Journal articles: 'Mantle weakening'

1

Muir, Joshua M. R., and John P. Brodholt. "Water distribution in the lower mantle: Implications for hydrolytic weakening." Earth and Planetary Science Letters 484 (February 2018): 363–69. http://dx.doi.org/10.1016/j.epsl.2017.11.051.

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Chen, Jiuhua, Toru Inoue, Donald J. Weidner, Yujun Wu, and Michael T. Vaughan. "Strength and water weakening of mantle minerals, olivine, wadsleyite and ringwoodite." Geophysical Research Letters 25, no. 4 (February 15, 1998): 575–78. http://dx.doi.org/10.1029/98gl00043.

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Mohiuddin, Anwar, Shun-ichiro Karato, and Jennifer Girard. "Slab weakening during the olivine to ringwoodite transition in the mantle." Nature Geoscience 13, no. 2 (January 20, 2020): 170–74. http://dx.doi.org/10.1038/s41561-019-0523-3.

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Liao, Jie, Qin Wang, Taras Gerya, and Maxim D. Ballmer. "Modeling Craton Destruction by Hydration-Induced Weakening of the Upper Mantle." Journal of Geophysical Research: Solid Earth 122, no. 9 (September 2017): 7449–66. http://dx.doi.org/10.1002/2017jb014157.

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Bercovici, David, and Elvira Mulyukova. "Evolution and demise of passive margins through grain mixing and damage." Proceedings of the National Academy of Sciences 118, no. 4 (January 19, 2021): e2011247118. http://dx.doi.org/10.1073/pnas.2011247118.

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How subduction—the sinking of cold lithospheric plates into the mantle—is initiated is one of the key mysteries in understanding why Earth has plate tectonics. One of the favored locations for subduction triggering is at passive margins, where sea floor abuts continental margins. Such passive margin collapse is problematic because the strength of the old, cold ocean lithosphere should prohibit it from bending under its own weight and sinking into the mantle. Some means of mechanical weakening of the passive margin are therefore necessary. Spontaneous and accumulated grain damage can allow for considerable lithospheric weakening and facilitate passive margin collapse. Grain damage is enhanced where mixing between mineral phases in lithospheric rocks occurs. Such mixing is driven both by compositional gradients associated with petrological heterogeneity and by the state of stress in the lithosphere. With lateral compressive stress imposed by ridge push in an opening ocean basin, bands of mixing and weakening can develop, become vertically oriented, and occupy a large portion of lithosphere after about 100 million y. These bands lead to anisotropic viscosity in the lithosphere that is strong to lateral forcing but weak to bending and sinking, thereby greatly facilitating passive margin collapse.

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Chen, Jiuhua, Toru Inoue, Donald J. Weidner, Yujun Wu, and Michael T. Vaughan. "Correction to “Strength and water weakening of mantle minerals, olivine, wadsleyite and ringwoodite”." Geophysical Research Letters 25, no. 7 (April 1, 1998): 1103–4. http://dx.doi.org/10.1029/98gl00733.

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Hirauchi, Ken-ichi, Sabine A. M. den Hartog, and Christopher J. Spiers. "Weakening of the slab–mantle wedge interface induced by metasomatic growth of talc." Geology 41, no. 1 (January 2013): 75–78. http://dx.doi.org/10.1130/g33552.1.

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Okuda, Hanaya, Ikuo Katayama, Hiroshi Sakuma, and Kenji Kawai. "Effect of normal stress on the frictional behavior of brucite: application to slow earthquakes at the subduction plate interface in the mantle wedge." Solid Earth 12, no. 1 (January 25, 2021): 171–86. http://dx.doi.org/10.5194/se-12-171-2021.

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Abstract. We report the results of friction experiments on brucite under both dry and wet conditions under various normal stresses (10–60 MPa). The final friction coefficients of brucite were determined to be 0.40 and 0.26 for the dry and wet cases, respectively, independent of the normal stress. Under dry conditions, velocity-weakening behavior was observed in all experiments at various normal stresses. Under wet conditions, velocity weakening was observed at low normal stress (10 and 20 MPa), whereas velocity strengthening was determined at a higher applied normal stress. Microstructural observations of recovered experimental samples indicate localized deformation within a narrow shear band, implying that a small volume of brucite can control the bulk frictional strength in an ultramafic setting. Among serpentinite-related minerals, weak and unstable frictional behavior of brucite under hydrated mantle wedge conditions may play a role in slow earthquakes at the subduction plate interface in the mantle wedge.

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Pysklywec, Russell N., and Christopher Beaumont. "Intraplate tectonics: feedback between radioactive thermal weakening and crustal deformation driven by mantle lithosphere instabilities." Earth and Planetary Science Letters 221, no. 1-4 (April 30, 2004): 275–92. http://dx.doi.org/10.1016/s0012-821x(04)00098-6.

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Wallner, Herbert, and Harro Schmeling. "Numerical models of mantle lithosphere weakening, erosion and delamination induced by melt extraction and emplacement." International Journal of Earth Sciences 105, no. 6 (June 7, 2016): 1741–60. http://dx.doi.org/10.1007/s00531-016-1343-y.

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Précigout, Jacques, and Frédéric Gueydan. "Mantle weakening and strain localization: Implications for the long-term strength of the continental lithosphere." Geology 37, no. 2 (February 1, 2009): 147–50. http://dx.doi.org/10.1130/g25239a.1.

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Wolstencroft, Martin, and J. Huw Davies. "Breaking supercontinents; no need to choose between passive or active." Solid Earth 8, no. 4 (August 7, 2017): 817–25. http://dx.doi.org/10.5194/se-8-817-2017.

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Abstract. Much debate has centred on whether continental break-up is predominantly caused by active upwelling in the mantle (e.g. plumes) or by long-range extensional stresses in the lithosphere. We propose the hypothesis that global supercontinent break-up events should always involve both. The fundamental principle involved is the conservation of mass within the spherical shell of the mantle, which requires a return flow for any major upwelling beneath a supercontinent. This shallow horizontal return flow away from the locus of upwelling produces extensional stress. We demonstrate this principle with numerical models, which simultaneously exhibit both upwellings and significant lateral flow in the upper mantle. For non-global break-up the impact of the finite geometry of the mantle will be less pronounced, weakening this process. This observation should motivate future studies of continental break-up to explicitly consider the global perspective, even when observations or models are of regional extent.

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13

Ni, Huaiwei, Yong-Fei Zheng, Zhu Mao, Qin Wang, Ren-Xu Chen, and Li Zhang. "Distribution, cycling and impact of water in the Earth's interior." National Science Review 4, no. 6 (October 27, 2017): 879–91. http://dx.doi.org/10.1093/nsr/nwx130.

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Abstract The Earth's deep interior is a hidden water reservoir on a par with the hydrosphere that is crucial for keeping the Earth as a habitable planet. In particular, nominally anhydrous minerals (NAMs) in the silicate Earth host a significant amount of water by accommodating H point defects in their crystal lattices. Water distribution in the silicate Earth is highly heterogeneous, and the mantle transition zone may contain more water than the upper and lower mantles. Plate subduction transports surface water to various depths, with a series of hydrous minerals and NAMs serving as water carriers. Dehydration of the subducting slab produces liquid phases such as aqueous solutions and hydrous melts as a metasomatic agent of the mantle. Partial melting of the metasomatic mantle domains sparks off arc volcanism, which, along with the volcanism at mid-ocean ridges and hotspots, returns water to the surface and completes the deep water cycle. There appears to have been a steady balance between hydration and dehydration of the mantle at least since the Phanerozoic. Earth's water probably originates from a primordial portion that survived the Moon-forming giant impact, with later delivery by asteroids and comets. Water could play a critical role in initiating plate tectonics. In the modern Earth, the storage and cycling of water profoundly modulates a variety of properties and processes of the Earth's interior, with impacts on surface environments. Notable examples include the hydrolytic weakening effect on mantle convection and plate motion, influences on phase transitions (on the solidus of mantle peridotite in particular) and dehydration embrittlement triggering intermediate- to deep-focus earthquakes. Water can reduce seismic velocity and enhance electrical conductivity, providing remote sensing methods for water distribution in the Earth's interior. Many unresolved issues around the deep water cycle require an integrated approach and concerted efforts from multiple disciplines.

