Pure Strike-slip Research Articles

Focal mechanisms of shallow earthquakes in the Aegean Sea and the surrounding lands determined by waveform modelling: a new database

Source parameters for 40 earthquakes with epicentres in the Aegean Sea and the surrounding lands have been determined using P and SH body waveform inversion of digital waveforms from the Global Seismograph Network (GSN). These results are combined with 145 previous solutions, 41 of which were determined by first motion polarities and 104 by waveform modelling. Thus, a new updated database of source parameters for earthquakes for the period 1953–1999 is available. These fault plane solutions confirm our previous knowledge about the seismotectonics of the Aegean Sea and the surrounding lands. They also provide strong evidence that the western part of the Peloponnese (Gulf of Patras to Gulf of Kyparissia) is mainly deforming by strike-slip faulting, parallel in strike to the Cephalonia Fault and are in favour of the models that suggest a connection of the North Anatolian Fault (NAF) zone with the transform zone of the Ionian Islands. The sedimentary arc (inner part of the Hellenic trench) is deforming by normal faulting with the T-axes trending ∼E–W. We did not determine any fault plane solutions at the eastern part of the Hellenic arc indicating pure strike-slip motions as expected from the presence of the Pliny and Strabo trenches. However, we did determine solutions that indicate normal faulting combined with a considerable strike-slip component. In the central Aegean Sea the newly determined focal mechanisms support the conclusion that the area, extending from the eastern coasts of Evia up to the coasts of Turkey, is exhibiting a clear strike-slip character. Thus, the effect of the propagating tip of the North Anatolian fault into the Aegean is very clearly pronounced. The focal mechanisms in the back-arc Aegean area confirm the existence of normal faulting with T-axes trending NNE–SSW in western Anatolia and NNW–SSE in Greece. A zone showing NNE–SSW extension that runs parallel to Northern Aegean branch of the NAF zone up to the Corinth Gulf is depicted from the mechanisms of few earthquakes.

Geochemistry, mineralization, structure, and permeability of a normal-fault zone, Casino mine, Alligator Ridge district, north central Nevada

We examine the geochemical signature and structure of the Keno fault zone to test its impact on the flow of ore-mineralizing fluids, and use the mined exposures to evaluate structures and processes associated with normal fault development. The fault is a moderately dipping normal-fault zone in siltstone and silty limestone with 55–100 m of dip-slip displacement in north-central Nevada. Across-strike exposures up to 180 m long, 65 m of down-dip exposure and 350 m of along-strike exposure allow us to determine how faults, fractures, and fluids interact within mixed-lithology carbonate-dominated sedimentary rocks. The fault changes character along strike from a single clay-rich slip plane 10–20 mm thick at the northern exposure to numerous hydrocarbon-bearing, calcite-filled, nearly vertical slip planes in a zone 15 m wide at the southern exposure. The hanging wall and footwall are intensely fractured but fracture densities do not vary markedly with distance from the fault. Fault slip varies from pure dip-slip to nearly pure strike-slip, which suggests that either slip orientations may vary on faults in single slip events, or stress variations over the history of the fault caused slip vector variations. Whole-rock major, minor, and trace element analyses indicate that Au, Sb, and As are in general associated with the fault zone, suggesting that Au- and silica-bearing fluids migrated along the fault to replace carbonate in the footwall and adjacent hanging wall rocks. Subsequent fault slip was associated with barite and calcite and hydrocarbon-bearing fluids deposited at the southern end of the fault. No correlation exists at the meter or tens of meter scale between mineralization patterns and fracture density. We suggest that the fault was a combined conduit-barrier system in which the fault provides a critical connection between the fluid sources and fractures that formed before and during faulting. During the waning stages of deposit formation, the fault behaved as a localized conduit to hydrocarbon-bearing calcite veins. The results of this study show that fault-zone character may change dramatically over short, deposit- or reservoir-scale distances. The presence of damage zones may not be well correlated at the fine scale with geochemically defined regions of the fault, even though a gross spatial correlation may exist.

