Imaging accretionary structures in the crust and mantle of the southeastern U.S.A.
Abstract:How is continental accretion accommodated in the crust and mantle lithosphere? We investigate this question in the southeastern U.S., where Mesozoic rifting left the exotic Suwannee Terrane behind on the Laurentian margin, and the accretionary structures of the Laurentia-Gondwana collision have been at least partially preserved.
We analyze Sp phases recorded by SESAME (Southeastern Suture of the Appalachian Margin Experiment), nearby Transportable Array and permanent stations. Sp receiver functions for individual waveforms were obtained by extended time multi-taper deconvolution, and migrated into a 3D volume using common conversion point stacking, a spline function representation of phase Fresnel zones, and 3D crust and mantle velocities.
Within the crust, we observe a pair of prominent phases (positive and negative, corresponding to a velocity increase and decrease with depth respectively). The lower, negative phase is continuous over more than 500 km of N-S distance. It is subhorizontal beneath the Blue Ridge and Inner Piedmont terranes, and south-dipping across the suture. The positive phase also dips south at the suture, then flattens in the lower crust. We observe such phases in both eastern and western Georgia, indicating along-strike continuity over at least ~170 km. Beneath SESAME stations in western Georgia, the negative phase is subhorizontal in the midcrust (~17 km depth) from 35.6°N, near the Blue Ridge Thrust, to 32.7°N. From 32.7°N to ~31°N, overlapping the suture zone inferred from COCORP reflection data, it dips south at a minimum angle of 8°. Further south, interference with the Moho may bias apparent phase depth. In the mantle, we observe a N-S transition in discontinuity structure across the suture. Ongoing work with wavefield migration (Bostock et al., 2001) should better resolve discontinuity dips and depths.
The crustal phase pair likely represents the top of the Laurentian plate, overthrust by allochthonous Appalachian terranes and the Gondwanan Suwannee fragment. The crustal phase pair is not explained by sedimentary basins in the region, as the depth, topography, and lateral extent of these phases is inconsistent with observations of basin structure. The N-S scale of this crustal discontinuity structure is similar to the decollement imaged beneath the Himalayas and southern Tibet.