T18 Multidisciplinary Studies of Cascade Volcanism and its Tectonic Setting I

Sunday, 18 October 2009
8:00 AM - 12:00 PM
E143/144

WHAT IS NEWBERRY VOLCANO?

Paper number: T18-3
Program: Technical Sessions
Time: 8:30 AM – 8:45 AM

Authors

Abstract

Newberry Volcano (NV) in central OR is variously included or excluded from the Cascades arc, described as the product of a moving hot spot, or a High Lava Plains volcano, or called a volcanic field. The km-high, shield-shaped volcanic edifice whose lavas (including extensive basalts) cover ~3000 km2 features a central caldera related to an ash-flow tuff eruption about 80 ka. Older ash-flow tuffs and widespread tephra indicate an intermittently explosive volcano that produced previous calderas. The similar Medicine Lake volcano (MLV) ~230 km to the S is also located just E of the Cascades volcanic axis. MLV is also a km-high shield-shaped volcano that produced a caldera, widespread basaltic lava flows, and numerous rhyolitic domes and flows. Petrologic studies indicate that each volcano has erupted dry tholeiitic and wet calcalkaline basalts, with the latter more common at NV. MLV is also commonly excluded from the Cascades arc and labeled instead as a Basin & Range (B&R) volcano. The center of MLV is 57 km ENE of Mt. Shasta (located W of the Cascades axis). The axis here and farther N is defined by small mafic shield volcanoes typical of much of the Cascades arc S of Mt. Rainier. The center of NV is 60 km SE of the South Sister stratocone that sits astride the Cascade axis. Farther N, in southern WA, Mt. St. Helens is located 57 km W of Mt. Adams and both are included without question among Cascade arc volcanoes.

If NV results from a traveling hot spot, then why has no such hot spot been proposed for MLV? If MLV is a B&R volcano, then where are the other MLV’s to the E? and where are the other NV’s to the E in the High Lava Plains? Does it make sense to describe a km-high volcanic edifice with a central caldera and multiple ash-flow tuffs as a volcanic field rather than a volcano? Or does the broad shield shape of MLV and NV simply go against the stereotype of what a major Cascade volcano should look like? NV, in the authors’ opinion, is a Cascade volcano located E of the arc axis where tectonically-focused crustal weakness allows magmas to erupt above the mantle wedge and downgoing slab. Mantle flow may enhance the subduction contribution and allow magmatism to be focused eastward from the main arc. Rhyolites at NV are not produced by a migrating hot spot, but rather by a combination of hydrous arc magmatic inputs, differentiation, and assimilation at NV itself.