Title

Structural Analysis of the Pioneer Thrust Fault, Big and Little Fall Creek Drainages, Pioneer Mountains, Blaine and Custer Counties, Idaho

Publication Date

2-2000

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Geology

Department

Geosciences

Major Advisor

David L. Schwarz

Abstract

The Pioneer thrust fault was previously proposed to be the continuous trace of a single, low-angle, Cretaceous thrust fault. The Pioneer thrust fault is mapped wherever the Ordovician Phi Kappa, Silurian Trail Creek, or Devonian Milligen formations are in fault contact above the Mississippian Copper Basin Group.

Geologic mapping and structural analysis of the Pioneer thrust fault in the Pioneer Mountains of south-central Idaho indicate that the mapped fault trace north of the Pioneer metamorphic core complex is not a single Cretaceous fault but a thrust fault offset by an assortment of normal faults. New mapping confirms the presence of the Pioneer thrust fault only in the drainages of Big and Little Fall creeks. The fault north of the Pioneer metamorphic core complex has many significant changes in strike and dip over its map trace. The fault as exposed in Little Fall Creek is typical of the unusual geometry. The Pioneer thrust fault in Little Fall Creek is truncated on the north and south by a series of northeast striking normal faults. Structure contour lines and stereo graphic analysis of the two types of faults differentiate between the northwest striking Pioneer thrust fault and the northeast striking normal faults.

The Pioneer thrust fault shows evidence of at least two phases of motion. The earliest phase of motion is recorded by folds in the hangingwall and footwall which verge to the northeast, an accepted direction for thrust faulting associated with regional compression during the Mesozoic Sevier orogeny. At other locations, the Pioneer thrust fault geometry and last recorded sense of motion are consistent with northwest-directed low-angle, oblique-slip motion, which is similar to other low-angle normal faults in the Pioneer Mountains. This direction is associated with Eocene extension. Evidence of this most recent motion is preseved by slickenlines in several locations on the fault surface. M-pole, M-plane analysis yields principal compressive stress axes that are incompatible with the formation of a low-angle oblique-slip fault, reinforcing the possibility of re-activation. Therefore it is suggested that the Pioneer thrust fault be renamed the Pioneer fault system to more accurately reflect its complex motion history.

The age of normal faulting in Big and Little Fall creeks is constrained by local and regional cross-cutting relationships. Based on the ages of regional extension, the Pioneer fault extensional re-activation may have occurred during either the late Early Eocene to earliest Middle Eocene (51 to 48 Ma) if it is similar to the Lake Creek and Trail Creek faults, or the early Middle Eocene (48 to 44 Ma) interval if motion is related to the low- angle normal fault in Big Fall Creek. The low angle normal fault in Little Fall Creek cuts the Summit Creek Stock (48 Ma, Silverburg, 1990a), indicating a post early Middle Eocene age of motion. Other normal faults in Little Fall Creek, interpreted as similar in age, also cut the Pioneer thrust fault. South of the Pioneer metamorphic core complex, the White Mountains fault, a fault interpreted as the same age as the normal faults of Little Fall Creek, cuts the Oligocene (33 Ma) low angle Wildhorse detachment fault. The age of normal faulting in Little Fall Creek is constrained to post early Middle Eocene (48 Ma), and pre-Quaternary (1.6 Ma).

Total displacement accommodated by the Pioneer thrust fault, either as a Cretaceous thrust fault or re-activated Eocene oblique-slip fault is unknown. There are no known pin-points to aid in reconstruction across the thrust fault. Estimations based on similar faults nearby would range from one hundred to several hundred meters.

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