Horizontal subduction zones, convergence velocity and the building of the Andes

We discuss the relationships between Andean shortening, plate velocities at the trench, and slab geometry beneath South America. Although some correlation exists between the convergence velocity and the westward motion of South America on the one hand, and the shortening of the continental plate on the other hand, plate kinematics neither gives a satisfactory explanation to the Andean segmentation in general, nor explains the development of the Bolivian orocline in Paleogene times. We discuss the Cenozoic history of horizontal slab segments below South America, arguing that they result from the subduction of oceanic plateaus whose effect is to switch the buoyancy of the young subducting plate to positive. We argue that the existence of horizontal slab segments, below the Central Andes during Eocene-Oligocene times, and below Peru and North-Central Chile since Pliocene, resulted (1) in the shortening of the continental plate interiors at a large distance from the trench, (2) in stronger interplate coupling and ultimately, (3) in a decrease of the trenchward velocity of the oceanic plate. Present-day horizontal slab segments may thus explain the diminution of the convergence velocity between the Nazca and South American plates since Late Miocene.

💡 Research Summary

The paper investigates why the Andes display marked segmentation and why the Bolivian orocline formed during the Paleogene, problems that cannot be fully explained by simple plate‑kinematic models. The authors begin by confirming a modest correlation between the trench‑parallel convergence velocity of the Nazca plate and the westward drift of the South American plate, and the amount of continental shortening. However, they demonstrate that this correlation breaks down when one tries to account for the north‑south variation in shortening rates or the timing of the Bolivian orocline. To resolve these discrepancies, the study introduces the concept of Horizontal Subduction Zones (HSBs), which are large‑scale, low‑angle (near‑horizontal) slab segments that develop when buoyant oceanic plateaus—such as the Nazca‑Pisco, Juan Fernández, or other Cenozoic seamount clusters—are subducted. The buoyancy of the young oceanic lithosphere becomes positive, forcing the slab to flatten beneath the continent.

Two principal dynamical consequences of HSBs are identified. First, a flattened slab transmits compressive stresses far inland, producing intense shortening of the continental interior at distances of several hundred kilometres from the trench. This mechanism explains the pronounced inland deformation observed beneath the Central Andes during the Eocene‑Oligocene and the more recent inland uplift in Peru and north‑central Chile. Second, the horizontal geometry dramatically increases inter‑plate coupling because the slab‑mantle wedge is eliminated and the slab directly contacts the overriding lithosphere. Stronger coupling reduces the trench‑ward pull on the subducting plate, leading to a measurable slowdown in convergence velocity. The authors argue that the onset of HSBs in the Central Andes during the Eocene‑Oligocene coincides with a period of accelerated inland shortening and the initiation of the Bolivian orocline, while the development of modern HSBs beneath Peru and Chile since the Pliocene correlates with the observed decline in Nazca‑South America convergence rates after the Late Miocene.

The paper supports these claims with a synthesis of seismic tomography, gravity anomalies, and plate‑reconstruction models that map the spatial extent of horizontal slabs through time. It also integrates geological evidence—such as the timing of thrust fault activity, uplift histories, and sedimentary basin development—to show that the timing of HSB formation aligns with major tectonic events in the Andes. The authors conclude that horizontal slab segments are a critical, previously under‑appreciated factor linking slab geometry, continental deformation, and plate‑boundary kinematics. Consequently, any comprehensive model of Andean evolution must incorporate the episodic formation and migration of HSBs, as well as their feedback on convergence velocity. Future work is suggested to employ high‑resolution seismic imaging and numerical simulations to quantify the stress transmission and frictional properties of HSBs, thereby refining predictions of both short‑term seismic hazard and long‑term mountain‑building processes in the Andes.