bailey 2007
DESCRIPTION
Bailey 2007TRANSCRIPT
American Journal of ScienceJANUARY 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 Forma-tion crop out in an elongate northeast-southwest trending belt in the central VirginiaBlue Ridge province. The northwestern contact of the Mechum River Formation isnonconformable above Mesoproterozoic basement, whereas the southeastern contactis a set of steeply dipping, brittle reverse faults that juxtapose basement againstMechum River rocks. The internal structure of the belt consists of northwest-verging,open to tight, moderately to steeply plunging asymmetric folds. Although previousworkers have interpreted the outcrop belt of the Mechum River Formation as a grabenformed during Laurentian rifting, structural relations require that its present geometryis a structural inlier related to Paleozoic contractional deformation. Strain analysisreveals that grain-scale deformation processes produced up to 35 percent shorteningduring foliation development whereas map-scale folds account for an additional 30 to50 percent shortening. Restoration of the Mechum River Formation to its pre-contractional geometry reveals little about the geometry of the original depositionalbasin. The bounding reverse faults on the east side of the Mechum River Formationare interpreted as out-of-sequence structures that developed after regional folding,metamorphism and foliation development that may be related to the emplacement ofthe Blue Ridge thrust sheet over a tectonic ramp. Sediment transport indicators areconsistent with a source area to the east, suggesting that the Mechum River Formationwas separated from similar units in the eastern Blue Ridge by an asymmetric basementhigh that may have been produced by block rotation above listric normal faults.
introduction
Neoproterozoic rocks form a distinctive and important component of the Appala-chian orogen. Early plate tectonic models for the Appalachian orogen recognized thatan ocean basin developed at the end of the Precambrian and that in the LaurentianAppalachians, late Precambrian sedimentary rocks sit with considerable unconformityabove mid-crustal Grenvillian rocks and form the base of the cover sequence at manylocations (Wilson, 1966; Bird and Dewey, 1970; Rodgers, 1972; Rankin, 1975). Thelatest Neoproterozoic to Early Cambrian units provide the key evidence for rifting insoutheastern Laurentia and for the development of the Iapetus Ocean. The Neopro-terozoic was a unique period in earth history characterized by climatic extremes fargreater than those experienced during the Phanerozoic (Kirschvink, 1992; Hoffmanand others, 1998), understanding the significance of Neoproterozoic sedimentarysequences 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
[American Journal of Science, Vol. 307, January, 2007, P. 1–22, DOI 10.2475/01.2007.01]
1
Neoproterozoic units crop out from Georgia to Newfoundland, but it is in thecentral and southern Appalachian Blue Ridge province (fig. 1) that these units areextensive and provide a detailed record of rifting and the transition to a passivemargin. In the southern Appalachians, this sequence includes thick sections of clasticrocks (Ocoee basin) that in some areas are interlayered with felsic and mafic volcanics(Grandfather Mountain and Mount Rogers). In Virginia, Neoproterozoic units arewidespread, but generally thinner than those exposed in the southern Blue Ridge (fig.1). Thomas (1976, Thomas, 1977, 1993) proposed that the northeast-striking riftedmargin of southeastern Laurentia is segmented by a number of northwest-strikingtransform faults; he further speculated that the Virginia-Tennessee transform sepa-rates a highly extended lower-plate rifted margin in the southern Blue Ridge from aless extended upper-plate rifted margin in the northern Blue Ridge (fig. 1).
In central and northern Virginia, the Blue Ridge province includes a largebasement massif of Mesoproterozoic rocks flanked by Neoproterozoic to early Paleo-zoic rocks (fig. 2). Mesoproterozoic rocks include a suite of granitoids formed duringthe 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 to730 Ma granitoid plutons (including the Robertson River Igneous Suite in northernVirginia) intrudes the Mesoproterozoic rocks (Bartholomew and Lewis, 1984; Tolloand Aleinikoff, 1996; Tollo and others, 2004b). Collectively, the Proterozoic granitoidsare unconformably overlain by a sequence of Neoproterozoic to early Cambrianmetasedimentary and metavolcanic rocks that record sedimentation and magmatism
Fig. 1. Regional overview map of the central and southern Appalachians highlighting the Blue Ridgeprovince, Laurentian rift-related features, and the Neoproterozoic Mechum River Formation. Location ofthe Virginia-Tennessee and Georgia transforms are based on Thomas (1991).
2 C. M. Bailey and others—The Mechum River Formation, 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 haveoccurred during two temporally distinct episodes; an early extensional event between680 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 ofIapetus 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 metasedimentaryrocks in the Virginia Blue Ridge exposed as an elongate belt surrounded by olderProterozoic granitoid and gneiss (fig. 2). Based on its elongate outcrop pattern and thenature of its sedimentary sequence many workers interpreted the Mechum RiverFormation to be a graben filled with terrestrial deposits that accumulated duringLaurentian rifting (Nelson, 1962; Schwab, 1974; Harris and others, 1986; Tollo andHutson, 1996). The Mechum River graben interpretation has been further cited inoverview 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 Ridgeanticlinorium is, however, a late Paleozoic contractional structure generated duringthe emplacement of the basement complex and its cover sequence over a footwallthrust ramp during late Paleozoic tectonism (Mitra, 1979; Harris, 1979; Evans, 1989). Ifthe Mechum River Formation was deposited in a graben how was it affected ormodified by later Paleozoic contractional deformation?
