Fault Population Analysis Research Articles

The significance of volcanic segments and rifts in faults characterization within the Amagmatic graben of the Afar Depression, Ethiopia

The Dobi graben is a northwestern trending, continental rift situated in the East Central Block (ECB) of the Afar Depression (AD), Ethiopia. Ongoing extensional rifting in the graben is evident from the swarm of intermediate magnitude earthquakes (5.7 < Ms < 6.3) in 1989. The graben's extension occurs on steeply dipping faults, where the maximum displacement and traced fault length spans four orders of magnitude. Using a 30 m resolution Shuttle Radar Topography Mission (SRTM) Digital Elevation Model (DEM), we conducted a fault population analysis in the Dobi graben. We traced 953 faults and conducted a size-frequency distribution analysis has been conducted for various spatially heterogeneous structural zones. Our results show that the frequency size distribution revealed a negative exponential fitting trend, indicating strong strain distribution within the Imbrication, the Fault Termination zone, and the active axial graben floor. However, a power law size distribution dominates most first-order border faults, suggesting that the strain is localized mainly at the graben flanks.The fault displacement-length profiles demonstrate that approximately 48% of the total fault traced lengths exhibit increasing slip rates towards the southeast, while about 40% display increasing slip rates towards the northwest. These suggest that ∼88% of the lateral propagation of the 953 faults in the Dobi graben is governed by the regional differential strain transfer of the Red Sea Rift (RSR) and Gulf of Aden Rift (GAR) to the central Afar. Most of the hmax/L aspect ratio of the Dobi graben's faults fits in Category II, which exhibits a constant hmax/L ratio, meaning that the hmax/L ratio increases with fault growth, presumably due to the graben's is in an active tectonic region. This implies that these faults are becoming more efficient at accommodating deformation as they grow. Additionally, the normalized average maximum displacement over maximum length (Dmax/Lmax) ratio for most faults is 0.03, which is in accord with the constant displacement length fault growth model.

Measurements of the extension required for crustal breakup on the magma-poor Iberia-Newfoundland conjugate margins

Rifted margins have been extensively studied, however there are still many important questions related to their formation processes. We use the Iberia-Newfoundland conjugate rifted margins as a natural laboratory to investigate how much extension is required to produce crustal rupture and separation. To achieve our aim we use; (i) gravity anomaly inversion to measure continental crustal thinning, (ii) subsidence analysis to measure continental lithosphere thinning and (iii) fault heave summation from seismic observations to obtain brittle continental crust extension. Integration of thinning from gravity anomaly inversion and subsidence analysis is used to determine continental crust extension and continental lithosphere extension respectively. These measurements have been made between the proximal continental crust and the distal Limit of Contiguous Continental Crust (LCCC) on the northern (SCREECH1 and ISE01) and the southern (SCREECH2 and TGS/LG12) conjugate seismic profiles. For the Iberia-Newfoundland conjugate rifted margins, extension values determined from the integration of crustal and lithosphere thinning are similar and suggest that on average 172 km of extension is required to produce crustal breakup. In contrast, measured extension from fault heave summation is on average 128 km which indicates an apparent extension discrepancy at the scale of the whole conjugate margin system when observed brittle continental crust extension is compared with lithosphere and crust extension. Fault population analysis shows that sub-seismic resolution faulting significantly contributes to an under-estimate of brittle crustal extension by up to 25%. Allowing for fault extension at sub-seismic resolution indicates that an extension discrepancy at the scale of the whole conjugate margin system does not exist. Fault geometries and the location of the distalmost contiguous continental crust are also important issues that should be carefully considered when comparing fault heave summation and extension from crust and lithosphere thinning. The absence of an extension discrepancy between fault extension, crustal extension and lithosphere extension at the scale of a whole conjugate margin system should not be confused with the presence of depth-dependent stretching and thinning at a given location on the margin which is both expected and observed at rifted continental margins. • Continental breakup and formation of the conjugate Iberia-Newfoundland rifted margins. • Extension required for rupture and separation of continental crust during breakup. • Measure extension from continental crust and lithosphere thinning and fault heaves. • No overall extension discrepancy but local depth-dependent stretching and thinning.

