Online First Geological Society of America Special Papers

K. Stephen Hughes* James P. Hibbard DelWayne R. Bohnenstiehl

Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, North Carolina 27695-8208, USA

ABSTRACT

Observations made during geologic mapping prior to the moment magnitude, M w

5.8 2011 Virginia (USA) earthquake are important for understanding the event. Because many Paleozoic ductile faults in the Piedmont of Virginia show signs of brittle overprint, relict faults in the epicentral area represent potential seismogenic surfaces in the modern stress regime. Three major faults that reportedly dissect the early- middle Paleozoic bedrock in the epicentral area are reviewed here: the Shores fault of uncertain age, which has been depicted as internal to the Early Ordovician or Cam- brian metaclastic Potomac terrane; the Late Ordovician Chopawamsic fault, which represents the Potomac-Chopawamsic terrane boundary; and the late Paleozoic Long Branch fault, which is internal to the Middle Ordovician Chopawamsic terrane.

Our mapping reveals no evidence for the Shores fault, as previously depicted, in the epicentral area, and has led to revision of the position and surface trace of the Chopawamsic fault. Both these features are considered to have no connection to the 2011 event. Ductile strain features in a previously unrecognized zone related to the Long Branch fault are considered with a simple analysis of aftershocks along the brit- tle Quail fault that followed the 2011 Virginia earthquake. Internal to the Chopawam- sic Formation, this Bend of River high-strain zone coincides in three dimensions with the aftershock-defi ned fault plane for the 2011 event. The spatial coincidence of the modern seismogenic surface (Quail fault) and Paleozoic metamorphic fabrics leads us to interpret that this zone of Paleozoic ductile strain, now located in the shallow crust, served as a guide to modern brittle intraplate rupture in 2011.

*Now at Department of Geology, University of Puerto Rico—Maygüez, Maygüez, Puerto Rico 00681, USA; kenneth.hughes@upr.edu. Hughes, K.S., Hibbard, J.P., and Bohnenstiehl, D.R., 2015, Relict Paleozoic faults in the epicentral area of the 23 August 2011 central Virginia earthquake:

Assessing the relationship between preexisting strain and modern seismicity, in Horton, J.W., Jr., Chapman, M.C., and Green, R.A., eds., The 2011 Miner- al, Virginia, Earthquake, and Its Signifi cance for Seismic Hazards in Eastern North America: Geologica l Society of America Special Paper 509, p. 331–343, doi:10.1130/2015.2509(19). For permission to copy, contact editing@geosociety.org. © 2014 The Geological Society of America. All rights reserved.

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332 Hughes et al.

INTRODUCTION

REGIONAL OVERVIEW

The 2011 main shock and aftershocks occurred in the north- earthquake occurred on 23 August 2011 and was likely felt by ern part of the CVSZ, which is one of many recognized intraplate more people than any other in U.S. history (Carter et al., 2012). seismic zones in eastern North America. Much of the CVSZ is This intraplate earthquake is the largest recorded event to occur in in the Piedmont of Virginia, and the 2011 event occurred below the Central Virginia seismic zone (CVSZ). Known events within the western Piedmont (Fig. 1). The western Piedmont of Virginia the CVSZ are temporally and spatially diffuse and most have (summarized by Hibbard et al., 2014) is mostly composed of not been convincingly associated with mapped geologic faults. metamorphosed Neoproterozoic to Paleozoic sedimentary and Most geologic maps produced for the Piedmont of Virginia are igneous rocks. The western Piedmont is bordered to the west by not detailed enough to identify features responsible for the 2011 the Appalachian Blue Ridge and to the east by both the eastern earthquake and previous shaking events. One of the most intrigu- Piedmont and Atlantic coastal plain sediments. Within and along ing questions pertaining to intraplate seismicity in the CVSZ is the margins of the Virginia Piedmont are numerous ductile faults if and how modern brittle fault ruptures (earthquakes) in the area that record some form of brittle overprint. Empirical data suggest are related to known Paleozoic faults, most of which record duc- that fault reactivation has been the norm along structural boundar- tile strain.

The moment magnitude, M w

5.8 Mineral, Virginia (USA)

ies in the Piedmont; furthermore, the overprint of brittle features As part of a separate study into the Paleozoic tectonic evolu- upon ductilely deformed rocks indicates that faults that originally tion of the Appalachian Piedmont of central and northern Vir- formed at mid-crustal depths are also active once transported to ginia, we undertook mapping of the bedrock geology in the north shallow crustal levels (e.g., Bourland, 1976; Bobyarchick and half of the Ferncliff 7.5 ′ quadrangle (Hughes, 2011) in the sum- Glover, 1979; Bourland et al., 1979; Weems, 1981; Gates, 1986, mer of 2010. While targeted studies have been conducted in the 1997; Pavlides, 1987, 1989, 2000; Pavlides et al., 1983, 1994; area following the earthquake, these original and objective obser- Spears and Bailey, 2002; Bailey et al., 2004; Spears et al., 2004; vations can be used in an attempt to analyze the surfaces upon Spears, 2010; Henika, 2012; Hollis and Bailey, 2012; Quinlan, which slip occurred during the main shock and subsequent after- 2012; Spears and Gilmer, 2012). For this reason it is important to shocks. In the course of mapping the Ferncliff quadrangle and examine zones of relict ductile strain in the epicentral area of the adjacent areas, relict Paleozoic faults in the epicentral region of 2011 Virginia earthquake. the 2011 Virginia earthquake were recognized. Because the fault

