Geologic history and thermal evolution in the hinterland region, western Himalaya, Pakistan

Abstract

The western Himalaya of Pakistan forms a classic fold and thrust belt with a foreland defined by the Salt Range thrust, Main Boundary thrust, and Panjal-Khairabad thrust, and a hinterland that stretches northward from the Panjal-Khairabad thrust across the Peshawar Basin, through the mountains of Swat and Hazara, to the Main Mantle thrust (MMT), which is the westward extension of the Indus suture zone. The MMT separates the Indian plate from the Kohistan island arc complex, which forms the southern margin of the amalgamated Asian tectonic plate. Rocks in the hinterland underwent deformation and regional metamorphism in early Cenozoic as a result of the collision between India and Kohistan along the MMT. The purpose of this contribution is to review the stratigraphy, structure, and thermal evolution of the western Himalayan hinterland region in order to clarify our present knowledge and to address inconsistencies regarding geologic history. The goal is to create a comprehensive understanding and working model of the hinterland region. We do this by synthesizing and evaluating all published isotopic age data from the region, and by placing each age into the stratigraphic and structural framework of a geologic map created during more than 1.5 years of traverse and reconnaissance field mapping over a 12-year period. We add 17 new isotopic ages that have bearing on the stratigraphic, intrusive, and metamorphic history. Stratigraphy is described as pre-. syn-, and post-rift, dependent on its age relative to a strong Carboniferous-Triassic rifting event that featured plutonism, normal faulting, deposition, the erosional removal of Paleozoic rocks from northern areas of the hinterland, and potentially multiple periods of Panjal Trap volcanism extending to the Late Triassic. We show that stratigraphy can be traced continuously across the hinterland region without disruption across Cenozoic faults except in the MMT zone where there are four separate areas of Indus mélange and three adjacent thrust slices of Indian plate rock, one of which is newly recognized. We provide evidence that all of the mélange and Indian plate thrust slices were emplaced at about the same time prior to peak Cenozoic metamorphism in underlying hinterland rocks, and prior to 50 Ma. We find no evidence for thrusting since that time. Geologic and isotopic evidence suggests that the Kohistan arc and underlying thrust slices were emplaced in a southwestward direction and that the arc was in place against hinterland rocks prior to ca. 48.1 Ma. Hinterland rocks had reached peak metamorphism and were cooling by ca. 50 Ma except in the Loe Sar and Kotah domes where cooling was delayed until ca. 39 Ma. The Kohistan arc underwent eastward translation post-45.8 Ma during folding and erosional exhumation of hinterland rocks. The Indus syntaxis is a long-lived anticlinal structure that it is currently active. Pre-Cenozoic history includes plutonism in the Paleoproterozoic, Neoproterozoic, Early and Middle Ordovician, and Permian, and possible tectonism in Late Cretaceous-Early Paleocene. There are indications of pre-Cenozoic regional metamorphism within metasedimentary rock, but they are masked by Cenozoic metamorphism such that their extent, if any, is unknown.