The Georgia Basin in southwest Canada and northwest USA is a Late Cretaceous to Cenozoic forearc basin that transitioned into a forearc depression in the Eocene following uplift of the forearc high (Vancouver Island). Both the tectonostratigraphic transition of the Georgia Basin from a conventional forearc basin to a forearc depression and the CO2 sequestration potential in the basin overall remain poorly understood due to limited subsurface data and complex tectonic histories. This study uses 2D seismic and well data to reconstruct the subsurface architecture of the Whatcom Sub-Basin, which is the southeastern depocenter of the Georgia Basin. The Whatcom Sub-Basin exhibits a wedge-shaped geometry and thickens to the south and west; the thickest strata exceed 7000 m and are located east of the Outer Island Fault and below the Strait of Georgia. Basin architecture is shaped by NE-SW and NW-SE normal faults, including the Outer Island Fault which exhibits 5.9 km of vertical displacement. Four key tectono-stratigraphic units are identified: the lower and upper Nanaimo Group, Huntingdon Formation, and Boundary Bay Formation. These units document episodes of erosion and tectonic reorganization and are separated by basin-margin disconformities that transition to correlative conformities with localized erosion toward the basin center. The Upper Cretaceous Nanaimo Group (both lower and upper) records the initial phase of deposition within a forearc basin and its thickness is controlled by faults and underlying structural highs and lows of the Coast Plutonic Complex. In contrast, the overlying Huntingdon and Boundary Bay formations display more tabular geometries that were deposited in a forearc depression. Both fault density and throw decrease upsection and record the shift to relative tectonic quiescence in the Cenozoic. An eastward reduction in deformation demonstrates asymmetric strain distribution. The evolutionary pathway of the Georgia Basin makes it an exceptional example of forearc depression development, and the architecture of strata therein suggests there is significant potential for CO2 storage. Specifically, the broad extents, consistent thickness and limited faulting of the Huntingdon and Boundary Bay formations suggest both intervals are favourable targets for CO2 storage. Future high-magnitude earthquakes may reactivate faults that crosscut the sedimentary fill; however, CO2 leakage risk may be reduced through mineral trapping and by injecting CO2-water solutions in areas where faulting is minimal. By linking tectonic events to stratigraphic architecture, this study not only provides new insights into the multi-phase evolution of the convergent-margin Georgia Basin, from forearc basin to forearc depression, but also demonstrates how such systems can provide favourable geological conditions for carbon sequestration.

Archean cratons represent stable continental domains which form the nuclei of the Earth's continents due to their thick ( >200 km), mechanically resistant keels. Cratons and their stable roots form through melt and fluid depletion processes. However, metasomatic refertilization may occur due to processes coeval with craton construction and/or overprinting episodes. Magnetotellurics, a geophysical method measuring subsurface electrical resistivity, is sensitive to the compositional and thermal states of the lithosphere and is useful in mapping depleted and refertilized cratonic domains. Here we show the results of a 3D anisotropic inversion to image the lithospheric resistivity structure of the western Superior Craton. The resistivity model reveals widespread (500×300 km2) anisotropy with a north-south conductive axis at depths ~100-200 km, inferred to represent phlogopite-bearing channels emplaced during mantle plume activity. The results have implications for our understanding of the modification and long-term stability of cratonic lithosphere, and the imaging and interpretation of their preserved geophysical signatures.

Tremor is a low-amplitude seismic signal that usually temporally coincides with episodic slow fault slip at plate boundaries worldwide. Since the discovery of tremor in Cascadia, significant effort has been devoted to understanding its relationship to slow slip. However, its source mechanism has been widely debated, owing in large part to the challenge of locating sources accurately in depth. We assemble a tremor catalog of 4,851 events for a ~ 10 X 20 km 2 area on southern Vancouver Island from slow slip episodes in 2003–2005 using a cross-station detection method adapted from previous studies, which we extend to use both P- and S- waves, thereby recovering accurate depths. Tremor occurs in distinct, quasi-planar clusters in the plate boundary region at a depth near 39 km, just beneath a layer of high reflectivity and within a zone of elevated Poisson’s ratio. We interpret this tremor to represent mafic underplating, wherein shearing generates tremor and continuously erodes basaltic material of the upper few hundred meters of the oceanic crust. Comminuted basalt with an increasingly anisotropic fabric is gradually plated onto the overriding lithosphere to form the highly reflective layer. Localized areas of material transfer within the subduction zone may manifest the distinct tremor clusters.

