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SURFACE OF THE EARTH: GLOBAL STUDIES 2006 IRIS 5-YEAR PROPOSAL Cooling History of the Pacific Lithosphere: Evidence for Thermal Boundary Layer Instabilities and Dislocation Creep Michael H. Ritzwoller, Nikolai M. Shapiro, Shijie Zhong • University of Colorado at Boulder Numerous studies based mostly on surface observables (e.g., topography, heat flux) have established that the oceanic lithosphere, particularly across the Pacific, does not cool continuously as it ages. Observations of surface wave dispersion used to construct a model of shear wave speeds and temperatures in the upper mantle establish that the Pacific lithosphere has experienced a punctuated cooling history (Figure 1), cooling diffusively at ages younger than ~70 Ma and then reheating in the central Pacific between ages of about 70 and 100 Ma. At ages from 100 Ma to about 135 Ma, the processes of reheating are substantially weaker than in the Central Pacific. Uppermost mantle temperatures can also be summarized in terms of the “apparent thermal age” of the lithsophere (Figure 2). Using numerical simulations of mantle convection with realistic plate motions and various rheologies, thermal boundary layer instabilities (TBI) are shown to develop naturally as the plate cools. The average thermal structure of the Pacific upper mantle can be matched if the mantle deformation mechanism is dislocation creep (n = 3) with an activation energy of about 360 kJ/mol, consistent with that for dislocation creep for olivine determined from laboratory studies. Average temperature profile of the Pacific upper mantle versus lithospheric age. The green lines are isotherms from a diffusively cooling half-space. An average temperature perturbation relative to the half-space cooling model develops in the Central Pacific due to processes of reheating that occur between 70 and 100 Ma, revealing the punctuated cooling history of the Pacific lithosphere. Apparent thermal age plotted versus lithospheric age across the Pacific. Apparent thermal age is defined as the lithospheric age at which a purely diffusively cooling temperature profile would most closely resemble the observed thermal structure. For a Newtonian mantle rheology governed by diffusion creep (n = 1), realistic activation energies do not reproduce the observed average thermal structure of the Pacific. But for a non-Newtonian rheology governed by dislocation creep (n = 3), realistic activation energies can reproduce the punctuated cooling history of the Pacific. Ritzwoller, M.H., N.M. Shapiro, and S. Zhong, Cooling history of the Pacific lithosphere, Earth Planet. Sci. Lett., 226, 69-84, 2004. Van Hunen, J., S. Zhong, N.M. Shapiro, and M.H.Ritzwoller, New evidence for dislocation creep from 3-D geodynamic modeling of the Pacific upper mantle, submitted 2005. 118
2006 IRIS 5-YEAR PROPOSAL SURFACE OF THE EARTH: GLOBAL STUDIES Advances in Active-Source Seismic Imaging Using Traveltime and Waveform Tomography Colin A. Zelt • Rice University John A. Hole • Virginia Polytechnic Institute R. Gerhard Pratt • Queen’s University, Canada In 2003, as part of a workshop funded by IASPEI, NSF, IRIS and Virginia Tech, a realistic active-source synthetic dataset was made available to the community for the purpose of testing inversion and imaging algorithms by workers who, at the time, did not know what the true model was. The synthetic wide-angle dataset, consisting of 51 shots and 2779 receivers, was calc hole/ccss/. The survey design is consistent with experiments that could be supported by the IRIS-PASSCAL Instrument Center in about three years. The true model contains large-scale features such as a low-velocity zone and regions where the crust-mantle boundary is sharp and smooth, as well as intermediate to wavelength-scale stochastic features. Both first arrival and simultaneous PmP/Pn traveltime tomography were applied to obtain smooth velocity models that compare favorably with the large-scale features of the true model. The model obtained from first-arrival traveltime tomography was used as a starting model for 2-D acoustic, frequency-domain waveform tomography. The final model from waveform tomography matches the large and intermediate-scale (down to ~1 km) features of the true model, including the low-velocity zone and the structure of the crust-mantle transition zone. The combined results from traveltime and waveform tomography show the complementary nature of these approaches and the potential for the analysis of real active-source crustal data in the future. True velocity model and inversion results. A) True model. B) Model derived from traveltime tomography using first arrivals. C) Model derived from traveltime inversion using first arrivals and wide-angle reflections from the Moho. D) Model derived from waveform tomography using (B) as the starting model. Hole, J.A., C. A. Zelt and R. G. Pratt, Advances in Controlled-Source Seismic Imaging, EOS, 86, No. 18, p. 177, 181, 2005. 119
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SCIENCE SUMMARIES 2006 IRIS 5-YEAR
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