查看更多>>摘要：After the devastating Tohoku-Oki earthquake (Mw 9.0, 2011) in Japan, the Korean Peninsula has experienced a higher number of large, plus Mw 5.0, earthquakes than recorded in the preceding half century of modern monitoring. In addition, seismicity has dramatically increased along with seismic waves arriving later than prior to the 2011 Tohoku-Oki earthquake, suggesting that the Korean crust has notably been perturbed. South Korea is densely populated, hence knowledge about active faults and earthquake mechanisms is of great relevance for public safety and risk mitigations. Quaternary faults, including the Chugaryeong crustal-scale fault, run through the Seoul metropolitan area and recent seismicity studies suggest that these faults are active. Based on two reflection seismic profiles, we provide compelling evidence that the depth clustered seismicity along the Chugaryeong fault is associated with the intersections of other fault systems. The two seismicity clusters, observed at two depth intervals of approximately 4.5???5 and 8???9 km, can be linked with two moderately-to-steeply-dipping bands of reflectivity interpreted to be splay faults and terminating at the Chugaryeong sub-vertical fault. We suggest that stress builds up at these fault intersections and is then released via strike-slip ruptures along the Chugaryeong fault. Time-clustered seismic events at the fault intersections support this hypothesis, indicating a start-stop mechanism is controlling the seismicity in the region at least based on nearly one decade of seismicity observations. The start-stop seismicity behaviour can possibly be used for forecasting earthquakes and their switching depth along the Chugaryeong fault.
查看更多>>摘要：Pores and cracks have an important role in the evolution of fault rocks because they strongly influence the behavior of the fluids that promote rock alteration and trigger the mechanical instability of faults. We used rock physics model inversion of measured elastic wave velocity and porosity to estimate the grain elastic moduli and crack aspect ratios of a range of fault rocks (intact rocks, fractured rocks, transition rocks, and fault gouge) from the Median Tectonic Line in southwest Japan. Our results show distinct gaps in the evolutionary trends of crack aspect ratios and grain elastic moduli from intact rocks to fault rocks. Crack aspect ratios show a nonlinear trend from intact rock to fault gouge, and then these values in fault gouge were considerably higher than in fractured rock and transition rock. In contrast, grain elastic moduli decreased as fracture evolved with the development and subsequent extinction of shear planes and then increased markedly with the formation of fault gouge. Our results show that crack aspect ratios and grain elastic moduli are clearly related to the evolution of shear fabrics in faults. Therefore, they might be useful indicators of fault activity and maturity.
查看更多>>摘要：The subsurface structure of the Himalaya is dominated by the seismically active Main Himalayan Thrust fault, which accommodates about half of the plate convergence between India and Asia. In Central Nepal, geological, geophysical, and geodetic studies indicate that the Main Himalayan Thrust has a flat-ramp-flat geometry, which causes cooling, erosion, and exhumation of rocks to be faster above the mid-crustal ramp than above the adjacent upper and lower flats. However, in the northern High Himalaya the ramp geometry, in particular its width and depth are not well resolved. To place further constraints on the northward extent and depth of the ramp, we present for the first time low-temperature thermochronological data and U-Pb zircon ages from the Gyimng region, China (northern High Himalaya). Our zircon U-Pb data show that the protolith of the High Himalayan orthogneisses is 478 +/- 4 Ma old, and was intruded by a large leucogranite at 19.5 +/- 0.7 Ma. The Pliocene-Quaternary cooling of the study area is constrained by apatite fission track ages and apatite and zircon (U-Th)/He ages between similar to 5 Ma and similar to 0.5 Ma, which show a marked trend of southward-younging ages. Together with published cooling ages from Nepal farther south, the ages define a U-shaped pattern across the Main Himalayan Thrust, with ages increasing to the north and south from a minimum in the High Himalaya. A thermokinematic model, in which the geometry of the mid-crustal ramp was varied, explains the age data and suggests that the ramp has a dip of similar to 22 degrees N. Compared to previous models, our new ages require that the ramp has a greater width (similar to 55 km) and reaches a greater depth (similar to 34 km) below the northern High Himalaya. Still farther north, the fault is presumably developed as a gently-dipping ductile shear zone, which forms the prominent reflector visible in seismic reflection data.
查看更多>>摘要：Complex orogenic belts typically display internal variations in deformational style, as is the case of the northern Eastern Cordillera of Colombia, which changed abruptly into a regional strike-slip system in the NNW-striking Santander Massif. We report paleomagnetic data, anisotropy of magnetic susceptibility (AMS) data, and magnetic mineralogy from 47 sites distributed in the northern Eastern Cordillera and Santander Massif to investigate and compare: (1) variations in the intensity of the deformation, (2) magnitude and sense of vertical axes rotations, and (3) the character and effect of strike-slip displacements along the Bucaramanga fault and NE-striking transverse faults within the Santander Massif. Rock magnetic experiments show that hematite is the principal magnetization carrier in all these rocks. Rocks of the axial zone of the Eastern Cordillera show intermediate magnetic fabrics. Magnetic fabrics of rocks from the Santander Massif, and the western flank of the Eastern Cordillera, are uniformly oblate with preservation of sedimentary fabrics. Directions of the ChRM from sites in the Eastern Cordillera (30 accepted of 35 sites) are interpreted to indicate that the axial zone has not been rotated, whereas sites located along the western flank of the northern Eastern Cordillera yield data suggesting clockwise rotations, which increase in magnitude up to 90 degrees in areas bounded by NE-striking transverse faults. In the Santander Massif (9 accepted of 12 sites) data suggest a mix of counterclockwise rotations (similar to 30) and clockwise rotations of similar to 40 degrees. The inferred clockwise vertical-axis rotations in the northern Eastern Cordillera and Santander Massif of up to 45 degrees are interpreted to be related to tectonic inversion of pre-existing Mesozoic age normal faults during the Neogene as right-lateral faults. Our corrected paleomagnetic inclination data are interpreted to support the hypothesis of northward translation of the studied tectonic elements during the Middle Jurassic to earliest Cretaceous time.
