A MID-CRUSTAL SHEAR ZONE ASSOCIATED WITH RODINIA'S FORMATION: IS IT TOO SOON TO WIPE THE GRENVILLE FRONT LINEAMENT FROM MAPS? (Invited Presentation)

While the present-day crustal structure of Grenville orogenesis has been extensively investigated in eastern Canada, evidence for contemporaneous crustal deformation is less well established beneath the eastern United States. The interpretation of a geophysical lineament through Ohio, Kentucky, Tennessee, and Alabama, typically inferred as the location of the Grenville deformation front, has recently been called into question; an alternative hypothesis is that this feature actually corresponds to the eastern arm of the Midcontinent Rift. Here we present P-to-S receiver functions computed for stations of the MAGIC array, a dense linear array of broadband seismometers across Ohio, West Virginia, and Virginia. We see evidence for a laterally continuous crustal negative velocity gradient extending from western Ohio (near the putative Grenville Front) to central West Virginia dipping to the southeast with an angle less than 10º. Given the position and geometry of the mid-crustal negative velocity gradient imaged beneath the MAGIC line, our preferred interpretation is that this feature corresponds to the main deformation front of the Grenville orogen beneath Ohio and West Virginia. The location and characteristics of this mid-crustal anomaly are consistent with a shallowly dipping, seismically-anisotropic intracrustal shear zone associated with contractional deformation associated with Rodinia’s formation, perhaps with later collision beneath what is now Ohio than further to the north in Canada. Our results do not contradict the interpretation of the potential field anomalies as the eastern arm of the Midcontinent Rift. The similarity between the geometry of this feature and similar mid-crustal detachments in other orogens, both ancient (Appalachians) and modern (Himalayas), suggests that this style of crustal deformation is common in continental collisional orogens and has persisted on Earth for at least one billion years.