Our purpose is to characterize the structural geometry of the Mechum RiverFormation and determine 1) if the Mechum River Formation occupies a Neoprotero-
vv
v
v
vvv
v v
v
v v
v
v
v v
v
v
v
v
vv
vv v
v
vv
v v
v
v v
v v
v
vv
vv
vv v
v
v v
v
v
v v
v
v v
v
v
v v
v
v
vv
vv v
v
v
vv
v
v v
v
v v
v
v
v v
v
v v
v
v
v
v v
v
v v
v
vvvv v
v
v v
v
v
v
v
v v
v
v v
v
v
v v
v
v v
v
v v
v
v
v v
v
v
v v
v
vv
vv
v v
v
v v
v
v
vv v
v
v v
v
v v
v
v
v v
v
v v
v
v
v
v
v v
v
v
v v
v
vv
v
v
v v
v
vv
v v
v
v v
v
v
v v
v
v v
v
v
v
v
v v
v
v
v v
vvvv v
v
v v
v
v
v
v
v v
v
v v
v
v
v v
v
v v
v
v v
v
v
v v
v
v
v v
v
v
v
vv
v v
v
v v
v
v
v v
v
v v
v
v v
v
v
v v
v
v v
v
v v
v
v
v v
v
v
v v
vv
v
v
v
vvv
v v
v
v v
v
v
v v
v
v v
v
v v
v
v
v v
v
v v
v
v
v
v
v v
v
v
v v
vv
v
v
vvv
v v
v
v v
v
v
v v
v
v v
v
v v
v
v
v v
v
v v
v
v
v
v
v v
v
v
v v
v
vv
vv v
v
vv
v v
v
v v
v
v
v v
v
v v
v
v v
v
v
v v
v
v v
v
v v
v
v
v v
v
v v
v
v v
vvvv v
v
v v
v
v
v
v
v v
v
v v
v
v
v v
v
v v
v
v v
v
v
v v
v
v v
v
v v
v
v
v v
v
v v
v
v
v v
v
v
v
vv
v v
v
v v
v
v
v v
v
v v
v
v v
v
v
v v
v
v v
v
v v
v
v
v v
v
v v
v
v v
v
v
v v
v
v v
v
v v
v
vv
vv v
v
vv
v v
v
v v
v
v
v v
v
v v
v
v v
v
v
v v
v
v v
v
v v
v
v
v v
v
v v
v
v v
v v
v
v
vvvv v
v
v v
v
v
v
v
v v
v
v v
v
v
v v
v
v v
v
v v
v
v
v v
v
v v
v
v v
v
v
v v
v
v v
v
v v
v
v
vv
v v
v
v v
v
v
vv v
v
v v
v
v v
v
v
v v
v
v v
v
v v
v
v
v v
v
v v
v v
v v
v
v
vv
v
v
vvv
v v
v
v v
v
v
v v
v
v v
v
v v
v
v
v v
v
v v
v vv
v
v v
v
vvvv v
v
v v
v
v
v
v
v v
v
v v
v
v
v v
v
v v
v v
v v
v
vv
v
v
vvvv
v v
v
v v
v
v
v v
v
v v
v v
v v
v
vv
vvv v
v
v v
v
v
v v
v
v v
v v
v v
v
v v
v
vv v
v
v
vv v
v
v v
v v
v v
v
vv
vv
vv
vv vvv
v
vv
v v
vv
vvv
vvv v
v
v v
v
v
v v
v
v v
v
v v
v
v
v
v vv
v
v
vv
vv
v v
v
v v
v
v v
v v
v
v
v
v
vvvvv
v v
v
v v
v
v
vvv v
v
v v
v
v v
v
v
v v
v
v v
v
v
v v
v
v
vvv
v
v
vvv
v v
v
v v
vv
v
vv vv
v
vv v
vv
v
vv vv
v
v v
v
v
vv
v
v v
v
v
vv vv
v
v v
v vvv v
v
v v
v
v
v
v
v v
v
v v
v v
v
v
v
v
v v
v
v v
v
v
v v
v
v
v v
v
v
v v
v
v
v
v
v v
v v
v
v
v v
v
v vv
v
v
v
v
vv
v v
v v
v v
v
vv v
v
v v
v
v
v v
v
v
v
v
v
v
v v
v
v v
v
v v
v
v
v v
v
v v
v
v v
v
v
v
v vv
v
v
v
v
v
v
v v
v
v
v
vv
v v
v v
v vv
vvvv v v
v
v
vv v
v
v
v
vv
v v
v
v v
v
v v
v
v
vv
v
v v
v
v vv
vv vv
v
v vv
v
v
v
vv
v v
v
v
v v
v v
v
v v
v
v
vv v
v
v v
v vv
v
v
v
v
v
vv
v v
v
vv v
v
v
v v
v
vv vv
v
v
v
v
v v
v
vv
v v
v
v
v
vv
v
v
v
v v
v
v v
v v
v
v v
v
v v
v
v
v v
v
v v
v
v v
v
v
v v
v
v v
v
v
v v
v v
vv v
v v
v
v
v
v
v v
v
v v
v
v
vv v
v
v v
v
v
v
v v
v
v
vv vv
v
vv
v
vv
vv
vv v
v
v v
v
v
vv
v
v v
v
v v
vv
vv
v
vvv v
v vv
v vv
vv vv
vvv
vv
vv
vv
vv
vv
vv
vv
vvvv
vvvv
vv
vv
vv vv
vvv
v vvvv
vv
vv
v
vv
v
vv
vv
v
vv
vv
vv
v
vv
vv
vv vv
vv
v
v
v
vv v
v
v
v v
v v
v
v
v
v
v
v
v
v
v v
v
v v
v
v
v v
v
v v
v
v
vv
v
vv
v
vv
vv vv vv
Fig. 2. Generalized geological map of the central Virginia Blue Ridge.
3A record of Neoproterozoic and Paleozoic tectonics in southeastern Laurentia
zoic graben, 2) if original Neoproterozoic structures are discernible and overprintedby later contractional structures, and 3) quantify the amount of deformation at boththe map-scale and at the meso- to micro-scale. We also seek to establish a betterregional framework for Neoproterozoic metasedimentary units in the Blue Ridge inorder to place constraints on tectonic models for Iapetan rifting and the Neoprotero-zoic paleogeography of southeastern Laurentia.
mechum river formation
Previous WorkMetasedimentary 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 asequence of metaconglomerate, metasandstone, and metasiltstone infolded into thebasement complex and, in places, bounded by high-angle reverse faults. At thesouthern end of the Mechum River Formation, Nelson (1962) depicted a graben withsteep 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 weresheared and reactivated as reverse faults during the Paleozoic. Harris and others(1986) interpreted the Mechum River Formation on the Interstate 64 seismic reflec-tion profile across central Virginia as a graben complex with subordinate horsts. Tolloand Hutson (1996) reported metarhyolite interlayered with Mechum River metasedi-mentary rocks near the northern termination and correlated these metarhyolites withthe 705 Ma Battle Mountain felsite (the youngest unit in the Robertson River IgneousSuite) (fig. 2).
Stratigraphy and Depositional SettingThe Mechum River Formation is a �500-m-thick sequence of low-grade metamor-
phosed clastic rocks that range from mudstones to boulder conglomerates and situnconformably above Mesoproterozoic granitoid and gneiss. The top of the MechumRiver Formation is not exposed. Gooch (ms, 1954; 1958) originally interpreted theMechum River Formation to be a deep-water deposit. In contrast, Schwab (1974)proposed that the Mechum River Formation is an alluvial sequence and noted thatcross bed azimuths (measured throughout the belt) define a centripetal patternconsistent with deposition from the west and east side of an elongate basin. Conley(1989) suggested that the Mechum River Formation includes marine deposits in thesouth that pass into non-marine deposits in the northern part of the belt. Bailey andPeters (1998) recognized glaciogenic marine deposits at the base of the Mechum RiverFormation in the south and noted that these rocks pass upwards into marine turbiditesthat 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 southwesternend of the belt and the unit generally becomes younger to the northeast (fig. 3; Baileyand Peters, 1998). At the base of the Mechum River Formation different rock typesnonconformably overlie the basement complex; near the southwestern end of the belt,boulder conglomerate occurs at the base, whereas 10 km to the northeast, phylliticsiltstone occurs at the base. The boulder conglomerate is stratigraphically below thephyllitic siltstone and we interpret the unconformity to be time transgressive. Werecognize seven subunits within the Mechum River Formation based on stratigraphicposition and rocktype (fig. 3).