Analysis of normal fault populations in the Kumano Forearc Basin, Nankai Trough, Japan: 1. Multiple orientations and generations of faults from 3‐D coherency mapping

Analyses of normal faults in the Kumano forearc basin of the Nankai Trough reveal multiple normal fault populations in a region generally thought to be under compression. Most faults have offsets of less than 20 m and dips of 60–70° and show no growth structures, indicating that the faults were active for short periods of time. The oldest generation of faults is older than ~0.9 Ma and strikes ~50–60°. The next oldest faults strike ~160–170°, are older than 0.44 Ma, and are related to local uplift along the western edge of the region. The youngest faults cut the seafloor; shallow faults near the SE margin of the basin curve from ~100° in the middle of the survey area to ~145° at the SE corner of the area. The pattern of the two youngest fault populations is consistent with the regional stress pattern (maximum horizontal stress subparallel to the trench). Orientations of older fault populations are caused by uplift of the underlying accretionary prism, implying that the forearc basin region is not as stable as previously thought. Reconstruction of displacements on the youngest faults shows that the overall horizontal extension is less than 2%, concentrated near the seaward edge of the basin. The active normal faults distributed throughout the basin support the idea that the horizontal stress parallel to the plate convergence direction does not reach the critical stress to activate or form thrust faults and produce horizontal shortening within the shallow portion of the inner wedge.

Analysis of normal fault populations in the Kumano forearc basin, Nankai Trough, Japan: 2. Principal axes of stress and strain from inversion of fault orientations

AbstractWe use a high‐resolution 3‐D seismic survey to map a population of recent normal faults within the Kumano Basin of the Nankai subduction zone, in order to quantify patterns of strain and stress state over the last 0.44 Myr. We identify distinct fault populations that define three phases of extension. Phases 1 and 2 comprise NW‐SE striking faults located along the western basin edge and in the northwestern portion of the study area, respectively. The NE‐SW striking faults of phase 3 comprise the largest population, and extend ~20 km landward from the basin's seaward edge. Phase 2 faults typically terminate within a few reflectors of the seafloor, whereas most phase 3 faults form seafloor scarps. Inversion of the fault populations documents NE‐SW extension during phases 1 and 2 and NW‐SE extension during phase 3, consistent with both core‐scale structures and horizontal stress orientations observed at Integrated Ocean Drilling Program (IODP) boreholes. Slip on phase 3 faults accommodates strain of up to ~1–2%, concentrated near the basin's seaward edge. Inversion for a best‐fit stress tensor yields a subvertical σ1 and subhorizontal σ2 and σ3 for all faulting phases. We find that during phase 3 in most portions of the basin, σ2 = σ3 (SHmax = Shmin), reflecting widely varying fault strikes. This contrasts with distinct SHmax and Shmin magnitudes inferred from IODP borehole data; these observations may be reconciled if the orientation of maximum horizontal stress fluctuates due to variation of subduction parallel compression through the seismic cycle.

Structural analysis of fault populations along the oblique, ultra-slow spreading Knipovich Ridge, North Atlantic Ocean, 74°30′N-77°50′N

