Geophysics and Tectonics Seminar - winter-2020
Drainage and sedimentary responses to dynamic topography: insights from source-to-sink landscape evolution modelling
Jan. 8, 2020
noon - 1 p.m.
Geology 1707
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Dynamic topography induced by mantle flow interacts with surface processes and can affect all segments (from fluvial to deep marine) of the sediment routing systems. Field observations and numerical investigations suggest that interactions between dynamic topography and surface processes can be preserved in the geological record. However, it remains challenging to isolate the fingerprints of dynamic topography in the geological record that primarily reflects tectonic processes. Landscape Evolution Modeling (LEM) is useful to evaluate the landscape responses to dynamic topography. We design generic source-to-sink models using the landscape evolution model pyBadlands to investigate the influence of dynamic topography on landscape evolution and stratigraphic formations. Our modeling results show that a migrating dynamic topography can induce significant drainage reorganizations and affect sediment routing from source to sink. Variations in sediment supply driven by the lateral migrating dynamic topography contribute to the formation of diachronous unconformities along the margin. The predicted sediment flux histories are then put into perspective with the Cretaceous sedimentary records along the Southern African margins. Finally, we demonstrate that correlating offshore depositional hiatuses and unconformities has the potential to constrain the spatio-temporal evolution of past dynamic topography events.
Turning fiber optic cables into the next-generation seismic networks: on land, submarine, glaciers, and beyond
Jan. 15, 2020
noon - 1 p.m.
Geology 1707
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Seismology is one of the main approaches to study quakes and image Earth and planetary interiors. However, deploying large-scale dense seismic networks has been challenging. Distributed acoustic sensing (DAS) is an emerging technology that converts every few meters of a long (currently 10’s of km) optical fiber into a seismic strainmeter. At its most basic level, DAS works by shining a laser pulse into the fiber from one end and interrogating the “echo” of Rayleigh scattering from intrinsic fiber defects. DAS provides a scalable and affordable way to deploy a dense seismic network, by installing dedicated fiber cables or leveraging existing telecommunication fiber networks. In the last two years, we have been exploring the potential of DAS in the next generation seismic networks on different scales. More specifically, we test DAS in earthquake detection, structure inversion, and hazard assessment. In this talk I will give an overview of these efforts and our vision for the future.
A craton is not forever: The formation, modification and possible destruction of the Wyoming craton
Jan. 22, 2020
noon - 1 p.m.
Geology 1707
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Cratons are often described as rigid lithospheric blocks, capable of resisting deformation over extended periods of time, and to first order this narrative appears to be true. And yet, uplift occurred across the Wyoming craton during the Laramide Orogeny, creating the Black Hills in western South Dakota. Despite this evidence for decratonization, seismic tomography reveals high velocities in the upper mantle between the Bighorn Mountains and the Black Hills at depths of up to 250 km, a region we refer to as the Thunder Basin Block (TBB). The anomaly has been interpreted as either a stable piece of lithosphere, which may or may not be original to the craton, or a weakened remnant in the process of delamination. The proximity of the block’s edge to the Black Hills suggests that the block may have played a role in their uplift. Our seismic experiment, nicknamed CIELO (Crust and lithosphere Investigation of the Easternmost expression of the Laramide Orogeny), operated from September 2017-2019 in order to better constrain the geographic limits and physical properties of the TBB. Preliminary teleseismic P delay times within the region, corrected for topography and basin structure, find early arrivals related to high-velocity mantle structure. These early results are consistent with a +2.5% anomaly over the upper 200 km of the mantle that sharply terminates at the western edge of the Black Hills and may mark the limit of the TBB. Ps receiver functions show rapid lateral variations in crustal thickness beneath the Black Hills, with a minimum of 39 km along the western edge, which is co-located with the margin of the high-velocity feature, thickening to a maximum of 58 km along the eastern edge of the mountains. There is also strong evidence for anomalous discontinuities within the mid-crust across the Black Hills. These preliminary results show that there is pronounced and complex structure at the eastern edge of the Thunder Basin Block throughout both the crust and upper mantle, suggesting a connection between the uplift of the Black Hills and the internal, potentially modified structure of the craton.