In central and northern Virginia, the western Piedmont is plane (Quail fault) assigned to the main cluster of aftershocks composed of the Potomac and Chopawamsic terranes, which are does not appear to directly correlate to a previously identifi ed separated by the Chopawamsic fault (Pavlides, 1989, 1990, 1995; structure, all relict zones of deformation in the area should be Pavlides et al., 1994; Mixon et al., 2000, 2005; Hughes et al., scrutinized with regard to the 2011 event. Field observations are 2013a; Hibbard et al., 2014). The metaclastic Potomac terrane supplemented with an analysis of the aftershocks that occurred in is to the northwest of the fault and has been interpreted to repre- the week immediately after the main shock.

sent an early Paleozoic accretionary complex that formed on the

Virginia Western Piedmont

Mesozoic Culpeper and Danville basins and related rocks

Ordovician to Early Silurian intrusive bodies. Stipple = felsic, black = mafic

Area of C M Ordovician rocks of the Chopawamsic

Figure 2 Terrane (C) and Milton Terrane (M).

VS Z

Cambrian to Ordovician rocks of the Potomac

SRA Terrane (P) and Smith River Allochthon (SRA).

idg

ge id

in

eR

Pla Blue Ridge Blu Blue Ridge Valley and Ridge Valley and R Valley and Ridge

edmont

stal

SRA

. Pi

E E. Piedmont E. Piedmont Coastal Plain C Coastal Plain

Figure 1. Geology of the western Piedmont of Virginia. R is Richmond. Dashed line indicates the approximate boundary of the Central Virginia seismic zone (CVSZ). Focal mechanism symbol shows the location of the August 2011 earthquake. Geology is modifi ed after Hibbard et al. (2006).

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Relict Paleozoic faults in the epicentral area of the 23 August 2011 central Virginia earthquake

eastern margin of Laurentia (Drake, 1989; Pavlides, 1989; Hor- cliff quadrangle. Signifi cant faulted contacts adjacent to the west ton et al., 1989; Hibbard et al., 2014) during the early Paleozoic. and east of the main cluster of seismicity are reviewed herein, The major subdivision of the terrane in the study area is the Mine including the Shores fault, the Chopawamsic fault, and the Long Run Complex; within the complex numerous faults have been Branch fault. interpreted to separate four metaclastic units, numbered I through

The fi rst major fault that we consider here, the Shores fault,

IV, from east to west (Pavlides, 1989). These fault boundaries was originally identifi ed ~40 km southwest of the epicentral area, have been inferred to exist based upon regional-scale geophysical along the James River as the western limit of deformation associ- characteristics of the metaclastics as well as the type and number ated with the Shores mélange (Brown, 1976, 1979; Evans, 1984). of bodies interpreted to represent exotic blocks within the meta- There, this boundary is coincident with the western margin of a clastic zones.

zone of high magnetic susceptibility known as the Shores linea- The metavolcanic Chopawamsic terrane is to the southeast ment (Brown, 1986), which extends northward into the epicentral of the Potomac terrane and has been interpreted as a Middle area. At the latitude of the James River, the Shores fault is marked Ordovician volcanic arc that developed on some form of con- by a change in metamorphic grade, from greenschist facies to the tinental crust (Pavlides, 1981; Coler et al., 2000). Rocks of the west of the fault to migmatitic facies on its east side, justifying Chopawamsic terrane throughout central and northern Virginia the interpretation that the contact is a fault. Northeast along strike have been dated by U-Pb zircon thermal ionization mass spec- from the Shores area, in the epicentral area, the boundary of the trometry as 474–65 Ma (Coler et al., 2000; Hughes et al., 2013b). Shores lineament has been interpreted, with no documented fi eld The Chopawamsic terrane is locally overlain by Late Ordovician– observations, as a faulted boundary between Mine Run Complex Early Silurian successor basins (Pavlides, 1990, 1995; Mixon et Units III and IV (Duke, 1983; Pavlides, 1989; Glover, 1989; Vir- al., 2000, Hibbard et al., 2014); adjacent to our original fi eld area, ginia Division of Mineral Resources, 1993; Spears et al., 2004; this assemblage of rocks includes the Quantico Formation, which Bailey and Owens, 2012). The name Shores fault has come to be is made up of mostly metaclastic rocks with minor metavolcanic applied and extended to this boundary as well (Spears et al., 2004, strata. Ordovician or younger faunal assemblages in the Quantico Fig. 1 therein; Bailey and Owens, 2012, Fig. 2 therein). However, Formation (Pavlides, 1980; Kolata and Pavlides, 1986), a 448 ± such northerly extrapolation of the Shores fault along the Shores

