Integrating seismic reflection and geological data and interpretations across an internal basement massif: The southern Appalachian Pine Mountain window, USA

Abstract

The southern Appalachian Pine Mountain window exposes 1.1 Ga Grenvillian basement and its metasedimentary Paleozoic(?) cover through the allochthonous Inner Piedmont. The issue of whether the crustal block inside the window was either transported above the master Appalachian (late Alleghanian) décollement or is an autochthonous block that was overridden by the décollement has been debated for some time. New detrital zircon geochronologic data from the cover rocks inside the window suggest this crustal block was derived from Gondwana but docked with Laurentia before the Alleghanian event. Reprocessed deep seismic refl ection data from west-central Georgia (pre-and poststack noise reduction, amplitude variation analysis, and prestack depth migration) indicate that a signifi cant band of subhorizontal refl ections occurs almost continuously beneath the window collinear with the originally recognized décollement refl ections north of the window. A marked variation in the décollement image, from strong and coherent north of the window to more diffuse directly beneath the window, is likely a partial consequence of the different geology between the Inner Piedmont and the window. The more diffuse image beneath the window may also result from imaging problems related to changes in topography and fold of cover (i.e., signal-to-noise ratio). Two alternative tectonic models for the Pine Mountain window can partially account for the observed variation in the décollement refl ectivity. (1) The Pine Mountain block could be truncated below by a relatively smooth continuation of the décollement. The window would thus expose an allochthonous basement duplex or horse-block thrust upward from the south along the Late Proterozoic rifted continental margin. (2) The window represents localized exhumation of autochthonous basement and cover along a zone of distributed intrabasement shearing directly beneath the window. Either model is viable if only refl ector geometry is considered; model (1) is favored if both geometry and kinematics of Blue Ridge-Piedmont thrust sheet emplacement are incorporated. In either model, the southern margin of the window merges to the west with the Iapetan early Alleghanian Central Piedmont suture, which juxtaposes North American-affi nity Piedmont rocks to the north and exotic Panafrican rocks of the Carolina (Avalon) terrane to the south. Immediately south of the window, this suture dips southward and merges in the lower crust with the late Alleghanian suture joining the Appalachians with Gondwana.

Gondwanan/peri-Gondwanan origin for the Uchee terrane, Alabama and Georgia: Carolina zone or Suwannee terrane(?) and its suture with Grenvillian basement of the Pine Mountain window

“…The Bartletts Ferry, Goat Rock, and Box Ankle fault zones have juxtaposed the Uchee terrane with Grenville basement and its stratigraphic cover within the Pine Mountain window (Figs. 1 and 2, this paper; Sears et al, 1981a;Hooper and Hatcher, 1988;West et al, 1995;McBride et al, 2005). Northeast of the Pine Mountain window, the Modoc fault zone separates the Uchee from the Charlotte terrane (Fig.…”

“…In addition to being a suspect for the Carolina zone suture, the Box Ankle fault zone has also been interpreted as an exposed segment of the "Appalachian décollement" Sears et al, 1981a;Nelson et al, 1987;Higgins et al, 1988;Steltenpohl and Kunk, 1993;Steltenpohl and Moore, 1988;Steltenpohl et al, 1992;West et al, 1995) that had passed above an autochthonous/parautochthonous Pine Mountain terrane. McBride et al (2005) recently resynthesized and reinterpreted seismic data to provide evidence for scattered and weak subhorizontal refl ectors beneath the Pine Mountain window, but the controversy remains unresolved. It is critical to understand how surface faults exposed around the Pine Mountain window relate to the suture and the décolle-ment.…”

“…1). In general, two contrasting schools of thought have held sway for the origin of the Uchee terrane: (1) Laurentianthat is, it may be a septum of Piedmont rocks that once was continuous across the now erosionally breached Pine Mountain anticlinorium ; or (2) Exotic-that is, it might be part of the peri-Gondwanan Carolina zone (Russell, 1978(Russell, , 1985Hatcher, 1988, 1990;Hibbard et al, 2002;McBride et al, 2005) or the Gondwanan Suwannee terrane. Neither the strongly metamorphosed lithologies nor the existing sparse isotopic age information, however, allows for distinguishing which proposal is correct.…”

Reanalysis of the COCORP Utah Line 1 deep seismic reflection profile: Toward an improved understanding of the Sevier Desert detachment question

“…These processes are intended to reduce the effect of noise caused by scattering, but may smooth over small structural details. The reader is directed to McBride et al (2005) for a discussion of this fi lter applied to another COCORP data set. Amplitude balancing with a 1 s window automatic gain control was applied before and after CDP stacking.…”

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“…The Pine Mountain window occurs in some of the more southern exposures of the Piedmont in Alabama, and is an east-plunging antiform exposing Grenvillian basement and platformal metasedimentary cover rocks (Galpin, 1915;Adams, 1933;Crickmay, 1933Crickmay, , 1952Clarke, 1952;Bentley and Neathery, 1970;Sears et al, 1981;McBride et al, 2005;Steltenpohl et al, 2010a). It is framed by the Towaliga fault that separates the window from the Inner Piedmont to the northwest, the Box Ankle fault that closes the east end of the window and is truncated by the Towaliga fault, and the Dean Creek and Bartletts Ferry-Goat Rock faults that separate the Pine Mountain window from the Uchee terrane (peri-Gondwanan; Steltenpohl et al, 2008) to the southeast (Bentley and Neathery, 1970;Sears et al, 1981;Hooper et al, 1997).…”