scholarly journals Fluid pressure and shear zone development over the locked to slow slip region in Cascadia

2018 ◽  
Vol 4 (3) ◽  
pp. eaar2982 ◽  
Author(s):  
Pascal Audet ◽  
Andrew J. Schaeffer
Keyword(s):  
Shear Zone ◽  
Fluid Pressure ◽  
Slow Slip ◽  
Slip Region ◽  
Zone Development

scholarly journals Control of Shear-Zone-Induced Pressure Fluctuations on Gold Endowment: The Giant El Callao District, Guiana Shield, Venezuela

Minerals ◽  
2018 ◽  
Vol 8 (10) ◽  
pp. 430 ◽  
Author(s):  
German Velásquez ◽  
Stefano Salvi ◽  
Luc Siebenaller ◽  
Didier Béziat ◽  
Daniel Carrizo
Keyword(s):  
Shear Zone ◽  
Pressure Fluctuations ◽  
Fluid Pressure ◽  
Fluid Phase ◽  
Shear Zones ◽  
Guiana Shield ◽  
Vein System ◽  
Transition Strain ◽  
Zone Development ◽  
Gold Bearing

The El Callao district, with a total endowment of more than 2000 t Au, is considered to be the most prolific gold resource in Venezuela. Mineralization is hosted by a vein system that is genetically associated with the El Callao transpressional shear zone. This vein system consists of a network of interconnected quartz–albite–ankerite veins enveloping a large number of metabasaltic fragments that host gold-bearing pyrites. Based on detailed mineralogical, microstructural, and fluid inclusion studies, a pressure-temperature pathway was established for the evolution of the mineralizing fluid during shear-zone development and exhumation. This path is characterized by repeated episodes of fluid pressure fluctuation from lithostatic (higher than 1.6 kbar) to near-hydrostatic values (<0.4 kbar), recorded throughout the transition from the quasi-plastic to frictional deformation cortical domains. Each successive pressure drop induced boiling of the hydrothermal fluid, with the resulting fluid phase separation controlling: (i) pyrite and invisible gold crystallization, which occurred during ductile and ductile-brittle transition strain conditions, and (ii) primary gold remobilization with consequent native-refined gold precipitation, occurring mainly under brittle conditions. The metallogenic framework that was proposed for the El Callao shear zone can be used as a vector to explore and characterize other mineralized shear zones in the Guiana Shield and analogous orogenic systems worldwide.

Episodic stress tensor and fluid pressure cycling in subducting oceanic crust during Northern Hikurangi slow slip events

2020 ◽  
Author(s):  
Emily Warren-Smith ◽  
Bill Fry ◽  
Laura Wallace ◽  
Enrique Chon ◽  
Stuart Henrys ◽  
...  
Keyword(s):  
Shear Zone ◽  
Oceanic Crust ◽  
Subduction Zones ◽  
Fluid Pressure ◽  
Fluid Migration ◽  
Ocean Bottom ◽  
Slow Slip ◽  
Interface Shear ◽  
Slow Slip Events ◽  
Fluid Reservoir

