SYNCONVERGENCE SPREADING OF THE HIGHER HIMALAYA CRYSTALLINE IN LADAKH

In Ladakh (Suru valley, NW Himalaya), regional fabrics of the Higher Himalaya Crystalline result from a major deformation event that (1) is restricted to the Indian crust above the Main Central Thrust (MCT), (2) postdates piling up of units above the MCT, (3) ended while thrusting was still continuing on the MCT, (4) developed under high-temperature conditions following thickening, (5) resulted in an asymmetric gneiss dome, and (6) is associated with ENEWSW to EW regional extension. We suggest that extension was accommodated by spreading of the hot basement accompanied and followed by its uplift within the overlying cover along a major detachment zone. The onset of the process appears closely related to strong thermal softening of the Higher Himalaya Crystalline and is broadly synchronous with the formation of the Himalayan leucogranites, around 25 Ma. The regional pattern suggests that the MCT zone could have acted as a decoupling zone between a thickening lower part and a thinning upper part of the Indian crust during Miocene times. The extension direction is consistent with bulk Himalayan kinematics involving NW-SE dextral wrenching along the western syntax, and eastward extusion of Tibet. We suggest that exhumation of gneissic ridges which characterize Indian units above the MCT all along the Himalayan belt could reflect large-scale crustal instability developed from thermally anomalous layers whose spreading ability can favor synconvergence extension.