Mitchell, Neil C.Hernandez, KarinaPreine, JonasLigi, Marco...
16页
查看更多>>摘要：Rayleigh-Taylor models for diapirism predict that diapirs should develop with characteristic spacings, whereas other models predict varied spacings. The deep-water Miocene evaporites in the Red Sea provide a useful op-portunity to quantify length scales of diapirism to compare with model predictions. We first review the stra-tigraphy of the uppermost evaporites in high-resolution seismic data, revealing tectonic growth stratigraphy indicating that halokinetic movements occurred while the evaporites were being deposited. In some places, movements continued after the Miocene evaporite phase. The S-reflection marking the top of the evaporites is an erosional surface, in places, truncating anticlines of layered evaporites. In others, reflections within the upper-most evaporites are conformable, suggesting a lack of erosion. The top of the evaporites therefore had relief at the end of the Miocene. We select for numerical analysis 14 long profiles of topography of the S-reflection. Variograms derived from them after detrending reveal minor periodicity, though with varied wavelength, and varied roughness of the surface. However, an average variogram computed from these profiles is nearly expo-nential, indicating that the evaporite surface is mostly stochastic with no uniform scale of diapirism. An expo-nential model fitted to that average variogram suggests a spatial range over which the S-reflection topography becomes decorrelated of 3 km, which is comparable with the mean vertical thickness of the evaporite body. Power spectra of the evaporite surface are flatter at long wavelengths, which we interpret as due to weakness of halite preventing large surface relief from developing. The results suggest only modest periodicity, so the Rayleigh-Taylor model does not explain deformation in the Red Sea evaporites studied here. Their topography may turn out to be useful for suggesting the vertical scales and lengthscales of relief to expect of early stages of other salt giants, such as that of the Santo Basin.
Oliveira, Magda E.Gomes, Afonso S.Rosas, Filipe M.Duarte, Joao C....
27页
查看更多>>摘要：During the early stages of continental rift, two main grabens are often formed in the upper crust. This double graben structural pattern is typically short-lived, only briefly accommodating rift-related extension. Although the formation and evolution of this structural pattern could be related to the existence of different types of (pre-rift) inherited crustal weaknesses, and possibly determined by their different number and spatial disposition, the process that causes the early nucleation of these grabens is still not fully understood. We hence carry out a set of numerical experiments to investigate the influence of lower crust mechanical weaknesses (weak-seeds) in the early rift nucleation of grabens, while simultaneously assuming different rheological configurations for the extending continental crust. Our results show that double graben structural patterns are generally favoured by crustal rheological configurations comprising a weak middle layer sandwiched between an upper brittle crust and a strong lower crust. In models in which this middle layer is relatively thick, two upper crustal main grabens nucleate above one single seed. In numerical experiments with a thinner middle layer this double graben pattern is observed to form even in the absence of any seeds, illustrating the critical role of the rheological configuration in early-rift double graben nucleation. We argue that different assumed crustal rheologies determine different modes of accommodating brittle deformation in the upper crust, resulting in different structural-mechanical grains imprinted in this layer, prior to the formation of the main grabens, and driving the subsequent nucleation of different structural configurations (including double grabens). The transient nature of the double graben configuration is also confirmed by our results, which consistently show that rift-related extensional strain is eventually fully concentrated in only one of these early formed structures.
