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 subduction 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

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 zoic. During the Alleghanian, the collision with

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. been suggested to lie along the New York–Ala- accretion occurred later in the mid-Devonian Crustal and subcrustal velocities are con- bama magnetic anomaly (Fisher et al., 2010). (Hatcher et al., 2007b; Hatcher, 2010). Both strained by a 1965 U.S. Geological Survey Post-Grenville rifting reached the area of the models are consistent with accretion of Caro- (USGS) refraction survey in eastern Tennessee present-day Carolinas in the latest Proterozoic linia to the north of its current location, beneath that was reexamined by Prodehl et al. (1984). (565 Ma; Aleinikoff et al., 1995), separating modern-day Delaware, Maryland, Virginia, and The survey consisted of two 400-km-long South America from Laurentia. Deposition of eastern North Carolina (Merschat and Hatcher, refraction lines with shot points on the ends and the sediments that comprise the Tugaloo terrane 2007). The closure of the Rheic Ocean from two intermediate points per line (Borcherdt and began during the Neoproterozoic–Cambrian, north to south resulted in the formation of the Roller, 1966). Prodehl et al. (1984) found over- followed by the incursion of the Iapetus Ocean, Cat Square terrane from trapped ocean-basin all fast crustal velocities in three distinct lay- resulting in widespread carbonate deposits. By sediments. Subsequent dextral translation of ers: a shallow 10–20-km-thick layer with Vp = the latest Cambrian, east-dipping subduction the Cat Square terrane, the eastern portion of 6.1 km/s, a midcrust extending to ~40 km depth (present-day orientation) of Iapetan oceanic the Tugaloo terrane, and Carolinia moved them with velocities of ~6.7 km/s, and a fast lower crust began the formation of arc complexes that 200–400 km to the south along the Brevard fault crust (7.3 km/s) underlain by a somewhat slow would later be accreted as parts of the Piedmont and other dextral faults during the Neoacadian upper mantle (7.9 km/s) at depths of ~50 km and eastern Blue Ridge. The vergence direc- orogeny and the early Alleghanian orogeny beneath the Appalachians. These crustal thick- tion of subduction is still debated and may have (Merschat and Hatcher, 2007; Hibbard and Wal- nesses are consistent with recent work by Haw- changed from east-dipping to west-dipping dur- dron, 2009). man (2008), who used wide-angle refl ections

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from quarry blasts to constrain crustal thickness S-waves that are produced when near-vertically receiver functions for each station sorted by and average velocities across much of the South- incident P-waves encounter a sharp velocity dis- back azimuth can be seen in Figure 3. ern Appalachians. He found average crustal continuity. This “scattered” S-wave will arrive velocities of ~6.6 km/s, and crustal thicknesses at the recording station slightly later than the Constraining Vp/Vs in our study area of between 47 and 53 km. direct through-going P-wave, and if the P- and In addition to seismic studies, several grav- S-wave velocities are known, the delay time One of the inherent limitations of receiver ity anomaly studies have been performed to can be used to calculate the depth to that dis- functions is the trade-off between velocity and help constrain the structures beneath the South- continuity. We used a time-domain pulse-strip- depth (Ammon et al., 1990). In particular, depth ern Appalachians. The dominant feature in this ping method, which minimizes the difference trades off strongly with the Vp/Vs ratio. How-

area is the coupled Bouguer gravity low and between the radial component Cr(t) and the con- ever, constraints on the average Vp/Vs ratio high, which extends along the full length of volution between an iteratively improved time of the layer between the discontinuity and the the Southern Appalachians. The gravity low is series of Gaussian pulses (the receiver function) surface can be obtained by looking at multiples.

located close to, but just east of, the Blue Ridge with the vertical component Cz(t) (Ligorria and Multiples are waves that bounce off the free Escarpment, an abrupt rise in elevation between Ammon, 1999). Iterations are stopped when surface and the discontinuity in question before the Piedmont and Blue Ridge geomorphic prov- improvements to fi t become too small, or some being recorded at the station as an SV wave. The inces. The escarpment locally coincides with the goodness of fi t is achieved. One benefi t of this differential arrival times are given by: Brevard fault, but over most of its length, there method is that it allows us to quantify the per- Δ =−=−−22 are rocks of similar composition and, presum- cent variance reduction of the receiver function TTPpPms T Ps 2 hVpp, (1) ably, erodibility on both sides. This fact has led at each iteration, giving us some quantitative Δ =−=−−22 some to speculate that the escarpment coincides measure of error. TTPsPms T P 2 hVsp. (2) with a postorogenic normal fault (White, 1950; For this study, we calculated receiver func- Ackerman and Knapp, 2002), although others tions for 91 events with magnitude greater than However, because multiples are often small have proposed that it represents an erosionally 6.0 that occurred at distances greater than 30° arrivals on individual receiver functions, it is retreating rift-fl ank uplift (Spotila et al., 2004). from the array (Fig. 2; Table 1). We band-pass generally easier to identify them by noting how Karner and Watts (1983) noted that a similar fi ltered the records from 0.1 to 5 Hz, and then ΔT changes with changing ray parameter (inci- low/high-gravity pair is observed in the Alps calculated the receiver functions with Gaussians dence angle) for direct arrivals (i.e., P-s con- and Himalayas. They modeled their anomalies of a = 2.5 and a = 5 (corresponding to central verted phases) and multiples. For direct arriv- as the obduction of low-density crustal material frequencies of 1.2 and 2.4 Hz, respectively). We als, increasing the ray parameter (and incidence and the underthrusting of high-density lower considered only receiver functions with better angle) results in an increase in the differential crust beneath this. However, as Hutchinson et al. than 70% variance reduction during the itera- time between the through-going P-wave and the (1983) noted, all gravity models are inherently tive deconvolution, and, subsequently, all of the converted wave. For multiples, the differential nonunique, and they showed that the Appala- qualifying receiver functions were inspected for time decreases with increasing ray parameter. chian gravity low/high pair can be modeled with consistency. Requiring a higher percent vari- By plotting receiver functions as a function of variations in crustal thickness or with a high- ance reduction reduces our spatial coverage, ray parameter, we are able to identify which density dipping suture zone. but it does not appear to change the locations arrivals are direct arrivals and which are mul- It is important to note that despite the multiple or amplitudes of observed arrivals. In total, this tiples by their respective moveouts (Zandt et al., geophysical studies across the Southern Appala- left us with 247 receiver functions (see Table 2 1995). Moveout plots for our stations are shown chians, none has been able to image structures for individual station numbers). Plots of the raw in Figure 4. beneath the dominant Moho at ~45 km depth. As such, most interpretations of gravity data have been limited to crustal density variations only. We present evidence that deeper structures likely play a central role in the formation of these gravity anomalies, and they may help to answer some questions about the persistence of the topographic relief in this ancient orogen.

