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Thematic Articles

Ore Deposits of the Central Andes

The Central Andes has one of the richest base metal endowments on Earth. In 2017, the Central Andes accounted for the mine production of around 39% of the world’s copper, 23% of its silver, 20% of its molybdenum, 14% of its zinc, and 12% of its tin as well as significant shares of other metals including gold and lead (USGS 2018). These metals are found in a variety of ore deposits of which by far the most important are those that occur as part of the “porphyry system” in the sense of Sillitoe (2010). However, important ore deposits of other mineralization styles also occur in the Central Andes. This info box presents characteristics of main and/or representative Mesozoic ore deposits that occur between latitudes 11°S and 30°S.

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Nitrate Deposits of the Atacama Desert: A Marker of Long-Term Hyperaridity

The nitrate deposits of the Atacama Desert are unique when one considers that in most surface environments nitrate is produced or consumed by biological processes and is easily washed away by rain. Nitrate deposits have puzzled geologists since Charles Darwin’s visit to the Atacama in 1835 and several hypotheses have been proposed to explain their origin. Here, we review our current understanding of the nitrate deposits in the Atacama Desert and show that nitrate’s primary origin is predominantly atmospheric. However, its massive accumulation and preservation specifically in Atacama is due to the serendipitous convergence of climatic, tectonic and hydrologic conditions that are unique to the Central Andes.

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Turning up the Heat: High-Flux Magmatism in the Central Andes

The Neogene history of the Central Andes records one of Earth’s most productive periods of high-flux silicic magmatism. Subduction of an aseismic ridge, the Juan Fernández Ridge (JFR), led to changes in mantle melt productivity that initiated a transcrustal magmatic system culminating in massive caldera- and ignimbrite-forming eruptions. This volcanism is time transgressive, tracking the southward passage of the JFR beneath the Central Andes. The volcanic field is underlain by a composite, arc-long mid- and upper-crustal granodiorite batholith that represents extensive processing of the continental crust by mantle-derived magmas. This batholith stabilized the upper crust and contributed to the extreme elevations despite a net crustal loss beneath the Puna region.

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Magmatism in the Central Andes

Active continental margins are shaped by subduction-related magmatism, and the Central Andes of South America are a prime example. The Central Andean orogen has evolved over the past 25 My via magmas ascending from the mantle and interacting with increasingly thickened continental crust. This process is reflected in the volumes and compositional variations of the magmas that erupt at the surface. These compositional variations can be traced in time and space, and, herein, we provide explanations for their cause and explore the nature of the Central Andes transcrustal magma systems that feed the iconic stratovolcanoes today.

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The Topographic Evolution of the Central Andes

Changes in topography on Earth, particularly the growth of major mountain belts like the Central Andes, have a fundamental impact on regional and global atmospheric circulation patterns. These patterns, in turn, affect processes such as precipitation, erosion, and sedimentation. Over the last two decades, various geochemical, geomorphologic, and geologic approaches have helped identify when, where, and how quickly topography has risen in the past. The current spatio-temporal picture of Central Andean growth is now providing insight into which deep-Earth processes have left their imprint on the shape of the Earth’s surface.

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The Central Andes: Elements of an Extreme Land

The Central Andes and the Atacama Desert represent a unique geological, climatic, and magmatic setting on our planet. It is the only place on Earth where subduction of an oceanic plate below an active continental margin has led to an extensive mountain chain and an orogenic plateau that is second in size only to the Tibetan Plateau, which resulted from continental collision. In this article, we introduce the history of the Central Andes and the evolution of its landscape. We also discuss links between tectonic forces, magmatism, and the extreme hyperarid climate of this land that, in turn, has led to rich deposits of precious ores and minerals.

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The Science of Terroir

Terroir involves the complex interplay of climate, soil, geology, and viticulture, all of which influence the character and quality of a wine from a given grape variety, rootstock, and viticultural practice. Contrary to the assertions of some wine writers, the minerals and character of the soil cannot be tasted in the wine. Rather, it is their effect on the grape ripening process that gives certain wines a “sense of place”. Most important is water availability, which is a function of climate (rainfall and humidity) and soil water-holding capacity. The soil structure reflects the geologic history of a region and may have evolved over millions of years as influenced by faulting, weathering, and bedrock mineralogy. Far-field effects such as glaciation and resultant sea-level change can affect landscapes that are thousands of kilometers apart.

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Remote Sensing Applications for Viticultural Terroir Analysis

With the rise of remotely piloted aircraft systems, increasing computing power and advances in image processing software, the opportunities for vineyard observations through remote sensing are increasing. Remote sensing and image analysis techniques that are becoming more available include object-based image analysis, spatiotemporal analysis, hyperspectral analysis, and topoclimatology. Each of these techniques are described and discussed as potential for development within a viticulture and terroir context. While remote sensing applications are well established at the smaller precision viticulture scale, the larger spatial scale of terroir analysis requires adaptation and new models of analysis.

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The Scale Dependence of Wine and Terroir: Examples from Coastal California and the Napa Valley (USA)

The physical parameters of terroir are scale dependent. At the regional scale, climate is paramount and relates to the grape varietals most suited to the setting. Intermediate factors include geologic setting, sun exposure, and topography, all of which influence grape quality and character. At the smaller scale, soil character and local climatic variation shape grape flavor and aroma. These notions are discussed in relation to four California (USA) wine regions: Sonoma County, Paso Robles, Santa Barbara County, and Napa Valley.

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Environmental and Viticultural Effects on Grape Composition and Wine Sensory Properties

The most important characteristics upon which wines are evaluated are the intensity and complexity of their flavors. Flavor describes the combined impression created by both the volatile compounds, which are responsible for wine aroma, and the nonvolatile components, which determine the taste sensation. Environmental factors (topography, soil, climate), termed terroir, influence the levels of grape metabolites related to wine organoleptic properties, i.e. properties that can be detected by the sense organs, such as taste, color, odor, and feel. However, modern vineyard management practices have the potential to modify a vine’s response to natural site influences and so modify the flavor of the resultant wine.

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