Lithospheric-scale geodynamic processes are interconnected with surface processes such as erosion and tectonic denudation, therefore the integration of geological and geophysical data is valuable when developing geodynamic models. One geodynamic problem that has gained worldwide interest is the contrast in lithospheric properties between the hot and thin Canadian Cordillera and cold and thick North American craton. This thesis focuses on characterizing two aspects of the crust in western Canada: 1) ancient surface and near-surface processes using low-temperature (U-Th)/He thermochronology; and 2) modern geothermal gradients using a wavelet analysis of magnetic anomalies. The first study resolves a differential, tectonically-driven Cretaceous exhumation history in the northern Canadian Cordillera. The second study finds that Curie depths (~580 °C) are shallow throughout the Cordillera, averaging ~15 km, compared to the Canadian Shield, averaging ~33 km. Both studies provide useful constraints for modeling the geodynamic evolution of the Canadian Cordillera.

From Introduction: The Canadian Cordilleran Orogenic Belt forms part of the circum-Pacific orogenic zone. It underlies an area of about 1,54 million sq. kilometres, is over 2400 kilometres long and 800 kilometres wide. The region is characteristically mountainous, much of it glaciated and alpine, containing plateaux, trenches, valleys, and fjords. The mountains, in general, rise to elevations between 2100 m and 3600 m above sea level, although Mount Logan in the St. Elias Mountains attains an altitude of 6000 m. The Canadian Cordillera is divided into two dominant orogenic belts: the eastern Columbian Orogenic Belt comprising defonned miogeosynclinal rocks and the western Pacific Orogenic Belt comprising allochthonous eugeosynclinal rocks. The Cordillera is further subdivided into five longitudinal tectonic belts within which rocks are broadly similar in type, age, and history. These belts are, from east to west: the Rocky Mountain Belt, the Omineca Crystalline Belt, the Intermontane Belt, the Coast Plutonic Complex, and the Insular Belt (Wheeler et al., 1972a). The Canadian Cordillera is important in that it contains: one of the world's largest lead-zinc-silver mine, Sullivan; the second-largest molybdenum mine, Endako; one of the most important concentrations of porphyry copper deposits, Highland Valley; Canada's largest tungsten mines, Cantung and Mactung; and Canada's second-largest silver district, Keno Hill (Sutherland Brown et a1., 1971). In addition, it contains several large massive sulphide and lead-zinc deposits.

A major change in seismic velocities between Earth’s crust and mantle is known as the Mohorovicic discontinuity (Moho). The depth of the Moho plays an important role in characterizing the overall structure of the crust and can be related to the tectonic setting of a region. Teleseismic P-wave receiver function techniques can provide estimates of the depth of the Moho and therefore crustal thickness under a broadband station. In this research we are interested in the structure of the crust and mantle across the northern Canadian cordillera, described by various tectonic settings. The teleseismic data recorded by broadband three-component seismic stations are used to perform receiver function analysis to determine the lateral variations of Moho depth under northern Canadian cordillera and map out the crustal thickness under the broadband stations. Based on visual inspection of receiver function results in the region, we find evidence of anisotropy or dipping reflectors in the crustal structure of the northern cordillera observed in back-azimuthal variations of transverse component receiver functions. We further provide a quantitative interpretation of receiver function in terms of anisotropy or dipping structure by decomposing the azimuthal variations of depth migrated receiver functions into back-azimuthal harmonics. This technique can be used to map out the orientation of anisotropy that may be related to cracks and/or rock texture caused by deformation. We resolve the Moho at an average depth of ~35 km along the western profile of the study area. Harmonic decomposition along the study area yields crustal anisotropy at depth 5-20 km, which does not extend in the lower crust. This can be the result of complex deformation at a detachment zone like a quasi-rigid displacement of the upper crust over a lower crust. The detected anisotropy over the study area is not coherent as the slow symmetry directions detected by harmonic decomposition are highly variable.

