Lower crustal earthquake facilitated by overpressurized frictional melts

2020 ◽  
Author(s):  
Xin Zhong ◽  
Arianne Petley-Ragan ◽  
Sarah Incel ◽  
Marcin Dabrowski ◽  
Niels Andersen ◽  
...  
Keyword(s):  
Wall Rock ◽  
Rapid Quenching ◽  
Elastic Model ◽  
Shear Rigidity ◽  
Geological Time ◽  
Differential Stress ◽  
Crustal Earthquakes ◽  
Fluid Pressurization ◽  
Frictional Melting ◽  
Lower Crustal

<p>Earthquakes are among the most catastrophic geological events that last only several to tens of seconds. During earthquakes, many processes may occur including rupturing, frictional sliding, pore fluid pressurization and occasionally frictional melting. However, little direct records of these fast processes remain preserved through geological time. During rapid shearing, frictional melt may form that lubricates the rocks and facilitates further sliding. The frictional melt layer may quench quickly within seconds to minutes depending on its thickness. After quenching, the product pseudotachylyte preserves valuable information about the conditions when the frictional melt was generated. Here, we study pseudotachylyte from Holsnøy Island in the Bergen Arcs of Western Norway, an exhumed portion of the lower continental crust. The investigated pseudotachylyte vein is ca. 1-2 cm thick and free of injection veins along the 2 m visible length of the fault. The pseudotachylyte matrix is made up of fine-grained omphacite (Jd<sub>38</sub>), sodic plagioclase (Ab<sub>83</sub>) and kyanite with minor rutile and sulphides. Many dendritic garnets are found within the pseudotachylyte showing gradual grain size reduction towards the wall rock. This suggests that the garnets crystallized during rapid quenching. The stability of epidote, kyanite and quartz in the wall rock plagioclase, and omphacite and albitic plagioclase together with quartz in the pseudotachylyte matrix constrains the ambient P ca. 1.5-1.7 GPa and T ca. 650-750°C. Using Raman elastic barometry, the constrained pressure condition from quartz inclusions in the dendritic garnets in the pseudotachylyte is > 2 GPa. Based on an elastic model (Eshelby’s solution), it is not possible to maintain 0.5 GPa overpressure within a thin melt layer by thermal pressurization or melting expansion. A potential explanation is that GPa level differential stress was present in the wall rocks and the melt pressure approached the normal stress when shear rigidity vanished during frictional melting. Our study illustrates how overpressure can be created within frictional melt veins under conditions of high differential stress, and offers a mechanism that facilitates co-seismic weakening during lower crustal earthquakes.  </p>

scholarly journals Lower crustal earthquake associated with highly pressurized frictional melts

2021 ◽  
Author(s):  
Xin Zhong ◽  
Arianne J. Petley-Ragan ◽  
Sarah H. M. Incel ◽  
Marcin Dabrowski ◽  
Niels H. Andersen ◽  
...  
Keyword(s):  
High Pressure ◽  
High Pressures ◽  
Maximum Principal Stress ◽  
Elastic Model ◽  
Differential Stress ◽  
Fault Surface ◽  
Seismic Fault ◽  
Crustal Earthquakes ◽  
Lower Crustal

AbstractEarthquakes at lower crustal depths are common during continental collision. However, the coseismic weakening mechanisms required to propagate an earthquake at high pressures are poorly understood. Transient high-pressure fluids or melts have been proposed as a viable mechanism, but verifying this requires direct in situ measurement of fluid or melt overpressure along fault planes that have hosted dynamic ruptures. Here, we report direct measurement of highly overpressurized frictional melts along a seismic fault surface. Using Raman spectroscopy, we identified high-pressure quartz inclusions sealed in dendritic garnets that grew from frictional melts formed by lower crustal earthquakes in the Bergen Arcs, Western Norway. Melt pressure was estimated to be 1.8–2.3 GPa on the basis of an elastic model for the quartz-in-garnet system. This is ~0.5 GPa higher than the pressure recorded by the surrounding pseudotachylyte matrix and wall rocks. The recorded melt pressure could not arise solely from the volume expansion of melting, and we propose that it was generated when melt pressure approached the maximum principal stress in a system subject to high differential stress. The associated palaeostress field demonstrates that a strong lower crust accommodated up to 1 GPa differential stress during the compressive stage of the Caledonian orogeny.

