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Abstract Understanding the effects of flat slab subduction on mountain building, arc magmatism, and basin evolution is fundamental to convergent‐margin tectonics, with implications for potential feedbacks among geodynamic, magmatic, and surface processes. New stratigraphic and geochronological constraints on Cenozoic sedimentation and magmatism in the southern Central Andes of Argentina (31°S) reveal shifts in volcanism, foreland/hinterland basin development, sediment accumulation, and provenance as the retroarc region was structurally partitioned during slab flattening. Detrital zircon U‐Pb age distributions from the western (Calingasta basin), central (Talacasto and Albarracín basins), and eastern (Bermejo foreland basin) segments of the retroarc basin system preserve syndepositional volcanism and orogenic unroofing of multiple tectonic provinces. Initial shortening‐related exhumation of the Principal Cordillera at 24–17 Ma was recorded by the accumulation of distal eolian deposits bearing Oligocene–Eocene zircons from the Andean magmatic arc. The Calingasta basin chronicled volcanism and basement shortening in the Frontal Cordillera at ~17–11 Ma, as marked by an upward coarsening succession of fluvial to alluvial fan deposits with a sustained zircon U‐Pb age component that matches pervasive Permian‐Triassic bedrock in the hinterland. An ~450 km eastward inboard sweep of volcanism at 11 Ma coincided with the inception of flat slab subduction, and subsequent thin‐skinned shortening in the Precordillera fold‐thrust belt that exhumed wedge‐top deposits and induced cratonward (eastward) advance of flexural subsidence into the Bermejo foreland basin. This foreland basin was structurally partitioned as basement uplifts of the Sierras Pampeanas transformed a fluvial megafan sediment routing network into smaller isolated alluvial fan systems fed by adjacent basement blocks. 

The south-central Chile and Argentina margin experienced a regional phase of extensional tectonics during the Oligocene–early Miocene, forming several basins across the forearc, Andean Cordillera, and retroarc regions. These basins accumulated thick successions of volcanic and sedimentary rocks. Subsequently, Neogene contractional tectonics led to the development of the current Andean Cordillera and the deposition of synorogenic clastic deposits in foreland basins. Traditionally, the Cura Mallín Formation, comprising a lower volcanic unit (CMV) and an upper sedimentary unit (CMS), has been interpreted to have formed during the Oligocene–early Miocene extensional phase. However, some studies propose deposition of the CMS in a foreland basin during the early–late Miocene. To unravel the transition from extensional to contractional tectonics in the Andes of south-central Chile and Argentina, we conducted new geochronological analyses (U-Pb, LA-ICP-MS) and integrated these results with structural, stratigraphic, and sedimentological observations in key sections within the CMS and the overlying Trapa-Trapa Formation in the Principal Cordillera along the Chile-Argentina border (37°–38°S). Our findings indicate that only the lower part of the CMS was deposited in an extensional setting, as evidenced by the presence of an inverted extensional wedge dated at ∼20 Ma. The middle-upper CMS (∼19 to 9 Ma) and contemporaneous units to the east exhibit evidence of syncontractional deformation, suggesting deposition in a foreland basin generated by shortening of the western Principal Cordillera. Around 9 Ma, uplift of the Agrio and Chos Malal fold and thrust belts, east of the Principal Cordillera, led to segmentation of the foreland basin. The Trapa Trapa Formation was deposited in a hinterland basin, with sediment sourced from the east. After ∼6.5 Ma, major contractional deformation shifted westward, resulting in intense folding of the CMS and Trapa Trapa Formation and subsequent thrusting of the western Principal Cordillera over the Central Depression. Our study suggests that deformation progressed toward the eastern foreland during the early to late Miocene and then shifted toward the western forearc during the late Miocene to Pleistocene. 

New U-Pb geochronological, Hf isotopic, heavy mineral, and sandstone petrographic results for Paleozoic clastic deposits of the Falkland/Malvinas Islands help address renewed debates on the plate tectonic history, regional paleogeography, and basin evolution of this geologic enigma prior to Mesozoic breakup of Gondwana. The Falkland/Malvinas Islands have been considered either an autochthonous part of the South American continent or part of an independent microplate displaced from the southeastern corner of Africa. We report detrital zircon U-Pb results (n = 1306 LA-ICPMS ages) for 11 sandstone samples from the Silurian-Devonian West Falkland Group (N = 7 samples, n = 837 grains) and Carboniferous-Permian Lafonia Group (N = 4 samples, n = 469 grains). Detrital zircon age distributions for the West Falkland Group point to consistent contributions from Neoproterozoic-Cambrian (650–520 Ma) and Mesoproterozoic (1100–1000 Ma) sources. Heavy mineral assemblages and sandstone petrographic data from these samples indicate significant input from recycled sediments. A potential shift in sediment sources during deposition of the Lafonia Group is indicated by the appearance of late Paleozoic (350–250 Ma) and Proterozoic (2000–1200 Ma) age populations, decreased proportions of stable heavy minerals, and a shift to juvenile Hf values for < 300 Ma zircons. The provenance change can be attributed to the onset of subduction-related arc magmatism and potential regional shortening and crustal thickening in southwestern Gondwana during the Permian transition of a passive margin into an active, retro-arc foreland basin. The detrital zircon age distributions identified here reflect potential source regions in southern Africa and/or the Transantarctic Mountains in Antarctica. These results are most readily accommodated within a Gondwana reconstruction that includes the Falkland/Malvinas Islands as a rotated microplate originating on the eastern side of southern Africa as part of the Gondwanide fold-thrust belt spanning from the Ventania region of Argentina through the Cape region of South Africa and into the Ellsworth and Pensacola mountains of Antarctica. 

