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Fliedner, M.M., 1997

Making of continental crust by arc magmatism; wide-angle seismic and gravity studies in the southern Sierra Nevada of California and the eastern Aleutian arc

Bibliographic Reference

Fliedner, M.M., 1997, Making of continental crust by arc magmatism; wide-angle seismic and gravity studies in the southern Sierra Nevada of California and the eastern Aleutian arc: Palo Alto, California, Stanford University, Ph.D. dissertation, 166 p., illust., map.

Abstract

This thesis combines two independent geophysical projects studying the structure of the continental crust on the Pacific margin of North-America. The first, was a crustal seismic survey of the eastern Aleutian Islands, carried out as multi-channel marine reflection profiling with accompanying wide-angle recordings with ocean-bottom and land seismometers. The second was the Southern Sierra Nevada Continental Dynamics experiment, two deep crustal seismic profiles of the Sierra Nevada batholith and vicinity. Both wide-angle seismic data sets were analyzed with a 3-D finite-difference traveltime tomography algorithm that allows the computation of both refracted and reflected arrivals. The traveltimes of the main crustal and upper-mantle phases $\rm P\sb{g},\ P\sb{n},\ P\sb{m}P,$ and in the Aleutians also the equivalent shear-wave phases were used to derive crustal seismic velocity models including the depth to the Moho. These models were used to infer composition by comparing them with laboratory measurements on various crustal rock types and, for the Sierra Nevada, converted to density models to study the isostatic compensation of the modern topography. The overarching theme of the two studies is how much arc magmatism contributes to continental growth. The Aleutians are an active island-arc in the process of becoming part of the North-American continent. Its crust is c. 30 km thick with an average velocity of 6.7 km/s. The Sierra Nevada was a continental arc, developed landward of the subduction margin of California in the Mesozoic. Its present-day crust is up to 42 km thick with an average velocity of 6 km/s. Its topography is supported by lateral density variations in the upper mantle. The net contribution of arc magmatism to the crust is basaltic in composition, the Aleutian arc is consequently more mafic than the average andesitic continent. A possible solution to this discrepancy can be found in the Sierra Nevada. Although the Mesozoic arc was probably not much different from the Aleutian Arc, only thicker, the modern Sierran crust is more felsic than the average continent. Its former mafic lower crust most likely underwent a metamorphic phase change into denser eclogite, which is being progressively replaced by more buoyant asthenospheric mantle.

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