scholarly journals The earthquake cycle in the dry lower continental crust: insights from two deeply exhumed terranes (Musgrave Ranges, Australia and Lofoten, Norway)

2021 ◽  
Vol 379 (2193) ◽  
pp. 20190416 ◽  
Author(s):  
Luca Menegon ◽  
Lucy Campbell ◽  
Neil Mancktelow ◽  
Alfredo Camacho ◽  
Sebastian Wex ◽  
...  
Keyword(s):  
Continental Crust ◽  
Lower Crust ◽  
Shear Zones ◽  
Wall Rock ◽  
Earthquake Cycle ◽  
Geological Time ◽  
Lower Continental Crust ◽  
Seismic Slip ◽  
Crustal Earthquakes ◽  
Deep Source

This paper discusses the results of field-based geological investigations of exhumed rocks exposed in the Musgrave Ranges (Central Australia) and in Nusfjord (Lofoten, Norway) that preserve evidence for lower continental crustal earthquakes with focal depths of approximately 25–40 km. These studies have established that deformation of the dry lower continental crust is characterized by a cyclic interplay between viscous creep (mylonitization) and brittle, seismic slip associated with the formation of pseudotachylytes (a solidified melt produced during seismic slip along a fault in silicate rocks). Seismic slip triggers rheological weakening and a transition to viscous creep, which may be already active during the immediate post-seismic deformation along faults initially characterized by frictional melting and wall-rock damage. The cyclical interplay between seismic slip and viscous creep implies transient oscillations in stress and strain rate, which are preserved in the shear zone microstructure. In both localities, the spatial distribution of pseudotachylytes is consistent with a local (deep) source for the transient high stresses required to generate earthquakes in the lower crust. This deep source is the result of localized stress amplification in dry and strong materials generated at the contacts with ductile shear zones, producing multiple generations of pseudotachylyte over geological time. This implies that both the short- and the long-term rheological evolution of the dry lower crust typical of continental interiors is controlled by earthquake cycle deformation. This article is part of a discussion meeting issue ‘Understanding earthquakes using the geological record’.

scholarly journals Dynamic earthquake rupture in the lower crust

2019 ◽  
Vol 5 (7) ◽  
pp. eaaw0913 ◽  
Author(s):  
Arianne Petley-Ragan ◽  
Yehuda Ben-Zion ◽  
Håkon Austrheim ◽  
Benoit Ildefonse ◽  
François Renard ◽  
...  
Keyword(s):  
Lower Crust ◽  
Failure Process ◽  
Wall Rock ◽  
Ductile Deformation ◽  
Earthquake Rupture ◽  
Seismic Slip ◽  
Crustal Earthquakes ◽  
Sequence Of Events ◽  
Deformation Processes ◽  
Lower Crustal

Earthquakes in the continental crust commonly occur in the upper 15 to 20 km. Recent studies demonstrate that earthquakes also occur in the lower crust of collision zones and play a key role in metamorphic processes that modify its physical properties. However, details of the failure process and sequence of events that lead to seismic slip in the lower crust remain uncertain. Here, we present observations of a fault zone from the Bergen Arcs, western Norway, which constrain the deformation processes of lower crustal earthquakes. We show that seismic slip and associated melting are preceded by fracturing, asymmetric fragmentation, and comminution of the wall rock caused by a dynamically propagating rupture. The succession of deformation processes reported here emphasize brittle failure mechanisms in a portion of the crust that until recently was assumed to be characterized by ductile deformation.

scholarly journals Lower-crustal earthquakes in southern Tibet are linked to eclogitization of dry metastable granulite

