Crust and Lithospheric Structure - Active Source Studies of Crust and Lithospheric Structure

1.08 - Crust and Lithospheric Structure – Active Source Studies of Crust and Lithospheric Structure

We combine observations of group and phase velocity dispersion of Rayleigh waves, of the waveform of a long‐period PL phase, of Pn and Sn velocities from unreversed refraction profiles using earthquakes, and of teleseismic S‐P travel time residuals to place bounds on the seismic wave velocity structure of the crust and upper mantle under Tibet. From surface wave measurements alone, the Tibetan crustal thickness can be from 55 to 85 km, with corresponding uppermost mantle shear wave velocities of about 4.4 to 4.9 km/s, respectively. The Pn and Sn velocities were determined to be 8.12±0.06 and 4.8±0.1 km/s, respectively, using travel time data at Lhasa from earthquakes in and on the margins of Tibet. Combining these results, the crustal thickness is most likely to be between 65–80 km with an average shear wave velocity in the upper crust less than 3.5 km/s. A synthesis of one PL waveform does not provide an additional constraint on the velocity structure but is compatible with the range of models given above. In contrast to observations obtained for eight earthquakes in the Himalaya, measurements of both teleseismic S and P wave arrival times for nine earthquakes within Tibet show unusually large intervals between P and S compared with the Jeffreys‐Bullen Tables. Thus the Pn and Sn velocities apparently do not reflect high velocities in the mantle to a great depth beneath Tibet. From the dependence of the seismic velocities of olivine on pressure and temperature and from the similarity of the measured Pn and Sn velocities beneath Tibet and beneath shields and platforms, the velocities at the Moho beneath Tibet are compatible with the temperature being 250°–300° higher than beneath shields and platforms, i.e., 750°C if the temperature beneath the platforms is close to 500°C. Such a temperature could reach or exceed the solidus of the lower crust. Simple one‐dimensional heat conduction calculations suggest that the volcanic activity could be explained by the recovery of the geotherm maintained by a mantle heat flux of about 0.9 HFU at the base of the crust. If the distribution of radioactive heat production elements were not concentrated at the top of the crust, radioactive heating could also contribute significantly to the recovery of the geotherm and thus lower the required mantle heat flow. Thus the idea of a thickened crust in response to horizontal shortening is compatible both with these data and with these calculations.Appendices are available with entire article on microfiche. Order from the American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009. Document J81‐004; $1.00. Payment must accompany order.

Near-vertical incidence and refraction – wide-angle reflection seismic data, recorded as part of Lithoprobe studies in the Paleoproterozoic–Archean domains of Canada’s Northwest Territories, show remarkable reflections from within the upper mantle. A parallel pair of reflectors imaged by the near-vertical data can be traced from Moho levels (∼33 km) down to ∼70 km depth. In a previous study, the reflectors were interpreted as the top and bottom of an ∼1.8 Ga subducted oceanic crust beneath the Hottah terrane. Further inboard, where the seismic line changes its direction from east–west to nearly north–south, another pair of reflectors extends subhorizontally for about 100 km at ∼70 km depth before dipping downward. The subhorizontal reflectors were not correlated with the dipping slab; instead they were interpreted as a separate feature. However, they roughly coincide with a horizontal interface modeled from wide-angle data by an earlier study. Considering the crooked line acquisition geometry, we re-examined both near-vertical incidence and wide-angle reflection data using 2-dimensional (2-D) and 3-D forward and inverse modeling algorithms. Our results demonstrate that the subhorizontal reflectors are the continuation of the relict subducted slab, which now extends laterally for 300 km. Its base is the source of the wide-angle data. The apparent flattening for the near-vertical data is most likely an artifact of projecting a 3-D geometry onto a 2-D cross section. The shallowly subducted slab probably contributed to the thickening and stabilization of the subcrustal lithosphere below the Wopmay orogen.

