To illustrate the Neogene shortening distribution in the back-arc units of the Central Andes and to estimate the contribution of the shortening to crustal thickening, two balanced crustal cross sections have been constructed across the northern and southern branches of the Bolivian orocline. Total Neogene shortening, which varies from 191 to 231 km, is accommodated by a crustal duplex below the Cordillera Oriental, but is insufficient to produce the 70 km of crustal thickness evidenced by geophysical data below the Altiplano. The best explanation for this anomalous thickening seems to be crustal underplating by material tectonically eroded from the continental margin; this process probably caused the Altiplano uplift. The subduction of oceanic lithosphere coupled with this underplating and a brief episode of gravity spreading of the Altiplano constituted the driving forces that produced Neogene shortening and development of the Central Andes.

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