Linkage Of Fault Segments Research Articles

Multiphase growth and basin control of main faults on Northern sub-basin, Beibuwan basin, Northwestern South China sea

The basement-involved faults and multiphase extension control the three-dimensional (3-D) fault geometries and kinematics, producing different fault growth models in continental rift basins. In this paper, we apply qualitative and quantitative fault analysis techniques to 3D high-resolution seismic data from the Northern sub-basin of Beibuwan basin, building the tectonic-stratigraphic framework and proposing the evolutionary meachanisms of main normal faults. The results suggest that extensional stress changed from NW-SE direction during Paleocene-Middle Eocene to N-S direction in Late Eocene-Oligocene, leading to the ENE or E-W directed propagation of fault networks together with lateral and vertical linkage of fault segments. This includes three conceptual models: (1) F1 originated from propagation, interaction, linkage and failure of initial seven ENE-directed isolated faults, and grows following isolated fault growth model; (2) F2 nucleated as a normal fault before buried in Late Eocene, and reactivated as dextral strike-slip fault in Oligocene, therefore experienced vertical segmentation and linkage; (3) F3 activated as a long-term constant-length fault with generally increased displacement in Paleogene. Our study proposes that the T83 interface (the top of the second member of the Liushagang Formation) is a tectonic stress transition interface, while the T60 interface is both a tectonic inversion interface and a post-rift unconformity interface. Furthermore, the growth of extensional faults commonly accompanied single breached, transfer-fault breached or double breached relay ramps and fault-related folds, thereby exerting a significant influence on the basin architecture and sediment distribution.

Assessment of fault damage zones in carbonate rocks based on numerical and sensitivity analyses

Fault damage zones are structural geological domains that can impact a reservoir's fluid flow and geomechanical behavior during the exploration and production phases in the oil and gas industry. Several methodologies have been proposed in the literature to characterize fault damage zones considering earthquake data, geophysical methods and outcrop observations. Based on those data, statistical relationships have been proposed to predict and constrain the width of fault damage zones in the subsurface. Unfortunately, those relationships show significant data dispersion and raise uncertainties regarding the extension of damage zones. A possible explanation for such distribution is the lack of knowledge of other parameters such as lithology and associated diagenesis, depth of faulting, tectonic environment, fault propagation, fault segment linkage and deformation mechanisms. Numerical modeling has emerged as a promising alternative for understanding geological problems. However, few studies have focused on characterizing fault damage zones at depth. This study presents a novel methodology based on the finite element method (FEM) combined with elastoplastic models and fault displacement data to forecast damage zone widths around geological faults on carbonate rocks. Despite the assumptions and simplifications related to the fault shape and displacement profile, this study shows that the proposed models provide a better understanding of the deformation mechanisms that trigger damage zones. Moreover, a sensitivity study considering the proposed methodology shows that in addition to the fault displacements and the carbonate rocks' geomechanical properties, the location of measured damage zone with respect to the fault can significantly impact the assessment of damage zone widths.

The Arkalochori Mw = 5.9 Earthquake of 27 September 2021 Inside the Heraklion Basin: A Shallow, Blind Rupture Event Highlighting the Orthogonal Extension of Central Crete

A strong, shallow earthquake occurred near Heraklion (Crete, Greece) on 27 September 2021. The earthquake produced significant ground deformation in the vicinity of Arkalochori village but without any evidence for surface ruptures of primary origin. We used geodetic (InSAR and GNSS) data to map motions of the Earth’s surface that occurred during and shortly after the earthquake. A 14 cm subsidence of the GNSS station ARKL and a maximum of 19 cm distance from the SAR satellite were recorded. The measured surface displacements were used to constrain the rupture geometry and slip distribution at depth. Our best-fitting inversion model suggests that the rupture occurred on a 13 km-long planar normal fault striking N195° E dipping 55° to the northwest, with major slip occurring to the east and updip of the hypocentre. The fault tip is located 1.2 km beneath the surface. The maximum coseismic slip occurred in the uppermost crust, in the depth interval of 4–6 km. A decrease in the fault offsets toward the Earth’s surface is likely caused by an increased frictional resistance of the shallow layers to rapid coseismic slip. Satellite observations made in the first month after the earthquake detected no post-seismic deformation (i.e., below one fringe or 2.8 cm). The seismic fault may be identified with the Avli (Lagouta) segment of the NNE-SSW striking, west-dipping, 23 km-long neotectonic Kastelli Fault Zone (KFZ). Part of the rupture occurred along the Kastelli segment, indicating a fault segment linkage and a history of overlapping ruptures along KFZ. Based on geological data and footwall topography we estimate an average slip rate between 0.17–0.26 mm/yr for the KFZ. The Arkalochori earthquake is a paradigm example for the on-going extension of Heraklion basin (central Crete) in the WNW-ESE direction, which is almost orthogonal to the E-W Messara graben and other active faults along the south coast of Crete.

