The structure of the Archaean crust of the North America has been studied based on the synthesis of geological and geophysical data, including seismic sections along LITHOPROBE Geotransects, magnetic and gravity anomaly maps, and seismic tomography data. The authors rely on the experience gained in the Russian Program of the deep geological and geophysical studies of the East European Craton. The juvenile Neoarchaean crust, containing the fragments of reworked Meso-and Paleoarchaean rocks, forms an asymmetric round-oval-shaped domain, wherein the geophysical, structural, and metamorphic parameters display a concentric zoning pattern. The Central zone occupies the Hudson Bay basin. The Internal zone (the northeastern and northern Superior Province) is mainly composed of the granulite facies of metaplutonic, metavolcanic and metasedimentary rocks. The External zone encompasses the southern Superior Province together with Hearne and Rae Provinces. This paper presents 3D crustal models of southern Superior Province. The crust development resulted from rifting and a partial disruption of the continental crust, short-term opening of the linear oceans, successive northward subduction and accretion of the ancient continental and juvenile Neoarchaean oceanic and island-arc terranes between ~2.78 and ~2.70 Ga. Subsequent events in the epicontinental environment, including formation of the metasedimentary belts, granulite facies metamorphism and intense ore formation processes, took place within the range from ~2.71 to ~2.63 Ga. The SCLM morphology within the limits of the Archaean North American Craton can be represented as a flattened overturned cone with a vertical axis (down to a depth of ~350 km). The Hudson Bay basin is located right above the lithospheric keel. A number of the main features of the structure and evolution of the Archaean crust of the North American Craton, primarily the ovalconcentric zoning, the important role of high-temperature magmatic and metamorphic processes and mainly intracontinental magmatism and sedimentation, indicates the leading role of the mantle-plume type processes. The Neoarchaean evolution of the North American craton represents the plate-tectonic processes initiated by a superplume. The Neoarchaean North American Craton is one of a series of similar phenomena that occurred ~2.75 Ga ago in a number of continental regions. The most important features, repeated to a certain degree in tectonic units of this type, are: (1) synchronous formation between 2.79 and 2.58 Ga; (2) mainly intracontinental development; (3) the prevalence of oval-shaped synformal tectonic structures of different ranks with some form of concentric zoning; (4) high-temperature magmatism (usually with the participation of enderbite-charnockites and gabbro-anorthosites) and metamorphism of the granulite facies; (5) a frequently repeated combination of high-grade (granulite and hightemperature amphibolites facies) and low-or moderate-grade (greenschist and epidote-amphibolite facies) metamorphic rocks; (6) the lower-c...

S U M M A R YAn iterative tomographic inversion scheme is presented for determination of 2-D velocity structure from seismic refraction first-arrival traveltimes. The method is suited to refraction profiles where source/receiver spacings are denser than for conventional profiles. The inversion method is based on an iterative solution of the linearized problem, and allows for determination of continuous velocity variations as well as geometry of subhorizontal interfaces. In each iteration, two-point ray tracing is performed using a shooting method to construct the linear system. The velocity field is defined using triangular cells within which the velocity gradient is constant, allowing analytic calculation of ray paths. Two different inversion techniques are considered, based on distinct linearized formulations of the forward problem. Inversion using a linearized traveltime-velocity Jacobian produced better results than a slowness formulation more akin to common series expansion techniques. Resolution examples reveal horizontal smearing due to ray geometry, drop-off in resolution with depth, as well as the effect of source-receiver geometry and velocity structure on resolution.Inversion examples indicate that a global norm produces solutions closer to the true model than solutions calculated using a smallest perturbation approach, when a good starting model is available. Streak effects caused by inhomogeneous ray coverage and the removal of these effects are demonstrated.

