SLAB DETACHMENT CONTROL ON TERTIARY MAGMATIC AND TECTONIC PULSES IN MEXICO

Seismic tomography studies imaged high velocities anomalies interpreted as subducted fragments of the Farallon plate in the upper mantle beneath Mexico. These anomalies are unconnected with the still subducting part of the Farallon remnants (Rivera and Cocos plates) implying that they detached sometime in the Tertiary. On the other hand, recent studies in the southern Sierra Madre Occidental (SMO) as well as the integration of the geology of the whole Trans-Mexican Volcanic Belt (TMVB) showed that the geologic record is characterized by short (4-5 m.y.) pulses of more intense volcanic and tectonic activity. I propose that two of these pulses are related to slab detachment episodes and that, in this way, we may infer the timing of these major geodynamic events. As in the well studied cases of the Mediterranean area the detachments start in one point and propagated laterally over some m.y. Initiation of slab detachment beneath Mexico is a consequence of the approach of the East Pacific Rise to the paleotrench west of Baja California because oceanic crust younger than 10 m.y. is more buoyant than the asthenosphere and, therefore, increasingly difficult to subduct. A first detachment of the lower part of the Farallon plate is proposed as the dynamic consequence of the first contact between the Pacific and North America plate at the end of Oligocene. A tear in the slab propagated SSE and produced a transient thermal anomaly and massive mafic melting at the base of the crust in the southern SMO. This, in turn, resulted in partial melting of the crust, ingnimbrite flare up and and extension at ~24-20 Ma. A second detachment event is inferred between ~14 and ~8 Ma as a result of the slowing subduction that preceded the final capture of the Magdalena microplate by the Pacific plate at 12.5 Ma. In this case the slab detached from the southern Gulf of California toward the ESE, paralleling the southern Mexico trench system. This detachment is considered to have caused a migrating mafic pulse of volcanism, presently observed at the base of the western and central TMVB, along the propagating tear. This time, the extensional reactivation of previous crustal discontinuities and the very thin mantle lithosphere left over by the previous detachment episode allowed rapid upraise of mantle melts rather than the formation of silicic crustal melts.