Nonlinear fault damage zone scaling revealed through analog modeling

AbstractFault damage zones strongly influence fluid flow and seismogenic behavior of faults and are thought to scale linearly with fault displacement until reaching a threshold thickness. Using analog modeling with different frictional layer thicknesses, we investigate damage zone dynamic evolution during normal fault growth. We show that experimental damage zone growth with displacement is not linear but progressively tends toward a threshold thickness, being larger in the thicker models. This threshold thickness increases significantly at fault segment relay zones. As the thickness threshold is approached, the failure mode progressively transitions from dilational shear to isochoric shear. This process affects the whole layer thickness and develops as a consequence of fault segment linkage as inferred in nature when the fault matures. These findings suggest that fault damage zone widths are limited both by different scales of mechanical unit thickness and the evolution of failure modes, ultimately controlled in nature by lithology and deformation conditions.

Along‐strike structural linkage and interaction in an active thrust fault system: A case study from the western Sichuan foreland basin, China

AbstractAlong‐strike structural linkage and interaction between faults is common in various compressional settings worldwide. Understanding the kinematic history of fault interaction processes can provide important constraints on the geometry and evolution of the lateral growth of segmented faults in the fold‐and‐thrust belts, which are important to seismic hazard assessment and hydrocarbon trap development. In this study, we study lateral structural geometry (fault displacement and horizon shortening) of thrust fault linkages and interactions along the Qiongxi anticline in the western Sichuan foreland basin, China, using a high‐resolution 3D seismic reflection dataset. Seismic interpretation suggests that the Qiongxi anticline can be related to three west‐dipping, hard‐linked thrust fault segments that sole onto a regional shallow detachment. Results reveal that the lateral linkage of fault segments limited their development, affecting the along‐strike fault displacement distributions. A deficit between shortening and displacement is observed to increase in linkage zones where complex structural processes occur, such as fault surface bifurcation and secondary faulting, demonstrating the effect of fault linkage process on structural deformation within a thrust array. The distribution of the geometrical characteristics shows that thrust fault development in the area can be described by both the isolated fault model and the coherent fault model. Our measurements show that new fault surfaces bifurcate from the main thrust ramp, which influences both strain distribution in the relay zone and along‐strike fault slip distribution. This work fully describes the geometric and kinematic characteristics of lateral thrust fault linkage, and may provide insights into seismic interpretation strategies in other complex fault transfer zones.

Fault failure modes, deformation mechanisms, dilation tendency, slip tendency, and conduits v. seals

Abstract Faults have complicated shapes. Non-planarity of faults can be caused by variations in failure modes, which in turn are dictated by mechanical stratigraphy interacting with the ambient stress field, as well as by linkage of fault segments. Different portions of a fault or fault zone may experience volume gain, volume conservation and volume loss simultaneously depending on the position along a fault's surface, the stresses resolved on the fault and the associated deformation mechanisms. This variation in deformation style and associated volume change has a profound effect on the ability of a fault to transmit (or impede) fluid both along and across the fault. In this paper we explore interrelated concepts of failure mode and resolved stress analysis, and provide examples of fault geometry in normal faulting and reverse faulting stress regimes that illustrate the effects of fault geometry on failure behaviour and related importance to fluid transmission. In particular, we emphasize the utility of using relative dilation tendency v. slip tendency on fault patches as a predictor of deformation behaviour, and suggest this parameter space as a new tool for evaluating conduit v. seal behaviour of faults.