Mapping the deep mantle influence in Ethiopia and Afar
Jan. 29, 2020
noon - 1 p.m.
Geology 1707
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Some of the major rifting events that led to the current continental configuration appear to have commenced around the same time as the local emplacement of large igneous provinces, which are thought to result from deep mantle upwelling. This suggests that mantle plumes may play a role in starting or sustaining continental breakup. A mantle plume has been implicated in initial rifting of the Arabian, Nubian, and Somalian plates in the northern section of the East Africa Rift System, where the transition from a continental to mid-ocean ridge style spreading regime is nearly complete. However, the ongoing role of the mantle plume in sustaining rifting is unclear, particularly given recent studies questioning the role of deep mantle upwelling in melt production. In this talk, I’ll present results from a project comparing the spatial distribution of 3He/4He measurements — which reveal a continued connection with the deep mantle — and tomographic velocity models that constrain regions of melt storage in the upper mantle. Together, these results show that deep mantle input, which leads to high 3He/4He ratios, correlates with regions of high partial melt, suggesting that the mantle plume has an ongoing role in melt production and rifting. The most direct deep mantle influence is in the narrow rift valley of the Main Ethiopian Rift while the deep mantle He isotope signature is less pronounced in Afar, where rifting is more advanced and less localized. These results are consistent with a model in which the “mantle wind” transports plume material to the northeast from the Main Ethiopian Rift to Afar while incorporation of shallow mantle/lithsopheric He and alpha decay dampen the deep mantle He isotope signal. A region of particularly high He isotope ratios near the Erta Ale volcano suggests geographically restricted upwelling of more pristine plume material, while a region of exceptionally low 3He/4He ratios, indicative of continental crust, correlates well with a high velocity anomaly. This work shows the unique utility of combining geochemical and geophysical datasets in seeking to understand the earth’s functioning.
The sixth mass extinction: a geological perspective on our current biodiversity crisis
Feb. 5, 2020
noon - 1 p.m.
The Hershey Hall Grand Salon, Room 158, Hershey Hall
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Earth is currently experiencing an accelerating biodiversity crisis that could rival past mass extinctions in terms of rate, magnitude, and selectivity. What lessons does the fossil record offer for how ecosystems will respond to massive loss of biodiversity? In this talk, I will compare the intensity and ecological selectivity of past mass extinction events to the current biodiversity crisis using a new database of animal sizes and ecological traits spanning both fossil and living species. Both on land and in the ocean, the strongly selective removal of large-bodied animals across many taxonomic groups is unique to the current diversity crisis and appears to be a unique signature of human influence on the biosphere. The geological record provides many past examples of climate warming, ocean acidification, and sea level change that can help to inform projections of future environmental conditions. However, it does not contain a biodiversity crisis with a similar pattern of extinction, adding to the challenge of forecasting future ecosystem function.
Exploring the interior of the Earth & Mars: Seismology with big & small data on HPC systems
Feb. 12, 2020
noon - 1 p.m.
Geology 1707
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The dramatic increase in the amount and quality of seismic data from global seismographic networks in recent years and advances in numerical solvers, as well as high-performance computing (HPC) systems, offer us unprecedented opportunities to refine our understanding of the inner dynamics of our planet. Global adjoint tomography, a full-waveform inversion technique, is one of the extreme projects in seismology due to the intense computational requirements and big data that can potentially be assimilated in inversions. After the first-generation global adjoint tomography model GLAD-M15 (Bozdag et al. 2016), and its successor GLAD-M25 (Lei et al., in revision), we have three major goals: 1) take better physics into account in adjoint inversions (i.e., anelasticity, general anisotropy, source parameters, etc.), 2) use all available data from data repositories in inversions (big data in seismology) while extracting maximum information from each seismic trace with appropriate measurement techniques and strategies, 3) go down to 1 Hz in global simulations to perform whole-Earth inversions including the core (exascale computing). On the other hand, we have the "small data" problem to explore the interior of other planetary bodies such as Mars where we now have data from the single InSight seismometer. I will talk about the current and future directions in global seismology as well as our initial results from Earth and Mars simulations.
