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The eastern margin of the Tibetan Plateau has given rise to much debate about mechanisms of plateau uplift and evolution and, in particular, the role of the lower crust in crustal thickening. Knowledge of the middle to lower crust conditions is critical for evaluating various models of crustal deformation, but data on crustal evolution through time are lacking. Here, we turn to the Gongga Shan granite, an intrusion along the Xianshuihe fault in easternmost Tibet that directly records local Cenozoic crustal conditions. We present 124 U-Pb samples from the Gongga Shan granite (GSG) that prove that the crust has been stepwise producing partial melt from 56 Ma to 4 Ma. According to the age distribution, the GSG can be separated into four major groups with ages of 4–10 Ma, 12–20 Ma, 25–40 Ma, and 43–56 Ma. Combining the timing information with geophysics and low-temperature thermochronology data, we suggest that events younger than 10 Ma may indicate the onset of recent crustal channel flow in the middle to lower crust. In contrast, the youngest 4 Ma ages indicate the ongoing partial melting of the middle crust. The 12–20 Ma events could be related to an earlier stage of crustal channel flow, consistent with the regional large-scale crustal channel flow in central Tibet.

In Taiwan’s Central Range mountains, fundamental constraints on depositional ages and the timing of deformation and metamorphism remain a problematic issue preventing a consensual chronology of orogenic events to be established. In this contribution, we report detrital zircon U-Pb detrital ages for the Chulai Formation, the easternmost strip of metamorphic sediments depositionally overlying the metamorphic, high-pressure Yuli belt. We demonstrate that the maximum depositional age of this unit is 11.2 ± 0.2 Ma (Upper Miocene, Tortonian), making it the youngest pervasively deformed and metamorphosed unit of the Central Range. Detrital zircon ages suggest an almost exclusively continental origin of the sediments similar to the Yuli belt’s matrix detrital age zircon spectra. Sedimentary relationships and structural considerations indicate that the Chulai Formation underwent essentially the same deformation history as the underlying Yuli belt, and thus the maximum depositional age of 11.2 ± 0.2 Ma is interpreted as the upper limit for the start of pervasive deformation of the eastern Central Range geological units. When considering the existing geochronological constraints on the metamorphism, we argue that the timing for the Cenozoic metamorphism of the Taiwan orogen is likely to be ~6 - 8 Ma.

Taiwan is located on northern margin of South China Sea, which evolved from rifting basin to passive margin from Paleocene to Miocene. The northern Hsuehshan Range exposes the Eocene to Miocene strata, which offers a unique place to study the rifting basin history. However, the strata ages in the northern Hsuehshan Range are controversial, particularly for the Hsitsun Formation and Szeleng Sandstone that may correlate with a proposed break-up unconformity in the Eocene and Oligocene. Previously, the Hsitsun Formation was considered to be pre-Early to Middle Eocene in age based on the presence of Nummulites junbarensis in the overlying Szeleng Sandstone. In this paper, we present new U-Pb ages of detrital zircon grains from the Hsitsun Formation and Szeleng Sandstone to constrain the depositional ages of these units. In the Hsitsun Formation, ca. 8% of zircon U-Pb grains are Cenozoic in age and have a mean age of 35 Ma indicating that the maximum deposition age of late Eocene for this formation. Combining these results with the ca. 30 Ma age for Paling Formation indicates the Szeleng Sandstone is between 30 - 35 Ma. The new ages determinations suggest that the deposition was continuous with no evidence of a break-up unconformity from Szeleng Sandstone to Paling Formation. Finally, because much of the stratigraphic interpretations and correlations are based on what were interpreted to be “in situ” biostratigraphic markers, the new U-Pb dates argue for recycling and reworking of several key fossil assemblages. This result has important implications for other stratigraphic interpretations both in Taiwan and in other orogenic systems around the world.

