We build a new radially anisotropic shear wave velocity model of Southern California based on ambient noise adjoint tomography to investigate crustal deformation associated with Cenozoic evolution of ...the Pacific‐North American plate boundary. Pervasive positive radial anisotropy (4%) is observed in the crust east of the San Andreas Fault (SAF), attributed to subhorizontal alignment of mica/amphibole foliation planes resulting from significant crustal extension. Substantial negative anisotropy (6%) is revealed in the middle/lower crust west of the SAF, where high shear wave speeds are also observed. The negative anisotropy could result from steeply dipping amphibole schists in a shear zone developed during Laramide flat slab subduction. Alternatively, it could be caused by the crystal preferred orientation (CPO) of plagioclase, whose fast axis aligns orthogonally to a presumed subhorizontal foliation. The latter new mechanism highlights potentially complex CPO patterns resulting from different lithospheric mineralogy, as suggested by laboratory experiments on xenoliths from the region.
Plain Language Summary
The crust of Southern California has been shaped by complex tectonic processes through the evolution of the Pacific‐North America plate boundary. The mechanisms of crustal deformation in this area are not fully understood. We investigate the deformation regime by studying the seismic radial anisotropy of shear wave speed associated with mineral or structural orientations. Our work reveals pervasive positive radial anisotropy (VSH > VSV) in the crust and uppermost mantle, which is consistent with the tectonic setting of widespread and long‐term crustal extension of the western United States through the Cenozoic. Interestingly, we also observe strong negative anisotropy (VSH < VSV) in the lower crust west of the San Andreas Fault that has not been reported before. We interpret the positive anisotropy to be caused by the subhorizontal alignment of foliation planes of mica/amphibole whereas the negative one is potentially created by either steeply dipping amphibole schists or subhorizontal alignment of plagioclase. The distinct radial anisotropies across the transform plate boundary might indicate the importance of complex CPO patterns, resulting from different lithospheric mineralogy under the same strain regime.
Key Points
A radially anisotropic shear wave velocity model of Southern California is constructed from ambient noise adjoint tomography
Positive radial anisotropy in the crust is caused by subhorizontal alignment of mica and amphibole associated with extensional tectonics
Negative anisotropy west of the San Andreas Fault is attributed to steeply dipping amphibole schists or subhorizontally foliated plagioclase
We present a high‐resolution P‐wave azimuthally anisotropic velocity model for the upper and middle crust beneath southern California by a novel adjoint‐state traveltime tomography technique. Our ...model reveals significant anisotropy variations between tectonic blocks that clearly reflect both past and current plate boundary deformation. In the shallow crust, seismic anisotropy is mostly controlled by the preferred alignment of microcracks related to the present N‐S compressive stress; while at deeper depths (>∼6 km), seismic anisotropy mainly records paleofabrics formed during the long‐lived Farallon subduction and later extension that have not been fully reset by the present transform motion. Interestingly, our model demonstrates distinct fast axes beneath the western Transverse Ranges from its neighboring blocks, probably reflecting the large‐scale vertical axis clockwise rotation of the block. In addition, we identify layered structures with distinct anisotropy features beneath the Salton Trough, which could be a result of the current transtension.
Plain Language Summary
Seismic anisotropy, which is defined as the directional dependence of the speed of seismic waves in a rock matrix, provides important constraints on the dynamic processes of the Earth. Here, we investigate this parameter in the upper to middle crust beneath southern California where the Pacific Plate slides northwestward with respect to the North American Plate by a novel seismic imaging technique. We find that present crustal anisotropy in southern California reflects both the past and current plate boundary deformation. While the shallow crustal anisotropy is dominantly controlled by the present stress, seismic anisotropy at larger depths (>∼6 km) mainly reflects paleofabrics associated with Mesozoic subduction and later extension. The clockwise rotation of the western Transverse Ranges is clearly resolved by its distinct anisotropy as compared to its neighboring blocks. On the other hand, we show that seismic anisotropy beneath the Salton Trough may be fully controlled by the current deformation, forming layered structures with distinct anisotropy features.
