Tectonic and Thermal Structure of the Middle Valley Sedimented Rift, Northern Juan de Fuca Ridge

Abstract

Recently acquired single-and multichannel seismic reflection profiles and over 500 heat flow measurements augment SeaBeam bathymetric and SeaMARC II and I side-scan acoustic data to provide new constraints on the tectonic setting and thermal structure of the sediment-filled rift, Middle Valley, of the northern Juan de Fuca Ridge. Over most of the length of the Juan de Fuca Ridge, spreading takes place along high-standing volcanic ridges, which are broken only by relatively small axial rift grabens. Near the northern end of the ridge south of the Sovanco fracture zone intersection, the volcanic supply is diminished, probably because of the lateral heat loss at the end of the ridge and the northern migration of the ridge in the wake of the lithospheric Explorer plate. This has resulted in the formation of deep rift valleys at the spreading axis. The deepest of these, Middle Valley, has been buried syntectonically by Pleistocene turbidite sediment. Heat flow in this valley varies inversely with sediment thickness, suggesting that the sediment forms a hydrologic seal over permeable igneous crust, where efficient hydrothermal circulation maintains relatively uniform temperatures. This simple model is investigated by comparing directly the thermal regime at depth and the seismic structure of the valley. Temperatures at depth are estimated from seafloor heat flow measurements using sediment physical properties derived from multichannel seismic (MCS) velocities. Although it is transitional and poorly defined in places, the acoustically defined sediment-basement contact is estimated to be typically about 300° C. Numerous local heat flow anomalies are observed in the valley. Two are associated with known hydrothermal vents also of roughly 300° C. Another lies near the normal-fault scarp that forms the eastern boundary of the valley, where basement is exposed at the seafloor, although in general, heat flow in the vicinity of the fault is relatively low, as are estimated basement temperatures. This suggests that the fault and the exposure of basement may serve primarily as a conduit for diffuse recharge of crustal fluids. Other heat flow anomalies and associated vents or vent fields overlie buried basement edifices where the sediment cover is locally attenuated. There is no significant heat flow anomaly and apparently no hydrothermal discharge associated with the most recent (Holocene) intrusive activity in the valley. The location of hydrothermal upflow zones appears to be influenced more by permeability structure, which is inferred to be controlled primarily by basement topography and variations in sediment thickness, rather than by the location of heat sources. The generally continuous, low-permeability sediment cover allows local vents to tap large areas of high-temperature permeable crust. The 300° C hydrothermal fluid temperatures currently present in Middle Valley are low compared to those required to produce solutions of high metal concentration. The presence of base-metal sulfide deposits in the valley suggests eith...

Fluid Inclusion Petrography and Microthermometry of the Middle Valley Hydrothermal System, Northern Juan de Fuca Ridge

“…wt% NaCl [ 1 σ]; n = 28). These similarities suggest a common origin for the fluids at these two sites, a conclusion also supported by the similarities between the high temperature alteration mineralogy of these two sites (Leybourne and Goodfellow, this volume) and by location of these sites along the same axis-parallel thermal anomaly (Davis and Villinger, 1992). The slightly higher mean temperature and salinity for Site 857 may indicate that the fluids at this site have mixed less with seawater and more closely approach endmember compositions.…”

Heat flow through the West Coast, South Island, New Zealand

“…Sonic velocity logs have been used to estimate porositydepth profiles within each well using a cubic polynomial transform similar to that of Davis & Villinger (1992). The observed porosity data were fitted with modified porositydepth relationships of the form…”

Effect of sedimentary facies and geological properties on thermal conductivity of Pleistocene volcanic sediments in Tokyo, central Japan

“…Fig. 8 Relationship between thermal conductivity measured using the box-type probe k BP and that measured using the needle-type probe k NP For example, Davis and Villinger (1992) estimated the depth profile of the thermal conductivity of Middle Valley sediments down to 2000 m using thermal conductivity of seawater and minerals (mica and clay minerals, quartz, feldspar, calcite, dolomite and chlorite). Midttømme et al (1998) discussed the effect of mineralogy on thermal conductivity of claystones and mudstones of London Clay.…”