Oblique-slip Faults Research Articles

Kinematics of the Danakil microplate

A refinement and extrapolation of recent motion estimates for the Danakil microplate, based on ancient kinematic indicators in the Afar region, describes the evolution of a microplate in the continental realm. The Danakil horst is an elevated part of this microplate, exposing a Precambrian basement within the Afar depression, the site of the Nubia–Somalia–Arabia triple junction. We compare evidence for strike- or oblique-slip faults in data from the Afar depression and southern Red Sea to small circles about published poles of rotation for the Danakil microplate with respect to Nubia. A reconstruction about the preferred pole reunites lengths of a Precambrian shear zone on the Nubia and Danakil sides and preserves a uniform basement fabric strike through Nubia, Danakil and Yemen. Since at least magnetic chron C5 (∼11 Ma) Danakil rotated about a different pole with respect to Nubia than either Somalia or Arabia, but between chrons C5 and C2A Nubia–Danakil motion was a close approximation to Nubia–Somalia motion. Since C2A relative motions of the Danakil microplate have been independent of movements on any of the neighbouring plate boundaries. We relate this to the onset of oceanic-type accretion within Afar. The resulting eastwards acceleration of Danakil was accommodated by westwards propagation of the Gulf of Aden rift that became the new, discrete, plate boundary between the Danakil microplate and the Somalia plate. Present-day activity suggests that the Red Sea and Aden rifts will link through Afar, thereby isolating the Danakil horst as a microcontinent on the Arabian margin.

Stress field variation related to fault interaction in a reverse oblique-slip fault: the Alhama de Murcia fault, Betic Cordillera, Spain

The interpretation of kinematic data from microstructural observations has led many authors to propose a complex dynamic development for the eastern Betic Cordillera from the Late Miocene to the present in which different—and sometimes contradictory—stress fields explain the generation of the same structures. The main aim of this work is to determine whether the dynamic variability of the study area, observed during the neotectonic period, is coherent with a single regional stress field. The structure of the area was analysed through the detailed mapping of the Alhama de Murcia fault (AMF) and the associated structures. Further, microtectonic data from shear veins in an interaction zone between the main and secondary faults were studied by the stress inversion method of Reches [Tectonics 6 (1987) 849]. A hierarchy is proposed for stress fields and their interpretation with respect to the interaction between structures of different scales. Relative movements of fault-bounded blocks with different sizes produce this hierarchy. These movements depend on boundary conditions of blocks and their position relative to the Alhama fault zone. The study shows that adequate cartography and understanding the context of micro- and mesotectonic data is necessary for determining the dynamic significance of microtectonic data. The use of stress inversion methods without taking the local tectonic context into consideration can give rise to incorrect interpretations, incorrect processing of previous data, and extrapolations inappropriate at other scales.

Lithosphere strength controls oceanic transform zone structure: insights from analogue models

Oceanic transform zones have often been regarded as plate boundaries. The origin of their structural variability is poorly constrained. A simple observation indicates that the transform zone is narrow and linear when the offset is large; while it is wide with a complex faulting pattern in the case of a small offset. On the other hand, for a given offset, large structural differences exist between transform zones located on the fast-spreading South-East Pacific Rise and on the slow-spreading Mid-Atlantic ridge. In general, the transform zones in slow-spreading environments are linear with a simple fault pattern, while in fast-spreading systems they are wide with a complex pattern of deformation. We perform small-scale analogue modelling to constrain the influence of lithospheric strength on the development of deformation above a transform boundary. The models are made up of sand and silicone putty as analogues of the brittle layer and the viscous layer of the lithosphere, respectively. Two plastic sheets coming from shifted gashes form a set-up of two diverging discontinuities connected by a transform boundary. The rheological layering and strength of the model were modified using different shapes of the viscous layer placed on the transform boundary. Above the divergent discontinuities, the faulting pattern is always formed by parallel normal faults. When no viscous layer is placed on the transform boundary (strong discontinuity), the deformed zone is narrow and has few linear faults. By adding a narrow and thin viscous layer, the deformed zone becomes wider with a complex faulting pattern formed by oblique-slip faults on the limits and by pure strike-slip faults in the inner part. These latter strike-slip faults trend oblique to the transform boundary. When a viscous layer with a wide lateral extent overlays the transform discontinuity (weak strength), the faulting is dominated by obliquely normal faults extending over a wide zone, and the strike-slip is restricted to the inner part of the deformed zone. Therefore, the mechanical strength of the small scale-model controls the shape of the deformed zone and the deformation partitioning. These results were applied to 24 oceanic transforms zones: we point out that the spreading rate and the transform offset are the two dominant parameters controlling the deformation pattern. These two factors directly control the lithospheric strength at the transform boundary. However, the distance to the nearest hotspot, which may generate warmer thermal conditions even in slow-spreading environments, should modify this result.

