Advances in Quantifying Climatic Influences on Mountain Landscape Evolution with Applications to the Tectonic Geomorphology of the Northern-Central Andes

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Mountain landscapes reflect competition between tectonic processes acting to build topography and erosive processes acting to wear it down. In temperate mountain landscapes, bedrock rivers are the primary erosional agent, setting both the pace of landscape evolution and form of…

Mountain landscapes reflect competition between tectonic processes acting to build topography and erosive processes acting to wear it down. In temperate mountain landscapes, bedrock rivers are the primary erosional agent, setting both the pace of landscape evolution and form of the surrounding topography. Theory predicts that river steepness is sensitive to climatic, tectonic, and lithologic factors, which dictate the rates and mechanics of erosional processes. Thus, encoded into topography is an archive of information about forces driving landscape evolution. Decoding this archive, however, is fraught and climate presents a particularly challenging conundrum: despite decades of research describing theoretically how climate should affect topography, unambiguous natural examples from tectonically active landscapes where variations in climate demonstrably influence topography are elusive. In this dissertation, I first present a theoretical framework describing how the spatially varied nature of orographic rainfall patterns, which are ubiquitous features of mountain climates, complicate expectations about how climate should influence river steepness and erosion. I then apply some of these ideas to the northern-central Andes. By analyzing river profiles spanning more than 1500 km across Peru and Bolivia, I show that the regional orographic rainfall pattern this landscape experiences systematically influences fluvial erosional efficiency and thus topography. I also show how common simplifying assumptions built into conventional topographic analysis techniques may introduce biases that undermine detection of climatic signatures in landscapes where climate, tectonics, and lithology all covary – a common condition in mountain landscapes where these techniques are often used. I continue by coupling this analysis with published erosion rates and a new dataset of 25 cosmogenic 10Be catchment average erosion rates. Once the influence of climate is accounted for, functional relationships emerge among channel steepness, erosion rate, and lithology. I then use these functional relationships to produce a calibrated erosion rate map that spans over 300 km of the southern Peruvian Andes. These results demonstrate that accounting for the effects of climate significantly enhances the ability to decode channel steepness patterns. Along with this comes the potential to better understand rates and patterns of tectonic processes, and identify seismic hazards associated with tectonic activity using topography.
Date Created
2022
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The Evolution of Infrastructure-Superstructure Interactions in the Annapurna Region, Central Himalaya

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The South Tibetan Detachment System (STDS) marks a major decoupling horizon in the Himalaya, separating the highly metamorphosed infrastructure in the footwall from the weakly to unmetamorphosed superstructure in the hanging wall. The STDS stretches the entire range and is…

The South Tibetan Detachment System (STDS) marks a major decoupling horizon in the Himalaya, separating the highly metamorphosed infrastructure in the footwall from the weakly to unmetamorphosed superstructure in the hanging wall. The STDS stretches the entire range and is likely one of the most significant deformational features of the orogen, but its spatial and temporal evolution remain relatively unconstrained. As its name suggests, the STDS is a system of faults which occur at slightly different structural levels and are often diachronous. Detailed studies on the different strands are needed to understand the slip history of the system as a whole, which in turn will improve understanding of Himalayan orogenesis, thus informing tectonic models for continental orogenesis in general. I focus on some of the best exposed strands of the STDS which are located in the Annapurna region of Nepal. Outcrops within the shear zones of basal structures in the Kali Gandaki and Marsyandi valleys – the Annapurna and Chame detachments – contain leucogranites that are variably deformed via ductile slip on the detachments. I used U/Pb zircon and Th/Pb monazite geochronology to constrain emplacement ages of these leucogranites, which suggest ductile slip ceased prior to 14.95 ± 0.78 Ma and 16.0 ± 1.1 Ma on the Annapurna and Chame detachments respectively. 40Ar/ 39Ar muscovite and biotite, (U-Th)/He zircon and apatite thermochronology data and resulting thermal-kinematic models for samples I collected in the shear zones and footwalls of these detachments suggest further slip was ongoing on both detachments until ca. 12 Ma, although the majority of slip on the Chame detachment likely ceased by ca. 15-14 Ma. I also collected samples in the footwall of a structurally higher detachment in the Marsyandi and the resulting cooling ages and thermal-kinematic models suggest slip was contemporaneous with that on the lower Chame detachment. The new constraints on N-S extension on the STDS in the Annapurna region presented in this dissertation call into question the popular idea of a geodynamic change from N-S to E-W extension in the central Himalaya during the early Miocene, and emphasize the importance of the STDS as a major decoupling horizon.
Date Created
2022
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Plio-Pleistocene Sediment Provenance and Erosion Rates Along the East African Rift System

