Slab segmentation of the Nazca plate across the Juan Fernández Ridge

We report preliminary results based on the construction of a new 3D model for the geometry of the subducted lithosphere of the Nazca Plate. This new 3D model differs from Slab2 in that it enables capture of the slab geometry in greater detail and allows the identification of previously unrecognised potential slab tears, both down-dip and along strike, as well as slab gaping as tears open. These differences in the inferred 3D structure emerge because previous models were excessively smoothed. Here we use the interpolation of line strings derived from the interpretation of individual cross-sections: a method that has the capacity to capture detail, and to accurately drape and thus derive a 3D geometry consistent with the observed hypocenter patterns, by using Delaunay triangulation alongside with 3D grid interpolation. The 3D slab morphology obtained provides insight into the interplay between subduction earthquakes at different depths. The variation in slab morphology reinforces the concept that the megathrust comprises distinct rupture segments that behave differently in terms of their overall seismotectonic behaviour. The slab morphology also links to changes in the broad geodynamics of the subduction zone, with links between shallow, intermediate, and deep seismicity that are consistent with variation in 3D slab morphology. It is also possible to explain the variation in surficial crustal tectonic processes in the context of geodynamic processes inferred to be taking place at depth in specific segments of the descending slab.

Surficial structures in the form of megathrust ruptures can be seen to be both a consequence of the highly segmented nature of the overriding plate, particularly in the Peruvian Andes, as well as the deeper variation in 3D slab morphology. For example, the inferred slab tears at the northern (Puna) edge and the southern (Payenia) edge of the Pampean segment may be explained as due to significant variation of the slab morphology and are reflected in changes in the characteristics of earthquake ruptures, and different tectonic modes in the supra-subduction zone lithosphere. Lineaments that separate the segments of the frontal megathrust also coincide with inherited surface fault systems on the South American plate. Clusters of intermediate extensional earthquakes weaken the lithosphere south of the Taltal Ridge (TR) and north of the Juan Fernandez Ridge (JFR). These structures mark the edge of the magmatically inactive plate interface due to the termination of magmatism in shallow dipping slab segments associated with the JFR, or in the flat slab of the Pampean segment.