THE SWAKANE GNEISS WAS UNDERTHRUST BENEATH WASHINGTON OR OREGON, WHILE THE PELONA-OROCOPIA-RAND SCHISTS WERE UNDERTHRUST SEPARATELY BENEATH SOUTHERN CALIFORNIA

The Swakane Gneiss (SwkG) in central WA and the Pelona-Orocopia-Rand Schists (PORS) in southern CA (SCal) are metasedimentary bodies that were underthrust deep beneath arc batholith rocks in Late Cretaceous time. Based mainly on old detrital zircon (1.38 and 1.65-1.80 Ga), Sauer et al (2019) proposed that the SwkG formed in SCal correlative with the PORS, then moved ~1600 km N to WA. Related to this, Coutts et al (2019 GSA Cordilleran Sec. abstr.) proposed that 84-62 Ma strata in the upper Nanaimo basin (UpNanB, Vancouver Island) were sourced from a separate, cryptic body correlative with the PORS which had been underthrust, metamorphosed, exhumed, and was eroding by 84 Ma, based on prominent Late Cretaceous rims on old detrital zircon (DZ). We prefer a simpler model, in which the old DZ in the SwkG and UpNanB was delivered to WA or OR by paleorivers draining the intensely deforming Proterozoic Belt Basin in nearby Idaho (Dumitru et al 2016). Key evidence that SwkG and UpNanB protoliths were mainly sourced from ID include: (1) Both exhibit sharp 1.38 Ga DZ age peaks which match better to 1.38 Ga plutons that intruded Belt strata than to the 1.40-1.45 Ga plutonic suite characteristic of SCal. (2) Sharp 1.38 Ga peaks are absent in SCal forearc basin samples, evidence against major 1.38 Ga source plutons in SCal. (3) Our new depth-profiling data show that Eocene Tyee strata in Oregon (which were definitely sourced from ID) contain 1.38 and 1.65-1.80 Ga DZ with prominent Late Cretaceous rims, showing ID is probably the main source for UpNanB strata. (4) DZ Hf isotope data from the SwkG are a poor match to an updated compilation of sources in SCal, but a good match to sources in ID plus northern CA. A key complication in our model is that a few PORS samples do exhibit 1.38 Ga DZ peaks and Idaho-like Hf patterns (Sauer et al). At ca. 85-80? Ma, the Kione paleoriver transported detritus SW from ID to the Great Valley forearc basin and thence to the Franciscan trench in central CA. We hypothesize that some of this ID detritus traveled a further ~500 km S down the trench axis via sediment gravity flows to SCal to source small portions of the PORS protolith with ID zircon, thereby confusing comparisons of PORS, SwkG, and UpNanB data. We infer that the SwkG and the PORS represent independent underthrusting episodes, in separate locations beneath WA-OR and SCal respectively.