ACS PRF | ACS
All e-Annual Reports
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 a source of numerous controversies that stem from a poor understanding of the geometry of first-order features associated with Jurassic rifting of the Maya block from continental North America. Since the mid-Jurassic, the Maya block has defined the southern margin of North America from the Pacific Ocean to the Yucatan Peninsula. Cenozoic sedimentary rocks obscure many of key tectonic features features, and Neogene tectonic modification of Maya block introduces an additional level of complexity. Central to this debate is the consideration that the Chiapas Massif, a Permian suite of subduction related igneous and metamorphic rocks, has formed a stable margin for the Maya block since it was translated to southern Mexico without any post-rifting modification of this part of the North American plate margin. Models that describe Gulf rifting have two deficiencies. First, motion of the Maya Block is generally described as a net counter-clockwise rotation of 35-70° about one or two Euler poles. None of these proposed rotations honor any paleomagnetic data from the southern Mexico that indicates an additional 20-30º or a total of 55-100° of counter-clockwise rotation. Second, none of these models consider the possibility that the Maya block has been tectonically modified with the formation of the Neogene Chiapas fold and thrust belt. The proposed research is making use of recently reported data regarding transform geometry and is acquiring a robust paleomagnetic data set to demonstrate an alternative hypothesis for Gulf rifting and Neogene Fragmentation of the Maya Block. Sampling will focus on Permian, Jurassic, and Tertiary igneous rocks of the massif and Triassic-Jurassic strata of the Todos Santos formation to rigorously test this alternative model for the post-Permian tectonic evolution of the Maya Block. Such work will also have profound effects on our understanding of the tectonic evolution of the Gulf of Mexico.
In February 2007, John Geissman (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 three field excursions to collect samples from and around the massif. A total of 22 sites from a range of materials were collected for paleomagnetic analysis along with 10 block samples for low-temperature thermochronlogic analysis to establish the timing and pattern of uplift of the Permian massif. Combined with 55 sites collected prior to the start of the funded project we have a total of 77 sites that have provided material for paleomagnetic and rock magnetic analysis. Since February, the PI, co-PI, and Scott Muggleton (UNM PhD student) have processed most of the paleomagnetic samples. Mr. Muggleton has prepared the block samples for dating uplift of the massif and has separated both abundant zircons and apatite grains. Working with UNAM, several samples are also being processed for dating using Ar/Ar and Shrimp U/Pb techniques to establish a geochronology of intrusive materials. Also, we have prepared dozens of petrographic thin-sections for descriptive and compositional analysis.
The results to date have been obtained from Miocene plutons, mylonites in the shear zone, Permian host plutonic rocks, and laterally extensive, well-exposed swarms of pseudotachylite veins, likely to be of mid-Cretaceous age, hosted by Permian rocks both south and north of the crystalline massif. Many sites exhibit demagnetization behavior characterized by the well-defined isolation of a north-seeking, shallow positive inclination magnetization, that is typically well-grouped, over a wide range of coercivities and laboratory unblocking temperatures, typically below about 580°C. In general, the accepted data are interpreted to suggest that, since the mid-Cretaceous, no large magnitude rotation of the Chiapas massif has taken place, and that the late Miocene intrusions have not been affected by appreciable local tectonism related to the Tonala shear zone. Our interpretations are compromised by the lack of adequate, reliable understanding of the paleohorizontal at the time of remanence acquisition. The low-temperature thermochronologic studies underway should further address this critical concern. Once properly tilt corrected the paleomagnetic data should serve to adequately test our hypothesis that the Maya Block has been fragmented into two separate tectonic blocks since the Neogene, which should in turn aid in establishing a tectonic model for the opening of the Gulf of Mexico that addresses all of the available data.
