Diapirism and dike-fed expansion are two popular models for the ascent and emplacement of large batches of granitoid magma into the middle and upper crust. Many structural and petrologic relationships in and around plutons can be cited as permissive evidence for either model, and so it has been difficult to differentiate between them on these grounds. Here we consider wall-rock strain rates resulting from dike-fed expansion at a constant-volume pluton-filling rate. Considering end-member pluton shapes and dike flow rates, and dikes 1 km in plan length, the dike-fed expansion model predicts wall-rock strain rates over the entire pluton-growth period that are up to six orders of magnitude faster than the fastest accepted regional tectonic strain rates (10−13 s−1). Strain rates in the very early stages of growth can reach values several orders of magnitude faster. Our results suggest that evidence for or against very fast strain rates in deformation aureoles around plutons may form a key criterion for identifying dike-fed expansion. On the other hand, our ability to extract strain-rate information from pluton aureoles is currently extremely limited.

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