GEOMETRY, TIMING, AND THERMAL EVOLUTION OF THE PARIS THRUST SHEET, IDAHO-WYOMING THRUST BELT

The Paris thrust fault is the oldest and westernmost fault in the Idaho-Wyoming foreland thrust belt. Using a regional Tertiary unconformity and seismic line data, the subsurface geometry of the Paris thrust is interpreted to be two extensive footwall flats separated by a footwall ramp. Because hanging wall rocks were uplifted several kilometers as they moved up and over the thrust ramp, fission track thermochronology of apatite and zircon mineral grains within the hanging wall was completed to measure the age and rate of fault movement.

Apatite within the Worm Creek member of the St. Charles Formation in the Bear River Range yields an age of 113.5 +/-8.75 Ma. Zircon in the Late Proterozoic Caddy Canyon Formation from the south Portneuf Range yields an age of 116.8 +/-3.3 Ma. Higher stratigraphic units, including the Cambrian Worm Creek member of the St. Charles Formation, the Cambrian/Late Proterozoic Camelback Formation, the Late Proterozoic Mutual Formation, and the Late Proterozoic Caddy Canyon Formation yield sixteen zircon ages from 398 Ma to 172 Ma interpreted to reflect partial annealing. These partially annealed samples are oldest in the west and east, and youngest in the center of the thrust sheet.

A new thermal model was created to interpret fission track ages and predict the pattern of isotherms during and after thrusting. The model incorporates variable durations and rates of thrusting and erosion as well as variable geothermal gradients, ramp heights and ramp angles. The most significant discovery to date is that the hanging wall temperatures remain 15°-30°C above the pre-thrusting temperature as it moves east of the thrust.