Geophysical and geothermal data are examined from the three southernmost sections of the Chile Ridge, starting at 44°S and continuing south to the triple junction of the Nazca, Antarctic, and South America plates at 47°S. These sections represent three progressively younger stages in a ridge‐trench collision event, corresponding to 3 m.y. before the collision, 1 m.y. before the collision, and culminating in an ongoing collision at the triple junction. Magnetic and bathymetric data across the Chile Rise indicate that there is little change in the configuration of the spreading center as the ridge approaches and then collides with the trench. In the collision zone a “normal” looking rift valley can be traced for 40 km before it disappears beneath the toe of the landward trench slope. There is no evidence for the complex pattern of ridge jumps and spreading center rotations that occurred when the Pacific‐Farallon spreading center collided with North America. In contrast, the overriding plate is greatly affected by the ridge collision. The landward trench slope steepens and narrows as the collision zone is approached. At the southern end of the collision zone a ridge associated with the Taitao Fracture Zone is colliding with the trench and may be in the process of being obducted onto the landward trench slope. Geothermal measurements were made along three transects of the margin, corresponding to the time 3 m.y. before the collision, during the collision, and 6 m.y. after the collision. The heat flow measurements in the collision zone document a large pulse of heat associated with the subduction of the ridge: Values as high as 345 mW/m2 were recorded on the lower trench slope. A prominent bottom‐simulating reflector (BSR) observed over a wide area of the landward trench slope north of the triple junction, and, more locally, south of the triple junction, is used to expand the grid of heat flow observations. Excellent agreement is found between measured heat flow and estimates of heat flow based on the depth to the BSR. The heat flow measurements compare favorably with a theoretical model assuming conductive heat flow. We estimate that the accretionary prism has been substantially removed in the collision zone and conclude that the landward trench slope is undergoing an episode of rapid tectonic erosion. Periodic ridge collisions in the past may account for the apparent truncation of the Andean forearc region.

A detailed survey of heat flow and geochemical gradients in pore water within a 10 x 10 km area surrounding Site 501/504 revealed a broad and undulating variation in heat flow. The average heat flow weighted by area is 216 mW/m 2 , and individual values range from 166 to 391 mW/m 2. The highest values were found in three localized zones. Gradients of Ca 2+ and Mg 2+ in pore waters of the upper 12 m of sediment were found to be extremely high within these high heat-flow zones. The profiles of calcium and magnesium have a characteristic exponential shape due to upward vertical movement of pore water through the sedimentary layer. Flux rates as high as 6 mm/yr were estimated from the shape of the curves. The discovery of significant flux through the sediments implies that there are reduced pressure variations in the top of the igneous crust on the order of a few bars and that the pressure field relative to hydrostatic probably drives lateral flow in upper basement at rates of 10-30 cm/yr. The survey and subsequent drill data verify that this flow attenuates variations in temperature and pore-water chemistry in the upper few hundred meters of basement. The variations in heat flow, pore-water chemical gradients, and implied fluxes of pore water are associated with hydrothermal convection in basement at a lateral scale of 3.5-4 km that may penetrate deeply into the crust.

Heat flow measurements in the east-use of magnetic, topographic, and deep-sea drillern Pacific now total over 800, a sufficient hum-ing data has established the overall tectonic bet to petit the analysis of their distribution structure and age trends in this area, while within a wide range of age zones. Major results leaving some ambiguity in the ages of certain reer of sediment much more quickly on the Galapagos flow measurements which would not be apparent if Spreading Center than on the East Pacific Rise. Quantitatively, the degree of heat transfer by convection appears to correlate inversely with the ratio of sediment thickness to topographic relief. past between the predicted and observed heat flow thermal activity dominates the thermal regime in distributions of lithospheric plates [cf., 0-to 10-m.y.-old sea floor on the East Pacific McKenzie, 1967; McKenzie and Sclater, 1969; Rise (EPR), and in 0-5-m.y.-old sea floor on Le Pichon and Lan•seth, 1969; Sclater and the Galapagos Spreading Center. The cessation Francheteau, 1970; Von Herzen and Anderson, 1972]. of this circulation with aging of the sea floor This paper addresses the variation of heat flow as a function of sediment thickness and age in the eastern Pacific in an attempt to aid in resolving these discrepancies which have consistently shown observed heat flow values in the crestal regions of midocean ridges to be lower appears to be controlled by sediment thickness and topographic relief. The Galapagos Spreading Center The east-trending Galapagos Spreading Center than predicted. Two problems complicate this extends from a triple junction with the EPR at analysis: the tectonic complexity of this part of 2øN, 102øW to the Panama fracture zone, a transthe Pacific and the local variability of heat form fault forming the eastern boundary separatflow measurements near spreading centers. The ing the Cocos plate and the Panama basin, which is thought to be attached to the Nazca plate

Closely spaced heat flow surveys at four sites on the flanks of the Central Indian Ridge and the Southeast Indian Ridge delineate a pattern of oscillatory heat flow which can only result from cellular convection of oceanic bottom water through the oceanic crust and overlying sediment. These cells have a wavelength of 5 to 10 kilometers and are presently active in sea floor 18 x 10(6), 25 x 10(6), and 45 x 10(6) years old of the Crozet Basin and in sea floor 55 x 10(6) years old of the Madagascar Basin. The precise measurement of nonlinear temperature profiles makes it possible to calculate the conductive and convective heat transfer components through the sea floor. Even in the oldest sites, geothermal convection is still a major component of heat transfer through both the crust and sedimentary layers. These observations coupled with the results of earlier oceanwide geothermal studies indicate that more than one-third of the entire surface area of the world's ocean floor contains presently active geothermal convection that is cellular in plan form.