Kinematic dilatancy effects on orogenic extrusion

Abstract

We undertake kinematic modelling to explore the role of volume increase in a slab extruding from an orogenic wedge with constant or decreasing slab width. Using a dilatancy term, we modify the velocity gradient tensor dependent on the stretching-rate factor, kinematic dilatancy and vorticity number. We use this to explore the previously largely ignored role of volume change in kinematic evolution of extrusive flow, considering area change for non-isochoric flow types with no deformation in the intermediate direction. By keeping individual parameters constant for geologically simple scenarios (e.g. finite strain, steady-state flow) we examine the interdependence of the reciprocal parameters (kinematic vorticity and dilatancy number) and note model situations where degrees of freedom are limited. These interdependent parameters thereby provide a set of rules for integrating and modelling real field data. In particular we observe that for extrusion flow with a constant slab (or 'channel') width, degrees of freedom in kinematic vorticity and volume change at given finite strains are very restricted. We compare scenarios of low and high strain and low and high volume change on anatexis (related to partial melting of fertile sedimentary rocks and release of water upon crystallization) for different parts of the Himalaya.

Implications of channel flow analogue models for extrusion of the Higher Himalayan Shear Zone with special reference to the out-of-sequence thrusting

“…This means there is a possibility that the MCT U , the STDS L and the STDS U were active between 19 and 14 Ma; and the MCT L , the STDS L and the STDS U at 14-12 Ma. (ix) A heterogeneous ductile shear regime was dominated by central pure shear and marginal simple shear (Grasemann et al 2006;Law et al 2004;review by Exner 2005;Jessup et al 2006;Carosi et al 2007;Larson and Godin 2009). (x) The sub-horizontal channel at mid-crustal depth beneath the Tibetan plateau contains 3-7% partially molten rocks (Caldwell et al 2009).…”

“…Grid lines acted as passive markers in all the experiments. PDMS being an incompressible fluid, these experiments implicitly neglected any kinematic dilatancy due to partial melting (Grasemann et al 2006) during extrusion of the HHSZ related to channel flow. Using PDMS also precluded modelling the brittle behaviour of rocks at depths of B8 km in the natural prototype.…”

Higher Himalayan Shear Zone, Sutlej section: structural geology and extrusion mechanism by various combinations of simple shear, pure shear and channel flow in shifting modes

“…Beaumont et al 2001Beaumont et al , 2004Jamieson et al 2004) that involves input of additional parameters such as density-, thermal conductivity, and the rate of erosion of the extruding rocks. However, those parameters do not have any direct relation with the first order constraints of extrusion (Grasemann et al 2006).…”

“…The fluid inside the channel, an analogue to the rocks of the HHSZ, is considered to be incompressible. In other words, the kinematic effects of dilatancy due to partial melting (Grasemann et al 2006) within 19-14 Ma in the upper parts of the HHSZ (Godin et al 2006) are neglected.…”

Simple shear is not so simple! Kinematics and shear senses in Newtonian viscous simple shear zones

“…Beaumont et al 2004;also Kellett et al 2010 as the latest example) considered a number of additional parameters such as geothermal gradient, radioactive heat production, thermal expansion coefficients and power law behaviour, together with density changes due to phase transition during metamorphism and extrusion augmented by focused erosion. In addition, the following factors were also ignored: gravitational spreading or erosion of materials extruded from the top end of the shear zone; kinematic dilatancy (Grasemann, Edwards & Wiesmayr, 2006); strain partitioning (Mancktelow, 2006); and changes in rock rheologies (von Huene, Ranero, & Scholl, 2009). In addition, the following factors were also ignored: gravitational spreading or erosion of materials extruded from the top end of the shear zone; kinematic dilatancy (Grasemann, Edwards & Wiesmayr, 2006); strain partitioning (Mancktelow, 2006); and changes in rock rheologies (von Huene, Ranero, & Scholl, 2009).…”