Reconstructing the end of the Appalachian orogeny

Abstract

We downloaded data from the Incorporated Research Institutions for Seismology (IRIS) Data Management Center, choosing events with M w >5.8 in the USGS National Earthquake Information Center catalogue. The epicentral distance of the sources was restricted to 55º-85º, which limits postcritical conversions and overlap with other teleseismic phases. The final stacks incorporate 28,123 S-toP waveforms sourced from 6,173 events. Sp Common Conversion Point Stack Method S-toP (Sp) converted waves are generated at sharp seismic velocity gradients, such as lithological contrasts or changes in anisotropy. The offset in time of the Sp conversions depends on the depth of the boundary, and phase amplitude on the size of the velocity contrast. As P waves travel faster than S waves, these converted waves arrive before the main S arrival. As such, they are uncontaminated by sedimentary basin multiples and are more suitable for this region, with its extensive sedimentary cover, than the more commonly used P-to-S converted waves. To isolate these converted phases, data was processed as follows. Phase windows were defined using an automated array-based procedure (Lekić and Fischer, 2014) to account for varying mantle velocities. Waveforms were rotated using a free-surface transfer matrix (Kennett, 1991; Bostock, 1998), with free-surface velocity structure found by grid search (Abt et al., 2010). We deconvolved S components from P components using the extended-time multitaper cross-correlation method (Helffrich, 2006), with optimized parameters (Lekić and Fischer, 2014). In order to be consistent with Ps studies, we reversed the polarities of observed Sp receiver functions such that a positive

Imaging crustal structure beneath the southern Appalachians with wavefield migration

“…(d) Mode inversion for Vp perturbation, using the backscattered PPp mode. Pink dashed line marks the PVG of the dipping PVG/NVG pair interpreted as the crustal suture from Sp observations [ Hopper et al, ].…”

“…Beneath the W line, there is a robust dipping crustal feature at comparable depths to the suture phase observed with Sp waveforms [ Hopper et al, ]. No structures are well imaged at mantle depths, consistent with the moderately gradual negative velocity gradient (NVG) at the lithosphere‐asthenosphere boundary in this region [ Hopper et al, ].…”

“…The multiple phases apparent on the mean mode Vs image arise due to atypically incomplete canceling of mismigrated energy. The Vp perturbation image, based solely on the PPp mode, clearly shows a dipping PVG that matches the position of this interface inferred from Sp phases [ Hopper et al, ]. Not only do the depth and lateral extent match well, the amplitudes appear comparable: the midcrustal suture phase has a larger amplitude than either the Moho or any phase associated with the base of the sediments.…”

“…The thickened crust beneath the Blue Ridge Mountains and the Piedmont imaged by Ps receiver functions [Parker et al, 2013;Schmandt et al, 2015] strikes northeastward (normal to the D line inversion plane). A low-angle south-southeastward dipping interface associated with the Late Paleozoic Gondwana-Laurentia suture strikes ENE over~360 km [Hopper et al, 2016] (normal to the W line inversion plane). To test for artifacts from non-2-D structure, we also migrated SESAME data only ( Figure S3).…”

“…Geological map with topographic shading (Laurentian affinity or suspect terranes, blue; exotic terranes, green; Cenozoic sediments, yellow) [ Steltenpohl et al, ]; subsurface (Mesozoic rift basin, dashes; Gondwanan affinity Suwannee terrane, carets) [ Chowns and Williams , ]; intersection of Suwannee suture with surface (blue dash‐dotted line) [ Hopper et al, ]. Circles and triangles are SESAME and Transportable Array/other stations, respectively.…”