Evolution of the northern and western Dinarides: a tectonostratigraphic approach

The Dinarides are a Late Jurassic to recent, mainly SW-vergent fold and thrust belt that extends from the Southern Alps in the NW to the Albanides/Hellenides in the SE. Main tectonostratigraphic units of the Dinarides are related to: a) Early and Middle Triassic rifting: was marked by strong magmatism and horst to graben related deposition overlying Variscian basement. Rift-related magmatism migrated westward through the Early to Late Triassic. Deposition was fault controlled and sedimentation patterns demonstrate a strong differentiation between shelf, slope and basin lithologies. Along the "eastern" Apulian margin toward the Sava-Vardar ocean rifting was followed by subduction-generated extension during Early Triassic till Late Jurassic. Subduction-related attenuation of continental crust along the eastern margin of Apulia caused a generation of the back-arc basin type of the oceanic crust. The remnants of that active continental margin lithologies are found within the Eastern thrust belt as the ophiolite melange of the Central Dinarides Ophiolite Belt. b) Late Jurassic to present day compression that generated: -Eastern thrust belt, foredeep and foreland. Continental convergence was progressive from Late Jurassic and was first expressed on the eastern margin to the Dinarides as the Eastern thrust belt and its foredeep. During Early Cretaceous compressional stresses began to be transmitted westward through the Dinarides, causing the migration of the foredeep basin and regional uplift of the Eastern thrust belt. - Northern Dinarides accretionary wedge. Subduction of the oceanic plate along the northern margin of the Dinarides culminated from Maastrichtian to Eocene. That is evidenced by accretionary wedge deposits located by oil wells in the basement of the South Pannonian Basin, an east-west trending remnants of the magmatic arc and well exposed retroarc flysch-like deposits. - Western thrust belt, foredeep and foreland: At the end of the Cretaceous the entire carbonate platform was uplifted until the Early Eocene. During Eocene the Dinaridic carbonate platform was finally drowned under the flysch deposits in the broad foredeep basin of the Western thrust belt. -Eastern Adria imbricated structures: At the beginning of the Oligocene collision and progressive underthrusting of the Adria below the Dinarides created the imbricare structures of Adria provenience in front of the Western thrust belt. The structural style of the Dinaridic thrust belt depends on the polyphase tectonic compression and the competence of the sedimentary units involved. The decollements are Permian shales locally interbedded with gypsum, fine-grained Early/Mid Triassic clastics and Late Jurassic-Early Crataceous anhydrites. The competent carbonate rocks are the most responsible for the structural style of the thrust belt. The compression started with ramping along the deep decollement from the root zone with a southwestern tectonic transport. In this way, by progressive overstepping of the thrust faults the various structural forms were created along the Eastern and Western thrust belt such as fault bend folds, tear fault related folds and folded thrust structures reworked by footwall deformations. - Wrenching and tectonic inversion: NE-SW striking system of the dextral strike slip faults during Oligocene Miocene was followed by NW-SE striking wrenching in Early and Middle Miocene, affecting South Pannonian Basin. Western thrust belt and Adriatic foreland. It is reflected as tectonic inversion in Mid-Bosnian Schists Mts. and as the system of the right-lateral strike and oblique slip faults, creating the large Sava and Drava depressions of the South Pannonian Basin and narrow pull-apart basins and large flower structures of the Dinaridic thrust belt.

