Overlapping Fault Segments Research Articles

Seismic cycle controlled by subduction geometry: novel 3-D quasi-dynamic model of Central Chile megathrust

SUMMARY Subduction earthquakes show complex spatial and temporal rupture patterns, exhibiting events of varied sizes, which rupture distinct or overlapping fault segments. Elucidating first-order controlling conditions of rupture segmentation and return periods of large earthquakes is therefore critical for seismic and tsunami hazard estimations. The Chilean subduction zone frequently hosts several Mw > 8 earthquakes, with heterogeneous recurrence rates and locations. Here, we implement 3-D quasi-dynamic rate and state frictional models to investigate the role of plate interface geometry on the distribution of interseismic coupling and coseismic ruptures in Central Chile. First, we develop synthetic-parametric models that show how dip and strike variations may increase the probabilities to produce partial seismic barriers, which tend to avoid the production of large earthquake ruptures and modulate rupture lengths. Then, we simulate the subduction seismic cycle processes on Central Chile (25°S–38°S), imposing depth-dependent frictional properties on a realistic non-planar 3-D subduction interface geometry. Similar to results obtained for synthetic-parametric models, after 5000 yr of simulation, regions with abrupt dip or strike changes increase the probabilities of stopping coseismic propagation of simulated Mw 8.0–9.0 earthquakes. Our simulated earthquake sequences on the Central Chile subduction zone delimit rupture areas that match geometrical interface features and historical earthquakes, results that point to the crucial role of fault interface geometry on seismic cycle segmentation along strike.

Lateral and vertical growth and linkage of normal faults in Bozhong8-4 structure of Western Bozhong Sag, Bohai Bay Basin, China

Many previous studies have shown that normal faults are formed by growth and linkage of shorter fault segments through time and have suggested various fault growth models. Although these models have yielded useful information on the evolution of normal fault systems in map view, the three-dimensional (3D) evolution of normal faults is poorly understood. Here, we employ an integrated 3D seismic reflection and well dataset to investigate the 3D growth process of fault zone F1 in the Bozhong 8–4 (BZ8-4) structure of the Western Bozhong Sag (WBS). We i) constructed 3D structural models of individual fault surfaces, ii) calculated the dip angles and dip azimuth distributions on fault surfaces, iii) produced D-d profiles and throw contour maps of fault surfaces, and iv) investigated the 2D/3D segmentation characteristics of geometry and displacement distribution. We found that Fault F1 exhibits segmentation characteristics of geometry and displacement distribution both along strike and dip oriented directions. Vertically, fault surfaces can be divided into three areas (lower, middle and upper parts) that correspond to three stages of regional tectonic evolution (intense faulting, relatively quiescent faulting and post-rifting). The coalescence zone of the upper and lower fault surfaces shows as a sub-horizontal anomaly of geometry and displacement distribution that is sub-parallel to the traces of the horizons. The lower and upper parts of fault surfaces also show segmentation characteristics along strike, which are identified by vertically anomalous zones of geometry and displacement distribution. However, the origins of the segmentation in this case are different. The segmentation of the lower part of the fault developed during the rifting stage is caused by the intersection of faults with different strikes, whereas the upper part of the fault is the result of the lateral growth and linkage of overlapping fault segments. Therefore, not all fault segmentation characteristics along strike can be attributed to fault growth and linkage. Based on 3D fault segment identification, we therefore establish a 3D evolution model of fault zone F1. During the deposition of the Shahejie Fm up to the member (Mbr) 3 of the Dongying Fm in the Paleogene, the faults in this area were highly active, with the development of the NE-trending Fault F1, the NEE-trending Fault Fa and the NW-trending Fault F2. The intersection of Faults Fa and F2 with Fault F1 results in Fault F1 exhibiting segmentation in geometry and displacement distribution. From the deposition of the Mbr 2 of the Dongying Fm, these faults ceased to be active. Since the Pliocene (5.2 Ma), under the influence of transtensional stress field, an en-echelon fault zone was developed in the Neogene strata above fault zone F1 active in the rifting stage. While these faults grow and link laterally, their tip-lines propagate along dip-oriented directions and activate the lower faults and link vertically with them to form the present fault zone F1.

