Early jointing in coal and black shale: Evidence for an Appalachian-wide stress field as a prelude to the Alleghanian orogeny

Early ENE-striking joints (present coordinates) within both Pennsylvanian coal and Devonian black shale of the Central and Southern Appalachians reflect an approximately rectilinear stress field with a dimension >1500 km. This Appalachian-wide stress field (AWSF) dates from the time of joint propagation, when both the coal and shale were buried to the oil window during the 10-15 m.y. period straddling the Pennsylvanian Permian boundary. The AWSF was generated during the final assembly of Pangea as a consequence of plate-boundary tractions arising from late-stage oblique convergence, where maximum horizontal stress, S_H, of the AWSF was parallel to the direction of closure between Gondwana and Laurentia. After closure, the AWSF persisted during dextral slip of peri-Gondwanan microcontinents, when S_H appears to have crosscut plate-scale transcurrent faults at ∼30°. Following >10 m.y. of dextral slip during tightening of Gondwana against Laurentia, the AWSF was disrupted by local stress fields associated with thrusting on master basement decollements to produce the local orocline-shaped Alleghanian map pattern seen today.