Behn
Mark D.
Behn
Mark D.
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ArticleConstraints on the depth, thickness, and strength of the G Discontinuity in the Central Pacific from S Receiver Functions(American Geophysical Union, 2021-03-09) Mark, Hannah F. ; Collins, John A. ; Lizarralde, Daniel ; Hirth, Greg ; Gaherty, James B. ; Evans, Rob L. ; Behn, Mark D.The relative motion of the lithosphere with respect to the asthenosphere implies the existence of a boundary zone that accommodates shear between the rigid plates and flowing mantle. This shear zone is typically referred to as the lithosphere-asthenosphere boundary (LAB). The width of this zone and the mechanisms accommodating shear across it have important implications for coupling between mantle convection and surface plate motion. Seismic observations have provided evidence for several physical mechanisms that might help enable relative plate motion, but how these mechanisms each contribute to the overall accommodation of shear remains unclear. Here we present receiver function constraints on the discontinuity structure of the oceanic upper mantle at the NoMelt site in the central Pacific, where local constraints on shear velocity, anisotropy, conductivity, and attenuation down to ∼300 km depth provide a comprehensive picture of upper mantle structure. We image a seismic discontinuity with a Vsv decrease of 4.5% or more over a 0–20 km thick gradient layer centered at a depth of ∼65 km. We associate this feature with the Gutenberg discontinuity (G), and interpret our observation of G as resulting from strain localization across a dehydration boundary based on the good agreement between the discontinuity depth and that of the dry solidus. Transitions in Vsv, azimuthal anisotropy, conductivity, and attenuation observed at roughly similar depths suggest that the G discontinuity represents a region of localized strain within a broader zone accommodating shear between the lithosphere and asthenosphere.
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ArticleCompositional dependence of lower crustal viscosity(John Wiley & Sons, 2015-10-23) Shinevar, William J. ; Behn, Mark D. ; Hirth, GregWe calculate the viscosity structure of the lower continental crust as a function of its bulk composition using multiphase mixing theory. We use the Gibbs free-energy minimization routine Perple_X to calculate mineral assemblages for different crustal compositions under pressure and temperature conditions appropriate for the lower continental crust. The effective aggregate viscosities are then calculated using a rheologic mixing model and flow laws for the major crust-forming minerals. We investigate the viscosity of two lower crustal compositions: (i) basaltic (53 wt % SiO2) and (ii) andesitic (64 wt % SiO2). The andesitic model predicts aggregate viscosities similar to feldspar and approximately 1 order of magnitude greater than that of wet quartz. The viscosity range calculated for the andesitic crustal composition (particularly when hydrous phases are stable) is most similar to independent estimates of lower crust viscosity in actively deforming regions based on postglacial isostatic rebound, postseismic relaxation, and paleolake shoreline deflection.
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ArticleThermal-mechanical behavior of oceanic transform faults : implications for the spatial distribution of seismicity(American Geophysical Union, 2010-07-01) Roland, Emily C. ; Behn, Mark D. ; Hirth, GregTo investigate the spatial distribution of earthquakes along oceanic transform faults, we utilize a 3-D finite element model to calculate the mantle flow field and temperature structure associated with a ridge-transform-ridge system. The model incorporates a viscoplastic rheology to simulate brittle failure in the lithosphere and a non-Newtonian temperature-dependent viscous flow law in the underlying mantle. We consider the effects of three key thermal and rheological feedbacks: (1) frictional weakening due to mantle alteration, (2) shear heating, and (3) hydrothermal circulation in the shallow lithosphere. Of these effects, the thermal structure is most strongly influenced by hydrothermal cooling. We quantify the thermally controlled seismogenic area for a range of fault parameters, including slip rate and fault length, and find that the area between the 350°C and 600°C isotherms (analogous to the zone of seismic slip) is nearly identical to that predicted from a half-space cooling model. However, in contrast to the half-space cooling model, we find that the depth to the 600°C isotherm and the width of the seismogenic zone are nearly constant along the fault, consistent with seismic observations. The calculated temperature structure and zone of permeable fluid flow are also used to approximate the stability field of hydrous phases in the upper mantle. We find that for slow slipping faults, the potential zone of hydrous alteration extends greater than 10 km in depth, suggesting that transform faults serve as a significant pathway for water to enter the oceanic upper mantle.
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PreprintDiapirs as the source of the sediment signature in arc lavas( 2011-05-31) Behn, Mark D. ; Kelemen, Peter B. ; Hirth, Greg ; Hacker, Bradley R. ; Massonne, Hans-JoachimMany arc lavas show evidence for the involvement of subducted sediment in the melting process. There is debate whether this “sediment melt” signature forms at relatively low temperature near the fluid-saturated solidus or at higher temperature beyond the breakdown of trace-element-rich accessory minerals. We present new geochemical data from high- to ultrahigh-pressure rocks that underwent subduction and show no significant depletion of key trace elements in the sediment melt component until peak metamorphic temperatures exceeded ~1050ºC from 2.7 to 5 GPa. These temperatures are higher than for the top of the subducting plate at similar pressures based on thermal models. To address this discrepancy, we use instability calculations for a non-Newtonian buoyant layer in a viscous half-space to show that, in typical subduction zones, solid-state sediment diapirs initiate at temperatures between 500–850ºC. Based on these calculations, we propose that the sediment melt component in arc magmas is produced by high degrees of dehydration melting in buoyant diapirs of metasediment that detach from the slab and rise into the hot mantle wedge. Efficient recycling of sediments into the wedge by this mechanism will alter volatile fluxes into the deep mantle compared to estimates based solely on devolatilization of the slab.
