Paleozoic and Grenvillian Structures in the southern Appalachians: Extended interpretation of seismic reflection data

Abstract

Interpretive reprocessing of seismic reflection data has elucidated Paleozoic and Grenvillian structures in the southern Appalachians. The seismic data include a 7500‐km² grid of ADCOH, Seisdata, and COCORP reflection profiles that traverse the Blue Ridge and Inner Picdmont geologic provinces of North Carolina, South Carolina, and Georgia. Surface geology and potential field data were used to constrain the interpretation. The reprocessed seismic reflection data have delineated the internal and external geometry of the crystalline Blue Ridge‐Inner Piedmont allochthon, including the locations of the Blue Ridge master décollement, Haycsville fault, and Brevard fault zone. On the basis of the reprocessed data, all of the major faults within the allochthonous upper crust sole in the Blue Ridge master décollement. Reflections extending to the southeast from beneath the surface location of the Hayesville fault to the Blue Ridge thrust might be the seismic signature of a high strain zone. This implies that internal deformation of the Blue Ridge allochthon associated with the Alleghanian orogeny might have occurred farther to the west than has been previously documented from field studies. Relative amplitude seismic data enabled the discrimination between Blue Ridge‐Inner Piedmont crystalline rocks and underlying lower Paleozoic shelf strata, thereby delineating the Blue Ridge thrust. The interpreted geometry constrains the top of the shelf sequence beneath the Blue Ridge to depths of less than 3 km. This relatively shallow depth of the shelf strata together with the presence of duplex structures and bright spots that are imaged within the sequence might imply favorable conditions for hydrocarbon exploration beneath the Blue Ridge. Midcrustal reflections from within the upper‐to‐Iower crust are interpreted to originate from preserved Grenvillian structures that were reactivated at the basement surface during Late Proterozoic‐Early Cambrian extension. Reflection continuity is occasionally disrupted by interpreted post‐Grenvillian, pre‐Early Cambrian low‐density intrusions. Topography at the basement surface, possibly caused by the intrusions, is interpreted to have controlled the formation of some of the structures within the overlying allochthon, including Blue Ridge and Brevard fault zone ramps. Correlation of seismic time‐structure contour maps with available gravity data and two‐dimensional gravity modeling suggest that anomalies in the gravity field can be attributed to low‐density sources within the autochthonous crust. Discontinuous reflection packages from depths of 36–42 km are interpreted to originate from the Mohoroviĉiĉ discontinuity. The reflectors trend about N15°E with a true dip of approximately 15°NW.

Crustal Thickness Variations across the Blue Ridge Mountains, Southern Appalachians: An Alternative Procedure for Migrating Wide-Angle Reflection Data

“…These findings differ significantly from previous models based on CMP data (Cook et al, 1979;Hubbard et al, 1991) that show a flat Moho dipping gently to a maximum depth of 43 km beneath the Blue Ridge. They are consistent, however, with regional gravity data (Hawman, 1996) and with the occurrence of crustal roots imaged by profiles crossing other Paleozoic orogens such as the Ural Mountains (Thouvenot et al, 1995;Knapp et al, 1998).…”

“…These correlate with conversion depths in the uppermost mantle for events that arrive shortly after the P s conversion from the Moho in receiver functions for USNSN station MYNC (Baker and Hawman, 2006). An apparent increase in reflectivity also occurs at 8 km, near the projected depth of the top of North American basement (Hatcher et al, 1987;Hubbard et al, 1991).…”

New physical evidence of the role of stream capture in active retreat of the Blue Ridge escarpment, southern Appalachians

“…White (1950) proposed that normal-sense reactivation of the nearby Brevard/Bowens Creek fault zone produced the escarpment. Numerous workers have cited isostatic rebound related to thickened Appalachian crust or flexural response to offshore sediment loading as drivers of BRE formation and evolution (Wright, 1927;Pratt et al, 1988;Battiau-Queney, 1989;Hubbard et al, 1991;Pazzaglia and Brandon, 1996;Pazzaglia and Gardner, 2000). Spotila et al (2004) presented thermochronological constraints on BRE evolution.…”

Contrasts in Tectonic style of the Central and Southern Appalachians, United States: Insights from seismic reflection data

“…4). The most unambiguous examples of this are those shown by Costain and Ç oruh (1989) and Hubbard et al (1991). Comparisons can be made between different sets of seismic reflection data, however, if relative line orientation and differences in recording and processing parameters, and the local geology are taken into account.…”

“…These data have revealed the reflection properties and geometries of the subsurface, and initiated debate on the origin and preservation of crustal reflectivity that records the tectonic history of these regions. In the southern and central Appalachians, regional reflection data in Georgia (Cook et al, 1979Peterson et al, 1984;Nelson et al, 1985), South Carolina (Behrendt, 1986;Williams et al, 1987;Hubbard et al, 1991;Domoracki, 1995;Domoracki, et al, 1999), and Virginia (Harris et al, 1982;Pratt et al, 1988;Lampshire et al, 1994;Glover et al, 1997), in conjunction with detailed geologic mapping, have aided in the understanding of the tectonic processes that formed the complex geology, which we observe today. This paper will focus on two reflection seismic transects in the central and southern Appalachians: (1) COCORP Georgia lines l, 5, and 8 in the southern Appalachians, and (2) reprocessed industry line WV2 and USGS I-64 in the central Appalachians (Fig.…”