Geology and Geomorphology of the European Alps and the Southern Alps of New Zealand

The European Alps (Alps) and Southern Alps of New Zealand (Southern Alps) are both high mountain ranges formed by the collision of tectonic plates. The Alps resulted from collision of the African and European Plates, which produced complex lithological and structural patterns associated with the development of a series of overthrusted nappes. In contrast, the plate margin deformation that created the Southern Alps produced a relatively simple structural and lithological pattern dominated by a single right lateral oblique slip fault zone known as the Alpine Fault. Strong contrasts are also apparent in the contemporary rates of landscape development. The Alps currently experience modest rates of uplift and denudation because deformation along the plate boundary has slowed. High rates of compressional strain along the Alpine Fault in New Zealand result in very high rates of uplift. These processes and the position of the mountain range across the prevailing atmospheric westerly circulation system result in exceptionally high rates of denudation. Although there are strong contrasts in the lithology and structure of the Alps and Southern Alps, both experienced the growth and decay of expanded valley and piedmont glaciers during the Quaternary. The impact of multiple Quaternary ice advances has left a strong imprint on the landscapes. Both mountain ranges have particularly well-developed, over-deepened troughs and widespread glacial sediments and landforms, which heavily influence modern geomorphic processes and land use. Today numerous glaciers in both regions show strong reactions to global warming since the end of the Little Ice Age.

Tectonic evolution of the northeastern part of the African continental margin, Egypt

The area between Manzalah Lake and the southern Galala Plateau in northeast Egypt constitutes the Galalas, Cairo-Suez, southern Nile Delta and northern Nile Delta structural provinces. The northern Galala Fault separates the Galalas Province from the Cairo-Suez Province and is considered to be the westward extension of the Themed Fault in central Sinai. The pre-Eocene rocks are affected by northeast to east-northeast-orientated folds and reverse faults, as well as east-west-orientated oblique-slip faults with dextral and normal components. Some folds and reverse faults are interpreted to have been formed by northwest to north-northwest-orientated compression related to the Syrian Arc movement, whereas the others by the secondary northwest orientated shortening, which accompanied dextral strike-slip component along the planes of the east-west-orientated faults. The east-west-orientated faults were initially formed during the Late Triassic/Early Jurassic extension related to the drifting of the African/Arabian Plate away from the Eurasian Plate as a result of opening of the Neotethyan Sea. The Neotethyan began to close due to convergence between the two plates, leading to the Syrian Arc deformation. This deformation mildly started in Late Cenomanian and followed by a more intensive phase in Conacian/Santonian. It mildly continued in the Maastrichtian, Early Palæocene and Late Palæocene/Early Eocene. The southward thinning of the pre-Eocene rocks controlled the intensity and style of deformation. Two deformational mechanisms are proposed for the Nile Delta hinge zone. The first is related to Late Oligocene—Early Miocene north-northwest-orientated Alpine compression. The second is related to northward gravitational sliding of the post-Oligocene shale and sandstone over Cretaceous-Eocene carbonates.

Temporal changes in fault strike (to 90°) and extension directions during multiple episodes of extension: An example from eastern Nevada

In areas with prolonged extensional histories or multiple episodes of extension, temporally distinct extensional fault systems, including dip-slip-normal, oblique-slip, and/or strike-slip faults, may form. These fault systems may be spatially distinct, overlapped, or completely superimposed, and different fault systems may have different fault strikes and extension directions. Although fault strike is not a very sensitive indicator of principal stress directions, changes through time in fault strike of as much as 90° exceed the uncertainty. Overprinted extensional fault systems of different ages with significantly different (to 90°) fault strikes and extension directions provide strong evidence of changes in the regional stress field with time. Tens of millions of years of extension occurred in the Basin and Range Province of the United States and many temporally distinct fault sets formed through that time. We document at least four extensional episodes and the fault kinematics of each, where possible, in the central Hiko Range, eastern Nevada. The four Cenozoic extensional events are prevolcanic, synvolcanic, postvolcanic Miocene–Pliocene (?), and postvolcanic Pliocene (?)–Quaternary. The prevolcanic faults strike north-south and are interpreted as footwall faults to the regional Eocene–Oligocene Snake-Stampede extensional system, which caused approximately east-west extension. The synvolcanic faults are late Oligocene–Miocene age and strike east-west. Although they are rare and have minor offset, they accommodated north-south–directed extension. Postvolcanic faults, part of the Timpahute lineament, are approximately east-west– striking normal and northeast-striking oblique-slip faults. They also accommodated approximately north-south extension during Miocene time. The northeast-striking left-normal oblique-slip fault set formed as new faults and records a change between north-south and east-west extension in Miocene time. Later east-west extension occurred along the Pliocene (?) to Quaternary Hiko fault zone. These four overprinted extensional systems show two separate changes in normal fault strike of ∼90°, from north-south to east- west and from east-west back to north-south. Consequently, the extension direction similarly changed through time. We suggest that causes for the changes in direction of upper crustal extension include a change from stresses created by (1) plate interactions, which generated approximately east-west extension, to (2) mantle motions, which generated approximately north-south extension, and (3) back again. The extension related to mantle motions is associated with a band of volcanism that passed across the northern Basin and Range Province, moving generally from north to south, during the protracted period of extension. Therefore, a change from plate boundary– or interaction-derived stresses to mantle- or volcanism-related stresses and back again is possible.