4 Ma crystallization age from a tuffaceous layer in the Quan- lineament was questioned by those who originally recognized the tico Formation (sensitive high-resolution ion microprobe U-Pb fault (Evans, 1984; Brown, 1986). Their doubt has been borne zircon; Horton et al., 2010), detrital zircon data from the Quan- out by recent mapping (Hughes, 2011). An alternate explanation tico Formation (Bailey et al., 2008), and other fi eld relationships for the boundary between Mine Run Complex Units III and IV (Pavlides et al., 1980) indicate that the Quantico Formation is is a multiply folded, conformable and gradational contact, lack- younger than Middle–Late Ordovician rocks of the Chopawam- ing the increase in metamorphic grade that was used to defi ne the sic terrane. With respect to fi eld relationships, the nature of the Shores fault in its type locality. This interpretation also is sup- contact between the Chopawamsic terrane and the Quantico For- ported by previous work in the vicinity (Hopkins, 1960; Smith et mation apparently varies along strike. In northern Virginia, the al., 1964) that showed no structure that would correspond to the Chopawamsic-Quantico contact has been interpreted as a con- Shores fault. Although this work has shown the Shores fault does formable interlayered contact (Southwick et al., 1971; Seiders not follow the margin of the Shores lineament (Virginia Division et al., 1975; Horton et al., 2010). However, fi eld observation of of Mineral Resources, 1971; Zietz et al., 1977; Snyder, 2005), the

a nonconformity (Pavlides et al., 1980) between the Quantico trace of the Shores fault between the epicentral latitude and that of Formation and the 459 ± 4 Ma Dale City pluton (Aleinikoff et the James River remains unclear. Interpretations of seismic data al., 2002) supports the interpretation that the Quantico Forma- along Interstate Highway 64 (e.g., Harris et al., 1986) originally tion, at least locally, unconformably overlies some rocks of the showed no refl ector that would correspond to the Shores fault, Chopawamsic terrane.

although recent reprocessing of this data (Pratt, 2012) portrayed it as a near-surface splay off of the more eastern Chopawamsic

MAJOR FAULTS LOCAL TO THE EPICENTRAL AREA fault. In addition, the most recent mapping (Burton et al., this vol- ume) shows a fault—the Byrd Mill fault—that may be related to The epicenters for the main shock and majority of after-

a greater Shores/Chopawamsic fault system (Hughes et al., 2014). shocks are in the Pendleton quadrangle, which is immediately Due to the potential of the Shores fault being overridden by the adjacent to our original mapping in the Ferncliff quadrangle (Fig. Chopawamsic fault just to the east, this relationship requires a 2). Prior to the earthquake, we mapped into the epicentral area focused study to elucidate the nature of its northern extent. Even in the Pendleton quadrangle following outcrop exposure along a considering the dearth of information concerning the Shores fault,

3 km segment of the South Anna River. Data from the main shock its position within the Potomac terrane, which is well west of the and aftershock sequence indicate that the rupture plane dips to epicentral area, is enough to eliminate it as a candidate for reacti- the southeast and therefore, the surface trace of this fault, if it vation with respect to the 2011 Virginia earthquake. extends beyond the 2011 rupture zone, must be to the northwest

The Chopawamsic fault traditionally has been interpreted as (updip) of the epicentral area, likely trending through the Fern-

a thrust fault that transported the Chopawamsic terrane westward

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334 Hughes et al.

Kilometers Kilometers

IV III 0 0 2 2 4 4 6 6 8 8

38°15'N

ault

II I

M ountain Run F

Lahore Pluton

Gordonsville

S outh

Long Branch Fault Anna Quantico Formation

E Green Lake

Rive r

Intrusive Complex

Pendleton Quad.

Quail Fault

Area of ? ault ?

Fault

Spotsylvania

es F

Fig. 4A

Shear Zone

Shor

Quad.

Geologic Units

Chopawamsic

Successor Basins

Late Ordovician - Early Silurian slate, phyllite, and schist

Late Ordovician granodiorite to granite E= Ellisville pluton, C= Columbia pluton

Chopawamsic Terrane

Middle Ordovician metavolcanic rocks of the Dixie

Chopawamsic formation

Columbia

C Middle Ordovician or older Mine Run Complex block-

Potomac Terrane

37°45'N

in-phyllite units I, II, III, & IV

James River

Magnitude

Depth (km)

Figure 2. Simplifi ed geology in the epicentral area (geology is modifi ed from Pavlides, 1989, 1990; Virginia Division of Mineral Resources, 1993; Bailey et al., 2005; Hughes, 2011; Spears et al., 2013; and includes our reconnaissance mapping). The main shock occurred at 8 km depth (Chapman, 2013) and is represented by the focal mechanism symbol. Quad.—geological quad- rangles (see text).

