EPSS Colloquium - fall-2018
Does Mars still have a pulse?: InSight explores the heat engine of the Red Planet
Oct. 2, 2018
3:30 p.m. - 4:30 p.m.
Geology 3656
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Former EPSS Grad student Matt Siegler (PSI/SMU) returns on sabbatical from Texas to give an overview of the upcoming Mars InSight Geophysics Mission. He will focus primarily on his role as part of the geothermal heat flow probe team and what we might learn about the vital signs of Mars.
Sulfur systematics illuminate the elusive record of magma redox evolution
Oct. 9, 2018
3:30 p.m. - 4:30 p.m.
Geology 3656
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Sulfur is an important element in igneous systems due to its impact on magma redox, its role in the formation of economically valuable ore deposits, and the influence of catastrophic volcanogenic sulfur degassing on global climate. The mobility and geochemical behavior of sulfur in magmas is complex due to its multi-valent (from S2- to S6+) and multi-phase (solid, immiscible liquid, gaseous, dissolved ions) nature. Sulfur behavior is closely linked with the evolution of oxygen fugacity (fO2) in magmas; the record of fO2 evolution is often difficult to extract from rock records, particularly for intrusive systems that undergo cyclical magmatic processes and crystallize to the solidus. We apply a novel method of measuring sulfur isotopic ratios via secondary ion mass spectrometry (SIMS) in zoned apatite crystals that we interpret as a record of open-system magmatic processes. We analyzed the sulfur concentration and isotopic variations preserved in multiple apatite crystals from single hand specimens from the Cadiz Valley Batholith, CA via electron microprobe and ion microprobe at UCLA. Isotopic variations in single apatite crystals ranged from 0 to 3.8‰ δ34S and total variation within a single hand sample was 6.1‰ δ34S. High S concentration cores yielded high isotopic ratios while low S concentration rims yielded low isotopic ratios. We favor an explanation of a combination of magma mixing and open-system, ascent-driven degassing under moderately reduced conditions: fO2 at or below NNO +1, although the synchronous crystallization of apatite and anhydrite is also a viable scenario. These findings have implications for the coupled sulfur and fO2 evolution of granitic plutons and suggest that in-situ apatite sulfur isotopic measurements could be a powerful new tool for evaluating coupled redox and sulfur behavior in magmatic systems.
Insights into Earthquake Physics Revealed by Advanced Seismic Observations
Oct. 16, 2018
3:30 p.m. - 4:30 p.m.
Geology 3656
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An improved understanding of the earthquake physics relies on better knowledge of earthquake rupture processes (earthquake nucleation, its complex rupture propagation, and the final arrest) and how faults release stresses through seismic/aseismic slip. Currently, the greatest challenge in this field is that the observations are behind the modeling efforts, making testing and validations of the ever-increasing rupture models impossible. In this talk, I will give an overview of the research activities of my team. We have improved the resolution and reduced the uncertainty of earthquake and tsunami source imaging which allows us to address the open questions of earthquake source dynamics. In the case study of the 2015 Mw 8.3 Chile earthquake, we observed splitting of rupture fronts around the rim of a large barrier. This encircling pattern is analogous to the double-pincer movement in military tactics. Such degree of complexity is previously only seen in simulations and it is observed for the first time in real earthquakes. In the 2011 Mw 9.0 Tohoku earthquake, we find that the coseismic rupture is bounded by the bottom of the seismogenic zone, which contradicts the claims that dynamic frictional weakening enables deep penetrations of large earthquakes into the ductile creeping zone. My team is also enhancing and combining the next-generation detection techniques (e.g. template matching, machine learning, array processing) to monitor microseismicity and tremors. In several case studies, we detect wide distributions of undocumented foreshocks and repeating earthquakes ahead of large earthquakes, suggesting early nucleations driven by large-scale aseismic slow slip. We also identify two episodes of deep clusters of seismicity below the Kilauea volcano crater ahead of its April 2018 eruption. The episodic activation of deep seismicity is indicative of movement of pressurized magma, an early precursor of the volcanic eruption. These studies demonstrate the capability of the enhanced detections to illuminate the underlying physical mechanisms behind various micro-seismic processes.
