William T. Parry

Fracturing and hydrothermal alteration in normal fault zones

Large normal fault zones are characterized by intense fracturing and hydrothermal alteration. Displacement is localized in a slip zone of cataclasite, breccia and phyllonite surrounding corrugated and striated fault surfaces. Slip zone rock grades into fractured, but less comminuted and hydrothermally altered rock in the transition zone, which in turn grades abruptly into the wall rock. Fracturing and fluid flow is episodic, because permeability generated during earthquakes is destroyed by hydrothermal processes during the time between earthquakes.Fracture networks are described by a fracture fabric tensor (F). The permeability tensor (k) is used to estimate fluid transport properties if the trace of F is sufficiently large. Variations in elastic moduli and seismic velocities between fault zone and wall rock are estimated as a function of fracture density (ε). Fracturing decreases elastic moduli in the transition zone by 50–100% relative to the country rock, and similar or even greater changes presumably occur in the slip zone.P-andS-wave velocity decrease, andVp/Vs increases in the fault zone relative to the wall rock. Fracture permeability is highly variable, ranging between 10−13 m2 and 10−19 m2 at depths near 10 km. Changes in permeability arise from variations in effective stress and fracture sealing and healing.Hydrothermal alteration of quartzo-feldspathic rock atT>300°C creates mica, chlorite, epidote and alters the quartz content. Alteration changes elastic moduli, but the changes are much less than those caused by fracturing.P-andS-wave velocities also decrease in the hydrothermally altered fault rock relative to the country rock, and there is a slight decrease inVp/Vs, which partially offsets the increase inVp/Vs caused by fracturing.Fracturing and hydrothermal alteration affect fault mechanics. Low modulus rock surrounding fault surfaces increases the probability of exceeding the critical slip distance required for the onset of unstable slip during rupture initiation. Boundaries between low modulus fault rock and higher modulus wall rock also act as rupture guides and enhance rupture acceleration to dynamic velocity. Hydrothermal alteration at temperatures in excess of 300°C weakens the deeper parts of the fault zone by producingphyllitic mineral assemblages. Sealing of fracture in time periods between large earthquakes generates pods of abnormally pressured fluid which may play a fundamental role in the initiation of large earthquakes.

Diagenetic Hematite and Manganese Oxides and Fault-Related Fluid Flow in Jurassic Sandstones, Southeastern Utah

A variety of diagenetic hematite and manganese oxide deposits occur within well-exposed Jurassic eolian and related deposits of southeastern Utah. Hematite concretions (millimeters to tens of meters in size) and strata-bound layers occur in the permeable Navajo, Page, and Entrada sandstones. Localized manganese oxide deposits without significant iron oxide occur in the overlying rocks covering the Summerville-Tidwell interval. Field, lab, and numerical modeling studies indicate the diagenetic deposits are related to the Moab fault. Fluid inclusion studies show salinities of fault fluids range from 0 to 19.7 NaCl equivalent weight percent. The d18O (SMOW) and d13C (PDB) values of cements and veins range from 7 to 27o/oo and -12 to +5o/oo, respectively. The d87Sr (SMOW) values of these cements and veins range from 0.210 to 2.977o/oo, values substantially more radiogenic than Pennsylvanian seawater. Saline brines formed from solution of Pennsylvanian salts by meteoric water and are interpreted to have flowed up the Moab fault and outward into adjacent permeable rocks. These brines are reducing from interaction with hydrocarbon, methane, organic acids, or hydrogen sulfide, and thus remove iron, manganese, and 87Sr, and bleach the sandstones near the fault. The isotopic evidence suggests multiple episodes of fluid flow up the Moab fault system. When saline, reduced brines mixed with shallow oxygenated groundwater, iron and manganese oxides were precipitated as cements to form concretions and tabular deposits in the porous sandstones. Multiple episodes of iron oxide mineralization and concretionary geometries are evident and can be explained as the result of permeability heterogeneities in the host rock, presence of favorable nucleii for precipitation, a self-organization process, or the influence of microbes.

A possible terrestrial analogue for haematite concretions on Mars

Recent exploration has revealed extensive geological evidence for a water-rich past in the shallow subsurface of Mars. Images of in situ and loose accumulations of abundant, haematite-rich spherical balls from the Mars Exploration Rover ‘Opportunity’ landing site at Meridiani Planum bear a striking resemblance to diagenetic (post-depositional), haematite-cemented concretions found in the Jurassic Navajo Sandstone of southern Utah. Here we compare the spherical concretions imaged on Mars to these terrestrial concretions, and investigate the implications for analogous groundwater-related formation mechanisms. The morphology, character and distribution of Navajo haematite concretions allow us to infer host-rock properties and fluid processes necessary for similar features to develop on Mars. We conclude that the formation of such spherical haematite concretions requires the presence of a permeable host rock, groundwater flow and a chemical reaction front.

