Extensional Processes at Mid-Ocean Ridges and Continental Rifts: We are investigating interactions between faulting, magmatism, and surface processes during lithospheric deformation. Our goal is to understand how these processes, in conjunction with lithospheric rheology, act together to shape the evolution of extensional environments. This research combines geodynamic modeling, topographic/bathymetric analyses, and active source seismic techniques. Focus sites include slow- and ultra-slow spreading ridges, the Okavango and Malawi rifts, and the Galapápgos Spreading center.
Origin of the Continental Crust: Despite decades of research, the origin of continental crust remains a matter of considerable debate. The basic paradox is that although the continental crust has an andesitic bulk composition, the majority of melts derived from the mantle are basaltic. This requires some process by which dense mafic/ultramafic residues of melting can be recycled back into the mantle, leaving behind only the lighter, more felsic rocks to form the continental crust. To date, two primary density sorting mechanisms have been proposed: (1) delamination, in which mafic/ultramafic rocks near the crust-mantle interface become density unstable with respect to the underlying mantle and sink back into the upper mantle; and (2) relamination, in which buoyant, felsic lavas and plutonic rocks in subducting, arc crust become unstable and rise through the mantle wedge and/or subduction channel and accumulate in the overlying crust. A key discriminant between these mechanisms is the average composition of the lower continental crust—delamination predicts a basaltic lower crust, while relamination predicts an intermediate to felsic composition. I am currently collaborating with colleagues Peter Kelemen (LDEO), Brad Hacker (UCSB), and Oli Jagoutz (MIT) to develop quantitative geochemical and geophysical tests to distinguish between delamination and relamination and determine how both processes have influenced the evolution of the Earth’s continental crust.
Mantle Melting and Melt Migration at Mid-Ocean Ridges: Mid-ocean ridges produce a majority of Earth’s volcanism and are the locus for the creation of oceanic crust. The volume and composition of melts formed at mid-ocean ridges are controlled by a range of parameters, including mantle temperature and composition, spreading rate, and the patterns and length scales of melt migration. To deconvolve these different parameters, we couple 2-D and 3-D geodynamic models of mantle flow and temperature structure with thermodynamic parameterizations of melting to make specific predictions of crustal thickness and the compositional variability of magmas. These predictions are then compared with geophysical and geochemical observations to constrain mantle melting parameters. An important conclusion of our work is that melt can only be transported to the ridge axis over relatively short horizontal length scales (~25km). This implies that although melting occurs over a wide region beneath the ridge axis, up to 20–40% of the total melt volume is not extracted and will eventually refreeze and refertilize the lithosphere.
Ice sheet Dynamics: During the summer months much of the ablation zone of the Greenland Ice Sheet is covered with supraglacial melt water lakes. Geophysical data collected in collaboration with S. Das (WHOI) and I. Joughin (Univ. Washington) over the last 9 years (2006–2014) show that large (~5.5 km2) lakes formed during the summer melt season can drain to the bed in < 2 hours. Our group has captured many of these drainage events using geodetic instruments, and we have shown that the drainage coincides with increased seismicity, rapid ice sheet uplift, and horizontal acceleration in the down-flow direction. More recently, we have used a dense GPS network surrounding 2 supraglacial lakes to (1) constrain crack geometry, (2) invert ice sheet response to lake drainage events, and (3) evaluate the evolution of the basal water system in time and space during (and after) lake drainage. An exciting result of this study is the identification of precursory uplift of the ice sheet associated with meltwater entering the sub-glacial hydrologic system before lake drainage events. These precursors generate tensile stresses beneath the lake that promote hydrofracture initiation and lake drainage. We hypothesize that these precursors are associated with the introduction of meltwater to the bed through neighbouring moulin systems (vertical conduits connecting the surface and base of the ice sheet). Our results imply that as lakes form in less crevassed, interior regions of the ice sheet water at the bed is currently less pervasive the creation of new surface-to-bed conduits caused by lake-draining hydro-fractures may be limited.
Grain size evolution in the Earth’s mantle: Experimental studies show deformation in the upper mantle occurs via two dominant deformation mechanisms: diffusion and dislocation creep. While both mechanisms act simultaneously, diffusion creep typically dominates at low stress and/or small grain size, while at high stress and/or large grain size dislocation creep is the dominant deformation mechanism. The effective mantle rheology is therefore dependent on both stress and grain size, as well as pressure, temperature, and mantle composition. Grain-size is a particularly important microstructural property because it influences seismic attenuation and wave-speeds, electrical conductivity, and melt permeability. Moreover, the grain size sensitivity of rheology has led to the hypothesis that it may play a key role in shear localization and the generation of plate tectonics. Because grain size is sensitive to the dislocation creep rate, grain size will evolve with deformation producing an additional feedback on mantle rheology. Thus, convection in the Earth’s mantle is coupled across scales ranging from global flow (100s to 1000s km) to regional rheologic variations (10s to 100s km) to the grain size scale (cm and smaller). To address this coupling, we have incorporated laboratory-based grain-size evolution models into 1-D and 2-D models of deformation in the oceanic upper mantle beneath mid-ocean ridges and subduction zones. Our current research focuses on how the predicted variations in grain size will influence melt migration in these settings, as well as how grain size will evolve in other geologic settings such as ice sheets.
Global Mantle Circulation and Upper Mantle Seismic Structure: While the present-day motions of Earth’s tectonic plates are well known from geodetic data, the direction and magnitude of the underlying mantle flow field remains largely unconstrained. This has led to a spectrum of models ranging from layered to whole mantle convection, with different implications for Earth’s thermal and chemical evolution. Our group uses a combination of geodynamic models and seismic data to unravel the relationship between plate tectonics and mantle convection today and in Earth’s past. Seismic anisotropy provides a powerful tool for constraining flow at depth within the Earth, because it can provide a direct measure of mantle velocity. In the upper mantle, the preferential alignment of olivine crystals by dislocation creep produces an anisotropic fabric that can be measured through a variety of seismic techniques, and these observations can be used to test regional and global models of mantle flow. In collaboration with Clint Conrad (U. Hawaii) and Paul Silver (DTM), we developed numerical models of global mantle flow that are driven by surface plate motions and mantle density heterogeneity inferred from seismic tomography. These models are used to predict the development of anisotropy throughout the asthenosphere. By comparing our predictions to a global compilation of shear-wave splitting data, we demonstrated that (1) asthenospheric flow contributes significantly to seismic anisotropy in oceanic regions, (2) asthenospheric anisotropy may be important in some, but not all, continental environments, and (3) the global net rotation of the lithosphere must be <60% the HS3 hotspot reference frame.
