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Late to Post-Appalachian Strain Partitioning and Extension in the Blue Ridge of Alabama and Georgia

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Late to Post-Appalachian Strain Partitioning and Extension in the Blue Ridge of Alabama and Georgia Late to post-Appalachian strain partitioning and extension in the Blue Ridge of Alabama and Georgia Mark G. Steltenpohl1, Joshua J. Schwartz2, and B.V. Miller3 1Department of Geology and Geography, Auburn University, Auburn, Alabama 36849, USA 2Department of Geological Sciences, California State University, Northridge, Northridge, California, 91330, USA 3Department of Geology & Geophysics, Texas A&M University, College Station, Texas 77843-3115, USA ABSTRACT margin, possibly refl ecting its reactivation its position far toward the foreland. Loose during Mesozoic rifting of Pangea. timing constraints for this extensional event Structural observations and U-Pb and The Alexander City fault zone is a middle (late Carboniferous to Early Jurassic) leave 40Ar/39Ar isotopic age dates are reported greenschist facies, dextral strike-slip fault room for several tectonic explanations, but for shear zones and metamorphic rocks in rather than a west-vergent thrust fault, as we favor the following. (1) Late Pennsyl- the southernmost Appalachian Blue Ridge. was previously thought. This fault zone is vanian to Early Permian crustal thickening Two major mylonite zones, the Goodwater- obliquely cut and extended by more east- created a wedge of Blue Ridge rocks bound Enitachopco and Alexander City fault zones, trending, subvertical, cataclastic faults above by the Good water-Enitachopco, have retrograded peak amphibolite facies fab- (Mesozoic?) characterized by intense quartz below by the décollement, and to the north- rics and assemblages in rocks of the ancient veining. These brittle faults resemble those west (present-day direction) by a topograph- Laurentian margin. Both faults are within in other parts of the Blue Ridge, Inner ically steep mountain front. (2) Further a zone of transition between Carboniferous Piedmont, and Pine Mountain terrane and, convergence and crustal thickening caused (Alleghanian) west-directed thrusts in the together with the Goodwater-Enitachopco this wedge to gravitationally collapse with foreland and synchronous strike-parallel and Towaliga faults, they appear to form a southward-driven motion. (3) Mesozoic rift- dextral shear zones in the hinterland. The broad graben-like structure across the entire ing reactivated some of the faults as the Gulf 40Ar/39Ar hornblende and muscovite dates piedmont. Shoulder rocks fl anking the Alex- of Mexico began to open. record late Mississippian cooling and exhu- ander City fault zone contain earlier formed mation from the Late Devonian (Neoacadian (peak to late peak metamorphic) dextral INTRODUCTION orogeny, 380–340 Ma) peak. Retrograde shear zones. A 369.4 ± 4.8 Ma U-Pb ther- mylonites of the Goodwater-Enitachopco mal ionization mass spectrometry date on Following the paradigm shift to plate tectonic fault are of two types. Earlier formed, type zircon records the time of crystallization of thinking in the mid-1960s, which explained 1, upper greenschist to lower amphibolite a dike that cuts one of the shears bracketing how layer-parallel compressive stresses were facies shears are roughly coplanar with the the peak metamorphic fabric to between ca. derived from colliding continents, most south- dominant schistosity of the country rock, and 388 and 369 Ma and places a minimum age ern Appalachian (eastern USA) mylonite zones show northwest-southeast stretching. Later for right-slip shearing. Similar kinematics, were interpreted as west-directed thrusts. formed, type 2 shears are discrete, steeply geometries, tectonostratigraphic positions, Thrust faults are classically documented in dipping, middle to upper greenschist facies and timing indicate that these Devonian the southern Appalachian foreland fold and shear zones that cut across the type 1 shears, shears are more southern counterparts to the thrust belt (i.e., the Valley and Ridge physio- displacing them in an oblique dextral and system of Neoacadian dextral faults exposed graphic province; Fig. 1), where fossiliferous normal slip sense. A 366.5 ± 3.5 Ma U-Pb in the North Carolina Blue Ridge. Paleozoic sedimentary rocks indicate them SHRIMP-RG (sensitive high-resolution Kinematic analysis of the Goodwater- to have formed during the late Carboniferous ion microprobe–reverse geometry) date on Enitachopco and Alexander City faults doc- to Permian (the Alleghanian orogenic phase; zircon from a prekinematic trondhjemite ument that dextral strains in the Alabama Roeder et al., 1978; Woodward, 1957; Hatcher dike that is cut by a type 2 shear zone places and western Georgia Blue Ridge are par- et al., 1989a). Development of methods to con- a maximum age on the time of