Syntectonic Fluid Infiltration and Paleohydrology in the Tendoy Thrust Sheet, Beaverhead Mts., Montana

A. C. Johnson
Lehigh University, Department of Earth and Environmental Sciences, Bethlehem, PA

This structural and geochemical study will elucidate the evolution of fluid-rock interaction in the Sevier foreland. Crustal fluids have been shown to affect deformation, ore and hydrocarbon deposition, and the earthquake cycle. It is therefore essential to study pathway development and evolution, fluid composition and sources, and the environmental conditions in the thrust sheet, which impact fluidrock interactions. Fluids may be derived from infiltration of shallow meteoric water, from deep within the foreland interior, or from a mixture of these two fluid regimes. Key environmental conditions in fluid flow and fluid-rock interactions are temperature, pressure, and salinity.

Understanding fluid pathway evolution in the Tendoy thrust sheet requires field and petrographic analyses (e.g., cathodoluminescence) of veins, cleavage, shear zones, and protolith. The d¹⁸O, d¹³C, and dD of fluids associated with each vein generation will be determined by stable isotopic analysis calculated for whole-rocks, veins, and cleavage at estimated temperatures, thus determining fluid composition and demonstrating fluid-rock interactions through isotopic exchange. Topographic or tectonic driving mechanisms for fluid flow and potential fluid sources will be evaluated after establishment of fluid composition. X-ray diffraction analysis on authogenic illites will provide the peak rock metamorphic temperatures. Fluid inclusion analyses will provide the fluid trapping temperatures and freezing point depressions will allow salinity and fluid pressure estimates. This multidisciplinary approach will develop a history of syntectonic fluid infiltration and paloehydrology of the Tendoy thrust sheet.

AAPG Search and Discovery Article #90902©2001 AAPG Foundation Grants-in-Aid