Near-lithostatic Fluid Pressures Research Articles

Fluid-mediated transition from dynamic rupturing to aseismic slip at the base of the seismogenic continental crust

Chemo-mechanical fluid-rock interactions are critical in controlling the frictional-viscous transition in the continental crust and the competition between seismic and aseismic deformation in fault zones. In this study, we provide quantitative constraints on the timing and magnitude of weakening, and associated changes in slip rates, due to fluid-rock interactions at the base of the seismogenic continental crust. Integrating field, microstructural analyses, and micromechanical modelling we constrain the microstructural and mechanical evolution of phyllosilicate-rich, carbonate-bearing brittle-ductile faults/shear zones developed in the Rieserferner granitoid pluton (Eastern Alps). Here, transient overpressure of (H2O + CO2)-rich fluids triggered dynamic rupturing in the strong host rock (>100 MPa), and promoted the development of weak phyllonites through long-term fluid-mediated feldspar-to-mica reactions. These phyllosilicate-rich fault rocks accommodated frictional-viscous aseismic creep at very low differential stresses (<10 MPa) and near-lithostatic fluid pressure conditions. Microscale vein networks overprinting the phyllonite indicate cyclical embrittlement related to increased creep rates (up to 10−7 s−1) that occurred over a timeframe of days to months and potentially related to slow earthquakes (slip rates of 10−8 m/s). These findings offer new constraints on the development and seismogenic potential of phyllosilicate-rich fault zones and on the effect of fluid chemistry on fault zone rheology. Fluid-mediated fault weakening can occur in rather short time (months-to-years) comparable to the interseismic period, progressively promoting long-term, viscous aseismic creep on a previously strong fault zone developed by dynamic rupturing. The combined effect of strain localization, the low permeability of the phyllonitic cores, as well as of fluid chemistry evolution and CO2-enrichment, may lead to the development of brittle-frictional instabilities during transient accelerated-creep events. Therefore, the fluid-mediated microstructural evolution of phyllosilicate-rich fault rocks controls their seismogenic behaviour, potentially leading to accelerated creep, slow earthquakes and slow slip on otherwise aseismically creeping faults.

Deformation mechanisms and fluid conditions of mélange shear zones associated with seamount subduction

Lithologic heterogeneity and the presence of fluids have been linked to seamount subduction and collocated with slow earthquakes. However, the deformation mechanisms and fluid conditions associated with seamount subduction remain poorly understood. The exhumed Chichibu accretionary complex on Amami-Oshima Island preserves mélange shear zones composed of mudstone-dominated mélange and basalt–limestone mélange deformed under sub-greenschist facies metamorphism. The mudstone-dominated mélange contains sandstone, siliceous mudstone, and basalt lenses in an illitic matrix. The basalt–limestone mélange contains micritic limestone and basalt lenses in a chloritic matrix derived from the mixing of limestone and basalt at the foot of a seamount. The basalt–limestone mélange overlies the mudstone-dominated mélange, possibly representing a submarine landslide from the seamount onto trench-fill terrigenous sediments. The asymmetric S–C fabrics in both mélanges show top-to-SE shear consistent with megathrust-related shear. Quartz-filled shear and extension veins in the mudstone-dominated mélange indicate brittle failure at near-lithostatic fluid pressure and low differential stress. Microstructural observations show that deformation in the mudstone-dominated mélange was accommodated by dislocation creep of quartz and combined quartz pressure solution with frictional sliding of illite, whereas the basalt-limestone mélange was accommodated by frictional sliding of chlorite and dislocation creep of coarse-grained calcite, with possible pressure solution creep and diffusion creep of fine-grained calcite. The mélange shear zones formed in association with seamount subduction record temporal changes in deformation mechanisms, fluid pressure, and stress state during megathrust shear with brittle failure under elevated fluid pressure, potentially linking tremor generation near subducting seamounts.

What's down there? The structures, materials and environment of deep-seated slow slip and tremor.

Deep-seated slow slip and tremor (SST), including slow slip events, episodic tremor and slip, and low-frequency earthquakes, occur downdip of the seismogenic zone of numerous subduction megathrusts and plate boundary strike-slip faults. These events represent a fascinating and perplexing mode of fault failure that has greatly broadened our view of earthquake dynamics. In this contribution, we review constraints on SST deformation processes from both geophysical observations of active subduction zones and geological observations of exhumed field analogues. We first provide an overview of what has been learned about the environment, kinematics and dynamics of SST from geodetic and seismologic data. We then describe the materials, deformation mechanisms, and metamorphic and fluid pressure conditions that characterize exhumed rocks from SST source depths. Both the geophysical and geological records strongly suggest the importance of a fluid-rich and high fluid pressure habitat for the SST source region. Additionally, transient deformation features preserved in the rock record, involving combined frictional-viscous shear in regions of mixed lithology and near-lithostatic fluid pressures, may scale with the tremor component of SST. While several open questions remain, it is clear that improved constraints on the materials, environment, structure, and conditions of the plate interface from geophysical imaging and geologic observations will enhance model representations of the boundary conditions and geometry of the SST deformation process.This article is part of a discussion meeting issue ‘Understanding earthquakes using the geological record’.