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14

Newton, David E., Maya G. Kopylova, Jennifer Burgess, and Pamela Strand. "Peridotite and pyroxenite xenoliths from the Muskox kimberlite, northern Slave craton, Canada." Canadian Journal of Earth Sciences 53, no. 1 (January 2016): 41–58. http://dx.doi.org/10.1139/cjes-2015-0083.

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We present petrography, mineralogy, and thermobarometry for 53 mantle-derived xenoliths from the Muskox kimberlite pipe in the northern Slave craton. The xenolith suite includes 23% coarse peridotite, 9% porphyroclastic peridotite, 60% websterite, and 8% orthopyroxenite. Samples primarily comprise forsteritic olivine (Fo 89–94), enstatite (En 89–94), Cr-diopside, Cr-pyrope garnet, and chromite spinel. Coarse peridotites, porphyroclastic peridotites, and pyroxenites equilibrated at 650–1220 °C and 23–63 kbar (1 kbar = 100 MPa), 1200–1350 °C and 57–70 kbar, and 1030–1230 °C and 50–63 kbar, respectively. The Muskox xenoliths differ from xenoliths in the neighboring and contemporaneous Jericho kimberlite by their higher levels of depletion, the presence of a shallow zone of metasomatism in the spinel peridotite field, a higher proportion of pyroxenites at the base of the mantle column, higher Cr2O3 in all pyroxenite minerals, and weaker deformation in the Muskox mantle. We interpret these contrasts as representing small-scale heterogeneities in the bulk composition of the mantle, as well as the local effects of interaction between metasomatizing fluid and mantle wall rocks. We suggest that asthenosphere-derived pre-kimberlitic melts and fluids percolated less effectively through the less permeable Muskox mantle, resulting in lower degrees of hydrous weakening, strain, and fertilization of the peridotitic mantle. Fluids tended to concentrate and pool in the deep mantle, causing partial melting and formation of abundant pyroxenites.

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15

Fuchs, Lukas, and Thorsten W. Becker. "Role of strain-dependent weakening memory on the style of mantle convection and plate boundary stability." Geophysical Journal International 218, no. 1 (April 9, 2019): 601–18. http://dx.doi.org/10.1093/gji/ggz167.

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16

Kaislaniemi, L., J. Hunen, and Pierre Bouilhol. "Lithosphere Destabilization by Melt Weakening and Crust‐Mantle Interactions: Implications for Generation of Granite‐Migmatite Belts." Tectonics 37, no. 9 (September 2018): 3102–16. http://dx.doi.org/10.1029/2018tc005014.

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17

Dijkstra, Arjan H., Martyn R. Drury, and René M. Frijhoff. "Microstructures and lattice fabrics in the Hilti mantle section (Oman Ophiolite): Evidence for shear localization and melt weakening in the crust-mantle transition zone?" Journal of Geophysical Research: Solid Earth 107, B11 (November 2002): ETG 2–1—ETG 2–18. http://dx.doi.org/10.1029/2001jb000458.

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18

Dobson, David P., Alexander Lindsay-Scott, Simon A. Hunt, Edward Bailey, Ian G. Wood, John P. Brodholt, Lidunka Vocadlo, and John Wheeler. "Anisotropic diffusion creep in postperovskite provides a new model for deformation at the core−mantle boundary." Proceedings of the National Academy of Sciences 116, no. 52 (December 11, 2019): 26389–93. http://dx.doi.org/10.1073/pnas.1914826116.

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The lowermost portion of Earth’s mantle (D″) above the core−mantle boundary shows anomalous seismic features, such as strong seismic anisotropy, related to the properties of the main mineral MgSiO3postperovskite. But, after over a decade of investigations, the seismic observations still cannot be explained simply by flow models which assume dislocation creep in postperovskite. We have investigated the chemical diffusivity of perovskite and postperovskite phases by experiment and ab initio simulation, and derive equations for the observed anisotropic diffusion creep. There is excellent agreement between experiments and simulations for both phases in all of the chemical systems studied. Single-crystal diffusivity in postperovskite displays at least 3 orders of magnitude of anisotropy by experiment and simulation (Da= 1,000Db;Db≈Dc) in zinc fluoride, and an even more extreme anisotropy is predicted (Da= 10,000Dc;Dc= 10,000Db) in the natural MgSiO3system. Anisotropic chemical diffusivity results in anisotropic diffusion creep, texture generation, and a strain-weakening rheology. The results for MgSiO3postperovskite strongly imply that regions within the D″ region of Earth dominated by postperovskite will 1) be substantially weaker than regions dominated by perovskite and 2) develop a strain-induced crystallographic-preferred orientation with strain-weakening rheology. This leads to strain localization and the possibility to bring regions with significantly varying textures into close proximity by strain on narrow shear zones. Anisotropic diffusion creep therefore provides an attractive alternative explanation for the complexity in observed seismic anisotropy and the rapid lateral changes in seismic velocities in D″.

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19

Albers, Elmar, Timothy Schroeder, and Wolfgang Bach. "Melt Impregnation of Mantle Peridotite Facilitates High‐Temperature Hydration and Mechanical Weakening: Implications for Oceanic Detachment Faults." Geochemistry, Geophysics, Geosystems 20, no. 1 (January 2019): 84–108. http://dx.doi.org/10.1029/2018gc007783.

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20

Kwoka, John. "You Say You Want an Antitrust Revolution? The Paradox of Modern Merger Control." Antitrust Bulletin 65, no. 4 (August 17, 2020): 568–78. http://dx.doi.org/10.1177/0003603x20950201.

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Over the past twenty years, merger control has made what appear to be substantial advances in concepts and methodology, claiming the mantle of a “revolution.” This essay observes, however, that over this very same time period, merger control has in fact weakened and concentration risen throughout the economy. It reexamines two of the most heralded new methodologies—market definition and unilateral effects. It shows ways in which these have had the paradoxical effect of actually weakening merger control rather than strengthening it.

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21

Neumann, Wladimir. "Towards 3D modelling of convection in planetesimals and meteorite parent bodies." Monthly Notices of the Royal Astronomical Society: Letters 490, no. 1 (October 3, 2019): L47—L51. http://dx.doi.org/10.1093/mnrasl/slz147.

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ABSTRACT Observations of asteroid belt members, investigations of meteorites and thermal evolution models converge on the paradigm of the ubiquity of melting processes in the planetesimals of the early Solar system. At least partial melting of planetesimals that fulfilled size and accretion time requirements to surpass the solidus temperatures of metal and silicates led to the weakening of the rock due to the interstitial melt. A decrease of the viscosity relative to melt-free material facilitates solid-state convection on partially molten bodies. Additional melting can produce liquid-like layers with suspended particles, i.e. magma oceans. Thermal evolution models indicate that partially molten layers can occur in the interior of undifferentiated bodies and in silicate mantles of differentiated ones. They can exist before a magma ocean forms or after it solidifies and above a whole-mantle magma ocean or below a shallow magma ocean. Thus, convection is likely. Attempts to model and to quantify the effects of convection in planetesimals remain rare. This study discusses the possibility of solid-state convection in partially molten planetesimals, presents a first-order comparison of a 3D mantle convection model with a conduction model taking a Vesta-sized body as an example and illustrates the importance of convection for meteorite parent bodies.

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22

Stein, C., A. Finnenkötter, J. P. Lowman, and U. Hansen. "The pressure-weakening effect in super-Earths: Consequences of a decrease in lower mantle viscosity on surface dynamics." Geophysical Research Letters 38, no. 21 (November 2011): n/a. http://dx.doi.org/10.1029/2011gl049341.

