Impact of the Orozco Fracture Zone on the central Mexican Volcanic Belt

We present a compilation of available geochemical data and new XRF major and trace element analyses of laves erupted adjacent to the Tzitzio Gap (TG), a prominent indentation in the central Mexican Volcanic Belt (MVB). In this region, centered at 101oW, Quaternary volcanism is displaced approximately 100 km toward the back of the arc. This embayment in the volcanic front is roughly aligned with the projected position of the Orozco Fracture Zone (OFZ), which separates segments of the Cocos Plate that differ in age by approximately 5 million years. The fracturing of the oceanic lithosphere associated with the OFZ appears to have caused alteration and increased the buoyancy of this ~100 km wide linear feature. Several indicators of the buoyancy of the OFZ have been reported, including shallowing of the Middle America Trench (MAT), seaward advancement of the shoreline, and the uplift and exposure of deeply buried and deformed Jurassic to Oligocene rocks that comprise the TG region. Arc volcanism in the regions surrounding the TG provides constraints on the subduction of the OFZ. In the central MVB, enrichment in fluid mobile elements such as Ba, Sr, P2O5, Pb and K2O typically decreases with distance from the Middle America Trench, consistent with progressive dehydration of the downgoing slab. Numerous studies in the central MVB have demonstrated that this fluid-mobile element enriched signature results from flux-induced melting at the front of the arc, while decompression melting of asthenospheric mantle peridotite is dominant at the back of the arc. Our data show that volcanism behind the TG exhibits an anomalous fluid signature (high K2O, P2O5, Ba, and Pb), more commonly associated with the front of the MVB, superimposed on compositions similar to those from extensional regions in northern Mexico. We propose that these distinctive structural and geochemical characteristics arise from subduction of the OFZ. Increased buoyancy of the fracture zone may cause it to subduct at a shallower angle. Therefore, the OFZ portion of the slab reaches the P/T conditions for de-volatilization further from the Middle America Trench, resulting in the profound offset of the arc toward the back and the fluid signature from the slab superimposed on back-arc, extension-related intraplate basalts.