Fluids, Geochemical Cycles, and Mass Transport in Fault Zones
Lukas P. BaumgartnerB. BosJ. A. D. ConnollyJean‐Pierre GratierFredéric GueydanStephen A. MillerClaudio RosenbergJános L. UraiB. W. D. YardleyMark Person
3
Citation
0
Reference
20
Related Paper
Citation Trend
Keywords:
Mass transport
Cite
Abstract Thermal springs commonly occur along faults because of the enhanced vertical permeability afforded by fracture zones. Field and laboratory studies of fault zone materials document substantial heterogeneities in fracture permeabilities. Modeling and field studies of springs suggest that spatial variations in permeability strongly influence spring locations, discharge rates and temperatures. The impact of heterogeneous permeability on spring geochemistry, however, is poorly documented. We present stable isotope and water chemistry data from a series of closely spaced thermal springs associated with the Hayward Fault, California. We suggest that substantial spatial variations observed in δ 18 O and chloride values reflect subsurface fluid transport through a poorly connected fracture network in which mixing of subsurface waters remains limited. Our measurements provide insight into the effect of fracture zone heterogeneities on spring geochemistry, offer an additional tool to intuit the nature of tectonically induced changes in fault zone plumbing, and highlight the need to consider local variations when characterizing fracture zone fluid geochemistry from spring systems with multiple discharge sites.
Fracture zone
Hot spring
Cite
Citations (27)
Isotopes of strontium
Cite
Citations (16)
The sediment-hosted McArthur River Zn-Pb-Ag deposit is located in the southern McArthur Basin of northern Australia. 3D numerical models are used to explore the role of thermal convection in the mineral system that formed the McArthur River deposit. The model geometry is a simplified representation of the area, comprising gently dipping hydro-stratigraphic units intersected by a steeply dipping fault. An aquifer represents the source of mineralising fluid, while the fault provides a pathway for this fluid to reach the site of mineralisation, either on the seafloor (syngenetic mineralisation) or within sedimentary rocks adjacent to the fault (diagenetic/epigenetic mineralisation). Two fault permeability scenarios are investigated, representing an open fault with high permeability, and a closed fault with low permeability in the top 300 m of the model and high permeability below. In both scenarios, thermal convection occurs within the fault and aquifer due to their high permeability, with upwellings of hot fluid spaced at ~ 14 to 23 km along the fault. The results show that convection results in sufficient fluid exchange between the aquifer and fault to account for known mineralisation at McArthur River (~20 Mt Zn). In the open fault scenario, the convective upwellings provide sufficient flow of hot fluid onto the seafloor to form a 20 Mt syngenetic deposit in ~ 0.4 to 0.9 Myr. Syngenetic mineralisation would be accompanied by minor diagenetic mineralisation within the sediments adjacent to convective upwellings in the fault. In the closed fault scenario, some of the hot upwelling fluid flows into sediments adjacent to the fault at ~ 300 m depth, sufficient to create a 20 Mt diagenetic/epigenetic deposit in ~ 2.3 Myr. The rate and temperature of fluid flowing onto the seafloor or into the host rocks increases with fault permeability and heat flux, with higher heat flux and/or permeability being required to generate a diagenetic/epigenetic deposit than a syngenetic deposit within a geologically reasonable timeframe. Geometric complexities (e.g. fault intersections, bends or offsets) and areas of anomalously high heat flow or permeability are likely to focus convective upwelling, and are therefore suggested as targets for mineral exploration.
Cite
Citations (8)
Abstract Many faults in active and exhumed hydrocarbon‐generating basins are characterized by thick deposits of carbonate fault cement of limited vertical and horizontal extent. Based on fluid inclusion and stable isotope characteristics, these deposits have been attributed to upward flow of formation water and hydrocarbons. The present study sought to test this hypothesis by using numerical reactive transport modeling to investigate the origin of calcite cements in the Refugio‐Carneros fault located on the northern flank of the Santa Barbara Basin of southern California. Previous research has shown this calcite to have low δ 13 C values of about −40 to −30‰PDB, suggesting that methane‐rich fluids ascended the fault and contributed carbon for the mineralization. Fluid inclusion homogenization temperatures of 80–125°C in the calcite indicate that the fluids also transported significant quantities of heat. Fluid inclusion salinities ranging from fresh water to seawater values and the proximity of the Refugio‐Carneros fault to a zone of groundwater recharge in the Santa Ynez Mountains suggest that calcite precipitation in the fault may have been induced by the oxidation of methane‐rich basinal fluids by infiltrating meteoric fluids descending steeply dipping sedimentary layers on the northern basin flank. This oxidation could have occurred via at least two different mixing scenarios. In the first, overpressures in the central part of the basin may have driven methane‐rich formation waters derived from the Monterey Formation northward toward the basin flanks where they mixed with meteoric water descending from the Santa Ynez Mountains and diverted upward through the Refugio‐Carneros fault. In the second scenario, methane‐rich fluids sourced from deeper Paleogene sediments would have been driven upward by overpressures generated in the fault zones because of deformation, pressure solution, and flow, and released during fault rupture, ultimately mixing with meteoric water at shallow depth. The models in the present study were designed to test this second scenario, and show that in order for the observed fluid inclusion temperatures to be reached within 200 m of the surface, moderate overpressures and high permeabilities were required in the fault zone. Sudden release of overpressure may have been triggered by earthquakes and led to transient pulses of accelerated fluid flow and heat transport along faults, most likely on the order of tens to hundreds of years in duration. While the models also showed that methane‐rich fluids ascending the Refugio‐Carneros fault could be oxidized by meteoric water traversing the Vaqueros Sandstone to form calcite, they raised doubts about whether the length of time and the number of fault pulses needed for mineralization by the fault overpressuring mechanism were too high given existing geologic constraints.