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The tectonism, volcanism, and sedimentation along the East African Rift System (EARS) produced a series of rift basins with a rich paleoanthropological record, including a Late Miocene–present record of hominin evolution. To better understand the relationship between Earth system history…

The tectonism, volcanism, and sedimentation along the East African Rift System (EARS) produced a series of rift basins with a rich paleoanthropological record, including a Late Miocene–present record of hominin evolution. To better understand the relationship between Earth system history and human evolution within the EARS, the Hominin Sites and Paleolakes Drilling Project (HSPDP) collected paleolake sediments near key paleoanthropological sites in Ethiopia and Kenya, compiling a multi-proxy, high-resolution geological and environmental record. As part of the HSPDP, I studied the detrital mineral record of the basins and evaluated tectonic and climatic controls on East African landscapes during the Plio-Pleistocene using samples from three of the drill sites, Chew Bahir: (CHB, ~620–present; Ethiopia), Northern Awash (NA, ~3.3–2.9 Ma; Ethiopia,), and West Turkana (WTK, ~1.9–1.4 Ma; Kenya). I employed laser ablation U/Pb and (U-Th)/He double dating (LADD) of detrital zircons, which yields paired U/Pb and (U-Th)/He dates, and (U-Th)/He dating of detrital apatites to evaluate sediment provenance and the cooling history of the source rocks. In addition, I used in situ 10Be cosmogenic radionuclide analyses to determine paleoerosion rates. Two chapters of this dissertation focus on results from the NA and WTK drill sites. Source units for the NA and WTK drill sites are largely Cenozoic volcanic rocks, and the detrital zircon record yields an extensive record of the timing of various phases of volcanism within the EARS. Exceptionally young zircon (U-Th)/He dates reflect partial resetting associated with late mafic volcanism and/or hydrothermal activity. Erosion rates are consistent and relatively low across the Plio-Pleistocene, despite significant tectonic and geomorphic shifts in the landscape. Two other chapters of this dissertation cover results from the CHB drill site. The Chew Bahir basin has significant exposures of Neoproterozoic and Early Paleozoic crystalline basement units, and the detrital zircon record yields one singular phase of volcanism in the EARS. The CHB erosion rates show an overall decreasing trend over time, consistent with an aridifying climate, and increased environmental variability after ~200 ka.
Date Created
2021
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Chronology of Planetesimal Differentiation Based on the Timing of Achondrite Formation in the Early Solar System

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During the early Solar System many physiochemical processes were taking place that would shape the formation and evolution of rocky bodies. Growth of these rocky objects was rapid, with some growing to sizes of 10s – 1000s km (“planetesimals”) in…

During the early Solar System many physiochemical processes were taking place that would shape the formation and evolution of rocky bodies. Growth of these rocky objects was rapid, with some growing to sizes of 10s – 1000s km (“planetesimals”) in the first few million years. Because these objects formed early, they contained sufficient 26Al (an isotope of Al with a short half-life of ~705,000 yrs) to heat the interiors to melting temperatures, resulting in the formation of the first igneous rocks in nascent Solar System. Depending on the size and time of accretion, some bodies experienced high degrees of melting (with some having global magma oceans) while others experienced lower degrees of partial melting, and yet others did not experience any melting at all. These varying degrees of heating and melting processes on early-formed planetesimals produced a variety of achondritic meteorite types. These achondrites have bulk compositions ranging from ultramafic to basaltic, with some rare types having more highly “evolved” (i.e., high-SiO2) compositions. Determining the detailed chronology of their formation with fine time resolution is key for understanding the earliest stages of planet formation, and there are high resolution chronometers that are ideally suited for this application. Three such chronometers (i.e., the 26Al-26Mg, 53Mn-53Cr, and 207Pb-206Pb chronometers) are the focus of this work. Based on investigations of these chronometers in several achondritic meteorites, the implications for the formation and evolution of planetesimals in the early Solar System will be discussed.
Date Created
2020
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Radiation Damage and Helium Diffusion in Mineral Chronometers

