Lithospheric Structure of the Central Andes Forearc from Gravity Data Modeling: Implication for Plate Coupling

Greg Gushurst,Rezene Mahatsente,Laurent Godin

doi:10.2113/2020/8843640

Abstract

Abstract Geodetic and seismological data indicates that the Central Andes subduction zone is highly coupled. To understand the plate locking mechanism within the Central Andes, we developed 2.5-D gravity models of the lithosphere and assessed the region’s isostatic state. The densities within the gravity models are based on satellite and surface gravity data and constrained by previous tomographic studies. The gravity models indicate a high-density (~2940 kg m-3) forearc structure in the overriding South American continental lithosphere, which is higher than the average density of the continental crust. This structure produces an anomalous pressure (20-40 MPa) on the subducting Nazca plate, contributing to intraplate coupling within the Central Andes. The anomalous lithostatic pressure and buoyancy force may be controlling plate coupling and asperity generation in the Central Andes. The high-density forearc structure could be a batholith or ophiolite emplaced onto the continental crust. The isostatic state of the Central Andes and Nazca plate is assessed based on residual topography (difference between observed and isostatic topography). The West-Central Andes and Nazca ridge have ~0.78 km of residual topography, indicating undercompensation. The crustal thickness beneath the West-Central Andes may not be sufficient to isostatically support the observed topography. This residual topography may be partially supported by small-scale convective cells in the mantle wedge. The residual topography in the Nazca ridge may be attributed to density differences between the subducting Nazca slab and the Nazca ridge. The high density of the subducted Nazca slab has a downward buoyancy force, while the less dense Nazca ridge provides an upward buoyancy force. These two forces may effectively raise the Nazca ridge to its current-day elevation.

Highlights

Large earthquakes commonly occur in Pacific subduction zones where the greatest tectonic strain accumulates [1]
In addition to the Valdivia earthquake, the South American subduction zone has had numerous large earthquakes in the past few centuries resulting in multiple full-length ruptures of the entire margin [7]
The northern model, 16°S, is located in the flat slab region where the Nazca ridge subducts beneath the South American plate

Summary

Introduction

Large earthquakes commonly occur in Pacific subduction zones where the greatest tectonic strain accumulates [1]. Even with the energy released from these recent earthquakes, it is expected that the zone still possesses enough elastic energy to experience similar events compared to the previous megaquakes in the region [15]. Earthquakes in this subduction zone are primarily caused by the Nazca plate subducting under the South American plate (Figure 1). Geodetic and seismological measurements in the area indicate regions of high plate coupling, centered around the Central Andes seismic gap zone up to a depth of 50 km, assuming elastic behavior [15]. We examine the locking mechanism of the plate interface in the Central Andes based on gravity data modeling. The density models are based on terrestrial and satellite gravity data and constrained by velocity models from seismic tomography and receiver function

Geologic Setting

Gravity Database and Methodology

E Kilometers mGal

Initial Model and Constraints

Results and Discussion

Conclusions