The Knipovich Ridge (73°30′–78°40′N) is an extreme end-member of the mid-ocean ridge spreading system, both in terms of spreading rate (<1.5 cm/yr) and angle between the ridge and the spreading direction (40°–53°). Structural analysis of side-scan sonar images obtained along ∼400 km of the ridge axis reveals systematic relationships between fault population parameters, obliquity, and axial segmentation along the ridge. Fault population characteristics conform to observational and experimental analyses of oblique rifting and spreading systems. For the ridge as a whole, faults in the axial region are short, straight, isolated (not linked into complex fault zones), and exhibit length-scaling relationships characteristic of young and active fault systems. Faults are generally oblique to both the ridge axis and the spreading direction, and orientations vary systematically with angle between ridge and spreading direction. Along-axis analysis reveals the influence of axial segmentation on fault population characteristics. Segment centers are dominated by faults perpendicular and sub-perpendicular to plate motion with longer characteristic length and generally lower fracture density. Conversely, segment ends are dominated by faults striking oblique to plate motion with shorter characteristic length and generally higher fracture density. We infer that faulting in segment centers is strongly influenced by the mechanical effects of dike intrusion perpendicular to plate motion, while faulting in segment ends is controlled by the mechanics of oblique rifting, non-transform discontinuities, and/or accommodation zones. The contrasts between these distinct structural and mechanical settings along the ridge axis are accentuated by the high obliquity and ultra-slow spreading rate of this spreading system.

Speleoseismology and palaeoseismicity of Benis Cave (Murcia, SE Spain): coseismic effects of the 1999 Mula earthquake (m b 4.8)

Abstract This work describes the coseismic ceiling block collapse within Benis Cave (−213 m; Murcia, SE Spain), associated with the 1999 Mula earthquake (m b =4.8, MSK VII). The collapse occurred at −156 m into the Earthquake Hall, and as a consequence one small gallery became blind. We studied the geology, topography and active tectonic structures relevant to the cave. In addition, we carried out a seismotectonic analysis of the focal mechanism solutions, and also a fault population analysis on slickensides measured in fault planes in the cave. The stress and strain regime is interpreted as being congruent with the palaeoseismic evidence, and agrees with the fault kinematics established for cave galleries developed within fault planes and growth anomalies of coral flowstone. Our analysis suggests that one active segment (NNE–SSW) determined the morphology and topography of the Benis Cave, where strong to moderate palaeoearthquakes (6≤M≤7) took place. As a consequence of this intense seismic activity a small gallery collapsed. A new palaeoseismic structure, or seismothem, has been recognized, namely the effect of palaeoearthquakes affecting the pattern of development of the spatial coral flowstone distribution located at the bottom of the cave.

Present-day strain field on the South American slab underneath the Sandwich Plate (Southern Atlantic Ocean): a kinematic model

Abstract This work analyses the present-day principal strain orientation on the downgoing slab of the South America Plate (SAM) beneath the Sandwich Plate (SAND). The strain regime was deduced from the study of 331 earthquake focal mechanism solutions examined by fault population analysis methods. In the slab, the maximum horizontal shortening direction (ey) rotates in trend in a clockwise direction from NE in the north, to SE in the south. Based on this rotation, three different areas were defined according to the prevailing focal mechanism type: (1) the North Zone, with ey oriented N058°E and reverse and strike–slip focal mechanisms; (2) the Central Zone, with only reverse focal mechanisms and ey striking N080°E; and (3) the South Zone, with ey oriented N106°E and reverse and strike–slip focal mechanisms. The strain field in the North Zone of the SAND involves decoupling of the slab at approximately 70 km depth. In contrast, the South Zone edge slab exhibits no decoupling and it exhibits different geometry (hook-like shaped) from the North Zone. Finally, we define the dextral strike–slip component acting at the South Sandwich Fracture Zone (SSFZ), according to focal mechanism solutions and the regional tectonic configuration.

