Footwall Damage Zone Research Articles

Hot faults: Iridescent slip surfaces with metallic luster document high-temperature ancient seismicity in the Wasatch fault zone, Utah, USA

and localized iridescence in the footwall dam- age zone of the Wasatch fault (Utah, USA) and document elevated temperatures on these faults. We propose that the formation of these iridescent slip surfaces requires frictional heat generated at geometric asperities at seismic slip rates. WASATCH FAULT ZONE ABSTRACT We document new geological indicators of ancient seismicity in the form of highly reflec- tive, iridescent, hematite-coated fault surfaces. Small faults that cut the Paleoproterozoic Farmington Canyon Complex in the footwall damage zone of the Brigham City segment of the Wasatch fault (Utah, USA) are smooth to striated surfaces, tens of square centimeters to 30 m 2 in area. The dull-rusty to high-metallic luster and moderate- to high-gloss surfaces exhibit multicolored elliptical iridescent patches ~0.5-3 cm across. Preexisting hematite crystals were deformed during slip on 1-200-mm-thick slip surfaces. Textural observations, X-ray diffraction, X-ray photoelectron spectroscopy, electron backscattered diffraction analysis, surface metrology, and similarity to experimentally formed iridescent spots in rocks and metals indicate that iridescence is associated with high-temperature (>300 °C) reduction of iron (Fe 3+ to Fe 2+ ) and associated conversion of hematite to magnetite. We pro- pose that the iridescent slip surfaces in the Wasatch fault damage zone are the result of seismic slip and flash heating at asperities along the small faults. The thousands of these sur - faces represent coseismic or aftershock deformation down to magnitude -3 in the exhumed footwall damage zone of the Wasatch fault.

Contrasts in physical properties between the hanging wall and footwall of an exhumed seismogenic megasplay fault in a subduction zone—An example from the Nobeoka Thrust Drilling Project

We examined the physical properties of an exhumed and fossilized subduction zone megasplay fault by analyzing geophysical logging data obtained by the Nobeoka Thrust Drilling Project, which provide a high‐resolution transect of properties across the main fault zone. The footwall cataclasite exhibits higher averages of neutron porosity (∼7.6%) and lower values of electric resistivity (∼232 Ωm) compared to the hanging wall phyllite (∼4.8%, ∼453 Ωm). This clear contrast between the hanging wall and footwall may account for the difference in maximum burial and structural variation. Despite the contrast observed between the hanging wall and footwall in macroscopic scale, the resistivity and porosity data from both the hanging wall and footwall can be fit with a single curve using Archie's law, suggesting the similarities in microstructures and mineralogy in this low porosity range. Above the main fault core of the Nobeoka Thrust a brittle damage zone in the hanging wall contains pseudotachylyte as evidence of the seismogenic slip and does not follow Archie's law. Damage zones in the hanging wall are also observed in the modern splay fault at shallow depth in the Nankai Trough but with much thicker width, whereas the footwall damage zone is more extensive in the Nobeoka Thrust. Splay faults may exhibit strong deformation in the hanging wall in the early stage, and as fault rocks get buried deeper and as displacement and physical property contrast increase across the fault, the damage effect may eventually be enlarged in the footwall.

Scientific drilling and downhole fluid sampling of a natural CO<sub>2</sub> reservoir, Green River, Utah

Abstract. A scientific borehole, CO2W55, was drilled into an onshore anticline, near the town of Green River, Utah for the purposes of studying a series of natural CO2 reservoirs. The objective of this research project is to recover core and fluids from natural CO2 accumulations in order to study and understand the long-term consequences of exposure of supercritical CO2, CO2-gas and CO2-charged fluids on geological materials. This will improve our ability to predict the security of future geological CO2 storage sites and the behaviour of CO2 during migration through the overburden. The Green River anticline is thought to contain supercritical reservoirs of CO2 in Permian sandstone and Mississippian-Pennsylvanian carbonate and evaporite formations at depths > 800 m. Migration of CO2 and CO2-charged brine from these deep formations, through the damage zone of two major normal faults in the overburden, feeds a stacked series of shallow reservoirs in Jurassic sandstones from 500 m depth to near surface. The drill-hole was spudded into the footwall of the Little Grand Wash normal fault at the apex of the Green River anticline, near the site of Crystal Geyser, a CO2-driven cold water geyser. The hole was drilled using a CS4002 Truck Mounted Core Drill to a total depth of 322 m and DOSECC’s hybrid coring system was used to continuously recover core. CO2-charged fluids were first encountered at ~ 35 m depth, in the basal sandstones of the Entrada Sandstone, which is open to surface, the fluids being effectively sealed by thin siltstone layers within the sandstone unit. The well penetrated a ~ 17 m thick fault zone within the Carmel Formation, the footwall damage zone of which hosted CO2-charged fluids in open fractures. CO2-rich fluids were encountered throughout the thickness of the Navajo Sandstone. The originally red sandstone and siltstone units, where they are in contact with the CO2-charged fluids, have been bleached by dissolution of hematite grain coatings. Fluid samples were collected from the Navajo Sandstone at formation pressures using a positive displacement wireline sampler, and fluid CO2 content and pH were measured at surface using high pressure apparatus. The results from the fluid sampling show that the Navajo Sandstone is being fed by active inflow of CO2-saturated brines through the fault damage zone; that these brines mix with meteoric fluid flowing laterally into the fault zone; and that the downhole fluid sampling whilst drilling successfully captures this dynamic process.

