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.