Bleaching of Jurassic Navajo Sandstone on Colorado Plateau Laramide highs: Evidence of exhumed hydrocarbon supergiants?

Spectacular color variations in the Lower Jurassic Navajo Sandstone reflect stratigraphic and structural control on the spatial distribution of fluid-driven alteration. Field observations and supervised classification of Landsat 7 Enhanced Thematic Mapper (ETM+) satellite imagery show that the most extensive regional bleaching of the Navajo Sandstone occurs on eroded crests of Laramide uplifts on the Colorado Plateau in southern Utah. Alteration patterns suggest that the blind reverse faults that core the eastern monoclines associated with these uplifts were carriers for hydrocarbons and brought the buoyant fluids to the crests of monoclines and anticlines, where they bleached the sandstone in both structural and stratigraphic traps. The extent of bleaching indicates that the Navajo Sandstone (Navajo Sandstone, Aztec Sandstone, and Nugget Sandstone) may have been one of the largest hydrocarbon reservoirs known. Rapid incision and breaching of this reservoir during Tertiary uplift and erosion of the Colorado Plateau could have released enough carbon into the atmosphere to significantly contribute to global carbon fluxes and possibly influence climate.

Chemical bleaching indicates episodes of fluid flow in deformation bands in sandstone

Jurassic sandstones on the Colorado Plateau have been variably bleached through interaction with hydrocarbon-bearing solutions or other reducing agents. Deformation bands in the Navajo Sandstone have a variety of colors in comparison with the host rock color that indicate the timing of bleaching relative to deformation-band formation. White deformation bands in red sandstone indicate that deformation bands were likely permeable at an early dilatant stage in their development history. Field characteristics, petrography, bulk rock chemistry, clay mineralogy, and geochemical modeling show that bleached deformation bands experienced an episode of chemical reduction where fluids removed some iron and left the remaining iron as pyrite and magnetite. Mass-balance calculations show that as much as 10 kg of chemically reducing fluid per 100 g of rock (1500 pore volumes of fluid) are necessary to remove 0.1 wt.% iron from a deformation band. These large pore volumes suggest that moving, reducing solutions regionally bleached the sandstone white, and bleached deformation bands resulted where deformation bands provided localized fluid access to unbleached, red sandstone during an initial dilatant stage. Alternatively, access of reducing soil solutions may be provided by gravity-driven, unsaturated flow in arid to semiarid vadose zones. Color and chemical composition is a valuable index to the pathway and timing of hydrocarbon movement through both host rocks and deformation bands.

Fault-fluid compositions from fluid-inclusion observations and solubilities of fracture-sealing minerals

Abstract Host-rock chemical alteration and syntectonic veins in and near fault zones are evidence for episodic fracturing and fluid transport during faulting. Alteration minerals, vein fillings, and fluid inclusions may be used to estimate fault-fluid chemistry, temperature, and pressure. Fluid inclusions in thrust faults, reverse faults hosting mesothermal gold deposits, and exhumed footwall rocks of normal faults show that fluid components include NaCl, CO 2 , CH 4 and CaCl 2 in addition to H 2 O. Fluid composition, temperature, and pressure are spatially and temporally variable on most faults; a typical fault fluid does not exist. NaCl concentrations in fault fluids vary from 0 to 39 wt.%, CaCl 2 concentrations range up to 19 wt.% and CO 2 concentrations range up to 32 mole% in fluid inclusions, but some inclusions are present that are 100 mole% CO 2 . Homogenization temperature measurements and pressure estimates confirm that these fluids were trapped at elevated pressure at depth on the faults. In CO 2 -bearing fault fluids, pressures fluctuated, and a range of CO 2 contents indicate effervescence. Varying solution densities of NaCl–H 2 O fluids have been interpreted to result from entrapment of fluids in inclusions at constant temperature and varying pressures. Diverse fluid compositions are present on some faults with similar homogenization temperatures and estimated pressures suggesting similar depths on the faults. Pressure, temperature and fluid composition determine the solubilities of fracture-filling minerals calcite and quartz and the formation of alteration minerals that are related to the mechanical behavior of the rock. Quartz may precipitate as a result of cooling or pressure reduction, but calcite solubility increases with cooling and decreases with decreased P CO 2 . Higher salinities increase solubilities of calcite and quartz and decrease the pH for equilibrium among feldspars, muscovite and solution. Mineral assemblages provide evidence of depressurization of the fluid as fluid moves from higher- to lower-pressured reservoirs. Precipitation of quartz, calcite, and K-feldspar or albite in fractures may result from fluid depressurization. Fault-zone rocks containing stilbite and laumontite reacted with fluid that contained little CO 2 at comparatively low temperature and pressure; kaolinite, prehnite, muscovite, epidote, and chlorite formed from fluids at higher temperature and pressure. Variations in mineralogy and fluid-inclusion characteristics on individual faults suggest separate fluids that differ in chemical composition, temperature, and pressure.