查看更多>>摘要：Exploring spatiotemporal variations in the regional stress field is critical for understanding the interactions between active tectonic plates. On July 4 and 6, 2019, strike-slip earthquakes of Mw 6.4 sinistral and 7.1 dextral, respectively, occurred in Ridgecrest, California, which provided an appropriate condition for studying conjugate earthquake stress variation. In this study, we used GNSS coseismic displacement to invert the coseismic slip distributions of each earthquake. According to the spatiotemporal distribution, the focal mechanisms were divided into three time periods and five subareas. Therefore, spatiotemporal variations of the stress field were calculated. Results indicate that the two earthquakes were not purely strike-slip, but had minor dip-slip components. The maximum slips of the Mw 6.4 foreshock and the Mw 7.1 mainshock were 0.7 and 3.1 m, respectively. Areas of significant stress variations corresponded to areas of larger slip distributions. During the three time periods, the maximum horizontal stress (SHmax) direction in each subarea first experienced counterclockwise and then clockwise rotation; the SHmax direction of the foreshock epicenter region changed from 3.8 degrees to 2.8 degrees to 5.8 degrees. After the mainshock, the SHmax of the foreshock epicenter region rotated clockwise by similar to 3.0 degrees, the deviatoric stress was similar to 4.5 MPa, and the stress drop in the epicenter region of the mainshock was similar to 4.3 MPa, from which we can judge that the stress was almost completely released. Six months before the earthquakes, the b value began to change markedly, while it stabilized after the earthquakes. In the dextral shear zone formed by the relative motions of the Pacific and North American plates, the cumulative stress tends to be jointly regulated by NW-SE and NE-SW oriented conjugate fractures. These two Ridgecrest earthquakes are a phenomenon of stress regulation.
查看更多>>摘要：In our study we examined the possibility of using strain rate data, determined from geodetic satellite measurements for estimations of magnitude of contemporary stress in a relatively stable area of northern Poland, comprised of fragments of the East European Craton, the Teisseyre-Tornquist Tectonic Zone and the Palaeozoic Platform. The strain rates and their principal directions were determined from 63 GNSS stations lacking special stabilization. The method for filtrating the unstable stations allowed us to eliminate half of these stations from the final strain rate calculations that included data averaging in a correlation radius of 150 km. A comparison of the horizontal strain directions determined by geodetic methods with the directions of contemporary stress based on borehole data (four of which were determined in this study) showed a high degree of convergence. Also the trends of strain rate variations showed consistency with integrated lithospheric strength of main tectonic units. The strain rates obtained are consistent with those published for the adjacent areas. Stress magnitudes were estimated by calibrating the timing of elastic strain accumulation by earthquakes in the Sambia Peninsula (Kaliningrad Oblast) and assuming a transient state of elastic strain on the craton. Our results indicate that the crystalline upper crust in the analysed part of the craton is in the strike-slip faulting stress regime prevailing over the normal faulting stress regime. The present state of an elastic strain has been accumulated over the last few million years causing stress being far from the critical state in the Mohr-Coulomb frictional criteria, except of the seismically active Sambia Peninsula.
Parcutela, Nathaniel E.Austria, Rurik S. P.Dimalanta, Carla B.Armada, Leo T....
14页
查看更多>>摘要：The occurrence of long wavelength Moho undulations in continental and oceanic lithospheres is attributed to lithospheric buckling. This has been reported for Cretaceous or older lithospheres under compressional regimes. However, there is very limited data on lithospheric buckling of oceanic island arcs. The region where the Cretaceous Cebu arc is under compression due to the impingement of the Palawan micro-Continental Block (PCB) to the Philippine Mobile Belt (PMB). The configuration of the inferred buckled Cretaceous Cebu arc lithosphere is derived from geologic and combined onshore and airborne gravity and magnetics datasets. Subsurface modeling using gravity data revealed the presence of mantle upwelling beneath Central Cebu. This could be considered as one of the crests of long-wavelength Moho undulations commonly attributed to a buckled lithosphere. The oceanic basement was also modelled to determine its configuration. Euler solutions derived from magnetic data showed that the contact between the arc crustal units of the Cansi Volcanics and Pandan Formation is affected by short wavelength folding with wavelengths ranging from 5 to 15 km. The younger arc crustal units are affected by tighter folding with wavelengths ranging from 1 to 5 km. The occurrence of a buckled arc lithosphere is being considered for the first time in this area. The consistent NE-SW folding axes inferred for the folding events strongly suggest that these are consequences of the Middle Miocene collision involving the Philippine Mobile Belt and the Palawan micro-Continental Block.
