Jan. 8, 2019

3:30 p.m. - 4:30 p.m.
3656 Geology

Presented By: Victor Tsai,
CalTech

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"Quantifying Natural Hazards: From Earthquake Damage to Debris Flows"

Many natural hazards have been well known and qualitatively understood for decades, but still lack accurate measures of how damaging future events will be. For example, despite many years of research, it still remains a question as to how much variability in ground motions one should expect of a large San Andreas type earthquake, and whether early warning for debris flows can be successfully implemented. In this talk, I address both of these questions by using simple but physically sound mechanical principles to quantify certain aspects of these hazards.

Jan. 15, 2019

3:30 p.m. - 4:30 p.m.
3656 Geology

Presented By: Max Rudolph,
UC Davis

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Earth's mantle viscosity and large-scale structure

The viscosity structure of Earth's deep mantle affects the thermal evolution of Earth, the ascent of mantle upwellings, sinking of subducted oceanic lithosphere, and the mixing of compositional heterogeneities in the mantle. Modeling the long-wavelength dynamic geoid allows us to constrain the radial viscosity profile of the mantle. I will discuss new solutions for the mantle viscosity structure and its uncertainty based on a suite of joint whole-mantle tomographic models of both S- and P-wave velocity as well as density. The resulting density variations in the lowermost mantle span scenarios in which the net buoyancy of the Large Low Shear Velocity Provinces (LLSVPs) appears to be primarily controlled by temperature, as well as those in which the LLSVPs represent intrinsically dense material. Finally, I will show results from kinematically-constrained mantle convection models with idealized viscosity profiles and make a comparison between emergent large-scale structures and structures observed in seismic tomography.

Jan. 22, 2019

3:30 p.m. - 4:30 p.m.
3656 Geology

Presented By: Seulgi Moon,
UCLA

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Uplift, stress, and erosion: Tectonic controls on landscape evolution

In tectonically active landscapes, both tectonically driven base level changes and bedrock damage can influence the spatial and temporal patterns of erosion. Though the influence of base level changes from differential rock uplift rates on erosion has been examined extensively in previous studies, few studies have examined whether the tectonic influence on bedrock damage may influence landscape evolution. In this talk, I show how tectonic stress interacts with topography and influences landscape evolution by altering the rates and patterns of bedrock fracturing, weathering, and erosion. First, I show how the present-day topographic stress fields influence bedrock fracture patterns in Forsmark, Sweden. The population of existing fractures likely reflects stress history, but the present-day topographic stress field influences the relative abundance of open fractures near the surface and at depths of hundreds of meters. Second, I present my group’s work in the eastern Tibet, which shows that the tectonic control on bedrock damage may explain the measured changes in rock erodibility and landslide characteristics.

Jan. 29, 2019

3:30 p.m. - 4:30 p.m.
3656 Geology

Presented By: Bin Chen,
University of Hawaii

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Experimental Simulations of Planetary Interiors

Geoscientists have long recognized the importance of generating extreme pressures and temperatures to reproduce in laboratory settings the conditions present in planetary interiors. Such experiments provide the basis for understanding the nature and mechanisms of dynamic processes taking place within deep interiors of Earth and other planetary bodies. Since early 1960s, mineral physics has rapidly emerged and been recognized as an important interdisciplinary field in Earth sciences, providing an essential link between laboratory measurements of the physical and chemical properties of minerals and rocks under extreme conditions and the geophysical and geochemical observations of the Earth’s interiors. Advanced high-pressure and synchrotron X-ray techniques have permitted experimental mineral physicists to probe the micro-scale properties of planetary materials that govern macro-scale behaviors of the complex planetary systems. Here, I will briefly describe the past, present, and future of the field, followed by recent research on the viscoelastic properties of iron-carbon liquids, elastic and thermal transport properties of high-pressure ices, and the implications on the internal structure and dynamics of planetary interiors.

Feb. 5, 2019

3:30 p.m. - 4:30 p.m.
3656 Geology

Presented By: Rita Parai,
Washington University, St Louis

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Xenon isotopic constraints on the history of volatile recycling into the mantle

Xenon is a unique tracer of volatile transport. The modern deep Earth Xe budget reflects primordial volatiles delivered during accretion, radiogenic ingrowth, outgassing in association with mantle processing, and regassing as Xe is carried within hydrous phases in downwelling lithologies. The Xe isotopic composition of the mantle thus reflects the integrated long-term history of volatile transport between the deep Earth and surface reservoirs. We present a numerical model of concurrent mantle degassing, regassing and fissiogenic production. We test a wide variety of outgassing and regassing rates and take the sequestration of Pu and U into the continental crust and the evolution of the atmospheric Xe isotopic composition into account. Model realizations that satisfy Xe isotopic constraints from mantle-derived rocks indicate that significant recycling of atmospheric Xe into the deep Earth could not have occurred prior to 2.5 Ga. Because Xe is carried into Earth’s interior in hydrous mineral phases, our results indicate that downwellings were drier in the Archean era relative to the present. Our results indicate that the mantle experienced net degassing throughout the Archean and transitioned to net regassing at some point after 2.5 Ga. Progressive drying of the Archean mantle would allow for slower convection and decreased heat transport out of the mantle, suggesting non-monotonic thermal evolution of Earth’s interior. If plate tectonics and plate subduction were initiated before 2.5 Gyr ago, then early downwelling subducted material was either hydrated to a lesser extent at the surface than in the modern-day, or volatiles were more efficiently expelled from Archean downwellings slabs at shallow depths and returned to the surface.