Late Cenozoic geomorphic and tectonic evolution of the Patagonian Andes between latitudes 42°S and 46°S: An appraisal based on fission-track results from the transpressional intra-arc Liquiñe-Ofqui fault zone

Abstract Fission-track (FT) thermochronology has been applied to investigate the low-temperature cooling and denudation history of the Patagonian Andes along the southern part of the intra-arc transpressional Liquiñe-Ofqui fault zone between 42° and 46°S. The Liquiñe-Ofqui fault is shown to have been the focus of enhanced cooling and denudation initiated between ca. 16 and 10 Ma. Several fault blocks with different cooling histories are identified; these are separated by major oblique- or reverse-slip faults proposed to form the eastern part of a major (crustal-scale) dextral transpression zone. Local very fast rates of cooling and denudation between ca. 7 and 2 Ma were coeval with collision of the Chile Rise (an active mid-oceanic ridge) with the Peru-Chile Trench between ∼47° and 48°S. This location is close to the southern termination of the Liquiñe-Ofqui fault, implying that the collision of the ridge was a major force driving late Cenozoic transpression. The lack of significant cooling and denudation before ca. 16 Ma is indicative of pure strike-slip or transtensional movement along the Liquiñe-Ofqui fault before the collision of the ridge. Digital landscape analysis supports glacial and periglacial erosion as the main contributor to denudation since ca. 7 Ma, leading to restriction of topographic development. The combination of transpression-induced rock uplift and glacial erosion is shown to be very effective at causing localized denudation. Anomalously young FT ages along the Liquiñe-Ofqui fault are attributed to the existence of a late Cenozoic localized heat-flow anomaly along the fault.

The 1999 Hector Mine Earthquake, Southern California: Vector Near-Field Displacements from ERS InSAR

Two components of fault slip are uniquely determined from two line-of-sight (LOS) radar interferograms by assuming that the fault-normal component of displacement is zero. We use this approach with ascending and descending interferograms from the ERS satellites to estimate surface slip along the Hector Mine earthquake rupture. The LOS displacement is determined by visually counting fringes to within 1 km of the outboard ruptures. These LOS estimates and uncertainties are then transformed into strike- and dip-slip estimates and uncertainties; the transformation is singular for a N-S oriented fault and optimal for an E-W oriented fault. In contrast to our previous strike-slip estimates, which were based only on a descending interferogram, we now find good agreement with the geological measurements, except at the ends of the rupture. The ascending interferogram reveals significant west-sidedown dip-slip (approximately 1.0 m) which reduces the strike-slip estimates by 1 to 2 m, especially along the northern half of the rupture. A spike in the strike-slip displacement of 6 m is observed in central part of the rupture. This large offset is confirmed by subpixel cross correlation of features in the before and after amplitude images. In addition to strike slip and dip slip, we identify uplift and subsidence along the fault, related to the restraining and releasing bends in the fault trace, respectively. Our main conclusion is that at least two look directions are required for accurate estimates of surface slip even along a pure strike-slip fault. Models and results based only on a single look direction could have major errors. Our new estimates of strike slip and dip slip along the rupture provide a boundary condition for dislocation modeling. A simple model, which has uniform slip to a depth of 12 km, shows good agreement with the observed ascending and descending interferograms.

