Cover deformation above a blind duplex: An example from West Virginia, U.S.A.

Abstract

Pretectonic cover above blind thrust systems responds to underlying thrusting by combinations of forethrusting, backthrusting and coupling. The role of these responses was examined in the Hanging Rock-Cacapon Mountain anticlinorium of the central Appalachians. This cover fold consists of Middle Ordovician and younger sedimentary rocks above a blind horse of Cambro-Ordovician carbonates. The cover has abundant folding and subsidiary faulting. Cover cleavage in carbonate and fine-grained clastic rocks formed before regional folding in response to layer-parallel shortening (LPS). This 15% LPS also produced grain-suturing and fluid-inclusion trails in quartzarenites. The LPS formed during cover forethrusting, accommodating older blind horses in the hinterland. With the formation of the underlying horse, cover coupling occurred with bending-folding, additional secondary buckling and cleavage rotation by flexural flow with perhaps minor flattening. This cover coupling was insufficient to accommodate horse formation. No evidence exists for backthrusting, but forelandwards, abundant subsurface evidence exists for cover forethrusting with imbricate thrusting. This thrusting accommodates horse formation and differs in style from the LPS of the older forethrusting event. So, for this blind system, the cover responded with a combination of coupling and forethrusting.