integrating seismic reﬂ ection and geological data and...

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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 Allegha-nian event. Reprocessed deep seismic refl ection data from west-central Georgia (pre- and poststack noise reduction, ampli-tude 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 Pro-terozoic 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 con-sidered; model (1) is favored if both geom-etry 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 n-ity 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.

Keywords: Appalachians, seismic refl ection, tectonics, décollement, faulting.

INTRODUCTION

The southern Appalachians are one of the most studied orogens in the world (Rodgers, 1970). Restoration of cross sections based on regional seismic profi les and potential fi eld data in the southern Appalachians suggests that most of the metamorphic core of the orogen has been transported northwestward along the master Appalachian décollement for at least 260 km (Cook et al., 1980; Hatcher and Zietz, 1980). Transport of the Blue Ridge–Piedmont allochthons occurred during the late Allegha-nian (Permian) orogeny over the autochtho-nous Grenvillian (ca. 1.1 Ga) basement and its cover of Late Proterozoic rifted margin and Paleozoic platform rocks (Hatcher, 1987). Base-ment blocks caught up and transported toward the trailing edge of the overriding allochthon are referred to as internal basement massifs (Hatcher, 1983). Understanding the processes by which these basement massifs are incorporated into allochthons can provide clues as to how a preexisting continental margin is destroyed and “recycled” into the core of the orogen. Although the importance of 1.1 Ga Grenvillian rocks has been recognized for decades throughout the Appalachians, the degree to which these rocks were accreted to the orogen by distal lateral transport remains elusive. If, as we argue herein, these basement massifs were laterally accreted from another continental margin, this would indicate that the lithologic assemblage in the orogen is strongly infl uenced by the crustal composition of the other continent. In this paper, we focus on the origin of one of the most

Integrating seismic refl ection and geological data and interpretations across an internal basement massif: The southern Appalachian Pine

Mountain window, USA

John H. McBride†

Department of Geology, Brigham Young University, P.O. Box 24606, Provo, Utah 84602-4606, USA

Robert D. Hatcher Jr.‡

Department of Earth and Planetary Sciences, University of Tennessee, 306 Earth and Planetary Sciences Building, Knoxville, Tennessee 37996-1410, USA

William J. Stephenson§

U. S. Geological Survey, M.S. 966, P.O. Box 25046, Denver, Colorado 80225, USA

Robert J. Hooper#

ConocoPhillips, P.O. Box 2197, Houston, Texas 77252-2197, USA

GSA Bulletin; May/June 2005; v. 117; no. 5/6; p. 669–686; doi: 10.1130/B25313.1; 12 fi gures; 2 tables.

†E-mail: [email protected].‡E-mail: [email protected].§E-mail: [email protected].#E-mail: [email protected].

McBRIDE et al.

670 Geological Society of America Bulletin, May/June 2005

intensely studied internal massifs in the Appa-lachians, the Pine Mountain window (Fig. 1A), which contains the southernmost occurrence of 1.1 Ga basement in the orogen (Sears et al., 1981; Hatcher, 1983; West et al., 1995). The Pine Mountain window provides a rare oppor-tunity to study rocks of the outboard (possibly exotic) portion of the Laurentian margin that are elsewhere buried beneath the Blue Ridge–Pied-mont allochthon (Carrigan et al., 2003; Yokel and Steltenpohl, 2001).

The Pine Mountain window trends WSW from west-central Georgia into eastern Alabama before disappearing beneath the Coastal Plain (Fig. 1B). It is a window through allochthonous high-grade metamorphic rocks of the Piedmont terrane that exposes high-grade 1.1 Ga rocks and their medium-grade early Paleozoic(?) sedimentary cover (Sears et al., 1981; Hooper and Hatcher, 1988) (Fig. 1B and 1C). Despite the considerable attention that has been focused on the Pine Mountain window (Schamel and Bauer, 1980; Bartholomew, 1984; Sears and Cook, 1984; Steltenpohl, 1988; West et al., 1995; Hooper et al., 1997), a basic issue remains immersed in controversy. Do Grenvillian rocks exposed in the window represent: (1) an autoch-thonous massif that has been displaced upward by normal-sense motion along the Towaliga fault; or (2) an allochthonous or exotic block plucked from the preorogenic rifted Laurentian margin and transported a signifi cant distance landward above the master décollement (Nelson et al., 1987; Hooper and Hatcher, 1988; West et al., 1995; Hooper et al., 1997). Resolving this issue is fundamental to understanding the rela-tive importance of vertical exhumation versus lateral transport on the crustal evolution of the Appalachians. Our study reanalyzed a deep seis-mic refl ection profi le across the Pine Mountain window, providing critical new constraints on interpreting the upper crustal structure of the décollement beneath and immediately around the window. The results are integrated with cur-rent information on the geology of the window (including a new compilation of the surface geology) in order to provide two alternative tec-tonic interpretations. This study also illustrates the value of reanalyzing “old” seismic refl ection data sets, which may yet reveal information use-ful to the geological community.

REGIONAL GEOPHYSICAL AND GEOLOGICAL SETTING

Hundreds of kilometers of seismic profi les traverse the southern Appalachians (Cook et al., 1979; Cook et al., 1983; Nelson et al., 1985; Pratt et al., 1988; Costain et al., 1989; Phinney and Roy-Chowdhury, 1989; McBride and Nelson,

1991). A common element to emerge from vir-tually all of these data has been the existence of a shallow crustal (usually 6–12 km depth [2–4 s refl ection traveltime]) subhorizontal refl ection package beneath the exposed core of the oro-gen, which is generally interpreted to track the position of the master Appalachian décollement (Cook et al., 1979; Lampshire et al., 1994) or the basal foreland stratigraphy beneath the detach-ment (Clark et al., 1978; Costain et al., 1989; Hatcher et al., 1989). This refl ection package

can be traced from beneath the Appalachian foreland (Appalachian Plateau and Valley and Ridge provinces) at least as far oceanward as the westward edge of the Atlantic Coastal Plain (Cook et al., 1981; Lampshire et al., 1994). Most investigators interpret the subhorizontal refl ection sequence itself as originating from relatively undeformed Lower and Middle Cam-brian continental shelf strata (Rome Formation, Conasauga Group, and lateral equivalents) just beneath the décollement (Clark et al., 1978;

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A

PINE MOUNTAIN WINDOW SEISMIC REFLECTION

Geological Society of America Bulletin, May/June 2005 671

Cook et al., 1979; Lampshire et al., 1994). Alter-natively, some have argued that a mylonitic zone directly associated with the décollement may contribute to the refl ectivity (Iverson and Smith-son, 1982; Hurich et al., 1985). Hatcher (1972) fi rst interpreted the Blue Ridge and Inner Pied-mont to be allochthonous. This explanation was revived with the “thin-skinned” interpretation of the Appalachian core engendered by the fi rst deep seismic refl ection observations (Cook et al., 1979), which implied that prominent early-to-mid-Paleozoic tectonic boundaries mapped at the surface were truncated and laterally transported above the Appalachian décollement. In contrast with the thin-skinned interpretation, Sears et al. (1981) concluded from surface map-ping that vertical uplift of Grenvillian basement in the Pine Mountain window had occurred along steep normal faults on the northern fl ank of the window and within it.