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Relict Paleozoic faults in the epicentral area of the 23 August 2011 central Virginia earthquake

and northward onto the Potomac terrane (Evans, 1984; Brown, within the Chopawamsic terrane, because the metavolcanic rocks 1986; Glover, 1989; Pavlides, 1989, 1990, 1995; Pavlides et al., to the east of the Quantico Formation are considered high-grade 1994; Mixon et al., 2000, 2005; Hughes et al., 2013a). Well to the equivalents to those west of the Quantico Formation (Pavlides et southwest of the 2011 epicentral area, near Dillwyn, Virginia, the al., 1994). These fi eld observations are supported by case stud- Chopawamsic fault has been interpreted to merge with the multi- ies that show an increase in metamorphic grade from northwest ply reactivated Brookneal shear zone (Virginia Division of Mineral to southeast across the Quantico Formation (Sutter et al., 1985;

Resources, 1993), but there is no evidence for similar reactiva- Flohr and Pavlides, 1986) and 40 Ar/ 39 Ar cooling ages in early tion in the epicentral study area. Detailed investigation along the Paleozoic rocks near and to the east of the Long Branch fault that Chopawamsic fault in the epicentral area suggests that it represents were reset in the late Paleozoic (ca. 330–290 Ma; Pavlides et al.,

a transpressional boundary with a sinistral component of shear and 1994; Jenkins, 2011; Jenkins et al., 2012; Hughes et al., 2014). that it was stitched ca. 444 Ma by the Ellisville pluton, a feature that In the western Piedmont, 40 Ar/ 39 Ar cooling ages to the west of shows no sign of fault displacement (Hughes et al., 2013a). Seismic the Long Branch fault are more typically Early Silurian or older refl ection studies along Interstate Highway 64 support interpreta- (summarized in Jenkins, 2011) with the fi nal thermal imprint and tions that the Chopawamsic terrane is structurally emplaced over subsequent cooling locally likely related to the intrusion of the the Potomac terrane along the Chopawamsic fault (Harris et al., Ellisville pluton (Pavlides et al., 1994; Hughes et al., 2013a). The 1982, 1986; Pratt, 2012). Our mapping and the subsequent mapping Long Branch fault previously has been mapped only as far south by the Virginia Division of Geology and Mineral Resources and as 38°N (Pavlides, 1990; Mixon et al., 2000) and has not been the U.S. Geological Survey (USGS) in and around the epicentral shown to extend southward toward the earthquake epicenter, at area confi rms that the bedrock geology above the main cluster of 37.905°N. The abrupt termination of the Long Branch fault sur- seismicity is composed of moderately east and southeast-dipping face trace at 38°N is clearly an artifact of regional mapping. discontinuous lenses and layers within the Chopawamsic Forma-

tion (Fig. 3A). These layers are compositionally variable, most FIELD OBSERVATIONS—LONG BRANCH FAULT commonly consisting of felsic epiclastic fi ne-grained metasedi- AND BEND OF RIVER HIGH-STRAIN ZONE

mentary assemblages, with minor biotite-chlorite schists as well as local felsic metavolcanic strata (Hughes, 2011; Spears et al., 2013).

Our initial investigation focused mainly on the Chopawam-

A weak to moderate foliation is parallel to the compositional layer- sic fault and its nature in what became the epicentral area. This ing where both are observed. Rocks immediately to the east of the mapping (Hughes, 2011) and subsequent investigations (e.g., southern tail of the Ellisville pluton are all part of the Chopawamsic Spears et al., 2013) has also lead to an improved understanding Formation. This observation is important because it indicates that of the Long Branch fault south of its previously mapped extent the western border of the Chopawamsic terrane, the Chopawamsic (Mixon et al., 2000). Rocks of the Chopawamsic Formation in fault, is farther to the west than shown on previous geological maps the epicentral area are generally fi ne grained and usually devoid (Pavlides, 1989; Virginia Division of Mineral Resources, 1993). of any macroscopic structural features other than a weak folia- The most recent state geologic map (Virginia Division of Mineral tion parallel to layering. However, locally, along the South Anna Resources, 1993) shows the Chopawamsic fault to be tightly folded River, some of the rocks containing larger clasts display a promi- across the Ellisville pluton tail in the area just west of the main nent linear fabric ranging from L = S to L >> S. At these locales, aftershock cluster; however, this complex folded geometry does not referred to here as the Yanceyville, Bend of River, and Big Bluff exist and is likely an artifact of regional mapping and compilation sites (Fig. 4A), the lineation trends east-northeast and the plunge across the lat 38°N boundary. The surface trace of the fault (Fig. 2) is shallow to moderate. Based upon the evidence presented here is less complex than previously proposed and trends to the west of and mapping by Spears et al. (2013), these deformed rocks the tail of the Ellisville pluton (Hughes, 2011).

appear to be related to a previously unmapped southern exten- North of the epicentral area of the 2011 event, the sion of the Long Branch fault and a related zone of strain to the Chopawamsic-Quantico contact is marked by the Long Branch west of this extension. fault (e.g.: Pavlides, 1990; Pavlides et al., 1994; Mixon et al.,

2000, 2005) with the unusual confi guration of placing the Southern Extension of the Long Branch Fault

younger Quantico Formation structurally above the older Chopawamsic Formation, evidenced by mylonite at the west-