Neotectonic and volcanic characteristics of the Karasu fault zone (Anatolia, Turkey): The transition zone between the Dead Sea transform and the East Anatolian fault zone

The Karasu Rift (Antakya province, SE Turkey) has developed between east-dipping, NNE-striking faults of the Karasu fault zone, which define the western margin of the rift and west-dipping, N–S to N20°–30°E-striking faults of Dead Sea Transform fault zone (DST) in the central part and eastern margin of the rift. The strand of the Karasu fault zone that bounds the basin from west forms a linkage zone between the DST and the East Anatolian fault zone (EAFZ). The greater vertical offset on the western margin faults relative to the eastern ones indicates asymmetrical evolution of the rift as implied by the higher escarpments and accumulation of extensive, thick alluvial fans on the western margins of the rift. The thickness of the Quaternary sedimentary fill is more than 465 m, with clastic sediments intercalated with basaltic lavas. The Quaternary alkali basaltic volcanism accompanied fluvial to lacustrine sedimentation between 1.57 ± 0.08 and 0.05 ± 0.03 Ma. The faults are left-lateral oblique-slip faults as indicated by left-stepping faulting patterns, slip-lineation data and left-laterally offset lava flows and stream channels along the Karasu fault zone. At Hacılar village, an offset lava flow, dated to 0.08 ± 0.06 Ma, indicates a rate of left-lateral oblique slip of approximately 4.1 mm·year –1. Overall, the Karasu Rift is an asymmetrical transtensional basin, which has developed between seismically active splays of the left-lateral DST and the left-lateral oblique-slip Karasu fault zone during the neotectonic period.

Geology of Sedimentary Rock-Hosted Disseminated Gold Deposits in Northwestern Sichuan, China

