Abstract

Structural telescoping of argillaceous lower Paleozoic rocks with coeval calcareous and quartzitic rocks in central Idaho has commonly been presumed to represent the latest Devonian to earliest Mississippian Roberts Mountains thrust by analogy with relations in Nevada. However, all datable thrusts in the Pioneer Mountains are post-middle Permian to pre-Eocene in age; if Antler-age (Mississippian) thrusts existed, they have been reactivated or obscured by major post-Antler movement. Furthermore, differences in fabric or structural style between thrusted argillaceous, pre-Mississippian sequences and calcareous-arenaceous, Pennsylvanian-Permian sequences, which have been cited in support of Antler orogenesis, are equivocal; these differences more likely resulted from disharmonic response to entirely later thrusting in rocks of different competencies or at different structural levels.

The Copper Basin Formation, a dominantly clastic deposit requiring an argillite-chert-quartzite western-highland source in Mississippian time, remains the only evidence in central Idaho for an Antler highland. The Copper Basin Formation now occurs in two superimposed allochthons which, when palinspastically restored, indicate that its original north-trending depositional basin extended at least 50-75 km west from the present Pioneer Mountains. Thus, in Mississippian time, the Antler highland reached no farther east than westernmost Idaho.

Emplacement of the Pioneer Mountains allochthons during mainly Mesozoic time involved (1) tectonic slices of high-grade metamorphic continental-shelf rocks and Precambrian crystalline basement, (2) eastward movement and imbrication of Antler detrital rocks, and (3) thrusting of lower Paleozoic argillaceous rocks from the Antler highland over the tectonic remnants of the Antler detritus. At least 100 km of post-Antler eastward translation is estimated; a comparable amount of pre-Mississippian facies telescoping within the Antler highland could be accommodated, based on reasonable reconstructions of lower Paleozoic depositional facies, but evidence that thrusting accompanied Antler highland development in Idaho has not yet been demonstrated.