Regional rigid-block rotation, small domain rotations and distributed deformation within the Acambay graben, central Trans-Mexican Volcanic Belt: paleomagnetic implications

Abstract

Paleomagnetic declinations for volcanic units within the Acambay graben, are rotated counterclockwise by various amounts with respect to expected reference directions. The average overall mean rotation is from -18.5 ± 7.5 to -15.8 ± 7.2 degrees, assuming the region behaves as a coherent rigid tectonic domain. Deformation on a local scale seems more complex and several tectonic domains can be identified. Site-mean rotation estimates are as high as -49 degrees, with rotations within the graben around -40 degrees. The angular differences among the site-mean paleomagnetic directions mainly result from: (a) paleosecular variation, (b) differential rotation of small tectonic domains, (c) internal deformation within the rotated domain, (d) age differences and timing of rotation, (e) sampling and measurement effects, (f) local structural complexities, apparent rotations due to improper structural correction, and (g) a combination of these factors. The paleomagnetic results are analyzed for geographic distribution within the graben, relative position of sampling sites with respect to fault scarps, etc. Factors (a), (b), (c) and (d) are considered important. Results support the occurrence of a counterclockwise rotation of a large domain, possibly related to regional sinistral shear and transtension within the magmatic arc. Locally, deformations within the graben seem distributed among various small domains, which show different amounts of counterclockwise rotation.