The structure and evolution of the lithosphere and underlying mantle control tectonics, magmatism, and natural resource development and distribution. Seismic tomography offers essential information on mantle temperature, the thickness and strength of the lithosphere, and the convection below it. Here, we report on the completion of a series of continent- and plate-scale waveform tomography studies using the automated multimode inversion of surface and S-wave waveforms. Together, the models cover all of the Earth’s continents and their surroundings and reveal a fascinating diversity of structures, while also indicating common mechanisms of lithospheric dynamics and evolution. Each model used global data coverage that was maximised witin the hemisphere centred at the continent. Structural information was extracted from Rayleigh waves and S and multiple S waves on over a million vertical-component-seismogram waveform fits. The effects of errors were minimised by statistical and targeted outlier analyses and the removal of the least mutually consistent data. The models advance the resolution of the imaging compared to the state of the art at the scale of the continents.Cratonic lithosphere on all continents shows complexities and fragmentation. The lithosphere beneath most diamondiferous kimberlites—originally emplaced on thick cratonic lithosphere—is still thick at present. Relatively low seismic velocities at kimberlite locations are indicative of craton-lithosphere thinning and are detected at multiple locations in all continents with known kimberlites. Cenozoic basalts are found exclusively where the lithosphere is observed to be thin. Beneath some of the volcanic regions, low-velocity anomalies extend deep into the mantle, consistent with deep-mantle upwellings feeding the magmatism. Sediment-hosted metal deposits tend to be located near contrasts in the thickness of the lithosphere, including both craton boundaries and other substantial heterogeneities. Intraplate seismicity is controlled by plate-boundary stresses and by the lateral variations of the lithospheric thickness and strength. Areas with relatively thin lithosphere localise deformation and seismicity, provided that sufficient tectonic stress is transmitted into the plate interior.ReferencesChagas de Melo, B, Lebedev, S, Celli, NL, et al., 2025. The lithosphere of South America from seismic tomography: Structure, evolution, and control on tectonics and magmatism, Gondwana Research, GR Focus - Invited Review, 138, 139–167. doi:10.1016/j.gr.2024.10.012.Chua, EL, Lebedev, S. Waveform tomography of the Antarctic Plate, Geophysical Journal International, submitted.Celli, NL, Lebedev, S, Schaeffer, AJ, Gaina, C, 2021. The tilted Iceland Plume and its effect on the North Atlantic evolution and magmatism, Earth and Planetary Science Letters 569, doi:10.1016/j.epsl.2021.117048.Celli, NL, Lebedev, S, Schaeffer, AJ, et al., 2020. The upper mantle beneath the South Atlantic Ocean, South America and Africa from waveform tomography with massive data sets, Geophysical Journal International 221, 178–204. doi:10.1093/gji/ggz574.de Laat, JI, Lebedev, S, Celli, NL, et al., 2023. Structure and evolution of the Australian plate and underlying upper mantle from waveform tomography with massive data sets, Geophysical Journal International 234, 153–189. doi:10.1093/gji/ggad062Dou, H, Xu, Y, Lebedev, S, et al., 2024. The upper mantle beneath Asia from seismic tomography, with inferences for the mechanisms of tectonics, seismicity, and magmatism, Earth-Science Reviews, 255, doi:10.1016/j.earscirev.2024.10484

On the Pacific margin off central Costa Rica, an anomalous lens-shaped zone is located between the overriding plate and the subducting oceanic lithosphere approximately 25 km landward of the deformation front. This feature was previously recognized in reflection seismic data when it was termed 'megalens'. Its origin and seismic velocity structure, however, could not unambiguously be derived from earlier studies. Therefore during RV SONNE cruise SO163, seismic wide-angle data were acquired in 2002 using closely spaced ocean bottom hydrophones and seismometers along two parallel strike and two parallel dip lines above the 'megalens', intersecting on the middle slope. The P-wave velocities and structure of the subducting oceanic Cocos Plate and overriding Caribbean Plate were determined by modelling the wide-angle seismic data in combination with the analysis of coincident reflection seismic data and the use of synthetic seismograms. The margin wedge is defined by high seismic velocities (4.3-6.1 km s(-1)) identified within a wedge-shaped body covered by a slope sediment drape. It is divided into two layers with different velocity gradients. The lower margin wedge is clearly constrained by decreasing velocities trenchward and terminates beneath the middle slope at the location of the 'megalens'. Seismic velocities of the 'megalens' are lower (3.8-4.3 km s(-1)) relative to the margin wedge. We propose that the 'megalens' represents hybrid material composed of subducted sediment and eroded fragments from the base of the upper plate. Upward-migrating overpressured fluids weaken the base of the margin wedge through hydrofracturing, thus causing material transfer from the upper plate to the lower plate. Results from amplitude modelling support that the 'megalens' observed off central Costa Rica is bound by a low-velocity zone documenting fluid drainage from the plate boundary to the upper plate.

