Coherent Deformation Research Articles

Morphological instabilities of stressed and reactive geological interfaces

Interfaces between contacting rocks of the Earth's crust are shown to be unstable and corrugating and develop roughness at various scales when submitted to nonhydrostatic stress. This instability may occur in various geological settings as long as a coherent deformation of the interface is allowed and the bodies that the interface separates have different material properties (i.e., viscosity, density, or elastic moduli). Relevant examples include fault planes, dissolution interfaces, or grain boundaries. Performing a two‐dimensional linear stability analysis, we consider two cases: one solid in contact with a viscous layer and two solids separated by a thin viscous layer. In both cases either shear and/or normal loads are imposed on the interface and thermodynamical conditions for the initiation of roughening are established. Applied on several geological systems such as grain contacts and fault planes, we propose that our analysis can explain how complex patterns may emerge at rock‐rock interfaces. Finally, we provide an analysis of the evolution of the static friction coefficient along sheared interfaces. The evolution is shown to depend solely on Poisson's ratio of the solid and the ratio of the shear and compressional stresses along the interface.

Mechanisms of periodic deformation of the film-substrate system under compressive stress

The paper considers elastic and plastic deformation of metal, oxide and semiconductor films during their growth, thermal annealing and mechanical loading. It is shown that in a compressed thin film-compliant substrate system, wrinkling of the film surface and coherent deformation of the substrate take place. With a rigid substrate, compressive stress leads to elastic buckling of the film and its local or periodic delamination from the substrate. Plastic deformation of thin films is governed by competitive changes in their surface energy and strain energy. It is found that the periodic stress-strain distribution at the interface of two media plays a fundamental role and underlies the mechanisms for degradation of thin films under various external actions.

Lithospheric Deformation of Continental China from Rayleigh Wave Azimuthal Anisotropy

AbstractThe tectonic history of continental China is complicated and the associated lithospheric deformation is large; the use of the anisotropic propagation of surface waves is an important way to investigate the history of lithospheric deformation pattern. We measured inter‐station Rayleigh‐wave phase velocity dispersion from cross‐correlation of narrow band‐pass filtered surface wave records and an image analysis technique. The dispersion data were then used to invert for the distribution of azimuthal anisotropy at periods 20~120 s of continental China and adjacent regions. Checkerboard tests show that the lateral resolution of phase velocity azimuthal anisotropy is about 5° (~500 km) in most area of continental China. Our results show that the pattern of azimuthal anisotropy in the study area displays clear spatial variation. The tectonic deformation of the crust and upper‐mantle east of 104°E is generally weaker than that to the west as inferred from the relative magnitude of azimuthal anisotropy, and strong tectonic deformation had occurred in the crust and upper‐mantle beneath Qinghai‐Tibet Plateau and its eastern margin. In eastern China relatively large amplitude of azimuthal anisotropy appears in South China block and Pearl River Estuary region, southwestern margin of Ordos basin and Qinling‐Dabie orogenic belt, indicating strong tectonic deformation occurred there in the geological history. At short and intermediate periods the fast propagation direction shows a rotational pattern from the west to the east within the Qinghai‐Tibet block consistent with the clockwise extrusion (or escape) of the crustal material in the block after the Eurasian and Indian collision. The deformation pattern in the mid‐lower crust of western Qinghai‐Tibet block is similar to that of the upper mantle, inferring possible vertically coherent deformation across the Moho interface. However, the central and eastern Qinghai‐Tibet block and its eastern margin show different deformation patterns in the crust and upper mantle, which suggests that deformation is not vertically coherent in the crust and upper mantle in eastern plateau region. Strong tectonic deformation had occurred within Tarim and Qaidam basins as well as Qilian fold belt north of Qinghai‐Tibet block. The fast propagation direction at the middle crustal depth is clearly different from those both in the lower crust and upper mantle underlying the upper Yangtze block including Sichuan basin. This suggests different tectonic deformation processes between the middle crust and lower crust beneath this block, whereas the lower crust and upper mantle appears to be coupled as inferred from the similar fast propagation direction at periods above 30 s. The Longmen Shan tectonic zone can be divided into southwestern and northeastern segments at about 103°E. The southwestern segment has relatively lower phase velocity while the northeastern area has higher phase velocity and stronger anisotropy. This suggests that the NE striking unilateral rupture propagation of the WenchuanMs8.0 earthquake on May 12, 2008, may be related not only to the cumulated high stress of the northeastern section but also to the underlying medium property along the segment with high phase velocity suitable for strain energy accumulation and concentrated release.