PINE MOUNTAIN WINDOW

The rocks exposed (Fig. 1B) in the Pine Mountain window comprise two assemblages: a basement gneiss complex and an overlying metasedimentary cover sequence that resemble known Grenvillian basement and cover rocks exposed in the Blue Ridge to the northwest (Sears and Cook, 1984; Odom et al., 1985; Carrigan et al., 2003; Steltenpohl et al., 2004). Isotopic age determinations indicate ca. 1.1 Ga for the granulite-facies basement gneiss similar to ages determined from other basements in the Blue Ridge (Odom et al., 1985; Carrigan et al., 2003). The Pine Mountain basement and its cover were deformed during the Paleozoic into a series of northwest-vergent recumbent fold nappes (Schamel and Bauer, 1980), which, as indicated by the outcrop pattern inside the window, were subsequently arched into a gently east-plunging antiform (Fig. 1B).

The window is framed by three major faults—the Towaliga fault to the north, the Box Ankle fault to the east, and the Goat Rock–Bartletts Ferry fault system to the south—which have distinctly different relative ages, rheology, and kinematics (Fig. 1C) (Hooper and Hatcher, 1988, 1992; West et al., 1995). Russell (1976) reported a Rb-Sr 375 Ma (Devonian) age for mylonite along the Bartletts Ferry fault. Student and Sinha (1992) determined lower-intercept U/Pb ages for discordant zircons of 303 Ma for Box Ankle fault mylonite and 330 Ma for Ocmulgee fault mylonite, indicating that all are Alleghanian faults. The Towaliga fault truncates the Box Ankle, so the Towaliga fault, with its garnet-grade mineral assemblage, is also an Alleghanian fault with dextral strike-slip move-ment coeval with mylonite formation (Hooper

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McBRIDE et al.


and Hatcher, 1992). At the eastern end of the window, the Box Ankle fault is a northwest-vergent thrust that developed close to the sil-limanite grade metamorphic peak. It separates Grenvillian basement in the footwall from Inner Piedmont rocks in the hanging wall (Hooper and Hatcher, 1988; Hooper et al., 1997; West et al., 1995). The Goat Rock fault, which marks the southern limit of the window from eastern Alabama to westernmost Georgia, separates basement and cover inside the window from (Inner) Piedmont (Uchee belt) rocks to the south (see Fig. 1B for these relationships). It likewise developed prior to the thermal peak but after the Box Ankle fault. The Goat Rock fault truncates the Box Ankle fault in west-central Georgia and then abruptly ends. Steltenpohl et al. (1992) reported Rb-Sr whole-rock and Ar40/Ar39 ages of 297 and 278 Ma for Goat Rock mylonite. While these dates could record movement on the fault, consistent with the Alleghanian kinematic model for the Pine Mountain window outlined below (Hatcher et al., 2003), they could also represent uplift ages.

Farther east (near Bolingbroke, Fig. 1B) the dextral Dean Creek fault cuts the high tempera-ture Ocmulgee fault and passes into the Carolina terrane, probably joining the dextral Modoc fault in central Georgia, as suggested by West et al. (1995). Southwestward the Dean Creek remains the boundary between the Carolina terrane rocks to the southeast and Inner Piedmont rocks to the northwest, maintaining its dextral movement sense and greenschist facies retrograde fabric. The Goat Rock fault appears to truncate the southwestward continuation of the Box Ankle fault, but this and other relationships between these large faults are less certain in the absence of high-quality geologic mapping from the Flint River westward into Alabama (Fig. 1B). Move-ment on both the Goat Rock and Dean Creek faults is dextral strike slip with a minor thrust component. The northwestern margin of the window is bounded by the post-thermal peak Towaliga fault, which postdates the other fram-ing faults and contains retrograde (sillimanite to garnet) mylonites and dextral shear-sense indi-cators (Hooper and Hatcher, 1988, 1992). On the basis of K-Ar muscovite crystallization ages from the foliated cataclasite along the Towaliga fault zone in Georgia, at least two episodes of deformation are known: cataclastic fl ow at ca. 296 Ma (i.e., Alleghanian) and brecciation at ca. 237 Ma (i.e., Middle Triassic) (Babaie et al., 1991; Hadizadeh et al., 1991). Some of the earliest structural interpretations of the Pine Mountain window depict the Towaliga fault as having signifi cant down-to-the-northwest normal displacement (e.g., Sears et al., 1981). The framing faults of the window at the east end

are all dextral strike-slip faults, except the Box Ankle fault, which is a thrust. Farther west there appears to be some evidence that the Goat Rock and Towaliga faults may have had an early his-tory of thrusting, although kinematic indicators here also suggest dextral strike slip (probably recording last motion).

