Micro- and nano-porosity of the active Alpine Fault zone, New Zealand

Kirilova, Martina; Toy, Virginia; Sauer, Katrina; Renard, François; Gessner, Klaus; Wirth, Richard; Xiao, Xianghui; Matsumura, Risa

doi:10.5194/se-11-2425-2020

Cited by 2 publications

(5 citation statements)

References 47 publications

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“…The data sets were processed, segmented, analyzed and visualized in Avizo (Thermo Fischer Scientific). Depending on the noise filtering algorithm, the segmentation method, and the interconnectivity options between voxels employed, final porosity values can vary (see Kirilova et al., 2020; McBeck et al., 2021 and references therein). All SμCT volumes were processed similarly with the same workflow recipe.…”

Section: Methodsmentioning

confidence: 99%

“…The earthquake rupture propagation resulted in a damage zone that transiently hosted a network of well-interconnected fracture porosity, before it was efficiently sealed and healed by solution-precipitation mechanisms, as is commonly seen in the upper crust (e.g., Evans et al, 1997;Kirilova et al, 2020;Mitchell & Faulkner 2012; Figure 9). Remnants of these interconnected pores are found in small clusters (Figure 6a; Figure S10 in Supporting Information S1), and in survivor clasts within the pseudotachylyte (Figure 6g).…”

Section: Preservation Of Coseismic Porosity and Of Fluid-rock Interac...mentioning

confidence: 99%

“…Thus far, porosity has been characterized to control fault strength in upper‐crustal cataclasites and fault gouge (Kirilova et al., 2020), or to be related to melt degassing during coseismic slip in fluid‐rich upper‐crustal pseudotachylytes (Boullier et al., 2001; Gomila et al., 2021; Magloughlin, 2011; Rowe et al., 2005). In this contribution we report a detailed microstructural account of porosity in pristine and mylonitized pseudotachylytes from the lower‐crustal anorthosites of Lofoten, Norway.…”

Section: Introductionmentioning

confidence: 99%

“…Although the microstructures signifying fracture-induced fluid pathways and infiltration of fluids promoting metamorphic and rheological changes have been well-documented (Austrheim, 1987), the evolution of this associated porosity as the coseismic pseudotachylyte becomes mylonitized in the post-and interseismic stages needs to be characterized. Thus far, porosity has been characterized to control fault strength in upper-crustal cataclasites and fault gouge (Kirilova et al, 2020), or to be related to melt degassing during coseismic slip in fluid-rich upper-crustal pseudotachylytes (Boullier et al, 2001;Gomila et al, 2021;Magloughlin, 2011;Rowe et al, 2005). In this contribution we report a detailed microstructural account of porosity in pristine and mylonitized pseudotachylytes from the lower-crustal anorthosites of Lofoten, Norway.…”

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confidence: 98%

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Dynamic Evolution of Porosity in Lower‐Crustal Faults During the Earthquake Cycle

et al. 2023

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Earthquake‐induced fracturing of the dry and strong lower crust can transiently increase permeability for fluids to flow and trigger metamorphic and rheological transformations. However, little is known about the porosity that facilitates these transformations. We analyzed microstructures that have recorded the mechanisms generating porosity in the lower crust from a pristine pseudotachylyte (solidified earthquake‐derived frictional melt) and a mylonitized pseudotachylyte from Lofoten, Norway to understand the evolution of fluid pathways from the coseismic to the post‐ and interseismic stages of the earthquake cycle. Porosity is dispersed and poorly interconnected within the pseudotachylyte vein (0.14 vol%), with a noticeably increased amount along garnet grain boundaries (0.25 ‐ 0.41 vol%). This porosity formed due to a net negative volume change at the grain boundary when garnet overgrows the pseudotachylyte matrix. Efficient healing of the damage zone by fluid‐assisted growth of feldspar neoblasts resulted in the preservation of only a few but relatively large interconnected pores along coseismic fractures (0.03 vol% porosity). In contrast, porosity in the mylonitized pseudotachylyte is dramatically reduced (0.02 vol% overall), because of the efficient precipitation of phases (amphibole, biotite and feldspars) into transient pores during grain‐size sensitive creep. Porosity reduction on the order of >85 % may be a contributing factor in shear zone hardening, potentially leading to the development of new pseudotachylytes overprinting the mylonites. Our results show that earthquake‐induced rheological weakening of the lower crust is intermittent and occurs when a fluid can infiltrate a transiently permeable shear zone, thereby facilitating diffusive mass transfer and creep.

