How significant is erosion in extrusion- insights from analogue and analytical models

Abstract

The absolute role of climate/erosion in extrusion of mountains has been debated for a long time. Field-studies, micro-structural observations and few analytical trials confirm that the extrusion of the Higher Himalayan Shear Zone, Sutlej section took place initially by simple shearing and was followed by combined simple shear and channel flow in a shifting mode (Mukherjee 2007). The extrusion mechanism of the HHSZ is studied with 10 analogue models of channel flow initiating from a horizontal channel and extrusion through a linked inclined channel. The inclined channel is the model HHSZ and is of parallel, gently diverging-up and strongly diverging-up geometries in different considerations. In these experiments, Polydimethylsiloxane (PDMS), a transparent Newtonian viscous polymer is used as the model material and geometric- and dynamic similarities are maintained with the prototype. Six flow zones are deciphered in the two channels in the mature stage of extrusion of the PDMS. Parabolic profiles are produced at the middle of both the inclined- and the horizontal channel during a Poiseuille and Jeffery Hamel flows. The part of the PDMS originally inside the horizontal channel starts moving through the inclined channel at a faster rate than the part of the PDMS initially residing in the inclined channel. This in effect gives rise to thrust movement of the former part of the PDMS. The thrust plane originates at the corner joining the inclined and the horizontal channel and rotates while coming closer to the free surface. The tectonic insights gained from these experiments are (i) secondary ductile thrusting took place in the HHSZ as a delayed response to its channel flow mode of extrusion, (ii) the genesis of the thrust seems to be related to the change in the direction of extrusive flow– from horizontal to inclined up; (iii) as these models were performed without any erosion of the extruded PDMS, erosion induced by climate did not trigger nor was a deciding factor in the genesis of the secondary thrust and concomitant extrusion of the HHSZ. However, erosion induced by a disparity in precipitation on the HHSZ might have augmented the extrusion process i.e. it had a passive role. In Sutlej section of the HHSZ, the simulated thrust can be correlated with the Chaura Thrust with the recorded activation at least 13 Ma after the ongoing extrusion of the HHSZ by channel flow mechanism around 18 Ma.