Evolution of crust and mantle beneath the Bering Sea region: Evidence from xenoliths and late Cenozoic basalts

Abstract

Late Cenozoic volcanic centers throughout the Bering Sea region of Alaska consist of small-volume eruptions of ×ows and cones of tholeiitic to alkaline basalt. Peridotite (spinel lherzolite, websterite, wehrlite, and pyroxenite) and gabbro xenoliths are abundant in some of the most alkaline ×ows and cones. Spinel lherzolite xenoliths are characterized by granuloblastic-equant and coarse-equant textures typical of mantle peridotites and yield equilibration temperatures of 1000-1200 °C (1.5 GPa). Relative to other spinel lherzolites worldwide, these xenoliths have low MgO and high Fe 2 O 3 T and are similar to some modeled compositions for fertile mantle. However, these xenoliths are relatively enriched in the light rare earth elements (REE), Th, U, K, Nb, and Ta. Rare amphibole peridotite xenoliths have textures and compositions that are consistent with an origin as pyroxenite veins in the mantle that were variably deformed and metasomatized with potassium-rich ×uids. These xenoliths have concave-up light REE patterns similar to those of the host basalts, suggesting that the alkaline melts may have been produced by melting of amphibole-bearing mantle peridotites. Gabbroic xenoliths are variably deformed and recrystallized at relatively high pressure (0.4-0.6 GPa). Trace element ratios (e.g., Ba/Nb, La/Nb) indicate that these xenoliths crystallized from melts that were produced in a suprasubduction-zone setting and suggest that they represent cumulates that formed in the lower crust during Late Cretaceous subduction. We suggest that these cumulates are the source of the strong re×ec-tions that are observed in seismic re×ection proµle images of the Bering Sea region.Volcanic ×ows consist of tholeiite, olivine tholeiite, alkali-olivine basalt, and basanite, whereas cones consist mostly of alkali-olivine basalt and basanite. Geochemical variations within each volcanic µeld largely re×ect the effects of fractionation and melting (depth and fraction of melting), whereas between-µeld geochemical differences are largely due to source composition differences. The enriched incompatible trace element compositions (light REE, Ba, Th, Nb, Ta) and ratios (Th/Ta, Rb/Zr, Nb/La, La/Yb) are similar to intraplate basalts from ocean island and continental extensional settings. In particular, the compositions of Bering Sea basalts are similar to those of the diffuse volcanic province of Southeast Asia, a region where late Cenozoic volcanism has been related to tectonic extrusion along reactivated strike-slip faults.

A new Late Cretaceous to Present APWP for Asia and its implications for paleomagnetic shallow inclinations in Central Asia and Cenozoic Eurasian plate deformation

“…We demonstrate the compatibility of our model with Cenozoic Arctic Ocean opening along the Gakkel Ridge, and also the consistency between inferred Siberia-Alaska convergence with transpressive right-lateral tectonics of the Brooks Range in Arctic Alaska (e.g. Wirth et al, 2002;Golonka et al, 2003). far as northern Alaska.…”

“…This convergence appears to be oblique between the Chukotka subplate (Golonka et al, 2003;Fig this range (e.g. Wirth et al, 2002;Golonka et al, 2003), and (2) the Cenozoic transpressive deformation along the Moma rift system (e.g. Gaina et al, 2002).…”

Seismic and Potential Field Constraints on Upper Crustal Architecture of Inner Bering Shelf, Offshore Southwestern Alaska

“…The Kaltag and the Fairweather faults postdate the era of terrane accretion, which was mostly middle Cretaceous in the area of these faults (Klemperer et al, 2002a). Many of the volcanic centers in western Alaska occur near the ends of the surface expressions of major strike-slip faults or along their projected traces (Wirth et al, 2002); for example, Figure 1 shows the Quaternary Togiak basalt field at the western terminus of the Denali Fault and the Saint Michael volcanic field near the end of the Kaltag Fault. The Denali fault system extends ~1500 km from western Canada to near the edge of the Bering Sea.…”

“…Major onshore strike-slip faults (e.g. Denali and Tintina) whose orientation varies from southeast to southwest (Wirth et al, 2002) can be clearly identified on geological maps of southwestern Alaska. Though several models (Cross and Freymuller, 2008;Finzel (2011)) of Alaskan evolution have been proposed, it remains uncertain whether these faults (1) extend offshore across the entire Bering shelf from northeast to southwest, (2) rotate to form a trend parallel to the shelf edge, or (3) terminate under the inner Bering shelf.…”