|U.S. Geological Survey Bulletin 2136
Available online only
The west limb of the Blue Ridge-South Mountain anticlinorium (BR-SMA), in northern Virginia and southern Maryland, is repeated by the Short Hill fault (SHF). Field relations, fabric diagrams of structural elements, drill core data, and microstructures show that the SHF is a younger-over-older thrust fault that has been folded with the rocks of the northeast-plunging BR-SMA. The SHF can be traced for over 60 km from a shear zone in Middle Proterozoic basement rocks in the core of the BR-SMA to the west limb where the fault places younger Lower Cambrian rocks over older Lower Cambrian rocks. Hanging-wall rocks (Swift Run, Catoctin, Weverton, Harpers, Antietam, and Tomstown Formations) lie in a regional syncline that is interpreted to be genetically related to contractional motion on the SHF. The hanging-wall syncline was cut by the SHF during contractional reactivation. The SHF is transected by pressure solution crenulation cleavage that is axial planar to the BR-SMA.
The SHF is an early structure that is parallel to cleavage in greenschist-facies rocks. When the contractional structures are restored, the SHF can be interpreted as an extensional fault. Contractional reactivation of the extensional fault is inferred to have occurred during the Alleghanian orogeny. The extensional fault formed after deposition of the Lower Cambrian Tomstown Formation, but before late Paleozoic metamorphism (Mississippian?), that obscured evidence of the extensional structures. The SHF may be part of a late Proterozoic rift zone in the axial region of the BR-SMA that was subjected to post-Cambrian(?) extension. Similar faults in the Blue Ridge province from Virginia to Pennsylvania may mark the Late Proterozoic to Early Cambrian rifted continental margin of North America that was thrust and uplifted during the Alleghanian orogeny.
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