Anisotropy of anhysteretic susceptibility (AAS) is a recently developed high-resolution method of measuring the magnetic fabric of rocks. In order to test the applicability and limitations of AAS for estimation of strain orientations in weakly-deformed and weakly magnetic rocks, we have used the method to examine the magnetic fabric of samples from a series of sites in limestones of the Helderberg and Onondaga formations along a 500-km E-W transect across New York State. Two distinct shortening directions have been previously identified and interpreted in terms of two separate phases of Alleghenian deformation. Over most of the transect, minimum anhysteretic susceptibility axes within the plane of bedding closely parallel the compression direction of the earlier ("Lackawanna") phase. A few sites show minimum anhysteretic susceptibility parallel to the later ("Main") phase. The threshold for resolution of the tectonic signal by AAS is at anhysteretic susceptibilities of about 2 × 10-5 (SI) and strain magnitudes of 0.5 to 1% as recorded by twinning in calcite. The central part of the transect exhibits minimum horizontal anhysteretic susceptibility perpendicular to the inferred tectonic compression, rather than parallel to it. We attribute this to either: (a) anisotropic transmission of stresses from the larger calcite matrix grains to the smaller magnetite grains during twinning; or (b) preferential recording of a late-stage non-coaxial stress direction in this area by late diagenetic magnetite.
Bibliographical noteFunding Information:
For assistance in the field, we are grateful to Sanford Ballard; for discussion of various related topics, we thank Andy Schedl, Ben van der Pluijm, and Don Peacor. The review by Graham Bor-radaile helped to clarify several points. Partial funding was provided by Conoco, Texaco, and NSF grants EAR 84-07007 and EAR 84-07083.