2018 ◽  
Vol 9 (1) ◽  
Author(s):  
Feng Shi ◽  
Yanbin Wang ◽  
Tony Yu ◽  
Lupei Zhu ◽  
Junfeng Zhang ◽  
...  
Keyword(s):  
Southern Tibet ◽  
Crustal Earthquakes ◽  
Lower Crustal

Tonalites in crustal evolution

1981 ◽  
Vol 301 (1461) ◽  
pp. 293-303 ◽  
Keyword(s):  
Partial Melting ◽  
Plate Tectonics ◽  
Mafic Magma ◽  
Wall Rock ◽  
Continental Margins ◽  
Geological Time ◽  
Roof Rocks ◽  
Basaltic Crust ◽  
Dominant Part ◽  
Mantle Magma

Tonalites, including trondhjemite as a variety, played three roles through geological time in the generation of Earth’s crust. Before about 2.9 Ga ago they were produced largely by simple partial melting of metabasalt to give the dominant part of Archaean grey gneiss terranes. These terranes are notably bimodal; andesitic rocks are rare. Tonalites played a crucial role in the generation of this protocontinental and oldest crust 3.7- 2.9 Ga ago in that they were the only low-density, high-SiO 2 rocks produced directly from basaltic crust. In the enormous event giving the greenstone-granite terranes, mostly 2.8-2.6 Ga ago, tonalites formed in lesser but still important proportions by partial melting of metabasalt in the lower regions of down-buckled greenstone belts and by remobilization of older grey gneisses. Tectonism in the Archaean (3.9- 2.5 Ga ago) perhaps was controlled by small-cell convection (McKenzie & Weiss 1975). Little or no ophiolite or eclogite formed, and only minor andesite. Plate tectonics of modern type (involving large, rigid plates) commenced in the early Proterozoic. Uniformitarianism thus goes back one-half of the age of the earth. Tonalites compose about 5-10 % of crust generated in Proterozoic and Phanerozoic time at convergent oceanic-continental margins. They occur here as minor to prominent members of the compositionally continuous continental-margin batholiths. A simple model of generation of these batholiths is offered: mantle-derived mafic magma pools in the lower crust above a subduction zone reacts with and incorporates wall-rock components (Bowen 1922), and breaches its roof rocks as an initial diapir. This mantle magma also develops a gradient of partial melting in its wall rocks. This wall-rock melt accretes in the collapsed chamber and moves up the conduit broached by the initial diapir, the higher, less siliceous fractions of melting first, the lower, more siliceous (and further removed) fractions of melting last. The process gives in the optimum case a mafic-to-siliceous sequence of diorite or quartz diorite through tonalite or quartz monzodiorite to granodiorite and granite. The model implies that great masses of cumulate phases and refractory wall rock form the roots of continentalmargin batholiths, and that migmatites overlie that residuum and underlie the batholiths.

scholarly journals Fault rheology beyond frictional melting

2015 ◽  
Vol 112 (30) ◽  
pp. 9276-9280 ◽  
Author(s):  
Yan Lavallée ◽  
Takehiro Hirose ◽  
Jackie E. Kendrick ◽  
Kai-Uwe Hess ◽  
Donald B. Dingwell
Keyword(s):  
Glass Transition ◽  
High Strain ◽  
Frictional Heating ◽  
Wall Rock ◽  
Dissipative Heating ◽  
Strain Rates ◽  
Stored Energy ◽  
Slip Resistance ◽  
Frictional Melting ◽  
Brittle Fragmentation

During earthquakes, comminution and frictional heating both contribute to the dissipation of stored energy. With sufficient dissipative heating, melting processes can ensue, yielding the production of frictional melts or “pseudotachylytes.” It is commonly assumed that the Newtonian viscosities of such melts control subsequent fault slip resistance. Rock melts, however, are viscoelastic bodies, and, at high strain rates, they exhibit evidence of a glass transition. Here, we present the results of high-velocity friction experiments on a well-characterized melt that demonstrate how slip in melt-bearing faults can be governed by brittle fragmentation phenomena encountered at the glass transition. Slip analysis using models that incorporate viscoelastic responses indicates that even in the presence of melt, slip persists in the solid state until sufficient heat is generated to reduce the viscosity and allow remobilization in the liquid state. Where a rock is present next to the melt, we note that wear of the crystalline wall rock by liquid fragmentation and agglutination also contributes to the brittle component of these experimentally generated pseudotachylytes. We conclude that in the case of pseudotachylyte generation during an earthquake, slip even beyond the onset of frictional melting is not controlled merely by viscosity but rather by an interplay of viscoelastic forces around the glass transition, which involves a response in the brittle/solid regime of these rock melts. We warn of the inadequacy of simple Newtonian viscous analyses and call for the application of more realistic rheological interpretation of pseudotachylyte-bearing fault systems in the evaluation and prediction of their slip dynamics.

