Gokul, V. S.Sreejith, K. M.Rao, G. SrinivasaRadhakrishna, M....
12页
查看更多>>摘要：The Indian Ocean Geoid Low (IOGL) in the Central Indian Ocean is the largest geoid depression on the globe. Spectral decomposition of geoid anomalies reveals that a significant fraction (up to similar to 90%) of the IOGL is due to mass anomalies lying within the lower mantle (similar to > 700 km). The crustal density structure and Moho depth variations obtained from 3-D gravity inversion along with the upper mantle density structure derived from SL2013sy tomographic model are used to compute the total geoid anomaly up to 700 km depth and compare it with degree-10 residual geoid anomaly map. Comparison of these two maps necessitated the presence of relatively shallow low- density sources within the upper mantle that were not resolved in the 3-D modelling. The presence of two anomalous sources: i) low S-wave velocities at the base of the LAB (4.4 km/s) and ii) an anomalous source of Vs = -1.0% at a depth of 320-400 km within the sub-lithospheric mantle observed in the tomographic model accounted for this difference. We model the geometry of these low-density anomalous bodies using 2-D joint gravity-geoid modelling, which reveals the total contribution of density structure up to the upper mantle depths (700 km) accounts for only 10% of the IOGL whereas the remaining part is primarily caused by mass anomalies in the lower mantle probably related to high-density subducted slabs or plume sources at the core-mantle boundary as proposed by earlier studies.
查看更多>>摘要：The Kalabagh Fault Zone in the western Sub-Himalayas is a dextral strike slip fault zone that exhibits a push-up block in the stepover zone of the two fault segments. The inception of the push-up block occurred along the restraining faults that form two morphotectonic bends along the mountain front at Zaluch and in the South of Thathi. The landform was further developed by en-echelon fold and thrust sequence up to Khairabad in the North and Ghundi village in the South. The aseismically moving fault zone has experienced earthquakes during late Quaternary forming surface expressions in the West of the push-up block at Khairabad, Larkakki and Ghundi. The Khairabad and Ghundi sections are the active tectonic fronts of the stepover zone. Displacements caused by these seismic activities has deformed Quaternary sediments creating stream deflections, surface ruptures, pressure ridges, uplifted sediments at the mountain front, foreland ridges and uplifted alluvial fan. Offsets in the late Quaternary sediments indicate the Kalabagh Fault is active and seismic events of M-w >= 6 has formed these ground effects. Furthermore, stratigraphic correlations of fluvial sediments indicate the Indus River has been flowing in the NS direction along the northern segment of the Kalabagh Fault. Uplift of the push-up block in the Kalabagh Fault Zone and subsequent deposition of the alluvial fans has caused westward diversion of the Indus River.
查看更多>>摘要：To investigate lateral and depth variations of seismic anisotropy beneath the central-western United States, we determined a detailed 3-D model of P-wave anisotropic tomography by inverting a large number of arrival-time data of local and teleseismic events. Our results reveal significant azimuthal anisotropies in the crust and lithosphere, which are associated with ancient orogenic collisional and magmatic activities. As depth increases, the fast-velocity direction (FVD) pattern becomes gradually trended and small features fade away. There is a boundary in the FVD distribution, which separates the tectonically active region in the west from the stable cratonic region in the east. Frozen-in anisotropy with a NW-SE FVD is preserved in the thick Wyoming cratonic lithosphere that exhibits as a high-velocity (high-V) anomaly to a depth of similar to 250 km. In the asthenosphere beneath the western thin lithosphere, FVDs are generally parallel with the absolute motion direction of the North American plate due to shearing between the plate and the asthenosphere. In the deeper areas, the subducted and fragmented slab exhibiting as high-V anomalies leads to slab-related mantle flows. These results indicate that seismic anisotropies exist in both the lithosphere and asthenosphere with different geodynamic mechanisms and it is feasible to link the P-wave azimuthal anisotropy to lithospheric deformations, fossil anisotropy in the lithosphere, and flows in the asthenosphere.
