Stella M. Moreiras

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.

Simulation of debris flows in the Central Andes based on Open Source GIS: possibilities, limitations, and parameter sensitivity

A GIS-based model framework, designed as a raster module for the Open Source software GRASS, was developed for simulating the mobilization and motion of debris flows triggered by rainfall. Designed for study areas up to few square kilometres, the tool combines deterministic and empirical model components for infiltration and surface runoff, detachment and sediment transport, slope stability, debris flow mobilization, and travel distance and deposition. The model framework was applied to selected study areas along the international road from Mendoza (Argentina) to Central Chile. The input parameters were investigated at the local scale. The model was run for a number of rainfall scenarios and evaluated using field observations and historical archives in combination with meteorological data. The sensitivity of the model to a set of key parameters was tested. The major scope of the paper is to highlight the capabilities of the model—and of this type of models in general—as well as its limitations and possible solutions.

Determination of seismogenic structures and earthquake magnitud from seismites in the Acequion river, Precordillera Range, central-western Argentina

Evidences for paleoearthquake induced liquefaction features in the Central Western Argentina were determined in the Acequion river valley. Well-preserved liquefaction structures were found in a Holocene lake dammed by rock avalanches. At least five paleoearthquakes affected this region during the Pleistocene-Holocene according to the temporal sequence established for these seismic structures and the rock avalanches recognized in the Acequion river. The magnitude of these triggering paleoevents and probable seismic source are analysed and discussed, concluding that liquefaction features and rock avalanches should be generated by M>5 earthquakes related to the nearby Cerro Salinas Fault belonging to Precordillera Oriental fault system. These findings allow us to extend the record of moderate to high magnitude-earthquakes to the Pleistocene-Holocene.

Megalandslides in the Andes of central Chile and Argentina (32°–34°S) and potential hazards

Abstract This review deals with an integration and update of the knowledge about large-volume landslides in the Central Andes at 32–34°S. An integrated landslide inventory for megalandslides in central Chilean and Argentinean Andean basins was developed, and dispersed chronological data on palaeolandslides were compiled, showing a dominance of Late Pleistocene and Holocene ages. Traditional hypotheses adopted for explaining landslide occurrence in the Central Andes are contrasted. Whereas seismic tremors have been widely suggested as the main triggering mechanism in Chilean collapses, palaeoclimatic conditions are considered as the main cause of Argentinean giant landslides. These different approaches denote the lack of multidisciplinary studies focused on the controversy about seismic or climate trigger mechanisms in the Central Andes. These studies are also essential to understand failure mechanisms and assessment of the related hazard and risk, which are essential to reduce social and economic impacts on vulnerable communities from future landslide events.

Landslide Processes in Argentina

Abstract This chapter reviews the status of knowledge on landslides in Argentina in order to understand their behavior and their implications for landscape evolution. This basic understanding allows an analysis of both the vulnerability of mountain communities and the potential risk for regional society and economies. Slope instability processes are analyzed as related to the Argentinean geological provinces. The main landslide processes throughout Argentina and detailed events that have occurred in each region of the country are reported. Landslides are widespread in Argentina. The most catastrophic events have been described for mountain regions, which comprise one-third of the country, extending from the north to the south border, involving mountains, ranges, tablelands, and sea-cliffs. Atypical rainstorms and seismic events affecting geological structures and weathered rocks are identified as the main landslides triggering forces in Argentina. Winter precipitation caused by the westerlies generate landslides, together with fall and flux mechanisms. Overaccumulation of snow and fast melting of snows during exceptionally warm spring days cause avalanches and debris flows. Landsliding triggered by earthquakes can occur all along the Andes, especially in the central and northern geological units. Volcanic activity promotes lahar formations. Over the Patagonian tablelands, landsliding is initiated mainly by fluvial undercutting after heavy summer storms, wave erosion at the footslopes, or groundwater-level variations. Snow avalanches and mudflows seem to be climatically conditioned in specific regions, while the other processes have a wide distribution. Antropic activities such as slope urbanization, irrigation, roads development, ski centers as well as exotic fauna that block and affect mountain rivers should also be considered landslide-triggering factors.

