![]() This work supports previous hypotheses that the magnitude of shortening decreases significantly along strike away from a maximum in southern Bolivia, largely as a result of the distribution of pre-Cenozoic basins that are able to accommodate a large magnitude of thin-skinned shortening. ![]() The minimum estimate for the magnitude of shortening at this latitude is ∼142 km, which is moderate in magnitude compared to the 250–350 km of shortening accommodated in the retroarc thrust belt of southern Bolivia to the north. The sporadic foreland-ward propagation documented here may be common in basement-involved thrust systems where inherited weaknesses due to previous crustal deformation are preferentially reactivated during later shortening. During Pliocene time, deformation again migrated >100 km eastward to a Cretaceous synrift depocenter in the Santa Bárbara Ranges. At 12–10 Ma, the thrust front jumped ∼75 km toward the east to bypass the central portion of a horst block of the Cretaceous Salta rift system, followed by initiation of new faults in a subsystem that propagated toward the west into this preexisting structural high. 40 Ma, when deformation and exhumation were restricted to the western margin of the Eastern Cordillera and eastern margin of the Puna Plateau. Each eastward jump in the deformation front was apparently followed by local westward deformation migration, likely reflecting a subcritically tapered orogenic wedge. At the latitude of the regional transect (24–25°S), results demonstrate that the thrust belt propagated in an overall eastward direction in three distinct pulses during Cenozoic time. Regional mapping, a cross section, and U-Pb and (U-Th)/He age dating of apatite and zircon presented here build upon the preexisting geological framework for the region. These variations have been proposed to explain the considerable southward decrease in the observed magnitude of Cenozoic shortening. The retroarc fold-and-thrust belt of the Central Andes exhibits major along-strike variations in its pre-Cenozoic tectonic configuration.
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