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~ 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.

Models of large, hot orogens containing a collage of reworked and accreted terranesThis article is one of a series of papers published in this Special Issue on the themeLithoprobe — parameters, processes, and the evolution of a continent.

“…9g, equivalent to GO-3 of Jamieson et al 2010) is generally consistent with that of the interpreted tectonic evolution of the southern Omineca belt of the Canadian Cordillera (Fig. 9;Parrish 1995;Carr and Simony 2006;Brown and Gibson 2006;Simony and Carr, in preparation). Although models with less syntectonic erosion (Figs.…”

“…The east-dipping limb may have developed during crustalscale extension on the Columbia River fault system that postdates the main phase of eastward transport. Similarly, in the core of the orogen to the south, somewhat earlier (85-60 Ma) displacement on the structurally higher Gwillim Creek shear zone, which carried the Valhalla complex and surrounding rocks, correlates with the Lewis thrust system to the east (Carr and Simony 2006;Fig. 9d).…”

“…5Bd-5Bf). Although Carr and Simony (2006) and Brown and Gibson (2006) agreed that a large composite crystalline sheet was transported in this manner, Brown and Gibson (2006) also interpreted this transport to have involved channel flow. The generic models cannot predict whether channel flow occurs in any specific natural example; however, it is interesting to note that incipient channel flow is developing in the middle crust of GRL1a through GRL1 (Fig.…”

“…The weaker material in the hangingwall subsequently flows over the block. In such cases, two or more upwind ramps, equivalent to those beneath the Monashee and Valhalla complexes (e.g., Carr and Simony 2006), will develop. Although ductile ramps play a similar transport role to decollement ramps in thin-skinned thrust belts, they develop in a different way because they involve viscous flow as opposed to brittle detachment.…”

“…9. Variations of model GRL1 (shown reversed) with different syn-orogenic erosion rates (parts (c), (e), and (g)) shown compared with interpreted cross section of the southern Canadian Cordillera: (a) modified from Cook (1995b), (b) modified from Hinchey (2005), (d) and (f) modified from Carr and Simony (2006). Note the models are shown in a configuration corresponding to the end of orogenesis and do not include the effects of postorogenic erosion and isostatic rebound.…”