T11D-2929
New insights on the final stages of the Appalachian orogeny in the southeast U.S.A.

Monday, 14 December 2015
Poster Hall (Moscone South)
Emily Hopper, Brown University, Providence, RI, United States, Karen M. Fischer, Brown University, Dept. of Earth, Environmental and Planetary Sciences, Providence, RI, United States, Lara S Wagner, Carnegie Institution for Science Washington, Washington, DC, United States, Robert B Hawman, Univ Georgia, Athens, GA, United States and Stephane Rondenay, University of Bergen, Bergen, Norway
Abstract:
The processes of continental collision that built the Appalachian Mountains are at least partially preserved by the suture between Laurentia and the Gondwanan Suwannee terrane, left behind in the southeastern U.S. by Mesozoic rifting. High resolution imaging of this suture provides insight into collisional deformation, important both to local tectonic history and as case study of continental collision that helps us to understand ancient and active orogenies worldwide.

We analyze Sp phases recorded by SESAME (Southeastern Suture of the Appalachian Margin Experiment) and other nearby broadband stations. Sp receiver functions were common conversion point stacked using local crust and mantle velocity models.

In the crust, we observe a south-dipping positive velocity gradient with depth (PVG) that extends from the surface to the mid-crust. Further north, we observe a positive/negative gradient pair. These phases exist in eastern and western Georgia, indicating along-strike continuity for at least 170 km. Modeling with synthetic waveforms shows they are not artefacts of shallow sediments.

The dipping PVG is spatially correlated with COCORP reflectors that were previously interpreted as a steep suture intersecting the Moho. However, the dip and greater horizontal extent of the PVG leads to a different interpretation: the existence of a large-scale detachment with a down-dip continuity of >330 km. This structure implies significant shortening and underthrusting of deep Laurentian or peri-Gondwanan basement beneath Suwannee crust during the final stages of the collision on a scale comparable to India-Eurasia crustal detachments.

In the mantle, we observe a sharp lithosphere-asthenosphere velocity gradient in the northwest part of the study region only, consistent with disruption of lithosphere in the southeast by Mesozoic rifting. Ongoing work with wavefield migration (Bostock et al., 2001) should better resolve discontinuity dips and depths.

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