Genesis and evolution of the structurally controlled vein mineralization (Sb–Hg) in the Escarlati deposit (León, Spain): Evidence from fault population analysis methods, fluid-inclusion research and stable isotope data

The Escarlati deposit is located in the Cantabrian Zone of the Variscan Massif and is one of the best examples in the Iberian Peninsula of Sb and Hg both coexisting in the same paragenesis. The Sb–Hg mineralization appears filling hydraulic and collapse breccias hosted in Late-Variscan fractures affecting Carboniferous black limestones. The structural analysis and mineralogical, fluid-inclusion and stable isotopic characterization of the Escarlati Sb–Hg deposit (Señales pass, León province), have been performed. In this contribution we aim to show the close relationships between the structural control of the mineralization and the evolution of its mineralogical and thermodynamic formation conditions. An analysis of calcite vein sets and striated fault planes exposed in the old mine working areas using several methods of fault population analysis (FPA), in particular the right dihedra method (RDM) and the slip model (SM), enables us to determine that the ore concentration was favoured by reactivation of previous structures during the latest-tectonic regime. There was a first strike-slip tectonic event with a maximum compression orientation of NW-SE ( σ 1 = 00°/300°) and maximum extension trending NE-SW ( σ 3 = 00°/030°), which generated early unmineralized calcite veins. A second strike-slip reverse movement, which reactivated early structures, produced an intense stage of fracturing developing a first stage with mylonites and a second one with breccias (hydraulic and collapse calcareous breccias) and the rotation of the stress axes in a clockwise sense, with a maximum shortening direction N–S ( σ 1 = 00°/010°) and extension direction E–W ( σ 3 = 00°/100°). The final part of the younger tectonic event is recorded by striated fault planes, which indicate a latest strike-slip reverse tectonic event with maximum shortening direction NNE-SSW ( σ 1 = 17°/200°) and extension direction WNW-ESE ( σ 3 = 28°/100°), and also a new clockwise rotation is registered. Ore mineralogy is related to the second strike-slip reverse movement tectonic event and shows two different mineralogical associations, the first of which is related to the initial movements of the strike-slip reverse tectonics and is made up of quartz and pyrite followed by stibnite, fluorite and calcite filling hydraulic breccias. The final part of this tectonic regime promoted high fluid flow and is characterized by the deposition of cinnabar together with a second generation of pyrite and fluorite, mostly in collapse breccias. Fluid inclusions of the mineralizing event reveal a drop in temperature between the two mineralizing stages (from approximately 190 °C to less than 150 °C) and a remarkable pressure drop from lithostatic (1125 bar) to hydrostatic conditions (250 bar) during the first stage as well as sub-superficial pressures during Hg deposition, in agreement with the evolution in two parts of the second strike-slip reverse movement. The carbon of the hydrothermal carbonates is influenced by the organic matter of the Barcaliente Formation and their δ 13C and δ 18O isotopic composition reveal that we can not distinguish two different populations of hydrothermal carbonates. The heavy δ 34S of sulphides indicates its leaching from sedimentary sulphur that had its origin in an abiogenic fractionation process. The lighter δ 34S values in cinnabar than in stibnite indicate changes to more oxidant conditions during cinnabar deposition. These changes probably occurred as a result of the loss of volatile components from the mineralizing fluid during the transition from lithostatic to hydrostatic conditions, as revealed by the fluid-inclusion study.

Distributed and localized faulting in extensional settings: Insight from the North Ethiopian Rift–Afar transition area

Extensional fault systems in the Earth's crust can exhibit two end‐member geometries that we identify as distributed and localized faulting regimes. A satellite image analysis of fault populations from the Main Ethiopian Rift‐Afar area reveals that the rift architecture contains these two faulting regimes. The occurrence of these regimes reveals a jump in the scale of fault segmentation and linkage. Strain localization at rift border zones exhibits particularly large‐scale fault linkage and a power law size distribution. This regime replaces prior distributed fault systems, showing small‐scale fault linkage and an exponential size distribution. The distributed faulting is interpreted as confined to the thick trap basalt carapace. We show that continental fault systems can develop by a combination of these two geometries, and we demonstrate how to quantitatively decipher the jump between them.

