Mapping Variability in the Thermo-mechanical Structure of the North American Plate and Upper Mantle

Lead PI: Dr. Ben Holtzman, Christopher Havlin

Unit Affiliation: Seismology, Geology and Tectonophysics, Lamont-Doherty Earth Observatory (LDEO)

September 2017 - August 2019
Inactive
North America ; United States
Project Type: Research

DESCRIPTION: EarthScope provides an unprecedented view into the Earth, enabling the emergence of new interpretations and questions concerning the physics of the Earth's interior. In addition to the purely basic research questions of earth structure, dynamics and evolution, understanding the thermal and mechanical structure is central to understanding societally relevant problems such as the Earth's response to melting ice sheets and rising sea levels, the volcanic potential beneath Yellowstone, and the regional origin of the geothermal gradient for high temperature geothermal energy production. The tectonic plate, or lithosphere, beneath Central and Eastern North America is tectonically stable, but Western North America has a complex recent history (60 million years) of magmatism and deformation, and a correspondingly thin and varied lithosphere structure. The PIs are developing computational methods for interpreting the thermo-mechanical structure (the temperature and the strength) of the upper mantle based on seismic data from the USArray. The core of the method is the systematic calculation of mechanical properties across a vast range of time scale, from seismic wave propagation (seconds) to mantle convection (millions of years) and everything in between. These properties depend on temperature, stress and other thermodynamic "state" variables. When we identify models that are consistent with observations at one time scale, those models constrain physical properties at other time scales. This particular project focuses on how heat from the mantle can corrode the tectonic plates from their underside, influencing where volcanoes and earthquakes occur closer to the surface. The broader impacts of this project fall into three categories: (1) the benefits of development of a unique, very flexible and integrative community code and its uptake by the seismology community; (2) advancement of three early career scientists; and (3) new development of visualization of EarthScope data products for the "SeismoDome" program in the Hayden Planetarium, school curricula and the general public. The aim of this project is to advance the quantitative interpretation of seismic measurements to constrain the thermo-mechanical structure of the shallow upper mantle beneath North America. In particular, the PIs are exploring the possibility that thermal-chemical corrosion occurs by melt infiltration into the plate. Central to this project, the Very Broadband Rheology (VBR) Calculator comprises computational methods intended to form practical and transparent bridges between petrology, seismology, mineral/rock physics and geodynamics. This project expands upon the VBR Calculator for calculation of frequency dependent mechanical properties across the broad range of time scales of geophysical interest, from seismic wave propagation to glacial isostatic adjustment to tectonic plate motions. By using a single approach at multiple locations around the continental US, we enable a direct comparative analysis of 1-D models of regional averages of measurements derived from the seismic wave field, with an emphasis on velocity, attenuation, and receiver functions. Initial results show that there are single forward models that can fit the average Vs "Stable North American" (SNA) lithosphere and asthenosphere. However, the "Tectonic North America" (TNA) velocity profile cannot be fit by a single, simple, steady-state thermal profile, indicating a more complex thermal history that has not reached a steady state. In the proposed research, new 1D open-system two phase flow models with heat advection by melt migration are being developed and added to the VBR library. These models characterize this kind of disequilibrium much further, producing lithospheric corrosion rates with reduced but realistic physics. The open system model results is being used to predict receiver function amplitude and width, enabling the seismic measurements to provide more detailed constraints on the thermal-mechanical structure in each setting.

SPONSOR:

National Science Foundation (NSF)

FUNDED AMOUNT:

$181,525

WEBSITE:

https://www.nsf.gov/awardsearch/showAward?AWD_ID=1736165&HistoricalAwards=false

PUBLICATIONS:

Lau, Harriet C. and Holtzman, Benjamin K.. "?Measures of Dissipation in Viscoelastic Media? Extended: Toward Continuous Characterization Across Very Broad Geophysical Time Scales," Geophysical Research Letters, v.46, 2019. doi:10.1029/2019gl083529

KEYWORDS

temperature mantle convection geodynamics magmatism thermo-mechanical structure lithosphere earthscope plate tectonics tectonic plate very broadband rheology calculator earth structure seismic waves

THEMES

Earth fundamentals