Similarities between the thick-skinned Blue Ridge anticlinorium and the thin-skinned Powell Valley anticline LEONARD D. HARRIS U.S. Geological Survey, Reston, Virginia 22092 ABSTRACT a nearly continuous sequence with sedimentary rocks of the Valley and Ridge. South of Roanoke, Virginia, in the southern Appalachi- The Blue Ridge anticlinorium in northern Virginia is a part of an ans, the continuity of the sequence is broken by a series of great integrated deformational system spanning the area from the Pied- thrust faults that have transported Precambrian rocks of the Blue mont to the Appalachian Plateaus. Deformation intensity within Ridge westward in Tennessee at least 56 km (35 mi), burying rocks the system decreases from east to west. Differences of opinion have of the Valley and Ridge province. Although surface relations in the emerged concerning the central Appalachians as to whether the southern Appalachians clearly demonstrate that basement rocks basement rocks exposed in the core of the Blue Ridge anticlinorium are involved in thrusting, surface relations in the central Appala- are rooted or are allochthonous. Available surface and subsurface chians are less definitive. Consequently, differences of opinion have stratigraphie and structural data suggest that the anticlinorium may emerged in the central Appalachians concerning whether, in the be a rootless thick-skinned analogue to the rootless thin-skinned subsurface, basement rocks beneath the Blue Ridge are rooted or Powell Valley anticline in the Valley and Ridge. Both structures involved in thrusting. As an example, Cloos (1947, 1972) consid- were produced during the Alleghenian orogeny by similar defor- ered that the Blue Ridge anticlinorium in northern Virginia and mational processes. The form of the Powell Valley anticline is at- Maryland is rooted and was produced by shear folding. He tributed to duplication of about 4,575 m (15,000 ft) of sedimentary suggested that distribution of slip along many closely spaced cleav- rock during approximately 16 km (10 mi) of northwest movement age planes and large shear zones is the mechanism responsible for above a subhorizontal décollement. Similarly, the form of the Blue uplift and transport of the Blue Ridge anticlinorium about 16 km Ridge anticlinorium is attributed to duplication of about 9,150 m (10 mi) westward. In contrast, Root (1970, 1973) suggested that (30,000 ft) of igneous, metamorphic, and sedimentary rock during basement rocks beneath the Blue Ridge in Pennsylvania are a minimum of 59 km (37 mi) of northwest movement above an allochthonous and are displaced westward about 80 km (50 mi) eastward continuation of a subhorizontal décollement within above an eastward continuation of the master décollement that is Cambrian sedimentary rocks beneath the Valley and Ridge. Thus, beneath the Valley and Ridge. in northern Virginia there is a mixing of structural styles: the Whether the Blue Ridge anticlinorium in the central Appalachi- thick-skinned rootless Blue Ridge anticlinorium sits above a thin- ans is nearly autochthonous, as viewed by Cloos, or allochthonous, skinned detachment. This relationship implies that thin- and thick- as viewed by Root, has a direct bearing on the fundamental role of skinned styles are simply end members of a complex deformational the Blue Ridge in the development of the entire Appalachian Moun- process that includes a transition zone, where characteristics of tain system. If autochthonous, the uplift of the rooted Blue Ridge as both styles commingle. a shear fold was the principal cause for the deformation of the Val- ley and Ridge in the central Appalachians. If allochthonous, the INTRODUCTION Blue Ridge, rather than being the cause of the deformation of the Valley and Ridge, was actually a part of the shortening process. It is In the eastern part of the United States there is a noticeable the purpose of this paper to evaluate current data concerning the change in the style of deformation at the surface from the Valley origin of the Blue Ridge in northern Virginia, where it forms a con- and Ridge province eastward into the Blue Ridge province. Style in tiguous sequence with rocks of the Valley and Ridge, in an attempt the Valley and Ridge, documented by drilling and seismic data to more clearly understand the place of the Blue Ridge anti- along the west edge of the Valley and Ridge from Pennsylvania to clinorium in the development of the Appalachian Mountain sys- Tennessee (Gwinn, 1964; Jacobeen and Kanes, 1974, 1975; Perry, tem. 1964, 1975; Harris, 1976) is characterized by flexure folding and thrust faults rooted in a master décollement that is entirely within REGIONAL STRUCTURAL RELATIONS the sedimentary cover (thin-skinned). Style in the Blue Ridge, on the basis of surface relations, is thought to be shear folding and re- Northern Virginia and the adjacent parts of West Virginia (along lated faulting involving both basement and cover as a unit (thick- the line of section A—A', Fig. 1) are uniquely suited for studying the skinned). This style change is best seen in the central Appalachians change in style of deformation from the Appalachian Plateaus in northern Virginia, Maryland, and southern Pennsylvania, where through the Blue Ridge anticlinorium. Rocks in that area form a the Blue Ridge anticlinorium, cored with Precambrian rocks, forms nearly complete lateral sequence that has been adequately de- Geological Society of America Bulletin, Part I, v. 90, p. 525-539, 7 figs., June 1979, Doc. no. 90605. 