An active spreading center collides with a subduction zone: A geophysical survey of the Chile Margin triple junction

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Abstract
The triple junction between the Nazca, Antarctic, and South American crustal plates has been mapped at lat 46.4°S, long 75.7° W. The Chile Ridge, an active spreading center opening at a half-rate of 28 mm/yr, can be traced into the axis of the Chile Trench; at the triple junction itself, the axial rift valley abuts the inner wall of the trench. The spreading process does not appear to be modified by proximity to a subduction zone until the actual point of collision: magnetic anomalies formed by sea-floor spreading can be traced to the inner wall of the trench; the depth of the axis of the Chile Ridge does not change with distance from the trench; the free-air gravity minimum associated with the trench decreases in amplitude owing to the elevated bathymetry at the triple junction, but this gravity anomaly continues across the junction; the thick wedge of trench-floor sediments is displaced only at the triple junction itself; and evidence for uplift of the continent also appears to be restricted to the immediate vicinity of the junction. We estimate that the present trench-ridge-trench triplejunction has existed for about 300,000 yr. Before the collision of this segment of the Chile Ridge axis with the trench, the triple junction was of the trench-transform-trench type.

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