TECTONIC REACTIVATION OF MAJOR PRE-EXISTING STRUCTURES AND THE RELATIONSHIP WITH TERTIARY VOLCANISM IN WESTERN OAXACA

The western Oaxaca volcanic field is part of the Tertiary magmatic province of southern Mexico. It is constituted by late Eocene to early Oligocene age volcaniclastic deposits, lavas and autobreccias, intruded by subvolcanic hypabyssal rocks, that accumulated in roughly north-south trending tectonic basins. Field and geochronological studies suggest that the formation of these basins as well as many of the Tertiary structures are related to the reactivation of major Paleozoic and Mesozoic structures.

Prior to the onset of volcanic activity, a major WNW-trending left lateral strike-slip fault system dragged north-plunging Mesozoic fold axes to the west. Subsequent reactivation of these structures with the same kinematics affected the entire volcanic sequence but apparently produced minor displacement.

Based on stratigraphical observations we infer that the main basin was formed by E-W extension and normal faulting along pre-existing roughly N-S trending structures. This stage predated and was partially contemporaneous with the initial calc-alkaline volcanism in early Oligocene time. One of the normal fault zones associated with the formation of the basin is the southern prolongation of the Caltepec fault zone, which is located on its eastern margin. This fault zone has been recognized as a dextral transpressional NNW-trending ductile fault zone of Early Permian age, which corresponds to the tectonic contact between the Paleozoic Mixteco terrane and the Precambrian Oaxaca terrane. Along the western margin of the basin, NNW-trending normal faults affected Cretaceous units previous to the initial stages of volcanic activity. Hypabyssal rocks dated as early Oligocene are aligned along these structures, suggesting a structural control with extension providing conduits for magma ascent. Later left-lateral strike-slip faults trending NW to N are observed in all the volcanic units.

Orientations of dikes and faults with slickenlines in the Tertiary volcanic units reveal paleostress reorientation from initial E-W extension to left lateral strike slip movement on NW-SE to N-S oriented faults.