Complex geometry and kinematics of subsidiary faults within a carbonate-hosted relay ramp

Minor fault geometry and kinematics within relay ramps is strongly related to the stress field perturbations that can be produced when two major fault segments overlap and interact. Here we integrate classical fieldwork and interpretation of a virtual outcrop to investigate the geometry and kinematics of subsidiary faults within a relay ramp along the Tre Monti normal fault in the Central Apennines. Although the Tre Monti fault strikes parallel to the regional extension (NE-SW) it shows predominant dip-slip kinematics, suggesting a NW-SE oriented extension acting at sub-regional scale (1–10 km). Conversely, the slickenlines collected on the front segment of the relay ramp highlight right-lateral kinematics. The subsidiary faults in the relay ramp show a complex geometry (variable attitudes) and slickenlines describe multiple kinematics (left-lateral, dip-slip, right-lateral), independently of their orientation. Our fault slip analysis indicates that a local stress field retrieved from the kinematic inversion of the slickenlines collected on the front segment, and likely promoted by the interaction between the overlapping fault segments that bound the relay zone, can explain most of the geometry and kinematics of the subsidiary faults. Further complexity is added by the temporal interaction with both the regional and sub-regional stress fields.

The role of bed-parallel slip in the development of complex normal fault zones

Normal faults exposed in Kardia lignite mine, Ptolemais Basin, NW Greece formed at the same time as bed-parallel slip-surfaces, so that while the normal faults grew they were intermittently offset by bed-parallel slip. Following offset by a bed-parallel slip-surface, further fault growth is accommodated by reactivation on one or both of the offset fault segments. Where one fault is reactivated the site of bed-parallel slip is a bypassed asperity. Where both faults are reactivated, they propagate past each other to form a volume between overlapping fault segments that displays many of the characteristics of relay zones, including elevated strains and transfer of displacement between segments. Unlike conventional relay zones, however, these structures contain either a repeated or a missing section of stratigraphy which has a thickness equal to the throw of the fault at the time of the bed-parallel slip event, and the displacement profiles along the relay-bounding fault segments have discrete steps at their intersections with bed-parallel slip-surfaces. With further increase in displacement, the overlapping fault segments connect to form a fault-bound lens. Conventional relay zones form during initial fault propagation, but with coeval bed-parallel slip, relay-like structures can form later in the growth of a fault. Geometrical restoration of cross-sections through selected faults shows that repeated bed-parallel slip events during fault growth can lead to complex internal fault zone structure that masks its origin. Bed-parallel slip, in this case, is attributed to flexural-slip arising from hanging-wall rollover associated with a basin-bounding fault outside the study area.

Fault Model for the 2015 Leucas (Aegean Arc) Earthquake: Analysis Based on Seismological and Geodetic Observations

The 17 November 2015 M w 6.6 earthquake in Leucas (Leukas, Lefkas, or Lefkada) Island in the Ionian Sea, western Aegean arc, was modeled using teleseismic long‐period P and SH waveforms and Global Positioning System (GPS) slip vectors. Detailed fault modeling in this region, characterized by intense seismicity and deformation rates, usually assigned to the Cephalonia Transform fault, is a challenge because of the unfavorable observation system. To overcome this problem, we independently analyzed seismological and geodetic data and then jointly evaluated the results. The adopted model indicates that the 2015 earthquake can be assigned to a shallow strike‐slip fault, with a minor component of thrusting, along the southwest coasts of Leucas and with relatively high slip for the area. Additionally, mostly low‐angle fault solutions satisfying geodetic observations were identified but were not further investigated. The preferred fault model permits recognition that recent M w>6.0 earthquakes in the area, some marked by extreme peak ground accelerations, are associated with a string of strike slip (or oblique slip), occasionally overlapping fault segments with variable characteristics, along or close to the west coasts of Leucas and Cephalonia (Keffalinia, Kefalonia) Islands, whereas the catastrophic 1953 M w 7.2 Cephalonia and other previous major earthquakes were associated with thrust faulting. [Electronic Supplement:][1] Table of the Global Positioning System (GPS)‐derived displacements and figures of P ‐wave first‐motion polarities, comparison of earthquake source parameters, GPS time series, 2D projections of geodetic solutions, the variance–covariance matrix of the geodetic solution, and the geodetic variable slip model. [1]: http://www.bssaonline.org/lookup/suppl/doi:10.1785/0120160080/-/DC1

