CONTINENTAL MARGIN TRUNCATION, ALONG-TRENCH TERRANE TRANSPORT, LATERAL MIGRATION OF IGNEOUS ACTIVITY AND DEFORMATION IN SOUTHWESTERN MEXICO

Results of a paleomagnetic study of Cretaceous limestones, volcanic and intrusive rocks from the area of Zihuatanejo-Papanoa-Acapulco in the southern Mexico continental margin are reported. Thermal and alternating field demagnetization permit isolation of a well-grouped characteristic magnetization (ChNRM) after removal of a soft secondary overprint. The ChNRM is of dominant normal polarity. Coercivity and unblocking temperature spectra, low-temperature susceptibility and IRM acquisition experiments suggest that magnetic carriers are both magnetite and hematite. Discordant paleodeclinations suggest the occurrence of vertical-axis local rotations. Comparison of the ChNRM for the limestone sequences and for the dikes and intrusive bodies point to the possibility of a regional remagnetization event. This event may be related to the Paleogene truncation of the continental margin and lateral migration of magmatism and tectonic activity associated to large-scale transport of the Chortis block along the margin from a paleoposition near the Zihuatanejo area. The truncated character of the margin is expressed by the absence of high-pressure/low-temperature belt and of Mesozoic accreted assemblages, and the anomalous coastal location of the late Cretaceous-early Tertiary magmatic belt. Further studies are needed to establish the nature and acquisition age of the ChNRM and to determine the timing of the tectonic and magmatic activity.