doi:10.1130/2014.2509(19)
, published online November 12, 2014;
Geological Society of America Special Papers
K. Stephen Hughes, James P. Hibbard and DelWayne R. Bohnenstiehl
seismicity
earthquake: Assessing the relationship between preexisting strain and modern
Relict Paleozoic faults in the epicentral area of the 23 August 2011 central Virginia
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Notes
331
The Geological Society of America Special Paper 509
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
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, Mw 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.
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.
INTRODUCTION
The moment magnitude, Mw 5.8 Mineral, Virginia (USA) earthquake occurred on 23 August 2011 and was likely felt by more people than any other in U.S. history (Carter et al., 2012). This intraplate earthquake is the largest recorded event to occur in the Central Virginia seismic zone (CVSZ). Known events within the CVSZ are temporally and spatially diffuse and most have not been convincingly associated with mapped geologic faults. Most geologic maps produced for the Piedmont of Virginia are not detailed enough to identify features responsible for the 2011 earthquake and previous shaking events. One of the most intrigu-ing questions pertainintrigu-ing to intraplate seismicity in the CVSZ is if and how modern brittle fault ruptures (earthquakes) in the area are related to known Paleozoic faults, most of which record duc-tile strain.
As part of a separate study into the Paleozoic tectonic evolu-tion of the Appalachian Piedmont of central and northern Vir-ginia, we undertook mapping of the bedrock geology in the north half of the Ferncliff 7.5′ quadrangle (Hughes, 2011) in the sum-mer of 2010. While targeted studies have been conducted in the area following the earthquake, these original and objective obser-vations can be used in an attempt to analyze the surfaces upon which slip occurred during the main shock and subsequent after-shocks. In the course of mapping the Ferncliff quadrangle and adjacent areas, relict Paleozoic faults in the epicentral region of the 2011 Virginia earthquake were recognized. Because the fault plane (Quail fault) assigned to the main cluster of aftershocks does not appear to directly correlate to a previously identifi ed structure, all relict zones of deformation in the area should be scrutinized with regard to the 2011 event. Field observations are supplemented with an analysis of the aftershocks that occurred in the week immediately after the main shock.
REGIONAL OVERVIEW
The 2011 main shock and aftershocks occurred in the north-ern part of the CVSZ, which is one of many recognized intraplate seismic zones in eastern North America. Much of the CVSZ is in the Piedmont of Virginia, and the 2011 event occurred below the western Piedmont (Fig. 1). The western Piedmont of Virginia (summarized by Hibbard et al., 2014) is mostly composed of metamorphosed Neoproterozoic to Paleozoic sedimentary and igneous rocks. The western Piedmont is bordered to the west by the Appalachian Blue Ridge and to the east by both the eastern Piedmont and Atlantic coastal plain sediments. Within and along the margins of the Virginia Piedmont are numerous ductile faults that record some form of brittle overprint. Empirical data suggest that fault reactivation has been the norm along structural boundar-ies in the Piedmont; furthermore, the overprint of brittle features upon ductilely deformed rocks indicates that faults that originally formed at mid-crustal depths are also active once transported to shallow crustal levels (e.g., Bourland, 1976; Bobyarchick and Glover, 1979; Bourland et al., 1979; Weems, 1981; Gates, 1986, 1997; Pavlides, 1987, 1989, 2000; Pavlides et al., 1983, 1994; Spears and Bailey, 2002; Bailey et al., 2004; Spears et al., 2004; Spears, 2010; Henika, 2012; Hollis and Bailey, 2012; Quinlan, 2012; Spears and Gilmer, 2012). For this reason it is important to examine zones of relict ductile strain in the epicentral area of the 2011 Virginia earthquake.
In central and northern Virginia, the western Piedmont is composed of the Potomac and Chopawamsic terranes, which are separated by the Chopawamsic fault (Pavlides, 1989, 1990, 1995; Pavlides et al., 1994; Mixon et al., 2000, 2005; Hughes et al., 2013a; Hibbard et al., 2014). The metaclastic Potomac terrane is to the northwest of the fault and has been interpreted to repre-sent an early Paleozoic accretionary complex that formed on the
R
SRA M
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Valley and R
idg
e
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Coastal Plain
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o
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stal
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in
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. Pi
edmont
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Blue Ridge
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e R
id
ge
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Area of Figure 2
Virginia Western Piedmont
Mesozoic Culpeper and Danville basins and related rocks
Ordovician to Early Silurian intrusive bodies. Stipple = felsic, black = mafic
Ordovician rocks of the Chopawamsic Terrane (C) and Milton Terrane (M). Cambrian to Ordovician rocks of the Potomac Terrane (P) and Smith River Allochthon (SRA).
