American Journal of Science

[American Journal of Science, Vol. 307, January, 2007, P. 1–22, DOI 10.2475/01.2007.01]

American Journal of Science JANUARY 2007

THE MECHUM RIVER FORMATION, VIRGINIA BLUE RIDGE: A RECORD OF NEOPROTEROZOIC AND PALEOZOIC TECTONICS IN SOUTHEASTERN LAURENTIA CHRISTOPHER M. BAILEY*†, SHANAN E. PETERS**, JOHN MORTON*, and NATHAN L. SHOTWELL* ABSTRACT. Neoproterozoic metasedimentary rocks of the Mechum River Formation crop out in an elongate northeast-southwest trending belt in the central Virginia Blue Ridge province. The northwestern contact of the Mechum River Formation is nonconformable above Mesoproterozoic basement, whereas the southeastern contact is a set of steeply dipping, brittle reverse faults that juxtapose basement against Mechum River rocks. The internal structure of the belt consists of northwest-verging, open to tight, moderately to steeply plunging asymmetric folds. Although previous workers have interpreted the outcrop belt of the Mechum River Formation as a graben formed during Laurentian rifting, structural relations require that its present geometry is a structural inlier related to Paleozoic contractional deformation. Strain analysis reveals that grain-scale deformation processes produced up to 35 percent shortening during foliation development whereas map-scale folds account for an additional 30 to 50 percent shortening. Restoration of the Mechum River Formation to its precontractional geometry reveals little about the geometry of the original depositional basin. The bounding reverse faults on the east side of the Mechum River Formation are interpreted as out-of-sequence structures that developed after regional folding, metamorphism and foliation development that may be related to the emplacement of the Blue Ridge thrust sheet over a tectonic ramp. Sediment transport indicators are consistent with a source area to the east, suggesting that the Mechum River Formation was separated from similar units in the eastern Blue Ridge by an asymmetric basement high that may have been produced by block rotation above listric normal faults. introduction

Neoproterozoic rocks form a distinctive and important component of the Appalachian orogen. Early plate tectonic models for the Appalachian orogen recognized that an ocean basin developed at the end of the Precambrian and that in the Laurentian Appalachians, late Precambrian sedimentary rocks sit with considerable unconformity above mid-crustal Grenvillian rocks and form the base of the cover sequence at many locations (Wilson, 1966; Bird and Dewey, 1970; Rodgers, 1972; Rankin, 1975). The latest Neoproterozoic to Early Cambrian units provide the key evidence for rifting in southeastern Laurentia and for the development of the Iapetus Ocean. The Neoproterozoic was a unique period in earth history characterized by climatic extremes far greater than those experienced during the Phanerozoic (Kirschvink, 1992; Hoffman and others, 1998), understanding the significance of Neoproterozoic sedimentary sequences is critical for reconstructing prevailing conditions during this era.

*Department of Geology, College of William and Mary, P. O. Box 8795, Williamsburg, Virginia 23187, USA **Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 34278, USA † Corresponding author: cmbail @ wm.edu

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C. M. Bailey and others—The Mechum River Formation, Virginia Blue Ridge:

Fig. 1. Regional overview map of the central and southern Appalachians highlighting the Blue Ridge province, Laurentian rift-related features, and the Neoproterozoic Mechum River Formation. Location of the Virginia-Tennessee and Georgia transforms are based on Thomas (1991).

Neoproterozoic units crop out from Georgia to Newfoundland, but it is in the central and southern Appalachian Blue Ridge province (fig. 1) that these units are extensive and provide a detailed record of rifting and the transition to a passive margin. In the southern Appalachians, this sequence includes thick sections of clastic rocks (Ocoee basin) that in some areas are interlayered with felsic and mafic volcanics (Grandfather Mountain and Mount Rogers). In Virginia, Neoproterozoic units are widespread, but generally thinner than those exposed in the southern Blue Ridge (fig. 1). Thomas (1976, Thomas, 1977, 1993) proposed that the northeast-striking rifted margin of southeastern Laurentia is segmented by a number of northwest-striking transform faults; he further speculated that the Virginia-Tennessee transform separates a highly extended lower-plate rifted margin in the southern Blue Ridge from a less extended upper-plate rifted margin in the northern Blue Ridge (fig. 1). In central and northern Virginia, the Blue Ridge province includes a large basement massif of Mesoproterozoic rocks flanked by Neoproterozoic to early Paleozoic rocks (fig. 2). Mesoproterozoic rocks include a suite of granitoids formed during the long-lived Grenvillian orogeny between 1.0 and 1.2 Ga (Bartholomew and Lewis, 1984; Aleinikoff and others, 2000; Tollo and others, 2004a). A distinctive suite of 680 to 730 Ma granitoid plutons (including the Robertson River Igneous Suite in northern Virginia) intrudes the Mesoproterozoic rocks (Bartholomew and Lewis, 1984; Tollo and Aleinikoff, 1996; Tollo and others, 2004b). Collectively, the Proterozoic granitoids are unconformably overlain by a sequence of Neoproterozoic to early Cambrian metasedimentary and metavolcanic rocks that record sedimentation and magmatism

