Fault reconstructions using aeromagnetic data in the Great Bear magmatic zone, Northwest Territories, Canada (original) (raw)
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Journal of Geophysical Research, 1995
The Spences Bridge Group is a mid-Cretaceous (104 Ma) volcanic succession in the southern Intermontane Belt of the Canadian Cordillera (50.5øN,121øW). It comprises the Pimainus Formation (mafic to felsic lava, volcaniclastic and interbedded epiclastic rocks) and the overlying Spius Formation (andesitic lava flows). Including previous work, we have 55 sites distributed among 15 localities representing most of the > 3000 m thickness. Forty-seven sites (286 oriented cores, 457 specimens), mainly andesites, yielded acceptable data. The beds are gently to moderately tilted, partly due to synvolcanic deformation. Samples taken from a sequence of flows at any one locality have, with one exception, well-grouped magnetization directions. Polarities are all normal, as expected for rocks laid down in the Cretaceous Normal Superchron. Declinations always are clockwise of that expected of cratonic North America, indicating 60 ø rotation of the Spences Bridge Group as a whole. However, declinations differ from locality to locality, implying relative interlocality rotations about vertical axes. Hence inclination-only analysis has been used to estimate dispersion, mean inclination, and paleolatitude. Minimum dispersion for the Spius Formation (27 sites) was achieved after 80% untilting, but the changes between 80 and 100% untilting are insignificant, indicating that magnetization was acquired predominantly before tilting. By contrast, minimum dispersion for the Pimainus Formation (20 sites) was achieved after 50% untilting, indicating that magnetization was acquired after synvolcanic tilting when buried beneath the overlying Spius Formation. Polished thin section studies show that magnetite in the Pimainus Formation has undergone extensive low temperature hydrothermal alteration, whereas magnetite in the Spius Formation shows little alteration. Throughout the range of tilt correction, from 0% to 100%, the mean inclinations of both formations were less than expected from observations obtained from mid-Cretaceous rocks of cratonic North America. The best estimate of paleolatitude (from the Spius Formation) is 50.8 ø q-5.0 ø (P=0.05), which is 9.5ø-t-5.7 ø less than would be expected had the rocks had been rigidly attached to North America. This corresponds to displacement from the south of 1100 q-600 km. Displacement between the northern Intermontane Belt and craton probably was accommodated along major strike-slip faults (Northern Rocky Mountain Trench, Finlay, Pinchi etc.). In the south, our results require a major dextral fault (the Intra-Quesnellia fault) to be situated during Late Cretaceous or Paleocene time within or marginal to the Omineca Belt, along which about 1000 km of dextral motion occurred. This could be a southern extension of the Pinchi Fault whose trace is now obscured by Eocene extension and tectonic denudation. The results also indicate that the largest tectonic discontinuity in the Canadian Cordillera occurs not to the east of the Intermontane Belt, as commonly assumed, but to the west, because the displacements relative to cratonic North America observed from the Intermontane Belt are only about one third of those observed from the Coast Belt. Introduction The Canadian Cordillera is divisible into five morphotectonic belts [Wheeler et al., 1991]. The Foreland Belt comprises mainly Precambrian to Mesozoic miogeoclinal strata which were folded and thrust toward the continent in
ASEG Extended Abstracts
New geophysical and geological results shed light on the tectonic history of the Montresor belt, located on the Rae craton of northern Canada -an Archean terrane that has been reworked by four Proterozoic orogenies. In this contribution we use forward modelling of high-resolution aeromagnetic data to explore the 3D geometry and structural history of the Montresor belt, part of the Rae cover sequence. Previously thought to be a simple syncline, our re-analysis of the aeromagnetic data has outlined a set of earlier structures that provide new insight on the deformation history of the belt. Five cross-sections model discrete magnetic-lithologic units truncated by a series of low-angle faults. Reconstruction of the magnetic map features and forward models reveals a pre-fold geometry analogous to foreland fold and thrust belts, produced by D1 deformation during the Trans-Hudson orogeny, bracketed by available geochronology between 1.94 and 1.864 Ga. The Montresor belt rocks have potential for a variety of mineral deposit types, including precious metals in hydrothermal settings, and are under study as part of the Geo-mapping for Energy and Minerals program in Canada.