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.

Indicative Structures of Paleoseismicity in the Acequion River Valley, San Juan Province, Central-Western Argentina

Evidence for paleo-seismicity has been discovered in the Acequión river valley, in West central Argentina. Two Holocene rock avalanches have been observed; the most recent of these dammed a lake, whose sediments contain liquefaction structures. At least five paleo-earthquakes affected this region during the late Quaternary, as deduced from the succession of their secondary effects. The magnitude and the probable tectonic source of these paleo-events are discussed. The observed liquefaction features associated with slumps, joints, fractures, and faults, should be generated by M>5 earthquakes related to the nearby quaternary Cerro Salinas fault, which belongs to the Eastern Precordillera fault system. These data extend the regional seismicity record to the Holocene and highlight the high seismic hazard in this part of Argentina.

Historical damage and earthquake environmental effects related to shallow intraplate seismicity of central western Argentina

Abstract Central western Argentina is identified as the most hazardous seismic zone in the country. Historical earthquakes with magnitudes greater than Ms>6.0 frequently occur in this territory and are associated with the subduction of the Nazca plate. However, seismic hazards have not been fully assessed in this region. No secondary seismic effects of a potential earthquake with destructive consequences have been considered, nor has the existence of shallow Quaternary blind faults been identified by seismic surveys. Neotectonic studies performed up to the present describe only those Quaternary faults with some surficial expression. Lacking proper hazard assessment limits strategies to reduce the economic impact of drastic seismic events. This chapter is focused on the impact of major destructive earthquakes that have occurred in central western Argentina in order to understand the incidence of these phenomena in the past and to consider the vulnerability of the region in light of its increased urbanization and changing agricultural practices.

Geodynamic processes in the Andes of Central Chile and Argentina: an introduction

Abstract The Andes, the worlds largest non-collisional orogen, is considered the paradigm for geodynamic processes associated with the subduction of an oceanic plate below a continental plate margin. In the framework of UNESCO-sponsored IGCP 586-Y project, this Special Publication includes state-of-the-art reviews and original articles from a range of Earth Science disciplines that investigate the complex interactions of tectonics and surface processes in the subduction-related orogen of the Andes of central Chile and Argentina (c. 27–39°S). This introduction provides the geological context of the transition from flat slab to normal subduction angles, where this volume is focused, along with a brief description of the individual contributions ranging from internal geodynamics and tectonics, Quaternary tectonics and related geohazards, to landscape evolution of this particular segment of the Andes.

Were merely storm-landslides driven by the 2015-2016 Niño in the Mendoza River valley?

Since Holocene time, above-mean precipitations recorded during the El Niño warm ENSO phase have been linked to the occurrence of severe debris flows in the arid Central Andes. The 2015–2016 El Niño, for its unusual strength, began driving huge and dangerous landslides in the Central Andes (32°) in the recent South Hemisphere summer. The resulting damages negatively impacted the regional economy. Despite this, causes of these dangerous events were ambiguously reported. For this reason, a multidisciplinary study was carried out in the Mendoza River valley. Firstly, a geomorphological analysis of affected basins was conducted, estimating morphometric parameters of recorded events such as velocity, stream flow, and volume. Atmospheric conditions during such events were analyzed, considering precipitations, snow cover, temperature range, and the elevation of the zero isotherm. Based on our findings, the role of El Niño on the slope instability in the Central Andes is more complex in the climate change scenario. Even though some events were effectively triggered by intense summer rainstorm following expectations, the most dangerous events were caused by the progressive uplifting of the zero isotherm in smaller basins where headwaters are occupied by debris rock glaciers. Our research findings give light to the dynamic coupled system ENSO–climate change–landslides (ECCL) at least in this particular case study of the Mendoza River valley. Landslide activity in this Andean region is driven by wetter conditions linked to the ENSO warm phase, but also to progressive warming since the twentieth century in the region. This fact emphasizes the future impact of the natural hazards on Andean mountain communities.