查看更多>>摘要：What are the geodynamic processes during the transition from the onshore NE Taiwan collision to the offshore southwest Okinawa back-arc basin opening associated to the Ryu-Kyu subduction? What is the local neotectonic scheme of this transition (e.g.: structural sketch map of the outcropping volcanic edifice highlighting major faults and their associated earthquakes)? These are some of the fundamental questions adressed to the Kuei-Shan Tao volcanic island (KST hereafter) which is the unique emerged volcanic outcrops situated within this geodynamic transition area. Several incompatible KST geological mappings had been published without any faults, nor dykes, nor feeders (Ichikawa, 1934; Hsu, 1963; and Chiu et al., 2010) that needed to be updated and completed on the structural point of view. In order to do so, we acquired a new high resolution UAS-drone topography (Digital Surface Model) through photogrammetric processing, with a ground resolution <10 cm. We analyse and interprete it in detail using morphostructural photo-interpretation methods. Field works on KST is restricted due to technical and administrative reasons, so we compare our morphostructural map to the shoreline outcrops observed from a boat survey. Then, we have updated the Kuei-Shan Tao geological mapping (lava flows and pyroclastic falls), and the structural scheme as well as the major erosionnal landslide processes. Taking into account the reinterpretation of surroundings offshore bathymetry, old and new seismic profiles, the different drillings done in the Kuei-Chia northern flank, previous geophysical works, the existing massive andesite datings), as well as the inferred stress regimes deduced from the earthquake's focal mechanisms, we propose a KST neotectonic map. We propose also a new scenario for the recent KST volcanic evolution. Kuei-Shan Tao geology and geodynamics may have a so great importance for the 0.5 millions citizens of the so close flat lying Ilan Plain in terms of natural hazards (eruptions, tsunamis, earthquakes...).
查看更多>>摘要：The Qinling Orogenic Belt is located at the central part of the China Central Orogenic Belt and links the Qilian and Kunlun Orogenic Belts to the west with the Dabie-Sulu Orogenic Belt to the east, which is sandwiched between the South China Block and North China Block. There have been several geodynamic models developed to explain the evolution process of the Qinling Orogenic Belt, but they remain controversial. The magnetotelluric (MT) method can study subsurface electrical structures. In order to have a better understanding of the tectonic evolution of the Qinling Orogenic Belt, a study of broadband and long period MT profile across the middle Qinling Orogenic Belt is carried out. The MT profile starts from the northeastern margin of the Sichuan basin, crosses the Qinling, and finally reaches the Weihe basin at about 108.5 degrees E. Based on data analysis results, we have finally obtained a lithospheric electrical resistivity model of the middle Qinling Orogenic Belt through three-dimensional (3-D) inversion of magnetotelluric data. The MT result reveals that the Sichuan and Weihe basins may have remnants of the ancient craton basement in our study area. The electrical structure characteristics of the South Qinling Belt and North Qinling Belt are obviously different, the South Qinling Belt is relatively low resistivity, while the North Qinling Belt is relatively high resistivity. This may indicate that the Southern Qinling Belt is affected by the later strike slip extrusion and the Northern Qinling Belt is a relatively stable block. The lithospheric mantle of middle Qinling Orogenic Belt is characterized by overall high resistivity, which may indicate that the lateral extrusion effect of the Tibetan Plateau material is negligible in this region.
查看更多>>摘要：The West Iberian Margin (WIM) is a key example of a magma-poor passive margin, punctuated by several postrift magmatic manifestations that are part of the Late Cretaceous Atlantic Alkaline Province. In this work, potential field (gravity and magnetics) data, constrained by 3D multichannel seismic reflection data, are used to describe and characterise the geometry and nature of magmatic features located offshore the central segment of the margin, the Estremadura Spur. The estimated geometry and nature of the magmatic features was achieved through the integration of 3D gravity and magnetic inversion and 2D magnetic forward modelling. The results provide an insightful 3D subsurface model revealing that: 1) the Estremadura Spur Intrusion represents a 28 x 15 km wide laccolith with an overall granitic nature and an estimated density of 2490-2640 kg/m3 and 0.01-0.05 SI magnetic susceptibility, 2) the 26 x 17 km Fontanelas buried volcano is dominantly basaltic, with density values of 2500-2821 kg/m3 and magnetic susceptibility of 0.01 to 0.0875 SI, and 3) multiple sill complexes intruded the region, thus producing a higher magnetic background on otherwise inconspicuous anomalies The models allowed achieving a confident fit suggesting that both the ESI and the Fontanelas volcano are coeval with the outcropping magmatic features from this same magmatic event. Additionally, the results support that Late Cretaceous alkaline magmatism on the West Iberian Margin is more significant than anticipated and provide further evidence to clarify unclear geometrical aspects of similar intrusions observed onshore. Acknowledging the geometry and nature of these magmatic entities allows to better understand the role of postrift intra-plate magmatism on continental hyper-extended rifted margins by clarifying how shallow plumbing systems evolve in these settings.
