Results of a sea-floor electromagnetic survey over a sedimented hydrothermal area on the Juan de Fuca Ridge

Abstract

SUMMARY The eastern part of Middle Valley, on the northern Juan de Fuca Ridge, is characterized by locally high heat flow, evidence of hydrothermal activity, and near-surface mineralization. A magnetometric offshore electrical sounding (MOSES) survey was conducted in this area to determine the electrical structure of the sediment and upper crust. Two perpendicular lines of data were obtained across a 'mound', a 500m diameter uplifted hill where sulphide mineralization and hydrothermal activity have been observed. The best-fitting model for the east-west regional line consists of a sediment layer of resistivity 0.70 f 0.06 Qm and thickness 500 f 100 m overlying basaltic basement of resistivity 2.3 f 0.6 Qm. Basement porosities are estimated to be of the order of 10-14 per cent, using measured vent fluid temperatures of 275-300 C, consistent with previous estimates of the porosity in fractured oceanic crust. Results for the north-south line along the mound indicate the presence of a 1.35 f 0.15 Qm, 100 f 20 m thick resistive cap overlying deeper sediments of much lower resistivity, of the order of 0.1 Qm. The resistivity structure across a localized high heat flow anomaly is similar but the resistive cap is absent. The phase lags calculated from the resistivity model deviate significantly from the measured lags and provide the clearest evidence for transverse isotropy, the interbedding of very conductive with less conductive layers. The conductive thin layers are two orders of magnitude less resistive than the resistive layers because of massive sulphide mineralization, a result now confirmed by drilling. Additional data were gathered at a site centred about the southern end of a vent field 3-4km northwest of the hydrothermal mound; the resistivities can be fit using either the regional model or a 1.1 Qm halfspace. There are some anomalous values, which appear to be spatially correlated with basement topographic highs.