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Crustal-Scale Shortening Structures Beneath the Blue Ridge Mountains, North Carolina, USA

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Crustal-Scale Shortening Structures Beneath the Blue Ridge Mountains, North Carolina, USA Crustal-scale shortening structures beneath the Blue Ridge Mountains, North Carolina, USA Lara S. Wagner*, Kevin Stewart, and Kathryn Metcalf† DEPARTMENT OF GEOLOGICAL SCIENCES, CB# 3315, UNIVERSITY OF NORTH CAROLINA AT CHAPEL HILL, CHAPEL HILL, NORTH CAROLINA 27599, USA ABSTRACT We present results from a new seismic data set that show evidence for crustal-scale shortening structures beneath the Blue Ridge Moun- tains in the Southern Appalachians. The data come from six broadband seismic stations deployed on a transect across the Piedmont and Blue Ridge of western North Carolina. The observed structures appear as both a Moho hole and doubled Moho in receiver function CCP (Common Conversion Point) stacks oriented roughly perpendicular to the trend of the Appalachian orogen. We interpret these features as evidence for tectonic wedging and associated delamination and underthrusting of Laurentian lithosphere beneath a crustal indenter. The Moho hole and underlying deeper Moho correspond closely to a signifi cant regional Bouguer gravity anomaly low, which we interpret as being due to overthickened, normal-density crustal material. Beneath the indenter, we observe a double Moho, which may correspond to the partial eclogitization of the underthrust material. This would be consistent with the sharp increase in the observed gravity above this feature. In addition to these crustal structures, we see evidence for a mantle lithospheric discontinuity at 90–100 km depth. This increase in velocity with depth is spatially limited and may dip slightly to the west, though more data are needed to verify this result. We interpret this anomaly to be a fossil slab accreted onto Laurentian lithosphere. If the westward dip is accurate, this slab may be a remnant of a west-vergent sub- duction zone that was active during the accretion of Carolinia. LITHOSPHERE; v. 4; no. 3; p. 242–256. doi: 10.1130/L184.1 INTRODUCTION during the Alleghanian orogeny (Iverson and evidence for crustal-scale tectonic wedging as Smithson, 1983; Rankin et al., 1991). Informa- well as mantle lithospheric discontinuities that The Southern Appalachian Mountains are tion about structures located beneath this Blue may represent fossil accreted slabs at 100 km composed of rocks and structures from repeated Ridge–Piedmont allochthon comes from a lim- depth. These results help to explain the observed tectonic events beginning with the assembly of ited number of refl ection and refraction studies, gravity anomalies in the area, and they provide Rodinia during the Proterozoic Grenville orog- most of which do not cross the rugged terrain data that can be used to test tectonic models for eny and ending with the Mesozoic rifting of of the Appalachian escarpment and Blue Ridge Southern Appalachian evolution. Pangea following the Alleghanian orogeny (e.g., Mountains (Nelson et al., 1985; McBride and Hibbard, 2000; Hibbard et al., 2002; Hatcher et Nelson, 1988; Hubbard et al., 1991; Cook and GEOLOGIC SETTING al., 2007a; Hatcher, 2010). What is known about Vasudevan, 2006). While there are a handful of these tectonic events is inferred from geological permanent broadband stations in the southeast- Along the east coast of North America, and geochemical observations, gravity and mag- ern United States, they are widely spaced and the tectonic events spanning the formation of netic anomaly maps, and from a limited num- generally located at lower elevations. Rodinia during the Grenville orogeny (1.2– ber of seismic-refl ection and seismic-refraction This study uses a newly collected data set 1.0 Ga) to the formation of Pangea during the studies (e.g., Hack, 1982; Hutchinson et al., composed of recordings from six broadband Alleghanian orogeny (330–280 Ma) represent 1983; Iverson and Smithson, 1983; Prodehl et seismic stations deployed across the Piedmont one complete Wilson cycle (Hatcher et al., al., 1984; Nelson et al., 1985; Hubbard et al., and the Blue Ridge Mountains in North Caro- 2007a). In the southeastern United States, the 1991; Rankin et al., 1991; Aleinikoff et al., 1995; lina (Fig. 1). This transect fi lls an important hole Appalachian Mountains are the product of four Loewy et al., 2003; Cook and Vasudevan, 2006; in the much-analyzed Consortium for Continen- orogenic events during this cycle: the Gren- Miller et al., 2006; Hawman, 2008; Anderson tal Refl ection Profi ling (COCORP) refl ection ville orogeny, the Ordovician Taconic orogeny, and Moecher, 2009; Fisher et al., 2010). Geo- profi le across Georgia and Tennessee (Prodehl the Late Devonian–Mississippian Neoacadian logic structures observed at the surface in the et al., 1984; Nelson et al., 1985; McBride and orogeny, and the Pennsylvanian Alleghanian Southern Appalachians are diffi cult to