ALONG-STRIKE DIFFERENCES IN RATES OF EARLY EOCENE CORE COMPLEX DEVELOPMENT IN THE WESTERN NORTH AMERICAN CORDILLERA AND POTENTIAL GEODYNAMIC CONTROLS

In the western Cordillera of North America, post-orogenic large-magnitude extension is recorded by the development of metamorphic core complexes within the hinterland. Existing data from core complexes of British Columbia, Washington, and northern Idaho (Bitterroot/Anaconda complexes) indicate Early Eocene extension and core complex development that immediately followed the end of crustal shortening at ~55-53 Ma. This extension and related exhumation is recorded by a variety of metamorphic, geochronologic, and thermochronologic data. Farther south, in central Idaho, our recent work on the Pioneer core complex shows that extension occurred synchronously with the core complexes to the north. In contrast, studies on the core complexes south of the Snake River Plain (such as the Albion-Raft River and Ruby - East Humbolt) have yielded no compelling evidence that extension began before ~41 Ma.

Despite the synchroneity of core complex development north of the Snake River Plain, comparison of metamorphic and thermal histories suggest that a much smaller cross-sectional area of crust was exhumed during the Early Eocene in the southern core complexes (Pioneer/Bitterroot/Anaconda) and that rates of exhumation were distinctly lower. This southward decrease coincides with the Lewis and Clark line, perhaps suggesting that this zone served as a transfer zone that accommodated a change in extension rates.

The along-strike changes in exhumation rates in synchronously extending regions have important implications for the geodynamic causes of post-orogenic extension. One set of geodynamic factors relate extension rates to north-south differences in the characteristics of the subducting slab to the west (e.g., differences in slab dip, differences in convergence vectors of adjacent subducting plates, the foundering of Siletzia lithosphere beneath Oregon and Idaho). Alternatively, extension rates may have been controlled by differences in character of the orogenic belt, such as crustal thickness, width of the orogen, and/or Moho temperature. We document the north-south differences in rates of core complex development and explore the potential contributions of each of these geodynamic factors.