PROVENANCE ANALYSIS OF MID-CRETACEOUS STRATA IN THE METHOW BASIN, SOUTHERN BRITISH COLUMBIA: EVIDENCE FOR RAPID ARC EXHUMATION

Accretionary events, abundant deformation, and subsequent erosion and burial of Mesozoic topography mean that the sedimentary record may contain the most complete account of complex North American Cordilleran paleogeography. Sandstone petrography and detrital zircon U-Pb and Lu-Hf analysis of east-derived strata of the Methow Basin in southern British Columbia suggest that the basin received sediment from a nearby, high-standing plutonic source throughout the mid-Cretaceous period of deposition. The east-derived strata of the Methow Basin in Manning Provincial Park include the Albian-Cenomanian Jackass Mountain Group (JMG) and the Cenomanian-Turonian Winthrop Formation (WF). All 12 JMG and WF samples have an entirely Mesozoic detrital zircon age signature, with no Precambrian or Paleozoic ages, and only a few Triassic ages. All samples contain an Aptian-Albian (125-105 Ma) age peak, which includes 82% of the detrital zircon dated from the WF (637 of 777grains). A Middle to Late Jurassic (170-145 Ma) peak occurs in the JMG samples, and is most significant in the older JMG. Detrital zircon ε_Hf values determined for 125-105 Ma and 170-145 Ma grains range from +6 to +12, regardless of age or location within the basin, indicating a juvenile source. Sandstone petrography indicates low potassium feldspar and volcanic lithic content, along with consistently poor sorting. Taken together, these data suggest that east-derived strata in Manning Park came from a largely tonalitic plutonic source that was isotopically juvenile. This plutonic source experienced active magmatism during the Middle to Late Jurassic and Early Cretaceous, and was uplifted and exposed by the mid-Cretaceous. The arc source must have remained a topographic barrier throughout the mid-Cretaceous, shedding abundant sediment into the Methow basin and blocking westward transport of Precambrian continental detritus. Potential large-scale post-Cretaceous translation of the Methow Basin means possible source areas include magmatic arcs along the length of western North America. However, the relatively proximal Coast Plutonic Complex (CPC) in the Southern Canadian Cordillera provides a close match with the Methow provenance data, whereas other potential sources do not match the zircon ages, ε_Hf values, and/or petrography of the Methow strata.