Impact of tectonic rafts’ gravitational instability on fault reactivation and geometry

The downslope gravitational instability of tectonic rafts can deform large volumes of supra-salt strata on continental margins. Detailed mapping of fault throw and geometry using three-dimensional (3D) seismic reflection data from the salt-rich Espírito Santo Basin (SE Brazil) shows that the complex fault geometries observed are primarily due to downslope gravitational instability of tectonic rafts. Three main tiers of faults were identified and their geometry analysed in detail. Tier 1 faults are associated with the initial stage of fragmentation of supra-salt strata into individual blocks, or rafts, which are separated by listric (roller) faults and associated minibasins. These faults comprise closely spaced normal faults that resulted from outer-arc stretching of strata overlaying discrete tectonic rafts. The faults accommodate a significant part of the bending strain occurring in the hanging-wall blocks of roller faults, thus forming a curved-polygonal planform geometry adjacently to large roller faults. Tier 2 faults are associated with a renewed stage of downslope gravitational gliding of tectonic rafts. The progressive bending of overburden strata during this stage led to the progressive faulting of overburden strata by these faults. The faults comprise closely spaced normal faults with a restricted range in fault strikes, forming a rectangular-polygonal planform geometry over rollover anticlines. Tier 3 faults are associated with a later stage of diachronous grounding of tectonic rafts. Differences in the degree of diachronous grounding of tectonic rafts are responsible for the development of these faults and subsequent shortening of the overburden strata. These faults comprise densely spaced set of normal faults with diverse range of fault strikes, forming an irregular-polygonal planform geometry over rollover anticlines. The interpreted fault tiers show fault throw maxima ranging from 50 m to 60 m, spacings of 180–420 m and trace lengths of 120–650 m. The interpreted fault tiers were reactivated and their growth is characterized by fault segment linkage. The diachronous downslope translation and grounding (welding) of tectonic rafts on sub-salt strata was an important process controlling the degree of deformation off Espírito Santo. Successive episodes of fault reactivation, and the subsequent generation of salt welds below tectonic rafts, potentially formed large conduits favouring fluid migration and the generation of hydrocarbon traps in supra-salt strata.

Source‐to‐sink analysis in an active extensional setting: Holocene erosion and deposition in the Sperchios rift, central Greece

AbstractWe present a source‐to‐sink analysis to explain sediment supply variations and depositional patterns over the Holocene within an active rift setting. We integrate a range of modelling approaches and data types with field observations from the Sperchios rift basin, Central Greece that allow us to analyse and quantify (1) the size and characteristics of sediment source areas, (2) the dynamics of the sediment routing system from upstream fluvial processes to downstream deposition at the coastline, and (3) the depositional architecture and volumes of the Holocene basin fill. We demonstrate that the Sperchios rift comprises a ‘closed’ system over the Holocene and that erosional and depositional volumes are thus balanced. Furthermore, we evaluate key controls in the development of this source‐to‐sink system, including the role of pre‐existing topography, bedrock erodibility and lateral variations in the rate of tectonic uplift/subsidence. We show that tectonic subsidence alone can explain the observed grain size fining along the rift axis resulting in the downstream transition from a braided channel to an extensive meander belt (>15 km long) that feeds the fine‐grained Sperchios delta. Additionally, we quantify the ratios of sediment storage to bypass for the two main footwall‐sourced alluvial fan systems and relate the fan characteristics to the pattern and rates of fault slip. Finally, we show that ≥40% of the sediment that builds the Sperchios delta is supplied by ≤22% of the entire source area and that this can be primarily attributed to a longer‐term (~106 years) transient landscape response to fault segment linkage. Our multidisciplinary approach allows us to quantify the relative importance of multiple factors that control a complex source‐to‐sink system and thus improve our understanding of landscape evolution and stratigraphic development in active extensional tectonic settings.

