scholarly journals Sustainable densification of the deep crust

Geology ◽  
2020 ◽  
Vol 48 (7) ◽  
pp. 673-677 ◽  
Author(s):  
Benjamin Malvoisin ◽  
Håkon Austrheim ◽  
György Hetényi ◽  
Julien Reynes ◽  
Jörg Hermann ◽  
...  
Keyword(s):  
Lower Crust ◽  
Subduction Zones ◽  
Numerical Models ◽  
Fluid Pressure ◽  
Unstable System ◽  
Porosity Formation ◽  
Deep Crust ◽  
The Earth ◽  
Buoyancy Forces ◽  
Wave Emission

Abstract The densification of the lower crust in collision and subduction zones plays a key role in shaping the Earth by modifying the buoyancy forces acting at convergent boundaries. It takes place through mineralogical reactions, which are kinetically favored by the presence of fluids. Earthquakes may generate faults serving as fluid pathways, but the influence of reactions on the generation of seismicity at depth is still poorly constrained. Here we present new petrological data and numerical models to show that in the presence of fluids, densification reactions can occur very fast, on the order of weeks, and consume fluids injected during an earthquake, which leads to porosity formation and fluid pressure drop by several hundreds of megapascals. This generates a mechanically highly unstable system subject to collapse and further seismic-wave emission during aftershocks. This mechanism creates new pathways for subsequently arriving fluids, and thus provides a route for self-sustained densification of the lower crust.

Bringing slow slip processes into focus

2020 ◽  
Author(s):  
Rebecca Bell
Keyword(s):  
High Resolution ◽  
Subduction Zones ◽  
Seismic Velocity ◽  
Numerical Models ◽  
Fluid Pressure ◽  
Geophysical Methods ◽  
Rock Properties ◽  
Slow Slip ◽  
Velocity Models ◽  
The North

<p>The discovery of slow slip events (SSEs) at subduction margins in the last two decades has changed our understanding of how stress is released at subduction zones. Fault slip is now viewed as a continuum of different slip modes between regular earthquakes and aseismic creep, and an appreciation of seismic hazard can only be realised by understanding the full spectrum of slip. SSEs may have the potential to trigger destructive earthquakes and tsunami on faults nearby, but whether this is possible and why SSEs occur at all are two of the most important questions in earthquake seismology today. Laboratory and numerical models suggest that slow slip can be spontaneously generated under conditions of very low effective stresses, facilitated by high pore fluid pressure, but it has also been suggested that variations in frictional behaviour, potentially caused by very heterogeneous fault zone lithology, may be required to promote slow slip.</p><p>Testing these hypotheses is difficult as it requires resolving rock properties at a high resolution many km below the seabed sometimes in km’s of water, where drilling is technically challenging and expensive. Traditional geophysical methods like travel-time tomography cannot provide fine-scale enough velocity models to probe the rock properties in fault zones specifically. In the last decade, however, computational power has improved to the point where 3D full-waveform inversion (FWI) methods make it possible to use the full wavefield rather than just travel times to produce seismic velocity models with a resolution an order of magnitude better than conventional models. Although the hydrocarbon industry have demonstrated many successful examples of 3D FWI the method requires extremely high density arrays of instruments, very different to the 2D transect data collection style which is still commonly employed at subduction zones.</p><p> The north Hikurangi subduction zone, New Zealand is special, as it hosts the world’s most well characterised shallow SSEs (<2 km to 15 km below the seabed).  This makes it an ideal location to collect 3D data optimally for FWI to resolve rock properties in the slow slip zone. In 2017-2018 an unprecedentedly large 3D experiment including 3D multi-channel seismic reflection, 99 ocean bottom seismometers and 194 onshore seismometers was conducted along the north Hikurangi margin in an 100 km x 15 km area, with an average 2 km instrument spacing. In addition, IODP Expeditions 372 and 375 collected logging-while drilling and core data, and deployed two bore-hole observatories to target slow slip in the same area. In this presentation I will introduce you to this world class 3D dataset and preliminary results, which will enable high resolution 3D models of physical properties to be made to bring slow slip processes into focus.  </p>

Fluid Infiltration Through Oceanic Lower Crust in Response to Reaction‐Induced Fracturing: Insights From Serpentinized Troctolite and Numerical Models

