CONTROL OF SHALOW AND STEEP SUBDUCTION ON INTRA-ARC DEFORMATION IN THE TRANS MEXICAN VOLCANIC BELT

The Trans-Mexican volcanic belt is a subduction-related arc dissected by a field of seismically active normal faults clustered in its western portion. GPS observations show that southern Mexico, by contrast, is under compression. This field of normal faults is an enigmatic feature of the Trans-Mexican volcanic belt and the nature of the mechanism driving extension has be the subject of debate for more than 25 years. Here is shown, using a finite element model, that extension is the direct result of subduction. A 40 km-thick elastic plate models the North America plate with a 2 km-high plateau sitting on its top. The underlying mantle is modeled by Newtonian incompressible fluid forced in convection along the Wadati-Benioff zone. Boundary conditions are consistent with heat-flow measurements, gravity modeling, and geological and seismological observations. Numerical experimentation shows that viscous coupling between tectonic plates and mantle allows for changes in momentum of the subducted plates (Rivera and Cocos) to be transmitted to the base and leading edge of North America. Depending on the geometry of the subduction zone this effect gives rise either to tension or compression in the volcanic arc. The high subduction angle prevailing in the northern Middle America trench concentrates viscous drag at the leading edge of the overriding plate; this bends downward the forearc and produces flexural tension at a distance of 100-200 km away from the trench in good agreement with the location of the active normal faults in the western Trans-Mexican volcanic belt. Farther south a sallow subduction angle concentrates viscous drag at the leading edge and under the arc. This puts south Mexico and the eastern part of the volcanic arc in compression, a result that is consistent the GPS observations. This explains why faulting is concentrated in the west-central portion of the Trans-Mexican volcanic belt.