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Now showing items 1 - 16 of 2149

  • Archean crustal compositions promote full mantle convection

    Klein, Benjamin Z.   Jagoutz, Oliver   Behn, Mark D.  

    Higher mantle potential temperatures characterized the early Earth, resulting in thicker, more mafic oceanic crust entering subduction systems. This change in the composition of subducted slabs, combined with the enhanced temperature contrast between the slab and ambient mantle, would have altered the buoyancy forces driving subduction in the early Earth. Here we investigate this "compositional effect" through a combination of petrologic and thermal modeling. Specifically, we construct density profiles for sinking slabs under modern and early Earth conditions based on a range of mafic crust and mantle compositions. Slab and mantle densities are then determined from mineral assemblages calculated using the thermodynamic modeling program Perple_X along slab geotherms estimated from an analytic thermal model. Consistent with previous studies, we find that modern MORB compositions are typically less dense than the ambient mantle in the basalt barrier zone, located immediately beneath the mantle transition zone. By contrast, possible early Earth oceanic crust compositions are denser than ambient mantle at all depths down to 1000 km. This compositional effect results in slabs that would have more readily penetrated the transition zone, promoting single-layered convection and effective mantle mixing in the early Earth. (C) 2017 Elsevier B.V. All rights reserved.
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  • Thermal segmentation of mid-ocean ridge-transform faults

    Wolfson-Schwehr, Monica   Boettcher, Margaret S.   Behn, Mark D.  

    3-D finite element simulations are used to calculate thermal structures and mantle flow fields underlying mid-ocean ridge-transform faults (RTFs) composed of two fault segments separated by an orthogonal step over. Using fault lengths and slip rates, we derive an empirical scaling relation for the critical step over length ( LS), which marks the transition from predominantly horizontal to predominantly vertical mantle flow at the base of the lithosphere under a step over. Using the ratio of step over length (L-S) to LS, we define three degrees of segmentation: first-degree, corresponding to type I step overs ( LS/L <1). In first-degree segmentation, thermal structures and mantle upwelling patterns under a step over are similar to those of mature ridges, where normal mid-ocean ridge basalts (MORBs) form. The seismogenic area under first-degree segmentation is characteristic of two, isolated faults. Second-degree segmentation creates pull-apart basins with subdued melt generation, and intratransform spreading centers with enriched MORBs. The seismogenic area of RTFs under second-degree segmentation is greater than that of two isolated faults, but less than that of an unsegmented RTF. Under third-degree segmentation, mantle flow is predominantly horizontal, resulting in little lithospheric thinning and little to no melt generation. The total seismogenic area under third-degree segmentation approaches that of an unsegmented RTF. Our scaling relations characterize the degree of segmentation due to step overs along transform faults and provide insight into RTF frictional processes, seismogenic behavior, and melt transport. Plain Language Summary Mid-ocean ridge-transform faults (i.e., strike-slip faults that accommodate lateral motion associated with seafloor spreading) are typically viewed as geometrically simple structures, where a continuous fault is located between two spreading ridges. However, high-resolution seafloor mapping has shown that the structure of these fault systems is often quite complex. Mid-ocean ridge-transform faults may be composed of two or more individual fault strands separated by a step over. Using results of numerical simulations, we show that the form of the step over is expected to vary systematically from small extensional basins to active spreading ridge segments, depending on the length of the step over, the length of the adjacent fault segments, and the plate tectonic spreading rate. Additionally, our results suggest that the chemistry of mid-ocean ridge basalts produced at the step over is expected to change systematically with the step over length. This work shows that while the structure of transform faults can significantly affect the thermal structure of the region, it can readily be determined from regional plate tectonic parameters (fault lengths, step over length, and spreading rate). Furthermore, this work provides key insights into frictional processes, seismic behavior, and melt transport along oceanic transform plate boundary faults.
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  • Constraints on the composition of the Aleutian arc lower crust from V-P/V-S

    Shillington, Donna J.   Van Avendonk, Harm J. A.   Behn, Mark D.   Kelemen, Peter B.   Jagoutz, Oliver  

