EPISODIC DEVELOPMENT OF A CONVERGENT MARGIN INFERRED FROM STRUCTURES AND PROCESSES ALONG THE SOUTHERN CHILE MARGIN

Seismic reflection data acquired in the vicinity of Isla Mocha across the southern coast of Chile image structures formed along the continen tal margin and reveal an episodic history of accretion, nonaccretion, and possibly erosion. Structures formed at the toe of the continental slope suggest frontal accretion of 3/4 to 1 3/4 km of trench fill. Sei smic images also reveal that a small accretionary wedge, 20-30 km wide , abuts the truncated continental metamorphic basement that extends se award from beneath the shelf. The small size of the accretionary wedge on three profiles examined here is not consistent with a long history of accretion with the current deformational style, as current rates o f frontal accretion could have accumulated all of the existing accreti onary wedge in less than 1-2 m.y. This is a small fraction of converge nce history along this margin, and the current accretionary mode has n ot been consistently maintained in the past. The Isla Mocha region is located between the temperate climate of central Chile and the glacial climate of southern Chile, and climatic conditions in this region hav e likely fluctuated sufficiently to cause significant variation in tre nch sediment supply. Accretionary and nonaccretionary or erosional epi sodes are probably linked to temporal variations in trench sediment th ickness, as suggested by observations along the Chile margin. Currentl y, thick trench sediment correlates with accretion along the southern Chile margin, and thin trench sediment correlates with nonaccretion/te ctonic erosion as near the Chile Ridge and from the Juan Fernandez Rid ge to northern Chile. The Isla Mocha region also lies 900-1000 km nort h of the Chile triple junction, and the Chile Ridge lies approximately 2000 km to the west and has not yet collided and affected the margin near Isla Mocha. This part of the precollision zone provides an excell ent reference to examine the effects of Chile Ridge collision in the d evelopment of the Chile margin. The most apparent effect of subduction of the buoyant, young crust of the Chile Ridge is a shallow trench th at is nearly devoid of sediment. Consequently, the triple junction is undergoing nonaccretion or erosion, and the accretionary complex near the triple junction remains smaller than to the north or south because the current phase of rapid accretion elsewhere in the trench has bypa ssed the triple junction region. The interplay of subduction zone proc esses, such as trench sedimentation and ridge collision, has resulted in an episodic development of the margin and produced a discontinuous record of convergence history within the accretionary wedge.