Sixteen sedimentary rock-hosted disseminated gold deposits (e.g., Dongbeizhai, Qiuluo, Manaoke, Laerma, Gala and Lianhecun) and about 50 prospects have been discovered in northwestern Sichuan Province, China since the late 1970s. They are hosted in complex rock series such as fine-grained clastic rocks, subvolcanic rocks, and ophiolitic melange, mainly of Triassic age. Mineral associates include pyrite, arsenopyrite, marcasite, realgar, orpiment, stibnite, cinnabar, quartz, cal-cite, and barite, and the elements gold, arsenic, antimony, mercury, thallium, and barium. In addition, scheelite, uranite, and bismuth minerals are present in some deposits. Submicron- and micron-sized gold is disseminated within black, carbonaceous, pelitic to sandy clastic and volcaniclastic rocks. Hydrothermal alteration includes fine-grained silica, calcite, dolomite, and siderite. Organic material occurs in most clastic host rocks and ores. Ore bodies are obviously controlled by fractures. Fluid inclusion data show that the ores were formed at temperatures ranging mostly from 150 to 250°C, at pressures less than 400 bars, and from fluids with salinities from 3 to 14 wt% NaCl and densities from 0.82 to 1.00 g/cm3. Isotopic data reveal that the ore-forming ages are from the middle-late Mesozoic to Tertiary and that the ore fluids vary from −117.8 to −52.9%c (average −83.6 + 16.5‰) in 5D value and from −11.1 to 16.2‰ (average 11.1 ± 9.2‰) in δ18O values. Sulfur isotopes from epigenetic sulfide change from −25.6 to 7.8‰, mostly between ‰6.6 and −1.2‰. In view of the element associations, sedimentary rock-hosted disseminated gold deposits in northwestern Sichuan can be divided into six subtypes as follows: Au, Au-As, Au-Sb, Au-Hg, Au-W, and Au-U. These deposits have similar features to those in Nevada in the western United States, including tectonic setting, mineral association, element association, gold occurrence, alteration (such as silicification and carbonatization), ore-forming temperature, metallogenic age, as well as spatial relationship with igneous rock bodies. But differences exist among them. Scheelite, uranite, and bismuth minerals are present only in some of the deposits of northwestern Sichuan. High-angle normal faults and regional oblique-slip (thrust) faults are responsible for the localization of mineralization in the Nevada and northwestern Sichuan deposits, respectively. Known northwestern Sichuan deposits are smaller than most United States analogues. In Nevada, the gold deposits occur chiefly in impure carbonate rock series of Paleozoic age, but in northwestern Sichuan they are hosted in complex rock series mainly of Triassic age. In the northwestern Sichuan deposits, decarbonatization and shallow acid-leaching alterations are much weaker than those in Nevada. Estimated pressures at the northwestern Sichuan deposits are lower than those for the Nevada deposits. The ore fluids in the northwestern Sichuan deposits have relatively higher salinities and slightly lower 8D values than those in the Nevada deposits. A possible model for the northwestern Sichuan deposits involves ore-forming components scavenged mainly from fine-grained clastic rocks by polygenetic fluids consisting of connate and meteoric water of moderate salinities. Regional geothermal anomalies were caused by tectonic and igneous activities during the Yanshanian intracontinental orogeny. The fluid, in which gold was transported mainly as Au (HS)-2, rose along deep fractures. At shallow depths, temperature and pressure decreased, the fluid was neutralized and oxidized, and ore was deposited.

Tectonics, fluid migration, and fluid pressure in a deformed forearc basin, Cook Inlet, Alaska

Faulting controls fluid migration within transpressive fault-propagation folds in the Cook Inlet forearc basin of south-central Alaska. Na-Ca-Cl brine migrates out of Mesozoic rocks through reverse and oblique-slip faults into the cores of anticlines, where the fluid spreads laterally outward into lower Tertiary strata by flow through cross faults and permeable beds. Precipitation of zeolite and carbonate cement and veins reduces the permeability of folded bedding and faults. Zeolite minerals are formed by chemical reactions between Na-Ca-Cl brine and sedimentary rocks. Carbonate minerals are precipitated when Na-HCO3 connate fluid in the Tertiary section reacts with rocks during diagenesis, and by mixing of migrated Na-Ca-Cl brine with the Na-HCO3 pore fluid. Carbonate cement is also precipitated by fluctuations in PCO2 during faulting and jointing. High fluid pressure is encountered while drilling through lower Tertiary and Mesozoic strata in some anticlines. High-pressure fluid is contained within porous beds that are intercalated with strata cemented by carbonate and zeolite minerals. Zeolite and carbonate cemented beds retard the dissipation of high fluid pressure, and channel fluid flow parallel to bedding within the anticlines. High fluid pressure may be generated by several processes, acting either alone or together. The evidence for fault-controlled migration of fluid out of the basement suggests that volumetric strain related to deformation is most important, but may be augmented by dynamo-thermal metamorphism, sedimentary compaction, alteration of organic-rich rock and hydrocarbons, and possibly glacial loading.

When Did the Western Anatolian Grabens Begin to Develop?

Abstract To solve a long-lasting controversy on the timing and mechanism of generation of the western Anatolian graben system, new data have been collected from a mapping project in western Anatolia, which reveal that initially north-south trending graben basins were formed under an east-west extensional regime during Early Miocene times. The extensional openings associated with approximately north-south trending oblique slip faults provided access for calc-alkaline, hybrid magmas to reach the surface. A north-south extensional regime began during Late Miocene time. During this period a major breakaway fault was formed. Part of the lower plate was uplifted and cropped out later in the Bozdağ, Horst, and above the upper plate approximately north-south trending cross-grabens were developed. Along these fault systems, alkaline basalt lavas were extruded. The north-south extension was interrupted at the end of Late Miocene or Early Pliocene times, as evidenced by a regional horizontal erosional surface which developed across Neogene rocks, including Upper Miocene-Lower Pliocene strata. This erosion nearly obliterated the previously formed topographic irregularities, including the Bozdağ elevation. Later, the erosional surface was disrupted and the structures which controlled development of the Lower-Upper Miocene rocks were cut by approximately east-west trending normal faults formed by rejuvenated north-south extension. This has led to development of the present-day east-west trending grabens during Plio-Quaternary time.

