Lithospheric Evolution of the Central Andean Fold-Thrust Belt: Making a High Elevation Plateau

Through integrating geophysical results of upper mantle structure, crustal thickness, and composition with new structural, stratigraphic and thermochronologic data, we develop a model for the construction of the central Andean plateau that is tied to the evolution of the Andean fold-thrust belt. Using foreland basin deposits as a signal of the initiation of mountain building, several authors have proposed that a topographically high fold-thrust belt existed in the western portion of the Central Andes as early as Cretaceous to early Paleocene time. The migration history of the foreland basin is a robust record of long-term fold-thrust belt migration and thus presents a coarse but testable evolutionary history of the system as a whole. Additional age constraints are obtained by integrating thermochronologic cooling ages with sequentially balanced cross sections across the fold-thrust belt. Combining the basin migration history with the structural evolution of the fold-thrust belt suggests an internally consistent model for the growth of the central Andes with time. Although crustal shortening has been identified as the dominant process in the development of the Andean Plateau, the kinematic evolution of lithospheric-scale shortening remains elusive. The present-day mantle picture provides important constraints on how the lithosphere as a whole evolved with time to produce the central Andean Plateau. In this model, we emphasize the importance of both the crust and upper mantle in the evolution of the central Andean plateau. Key steps in the evolution of the Andean plateau are as follows: 1) Initiation of mountain building by 70 Ma suggested by the associated foreland basin depositional history. 2) Eastward propagation of a narrow, early fold-thrust belt at 40 Ma through the eastward advance of a 200-400 km long basement thrust sheet. 3) Continued shortening within the Eastern Cordillera from 40 -15 Ma thickened the crust and mantle and established the eastern boundary of the modern central Andean plateau. Removal of excess mantle through lithospheric delamination at the Eastern Cordillera-Altiplano boundary during the Early Miocene appears necessary to accommodate underthrusting of the Brazilian shield. Replacement of delaminated mantle lithosphere by hot asthenosphere may have provided the heat source for a pulse of mafic volcanism in the eastern Cordillera and Altiplano at 24-23 Ma, and further volcanism recorded by 12-7 Ma crustal ignimbrites. 4) After ~20 Ma, deformation waned in the Eastern Cordillera and Interandean zone and began to be transferred into the Subandean zone. Simultaneous growth of a crustal-scale duplex in the hinterland is proposed to have fed slip into the Subandean zone and initiated deformation within the Altiplano between 20 and 10 Ma. We suggest that Cenozoic deformation within the mantle lithosphere (both shortening and removal) has been focused at the Eastern Cordillera-Altiplano boundary where mantle most likely continues to be removed through piecemeal delamination.