Shallow Curie‐point isotherm depths, indicated by the analysis of magnetic anomalies, young silicic volcanism, hot springs, and high heat flow mark the Cascade Range of central Oregon as a potentially important geothermal resource area. Aeromagnetic measurements in the central Cascades between 43°00′ and 44°15′N latitude and 121°00′ and 122°30′W longitude exhibit predominant northwest‐southeast lineations and less prominent north‐northeast by south‐southeast lineations. Longer wavelength components of the magnetic anomalies (greater than 25 km), possibly related to deeper sources, trend approximately north‐south parallel to the Cascade Range. Magnetic anomalies show a right‐lateral offset of the High Cascades Volcanism at 43°30′N latitude, which is consistent with the tectonic structures of the Basin and Range province southeast of the area. Magnetic source depth calculations show that the High Cascades occupy a structural depression or graben on the eastern side of the Western Cascades. Spectral analysis of the magnetic anomalies suggests that the High Cascades are underlain by Curie‐point isotherm depths as shallow as 9 km. These shallow Curie‐point isotherm depths imply thermal gradients greater than 50°C/km and surface heat flow greater than [Formula: see text].
Seismic and gravity data were used to construct crustal and subcrustal sections from the Tufts Abyssal Plain, west of the coast of Oregon, to southern Idaho. Nearly the entire area appears to be in isostatic equilibrium. Mantle depths west of the Juan de Fuca Ridge, in eastern Oregon, and in southern Idaho are normal for the surface elevations or water depths at these locations, and the corresponding mantle densities and velocities appear to be 'normal'. The mantle under the Juan de Fuca Ridge and the Cascadia Basin (west of Oregon) is relatively shallow and its density appears to be below normal to a considerable depth. Two different mantle structures are considered that are consistent with the seismic and gravity data. Magnetic data were not found to be diagnostic in differentiation between these two structures.Across the continental margin the Mohorovicic discontinuity dips sharply under the continents, being at a typical depth of 18 km near the coast line. The discontinuity is relatively shallow under western Oregon (as has been proposed previously), and the corresponding mantle is described in terms of apparent velocity and density.
Gravity measurements obtained with a surface‐ship gravity meter operated over a non‐linear response range are in error when vertical accelerations are substantial. Errors in measurement due to the effect of vertical accelerations were calculated for all gravity observations that Oregon State University made during the period 1963–1965. The root‐mean‐square (rms) corrections applicable to these measurements were determined for two different non‐linear responses of LaCoste and Romberg gravity meter S‐9. The rms error in free‐air anomalies corrected for vertical accelerations, based on statistical analyses of differences of crossings of track lines, comparisons with values at nearby pendulum stations, and errors in anomalies over the Newport (Oregon) Gravity Range, was ±5½ mgal when navigation was determined with Loran A.
Crustal thickness determinations were made from seismic refraction measurements along two profiles exLending north-northwest along the Inside Passage of Southeast Alaska and western British Columbia. The source for the seismic waves was a series of 10 explosions detonated by personnel of the Earth Physics Branch. 1)epartment of Energy, Mines and Resources of Canada. Arrivals recorded in Southeast Alaska from explosions in Bird Lake on Graham Island indicate a 26 km thick crust over a mantle with a P,, velocity of 7.9 km/s. Measurements of P,, wave attenuation suggest that the uppermost mantle has a (;! value of 650 z t 120. Arrivals recorded along the mainland coast of British Columbia from explosions in Ripley Bay indicate a 30 km thick crust over a mantle with a PI, velocity of 8.1 km/s. On the basis of compiled gravity and seismic refraction data, the crust of the lnside Passage region from Queen Charlotte Sound to Cross Sound is believed to be anomalously thin. Crustal thickening from ocean to continent appears to occur in a stepwise, rather than continuous, fashion in the vicinity of the Inside Passage. --L'dpaisseur de la croilte a ttd CvaluCe par des mcsures obtenues par rkfraction stismiyue selon deux profiles vers le NNW, suivant Inside Passage au sud-est de ]'Alaska et ri l'ouest de la Colombie-Britannique. Une skrie de 10 explosions prkparkes par le personnel de la section Physique de la 'Terre, dkpartement de l'Energie, des Mines el dcs Kessources du Canada, crbaient les ondes siismiques. Ides arrivkes enregistrees clans le sud-est de 1'Alaska 2 partir d'explosions au lac Bird sur 1'Ile Graham, indiyuelat une kpaisseur de 26 km pour la croi~te recouvrant le manteau, avec une vitesl;e PI, de 7.9 km/s. Les mesures de l'atttnuation des ondes PI, sugg6rent une valeur Q de 650 2 120 pour la partie supCrieure du manteau. Les arrivdes enregistrkes lc long de la cate de la Colombie-Rritannique et provoyilCes par les explosions B la Baie Ripley, montrent une epaisseur de la croilte de 30 km au-dessus du manteau et avec une vitesse PI, de 8.
A surface‐ship gravity survey was made over the eastern part of the Mendocino escarpment. Free‐air and Bouguer gravity anomaly maps are presented and, also, three hypothetical crustal and subcrustal cross sections over the escarpment (using published refraction data for control). East of about 130°W the Mendocino escarpment is found to be an escarpment in the Mohorovicic discontinuity; north of the escarpment there appears to be a thick low‐density upper mantle layer which is interpreted as being chemically different from that south of the escarpment. The escarpment appears to be a result of the juxtaposition of two different density mantles, the entire region being essentially in isostatic equilibrium. West of about 130°W the escarpment is relatively small, and the mantle densities on both sides of it appear to be nearly the same. The significance of the gravity data is considered in terms of previous interpretations of the region.
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