SRA
C M
CVSZ P
eastern margin of Laurentia (Drake, 1989; Pavlides, 1989; Hor-ton et al., 1989; Hibbard et al., 2014) during the early Paleozoic. The major subdivision of the terrane in the study area is the Mine Run Complex; within the complex numerous faults have been interpreted to separate four metaclastic units, numbered I through IV, from east to west (Pavlides, 1989). These fault boundaries have been inferred to exist based upon regional-scale geophysical characteristics of the metaclastics as well as the type and number of bodies interpreted to represent exotic blocks within the meta-clastic zones.
The metavolcanic Chopawamsic terrane is to the southeast of the Potomac terrane and has been interpreted as a Middle Ordovician volcanic arc that developed on some form of con-tinental crust (Pavlides, 1981; Coler et al., 2000). Rocks of the Chopawamsic terrane throughout central and northern Virginia have been dated by U-Pb zircon thermal ionization mass spec-trometry as 474–65 Ma (Coler et al., 2000; Hughes et al., 2013b). The Chopawamsic terrane is locally overlain by Late Ordovician– Early Silurian successor basins (Pavlides, 1990, 1995; Mixon et al., 2000, Hibbard et al., 2014); adjacent to our original fi eld area, this assemblage of rocks includes the Quantico Formation, which is made up of mostly metaclastic rocks with minor metavolcanic strata. Ordovician or younger faunal assemblages in the Quantico Formation (Pavlides, 1980; Kolata and Pavlides, 1986), a 448 ± 4 Ma crystallization age from a tuffaceous layer in the Quan-tico Formation (sensitive high-resolution ion microprobe U-Pb zircon; Horton et al., 2010), detrital zircon data from the Quan-tico Formation (Bailey et al., 2008), and other fi eld relationships (Pavlides et al., 1980) indicate that the Quantico Formation is younger than Middle–Late Ordovician rocks of the Chopawam-sic terrane. With respect to fi eld relationships, the nature of the contact between the Chopawamsic terrane and the Quantico For-mation apparently varies along strike. In northern Virginia, the Chopawamsic-Quantico contact has been interpreted as a con-formable interlayered contact (Southwick et al., 1971; Seiders et al., 1975; Horton et al., 2010). However, fi eld observation of a nonconformity (Pavlides et al., 1980) between the Quantico Formation and the 459 ± 4 Ma Dale City pluton (Aleinikoff et al., 2002) supports the interpretation that the Quantico Forma-tion, at least locally, unconformably overlies some rocks of the Chopawamsic terrane.
MAJOR FAULTS LOCAL TO THE EPICENTRAL AREA
The epicenters for the main shock and majority of after-shocks are in the Pendleton quadrangle, which is immediately adjacent to our original mapping in the Ferncliff quadrangle (Fig. 2). Prior to the earthquake, we mapped into the epicentral area in the Pendleton quadrangle following outcrop exposure along a 3 km segment of the South Anna River. Data from the main shock and aftershock sequence indicate that the rupture plane dips to the southeast and therefore, the surface trace of this fault, if it extends beyond the 2011 rupture zone, must be to the northwest (updip) of the epicentral area, likely trending through the
Fern-cliff quadrangle. Signifi cant faulted contacts adjacent to the west and east of the main cluster of seismicity are reviewed herein, including the Shores fault, the Chopawamsic fault, and the Long Branch fault.
