The Upper Mantle Structure of Northwestern Canada From Teleseismic Body Wave Tomography (original) (raw)
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Sharp mantle transition from cratons to Cordillera in southwestern Canada
Journal of Geophysical Research: Solid Earth, 2015
The Western Canada Sedimentary Basin marks the transition from the old North American continental lithosphere to young accreted terranes. Earlier studies in this region have suggested a large number of intricate basement domains as well as major seismic velocity gradients in the mantle. To investigate the effect of the accretion and subduction on the mantle structure beneath the western margin of the North American Craton, we analyze P-to-S converted waves from upper mantle discontinuities from the Canadian Rockies and Alberta Network, a regional broadband seismic array based in Alberta, Canada. The depths of the 410 and 660 km seismic discontinuities are correlated and, on average, are 9 km and 7 km greater than their respective global estimates. The largest depression is observed beneath the Rocky Mountain foreland belt in southern Alberta, which highlights a steep southward/westward structural gradient from the cratons to Cordillera at lithospheric mantle depths. The severity of the depressions, especially in the southernmost Alberta, may be triggered by diffuse partial melt or increased water content above the 410 km discontinuity. This result is corroborated by locally increased impedance contrast across the 410 km discontinuity and a strongly depressed 660 km discontinuity. A relic Mesozoic slab fragment may be partially responsible for the deep olivine phase boundary at the base of the upper mantle.
Journal of Geophysical Research: Solid Earth, 2014
New magnetotelluric soundings at 64 locations throughout the central Rae craton on mainland Nunavut constrain 2-D resistivity models of the crust and lithospheric mantle beneath three regional transects. Responses determined from colocated broadband and long-period magnetotelluric recording instruments enabled resistivity imaging to depths of > 300 km. Strike analysis and distortion decomposition on all data reveal a regional trend of 45-53°, but locally the geoelectric strike angle varies laterally and with depth. The 2-D models reveal a resistive upper crust to depths of 15-35 km that is underlain by a conductive layer that appears to be discontinuous at or near major mapped geological boundaries. Surface projections of the conductive layer coincide with areas of high grade, Archean metasedimentary rocks. Tectonic burial of these rocks and thickening of the crust occurred during the Paleoproterozoic Arrowsmith (2.3 Ga) and Trans-Hudson orogenies (1.85 Ga). Overall, the uppermost mantle of the Rae craton shows resistivity values that range from 3000 Ω m in the northeast (beneath Baffin Island and the Melville Peninsula) to~10,000 Ω m beneath the central Rae craton, to >50,000 Ω m in the south near the Hearne Domain. Near-vertical zones of reduced resistivity are identified within the uppermost mantle lithosphere that may be related to areas affected by mantle melt or metasomatism associated with emplacement of Hudsonian granites. A regional decrease in resistivities to values of~500 Ω m at depths of 180-220 km, increasing to 300 km near the southern margin of the Rae craton, is interpreted as the lithosphere-asthenosphere boundary.
Lithospheric structure across the craton-Cordilleran transition of northeastern British Columbia
… Journal of Earth …, 2001
The lithospheric structure of the transition from the craton to the Cordillera in northeastern British Columbia is interpreted from inversion of seismic refraction -wide-angle reflection data along a 460-km profile, and from 3-d (3-dimensional) inversion and 2.5-d forward modelling of Bouguer gravity data. The seismic profile extends westward from the sediment-covered edge of cratonic North America across the Foreland and Omineca morphogeological belts to the eastern boundary of accreted terranes, beyond the Tintina Fault. Across the ancient cratonic margin, the resultant models reveal a westward-thickening package of low upper crustal velocities (6.2 km/s and less) and low densities to almost 20 km depth below the Western Canada Sedimentary Basin, overlying a west-facing ramp of higher velocities and densities in the middle and lower crust. These features are inferred to represent passive-margin sediments deposited on the ancient rifted margin during the mid-to-late Proterozoic and early Paleozoic. A wedge-shaped high-velocity (7.3 km/s) crustal layer at the base of the crust beneath the edge of cratonic North America is interpreted to be the result of magmatic underplating during rifting. In the Cordilleran Foreland Belt, high velocities (6.4 km/s) in the upper 5 km of the crust indicate rocks upthrust from the middle crust. A narrow trench of low velocities in the near-surface, which is imaged~20 km to the west of the inferred location of the Tintina Fault, is interpreted to represent the actual location of the fault or a major splay. From east to west, the Moho decreases in depth from~40 km to~34 km below the rifted margin of ancestral North America, then defines a small root at~38 km depth below the high topography and upper crustal velocities of the eastern Foreland Belt, and gradually shallows to~34 km beneath