Volcanism in the eastern Aleutian arc: Late quaternary and holocene centers, tectonic setting and petrology

Abstract

Cale-alkaline volcanism and oceanic plate subduction are intimately linked in the eastern Aleutian arc. The volcanic arc is segmented: larger caldera-forming volcanic centers tend to be located near segment boundaries. Intrasegment volcanoes form smaller stratocones. Ten of the 22 volcanoes that make up the 540 km long volcanic front in the eastern Aleutian are have erupted in recorded history and another six show hydothermal activity. The geometry of the Benioff zone in the eastern Aleutian arc has been defined by earthquake data from a local, high-gain short-period seismograph network. The Benioff zone dips at an angle of about 45° beneath the volcanic arc and reaches a maximum depth of 200 km. Based on the alignment of volcanoes, the eastern Aleutain arc can be subdivided into two main segments, the Cook and Katmai segments. A misorientation of 35° of the two segments reflects a change in strike of the underlying Benioff zone and implies a lateral warping of the subducting plate. The Cook segment volcanoes line up closely on the 100 km isobath of the Benioff zone. The Katmai segment volcanoes lie on a cross-cutting trend with respect to the strike of the underlying Benioff zone. Depths to the dipping seismic zone beneath volcanoes of the Katmai segment vary by 25% from 100 to 75 km. In the Katmai segment there is also good geophysical evidence that crustal tectonics plays an important role in localizing volcanism. Narrowly spaced linear groups of volcanoes appear to be positioned over a deep crustal fault that underlies the volcanic front. Transverse arc elements divide the arc into subsegments and localize larger magma reservoirs at shallow levels in the crust. Intrasegment volcanoes in both the Cook and Katmai segments erupt andesite and minor dacite of remarkably uniform composition despite differences in depths to the Benioff zone. Segment boundary volcanoes erupt lavas with a wider range of compositions (basalt to rhyolite) but are still calc-alkaline, in contrast to volcanoes in similar tectonic settings near segment boundaries in the central Aleutains. Greater crustal thickness in the eastern Aleutian arc, coupled with structural traps in the crust, allow magma ponding at shallow crustal levels. Differentiation at shallow depths yields dacite and even rhyolite.