The fi rst major fault that we consider here, the Shores fault, was originally identifi ed ~40 km southwest of the epicentral area, along the James River as the western limit of deformation associ-ated with the Shores mélange (Brown, 1976, 1979; Evans, 1984). There, this boundary is coincident with the western margin of a zone of high magnetic susceptibility known as the Shores linea-ment (Brown, 1986), which extends northward into the epicentral area. At the latitude of the James River, the Shores fault is marked by a change in metamorphic grade, from greenschist facies to the west of the fault to migmatitic facies on its east side, justifying the interpretation that the contact is a fault. Northeast along strike from the Shores area, in the epicentral area, the boundary of the Shores lineament has been interpreted, with no documented fi eld observations, as a faulted boundary between Mine Run Complex Units III and IV (Duke, 1983; Pavlides, 1989; Glover, 1989; Vir-ginia Division of Mineral Resources, 1993; Spears et al., 2004; Bailey and Owens, 2012). The name Shores fault has come to be applied and extended to this boundary as well (Spears et al., 2004, Fig. 1 therein; Bailey and Owens, 2012, Fig. 2 therein). However, such northerly extrapolation of the Shores fault along the Shores lineament was questioned by those who originally recognized the fault (Evans, 1984; Brown, 1986). Their doubt has been borne out by recent mapping (Hughes, 2011). An alternate explanation for the boundary between Mine Run Complex Units III and IV is a multiply folded, conformable and gradational contact, lack-ing the increase in metamorphic grade that was used to defi ne the Shores fault in its type locality. This interpretation also is sup-ported by previous work in the vicinity (Hopkins, 1960; Smith et al., 1964) that showed no structure that would correspond to the Shores fault. Although this work has shown the Shores fault does not follow the margin of the Shores lineament (Virginia Division of Mineral Resources, 1971; Zietz et al., 1977; Snyder, 2005), the trace of the Shores fault between the epicentral latitude and that of the James River remains unclear. Interpretations of seismic data along Interstate Highway 64 (e.g., Harris et al., 1986) originally showed no refl ector that would correspond to the Shores fault, although recent reprocessing of this data (Pratt, 2012) portrayed it as a near-surface splay off of the more eastern Chopawamsic 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 a greater Shores/Chopawamsic fault system (Hughes et al., 2014). Due to the potential of the Shores fault being overridden by the Chopawamsic fault just to the east, this relationship requires a focused study to elucidate the nature of its northern extent. Even considering the dearth of information concerning the Shores fault, its position within the Potomac terrane, which is well west of the epicentral area, is enough to eliminate it as a candidate for reacti-vation with respect to the 2011 Virginia earthquake.
0 2 4 6 8 Kilometers
E
C
I
II
III
IV
77°45'W 78°W
78°15'W
38°15'N
38°N
37°45'N
0 2 4 6 8
Kilometers
15
Columbia Dixie Gordonsville
Louisa
Mineral
Spotsylvania Shear Zone
<2 2-3 3-4 4-5 5.8
Magnitude
Depth (km)
8
0 2 4 6
Chopawamsic Fault
Long Branch Fault
James River
Late Ordovician granodiorite to granite E= Ellisville pluton, C= Columbia pluton
Middle Ordovician metavolcanic rocks of the Chopawamsic formation
Middle Ordovician or older Mine Run Complex block-in-phyllite units I, II, III, & IV
Late Ordovician - Early Silurian slate, phyllite, and schist
Potomac Terrane
Chopawamsic Terrane
Successor Basins
Geologic Units
Quad.Pendleton Quad.
Area of Fig. 4A
Lahore Pluton
Green Springs Intrusive Complex
Quail Fault
Mountain Run F
ault
Lake Anna S
outh
Anna
River
Quantico Formation
Shor es F
ault
Boundar
y
Conf ormable
?
? ?
?
