查看更多>>摘要：We applied a 2-dimensional, non-spectral technique to investigate the spatial variations of the lithospheric effective elastic thickness (T-e) in the eastern Bayan Har block and its adjacent areas. A program was designed to calculate the Moho flexure induced by topography loading, as well as the Bouguer gravity anomalies caused by that Moho flexure. The T-e values were determined by minimizing the RMS differences between the observed and calculated Bouguer gravity anomalies. The results suggested that T-e varies significantly from eastern Tibet to the Sichuan Basin. The eastern Bayan Har block and northern Sichuan-Yunnan block have low T-e values (0 < T-e < 20 km), indicating an easily deformable lithosphere. The Sichuan Basin has high T-e values (40 km < T-e < 100 km) acts as a rigid block that resists the eastward extrusion of plateau materials. The moderate T-e values (30 km < T-e < 40 km) found under the Longmen Shan fault belt indicate that the lithosphere of the Sichuan Basin plays an important role in supporting the Longmen Shan topography. The extremely low T-e (T-e < 10 km) observed in the seismogenic zone of the Ms7.0 Jiuzhaigou earthquake indicated that the topography is compensated locally, which is significantly different from the Longmen Shan topography. A banded region of relatively low T-e values (< 40 km) stretching from the northeastern Bayan Har block to the southwestern Ordos block sketches out an escape channel for plateau materials, and challenges the existing of crustal flow on the north side of the Sichuan Basin.
查看更多>>摘要：The Xing'an and Songnen Blocks are microcontinents in the eastern Central Asian Orogenic Belt (CAOB). The Heihe-Hegenshan suture has been interpreted to represent the boundary between the blocks. However, the lack of ophiolite exposure and deformation towards the northern extent of the suture mean that its precise nature and location remain controversial. To better understand the position and structure of the suture, magnetotelluric data were acquired at 104 stations along five profiles in the northern Songnen and Xing'an Blocks. A three-dimensional nonlinear conjugate gradient algorithm was used to image the resistivity structure of the lithosphere. Our resistivity models indicate that the lithospheres of both the Xing'an and Songnen Blocks are characterized by overall high resistivity and have heterogeneous electrical structures. We define a highly conductive anomaly (C2) to the east of the Duobaoshan Island Arc that divides lithosphere of high resistivity and controls the distribution of Carboniferous magmatic rocks related to collision. We suggest that C2 is the northern extension of the Heihe-Hegenshan suture. The highly conductive anomaly (C1) is located in the Xing'an Block on the west side of the island arc. While the exposed lithology on the surface is Jurassic intrusions, usually with high resistivity. We suggest that C1 may be an early structure, which was transformed by the closure of the Mongolia-Okhotsk Ocean. No large-scale Mesozoic intrusive bodies occur above C3, a highly conductive anomalous region in the Songnen Block. And the seismic reflection characteristics have changed significantly in the region C3. Therefore, we interpret C3 as a shear zone in the crust related to a change in the subduction direction of the paleo-Pacific plate. The resistivity model provides new geophysical evidence for determining the suture and the evolutionary traces of the Mongolia-Okhotsk Ocean and the Paleo-Pacific Ocean.
查看更多>>摘要：The Tan-Lu Fault Zone (TLFZ) is the largest strike-slip system in East Asia. The central TLFZ has four main branch faults across a width of 20- 40 km, and both the great 1668 Tancheng Earthquake (M8.5) and the Anqiu Earthquake (M7.0) occurred on its two eastern branch faults. Due to the lack of recent earthquakes and stations around a seismic-quiescent segment of the TLFZ centered near the 70 BCE Anqiu Earthquake, previous tomographic images have resolutions coarser than 20 km in the middle and lower crust, unable to show how the crust is faulted along the TLFZ. To improve the resolution, we deployed a 70-km-long W-E-trending seismic array with 38 portable seismographs across the TLFZ at 36.1 degrees N. Using one month of teleseismic records and a newly developed multiscale teleseismic tomography method, our P-wave velocity profile has achieved a resolution of 5 km x 5 km in shallow crust, and 10 km x 10 km in deep crust, as shown by the resolution tests. The profile's velocity variation correlates well with surface geology and crustal electrical resistivity profiles. The new profile provides clues for the depth distribution of major TLFZ branches, and indicates the two eastern TLFZ branch faults cut through the crust along a column-shaped low-velocity anomaly. This low-velocity column is associated with low-resistivity anomalies in the upper and lower crust, indicating a likely presence of fluids in the faulted rocks. This crustal faulting interpretation is consistent with the Moho geometry of previous receiver function studies, with an alignment between a receiver function Moho dome and a large low-velocity anomaly. Though seismicity is sparse near the profile, our interpreted crust-cutting location is in line with a linear cluster of recent TLFZ earthquakes about 20 km south of the profile, indicating that this quiescent segment could be a 150 km seismic gap along the central TLFZ.
