Print this pageConstraints on Neogene Himalayan Kinematic History from Modal Petrographic and Detrital Zircon Provenance Data, Western Nepal
DeCELLES, P. G., G. E. GEHRELS, and P. A. KAPP
The Miocene-Pliocene erosional record of Himalayan thrusting in western Nepal is preserved in foreland basin deposits of the Dumri Formation and the Siwalik Group. Sandstones were analyzed for modal framework composition and U-Pb ages of detrital zircons as a means of determining the erosional unroofing history of the Himalayan thrust belt. Petrographic data demonstrate an upsection enrichment in feldspar and higher grade metamorphic lithic grains at the expense of quartzose grains and low-grade meta-sedimentary and sedimentary lithic grains. K-feldspar increases abruptly in the middle and upper Siwaliks, suggesting that granitic source rocks became widely exposed at approx. 8 Ma. Possible sources of the feldspathic sand are high-grade metamorphic rocks in the Main Central thrust (MCT) sheet, Cenozoic leucogranites in the higher Himalaya, and coarse augen gneisses and Cambrian-Ordovician granites in the Dandeldhura thrust (DT) sheet. Ninety-five U-Pb ages on detrital zircons from Siwaliks sandstones indicate age clusters of 480-520 Ma, approx. 850-1200 Ma, approx. 1.8-2.0 Ga, and approx. 2.5 Ga. An abundance of Cambrian-Ordovician grains strongly indicates a major source for Siwaliks detritus in the granites of the DT sheet. The "Grenville" ages are consistent with sources in the MCT sheet and the older Precambrian grains were probably derived from the DT and MBT sheets. None of the detrital zircon ages indicates derivation from the Cenozoic higher Himalayan leucogranites. The combined provenance data indicate that a major phase of MBT displacement and passive uplift of the overlying DT sheet occurred approx. 8 Ma. This event most likely constructed a large duplex in the MBT hanging wall. The MBT trailing ramp remains active, while post-Pliocene frontal thrust displacement has been transferred to thrust faults within the Siwaliks.