    Determining the bulk composition of island arc lower crust is essential for distinguishing between competing models for arc magmatism and assessing the stability of arc lower crust. We present new constraints on the composition of high P-wave velocity (V-P=7.3-7.6 km/s) lower crust of the Aleutian arc from best-fitting average lower crustal V-P/V-S ratio using sparse converted S-waves from an along-arc refraction profile. We find a low V-P/V-S of similar to 1.7-1.75. Using petrologic modeling, we show that no single composition is likely to explain the combination of high V-P and low V-P/V-S. Our preferred explanation is a combination of clinopyroxenite (similar to 50-70%) and alpha-quartz bearing gabbros (similar to 30-50%). This is consistent with Aleutian xenoliths and lower crustal rocks in obducted arcs, and implies that similar to 30-40% of the full Aleutian crust comprises ultramafic cumulates. These results also suggest that small amounts of quartz can exert a strong influence on V-P/V-S in arc crust.
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  • Differentiation of the continental crust by relamination RID E-7750-2011

    Hacker, Bradley R.   Kelemen, Peter B.   Behn, Mark D.  

    Crust extracted from the mantle in arcs is refined into continental crust in subduction zones. During sediment subduction, subduction erosion, arc subduction, and continent subduction, mafic rocks become eclogite and may sink into the mantle, whereas more silica-rich rocks are transformed into felsic gneisses that are less dense than peridotite but more dense than the upper crust. These more felsic rocks rise buoyantly, undergo decompression melting and melt extraction, and are relaminated to the base of the crust. As a result of this process, such felsic rocks could form much of the lower crust. The lower crust need not be mafic and the bulk continental crust may be more silica rich than generally considered. (C) 2011 Elsevier B.V. All rights reserved.
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  • Diapirs as the source of the sediment signature in arc lavas

    Behn, Mark D.   Kelemen, Peter B.   Hirth, Greg   Hacker, Bradley R.   Massonne, Hans-Joachim  

    Island arc lavas, erupted above subduction zones, commonly contain a geochemical component derived from partial melting of subducted sediment. It is debated whether this sediment melt signature, with enriched trace element concentrations and isotope ratios, forms at relatively low or high temperatures. Here we compile and analyse the geochemistry of metamorphosed sedimentary rocks that have been exposed to pressures between 2.7 and 5 GPa during subduction at a range of locations worldwide. We find that the trace elements that form the sediment melt signature are retained in the sediments until the rocks have experienced temperatures exceeding 1,050 degrees C. According to thermal models, these temperatures are much higher than those at the surface of subducted slabs at similar pressures. This implies that the sediment melt signature cannot form at the slab surface. Using instability calculations, we show that subducted sediments detach from the downgoing slab at temperatures of 500-850 degrees C to form buoyant diapirs. The diapirs rise through the overlying hot mantle wedge, where temperatures exceed 1,050 degrees C, undergo dehydration melting, and release the trace elements that later form the sediment melt signature in the erupted lavas. We conclude that sediment diapirism may reduce the transport of trace elements and volatiles such as CO(2) into the deep mantle.
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  • Grain-size distribution in the mantle wedge of subduction zones

    Wada, Ikuko   Behn, Mark D.   He, Jiangheng  

    Mineral grain size plays an important role in controlling many processes in the mantle wedge of subduction zones, including mantle flow and fluid migration. To investigate the grain-size distribution in the mantle wedge, we coupled a two-dimensional (2-D) steady state finite element thermal and mantle-flow model with a laboratory-derived grain-size evolution model. In our coupled model, the mantle wedge has a composite olivine rheology that incorporates grain-size-dependent diffusion creep and grain-size-independent dislocation creep. Our results show that all subduction settings lead to a characteristic grain-size distribution, in which grain size increases from 10 to 100 mu m at the most trenchward part of the creeping region to a few centimeters in the subarc mantle. Despite the large variation in grain size, its effect on the mantle rheology and flow is very small, as >90% of the deformation in the flowing part of the creeping region is accommodated by grain-size-independent dislocation creep. The predicted grain-size distribution leads to a downdip increase in permeability by similar to 5 orders of magnitude. This increase is likely to promote greater upward migration of aqueous fluids and melts where the slab reaches similar to 100 km depth compared with shallower depths, potentially providing an explanation for the relatively uniform subarc slab depth. Seismic attenuation derived from the predicted grain-size distribution and thermal field is consistent with the observed seismic structure in the mantle wedge at many subduction zones, without requiring a significant contribution by the presence of melt.
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  • Cellular convection in a chamber with a warm surface raft (vol 23, 104103, 2011)

    Whitehead, J. A.   Shea, Erin   Behn, Mark D.  

