Rock record constraints on the seismic signature of subduction interface shear zones

Mapping Intimacies ◽

10.5194/egusphere-egu2020-3285 ◽

2020 ◽

Author(s):

Carolyn Tewksbury-Christle ◽

Whitney Behr ◽

Mark Helper

Keyword(s):

Shear Zone ◽

Subduction Zones ◽

Seismic Velocity ◽

Shear Zones ◽

Seismic Velocities ◽

Interface Shear ◽

Fracture Porosity ◽

Ductile Shear Zones ◽

Ductile Shear ◽

Seismic Signature

The low velocity layer (LVL) in modern subduction zones is a 3-5 km thick region that parallels the top of the downgoing slab and is characterized by anomalously high Vp/Vs ratios (1.8-2.5) consistent with 2.5-4% fracture porosity at near-lithostatic pore fluid pressures. The LVL has been previously interpreted as partially hydrated, relatively undeformed oceanic crust at the top of the downgoing slab, but collocation of the LVL with episodic tremor and slow slip events (ETS) in modern subduction zones suggests that the LVL may alternatively represent the seismic signature of a subduction interface shear zone.&#160;To test this hypothesis, we use field & structural observations, geochronology, and seismic velocity calculations to compare and contrast the bulk seismic properties of a fossil subduction interface shear zone (Condrey Mountain Schist, CMS, northern CA) to properties of modern LVLs. Specifically, we 1) determined thicknesses of underplated packages (interpreted to represent the maximum thickness of the actively deforming interface) using depositional age discontinuities and high resolution structural mapping, 2) averaged the bulk rock seismic velocities weighted by mapped lithologic proportions and corrected for pressure-temperature effects, and 3) used field evidence of modifying factors (e.g., microcracks, fluid-filled veins, mineral anisotropy) to further refine the possible range of seismic velocities and effects on Vp/Vs ratio.The CMS greenschist- to blueschist-facies units were subducted to ~25-35 km (450&#176;C, 0.8-1.0 GPa) with limited retrogression or exhumational overprint. These rocks were underplated episodically at depth in three packages individually up to 4.5 km thick from 155-135 Ma, based on detrital zircon data. Each package is dominantly composed of metasedimentary rocks with m- to km-scale metamafic and serpentinized ultramafic lenses. Strain localization to ~1 km thick ductile shear zones between underplating episodes is collocated with km-scale serpentinized ultramafic lenses at the base of each package. Deformation was distributed and ductile with rare macro- or micro-scale prograde brittle failure in the metasedimentary or metamafic units. In the serpentinized ultramafics, ductile shear zones wrap massive blocks with prograde brittle fracture. Maximum fracture porosity estimated from relict veins is ~10%. Average Vp/Vs for the CMS is ~1.6 (lithology alone) but up to 3.0 (accounting for maximum fracture porosity).The fossil subduction interface shear zone preserved in the CMS is consistent in both thickness and seismic signature with the LVL in modern subduction zones. Estimated Vp/Vs is higher than the LVL but assumes that all fractures are simultaneously open. The total thickness of the CMS (10+ km) is greater than the LVL, however, so previously underplated material must lose its anomalous seismic signature during underplating (e.g., due to fluid loss and transport up the slab during or after underplating). Our results support the hypothesis that LVLs in modern subduction zones represent the seismic signature of the subduction interface shear zone.

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Ductile shear zones – future perspectives

10.5194/egusphere-egu2020-4273 ◽

2020 ◽

Author(s):

Christoph Schrank

Keyword(s):