&lt;p&gt;&lt;span&gt;The occurrence of slow slip events (SSEs) in subduction zones has been proposed to be linked to the presence of, and fluctuations in near-lithostatic fluid pressures (P&lt;/span&gt;&lt;sub&gt;&lt;span&gt;f&lt;/span&gt;&lt;/sub&gt;&lt;span&gt;) within the megathrust shear zone and subducting oceanic crust. In particular, the 'fault-valve' model is commonly used to describe occasional, repeated breaching of a low-permeability interface shear zone barrier, which caps an overpressured hydrothermal fluid reservoir. In this model, a precursory increase in fluid pressure may therefore be anticipated to precede megathrust rupture. Resulting activation of fractures during slip opens permeable pathways for fluid migration and fluid pressure decreases once more, until the system becomes sealed and overpressure can re-accumulate. While the priming conditions for cyclical valving behaviour have been observed at subduction zones globally, and evidence for post-megathrust rupture drainage exists, physical observations of precursory fluid pressure increases, and subsequent decreases, particularly within the subducting slab where hydrothermal fluids are sourced, remain elusive. &lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;Here we use earthquake focal mechanisms recorded on an ocean-bottom seismic network to identify changes in the stress tensor within subducting oceanic crust during four SSEs in New Zealand&amp;#8217;s Northern Hikurangi subduction zone. We show that the stress, or shape ratio, which describes the relative magnitudes of the principal compressive stress axes, shows repeated decreases prior to, and rapid increases during the occurrence of geodetically documented SSEs. We propose that these changes represent precursory accumulation and subsequent release of fluid pressure within overpressured subducting oceanic crust via a &amp;#8216;valving&amp;#8217; model for megathrust slip behaviour. Our observations indicate that the timing of slow slip events on subduction megathrusts may be controlled by cyclical accumulation of fluid pressure within subducting oceanic crust.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;Our model is further supported by observations of seismicity preceding a large SSE in the northern Hikurangi Margin in 2019, captured by ocean-bottom seismometer&lt;/span&gt;&lt;span&gt;s&lt;/span&gt;&lt;span&gt; and &lt;/span&gt;&lt;span&gt;absolute &lt;/span&gt;&lt;span&gt;pressure &lt;/span&gt;&lt;span&gt;recorders.&lt;/span&gt; &lt;span&gt;O&lt;/span&gt;&lt;span&gt;bservations of microseismicity &lt;/span&gt;&lt;span&gt;during this period &lt;/span&gt;&lt;span&gt;indicate that a stress state conducive to vertical fluid flow was present in the downgoing plate prior to SSE initiation, before subsequently returning to a&lt;/span&gt;&lt;span&gt; down-dip&lt;/span&gt;&lt;span&gt; extensional state following the SSE. We propose this precursory seismicity is indicative of fluid migration towards the interface shear zone from the lower plate fluid reservoir, which may have helped triggering slip on the megathrust. &lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;We also present preliminary results of a moment tensor study to investigate spatial and temporal patterns in earthquake source properties in SSE regions along the Hikurangi Margin. In particular, earthquakes near Porangahau &amp;#8211; a region susceptible to dynamic triggering of tremor and where &lt;/span&gt;&lt;span&gt;shallow &lt;/span&gt;&lt;span&gt;SSEs occur every 5 years or so &amp;#8211; exhibit distinctly lower double couple components than elsewhere along the margin. We &lt;/span&gt;&lt;span&gt;attribute this to elevated fluid pressures within the crust here, which is consistent with recent observations of high seismic reflectivity from an autocorrelation study. Such high fluid pressure may control the broad range of seismic and aseismic phenomena observed at Porangahau. &lt;/span&gt;&lt;/p&gt;

Free gas distribution and basal shear zone development in a subaqueous landslide – Insight from 3D seismic imaging of the Tuaheni Landslide Complex, New Zealand

2018 ◽  
Vol 502 ◽  
pp. 231-243 ◽  
Author(s):  
Felix Gross ◽  
Joshu J. Mountjoy ◽  
Gareth J. Crutchley ◽  
Christoph Böttner ◽  
Stephanie Koch ◽  
...  
Keyword(s):  
New Zealand ◽  
Shear Zone ◽  
Seismic Imaging ◽  
3D Seismic ◽  
Gas Distribution ◽  
Free Gas ◽  
Zone Development ◽  
Basal Shear

The influence of dikes on auriferous shear zone development within granitoid intrusions: the Bourlamaque pluton, Val-d'Or district, Abitibi greenstone belt

1993 ◽  
Vol 30 (9) ◽  
pp. 1924-1933 ◽  
Author(s):  
Abdelhay Belkabir ◽  
François Robert ◽  
L. Vu ◽  
C. Hubert
Keyword(s):  
Shear Zone ◽  
Shear Zones ◽  
Slip Direction ◽  
Principal Stresses ◽  
Quartz Veins ◽  
Profound Influence ◽  
Complex Shear ◽  
Zone Development ◽  
Granitoid Intrusions ◽  
Geometry And Kinematics

