VILLARD S. GRIFFIN, JR. Department of Chemistry and Geology, Clemson University, Clemson, South Carolina 29631 Fabric Relationships Across the Catoctin Mountain- Blue Ridge Anticlinorium in Central Virginia ABSTRACT a subhorizontal northeast-bearing lineation. A diagram of poles to mica cleavages defines Study of planar and linear subfabrics across a subfabric characterized by a complete girdle the Catoctin Mountain-Blue Ridge anticlin- similar to the s-pole T-girdle, but having a orium in central Virginia suggests distinct /3 pole bearing 17 degrees closer to due north. differences in the kinematic history of the The Catoctin Mountain-Blue Ridge anti- core and flanks of the anticlinorium. The clinorium in central Virginia is similar to its Precambrian basement complex gneisses in extension in Maryland and Pennsylvania and the core of the anticlinorium record the possesses the same cleavage and foliation fan effects of penetrative deformation in the a evident in the anticlinorium there. The anti- direction. Maxima of mica cleavage poles clinorium in central Virginia, however, has form a complete girdle subnormal to the fewer similarities with the Grandfather pronounced down-dip lineation of elongated Mountain Window area southwestward in minerals. Northeast-striking foliation pro- North Carolina. Like the rocks in the Window duces well-defined maxima that have a poorly area, the Precambrian basement rocks in the developed partial ir-girdle. anticlinorium are profoundly deformed Piedmont rocks in the James River syn- cataclastically and have a pronounced cata- clinorium, on the eastern limb of the Catoctin clastic a lineation. Unlike the rocks in the Mountain-Blue Ridge anticlinorium have an Window area, those in the Piedmont part of s-surface pattern somewhat similar to that of the anticlinorium do not have a pronounced the basement gneisses, but with a better de- cataclastic fabric. veloped partial ir-girdle. However, the linea- tion and mica cleavage pole patterns differ INTRODUCTION markedly from those in the basement. Linea- This study attempts to characterize in a tion is predominantly crenulation in the older general sense the structural style and, to some foliation and is either subhorizontal or steeply degree, the deformational history of the three plunging. These lineations define a great circle tectonic domains within the Catoctin Moun- girdle. A plot of poles to mica cleavage shows tain-Blue Ridge anticlinorium in central a partial 7r-girdle almost coincident with that Virginia (Fig. l) through selective analysis of of the s-poles, indicating mica growth in the s-surfaces, lineations, and mica cleavage poles. bedding and axial plane cleavage. Crenula- The tectonic characteristics of the anti- tion lineations probably have resulted from clinorium have been well documented by intersections of younger cleavages having similar fabric analysis techniques and detailed different strikes than the older foliations conventional mapping in Maryland and they cut. Pennsylvania (Cloos, 1947; Whitaker, 1955; A plot of poles to bedding and cleavage Root, 1970) and in northern Virginia parallel to bedding in the Chilhowee Group (Nickelsen, 1956). Tectonic studies of the on the northwestern flank of the anticlinorium northeast continuation in eastern Pennsyl- shows a well-defined great circle 7r-girdle. vania, New Jersey, and New York have been This girdle indicates that the folds are over- outlined recently by Drake (1970). To the turned northwestward and are cylindroidal. southwest, along the continuation of the A later cross-cutting cleavage parallels the anticlinorium in Virginia, no similar fabric statistical axial plane of the s-pole rr-girdle. studies have been carried out, although Reed The intersections of this late cleavage with and Bryant (1964; Reed and others, 1970) bedding and cleavage parallel to it produce have given attention to fabric analysis in the Geological Society of America Bulletin, V. 82, p. 417-432, 6 figs., February 1971 417 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/2/417/3428328/i0016-7606-82-2-417.pdf by guest on 02 October 2021 418 V. GRIFFIN-FABRIC RELATIONSHIPS, CENTRAL VIRGINIA Grandfather Mountain Window area in North Carolina. The central Virginia area was selected for this analysis because it has been studied in detail by conventional means through the efforts of many workers (Furcron, 1935; Bloomer, 1950; Espenshade, 1954, 1970; Brown, 1958, 1970; Bloomer and Werner, 1955; Werner, 1966; Nelson, 1962; Allen, 1963; Redden, 1963; Abdel-Khalek and Khoury, 1969). No published work is pres- ently available on the anticlinorium south- west of central Virginia, with the recent exceptions of the Philpott Reservoir quad- rangle (Conley and Henika, 1970) and