The basal boulder conglomerate and diamictite pass upward into rhythmicallybedded 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
4 C. M. Bailey and others—The Mechum River Formation, Virginia Blue Ridge:
overlain by arkose and laminated arkosic wacke (Zm2) with outsized clasts of granitoidgneiss; these are overlain by a thick sequence of thinly laminated siltstone andmudstone (Zm3). Further to the northeast, fine-grained rocks pass upward into adistinct sequence of arkose and arkosic wacke (Zm4). Both graded and ungraded bedsare present (up to 2 m thick) and the average bed thickness decreases across the beltfrom east to west. Bed bottoms are characteristically scoured, graded beds fine upwardsinto parallel laminations, and cross stratification is rare or absent (fig. 4B). Thissubunit is interpreted to have been deposited by subaqueous gravity flows. A monoto-nous 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 andarkosic wacke (Zm6) are underlain by coarse- to medium-grained arkose and conglom-erate (Zm5 and Zm7). These subunits are matrix-poor, relative to the sandy units inthe 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 andZm7. The northern subunits in the Mechum River Formation are interpreted toinclude alluvial fan, fluvial braid plain, and deltaic deposits that may have formed in aglacial outwash environment.
The Mechum River Formation is primarily an arkosic sequence with a clastassemblage that includes abundant quartz and alkali feldspar (predominately per-thite) with lesser amounts (�5 to 10%) of plagioclase and ilmenite and minor zircon,magnetite, apatite, and rutile. Schwab (1974) reported that Mechum River frameworkclasts are indicative of intra-cratonic setting. The provenance for the Mechum RiverFormation is alkali feldspar rich granitoids of the Blue Ridge basement complex. Themetamorphic minerals in the Mechum River Formation are dominated by muscovite,biotite, and epidote.
AgeClasts within Mechum River Formation metaconglomerates are primarily Mesopro-
terozoic granitoids, but in the north, clasts of the �730 Ma Laurel Mills granite of the
Fig. 3. Schematic stratigraphic cross section through the Mechum River Formation from southwest tonortheast (parallel to the long axis of the formation) with subunit descriptions and depositional interpreta-tions.
5A record of Neoproterozoic and Paleozoic tectonics in southeastern Laurentia
Robertson River Igneous Suite are common and yield a maximum depositional age(Hutson, 1992; Morton and Bailey, 2004). Dikes and sills of foliated metabasalt withcompositions similar to the �570 Ma Catoctin Formation intrude the Mechum RiverFormation and loosely bracket a minimum depositional age (Gooch, 1958; Bailey andothers, 2003). Tollo and Hutson (1996) described metarhyolites with compositionsidentical to �705 Ma felsites of the Robertson River Igneous suite interlayered withMechum River rocks near Castleton; they interpreted that deposition of the MechumRiver Formation was ongoing at �705 Ma. However, metarhyolites at Castleton cropout to the east of the Mechum River belt and are in tectonic contact with the MechumRiver Formation (Morton and Bailey, 2004). In the Castleton area, the Mechum Riverbelt contains no volcanic detritus. The extrusion and deposition of the �705 MaCastleton metarhyolites and their associated sedimentary rocks probably postdate theMechum River Formation. Based on the available data the Mechum River Formation
Fig. 4. (A) Boulder conglomerate/diamictite (Zm1) near Batesville (37.97° N, 78.74° W), (B) gradedpackages with scoured bed bottoms in arkosic wacke (Zm4) (38.12° N, 78.58° W), (C) coarse-grained arkosewith 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).
6 C. M. Bailey and others—The Mechum River Formation, Virginia Blue Ridge:
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 geometryThe 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 fewkey areas within the Mechum River belt. In addition to our mapping we have utilizeddata presented on maps by Mitra (ms, 1977), Lukert and Halladay (1980), Hutson(1992), Tollo and Lowe (1994). Cross sections were constructed, in part from down-plunge projection of structural data collected at the surface in Mechum River metasedi-mentary rocks as well as from available gravity and aeromagnetic data.
Map-scale RelationsThe southern end of the Mechum River Formation is �20 km southwest of
Charlottesville (figs. 1 and 5). The southwestern contact between the Mechum RiverFormation and Mesoproterozoic granitoid gneiss is an irregular, hummocky surfacewith 5 to 25 meters of relief and is overlain by massive conglomerate and diamictitewith discontinuous lenses of arkosic sandstone (fig. 4A). This contact is interpreted asan unconformity. These coarse-grained deposits grade upward into arkosic wacke andlaminated siltstone. The structural geometry is an open to tight asymmetric fold that
Fig. 5. Geologic map and cross section of the Mechum River Formation at its southwestern terminationnear Batesville. Covesville, Virginia, 7.5-minute quadrangle.
7A record of Neoproterozoic and Paleozoic tectonics in southeastern Laurentia
plunges to the northeast. Locally, the eastern limb is overturned. At the northwesterncontact, Mechum River rocks are upright and nonconformably overlie Mesoprotero-zoic biotite-bearing granitoid (fig. 5). The southeastern contact dips steeply to thesoutheast and Mesoproterozoic granitoids structurally overlie Mechum River rocks(fig. 5).
Northwest of Charlottesville, arkose and arkosic wacke (Zm4) are exposed in aseries of northwest-verging overturned folds (fig. 6). Map-scale folds plunge at moder-ate angles to the northeast with hinge lines that trend more easterly than thesoutheastern contact (fig. 6). A penetrative foliation defined by aligned phyllosilicatesand elongated detrital minerals strikes to the northeast, dips uniformly to the south-east, 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, thesoutheastern contact is a reverse fault that cuts overturned beds on the southeast limbof 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 penetra-tive foliation in the Mechum River Formation.
15 km north of Charlottesville, the Mechum River Formation crops out in a beltover 1 km wide (figs. 2 and 7). At the northwestern contact, coarse-grained arkose sitsunconformably above granitoid gneiss. Bedding is upright and dips uniformly to thesoutheast; the penetrative foliation dips more steeply to the southeast. The southeast-ern contact is a steeply dipping reverse fault that places Mesoproterozoic rocks onupright Mechum River metasedimentary rocks. In this area the Mechum River beltforms a homoclinal sequence of southeast dipping beds. The bounding reverse faultcuts through a map-scale fold hinge.
In the Madison area, the Mechum River Formation crops out in two northeast-southwest trending belts, 0.25 to 1.2 km wide, separated by two en-echelon reversefaults (figs. 1 and 8). Gooch (1958) first recognized this structure and interpreted it tobe an anticline that exposes basement in its core. Our mapping does not support theanticline interpretation; rather the two bounding reverse faults duplicate the MechumRiver Formation. Within the Mechum River belt, bedding is folded into steeply dippingpanels with different strikes (fig. 9). Folds are open to tight and commonly havesoutheast 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 obviouslylinked with simple northwest-southeast contraction; these folds are consistent with acomponent of transpressional deformation. Bailey and others (2002a) reported evi-dence for triclinic deformation in nearby Blue Ridge mylonite zones and these foldsappear to be kinematically compatible with triclinic structures (for example, NW/SEdirected shortening with strike-parallel displacement). The penetrative foliation in theMadison 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 �5km (fig. 9). At the northwestern contact cobble to boulder conglomerate and coarse-grained arkose overlie Mesoproterozoic biotite-bearing granitoid gneiss. At Bessie BellMountain (fig. 9), the northwestern contact is not planar and appears to be anerosional surface with as much as 50 m of relief (Morton and Bailey, 2004). At thislocation the northwestern contact is also cut by a subvertical, north-northwest strikingfault; 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 totight, asymmetric northwest-verging anticlines and synclines (fig. 9). At a number oflocations, bedding in the Mechum River Formation is truncated at the southeastern
8 C. M. Bailey and others—The Mechum River Formation, Virginia Blue Ridge:
Fig. 6. Geologic map, cross sections, and stereograms of the Mechum River Formation to the northwestof Charlottesville. Charlottesville West, Virginia, 7.5-minute quadrangle. Data portrayed on stereogramscollected from the map area and adjacent regions.
