Tung Yi Lee

Adakites from continental collision zones: Melting of thickened lower crust beneath southern Tibet

Adakites are geochemically distinct intermediate to felsic lavas found exclusively in subduction zones. Here we report the first example of such magmas from southern Tibet in an active continental collision environment. The Tibetan adakites were emplaced from ca. 26 to 10 Ma, and their overall geochemical characteristics suggest an origin by melting of eclogites and/or garnet amphibolites in the lower part (≥50 km) of thickened Tibetan crust. This lower-crustal melting required a significantly elevated geotherm, which we attribute to removal of the tectonically thickened lithospheric mantle in late Oligocene time. The identification of collision-type adakites from southern Tibet lends new constraints to not only the Himalayan-Tibetan orogenesis—how and when the Indian lithosphere started underthrusting Asia can be depicted—but also the growth of the early continental crust on Earth that consists dominantly of the tonalite-trondhjemite-granodiorite suites marked by adakitic geochemical affinities.

Cenozoic plate reconstruction of Southeast Asia

Abstract The India-Eurasia collision alone set up a series of chain reactions and caused the formation and destruction of sedimentary basins within the domain of the collision belt. Changes in the rate and angle of convergence between the India and Eurasia plates reflect different stages of tectonic development in Southeast Asia. For example, extrusion of the Indochina block induced the consumption of the pre-existing proto-South China Sea along northeastern Kalimantan and led to the eventual opening of the South China Sea along the South China margin. Subsequent motions of the Sino-Burma-Thailand, Malay Peninsula, Sumatra, and Kalimantan blocks have produced a succession of basins stretching from north Sumatra to central Thailand and on to the Natuna area. We present reconstructions of the Southeast Asia region at 60 Ma, 50 Ma, 40 Ma, 30 Ma, 20 Ma, 15 Ma, 10 Ma, and 5 Ma. It is clear, from the reconstructions, that the impact between Greater India and Southeast Asia took place in the northwestern part of Southeast Asia. The duration for the impact was probably from the Middle Eocene to Early Miocene. This timing is in good agreement with the dating of the main Red River Fault motion (Wu et al., 1989; Scharer et al., 1990). Since the impact between Greater India and Southeast Asia was basically west of the Burma block, there is no reason to assume that the Sumatra, Malay Peninsula, and Kalimantan should extrude to the southeast first along the left-laterally displaced Mae Ping and Three Pagodas fault zones as suggested by Peltzer and Tapponnier (1988). If the opening of the central Thailand basins, the Gulf of Thailand, and the Malay Basin are taken into consideration, a dextral megashear zone is required to compensate the relative motion between Indochina and the Malay Peninsula. This dextral megashear zone might even extend into western Kalimantan and serve as a boundary between the Indochina block and Kalimantan.

Diachronous uplift of the Tibetan plateau starting 40 Myr ago

The uplift of the Tibetan plateau is generally regarded as a response to the convective removal of the lower portion of the thickened Asian lithosphere. This removal is also thought to be responsible for the east–west extension that took place during the India–Asia collision. The timing of these events has been a subject of great interest for understanding mountain-building processes, collisional tectonics and the influence of these processes on climate change,. In western Tibet, potassic lavas related to east–west extension were found to have been extruded over the past 20 Myr (refs 5, 6). Here we report the widespread occurrence of magmas in eastern Tibet which show similar geochemical signatures to the potassic lavas to the west but formed 40–30 Myr ago. These magmatic activities suggest a diachronous uplift history for the Tibetan plateau, with the convective removal of the lower lithosphere inducing rapid uplift in the east beginning some 40 Myr ago and in the west about 20 Myr later. This observation is consistent with sedimentation records from the Ganges–Brahmaputra delta to the Bengal fan, and can better account for the tectonically driven models for strontium isotope evolution in the ocean and global cooling over the past 40 Myr.

Intraplate extension prior to continental extrusion along the Ailao Shan-Red River shear zone

Left-lateral movement of the Ailao Shan–Red River shear zone lends support to the hypothesis of continental extrusion resulting from the collision of India with Asia. Our new observations from northwestern Yunnan, China, and northwestern Vietnam on different sides of the shear zone demonstrate that the sinistral offset was ∼600 km according to correlations of Permian-Triassic flood basalt successions and late Paleogene highly potassic mafic magmas. We conclude that the shear was propagating on the South China continental margin and does not correspond to a suture between South China and Indochina. Furthermore, the highly potassic magmas were emplaced from ca. 40 to 30 Ma, before the shear movement, which was caused by the late Oligocene to early Miocene (ca. 27–22 Ma) extrusion activity. This suggests that a late Eocene to early Oligocene intraplate extension, possibly induced by delamination of thickened continental lithosphere, took place in northwestern Yunnan (or eastern Tibet) as a response to the India-Asia collision. This extension, and sea-floor spreading of the South China Sea that began ca. 30 Ma, could have accounted for the initiation of the Ailao Shan–Red River shear zone.

