Appalachian Ultradeep Core Hole Research Articles

Shallow seismic reflection profiling over the Brevard zone, South Carolina

Shallow reflection profiles over crystalline rocks of the Brevard zone show events that correlate with projections of mapped lithologic contacts, in spite of strong attenuation and statics effects associated with a zone of severe chemical weathering (saprolite). The profiles, which were collected in western South Carolina about 5 km from Appalachian ultra Deep Core Hole (ADCOH) regional lines 1 and 3, straddle the contact between the Brevard fault zone and the wider Brevard mylonitic shear zone to the southeast. The principal goal of these experiments was to test the feasibility of using a hammer source and 24-channel seismograph to image near‐surface brittle and ductile structures to a depth of several hundred meters. We recorded two dip lines and one strike line with a total common midpoint (CMP) coverage of 1 km. Processing focused on eliminating static shifts and separating reflections from overlapping refracted S-waves. A statics approach based on the alignment of first arrivals on common‐offset gathers effectively removed most of the statics variations. Suppressing S-waves by apparent velocity filtering of shot gathers caused considerable smearing in the stacked section because of the short receiver spreads. Moreover, for shot gathers with high levels of incoherent noise and/or residual statics, velocity filtering imposed a level of coherency on the output that was not present in the original gather. Better results were obtained by applying severe surgical mutes to isolate the most reliable portions of the velocity‐filtered shot gathers prior to CMP stacking. Final CMP stacked sections show distinct packages of moderately dipping reflections with a lateral continuity of 25 to 100 m over a depth range of 85 to 400 m. The reflections are attributed to compositional layering, variations in degree of anisotropy, near‐horizontal expansion joints, and variations in fracture density. Southeast‐dipping reflections imaged for a 450-m dip line straddling the southeast flank of the brittle Brevard fault zone correlate with projected lithologic contacts of thrust‐faulted units mapped at the surface. Reflections show poorer continuity for lines recorded over a thick sequence of more uniform lithology within the adjacent Brevard mylonite zone. Reflections with apparent dips to the northwest are consistent with images in the ADCOH regional profiles and may reflect late‐stage folding and normal faulting.

Seismotectonic structures along the Savannah River Corridor, South Carolina, U.S.A.

The Savannah River Corridor is defined herein to extend from the Brevard zone in South Carolina to just northwest of Charleston, South Carolina. The length of the Corridor is 400 km, the width 300 km, and the strike is S 40°E. The Corridor includes (1) the Savannah River Site, which is the locus of the most detailed seismic reflection data set in the eastern United States, and is the keystone for the extrapolation of regional reflection seismic data away from the Site, and (2) the South Carolina–Georgia seismic zone. The principal data used for the conclusions of this study are reflection seismic data acquired by the Consortium for Continental Reflection Profiling (COCORP), the Appalachian Ultra-Deep Core Hole (ADCOH) project, and Conoco, Inc. (SRS). Relatively higher seismicity appears to be associated with a crustal scale antiform that has been imaged in the Savannah River Corridor as well as to the northeast along strike in Virginia. An increase in earthquake activity occurs in that part of the crystalline allochthon that overlies and conceals a foreland-dipping imbricate stack that has developed in early Paleozoic rocks beneath the overthrust Piedmont crystalline allochthon. To the southeast, hypocenters of two SRS earthquakes (1985 and 1988), projected along the strike of the Pen Branch northwest-bounding fault of the Dunbarton Triassic rift basin plot on a highly reflective southeastward-dipping shear zone that underlies the basin. Here, the proximity of the hypocenters to the border fault of the Dunbarton basin suggests that syntectonic structures associated with the Pen Branch border fault might be involved. Some of the seismicity is probably associated with reactivation of older Paleozoic and Mesozoic structures.

