Search

The Lithoprobe seismic reflection transect across the southern Coast Mountains of the Canadian Cordillera images fundamental crustal structures presumably related to collision of the Intermontane and Insular composite terranes, and deep levels in the upper plate of the offshore Cascadia subduction belt. The eastern part of the Coast Mountains are characterized by east dipping upper crustal reflectors that project to exposed faults and east dipping lower crustal reflectors; they are truncated by subhorizontal to west dipping middle and upper crustal reflectors. These geometric relationships are interpreted to have formed during an early phase of primarily west directed contraction that created the east dipping structures of the upper and lower crust, and a later phase of east directed shortening caused by wedging of the Intermontane belt into the lower and middle crust of the tectonic stack. Subsequently, the Coast belt may have been displaced eastward on contractional faults that ascend from the lower crust beneath the Intermontane belt and surface in the Omineca and Foreland belts. Extensional faults bounding the east flank of the Coast Mountains and west flank of the central Nicola horst in the Intermontane belt flatten into the middle and lower crust of the intervening region and geometrically outline crustal boudinage. Within the western Coast Mountains, east dipping reflections spanning the middle crust to upper mantle are traced updip to Vancouver Island and the underlying Cascadia subduction zone. The C reflector on Vancouver Island is believed to separate Wrangellia from underlying accreted terranes and is correlated to the mainland where it forms the upper boundary of a reflective lower crustal wedge that flattens into the Moho. If the Moho is not a young feature, then some accreted material appears to have wedged into the continental framework above the crust-mantle boundary, possibly causing shortening in the overlying crust and creating midcrustal ramps observed on the reflection data. The structurally lower E reflections, interpreted as shear zones, originate at the subduction contact offshore and project landward into sub-Moho reflections within the upper plate on the Mainland. The region between the E reflector and the descending oceanic plate is interpreted to be subducted lower continental crust and mantle.

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.

[1] Factors controlling widespread cratonic arch formation across western North America from the Early Paleozoic to the Middle Mesozoic remain poorly understood. While a number of causes, such as mantle hot spots, lithospheric contraction, mantle flow, and lithospheric flexure due to surface loading, have been suggested to explain the localized uplift of basement blocks on otherwise tectonically quiescent margins, a variety of mechanisms may be at work. Here, we test the hypothesis that seismically detected packages of discrete midcrustal igneous sheet intrusions impart rheological contrasts within the crust, which act as seeds for cratonic arch formation in the presence of weak intraplate compression, using finite element modeling of intraplate deformation. We find that the seismically resolved sills can generate significant surface uplift in the presence of modest strain rates over reasonable orogenic timescales. Cratonic arch formation would be enhanced in the presence of additional thin, seismically unresolvable sills. While midcrustal sheet intrusions may be just one of many factors influencing cratonic arch formation, their presence in northern Alberta, Canada, can help explain the asymmetric configuration of the well-studied Peace River Arch.

[1] The Cordillera in northern Canada is underlain by westward tapering layers that can be followed from outcrops of Proterozoic strata in the Foreland belt to the lowermost crust of the orogenic interior, a distance of as much as 500 km across strike. They are interpreted as stratified Proterozoic rocks, including ∼1.8–0.7 Ga supracrustal rocks and their basement. The layering was discovered on two new deep seismic reflection profiles in the Yukon (Line 3; ∼650 km) and northern British Columbia (Line 2; ∼1245 km in two segments) that were acquired as part of the Lithoprobe Slave-Northern Cordillera Lithospheric Evolution (SNORCLE) transect. In the Mackenzie Mountains of the eastern Yukon, the layering in Line 3 is visible between 5.0 and 12.0 s (∼15 to 36 km depth). It is followed southwestward for nearly 650 km (∼500 km across strike) and thins to less than 1.0 s (∼3.0–3.5 km thickness) near the Moho at the Yukon-Alaska international boundary. In the northern Rocky Mountains of British Columbia, the upper part of the layering on Line 2 correlates with outcrops of Proterozoic (1.76–1.0 Ga) strata in the Muskwa anticlinorium. At this location, the layering is at least 15 km thick and is followed westward then southward into the middle and lower crust for ∼700 km (∼300 km across strike). It disappears as a thin taper at the base of the crust ∼150 km east of the coast of the Alaskan panhandle. The only significant disruption in the layering occurs at the Tintina fault zone, a late to postorogenic strike-slip fault with up to 800 km of displacement, which appears as a vertical zone of little reflectivity that disrupts the continuity of the deep layering on both profiles (∼300 km apart). The base of the layered reflection zone coincides with the Moho, which exhibits variable character and undulates in a series of broad arches with widths of ∼150 km. In general, the mantle appears to have few reflections. However, at the southwest end of Line 3 near the Alaska-British Columbia border, a reflection dips eastward from ∼14.0 s to ∼21.0 s (∼45 to 73 km depth) beneath exposed Eocene magmatic rocks. It is interpreted as a relict subduction surface of the Kula plate. Our interpretation of Proterozoic layered rocks beneath most of the northern Cordillera suggests a much different crustal structure than previously considered: (1) Ancient North American crust comprising up to 25 km of metamorphosed Proterozoic to Paleozoic sediments plus 5–10 km of pre-1.8 Ga crystalline basement projects westward beneath most of the northern Canadian Cordillera. (2) The lateral (500 km by at least 1000 km) and vertical (up to 25 km) extent of the Proterozoic layers and their internal deformation are consistent with a long-lived margin for northwestern North America with alternating episodes of extension and contraction. (3) The detachments that carry deformed rocks of the Mackenzie Mountains and northern Rocky Mountains are largely confined to the upper crustal region above the layering. (4) Accreted terranes include thin klippen that were thrust over North American pericratonic strata (e.g., Yukon-Tanana), and terranes such as Nisling and Stikinia that thicken westward as the underlying Proterozoic layers taper and disappear. (5) The ages of exposed rocks are not necessarily indicative of the ages of underlying crust, a frequent observation in Lithoprobe interpretations, so that estimates of crustal growth based on surface geology may not be representative.