Crustal Properties Across the Mid-Continent Rift via Transfer Function Analysis

Abstract:

The Mid-Continent Rift (MCR), a failed Proterozoic rift structure in central North America, is a dominant feature of North American gravity maps. The rift underwent a combination of extension, magmatism, and later compression, and it is difficult to predict how these events affected the overall crustal thickness and bulk composition in the vicinity of the rift axis, though the associated gravity high indicates that large-volume mafic magmatism took place. The Superior Province Rifting Earthscope Experiment (SPREE) project instrumented the MCR with Flexible Array broadband seismographs from 2011 through 2013 in Minnesota and Wisconsin, along two lines crossing the rift axis as well as a line following the axis.

We examine teleseismic P-coda data from SPREE and nearby Transportable Array instruments using a new technique: transfer-function analysis. In this approach, possible models of crustal structure are used to generate a predicted transfer function relating the radial and vertical components of the P coda at a particular site. The transfer function then allows generation of a misfit (between the true radial component and a synthetic radial component predicted from the vertical trace) without the need to perform receiver-function deconvolution, thus avoiding the deconvolution problems encountered with receiver functions in sedimentary basins. We use the transfer-function approach to perform a grid search over three crustal properties: crustal thickness, crustal P/S velocity ratio, and the thickness of an overlying sedimentary basin.

Results for our SPREE/TA data set indicate that the crust is significantly thickened along the rift axis, with maximum thicknesses approaching 50 km; the crust is thinner (ca. 40 km) outside of the rift zone. The crustal thickness structure is particularly complex beneath southeastern Minnesota, where very strong Moho topography is present, as well as up to 2 km of sediment; further north, the Moho is smoother and the basin is not present. P/S ratio varies along the rift axis, suggesting a higher mafic component (higher ratio) in southern Minnesota. The complexity we see along the MCR is consistent with the results obtained by Zhang et al. (this conference) using receiver function analysis.