Horizontal slip along Alleghanian joints of the Appalachian plateau: evidence showing that mild penetrative strain does little to change the pristine appearance of early joints

Some Alleghanian joints in black shales of the Geneseo and Middlesex. Formations of the Catskill Delta complex, Finger Lakes district, New York, slipped horizontally up to 8 cm. Horizontal slip is measured by the offset of ENE-striking joints. Alleghanian joints striking 330-350 degrees display a right-lateral slip with an average value of 1.9 cm, while joints striking 004-010 degrees slip in the left-lateral sense with an average value of 1.3 cm. The maximum horizontal stress (S-H) driving this slip falls between 350 degrees and 004 degrees, the orientation of local Alleghanian layer-parallel shortening as indicated by both disjunctive and pencil cleavage. By commonality of orientation, we infer that slip on Alleghanian joints is driven contemporaneously with layer-parallel shortening. If so, the offset ENE-striking joints predate the Alleghanian stress field. These observations mean that both pre-Alleghanian and early Alleghanian joints persist through a period of penetrative strain. (C) 2001 Elsevier Science B.V. All rights reserved.