Flat‐slab subduction, topography, and mantle dynamics in southwestern Mexico

Abstract

Topography above subduction zones arises from the isostatic contribution of crustal and lithospheric buoyancy, as well as the dynamic contribution from slab‐driven mantle flow. We evaluate those effects in southwestern Mexico, where a segment of the Cocos slab subducts horizontally. The eastern part of the volcanic arc—the Trans‐Mexican Volcanic Belt—stands at an average elevation of 2.3 km, nearly 1.3 km above the fore‐arc. Lateral changes in bulk crustal density are relatively small, and seismic imaging shows that there is little variation in crustal thickness between these two regions. Thus, the elevation difference between the arc and the fore‐arc should arise from differences in mantle properties. We present finite element models of flat‐slab subduction that provide a simultaneous match to topography, plate velocities, and stress state in the overriding plate. We find that the dynamic effects are primarily controlled by the amount of coupling at the subduction interface and in the mantle wedge, the lack of slab anchoring into the lower mantle, and the absence of continental mantle lithosphere. With a mantle wedge and a subduction interface that are, respectively, 2 and 4 orders of magnitude weaker than the asthenosphere, the flat slab exerts a downward pull that can explain most of the elevation difference between the fore‐arc and the arc. We infer that lateral viscosity variations play a significant role in shaping dynamic topography in complex tectonic settings and that sublithospheric dynamics can influence the topography at wavelengths that are significantly shorter than previously recognized.

The dynamical control of subduction parameters on surface topography

“…Slab length varies strongly in nature between different subduction zones and through time and might have an important impact on surface topography through, for example, slab anchoring in the lower mantle [see e.g., Gérault et al ., ]. Here, the purely geometric aspect of a variable slab length (neglecting a possible interaction with the upper mantle transition zone) is tested.…”

“…Key features distinguishing the isoviscous model from the control model with a high‐viscosity plate are a more symmetric surface depression that laterally aligns approximately with the lateral extent of the shallow slab, no viscous fore‐bulge and a shallower trench (supporting information Figure S9). This impossibility of reproducing characteristic regional subduction‐zone topography without lateral viscosity variations has been pointed out already previously [e.g., Kaban et al ., ; Gérault et al ., ].…”

“…The mantle wedge is believed to have a lower viscosity by at least an order of magnitude compared to the rest of the upper mantle due to the presence of volatiles released by the sinking plate [ Hirth and Kohlstedt , ]. It is further believed to have an important impact on the surface deflection above [ Billen and Gurnis , ; Billen et al ., ; Gérault et al ., ]. We tested therefore additionally a suite of experiments using a model with a low‐viscosity (i.e., two‐orders of magnitude lower) mantle wedge (supporting information Figure S17).…”

Deformation in the mantle wedge associated with Laramide flat‐slab subduction

“…Calculated effective viscosities of >10 19 Pa‐s are within the range expected for cold but hydrated lithospheric mantle, but are several orders of magnitude larger than would be expected for a weak and decoupled subduction interface shear zone. The Cocos flat‐slab in southwestern Mexico, for example, is thought to be decoupled from the overriding plate based on observations of extension in the arc and a neutral state of stress in the fore‐arc [ Gérault et al ., ]. Numerical models of slab dynamics in this region suggest interface viscosities on the order of 10 14 to 10 17 Pa‐s [ Gérault et al ., ].…”

“…The Cocos flat‐slab in southwestern Mexico, for example, is thought to be decoupled from the overriding plate based on observations of extension in the arc and a neutral state of stress in the fore‐arc [ Gérault et al ., ]. Numerical models of slab dynamics in this region suggest interface viscosities on the order of 10 14 to 10 17 Pa‐s [ Gérault et al ., ]. Similarly, Wada et al .…”