JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 93, NO. B9, PAGES 10,081-10,094, SEPTEMBER 10, 1988 Crustal Structure of East Central Oregon' Relation Between Newberry Volcano and Regional Crustal Structure R. D. CATCHINGS U.S. Geological Survey,Menlo Park, California Department of Geophysics, Stanford University, California W. D. MOONEY U.S. Geological Survey, Menlo Park, California A 180-km-longseismic refraction transect from the easternHigh Cascades, across Newberry Volcano, to the easternHigh Lava Plains is used to investigate the subvolcanic crustal and upper mantle velocity structure there. Near-surface volcanic flows and sedimentary debris (1.6-4.7 km/s), ranging from 3 to 5 km in thickness, overlie subvolcanic Basin and Range structures. East and west of Newberry Volcano, the subvolcanicbasement(5.6 km/s) has been downwarped, producing 5-km-deep basins.The midcrust (8- to 28-km depth) is characterizedby velocitiesranging from 6.1 to 6.5 km/s and varies laterally in thicknesses. The lower crust is characterizedby an unusually high velocity (about 7.4 km/s), and its geometry mirrors the subvolcanic basementgeometry. The Moho is located at a depth of 37 km and represents a transition to an upper mantle velocity of 8.1 km/s. The shallowsubsurface (1.2 km) beneath Newberry Volcano is characterized by high-velocity(5.6 km/s, versus 4.1 km/s for the surroundingarea) intrusions and appears to be located on a basement high. Beneath the seismic refraction array at Newberry Volcano, an absence of low-velocityanomalies suggests that large silicicmagma chambers do not exist in the upper crust, but apparent high attenuation of the seismic wave field may be consistent with either partial melts in small volumes, elevated crustal temperatures, and/or poor geophone- recording site coupling. The east central Oregon velocity structureis nearly identical to that of the northwestern Nevada Basin and Range and the Modoc Plateau of northeastern California, and vari- ations in the deep crustal structureabout Newberry Volcano are consistent with tectonism involving crustal underplating,melting, and extension. If paleomagnetic estimates of extensionin the east central Oregon area are correct, the North American continent experienced significantgrowth through exten- sion,underplating, and volcanism in eastcentralOregon.Comparison of eastcentralOregon with other continental back arc areasshowssimilaritiesin crustal thickness, velocity, and heat flow. INTRODUCTION East central Oregon is divided into several physiographic provinces includingthe High Cascades, High Lava Plains,and the Basin and Range. Our study area is within this region and is bordered by the Western Cascade Range, the Blue Moun- tains and Columbia Plateau, the Snake River Plain, and the Basin and Range on the west, north, east, and south, respec- tively (Figure la). Two of theseprovinces, the Columbia Pla- teau and the Snake River Plain, along with the Yellowstone Plateau jointly form one of the world's largest continental basalticprovinces, estimated at more than 280,000 km 2 [Thompson,1977]. Similarly, much of the remainder of east central Oregon is covered by basaltic volcanic and sedi- mentary debris which make the subbasaltupper crust inac- cessible to direct geologicinvestigation.Continental volcanic plateaus, suchas thoseof the PacificNorthwestern United States, represent a key elementin understanding crust-mantle interactionsand processes of continental growth and exten- sion; yet the geologic inaccessibility, combined with the fact that little seismic exploration of the crust has been attempted, makes the subvolcanic crustal structure in these areas largely unknown. In order to investigate the regional crustal structureof east central Oregon, the U.S. Geological Survey conducted a seis- mic investigation of the crustal structurethere concurrently with an effort to investigate the local crustal structureof New- berry Volcano, a large shieldvolcanoon the eastern boundary of the High Cascade Rangeand a potential source of geother- mal energy. This eastcentralOregon(ECO) regional profile originatedin the High Cascades roughly 30 km west of New- berry Volcano and trended approximately 180 km east across the High Lava Plains (Figure 1). The ECO seismictransect was centeredon the Newberry area because, in addition to its importance as a geothermal site, NewberryVolcano is of great tectonic interest, as it is situated at the intersection of Cascade and Basin and Range tectonism, a continental arc to back arc transition. Furthermore, Newberry Volcano marks the west- ernmost point of a complex pattern of volcanismwhich pro- gressively increases in age as it trends toward the Nevada Basin and Range and toward the Snake River Plain [Christiansen and McKee, 1978]. In this paper we present crustal and upper mantle velocity and densitymodelsfor the High Cascades/High Lava Plains region of east central Oregon and discuss the relation betweenNewberry Volcano and the regional crustal structure. REGIONAL GEOLOGIC AND TECTONIC SETTING This paperis not subject to U.S. copyright. Published in 1988 by the American GeophysicalUnion. Paper number 7B 1051. The regionalgeology and tectonics of east central Oregon are dominated by continental flood basalts and extensional faulting,which are attributedto subduction-related (back arc) extension and volcanism associated with the subduction of the 10,081