Heat flow, hydrothermal circulation and basalt intrusions in the Guaymas Basin, Gulf of California

The Guaymas Basin is part of an en echelon series of short, seafloor-spreading segments in the Gulf of California. The basin is separated from adjacent deeps by long, northwest-southeast-striking transform faults, and divided into subparallel, northern and southern troughs which trend northeast to southwest. Rapid sedimentation (1 to 5 m ka 1) results in a striking morphological contrast between seafloor spreading in the Guaymas Basin and that in more mature, mid-ocean settings. Basalt does not extrude onto the seafloor in the basin as flows and pillows, but instead intrudes into a thick sediment layer. The presence of relatively unconsolidated sediments lining the floor of the basin make this setting ideal for the study of axial heat flow and the thermal budget of oceanic crustal formation. Thermal data delineate several modes of heat transfer in both troughs of the basin, with negligible heat flow in some areas and values greater than 3 W m 2 in others. Broad heat flow highs, several square kilometers in area, result from a combination of deep, conductive heat flow, slow, intergranular fluid circulation and vigorous hydrothermal venting. Areas of low heat flow between heat-flow highs and near normal faults which bound the basin may be indicative of hydrothermal recharge. The geometry of faults, shallow sills and broad northeast-southwest structural trends in the basin largely seems to govern the distribution of heat flow. The highest values are clustered within a few isolated zones in each trough, with a total area of about 30 km 2. Variations in heat flow of two orders of magnitude occur over distances as small as a few hundred meters. Fourteen hydrothermal vents in the southern trough of the Guaymas Basin have been sampled directly, and found to have temperatures in excess of 250°C. Locally, conductive heat flow at the seafloor is significantly raised in the vicinity of these vents, but the general pattern appears to require more deeply seated, conductive heat sources. The mean heat flow in the southern trough is 453 ± 342 (s.d.) mW m 2. A simple analysis of the heat budget in the southern trough suggests that the total intruded section between the top of the magma chamber and the seafloor may be unusually thin, perhaps only several hundred meters in thickness.