Three key interpretations for the origin of the Pine Mountain window (Fig. 2) have been pro-posed, all hinging specifi cally on the fate of the master Appalachian décollement as it approaches the northern margin of the window from the northwest beneath the allochthonous Blue Ridge and Inner Piedmont (Fig. 1A). As fi rst reported by Nelson et al. (1985), initial observations from a deep seismic refl ection transect acquired by the Consortium for Continental Refl ection Profi ling (COCORP) extending from north-eastern Alabama southward to northern Florida suggested that the décollement ends abruptly at the northern margin of, and does not pass beneath, the Pine Mountain window (Nelson et al., 1985). Based on these observations, Nelson et al. (1987) (Fig. 2A) interpreted the window as an autochthonous block of Grenvillian basement (and not an accreted terrane) that had been dis-placed upward by ~9 km of down-to-the-north-west normal faulting along the Towaliga fault during late Paleozoic or early Mesozoic crustal extension (e.g., during the 237 Ma deformation along the Towaliga fault; Babaie et al., 1991). In this interpretation, the Bartletts Ferry and Goat Rock faults represent remnants of the décol-lement, which had been lifted up and eroded from above the window (Fig. 1B). Alternatively, Hooper and Hatcher (1988) (Fig. 2B) indicated that the framing faults all have different ages, rheology, and kinematics and argued that they thus cannot be part of the same fault or the same décollement. In their interpretation (Hooper and Hatcher, 1988), the décollement passes beneath the window at depth, making the Pine Mountain window a suspect terrane. In this model, the lack of subhorizontal refl ectivity beneath the window was explained by the décollement beneath the window tectonically stripping off the generally refl ective sedimentary cover sequence (or it was never present?) during thrusting, thereby juxtaposing granitic basement on basement. In a third interpretation, West et al. (1995) (Fig. 2C) likewise discounted the notion that the Goat Rock fault represents the exhumed décollement (Nelson et al., 1987) and proposed instead that the master décollement was the Box Ankle fault (Fig. 1B). In this model, the décollement does not undercut the window but is exposed at the surface in an ~10-km-thick shear zone. Hooper et al. (1997) agreed that the Box Ankle fault might locally be a refolded segment of an earlier Alleghanian décollement that was abandoned

as the Pine Mountain block was plucked from the continental margin, forming a new deeper décollement.

Some models for the origin of the Pine Mountain window are thus strongly predicated on the observation from deep seismic refl ection data that the Appalachian décollement does not extend beneath the window (Nelson et al., 1985). Two models take the refl ection data at face value (Fig. 3) and offer explanations for why the detachment does not pass beneath the window (Nelson et al., 1987; West et al., 1995). The alternative model of Hooper and Hatcher (1988) offers a geophysical solution to the lack of refl ectivity beneath the window by suggesting the detachment continues beneath the window but juxtaposes low-impedance-contrast granitic basement on basement yielding low refl ectivity.

Sensitive high-resolution ion microprobe (SHRIMP) U-Pb ages of detrital zircons have yielded inherited core ages in the range of 2.0–2.4 Ga, suggesting the Pine Mountain cover metasedimentary rocks and assemblage were derived from Amazonia (Steltenpohl et al., 2004). Both the metasedimentary cover rock and the basement on which it was deposited contain 1.1 Ga U-Pb zircon ages, but these ages are com-mon to both Laurentian and Gondwanan Gren-villian terranes. These data require that, if the Pine Mountain basement-cover assemblage rests on Laurentian crust, a subhorizontal thrust must separate the two crustal assemblages. Palinspas-tic considerations preclude simple truncation of the southeast-dipping Laurentian basement-cover assemblage beneath the Inner Piedmont, Blue Ridge, and Valley and Ridge, so thoroughly imaged on COCORP, ADCOH (Costain et al., 1989), and industry seismic refl ection profi les.

COCORP GEORGIA LINE 15

The conclusion of Hooper and Hatcher (1988) that the Appalachian décollement passes beneath the Pine Mountain window but has no refl ectivity is diffi cult to test because it was based on “lack” of evidence. Considering the distinct variation in surface geology across the Towaliga fault, in the degree of data coverage (fold of cover), and in the topography, we suspect that the variability in expression of the décollement level on COCORP Georgia Line 15 (Fig. 3) is likely a combination of changes in both deep geology and acquisition conditions. We thus pursued a two-pronged approach of analyzing possible sources of noise (Fig. 4), signal loss, and imaging problems along COCORP Georgia line 15 while completely reprocessing the data.

The data comprise ~91.4 km of coverage, with two small gaps, and were acquired in 1984 by Geosource Inc. Five truck-mounted vibrators pro-



vided the energy source (16 vibroseis sweeps per station) that was recorded into 96 channels with 24 geophones per group. The source and group intervals were 100.6 m, deployed mostly as an off-end receiver spread. A 16-s source sweep of 8–32 Hz was correlated with a 32-s recording to produce a 16-s fi nal record sampled at 8 ms. The profi le begins to the south in the Coastal Plain and trends NNW across the Carolina terrane,

then the Uchee (Inner Piedmont?) belt, crosses the Pine Mountain window, and extends into the Inner Piedmont (Fig. 1B). Although the profi le crosses the window west of the best fault expo-sures (West et al., 1995), it nonetheless traverses the central portion of the window including the Towaliga and Goat Rock framing faults.

The original fi nal coherency-fi ltered stacked refl ection record (Fig. 3) represents the best

processing and display of the stacked data (Nelson, 1988) available until now. This sec-tion shows two prominent refl ection packages in the upper crust (0–5 s) north of the Towaliga fault (i.e., north of the window) interpreted by Nelson et al. (1985, 1987) to represent the décollement. The northernmost refl ections comprise a simple gently southeast-dipping set ~8.6 km long (stations 775–860). This is

Piedmont terrane Pine Mtn. terranePine Mtn. terrane

Uchee terrane

Figure 2. Three principal hypotheses on the structure of the Pine Mountain window that differ according to the position of the master Appalachian décollement beneath the window. (A) The décollement is normal faulted and exhumed above the window (Nelson et al., 1987). (B) The décollement passes at depth smoothly beneath the window with some offset beneath the Towaliga fault (Hooper and Hatcher, 1988). (C) The décollement is presently at or just below the surface and is represented by the Box Ankle fault (West et al., 1995). See text and respective cited papers for further explanation. Abbreviations in this and subsequent fi gures are SL—sea level; TF—Towaliga fault; SF—Shiloh fault; BFF—Barletts Ferry fault; GRF—Goat Rock fault; BAF—Box Ankle fault; DCF—Dean Creek fault. In this and subse-quent fi gures, “⊗” and “ ” symbols indicate away and toward strike-slip fault motion, respectively.

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McBRIDE et al.


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separated by a 6 km gap from a ~12-km-long complex refl ection package (station 600–715) expressed as an asymmetric synform that appears to be cut off abruptly to the south, ~15 km north of the surface position of the Towaliga fault. The remainder of the sec-tion to the south shows little or no coherent refl ectivity at the level of the décollement beneath the Inner Piedmont, although short refl ection segments are weakly visible beneath the window between 3 and 4 s.

RESULTS

The details of our reprocessing fl ow are pre-sented in Appendix A. For the sake of complete-ness, we have displayed the entire original and reprocessed sections of COCORP Georgia line 15; however, the target of our reprocessing and discussion herein will be focused on the upper crust beneath the Pine Mountain window and immediate vicinity. In order to demonstrate the improvements in the stacked section from our

reprocessing, we present the original COCORP Atlas (Nelson, 1988) line 15 section (Fig. 3) and our fi nal reprocessed section (i.e., including all steps in Table 1 except migration) (Fig. 5A) at the same scale and section coverage. So as to furnish a more direct comparison, we also show the digitally redisplayed original section using data from the COCORP digital database (Fig. 6A) and an intermediate reprocessed stack without the “blend” and “migration” steps in Table 1 (Fig. 6B), both displayed as a traditional variable-area no-wiggle trace record.