The Paleoarchean East Pilbara Terrane contains some of the oldest rocks on Earth, and has a distinctive surface architecture of granite domes surrounded by arcuate greenstone belts. Explanations for creation of the domes include fold interference, core complex formation and the subsidence of a dense, mafic crust between relatively buoyant granitic complexes. In the latter "sagduction" process, greenstones are often viewed as descending to the base of the crust. Here we present a 3-D inversion of Bouguer gravity data for density contrast constrained by shear wave velocities and a seismic reflection profile. We show that the East Pilbara Terrane is predominantly characterized by a two-layer crust with high-density greenstone rocks largely confined to the upper crust, which is inconsistent with vertical tectonic models requiring large-scale greenstone preservation in the lower crust. The eastern margin of the terrane is marked by a west-to-east transition from low-density ovoid bodies to linear northwest-trending sources associated with the Proterozoic Paterson Orogen. We identify the previously unknown 10-15 km wide Tumbinna Pool Dome, between the Mount Edgar and Yilgalong domes. Higher density sources related to the surface distribution of greenstone rocks largely occur above 10-15 km depth, though they may locally reach >15 km depth on the north side of the Mount Edgar Dome. Some granitic domes have higher density cores related to distinct granite suites, while the Split Rock Supersuite appears to exhibit systematically lower density at depths of 5-10 km. Low-density sources due to small intrusions such as the North Pole Dome only extend to depths up to 10 km, while the large Mount Edgar Dome can be identified at depths as great as 15-20 km. Some low-density bodies in the lower crust, one of which underlies the Yilgalong Dome, correlate with regions of reduced seismic reflectivity, which we interpret to be possible relict migmatisation zones or melt pathways. A weakly constrained ESE-trending high-density region immediately below the Moho correlates with isolated sub-Moho reflectors, and may be a relict zone of rifting and delamination.

A data set consisting of 376 broadband and long-period MT measurements was used to generate the first ever 3D resistivity model of the Archean western Superior Craton. The modeled resistivity structure is compared to coincident seismic reflection data. The observed geophysical signatures are interpreted within the context of the late stages of crustal growth and cratonization of the region via the progressive accretion of terranes against the initial cratonic core. The northern-most terranes comprising the cratonic core exhibit a nearly homogenous highly resistive crust. The lower crust of the southern terranes contains largely continuous low resistivity bands which run subparallel to major terrane boundaries and corresponding fault systems. In some cases, low resistivity features are coincident with dense packages of sub-horizontal to listric reflections within the mid- to lower crust. These resistivity structures are inferred to represent preserved geoelectric signatures of late to post-orogenic magmatic pulses likely related to delamination of locally overthickened crust. Increased mantle heat flow resulted in partial melting of the lower crust and upper mantle and upward migration of CO2-rich melts and fluids through crustal weak zones corresponding to shear and/or suture zones formed during terrane amalgamation. Thermal softening of the mid- to lower crust led to orogenic collapse and reactivation of the crustal shear zones, resulting in formation and interconnection of graphitic films which were preserved within the stable craton. These results have implications for the tectono-magmatic history of the western Superior Craton, as well toward the understanding of the geodynamic regime of the Archean Earth. The present day geophysical signatures of cratons, ancient continents which formed and have remained stable since the Archean roughly 2.5-4.0 billion years ago, may be used to investigate the tectonic processes that were active during their formation. Here, we combine magnetotelluric (MT) and seismic data to investigate the crust and upper-most mantle structure of the western Superior Craton of Canada. A 3D resistivity volume of the region derived from the MT data maps electrically conductive zones within the lower crust which overlap with zones of high seismic reflectivity, and are spatially correlated with major geological terrane boundaries. These signatures are interpreted as representing electrically conductive graphite and/or sulphidic films emplaced within deep crustal shear zones as a result of crustal collapse and magmatic activity during the late stages of construction and stabilization of the Superior Craton. Our results have implications for our understanding of the processes responsible for the formation of these ancient continents. First large-scale 3D resistivity model of the Archean western Superior Craton reveals similar to east-west trending low resistivity features at lower-crustal depths Conductive features coincident with listric to sub-horizontal seismic reflections, suggestive of mid- to lower crustal shear zones Preserved geophysical signatures are indicative of late Archean tectono-magmatic processes

