Intermediate-depth earthquake generation and shear zone formation caused by grain size reduction and shear heating

Geology ◽  
2015 ◽  
Vol 43 (9) ◽  
pp. 791-794 ◽  
Author(s):  
M. Thielmann ◽  
A. Rozel ◽  
B.J.P. Kaus ◽  
Y. Ricard
Keyword(s):  
Grain Size ◽  
Shear Zone ◽  
Size Reduction ◽  
Zone Formation ◽  
Intermediate Depth ◽  
Shear Heating ◽  
Earthquake Generation ◽  
Intermediate Depth Earthquake

Earthquake cycles and shear zones: interplay between earthquakes, aseismic fault slip, and bulk viscous deformation

2021 ◽  
Author(s):  
Kali Allison ◽  
Laurent Montesi ◽  
Eric Dunham
Keyword(s):  
Grain Size ◽  
Viscous Flow ◽  
Shear Zone ◽  
Shear Zones ◽  
Strike Slip ◽  
Fault Slip ◽  
Coseismic Slip ◽  
Bulk Viscous ◽  
Shear Heating ◽  
Seismic Cycles

<p>The interaction between the seismogenic portion of faults and their ductile roots is central to understanding the mechanics of seismic cycles. It is well established that faults are highly localized within the cold and brittle upper crust, but less is known about fault and shear zone structure in the warmer, more ductile, lower crust and in the upper mantle. Increasing temperature with depth causes two transitions in behavior: a frictional transition from seismic to aseismic fault behavior and a transition from brittle to ductile off-fault deformation (BDT). To explore the effects of these two transitions on seismic cycle characteristics (e.g., recurrence interval, nucleation depth, and down-dip limit of coseismic rupture), we simulate seismic cycles on a 2D strike-slip fault. All phases of the earthquake cycle are simulated, allowing the model to spontaneously generate earthquakes and to capture aseismic fault slip and off-fault viscous flow in the interseismic period. The fault is represented with rate-and-state friction. In the off-fault material, distributed viscous flow occurs through dislocation creep. We also consider two possible weakening mechanisms that may be active in lower crustal shear zones: shear heating and grain size reduction, which changes the ductile rheology from dislocation to diffusion creep. This model makes it possible to self-consistently simulate the variations of stress, strain rate, and grain size in the vicinity of a strike-slip fault.</p><p>We find that the viscous shear zone beneath the fault (defined as the region of elevated viscous strain rate) is roughly elliptically shaped, extending up to 10 km below the fault and with a width of 1 to 3 km. When weakening mechanisms are neglected, the BDT occurs below the depth of the transition from seismic to aseismic fault slip. In these cases, seismic cycle characteristics are similar to those of a traditional elastic cycle simulation that neglects viscoelastic deformation. However, the inclusion of shear heating, which produces a thermal anomaly relative to the background geotherm, shallows the BDT enough to limit the down-dip propagation of coseismic slip in some cases. In these cases, earthquakes penetrate 1-2 km into the shear zone, consistent with observations of zones in which both viscous flow and coseismic slip occur. Also, in these simulations, very little aseismic fault slip occurs. Instead, tectonic plate motion is accommodated primarily through coseismic slip and bulk viscous flow. Preliminary simulations that include the effects of grain size reduction within the shear zone show similar effects. Both weakening mechanisms narrow the shear zone by up to 20%, suggesting that the fault also plays a large role in controlling shear zone localization.</p>

scholarly journals Brittle grain size reduction of feldspar, phase mixing and strain localization in granitoids at mid-crustal conditions (Pernambuco shear zone, NE Brazil)

2015 ◽  
Vol 7 (4) ◽  
pp. 2953-2998
Author(s):  
G. Viegas ◽  
L. Menegon ◽  
C. J. Archanjo
Keyword(s):  
Grain Size ◽  
Shear Zone ◽  
Strain Localization ◽  
Shear Bands ◽  
Preferred Orientation ◽  
Size Reduction ◽  
Diffusion Creep ◽  
Crystallographic Preferred Orientation ◽  
Fine Grained

