Klein Benjamin Z.

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Last Name
Klein
First Name
Benjamin Z.
ORCID
0000-0001-5206-3490

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  • Article
    The role of elasticity in simulating long-term tectonic extension
    (Oxford University Press, 2016-01-27) Olive, Jean-Arthur ; Behn, Mark D. ; Mittelstaedt, Eric ; Ito, Garrett T. ; Klein, Benjamin Z.
    While elasticity is a defining characteristic of the Earth's lithosphere, it is often ignored in numerical models of long-term tectonic processes in favour of a simpler viscoplastic description. Here we assess the consequences of this assumption on a well-studied geodynamic problem: the growth of normal faults at an extensional plate boundary. We conduct 2-D numerical simulations of extension in elastoplastic and viscoplastic layers using a finite difference, particle-in-cell numerical approach. Our models simulate a range of faulted layer thicknesses and extension rates, allowing us to quantify the role of elasticity on three key observables: fault-induced topography, fault rotation, and fault life span. In agreement with earlier studies, simulations carried out in elastoplastic layers produce rate-independent lithospheric flexure accompanied by rapid fault rotation and an inverse relationship between fault life span and faulted layer thickness. By contrast, models carried out with a viscoplastic lithosphere produce results that may qualitatively resemble the elastoplastic case, but depend strongly on the product of extension rate and layer viscosity U × ηL. When this product is high, fault growth initially generates little deformation of the footwall and hanging wall blocks, resulting in unrealistic, rigid block-offset in topography across the fault. This configuration progressively transitions into a regime where topographic decay associated with flexure is fully accommodated within the numerical domain. In addition, high U × ηL favours the sequential growth of multiple short-offset faults as opposed to a large-offset detachment. We interpret these results by comparing them to an analytical model for the fault-induced flexure of a thin viscous plate. The key to understanding the viscoplastic model results lies in the rate-dependence of the flexural wavelength of a viscous plate, and the strain rate dependence of the force increase associated with footwall and hanging wall bending. This behaviour produces unrealistic deformation patterns that can hinder the geological relevance of long-term rifting models that assume a viscoplastic rheology.
  • Article
    The ascent of subduction zone mélanges: Experimental constraints on mélange rock densities and solidus temperatures
    (Elsevier, 2023-09-27) Codillo, Emmanuel A. ; Le Roux, Veronique ; Klein, Benjamin Z. ; Behn, Mark D. ; Marschall, Horst R. ; Bebout, Gray E.
    Mélanges are mixtures of subducted materials and serpentinized mantle rocks that form along the slab-mantle interface in subduction zones. It has been suggested that mélange rocks may be able to ascend from the slab-top into the overlying mantle, as solid or partially molten buoyant diapirs, and transfer their compositional signatures to the source regions of arc magmas. However, their ability to buoyantly rise is in part tied to their phase equilibria during melting and residual densities after melt extraction, all of which are poorly constrained. Here, we report a series of piston-cylinder experiments performed at 1.5–2.5 GPa and 500–1050 °C on three natural mélange rocks that span a range of mélange compositions. Using phase equilibria, solidus temperatures, and densities for all experiments, we show that melting of mélanges is unlikely to occur along the slab-top at pressures ≤ 2.5 GPa, so that diapirism into the hotter mantle wedge would be required for melting to initiate. For the two metaluminous mélange compositions, diapir formation is favored up to pressures of at least 2.5 GPa. For the peraluminous mélange composition investigated, diapir buoyancy is possible at 1.5 GPa but limited at 2.5 GPa due to the formation of high-density garnet, primarily at the expense of chlorite. We also evaluate whether thermodynamic modeling (Perple_X) can accurately reproduce the phase equilibria, solidus temperatures, and density evolution of mélange compositions. Our analysis shows good agreement between models and experiments in mélange compositions with low initial water contents and low-pressure (≤ 1.5 GPa) conditions. However, discrepancies between the thermodynamic models and experiments become larger at higher pressures and high-water contents, highlighting the need for an improved thermodynamic database that can model novel bulk compositions beyond the canonical subducting lithologies. This study provides experimental constraints on mélange buoyancy that can inform numerical models of mélange diapirism and influence the interpretations of both geophysical signals and geochemical characteristics of magmas in subduction zones.