Age of the Emeishan flood magmatism and relations to Permian–Triassic boundary events

The Permian^Triassic (P^T) mass extinction, the greatest biological mortality event in the Earth’s history, was probably caused by dramatic and global forcing mechanisms such as the Siberian flood volcanism. Here we present the first set of high-precision 40 Ar/ 39 Ar dating results of volcanic and intrusive rocks from the Emeishan Traps, South China, which define a main stage of the flood magmatism at V251^253 Ma and a subordinate precursory activity at V255 Ma. This time span is generally coeval with, or slightly older than, the age of the P^T boundary estimated by the ash beds in the Meishan stratotype section and the main eruption of the Siberian Traps. Our data reinforces the notion that the eruption of the Emeishan Traps, rather than eruption of the Siberian Traps, accounted for the formation of the P^T boundary ash beds in South China. The Emeishan flood magmatism, which occurred in the continental margin comprising thick marine limestone formations, moreover, may have triggered rapid release of large volumes of methane and carbon dioxide that could have been responsible for the global N 13 C excursion and associated environmental crisis leading to the mass extinction at the P^T boundary. fl 2002 Elsevier Science B.V. All rights reserved.

Thermochronological evidence for the movement of the Ailao Shan-Red River shear zone: A perspective from Vietnam

In order to explore the timing of strike-slip movement along the Ailao Shan–Red River shear zone, an 40Ar/39Ar thermochronological study of the Day Nui Con Voi metamorphic massif in northern Vietnam was undertaken. The massif, exposed in the southeastern segment of the Ailao Shan–Red River shear zone, reveals a rapid cooling in the early Miocene (25–21 Ma) after a very slow cooling ca. 34–25 Ma. The slow cooling period most likely resulted from a geothermal perturbation in the lithosphere owing to the late Paleogene (40–30 Ma) extension in eastern Tibet and western Yunnan, China. The rapid cooling, consistent with evidence of a wider rapid cooling span from 27 to 17 Ma compiled from the entire Ailao Shan–Red River shear zone, constrains the duration of left-lateral shearing, which eventually offset Indochina from South China by about 600 km. The time constraints we have established reinforce the argument that the onset of the Ailao Shan–Red River shear zone postdates the opening of the South China Sea that began ca. 30 Ma. This result highlights the necessity for reevaluating the Cenozoic tectonic models of Southeast Asia.

Onset timing of left-lateral movement along the Ailao Shan–Red River Shear Zone: 40Ar/39Ar dating constraint from the Nam Dinh Area, northeastern Vietnam

Left-lateral motion along the Ailao Shan‐Red River (ASRR) Shear Zone has been widely advocated to be the result of the collision between the Indian and Eurasian plates and to account for sea-floor spreading in the South China Sea. Our new 40 Ar/ 39 Ar data on the south-easternmost outcrop of the Day Nui Con Voi metamorphic massif, northern Vietnam, suggest that the exhumation of metamorphic massif by shearing along the ASRR zone began 027 Ma and lasted until 022 Ma. A perfect correlation between location and cooling path for the samples along the shear zone suggests that the transtensional deformation may have propagated northwestward at a rate of 0 6c m y ˇ1 . Such a good correlation also indicates that the onset of the leftlateral movement of the shear zone may have occurred later than 027.5 Ma. This conclusion is consistent with our previous interpretation that collision-induced southeastward extrusion of Indochina along the ASRR Shear Zone postdates the opening of the South China Sea, and that extrusion tectonics in SE China may not be responsible for the opening of the South China Sea. # 2000 Elsevier Science Ltd. All rights reserved.