Research Article| October 01, 2013 Three‐Dimensional Seismic Velocity Structure and Precise Earthquake Relocations in the Salton Trough, Southern California Guoqing Lin Guoqing Lin Division of Marine Geology and Geophysics, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida 33149 Search for other works by this author on: GSW Google Scholar Author and Article Information Guoqing Lin Division of Marine Geology and Geophysics, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida 33149 Publisher: Seismological Society of America First Online: 14 Jul 2017 Online ISSN: 1943-3573 Print ISSN: 0037-1106 Bulletin of the Seismological Society of America (2013) 103 (5): 2694–2708. https://doi.org/10.1785/0120120286 Article history First Online: 14 Jul 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Guoqing Lin; Three‐Dimensional Seismic Velocity Structure and Precise Earthquake Relocations in the Salton Trough, Southern California. Bulletin of the Seismological Society of America 2013;; 103 (5): 2694–2708. doi: https://doi.org/10.1785/0120120286 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyBulletin of the Seismological Society of America Search Advanced Search Abstract I present a new 3D seismic velocity model and high‐precision earthquake relocations between 1981 and 2010 near the Salton Trough and the San Jacinto fault zone. The simul2000 tomography algorithm is applied to derive the 3D VP and VP/VS models using first arrivals of a composite event data set. The final VP model correlates with geological features at shallow depths and is more consistent with the refraction studies than the starting 3D regional‐scale model. The VP/VS model shows strong variations in the upper crust and is dominated by relatively low values (below 1.7) in the middle crust except for the Salton Trough and the Imperial Valley. The newly resolved model is used to relocate all the seismicity in the study area. I then apply the similar‐event cluster analysis and differential‐time relocation approach based on waveform cross‐correlation data to the 3D relocated events. A dramatic sharpening of seismicity patterns is obtained after using these methods. The new locations are more tightly clustered than the previous catalogs mainly due to the different absolute locations. The depth distribution along a basal surface may suggest the existence of a detachment fault from the north end of the Salton Trough to United States–Mexico border. The fine‐scale velocity structure and precise earthquake relocations will be helpful for investigating the fault geometry, seismicity, stress interaction, and other seismological studies for southern California.Online Material: Figures of model resolution tests, cross‐sectional comparisons of different P‐velocity models, and earthquake depth distributions. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Regional variations of the intrinsic shear wave quality factor Qµ in both the upper crust and upper mantle of continents are large, with values in old, stable cratons exceeding those in tectonically active regions in both depth ranges by as much as an order of magnitude or more. Qµ depends upon frequency, at least near 1 Hz, and that frequency dependence also varies regionally in the upper crust. It is typically low in tectonically active regions and higher in stable regions. Because of the large variations in Qµ from region to region, it is easy to map regional variations of both upper crustal Qµ and Q estimated from the coda of Lg waves (QLgc), even though both measurements may be marked by large uncertainties. Although coda Q of direct body waves may be strongly affected by scattering, QLgc appears to be primarily governed by intrinsic Qµ in the upper crust. Both upper crustal Qµ and QLgc values correlate with the time that has elapsed since the most recent tectonic activity in continental regions. A tomographic image of the variation of QLgc values across Africa shows reduced Q values which correspond to recent tectonic activity in the East African rift system and other regions of Mesozoic or younger age. Reductions of QLgc that correlate with tectonic activity that occurred in the early Paleozoic during the coalescence of the cratons which formed that continent can also be detected. Qµ increases rapidly at midcrustal depths, in a range which appears to coincide with the transition to the plastic lower crust. In the lower crust and upper mantle, Qµ decreases with increasing depth, possibly by progressive unpinning of dislocations with increasing temperature. Observed regional variations in upper mantle Qµ at depths of about 150 km can be explained by differences in temperature alone, but those at crustal depths cannot. Regional variations of Qµ in the upper crust are most easily explained by differences in the density of fluid‐filled fractures in which fluids can move during the propagation of seismic waves. Studies of the regional variation of Qµ and QLgc indicate that crack density is greatest during and immediately following tectonic activity in a region and that it decreases with time. Permeability determinations in deep wells show that fluid movements in those cracks may be largely restricted to zones of crustal fracturing. That situation will produce widely differing values of Q in local studies, depending on the location of the study relative to the fractures. The fluid volume in cracks appears to decrease with time by loss to the surface or by retrograde metamorphism, causing a reduction in the number of open cracks and a concomitant increase in Qµ.