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A mineral’s helium content reflects a balance between two competing processes: accumulation by radioactive decay and temperature-dependent diffusive loss. (U-Th)/He dating of zircon and other uranium and thorium-bearing minerals provides insight into the temperature histories of rocks at or near…

A mineral’s helium content reflects a balance between two competing processes: accumulation by radioactive decay and temperature-dependent diffusive loss. (U-Th)/He dating of zircon and other uranium and thorium-bearing minerals provides insight into the temperature histories of rocks at or near Earth’s surface that informs geoscientists’ understanding of tectonic and climate-driven exhumation, magmatic activity, and other thermal events. The crystal structure and chemistry of minerals affect helium diffusion kinetics, recorded closure temperatures, and interpretations of (U-Th)/He datasets. I used empirical and experimental methods to investigate helium systematics in two minerals chronometers: zircon and xenotime.

The same radioactivity that makes zircon a valuable chronometer damages its crystal structure over time and changes zircon helium kinetics. I used a zircon, titanite, and apatite (U-Th)/He dataset combined with previously published data and a new thermal model to place empirical constraints on the closure temperature for helium in a suite of variably damaged zircon crystals from the McClure Mountain syenite of Colorado. Results of this study suggest that the widely-used zircon damage accumulation and annealing model (ZRDAAM) does not accurately predict helium closure temperatures for a majority of the dated zircons. Detailed Raman maps of Proterozoic zircon crystals from the Lyon Mountain Granite of New York document complex radiation damage zoning. Models based on these results suggest that most ancient zircons are likely to exhibit intracrystalline variations in helium diffusivity due to radiation damage zoning, which may, in part, explain discrepancies between my empirical findings and ZRDAAM.

Zircon crystallography suggests that helium diffusion should be fastest along the crystallographic c-axis. I used laser depth profiling to show that diffusion is more strongly anisotropic than previously recognized. These findings imply that crystal morphology affects the closure temperature for helium in crystalline zircon. Diffusivity and the magnitude of diffusive anisotropy decrease with low doses of radiation damage.

Xenotime would make a promising (U-Th)/He thermochronometer if its helium kinetics were better known. I performed classic step-wise degassing experiments to characterize helium diffusion in xenotime FPX-1. Results suggest that this xenotime sample is sensitive to exceptionally low temperatures (∼50 °C) and produces consistent (U-Th)/He dates.
Date Created
2019
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VOLUME APPROXIMATIONS OF LASER PIT ABLATIONS

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Radiometric dating estimates the age of rocks by comparing the concentration of a decaying radioactive isotope to the concentrations of the decay byproducts. Radiometric dating has been instrumental in the calculation of the Earth's age, the Moon's age, and the…