Southwestern Alaska encompasses a group of fault-bounded tectonostratigraphic terranes that were accreted to North America during the Mesozoic and Paleogene. To characterize the offshore extension of these terranes and several significant faults identified onshore, we reprocessed three intersecting multichannel deep seismic reflection profiles totaling ~750 line-km that were shot by the R/V Ewing across part of the inner Bering continental shelf in 1994. Since the uppermost seismic section is often contaminated by high amplitude water layer multiples from the hard and shallow seafloor, the migrated reflection images are supplemented with high-resolution P wave velocity models derived by traveltime tomography of the recorded first-arrivals to depths of up to 2000 m. Additionally, other geophysical datasets such as well logs, ship-board gravity, ship-board magnetics, satellite-altimetry gravity and air-borne magnetics are also incorporated into an integrated regional interpretation. We delineate the offshore extension of the major mapped geological elements, including the Togiak-Tikchik fault, East Kulukak fault, Chilchitna fault, Lake Clarke fault, Togiak terrane, Goodnews terrane, Peninsular terrane, Northern and Southern Kahiltna flysch deposits, and the Regional Suture Zone. We interpret the offshore Togiak-Tikichik fault to be a terrane bounding fault separating the Togiak terrane and Goodnews terrane. We also locate the offshore boundaries of the Regional Suture Zone using satellite gravity anomaly and air-borne magnetic data. Furthermore, we suggest that the sedimentary fill in the graben-like features offshore, as identified by seismic tomographic velocity models, is constituted by the deposits of Northern and Southern Kahiltna flysch.

The Narryer Terrane, which is located on the northwest margin of the Yilgarn Craton, is one of the oldest preserved blocks of continental crust on Earth with rocks as old as 3.73 Ga and detrital zircons up to 4.4 Ga. In 2010‐11, three deep seismic reflection profiles were acquired over the terrane, and interpreted to show that during the Neoarchean the Narryer Terrane was thrust over the northwest edge of the Youanmi Terrane, which represents the core of the Yilgarn Craton, and in turn underthrust from the north by the Glenburgh Terrane during the Paleoproterozoic. We have reprocessed the seismic data to improve the imaging, extract 3D reflector orientations, and determine near‐surface velocity models. We locate the shear zone along which the Narryer Terrane was exhumed and correlate upper crustal faults interpreted in the seismic data with near‐surface faults and shear zones identified in a regional map of the first vertical derivative of the total magnetic field. One >150 km and two ∼50 km long shear zones divide the exposed Narryer Terrane into distinct structural domains that were probably juxtaposed relatively late in the thrusting of the Narryer Terrane over the Youanmi Terrane in the Paleoproterozoic. While most supracrustal rocks occur in the Northern and Southern domains, the Western Domain where magmatic ages are <3.0 Ga, high mass density, consistent with exhumation from the middle‐lower crust, and is made up of four blocks that exhibit differing patterns of seismic reflectivity, indicating they were likely combined prior to their exhumation.