Active tectonics of the South Caspian Basin

We use observations of surface faulting, well-constrained earthquake focal mechanisms and centroid depths, and velocity structure determined by surface wave propagation and teleseismic receiver functions to investigate the present-day deformation and kinematics in and around the South Caspian Basin. The lack of earthquakes within the basin itself indicates that it behaves as a rigid block, though its sedimentary cover is deformed by numerous folds that are decoupled from its rigid basement by overpressured mud. The basin contains a sedimentary sequence almost 20 km thick above a relatively high-velocity basement that is thinner within the basin than on its margins. The basement beneath the basin could be either unusually thick oceanic crust or thinned, but relatively high-velocity, continental crust. The South Caspian Basin is surrounded by active earthquake belts on all sides. No earthquakes deeper than 30 km can be confirmed in the Kopeh Dag, Alborz and Talesh, which bound the NE, S and W sides of the basin. In contrast, earthquakes occur to depths of at least 80 km on the Apsheron–Balkhan sill, which bounds the N side of the basin and where no earthquakes can be confirmed that are shallower than 30 km. We interpret these deeper earthquakes to indicate the onset of subduction of the South Caspian Basin beneath the central Caspian, a process that appears to occur aseismically at shallow levels. Although oblique shortening is partitioned into pure strike-slip and pure thrust in many areas, conjugate right-lateral and left-lateral components in the Kopeh Dag and eastern Alborz suggest that the South Caspian Basin has a westward component of motion relative to both Eurasia and Iran. This motion enhances westward underthrusting of the basin beneath the Talesh mountains of Iran and Azerbaijan. We estimate the present motions of the South Caspian Basin to be ∼ 13–17 mm yr−1 to the SW relative to Iran (a maximum value) and ∼ 8–10 mm yr−1 to the NW or NNW relative to Eurasia. We suspect that these motions are all relatively recent, and may have begun only in the Pliocene (3–5 Ma). The South Caspian Basin will ultimately be destroyed by subduction or underthrusting and its present situation may represent an intermediate stage between that of the eastern Mediterranean and that of the seismically active slab beneath the Hindu Kush.

Active tectonics of the South Caspian Basin

We use observations of surface faulting, well-constrained earthquake focal mechanisms and centroid depths, and velocity structure determined by surface wave propagation and teleseismic receiver functions to investigate the present-day deformation and kinematics in and around the South Caspian Basin. The lack of earthquakes within the basin itself indicates that it behaves as a rigid block, though its sedimentary cover is deformed by numerous folds that are decoupled from its rigid basement by overpressured mud. The basin contains a sedimentary sequence almost 20 km thick above a relatively high-velocity basement that is thinner within the basin than on its margins. The basement beneath the basin could be either unusually thick oceanic crust or thinned, but relatively high-velocity, continental crust. The South Caspian Basin is surrounded by active earthquake belts on all sides. No earthquakes deeper than 30 km can be confirmed in the Kopeh Dag, Alborz and Talesh, which bound the NE, S and W sides of the basin. In contrast, earthquakes occur to depths of at least 80 km on the Apsheron-Balkhan sill, which bounds the N side of the basin and where no earthquakes can be confirmed that are shallower than 30 km. We interpret these deeper earthquakes to indicate the onset of subduction of the South Caspian Basin beneath the central Caspian, a process that appears to occur aseismically at shallow levels. Although oblique shortening is partitioned into pure strike-slip and pure thrust in many areas, conjugate right-lateral and left-lateral components in the Kopeh Dag and eastern Alborz suggest that the South Caspian Basin has a westward component of motion relative to both Eurasia and Iran. This motion enhances westward underthrusting of the basin beneath the Talesh mountains of Iran and Azerbaijan. We estimate the present motions of the South Caspian Basin to be ∼13–17 mm yr−1 to the SW relative to Iran (a maximum value) and ∼8–10 mm yr−1 to the NW or NNW relative to Eurasia. We suspect that these motions are all relatively recent, and may have begun only in the Pliocene (3–5 Ma). The South Caspian Basin will ultimately be destroyed by subduction or underthrusting and its present situation may represent an intermediate stage between that of the eastern Mediterranean and that of the seismically active slab beneath the Hindu Kush.