The collection of a range of different seismic data types has greatly improved our understanding of the crustal architecture of Australia's Archaean Yilgarn Craton over the last few years. These seismic data include broadband seismic studies, seismic receiver functions, wide-angle recordings and mine-scale to deep seismic reflection transects. Each data set provides information on the three-dimensional (3D) tectonic model of the Yilgarn Craton from the craton scale through to the mine scale. This paper demonstrates that the integration and rationalisation of these different seismic data sets into a multi-scale 3D geological/seismic model, that can be visualised at once in a single software package, and incorporating all available data sets, significantly enhances this understanding. This enhanced understanding occurred because the integrated 3D model allowed easy and accurate comparison of one result against another, and facilitated the integrated questioning and interrogation across scales and seismic method. As a result, there are feedback questions regarding understanding of the individual seismic data sets themselves, as well as the Yilgarn Craton as a whole. The methodology used, including all the data sets in the model range, had to allow for the wide range of data sets, frequencies and seismic modes. At the craton scale, P-wave, S-wave and surface wave variations constrained the 3D lithospheric velocity model, revealing noticeable large-scale velocity variations within and across the craton. An interesting feature of the data, easily identified in 3D, is the presence of a fast S-wave velocity anomaly (> 4.8 km s − 1 ) within the upper mantle. This velocity anomaly dips east and has a series of step-down offsets that coincide approximately with province and terrane boundaries of the Yilgarn Craton. One-dimensional receiver function profiles show variations in their crustal velocity across the craton. These crustal velocity variations are consistent with the larger-scale geological subdivision of the craton, and provide characteristic profiles for provinces and terranes. The receiver function results and the deep seismic reflection data both agree on the depth to the Moho, and both indicate an increase in Moho depth to the east. The 2D seismic refraction results in the south-west of the craton provide crustal thickness information, an indication of middle and lower crustal compositions, and information regarding the broad-scale architectural framework. At the province- and terrane-scale, the deep seismic reflection data and the mine-scale seismic data provide geometric constraints on crustal architecture, in particular the orientation of the region's fault systems as well as variations in the thickness of the granite–greenstone succession. Integration of the results from wide-angle seismic refraction data coincident with the deep seismic reflection data provided additional constraints on likely upper crustal lithologies. The integrated 3D seismic model implies the dominant geodynamic process involved the development of an orogenic belt that developed with a series of contractional (folding and thrusting) events, separated by equally important extensional events. The seismic reflection data in particular suggests that extensional movement on many shear zones was more common than previously thought. The seismic reflection data suggest that the dominant mineral systems involved deeply sourced fluid flowing up crustal-penetrating shear zones. These deeply sourced fluids were further focussed into sites located above fault-breached domal regions in the upper crust.

Understanding the thermal and compositional structure of the Antarctic lithosphere is fundamental for assessing its tectonic stability, geodynamic evolution, and mantle processes beneath East and West Antarctica. However, interpretations based on single geophysical observables remain highly non-unique due to the coupled effects of temperature and composition on seismic velocity and density. Here we present a multi-physics framework that integrates gravity, seismic velocity, heat flow, and thermodynamic modeling to derive high-resolution density, temperature, and compositional models of the Antarctic lithosphere and lithospheric mantle. We first perform a three-dimensional parallel gravity inversion constrained by seismic shear-wave velocity structure, using a structurally coupled objective function that combines data misfit, model regularization, and Gramian-based structural consistency. Structural similarity between density and velocity is enforced in the mantle, where seismic constraints are strongest, while thermally corrected density relationships are incorporated within the crust. The inversion is accelerated through a matrix-free implementation with CUDA-enabled forward and adjoint modeling and MPI–GPU parallelization, enabling continental-scale imaging at a resolution of 5 km × 5 km. The resulting absolute density model reproduces observed Bouguer gravity anomalies with low residuals and reveals pronounced lateral heterogeneity across Antarctica. Building on these results, we further decouple temperature (T) and composition (Mg#) in the upper mantle through joint simulation of seismic velocity and density. Forward models are constructed using Gibbs free energy minimization with Perple_X, incorporating phase equilibria, anelastic attenuation, and rheological effects. A probabilistic grid-search approach with Monte Carlo uncertainty analysis enables robust estimation of T and Mg# and identification of regions where standard solid-state physics fails to explain observations. Our results indicate a cold, thick, and chemically depleted lithospheric root beneath East Antarctica, consistent with a stable cratonic mantle, while West Antarctica is characterized by elevated temperatures, fertile compositions, and widespread regions exceeding solid-state limits, suggesting active asthenospheric upwelling and possible decompression melting beneath the West Antarctic Rift System. This study demonstrates the power of integrated geophysical–thermodynamic approaches for resolving the thermo-compositional state of continental lithosphere.