At the Yanceyville site (37.93861N, 77.98218W; Fig. 4A), ern margin of the Quantico Formation (Pavlides, 1973, 1976). the L >> S fabric (070°, 35°) is made up of pebble- to boulder- The Long Branch fault has previously been interpreted to be a sized fragments of fi ne-grained felsic–intermediate volcanic late Paleozoic Alleghanian structure, and there may be evidence clasts in an intermediate composition volcanic matrix (Fig. 3B, for at least two episodes of motion along the fault during that 3C). The site is within the zone of most intense damage from time (Pavlides et al., 1994; Pavlides, 2000). Even though the the 2011 Virginia earthquake (Heller and McGowan, 2012). Quantico Formation is not considered part of the Chopawamsic The intense prolate fabric also includes a weak foliation, and terrane, the Long Branch fault appears to demarcate the bound- clast aspect ratios are 15+:2.6:1. Exact ellipsoidal measure- ary between greenschist and amphibolite facies metamorphism ments are diffi cult to acquire due to the exceptional length of

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336 Hughes et al.

Relict Paleozoic faults in the epicentral area of the 23 August 2011 central Virginia earthquake

337

lineated features. In thin section, the volcanic clasts and matrix of the rock consist mostly of fi ne-grained quartz with additional orthoclase, biotite, and garnet. Quartz crystals have been strained and deformed into ribbons parallel to the maximum stretching direction (Fig. 5A). Orthoclase crystals are not elongated but are commonly dimensionally aligned parallel or subparallel to the L fabric (Figs. 5A, 5B).

This linear fabric is in a position close to the Quantico- Chopawamsic contact. However, unlike the relationship to the north of 38°N, in the epicentral area the zone of strain related to the Long Branch fault is centered to the west of the Chopawamsic- Quantico Formation contact (Spears et al., 2013). This mapping confi guration is shown in Figures 2 and 4; it places the Yanc- eyville site at or extremely near the surface trace of the Long Branch fault. From the data available, it is most likely that the strong prolate fabric at the Yanceyville site is deformation related to a southern extension of the Long Branch oblique thrust fault. In support of the proposed southern extension of the Long Branch fault (this work; Spears et al., 2013), trends in regional and local geophysical data continue uninterrupted into the epicentral area from the northeast (Snyder, 2005; Shah et al., 2012), suggesting that crustal features present to the northeast, including the Long Branch fault, extend into the epicentral area.

Newly Recognized Bend of River High-Strain Zone

The Bend of River site (37.95984N, 78.00155W; Fig. 4A) on the South Anna River is ~3 km to the northwest of the Yanc- eyville site. Outcrops here are characterized by meter-scale inter- layered felsic to mafi c volcaniclastic rocks. Pebble-sized volca- nic clasts (Fig. 3D) at the bases of some of these layers (Fig. 3E) are lineated (062°, 35°) in a style and orientation similar to the features found at the Yanceyville site. At the Big Bluff site (37.95825N, 77.99930W; Fig. 4A) southeast of the Bend of River site, and upsection in the Chopawamsic Formation, other layered rocks also exhibit linear mineral and clast lineations (085°, 25°; Fig. 3F) in mafi c layers (Fig. 3G) that crop out along the South

Figure 3. Photographs from the study area. (A) Shallowly dipping compositional layering in the Chopawamsic Formation near the east- ern boundary of the Ferncliff quadrangle. For scale, the thickest maf- ic layer is ~1 m thick. Layering has a strike of N34°E and a dip of 38°SE. Location is in Ferncliff quadrangle, 37.94007N, 78.00709W. (B, C) Views of the lineated Yanceyville metavolcanic rock in the Chopawamsic Formation. B is looking parallel with the elongation direction (pick is ~1 m long); C is looking perpendicular to the maxi- mum stretching direction. Location is in the Pendleton quadrangle at 37.93861N, 77.98218W. (D, E) Outcrop at the Bend of River site showing lineated clasts within meter-scale layers, which are shown in

E. Lineation is oriented N62°E, 35°; layering is oriented N5°E, 39°E. Location in the Ferncliff quadrangle is 37.95984N, 78.00155W. (F, G) Outcrop at the Big Bluff site. F shows the mineral-aggregate lineations found in mafi c layers shown in G. Lineation is oriented N85°E, 25°; layering is oriented N0°E, 24°E. Location in the Pendleton quadrangle is 37.95825N, 77.99930W.

Anna River. The prominent L fabrics here indicate that there is a zone of intensely strained rocks related to the Long Branch fault, which we refer to as the Bend of River high-strain zone.

Only one foliation is recorded in the rocks located in and around the Bend of River high-strain zone, and it is consistently observed to be parallel with compositional layering within the Chopawamsic Formation. Measured foliation orientations in the study area are shown as poles to planes in Figure 6. The popula- tion of poles to foliation is bimodal, with two populations that correspond to average planar foliation orientations of N37°E, 42S°E and N01°E, 25°E. The foliation orientation of N37°E, 42°SE is consistent with regional fabrics but the N01°E, 25°E orientation has possibly been rotated from the regional orienta- tion. In Figure 6, we refer to this north-striking orientation as a modifi ed foliation. There is no systematic geographic distribu- tion of the two foliation orientations (Hughes, 2011). In the fol- lowing discussion, we consider the origin of these observations.

