Types of heterogeneities and deformation mechanisms in blueschist rocks: an example from an exhumed subduction complex in Ishigaki Island, Ryukyu Arc

2021 ◽  
Author(s):  
Sara De Caroli ◽  
Ake Fagereng ◽  
Kohtaro Ujiie ◽  
Francesca Meneghini
Keyword(s):  
Electron Backscatter Diffraction ◽  
Deformation Mechanisms ◽  
Seismogenic Zone ◽  
Depth Range ◽  
Plate Interface ◽  
Slow Slip Events ◽  
Ishigaki Island ◽  
Wide Range ◽  
Ryukyu Arc ◽  
Mineral Scale

<p>The geological properties of the subduction interface, such as stable metamorphic assemblages and the rheology of shear zone rocks, change with depth. Studies based on seismic and geodetic observations suggest that these changes can be accompanied by differences in seismic styles. In this realm, slow slip events (SSEs) and related tremor signals, grouped as episodic tremor and slip (ETS) events, have been detected down-dip of the subduction megathrust seismogenic zone. A wide range of mechanisms, some invoking rheological heterogeneity, has been proposed to explain ETS occurrence. Given that ETS events accommodate most of the plate interface displacement in a depth range below the seismogenic zone, it is of great interest to understand the rheology of the rock lithologies that are likely to host ETS along the deep subduction interface.</p><p>Here, we present data from an exhumed subduction complex in Ishigaki Island, Ryukyu Arc. In particular, we analyse the Triassic high pressure-low temperature Tomuru metamorphic rocks, which comprise blueschist and greenschist facies metabasites that underwent subduction-related deformation. These rocks offer an important natural laboratory in which to study the characteristics of blueschist deformation structures to infer rheology and, in particular, the role played by heterogeneities in an environment comparable to modern ETS down-dip of the seismogenic zone.</p><p>Through multiscale and multidisciplinary, field- and laboratory-based studies, including quantitative microstructural and image analyses, we focus on two main topics. Firstly, we aim to understand blueschist rheology, by documenting the deformation mechanisms active in blueschist rocks through electron backscatter diffraction (EBSD), in order to quantify intracrystalline deformation and lattice preferred orientation (LPO) development. Secondly, we study the effect of grain size on blueschist foliation development and, ultimately, on blueschist deformation.  Through these analyses, we hope to constrain both subduction interface strength and dominant mineral- scale deformation mechanisms at blueschist conditions.</p>

scholarly journals Shallow slow earthquakes to decipher future catastrophic earthquakes in the Guerrero seismic gap.

2020 ◽  
Author(s):  
Raymundo Plata-Martínez ◽  
Satoshi Ide ◽  
Masanao Shinohara ◽  
Emmanuel Soliman Garcia Mortel ◽  
Naoto Mizuno ◽  
...  
Keyword(s):  
Seismogenic Zone ◽  
Seismic Gap ◽  
Slow Slip ◽  
Dynamic Rupture ◽  
Plate Interface ◽  
Slow Slip Events ◽  
Repeating Earthquakes ◽  
Slow Earthquakes ◽  
Slip Area ◽  
Large Earthquakes

Abstract The Guerrero seismic gap is presumed to be a major source of seismic and tsunami hazard along the Mexican subduction zone. Until recently, there were limited observations to describe the shallow portion of the plate interface in Guerrero. For this reason, we deployed offshore instrumentation to gain new seismic data and identify the extent of the seismogenic zone inside the Guerrero gap. We discovered episodic shallow tremors and potential slow slip events which, together with repeating earthquakes, seismicity, residual gravity and residual bathymetry suggest that a portion of the shallow plate interface in the Guerrero seismic gap undergoes stable slip. This mechanical condition may not only explain the long return period of large earthquakes with origins inside the Guerrero seismic gap, but also reveal why the rupture from past M<8 earthquakes on adjacent megathrust fault segments did not propagate into the gap to encompass a larger slip area. Nevertheless, a large enough earthquake initiating nearby could rupture through the entire Guerrero seismic gap if driven by dynamic rupture effects.

scholarly journals Excitation of San Andreas tremors by thermal instabilities below the seismogenic zone