The integration of low-temperature thermochronometers, including apatite and zircon (U-Th)/He (AHe, ZHe) and apatite fission-track (AFT) methods, allows for a quantification of the thermal history experienced by rocks as they heat and cool through upper crustal temperature regimes (<200°C). Whereas these methods are practical in geologic terranes that have undergone rapid cooling, application to strata with protracted cooling histories is complicated by the enhanced role of grain-specific parameters (volume, chemistry, radiation damage) on the kinetics of helium diffusion and fission track annealing. The effects of these variables are most prevalent in sedimentary samples, where natural variance in detrital accessory mineral populations results in a broad range of diffusion kinetics and great dispersion in corresponding cooling dates. This thesis integrates contemporary thermochronometer diffusion and annealing kinetics to investigate the burial and exhumation history of two natural laboratories. In the Mackenzie Mountains and Plain of the Northwest Territories, long-term radiation damage accumulation in zircon from Neoproterozoic siliciclastic units produces ZHe dates that track Albian to Paleocene burial and exhumation in front of the foreland-propagating fold-thrust belt. For the Phanerozoic stratigraphic section, AFT annealing kinetics are calculated from Devonian and Cretaceous samples, and are incorporated into multi-kinetic AFT modeling. These kinetics also constrain AHe date-radiation damage trends, and when combined allow for an estimation on the magnitude of eroded sediment across regional pre-Albian and post-Paleocene unconformities. Finally, conodont (U-Th)/He data from Anticosti Island, Québec in the Gulf of the St. Lawrence are compared with ZHe, AHe and AFT data to test their utility as a thermochronometer for carbonate basin analysis. These data evince a Mesozoic thermal history previously unattributed to the region. Ultimately, this thesis provides a novel assessment on the ways in which thermochronometer date dispersion can be quantified to assess the thermal evolution of sedimentary basins from burial through to inversion.

The lithosphere of northwestern Canada recorded more than 2.5 Gy of complex tectonic evolution, from the formation of the ancient cores of the continental lithosphere such as the Slave craton to the Phanerozoic Cordilleran orogeny with substantial variations in crust and upper mantle structures that led to the concentration of natural resources (i.e., diamonds in cratons). Present-day northwestern Canada juxtaposes a thin and hot Cordilleran lithosphere to the thick and cold cratonic lithosphere, which has important implications for regional geodynamics. Recently, seismic station coverage has drastically increased across northwestern Canada, allowing the development of seismic tomography models and other passive-source seismic methods at high resolution in order to investigate the tectonic evolution and dynamics of the lithosphere in this region. The P- and S-wave upper mantle structures of northwestern Canada reveal that the distribution of kimberlite fields in the Slave craton correlates with the margin of fast and slow seismic mantle anomalies, which could delineate weak zones in the lithosphere. Based on our tomographic models we identify two high-velocity seismic anomalies straddling the arcuate Cordillera Deformation Front that have controlled its regional deformation, including a newly identified Mackenzie craton characterized by high seismic velocities extending from the lower crust to the upper mantle to the north of the Mackenzie Mountains. Furthermore, our P-wave tomography model shows sharp velocity contrasts beneath the surface trace of the Tintina Fault. Estimates of seismic anisotropy show a progressive rotation of fast-axis directions when approaching the fault zone. Together, they provide seismic evidence for the trans-lithospheric nature of the Tintina Fault. We further propose that the Tintina Fault has chiseled off small pieces of the Laurentian craton between the Late Cretaceous and the Eocene, which would imply that large lithospheric-scale shear zones are able to cut through small pieces of refractory cratonic mantle and transport them over several hundred kilometers.

No description available.

