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 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

Himalayan metamorphic sequence as an orogenic channel: Insight from Bhutan

This paper presents quantitative data on strain, deformation temperatures and vorticity of flow at the top of the Greater Himalayan Slab. The data were collected from the Tibetan side of the Everest Massif where two low-angle normal faults bound the upper surface of the Greater Himalayan Slab, the earlier and structurally lower Lhotse Detachment and the later and structurally higher Qomolangma Detachment. Greenschist- to sillimanite-grade quartz-rich metasedimentary rocks exposed in the Rongbuk to North Col region of the Everest Massif are characterized by cross-girdle quartz c -axis fabrics indicating approximate plane strain conditions. Fabric opening angles progressively increase with depth beneath the overlying Lhotse Detachment, and indicate progressively rising deformation temperatures of 525–625 ± 50 °C at depths of 300–600 m beneath the detachment. Deformation temperatures of c . 450 °C are indicated by fabric opening angles in epidote amphibolite-facies mylonites located closer to the overlying detachment. A top down-to-the-north (normal) shear sense is indicated by the asymmetry of microstructures and c -axis fabrics, but the degree of asymmetry is low at distances greater than 400 m beneath the detachment, and sillimanite grains are drawn into adjacent conjugate shear bands but still appear pristine, indicating that deformation occurred at close to peak metamorphic temperatures. These ‘quenched’ fabrics and microstructures indicate rapid exhumation in agreement with previous isotopic dating studies. Mean kinematic vorticity numbers ( W m ) were independently calculated by three analytical methods. Calculated W m values range between 0.67 and 0.98, and indicate that although a simple shear component is generally dominant, particularly in greenschist-facies mylonites located between the Lhotse and overlying Qomolangma detachments, there is also a major component of pure shear in samples located at 400–600 m beneath the Lhotse Detachment (pure and simple shear make equal contributions at W k =0.71). Our integrated strain and vorticity data indicate a shortening of 10–30% perpendicular to the upper surface of the Greater Himalayan Slab and confirm that the upper surface of the slab is a ‘stretching fault’ with estimated down-dip stretches of 10–40% (assuming plane strain deformation) measured parallel to the flow plane–transport direction.

/pdf/strain-deformation-temperatures-and-vorticity-of-flow-at-the-pxyiv9jgj4.pdf

Strain, deformation temperatures and vorticity of flow at the top of the Greater Himalayan Slab, Everest Massif, Tibet

Quantitative kinematic flow analysis from the Main Central Thrust Zone (NW-Himalaya, India): implications for a decelerating strain path and the extrusion of orogenic wedges

Harry Fritz

Papers

Quantitative kinematic flow analysis from the Main Central Thrust Zone (NW-Himalaya, India): implications for a decelerating strain path and the extrusion of orogenic wedges