REMNANTS OF JURASSIC FOREARC FROM THE SOUTHERN MARGIN OF ASIA EXPOSED IN THE INDUS-YARLUNG SUTURE ZONE, SOUTHERN TIBET

The Lhasa terrane is southern Tibet has a magmatic history extending back to the Jurassic or even Triassic. However, the forearc basin record extends only to the early Cretaceous, in depositional contact with ophiolite massifs and opiolitic mélanges formed during early Cretaceous forearc hyperextension. No record of Jurassic forearc strata from the southern margin of Asia has been documented until now. We have identified two exposures of continuous stratigraphy exposed in the Indus-Yarlung Suture Zone between ophiolitic mélanges and sedimentary matrix mélanges. The stratigraphy includes red chert, black and grey chert interbedded with shale, intraclastic green shale, intraclastic limestone, and calcareous lithic sandstones containing angular clasts of limestone and volcanics. Detrital zircon geochronology on the sandstone reveals a single age population with a peak at 160 Ma, a signature previously undocumented in the suture zone. We interpret that the sandstone was sourced from a carbonate platform and undissected arc. There was a lull in Gangdese arc magmatism 174-156 Ma, concurrent with ophiolite generation and Zedong arc magmatism. We propose that the forearc extended beginning at ~174 Ma, and magmatism migrated southward in the Zedong arc. The Jurassic strata were deposited along the southern margin of Asia in the extended forearc and survived reaccretion to the Lhasa terrane, late Cretaceous forearc hyperextension, and another episode of reaccretion. This is the first documented pre-Cretaceous forearc strata from the southern margin of the Lhasa terrane.