Abstract. The Pernambuco shear zone (northeastern Brazil) is a large-scale strike-slip fault that, in its eastern segment, deforms granitoids at mid-crustal conditions. Initially coarse (> 50 μm) grained feldspar porphyroclasts are intensively fractured and reduced to an ultrafine-grained mixture consisting of plagioclase and K-feldspar grains (~ < 15 μm in size) localized in C' shear bands. Detailed microstructural observations and EBSD analysis do not show evidence of intracrystalline plasticity in feldspar porphyroclasts and/or fluid-assisted replacement reactions. Quartz occurs either as thick (~ 1–2 mm) monomineralic bands or as thin ribbons dispersed in the feldspathic mixture. The microstructure and c axis crystallographic preferred orientation are similar in the thick monomineralic band and in the thin ribbons, and suggest dominant subgrain rotation recrystallization and activity of prism ⟨a⟩ and rhomb ⟨a⟩ slip systems. However, the grain size in monophase recrystallized domains decreases when moving from the transposed veins to the thin ribbons embedded in the feldspathic C' bands (14 μm vs. 5 μm, respectively). The fine-grained feldspar mixture has a weak crystallographic preferred orientation interpreted as the result of oriented growth during diffusion creep, as well as the same composition as the fractured porphyroclasts, suggesting that it generated by mechanical fragmentation of rigid porphyroclasts with a negligible role of chemical disequilibrium. Assuming that the C' shear bands deformed under constant stress conditions, the polyphase feldspathic aggregate would have deformed at a strain rate one order of magnitude faster than the monophase quartz ribbons. Overall, our dataset indicates that feldspar underwent a brittle-viscous transition while quartz was deforming via crystal plasticity. The resulting rock microstructure consists of a two-phase rheological mixture (fine-grained feldspars and recrystallized quartz) in which the polyphase feldspathic material localized much of the strain. Extensive grain-size reduction and weakening of feldspars is attained in the East Pernambuco mylonites mainly via fracturing under relatively fluid-absent conditions which would trigger a switch to diffusion creep and further strain localization without a prominent role of metamorphic reactions.

Grain-size reduction of feldspar and flow of deformed granites within the Gaoligong shear zone, southwestern Yunnan, China

2019 ◽  
Vol 62 (9) ◽  
pp. 1379-1398
Author(s):  
Yanlong Dong ◽  
Shuyun Cao ◽  
Xuemei Cheng ◽  
Junlai Liu ◽  
Hanchen Cao
Keyword(s):  
Grain Size ◽  
Shear Zone ◽  
Size Reduction

scholarly journals Brittle grain-size reduction of feldspar, phase mixing and strain localization in granitoids at mid-crustal conditions (Pernambuco shear zone, NE Brazil)

Solid Earth ◽  
2016 ◽  
Vol 7 (2) ◽  
pp. 375-396 ◽  
Author(s):  
Gustavo Viegas ◽  
Luca Menegon ◽  
Carlos Archanjo
Keyword(s):  
Grain Size ◽  
Shear Zone ◽  
Strain Localization ◽  
Shear Bands ◽  
Preferred Orientation ◽  
Size Reduction ◽  
Diffusion Creep ◽  
Crystallographic Preferred Orientation ◽  
Fine Grained

Abstract. The Pernambuco shear zone (northeastern Brazil) is a large-scale strike-slip fault that, in its eastern segment, deforms granitoids at mid-crustal conditions. Initially coarse-grained (> 50 µm) feldspar porphyroclasts are intensively fractured and reduced to an ultrafine-grained mixture consisting of plagioclase and K-feldspar grains (< 15 µm) localized in C' shear bands. Detailed microstructural observations and electron backscatter diffraction (EBSD) analysis do not show evidence of intracrystalline plasticity in feldspar porphyroclasts and/or fluid-assisted replacement reactions. Quartz occurs either as thick (∼ 1–2 mm) monomineralic veins transposed along the shear zone foliation or as thin ribbons ( ≤ 25 µm width) dispersed in the feldspathic mixture. The microstructure and c axis crystallographic-preferred orientation are similar in the thick monomineralic veins and in the thin ribbons, and they suggest dominant subgrain rotation recrystallization and activity of prism < a > and rhomb < a > slip systems. However, the grain size in monophase recrystallized domains decreases when moving from the quartz monomineralic veins to the thin ribbons embedded in the feldspathic C' bands (14 µm vs. 5 µm respectively). The fine-grained feldspar mixture has a weak crystallographic-preferred orientation interpreted as the result of shear zone parallel-oriented growth during diffusion creep, as well as the same composition as the fractured porphyroclasts, suggesting that it generated by mechanical fragmentation of rigid porphyroclasts with a negligible role of chemical disequilibrium. Once C' shear bands were generated and underwent viscous deformation at constant stress conditions, the polyphase feldspathic aggregate would have deformed at a strain rate 1 order of magnitude faster than the monophase quartz monomineralic veins, as evidenced by applying experimentally and theoretically calibrated flow laws for dislocation creep in quartz and diffusion creep in feldspar. Overall, our data set indicates that feldspar underwent a brittle-viscous transition while quartz was deforming via crystal plasticity. The resulting rock microstructure consists of a two-phase rheological mixture (fine-grained feldspars and recrystallized quartz) in which the polyphase feldspathic material localized much of the strain. Extensive grain-size reduction and weakening of feldspars is attained in the East Pernambuco mylonites mainly via fracturing which would trigger a switch to diffusion creep and strain localization without a prominent role of metamorphic reactions.