Key Points
We build a 3‐D high‐resolution upper to middle crust azimuthally anisotropic model of southern California using local P‐wave arrival times
Present anisotropy mostly reflects shape preferred orientation of stress‐related microcracks in the shallow crust and crystallographic preferred orientation of paleo mineral fabrics at larger depths
Distinct anisotropy beneath the western Transverse Ranges from its adjacent blocks reflects its history with rotation up to 110°
Seismic anisotropy is controlled by aligned rock‐forming minerals, which most studies attribute to solid‐state shear with less consideration for magmatic fabric in plutonic rocks (rigid‐body rotation ...of crystals in the presence of melt). Our study counters this traditional solid‐state bias by evaluating contributions from fossil magmatic fabric. We collected samples from various tectonic settings, identified mineral orientations with electron backscatter diffraction and neutron diffraction, and calculated their bulk rock elastic properties. Results indicate that magmatic fabric may lead to moderate to strong anisotropy (3%–9%), comparable to solid‐state deformation. Also, magmatically aligned feldspar may cause foliation‐perpendicular fast velocity, a unique orientation that contrasts with a fast foliation typical of solid‐state deformation. Therefore, magmatic fabric may be more relevant to seismic anisotropy than previously recognized. Accordingly, increased considerations of magmatic fabric in arcs, batholiths, and other tectonic settings can change and potentially improve the prediction, observation, and interpretation of crustal seismic anisotropy.
Plain Language Summary
Seismic waves that travel through Earth have directionally varying velocities, which depend on the type and orientation of minerals that make up a rock. A common assumption is that the processes that align these minerals occur when the rock is solid, by either brittle fracturing or by plastic deformation at the subatomic scale. However, a similar alignment can form in a magmatic rock by the rotation of elongated crystals in a partially molten rock mush, prior to solidification, and that alignment remains after cooling. In this study, we measured the orientation of such magmatically aligned minerals in rocks and calculated how fast seismic waves travel through the rocks. In some cases seismic properties are similar, whether planar minerals like mica were aligned in the solid‐state or magmatic‐state, with faster seismic waves traveling parallel to the rock's planar fabric. However, where tabular feldspar is strongly aligned in magmatic rocks, seismic waves instead travel faster perpendicular to the rock's planar fabric. Scientists who use seismic stations on Earth's surface to measure differences in crustal seismic velocities can therefore make more accurate interpretations of subsurface rock fabric by considering whether minerals were aligned in a solid‐state or magmatic‐state.
Key Points
Magmatic fabric may lead to moderate to strong (3%–9%) anisotropy, comparable to that commonly produced by solid‐state shear deformation
Magmatically aligned feldspar causes foliation‐perpendicular fast velocities, contrasting to foliation‐parallel as typical of solid‐state
Magmatic fabrics provide complementary or alternative explanations (compared to solid‐state) for observed crustal anisotropy
The convergent plate boundary in eastern Indonesia and Timor‐Leste captures an active oblique collision between the Banda Arc and the Australian plate. We analyzed ∼5 years' worth (2014–2019) of ...radial and tangential teleseismic Ps receiver functions (RFs) observed at 30 temporary broadband seismic stations across the area. Azimuthal variations in RF arrivals are observed throughout the region, indicative of the presence of oriented tectonic fabrics (dipping contrasts or plunging axis anisotropy) from a variety of crustal depths. The two main strikes of these fabrics are roughly parallel to the orogen and the plate convergence across the outer arc islands, likely associated with orogenic and strike‐slip structures. We observe distinct double polarity‐reversal arrivals with opposite polarity that reflect an anisotropic layer with orogen‐parallel strikes in the shallow crust beneath Timor and Savu, interpreted as metamorphic rocks. Fabrics oriented E‐W are imaged beneath the Flores and Lomblen that host active volcanoes, where we find interesting correlations with magmatic structures. NNW‐SSE striking fabric is imaged at ∼13 km depth beneath central Flores, which relates to a connected dike magmatic system that feeds the aligned cinder cones exposed on the surface. Finally, we identify convergence‐parallel fabrics on the volcano‐extinct islands of Alor and Atauro, consistent with one main fabric orientation imaged in Timor. We suggest all convergence‐parallel fabrics might accommodate strike‐slip motion generated by the overall NNE convergence of the Australian plate with respect to Eurasian plate and contribute to strain partitioning between the trough and backarc resulting from the collision.