the Mount Rogers area (Rankin, 1970). Much disagreement exists on many im- portant aspects of this area (Redden, 1963; Furcron, 1969). GEOLOGIC SETTING Lithology and Stratigraphy The anticlinorium consists of three major northeast-trending belts (Fig. 2). The central belt is the basement complex, underlain by crystalline rocks of Precambrian age. Pied- mont rocks lie to the east of the basement complex, whereas late Precambrian and Chilhowee rocks of the Blue Ridge highlands lie to the west. Several gneissose, schistose Figure 1. Map of Virginia showing location and granitoid units are included within the of study area (stippled) in relation to the major basement complex (Bloomer and Werner, geologic provinces. 1955). These are the Lovingston Granite Gneiss, Marshall and Pedlar Formations, and tion as a charnockite where it contains hypers- the Roseland Anorthosite. Minor lensoid thene. This unit is a granulite of highly bodies of dark, fine-grained amphibole and variable composition similar in field appear- biotite gneiss, termed the Moneta Forma- ance to the classical granulites of Lapland. tion, occur within the other basement units. The Roseland Anorthosite is also roughly Coarse-grained metagabbroic hornblende layered to coarsely crystalline rock, similar gneiss and soapstone bodies occur within the in appearance to anorthosites from granulite Lovingston Granite Gneiss along the eastern terranes. Ross (1941), and recently, Herz side of the basement complex. Triassic(P) (1969), discuss in detail the characteristics of diabase dikes and Precambrian greenstone the Roseland Anorthosite. feeder dikes cross-cut basement units also. The eastern flank of the anticlinorium con- None or these minor units is shown in sists of metasedimentary and metavolcanic Figure 2. rocks which overlap the basement complex The Lovingston Granite Gneiss and nonconformably (Bloomer and Werner, 1955; Marshall Formation are primarily micaceous Brown, 1958). Most workers consider the gneisses of similar composition, and each Lynchburg Gneiss the oldest Piedmont unit contains poorly foliated granitoid phases. The in this area within the James River synclin- Lovingston unit contains a significant amount orium of Brown (1958). The Lynchburg con- of the granitoid phase (Fig. 2). tains several different and distinctive rock Watson and Cline (1916) and Bloomer types, such as graywacke, slate, quartzite, (unpub. ms.) characterize the Pedlar Forma- mafic and ultramafic intrusive rocks. The Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/2/417/3428328/i0016-7606-82-2-417.pdf by guest on 02 October 2021 GEOLOGIC SETTING 419 Rockfish Conglomerate marks the base of northeast, and Furcron (1969) recognizes this formation in places (Nelson, 1932; another in the James River area. Bloomer and Werner, 1955, p. 583) and con- The Unicoi Formation lies at the base of sists of clastic materials derived from the the Chilhowee Group in this area and con- underlying basement. The Catoctin Green- sists of greenstone in the lower part, and stone lies both within and partially above quartzwacke, pebbly arkose and pebbly the upper Lynchburg and is a thin unit in quartzite higher in the section. The overlying this area. Harpers Formation is composed of quartz- The Evington Group overlies both the wacke interbedded irregularly with quartzite Lynchburg and Catoctin Formations. Espen- and grades into the Antietam Quartzite above. shade (1954) and Brown (1958) consider the The Antietam Quartzite contains at least 95 contact gradational; the contact zone later percent quartz. being affected by minor reverse faulting. On Rankin and others (1969) consider the the other hand, Jonas (1929), Furcron (1935, Lynchburg, Swift Run and Catoctin Forma- 1969), and Redden (1963) interpret the con- tions to be late Precambrian in age, and the tact largely in terms of major normal faults Chilhowee and Evington Groups to be Early postdating great northwestward thrusting Cambrian. Correlation across the anticlinori- along the Martic line. Espenshade (1954) um is problematical, but if the Catoctin Green- and Brown (1958) consider the Candler For- stone is exposed on both the northwestern mation the oldest unit of the overlying and southeastern flanks, then the Chilhowee Evington Group and a Catoctin-like green- and Evington Groups probably are equiva- stone (Slippery Creek Greenstone) to be the lent, in part at least. youngest. Much disagreement about the stratigraphic sequence and existence of the Metamorphism James River Synclinorium is centered on the equivalence or nonequivalence of the Catoctin The metamorphic rank varies across the and Slippery Creek Greenstones. map area in such a manner as to suggest at Low-grade metasedimentary and metavol- least