Seismic shear wave splitting in upper crust characterized by Taiwan tectonic convergence
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This study conducts a comprehensive investigation of crustal seismic anisotropy over varied geological regimes of Taiwan. With a large amount of earthquake data, the lateral variation of seismic shear wave splitting (SWS) is fully examined in terms of crustal deformation process. As the well-known vigorous orogeny subjected to the Eurasian—Philippine plate collision, tectonic convergence of Taiwan is presumably propagating from east to west. The acquired SWS waveform data cover areas from the slightly deformed Western Plains to the intermediate-to-high metamorphic Western Foothills and central mountain ranges. By means of waveform cross-correlation, the SWS parameters—the fast-wave polarization orientation and delay time—infer that the mechanism of lithologic deformation of Taiwan convergence can be classified into two domains: the convergence-parallel laminating west of the Deformation Front and the convergence-perpendicular striking east of the Deformation Front. The convergence-parallel SWS measurement presents the internal fabrics consisting of microfractures subject to lateral compression before the yielding of the lithologic strength, whereas the convergence-perpendicular measurements reveal the lateral accommodation of deformation as the stress/strain surpass the yielding strength of rock, where the predominant SWS polarization is in the NE—SW direction similar to the general trend of Taiwan's mountain ranges. It is remarkable that there is no correlation between metamorphic degrees with SWS parameters. The geological province which corresponds to higher metamorphism is not consistent with large SWS parameters. This may be because of anisotropic weakness caused by multiple tectonic processes at considerable metamorphic zone. Furthermore, comparison of the SWS delay times with corresponding focal depths suggests that seismic anisotropy in the upper crust may come from multiple layers, and the fabric lamination causing the anisotropy may be confined only within the shallow crust.Keywords:
Seismic anisotropy
Orogeny
Shear wave splitting
Lithology
Terra Nova, 25, 57–64, 2013 Abstract Characterizing the interaction of a fault with its surroundings is vital to fully understand the tectonic processes involved and predict future behaviour. Regional and local stress orientations affect different fracture length scales, manifested by numerous associated fault, fracture and crack structures. We use seismic anisotropy to constrain the dominant orientation of aligned rupture planes of various length scales. In particular, we study shear‐wave splitting of regional seismic events in Trans‐Alboran Shear Zone (TASZ), south‐east Spain. The TASZ consists of three major left‐lateral strike‐slip faults and numerous secondary strike‐slip and thrust faults. The observed orientations for S‐waves vary from roughly N–S in the northern segment of TASZ, to E–W in the centre, to NNW–SSE and NNE–SSW in the south. We show that the strikes of fast polarizations reflect both structural and lithological differences, indicating complex interactions of principal and secondary faults within the crust to accommodating tectonic stresses.
Thrust fault
Principal stress
Seismic anisotropy
Shear wave splitting
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The Grenville Orogen is a crustal-scale thrust stack composed of longitudinal belts with distinctive P-T conditions and/or timing of metamorphism. Recent work has shown that adjacent belts with Ottawan metamorphism (ca. 1090–1020 Ma) exhibit metamorphic signatures implying development at very different levels in the orogen. The HP Belt, which is characterized by peak metamorphic conditions of ca. 1800 MPa/850 °C, is overlain by an MP Belt in which metamorphic conditions were ca. 800–1000 MPa/850 °C. Both belts are locally juxtaposed against the Non-Metamorphic Lid in which penetrative Grenvillian fabrics are absent and metamorphic temperatures were 500>T>350 °C. Thus sections of the lower crust (60–70 km depth), mid crust (~ 30 km depth) and upper orogenic crust (depth undetermined) are preserved in close proximity to each other. The elevated temperature of regional metamorphism in the Ottawan HP and MP belts was associated with the emplacement of mantle-derived magmas locally, implying a mantle contribution to the thermal budget of the orogen. Midto late Ottawan juxtaposition of the lower-crustal HP Belt with the upper-crustal Non-Metamorphic Lid took place by the combined processes of tectonic extrusion and orogenic collapse, the latter probably facilitated by a ductile melt-weakened mid-crust. By the time of renewed convergence at ca. 1005 Ma during the Rigolet Orogeny, the HP and MP belts were in the upper crust and active ductile deformation in the orogen took place principally beneath them and nearer the foreland. Metamorphism during the Rigolet Orogeny was Barrovian in character and led to substantial crustal thickening in the vicinity of the Grenville Front and the parautochthonous northern margin of the orogen. The orogen thus grew by crustal-scale understacking resulting in preservation of the older metamorphic rocks in the orogenic superstructure. Post-Rigolet normal faulting affected the full thickness of the orogenic crust and displaced the Moho locally, implying orogen-scale orogenic collapse. The great width of the Grenville Orogen (> 600 km on the Laurentian side alone) and the prevailing high temperatures of Ottawan metamorphism reflect a Himalayan-scale orogen characterized by massive horizontal tectonic transport within a ductile lower and mid crust.
