Research School of Earth Sciences
Testing the Plume Hypothesis: Laboratory models of Subduction-plume interaction for the Cascades and Tonga-Lau Convergent Margins
Chris Kincaid1, Ross Griffiths2, the High Lava Plains
Working Group3 and the Somoan Plume working group4
1 Graduate School of Oceanography, University
of Rhode Island, Narragansett, RI, USA
Subduction of lithospheric plates back into the mantle at subduction zones (ocean trenches) provides the dominant driving force for plate tectonics and causes thermal and chemical exchange with the Earth's interior. We have developed a laboratory apparatus for modeling 3D aspects of flow in subduction zones in response to various modes by which plates move and subduct into the mantle. These include slab rollback, when the slab sinks with a backward retreating or horizontal component of motion, and periods where the angle of descent (slab dip) either increases or decreases with time. Previous work has documented the importance of these modes of plate motion on 3D shallow mantle return flow and both slab and mantle wedge temperatures (Kincaid and Griffiths, 2003; 2004).
Kincaid and Griffiths are involved with two NSF funded projects to look at aspects of plume vs. non-plume models for reconciling patterns in geophysical and geochemical data collected within subduction zones. One project (the High Lava Plains project) involves a combination of efforts (seismology, geology/geochemistry and geodynamics) to better understand the evolution of the Cascades subduction system, spatial-temporal patterns in melt production and continental growth in the northwestern USA. In addition to field geology, this effort involves an a spatially detailed broadband seismic experiment coupled with a recent large scale active source seismic experiment. In terms on modeling, Kincaid and Griffiths are exploring aspects of 3D mantle flow and both thermal and compositional evolution of the mantle, and spatial-temporal patterns melt production for a subduction zone with representative plate motions for the Cascades-Pacific northwest USA system.
Plume-subduction interaction experiments show that plumes can be strongly deformed by rollback subduction and can be efficiently drawn into the arc wedge corner over large horizontal distances (~1000 km). The combination of rollback subduction and backarc extension deform the plume head and tail in a way that produces an early, circular large volume melt feature and two subsequent linear melt production features which are offset in space and time (~15 Ma). Two tracks are formed with time-transgressive rhyolite melting (e.g. reheating of the plate) and basaltic melt production, which trend in opposite directions. One is similar to the High Lava Plains in central Oregon, and the other is similar to the Snake River Plain. The initial, circular magma production feature is offset from the linear features by 300-400 km in a trench-parallel direction. The style of back-arc spreading changes the offsets and orientations of these three basic features. The influence of plate steepening is to deform the plume material into a very narrow feature. The severe, rapid deformation of plume material works to limit rise rates, essentially trapping much of this material deep in the wedge. The combined effects of increased diffusion and small length scales, partial melting and severe distortion in 3D, will make these remnant plume features difficult to image with seismic techniques.
In 2008 a URI PhD student, Ms Kelsey Druken, is working to extend these models of Cascades subduction system, including the effect of a keel-like morphology (Figure 1a) for the base of the overriding plate. An important part of the project is to also test non-plume models. Three-dimensional flow-fields (Figure 1) are being analysed in combination with temperature data and a melt production model to calculate spatial patterns in vertical heat and melt flux through time for non-plume cases. In addition, finite strain is being observed within the flow through the use of whiskers. These act as a proxy for olivine alignment within the upper mantle and are being compared with seismic anisotropy data collected from the High Lava Plains.
Kincaid, C., and R. W. Griffiths (2003) Thermal evolution of the mantle
during rollback subduction, Nature, 425, 58-62