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23

Handy, M. R., and H. Stünitz. "Strain localization by fracturing and reaction weakening — a mechanism for initiating exhumation of subcontinental mantle beneath rifted margins." Geological Society, London, Special Publications 200, no. 1 (2002): 387–407. http://dx.doi.org/10.1144/gsl.sp.2001.200.01.22.

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24

Linckens, Jolien, and Sören Tholen. "Formation of Ultramylonites in an Upper Mantle Shear Zone, Erro-Tobbio, Italy." Minerals 11, no. 10 (September 24, 2021): 1036. http://dx.doi.org/10.3390/min11101036.

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Deformation in the upper mantle is localized in shear zones. In order to localize strain, weakening has to occur, which can be achieved by a reduction in grain size. In order for grains to remain small and preserve shear zones, phases have to mix. Phase mixing leads to dragging or pinning of grain boundaries which slows down or halts grain growth. Multiple phase mixing processes have been suggested to be important during shear zone evolution. The importance of a phase mixing process depends on the geodynamic setting. This study presents detailed microstructural analysis of spinel bearing shear zones from the Erro-Tobbio peridotite (Italy) that formed during pre-alpine rifting. The first stage of deformation occurred under melt-free conditions, during which clinopyroxene and olivine porphyroclasts dynamically recrystallized. With ongoing extension, silica-undersaturated melt percolated through the shear zones and reacted with the clinopyroxene neoblasts, forming olivine–clinopyroxene layers. Furthermore, the melt reacted with orthopyroxene porphyroclasts, forming fine-grained polymineralic layers (ultramylonites) adjacent to the porphyroclasts. Strain rates in these layers are estimated to be about an order of magnitude faster than within the olivine-rich matrix. This study demonstrates the importance of melt-rock reactions for grain size reduction, phase mixing and strain localization in these shear zones.

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Tackley, Paul J. "Self-consistent generation of tectonic plates in time-dependent, three-dimensional mantle convection simulations 2. Strain weakening and asthenosphere." Geochemistry, Geophysics, Geosystems 1, no. 8 (August 2000): n/a. http://dx.doi.org/10.1029/2000gc000043.

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26

Girard, Jennifer, Jiuhua Chen, Paul Raterron, and Caleb W. Holyoke. "Hydrolytic weakening of olivine at mantle pressure: Evidence of [100](010) slip system softening from single-crystal deformation experiments." Physics of the Earth and Planetary Interiors 216 (March 2013): 12–20. http://dx.doi.org/10.1016/j.pepi.2012.10.009.

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27

Reid, M. R., J. R. Delph, M. A. Cosca, W. K. Schleiffarth, and G. Gençalioğlu Kuşcu. "Melt equilibration depths as sensors of lithospheric thickness during Eurasia-Arabia collision and the uplift of the Anatolian Plateau." Geology 47, no. 10 (August 20, 2019): 943–47. http://dx.doi.org/10.1130/g46420.1.

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Abstract A co-investigation of mantle melting conditions and seismic structure revealed an evolutionary record of mantle dynamics accompanying the transition from subduction to collision along the Africa-Eurasia margin and the >1 km uplift of the Anatolian Plateau. New 40Ar/39Ar dates of volcanic rocks from the Eastern Taurides (southeast Turkey) considerably expand the known spatial extent of Miocene-aged mafic volcanism following a magmatic lull over much of Anatolia that ended at ca. 20 Ma. Mantle equilibration depths for these chemically diverse basalts are interpreted to indicate that early to middle Miocene lithospheric thickness in the region varied from ∼50 km or less near the Bitlis suture zone to ∼80 km near the Inner Tauride suture zone. This southward-tapering lithospheric base could be a vestige of the former interface between the subducted (and now detached) portion of the Arabian plate and the overriding Eurasian plate, and/or a reflection of mantle weakening associated with greater mantle hydration trenchward prior to collision. Asthenospheric upwelling driven by slab tearing and foundering along this former interface, possibly accompanied by convective removal of the lithosphere, could have led to renewed volcanic activity after 20 Ma. Melt equilibration depths for late Miocene and Pliocene basalts together with seismic imaging of the present lithosphere indicate that relatively invariant lithospheric thicknesses of 60–70 km have persisted since the middle Miocene. Thus, no evidence is found for large-scale (tens of kilometers) Miocene delamination of the lower lithosphere from the overriding plate, which has been proposed elsewhere to account for late Miocene and younger uplift of Anatolia.

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28

Pleus, Alexandra, Garrett Ito, Paul Wessel, and L. Neil Frazer. "Rheology and thermal structure of the lithosphere beneath the Hawaiian Ridge inferred from gravity data and models of plate flexure." Geophysical Journal International 222, no. 1 (April 2, 2020): 207–24. http://dx.doi.org/10.1093/gji/ggaa155.

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SUMMARY We examine the rheology and thermal structure of the oceanic lithosphere, expressed in situ by plate flexure beneath the Hawaiian Ridge, where volcanoes of variable sizes have loaded seafloor of approximately the same age, and thus where the lithosphere is expected to have had an approximately uniform age-dependent thermal structure at the time of loading. Shipboard and satellite-derived gravity, as well as multibeam bathymetry data are used in models of plate flexure with curvature-dependent flexural rigidity, the strength of which is limited, in the shallow lithosphere, by brittle failure, and in the deeper lithosphere, by low-temperature plasticity (LTP). We compute relative likelihoods and posterior probabilities for four model parameters: average crustal density ρc, friction coefficient for brittle failure ${\mu _f}$, a pre-exponential weakening factor F controlling the strength of LTP and lithospheric geotherm age t. Results show that if the lithosphere temperatures were as is expected for normal (t = ) 90-Myr-old seafloor at the time of volcano loading, the rheology must be significantly weaker than expected. Specifically, weak brittle strengths (μf ≤ 0.3) show relatively high probabilities for three of the six published LTP flow laws examined. Alternatively, moderate-to-large brittle strengths (μf ≥ 0.5) require all LTP flow laws to be substantially weakened with F = 102 to > 108 or, equivalently, activation energy reduced by 10–35 per cent. In contrast, if the lithosphere has been moderately reheated by the Hawaiian hotspot, represented by geotherms for t = 50–70 Myr, then the flow laws of Evans & Goetze, Raterron et al. and Krancj et al. require little or no weakening. Such modest thermal rejuvenation is allowed by heatflow constraints, supported by regional mantle seismic tomography imaging as well as compositions of mantle xenoliths, and reconciles previously noted discrepancies between the LTP strengths of lithosphere beneath Hawaii versus that entering the Pacific subduction zones.

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Mameri, Lucan, Andréa Tommasi, Javier Signorelli, and Riad Hassani. "Olivine-induced viscous anisotropy in fossil strike-slip mantle shear zones and associated strain localization in the crust." Geophysical Journal International 224, no. 1 (August 25, 2020): 608–25. http://dx.doi.org/10.1093/gji/ggaa400.

