Development Of Fault Rocks Research Articles

The importance of structural complexity in the localization of geothermal systems: A case study along the Vallès-Penedès Fault in the Catalan Coastal Ranges (NE Spain)

The Vallès-Penedès Fault is a Neogene normal fault marked by the presence of two established geothermal systems at La Garriga-Samalús and Caldes de Montbuí, within the Catalan Coastal Ranges (NE Spain). The analysis and collation of existing and new geological and geophysical datasets provide the basis for the development of an improved conceptual model that explains the presence and localization of hot geothermal fluid systems at relatively shallow depths (e.g., 60 °C at surface and 90 °C at 1 km). Geothermal flow is concentrated within Paleozoic granodiorites of the immediate footwall of the V-P fault, host rocks that are susceptible to fault-related fracturing, and the generation of both extension and hybrid fracture systems in association with active fault displacements. Flow localization is enhanced further by the presence of fault-related structural complexities, with both systems marked by 300 m wide steps in the main fault trace. These are attributed to relay development and breaching characterizing host rocks by high fracture intensities and fault rock development on a fault that locally has a vertical displacement of over 1.5 km. Accentuated fracturing and deformation are consistent with strain localization predicted by existing models for the development of fault zones along normal faults. The plumbing of the geothermal systems is attributed to up-fault flow in combination with lateral flow controlled by the intersection of the V-P fault with a low-angled Paleogene thrust defining the base of the host-rock granodiorites, with the geothermal systems localized at the distal end of the thrust. Sustained geothermal flow is attributed to groundwater flow circulation associated with seismic pumping and valving of warmer and deeper fluids, and the ingress of groundwater along faults and within fractured basement rocks.

How Fault Rocks Form and Evolve in the Shallow San Andreas Fault

AbstractWe document the mechanical and geochemical processes of fault rock development in the shallow San Andreas fault (Mojave segment), and quantify their importance in shaping the mineralogy, grain size, fabric, and frictional characteristics of gouge. Through a combination of field and laboratory analysis of an extensive suite of shallow (<150 m) drill cores, we show that fault rocks evolved from a granodiorite protolith via three main processes: distributed microfracturing/pulverization; cataclastic flow and incipient fabric development; and production of authigenic illite/smectite during fluid‐rock interaction. The interdependence of these mechanical and geochemical processes results in a diverse suite of fault rocks, and causes significant changes in frictional strength. Spatial variations in the effects of these mechanisms, as manifested in fault‐rock mineralogy and geochemistry, indicate marked variations in their relative contribution to fault‐rock evolution. These data reveal a complex San Andreas fault with multiple principal slip zones and damaged and altered rock hosting numerous interconnected secondary slip surfaces. The resulting picture of the San Andreas Fault zone suggests a substantial departure from the simple structures envisioned for near‐surface seismogenenic faults in numerical models is required, and may inform future efforts to forecast peak ground accelerations during southern California earthquakes.

Key controls on the hydraulic properties of fault rocks in carbonates

A significant knowledge gap exists when analysing and predicting the hydraulic behaviour of faults within carbonate reservoirs. To improve this, a large database of carbonate fault rock properties has been collected from 42 exposed faults, from seven countries. Faults analysed cut a range of lithofacies, tectonic histories, burial depths and displacements. Porosity and permeability measurements from c. 400 samples have been made, with the goal of identifying key controls on the flow properties of fault rocks in carbonates. Intrinsic and extrinsic factors have been examined, such as host lithofacies, juxtaposition, host porosity and permeability, tectonic regime, displacement, and maximum burial depth, as well as the depth at the time of faulting. The results indicate which factors may have had the most significant influence on fault rock permeability, improving our ability to predict the sealing or baffle behaviour of faults in carbonate reservoirs. Intrinsic factors, such as host porosity, permeability and texture, appear to play the most important role in fault rock development. Extrinsic factors, such as displacement and kinematics, have shown lesser or, in some instances, a negligible control on fault rock development. This conclusion is, however, subject to two research limitations: lack of sufficient data from similar lithofacies at different displacements, and a low number of samples from thrust regimes. Thematic collection: This article is part of the Fault and top seals collection available at: https://www.lyellcollection.org/cc/fault-and-top-seals-2019

To D or not to D? Re-evaluating particle-size distributions in natural and experimental fault rocks

