Tectonometamorphic evolution of the central Karakorum (Baltistan, northern Pakistan)

Abstract

New structural and metamorphic data from the Chogo Lungma glacier area and Basha valley enlighten the polyphased tectonometamorphic evolution of the Metamorphic Complex of Karakorum. A pre- to early D1 event is defined by a metamorphic cleavage in the core of some zoned garnet and staurolite porphyroblasts. The main tectonometamorphic event (D1) is characterized by N110°E south-vergent isoclinal folds associated with a metamorphic axial-plane cleavage formed in a predominantly flattening regime of deformation. It corresponds to a Grt-Bt-Mu-Ky peak metamorphism (M1: 620–730°C and 7.5–11 kbar). The D2 tectonometamorphic event corresponds to the development of several conical domes elongated striking N110°E to N140°E. The S2 cleavage is seldom individualized as it usually corresponds to a reworking of S1. In some areas, late evolution of D2 doming deformation is marked by a crenulation. Petrologically, D2 is characterized by shearing and boudinage of M1 minerals and by development of sillimanite. It corresponds to a decompressional evolution (7.5 down to 4 kbar) with only slight changes in temperature. A late event corresponds to large undulations and open folds which affect the D2 crenulation. We propose here a model of vertical plus dextral extrusion of the middle crust to explain the dome structures in Karakorum. The steep pattern of the metamorphic P-T path implies rapid exhumation rates and suggests that the different phases could be part of a continuum of deformation. Preliminary results by 40Ar39Ar method give young cooling ages (<10 Ma) for the D2 event (Villa et al., 1996, this issue). Thus, most of the observed tectonometamorphic evolution in central Karakorum could be fairly young.