Philip S. Simony

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.

Geometry and pressure-temperature significance of the kyanite-sillimanite isograd in the Mica Creek area, British Columbia

Late Precambrian and Lower Paleozoic metapelitic rocks near Mica Creek, British Columbia, range in metamorphic grade from biotite zone to sillimanite zone. The kyanite-sillimanite isograd was established after phase 2 folding but was deformed by phase 3 folding. Topographic relief of about 2 km, combined with phase 3 folding, permits reasonably precise determination of the geometry of the isogradic surfaces. The effects of phase 3 folding have been accounted for and the isobaric surfaces at the time of metamorphism are inferred to have been dipping gently. Using the kyanite-sillimanite experimental phase diagram, intersection of the isobars and the isograd permits estimates of differences in temperature and pressure along a cross section. These estimates are about 400 bars and 20° C. Mineral geothermometry (garnet-biotite) and geobarometry (garnetplagioclase-Al 2SiO5-quartz) does not provide a fine enough resolution to detect these predicted differences.

Strain and folding mechanisms in a similar style fold from the northern Selkirks of the Canadian Cordillera

Abstract Minor structures and intragranular strain in calcite have been examined in a folded marble unit near Mica Dam, 130 km north of Revelstoke, British Columbia. The fold forms part of a train of polyclinal folds in the core of the major Mica Dam Antiform which formed in a third folding phase, F 3 Metamorphism had reached kyanite grade before F 3 , biotite quartz and calcite were recrystallizing early in F 3 but no longer did so in the later stages. Pressure of 4.0–5.0 kbar and temperatures of 300–400°C are estimated to have prevailed during folding. The fold is an asymmetric, approximately cylindrical antiform with axial surface dipping southwest and hinge plunging southeast at 12°. The parameters t ′ α and T ′ α determined on a right section indicate the fold to be approximately similar. Calc-silicate layers and pegmatite sheets which outline the fold are both buckled and boudine and the adjacent synform is limited by an upper detachment surface. Other F 3 minor folds, related to the Mica Dam Antiform, exhibit the same characteristics and all this indicates that there was a buckle component in the folding history. Using the least-squares strain-gage technique, intragranular twinning strains have been determined for five samples of marble. The maximum extensional strain, ϵ 3 , is subparallel to layering and at a high angle to the fold hinge. All of the samples exhibit a compression strain nearly parallel to the fold hinge. These strains do not reflect the buckling mechanism suggested by field observations. The late-stage three-dimensional strains observed in this fold are not compatible with existing two-dimensional, theoretical models for the origin of similar style folds.