2020 ◽  
Vol 125 (11) ◽  
Author(s):  
Kazuki Yoshida ◽  
Atsushi Okamoto ◽  
Hiroyuki Shimizu ◽  
Ryosuke Oyanagi ◽  
Noriyoshi Tsuchiya ◽  
...  
Keyword(s):  
Lower Crust ◽  
Numerical Models ◽  
Fluid Infiltration

scholarly journals The Role of Lower Crustal Rheology in Lithospheric Delamination During Orogeny

2021 ◽  
Vol 9 ◽  
Author(s):  
Lin Chen
Keyword(s):  
Lower Crust ◽  
Numerical Models ◽  
The Tibetan Plateau ◽  
Mantle Lithosphere ◽  
Laboratory Studies ◽  
Surface Uplift ◽  
Continental Lower Crust ◽  
Lower Crustal ◽  
Flow Laws

The continental lower crust is an important composition- and strength-jump layer in the lithosphere. Laboratory studies show its strength varies greatly due to a wide variety of composition. How the lower crust rheology influences the collisional orogeny remains poorly understood. Here I investigate the role of the lower crust rheology in the evolution of an orogen subject to horizontal shortening using 2D numerical models. A range of lower crustal flow laws from laboratory studies are tested to examine their effects on the styles of the accommodation of convergence. Three distinct styles are observed: 1) downwelling and subsequent delamination of orogen lithosphere mantle as a coherent slab; 2) localized thickening of orogen lithosphere; and 3) underthrusting of peripheral strong lithospheres below the orogen. Delamination occurs only if the orogen lower crust rheology is represented by the weak end-member of flow laws. The delamination is followed by partial melting of the lower crust and punctuated surface uplift confined to the orogen central region. For a moderately or extremely strong orogen lower crust, topography highs only develop on both sides of the orogen. In the Tibetan plateau, the crust has been doubly thickened but the underlying mantle lithosphere is highly heterogeneous. I suggest that the subvertical high-velocity mantle structures, as observed in southern and western Tibet, may exemplify localized delamination of the mantle lithosphere due to rheological weakening of the Tibetan lower crust.

scholarly journals Magnetar birth: rotation rates and gravitational-wave emission

2020 ◽  
Vol 494 (4) ◽  
pp. 4838-4847 ◽  
Author(s):  
S K Lander ◽  
D I Jones
Keyword(s):  
Viscous Dissipation ◽  
Radio Burst ◽  
Stellar Rotation ◽  
Radio Pulsars ◽  
Rotation Rates ◽  
Buoyancy Forces ◽  
Wave Emission ◽  
Fast Radio Burst ◽  
Electromagnetic Signals ◽  
First Time

ABSTRACT Understanding the evolution of the angle χ between a magnetar’s rotation and magnetic axes sheds light on the star’s birth properties. This evolution is coupled with that of the stellar rotation Ω, and depends on the competing effects of internal viscous dissipation and external torques. We study this coupled evolution for a model magnetar with a strong internal toroidal field, extending previous work by modelling – for the first time in this context – the strong protomagnetar wind acting shortly after birth. We also account for the effect of buoyancy forces on viscous dissipation at late times. Typically, we find that χ → 90° shortly after birth, then decreases towards 0° over hundreds of years. From observational indications that magnetars typically have small χ, we infer that these stars are subject to a stronger average exterior torque than radio pulsars, and that they were born spinning faster than ∼100–300 Hz. Our results allow us to make quantitative predictions for the gravitational and electromagnetic signals from a newborn rotating magnetar. We also comment briefly on the possible connection with periodic fast radio burst sources.

scholarly journals Accelerated non-linear destruction of the earth's crust

2001 ◽  
Vol 6 (4) ◽  
pp. 281-290
Author(s):  
E. V. Artyushkov
Keyword(s):  
Lower Crust ◽  
Sedimentary Basins ◽  
Earth’S Crust ◽  
Mountain Ranges ◽  
Crustal Rocks ◽  
The Earth ◽  
Lithospheric Plates ◽  
Temperature And Pressure ◽  
Earth's Crust ◽  
Major Hydrocarbon

