Imaging the Eastern Trans-Mexican Volcanic Belt

Imaging the Eastern Trans-Mexican Volcanic Belt

The abrupt end of the Trans-Mexican Volcanic Belt (TMVB) in central Mexico marks a dramatic change in the subduction system that defines the western margin of North America. To the west, the subducted slab is horizontal at 45 km depth until it reaches the TMVB (at the latitude Mexico City) where it plunges into the mantle to a depth of 500 km. To the east the slab descends with a dip of 30-degrees to a depth of 150 km where it is cut off by an unexplained structure. The goal of this study is to understand why these systems are different and how they function together. The larger goal is to find a unified model for the subduction system from the western coast of the United States to Central America. Determining the tectonic structure in this region is important, because it is a transition point in that model. The study will also continue cooperative studies with Mexican scientists that have been ongoing for the past decade.

We propose to make a detailed image the structure beneath the eastern end of the TMVB, using receiver functions to map subsurface interfaces in the structure. This method uses the conversion of P-waves to S-waves to determine the location and contrast of the interfaces. To determine the velocity structure of the region we will use surface waves from earthquakes and ambient noise correlations. Both of these techniques can also be used to estimate the anisotropic behavior of the subsurface, which can be used to determine the present and past flow of material. The goal is to integrate the structure and flow into models that can be used to explain the tectonic evolution of this area. The study will exploit several existing seismic datasets in the region, plus a current Mexican project (GECO) that is operating a dozen seismic stations in the region that we plan to image.

To download the NSF proposal clock here.

Model of flow beneath eartern end of the TMVB

Three-dimensional schematic illustration of the proposed tectonic setting and inferred mantle flow (green arrows). Asthenospheric mantle materials are flowing almost trench-parallel beneath the slab in the forearc region untila they are redirected to the mantle wedge through a tear that separates Central and South Cocos. The accelerated rollback rate of South Cocos relative to Central Cocos introduces a suction force that further displaces the mantle materials laterally toward the south of Mexico. The shear stress exerted by the toroidal flow around the slab tear may be transporting the melts and fluids that are feeding the volcanoes in the LTVF from central Mexico, thus allowing their existence without any slab material at an appropriate depth directly underneath them. Some of the active volcanoes composing the eastern TMVB volcanic chain, however, might still be fed by some slab edge melting mechanism. Anisotropy in the backarc of central Mexico (i.e., Central Cocos) is primarily controlled by a trench-perpendicular 2-D corner flow that is induced by the downdip motion of the slab. The flow of mantle materials through a slab tear, with the accompanying slab edge melting, may explain the exceptional spatial distribution of stratovolcanoes in the eastern sector of the TMVB, as well as the abrupt change in the source of melts observed in young rocks in this segment of the MAT.

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