TRANSPORT OF HEAT BY HYDROTHERMAL CIRCULATION IN A YOUNG RIFT SETTING: OBSERVATIONS FROM THE AUKA AND JAICHMAA JA'AG' VENT FIELD IN THE PESCADERO BASIN, SOUTHERN GULF OF CALIFORNIA

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 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. It predicts a pattern of hydrothermal circulation that is fast and vertical on the fault zone and slower and horizontal throughout the sediment produced by a source that is ~5 km deep. The model also predicts that the upwelling plumes cool mostly by lateral advection of heat by seepage parallel to the stratification, along fine-sand layers deposited by density currents and siliceous oozes. We use an optimal fitting method to estimate the reservoir depth, permeability, and circulation rate; 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. Model scaling relationships and observations elsewhere indicate that only faults <3 km in length appear to develop the conditions required to sustain the temperature gradients observed in the sediment-hosted Auka vent field. This could explain why other longer neighboring faults do not show signs of hydrothermal activity. Moreover, the relative transmissivity of the fault's damage zone is the strongest control on the width of the captured plume, and therefore on the heat flow decay length.