Comparing the two large-format sections (Figs. 3 and 5A), one can see that the dominant frequency has shifted somewhat toward the high end on the reprocessed section, provid-ing enhanced spatial resolution on the stack. A strong and more coherent refl ectivity for the décollement north of the Pine Mountain window appears on both the original (Fig. 3) and the reprocessed (Fig. 5A) versions (stations 600–905). Moreover, the new section exhibits a richer refl ectivity beneath the décollement level for this area between 4 and 7 s. Unlike the origi-nal section, horizontal bands of refl ectivity at and just below the décollement level continue further southward (beginning at station 600) toward and passing beneath the northern boundary of the window (Towaliga fault). This refl ectivity band, although present between 3.6 and 3.8 s (Fig. 5B), is weakest beneath either side of the fault (sta-tions 420–480) where CMP fold is low and the topography associated with Pine Mountain is the most rugged and could cause increased raypath distortion (Fig. 3). This band increases in dura-tion (3.0–4.2 s) and internal coherency further south beneath the window where the CMP fold is high and the variation in topography is lower (stations 400–210). South of the Goat Rock fault (near station 210), which is the southern framing fault of the window where the profi le passes into the Uchee (Inner Piedmont?) belt, the horizontal refl ective band loses coherency and increases in duration to 3.0–5.0 s. Moho discontinuity refl ec-tions also die out northward approximately where the rugged topography and low fold of the win-dow begin (station 410), which independently implies that the overall decrease in refl ectivity beneath the station 410–460 interval is partly an artifact of the unfavorable acquisition conditions. A comparison of the close-up excerpt for the two original and reprocessed variable-area wiggle trace sections (Fig. 6A and 6B, respectively) demonstrates poorly developed and/or incoherent subhorizontal refl ectivity beneath the Pine Moun-tain window on the original section as opposed to a relatively prominent subhorizontal refl ection band on the new section.

The highly variable refl ectivity quality along the line (Fig. 5C) prompted us to consider the

TABLE 1. GENERAL DATA PROCESSING FLOW FOR COCORP GEORGIA LINE 15 USED IN THIS STUDY

Input vibroseis correlated SEG-Y shot fi lesKill bad or blank tracesApply 0-phase antialias fi lterGeometry assignment (use inline geometry)†

Test fk fi lter (polygonal accept region) (apply only prestack migration)Air blast attenuation (331 m/s, 400-ms gate)Elevation static correction (250 m datum, 4 km/s velocity)Test residual static correction (do not apply)Mute test and apply; use mute just below shallow refractionsOrmsby bandpass minimum phase fi lter (10–17–28–32 Hz)†

L1 Norm predictive adaptive deconvolution (400-ms operator length, 100-ms prediction distance)†

NMO correction (50% stretch mute) using modifi ed original velocity function500-ms automatic gain controlCDP stack, apply fi nal datum staticsPhase-shift time migration (constant velocity = 4 km/s) (8–32 Hz) (not shown)†

Tau-p-based transformation (–2, 2 ms/trace preserved; 15-trace aperture)†

Eigenvector fi lter (0.1–15.0 s design/application window 0%–20%)†

Blend original with absolute value amplitudes squared†

Display as black and white variable area; no wiggle trace

†Steps that went beyond original data processing.

Figure 4. Analysis of ambient noise along Georgia line 15 expressed as averaged channels from records where no vibrations were made (“wind strips”) for 1–2-s window compared with the relative refl ectivity of the décollement level (boxes).

McBRIDE et al.


Time (s)

Depth (km for 6 km/s)

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Figure 5 (continued). (B) Close-up of Figure 5A showing refl ectivity levels (arrows) beneath the Pine Mountain window. (C) Interpretation of seismic line showing principal features and variation along the profi le. (Continued on following page).

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McBRIDE et al.


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degree of variation in ambient noise levels recorded during the fi eld program by periodi-cally listening for 16 s (correlated traveltime) without operating the vibrators (“wind strips”). We averaged the root-mean-square trace value in a 1-s traveltime window for every 12 chan-nels and compared the variation in ambient noise to the relative refl ectivity in the expected décollement time window (2–4 s) along the profi le (Fig. 4). The results indicate a tendency for weak refl ectivity to correlate with elevated noise levels, even including the short gap in high refl ectivity of the décollement along the northern half of the profi le (i.e., stations 715–775, Fig. 4). Thus, although varying noise level is certainly not the only cause of observed refl ectivity varia-tion, it is a likely contributing factor.

Amplitude decay curves have become a typical means of assessing refl ectivity variations in deep seismic refl ection data (Mayer and Brown, 1986; Barnes, 1991). Applying such an analysis to the Pine Mountain window data (Fig. 7) reveals an amplitude anomaly at the traveltime correspond-ing to the expected décollement level (3.6–3.8 s) both beneath and north and south of the window. Except for the region of rugged topography and low fold around the Towaliga fault (stations

410–500), the expected décollement traveltime marks both a positive amplitude excursion (i.e., a defl ection to the right, Fig. 7) and the onset of increased refl ectivity. The degree of amplitude variation also increases below the inferred décol-lement. These relationships suggest a fundamen-tal boundary controlling the refl ection image (i.e., the décollement) exists on either side of the Towaliga fault but that the image is weaker south of the fault beneath the window.

Horizontal and gently dipping refl ections are also apparent on the newly reprocessed section between ~1 s and the upper surface of the décollement level (3–4 s generally) (Fig. 5A and 5C). Flat-lying or gently southeast-dipping refl ections are best developed beginning near the Towaliga fault and continuing to the south beneath the window to about the surface loca-tion of the Goat Rock fault (stations 480–220). A two-cycle event (“A,” Fig. 5A) appears at 2.4 s above the décollement at station 525 and dips gently northwest (9º maximum) with an asymmetric concave-upward listric geometry to station 700, a lateral distance of ~18 km, where it apparently merges with the shallow part of the décollement refl ection. This refl ection occurs above the symmetric concave-upward décol-

lement and just downdip from the interpreted (Fig. 5C) northwest-dipping Towaliga fault.

