Wave Speed Structure Research Articles

Seismic imaging of the lithosphere beneath Hudson Bay: Episodic growth of the Laurentian mantle keel

The Hudson Bay basin in northern Canada conceals one of the major collisional zones of the Canadian Shield, the Trans-Hudson Orogen (THO), which marks the Paleoproterozoic collision between the Archean Superior and Western Churchill cratons at ∼1.9–1.8Ga. Improved knowledge of upper mantle structure beneath the region is essential to establish the nature of the THO, specifically whether Himalayan-style plate tectonics operated in Paleoproterozoic times. Detailed seismological constraints on lithospheric architecture are also required to advance our understanding of the mechanism and timing of keel formation.We use surface wave tomography to illuminate new details of the lithospheric architecture of the Hudson Bay region, resolving both seismic wavespeed and azimuthal anisotropy. Phase velocity maps are calculated from fundamental-mode Rayleigh wave dispersion curves, then used to construct a 3D model exploring upper mantle structure to depths of ∼300km. Fast shear wavespeeds suggest a lithospheric thickness varying from ∼180km to almost 280km beneath the Hudson Bay region. The new study confirms previous inferences that there is no correlation between crustal ages and lithospheric thickness.Patterns of shear wavespeed and azimuthal anisotropy indicate a layered lithosphere. In the uppermost mantle, both the highest velocities and the anisotropic fast directions wrap around the Bay. This structure is likely related to the formation processes of the Paleozoic intracratonic basin. At greater depth (∼70–150km) we resolve two high-wavespeed cores separated by a relatively narrow near-vertical lower-velocity curtain. This internal architecture is suggested to result from the terminal phase of a modern-style plate-tectonic collision between the Archean Superior and Churchill cratons during the Trans-Hudson orogeny, entrapping juvenile Proterozoic material.The lower lithosphere (≥160km depth) has a relatively homogeneous wavespeed structure across the region, with distinct patterns of anisotropy closely resembling the subsurface geometry of the THO. We interpret this basal layer as juvenile or reworked material accreted to the base of the existing cratonic lithosphere during or soon after the Trans-Hudson orogeny. The formation of the Laurentian keel thus likely occurred in multiple phases, with a basal layer developing in post-Archean times, during the THO.

P‐wave tomography of eastern North America: Evidence for mantle evolution from Archean to Phanerozoic, and modification during subsequent hot spot tectonism

The unique physical and chemical properties of cratonic lithosphere are thought to be key to its long‐term survival and its resistance to pervasive modification by tectonic processes. Study of mantle structure in southeast Canada and the northeast US offers an excellent opportunity to address this issue because the region spans 3 billion years of Earth history, including Archean formation of the Superior craton and younger accretion of terranes to eastern Laurentia during the Proterozoic Grenville and Phanerozoic Appalachian orogenies. Trending NW–SE through each of these terranes is the track of the Great Meteor hot spot, which affected the region during the Mesozoic. Here we study mantle seismic velocity structure beneath this region of eastern North America using tomographic inversion of teleseismic P‐wave relative arrival‐times recorded by a large‐aperture seismograph network. There are no large‐scale systematic differences between Superior and Grenville mantle wave speed structure, which may suggest that tectonic stabilization of cratons occurred in a similar fashion during the Archean and Proterozoic. Cratonic lithosphere is largely thought to be resistant to modification by hot spot processes, in contrast to younger terranes where lithospheric erosion and significant magmatism are expected. Low velocities beneath the regions affected by the Great Meteor hot spot are broadest beneath the Paleozoic Appalachian terranes, indicating pervasive modification of the lithosphere during magmatism. The zone of modification narrows considerably into the Proterozoic Grenville province before disappearing completely in the Archean Superior craton, where the surface signature of Mesozoic magmatism is limited to kimberlite eruptions.