Seismic anisotropy of upper mantle in eastern China

Based on the polarization analysis of teleseismic SKS waveform data recorded at 65 seismic stations which respectively involved in the permanent and temporary broadband seismograph networks deployed in eastern China, the SKS fast-wave direction and the delay time between the fast and slow shear waves at each station were determined by use of SC method and the stacking analysis method, and then the image of upper mantle anisotropy in eastern China was acquired. In the study region, from south to north, the fast-wave polarization directions are basically EW in South China, gradually clockwise rotate to NWW-SEE in North China, then to NW-SE in Northeast China. The delay time falls into the interval [0.41 s, 1.52 s]. Anisotropic characteristics in eastern China indicate that the upper mantle anisotropy is possibly caused by both the collision between the Indian and Eurasian Plates and the subduction from the Pacific and Philippine Sea Plates to the Eurasian Plate. The collision between two plates made the crust of western China thickening and uplifting and the material eastwards extruding, and then caused the upper mantle flow eastwards and southeastwards. The subduction of Pacific Plate and Philippine Sea Plate has resulted in the lithosphere and the asthenosphere deformation in eastern China, and made the alignment of upper mantle peridotite lattice parallel to the deformation direction. The fast-wave polarization direction is consistent with the direction of lithosphere extension and the GPS velocity direction, implying that the crust-upper mantle deformation is possibly a vertically coherent deformation.

Evidence for ancient lithospheric deformation in the East European Craton based on mantle seismic anisotropy and crustal magnetics

We present new shear-wave splitting measurements performed at 16 stations on the East European Craton, and discuss their implications in terms of upper-mantle anisotropy for this geophysically poorly-known region. Previous investigations of mantle anisotropy in Central Europe have shown fast directions aligning smoothly with the craton's margin and various suggestions have been proposed to explain their origin such as asthenospheric flow or lithospheric frozen-in deformation. Here, we aim at investigating the continuation of this shear-wave splitting pattern further to the East, into the East European Craton. For the craton, the interpretation appears to be less ambiguous than for central Europe since several arguments support lithospheric anisotropy in this region: 1) The large-scale coherence within either of the four constituting blocks and the significant variations between the blocks on a small-scale, 2) the weak correlation with absolute plate motion vectors, and 3) the good correlation between anisotropy and crustal features, for which we use magnetic field alignments as a proxy. Rather good correlation of these magnetic features with seismic fast orientations strongly supports the idea of vertically coherent deformation throughout upper mantle and crust. The observed splitting orientations thus reflect the last tectonic events of each block, frozen-in into the lithosphere for hundreds of millions of years.

Seismic anisotropy of upper mantle in Sichuan and adjacent regions

Based on the polarization analysis of teleseismic SKS waveform data recorded at 94 broadband seismic stations in Sichuan and adjacent regions, the SKS fast-wave direction and the delay time between the fast and slow shear waves were determined at each station using the grid searching method of minimum transverse energy and the stacking analysis method, and the image of upper mantle anisotropy was acquired. The fast-wave polarization directions are mainly NW-SE in the study area, NWW-SEE to its northeast and NS to its west. The delay time falls into the interval [0.47 s, 1.68 s]. The spatial variation of the fast-wave directions is similar to the variation of GPS velocity directions. The anisotropic image indicates that the regional tectonic stress field has resulted in deformation and flow of upper mantle material, and made the alignment of upper mantle peridotite lattice parallel to the direction of material deformation. The crust-upper mantle deformation in Sichuan and adjacent regions accords with the mode of vertically coherent deformation. In the eastern Tibetan Plateau, the crustal material was extruded to east or southeast due to SE traction force of the upper mantle material. The extrusion might be obstructed by a rigid block under the Sichuan Basin and the crust has been deformed. After a long-term accumulation of tectonic strain energy, the accumulative energy suddenly released in Yingxiu town of the Longmenshan region, and Wenchuan MS8.0 earthquake occurred.