For prestack depth migration, the premi-gration fi ltering parameters and fi nal display parameters were chosen to emphasize only the strongest parts of the signal along the expected décollement interval (Fig. 5D). The depth migration is suffi cient to verify the main results of the fi rst processing stream (Table 1) and to provide an actual depth image based on a 2-D velocity function derived directly from the traveltime data. Starting on the northwest, the results show a complex sequence of horizontal refl ections within a 8.2–9.8-km-depth range that then dips downward at an angle of 12º into a mostly fl at refl ection band at an 11.8–12.4-km-depth range, or total depth change of ~3 km.

INTERPRETATION AND DISCUSSION

Continuity of Horizontal Refl ectivity at the Décollement Level

Our results indicate that a subhorizontal band of refl ectivity passes from beneath the Inner Pied-mont, where it has previously been interpreted as the master Appalachian décollement, to beneath

Dept

h (k

m)

Figure 5 (continued). (D) Portion of Georgia line 15 with prestack depth migration applied (Table 2). Moho is Moho discontinuity; A, X, and Y are refl ectors referred to in the text.



Figure 6. (A) Excerpt of original COCORP-processed version of Georgia line 15 (directly from the COCORP database [http://www.geo.cornell.edu/geology/cocorp/COCORP.html] with no additional processing) replotted and displayed as variable-area no-wiggle trace over the area of the Pine Mountain window and immediate vicinity. (B) As above, but using our reprocessed version. For both sections, the display gain used (−3 dB) was identical. See text for additional explanation.

Tim

e (s

)T

ime

(s)

CMP Station Location number

A

B

the Pine Mountain window and the Uchee belt and Carolina terrane, where it previously has not been observed. We interpret the refl ections previously attributed to the décollement beneath the Inner Piedmont and Blue Ridge to continue beneath the Towaliga fault without a signifi cant elevation change. However, our new processing

also indicates a difference in the expression of the refl ectivity, implying a sharp, coherent bound-ary beneath the Inner Piedmont and a clear but relatively diffuse and segmented zone beneath the window. Beneath the southern part of the window (stations 210–400), the décollement consists of two or three levels of more coherent

refl ectivity with less coherent intervals between (Fig. 5B). The weakest expression of the hori-zontal refl ectivity lies between the Towaliga fault and station 410 where there is both low fold and rugged topography.

We have discussed purely geophysical factors that could contribute to variations in refl ectivity

McBRIDE et al.


along line 15. In geological terms, the stark con-trast in refl ectivity north and south of the Towa-liga fault may simply represent a change along the décollement level from buried stratifi ed (and thus refl ective) early Paleozoic sediments beneath the Inner Piedmont allochthon to the north (Cook et al., 1983) to less of an acoustic contrast directly beneath the window due to Grenvillian granitic basement immediately above and below the décollement (Hooper and Hatcher, 1988). It is possible that the original early Paleozoic cover was locally stripped off during early thrusting of the Inner Piedmont allochthon over Grenvillian basement (Hooper and Hatcher, 1988). The more diffuse band of refl ectivity beneath the window, therefore, may indicate a dependence of refl ectiv-ity on the lesser effects of localized mylonitiza-tion, on other large-scale shear fabrics involving local ductile deformation, or on mineralization along a faulted zone rather than the strong effects of fl at-lying, well-stratifi ed sediments (cf. Iverson and Smithson, 1982). Jones and Nur (1984) con-sidered anistropy and the degree of lamination within mylonitized fault zones, including those of the Pine Mountain window, to be capable of producing strong refl ectivity. Others have shown that the effects of changes in anistropy (10%) and bulk seismic velocity contrasts (>0.5 km/s) asso-ciated with mylonites can combine to enhance fault-zone refl ectivity (Law and Snyder, 1997). Jones (1986) used P-wave velocity data reported in Jones and Nur (1984) to calculate a refl ection coeffi cient, based on bulk velocity contrasts

alone, of 0.022 for the Appalachian Brevard fault zone. Finally, the sedimentary cover sequence may have been originally relatively thin or even absent beneath the present-day Pine Mountain window. In western Georgia, the southeastern limit of early Paleozoic shelf sedimentation would be expected to lie at or just southeast of the Pine Mountain window as shown by palinspastic restorations of the thrust belt over the Cambrian Ouachita-Appalachian rifted margin of North America (Ferrill and Thomas, 1988). According to Thomas (1991), the window must be located near the autochthonous edge of Laurentian continental crust and the north-to-south transi-tion from passive shelf margin to rifted margin (off-shelf) facies (Fig. 8). Therefore, we would expect that portion of the décollement that is highly refl ective due to thick, stratifi ed sediments to be limited to the southeast near the Towaliga fault. The décollement would continue further south (e.g., as previously shown by Hooper and Hatcher [1988]) toward the root zone beneath the Coastal Plain (McBride and Nelson, 1991) but without the localized refl ection enhancement afforded by a thick underlying sequence of early Paleozoic shelf sediments.

Interpretation of Refl ectivity Variations Associated with the Décollement

Understanding other refl ectivity patterns associated with the décollement could aid in reducing the number of viable tectonic explana-

tions for development of the window. The most noticeable refl ectivity variation occurring from south to north across the Towaliga fault, other than the décollement itself, is the appearance of complex dipping refl ections above and below the décollement north of the fault. Beneath the dish-shaped décollement north of the fault, the inward-dipping pair of refl ections (X and Y) (Fig. 5A) may represent deeply buried rift sedi-ments emplaced on the early Paleozoic conti-nental edge (Fig. 8) (Lillie, 1984) and/or layered volcanic sequences in what was originally early Paleozoic transitional crust along the old margin (Cook et al., 1983) that may have subsequently been deformed by Paleozoic compression. Else-where, in the central Appalachians, Lampshire et al. (1994) documented examples of synformal undulations of the master décollement accompa-nied by variations in the thickness and geometry of the underlying shelf sediment sequence that are very similar to the synformal refl ectivity pat-tern north of the Towaliga fault. On the northern part of line 15, the 3-km north-to-south increase in the depth of the décollement (i.e., from sta-tions 850–775–700–600, respectively, Fig. 5A) is most simply interpreted as the effect of a pre-existing down-to-the-south normal fault in the Laurentian basement that was created originally along the Late Proterozoic–early Paleozoic rift-related passive continental margin (Fig. 8). Thomas (1991), using well data and seismic refl ection profi les, has documented localized thickening of early Paleozoic shelf strata in

Figure 7. Amplitude decay curves for main portion of Georgia line 15 computed from cmp-stacked data with no amplitude correction except for a long-window (5000 ms) auto-matic gain control. Each curve was computed for 10 cmp traces summed around the location shown (i.e., covering an ~500 m span of the profi le). Locations were selected so as to provide as even a spacing as possible (i.e., every ~5 km) while avoiding areas of no data, low cmp fold, and apparently high-noise areas. The décol-lement level, as mapped inde-pendently from Figure 5A, is indicated.



narrow grabens beneath the Inner Piedmont, Blue Ridge, and Valley and Ridge in Alabama and Georgia. Abrupt changes in the depth of the décollement have been interpreted throughout the core of the Appalachian orogen as being controlled by early Paleozoic (Cook et al., 1983) or more likely Neoproterozoic (Hatcher, 1989; Aleinikoff et al., 1995) rifting.

