Joint Inversion of Mantle Viscosity and Thermal Structure: Applications of the Adjoint of Mantle Convection with Observational Constraints

Details Joint Inversion of Mantle Viscosity and Thermal Structure: Applications of the Adjoint of Mantle Convection with Observational Constraints Joint Inversion of Mantle Viscosity and Thermal Structure: Applications of the Adjoint of Mantle Convection with Observational Constraints

Dec 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufmdi21a0339l&link_type=abstract

American Geophysical Union, Fall Meeting 2007, abstract #DI21A-0339

Other

8121 Dynamics: Convection Currents, And Mantle Plumes, 8124 Earth'S Interior: Composition And State (1212, 7207, 7208, 8105), 8125 Evolution Of The Earth (0325), 8147 Planetary Interiors (5430, 5724, 6024), 8162 Rheology: Mantle (8033)

Scientific paper

The adjoint method widely used in meteorology and oceanography was introduced into mantle convection by Bunge et al (2003) and Ismail et al (2004). We implemented the adjoint method in CitcomS, a finite-element code that solves for thermal convection within a spherical shell. This method constrains the initial condition by minimizing the mismatch of prediction to observation. Since the present day mantle thermal structure is inferred from seismic tomography, we converted seismic velocity to temperature, an uncertain conversion. Moreover, since mantle viscosity is also uncertain, the inference of mantle initial conditions from tomography is not unique. We have developed a method that incorporates dynamic topography as an additional constraint and are able to jointly invert for mantle viscosity and the seismic to thermal scaling. We assume the thermal structure of present day mantle has the same ¡°pattern¡± as inferred from tomography, but leave the scaling to temperature as an unknown. The other constraint is the evolving dynamic topography recorded at specific points on earth's surface. From the governing equations of mantle convection, we derive the relations between dynamic topography, thermal anomaly and mantle viscosities. These relations allow a two- layer looping algorithm that inverts for viscosity and thermal anomaly: the inner loop takes the tomographic image as a constraint and the outer loop takes dynamic topography and its rate of change. Starting with incorrect values of thermal anomaly and viscosities, we show with synthetic experiments that all variables converge to their correct values after a finite number of iterations. Our method is examined both in a uniformly viscous mantle and a mantle with depth- and temperature-dependent viscosity. The method has been applied to the descent of the Farallon slab beneath North America.

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