Relative arrival-time upper-mantle tomography and the elusive background mean

SUMMARY The interpretation of seismic tomographic images of upper-mantle seismic wave speed structure is often a matter of considerable debate because the observations can usually be explained by a range of hypotheses, including variable temperature, composition, anisotropy, and the presence of partial melt. An additional problem, often overlooked in tomographic studies using relative as opposed to absolute arrival-times, is the issue of the resulting velocity model’s zero mean. In shield areas, for example, relative arrival-time analysis strips off a background mean velocity structure that is markedly fast compared to the global average. Conversely, in active areas, the background mean is often markedly slow compared to the global average. Appreciation of this issue is vital when interpreting seismic tomographic images: ‘high’ and ‘low’velocityanomaliesshouldnotnecessarilybeinterpreted,respectively,as‘fast’and‘slow’ compared to ‘normal mantle’. This issue has been discussed in the seismological literature in detail over the years, yet subsequent tomography studies have still fallen into the trap of mis-interpreting their velocity models. I highlight here some recent examples of this and provide a simple strategy to address the problem using constraints from a recent global tomographic model, and insights from catalogues of absolute traveltime anomalies. Consultation of such absolute measures of seismic wave speed should be routine during regional tomographic studies, if only for the benefit of the broader Earth Science community, who readily follow the red = hot and slow, blue = cold and fast rule of thumb when interpreting the images for themselves.

Local Helioseismology of Sunspots: Current Status and Perspectives

Mechanisms of the formation and stability of sunspots are among the longest-standing and intriguing puzzles of solar physics and astrophysics. Sunspots are controlled by subsurface dynamics hidden from direct observations. Recently, substantial progress in our understanding of the physics of the turbulent magnetized plasma in strong-field regions has been made by using numerical simulations and local helioseismology. Both the simulations and helioseismic measurements are extremely challenging, but it becomes clear that the key to understanding the enigma of sunspots is a synergy between models and observations. Recent observations and radiative MHD numerical models have provided a convincing explanation to the Evershed flows in sunspot penumbrae. Also, they lead to the understanding of sunspots as self-organized magnetic structures in the turbulent plasma of the upper convection zone, which are maintained by a large-scale dynamics. Local helioseismic diagnostics of sunspots still have many uncertainties, some of which are discussed in this review. However, there have been significant achievements in resolving these uncertainties, verifying the basic results by new high-resolution observations, testing the helioseismic techniques by numerical simulations, and comparing results obtained by different methods. For instance, a recent analysis of helioseismology data from the Hinode space mission has successfully resolved several uncertainties and concerns (such as the inclined-field and phase-speed filtering effects) that might affect the inferences of the subsurface wave-speed structure of sunspots and the flow pattern. It becomes clear that for the understanding of the phenomenon of sunspots it is important to further improve the helioseismology methods and investigate the whole life cycle of active regions, from magnetic-flux emergence to dissipation.

The effects of 3-D anelasticity (Q) structure on surface wave amplitudes

SUMMARY We investigate the effects of lateral perturbations in velocity and anelasticity (Q) on surface wave amplitudes based upon wave propagation simulations in 3-D earth models using a spectral element method (SEM). We construct 3-D Q models based upon a wave speed model S20RTS using a set of reasonable mineralogical parameters assuming lateral variations in both wave speed and anelasticity are due to temperature perturbations. We measure and compare amplitude perturbations of surface waves caused by 3-D wave speed (elastic) structures and those caused by 3-D anelastic (Q) structures at a period range of 50–200 s. The measurements show that influence of 3-D wave speed structures on amplitudes is comparable to that of 3-D Q structures at short period (50 s), but becomes dominant at longer periods. In ray theoretical framework, surface wave amplitudes can be decomposed into three terms—elastic focusing, anelastic attenuation and anelastic focusing—which depend, respectively, upon the roughness of phase velocity perturbations (), perturbations in anelasticity (δ ln Q−1) and the roughness of perturbations in anelasticity (). Ray-theoretical calculations confirm the relative importance of 3-D Q and 3-D wave speed in perturbing surface wave amplitudes and show that anelastic focusing effects, which have been ignored in present-day tomographic studies, have a more significant effect than attenuation. Therefore, the effects of 3-D Q structures can be ‘counter-intuitive’ at long period for surface waves travelling through a low-Q region may experience amplitude increase.

