Bimodal volcanism of the High Lava Plains and Northwestern Basin and Range of Oregon: Distribution and tectonic implications of age‐progressive rhyolites

Multiple episodes of Oligocene and younger silicic volcanism are represented in the high lava plateau of central and southeastern Oregon. From 12 Ma to Recent, volcanism is strongly bimodal with nearly equal volumes of basalt and rhyolite. It is characterized by moderate to high silica (SiO2 >72 wt. %) rhyolitic tuffs and domes that are younger to the west, and widespread, tholeiitic basalts that show no temporal pattern. We report 18 new 40Ar/39Ar incremental heating ages on rhyolites, and establish that the timing of the age‐progressive rhyolites is decoupled from basaltic pulses. This work expands on that of previous workers by clearly linking the High Lava Plains (HLP) and northwestern‐most Basin and Range (NWBR) rhyolite volcanism into a single age‐progressive trend. The spatial‐temporal relationship of the rhyolite outcrops and regional tectonics indicate that subsidence due to increasingly dense crust creates large, primarily sediment‐filled basins within the more volcanically active HLP. The west‐northwest age progression in rhyolitic volcanism is counter to the trend expected for a quasi‐stationary mantle upwelling relative to North American plate motion. We attribute the rhyolitic age progression to mantle upwelling in response to slab rollback and steepening, and this is consistent with mantle anisotropy under the region and analog slab rollback models. This removes the necessity of deep mantle plume involvement. Laboratory experimental studies indicate that the geometry of the downgoing slab can focus upwelling or asthenospheric counterflow into a constricted band, resulting in greater volcanic volumes in the HLP as compared to the NWBR.