Oblique rifting at Tempe Fossae, Mars

This work shows the results of a structural study of the faults observed at the Tempe Rift (northeastern Tharsis region), Mars. A new, detailed map of faults and fault systems was used to geometrically characterize the fracture architecture of the Tempe Rift and to measure fault length, displacement, and spacing data, to analyze the spatial distribution of fault centroids, and to investigate the fractal nature of fault trace maps. A comparison with analog models and the use of conventional techniques of fault population analysis show that the Tempe Rift was most probably generated under sinistral oblique‐rifting processes, which highlights the importance of the presence of inherited fractures in the tectonic evolution of the Noachian crust. The angle between the extension direction and the rift axis varies along the Tempe Rift, ranging from 50°–60° at its central southern part to 66°–88° to the southwest. Fault scaling relationships are similar to those found at mid‐ocean ridges on Earth with exponential fault length‐frequency distributions. Localized, inhomogeneous deformation generated weakly interacting faults, spanning the entire thickness of the mechanical layer. This thickness decreased from southwest to northeast along the rift, along with distance from the central part of the Tharsis dome.

Calcite e-twins as markers of recent tectonics: insights from Quaternary karstic deposits from SE Spain

Superficial Pleistocene karstic coatings found in the boundary region between the Iberian and Betic Chains (Spain) are often offset by normal faults. These travertines were sampled to investigate whether mechanical twins in calcite can develop at low paleodepths. Surprisingly, twinned grains are very abundant in sparry grains, which are found in the proximity of the fault planes (mean twin densities 28 twins/mm). Paleostress determinations based on the analysis of calcite e-twinning yielded the same principal stress orientations as the fault population analysis, i.e. a NW–SE extension tectonic regime. Therefore, calcite e-twin analysis is a useful method with which to establish the orientation of recent stress tensors. Calcite e-twins develop under conditions of minimum overburden (∼300 m) during fault slip. The low differential stresses during the fault slip event suggest that the critical resolved shear stress on the twin plane must be lower than the expected mean value of 10 MPa or that local stresses experienced by rock in and near the fault zone exceed ambient stress conditions during fault slip.

The Denizli graben-horst system and the eastern limit of western Anatolian continental extension: basin fill, structure, deformational mode, throw amount and episodic evolutionary history, SW Turkey

