Dynamic Coupling of Sedimentation and Convergence Tectonics in Peru-Chile Trench and Outer Andean Margin

Laverne D. Kulm, Todd M. Thornburg

The convergence rate and sediment supply to the trench control the evolution of trench deposits as well as the subduction processes of accretion and erosion along the adjacent margin. South of 41 °S latitude, where Pleistocene cordilleran glaciation was severe, turbidity current deposition was unchannelized, producing sheeted basin deposits. Between 41 °S and 33 °S, trench fans (^approx 20 km wide) exhibit both depositional and erosional morphologies in response to dynamic tectonism within a prevailing axial gradient. An outboard axial channel transports massive quantities of remobilized sediments to the north. Subsurface lenticular bodies seen on seismic reflection profiles represent buried channel deposits. Sea MARC-II records show complex dispersal patte ns and erosional features on the fans, extensional structures on the descending plate, and anastomosed accreted ridges on the inner trench wall. Lithofacies include channel (amalgamated laminated to massive sand), levee (rhythmic thin-bedded graded sand and silt), and basin (low-energy graded and laminated silt) deposits. At 33 °S, San Antonio canyon feeds an axial sediment lobe at the base of a 1,400-m vertical discontinuity in the subducting slab. The canyon captures littoral sands and represents the last major source of sediment supply. North of 33 °S, the trench consists of small basins ponded within block-faulted depressions on the oceanic plate. Lithofacies include contourite (winnowed silt and sand laminae) and basin deposits. Large offsets in the descending plate, a ste p inner trench wall and the lack of slope basins indicate the northern Chile margin is undergoing subduction erosion.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.