the Omineca belt. An enigmatic laterally heterogeneous upper mantle has anomalously high velocities (up to 8.3 km/s) beneath the Foreland Belt, flanked by regions of low velocities (7.7-7.8 km/s). Results indicate that the location of the Cordilleran deformation front west of the ramped cratonic margin directly affected the tectonic evolution of the region. du Moho diminue d'environ 40 km à 34 km sous l'ancienne marge du craton nord-américain, définie ensuite une racine crustale à environ 38 km de profondeur sous la haute topographie et les hautes vitesses dans la croûte supérieure de la partie orientale du Domaine de l'avant-pays, pour ensuite remonter graduellement jusqu'à environ 34 km sous le Domaine d'Omineca. Un manteau supérieur énigmatique et hétérogène est caractérisé par des vitesses anormalement élevées (jusqu'à 8,3 km/s) sous le Domaine de l'avant-pays flanquées par des régions de basses vitesses (7,7-7,8 km/s). Les résultats indiquent que la position du front de déformation de la Cordillère à l'ouest de la rampe de la marge cratonique a affecté directement l'évolution tectonique de la région.
The teleseismic signature of fossil subduction: Northwestern Canada
Journal of Geophysical Research, 2008
broadband, three-component seismometers were deployed along the MacKenzie-Liard Highway in Canada's Northwest Territories as part of the joint Lithoprobe-IRIS Canada Northwest Experiment (CANOE). These stations traverse a paleo-Proterozoic suture and subduction zone that has been previously documented to mantle depths using seismic reflection profiling. Teleseismic receiver functions computed from 250earthquakesclearlyrevealtheresponseoftheancientsubductionzone.Ontheradialcomponent,thesutureisevidentasadirectconversionfromtheMoho,thedepthofwhichincreasesfrom250 earthquakes clearly reveal the response of the ancient subduction zone. On the radial component, the suture is evident as a direct conversion from the Moho, the depth of which increases from 250earthquakesclearlyrevealtheresponseoftheancientsubductionzone.Ontheradialcomponent,thesutureisevidentasadirectconversionfromtheMoho,thedepthofwhichincreasesfrom30 km to 50kmoverahorizontaldistanceof50 km over a horizontal distance of 50kmoverahorizontaldistanceof70 km before its signature disappears. The structure is still better defined on the transverse component where the Moho appears as the upper boundary of a 10 km thick layer of anisotropy that can be traced from 30 km to at least 90 km depth. The seismic response of this layer is characterized by a frequency dependence that can be modeled by upper and lower boundaries that are discontinuous in material properties and their gradients, respectively. Anisotropy can be characterized by a ±5% variation in shear velocity and hexagonal symmetry with a fast axis that plunges at an oblique angle to the subduction plane. The identification of this structure provides an unambiguous connection between fossil subduction and fine-scale, anisotropic mantle layering. Previous documentation of similar layering below the adjacent Slave province and from a range of Precambrian terranes across the globe provides strong support for the thesis that early cratonic blocks were stabilized through processes of shallow subduction.
Imaging the lithospheric mantle in northwestern Canada with seismic wide-angle reflections
Geophysical Research Letters, 1999
Normal-moveout-corrected and stacked sections of wide-angle reflection data collected in the Archcan Slave province and Paleoproterozoic Wopmay orogen in northwestern Canada image the crust-mantle boundary as a subhorizontal discontinuity at about 32-36 km depth along a 450 km lateral extent, and, for the first time from such data, layering within the lithospheric mantle at depths between 75 and 90 km over a lateral distance of 750 km. These images correspond to Moho and mantle reflections observed on a colocated multichannel seismic (MCS) reflection profile. Structural features and/or the basalt-eclogite phase change, associated with subduction of oceanic and/or delaminated lower crust, are possible sources of the deep reflectivity. One mantle reflection group lies west of the MCS data and east of the Cordilleran deformation front. It could have similar sources or result from the Mesoproterozoic Racklan orogen or Meso-Neoproterozoic rifting of the ancient margin. LITHOPROBE'S SNORCLE (•lave NORthern Cordillera
Upper mantle structure in the Southeastern Canadian Cordillera
Geophysical Research Letters, 1997
A seismic broadside wide-angle experiment in Canada's southeastern Cordillera has revealed an anomalous and unexpected seismic arrival. Analysis of the data shows that the arrival is unlikely to be a crustal or shallow mantle phase or multiple or converted waves. The kinematic characteristics of this arrival dictate that it is a reflection from within, or from the base of, a low velocity zone (< 7.8-8.0 km s -1) situated at a depth of 59 to 62 km that we identify as the top of the asthenosphere. This leads to the conclusion that the base of the lithosphere is at a depth of 50 _+ 2 km in the region. The interpreted shallow asthenosphere bolsters earlier conclusions of a heat source at depth in the southeastern Canadian Cordillera. The interpretation also shows that stratification in the upper mantle is resolvable by current wide-angle seismic experiments.