Seismic imaging of fault damaged zone and its scaling relation with displacement

We have studied seismically resolved damaged zone of normal faults in siliciclastic rocks of the Norwegian continental shelf. The workflow we have developed reveals structural details of the fault damaged zone and in particular, the subsidiary synthetic faults, horsetail at the main lateral fault tips at different depths and fault bend. These subsidiary or small fault segments form an area that can be clearly followed laterally and vertically. We call this area fault damaged zone. The studied damaged zone on seismic data comprises the fault core and the fault damage zone, as defined in outcrop studies. Spectral decomposition (short-time Fourier transform for time-frequency resolution and continuous wavelet transform) was performed on the data centered around faulted intervals. The magnitude of higher frequencies was used to generate coherence attribute volumes. Coherence attributes were filtered to enhance fault images. This integrated workflow improves fault images on reflection seismic data. Our approach reveals details of damaged zone geometry and morphology, which are comparable with the outcrop studies of similar examples conducted by previous researchers or us. We have extracted the fault geometry data including the segment length, displacement, and damaged zone width at different depths. Our results show that subsidiary faults, fault bends, linkage of fault segments, and branching in the fault tip (horsetail structure or process zone) all affect the width of the damaged zone and the distribution of displacement. We have seen a distinct increase in the fault damaged zone width near the fault bend locations. The fault segment length decreases with depth toward the lower fault tip, which is below the base Cretaceous unconformity. In addition, the displacement increases below the unconformity. In general, there is a positive correlation between fault displacement and the corresponding damaged zone width measured in this study, which is in agreement with previous studies.

The role of gravitational collapse in controlling the evolution of crestal fault systems (Espírito Santo Basin, SE Brazil) – Reply

A high-quality 3D seismic volume from offshore Espirito Santo Basin (SE Brazil) is used to assess the importance of gravitational collapse to the formation of crestal faults above salt structures. A crestal fault system is imaged in detail using seismic attributes such as curvature and variance, which are later complemented by analyses of throw vs. distance (T-D) and throw vs. depth (T-Z). In the study area, crestal faults comprise closely spaced arrays and are bounded by large listric faults, herein called border faults. Two episodes of growth are identified in two opposite-dipping fault families separated by a transverse accommodation zone. Statistical analyses for eighty-four (84) faults show that fault spacing is < 250 m, with border faults showing the larger throw values. Fault throw varies between 8 ms and 80 ms two-way time for crestal faults, and 60–80 ms two-way time for border faults. Fault length varies between ∼410 m and 1750 m, with border faults ranging from 1250 m to 1750 m. This work shows that border faults accommodated most of the strain associated with salt growth and collapse. The growth history of crestal faults favours an isolated fault propagation model with fault segment linkage being associated with the lateral propagation of discrete fault segments. Importantly, two episodes of fault growth are identified as synchronous to two phases of seafloor erosion, rendering local unconformities as competent markers of fault reactivation at a local scale. This paper has crucial implications for the understanding of fault growth as a means to assess drilling risk and oil and gas migration on continental margins. As a corollary, this work demonstrates that: 1) a certain degree of spatial organisation occurs in crestal fault systems; 2) transverse accommodation zones can form regions in which fault propagation is enhanced and regional dips of faults change in 4D.

The role of gravitational collapse in controlling the evolution of crestal fault systems (Espírito Santo Basin, SE Brazil)

A high-quality 3D seismic volume from offshore Espírito Santo Basin (SE Brazil) is used to assess the importance of gravitational collapse to the formation of crestal faults above salt structures. A crestal fault system is imaged in detail using seismic attributes such as curvature and variance, which are later complemented by analyses of throw vs. distance (T-D) and throw vs. depth (T-Z). In the study area, crestal faults comprise closely spaced arrays and are bounded by large listric faults, herein called border faults. Two episodes of growth are identified in two opposite-dipping fault families separated by a transverse accommodation zone. Statistical analyses for eighty-four (84) faults show that fault spacing is < 250 m, with border faults showing the larger throw values. Fault throw varies between 8 ms and 80 ms two-way time for crestal faults, and 60–80 ms two-way time for border faults. Fault length varies between ∼410 m and 1750 m, with border faults ranging from 1250 m to 1750 m. This work shows that border faults accommodated most of the strain associated with salt growth and collapse. The growth history of crestal faults favours an isolated fault propagation model with fault segment linkage being associated with the lateral propagation of discrete fault segments. Importantly, two episodes of fault growth are identified as synchronous to two phases of seafloor erosion, rendering local unconformities as competent markers of fault reactivation at a local scale. This paper has crucial implications for the understanding of fault growth as a means to assess drilling risk and oil and gas migration on continental margins. As a corollary, this work demonstrates that: 1) a certain degree of spatial organisation occurs in crestal fault systems; 2) transverse accommodation zones can form regions in which fault propagation is enhanced and regional dips of faults change in 4D.

