THE IMPACT OF VERTICAL-AXIS ROTATIONS ON SHORTENING AND THRUST SEGMENTATION: AN EXAMPLE FROM THE WYOMING SALIENT, SEVIER FOLD-THRUST BELT, USA

Shortening is the total amount of deformation between two converging bodies with the displacement field comprised of three components: translation, rotation, and strain. Translations along faults and folding strain are commonly the only components of tectonic shortening incorporated into regional studies. The total amount of shortening between two converging plates is referred to as ‘tectonic shortening’ to distinguish it from the amount of shortening contributed independently from translation, strain and rotation. Previous investigations have shown that small-scale strain shortening and rotations about a horizontal axis (i.e. folding) has an important influence on the total estimated shortening in fold-thrust belts. Determinations of vertical-axis rotations through structural and paleomagnetic analyses is a key element in deciphering the rotational component of shortening within orogenic systems. In particular, curved fold-thrust belts are areas in which vertical-axis rotations have been documented. The Prospect thrust of the Wyoming salient serves as a case example to demonstrate that thrust segmentation: (1) may suggest an episode of vertical-axis rotation, (2) can aid in determining the relative timing of rotation with strain or translation, and (3) may also constrain the location of the vertical axis, or pivot. In addition, the impact of rotational deformation on shortening is illustrated by applying simple geometric calculations. Shortening estimates which take vertical-axis rotations into account differ from those estimates which do not incorporate such rotations.