SKS splitting beneath Capital area of China

Based on the polarization analysis of teleseismic SKS waveform data recorded at 49 seismic stations in Capital Area Seismograph Network, the SKS fast-wave direction and the delay time between the fast and slow shear waves at each station were determined by using the grid searching method of minimum transverse energy and the stacking analysis method, and then we acquired the image of upper mantle anisotropy in Capital area. In the study area, the fast-wave polarization direction is basically WNW-ESE, and the delay time falls into the interval from 0.56 s to 1.56 s. The results imply that the upper mantle anisotropy in Capital area is mainly caused by the subduction of the Pacific plate to Eurasian plate. The subduction has resulted in the asthenospheric material deformation in Capital area, and made the alignment of upper mantle peridotite lattice parallel to the deformation direction. And the collision between the Indian and Eurasian plates made the crust of western China thickening and uplifting and material eastwards extruding, and then caused the upper mantle flow eastwards, and made the upper mantle deformation direction parallel to the fast-wave direction. The deformation model of the crust and upper mantle is possibly vertically coherent deformation by comparing the fast-wave polarization direction with the direction of lithospheric extension and the GPS velocity direction.

Anisotropy beneath an active collision orogen of Taiwan: Results from across islands array observations

We examined shear wave splitting in SKS phases from a large event recorded by a temporary array across southern Taiwan. The span extends from the western plain to the east coast. We applied particle motion and cross correlation methods to estimate its polarization directions and delay times. Analysis shows clear evidence of splitting except stations on the east coast. The fast split‐shear wave along this transect are approximately parallel to the strike of the mountain belt (NE‐SW). The delay times show a short‐wavelength variation and are well correlated to the surface geology. The largest split time is about 1.6 sec located at Eastern Central Mountain Range and indicates its possible mantle origin. Implications suggest that anisotropy is related to the collision tectonics which built the Taiwan islands, and that this tectonic compression involved the lithosphere and is characterized by a strong coherent deformation of the upper mantle and the crust.

Shear wave splitting beneath the central Tien Shan and tectonic implications

Shear‐wave splitting analyses were performed at 30 seismic stations in the central Tien Shan and its vicinity. Fast orientation at most of the stations on the Tien Shan range is ENE‐WSW, parallel to the strike of the Tien Shan. It largely reflects a coherent deformation of the lithosphere in response to the NS shortening. Anisotropy with NNE‐SSW fast axes is observed within adjacent basins, around the Issyk Kul and near the northwestern Tarim, where the lithosphere is presumably strong, less deformed. The NNE‐SSW orientation, which coincides with the surface velocity of the central Tien Shan, can be best explained by shear between the lithosphere and asthenosphere on a regional scale rather than small‐scale convection as proposed in previous studies. The dextral Talasso‐Fergana fault seems to have little effect on anisotropy at its nearby stations, suggesting this fault is probably confined to the shallow part of the lithosphere.

Seismic anisotropy in Eastern Tibet from shear wave splitting reveals changes in lithospheric deformation

