Alumno

183

by Victor A. Ramos

Plate tectonic setting of the Andean Cordillera
Laboratorio de Tectónica Andina, Universidad de Buenos Aires, Argentina. E-mail: andes@gl.fcen.uba.ar

The Andes are a natural laboratory for the study of the interaction between subduction of the oceanic plate and active geological processes. Inter- and intraplate seismicity, volcanic activity, thick- andthin-skinned fold and thrust belts, and foreland basin subsidence, in conjunction with space geodetic observations, contribute to characterize the present plate tectonic setting of discrete segments of the Andes. The inherited geological history, as well as the present tectonic setting, is responsible for the unique geology of the Northern, Central, and Southern Andes. The Northern Andes are the result ofMesozoic and Cenozoic collisions of oceanic terranes, prior to the present Andean-type setting. The Central Andes have a long history of subduction and volcanic arc activity, while the Southern Andes record the closing of a back-arc oceanic basin, and almost no volcanic arc activity. These major geological units have along-strike variations in the subduction geometry that controls the differentvolcanic zones. The link between trench collision of aseismic ridges and flat-slab segments plays an important role in the conclusion of the volcanic activity. The collision of seismic ridges also produces a volcanic arc gap, but with an unusual suite of near trench magmatism, plateau basalts, and adakites.

acterize the different segments are widely variable. The present overview will focus on themajor geological differences among these segments, based on today’s plate-tectonic knowledge of this mountain chain.

Major geological provinces
The Andes north of the Golfo de Guayaquil are unique, as established by Gansser (1973). The Northern Andes record an important accretion of oceanic crust during Jurassic, late Cretaceous, and Paleogene times. As a result, the Western Cordillera ofColombia and Ecuador is mainly constituted of an oceanic basement that during accretion was related to ophiolite obduction, important penetrative deformation and metamorphism, in cases up to blue schist facies. Further north, the emplacement of the Caribbean nappes was related to the collision of an island arc system, the Bonaire block, during Paleogene times (Bosch and Rodríguez, 1992, Kellogg andBonini, 1982). Subsequent tectonic inversion of normal faults during Neogene times uplifted the Mérida Andes (Colleta et al., 1997) and overimposed strike-slip partitioning to dominate present tectonics (Schubert, 1982). The Central Andes between the Golfo de Guayaquil (4˚S) and the Golfo de Penas (46˚30'S) are a typical Andean-type orogen where tectonics was driven by subduction. Within this generalregion several sectors record a complex tectonic history, punctuated by changes in the Wadati-Benioff geometry and different uplift mechanisms. The geology north of Arequipa (14˚S), the northern sector of the Central Andes, is the result of extensional tectonics and subduction during early Mesozoic times (Mégard, 1987). Subsequent compression and migration of magmatism, and deformation towardsthe foreland uplifted the Peruvian Andes, with the development of the present flat-slab subduction in the last 5 My (Sébrier and Soler, 1991). The central sector of the Central Andes comprises southern Peru, Bolivia, and northern Argentina (14˚–27˚S). It is characterized by normal subduction and an active volcanic arc. The present uplift of the Altiplano-Puna plateau is thermally induced (Isacks,1988). Variation in the Wadati-Benioff geometry during Neogene times was responsible for a wider magmatic arc during early Miocene times and therefore higher thermal gradients that favored softening of the lower crust and later important thickening and uplift of the Eastern Cordillera and the Subandean belt during late Miocene times (Allmendinger et al., 1997). The southern sector of the Central...