Diagenetic controls on the evolution of fault-zone architecture and permeability structure: Implications for episodicity of fault-zone fluid transport in extensional basins

Diagenesis demonstrably changes both the mechanical and hydrologic properties of sediments. However, the effect of these changes on the distribution and types of structures that develop in fault zones over time, and their impact on fault-zone fluid flow has not previously been systematically investigated. We explored the impact of diagenesis on the mechanical and hydrologic properties of initially unlithified sand cut by a syndepositional normal fault in an extensional basin. Field, microstructural, and geochemical data document a diagenetic record of initially continuous fluid flow through a hanging-wall damage zone up to 10 m wide. In this zone, initial deformation via particulate flow is recorded by a foliation defined by a grain shape preferred orientation, preserved by subsequently precipitated, pore-filling calcite cement. A transition to episodic fluid flow is demonstrated by calcite veins that crosscut the previously cemented, foliated sandstone within relatively narrow (≤5 m from the fault core) segments of the older damage zone. Unlike the widespread record of particulate flow, veins are restricted to the vicinity of a mapped relay zone between overlapping fault segments, and an inferred, partially covered relay zone. Vein microstructures record repeated fracture opening and sealing. Veins in breccia zones have δ 13 C values as high as +6.0‰, suggesting degassing of CO 2 - and/or CH 4 -charged fluids. These data collectively suggest that relatively early damage zone cementation strengthened and stiffened the foliated damage zone, affecting the localization of brittle fractures and fluid flow in relay zones. Our results highlight systematic changes in the character and locus of deformation and fluid flow during the development of normal faults and provide a basis for predicting how these structures may act to trap or transmit fluids during the development of extensional basins.

Extensional strain and displacement distribution due to mesoscale normal faults in Late Miocene-Pliocene sedimentary rocks along the northwestern side of the Red Sea, Egypt

Field observations are presented on the NW-SE mesoscale, dip-slip, normal faults in the Late Miocene–Pliocene sedimentary rocks, along the northwestern part of the SW side of the Red Sea, Egypt. These faults were initiated parallel to the Red Sea, and were originated by the NE-SW extension associated with the Red Sea opening in the Late Oligocene-Early Miocene time. About 100 mesoscale normal faults were measured in the Late Miocene-Pliocene sedimentary rocks along seven scan-lines. The extensional strain determined in five scan-lines ranges from 2.6393 to 5.12% with an average of 3.53145%. The other two scan-lines have anomalous values of 6.2988 and 15.53%. The represented data demonstrate that the extensional strain varies significantly from profile to profile and even along the fault because of several surficial factors. The first factor is a difference in lithology. The second and third factors are the local stress and the difference between perpendicular to the direction of maximum lateral extension of area and strike of faults. The L-D (Length-Displacement) diagrams along twelve selected faults reveal three patterns. These patterns include a cone-shaped (C-type), meso-shaped (M-type), and a zigzag-shaped (Z-type). The remarkable variation of displacement (D) along the fault plane (L) is a result of the difference in lithology, and/or the overlapping fault segments, as well as the local stress along the faults.

Sampling the uncertainty associated with segmented normal fault interpretation using a stochastic downscaling method

A large-scale normal fault may be composed of several overlapping fault segments separated by relay zones at a finer scale. Fault segmentation may be critical to the understanding and the forecast of physical phenomena at the reservoir or basin scale (e.g. fluid flow, seismic rupture propagation). In this paper, we propose an automatic and stochastic method to sub-divide (downscale) below the data resolution a segmented normal fault into en-echelon segments that may be linked by connecting faults, based on variations in the orientation of the fault. The downscaling algorithm is composed of three main steps. (1) The first step involves detecting the segments using geometrical criteria. (2) Then the overlapping segments are modeled using isolated fault descriptions and statistics. (3) Lastly, the maturity of the simulated relay zones is evaluated based on relay geometry. If a given maturity threshold is reached, the relay ramp is breached. These three downscaling steps depend on seven parameters that can be defined as constant or randomly chosen from probability distributions to sample uncertainties. The method was applied to a large normal fault that laterally limits a hydrocarbon reservoir and is poorly imaged from seismic data. This resulted in one hundred possible 3D fault array models. The analysis of the emerging parameters shows a strong variability of the number and length of segments, and of the overlap and spacing between segments.