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SUMMARY We propose that strain localization in plate interiors, such as linear belts of intraplate seismicity, may arise from spatial variations in viscous anisotropy produced by preferred orientation of olivine crystals (CPO or texture) inherited from previous deformation episodes in the lithospheric mantle. To quantify this effect, we model the deformation of a plate containing a fossil strike-slip mantle shear zone at different orientations relative to an imposed horizontal shortening, but no initial heterogeneity in the crust. The fossil shear zone is characterized by different orientation and intensity of the olivine CPO relatively to the surrounding mantle, which is isotropic in most simulations. The anisotropy in viscosity produced by the CPO, which remains fixed throughout the simulations, is described by an anisotropic (Hill) yield function parametrized based on second-order viscoplastic self-consistent (SO-VPSC) models. The results indicate that lateral variations in viscous anisotropy in the mantle affect the strain distribution in the entire lithosphere. Reactivation of the strike-slip mantle shear zone and strain localization in the crust above it occur for horizontal compression at 35–55° to the fossil shear plane, with a maximum at 45°. The magnitude of strain localization depends on (i) the contrast in viscous anisotropy and, hence, on the variations in CPO orientation and intensity in the mantle, (ii) the boundary conditions and (iii) the feedbacks between mantle and crustal deformation. For a strong olivine CPO, when the boundary conditions do not hinder shear parallel to the fossil mantle shear zone, strain rates within it are up to a factor 30 higher than in an isotropic surrounding mantle or up to a factor 200 when the surrounding mantle is anisotropic, which results in strain rates up to a factor 10 or up to a factor 100 higher in the crust right above the fossil shear zone. Frictional weakening in the crust faults increases strain localization in the entire lithospheric column. High strength contrasts between the mantle and the ductile crust result in less efficient mechanical coupling, with strong localization in the mantle and lower crust, but weak in the brittle upper crust. Decrease in the crust–mantle strength contrast enhances the coupling and produces more homogenous strain distribution with depth, as well as a time-dependent evolution of strain localization, which reaches a peak and decreases before attaining steady-state. Comparison of seismic anisotropy, regional stress and focal mechanism data in linear arrays of intraplate seismicity, like the New Madrid and South Armorican seismic zones, to our models' predictions corroborates that olivine CPO preserved in fossil lithospheric-scale shear zones may be key for the development of such structures.

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Quinquis, M. E. T., and S. J. H. Buiter. "Testing the effects of the numerical implementation of water migration on models of subduction dynamics." Solid Earth Discussions 5, no. 2 (October 24, 2013): 1771–815. http://dx.doi.org/10.5194/sed-5-1771-2013.

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Abstract. Subduction of oceanic lithosphere brings water into Earth's upper mantle. Previous numerical studies have shown how slab dehydration and mantle hydration can impact the dynamics of a subduction system by allowing a more vigorous mantle flow and promoting localisation of deformation in lithosphere and mantle. The depths at which dehydration reactions occur in the hydrated portions of the slab are well constrained in these models by thermodynamic calculations. However, the mechanism by which free water migrates in the mantle is incompletely known. Therefore, models use different numerical schemes to model the migration of free water. We aim to show the influence of the numerical scheme of free water migration on the dynamics of the upper mantle and more specifically the mantle wedge. We investigate the following three migration schemes with a finite-element model: (1) element-wise vertical migration of free water, occurring independent of the material flow; (2) an imposed vertical free water velocity; and (3) a Darcy velocity, where the free water velocity is calculated as a function of the pressure gradient between water and the surrounding rocks. In addition, the material flow field also moves the free water in the imposed vertical velocity and Darcy schemes. We first test the influence of the water migration scheme using a simple Stokes flow model that simulates the sinking of a cold hydrated cylinder into a hot dry mantle. We find that the free water migration scheme has only a limited impact on the water distribution after 1 Myr in these models. We next investigate slab dehydration and mantle hydration with a thermomechanical subduction model that includes brittle behaviour and viscous water-dependent creep flow laws. Our models show how the bound water distribution is not greatly influenced by the water migration scheme whereas the free water distribution is. We find that a water-dependent creep flow law results in a broader area of hydration in the mantle wedge which feeds back to the dynamics of the system by the associated weakening. This supports using dynamic time evolution models to investigate the effects of (de)hydration. We also show that hydrated material can be transported down to the base of the upper mantle at 670 km. Although (de)hydration processes influence subduction dynamics, we find that the exact numerical implementation of free water migration is not important. This implies that a simple implementation of water migration would be sufficient for studies that focus on larger-scale features of subduction dynamics.

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31

Kellermann Slotemaker, A., J. H. P. de Bresser, C. J. Spiers, and M. R. Drury. "Microstructural Evolution of Synthetic Forsterite Aggregates Deformed to High Strain." Materials Science Forum 467-470 (October 2004): 579–84. http://dx.doi.org/10.4028/www.scientific.net/msf.467-470.579.

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Microstructures provide the crucial link between solid state flow of rock materials in the laboratory and large-scale tectonic processes in nature. In this context, microstructural evolution of olivine aggregates is of particular importance, since this material controls the flow of the Earth’s upper mantle and affects the dynamics of the outer Earth. From previous work it has become apparent that if olivine rocks are plastically deformed to high strain, substantial weakening may occur before steady state mechanical behaviour is approached. This weakening appears directly related to progressive modification of the grain size distribution through competing effects of dynamic recrystallization and syn-deformational grain growth. However, most of our understanding of these processes in olivine comes from tests on coarse-grained materials that show grain size reduction through dynamic recrystallization. In the present study we focused on fine-grained (~1 µm) olivine aggregates (i.e., forsterite/Mg2SiO4), containing ~0.5 wt% water and 10 vol% enstatite (MgSiO3), Samples were axially compressed to varying strains up to a maximum of ~45%, at 600 MPa confining pressure and a temperature of 950°C. Microstructures were characterized by analyzing full grain size distributions and textures using SEM/EBSD. We observed syndeformational grain growth rather than grain size reduction, and relate this to strain hardening seen in the stress-strain curves.

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Quinquis, M. E. T., and S. J. H. Buiter. "Testing the effects of basic numerical implementations of water migration on models of subduction dynamics." Solid Earth 5, no. 1 (June 26, 2014): 537–55. http://dx.doi.org/10.5194/se-5-537-2014.

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Abstract. Subduction of oceanic lithosphere brings water into the Earth's upper mantle. Previous numerical studies have shown how slab dehydration and mantle hydration can impact the dynamics of a subduction system by allowing a more vigorous mantle flow and promoting localisation of deformation in the lithosphere and mantle. The depths at which dehydration reactions occur in the hydrated portions of the slab are well constrained in these models by thermodynamic calculations. However, computational models use different numerical schemes to simulate the migration of free water. We aim to show the influence of the numerical scheme of free water migration on the dynamics of the upper mantle and more specifically the mantle wedge. We investigate the following three simple migration schemes with a finite-element model: (1) element-wise vertical migration of free water, occurring independent of the flow of the solid phase; (2) an imposed vertical free water velocity; and (3) a Darcy velocity, where the free water velocity is a function of the pressure gradient caused by the difference in density between water and the surrounding rocks. In addition, the flow of the solid material field also moves the free water in the imposed vertical velocity and Darcy schemes. We first test the influence of the water migration scheme using a simple model that simulates the sinking of a cold, hydrated cylinder into a dry, warm mantle. We find that the free water migration scheme has only a limited impact on the water distribution after 1 Myr in these models. We next investigate slab dehydration and mantle hydration with a thermomechanical subduction model that includes brittle behaviour and viscous water-dependent creep flow laws. Our models demonstrate that the bound water distribution is not greatly influenced by the water migration scheme whereas the free water distribution is. We find that a bound water-dependent creep flow law results in a broader area of hydration in the mantle wedge, which feeds back to the dynamics of the system by the associated weakening. This finding underlines the importance of using dynamic time evolution models to investigate the effects of (de)hydration. We also show that hydrated material can be transported down to the base of the upper mantle at 670 km. Although (de)hydration processes influence subduction dynamics, we find that the exact numerical implementation of free water migration is not important in the basic schemes we investigated. A simple implementation of water migration could be sufficient for a first-order impression of the effects of water for studies that focus on large-scale features of subduction dynamics.

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Khromykh, S. V., P. D. Kotler, A. E. Izokh, and N. N. Kruk. "A REVIEW OF EARLY PERMIAN (300–270 MA) MAGMATISM IN EASTERN KAZAKHSTAN AND IMPLICATIONS FOR PLATE TECTONICS AND PLUME INTERPLAY." Geodynamics & Tectonophysics 10, no. 1 (March 23, 2019): 79–99. http://dx.doi.org/10.5800/gt-2019-10-1-0405.

