Conrad Group Research Projects

My research is directed toward gaining a better understanding of the Earth's dynamic interior and mobile surface. To do this, I use geological and geophysical observations to constrain numerical models of mantle and lithosphere dynamics. Below I describe some of the types of observations that I have used recently.

Tectonic Plate Motions The Earth's surface is broken up into several tectonic plates that move as rigid blocks relative to each other. Convection in the Earth's mantle ultimately drives the motions of these plates. I have used the observed motions of Earth's tectonic plates as a constraint on viscous flow in the Earth's mantle. In doing so I have constrained the interaction between mantle flow and plate motions, both today and for times in the past. How Mantle Slabs Drive Plate Motions Cenozoic Plate-Driving Forces S.A. Steiner1 and C.P. Conrad, Does active mantle upwelling help drive plate motions?, Physics of the Earth and Planetary Interiors, 161, 103-114, 2007. [online version] [reprint] C.P. Conrad and C. Lithgow-Bertelloni, The temporal evolution of plate driving forces: Importance of "slab suction" versus "slab pull" during the Cenozoic, Journal of Geophysical Research, 109, B10407, doi:10.1029/2004JB002991, 2004. [online version] [reprint] C.P. Conrad and C. Lithgow-Bertelloni, How mantle slabs drive plate tectonics, Science, 298, 207-209, 2002. [online version] [reprint] [supporting material] [U. Michigan press release] [Geotimes article]

Patterns of Global Seismicity As the tectonic plates move past each other at plate boundaries, they deform. Part of this deformation involves occurs suddenly and catastrophically in the form of earthquakes. Because these earthquakes occur in response to the forces acting to drive plate motions, spatial variations in the size and frequency of earthquakes place constraints on these forces. I have used variations in the incidence of great earthquakes (greater than magnitude 9.0) and deep earthquakes (deeper than 100 km) to place constraints on plate forces and deformation mechanisms. Great Earthquakes, Slab Weakening, and Slab Pull S.L. Bilek, C.P. Conrad, and C. Lithgow-Bertelloni, Slab pull, slab weakening, and their relation to deep intra-slab seismicity, Geophysical Research Letters, 32, L14305, doi:10.1029/2005GL022922, 2005. [online version] [reprint] C.P. Conrad, S. Bilek, and C. Lithgow-Bertelloni, Great earthquakes and slab pull: interaction between seismic coupling and plate-slab coupling, Earth and Planetary Science Letters, 218, 109-122, 2004. [online version] [reprint]

Earth's Surface Topography Topography at the Earth's surface is controlled by near-surface density variations (isostatic topography) and by the dynamic response of the surface to viscous flow in the Earth's mantle (dynamic topography). Dynamic topography can provide a constraint on mantle flow models, but it can be difficult to observe because it is obscured by isostatic topography. I have used mantle flow models to confirm geological observations of dynamic topography on the seafloor of the present-day North Atlantic Ocean and on the African continent during the Cenozoic. C.P. Conrad, C. Lithgow-Bertelloni, and K.E. Louden, Iceland, the Farallon slab, and dynamic topography of the North Atlantic, Geology, 32, 177-180, 2004. [abstract] [reprint] C.P. Conrad and M. Gurnis, Mantle flow, seismic tomography and the breakup of Gondwanaland: Integrating mantle convection backwards in time, Geochemistry, Geophysics, and Geosystems, 4, 1031, doi:10.1029/2001GC000299, 2003. [online version] [reprint]

Climate Change and Sea Level Convection in the Earth's mantle, expressed at the surface by the tectonic plate motions, can influence the global climate and environment in several ways. I have examined several links between the geodynamics of Earth's interior and changes to Earth's surface environment. For example, changes in the volume of the ocean basins can cause sea level rise. Changes in rates of seafloor spreading or subduction can influence the chemistry and composition of the atmosphere and oceans. Tectonic deformation of the Earth's surface clearly influences regional environments, and can influence global climate in several ways. Subduction Zone Uplift and Methane Hydrates S.J. Loyd, T.W. Becker, C.P. Conrad, C. Lithgow-Bertelloni, and F.A. Corsetti, Time variability in Cenozoic reconstructions of mantle heat flow: Plate tectonic cycles and implications for Earth's thermal evolution, Proceedings of the National Academy of Sciences, 104, 14266-14271, 2007. [abstract] [reprint] C.P. Conrad and C. Lithgow-Bertelloni, Faster seafloor spreading and lithosphere production during the mid-Cenozoic, Geology, 35, 29-32, 2007. [online version] [reprint] [highlight in Nature] [download data] L. Husson, and C.P. Conrad, Tectonic velocities, dynamic topography, and relative sea level, Geophysical Research Letters, 33, L18303, doi:10.1029/2006GL026834, 2006. [online version] [reprint] X. Xu, C. Lithgow-Bertelloni, and C.P. Conrad, Global reconstructions of Cenozoic seafloor ages: Implications for bathymetry and sea level, Earth and Planetary Science Letters, 243, 552-564, 2006. [online version] [reprint] A.H. Jahren, C.P. Conrad, N.C. Arens, G. Mora, and C. Lithgow-Bertelloni, A plate tectonic mechanism for methane hydrate release along subduction zones, Earth and Planetary Science Letters, 236, 691-704, 2005. [online version] [reprint]

Seismic Anisotropy Viscous flow in the mantle deforms mantle rocks and minerals. Below Earth's surface plates, this deformation results in the alignment of olivine crystals in the direction of mantle flow. This alignment can be detected seismically by measuring differences in the rates of seismic wave propagation in different directions. Thus, these measurements of "seismic anisotropy" can yield constraints on the direction of mantle flow beneath Earth's surface plates. I have used measurements of seismic anisotropy to constrain models of mantle flow, looking at upwelling flow beneath the Africa and downwelling flow beneath North America in particular. C.P. Conrad, M.D. Behn, and P.G. Silver, Global mantle flow and the development of seismic anisotropy: Differences between the oceanic and continental upper mantle, Journal of Geophysical Research, 112, B07317, doi:10.1029/2006JB004608, 2007. [online version] [reprint] [download flow model] M.A. Behn, C.P. Conrad, and P. Silver, Detection of upper mantle flow associated with the African superplume, Earth and Planetary Science Letters, 224, 259-274, 2004. [online version] [reprint]

Lithospheric Stresses Mantle flow exerts shear tractions on the base of the Earth's surface plates. These tractions drive the surface plate motions, but are also the ultimate source of the seismicity, mountain building, and crustal deformation that we observe at the surface. I have developed models of mantle flow that predict the lithospheric stresses responsible for these deformation processes. Using observations of lithospheric stress as a constraint on these models, we can begin to understand how the mantle controls the deformation processes that we observe geologically and seismically. C.P. Conrad and C. Lithgow-Bertelloni, Influence of continental roots and asthenosphere on plate-mantle coupling, Geophysical Research Letters, 33, L05312, doi:10.1029/2005GL025621, 2006. [online version] [reprint] [download model]

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