SEISMIC IMAGING OF CRUSTAL STRUCTURE BENEATH OHIO: POSSIBLY THE ELUSIVE GRENVILLE DEFORMATION?

The ca. 1 Ga Grenville orogeny was a protracted mountain-building event that culminated in the collision of Laurentia and Amazonia and the formation of the Rodinia supercontinent. While the expression of Grenville orogenesis in present-day crustal structure has been extensively investigated in eastern Canada, evidence for contemporaneous crustal deformation is less well established beneath the eastern United States. Furthermore, the interpretation of a geophysical lineament through the U.S. mid-continent, typically inferred to be the Grenville deformation front, has recently been called into question; an alternative hypothesis is that this feature actually corresponds to an eastern arm of the Midcontinent Rift. Here we present P-to-S receiver functions computed for stations of the MAGIC experiment, a dense array of broadband seismometers across Virginia, West Virginia, and Ohio. We see evidence for a crustal negative velocity gradient that dips gently (dip angle less than 10 degrees) to the southeast and extends east from a location near the putative Grenville Front, terminating near the Appalachian Mountains. While we cannot date this feature, its location and characteristics are consistent with a shallowly dipping, seismically anisotropic intracrustal shear zone associated with collisional deformation, perhaps during Grenville orogenesis. The similarity between this feature and similar mid-crustal detachments in other orogens, both ancient (Appalachians) and modern (Himalayas), suggests that this style of crustal deformation has been common in continental collisional orogens. The imaging results presented here inform the ongoing scientific debate about the relative importance and timing of Midcontinent Rift and Grenville Orogen processes in the eastern U.S.