Potential seismic hazards and tectonics of the upper Cook Inlet basin, Alaska, based on analysis of Pliocene and younger deformation

The Cook Inlet basin is a northeast-trending forearc basin above the Aleutian subduction zone in southern Alaska. Folds in Cook Inlet are complex, discontinuous structures with variable shape and vergence that probably developed by right-transpressional deformation on oblique-slip faults extending downward into Mesozoic basement beneath the Tertiary basin. The most recent episode of deformation may have began as early as late Miocene time, but most of the deformation occurred after deposition of much of the Pliocene Sterling Formation. Deformation continued into Quaternary time, and many structures are probably still active. One structure, the Castle Mountain fault, has Holocene fault scarps, an adjacent anticline with flower structure, and historical seismicity. If other structures in Cook Inlet are active, blind faults coring fault-propagation folds may generate M w 6–7+ earthquakes. Dextral transpression of Cook Inlet appears to have been driven by coupling between the North American and Pacific plates along the Alaska-Aleutian subduction zone, and by lateral escape of the forearc to the southwest, due to collision and indentation of the Yakutat terrane 300 km to the east of the basin.

Active tectonics and morphostructure at the northern margin of central Bransfield Basin, Hurd Peninsula, Livingston Island (South Shetland Islands)

Geophysical, structural, geomorphological, topographical and bathymetric data from the Hurd Peninsula area, Livingston Island, South Shetland archipelago, suggest that an extensional fault system, orientated NW–SE, together with a conjugate group of NE–SW normal oblique-slip faults, control the landforms in this area. These structures separate fault-bounded blocks of different heights, giving rise to a horstgraben structure. The depressed blocks were filled by glaciers and flooded in part by the sea. The recent movement of these faults can be established from the calculated isopachs of a small Quaternary sedimentary basin, related to this extensional fault system, which shows that sedimentary bed thickness is controlled mainly by the NE–SW fault system. Geomorphological analysis also shows that the NW–SE faults control the main morphostructures of this region. The character of the recent stress tensor has been established from fault-slip data, taking into account only those faults that are related to morphostructures. The calculated palaeostress tensor is extensional, with a N46°E main extension direction, and an average stress ratio of 0.17.

An example of neotectonism in a continental interior — Thebes Gap, Midcontinent, United States

Some of the most intense neotectonic activity known in the continental interior of North America has been recently discovered on a fault zone in the Thebes Gap area, Missouri and Illinois. This faulting almost assuredly was accompanied by large earthquakes. The zone is located approximately 30 km north of the New Madrid seismic zone and consists of complex north-northeast- to northeast-striking, steeply dipping faults that have had a long-lived history of reactivation throughout most of the Phanerozoic. Geophysical studies by others suggest that the faults are rooted in the deeply buried Late Proterozoic and Early Cambrian Reelfoot rift system. Quaternary deposits are cut by at least four episodes of faulting, two of which occurred during the Holocene. The overall style of neotectonic deformation is interpreted as right-lateral strike-slip faulting. At many locations, however, near-surface displacements have stepped from one fault strand to another and produced normal and oblique-slip faults in areas of transtension and high-angle reverse faults, thrust faults, and folds in areas of transpression. There is evidence of reactivation of some near-surface fault segments during the great 1811–1812 New Madrid earthquakes. Quaternary faulting at Thebes Gap demonstrates that there are additional seismic-source zones in the Midcontinent, U.S., other than New Madrid, and that even in the absence of plate-margin orogenesis, intense neotectonic activity does occur over long time periods along crustal weaknesses in continental interiors.