Yildirim, CengizAksoy, M. ErsenOzcan, OrkanIsiler, Mehmet...
18页
查看更多>>摘要：The Gulf of Gokova is one of the earthquake-prone areas in the Aegean Sea and southwestern Turkey. The paleoseismology of the region is not well known because of the lack of robust field data. In this study, we focused on marine tidal notches as geomorphic markers of modern and paleoseismic deformations. Our geomorphic surveys suggest that the 20 July 2017 Bodrum-Kos Earthquake coseismically uplifted the Karaada Islet at the Turkish coast. We also provide new observations for the entire coastal area of the Gulf of Gokova. The six well preserved uplifted notch levels up to 1.5 m a.s.l. at the east of Oren town are geomorphic markers of offshore earthquakes along the northern shores. Radiocarbon dating C-14 results suggest that the last 0.5 m of this uplift occurred in the last 2314 +/- 32 years. The four well-preserved marine tidal notches up to 1.2 m a.s.l. indicate the earthquake-related uplift along the southern shores. There are also submerged shorelines along the southern shores at 1 m and 3-4 m below sea level as geomorphic markers of relative sea-level change. The evidence of a longer-term deformation is the presence of an uplifted wave-cut marine terrace on the southern shore. We do not have age data, but the inner edge elevation of the terrace is 238 m above modern sea level. In this study, we reveal that the shores of the Gulf of Gokova have been uplifting since the Pleistocene and Holocene, with a high probability of large magnitude offshore earthquakes occurring on normal faults very close to the modern shoreline.
查看更多>>摘要：The Bahia massif exposes the lower crustal section of the Oman ophiolite located close to the thrust front of the Semail nappe. It is affected by intense faulting previously attributed to tectonic events that dismembered a classical ophiolitic sequence during or after the obduction. Here we show that most of this complexity is primary, inherited from syn-accretion tectonics. The crustal section is exposed in a 15 by 8 km tectonic enclave surrounded by mantle peridotite. Its northern boundary corresponds to a major, steeply dipping normal fault striking WNWESE, at low angle to the paleo-ridge axis. Movement along this fault was accommodated by intense plastic deformation of the crustal cumulates and adjacent mantle peridotites at temperature conditions >= 900 degrees C. The thickness of the deformed zone reaches several hundred meters. The flattening of the cumulate layering away from the fault is correlated to a decrease in the deformation intensity. Undeformed olivine-gabbro dykes crosscut this "tectonic Moho" indicating that the tilting occurred before the end of the igneous activity. To the southwest, the crustal enclave is bounded by a NW-SE trending transtentional shear zone that was active in the amphibolite to greenschist facies and was intensely injected by syn- to post-kinematic gabbronorite and tonalite/ trondhjemite dykes and plugs. The age of one felsic sample (95.214 +/- 0.032 Ma, high-precision U-Pb zircon dating) is within error of the age of intrusive felsic intrusions into the mantle and lowermost axial crust from the length of the Oman ophiolite, which slightly post-dates the mean crystallization age of the Semail crust (V1 magmatism; 96.1-95.6 Ma). Other contacts are low temperature features including cataclastic faults, serpentine-carbonate breccias and flat-lying decollements. Parent melts of the Bahia crustal cumulates were more siliceous and hydrous, i.e. more andesitic, than typical mid-ocean ridge basalt (MORB) as deduced from the frequent occurrence of early crystallizing orthopyroxene (opx) and late crystallizing amphibole. Some facies such as cumulate harzburgite and opx-troctolite have not been documented elsewhere in the Oman ophiolite and may be specific to the tectonic context in which the frontal massifs accreted. The chemical composition of the lower crustal cumulates can be accounted for by the hybridization in various proportions between MORB and a primitive andesite from a depleted source whose origin can be looked for in melts from a nascent subduction zone or from high temperature hydrothermal processes. The structure of the Bahia lower crustal section is reminiscent of the plutonic growth faults documented along present-day slow-spreading centres in both mid-ocean ridge and back arc settings. The distinctive characteristics of the Moho and lower crustal section in the Bahia massif are tentatively related to their position at the leading edge of the ophiolite, i.e. closer to the Arabian continental margin at the time of accretion than the massifs from the internal part of the ophiolite that have a more continuous and less deformed lower crust. It indicates that the style of crustal accretion may have changed during the opening of the oceanic basin from which the Oman ophiolite issued.
