Tectonomagmatic similarities between the modern Chilean flat‐slab region and pre‐Neogene magmatic episodes suggest that they represent analogues to flat subduction. Evolutionary patterns in each magmatic suite include (i) increasing La/Yb ratios and Sr‐and Nd‐isotopic enrichment through time, (ii) eastward‐migration of magmatism after periods of transpressional/transtensional intra‐arc deformation, and (iii) subsequent termination and virtual absence of main‐arc activity for 5–10 Myr. These patterns may reflect slab shallowing followed by flat subduction and thickening of the overlying crust. If repeated, they require interchanging episodes of slab steepening. Increasing convergence rates force slab kinking and eventual failure of the oversteepened slab, followed by rebound of the slab tip (owing to lack of further slab pull), flat subduction and termination of subduction‐related magmatism. Rapid subduction leads to shallow overriding of the detached slab fragment. Eclogitization of the gradually steepening slab tip at depth and subsequent slab pull permits asthenospheric corner flow and subduction‐related magmatism.

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Understanding Neogene arc crustal thickening in the central Andes requires (1) some estimate of initial pre‐Neogene (prior to 26 Ma) crustal thicknesses and (2) mechanisms that account for the remaining deficit in crustal thickening (10–30%). Mid‐Miocene horizontal crustal shortening can explain most but not all crustal thickening in the modern central Andean arc. Systematic changes in geochemical and Sr, Nd, and selected Pb isotopic data of Late Cretaceous–Eocene (∼78–37 Ma) and older arc magmatic episodes from north Chile provide new constraints on both. First, Andean crust may have been significantly thickened by long‐term underplating of mantle‐derived basalt from Jurassic to present. Second, estimated initial (late Eocene) crustal thicknesses of ∼45 km are consistent with (1) amphibole‐ and garnet‐bearing residual mineralogies for late Eocene syntectonic/posttectonic granitoids, (2) lower crustal P wave velocities of 7.3–7.7 km s−1 compatible with underplated mafic crust, and (3) results from recent experimental petrologic work showing garnet stability in mafic mineralogies ≥12 kbar (≥40 km crustal thickness). Analogous to older Andean magmatic episodes in north Chile, newly underplated basaltic crust may account for the remaining deficit in Neogene crustal thickening. Similar evolutionary patterns in geochemistry and initial Sr and Nd isotopic characteristics of Andean (200 Ma to present) magmatic rocks suggest that the Andean orogeny in this region evolved by a combination of processes of repeated arc migration, tectonic and magmatic crustal thickening, and igneous recycling which was controlled by periodically changing plate convergence rates and obliquity and corresponding changes in the rheologic behavior of the continental crust.

referenced classic type of orogen induced by non-collisional subduction of oceanic beneath continental lithosphere. Yet, plate convergence and subduction-induced magmatism since at least 200 Ma generated two fundamentally different types of orogens along western South America: (1) a plateau-orogen with anomalous thick orogenic crust in the Central Andes, and (2) a non-plateau orogen in the Southern Andes with thinner crust. Both arc systems show evidence of contemporaneous igneous activity for at least 200 Myr, yet it is unclear why they developed into fundamentally different arc orogens. Although much previous work focused on the Central Andean orogenic arc, few hints exist on which segment the evolution of the Andean orogen is typical, and which one is not.We address this issue by exploring long-term evolutionary tectonic, magmatic, geochemical and isotopic patterns of two contrasting Andean arc segments (Fig. 16.1):(1) the foreland and modern volcanic arc of the Central Andes in north Chile (21-26°S), and (2) the Southern Volcanic Zone (SVZ) in south Chile (41-46°S).The north Chilean arc segment is a wide mature volcanic arc system with shortened, thick orogenic crust (up to 70 km, Wigger et al. 1994), whereas the south Chilean arc segment is a narrow segment with relatively normal crustal thicknesses (30-40 km, Hildreth and Moorbath 1988, Yuan et al., in review). Spatial and temporal changes in evolutionary magmatic, geochemical and isotopic patterns during this time presumably recorded changes in both the geometry of the subducting slab and the tectonic conditions in the overriding plate. Hence they provide ideal opportunities to explore the cause and effect relationships of tectonics and magmatism along this classic non-collisional convergent margin. Moreover, these patterns offer efficient yet underestimated tools for scaling and balancing orogenic and magmatic processes, including subduction erosion (

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