Geology of the Crust and Mantle, Western United States

Abstract

Seismic refraction, gravity, phase velocity, and magnetic data, coupled with the geologic record, are all approximately satisfied by the structure shown in Fig. 9. A 20-kilometer crust under the Coast Ranges and Great Valley thickens to more than 30 kilometers under the Sierra Nevada and parts of the Basin and Range province; this whole area is underlain by an anomalous upper mantle with a velocity and density about 3 percent less than normal. It is not likely that the anomalous mantle extends much deeper than 50 kilometers, and the lower boundary may be gradational. The thicker crust or "root" under the Sierran highland region (Sierra Nevada and western Basin Ranges) is not limited to the Sierra Nevada proper. The root and the voluminous plustonic rocks originated in the Mesozoic era, and they constitute the now consolidated core of the Cordilleran eugeosyncline. But it must not be supposed that the root has persisted unchanged. The great mountain-building uplifts in the Cenozoic era must have been accompanied by large changes in the root and adjacent mantle. A zone of positive gravity and magnetic anomalies extending the length of the Great Valley is associated with mafic rocks of the western Sierra greenstone belt, an element of the Cordilleran eugeosyncline. Belts of maficto-intermediate lavas, accompanied by mafic and ultramafic intrusions, are marked by similar anomalies in other ancient geosynclines. An anomalous upper mantle of plagioclase peridotite, an expanded phase of the normal mantle, could explain about 1 kilometer of the uplift that took place over much of the region in Cenozoic time. To explain all of the Cenozoic uplift in the Sierra Nevada and Basin Ranges by this means would require the hypothesis of a separation of the anomalous mantle into crust and normal mantle fractions, followed by a renewal of the anomalous mantle through the action of regional convection currents or local overturning in the upper mantle. The low-velocity zones for compressional and transverse waves in the upper mantle may be related to this problem. Whatever its origin and composition, an anomalous upper mantle characterizes many regions of present or recent tectonic activity, such as Japan and the Mid-Atlantic Ridge (39). The anomalous mantle of western North America might form a continuous belt to the south, with anomalous mantle beneath the crest of the East Pacific Rise (40). The anomalous upper mantle may thus be an essential part of the heat engine driving the tectonic activity of these regions. The Basin and Range region was broken into blocks and laterally extended during the Cenozoic uplift, so that some blocks lagged behind, or sank. Some of the intricate disruption of the upper crust may be related to shallow Cenozoic volcanism. The relatively large and rigid Sierra Nevada block may have been tilted westward during Basin-Range deformation because of the high density of greenstones on the west side and the lower density of granitic rocks to the east. Man's environment, in the longer view of geologic time, is strongly influenced by mountain-building processes originating in the earth's crust and mantle. In the scale of a few lifetimes, climate, sea level, and the shape of the land are appreciably altered. How this comes about, and whether man can hope to influence the processes, are challenging, unsolved problems. But enough has now been learned about the crust and mantle to suggest precisely what questions must be answered and what critical experiments performed. Note added in proof: Osborne (42) has directed our attention to the possibility that the granitic rocks and also the andesites and dacites were formed by fractional crystallization of basaltic magma under conditions of high oxygen pressure. This possibility in no way conflicts with the geophysical data. In fact, such direct additions to the silicic upper crust from the mantle or lower crust would simplify the perplexing problem of how the crust is replenished in areas of great erosion.