Fault zone deformation and displacement partitioning in mechanically layered carbonates: The Hidden Valley fault, central Texas

The Hidden Valley fault is exposed in Canyon Lake Gorge (central Texas) and cuts the Cretaceous Glen Rose Formation. This exposure provides an opportunity to explore the relationship between deformation mechanisms and fault displacement along 830 m (2723 ft) of a normal fault typical of those in carbonate reservoirs and aquifers around the world. The fault zone has five domains: gently deformed footwall damage zone, intensely deformed footwall damage zone, fault core, intensely deformed hanging-wall damage zone, and gently deformed hanging-wall damage zone. Footwall deformation is more intense and laterally extensive than hanging-wall deformation, and the intensely deformed hanging-wall damage zone is narrow and locally absent. The fault core contains thin clay-rich gouge or smear in most places but is locally represented by only a slickensided surface between limestone layers. The 55- to 63-m (180–207-ft) fault throw across a 43- to 98-m (141- to 322-ft)-wide fault zone is accommodated by slip along the fault core, layer tilting (synthetic dip development) in footwall and hanging-wall damage zones, and distributed faulting in footwall and hanging-wall damage zones. Total offset across the fault overestimates actual stratigraphic offset by 8 to 12 m (26–39 ft) or about 14 to 21%. In our interpretation, the Hidden Valley fault zone records both early extensional folding of the Glen Rose Formation and subsequent normal faulting that propagated downward from the overlying competent Edwards Group. The damage zone width is thus established before fault breakthrough.

Relationships between carbonate fault rocks and fluid flow regime during propagation of the Neogene extensional faults of the Penedès basin (Catalan Coastal Ranges, NE Spain)

Different types of fractures and fault rocks occur within the major normal fault zones bordering the NW of the Neogene Penedès basin (NE Spain). In the southwest part of the basin, the main fault is the Baix Penedès fault that places the Upper Jurassic–Lower Cretaceous dolomicrites of the footwall in contact with the Miocene siliciclastic sediments of the hangingwall. Within the footwall damage zone, different fracture systems and their respective associated breccias and cements have been recognized. Using a combination of structural, petrologic and geochemical analyses, we show that these fracture systems evolved from mainly tectonic to mainly karstic features during the progressive development and upward propagation of the fault. The earlier fractures and associated breccias represent the initial stages of the fault propagation, when the process zone formed around the fault tip was dominated by dilatant fracturing (fractures 1 and dilatant breccias B1). The fluid involved in this early deformation precipitated the dolomite cement C1, in equilibrium with the host-rock in a closed hydrogeological system. Localization of frictional processes generated cataclasites and ultracataclasites along distinct second-order faults (fractures 2). In these fractures, the dolomite cement C2 precipitated in relation to the incipient opening of the hydrological system to percolating meteoric waters. The later fractures and associated breccias represent the late stages of the fault propagation. These structures (fractures 3, fractures 4 and the dilatant breccias B2) were formed when the extensional deformation reached the surface, resulting in an increasingly open hydrogeological system. At these stages, deposition of the dolomitic sediment DS and precipitation of the calcite cements C3, C4 and C5 show the circulation of freshwaters in a vadose meteoric environment, i.e. in karstic conditions.

Fluid‐rich damage zone of an ancient out‐of‐sequence thrust, Kodiak Islands, Alaska

The Uganik Thrust is a fossil out‐of‐sequence thrust fault which was active over a period of ∼3 Ma during the early Tertiary until activity ceased with the subduction of the Kula‐Farallon spreading ridge at ∼57 Ma. During this period the fault experienced at least ∼1 km of throw and developed a strongly asymmetric damage zone. The brittle damage zone in the footwall of the fault acted as a conduit for fluid advection during the active faulting. A similar asymmetrical footwall damage zone has been interpreted as a fluid conduit at the Nobeoka Thrust, Shimanto Belt, SW Japan. Thermal indicators in the uppermost footwall give similar maximum paleotemperatures to those in the hanging wall (∼280°C), while previous work elsewhere in the footwall formation suggests maximum burial temperatures of ∼240°C. In this case, similar to the Irish Canyon thrust in the Franciscan accretionary complex, the location of the thermal anomaly is spatially offset from the structural fault which caused it owing to thermal overprinting in the vicinity of the fault.