The geophysics, geology and mechanics of slow fault slip

Modern geodetic and seismologic observations describe the behavior of fault slip over a vast range of spatial and temporal scales. Slip at sub-seismogenic speeds is evident from top to bottom of lithospheric faults and plays an important role throughout the earthquake cycle. Where earthquakes and tremor accompany slow slip, they help illuminate the spatiotemporal evolution of fault slip. Geophysical subsurface imaging and geologic field studies provide information about suitable environments of slow slip. In particular, exhumed fault and shear zones from various depths reveal the importance of multiple deformation processes and fault-zone structures. Most geologic examples feature frictionally weak and velocity-strengthening materials, well-developed mineral fabrics, and abundant veining indicative of near-lithostatic fluid pressure. To produce transient slow slip events and tremor, in addition to the presence of high-pressure fluids a heterogeneous fault-zone structure, composition, and/or metamorphic assemblage may be needed. Laboratory and computational models suggest that velocity-weakening slip patches smaller than a critical dimension needed for earthquake nucleation will also fail in slow slip events. Changes in fluid pressure or slip rate can cause a fault to transition between stable and unstable fault slip behavior. Future interdisciplinary investigations of slow fault slip, directly integrating geophysical, geological and modeling investigations, will further improve our understanding of the dynamics of slow slip and aid in providing more accurate earthquake hazard characterizations.

Slow wavespeeds and fluid overpressure in a region of shallow geodetic locking and slow slip, Hikurangi subduction margin, New Zealand

Travel times recorded onshore from active-source marine seismic surveys were used to determine Hikurangi forearc wavespeeds. Ray-path midpoints sample forearc crust above the shallow (<10–12 km) subduction thrust. The southern region is locked to c. 30 km depth, exhibits slow slip at 30-45 km depth, and is similar to other subduction zones. Our 1D southern model has a rapid increase in seismic velocity (Vp) from the seabed to 4 km depth, consistent with rapidly-deposited clastic sediments, and near-constant Vp=5.0±0.2 km/s at depths of 4–10 km within the forearc. The northern region has slow-slip events at shallow depths of c. 5–15 km, beneath the volume sampled by our analysis. Average travel times at offsets of 20–80 km are >1 s more than at equivalent offsets in the south, and Vp increases with depth within the forearc from 3.5±0.1 km/s at 4 km depth to 4.5±0.2 km/s at 10 km depth. We interpret the low wavespeeds in terms of compaction disequilibrium and compare seismic inferences of anomalously-high porosity (>10%) with hydrostatic reference compaction models to show effective stress is low in both the north (27±10 Mpa) and south (36±14 MPa). In the south, pore space and conduits have a high chance of being localized on faults or sand layers, the wedge is clearly compressionally critical, and we suggest the higher seismic velocities recorded primarily reflect higher effective stress levels transmitted through the rock framework. The observed wedge geometry and our estimate of fluid pressure within the wedge (λ=0.87±0.05) suggest a weak overpressured subduction thrust. In the north, we suggest high fluid pressure is maintained by a large fluid inventory from subducting sediment, a pore-space geometry characterized by pervasively-fractured rock and mudstone, and a lower mean stress due to its stable and non-critical wedge geometry. We consider both end-member assumptions of convergent and tensile failure, the latter of which reveals the possibility of near-lithostatic fluid pressure on the north Hikurangi subduction interface, although this remains unproven. Our observations are consistent with high fluid pressures encountered in petroleum wells at depths <3 km and we observe a clear spatial correlation between the distribution of residual travel-times and the maximum depth of geodetic coupling and slow-slip. Our observations provide evidence for a causative link between high fluid pressure and anomalously-shallow slow slip events and depth of geodetic locking.

Structural Controls on Orogenic Au Mineralization During Transpression: Lupa Goldfield, Southwestern Tanzania

Au mineralization in the western Lupa goldfield, southwestern Tanzania was associated with transpression and reverse sinistral slip along a network of steeply S dipping shear zones with non-Andersonian geometries. Slip was accommodated by: (1) frictional failure and sliding during emplacement of quartz ± Au-bearing veins; and (2) crystal plasticity and fluid-assisted diffusive mass transfer. The Kenge mineral system is situated along a NW-SE-trending shear zone and is characterized by ≤10-m thick, Au-bearing fault-fill veins hosted by well-developed phyllosilicate-rich mylonites. The broadly contemporaneous Porcupine mineral system is situated along an ENE-WSW– to E–W-trending shear zone, which is characterized by narrow, discontinuous mylonitic shear zone within a silicified and nonfoliated granitoid protolith. Au mineralization at Porcupine occurs within steeply dipping fault-fill and subhorizontal extension/oblique-extension veins. Three-dimensional frictional reactivation theory provides a self-consistent explanation for the different vein styles at Kenge and Porcupine and extends the classic fault valve model to the general case of oblique slip along multiple, arbitrarily oriented shear zones. Analysis of the differential stress required for frictional reactivation suggests the following: (1) the Kenge shear zone was intrinsically weaker than the Porcupine shear zone, consistent with the lack of well-developed mylonites at Porcupine; and (2) frictional reactivation of the Kenge shear zone occurred under suprahydrostatic but sublithostatic pore fluid pressures, whereas frictional reactivation of the Porcupine shear zone occurred under near-lithostatic fluid pressures. We hypothesize that near-lithostatic pore fluid pressures relieved effective normal stresses at grain-grain contacts, helping to preserve intragranular and fracture porosity at the Porcupine orebody. As such, these pore spaces may be important microstructural sites for Au mineralization. Low effective normal stresses can also explain the poorly developed phyllosilicate-rich mylonites and limited degree of shear zone weakening at Porcupine.