DATA AND METHODS Figure 2. Map of events used to calculate receiver functions in Data from this study come from our Appala- this study. White circles are event locations. White lines show great chian Seismic Transect (AST) (Fig. 1). Between circle paths from the events to April 2009 and June 2010, we recorded continu- station WWH in the center of our ous broadband seismic data along a six-station, transect. 75-km-long transect across the Blue Ridge Mountains of North Carolina. The stations were Nanometrics Trillium-120 sensors paired with Nanometrics Taurus digitizers owned by the University of North Carolina–Chapel Hill. Receiver functions are useful for studying lithospheric structures because they highlight

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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 ) continued TABLE 1. EARTHQUAKES USED TO CALCULATE RECEIVER FUNCTIONS ( CALCULATE TO USED 1. EARTHQUAKES TABLE 2010 1692010 2 147 35 17 44.44900131°N 34 148.6909943°E 13.69799995°S 166.6430054°E 20092009 207 245 23 8 29 14 4.796999931°S 102.9089966°E 7.782000065°S 107.2969971°E 2010201020102010 59 742010 58 75 112010 11 1132010 6 36 2 111 19 10 45 34.90299988°S 57 33 17 35.80199814°S 14 36.12200165°S 71.61699677°W 20 36.21699905°S 73.15799713°W 38 37.52899933°S 72.89800262°W 73.25700378°W 50 15.27099991°S 72.96900177°W 173.2189941°W 25.93000031°N 128.4250031°E 2009200920092009 2282009 228 3122009 274 13 105 72009 202010 261 8 22010 20 572010 250 12010 23 10 11 1.447999954°S2010 21 35 1.478999972°S2010 139 16 8.206999779°S 99.43299866°E 2010 144 26 2.514999866°S 99.48999786°E 2010 144 0 3.117000103°S 118.6309967°E 202010 143 32 101.5009995°E 4 100.4690018°E 2010 16 9.137999535°S 632010 34 16 66 26 10.19799995°S2010 101 115.5930023°E 22 232010 25 58 22 40.65200043°N 110.6279984°E 25 40.41199875°N 85 7 55 22 58 5.073999882°S 124.6920013°W 8.086999893°S 124.9609985°W 70 49 15 8.086999893°S 15 77.53600311°W 13.9279995°S2010 15 17 71.55799866°W 19 22.22699928°S 71.55799866°W 55 14 16.24300003°S 74.35199738°W 36.96500015°N 2 58 11 68.32800293°W 2010 24.87199974°S 50 115.2969971°W 27.95299911°S 3.542000055°W 33.42200089°S 65.60199738°W 146 8 34.29000092°S2010 70.82099915°W 71.82800293°W 71.89099884°W 2010 12 9 1012010 37.77299881°S 79 122010 10 167 75.04799652°W 2010 25.77300072°N 14 163 02010 129.9440002°E 3 129 20 19 10.87800026°S 64 27 161.1159973°E 6 3.361000061°S 48 152.2449951°E 16 2.385999918°S 21 7.881000042°N 136.6349945°E 26 91.93599701°E 3.748000145°N 3.762000084°S 96.01799774°E 100.9909973°E 20102010 1162010 52010 3 1752010 151 12 192010 5 95 35 20 22.18000031°N 96 52 123.6299973°E 10 9.019000053°S 13 22 5.513999939°S 157.5509949°E 27 11.13199997°N 151.1609955°E 37 93.47100067°E 0.187999994°S 125.0059967°E 2.382999897°N 97.04799652°E Year Julian day Hour Minute Latitude Longitude ) continued ( TABLE 1. EARTHQUAKES USED TO CALCULATE RECEIVER FUNCTIONS CALCULATE TO USED 1. EARTHQUAKES TABLE 20092009 2212009 2952009 11 3132009 20 136 13 112009 222 9 33.16699982°N 1 2 137.9409943°E 229 36.51699829°N 42009 12 17.23600006°S 70.94999695°E 25 0 196 31.5189991°S 178.3350067°E 2009 11.61199951°S 25 178.7920074°W 9 277 166.0899963°E 23.50099945°N 43 11 123.4990005°E 45.76200104°S 18 166.5619965°E 6.739999771°N 123.3779984°E 2009 157200920092009 202009 1882009 193 232 392009 317 19 107 6 23.86400032°N2009 6 286 19 32009 46.10499954°W 21 2 96 44 75.35099792°N2009 20 15 253 15.04100037°S 172009 72.19999695°N 72.45300293°W 1 31 19.3939991°S 97 70.44499969°W 2009 19.58399963°S 2 156 0.944000006°E 52.60400009°N 43 70.32099915°W 70.48300171°W 2009 242 4 58 167.1179962°W 3 42.33399963°N2009 328 35 15 48.31700134°N2009 13.33399963°E 43 106 154.1920013°E 46.04899979°N2009 13 9 303 41.82400131°N 151.5480042°E 2009 152009 187 15.22299957°S 143.4450073°E 5 72009 280 15 172.5709991°W 217 22 20.70800018°S2009 22 60.20299911°S 229 22 174.0350037°W 2009 54 0 196 29.21800041°N 26.8579998°W 2009 22 10 24.87400055°N 217 129.7819977°E 372009 20 13.00599957°S 128.0269928°E 165 302009 24.2329998°N 8 166.5099945°E 297 30 23.42399979°N 6 125.0950012°E 273 52 123.5279999°E 15 3.375°S 19 10 45.55400085°S 2 166.3560028°E 150.5059967°E 5.361000061°N 35 126.4380035°E 6.132999897°S 0.720000029°S 130.3849945°E 99.86699677°E 200920092009 130 321 288 1 152009 17 222009 37 286 552009 1.393000007°N 52.13100052°N 1872009 3.272000074°N 5 85.16899872°W 131.3970032°W 260 103.822998°W 2009 15 182 472009 23 108 42009 52.75400162°N 9 272 32 19 50.43500137°N 166.996994°W 224 42 29.1439991°S 18 176.9920044°E 30 23 34.16400146°N 112.2669983°W 5 46.01499939°N 25.47100067°E 6 151.427002°E 15.48900032°S 32.82099915°N 172.0950012°W 2009 140.3950043°E 1742009 14 2222009 39 20 240 5.157000065°S 16 153.7819977°E 2 14.09899998°N 9 92.88800049°E 7.145999908°S 123.427002°E Year Julian day Hour Minute Latitude Longitude

