Constraining Andean Propagation of Exhumation at the Limit of the Eastern Cordillera, NW Argentina, Using Low‐Temperature Thermochronology in a Structural Context

Abstract

Within the Central Andes of NW Argentina, the spatiotemporal distribution and style of deformation is strongly influenced by pre‐Cenozoic heterogeneities, mostly related to the Salta rift extension in the Cretaceous. At the enigmatic junction of the thin‐skinned Subandean belt and the thick‐skinned Santa Barbara System, the Tilcara Range and adjacent San Lucas block, located within the Eastern Cordillera, show thermochronological and field evidence of multiple exhumation events. Mesozoic (140‐115 Ma), pre‐Andean exhumation of basement highs is constrained by unconformities between basement and syn‐rift strata, as well as zircon (U‐Th‐Sm)/He cooling ages. Cenozoic Andean exhumation is quantified by apatite (U‐Th‐Sm)/He and fission track cooling ages, which were reset between the Late Cretaceous and Miocene. These data show that the westernmost Tilcara Range began exhuming in the late Oligocene‐early Miocene (26‐16 Ma), after which exhumation propagated to the border of the Eastern Cordillera in the middle Miocene (22‐10 Ma). The onset of rapid exhumation in the San Lucas block, which is located east of the Tilcara Range, occurred in the late Miocene (10‐8 Ma) in its western part, and in the late Miocene‐early Pliocene (6‐4 Ma) in its eastern part. Internal deformation of the San Lucas block, disturbing zircon (U‐Th‐Sm)/He and apatite fission track age patterns, predates propagation of rapid exhumation. The here presented low‐temperature thermochronology data set thus quantifies the multi‐phase exhumation history of the Eastern Cordillera of NW Argentina and constrains the timing of Andean propagation of exhumation within the Eastern Cordillera and the adjacent structural transition zone.

Structural Inheritance in the Eastern Cordillera, NW Argentina: Low‐Temperature Thermochronology of the Cianzo Basin

“…Our youngest (Pliocene) AHe cooling ages, located at the eastern margin of the study area, overlap with cooling ages of Van Kooten, Sobel, et al. (2022) and fall into the 6.5–2.1 Ma time step of Henríquez et al. (2023).…”

“…Our thermal models of both limbs show an onset of cooling as a result of exhumation between the latest Oligocene to middle Miocene (24–15 Ma), as well as similar exhumation rates of ∼0.25 mm/yr for both limbs (Table 3; Models CO‐AE, CA‐AE, HPT‐AS: Figures 6–8). The proposed onset of exhumation for the Hornocal fault corresponds to the timing suggested for the Tilcara Range (Van Kooten, Sobel, et al., 2022), located along strike south of the Cianzo basin. Growth strata in the footwall of the Hornocal fault (Figure 4a) provide a middle–late Miocene age constraint for fault activity, based on 40 Ar/ 39 Ar analyses of tuffs (Siks & Horton, 2011).…”

“…Propagation of the thrust front and exhumation of the ranges east of the Cianzo basin occurred after ∼6.5 Ma (Henríquez et al., 2023). Pliocene and Pleistocene AHe cooling ages in the easternmost part of the Eastern Cordillera (Van Kooten, Sobel, et al., 2022) and recent earthquake focal mechanisms (Heidbach et al., 2018) show that shortening and exhumation are ongoing in this part of the Central Andes.…”