Geometric scaling of felsic sheet intrusions in the brittle upper crust, eastern Elba Island, Italy, with implications for host-rock strain distribution and strain rates

Abstract

Sheet intrusions (dikes and sills) represent the most viable mode of magma transport and emplacement within the brittle upper crust. The thickness-to-length aspect ratio of such intrusions provides information about magma emplacement mechanisms and provides information on intruded magma volumes. Within the brittle upper crust, the mobility of magma is governed by magma cooling and solidification rates. The cooling rate of sheet intrusions depends on the temperature difference between magma and its host rocks, the thickness of the intrusion and the thermal diffusivity of the surrounding crust. The cooling rate of magma in the upper crust can be expected to be faster than the tectonic strain rate. In eastern Elba Island Italy, well exposed, late Miocene leucogranite sheets that intrude schistose host rocks within the contact aureole of the Porto Azzurro pluton provide a natural laboratory for analysing sheet intrusion geometries (length, thickness, spacing between intrusions). The thickness-to-length ratios of the intrusions define a power law dimensional scaling relationship with exponent ~1. Using this length versus thickness scaling we estimate that the total volume of leucogranite that intruded the Porto Azzurro pluton contact aureole was 4‑15 × 106 m3.
Moreover, by analysing the spacing between sheet intrusions along with the size distribution of their thicknesses, we estimate that their emplacement was accommodated by an average bulk strain (volumetric strain) of 13-15% of the Porto Azzurro pluton contact aureole host rocks. Our observations also highlight how host rock brittle structures may control the location and mode of magma emplacement. Well exposed sections of sheet intrusions and their host rocks, as observed on Elba Island, are natural laboratories where geometries of intrusions can be analysed to provide important clues on the mechanisms of magma emplacement within the brittle crust, and interactions between magmatic and deformation processes.