9A record of Neoproterozoic and Paleozoic tectonics in southeastern Laurentia
contact. To the north, the southeastern contact steps to the west and the cross-strikewidth of the Mechum River belt dramatically decreases (fig. 9). South of Castleton thebasal conglomerate in the Mechum River Formation nonconformably overlies the�730 Ma Laurel Mills granite of the Robertson River suite; the conglomerate containsabundant clasts of the Laurel Mills granite.
Felsic metavolcanic rocks, metavolcanic conglomerates, and phyllitic rocks areexposed in a 0.2 to 0.5 km wide belt to the southeast of Castleton. These volcanic rocksare 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 ofLaurel Mills granite. Foliation and bedding orientations in the Castleton metavolcanicsrange from steeply dipping to subvertical and are structurally distinct from attitudes inthe Mechum River belt to the west (fig. 9). We have not directly observed the boundingcontacts of the Castleton rocks, but the map pattern indicates that they are steeplydipping. The Castleton rocks could have been extruded/deposited in a basin relatedto foundering of an older Robertson River magma chamber. Later contractional
Fig. 7. Geologic map, cross section, and stereograms of the Mechum River Formation to the north ofCharlottesville. Earlysville, Virginia, 7.5-minute quadrangle.
10 C. M. Bailey and others—The Mechum River Formation, Virginia Blue Ridge:
deformation has folded these units into a series of subvertical upright isoclinal folds(fig. 9). Alternatively, the bounding structures at Castleton may be steeply-dippingimbricate reverse faults of Paleozoic age that are similar to the bounding faults on thesoutheast 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 MechumRiver rocks, however, new mapping and structural attitudes do not demonstrate this.
Contact RelationsAlthough geologic contacts are rarely well exposed in the Virginia Blue Ridge, we
observed the Mechum River Formation contact at approximately 15 locations, and
Fig. 8. Geologic map, cross sections, and stereograms of the Mechum River Formation to the west ofMadison. Madison, Virginia, 7.5-minute quadrangle.
11A record of Neoproterozoic and Paleozoic tectonics in southeastern Laurentia
delineated the contact to within a few meters at many sites. Both the southeasterncontact and northwestern contacts of the Mechum River belt dip moderately to steeply(�55° to subvertical) to the southeast. At the southeastern contact, Mesoproterozoicgranitoid gneiss and in the north, Robertson River granitoids, structurally lie aboveMechum River metasedimentary rocks. At many locations along the southeasterncontact Mechum River rocks are upright. The southeastern contact is a family ofhigh-angle reverse faults that places older Proterozoic granitoids on NeoproterozoicMechum River rocks. At a number of locations the bounding fault tips out and isreplaced by another en-echelon fault (figs. 8 and 9). The bounding Mechum Riverfaults are not mylonitic, but are discrete brittle structures. At some locations the 10 to
v v v
vv
v v
v
v v
v
v v
v
v
v
vv
v v
v
v
v v
v v
v
v v
v
v
v v
v
v v
v
v v
v
v
v v
v
v
v v
vv v
v
v v
v
v
v
v
v v
v
v v
v
v
v v
v
v v
v
v v
v
v
v v
v
v v
v
v
v
v
v v
v
v
v
v
v
v
vv
v v
v
v v
v
v
v v
v
v v
v
v v
v
v
v v
v
v v
v
v v
v
v
v v
v
v v
v
v v
v v
v
v
v
v v
v
v
v
v
v v
v
v v
v
v
v v
v
v v
v
v v
v
v
v v
v
v v
v
v v
v
v
v v
v
v v
v
v v
v
vv v
v
v v
v
v
v v
v
v v
v
v v
v
v
v v
v
v v
v
v v
v
v
v v
v
v v
v
v
v
v
v v
v
v
v
v
v
v
vv
v v
v
v v
v
v
v v
v
v v
v
v v
v
v
v v
v
v v
v
v v
v
v
v v
v
v v
v
v v
vv v
v
v v
v
v
v
v
v v
v
v v
v
v
v v
v
v v
v
v v
v
v
v v
v
v v
v
v v
v v
v
v
v
v
v
v v
v
v v
v
v
v v
v
v v
v
v v
v
v
v v
v
v v
v
v v
v
v
v v
v
v v
v
v v
v
v
v
v
v v
v
v v
v
v
v v
v
v v
v
v v
v
v
v v
v
v
v
v
v
v
v
vv
v v
v
v v
v
v
v v
v
v v
v
v v
v
v
v v
v
v
v
v
v
vv v
v
v v
v
v
v v
v
v v
v
v v
v
v
v v
v
v v
v
v
v
v
v
v
v
v
v
v v
v
v v
v
v
v v
v
v v
v
v v
vv
vv v
v
v v
v
v
v v
v
v v
v v
v v
v
v
v
v v
v
vv
vv v
v v
v v
vv
vv v
v
v
v
v
v
v
vv
v v
v v
v v
v vv v vv
v v
v vv
v
v
v
vv v
v v
v v vv
Fig. 9. Geologic map, cross sections, and stereograms of the Mechum River Formation near Castleton.Castleton and Woodville, Virginia, 7.5-minute quadrangles.
12 C. M. Bailey and others—The Mechum River Formation, Virginia Blue Ridge:
30 m wide fault zone includes abundant vein quartz that displays no evidence of crystalplastic deformation. Faulting along the southeastern margin of the Mechum River belttruncates the folds and penetrative greenschist facies fabrics (figs. 5 – 9). We estimatethat throw across these faults is no more than a few kilometers based on theobservation that basement rocks in the hanging wall are at the same grade as theMechum River metasedimentary rocks.
At the northwestern contact Mechum River metasedimentary rocks structurallyoverlie Mesoproterozoic basement units. Everywhere along this contact the MechumRiver 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 atthis contact and typically grade upward into finer grained rocks. We interpret thiscontact to be a tilted unconformity and not a tectonic contact. However, at manylocations rocks on both sides of the contact are penetratively deformed and at somelocations the basement rocks are mylonitic.