Two‐dimensional (2‐D) viscoelastic finite element models were used to calculate the time‐dependent changes in Coulomb failure stresses following thrust earthquakes due to respective effects of relaxation of viscous lower crust or upper mantle and postseismic creep on the main fault or its downdip extension. Results suggest that thrust earthquakes cause a coseismic increase in Coulomb stress along antithetic lobes normal to the slip plane. Following a quake, creep processes that reduce stresses in a ductile lower crust or upper mantle are calculated to cause a transfer of stress to the upper crust. Under certain conditions, transfer of stress may lead to a further buildup of high Coulomb stress along the base of the upper crust, potentially shortening the time to failure of other faults in the region. The conditions under which an antithetic lobe of high Coulomb stress are favored to expand at the base of the upper crust postseismically within a few decades include the following: the lower crust or upper mantle has an effective viscosity not greater than 1019 Pa s; the thrust fault has a moderate dip angle (40°–50°); the brittle/ductile transition is deep enough to provide a corridor at the base of the upper crust for expansion; and the crust has a low apparent coefficient of friction (<0.2). Postseismic increases in Coulomb stress within the upper crust may also be caused by aseismic creep on the fault. Stress changes due to this mechanism are maximized with a high apparent coefficient of friction. Analysis of experimentally determined non‐Newtonian flow laws suggests that wet granitic, quartz, and feldspar aggregates may have a viscosity of the order of 1019 Pa s. The calculated rate of stress transfer from a viscous lower crust or upper mantle to the upper crust becomes faster with increasing values of the power law exponent and the presence of a regional compressive strain rate. Results of this 2‐D analysis suggest a potentially important role of viscous flow in controlling time‐dependent postseismic stress changes that warrant further investigation using 3‐D viscoelastic analysis.

Reflectivity characteristics of the crust from modelling refraction/wide-angle and normal-incidence reflection data on BABEL Line 1

From tectonic reconstruction to upper mantle model: An application to the Alpine-Mediterranean region

The Central and East Java region, which is part of the Sunda Arc, has an important role in producing destructive earthquakes and volcanic complexes in Indonesia as a result of the convergence between the Indo-Australian plate that subducts under the Eurasian plate. In this study, the 3-D seismic velocity structure (Vp, Vs, and Vp/Vs) of the crust and upper mantle was determined to reveal the presence of the subducted slab, volcanic sources, and seismogenic features in the Central and East Java region. We have manually re-picked P- and S-arrivals of 1,488 events from January 2009 to September 2017 recorded at 27 stations of the BMKG network. An iterative damped least-squares inversion method was applied to simultaneously calculate both hypocenter relocations and velocity structure beneath this high-risk region to a depth of 200 km. We then compare the tomographic results and seismicity to interpret structural features in the seismic zones. The subducted slab is dipping toward the north, imaged by the high-velocity regions with low Vp/Vs at depths of about 50–100 km. Low-velocity anomalies with high Vp/Vs above the slab at a depth of ∼100 km, imply the possible location of partial melting from slab dehydration. Fluids and melts are ascending to feed the volcanoes i. e., Merapi-Merbabu, Wilis, Pandan, Semeru, Bromo, and Ijen that also have similar low-velocity anomalies at 10–30 km depths, suggesting the presence of the sedimentary basin or magma reservoir. We also have redetermined the hypocenter location of the 2021 (Mw 6.1) Malang earthquake at 8.94oS, 112.45oE, with a depth of 59.7 km. The location error in the x, y, and z directions are 3.08, 6.39, and 11.91 km, respectively. This intraslab event with a thrusting mechanism is located in the high-velocity region and close to the intermediate-depth seismic clusters, which indicates the geometry of the oceanic slab. In the region of 1994 (Mw 7.8) Banyuwangi earthquake, we found a low-velocity anomaly at ∼ 50 km depth that might be associated with the presence of subducting seamount that is more hydrated than the surrounding slab. The slip over this subducting seamount caused the tsunamigenic earthquake.