Feb. 12, 2019

3:30 p.m. - 4:30 p.m.
3656 Geology

Presented By: Jim Zachos,
UC Santa Cruz

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Intensification of the hydrological cycle during Early Eocene Hyperthermals: Theory versus observations

The early Eocene hyperthermals, a series of transient global warming events (∆T=+2 to +6°C), provide a unique opportunity to assess the sensitivity of the hydrologic cycle to the scale of greenhouse forcing expected over the next several centuries. A growing body of evidence from the most prominent of the hyperthermals, the Paleocene Thermal Maximum (PETM; ~56 Ma), points toward a major mode shift in the intensity and patterns of precipitation. Regionally, the shift in hydrology differs notably, with some regions becoming drier, others wetter. In many regions both sedimentologic and paleontologic evidence indicate that precipitation became much more seasonal or episodic in character. In continental fluvial and coastal sections, changes in siliciclastic depositional facies reflect on increased frequency of high-energy events (e.g., extreme flooding), possibly from monsoon-like seasonal rains, and/or from unusually intense and/or sustained extra-tropical storms. In the open ocean, geochemical data, though still relatively sparse, suggests that the sub-tropical ocean became saltier as a consequence of locally reduced precipitation and/or increased evaporation suggestive of increased meridional vapor transport from low to high latitudes. Indeed evidence, from high latitude oceans suggests reduced salinity. New data emerging for subsequent smaller hyperthermals (ie., Eocene Thermal Maximum 2) show similar patterns. Such observations are consistent with and thus support general theory on the sensitivity of large-scale vapor transport and regional cycle of precipitation to extreme greenhouse warming.

Feb. 19, 2019

3:30 p.m. - 4:30 p.m.
3656 Geology

Presented By: Ryan Ewing,
Texas A&M

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Linking transport thresholds and biosignatures for exploration of life in martian eolian Environments

Biosignatures are usefully thought of as anything observable that provides evidence of current or past life. Biosignature detection efforts focus on ancient Earth and planetary exploration. The Mars 2020 mission addresses key questions about the potential for life on Mars and aims to search for signs of past microbial life with the ultimate goal of caching samples for return to Earth. Should the Mars 2020 mission sample ancient rocks deposited by wind? We address this question by linking wind tunnel studies in simulated martian conditions and analog studies of modern, microbially influenced eolian environments at Padre Island, Texas. This talk presents results from wind tunnels experiments that determine a threshold for sporadic bursting. This threshold is lower than traditional continuous motion threshold models for Mars and lower than measured in situ wind speeds at Viking Lander 2 and Curiosity rover sites. This new threshold model paired with recent theory on sediment motion on Mars is consistent with orbital and in situ observations of active sand transport on Mars. The second part of the talk presents the role of thresholds in concentrating heavy minerals that are bound by microbial mats that occupy sabkhas in wet eolian systems; an explored environment on Mars. Upon burial, the heavy mineral horizons play a key role in identifying an eolian-environment biosignature. Handheld and imaging micro x-ray florescence (XRF) analysis of trench walls and sediment peels from trenches show distinctive textural and geochemical signatures linked to heavy minerals concentrated within the microbial mats at the surface.

March 5, 2019

3:30 p.m. - 4:30 p.m.
3656 Geology

Presented By: Quanzhi Ye,
California Institute of Technology

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Asteroid Disruptions Close to the Sun

The under-abundance of asteroids on orbits with small perihelion distances suggests that thermally-driven disruption may be an important process in the removal of rocky bodies in the Solar System. On a separate front, transit observation of various planetary systems indicates that near-star disruption of small planetesimals is common in the Galaxy, prompting the need to understand such a process using our own Solar System as an example. Here I will discuss how the debris streams arise from possible thermally-driven disruptions in the near-Sun region based on simulations of the disruption of near-Sun asteroids, and how can we use meteor data to understand the asteroid disruptions near the Sun. I will show that there is a clear overabundance of Sun-approaching meteor showers, which is best explained by a combining effect of comet contamination and an extended disintegration phase that lasts up to a few thousand years. Finally, I will briefly discuss the implication of our finding for exoplanetary systems.

March 12, 2019

3:30 p.m. - 4:30 p.m.
3656 Geology

Presented By: Emily Chin,
UC San Diego

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Beyond the volcanic-plutonic connection: melt-mediated processes in the deep lithosphere and their impact on shallow magmatism

Volcanic rocks ultimately share a common origin as partial melts of the mantle, subsequently traveling through the lithosphere before eruption. Using a database of ~60,000 natural samples, I show that major element systematics of volcanic rocks and their complementary crystalline cumulates are broadly consistent with fractional crystallization as a first order process. Yet, primitive lavas worldwide exhibit large compositional variability, most of which could not have been inherited from a typical mantle source alone, but require secondary processes such as melting chemical heterogeneities, melt-rock reaction, fractional crystallization, and magma mixing. Increasingly, it is recognized that many of these processes extend deeper, and thus earlier in a magma’s evolution, than previously thought. In this talk, I present results from case studies from the “bottom up”, focusing on deep lithospheric melt-related processes and their impact on the geochemical and rheological evolution of mantle and lower crust.