Refl ections above the décollement north of the Towaliga fault represent structure preserved within the overriding Blue Ridge–Piedmont allochthon. The asymmetric synformal shape of refl ection “A” (Fig. 5A) is characteristic of lithologic contrasts within the allochthon, as imaged on higher-quality refl ection profi les in Virginia and attributed to metamorphosed basaltic or other mafi c igneous bodies (Costain et al., 1989). Alternatively, these and other subhorizontal or gently dipping refl ections could represent nappes within the allochthon as shown, for example, by Sears et al. (1981) in a cross section through the Inner Piedmont just north of the Pine Mountain window.

Two Tectonic Scenarios for the Pine Mountain Window

The results of reprocessing COCORP Geor-gia line 15 help reconcile more than fi fteen

years of controversy over the fate of the master décollement beneath the Pine Mountain window and its spatial and temporal relationships to the framing faults of the window. Our results also allow us to harmonize geological and geophysi-cal observations from the window with other deep seismic refl ection profi les in the Coastal Plain directly south of the window (Fig. 1A). These or any subsequent models must incor-porate the following observations: (1) a hori-zontal band of refl ectivity passes continuously from beneath the Inner Piedmont to beneath the Pine Mountain window, the Uchee belt, and Carolina terrane; (2) the refl ectivity of the horizontal refl ections at ~12 km depth (Fig. 5C) varies from strong to diffuse north and south of the Towaliga fault, respectively; (3) only small amounts of displacement (i.e., a few hundred meters at most) occur downdip of the Towaliga fault, although none is required by the data; and (4) a sharp break occurs in the strong refl ectivity below station 600 (Fig. 5C). We consequently present two tectonic models for the emplace-ment of the Pine Mountain window—a primary model and a modifi cation thereof.

Primary ModelOur primary model (Fig. 9A) is based partly

on westward extrapolation of the data of Hooper

and Hatcher (1988) for the eastern portion of the Pine Mountain window (profi le A–A’; Fig. 1B). Here the faults framing the window on the northwest and southeast (e.g., Towaliga and Goat Rock or Dean Creek faults, respectively) may have been initiated during the late Paleozoic as northwest-verging thrusts, intimately associated with the emplacement of the massif, and were later reactivated as dextral strike-slip faults. The Box Ankle fault, which frames the east end of the window (Fig. 1B), is truncated by both of these high-angle faults or their lateral equivalents. Hooper and Hatcher (1988) mapped the end of the Towaliga fault just east of this truncation. Although all the framing faults are Alleghanian structures, all ultimately root into or are truncated by the master décollement. The rocks inside the window appear to comprise a horse derived from the middle Paleozoic continental margin to the southeast, and the internal structure of the Pine Mountain window may consist of a stack of duplex imbricates (Fig. 9A). Faults like the Shi-loh that appear to be normal faults (Fig. 1B) may simply be late, east-vergent imbricates produced during compression and crowding of the Pine Mountain block against the Inner Piedmont to the northwest. Our cross section through the eastern part of the window (Fig. 9A) shows a southeast-dipping to near-vertical Towaliga fault, consistent with the kinematics of this fault and the other framing faults of the window. The geometry revealed from reprocessing line 15 accommo-dates only a very small (<200 m) displacement of the décollement downdip from the Towaliga fault where the two intersect, because greater dis-placement would be clearly resolved, and none is actually required. Such a revision is consistent with the overall interpretation of the window as a northwest-vergent duplex (pre–strike-slip his-tory). In this model, the termination of bright horizontal refl ectivity beneath station 600 likely represents abrupt thinning of the early Paleozoic cover sequence along the original continental margin (Fig. 9A), while the more diffuse refl ec-tivity beneath the window represents a very thin or absent cover sequence or a zone where the sequence has been tectonically stripped, leav-ing a granitic basement-to-basement contact as described earlier. Northwest of the Towaliga fault, the inferred early Paleozoic normal fault (below station 800; Fig. 5C), across which the décollement drops down to the southeast, could have formed a buttress against which early Paleo-zoic shelf strata were synformally folded and deformed by the overriding allochthon just ahead of the growing Pine Mountain duplex.

Modifi ed Primary ModelWe also present a modifi ed version (B–B’;

Fig. 1B) of the foregoing scenario based more

Figure 8. Interpreted late Precambrian–early Paleozoic continental margin marked by rift segments and transform faults from Thomas (1991) with position of Pine Mountain window indicated.

McBRIDE et al.


directly on the seismic profi le in the western part of the window (Fig. 9B). In this model the décol-lement beneath the window originated as a zone of distributed shearing within Grenvillian base-ment that ultimately cuts upward and is captured at the level of the early Paleozoic shelf sequence below the Inner Piedmont allochthon (Fig. 9B). The localization of thrusts cutting down into basement may have been controlled by a preex-isting buttress created by a down-dropped block

during late Proterozoic to early Paleozoic rifting (Fig. 9B). The northern limb of the asymmetric anticline dips steeply north (Fig. 9B) rather than near vertical or southward as in Figure 9A. The bright refl ection termination below station 600 results from the abrupt steepening of the north-ern limb, while the relatively diffuse horizontal refl ectivity beneath the window results from the base of a low-angle shear zone within basement. Late Paleozoic or Mesozoic normal fault reacti-

vation of the Towaliga fault (Nelson et al., 1987) (Fig. 2A) is admissible along the northern limb of the anticline (Fig. 9B). This alternative model explains the refl ectivity beneath the Pine Moun-tain window, but describes a less likely scenario because of what we know about the timing and kinematics of the faults from micro- to map-scale structural relationships observed on the surface during detailed fi eld studies at the east end of the window.