The Denizli graben-horst system (DGHS) is located at the eastern-southeastern converging tips of three well-identified major grabens, the Gediz, the Küçük Menderes and the Büyük Menderes grabens, in the west Anatolian extensional province. It forms a structural link between these grabens and the other three NE-NW-trending grabens—the Çivril, the Acıgöl and the Burdur grabens—comprising the western limb of the Isparta Angle. Therefore, the DGHS has a critical role in the evolutionary history of continental extension and its eastward continuation in southwestern Turkey, including western Anatolia, west-central Anatolia, and the Isparta Angle. The DGHS is a 7-28-km wide, 62-km long, actively growing and very young rift developed upon metamorphic rocks of both the Menderes Massif and the Lycian nappes, and their Oligocene-Lower Miocene cover sequence. It consists of one incipient major graben, one modern major graben, two sub-grabens and two intervening sub-horsts evolved on the four palaeotectonic blocks. Therefore, the DGHS displays different trends along its length, namely, NW, E-W, NE and again E-W. The DGHS has evolved episodically rather than continuously. This is indicated by a series of evidence: (1) it contains two graben infills, the ancient graben infill and the modern graben infill, separated by an intervening angular unconformity; (2) the ancient graben infill consists of two Middle Miocene-Middle Pliocene sequences of 660 m thickness accumulated in a fluvio-lacustrine depositional setting under the control of first NNW-SSE- and later NNE-SSW-directed extension (first-stage extension), and deformed (folded and strike-slip faulted) by a NNE-SSW- to ENE-WSW-directed phase of compression in the latest Middle Pliocene, whereas the modern graben infill consists of 350-m thick, undeformed (except for local areas against the margin-bounding active faults), nearly flat-lying fanapron deposits and travertines of Plio-Quaternary age; (3) the ancient graben infill is confined not only to the interior of the graben but is also exposed well outside and farther away from the graben, whereas the modern graben infill is restricted to only the interior of the graben. These lines of evidence imply an episodic, two-stage extensional evolutionary history interrupted by an intervening compressional episode for the DGHS. Both the southern and northern margin-bounding faults of the DGHS are oblique-slip normal faults with minor right- and/or left-lateral strike-slip components. They are mapped and classified into six categories, and named the Babadağ, Honaz, Aşağıdağdere, Küçükmalıdağ, Pamukkale and Kaleköy fault zones, and composed of 0.5-36-km long fault segments linked by a number of relay ramps. Total throw amounts accumulated on both the northern and southern margin-bounding faults are 1,050 m and 2,080 m, respectively. In addition, the maximum width of the DGHS and the thickness of the crust beneath it are more or less same (~ 28 km). The total of these values indicate a vertical slip rate of 0.15-0.14 mm/year and averaging 7% extension for the asymmetrical DGHS. The master faults of the Babadağ, Honaz, Küçükmalıdağ, Pamukkale and Kaleköy fault zones are still active and have a potential seismicity with magnitudes 6 or higher. This is indicated by both the historical (1703 and 1717 seismic events) to recent (1965, 1976, 2000 seismic events) earthquakes sourced from margin-bounding faults and some diagnostic morphotectonic features, such as deflected drainage system, degraded alluvial fans with apices adjacent to fault traces, back-tilting of fault-bounded blocks, and actively growing travertine occurrences. The kinematic analyses of main fault-slip-plane data, Upper Quaternary fissure ridges and focal-mechanism solutions of some destructive earthquakes clearly indicate that the current continental extension (second-stage extension) by normal faulting in the DGHS continues in a (mean) 026° to 034° (NNE-SSW) direction. Detailed and recent field geological mapping, stratigraphy of the Miocene-Quaternary basins, palaeostress analysis of fault populations and main margin-bounding faults of these basins, extensional gashes to fissures, and focal-mechanism solutions of destructive earth-quakes that have occurred in last century strongly indicate that extension is not unidirectional and confined only to western Anatolia, but also continues farther east across the Isparta Angle and west-central Anatolia, up to the Salt Lake fault zone in the east and the inönü-Eskişehir fault zone in the north-northeast. Therefore, the term “southwest Turkey extensional province” is proposed in lieu of the term “west Anatolian extensional province”.

Application of GIS to spatial analysis of mesofault populations

This paper presents a GIS approach to spatial analysis of fault populations. Coherence between fault moment tensors, a concept borrowed from the geophysical works of Kagan and Knopoff [1985a. Geophysical Journal of the Royal Astronomical Society 81(2), 429–444; 1985b. Geophysical Journal of the Royal Astronomical Society 83(3), 637–656], is the parameter used for quantifying the degree of similarity between faults. Fault datasets from various structural sites can be imported into a GIS software such as ArcView®; using a series of customized GIS tools, data can then be checked and their spatial dependence may be investigated. This customization may be used to investigate the distribution of coherence values based on fault types (i.e. normal, left-lateral, etc.) and/or on the distance between faults. The analysis of a natural dataset reveals that, depending on the movement type, faults can display different coherence distributions with increasing distance ranges, and in some cases also a cyclic behavior. These findings can help reconstruct the history and understand the kinematic significance of fault populations.