Numerical modelling of strain localisation and fluid flow during extensional fault reactivation: Implications for hydrocarbon preservation

A numerical modelling approach is used to investigate the main controls of strain localisation on faults and fluid flow in fault-bound reservoir rocks during extensional reactivation. A series of 3D coupled mechanical deformation and fluid flow models were performed to simulate the response of simple fault and reservoir-seal geometries to extensional reactivation. The model results demonstrate that initial fault length is a primary control on strain distribution between faults and on fluid transport from the reservoir through the top seal. During extensional reactivation, longer faults in the population tended to accommodate greater shear strain and accumulate greater throw than smaller faults. With greater length contrast, there is an increased throw contrast for the same amount of extension. This behaviour can be attributed to the mechanism of strain sharing among fault elements. Spacing between adjacent faults is a secondary factor affecting strain partitioning. For a given fault length contrast, increasing the spacing increases the freedom of adjacent faults to move and leads to a more even distribution of strain. Fault strike overlap styles also affect strain distribution along faults. This is mainly expressed as the reduction of throws in the overlapping fault segments and partially overlapped or adjacent fault ends. Our models further demonstrate that fault lengths are likely to control fluid flow during extensional reactivation. Large faults in the models are the main conduit for fluid transport, expressed as strong upward fluid flow along these faults from the sandstone reservoir horizon through the shale seal.

Geophysical evidence of crustal-heterogeneity control of fault growth in the Neocomian Iguatu basin, NE Brazil

Models of fault growth propose that rift initiation starts with short fault segments. Knowledge of the growth of these segments and their interactions is important to understanding rift geometry and evolution. In the northern part of the Borborema Province, northeastern Brazil, a continental-scale, Cretaceous extensional system of faults has been observed to have reactivated ductile Precambrian shear zones. The faults form small grabens that represent the rift stage of the sedimentary basins. We integrated airborne radiometric and magnetic data with terrestrial gravity survey to investigate the influence of crustal heterogeneity on fault growth and the development of the extensional faults in one of these grabens, the Iguatu basin. Previous studies presented geophysical data, which provide evidence that the Iguatu basin contains a half-graben geometry. In our study, gravity and airborne geophysical data indicate that the basement of the Iguatu basin is part of a heterogeneous structural framework composed of two structural domains, is affected by several ductile shear zones and intruded by a few granite bodies. The gravity modeling reveals that this basin is composed of three right-bend en echelon fault segments. They form a sigmoid system of normal faults that accommodate the strong ∼90° bend of the Precambrian shear zones from E–W to roughly N–S. The growth of these segments led to the generation of two isolated depocenters. The overlapping fault segments link through relay ramps. Release faults that are nearly perpendicular or oblique to the three main fault segments form marginal strike ramps and horst structures in both depocenters. 3D-gravity modeling incorporates the presence of interfering sources of a heterogeneous structural framework. The modeling reveals a maximum sedimentary cover 1620 m thick, which occurs at the bend of the reactivated shear zones. The gravity signature of a possible granite body, after removal of the gravity effect of the basin-filling deposits, indicates that this possible intrusion forms a horst oriented roughly parallel to the basement fabric. The relationship between maximum displacement and fault length indicates that the fault segments formed independently during the initiation of the half-graben. But the fault segments, linked by relay ramps, influence the development of one another. We interpret the segments to represent a pre-linkage, multisegmented stage of an aborted rift system. This pattern of fault growth may have also influenced the rift stage of other sedimentary basins in the region.

