Abstract The Andes of the Neuquén Mesozoic basin have experienced multiple episodic tectonic events as a consequence of the changes of the plate tectonic boundary configuration. Each episode of deformation has overprinted the previous one, making it difficult to unravel the Andean tectonic history. The first deformation event took place in the uppermost Cretaceous with the formation of the Agrio fold-and-thrust belt. This event was related to the shallowing of the subducting plate recorded by the migration of the volcanic arc toward the foreland. During the late Oligocene–early Miocene, an extensional event, related to the steepening of the subducted plate, affected only the hinterland region causing the opening of the Cura Mallín basin. This basin was closed during the late Miocene, together with the development of a new fold-and-thrust belt that reactivated the previous structures. During the Late Tertiary, two more episodes of extension and compression affected the Andean area. The trip focuses on the field evidence that documents this complex history of evolution by looking at evidence of the sequence of the distinct tectonic events.

ABSTRACT The different segments of the Andean thrust system have distinctive topography and inferred crustal roots. These two characteristics both depend upon crustal shortening, and on this basis they provide independent constraints for evaluating estimates of Cenozoic shortening obtained by balanced structural cross-sections of different segments of the fold-and-thrust systems. Three transects in the Central Andes are analyzed: a northern (22–23°S), a central (32–33°S), and a southern segment (37–39°S). Each segment shows different amounts of orogenic shortening, generated through a complex combination of thin- and thick-skinned thrusting. Based on known age constraints, different shortening rates are calculated for each segment. Estimates of crustal shortening derived from gravity and seismic-refraction data are used to evaluate interpretations of the structural style. In some segments, where alternative styles were proposed, the crustal-shortening estimates are used to identify the more realistic models. Crustal shortening, shortening rates, and the resulting topography decrease progressively from north tosouth. These variations cannot befully explainedbydifferential fore-arc rotation, as in the Bolivian orocline model. Instead, a close correlation is suggested between the age of oceanic crust being subducted and the amount of shortening and propagation of the orogenic front toward the foreland. This fact becomes more important than fore-arc rotation farther south of the Bolivian orocline. On this basis, the present topography of the Andes, along the Nazca plate boundary, can be correlated with the age of adjacent oceanic crust.

ABSTRACT We have expanded previous trishear fault-propagation-fold forward models by allowing additional structural complexity in the form of multiple ramps and flats, variable propagation-to-slip ratio (P/S) and trishear angle, as well as multiple faults in a single section. The resulting forward models simulate characteristics of real structures very well. Our six-parameter grid search overcomes a longstanding obstacle to the application of trishear by providing a scientifically objective way of choosing the correct parameters to apply to real structures. By grid searching forward models with known initial parameters, we investigate the sensitivity of trishear to various parameters. These experiments highlight the importance of P/S in determining fold shape: A change in P/S of just 0.3 produces a change in fold shape that is equivalent to that produced by a 15-208 change in trishear angle. Application of the trishear model to contractional and extensional structures of the Neuquén Basin highlights its utility for predicting (1) strain and strain path, (2) fracture orientation and distribution, (3) fault-slip magnitude, and (4) fault nucleation point/děcollement depth. Our study of large basement-cored producing anticlines such as Filo Morado-Pampa Tril emphasizes an important point: Trishear and parallel kink-fold geometries can be compatible when applied at different scales. Trishear provides a bulk description of the deforming zone on the thickened, triangular eastern flank of the fold but makes no explicit prediction about how the strain is accommodated. In these structures, the strain is variably accommodated by tight folding, duplexing, and flow of evaporites and depends significantly on the thickness of the “competent” units in the section.