Silvana Spagnotto

Evolution of shallow and deep structures along the Maipo-Tunuyán transect (33°40'S): From the Pacific coast to the Andean foreland

Abstract We propose an integrated kinematic model with mechanical constrains of the Maipo–Tunuyán transect (33°40′S) across the Andes. The model describes the relation between horizontal shortening, uplift, crustal thickening and activity of the magmatic arc, while accounting for the main deep processes that have shaped the Andes since Early Miocene time. We construct a conceptual model of the mechanical interplay between deep and shallow deformational processes, which considers a locked subduction interface cyclically released during megathrust earthquakes. During the coupling phase, long-term deformation is confined to the thermally and mechanically weakened Andean strip, where plastic deformation is achieved by movement along a main décollement located at the base of the upper brittle crust. The model proposes a passive surface uplift in the Coastal Range as the master décollement decreases its slip eastwards, transferring shortening to a broad area above a theoretical point S where the master detachment touches the Moho horizon. When the crustal root achieves its actual thickness of 50 km between 12 and 10 Ma, it resists further thickening and gravity-driven forces and thrusting shifts eastwards into the lowlands achieving a total Miocene–Holocene shortening of 71 km.

Temporal variation of the stress field during the construction of the Central Andes: Constrains from the volcanic arc region (22°- 26°S), Western Cordillera, Chile, during the last 20 Ma†

In order to understand the response of the stress field state to intrinsic processes during the construction of the Andes, such as thickening of the continental crust, lithospheric delamination, and/or thermal weakening, we investigate the stress field evolution of the arc region since the last 20 my, in the Central Andes (22°-26.5°S). The 43 reduced paleostress tensors derived from inversion of 682 fault-slip data reveal a complex pattern of stress states during the last episode of orogenic construction and topographic uplift. We identify two geodynamic stages; the first stage corresponds to the construction of the Altiplano/Puna plateau and the second one to its gravitational collapse. Four stress states that have prevailed in the Altiplano/Puna plateau since middle Miocene times, characterize the transition from one stage to the other. Along the study latitudes, a spatiotemporal change in stress state is clearly observed, which led to an understanding that a change in the stress field may be related not only to the boundary conditions but also to intrinsic factors associated with the construction of the Andean orogeny. Our results suggest that ca. 13-10 Ma and ca. 8-5 Ma, in the southern Altiplano and northern Puna, and in the southern Puna, respectively, regional elevation and crustal thicknesses reached threshold values necessary to generate the orogenic collapse.

Crustal structure and tectonic setting of the south central Andes from gravimetric analysis

Based on terrestrial gravity data, in this paper we prepared a map of Bouguer anomalies, which was filtered to separate shallow and deep gravity sources. Based on a density model and gravimetric inversion techniques, the discontinuous crust- mantle boundary and the top of crystalline basement were modeled. Subsequently, the equivalent elastic thickness (Te) was evaluated, considering information from the crust-mantle discontinuity and topographic load, finding high Te values in the eastern Andean foothills and west of the Velasco range. These results are consistent with the positive isostatic and residual Bouguer anomaly values, which suggest the presence of high-density rocks in the mid-to upper crust. In addition, petrographic and geochemical analysis conducted in surface outcrops suggest a mantle origin.

Cenozoic Orogenic Evolution of the Southern Central Andes (32–36°S)

This review explores the complex interactions of endogenic and exogenic processes in the segment of the Andes that straddle a transition from the Pampean flat slab to a normal subduction segment (32°–36°S). This segment shows remarkable along-strike variations in topographic uplift, structural elevation, amount and rate of shortening, and crustal root geometry. In the flat-slab segment, high elevations, the development of several tectonic provinces (mountain systems) and the lack of active volcanism characterize the orogen. Deformation and uplift advanced to the east, together with arc-related magmatic activity, sequentially uplifting the Principal Cordillera (20 to ~8 Ma), the Frontal Cordillera (12–5 Ma), the Precordillera (<10 Ma) and the Sierras Pampeanas (<5 Ma). In the normal subduction segment to the south, the Andes are characterized by a decrease in elevation, with a big step in topography at ~35°S and the development of an active magmatic arc straddling the Argentina-Chile border. The Frontal Cordillera is only in the northern part of the normal subduction segment, disappearing at 34°S; only the Principal Cordillera remains south of this latitude. Similarly, deformation progressively advanced to the east, uplifting the Principal Cordillera (20–8 Ma), the Frontal Cordillera (<10 Ma) and the San Rafael basement block (<5 Ma). The amount of shortening systematically decreases from north to south along these two segments, but at the transitional zone between flat and normal subduction zones, there is a sharp decline from ~180 km of shortening (32°S) to ~70 km (33°40′S). South from this latitude, the amount of shortening lineally decreases until it reaches ~30 km at 35°S. Yet, interestingly, the amount of late Miocene surface uplift is opposite to the trend in crustal shortening . These along-strike variations are best explained by boundary conditions of the subduction system related to interplate dynamics controlling the overall pattern of tectonic shortening. However, local variations in mean topographic elevation, deformation styles and crustal root geometry are more likely to be due to upper-plate lithospheric strength variations. These strength variations have governed the coupling degree between brittle upper crust and ductile lower crust deformation. In the flat-slab segment, an initial thick and felsic crust favors the coupling model; while in the normal subduction segment, a thin and mafic lower crust allows the uncoupling model.