Deciphering Magma-Faulting Interactions in Rift, Arc and Ocean Island Volcanoes with Satellite Geodesy
Oct. 23, 2018
3:30 p.m. - 4:30 p.m.
Geology 3656
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Magma-tectonic interactions occur at scales from individual magmatic systems to plate boundaries. Numerous studies reveal spatial and temporal relationships between magma intrusions, earthquakes, and volcanic eruptions. Field, geophysical, and modeling studies suggest earthquakes can trigger intrusions and volcanic activity; conversely, magma transport and storage can generate earthquakes via stress changes in surrounding country rock. In rifting events, magmatic fluids can also help release tectonic stresses. Volume change/amount of slip, source geometry and location of magma pathways and tectonic sources may be assessed through modeling of geodetically-imaged deformation sources. The physical processes that lead to ground deformation at restless volcanoes are also understood to drive microseismic activity in the form of volcano-tectonic earthquakes. Even if geodesy and seismology are recognized as the most useful geophysical tools for volcano monitoring, they are very rarely used synergistically. Here, we will show example of joint analyses of independent seismic and geodetic datasets to better constrain magma and faulting source characteristics, as well as their interactions, at active volcanoes located in several tectonic settings.
Generating plate tectonics from the grain to global scale
Oct. 30, 2018
3:30 p.m. - 4:30 p.m.
Geology 3656
Presented By:
- David Bercovici - Yale University
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The generation of plate tectonics on Earth relies on complex mechanisms for shear-localization, as well as for the retention and reactivation of weak zones in the cold ductile lithosphere. Pervasive mylonitization, wherein zones of high deformation coincide with extensive mineral grain-size reduction, is an important clue to this process. In that regard, the grain-damage model of lithospheric weakening provides a physical framework for both mylonitization and plate generation, and accounts for the competition between grain-reduction by deformation and damage, and healing by grain growth. Zener pinning at the evolving interface between mineral components, such as olivine and pyroxene, plays a key role in helping drive grains to small mylonitic sizes during deformation, and then retards their growth once deformation ceases. The combined effects of damage and pinning, however, rely on the efficiency of inter-grain mixing between phases (e.g., olivine and pyroxene) and grain dispersal. Grain-scale mixing can be represented as a complex anisotropic, stress-driven chemical diffusion model. This model predicts mylonitization, self-weakeningand shear-localization in mixing zones, while neighboring unmixed zones remain strong. This shows that even with uniform stress and temperature at a given layer in the lithosphere, two states of deformation can emerge and coexist in a hysteretic state, just as weak plate boundaries and strong plates coexist on Earth.
Hydroclimatic variability in Mainland Southeast Asia since the Last Glacial Maximum
Nov. 6, 2018
3:30 p.m. - 4:30 p.m.
Geology 3656
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The Asian Monsoon system is an important component of the global climate system that plays a major role in the transport of heat and moisture from the tropics to higher latitudes. Even small variations in the strength and timing of seasonal rainfall can have significant impacts on the billions of people living within the AM domain, yet climate model projections of future regional-scale hydrologic change still remain uncertain. Paleoclimate records from speleothems have substantially improved our understanding of the timing and mechanisms of past AM variability on orbital to decadal time-scales, but the impact of these variations on regional precipitation patterns remains unclear. This is due in part to the multitude of potential controls on speleothem oxygen isotope composition, but also to the sparse coverage of the paleoclimate record over certain regions, such as Mainland Southeast Asia. To address this, I will present new multi-proxy speleothem data from northern Laos spanning the last 38,000 years. We find that oxygen isotope variations are primarily driven by rainout upstream from our study site, whereas the carbon isotope, trace element, and crystal fabric records reflect local water balance. Our results confirm that rainfall in northern Laos decreased during previously identified last millennium drought events and during weak monsoon periods such as Heinrich Stadial 1. We also show that, consistent with paleoclimate model simulations, Southeast Asia experienced dry conditions during the early Holocene summer insolation maxima, despite the strong Asian Monsoon at that time, thus highlighting that Asian monsoon strength and regional precipitation do not always co-vary.
More soil, less storage: The influence of soil characteristics on subsurface water storage and runoff generation
Nov. 13, 2018
3:30 p.m. - 4:30 p.m.