HMI time-distance pipeline: An overview and data products

The Helioseismic and Magnetic Imager onboard Solar Dynamics Observatory provides uninterrupted high-resolution observations of solar oscillations over the entire disk. This gives a unique opportunity for mapping subsurface flows and wave-speed structures and investigating their role in the Sun's dynamics and magnetic activity on various scales by methods of local helioseismology. A data analysis pipeline for the time-distance helioseismology analysis has been developed and implemented at the SDO Joint Science Operation Center (JSOC) at Stanford. It provides near-real time processing of the helioseismology data. We provide an overview of this pipeline, including the data flow procedures, measurement and inversion codes, and our data products.

Towards physics-based helioseismic inversions of subsurface sunspot structure

AbstractNumerical computations of wave propagation through sunspot-like magnetic field structures are critical to developing and testing methods to deduce the subsurface structure of sunspots and active regions. We show that helioseismic analysis applied to the MHD sunspot simulations of Rempel and collaborators, as well as to translation-invariant models of umbral-like fields, yield wave travel-time measurements in qualitative agreement with those obtained in real sunspots. However, standard inversion methods applied to these data fail to reproduce the true wave-speed structure beneath the surface of the model. Inversion methods which incorporate direct effects of the magnetic field, including mode conversion, may be required.

3D upper mantle structure beneath Japan and its surrounding region from inter-station dispersion measurements of surface waves

Three-dimensional shear wave speed structure of the upper mantle beneath the Japanese Islands and the surrounding regions is constructed from the measurements of inter-station phase speeds of surface waves. We employ the dense broad-band seismic network (F-net), deployed throughout Japan, as well as permanent stations of global seismic network in East Asia. A temporary broad-band seismic network in Far-East Russia, which has been constructed as a part of the Stagnant Slab Project since 2004, is also used in combination with the permanent stations. Using seismic events from 2005 to 2007 with moment magnitude greater than 6.0 and depth shallower than 100km, we measure the phase speeds of the fundamental-mode Rayleigh and Love waves, working with the conventional two-station technique. Phase speed maps of the Japanese Islands and the Sea of Japan are obtained from the inversions of inter-station phase speeds that are measured for a large number of station-pairs. We have collected over 5300 paths for Rayleigh waves and 3800 paths for Love waves. The phase speed maps of Rayleigh waves, in the period range from 25 to 140s, are inverted to obtain an isotropic 3D shear wave speed model in the depth range from 40 to 200km. The isotropic S wave model represents prominent fast wave speed anomalies in northeastern Japan, associated with the subducting Pacific Plate. Fast anomalies of the lithosphere beneath the Sea of Japan are also imaged, implying that the average thickness of lithosphere beneath the Yamato Rise and Japan Basin is about 60±10km. Immediately below this oceanic lithosphere, slow anomalies are found in the asthenosphere beneath the entire region of the Sea of Japan in the depth down to about 200km. In southwestern Japan, the low-angle subduction of the Philippine Sea Plate is mapped clearly, indicating that the northern end of the plate extends beyond the northern coast of the Chugoku region, reaching beneath the Oki Islands in the Sea of Japan. A strong low velocity anomaly off the west coast of Kyushu is found in the depth down to about 130km, which supports the existence of an upwelling flow beneath this area. We also invert the phase speed maps of Rayleigh and Love waves in the period range from 30 to 80s simultaneously, to construct the radially anisotropic shear wave speed model. A remarkable anisotropy with SH>SV is found in the mantle wedge beneath northeastern Japan, while only weak anisotropy is seen beneath the Sea of Japan. Such anisotropic properties, which cannot readily be obtained from body wave studies only, will be an important key to understanding the dynamic processes in this tectonically actively region.

