Cleavage duplexes in the Marcellus Shale of the Appalachian foreland

Abstract

Cleavage duplexes are zones of platy, spaced cleavage, that either parallel bedding or ramp through shale sections. Examples are 2–60 m thick. The cleavage within the zones is generally subperpendicular to the zone boundaries but sigmoidally dragged against floor, roof and internal thrusts. Between the well-developed floor thrust and the more obscure roof thrusts of the duplexes the shale has been shortened >30% perpendicular to cleavage. The thrusts are sharp strain discontinuities because both overlying and underlying shale is uncleaved and less deformed (<10% layer parallel shortening). Cleavage in the duplexes was initiated perpendicular to bedding by pure shear in front of a propagating tip line. Cleavage halos at the ends of associated stiff carbonate concretions are small scale models illustrating initiation of cleavage. The cleavage is a primary crenulation cleavage with dissolution of limbs indicated by clay carbon partings. Transfer of thrusting from the floor to the roof of the cleavage duplex proceeded incrementally toward the foreland, imposing the simple shear that led to sigmoidal cleavage traces as the cleavage continuously evolved. Environmental conditions for formation of cleavage duplexes during the Alleghanian Orogeny are estimated as temperature 200–250°C, and pressure 1.1–1.3 kb, based upon conodont coloration (CA1 4), fluid inclusions and restored stratigraphic thicknesses. Cleavage duplexes or similar fold duplexes have been recognized in suitable black shales of Ordovician to Carboniferous age. They are manifestations of the progressive transfer of slip from floor to roof through a disturbed zone that serves as a shear boundary between large, more internally passive, thrust sheets.