Cataclasis is a first-order mechanism shaping fault-rock development in the brittle crust. The most accepted model for cataclasis is “constrained comminution” which predicts a power-law distribution of particle size described by the fractal dimension, D. In this paper, we statistically test for the presence of power-law scaling in 61 naturally and experimentally deformed fault rocks, thereby assessing the validity of the constrained comminution model. We find that no data set exhibits compelling evidence for power-law scaling, even when a lower threshold is applied to promote the appearance of power-law scaling. Log-normal distributions provide better fits to statistically conclusive test cases, both when using a lower threshold (91% of cases) and when examining the full data sets. Thus, our results are not consistent with constrained comminution. We propose that log-normal distributions provide a more useful description of fault-rock particle-size distributions and discuss the implications for the mechanisms of cataclasis.

Implications of meso- to micro-scale deformation for fault sealing capacity: Insights from the Lenghu5 fold-and-thrust belt, Qaidam Basin, NE Tibetan Plateau

As faults can be barriers to or conduits for fluid flow, it is critical to understand fault seal processes and their effects on the sealing capacity of a fault zone. Apart from the stratigraphic juxtaposition between the hanging wall and footwall, the development of fault rocks is of great importance in changing the sealing capacity of a fault zone. Therefore, field-based structural analysis has been employed to identify the meso-scale and micro-scale deformation features and to understand their effects on modifying the porosity of fault rocks. In this study, the Lenghu5 fold-and-thrust belt (northern Qaidam Basin, NE Tibetan Plateau), with well-exposed outcrops, was selected as an example for meso-scale outcrop mapping and SEM (Scanning Electron Microscope) micro-scale structural analysis. The detailed outcrop maps enabled us to link the samples with meso-scale fault architecture. The representative rock samples, collected in both the fault zones and the undeformed hanging walls/footwalls, were studied by SEM micro-structural analysis to identify the deformation features at the micro-scale and evaluate their influences on the fluid flow properties of the fault rocks. Based on the multi-scale structural analyses, the deformation mechanisms accounting for porosity reduction in the fault rocks have been identified, which are clay smearing, phyllosilicate-framework networking and cataclasis. The sealing capacity is highly dependent on the clay content: high concentrations of clay minerals in fault rocks are likely to form continuous clay smears or micro- clay smears between framework silicates, which can significantly decrease the porosity of the fault rocks. However, there is no direct link between the fault rocks and host rocks. Similar stratigraphic juxtapositions can generate fault rocks with very different magnitudes of porosity reduction. The resultant fault rocks can only be predicted only when the fault throw is smaller than the thickness of a faulted bed, in which scenario self-juxtaposition forms between the hanging wall and footwall.

Fault core process and clay content derived from XRF analysis: Salina Creek Fault, Utah

Abstract The distribution of fault rocks interpreted across a modelled fault in oil or gas reservoirs is most often described by its clay content derived from standard industry algorithms such as shale gouge ratio and clay smear factor. These distributions are below the mapping resolution in seismic data, and the actual processes and mechanisms for the fault-rock development are not well understood. A well-exposed and well-preserved low-throw fault in an old railroad tunnel near Salina, Utah provides the access and scale to interpret fault-rock distributions, measure their clay contents and describe the fault-rock development. A significant number of fault-rock elemental compositions were measured quickly on the outcrop surface using a hand-held X-ray fluorescence (XRF) elemental analyser with a novel surface preparation and analysis strategy. The elemental data were converted to clay contents using a small set of samples where elemental composition was calibrated to X-ray diffraction mineralogy. The mineralogy data provide a basis for evaluating the degree of mixing of protolith beds during fault-rock development in the fault core. The fault core is not randomly mixed fragments derived from the protolith (gouge or breccia), but rather discrete, thin layers parallel to the fault surface, many of which can be traced back to a source sandstone or mudstone bed. The mineralogical composition of some fault-rock layers are unchanged from their protolith source bed, but other layers are mechanical mixtures of several source beds. The shale gouge ratio algorithm under-represents the average measured fault-rock clay content. The clay smear algorithm more accurately describes the clay content distribution, but underestimates the clay content heterogeneity along the smear length. A key uncertainty for predicting fault sealing remains prediction of the lengths and continuity of smears.

