Wide-angle seismic reflections as direct indictors of partial melt beneath an andesite arc
Abstract
Quasi-linear arcs of andesite volcanoes that erupt above 120 ± 40 km deep Waditi-Benioff zones are one of the most striking surface manifestations of plate tectonics. The prevailing view is that andesite magma that feed these arcs are an end-product of fluid-assisted melting in the mantle wedge1. Two key questions remain unresolved: why is the line of an active arc so sharply defined? and where do parental mantle-derived melts pool and differentiate into mafic restite and more felsic magma components that are so distinctive of continental magmatic arc systems ? Here we present seismic evidence from the MORC (mantle or crust?) project for a focused zone of melt below the andesite arc within northern New Zealand. Nine ~ 0.4 - 1.3 tonne dynamite shots were recorded on ~ 700 seismographs spaced along a 120 km long array across, and normal to the axis of the Taupo Volcanic Zone (TVZ). Two important features of the ray tracing solution are: (i) a lower-crustal, 10 km thick, "rift-pillow" of rocks where the P-wave seismic velocities (Vp) are ~ 6.8-7.1 km/s. Seismic reflections from top and bottom of the pillow are termed R1 and R2 respectively. (ii) Strong seismic reflections (termed R3 reflector) are recorded from a relatively short (12 -18 km-long) reflector at a depth of ~ 32 ± 2 km. An analysis of interval velocities between the R2 and R3 reflectors show a P-wave speed of 7.6 +/- 0.2 km/s, which is interpreted as anomalous upper mantle. Thus R3 is regarded as being a reflector within the upper mantle. Ray-tracing locates R3 beneath the eastern margin of the TVZ and directly beneath the active volcanic (andesite) front and geothermal fields. A causal relationship between the R3 reflector and the andesite arc is therefore implied. The amplitude ratio of the R3/R2 reflections along common traces is as high as 4-6 for incidence angles of reflection of 45-60 degrees. These data suggest R3 is best explained with numerical methods by an interface across which there is a ~ 80% drop in the S-wave seismic velocity (Vs) and a modest (10%) drop in Vp. Such a change in wave-speeds is consistent with R3 representing the upper surface of a volume where the partial melt content may be as high as 12%. We propose this volume to be primitive basaltic melt from the mantle wedge that has penetrated under the arc into a relatively narrow zone and has become stalled at a thermal boundary layer in the upper mantle. Fractionation of the mantle melt is proposed to occur at this level.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.S34B..05S
- Keywords:
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- 7240 SEISMOLOGY / Subduction zones;
- 8178 TECTONOPHYSICS / Tectonics and magmatism;
- 8185 TECTONOPHYSICS / Volcanic arcs