Crustal-scale shortening structures beneath the Blue Ridge Mountains, North Carolina, USA

Abstract

We present results from a new seismic data set that show evidence for crustal-scale shortening structures beneath the Blue Ridge Mountains in the Southern Appalachians. The data come from six broadband seismic stations deployed on a transect across the Piedmont and Blue Ridge of western North Carolina. The observed structures appear as both a Moho hole and doubled Moho in receiver function CCP (Common Conversion Point) stacks oriented roughly perpendicular to the trend of the Appalachian orogen. We interpret these features as evidence for tectonic wedging and associated delamination and underthrusting of Laurentian lithosphere beneath a crustal indenter. The Moho hole and underlying deeper Moho correspond closely to a signifi cant regional Bouguer gravity anomaly low, which we interpret as being due to overthickened, normal-density crustal material. Beneath the indenter, we observe a double Moho, which may correspond to the partial eclogitization of the underthrust material. This would be consistent with the sharp increase in the observed gravity above this feature. In addition to these crustal structures, we see evidence for a mantle lithospheric discontinuity at 90-100 km depth. This increase in velocity with depth is spatially limited and may dip slightly to the west, though more data are needed to verify this result. We interpret this anomaly to be a fossil slab accreted onto Laurentian lithosphere. If the westward dip is accurate, this slab may be a remnant of a west-vergent subduction zone that was active during the accretion of Carolinia.

Shallow mantle velocities beneath the southern Appalachians from Pn phases

“…In an alternative model, Hibbard et al [] suggest that northwest directed subduction created a suture between the lower crusts of Laurentia and the Carolina terrane in the vicinity of the transition from high to low Pn mantle velocities. This model also provides a possible explanation for a northwest dipping mantle discontinuity observed beneath the Inner Piedmont and Blue Ridge terranes at stations ~250 km to the northeast of our study region [ Wagner et al , ]. However, while this latter scenario implies the possibility of a suture between the Laurentian and Carolina mantle lithospheres, the low‐velocity zone resolved by the Pn phases in this study would still lie within the Laurentian lithosphere.…”

Estimating crustal thickness using SsPmp in regions covered by low‐velocity sediments: Imaging the Moho beneath the Southeastern Suture of the Appalachian Margin Experiment (SESAME) array, SE Atlantic Coastal Plain

“…The second question can be explained at least in part by enhanced erosion in valleys compared with mountain summits. Alternative mechanisms include dynamic processes such as thermal uplift and rebound associated with delamination of a portion of the lithosphere [Wagner et al, 2012]. Low Pn velocities (7.9 km/s) beneath the Blue Ridge suggest that structures within the uppermost mantle contribute to the support of topography [MacDougall et al, 2015].…”

Crustal evolution across the southern Appalachians: Initial results from the SESAME broadband array

“…The location of maximum crustal thickness (~55 km) appears localized beneath stations D20, W34, W35, and W53A, in agreement with previous wide‐angle results (54–56 km) and a receiver function estimate of 50–52 km at MYNC. To the northeast at V53A, crustal thickness decreases to 46 km (5.7 s), which is consistent with estimates of 46 km from the AST array [ Wagner et al ., ]. In the Tennessee Valley and Ridge, estimates of H indicate the Moho shallows to 45–48 km (5.5–6.0 s), but the crust thickens again to 54 km (6.6 s) to the northwest at station V50A along the southeast flank of the Cumberland Plateau.…”

“…The Fall Line marks the onlap of Coastal Plain sediments onto exposed crystalline rock of the southern Appalachians. FLED: Florida‐to‐Edmonton deployment, including station FA07 [ French et al ., ]; AST: Appalachian Seismic Transect [ Wagner et al ., ]; USNSN: U. S. National Seismic Network. (b) Locations of stations analyzed in this study with respect to major terrane boundaries.…”