New and existing U–Pb and 40Ar/39Ar geochronological data, and oxygen and hydrogen stable isotope data, are combined with structural and metamorphic data from Thor–Odin, the southern culmination of the Monashee Complex. This leads to a new interpretation of the timing of deformation and metamorphism. Amphibolites in Thor–Odin with hornblende 40Ar/39Ar dates between ~75–70 and ~51 Ma experienced more 18O- and D-depletion than amphibolites with older dates. The younger dates that were previously interpreted as cooling ages, may have resulted from complete or partial Ar loss in the presence of meteoric fluids that were introduced into the rock during extension. <br><br> Monazite crystals in pelitic schist, quartzite and orthogneiss, which have U–Pb ages younger than 40Ar/39Ar hornblende ages in amphibolite in northwest Thor–Odin, may have grown during tension in the presence of fluids. Titanite, xenotime and zircon dates may be interpreted in the same way. Thus, the U–Pb dates that were previously interpreted as representing peak of metamorphism and the hornblende 40Ar/39Ar dates that were previously interpreted as representing cooling ages, may be interpreted as reflecting meteoric fluid penetration of the crust during regional extension. This implies that the age of the thermal peak of metamorphism is older than ~75–70 Ma. Migmatisation in a basement orthogneiss in Thor–Odin occurred at ~1.8 Ga. Dissolution rims are preserved in zircon between ~1.8 Ga domains and 52 Ma overgrowths. Because growth of new zircon (and possibly other U–Pb accessory phases) did not take place, any geological event that occurred during the ~1.8 Ga to 52 Ma time interval is not recorded. Cordilleran deformation and metamorphism may have taken place within that time interval, e.g. in the Middle Jurassic and/or mid- to Late Cretaceous, the time of Cordilleran deformation and metamorphism in the rocks overlying the Monashee Complex. <br><br> The Joss Mountain orthogneiss, west of the Monashee Complex in the Selkirk Allochthon, is dated at 362 +/– 13 Ma. F3 folding in pelitic schist at Joss Mountain is constrained between ~73 and ~70 Ma. Existing structural, metamorphic and geochronological data in, and close to, the Shuswap Metamorphic Complex in the southern Canadian Cordillera are shown to be consistent with a channel flow model.

The Monashee Complex is a structural culmination which exposes rocks from the lowest stratigraphic levels of the Canadian Cordillera. The Monashee Complex is subdivided into two lesser structural culminations; the Frenchman Cap and Thor–Odin culminations. The lithostratigraphic succession of the Thor–Odin Culmination is completely transposed by penetrative isoclinal folds with amplitudes from microscopic (<1 mm) to regional (10’s km). Lower structural levels are occupied by Proterozoic gneisses and migmatites of the Monashee basement assemblage. These are infolded with overlying metasedimentary rocks of the Monashee cover assemblage, which are Proterozoic to possibly Paleozoic in age. The basement and cover assemblages were subsequently intruded by Eocene granitic pegmatite, aplite and lamprophyre dykes. Regional metamorphism of the basement and cover assemblages reached upper amphibolite to lower granulite facies. The northeastern portion of the Thor–Odin Culmination of the Monashee Complex contains a suite of structures and fabrics, which are classified into four sets, based on their interpreted kinematic significance. These are: 1) transposition related structures (DT); 2) open, upright folds (DO); 3) exhumation related structures (DE); and 4) brittle faults (DB). Each successive set of structures exerted a control on the geometry of the next set. The large-scale geometry of the culmination is an interference structure between DT folds, a DE arch and high-strain zones, and a DB brittle horst. Early, DT fold style varies from intrafolial isoclinal “mature” style folds to upright or inclined asymmetric “immature” folds. This continuum of fold styles, along with evidence of anticlockwise rotation (looking down a vertical axis toward the shear plane) of fold axes and lineations is interpreted as being a result of penetrative triclinic non-coaxial flow. DO upright, symmetrical folds overprint early structures and fabrics, but are only preserved at low structural levels in the culmination where the DE coaxial stretching overprint is weak. DE normal shear bands and boudins overprint all earlier structures. A complex high-strain zone, the Thor–Odin High-Strain Zone, outcrops at high structural levels and along the margins of the culmination. The Thor–Odin High- Strain Zone developed as a result of material moving away from the crest of the culmination, outwards toward the flanks. Eocene brittle faults (DB) and fractures within the Thor–Odin Culmination of the Monashee Complex are divisible into three distinct sets. Initial 340–010º trending strikeslip faults (Set 1) were locally overprinted and reactivated by normal faults with a 325– 020º trend (Set 2). A third set of 255–275º 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 Thor–Odin Culmination, reveals piercement points (fold hinges) on the east side of the fault, which are not readily matched on the west side. The minimum displacement required on the Victor Creek Fault to down-drop the fold hinge below the level of exposure on the west side is 1370 m, assuming normal down-to-the west displacement. However, the geometry of the fault is consistent with a Set 1 dextral strike-slip fault. Matching the piercement points in the study area with possible equivalents to the north indicates 55–60 kms of dextral strike-slip displacement. The Monashee Reflection (MR) is a major crustal-scale, cross-cutting reflection appearing on two mutually perpendicular Lithoprobe seismic profiles in the southern Omineca Belt of the Canadian Cordillera. It has previously been interpreted as the downplunge extension of an arched regional ductile thrust fault, the Monashee Décollement, and is described as separating the Monashee Complex from the overlying Selkirk Allochthon. Recent mapping demonstrates that this boundary is not a discrete ductile thrust, but rather transposed and gradational. Overprinting the transition zone is a complex, outward-dipping, normal, structure; the Thor−Odin High-Strain Zone. Three alternative 3-D geometric models have been developed for the MR in order to project the reflection to the surface. The favoured model correlates the surface trace of the Thor−Odin High-Strain Zone with MR. Normal shear sense kinematics are interpreted for the MR based on: 1) the overall geometry and asymptotic relationship between the MR and reflections in the hanging wall and footwall; 2) offset of metamorphic and geochronological gradients, consistent with an extensional zone, rather than with thrust fault interpretation and 3) the cross-cutting nature of the MR is consistent with normal structures throughout the region.

