Abstract
At two trench segments below the Andes, the Nazca Plate is subducting sub-horizontally over ∼200–300 km, thought to result from a combination of buoyant oceanic-plateau subduction and hydrodynamic mantle-wedge suction. Whether the actual conditions for both processes to work in concert existed is uncertain. Here we infer from a tectonic reconstruction of the Andes constructed in a mantle reference frame that the Nazca slab has retreated at ∼2 cm per year since ∼50 Ma. In the flat slab portions, no rollback has occurred since their formation at ∼12 Ma, generating ‘horse-shoe’ slab geometries. We propose that, in concert with other drivers, an overpressured sub-slab mantle supporting the weight of the slab in an advancing upper plate-motion setting can locally impede rollback and maintain flat slabs until slab tearing releases the overpressure. Tear subduction re-establishes a continuous slab and allows the process to recur, providing a mechanism for the transient character of flat slabs.
Highlights
At two trench segments below the Andes, the Nazca Plate is subducting sub-horizontally over B200–300 km, thought to result from a combination of buoyant oceanic-plateau subduction and hydrodynamic mantle-wedge suction
While the formation of the Andes has been viewed in context of absolute plate motion of the South American plate[23], that is, in a mantle frame of reference, flat-slab formation has not, even though it is essential for determining the role of, for example, slab rollback in the process
In combination with the absolute motion of South American trench estimated from mantle reference frames, this reconstruction will provide the absolute motion of the Andean trench
Summary
At two trench segments below the Andes, the Nazca Plate is subducting sub-horizontally over B200–300 km, thought to result from a combination of buoyant oceanic-plateau subduction and hydrodynamic mantle-wedge suction. With constraints on the age of flat slab formation (11–12 Ma)[9,10], the Nazca Plate subduction rate through time from the SAM-Antarctica(-Pacific)Nazca plate circuit[24], the current dimensions of the flat slabs[5,6], and the absolute (African) plate motion evolution[25], we will quantify the relationship between the flat slab length, rollback of the slab bend, and the amount of subduction since the onset of flat slab formation This will be used as basis to evaluate possible dynamics triggering and supporting flat slab subduction
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