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2018
Harmon, N, Rychert C, Agius M, Tharimena S, Le Bas T, Kendall JM, Constable S.  2018.  Marine geophysical investigation of the chain fracture zone in the equatorial Atlantic from the PI-LAB experiment. Journal of Geophysical Research-Solid Earth. 123:11016-11030.   10.1029/2018jb015982   AbstractWebsite

The Chain Fracture Zone is a 300-km-long transform fault that offsets the Mid-Atlantic Ridge. We analyzed new multibeam bathymetry, backscatter, gravity, and magnetic data with 100% multibeam bathymetric data over the active transform valley and adjacent spreading segments as part of the Passive Imaging of the Lithosphere Asthenosphere Boundary (PI-LAB) Experiment. Analyses of these data sets allow us to determine the history and mode of crustal formation and the tectonic evolution of the transform system and adjacent ridges over the past 20Myr. We model the total field magnetic anomaly to determine the age of the crust along the northern ridge segment to better establish the timing of the variations in the seafloor fabric and the tectonic-magmatic history of the region. Within the active transform fault zone, we observe four distinct positive flower structures with several en echelon fault scarps visible in the backscatter data. We find up to -10mGal residual Mantle Bouguer Anomaly in the region of the largest positive flower structure within the transform zone suggesting crustal thickening relative to the crustal thinning typically observed in fracture zones in the Atlantic. The extensional/compressional features observed in the Chain Transform are less pronounced than those observed further north in the Vema, St. Paul, and Romanche and may be due to local ridge segment adjustments.

Constable, S, Kowalczyk P, Bloomer S.  2018.  Measuring marine self-potential using an autonomous underwater vehicle. Geophysical Journal International. 215:49-60.   10.1093/gji/ggy263   AbstractWebsite

The marine self-potential (SP) method is used to explore for hydrothermal venting and associated seafloor mineralization. Measurements are commonly made in deep water using instruments towed close to the seafloor, which requires dedicated ship time, is limited to slow speeds, and is subject to navigation errors. Instead, we mounted a three-axis electric field receiver on an autonomous underwater vehicle (AUV), and tested the method with data collected in the Iheya area of the Okinawa Trough, off Japan. Parts of this prospect have documented hydrothermal venting and seafloor massive sulfide (SMS) deposits. An International Submarine Engineering Limited explorer-class AUV was fitted with a controlled-source electromagnetic (CSEM) amplifier and logging system, modified to collect DC SP data using silver chloride electrodes on approximately 1 m dipoles. A 1 km x 1 km area was surveyed with a flight pattern of six lines, collected three times to assess repeatability and noise levels. The entire data set was collected in a single day on station with a 10 hr AUV deployment. Flying height was 70 m, navigation errors were less than 3 m, collection speed was 1.1 m s(-1) and electric field noise levels were less than 5 mu V m(-1). Localized anomalies of 0.3 mV m(-1) were observed, from which potentials were estimated using regularized inversion, yielding negative SP anomalies of 15-25 mV. Modelling electric field data as dipoles shows that the negative poles causing the anomalies are localized near the seafloor with a diffuse return current deeper than 1000 m below seafloor. Apparent conductivities as high as 30 S m(-1) were derived from CSEM data collected during the same deployment, which strongly suggests that SMS mineralization is associated with one of the SP anomalies, although the localization near the seafloor and the lack of a dipolar signal suggest that the causative mechanism for the SP anomalies is due to hydrothermal venting. In either case, we have demonstrated that AUV-mounted instrument systems are an efficient, effective and low noise means of collecting marine SP data.

1998
MacGregor, LM, Constable S, Sinha MC.  1998.  The RAMESSES experiment - III. Controlled-source electromagnetic sounding of the Reykjanes Ridge at 57 degrees 45 ' N. Geophysical Journal International. 135:773-789.   10.1046/j.1365-246X.1998.00705.x   AbstractWebsite

A controlled-source electromagnetic sounding survey centred on an axial volcanic ridge (AVR) segment of the Reykjanes Ridge at 57 degrees 45'N was performed as part of the RAMESSES experiment. Low-frequency (0.35-11 Hz) electromagnetic signals were transmitted through the crust to an array of horizontal electric field recorders at the seafloor to ranges of 15 km from the source, which was a 100 m long horizontal electric dipole towed at heights of 50-80 m from the seafloor. Coincident seismic and magnetotelluric studies were conducted during the rest of the RAMESSES experiment. Data were interpreted using a combination of 1-D forward modelling and inversion, and iterative forward modelling in two dimensions. On the axis of the AVR, the resistivity at the seafloor is 1 Ohm m. There is a steep resistivity gradient in the upper few hundred metres of the crust, with the resistivity reaching approximately 10 Ohm m at a depth of 500 m. In order to explain the low resistivities, the upper layer of the crust must be heavily fractured and saturated with sea water. The resistivity increases with distance from the axis as the porosity decreases with increasing crustal age. The most intriguing feature in the data is the large difference in amplitude between fields transmitted along and across the AVR axis. A significant zone of low-resistivity material is required at approximately 2 km depth beneath the ridge crest in order to explain this difference. It is coincident with the low-velocity zone required by the seismic data, and has a total electrical conductance in excellent agreement with the results of the magnetotelluric study. The low-resistivity zone can be explained by the presence of a body of partially molten basalt in the crust. Taken together, these results provide the first clear evidence for a crustal magma chamber at a slow spreading mid-ocean ridge. The data constrain the melt fraction within the body to be at least 20 per cent, with a melt volume sufficient to feed crustal accretion at this segment of the ridge for of the order of 20 000 years. Since this body would freeze in the order of 1500 years, this finding lends support to the hypothesis that, at slow spreading rates, crustal accretion is a cyclic process, accompanying periodic influxes of melt from the mantle to a crustal melt reservoir.