A record of Neoproterozoic and Paleozoic tectonics in southeastern Laurentia v

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Fig. 2. Generalized geological map of the central Virginia Blue Ridge.

associated with Laurentian rifting and the development of Iapetus (Rankin, 1975; Wehr and Glover, 1985). Neoproterozoic rifting in the Blue Ridge appears to have occurred during two temporally distinct episodes; an early extensional event between 680 and 765 Ma (Aleinikoff and others, 1995; Li and Tull, 1998; Tollo and others, 2004b) and a second event between ⬃540 and 575 Ma, followed by the opening of Iapetus and the development of a southeast-facing passive margin (Wehr and Glover, 1985; Badger and Sinha, 1988; Simpson and Eriksson, 1989; Aleinikoff and others, 1995). The Mechum River Formation is a sequence of Neoproterozoic metasedimentary rocks in the Virginia Blue Ridge exposed as an elongate belt surrounded by older Proterozoic granitoid and gneiss (fig. 2). Based on its elongate outcrop pattern and the nature of its sedimentary sequence many workers interpreted the Mechum River Formation to be a graben filled with terrestrial deposits that accumulated during Laurentian rifting (Nelson, 1962; Schwab, 1974; Harris and others, 1986; Tollo and Hutson, 1996). The Mechum River graben interpretation has been further cited in overview articles (Fichter and Diecchio, 1986; Rankin and others, 1989; Thomas, 1991, 1993; Rast, 1992), textbooks (Hatcher, 1995), and educational resources (Fichter, 1993), in essence becoming embedded in the geologic literature. The Blue Ridge anticlinorium is, however, a late Paleozoic contractional structure generated during the emplacement of the basement complex and its cover sequence over a footwall thrust ramp during late Paleozoic tectonism (Mitra, 1979; Harris, 1979; Evans, 1989). If the Mechum River Formation was deposited in a graben how was it affected or modified by later Paleozoic contractional deformation? Our purpose is to characterize the structural geometry of the Mechum River Formation and determine 1) if the Mechum River Formation occupies a Neoprotero-

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zoic graben, 2) if original Neoproterozoic structures are discernible and overprinted by later contractional structures, and 3) quantify the amount of deformation at both the map-scale and at the meso- to micro-scale. We also seek to establish a better regional framework for Neoproterozoic metasedimentary units in the Blue Ridge in order to place constraints on tectonic models for Iapetan rifting and the Neoproterozoic paleogeography of southeastern Laurentia. mechum river formation

Previous Work Metasedimentary rocks in the core of the Blue Ridge anticlinorium were first noted on the 1928 Geologic Map of Virginia (Nelson, 1928), but Gooch (ms, 1954; 1958) provides the first detailed description of the Mechum River Formation; as a sequence of metaconglomerate, metasandstone, and metasiltstone infolded into the basement complex and, in places, bounded by high-angle reverse faults. At the southern end of the Mechum River Formation, Nelson (1962) depicted a graben with steep normal faults along its margins, but also noted the synclinal nature of the belt (Batesville syncline). Mitra and Lukert (1982), in the northern part of the belt, interpreted it to have been an original graben, in which the basin bounding faults were sheared and reactivated as reverse faults during the Paleozoic. Harris and others (1986) interpreted the Mechum River Formation on the Interstate 64 seismic reflection profile across central Virginia as a graben complex with subordinate horsts. Tollo and Hutson (1996) reported metarhyolite interlayered with Mechum River metasedimentary rocks near the northern termination and correlated these metarhyolites with the 705 Ma Battle Mountain felsite (the youngest unit in the Robertson River Igneous Suite) (fig. 2). Stratigraphy and Depositional Setting The Mechum River Formation is a ⬎500-m-thick sequence of low-grade metamorphosed clastic rocks that range from mudstones to boulder conglomerates and sit unconformably above Mesoproterozoic granitoid and gneiss. The top of the Mechum River Formation is not exposed. Gooch (ms, 1954; 1958) originally interpreted the Mechum River Formation to be a deep-water deposit. In contrast, Schwab (1974) proposed that the Mechum River Formation is an alluvial sequence and noted that cross bed azimuths (measured throughout the belt) define a centripetal pattern consistent with deposition from the west and east side of an elongate basin. Conley (1989) suggested that the Mechum River Formation includes marine deposits in the south that pass into non-marine deposits in the northern part of the belt. Bailey and Peters (1998) recognized glaciogenic marine deposits at the base of the Mechum River Formation in the south and noted that these rocks pass upwards into marine turbidites that grade laterally into fluvio-deltaic deposits at the northern end of the belt. The oldest rocks in the Mechum River Formation are exposed at the southwestern end of the belt and the unit generally becomes younger to the northeast (fig. 3; Bailey and Peters, 1998). At the base of the Mechum River Formation different rock types nonconformably overlie the basement complex; near the southwestern end of the belt, boulder conglomerate occurs at the base, whereas 10 km to the northeast, phyllitic siltstone occurs at the base. The boulder conglomerate is stratigraphically below the phyllitic siltstone and we interpret the unconformity to be time transgressive. We recognize seven subunits within the Mechum River Formation based on stratigraphic position and rocktype (fig. 3). The basal boulder conglomerate and diamictite pass upward into rhythmically bedded arkose and diamictite (fig. 4A, Zm1) interpreted to be glaciomarine deposits (Bailey and Peters, 1998). To the northeast the basal subunit (Zm1) grades into and is