Although magmatic- and metamorphic- derived fluids are widely recognized mineralizing agents, the role of crustal architecture in defining source and sink zones within the middle to lower crust and upper mantle of ancient orogens remains enigmatic. The globally largest and best-preserved Archean greenstone belts lie in the Superior Province, Canada. They provide an ideal location to investigate the influence of igneous construction and subsequent syn-deformational plutonism and metamorphism on the localization of metal-rich melts and fluids throughout the crustal column. Integration of three-dimensional magnetotelluric modelling and seismic reflection sections across the Abitibi subprovince reveals details of a 'whole-of-crust' magmatic and hydrothermal system. East-west low resistivity structures broadly underlie the surface traces of the major deformation zones that are host to significant gold endowment (>200 Moz), while mid-crustal cross trends suggest mineralized fluids flowed along 'pipes' within fault planes. Most low resistivity structures are inferred to represent domains containing interconnected zones of graphite and/or sulfide. These delineate relict mantle source/transit domains and crustal pathways enriched by the flow of magmas or metamorphic fluids genetically related to a late-stage pulse of ore-bearing magmatism, possibly as a result of slab break-off or delamination. Using the combined MT and seismic data, we develop a 3-D crustal-scale model which highlights how evolving orogenic architecture-controlled mass transfer and metallogenic processes developed.

Seismic reflectors in the uppermost mantle, which can indicate past plate tectonic subduction, are exceedingly rare below Archaean cratons, and restricted to the Neoarchaean. Here we present reprocessed seismic reflection profiles from the northwest Archaean Yilgarn Craton and the Palaeoproterozoic Capricorn Orogen of western Australia that reveal the existence of a ~4 km thick south-dipping band of seismic reflectors that extends from the base of the Archaean crust to at least 60 km depth. We interpret these reflectors, which lie south of a ~50 km deep crustal root, as a relict suture zone within the lithosphere. We suggest that the mantle reflectors were created either by subduction of an oceanic plate along the northern edge of the Yilgarn Craton, which started in the Mesoarchaean and produced the rocks in northern Yilgarn greenstone belts that formed in a supra-subduction zone setting, or, alternatively, by underthrusting of continental crust deep into the lithosphere during the Palaeoproterozoic.

The role of melts and magmatic/metamorphic fluids in mineralization processes is well established. However, the role of crustal architecture in defining source and sink zones in the middle to lower crust remains enigmatic. Integration of three dimensional magnetotelluric (MT) modelling and seismic reflection data across the Archean Abitibi greenstone belt of the Superior Province, Canada, reveals a ‘whole-of-crust’ mineralizing system and highlights the controls by crustal architecture on metallogenetic processes. Electrically conductive conduits in an otherwise resistive upper crust are coincident with truncations and offsets of seismic reflections that are mostly interpreted as major brittle-ductile fault zones. The spatial association between these features and low resistivity zones imaged in the 3D models suggest that these zones acted as pathways through which fluids and melts ascended toward the surface. At mid-crustal levels, these ‘conduit’ zones connect to ~50 km long, north-south striking conductors, and are inferred to represent graphite and/or sulphide deposited from cooling fluids. At upper mantle to lower crustal depths, east-west trending conductive zones dominate and display shallow dips. The upper mantle features are broadly coincident with the surface traces of the major deformation zones with which a large proportion of the gold endowment is associated. We suggest that these deep conductors represent interconnected graphitic zones perhaps augmented by sulphides that are relicts from metamorphic fluid and melt emplacement associated primarily with the later stages of regional deformation. Thus, from the combined MT and seismic data, we develop a crustal-scale architectural model that is consistent with existing geological and deformational models, providing constraints on the sources for and signatures of fluid and magma emplacement that resulted in widespread metallogenesis in the Abitibi Subprovince.