The upper part of the Earth—the lithospheric layer,∼100 km thick, is rigid. Segments of this spherical shell–lithospheric plates are drifting over a ductile asthenosphere. On the continents, the lithosphere includes the Earth's crust,∼40 km thick, which is underlain by peridotitic rocks of the mantle. In most areas, at depths∼20–40 km the continental crust is composed of basalts with density∼2900kg m−3. At temperature and pressure typical for this depth, basalts are metastable and should transform into another assemblage of minerals which corresponds to garnet granulites and eclogites with higher densities 3300–3600 kgm−3. The rate of this transformation is extremely low in dry rocks, and the associated contraction of basalts evolves during the time≥108a. To restore the Archimede's equilibrium, the crust subsides with a formation of sedimentary basins, up to 10–15 km deep.Volumes of hot mantle with a water-containing fluid emerge sometimes from a deep mantle to the base of the lithosphere. Fluids infiltrate into the crust through the mantle part of the lithosphere. They catalyze the reaction in the lower crust which results in rock contraction with a formation of deep water basins at the surface during∼106a. The major hydrocarbon basins of the world were formed in this way. Infiltration of fluids strongly reduces the viscosity of the lithosphere, which is evidenced by narrow-wavelength deformations of this layer. At times of softening of the mantle part of the lithosphere, it becomes convectively replaced by a hotter and lighter asthenosphere. This process has resulted in the formation of many mountain ranges and high plateaus during the last several millions of years. Softening of the whole lithospheric layer which is rigid under normal conditions allows its strong compressive and tensile deformations. At the epochs of compression, a large portion of dense eclogites that were formed from basalts in the lower crust sink deeply into the mantle. In some cases they carry down lighter blocks of granites and sedimentary rocks of the upper crust which delaminate from eclogitic blocks and emerge back to the crust. Such blocks of upper crustal rocks include diamonds and other minerals which were formed at a depth of 100–150 km.

Advances in understanding localised variations in deformation experiments using numerical models

2020 ◽  
Author(s):  
Sebastian Cionoiu ◽  
Lucie Tajčmanová ◽  
Lyudmila Khakimova
Keyword(s):  
Grain Size ◽  
Phase Transitions ◽  
Subduction Zones ◽  
Numerical Models ◽  
3D Models ◽  
Rock Properties ◽  
Stress And Strain ◽  
Transition Distribution ◽  
Pressure Stress ◽  
2D And 3D

<p>Phase transitions affect the physical properties of rocks (e.g. rheology) and control geodynamic processes at different spatial and time scales. However, the influence of deformation on phase transitions and their coupling is not well understood. <br>Previous experiments, with both assembly-induced and additionally placed mechanical heterogeneities, have shown patterns in the phase transition distribution. Numerical modelling (2D, viscous finite difference models) have been used to correlate the experimental observations with the mechanic stress state. The locally increased mean stress in the models shows the best correlation with the formation of high-pressure polymorphs in experiments (Cionoiu et al. 2019).<br>Besides the distribution of polymorphs, grain-size and deformation patterns also vary across the samples due to stress, strain and pressure variations. To better understand the mechanisms contributing to these variations, we used advanced numerical models (3D, viscoelastic) to calculate the local distribution of first order parameters as pressure, stress and strain. The modelled stress and strain patterns are compared to the experimentally produced phase transformation distribution and previous (2D) modelling results. The 2D and 3D models differ partially regarding the quantification of local stresses – an effect that mainly depends on sample geometry (coaxial vs. general-shear). However, the qualitative fit between experiments, 2D and 3D models persists (i.e. the localisation of increased stresses or strain).<br>This contribution shows how numerical models, that closely represent the sample, can further improve the understanding of processes occurring in deformation experiments. Our new results emphasize that mechanically-induced stress-variations influence the grain-size and mineralogy of rocks which feeds back on their rheology.</p><p>References: <br>Cionoiu, S., Moulas, E. & Tajčmanová, L. Impact of interseismic deformation on phase transformations and rock properties in subduction zones. Sci Rep 9, 19561 (2019)</p>