Abstract Knowledge about seismic anisotropy can provide important insight into the deformation of the crust and upper mantle beneath tectonically active regions. Here we focus on the southeastern part of the Tibetan plateau, in Sichuan and Yunnan provinces, SW China. We measured shear wave splitting of core-refracted phases (SKS and SKKS) at a temporary array of 25 IRIS-PASSCAL stations. We calculated splitting parameters using a multi-channel and a single-record cross-correlation method. Multiple layers of anisotropy cannot be ruled out but are not required by the data. A Fresnel zone analysis suggests that the shallow mantle (between 60 and 160 km depth) is the most likely source of anisotropy. The polarization directions reveal a pronounced transition from primarily north–south in the north (Sichuan) to mostly east–west orientations in the south (Yunnan). In the southern part of the study region, that is, south of ∼ 26°N, the fast polarization directions do not correlate well with known surface features and geodetic estimates of the crustal displacement fields. Whereas GPS campaigns provide evidence suggesting north–south crustal flow across the Red River Fault, the pattern of anisotropy argues against such flow in the upper mantle. These observations support models that allow differential movement of upper crust relative to lithospheric mantle. In the northern part of the study region the relationships are more ambiguous and coherent deformation of the crust and mantle lithosphere cannot be excluded. The interpretation of the shear wave splitting results is non-unique, but we suggest that the observed N–S transition reflects a fundamental change in deformation regime across our study region. It may be related to lateral variations in lithospheric rheology, or may mark a transition from the direct impact of the continental collision to dominance of the far-field strain field associated with regional subduction processes. Understanding the nature of the lateral change in deformation regime may prove critical for our understanding the geotectonic evolution of (eastern) Tibet.

Properties of Nimonic 80A alloy with change in the chemical compositions and Heat Treatment Paths

Properties of Ni-base superalloys of Nimonic 80A alloy system were investigated by the observation of microstructures, precipitates ana hardness as a function of the chemical compositions ana the paths of heat treatment. The higher hardness values showed, the higher Ti/Al ratio among high compositions of Cr and Co element. The lower (Ti+Al) and Fe contents decreased in the same Ti/Al ratio, the higher hardness values showed. This results are considered that coherent deformation was increased with increasing Ti/Al ratio. Hardness showed higher value when Cr contents was <TEX>$18 wt\%$</TEX> less than <TEX>$21wt\%Cr$</TEX>. In <TEX>$3.15 wt\%$</TEX>Co alloy, <TEX>$\gamma'$</TEX> phase was very fine as around 50nm and, its volume fraction and hardness showed the highest value by 2step-aging treatment.

Constraining the extent of crust–mantle coupling in central Asia using GPS, geologic, and shear wave splitting data

We have obtained constraints on mechanical crust–mantle coupling for Tibet and Yunnan/Indo China, by comparing the observed surface deformation field inferred from GPS and Quaternary fault slip rate data, with the mantle deformation field inferred from several SKS shear wave splitting data sets. We first determined whether the anisotropy is dominantly asthenospheric or lithospheric by testing simple models of both types against the observed values of the fast polarization direction, ϕ, which is assumed to be parallel to the horizontal projection of the direction of maximum shear. For asthenospheric flow, we solved for a best-fitting uniform sub-asthenospheric velocity model for Eurasia. The fit, however, was not satisfactory (RMS misfit Δ ϕ = 20°). Solving for separate uniform flow fields in each region improves the fit (Δ ϕ = 15° for Tibet, Δ ϕ = 11° for Yunnan), although the resulting flow fields are inconsistent with several geophysical and geological constraints and thus considered unlikely. We then considered lithospheric models. For Tibet, vertically coherent deformation (i.e., maximum shear direction from surface deformation is parallel to ϕ) yields an improved match (Δ ϕ = 11°) for left-lateral shear. Both the goodness of fit and the dominance of left-lateral surface faulting in Tibet, argue for a lithospheric source of anisotropy. The misfit for Yunnan is large for either right- (Δ ϕ = 53°) or left-lateral (Δ ϕ = 49°) shear, which argues for complete crust–mantle decoupling in Yunnan. We show that the fast polarization directions throughout both the Tibet and Yunnan region can be fit by a single lithospheric dynamic model in which there is strong coupling between crust and mantle beneath Tibet, but a complete decoupling between crust and mantle beneath Yunnan crust. This dynamic model predicts left-lateral maximum shear directions within the mantle that align with fast polarization directions in both regions (Δ ϕ = 9°). These maximum shear directions within the mantle align with the left-lateral maximum shear directions in the crustal deformation field in Tibet, but are not in Yunnan. Our results have the following implications. First, the coherence between crust and mantle deformation in Tibet implies strong crust–mantle mechanical coupling, since this is the only way that crustal buoyancy forces, required to account for the surface deformation field, can be transmitted into the mantle. This behavior is consistent with a uniform-strength lithosphere or strong crust, but not with a substantially weaker crust. For example, it is inconsistent with the popular “jelly-sandwich” rheology, and thus precludes behavior such as large-scale lower crustal flow in Tibet. The crust–mantle coherence is also incompatible with mantle delamination. Second, crust–mantle decoupling within the Yunnan lithosphere argues that mantle deformation there is controlled only by boundary conditions; crustal buoyancy forces are not transmitted into the mantle beneath Yunnan. Moreover, the dynamic model for the mantle shows that the Yunnan crust is moving south with respect to the mantle at rates as high as ∼30 mm/yr. Third, there is a fundamental rheological lithospheric transition between Tibet and Yunnan that may provide a key to understanding this significant orogen.

