological Society of London at Burlington House, in December 2004. Because most of the ongoing debate on crustal flow focuses on the Cenozoic age Himalaya–Tibet collisional system, some of the key questions that are addressed in this volume include the following. . Does the model for channel flow in the Himalaya–Tibet system concur with all available geological and geochronological data? From: LAW, R. D., SEARLE, M. P. & GODIN, L. (eds) Channel Flow, Ductile Extrusion and Exhumation in Continental Collision Zones. Geological Society, London, Special Publications, 268, 1–23. 0305-8719/06/$15.00 # The Geological Society of London 2006. . How do the pressure–temperature-time (P-T-t) data across the crystalline core of the Himalaya fit with the proposed channel flow? . Are the microstructural fabric data (pure shear and simple shear components) compatible with crustal extrusion (thickening or thinning of the slab)? . If the channel flow model is viable for the Himalaya–Tibet system, what may have initiated channel flow and ductile extrusion? . Why did the extrusion phase of the Himalayan metamorphic core apparently cease during the late Miocene–Pliocene? . Are some of the bounding faults of the potential channel still active, or were they recently active? . Is the Himalayan channel flow model exportable to other mountain ranges? This introductory paper addresses the historical, theoretical, geological and modelling aspects of crustal flow in the Himalaya–Tibet orogen. Critical tests for crustal flow in the Himalaya, and possible applications to other orogenic belts, are presented and difficulties associated with applying these tests are discussed. Personal communication citations (pers. comm. 2004) identify comments expressed during the conference. The Himalaya–Tibetan plateau system The Himalaya–Tibet system initiated in Early Eocene times, following collision of the Indian and Eurasian plates (see Hodges (2000) and Yin & Harrison (2000) for reviews). The collision resulted in closure of the Tethyan Ocean, southward imbrication of the Indian crust, and northward continental subduction of Indian lower crust and mantle beneath Asia. The collision thickened the southern edge of the Asian crust to 70 km, and created the Tibetan Plateau, the largest uplifted part of the Earth’s surface with an average elevation of 5000 m (Fielding et al. 1994). The Himalayan orogen coincides with the 2500km-long topographic front at the southern limit of the Tibetan Plateau. It consists of five broadly parallel lithotectonic belts, separated by mostly north-dipping faults (Fig. 1). The Himalayan metamorphic core, termed the Greater Himalayan sequence (GHS), is bounded by two parallel and opposite-sense shear zones that were both broadly active during the Miocene (Hubbard & Harrison 1989; Searle & Rex 1989; Hodges et al. 1992, 1996). The Main Central thrust (MCT) zone marks the lower boundary of the GHS, juxtaposing the metamorphic core above the underlying Lesser Himalayan sequence. The South Tibetan detachment (STD) system defines the upper boundary roof fault of the GHS, marking the contact with the overlying unmetamorphosed Tethyan sedimentary sequence. The apparent coeval movement of the MCT and STD, combined with the presence of highly sheared rocks and high grade to migmatitic rocks within the GHS, has led many workers to view the GHS as a north-dipping, southward-extruding slab of mid-crustal material flowing away from the thick southern edge of the Tibetan Plateau, towards the thinner foreland fold-thrust belt. Dynamics of channel flow The concepts of crustal extrusion and channel flow originated in the continental tectonics literature in the early 1990s. Unfortunately, these two processes are often referred to interchangeably without justification. One of the main points that emerged from the Burlington House conference was that a distinction between channel flow and crustal extrusion must be made. Parallel versus tapering bounding walls on channel flow and/or extrusion processes, and how these processes may replenish over time, are two resolvable parameters that are critical for distinguishing channel flow from extrusion. Brief definitions and overviews of the two processes are presented below. A more detailed overview of the mechanics of the related processes is provided by Grujic (2006).

/pdf/channel-flow-ductile-extrusion-and-exhumation-in-continental-wontz6tvla.pdf

Channel flow, ductile extrusion and exhumation in continental collision zones: an introduction

The south Tibetan detachment system in the Rongbuk valley, Everest region. Deformation features and geological implications

Abstract The Lesser Himalayan units, the base of the Tethyan sedimentary sequence and the intervening High Himalayan Crystallines have been studied along a transect in the eastern Nepal Himalaya and southern Tibet. The lowest Lesser Himalayan tectonic unit exposed along the transect is the parautochthonous Tumlingtar Unit, a thick sequence of phyllite and quartzite of presumed Late Proterozoic age. Metamorphic grade is low (chlorite and biotite zones) and deformation much weaker than in the overlying unit, the Lesser Himalayan Crystallines, a unit including large bodies of granitic orthogneiss of probable Early Palaeozoic age. Metamorphic grade of the Lesser Himalayan Crystallines is higher than in the Tumlingtar Unit, being characterized by garnet-biotite-muscovite-kyanite-staurolite assemblages in metapelites and biotite-muscovite-kyanite in blastomylonitic orthogneiss. A belt of metasedimentary and metagranitic thrust sheets separate the Lesser Himalayan Crystallines from the overlying High Himalayan Crystallines (HHC). Its position corresponds to that of the Main Central Thrust (MCT) as defined in central Nepal. The HHC are a gneiss sequence of 6–7 km visible thickness in which metasediments of upper amphibolite facies (Barun Gneiss) are associated with large tabular bodies of granitic orthogneiss that occur especially in the middle part of the sequence and which display evidence of water-deficient partial melting. The age of the granite protolith from which the orthogneiss derives is Early Paleozoic, possibly Cambrian. The top of the HHC consists of biotite paragneiss and micaschist (Black Gneiss) hosting networks and lens-shaped bodies of tourmaline leucogranite which may attain a thickness of 1-2 km (Makalu, Baruntse) and evolve into cylindrical diapirs with gently north-plunging long axes (Nuptse). At the regional scale the top of the HHC coincides with the muscovite-in isograd and appears as a major intracrustal detachment zone which brought the HHC into contact with low-grade metasediments of the Tethyan Zone (North Col Fm) during the Miocene uplift of the Himalayan nappe pile, with attendant muscovite dehydration melting of the metasediments occurring at the top of the HHC. The top of the HHC, the overlying North Col Fm. and the leucogranite sills hosted in both, are affected by a ductile extensional tectonics which appears broadly synchronous with granite emplacement and is characterized by extension lineations trending 20° to 40°. Extensional structures occur as far south as Makalu, whereas further south ductile deformations have the same orientation but are compressional. In the area south of Makalu the ductile deformation involves some of the Miocene leucogranite bodies, suggesting that Miocene compression in the lower and middle HHC may have been synchronous with ductile extension in the upper HHC. Relations between metamorphism and deformation are inconsistent with a wholly post-metamorphic thrusting of the HHC over the Lesser Himalayan units and rather suggest that the Miocene exhumation of the HHC may be better explained by changes in the internal dynamics of the orogenic wedge produced by the Early Himalayan collision between northward-drifting India and Eurasia.

Geology and tectonomagmatic evolution of the eastern Himalaya along the Chomolungma-Makalu transect

Geology of the Higher Himalayan Crystallines in Khumbu Himal (Eastern Nepal)

Late Hercynian shear zones in Sardinia

'Guide - book to the excursion on the paleozoic basement of Sardinia.'

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