8.2 Gravity collapse
Burchfiel and Royden (1985) first suggested that movements along the MCT and the STDS might have been broadly contemporaneous, and therefore presented a model where the metamorphic core of the orogen (HHCS) was represented as a northwards-tapering prism bound by these two structures and extruded southwards toward the Indian foreland (Fig. 8.1).
These authors interpret the extensional movements at the top of the HHCS as a consequence of gravitational collapse of the Himalayan topographical front. Events leading to this gravitational collapse include continental crustal thickening at the leading (southern) edge of Tibet by the underplating of the Tibetan crust with incompletely subducted material from the Indian crust. As the crustal thickness increases beneath the leading edge of Tibet, the topographic elevation increases as well and provides increased vertical stress to drive extension. Burchfiel and Royden suggest that eventually, the difference in topographic elevation between the Indian foreland and the southern end of Tibet reaches a point where the generated stress can no longer be supported by the cohesive strength of the rocks within the upper crust and gravitational collapse occurs.
Burchfiel et al. (1992) suggest that the primary trigger for the gravitation driven extensional collapse of the orogen was a major reduction in the crustal strength due to melting of the hanging wall rocks within the lower part of the upper plate. They take as a proof for this weakening of the cohesive strength of the rocks the widespread presence of leucogranites in the footwall of the detachment fault.
Hodges et al. (1996) take up the gravity collapse model of Burchfield but propose that episodic displacement occurred on both the MCT and the STDS systems long after their initiation (Fig. 8.2), which contradicts the concept of a single extensional phase of deformation (England and Molnar, 1993). For Hodges et al., the STDS must consequently have been capable of accommodating continual cycling between shortening and extension for a substantial portion (whatever that means) of the history of the orogen. In this model, extensional movements along the STDS correspond to a physical response which compensates the instability caused by over steepening during mountain building. In a continually evolving orogen, where many physical parametres are constantly changing, there is a continual alternation between extension and contraction in the wedge, as the system oscillates about the stable state. This model which is termed dynamic compensation by Hodges et al. (1996), interprets the extensional fault systems as gravitationally driven compensational structures that, along with physical erosion, helped to maintain a critical crustal profile in the orogen during continued convergence between India and Asia.
These models, which see syn-orogenic extension as a consequence of gravitational instability, are very popular among the Himalayan geologist community. Such models do however require that an over-steepening state can be reached in an orogen. Reaching such a state of instability would require that the upheaval of the topographic front could not be compensated solely by erosional processes and thus had to regain equilibrium in «collapsing» along normal faults. We believe that gravity-driven extension is a process that did certainly occur in the Himalaya to produce the late D6 high-angle normal faults which compensate the gravitational instability created by the D5 doming phase. We do however think that this process could not lead to the formation of ductile low-angle extensional structures as the STDS. Our data show that crustal melting and the production of leucogranitic melts are a consequence of extension and thus cannot be the trigger necessitated to induce extensional shearing as proposed by Burchfiel et al. (1992). Moreover, multicycling between over-steepening and gravity collapse along low-angle structures as proposed by Hodges et al. (1996) could explain a certain amount of exhumation but this process is largely insufficiant to explain the juxtaposition of rocks that equilibriated at depths greater than 30 kilometers with nearly unmetamorphosed sediments (Fig. 8.1 (Hodges et al. 1996)).
Finally, models based on gravity collapse do not take into account the reversal in shear sense observed along the STDS.