Cenozoic plate reconstruction of the South China Sea region

Abstract Reconstructions of the South China Sea region at 60 Ma, 40 Ma, 30 Ma, 20 Ma, 10 Ma and 5 Ma are presented. We have attempted to place the South China Sea Basin in a regional tectonic framework. The tectonic evolution of the major blocks surrounding the South China Sea were analyzed, as well as the relative motions of the Indian and Australian plates. We have tried to correct the tectonic models available in this region. A 3-D graphics terminal was used to derive rotation poles for the different tectonic blocks and our model was then tested to determine its self-consistency. When the model conflicted with previous interpretations the input data were evaluated for alternative explanations. At least two, and possibly three, stages of extension can be recognized in this region. The earliest one, active in the Late Cretaceous to Eocene, involved NW-SE extension. The second one, active from the Late Eocene to Early Miocene involved north-south extension. The third stage of extension, which probably trended NW-SE, can be dated as post-Oligocene. The first extensional event produced the NE-SW trending proto-South China Sea and a series of sedimentary basins along the South China margin. Following the southeastward extrusion of Indochina, the proto-South China Sea was mostly consumed at the Palawan Trough. Renewed north-south extension in the South China continental margin started the present-day South China Sea spreading in the Oligocene. The southeastward extrusion of Indochina, blocked by Sundaland, resulted in the NW-SE trending opening of the South China Sea Basin in the Early Miocene. Collision of the North Palawan microcontinental block with the West Philippines block stopped the opening of the South China Sea at the end of Early Miocene. Spreading activity switched to the Sulu Sea Basin in the Middle Miocene but collision between the Sulu Ridge and the West Philippines at Mindanao halted the opening of the Sulu Sea at the end of the Middle Miocene. In the Late Miocene, Greater India continued its northward path and seems to have ripped open the Andaman Sea. In the Pliocene, subduction along the northern Manila Trench placed the North Luzon Arc on a collision path with the East Asia continental margin at Taiwan. Our reconstructions, along with detailed geological and geophysical information, may be used as a predictive tool for basin evolution models and block interactions in this region. The development of the South China Sea Basin, the Gulf of Thailand, the Malay Basin and the central Thailand basins are the result of collision-induced extensional forces. The Sulu, Celebes and Sumatra basins were formed as a consequence of prolonged subduction. The opening of the Pearl River Mouth, West Natuna, South China Sea, Sulu, and possibly Celebes, basins were terminated by various plate collisions. During the course of plate reorganizations major boundary faults have changed their slip senses during different stages of evolution.

Extreme storm events, landscape denudation, and carbon sequestration: Typhoon Mindulle, Choshui River, Taiwan

We have performed the first known semicontinuous monitoring of particulate organic carbon (POC) fluxes and dissolved Si concentrations delivered to the ocean during a typhoon. Sampling of the Choshui River in Taiwan during Typhoon Mindulle in 2004 revealed a POC flux of 5.00 × 105 t associated with a sediment flux of 61 Mt during a 96 h period. The linkage of high amounts of POC with sediment concentrations capable of generating a hyperpycnal plume upon reaching the ocean provides the first known evidence for the rapid delivery and burial of POC from the terrestrial system. These fluxes, when combined with storm-derived CO2 consumption of 1.65 × 108 mol from silicate weathering, elucidate the important role of these tropical cyclone events on small mountainous rivers as a global sink of CO2.

SOUTHERNMOST SOUTH-AMERICA ANTARCTIC PENINSULA RELATIVE PLATE MOTIONS SINCE 84 MA - IMPLICATIONS FOR THE TECTONIC EVOLUTION OF THE SCOTIA ARC REGION

We have attempted to quantify the relative motion history between southernmost South America (SSA) and the Antarctic Peninsula (AP) by calculating and comparing SSA-Africa, AP-Africa and SSA-AP synthetic flow lines for 84–0 Ma. The flow lines were created using published poles of rotation and plate reconstruction software. The results indicate that since 84 Ma, SSA and AP have moved approximately westward relative to a fixed Africa; however, SSAs rate of westerly motion in that reference frame has been significantly more rapid than APs rate. Approximately 1320 km of east-west, left-lateral strike-slip displacement and 490 km of north-south, divergent displacement have occurred between the southern tip of SSA and the northern tip of AP since 84 Ma. Increased rates of SSA-AP interplate separation and a change in the angle of plate divergence at approximately 55–40 Ma marked the onset of accelerated continental separation that eventually led to seafloor spreading in the western Scotia Sea at 30 Ma and the development of the Scotia Arc. Increased separation rates between SSA and AP at 55–40 Ma may be related to a global Eocene plate reorganization event. The northeast-southwest oriented western Scotia Sea spreading centers appear to have accommodated all of the SSA-AP interplate motion between 30 and 9 Ma. We suggest that prior to 30 Ma and the opening of Drake Passage, components of interplate strike-slip and divergent motion were accommodated by intracontinental deformation that included strike-slip faulting, counterclockwise tectonic rotation, and continental extension in the southernmost Andes. The results indicate that the opening of the Scotia Sea was caused by plate-scale motions as SSA and AP drifted away from Africa at different velocities along different, nonparallel trajectories. Subduction retreat along the South Scotia Ridge and South Sandwich arc and back arc spreading in the Scotia Sea contributed to the width of separation between SSA and AP across Drake Passage. The results place limits on how SSA-AP relative motion has been temporally and spatially partitioned in the Scotia Arc region.