Penetrative Fabrics and Strain AnalysisMechum River metasedimentary rocks commonly display a penetrative foliation
defined by aligned biotite, muscovite, and elongate detrital grains of quartz andfeldspar. This foliation invariably strikes northeast-southwest, dips moderately tosteeply to the southeast, and is axial planar with respect to folds in the Mechum RiverFormation (figs. 5 – 9). Foliation is best developed in fine-grained rocks, and maycompletely obliterate primary structures. In graded arkosic wacke layers, the foliationis commonly refracted across fining upward sedimentary packages producing theappearance of cross bedding (fig. 10A). Cuspate folds are developed at the lowercontact of coarser graded packages due to the competence contrast with the underly-ing finer grained rocks (fig. 10B).
Fig. 10. (A) Graded beds and refracted foliation in Zm4 in the southern part of the belt. (B) Refractedfoliation and cuspate fold at the base of coarse-grained arkose from Zm4.
13A record of Neoproterozoic and Paleozoic tectonics in southeastern Laurentia
Metamorphic minerals in Mechum River metasedimentary rocks include abun-dant biotite, muscovite, epidote, granular sphene (after ilmenite) and chlorite. Phyl-litic rocks commonly include biotite porphyroblasts and at some locations garnet andrare graphite laths. Porphyroblast microstructures are consistent with pre- to syntec-tonic growth relative to the foliation development. Microstructures in detrital quartzgrains include undulose extinction, core and mantle textures, and indented grainboundaries; recrystallization in sand-sized grains ranges from extensive to minor (fig.11A). Alkali feldspar clasts display abundant fractures with both fracture surfaces andgrain 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 inquartz and predominantly brittle deformation and dissolution of feldspar that isconsistent with deformation during greenschist facies conditions. The Mechum RiverFormation experienced regional metamorphism at the greenschist facies that wascontemporaneous 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. Grainshapes were digitized from thin sections, individual grain shape ratios and long axisorientations measured (30 to 80 grains per section), and then plotted on a hyperbolicstereogram (De Paor, 1988) to determine sectional strain ratios. For each sample, twoor three orthogonal planes were measured and the sectional data used to calculatethree-dimensional strains. Principal strains are subparallel to fabric elements and the
Fig. 11. Photomicrographs of (A) deformed detrital quartz and feldspar grains. Quartz includesmonocrystalline (mq) grains with undulose extinction and polycrystalline grains (rq), feldspar (f) grains aremore angular, commonly fractured and mantled by a selvage of muscovite. Metamorphic minerals includemuscovite, biotite, and epidote. (B) weakly deformed arkosic wacke. (C) strongly deformed arkosic wackewith steeply inclined foliation. Metamorphic minerals include muscovite, biotite, and epidote. Photos ofMechum River Formation arkosic wackes exposed northwest of Charlottesville (fig. 6). (D) XZ sectionalstrain ratio of 2.5:1 for arkosic wacke illustrated in C, that corresponds with �35% shortening normal tofoliation.
14 C. M. Bailey and others—The Mechum River Formation, Virginia Blue Ridge:
foliation plane approximates the XY plane. Three-dimensional strain geometriesinclude plane strain to oblate ellipsoids (Shotwell and Bailey, 2000). Whole-rock strainratios in XZ sections range from 1.1 to 3.4 (fig. 11 B-D) with quartz clasts generallyrecording higher strain ratios (10 – 30%) than feldspar clasts. The higher strain ratiosrecorded by quartz clasts is likely a function of the crystal plastic strain experienced byquartz relative to the brittle deformation and dissolution experienced by feldspar.These strain estimates do not account for strain in the matrix or grain boundary slidingalong phyllosilicates and as such are minimum values. There was no tangible differ-ence in grain-scale strain based on position within the meso-scale structures (that is- nogreater 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 35percent shortening.
Restoration of the Mechum River BeltThe Mechum River belt was restored to it pre-contractional deformation geometry
by anchoring a pin line at the unconformable northwestern contact, retrodeformingthe penetrative strain, and then unfolding the map-scale structures to create horizontallayering (fig. 12A). Retrodeformation of the penetrative strain was accomplished byusing average whole-rock strains (Rs- 1.3 to 2.4) and applying a pure shear graphicaltransformation parallel to foliation and axial planes such that rocks were elongatednormal to foliation. Grain-scale strain may have accumulated by general or simpleshear, and our retrodeformation method is intended only to place broad limits on theamount of shortening. Map-scale folding accounts for up to 50 percent shorteningacross the belt. When the layering is unfolded to a subhorizontal orientation thereverse faults at the southeastern contact become northwest dipping faults and theirorigin as post folding structures becomes evident (fig. 12A). In its present geometrythe Mechum River belt ranges from 0.5 to 5 km wide, after retrodeformation andunfolding the belt restores to a width of 1.5 to 8 km. However, the southeastern contactis a reverse fault that cuts through existing folds, thus even the restored Mechum Riverbelt preserves an unknown, but probably a small part of the complete depositionalbasin (fig. 12).
discussion
Neoproterozoic Geometry of the Mechum River BasinThe Mechum River Formation is a structural inlier within the Blue Ridge
anticlinorium and is not, in its present configuration a graben. Restoration of theMechum River Formation to its pre-Paleozoic contractional orientation does notdefine a graben or any definite rift-related structures (fig. 12); the restored width of theMechum River belt still forms a relatively narrow belt (5 – 8 km). The eastern margin ofthe Mechum River belt is a reverse fault and clearly does not represent the limits of thedepositional basin. Although Neoproterozoic to early Paleozoic rift-related faults occurin 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 RiverFormation. If the Mechum River Formation were bounded by original normal faultsthere would likely be a discordance or truncation between the fault contact and riftdeposits along both margins of the belt. Conglomerate is common at the nonconform-able contact along the western margin of the Mechum River Formation, but is rare onthe eastern side of the belt; if the belt were a graben conglomerate would be likely tocrop out along both margins. Mitra and Lukert (1982) proposed that originalbasin-bounding faults were rotated and sheared into reverse faults during Paleozoicdeformation. The concordance between the sedimentary layering and the western
15A record of Neoproterozoic and Paleozoic tectonics in southeastern Laurentia
contact of the Mechum River Formation is consistent with an unconformable contactrather than a normal fault reactivated as a thrust. Furthermore, the overall finite strainrecorded in these rocks is low (�3:1) and likely insufficient to significantly rotateoriginal structures.
Schwab’s (1974) cross bed azimuth data from throughout the Mechum River beltsuggests a centripetal pattern consistent with deposition from either side of anelongate basin. We have observed cross bedding in Mechum River arkoses located onlyin the fluvio-deltaic deposits exposed in the northern part of the belt (subunits Zm5, 6,
Fig. 12. (A) Deformed and restored state cross section of the Mechum River Formation with sectionalstrain data projected onto cross section, unstrained section retrodeformed back by applying the averagestrain 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 indicatorlocalities.
