Structural geometry and metamorphic evolution of the Karakoram crust, Biafo-Panmah Karakoram, North Pakistan

The Karakoram metamorphic complex (KMC) in the Braldu valley and Biafo glacier regions of northern Pakistan lies between the Main Karakoram Thrust (MKT) in the south and the Baltoro Plutonic complex in the north, and represents the middle or deep crust of the South Asian continental margin. Previous work has shown that it is characterised by a complex, polyphase history of deformation, metamorphism and leucogranite dyke intrusion (Searle and Tirrul, 1991; Fraser et al., 2001). We have carried out new field studies along the Braldu valley, the Panmah valley, the Biafo Glacier and from Askole to Shigar, via Skoro-La, which give a complete transect across the complex from the 25-21 Ma Baltoro granite to the MKT. The main structures in the area are two large isoclinal, recumbent fold nappes, 1000-1300 m thick, which have been folded around lower, later domes (Bullah and Dassu gneiss domes) in the Braldu, Biafo and Panmah valleys. The Biafo nappe is a southward-verging recumbent fold thrust over gneissic basement and is overlain by the Panmah nappe. The axial planes of the recumbent folds dip to the NE at ∼15°. They are refolded along an E-W axis by an elongated, domal structure, the Bullah dome, which exposes sillimanite grade gniess in its core and is equivalent to the Dassu gneiss dome. The basement gneisses are Precambrian with a sillimanite grade metamorphism at 5.4 ± 0.3 Ma (Fraser et al., 2001) and intruded by younger leucogranite dykes. The domes are thought to be thrust-related culminations probably associated with movement on the MKT and related structures. An E-W trending thrust parallel to the MKT in the Braldu valley separates the Biafo and Panmah nappes from the rocks to the south. Previous studies divided the Karakoram metamorphic complex into Dumordu and Ganchen units. It is thought that this division is inappropriate due to the highly interbedded nature of the rocks. The complex consists mostly of biotite-rich schists and gneisses with more minor marble, pelitic schist and amphibolite. P-T work (Allen and Chamberlain 1991; Searle and Tirrul, 1991) has shown that peak metamorphic temperatures during the main phase of Barrovian metamorphism were 650-700°C and peak pressures around 8.5 1 kbar (850 MPa). Metamorphism preceded intrusion of the early Miocene Baltoro granite batholith. Thermobarometry on samples collected from throughout the area will further constrain metamorphic conditions. Together with U-Pb dating of leucogranite dykes which intrude the complex we hope to provide a detailed history of the metamorphic and tectonic events which have affected the Karakoram crust