Imaging the midcontinent rift beneath Lake Superior using large aperture seismic data

Abstract

We present a detailed velocity model across the 1.1 billion year old Midcontinent Rift System (MRS) in central Lake Superior. The model was derived primarily from onshore‐offshore large‐aperture seismic and gravity data. High velocities obtained within a highly reflective half‐graben that was imaged on coincident seismic reflection data demonstrate the dominantly mafic composition of the graben fill and constrain its total thickness to be at least 30km. Strong wide‐angle reflections are observed from the lower crust and Moho, indicating that the crust is thickest (55–60km) beneath the axis of the graben. The total crustal thickness decreases rapidly to about 40 km beneath the south shore of the lake and decreases more gradually to the north. Above the Moho is a high‐velocity lower crust interpreted to result from syn‐rift basaltic intrusion into and/or underplating beneath the Archean lower crust. The lower crust is thickest beneath the axis of the main rift half‐graben. A second region of thick lower crust is found approximately 100km north of the axis of the rift beneath a smaller half graben that is interpreted to reflect an earlier stage of rifting. The crustal model presented here resembles recent models of some passive continental margins and is in marked contrast to many models of both active and extinct Phanerozoic continental rift zones. It demonstrates that the Moho is a dynamic feature, since the pre‐rift Moho is probably within or above the high‐velocity lower crust, whereas the post‐rift Moho is defined as the base of this layer. In the absence of major tectonic activity, however, the Moho is very stable, since the large, abrupt variations in crustal thickness beneath the MRS have been preserved for at least a billion years.

Seismic Attenuation Computed from GLIMPCE Reflection Data and Comparison with Refraction Results

“…The refraction data along Line A were used in previous studies to determine the velocity structure (TRÉ HU et al, 1991;Hamilton and Mereu, 1993;Lutter et al, 1993). In addition, MATHENEY et al (1997) simultaneously inverted for velocity and attenuation along line A, using the method presented by NOWACK and MATHENEY (1997a) for the inversion of multiple seismic attributes.…”

“…The surrounding Archean crystalline rocks noted by AG in Figure 13 have Q −1 values similar to those of the Osler volcanics which have interbed sedimentary rocks. In the velocity model of TRÉ HU et al (1991), there are correspondingly lower velocities at these depths near shot locations 800 and 3600 for the crystalline rocks. While lithologies seem to be a significant factor in the apparent Q −1 values of Figure 13, scattering, interference, and thin bed layering may also have effects on apparent attenuation.…”

Interprétation des données de flux de chaleur et de gravité dans le Bouclier Canadien /

“…(1984-1985; 1987-1990), le projet GLIMPCE était concerné par la structure profonde des Grands Lacs et du (Behrendt et al, 1988). L'épaisseur de la croûte déterminée par GLIMPCE, entre 30 et 36 km, est différente de celle de l'interprétation précédente (Epili et Mereu, 1989;Mereu et al, 1990;Trehu et al, 1991). La profondeur du Moho sous le Front de Grenvilîe déterminée par une réflexion prolongée vers l'Est près de la latitude 46° est estimée à environ 51 km (Milkereit et al, 1990b).…”

DISTINCT CRUSTAL STRUCTURE OF THE NORTH AMERICAN MID-CONTINENT RIFT FROM P WAVE RECEIVER FUNCTIONS

“…The depth of the deepest imaged interface changes along the line from 40 to 55 km beneath the center of the MCR, while a secondary deep interface rises beneath the rift to as shallow as 25 km [e.g., Cannon et al, 1989;Tréhu et al, 1991;Thomas and Teskey, 1994;Miller and Nicholson, 2013]. Tens-of-kilometers thick stacks of volcanic rocks were inferred to lie beneath sediments [Green et al, 1989;Ojakangas et al, 2001].…”

“…The Great Lakes International Multidisciplinary Program on Crustal Evolution (GLIMPCE) was an active source seismic reflection and refraction study carried out in the 1980s Tréhu et al, 1991]. Several GLIMPCE lines were shot in Lake Superior, which is underlain by the MCR.…”