Plain Language Summary
We used ∼5 years of seismic data collected at 30 temporary seismic stations to aid in understanding the evolution of the Banda Arc‐Australian plate collision. The seismometers were deployed between 2014 and 2019 across the islands of Eastern Indonesia and Timor‐Leste in order to seismically image the structure of the deep Earth and infer more about the processes of plate collision. Here, we used a technique named receiver function analysis that images boundaries and changes in fabric of the rocks in the Earth's crust. The layers and structures we image at depth across the region reflect the complex tectonic history and variation in rock types that can be interpreted with complementary geological data. We find that the majority of the orientations of the deep crustal structures are roughly parallel to the orientation of the islands and new mountain formation. However, some of the imaged structures and fabrics are clearly related to active volcanoes and others are associated with the complex deformation of the crust during the recent collision of the Australian plate with the active volcanic arc.
Key Points
Azimuthal variations in receiver functions reveal oriented tectonic fabrics across the arc‐continent collision zone in Banda
Orogen‐ and convergence‐parallel oriented fabrics imaged on Timor and Alor are interpreted as orogenic and strike‐slip structures
Oriented fabrics imaged on inner arc islands of Flores and Lomblen are likely associated with variable volcanic structures
Deep continental crustal structures are enigmatic due to lack of direct exposures and limited tools to investigate them remotely. Seismic waves can sample these rocks, but most seismic methods focus ...on coarse crustal structures while laboratory measurements concentrate on crystal‐scale rock properties, and little work has been conducted to bridge this interpretation gap. In some places, geologic maps of crystalline basement provide samples of the intermediate‐scale fabrics and structures that may represent in situ deep crust. However, previous research has not considered natural geometric variations from map data, nor is this heterogeneity typically included in map‐scale seismic property calculations. Here, we test how map‐scale fabrics influence crustal seismic anisotropy in Colorado by analyzing structural data from geologic maps, combining those data with bulk rock elastic tensors to calculate map‐scale seismic properties, and evaluating the resulting comparisons with observed receiver function A1 (360° periodic) arrivals. Crystalline fabrics, predicted seismic properties, and tectonic structures positively correlate with shallow and deep crustal A1 arrivals. Additionally, widespread correlations occur between mapped fault traces and regional foliations, implying that preexisting mechanical heterogeneity may have strongly influenced subsequent reactivation. We interpret that various mapped geologic contact types (e.g., lithologic and structural) generate A1 arrivals and that multiple parallel features (e.g., faults, foliations, and intrusions) contribute to a seismically visible tectonic grain. Therefore, Colorado's exhumed basement, as expressed in outcrops and maps, offers insight into modern deep crustal geological and geophysical structure.
