Sharon D. Carr

Lithoprobe crustal reflection cross section of the southern Canadian Cordillera, 1, Foreland thrust and fold belt to Fraser River Fault

Seismic reflection data from the south central Canadian Cordillera covering the interval from the easternmost metamorphic core complexes near Arrow Lakes to the Fraser River fault system along the Fraser River reveal a highly reflective and complex crust. The base of the crustal reflectivity, interpreted as the reflection Moho, is clearly delineated by a continuous sharp boundary that is essentially planar and slopes uniformly over a distance of 250 km from about 12.0 s in the east to about 10.5 s in the west. This virtual lack of relief at the base of the crust contrasts sharply with surface structures that involve 25 km or more of structural relief. Some of these surface structures can be readily correlated to structures that are outlined by the reflection data and that can be followed into the middle and lower crust. Even though part of this area was subjected to large amounts of Eocene extension, the crust is not divisible into transparent upper and reflective lower layers as it is in parts of the U.S. Cordillera. Three structural culminations, the Monashee complex, the Vernon antiform, and the Central Nicola horst, are interpreted on the basis of the reflection configuration and the surface geological relationships to have formed initially during Jurassic to Eocene compression and then to have been modified and exposed during early and middle Eocene extension. An example of a compressional structure observed on the profiles is the Monashee decollement, which can be traced from the surface westward into the lower crust. Extension is manifested along a variety of normal faults, including the regionally extensive low angle Okanagan Valley-Eagle River fault system, moderately dipping faults such as the Columbia River and Slocan Lake faults, and high-angle faults such as the Quilchena Creek and Coldwater faults. Both Jurassic to Eocene compressional shear zones and early to middle Eocene extensional shear zones are listric into the lower crust or Moho under the Intermontane belt.

Tectonic setting and U‐Pb geochronology of the Early Tertiary Ladybird Leucogranite Suite, Thor‐Odin ‐ Pinnacles Area, Southern Omineca Belt, British Columbia

The Thor-Odin - Pinnacles area is a structural culmination in the Shuswap complex of the southern Omineca Belt of the Canadian Cordillera. It comprises amphibolite-facies rocks that were deformed during Mesozoic-Paleocene compression and were exhumed in the footwalls of Eocene normal faults during crustal extension. The Ladybird leucogranite suite coincides with the extended terrane in the southern Omineca Belt. It is generally restricted to a midcrustal level which lies in the hanging walls of deep-seated thrust faults and the footwalls of extensional faults. Field relationships of the leucogranites and U-Pb geochronology place timing constraints on compressional and extensional shear zones. The last thrust motion on the Monashee decollement occurred in the latest Paleocene, and the shear zone had stopped by 58 Ma. Crustal-scale normal faults were active in the early Eocene, indicating that crustal extension closely followed the compressional regime. Geological and geochronological data are consistent with an anatectic crustal origin for the Ladybird granite. The granites apparently postdate the thermal peak of metamorphism (Carr, 1990) and were generated during the final stages of thrusting, perhaps due to decompression melting as the midcrustal rocks were carried up a thrust ramp and unroofed and/or due to the introduction of hydrous fluids into the system. In situ magma and hot intrusions probably played an important role in the nucleation of extensional shear zones. The extensional regime then facilitated the intrusion of vast late-synkinematic to posttectonic plutons. U-Pb systematics reveal that zircons in high-temperature shear zones may have suffered high-temperature Pb loss, perhaps due to deformation- or fluid-enhanced diffusion, and that monazite systematics from samples from high-grade terranes are complex. Magmatic monazite populations contain crystals of different ages that do not coincide with zircon ages and apparently represent neither a crystallization age nor a cooling age.

The Monashee decollement of the southern Canadian Cordillera: a crustal-scale shear zone linking the Rocky Mountain Foreland belt to lower crust beneath accreted terranes

The Monashee decollement, a crustal-scale shear zone in the hinterland (Omineca belt) of the southern Canadian Cordillera is interpreted as correlating with the sole thrust of the Rocky Mountain Foreland belt. LITHOPROBE seismic reflection data indicate that the shear zone is rooted in the lower crust beneath the Intermontane Superterrane. The decollement is exposed on the margins of the Monashee complex, an elongate domal culmination within the Omineca belt. This complex includes Early Proterozoic gneisses of North American basement. The hangingwall of the decollement consists of deformed and metamorphosed North American continental margin and oceanic accreted terranes of the Selkirk allochthon. Layered reflections observed in LITHOPROBE seismic reflection data correlate with sheared and transposed rocks in both the hangingwall and the footwall of the Monashee decollement; the decollement is imaged as reflections, cutoffs and correlatable reflection segments. Field relations, kinematics and geochronology indicate that the shear zone has experienced a complex history that included protracted easterly directed shear of hangingwall rocks and imbrication of basement rocks in its footwall. U-Pb zircon dating of syn- and post-kinematic leucogranites in the decollement zone demonstrates that the final stages of thrusting occurred in the Late Palaeocene. The cessation of thrusting and the onset of crustal extension in the southern Omineca belt correspond to the end of thrusting in the Foreland belt.

Ductile thrusting versus channel flow in the southeastern Canadian Cordillera: evolution of a coherent crystalline thrust sheet

Abstract The Late Cretaceous Gwillim Creek shear zone (GCSZ) exposed in the core of the Valhalla complex, and located in the hinterland of the southern Canadian Rocky Mountain thrust belt, is a 5–7 km thick, easterly verging, ductile thrust zone. It was active after c. 90 Ma and during anatexis (800°C and 800 MPa), rose eastward in the direction of transport, and its base was refrigerated from below at c. 60 Ma by thrust translation onto a cold footwall. Extensional shear zones are younger than the GCSZ, and there is no evidence of channel flow or ductile extrusion. Instead, a 30 km thick, coherent sheet was translated on the GCSZ, which at depth was linked to the Foreland thrust belt such as to form a composite crystalline thrust sheet. Doming of the Valhalla complex may be related to Eocene thrusting beneath the complex during the last stage of shortening. A channel flow, proposed by others for the region north of the Valhalla complex, could have evolved within the crystalline sheet by activation of lateral transition zones and an upper detachment, but is no wider than 250 km, does not represent the dominant orogenic process and may represent a nascent channel.