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[1] We forward modeled the Bouguer anomaly in a region encompassing the Pacific Ocean (85°W) and the Andean margin (60°W) between northern Peru (5°S) and Patagonia (45°S). The three-dimensional density model that reproduces the gravity field is a continental-scale representation of density structure to 410 km depth that characterizes the mantle and crust of the oceanic Nazca plate, subducted slab and continental margin with a minimum number of bodies. We predefined the density of each body after studying the dependency of density on composition of crustal and mantle materials and pressuretemperature conditions appropriate for the Andean setting. A database of independent geophysical information constrains the geometry of the top of the subducted slab, locally the Moho of the oceanic and continental crusts and, indirectly, the lithosphereasthenosphere boundary underneath the continental plate. Other boundaries, notably the intracrustal density discontinuity separating upper from lower crust below the continent, were not constrained and their geometry is the result of fitting the observed and calculated Bouguer anomaly during forward modeling. This contribution presents the model to the Andean geoscientific community and contains some tools, like a sensitivity analysis, that helps potential users of the model to interpret its results. We describe and discuss some of these results in order to illustrate the application of the model to the study of a wide range of phenomena (e.g., modification of oceanic plate structure by hot spots, shape of the subducted slab, thermal structure of the continental lithosphere, compensation mechanism and formation of orogenic relieve, causes of Andean segmentation).Citation: Tassara, A., H.-J. Götze, S. Schmidt, and R. Hackney (2006), Three-dimensional density model of the Nazca plate and the Andean continental margin,

SUMMARY We present an upgraded version of a previously published 3‐D density model of the Andean subduction zone between 18°S and 45°S. This model consists of 3‐D bodies of constant density, which geometry is constrained by independent seismic data and is triangulated from vertical cross‐sections. These bodies define the first‐order morphology and internal structure of the subducted Nazca slab and South American Plate. The new version of the density model results after forward modelling the Bouguer anomaly as computed from the most recent version of the Earth Gravitational Model (EGM2008). The 3‐D density model incorporates new seismic information to better constrain the geometry of the subducted slab and continental Moho (CMH) and has a trench‐parallel resolution doubling the resolution of the previous model. As an example of the potential utility of our model, we compare the geometry of the subducted slab and CMH against the corresponding global models Slab1.0 and Crust2.0, respectively. This exercise demonstrates that, although global models provide a good first‐order representation of the slab and upper‐plate crustal geometries, they show large discrepancies (up to ±40 km) with our upgraded model for some well‐constrained areas. The geometries of the slab, lithosphere–asthenosphere boundary below the continent, CMH and intracrustal density discontinuity that we present here as Supporting Information can be used to study Andean geodynamic processes from a wide range of quantitative approaches.

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[1] We document a crustal-scale structural model for the central Chile Andes based on seismicity and surface geology, which consists in a major east verging ramp-detachment structure connecting the subduction zone with the cordillera. The ramp rises from the subducting slab at ∼60 km depth to 15-20 km below the western edge of the cordillera, extending eastward as a 10 km depth flat detachment. This structure plays a fundamental role in the Andean orogenesis because most of the shortening has been accommodated by structures rooted in it and allows the distribution of crustal thickening in a "simple shear deformation mode." Indeed, despite shortening distribution being very asymmetric (∼16 km versus ∼70 km in the western and eastern side, respectively), the western side is higher and thicker than what is expected. Yield strength envelopes show strong rheological control on this structure. V p and V p /V s variations in the upper mantle and in the deepest limit of the seismogenic interplate contact mark the intersection of the ramp with the slab, which coincides with the blueschist-eclogite transition. Therefore, subduction processes would control the depth where the major east verging structure may merge with the slab. Such a ramp-flat structure is observed in other parts of the Chilean margin; hence, it seems to be a first-order feature in the Andean subduction zone. This structure delimitates upward the rocks, transmitting part of the plate convergence stress from the plate interface, and controls mountain-building tectonics, thus playing a key role in the Andean orogeny.

We propose an integrated kinematic model with mechanical constrains of the Maipo–Tunuyán transect (33°40′S) across the Andes. The model describes the relation between horizontal shortening, uplift, crustal thickening and activity of the magmatic arc, while accounting for the main deep processes that have shaped the Andes since Early Miocene time. We construct a conceptual model of the mechanical interplay between deep and shallow deformational processes, which considers a locked subduction interface cyclically released during megathrust earthquakes. During the coupling phase, long-term deformation is confined to the thermally and mechanically weakened Andean strip, where plastic deformation is achieved by movement along a main décollement located at the base of the upper brittle crust. The model proposes a passive surface uplift in the Coastal Range as the master décollement decreases its slip eastwards, transferring shortening to a broad area above a theoretical point S where the master detachment touches the Moho horizon. When the crustal root achieves its actual thickness of 50 km between 12 and 10 Ma, it resists further thickening and gravity-driven forces and thrusting shifts eastwards into the lowlands achieving a total Miocene–Holocene shortening of 71 km.

Editor: P. ShearerKeywords: subduction post-seismic megathrust afterslip GPS MauleThe excellent spatial coverage of continuous GPS stations in the region affected by the Maule 8.8 2010 earthquake, combined with the proximity of the coast to the seismogenic zone, allows us to model megathrust afterslip on the plate interface with unprecedented detail. We invert post-seismic observations from continuous GPS sites to derive a time-variable model of the first 420 d of afterslip. We also invert co-seismic GPS displacements to create a new co-seismic slip model. The afterslip pattern appears to be transient and non-stationary, with the cumulative afterslip pattern being formed from afterslip pulses. Changes in static stress on the plate interface from the co-and post-seismic slip cannot solely explain the aftershock patterns, suggesting that another process -perhaps fluid related -is controlling the lower magnitude aftershocks. We use aftershock data to quantify the seismic coupling distribution during the post-seismic phase.Comparison of the post-seismic behaviour to interseismic locking suggests that highly locked regions do not necessarily behave as rate-weakening in the post-seismic period. By comparing the inter-seismic locking, co-seismic slip, afterslip, and aftershocks we attempt to classify the heterogeneous frictional behaviour of the plate interface.