THE EOHIMALAYAN FOLD-THRUST BELT: IMPLICATIONS FOR THE GEODYNAMIC EVOLUTION OF THE NW-HIMALAYA (INDIA)

In the Himalayas the Eocene deformation phase is called the Eohimalayan event. By means of a forward and backward balanced cross-section along a classical traverse through the Tethyan Himalaya we constrain Eohimalayan SW vergent folding and SW directed thrusting during the Middle Eocene in a belt above a basal thrust at 10km depth. Deformation has been partitioned into large-scale, SW-vergent fault-propagation folds together with translation along imbricate thrusts in the basal units (i.e. the Basal Fold-Thrust Belt) and predominantly short wavelength lower amplitude folding within the overlying multilayered sequence of competent and incompetent units (i.e. the Upper Fold Belt). Ar/Ar ages from newly recrystallized illlites within fold axial plane cleavage domains indicate a deformational age between 42-45 Ma. The basal thrust is located at the structural position of the Southern Tibetan Detachment System with a NE directed normal sense displacement suggesting Miocene reactivation of a major Eocene contractional fault. Stepwise palinspastic restoration demonstrates that the southwestern part of the Tethyan Himalaya above this Eocene contractional fault has subsequently been openly folded due to in sequence southwestward propagation of the Himalayan Thrust System.