Geological storage of CO 2 that has been captured at large, point source emitters represents a key potential method for reduction of anthropogenic greenhouse gas emissions. However, this technology will only be viable if it can be guaranteed that injected CO 2 will remain trapped in the subsurface for thousands of years or more. A significant issue for storage security is the geomechanical response of the reservoir. Concerns have been raised that geomechanical deformation induced by CO 2 injection will create or reactivate fracture networks in the sealing caprocks, providing a pathway for CO 2 leakage. In this paper, we examine three large-scale sites where CO 2 is injected at rates of ∼1 megatonne/y or more: Sleipner, Weyburn, and In Salah. We compare and contrast the observed geomechanical behavior of each site, with particular focus on the risks to storage security posed by geomechanical deformation. At Sleipner, the large, high-permeability storage aquifer has experienced little pore pressure increase over 15 y of injection, implying little possibility of geomechanical deformation. At Weyburn, 45 y of oil production has depleted pore pressures before increases associated with CO 2 injection. The long history of the field has led to complicated, sometimes nonintuitive geomechanical deformation. At In Salah, injection into the water leg of a gas reservoir has increased pore pressures, leading to uplift and substantial microseismic activity. The differences in the geomechanical responses of these sites emphasize the need for systematic geomechanical appraisal before injection in any potential storage site.carbon sequestration | geomechanics | InSAR | microseismic monitoring C arbon capture and storage (CCS)-where CO 2 is captured at large point source emitters (such as coal-fired power stations) and stored in suitable geological repositories-has been touted as a technology with the potential to achieve dramatic reductions in anthropogenic greenhouse gas emissions (1, 2). However, its success is dependent on the ability of reservoirs to retain CO 2 over long timescales (a minimum of several thousand years). If CCS is to make a significant impact on global emissions, more than 3.5 billion tons of CO 2 per year must be stored (3), which at reservoir conditions will have a volume of ∼30 billion barrels (4).Secure storage of such large volumes of CO 2 requires more than just the availability of the appropriate volumes of pore space. CO 2 is buoyant in comparison with the saline brines that fill the majority of putative storage sites. Therefore, injected CO 2 will rise through porous rocks and return to the surface, unless trapped by impermeable sealing layers (such as shales and evaporites). Preliminary estimates have tended to indicate that, from a volumetric perspective at least, sufficient storage capacity exists for many decades of CO 2 emissions in deep-lying saline aquifers that have suitable sealing capability (5).It is equally important that the integrity of the seal is not compromised by injection activitie...

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The Snowbird tectonic zone (STZ) is a fundamental boundary within Canada's Western Churchill Province, one of the world's largest yet poorly‐known fragments of Archean crust. Geophysical data from a collocated magnetotelluric and teleseismic transect across the northeastern segment of the STZ provide an image of its subsurface geometry and indicate that it may have been previously mislocated. The model suggests that (1) the STZ has played a major role in the Neoarchean assembly and Paleoproterozoic reworking of the western Canadian Shield, (2) it was reactivated in a manner comparable to other crustal‐scale features such as the Kapuskasing zone of the Superior Province, Canada, and the Redbank thrust of the Arunta block, central Australia, and (3) it juxtaposes mantle blocks with contrasting geophysical properties, revealing a lithosphere‐scale overlap of the leading edges of the Rae and Hearne domains. The STZ thus records plate interactions in the Neoarchean comparable in scale with that of modern orogenic belts.

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The formation of oceanic lithosphere along ocean ridges, and the role that crustal magma chambers play in the accretionary process, continue to be fundamental issues in plate tectonics. To address these issues, a multireceiver airgun/oceanbottom seismograph refraction line was shot across the Endeavour segment of the Juan de Fuca Ridge near 48" N, 129" W. 2-D traveltime tomography applied to the data results in a three-layer model for the upper crust. Layer 1 is 250-650m thick, with u1 = 2.5 km s-' and Vzvl = 0.5 s-'. Layer 2 is -800 m thick, u2 = 4.8 km s-' and Vzuz = 1 . 0~~' .Layer 1 and layer 2 probably represent the sequence of extrusives and the transition to layer 3 (u3 = 5.8 km s-l, Vzv3 = 0.5 s-') is associated with the extrusives-dike complex transition. An abrupt velocity transition between layer 1 and layer 2 may be a metamorphic front within the basalt pillows or it may be the depth to which surface fissures penetrate. A low-velocity anomaly (velocities decreased by <0.45 km s-') exists beneath the ridge in layer 2 and upper layer 3. It is interpreted as a zone of increased fracture porosity and/or permeability associated with axial hydrothermal circulation, and correlates reasonably well with a sub-axial reflector. No evidence is found for the existence of a crustal magma chamber in the depth range of 1.5-3.5 km sub-bottom. However, the velocity anomalies observed in layer 3 suggest that crustal temperatures in this layer are elevated by 150-250 "C beneath and to the east of the ridge relative to temperatures west of the ridge.