Stress-state evolution of the brittle upper crust during compressional tectonic inversion as defined by successive quartz vein types (High-Ardenne slate belt, Germany)

Abstract: In the frontal part of the Rhenohercynian fold-and-thrust belt (High-Ardenne slate belt, Germany), two successive types of quartz veins, oriented normal and parallel to bedding respectively, are interpreted to reflect the early Variscan compressional tectonic inversion of the Ardenne–Eifel sedimentary basin. Fracturing and sealing occurred in Lower Devonian siliciclastic multilayers under very low-grade metamorphic conditions in a brittle upper crust. A geometrical and microthermometric analysis of these veins has helped to constrain the kinematic and pressure–temperature conditions of both vein types, allowing the reconstruction of the stress-state evolution in a basin during tectonic inversion. It is demonstrated that bedding-normal extension veins, which developed under low differential stresses and repeatedly opened and sealed (crack-seal) under near-lithostatic fluid pressures, reflect the latest stage of an extensional stress regime. Bedding-parallel veins, which developed at differential stresses that were still low enough to allow the formation of extension veins, cross-cut the bedding-normal veins and preceded the regional fold and cleavage development. These veins show a pronounced bedding-parallel fabric, reflecting bedding-normal uplift and bedding-parallel shearing under lithostatic to supra-lithostatic fluid pressures during the early stages of a compressional stress regime. This kinematic history corroborates that fluid overpressures are easy to maintain during compressional tectonic inversion at the onset of orogeny.

Imaging the source region of Cascadia tremor and intermediate-depth earthquakes

Research Article| December 01, 2009 Imaging the source region of Cascadia tremor and intermediate-depth earthquakes Geoffrey A. Abers; Geoffrey A. Abers * 1Lamont-Doherty Earth Observatory of Columbia University, Route 9W, P.O. Box 1000, Palisades, New York 10964, USA *E-mail: abers@ldeo.columbia.edu. Search for other works by this author on: GSW Google Scholar Laura S. MacKenzie; Laura S. MacKenzie 2Schlumberger Data and Consulting Services 6501 S. Fiddler's Green, Greenwood Village, Colorado 80111, USA Search for other works by this author on: GSW Google Scholar Stéphane Rondenay; Stéphane Rondenay 3Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA Search for other works by this author on: GSW Google Scholar Zhu Zhang; Zhu Zhang 4Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82070, USA Search for other works by this author on: GSW Google Scholar Aaron G. Wech; Aaron G. Wech 5Department of Earth and Space Science, University of Washington, Seattle, Washington 98195, USA Search for other works by this author on: GSW Google Scholar Kenneth C. Creager Kenneth C. Creager 5Department of Earth and Space Science, University of Washington, Seattle, Washington 98195, USA Search for other works by this author on: GSW Google Scholar Geology (2009) 37 (12): 1119–1122. https://doi.org/10.1130/G30143A.1 Article history received: 16 Feb 2009 rev-recd: 17 Jul 2009 accepted: 21 Jul 2009 first online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Geoffrey A. Abers, Laura S. MacKenzie, Stéphane Rondenay, Zhu Zhang, Aaron G. Wech, Kenneth C. Creager; Imaging the source region of Cascadia tremor and intermediate-depth earthquakes. Geology 2009;; 37 (12): 1119–1122. doi: https://doi.org/10.1130/G30143A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The subduction of hydrated oceanic crust releases volatiles that weaken the plate boundary interface, trigger earthquakes, and regulate transient phenomena such as episodic tremor and slip (ETS). It is not clear how dehydration can separately induce earthquakes within the subducting plate and ETS, partly because few data exist on their relationship to subduction zone structures. We present results of a seismic experiment in the Washington Cascades, United States, that images a region producing both earthquake types. Migration of scattered teleseis-mic waves provides images of low-velocity subducting crust at depths <40–45 km with sharp boundaries above and below it. The sharp upper boundary indicates a layer of weak sediment or an overpressured fault zone that terminates abruptly downdip at 40–45 km depth. Regular earthquakes are at the top of the mantle within the downgoing plate everywhere the plate is <95 km deep, but ETS only exists where the sharp upper boundary occurs. The ETS location supports models of slow slip that require near-lithostatic fluid pressure, whereas regular earthquakes nucleate closer to the origin of metamorphic dehydration. Very low shear stresses on the plate boundary may limit seismicity to ETS and similar phenomena. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The “boudinage” question in the High-Ardenne Slate Belt (Belgium): a combined structural and fluid-inclusion approach