Recent and active deformation pattern off the easternmost Algerian margin, Western Mediterranean Sea: New evidence for contractional tectonic reactivation

We describe for the first time a set of large active thrusts and folds near the foot of the easternmost Algerian margin, Western Mediterranean, from swath bathymetry and high-resolution seismic data acquired in 2005 during the Maradja2/Samra cruise. This active system resumes a previous passive margin and creates growth strata deposition on the limbs of large folds, resulting in the development of perched basins at the foot of the margin since less than ~ 1 Ma. They form a set of overlapping fault segments verging toward the Algerian basin, in a way similar to what has been observed off eastern Algiers on the rupture zone of the 2003 Mw 6.8 Boumerdes earthquake. The horizontal shortening rate across large folds is estimated to be of the order of 1 mm/yr. Although no historical earthquakes are reported here, these fault segments could have been responsible for large ( M ~ 7.5) events in the past. This young tectonic system further supports the hypothesis of subduction inception of the Neogene oceanic lithosphere in the context of the Africa–Eurasia convergence.

Relay and accommodation zones in the Dobe and Hanle grabens, central Afar, Ethiopia and Djibouti

The right-stepping Dobe and Hanle grabens display a variety of structures that serve to transfer extensional displacement. These structures range from relay zones between overlapping fault segments to accommodation zones between interacting rift segments. The study reveals the presence of three examples of displacement transfer structures: a breached relay zone, a large-scale accommodation zone that is partially breached, and a composite zone that combines elements of both. All three examples exhibit common structural elements. First, dipping ramps develop between horizontal horst blocks and graben floors. Second, these ramps are cut by numerous faults, most of which are antithetic to the ramps and the graben boundary faults. The antithetic faults bound elongate blocks that are rotated into the grabens. Third, crosscutting faults partially or completely link the en echelon or overlapping graben boundary faults. The identification of precursory structures (mode I fractures) at the leading edge of the Dobe–Hanle accommodation zone breaching faults suggest that the breaching process may be continuing. The spatial alignment from north to south of the crosscutting faults, open fractures and lineaments indicates that the breaching process is progressing from the Dobe graben towards the Hanle graben.

Faulting, mass-wasting and deposition in an active dextral shear zone, the Gulf of Saros and the NE Aegean Sea, NW Turkey

Structural, mass-wasting and sedimentation processes along an active dextral shear zone beneath the Gulf of Saros and the NE Aegean Sea were investigated on the basis of new high-resolution swath bathymetric data and multi-channel seismics. A long history of dextral shearing operating since the Pliocene culminated in the formation of a NE-SW-trending, ca. 800-m-deep basin (the so-called inner basin) in this region, which is bordered by a broad shelf along its northern and eastern sides and a narrow shelf at the southern side. The western extension of the North Anatolian Fault Zone (the Ganos Fault) cuts the eastern shelf along a narrow deformation zone, and ends sharply at the toe of the slope, where the strain is taken up by two NE-SW-oriented fault zones. These two fault zones cut the basin floor along its central axis and generate a new, Riedel-type pull-apart basin (the so-called inner depression). According to the bathymetric and seismic data, these basin boundary fault zones are very recent features. The northern boundary of the inner depression is a through-going fault comprising several NE-SW- and E-W-oriented, overlapping fault segments. The southern boundary fault zone, on the other hand, consists of spectacular en-echelon fault systems aligned in NE–SW and WNW–ESE directions. These en-echelon faults accommodate both dextral and vertical motions, thereby generating block rotations along their horizontal axis. As the basin margins retreat, the basin widens continuously by mass-wasting of the slopes of the inner basin. The mass-wasting, triggered by active tectonics, occurs by intense landsliding and channel erosion. The eroded material is transported into the deep basin, where it is deposited in a series of deep-sea fans and slumps. The high sedimentation rate is reflected in an over 1,500-m-thick basin fill which has accumulated in Pliocene–Quaternary times.

The 3D geometry of small-scale relay zones between normal faults in soft sediments

Serial cross-sections of faults within soft sediments of Miocene age, on the East coast of Denmark, were collated to provide 3D data sets with which to study the geometry of minor faults (displacements < 10 cm). From the wide variety of geometrical complexities displayed by these faults, many due to mutually cross-cutting relationships between opposed dipping faults, we present relatively simple structures considered to demonstrate stages in the evolution of relay zones between overlapping fault segments. The structures described are: (1) initial embayments in fault tip-lines, (2) established relay zones displaying transfer of displacement between overlapping fault segments, (3) breached relay zones, and (4) fault bound lenses. An individual relay zone may display more than one of these structures along its length and relay zones that are intact on one cross-section may be breached along strike on another. Examples of relay zones that are isolated from the fault tip-line and are effectively holes within a continuous fault are also observed. The observed relay zones breach by propagation of the initial parent fault through the relay zone rather than by the formation of a new breaching fault.