In southern central Yukon Territory, Canada, a northwest-trending 267 Ma ophiolitic assemblage defines the Tummel fault zone (TFZ), which juxtaposes Paleozoic miogeoclinal strata of Cassiar terrane with metavolcanic and metasedimentary rocks of Yukon-Tanana terrane. Basalt, greenstone, and chert occur in the TFZ and are correlated with Slide Mountain terrane. Northeast of the TFZ, pelitic and semipelitic rocks of the Kechika Group are overlain by carbonate of the Askin Group in Cassiar terrane. Southwest of the TFZ, Yukon-Tanana terrane comprises Devonian-Mississippian quartzofeldspathic basement (the Snowcap Complex) overlain by Mississippian clastic and arc-derived rocks of the Drury, Pelmac, and Little Salmon formations. The ca. 105 Ma Glenlyon Batholith and its satellite plutons intrude Cassiar terrane and the TFZ, imposing a contact metamorphic aureole that overprints earlier metamorphic features in rocks of Cassiar and Yukon-Tanana terranes and the TFZ, and indicates pre-105 Ma juxtaposition of these three tectonic assemblages.

In southern central Yukon Territory, Canada, a northwest-trending 267 Ma ophiolitic assemblage defines the Tummel fault zone (TFZ), which juxtaposes Paleozoic miogeoclinal strata of Cassiar terrane with metavolcanic and metasedimentary rocks of Yukon-Tanana terrane. Basalt, greenstone, and chert occur in the TFZ and are correlated with Slide Mountain terrane. Northeast of the TFZ, pelitic and semipelitic rocks of the Kechika Group are overlain by carbonate of the Askin Group in Cassiar terrane. Southwest of the TFZ, Yukon-Tanana terrane comprises Devonian-Mississippian quartzofeldspathic basement (the Snowcap Complex) overlain by Mississippian clastic and arc-derived rocks of the Drury, Pelmac, and Little Salmon formations. The ca. 105 Ma Glenlyon Batholith and its satellite plutons intrude Cassiar terrane and the TFZ, imposing a contact metamorphic aureole that overprints earlier metamorphic features in rocks of Cassiar and Yukon-Tanana terranes and the TFZ, and indicates pre-105 Ma juxtaposition of these three tectonic assemblages.