Newly identified segments of the Pacific–Australia plate boundary along the North Fiji transform zone

The North Fiji transform zone, a 1500 km long and 200 km wide transform segment of the Pacific–Australia plate boundary, is one of the major transform fault systems of the Earth. New data collected during the ALAUFI cruise (March 2000) on board the R/V L’Atalante make it possible to define more accurately the geometry and kinematics of this transform plate boundary. Three spreading centers or extensional zones (the North Cikobia spreading center, the Futuna spreading center and the southeast Futuna volcanic zone) and a strike-slip fault zone (the Futuna transform fault) have been discovered over a distance of 500 km along the eastern North Fiji transform zone, from the north of the Fiji platform to the east of the Futuna archipelago. The Futuna transform fault oriented 100° has been mapped over a distance of 250 km. It must be considered to be an important tectonic element of the transform plate boundary. Pure strike-slip as well as transpression and transtension motions are responsible for the complex morphology of this feature. The uplifted Futuna–Alofi ridge represents a major compressional relay along the Futuna transform fault. The Futuna spreading center trending 20–30° is composed of a series of en échelon left-stepping spreading segments. It represents a 200 km long extensional relay between the Futuna transform fault and the western part of the North Fiji transform zone, the Fiji transform fault, which bounds the Fiji platform to the north. The opening rate at the Futuna spreading center is estimated at 4 cm/yr. Although the North Cikobia spreading center and the southeast Futuna volcanic zone have been only partly mapped, bathymetric and reflectivity data clearly reveal that active extension also takes place along these two features. A spreading rate of 2 cm/yr is inferred at the North Cikobia spreading center. Therefore, the North Fiji transform zone appears to be composed of two main overlapping transform segments relayed by parallel extensional zones. The three active extensional zones have an ENE–WSW to NNE–SSW orientation, while compressive features along the Futuna transform fault are NW–SE to NNW–SSE oriented, in accordance with the present-day left-lateral transform motion along this part of the Pacific–Australia plate boundary.

Concentration of shearing deformation related to changes in strike of monoclinal fold axes: the Waterpocket monocline, Utah

Detailed field investigation of the Waterpocket monocline within the Utah portion of the Colorado Plateau reveals a strong correlation between abrupt changes in the trend of the monoclinal fold axis and enhanced fault development within the Navajo Sandstone. Faults within the bends of the fold axis range in scale from individual deformation bands to large, complex fault zones visible on 1:40,000 scale aerial photographs. Slip vectors on faults within the bends examined range from nearly pure strike-slip to subequal amounts of strike-slip and dip-slip offset. The spatial density and size of individual faults decrease rapidly away from the fold axis bends. We interpret the abrupt variations in fold axis orientations to result from segmentation of the basement faults underlying the monocline or similarly oriented bends in the basement faults. In this regard, the structures observed within the Navajo Sandstone are transfer structures developed above relay zones of the basement fault segments. The exact nature of the deformation at a given bend depends upon the geometry and kinematic history of the fault segments at depth. Variations in the proportion of strike-slip and dip-slip offset on these faults suggest that variables such as the orientation of the bend with respect to the direction of maximum compressive stress during the Laramide and details of the geometry of adjacent basement fault segments may form important controls on the nature and distribution of secondary structures in the folded strata. The results of this study highlight the need to consider along-strike variations in fold geometry in addition to considering the effects of cross-sectional fold shape when assessing fold-related fracturing.

Neogene wrench reactivation of the Barcoo Sub-basin, northwest Australia: implications for Neogene tectonics of the northern Australian margin

The Barcoo Sub-basin forms the southern part of the Browse Basin of Australia’s northwestern margin. Miocene reactivation of older Precambrian through Mesozoic-aged structures has resulted in a present-day complex right-lateral wrench zone. The fault system displays restraining and releasing bends separated by areas of almost pure strike-slip displacement, along its 180 km length. Reactivation and inversion varies along the system, with the southern end structurally highest, offsetting beds up to and including the seafloor. The location of restraining and releasing bends controls the occurrence of oil and gas shows in nine wells drilled in the sub-basin, with Neogene inversion causing trap leakage in a number of cases. Right-lateral motion on the Barcoo Fault system, in contrast to a proposed regional-scale left-lateral movement component on the northwestern margin as a whole, implies strong strain partitioning along the margin with regional tectonic control. We propose that the structural domain boundaries may be controlled in part by the location of the continental collision zone between the northernmost limit of Australian continental crust with the southernmost continental fragments of the Eurasian plate, in the vicinity of the island of Sumba.

