The crust of the northern U. S. Craton: A search for beginnings

Abstract

Except for limited areas, the basement of the northern U.S. midcontinent is cov ered by mildly deformed Phanerozoic sedimentary strata. Thus, geophysical studies, together with petrologic and isotopic age information obtained from deep drill hole samples, are the primary source of our knowledge of the nature of the crystalline basement crust. These rocks record the complex evolution of a large segment of the North American continental nucleus over Precambrian time and provide significant clues to subsequent deformation and sedimentation of the overlying Phanerozoic strata, as well as to modern tectonism. Significant progress has been made in charac terizing the crust of the craton over the past half century as a result of regional-scale gravity and magnetic anomaly data acquisition, seismic profiling that has imaged the crust and in some cases the upper mantle, and geochemical data that have identified the source and age of the basement. Despite this progress, the available geophysical data are generally inadequate even for regional studies and many critical geologic questions remain unanswered. However, the technology is available to dramatically improve data bases and our knowledge base of the nature, evolution, processes, and architecture of this region. The combined efforts of the geoscience disciplines are required to take advantage of this technology and apply it in an integrated manner to the search for the beginnings of the U.S. craton.

Crust and upper mantle discontinuity structure beneath eastern North America

“…Simple one‐dimensional velocity models were constructed including one or two layers of sedimentary rocks and one crustal basement layer. The total sedimentary rock thickness beneath each station was obtained from published maps [ Flawn et al , 1967; Shumaker and Wilson , 1996; Hinze , 1996] and varied from 0.6 to 6.0 km (Figure 3). P wave velocity was assumed to be 4.5 km/s in the top sedimentary rock layer and 5.3 km/s in the bottom layer.…”

“…Simple one-dimensional velocity models were constructed including one or two layers of sedimentary rocks and one crustal basement layer. The total sedimentary rock thickness beneath each station was obtained from published maps [Flawn et [Flawn et al, 1967;Shumaker and Wilson, 1996;Hinze, 1996]; their relative thicknesses and total crustal thickness were determined by modeling P m s phases and Moho reverberations beneath each station. Three crustal basement velocity combinations were used for the western stations, and two were used for the eastern stations.…”

“…The heavy line indicates the models that provide the best fit to regional groupings of stations. al., 1967;Shumaker and Wilson, 1996;Hinze, 1996] and varied from 0.6 to 6.0 km ( Figure 3). P wave velocity was assumed to be 4.5 km/s in the top sedimentary rock layer and 5.3 km/s in the bottom layer.…”