OROGEN-PARALLEL EXTENSION AS EXPRESSED BY THE DEVELOPMENT OF GNEISS DOMES: AN EXAMPLE FROM THE HIMALAYA

Hypotheses advanced to explain the relationship between orogen-parallel extension and orogen-normal contraction within the Himalayan orogen broadly predict the present kinematic pattern. However, these models do not predict their coeval evolution on a time scale over millions of years. Recent investigations in western Tibet and western Nepal reveal a linked system of late Miocene strike-slip and low-angle normal faults associated with gneiss dome complexes, which accommodated orogen parallel extension and vertical thinning in the hinterland of the Himalayan fold-thrust belt. Deformation within the gneiss dome complexes is constrictional whereby the maximum elongation direction parallels the strike of the curved Himalayan orogenic front. The largest of these gneiss dome complexes is the Gurla Mandhata metamorphic core complex (120 km x 40 km). Its development is associated with a pair of top-to-the-west low-angle normal faults which have assisted in the exhumation of a >3 km-think sequence of mid-crustal rocks and syn-extensional granite bodies and migmatites in its footwall. Related to the Gurla Mandhata metamorphic core complex is a large (180 km x 40 km) Pliocene supradetachment (?) basin (Zada basin). Both the Gurla Mandhata metamorphic core complex and the Zada basin formed within the hanging wall of an early to middle Miocene regional thrust system (Great Counter thrust) whose trace parallels that of the active orogenic front.

The geologic history of the study area indicates that a change in deformation style occurred during the development of the Himalayan orogeny, an early contractional event (orogen-normal contraction) and a younger extensional event (orogen-parallel extension). This deformation history may be explained by a model involving foreland propagating structural systems facilitating orogen-normal contraction in the orogenic front and orogen-parallel extension in the hinterland. Propagation of these structural systems is hypothesized to occur in response to either outward radial expansion of the Himalayan orogen or oblique convergence along a curved margin. I further hypothesize that crustal thickening, which will decrease the strength of the deforming orogenic front, is a necessary precondition for orogen-parallel extension accompanied by the development of gneiss domes.