16 C. M. Bailey and others—The Mechum River Formation, Virginia Blue Ridge:
and 7, fig. 12B). Restoration of cross bedding to its original horizontal position (byrotation 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). Weobserve no cross beds in the southern part of the Mechum River Formation, herearkosic wackes form graded packages that lack cross stratification. Refracted foliationhere superficially resembles cross bedding (fig. 10). Average bed thicknesses in thegravity flow deposits (Zm2 and Zm4) as measured at the outcrop-scale decrease fromeast to west across the Mechum River belt, an observation consistent with sedimenttransport from the east. Clasts in the Mechum River Formation are derived primarilyfrom Mesoproterozoic leucocratic granitoids and the Neoproterozoic Robertson Rivergranitoids that crop out primarily to the east of the Mechum River belt. Charnockite acommon rocktype from the western Blue Ridge is almost entirely absent from theMechum River clast assemblage. Collectively, these data indicate a source area for theMechum River Formation that was to the east/northeast of the present belt.
Deformation Timing and SequenceThe age of regional metamorphism and deformation in the Blue Ridge anticlino-
rium is broadly constrained to the Paleozoic, but a detailed chronology of thedeformation history awaits further geochronologic research. Previous workers haveargued that ductile deformation occurred during the Ordovician and was related toTaconic tectonism, (Wehr and Glover, 1985; Glover and others, 1989; Rankin andothers, 1989; Evans, 1991; Bailey and Simpson, 1993) whereas others have ascribedductile deformation to late Paleozoic tectonism associated with the Alleghanianorogeny (Mitra and Elliot, 1980; Evans, 1989; Burton and others, 1992). Recent Ar-Argeochronology in the central Virginia Blue Ridge yield muscovite cooling agesbetween 310 and 355 Ma for basement and cover rocks that experienced greenschistfacies metamorphism and deformation (300 to 450° C) (Polvi, ms, 2003; Wooton andothers, 2005; Bailey and others, 2007). These new data are compatible with earlyAlleghanian to Acadian ductile deformation in the Virginia Blue Ridge.
We propose that the central Virginia Blue Ridge was deformed at the greenschistfacies between 310 and 360 Ma; this contractional deformation event producedhigh-strain zones in the basement, penetrative fabrics in both the basement and coversequence, and folds in the cover sequence (fig. 13B). During the late Alleghanian (280to 300 Ma) the Blue Ridge thrust sheet was emplaced over a ramp(s) in the underlyingCambro-Ordovician sequence (Harris, 1979; Evans, 1989). The exact location of thisramp(s) and the extent of Cambro-Ordovician rocks beneath the Blue Ridge basementis 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 ahorizontal axis to accommodate this translation (fig. 13). The geometric and mechani-cal difficulties of translating a cooling basement-cored thrust sheet up a tectonic rampand onto a flat produced brittle out-of-sequence faults in the basement (fig. 13C). Thefamily of steeply dipping reverse faults that place Proterozoic granitoids on theNeoproterozoic Mechum River Formation are out-of-sequence brittle structures thatdeveloped 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 Riverbelt.
Regional Tectonic Considerations and SpeculationThe Mechum River Formation is lithologically similar to the Lynchburg Group
exposed in the eastern Blue Ridge (fig. 2). Wehr (1986) describes a transition in theLynchburg Group from fluvio-deltaic deposits in the north (originally the Fauquier
17A record of Neoproterozoic and Paleozoic tectonics in southeastern Laurentia
Fig. 13. Deformation model for the Mechum River Formation, (A) Neoproterozoic deposition inasymmetric 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 earlyAlleghanian to Acadian. (C) Rotation of the basement and cover sequence due to the emplacement of theBlue Ridge thrust sheet over a tectonic ramp. Steepening of pre-existing structures and out-of-sequencebrittle faulting near the eastern contact. (D) Present day surface level.
18 C. M. Bailey and others—The Mechum River Formation, Virginia Blue Ridge:
Formation and later renamed the Bunker Hill and Monumental Mills Formations) tosubaqueous gravity flow deposits in the south (Thorofare Mountain and Ball MountainFormations) that is similar to changes in Mechum River lithofacies (alluvial deposits inthe north and marine deposits in the south). As discussed above sediment transportindicators suggest that the Mechum River Formation was derived from a Proterozoicgranitoid 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 sourcearea to the west. The Mechum River Formation may be a western outlier of theLynchburg 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 tothe west (present day coordinates) of the main Lynchburg depocenter. The basementhigh may have been produced by rotation of at least two listric normal fault blocks ofNeoproterozoic age; one along the western edge of the Lynchburg Group and anotherto the west (present day coordinates) of the Mechum River belt. The Lynchburg Groupis an �5 km thick clastic sequence that required significant tectonic subsidence- likelyaccommodated by normal faulting. Chapman and others (2003) presented evidencefor 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 requiredto produce the necessary tilt to provide an eastern source for the Mechum Riverdeposits. Wehr and Glover (1985) and Evans (1991) proposed a hinge zone separatinghighly extended crust in the eastern Blue Ridge from less extended crust in the westernBlue Ridge; their model is consistent with our Neoproterozoic reconstruction (fig. 13).However, evidence for a specific Neoproterozoic rift-related fault has not beenidentified to the west of the Mechum River belt, although overprinting by Paleozoicductile deformation would likely obscure a Neoproterozoic brittle structure.
Clast compositions and cross cutting relationships suggest that the Mechum RiverFormation and Lynchburg Group were deposited during an early episode (700 – 730Ma) of Neoproterozoic rifting in southeastern Laurentia. In central Virginia, this eventinvolved significant felsic magmatism (Robertson River granitoids) and contemporane-ous sedimentation. This early episode of rifting involved considerable vertical tecto-nism that facilitated both the upperward movement of felsic magmas as well as basinsubsidence for the thick rift sequence. Deposition and magmatism had waned by 680Ma (Tollo and others, 2004b). Although the upper- and lower-plate rift margin modelof Thomas (1993) might be applicable to the successful, second phase of Iapetanrifting in the southern Appalachians, the older pulse of rifting generated a thicksedimentary 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 ofthin terrestrial deposits in the Swift Run Formation, and extrusion of the Catoctinflood basalts and rhyolite (Southworth and Brezinski, 1996; Bailey and others, 2002b).A significant unanswered question remains: what happened along the Laurentian edgeof Rodinia during the interval between 680 and 570 Ma?
conclusionsThe 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 centralVirginia Blue Ridge province. In its present geometry the Mechum River belt is not agraben and restoration of the belt to its pre-contractional geometry does not reveal anydefinitive rift-related structures. The reverse faults that bound the eastern edge of theMechum River belt are interpreted as out-of-sequence structures developed afterregional ductile deformation during the emplacement of the Blue Ridge thrust sheetin the Alleghanian. The Mechum River basin received sediment from a Blue Ridge
19A record of Neoproterozoic and Paleozoic tectonics in southeastern Laurentia
basement source area located to the east, possibly a rotated fault block above a listricrift-related fault.
acknowledgmentsThis 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 providedby 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 ofmetarhyolites 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 geochronologyof zircon and monazite from Mesoproterozoic granitic gneisses of the northern Blue Ridge, Virginiaand Maryland, USA: Precambrian Research, v. 99, p. 113–146.