Fault structure and slip localization in carbonate-bearing normal faults: An example from the Northern Apennines of Italy

Carbonate-bearing normal faults are important structures for controlling fluid flow and seismogenesis within the brittle upper crust. Numerous studies have tried to characterize fault zone structure and earthquake slip processes along carbonate-bearing faults. However, due to the different scales of investigation, these studies are not often integrated to provide a comprehensive fault image. Here we present a multi-scale investigation of a normal fault exhumed from seismogenic depths. The fault extends for a length of 10 km with a maximum width of about 1.5 km and consists of 5 sub-parallel and interacting segments. The maximum displacement (370–650 m) of each fault segment is partitioned along sub-parallel slipping zones extending for a total width of about 50 m. Each slipping zone is characterized by slipping surfaces exhibiting different slip plane phenomena. Fault rock development is controlled by the protolith lithology. In massive limestone, moving away from the slip surface, we observe a thin layer (<2 cm) of ultracataclasite, cataclasite (2–10 cm) and fault breccia. In marly limestone, the fault rock consists of a cataclasite with hydrofractures and smectite-rich pressure solution seams. At the micro-nanoscale, the slip surface consists of a continuous and thin (<300 μm) layer composed of coarse calcite grains (∼5–20 μm in size) associated with sub-micrometer grains showing fading grain boundaries, voids and/or vesicles, and suggesting thermal decomposition processes. Micrometer-sized calcite crystals show nanoscale polysynthetic twinning affected by the occurrence of subgrain boundaries and polygonalized nanostructures. Investigations at the kilometres-tens of meter scale provide fault images that can be directly compared with high-resolution seismological data and when combined can be used to develop a comprehensive characterization of seismically active fault structures in carbonate lithologies. Micro and nanoscale investigations along the principal slipping zone suggest that different deformation processes, including plastic deformation and thermal decomposition, were active during seismic slip.

Dynamic development of fault rocks in a crustal‐scale detachment: An example from western Norway

The Nordfjord‐Sogn Detachment (NSD) zone of western Norway juxtaposes eclogites and gneisses of lower crustal affinity with sedimentary deposits, suggesting that the zone experienced substantial displacement during Devonian extensional denudation of the Caledonides. Renewed activation occurred in Permian and Jurassic‐Cretaceous times. The detachment zone consists of a succession of fault‐related rocks reflecting various crustal positions and deformation mechanisms during activation and unroofing. In more detail, the lower 1–2 km thick section of the NSD zone consists of mylonites with various protoliths, showing a general upward decrease in metamorphic grade. The mylonitic section is truncated by the NSD fault, defining the boundary toward hanging wall nappe rocks that are topped by sedimentary basins. In an attempt to provide precise descriptions of the observed fault rocks, we classify these rocks into (1) noncohesive breccias, (2) cemented and indurated breccias of secondary cohesion, and (3) cataclasites, phyllonites, and mylonites formed with primary cohesion. Along and across‐strike sections of the fault reveal the spatial and temporal relationships of the NSD fault. The fault zone consists of a lower section of phyllonites and cataclasites, reaching a total thickness of 50–70 m. Significant volumes of pseudotachylyte were injected into these fault rocks, and were later reworked. Reactivation of the fault is suggested by overprinting carbonate‐cemented breccias that are cut by a discrete fault itself associated with breccias and gouge. By comparing the various fault rocks and their formation mechanisms, some constraints can be applied to the unroofing history. The NSD fault developed in the ductile‐brittle transition zone of the crust, where it experienced an initial shift from mainly frictional to plastic flow caused by reaction softening, before hardening caused a return to frictional flow. At the same time, repeated pulses of pseudotachylyte were generated, possibly lubricating the fault zone and contributing to strain localization. These pseudotachylyte pulses also show that the fault was affected by seismic slip events, before returning to a creep regime within a wider section than that ruptured by earthquakes. Younger cemented breccias and noncohesive breccias and gouge reflect earthquake‐rupturing and frictional flow within the seismogenic crust.

Role of seismogenic processes in fault-rock development: An example from Death Valley, California

Fault rocks developed along the Mormon Point turtleback of southern Death Valley suggest that a jog in the oblique-slip Death Valley fault zone served as an ancient seismic barrier, where dominantly strike-slip ruptures were terminated at a dilatant jog. Dramatic spatial variations in fault-rock thickness and type within the bend are interpreted as the products of: (1) fault overshoot, in which planar ruptures bypass the intersection of the two faults composing the bend and slice into the underlying footwall; and (2) implosion brecciation, in which coseismic ruptures arrested at a releasing bend in the fault lead to catastrophic collapse brecciation, fluid influx, and mineralization.