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  • Relationship Between Greenland Ice Sheet Surface Speed and Modeled Effective Pressure

    Stevens, Laura A.   Hewitt, Ian J.   Das, Sarah B.   Behn, Mark D.  

    We use a numerical subglacial hydrology model and remotely sensed observations of Greenland Ice Sheet surface motion to test whether the inverse relationship between effective pressure and regional melt season surface speeds observed at individual sites holds on a regional scale. The model is forced with daily surface runoff estimates for 2009 and 2010 across an similar to 8,000-km(2) region on the western margin. The overall subglacial drainage system morphology develops similarly in both years, with subglacial channel networks growing inland from the ice sheet margin and robust subglacial pathways forming over bedrock ridges. Modeled effective pressures are compared to contemporaneous regional surface speeds derived from TerraSAR-X imagery to investigate spatial relationships. Our results show an inverse spatial relationship between effective pressure and ice speed in the mid-melt season, when surface speeds are elevated, indicating that effective pressure is the dominant control on surface velocities in the mid-melt season. By contrast, in the early and late melt seasons, when surface speeds are slower, effective pressure and surface speed have a positive relationship. Our results suggest that outside of the mid-melt season, the influence of effective pressures on sliding speeds may be secondary to the influence of driving stress and spatially variable bed roughness.
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  • Formation of lower continental crust by relamination of buoyant arc lavas and plutons

    Kelemen, Peter B.   Behn, Mark D.  

    The formation of the Earth's continents is enigmatic. Volcanic arc magmas generated above subduction zones have geochemical compositions that are similar to continental crust, implying that arc magmatic processes played a central role in generating continental crust. Yet the deep crust within volcanic arcs has a very different composition from crust at similar depths beneath the continents. It is therefore unclear how arc crust is transformed into continental crust. The densest parts of arc lower crust may delaminate and become recycled into the underlying mantle. Here we show, however, that even after delamination, arc lower crust still has significantly different trace element contents from continental lower crust. We suggest that it is not delamination that determines the composition of continental crust, but relamination. In our conceptual model, buoyant magmatic rocks generated at arcs are subducted. Then, upon heating at depth, they ascend and are relaminated at the base of the overlying crust. A review of the average compositions of buoyant magmatic rocks - lavas and plutons - sampled from the Aleutians, Izu-Bonin-Marianas, Kohistan and Talkeetna arcs reveals that they fall within the range of estimated major and trace elements in lower continental crust. Relamination may thus provide an efficient process for generating lower continental crust.
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  • Response to comment on "intermittent plate tectonics?"

    Silver, Paul G.   Behn, Mark D.  

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  • Intermittent plate tectonics?

    Silver, Paul G.   Behn, Mark D.  

    Although it is commonly assumed that subduction has operated continuously on Earth without interruption, subduction zones are routinely terminated by ocean closure and supercontinent assembly. Under certain circumstances, this could lead to a dramatic loss of subduction, globally. Closure of a Pacific- type basin, for example, would eliminate most subduction, unless this loss were compensated for by comparable subduction initiation elsewhere. Given the evidence for Pacific- type closure in Earth's past, the absence of a direct mechanism for termination/ initiation compensation, and recent data supporting a minimum in subduction flux in the Mesoproterozoic, we hypothesize that dramatic reductions or temporary cessations of subduction have occurred in Earth's history. Such deviations in the continuity of plate tectonics have important consequences for Earth's thermal and continental evolution.
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  • Thermal structure of oceanic transform faults

    Behn, Mark D.   Boettcher, Margaret S.   Hirth, Greg  

    We 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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  • Trench-parallel anisotropy produced by foundering of arc lower crust

    Behn, Mark D.   Hirth, Greg   Kelemen, Peter B.  

    Many volcanic arcs display fast seismic shear-wave velocities parallel to the strike of the trench. This pattern of anisotropy is inconsistent with simple models of corner flow in the mantle wedge. Although several models, including slab rollback, oblique subduction, and deformation of water-rich olivine, have been proposed to explain trench-parallel anisotropy, none of these mechanisms are consistent with all observations. Instead, small-scale convection driven by the foundering of dense arc lower crust provides an explanation for the trench-parallel anisotropy, even in settings with orthogonal convergence and no slab rollback.
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  • Spreading rate dependence of gravity anomalies along oceanic transform faults

    Gregg, Patricia M.   Lin, Jian   Behn, Mark D.   Montesi, Laurent G. J.  