The Middle Jurassic–Early Cretaceous evolution of the Neuquén Basin is traditionally attributed to a long phase of thermal subsidence. However, recent works have challenged this model. In view of this, we study the Late Jurassic Tordillo Formation, a non‐marine depositional unit that marks a shift to regional regression across the basin. Previous studies propose different causes for this regression, including the growth of the magmatic arc in the west, uplift in the south or extension in the north. We studied the Tordillo Formation in sections located at an intermediate position in the Neuquén Basin, in order to understand the tectonic processes active during sedimentation. We present evidence of normal faulting within the Tordillo Formation and the base of the overlying Vaca Muerta Formation. Some of these faults can be attributed as syndepositional. We characterize the Tordillo Formation as part of a distal fan‐playa lake depositional system with a contemporaneous western magmatic arc as the main source of sediment. When compared to the Late Triassic–Early Jurassic NE to NNE‐oriented rifting, which marks the opening of the Neuquén Basin, the Late Jurassic extension shows a switch in stress orientation; the latter is orthogonal to the north‐trending subduction zone. We interpret this change as a renewed phase of back‐arc extension induced by slab rollback along with minor distributed intraplate extension prior to opening of the South Atlantic Ocean. 

Unconformities in foreland basins may be generated by tectonic processes that operate in the basin, the adjacent fold–thrust belt or the broader convergent margin. Foreland basin unconformities represent shifts from high accommodation to non-depositional or erosional conditions in which the interruption of subsidence precludes the net accumulation of sediment. This study explores the genesis of long-duration unconformities (>1–20 myr) and condensed stratigraphic sections by considering modern and ancient examples from the Andes of western South America. These case studies highlight the potential geodynamic mechanisms of accommodation reduction and hiatus development in Andean-type retroarc foreland settings, including: (1) shortening-induced uplift in the frontal thrust belt and proximal foreland; (2) the growth and advance of a broad, low-relief flexural forebulge; (3) the uplift of intraforeland basement blocks; (4) tectonic quiescence with regional isostatic rebound; (5) the end of thrust loading and flexural subsidence during oblique convergence; (6) diminished accommodation or sediment supply due to changes in sea-level, climate, erosion or transport; (7) basinwide uplift during flat-slab subduction; and (8) dynamic uplift associated with slab window formation, slab break-off, elevated intraplate (in-plane) stress, or related mantle process. These contrasting mechanisms can be distinguished on the basis of the spatial distribution, structural context, stratigraphic position, palaeoenvironmental conditions, and duration of unconformities and condensed sections. Thematic collection: This article is part of the Fold-and-thrust belts collection available at: https://www.lyellcollection.org/cc/fold-and-thrust-belts 

Abstract The spatial and temporal distribution of arc magmatism and associated isotopic variations provide insights into the Phanerozoic history of the western margin of South America during major shifts in Andean and pre-Andean plate interactions. We integrated detrital zircon U-Th-Pb and Hf isotopic results across continental magmatic arc systems of Chile and western Argentina (28°S–33°S) with igneous bedrock geochronologic and zircon Hf isotope results to define isotopic signatures linked to changes in continental margin processes. Key tectonic phases included: Paleozoic terrane accretion and Carboniferous subduction initiation during Gondwanide orogenesis, Permian–Triassic extensional collapse, Jurassic–Paleogene continental arc magmatism, and Neogene flat slab subduction during Andean shortening. The ~550 m.y. record of magmatic activity records spatial trends in magma composition associated with terrane boundaries. East of 69°W, radiogenic isotopic signatures indicate reworked continental lithosphere with enriched (evolved) εHf values and low (<0.65) zircon Th/U ratios during phases of early Paleozoic and Miocene shortening and lithospheric thickening. In contrast, the magmatic record west of 69°W displays depleted (juvenile) εHf values and high (>0.7) zircon Th/U values consistent with increased asthenospheric contributions during lithospheric thinning. Spatial constraints on Mesozoic to Cenozoic arc width provide a rough approximation of relative subduction angle, such that an increase in arc width reflects shallower slab dip. Comparisons among slab dip calculations with time-averaged εHf and Th/U zircon results exhibit a clear trend of decreasing (enriched) magma compositions with increasing arc width and decreasing slab dip. Collectively, these data sets demonstrate the influence of subduction angle on the position of upper-plate magmatism (including inboard arc advance and outboard arc retreat), changes in isotopic signatures, and overall composition of crustal and mantle material along the western edge of South America.