Gileva, Nadezhda A.Filippova, Alena, IBukchin, Boris G.Fomochkina, Anastasiya S....
15页
查看更多>>摘要：We study the M-w 5.6 Bystraya earthquake, occurred on September 21, 2020 at the SW flank of the Baikal rift zone in the area, in which only few small seismic events have been detected during the whole period of instrumental observations. Source parameters of the mainshock and its largest aftershock (M-w 4.7) are inverted from intermediate-period surface wave amplitude spectra. Hypocentral depths and seismic moment tensors in a double-couple approximation are calculated for both earthquakes, while integral source characteristics, describing its spatio-temporal development, are estimated only for the mainshock. For the Bystraya earthquake, directivity effects (rupture direction and velocity) are also modelled. The obtained results show that the study seismic event can be associated with activation of the SE segment of the Main Sayan fault. The faults of the mainshock and its largest aftershock are formed under the influence of the NW-SE extension and NE-SW compression. The predominance of left-lateral strike-slip motions agrees well with previous studies at the SW flank of the Baikal rift zone. A small thrust-fault component could reflect the inversion uplift of some blocks of the Bystraya basin bottom as well as the Bystrinskaya Sopka massif. The dipping of the earthquake fault plane could indicate flattening of the considered fault segment at some depth or the existence of differently oriented structural discontinuities inside the fault zone of a finite width. The long rupture of the Bystraya earthquake could be connected with its almost pure strike-slip focal mechanism and orientation of the rupture plane approximately along the Main Sayan fault strike.
查看更多>>摘要：The Indian Ocean geoid low (IOGL) is one of the lowest geoid anomalies on Earth. Several theories have been proposed to explain this geoid low, including past subduction (Nerlich et al., 2016; Rao and Kumar, 2014), subduction coupled with low velocity anomalies in the upper mantle (Spasojevic et al., 2010) and hot, low density anomaly in the upper to mid mantle depths (Reiss et al., 2017; Ghosh et al., 2017). It was also argued by Ghosh et al. (2017) that subducted slabs in the lower mantle have minimal role to play in contributing to this geoid signal. We further investigate that claim here by looking at the geoid contributed from various processes (density vs dynamic topography) as well as by inspecting the contribution from different spherical harmonic degrees. Our current findings substantiate the earlier claim that lower mantle slabs indeed play a minimal role in creating this anomalous geoid low.
McPhee, Peter J.Koc, Aytenvan Hinsbergen, Douwe J. J.
22页
查看更多>>摘要：Central Anatolia (Turkey) is a small and nascent example of a high orogenic plateau, providing a natural laboratory to study processes driving plateau rise. The 1-km-high plateau interior uplifted since c. 8-5 Ma, with a further phase of kilometre-scale uplift affecting the southern plateau margin since 0.45 Ma. Several causes of plateau rise have been proposed: peeling or dripping delamination of the lithospheric mantle; asthenospheric upwelling through slab gaps created by slab fragmentation or break-off, and; continental underthrusting and crustal shortening below the southern plateau margin. The Neogene history of the plateau has not been diagnostic of the causes of plateau rise. We thus evaluate proposed uplift causes in the context of the Anatolian orogenesis, which formed the plateau lithosphere during subduction since the Cretaceous. We combine this analysis with available constraints on uplift, and geophysical data that illuminate the modern mantle (and crustal) structure. Our analysis suggests that lithospheric dripping, which followed arc magmatism and shortening in the Kirsehir Block (eastern Central Anatolia), is the most likely cause of plateau interior uplift. Lithospheric dripping is, however, an unlikely sole driver of multi-phase uplift along the southern plateau margin. There, underthrusting of the African continental margin, recorded by c. 11-7 Ma thrusting on Cyprus, is a viable cause of uplift since 0.45 Ma, but cannot account for earlier uplift since c. 8-5 Ma. Instead, slab break-off below the southern plateau margin is likely in light of geophysical data. On the SW plateau margin, small-scale peeling delamination of the Central Taurides by the Antalya slab since early Miocene times accounts for >150 km slab retreat with no corresponding upper-plate deformation. A southwest-travelling wave of subsidence and uplift signalled this retreat and may have contributed to coeval oroclinal bending of the western Central Taurides and southeastward thrusting of the Lycian Nappes.
