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Toffol G, Pennacchioni G, Menegon L, Wallis D, Faccenda M, Camacho A, Bestmann M. On-fault earthquake energy density partitioning from shocked garnet in an exhumed seismic midcrustal fault. SCIENCE ADVANCES 2024; 10:eadi8533. [PMID: 38427735 PMCID: PMC10906929 DOI: 10.1126/sciadv.adi8533] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/22/2023] [Accepted: 01/26/2024] [Indexed: 03/03/2024]
Abstract
The energy released during an earthquake is mostly dissipated in the fault zone and subordinately as radiated seismic waves. The on-fault energy budget is partitioned into frictional heat, generation of new grain surface by microfracturing, and crystal-lattice distortion associated with dislocation defects. The relative contribution of these components is debated and difficult to assess, but this energy partitioning strongly influences earthquake mechanics. We use high-resolution scanning-electron-microscopy techniques, especially to analyze shocked garnet in a fault wall-rock, to provide the first estimate of all three energy components for a seismic fault patch exhumed from midcrustal conditions. Fault single-jerk seismicity is recorded by the presence of pristine quenched frictional melt. The estimated value of energy per unit fault surface is ~13 megajoules per square meter for heat, which is predominant with respect to both surface energy (up to 0.29 megajoules per square meter) and energy associated with crystal lattice distortion (0.02 megajoules per square meter).
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Affiliation(s)
- Giovanni Toffol
- Department of Geosciences, University of Padova, Padova, Italy
| | | | - Luca Menegon
- Njord Centre, Department of Geosciences, University of Oslo, Oslo, Norway
| | - David Wallis
- Department of Earth Sciences, University of Cambridge, Cambridge, UK
| | | | - Alfredo Camacho
- Department of Geological Sciences, University of Manitoba, Winnipeg, Canada
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Craiu A, Ferrand TP, Manea EF, Vrijmoed JC, Mărmureanu A. A switch from horizontal compression to vertical extension in the Vrancea slab explained by the volume reduction of serpentine dehydration. Sci Rep 2022; 12:22320. [PMID: 36566238 DOI: 10.1038/s41598-022-26260-5] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/08/2022] [Accepted: 12/13/2022] [Indexed: 12/25/2022] Open
Abstract
The Vrancea slab, Romania, is a subducted remnant of the Tethyan lithosphere characterized by a significant intermediate-depth seismicity (60-170 km). A recent study showed a correlation between this seismicity and major dehydration reactions, involving serpentine minerals up to 130 km depth, and high-pressure hydrated talc deeper. Here we investigate the potential link between the triggering mechanisms and the retrieved focal mechanisms of 940 earthquakes, which allows interpreting the depth distribution of the stress field. We observe a switch from horizontal compression to vertical extension between 100 and 130 km depth, where the Clapeyron slope of serpentine dehydration is negative. The negative volume change within dehydrating serpentinized faults, expected mostly sub-horizontal in the verticalized slab, could well explain the vertical extension recorded by the intermediate-depth seismicity. This apparent slab pull is accompanied with a rotation of the main compressive stress, which could favour slab detachments in active subduction zones.
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Affiliation(s)
- Andreea Craiu
- National Institute for Earth Physics, Calugareni, 12, Măgurele, Ilfov, Romania
| | - Thomas P Ferrand
- Institut für Geologische Wissenschaften, Freie Universität Berlin, Malteserstraße 74-100, 12249, Berlin, Germany. .,Institut des Sciences de la Terre d'Orléans, UMR-7327, Université d'Orléans - CNRS, 1A Rue de la Ferollerie, 45100, Orléans, France.
| | - Elena F Manea
- National Institute for Earth Physics, Calugareni, 12, Măgurele, Ilfov, Romania.,GNS Science, PO Box 30-368, Lower Hutt, New Zealand
| | - Johannes C Vrijmoed
- Institut für Geologische Wissenschaften, Freie Universität Berlin, Malteserstraße 74-100, 12249, Berlin, Germany
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Menegon L, Campbell L, Mancktelow N, Camacho A, Wex S, Papa S, Toffol G, Pennacchioni G. The earthquake cycle in the dry lower continental crust: insights from two deeply exhumed terranes (Musgrave Ranges, Australia and Lofoten, Norway). PHILOSOPHICAL TRANSACTIONS. SERIES A, MATHEMATICAL, PHYSICAL, AND ENGINEERING SCIENCES 2021; 379:20190416. [PMID: 33517876 PMCID: PMC7898122 DOI: 10.1098/rsta.2019.0416] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Accepted: 09/14/2020] [Indexed: 05/31/2023]
Abstract
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'.
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Affiliation(s)
- Luca Menegon
- The Njord Centre, Department of Geoscience, University of Oslo, 1048 Blindern, Norway
- School of Geography, Earth and Environmental Sciences, University of Plymouth, Drake Circus, PL4 8AA Plymouth, UK
| | - Lucy Campbell
- School of Geography, Earth and Environmental Sciences, University of Plymouth, Drake Circus, PL4 8AA Plymouth, UK
| | - Neil Mancktelow
- Department of Earth Sciences, ETH Zurich, 8092 Zurich, Switzerland
| | - Alfredo Camacho
- Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada
| | - Sebastian Wex
- Department of Earth Sciences, ETH Zurich, 8092 Zurich, Switzerland
| | - Simone Papa
- Department of Geosciences, University of Padova, Via Gradenigo 6, 35131 Padua, Italy
| | - Giovanni Toffol
- Department of Geosciences, University of Padova, Via Gradenigo 6, 35131 Padua, Italy
| | - Giorgio Pennacchioni
- Department of Geosciences, University of Padova, Via Gradenigo 6, 35131 Padua, Italy
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Fagereng Å, Beall A. Is complex fault zone behaviour a reflection of rheological heterogeneity? PHILOSOPHICAL TRANSACTIONS. SERIES A, MATHEMATICAL, PHYSICAL, AND ENGINEERING SCIENCES 2021; 379:20190421. [PMID: 33517872 PMCID: PMC7898124 DOI: 10.1098/rsta.2019.0421] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Accepted: 08/18/2020] [Indexed: 05/26/2023]
Abstract
Fault slip speeds range from steady plate boundary creep through to earthquake slip. Geological descriptions of faults range from localized displacement on one or more discrete planes, through to distributed shearing flow in tabular zones of finite thickness, indicating a large range of possible strain rates in natural faults. We review geological observations and analyse numerical models of two-phase shear zones to discuss the degree and distribution of fault zone heterogeneity and effects on active fault slip style. There must be certain conditions that produce earthquakes, creep and slip at intermediate velocities. Because intermediate slip styles occur over large ranges in temperature, the controlling conditions must be effects of fault properties and/or other dynamic variables. We suggest that the ratio of bulk driving stress to frictional yield strength, and viscosity contrasts within the fault zone, are critical factors. While earthquake nucleation requires the frictional yield to be reached, steady viscous flow requires conditions far from the frictional yield. Intermediate slip speeds may arise when driving stress is sufficient to nucleate local frictional failure by stress amplification, or local frictional yield is lowered by fluid pressure, but such failure is spatially limited by surrounding shear zone stress heterogeneity. This article is part of a discussion meeting issue 'Understanding earthquakes using the geological record'.
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