FlankFlux: an experiment to study the nature of hydrothermal circulation in young oceanic crust

Abstract

The sediment-buried eastern flank of the Juan de Fuca Ridge provides a unique environment for studying the thermal nature and geochemical consequences of hydrothermal circulation in young ocean crust. Just 18 km east of the spreading axis, where the sea-floor age is 0.62 Ma, sediments lap onto the ridge flank and create a sharp boundary between sediment-free and sediment-covered igneous crust. Farther east, beneath the nearly continuous turbidite sediment cover of Cascadia Basin, the buried basement topography is extremely smooth in some areas and rough in others. At a few isolated locations, small volcanic edifices penetrate the sediment surface. An initial cruise in 1978 and two subsequent cruises in 1988 and 1990 on this sedimented ridge flank have produced extensive single-channel seismic coverage, detailed heat flow surveys co-located with seismic lines, and pore-fluid geochemical profiles of piston and gravity cores taken over heat flow anomalies. Complementary multichannel seismic reflection data were collected across the ridge crest and eastern flank in 1985 and 1989. Preliminary results of these studies provide important new information about hydrothermal circulation in ridge flank environments. Near areas of extensive basement outcrop, ventilated hydrothermal circulation in the upper igneous crust maintains temperatures of less than 10–20 °C; geochemically, basement fluids are virtually identical to seawater. Turbidite sediment forms an effective hydrologic and geochemical seal that restricts greatly any local exchange of fluid between the igneous crust and the ocean. Once sediment thickness reaches a few tens of metres, local vertical fluid flux through the sea floor is limited to rates of less than a few millimetres per year. Fluids and heat are transported over great distances laterally in the igneous crust beneath sediment however. Heat flow, basement temperatures, and basement fluid compositions are unaffected by ventilated circulation only where continuous sediment cover extends more than 15–20 km away from areas of extensive outcrop. Where small basement edifices penetrate the sediment cover in areas that are otherwise fully sealed, fluids discharge at rates sufficient to cause large heat flow and pore-fluid geochemical anomalies in the immediate vicinity of the outcrops. After complete sediment burial, hydrothermal circulation continues in basement. Estimated basement temperatures and, to the limited degree observed, fluid compositions are uniform over large areas despite large local variations in sediment thickness. Because of the resulting strong relationship between heat flow and sediment thickness, it is not possible, in most areas, to detect any systematic pattern of heat flow that might be associated with cellular hydrothermal circulation in basement. However, an exception to this occurs at one location where the sediment thickness is sufficiently uniform to allow detection of a systematic variation in heat flow that can probably be ascribed to cellular circulation. At that location, temperatures at the sediment–basement interface vary smoothly between about 40 and 50 °C, with a half-wavelength of about 700 m. A permeable-layer thickness of similar dimension is inferred by assuming that circulation is cellular with an aspect ratio of roughly one. This thickness is commensurate with the subbasement depth to a strong seismic reflector observed commonly in the region. Seismic velocities in the igneous crustal layer above this reflector have been observed to be low near the ridge crest and to increase significantly where the transition from ventilated to sealed hydrothermal conditions occurs, although no associated reduction in permeability can be ascertained from the thermal data.

Design, deployment, and status of borehole observatory systems used for single-hole and cross-hole experiments, IODP Expedition 327, eastern flank of Juan de Fuca Ridge

“…Site map showing locations of ODP/IODP Holes 1026B, 1027C, U1301A, U1301B, U1362A, and U1362B (see the "Site U1362" chapter; Expedition 301 Scientists, 2005b;Shipboard Scientific Party, 1997). Gold contours show locations of basement outcrops (Davis et al, 1992b;Zühlsdorff et al, 2005). Sites 1026, U1301, and U1362 are aligned above a buried basement high, where sediment thickness is 236-265 m, whereas Site 1027 is located above an area of thicker sediment and deeper basement.…”

Expedition 301 synthesis: hydrogeologic studies

“…F1A), then flowing toward the east. Some results from Leg 168 were consistent with this view, including seafloor heat flow and basement temperatures that increased and basement fluids that were warmer and more altered with greater distance to the east along the drilling transect (e.g., Davis et al, 1992Davis et al, , 1999Elderfield et al, 1999). But there were inconsistencies with this conceptual model of largescale hydrogeologic flow, especially after Leg 168 results were considered.…”

Site U1301

“…Sediment structures and the sediment/basalt interface are clearly visible, as are steeply dipping normal faults to the west of the holes (indicated with vertical lines that show total depth penetration). Both holes are located over the peak of a buried basement high, near the western edge of a major distributary channel for turbidites that flowed off the North American continental shelf (Davis et al, 1992;Zühlsdorff et al, 2005;Underwood et al, 2005;Hutnak et al, 2006). Proc.…”