The spatial-temporal evolution of intracontinental faults and the forces that drive their style, orientation, and timing are central to understanding tectonic processes. Intracontinental NW-striking dextral faults in the Gabbs Valley–Gillis Ranges (hereafter referred to as the GVGR), Nevada, define a structural domain known as the eastern Central Walker Lane located east of the western margin of the North American plate. To consider how changes in boundary type along the western margin of the North American plate influenced both the initiation and continued dextral fault slip to the present day in the GVGR, we combine our new detailed geologic mapping, structural studies, and 40Ar/39Ar geochronology with published geologic maps to calculate early to middle Miocene dextral fault-slip rates. In the GVGR, Mesozoic basement is nonconformably overlain by a late Oligocene to Miocene sequence dominated by tuffs, lavas, and sedimentary rocks. These rocks are cut and offset by four primary NW-striking dextral faults, from east to west the Petrified Spring, Benton Spring, Gumdrop Hills, and Agai Pah Hills–Indian Head faults. A range of geologic markers, including tuff- and lava-filled paleovalleys, the southern extent of lava flows, and a normal fault, show average dextral offset magnitudes of 9.6 ± 1.1 km, 7.0 ± 1.7 km, 9.7 ± 1.0 km, and 4.9 ± 1.1 km across the four faults, respectively. Cumulative dextral offset across the GVGR is 31.2 ± 2.3 km. Initiation of slip along the Petrified Spring fault is tightly bracketed between 15.99 ± 0.05 Ma and 15.71 ± 0.03 Ma, whereas slip along the other faults initiated after 24.30 ± 0.05 Ma to 20.14 ± 0.26 Ma. Assuming that slip along all four faults initiated at the same time as the Petrified Spring fault yields calculated dextral fault-slip rates of 0.4 ± 0.1–0.6 ± 0.1 mm/yr, 0.4 ± 0.1–0.5 ± 0.1 mm/yr, 0.6 ± 0.1 mm/yr, and 0.3 ± 0.1 mm/yr on the four faults, respectively. Middle Miocene initiation of dextral fault slip across the GVGR overlaps with the onset of normal slip along range-bounding faults in the western Basin and Range to the north and the northern Eastern California shear zone to the south. Based on this spatial-temporal relationship, we propose that dextral fault slip across the GVGR defines a kinematic link or accommodation zone between the two regions of extension. At the time of initiation of dextral slip across the GVGR, the plate-boundary setting to the west was characterized by subduction of the Farallon plate beneath the North American plate. To account for the middle Miocene onset of extension across the Basin and Range and dextral slip in the GVGR, we hypothesize that middle Miocene trench retreat drove westward motion of the Sierra Nevada and behind it, crustal extension across the Basin and Range and NW-dextral shear within the GVGR. During the Pliocene, the plate boundary to the west changed to NW-dextral shear between the Pacific and North American plates, which drove continued dextral slip along the same faults within the GVGR because they were fortuitously aligned subparallel to plate boundary motion.

We present new seismic tomography of the Washington forearc using a suite of manually picked regional earthquake phase arrivals. The recovery of similarly sampled P and S velocity permits the robust calculation of Poisson's ratio throughout the region. The seismological signature of Siletzia, an accreted oceanic plateau that crops out in Washington as the Crescent Formation, is evident in our models as a continuous high Poisson's ratio body that coincides with subsurface structures estimated from potential field maps. Relocated earthquakes preferentially locate in low Poisson's ratio regions in the forearc crust and, in particular, in a diffuse layer located at 15–25 km depth in the crust beneath relatively aseismic Siletzia. Our imaging of the Puget Sound is consistent with previous interpretations of the architecture of major faults and blocks, as evident by distinct Poisson's ratio signatures that distinguish sedimentary basins from mafic rocks of the Crescent Formation. We speculate that seismicity below the Puget Sound is promoted by slab‐derived fluids that are localized beneath Siletzia as a result of intrinsic low vertical permeability.

In subduction zones, landward dipping regions of low shear wave velocity and elevated Poisson’s ratio, which can extend to at least 120 km depth, are interpreted to be all or part of the subducting igneous oceanic crust. This crust is considered to be overpressured, because fluids within it are trapped beneath an impermeable seal along the overlying inter-plate boundary. Here we show that during slow slip on the plate boundary beneath southern Vancouver Island, low frequency earthquakes occur immediately below both the landward dipping region of high Poisson’s ratio and a 6–10 km thick shear zone revealed by seismic reflections. The plate boundary here either corresponds to the low frequency earthquakes or to the anomalous elastic properties in the lower 3–5 km of the shear zone immediately above them. This zone of high Poisson’s ratio, which approximately coincides with an electrically conductive layer, can be explained by slab-derived fluids trapped at near-lithostatic pore pressures.