Geology 3656
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Hydrologic first principles indicate that steeper slopes and thinner soils drive faster streamflow production, shorter internal catchment water residence times, and thus less catchment water storage. Yet, in our recent analysis of long-term (10+ years) daily runoff from 73 USGS gaging stations across the Appalachian Mountain and Piedmont physiographic provinces of North Carolina, USA, we found the opposite response. In this seminar talk, I present a synthesis of observational and empirical data analysis results from hillslope to regional scale sites to demonstrate how shallow subsurface structure and stratigraphy may drive counter-intuitive hydrological behaviors. I demonstrate how differential subsurface development in watersheds with varying topography have varying implications for predicted runoff responses to land-use development, natural hazards (flash flooding, droughts) and other environmental change.
The hidden sides of the olivine-wadsleyite phase transition: linking geochemical, geophysical and geodynamical observations
Nov. 20, 2018
3:30 p.m. - 4:30 p.m.
Geology 3656
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Olivine is widely viewed as the most abundant mineral in the Earth’s upper mantle, with contents ranging from 30% to 65% for different mantle petrological models. The olivine-wadsleyite transition at ~13-14GPa is most likely the cause of the 410 km discontinuity. For a long time, the magnitude and topography of this discontinuity have been used to calculate the averaged olivine content and temperature variations near the 410 km depth. Although the olivine-wadsleyite phase transformation has been studied intensely over the past several decades, there are many other unrevealed aspects of this phase transition, beyond the Clapeyron slope, the compositional/thermal effect, and the isotropic velocity jumps. In this talk, I am going to focus on two “hidden” aspects: the first one is utilizing olivine-wadsleyite transition and 410 discontinuity as a geochemical/petrological marker; the second one is investigating the possibility of using the anisotropy change across the olivine-wadsleyite transition as a deformation/flow field indicator. These studies are very important for linking and understanding the geochemical data, geodynamic simulations and geophysical observations at upper mantle transition zone depth range in the Earth’s interior.
Sounding Ice: Exploring For Liquid Water in the Frozen Subsurfaces of Greenland and Europa
Nov. 27, 2018
3:30 p.m. - 4:30 p.m.
Geology 3656
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The polar ice caps of Earth are rapidly melting under the influence of global warming. On the Greenland ice sheet, the resulting melt water is creating many new types of glacial features, including lakes deep beneath the overlying ice, liquid water stored in aquifers in the near surface ice and snow, and stunning aquamarine ponds of melt at the surface. My team and I are investigating the properties and size of these melt pockets using geophysical tools to look through the ice and determine where and how much liquid water is present. We are using this unique Greenland environment to plan for exploring the interior of Jupiter's icy moon Europa. Our techniques for studying liquid water within the Greenland ice sheet provide crucial inputs for future NASA missions that plan to investigate the properties of the subsurface Europa ocean (and other icy ocean worlds like it) and determine if Europa could potentially harbor life.
Redox variations in ocean island volcanoes and the solid Earth oxygen cycle
Dec. 4, 2018
3:30 p.m. - 4:30 p.m.
Geology 3656
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Subduction zone lavas are more oxidized than mid-ocean ridge lavas, and this has been attributed to recycling of seafloor sediments and altered oceanic crust exposed to the oxygen- and water-rich conditions at Earth’s surface. A mass balance of oxidized materials input and output to the Marianas subduction system demonstrates that there is more oxygen contained in seafloor sediments and altered oceanic crust than is necessary to produce oxidized arc lavas along the arc, and suggests that a significant portion of surface-derived oxygen is transported beyond the arc, into the deep mantle. Some portion of this oxygen is returned to the surface in the form of oxidized lavas at hotspots where the plume contains materials that were at the surface after the Great Oxidation Event (e.g., Hawaii). Lavas erupted at plumes that contain surface-derived materials that pre-date the Great Oxidation Event may not be oxidized above levels expected from ambient upper mantle (e.g, Reunion). This suggests that heterogeneity in the oxidation state of basaltic lavas on Earth is intimately linked to (a) the arrival of abundant O2 at Earth’s surface and (b) modern day tectonic setting. It also supports previous studies using V-partitioning behavior in olivine, that the mantle that has not been significantly impacted by material recycling has maintained a relatively constant oxidation state from Archean to present day.