A record of Neoproterozoic and Paleozoic tectonics in southeastern Laurentia

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Fig. 3. Schematic stratigraphic cross section through the Mechum River Formation from southwest to northeast (parallel to the long axis of the formation) with subunit descriptions and depositional interpretations.

overlain by arkose and laminated arkosic wacke (Zm2) with outsized clasts of granitoid gneiss; these are overlain by a thick sequence of thinly laminated siltstone and mudstone (Zm3). Further to the northeast, fine-grained rocks pass upward into a distinct sequence of arkose and arkosic wacke (Zm4). Both graded and ungraded beds are present (up to 2 m thick) and the average bed thickness decreases across the belt from east to west. Bed bottoms are characteristically scoured, graded beds fine upwards into parallel laminations, and cross stratification is rare or absent (fig. 4B). This subunit is interpreted to have been deposited by subaqueous gravity flows. A monotonous sequence of laminated mudstone (fig. 4D) and thinly bedded arkosic wacke (Zm6) overlies Zm4. In the northern part of the Mechum River belt the mudstone and arkosic wacke (Zm6) are underlain by coarse- to medium-grained arkose and conglomerate (Zm5 and Zm7). These subunits are matrix-poor, relative to the sandy units in the rocks to the south. Planar, trough and festoon cross stratification is common (figs. 4E and 4F). Cobble and pebble conglomerates are common at the base of Zm5 and Zm7. The northern subunits in the Mechum River Formation are interpreted to include alluvial fan, fluvial braid plain, and deltaic deposits that may have formed in a glacial outwash environment. The Mechum River Formation is primarily an arkosic sequence with a clast assemblage that includes abundant quartz and alkali feldspar (predominately perthite) with lesser amounts (⬃5 to 10%) of plagioclase and ilmenite and minor zircon, magnetite, apatite, and rutile. Schwab (1974) reported that Mechum River framework clasts are indicative of intra-cratonic setting. The provenance for the Mechum River Formation is alkali feldspar rich granitoids of the Blue Ridge basement complex. The metamorphic minerals in the Mechum River Formation are dominated by muscovite, biotite, and epidote. Age Clasts within Mechum River Formation metaconglomerates are primarily Mesoproterozoic granitoids, but in the north, clasts of the ⬃730 Ma Laurel Mills granite of the

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Fig. 4. (A) Boulder conglomerate/diamictite (Zm1) near Batesville (37.97° N, 78.74° W), (B) graded packages with scoured bed bottoms in arkosic wacke (Zm4) (38.12° N, 78.58° W), (C) coarse-grained arkose with subangular fragments of feldspar and quartz (Zm5) exposed near Madison (38.38° N, 78.32° W), (D) laminated mudstone (Zm6) (38.35° N, 78.36° W), (E) planar cross stratification in arkosic wacke (Zm6) (38.53° N, 78.19° N), (F) trough cross bedding in arkose (Zm7) near Castleton (38.60° N, 78.12° W).

Robertson River Igneous Suite are common and yield a maximum depositional age (Hutson, 1992; Morton and Bailey, 2004). Dikes and sills of foliated metabasalt with compositions similar to the ⬃570 Ma Catoctin Formation intrude the Mechum River Formation and loosely bracket a minimum depositional age (Gooch, 1958; Bailey and others, 2003). Tollo and Hutson (1996) described metarhyolites with compositions identical to ⬃705 Ma felsites of the Robertson River Igneous suite interlayered with Mechum River rocks near Castleton; they interpreted that deposition of the Mechum River Formation was ongoing at ⬃705 Ma. However, metarhyolites at Castleton crop out to the east of the Mechum River belt and are in tectonic contact with the Mechum River Formation (Morton and Bailey, 2004). In the Castleton area, the Mechum River belt contains no volcanic detritus. The extrusion and deposition of the ⬃705 Ma Castleton metarhyolites and their associated sedimentary rocks probably postdate the Mechum River Formation. Based on the available data the Mechum River Formation


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Fig. 5. Geologic map and cross section of the Mechum River Formation at its southwestern termination near Batesville. Covesville, Virginia, 7.5-minute quadrangle.

was deposited prior to 705 Ma. The northern part of the belt can be no older than ⬃730 Ma, but at its southern limits may be somewhat older. structural geometry

