OROGENIC CURVATURE IN THE NORTHERN TACONIC ALLOCHTHON AND ITS RELATION TO FOOTWALL GEOMETRY

The Taconic allochthon, which is an approximately 200-km-long overthrust sheet on the western margin of the Appalachian orogen, trends north-northeast. The axial traces of folds within it, however, are not everywhere north-northeast trending. In the northern Taconic allochthon, the axial traces of folds change orientation from north-northeast to north-northwest and define a salient and recess. Structural and stratigraphic data indicate that this map-view curvature is not a result of oroclinal bending or lateral variation in stratigraphic thickness. Instead, the data suggest that the salient and recess formed in response to transport of the strata across an oblique ramp.

The Mount Hamilton/Tanner Hill syncline extends continuously from north to south across the region of orogenic curvature, and its axial trace is gently curving in the salient and more angular in the recess. Three anticline-syncline pairs lie directly east of the Tanner Hill syncline in the north-northwest trending part of the salient but are not present to the east of the Mount Hamilton syncline where the salient trends north-northeast. The anticline-syncline pairs form a panel of gently south-southeast plunging folds. Strain data and the orientations of the mineral lineation and axial planar slaty cleavage in the region where the folds are north to north-northwest trending are consistent with transpressional deformation within a shear zone that strikes northwest and dips approximately 30° to the northeast. The Taconic sequence is composed mainly of incompetent units, and experimental studies have shown that a high strength contrast across a ramp leads to ductile thickening of the incompetent strata in the hangingwall adjacent to the ramp and the formation of a ductile ramp that smoothes the thrust-fault trajectory. Reactivation of an obliquely oriented, steeply dipping, preexisting fracture zone in the basement, therefore, can account not only for the evidence for transpression but also for the panel of folds to the east of the Tanner Hill syncline and for the relatively low dip of the transpressional shear zone. Transpressional deformation in the vicinity of the oblique ramp resulted in lateral variation in the orientation of the principal strain axes and the observed curvature of the axial traces of the folds in the northern Taconic allochthon.