Shear Zone ◽

Research Effort ◽

Rheological Behaviour ◽

Shear Zones ◽

Ductile Deformation ◽

Field Observations ◽

Ductile Shear Zones ◽

In Situ Experiments ◽

Ductile Shear ◽

Physical Consequences

About 50 years ago, John Ramsay and colleagues established the thorough foundation for the field-scale observational and mathematical description of the structures, deformation, and kinematics in ductile shear zones. Since then, these probably most important instabilities of the ductile lithosphere enjoyed an almost explosive growth in scientific attention. It is perhaps fair to say that this tremendous research effort featured four main themes:&#160;[1] The historic scientific nucleus &#8211; quantification of shear-zone geometry, strain and associated kinematic history from field observations&#160;[2] Qualitative and quantitative analysis of microphysical deformation mechanisms in the field and the laboratory&#160;[3] Shear-zone rheology&#160;[4] The development of physically consistent mathematical models for shear zones, mainly using continuum mechanics.&#160;In concert, these four cornerstones of shear-zone research enabled tremendous progress in our understanding of why and how ductile shear zones form. So, what are some of the outstanding problems?&#160;A truly comprehensive model for ductile shear zones must account for the vast range of length and time scales involved, each easily covering ten orders of magnitude, as well as the associated intimate coupling between thermal, hydraulic, mechanical, and chemical processes. The multi-scale and multi-physics nature of ductile shear zones generates scientific challenges for all four research themes named above. This presentation is dedicated to highlighting exciting challenges in themes 2, and 3 and 4.&#160;In the microanalytical arena [2], the nano-scale is an exciting new frontier, especially when it comes to the interplay between metamorphism and ductile deformation. The nano-frontier can be tackled with new synchrotron methods. I showcase some applications to fossil shear-zone samples and discuss opportunities for in-situ experiments. In the domain of rheology [3], I present some simple experiments with strain-softening materials and field observations that support the notion: transient rheological behaviour is very important for shear localisation. In the modelling domain [4], some recent examples for the intriguing physical consequences predicted by new multi-physics and cross-scale coupling terms in ductile localisation problems are illustrated.

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Development of C- shear bands in brittle-ductile shear zones: Insights from analogue and numerical models.

10.5194/egusphere-egu2020-960 ◽

2020 ◽

Author(s):

Arnab Roy ◽

Nandan Roy ◽

Puspendu Saha ◽

Nibir Mandal

Keyword(s):

Shear Zone ◽

Numerical Models ◽

Shear Bands ◽

Shear Zones ◽

Rheological Parameters ◽

Comprehensive Understanding ◽

Experimental Conditions ◽

Ductile Shear Zones ◽

Analogue Models ◽

Ductile Shear

Development of brittle and brittle-ductile shear zones involve partitioning of large shear strains in bands, called C-shear bands (C-SB) nearly parallel to the shear zone boundaries. Our present work aims to provide a comprehensive understanding of the rheological factors in controlling such SB growth in meter scale natural brittle- ductile shear zones observed in in Singbhum and Chotonagpur mobile belts.&#160; The shear zones show C- SB at an angle of 0&#176;- 5&#176; with the shear zone boundary. We used analogue models, based on Coulomb and Viscoplastic rheology to reproduce them in experimental conditions.These models produce dominantly Riedel (R) shear bands. We show a transition from R-shearing in conjugate to single sets at angles of ~15o by changing model materials. However, none of the analogue models produced C-SB, as observed in the field. To reconcile the experimental and field findings, numeral models have been used to better constrain the geometrical and rheological parameters. We simulate model shear zones replicating those observed in the field, which display two distinct zones: drag zone where the viscous strains dominate &#160;and the core zone, where both viscous and plastic strains come into play. &#160;Numerical model results suggest the formation of &#160;C- SB for a specific rheological condition. We also show varying shear band patterns as a function of the thickness ratio between drag and core zones.

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The influence of the heterogeneous asperity distribution on induced seismicity and permeability evolution during hydraulic fault zone stimulation

10.5194/egusphere-egu2020-18774 ◽

2020 ◽

Author(s):

Linus Villiger ◽

Dominik Zbinden ◽

Antonio Pio Rinaldi ◽

Paul Antony Selvadurai ◽

Hannes Krietsch ◽

...

Keyword(s):

Shear Zone ◽

Induced Seismicity ◽

Shear Zones ◽

Ductile Shear Zone ◽

Permeability Evolution ◽

Permeability Enhancement ◽

Ductile Shear Zones ◽

Injection Interval ◽

Ductile Shear ◽

Asperity Distribution

Several decameter-scale in-situ stimulation experiments were conducted in crystalline rock at the Grimsel Test Site, Switzerland, with the aim to advance our understanding of the seismo-hydro-mechanical processes associated with deep geothermal reservoir stimulation. To allow comparability between the experiments, a standardized injection protocol was applied for all experiments. Induced seismicity was recorded using acoustic emission sensors and accelerometers, which were distributed along tunnel walls and within four boreholes. Hydro-mechanical responses of the fault zones were measured using grouted longitudinal fiberoptic strain sensors and open pressure monitoring borehole intervals. A total of four ductile shear zones (with brittle overprint) and two brittle-ductile shear zones have been stimulated during these experiments.Here we present an analysis of heterogeneous permeability evolution within a target shear zone during ongoing stimulation. The shear zone in question is an originally ductile shear zone which contains a single fracture in the injection interval. The observed planar seismicity cloud indicates that most of the stimulation process was confined within the target shear zone. Hydraulic characterization of the injection interval before and after stimulation revealed an enhancement in interval transmissivity from 8.3-10-11 m2/s to 1.5-7 m2/s. Within the reservoir, the seismo-hydro-mechanical data (i.e. seismicity cloud, pressure peaks and local deformation) spatiotemporally coincide, suggesting that permeability enhancement along the shear zone is highly localized and heterogeneous. Thus, we argue that the permeability evolution is linked to asperity distribution and breakdown within the shear zone.The conceptual model developed from the experimental analysis is implemented in a three-dimensional numerical model, with which we attempt to simulate the directional permeability creation observed in the experiment. The model accounts for a discrete planar fault zone of finite thickness with distributed low-permeability, brittle asperities embedded in a more permeable damage zone mimicking the ductile shear zone at Grimsel. The hydro-mechanical processes are modeled with the TOUGH-FLAC simulator, which sequentially couples fluid flow and poroelastic deformation within the fault and the surrounding medium. A Mohr-Coulomb failure criterion is used to simulate asperity reactivation, which can lead to permeability enhancement of the reactivated area.

