Accretion and lateral variations in tectonic structure along the Peru-Chile Trench

Abstract

Extensive marine geophysical data sets on the deformational characteristics of the oceanic crust and of sediments in and near the Peru-Chile Trench (notably those presented by Schweller, Kulm and co-workers) have established the presence of significant north-south variations along the trench. In this paper we investigate the physical basis for these variations by studying the stress field in the Nazca plate with emphasis on the region near the trench. Finite element methods were employed to calculate the stress distribution. The resulting regional stress field near the trench is characterized by compression normal to the trench at latitudes between 1°S and approximately 16°S, tension in the range 16°S–27°S and again compression from 27°S to 45°S. Many conspicuous and hitherto unexplained variations along the trench system can be understood by taking into account these lateral changes. A close correlation appears to exist between the latitude ranges with compression and tension normal to the trench and the accreting and non-accreting provinces of the Peru-Chile Trench, respectively. The basis for this correlation must be sought in the role which grabens in the seaward trench slope play in the subduction (or accretion) of sediments. Deep grabens can trap the sediments and carry them down into the subduction zone, leaving little or no material to be accreted onto the margin. The regional stress field is superimposed on the stresses generated in bending the Nazca plate prior to subduction. Tension in the direction normal to the trench, as inferred for the Nazca plate off southern Peru and northern Chile (16°S–27°S), lowers the neutral plane in the bending plate and increases the tensional stresses near the surface. This promotes the formation of grabens and, consequently, the subduction of sediments. Similarly, compression normal to the trench counteracts the formation of deep grabens in the latitude ranges 1°S–16°S and 27°S–45°S, thus favouring accretion of sediments.From this study we infer that the regional stress field may have a significant effect on the topography of the seaward trench slope. Taking into account the role of regional stress in graben formation not only explains first-order latitudinal variations in accretion and subduction of sediments along the Peru-Chile Trench, but it is also expected to contribute significantly to understanding similar variations observed in other subduction zones.