    Mid-ocean ridge morphology and crustal accretion are known to depend on the spreading rate of the ridge. Slow-spreading mid-ocean-ridge segments exhibit significant crustal thinning towards transform and non-transform offsets(1-12), which is thought to arise from a three-dimensional process of buoyant mantle upwelling and melt migration focused beneath the centres of ridge segments(1,2,4-7,9,10,12). In contrast, fast-spreading mid-ocean ridges are characterized by smaller, segment-scale variations in crustal thickness, which reflect more uniform mantle upwelling beneath the ridge axis(13-15). Here we present a systematic study of the residual mantle Bouguer gravity anomaly of 19 oceanic transform faults that reveals a strong correlation between gravity signature and spreading rate. Previous studies have shown that slow-slipping transform faults are marked by more positive gravity anomalies than their adjacent ridge segments(1,2,4,6), but our analysis reveals that intermediate and fast-slipping transform faults exhibit more negative gravity anomalies than their adjacent ridge segments. This finding indicates that there is a mass deficit at intermediate- and fast-slipping transform faults, which could reflect increased rock porosity, serpentinization of mantle peridotite, and/or crustal thickening. The most negative anomalies correspond to topographic highs flanking the transform faults, rather than to transform troughs (where deformation is probably focused and porosity and alteration are expected to be greatest), indicating that crustal thickening could be an important contributor to the negative gravity anomalies observed. This finding in turn suggests that three-dimensional magma accretion may occur near intermediate- and fast-slipping transform faults.
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  • A community benchmark for subduction zone modeling RID B-1220-2008 RID D-3364-2009

    van Keken, Peter E.   Currie, Claire   King, Scott D.   Behn, Mark D.   Cagnioncle, Amandine   He, Jiangheng   Katz, Richard F.   Lin, Shu-Chuan   Parmentier, E. Marc   Spiegelman, Marc   Wang, Kelin  

    We have developed a suite of benchmarks to facilitate the comparison of numerical models for the dynamics and thermal structure of subduction zones. The benchmark cases are based on a thermomechanical approach in which the slab is prescribed kinematically and the wedge flow is computed dynamically. We propose various cases to investigate the influence of boundary conditions and rheology on wedge flow and resulting thermal structure. A comparison between the codes suggest that accurate modeling of the thermal field requires a good implementation of the velocity discontinuity along the seismogenic zone and high resolution in the thermal boundary layers. A minor modification to the boundary conditions of the wedge flow is also necessary to avoid a pressure singularity that exists in analytical solutions of the cornerflow model. (C) 2008 Elsevier B.V. All rights reserved.
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  • Spreading rate-dependent variations in crystallization along the global mid-ocean ridge system

    Wanless, V. Dorsey   Behn, Mark D.  

    We investigate crustal accretion at mid-ocean ridges by combining crystallization pressures calculated from major element contents in mid-ocean ridge basalt (MORB) glasses and vapor-saturation pressures from melt inclusions and MORB glasses. Specifically, we use established major element barometers and pressures estimated from 192 fractional crystallization trends to calculate crystallization pressures from >9000 MORB glasses across the global range of mid-ocean ridge spreading rates. Additionally, we estimate vapor-saturation pressures from >400 MORB glasses from PETDB and >400 olivine-hosted melt inclusions compiled from five ridges with variable spreading rates. Both major element and vapor-saturation pressures increase and become more variable with decreasing spreading rate. Vapor saturation pressures indicate that crystallization occurs in the lower crust and upper mantle at all ridges, even when a melt lens is present. We suggest that the broad peaks in major element crystallization pressures at all spreading rates reflects significant crystallization of on and off-axis magmas along the base of a sloping lithosphere. Combining our observations with ridge thermal models we show that crystallization occurs over a range of pressures at all ridges, but it is enhanced at thermal/rheologic boundaries, such as the melt lens and the base of the lithosphere. Finally, we suggest that the remarkable similarity in the maximum vapor-saturation pressures (approximate to 3 kbars) recorded in melt inclusions from a wide range of spreading rates reflects a relatively uniform CO2 content of 50-85 ppm for the depleted upper mantle feeding the global mid-ocean ridge system.
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