Himalayan structure and metamorphism south of the Main Mantle Thrust, Lower Swat, Pakistan

Abstract

Abstract During the Eocene‐Oligocene, the Indian plate collided with the Kohistan arc along the Main Mantle Thrust (MMT) zone. The structure of the Lower Swat rock sequence, on the Indian plate directly south of the MMT, is a dome with a basement of granitic gneiss and quartz‐rich schist unconformably overlain by amphibolitic and calcareous schist. The earliest superposed small‐scale folds (F1 & F2) represent a progressive F1/F2 deformation that is associated with a single set of WSW‐vergent large‐scale folds (termed F2). These folds are inferred to have developed during oblique, WSW‐directed overthrusting of the MMT suture complex onto the Lower Swat rock sequence. Metamorphism began during F1/F2 as indicated by an S1 foliation that developed during biotite‐grade metamorphism. S1 is preserved as a relict texture in porphyroblasts that grew during a subsequent interkinematic phase during garnet‐ and higher grade metamorphism. The dominant, regional foliation (S2) developed following the interkinematic phase. S2 is associated with transposition of S1 and rotation or dismemberment of porphyroblasts. Annealing recrystallization followed S2 and continued during F3 thereby destroying or masking possible pre‐existing stretching fabrics. Superposed F3 folds are upright and open with N‐S axial trends. They may correlate with early doming of the Lower Swat rock sequence and with strike‐slip displacement in the northern part of the MMT zone, north of the Lower Swat area. F3 was followed by retrograde metamorphism and development of E‐W‐trending, S‐vergent F4 folds. F4 may be associated with a final phase of southward directed thrusting and inactivity in the MMT zone. Correlation of published 40Ar/39Ar ages with the metamorphic fabrics suggests that F1/F2 and F3 occurred in the Eocene, and that F4 developed in the Oligocene. F4 is the earliest indication of southward verging structures on this part of the Indian plate.