Rates of processes in magmatic arcs: implications for the timing and nature of pluton emplacement and wall rock deformation

Abstract

The construction of arcs and in many cases the emplacement of plutons occur in tectonically active regions. It is critical, therefore, to evaluate the rates of structural and magmatic processes when trying to understand the evolution of arcs and the associated pluton-wall rock systems. Our best estimates of average rates or durations of processes in shallow to moderate level arc environments are the following: (1) crystal growth rates in magma of 10 −4 cm year −1; (2) growth rates of metamorphic porphyroblasts between 10 −5 and 10 −2 cm year −1; (3) long-term magma supply rates of 10 −1 km 3 year −1 and short-term rates of up to 350 km 3 year −1; (4) diapiric ascent rates for mafic plutons of 1–3 m year −1; (5) cooling of plutons to ambient wall rock temperatures in 10 5–10 6 years; (6) final crystallization of plutons in a small fraction of the time needed for complete cooling; (7) fault displacements of 3 cm year −1; (8) development of cleavages in fault zones in less than 10 6 years at strain rates of 10 −13 s −1 or higher; and (9) the development of regional cleavages in 10 6 years at strain rates of 10 −14 s −1 These rates indicate that processes operating in magmatic arcs are relatively rapid: pluton emplacement, cleavage development, etc., occur over time spans of tens of thousands to no more than a few million years. However, the rate of ascent and crystallization of plutons at shallow levels is generally shorter than that needed to get large displacements on faults or widespread cleavages developed. At deeper levels, or in zones undergoing faster strain rates, the time spans of the various processes approach one another. Thus plutons, with otherwise similar characteristics, emplaced in regions undergoing fast strain rates, or at deeper crustal levels, may appear quite different structurally from those emplaced at shallow levels or in regions undergoing slower strain rates. Comparison of these data also suggests that the rate at which wall rock deforms is the limiting factor controlling the rates of other structural processes during emplacement of plutons unless fast strain rates or multiple deformation mechanisms are considered. Thus emplacement mechanisms that rely on the transport of magma over 10 5 to 10 6 years are favored and need further consideration. Finally, we argue that the structural and other characteristics of pluton-wall rock systems will depend on rates of various processes involved and that these rates at the very least influence, and sometimes invalidate, the timing criteria previously published by us and others.