查看更多>>摘要：In the past decades, the arc-continent collision in Taiwan was commonly interpreted as a relatively continuous process with an invariant plate convergence vector for at least the last 6???5 Ma. This steady convergence, including the rate and the obliquity, between the subducting continental margin and the plate boundary suggest a propagating collision with a space-time equivalence along the developing orogen. More recently, detailed lowtemperature geochronologic and plate reconstructions suggest that the plate convergence changed from highly oblique to nearly orthogonal in the last 2 to 1 Ma. This early phase of oblique convergence, driven primarily by the northward motion of the Philippine Sea Plate wrt the Eurasian Plate, implies a significant ???fossil??? component of left-lateral, strike-slip motion along the presumably north-trending plate boundary. A synthesis of available data compiled in this study suggests that previously uncharacterized zones of strike-slip deformation exist in the Tailuko Belt, and here we document: 1) the distribution of horizontal shear, 2) the kinematics of deformation, 3) the age of deformation, and 4) regional consistency between geologic studies and plate reconstructions. Horizontal shear may also be recorded at shallow structural levels in southern Taiwan by brittle faults and block rotations. Integration of these new data with previously published kinematic data across strike also suggests plate convergence was partitioned with strike-slip motion in the retrowedge (i.e., the Tailuko Belt) and shortening in the prowedge (i.e., the Slate Belt).
查看更多>>摘要：The West Helanshan fault is situated at the junction of the northeastern Tibetan Plateau, Alxa, and Ordos blocks, providing an opportunity to investigate the deformation in the transitional zone between the contractional northeastern Tibetan Plateau and extensional North China. Using field investigations and satellite imagery interpretations, we determined detailed geometry and kinematics of the West Helanshan fault. The West Helanshan fault is divided into three segments, including the northern, middle, and southern segments, based on varying fault strike, slip sense, and geomorphic expression. The northern and middle segments are dominated by rightlateral strike-slip motion as indicated by linear fault traces, horizontally offset streams and terrace risers, relatively insignificant vertical offset, and frequently changing facing direction of fault scarps. The late Quaternary right-slip rate is estimated to be 0.2-0.4 mm/yr, which is relatively low comparing with other strike-slip faults in the northeastern Tibetan Plateau, based on Be-10-dated offset fluvial/alluvial surfaces. The southern segment shows conspicuous fault scarps with significant systematic vertical offset, which has a reverse component as revealed in the trench. The timing of the most recent earthquake revealed in the trench is constrained to be between 6.2 +/- 0.7 ka and 5.9 +/- 0.4 ka. Notably, the West Helanshan fault cuts through the city center of Azuoqi town, which is the capital of Azuoqi county and has about 100,000 residents, carrying high potential earthquake hazard in the future. Finally, combining the regional fault geometry and kinematics with GPS observations, we propose a limited north-northeastward extrusion model for the southern Alxa Block.
查看更多>>摘要：Gravity changes related to large earthquake preparation are reported in broad seismogenic source regions, and crustal fluids may change the gravity potential and trigger earthquakes. However, what is the major factors affecting crustal fluids movement in the upper 10 km of the continental crust and altering the gravity potential is still poorly understood. In this study, we evaluated the gravity changes caused by invasion of crustal fluids into the depth of 6???10 km using finite element based numerical modeling. We conducted a series of numerical ex-periments to investigate the influences of permeability distribution, the fluid source, and the invasion depth on gravity changes. The sensitivity analysis and comparison results suggest that the invasion of high-pressure fluids increases fluid density and produces positive gravity changes within a decade. Fluid pressure is the key factor for the variations of fluid density, while temperature exerts a minor influence. Consistent with the dissipation of fluid pressure, the corresponding gravity changes are fast (>3 ??Gal/year) in the first 2???3 years of fluid invasion and become increasingly slow later. Therefore, the first few years are the more suitable time for monitoring the gravity changes caused by crustal fluids invasion. The fluid-related gravity changes are positively correlated to the porosity and permeability of rocks adjacent to the fluid source, the salinity and the scale of crustal fluids, and the initial temperatures, and are negatively correlated to the invasion depth. Transient invasion of crustal fluids can be detected by means of the high precision gravity observation under certain conditions subjected to the initial geological characteristics of the continental crust and the geochemical evolution of the fluid sources.
查看更多>>摘要：On 20 July 2017 (22:31:10 UTC), an M(w)6.6 earthquake took place offshore Kos Island, in the southeastern (SE) Aegean Sea, producing severe damage and loss of life in the city of Kos and several smaller cities and villages both in Kos Island (Greece) and in the Bodrum peninsula (Turkey). All available seismological data until the end of October 2017 were gathered from seismological stations located in Greece and Turkey for the relocation process. The relocated aftershocks are clustered at least in three distinctive patches, creating a zone reaching a total length of about 40 km, elongated in a nearly east-west direction, and are mainly concentrated at depths 8-15 km, with the main shock hypocenter placed at similar to 13 km, implying a seismogenic layer of 7 km thickness, indicative for normal faulting earthquakes with M-max similar to 6.5. The aftershock fault plane solutions are predominantly normal faulting in response to the north-south extension of the back arc Aegean area and consistent with the broader regional stress field. We also applied the satellite radar interferometry (InSAR) technique to define the coseismic surface displacements, revealing the range of deformation on the Island and the surrounding mainland. We combined this deformation field along with the available vectors of displacement measured by the Global Navigation Satellite System (GNSS) technique with the seismological data to determine the fault geometry and rupture process. The resulted variable slip model indicates a rather elliptical rupture area of 306 km(2), with the coseismic slip ranging between 0.5 and 2.3 m. The peak moment release occurred in the depth interval of 9-11 km, consistent with the depth distribution of seismicity in the study area. We used the variable slip model to calculate Coulomb stress changes and investigate possible triggering due to stress transfer to the nearby fault segments.