2020 ◽  
Vol 6 (36) ◽  
pp. eabb2057 ◽  
Author(s):  
Lifeng Wang ◽  
Sylvain Barbot
Keyword(s):  
Low Frequency ◽  
Frictional Resistance ◽  
Seismogenic Zone ◽  
Slow Slip ◽  
Slow Slip Events ◽  
Tectonic Plates ◽  
San Andreas ◽  
Wide Range ◽  
Shear Heating ◽  
Increasing Temperature

The relative motion of tectonic plates is accommodated at boundary faults through slow and fast ruptures that encompass a wide range of source properties. Near the Parkfield segment of the San Andreas fault, low-frequency earthquakes and slow-slip events take place deeper than most seismicity, at temperature conditions typically associated with stable sliding. However, laboratory experiments indicate that the strength of granitic gouge decreases with increasing temperature above 350°C, providing a possible mechanism for weakening if temperature is to vary dynamically. Here, we argue that recurring low-frequency earthquakes and slow-slip transients at these depths may arise because of shear heating and the temperature dependence of frictional resistance. Recurring thermal instabilities can explain the recurrence pattern of the mid-crustal low-frequency earthquakes and their correlative slip distribution. Shear heating associated with slow slip is sufficient to generate pseudotachylyte veins in host rocks even when fault slip is dominantly aseismic.

scholarly journals Shallow slow earthquakes to decipher future catastrophic earthquakes in the Guerrero seismic gap

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
R. Plata-Martinez ◽  
S. Ide ◽  
M. Shinohara ◽  
E. S. Garcia ◽  
N. Mizuno ◽  
...  
Keyword(s):  
Seismogenic Zone ◽  
Seismic Gap ◽  
Slow Slip ◽  
Dynamic Rupture ◽  
Plate Interface ◽  
Slow Slip Events ◽  
Repeating Earthquakes ◽  
Residual Gravity ◽  
Slow Earthquakes ◽  
Large Earthquakes

AbstractThe Guerrero seismic gap is presumed to be a major source of seismic and tsunami hazard along the Mexican subduction zone. Until recently, there were limited observations at the shallow portion of the plate interface offshore Guerrero, so we deployed instruments there to better characterize the extent of the seismogenic zone. Here we report the discovery of episodic shallow tremors and potential slow slip events in Guerrero offshore. Their distribution, together with that of repeating earthquakes, seismicity, residual gravity and bathymetry, suggest that a portion of the shallow plate interface in the gap undergoes stable slip. This mechanical condition may not only explain the long return period of large earthquakes inside the gap, but also reveals why the rupture from past M < 8 earthquakes on adjacent megathrust segments did not propagate into the gap to result in much larger events. However, dynamic rupture effects could drive one of these nearby earthquakes to break through the entire Guerrero seismic gap.

scholarly journals The rise and demise of deep accretionary wedges: A long-term field and numerical modeling perspective

Geosphere ◽  
2021 ◽  
Author(s):  
Samuel Angiboust ◽  
Armel Menant ◽  
Taras Gerya ◽  
Onno Oncken
Keyword(s):  
Numerical Modeling ◽  
Large Scale ◽  
Plate Boundary ◽  
Transport Processes ◽  
Dynamical Instability ◽  
Seismogenic Zone ◽  
Plate Interface ◽  
Wide Range ◽  
Tectonic Transport