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TABLE 2. NUMBER OF RECEIVER FUNCTIONS USED FOR EACH STATION IN THIS STUDY based on the results of Prodehl et al. (1984) and AND AVERAGE CRUSTAL VALUES DETERMINED FROM H-K STACKING Hawman (2008). Results of our H-K stacks can Station name Number of receiver functions Average crustal thickness Average crustal Vp/Vs be seen in Figure 5. (km) RGR 31 46 1.78 Common Conversion Point Stacks BEE 58 46 1.74 WWH 55 46 1.74 While we assume near-vertical incidence for AFH 19 46 1.72 MFH 45 46 1.74 our incoming P-waves, in reality, the incidence BSC 40 46 1.73 angle i is always greater than zero. As such, the point at which the incoming ray pierces a given horizon is not directly below the station and may These observations can be used to predict the where W values are weights applied to the direct in fact be closer to or directly below an adjacent range of thicknesses (H) and Vp/Vs ratios (K) arrivals and multiples (we use 1/3 for each), station. The deeper the horizon in question, the that best match the observed moveouts of direct and r(t) is the amplitude of the receiver func- farther the pierce point will be from the record- and multiple arrivals from the Moho (Zhu and tion at that time. Note that the amplitudes of the ing station. Figure 6 shows pierce points for our Kanamori, 2000). This can be done by calculat- PpSms and PsPms arrivals are subtracted due receiver functions at 45 and 100 km depth. If ing predicted arrival times for the Moho direct to their predicted negative amplitudes (a conse- lateral heterogeneities exist along a given dis-

arrivals (tPs) and multiples (tPpPms, tPsPms, tPpSms) for a quence of the free-surface–S-wave interaction). continuity, it is important that we stack, not by given crustal thickness and Vp/Vs ratio and sum- This sum is calculated over a range of H and station, but by pierce point at that depth. This ming the amplitudes for all receiver functions: K values, and the amplitudes are plotted and is the goal of common conversion point (CCP) contoured. The highest amplitude corresponds stacking (Dueker and Sheehan, 1997; Gilbert, ( κ=) ( ) + SH, WrtPs Ps W PpPms rt( PpPms ) to the best-ﬁ tting thickness and Vp/Vs ratio 2003, 2004). This methodology back-projects for a given ﬁ xed P-wave velocity. We assume the incoming ray paths and stacks rays that pass − WrtPsPms+PpSms( PsPms+PpSms ), (3) a crustal average P-wave velocity of 6.6 km/s within a given distance of a stacking point. We

0 0 0

10 10 10

20 20 20 Time after P-arrival (s) Time Time after P-arrival (s) Time after P-arrival (s) Time

RGR BEE WWH 30 30 30 0 60 120 180 240 300 360 0 60 120 180 240 300 360 0 60 120 180 240 300 360 Figure 3. Plot of raw receiver func- Back azimuth (degrees) Back azimuth (degrees) Back azimuth (degrees) tions calculated with a Gaussian width of a = 2.5, sorted by back 0 0 0 azimuth at each station.

