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National Science Foundation Graduate Research Fellowship; National Geographic Young Explorer's grant [9744-15]; Geological Society of America Graduate Student Research grant

Deflections of slip vectors of interplate thrust earthquakes from expected directions are used to estimate arc-parallel strain rates within the overriding plates at the world9s major convergent plate margins. Arc-parallel extension strain rates, between 10 −8 /yr and 10 −7 /yr and significant at 2 standard deviations, are observed in the fore arcs of the Aegean, Aleutian, Mariana, Sumatran, southern Kuriles, New Hebrides, Scotia, and southern Central American (lat 8° to 12°N) subduction zones and the Himalayas. Northern Chile (lat 17° to 31°S) and Central America (lat 11° to 18°N) show arc-parallel compression. Available geologic and geodetic estimates of fore-arc slip and strain rates agree within a factor of two with slip-vector estimates. Arc-parallel strain in fore arcs is rapid enough to produce geologically significant effects, such as unroofing of high-grade metamorphic rocks and disruption of transported fore-arc terranes. Fore arcs deform even where convergence is perpendicular to curved margins, demonstrating that head-on subduction can produce a three-dimensional strain field.

Existing trace element and isotopic data, as well as new U-series data for Nicaragua and Costa Rica, are examined in light of the recent suggestion that asthenospheric mantle is flowing northward behind the subducting Nazca plate around the northwest corner of South America. Lavas from central Nicaragua have trace element and isotopic compositions suggesting that they are derived from relatively depleted mantle with a long history of interaction with the subducting Cocos plate. Lavas from northern and central Costa Rica have compositions indicating a more enriched mantle source that has been influenced little by the subducting slab. These compositional traits vary irregularly between the two regions, suggesting that mantle compositions and subduction influences intermingle in a complex manner beneath northern Costa Rica. The dip of the subducting slab decreases abruptly southward, and accretion of sediment begins in this same vicinity. These changes in composition and physical parameters may all be induced or influenced by an influx of asthenospheric material from behind the retreating Nazca plate.