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Microstructural and Geochronological Analyses of Mesozoic Ductile Shear Zones in the Western Gyeonggi Massif, Korea: Implications for an Orogenic Cycle in the East Asian Continental Margin

Minerals ◽

10.3390/min10040362 ◽

2020 ◽

Vol 10 (4) ◽

pp. 362

Author(s):

Jeong-Yeong Park ◽

Seung-Ik Park ◽

Taejin Choi

Keyword(s):

Shear Zone ◽

Continental Margin ◽

East Asian ◽

Shear Zones ◽

Late Triassic ◽

Top Down ◽

Orogenic Collapse ◽

Ductile Shear Zones ◽

Gyeonggi Massif ◽

Ductile Shear

In response to orogenic cycles, the ductile shear zone records a complex crustal deformation history. In this study, we conducted a microstructural analysis of two NW–SE trending ductile shear zones (Deokjeok Shear Zone (DSZ) and Soya Shear Zone (SSZ)) in the Late Triassic post-collisional granites along the western Gyeonggi Massif in the Korean Peninsula. The DSZ, overlain by the Late Triassic to the Early Jurassic post-collisional basin fill (Deokjeok Formation), has asymmetric microstructures indicative of a top-down-to-the-northeast shear. Depending on the structural position, the SSZ, which structurally overlies the Deokjeok Formation, exhibits two contrasting styles of deformation. The lower portion of the SSZ preserves evidence of top-up-to-the-southwest shearing after top-down-to-the-northeast shearing; on the other hand, the upper portion only indicates a top-up movement. Given the primary deformation mechanisms of both quartz and feldspar, the deformation temperatures of DSZ and SSZ were estimated at ~300–350 °C and ~350–400 °C, respectively, indicative of the mid-crustal condition. New zircon U-Pb isotopic ages from mylonitic granite in the SSZ and volcanic rocks in the Deokjeok Formation, combined with previously published geochronological data, indicate that the post-collisional granites and volcano-sedimentary sequence were nearly contemporaneous (ca. 223–217 Ma) and juxtaposed because of the Late Triassic orogenic collapse and subsequent new orogenic event. In this study, we highlight the role of the extensional DSZ as a detachment propagated into the middle crust during the Late Triassic orogenic collapse. Our results report a deformational response to a transition from the collisional Songrim Orogeny to the subduction-related Daebo Orogeny in the western Gyeonggi Massif. This, in turn, provides essential insight into cyclic mountain building/collapse in the East Asian continental margin during the Mesozoic time.

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On the causes of brittle nucleation of shear zones: the example of Cap de Creus, Eastern Pyrenees (Spain)

10.5194/egusphere-egu2020-19297 ◽

2020 ◽

Author(s):

Claudio Rosenberg ◽

Loïc Labrousse ◽

Nicolas Landry ◽

Elena Druguet ◽

Jordi Carreras

Keyword(s):