The structural evolution of dilational stepovers in regional transtensional zones

Abstract We propose a theoretical model, supported by a field study, to describe the patterns of fault/fracture meshes formed within dilational stepovers developed along faults accommodating regional scale wrench-dominated transtension. The geometry and kinematics of the faulting in the dilational stepovers is related to the angle of divergence (α), and differs from the patterns traditionally predicted in dilation zones associated with boundary faults accommodating strike-slip displacements (where α = 0°). For low values of oblique divergence (α&lt;30°) and low strain, the fault–fracture mesh comprises interlinked tensile fractures and shear-extensional planes, consistent with wrench-dominated transtension. At higher values of strain, a switch occurs from wrench- to extension-dominated transtension, leading to the reactivation and/or disruption of the early-formed structures. These structural processes lead to the development of a geometrically complex and kinematically heterogeneous fault pattern, which may affect and/or perturb the development of a through-going fault linking and facilitating the slip transfer between the two overlapping fault segments. As a result, dilational stepover zones will tend to form long-lived sites of localized extension and subsidence in regional transtensional tectonic settings. Cyclic increases/decreases of structural permeability will be related to slip on the major boundary faults that control the distribution of fluid-flow paths and, consequently, the long- and short-term structural evolution of these sites. Our model also predicts complex and more realistic subsurface fluid migration pathways relevant to our current understanding of hydrothermal ore deposits and hydrocarbon migration and storage.

Geometry, segmentation pattern and displacement variations along a major Apennine thrust zone, central Italy

Hundreds of kilometres-long, arc-shaped, continuous thrust faults commonly imply very large displacements by detachment-dominated thrusting. Therefore, their occurrence has a large impact on fold–thrust belt structure. One of the major thrust faults of the central-northern Apennines, the Umbria–Marche–Sabina Thrust Zone, is traditionally believed to represent one such feature, characterised by displacements of several tens of kilometres. However, detailed studies of this structure revealed that it is actually composed of a series of partially overlapping fault segments, rather than consisting of a single, continuous thrust. Cross-section balancing and restoration, carried out by the integration of surface geological data with available sub-surface information, points out relatively limited amounts of thrust displacement (<10 km). Displacement–distance profiles show moderate displacement gradients well compatible with those reported for coherent thrust sheets. They also suggest that individual thrust segments were originally isolated and then grew by lateral propagation, leading to overlap and variable fault interaction within relay zones. The relationship between maximum displacement and fault trace length tends to follow a power-law distribution, as it commonly occurs for fault populations. However, a better correlation could be obtained by further segmentation of the northernmost (blind) fault in an area of no seismic data, thus confirming that fault scaling relationships may be useful for pointing out possible problems with the structural interpretation of poorly constrained areas and for exploring viable alternative solutions.

Layer rotation around vertical fault overlap zones: observations from seismic data, field examples, and physical experiments

Vertically overlapping fault segments are common structures in faulted hydrocarbon reservoirs. Experimental work and field observations show a close relationship between the rotation of layers in the region of overlap, the type of overlap (restraining vs. releasing) and fault curvature. In general, releasing overlap zones (where the normal fault steps upward into the hanging-wall) show normal rotation or drag, thus decreasing the effective throw on the fault. In contrast, restraining overlaps tend to develop reverse rotation in the overlap zone, particularly if the normal fault tips curve toward each other. Releasing overlap zones seem to be more common than the restraining zones, and the overlaps tend to form in shaly layers between thicker sandstones. Narrow overlaps of this type typically develop zones of drag or shale smear that could seal or reduce communication across the adjacent sandstone layers. Hence, overlap zones may significantly influence communication in a reservoir, depending on the fault arrangement, geometry, and lithological properties. Seismic interpreters and structural geologists should pay particular attention to layer rotation to identify vertical overlap structures and to evaluate their influence on reservoir performance.

