The largest geophysics conference in the world will be held in Washington DC this year. I'll be there presenting some new results on North American lithospheric structure - see abstract below.
If you are a prospective student or post-doc (or simply interested in chatting about fun science) attending AGU then I encourage you to get in touch with me; I would be happy to make time to meet with you.
In case you're interested, here are some sessions that I will be following with alert interest and great concern (in no particular order):
The deep structure of North American lithosphere illuminated by complementary seismic data types
Zachary Eilon, Karen M. Fischer, Colleen A. Dalton
The internal structure of continental lithosphere holds the key to our understanding of plate tectonic cycles and the long-term stability of cratons. Detailed imaging of the upper mantle has the potential to reveal chemical alteration, deformation fabric, thermal structure, and melt distribution, providing not only a contemporary snapshot but also an integrated history of the interactions between the Earth’s surface and its deep interior. We apply a transdimensional, hierarchical Bayesian inversion parameterised using piecewise discontinuous splines to solve for 1-D radially anisotropic shear velocity models of crust and upper mantle beneath long-lived stations across North America. The advantages of a Bayesian approach include quantitative uncertainty estimation and the ability to modularly incorporate multiple data types; we use Rayleigh wave and Love wave phase velocities from earthquakes and ambient noise, Rayleigh wave ellipticity ratios, and both P-sandS-pconverted phases. Taken jointly, this combination of data types provides excellent constraints on absolute velocities, crustal structure, and sharp gradients in velocity within or at the base of the lithosphere. We find that S-pphases are essential to capture accurately the characteristics of deep velocity gradients, such as those seen widely but discontinuously in the cratonic mid-lithosphere. Observed structures include shallow melt and adiabatic geotherms immediately beneath thin lithosphere in portions of the western US, as well as intra-lithospheric low-velocity zones beneath the cratons that have amplitude and depth consistent with new data on exotic lithologies associated with subduction-derived fluid reaction products.
Non-specialist/jargon version: At the heart of many continents lie large regions of the Earth’s tectonic plates that appear to be much older than their surroundings. We don’t yet understand how these stable regions were formed billions of years ago, or exactly why they are so resistant to being broken up or squished into mountains as vast forces act upon them. Here we use a new combination of types of seismic waves – Earth vibrations set off by distant earthquakes – to assess the properties of these ancient continental regions and the younger, weaker material that surrounds them. Different types of seismic waves are good for pinpointing certain individual properties of the Earth; by taking into account the information from multiple data types simultaneously, we can get a much better understanding of the internal structure of the tectonic plates and try to answer questions about how they were produced and how they have evolved – or not – over time.
The CIDER 2018 summer program returns to the theme of the original CIDER Program on Relating Geophysical and Geochemical Heterogeneity in the Deep Earth. Significant advances and discoveries since 2004 motivate a return to this long-standing question. Improvements in the quality and quantity of observations have combined with computational advances in modeling seismic-wave propagation to turn blurry images into sharply focused snapshots of the present-day structure. Meanwhile, advances in experimental and theoretical mineral physics have brought new insights into the crystal structure and transport properties of materials at high pressure and temperature. Growing confidence in the predictions for representative minerals informs our interpretation of geophysical heterogeneity in terms of the primary variables (e.g. temperature, pressure, major-element chemistry, trace-element chemistry and volatiles). Separately, advances in geochemical analysis reveal growing evidence for short-lived isotopes in the early Earth. These new observations have transformed our understanding of the Earth’s initial condition and raised new questions about the preservation of isotopic anomalies in a dynamic planet. Even the recent advances in our understanding of the formation of the Moon bear on this topic because it has important consequences for the possible thermal and compositional states that emerge in the aftermath of a Moon-forming collision.
The goal of this Summer Program is to bring together junior and senior scientists from different disciplines to cross-educate each other and help advance our understanding of the processes that govern the long-term evolution of our planet.
Sailing aboard the R/V Kilo Moana, we plan to deploy 30 broadband ocean bottom seismometers in the central Pacific Ocean. Distributed in a large (~500x500 km) array on the seafloor hundreds of miles from the nearest land-based seismic station, these instruments will passively record global seismicity for 15 months. When we recover them in 2019, they will (hopefully!) hold data that sheds light on convection within the Earth's mantle, as well as the structure and development of oceanic plates.