The structural geometry of the Mechum River belt was delineated by new 1:24,000 scale mapping, cross section construction and restoration, as well as structural, strain, and petrographic analyses. Here we present detailed maps and cross sections of a few key areas within the Mechum River belt. In addition to our mapping we have utilized data presented on maps by Mitra (ms, 1977), Lukert and Halladay (1980), Hutson (1992), Tollo and Lowe (1994). Cross sections were constructed, in part from downplunge projection of structural data collected at the surface in Mechum River metasedimentary rocks as well as from available gravity and aeromagnetic data. Map-scale Relations The southern end of the Mechum River Formation is ⬃20 km southwest of Charlottesville (figs. 1 and 5). The southwestern contact between the Mechum River Formation and Mesoproterozoic granitoid gneiss is an irregular, hummocky surface with 5 to 25 meters of relief and is overlain by massive conglomerate and diamictite with discontinuous lenses of arkosic sandstone (fig. 4A). This contact is interpreted as an unconformity. These coarse-grained deposits grade upward into arkosic wacke and laminated siltstone. The structural geometry is an open to tight asymmetric fold that

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plunges to the northeast. Locally, the eastern limb is overturned. At the northwestern contact, Mechum River rocks are upright and nonconformably overlie Mesoproterozoic biotite-bearing granitoid (fig. 5). The southeastern contact dips steeply to the southeast and Mesoproterozoic granitoids structurally overlie Mechum River rocks (fig. 5). Northwest of Charlottesville, arkose and arkosic wacke (Zm4) are exposed in a series of northwest-verging overturned folds (fig. 6). Map-scale folds plunge at moderate angles to the northeast with hinge lines that trend more easterly than the southeastern contact (fig. 6). A penetrative foliation defined by aligned phyllosilicates and elongated detrital minerals strikes to the northeast, dips uniformly to the southeast, and is axial planar to the map-scale folds (fig. 6). At the northwestern contact, bedding is upright and overlies the basement complex. In cross section B, the southeastern contact is a reverse fault that cuts overturned beds on the southeast limb of a syncline, whereas in cross section C the fault cuts the hinge of a syncline (fig. 6). The southeastern contact is, thus, a reverse fault that truncates folds and the penetrative foliation in the Mechum River Formation. 15 km north of Charlottesville, the Mechum River Formation crops out in a belt over 1 km wide (figs. 2 and 7). At the northwestern contact, coarse-grained arkose sits unconformably above granitoid gneiss. Bedding is upright and dips uniformly to the southeast; the penetrative foliation dips more steeply to the southeast. The southeastern contact is a steeply dipping reverse fault that places Mesoproterozoic rocks on upright Mechum River metasedimentary rocks. In this area the Mechum River belt forms a homoclinal sequence of southeast dipping beds. The bounding reverse fault cuts through a map-scale fold hinge. In the Madison area, the Mechum River Formation crops out in two northeastsouthwest trending belts, 0.25 to 1.2 km wide, separated by two en-echelon reverse faults (figs. 1 and 8). Gooch (1958) first recognized this structure and interpreted it to be an anticline that exposes basement in its core. Our mapping does not support the anticline interpretation; rather the two bounding reverse faults duplicate the Mechum River Formation. Within the Mechum River belt, bedding is folded into steeply dipping panels with different strikes (fig. 9). Folds are open to tight and commonly have southeast dipping axial planes and fold axes that plunge moderately to steeply south (Shotwell and Bailey, 2000). Folds in the Madison area are drape-like and not obviously linked with simple northwest-southeast contraction; these folds are consistent with a component of transpressional deformation. Bailey and others (2002a) reported evidence for triclinic deformation in nearby Blue Ridge mylonite zones and these folds appear to be kinematically compatible with triclinic structures (for example, NW/SE directed shortening with strike-parallel displacement). The penetrative foliation in the Madison area strikes northeast-southwest, dips moderately to steeply to the southeast, and is axial planar with respect to folds (fig. 8). In the Castleton area the Mechum River belt achieves its maximum width of ⬃5 km (fig. 9). At the northwestern contact cobble to boulder conglomerate and coarsegrained arkose overlie Mesoproterozoic biotite-bearing granitoid gneiss. At Bessie Bell Mountain (fig. 9), the northwestern contact is not planar and appears to be an erosional surface with as much as 50 m of relief (Morton and Bailey, 2004). At this location the northwestern contact is also cut by a subvertical, north-northwest striking fault; the offset across this fault is minor and post-dates folding of the unconformity (fig. 9). Coarse arkosic rocks (Zm7) are overlain by arkosic graywacke and siltstone (Zm6). Collectively, the Mechum River Formation is folded into a sequence of open to tight, asymmetric northwest-verging anticlines and synclines (fig. 9). At a number of locations, bedding in the Mechum River Formation is truncated at the southeastern


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Fig. 6. Geologic map, cross sections, and stereograms of the Mechum River Formation to the northwest of Charlottesville. Charlottesville West, Virginia, 7.5-minute quadrangle. Data portrayed on stereograms collected from the map area and adjacent regions.

