Quaternary alluvial lineaments in the Atacama Desert, northern Chile: morphology, relationships to bedrock structures, and link to the seismic cycle of the Andean subduction margin
Date
2023
Authors
Perman, Emily A., author
Singleton, John, advisor
Gallen, Sean, advisor
Bhaskar, Aditi, committee member
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Abstract
Located in the upper plate of the modern Nazca-South American subduction zone, the hyperarid Atacama Desert is an ideal place to study forearc deformation through surface geomorphology. We studied neotectonic lineaments in alluvium between ~25.5° and 26° S in the Coastal Cordillera with the goal of understanding modern forearc strain. Visible in satellite imagery, these lineaments are defined by linear to curvilinear structures consisting of subparallel ridges that range from tens of meters to kilometers in length. Field observations and 10 cm-resolution digital elevation models (DEMs) derived from drone imagery record a consistent, asymmetrical "ridge-trough-ridge" morphology that commonly traces into ~1–2 m-wide bedrock fissures containing gypsum and calcite. Most lineaments in this region trend ~N-S to NW-SE, parallel to the dominant bedrock structural grain and the Cretaceous Atacama and Taltal fault systems. Small-scale faults found in the lineament ridges have cm- to mm-scale apparent normal-sense displacement and consistently dip moderately to steeply (50–70°) towards the lineament troughs, defining a graben-like structure. In outcrop, these normal fault zones are enhanced by differential erosion, and in thin section faulted material is distinguishable by increased cementation within fractures penetrating grain boundaries. A tuff deposit within an alluvial fan containing several lineaments yields a zircon U-Pb age of 2.2 ± 0.1 Ma, indicating that lineaments in this fan are Quaternary in age and likely related to upper plate strain due to modern subduction along the Nazca-South American plate boundary. Older alluvial surfaces tend to have lineaments with broader and taller ridges than those formed on relatively younger alluvial surfaces, indicating that these structures formed progressively through time. In addition, profile data gathered from DEMs show a weak linear correlation between lineament trough width and ridge height, meaning that wider lineaments tend to have taller ridges. Along the flanks of two trenched ridges, we observed shallowly-dipping planes that resemble thrust faults, suggesting the ridges may have formed in response to contractional deformation. We propose the lineaments record alternating forearc shortening and extension related to interseismic and coseismic phases of the earthquake cycle, with the development of ridges and thrust faults recording interseismic shortening and normal faults and fissures which form the central troughs recording coseismic extension.
Description
Zip file contains data spreadsheet and map 1.
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Subject
forearc strain
lineaments
seismic cycle
lineament
Atacama
neotectonics