Dilatant normal faulting in jointed cohesive rocks: a physical model study

Abstract. Dilatant faults often form in rocks containing pre-existing joints, but the effects of joints on fault segment linkage and fracture connectivity are not well understood. We present an analogue modeling study using cohesive powder with pre-formed joint sets in the upper layer, varying the angle between joints and a rigid basement fault. We analyze interpreted map-view photographs at maximum displacement for damage zone width, number of connected joints, number of secondary fractures, degree of segmentation and area fraction of massively dilatant fractures. Particle imaging velocimetry provides insight into the deformation history of the experiments and illustrates the localization pattern of fault segments. Results show that with increasing angle between joint-set and basement-fault strike the number of secondary fractures and the number of connected joints increase, while the area fraction of massively dilatant fractures shows only a minor increase. Models without pre-existing joints show far lower area fractions of massively dilatant fractures while forming distinctly more secondary fractures.

Structural evolution of a gravitationally detached normal fault array: analysis of 3D seismic data from the Ceduna Sub‐Basin, Great Australian Bight

AbstractThe growth, interaction and controls on normal fault systems developed within stacked delta systems at extensional delta‐top settings have not been extensively examined. We aim to analyse the kinematic, spatial and temporal growth of a Cretaceous aged, thin‐skinned, listric fault system in order to further the understanding of how gravity‐driven fault segments and fault systems develop and interact at an extensional delta‐top setting. Furthermore, we aim to explore the influence of a pre‐existing structural framework on the development of gravity‐driven normal faults through the examination of two overlapping, spatially and temporally distinct delta systems. To do this, we use three‐dimensional (3D) seismic reflection data from the central Ceduna Sub‐basin, offshore southern Australia. The seismic reflection data images a Cenomanian‐Santonian fault system, and a post‐Santonian fault system, which are dip‐linked through an intervening Turonian‐early Campanian section. Both of these fault systems contain four hard‐linked strike assemblages oriented NW–SE (127–307), each composed of 13 major fault segments. The Cenomanian‐Santonian fault system detaches at the base of a shale interval of late Albian age, and is characterised by kilometre‐scale growth faults in the Cenomanian‐Sanontian interval. The post‐Santonian fault system nucleated in vertical isolation from the Cenomanian‐Santonian fault system. This is evident through displacement minima observed at Turonian‐early Campanian levels, which is indicative of vertical segmentation and eventual hard dip‐linkage. Our analysis constrains fault growth into four major evolutionary stages: (1) early Cenomanian nucleation and growth of fault segments, resulting from gravitational instability, and with faults detaching on the lower Albian interval; (2) Santonian cessation of growth for the majority of faults; (3) erosional truncation of fault upper tips coincident with the continental breakup of Australia and Antarctica (ca. 83 Ma); (4) Campanian‐Maastrichtian reactivation of the underlying Cenomanian‐Santonian fault system, inducing the nucleation, growth and consequential dip‐linkage of the post‐Santonian fault system with the underlying fault system. Our results highlight the along‐strike linkage of fault segments and the interaction through dip‐linkage and fault reactivation, between two overlapping, spatially and temporally independent delta systems of Cenomanian and late Santonian‐Maastrichtian age in the frontier Ceduna Sub‐Basin. This study has implications regarding the growth of normal fault assemblages, through vertical and lateral segment linkage, for other stacked delta systems (such as the Gulf of Mexico) where upper delta systems develop over a pre‐existing structural framework.

