The geology of the roof-zone of the Kohistan batholith, northwestern Pakistan.
thesisposted on 19.11.2015, 09:10 by Michael Sullivan
The Kohistan arc in the northwestern Himalayas records a dual history of intra-oceanic and continental subduction and represents a juvenile addition to the continental crust. Within western Kohistan, the roof-zone of the Kohistan batholith has a complex tectonomagmatic history that reflects fluctuations in the mechanisms of subduction. Five separate magmatic episodes are identified and although being temporally, spatially and morphologically distinct, each retains similar subduction-related chemical and petrological characteristics. Two calc-alkaline intrusive suites, each consisting of nested gabbroic-granitic plutons represent high level intrusions emplaced by predominantly brittle fracture mechanisms. The earliest, Stage 1, date from ca. 78-75 Ma and intrude into a variably deformed and metamorphosed island-arc forearc succession (Kalam Group). Unconformably overlying the earlier suite, but intruded by the Stage 2 suite (ca. 48-45 Ma) are the dismembered remnants of a subaerial volcanic arc (ca. 60-55 Ma). The Dir Group and its northern continuation the Shamran Volcanics, represent resumption of volcanic activity along the margin of Eurasia after a prolonged period of uplift and erosion. Two contrasting environments of deposition are identified; a distal deep-water marine forearc (Baraul Banda Slate Fm.) and a subaerial/rifted volcanic cone facies (Utror Volcanic Fm. and Shamran Volcanics). Sedimentary forearc thicknesses and the abundance of rhyolite lavas and pyroclastic-flows imply extensional tectonic control even though the arc developed at a convergent margin. Juxtaposition of the two facies along the Dir Thrust reveals that between ca. 75 Ma and 55 Ma the focus of magmatic activity within Kohistan migrated northwards. During the India-Eurasia collision the arc was tectonically thickened by SSE-directed thrusts which reworked unconformable and intrusive contacts. Calc-alkaline basaltic dykes exploited these collision-related fabrics. The final magmatic episode cross-cuts all structures and lithologies and consists of peraluminous leucogranite sheets. In adopting a multi-disciplinary approach, this study has not only documented the geotectonic history of the region but has also emphasised the importance of studying surface processes when investigating structurally complex magmatic arcs.