Radiometric dating estimates the age of rocks by comparing the concentration of a decaying radioactive isotope to the concentrations of the decay byproducts. Radiometric dating has been instrumental in the calculation of the Earth's age, the Moon's age, and the age of our solar system. Geochronologists in the School of Earth and Space Exploration at ASU use radiometric dating extensively in their research, and have very specific procedures, hardware, and software to perform the dating calculations. Researchers use lasers to drill small holes, or ablations, in rock faces, collect the masses of various isotopes using a mass spectrometer, and scan the pit with an interferometer, which records the z heights of the pit on an x-y grid. This scan is then processed by custom-made software to determine the volume of the pit, which then is used along with the isotope masses and known decay rates to determine the age of the rock. My research has been focused on improving this volume calculation through computational geometry methods of surface reconstruction. During the process, I created an web application that reads interferometer scans, reconstructs a surface from those scans with Poisson reconstruction, renders the surface in the browser, and calculates the volume of the pit based on parameters provided by the researcher. The scans are stored in a central cloud datastore for future analysis, allowing the researchers in the geochronology community to collaborate together on scans from various rocks in their individual labs. The result of the project has been a complete and functioning application that is accessible to any researcher and reproducible from any computer. The 3D representation of the scan data allows researchers to easily understand the topology of the pit ablation and determine early on whether the measurements of the interferometer are trustworthy for the particular ablation. The volume calculation by the new software also reduces the variability in the volume calculation, which hopefully indicates the process is removing noise from the scan data and performing volume calculations on a more realistic representation of the actual ablation. In the future, this research will be used as the groundwork for more robust testing and closer approximations through implementation of different reconstruction algorithms. As the project grows and becomes more usable, hopefully there will be adoption in the community and it will become a reproducible standard for geochronologists performing radiometric dating.
Date Created
2016-05
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(U-Th)/He Geochronology of Grains in Baked Zones to date Volcanism

Description
Many radioactive decay schemes employed in geochronology prove imprecise when placing accurate age constraints on young basalt flows. The (U-Th)/He systematics of detrital zircon and apatite within baked zones is examined as an alternative. Parent-daughter radioisotope ratios within grains from…

Many radioactive decay schemes employed in geochronology prove imprecise when placing accurate age constraints on young basalt flows. The (U-Th)/He systematics of detrital zircon and apatite within baked zones is examined as an alternative. Parent-daughter radioisotope ratios within grains from baked zones can completely reset if subjected to temperatures high enough and long enough for bulk diffusive loss. Presented here is the reproducibility of initial attempts to date flows by examining the (U-Th)/He geochronology of grains within baked zones. We examine grains from two localities within the San Francisco Volcanic Field and the Mormon Volcanic Field in northern Arizona. Thirteen zircon and apatite grains yielded from locality 2 collected from the uppermost 10 cm beneath a 7m flow of a basalt yield an apparent age of 4.39 ± 0.28 Ma (2σ), which is within range of published Middle Pliocene ages. Twenty-nine grains from locality 1 collected from the uppermost 20 cm beneath a 2 to 5m flow yield dates ranging from 0.47 ± 0.02 Ma to 892.77 ± 27.02 Ma, indicating the grains were partially reset or not reset at all. The degree to which grains are reset depends on a variety of factors detailed in this study. With these factors accounted for however, our study confirms application of this indirect dating technique is a useful tool for dating basaltic flows.
Date Created
2014-05
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Refining Lunar Impact Chronology Through High Spatial Resolution 40Ar/39Ar Dating of Impact Melts

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Quantitative constraints on the ages of melt-forming impact events on the Moon are based primarily on isotope geochronology of returned samples. However, interpreting the results of such studies can often be difficult because the provenance region of any sample returned…

Quantitative constraints on the ages of melt-forming impact events on the Moon are based primarily on isotope geochronology of returned samples. However, interpreting the results of such studies can often be difficult because the provenance region of any sample returned from the lunar surface may have experienced multiple impact events over the course of billions of years of bombardment. We illustrate this problem with new laser microprobe 40Ar/39Ar data for two Apollo 17 impact melt breccias. Whereas one sample yields a straightforward result, indicating a single melt-forming event at ca. 3.83 Ga, data from the other sample document multiple impact melt–forming events between ca. 3.81 Ga and at least as young as ca. 3.27 Ga. Notably, published zircon U/Pb data indicate the existence of even older melt products in the same sample. The revelation of multiple impact events through 40Ar/39Ar geochronology is likely not to have been possible using standard incremental heating methods alone, demonstrating the complementarity of the laser microprobe technique. Evidence for 3.83 Ga to 3.81 Ga melt components in these samples reinforces emerging interpretations that Apollo 17 impact breccia samples include a significant component of ejecta from the Imbrium basin impact. Collectively, our results underscore the need to quantitatively resolve the ages of different melt generations from multiple samples to improve our current understanding of the lunar impact record, and to establish the absolute ages of important impact structures encountered during future exploration missions in the inner Solar System.