On 12 May 2008, the MW 7.9 Wenchuan earthquake triggered widespread damaging landslides in many parts of the Longmen Shan area. Among these landslides, the Donghekou ejection landslide is quite special. It is located at the northeast end of the Beichuan rupture, and it has caused a great loss of life at the villages of Donghekou, Qinchuan, and Sichuan Provinces. Because of its special location, this ejection landslide differs from landslides caused by gravity or rainstorms only; the sliding surface is not uniformly continuous. Instead, two sections can be distinguished: an upper section with a step sliding surface and a lower section more gently dipping. The landslide started with material ejection caused by large local seismic acceleration, throwing rocks into the air with a parabolic trajectory before they fell back to the ground. In this paper, we analyze geologic and geomorphologic conditions that favored the occurrence of this landslide, and we introduce a simple tectonicgeomorphology model to explain the mechanism that led to the ejection landslide. We find that the location of the landslide zone, along with the domino-like ground tension cracks observed on both sides of the Beichuan rupture, is controlled by the propagation of the rupture. Our result also suggests that, in addition to local seismic shaking intensity, horizontal acceleration, and geomorphologic and geologic conditions, the vertical acceleration and the style of faulting could also play an important role in the occurrence of earthquake-triggered landslides. [ABSTRACT FROM AUTHOR]

In Taiwan's Central Range mountains, fundamental constraints on depositional ages and the timing of deformation and metamorphism remain a problematic issue preventing a consensual chronology of orogenic events to be established. In this contribution, we report detrital zircon U-Pb detrital ages for the Chulai Formation, the easternmost strip of metamorphic sediments depositionally overlying the metamorphic, high-pressure Yuli belt. We demonstrate that the maximum depositional age of this unit is 11.2 ± 0.2 Ma (Upper Miocene, Tortonian), making it the youngest pervasively deformed and metamorphosed unit of the Central Range. Detrital zircon ages suggest an almost exclusively continental origin of the sediments similar to the Yuli belt's matrix detrital age zircon spectra. Sedimentary relationships and structural considerations indicate that the Chulai Formation underwent essentially the same deformation history as the underlying Yuli belt, and thus the maximum depositional age of 11.2 ± 0.2 Ma is interpreted as the upper limit for the start of pervasive deformation of the eastern Central Range geological units. When considering the existing geochronological constraints on the metamorphism, we argue that the timing for the Cenozoic metamorphism of the Taiwan orogen is likely to be ~6 - 8 Ma. [ABSTRACT FROM AUTHOR]

The Gongga Shan batholith of eastern Tibet, previously documented as a ca. 32-12.8 Ma granite pluton, shows some of the youngest U-Pb granite crystallization ages recorded from the Tibetan Plateau, with major implications for the tectonothermal history of the region. Field observations indicate that the batholith is composite; some localities show at least seven crosscutting phases of granitoids that range in composition from diorite to leucocratic monzogranite. In this study we present U-Pb ages of zircon and allanite dated by laser ablation-inductively coupled plasma-mass spectrometry on seven samples, to further investigate the chronology of the batholith. The age data constrain two striking tectonic-plutonic events: a complex Triassic-Jurassic (ca. 215-159 Ma) record of biotite-hornblende granodiorite, K-feldspar megacrystic granite and leucogranitic plutonism, and a Miocene (ca. 14-5 Ma) record of monzonite-leucogranite emplacement. The former age range is attributed to widespread Indosinian tectonism, related to Paleo-Tethyan subduction zone magmatism along the western Yangtze block of south China. The younger component may be related to localized partial melting (muscovite dehydration) of thickened Triassic flysch-type sediments in the Songpan-Ganze terrane, and are among the youngest crustal melt granites exposed on the Tibetan Plateau. Zircon and allanite ages reflect multiple crustal remelting events; the youngest, ca. 5 Ma, resulted in dissolution and crystallization of zircons and growth and/or resetting of allanites. The young garnet, muscovite, and biotite leucogranites occur mainly in the central part of the batholith and adjacent to the eastern margin of the batholith at Kangding, where they are cut by the left-lateral Xianshui-he fault. The Xianshui-he fault is the most seismically active strike-slip fault in Tibet and is thought to record the eastward extrusion of the central part of the Tibetan Plateau. The fault obliquely cuts all granites of the Gongga Shan massif and has a major transpressional component in the Kangding-Moxi region. The course of the Xianshui Jiang river is offset by ~62 km along the Xianshui-he fault and in the Kangding area granites as young as ca. 5 Ma are cut by the fault. Our new geochronological data show that only a part of the Gongga Shan granite batholith is composed of young (Miocene) melt, and we surmise that as most of eastern Tibet is composed of Precambrian-Triassic Indosinian rocks, there is no geological evidence to support regional Cenozoic internal thickening or metamorphism and no evidence for eastward-directed lower crustal flow away from Tibet. We suggest that underthrusting of Indian lower crust north as far as the Xianshui-he fault resulted in Cenozoic uplift of the eastern plateau. [ABSTRACT FROM AUTHOR]