Interactive property of large thrust sheets with footwall rocks—the Sub thrust interactive duplex hypothesis: A mechanism of dome formation in thrust sheets

Recently acquired Appalachian Ultradeep Core Hole (ADCOH) Project site investigation seismic reflection data and geologic data from the Appalachians and several other orogenic belts suggest an important mutually interdependent relationship exists between emplacement of large crystalline thrust sheets and the deforming foreland rocks beneath. This relationship suggests isolated domes beneath crystalline thrust sheets may be produced by passive folding of the sheet as a result of formation of an antiformal stack duplex in the platform sedimentary sequence beneath. Suggestions that domes in crystalline thrust sheets formed by interference of late open folds is doubtlessly still valid in places, but the platform duplex mechanism is probably also valid to explain the late doming of many crystalline and other large thrust sheets. The dome beneath the Shooting Creek and Brasstown Bald windows in the ADCOH site region is imaged as an antiformal stack duplex at depth. The Tallulah Falls dome, Grandfather Mountain and Mountain City windows, and Smokies Foothills duplex in the site region and elsewhere in the southern Blue Ridge are all late isolated domes and all are probably or demonstrably underlain by antiformal stack duplexes beneath the Blue Ridge-Piedmont composite crystalline thrust sheet. The Assynt window and footwall duplex benath the Arnabol and Moine thrusts in Scotland, and the Engadine window in the Alps may be similar structures.

Alternative Stratigraphic Interpretations for Thrust Sheets Beneath Southern Appalachian Crystalline Overthrust: ABSTRACT

In eastern Tennessee, strata as young as Mississippian lie directly beneath the Great Smoky thrust. There, a stack of three sheets (Pulaski, Saltville, and Copper Creek) contains an estimated thickness of 19,000 ft of sedimentary rocks above Precambrian basement. Seismic profiles obtained as part of the site study for the proposed Appalachian Ultra-deep Core Hole (ADCOH), 85 mi southeast of the Great Smoky fault, have been interpreted to reflect subhorizontal Paleozoic strate about 5,000 ft thick beneath overthrust crystalline rocks and above autochthonous crystalline basement. The most remarkable relationship to be explained is not that subhorizontal Paleozoic strate extend southeastward to the ADCOH site east of the Brevard zone, but rather by what mechanism such a great thickness of Paleozoic strata in several thrust sheets is lost southeastward. Six alternative scenarios of a structural cross section from eastern Tennessee to the ADCOH site show the proposed core piercing a single subhorizontal stratigraphic succession formed by (1) attached platform strata, (2) the Copper Creek thrust sheet, (3) the Saltville thrust sheet, (4) the Pulaski thrust sheet, (5) a previously unknown thrust sheet that came from the eastern platform margin slope of North America, and (6) a thrust sheet of terrane accretedmore » from another continent, probably Africa. The subhorizontal strata at the ADCOH site also may represent a stacking of thrust sheets, each containing only part of the stratigraphic thickness preserved farther northwest. Stratigraphic criteria will permit identification of a specific thrust sheet or of stacked thrust sheets in the core hole. The indicated southeastward thinning of strata in thrust sheets beneath the crystalline overthrust raises questions about foreland thrusting mechanisms.« less

Tectonic implications of new Appalachian Ultradeep Core Hole (ADCOH) seismic reflection data from the crystalline southern Appalachians

Summary. Some 180 km of new VIBROSEIS profiles have been acquired in the southern Appalachian Inner Piedmont, Brevard fault zone and eastern Blue Ridge as part of the ADCOH Project site investigation. These data are of the highest quality yet obtained in a crystalline terrane in the US, perhaps in the world, and reveal several conclusions that should have a direct bearing upon the world-wide nature of composite crystalline thrust sheets and their modes of interaction with the platform rocks beneath. Strong reflections previously interpreted as the base of the crystalline sheet are clearly part of the platform sedimentary (clastic rocks) sequence resting upon the autochthonous basement and early Palaeozoic rift basins. This reflection package and related transparent zones are clearly repeated beneath the crystalline sheet indicating a complex of thrusts repeating units within the platform succession. Reflectors (granitoid-amplibolite contacts) in the crystalline sheet in the Inner Piedmont represent recumbent folds of similar wavelengths and amplitudes to folds mappable on the surface. Duplexing of platform rocks beneath the crystalline sheet appears to have resulted in doming of the crystalline sheet. Similarly, duplex formation in the platform was probably controlled by both the thickness of the crystalline sheet and the rheological properties of the platform succession.