The terms “boudin” and “boudinage” 1 1 “Boudin”, “boudinage”, “interboudin”: The terms between quotes apply to the particular structures in the High-Ardenne Slate Belt; the terms without quotes apply to the process of boudinage. were first introduced, as descriptive terms, for particular geometric features associated with quartz veins in competent metaquartzitic layers in the High-Ardenne Slate Belt (Belgium). The occurrence of the “boudins” has subsequently been used to draw kinematic conclusions on a pre-Variscan extensional event in the Rhenohercynian belt. In this paper, a combined structural and fluid-inclusion approach allows proposing a new and more representative kinematic model for the development of the veins and associated structures. Moreover, the kinematic significance of “interboudin” veins is emphasized. In this regard, microthermometry of primary CO 2–N 2 fluid inclusions in the quartz of the “interboudin” veins suggests that the origin of vein development is hydraulic fracturing in an already compressional setting. This suggestion implies the presence of fluid and near-lithostatic fluid pressures to permit fracturing at great depths where the rocks otherwise would be either stable or in the realm of ductile behaviour. The vein filling fluid had a local origin and was most likely related to the formation of bedding-parallel dissolution seams within the host rock. Veining is followed by layer-parallel shortening, resulting in the development of “double-sided” mullions at both interfaces of metaquartzitic layers, pinned by pre-existing veins. Subsequently, these polyphase structures behave passively in the Variscan deformation.

Reaction-enhanced permeability during decarbonation of calcite + quartz → wollastonite + carbon dioxide

Permeability was measured during decarbonation reaction of calcite + quartz → wollastonite + CO2 in the presence of water. The aggregate used for measurement was initially composed of 90 wt% calcite and 10 wt% quartz. At an effective pressure (confining pressure minus pore fluid pressure) of 25 MPa, the reaction proceeds to completion in <40 h at temperatures of 600–700 °C. Permeability remains nearly constant at about 10−16.5 m2 at 600 °C, but it evolves from 10−16.5 to 10−19 m2 at 700 °C as quartz and calcite react to form porous wollastonite aggregates. Over the same temperature range, but at an effective pressure of 100 MPa, the permeability reduces rapidly below 10−19 m2, and only limited volumes of dense wollastonite aggregates were produced on calcite grain surfaces. Our experiments demonstrate the importance of effective pressure in influencing reaction progress and permeability evolution during metamorphic reactions. The experiments clearly suggest that where water-infiltration–driven decarbonation in siliceous carbonates occurs under near-lithostatic fluid pressure conditions, reaction kinetics can be very fast and the release of carbon dioxide is continuous.

Fault-valve behaviour in optimally oriented shear zones: an example at the Revenge gold mine, Kambalda, Western Australia

Quartz vein systems developed in and adjacent to shear zones host major gold deposits in the Kambalda region of the Norseman–Wiluna greenstone belt. At the Revenge Mine, two groups of mineralised reverse shear zones formed as conjugate, near-optimally oriented sets during ESE subhorizontal shortening adjacent to a major transpressional shear system. The shear zones developed at temperatures of about 400°C in a transitional brittle–ductile regime. Deformation was associated with high fluid fluxes and involved fault-valve behaviour at transiently near-lithostatic fluid pressures. During progressive evolution of the shear system, early brittle and ductile deformation was overprinted by predominantly brittle deformation. Brittle shear failure was associated with fault dilation and the formation of fault-fill veins, particularly at fault bends and jogs. A transition from predominantly brittle shear failure to combined shear along faults and extension failure adjacent to faults occurred late during shear zone evolution and is interpreted as a response to a progressive decrease in maximum shear stress and a decrease in effective stresses. The formation of subhorizontal stylolites, locally subvertical extension veins and minor normal faults in association with thrust faulting, indicates episodic or transient reorientation of the near-field maximum principal stress from a subhorizontal to a near-vertical attitude during some fault-valve cycles. Local stress re-orientation is interpreted as resulting from near-total shear stress release and overshoot during some rupture events. Previously described fault-valve systems have formed predominantly in severely misoriented faults. The shear systems at Revenge Mine indicate that fault-valve action, and associated fluctuations in shear stress and fluid pressure, can influence the mechanical behaviour of optimally-oriented faults.