movement titioned much farther toward the foreland strain the kinematics of fault zones based on along the Goodwater-Enitachopco fault. The than is reported to the northeast, likely as shear-sense indicators in mylonitic rocks in 40Ar/39Ar cooling dates place a minimum on a consequence of the southern Appala- the late 1970s and early 1980s (e.g., see ref- the timing of extensional movement along chian master décollement having passed erences cited in Steltenpohl, 1988) led to the the type 1 shears of between ca. 334 and obliquely across a several kilometer step up paradoxical discovery that practically all of 327 Ma. The Goodwater-Enitachopco fault along the Cartersville transform. The top- the major Carboniferous to Permian mylonite coincides at depth with a basement step up to-the-south-southeast normal-slip compo- zones within the exposed southern Appalachian that has been interpreted as a Cambrian rift nent of movement along the Goodwater- hinterland record right-slip movement rather fault formed along the ancient Laurentian Enitachopco fault is unusual, considering than thrusting (i.e., the Brevard, Towaliga, Geosphere; June 2013; v. 9; no. 3; p. 647–666; doi:10.1130/GES00738.1; 13 fi gures; 3 tables; 1 supplemental fi le. Received 25 July 2011 ♦ Revision received 31 January 2013 ♦ Accepted 19 February 2013 ♦ Published online 7 May 2013 For permission to copy, contact [email protected] 647 © 2013 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/9/3/647/3345404/647.pdf by guest on 29 September 2021 Steltenpohl et al. Modoc, BoxAnkle & WVA hornblende thermochronological analyses. Our Goat Rock shear zones KY results are surprising in that they indicate some- VA Grenville basement 84° 37° A what peculiar kinematic and timing histories t SM that we believe hold important new insights 0 100 km into the late stages of tectonic evolution of the southernmost Appalachians. W dow Cartersvile win transform TN GM Sauratown Mtns GEOLOGIC SETTING BF ennessee Embaymen T ? At the scale of a geologic map of eastern North HF America, stark differences in southern Appa- 86° NC 80° lachian structures are clearly seen between the HF 35° 35° SC Tennessee–western North and South Carolina– GA GS northern Georgia and the Alabama segments of the orogen (Fig. 1). Eastern Tennessee contains AL Brevard zone the classic, thin-skinned, southern Appalachian Laurentian platform foreland fold-and-thrust belt with as many as 13 CF Laurentian margin (E & W BR) labama Promontory AF different, generally coplanar, northwest-directed A ? Dahlonega belt (E & W BR) HL thrusts. In sharp contrast, the Valley and Ridge TC ? olinia Cartoogechaye terrane (EBR) GE Car Cowrock terrane (EBR) of Alabama and western Georgia contains many TF MZ fewer northwest-directed thrusts (see Fig. 1). SWL Tugaloo terrane (EBR & IP) Coastal Plain ACFZ W Emuckfaw Group (EBR) The Cartersville transform corresponds to this PM Uchee terrane Cat Square terrane (IP) transition and is interpreted to mark an ancient GR/BF FZ thermochronology transect in Figure 10 transform fault along the rifted Laurentian mar- gin, separating the Tennessee embayment from Figure 1. Tectonic map of the Southern Appalachians (modifi ed from Hatcher et al., the Alabama promontory (Fig. 1), which served 1989b, 2007a; Hatcher, 2004; Hibbard et al., 2002, 2006; Steltenpohl, 2005; Steltenpohl as a template around which later-emplaced et al., 2008). The Cartersville transform is dashed where we have extended it. Abbre- Appalachian sheets conformed (Thomas, viations: ACFZ—Alexander City fault zone; AF—Allatoona fault; BF—Burnsville fault; 1991, 2006; Tull et al., 1998a, 1998b; Tull and CF—Chattahoochee fault; E BR and W BR—Eastern and Western Blue Ridge; GE— Holm, 2005; Thomas and Steltenpohl, 2010). Goodwater-Enitachopco fault; GMW—Grandfather Mountain window; GR/BF FZ— This thrust stack is directly west of the Great Goat Rock–Bartletts Ferry fault zone; GS—Great Smoky thrust; HL—Hollins Line Smoky and Hayesville thrusts (Fig. 1) that have fault; HF—Hayesville-Fries fault; IP—Inner Piedmont; MZ—Modoc zone; PMW—Pine emplaced the western and eastern Blue Ridge Mountain window; SMA—Smith River allochthon; SWL—Stonewall Line shear zone; terranes, respectively, upon the Laurentian plat- TC—Talladega-Cartersville fault; TF—Towaliga fault. Other letters in bold are state form (Fig. 1; Hatcher, 1987, 2010). The Talla- abbreviations. dega-Cartersville fault is the frontal Blue Ridge fault in Alabama and western Georgia and, like its structural equivalent the Great Smoky fault, Bartletts Ferry, Goat Rock, and Modoc fault from a thickened collisional welt into a col- is the southern Appalachian master décollement zones depicted in Fig. 1; Secor et al., 1986; lapsed and extending rift margin by the end of (Cook et al., 1979). The Talladega-Cartersville Steltenpohl, 1988, 2005; Hooper and Hatcher, Triassic time
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