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The history of the Central Asian Orogenic Belt (CAOB) was marked by several major events of magmatism which produced large volumes of volcanic and intrusive (mafic-ultramafic and granitic) rocks within a relatively short time span (30–40 Ma) over a vast area. The magmatic activity postdated the orogenic stages of accretionary-collisional belts in Central Asia and likely resulted from the impact of mantle plumes that formed Large Igneous Provinces (LIPs). The formation of the Tarim–South Mongolia LIP at 300–270 Ma is the best known among the major Permian events of basaltic and granitic magmatism. Early Permian igneous rocks (volcanic, subvolcanic and intrusive suites that vary from ultramafic to felsic compositions) of the same age range (300 to 270 Ma) have been recently found also in Eastern Kazakhstan, within the late Paleozoic Altai collisional system. The compositions and ages of the rocks suggest that the Eastern Kazakhstan magmatism was the northward expansion of the Tarim LIP. The spread of the Tarim LIP was apparently facilitated by lithospheric extension after the Siberia-Kazakhstan collision. The extension led to rheological weakening of the lithosphere whereby deep mantle melts could penetrate to shallower depths. The early Permian history of Eastern Kazakhstan was controlled by the interplay of plate tectonic and plume processes: plate-tectonic accretion and collision formed the structural framework, and the Tarim mantle plume was a heat source maintaining voluminous magma generation.

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Hidas, Károly, Carlos J. Garrido, Guillermo Booth-Rea, Claudio Marchesi, Jean-Louis Bodinier, Jean-Marie Dautria, Amina Louni-Hacini, and Abla Azzouni-Sekkal. "Lithosphere tearing along STEP faults and synkinematic formation of lherzolite and wehrlite in the shallow subcontinental mantle." Solid Earth 10, no. 4 (July 10, 2019): 1099–121. http://dx.doi.org/10.5194/se-10-1099-2019.

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Abstract. Subduction-transform edge propagator (STEP) faults are the locus of continual lithospheric tearing at slab edges, resulting in sharp changes in the lithospheric and crustal thickness and triggering lateral and/or near-vertical mantle flow. However, the mechanisms at the lithospheric mantle scale are still poorly understood. Here, we present the microstructural study of olivine-rich lherzolite, harzburgite and wehrlite mantle xenoliths from the Oran volcanic field (Tell Atlas, northwest Algeria). This alkali volcanic field occurs along a major STEP fault responsible for the Miocene westward slab retreat in the westernmost Mediterranean. Mantle xenoliths provide a unique opportunity to investigate the microstructures in the mantle section of a STEP fault system. The microstructures of mantle xenoliths show a variable grain size ranging from coarse granular to fine-grained equigranular textures uncorrelated with lithology. The major element composition of the mantle peridotites provides temperature estimates in a wide range (790–1165 ∘C) but in general, the coarse-grained and fine-grained peridotites suggest deeper and shallower provenance depth, respectively. Olivine grain size in the fine-grained peridotites depends on the size and volume fraction of the pyroxene grains, which is consistent with pinning of olivine grain growth by pyroxenes as second-phase particles. In the coarse-grained peridotites, well-developed olivine crystal-preferred orientation (CPO) is characterized by orthorhombic and [100]-fiber symmetries, and orthopyroxene has a coherent CPO with that of olivine, suggesting their coeval deformation by dislocation creep at high temperature. In the fine-grained microstructures, along with the weakening of the fabric strength, olivine CPO symmetry exhibits a shift towards [010] fiber and the [010] and [001] axes of orthopyroxene are generally distributed subparallel to those of olivine. These data are consistent with deformation of olivine in the presence of low amounts of melts and the precipitation of orthopyroxenes from a melt phase. The bulk CPO of clinopyroxene mimics that of orthopyroxene via a topotaxial relationship of the two pyroxenes. This observation points to a melt-related origin of most clinopyroxenes in the Oran mantle xenoliths. The textural and geochemical record of the peridotites are consistent with interaction of a refractory harzburgite protolith with a high-Mg no. melt at depth (resulting in the formation of coarse-grained clinopyroxene-rich lherzolite and wehrlite) and with a low-Mg no. evolved melt in the shallow subcontinental lithospheric mantle (forming fine-grained harzburgite). We propose that pervasive melt–peridotite reaction – promoted by lateral and/or near-vertical mantle flow associated with lithospheric tearing – resulted in the synkinematic crystallization of secondary lherzolite and wehrlite and had a key effect on grain size reduction during the operation of the Tell–Rif STEP fault. Melt–rock reaction and secondary formation of lherzolite and wehrlite may be widespread in other STEP fault systems worldwide.

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Abers, Geoffrey A., Peter E. van Keken, and Cian R. Wilson. "Deep decoupling in subduction zones: Observations and temperature limits." Geosphere 16, no. 6 (October 27, 2020): 1408–24. http://dx.doi.org/10.1130/ges02278.1.

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Abstract The plate interface undergoes two transitions between seismogenic depths and subarc depths. A brittle-ductile transition at 20–50 km depth is followed by a transition to full viscous coupling to the overlying mantle wedge at ∼80 km depth. We review evidence for both transitions, focusing on heat-flow and seismic-attenuation constraints on the deeper transition. The intervening ductile shear zone likely weakens considerably as temperature increases, such that its rheology exerts a stronger control on subduction-zone thermal structure than does frictional shear heating. We evaluate its role through analytic approximations and two-dimensional finite-element models for both idealized subduction geometries and those resembling real subduction zones. We show that a temperature-buffering process exists in the shear zone that results in temperatures being tightly controlled by the rheological strength of that shear zone’s material for a wide range of shear-heating behaviors of the shallower brittle region. Higher temperatures result in weaker shear zones and hence less heat generation, so temperatures stop increasing and shear zones stop weakening. The net result for many rheologies are temperatures limited to ≤350–420 °C along the plate interface below the cold forearc of most subduction zones until the hot coupled mantle is approached. Very young incoming plates are the exception. This rheological buffering desensitizes subduction-zone thermal structure to many parameters and may help explain the global constancy of the 80 km coupling limit. We recalculate water fluxes to the forearc wedge and deep mantle and find that shear heating has little effect on global water circulation.

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Melekestsev, I. V. "The expected future super-eruption of the Yellowstone super volcano (USA) is “cancelled” by the Pleistocene glaciation and by the inversion of caldera complex development." Geomorphology RAS, no. 2 (April 26, 2019): 18–36. http://dx.doi.org/10.31857/s0435-42812019218-36.

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The review of the reconstructions of the eruptive activity of the Yellowstone Caldera Complex (YCC) in the USA allows to suggests three groups of arguments supporting that the “volcanic super-eruption of Yellowstone” is not likely to occur in the coming hundreds or thousands of years. First is the gradual weakening of the volcanic potential of the magmatic source (which is the frontal lobe of the magmatic super-flow, and not the mantle plume) during the last 2 million yeats. Second is the impact of the repeated occurrence of ice sheets in the YCC area during the past 640 thousand years. Finally, the equivalent super-eruption, in terms of energy released and the mass of exploded material, had already occurred at about 70 thousand years ago, and since that time, the YCC has passed from the volcanic to the hydrothermal evolutionary stage.

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ZHANG, XIAOHUI, HONGFU ZHANG, NENG JIANG, and SIMON A. WILDE. "Contrasting Middle Jurassic and Early Cretaceous mafic intrusive rocks from western Liaoning, North China craton: petrogenesis and tectonic implications." Geological Magazine 147, no. 6 (May 7, 2010): 844–59. http://dx.doi.org/10.1017/s0016756810000373.

