Don White

Images of a lower-crustal oceanic slab: Direct evidence for tectonic accretion in the Archean western Superior province

The Archean western Superior province in Canada is the type area for proposed Archean plate tectonics. Seismic images from this region provide direct evidence for assembly of the craton by terrane accretion and for a large slab of remnant oceanic crust preserved at the base of the crust. This slab, with inferred garnet amphibolite composition, adds a critical piece of evidence to previous suggestions that Archean subduction was at a shallow angle and that some Neoarchean tonalite-trondhjemite-granodiorite suites, distinct from most modern-day suprasubduction magmas, are melts primarily derived directly from subducted slabs.

Structure of a Paleoproterozoic continent-continent collision zone: a LITHOPROBE seismic reflection profile across the Trans-Hudson Orogen, Canada

Abstract An ~ 800 km reflection seismic profile across the Trans-Hudson Orogen, northern Saskatchewan and Manitoba, images crustal-scale tectonic imbrication in an unprecedented picture of Paleoproterozoic crustal accretion and continent-continent collisional tectonism. The profile is crudely symmetric about a crustal-scale culmination in the western part of an accreted juvenile collage (Reindeer Zone). Geologic and isotopic data suggest that this culmination is cored by microcontinental Archean basement. West of the culmination, highly reflective juvenile crustal elements dip westward into the lower crust, beneath the Wathaman Batholith and Archean continental crust of Hearne craton. To the east, strong reflections in the juvenile Reindeer Zone crust and reworked Archean foreland of the Thompson belt have eastward dips persisting into the middle crust and extending beneath the Superior craton. A continuous reflection Moho, well-defined for > 500 km in the western part of the profile, shows marked relief (12- > 15 s), including a prominent root below the crustal culmination. These imaged structures give evidence of substantial crustal shortening and thickening via large-scale imbrication consistent with collisional orogeny. W-dipping structures below the Wathaman Batholith and reworked Hearne craton may reflect subduction polarity in this part of the orogen. However, geological evidence suggests that E-dipping structures below Superior craton are largely related to late/post-collisional deformation, rather than to prior oceanic subduction polarity.

Comparison of geomechanical deformation induced by megatonne-scale CO2 storage at Sleipner, Weyburn, and In Salah

Significance The economic and political viability of carbon capture and sequestration (CCS) is dependent on the secure storage of CO2 in subsurface geologic reservoirs. A key leakage risk is that posed by geomechanical deformation generating fractures in otherwise sealing caprocks. This study examines this risk, comparing and contrasting deformation induced at three large-scale CCS sites—Sleipner (Norwegian North Sea), Weyburn (Canada), and In Salah (Algeria). These sites show very different geomechanical responses, highlighting the importance of systematic geomechanical appraisal prior to injection, and comprehensive, multifaceted monitoring during injection at any future large-scale CCS operations. Geological storage of CO2 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 CO2 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 CO2 injection will create or reactivate fracture networks in the sealing caprocks, providing a pathway for CO2 leakage. In this paper, we examine three large-scale sites where CO2 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 CO2 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.

Lithospheric assembly and modification of the SE Canadian Shield: Abitibi-Grenville teleseismic experiment

This paper presents the results of a joint Lithoprobe-Incorporated Research Institutions for Seismology (IRIS)/Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) teleseismic experiment that investigates portions of the Grenville and Superior Provinces of the Canadian Shield along the Quebec-Ontario border. Data from a 600-km-long, N-S array of 28 broadband seismographs deployed between May and October 1996 have been supplemented with additional recordings from an earlier 1994 deployment and from stations of the Canadian National Seismograph Network and the Southern Ontario Seismic Network. Relative delay times of P and S waves from 123 and 40 teleseismic events, respectively, have been inverted for velocity perturbations in the upper mantle and reveal a low-velocity, NW-SE striking corridor that crosses the southern portion of the line at latitude 46°N and lies between 50 and 300 km depth. Multievent S K S-splitting results yield an average delay time of 0.57±0.22 s and a direction of fast polarization of N93°E±18°, which is consistent with an earlier interpretation as being due to fossil strain fields related to the last major regional tectonic event. Subtle variations in splitting parameters over the low-velocity corridor may suggest an associated disruption in mantle fabric. Profiling of radial receiver functions reveals large and abrupt variations in Moho topography, specifically, a gradual thickening in crust from 40 to 45 km between latitudes 45°N and 46°N, which is followed by an abrupt thinning to 35 km at 46.6°N, some 65 km southeast of the Grenville Front. This structure is interpreted as a subduction suture extending the full length of the Front and punctuating a major pre-Grenvillian (Archean-Proterozoic) episode of lithospheric assembly in the southeast Canadian Shield. The low-velocity mantle corridor, by contrast, is better explained as the extension of the Monteregian-White Mountain-New England seamount hotspot track below the craton and is here postulated to represent interaction of the Great Meteor plume with zones of weakness within the craton developed during earlier rifting episodes.

Paleoproterozoic collisional orogen beneath the western Canada sedimentary basin imaged by Lithoprobe crustal seismic-reflection data

Exceptionally clear images of crustal structure of the Canadian Shield that underlies the western Canada sedimentary basin beneath 3.5–2.2 km of Phanerozoic sedimentary strata have been obtained on a seismic-reflection profile acquired by Lithoprobe. The profile crosses tectonic domains of central Alberta and delineates a major buried orogenic belt of Paleoproterozoic (∼1.8 Ga) age associated with crustal scale thrust imbrication and deflections in the crust-mantle boundary. Available geochronologic data suggest that crustal imbrication observed in the Alberta basement was coeval with that documented in the Trans-Hudson orogen to the east (1.80–1.83 Ga) and implies that a large region of continental crust, extending >1000 km from the western Superior province to the Snowbird tectonic zone, underwent considerable shortening during assembly of this part of the Canadian Shield.