STORTI, F., HOLDSWORTH, R. E. &amp; SALVINI, F. (eds) 2003. Intraplate Strike-Slip Deformation Belts. Geological Society Special Publication no. 210. viii+234 pp. London, Bath: Geological Society of London, Price £65.00, US $108.00; members' price £32.50, US $54.00; AAPG members' price £39.00, US $65.00 (hard covers). ISBN 1 86239 132 7

Geological Society Special Publication 210 contains a collection of papers devoted entirely to strike-slip tectonics –specifically intra-plate strike-slip deformation belts. The introductory paper by Storti et al. contains a very useful review of current ideas concerning intra-plate strike-slip deformation belts. The paper is well referenced and focuses the reader on the importance of these structures both in terms of localizing deformation and their role as major sources of long-term mechanical anisotropy in the lithosphere. The following paper by Vauchez & Tommasi retruns to the topic of lithospheric anisotropy and coherent deformation of the entire lithosphere using a combination of geological and geophysical observations, from a wide variety of examples around the globe. This is followed by the lone analogue modelling paper by Guido Schreurs, detailing experiments of distrubuted strike-slip shear zones. This paper presents both a useful summary of previous modelling work combined with well-illustrated models using a novel and original approach to generate distributed shear in the overburden. The remaining 10 papers deal with natural examples of intra-plate strike-slip deformation belts from the India–Eurasia collision-escape belt, Antarctica, North America to South America. Buslov et al. document recent deformation in the …

Spatiospectral localization of isostatic coherence anisotropy in Australia and its relation to seismic anisotropy: Implications for lithospheric deformation

We investigate the two‐dimensional (2‐D) nature of the coherence between Bouguer gravity anomalies and topography on the Australian continent. The coherence function or isostatic response is commonly assumed to be isotropic. However, the fossilized strain field recorded by gravity anomalies and their relation to topography is manifest in a degree of isostatic compensation or coherence which does depend on direction. We have developed a method that enables a robust and unbiased estimation of spatially, directionally, and wavelength‐dependent coherence functions between two 2‐D fields in a computationally efficient way. Our new multispectrogram method uses orthonormalized Hermite functions as data tapers, which are optimal for spectral localization of nonstationary, spatially dependent processes, and do not require solving an eigenvalue problem. We discuss the properties and advantages of this method with respect to other techniques. We identify regions on the continent marked by preferential directions of isostatic compensation in two wavelength regimes. With few exceptions, the short‐wavelength coherence anisotropy is nearly perpendicular to the major trends of the suture zones between stable continental domains, supporting the geological observation that such zones are mechanically weak. Mechanical anisotropy reflects lithospheric strain accumulation, and its presence must be related to the deformational processes affecting the lithosphere integrated over time. Three‐dimensional models of seismic anisotropy obtained from surface wave inversions provide an independent estimate of the lithospheric fossil strain field, and simple models have been proposed to relate seismic anisotropy to continental deformation. We compare our measurements of mechanical anisotropy with our own model of the azimuthally anisotropic seismic wave speed structure of the Australian lithosphere. The correlation of isostatic anisotropy with directions of fast wave propagation gleaned from the azimuthal anisotropy of surface waves decays with depth. This may support claims that above ∼200 km, internally coherent deformation of the entire lithosphere is responsible for the anisotropy present in surface wave speeds or split shear waves.

