EPISODIC MAGMATIC FRONT MIGRATION AND MOUNTAIN BUILDING PROCESSES IN THE CENTRAL ANDES OF NORTHERN CHILE: THE LAST 180 MA

Numerous studies concerning Central Andean tectonics have emphasized the Neogene horizontal shortening along the Andean belt and the creation/removal of the crustal root bellow the western Cordilleran arc and Altiplano. Little attention has been paid to the Chilean "hinterland" which record a protracted history of magmatism and landscape formation and destruction during the last 180 my. In northern Chile, magmatic arcs have episodically migrated eastward during events of plate readjustment, accompanied by massive losses of forearc slivers by tectonic (subduction) erosion. From 180 to 90 Ma the magmatic arc, located at the present-day coastal Cordillera evolved under extensional /transtensional conditions while magmas became more primitive and the crust thinner through time. At 85 Ma an orogen-wide compressional episode was followed by an eastward shift of the magmatic front, and when magmatism reassumed along the Central Valley at 80-65 Ma lavas erupted in a series of discontinuous fault-bounded, pull-apart (?) basins. After a new episode of shortening near the K-T boundary, these lavas where covered by high K rhyolitic ignimbrites and intermediate lavas showing within-plate signatures which erupted during a period of subdued plate convergence. A new eastward shift in the magmatic front occurred in the Eocene at the onset of a period of fast oblique plate convergence . An Eocene mountain range (Precordillera or Cordillera de Domeyko) was rapidly uplifted, exhumed and eroded. Magmas became trapped in the heated crust submitted to compression, except small batches of "adakitic" melt canalized trough faults, which were able to rise to shallow-level magma chambers to form the giant porphyry coppers of Northern Chile. Trace element signatures indicate that the continental crust exceeded 50 km thickness. A magmatic lull, extension and crustal thinning occurred afterwards in the Oligocene (35 to 26 Ma) before the last shift of the magmatic front, which coincided, with the breakup of the Farallon plate when the modern central Andean arc started to develop in the Western Andean Cordillera.