525 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/6/525/3434014/i0016-7606-90-6-525.pdf by guest on 29 September 2021 526 L. D. HARRIS scribed in several reports (Cloos, 1947, 1972; King, 1950; Brent, placement above a décollement limited to the area west of the Blue 1960; Nelson, 1962; Gwinn, 1964; Perry, 1964, 1975; Smith and Ridge. Seismic and drilling data (Perry, 1964, 1975; Gwinn, 1964,' others, 1964; Rader and Perry, 1976). Collectively, these papers 1970; Jacobeen and Kanes., 1974, 1975) confirms that in this area demonstrate the decrease in intensity of deformation from east to the Valley and Ridge is underlain by a décollement at the Cambrian west. level. Cloos (1972), who studied in detail the northwest limb of the The construction of my regional cross section (Fig. 2) is based on Blue Ridge anticlinorium, concluded that uplift and westward the concept that lateral movement above a master décollement, movement of 13 to 16 km (8 to 10 mi) of the basement core of the which changes its stratigraphie position westward from Cambrian Blue Ridge would be sufficient to produce the structural pattern shown in the present Valley and Ridge in northern Virginia, Mary- land, and southern Pennsylvania. Although he did not construct a cross section to illustrate his concept of the subsurface relations from the Blue Ridge to the Appalachian Plateaus, he did suggest that folding in the Valley and Ridge was probably related to dis- Figure 1. Generalized structure map of central and southern Appalachians (modi- fied from King and Beikman, 1974). / Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/6/525/3434014/i0016-7606-90-6-525.pdf by guest on 29 September 2021 BLUE RIDGE ANTICLINORIUM AND POWELL VALLEY ANTICLINE 527 to younger levels, is responsible for the development of an integ- tended to form the same fan patterns observed for cleavage, circles rated deformation system spanning the area from the Piedmont to became ellipses (deformed oolites), intensity of deformation the Appalachian Plateaus. Near Wills Mountain at the west margin gradually decreased away from the hinged area, and incipient de- of the Valley and Ridge, the décollement changes stratigraphie po- velopment of a bedding-plane thrust took place. In addition, sition from Cambrian along a major tectonic ramp to the Mar- although not intended, his experiment shows that he was dealing tinsburg Formation (Ordovician). The detachment continues at the with a flexure fold adjusting to compression, as indicated by the level of the Martinsburg beneath the Appalachian Plateaus to the offsetting of tops of circles in opposite directions, which took place Elkins Valley anticline, where it tectonically ramps upward to the between the layers of clay separated by Saran Wrap (Cloos, 1972, Silurian. Displacement of the northwest limb of the Wills Mountain Fig. 5). Thus, surprisingly, Cloos experimentally demonstrated that anticline from its ramp zone appears to be about 21 km (13 mi). all structures he assumed to be the result of shear folding can be The splay anticlines west of Wills Mountain are the principal sub- produced by a flexure fold. If the Blue Ridge anticlinorium is a kind surface take-up zones that compensate for the Wills Mountain dis- of thick-skinned flexure fold, it may be instructive to compare the placement. East of Wills Mountain the surface of the Valley and similarities and the differences between it and the rootless thin- Ridge is cut by two major thrust faults — the Little North Moun- skinned Powell Valley anticline in the Pine Mountain thrust sheet in tain and the Pulaski-Staunton. Seismic profiles, deep drilling, and the Valley and Ridge. geologic relations (Jacobeen and Kanes, 1975; Rader and Perry, 1976) suggest that rocks above the Little North Mountain fault COMPARISON OF THE POWELL VALLEY ANTICLINE have moved about 27 km (17 mi). A klippe 6 km (4 mi) west of the AND THE BLUE RIDGE ANTICLINORIUM Pulaski-Staunton fault indicates that displacement on the Pulaski- Staunton is at least 6 km (4 mi). In addition, the apparent subsur- The Pine Mountain thrust fault, which is the standard model face duplication and folding may account for an additional 5 km (3 (Rich, 1934) for thin-skinned tectonics in the southern Appalachi- mi) of displacement. ans, is thought to have been initiated as a subhorizontal décolle- Summing of the displacements on all thrust faults suggests that a ment in an incompetent zone just above basement, and to have minimum of shortening in the Valley and Ridge immediately in shifted upward across competent units along a steeply dipping front of the Blue Ridge is about 59 km (37 mi).