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Fig. 7. Geologic map, cross section, and stereograms of the Mechum River Formation to the north of Charlottesville. Earlysville, Virginia, 7.5-minute quadrangle.

contact. To the north, the southeastern contact steps to the west and the cross-strike width of the Mechum River belt dramatically decreases (fig. 9). South of Castleton the basal conglomerate in the Mechum River Formation nonconformably overlies the ⬃730 Ma Laurel Mills granite of the Robertson River suite; the conglomerate contains abundant clasts of the Laurel Mills granite. Felsic metavolcanic rocks, metavolcanic conglomerates, and phyllitic rocks are exposed in a 0.2 to 0.5 km wide belt to the southeast of Castleton. These volcanic rocks are not in direct stratigraphic contact with the Mechum River metasedimentary rocks, rather they are separated from the Mechum River belt by a 0.2 to 0.5 km wide belt of Laurel Mills granite. Foliation and bedding orientations in the Castleton metavolcanics range from steeply dipping to subvertical and are structurally distinct from attitudes in the Mechum River belt to the west (fig. 9). We have not directly observed the bounding contacts of the Castleton rocks, but the map pattern indicates that they are steeply dipping. The Castleton rocks could have been extruded/deposited in a basin related to foundering of an older Robertson River magma chamber. Later contractional


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Fig. 8. Geologic map, cross sections, and stereograms of the Mechum River Formation to the west of Madison. Madison, Virginia, 7.5-minute quadrangle.

deformation has folded these units into a series of subvertical upright isoclinal folds (fig. 9). Alternatively, the bounding structures at Castleton may be steeply-dipping imbricate reverse faults of Paleozoic age that are similar to the bounding faults on the southeast side of the Mechum River belt (Morton and Bailey, 2004). Tollo and Hutson (1996) argued for a direct connection between the Castleton rocks and the Mechum River rocks, however, new mapping and structural attitudes do not demonstrate this. Contact Relations Although geologic contacts are rarely well exposed in the Virginia Blue Ridge, we observed the Mechum River Formation contact at approximately 15 locations, and


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Fig. 9. Geologic map, cross sections, and stereograms of the Mechum River Formation near Castleton. Castleton and Woodville, Virginia, 7.5-minute quadrangles.

delineated the contact to within a few meters at many sites. Both the southeastern contact and northwestern contacts of the Mechum River belt dip moderately to steeply (⬃55° to subvertical) to the southeast. At the southeastern contact, Mesoproterozoic granitoid gneiss and in the north, Robertson River granitoids, structurally lie above Mechum River metasedimentary rocks. At many locations along the southeastern contact Mechum River rocks are upright. The southeastern contact is a family of high-angle reverse faults that places older Proterozoic granitoids on Neoproterozoic Mechum River rocks. At a number of locations the bounding fault tips out and is replaced by another en-echelon fault (figs. 8 and 9). The bounding Mechum River faults are not mylonitic, but are discrete brittle structures. At some locations the 10 to


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Fig. 10. (A) Graded beds and refracted foliation in Zm4 in the southern part of the belt. (B) Refracted foliation and cuspate fold at the base of coarse-grained arkose from Zm4.

30 m wide fault zone includes abundant vein quartz that displays no evidence of crystal plastic deformation. Faulting along the southeastern margin of the Mechum River belt truncates the folds and penetrative greenschist facies fabrics (figs. 5 – 9). We estimate that throw across these faults is no more than a few kilometers based on the observation that basement rocks in the hanging wall are at the same grade as the Mechum River metasedimentary rocks. At the northwestern contact Mechum River metasedimentary rocks structurally overlie Mesoproterozoic basement units. Everywhere along this contact the Mechum River rocks are upright. At many locations this contact is non-planar at the map-scale (fig. 9), and consistent with an erosional contact. Conglomeratic rocks are localized at this contact and typically grade upward into finer grained rocks. We interpret this contact to be a tilted unconformity and not a tectonic contact. However, at many locations rocks on both sides of the contact are penetratively deformed and at some locations the basement rocks are mylonitic. Penetrative Fabrics and Strain Analysis Mechum River metasedimentary rocks commonly display a penetrative foliation defined by aligned biotite, muscovite, and elongate detrital grains of quartz and feldspar. This foliation invariably strikes northeast-southwest, dips moderately to steeply to the southeast, and is axial planar with respect to folds in the Mechum River Formation (figs. 5 – 9). Foliation is best developed in fine-grained rocks, and may completely obliterate primary structures. In graded arkosic wacke layers, the foliation is commonly refracted across fining upward sedimentary packages producing the appearance of cross bedding (fig. 10A). Cuspate folds are developed at the lower contact of coarser graded packages due to the competence contrast with the underlying finer grained rocks (fig. 10B).

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Fig. 11. Photomicrographs of (A) deformed detrital quartz and feldspar grains. Quartz includes monocrystalline (mq) grains with undulose extinction and polycrystalline grains (rq), feldspar (f) grains are more angular, commonly fractured and mantled by a selvage of muscovite. Metamorphic minerals include muscovite, biotite, and epidote. (B) weakly deformed arkosic wacke. (C) strongly deformed arkosic wacke with steeply inclined foliation. Metamorphic minerals include muscovite, biotite, and epidote. Photos of Mechum River Formation arkosic wackes exposed northwest of Charlottesville (fig. 6). (D) XZ sectional strain ratio of 2.5:1 for arkosic wacke illustrated in C, that corresponds with ⬃35% shortening normal to foliation.