HIGH-RESOLUTION HELIOSEISMIC IMAGING OF SUBSURFACE STRUCTURES AND FLOWS OF A SOLAR ACTIVE REGION OBSERVED BYHINODE

We analyze a solar active region observed by the Hinode CaII H line using the time-distance helioseismology technique, and infer wave-speed perturbation structures and flow fields beneath the active region with a high spatial resolution. The general subsurface wave-speed structure is similar to the previous results obtained from SOHO/MDI observations. The general subsurface flow structure is also similar, and the downward flows beneath the sunspot and the mass circulations around the sunspot are clearly resolved. Below the sunspot, some organized divergent flow cells are observed, and these structures may indicate the existence of mesoscale convective motions. Near the light bridge inside the sunspot, hotter plasma is found beneath, and flows divergent from this area are observed. The Hinode data also allow us to investigate potential uncertainties caused by the use of phase-speed filter for short travel distances. Comparing the measurements with and without the phase-speed filtering, we find out that inside the sunspot, mean acoustic travel times are in basic agreement, but the values are underestimated by a factor of 20-40% inside the sunspot umbra for measurements with the filtering. The initial acoustic tomography results from Hinode show a great potential of using high-resolution observations for probing the internal structure and dynamics of sunspots.

V p and V p/V s structures in the crust and upper mantle of the Taiwan region, China

A tomographic study of the Vp and Vp/Vs structures in the crust and upper mantle beneath the Taiwan region of China is conducted by simultaneous inversion of P and S arrival times. Compared with the previous tomographic results, the spherical finite difference technique is suitable for the strong heterogeneous velocity structure, and may improve the accuracy in the travel time and three-dimensional ray tracing calculations. The Vp and Vp/Vs structures derived from joint inversion and the relocated earthquakes can provide better constraints for analyzing the lateral heterogeneity and deep tectonic characters in the crust and upper mantle. Our tomographic results reveal significant relations between the seismic wavespeed structure and the tectonic characters. In the shallow depth, sedimentary basins and orogen show distinct wavespeed anomalies, with low Vp, high Vp/Vs in basins and high Vp, low Vp/Vs in orogen. As the suture zone of Eurasian Plate and Philippine Sea Plate, Longitudinal Valley is characterized by a significant high Vp/Vs anomaly extending to the middle-lower crust and upper mantle, which reflects the impact of rock cracking, partial melting, and the presence of fluids. In the northeast Taiwan, the Vp, Vp/Vs anomalies and relocated earthquakes depict the subducting Philippine Sea Plate under the Eurasian Plate. The high Vp of oceanic plate and the low Vp, high Vp/Vs atop the subducted oceanic plate extend to 80 km depth. Along the east-west profiles, the thickness of crust reaches 60 km at the east of Central Range with eastward dipping trend, which reveals the eastward subduction of the thickened and deformed crust of the Eurasian continental plate.

Seismic structure of the upper mantle beneath the Philippine Sea from seafloor and land observation: Implications for mantle convection and magma genesis in the Izu–Bonin–Mariana subduction zone

We have determined the three-dimensional shear wave speed structure of the upper mantle in and around the Philippine Sea region using seismograms recorded by dense land-based and long-term broadband ocean bottom seismographic stations. We used a surface wave tomography technique in which multimode phase speeds are measured and inverted for a 3-D shear wave speed structure by incorporating the effects of a finite frequency and ray bending. The new ocean bottom data provided us with improved spatial resolution (~ 300 km) in the Philippine Sea region. In the upper 120 km, the shear wave speed structure is well correlated with seafloor age. At depths greater than 160 km, fast anomalies of the subducting Pacific Plate are clearly defined. We also found slow speed anomalies beneath the Kyushu–Palau ridge at depths greater than 120 km. Along the Izu–Bonin(Ogasawara)–Mariana arc, we have detected three separate slow anomalies in the mantle wedge at depths shallower than 100 km beneath the back arc. Each anomaly has a width of ~ 500 km. Moreover, these three anomalies have a close relationship with the three groups of frontal and rear arc volcanoes having distinct Sr, Nd, and Pb isotope ratios. We suggest that each of the anomalies is a site of large-scale flow of upper mantle into the mantle wedge, and that each already contains a component from the adjacent subducting slab.

