DELAMINATION MAGMATISM AND EXHUMATION: REFINED CHRONOSTRATIGRAPHY OF THE CHALLIS-KAMLOOPS GROUP AND THE EARLY EOCENE TECTONIC EVOLUTION OF THE PACIFIC NORTHWEST

The Challis-Kamloops group (CKG) is the supracrustal record of Early Eocene crustal collapse in the retro-arc region of the Pacific Northwest Cordillera. It comprises volcanic and non-marine sedimentary deposits in basins formed in the upper plates of developing metamorphic core complexes (MCCs) and peripheral terrain along a vast linear belt from the Yukon Territory to Montana.

Nineteen new tandem LA-ICP-MS/CA-TIMS zircon ages redefine the legacy stratigraphic framework, revealing that the CKG comprised two magmatic phases separated by a lull. The 1^st phase (~52.7 Ma to ~51.5 Ma) was coincident with flow and ductile exhumation of the lower crust, forming MCCs. The onset of magmatism was oldest in the central Republic and Toroda Creek grabens and propagated outward in a radial pattern, consistent with strain indicators in the lower plates of the MCCs and the regional magnitudes of extension.

A similar shoshonitic, calc-alkaline, arc-like magmatic sequence occurs in each CKG basin. This common sequence is time-transgressive. It is oldest in the center of, and youngs toward the margins of the highly extended region, suggesting that a common tectonomagmatic process propagated across the entire highly extended region within ~1.1 My, coupled with rapid lower crustal exhumation. The 1^st phase was followed by a magmatic lull characterized by regional erosion, minor felsic volcanism and sedimentation in basins along detachment faults bounding the highly extended region. The 2^nd phase (~49-45 Ma) comprised mainly felsic volcanic flows and plugs, and was accompanied by brittle extension with a lower exhumation rate. It is yet unclear if the 2^nd phase involved delamination, or culminated the first delamination cycle.

The pattern of onset ages appears to be less consistent with slab rollback or slab breakoff than with delamination of mantle lithosphere/lower crust, as was previously inferred from geophysics, but both mechanisms may have contributed. Which tectonic processes triggered delamination and how it relates to CKG geochemistry, magma sources and radiogenic isotopes are currently under study.

The two CKG magmatic phases correspond well to periods of plate boundary reorganization in the Pacific basin, to the timing of Siletzia terrane accretion at the trench and to tectonic events in the Washington Cascades orogen.