Late Cenozoic Extensional Formation of the Antofalla Depression, Southern Puna Plateau, Argentina: An Effect of Lithospheric Foundering?

Abstract

Deformation within orogenic plateaus functions to establish a dynamic equilibrium between tectonic boundary stresses and plateau gravitational potential energy. Temporal changes in deformation kinematics record perturbations to boundary stresses or internal plateau processes, such as lithospheric foundering. We integrate new mapping, field observations, and geochronologic ages with published data to document complex late Cenozoic upper crustal deformation in the region of the Antofalla depression, a ∼125 km long, sublinear basin within the southern Puna orogenic plateau, Argentina. The juxtaposition of stratal ages across the depression requires >900 m vertical offset on a surface‐breaking fault. Regional geologic structure, basin geomorphology, and our observation of a breccia paralleling the depression margin suggest formation of the depression by normal faulting. We interpret published stratigraphic logs to suggest that the depression formed between ca. 16 and 11 Ma following Andean shortening. Folded Late Miocene to Quaternary strata on the eastern depression margin indicate that extension ended and shortening resumed before present, revealing toggling between extensional and contractional kinematic regimes. The kinematic evolution of the Antofalla depression contrasts with the rest of the southern Puna plateau, which underwent shortening until latest Miocene to Quaternary time, followed by extension and strike‐slip deformation. Taken together, the spatial and temporal variations in late Cenozoic deformation of the southern Puna plateau are inconsistent with mechanisms that would affect the entire orogen, such as slowing convergence, but are compatible with lithospheric foundering.

Eocene to Quaternary Deformation of the Southern Puna Plateau: Thermochronology, Geochronology, and Structural Geology of an Andean Hinterland Basin (NW Argentina)

“…Moreover, divergence of the upper crust above the center of the foundering lithosphere is accommodated by upper‐crustal compression in the surrounding regions (Molnar, 2015). This pattern roughly matches the inferred kinematics of deformation in the Southern Puna during the Late Miocene (Tye et al., 2022), when the Antofalla region was undergoing horizontal extension, but surrounding regions such as Pasto Ventura were being shortened (Schoenbohm & Carrapa, 2015; Zhou et al., 2015). If lithospheric foundering occurred in the Southern Puna, therefore, it likely involved overall thickening of the weak, buoyant crust, even though the upper crust was in extension.…”

“…When possible, we prefer multiple-grain estimates of maximum depositional age over the YSG age. We applied a discordance filter based on the log ratio distance from the concordia composition ("concordia distance"; Vermeesch, 2021) a Age reported by Tye et al (2022).…”

“…The Escondida Formation consists of massive, poorly sorted, coarse conglomerates (Kraemer et al, 1999) and displays evidence of syntectonic sedimentation (Adelmann, 2001). The Escondida Formation was previously associated with the onset of horizontal extension in the region (Kraemer et al, 1999), but is alternatively associated with re-establishment of horizontal contraction after an episode of Late Miocene horizontal extension in the Antofalla region (Tye et al, 2022), an idea that we explore below.…”

“…Escarpments range from 5 to 10 km in length and 20–120 m in height. The largest escarpments are associated with normal‐sense displacement in bedding contacts, visible in the Salina del Fraile (Tye et al., 2022). We therefore map the horst‐and‐graben structures as normal faults (Figures 4 and 7), though it is possible that some or all the faults accommodated a significant component of strike slip.…”

“…A layer of volcanic ash that unconformably overlies hanging wall strata of Fault D yields a depositional age of ∼1.2 Ma (AF20A-01; Table 1) and is tilted eastward. Zircon dating of Quaternary strata in the footwall was reported by Tye et al (2022) and are summarized as follows in the context of Fault D: a reworked volcanic ash deposit yields 14 zircons younger than 9 Ma, and after applying a nominal common Pb correction yields a maximum depositional age of 1.47 ± 0.04 Ma (AF20A-02; Tye et al, 2022, Table 1). Unlike the volcanic ash in the hanging wall of Fault D, the reworked ash in the footwall was subject to intense folding (Tye et al, 2022).…”