查看更多>>摘要：As the largest oil-bearing basin in northwest China, the Tarim basin records multiple major tectonic-sedimentary transitions during the Early Paleozoic, which play an important role in understanding assembly processes of Gondwana. An important transition from the Middle to Late Ordovician was development of important unconformity T-7(4), the end of a unified carbonate platform, and development of turbidites in eastern basin. However, due to unobvious terrigenous clastic record, there is still a lack of understanding of driving mechanism of the tectonic transition. Marine carbonate red beds (MCRBs) with detritus components are developed in the Tumuxiuke/Kanling Formation at the bottom of the Upper Ordovician. By detailed field observation, zircon U-Pb dating and Hf-isotope analyses, provenance and the tectonic implication of the MCRBs are studied. New zircon U-Pb dating results constrain the depositional age of the MCRBs at ca. 454Ma. Provenance tracing shows the Precambrian detrital zircons were mainly sourced from the basement of the Tarim block, and the early Late Ordovician detrital zircons were probably derived from ash from the Altyun Tagh orogenic belt. Based on syn-depositional magmatic zircon records and the magmatic-metamorphic records in the Altyn Tagh orogenic belt, the closure age of North Altyn Ocean, one branch of Proto-Tethys Ocean, can be constrained to the latest Middle Ordovician (similar to ca. 460 Ma). The collison of the Tarim block with the Central Altyn terrane occurred (ca. 460 Ma - 454 Ma), leading to the major tectonic-sedimentary transition in the Tarim block. The U-Pb age spectrum of detrital zircons also indicates that the Tarim block is closely related to the North Indian block in Gondwana. This study shows that carbonate rocks with low clastic flux can be applied as provenance research objects.
查看更多>>摘要：Some conceptual models proposed for explaining the Hindu Kush intermediate-depth seismicity assumed that a subhorizontal fracture was migrating laterally, from east toward west, along the strike of a subducted tectonic plate. The present study has identified a distinct cluster of earthquake hypocenters, likely corresponding to the advancing tip of the subhorizontal fracture. Those hypocenters belonged to a set of fairly moderate earthquakes (5.6 <= M <= 6.5), which had not been, so far, explicitly addressed in the Hindu Kush seismo-tectonic interpretations, and which were proven to be virtually the only events displaying a failure regime characteristic to a laterally propagating breakoff - namely, reverse-fault focal mechanisms with subhorizontal compression axes parallel to the slab strike. A separate, upper-positioned hypocenter cluster was noticed to include all the M-w >= 7 intermediate-depth earthquakes: it was conjectured, relying on a previously published numerical model, that those strong events could be triggered by upward transfer of the viscoelastic stress induced by the slab pull increase caused by an episode of lateral breakoff advance. At the same time, as the inferred breakoff fracture enables the deeper segment of the plate to detach from the shallower segment, the plate region situated above the severed region is relieved of the gravitational pull that was imposed by the deeper, and currently detached, plate segment. In response to the resulting crustal rebound, the surface topography is expected to exhibit down-tilting to the west. Such an anticipation seems to be confirmed by the catchments of several streams which flow roughly perpendicular to the intermediate-depth earthquakes epicentral domain: relevant geomorphic indices (the swath profile average and maximum elevation curves; the transverse hypsometric integral - THi; the transverse topographic symmetry factor - T-vector) computed for the concerned valleys consistently suggested larger uplift rates in the east, as compared to those manifest further to the west.
查看更多>>摘要：It remains unknown whether middle-lower crustal flow exists in the eastern Tibetan Plateau. More and more geodetic data have been acquired during the past several tens of years, especially after the 2008 Mw 7.9 Wenchuan earthquake. Some important questions are proposed: can we use the geodetic data to determine if middle-lower crustal flow exists in the eastern Tibetan Plateau, and can we estimate the patterns and magnitudes of the crustal flow in this region if it exists? To investigate these questions, we construct a three-dimensional viscoelastoplastic finite-element model to simulate regional crustal deformation during the seismic cycle of 2008 Wenchuan earthquake in cases with and without middle-lower crustal flow, and analyze and compare these model results with geodetic observations. From modeling the co- and post-seismic deformation, we have suggested a steady-state (Maxwell) viscosity of 1.1 ?? 1019 Pa s within a 40-km-thick middle-lower crust beneath the eastern Tibetan Plateau. Through inter-seismic deformation, we find that the eastern Tibetan Plateau, in crustal shortening model, has undergone pure shear deformation, leading to an even-distributed surface uplift rate of 0.5 mm/yr. It is significantly lower than the average uplift rate of 2.2 mm/yr from the observed leveling data. On the other hand, the hybrid crustal flow model can predict a faster surface uplift rate with the fastest uplift rate localized at the flow front, which is more consistent with the observed leveling data. Based on the model results and the leveling data, we infer that middle-lower flow may exist within a wide area beneath the eastern Tibetan Plateau.
查看更多>>摘要：The North China Craton is one of the oldest continental nuclei in the world. However, extreme lithospheric modification has occurred since the Mesozoic, of which the most typical area is the Yanshan orogenic belt. In this research, a 3-D lithospheric electrical structure beneath the Yanshan belt and adjacent areas was obtained using magnetotelluric array data, which reveals the regional heterogeneity. Notably, the crust and upper mantle (< 100 km) within the Yanshan belt, especially in its southern part, are characterized by high resistivity (> 1000 Omega m). These features are interpreted to be a relic cratonic lithosphere mainly composed of highly depleted olivine. A lithosphere-scale relatively high-conductivity zone (similar to 100 Omega m, near EW-trending), which roughly lies beneath the Miyun-Xifengkou-Qinglong fault coincides with the boundary of Archean greenstone belt and also the Mesozoic granitic zone, probably indicating a reactivated Precambrian lithospheric weak/shear zone within Yanshan belt. In contrast, the lower crust and upper mantle of the Bohai Bay basin show large areas of high conductivity, which may result from the modification of the crust and mantle by the upwelled asthenospheric materials. Additionally, the epicenters of several historic destructive earthquakes, including the 1976 Tangshan earthquake (M7.8), are mainly located near the high- and low-resistivity boundary at the conjunction of the Yanshan belt and the Bohai Bay basin. The fragmentation of brittle crustal rocks mainly happened in where the tectonic stresses concentrated, which might result from the rheological difference between the high-resistivity ancient and the high-conductivity modified lithosphere. In these places, intense seismicity would be induced.
NSTL
Elsevier