Does tectonic deformation control episodic continental arc magmatism? Evidence from granitic magnetic fabrics (AMS)

Abstract

This paper applies magnetic fabric analyses to plutons of the East Pacific continental arc. Continental arc magmatism is strongly episodic, with voluminous granitic magma addition occurring during discrete high-flux events (‘flare-ups’). The cause of these flare-ups is debated, variously invoking tectonic, mantle, or crustal controls. To understand how the syn-magmatic strain history changes during a flare-up, we compare granitic magnetic fabric (Anisotropy of Magnetic Susceptibility, AMS) and geochronological data from the Antarctic Peninsula (Lassiter Coast), Sierra Nevada, and Chile. This comparison indicates a common pattern in orientation and magnitude of syn-magmatic deformation, showing flare-up events occur during increased tectonic compression driven by enhanced interplate coupling and fast subduction. Flare-ups terminate as tectonic compression reduces or the regime becomes extensional, even if convergence rates remain high. As with enhanced seismicity, magmatic flare-ups result from high tectonic compression, during discrete periods of enhanced interplate coupling within broader periods of increased subduction rates. The enhanced magmatic flux results either from crustal thickening leading to partial melting of a newly accreted, hydrous mafic underplate, enhanced melt segregation in the source, or in response to high tectonic compression rendering lithostatic compression the weakest compressive force, enhancing magma extraction and ascent from the mantle.