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ArticleThermal structure of oceanic transform faults(Geological Society of America, 2007-04) Behn, Mark D. ; Boettcher, Margaret S. ; Hirth, GregWe use three-dimensional finite element simulations to investigate the temperature structure beneath oceanic transform faults. We show that using a rheology that incorporates brittle weakening of the lithosphere generates a region of enhanced mantle upwelling and elevated temperatures along the transform; the warmest temperatures and thinnest lithosphere are predicted to be near the center of the transform. Previous studies predicted that the mantle beneath oceanic transform faults is anomalously cold relative to adjacent intraplate regions, with the thickest lithosphere located at the center of the transform. These earlier studies used simplified rheologic laws to simulate the behavior of the lithosphere and underlying asthenosphere. We show that the warmer thermal structure predicted by our calculations is directly attributed to the inclusion of a more realistic brittle rheology. This temperature structure is consistent with a wide range of observations from ridge-transform environments, including the depth of seismicity, geochemical anomalies along adjacent ridge segments, and the tendency for long transforms to break into small intratransform spreading centers during changes in plate motion.
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PreprintImplications of grain size evolution on the seismic structure of the oceanic upper mantle( 2009-03-04) Behn, Mark D. ; Hirth, Greg ; Elsenbeck, James R.We construct a 1-D steady-state channel flow model for grain size evolution in the oceanic upper mantle using a composite diffusion-dislocation creep rheology. Grain size evolution is calculated assuming that grain size is controlled by a competition between dynamic recrystallization and grain growth. Applying this grain size evolution model to the oceanic upper mantle we calculate grain size as a function of depth, seafloor age, and mantle water content. The resulting grain size structure is used to predict shear wave velocity (VS) and seismic quality factor (Q). For a plate age of 60 Myr and an olivine water content of 1000 H/106Si, we find that grain size reaches a minimum of ~15 mm at ~150 km depth and then increases to ~20–30 mm at a depth of 400 km. This grain size structure produces a good fit to the low seismic shear wave velocity zone (LVZ) in oceanic upper mantle observed by surface wave studies assuming that the influence of hydrogen on anelastic behavior is similar to that observed for steady state creep. Further it predicts a viscosity of ~1019 Pa s at 150 km depth and dislocation creep to be the dominant deformation mechanism throughout the oceanic upper mantle, consistent with geophysical observations. We predict larger grain sizes than proposed in recent studies, in which the LVZ was explained by a dry mantle and a minimum grain size of 1 mm. However, we show that for a 1 mm grain size, diffusion creep is the dominant deformation mechanism above 100– 200 km depth, inconsistent with abundant observations of seismic anisotropy from surface wave studies. We therefore conclude that a combination of grain size evolution and a hydrated upper mantle is the most likely explanation for both the isotropic and anisotropic seismic structure of the oceanic upper mantle. Our results also suggest that melt extraction from the mantle will be significantly more efficient than predicted in previous modeling studies that assumed grain sizes of ~1 mm.
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ArticleUsing short-term postseismic displacements to infer the ambient deformation conditions of the upper mantle(American Geophysical Union, 2012-01-31) Freed, Andrew M. ; Hirth, Greg ; Behn, Mark D.To interpret short-term postseismic surface displacements in the context of key ambient conditions (e.g., temperature, pressure, background strain rate, water content, creep mechanism), we combined steady state and transient flow into a single constitutive relation that can explain the response of a viscoelastic material to a change in stress. The flow law is then used to investigate mantle deformation beneath the Eastern California Shear Zone following the 1999 M7.1 Hector Mine earthquake. The flow law parameters are determined using finite element models of relaxation processes, constrained by surface displacement time series recorded by 55 continuous GPS stations for 7 years following the earthquake. Results suggest that postseismic flow following the Hector Mine earthquake occurs below a depth of ~50 km and is controlled by dislocation creep of wet olivine. Diffusion creep models can also explain the data, but require a grain size (3.5 mm) that is smaller than the inferred grain size (10–20 mm) based on the mantle conditions at these depths. In addition, laboratory flow laws predict dislocation creep would dominate at the stress/grain size conditions that provide the best fit to diffusion creep models. Model results suggest a transient creep phase that lasts ~1 year and has a viscosity ~10 times lower than subsequent steady state flow, in general agreement with laboratory observations. The postseismic response is best explained as occurring within a relatively hot upper mantle (e.g., 1200–1300°C at 50 km depth) with a long-term background mantle strain rate of 0.1–0.2 μstrain/yr, consistent with the observed surface strain rate. Long-term background shear stresses at the top of the mantle are ~4 MPa, then decrease with depth to a minimum of 0.1–0.2 MPa at 70 km depth before increasing slowly with depth due to the pressure dependence of viscosity. These conditions correspond to a background viscosity of 1021 Pa s within a thin mantle lid that decreases to ~5 × 1019 Pa s within the underlying asthenosphere. This study shows the utility of using short-term postseismic observations to infer long-term mantle conditions that are not readily observable by other means.