Stress/fault controls on the containment and release of overpressured fluids: Examples from gold-quartz vein systems in Juneau, Alaska; Victoria, Australia and Otago, New Zealand

The brittle carapace overlying a prograding metamorphic belt embraces the transition between a hydrostatically pressured near-surface fluid regime and the near-lithostatic fluid pressures characterising prograde metamorphism at depth, with the lower portion of the carapace acting as a low permeability barrier. During exhumation, the base of the carapace migrates downwards into the core of a cooling orogen. Mesozonal gold-quartz vein systems hosted in fault-fracture meshes (comprising low-displacement shears interlinked with hydraulic extension fractures) may develop towards the base of, or within, the brittle carapace. Such mesh structures, `self-generated' by the infiltration of overpressured fluids at pressures locally exceeding the least principal compressive stress (i.e. P f> σ 3), form high-permeability conduits for episodic large-volume fluid discharge by fault-valve action. These concepts are explored principally through comparison of gold-quartz vein systems hosted in fault-fracture meshes within the Bendigo–Ballarat zone of the Paleozoic Lachlan Fold Belt in Victoria, Australia, with those developed within the Mesozoic Otago Schists of southern New Zealand. The stress state within the carapace critically affects the containment of overpressured fluids within the mid-crust and the mode of fluid release. Fluids are most easily trapped beneath the carapace in compressional–transpressional regimes where the highest levels of overpressuring may be attained. Supralithostatic fluid pressures may then develop locally beneath permeability barriers over depth intervals determined by the tensile strength of sealing horizons. Mesh development is favoured by heterogeneity and high competence contrast between adjacent rock units but is inhibited by the presence of low-cohesion faults that are favourably oriented for reactivation within the carapace stress field, because such faults reactivate in shear at fluid pressure levels less than those required for hydraulic extension fracturing. Attainment of extreme overpressures ( P f> σ 3) and the formation of high-permeability meshes in any given stress regime therefore requires either intact crust (perhaps reconstituted through metamorphism), or crust where faults have become severely misoriented in the prevailing stress field through progressive deformation. In this latter case, the existing faults may be reactivated by the extreme overpressure and incorporated into the mesh structure. The combination of these circumstances explains why mesh structures associated with steep reverse faults promote particularly vigorous fault-valve action and form good mineralising sites. Major episodes of fluid loss and mineralisation may, however, also be effected by an abrupt transition from a compressional to an extensional stress field, as occurred locally within the Juneau gold-belt of SE Alaska.

The San Gabriel Mountains bright reflective zone: possible evidence of young mid-crustal thrust faulting in southern California

During the Los Angeles Region Seismic Experiment (LARSE), a reflection/refraction survey was conducted along a line extending northeastward from Seal Beach, California, to the Mojave Desert, crossing the Los Angeles basin and San Gabriel Mountains. Shots and receivers were spaced most densely through the San Gabriel Mountains for the purpose of obtaining a combined reflection and refraction image of the crust in that area. A stack of common-midpoint (CMP) data reveals a bright reflective zone, 1-s thick, that dominates the stack and extends throughout most of the mid-crust of the San Gabriel Mountains. The top of this zone ranges in depth from 6 s (∼18-km depth) in the southern San Gabriel Mountains to 7.5 s (∼23-km depth) in the northern San Gabriel Mountains. The zone bends downward beneath the surface traces of the San Gabriel and San Andreas faults. It is brightest between these two faults, where it is given the name San Gabriel Mountains ‘bright spot’ (SGMBS), and becomes more poorly defined south of the San Gabriel fault and north of the San Andreas fault. The polarity of the seismic signal at the top of this zone is clearly negative, and our analysis suggests it represents a negative velocity step. The magnitude of the velocity step is approximately 1.7 km/s. In at least one location, an event with positive polarity can be observed 0.2 s beneath the top of this zone, indicating a thickness of the order of 500 m for the low-velocity zone at this location. Several factors combine to make the preferred interpretation of this bright reflective zone a young fault zone, possibly a ‘master’ decollement. (1) It represents a significant velocity reduction. If the rocks in this zone contain fluids, such a reduction could be caused by a differential change in fluid pressure between the caprock and the rocks in the SGMBS; near-lithostatic fluid pressure is required in the SGMBS. Such differential changes are believed to occur in the neighborhood of active fault zones, where ‘fault-value’ action has been postulated. Less likely alternative explanations for this velocity reduction include the presence of magma and a change in composition to serpentinite or metagraywacke. (2) It occurs at or near the brittle-ductile transition, at least in the southern San Gabriel Mountains, a possible zone of concentrated shear. (3) A thin reflection rising from its top in the southern San Gabriel Mountains projects to the hypocenter of the 1987 M 5.9 Whittier Narrows earthquake, a blind thrust-fault earthquake with one focal plane subparallel to the reflection. Alternatively, one could argue that the bends or disruptions in the reflective zone seen at the San Gabriel and San Andreas faults are actually offsets and that the reflective zone is therefore an older feature, possibly an older fault zone.

