FORMATION OF MAGMATIC CRUST AT THE ANDEAN CONTINENTAL-MARGIN DURING EARLY MESOZOIC - A GEOLOGICAL AND THERMAL-MODEL OF THE NORTH CHILEAN COAST RANGE

The Pacific margin of South America has existed since at least the Ear ly Paleozoic, developing a diversified continental crust. In the Trias sic and Jurassic, mantle-derived plutonic and Volcanic rocks, formed a ca. 100-km-wide belt of new magmatic crust in the coast range of nort hern Chile. The tectonic regime was extensional. Surface geology, seis mic velocities and the gravity field indicate that the magmatic rocks in the upper 20 km of the crust have a mainly basic composition. The P re-Andean continental crust is only preserved in isolated small areas, but the rifting had no proveable influence on the surface elevation o f 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 in tensive phase of magmatic additions to the crust. Numerical experiment s were performed using a two-dimensional finite difference grid, with spatial relations derived from the geological model. The duration of t wo phases of extension and contemporaneous compensation of the crustal thinning by magmatic accretion and the location of the extension in t he lithosphere were varied in order to evaluate the duration of the te ctonic-magmatic phases, and to distinguish between the effects of tect onic movements and magmatic additions on temperature gradient and elev ation. The effect of these two first order heat advection processes on the thermal structure of the lithosphere depends on their duration. I t is possible to set reasonable constraints on the duration of extensi on by comparing calculated temperature distributions with geological d ata. The temperature distribution in the crust is dominated by process es in the crust and only to a minor extent by deep-seated thermal anom alies. 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 tectoni c-magmatic processes has minor influence on the calculated elevation.