Kinematics and geometry of active detachment faulting beneath the Trans-Atlantic Geotraverse (TAG) hydrothermal field on the Mid-Atlantic Ridge

Abstract

Newly acquired seismic refraction and microearthquake data from the TransAtlantic Geotraverse (TAG) segment of the Mid-Atlantic Ridge at 26°N reveal for the fi rst time the geometry and seismic character of an active oceanic detachment fault. Hypocenters from 19,232 microearthquakes observed during an eight month ocean bottom seismometer deployment form an ~15-km-long, dome-shaped fault surface that penetrates to depths >7 km below the seafl oor on a steeply dipping (~70°) interface. A tomographic model of compressional-wave velocities demonstrates that lower crustal rocks are being exhumed in the detachment footwall, which appears to roll over to a shallow dip of 20° ± 5° and become aseismic at a depth of ~3 km. Outboard of the detachment the exhumed lithosphere is deformed by ridge-parallel, antithetical normal faulting. Our results suggest that hydrothermal fl uids at the TAG fi eld exploit the detachment fault to extract heat from a region near the crust-mantle interface over long periods of time.

The Trans-Atlantic Geotraverse hydrothermal field: A hydrothermal system on an active detachment fault

“…The absence of a crustal melt reservoir and the extension of microearthquake activity (and thus the brittle-plastic rheological transition) to depths greater than 7 km suggest that hydrothermal fluids penetrate much deeper into the lithosphere than previously supposed deMartin et al, 2007).…”

“…Over the last ten years, however, geophysical studies have revealed that the TAG hydrothermal field is located on the hanging wall of an active detachment fault (Tivey et al, 2003;Canales et al, 2007;deMartin et al, 2007). This finding has major implications for our understanding of hydrothermal circulation at slow-spreading mid-ocean ridges.…”

“…A study of microearthquakes over an eight month period showed that detachment faulting is accommodated on an ~15 km-long, dome-shaped fault surface that penetrates into the mantle at a steep dip of ~70° over the depth interval of ~3 to >7 km below the seafloor, and rolls over to a low angle of ~20°, becoming aseismic at shallower depths of 2-3 km below the seafloor (Figure 2) ). An active-source seismic survey revealed that there are no mid-crustal magma bodies beneath the hydrothermal field , so deMartin et al (2007) inferred that hydrothermal fluids must penetrate to great depths (>7 km) to extract heat out of a gabbroic intrusion at the root zone of the detachment fault (Figure 2). They suggested that circulation in the middle to lower crust may be focused on the relatively permeable, three-dimensional detachment fault surface, while in the upper crust high temperature fluids rise vertically through the highly tectonized hanging wall .…”

“…Since the discovery in 1985 of the first high temperature vents, massive sulfide deposits, and new vent ecosystems in the Atlantic Ocean again by Peter Rona (Rona at el., 1986), the TAG hydrothermal field has served as a major 'type' example of a massive sulfide deposit on a slow-spreading ridge, particularly with its similarities to sulfide deposits in Cyprus (e.g., Herzig et al, 1991;Hannington et al, 1998;Humphris and Cann, 2000). It is one of the most comprehensively studied seafloor hydrothermal fields and has been the focus of numerous geophysical (e.g., Kong et al, 1992, Tivey et al, 1993, 1996Goto et al, 2003;deMartin et al, 2007;Canales et al, 2007;Zhao et al, 2012;Pontbriand and Sohn, 2014), geochemical (e.g., Edmond et al, 1995;Humphris et al, 1998;Tivey et al, 1995Humphris and Bach, 2005), and biological (e.g., Van Dover et al, 1988;Galkin and Moskalev, 1990;Copley et al, 1999) investigations. In addition, it is one of only three actively venting seafloor hydrothermal systems to be sampled in the third dimension as part of the Ocean Drilling Program (ODP) Herzig et al, 1998).…”

Central role of detachment faults in accretion of slow-spreading oceanic lithosphere

“…An 8-month microseismic experiment at the TAG hydrothermal field (26.1°N) shows deformation along the active detachment fault accommodated by continuous creep associated with steady hydroacoustic event rates (Figure 1d). Microseismicity events at ridge sections with active detachments (TAG 13,15 , 23°N 16 ) are observed down to 7-8 km below seafloor, with the interpreted fault rooting directly below the neovolcanic zone 13 . In contrast, at the centre of symmetrical segments (35°N 17 and Figure 1b, 29°N 18 ) events extend to maximum depths of 5-6 km, 1-3 km shallower than near detachments (see There is a close association of hydrothermal activity and asymmetrical accretion.…”