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Section: Methodsmentioning

confidence: 99%

Section: Preservation Of Coseismic Porosity and Of Fluid-rock Interac...mentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

mentioning

confidence: 98%

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Dynamic Evolution of Porosity in Lower‐Crustal Faults During the Earthquake Cycle

et al. 2023

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“…The data sets were processed, segmented, analyzed and visualized in Avizo (Thermo Fischer Scientific). Depending on the noise filtering algorithm, the segmentation method, and the interconnectivity options between voxels employed, final porosity values can vary (see Kirilova et al, 2020;McBeck et al, 2021 and references therein). All SμCT volumes were processed similarly with the same workflow recipe.…”

Section: Synchrotron X-ray Microtomography (Sμct)mentioning

confidence: 99%

Dynamic evolution of porosity in lower crustal faults during the earthquake cycle

Michalchuk

Menegon

Renard

et al. 2022

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Fractures derived from earthquakes can create permeable conduits for fluids to flow, enhancing fluid-rock interactions, and potentially altering the strength and rheology of fault systems. In the dry lower crust, numerous field examples show mutually overprinting pseudotachylytes (solidified melts produced during seismic slip) and mylonitized pseudotachylytes (produced during the post- and interseismic viscous creep). The mylonites contain hydrous mineral assemblages, suggesting episodic pulses of fluids infiltration and rheological weakening triggered by the earthquake. Here, our aim is to understand the porosity generating mechanisms during the earthquake cycle and characterize the intermittent evolution of porosity.The Nusfjord East shear zone network (Lofoten, Norway) is an exhumed lower crustal section composed largely of anhydrous anorthosites that contain mutually overprinting pseudotachylytes and mylonitized pseudotachylytes. We present a microstructural analysis focusing on the mechanisms generating, maintaining, and destroying porosity from an exhumed network of lower crustal coeval pseudotachylytes and mylonites using synchrotron X-ray microtomography (S&#956;CT), focused ion beam scanning electron microscopy (FIB-SEM) nanotomography, electron backscatter diffraction (EBSD) analysis, and SEM imaging.In the pristine pseudotachylyte, S&#956;CT data show that porosity is concentrated within the pseudotachylyte vein (0.16 vol% porosity), especially around framboidal garnet clusters and single garnet grains. SEM observations reveal that garnets within the vein often contain an asymmetric rim of barium-enriched K-feldspar. The damage zone of the host anorthosite on the other hand is efficiently healed (0.03 vol% porosity) primarily with the growth of plagioclase neoblasts nucleated from pulverized fragments of the host anorthosite, and secondly with the precipitation of barium-enriched K-feldspar found lining intragranular microfractures. A FIB-SEM transect along one of these microfractures shows a myrmekite microstructure formed during fluid-rock interaction that completely sealed the porosity.In the mylonitized pseudotachylyte, S&#956;CT data show a porosity of 0.03 vol%, mainly concentrated within monomineralic domains of plagioclase, which are interpreted as recrystallized, sheared survivor clasts of wall-rock fragments. EBSD analyses indicate that deformation in these monomineralic domains was accommodated by diffusion creep and grain boundary sliding. Polymineralic domains along the mylonitic foliation, which primarily derive from the overprint of the original pseudotachylyte veins, also deformed by diffusion creep and grain boundary sliding. However, unlike in the monomineralic domains, they lack detectable porosity. We interpret these observations to reflect the efficient precipitation of hydrous phases into the pores during creep cavitation.Dynamic fracturing during earthquakes is the primary mechanism for porosity generation in the lower crust. Our study shows that porosity is further reduced by up to 90% when a pristine pseudotachylyte is viscously re-worked under deformation conditions promoting grain-size sensitive creep and grain boundary sliding. We suggest that such porosity reduction eventually results in shear zone hardening, which may evolve in the development of new pseudotachylytes overprinting the mylonites, as frequently observed in Nusfjord. Thus, earthquake-induced rheological weakening of the lower crust is intermittent, and occurs only as long as the fluid can infiltrate in the shear zone, thereby facilitating diffusive mass transfer.

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Micro- and nano-porosity of the active Alpine Fault zone, New Zealand

Cited by 2 publications

References 47 publications

Dynamic Evolution of Porosity in Lower‐Crustal Faults During the Earthquake Cycle

Dynamic Evolution of Porosity in Lower‐Crustal Faults During the Earthquake Cycle

Dynamic evolution of porosity in lower crustal faults during the earthquake cycle

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