8.3 Ductile extrusion-channel flow model
In this model (Fig 8.3) proposed by Grujic et al. (1996), the HHCS is considered as a ductile extruded wedge sandwiched between a rigid underthrusting plate (e.g. Lesser Himalaya) and a rigid buttress (Tibet-Tethyan Himalaya). The HHCS did not act as a rigid block but was deformed pervasively during its extrusion. The STDS is considered as a hinterland-dipping backstop that did not result from large-scale crustal thinning of an overthickened orogenic wedge. The normal fault geometry of the STDS is interpreted as resulting from the south-directed extrusion of the HHCS rather than by north-south extension. Extensional movements along the STDS are only relative, as the Himalaya was under compressive stress since 50 Ma. and did never undergo net crustal extension during that period (see also Searle, 1995). Ductile extrusion of the HHCS is also interpreted to be related with leucogranite intrusions.
Basically, in this model (Fig 8.3) the MCT zone is not restricted to the base of the HHCS but is extended to its whole thickness. The process of extrusion proposed by Grujic et al. (1996), can be approximated quantitatively by channel flow models as have been used to describe subduction zone processes (Mancktelow, 1995). Such models characterize a thrust system (e.g. the whole HHCS) as a viscous material-filled channel lying between two rigid sheets that deform the viscous material between them through induced shear and pressure gradient within the channel. They see the resulting deformation in the HHCS as an hybrid between simple shear and pipe-flow. Pipe-flow effect is basically what happens with channelized water: highest velocities are reached in the centre of the channel and opposite vorticity occur at the boundaries. Reverse shear sense at the top of the channel is the flow-pattern inferred across the top of the HHCS, along the STDS, whereas south-directed ductile thrusting dominates at the base of the HHCS, along the MCT.
It is proposed by Grujic et al. (1996), that the extrusion of the HHCS must be the consequence of a lateral pressure gradient associated with decreasing viscosities in a channel bound by two non-parallel walls. The lateral pressure gradient develops due to the building of topography during collisional tectonics. It is also suggested that melting at the top of the HHCS lubrificated and enhanced rapid tectonic exhumation.
An interesting point in this model is, that it considers the HHCS as a ductile body rather than a rigid slab and that deformation is considered as pervasive through the whole HHCS rather than concentrated at its base (MCT) and top (STDS). An other interesting point is that extensional movements along the STDS are considered as relative within an overall compressive system. It also takes into account the reversal in shear sense (top to the NE «relative extension» superposed over top to the SW simple shear) along the STDS. This model does however lack a plausible explanation for the mechanisms that lead to the extrusion of the HHCS.