Seismically Inferred Fault Zone Properties and Fluid Migration along the Barbados Ridge Decollement: ABSTRACT

Three-dimensional seismic reflection data acquired in a 5 x 25 km area of the toe of the Barbados Ridge accretionary complex reveal the amplitude and waveform associated with the reflection from the decollement plate boundary fault. Reflections are analyzed using computer-automated modeling of amplitudes and waveforms developed to quantitatively measure reflection coefficients and examine fault zone physical properties related to the behavior of pore fluids migrating along the decollement. Modeling shows that the decollement reflection is usually caused by a thin low-velocity, low-density (and presumably high porosity) layer. A map of layer thickness indicates smooth thickness variations from 8 to 20 m over 500 m distances and trends that parallel the strike of the Barbados Ridge. In a 2 x 6 km region of high amplitude, a 100-200 m/s low-velocity anomaly and a corresponding low-density anomaly is inferred in the fault zone from reflection coefficients at the top of the layer. The map patterns of reflection properties reveal km-wide areas of connected high-porosity, and presumably high-permeability zones. The magnitude of the reflection coefficients at the base of the layer reveal connected km-wide areas where the amplitude is equal to that at the top of the layer and other areas wheremore » it is only half as large. This variation may indicate differences in the history of fluid movements through the fault zone. The fault probably develops near-lithostatic fluid pressure in the km-wide high-amplitude areas. The high fluid pressure maintains a weak fault and dilates the fault zone allowing focussed flow along a permeable fault plane.« less

Geochemistry of tectonically expelled fluids from the northern Coast ranges, Rumsey Hills, California, USA

Abstract Tectonic compression has created abnormally high pressure on deep basinal fluids causing their expulsion from areally exposed Upper Cretaceous rock along the eastern margin of the California Coast ranges. The fluids emerge as near-neutral, perennial sodium chloride springs at high elevations with flow rates as high as 10 L per min. Higher spring discharges are more common around the exposure of a west-vergent fault propagation fold axis. Spring waters range from ~1000 to 27,000 mg/L TDS. The least saline water ( δ 18 O = −7.5‰ ) closely represents local meteoric water that mixes with saline fluid ( δ 18 O = +5.3‰ ) and forms a slope of ~3.5 on a δD vs. δ18O plot. A Na (125 to 8000 mg/L) vs. Cl (150 to 17,000 mg/L) plot shows a linear dilution trend that extends close to, but below, the values for modern seawater. Calcium (75–3000 mg/L) is considerably enriched relative to seawater and forms a nonlinear trend with chloride. In detail, the “Na deficit,” defined by the difference between the measured Na content and the Na concentration on a hypothetical seawater dilution line, is approximately balanced by the Ca excess, similarly defined by the seawater dilution line. This relationship strongly suggests that the fluid is diluted seawater that is being modified by active albitization of plagioclase at different depths. Simultaneous B and 18O enrichment of the fluids, accompanied by deuterium depletion, further suggest that the seawater modification is influenced by clay diagenesis. Bicarbonate and SiO2 concentrations show an inverse correlation with Cl, with most waters being saturated or slightly oversaturated with calcite and quartz at the discharge temperatures. Some freshwater springs with near-meteoric stable isotope values may represent mixing of young groundwater from perched aquifers, but in many cases, the freshwater springs emerge along the same structures and have the same perennial nature as the saline fluids, and expulsion of an older fresh groundwater component that is under abnormal fluid pressures cannot be ruled out. Basinal fluids elsewhere commonly show dilution trends with local meteoric water, and in the case of the Rumsey Hills, some of the dilute saline waters may indicate deep penetration of meteoric water (> 1 km) in the Pleistocene before the latest tectonic uplift. Geothermometry of the spring waters (maximum ~90°C) suggest an origin from as deep as 4.0 km. This depth is consistent with the depth of the core of a fault propagation anticline below the surface of the Rumsey Hills developed by active internal deformation of an east-tapering wedge beneath the southwestern Sacramento Valley. Active tectonic compression causes near-lithostatic fluid pressures in the shallow subsurface below the Rumsey Hills and volume strain within the core of the anticline that results in upward expulsion of the saline fluids from the indicated depths.

Fluid flow along a strike-slip fault at the toe of the Oregon accretionary prism: Implications for the geometry of frontal accretion

