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 collison zone and conclude that the landward trench slope is undergoing an eposode of rapid tectonic erosion. Periodic ridge collisions in the past may account for the apparent truncation of the Andean forearc region. ¿ American Geophysical Union 1986