Formation of magmatic crust at the Andean continental margin during early Mesozoic: a geological and thermal model of the North Chilean Coast Range

Abstract

The Pacific margin of South America has existed since at least the Early Paleozoic, developing a diversified continental crust. In the Triassic and Jurassic, mantle-derived plutonic and volcanic rocks, formed a ca. 100-km-wide belt of new magmatic crust in the coast range of northern Chile. The tectonic regime was extensional Surface geology, seismic velocities and the gravity field indicate that the magmatic rocks in the upper 20 km of the crust have a mainly basic composition. The Pre-Andean continental crust is only preserved in isolated small areas, but the rifting had no proveable influence on the surface elevation of the area, which was always above or close to sea level. The coastal magmatic belt marks the onset of the Andean Cycle and was its most intensive phase of magmatic additons to the crust. Numerical experiments were performed using a two-dimensional finite difference grid, with spatial relations derived from the geological model. The duration of two phases of extension and contemporaneous compensation of the crustal thinning by magmatic accretion and the location of the extension in the lithosphere were varied in order to evaluate the duration of the tectonic-magmatic phases, and to distinguish between the effects of tectonic movements and magmatic additions on temperature gradient and elevation. The effect of these two first order heat advection processes on the thermal structure of the lithosphere depends on their duration. It is possible to set reasonable constraints on the duration of extension by comparing calculated temperature distributions with geological data. The temperature distribution in the crust is dominated by processes in the crust and only to a minor extent by deep-seated thermal anomalies. Surface elevation is largely controlled by deep-seated thermal anomalies in the mantle if the thickness of the crust is held constant by magmatic accretion during extension. The time scale of the tectonic-magmatic processes has minor influence on the calculated elevation.