Crustal rupture and the initiation of imbricate thrusting in the Peru-Chile Trench

Abstract

A detailed survey of parts of the Peru-Chile Trench between lat 6° and 10°S shows that it has undergone extensive deformation within the past several thousand years. The tholeiitic basalt crust and overlying sedimentary deposits have ruptured, forming ridges within the trench axis and uplifted turbidite basins of former trench fill on the seaward wall of the trench. Several displaced terrigenous turbidite units show that 700 to 910 m of uplift has occurred within the past 3,155 yr on the most prominent ridge, while 700 m of uplift has occurred during the past 5,100 yr in the basins on the seaward wall of the trench. Average rates of relative vertical movement may range from 14 to 22 cm/yr in this area. This study suggests that the oceanic plate is fractured into three segments (N11°W, segment 1; N24°W, segment 2; and N31°W, segment 3) between lat 6° and 10°S. The most intense deformation is found at lat 7°40′S and 9°20′S, between segments 1 and 2 and 2 and 3, respectively; major tear faults are postulated for these two regions of extreme crustal rupture. The bathymetry, structure, nature of vertical movements, sediments of the Peru-Chile Trench and lower continental slope, and first motions of earthquakes on the oceanic plate suggest that imbricate thrusting plays an important role in the development of the features observed at the convergent boundary off Peru. The structural elements are best explained by a five-stage imbricate thrust model: (1) normal faulting of the descending oceanic plate resulting from tensional stress along the line of flexure; (2) reverse faulting resulting from a change from tensional to compressional stress within the trench; (3) initiation of thrust faults within the trench; (4) extensive rupture forming axial ridges and basins within the trench; and (5) accretion to the lower slope of the thrust sheet as it moves forward. Reactivation of movement along older imbricate thrusts in the continental slope may occur, particularly in stages 3, 4, and 5. The prominent benches on the continental slope probably develop as a consequence of the imbricate thrusting. The onshore and offshore region from lat 6° to 10°S is marked by a seismic gap (absence of shallow-focus earthquakes with magnitude > 7.7) and the onshore region by a paucity of late Cenozoic plutonic and volcanic activity. We postulate that the seismic gaps occur because extensive fracturing of the descending oceanic plate and motion along old imbricate thrust faults in the continental slope preclude the build-up of the large stress fields necessary to produce large-magnitude earthquakes. If imbricate thrusting and accretion is more prevalent than subduction in this region, it may also help to explain the lack of late Cenozoic volcanism in the area of the Huancabamba deflection.