Faulting Processes of Historic (1917–1962) M≥ 6.0 Earthquakes Along the North-central Caribbean Margin

—The plate boundary along the north-central Caribbean margin is geologically complex. Our understanding of this complexity is hampered by the fact that plate motions are relatively slow (1 to 2 cm/yr), so that recent seismicity often does not provide a complete picture of tectonic deformation. Studies of the faulting processes of instrumentally recorded earthquakes occurring prior to 1962 thus provide important information regarding the nature and rate of seismic deformation within the region, and are essential for a comprehensive assessment of seismic hazard. We have conducted body waveform modeling studies of eight earthquakes which occurred along the north-central Caribbean plate margin, extending from southeastern Cuba to the Swan Island fracture zone (75 to 83°W). None of these earthquakes has been previously studied and several occurred in regions where no recent (post-1962) seismicity has been recorded. The plate margin in the western portion of our study area is characterized by a transform fault-spreading center system. In the central and eastern portions of our study area the plate margin is a complex, diffuse region of deformation that couples transform motion in the Cayman trough to subduction along the Lesser Antilles arc. Our results show that the western portion of the study area has only experienced large strike-slip earthquakes. Off southeastern Cuba two earthquakes appear to have occurred on high angle, northward dipping, reverse faults with south to southeastward directed slip vectors. An earthquake in northern Jamaica in 1957 shows pure strike-slip faulting, most likely along an east-west trending fault. Finally, an unusual sequence of events located in the Pedro Bank region ∼70 km southwest of Jamaica has a mainshock with a reverse-oblique mechanism, suggesting continuity of the plate interface stress field well south of the northern Caribbean margin.

Basement Reactivation and Control of Neogene Structures in the Outer Browse Basin, North West Shelf

Basement control on reactivation along the North West Shelf has been broadly postulated, but direct links with Neogene events have yet to be substantiated. One reason for this is that styles of Neogene deformation vary around the North West Shelf, and have been found to be tightly geographically constrained to certain areas, such as the Barcoo Sub-basin. In the Barcoo Sub-basin of the Browse Basin structures initiated during Late Permian to Early Triassic extension have strongly influenced patterns of subsequent Neogene deformation. Recent structural and stratigraphic analyses of Cenozoic and Mesozoic horizons in the Barcoo Sub-basin have revealed the strong influence of deep structures upon the location and nature of structures within the Miocene section. In particular NE-SW extensional faults have been reactivated at several times since the early Triassic, with the subsequent formation of inversion structures such as open folds developed in Triassic half graben growth sequences. Reactivation of these Early Triassic extensional faults continued during the Cretaceous and Cenozoic. Minor fault movement is recorded in the Cenomanian-Turonian, whilst more pronounced faulting occurred in the Middle Oligocene and Middle to Late Miocene. The latter was associated with pure strike-slip and transpressional reactivation of NE-SW faults and formation of extensive inversion structures such as the Barcoo and Lombardina-Lynher structures. Other effects of continued motion on these faults can be seen from the location of drape anticlines in the Cretaceous section and the preferential location of shallow marine reefs in the Miocene section directly over the edge of Triassic horsts and crests of footwall blocks.

