Abstract: Palinspastic Map of the Orogenic Belt in Western Montana, Northwestern Wyoming and Idaho

STERNE, EDWARD J.

The orogenic belt of Montana, Idaho and Wyoming, consists of four major thrust systems which transport Proterozoic rocks. Three systems carry sedimentary rocks of the Precambrian Belt Supergroup, while the fourth involves Archean crystalline basement. Deformation progressed generally from west to east above a basal detachment which is common to all of, the thrust systems. The orogenic belt encountered a profound cross-strike discontinuity where it traversed the thick, eastward embayment of the Belt sedimentary basin. The thrust systems underwent large-scale displacement transfer as they crossed the embayment. In addition, the boundary between the basement- and sedimentary-carrying systems is largely discordant because thrusts carrying sedimentary rocks verge predominantly to the east, while the basement-involved thrusts are both east and west vergent. For this reason, only two places, the Bridger Range and the Highland Mountains illustrate displacement transfer between basement- and sedimentary-carrying thrust systems.

The palinspastic map incorporates surface geology, well control, seismic data, restored cross sections, paleomagnetic data and kinematic indicators to estimate the distance and direction of movement on the major thrust systems. Finite displacement vectors, which connect current and restored positions of any point, show east-northeast transport in the orogenic belt. In contrast, relative motion vectors, which track the relative motion of adjacent thrust blocks and parallel the slip: direction in thrust planes, diverge significantly from the general transport direction, especially around thrust system terminations. Such "out of the plane" movements must be factored into the positioning of cross sections to anticipate restoration problems.

AAPG Search and Discovery Article #90946©1997 AAPG Rocky Mountain Section Meeting, Denver, Colorado