2. Mineralogy – Petrology – Geochemistry - SWISS GEOSCIENCE ...

48
Symposium 1: Structural Geology, Tectonics and Geodynamics
1.B.5
Crustal Growth at Active Continental Margins : Numerical studies
Guizhi Zhu*, Taras Gerya*, Paul Tackley*, Eduard Kissling *
1 Department of Earth Sciences, ETH Zurich, Sonneggstrasse 5,CH-8092 Zurich (guizhi.zhu@erdw.ethz.ch)
Active margins are important sites of new continental crust formation by magmatic processes related to the subduction
of oceanic plates. We investigate these phenomena using a three-dimensional coupled petrological-geochemical-thermomechanical
numerical model, which combines a finite-difference flow solver with a non-diffusive marker-in-cell technique
for advection (I3ELVIS code, Gerya and Yuen, PEPI,2007). The model includes mantle flow associated with the
subducting plate, water release from the slab, fluid propagation that triggers partial melting at the slab surface, melt
extraction and the resulting volcanic crustal growth at the surface. The model also accounts for variations in physical
properties (mainly density and viscosity) of both fluids and rocks as a function of local conditions in temperature, pressure,
deformation, nature of the rocks, and chemical exchanges.
Our results show different patterns of crustal growth and surface topography, which are comparable to nature, during
subduction at active continental margins. Often, two trench-parallel lines of magmatic activity, which reflect two maxima
of melt production atop the slab, are formed on the surface, see Figure 1. The melt extraction rate controls the patterns
of new crust at different ages (Zhu et al., Journal of Earth Science,2011). Moving free water reflects the path of fluids, and
the velocity of free water shows the trend of two parallel lines of magmatic activity. The formation of new crust in particular
time intervals is distributed in finger-like shapes, corresponding to finger-like and ridge-like cold plumes developed
atop the subducting slabs (Zhu et al., G-cubed,2009; PEPI,2011). Most of the new crust is basaltic, formed from peridotitic
mantle. Granitic crust extracted from melted sediment and upper crust forms in a line closer to the trench, and its distribution
reflects the finger-like cold plumes. Dacitic crust extracted from the melted lower crust forms in a line farther
away from the trench, and its distribution is anticorrelated with the finger-like plumes.
Figure 1. Topography evolution at (a) 7.2 Myr and (b) 13.5 Myr since the beginning of the model calculation and new crust development
Swiss Geoscience Meeting 2011
Platform Geosciences, Swiss Academy of Science, SCNAT