The age and degree of diachroneity of India-Asia collision is critical to construction of models of orogenesis and to understanding the causes of spatial variations in Himalayan evolution along strike. The age of collision is quoted between 65-34 Ma (Jaeger et al 1989; Aitchison et al 2007) and the degree of dichroneity is considered negligible (Searle et al 1997) to substantial (Rowley 1998). We studied the youngest Tethyan succession in the east (Tingri, Tibet) and west (Ladakh, India) of the orogen and used two approaches to date collision: 1) timing of closure of Tethys, by dating the youngest marine strata and 2) first evidence of Asian detritus deposited on the Indian plate, using U-Pb ages of detrital zircon to assess provenance. Both these approaches provide a minimum age to collision. In Ladakh, Indian plate passive margin limestones of the Paleocene Dibling Fm are overlain by the youngest marine facies of the region, the marine Kong Fm and fluvio-deltaic Chulung La Fm (Garzanti et al 1987). The age of the Kong and Chulung La Formations is disputed, from P5/6 (Fuchs & Willems 1990) to P8 (Garzanti et al 1987) the discrepancy possibly the result of research at different locations. Provenance is considered to be either ophiolitic from the Indian plate (Fuchs & Willems 1990) or containing detritus from the Trans-Himalayan arc of the Asian plate (Garzanti et al 1987; Critelli & Garzanti 1994). Our samples from the Kong Fm contained planktic foraminifera indicating a Middle to Early P6 age (54-56 Ma) and larger benthic foraminifera indicating Middle SBZ8 age (53-54 Ma). U-Pb dating of detrital zircons allows discrimination between Asian provenance (dominated by Mesozoic grains from the Trans-Himalayan arc) and Indian provenance (characterized by Precambrian grains and an absence of Mesozoic grains). Our data from the Kong and Chulung La Fms shows a primary provenance from the Asian plate. Thus collision is constrained by arrival of Asian detritus on the Indian plate by 54 Ma. In Tingri, Tibet, Indian plate passive margin limestones of the Zephure Shan Fm extend to the early Eocene, overlain by marine facies of the Pengqu Fm. The youngest marine facies have been dated at 34 Ma (Wang et al. 2002), but this age is disputed by other workers who assign an age of 50 Ma (Zhu et al. 2005). Our new biostratigraphic data from the Pengqu Fm show that calcareous nannofossil species are compatible with an age corresponding to Zones NP11-12 (50.6-53.5 Ma). The dominant population of detrital zircons have Cretaceous-Paleocene ages, derived from the Asian plate, thus indicating that contact between India and Asia had occurred by this time. We therefore conclude that India-Asia collision occurred by 54 Ma in the west, with only extremely limited, if any diachroneity eastward.
The Indus Group includes marine and terrestrial Tertiary sedimentary rocks which were deposited in an evolving late-forearc to intermontane basin setting during the closure of Neo-Tethys and onset of India-Asia collision (Brookfield and Andrews-Speed 1984, Van Haver 1984, Searle 1990, Sinclair and Jaffey 2001, Clift et al. 2002). Clift et al. (2002) have constrained the age of collision by determining the lowermost stratigraphic point in the Indus Group that contains detritus from both Indian and Asian plates, and also by identifying where the Asian margin derived Indus Group unconformably overlies Indian margin sediments. The Chogdo Formation, dated by an overlying limestone at older than 54.9 Ma (O. Green, unpublished data cited in Sinclair and Jaffey 2001) is identified by Clift et al. (2001), to be the oldest unit of the Indus Group to contain detritus from both the Indian and Asian plates, and to stratigraphically overly Lamayuru Group Indian slope turbidites and Jurutze forearc basin rocks, thereby pinpointing the timing of continental collision to the Late Paleocene. However, despite its importance, these previous evaluations of the Indus Group have been hampered by poor stratigraphic knowledge and uncertain lateral correlations, largely due to the relatively complex deformation of the rocks and poor biostratigraphic control, and by application of only a limited number of techniques to assess provenance. We use a combination of geological mapping, biostratigraphy, facies analysis, petrography, bulk rock geochemistry, and isotopic characterisation of single detrital grains to 1) create an accurate and more widely representative stratigraphy for the Indus Group, 2) determine the nature of the contacts which separate the overlying Indus Group from underlying Indian and Asian Plate formations and 3) determine the provenance of the Group, in particular the stratigraphic level within the Indus Group at which both Indian and Asian plate detrital minerals occur together, in order to constrain the time of collision and discover which geological terranes where exhumed and actively eroded during the early stages of the Himalayan orogeny. Our initial analyses indicate that 1) the Chogdo Formation may not be as widely occurring as previous interpretations have led to believe; partly due to obscured tectonic contacts and problems with lateral correlations along strike. Therefore certain stratigraphic intervals which are currently identified as belonging to the Chogdo Formation may well be younger (<48.6Ma; Wu et al. 2007) Indus Group formations; 2) The Chogdo Formation is overwhelmingly Asian-derived, and there is no unequivocal evidence for input from the Indian plate. Reassessment of constraints to the timing of closure of Neotethys, and consequent India-Asia collision as determined from the Chogdo Formation is therefore required.
In the Yelverton Bay region of northwestern Ellesmere Island, bimodal intrusive and volcanic rocks are associated with a major fault in the Proterozoic–Cambrian rocks of the Pearya Terrane. The Wootton intrusion consists mainly of gabbro with lesser amounts of granitic and hybrid rocks; the Hansen Point volcanics are composed of felsic rocks and basalt. Plutonic zircons are very slightly discordant, but volcanic zircons have unusually high degrees of inheritance. Interpreted U/Pb zircon ages of 92.0 ± 1.0 Ma for the Wootton intrusion (assuming a wide range of inheritance ages) and of [Formula: see text] for the Hansen Point volcanics are close to the 93 Ma average of hornblende K/Ar dates obtained earlier for a small quartz diorite pluton in central northernmost Ellesmere Island. All fall into the early Late Cretaceous and indicate correlation with mafic volcanics of the Cenomanian–Turonian Strand Fiord Formation of eastern Axel Heiberg Island. The upper intercept age for the Hansen Point volcanics ([Formula: see text]) suggests that the felsic component in the bimodal suites was in part derived from the upper Middle Proterozoic (Neohelikian) basement gneiss. Limited field observations on the Wootton intrusion also are compatible with the hypothesis that the granitic component represents sialic basement, melted by mafic intrusion at depth during an extensional tectonic regime.
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