Patterns and timing of exhumation and foreland basin deformation in the Eastern Cordillera of NW Argentina

Foreland basin segmentation in retroarc orogenic systems like the Central Andes of South America can be associated with two very different modes of deformation: thin-skinned and thick-skinned. The Eastern Cordillera (EC) and related ranges of Bolivia and Argentina exhibits a wide variety of structural features, both thick- and thin-skinned, that make this region a prime area to study the factors that affect fold-and-thrust belts as a whole. This study uses a systematic combination of structural, geochronologic, and thermochronological techniques to investigate how and in what order the various structures of the Argentinian EC from 25º--26ºS have developed in response to the eastward expansion of regional deformation. Detailed geological mapping in the Calchaqui Valley reveals a complex interplay of coeval thick-skinned and thin-skinned deformation that has brittlely deformed a thick Cenozoic succession of synorogenic siliciclastic rocks during Neogene inversion of the Salta Rift system. Detrital zircon U-Pb geochronology determines the maximum depositional age of the uppermost San Felipe Formation to be 2.3 +/- 0.1 Ma, and constrains growth in the lower Angastaco Formation by 14.1 +/- 0.5 Ma. Apatite (U-Th)/He thermochronology on detrital samples collected across the width of the southernmost EC (~80km) demonstrates a sequential eastward propagation of exhumation (and inferred deformation) from ca. 14--3 Ma. When coupled with published apatite fission-track cooling ages from the Puna-EC margin (Deeken et al., 2006), these new data indicate that the major structures of EC at ∼25.5ºS developed sequentially eastward during the Miocene-Pliocene at a rate of ∼8.3 mm/a. This study indicates that the propagation of deformation across the southern Central Andes can be summarized as having four major evolutionary stages, including: (1) development of the Western Cordillera from ca. 90--40 Ma; (2) rapid eastward migration of deformation across the Puna province from ca. 40--38 Ma; (3) distributed shortening and thickening within the Puna province from ca. 38--22 Ma; and (4) eastward migration of deformation and exhumation across the EC from ca. 22--3 Ma. These stages of eastward development of the southern Central Andes appear to be largely controlled by variations in plate convergence, but there is also considerable evidence that the orogen is profoundly affected by pre-existing basement anisotropy and dynamic mantle processes. Similar propagation rates of the thin-skinned EC of Bolivia, the thin-skinned Subandes of NW Argentina, and the thick-skinned southern EC of Argentina suggests that the rate at which the Central Andean orogenic system expands eastward is a factor that is independent of the magnitude of basement involvement.