THE EFFECTS OF SUBDUCTING AN ACTIVE SPREADING RIDGE ON THE SOUTHERN CHILE TRENCH

The Southern Chile triple junction is a classic example of the subduction of an actively spreading, mid-ocean ridge. Because of good marine geophysical constraints and the relatively simple geometry associated with the plate motions, the basic tectonic evolution of the plate system over the last 20 to 30 million years is well documented. Plate reconstructions show that a 700 km long section of the Chile Ridge collided with the Chile Trench starting near Tierra del Fuego about 14 Ma and migrated northward to the Golfo de Penas by 10 Ma. Additional sections of the ridge collided with the trench around 6 and 3 Ma and a segment of the ridge is currently being subducted northwest of the Taitao Peninsula. Because of the geometry of the transform offsets on the Chile Ridge, some sections of the trench, notably that seaward of the Tres Montes Peninsula, were probably subjected to multiple collision events. The simple geometry of the ridge-trench collisions also makes it possible to estimate the shape of the slab window that developed beneath Patagonia following ridge subduction. The ridge collision events have had a profound effect on the landward trench slope of the Chile trench. Seismic profiling expeditions and a deep sea drilling cruise (ODP Leg 141) have documented an extensive amount of tectonic erosion as the ridge approaches the trench followed by the rapid rebuilding of the landward trench slope after the passage of the triple junction. The balance of tectonically accreted versus subducted sediments along the margin has been determined from the drilling data by Behrmann and Kopf (2001). They find that north of the triple junction, prior to ridge subduction, only about 20% of the sediments on the Nazca plate are frontally accreted whereas south of the triple junction, following ridge subduction, 60% of the sediments are being accreted. Recent marine geophysical work by Bourgois et at. (2000) focused on understanding the effects of climate-induced variations in the sediment supply on the tectonics of the landward trench slope. They also propose that there was an apparent post-subduction, westward jump of the ridge north of the Taitao Fracture Zone associated with the development of a microplate, the Chonos microplate, just north of the present day triple junction. However, the shape of the central magnetic anomaly in the collision zone would seem to preclude such a ridge jump.