Neogene magmatic expansion and mountain building precesses at the southern Central Andes, 36º-37ºS, Argentina.

SPAGNUOLO, M., LITVAK, V., FOLGUERA, A., BOTTESI, G., RAMOS V.A.

JOURNAL OF GEODYNAMICS

PERGAMON-ELSEVIER SCIENCE LTD

Año: 2012 p. 1 - 14

0264-3707

The eastern Andean slope from 34◦ to 37◦S has a unique character when compared to neighboring sectors:Lower Miocene volcanic rocks cover upper Cretaceous to Eocene contractionally deformed depositsin the retroarc area. Remnants of the previous upper Oligocene arc front are found at the present-daywestern Andean slope, west of the mentioned Miocene arc sequences. These lavas and ignimbrites ofthe Charilehue Formation, typically described as an arc suite, have ages between 18 and 14.5 Ma andthickness over 1000 m, and are delimited by normal faults. Chemical analyses reveal that they have ananomalous chemistry characterized by poorly evolved mantle-derived rocks, which differ from Eoceneand Late Miocene typical arc magmas in the region. Their spatial distribution depicts a NE trend morethan 100 km long, whose outcrops were affected by Middle to Late Miocene compressional deformationand subsequent exhumation, as shown by field and fission track data. The Charilehue volcanic sequencesrepresent the westernmost products of an incipient and poorly evolved arc that migrated and expandedto the foreland, initially coexisting with an extensional setting inherited from late Oligocene times. Thiscomplex evolution is interpreted as the result of a slab shallowing related to the time when the SouthAmerican plate started its westward absolute shift after a quasistatic period in the Oligocene. The shiftof the magmatism to the foreland probably caused a higher thermal gradient and, therefore, a shallowerbrittle?ductile transition that triggered thick-skinned basement tectonics in the retroarc region,reactivating Late Cretaceous to Eocene décollements.