A magnetotelluric study of the High Cascades graben in central Oregon (original) (raw)

1989, Journal of Geophysical Research

We have analyzed wideband magnetotelluric data from three east-west transits across the Central Oregon High Cascades. The data were modelled using preliminary onedimensional inversions of rotationally invariant parameters followed by conventional two-dimensional analysis. The resulting electrical profries show offsets at the boundaries of the High Cascades that are consistent with downfaulting beneath the High Cascades Graben. Conductive and resistive layers in the upper crust are interpreted in terms of the known and inferred volcanotectonic history of the range. A pervasive conductor is present in the lower crust. It deepens eastward across the Western Cascades, High Cascades, and adjoining Deschutes Plateau and may indicate concomitant eastward crustal thickening. Regional heat flow and gravity studies have been interpreted as evidence of a transition from thinner crust on the west to thicker crust on the east. This transition may mark a "suture" between pre-Tertiary continental crust and an accreted oceanic terrane that underlies western Oregon, or it may indicate the westward limit of the anomalous lithosphere that is thought to underlie the Basin and Range province. Introduction As part of the EMSLAB experiment [EMSLAB Group, 1988], the University of Oregon geophysics group has collected wideband magnetotelluric (MT) data at more than 40 sites along the main EMSLAB transect, extending from the Oregon coast to central Oregon and crossing the crest of the Cascade Range at Mount Jefferson (Figure 1). Results from the Coast Range portion of the transect are reported elsewhere [Waff et al., 1988]. Briefly, data from the Coast Range reveal the presence of a conductive layer located at a depth of 22-32 km that dips downward to the east at an angle of about 15 ø. We believe that this eastward dipping conductor is the result of fluids present at and/or near the thrust surface of the subducted Juan de Fuca plate. In this paper, we report our findings from the High Cascades portion of the EMSLAB transect. We have included data from two other transects across the High Cascade Range to the south of the main EMSLAB transect (Figure 1). Together, these three transects provide an electrical profile of a major volcanotectonic feature known as the High Cascades Graben. Cascade Range Geology From late Eocene to middle Miocene time, the oregon Cascade Range developed as a fairly typical continental margin volcanic arc related to subduction of the Farallon/Juan de Fuca plate beneath the North American continent. During this span of time, the volcanic arc, along with a forearc basin underlain by a block of accreted oceanic crust, rotated clockwise at an average rate of lø-2ø/m.y. about a pole that was probably located near the present-day course of the lower Columbia River [Magill and Cox, 1980]. Magmas erupted during this time consisted predominantly of calc-alkaline andesitic basalt to