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

Abstract

Abstract: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 ttuqast 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.

Crustal–lithospheric structure and continental extrusion of Tibet

“…7) developed for the Greater Himalaya along the southern margin of the Tibetan Plateau was proposed initially from field geological mapping and strain data combined with P-T constraints and U-Pb dating of metamorphic rocks and leucogranites (Searle & Rex 1989;Grujic et al 2002;Searle et al 2003Searle et al , 2006Law et al 2004Law et al , 2006Searle & Szulc 2005;Godin et al 2006). The channel flow model infers that a partially molten middle crust layer was extruded south from beneath southern Tibet to the Greater Himalaya during the Early Miocene, at c. 23-15 Ma.…”

“…Clark & Royden 2000;Haines et al 2003) and (b) the Himalayan mid-crustal 'channel flow' model (e.g. Beaumont et al 2001Beaumont et al , 2004Grujic et al 2002;Searle et al 2003Searle et al , 2006Searle et al , 2010bLaw et al 2004Godin et al 2006).…”

The Nature of Ductile Deformation in the Phyllite-Quartzite Unit (External Hellenides)

“…The roof stretching fault may display normal or thrust sense, depending on the motion of the crust above the extruding material (e.g. Godin et al, 2006).…”

Middle-Miocene transformation of tectonic regime in the Himalayan orogen

“…This confirms the view that NSTR is probably the result of N-S contraction [29] and provides unequivocal evidence for the transformation of tectonic regime. In order to accurately constrain the time of the transformation of tectonic regime from N-S extension to N-S contraction, we summarize and analyze the ages of STDS and NSTRs in Gyirong and other areas in Himalayan orogen ( Figure 7) [4][5][6][7][8][9][10][12][13][14][15][16][17]22,28,[30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48]. The result shows that the STDS was active from ca.…”

“…(2) After early Oligocene, the tectonic regime transformed from N-S contraction to N-S extension. Between late Oligocene and early Miocene (28.4-16.2 Ma), the northern Himalayas experienced widespread extension, resulting in large scale extensional tectonics, such as the north-dipping STDS [4][5][6][7][8][9][10][11][12][13][14][15][16]. (3) In middleMiocene (~13 Ma), the tectonic regime shifted to E-W extension [17,18], which formed the NSTR cutting the STDS, such as Yadong-Gulu rift, Nima-Dingri rift and XainzaDinggye rift.…”