Peering beneath the Canadian crust
Astronomy & Geophysics, 2016
Recent modification Recent studies have also suggested that cratonic regions can be modified by relatively recent tectonic processes. Modification often involves a subducting plate descending beneath the craton: water expelled from the subducting slab can migrate into the cratonic root and alter rock chemistry (e.g. in southeast Canada, Boyce et al. 2016). Sometimes the rock chemistry can be altered so dramatically that the root weakens and sinks into the
Tectonophysics, 2006
Observations of upper mantle reflectivity at numerous locations around the world have been linked to the presence of a heterogeneous distribution of rock types within a broad layer of the upper mantle. This phenomenon is observed in wide-angle reflection data from Lithoprobe's Alberta Basement Transect [the SAREX and Deep Probe experiments of 1995] and Trans-Hudson Orogen Transect [the THoRE experiment of 1993]. SAREX and Deep Probe image the Archaean lithosphere of the Hearne and Wyoming Provinces, whereas THoRE images the Archaean and Proterozoic lithosphere of the Trans-Hudson Orogen and neighbouring areas. Finite-difference synthetic seismograms are used to constrain the position and physical properties of the reflective layer. SAREX/Deep Probe modelling uses a 2-D visco-elastic finite-difference routine; THoRE modelling uses a pseudospectral algorithm. In both cases, the upper mantle is parameterized in terms of two media. One medium is the background matrix; the other is statistically distributed within the first as a series of elliptical bodies. Such a scheme is suitable for modelling: (1) variations in lithology (e.g., a peridotite matrix with eclogite lenses) or (2) variations in rheology (e.g., lenses of increased strain within a less strained background). The synthetic seismograms show that the properties of heterogeneities in the upper mantle do not change significantly between the two Lithoprobe transects. Beneath the Trans-Hudson Orogen in Saskatchewan, the layer is best modelled to lie at depths between 80 and 150 km. Based on observations from perpendicular profiles, anisotropy of the heterogeneities is inferred. Beneath the Precambrian domains of Alberta, 400 km to the west, upper mantle heterogeneities are modelled to occur between depths of 90 and 140 km. In both cases the heterogeneous bodies within the model have cross-sectional lengths of tens of kilometers, vertical thicknesses less than 1 km, and velocity contrasts from the background of − 0.3 to − 0.4 km/s. Based on consistency with complementary data and other results, the heterogeneous layer is inferred to be part of the continental lithosphere and may have formed through lateral flow or deformation within the upper mantle.
Geophysical Research Letters, 1997
Three seismic refraction/wide-angle reflection profiles were recorded over the Trans-Hudson Orogen. The profiles display high quality coherent seismic signals associated with the Moho and lithospheric mantle to depths in excess of 160 km. As outlined by wide-angle reflections, depth to Moho varies between 40-54 km with a number of well-defined structural reliefs. Immediately beneath the Moho, in the central part of the orogen, an anomalous high-velocity zone (-8.45 km/s) with lateral dimensions of 100 km (north-south) by 130 km (east-west) and maximum thickness of 50 km was delineated by mantle refraction arrivals. This zone may represent a locally preserved remnant of older mantle from micro-continental collisions. Mantle discontinuities with some structural relief are interpreted at average depths of 75 and 158 km. Based on wideangle reflections at offsets beyond 400 km, they bound a layered zone with variable velocities, probably representing imbricated sequences of peridofite and eelogite. We postulate that these unusual features of the upper mantle are the result of lithospheric convergence of bounding Archcan eratons and closure of the intervening ocean basin. HAJNAL ET AL.' MANTLE INVOLVEMENT 1N LITHOSPHERIC COLLISION Observations and Results