A

B

Figure 9. Two tectonic scenarios for the emplacement of the Pine Mountain window. (A) Interpreted structural cross section for the eastern end of the window mainly based on geological observations and the reprocessed line 15 refl ection profi le over the western end of the window. (B) Alternative cross section for the vicinity of the seismic profi le further west. This model is based on ideas kindly provided by K. D. Nelson. Horizontal ruled pattern represents interpreted early Paleozoic shelf strata. Abbreviations are as in Figure 1B.



Relationships to Other Data—Regional Implications

In western Georgia, our observation that the décollement passes beneath an allochthonous Pine Mountain window can be reconciled with the interpretation of the COCORP deep refl ec-tion transect over the Coastal Plain to the south (Nelson et al., 1985; McBride and Nelson, 1991) (Fig. 10). Our position for the décolle-ment beneath the window at 3–4 s (11–12 km) approximately matches the shallowest level of coherent dipping refl ectors, or other complex structure in the middle and lower crust, on the Coastal Plain refl ection profi les (specifi -cally, line 14) (Fig. 1A). The décollement may therefore continue into the dipping refl ections interpreted as thrusts or shears within the root zone from which the décollement ultimately emerged during late stages of the Alleghanian orogeny. This relationship has implications for the division between North American–affi nity Piedmont terrane, the exotic Pine Mountain crustal block (Steltenpohl et al., 2004), and more outboard exotic terranes. The Ocmulgee fault, which parallels and probably merges at depth with the Goat Rock fault where crossed by line 15, is a fundamental intra-Piedmont ter-rane boundary separating North American Pied-mont rocks and Pine Mountain block crust from the exotic Carolina Panafrican Neoproterozoic to Ordovician volcanic-arc terrane to the south (Figs. 1C and 10) (Hooper and Hatcher, 1988, 1990). Foliation orientations related to the Ocmulgee fault dip moderately to steeply south-east toward the south-dipping refl ections that penetrate the middle and lower crust immedi-ately to the south (Fig. 10). This implies that the

leading edge of the root zone would correspond to the Carolina–Inner Piedmont terrane bound-ary (possible Neoacadian or Alleghanian suture; Bream et al., 2001; Bream et al., 2004; Hatcher, 2002) in the southern Appalachians (Hooper and Hatcher, 1990), which was later reactivated as the root zone for the Alleghanian décolle-ment (McBride and Nelson, 1991) (Fig. 10). The reprocessing results, showing a continuous décollement beneath the window, therefore sup-port a process in the construction of a mountain belt where preexisting crust is reorganized by “recycling” old basement by lateral accretion from deeper levels into the evolving allochthon. This is consistent with derivation of the Pine Mountain window crustal block from a Gond-wanan source, as suggested by Steltenpohl et al. (2004).

Proposed Kinematic Model

A possible kinematic scenario for formation of the Pine Mountain window is: (1) Early Paleozoic(?) deposition near Gondwana of the Pine Mountain window cover sequence on 1.1 Ga crust; (2) Middle Paleozoic (Devonian?) accretion of the Pine Mountain terrane to Lau-rentia; (3) Middle Paleozoic (Devono-Mississip-pian, ca. 350 Ma) accretion of Carolina terrane, partial subduction beneath Carolina of Tugaloo and Cat Square terranes (initial formation of the Ocmulgee fault and central Piedmont suture ca. 330 Ma), and 305 Ma Box Ankle thrust-ing and formation of the Rumble shear zone (Fig. 1A and 1B); (4) Upper midcrustal dextral strike-slip escape and imbrication of the Pine Mountain block along the Towaliga, Bartletts Ferry, and Goat Rock faults (ca. 300 Ma) with

localized apparent normal slip along Towaliga fault related to upward compressional (not tensional or rift-related) buttressing and escape of the Pine Mountain block; dextral southwest propagation of the eastern Piedmont fault sys-tem and the Dean Creek–Modoc fault system (ca. 280 Ma?); and (5) Permian (ca. 270 Ma) head-on collision of Gondwana with Laurentia, producing the Blue Ridge–Piedmont megathrust sheet that propagated along the Permian ductile-brittle transition beheading all earlier terranes, including the Pine Mountain block (imaged in reprocessed COCORP data), transporting them onto the Laurentian platform.

APPENDIX A: REPROCESSING STRATEGY

The reprocessing of COCORP Georgia line 15 was aimed mainly at attenuating incoherent and low-apparent velocity noise and at producing a more accurate poststack image (Table 1). In order to reduce common midpoint (CMP) scattering, we chose a sinu-ous CMP geometry assignment closely following the curved string of stations (Fig. A1) rather than using several long straight-line segments, as was the case in the original processing. This approach is justifi ed because our target refl ectors are fl at (i.e., they have no strike). A frequency-wavenumber fi lter was applied in the source domain to attenuate low-apparent velocity noise (i.e., “steeply dipping” events on source records) and to visually inspect individual source records (e.g., Fig. A2), but was only employed for prestack migra-tion (Table 2), as discussed below. In addition to applying the usual elevation static corrections, sev-eral attempts at automatic residual static correction were made but were rejected due to the possibility of overenhancement (or creation) of artifi cial lateral refl ection coherency. The exceptionally low frequency content and the narrow frequency band (~8–24 Hz) of data together with the high degree of noise prob-ably resulted in the failure of a credible residual static correction solution. We also tested refraction static corrections; however, the long shot record spread

Figure 10. Simplifi ed interpretive drawing through the southern Appalachians in western Georgia based on COCORP deep seismic refl ec-tion profi les of Georgia lines 10–15 and Florida line 1 (see Fig. 1A) (modifi ed from McBride and Nelson, 1991). MAD—master Appalachian décollement; TF—Towaliga fault; OF—Ocmulgee fault. Dipping fault or shear zone is based on dipping refl ection zone observed on Geor-gia lines 13 and 14 (Fig. 1A).

McBRIDE et al.


lengths (~10 km) showed no long-wavelength varia-tions that would translate into noticeable static shifts for the refl ection wavelengths of interest. Further, the receiver spacing (~100 m) was usually much greater than the elevation variation for any particular spread. In fact, due to the limited frequency content, even the application of elevation static corrections produced little improvement in coherency. Refl ected arrivals are variable over the 3–4 s traveltime interval beneath the window (south of the Towaliga fault), and while they are in places best expressed in the near offset ranges, elsewhere they may also be expressed in intermedi-ate-to-far offset ranges (e.g., Fig. A2). The effects of low-apparent velocity noise plus nonsystematic source-generated noise on the near traces (especially channels 1 to about 20, e.g., Fig. A2) are severe in masking coherent refl ected arrivals beneath the Pine Mountain window. In reprocessing the entire data set, we utilized two strategies, one poststack and the other prestack, for attacking this noise.