10 10 10 Time after P-arrival (s) Time Time after P-arrival (s) Time 20 20 after P-arrival (s) Time 20

AFH MFH BSC 30 30 30 0 60 120 180 240 300 360 0 60 120 180 240 300 360 0 60 120 180 240 300 360 Back azimuth (degrees) Back azimuth (degrees) Back azimuth (degrees)

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30 30 30 RGR BEE WWH

46 46 46

20 20 20

46 46 46 Time after P-arrival (s) Time 10 after P-arrival (s) Time 10 after P-arrival (s) Time 10

46 46 46

0 0 0 0.040.05 0.06 0.07 0.08 0.040.05 0.06 0.07 0.08 0.040.05 0.06 0.07 0.08 Ray parameter (s/km) Ray parameter (s/km) Ray parameter (s/km) 30 30 30 AFH MFH BSC

46 46 46

20 20 20

46 46 46

90 90 Time after P-arrival (s) Time Time after P-arrival (s) Time 10 10 after P-arrival (s) Time 10 60 60 46 46 46

0 0 0 0.040.05 0.06 0.07 0.08 0.040.05 0.06 0.07 0.08 0.040.05 0.06 0.07 0.08 Ray parameter (s/km) Ray parameter (s/km) Ray parameter (s/km) Figure 4. Moveout plots for receiver functions calculated with a Gaussian width of a = 2.5 (~1.2 Hz). Red indicates positive receiver function arrivals. Blue indicates negative receiver function arrivals. Lines with positive moveout, labeled with black numbers, show predicted times for direct P-s conversions. Negative moveout lines labeled with blue or red numbers show predicted arrival times for multiples (PpPms has blue numbers, PsPms + PpSms has red numbers).

create a two-dimensional (2-D) grid of stacking whereas incoherent arrivals from small struc- and an average Vp/Vs ratio of 1.74. Constraints points with 5-km spacing that spans the area tures and ambient noise are reduced. Stacks on both depend on the ability to identify direct sampled by our pierce points at 100 km depth are calculated for both the higher- and lower- arrivals and multiples on the individual sta- (Fig. 6). Smoothing is achieved by including frequency receiver functions calculated earlier. tion moveout plots (Fig. 4). All stations show a pierce points from distances equal to twice the very clear direct arrival (positive moveout with stacking node spacing. Stacked bins at a given RESULTS respect to p) for a signiﬁ cant increase in velocity depth are only plotted if they include data from with depth (positive receiver function arrival) at a minimum of ﬁ ve receiver functions. We sub- Crustal Structure 5–6 s after the direct P-wave arrival. This cor- sequently apply the phase-weighting algorithm responds well with the predicted times for a dis- of Frassetto et al. (2010) in order to empha- Average crustal thicknesses and Vp/Vs ratios continuity at 46 km depth, assuming a crustal size coherent arrivals (Schimmel and Paulssen, beneath the AST stations can be seen in the H-K P-wave velocity of 6.6 km/s and a Vp/Vs ratio of 1997). This methodology sums the instanta- stacks in Figure 5. The contours show 80% and 1.74. The negative moveout, positive-amplitude neous phase of the receiver functions in a given 90% of the maximum stacked amplitude for a arrival associated with the PpPms multiple is bin and uses this value to weight the stacked given station. Vp/Vs is much more poorly con- also clear at all stations, with the possible excep- receiver functions. The result is that coher- strained than crustal thickness, but within error, tion of RGR. The negative moveout, negative- ent arrivals such as the Moho are emphasized, all stations show a crustal thickness of ~46 km amplitude receiver function arrival associated

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from beneath the Moho hole (or weakening) to the SSE edge of our resolved area. The locations of the Moho hole and double Moho are shown as ovals in the CCP cross sections in Figure 7, and as stars and circles, respectively, in map view in Figure 8. Figure 8 also shows the correlation between these features and regional Bouguer gravity anomalies. In general, where the Moho is weakened or missing, the Bou- guer anomalies are markedly low, but where the Moho is doubled (i.e., clear arrival at both 45 and 60 km depth), the Bouguer gravity anomaly values increase, becoming much less negative. In order to determine if these observations could be a local anomaly or artifact caused by one station, we restacked the CCP proﬁ les six times, each time omitting the receiver functions from one of our stations. Notably, removing the receiver functions recorded at BSC did not elim- inate any of the features discussed (the Moho hole or double Moho). Figure 9 shows crustal proﬁ les 7 and 8 both with and without data from BSC. The absence of evidence for the double Moho in the moveout plots must therefore be due to stacking across heterogeneous structures at adjacent stations.