Geology, 1986
The Tintina trench-northern Rocky Mountain trench (TT-NRMT) fault zone and the Fraser River-Straight Creek (FR-SC) fault zone are separate, en echelon, concentric, smallcircle fault segments of a composite intracontinental transform fault zone more than 2 500 km long that cuts diagonally across the Canadian Cordillera, from the outboard part of a tectonic collage of accreted foreign terranes in the south into the North American preaccretionary continental margin in the north. Most of the 4S0 km of right-hand slip on the TT-NRMT fault zone was transformed southward during the Late Cretaceous and Paleocene into oblique convergence in the southern Canadian Rockies; the remainder, probably com prising less than 100 km, was transformed southwestward during early and middle Eocene time via a zone of distributed shear and east-west crustal stretching into right-hand slip on the en echelon FR-SC fault zone. These interpretations, based on regional systematic mapping of geologic structures, are in conflict with interpretations of paleomagnetic measurements that call for more than 1000 km of post-mid-Cretaceous, right-hand displacement along the general locus of the TT-NRMT fault zone, involving foreign terranes and the parts of North America to which they had been accreted. This paradox must be resolved.
Tectonics, 1999
The Coast Plutonic Complex is the largest magmatic arc of the North American Cordillera, extending from northwestern Washington State to eastern Alaska. It forms the transition between two tectonic domains that are suspected to have undergone several phases of large (several thousands of kilometers) orogen-parallel displacement during the Mesozoic and early Cenozoic. A compilation of fabric data, published isotopic ages, and new structural observations shows that the western Coast Plutonic Complex was affected by subvertical, orogen-parallel, crustal-scale shear zones. These shear zones mainly reflect sinistral transpression and were sequentially active from-110 to 87 Ma during the intrusion of voluminous batholiths. Sinistral shearing was roughly coeval with the development of the thrust belts flanking the Coast Plutonic Complex (between -101 and-85
Canadian Journal of Earth Sciences, 2005
Regionally recognized dextral strike-slip faulting is present in the Monashee complex of the southern Canadian Cordillera but is overprinted and partially obscured by subsequent extension. Eocene brittle faults and fractures within the ThorOdin culmination of the Monashee complex are divisible into three distinct sets. Initial 340°010° trending strike-slip faults (set 1) were locally overprinted and reactivated by normal faults with a 325°020° trend (set 2). A third set of 255°275° trending fractures (set 3) are interpreted as conjugates to set 1, reactivated as transfer faults to the set 2 normal faults. Large regional faults weather recessively, forming topographic lineaments that transect the Monashee complex. The Victor Creek Fault defines one such lineament. Detailed mapping within the northern ThorOdin culmination reveals piercement points (fold hinges) on the east side of the fault that are not readily matched on the west side. The minimum displacement required on the Vi...
… Journal of Earth …, 1998
The 70 Ma Carmacks Group, a subaerial volcanic succession which once covered much of central southwest Yukon, has a paleomagnetic remanent direction which passes the fold test and the reversal test. A new collection of 13 sites, combined with 13 sites from a previous study, renders a pole (088.6°E, 78.4°N, A 95 = 7.8°) which is far-sided with respect to the pole for cratonic North America and implies a displacement from the south of 1900 ± 700 km. Late Triassic Mandanna Member red beds and Early Jurassic Nordenskiöld Formation tuffs, deformed in the Late Jurassic, fail the fold test and conglomerate test but pass a contact test with Eocene dykes. The postdeformational remanent direction is identical to that isolated from the Carmacks Group. The magnetic signature contained in these older formations is probably an overprint produced by an extensive hydrothermal system active during Carmacks extrusion. Geological work indicates that the Carmacks Group is plume related. Given its paleomagnetic latitude and geological nature, we hypothesize that the Carmacks Group is a displaced segment of the Yellowstone hot-spot track, and the hydrothermal system which remagnetized the older rocks was established by mantle upwelling below the region.
