RHEOLOGICAL TRANSITIONS AND STRUCTURAL OVERPRINTING ASSOCIATED WITH CRETACEOUS PARTIAL MELTING AND STRAIN LOCALIZATION IN THE POTTERS POND MIGMATITE DOMAIN, WEST-CENTRAL IDAHO

Migmatites, representing former partially molten rocks and magmas, provide important constraints on the tectonic evolution of orogens. Partial melt migration is intrinsically linked to deformation and, as such, migmatites — particularly in shear zones — inform on the timescales and interaction of crustal melting, deformation processes, and the transient rheology of the lithosphere. We combine structural mapping and the results of U-Pb zircon and monazite geochronology to determine the deformation history and the timing and duration of partial melting in a newly recognized migmatite terrain in west-central Idaho—the Potters Pond migmatite domain (PPMD). The PPMD is the only known exposure of migmatites within the western Idaho shear zone (WISZ) over its ~300 km length. Migmatites within the PPMD are structurally and compositionally heterogeneous, consisting primarily of stromatic and folded metatexite, interlayered or intruded by voluminous bodies of heterogeneous diatexite. Structures indicative of melt present deformation (e.g., leucosome accumulation in dilatant structural sites) are abundant in metatexite and diatexite domains. Early-formed units with synmigmatitic, melt-present deformation structures are overprinted by solid-state deformation fabrics that are, in turn, crosscut and deformed by a younger generation of migmatites not deformed in the solid state. U-Pb age determinations document two protracted periods of migmatite crystallization during the Early and Late Cretaceous. Early Cretaceous (ca. 145–128 Ma) crystallization ages are coeval with the collision and suturing of oceanic terranes of the Blue Mountains province with North America and the formation of the Salmon River suture zone (SRSZ). Migmatite crystallization ages from ca. 104–90 Ma are associated with Late Cretaceous dextral transpression in the WISZ. Field observations and geochronology of crosscutting migmatite generations are thus interpreted to record deep crustal deformation and anatexis associated with formation of the SRSZ, subsequently overprinted by solid-state deformation and renewed anatexis during the formation of the WISZ. Deformation conditions in the PPMD were variable through time, resulting in corresponding rheological transitions and structural overprinting in deep crustal migmatites.