Continental arc rocks located further away from the trench are typically characterized by more evolved radiogenic isotopic compositions. Episodic arc migration away from the trench would produce a temporal record distinguished by episodic shifts to more evolved compositions. In most continental arcs, temporal shifts to evolved isotopic compositions correlate with magmatic high-flux events, which are the basis for cyclicity models in Cordilleran orogenic systems. Landward arc migration into more melt-fertile regions of the continental lithosphere can explain both episodic shifts in isotopic composition and high-flux events without requiring underthrusting of retroarc lithosphere. If arc migration is predominantly autocyclic, controlled by intra-arc processes, then arc migration itself may drive orogenic cyclicity. Conversely, periodic arc migration may be controlled by allocyclic processes like slab anchoring or folding in the mantle transition zone. In either case, arc migration may be the key to understanding what drives high-flux events and cyclicity in Cordilleran orogens.
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