Estimation of Shear Wave Speed in Ocean-Bottom Sediment Using Electromagnetic Induction Source

An experiment for studying the shear wave speed in ocean-bottom sediment using Scholte pulse waves was conducted on a shelf in the East China Sea. To generate Scholte pulse waves, we developed a new electromagnetic induction source, which was placed at 100-m depth on the seafloor and was remotely controlled on board through a 1000-m-long cable. A Scholte pulse propagating in sandy sediment was then measured using a couple of geophones placed on the seafloor at a distance of 100 m from the source. The dispersion property of the Scholte waves was obtained by applying a multiple filter technique (MFT) to the observed waves. We then obtained the shear wave speed structure, i.e., the depth dependency of the shear wave speed in the sediment, by applying an inversion method based on a genetic algorithm (GA). The estimated speed was about 100 m/s near the bottom and increased with depth. The power was a close match to the estimated depth-speed profile, as reported previously by Hamilton [E. L. Hamilton, Geophysics, vol. 41, pp. 985-996, 1976].

The P wavespeed structure in the mantle to 800 km depth below the Philippines region: geodynamic implications

P waves from earthquakes south of Taiwan, recorded by the BATS seismic array and CWB seismic network, were used define the P wavespeed structure between depths of 100 and 800 km below the Philippines region. The presence of a low wavespeed zone in the upper mantle is inferred, although the details are unclear. Wavespeeds in the uppermost mantle are low, as expected for seismic energy propagating within an oceanic plate. The estimated depths of the 410- and 660-km discontinuities are 325 and 676 km respectively. The unusually shallow depth of the upper discontinuity below and to the east of Luzon is inferred by clearly resolving the travel-time branch produced by refraction through the transition zone. A possible explanation for the northern part of the region covered is that seismic energy reaches its maximum depth within or close to the cool, subducted oceanic South China Sea slab where subduction has been slow and relatively recent. Further south, however, the presence of a broken remnant of the South China Sea slab, formed during a period of shallower subduction, is suggested at depths below 300 km to explain the broad extent of the elevated 410-km discontinuity. The 660-km discontinuity is slightly deeper than usual, implying that low temperatures persist to lower mantle depths. The wavespeed gradients within the transition zone between depths of 450 and 610 km are higher than those predicted by both the pyrolite and piclogite models of the mantle, possibly due to the presence of water in the transition zone.

Support of high elevation in the southern Basin and Range based on the composition and architecture of the crust in the Basin and Range and Colorado Plateau

To explore the nature of how the structure and physical properties of the crust vary from extended to relatively unextended domains we present teleseismic receiver functions which measure crustal thickness, shear wavespeed structure and the Vp/Vs ratio at 12 seismic stations in eastern Arizona. The crustal thickness is ∼ 28 km, increases slightly eastward, and remains nearly uniform beneath the varying elevations in the southern Basin and Range. The observed Vp/Vs ratio in the Basin and Range (∼ 1.78) exceeds the global average. The southern Colorado Plateau exhibits thicker crust (∼ 40 km) and a slightly greater observed Vp/Vs (∼ 1.81). A discrete region in the Colorado Plateau generates an unusually high Vp/Vs ratio (1.90) and contains low wavespeed zones which serve as evidence of partial melt related to Quaternary volcanism. The metamorphic core complexes in the southern Basin and Range likewise exhibit anomalously high Vp/Vs values (1.79–1.87) and lack locally compensating crustal roots. Density models show that ∼ 85 kg/m 3 lighter crust or ∼ 35 kg/m 3 lighter mantle than that of the surrounding Basin and Range helps these metamorphic core complexes maintain their high elevation. Compositional modeling of intrusive bodies exposed throughout the Catalina–Rincon metamorphic core complex indicates that the observed high Vp/Vs ratio and modeled low density could result from substantial amounts of a plagioclase-rich, quartz-poor rock. These Vp/Vs data are evidence that significant compositional heterogeneity of the crust can occur over a short distance and provide a clue as to how these areas that underwent significant Tertiary extension may have been preconditioned for orogenic collapse.

