The Shyok Suture Zone and Main Karakoram Thrust System in the Saltoro Range of Ladakh, Northwestern India

Prior to to its collision with India, the southern margin of Eurasia had already experienced a long history of accretionary tectonics. Several continental fragments can be identified in southern Tibet, and they are separated from one another by suture zones that are less well understood with regard to their structural and metamorphic evolution than the more familiar Indus-Yarlung suture separating India from the Eurasian ensemble to the north. One example is the Shyok suture zone (SSZ) of northwest India and Pakistan, which separates a Jurassic-Cretaceous continental arc terrain to the north (the Karakoram block) from a second arc complex represented by the predominately Paleocene-Eocene Ladakh batholith. The structural characteristics of the SSZ are important from the standpoint of better understanding the accretionary history of southern Tibet and also from the standpoint of helping constrain correlations with other, possibly equivalent suture zones farther east that are offset across the Cenozoic Karakoram fault (KF). Unfortunately, there is no consensus on the geology of the poorly studied SSZ, which has prompted us to undertake an integrated structural, geochronologic, and thermochronometric study of the feature. Mapping over two field seasons was supplemented with the use of visible to thermal infrared ASTER and Google Earth satellite imagery, allowing for study of areas otherwise not accessible. Three major lithotectonic units were mapped: 1) basal greenschist facies mafic volcanic rocks with intercalated limestones and mudstones; 2) coarse conglomerates and intercalated sandstones of the overlying Saltoro molasse; and 3) ophiolitic mélange at the top of the section. The three packages are separated by thrust faults. A 40Ar/39Ar hornblende date of 110 Ma for a feeder dikes of the mafic volcanic rocks in the basal unit, interpreted to be the intrusive age of the dike, provides a maximum age constraint for the SSZ. A minimum constraint is provided by a 75 Ma by a 40Ar/39Ar muscovite date for an unmetamorphosed granite. The SSZ was modified by two major fault systems in late Cenozoic time: the Main Karakoram thrust (MKT) system, which accommodated southward thrusting of the Karakoram block over the Ladakh block (and its Kohistan block equivalent to the NW in the Pakistan Karakoram ranges), and the KF system. The MKT system has been most thoroughly studied in Pakistan and its position in Ladakh has been controversial. Our mapping has led to the identification of a previously unrecognized, low-angle fault system at high elevations within the Saltoro range that places migmatitic granitic rocks and high-grade metasedimentary rocks over SSZ units. These hanging wall units appear to be correlative with Karakoram rocks exposed farther north, across an active strand of the KF. Given the relationships observed and previous descriptions of the MKT system in Pakistan, we tentatively assign this structure with the MKT system. If correct, this observation extends the trace length of the MKT by 100 km.