Abstract The present-day Andes have formed in response to subduction-related processes operating continuously along the western margin of South America since the Jurassic period. When these processes started, the continental margin was mainly formed of metamorphic complexes and associated magmatic rocks which evolved during Proterozoic (?), Palaeozoic and Triassic times, and which now constitute the basement to the Mesozoic and Cenozoic Andean sequences. These older units are commonly referred to in the Chilean geological literature as the ‘basement’ or the ‘crystalline basement’. The basement rocks crop out discontinuously ( Fig. 2.1 ) in northern Chile, both in the coastal areas and in the main cordillera. In contrast, from latitude 34°S southwards, they form an almost continuous belt within the Coastal Cordillera extending to the Strait of Magellan. In addition, sparse outcrops occur both in the main Andean cordillera as well as further east in the Aysen and Magallanes regions. In the first maps and syntheses of the geology of Chile (e.g. Ruiz 1965 ) these rocks were generally considered to be of Precambrian age, forming a western continuation of the Brasilian craton. Later work has demonstrated that rocks first described as metamorphic basement units show a wide range of metamorphic grades and ages extending from possible Late Proterozoic through Palaeozoic and even, in some cases, to Jurassic–Cretaceous. With regard to previous works that have attempted to synthesize data on Chilean basement geology, the reader is referred to those by González-Bonorino ( 1970 , 1971 ), González-Bonorino & Aguirre (1970) , Aguirre et al .

Modeling of in situ rock properties based on a Gibbs free energy minimization approach shows that regional metamorphism of granulite facies may critically enhance the decrease of crustal density with depth. This leads to a gravitational instability of hot continental crust, resulting in regional doming and diapirism. Two types of crustal models have been studied: (1) lithologically homogeneous crust and (2) heterogeneous , multilayered crust. Gravitational instability of relatively homogeneous continental crust sections is related to a vertical density contrast developed during prograde changes in mineral assemblages and the thermal expansion of minerals with increasing temperature. Gravitational instability of lithologically heterogeneous crust is related to an initial density contrast of dissimilar intercalated layers enhanced by high-temperature phase transformations. In addition, the thermal regime of heterogeneous crust strongly depends on the pattern of vertical interlayering: A strong positive correlation between temperature and the estimated degree of lithological gravitational instability is indicated. An interrelated combination of two-dimensional, numerical thermomechanical experiments and modeling of in situ physical properties of rocks is used to study the processes of gravitational redistribution within a doubly stacked, heterogeneously layered continental crust. It is shown that exponential lowering of viscosity with increasing temperature, in conjunction with prograde changes in metamorphic mineral assemblages during thermal relaxation after collisional thickening of the crust, provide positive feedback mechanisms leading to regional doming and diapirism that contribute to the exhumation of high-grade metamorphic rocks.