Role of a ductile décollement in the development of pull-apart basins: Experimental results and natural examples

Traditional models describe pull-apart basins as graben or half-graben basins with normal or normal-oblique slip master faults, analogous to Death Valley, California. Yet many pull-aparts are characterized by asymmetric basins with strike-slip master faults indicating that not all pull-apart basins conform to the simple Death Valley models. We present analogue modelling results that show developmental sequences and structural styles of pull-aparts are dramatically different when overburden rides over a ductile horizon, and that thickness of the ductile horizon exerts control on basin development. In our models, synthetic and antithetic strike-slip faults control basin geometries, while localized normal faulting and local oblique slip on strike-slip faults accommodate basin subsidence. Faults evolve from initial strike-slip to normal-oblique and normal dip slip to form a system of either isolated sub-basins in the case of thick ductile layers, or coalescing sub-basins in the case of thin ductile layers. These results demonstrate distinct differences between non-ductile and ductile décollement pull apart structures. Basin boundaries dominated by normal faults suggest a décollement within or at the base of a non-ductile layer similar to Death Valley, California. Basin bounding faults dominated by strike-slip and oblique-slip faults indicate basin formation over a ductile layer, similar to the Gulf of Elat (Aqaba) or Gulf of Paria (Venezuela and Trinidad).

Structural styles of Paleozoic intracratonic fault reactivation: a case study of the Grays Point fault zone in southeastern Missouri, USA

A pronounced, subparallel set of northeast-striking faults occurs in southeastern Missouri, but little is known about these faults because of poor exposure. The Commerce fault system is the southernmost exposed fault system in this set and has an ancestry related to Reelfoot rift extension. Recent published work indicates that this fault system has a long history of reactivation. The northeast-striking Grays Point fault zone is a segment of the Commerce fault system and is well exposed along the southeast rim of an inactive quarry. Our mapping shows that the Grays Point fault zone also has a complex history of polyphase reactivation, involving three periods of Paleozoic reactivation that occurred in Late Ordovician, Devonian, and post-Mississippian. Each period is characterized by divergent, right-lateral oblique-slip faulting. Petrographic examination of sidewall rip-out clasts in calcite-filled faults associated with the Grays Point fault zone supports a minimum of three periods of right-lateral oblique-slip. The reported observations imply that a genetic link exists between intracratonic fault reactivation and strain produced by Paleozoic orogenies affecting the eastern margin of Laurentia (North America). Interpretations of this link indicate that right-lateral oblique-slip has occurred on all of the northeast-striking faults in southeastern Missouri as a result of strain influenced by the convergence directions of the different Paleozoic orogenies.

4D evolution of segmented strike-slip fault systems: applications to NW Europe

Scaled, physical sandbox modelling has been used to simulate the progressive deformation of segmented strike-slip fault systems at both releasing (pull-apart basins) and restraining oversteps (‘pop-up’ structures) developed in a weak sedimentary cover above rigid basement. Pull-apart basin evolution is characterized by the initial development of a spindle graben bounded by oblique-extensional faults that cross the ‘basement’ overstep, and subsequent lengthening of the basin as displacement on the master faults increases. The pull-apart basins are bounded by terraced oblique slip extensional sidewall fault systems that link the laterally offset principal displacement zones (PDZ). The sidewall faults show changes in kinematics from dominantly dip-slip extension in their central sections to oblique slip and strike-slip at either end where they merge with the PDZ. The pull-apart basin models are flat-bottomed in the centre of the basin and become asymmetric at either end where the sidewall fault systems join the PDZ. Cross-basin fault systems, which are characteristic of all models, cut the floor of the pull-apart basins and link the offset PDZs. Deformation above restraining offsets is characterized by the formation of rhombic to lozenge-shaped uplifts or ‘pop-ups’ – structures bounded by oblique-slip reverse faults that fan outwards from the PDZ. With increased displacement on the master faults the rhombic-uplifts grow in amplitude. The upper surfaces of the ‘pop-up’ structures are dissected by both synthetic and antithetic strike- and oblique-slip faults. Distinctive asymmetric positive flower structures characterize the lateral margins of the ‘pop-up’ structures. The analogue models are compared with natural examples of strike-slip pull-apart and ‘pop-up’ fault systems from NW Europe. The models show many strong similarities in structural geometry and stratal architectures with many natural examples, although it must be emphasized that similarities between structures of purely extensional origin (multiphase) and those associated with inversion do exist. Therefore, these analogue models of complex strike-slip structures may provide useful templates for seismic interpretation in petroleum basins provided there is a demonstrable link between strike-slip motion on basement structures and deformation in the cover.

