Behn Mark D.

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Mark D.

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  • Article
    Relationship between Greenland Ice Sheet surface speed and modeled effective pressure
    (John Wiley & Sons, 2018-09-27) 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 ~8,000‐km2 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.
  • Preprint
    Mid-ocean ridge jumps associated with hotspot magmatism
    ( 2007-10-11) Mittelstaedt, Eric ; Ito, Garrett T. ; Behn, Mark D.
    Hotspot-ridge interaction produces a wide range of phenomena including excess crustal thickness, geochemical anomalies, off-axis volcanic ridges and ridge relocations or jumps. Ridges are recorded to have jumped toward many hotspots including, Iceland, Discovery, Galapagos, Kerguelen and Tristan de Cuhna. The causes of ridge jumps likely involve a number of interacting processes related to hotspots. One such process is reheating of the lithosphere as magma penetrates it to feed near-axis volcanism. We study this effect by using the hybrid, finite-element code, FLAC, to simulate two-dimensional (2-D, cross-section) viscous mantle flow, elasto-plastic deformation of the lithosphere and heat transport in a ridge setting near an off-axis hotspot. Heating due to magma transport through the lithosphere is implemented within a hotspot region of fixed width. To determine the conditions necessary to initiate a ridge jump, we vary four parameters: hotspot magmatic heating rate, spreading rate, seafloor age at the location of the hotspot and ridge migration rate. Our results indicate that the hotspot magmatic heating rate required to initiate a ridge jump increases non-linearly with increasing spreading rate and seafloor age. Models predict that magmatic heating, itself, is most likely to cause jumps at slow spreading rates such as at the Mid-Atlantic Ridge on Iceland. In contrast, despite the higher magma flux at the Galapagos hotspot, magmatic heating alone is probably insufficient to induce a ridge jump at the present-day due to the intermediate ridge spreading rate of the Galapagos Spreading Center. The time required to achieve a ridge jump, for fixed or migrating ridges, is found to be on the order of 105-106 years. Simulations that incorporate ridge migration predict that after a ridge jump occurs the hotspot and ridge migrate together for time periods that increase with magma flux. Model results also suggest a mechanism for ridge reorganizations not related to hotspots such as ridge jumps in back-arc settings and ridge segment propagation along the Mid-Atlantic Ridge.
  • Article
    Role of melt supply in oceanic detachment faulting and formation of megamullions
    (Geological Society of America, 2008-06) Tucholke, Brian E. ; Behn, Mark D. ; Buck, W. Roger ; Lin, Jian
    Normal faults are ubiquitous on mid-ocean ridges and are expected to develop increasing offset with reduced spreading rate as the proportion of tectonic extension increases. Numerous long-lived detachment faults that form megamullions with large-scale corrugations have been identified on magma-poor mid-ocean ridges, but recent studies suggest, counterintuitively, that they may be associated with elevated magmatism. We present numerical models and geological data to show that these detachments occur when ~30%–50% of total extension is accommodated by magmatic accretion and that there is significant magmatic accretion in the fault footwalls. Under these low-melt conditions, magmatism may focus unevenly along the spreading axis to create an irregular brittle-plastic transition where detachments root, thus explaining the origin of the enigmatic corrugations. Morphological and compositional characteristics of the oceanic lithosphere suggested by this study provide important new constraints to assess the distribution of magmatic versus tectonic extension along mid-ocean ridges.
  • Preprint
    Spatio-temporal evolution of strain accumulation derived from multi-scale observations of Late Jurassic rifting in the northern North Sea : a critical test of models for lithospheric extension
    ( 2005-01-23) Cowie, Patience A. ; Underhill, John R. ; Behn, Mark D. ; Lin, Jian ; Gill, Caroline E.
    We integrate observations of lithospheric extension over a wide range of spatial and temporal scales within the northern North Sea basin and critically review the extent to which existing theories of lithospheric deformation can account for these observations. Data obtained through a prolonged period of hydrocarbon exploration and production has yielded a dense and diverse data set over the entire Viking Graben and its flanking platform areas. These data show how syn-rift accommodation within the basin varied in space and time with sub-kilometer-scale spatial resolution and a temporal resolution of 2–3 Myr. Regional interpretations of 2D seismic reflection, refraction and gravity data for this area have also been published and provide an image of total basin wide stretching for the entire crust. These image data are combined with published strain rate inversion results obtained from tectonic subsidence patterns to constrain the spatio-temporal evolution of strain accumulation throughout the lithosphere during the 40 Myr (170–130 Ma) period of Late Jurassic extension across this basin. For the first 25–30 Myr, strain localisation dominated basin development with strain rates at the eventual rift axis increasing while strain rates over the flanking areas declined. As strain rates across the whole basin were consistently very low (< 3 × 10- 16 s- l), thermally induced strength loss cannot explain this phenomenon. The strain localisation is manifest in the near-surface by a systematic migration of fault activity. The pattern and timing of this migration are inconsistent with flexural bending stresses exerting an underlying control, especially when estimates of flexural rigidity for this area are considered. The best explanation for what is observed in this time period is a coupling between near-surface strain localisation, driven by brittle (or plastic) failure, and the evolving thermal structure of the lithosphere. We demonstrate this process using a continuum mechanics model for normal fault growth that incorporates the strain rate-dependence of frictional strength observed in laboratory studies. During the final 10 Myr of basin formation, strain accumulation was focused within the axis and strain rates declined rapidly. Replacement of weak crust by stronger mantle material plus crustal buoyancy forces can adequately explain this decline.
  • Article
    Magmatic and tectonic extension at mid-ocean ridges : 1. Controls on fault characteristics
    (American Geophysical Union, 2008-08-02) Behn, Mark D. ; Ito, Garrett T.
    We use 2-D numerical models to explore the thermal and mechanical effects of magma intrusion on fault initiation and growth at slow and intermediate spreading ridges. Magma intrusion is simulated by widening a vertical column of model elements located within the lithosphere at a rate equal to a fraction, M, of the total spreading rate (i.e., M = 1 for fully magmatic spreading). Heat is added in proportion to the rate of intrusion to simulate the thermal effects of magma crystallization and the injection of hot magma into the crust. We examine a range of intrusion rates and axial thermal structures by varying M, spreading rate, and the efficiency of crustal cooling by conduction and hydrothermal circulation. Fault development proceeds in a sequential manner, with deformation focused on a single active normal fault whose location alternates between the two sides of the ridge axis. Fault spacing and heave are primarily sensitive to M and secondarily sensitive to axial lithosphere thickness and the rate that the lithosphere thickens with distance from the axis. Contrary to what is often cited in the literature, but consistent with prior results of mechanical modeling, we find that thicker axial lithosphere tends to reduce fault spacing and heave. In addition, fault spacing and heave are predicted to increase with decreasing rates of off-axis lithospheric thickening. The combination of low M, particularly when M approaches 0.5, as well as a reduced rate of off-axis lithospheric thickening produces long-lived, large-offset faults, similar to oceanic core complexes. Such long-lived faults produce a highly asymmetric axial thermal structure, with thinner lithosphere on the side with the active fault. This across-axis variation in thermal structure may tend to stabilize the active fault for longer periods of time and could concentrate hydrothermal circulation in the footwall of oceanic core complexes.