Key Points
Map‐scale seismic properties are calculated using combinations of geologic map data and bulk rock elastic tensors
Exhumed crystalline fabrics and tectonic structures correlate with in situ deep crustal features as imaged with receiver functions
Proterozoic regional foliations and their mechanical heterogeneity influenced the orientations of subsequent faulting
Rayleigh wave phase velocity maps from ambient noise and earthquake data are inverted jointly with receiver functions observed at 828 stations from the USArray Transportable Array west of 100°W ...longitude for data recorded in the years 2005 through 2010 to produce a 3‐D model of shear wave speeds beneath the central and Western US to a depth of 150 km. Eikonal tomography is applied to ambient noise data to produce about 300,000 Rayleigh wave phase speed curves, and Helmholtz tomography is applied to data following 1550 (Ms > 5.0) earthquakes so that Rayleigh wave dispersion maps are constructed from 8 to 80 s period with associated uncertainties across the region. Harmonic stripping generates back‐azimuth independent receiver functions with uncertainty estimates for each of the stations. A nonlinear Bayesian Monte Carlo method is used to estimate a distribution of shear wave speed (Vs) models beneath each station by jointly interpreting surface wave dispersion and receiver functions and their uncertainties. The assimilation of receiver functions improves the vertical resolution of the model by reducing the range of estimated Moho depths, improving the determination of the shear velocity jump across Moho, and improving the resolution of the depth of anomalies in the uppermost mantle. A great variety of geological and tectonic features are revealed in the 3‐D model that form the basis for future detailed local to regional scale analysis and interpretation.
Surface wave ambient noise tomography and earthquake tomography are performed
Joint inversion of surface wave and receiver functions are applied
Obtain a 3‐D Vsv model of the crust and uppermost mantle for Western US
We study how numerically predicted seismic anisotropy in the upper mantle is affected by several common assumptions about the rheology of the convecting mantle and deformation‐induced lattice ...preferred orientations (LPO) of minerals. We also use these global circulation and texturing models to investigate what bias may be introduced by assumptions about the symmetry of the elastic tensor for anisotropic mineral assemblages. Maps of elasticity tensor statistics are computed to evaluate symmetry simplifications commonly employed in seismological and geodynamic models. We show that most of the anisotropy predicted by our convection‐LPO models is captured by estimates based on a best fitting hexagonal symmetry tensor derived from the full elastic tensors for the computed olivine:enstatite LPOs. However, the commonly employed elliptical approximation does not hold in general. The orientations of the best fitting hexagonal symmetry axes are generally very close to those predicted for finite strain axes. Correlations between hexagonal anisotropy parameters for P and S waves show simple, bilinear relationships. Such relationships can reduce the number of free parameters for seismic inversions if this information is included a priori. The match between our model predictions and observed patterns of anisotropy supports earlier, more idealized studies that assumed laboratory‐derived mineral physics theories and seismic measurements of anisotropy could be applied to study mantle dynamics. The match is evident both in agreement between predicted LPO at selected model sites and that measured in natural samples, and in the global pattern of fast seismic wave propagation directions.
The style of convective force transmission to plates and strain‐localization within and underneath plate boundaries remain debated. To address some of the related issues, we analyze a range of ...deformation indicators in southern California from the surface to the asthenosphere. Present‐day surface strain rates can be inferred from geodesy. At seismogenic crustal depths, stress can be inferred from focal mechanisms and splitting of shear waves from local earthquakes via crack‐dependent seismic velocities. At greater depths, constraints on rock fabrics are obtained from receiver function anisotropy, Pn and P tomography, surface wave tomography, and splitting of SKS and other teleseismic core phases. We construct a synthesis of deformation‐related observations focusing on quantitative comparisons of deformation style. We find consistency with roughly N‐S compression and E‐W extension near the surface and in the asthenospheric mantle. However, all lithospheric anisotropy indicators show deviations from this pattern. Pn fast axes and dipping foliations from receiver functions are fault‐parallel with no localization to fault traces and match post‐Farallon block rotations in the Western Transverse Ranges. Local shear wave splitting orientations deviate from the stress orientations inferred from focal mechanisms in significant portions of the area. We interpret these observations as an indication that lithospheric fabric, developed during Farallon subduction and subsequent extension, has not been completely reset by present‐day transform motion and may influence the current deformation behavior. This provides a new perspective on the timescales of deformation memory and lithosphere‐asthenosphere interactions.
