Arie J. van der Velden

Frozen subduction in Canada's Northwest Territories: Lithoprobe deep lithospheric reflection profiling of the western Canadian Shield

Lithoprobe deep seismic reflection data from the northwestern Canadian Shield provide images of structures to the base of the lithosphere between the Archean Slave Province on the east and the Cordillera on the west. Mantle reflections dip eastward from the lower crust to about 100 km depth beneath the ∼1.88–1.84 Ga Great Bear magmatic arc and almost certainly represent a subduction surface associated with arc development. Where the mantle reflections flatten into the lower crust, they merge with prominent west dipping crustal reflections thus delineating a lithospheric-scale wedge that formed as a result of Proterozoic plate convergence between the Slave craton on the east and the ∼1.84 Ga Fort Simpson terrane on the west. The crust throughout the 725 km survey is highly reflective, and the Moho remains at a constant level between about 11.0 and 12.0 s beneath both Archean craton and Proterozoic accreted rocks; the most significant Moho deflection occurs beneath the ∼1.8–0.7 Ga Fort Simpson basin where it appears to rise to about 9.0 s (about 27 km). Within the crust of the Slave Province, east dipping, low-angle reflections near the surface beneath the Yellowknife basin may be shallow detachments, and lower crustal geometry is consistent with low-angle imbrication during Late Archean (∼2.65 Ga) tectonism associated with development of the Yellowknife basin volcanic rocks. West of the Slave craton, accreted Proterozoic crust is characterized by gently folded upper crustal layers overlying apparent thrust duplexes above detachment surfaces that flatten near the Moho. These lower crustal rocks were likely inserted as a tectonic wedge above the Moho and beneath the Slave Province during the contractional phase (∼1.90–1.88 Ga Calderian orogeny) of the Wopmay orogen. On the western end of the profile, the Fort Simpson extensional basin has up to 20–25 km of Proterozoic (meta) sedimentary rocks and sills(?) that are younger than 1.84 Ga.

The big picture: A lithospheric cross section of the North American continent

A lithospheric cross section constructed within a 6000-kmlong corridor across southern Canada and its margins at 45– 55°N illuminates the assembly of the North American continent at an unprecedented scale. Based on coordinated, multidisciplinary research, the profile emphasizes lithospheric-scale relationships between orogens—plate collisions and accretions have sequentially stacked orogen upon orogen such that the older crust forms basement to the next younger. This largescale perspective highlights the similarities among crustal structures produced by orogenic processes despite the broad range of age from the Mesoarchean to the present. Heterogeneities in the lithospheric mantle suggest that, in certain situations, relict subducted or delaminated lithosphere can remain intact beneath, and eventually within, cratonic lithospheric mantle. In contrast, the dominantly subhorizontal Moho appears to be reequilibrated through mechanical and/or thermal processes; few crustal roots beneath orogens are preserved. INTRODUCTION A unique cross section of the North American continent represents a synthesis of more than two decades of coordinated research conducted by Lithoprobe, Canada’s national geoscience project. Based on existing interpretations within eight study regions, or transects, that are linked directly or by projection along strike, we have constructed a transcontinental lithospheric profile (Fig. 1 and poster insert). From west to east, this 6000-km profile crosses the Juan de Fuca oceanic plate, the active Cascadia subduction zone, the southern Cordillera (0.19 Ga–present), the Alberta and Trans-Hudson orogens (1.92–1.8 Ga), the Superior Province (3.82–2.60 Ga), the Mid-Continent Rift System (1.1–1.0 Ga), the Grenville orogen (1.19–0.99 Ga), the Newfoundland Appalachian orogen (0.47–0.28 Ga), the Grand Banks continental shelf, and the Atlantic passive margin (0.2 Ga). The diversity of tectonic history and ages included in the section facilitates direct comparison of the secular and spatial variation of orogenic processes. Data and interpretations are based on coordinated multidisciplinary research combined with a strong, steadily improving base of regional geotectonic knowledge. The structures displayed are primarily based on active-source seismic (reflection and refraction) data. However, the regional geometry and interpretations of the structure and tectonic processes utilize the full array of geological, geochemical, and geophysical data available for that region. Appendix 1 (see GSA’s supplemental data repository) summarizes how the cross section was constructed. A complete listing of references used to construct the cross section is provided in Appendix 2 (see footnote 2). In addition, Hammer et al. (2010) provide an in-depth description and two complementary lithospheric cross sections. The cross section is portrayed in terms of the “tectonic age” within the crust. We define this as the time since the most recent episode of significant tectonic deformation (Fig. 1 and insert [see footnote 1]). Tectonic age was chosen over more typical designations (e.g., geology or terranes/domains) because it simplifies the interpreted cross section to highlight comparative structures and to convey the sequence of orogenic development based on the current structural interpretations. In some areas, we chose to modify the tectonic age designations in order to convey key aspects of structure as well as the sequence of orogenic development based on current structural interpretations. For example, the Archean Sask, Hearne, and Superior continents were welded together in the Paleoproterozoic Trans-Hudson Orogen (1.92–1.80 Ga), yielding the core of the Laurentian craton. The largely unexposed Sask craton, discovered by Lithoprobe seismic studies (e.g., Lucas et al., 1993; Lewry et al., 1994; Hajnal et al., 2005), lies almost entirely beneath juvenile crustal imbricate structures. Although the Sask craton dates to 2.45–3.3 Ga, the lithospheric fragment was likely deformed by the Paleoproterozoic orogeny. However, to clarify its role in the assembly of Laurentia, we have chosen to label it with an Archean tectonic age but stippled to indicate Paleoproterozoic modification. Similar display procedures have been applied in other parts of the lithospheric cross section.

