Andrés Echaurren

Quantification of the timing and mechanisms of crustal thickening in Cordilleran orogenic hinterlands remains challenging. We address this in the Andes using whole‐rock geochemistry and zircon petrochronology from a >70 Myr volcanic record in the Tinguiririca valley of Chile (∼35S), which captures a pivotal transition from extensional to contractional deformation. Zircon (Sm, Gd, Dy)/Yb ratios generally track trends in whole‐rock geochemical crustal thickness (paleomohometry) models across this record, with Dy/Yb showing the most consistent correspondence. Our results indicate modest crustal thinning coeval with Late Cretaceous normal faulting and oblique plate convergence. Eocene and Early Miocene samples yield similar crustal thicknesses (∼35 km), suggesting limited local net attenuation during proposed intra‐arc rifting (Abanico basin). The crust then thickened by ∼10 km to near‐modern values (∼46 km) during the Neogene; Quaternary estimates match modern geophysical constraints, validating the paleomohometry approach. Thickening was most rapid between ca. 17 and 13 Ma, coinciding with accelerated retroarc shortening and plate convergence. Additional zircon geochronologic and geochemical proxies (Th/U, phosphorus content, and oxygen fugacity) indicate enhanced Miocene crustal melting and magmatic productivity. Along‐strike comparisons reveal north‐to‐south younging and decreasing magnitude of thickening, mirroring trends in retroarc shortening. These findings indicate that underthrusting of foreland lithosphere during retroarc shortening drove crustal thickening, magmatic flare‐up, and generation of orogenic topography in the southern Central Andes.

Flat subduction, an intermittent phenomenon along active margins, arises from well-known causes, yet the mechanisms driving its expansion remain poorly understood. The prevailing view suggests that trenchward continental motion drives slab overthrusting, causing the flat slab to expand oceanward. Here, we explore an alternative mechanism: underthrusting of the subducting plate through forward propagation of the flat-slab hinge. We directly evaluate both hypotheses through a kinematic analysis of trench and flat-slab motions using a global flat subduction database cast into multiple absolute plate motion models. Our results indicate that flat-slab expansion reflects distinct end-member processes, with forward propagation emerging as the dominant mode. We present a framework for flat-slab propagation that emphasizes the dynamic interaction between lower-plate motion and slab pull from adjacent subduction zones, an interplay validated through numerical modeling. These findings challenge conventional assumptions and underscore the need to reconsider the role of lower-plate kinematics in flat-slab dynamics.

Sedimentary basins in the distal Cenozoic Andean retroarc yield an important geological archive that provides crucial insights into the tectonic and sedimentary processes associated with the different stages of mountain building. At 33° S, the tectonic and sedimentary processes that have operated during the Neogene and Quaternary periods of Andean orogenesis are well documented, whereas information on the Paleogene period remains fragmentary and partly enigmatic. The Paleogene sedimentation in the distal retroarc at this latitude is represented by the Divisadero Largo Formation, a 70‐m‐thick sedimentary unit that has been extensively studied for its fossil content, leading to the controversial definition of the late Eocene Divisaderan South American Land Mammal Age (SALMA). New zircon U–Pb geochronological data provide a valuable age constraint for Paleogene tectonic and sedimentary processes in the Southern Central Andes. Furthermore, we present the first detailed facies analysis of the Divisadero Largo Formation, combined with a sedimentary provenance study and a seismic subsurface characterisation of this unit. Our results indicate that the age of the Divisadero Largo Formation is Palaeocene to early Eocene (~65–41 Ma). Deposition of this unit occurred in a shallow, lacustrine depositional environment with variable water depths and was characterised by a low accumulation rate of 3 m/Myr. During this time, the sediment source was predominately located in the Andean magmatic arc; however, no conclusive evidence of significant Paleogene deformation exists. These characteristics (age, depositional environment, low accumulation rate and provenance) enable a regional correlation with Paleogene deposits farther south in the Neuquén Basin. In addition, based on U–Pb geochronology and sedimentary features, a 20 Myr hiatus could be defined between the Divisadero Largo Formation and overlying synorogenic deposits, as has been proposed farther south, reflecting the northernmost record of this hiatus. Taken together, these new observations help to refine a tectono‐sedimentary model for the evolution of the Southern Central Andes retroarc basin at 33° S that comprises four stages preceding the well‐documented Miocene contraction phase: (i) Late Jurassic–Early Cretaceous extension; (ii) Late Cretaceous contraction; (iii) Palaeocene–middle Eocene tectonic quiescence; and (iv) a renewed phase of late Eocene–Oligocene extension.