查看更多>>摘要：High-resolution images of the crust were obtained along two high-density temporary seismic arrays that cross tectonic blocks in the southwest boundary of the Ordos block and its adjacent regions. Dense stations with <1-km station spacings allowed teleseismic receiver functions to unveil the unprecedented details of crustal structures from the northeast margin of the Tibetan Plateau and the Qinling orogenic belt to the southwest Ordos block. The observations from nearby broadband seismic stations and two-dimensional numerical simulation were used to verify the reliability and validity of our results from the temporary dense array data. The receiver function profiles and the migration images revealed the contrasting crustal structures beneath the southwest Ordos block and the adjacent regions. Liupanshan mountain is a boundary that resists the lateral growth of the Tibetan Plateau; however, the slight bending of the Moho indicates absence of large-scale deformation in west Liupanshan. The Moho transition zone beneath the Weihe graben and southwest corner of the Ordos block represented by the complicated P-to-S converted phases can be contributed to the eastward extrusion of hot uppermost mantle from the Tibetan Plateau driven by the mantle flow.
查看更多>>摘要：We present a high-resolution 3-D model of P wave tomography of the crust and mantle down to 1600 km depth under Southeast Asia (SE Asia), which is determined by inverting more than 1.3 million P wave arrival time data of local, regional and teleseismic events recorded at 1241 seismic stations deployed in SE Asia and mainland China. Strong and weak smoothing inversions are conducted to reveal different characteristic scales of velocity anomalies in the mantle. Both seismic and aseismic portions of the subducted plates in different parts of the circular subduction system in SE Asia are clearly revealed as high-velocity bodies in our tomographic images. The intraplate volcanoes in Borneo are underlain by significant low-velocity anomalies in the upper mantle, whereas a clear high-velocity anomaly is visible in the mantle transition zone and the lower mantle. We deem that hot and wet upwelling flows in the big mantle wedge above the stagnant slab in the mantle transition zone may have caused the intraplate volcanism in Borneo. A large low-velocity anomaly consisting of a flat head and a twisted tail extending to the bottom of our model is imaged beneath the southeastern Asian basalt province. The low velocity anomaly possibly represents a hot zone whose shape may have been modified by mantle wind associated with a plate rotation. The hot zone has a lower-mantle origin and probably contains some small mantle upwellings inside, with the Hainan plume being a major trunk.
查看更多>>摘要：The most pronounced high-amplitude (-40 to -106 m) geoid deficit with respect to the Earth's reference shape straddles the Indian Ocean. The existing hypotheses suggest a possible linkage of low density and low-velocity anomalies in the mid-and upper mantle and high-velocity anomalies near the base of the mantle to the source behind the lowest geoid. We obtained P to S wave radial receiver functions to investigate these mantle anomalies from an extensive array of Ocean Bottom Seismometers (OBSs) positioned in the centre of the Indian Ocean Geoid Low (IOGL). Our modelling suggests that the mean depths for d410 and d660 range from 386.0 +/- 13.6 to 459.7 +/- 2.9 km and 643.1 +/- 7.4 to 710.2 +/- 5.4 km with an average of 432.6 km and 680.2 km, respectively. The average thickness of the transition zone varies from 199.6 to 289.8 km. New results confirm an extensive similar to 800 km wide depression at d410 and d660 towards the centre of the geoid. The depression potentially implies a rather hot mantle material, in which majorite garnet to perovskite transition may become dominant at 660 km depth. The excess temperature calculated for d410 and d660 topography ranges from 139.5 to 557.5 K and 206 to >1000 K, respectively. The shear velocity anomalies derived from the excess temperature range from -0.89 to -3.52 (%) at d410, and at dd60 range from -1.13 to -7.87 (%). The velocity anomalies are highly underestimated at d660 and could be explained by hydrous mantle upwelling at the lower mantle transition zone. The present study outcomes offer new avenues towards exploring a mantle upwelling in the Indian Ocean and its linkage with geoid undulations.
NSTL
Elsevier