SUBDUCTION EROSION AND CRUSTAL THICKENING IN LA RAMADA REGION, CORDILLERA PRINCIPAL AND FRONTAL (32°30'LS)

The structure and magmatic setting of La Ramada and Espinacito region can be used as tools to decipher the tectonic evolution of a segment of the Frontal and Principal Cordilleras at ~32°S. This segment is located over the modern zone of shallow subduction (28-33°S flat slab).

The evolving characteristics of these magmatic rocks, suggest two possible mechanisms for the Miocene evolution of these segment: subduction erosion for the period previous to early late Miocene, and crustal thickening for the periods after early Late Miocene. The Neogene evolution on the La Ramada and Espinacito region, have three main periods of eruption:

Andesitic Breccia deposited on the base of the Manantiales foreland basin, developed at ~20 Ma during the uplift of the La Ramada fold and thrust belt. This retroarc volcanic sequences have La/Yb ratios between 12 and 14, suggesting a non-thickened crust, indicating that at this time the mountain range had not still been structured.

Laguna Andesite located to the southeast of La Ramada and Espinacito; it has 15,45 ± 0.30 Ma; and 57,35% SiO2; La/Yb (11,82); La/Sm (5.7). These rocks have very high anomalous, K (21832 ppm), Th (8,30) and U (2,30). This chemistry indicates subcrustal contamination in the magmatic source probably by forearc subduction erosion. At this time the magamtic arc front shifted toward the east along the Chile-Argentina boundary.

La Ramada Volcanic complex is composed of andesites-basaltic andesites (57,63-61,44 % SiO2) with 10.7 ± 0.7 to 12.7 ± 0.6 Ma. They have Ba/La (23,71-33,45), La/Ta (28,11-55,47), La/Yb (14,36-22,09) and La/Sm (4,56-5,82) ratios typical signature of a magamtic arc. They also have ⁸⁷Sr/⁸⁶Sr initial ratio of ~0.7047, and Sr ~732. These higher REE values are consistent with a higher pressure residual assemblage in a thickened crust.

Based on these facts it is assumed that during the Andesitic Breccia (Early Miocene) the crust had a normal thickness. During the Laguna Andesitic times (Middle Miocene), the crust had a normal thickness, but occur subduction erosion, during this shifting of the frontal magmatic arc toward east. Finally the crust had an important thickening during the La Ramada volcanic complex times (early Late Miocene). The Juan Fernandez Ridge was subducted at this time beneath La Ramada region (32°S) and shallowing of the “flat slab” segment begun.