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AbstractZircon U–Pb dating, whole-rock major oxide, trace element and Sr–Nd isotopic data are presented for the Late Mesozoic mafic intrusive rocks from Yiwulüshan of western Liaoning along the eastern segment of the Yanshan belt, North China craton, with two episodes of magmatism documented. Middle Jurassic hornblende-rich gabbros show enrichment of large ion lithophile elements and light REE, and prominent depletion in high field strength elements, and possess moderately enriched isotopic compositions with (87Sr/86Sr)i ranging from 0.7056 to 0.7065 and ɛNd(t) from −5.0 to −7.1. These features suggest that the gabbros were derived from an amphibole-bearing harzburgitic lithospheric mantle source metasomatized recently by slab-derived fluids. By contrast, Early Cretaceous mafic dykes are gabbroic dioritic to dioritic in composition, with comparable trace element characteristics to continental crust and depleted isotopic signatures ((87Sr/86Sr)i = 0.7048–0.7055, ɛNd(t) = 0 to −3.0). They probably originated from partial melting of a relatively fertile asthenospheric mantle in the spinel stability field, with subsequent lower crustal assimilation and fractional crystallization. These two contrasting mafic intrusive suites, together with multiple Mesozoic mafic volcanic rocks from western Liaoning, documented a localized lithospheric thinning process, mainly through prolonged hydro-weakening or melt–rock interaction and triggered by gravitational collapse, possibly within an evolved post-collisional to within-plate extensional regime.

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Gasc, Julien, Blandine Gardonio, Damien Deldicque, Clémence Daigre, Arefeh Moarefvand, Léo Petit, Pamela Burnley, and Alexandre Schubnel. "Ductile vs. Brittle Strain Localization Induced by the Olivine–Ringwoodite Transformation." Minerals 12, no. 6 (June 4, 2022): 719. http://dx.doi.org/10.3390/min12060719.

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As it descends into the Earth’s mantle, the olivine that constitutes the lithosphere of subducting slabs transforms to its high-pressure polymorphs, wadsleyite and ringwoodite, in the so-called transition zone. These transformations have important rheological consequences, since they may induce weakening, strain localization, and, in some cases, earthquakes. In this study, germanium olivine (Ge-olivine) was used as an analogue material to investigate the rheology of samples undergoing the olivine–ringwoodite transformation. Ge-olivine adopts a ringwoodite structure at pressures ~14 GPa lower than its silicate counterpart does, making the transformation accessible with a Griggs rig. Deformation experiments were carried out in a new-generation Griggs apparatus, where micro-seismicity was recorded in the form of acoustic emissions. A careful analysis of the obtained acoustic signal, combined with an extensive microstructure analysis of the recovered samples, provided major insights into the interplay between transformation and deformation mechanisms. The results show that significant reaction rates cause a weakening via the implementation of ductile shear zones that can be preceded by small brittle precursors. When kinetics are more sluggish, mechanical instabilities lead to transformational faulting, which stems from the unstable propagation of shear bands localizing both strain and transformation. The growth of these shear bands is self-sustained thanks to the negative volume change and the exothermic nature of the reaction, and leads to dynamic rupture, as attested by the acoustic emissions recorded. These micro-earthquakes share striking similarities with deep focus earthquakes, which may explain several seismological observations such as magnitude frequency relations and the occurrence of deep repeating earthquakes and foreshocks.

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Ootes, Luke, Valerie A. Jackson, William J. Davis, Venessa Bennett, Leanne Smar, and Brian L. Cousens. "Parentage of Archean basement within a Paleoproterozoic orogen and implications for on-craton diamond preservation: Slave craton and Wopmay orogen, northwest Canada." Canadian Journal of Earth Sciences 54, no. 2 (February 2017): 203–32. http://dx.doi.org/10.1139/cjes-2016-0059.

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The Wopmay orogen is a Paleoproterozoic accretionary belt preserved to the west of the Archean Slave craton, northwest Canada. Reworked Archean crystalline basement occurs in the orogen, and new bedrock mapping, U–Pb geochronology, and Sm–Nd isotopic data further substantiate a Slave craton parentage for this basement. Detrital zircon results from unconformably overlying Paleoproterozoic supracrustal rocks also support a Slave craton provenance. Rifting of the Slave margin began at ca. 2.02 Ga with a second rift phase constrained between ca. 1.92 and 1.89 Ga, resulting in thermal weakening of the Archean basement and allowing subsequent penetrative deformation during the Calderian orogeny (ca. 1.88–1.85 Ga). The boundary between the western Slave craton and the reworked Archean basement in the southern Wopmay orogen is interpreted as the rifted cratonic margin, which later acted as a rigid backstop during compressional deformation. Age-isotopic characteristics of plutonic phases track the extent and evolution of these processes that left penetratively deformed Archean basement, Paleoproterozoic cover, and plutons in the west, and “rigid” Archean Slave craton to the east. Diamond-bearing kimberlite occurs across the central and eastern parts of the Slave craton, but kimberlite (diamond bearing or not) has not been documented west of ∼114°W. It is proposed that while the crust of the western Slave craton escaped thermal weakening, the mantle did not and was moved out of the diamond stability field. The Paleoproterozoic extension–convergence cycle preserved in the Wopmay orogen provides a reasonable explanation as to why the western Slave craton appears to be diamond sterile.

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40

Ohuchi, T. "Grain-size-sensitive creep of olivine induced by oxidation of olivine in the Earth's deep upper mantle: Implications for weakening of the subduction interface." Physics of the Earth and Planetary Interiors 326 (May 2022): 106865. http://dx.doi.org/10.1016/j.pepi.2022.106865.

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41

Gay, Jeffrey P., Lowell Miyagi, Samantha Couper, Christopher Langrand, David P. Dobson, Hanns-Peter Liermann, and Sébastien Merkel. "Deformation of NaCoF3 perovskite and post-perovskite up to 30 GPa and 1013 K: implications for plastic deformation and transformation mechanism." European Journal of Mineralogy 33, no. 5 (September 30, 2021): 591–603. http://dx.doi.org/10.5194/ejm-33-591-2021.

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Abstract. Texture, plastic deformation, and phase transformation mechanisms in perovskite and post-perovskite are of general interest for our understanding of the Earth's mantle. Here, the perovskite analogue NaCoF3 is deformed in a resistive-heated diamond anvil cell (DAC) up to 30 GPa and 1013 K. The in situ state of the sample, including crystal structure, stress, and texture, is monitored using X-ray diffraction. A phase transformation from a perovskite to a post-perovskite structure is observed between 20.1 and 26.1 GPa. Normalized stress drops by a factor of 3 during transformation as a result of transient weakening during the transformation. The perovskite phase initially develops a texture with a maximum at 100 and a strong 010 minimum in the inverse pole figure of the compression direction. Additionally, a secondary weaker 001 maximum is observed later during compression. Texture simulations indicate that the initial deformation of perovskite requires slip along (100) planes with significant contributions of {110} twins. Following the phase transition to post-perovskite, we observe a 010 maximum, which later evolves with compression. The transformation follows orientation relationships previously suggested where the c axis is preserved between phases and hh0 vectors in reciprocal space of post-perovskite are parallel to [010] in perovskite, which indicates a martensitic-like transition mechanism. A comparison between past experiments on bridgmanite and current results indicates that NaCoF3 is a good analogue to understand the development of microstructures within the Earth's mantle.

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Sutherland, R., G. R. Dickens, P. Blum, C. Agnini, L. Alegret, G. Asatryan, J. Bhattacharya, et al. "Continental-scale geographic change across Zealandia during Paleogene subduction initiation." Geology 48, no. 5 (February 6, 2020): 419–24. http://dx.doi.org/10.1130/g47008.1.

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Abstract Data from International Ocean Discovery Program (IODP) Expedition 371 reveal vertical movements of 1–3 km in northern Zealandia during early Cenozoic subduction initiation in the western Pacific Ocean. Lord Howe Rise rose from deep (∼1 km) water to sea level and subsided back, with peak uplift at 50 Ma in the north and between 41 and 32 Ma in the south. The New Caledonia Trough subsided 2–3 km between 55 and 45 Ma. We suggest these elevation changes resulted from crust delamination and mantle flow that led to slab formation. We propose a “subduction resurrection” model in which (1) a subduction rupture event activated lithospheric-scale faults across a broad region during less than ∼5 m.y., and (2) tectonic forces evolved over a further 4–8 m.y. as subducted slabs grew in size and drove plate-motion change. Such a subduction rupture event may have involved nucleation and lateral propagation of slip-weakening rupture along an interconnected set of preexisting weaknesses adjacent to density anomalies.

