Imaging the Eastern Trans-Mexican Volcanic Belt With Ambient Seismic Noise: Evidence for a Slab Tear

The eastern sector of the Trans-Mexican Volcanic Belt (TMVB) is an enigmatic narrow zone that lies just above where the Cocos plate displays a sharp transition in dipping angle in central Mexico. Current plate models indicate that the transition from flat to steeper subduction is continuous through this region, but the abrupt end of the TMVB suggests that the difference in subduction styles is more likely to be accommodated by a slab tear. Based on a high-resolution shear wave velocity and radial anisotropy model of the region, we argue that a slab tear within South Cocos can explain the abrupt end of the TMVB. We also quantify the azimuthal anisotropy beneath each seismic station and present a well-defined flow pattern that shows how mantle material is being displaced from beneath the slab to the mantle wedge through the tear in the subducted Cocos plate. We suggest that the toroidal mantle flow formed around the slab edges is responsible for the existence of the volcanic gap in central Mexico. Moreover, we propose that the temperature increase caused by the influx of hot, less dense mantle material flowing through the tear to the Veracruz area may have significant implications for the thermomechanical state of the subducted slab and explain why the intermediate-depth seismicity ends suddenly at the southern boundary of the Veracruz basin. The composite mantle flow formed by the movement of mantle material through the slab tears in western and southern Mexico may be allowing the Cocos plate to roll back in segments. The Trans-Mexican Volcanic Belt (TMVB) is a prominent and enigmatic feature of the subduction system in Mexico. Its volcanic style diversity and oblique orientation to the trench are explained by the large along-strike variations in the subduction parameters of the Rivera and Cocos plates. However, the abrupt termination of the TMVB on its eastern end with the Pico de Orizaba volcano is puzzling as the current slab model suggests that the transition of the Cocos flat-slab geometry to normal subduction is smooth through this region. There is evidence that suggests that a tear in the slab might be developing, but it is unclear how this feature can support the unusually large topographic gradient that connects the volcanic high peaks with the Veracruz basin just south of the volcanic front. To provide further insight into the transition anatomy of this portion of the subducted slab and its relation with surface topography, we present a detailed and unified model of the velocity structure of the crust and uppermost mantle of central Mexico.