Introduction

Oregon is located on the North American Plate. The Juan de Fuca plate is subducting beneath the North American Plate from the west. The Newberry volcanic track, located in Oregon, is consisted of a sequence of volcanic domes and lava flows, that started near the McDermitt Caldera and have gradually moved westward to Newberry over the last ~12 million years. Question 1: Although the slab has been imaged to depths of ~300 to 400 km beneath Washington, there is little evidence for the presence of a slab in the mantle east of the Cascades beneath Oregon. The question is however, does the Juan de Fuca slab extend down into the mantle east of the Cascade beneath Oregon, or does it break up and disintegrate due to its young age or the passage of the proposed Yellowstone Plume? Question 2: The Newberry volcanic track orients at ~120 degrees to the North American Plate motion. While mantle plumes are often called upon to explain hotspot tracks, the Newberry hotspot track cannot be the product of plate motion over a stationary mantle source. Then what is the causal mechanism of the Newberry track?

The Deployment of the OATS

The Oregon Array for Teleseismic Study (OATS) consists of 12 broadband stations, eleven of which （Guralp CMG3 T series) were deployed in May 2003 by Prof. Allen's research group and the other one (OT02) is the permanent GSN station COR. The line extends NW from the McDermitt Caldera along the Newberry hotspot track to the coast.

Now all stations have been pulled out in the early May 2006 except.OT01 and OT03 which are still in operation to study tremors related with the Cascadia subduction zone. See some photos from field work...

OATS newsletters

-- Newsletter 5 - July 2006 pdf
-- Newsletter 4 - October 2005 pdf
-- Newsletter 3 - October 2004 pdf
-- Newsletter 2 - March 2004 pdf
-- Newsletter 1 - June 2003 pdf
Note: Drew is the editior of Newsletter 1, and Mei is the editior for all others

Velocity structure (JdF07-S) along the OATS from seismic tomography

The dataset from OATS was complemented by data from 9 permanent networks and 3 temporary deployments. The permanent networks are Berkeley Digital Seismograph Network (BDSN), Cascade Chain Volcano Monitoring (CC), Global Seismograph Network (GSN), Laser Interferometer Gravitational-Wave Experiment (LIGO), Princeton Earth Physics Project-Indiana (PEPP), Pacific Northwest Regional Seismic Network (PNSN), USArray Transportable Network (TA), University of Oregon Regional Network (UO), and the United States National Seismic Network (USNSN). The temporary deployments are Eastern Snake River Plain Experiment (ESRP), Wallowa Mountains (Wallowa), and the Grants Pass Well Experiment (Grants Pass ).

Contributions to Question 1 and 2: Our tomographic images clearly show that the Juan de Fuca plate dives into the mantle beneath Oregon and continues east of the High Cascades with a dip of ~50º reaching a depth of ~400 km but no deeper. The low-velocity belt beneath the slab in our Vs model is a feature not commonly observed beneath subducting slabs. Resolution tests show that the low velocity zone is not an artifact and can be resolved by our dataset. We propose that the absence of the slab below 400 km today is due to the arrival of the Yellowstone plume head ~17 Ma, which destroyed the Juan de Fuca slab at greater depths. The low velocity anomaly beneath the slab is likely the remnant plume head material. We also imaged low velocity anomaly beneath the Newberry volcano reaching a depth of only ~150 km.

Anisotropy structure from SKS splitting

Contributions to Question 2: SKS splitting observations across Oregon suggest a simple pattern of anisotropy which varies smoothly across the region. We infer a single layer of anisotropy beneath the Newberry Hotspot Track that is most likely in the asthenosphere. The fast directions are oriented ENE-WSW to the west of the Cascades which is consistent with mantle shear parallel to subduction of the Juan de Fuca Plate. To the east, fast directions are more E-W, perhaps due to shear caused by Basin and Range. Since the observed fast directions are not parallel to the Newberry track, it is unlikely that asthenospheric flow is responsible for the age-progression volcanism. Instead, we suggest the Newberry track is the product of a lithosphere-controlled process. While progressive lithospheric faulting and the extension of Basin and Range have also been called upon to explain the formation of the Newberry track previously, supporting field evidence is still lacking.

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