Monitoring CO2 storage during EOR at the Weyburn-Midale Field

Carbon dioxide ( CO2 ) and hydrocarbons both occur as natural accumulations within the Earth and share an intertwined industrial history. Research in the 1950–1960s demonstrated that CO2 had potential as a miscible agent for enhanced oil recovery (EOR), but commercial-scale implementation of this method was limited by the availability of a large supply of inexpensive CO2 . By the mid-1980s, CO2 from large natural occurrences in the southwestern U.S. was being transported by pipeline to oil fields in the Permian Basin of Texas, which allowed deployment of CO2 flooding as a large-scale tertiary EOR method. It is estimated that tertiary oil recovery using CO2 could add as much as 13 billion barrels to existing recoverable resources in the U.S. (USDOE, 2002).

Three-dimensional collisional structure of the Trans-Hudson Orogen, Canada

Abstract The three-dimensional structure of the eastern Trans-Hudson Orogen (THO), part of a Paleoproterozoic continent-continent collision zone in central North America, is revealed through a network of LITHOPROBE seismic reflection profiles. The seismic images are interpreted to delineate a series of stacked thrust sheets essentially confined to the crust. E-W profiles show strong, E-dipping reflections extending throughout the crust while N-S profiles record events outlining antiformal and synformal structures. This allows the identification of decollements that may have localized along pre-existing structures (e.g. possible basin-bounding extensional faults) and at major rheological boundaries (e.g. basement-cover contact, upper-middle crust transition). The present topographic surface displays oblique crustal sections with 10–15 km of structural relief, generated during post-collisional, intracontinental transpression of THO as a result of crustal-scale folding and faulting.

How the crust meets the mantle: Lithoprobe perspectives on the Mohorovičić discontinuity and crust–mantle transition

Application of regional geophysical and geological methods throughout two decades of Canada’s Lithoprobe project provides new opportunities to analyze the Mohorovicic discontinuity (Moho) and crust–mantle transition. The transect format employed during Lithoprobe, in which 10 specified regions of Canada were targeted for approximately a decade each, between 1984 and 2003, permitted teams of scientists to focus on geological, geophysical, and tectonic issues for each transect. As a primary objective was to enhance knowledge of the structure of the crust and lithosphere, an obvious target in each transect was the nature and origin of the Moho and crust–mantle transition. Accordingly, the combined results provide new perspectives on the Moho and the relationship of the Moho to the crust–mantle transition. Perhaps the most important result is that the continental geophysical Moho is a deceptively simple feature; it has a variety of signatures at different scales that preclude a single, universally applicable interpretation. In methods that provide large-scale information, such as regional seismic studies, it is a relatively abrupt refraction velocity contrast that often displays a dramatic downward decrease in seismic reflectivity. However, its origin in a geological or tectonic sense is perhaps best determined by careful analyses of structural details near the geophysical Moho, which are complex and varied. In some areas within Canada, it appears that the geophysical Moho may be old and perhaps remains from the time the crust formed; in other areas, it appears to be a relatively young feature that was superimposed onto older crustal fabrics.

Seismic reflection images of high‐angle faults and linked detachments in the Trans‐Hudson Orogen

Postcollisional (1.8–1.7 Ga) intracontinental deformation in the Trans-Hudson Orogen (Canada) produced a series of orogen-parallel high-angle faults and folds. In seismic reflection profiles, the faults are imaged by subvertical zones of diffractions and truncated reflections that extend to 4–8 s (12–24 km). The folded and faulted upper part of the crust is underlain by laterally coherent shallow-dipping reflections that are locally bounded by discrete, highly reflective zones. These zones are interpreted as detachments (shear zones) and can be traced from the upper to lower crust, where some of them appear to pass into laterally continuous reflections that define the Moho. Two distinct regimes of postcollisional crustal deformation are inferred from the seismic images: high-angle faulting and lateral block extrusion in the upper crust and low-angle ductile shearing in the mid/lower crust. The surface geology indicates that the faults resulted in southwest (orogen-parallel) extrusion of the orogens internal zone relative to the bounding Archean Hearne and Superior cratons. Faulting was concurrent with the development of upright folds with trends that are subparallel to the extrusion direction. The seismic images suggest that the high-angle fold/fault structures are kinematically linked to low-angle detachments represented by laterally coherent, highly reflective zones. The detachment shear zones are inferred to have a top-to-the-southwest sense of shear associated with a subhorizontal, northeast-southwest extension direction, parallel to those observed for 1.83–1.80 Ga collisional shear zones exposed in major postcollisional fold culminations. Long-lived orogen-parallel extension is interpreted as a consequence of the boundary conditions imposed by the northeast trend of both the Superior and Hearne margins.

Shear wave splitting observations in the Archean Craton of western Superior

Shear wave splitting observations in the Archean Superior Province of the Canadian Shield show moderate to large delay times (1.1–2.1 s) with azimuths suggesting upper-mantle anisotropy is subparallel to the structural grain of the craton. Two regions with uniform anisotropy azimuth (62°±7° and 87°±3°) are separated by a transitional zone showing strong dependence of observed azimuth on source direction, suggesting that lateral structure is being observed. No splitting was observed at stations near the coast of Hudson Bay, continuing a pattern of weak splitting in surrounding Proterozoic orogenic belts. Current absolute plate motion is consistent with the direction of anisotropy, but would not explain the regional contrasts. Instead, the anisotropy appears to be related to Archean structure and tectonic history.