Badger, R. L., and Sinha, A. K., 1988, Age and Sr isotopic signature of the Catoctin volcanic province:Implications for subcrustal mantle evolution: Geology, v. 16, p. 692–695.
Bailey, C. M., ms, 1994, Temporal, kinematic, and strain analysis of granitic tectonites from the centralAppalachians: Baltimore, Maryland, Johns Hopkins University, Ph. D. thesis, 254 p.
Bailey, C. M., and Peters, S. E., 1998, Glaciogenic sedimentation in the late Neoproterozoic Mechum RiverFormation, Virginia: Geology, v. 26, p. 623–626.
Bailey, C. M., and Simpson, C., 1993, Extensional and contractional deformation in the Blue Ridge Province,Virginia: Geological Society of America Bulletin, v. 105, p. 411–422.
Bailey, C. M., Bobyarchick, A. R., and Jiang, D., 2002a, Kinematics and Vorticity of high-strain zones: VirginiaBlue Ridge and Piedmont: Geological Society of America Field Forum Guidebook, 26 p.
Bailey, C. M., Giorgis, S., and Coiner, L. V., 2002b, Tectonic inversion and basement buttressing; an examplefrom the Central Appalachian Blue Ridge Province: Journal of Structural Geology, v. 24, p. 925–936.
Bailey, C. M., Berquist, P. J., Mager, S. M., Knight, B. D., Shotwell, N. L., and Gilmer, A. K., 2003, BedrockGeology of the Madison 7.5’ quadrangle, Virginia: Virginia Division of Mineral Resources Publication157, 22 p.
Bailey, C. M., Kunk, M. J., Southworth, S., and Wooton, K. M., 2007, Late Paleozoic orogenesis in the VirginiaBlue Ridge and Piedmont: a view from the south: Geological Society of America Abstracts withPrograms.
Bartholomew, M. J., and Lewis S. E., 1984, Evolution of the Grenville massifs in the Blue Ridge geologicprovince, southern and central Appalachians, in Bartholomew, M. J., editor, The Grenville event in theAppalachians and other related topics: Geological Society of America Special Paper 194, p. 229–254.
Bird, J. M., and Dewey, J. F., 1970, Lithosphere plate-continental margin tectonics and the evolution of theAppalachian orogen: Geological Society of America Bulletin, v. 81, p. 1031–1061.
Burton, W. C., Kunk, M. J., and Lyttle, P. T., 1992, Age constraints on the timing of regional cleavageformation in the Blue Ridge anticlinorium, northernmost Virginia: Geological Society of AmericaAbstracts with Programs, v. 24, n. 2, p. 5.
Chapman, J. B., Bailey, C. M., and Griffith, A., 2003, Structural geometry of the eastern Blue Ridge province,central and northern Virginia: Geological Society of America Abstracts with Programs, v. 35, n. 1, p. 7.
Conley, J. F., 1989, Geology of the Blue Ridge anticlinorium: 28th International Geological Congress, FieldTrip Guidebook T 356, 24 p.
De Paor, D. G., 1988, Rf/� strain analysis using an orientation net: Journal of Structural Geology, v. 10,p. 323–333.
Evans, M. A., 1989, Structural geometry and evolution of foreland thrust systems, northern Virginia:Geological Society of America Bulletin, v. 101, p. 339–354.
Evans, N. H., 1991, Latest Precambrian to Ordovician metamorphism in the Virginia Blue Ridge: origin ofthe contrasting Lovingston and Pedlar basement terranes: American Journal of Science, v. 291,p. 425–452.
Fichter, L. S., 1993, The geologic evolution of Virginia: National Association of Geology Teachers ShortCourse, 17 p.
Fichter, L. S., and Diecchio, R. J., 1986, Stratigraphic model for timing the opening of the proto-AtlanticOcean in northern Virginia: Geology, v. 14, p. 307–309.
Glover, L., III, Evans, N. H., Patterson, J. G., and Brown, W. R., 1989, Tectonics of the Virginia Blue Ridgeand Piedmont: 28th International Geological Congress, Field Trip Guidebook T363, p. 59.
Gooch, E. O., ms, 1954, Infolded metasedimentary rocks near the axial zone of the Catoctin mountain-BlueRidge anticlinorium in Virginia: Chapel Hill, University of North Carolina, Ph. D. thesis, 29 p.
–––––– 1958, Infolded metasedimentary rocks near the axial zone of the Catoctin Mountain - Blue Ridgeanticlinorium in Virginia: Geological Society of America Bulletin, v. 69, p. 569–574.
Harris, L. D., 1979, Similarities between the thick-skinned Blue Ridge anticlinorium and the thin-skinnedPowell Valley anticline: Geological Society of America Bulletin, v. 90, p. 525–539.
20 C. M. Bailey and others—The Mechum River Formation, Virginia Blue Ridge:
Harris, L. D., de Witt, W., Jr., and Bayer, K. C., 1986, Interpretative seismic profile along Interstate I-64 incentral Virginia from the Valley and Ridge to the Coastal Plain: Virginia Division of Mineral ResourcesPublication 66.
Hatcher, R. D., Jr., 1995, Structural Geology, Principles, Concepts, and Problems: New Jersey, Prentice Hall,2nd Edition, 525 p.
Hoffman, P. F., Kaufman, A. J., and Galverson, P. J., 1998, A Neoproterozoic snowball Earth: Science, v. 281,p. 1342–1347.
Hutson, F. E., ms, 1992, Provenance and tectonic history of the Mechum River Formation: Washington D. C.,George Washington University, M.S. thesis, 321 p.
Kirschvink, J. L., 1992, Late Proterozoic low-latitude global glaciation: The snowball Earth, in Schopf, J. W.,and Klein, C., editors, The Proterozoic biosphere: A multidisciplinary study: Cambridge, UnitedKingdom, Cambridge University Press, p. 51–52.
Knight, B. D., and Bailey, C. M., 1999, The White Oak Run fault zone: a Neoproterozoic extensionalstructure in the eastern Blue Ridge, Madison County, Virginia: Geological Society of America Abstractswith Programs, v. 31, n. 3, p. 26.
Lampshire, L. D., Coruh, C., and Costain, J. K., 1994, Crustal structures and the eastern extent of the lowerPaleozoic shelf strata within the central Appalachians: a seismic reflection interpretation: GeologicalSociety of America Bulletin, v. 106, p. 1502–1511.
Li, L., and Tull, J. F., 1998, Cover stratigraphy and structure of the southernmost basement massifs in thesouthern Appalachian Blue Ridge: evidence for a two-stage Late Proterozoic rifiting: American Journalof Science, v. 298, p. 829–867.
Lukert, M. T., and Halladay, C. R., 1980, Geology of the Massies Corner quadrangle, Virginia: VirginiaDivision of Mineral Resources Publication 17, scale 1:24,000.
Mitra, G., ms, 1977, The mechanical processes of deformation of granitic basement and the role of ductiledeformation zones in the deformation of Blue Ridge basement in northern Virginia: Baltimore,Maryland, Johns Hopkins University, Ph. D. thesis, 219 p.
