Imaging a magma plumbing system from MASH zone to magma reservoir


Delph, Jonathan R.
Ward, Kevin M.
Zandt, George
Ducea, Mihai N.
Beck, Susan L.

The Puna Plateau of the Central Andes is a well-suited location to investigate the processes associated with the tectono-magmatic development of a Cordilleran system. These processes include long-lived subduction (including shallow and steep phases), substantial crustal thickening, the emplacement of large volumes of igneous rocks, and probably delamination. To elucidate the processes associated with the development of a Cordilleran system, we pair Common Conversion Point-derived receiver functions with Rayleigh wave dispersion data from Ambient Noise Tomography. The resulting high-resolution shear wave velocity model of the southern Puna Plateau reveals the details of a lithospheric-scale magma plumbing system. Slow velocities near the crust–mantle transition are interpreted as a MASH zone (a partially molten zone where mantle-derived melts interact with the lithosphere and undergo density differentiation) with ∼4–9% melt. After differentiation, less dense and presumably more felsic melts propagate to shallower depths within the crust (∼20 km below surface) and comprise vertically (∼10 km) and laterally (∼75 km) extensive slow velocity bodies that span the frontal arc and plateau interior. These large slow velocity bodies represent a partially molten mid-crust (up to 22%) where magma can further evolve to higher silica concentrations. The periodic influx of melt from the underlying MASH zone into these mid-crustal bodies may serve as a trigger to the eruption of the voluminous ignimbrites observed in the southern Puna Plateau. Many of the active tectonic processes operating along the southern Puna Plateau are thought to be analogous to the processes that formed the North American Cordillera. Thus, these results could provide insight into some of the processes associated with the development of a Cordilleran margin.

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Fig. 1.
The seismic station distribution (yellow squares: intermediate or broadband stations; inverted triangles: short-period stations) in the southern Puna Plateau and surrounding region. Elevations >3 km represent the southern Puna Plateau (unshaded). Only stations that had high-quality receiver functions are plotted. Irregularly shaped polygons represent ignimbrite deposits colored by eruptive age, and black semi-circular lines represent mapped ignimbrite calderas (from the Andes Ignimbrite Database, Freymuth et al., 2015 and refs. therein). Black units are basaltic deposits mapped at the surface, many of which show a geochemical signature for delamination (Kay et al., 1994, Ducea et al., 2013 and Murray et al., 2015). Red star represents location of shear velocity profile in Fig. 4 within the Cerro Galan Caldera. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Earth and Planetary Science Letters