scholarly journals Lower-crustal earthquakes in the West Kunlun range

2011 ◽  
Vol 38 (1) ◽  
pp. n/a-n/a ◽  
Author(s):  
Guo-Chin Dino Huang ◽  
Steven W. Roecker ◽  
Vadim Levin
Keyword(s):  
The West ◽  
Crustal Earthquakes ◽  
West Kunlun ◽  
Lower Crustal

scholarly journals Pseudotachylyte as field evidence for lower crustal earthquakes during the intracontinental Petermann Orogeny (Musgrave Block, Central Australia)

2017 ◽  
Author(s):  
Friedrich Hawemann ◽  
Neil Mancktelow ◽  
Sebastian Wex ◽  
Alfredo Camacho ◽  
Giorgio Pennacchioni
Keyword(s):  
Crustal Earthquakes ◽  
Field Evidence ◽  
Central Australia ◽  
Lower Crustal

Highly stressed lower crust: Evidence from dry pseudotachylytes in granulites, Lofoten, Norway

2020 ◽  
Author(s):  
Kristina G. Dunkel ◽  
Xin Zhong ◽  
Luiz F. G. Morales ◽  
Bjørn Jamtveit
Keyword(s):  
Lower Crust ◽  
Electron Backscatter Diffraction ◽  
Alkali Feldspar ◽  
Mineralogical Composition ◽  
Plastic Instability ◽  
Wall Rock ◽  
Seismogenic Zone ◽  
Hydrous Minerals ◽  
Brittle Ductile Transition ◽  
Lower Crustal

<p>Due to the high confining pressures in the lower crust, the generating mechanisms of lower crustal earthquakes, occurring below the standard seismogenic zone, are puzzling. Their investigation is difficult because the records of such earthquakes, pseudotachylytes, are typically reacted and/or deformed. Here we describe exceptionally pristine pseudotachylytes in lower crustal granulites from the Lofoten Vesterålen Archipelago, Norway. The pseudotachylytes have essentially the same mineralogical composition as their host (plagioclase, alkali feldspar, orthopyroxene) and contain microstructures indicative of rapid cooling (microlites, spherulites, ‘cauliflower’ garnet). Neither the wall rock nor the pseudotachylytes themselves contain hydrous minerals, and no mylonites are associated with the pseudotachylytes. This excludes the most commonly suggested weakening mechanisms that may cause earthquakes below the brittle-ductile transition: dehydration- or reaction-induced embrittlement, plastic instability, thermal runaway, and downward propagation of seismic rupture from shallow faults into their deeper ductile extensions. Hence, we suggest that transient stress pulses caused by shallower earthquakes are the most likely explanation for the occurrence of fossil earthquakes in the analysed rocks from Lofoten.</p><p>Earthquakes are short events, but their effects on the tectonic and metamorphic development of their host can be long-lasting. The initial deformation features related to seismic events, which potentially determine these effects, are often overprinted by metamorphism driven by fluids infiltrating the rock along the seismic fault. Because of the anhydrous conditions in the present case, those structures are preserved. The wall rocks to the investigated pseudotachylytes appear undamaged in optical and backscatter electron observation; however, cathodoluminescence imaging of feldspar and quartz reveals healed fractures and alteration zones. Those areas are further investigated with electron backscatter diffraction and transmission electron microscopy to better understand the microstructural and chemical changes during and after the seismic event.</p>

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