VOLCANIC AND STRUCTURAL SETTING OF THE HIGH LAVA PLAINS, AN INTRACONTINENTAL TRANSFORM ZONE IN THE NORTHWESTERN USA

The High Lava Plains (HLP) of south-central Oregon share volcanic and structural characteristics with transform fault zones in Iceland. Late Cenozoic (~ 12 Ma -Recent) volcanism is strongly bimodal, composed of tholeiitic basalts and high-silica rhyolites. Rhyolites focused on the High Lava Plain are systematically younger westwards, while basalts have a main pulse of activity at ~8 Ma, with activity persistent to the present. They are Fe-rich, like Icelandic rhyolites, and distinct from coeval calcalkaline rhyolites occurring more broadly in the northwest Basin and Range and Cascades. Among the Fe-rich, tholeiitic type, are rhyolites with compositions consistent with a crystal fractionation dominated history as well as rhyolites derived by partial melting of mafic crust. Persistent injection and differentiation of basalt, in concert with extension has led to development of a mafic 35-40 km thick crust.

The intense volcanism of the HLP straddles a broad array of steep NW-striking faults called the Brothers Fault zone (BFZ) at the northern terminus of major NNE-striking Basin and range faults of SE Oregon. Structural data indicate that the BFZ formed due to the combined effects of northward propagation of the Basin and Range faults and, importantly, in response to the rotational deformation field in the Cascade backarc due to the clockwise motion of the south-central Oregon block around an Euler pole near 118°W and 45°N. Such a model is consistent with the normal separation that dominates the oblique slip of the BFZ and that Basin and Range faults have small vertical separations at the latitude of the Brothers Fault Zone. An instantaneous velocity of 2 mm/yr falls within the uncertainties of GPS measurements and essentially results in a possible range of 0-2mm/yr for Basin and Range extension in Oregon at a latitude of 43°N.

We interpret the HLP to be a magmatically active (leaky) transform margin separating the northwest Basin and Range from less extended basement to the north. The HLP and BFZ merge northwestward into the Cascades arc, that itself has undergone rifting which has propagated northward since ~8 Ma. We suggest that the HLP and BFZ transfer extension from the Basin and Range into the Cascades arc in concert with northwestward motion of the Cascades forearc, slab roll-back and oblique subduction.