Philip T. C. Hammer

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.

Jasper Seamount structure: Seafloor seismic refraction tomography

The velocity structure of Jasper Seamount was modeled using one- and three-dimensional inversions of P wave travel times. The results represent the first detailed seismic images of a submerged, intraplate volcano. Two seismic refraction experiments were completed on Jasper Seamount, incorporating ocean bottom seismometers and navigated seafloor shots. The P wave travel times were first used to compute a one-dimensional velocity profile which served as a starting model for a three-dimensional tomographic inversion. The seamount P velocities are significantly slower than those observed in typical oceanic crust at equivalent subbasement depths. This suggests that Jasper Seamount is constructed predominantly of extrusive lavas with high average porosity. The velocity models confirm morphological predictions: Jasper Seamount is a shield volcano with rift zone development. High seismic velocities were detected beneath the large radial ridges while low velocities characterize the shallow summit and flanks. Comparisons between P velocity models of Jasper Seamount and the island of Hawaii reveal that these two shield volcanoes are not structurally proportional. Jasper Seamount is far smaller than Hawaii, yet both volcanoes exhibit an outer extrusive layer of similar thickness. This suggests that seamount size influences the intrusive/extrusive proportions; density equilibrium between melt and country rock may explain this behavior.

Crustal structure of NW British Columbia and SE Alaska from seismic wide‐angle studies: Coast Plutonic Complex to Stikinia

Crustal structure beneath the transition from the Coast belt to the Intermontane superterrane of the northern Canadian Cordillera is interpreted from the inversion of refraction and wide-angle reflection seismic data. The profile traverses an accretionary suture zone (Coast Plutonic Complex) to continental crust deformed by the transpressive collision (Stikine terrane). Using data acquired by the Accrete onshore/offshore experiment and by a partially overlapping Lithoprobe onshore experiment, P wave travel time inversion and forward amplitude modeling are employed to determine crustal velocity structure. The model exhibits a well-defined transition throughout the crust that distinguishes the Coast Plutonic Complex (CPC) from Stikinia. Average crustal velocities beneath the CPC (6.45 km/s) are considerably faster than those beneath Stikinia (6.25 km/s). Crustal thickness also changes across the transition; thin crust beneath the Coast belt (30–32 km) thickens beneath Stikinia (35–37 km). The observations within the Coast belt are consistent with a tectonic history most recently dominated by extensional deformation. Primary structural control could be associated with either Neogene extension and/or the processes that are responsible for exhuming the Coast belt during the early Paleogene and have been inferred from geological studies. Slow mantle velocities (7.8–7.9 km/s) beneath the entire profile are indicative of high upper mantle temperatures. Comparison with the southern Cordilleran Coast belt reveals similar velocity structure within the massive plutonic complexes. However, substantial differences between the northern and southern Coast belts emphasize along-strike variations in terranes, orogen geometry and postorogenic tectonics.

High‐resolution seismic reflection imaging of a thin, diamondiferous kimberlite dyke

Seismic reflection techniques are, for the first time, used to image a thin, diamondiferous kimberlite dyke from subcrop to depths greater than 1300 m. Geophysical exploration for kimberlite deposits typically involves airborne potential field surveys that are well suited for detecting vertical outcropping pipes but often fail to reveal thin, subhorizontal dykes and sills. Because seismic techniques are especially well suited for mapping structures that have shallow dips and strong impedance contrasts, a feasibility study and seismic reflection survey were undertaken on the diamondiferous Snap Lake dyke (Northwest Territories, Canada) to evaluate the potential for using seismic techniques on these targets. The dyke (average thickness 2–3 m) provides an excellent test site because a drilling program has defined the gross dyke geometry and provides core samples from the kimberlite and host rocks. The feasibility study involved measuring P‐velocity and density of selected cores. Using these data, reflectivit...