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Reconstructing Farallon Plate Subduction Beneath North America Back to the Late Cretaceous

Science
7 Nov 2008
Vol 322, Issue 5903
pp. 934-938

Abstract

Using an inverse mantle convection model that assimilates seismic structure and plate motions, we reconstruct Farallon plate subduction back to 100 million years ago. Models consistent with stratigraphy constrain the depth dependence of mantle viscosity and buoyancy, requiring that the Farallon slab was flat lying in the Late Cretaceous, consistent with geological reconstructions. The simulation predicts that an extensive zone of shallow-dipping subduction extended beyond the flat-lying slab farther east and north by up to 1000 kilometers. The limited region of flat subduction is consistent with the notion that subduction of an oceanic plateau caused the slab to flatten. The results imply that seismic images of the current mantle provide more constraints on past tectonic events than previously recognized.

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Supplementary Material

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References and Notes

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Both shear (4) and compressional (5, 6) wave tomography show a prominent positive slablike seismic anomaly at mid-mantle depths below the east coast of North America. The high-velocity component of the shear wave seismic tomography (4) is converted to effective temperature (21) with a scaling factor of 2 × 103 °C km–1 s. The upper 250-km signal associated with the North American craton is removed because it is likely neutrally buoyant (28). We also remove structures below 2400-km depth, where there is a clear gap in the tomographic image. For the rest, we assume a constant seismic-to-temperature scaling that we constrain by fitting our models to stratigraphic data.
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The adjoint method, a gradient-based inversion, optimizes the initial condition by minimizing the mismatch between a model prediction and observation. For the nonlinear mantle convection, the observational constraint is seismic tomography. A simple backward integration of present-day mantle structures to the initial time provides a first-order estimate of the initial condition (9). In a forward-adjoint looping scheme, the forward run—which solves for conservation of mass (ui,i = 0, where u is velocity), momentum [–P,i +ui,j + ηuj,i),j + ρoαΔTgδir = 0, where P is dynamic pressure, η viscosity, ΔT effective temperature, α thermal expansion, ρo reference density, and g gravitational acceleration], and energy (T,t + uiT,i = κT,ii, where T is temperature, t time, and κ thermal diffusivity) for an incompressible Newtonian fluid—predicts the present-day mantle thermal structure, whereas the adjoint run, which solves for the kinematic adjoint equation (λ,t + ui λ,i + κλ,ii = 0, where λ is adjoint temperature) with u stored from the forward run—back-propagates the mismatch to the initial time. The past mantle thermal state is iteratively updated until the mismatch with the observation (tomography) is small. The adjoint method has been incorporated (9) in a finite-element incompressible model of mantle convection (17).
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This is contribution no. TO 88 of the Caltech Tectonics Observatory. The work was partially supported by the Gordon and Betty Moore Foundation through the Tectonics Observatory and the NSF through EAR-0609707. We appreciate discussions with J. Saleeby and R. D. Müller. The original CitcomS software was obtained from the Computational Infrastructure for Geodynamics (CIG) (http://geodynamics.org).

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Information & Authors

Information

Published In

Science
Volume 322 | Issue 5903
7 November 2008

Submission history

Received: 8 July 2008
Accepted: 8 October 2008
Published in print: 7 November 2008

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Notes

Supporting Online Material
www.sciencemag.org/cgi/content/full/322/5903/934/DC1
Materials and Methods
Figs. S1 to S3
References

Authors

Affiliations

Lijun Liu*
Seismological Laboratory, California Institute of Technology, Pasadena, CA 91125, USA.
Sonja Spasojević
Seismological Laboratory, California Institute of Technology, Pasadena, CA 91125, USA.
Michael Gurnis
Seismological Laboratory, California Institute of Technology, Pasadena, CA 91125, USA.

Notes

*
To whom correspondence should be addressed. E-mail: [email protected]

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