The Bend of River site appears to delineate the western boundary of the Bend of River high-strain zone, as no similar high-strain L > S features were found to the west of this locality during geologic mapping in the Ferncliff quadrangle (Hughes, 2011). Due to their proximity to each other and the Yanceyville site, deformational features observed at and around the Bend of River and Big Bluff sites are interpreted to record deformation related to a localized zone of ductile strain situated along and just west of the Long Branch fault. The continuity and homogeneity of this deformed zone are not known at this time; future mapping is required to determine the width of this zone and its continuity along the Long Branch fault.

2011 EVENT AND AFTERSHOCKS: POSITION OF THE QUAIL FAULT RELATIVE TO DUCTILE HIGH-STRAIN FEATURES

The main shock of the 2011 event occurred at 1:51 p.m. local time on 23 August 2011. The location (37.905N, 77.975W) and depth (8.0 km) used for the main shock in Figures 2 and 4 are those reported by Chapman (2013). The moment tensor solution produced by the USGS for the main shock show a nodal plane striking N28°E and dipping 50°SE along a previously unrecog- nized structure (Horton et al., 2012a). The main causative fault has been named the Quail fault (Horton et al., 2012a) for the local community in southern Louisa County.

The aftershock record for the 2011 event permits a focused examination of potential slip surfaces in the subsurface bed- rock that goes beyond focal mechanics determined for the main shock. The aftershock data set used here was recorded by an array of temporary seismometers deployed and made publically available online by the Virginia Tech Seismological Observa- tory (2011). After removing earthquakes far away from the main event cluster, the subset of data consists of 64 aftershocks that were recorded 24 August–1 September 2011. This data set does not include the main shock, which was not located as precisely as the aftershock data.

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338 78°W Hughes et al. ?

Fault

S = 034° D = 52°SE

Chopawamsic Formation

Ellisville Pluton

Long Branch

Bend of R str ain z one iver Quail Fault

Quail River

A′

Big Bluff Site Yanceyville Site

Kilometers

Bend of River Site

0 0.5 1 1.5 A 2

Bend of River

Big Bluff

Yanceyville

A Site

Site

Site

Quail Fault

Quantico Successor Basin

2 km

Long Branch Chopawamsic

Ellisville Pluton

Chopawamsic Fault

Zone

Potomac Terrane (Mine Run Complex)

Mountain Run Fault

Undivided Cambrian and Undivided Cambrian and Undivided Cambrian and ? ? ? Precambrian Rocks Precambrian Rocks Precambrian Rocks

No Vertical Exaggeration No No Vertical Exaggeration ertical Exaggeration

Figure 4. (A) Local map of the epicentral area with the same geologic units as Figure 2. Position of Long Branch fault is after Spears et al. (2013). Note locations of the Yanceyville, Bend of River, and Big Bluff sites. Black lines on map show local roads. Color and size of aftershock locations are as in Figure 2. S—strike; D—dip. (B) Interpretive cross- section view along line A-A ′ in A.

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Relict Paleozoic faults in the epicentral area of the 23 August 2011 central Virginia earthquake

Figure 5. Photomicrographs of the Yanceyville lineated metavolcanic rock. (A) Ribboned quartz clasts within a mostly fi ne-grained quartzose matrix. (B) Orthoclase feldspars are commonly dimensionally aligned parallel with the clast lineation. Field of view diameter is ~9.5 mm.

An orthogonal regression plane was produced by principal component analysis in order to obtain an attitude for the best-fi t fault plane through the selected subset of aftershock data. Follow- ing Horton et al. (2012a), we refer to this fault plane as the Quail fault. Three-dimensional analysis was conducted with an aspect ratio of 1:1:1 with all units being in meters (depth, Northings, and Eastings). The resultant best-fi t plane for the aftershock data has an attitude with a strike of N33.6°E and a dip of 51.5°SE (Figs. 7A, 7B). The standard error for this plane is ±258 m in a Figure 6. Comparison of modern brittle aftershock plane and inherited

direction normal to the plane, potentially refl ecting the width of strain features of the Bend of River strain zone. Aftershock plane is shown as black great circle with an uncertainty girdle derived from

a brittle damaged zone or the resolution of the aftershock data. A bootstrap analysis (shown in Fig. 7). P—plunge; T—trend; S—strike; random-sampling bootstrap function was employed to quantify D—dip. There are 37 bimodal poles to foliation orientations plotted. the error that can be attributed to the scatter of the aftershock The solid black line is the regional, unaltered foliation orientation data; this exercise yields a strike of N33.6°E ± 4.1° and a dip of (N37°E, 42°SE) and the dashed line is the modifi ed foliation orienta-