Date Created
2015-02-12
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Comparative evolution of the Shyok and Yarlung suture zones: implications for the collision Between India and Eurasia

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The collision between the Indian and Eurasian tectonic plates marked the onset of the rise of the Himalayan-Tibetan orogen, but also brought about profound changes to the Earth's oceans and climate. The exact sequence of events that occurred during this…

The collision between the Indian and Eurasian tectonic plates marked the onset of the rise of the Himalayan-Tibetan orogen, but also brought about profound changes to the Earth's oceans and climate. The exact sequence of events that occurred during this collision is poorly understood, leading to a wide range of estimates of its age. The Indus and Yarlung sutures are generally considered to represent the final collision between India and Eurasia, and together form a mostly continuous belt that can be traced over 2000 km along strike. In the western portions of the orogen the Karakoram Fault introduces a key complexity to the study of timing of collision by offsetting the Indus and Yarlung sutures. Recent work has used the complexities introduced by the Karakoram Fault to suggest that the more northerly Shyok suture, not the Indus suture, represents the India-Eurasia collision zone. Estimates for timing of the India-Eurasia collision fall into one of three groups: 40-34 Ma, 55-50 Ma, and 66-60 Ma. Attempts to reconcile these models have thus far been unsuccessful. In order to provide additional data that might further clarify the timing and location of collision, studies have been performed along the Shyok suture in India and along the Yarlung suture in Tibet at Sangsang. A study along the Shyok suture argues that the suture formed between 92-85 Ma. This timing precludes an interpretation that the Shyok suture marks the location of the India-Eurasia collision. A second study demonstrates the utility of two new geochronometers, (U-Th)/Pb joaquinite and 40Ar/39Ar neptunite, that play an important role in unraveling the tectonic history of the Yarlung suture. A third study is an investigation of the structure and geochronology of the Sangsang ophiolite complex. Here, multiple (U-Th)/Pb and 40Ar/39Ar systems record magmatism and metamorphism spanning ca. 125-52 Ma. By tying these chronometers to tectonic process, a history is reconstructed of the southern margin of Tibet that includes Early Cretaceous to Late Cretaceous forearc rifting associated with mid ocean ridge subduction, Paleocene accretionary wedge uplift and erosion, and finally Eocene metasomatism and collision.
Date Created
2016
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Evidence for Pleistocene Low-Angle Normal Faulting in the Annapurna-Dhaulagiri Region, Nepal

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North-south-directed extension on the South Tibetan Fault System (STFS) played an important role in Himalayan tectonics of the Miocene Period, and it is generally assumed that orogen-perpendicular extension ceased in this orogenic system before the Pliocene. However, previous work in…

North-south-directed extension on the South Tibetan Fault System (STFS) played an important role in Himalayan tectonics of the Miocene Period, and it is generally assumed that orogen-perpendicular extension ceased in this orogenic system before the Pliocene. However, previous work in the Annapurna and Dhaulagiri Himalaya of central Nepal revealed evidence for local Pleistocene reactivation of the basal STFS structure in this area (the Annapurna Detachment). New structural mapping and (U-Th)/He apatite and zircon thermochronology in this region further document the significance of Pleistocene N-S extension in this sector of the Himalaya. Patterns of (U-Th)/He accessory-mineral ages are not disrupted across the reactivated segment of the STFS basal detachment, indicating that Pleistocene offset was limited. In contrast, the trace of a N-dipping, low-angle detachment in the hanging wall of the reactivated Annapurna Detachment - formerly linked to the STFS, but here named the Dhaulagiri Detachment - coincides with an abrupt break in the cooling-age pattern in two different drainages ∼20 km apart, juxtaposing Miocene hanging-wall dates against Pleistocene footwall dates. Our observations, combined with previous fission-track data from the region, provide direct evidence for significant N-S extension in the central Himalaya as recently as the Pleistocene.

Date Created
2015-03-01
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