Variations in the reflectivity of the moho transition zone beneath the Midcontinent Rift System of North America: Results from true amplitude analysis of GLIMPCE data

Abstract

True amplitude processing of The Great Lakes International Multidisciplinary Program on Crustal Evolution seismic reflection data from the Midcontinent Rift System of North America shows large differences in the reflectivity of the Moho transition zone beneath the axial rift, beneath the rift flanks, and outside of the rift. The Moho reflection from the axial rift has a discontinuous, diffractive character marginally stronger (several decibels) than an otherwise transparent lower crust and upper mantle. Beneath the axial rift, Moho is interpreted to be a synrift igneous feature. Beneath the rift flanks, the reflectivity of the Moho transition is generally well developed with two identifiable boundaries, although in places it is weakly reflective to nonreflective, similar to Moho outside the rift. The two boundaries are interpreted as the base of essentially intact, although stretched, prerift Archean crust (upper boundary) and new synrift Moho 1–2 s (6–7 km) deeper (lower boundary). Beneath the rift flanks, the layered reflection Moho transition results from the preexisting crustal composition and fabric modified by synrift igneous processes and extensional tectonic/metamorphic processes. The geologic evidence for extensive basaltic magmatism in the rift is the basis for interpreting the Moho signature as a Keweenawan structure that has been preserved for 1.1 b.y. Extension and magmatism appear to enhance reflectivity in the lower crust and Moho transition zone only where stretching factors are moderate (rift flanks) and not where they are extreme (axial rift). This leads to the prediction that the reflectivity across analogous volcanic passive continental margins should be greatest beneath the moderately stretched continental shelves and should decrease towards the ocean‐continent boundary.

Possible extension of the Midcontinent Rift in west Texas and eastern New Mexico

“…This program involved multichannel seismic reflection studies in the Great Lakes (e.g., Behrendt et al 1988;Green et al 1989;Cannon et al 1989;Hutchinson et al 1992), wide-angle seismic reflection and refraction recordings from air-gun sources (Epili and Mereu 1989;Tr6hu et al 1991), and aeromagnetic surveying (Teskey et al 1991). The reflection profiles show that the rift basins are asymmetric and filled with up to 30 km of rift-related volcanic and sedimentary rocks (Behrendt et al 1988;Green et al 1989).…”

“…T h i s study found thickened crust and highly disrupted Moho under the rift axis compared with the rift flanks (Epili and Mereu 1989). Hutchinson et al (1992) reprocessed several GLIMPCE reflection lines using relative true amplitude processing techniques. They found well-developed Moho reflectivity under the rift flanks, with two identifiable boundaries.…”

“…They found well-developed Moho reflectivity under the rift flanks, with two identifiable boundaries. The two boundaries are interpreted to represent the Moho of the prerift Archean Crust (upper boundary) and the new synrift Moho (lower boundary) (Hutchinson et al 1992). New aeromagnetic data were recorded and combined with existing data to form a new aeromagnetic data set for the Lake Superior region (Teskey et al 1991).…”

Joint Inversion of SPREE Receiver Functions and Surface Wave Dispersion Curves for 3‐D Crustal and Upper Mantle Structure Beneath the U.S. Midcontinent Rift

“…For the MCR, the combination of seismic and gravity anomalies of Lake Superior has been interpreted as underplating in the lower crust up to 15 km thick (Behrendt et al., 1990; Hamilton & Mereu, 1993; Hinze et al., 1992; Hutchinson et al., 1990, 1992; Shay & Tréhu, 1993; Zhang et al., 2016). Behrendt et al.…”

“…(1990) and Hutchinson et al. (1992) proposed that the high‐velocity material that makes up the present‐day lower crust was probably originally mantle material that now appears to have crustal properties because of the large volume of intruded gabbroic rocks. Although there are often multiple seismic reflectors within the underplated layer, its upper and lower boundaries usually display distinct seismic reflectors that are seen as both normal‐incidence and wide‐angle reflections (Thybo & Artemieva, 2013).…”