Tectonic and erosional history of southern Tibet recorded by detrital chronological signatures along the Yarlung River drainage

Title of Publication: 
Tectonic and erosional history of southern Tibet recorded by detrital chronological signatures along the Yarlung River drainage
Carrapa, Barbara, bin Hassim, Mohd Faiz, Kapp, Paul A., DeCelles, Peter G., and Gehrels, George
Publication Info: 
GSA Bulletin; Month/Month 2016; v. 128; no. X/X; p. 1–12; doi: 10.1130/B31587.1; 6 figures; 1 table; Data Repository item 2016356.

The Indus-Yarlung suture zone, within the larger Indo-Asia collision zone in southern Tibet, is characterized by a central depression with two oppositely directed axial rivers: the eastward-flowing Yarlung River and the
westward-flowing Indus River. The axial valley is flanked by high-elevation ranges of the southern Lhasa terrane to the north and the Tethyan Himalaya to the south. This study analyzes the detrital geochronological
and thermochronological signatures of rivers draining into the Indus-Yarlung suture zone as a proxy for the timing of tectonic, magmatic, and erosional processes in southern Tibet. Zircon U-Pb ages reflect source area crystallization ages, and their distribution is proportional to the relative area of source rocks exposed in the catchment areas. Rivers draining the northern side of the collision zone are dominated by ages between ca. 40 Ma and ca. 60 Ma, similar to ages of rocks in the Gangdese arc, whereas one river sample draining the southern side records a Tethyan Himalayan signature characterized by age clusters at ca. 500 Ma and ca. 1050 Ma. U-Pb zircon ages from the modern drainages are similar to signals preserved in the Oligocene−Miocene Liuqu and Kailas basins, suggesting that the geology of the Cenozoic drainages was similar to modern, albeit with significant erosion in the Miocene. New apatite fission-track (AFT) ages from some of the same rivers show an early Miocene regional exhumation signature, which is interpreted to record regional uplift-induced erosion coupled with efficient river incision by the Indus-Yarlung fluvial system as a result of renewed underthrusting (following rollback) of India under Asia. Late Miocene (ca. 8 Ma) apatite (U-Th)/He (AHe) ages are consistent with cooling and exhumation associated with E-W extension followed by a decrease in erosion after ca. 6 Ma.

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Figure 1. (A–B) Digital topographic maps of the southern Tibetan Plateau and the Himalayan orogen based on Global Multi-Resolution Topography (GMRT) data. Main streams of the rivers are marked by blue lines; their tributaries are marked by red lines. Rivers 1–7 mark locations of sand samples collected from tributaries of the Yarlung River in this study. (C) Simplified cross section across southern Tibet and the Himalaya, modified after Murphy (2007). (D) Diagrammatic maps of the bedrock geology (from 1:1,500,000 geological map of Tibet; Pan et al., 2004) in each of the analyzed river catchment areas; pie charts indicate relative proportions of different rock types.