Structural controls on drainage development in the Canyonlands grabens of southeast Utah

The Canyonlands grabens in southeast Utah form an active exten sional fault array covering 200 km2 southeast of the Colorado River. The fault array formed as a result of gravity gliding above a thick layer of salt. Growth of this fault array within the last 0.5 m.y. (possibly last 0.1 m.y.) has resulted in major changes in the stream drainages across the area through processes of stream capture and diversion. During growth of the fault array, relay ramps between overlapping fault segments form topographic lows along the graben margins. These commonly act as access points for captured streams to enter a graben system. As fault segments continue to propagate laterally, linkage leads to breaching of the relay ramp structures. This causes changes in the course and gradients of the streams, com monly shifting the locus of alluvial sediment deposition away from grabens that were previously infilling. This complex evolution of drainage networks in a growing fault array may provide a valuable analog to the early structural and stratigraphic development of larger continental rift systems. Reservoir distribution is an essential element of many hydrocarbon plays, and understanding the rela tionship between active fault growth and drainage evolution may help predict reservoir distribution and quality.

Faulting and fault scaling on the median valley floor of the trans‐Atlantic geotraverse (TAG) segment, ∼26°N on the Mid‐Atlantic Ridge

A quantitative assessment of faulting on the median valley floor of a slow spreading ridge is accomplished through the analysis of high‐resolution DSL‐120 sidescan sonar and coregistered bathymetric data from the TAG segment near 26°N on the Mid‐Atlantic Ridge. At this location, faulting is exposed within a 3–5 km wide ridge‐parallel zone lying asymmetrically on the eastern half of the median valley floor. Mapped faults have a normal sense of displacement, are &lt;2 km in length, and accommodate ∼1.5% brittle extension. Evidence of fault linkage within the fault population includes kinked and bent fault traces in map view, the development of overlapping fault segments or relay ramps, and the presence of multiple local maxima in the displacement‐distance profiles of some faults. Faults have a slight tendency to dip to the east, or outward relative to the valley axis, and exhibit little symmetry of fault dip about the axis of the faulted zone or any other ridge‐parallel line. Faults exhibit a roughly linear relationship between maximum fault throw and fault length, with a mean ratio of 0.030 for the population. Regression of length‐frequency data indicates a power‐law distribution, with an exponent of 1.64–1.96, demonstrating that fractal populations can exist in the mid‐ocean ridge environment. The fractal nature of this length‐frequency distribution and the ratio of maximum fault throw to fault length differ significantly from those described previously for populations of larger abyssal hill faults in the fast spreading environment, where the distribution is exponential and the throw‐to‐length ratio is ∼5 times lower. These results suggest that the scaling of fault populations in the midocean ridge setting may vary as a function of spreading rate and/or fault size.

Four-Dimensional Evolution of a Salt-Related Fault Network: Eugene Island Block 330 Field, Offshore Louisiana: ABSTRACT

The Eugene Island Block 330 field consists of two anticlinal rollovers in the hanging wall of a listric normal fault system that detaches along a deep salt weld. We use interpretation of well and 3D seismic data, 3D structural restoration, and fault analysis techniques to investigate the evolution of the three-dimensional geometry through time and its impact on hydrocarbon migration and entrapment. The master fault that currently forms the northeastern boundary of the field originated as two distinct, overlapping fault segments separated by a relay ramp. The segments linked by breaching of the ramp prior to 2.2 Ma. The resultant footwall splay continued to accommodate a significant proportion of the net slip, but became essentially inactive by 1.5 Ma. In the hanging wall of the master fault, interaction between two synthetic splays and an antithetic fault resulted in a similar, complex history of varying displacement distribution. Analysis of evolving hanging and footwall cutoff geometries on the various faults suggests that cross-fault hydrocarbon migration at sand-on-sand contacts may have contributed to reservoir filling. This scenario requires that Lentic (>2.2 Ma) sands in the footwall were connected to deeper hydrocarbon sources, which cannot be demonstrated and may be problematic because of themore » discontinuous nature of these sands. Moreover, the present distribution of hydrocarbons and the sealing nature of faults at some sand-on-sand contacts imply that another mechanism, such as episodic vertical migration along fault zone is required to introduce hydrocarbons into the shallow reservoirs.« less