CONTINENTAL COLLISIONS DURING APPALACHIAN OROGENESIS AS REVEALED BY NEW GEOPHYSICAL OBSERVATIONS FROM THE SOUTHEASTERN U.S (Invited Presentation)

In the Southern Appalachians, EarthScope’s Transportable Array was supplemented by targeted EarthScope seismic experiments: SESAME (passive) and SUGAR (active). Newly available geophysical and other data forces us to reinterpret long accepted answers to fundamental questions. How was collisional deformation accommodated? The processes of collision are at least partially preserved by the suture with the Gondwanan Suwannee terrane, left in the southeastern U.S. by Mesozoic rifting. This suture lies beneath the Coastal Plain, its near-surface location broadly constrained by peri-Gondwanan and Laurentian affinity rocks outcropping to the northwest and sparse borehole samples of Gondwanan affinity in southern Georgia. COCORP reflection images from the 1980s highlighted a steeply dipping structure interpreted as the Alleghanian (Suwannee) Suture that could accommodate significant strike-slip motion. However, recent Sp imaging recorded by SESAME stations revealed a south-dipping structure extending from the near-surface at the northern limit of the Gondwanan suture zone to the mid-crust in southern Georgia and beyond. We interpret this as the Suwannee suture, indicating the Gondwanan crust overthrust the Laurentian margin by at least 330 km in a collision that was near head-on. Remigration of the COCORP data highlights a flat-lying feature at a similar depth to this Sp phase, although the dipping feature originally interpreted as an Alleghanian suture remains relatively prominent; this may be related to older tectonism. Further north, a shallow, flat-lying reflector has long been interpreted as the Southern Appalachian Detachment that accommodated the thin-skinned overthrusting of the allocthonous Blue Ridge terrane. Detailed velocity modeling of SUGAR refraction data reveals this to be a low velocity layer, compatible with this thrusting localized in a metasedimentary layer. We are also compiling estimates of paleo-topography and crustal thickness during orogenesis, e.g. from peak P-T conditions, and how these compare with currently observed values from modern seismic imaging. Combining our deeper understanding of how the collision occurred with estimates of crustal thickness evolution helps us to build a complete picture of the evolution of the lithosphere from pre-orogenesis to today.