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Metamorphic core complex
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Abstract Madagascar occupies a key position in the assembly and breakup of the supercontinent Gondwana. It has been used in numerous geological studies to reconstruct its original position within Gondwana and to derive plate kinematics. Seismological observations in Madagascar to date have been sparse. Using a temporary, dense seismic profile across southern Madagascar, we present the first published study of seismic anisotropy from shear wave splitting analyses of teleseismic phases. The splitting parameters obtained show significant small‐scale variation of fast polarization directions and delay times across the profile, with fast polarization rotating from NW in the center to NE in the east and west of the profile. The delay times range between 0.4 and 1.5 s. A joint inversion of waveforms at each station is applied to derive hypothetical one‐layer splitting parameters. We use finite‐difference, full‐waveform modeling to test several hypotheses about the origin and extent of seismic anisotropy. Our observations can be explained by asthenospheric anisotropy with a fast polarization direction of 50°, approximately parallel to the absolute plate motion direction, in combination with blocks of crustal anisotropy. Predictions of seismic anisotropy as inferred from global mantle flow models or global anisotropic surface wave tomography are not in agreement with the observations. Small‐scale variations of splitting parameters require significant crustal anisotropy. Considering the complex geology of Madagascar, we interpret the change in fast‐axis directions as a ~150 km wide zone of ductile deformation in the crust as a result of the intense reworking of lithospheric material during the Pan‐African orogeny. This fossil anisotropic pattern is underlain by asthenospheric anisotropy induced by plate motion.
Shear wave splitting
Seismic anisotropy
Asthenosphere
Supercontinent
Seismic Tomography
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Geosyncline
Orogeny
Basement
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The Scrip Nappe, a large recumbent anticline that occupies the northern Selkirk and northern Monashee Mountains, has an inverted lower limb, some 50 km in length across strike, and comprises stratigraphic divisions of the Hadrynian Horsethief Creek Group, which can be traced southward with decreasing metamorphic grade through the Selkirk Mountains to the northern Purcell Mountains. The Scrip Nappe has a southwesterly vergence and it formed that way during the first folding phase of the Mesozoic Columbian Orogeny. Metamorphism no greater than biotite zone accompanied that first deformation. The nappe was subsequently refolded into tight northeast verging folds. Metamorphism rose to upper amphibolite facies late in the second deformation phase. After the metamorphic climax, northeast verging buckle folds and associated crenulation cleavage formed locally during a third folding episode. The entire nappe complex was then carried northeastward, on the Purcell thrust, over the folds and thrusts of the western Rocky Mountains.