link with Nelson, 1988; Cook and Vasudevan, 2006). We orogeny (Hibbard et al., 2002; Hatcher et al., structures at depth because the upper ~10 km of use these data to calculate receiver functions, 2007a; Hatcher, 2010). Subsequent to these crust was transported at least tens of kilometers which can identify seismic discontinuities in orogenic events, Triassic rifting resulted in the and possibly hundreds of kilometers westward the crust and upper mantle. Unlike previous opening of the Atlantic Ocean and the formation active-source refl ection studies, we image the of the modern passive margin (Schettino and Moho clearly beneath the highest elevations in Turco, 2009). *E-mail: [email protected]. †Current address: Department of Geosciences, the Southern Appalachians. Furthermore, this The Proterozoic Grenville orogeny in the University of Arizona, Gould-Simpson Building #77, is the fi rst study to image sub-Moho structures Southern Appalachians was likely due to a col- 1040 E. 4th Street, Tucson, Arizona 85721, USA. down to ~120 km depth in this area. We fi nd lision between Laurentia and South America 242 For permission to copy, contact [email protected] | |Volume © 2012 4 Geological| Number Society3 | LITHOSPHERE of America Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/4/3/242/3045114/242.pdf by guest on 28 September 2021 Crustal-scale shortening structures beneath the Blue Ridge Mountains, North Carolina, USA | RESEARCH v v v Either model results in Carolinia being 83° W v v 81° W v v v located due east of its current surface location in TN v v v v v central North Carolina (Fig. 1) by the late Paleo- v v v v v zoic. During the Alleghanian, the collision with v v v v v v v v v Africa and formation of Pangea completed the v GR v v v v RGRR v v v Valley and Ridge v v E Wilson cycle and resulted in widespread craton- E v GMW v v BEEB v v v v v v v v ward thrusting of the uppermost crust, thereby v v v H forming the Blue Ridge–Piedmont allochthon v Wv v WWHW v (Hatcher, 1972). Displacement estimates for the H v v v MFHMF v v H NC allochthon range from <100 km (Keller, 1999) Fv Laurentianv rocks AFHA v to over 250 km (Rankin et al., 1991). The loca- v v v Brevard faultC tion of the suture between Africa and Laurentia BSCBS v is believed to lie offshore along the East Coast v v magnetic anomaly (McBride and Nelson, 1988). v v CPSZ v v The allochthonous nature of the upper crust Cat Square terrane v Tugaloo terrane makes it diffi cult to link seismically imaged v v v structures beneath the Blue Ridge–Piedmont v v Carolinia allochthon with terrane boundaries observed at Tugaloo terranev 35° N 35° N v the surface. v v v 81° W v GA v PREVIOUS WORK SC v v Constraints on the geometry of the Blue v Ridge–Piedmont allochthon and the structure v of the underlying basement come from a num- v 50 km v v v v ber of active-source studies. COCORP acquired v v v 83° W seismic-refl ection data along several NW-SE– trending profi les in northern Georgia. These Figure 1. Map of study area. Appalachian Seismic Transect (AST) seismic stations are shown as black tri- data were collected between 1978 and 1985 angles. Dotted box shows area covered by our common conversion point (CCP) stacking nodes. Dashed and were reprocessed by Cook and Vasudevan line shows the approximate location of the Blue Ridge Escarpment. Bold lines are terrane boundaries and faults, and gray lines are state boundaries. CPSZ—Central Piedmont Suture Zone; GMW—Grandfa- (2006). Their results show a highly refl ective ther Mountain Window; SC—South Carolina; NC—North Carolina; GA—Georgia; TN—Tennessee. lower crust and Moho beneath the coastal plains and Carolinia, but little to no refl ectivity in the mid- or lower crust beneath the Inner Piedmont (Tohver et al., 2005). Pieces of Laurentian litho- ing the accretion of Carolinia (Hibbard, 2000; or Blue Ridge. In contrast, the 1985 Appala- sphere are believed to be present in the Cuyania Miller et al., 2006; Anderson and Moecher, chian ultradeep core hole (ADCOH) profi les, terrane of northwestern Argentina (Thomas and 2009). The Taconic orogeny (ca. 460 Ma) located slightly farther to the north, do show Astini, 1996; Loewy et al., 2003), while mate- resulted in the emplacement of the Ordovician signifi cant refl ectivity in the mid- and lower rial of South American affi nity is believed to Tugaloo terrane plutons, and the accretion of crust beneath the Inner Piedmont (Hubbard et have been accreted to the southeastern (present arc terranes, onto the Laurentian margin. There al., 1991). While the Moho is not resolvable on orientation) margin of Laurentia (Tohver et al., is some debate over whether or not Carolinia most of the ADCOH lines, a number of mid- 2005). The suture for such an accreted terrane accreted during the Late Ordovician–Silurian crustal arrivals are visible, indicating generally has not been defi nitively identifi ed, but it has (Hibbard et al., 2002, 2010), or whether this high refl ectivity throughout the region.
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