–––––– 1979, Ductile deformation zones in Blue Ridge basement and estimation of finite strains: GeologicalSociety of America Bulletin, v. 90, p. 935–951.
Mitra, G., and Elliott, D., 1980, Deformation of the basement in the Blue Ridge and the development of theSouth Mountain cleavage, in Wones, D. R., editor, The Caledonides in the USA: Virginia PolytechnicInstitute and State University Department of Geological Sciences Memoir 2, p. 307–312.
Mitra, G., and Lukert, M. T., 1982, Geology of the Catoctin-Blue Ridge anticlinorium in northern Virginia, inLyttle, P. T., editor, Central Appalachian Geology: Geological Society of America Northeastern/Southeastern Field Trip Guidebook, p. 83–109.
Morton, J., and Bailey, C. M., 2004, Structural Geometry of the Mechum River Belt, Blue Ridge province,Virginia: Geological Society of America Abstracts with Programs, v. 36, n. 2, p. 139.
Nelson, W. A., 1928, Geologic Map of Virginia: Virginia Division of Mineral Resources, scale 1:500,000.–––––– 1962, Geology and mineral resources of Albemarle County: Virginia Division of Mineral Resources
Bulletin 77, 99 p., scale 1:62,500.Polvi, L. E., ms, 2003, Temporal, kinematic and geochemical analysis of the Lawhorne Mill high-strain zone,
Blue Ridge province Nelson County, Virginia: Williamsburg, Virginia, College of William and Mary, B. S.thesis, 54 p.
Rankin, D. W., 1975, The continental margin of eastern North America in the southern Appalachians: Theopening and closing of the proto-Atlantic Ocean: American Journal of Science, v. 275-A, p. 298–336.
Rankin, D. W., Drake, A. A., Jr., Glover, L., III, Goldsmith, R., Hall, L. M., Murray, D. P., Ratcliffe, N. M.,Read, J. F., Secor, D. T., Jr., and Stanley, R. S., 1989, Pre-orogenic terranes, in, Hatcher, R. D., Thomas,W. A., and Viele, G. W. editors, The Appalachian - Ouachita Orogen in the United States: GeologicalSociety of America, The Geology of North America, v. F-2, p. 7–100.
Rast, N., 1992, Late Precambrian tectonism- the opening of the Iapetus Ocean: Basement Tectonics, v. 8,p. 395–406.
Rodgers, J., 1972, Latest Precambrian (post-Grenville) rocks of the Appalachian region: American Journal ofScience, v. 272, p. 507–520.
Schwab, F. L., 1974, Mechum River Formation: Late Precambrian (?) alluvium in the Blue Ridge Province ofVirginia: Journal of Sedimentary Petrology, v. 44, p. 862–871.
Shotwell, N. L., and Bailey, C. M., 2000, Structural geometry and strain in the Neoproterozoic Mechum RiverFormation, Blue Ridge province, Virginia: Geological Society of America Abstracts with Programs, v. 32,n. 2, p. 73.
Simpson, E. L., and Eriksson, K. A., 1989, Sedimentology of the Unicoi Formation in southern and centralVirginia: Evidence for Late Proterozoic to Early Cambrian rift-to-passive margin transition: GeologicalSociety of America Bulletin, v. 101, p. 42–54.
Southworth, C. S., and Brezinski, D. K., 1996, How the Blue Ridge anticlinorium in Virginia becomes theSouth Mountain anticlinorium in Maryland, in Brezinski, D. K., and Reger, J., editors, Studies inMaryland Geology: Maryland Geological Survey Special Publication n. 3, p. 253–275.
Thomas, W. A., 1976, Evolution of the Ouachita-Appalachian continental margin: Journal of Geology, v. 84,p. 323–342.
–––––– 1977, Evolution of Appalachian-Ouachita salients and recesses from reentrants and promontories inthe continental margin: American Journal of Science, v. 277, p. 1233–1278.
–––––– 1991, The Appalachian-Ouachita rifted margin of southeastern North America: Geological Society ofAmerica Bulletin, v. 103, p. 415–431.
–––––– 1993, Low-angle detachment geometry of the late Precambrian-Cambrian Appalachian-Ouachitarifted margin of southeastern North America: Geology, v. 21, p. 921–924.
21A record of Neoproterozoic and Paleozoic tectonics in southeastern Laurentia
Tollo, R. P., and Aleinikoff, J. N., 1996, Petrology and U-Pb geochronology of the Robertson River IgneousSuite, Blue Ridge province, Virginia: evidence for multistage magmatism associated with an earlyepisode of Laurentian rifting: American Journal of Science, v. 296, p. 1045–1090.
Tollo, R. P., and Hutson, F. E., 1996, 700 Ma age for the Mechum River Formation, Blue Ridge province,Virginia: A unique time constraint on pre-Iapetan rifting of Laurentia: Geology, v. 24, p. 59–62.
Tollo, R. P., and Lowe, T. K., 1994, Geologic map of the Robertson River Igneous Suite, Blue Ridge province,northern and central Virginia: United States Geological Survey Miscellaneous Field Studies Map,MF-2229, 1:100,000 scale.
Tollo, R. P., Aleinikoff, J. N., Borduas, E. A., and Hackley, P. C., 2004a, Petrologic and geochronologicevolution of the Grenville orogen, northern Blue Ridge province, Virginia, in Tollo, R. P., Corriveau, L.,McLelland, J., and Bartholomew, M. J., editors., Proterozoic tectonic evolution of the Grenville orogenin North America: Geological Society of America Memoir 197, p. 647–678.
Tollo, R. P., Aleinikoff, J. N., Bartholomew, M. J., and Rankin, D. W., 2004b, Neoproterozoic A-typegranitoids on the central and southern Appalachians: intraplate magmatism associated with episodicrifting of the Rodinian supercontinent: Precambrian Research, v. 128, p. 3–38.
Virginia Division of Mineral Resources, 1993, Geologic Map of Virginia: Virginia Division of MineralResources, scale 1:500,000.
Wehr, F., ms, 1983, Geology of the Lynchburg Group in the Culpeper and Rockfish River areas: Blacksburg,Virginia Polytechnic Institute and State University, Ph. D. thesis, 254 p.
–––––– 1986, Stratigraphy of the Lynchburg group and Swift Run Formation, Late Proterozic (730-570 Ma),central Virginia: Southeastern Geology, v. p. 225–239.
Wehr, F., and Glover, L., 1985, Stratigraphy and tectonics of the Virginia-North Carolina Blue Ridge:Evolution of a late Proterozoic-early Paleozoic hinge zone: Geological Society of America Bulletin, v. 96,p. 285–295.
Wilson, J. T., 1966, Did the Atlantic close and then re-open?: Nature, v. 211, p. 676–681.Wooton, K. M., Bailey, C. M., and Kunk, M. J., 2005, The nature and timing of deformation in the Blue Ridge
province, Greene County Virginia: Geological Society of America Abstracts with Programs, v. 37, n. 2,p. 36.
22 C. M. Bailey and others22