A GAP IN THE CENTRAL MEXICAN VOLCANIC BELT: A RESULT OF SHALLOW SUBDUCTION OF THE OROZCO FRACTURE ZONE?

Pronounced gaps in the Late Miocene to Recent volcanism of the Mexican Volcanic Belt (MVB) have been recognized since early systematic study of the region (e.g. Nixon, 1982), but the distribution and origin of these gaps have not been explained. This study is focused on the largest gap, which is located in central Mexico and centered on the Tzitzio anticline (101°W). This gap roughly corresponds to the subducted trace of the Orozco Fracture Zone (OFZ), which separates northern Cocos Plate oceanic crust of different ages and therefore densities. The older oceanic crust (17.6 Ma) to the northwest of the OFZ is colder, thicker, and denser than the younger oceanic crust (14.5 Ma) to the southeast of the OFZ. This causes a buoyancy contrast that appears to be reflected in the dip of the subduction angle on either side of the OFZ, with the northwest side subducting at a 30 degree angle and the southeast side subducting at a 5-10 degree angle within the seismogenic zone (Pardo and Suarez, 1995). Where the OFZ intersects the Middle America Trench (MAT), bathymetry shows a bulge on the southeast side of the OFZ. This bulge and the bay north of Ixtapa Zihuatanejo may be associated with fore-arc deformation where the fracture zone is consumed.

We have sampled a transect of the Zitacuaro-Valle de Bravo volcanic field (ZVB) directly to the east of this gap, and of the Michoacan-Guanajuato Volcanic Field (MGVF) which occurs directly to the west. Geochemical and isotopic data from the Quaternary volcanism of these two regions indicates that fluids and fluid-mobile elements (e.g. K₂O, Rb, Ba, P₂O₅, and radiogenic ⁸⁷Sr/⁸⁶Sr ratios) are enriched in the volcanism along the front of the arc compared to the middle and back portions of the arc. However, preliminary investigation behind the gap has shown that the Quaternary lavas that have erupted there are enriched in fluid-mobile elements, similar to the Quaternary lavas found along the adjacent arc front, approximately 150 kms closer to the Middle America Trench (MAT). This enrichment in fluid-mobile elements in lavas behind the gap may indicate that due to shallow subduction of the buoyant fracture zone, fluids from the subducting slab are released further from the MAT, thus shifting the mantle wedge melting zone approximately 150 kilometers further from the MAT behind the gap.