Two critical steps were the application of an Ormsby bandpass fi lter (100% pass for 17–28 Hz) followed by a predictive deconvolution. These two steps were aimed at reducing incoherent noise in the prestack domain just prior to the CMP sorting and normal move-out correction. We adopted the two-

Figure A1. Common midpoint scatter diagram for COCORP Georgia line 15 with line of receiver stations plotted. Intersections with major faults crossed by the profi le are plotted. F

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dimensionally varying velocity function originally derived by Cornell University from standard normal move-out analysis of CMP records and constant-velocity stacks (Arnow, 1987) with some modifi ca-tion in the critical area of the northern Pine Mountain window. We employed a limited-aperture tau-p (zero-intercept time [tau] and apparent slowness [p]) transform of the data in order to selectively attenuate this systematic noise in the poststack domain. This process is a way of velocity (“dip”) fi ltering (Wang and Houseman, 1997). Following the transformation of the input traces to a selected range of dips (Table 1), each dip trace is weighted by its semblance. The weighted dip traces are then inverse tau-p transformed back to x-t (horizontal distance–traveltime) space with the more coherent events contributing more in the transformation. This transformation was followed by application of an eigenvector decomposition of the trace data after which the original data were recon-structed by adding the low number eigenimages (20% in our case), which tends to preferentially reproduce horizontal refl ection images (Jones and Levy, 1987). Retaining the low number eigenimages attenuates random and coherent noise (Ulrych et al., 1999). In summary, the tau-p process was the most powerful for coherent (low-apparent velocity) noise attenuation in the stack, whereas the eigenvector decomposition was useful for further random noise reduction and cosmetic improvement for display purposes. Finally, in order to display the fi nal CMP stacked sections at small scale and further reduce noise, we blended (1:1) the output data with the absolute value of the ampli-tude squared and displayed as variable area with no wiggle trace (Fig. 5A).

A constant-velocity poststack phase-shift time migration (Gazdag, 1978) was also applied between the fi nal fi lter and the blending step (Table 1) (not shown). In order to obtain an approximate depth image of the décollement north and south of the Towaliga fault, we performed several trials of prestack depth migration (e.g., Fig. 5D). Depth migration was applied to individual shot records using fi nite-differ-ence extrapolators, with the fi ltering processes all applied prior to the migration step (Table 2). Unlike the fi rst processing stream (Table 1), we used a fre-

quency-wavenumber fi lter (Table 2). The shot-record migration uses a vertically and laterally varying interval velocity function, in this case derived from the stacking velocity function converted to gradients, as part of a wavefi eld downward continuation scheme (Soubaras, 1992). Obviously, depth migration directly transforms any errors in the interval velocity fi eld into depth errors; however, our results represent the best representation of depth possible and provide an alter-native processing procedure that confi rms the results of the CMP stack beneath the window (Fig. 5A). After migration, the shot records are CMP-sorted, stacked, and then displayed with an eigenvector decomposi-tion fi lter (Fig. 5A) followed by a trace mixing fi lter (Table 2).

APPENDIX B: REGIONAL GEOLOGIC/TECTONIC MAP COMPILATION

The geologic/tectonic map of the Pine Mountain window (Fig. 1B) was compiled from the sources indicated in the caption and modifi ed from West et al. (1995). Carolina terrane rocks (south and east of the Ocmulgee fault and south of the Dean Creek fault) are typical felsic to mafi c volcanic and volca-niclastic rocks belonging to this terrane (see Hibbard et al. [2002] for descriptions of these assemblages in the Carolinas). Rocks between the Ocmulgee, Dean Creek, and Box Ankle faults to the east, south of the Goat Rock fault, and further west and north of the Towaliga fault belong to the Inner Piedmont assem-blage of dominantly migmatitic metasandstone (much of it mylonitic) and lesser amounts of amphibolite, quartzite, and pelitic schist. This assemblage also belongs to the Uchee belt further west but possibly with a larger volcanic component (Wilson et al., 2002). The rocks inside the window, north of the Goat Rock and Box Ankle faults, west of the Box Ankle fault at the eastern end of the window, and south of the Towaliga fault, consist of the basement assemblage (Woodland facies, Jeff Davis Granite, Moffi tts Mill Gneiss) with its cover of the Pine Mountain series (Sparks Schist, Hollis Quartzite/Chewacla Marble, and Manchester Schist).

ACKNOWLEDGMENTS

Preparation of this paper has greatly benefi ted from discussions with the late K.D. Nelson, who graciously shared his ideas with us. This work was partially supported by the Earthquake Engineering Research Centers Program of the National Science Foundation under Award Number EEC-9701785 (JHM). Field work in the Pine Mountain window was supported by National Science Foundation Grant EAR-7911802 to RDH. Data processing for this study was performed by the authors using Landmark’s ProMAX 2-D™. We acknowledge the support of this research by Land-mark Graphics via the Landmark University Grant Program at the University of Illinois at Urbana-Cham-paign and Brigham Young University. We are grateful to P.A. Mueller for providing us with a draft of the Steltenpohl et al. (2004) paper. Reviews of previous versions by K.D. Nelson, and by J.J. Miller and J.K. Odum for the U.S. Geological Survey, and by W.S. Holbrook, K.C. Miller, and an anonymous referee for GSA Bulletin greatly improved the manuscript.

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TABLE 2. ADDITIONAL DATA PROCESSING USED FOR PRESTACK DEPTH MIGRATION FOR

COCORP GEORGIA LINE 15

Input vibroseis correlated SEG-Y shot fi lesKill bad or blank tracesApply 0-phase antialias fi lterGeometry assignment (use inline geometry)Test and apply fk fi lter (polygonal accept region)16-s resample w/ hi-fi antialias fi lterMute application; use mute just below shallow

refractionsElevation static correction

(400 m datum, 4 km/s velocity)Ormsby bandpass minimum phase fi lter

(10–17–28–32 Hz)400-ms automatic gain controlPrestack fi nite-difference shot migration (400 m

reference datum; 8–32 Hz; 30° maximum dip; 24-tr. Padding; 0–22 km; grad. 2-D velocity func.)

CDP stack, apply fi nal datum statics after stackEigenvector fi lter (2–18 km design window 0%–10%)Trace Mixing (9 traces: 1, 2, 3, 4, 5, 4, 3, 2, 1)Display as black and white variable area; no wiggle trace

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