SYNCHRONOUS ARC HIATUS AND EXHUMATION: THE RESULT OF SUBDUCTION EROSION OF FOREARC IN THE LATE OLIGOCENE - EARLY MIOCENE OF SOUTHWESTERN MEXICO

Available geochronologic data from Paleogene magmatic rocks in southwestern Mexico portray arc magmatism whose trend varied in time. For the late Eocene - early Oligocene the arc was near parallel to the present day truncated margin. The axis of this arc is now represented by an extensive batholithic belt along the continental margin but volcanic counterparts extend up to 220 km inland. The late Oligocene - lower Miocene (28.5-20 Ma) was a time of major change in the evolution of arc magmatism and the subduction regime: (i) cessation of WNW-trending arc magmatism to be reinitiated farther north at ~20 Ma in the E-W Trans-Mexican Volcanic Belt (TMVB); (ii) removal of an approximately 200 km wide late Eocene - early Oligocene forearc; (iii) exhumation of 13-20 km of the late Eocene - early Oligocene arc along the present day continental margin; and (iv) a switch, in the northern area, of extension and shortening directions associated with strike slip tectonics. To explain the removal of the forearc region, two alternative interpretations have been proposed: lateral displacement of the Chortis block and subduction erosion. This latter model is more compatible with key tectonic features observed along the continental margin and in northern Middle America. A ~200 km step between the top of the Cocos and Farallon plates beneath the eastern TMVB seen in tomography is estimated to be ~25 Ma, synchronous with subduction erosion and fragmentation of the Farallon Plate. This step is interpreted as a structural ramp at the top of material that was removed from the base of the overriding plate in the forearc region. The hiatus in arc magmatism suggests a temporary cessation of subduction of the Farallon Plate with Cocos Plate subduction taken up by thrust ramping and subduction erosion of the overriding plate. The obliquity between the axes of relative plate motion and strain suggests a triclinic geometry that may be due to a combination of plate coupling with slip partitioning, mantle wedge convection and the absolute westward movement of the North American Plate.