Grain Boundaries ◽

Shear Zone ◽

Deformation Temperature ◽

Shear Zones ◽

Nucleation And Growth ◽

White Mica ◽

Fracture Plane ◽

Fracture Planes ◽

Ductile Shear Zones ◽

Ductile Shear

The area of Cap de Creus, at the eastern termination of the Axial Zone of the Pyrenean Belt, exposes some of the most famous outcrops of ductile shear zones and shear zone networks (Carreras, 2001). Recent studies proposed that the nucleation and growth of such shear zones may have taken place by brittle processes (Fusseis et al., 2006; Fusseis and Handy, 2008).The present study investigates the geometrical relationships between fracture systems and some shear zones, the deformation temperature of these shear zones, and the processes leading to the nucleation and growth of shear zones along fracture planes. We selected two areas of the Cap de Creus, the Cala d&#8217;Agulles, and the Punta de Cap de Creus, because they are most intensely dissected by subparallel sets of shear zones and fractures. The orientation of the average shear zone planes is sub-parallel to the orientation of the major set of fractures, and the great extent and close spacing of some shear zones that we characterized by aerial photos from a drone, is similar to the distribution and extent of the fracture planes. These observations, in addition to those of Fusseis et al. (2006) suggest that the shear zones nucleated on previous fracture planes.&#160;These fractures are surrounded by haloes of nearly 1 cm thickness affecting the fabric of the country rock, an amphibolite-facies, biotite-andalusite bearing schist. Microscopic observations show that the haloes correspond to the wide-spread presence of thin (less than 2&#181;m thickness) phosphate seams coating the grain boundaries, preferentially those oriented at low angle to the fracture plane, and to the alteration of plagioclase to white mica and sericite, and to the growth of tourmaline, also related to grain boundaries and micro-fractures.Deformation temperature in the shear zones is assessed by white mica thermometry and pseudosections. The calculated T of at least 350-400&#176; C is consistent with qualitative observations showing the presence of stable biotite within very fine-grained (<< 10 &#181;m) shear bands and the recrystallization of quartz by rotation of sub-grain boundaries.In summary, fractures formed at high temperature, possibly associated with the intrusion of tourmaline-bearing pegmatites and fluids, which predate the ductile mylonitic event (Druguet, 2001; Van Lichtervelde et al., 2017). Fluids altered and weakened a volume of approximately 2 cm thickness all along the fracture planes, whose extent may reach > 100 m. The inferred, relatively high T of ca. &#160;400&#176; C indicates that fracturing is not due to the proximity of the brittle-ductile transition. In addition, no significant micro-fracturing of the mylonites is observed in thin sections. Therefore, fracturing precedes the ductile shear zones, which nucleate on some of the &#8220;inherited&#8221; sets of thin, planar, weakened structures, the large majority of which remains undeformed. These observations raise the question on whether nucleation and propagation of ductile shear zones is mechanically unrelated to brittle fracturing. Their weakening of planar structures would originate from fluid migration along fracture planes, but fracturing would no longer be active during ductile deformation.

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Kinematics and Geochronology of Late Paleozoic–Early Mesozoic Ductile Deformation in the Alxa Block, NW China: New Constraints on the Evolution of the Central Asian Orogenic Belt

Lithosphere ◽

10.2113/2021/3365581 ◽

2021 ◽

Vol 2021 (1) ◽

Author(s):

Beihang Zhang ◽

Jin Zhang ◽

Heng Zhao ◽

Junfeng Qu ◽

Yiping Zhang ◽

...

Keyword(s):

Shear Zone ◽

Shear Zones ◽

Strike Slip ◽

Late Paleozoic ◽

Ductile Shear Zone ◽

Early Mesozoic ◽

Central Asian ◽

Ductile Shear Zones ◽

Dextral Shear ◽

Ductile Shear

Abstract Strike-slip faults are widely developed throughout the Central Asian Orogenic Belt (CAOB), one of the largest Phanerozoic accretionary orogenic collages in the world, and may have played a key role in its evolution. Recent studies have shown that a large number of Late Paleozoic–Early Mesozoic ductile shear zones developed along the southern CAOB. This study reports the discovery of a NW–SE striking, approximately 500 km long and up to 2 km wide regional ductile shear zone in the southern Alxa Block, the Southern Alxa Ductile Shear Zone (SADSZ), which is located in the central part of the southern CAOB. The nearly vertical mylonitic foliation and subhorizontal stretching lineation indicate that the SADSZ is a ductile strike-slip shear zone, and various kinematic indicators indicate dextral shearing. The zircon U-Pb ages and the 40Ar/39Ar plateau ages of the muscovite and biotite indicate that the dextral ductile shearing was active during Middle Permian to Middle Triassic (ca. 269–240 Ma). The least horizontal displacement of the SADSZ is constrained between ca. 40 and 50 km. The aeromagnetic data shows that the SADSZ is in structural continuity with the coeval shear zones in the central and northern Alxa Block, and these connected shear zones form a ductile strike-slip duplex in the central part of the southern CAOB. The ductile strike-slip duplex in the Alxa Block, including the SADSZ, connected the dextral ductile shear zones in the western and eastern parts of the southern CAOB to form a 3000 km long E-W trending dextral shear zone, which developed along the southern CAOB during Late Paleozoic to Early Mesozoic. This large-scale dextral shear zone was caused by the eastward migration of the orogenic collages and blocks of the CAOB and indicates a transition from convergence to transcurrent setting of the southern CAOB during Late Paleozoic to Early Mesozoic.

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