Central and Southern Andean Tectonic Evolution Inferred from Arc Magmatism

Abstract

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 (

Cenozoic uplift of the Central Andes in northern Chile and Bolivia—reconciling paleoaltimetry with the geological evolution

“…Figure 6 shows the timing and distribution of magmatism in the Central Andes between 16°S and 22°S. Volcanic activity between ϳ26 and ϳ37 Ma is rare in this part of the Andes, with the absence of a volcanic arc, interpreted to be the result of flat slab subduction in this period (James and Sacks 1999; Haschke et al 2006;Hoke and Lamb 2007). Since ϳ26 Ma, andesitic-felsic volcanic activity has been widespread across the Central Andes, extending up to 300 km behind the arc (Fig.…”

“…Early-mid Miocene (14-26 Ma) Sn-and Ag-bearing subvolcanic stocks and granitoid intrusions ( Fig. 6; Haschke et al 2006;Oncken et al 2006;Hoke and Lamb 2007), together with major Late Miocene ignimbrite fields (Figs. 4 and 6, Los Frailes ignimbrites, 2-12 Ma, and Morocoalla ignimbrites 6-9 Ma) are preserved on the western margin of the Eastern Cordillera (Schneider 1985;Baker and Francis 1978;Barke et al 2007).…”

“…The simplest way that this can happen is by the slab sinking, creating space for an inflow of hot and low viscosity asthenospheric mantle into the overlying mantle wedge. Hydration and adiabatic decompression could result in prolific mantle melting, with the rejuvenation of the volcanic arc and widespread behind arc mafic-felsic magmatism (James and Sacks 1999;Haschke et al 2006;Hoke and Lamb 2007). Figure 8 shows the topographic evolution of the Altiplano and Western and Eastern Cordilleras along the northern (ϳ17.5°S) and southern (ϳ20°S) profiles (Figs.…”

“…The abrupt onset of volcanism in the Bolivian Andes at ϳ26 Ma, extending up to 300 km behind the arc, suggests that the asthenospheric corner flow was re-established at this time, with rapid steepening of the subducted slab (James and Sacks 1999;Haschke et al 2006;Hoke and Lamb 2007). This would have decoupled the overlying lithosphere from the effects of the negative buoyancy of the subducted slab, resulting in ϳ1 km of rapid uplift (Figs.…”

The emerging field of geogenomics: Constraining geological problems with genetic data

“…First, the long-lived Andean arc shows evidence of protracted magmatism since the Late Cretaceous. The latest phase of magmatism, focused in the Western Cordillera over the past ~30 Myr (Haschke et al, 2006), must have been associated with significant positive topography. No detailed studies have addressed paleoelevation of the magmatic arc forming the western flank of the Andean chain because most indicators of past elevation are restricted to sedimentary materials (e.g., paleosol carbonates, fossil leaves, bone, and teeth) further east.…”