- 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.
Keywords: high lava plains; bimodal volcanism; intraplate volcanism; Ar-Ar geochronology; Oregon geology; slab rollback.
- Ford, M. T., A. L. Grunder, and R. A. Duncan (2013), Bimodal volcanism of the High Lava Plains and northwestern Basin and Range of Oregon: Distribution and tectonic implications of age-progressive rhyolites, Geochemistry Geophysics Geosystems, 14, 2836–2857. doi:10.1002/ggge.20175
|Funding Statement (additional comments about funding)
- This work is part of the High Lava Plains Project, with support provided by the NSF Continental Dynamics program through grant EAR-0506869 (ALG and RAD).
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