I was pleased to have the chance to visit UTIG (the University of Texas Institute for Geophysics) last week to talk about our new Bayesian inversion using multiple seismic data types. It was great to meet lots of the UTIG researchers and enjoy both science discussions and Austin TX local brews (especially in combination)!
Title and abstract:
An adaptive Bayesian inversion for upper mantle structure using surface waves and scattered body waves
I will present a methodology for 1-D imaging of upper mantle structure using a Bayesian approach that incorporates a novel combination of seismic data types and an adaptive parameterisation based on piecewise discontinuous splines. This inversion algorithm lays the groundwork for improved seismic velocity models of the lithosphere and asthenosphere by harnessing the recent expansion of large seismic arrays and computational power alongside sophisticated data analysis. Careful processing of P- and S-wave arrivals isolates converted phases generated at velocity gradients between the mid-crust and 300 km depth. This data is allied with ambient noise and earthquake Rayleigh wave phase velocities to obtain detailed VS and VP velocity models. Synthetic tests demonstrate that converted phases are necessary to accurately constrain velocity gradients, and S-p phases are particularly important for resolving mantle structure, while surface waves are necessary for capturing absolute velocities. We apply the method to several stations in the northwest and north-central United States, finding that the imaged structure improves upon existing models by sharpening the vertical resolution of absolute velocity profiles, offering robust uncertainty estimates, and revealing mid-lithospheric velocity gradients indicative of thermochemical cratonic layering. This flexible method holds promise for increasingly detailed understanding of the upper mantle.
I'm involved in convening three sessions at this year's American Geophysical Union Fall meeting (this year in New Orleans!):
The abstract portal is now open. Apply by August 2nd!
The three sessions I'm chairing/convening are linked below. It looks like it's going to be a fantastic meeting (and v. excited to explore New Orleans). Hope to see you there!
The breakup of continents is a fundamental process of plate tectonics. However, we have not yet identified the crucial ingredients that permit complete rupture of strong continental lithosphere. Studies of continental breakup are biased towards success stories - rifts that evolve to oceanic spreading. Some extension episodes cease before this point, presumably in the absence of some fundamental process, initial condition(s), or forcing. Investigations of “failed rifts” may help isolate key processes or conditions that enable continental breakup, particularly when compared to successful examples. Outstanding questions include: Does rift success/failure depend on intrinsic or far-field properties? How do pre-existing structure, magma, and volatiles influence rift initiation, continuation, and extinction? Are failed rifts actually “paused rifts” that can later be reactivated? Do analogous mechanical controls apply to extinct seafloor spreading centers? We solicit contributions from diverse geoscience perspectives, including geodesy, geodynamics, geochemistry/petrology, volcanology, structural geology and seismology.
The lithosphere-asthenosphere boundary (LAB) separates Earth’s rigid tectonic plates from the underlying convecting mantle. Tractions, thermal gradients, compositional differentiation, fluid/melt accumulation, crystallographic fabrics, and grain size heterogeneity have all been suggested to occur at (or near) this interface. How this boundary is expressed in different tectonic settings and how it relates to other seismic and magnetotelluric discontinuities including the Moho and mid-lithospheric discontinuities (MLDs) is a topic of ongoing vigorous debate. We will focus on the lithosphere-asthenosphere system in a variety of settings including but not limited to continents, oceans, margins, rifts, ridges, hotspots, plumes, and subduction zones. We welcome research contributions from diverse fields, including but not limited to seismology, magnetotellurics, petrology/mineralogy, dynamical modelling, and mineral physics.
Melting within the Earth, across a range of depths, has fundamental consequences for planetary dynamics and evolution. Yet, questions regarding source composition, melt migration and distribution, in situ melt fraction, and magma storage and evolution remain unanswered. We invite contributions from studies of asthenosphere/lithosphere melting in various tectonic environments (e.g., mid-ocean ridges, subduction zones, hotspots), as well as deep planetary melting (e.g., transition zone, magma ocean). Topics covered by this session include: (1) magmatic processes in the presence of volatiles and heterogeneous rock assemblages, (2) melt migration and melt-rock interaction, (3) melting and solidification at the core-mantle boundary, (4) imaging melt within the Earth through seismic or magnetotelluric approaches, (5) plume vs. non-plume origin of intraplate volcanism, and (6) magmatic processes and the evolution of magmas in the early Earth. We solicit research that combines the strengths of geochemistry, petrology, geology, geodynamics, geophysics, and mineral physics.