Metamorphic minerals in Mechum River metasedimentary rocks include abundant biotite, muscovite, epidote, granular sphene (after ilmenite) and chlorite. Phyllitic rocks commonly include biotite porphyroblasts and at some locations garnet and rare graphite laths. Porphyroblast microstructures are consistent with pre- to syntectonic growth relative to the foliation development. Microstructures in detrital quartz grains include undulose extinction, core and mantle textures, and indented grain boundaries; recrystallization in sand-sized grains ranges from extensive to minor (fig. 11A). Alkali feldspar clasts display abundant fractures with both fracture surfaces and grain boundaries typically mantled by fine-grained white mica and quartz neoblasts (fig. 11A). Plagioclase clasts are commonly saussauritized and fractured. Collectively, these microstructures are compatible with significant crystal plastic deformation in quartz and predominantly brittle deformation and dissolution of feldspar that is consistent with deformation during greenschist facies conditions. The Mechum River Formation experienced regional metamorphism at the greenschist facies that was contemporaneous with the development of penetrative fabrics and map-scale folding. Sectional strain ratios were estimated at the grain- to thin section-scale using Rf/␾ strain analysis of quartz and feldspar clasts in 12 arkosic metasandstone samples. Grain shapes were digitized from thin sections, individual grain shape ratios and long axis orientations measured (30 to 80 grains per section), and then plotted on a hyperbolic stereogram (De Paor, 1988) to determine sectional strain ratios. For each sample, two or three orthogonal planes were measured and the sectional data used to calculate three-dimensional strains. Principal strains are subparallel to fabric elements and the


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foliation plane approximates the XY plane. Three-dimensional strain geometries include plane strain to oblate ellipsoids (Shotwell and Bailey, 2000). Whole-rock strain ratios in XZ sections range from 1.1 to 3.4 (fig. 11 B-D) with quartz clasts generally recording higher strain ratios (10 – 30%) than feldspar clasts. The higher strain ratios recorded by quartz clasts is likely a function of the crystal plastic strain experienced by quartz relative to the brittle deformation and dissolution experienced by feldspar. These strain estimates do not account for strain in the matrix or grain boundary sliding along phyllosilicates and as such are minimum values. There was no tangible difference in grain-scale strain based on position within the meso-scale structures (that is- no greater strain ratios on the overturned limbs of folds versus the upright limbs; fig. 12A). XZ strain ratios of 2.5 correspond to ⬃35 percent shortening normal to foliation (fig. 11D). Penetrative deformation at the grain-scale contributed upwards of 35 percent shortening. Restoration of the Mechum River Belt The Mechum River belt was restored to it pre-contractional deformation geometry by anchoring a pin line at the unconformable northwestern contact, retrodeforming the penetrative strain, and then unfolding the map-scale structures to create horizontal layering (fig. 12A). Retrodeformation of the penetrative strain was accomplished by using average whole-rock strains (Rs- 1.3 to 2.4) and applying a pure shear graphical transformation parallel to foliation and axial planes such that rocks were elongated normal to foliation. Grain-scale strain may have accumulated by general or simple shear, and our retrodeformation method is intended only to place broad limits on the amount of shortening. Map-scale folding accounts for up to 50 percent shortening across the belt. When the layering is unfolded to a subhorizontal orientation the reverse faults at the southeastern contact become northwest dipping faults and their origin as post folding structures becomes evident (fig. 12A). In its present geometry the Mechum River belt ranges from 0.5 to 5 km wide, after retrodeformation and unfolding the belt restores to a width of 1.5 to 8 km. However, the southeastern contact is a reverse fault that cuts through existing folds, thus even the restored Mechum River belt preserves an unknown, but probably a small part of the complete depositional basin (fig. 12). discussion

Neoproterozoic Geometry of the Mechum River Basin The Mechum River Formation is a structural inlier within the Blue Ridge anticlinorium and is not, in its present configuration a graben. Restoration of the Mechum River Formation to its pre-Paleozoic contractional orientation does not define a graben or any definite rift-related structures (fig. 12); the restored width of the Mechum River belt still forms a relatively narrow belt (5 – 8 km). The eastern margin of the Mechum River belt is a reverse fault and clearly does not represent the limits of the depositional basin. Although Neoproterozoic to early Paleozoic rift-related faults occur in the Virginia Blue Ridge (Southworth and Brezinski, 1996; Bailey and others, 2002a), there is no evidence that rift-related structures are associated with the Mechum River Formation. If the Mechum River Formation were bounded by original normal faults there would likely be a discordance or truncation between the fault contact and rift deposits along both margins of the belt. Conglomerate is common at the nonconformable contact along the western margin of the Mechum River Formation, but is rare on the eastern side of the belt; if the belt were a graben conglomerate would be likely to crop out along both margins. Mitra and Lukert (1982) proposed that original basin-bounding faults were rotated and sheared into reverse faults during Paleozoic deformation. The concordance between the sedimentary layering and the western