查看更多>>摘要：Based on field geology, seismic reflection profiling data and isopach maps, the structural deformation pattern, timing of deformation, and mechanism of formation of the western part of the NE Qaidam Basin (including the Pingtai Uplift, Lenghu and Eboliang structural belts) during the Mesozoic and Cenozoic eras have been investigated. Several anticlines in the Lenghu and Eboliang structural belts are generally characterized by double fault systems at different structural levels. At shallower crustal levels, a thrust fault system dominates and at lower levels positive flower structures accommodate oblique shortening, and these are mainly separated by weak strata layers in the Upper Xiaganchaigou Formation, of late Eocene age. Left-step en echelon distributed normal faults and trailing extensional imbricate fan structures consistently demonstrate that the Lenghu and Eboliang structural belts are dominated by dextral transpressional deformation. The western part of the NE Qaidam Basin experienced an initial stage of extension in the Jurassic, which was associated with the formation of several NWstriking normal faults. We hypothesize that most of the deep compressional structures and shallow anticlines were formed due to the SW-directed thrusting of the Qilian Shan since the early Eocene, as a far-field effect of the Indo-Asian "hard" collision. The NNE-directed shortening was associated with dextral transpressional deformation within the western part of the NE Qaidam Basin that took place after the deposition of the Shangyoushashan Formation, of middle-late Miocene age. We interpret this phase of deformation to have been an integral part of the pulsed growth of the whole Tibetan Plateau.
查看更多>>摘要：SKS wave splitting measurements revealed strong and complex seismic anisotropy in the upper mantle beneath the southeastern Tibetan Plateau. To better understand lithospheric deformation and upper mantle anisotropy observed in this region, we performed studies on microstructures, seismic anisotropy and chemical compositions of the mantle peridotite xenoliths collected from Maguan, Yunnan Province, in the SE Tibetan Plateau. The mantle xenoliths show two types of olivine crystal preferred orientation (CPO), AG-type and A-type. AG-type fabric is characterized by a concentration of [010]-axis normal to the foliation and a large circle girdle of the [100] and [001] axes in foliation. A-type fabric shows point concentrations of [100] and [010] axes subparallel to lineation and subnormal to foliation, respectively. Orthopyroxene and clinopyroxene align their [001]-axis sub-parallel to the [100]-axis of olivine. Seismic anisotropy calculated from the fabric of xenoliths displays moderate to strong polarization anisotropy (AV(s)) ranging from 3.1% to 7.0%, with 4.6% on average. Based on the seismic properties of xenoliths, we propose that, in SE Tibetan Plateau (south of 26 degrees N), the SKS wave splitting is attributed to melt-enhanced anisotropy with a vertical structural frame lithosphere. Under this structural configuration, the large delay time could be expected with the fast wave polarization direction (FPD) in E-W direction. Our study also supports the model of double anisotmpic layers in this area. The fast orientation of lower layer caused by lithospheric mantle anisotropy is in E-W direction, while the upper one with the NNW-SSE FPD is related to the frozen anisotropy in the middle-lower crust.
NSTL
Elsevier