OROGENIC RECYCLING OF DETRITAL ZIRCONS IN THE NORTH AMERICAN CORDILLERA: IMPLICATIONS FOR SOURCE-TO-SINK STUDIES

Detrital zircon (DZ) analysis has become the standard tool for source-to-sink sediment routing studies, used over many temporal and spatial scales to pinpoint the ancient source areas of sedimentary deposits. Identifying the linkages between DZ signatures and their source areas is inherently limited by existing knowledge and/or assumptions regarding the locations of source areas, the lithologies and detrital compositions of those sources, and the topographic complexities of source-to-sink pathways. In North American source-to-sink studies, DZ distributions are commonly subdivided according to established age ranges of North American crustal provinces and accreted/emplaced Cordilleran terranes to assess spatial and/or temporal trends in data. Using a compilation of DZ data (>19,000 individual grain ages) for Mesoproterozoic-Paleogene strata of the Northern Rocky Mountains, we demonstrate a combination of periodic, first-cycle DZ derivation from crystalline sources and multi-stage recycling of poly-cycle DZ via orogenesis. Results show that 1) DZ age distributions become increasingly complex through time with the introduction of new DZ sources to the study area, but 2) once an age group appears in the Mesoproterozoic-Paleogene tectonostratigraphic sequence, grains of that age persist up-section. These trends show that pre-Cordilleran DZ age groups are spatiotemporally ubiquitous and non-unique. Subdividing DZ distributions according to North American basement ages to determine source areas is therefore misleading because grains of a given age may either be derived from the basement source itself or from sedimentary rocks containing grains of that age. Thus, subdividing DZ distributions in this way provides a means to describe the presence, absence, and proportions of different DZ age groups, but does not fingerprint exact source areas without additional information.