Crustal structure, fossil subduction, and the tectonic evolution of the Newfoundland Appalachians: Evidence from a reprocessed seismic reflection survey

Reprocessed Lithoprobe seismic reflection data across the Appalachian orogen in Newfoundland provide images of an Ordovician–Devonian collision zone that separates Laurentia from Ganderia, an accreted peri-Gondwanan microcontinent. Prominent reflectivity within Ganderian basement tapers westward and merges with reflections that project beneath the Moho, outlining a probable Ordovician to Devonian subduction zone. Reflectivity within Ganderian basement likely originates from transposed compositional layering within Cambrian–Neoproterozoic arc basement. Migmatites and other high-grade rocks of the Meelpaeg allochthon were likely extruded in the Devonian toward the northeast. The reflection Moho may have been established in the Devonian in parts of Newfoundland by partial melting of the lower crust. Reflection truncations outline a near-vertical Carboniferous strike-slip fault zone that cuts the entire crust.

Three-dimensional crustal structure of the Purcell anticlinorium in the Cordillera of southwestern Canada

The Purcell anticlinorium in southwestern Canada and the northwestern United States is situated at the transition from deformed North American rocks of the foreland thrust and fold belt to accreted rocks of the Kootenay arc. Nearly 1000 km of industry seismic reflection data were processed with extended VIBROSEIS correlation and coherency filtering and then combined with LITHOPROBE reflection data to outline the three-dimensional crustal structure of the anticlinorium. Reflection Moho is visible at ∼12.0 s (∼36 km) beneath the anticlinorium and deepens eastward to ∼15.0 s (∼45 km) beneath the Rocky Mountain thrust and fold belt. The transition from thick to thin crust coincides with westward deepening of North American basement and with the southern Rocky Mountain Trench, a basin or series of basins formed during Tertiary extension. Regionally extensive reflections in the upper crust correlate with ca. 1468 Ma gabbroic sills in the Aldridge Formation of the Mesoproterozoic Belt/Purcell Supergroup. Features outlined by the sills support an interpretation that the anticlinorium formed during Mesozoic contraction when imbricate thrust faults carried up to 15 km of Belt/Purcell and Paleozoic margin sedimentary rocks, and basement in some areas, eastward over a prominent basement ramp.

