Qingzhou Lai

New geological evidence of crustal thickening in the Gangdese block prior to the Indo-Asian collision

Recent mapping in the Gangdese block has revealed many leucogranites that are similar to those in the High Himalaya. These leucogranites formed at ∼140 Ma as indicated by monazite Th-Pb ion-microprobe dating and cooled at ∼130 Ma as indicated by muscovite40Ar/39Ar dating. In conjunction with previous structural and paleogeographic studies, the new data indicate that the Gangdese block underwent crustal thickening and associated exhumation during ∼140–130 Ma. In this regard, the southern margin of Eurasia continent was comparable to the modern South American Altiplano-Puna plateau, the prime example of active ocean-continent subduction and associated thickened crust. Specifically, the early stages of crustal thickening and uplifting of the Gangdese block may result from subduction of the Neo-Tethyan Ocean. If the Tibetan Plateau would form by accretion of a series of blocks with thickened crust, an elevated topographic plateau similar to the Altiplano-Puna plateau had formed before collision between the Indian and Eurasian plates. Then the Tibetan Plateau would have quickly thickened, uplifted, and begun to extend soon after onset of the collision. Thus, the deformational mechanism of the Tibetan Plateau is not distributed shortening, but rather concentrating deformation within regions of thin crust between the accreted blocks.

Quantifying the rise of the Himalaya orogen and implications for the South Asian monsoon

We reconstruct the rise of a segment of the southern flank of the Himalaya-Tibet orogen, to the south of the Lhasa terrane, using a paleoaltimeter based on paleoenthalpy encoded in fossil leaves from two new assemblages in southern Tibet (Liuqu and Qiabulin) and four previously known floras from the Himalaya foreland basin. U-Pb dating of zircons constrains the Liuqu flora to the latest Paleocene (ca. 56 Ma) and the Qiabulin flora to the earliest Miocene (21- 19 Ma). The proto-Himalaya grew slowly against a high (similar to 4 km) proto-Tibetan Plateau from similar to 1 km in the late Paleocene to similar to 2.3 km at the beginning of the Miocene, and achieved at least similar to 5.5 km by ca. 15 Ma. Contrasting precipitation patterns between the Himalaya-Tibet edifice and the Himalaya foreland basin for the past similar to 56 m.y. show progressive drying across southern Tibet, seemingly linked to the uplift of the Himalaya orogen.

Midcrustal shearing and doming in a Cenozoic compressive setting along the Ailao Shan-Red River shear zone

The Cenozoic Xuelong Shan antiformal dome is located along the northern segment of the Ailao Shan-Red River shear zone in Yunnan, China. Subhorizontal foliation in the gneiss core is recognized, representing a broad top-to-NE shear initiated under amphibolite facies conditions and propagating into greenschist facies in the mantling schist and strike-slip shear zone. Microfabrics of crystallographic-preferred orientations (CPOs) in quartz suggest that the deformation temperatures increased with increasing structural depth from the upper crust (300–500°C) in the mantling schist to the midcrust (15 km or more, ≥650°C) in the gneissic core. This trend is mirrored by variations in the metamorphic grade of the syn-kinematic mineral assemblages and microstructures, which range from garnet + amphibole + biotite + sillimanite + rutile + feldspar in the core to garnet + staurolite + biotite + epidote + muscovite within the limb units. The dome experienced the following deformation history: (1) a broad top-to-NE shear in the subhorizontal foliation of the gneiss core during the first stage of deformation (D1); (2) opposing reverse-sense shear along the two schist limbs of the dome during contraction-related doming (D2–D3); (3) sinistral strike-slip shearing within the eastern limb (D4); and (4) extensional deformation (D5). The structural-thermal patterns suggest the antiformal dome formation was roughly coeval with top-to-NE ductile shearing in the midcrust of Tibet at 32 Ma or earlier. A major implication is that there was a phase of contractional ductile deformation in the region prior to the initiation of strike-slip deformation.