and northward onto the Potomac terrane (Evans, 1984; Brown, 1986; Glover, 1989; Pavlides, 1989, 1990, 1995; Pavlides et al., 1994; Mixon et al., 2000, 2005; Hughes et al., 2013a). Well to the southwest of the 2011 epicentral area, near Dillwyn, Virginia, the Chopawamsic fault has been interpreted to merge with the multi-ply reactivated Brookneal shear zone (Virginia Division of Mineral Resources, 1993), but there is no evidence for similar reactiva-tion in the epicentral study area. Detailed investigareactiva-tion along the Chopawamsic fault in the epicentral area suggests that it represents a transpressional boundary with a sinistral component of shear and that it was stitched ca. 444 Ma by the Ellisville pluton, a feature that shows no sign of fault displacement (Hughes et al., 2013a). Seismic refl ection studies along Interstate Highway 64 support interpreta-tions that the Chopawamsic terrane is structurally emplaced over the Potomac terrane along the Chopawamsic fault (Harris et al., 1982, 1986; Pratt, 2012). Our mapping and the subsequent mapping by the Virginia Division of Geology and Mineral Resources and the U.S. Geological Survey (USGS) in and around the epicentral area confi rms that the bedrock geology above the main cluster of seismicity is composed of moderately east and southeast-dipping discontinuous lenses and layers within the Chopawamsic Forma-tion (Fig. 3A). These layers are composiForma-tionally variable, most commonly consisting of felsic epiclastic fi ne-grained metasedi-mentary assemblages, with minor biotite-chlorite schists as well as local felsic metavolcanic strata (Hughes, 2011; Spears et al., 2013). A weak to moderate foliation is parallel to the compositional layer-ing where both are observed. Rocks immediately to the east of the southern tail of the Ellisville pluton are all part of the Chopawamsic Formation. This observation is important because it indicates that the western border of the Chopawamsic terrane, the Chopawamsic fault, is farther to the west than shown on previous geological maps (Pavlides, 1989; Virginia Division of Mineral Resources, 1993). The most recent state geologic map (Virginia Division of Mineral Resources, 1993) shows the Chopawamsic fault to be tightly folded across the Ellisville pluton tail in the area just west of the main aftershock cluster; however, this complex folded geometry does not exist and is likely an artifact of regional mapping and compilation across the lat 38°N boundary. The surface trace of the fault (Fig. 2) is less complex than previously proposed and trends to the west of the tail of the Ellisville pluton (Hughes, 2011).
North of the epicentral area of the 2011 event, the Chopawamsic-Quantico contact is marked by the Long Branch fault (e.g.: Pavlides, 1990; Pavlides et al., 1994; Mixon et al., 2000, 2005) with the unusual confi guration of placing the younger Quantico Formation structurally above the older Chopawamsic Formation, evidenced by mylonite at the west-ern margin of the Quantico Formation (Pavlides, 1973, 1976). The Long Branch fault has previously been interpreted to be a late Paleozoic Alleghanian structure, and there may be evidence for at least two episodes of motion along the fault during that time (Pavlides et al., 1994; Pavlides, 2000). Even though the Quantico Formation is not considered part of the Chopawamsic terrane, the Long Branch fault appears to demarcate the bound-ary between greenschist and amphibolite facies metamorphism
within the Chopawamsic terrane, because the metavolcanic rocks to the east of the Quantico Formation are considered high-grade equivalents to those west of the Quantico Formation (Pavlides et al., 1994). These fi eld observations are supported by case stud-ies that show an increase in metamorphic grade from northwest to southeast across the Quantico Formation (Sutter et al., 1985; Flohr and Pavlides, 1986) and 40Ar/39Ar cooling ages in early
Paleozoic rocks near and to the east of the Long Branch fault that were reset in the late Paleozoic (ca. 330–290 Ma; Pavlides et al., 1994; Jenkins, 2011; Jenkins et al., 2012; Hughes et al., 2014). In the western Piedmont, 40Ar/39Ar cooling ages to the west of
the Long Branch fault are more typically Early Silurian or older (summarized in Jenkins, 2011) with the fi nal thermal imprint and subsequent cooling locally likely related to the intrusion of the Ellisville pluton (Pavlides et al., 1994; Hughes et al., 2013a). The Long Branch fault previously has been mapped only as far south as 38°N (Pavlides, 1990; Mixon et al., 2000) and has not been shown to extend southward toward the earthquake epicenter, at 37.905°N. The abrupt termination of the Long Branch fault sur-face trace at 38°N is clearly an artifact of regional mapping.
FIELD OBSERVATIONS—LONG BRANCH FAULT AND BEND OF RIVER HIGH-STRAIN ZONE
Our initial investigation focused mainly on the Chopawam-sic fault and its nature in what became the epicentral area. This mapping (Hughes, 2011) and subsequent investigations (e.g., Spears et al., 2013) has also lead to an improved understanding of the Long Branch fault south of its previously mapped extent (Mixon et al., 2000). Rocks of the Chopawamsic Formation in the epicentral area are generally fi ne grained and usually devoid of any macroscopic structural features other than a weak folia-tion parallel to layering. However, locally, along the South Anna River, some of the rocks containing larger clasts display a promi-nent linear fabric ranging from L = S to L >> S. At these locales, referred to here as the Yanceyville, Bend of River, and Big Bluff sites (Fig. 4A), the lineation trends east-northeast and the plunge is shallow to moderate. Based upon the evidence presented here and mapping by Spears et al. (2013), these deformed rocks appear to be related to a previously unmapped southern exten-sion of the Long Branch fault and a related zone of strain to the west of this extension.
