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].

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A relatively large density contrast between the Cenozoic and pre-Cenozoic rock that forms the continental margin of southern Peru causes gravity anomalies which outline the topography of the pre-Tertiary rock of the continental shelf of southern Peru. A coastal gravity high of +20 to +50 mgal extends from Mollendo to Lima and is associated with andesites and basalts of Mesozoic age and Precambrian gneisses and granodiorites that crop out along the coast. The gravity anomalies indicate that this coastal structural high extends nearly 100 km out to sea from the Paracas Peninsula southwest of Pisco. Two prominent gravity lows on the margin are those associated with the Pisco Basin on the shelf at lat 13°25'S and the Mollendo Basin on the slope at about lat 17° 10'S. Sediment thickness in the Pisco Basin indicated by the gravity anomalies is approximately 2.2 km, and the thickness of the sediments in the Mollendo Basin is approximately 4 km. A series of closed gravity lows occurs on the outer continental shelf and upper slope seaward of the coastal structural high. The lows outline relatively small depositional basins that have sediment thicknesses of approximately 1 km. Between lat 13° and 13°30'S, a marked change occurs in the character of the gravity field of the shelf and slope. The amplitude of the anomaly associated with the coastal structural high decreases abruptly, and a linear negative anomaly of less than-70 mgal, which extends northwestward, indicates a sedimentary basin on the upper continental slope.The northeastern end of the Nazca Ridge is isostatically compensated by a relatively thick crust whose layers have a slightly lower density than similar layers of the adjacent oceanic crust. Most of the increase in crustal thickness occurs in the basal crustal layer. Approximately 330 km west of southern Peru, the depth to the Mohorovicic discontinuity increases from approximately 10.5 km on the northeast side of the Nazca Ridge to 18 km under the center of the ridge, then rises again to 9.7 km southwest of the ridge. Measurements show that the free-air gravity anomaly along the trench axis has a maximum value 200 km northwest of the point of minimum bathymetric depth of the trench. This requires the rock of a mass column off the northeast end of the Nazca Ridge to be less dense than the 569 on July 15, 2015 memoirs.gsapubs.org Downloaded from 570 COUCH AND WHITSETT rock of a mass column under the axis of the trench for at least 200 km northwest of the Nazca Ridge, and is consistent with subduction of the Nazca Ridge beneath the continental margin of southern Peru.

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