Quantifying the timing and sense of fault dip slip: New application of biostratigraphy and geohistory analysis

The timing and sense of the dip-slip component of a fault9s motion can be quantified by using geohistory analysis. Geohistory analysis traditionally is used to quantify tectonic subsidence or uplift of marine basins, but is newly applied to evaluate phases of activity of individual faults, including blind faults. Intermittent fault activity and changing sense of dip slip emerge, particularly for faults reactivated under changing kinematic regimes. Through the use of outcrop or core data from two fault blocks on either side of a major fault, the vertical component of displacement can be isolated. Steps of geohistory analysis include generation of (1) a rock-accumulation curve consisting of stratal thicknesses as a function of age, (2) a decompacted sediment-accumulation curve as a function of age, and (3) a total-subsidence curve with paleobathymetry inferred from fossil assemblages and sedimentary structures. Because errors on the inferred water depths are considered to be 20% of the water depth, the method is limited to large-scale faults. The analysis is repeated on both sides of the fault to determine which block subsided or was uplifted faster, thus isolating both the sense of dip slip and its relative magnitude. If both blocks display parallel subsidence curves within the error margins, a period of fault inactivity or pure strike slip is inferred. If the sense of dip slip changed, the subsidence curves cross. If the dip direction of the fault plane is interpreted from field or subsurface data, a change—for example, from an extensional component to a contractional component—also emerges, and the timing of basin inversion is detectable. As a case study, fault activity was evaluated for a mosaic of faults cutting the Messaras basin of Crete, Greece, a Neogene forearc basin that underwent extension followed by transpression. This approach is applicable to neotectonics or ancient tectonics and pertains to subsurface or exhumed faults as well as active faults offshore that generate contrasting water depths across a fault zone.

The 11 December, 1995 earthquake (Mw=6.4): Implications for the present‐day relative motion on the Rivera‐Cocos Plate Boundary

The 11 December, 1995 earthquake is the largest and best constrained instrumentally recorded event which has occurred on the Rivera‐Cocos plate boundary. The reported focal mechanism for this event indicates almost pure strike‐slip faulting with nodal planes oriented north‐south and east‐west. A visual inspection shows that the seismograms recorded world‐wide strongly suggest a directivity effect indicative of a rupture propagating eastward from the epicenter. This observation is confirmed by a directivity analysis which shows a sharp reduction in the difference between observed and synthetic seismograms when the rupture direction is at an azimuth of approximately 90°. These results indicate that the east‐west trending nodal plane of the earthquake of 11 December, 1995 is the actual fault plane. Considering that this is the largest instrumentally recorded earthquake in the region, it strongly suggests that the relative motion of the Rivera plate with respect to the Cocos plate takes place along east‐west oriented faults, and that the sense of motion is right‐lateral, strike‐slip.

Recent stress field of the Sinai subplate region

Quaternary and recent tectonic stress data for the Sinai subplate region have been compiled including focal mechanism solutions, in-situ stress measurements, fault slip data and alignments of young volcanic feeders. The average direction of maximum horizontal stress ( S H) has been determined by means of a linear interpolation method. The results indicate that the direction of the average maximum horizontal stress is closely parallel to the direction of the absolute motion of Africa. Analysis of the orientation of principal stress directions suggests that a large part of the Sinai subplate region is subjected to a strike-slip regime characterized by an average maximum horizontal stress trend of NW (54°W). For example, out of 50 earthquake focal mechanism solutions 52% are pure strike-slip, and 10% are either normal or thrust fault with a remarkable strike-slip component. The remaining are pure normal fault (18%), thrust fault (12%), or undetermined (8%). Three different tectonic subprovinces can be delineated. Northwestern Egypt, the eastern Mediterranean, Israel, Lebanon, and northern Syria are characterized by strike-slip often with a remarkable thrust faulting component stress regime. The northern Red Sea, the Sinai peninsula, and a significant part of the Arabian plate are characterized by strike-slip often with a remarkable normal faulting component stress regime. This is also the case for southwestern Turkey.