查看更多>>摘要：A dense seismological array and profile reveal the deep structure across the Longmenshan from the Songpan-Ganzi terrane of the Tibetan plateau to the Sichuan basin. Receiver function and tomographic images reveal that the Pengguan Complex which cores the Longmenshan in the region where the Ms. 8 Wenchuan earthquake of 2008 occurred, is marked by high velocities in the upper 15 km of the crust. At about 15 km depth both P- and S-wave velocities decrease at a flat-lying boundary around which the aftershock hypocentres of the Wenchuan earthquake are concentrated. Thus, this boundary may be a faulted interface or detachment, marking the base of the Pengguan Complex. Moho depths change significantly in going from the Tibetan plateau to the Sichuan basin. At the location of the dense profile a Moho step occurs, located about 50 km NW of the surface trace of the Yingxiu-Beichuan fault (YBF). The boundary at about 15 km depth below the Pengguan Complex seems to deepen at around the Wenchuan-Maoxian fault (WMF) by about 3 km and merge to the NW with another interface at about 18 km depth. This interface, NW of the WMF, which correlates with the top of a zone of high conductivity is interpreted to represent the top of the Tibetan mid-crustal low velocity, high conductivity zone. The tomographic image indicates that the boundary between the low velocities of the Songpan-Ganzi terrane and the high velocities of the Sichuan basin in the middle and lower crust occurs NW of the surface trace of the YBF. Thus, it is proposed that a zone extending from the WMF at about 15 km depth to the Moho step about 25 km further NW marks the boundary between the Tibetan plateau and the Sichuan basin in the middle and lower crust.
查看更多>>摘要：The Yili block in the Central Asian Orogenic Belt (CAOB), forms the easternmost part of the Kazakhstan collage system. Exploring its thermo-tectonic history is important to reconstruct the intra-continental evolution of the Tianshan belt. In this contribution, we report new apatite fission track (AFT) data from the basement rocks from the northern (i.e. the Wenquan complex) and southern (i.e. the Dahalajunshan - Nalati range) margins of the Yili block. Thermal history modeling reveals that the Wenquan complex underwent moderate basement cooling in the Cretaceous, possibly due to far-field effects of the Tethys-deformation and the following Lhasa-Qiangtang collision. These events at the southern Eurasian margin propagated tectonic stress to the northern Yili and triggered localized deformation. Early Triassic-middle Jurassic moderate cooling is also identified in the Dahalajunshan - Nalati range, and is interpreted to be related to the post-orogenic strike-slip motion along the major shear zones and the effects of the Qiangtang and Kunlun-Qaidam collision. Combined with the published thermochronological data, it is suggested that the northern and southern parts of the Yili block experienced different Mesozoic thermo-tectonic evolution. Basement cooling of the northern Yili block generally took place before the Cretaceous, exhuming shallower crustal levels as compared with the southern one. The intermontane Yili basin may have accommodated substantial propagated contraction induced by the Cretaceous collisional events. Based on our new results and the previously published thermochronological data, it is suggested that the intra-continental reactivation of the North Tianshan and Nalati faults probably did not invoke significant regional exhumation during the Meso-Cenozoic. Instead, small-scale brittle faults controlled localized enhanced denudation.
查看更多>>摘要：The Lenglongling fault (LLLF) is an important part of the Qilian-Haiyuan fault (QHF) zone along the NE Tibetan Plateau. The Jinqianghe fault (JQHF) branches off of the LLLF and forms a fault junction that separates middle and eastern LLLF. Currently, it is not well understood how/if strain from the LLLF is transmitted to the JQHF over geologic time scales and how seismically active the eastern section of the LLLF is. Recent studies indicate that the 1927 M8 Gulang earthquake ruptured the middle LLLF and propagated eastward onto the eastern LLLF instead of propagating onto the JQHF. To assess if this concurrent rupture of middle and eastern LLLF is (or might be) a common phenomenon, we measured the Holocene slip-rate at the Huoshaotai site, east of the LLLF - JQHF junction, to compare it with known rates from the middle LLLF. Using displaced geomorphic markers and radiocarbon dating, we determined a Holocene left-lateral slip-rate of 6.0 +/- 0.8 mm/a for the eastern LLLF. This rate is only slightly below the consensus slip-rate for the middle LLLF (6.6 +/- 0.3 mm/a; 6.4 +/- 0.7 mm/a), suggesting a gradual strain transfer and similar seismic activity for both sections. Paleoseismic evidence further suggests that at least three preceding earthquakes may have ruptured both sections simultaneously during the Holocene, indicating that concurrent failure of both sections, as in 1927, may not be uncommon. Concurrent failure of middle LLLF and JQHF is however not reported. Hence, it appears that LLLF and JQHF act as independent seismo-tectonic entities, without substantial strain transfer or concurrent seismic activity. These observations contribute to a better understanding of the QHF zone and the potential for multi-fault rupture along it.
NSTL
Elsevier