Wrench faults down to the asthenosphere: geological and geophysical evidence and thermomechanical effects

Abstract We review a set of geological and geophysical observations that strongly support a coherent deformation of the entire lithosphere in major intracontinental wrench faults. Tectonic studies of wrench faults eroded down to the middle to lower crust show that, even in cases in which the lower to middle crust is partially melted, strain remains localized (although less efficiently) in transcurrent shear zones. Seismic profiling as well as seismic tomography and magnetotelluric soundings provide strong argument in favour of major wrench faults crosscutting the Moho and deforming the upper mantle. Pn velocity anisotropy, shear-wave splitting and electric conductivity anisotropy measurements over major wrench faults and in transpressional domains support that a wrench fault fabric exists over most or even the entire lithosphere thickness. These seismic and electrical anisotropies are generated by a crystallographic preferred orientation of olivine and pyroxenes developed in the mantle during the fault activity, which is frozen in the lithospheric mantle when the deformation stops. The preservation of such a ‘wrench fault type’ fabric within the upper mantle may have major effects on the subsequent tectonothermal behaviour of continents, because olivine is mechanically and thermally anisotropic. Indeed, the association of numerical models and laboratory data on textured mantle rocks strongly suggests that the orogenic continental lithosphere is an anisotropic medium with regards to its stiffness and to heat diffusion. This anisotropy may explain the frequent reactivation, at the continents scale, of ancient lithospheric-scale wrench faults and transpressional belts during subsequent tectonic events.

X-ray texture investigations of bacterial cellulose

Never dried highly swollen bacterial cellulose fleeces produced by the bacteria Acetobacter xylinum were deformed uniaxially and kept under strain during the drying process. The orientation of the crystalline nano-fibrils was determined by X-ray pole figure measurements of the (1–10) lattice plane in reflection mode and the (004) interference in transmission mode. Appropriate cuts through the pole figures were used for the determination of planar and axial degrees of orientation. Generally, a uniplanar texture with the (1–10) planes parallel to the fleece surface and an axial component in the draw direction were found. With increasing draw ratio both the axial orientation and the uniplanar orientation in draw direction were improved, whereas transversely the uniplanar texture component broadened slightly. Mechanical strength and modulus were almost doubled and tripled, respectively, for favoured cases. As compared to the wet aqueous samples, a higher coherent deformation of the bacterial cellulose membrane could be achieved by drawing the samples soaked in NaOH solutions with concentrations in the range of 8 to 10 wt.-%. By the presence of the lye significant improvements of the axial chain orientation of up to 100% could be obtained resulting in a maximum strength of 580 MPa. The improved orientability is likely to be explained by a NaOH-induced reduction in the number of inter-fibrillar bridging points formed by H-bonds

La déformation post-Miocène en Provence occidentale

The structure of western Provence (SE France) is the result of successive deformations connected to the building of the Pyrenees and the Alps. It is a seismically active region still undergoing deformation. The aim of this study is to characterize the recent deformation in western Provence and to integrate the cumulated displacements in a coherent deformation model. In order to do this, we identified the recent structures that concentrate the deformation. We used the Miocene as a sedimentary marker to estimate the discontinuous deformation over the last 20 Ma and geomorphic surfaces to evaluate the amount of the post-Miocene deformation. Miocene terrains are deformed along south-vergent thrusts such as Le Luberon, Les Costes, La Trévaresse or Les Alpilles, and along sinistral strike-slip faults such as the Durance and Nîmes faults. North-vergent Pyrenean thrusts such as L’Étoile-Sainte Baume, Sainte Victoire or the Eguilles thrusts were not reactivated during the Alpine phase. Field evidence shows that in the Luberon, the main folding phase occurred during, or immediately after the Burdigalian (20.5–16 Ma). The shortening measured on a regional N–S cross-section is of a few kilometres, implying a deformation of 0.1–0.2 mm·year –1 since the beginning of the Miocene. Geomorphic surfaces have been reported on cross-sections of the E–W thrusts. The intensity of the deformation decreases southward and through time during the Miocene. Pliocene surfaces are not deformed near the active structures, except at the front of the Digne thrust. Furthermore, Quaternary geomorphic markers such as alluvial fans are not affected by the Durance strike-slip fault. Our results show that from Miocene to the Present, Provence was not intensively deformed (0.1–0.2 mm·year –1), and occurred in a short period of time during the Miocene. It is coherent with the southward emplacement of the Alpine Digne thrust being the cause of this deformation. Since the end of the Miocene, there have been no major displacements on any of the active structures.