Results from regional vibroseis profiling: Appalachian ultra-deep core hole site study

In 1985, 180 km of regional vibroseis profiles were acquired in the Carolinas and Georgia, southeastern United States, as part of the Appalachian Ultra-Deep Core Hole (ADCOH) Site Study. The data quality is excellent, with large-amplitude reflections from faults and crystalline rocks, lower Palaeozoic shelf strata and from within autochthonous Grenville basement. The profiles image the subsurface more clearly than other available data and allow the possibility of alternative interpretations of important elements of the tectonic framework of the southern Appalachians. The major points in the interpretation are: 1) the Blue Ridge master decollement is at a depth of 2-3 km beneath the Blue Ridge. This thrust increases in dip just NW of the Brevard fault zone. 2) the Brevard fault zone appears to splay from the master decollement at 6 km (2.2 s) near Westminster, S.C., and defines the base of the crystalline Inner Piedmont allochthon. 3) Below the Blue Ridge thrust sheet are images of duplex and imbricate structures (‘duplex tuning wedges’) connected by other thrust faults that duplicate shelf strata to a thickness of 4-5 km. 4) Subhorizontal reflections from depths of 6 to 9 km may be from relatively undisturbed lower Palaeozoic strata as suggested by others. 5) Eocambrian-Cambrian(?) rift basins in the Grenville basement are also imaged. The ADCOH data were originally recorded with 14-56 Hz bandwidth and 8 s length, but an extended Vibroseis correlation was used to produce 17 s data length revealing reflections from within the upper crust. Below 8 s, reflections from within the Grenville basement become weak, but are observable as late as 13 s; however, these Moho (?) reflections are generally short segments.

Heat flow at the proposed Appalachian Ultradeep Core Hole (ADCOH) Site: Tectonic implications

The heat flow in northwestern South Carolina at the Appalachian Ultradeep Core Hole (ADCOH) site area is approximately 55 mW/m². This data supplements other data to the east in the Piedmont and Atlantic Coastal Plain provinces where heat flows > 55 mW/m² are characteristic of post‐ and late‐synmetamorphic granitoids. Piedmont heat flow and heat generation data for granites, metagranites, and one Slate Belt site, in a zone approximately parallel to major structural Appalachian trends, define a linear relation. Tectonic truncation of heat‐producing crust at a depth of about 8 km (a depth equal to the slope of the heat flow‐heat production line) is proposed to explain the linear relation. Using the value of reduced heat flow estimated from this empirical relation, and assuming thicknesses of heat‐producing crust defined by new ADCOH seismic data, the heat flow and heat production at the ADCOH site are consistent with a depth to the base of the Inner Piedmont crystalline allochthon of about 5.5 km. Seismic data at the ADCOH site confirm that the Inner Piedmont is tectonically truncated at about 5.5 km by the Blue Ridge master decollement. Temperatures at 10 km at the ADCOH site are predicted to be less than 200 °C.

