R. P. Von Herzen

Rifting origin for the vema fracture in the North Atlantic

The mid-Atlantic ridge crest is offset more than 300 km along the E-W trending Vema Fracture valley. The trend of the extremeties of this valley outside the region of recent earthquakes, as well as that of apparently inactive transverse valleys to the south, is somewhat south of east. These observations, and the fact that the valleys to the south apparently do not extend across the ridge crest, strongly suggest that a new pattern of ridge growth and sea-floor spreading, represented by the Vema Fracture valley, has been superimposed on an older one. A geometrical model is proposed for the recent development of the Vema Fracture valley, which is shown to be consistent with other geological and geophysical observations in this region.

Heat-Flow Values from the South-Eastern Pacific

Prior to these new measurements, twenty-five heat-fiow values from the Pacific Ocean Basin and eight from the Atlantic have been published1. The surprising fact brought out by these measurements is the approximate equality of average heat flow of the oceanic and continental areas, about 1 × 10−6 cal./cm.2 sec. This result was unexpected on the basis of values of radioactivity indicated for typical continental and oceanic crustal rocks existing near the Earths surface. It has usually been assumed that most of the surface heat flow in continental regions must originate in the relatively thick and radioactive continental crust. Under the oceans, nearly all the average heat flow must come from the mantle beneath the relatively thin oceanic crust.

Geologic form and setting of a hydrothermal vent field at lat 10°56′N, East Pacific Rise: A detailed study using Angus and Alvin

A hydrothermal vent field, here called the Feather Duster site, occurs on the eastern marginal high near the edge of a narrow (95-m) and shallow (15-20-m) axial graben, within an area dominated by sheet flows and collapse features. The sheet flows are intermediate in relative age between younger fluid-flow lavas on the floor of the axial graben and older pillow (constructional) lavas on the marginal highs. Hydrothermal activity occurs in two zones within a 65 by 45 m area. The main zone is located where a fissure system and sulfide-sulfate chimneys vent warm (9-47/sup 0/C) and hot (347/sup 0/C) hydrothermal fluids. Here, two mounds of massive sulfide totaling about 200 t are forming. One occurs at the base of a 3-m-high scarp which is the wall of a drained lava lake; the other is perched on top of the scarp. 19 references, 4 figures.

A constraint on the shear stress at the Pacific‐Australian plate boundary from heat flow and seismicity at the Kermadec forearc

New heat flow measurements and relocated hypocenters that constrain the subduction geometry of the Pacific plate at the Kermadec trench yield an estimate of shear stress on the thrust fault. With the exception of a few relatively high values (>60 mW m−2) on the upper forearc region near the active volcanic ridge, most of the 64 heat flow values along two profiles across the forearc range from 20 to 40 mW m−2. Corrections for bottom water temperature variations that caused nonuniform temperature gradients were required for most measurements. The means of the most reliable values for the northern and southern profiles are 28.9±5.8 mW m−2 (n = 33) and 28.9±7.2 mW m−2 (n = 19), respectively, and the measurements show no apparent systematic variation along the profiles over the range ∼50 to 150 km distance from the trench. The means of the 10 values at each of two sites on the Pacific plate seaward of the profiles are 57.2±6.3 and 60.2±6.6 mW m−2. Redeterminations of focal depths and fault plane solutions of earthquakes in the vicinity of the heat flow profiles indicate thrust faulting on a plane dipping 17°±2°. Calculated values of heat flow for a two-dimensional analytical approximation to conduction through the upper plate, diffusion into the downgoing slab, and advection by that slab are consistent with either a uniform stress of ∼40±17 MPa along the thrust fault or stress increasing linearly at ∼0.5±0.2 MPa km−1 with distance from the trench axis. The comparable scatter in the heat flow measurements about those calculated from these simple stress distributions does not show one to be a better approximation than the other.

A major geothermal anomaly in the Gulf of California

We have mapped a 3-km wide, high heat flow anomaly with a maximum value of 30 μcalorie cm −2 s−1 within a zone of seafloor extension in the central Gulf of California. From seismic reflection data and thermal modelling we suggest that the anomaly is caused by a 1-km wide basaltic intrusion which is roughly 100 m deep and less than 18,000 yr old.

Heat flow and age of the Gulf of Oman

We present the results of twenty heat flow stations in the Gulf of Oman which are used to infer the first reliable age estimates for the basin. A mean surface heat flux of 42.6±3.6 mW m−2 exhibits no significant regional variation. After correction for thick and rapidly deposited sediments this yields an age of 70 to 100 Ma according to oceanic thermal models. This age is also consistent with the sediment corrected basement depths of 5.5–6.0 km and with formation during the Cretaceous quiet zone. The latter can explain the absence of magnetic sea-floor spreading lineations. Heat flow measurements are also used to confirm the presence of gas hyrdate layers. The measured thermal gradient yields a depth for the solid to free gas phase transition which is the same as that deduced from “bright spots” seen on seismic reflection profiles.