Recent vertical offset and near‐surface structure of the Alpine Fault in Westland, New Zealand, from ground penetrating radar profiling

The Alpine Fault in central Westland is an oblique slip fault with a predominantly strike‐slip component of 25–35 mm/yr. The smaller vertical component has been variously estimated at between 5.5 and 14 mm/yr. Ground penetrating radar and level profiles of the Alpine Fault beside the Toaroha River in Westland indicate that there has been vertical movement of 21.75 ± 0.5 m of a fluvial surface dated at between 2420 and 3150 yr. This yields an average vertical movement of 7.8 ± 1 mm/yr, and is consistent with an estimate derived from regional studies at Paringa, which excludes the local effects of tilting. It is a little higher than uplift rates of 5.5 mm/yr previously estimated by Bull and Cooper, based on the current elevation of inferred uplifted Pleistocene marine terraces from the same general area, but is of the same order as their estimate.

A structural transect across the coastal mobile belt in the Brazilian Highlands (latitude 20°S): the roots of a Precambrian transpressional orogen

We present results of a detailed structural analysis from a 250 km-long, east–west-trending transect crossing the Coastal Mobile Belt, a part of the Precambrian orogen which lies between the eastern edge of the São Francisco craton and the Atlantic coast. The region exposes amphibolite–granulite grade metamorphic rocks and migmatites which formed at mid-lower crustal depths during the Brasiliano orogeny (0.63–0.52 Ga). This event marked the closure of the northernmost Adamastor Ocean and collision between the São Francisco and Congo Cratons during West Gondwana assembly. Brasiliano deformation resulted in W-vergent structures including thrust faults which accommodated kilometer-scale transport of crystalline basement, overturned kilometer-scale folds, sheath folds and penetratively developed gneissosity and schistosity. Isolated relics of an older folded fabric occur locally and may represent Transamazonian (2.2–2.0 Ga) deformation. The orogen is kinematically partitioned with the eastern 175 km dominated by moderately- to steeply dipping north-trending dextral strike-slip and oblique-slip faults and associated flower structures, whereas the western 75 km is dominated by W-vergent shallowly- to moderately east-dipping thrust faults. The boundary between these two provinces may mark a Brasiliano suture. Throughout the transect, quartzite and metasedimentary belts form strongly deformed zones between massive crystalline basement thrust sheets. The granulite-cored Serra do Caparaù massif, the highest mountains in South America outside of the Andes and Guyana shield, occupies a restraining bend between two Brasiliano dextral shear zones. The W-vergent Coastal Mobile Belt formed contiguously with the E-vergent Pan-African West Congo orogen now exposed along the conjugate margin of Africa. Thus an important late Precambrian boundary between structurally linked but kinematically opposed structural provinces must lie hidden in the extended offshore continental margins of either continent. Cretaceous opening of the South Atlantic and separation of the West Congo belt from the Coastal Mobile Belt may have been structurally influenced by this boundary.

Escape tectonics in the Los Angeles metropolitan region and implications for seismic risk

Recent damaging earthquakes in California, including the 1971 San Fernando1, 1983 Coalinga2, 1987 Whittier Narrows3 and 1994 Northridge4 events, have drawn attention to thrust faults as both potentially hazardous seismic sources and as a mechanism for accommodating shortening in many regions of southern California. Consequently, many geological studies5,6 have concluded that thrust faults in Southern California pose the greatest seismic hazard, and also account for most of the estimated 5–7 mm yr−1 of contraction across the greater Los Angeles metropolitan area7,8 indicated by Global Positioning System geodetic measurements9. Our study demonstrates, however, that less than 50% of the geodetically observed contraction is accommodated on the principal thrust systems across the Los Angeles region. We integrate the most recent geological, geodetic and seismological data to assess the spatial distribution of strain across the Los Angeles metropolitan region. We then demonstrate that a significant component of seismic moment release and shortening in this region is accommodated by east–west crustal escape ‘extrusion’ along known strike-slip and oblique-slip faults.