ROOF SEQUENCE RESPONSE TO EMPLACEMENT OF THE WILLS MOUNTAIN DUPLEX - THE ROLES OF FORETHRUSTING AND SCALES OF DEFORMATION

This paper focuses on the behavior of a roof sequence in the Appalachi an Plateau of West Virginia, U.S.A., and emplacement of the Wills Moun tain duplex with 17.5 km of displacement. Unlike the Plateau along str ike in Pennsylvania and New York where forethrusting was previously do cumented, this roof sequence lacks an underlying salt-dominated roof d ecollement. Kinematic analyses reveal that the roof sequence in the We st Virginian Plateau accommodated about two-thirds of the 17.5 km of s hortening by the adjacent Wills Mountain duplex, as a forethrusting ki nematic response. The remaining shortening imbalance of about 5 km bet ween the duplexes and younger roof sequence rocks is accommodated by a dditional forethrusting further into the foreland and local compensati on. This kinematic response matches that along strike in the central A ppalachians despite the loss of the salt decollement. We interpret tha t an Ordovician shale-dominated formation was sufficiently weak to sub stitute for the salt horizon. Thus, a weak mechanical unit rather than specifically a salt decollement is a necessary prerequisite for foret hrusting. A contributing factor to forethrusting may be the subvertica l front of the Wills Mountain duplex, which inhibited other responses by the roof sequence. Mesoscale and smaller processes, including grain -to-grain pressure solution, twinning and cleavage formation account f or over 75% of the shortening in the roof sequence, and, if ignored, w ould result in an erroneous interpretation of backthrusting or local c ompensation. This result suggests that failure to consider all deforma tion scales could lead to incorrect kinematic conclusions in other tec tonic systems. (C) 1997 Elsevier Science Ltd.