Cordilleran Section - 108th Annual Meeting (29–31 March 2012)

Paper No. 4
Presentation Time: 09:30


MOLINA GARZA, Roberto, Centro de Geociencias, UNAM, -Campus Juriquilla, Queretaro, Queretaro, 76230, Mexico, ROGERS, Rob, California State University Stanislaus, One University Circle, Turlock, CA 95382 and VAN HINSBERGEN, Douwe, Physics of Geological Processes, University of Oslo, Sem Sælands vei 24, Oslo, 0316, Norway,

Several workers have linked the Cretaceous-Cenozoic stratigraphy of the Chortis block and southern Mexico, suggesting proximity of these continental blocks during this interval. Also, patterns of magmatism and mylonite belts parallel to the modern mid-American trench have been interpreted to result from continental truncation of the continental margin of Mexico. According to these models the Chortis block moved southeastward with respect to southern Mexico since Eocene time. Paleomagnetic data for Chortis, however, have been interpreted to indicate a complex pattern of rotations with minor latitudinal transport. Paleomagnetic sampling in tectonically stable settings in east-central Honduras and progressive step-wise thermal demagnetization of the Upper Cretaceous – Paleogene Valle Angeles Group indicate that these rocks have complex multivectorial natural magnetization. Widespread, is a west-directed and moderate inclination magnetization (or its antipodal) with laboratory unblocking temperatures between about 280º and 580ºC. High coercivity and low distributed unblocking temperatures suggest that it resides in pigmentary hematite. This magnetization fails a local fold test and a regional tilt test. A multivectorial natural magnetization was not recognized in previous work of this unit, nor the failure to pass a fold test; those results are thus considered suspect. A preliminary mean direction for this secondary magnetization D=295º, I=29.4º (n=35 sites, k=80.5, alpha95=4.2º) indicates a post Paleocene counterclockwise rotation of 55.2º+/-6º. The rotation indicated by paleomagnetic data is larger than inferred in already published paleogeographic reconstructions of Chortis. Paleomagnetic data for the overlying Padre Miguel Ignimbrite suite indicate that the rotation occurred between about 55 and 18 Ma.