Anticline
Crenulation
Orogeny
Greenschist
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Abstract The structure of the metamorphic rocks forming the Southern Alps in South Westland, New Zealand, is described. Metamorphic grade increases north-westwards from greywacke and argillite east of the Main Divide through Chlorite Zone schists to high-grade schists and gneisses of the Biotite, Garnet, and Oligoclase Zones. The metamorphic zonal sequence is interrupted and repeated by post-metamorphic strike faults and folds, and terminated on the north-west by the Alpine Fault. After geosynclinal sedimentation in the late Paleozoic and early Mesozoic, the thick geosynclinal sediments were deformed and metamorphosed in the middle Jurassic to early Cretaceous Rangitata Orogeny. Three movement phases separated by relatively static periods of metamorphic crystallisation are distinguished. In the first movement phase (F1), movements were dominantly translational, forming large recumbent folds with axial surfaces dipping south and south-west. The F1 movements were accompanied by Chlorite Zone metamorphism and development of schistosity and lineation. In the first static phase (M1), garnet and biotite porphyroblasts grew in the schists. In the second movement phase (F2) , the F1 recumbent folds were isoclinally folded about steep axial planes, and a second schistosity and lineation produced by transposition of the first. The F2 folds and lineations plunge south and south-west in the same direction as the regional dip produced by the F1 folding. In the second static phase (M2), garnet, biotite, and amphibole grew in the schists, and the isograd pattern was established, and transgressed the F1 and F2 fold axes on a regional scale. In the postmetamorphic (F3) deformation, the orogenic belt was uplifted by folding and faulting, the Alpine Fault began to move, and some folding about steep axes took place under a wrench regime. Lamprophyre dike swarms, probably Cretaceous in age, are the only post-tectonic intrusives known. They have probably been displaced 110 miles from similar dikes in North Westland by dextral strike-slip movement on the Alpine Fault in the late Cainozoic Kaikoura Orogeny.
Lineation
Crenulation
Isograd
Orogeny
Andalusite
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The area of Damasi-Tyrnavos (Thessaloniki, Central Greece), in the vicinity of Larissa, was characterized by low seismic activity during the last decades. Two strong earthquakes of Mw = 6.3 and Mw = 6.0 The area of Damasi – Tyrnavos (Thessaly, Central Greece), in the vicinity of Larissa, was characterized occurred in early March 2021, followed by an intense aftershock sequence, related to WNW-ESE to NW-SE oriented faulting. This sequence was recorded by a dense local seismological network that provided a rich dataset and a unique opportunity to investigate upper crust shear-wave splitting for the first time in the study area. A fully automated technique, employing the eigenvalues method and cluster analysis, was implemented to measure the fast shear-wave polarization direction and the time-delay between the two split-shear-waves. This procedure yielded 655 results of adequate quality grade at 9 stations, after analyzing 1602 events and applying strict selection criteria, including the shear-wave window. The measured directions revealed a complex upper crust anisotropic regime. WNW-ESE to NW-SE, in accordance both with the APE model, being parallel to the local 𝜎 Hmax direction, and the strike of the fault planes. On the other hand, stations at the central part exhibit NNW-SSE and NNE-SSW anisotropy directions. An interesting feature is that the two northern stations are characterized by larger normalized time-delay values, possibly related to the migration of seismicity to the north during the initial stage of the seismic sequence.
Shear wave splitting
Upper crust
Sequence (biology)
Seismic anisotropy
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We systematically analysed shear wave splitting (SWS) for seismic data observed at a temporary array and two permanent networks around the San Andreas Fault (SAF) Observatory at Depth. The purpose was to investigate the spatial distribution of crustal shear wave anisotropy around the SAF in this segment and its temporal behaviour in relation to the occurrence of the 2004 Parkfield M 6.0 earthquake. The dense coverage of the networks, the accurate locations of earthquakes and the high-resolution velocity model provide a unique opportunity to investigate anisotropy in detail around the SAF zone. The results show that the primary fast polarization directions (PDs) in the region including the SAF zone and the northeast side of the fault are NW–SE, nearly parallel or subparallel to the SAF strike. Some measurements on the southwest side of the fault are oriented to the NNE–SSW direction, approximately parallel to the direction of local maximum horizontal compressive stress. There are also a few areas in which the observed fast PDs do not fit into this general pattern. The strong spatial variations in both the measured fast PDs and time delays reveal the extreme complexity of shear wave anisotropy in the area. The top 2–3 km of the crust appears to contribute the most to the observed time delays; however substantial anisotropy could extend to as deep as 7–8 km in the region. The average time delay in the region is about 0.06 s. We also analysed temporal patterns of SWS parameters in a nearly 4-yr period around the 2004 Parkfield main shock based on similar events. The results show that there are no appreciable precursory, coseismic, or post-seismic temporal changes of SWS in a region near the rupture of an M 6.0 earthquake, about 15 km away from its epicentre.
Shear wave splitting
Seismic anisotropy
Upper crust
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