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Fig. 12. (A) Deformed and restored state cross section of the Mechum River Formation with sectional strain data projected onto cross section, unstrained section retrodeformed back by applying the average strain in a pure shear fashion normal to foliation/fold axial planes, unfolded section. (B) Map of restored, pre-contraction geometry of the Mechum River Formation with rose diagrams for paleocurrent indicator localities.

contact of the Mechum River Formation is consistent with an unconformable contact rather than a normal fault reactivated as a thrust. Furthermore, the overall finite strain recorded in these rocks is low (⬍3:1) and likely insufficient to significantly rotate original structures. Schwab’s (1974) cross bed azimuth data from throughout the Mechum River belt suggests a centripetal pattern consistent with deposition from either side of an elongate basin. We have observed cross bedding in Mechum River arkoses located only in the fluvio-deltaic deposits exposed in the northern part of the belt (subunits Zm5, 6,


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and 7, fig. 12B). Restoration of cross bedding to its original horizontal position (by rotation about both the local fold axes and the strike of originally horizontal bedding) reveals azimuthal data consistent with west- to southwest-directed sediment transport (fig. 12B). These data are in sharp contrast to those presented by Schwab (1974). We observe no cross beds in the southern part of the Mechum River Formation, here arkosic wackes form graded packages that lack cross stratification. Refracted foliation here superficially resembles cross bedding (fig. 10). Average bed thicknesses in the gravity flow deposits (Zm2 and Zm4) as measured at the outcrop-scale decrease from east to west across the Mechum River belt, an observation consistent with sediment transport from the east. Clasts in the Mechum River Formation are derived primarily from Mesoproterozoic leucocratic granitoids and the Neoproterozoic Robertson River granitoids that crop out primarily to the east of the Mechum River belt. Charnockite a common rocktype from the western Blue Ridge is almost entirely absent from the Mechum River clast assemblage. Collectively, these data indicate a source area for the Mechum River Formation that was to the east/northeast of the present belt. Deformation Timing and Sequence The age of regional metamorphism and deformation in the Blue Ridge anticlinorium is broadly constrained to the Paleozoic, but a detailed chronology of the deformation history awaits further geochronologic research. Previous workers have argued that ductile deformation occurred during the Ordovician and was related to Taconic tectonism, (Wehr and Glover, 1985; Glover and others, 1989; Rankin and others, 1989; Evans, 1991; Bailey and Simpson, 1993) whereas others have ascribed ductile deformation to late Paleozoic tectonism associated with the Alleghanian orogeny (Mitra and Elliot, 1980; Evans, 1989; Burton and others, 1992). Recent Ar-Ar geochronology in the central Virginia Blue Ridge yield muscovite cooling ages between 310 and 355 Ma for basement and cover rocks that experienced greenschist facies metamorphism and deformation (300 to 450° C) (Polvi, ms, 2003; Wooton and others, 2005; Bailey and others, 2007). These new data are compatible with early Alleghanian to Acadian ductile deformation in the Virginia Blue Ridge. We propose that the central Virginia Blue Ridge was deformed at the greenschist facies between 310 and 360 Ma; this contractional deformation event produced high-strain zones in the basement, penetrative fabrics in both the basement and cover sequence, and folds in the cover sequence (fig. 13B). During the late Alleghanian (280 to 300 Ma) the Blue Ridge thrust sheet was emplaced over a ramp(s) in the underlying Cambro-Ordovician sequence (Harris, 1979; Evans, 1989). The exact location of this ramp(s) and the extent of Cambro-Ordovician rocks beneath the Blue Ridge basement is unclear (for example, Evans, 1989; Bailey, ms, 1994; Lampshire and others, 1994), however rocks in the central and eastern Blue Ridge have been rotated about a horizontal axis to accommodate this translation (fig. 13). The geometric and mechanical difficulties of translating a cooling basement-cored thrust sheet up a tectonic ramp and onto a flat produced brittle out-of-sequence faults in the basement (fig. 13C). The family of steeply dipping reverse faults that place Proterozoic granitoids on the Neoproterozoic Mechum River Formation are out-of-sequence brittle structures that developed during the emplacement of the late Alleghanian Blue Ridge thrust sheet. Lukert and Halladay (1980), Knight and Bailey (1999), and Chapman and others (2003) identified similar steeply dipping brittle faults to the east of the Mechum River belt. Regional Tectonic Considerations and Speculation The Mechum River Formation is lithologically similar to the Lynchburg Group exposed in the eastern Blue Ridge (fig. 2). Wehr (1986) describes a transition in the Lynchburg Group from fluvio-deltaic deposits in the north (originally the Fauquier

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Fig. 13. Deformation model for the Mechum River Formation, (A) Neoproterozoic deposition in asymmetric basin to the west of a basement high that is separated from the Lynchburg Group to the east. (B) Folding of the cover sequence during regional deformation and metamorphism in the prior to the early Alleghanian to Acadian. (C) Rotation of the basement and cover sequence due to the emplacement of the Blue Ridge thrust sheet over a tectonic ramp. Steepening of pre-existing structures and out-of-sequence brittle faulting near the eastern contact. (D) Present day surface level.