We present a new model of the lithosphere and asthenosphere structure down to 300 km depth beneath the Pyrenees from the joint inversion of recent gravity and teleseismic data. Unlike previous studies, crustal correction was not applied on teleseismic data in order (i) to preserve the consistency between gravity data, which are mainly sensitive to the density structure of the crust lithosphere, and traveltime data, and (ii) to avoid the introduction of biases resulting from crustal reductions. The density model down to 100 km depth is preferentially used here to discuss the lithospheric structure of the Pyrenees, whereas the asthenospheric structure from 100 to 300 km depth is discussed from our velocity model. The absence of a high density anomaly in our model between 30 and 100 km depth (except the Labourd density anomaly) in the northern part of the Pyrenees seems to preclude eclogitization of the subducted Iberian crust at the scale of the entire Pyrenean range. Local eclogitization of the deep Pyrenean crust beneath the western part of the Axial Zone (west of Andorra) associated with the positive central density anomaly is proposed. The Pyrenean lithosphere in density and velocity models appears segmented from east to west. No clear relation between the along-strike segmentation and mapped major faults is visible in our models. The Pyrenees’ lithosphere segments are associated with different seismicity pattern in the Pyrenees suggesting a possible relation between the deep structure of the Pyrenees and its seismicity in the upper crust. The concentration of earthquakes localized just straight up the central density anomaly can reflect the subsidence and/or delamination of an eclogitized Pyrenean deep root. The velocity model in the asthenosphere is similar to previous studies. The absence of a high-velocity anomaly in the upper mantle and transition zone (i.e. 125 to 225 km depth) seems to preclude the presence of a detached oceanic lithosphere beneath the European lithosphere.

Travel time inversion and amplitude modeling of a 350‐km Lithoprobe seismic refraction/wide‐angle reflection profile determined the velocity structure of the crust and upper mantle along strike in the Omineca Belt of the Canadian Cordillera. The upper crust to 12–18 km depth has velocities from 5.6 to 6.2 km s−1, and two shear zones, the Monashee Décollement and Gwillim Creek Shear Zone, are imaged by the wide‐angle reflections and velocity trends. Minor velocity differences on either side of the Monashee Décollement may be related to separate rock origins. Prominent reflections define the boundaries of a low‐velocity midcrustal layer from 10–15 km to 20–25 km depth with velocities less than 6.1 km s−1. The low velocities of the midcrust, associated with high electrical conductivities and high heat flow, may be considered as support for the hypothesis of fluids in the Cordilleran crust, though other possibilities, such as the effect of high temperatures on rock velocities are possible. In the lower crust velocities range from 6.4–6.5 km s−1 at the top of the lower crust to 6.6–6.8 km s−1 at its base. The Moho is very clearly defined by the refraction/wide‐angle reflection data and has a gentle southerly dip. Crustal thicknesses are 35–37 km. A thin crust‐mantle transition zone of 1–2 km thickness in which velocities vary between 7.6 and 7.7 km s−1 is consistent with coincident reflection data. Upper mantle velocities range from 7.9 to 8.1 km s−1 with indications from the data of upper mantle layering. In comparison with neighboring regions, the Omineca Belt has an anomalously thin crust, low crustal velocities, and a low‐velocity upper mantle, similar only to the Basin and Range province. The velocity structure may partly mirror the temperature profile which has overprinted the geological signature of the region as measured by the seismic refraction method. The characteristics of a thin crust and lithosphere, along with low velocities from midcrust to mantle suggests that both the Basin and Range and the southern Canadian Cordillera are currently being heated from a source within the mantle.