查看更多>>摘要：Exposed in the Coastal Range of eastern Taiwan is a portion of the Luzon volcanic arc that has collided with the South China continent since 6 Ma, as well as syn-orogenic sedimentary rocks deposited on the arc and subsequently intensely folded and thrusted. This study sampled syn-orogenic mudstones for the measurement of anisotropy of magnetic susceptibility (AMS) at four sites on the sides of the range. The AMS fabrics reveal a contribution from burial compaction and, less significantly, tectonic shortening which varies slightly across the range. This lateral variation is considered as, for simplicity, a result of two-phase deformation because these rocks were involved in an eastward propagating back thrust system on the colliding arc. A new strain analysis method is developed to estimate the overall tectonic strain by unstraining the pseudo-strains with respect to the AMS measurements at each site with the knowledge about the distribution of the susceptibility ellipsoids in the unstrained state. Strain results reveal that shortening phases occurred before and after the folding, with a shortening ratio of 0.177???0.195 and 0.181???0.248, respectively. The post-folding shortening on the western side is sub-parallel to the range, and the pre-folding shortening on both sides is layer-parallel and nearly perpendicular to the range. Therefore, the mudstones underwent layer-parallel shortening, fold-related tilting and eventually orogen-parallel shortening after deposition. This sheds light on the complexity in deformation of the colliding Luzon arc.
Batista, Joelson C.Sampaio, Edson E. S.Oliveira, Elson P.Barbosa, Johildo S. F....
22页
查看更多>>摘要：Geophysical techniques can improve knowledge about the deep structures of collisional orogens. Therefore, magnetotelluric and gravimetric data were combined in this study to enhance the poorly understood subsurface structure of the transition zone from the Sa??o Francisco Craton to the Neoproterozoic Sergipano Orogen in northeastern Brazil. The results along a 240-km-long, NE???SW trending geophysical transect revealed two main geoelectric domains beneath the Cretaceous Tucano Basin, which can be linked to two distinct tectonic blocks on the surface. The less resistive SW domain corresponds to the rocks of the Serrinha Block within the Sa??o Francisco Craton. In contrast, the more resistive NE domain matches the basement rocks of the northernmost units of the Sergipano Orogen. The results are discussed considering Paleoproterozoic and Neoproterozoic scenarios with different interpretations of the NE domain. Based on the surface geology and additional gravimetric and magnetotelluric data, the best scenario is to consider the NE domain as representing the geoelectric signature of the three northernmost geological domains of the Sergipano Orogen (Macurure??, Pogo Redondo???Maranc??o, and Caninde??) and that of the Pernambuco???Alagoas Block (PEAL). PEAL collided with the Sa??o Francisco paleolithosphere forming the Sergipano Orogen. The NE geoelectric domain can be interpreted as an indenter of the tectonically intruded PEAL beneath the Sa??o Francisco paleolithosphere. The new data properly define the northern limit of the Sa??o Francisco Craton and explain both indentation structures and contrasting sediment provenance observed in the Sergipano Orogen.
查看更多>>摘要：Broadband and long-period magnetotelluric (MT) soundings were collected along a 600-km-long EW-trending profile across the southwestern segment of the Jiangnan Orogenic Belt (JOB) to explore the possible remnants of ancient tectonic processes beneath the central South China Block (SCB). Beneath the JOB, the resistivity model reveals a west-dipping, trans-lithosphere conductor ( 20 ?? ??? m) embedded within the relatively thick (80???120 km) and highly resistive ( 1000 ?? ??? m) lithosphere, possibly associated with relics of the Neoproterozoic subduction zone between the Yangtze and Cathaysia blocks. The crustal portion of this conductor is interpreted as solid conducting materials (e.g., graphite, sulfide) that were emplaced into the crust during subduction and orogenesis, whereas its lithospheric mantle portion is more likely to be volatile-bearing minerals formed by subduction-related metasomatic processes. We attribute the greatly reduced asthenospheric resistivities (1-10 ?? ??? m) below the JOB to low-degree melting of the early-formed metasomatized mantle. In contrast, the lithosphere of the Cathaysia Block is evidently thinned (60???100 km) and segmented by several subvertical, lithospheric-scale conductors (1-30 ?? ??? m) that are likely associated with fossil fluid pathways along reactivated shear/fault zones. Notably, the thinnest lithosphere sits above a prominent low-resistivity (10 - 30 ?? ??? m) anomaly in the asthenospheric mantle and is spatially coincident with the location of Late Jurassic basaltic outcrops. We interpret these features as signs of lithospheric extension and asthenospheric upwelling driven by roll-back of the Paleo-Pacific Plate during the Late Mesozoic. The results provide key constraints on subduction???accretion tectonics associated with the Neoproterozoic assembly of the SCB and the subsequent lithospheric evolution during the Phanerozoic.
NSTL
Elsevier