Graduate Thesis Or Dissertation
 

A field and geochemical study of the boundary between the Nanga Parbat-Haramosh massif and the Ladakh arc terrane, northern Pakistan

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  • The Nanga Parbat-Haramosh massif (NPHM) is a north-south trending structural and topographic high, which interrupts the east-west trend of the Himalaya in northern Pakistan. Previously, the massif was thought to be bounded by the Main Mantle thrust (MMT), a north-dipping thrust along which the Kohistan-Ladakh arc was thrust south over the northern margin of the Indian continent. This study presents field and geochemical data suggesting that the eastern boundary of the massif, the Stak fault zone, is a young feature that displaces the suture zone. The Stak fault zone marks the boundary between Precambrian kyanite-sillimanite bearing biotite gneiss of continental affinity and Cretaceous (?) arc lithologies of the western Ladakh terrane. The arc complex consists of amphibolitic country rock that has been intruded by gabbroic to tonalitic plutons. The protolith of the amphibolite is immature oceanic island arc tholeiitic basalt. The mafic to intermediate plutons are dominantly calc-alkaline and could have formed in either a mature island arc setting or a continental margin setting. The Ladakh arc terrane exposes the upper section of an arc, below the sedimentary and volcanic cover. The Stak fault zone is a 3-5 km wide zone containing at least four major high angle faults that separate blocks of various lithologies. The only true mylonite zone occurs along the westernmost fault. A faulted late stage dike is evidence for recent activity along the easternmost fault. The units along the western side of the fault zone are analogous to deep oceanic arc lithologies; tholeiitic amphibolite, banded gneiss, and a section of a layered mafic complex. The units along the eastern side of the fault zone are mineralogically and chemically correlative to the mafic plutons exposed in the western Ladakh terrane. The geometry of the fault zone, the lack of suture zone lithologies, and the evidence for recent activity suggest that the Stak fault zone does not represent the suturing event, when the Kohistan- Ladakh arc was obducted onto the northern margin of India. Instead, the fault zone is likely formed in response to the recent uplift of the NPHM.
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