Geology of the Shenandoah National Park Region - Csmres Jmu ...

cover rocks are typically asymmetric northwest-verging structures. Axial planar foliation (cleavage) is well
developed in fine-grained rocks and dips gently to moderately southeast.
Mylonitic rocks occur in anastomosing zones, up to a kilometer thick, that cut basement rocks along the eastern
margin of the Park (Figs. 4 and 5). Mylonite zones in the Park were first recognized by Gathright (1976) and
aspects of these rocks have been discussed by Mitra (1977), Bailey and Simpson (1993), and Bailey and
others (2002). Asymmetric structures consistently record a top-to-the-northwest sense of shear (i.e. hanging
wall up movement). The reverse displacement across Blue Ridge high-strain zones accommodated crustal
contraction enabling the relatively stiff basement complex to shorten while cover rocks were folded.
A distinctive coarse foliation or compositional banding is developed in the older Mesoproterozoic basement
units (Groups 1 and 2, >1,150 Ma) and defined by aligned feldspars and quartz aggregates that formed at
upper amphibolite- to granulite-facies conditions. Younger Mesoproterozoic basement units (Group 3, 80°). Transverse faults cut previously folded contacts and
the apparent strike-slip offset in map view is accomplished by dip-slip movement with maximum displacements
of ~100 m. A number of transverse faults, including the recently recognized Simmons Gap and Sandy Bottom
faults in Shenandoah National Park, correspond to well-developed topographic lineaments (Fig. 4; Bailey
and others, 2009).
North-northwest striking transverse faults and extension fractures are subparallel to a regional suite of Jurassic
diabase dikes. Bailey and others (2006) interpreted many transverse faults in Shenandoah National Park
as Jurassic structures noting that these faults record minor east-northeast directed extension and possibly
developed in a transtensional stress regime. Wieczorek and others (2004) used LIDAR imagery to identify a
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