Mantle Structure

In addition to crustal direct arrivals and multiples, the moveout plot for station BSC (and to a lesser extent for station MFH) in Figure 4 shows a prominent direct arrival at ~10 s, which would correspond to a signifi cant increase in velocity with depth at ~90 km depth. Figure 9B shows CCP profi le 8 using receiver functions calculated with a Gaussian of a = 2.5. This profi le also clearly shows the Moho hole and double Moho observed in the higher-frequency receiver function stacks of Figure 7. The deep disconti- Figure 5. H-K stacks for all six Appalachian Seismic Transect (AST) stations. Note the nuity lies at depths between 90 and 100 km, and different scales for each plot. Contours show 80% and 90% of maximum amplitude it may be discontinuous beneath the center of for a given plot. Best fi t for each station is labeled above the fi gure. the array. It appears to dip slightly to the west, though more data are needed to evaluate this possibility. with the PsPms and PpSms arrivals can be seen frequency Gaussian of a = 5. These cross sec- As with the crustal results, we also restacked at BSC, AFH, and WWH, but it is less clear at tions all show a clear Moho arrival at ~45 km the CCP profi les omitting one station at a time in MFH, BEE, and RGR. depth, possibly deepening slightly at the NNW order to determine if any one station was control- Also noticeable on the moveout plot for BSC edge. The Moho arrival is not continuous, how- ling one of the deeper arrivals. The deep arrival is a second arrival just after the Moho arrival, cor- ever. All profi les show either a marked weaken- farthest to the NNW does appear to be largely responding to an increase in velocity at 60 km ing or hole, usually just SSE of the Blue Ridge controlled from data at station BEE. However, depth. The arrival fades at higher ray parameters, Escarpment. These Moho holes are most pro- the rest of the deeper discontinuity is still visible perhaps because these rays sample a larger area nounced on those transects that cross the escarp- even with data from any one station removed. around the station, resulting in incoherent stack- ment at near-normal angles (profi les 7–9), but Results of the stacking with station BSC omitted ing if the structures vary laterally. A faint arrival they are seen as reductions on profi les west of are shown in Figure 9B. Note that even though at the same time can be seen in the moveout plot our stations where the strike of the transects BSC is the only station showing a clear arrival for MFH. trends subparallel to the escarpment. In addi- at ~90 km depth in the moveout plots (Fig. 4), CCP stacking results for crustal-scale struc- tion to the Moho hole, we see a second “Moho” removing all of the BSC data from the CCP tures are shown in Figure 7. These stacks use arrival on the central and eastern profi les at stacking does not remove these anomalies, receiver functions calculated with our higher- ~60 km depth. This “double Moho” extends thereby demonstrating that the absence of these

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13 12 11 10 9 8 7 6 5 4 3 2 1

TN 36˚N NC

13 12 11 10 9 8 7 6 5 4 3 2 SC 1 20 km 35˚N 83˚W 82˚W

Figure 6. Map of common conversion point (CCP) nodes (black diamonds) and receiver function pierce points at 45 km (red dots) and 100 km depth (blue dots). Stations are shown as light-blue triangles. Numbers at the ends of the CCP node lines indicate pro- ﬁ le numbers referenced in Figures 5, 7, 8, and 9. Dashed line shows the Blue Ridge Escarpment. Solid lines show state boundaries.

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RGR WWH Profile 4 Profile 5 BEE AFH MFH BSC 0 0

20 20

40 Depth (km) Depth (km)

60 60

02040 6080 100 120 140 02040 6080 100 120 140 Distance (km) NNW-SSE Distance (km) NNW-SSE

Profile 6 Profile 7

0 0

20 20

40 Depth (km) Depth (km)

60 60

02040 6080 100 120 140 02040 6080 100 120 140 Distance (km) NNW-SSE Distance (km) NNW-SSE

Profile 8 Profile 9

0 0

20 20

40 Depth (km) Depth (km)

60 60

02040 6080 100 120 140 02040 6080 100 120 140 Distance (km) NNW-SSE Distance (km) NNW-SSE Figure 7. Common conversion point (CCP) stacked profi les using receiver functions calculated with a Gaussian width of a = 5 (~2.4 Hz). Red indicates positive receiver function arrivals; blue indicates negative receiver function arrivals. Black triangles show station locations, and gray profi les show exaggerated topography. The vertical blue line on the topography indicates the location of the escarpment on that profi le. Ovals show locations of weak Mohos, Moho holes, and the deeper Moho at 60 km depth.

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Figure 8. Bouguer gravity anomaly map (in color with contours). Black diamonds show common conversion point (CCP) node locations (same as in Fig. 4). Circles show locations of Moho holes (white circles), weak Mohos (gray circles), and double Mohos (orange circles). Dashed line shows the approximate location of the Blue Ridge Escarpment. Notice how the Moho holes line up with the lowest gravity values, whereas the double Moho coincides spatially with a strong increase in gravity, especially where the CCP proﬁ les are stacked roughly normal to the trend of the escarpment and gravity anomalies. White triangles are station locations (see Fig. 1 for station names).

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Profile 7 Profile 8 A RGR WWH RGR WWH BEE BEE AFH MFH BSC AFH MFH BSC 0 0

20 20

40 40 Depth (km) Depth (km)

60 60

02040 6080 100 120 140 02040 6080 100 120 140 Distance (km) NNW-SSE Distance (km) NNW-SSE Profile 7 Profile 8 RGR WWH RGR WWH BEE BEE AFH AFH MFH MFH 0 0 No BSC No BSC

20 data used 20 data used

40 40 Depth (km) Depth (km)

60 60

02040 6080 100 120 140 02040 6080 100 120 140 Distance (km) NNW-SSE Distance (km) NNW-SSE

RGR WWH Profile 8 RGR WWH Profile 8 B BEE BEE AFH AFH MFH BSC MFH 0 0

20 20

40 40

60 60 No BSC Depth (km) Depth (km)

Depth (km) 80 Depth (km) 80 data used

100 100

120 120

0601201402040 80 100 0601201402040 80 100 Distance (km) NNW-SSE Distance (km) NNW-SSE

Figure 9. Common conversion point (CCP) profi les with and without data from station BSC. Note that the Moho holes, the double Moho, and the 100-km-depth discontinuity are present even without using data from station BSC: (A) Profi les 7 and 8 from Figure 5 (Gaussian a = 5). Top plots show CCP stacks with all stations included. Bottom plots show CCP stacks calculated without BSC data. (B) Profi le 8 from Figure 8 (Gaussian a = 2.5). Left plot uses all data; right plot omits data from BSC.

