Thermal evolution of a sheared continental margin: Insights from the Ballenas transform in Baja California, Mexico

The Ballenas transform margin in central Baja California offers an unparalleled opportunity to study the thermal behaviour of a sheared continental margin during various stages of its evolution. Apatite fission track and (U–Th)/He results from two transects perpendicular to the coast reveal a pronounced latest Pliocene to Pleistocene (~ 1.8 Ma) heating event related to the Neogene opening of the Gulf of California. Proximity to a regional pre-rift unconformity indicates that samples remained at near-surface levels since Paleogene unroofing, despite having experienced reheating to maximum paleotemperatures within or above the fission track partial annealing zone. In general, maximum paleotemperatures during overprinting decrease from > 100–120 °C near the coast to below 60 °C ca. 5–8 km further inland, suggesting lateral heat flow from a source within the Gulf of California. Heat sources related to the structural development of the Ballenas transform fault, located approximately 1.5–4.5 km offshore from the two sample transects, most likely controlled the observed reheating. Overprinting patterns do not support conductive reheating due to reburial, magmatism or frictional shear. Instead, a pronounced thermal spike in between much less overprinted neighbouring samples strongly favours convective heating by hydrothermal fluids as the dominant overprinting process. Hydrothermal activity may be caused by either deep fluid circulation along newly formed shear zones of the transform fault or, more likely, magmatic leaking along the transform fault. Latest Pliocene to Pleistocene (~ 1.8 Ma) activity on the Ballenas transform fault is closely linked to extension in the Lower and Upper Delfín basins and provides a minimum age for the structural reorganisation and the relocation of extension in the northern Gulf of California. This study shows that hydrothermal activity can cause significant thermal events in a transform margin before the passage of the spreading centre. Paleotemperatures from the Ballenas transform are similar to those of other transform margins after passage of a spreading centre, which suggests that hydrothermal fluids may have an important thermal buffering effect, moderating the maximum temperatures of the ridge segments near their intersection with a continental transform margin.