51.5°SE ± 3.1°. The errors reported are two standard deviations tion (N01°E, 25°E). Bend of River, Big Bluff, and Yanceyville linea- tion orientations are shown as labeled stars. The stereonet plot was

from the mean; these data are displayed graphically in Figure partially prepared with the Stereonet v.7.3.0 program of Allmendinger 7B. Bootstrap estimates of the strike and dip are normally dis- et al. (2012). tributed about the mean, indicating that the aftershock data are well distributed along a single plane of interest, with no excessive scatter. Therefore, this best-fi t plane is interpreted to represent trace of the fault farther to the southeast. In addition, the presence the attitude of the Quail fault, and is shown in Figures 2 and 4. of a near-surface, subvertical set of aftershocks that occurred The orientation for the Quail fault plane determined here is simi- in December 2011–January 2012 (Horton et al., 2012b) would lar to other estimates for the orientation of the main shock and

be more indicative of a concave Quail fault if they occurred on subsequent aftershock plane: N29°E, 51°SE (Chapman, 2013); the same surface as the main shock and main set of aftershocks. N25°E, 55°SE (Davenport et al., 2012); N36°E ± 12°, 45°SE ± 6° The westernmost position of the extrapolated surface trace of the (McNamara et al., 2012); N28°E, 50°SE (Horton et al., 2012a); Quail fault (Figs. 2 and 4) demonstrates the improbability of rup- N25°E, 45°SE (Ellsworth et al., 2011).

ture along any potential structure along the southeast margin of The method chosen to delineate the Quail fault is based on the Ellisville pluton tail (Harrison, 2012). In support of this con-

a statistical best-fi t model that weights each aftershock equally clusion, the intrusive, nonfaulted, relationship between the tail of and the model also assumes that the aftershocks occurred along the pluton and the Chopawamsic Formation has been observed

a brittle fault zone, not preferentially within the hanging wall of directly in outcrop (Hughes et al., 2013a, Fig. 3E therein). Fur- the fault, a scenario proposed by others (e.g., Harrison, 2012). thermore, inclusions and screens of Chopawamsic Formation The extrapolated surface trace shown in Figures 2 and 4 repre- rocks at the southeast margin of the Ellisville pluton tail near sents the most western position likely for the surface trace of the South Anna River (Burton, 2013; Hughes et al., 2014, Stop the Quail fault, as the linear regression used produced a plane

9 therein) also reinforce its intrusive, rather than fault-controlled, devoid of any potential geologic concavity. A possible arcuate relationship with the Chopawamsic Formation. concave shape, a structural form commonly interpreted to exist

The extrapolated surface trace of the Quail fault, as pre- through the Virginia Piedmont (e.g., Harris et al., 1982, 1986; sented here (Fig. 4B), is 70 m southeast of the Bend of River site Pavlides, 1989; Pavlides et al., 1994; Mixon et al., 2000, 2005), and <200 m northwest of the Big Bluff site. These distances are to the southeast-dipping Quail fault, would only shift the surface well within the ±258 m standard error obtained from the analysis

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340 Hughes et al.

Figure 7. Aftershock analysis. (A) Principal component analysis of the set of 64 aftershocks analyzed yields a best-fi t orientation of the Quail fault at N34E°, 52°SE. The plane is shown in the three-dimensional scatterplot; data points above are shown as circles while data points below are shown as squares. The residual distances from the data points to the best-fi t plane yield a standard error of ±258 m. (B) Histograms that show the relative distribution of outcomes from a bootstrap model using 50,000 iterations. Errors for the strike and dip of the best-fi t plane for the Quail fault are given as two standard deviations from the mean. The strike yielded for this data set is N33.6°E ± 4.1° and the dip is 51.5°SE ± 3.1°.

of aftershocks. The placement of the Quail fault was arrived at fault exploited a preserved structural fabric within or along the independently and without consideration of any known strain margin of the strain zone (Fig. 4B). features and was based solely on the analysis of the subset of

There is also a local disruption of compositional layering

64 aftershocks. Therefore, we recognize that the Quail fault, the along the trace of the Quail fault; in numerous areas the regional main causative fault that ruptured in 2011, is spatially coincident attitude of N37°E, 42°SE has been locally modifi ed to N01°E, with, and projects directly through, the Paleozoic Bend of River 25°E (Fig. 6). The disrupted attitude of layering along and near high-strain zone. It seems likely that the ductile deformation here the trace of the Quail fault may be a result of previous brittle rota- is associated with the Long Branch fault due to the similarity tion and/or kinking along or near the same structure. Prior brittle in style and orientation of these features with those seen at the events with kinematics similar to the reported reverse motion Yanceyville site.

along the Quail fault could have modifi ed the regional N37°E, 42°SE in places to N01°E, 25°E (Fig. 6). Possible offset of Qua-

DISCUSSION

ternary terraces in the same vicinity along the South Anna River (Berti et al., 2015) may be consistent with a previously active

A focused examination of the aftershock data reveals a fault Quail fault. orientation of N34°E ± 4°, 52°SE ± 3° for the Quail fault. This