Conductive heat flow at the TAG Active Hydrothermal Mound: Results from 1993–1995 submersible surveys

We report 70 measurements of conductive heat flow at the 50-m-high, 200-m-diameter TAG active hydrothermal mound, made during submersible surveys with Alvin in 1993 and 1995 and Shinkai 6500 in 1994. The stations were all measured with 5-thermistor, 0.6- or 1-m-long Alvin heat flow probes, which are capable of determining both gradient and thermal conductivity, and were transponder-navigated to an estimated accuracy of ±5–10 m relative to the 10-m-diameter central complex of black smokers. Within 20 m of this complex, conductive heat flow values are extremely variable (0.1- > 100 W/m²), which can only be due to local spatial and possible temporal variability in the immediate vicinity of the vigorous discharge sites. A similar local variability is suggested in the “Kremlin” area of white smokers to the southeast of the black smoker complex. On the south and southeast side of the mound, there is very high heat flow (3.7- > 25 W/m²) on the sedimented terraces that slope down from the Kremlin area. Heat flow is also high (0.3–3 W/m²) in the pelagic carbonate sediments on the surrounding seafloor within a few tens of meters of the southwest, northwest, and northeast sides of the mound. On the west side of the sulfide rubble plateau that surrounds the central black smoker peak, there is a coherent belt of very low heat flow (<20 mW/m²) 20–50 m west of the smokers, suggestive of local, shallow recharge of bottom water. The three submersible surveys spanned nearly two years, but showed no indication of any temporal variability in conductive heat flow over this time scale, whether natural or induced by ODP drilling in 1994.

Heat flux through an old (∼175 Ma) passive margin: Offshore southeastern United States

New heat flow data on the southeastern United States passive margin show that measured, uncorrected flux averages 49±11.8 mW m−2 through old (∼175 Ma) oceanic crust. Nonuniform thermal gradients were measured at about half of the 114 penetrations that comprise the data set and over the full range of water depths (1900 m to 4250 m) at which data were collected. With the simplifying assumption that the nonuniform gradients were caused by a step function change in bottom water temperatures at some time before the heat flow cruises, concave down (decreasing dT/dz with depth) and concave up (increasing dT/dz) sediment thermal gradients can be explained by respective average temperature decreases and increases of 0.1–0.2 K amplitude occurring 28–35 days before the measurements. Thermal gradients throughout the entire region are strongly influenced by oceanographic phenomena and locally by the presence of subsurface diapiric structures, while sediment thickness variations and lateral differences in sedimentation rate and sediment lithology appear to exercise relatively little control over thermal regimes. Mean reduced heat flow in the study area is estimated at ∼49 mW m−2 by decompacting and back stripping the 5–8 km of sediment deposited on the margin since the formation of the underlying oceanic crust. This value agrees with previous measurements made on younger crust in the Blake Ridge area but is significantly higher than the reduced heat flow value in a similar passive margin setting at the Baltimore Canyon Trough. We use the calculated average thermal gradient value and an assumption of constant conductivity to estimate temperatures of 19.5°C to 24.5°C at the bottom-simulating reflector (BSR) on the Blake Ridge slope.

Ocean-floor heat-flow measurements west of the United States and Baja California☆

Abstract Heat-flow values west of North America range from 0.10-5.79 · 10 −6 cal./cm 2 sec. Most measurements at the same location repeat reasonably well. Some of the variations appear periodically distributed with distance over long lines, with characteristic “wavelengths” of the variations ranging from about 250 km off Baja California to 1,000 km off northern California. For the most part, the variations appear uninterrupted across the Mendocino fracture zone. High values occur near the coast, three or four times normal north of the Mendocino fault and twice normal in the borderland off southern and Baja California.

Geoelectrical measurements at the TAG Hydrothermal Mound

Direct current electrical experiments with electrode arrays deployed by a submersible (ALVIN) were used to determine the resistivity structure of the seafloor below, and vertical potential gradients in sea water above, the hydrothermally active TAG mound. Apparent resistivities of the sulfides at shallow depths (∼10 m) below seafloor over the mound ranged between 0.18–0.21 ohm-m, compared to nearby basalt pillow apparent resistivities of 2.1–2.4 ohm-m. The potential differences between heights of about 18 to 27 m above the mound averaged +3.7 mV (voltage increasing with height), with variations between about −2.9 and +12.7 mV.