Punctuated shortening and subsidence in the Altiplano Plateau of southern Peru: Implications for early Andean mountain building

Sedimentologic, provenance, geochronologic, and magnetostratigraphic results from clastic nonmarine deposits in the northern Altiplano Plateau of southern Peru (14-15 degrees S) demonstrate late Eocene-Oligocene (37-26 Ma) accumulation of the >4-km-thick San Jeronimo (Puno) Group within a retroarc foreland basin related to early Andean shortening and crustal thickening. Punctuated Oligocene (29-26 Ma) displacement along deep-seated contractional structures, as revealed by growth stratal relationships, abruptly partitioned this regional flexural basin and established the structural boundaries of the smaller intermontane Ayaviri Basin, which continued to evolve in a hinterland setting during late Oligocene-Miocene shortening. This brief episode of shortening along the Altiplano-Eastern Cordillera boundary is correlated with exceptionally rapid sediment accumulation (>1100-1800 m/m.y.), tightly constrained to 30-28 Ma on the basis of U-Pb geochronology and magnetic polarity stratigraphy. Provenance data from detrital zircon U-Pb age populations and sandstone compositions indicate derivation from a complex belt of Paleogene shortening and probable basin inversion in the Western Cordillera that was subsequently overprinted by Andean arc magmatism. This early Andean zone is interpreted as the along-strike continuation of the better-exposed Maranon fold-thrust belt to the north (5-13 degrees S) and a proposed belt of shortening to the south along the Chilean Precordillera and Western Cordillera of Bolivia and northern Argentina (17-25 degrees S). Subsequent focusing of late Oligocene shortening along the Eastern Cordillera-Altiplano boundary may have been linked to shallowing of the subducting slab and potential reactivation of crustal anisotropies.