ALTERNATIVE CENOZOIC RECONSTRUCTIONS FOR THE CHORTIS BLOCK RELATIVE TO SOUTHERN MEXICO

In the Eocene, the Chortis block has been juxtaposed against either southern Mexico (Model #1) or the Maya block (Model #2). A third option presented here rotates the Chortis block ~1100 km along the Cayman transform fault during the Cenozoic using the pole of rotation near Santiago, Chile: this locates the Chortis block southwest of its current position (Model #3). Evaluation of these three models provides only two crucial tests: the rotation pole of the Caribbean Plate and the presence of undeformed Late Cretaceous- Eocene sediments in the Gulf of Tehuantepec, both of which are incompatible with Models #1 and #2. Tectonic implications of Model #3 include: (a) the relatively small, Cenozoic, sinistral displacement (~170 km) observed across the Motagua fault zone is the result of oblique subduction of the Farallon Plate beneath southern Mexico; (b) the anticlockwise rotation of the Cenozoic arc magmatism in Mexico is congruent with a similar rotation of the trench on the trailing margin of the northeasterly migrating Chortis block; (c) the truncation of the ~100-25 Ma magmatic arc by the southern coast of Mexico is the result of subduction erosion; and (d) it suggests continuity between Motagua and Guerrero oceanic terranes through the Gulf of Tehuatepec in the Late Cretaceous.