Southern Extension of the Long Branch Fault
A
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B
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B
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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 Feast-erncliff quadrangle. For scale, the thickest maf-ic layer is ~1 m thmaf-ick. 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 foliapopula-tion is bimodal, with two populapopula-tions 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 foliadistribu-tion orientadistribu-tions (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.
0 0.5 1 1.5 2 Kilometers
Quail Fault
Long Branch
Fault
South Anna
River S = 034°
D = 52°SE
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Quail
Big Bluff Site
Yanceyville Site
Bend of River Site A
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′
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Chopawamsic Fault
Mountain Run Fault Long Branch
Fault
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Quail Fault
Undivided Cambrian and Undivided Cambrian and
Precambrian Rocks Precambrian Rocks Undivided Cambrian and
Precambrian Rocks
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Yanceyville SiteA
′
Bend of River Site
2 km
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10 Chopawamsic
Terrane
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Chopawamsic Formation Ellisville Pluton
Big Bluff Site
201 1 Aftershock
Zone
Ellisville Pluton ?
Bend of R
iver str
ain z one
Ellisville Pluton
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 direction normal to the plane, potentially refl ecting the width of a brittle damaged zone or the resolution of the aftershock data. A random-sampling bootstrap function was employed to quantify the error that can be attributed to the scatter of the aftershock data; this exercise yields a strike of N33.6°E ± 4.1° and a dip of 51.5°SE ± 3.1°. The errors reported are two standard deviations from the mean; these data are displayed graphically in Figure 7B. Bootstrap estimates of the strike and dip are normally dis-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 the attitude of the Quail fault, and is shown in Figures 2 and 4. The orientation for the Quail fault plane determined here is simi-lar to other estimates for the orientation of the main shock and subsequent aftershock plane: N29°E, 51°SE (Chapman, 2013); N25°E, 55°SE (Davenport et al., 2012); N36°E ± 12°, 45°SE ± 6° (McNamara et al., 2012); N28°E, 50°SE (Horton et al., 2012a); N25°E, 45°SE (Ellsworth et al., 2011).
The method chosen to delineate the Quail fault is based on a statistical best-fi t model that weights each aftershock equally and the model also assumes that the aftershocks occurred along a brittle fault zone, not preferentially within the hanging wall of the fault, a scenario proposed by others (e.g., Harrison, 2012). The extrapolated surface trace shown in Figures 2 and 4 repre-sents the most western position likely for the surface trace of the Quail fault, as the linear regression used produced a plane devoid of any potential geologic concavity. A possible arcuate concave shape, a structural form commonly interpreted to exist through the Virginia Piedmont (e.g., Harris et al., 1982, 1986; Pavlides, 1989; Pavlides et al., 1994; Mixon et al., 2000, 2005), to the southeast-dipping Quail fault, would only shift the surface 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.
Figure 6. Comparison of modern brittle aftershock plane and inherited strain features of the Bend of River strain zone. Aftershock plane is shown as black great circle with an uncertainty girdle derived from bootstrap analysis (shown in Fig. 7). P—plunge; T—trend; S—strike; D—dip. There are 37 bimodal poles to foliation orientations plotted. The solid black line is the regional, unaltered foliation orientation (N37°E, 42°SE) and the dashed line is the modifi ed foliation orienta-tion (N01°E, 25°E). Bend of River, Big Bluff, and Yanceyville linea-tion orientalinea-tions are shown as labeled stars. The stereonet plot was partially prepared with the Stereonet v.7.3.0 program of Allmendinger et al. (2012).