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features in the moveout plots of other stations is continuity in the Moho as evidence of a litho- preceded the formation of the Blue Ridge– due to stacking across heterogeneous structures. spheric downwelling or “drip.” Double Mohos Piedmont allochthon during the Alleghanian, have been reported in the Himalayas, where they so that the surface expression of this wedging DISCUSSION are interpreted as evidence of the partial eclogi- was subsequently deformed and displaced to the tization of the lower crust (Schulte-Pelkum et northwest along with all of the other adjacent Crustal Structure al., 2005). A similar explanation was presented terrane boundaries during the Alleghanian col- by Gilbert et al. (2006) to explain the presence lision. The second model is similar to models Contrary to the results of studies using of a double Moho beneath central Chile. Hansen presented in Keller and Hatcher (1999) and Hib- COCORP refl ection data, we fi nd the base of the and Dueker (2009) found evidence for a double bard et al. (2002). In this model, the indenter is crust beneath the Piedmont and Blue Ridge to Moho using both P-s receiver functions and S-p the part of Carolinia that has both under- and be highly refl ective. The average crustal thick- receiver functions recorded across the edge of overthrust the Laurentian basement. This wedg- ness of ~46 km is consistent with the results of the Wyoming craton. They suggested that this ing could either have occurred during the initial Hawman (2008) and Prodehl et al. (1984), but it double Moho represents tectonic wedging of the accretion of Carolinia or, more likely, subse- is somewhat deeper than that suggested by Cook Wyoming craton into younger Proterozoic crust. quently, during the Alleghanian orogeny and and Vasudevan (2006). This is likely due to the Wedge tectonics have become recognized formation of the Blue Ridge–Piedmont alloch- slow crustal velocity used by Cook and Vasude- as a common feature in accretionary margins thon. If the wedging did occur during the initial van (2006) to migrate time to depth (6 km/s). within continental interiors (Price, 1986; Mar- accretion of Carolinia, this would imply that the Locally, the topographic relief along our tran- tinez-Torres et al., 1994; Snyder, 2002; Moore accretion was the result of east-dipping subduc- sect reaches 1200 m, though in other parts of the and Wiltschko, 2004). Price (1986, p. 239) tion, because no structures continue past the Blue Ridge Province, it is greater than 1500 m. defi ned wedges as “bounded above and below indenter dipping west into the mantle as would The persistence of high topographic relief in by shear zones of opposite vergence,” similar to be expected with west-dipping subduction. If ancient mountain ranges is a long-standing prob- the conjugate surfaces observed in experimen- the thrusting occurred later during the Allegha- lem in geology because many models predict that tally compressed marbles and limestones. Sny- nian, however, subduction during the accretion most of the topographic relief in a mountain range der (2002) showed evidence for whole-crustal of Carolinia could have proceeded via either should have eroded away after ~100 m.y. (Ahn- tectonic wedging at the margins of the Archean east- or west-dipping subduction. ert, 1970; Tucker and Slingerland, 1994; Beau- cratons in Canada and Fennoscandia. Moore Given that the high elevations NNW of the mont et al., 2000). The Southern Appalachians and Wiltschko (2004) defi ned different wedging Blue Ridge Escarpment appear to be isostati- presumably attained their greatest elevation dur- geometries and attempted to ascribe these to all cally compensated by the 46-km-thick crust ing the Alleghanian orogeny ca. 300 Ma, and, major orogenic belts. observed beneath them, it is somewhat surpris- according to models like Ahnert (1970), almost Figure 10 shows two possible interpretations ing that the low-lying areas SSE of the escarp- all of the topographic relief should have been of the tectonic structures that may be responsible ment are underlain by a 60-km-thick crust. eliminated by ca. 200 Ma. Hack (1982) felt that for our observed Moho hole discontinuity and However, if the underthrust portion of the crust postorogenic uplift was responsible for much of double Moho. In both cases, part of the lower beneath the indenter is eclogitized, this low- the present-day topographic relief in the Southern crust of the Laurentian basement is underthrust lying area may be isostatically compensated. Appalachians. Matmon et al. (2003), however, beneath an indenter. This is consistent with the Assuming the same reference crust as before, calculated long-term erosion rates in the South- crustal-delamination model proposed by Moore and assuming the top 45 km section of crust ern Appalachians using cosmogenic isotopes and and Wiltschko (2004) for the Central Piedmont beneath these low-lying areas has the same den- concluded that the present-day topography is shear zone. The Moho hole is the result of the sity we calculated earlier for beneath the Blue the result of slow erosion of crust that had been downgoing portion of the Laurentian lower Ridge Mountains (2.77 g/cm3), the ~385 m thickened during the Alleghanian orogeny. The crust (similar to the Moho hole in the Sierras elevation SSE of the escarpment would be iso- thickness of our observed crust suggests that the being caused by sinking of the lower crust). statically compensated if the bottom 15 km of elevations of the Southern Appalachians are iso- The double Moho to the SSE suggests that this crust (the underthrust portion beneath the Moho statically compensated, which is consistent with lower crust is of higher seismic velocity than at 46 km depth) had an average density of 3.4 g/ Matmon’s interpretation. Using a reference crust the indenter above it, perhaps due to the partial cm3. This is well within the range of densities of 35 km thickness with a density of 2.7 g/cm3 eclogitization of the underthrust material. The expected for an eclogitized lower crust (3.1–3.6 and a mantle density of 3.3 g/cm3, and assuming close correlation of the location of the Moho g/cm3) (Austrheim, 1991). Given that there is a the crustal thickness is 46 km (applicable beneath hole with the pronounced gravity low in the area low-gravity anomaly collocated with the Moho the highest elevations in this study), the aver- (Fig. 8) may be due to the presence of overthick- hole immediately SSE of the escarpment, we age crustal density of the Blue Ridge Mountains ened, but not eclogitized, continental crust. The do not propose that this section is eclogitized. would have to be somewhat higher (~2.77 g/cm3) increase in the gravity to the SSE of this low However, the double Moho begins only 20 km to explain the observed roots and elevations. would be consistent with the presence of dense, farther SSE, coinciding with a marked increase To our knowledge, we are the fi rst to report eclogitic, lower crust beneath the indenter. in gravity. Detailed gravity modeling will be discontinuities in the Moho and the presence of The difference between the two models in required to determine if the precise anomalies a deeper “double Moho” at 60 km depth beneath Figure 10 lies in the affi nity of the indenter and observed can be explained with an underthrust the Southern Appalachians. These features have the timing of the deformation. In the fi rst model, eclogitic root. We do note that the location of the been seen in other convergent-margin settings, the observed wedge structures do not necessar- Blue Ridge Escarpment is aligned very closely both active and ancient. Zandt et al. (2004) ily correspond to a terrane boundary, but may with the NNW edge of the observed Moho hole reported a “Moho hole” beneath the southern instead be whole-crustal shortening structures (Fig. 10). This may be coincidental, given that Sierra Nevada, which they imaged with receiver due to a major collisional event. In this model, the high-density lower crust is believed to begin function CCP stacks. They interpreted this dis- we propose that the deformation would have 20 km farther SSE. However, the observed

