Abstract
This study investigates a prominent indentation in the central Mexican Volcanic Belt (MVB), referred to as the Tzitzio Gap (TG). In this region, centered at 101° W, Quaternary volcanism is displaced approximately 100
km toward the back of the arc. This embayment 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
Ma. 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, P
2O
5, Pb and K
2O typically decreases with distance from the Middle America Trench, consistent with progressive dehydration of the down-going 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. The data presented here (a compilation of available geochemical data and new XRF major and trace element analyses of lavas erupted adjacent to the TG) shows that the volcanism that occurs behind the TG exhibits an anomalous fluid signature (high K
2O, P
2O
5, Ba, and Pb) superimposed on compositions similar to those from the 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.