45402-AC8
A Collaborative Paleomagnetic, Remote Sensing, and Field Investigation of the Neogene Fragmentation of the Maya Block with Implications for Jurassic Opening of the Gulf of Mexico
The tectonic evolution of the Gulf of Mexico is controversial at best. A poor understanding of first-order features associated with Jurassic Rifting of the Maya block from the Texas Gulf Coast is permissive of several plate scale models. These issues are further complicated by the Neogene modification of the southern, North American plate margin, and prolonged accumulation of Cenozoic siliciclastic deposits and associated intrabasinal deformation. Central to this debate is the consideration that the Chiapas Massif, an exhumed Permian suite of subduction related igneous and metamorphic rocks, has formed a stable margin for the May block since it was translated to southern Mexico without any post-rifting modification of this part of the North American plate. Models that describe Gulf rifting have two deficiencies. First, motion of the Maya block is often described as a net counter-clockwise roation of 35-70° about one or two Jurassic-age Euler poles. None of these propose rotations are consistent with paleomagnetic data from southern Mexico, which indicate an 70-100° of counter-clockwise rotation of the Maya Block. Second, none of thes models consider the possibility that the Maya Block has been tectonically modified in association with the formation of the Chiapas fold and thrust belt and the lesser known Chiapas Ixtapa transtensional domain. The supported research considers recently reported data regarding transform geometry and is acquiring a robust paleomagnetic data set to demonstrate an alternative hypothesis for Gulf rifting and subsequent Neogene fragmentation of the Maya Block. Sampling has focused on Permian, Jurassic, and Tertiary intrusions of the massif and Triassic-Jurassic strata of the Todos Santos formation to rigorously test an alternative hypothesis that addresses the post-Permian tectonic evolution of the Maya Block. This work promises to make significant contributions to our understanding of the tectonic evolution of the Gulf of Mexico. Prior to receiving PRF support, the principle investigators and their UNAM collaborator Dr. Roberto Molina-Garza, discovered a previously unmapped fossil plate boundary shear zone along the west cost of the Chiapas Massif (Wawrzyniec et al., 2006). The Tonola shear zone is a multiply reactivated feature that functioned as a plate scale transform between continental Mexico and rifting gulf basin. It later may have served as strike-slip fault accommodating the transition of the Chortis block from the North American to the Caribbean plate. In February 2007, John Geissman (UNM, co-PI), and Linda Donohoo (UNM, co-PI), and Linda Donohoo (UNM, PhD candidate) traveled with Dr. Roberto Molina-Garza (UNAM, supporting collaborator) to southern Mexico to conduct the first of two field excursions to collect samples from around the massif. To date a total of 79 sites from a range of materials have been collected for paleomagnetic analysis along with 36 samples suitable for processing for dating the timing of uplift of the massif. Since February, 2007, the paleomagnetic and magnetic fabric analysis of these materials have been completed by research assistant Scott Muggleton. During 2008, the samples for dating have been sectioned, fully described, crushed, and are currently being picked to isolate materials for analysis at the University of Kansas facilities. Research assistant Jack Grow is aiding in the completion of these activities. Current results largely center on paleomagnetic data. These data are derived from three key suites of rock: the Miocene plutonic complexes found along and within the Tonola Shear Zone (Wawrzyniec et al., 2006; Molina-Garza et al., 2008); intrusive rocks of the Permian age plutonic complex including what we believe to be Jurassic age mafic dikes; and Jurassic and Permian strata and volcanic sequences found east and north of the Permian pluton. The latter is UNAM
