TIMING OF EXTENSION IN THE PIONEER CORE COMPLEX, SOUTH-CENTRAL IDAHO: IMPLICATIONS FOR THE SPATIAL-TEMPORAL PATTERNS OF THE CORDILLERAN COMPLEXES AND THE ONSET OF TERTIARY EXTENSION

The Pioneer core complex (PCC) of central Idaho lies along the boundary of two extensional domains defined by different ages for the dominant period of core-complex formation. To the north, in the U.S. and Canada, core complexes accommodated large-magnitude extension in the Early and Middle Eocene, while those in the Great Basin to the south show predominantly evidence for younger development. Thus, the PCC represents a key element in reconstruction of the spatial/temporal development of the cordilleran core complexes.

Previous workers have disagreed on the directions of Eocene extensional strain. Our new U-Pb ages of highly strained dikes and synmagmatic fabrics clearly indicate that WNW-directed ductile extension recorded in parts the footwall was ongoing by 50 Ma and continued to at least 47 Ma. Unstrained, cross-cutting dikes indicate that ductile strain at these crustal levels ceased before the final stages of dike injection, after which time strain may have become localized in the main detachment. These dikes are currently being dated. Extension in the PCC at this time was synchronous with the emplacement of NE-tending dikes in the region and with NE-trending normal faults in the ranges to the east. This event overlaps with the core-complex formation to the north. However, consideration of differences in thermochronologic and metamorphic data suggest that extensional rates during this phase of extension may have been much lower in central Idaho toward the southern end of the northern core-complex domain. Such a distribution of extensional rates may have important implications for understanding the geodynamic setting for the switch from contraction to extension and early core-complex formation. These implications will be discussed.

Existing thermochronology indicates that WNW-directed motion on the detachment occurred continuously or episodically until ~33 Ma, around which time cooling rates slowed drastically. This phase of Late Eocene extension overlapped with core-complex development in the northern Great Basin, as well as in the Anaconda core complex to the north. Thus, the development of the PCC may have formed near the southern end of an early extensional domain and near the northern end of a younger extensional domain.