The prevailing tectonic setting in the Gulf California suggests the presence of an undetermined number of short spreading centres with associated hydrothermal systems. However, to date, active seafloor spreading phenomena have been documented in only three of the eight tectonically active basins. Here we report heat flow values as high as 15,436 mW m(-2) in two of the northernmost basins of the Gulf of California, providing evidence of intense hydrothermal activity associated with the transition from continental rifting to seafloor spreading. The mean heat flow for the Wagner and Consag basins area is 1,875 mW m(-2), more than 15 times higher than the mean value for oceanic crust (105.4 mW m(-2)). Additional evidence for vigorous hydrothermal circulation and a shallow heat source includes intense gas discharge (CO(2) and CH(4)), widespread low pH (average 7.7), locally high (222)Rn concentrations in the bottom water and a high extent of organic matter maturation in the sediments.