Shear-zone-related gold–quartz veins in granitoid intrusions are commonly intimately associated with mafic dikes, which may have a profound influence on the localization, orientation, and kinematics of auriferous shear zones. The Bourlamaque pluton of the Val-d'Or district contains several economic auriferous shear zones, most of which follow and overprint diorite dikes. Mineralization in all deposits consists of quartz–tourmaline–pyrite veins in reverse- oblique orientation with a significant range of strike, dip, and slip direction. The geometry and kinematics of shear zone and vein array within the pluton is more complex than the simple conjugate pattern predicted for a deforming homogeneous intrusion. The stress tensor determined from the auriferous shear zones within the pluton indicates the same northerly-directed compression recorded by similar shear zones outside the pluton. This indicates that the complex shear zone and vein pattern within the pluton reflects the influence of diorite dikes, which acted as weak layers that were activated during subsequent deformation, showing the importance of layer anisotropy in auriferous shear zone development.The plunges of orebodies bear simple geometric relationships to the slip direction along a host shear zone: these are generally perpendicular to, or in some cases parallel to, the slip direction. Knowledge of the slip directions along activated dikes would therefore allow prediction of the possible plunge(s) of orebodies at early stages of exploration programs. Slip direction along an activated layer is controlled by the orientation of the layer with respect to the stress field and by the relative magnitudes of the three principal stresses. Using techniques developed for analysis of fault slip data, both parameters can be determined, provided there is a sufficient database, and slip direction can be predicted for activated layers of any orientations.

scholarly journals Fluid-mediated, brittle–ductile deformation at seismogenic depth – Part 2: Stress history and fluid pressure variations in a shear zone in a nuclear waste repository (Olkiluoto Island, Finland)

Solid Earth ◽  
2020 ◽  
Vol 11 (2) ◽  
pp. 489-511 ◽  
Author(s):  
Francesca Prando ◽  
Luca Menegon ◽  
Mark Anderson ◽  
Barbara Marchesini ◽  
Jussi Mattila ◽  
...  
Keyword(s):  
Shear Zone ◽  
Nuclear Waste ◽  
Strain Localization ◽  
Microstructural Analysis ◽  
Fluid Pressure ◽  
Rheological Parameters ◽  
Nuclear Waste Repository ◽  
Brittle Deformation ◽  
Differential Stress ◽  
Brittle Ductile Transition

Abstract. The microstructural record of fault rocks active at the brittle–ductile transition zone (BDTZ) may retain information on the rheological parameters driving the switch in deformation mode and on the role of stress and fluid pressure in controlling different fault slip behaviours. In this study we analysed the deformation microstructures of the strike-slip fault zone BFZ045 in Olkiluoto (SW Finland), located in the site of a deep geological repository for nuclear waste. We combined microstructural analysis, electron backscatter diffraction (EBSD), and mineral chemistry data to reconstruct the variations in pressure, temperature, fluid pressure, and differential stress that mediated deformation and strain localization along BFZ045 across the BDTZ. BFZ045 exhibits a mixed ductile–brittle deformation, with a narrow (<20 cm thick) brittle fault core with cataclasites and pseudotachylytes that overprint a wider (60–100 cm thick) quartz-rich mylonite. Mylonitic deformation took place at 400–500 ∘C and 3–4 kbar, typical of the greenschist facies metamorphism at the base of the seismogenic crust. We used the recrystallized grain size piezometry for quartz to document a progressive increase in differential stress, from ca. 50 to ca. 120 MPa, towards the shear zone centre during mylonitization and strain localization. Syn-kinematic quartz veins formed along the mylonitic foliation due to transiently high pore fluid pressure (up to lithostatic value). The overprint of the veins by dynamic recrystallization and mylonitic creep is further evidence of the occurrence of brittle events under overall ductile conditions. We propose a conceptual model in which the ductile–brittle deformation cycle was controlled by transient oscillations in fluid pressure and progressively higher differential stress, possibly occurring in a narrowing shear zone deforming towards the peak strength of the crust at the BDTZ.

Bringing slow slip processes into focus

2020 ◽  
Author(s):  
Rebecca Bell
Keyword(s):  
High Resolution ◽  
Subduction Zones ◽  
Seismic Velocity ◽  
Numerical Models ◽  
Fluid Pressure ◽  
Geophysical Methods ◽  
Rock Properties ◽  
Slow Slip ◽  
Velocity Models ◽  
The North

