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ATACAMA DESERT LANDSCAPE EVOLUTION IN RESPONSE TO SHORT-DURATION TRIGGERING EVENTS: INSIGHTS ACROSS SCALES OF TIME AND SPACE

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Abstract

The landscape evolution at varying temporal and spatial scales in the Atacamaremains an important issue that has not been fully addressed holistically. Previous studies have tended to focus on the effects of short-lived phenomena or the climatic and tectonic evolution of the area. This dissertation presents a multi-scale approach to investigating landscape change in the Atacama, with a focus on extreme rainfall events and their impact on different geological units, as well as the study of weathering controls in small scale objects. These objectives were achieved by integrating remote sensing and field observations which allowed us to enhance our understanding of the entire landscape. The first part of this study presents a simple method to identify areas of extreme surface change using Change Vector Analysis (CVA) and Principal Component Analysis (PCA) applied to Landsat 8 OLI imagery from two rain events in 2015 and 2019. The results were corroborated by comparing with previous studies and field observations, demonstrating that CVA and PCA can be used to identify various types of change, including salt development, soil moisture, and mass transfer, even up to 60 kilometers away from the loci of the rain. In the second part of this study, Synthetic Aperture Radar (SAR) images were used to study the effects of the 2019 rain event. Time series of InSAR coherence expressed as similarity matrices and time series of Sentinel 1-A backscattering were employed to reveal different types and rates of change in a wide range of geological materials. These included salt flats located in the hyperarid core of the Atacama that displayed transient and delayed change, and salt flats located in the high Andes that showed immediate and progressive change. Results also included gypsic soils displaying transient temporal change and mudflow deposition. All results were placed in the context of long-term changes that have been identified or estimated previously to provide a link between short-term and long-term evolutionary paths. In general, the study area needs more frequent rain events or infrequent but more intense rainfalls to generate the landscape to its current display. Finally, the study of a field of weathering boulders that decay to debris cones with small scale stratigraphic records in the hyperarid core of the Atacama Desert could lead to a rich Holocene and Pleistocene climatic or earthquake history for the area. These boulders result from rockfall from the Chuculay fault scarp in the Coastal Cordillera and weather in a semi-closed system where most of the material detached from the boulders is now in their surrounding detritus cones. The relative weathering stages within a collection of boulders are not necessarily a valid metric for their relative ages, meaning that boulders that have lost 50% (stage 3 out of 5) of their original volume can be older than boulders that are now a pile of sediments (stage 5 out of 5). The factors that control the weathering rates are still undetermined for the Atacama. Using a wide range of tools, including Structure from Motion (SfM), Raman spectroscopy, and field observations, this part of the study reveals that there are many complexities that need to be solved in order to extract environmental information from the boulder/detritus cone systems. These include weathering rates that are highly controlled by the fracture frequency, content of ferromagnesian minerals, and boulder size, in addition to the development of sulfate levels in the detritus cones that are the result of environmental phenomena that are present only under specific conditions related to water influx and the location of the boulders.

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Description

301 pages

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Date Issued

2023-08

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Keywords

Atacama; Boulders; Landscape evolution; Remote sensing; Surface change; Weathering

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Union Local

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Committee Chair

Jordan, Teresa

Committee Co-Chair

Committee Member

Lohman, Rowena
Philpot, William

Degree Discipline

Geological Sciences

Degree Name

Ph. D., Geological Sciences

Degree Level

Doctor of Philosophy

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Government Document

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Attribution-NonCommercial-NoDerivatives 4.0 International

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dissertation or thesis

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