A Primer on Core Dynamics
Feb. 19, 2020
noon - 1 p.m.
Geology 1707
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In this talk, I will present simple, almost lay level, experiments to explain the basic style of convection that develops in planetary cores. In doing so, the open dynamical questions in core dynamics and dynamo theory will be made clear(er!).
How did India break the plate tectonic speed limit?
Feb. 26, 2020
noon - 1 p.m.
Geology 1707
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During the Cretaceous, the Indian plate moved towards Asia at some of the fastest speeds ever recorded. The details of this journey are preserved in the Indian Ocean seafloor, which document two distinct pulses of fast motion, separated by a noticeable slowdown. The nature of this rapid acceleration, followed by an equally rapid slowdown and then succeeded by second, and longer duration speed-up is puzzling to explain. Using an extensive dataset, we confirm these observations and use numerical models of subduction to show that the arrival of the Deccan mantle plume-head at ~67Ma started a sequence of events that can explain this history of plate motion. The forces applied by the plume causes initiation of an intraoceanic subduction zone, which eventually adds enough additional force to drive the plates at the anomalously fast speeds. Our models support a two-stage India-Eurasia collision sequence starting with the 'soft' collision at ~50 Ma between the purported intra-oceanic arc above the middle plate accreting onto the Eurasian margin, followed by the 'hard' collision between continental India and Eurasia as early as 43 Ma or as late as ~25-20 Ma. The Kohistan-Ladakh Arc terranes in NW Himalayas previously suggested to represent accretion of an intra-oceanic arc are a candidate, however, the western portion of Indus-Yarlung suture zone hosts a belt of Jurassic to Early Cretaceous ophiolites that include additional potential candidates such as the Xigaze ophiolite. Either way, our hypothesis supports recent interpretations that Greater India was not an extension of the Indian continent, but instead was a micro-continent separated from northern cratonic India by an ocean basin.
Plumes and their interaction with a heterogeneous mantle: Insights from geodynamic modeling
March 4, 2020
noon - 1 p.m.
Geology 1707
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Many observations have shown that the lowermost mantle has a heterogeneous thermal and chemical structure. This heterogeneity is sampled when buoyant plumes rise from the core-mantle boundary and carry material towards the surface, providing a window to the composition of the Earth’s deep interior. Consequently, it is important to understand the processes that allow mantle plumes to inherit lower-mantle geochemical signatures and transport material from the core-mantle boundary to the surface. I will talk about how chemical and rheological heterogeneities likely present in the lowermost mantle may influence plume dynamics. Specifically, I will discuss how subducted slabs determine how mantle plumes sample chemical reservoirs in the lowermost mantle, the influence of an evolving mineral grain size and the associated variations in mantle rheology, and the effects of partial melting near the core-mantle boundary on chemical heterogeneities in rising plumes.
Coupled magma ocean-early atmosphere evolution
March 11, 2020
noon - 1 p.m.
Geology 1707
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Magma oceans were once ubiquitous in the early solar system,erent evolutionary paths of terrestrial planets and their moons. In particular, the re magma oceans may have profound influence on the redox state of subsequently formed mantles and the overlying atmospheres. The thermodynamics of redox buffering reactions in realistic magma ocean environments, however, remain poorly constrained. Our first-principles molecular dynamics results combined with thermodynamic modeling suggest that the magma oceans of Earth, Mars, and the Moon are likely characterized with a vertical gradient in oxygen fugacity with deeper magma oceans invoking more oxidizing surface conditions. This redox zonation may be the major cause for Earth’s upper mantle being more oxidized than Mars’ and the Moon’s upper mantles. The contrasting redox profiles between these planetary bodies also suggest that the early atmosphere in equilibrium with Earth’s surface may have been dominated by CO2 and H2O, in contrast to those enriched in H2O and H2 for Mars and H2 and CO for the Moon.