PALEOGENE CONTINENTAL-MARGIN TRUNCATION IN SOUTHWESTERN MEXICO - GEOCHRONOLOGICAL EVIDENCE

The reasons for, and mechanisms of, continental margin truncation in S W Mexico where Mesozoic-Cenozoic plutons are situated directly on the Pacific coast, are not yet well understood. Large-scale dextral and/or sinistral displacements of the continental margin terranes, now formi ng parts of Baja California or the Chortis block, have been proposed. The well-defined along-coast NW-SE decreasing granitoid intrusion age trend (similar to 1.2 cm/yr in the 100 Ma-40 Ma time interval) between Puerto Vallarta and Zihuatanejo is interpreted by us to be a geometri c artifact of oblique continental margin truncation rather than the co nsequence of a sinistral offset of the Chortis block from those latitu des toward the SE. Changes in the dip and velocity of the NNW-SSE tren ding Cretaceous-Tertiary subduction zone resulted in a landward migrat ion of the magmatic are. Taking into account certain stratigraphic aff inities of Chortis and the Oaxaca and Mixteca terranes, together with the known displacement rates along the North America-Caribbean Plate b oundary, the northwesternmost paleoposition of the Chortis block with respect to SW Mexico was near Zihuatanejo. In contrast, between Zihuat anejo and the Isthmus of Tehuantepec, the cessation of the Tertiary ma gmatism decreased more rapidly (similar to 7.7 cm/yr), although the tr end is not so obvious. Starting in the late Eocene, Chortis moved abou t 1100 km to the SE along a transform boundary associated with the ope ning of the Cayman Trough. Based on our geochronological data and stru ctural relationships between mylonite zones and plutons in the Acapulc o-Tehuantepec area, we propose an approximately 650 km SE movement of Chortis from about 40-25 Ma, with a velocity of 6.5-4.3 cm/yr. Since t his is considerably slower than the decreasing age trend obtained by u s using the geochronological data, we consider batholith formation in this segment to predate and postdate the offshore passage of the North America-Farallon-Caribbean triple junction. Geological observations a nd paleomagnetic data do not give strong support for large-scale right -lateral displacements of crustal blocks like the Baja California. Giv en the isotopic data presented, the continental margin truncation in S W Mexico seems to be the consequence of an interaction of mechanisms. Of these, we regard tectonic erosion associated with the subduction pr ocess to be the most important in the northwestern segment. On the oth er hand, the lateral removal of material associated with the displacem ent of Chortis is more important in the southeastern segment.