Tectonic regimes and stress patterns in the Vrancea Seismic Zone: Insights into intermediate-depth earthquake nests in locked collisional settings

2021 ◽  
Vol 799 ◽  
pp. 228688
Author(s):  
Laura Petrescu ◽  
Felix Borleanu ◽  
Mircea Radulian ◽  
Alik Ismail-Zadeh ◽  
Liviu Maţenco
Keyword(s):  
Seismic Zone ◽  
Intermediate Depth ◽  
Stress Patterns ◽  
Intermediate Depth Earthquake

Dynamic strain aging and grain size reduction effects on the fatigue resistance of SA533B3 steels

2004 ◽  
Vol 324 (2-3) ◽  
pp. 140-151 ◽  
Author(s):  
J.Y Huang ◽  
J.R Hwang ◽  
J.J Yeh ◽  
C.Y Chen ◽  
R.C Kuo ◽  
...  
Keyword(s):  
Grain Size ◽  
Dynamic Strain Aging ◽  
Fatigue Resistance ◽  
Strain Aging ◽  
Size Reduction ◽  
Dynamic Strain

Grain size reduction by utilizing a very thin CrW seedlayer and dry-etching process in CoCrTaNiPt longitudinal media

2000 ◽  
Vol 87 (9) ◽  
pp. 6860-6862 ◽  
Author(s):  
Satoru Yoshimura ◽  
D. D. Djayaprawira ◽  
Tham Kim Kong ◽  
Yusuke Masuda ◽  
Hiroki Shoji ◽  
...  
Keyword(s):  
Grain Size ◽  
Dry Etching ◽  
Size Reduction ◽  
Etching Process

Modeling and Numerical Simulation Research on Hollow Steel Ingot Forging Process of Heavy Cylinder Forging

2013 ◽  
Vol 712-715 ◽  
pp. 627-632
Author(s):  
Min Liu ◽  
Qing Xian Ma
Keyword(s):  
Numerical Simulation ◽  
Grain Size ◽  
Shear Zone ◽  
Grain Refinement ◽  
Steel Ingot ◽  
Effective Strain ◽  
Meridian Plane ◽  
Forging Process ◽  
Forming Load ◽  
Average Grain Size

Aiming at the disadvantages of low utilization ratio of steel ingot, uneven microstructure properties and long production period in the solid steel ingot forging process of heavy cylinder forgings such as reactor pressure vessel, a new shortened process using hollow steel ingot was proposed. By means of modeling of lead sample and DEFORM-3D numerical simulation, the deformation law and grain refinement behavior for 162 ton hollow steel ingot upsetting at different reduction ratios, pressing speeds and friction factors were investigated, and the formation rule of inner-wall defects in upsetting of hollow steel ingots with different shape factors was further analyzed. Simulation results show that the severest deformation occurs in the shear zone of meridian plane in the upsetting process of hollow steel ingot, and the average grain size in the shear zone is the smallest. As pressing speed increases, the forming load gradually increases and the deformation uniformity gets worse, while the average grain size decreases. An increase in friction factor can increase the peak value of effective strain, but it significantly reduces the deformation uniformity, increases the forming load and goes against grain refinement. Moreover, the four kinds of defects on the inner wall of steel ingot can be eliminated effectively by referring to the plotted defect control curve for hollow steel ingot during high temperature upsetting.

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