Crustal structure from pacific basin to central Nevada

Seismic refraction, gravity, and phase velocity data are generally consistent with an interpretation of crust and upper mantle structure from the Pacific basin across the edge of the continent, the Coast Ranges, the Great Valley of California, the Sierra Nevada, and part of the Basin and Range province. The derived structure consists of a thin oceanic crust beneath the Pacific basin, thickening at the continental slope to a little more than 20 km under the Coast Ranges and Great Valley, and thickening further under the Sierra Nevada and Basin and Range province. The ocean basin is underlain by a normal high-velocity mantle, but the entire continental area is underlain by an anomalous upper mantle whose velocity and density are about 3% less than normal. The anomalous mantle extends to a depth of about 50 km and its lower boundary may be gradational. The crust under the Sierran highland region (Sierra Nevada and western Basin Ranges) is thicker than that in the area to the east; this ‘root’ of the highland is not limited to the Sierra Nevada proper. The root and the voluminous plutonic rocks constitute the core of the Cordilleran eugeosyncline. The root derived from a simple gravity model is about 32 km deep and, from seismic refraction data, more than 40 km deep. To reconcile the two a denser root is required, and it is suggested that this represents a residue of partial melting to form the plutonic rocks. The anomalous upper mantle, which is probably composed of plagioclase peridotite derived by a phase change from garnet peridotite or pyroxene peridotite, can explain no more than about 1 km of the Cenozoic uplift in the region. To explain greater uplift by this means requires renewal of the anomalous mantle, possibly by differentiation into basalt and peridotite followed by convective overturn to bring fresh garnet peridotite to the upper mantle. An anomalous upper mantle characterizes many regions of recent tectonic activity. Superimposed upon the regional gravity anomalies are anomalies caused by large anomalous masses in the upper part of the crust: (1) a negative anomaly of about 50 mgal caused by sedimentary rocks of the Great Valley, (2) positive anomalies associated with the ‘greenstone belt’ beneath the Great Valley and the western Sierra Nevada, (3) negative anomalies due to granitic batholiths, and (4) a negative anomaly suggesting thick sedimentary rocks on the continental slope. The positive gravity and magnetic anomaly of the greenstone belt is comparable with anomalies marking the mafic belts of other geosynclines.