Normal fault growth and segment linkage in a gravitationally detached delta system: evidence from 3D seismic reflection data from the Ceduna Sub-basin, Great Australian Bight

The authors used three-dimensional (3D) seismic reflection data from the central Ceduna Sub-Basin, Australia, to establish the structural evolution of a linked normal fault assemblage at the extensional top of a gravitationally driven delta system. The fault assemblage presented is decoupled at the base of a marine mud from the late Albian age. Strike-linkage has created a northwest–southeast oriented assemblage of normal fault segments and dip-linkage through Santonian strata, which connects a post-Santonian normal fault system to a Cenomanian-Santonian listric fault system. Cenomanian-Santonian fault growth is on the kilometre scale and builds an underlying structural grain, defining the geometry of the post-Santonian fault system. A fault plane dip-angle model has been created and established through simplistic depth conversion. This converts throw into fault plane dip-slip displacement, incorporating increasing heave of a listric fault and decreasing in dip-angle with depth. The analysis constrains fault growth into six evolutionary stages: early Cenomanian nucleation and radial growth of isolated fault segments; linkage of fault segments by the latest Cenomanian; latest Santonian Cessation of fault growth; erosion and heavy incision during the continental break-up of Australia and Antarctica (c. 83 Ma); vertically independent nucleation of the post-Santonian fault segments with rapid length establishment before significant displacement accumulation; and, continued displacement into the Cenozoic. The structural evolution of this fault system is compatible with the isolated fault model and segmented coherent fault model, indicating that these fault growth models do not need to be mutually exclusive to the growth of normal fault assemblages.

5.9 Mw, 18th June 2010 earthquake and fault segment linkage at Andaman – A study based on macroseismic survey, GPS geodesy and Coulomb stress changes

18th June, 2010 5.9Mw earthquake at North Andaman triggered along NW–SE pre-existing fault with reverse fault mechanism. Macroseismic survey and GPS geodesy reveal maximum damages following NE–SW trend due to normal fault mechanism. Coulomb stress modeling for post- and inter-seismic earthquakes after the 2004 mega-earthquake show different stages of fault segment linkage at North Andaman. The present earthquake has been explained as co-shock due to asiesmic soft linkage of fault propagation.

Mountain front migration and drainage captures related to fault segment linkage and growth: The Polopos transpressive fault zone (southeastern Betics, SE Spain)

The Polopos E–W- to ESE–WNW-oriented dextral-reverse fault zone is formed by the North Alhamilla reverse fault and the North and South Gafarillos dextral faults. It is a conjugate fault system of the sinistral NNE–SSW Palomares fault zone, active from the late most Tortonian (≈7 Ma) up to the late Pleistocene (≥70 ky) in the southeastern Betics. The helicoidal geometry of the fault zone permits to shift SE-directed movement along the South Cabrera reverse fault to NW-directed shortening along the North Alhamilla reverse fault via vertical Gafarillos fault segments, in between. Since the Messinian, fault activity migrated southwards forming the South Gafarillos fault and displacing the active fault-related mountain-front from the north to the south of Sierra de Polopos; whilst recent activity of the North Alhamilla reverse fault migrated westwards. The Polopos fault zone determined the differential uplift between the Sierra Alhamilla and the Tabernas-Sorbas basin promoting the middle Pleistocene capture that occurred in the southern margin of the Sorbas basin. Continued tectonic uplift of the Sierra Alhamilla-Polopos and Cabrera anticlinoria and local subsidence associated to the Palomares fault zone in the Vera basin promoted the headward erosion of the Aguas river drainage that captured the Sorbas basin during the late Pleistocene.

Evolution and structural style of relay zones in layered limestone–shale sequences: insights from the Hammam Faraun Fault Block, Suez rift, Egypt

A fully breached relay zone was investigated to gain insight into relay growth and breaching in layered limestone–shale sequences. Associated fracture patterns were analysed to gain a qualitative understanding of fault- and damage-zone evolution and fracture-related fluid conduits associated with fault overlap zones. Internally, the relay zone is characterized by dilatant brittle deformation in mechanically strong limestone of the stratigraphically upper part of the ramp, and down-dip shear along extensional detachments in stratigraphically lower and mechanically weak shale layers at the base. The linking damage zone is characterized by multi-directional fracture patterns, including fracture corridors at high angles to the main (bounding) faults. The causal relationship between fault growth and the complex fracture patterns lies in the interaction and progressive rotation of the local stress fields of overlapping or linking fault segments. Increasingly complex fracture patterns may therefore be expected during growth and linkage of fault segments. The observed fracture patterns, in concert with complex juxtaposition relations generated by dipping relay beds, indicate that fault linkage points in carbonate rocks represent localized conduits for cross-fault as well as vertical along-fault fluid flow. These have implications for hydrocarbon migration pathways and for reservoir connectivity during production from carbonate reservoirs.