Cite
Citations (10)
Mud Volcanism and fluid seepage are widespread phenomena in the Gulf of Cadiz (SW Iberian Margin). In this seismically active region located at the boundary between the African and Eurasian plates, fluid flow is typically focused on deeply rooted active strike-slip faults. The geochemical signature of emanating fluids from various mud volcanoes (MVs) has been interpreted as being largely affected by clay mineral dehydration and recrystallization of Upper Jurassic carbonates. Here we present the results of a novel, fully-coupled 1D basin-scale reactive-transport model capable of simulating major fluid forming processes and related geochemical signatures by considering the growth of the sediment column over time, compaction of sediments, diffusion and advection of fluids, as well as convective and conductive heat flow. The outcome of the model is a realistic approximation to the development of the sediment pore water system over geological time scales in the Gulf of Cadiz. Combined with a geochemical reaction transport model for clay mineral dehydration and calcium carbonate recrystallization, we were able to reproduce measured concentrations of Cl, strontium and 87Sr/86Sr of emanating mud volcano fluids. These results support previously made qualitative interpretations and add further constraints on fluid forming processes, reaction rates and source depths. The geochemical signature at Porto MV posed a specific problem, because of insufficient constraints on non-radiogenic 87Sr/86Sr sources at this location. We favour a scenario of basement-derived fluid injection into basal Upper Jurassic carbonate deposits (Hensen et al., 2015). Although the mechanism behind such basement-derived flow, e.g. along permeable faults, remains speculative at this stage, it provides an additional source of low 87Sr/86Sr fluids and offers an idea on how formation water from the deepest sedimentary strata above the basement can be mobilized and eventually initiate the advection of fluids feeding MVs at the seafloor. The dynamic reactive-transport model presented in this study provides a new tool addressing the combined simulation of complex physical-geochemical processes in sedimentary systems. The model can easily be extended and applied to similar geological settings, and thus help us to provide a fundamental understanding of fluid dynamics and element recycling in sedimentary basins.
Mud volcano
Radiogenic nuclide
Cite
Citations (8)
Extensional fault
Cite
Citations (49)
Fault gouge
Cite
Citations (16)
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 δ18O–δ13C 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.
Cite
Citations (50)
Abstract Hydrothermal systems involving dormant faults within orogenic belts are rarely targeted for geothermal exploration, partly because of the complexity of the 3‐D topography, the unknown permeability of the fault zones and the basement lithology, and the lack of deep‐level data. This study brings together various types of surface information (spring features, geological data, topography, and hydrochemistry) to explain the alignment of 29 hot springs (29–73 °C) along the dormant Têt fault (Eastern Pyrénées, France). Water ion concentrations, stable water isotopes, and lithium isotopic ratios indicate that (i) fluids originating from meteoric water infiltrate above an altitude of 2,000 m, (ii) the rocks interacting with the fluids are similar for all the springs, and (iii) the maximum fluid temperatures at depth show similar variations along the fault and at the surface. A 3‐D numerical model of the system, assembled from field structural data and from a digital elevation model, explores the permeability combinations for the basement and for a three‐fault network. The models indicate that for a relatively permeable basement (10 −16 m 2 ), fluids are topography‐driven down to thousands of meters (until −3,700 m) before being captured by the more permeable Têt fault. Hot spring temperatures can be numerically reproduced when fault permeability is around 10 −14 m 2 , a value slightly lower than the critical permeability for which free convection would occur within the Têt fault. Our study shows that thermal anomalies are possible along dormant faults close to elevated topography in the core of an orogenic belt, thereby opening new perspectives for geothermal exploration.
Lithology
Basement
Cite
Citations (60)