EXTENSIONAL AND COOLING HISTORY OF THE PIONEER CORE COMPLEX, SOUTH-CENTRAL IDAHO: LONG-LIVED EPISODIC EXTENSION AND IMPLICATIONS FOR REGIONAL PATTERNS OF EXTENSION

The Pioneer core complex (PCC) of central Idaho lies along the boundary of two extensional domains of different ages. To the north, in the U.S. and Canada, core complexes accommodated large-magnitude extension in the Early Eocene, while those south of the Snake River Plain show predominantly evidence for younger development. Eocene extension in the PCC has been previously proposed, but the directions and exact timing of this event are ambiguous. The PCC also lies along the northwest boundary of the Miocene Basin and Range province. The age of Basin-and-Range extension north of the Snake River plain, however, is poorly constrained. We present new field, U-Pb, and U-Th/He data that allow reconstruction of the complete cooling and extensional history of the core complex. Identification of the ages of episodic extension in the core complex provides an increased resolution of regional extension patterns.

The cooling history was constructed by combining new U-Pb and U-Th/He data, with existing ⁴⁰Ar/³⁹Ar data. The salient features of the cooling history include: (1) rapid Middle Eocene cooling following intrusion & migmatization (rates >50°C/my), (2) cooling from 500°C to less than 200°C at rates of 25-40°C/my between ~44 and ~33 Ma., and (3) a cooling event at ~10 Ma, superimposed on slow cooling since ~33 Ma, (average rate of <5°C/my).

Field and U-Pb data demonstrate a WNW-directed extensional event from >50Ma to at least ~47 Ma, in contrast to previous interpretations of SW-directed extension. Furthermore, this work shows synchroneity of core complex development from Canada southward to the Snake River Plain. Rapid middle Eocene cooling is the result of both extensional exhumation during this event and conductive cooling following emplacement of syn-extensional ~50-47 Ma mid-crustal intrusions. Cooling from 48-33 Ma indicates long-lived or episodic extensional exhumation within the PCC.

An event at ~10 Ma in the PCC is documented with a U-Th/He apatite age-elevation profile, which suggests an exhumation rate of ~0.3 km/my. This event, superimposed on very slow exhumation since 33 Ma, is the result of motion on the NE-striking White Mtns. fault that bounds the SE-side of the complex, and possibly a NNE-striking fault that cuts through the PCC. This 10 Ma faulting may be related to passage of the Yellowstone hotspot to the south.