Dynamics and Preservation Potential of Subduction Complexes in Continental Sutures: A Case Study from the Sedimentary-Marix Mélange of the Indus-Yarlung Suture Zone in Southern Tibet

Abstract:

Intra-continental sutures are the geologic record of ancient subduction zones. Subduction complexes are a useful record of ancient continental collisions because they preserve sediments and/or blocks from units which have since eroded and are the first point of contact during collision. The India-Asia collision is one of the most-studied collisional orogens, but how much of the original subduction complex is preserved and what we can determine about the dynamics of the ancient subduction zone along the southern margin of Asia is poorly understood. Compared to other subduction complexes around the world, the complex preserved in the Indus Yarlung Suture Zone (IYSZ) of southern Tibet is anomalous. Blueschist facies metamorphism, a prominent mineral assemblage along intra-continental suture zones, is common in the northwest Himalaya, but not found along the central segment of the IYSZ. Most of the subduction complex is greenschist facies, inconsistent with the geotherm for a subduction zone. We present a metamorphic history for the greenschist facies rocks to reconcile this contradiction.

A deep forearc basin (~5-8 km) developed during the Cretaceous, requiring an accretionary subduction zone, a topographic or structural outer forearc high behind which to trap sediment, and/or basal subduction erosion. The preserved subduction complex is almost entirely tectonic sedimentary-matrix mélange with minor outcrops of overlying turbidites. We present evidence from detrital zircon U-Pb geochronology of sandstones that indicate along-strike variability in the provenance of the sedimentary-matrix mélange. For example, both lower and upper plate material are present near the town of Ngamring, while regions along-strike to the west contain little to no upper plate material. The blocks in the sedimentary-matrix mélange are well-mixed throughout kilometers of exposed width. Sandstone blocks of Tethyan affinity, which could have entered the trench only at the onset of collision, are well mixed with oceanic blocks offscraped from the subducting Neo-Tethyan plate.. Mixing must have occurred within a few Myr as deformation quickly stepped southward. This rapid juxtaposition is unexplained by current models of accretionary complex deformation.