Heat flow measurements collected throughout the Auka and JaichMaa Ja'ag' hydrothermal vent fields in the central graben of the Southern Pescadero Basin, southern Gulf of California, indicate upflow of hydrothermal fluids associated with rifting dissipate heat in excess of 10 W/m2 around faults that have a few kilometers in length. Paradoxically, longer faults do not show signs of venting. Heat flow anomalies slowly decay to background values of ∼2 W/m2 at distances of ∼1 km from these faults following an inverse square-root distance law. We develop a near-fault model of heat transport in steady state for the Auka vent field based on the fundamental Green's function solution of the heat equation. The model includes the effects of circulation in fracture networks, and the lateral seepage of geothermal brines to surrounding hemipelagic sediments. We use an optimal fitting method to estimate the reservoir depth, permeability, and circulation rate. Independently derived constraints for the model, indicate the heat source is at a depth of ∼5.7 km; from the model, permeability and flow rates in the fracture system are ∼10−14 m2 and 10−6 m/s, respectively, and ∼10−16 m2 and 10−8 m/s in the basin aquitards, respectively. Model results point to the importance of fault scaling laws in controlling sediment-hosted vent fields and slow circulation throughout low permeability sediments in controlling the brine's chemistry. Although the fault model seems appropriate and straightforward for the Pescadero vents, it does seem to be the exception to the other known sediment-hosted vent fields in the Pacific.

Hydrothermal vent fields located in the gap between known sites in Guaymas Basin and 21°N on the East Pacific Rise were discovered on the Alarcón Rise and in southern Pescadero Basin. The Alarcón Rise spreading segment was mapped at 1-m resolution by an autonomous underwater vehicle. Individual chimneys were identified using the bathymetric data. Vent fields were interpreted as active from temperature anomalies in water column data and observed and sampled during remotely operated vehicle dives. The Ja Sít, Pericú, and Meyibó active fields are near the eruptive fissure of an extensive young lava flow. Vent fluids up to 360 °C from Meyibó have compositions similar to northern East Pacific Rise vents. The Tzab-ek field is 850 m west of the volcanic axis, and active chimneys rise up to 33 m above a broad sulfide mound. The inactive field is 10 km north-northeast along the rift axis, and most sulfide chimneys are enriched in Zn and associated elements that are transported at lower temperature compared to the more Cu-rich active fields. In southern Pescadero Basin, the Auka field is on the margin of a sediment-filled graben at 3,670-m depth. Discharging fluids are clear, contain hydrocarbons, and have neutral pH, elevated salinity, and temperatures up to 291 °C. They have deposited massive mounds of calcite with minor sulfide. The fluids are compositionally similar to those in Guaymas Basin, produced by high-temperature basalt-seawater interaction followed by reaction with sediment. The paucity of sulfide minerals suggests subsurface deposition of metals.