No abstract

We report the first combined geochronologic and paleomagnetic study of volcanic rocks from the Shiquanhe and Yare Basins at the westernmost Lhasa Terrane, which aims to provide an accurate constraint on the shape and paleoposition of the southern margin of Asia prior to the India-Asia collision. Three new 40 Ar/ 39 Ar ages of 92.5 ± 2.9 Ma, 92.4 ± 0.9 Ma, and 79.6 ± 0.7 Ma determined by fresh matrix or feldspar from lava flows suggest a Late Cretaceous age for the investigated units. Characteristic remanent magnetizations have been successfully isolated from 38 sites which pass positive fold and/or reversal, conglomerate tests and are hence interpreted as primary in origin. The two paleopoles obtained from Yare and Shiquanhe yield consistent paleolatitudes of 13.6°N ± 9.6°N and 14.2°N ± 2.7°N, respectively (for a reference site of 31.5°N, 80°E), indicating that the southern margin of Asia near the western syntaxis was located far south during the Late Cretaceous time. A reconstruction of the Lhasa Terrane in the frame of Eurasia with paleomagnetic data obtained from its western and eastern parts indicates that the southern margin of Eurasia probably had a quasi-linear orientation prior to the collision formerly trending approximately 315°E. This is compatible with the shape of the Neo-Tethys slab observed from seismic tomographic studies. Our findings provide a solid basis for evaluating Cenozoic crustal shortening in the Asian interior and the size of Greater India near the western syntaxis.

The Wenchuan Ms8.0 earthquake of 2008 occurred on the Longmen Shan tectonic zone which seems not very active from available data. And it is reported that no noticeable precursors have been observed before this event. It might imply that the crustal deformation at depth associated with this gigantic shock had no expression on the surface or shallow subsurface which was detectable. In this article, we attempt to analyze the processes of brewing and generation of this event at depth that can account for the reason of this devastating earthquake. Tectonically, it is obvious that the India‐Eurasia plate collision serves as the most important dynamic setting of major earthquakes in southwestern China. The Eastern Himalayan Syntaxis is moving rapidly in NNE direction into the Tibetan plateau, forcing deep material to flow toward east. Due to the obstruction of the NE trending Longmen Shan tectonic zone in the northwestern margin of the Sichuan basin, the partial flow turns to southeast. Longmen Shan is a conspicuous boundary of topography and crust structure, across which the elevation difference is as large as 3500±500 m, with crustal thickness 60±5 km in its west and 40±2 km in its east, respectively. Thus it must experience intense deformation and stress accumulation, though very slow without appreciable manifestation. Based on available data, it is inferred that material of lower crust and covering strata of upper mantle is flowing eastward along two deep slide surfaces, of which one is the layer of low‐velocity and low‐resistivity (20~25 km depth), and the other is the top of mantle asthenosphere. Hindered by rigid matter beneath the Sichuan basin, the flow of lower crust and covering strata of upper mantle material turns overthrusting at high angles at the Longmen Shan. On the other hand, three thrust faults visible west of the Longmen Shan extend downward to depth 20 km, where they converge into one active zone. Thus the Longmen Shan tectonic zone is a place where occurs the coupling between slippage on faults of shallow subsurface and material flow of lower crust and covering strata of upper mantle. Under the intense exchange of matter and energy, the source medium ruptures suddenly at a large scale to release a huge amount of elastic strain, resulting in the Wenchuan Ms8.0 earthquake.

No abstract

No abstract

The drift of Gondwana‐derived blocks across the Tethys has remained unclear specifically because one of the largest components, Indochina, has remained unconstrained by paleomagnetism during the late Paleozoic, the interval comprising the first half of its journey towards the Eurasian landmass. Hence, we report a paleomagnetic study of Early‐Middle Permian limestones from central Thailand yielding a high‐quality data set for the Indochina Block with probable primary origin. The magnetization yields a paleomagnetic pole at 34.1°N, 331.7°E (K = 45.4, A95 = 5.7°) corresponding to a paleolatitude of 21.2 ± 5.7°S at the center of the study area (15°N, 101°E) and supporting a tropical restoration for late Paleozoic Indochina. A total of ~5,000 km northward transition from a proximity to Gondwana in the Early‐Middle Permian (ca. 280 Ma) to the southern margin of Eurasia by the Late Triassic (Norian, ca. 220 Ma) is identified and yields an average northward latitudinal drift rate of ~4.5 cm/year. A concurrent northward drifting history during the Permian is suggested for the Indochina, North Qiangtang, and South China blocks, and thus, the widely used model of Permian Cathaysialand is quantitively supported when combined with their paleobiogeographic affinities.

No abstract

Paleomagnetic studies on the syncollisional magmatic eruptions provide a direct way to understand when and where the Paleo-Tethys closed. We report a well-dated paleomagnetic pole from the southern Lancangjiang volcanic belt in western Yunnan of China. Stable Characteristic Remanent Magnetizations (ChRMs) were isolated from 21 sites at high temperatures following step-wise thermal demagnetization. The data pass fold, reversal, and conglomerate tests and are interpreted to be primary thermal remanent magnetizations. The new paleomagnetic pole (46.1°N/176.2°E, K=44.9, A 95 =4.8°, N=21) yields a paleolatitude of 26.0 ± 4.8°N at the reference site (23.