Fluid pressure transients on seismogenic normal faults

Abstract Fluid inclusions in hydrothermally altered footwall rocks of the Dixie Valley fault, Nevada, and the Wasatch fault, Utah, indicate that pore fluid pressure fluctuated. Minimum entrapment pressures for fluid inclusions consisting of H2O-CO2-NaCl ranged from 295 MPa to 60 MPa in the temperature range 350° to 170 ° C on the Wasatch fault, and from 158 to 35 MPa in the temperature range 350° to 200 ° C on the Dixie Valley fault. Scatter in the pressure estimates at constant temperature is interpreted as paleo-fluid pressure transients at depths of up to 11 km on the Wasatch fault and 3 to 5 km on the Dixie Valley fault. Observed pressure transients range from 5 MPa, within the limits of error in pressure determination, to 120 MPa on the Wasatch fault and 7 to 126 MPa on the Dixie Valley fault. The pressure transients are greatest on both faults in the temperature range 270 ° to 310 ° C. The fluids represented by fluid inclusions play a key role in nucleation and propagation of earthquake ruptures. High fluid pressures may initiate rupture, then dilatancy, pore-pressure reduction, and dilatant hardening may arrest the rupture. However, decompression of the fluids and phase separation produces a decrease in fluid bulk modulus of 41 to 90% which reduces the dilatant hardening effect and may permit ruptures to propagate.

Fluid inclusion evidence for minimum 11 km vertical offset on the Wasatch fault, Utah

The footwall of the Wasatch fault in the Corner Creek area near Salt Lake City is hydrothermally altered and deformed quartz monzonite of the Oligocene Little Cottonwood stock. Secondary fluid inclusions are associated with hydrothermal alteration minerals and structural deformation. Thermometric measurements of fluid inclusion characteristics indicate entrapment of a CO2-H2O-NaCl fluid at minimum temperatures of 223–353 °C and minimum fluid pressures of 900–2800 bar. The 2800-bar fluid pressure is near lithostatic pressure at a depth of 11 km, the minimum displacement of the fault that is required for exhumation of the observed alteration and fluid inclusions and more than three times greater than most previous estimates. The large displacement estimate is supported by the occurrence of pyrophyllite in shales in the footwall, by the petrology of metamorphosed shale surrounding an unroofed pluton in the footwall, and by geologic reconstruction of the eroded footwall.

Red rock and red planet diagenesis: Comparisons of Earth and Mars concretions

Compelling similarities between concretions on Earth and “blueberries” on Mars are used to suggest the blueberries are concretions that formed from a history of watery diagenesis. In the terrestrial examples, groundwater flow produces variations in sandstone color and iron oxide concretions in the Jurassic Navajo Sandstone of Utah. Variations in concretion mineralogy, form, and structure reflect different conditions at chemical reaction fronts, the influence of preferential fluid flow paths, the relative roles of advection and diffusion during precipitation, the presence of multiple events, fluid geochemistry, and time. The terrestrial concretions are analogs that can be used to understand the water-saturated conditions that formed spherical hematite concretions on Mars.

Structural and fluid-chemical properties of seismogenic normal faults

Abstract Structures in extensional fault zones can be classified using the configuration of fault branch lines and tip lines. Important classes include the segment bend, segment termination, segment branch, cross-fault intersection, and segment offset. The effect of these structures on rupture history is not necessarily consistent, neither between individual earthquakes nor between different fault zones. Rupture behavior is dependent on several other factors including loading conditions (regional and localized rupture tip stress fields), fluid-mechanical processes, and chemical processes. Fracture toughness is partly controlled by the angular discordance between slip directions on adjacent fault segments. Greater discordance between slip directions and intersection lines of fault segments results in greater strain incompatibility. An internal fracture network generally evolves within a segment boundary to maintain compatibility and transfer slip between the segments. The dimensions and structure of this fracture network may also partly control rupture propagation. Presumably, activation of a fault network with large angular discordances between slip directions and intersection lines will generate numerous asperities as the subsidiary faults mutually interact and offset each other. The geometry of a segment boundary may change with depth and the three-dimensional nature of the structure may be important in controlling rupture history. Fluids influence rupturing via fluid-pressure effects and time-dependent chemical processes. Fracture propagation by stress corrosion may favor instability, and chemical alteration may produce minerals of lower strengths, allowing time-dependent creep. Sealing and healing of fractures, however, may remove damage and increase strength. Elementary computations indicate that representative times for sealing and chemical alteration are between 1 and 1000 years for reasonable physical conditions, well within the recurrence intervals of most large earthquakes. Time to failure for stress corrosion cracking is more highly variable and strongly sensitive to applied stress and fluid pressure.