Geochemical and geochronological data reveal that late Oligocene‐early Miocene time is a break point in the evolution of Andean magmatism. The Patagonian Andes registered the onset of arc volcanism since the late Eocene forming part of the El Maitén Belt, whose development was driven by the subduction of the Farallon/Nazca plates beneath the Andean margin. During the Oligocene, the El Maitén Belt shows a change in the geochemical signature of its magmas from tholeiitic to calc‐alkaline compositions, reflecting a more mature stage in the magmatic arc evolution. Toward the early Miocene, a striking event is registered in Andean volcanic sequences as mafic tholeiitic lava flows of the El Maitén are interbedded with marine deposits, suggesting their development in the context of a fast subsiding regime. Geochemical analyses presented in this paper show that these rocks resemble enriched mid‐ocean ridge basalt‐like and ocean island basalt compositions, isotopically depleted, which strongly contrast with previous arc products. By this time, a global plate reorganization event had caused an increase in convergence rates, accelerated rollback, and a more orthogonal geometry of subduction, triggering widespread magmatism and the development of extensional basins in the overriding plate. Arc‐related volcanism during the early Miocene can be found only in the western slope of the Andes, suggesting the retreat of the volcanic front toward the trench. The proposed model highlights a strong linkage between the geochemical signature of magmatic products and changes in the subduction zone configuration and mantle dynamics during the evolution of the Patagonian Andes (41–44°S).

The Auca Mahuida volcanic field lies on the southernmost Payenia Volcanic Province, one of the broadest retroarc volcanic plateaux in the southern Central Andes (~38°S). This voluminous basaltic flooding of Quaternary age was originated from a deep asthenospheric source, interpreted as a mantle plume product of changing slab dynamics. The geometry of this source is deduced from magnetotelluric data, but the limited spatial coverage of this array does not allow a detailed resolution of this anomaly. In order to present a detailed geometry of the conductive anomaly and related crustal magmatic bodies, we used multiple data sources. We combined Magnetic and Bouguer anomalies, Curie isotherm depth (T_c), Elastic Thickness (T_e) and Moho depth derived from the Global Earth Magnetic Anomaly Grid (EMAG2) and terrestrial gravity measurements, all together in a holistic geophysical analysis. The magnetic data depict a nearly 200‐km‐in‐diameter circular anomaly that would correspond to a dense body according to the Bouguer anomaly. Geoid data from the Gravity Field Model (EIGEN‐6c4) have been filtered in order to isolate deeper mass influences and visualize the asthenospheric upwelling previously described from magnetotelluric data. Moho inversion yields a crustal attenuation at 36‐ to 32‐km depth coinciding with T_e below 20‐km depth and a shallow T_c (≤15‐km depth) at the site where Geoid positive undulation was calculated. Finally, surface analysis allowed defining a topographic swell, compatible with the dimensions of the identified magnetic anomaly, where the main rivers deviated, potentially due to a recent base level change.

The Andes in northern Patagonia are mainly formed by Mesozoic magmatic units: the mostly Jurassic–Cretaceous North Patagonian Batholith and volcanism of the Jurassic Lago La Plata (Ibáñez) Formation as well as the mid-Cretaceous Divisadero Group. These rocks represent the development of a magmatic belt through Jurassic–mid-Cretaceous time, during a switch of the tectonic regime from extension to compression. To study arc evolution during this transition, we carried out fieldwork and geochemical sampling at c. 43°S, clarifying structural relationships and characterizing the magmatic sources. Multi-element diagrams for both volcanic units suggest a slab-derived signature, whereas isotopic ratios (Sr–Nd–Pb) indicate parental melts sourced from the subduction-modified asthenospheric mantle interacting with crustal sources during their emplacement. An angular unconformity is identified between the synextensional Jurassic volcanic rocks and Lower Cretaceous sedimentary rocks beneath the mid-Cretaceous sequences. Although this deformational event was simultaneous with generalized overriding plate compression, geochemical ratios indicate an immature Aptian–Albian arc with no associated crustal thickening. Late Jurassic to mid-Cretaceous arc settlement after a trenchward retraction of magmatism from the foreland between c. 41 and 45°S, with an associated increase in slab dip angle, may have provoked crustal softening facilitating the subsequent initial fold–thrust belt growth. Supplementary material: Petrographic descriptions and geochemical–isotopic data are available at https://doi.org/10.6084/m9.figshare.c.3677974