Research Article| May 01, 1993 Fluid flow along a strike-slip fault at the toe of the Oregon accretionary prism: Implications for the geometry of frontal accretion HAROLD J. TOBIN; HAROLD J. TOBIN 1Department of Earth Sciences and Institute for Marine Sciences, University of California, Santa Cruz, California 95064 Search for other works by this author on: GSW Google Scholar J. CASEY MOORE; J. CASEY MOORE 1Department of Earth Sciences and Institute for Marine Sciences, University of California, Santa Cruz, California 95064 Search for other works by this author on: GSW Google Scholar MARY E. MACKAY; MARY E. MACKAY 2Department of Geology and Geophysics, School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, Hawaii 96822 Search for other works by this author on: GSW Google Scholar DANIEL L. ORANGE; DANIEL L. ORANGE 1Department of Earth Sciences and Institute for Marine Sciences, University of California, Santa Cruz, California 95064 Search for other works by this author on: GSW Google Scholar LAVERNE D. KULM LAVERNE D. KULM 3College of Oceanography, Oregon State University, Corvallis, Oregon 97331 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1993) 105 (5): 569–582. https://doi.org/10.1130/0016-7606(1993)105<0569:FFAASS>2.3.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation HAROLD J. TOBIN, J. CASEY MOORE, MARY E. MACKAY, DANIEL L. ORANGE, LAVERNE D. KULM; Fluid flow along a strike-slip fault at the toe of the Oregon accretionary prism: Implications for the geometry of frontal accretion. GSA Bulletin 1993;; 105 (5): 569–582. doi: https://doi.org/10.1130/0016-7606(1993)105<0569:FFAASS>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Sediment offscraping and accretion takes place predominantly by landward-vergent thrusting along most of the Oregon accretionary prism north of latitude 44°50'N. This indicates very low basal shear stress, probably due to near-lithostatic fluid pressure on the décollement horizon. Recent multichannel seismic and sidescan sonar studies have demonstrated the existence of an oblique, basement-involved, left-lateral strike-slip fault, termed the Wecoma fault. It cuts abyssal plain sediments and oceanic basement of the subducting plate. Where the Wecoma fault intersects the wedge thrust front at 45°09'N, accretion to the wedge occurs through an anomalous seaward-vergent thrust. Linear, fault-parallel erosional gullies, which trend obliquely across the slope on the seaward flank of the frontal anticline in the accretionary wedge, are interpreted as marking the surface traces of splays of the Wecoma fault. Data collected during six Alvin dives on and around these gullies demonstrate that fluid expulsion from the deforming sediments is preferentially concentrated along the fault traces. Evidence includes the presence of chemosynthetic biological communities, pervasive veining and fracturing of the rock, and extensive carbonate cementation, all forming preferentially within the gullies. This strike-slip fault apparently acts as a high-angle fluid escape conduit, channeling fluids from deep within the wedge to the surface. We propose that this fluid conduit causes the local inversion of thrusting to seaward vergence by allowing the escape of highly overpressured fluids from the incipient décollement horizon. The consequent increase in basal effective normal stress favors the development of a seaward-vergent thrust. The discovery of this strike-slip fault acting as an efficient fluid conduit to the surface of the wedge emphasizes the generally unappreciated importance of strike-slip (and normal) faults in dewatering accretionary prisms. Such dewatering features may significantly influence the development of large-scale structures, and they have broad applicability to other tectonic settings, including collision zones and continental fold-and-thrust belts. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The change in orientation of subsidiary shears near faults containing pore fluid under high pressure

Byerlee, J., 1992. The change in orientation of subsidiary shears near faults containing pore fluid under high pressure. In: T. Mikumo, K. Aki, M. Ohnaka, L.J. Ruff and P.K.P. Spudich (Editors), Earthquake Source Physics and Earthquake Precursors. Tectonophysics, 211: 295–303. The mechanical effects of a fault containing near-lithostatic fluid pressure in which fluid pressure decreases monotonically from the core of the fault zone to the adjacent country rock is considered. This fluid pressure distribution has mechanical implications for the orientation of subsidiary shears around a fault. Analysis shows that the maximum principal stress is oriented at a high angle to the fault in the country rock where the pore pressure is hydrostatic, and rotates to 45° to the fault within the fault zone where the pore pressure is much higher. This analysis suggests that on the San Andreas fault, where heat flow constraints require that the coefficient of friction for slip on the fault be less than 0.1, the pore fluid pressure on the main fault is 85% of the lithostatic pressure. The observed geometry of the subsidiary shears in the creeping section of the San Andreas are broadly consistent with this model, with differences that may be due to the heterogeneous nature of the fault.

Friction, overpressure and fault normal compression

More than twenty‐five years ago Miller and Low reported the existence of a threshold pore pressure gradient below which water would not flow through clay. Recent experimental observations of the shear strength of structured water on biotite surfaces have provided a physical basis for understanding this threshold gradient. The existence of this phenomenon has profound implications for the rheological properties of mature fault zones, such as the San Andreas, that contain large thicknesses of fault gouge. For example, a clay–filled fault zone about 1 km wide at the base of the seismogenic zone decreasing to zero width at the surface could support core fluid pressure equal to the maximum principal stress over the entire seismogenic zone. As a result, the fault would have near‐zero strength and the maximum principal stress measured on the flanks of the fault, would be oriented normal to the fault surface. Another consequence of the threshold gradient is that normal hydrostatic fluid pressures outside the fault zone could coexist with near‐lithostatic fluid pressures in the interior of the fault zone without the need for continual replenishment of the overpressured fluid. In addition, the pore pressure at any point should never exceed the local minimum principal stress so that hydrofracture will not occur.