&lt;p&gt;The discovery of slow slip events (SSEs) at subduction margins in the last two decades has changed our understanding of how stress is released at subduction zones. Fault slip is now viewed as a continuum of different slip modes between regular earthquakes and aseismic creep, and an appreciation of seismic hazard can only be realised by understanding the full spectrum of slip. SSEs may have the potential to trigger destructive earthquakes and tsunami on faults nearby, but whether this is possible and why SSEs occur at all are two of the most important questions in earthquake seismology today. Laboratory and numerical models suggest that slow slip can be spontaneously generated under conditions of very low effective stresses, facilitated by high pore fluid pressure, but it has also been suggested that variations in frictional behaviour, potentially caused by very heterogeneous fault zone lithology, may be required to promote slow slip.&lt;/p&gt;&lt;p&gt;Testing these hypotheses is difficult as it requires resolving rock properties at a high resolution many km below the seabed sometimes in km&amp;#8217;s of water, where drilling is technically challenging and expensive. Traditional geophysical methods like travel-time tomography cannot provide fine-scale enough velocity models to probe the rock properties in fault zones specifically. In the last decade, however, computational power has improved to the point where 3D full-waveform inversion (FWI) methods make it possible to use the full wavefield rather than just travel times to produce seismic velocity models with a resolution an order of magnitude better than conventional models. Although the hydrocarbon industry have demonstrated many successful examples of 3D FWI the method requires extremely high density arrays of instruments, very different to the 2D transect data collection style which is still commonly employed at subduction zones.&lt;/p&gt;&lt;p&gt;&amp;#160;The north Hikurangi subduction zone, New Zealand is special, as it hosts the world&amp;#8217;s most well characterised shallow SSEs (&lt;2 km to 15 km below the seabed).&amp;#160; This makes it an ideal location to collect 3D data optimally for FWI to resolve rock properties in the slow slip zone. In 2017-2018 an unprecedentedly large 3D experiment including 3D multi-channel seismic reflection, 99 ocean bottom seismometers and 194 onshore seismometers was conducted along the north Hikurangi margin in an 100 km x 15 km area, with an average 2 km instrument spacing. In addition, IODP Expeditions 372 and 375 collected logging-while drilling and core data, and deployed two bore-hole observatories to target slow slip in the same area. In this presentation I will introduce you to this world class 3D dataset and preliminary results, which will enable high resolution 3D models of physical properties to be made to bring slow slip processes into focus. &amp;#160;&lt;/p&gt;

scholarly journals Microstructural, modal and geochemical changes as a result of granodiorite mylonitisation – a case study from the Rolovská shear zone (Čierna hora Mts, Western Carpathians, Slovakia)

Geologos ◽  
2016 ◽  
Vol 22 (3) ◽  
pp. 171-190 ◽  
Author(s):  
Roman Farkašovský ◽  
Katarína Bónová ◽  
Marián Košuth
Keyword(s):  
Shear Zone ◽  
Shear Bands ◽  
Major And Trace Elements ◽  
White Mica ◽  
Western Carpathians ◽  
Lateral Mobility ◽  
Chemical Changes ◽  
Zone Development ◽  
Zone Centre

Abstract Strong tectonic remobilisation and shear zone development are typical features of the easternmost part of the Veporicum tectonic unit in the Western Carpathians. The granodiorite mylonites in the area of the Rolovská shear zone (Čierna hora Mts) underwent a complex polystage evolution during the Hercynian and Alpine orogenies. Deformation during the latter reached greenschist facies under metamorphic conditions. Mylonites are macroscopically foliated rocks with a stretching lineation and shear bands. Structurally different mylonite types, ranging from protomylonites to ulramylonites with typical grainsize reduction from the margins towards the shear zone centre, have been assessed. The modal mineralogy of the different mylonite types changes considerably. Typical is a progressive decrease in feldspar content and simultaneously the quartz and white mica content increases from protomylonites towards the most strongly deformed ultramylonites. The deformation had a brittle character in less deformed rocks and a ductile one in more deformed tectonites. Obvious chemical changes occur in mesomylonites and ultramylonites. During mylonitisation, the original biotite granodiorite was depleted of Mg, Fe, Na, Ca and Ba, while K, Rb and mainly Si increased considerably. Other (major and trace) elements reflect erratic behaviour due to lateral mobility. Chemical changes indicate the breakdown and subsequent recrystallisation of biotite and feldspars and, in turn, the crystallisation of albite and sericite. REE decrease in ultramylonites due to the breakup of accessory minerals during deformation and alteration.

Observation of shear zone development in ring-shear apparatus with a transparent shear box

Landslides ◽  
2006 ◽  
Vol 3 (3) ◽  
pp. 239-251 ◽  
Author(s):  
Hiroshi Fukuoka ◽  
Kyoji Sassa ◽  
Gonghui Wang ◽  
Ryo Sasaki
Keyword(s):  
Shear Zone ◽  
Ring Shear ◽  
Zone Development ◽  
Ring Shear Apparatus ◽  
Shear Box
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