Plain Language Summary
While structural geologists can interpret orientations of rock fabric in exposures at the surface to make inferences on how the rock deformed in the past, geophysical measurements usually only offer snapshots of present‐day conditions below the surface. An exception is offered by several geophysical methods that allow measurements of rock fabric below the surface. Such measurements are sensitive to different depths in the Earth's crust and mantle. We combine existing measurements from southern California to test how deep rock fabric compares to what we can measure in terms of present‐day surface deformation. While the region is currently under strike‐slip deformation, with the Pacific plate sliding horizontally northwestward relative to the North American plate along the San Andreas Fault, markers of deformation at depth do not consistently line up with the present day motion. We infer that deep rock fabric has an imprinted memory from past deformation episodes when the region underwent compression in a subduction zone and extension in subsequent episodes. This deep rock memory persists to this date and may influence how the region deforms currently.
Key Points
Seismicity, geodesy, and seismic anisotropy provide depth‐dependent deformation markers for southern California crust and mantle
Surface velocity gradients, coseismic strain, and SKS splitting are all broadly consistent with N‐S compression/E‐W extension
Shallow splitting, receiver functions, P, and Pn suggest fabric inherited from past tectonic episodes dominates in the lithosphere
SUMMARY
Observations of seismic anisotropy in continental crust are of primary importance to understanding deep crustal structure and dynamics. While many crustal minerals are elastically ...anisotropic, seismic studies commonly assume that only the most anisotropic minerals (e.g. micas) are responsible for the observed anisotropic signal while making the simplifying assumption that potential interaction of multiple mineral phases is insignificant. This study explores the interference effects between mica and quartz on the calculated response of compressional and shear waves in high‐strain tectonites (mylonites). Sample texture and mineralogy were evaluated via electron backscatter diffraction and the calculation of seismic response was carried out over a range of mica to quartz ratios. Both natural and synthetic crystallographic orientation data, which mimic quartz and mica orientations under increasing temperature in high, non‐coaxial strain settings, were evaluated. Not surprisingly, the results show that adding quartz to micaceous lithologies significantly reduces the magnitude of seismic anisotropy. However, the study also highlights the mutually destructive nature of this relationship and suggests a critical threshold proportion between the two phases across which the symmetry of the end‐member induced anisotropies becomes significantly altered, a result that may have significant implications for studies interpreting crustal dynamics from seismic anisotropy.
Through the Alaska Transportable Array deployment of over 200 stations, we create a 3‐D tomographic model of Alaska with sensitivity ranging from the near surface (<1 km) into the upper mantle ...(~140 km). We perform a Markov chain Monte Carlo joint inversion of Rayleigh wave ellipticity and phase velocities, from both ambient noise and earthquake measurements, along with receiver functions to create a shear wave velocity model. We also use a follow‐up phase velocity inversion to resolve interstation structure. By comparing our results to previous tomography, geology, and geophysical studies we are able to validate our findings and connect localized near‐surface studies with deeper, regional models. Specifically, we are able to resolve shallow basins, including the Copper River, Cook Inlet, Yukon Flats, Nenana, and a variety of other shallower basins. Additionally, we gain insight on the interaction between the upper mantle wedge, asthenosphere, and active and nonactive volcanism along the Aleutians and Denali volcanic gap, respectively. We observe thicker crust beneath the Brooks Range and south of the Denali fault within the Wrangellia Composite Terrane and thinner crust in the Yukon Composite Terrane in interior Alaska. We also gain new perspective on the Wrangell Volcanic Field and its interaction between surrounding asthenosphere and the Yakutat Terrane.
Key Points
We present a 3‐D shear velocity model of Alaska with constraint from near surface, via Rayleigh ellipticity (H/V), to upper mantle
We explore interaction of upper mantle wedge and asthenosphere and impact on volcanism in the Aleutians and the Denali volcanic gap
Basin structure throughout Alaska, including Colville Basin and Cook Inlet, is consistent with previous geophysical studies
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