trace of the fault farther to the southeast. In addition, the presence of a near-surface, subvertical set of aftershocks that occurred in December 2011–January 2012 (Horton et al., 2012b) would be more indicative of a concave Quail fault if they occurred on the same surface as the main shock and main set of aftershocks. The westernmost position of the extrapolated surface trace of the Quail fault (Figs. 2 and 4) demonstrates the improbability of rup-ture along any potential strucrup-ture along the southeast margin of the Ellisville pluton tail (Harrison, 2012). In support of this con-clusion, the intrusive, nonfaulted, relationship between the tail of the pluton and the Chopawamsic Formation has been observed directly in outcrop (Hughes et al., 2013a, Fig. 3E therein). Fur-thermore, inclusions and screens of Chopawamsic Formation rocks at the southeast margin of the Ellisville pluton tail near the South Anna River (Burton, 2013; Hughes et al., 2014, Stop 9 therein) also reinforce its intrusive, rather than fault-controlled, relationship with the Chopawamsic Formation.
of aftershocks. The placement of the Quail fault was arrived at independently and without consideration of any known strain features and was based solely on the analysis of the subset of 64 aftershocks. Therefore, we recognize that the Quail fault, the main causative fault that ruptured in 2011, is spatially coincident with, and projects directly through, the Paleozoic Bend of River high-strain zone. It seems likely that the ductile deformation here is associated with the Long Branch fault due to the similarity in style and orientation of these features with those seen at the Yanceyville site.
DISCUSSION
A focused examination of the aftershock data reveals a fault orientation of N34°E ± 4°, 52°SE ± 3° for the Quail fault. This surface is spatially distinct from both the Chopawamsic fault and the boundary interpreted by some to represent the Shores fault, which has positions and surface traces farther to the west. Ductile planar and linear fabrics in the Bend of River high-strain zone related to the Long Branch fault are geographically and spatially coincident with the brittle Quail fault (Fig. 6). From the data, we interpret the L ≥ S-tectonite fabrics within the Bend of River strain zone of the Chopawamsic Formation to be a product of deformation associated with a southern extension of the Long Branch fault as mapped by Spears et al. (2013). The geographic 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 Long Branch fault lead us to interpret that the rupture of the Quail
fault exploited a preserved structural fabric within or along the margin of the strain zone (Fig. 4B).
There is also a local disruption of compositional layering along the trace of the Quail fault; in numerous areas the regional attitude of N37°E, 42°SE has been locally modifi ed to N01°E, 25°E (Fig. 6). The disrupted attitude of layering along and near the trace of the Quail fault may be a result of previous brittle rota-tion and/or kinking along or near the same structure. Prior brittle events with kinematics similar to the reported reverse motion 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-ternary terraces in the same vicinity along the South Anna River (Berti et al., 2015) may be consistent with a previously active Quail fault.
While targeted post-event fi eld investigation, seismic data processing, and remote sensing efforts will lead to improved understanding of the 2011 Virginia earthquake, the recognition of modern brittle fault planes that correspond with relict ductile strain features cannot be overlooked as coincidental. The ductile fabrics we measured were recognized as part of an ongoing study not focused on modern seismicity, and these observations invite further investigation into the control of modern intraplate earth-quakes by inherited features in the CVSZ and elsewhere.
SUMMARY
fault and the associated Bend of River high-strain zone. The southern extension of the Long Branch fault and associated Bend of River strain zone appear to have controlled the orientation and rupture of the modern, brittle Quail fault. In the Piedmont of Vir-ginia, numerous regional examples show ductile fabrics that have been overprinted by brittle faulting as they make their way to the shallow crust. Even in the absence of observed brittle features at the surface, we consider the 2011 rupture of the Quail fault to represent a modern analog to this type of ductile to brittle reacti-vation commonly observed in the region.
ACKNOWLEDGMENTS
Geologic mapping in the epicentral area was supported by U.S. Geological Survey EDMAP grant G10AC00265 to Hibbard. Canoeing assistance was provided by Dillon Conner and Dil-lon Nance. We thank George Payne for access to his property. Development of the ideas presented in this paper benefi ted from discussions with Bill Burton, Mark Carter, Rich Harri-son, David Spears, Chuck Bailey, and others. We thank review-ers Bill Henika and Kevin Stewart for constructive criticism that led to an improved and more focused paper, and editors J. Wright Horton Jr., Martin C. Chapman, and Russell A. Green for their efforts and professionalism during the production of this volume.
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