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Formation and later renamed the Bunker Hill and Monumental Mills Formations) to subaqueous gravity flow deposits in the south (Thorofare Mountain and Ball Mountain Formations) that is similar to changes in Mechum River lithofacies (alluvial deposits in the north and marine deposits in the south). As discussed above sediment transport indicators suggest that the Mechum River Formation was derived from a Proterozoic granitoid source area exposed to the east/northeast of the current outcrop belt (fig. 12). Wehr (ms, 1983) suggested that the Lynchburg Group was derived from a source area to the west. The Mechum River Formation may be a western outlier of the Lynchburg Group, but the two rock units were separated by an original basement high (fig. 13A). The Mechum River Formation may have been deposited in embayment to the west (present day coordinates) of the main Lynchburg depocenter. The basement high may have been produced by rotation of at least two listric normal fault blocks of Neoproterozoic age; one along the western edge of the Lynchburg Group and another to the west (present day coordinates) of the Mechum River belt. The Lynchburg Group is an ⬃5 km thick clastic sequence that required significant tectonic subsidence- likely accommodated by normal faulting. Chapman and others (2003) presented evidence for a Neoproterozoic basin-bounding normal fault (later reactivated in the Paleozoic) along the western edge of the Lynchburg Group in central Virginia (fig. 2). A significant listric normal fault to the west of the Mechum River belt is required to produce the necessary tilt to provide an eastern source for the Mechum River deposits. Wehr and Glover (1985) and Evans (1991) proposed a hinge zone separating highly extended crust in the eastern Blue Ridge from less extended crust in the western Blue Ridge; their model is consistent with our Neoproterozoic reconstruction (fig. 13). However, evidence for a specific Neoproterozoic rift-related fault has not been identified to the west of the Mechum River belt, although overprinting by Paleozoic ductile deformation would likely obscure a Neoproterozoic brittle structure. Clast compositions and cross cutting relationships suggest that the Mechum River Formation and Lynchburg Group were deposited during an early episode (700 – 730 Ma) of Neoproterozoic rifting in southeastern Laurentia. In central Virginia, this event involved significant felsic magmatism (Robertson River granitoids) and contemporaneous sedimentation. This early episode of rifting involved considerable vertical tectonism that facilitated both the upperward movement of felsic magmas as well as basin subsidence for the thick rift sequence. Deposition and magmatism had waned by 680 Ma (Tollo and others, 2004b). Although the upper- and lower-plate rift margin model of Thomas (1993) might be applicable to the successful, second phase of Iapetan rifting in the southern Appalachians, the older pulse of rifting generated a thick sedimentary sequence (much of which is marine) in the central Virginia Blue Ridge. The onset of the second episode of rifting at the end of the Neoproterozoic (⬃570 Ma) is marked by the development of normal faults in the western Blue Ridge deposition of thin terrestrial deposits in the Swift Run Formation, and extrusion of the Catoctin flood basalts and rhyolite (Southworth and Brezinski, 1996; Bailey and others, 2002b). A significant unanswered question remains: what happened along the Laurentian edge of Rodinia during the interval between 680 and 570 Ma? conclusions

The 705 to 730 Ma Mechum River Formation is a sequence of rift-related sedimentary rocks preserved as a folded and faulted structural inlier in the central Virginia Blue Ridge province. In its present geometry the Mechum River belt is not a graben and restoration of the belt to its pre-contractional geometry does not reveal any definitive rift-related structures. The reverse faults that bound the eastern edge of the Mechum River belt are interpreted as out-of-sequence structures developed after regional ductile deformation during the emplacement of the Blue Ridge thrust sheet in the Alleghanian. The Mechum River basin received sediment from a Blue Ridge

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basement source area located to the east, possibly a rotated fault block above a listric rift-related fault. acknowledgments

This research was supported by grants from the College of William and Mary, Denison University, and the U.S Geological Survey. S. Giorgis, C. Lamon, P. Berquist, J. Weiss, and A. Forte assisted with fieldwork. We thank R. Tollo, S. Southworth, B. Henika, and W. Burton for productive discussions. Constructive reviews were provided by D. Rankin, S. Southworth, J. Tull, and R. P. Winstch. references Aleinikoff, J. N., Zartman, R. E., Walter, M., Rankin, D. W., Lyttle, P. T., and Burton, W. C., 1995, U-Pb ages of metarhyolites of the Catoctin and Mount Rogers Formations, central and southern Appalachians: evidence for two pulses of Iapetan rifting: American Journal of Science, v. 295, p. 428 – 454. Aleinikoff, J. N., Burton, W. C., Lyttle, P. T., Nelson, A. E., and Southworth, C. S., 2000, U-Pb geochronology of zircon and monazite from Mesoproterozoic granitic gneisses of the northern Blue Ridge, Virginia and Maryland, USA: Precambrian Research, v. 99, p. 113–146. Badger, R. 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