Geologic Investigations Associated with Appalachian Ultradeep Core Hole (ADCOH) Site Study: ABSTRACT

Geologic investigations related to the ADCOH site study are concentrated in several areas: (1) detailed geologic mapping of approximately 400 mi/sup 2/ of new area to add to the 3000 mi/sup 2/ of existing detailed geologic mapping, (2) description and interpretation of cores from four shallow boreholes ranging in depth from 500 ft to 1400 ft, and (3) structural interpretation of surface and reflection seismic data. The eastern Blue Ridge Mountains (BR) and inner Piedmont (IP) consist of an assemblage of late Precambrian rift-related sedimentary and mafic volcanic rocks. These were metamorphosed during the early Paleozoic and intruded by plutons of different ages. The Chauga belt (CB) consists of an assemblage of late Precambrian to early Paleozoic(.) greenschist-facies clastic, carbonate, and volcanic rocks separating the two higher grade terranes of the BR and IP. The Brevard fault zone (BZ) is localized along the western edge of the CB. The BZ is a complexly reactivated zone of faulting exhibiting an episodic movement history that may span much of the Paleozoic. One of its last movement episodes brought slices of platform carbonate (Knox.) from beneath the Blue Ridge-inner Piedmont thrust sheet to the surface along a brittle fault. Lithologic units of themore » BR (including the 1.1-Ga Wiley gneiss basement), CB, and IP were encountered in the four shallow drill holes. The basement-cover contact was crossed numerous times in the lower third of the BR hole, indicating isoclinal folding of this boundary. It is uncertain whether the basement-cover contact is a folded fault or an unconformity. New geologic mapping has revealed a major late shear zone near this basement-cover contact on the southeastern flank of the Tallulah Falls dome.« less

Regional Tectonic Implications of Seismic Reflection, Gravity, Structural, and Stratigraphic Data from New Appalachian Ultradeep Core Hole (ADCOH) Site Study: ABSTRACT

The Appalachian Ultradeep Core Hole (ADCOH) site study area is located in the eastern Blue Ridge Mountains and western Piedmont of the Carolinas and northeastern Georgia. The goal of the proposed project is an ultradeep core hole that would penetrate and prove the existence of the main detachment thrust of the Blue Ridge-Piedmont (BRP) thrust sheet. Intermediate objectives include study of higher level faults and rock units within the thrust sheet, as well as investigation of the Paleozoic platform cover and basement beneath the BRP thrust sheet. Site study data have revealed that the level of the main BRP detachment may have shallowed rapidly west of the Brevard fault zone, possibly bringing lower Paleozoic carbonate platform rocks to within 3-4 km of the surface near Hayesville, North Carolina, in a large duplex structure. If so, there are implications here and farther west for possible hydrocarbon accumulations. Duplex structure appears to be a dominant style of deformation both within and beneath the BRP thrust sheet. A recently discovered shear zone on the Tallulah Falls dome may represent an imbricate from one of the blind duplexes.

Heat Flow and Tectonic Implications: Appalachian Ultradeep Core-Hole Site Area: ABSTRACT

Heat Flow and Tectonic Implications: Appalachian Ultradeep Core-Hole Site Area: ABSTRACT

Seismic Signatures of Tectonic Lithofacies from Regional Lines, Appalachian Ultradeep Core-Hole Site Area: ABSTRACT

A highly reflective upper crust is imaged on regional seismic data between Hayesville, North Carolina, and Fair Play, South Carolina. Above Eocambrian(.) rift basins, reflections originate from probable lower Paleozoic shelf strata at 8-10 km (3 sec). Reflections at 1.2-2.6 sec east of Hayesville also might be from lower Paleozoic rocks, implying a (repeated) thickness of about 5 km of shelf strata. Among the most prominent seismic signatures are wedge-shaped packages of reflections opening to the northwest from depths of 6 km (2 sec) and 9 km (3.2 sec) between Hayesville, North Carolina, and Westminster, South Carolina. These reflections may indicate locations where decollements ramp up to shallow crustal levels, and where local stacking of thin beds has resulted in tuning of reflections with a two to three fold increase in reflection amplitudes. The horizontal bases of such packages may indicate the crustal levels of decollements that are usually acoustically transparent. By one interpretation, the Blue Ridge master (largest horizontal displacement) decollement lies above these signatures at a depth of 3-5 km. Just northwest of Westminster, reflections from the Brevard zone may be truncated near or at this master decollement at 5.5 km.