Reconstruction of Taconian and Acadian paleostress regimes in the Quebec and northern New Brunswick Appalachians

A numerical analysis of fault populations was applied in the Quebec and northern New Brunswick Appalachians to characterize paleostress fields attributed to the Taconian and Acadian orogenies. The first brittle deformation documented in the external domain of the Humber Zone is associated with the thin-skinned tectonics of the Taconian orogen. The Taconian brittle deformation, characterized by north–south to NE–SW reverse conjugated brittle faults, evolved under a pure shear compressional regime (vertical σ3 axes) with σ1 axes oriented ESE–WNW. The ENE–WSW dextral and NW-SE sinistral faults that are observed in the St.Lawrence Lowlands are also attributed to Taconian compression. The first brittle deformation recognized in the Dunnage Zone and the Gaspé Belt is attributed to an ESE–WNW compression during the Acadian Orogeny. In the Dunnage Zone of southern Quebec, the Acadian paleostress axes are similar to the directions of σ1 axes and to the pure shear compressional regime computed for the Silurian and Devonian rocks of the Gaspé Belt. In the Gaspé Peninsula, the Acadian stresses are oriented ESE–WNW and are mechanically compatible with transpressional regimes. Reverse brittle faults and pure shear stress are identified along regional and secondary sets of NE–SW brittle faults, whereas a strike-slip stress regime is reconstructed along major east–west faults. In the Humber Zone of the Quebec Appalachians, a second compression is identified. It is characterized by conjugate ENE–WSW dextral and NW–SE sinistral strike-slip faults that crosscut Taconian thrust faults. This late deformation event is attributed to a distal expression of the Acadian Orogeny.

Fault analysis and paleostress evolution in large strain regions: methodological and geological discussion of the southeastern Himalayan fold-and-thrust belt in Pakistan

We document the structure and kinematics of the southeastern part of the fold-and-thrust belt of the Pakistani Himalaya. Field analysis documents the importance of strike–slip faulting associated with folding. Accordingly, a transpression regime is inferred to be responsible for variable amounts of shortening, from fault block to fault block. The analysis of fault populations that affect the Mesozoic to early Miocene sediments allows distinguishing two paleostress tensor directions: a dominant NW–SE compression and a minor E–W compression are compatible with buckling around the N–S axis of the near-by Hazara-Kashmir syntaxis. From the lack of both systematic overprinting-relationships and spatial trend (the two tensors were obtained at different locations) we conclude that in each location any of these two shortening directions can dominate. The distribution of the paleostress tensors substantiates a transpressional regime due to far-field Himalayan compression and a lateral escape component of the allochthonous fold-and-thrust belt away from the growing Hazara-Kashmir anticline.

Changes in strain trajectories in three different types of plate tectonic boundary deduced from earthquake focal mechanisms

Principal strain orientations (minimum horizontal compression—ex and maximum horizontal compression—ey) were established at three different types of plate tectonic boundary: two transform faults, an oceanic ridge located on the Southeast Indian Ridge and a trench located close to the South Sandwich Archipelago. To establish the strain patterns in each zone, 104 earthquake focal mechanisms (centroid-moment tensor solutions for earthquakes with mb≥4; Harvard seismology data, CMT) were examined by fault population analysis. Despite the existence of only one tectonic process that controlled deformation in these zones (divergence, convergence or passive displacement), and only one main strain tensor, several coeval strain ellipsoids were found. These differed from the main strain tensor in the location of the principal strains. In general, permutations were observed between the principal strains, i.e., interchanges between the location of the principal strain axes maintaining the strain ellipsoids in the same 3D orientation. Only in some cases were changes in the ellipsoid orientation associated with major structures.

A kinematic model of the Scotia plate (SW Atlantic Ocean)

The focal mechanisms located around the Scotia plate were examined by fault population analysis to calculate both the trends of the maximum horizontal shortening on the plate and the orientations of its associated faults. The results show that both the northern and southern boundaries of the Scotia plate (the north and south Scotia ridges) are currently undergoing left-lateral movement. This movement induces a left-lateral shear couple in the whole plate, with a maximum horizontal NE–SW shortening direction. The established fault orientations and fault classes closely match the predicted structural pattern of a theoretical shear couple that includes the entire Scotia plate. The stress trajectories deduced from finite element modeling also agree with the proposed left-lateral shear couple model; the calculated stress trajectories are characterized by a homogeneous NE–SW compression direction over the whole Scotia plate. This shear model also explains the slip of recent tectonic structures found at the boundaries of the Scotia plate and suggests the possibility of reactivation of its former tectonic structures.

