The North American Midcontinent Rift beneath Lake Superior from Glimpce seismic reflection profiling

Abstract

The Midcontinent rift system is a 1.1‐b.y.‐old structure extending from Kansas, through the Lake Superior region, and into southern Michigan. The rift is filled with thick sequences of basaltic volcanic rocks and clastic sediments. For most of its extent it is buried beneath Paleozoic rocks but can be traced by its strong gravity and magnetic anomalies. The rocks of the rift system are exposed only in the Lake Superior region and comprise the Keweenawan Supergroup. Much of the geology of the Keweenawan is beneath Lake Superior and has only been inferred from potential field studies and seismic refraction studies and extrapolation from on‐shore geology. Seismic reflection surveys by the Great Lakes International Multidisciplinary Program on Crustal Evolution in 1986 imaged much of the deep structure of the rift beneath the lake in detail. The reflection profiles across the rift reveal a deep, asymmetrical central graben whose existence and magnitude was not previously documented. They show that, in addition to crustal sagging documented by previous investigations, normal faulting played a major role in subsidence of the axial region of the rift. A sequence of volcanic and sedimentary rocks, in places greater than 30 km thick, fills the graben. Thinner volcanic and sedimentary units lie on broad flanks of the rift outside of the graben. Near the axis, the prerift crust is thinned to about one fourth of its original thickness, apparently by a combination of low‐angle extensional faulting and ductile stretching or distributed shear. The sense of asymmetry of the central graben changes along the trend of the rift, documenting the segmented nature of the structure and suggesting the existence of accommodation zones between the segments. The location of the accommodation zones is inferred from abrupt disruptions in the Bouguer gravity signature of the rift. Uplift of the central graben occurred when the original graben‐bounding normal faults were reactivated as high‐angle reverse faults with throws of 5 km or more in places. The Midcontinent rift has some striking similarities to some younger passive continental margins. We propose that it preserves a record of nearly complete continental separation which, had it not been arrested, would have created a Middle Proterozoic ocean basin.

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

“…The seismic refraction and reflection data from this line have been used to invert for the velocity and reflectivity structure of the subsurface (MATHENEY et al, 1997;LUTTER et al, 1993;SHAY and TRÉ HU, 1993;HAMILTON and MEREU, 1993;CANNON et al, 1989). These prior results allow the attenuation model presented in this study to be related to the lithology and geologic features across the MCR.…”

“…For the deeper regions beneath Lake Superior, the Q −1 results shown in Figure 11 have been shaded and compared with the analysis of the reflectivity structure by CANNON et al (1989) in Figure 13. A logarithmic gray-shade scale is used to plot the model in order to emphasize the deeper portions of the model.…”

“…Overlain on the Figure 13 Gray-shade plot of the inverse-Q profiles derived from the seismic reflection data. The solid and dashed lines are adapted from the interpretation of the seismic reflection data from CANNON et al (1989). See text for an explanation of the abbreviations.…”

“…gray-shade map is a line drawing adaptation of the interpretation of CANNON et al (1989) along with the possible lithologies. The interpretation of CANNON et al (1989) was derived from the reflectivity structure on the stacked and processed seismic reflection profile.…”

“…The resulting Q −1 model is then compared to an independently derived attenuation model obtained from seismic refraction data by MATHENEY et al (1997) along the same transect. Finally, the derived attenuation structure is compared with the interpreted reflectivity structure from CANNON et al (1989).…”