A Tale of two Arcs: Wide-Angle Seismic Investigations of the Composition and Structure of the Sierra Nevada of California and the Aleutian Arc, Alaska

by M M Fliedner and S L Klemperer (Stanford University)

Two very different magmatic arcs on the Pacific margin of North America have been studied in recent years in large- scale seismic refraction/reflection experiments:

The Sierra Nevada batholith is a partially eroded Mesozoic continental arc, now situated between the modern San-Andreas transform margin and the extensional Basin and Range Province. The active Aleutian Arc is partially oceanic (Aleutian Island section) partially continental (Alaska Peninsula section). Although both formed in the same tectonic regime, their crustal structure as imaged by surface seismic is quite different.

3-D finite-difference modeling and tomographic inversion of P_g, P_n, and P_mP phases of the seismic data collected by the Southern Sierra Nevada Continental Dynamics Project show that the Sierra Nevada crust is thin (35 km, thickening by up to 7 km on the west side) and slow (<6.5 km/s). In spite of its low density (inferred from the low P-wave velocity), this crustal "root" cannot support the high topography isostatically: a lateral density contrast in the upper mantle is necessary for isostatic compensation.

Using the same techniques to analyze wide-angle recordings from a multi-channel marine reflection survey, we get a different picture in the Aleutian Arc: its crust is comparable in thickness to the Sierra Nevada although the volcanic chain is much lower than the High Sierra. Average P-velocities are higher (>6.7 km/s), increasing gradually to mantle velocities. P_mP is a less prominent phase because of the smaller velocity contrast between crust and mantle; indeed, the crust and arc-mantle are so transparent that the subducting Pacific slab shows up as a strong reflector.

Assuming that the modern Aleutian Arc is typical for magmatic arcs in general, our findings imply that arcs are not completely incorporated into the crust when they accrete to a continent: their bulk composition is more mafic than average continental crust. Parts of their lower crust must be recycled into the mantle. The Sierra Nevada in particular may have lost half of the original seismic crust by conversion of the lower crust into eclogite (seismic mantle). This petrologic root is now preserved only under the western Sierra, whereas buoyant asthenospheric mantle replaced it under the eastern Sierra providing the isostatic support for the modern mountain range.