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,2
1 Department of Earth Sciences, Cambridge University, Downing Street, Cambridge CB2 3EQ, United Kingdom, and Department of Geology and Geophysics, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, United Kingdom
2 Dipartimento di Scienze della Terra, Via Mangiagalli, 34-20133 Milano, Italy, and Dipartimento di Scienze Geologiche e Geotecnologie, Universitá di Milano-Bicocca, Via Emanueli 14, 20126 Milano, Italy
The latest Paleocene–middle Eocene Subathu Formation and the Oligocene–Miocene Dagshai and Kasauli Formations of the Indian foreland basin record the early evolution of the Himalayan fold-thrust belt. Sandstone petrography of the Subathu Formation shows a predominantly recycled sedimentary source, with a distinct ophiolitic and volcanic influence that was drastically reduced by the time of deposition of the Dagshai Formation. Sandstones in the Dagshai and Kasauli Formations consist predominantly of metapelitic detritus. The metamorphic grade of metapelitic lithic grains increases with time, from dominantly very low grade at the base of the Dagshai Formation to dominantly low grade in the Kasauli sandstones. Mudstone geochemistry documents the presence of a mafic-ultramafic source during the time of deposition of the Subathu Formation that becomes significantly less important by the time of deposition of the Dagshai Formation. Compositions of Subathu Formation detrital spinels show they were either derived from both mid-ocean ridge basalt–type and arc-type ophiolites or from an ophiolite of composite origin, and that southerly derivation from the Deccan Trap continental flood basalts is unlikely. Kasauli Formation garnet compositions suggest derivation from medium-grade metamorphic rocks to amphibolite facies.
Subathu Formation composition reflects provenance influence from the Indus suture zone during latest Paleocene–middle Eocene time, indicating initiation of continent-continent collision and development of the foreland basin by this time. Suture-zone influence was drastically reduced by the time of deposition of the Dagshai Formation, when the embryonic thrust belt provided a barrier sufficient to partially separate the suture zone from the Himalayan foreland basin. The first appearance of Himalayan metamorphic detritus occurs in the Dagshai Formation at the close of Oligocene time, whereas the Kasauli Formation records erosion to deeper metamorphic levels during earliest Miocene time. The occurrence of garnet and higher grade metamorphic lithic grains during early Neogene time is coincident with the timing of displacement along the Main Central thrust and South Tibetan detachment zone.
Composition of early foreland basin sediments from Pakistan (Balakot, Murree and Kamlial Formations) to Nepal (Bhainskati and Dumri Formations) and Bangladesh (Kopili, Barail Formations; Surma Group) indicates diachronous arrival of ophiolitic to low-grade metamorphic detritus derived respectively from the Indus Suture and early Himalayan thrust sheets in the north. This is consistent with progressively later closure of Neotethys along the suture, from latest Paleocene time in the west to Eocene time or even later in the east.
Key Words: Himalaya • foreland basin • sediment provenance • tectonics • paleogeography • continental collision
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