Hillslope erosion & morphology in Chile
PhD Project
Chile is a country with a vast range of climate zones, from the rainy south to the hyper-arid Atacama desert. Chile also hosts an active and destructive subduction zone, which produces earthquakes, and therefore fractures in the bedrock. In my PhD, I am investigating the influence of climate (precipitation and temperature), as well as fracturing, on weathering and erosion in the Chilean Coastal Cordillera (The Coastal Cordillera is a pre-Andean volcanic arc (~Jurassic-Carboniferous) which is now a granite batholith just to the west of the Andes). To investigate this question, I’m quantifying erosion rates on hillslopes and in streams using cosmogenic nuclide dating (Beryllium-10) of bedrock, boulders, and soil, and measuring fracture density in different locations. We predict fracture spacing dictates block size because blocks arrive on the surface already dissected by fractures from tectonically-induced brittle deformation; then block size is further reduced by chemical weathering and transported by water or landslides. On a hillslope of a given angle, we predict erosion rate will increase with higher fracture spacing and finer-grained material. I’m interested to see how bedrock fracturing and climate interact to produce different landscapes and erosional processes.
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Part of this story is now published in ESurf: https://esurf.copernicus.org/articles/11/305/2023/
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Recorded talk from AGU 2020:
https://www.youtube.com/watch?v=tTt-fpL-fLw&feature=youtu.be
Subduction erosion in northern Chile
PhD Project
Subduction has been ongoing in Chile since the Jurassic or earlier, but the style of subduction has changed with time and latitude. The margin experienced subduction erosion for a long time (net loss of material from the continental margin), but currently southern Chile has an accretive margin (net gain of material). Glaciation of Patagonia ~5 million years ago is thought to have triggered subduction accretion: the glaciers scrape off a massive amount of material from the Andes; this sediment is then transported to the continental margin and fills the trench. Subduction accretion is visible via seismic reflection, but the rate of subduction erosion is harder to quantify. To help solve this problem, I’m planning on determining erosion rates of sea cliffs in the Atacama desert using 10Be cosmogenic nuclide dating.
Lithologic control on landscape evolution
PhD Project
Just east of La Serena, Chile, is a nature reserve called Santa Gracia, where goats and wild donkeys nibble on cacti and roam over smooth granite hills of two different lithologies: the Santa Gracia monzogranite pluton and another pluton composed of diorite. The Santa Gracia pluton has been sculpted into low-lying hills dissected by dense channels, while the diorite lithology is host to larger hills and a lower drainage density. To understand why, I’m collecting samples of hillslope and fluvial material to obtain the bedrock chemistry, mineralogy and erosion rate, and mapping stream initiation points in the field. We predict that the landscape morphology has to do with varying chemical weathering rates of the two different lithologies due to mineralogical differences.
Permian subduction initiation
Master's Project
The Sierra Nevada mountain range in California was an active chain of volcanoes, like the Andes, that peaked in activity ~100 million years ago. The subduction zone that generated the volcanic chain initiated ~250 million years ago, but the earliest magmatic material has been eroded and deposited into basins surrounding the Sierra Nevada. For my Master’s project, I studied a sedimentary basin to the east of the Sierra Nevada (the Inyo Mountains) to understand the history of subduction initiation of the California margin.
I measured stratigraphic sections, determined sedimentary depositional environments and sea level throughout Permian time, and explored the provenance and maximum depositional age of sediment layers using detrital zircon U-Pb geochronology. I found that the sea level fluctuated drastically throughout the Permian, and hypothesized that this was (mostly) tectoncially-induced. I also measured the age of detrital zircons from the early Cordilleran magmatic arc and found a rare group of grains with ages of 260-270 million years old, meaning subduction initiated 10-20 million years older than we thought. This work was published in Lithosphere in 2020: https://doi.org/10.2113/2020/9406113.
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Working in the dusty hills directly between the highest mountain in the continental US (Mt. Whitney) and the deepest valley (Death Valley) was an extraordinary experience. I wrote about my field adventures in the series Inyo Field Notes under Science Writing.
Arc-to-rift transition in Baja, Mexico
Bachelor's Project
The Gulf of California is a young rift that separates Baja peninsula from mainland Mexico, akin to the Red Sea. From about 24-12 million years ago, a subduction zone emplaced a magmatic arc along Baja, and then the East Pacific Rise spreading center was subducted, causing a transition to continental rifting aided by thermal weakening of the crust from arc magmatism. Now, Baja peninsula drifts away from mainland Mexico and will continue to do so over the next few million years, perhaps eventually winding up in the Northwest!
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For my Bachelor's project, I helped map volcanic deposits in Santa Rosalia basin in Baja el Sur, and using geochemical and geochronological methods, we identified volcanic deposits as being either arc-related, rift-related, or transitional. We put together a volcanic and tectonic history of the transition to rifting, which was published in May 2020 in Geosphere: https://doi.org/10.1130/GES02094.1
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Baja is one of the most beautiful places I've been, with rugged desert juxtaposed with a serene sea and mind-blowing sunrises and sunsets. Much easier to digest than a Geosphere article: Read about my experiences in the series Baja Field Notes under Science Writing!