Fabric Relationships Across the Catoctin Mountain-Blue Ridge Anticlinorium in Central Virginia

摘要

Study of planar and linear subfabrics across the Catoctin Mountain-Blue Ridge anticlinorium in central Virginia suggests distinct differences in the kinematic history of the core and flanks of the anticlinorium. The Precambrian basement complex gneisses in the core of the anticlinorium record the effects of penetrative deformation in the a direction. Maxima of mica cleavage poles form a complete girdle subnormal to the pronounced down-dip lineation of elongated minerals. Northeast-striking foliation produces well-defined maxima that have a poorly developed partial -girdle. Piedmont rocks in the James River synclinorium, on the eastern limb of the Catoctin Mountain-Blue Ridge anticlinorium have an s-surface pattern somewhat similar to that of the basement gneisses, but with a better developed partial -girdle. However, the lineation and mica cleavage pole patterns differ markedly from those in the basement. Lineation is predominantly crenulation in the older foliation and is either subhorizontal or steeply plunging. These lineations define a great circle girdle. A plot of poles to mica cleavage shows a partial -girdle almost coincident with that of the s-poles, indicating mica growth in the bedding and axial plane cleavage. Crenulation lineations probably have resulted from intersections of younger cleavages having different strikes than the older foliations they cut. A plot of poles to bedding and cleavage parallel to bedding in the Chilhowee Group on the northwestern flank of the anticlinorium shows a well-defined great circle -girdle. This girdle indicates that the folds are overturned northwestward and are cylindroidal. A later cross-cutting cleavage parallels the statistical axial plane of the s-pole -girdle. The intersections of this late cleavage with bedding and cleavage parallel to it produce a subhorizontal northeast-bearing lineation. A diagram of poles to mica cleavages defines a subfabric characterized by a complete girdle similar to the s-pole -girdle, but having a β pole bearing 17 degrees closer to due north. The Catoctin Mountain-Blue Ridge anticlinorium in central Virginia is similar to its extension in Maryland and Pennsylvania and possesses the same cleavage and foliation fan evident in the anticlinorium there. The anticlinorium in central Virginia, however, has fewer similarities with the Grandfather Mountain Window area southwestward in North Carolina. Like the rocks in the Window area, the Precambrian basement rocks in the anticlinorium are profoundly deformed cataclastically and have a pronounced cataclastic a lineation. Unlike the rocks in the Window area, those in the Piedmont part of the anticlinorium do not have a pronounced cataclastic fabric.