Early Tertiary, coesite-bearing eclogites crop out in the internal zones of the Indian Plate in the Kaghan Valley of North Pakistan. Field and geochronological data suggest that UHP metamorphism, at c . 46 Ma, accompanied subduction of the leading edge of continental India beneath Kohistan. A new U–Pb rutile age of 44.1 ± 1.1 Ma can be interpreted as either a growth or a cooling age, given the uncertainty of the closure temperature of rutile for Pb, but clearly documents a very short-lived metamorphic peak, with cooling occurring between 44 and 40 Ma. Eclogitic assemblages are preserved in Permo-Triassic mafic rocks in both cover and basement sequences, with the best-preserved eclogites hosted in the anhydrous basement. The eclogite-facies rocks lie on the hanging wall of a major south-directed ductile thrust zone, which placed basement rocks that contain coesite eclogites southward onto lower-pressure cover metasediments. Shear criteria show that those sequences that contain eclogite-facies rocks are flanked above by a zone of pervasive north-directed extensional shearing. Mineral assemblages contained within both the thrust and extensional shear zones are consistent with them both having operated synchronously during the amphibolite- to greenschist-facies transition. Amphibole Ar–Ar cooling history data suggest that this was at c . 40–42 Ma. As in the Alps this implies exhumation from the metamorphic peak to greenschist-facies conditions within a few million years. This short time scale has important implications, as it is likely that both the thrust and extensional shear zones represent the late stage of a deformation continuum that commenced at UHP and brought the UHP rocks back to ‘mid’-crustal levels. This late Eocene extensional shearing, and associated exhumation of HP or UHP rocks, has not previously been recorded in the Pakistan Himalaya. The thrusts and shears were later folded by major east-trending folds, themselves cut by Miocene-aged, north-vergent extensional faults that currently separate the Indian Plate from the structurally overlying Kohistan island arc.
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). Such discrepancy is, to some extent, the result of the different definitions and methods used to define the collision. Here, we evaluate constraints from the sedimentary record preserved in the suture zone and Tethyan Himalaya where a minimum age to collision has been constrained by determining 1) the timing of cessation of marine facies, 2) first evidence of Asian detritus deposited on the Indian plate and 3) first evidence of mixed Indian-Asian detritus in the sedimentary record.
Research Article| February 01, 1992 Long-lived continent-ocean interaction in the Early Proterozoic Ungava orogen, northern Quebec, Canada S. B. Lucas; S. B. Lucas 1Geological Survey of Canada, Continental Geoscience Division, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada Search for other works by this author on: GSW Google Scholar M. R. St-Onge; M. R. St-Onge 1Geological Survey of Canada, Continental Geoscience Division, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada Search for other works by this author on: GSW Google Scholar R. R. Parrish; R. R. Parrish 1Geological Survey of Canada, Continental Geoscience Division, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada Search for other works by this author on: GSW Google Scholar J. M. Dunphy J. M. Dunphy 2Université de Montréal, Département de Géologie, C.P. 6128, Succursale A, Montréal, Québec H3C 3J7, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information S. B. Lucas 1Geological Survey of Canada, Continental Geoscience Division, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada M. R. St-Onge 1Geological Survey of Canada, Continental Geoscience Division, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada R. R. Parrish 1Geological Survey of Canada, Continental Geoscience Division, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada J. M. Dunphy 2Université de Montréal, Département de Géologie, C.P. 6128, Succursale A, Montréal, Québec H3C 3J7, Canada Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1992) 20 (2): 113–116. https://doi.org/10.1130/0091-7613(1992)020<0113:LLCOII>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation S. B. Lucas, M. R. St-Onge, R. R. Parrish, J. M. Dunphy; Long-lived continent-ocean interaction in the Early Proterozoic Ungava orogen, northern Quebec, Canada. Geology 1992;; 20 (2): 113–116. doi: https://doi.org/10.1130/0091-7613(1992)020<0113:LLCOII>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The Early Proterozoic Ungava orogen contains evidence for the development of a longlived (>80 m.y.) and presumably wide ocean that was subsequently destroyed during plate convergence leading to arc-continent collision. A magmatic arc was built on 2.00 Ga oceanic crust above a north-dipping subduction zone between about 1.90 and 1.86 Ga. Development of a south-verging continental thrust belt containing units associated with prior rifting of the Archean Superior province basement probably records impingement of the upper plate against the underthrust continental margin at ca. 1.87 Ga. Subduction of the Superior province continental crust is inferred to have led to a reversal in subduction polarity. A younger, south-dipping subduction zone is suggested by a suite of 1.85-1.83 Ga quartz diorite to granite plutons that intrude the upper- plate oceanic crust and the older arc. The arc-continent collision resulted in a minimum of 65 km of tectonic overlap of the composite arc's plutonic core on the autochthonous footwall basement. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
A range of ages have been proposed for the timing of India‐Asia collision; the range to some extent reflects different definitions of collision and methods used to date it. In this paper we discuss three approaches that have been used to constrain the time of collision: the time of cessation of marine facies, the time of the first arrival of Asian detritus on the Indian plate, and the determination of the relative positions of India and Asia through time. In the Qumiba sedimentary section located south of the Yarlung Tsangpo suture in Tibet, a previous work has dated marine facies at middle to late Eocene, by far the youngest marine sediments recorded in the region. By contrast, our biostratigraphic data indicate the youngest marine facies preserved at this locality are 50.6–52.8 Ma, in broad agreement with the timing of cessation of marine facies elsewhere throughout the region. Double dating of detrital zircons from this formation, by U‐Pb and fission track methods, indicates an Asian contribution to the rocks thus documenting the time of arrival of Asian material onto the Indian plate at this time and hence constraining the time of India‐Asia collision. Our reconstruction of the positions of India and Asia by using a compilation of published palaeomagnetic data indicates initial contact between the continents in the early Eocene. We conclude the paper with a discussion on the viability of a recent assertion that collision between India and Asia could not have occurred prior to ∼35 Ma.
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