Structural Framework of the Chile Margin at the Chile Ridge Collision Zone

Abstract

Three multichannel seismic reflection lines collected as part of the site survey for drilling Leg 141 examine the structure of the active Chile margin in the vicinity of the Chile Triple Junction. These data are used to compare the margin structure along three cross sections that characterize the interaction of the Chile seafloor-spreading ridge as it collides with the Chile Trench and is subducted beneath the South American continent. Line 745 is located where the ridge is expected to collide with the lower trench slope in approximately 100 k.y. At Line 751 the ridge was subducted at approximately 50 ka. Line 762 examines the Taitao Ridge, which is thought to be an ophiolite that has been obducted onto the margin at the Taitao Fracture Zone. The margin prior to ridge collision is characterized by a small accretionary complex and forearc basin that lie along the lower trench slope and abut attenuated continental crust underlying the middle and upper slopes. The tectonic interaction of the ridge with the margin appears minimal at this stage, with effects limited to those incurred by subducting a few large basement features that are not completely buried by the 100-400 m of trench-fill sediments. Where the ridge has recently been subducted, the tectonic interaction of the ridge is more pronounced. The toe of the small accretionary complex appears to have collapsed as the ridge passed beneath the lower trench slope and filled part of the large bathymetric depressions of the ridge axis that have not been filled by trench sediments. Comparison of the middle and upper slopes along the margin implies a large control by the continental crust on the morphology of the middle and upper slopes. Examination of the Taitao Ridge section shows considerable contrast in structure with the northern line. Here, much of the consinental slope is composed of a large igneous body thought to be ophiolite, and the thick trench-fill sequence shows no compressional deformation or active accretion.