Proterozoic crustal transition beneath the Western Canada sedimentary basin

New deep seismic reflection data outline a major crustal transition, from thick craton on the east to thin basinal crust on the west, that is buried beneath the Phanerozoic rocks of the Western Canada sedimentary basin. The transition formed prior to, or during, deposition of westward-thickening Mesoproterozoic sedimentary rocks between about 1.85 and 0.9 Ga and appears to be the southward continuation of the Fort Simpson structural trend, which is crossed by reflection data nearly 700 km to the north. It may be analogous to crustal ramps that are now commonly observed in many Phanerozoic orogens and, along with an associated decrease in Moho depth, has been preserved at this location for more than 1.0 b.y.

Proterozoic and Cenozoic subduction complexes: A comparison of geometric features

Mantle reflections along the Lithoprobe Slave Northern Cordillera Lithospheric Evolution (SNORCLE) deep seismic reflection transect in northwestern Canada are interpreted as a relict subduction zone. Geometric features of the data in the vicinity of the interpreted subduction zone are compared with those of the Vancouver Island profile, which images a modern convergent margin. Comparisons are made at both a regional scale, as in the case of the location of the magmatic arc with respect to the subduction zone, as well as a local scale, such as the geometry of structures within the accretionary wedge. The comparison provides a test for the paleosubduction hypothesis and aids the interpretation of the deep reflection profile where it is difficult to trace individual features to outcrop. The similarity of geometry and scales of the structures gives support to the interpretation that the lithospheric structures in the Wopmay orogen are the products of accretionary processes at ∼1.88–1.84 Ga and that these accretionary processes are similar to those observed today.

Structure and tectonic development of the southern Rocky Mountain trench

The Rocky Mountain trench is one of the youngest, most prominent, and most enigmatic structures of the Canadian Cordillera. Approximately 650 km of seismic-reflection data, providing regional three-dimensional coverage over an area of 10,000 km², include six crossings of the Rocky Mountain trench between 49°N and 50°15′N. Prominent reflections from mid-Proterozoic Moyie sills outline thrust-and-fold structures of a Late Jurassic to Early Cretaceous fault system that was cut by the Rocky Mountain trench fault in the Tertiary. The near-basement reflections outline a 10 km high west facing basement ramp, the hinge line of which spatially coincides with the Rocky Mountain trench in this area. This ramp is part of a mid-Proterozoic margin upon which the Belt-Purcell supergroup was deposited and is preserved beneath the trench. During Mesozoic contraction, the basal detachment of the Foreland belt closely followed the craton-cover contact across the basement ramp. Thrusting ceased and extension was initiated when a culmination of thick basinal strata was juxtaposed with the basement ramp. In the Eocene-Miocene, the basement ramp and the culmination above it focused stress, reactivating the basal detachment and causing extensional faulting in the southern Rocky Mountain trench. The Rocky Mountain trench fault may be linked via the basal detachment to the Flathead fault on the east and the Eocene extensional faults that flank the Omineca belt on the west.

Displacement of the Lewis thrust sheet in southwestern Canada: New evidence from seismic reflection data

Seismic reflection data across the Rocky Mountain trench in the southern Canadian Cordillera reveal prominent reflections from Mesoproterozoic (Aldridge Formation) sills that allow a direct stratigraphic correlation across the trench when 10 km of extension is restored. Trun cated reflections at ∼11-15 km depth on the west side of the trench are interpreted as a footwall cutoff of the Lewis thrust in Belt-Purcell strata that lies ∼115 km west of the leading edge of the Lewis thrust, exposed in the Waterton area; retrodeformation of the Lewis sheet requires a minimum of 75 km of transport on the Lewis thrust and 40 km of transport by the development of footwall-domain duplexes. The trench is located above a major change in slope of the top of the North American basement that coincides with a pronounced increase in thickness of lower Belt-Purcell strata in their retrodeformed position. This transition may represent part of an ancient margin that was initially established in Paleoproterozoic time.