Chapter 15: Crustal structure of the continental interior

Abstract

Seismic refraction profiles and deep seismic reflection profiling data have been compiled for the continental interior of the United States and adjacent southern Canada.A total of 58 refraction profiles and 17 deep reflection profiling studies are available in the midcontinental area. Statistics derived from the refraction data indicate that the crust of the continental interior is characterized by a crustal thickness of about 43 km, an average crustal compressional seismic velocity of 6.5 km/sec, and a nearly uniform uppermost mantle compressional velocity (P") of 8.1 km/sec. The crust displays a generally layered character with compressional velocities near 6.1 km/sec for the upper 10 to 20 km and about 6.8 km/sec for the lower crust. In some areas, generally near the southern and western margins of the relatively undeformed craton, a highvelocity (approximately 7.0 to 7.4 km/sec) lower crustal layer 10 to 20 km thick is present just above the Moho. These areas also display higher than average mean crustal velocity and crustal thickness. Contour diagrams of crustal seismic data also identify a crustal structure anomaly consisting of higher average crustal velocity and crustal thickness associated with the Midcontinent Rift System. Seismic reflection profiling data, which are primarily available in areas of the midcontinent that are known to be anomalous, provide improved resolution of structural features of the continental crust, particularly in areas characterized by the presence of thick (5 to 20 km or more) basins filled with Proterozoic sedimentary or volcanic rocks. For other parts of the crystalline crust, seismic reflections are, in general, laterally discontinuous and commonly dip moderately, relative to shallow reflections and reflections from the underlying crustmantle boundary.

Crustal modeling from spectrally correlated free‐air and terrain gravity data—A case study of Ohio

“…For example, our results tend to show higher frequency variations than are commonly observed in seismic studies of the other crustal regions of the midcontinent (e.g., Braile, 1989;Braile et al, 1989). This apparent disparity may well reflect limitations in our gravity inversion assumptions, although ambiguities related to the interpretation of the seismic data (e.g., Braile, 1989) may also contribute.…”

“…This reference depth corresponds to the mean value determined from Braile et al (1989). The reference depth can also be differentially adjusted for each block so that the inversion may be readily tuned to match better local constraints from seismic or other geophysical data when they become available.…”

Proterozoic geology of the east-central Midcontinent basement

“…This seismic activ ity is commonly associated with major rift basins underlying the embayment (e.g., Braile, 1989;Braile et al, 1986;Hildenbrand, 1985;Mooney et al, 1983;Kane et al, 1981; Reelfoot rift in Figs. 2,5, and 7).…”

Relation between alternations of uplift and subsidence revealed by Late Cenozoic fluvial sequences and physical properties of the continental crust

“…This would imply a mobile layer ~11.5 km thick, albeit thinner than that estimated by Westaway (2007b). Braile (1989), the crust is locally 52 km thick, the basal 14 km being interpreted (e.g., by Durrheim & Mooney, 1994) as mafic underplating. The tomography , 1997) indicates that the temperature is ~900 °C at 110 km depth.…”

“…5), for which modelling indicates W i ~8-10 km, making the mobile layer ~9-11 km thick. The crust beneath Denver was long regarded as ~50 km thick (e.g., Allenby & Schnetzler, 1983;Braile, 1989), with mafic underplating accounting for the lower part of the crust, at >30 km depth (e.g. Durrheim & Mooney, 1994;Karlstrom et _ al., 2002).…”