Subduction-triggered magmatic pulses: a new class of plumes?

C. Faccenna¹, T.W. Becker², S. Lallemand³, Y. Lagabrielle³, F. Funiciello¹ & C. Piromallo⁴

¹Dip. Scienze Geologiche, Università “Roma Tre” and  CNR, IGAG-Rome, Italy, faccenna@uniroma3.it ; ffunicie@uniroma3.it

²Dep. Earth Sciences, University Southern California, Los Angeles, USA, thorstinski@gmail.com

³Laboratoire Geosciences Montpellier, Université Montpellier II, CNRS, France, serge.lallemand@gm.univ-montp2.fr ; yves.lagabrielle@gm.univ-montp2.fr

⁴Istituto Nazionale di Geofisica e Vulcanologia, Roma, Italy, piromallo@ingv.it

This webpage is a summary of: Faccennaa, C., T.W. Beckerb, S. Lallemandc, Y. Lagabriellec, F. Funiciellod and C. Piromallo, Subduction-triggered magmatic pulses: A new class of plumes?, Earth Planet. Sci. Lett., 54-68, 2010.

Background

A variety of thermal plumes - splash (Davies & Bunge, 2006), baby (e.g., Wilson & Dowes, 2006) and edge (King & Ritsema, 2000) plumes - have been recently described as focused vertical upwellings that are not directly rooted in the lower mantle, as predicted by the original hotspot model (Morgan, 1971). Moreover, high-resolution seismological images have shown many apparent small-scale convective heterogeneities in the uppermost mantle at margins such as the western US (e.g., Sigloch et al., 2008; West et al., 2009) and the Mediterranean (Faccenna & Becker, 2010). Small scale mantle flow could explain also the velocity distribution beneath the Rio Grande Rift  (van Wijk et al., 2008), the uplift and magmatism of the Colorado plateau (Roy et al., 2009) and extension beneath the Great basin (West et al., 2009). Subcontinental small-scale convection may be excited by sharp temperature gradients at a craton’s edge, where decompression melting may cause volcanism (King & Anderson, 1995). In this work, we suggest an indirect connection to slab return flow, which may interact with a hydrated layer in the transition zone (Lehay & Bercovici, 2007) to facilitate localized upwellings.

Subduction-related upwelling

In several regions, there is evidence for volcanism that is spatially and temporally connected to subduction zones but not associated with mantle wedge melting. Related magmas show ocean island basalt (OIB)-type signatures developing from volcanoes located either far off the arc, ahead of the trench, or at slab edges. Relationships between subduction and anomalous volcanism, though already postulated to explain regional cases of intraplate magmatic activity (e.g., Changbai volcano, East Asia: Zhao et al., 2009; New Hebrides-North Fiji: Lagabrielle et al., 1997; Mediterranean-European: Goes et al., 1999, Piromallo et al., 2008, Lustrino & Wilson, 2007), have never been framed and modeled in a subduction-related convecting system.

We demostrate that subduction within the upper mantle triggering return flow can generate focused, sub-lithospheric, non-thermal mantle upwellings.  The time-dependent evolution of subduction-induced mantle circulation has been explored running 3D dynamically self-consistent numerical models using the finite element code CitcomCU (e.g., Zhong et al., 1998; Moresi & Solomatov, 1995). In the adopted simplified setup, an old single slab fixed in the far field sinks into the mantle. This is approximated by a visco-plastic material with temperature-dependent viscosity. The evolution of slab sinking into the mantle (Figure 1) shows a progressive increase of the subduction velocity during the fall of the slab into the upper mantle (Becker et al., 1999). The sinking slows once the slab reaches the base of the upper mantle and it temporarily ponds where the viscosity increases (Funiciello et al., 2003). Subsequently, the subduction process is taken up by trench rollback. As discussed by Funiciello et al. (2004), the pattern of mantle circulation is strongly variable during the three aforementioned evolutionary stages. To describe better subduction-induced mantle flow, it is instructive to perform decomposition into the toroidal and poloidal components (e.g., Tackley, 2000).

