THE ROLE OF MACROSCALE THRUSTS IN THE DEFORMATION OF THE ALLEGHANIAN ROOF SEQUENCE IN THE CENTRAL APPALACHIANS - A REEVALUATION

The roof sequence to the blind Alleghanian foreland thrust duplexes of the central Appalachians in Maryland, Virginia, and West Virginia is commonly interpreted to be dominantly deformed by macroscale thrusts. The present contribution argues that these faults are not abundant bec ause: (1) Most geologic map data indicate that macroscale thrusts are not abundant in the roof sequence. (2) A prediction that thrusts are u biquitous proves incorrect when tested with a set of seismic reflectio n data. (3) Thrusts are not common along strike for the roof sequence in Maryland and Pennsylvania. (4) The upper part of the roof sequence, which lacks the previously interpreted abundant macroscale thrusts of the lower part of the roof sequence, also lacks compensatory deformat ion at the micro- and outcrop scales. (5) If gas fields are indicators of macroscale-thrust frequency, their limited distribution implies th at the faults are rare. This revised interpretation has three major im plications: (1) Much less deformation is partitioned at the macroscale in the roof sequence, and no cross section through the central Appala chians can be balanced without the inclusion of shortening by structur es smaller than the scale of the cross section, where the cumulative e ffect of outcrop-scale deformation is the most difficult to quantify. (2) Comparisons of the previous structural interpretations with abunda nt macroscale thrusts to analog models and shortening assessments of t hose macroscale structural profiles by fractal analysis should be reco nsidered. (3) The roof sequence is not the displacement sink for the 6 0+ km translation of the North Mountain thrust sheet from the hinterla nd.