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Warren, C. J. "Exhumation of (ultra-)high-pressure terranes: concepts and mechanisms." Solid Earth 4, no. 1 (February 13, 2013): 75–92. http://dx.doi.org/10.5194/se-4-75-2013.

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Abstract. The formation and exhumation of high and ultra-high-pressure, (U)HP, rocks of crustal origin appears to be ubiquitous during Phanerozoic plate subduction and continental collision events. Exhumation of (U)HP material has been shown in some orogens to have occurred only once, during a single short-lived event; in other cases exhumation appears to have occurred multiple discrete times or during a single, long-lived, protracted event. It is becoming increasingly clear that no single exhumation mechanism dominates in any particular tectonic environment, and the mechanism may change in time and space within the same subduction zone. Subduction zone style and internal force balance change in both time and space, responding to changes in width, steepness, composition of subducting material and velocity of subduction. In order for continental crust, which is relatively buoyant compared to the mantle even when metamorphosed to (U)HP assemblages, to be subducted to (U)HP conditions, it must remain attached to a stronger and denser substrate. Buoyancy and external tectonic forces drive exhumation, although the changing spatial and temporal dominance of different driving forces still remains unclear. Exhumation may involve whole-scale detachment of the terrane from the subducting slab followed by exhumation within a subduction channel (perhaps during continued subduction) or a reversal in motion of the entire plate (eduction) following the removal of a lower part of the subducting slab. Weakening mechanisms that may be responsible for the detachment of deeply subducted crust from its stronger, denser substrate include strain weakening, hydration, melting, grain size reduction and the development of foliation. These may act locally to form narrow high-strain shear zones separating stronger, less-strained crust or may act on the bulk of the subducted material, allowing whole-scale flow. Metamorphic reactions, metastability and the composition of the subducted crust all affect buoyancy and overall strength. Future research directions include identifying temporal and spatial changes in exhumation mechanisms within different tectonic environments, and determining the factors that influence those changes.

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Mazzotti, Stéphane, Hervé Jomard, and Frédéric Masson. "Processes and deformation rates generating seismicity in metropolitan France and conterminous Western Europe." BSGF - Earth Sciences Bulletin 191 (2020): 19. http://dx.doi.org/10.1051/bsgf/2020019.

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Most of metropolitan France and conterminous Western Europe is currently located within the Eurasia intraplate domain, far from major plate boundaries (the Atlantic ridge and Nubia – Eurasia convergence zone). As in other intraplate regions, present-day deformation and seismicity rates are very slow, resulting in limited data and strong uncertainties on the ongoing seismotectonics and seismic hazards. In the last two decades, new geological, seismological and geodetic data and research have brought to light unexpected deformation patterns in metropolitan France, such as orogen-normal extension ca. 0.5 mm yr−1 in the Pyrenees and Western Alps that cannot be associated with their mountain-building history. Elsewhere, present-day deformation and seismicity data provide a partial picture that points to mostly extensive to strike-slip deformation regimes (except in the Western Alps foreland). A review of the numerous studies and observations shows that plate tectonics (plate motion, mantle convection) are not the sole, nor likely the primary driver of present-day deformation and seismicity and that additional processes must be considered, such as topography potential energy, erosion or glacial isostatic adjustment since the last glaciation. The exact role of each process probably varies from one region to another and remains to be characterized. In addition, structural inheritance (crust or mantle weakening from past tectonic events) can play a strong role in deformation localization and amplification up to factors of 5–20, which could explain some of the spatial variability in seismicity. On the basis of this review, we identify three research directions that should be developed to better characterize the seismicity, deformation rates and related processes in metropolitan France: macroseismic and historical seismicity, especially regarding moment magnitude estimations; geodetic deformation, including in regions of low seismicity where the ratio of seismic to aseismic deformation remains a key unknown; an integrated and consistent seismotectonic framework comprising numerical models, geological, seismological and geodetic data. The latter has the potential for significant improvements in the characterization of seismicity and seismic hazard in metropolitan France but also Western Europe.

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Grahame, Jarrad, and Victoria Cole. "Prospectivity of the Triassic successions of the North West Shelf of Australia: New insights from a regional integrated geoscience study." Leading Edge 40, no. 3 (March 2021): 172–77. http://dx.doi.org/10.1190/tle40030172.1.

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The North West Shelf (NWS) of Australia is a prolific hydrocarbon province hosting significant volumes of hydrocarbons, primarily derived from Jurassic and Cretaceous targets. A new regional, integrated geoscience study has been undertaken to develop insights into the paleogeography and petroleum systems of Late Permian to Triassic successions, which have been underexplored historically in favor of Jurassic to Cretaceous targets. Within the NWS study area, graben and half-graben depocenters developed in response to intracratonic rifting that preceded later fragmentation and northward rifting of small continental blocks. This, coupled with contemporaneous cycles of rising sea levels, brought about the development of large embayments and shallow, epeiric seas between the Australian continental landmass and outlying continental fragments in the early stages of divergence. Key elements of the study results discussed herein include the study methodology, the paleogeographic and gross depositional environment mapping, and the reservoir and source kitchen modeling. The study results highlight the presence of depocenters that developed within oblique rift zones due to regional Permo-Triassic strike-slip tectonics that bear compelling similarities to modern-day analogues. These intracratonic rift zones are well-known and prominent tectonic features resulting from mantle upwelling and weakening of overlying lithospheric crust, initiating the development of divergent intraplate depocenters. The comprehensive analysis of these depocenters from a paleogeographic and petroleum system perspective provides a basin evaluation tool for Triassic prospectivity.

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Andrei Bala, Mircea Radulian, and Dragos Toma-Danila. "Present-day stress field pattern in the Vrancea seismic zone (Romania) deduced from earthquake focal mechanism inversion." Annals of Geophysics 64, no. 6 (December 17, 2021): PE660. http://dx.doi.org/10.4401/ag-8632.

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Vrancea seismogenic zone in the South-Eastern Carpathians is characterized by localized intermediate-depth seismicity. Due to its complex geodynamics and large strain release, Vrancea represents a key element in the Carpatho-Pannonian system. Data from a recently compiled catalogue of fault plane solutions (REFMC) are inverted to evaluate stress regime in Vrancea on depth. A single predominant downdip extensive regime is obtained in all considered clusters, including the crustal layers located above the Vrancea slab. The prevalent stress regime confirms previous investigations and requires some mantle-crust coupling. The S3 principal stress is close to vertical, while S1 and S2 are horizontal, oriented perpendicularly and respectively tangentially to the Carpathians Arc bend. This configuration is present at any depth level. According to seismicity patterns, there are two main active segments in the Vrancea intermediate-depth domain, at 55 – 105 km and 105 – 180 km, both able to generate major events. The configuration of the tectonic stresses as resulted from inversion is similar in both segments. Also, high fault instability (I > 0.95) is characterizing the segments. The only notable difference is given by the friction and stress ratio parameters which drop down in the bottom segment from μ = 0.95 to μ = 0.55 and from R = 0.51 to R = 0.29. This variation is attributed to possible weakening processes activated below 100 km depth and can explain the intensification of seismicity production as earthquake rate and average energy release in the lower segment versus the upper segment.

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Bosworth, William, and Daniel F. Stockli. "Early magmatism in the greater Red Sea rift: timing and significance." Canadian Journal of Earth Sciences 53, no. 11 (November 2016): 1158–76. http://dx.doi.org/10.1139/cjes-2016-0019.