Geochemical traces of CO2-rich fluid flow along normal faults in central Italy

SUMMARY We studied the oxygen and carbon isotopic composition of fault rocks, as well as fractured and fragmented platform carbonates, along large basin-bounding normal fault zones in central Italy. The internal architecture of the principal fault segments found in these zones consists of faulted and brecciated Quaternary sediments in the hanging walls, and of four different carbonate structural domains (A‐D) in the footwalls. In the footwall damage zones, we identified fractured and fragmented platform carbonates (domain A) and pulverized carbonates (domain B). Within the fault cores, we recognized both matrix-supported (domain C) and cement-supported fault rocks (domain D). The latter structural domain is located primarily along the main slip surfaces of the fault cores. Analysing the geochemistry of powder samples of fault rocks, fractured and fragmented platform carbonates, Quaternary basinal sediments, and platform carbonate host rocks we observed two different trends in δ 18 O‐δ 13 C space. These two isotopic trends are interpreted as the signatures of two different fault fluids. The first, and most prominent, isotopic trend is related to a meteoric-derived fluid, the second one to a groundwater-derived fluid originated from the local South Marsica aquifer. Although we do not have any laboratory data on the T/P conditions of fluid mineralization, based on the isotopic difference between the fluid sources and the precipitated solutes, we predicted a mineralization temperature of ∼25 ◦ for the meteoric-derived fluid, and ∼100 ◦ for the groundwater-derived fluid. The stable isotope composition of the four structural domains shows the focussing and compartmentalization of the meteoric-derived fluid primarily at the hanging wall/footwall contacts. The groundwater-derived fluid also moved through the fragmented carbonates associated with small, intracarbonate normal faults to the east of the southernmost studied normal fault zone. Considering the high CO2 content of the South Marsica aquifer, we suggest that pockets of high pCO2 fluids may have formed within the fault cores during exhumation and ongoing normal faulting.

Fluid flow history from damage zone cements near the Dent and Rawthey faults, NW England

Fault-related breccias, in Carboniferous limestone, have been well documented in the footwall damage zone to the reverse-oblique Dent Fault. This study describes breccias in the more complicated hanging wall, hosted in pre-deformed Ordovician and Silurian lithologies along the Dent Fault or one of its splays such as the Rawthey Fault. Brecciation style is shown to be influenced by lithology, with the most extensive brecciation in mechanically strong units: siliceous or calcareous mudstone, cemented sandstone and fine tuff. Most brecciation involves dilational strain in the fault damage zones. Attritional breccias are rare, even in fault cores. Dilation breccias are cemented sporadically by quartz or barite, but mostly by carbonate minerals. Transition matrix analysis shows that the sequence of hanging-wall carbonate cements matches that in the footwall: first dolomite or calcite, then ferroan dolomite, then ferroan calcite. This similarity suggests hydrological connectivity from hanging wall to footwall and through tens of cubic kilometres across the Dent–Rawthey fault system. Such connectivity is predicted across upper crustal faults that cut strong mechanical stratigraphy. It is proposed that fluid flow though the fault system was driven mainly by syntectonic topography above the hanging wall, and possibly also by thermal convection caused by a buried granite pluton in the footwall.

Enhanced fracture permeability and accompanying fluid flow in the footwall of a normal fault: The Hurricane fault at Pah Tempe hot springs, Washington County, Utah

The Pah Tempe hot springs discharge ~260 L/s of water at ~40 °C into the Virgin River in the footwall damage zone of the Hurricane fault at Timpoweap Canyon, near Hurricane, Utah, USA. Although these are Na-Cl waters, they actively discharge CO2 gas and contain signifi cant quantities of CO2 (~34.6 mmol/kg), predominantly as H2CO3 and HCO3 –. Because of excellent exposures, Pah Tempe provides an exceptional opportunity to observe the effects of enhanced fracture permeability in an active extensional fault. Pah Tempe waters have been deeply circulated (>5 km; >150 °C) into basement rock as illustrated by the clear water-rock exchange of oxygen isotopes. Waters were probably recharged under colder climate conditions than present and therefore have a prolonged subsurface residence. Discharge of both water and gas in the springs correlates to the density of fractures in carbonate rocks above stream level. This observation suggests that clusters of high fracture density in the faultdamage zone act as pathways from the likely regional aquifer, the eolian Queantoweap Sandstone, through the overlying confining unit, the gypsiferous silty Seligman Member of the Kaibab Formation. Mass-balance modeling suggests that the majority of CO2 discharge is the product of the quantitative dissolution of CO2 gas at depth within the fault zone. Upon discharge, most of the carbon is released to the surface as dissolved species. It appears that the subsurface production rate of CO2 is relatively low because Pah Tempe waters are grossly undersaturated in CO2 at inferred minimum circulation depths and temperatures. Geological and geochemical data also suggest that the CO2 is dominated by a crustal component complemented by minor mantle contributions.