Two‐dimensional distinct element modeling of the structure and growth of normal faults in multilayer sequences: 1. Model calibration, boundary conditions, and selected results

The distinct element method is used for modeling the growth of normal faults in layered sequences. The models consist of circular particles that can be bonded together with breakable cement. Size effects of the model mechanical properties were studied for a constant average particle size and various sample widths. The study revealed that the bulk strength of the model material decreases with increasing sample size. Consequently, numerical lab tests and the associated construction of failure envelopes were performed for the specific layer width to particle diameter ratios used in the multilayer models. The normal faulting models are composed of strong layers (bonded particles) and weak layers (nonbonded particles) that are deformed in response to movement on a predefined fault at the base of the sequence. The modeling reproduces many of the geometries observed in natural faults, including (1) changes in fault dip due to different modes of failure in the strong and weak layers, (2) fault bifurcation (splaying), (3) the flexure of strong layers and the rotation of associated blocks to form normal drag, and (4) the progressive linkage of fault segments. The model fault zone geometries and their growth are compared to natural faults from Kilve foreshore (Somerset, United Kingdom). Both the model and natural faults provide support for the well‐known general trend that fault zone width increases with increasing displacement.

Characterization of strike-slip fault–splay relationships in sandstone

We document the length and angular relationships between strike-slip faults and their splays. The data indicate that the maximum splay length is correlated to the fault length by a power law but shows little correlation with the fault slip magnitude. The kink angle between faults and their splays is small for isolated faults (average ∼20°) and systematically larger for mechanically interacting faults (average ∼50°). Analytical models predict that splay length decreases with increasing confinement under biaxial compression. 2D numerical models of isolated faults show that small kink angles correspond to small values of the angle between the fault and the maximum compression ( β) whereas large kink angles require greater β values. Similar models of interacting faults confirm the critical role of β on the kink angles and suggest that fault overlap and fault separation can also induce important variations in the kink angles. Our results provide a basis for a better understanding of fault segment linkage via splaying process and for a better assessment of the maximum thickness of fault damage zones. The results may also help to predict the length and orientation of secondary faults formed by splaying associated with first order faults with resolvable slip magnitude in the subsurface.

Quantitative study of the deformation at Southern Explorer Ridge using high-resolution bathymetric data

We present preliminary results of a morphological study of the summit of the Southern Explorer Ridge (SER). The SER is an inflated intermediate-rate spreading center located in the northeast Pacific off the West coast of Canada, that hosts a large hydrothermal vent complex known as “Magic Mountain”. A quantitative assessment of faulting on the axial summit graben floor close to the ridge summit is accomplished through the analysis of high-resolution, near-bottom, bathymetric data. These data were acquired using a multibeam system mounted on an autonomous underwater vehicle operated a few tens meters above the seafloor. Structural mapping reveals numerous subvertical fissures and normal faults that nucleated from tension fissures. The ratio between the length and the maximum scarp height of normal faults is not constant contrary to what is generally observed on subaerial faults, highlighting the probable importance of fault segment linkage and fault growth processes within the relatively thin brittle layer. Populations of small faults exhibit an exponential size-frequency distribution that reflects the importance of linkage in the fault growth history and the relatively large amount of tectonic strain (3.7 to 18.4%) accommodated by the normal faults. We propose that the 500 to 600-m wide and ∼ 60-m deep asymmetric axial summit graben of the SER formed due to magma chamber deflation as well as normal faulting that initiated on the present eastern border of the graben. We find a well-defined geographic distribution in the types of lava flows, which indicates a general decrease of the eruption rate through time. We also find that the “Magic Mountain” hydrothermal field is located in the vicinity of the large eastern axial summit graben bounding fault whose dimensions suggest it may reach to the brittle ductile-transition depth. This fault likely has provided an efficient physical pathway for fluids from the subsurface to the seafloor for a significant period of time thus allowing the hydrothermal system to grow and mature.