POST-SUBDUCTION VOLCANISM IN THE BAJA CALIFORNIA PENINSULA, MEXICO: THE EFFECTS OF REGIONAL THERMAL ANOMALIES AND ITS IMPLICATIONS ON THE MAGMATIC EVOLUTION OF BAJA CALIFORNIA VOLCANISM FOLLOWING A MAJOR TECTONIC RECONFIGURATION

An intriguing feature of the Baja California peninsula is that volcanic activity thrived and continued along the

axis of the peninsula even after the cessation of subduction 10 Ma ago. Post-subduction volcanism in Baja

California occurred mainly in monogenetic volcanic fields comprising a variety of compositions, most of

them associated with high-temperature regimes and marked by a “slab” signature (i.e., adakites, Niobiumenriched

basalts and high-magnesium andesites). Several attempts have been made to explain the origin and

compositional diversity of such post-subduction volcanism. Many of them rely on the assumption that

anomalous magmas are formed in direct response to tectonic events such as slab window formation or slabtearing

processes. However, none of them can offer a satisfactory explanation as to why volcanism as young

as 1 Ma can be found along the Baja California peninsula. Observations elsewhere and in numerical

simulations have shown that the slab tearing process is a fast one lasting only a few million years. By contrast

the post-subduction volcanism in Baja California has lasted more than 10-million years. Here, we present a

physical model that shed light into the origin of this controversial phenomenon. The model calls upon viscous

dissipation or shear heating as the process responsible for the generation of a regional heat flow anomaly with

a maximum amplitude of 40 mW/m2 clearly observed in deep boreholes drilled in the area. We hypothesize

that at moderate depths it may have caused partial melting after the cessation of subduction along the Baja

California. Our results show that indeed is possible for rocks to increase their temperatures substantially in

this way. Preliminary numerical experimentation shows that the melt fraction could reach up to 10% and the

that the melt fraction could reach up to 10% and the maximum amount of shear heating could lead to a

temperature increase close to 200 °C at 35 km depth.

Moreover, the rise of magmas and/or hot fluids in the shear zone will enhance the temperature increase in

shallower parts, further promoting the production of melt.