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Throughout the greater Red Sea rift system the initial late Cenozoic syn-rift strata and extensional faulting are closely associated with alkali basaltic volcanism. Older stratigraphic units are either pre-rift or deposited during pre-rupture mechanical weakening of the lithosphere. The East African superplume appeared in northeast Africa ∼46 Ma but was not accompanied by any significant extensional faulting. Continental rifting began in the eastern and central Gulf of Aden at ∼31–30 Ma coeval with the onset of continental flood volcanism in northern Ethiopia, Eritrea, and western Yemen. Volcanism appeared soon after at Derudeb in southern Sudan and at Harrats Hadan and As Sirat in Saudi Arabia. From ∼26.5 to 25 Ma a new phase of volcanism began with the intrusion of a dike field reaching southeast of Afar into the Ogaden. At 24–23 Ma dikes were emplaced nearly simultaneously north of Afar and reached over 2000 km into northern Egypt. The dike event linked Afar to the smaller Cairo mini-plume and corresponds to initiation of lithospheric extension and rupture in the central and northern Red Sea and Gulf of Suez. By ∼21 Ma the dike intrusions along the entire length of the Red Sea were completed. Each episodic enlargement of the greater Red Sea rift system was triggered and facilitated by breakthrough of mantle-derived plumes. However, the absence of any volumetrically significant rift-related volcanism during the main phase of Miocene central and northern Red Sea – Gulf of Suez rifting supports the interpretation that plate–boundary forces likely drove overall separation of Arabia from Africa.

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Cui, Jing, Xuhui Shen, Jingfa Zhang, Weiyu Ma, and Wei Chu. "Analysis of spatiotemporal variations in middle-tropospheric to upper-tropospheric methane during the Wenchuan Ms = 8.0 earthquake by three indices." Natural Hazards and Earth System Sciences 19, no. 12 (December 17, 2019): 2841–54. http://dx.doi.org/10.5194/nhess-19-2841-2019.

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Abstract. This research studied the spatiotemporal variation in methane in the middle to upper troposphere during the Wenchuan earthquake (12 May 2008) using AIRS retrieval data and discussed the methane anomaly mechanism. Three indices were proposed and used for analysis. Our results show that the methane concentration increased significantly in 2008, with an average increase of 5.12×10-8, compared to the average increase of 1.18×10-8 in the previous 5 years. The absolute local index of change of the environment (ALICE) and differential value (diff) indices can be used to identify methane concentration anomalies. The two indices showed that the methane concentration distribution before and after the earthquake broke the distribution features of the background field. As the earthquake approached, areas of high methane concentration gradually converged towards the west side of the epicenter from both ends of the Longmenshan fault zone. Moreover, a large anomalous area was centered at the epicenter 8 d before the earthquake occurred, and a trend of strengthening, weakening and strengthening appeared over time. The gradient index showed that the vertical direction obviously increased before the main earthquake and that the value was positive. The gradient value is negative during coseismic or post-seismic events. The gradient index reflects the gas emission characteristics to some extent. We also determined that the methane release was connected with the deep crust–mantle stress state, as well as micro-fracture generation and expansion. However, due to the lack of any technical means to accurately identify the source and content of methane in the atmosphere before the earthquake, an in-depth discussion has not been conducted, and further studies on this issue may be needed.

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Roy, Mousumi. "Assessing the role of thermal disequilibrium in the evolution of the lithosphere–asthenosphere boundary: an idealized model of heat exchange during channelized melt transport." Solid Earth 13, no. 9 (September 5, 2022): 1415–30. http://dx.doi.org/10.5194/se-13-1415-2022.

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Abstract. This study explores how the continental lithospheric mantle (CLM) may be heated during channelized melt transport when there is thermal disequilibrium between (melt-rich) channels and surrounding (melt-poor) regions. Specifically, I explore the role of disequilibrium heat exchange in weakening and destabilizing the lithosphere from beneath as melts infiltrate into the lithosphere–asthenosphere boundary (LAB) in intraplate continental settings. During equilibration, hotter-than-ambient melts would be expected to heat the surrounding CLM, but we lack an understanding of the expected spatiotemporal scales and how these depend on channel geometries, infiltration duration, and transport rates. This study assesses the role of heat exchange between migrating material in melt-rich channels and their surroundings in the limit where advective effects are larger than diffusive heat transfer (Péclet numbers > 10). I utilize a 1D advection–diffusion model that includes thermal exchange between melt-rich channels and the surrounding melt-poor region, parameterized by the volume fraction of channels (ϕ), average relative velocity (vchannel) between material inside and outside of channels, channel spacing (d), and timescale of episodic or repeated melt infiltration (τ). The results suggest the following: (1) during episodic infiltration of hotter-than-ambient melt, a steady-state thermal reworking zone (TRZ) associated with spatiotemporally varying disequilibrium heat exchange forms at the LAB. (2) The TRZ grows by the transient migration of a disequilibrium-heating front at a material-dependent velocity, reaching a maximum steady-state width δ proportional to ϕvchannel(τ/d)n, where n≈2 for periodic thermal perturbations and n≈1 for a single finite-duration thermal pulse. For geologically reasonable model parameters, the spatiotemporal scales associated with establishment of the TRZ are comparable with those inferred for the migration of the LAB based on geologic observations within continental intra-plate settings, such as the western US. The results of this study suggest that, for channelized transport speeds of vchannel=1 m yr−1, channel spacings d≈102 m, and timescales of episodic melt infiltration τ≈101 kyr, the steady-state width of the TRZ in the lowermost CLM is ≈10 km. (3) Within the TRZ, disequilibrium heat exchange may contribute ≈10-5 W m−3 to the LAB heat budget.

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Soret, Mathieu, Philippe Agard, Benoît Ildefonse, Benoît Dubacq, Cécile Prigent, and Claudio Rosenberg. "Deformation mechanisms in mafic amphibolites and granulites: record from the Semail metamorphic sole during subduction infancy." Solid Earth 10, no. 5 (October 23, 2019): 1733–55. http://dx.doi.org/10.5194/se-10-1733-2019.

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Abstract. This study sheds light on the deformation mechanisms of subducted mafic rocks metamorphosed at amphibolite and granulite facies conditions and on their importance for strain accommodation and localization at the top of the slab during subduction infancy. These rocks, namely metamorphic soles, are oceanic slivers stripped from the downgoing slab and accreted below the upper plate mantle wedge during the first million years of intraoceanic subduction, when the subduction interface is still warm. Their formation and intense deformation (i.e., shear strain ≥5) attest to a systematic and transient coupling between the plates over a restricted time span of ∼1 Myr and specific rheological conditions. Combining microstructural analyses with mineral chemistry constrains grain-scale deformation mechanisms and the rheology of amphibole and amphibolites along the plate interface during early subduction dynamics, as well as the interplay between brittle and ductile deformation, water activity, mineral change, grain size reduction and phase mixing. Results indicate that increasing pressure and temperature conditions and slab dehydration (from amphibolite to granulite facies) lead to the nucleation of mechanically strong phases (garnet, clinopyroxene and amphibole) and rock hardening. Peak conditions (850 ∘C and 1 GPa) coincide with a pervasive stage of brittle deformation which enables strain localization in the top of the mafic slab, and therefore possibly the unit detachment from the slab. In contrast, during early exhumation and cooling (from ∼850 down to ∼700 ∘C and 0.7 GPa), the garnet–clinopyroxene-bearing amphibolite experiences extensive retrogression (and fluid ingression) and significant strain weakening essentially accommodated in the dissolution–precipitation creep regime including heterogeneous nucleation of fine-grained materials and the activation of grain boundary sliding processes. This deformation mechanism is closely assisted with continuous fluid-driven fracturing throughout the exhumed amphibolite, which contributes to fluid channelization within the amphibolites. These mechanical transitions, coeval with detachment and early exhumation of the high-temperature (HT) metamorphic soles, therefore controlled the viscosity contrast and mechanical coupling across the plate interface during subduction infancy, between the top of the slab and the overlying peridotites. Our findings may thus apply to other geodynamic environments where similar temperatures, lithologies, fluid circulation and mechanical coupling between mafic rocks and peridotites prevail, such as in mature warm subduction zones (e.g., Nankai, Cascadia), in lower continental crust shear zones and oceanic detachments.

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Journal articles on the topic 'Mantle weakening'

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