A fluid inclusion and stable isotope study of synmetamorphic copper ore formation at Mount Isa, Australia

Earlier structural studies indicate that the dolomitic and siliceous breccias and contained copper mineralization at Mount Isa (Queensland, Australia) formed during regional deformation and greenschist facies metamorphism of their host mid-Proterozoic metasediments. The metasomatic breccia body (locally called silica dolomite) is zoned, with an outer halo of sparry dolomitic alteration replacing finely laminated dolomitic-pyritic metasediments. The core of the dolomitic breccia next to the underlying altered greenstones is overprinted by siliceous breccia containing the bulk of the chalcopyrite ore.Fluid inclusions in quartz and dolomite were studied by microthermometry, micro-Raman spectrometry, and semiquantitative electron microprobe analysis of inclusion salts. Two types of aqueous inclusions are restricted to dolomitic breccia: CaCl 2 -rich (group 1) with a cation ratio Na/Ca/K/Mg nearly equal 30:10:3:1 and salinity ( approximately 25 wt %) similar to recent Salton Sea geothermal brines; and low-salinity with 10 to 20 mole percent CO 2 (CO 2 -rich, group 2). These two aqueous inclusion types are intimately associated in the same crystals and have probably both been trapped during dolomitic alteration.Siliceous alteration superimposed on dolomitic breccia contains NaCl-rich fluid inclusions (group 3: Na/Ca/K/Mg nearly equal 300:10:30:1, minor CH 4 ). Microstructures suggest that earlier higher salinity (10-20 wt %) and later lower salinity (4-9 wt %) variants of this NaCl-rich fluid bracket the main stage of chalcopyrite introduction. There are no low-density vapor inclusions indicative of any stage of fluid boiling. Rare high-density CH 4 (+ or -CO 2 )-rich carbonic inclusions are associated with NaCl-rich aqueous inclusions in a few samples. Secondary inclusions of a highly calcic low-temperature brine (group 4) occur along late healed cracks and postdate all major stages of mineral precipitation.The C, O, and H isotope compositions of dolomites, quartz, and hydrous silicates and of fluid inclusion extracts from samples of the siliceous alteration zone were imposed by the externally derived, weakly CH 4 + CO 2 -bearing, NaCl-rich fluid at high fluid/rock ratios and over a small range of temperatures. This fluid may have originated as a highly evolved basin brine or an evaporite-derived metamorphic fluid. The early CaCl 2 -rich fluid may have had an origin similar to the NaCl-rich ore fluid, but its different delta D and the mass balance constraints arising from its chemical composition suggest that it has not been derived from the NaCl-rich fluid by progressive rock interaction within the zoned alteration system. Similarly, an independent source is likely for the CO 2 -rich fluid.The absolute pressure-temperature ranges for the two alteration processes seem to be slightly different, but it is uncertain whether the quartz-chalcopyrite mineralization occurred at slightly lower temperature or higher pressure compared with the dolomitic alteration. The latter is confined to 270 degrees to 350 degrees C and 700 to 1,500 bars by isochore and mineral stability data.The following working hypothesis for the syntectonic and synmetamorphic fluid-rock interaction and copper ore formation at Mount Isa is proposed. In an initial stage, dolomitic alteration occurred by chemical interaction between the Urquhart Shale and moderate amounts of two fluids of different chemical and hydrological origin, represented by the CaCl 2 -rich and the CO 2 -rich fluid inclusions. Fluid mixing may have been aided by extension on the limb of a regional fold which temporarily lowered fluid pressures, thus leading to hydraulic brecciation and fluid influx from two fluid regimes, above and below. The CO 2 -rich fluid probably represents the local metamorphic fluid of the Urquhart Shale, whereas the CaCl 2 -rich brine was introduced from the underlying altered greenstones. The initial stage of brecciation and dolomitic alteration was followed by the main stage of silicification and copper introduction by the much more copious NaCl-rich fluid, probably at near-lithostatic fluid pressure. Quartz and chalcopyrite precipitation occurred by a combination of slight cooling, a pH increase, and possibly an increase in sulfur activity, as the initially reduced, acid, and possibly S-deficient, NaCl-rich fluids reacted with the previously dolomitized pyritic and dolomitic Urquhart Shale.

Coupled grain-scale dilatancy and mass transfer during deformation at high fluid pressures: examples from Mount Lyell, Tasmania

High fluid pressures have promoted the coupled operation of grain-scale dilatancy and solution-precipitation processes as dominant deformation mechanisms during cleavage development in a sequence of Cambrian silicic volcanics in the Mount Lyell area, Tasmania. Episodic opening of pervasive microcracks has localized the precipitation of material removed from dissolution sites, and has led to the growth of oriented crack-seal microstructures which are a major element of the overall deformation microfabric. It is argued that transient, dilatancy-driven fluid pressure gradients, and consequent fluid migration within the fluid-containing grain-boundary and microcrack network, can play an important role in contributing to mass transfer between dissolution sites and dilatant microfracture sites. The enhancement of grain-scale microfracture processes and coupled solution-precipitation processes, which accompanies the development of near-lithostatic fluid pressures, is expected to lead to high fluid pressure crustal regimes becoming substantially weaker than otherwise similar low fluid pressure regimes. The onset of such weakening processes in response to rising fluid pressures is probably a significant factor triggering pervasive regional deformation of the upper-crust.