Cenozoic extension in coastal Dhofar (southern Oman): implications on the oblique rifting of the Gulf of Aden

A detailed analysis of fault populations that affect the Paleocene to early Miocene sedimentary succession of southern Dhofar allows the definition of a time-sequence of paleostresses and the reconstruction of the rifting history of the eastern Gulf of Aden. Two distinct phases of N20°- and N160°-trending extensions took place during the deposition of the synrift Dhofar Group from late Priabonian (35 Ma) to middle Burdigalian (18 Ma). A period of tectonic instability preceded these events, marked by normal and strike–slip faulting as early as the mid-Eocene. The two main directions of extension, also recorded in southern Yemen, developed respectively oblique and normal to the N75E trend of the Gulf of Aden. An anticlockwise succession of the two episodes from N20 to N160, only established in Yemen, accounts for the observed synrift fault pattern, which is consistent with the type and geometry of faults predicted in analogue models of oblique rifting. A latest phase of NS compression, marked by strike–slip faulting, may represent a far echo of the Arabia–Eurasia collision.

THE FE DEPOSIT, WEST-CENTRAL SPAIN: TECTONIC-HYDROTHERMAL URANIUM MINERALIZATION ASSOCIATED WITH TRANSPRESSIONAL FAULTING OF ALPINE AGE

Primary uranium deposits (uraninite + coffinite + carbonates + “adularia” + iron sulfides) of Mina Fe, in west–central Spain, occur in fault-related rocks. These rocks have been formed by the effect of an aftershock fault cluster at the termination of a major strike-slip fault, the Grafitosa fault. The transpressional shear-zone termination presents two symmetrical brittle dilational jogs with reactivation of old Variscan faults and generation of other new Alpine faults. Lithogeochemical analyses show that the hydrothermal flow associated with the Grafitosa fault mobilized uranium and related elements during pressure solution in carbonaceous slates of the Variscan basement. The orientation of the paleo-stress field in the dilational jogs, deduced from an analysis of fault populations, has shown the range of fault surfaces with favorable orientation for mineral deposition during slate consumption in the restraining bend, which agrees with a extensional setting for the ore deposit. The detailed structural and geochemical data presented here support an Alpine age tectonic-hydrothermal model for the Fe deposit and probably other cases of uranium mineralization of the same type existing in carbonaceous metasedimentary rocks in the Iberian peninsula, most of them uncorformably overlain by Tertiary sedimentary units.

Transverse fold evolution in the External Sierra, southern Pyrenees, Spain

Fault-slip data are used to reconstruct varying tectonic regimes associated with transverse fold development along the eastern and southern margins of the Jaca basin, southern Pyrenees, Spain. The Spanish Pyrenean foreland consists of thrust sheets and leading-edge décollement folds which developed within piggyback basins. Guara Formation limestones on the margins of the Jaca basin were deposited synchronously with deformation and are exposed in the External Sierra. Within the transverse folds, principal shortening axes determined from P and T dihedra plots of fault-slip data show a shift from steep shortening in stratigraphically older beds to NNE–SSW horizontal shortening in younger beds. Older strata are characterized by extensional faults interpreted to result from halotectonic (salt tectonics) deformation, whereas younger strata are characterized by contraction and strike-slip faults interpreted to result from thrust sheet emplacement. The interpretation of the timing for the shortening axes in the younger strata is supported by the observation that these axes are parallel to shortening axes determined from finite strain analysis, calcite twins, and regional thrusting directions determined from fault-related folds and slickenlines. This study shows that fault population analysis in syntectonic strata provides an opportunity to constrain kinematic evolution during orogeny.