Mathematics – Logic
Scientific paper
Oct 1983
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1983e%26psl..65..133b&link_type=abstract
Earth and Planetary Science Letters, Volume 65, Issue 1, p. 133-144.
Mathematics
Logic
39
Scientific paper
In northern Pakistan, the extraordinary 40-km-thick Kohistan sequence of metamorphosed mafic, ultramafic and calc-alkaline layered plutonic and volcanic (mainly andesitic to rhyodacitic) rocks has been recently reinterpreted as the only complete vertical section of an intra-oceanic island arc presently exposed anywhere in the world. Plate scale-models have been suggested to explain the origin and the tectonic evolution of this arc in the Himalayan collision range. Despite some noticeable differences in the models, there is general agreement that the Kohistan sequence represents the crust of an arc obducted onto the northern edge of the Indian plate before the Oligocene collision of India against Asia. New data on the tectonometamorphic evolution of the Kohistan sequence have led to the conclusion that a first major D1 event developed with increasing metamorphic grades oriented downward the Kohistan pile but also toward ``ensandwiched'' basic-ultrabasic granulitic rocks. Rare geochronological data and petrological evidence indicate that the latter granulites (pyriclasites and various plagiopyrigarnites, pyrigarnites, metatroctolites, metawebsterites, metadunites, etc.) were parts of enormous Lower to Middle Cretaceous layered calc-alkaline plutons emplaced during the arc-building stages. As the D1 metamorphic event is correlated with the Upper Cretaceous obduction process of the Kohistan onto India, it is proposed that the D1 thermal structure was strongly controlled by remnant magmatic heat source(s) within the obducted arc. This interpretation fits nicely with recent theoretical thermal models since the obduction of the arc was probably shortly after (10-20 Ma) the emplacement of the former plutonic arc rocks. A blueschist ``tectonic mélange'' underlying the obducted arc was possibly synchronous to the obduction and not clearly linked to the older subductive process. The Oligocene collision of India against Asia was associated with a Barrovian overprinting metamorphism in the Kohistan arc. During this second event the dynamo-thermal structure was much more classical and not controlled by the occurrence of the Cretaceous metaplutonic rocks.
A tentative model of the thermal structures during the major events which affected the Kohistan arc is proposed.
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