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.

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To understand the behavior of the Earth's magnetic field, and possibly even predict its future behavior, it is paramount to understand its past. Our understanding of the behavior of the geomagnetic field arises from magnetic signals stored in geological and archeological materials. They acquire a magnetization when they cool in the Earth's magnetic field, and retain that magnetization over (geological) timescales. Igneous rocks, e.g., lavas, are the only recorders of the direction and the intensity of the field that are available throughout geologic history and all over the globe. Since lavas take snapshots of the state of the Earth's magnetic field for their location and point in time when they cool, frequently erupting volcanic regions with well-dated

To understand the behavior of the Earth's magnetic field, and possibly even predict its future behavior, it is paramount to understand its past. Our understanding of the behavior of the geomagnetic field arises from magnetic signals stored in geological and archeological materials. They acquire a magnetization when they cool in the Earth's magnetic field, and retain that magnetization over (geological) timescales. Igneous rocks, e.g., lavas, are the only recorders of the direction and the intensity of the field that are available throughout geologic history and all over the globe. Since lavas take snapshots of the state of the Earth's magnetic field for their location and point in time when they cool, frequently erupting volcanic regions with well-dated