Several decades of field, geophysical, analogue, and numerical modeling investigations have enabled documentation of the wide range of tectonic transport processes in accretionary wedges, which constitute some of the most dynamic plate boundary environments on Earth. Active convergent margins can exhibit basal accretion (via underplating) leading to the formation of variably thick duplex structures or tectonic erosion, the latter known to lead to the consumption of the previously accreted material and eventually the forearc continental crust. We herein review natural examples of actively underplating systems (with a focus on circum-Pacific settings) as well as field examples highlighting internal wedge dynamics recorded by fossil accretionary systems. Duplex formation in deep paleo–accretionary systems is known to leave in the rock record (1) diagnostic macro- and microscopic deformation patterns as well as (2) large-scale geochronological characteristics such as the downstepping of deformation and metamorphic ages. Zircon detrital ages have also proved to be a powerful approach to deciphering tectonic transport in ancient active margins. Yet, fundamental questions remain in order to understand the interplay of forces at the origin of mass transfer and crustal recycling in deep accretionary systems. We address these questions by presenting a suite of two-dimensional thermo-mechanical experiments that enable unravelling the mass-flow pathways and the long-term distribution of stresses along and above the subduction interface as well as investigating the importance of parameters such as fluids and slab roughness. These results suggest the dynamical instability of fluid-bearing accretionary systems causes either an episodic or a periodic character of subduction erosion and accretion processes as well as their topographic expression. The instability can be partly deciphered through metamorphic and strain records, thus explaining the relative scarcity of paleo–accretionary systems worldwide despite the tremendous amounts of material buried by the subduction process over time scales of tens or hundreds of millions of years. We finally stress that the understanding of the physical processes at the origin of underplating processes as well as the forearc topographic response paves the way for refining our vision of long-term plate-interface coupling as well as the rheological behavior of the seismogenic zone in active subduction settings.

scholarly journals Earthquake swarms and slow slip on a sliver fault in the Mexican subduction zone

2019 ◽  
Vol 116 (15) ◽  
pp. 7198-7206 ◽  
Author(s):  
Shannon L. Fasola ◽  
Michael R. Brudzinski ◽  
Stephen G. Holtkamp ◽  
Shannon E. Graham ◽  
Enrique Cabral-Cano
Keyword(s):  
Subduction Zone ◽  
Local Network ◽  
Seismogenic Zone ◽  
Slow Slip ◽  
Plate Interface ◽  
Slow Slip Events ◽  
Parallel Motion ◽  
Oblique Convergence ◽  
Seismicity Catalog ◽  
Mexican Subduction Zone

The Mexican subduction zone is an ideal location for studying subduction processes due to the short trench-to-coast distances that bring broad portions of the seismogenic and transition zones of the plate interface inland. Using a recently generated seismicity catalog from a local network in Oaxaca, we identified 20 swarms of earthquakes (M < 5) from 2006 to 2012. Swarms outline what appears to be a steeply dipping structure in the overriding plate, indicative of an origin other than the plate interface. This steeply dipping structure corresponds to the northern boundary of the Xolapa terrane. In addition, we observed an interesting characteristic of slow slip events (SSEs) where they showed a shift from trenchward motion toward an along-strike direction at coastal GPS sites. A majority of the swarms were found to correspond in time to the along-strike shift. We propose that swarms and SSEs are occurring on a sliver fault that allows the oblique convergence to be partitioned into trench-perpendicular motion on the subduction interface and trench-parallel motion on the sliver fault. The resistivity structure surrounding the sliver fault suggests that SSEs and swarms of earthquakes occur due to high fluid content in the fault zone. We propose that the sliver fault provides a natural pathway for buoyant fluids attempting to migrate upward after being released from the downgoing plate. Thus, sliver faults could be responsible for the downdip end of the seismogenic zone by creating drier conditions on the subduction interface trenchward of the sliver fault, promoting fast-slip seismogenic rupture behavior.

scholarly journals Low-frequency earthquakes along the Ryukyu Islands triggered by teleseismic earthquakes

2021 ◽  
Vol 73 (1) ◽  
Author(s):  
Ayumi Kinjo ◽  
Mamoru Nakamura
Keyword(s):  
Low Frequency ◽  
Stress Transfer ◽  
Ryukyu Islands ◽  
Plate Interface ◽  
Okinawa Island ◽  
Slow Slip Events ◽  
Arrival Times ◽  
Short Period ◽  
Ryukyu Arc ◽  
The Ryukyu Islands

AbstractTremors and low-frequency earthquakes (LFEs), which occur in the plate interface, can provide useful information about the state of aseismic stress transfer in mega-earthquake fault zones. We estimated the distribution of triggered LFEs in the subducted plate interface. Specifically, we detected LFEs in the Ryukyu Trench triggered by the surface waves of large teleseismic earthquakes by using the waveform records of broadband and short-period seismometers installed in the Ryukyu Arc. We selected a total of 45 teleseismic earthquakes with magnitudes of more than 7.5, which occurred between 2004 and 2017, for the analysis. We could detect the triggered LFEs for five teleseismic earthquakes. Then, we determined the hypocenters of LFEs by using the relative arrival times of LFEs for each station. The LFEs were distributed in the south of Okinawa Island and the Yaeyama area. Moreover, they were distributed around the source fault of the slow slip events. These were almost the same as and concentrated near to the locations of the most active LFE clusters accompanying very low-frequency earthquakes, suggesting higher sensitivity of inducing LFEs near these clusters. This indicates that the LFEs accompanying VLFEs are activated by stress acceleration in the Yaeyama and Okinawa areas.

