2009 Portland GSA Annual Meeting (18-21 October 2009)

Paper No. 3
Presentation Time: 9:00 AM-6:00 PM


CONTLA, Diego, Tectónica, Instituto de Geología, UNAM, Instituto de Geología, Universidad Nacional Autónoma de México Ciudad Universitaria, Mexico City, 04510, Mexico, diegocontla@ciencias.unam.mx

The non-volcanic tremor (NVT) activity registered in the Mexican subduction zone (MSZ) possesses two broad studied areas where these episodes are developing; the MASE (Middle American Subduction Experiment) allowed the detection of these higher spectral amplitude episodes. One of the primordial aims to comprehend this recently discovered phenomenon is to formulate a comprehensive model that includes all known processes and materials involved in this complex area. The geometry of the MSZ is a chief factor because the pressure and thermal conditions depend on it, and all mineralogical phases are essentially controlled by the P-T configuration. The distances between the trench and the nearest and farthest NVT zones are 150 and 225 km, respectively, and these two areas feature the same low-frequency events, with a low number of NVT events in the former, and a wider distribution for the latter zone. The change from a steep to horizontal geometry at the MSZ since the Miocene represents one of the most important events that have controlled its evolution. Using this information, and based on the hypothesis of a trapped “paleo-mantle wedge”, suggests the existence of two types of lithologies, the basaltic oceanic crust and the serpentinized ultramafic paleo-mantle wedge. Recently the relation dehydration-intermediate to deep earthquakes was established to explain this kind of seismic events; therefore, using the thermal arrangement of the MSZ, the location of 86 NTV, P-T stability diagrams and the water content for each considered lithology, our results show that the principal metamorphic facies that should be present in the studied zone are Lawsonite-Blueschist, Epidote-Blueschist and Zoizite-Amphibolite-Eclogite for MORB and Serpentinite-Chlorite-Brucite and Serpentinite-Chlorite-Olivine assemblages for this trapped mantle. However, the MORB assemblages dehydrate under the P-T conditions related to the NVT, whereas the metasomatized ultramafic rocks remain stable. We conclude that the MORB composition better fits the relation between NVT and dehydration reactions. This probably discards serpentinites as the most accepted dehydrating phase in several subduction zones.