查看更多>>摘要：The Late Jurassic tectonic environment of the Erguna-Xing???an blocks (in western Northeast China) remains poorly understood due to the complications arising from supposedly superimposed impacts on activities associated with the Mongol-Okhotsk Ocean and the Paleo-Pacific Ocean tectonic regimes. The present work investigated the latest Jurassic A-type granite (Chaganhada granite) in the southwestern Xing???an block. The Chaganhada granite was emplaced at 148 Ma. According to the observations made with the zircon saturation thermometer, the Chaganhada granitic magma has a greater magmatic temperature (>855 ???C) than common I-S type granites. In addition, the geochemical characteristics of Chaganhada granite suggest the magmatic affinity of A1-type, which can be supported by high total alkalis contents (8.55%???9.50%), Zr + Nb + Ce + Y values (380???602 ppm), and 10,000*Ga/Al ratios (2.74???3.22), and low Y/Nb ratios. Low CaO + FeO + MgO + TiO2 values, CaO/(FeOt + MgO + TiO2) and K/Rb ratios and initial 87Sr/86Sr ratios (0.7048???0.7055), positive ??Nd(t) values (0.90???1.22), and zircon ??Hf(t) values (1.2???5.1) were also observed in the Chaganhada granite. Hence, the study suggests that the latest Jurassic Chaganhada A-type granite might have originated from the crustal quartzfeldspathic igneous sources. Additionally, we approximately restrict a NE-SW trending Late Jurassic A-type granitic magmatic belt in the westernmost part of NE China using data collected by this study as well as publicly available data related to zircon ages, geochemical compositions, and location of the igneous rocks in NE China. Based on the observations above, the current study infers that a Late Jurassic intense extension impacted the westernmost NE China, presumably associated with the orogenic collapse following the closure of the western part of the Mongol-Okhotsk Ocean.
Ruiz Gonzalez, VRenda, E. M.Vizan, H.Ganerod, M....
18页
查看更多>>摘要：This work presents the analysis of paleomagnetic results obtained from four sampling areas of the Jurassic Bahia Laura Complex in the Deseado Massif and their implications on the regional deformation history during the breakup of Gondwana. Paleomagnetic data show cessation of tectonic block rotations about vertical axes around 160 Ma and a change from a transtensional to a mainly extensional tectonic regime. Two biotite samples yielded Ar-40-Ar-39 radiometric ages: one from the eastern outcrops of the Chon Aike Formation (plateau age of 184.66 +/- 0.55 Ma) and the other from the La Matilde Formation (plateau age of 157.40 +/- 0.65 Ma), located at the central part of the Deseado Massif. Finally, an apparent polar wander path (APWP) was calculated for the Jurassic of South America (200 to 140 Ma). This APWP indicates that South America experienced a northward drift between 200 Ma and 170 Ma, a clockwise rotation (similar to 10 degrees) between 170 Ma and 160 Ma, and a westward drift between 160 Ma and 140 Ma. The 170-160 Ma rotation could has been triggered by the combined effects of the uncoupling of the Antarctic Peninsula, the high rates of subduction of the Phoenix plate beneath Patagonia, the opening of the Rocas Verdes basin and the Weddell Sea, and the cessation of the Tethys slab-pull. Furthermore, the calculated APWP for South America does not support a Jurassic massive true polar wander event.
查看更多>>摘要：Active faults in the forearc of southern Peru pose a poorly understood hazard to the region. The Purgatorio Fault is a 60 km-long fault that extends between Moquegua and Tacna that has hosted several scarp-forming earthquakes over the last 6 ka. We present new measurements of the fault scarp geomorphology along the Purgatorio Fault, and use dating of the stratigraphy within a new paleoseismic trench excavated across the fault to establish the chronology of scarp formation. We find that the most recent surface-rupturing earthquake on the Purgatorio Fault occurred sometime between 1630C.E and 1790C.E and had a moment magnitude (M-w) of similar to 7. We propose that this most recent surface-rupturing earthquake on the Purgatorio Fault was the 1715C.E earthquake recorded in the historical catalogue of the region, which was previously attributed to the megathrust offshore. Our results highlight the importance of establishing a paleoseismic record of onshore faults to differentiate between major megathrust and forearc earthquakes. Given the proximity of these shallow, onshore faults to coastal communities in Peru, the shallow earthquakes they generate may pose a severe, yet often overlooked, seismic hazard.
NSTL
Elsevier