scholarly journals Friction of Metals: A Review of Microstructural Evolution and Nanoscale Phenomena in Shearing Contacts

2021 ◽  
Vol 69 (4) ◽  
Author(s):  
Michael Chandross ◽  
Nicolas Argibay
Keyword(s):  
Shear Strength ◽  
Grain Boundary Sliding ◽  
Deformation Mechanisms ◽  
Low Friction ◽  
Friction Behavior ◽  
Important Conclusion ◽  
Fundamental Properties ◽  
Wide Range ◽  
Boundary Sliding ◽  
Metal Interfaces

AbstractThe friction behavior of metals is directly linked to the mechanisms that accommodate deformation. We examine the links between mechanisms of strengthening, deformation, and the wide range of friction behaviors that are exhibited by shearing metal interfaces. Specifically, the focus is on understanding the shear strength of nanocrystalline and nanostructured metals, and conditions that lead to low friction coefficients. Grain boundary sliding and the breakdown of Hall–Petch strengthening at the shearing interface are found to generally and predictably explain the low friction of these materials. While the following is meant to serve as a general discussion of the strength of metals in the context of tribological applications, one important conclusion is that tribological research methods also provide opportunities for probing the fundamental properties and deformation mechanisms of metals.

Effects of heterogeneity on frictional-viscous deformation and the depth-extent of the seismogenic zone

2021 ◽  
Author(s):  
Ake Fagereng ◽  
Adam Beall
Keyword(s):  
Shear Stress ◽  
Yield Strength ◽  
Fault Zone ◽  
Fluid Pressure ◽  
Local Stress ◽  
Seismogenic Zone ◽  
Depth Range ◽  
Viscous Deformation ◽  
Viscosity Contrast ◽  
Depth Extent

&lt;p&gt;Current conceptual fault models define a seismogenic zone, where earthquakes nucleate, characterised by velocity-weakening fault rocks in a dominantly frictional regime. The base of the seismogenic zone is commonly inferred to coincide with a thermally controlled onset of velocity-strengthening slip or distributed viscous deformation. The top of the seismogenic zone may be determined by low-temperature diagenetic processes and the state of consolidation and alteration. Overall, the seismogenic zone is therefore described as bounded by transitions in frictional and rheological properties. These properties are relatively well-determined for monomineralic systems and simple, planar geometries; but, many exceptions, including deep earthquakes, slow slip, and shallow creep, imply processes involving compositional, structural, or environmental heterogeneities. We explore how such heterogeneities may alter the extent of the seismogenic zone.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;We consider mixed viscous-frictional deformation and suggest a simple rule of thumb to estimate the role of heterogeneities by a combination of the viscosity contrast within the fault, and the ratio between the bulk shear stress and the yield strength of the strongest fault zone component. In this model, slip behaviour can change dynamically in response to stress and strength variations with depth and time. We quantify the model numerically, and illustrate the idea with a few field-based examples: 1) earthquakes within the viscous regime, deeper than the thermally-controlled seismogenic zone, can be triggered by an increase in the ratio of shear stress to yield strength, either by increased fluid pressure or increased local stress; 2) there is commonly a depth range of transitional behaviour at the base of the seismogenic zone &amp;#8211; the thickness of this zone increases markedly with increased viscosity contrast within the fault zone; and 3) fault zone weakening by phyllosilicate growth and foliation development increases viscosity ratio and decreases bulk shear stress, leading to efficient, stable, fault zone creep. These examples are not new interpretations or observations, but given the substantial complexity of heterogeneous fault zones, we suggest that a simplified, conceptual model based on basic strength and stress parameters is useful in describing and assessing the effect of heterogeneities on fault slip behaviour.&amp;#160; &amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&lt;/p&gt;

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