Changes in Motion of the Nazca/Farallon Plate Over the Last 34 Million Years: Implications for Flat‐Slab Subduction and the Propagation of Plate Kinematic Changes

Abstract A high‐resolution sequence of finite‐rotation solutions spanning the past 34 Myr for the Nazca plate relative to the Pacific, Antarctic, and South American plates shows a pattern of 1–5‐Myr‐long intervals of nearly constant plate motion mostly separated by decelerations or accelerations lasting no longer than a few hundred thousand years. Sustained acceleration may have occurred over 25–19 Ma, but since 18 Ma, rapid decelerations occurred centered on 15.8, 13.7, ∼7.1, 5.9, and ∼0.6 Ma and accelerations occurred centered on 10.0 and ∼8.3 Ma. These changes are reasonably interpreted as resulting from changes between flat‐slab and normal subduction of the Nazca plate. For 18–16 Ma, an interval with at most one flat slab, the average Nazca speed relative to the nearly mantle‐stationary Antarctic plate was ∼135 mm/yr. In contrast, Nazca‐Antarctic geodetic rates, with at least three flat slab segments, average only ∼40 mm/yr. Some changes in South America motion are synchronous with changes in Nazca motion, supporting an interpretation that changes in interplate coupling at flat slab segments are a major cause of changes in plate motion. Possibly synchronous motion changes between plate pairs in the Atlantic and Pacific basins since 8 Ma may result from strong coupling between the North and South American plates across their diffuse boundary, and speculatively, between the North American and Pacific plates at the Yakutat flat slab segment beneath Alaska.