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RGR WWH Creek BEE Burnsville Miller Cove AFH Brevard BSC MFH Brindle Valley & Range Grenville-affinity Western-Tugaloo Eastern-Tugaloo Cat-Square CPS Carolinia Gr. Basem’t?

Grenville basement Grenville basement

Grenville eclogitic root

RGR WWH BEE Miller Cove Burnsville C AFH BS MFH Brindle Creek S Brevard Valley & Range Grenville-affinity Western-Tugaloo Eastern-Tugaloo Cat-Square CP Carolinia

Grenville basement

Carolinia

Grenville eclogitic root

Figure 10. Tectonic interpretation of proﬁ le 7 from Figure 7. Heavy red line above the topography shows the Bouguer gravity anomaly along this proﬁ le from Figure 8. CPS—Central Piedmont Suture.

elastic thickness beneath the Southern Appala- Ridge Mountains is uncertain. Seismic discon- kilometers, making it unlikely to be due to such chians is particularly high (40–70 km) (Stewart tinuities at these depths are often referred to as a phase transition. If, instead, this increase in and Watts, 1997), so it may be possible that the Hales discontinuities (Hales, 1969), which are velocity with depth is due to some persistent high-density lower crust in the area plays a role in believed to be due to the phase transition from tectonic feature, then this tectonic feature must perpetuating this long-lived topographic feature. spinel to garnet in the upper mantle—a transi- be part of the mantle lithosphere, and the mantle tion for which depths are highly variable and are lithosphere must in turn be over 100 km thick in Mantle Structures strongly controlled by the concentration of chro- this area. While we do not observe the decrease mium (Klemme, 2004). In our case, the observed in velocity that would be associated with the lith- The cause of the sharp increase in velocity discontinuity appears to be spatially limited, osphere-asthenosphere boundary, this thickness with depth at ~90−100 km beneath the Blue appearing and disappearing over a few tens of constraint is consistent with a number of studies