Exploration Geophysics, 2019
As part of the Natural Sciences and Engineering Research Council of Canada-Canada Mining Innovation Council (NSERC-CMIC) Mineral Exploration Footprints project, three selected magnetic inversion programs (VPmg, MAG3D and VINV) were used to process the same aeromagnetic data set from the Highland Valley Copper district, British Columbia, Canada. In each case, the inversion was constrained using available geological and physical property constraints. Analysis of magnetic susceptibility data suggests that the observed aeromagnetic anomaly pattern includes effects associated with boundaries between lithological units and fault zone alteration resulting from removal of magnetite. Susceptibility contrast associated with alteration is greater than that associated with changes in lithology. The inversions seek to define the threedimensional geometry of geological boundaries and the fractures are treated as high-frequency noise. Results from the three programs, although similar, are sensitive to attributes of the different algorithms. VPmg emphasises physical boundaries between geological domains, MAG3D produces a more blurred image, whereas VINV produces reasonable geological images. Computer performance using the different programs ranges from reasonable for VPmg to computer intensive for MAG3D and VINV. Differences in the results reflect the inherent uncertainty in producing inversions from "noisy" aeromagnetic data.
Journal of Geophysical Research, 2004
1] Two goals of Lithoprobe's geoscientific studies in the Phanerozoic accretionary cordillera of western North America were to define the subsurface geometries of the terranes and to infer the physical conditions of the crust. These questions were addressed in Canada's southern cordillera a decade ago and have more recently been addressed in the northern cordillera, of which one component of the new studies is magnetotelluric (MT) profiling from ancestral North American rocks to the coast. We present a resistivity cross section, and its interpretation, of the northern cordillera derived from modeling data from 42 MT sites along a 470-km-long NE-SW profile. Beneath the Coast Belt (southwestern end of the profile) a deep crustal low-resistivity layer dips inland; we interpret the crustal part of this conductor as being due to metasedimentary rocks emplaced and metamorphosed during Paleocene Kula plate subduction. A strong lateral transition in lithospheric mantle resistivity exists below the Intermontane Belt that is spatially coincident with changes in chemical and isotopic characteristics of Tertiary to recent alkaline lavas, suggesting that isotopically enriched lithosphere related to the Coast Belt basalts extends partly beneath the Intermontane Belt. The unusually high lower crustal resistivity in the Intermontane and Omineca Belts, similar in value to the resistivity found in the unextended part of central British Columbia, excludes the presence of fluids or conducting metasediments. Finally, our resistivity model displays strong lateral variation of the middle and lower crust between different terranes within the same belt, as a result of the complex structural evolution of the lithosphere.
Brittle faulting in the Canadian Appalachians and the interpretation of reflection seismic data
Journal of Structural Geology, 1995
Brittle faults in the Canadian Appalachians, mostly Late Devonian or younger in age, are divided into two groups on the basis of their movement vectors: a strike-slip group and a dip-slip group. The two groups are subdivided on the basis of orientation and sense of movement. They are interpreted in terms of Palaeozoic transpression and Mesozoic extension during Atlantic opening. The Palaeozoic faults comprise strike-slip and reverse faults which were active in Devono-Carboniferous tiines. Mesozoic strike-slip faults are interpreted as transfer faults and they are coupled with normal faults, many of which developed by reactivation of earlier reverse faults or steeply-dipping surfaces including bedding and earlier foliations. Together these Mesozoic faults comprise an adequate mechanism for crustal extension perpendicular to the orogen. Evidence for listric normal faults is rare in the older rocks of the orogen, and it is suggested that this is due to the adequacy of the deformation mechanisms afforded by the pre-existing planes of weakness.