Vp/Vs ratio and dehydration reactions in subduction zones

2020 ◽  
Author(s):  
Nicolas Brantut ◽  
Emmanuel David
Keyword(s):  
Experimental Data ◽  
Subduction Zones ◽  
Fluid Pressure ◽  
Closed Form Solution ◽  
Form Solution ◽  
Strong Increase ◽  
Seismological Data ◽  
Dehydration Reactions ◽  
Model Predictions ◽  
Effective Medium Approach

<p>High Vp/Vs ratio is a commonly used diagnostic for elevated fluid pressure when interpreting seismological data. The physical basis for this interpretation comes from rock physical data and models of isotropic, cracked rocks. Here, we establish precise conditions under which this interpretation is correct, by using an effective medium approach for fluid-saturated rocks. While the usual result of an increasing Vp/Vs with increasing fluid-saturated porosity holds for crack-like pores, we find that Vp/Vs ratio is not always monotonically increasing with increasing fluid content if the porosity shape deviates from thin cracks, and if the initial Vp/Vs of the rock (without porosity) is already quite high. This is specifically the case of dehydrating rocks, where initial Vp/Vs may already be high (>1.9 for lizardite, for instance), and where the porosity created by the dehydration reaction may be in the form of elongated needles. The model predictions are supported by existing experimental data obtained during dehydration experiments in gypsum and lizardite, which both show a significant decrease in Vp/Vs as dehydration proceeds. Although no experimental data is yet availbale on antigorite, we make a prediction that antigorite dehydration may not lead to any strong increase in Vp/Vs ratio under typical subduction zone conditions. We present our theoretical results in the form of simple closed-form solution (valid asymptotically for a range of limiting cases), which should help guide the interpretation of Vp/Vs ratio from seismological data.</p>

Plate speeds modulated by sediment subduction: insights from numerical models

2020 ◽  
Author(s):  
Whitney Behr ◽  
Adam Holt ◽  
Thorsten Becker ◽  
Claudio Faccenna
Keyword(s):  
Subduction Zones ◽  
Convergence Rates ◽  
Numerical Models ◽  
Finite Width ◽  
Dynamic Topography ◽  
Plate Interface ◽  
Interface Models ◽  
Low Viscosity ◽  
Uniform Interface ◽  
Sinking Velocities

<p>Tectonic plate velocities predominantly result from a balance between the potential energy change of the subducting slab and viscous dissipation in the mantle, bending lithosphere, and slab–upper plate interface. A range of observations suggest that slabs may be weak, implying a more prominent role for plate interface dissipation than previously thought. Behr & Becker (2018) suggested that the deep interface viscosity in subduction zones should be strongly affected by the relative proportions of sedimentary to mafic rocks that are subducted to depth, and that sediment subduction should thus facilitate faster subduction plate speeds. Here we use fully dynamic 2D subduction models built with the code ASPECT to quantitatively explore how subduction interface viscosity influences: a) subducting plate sinking velocities, b) trench migration rates, c) convergence velocities, d) upper plate strain regimes, e) dynamic topography, and f) interactions with the 660 km mantle transition zone.  We implement two main types of models, including 1) uniform interface models where interface viscosity and slab strength are systematically varied, and 2) varying interface models where a low viscosity sediment strip of finite width is embedded within a higher viscosity interface. Uniform interface models indicate that low viscosity (sediment-lubricated) slabs have substantially faster sinking velocities prior to reaching the 660, especially for weak slabs, and also that they achieve faster ‘steady state’ velocities after 660 penetration. Even models where sediments are limited to a strip on the seafloor show accelerations in convergence rates of up to ~5 mm/y per my, with convergence initially accommodated by trench rollback and later by slab sinking. We discuss these results in the context of well-documented plate accelerations in Earth’s history such as India-Asia convergence and convergence rate oscillations along the Andean margin.</p><p>References: Behr, W. M., & Becker, T. W. (2018). Sediment control on subduction plate speeds. <em>Earth and Planetary Science Letters</em>, <em>502</em>, 166-173.</p>

Verification, Validation, and Confirmation of Numerical Models in the Earth Sciences

Science ◽  
1994 ◽  
Vol 263 (5147) ◽  
pp. 641-646 ◽  
Author(s):  
N. Oreskes ◽  
K. Shrader-Frechette ◽  
K. Belitz
Keyword(s):  
Earth Sciences ◽  
Numerical Models ◽  
The Earth
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