While targeted post-event fi eld investigation, seismic data surface is spatially distinct from both the Chopawamsic fault and processing, and remote sensing efforts will lead to improved the boundary interpreted by some to represent the Shores fault, understanding of the 2011 Virginia earthquake, the recognition which has positions and surface traces farther to the west. Ductile of modern brittle fault planes that correspond with relict ductile planar and linear fabrics in the Bend of River high-strain zone strain features cannot be overlooked as coincidental. The ductile related to the Long Branch fault are geographically and spatially fabrics we measured were recognized as part of an ongoing study coincident with the brittle Quail fault (Fig. 6). From the data, not focused on modern seismicity, and these observations invite we interpret the L ≥ S-tectonite fabrics within the Bend of River further investigation into the control of modern intraplate earth- strain zone of the Chopawamsic Formation to be a product of quakes by inherited features in the CVSZ and elsewhere. deformation associated with a southern extension of the Long

Branch fault as mapped by Spears et al. (2013). The geographic SUMMARY

and spatial coincidence of the surface trace of the Quail fault and fabrics within the Bend of River strain zone to the west of the

Relict ductile L = S and L >> S fabrics in the Chopawamsic Long Branch fault lead us to interpret that the rupture of the Quail Formation appear to be related to the late Paleozoic Long Branch

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Relict Paleozoic faults in the epicentral area of the 23 August 2011 central Virginia earthquake

fault and the associated Bend of River high-strain zone. The Bourland, W.C., 1976, Tectonogenesis and metamorphism of the Piedmont from southern extension of the Long Branch fault and associated Bend

Columbia to Westview, Virginia, along the James River [M.S. thesis]:

of River strain zone appear to have controlled the orientation and Blacksburg, Virginia Polytechnic Institute and State University, 105 p.

Bourland, W.C., Glover, L., III, and Poland, F.B., 1979, The Lakeside fault zone

rupture of the modern, brittle Quail fault. In the Piedmont of Vir-

north of the Willis River, in Glover, L., III, and Tucker, R.D., eds., Virginia

ginia, numerous regional examples show ductile fabrics that have

Piedmont Geology Along the James River from Richmond to the Blue

been overprinted by brittle faulting as they make their way to the Ridge: Guides to Field Trips 1–3 for Southeastern Section Meeting, Geo-

logical Society of America: Blacksburg, Virginia Polytechnic Institute

shallow crust. Even in the absence of observed brittle features at

and State University, p. 17–18.

the surface, we consider the 2011 rupture of the Quail fault to Brown, W.R., 1976, Tectonic mélange(?) in the Arvonia slate district of Virginia: Geological Society of America Abstracts with Programs, v. 8, p. 142. represent a modern analog to this type of ductile to brittle reacti- Brown, W.R., 1979, Field guide to the Arvonia-Schuyler district, in Glover, L., vation commonly observed in the region.

III, and Tucker, R.D., eds., Virginia Piedmont Geology Along the James River from Richmond to the Blue Ridge: Guides to Field Trips 1–3 for

ACKNOWLEDGMENTS Southeastern Section Meeting, Geological Society of America: Blacks-

burg, Virginia Polytechnic Institute and State University, p. 24–41. Brown, W.R., 1986, Shores complex and mélange in the central Virginia Pied-

Geologic mapping in the epicentral area was supported by U.S.

mont, in Neathery, T.L., ed., Southeastern Section of the Geological Soci-

Geological Survey EDMAP grant G10AC00265 to Hibbard. ety of America, Centennial Field Guide Volume 6: Northeastern Section,

Geological Society of America, Centennial Field Guide, v. 6, p. 209–214.

Canoeing assistance was provided by Dillon Conner and Dil- Burton, B., 2013, Bedrock geology of the Ferncliff Quadrangle, Virginia: Vir- lon Nance. We thank George Payne for access to his property.

ginia Geological Research Symposium, April 11, 2013, Charlottesville,

Development of the ideas presented in this paper benefi ted Virginia: Virginia Division of Geology and Mineral Resources.

Burton, W.C., Harrison, R.W., Spears, D.B., Evans, N.H., and Mahan, S.A.,

from discussions with Bill Burton, Mark Carter, Rich Harri-

2015, this volume, Geologic framework and evidence for Neotectonism

son, David Spears, Chuck Bailey, and others. We thank review-

in the epicentral area of the 2011 Mineral, Virginia, earthquake, in Hor- ton, J.W., Jr., Chapman, M.C., and Green, R.A., eds., The 2011 Mineral,

ers Bill Henika and Kevin Stewart for constructive criticism

Virginia, Earthquake, and Its Signifi cance for Seismic Hazards in East-

that led to an improved and more focused paper, and editors

ern North America: Geological Society of America Special Paper 509,

J. Wright Horton Jr., Martin C. Chapman, and Russell A. Green

doi:10.1130/2015.2509(20). Carter, M.W., Blanpied, M.L., Leeds, A.L., Harp, E.L., McNamara, D.E., Har-

for their efforts and professionalism during the production of

rison, R.W., and Schindler, J.S., 2012, USGS response to the Mineral,

this volume.

Virginia MW5.8 earthquake of 23 August 2011: Geological Society of America Abstracts with Programs, v. 44, no. 4, p. 13.

Chapman, M.C., 2013, On the rupture process of the 23 August 2011 Virginia

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