3-D structure of island arcs and continental growth

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The accretion of oceanic arcs to continental margins has been a key contributor to continental growth since the Late Archean, and the remnants of island arcs, which are often the focus of mineral exploration activity, have been identified in several Canadian regions. However, the mechanisms by which arcs are accreted and the continental crust becomes stabilised are poorly understood. If island arcs contain significant volumes of continental-style crust, then continental growth through direct arc accretion may be feasible if the more mafic lower arc crust can be removed by some form of presently unknown delamination process. Alternatively, mafic arc crust requires syn- or post-accretion modification to produce the intermediate composition continental crust that exists today. An integrated multichannel seismic reflection and 3-D refraction survey, the only true 3-D survey yet acquired over an island arc, extends more than 300 km along the archetypical Mariana island arc, and is the focus of this proposal. One objective is to determine quantitatively the 3-D distribution of felsic-intermediate composition crust within the Mariana arc, and its relation to the underlying mafic lower crust. Variations of 5-10 percent in the volume of intermediate composition arc crust significantly alter petrologic predictions of the volumes of associated mafic restite and cumulates that require subsequent removal before or during arc-continent collision. A second objective is the identification using the high-quality multichannel seismic data of reflectors beneath the arc associated with arc magmatism, delamination of the lower arc crust, and the arc's structural evolution, e.g. arc rifting, extension, and subsidence. Waveform tomography will also be used to map the variability in composition of both volcanic rocks and intrusions in the shallower sections of the arc, and to determine their along-strike variability on a broader scale than is possible by geological sampling. At the largest scale, this project will constrain the process by which the proposed foundering of deep arc crust occurs every few million years.