The Peru-Chile Margin from Global Gravity Field Derivatives

Abstract

Deformation along the 3,500 km subduction Pacific margin of the Peru-Chile trench is partially controlled by ocean bathymetric heterogeneities and sediments. Oceanic highs (e.g. ridges, fracture zones, plateaus) influence deformation in the fore-arc zone where collision occurs, and control turbiditic flow dispersal and consequently the amount of sediments accreted at the frontal accretionary prism and subduction channel, compartmentalizing the trench into segments linked to seismic segmentation. Recent satellite missions (CHAMP, GRACE and GOCE) have introduced an extraordinary improvement in the reconstruction of the global gravity field. Earth gravity field models, mainly derived from satellite measurements, reflect mass inhomogeneities of the earth. This chapter focuses on the determination of mass heterogeneities over the oceanic plate and their relation to general distribution of sediments over the Peru-Chilean margin, seismic segmentation along the margin, and the relationship between trench sediment thickness and the variable Andean orogenic volume, by means of a gravimetric analysis. Using the gravity potential model EGM2008 and satellite GOCE data we calculated two functionals of the geopotential: the Bouguer anomaly and the vertical gravity gradient, both corrected for the topographic effect. The vertical gravity gradient field is of special interest as it highlights main geological features, and allows unraveling unknown structures that are concealed by sediments. From these, different features can be clearly depicted such as the contact between the Pacific oceanic crust and the South American plate, the Nazca Ridge, the Juan Fernandez Ridge and the Chile Rise, among others. The segmentation between a filled trench south of Juan Fernandez Ridge, a partially filled trench to the north up to the Copiapo Ridge, and a completely starved trench north of this latitude is depicted. Finally, the relationship between gravity derived fields, high oceanic features and seismic segmentation is discussed for the last megathrust earthquakes that affected this subductive plate boundary.