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indicating a deep lithosphere-asthenosphere underthrust basement has become eclogitized. If Austrheim, H., 1991, Eclogite formation and dynamics of crustal roots under continental collision zones: Terra boundary beneath the eastern United States the bottom 15 km section of the 60-km-thick Nova, v. 3, no. 5, p. 492–499, doi:10.1111/j.1365-3121.1991. (Rychert et al., 2005, 2007; Abt et al., 2010). crust is eclogitized, then the area would be iso- tb00184.x. One possible tectonic feature that might statically compensated. This might help explain Beaumont, C., Kooi, H., and Willett, S., 2000, Coupled tectonic-surface process models with applications to rifted fi t the observed discontinuity is a discontinu- the longevity of the Blue Ridge Escarpment. margins and collisional orogens, in Summerfi eld, M.A., ous accreted fossil slab. Similar small regions (4) We observe localized seismic scatterers ed., Geomorphology and Global Tectonics: Chichester, of seismic discontinuities at these depths have UK, John Wiley, p. 29–55. at depths between 90 and 100 km, suggesting Borcherdt, R.D., and Roller, J.C., 1966, A Preliminary Summary been observed beneath southern Finland and the presence of mantle lithosphere to at least of a Seismic Refraction Survey in the Vicinity of the Cum- have been interpreted as relics of ancient sub- these depths throughout our study area. This berland Plateau Observatory, Tennessee: U.S. Geological duction zones (Yliniemi et al., 2004). The age Survey Technical Letter, Crustal Studies 43, 21 p. seismic scatterer appears to dip slightly to the Cook, F., and Vasudevan, K., 2006, Reprocessing and and affi nity of such a fossil slab beneath the west, which may be consistent with westward- enhanced interpretation of the initial COCORP Southern Blue Ridge are hard to constrain without a more Appalachians traverse: Tectonophysics, v. 420, no. 1–2, dipping subduction during the accretion of Car- p. 161–174. spatially extensive seismic data set. If the appar- olinia, though more data are needed to confi rm Dueker, K., and Sheehan, A., 1997, Mantle discontinuity ent westward dip is verifi ed, this might suggest this result. structure from midpoint stacks of converted P to S that the relict slab is Iapetan, with subduction converted waves across the Yellowstone hotspot track: Journal of Geophysical Research, v. 102, p. 8313–8327, ending with the accretion of Carolinia either ACKNOWLEDGMENTS doi:10.1029/96JB03857. in the Ordovician or mid-Devonian. Hibbard Fisher, C.M., Loewy, S.L., Miller, C.F., Berquist, P., Van Schmus, (2000) proposed that the convergence and sub- W.R., Hatcher, R.D., Wooden, J.L., and Fullagar, P.D., Many thanks go to Emeritus Professor John Rog- 2010, Whole-rock Pb and Sm-Nd isotopic constraints on sequent accretion of Carolinia were accommo- ers, without whom this project would not have the growth of southeastern Laurentia during Grenvil- lian orogenesis: Geological Society of America Bulletin, dated by a westward-dipping subduction zone been possible. We would also especially like to that was active during the Late Ordovician and v. 122, no. 9–10, p. 1646–1659, doi:10.1130/B30116.1. thank Lepolt Linkimer and Andy Frassetto for Frassetto, A., Zandt, G., Gilbert, H.J., Owens, T.J., and Jones, Early Silurian. Anderson and Moecher (2009) C.H., 2010, Improved imaging with phase-weighted sharing their expertise on receiver function anal- proposed that the Piedmont terrane was accreted common conversion point stacks of receiver functions: yses with us. We would like to thank our anony- Geophysical Journal International, doi:10.1111/j.1365 during westward subduction during the Ordovi- mous reviewer for the helpful and constructive -246X.2010.04617.x. cian. Hatcher (2010) proposed that Carolinia Gilbert, H.J., 2003, Receiver functions in the western United comments we received. Also, many thanks to accreted in the Devonian and showed conver- States, with implications for upper mantle structure Jim Hibbard for his insight and helpful sugges- and dynamics: Journal of Geophysical Research, v. 108, gence being accommodated on an eastward- tions. We are greatly indebted to the Marion and no. B5, doi:10.1029/2001JB001194. dipping subduction zone. If the slab was associ- Gilbert, H.J., 2004, Images of crustal variations in the Inter- Armstrong Fish Hatcheries, the Mecklenburg mountain West: Journal of Geophysical Research, ated with accretion of the Carolinia or Piedmont Boy Scout Reservation, and the two private land- v. 109, no. B3, doi:10.1029/2003JB002730. terranes, its present-day geometry is consistent Gilbert, H.J., Beck, S.L., and Zandt, G., 2006, Lithospheric and with the westward-dipping subduction proposed owners who allowed us to install stations on their upper mantle structure of central Chile and Argentina: by Hibbard (2000) and Anderson and Moecher property. As with any fi eld deployment, many Geophysical Journal International, v. 165, no. 1, p. 383– students helped install, service, and demobilize 398, doi:10.1111/j.1365-246X.2006.02867.x. (2009). Hack, J.T., 1982, Physiographic divisions and differential uplift these stations. In particular, we are very grate- in the Piedmont and Blue Ridge: U.S. Geological Survey ful to Sara Hanson-Hedgecock for all of her hard Professional Paper 1265-P, p. 1–49. CONCLUSIONS Hales, A.L., 1969, A seismic discontinuity in the lithosphere: work collecting and creating the database for the Earth and Planetary Science Letters, v. 7, p. 44–46, (1) We present receiver function images data used here. doi:10.1016/0012-821X(69)90009-0. Hansen, S.E., and Dueker, K., 2009, P- and S-wave receiver from the fi rst broadband seismic transect across function images of crustal imbrication beneath the the Southern Appalachians. These cross sec- REFERENCES CITED Cheyenne belt in southeast Wyoming: Bulletin of the tions provide the fi rst look at subcrustal struc- Seismological Society of America, v. 99, no. 3, p. 1953– Abt, D.L., Fischer, K.M., French, S.W., Ford, H.A., Yuan, H., 1961, doi:10.1785/0120080168. tures beneath the ancient orogen and show evi- and Romanowicz, B., 2010, North American lithospheric Hatcher, R.D., 1972, Developmental model for the Southern dence for whole-crustal-scale compressional discontinuity structure imaged by Ps and Sp receiver Appalachians: Geological Society of America Bulletin, v. 83, p. 2735–2760, doi:10.1130/0016-7606(1972)83[2735: functions: Journal of Geophysical Research, v. 115, no. structures beneath the Blue Ridge Mountains of DMFTSA]2.0.CO;2. B9, doi:10.1029/2009JB006914. Hatcher, R.D., 2010, The Appalachian orogen: A brief sum- North Carolina. 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