Figure 1: Evolution of the reference numerical model. Each panel is composed of two parts. The upper part shows the lateral cross-section of the model taken through the middle of the plate. The color plot gives the magnitude of the non-dimensional lithospheric temperature, assuming the initial mantle temperature is fixed at 1. Arrows illustrate the x-z flow pattern in the mantle. The lower part shows the horizontal cross-sections taken at x km depths from the top. In this case, the color plot gives the magnitude of the vertical velocity component. Arrows illustrate the x-y flow pattern in the mantle. Non-dimensional units and unity time correspond to ~15 Ma. Click here or on Figure for enlargement.

Our modelling shows that the rapid sinking of the slab during the initial stages of subduction, with a dominance of poloidal flow, induces a convective cell ahead of the slab with a wavelength on the order of the upper mantle thickness. The maximum vigor of the poloidal flow is attained just before the slab encounters the 660-km discontinuity. The toroidal/poloidal ratio (TPR) is less than 0.5 (for selected model parameters) during slab subduction into the upper mantle and reaches maximum values on the order of 0.6 after the slab interaction with the 660-km discontinuity. The poloidal cell involves upwelling components:

These upwelling mantle currents, combined with decompression melting, could lead to a thinning of the lithospheric thermal boundary layer and explain the existence/timing of:

Combining modeling results with natural data from  key regions - western North America (Figure 2), the Central Mediterranean, the North Fiji/Lau Basin, the West Philippine basin, Mt. Etna (Figure 3) and others, we confirmed the physical relationships between subduction and anomalous volcanism.

Figure 2: a) Map of Western North America (redrawn from Xue & Allen, 2007). Three main magmatic provinces can be recognized: the Columbia River Basalts, the Yellowstone Hotspot Track (with red showing the location of magmatic centers and related ages) and the High Lava Plains of Oregon, or Newberry Hotspot Track (with red showing the location of magmatic centers and related ages, and black thin solid lines indicating the location of magmatic centers and related ages). Dashed lines show the depth of the subducting lithosphere. MC indicates McDermitt Caldera. The A-B line indicates the cross-section of panels b-g. b) The tomographic cross-section along the Western North America margin from Li et al. (2008). c-g) Reconstruction of the evolution of the Juan de Fuca subduction zone and of the position of the volcanic centers back over the last 17 Ma, starting from the distribution of the velocity anomaly in panel b). Arrows indicate the net motions of the subducting plate, the overriding plate and the trench. The age of the volcanic centers is expressed in Ma. This shows that the emergence of volcanism occurred just as the slab arrived at the 660-km discontinuity and that the position of the volcanism lines up with the slab tip (Figure 2b). Based on seismological observations and plate-tectonic reconstructions, we speculate that a mechanism of subduction-driven upwelling, similar to the one shown in Figure 1, could be adapted to the Yellowstone case. We also speculate that the massive onset of volcanism in the surrounding area could be triggered by a shallow, upper-mantle source, perhaps triggered by the separation of the oldest portion of the slab and the onset of the new subduction cycle (Sigloch et al., 2008).

Figure 3: Mantle structure beneath and around Mount Etna from the PM01 model (Piromallo & Morelli, 2003). Note the low velocity anomalies located around the Calabrian slab. a) Sections at 150 and 250 km depth. b) Topography and elevation of the MIS 5.5 marine terraces along the section (from Ferranti et al., 2006) illustrating uplift in correspondence of Mount Etna. c) AA' cross section shown in a) of the seismic tomography model, showing a marked low velocity anomaly around the slab. Our modeling results show that the contradiction between these classes of models, slab-induced asthenospheric flow and the hot spot model, is only just apparent. We propose that Mt. Etna can be considered an upwelling structure confined in the upper mantle as a consequence of the complex 3-D mantle circulation triggered by the subducting lithosphere.

Summary

The combination of numerical modeling and tectonic reconstructions reveals (Figure 4):

Figure 4: Cartoons showing scenarios able to trigger decompression melting produced by slab return flow, and the resulting development of off-axis volcanism. a) Slab-mantle interaction; b) Steady-state condition of subduction with the slab lying on the 660 km discontinuity.

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