Teleseismic shear wave splitting measurements in noisyenvironments

SUMMARY High noise levels hamper teleseismic shear wave splitting measurements, which bandpass filtering does not always help. To investigate how robust splitting measurements are to noise, we analysed a set of synthetic records with known splitting parameters and added fixed levels of noise. In the presence of weak anisotropy, single-waveform splitting measurements are unreliable when operating with noisy data sets. A practical rule in terms of S/N ratio and splitting delay time parameters is that splitting is confidently detectable at S/N>8, regardless of the wave’s original polarization orientation. However, for the evidence of weak anisotropy to be detectable and measurable at an S/N value of 4, the backazimuth separation of the phases from the fast polarization direction needs to be higher than 20°. Stacks of individual measurements consistently yield reliable results down to S/N values of 4. Applying stacking to data from DSB (Dublin, Ireland), the fast polarization direction w and lag time dt are 58° and 0.95 s. This orientation reflects surface trends of deformation in the area, as found elsewhere in the UK. Our result thus reinforces the proposed model that the detected anisotropy in the British Isles originates from lithospheric coherent deformation preserved from the last main tectonic episode.

Seismic anisotropy beneath the southern Himalayas‐Tibet collision zone

Shear wave phases ScS, SKS, and SKKS, collected from the Project International Deep Seismic Sounding of Tibet and Himalayas II (INDEPTH‐II) array in the eastern Himalayas and Tibet are analyzed for the orientation and extent of polarization seismic anisotropy beneath the southern Himalayas‐Tibet collision zone. No evidence is seen for any shear wave splitting for stations in this part of the collision zone except for two northern stations located about 100 km north of the Indus‐Tsangpo Suture. The lack of shear wave splitting beneath the stations in the vicinity of suture and the Tethyan Himalayas indicates that there is no appreciable large‐scale mantle polarization seismic anisotropy beneath this part of the collision zone. One explanation for the lack of polarization seismic anisotropy would be a subvertical mantle shear strain field resulting from a downwelling mantle flow of a thickened Indian lower continental lithosphere. North of the Gandese belt, the fast directions of the split shear waves are oriented in a northeast‐southwest direction, approximately parallel to the trend of a major left‐lateral strike‐slip fault zone that bounds the Gulu rift. The coincidence of fast polarization directions parallel to the surficial geologic shear zones and the direction of shear planes within the upper crustal strain rate field suggests lithospheric‐wide coherent deformation.

Lithospheric deformation inferred from teleseismic shear wave splitting observations in the United Kingdom

Observations of splitting in teleseismic shear waves recorded at stations in the United Kingdom (UK) seismic network operated by the British Geological Survey are related to the tectonic structures and history in the UK. The combination of network density and tectonic variability in the UK permits an assessment to be made of the relationship between splitting and geology. Fast polarization directions are most commonly parallel to geologic features resulting from compressional or strike‐slip events, and splitting delay times mirror the increasing involvement of basement in deformation as observed through field investigation. Different orogenic episodes may be distinguished in the splitting orientations. A peak in delay times is suggested in the observations at the Iapetus Suture, a site of Paleozoic subduction now in the UK interior. These observations are consistent with the idea that continental anisotropy arises through coherent deformation of the lithosphere and is preserved long after orogenic deformation ceases.