Three‐dimensional shear wave structure beneath the Philippine Sea from land and ocean bottom broadband seismograms

We obtained three‐dimensional (3‐D) shear wave speed structure beneath the Philippine Sea and the surrounding region from seismograms recorded by land‐based and long‐term broadband ocean bottom seismographic stations. The ocean bottom data gave us a better station coverage to obtain a higher spatial resolution (about 300–400 km) in the Philippine Sea than in previous studies. We employed a new technique of surface wave tomography, in which multimode phase speeds are measured and inverted for a 3‐D shear wave speed structure by incorporating the effects of finite frequency and ray bending. There is a sharp speed contrast along the Izu‐Bonin‐Mariana trench, across which the Philippine Sea side has a significantly slower upper mantle than the Pacific Ocean side. In the upper 120 km, the shear wave speed structure is well correlated with the age of the provinces. At depths greater than 160 km, the pattern is dominated by fast anomalies of the subducted slabs of the Pacific plate and two slow anomalies to the south of the Daito ridge and in the southernmost part of the Philippine Sea.

Shear wave speed structure beneath the South Pacific superswell using broadband data from ocean floor and islands

We determined three‐dimensional shear wave speed structure beneath the South Pacific superswell down to a depth of 200 km by analyzing Rayleigh wave records from broadband ocean bottom seismograph stations and island stations in the Pacific Ocean. The ocean bottom stations were deployed from 2003 to 2005 on the seafloor in the French Polynesian region, which enabled us to study the upper mantle structure beneath the Superswell with unprecedentedly high resolution. We measured the dispersions of fundamental mode Rayleigh waves at periods between 40 and 140 seconds by the two‐station method. We found pronounced slow anomalies near the hot spots and in the Lau Basin. The slow anomalies beneath the hot spots have deeper‐rooted than those associated with the Lau basin. The slow anomalies near the Society, Macdonald, Marquesas, and Pitcairn hot spots continue down to at least 200 km depth.

Upper mantle structure beneath the eastern Colorado Plateau and Rio Grande rift revealed by Bouguer gravity, seismic velocities, and xenolith data

The lithospheric architecture of the Colorado Plateau–Rio Grande rift transition zone reflects a narrow corridor of active tectonism and magmatism between the tectonically stable Colorado Plateau and the stable interior of North America. In this paper we analyze Bouguer gravity data, crustal thickness variations, xenolith data, and seismic wave speed structure to argue that the Rio Grande rift and southeastern Colorado Plateau is underlain by a low‐density upper mantle province that does not trend along upper crustal tectonic boundaries but, rather, is correlated with regions of middle to late Tertiary magmatism. We model uppermost mantle density contrasts using Bouguer gravity along five parallel profiles across the Colorado Plateau and validate our models by converting modeled density contrasts to seismic velocity variations for comparison with regional seismic data, as well as comparing modeled densities to calculated densities from xenoliths. We find that upper crustal features and crustal thickness variations do not explain long‐wavelength (>200 km) variations in Bouguer gravity across the Colorado Plateau–Rio Grande rift. A large 200–440 km wide region of low uppermost mantle density is modeled beneath the eastern Colorado Plateau margin, Jemez Lineament, Rio Grande rift, and parts of the westernmost Great Plains. This region has a density contrast of −90 to −30 kg/m3 relative to the adjacent Colorado Plateau and Great Plains provinces and coincides with previously imaged regions of low upper mantle seismic velocities. In situ densities inferred from xenolith data in the Colorado Plateau support the modeled upper mantle density variations and support the idea that the eastern and western margins of the plateau have been significantly modified by metasomatism. We suggest that the region of low upper mantle density likely reflects some combination of temperature variations, partial melt contents, and compositional variations related to Tertiary magmatic activity and/or thinned lithosphere. The relationship of this zone of modified upper mantle to extension in the Rio Grande rift remains enigmatic, however, particularly given the lack of spatial correlation between the trends of upper mantle structures and surficial extensional structures.

Mantle structure and composition to 800-km depth beneath southern Africa and surrounding oceans from broadband body waves

Average one-dimensional P and S wavespeed models from the surface to depths of 800 km were derived for the southern African region using travel times and waveforms from earthquakes recorded at stations of the Kaapvaal and South African seismic networks. The Herglotz–Wiechert method combined with ray tracing was used to derive a preliminary P wavespeed model, followed by refinements using phase-weighted stacking and synthetic seismograms to yield the final model. Travel times combined with ray tracing were used to derive the S wavespeed model, which was also refined using phase-weighted stacking and synthetic seismograms. The presence of a high wavespeed upper mantle lid in the S model overlying a low wavespeed zone (LWZ) around 210- to ∼345-km depth that is not observed in the P wavespeed model was inferred. The 410-km discontinuity shows similar characteristics to that in other continental regions, but occurs slightly deeper at 420 km. Depletion of iron and/or enrichment in aluminium relative to other regions are the preferred explanation, since the P wavespeeds throughout the transition zone are slightly higher than average. The average S wavespeed structure beneath southern Africa within and below the transition zone is similar to that of the IASP91 model. There is no evidence for discontinuity at 520-km depth. The 660-km discontinuity also appears to be slightly deeper than average (668 km), although the estimated thickness of the transition zone is 248 km, similar to the global average of 241 km. The small size of the 660-km discontinuity for P waves, compared with many other regions, suggests that interpretation of the discontinuity as the transformation of spinel to perovskite and magnesiowüstite may require modification. Alternative explanations include the presence of garnetite-rich material or ilmenite-forming phase transformations above the 660-km discontinuity, and the garnet–perovskite transformation as the discontinuity.

Seismic and mechanical anisotropy and the past and present deformation of the Australian lithosphere

We interpret the three-dimensional seismic wave-speed structure of the Australian upper mantle by comparing its azimuthal anisotropy to estimates of past and present lithospheric deformation. We infer the fossil strain field from the orientation of gravity anomalies relative to topography, bypassing the need to extrapolate crustal measures, and derive the current direction of mantle deformation from present-day plate motion. Our observations provide the depth resolution necessary to distinguish fossil from contemporaneous deformation. The distribution of azimuthal seismic anisotropy is determined from multi-mode surface-wave propagation. Mechanical anisotropy, or the directional variation of isostatic compensation, is a proxy for the fossil strain field and is derived from a spectral coherence analysis of digital gravity and topography data in two wavelength bands. The joint interpretation of seismic and tectonic data resolves a rheological transition in the Australian upper mantle. At depths shallower than ∼150–200 km strong seismic anisotropy forms complex patterns. In this regime the seismic fast axes are at large angles to the directions of principal shortening, defining a mechanically coupled crust–mantle lid deformed by orogenic processes dominated by transpression. Here, seismic anisotropy may be considered ‘frozen’, which suggests that past deformation has left a coherent imprint on much of the lithospheric depth profile. The azimuthal seismic anisotropy below ∼200 km is weaker and preferentially aligned with the direction of the rapid motion of the Indo-Australian plate. The alignment of the fast axes with the direction of present-day absolute plate motion is indicative of deformation by simple shear of a dry olivine mantle. Motion expressed in the hot-spot reference frame matches the seismic observations better than the no-net-rotation reference frame. Thus, seismic anisotropy supports the notion that the hot-spot reference frame is the most physically reasonable. Independently from plate motion models, seismic anisotropy can be used to derive a best-fitting direction of overall mantle shear.

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.