Analogue models of obliquely convergent continental plate boundaries

Analogue models are used to examine crustal‐scale faulting at obliquely convergent continental plate boundaries. A uniform Coulomb material is deformed with basal kinematic boundary conditions to model two obliquely convergent lithospheric plates. The mantle part of one plate is assumed to detach from its overriding crust and then be subducted beneath the other plate. The obliquity of the collision is assumed to remain constant throughout the deformation. Experiments are run with obliquities ranging from pure convergence (low obliquity) to pure strike slip (high obliquity). Reverse faults are observed for all obliquities with a nonzero convergent component. By contrast, only collisions with a large amount of strike slip motion exhibit wrench faulting. In experiments dominated by their convergent component, the strike slip motion is totally accommodated by oblique slip along the reverse faults. Strain partitioning between reverse faults and wrench faults is only observed for experiments run above a certain critical partitioning obliquity. Prom the observed initial faults, we can deduce the change in orientation in the principal stress triad as the obliquity is changed. We propose that the initial direction of maximum compressive stress (σ1) rotates horizontally as the obliquity is changed, which in turn affects the geometry of the initial faults formed in the material. In the case of reverse faults, the rotation increases their dip measured along the direction of pure convergence. The relative magnitude of the minimum horizontal stress and the vertical stress determine whether reverse faults or strike slip faults are the first to form. Although long term deformation is more difficult to analyze, a simple relationship for the angle at which strain partitioning occurs is derived.

Slip along the Hayward fault, California, estimated from space-based synthetic aperture radar interferometry

For 3–5 years following the 1989 M 7.1 Loma Prieta earthquake, creep along the southern Hayward fault, California, slowed or ceased. Slip apparently resumed pre-earthquake rates by 1994 except for a locked ∼3-km-long segment at the southern fault tip, which had consistently slipped at ∼9 mm/yr before 1989. We use repeated interferometric synthetic aperture radar (IntSAR) measurements to map active deformation along the Hayward fault while slip rates recovered between 1992 and 1995. If pure strike slip is assumed, then the slip rates estimated from IntSAR range changes between 1992 and 1995 are generally consistent with creepmeter and alignment-array measurements along much of the fault and confirm the temporary locking of the southernmost fault segment. However, along ∼6 km of the Fremont segment, IntSAR slip estimates appear to be at least twice those measured in the field. Transient vertical slip (northeast side up) of 2–3 mm/yr near the southern tip of the creep patch could explain this observation. First-order boundary-element models of a vertical frictionless fault in an elastic half-space predict some, but not all, of the inferred vertical slip.

Paleostress reconstructions and geodynamics of the Baikal region, Central Asia, Part 2. Cenozoic rifting

Investigations on the kinematics of rift opening and the associated stress field present a renewed interest since it has recently been shown that the control of the origin and evolution of sedimentary basins depends to a large extent on the interplay between lithospheric strength and applied stresses. It appears that changes of stress field with time are an important factor that either controls or results from the rifting process. The object of this paper is to study the changes of fault kinematics and paleostress field with time in the Baikal Rift System during the Cenozoic. Reduced paleostress tensors were determined by inversion from fault-slip data measured in the central part of the rift and its southwestern termination, between 1991 and 1995. Results show that the stress field varies as well in time as in space. Two major paleostress stages are determined, corresponding broadly to the classical stages of rift evolution: Late Oligocene-Early Pliocene and Late Pliocene-Quaternary. The first paleostress stage is related to the rift initiation and the second to the major stage of rift development. Similarities between the recent paleostress field and the present-day stress field inverted from focal mechanisms indicate that the second paleostress stage is still active. Therefore, we propose to use ‘proto rift’ for the Late Oligocene-Early Pliocene stage and ‘active rift’ for the Late Pliocene-Quaternary stage of rift development. During the ‘proto rift’ stage, the stress field was characterized by a compressional to strike-slip regime. A progressive change from transpression to transtension is suspected for the central part of the rift (Baikal and Barguzin basins) during this period. In the western termination of the rift (Sayan Massif, Tunka depression), a strongly compressional stress field with oblique thrusting kinematics is well constrained in the Late Miocene-Early Pliocene interval. The ‘active rift’ stage was initiated by a marked change in fault kinematics and stress regime in the Late Pliocene. In the central part of the rift, the stress regime changed into pure extension, while in the southwestern extremity, it changed into pure strike-slip. Fault kinematics suggests that rifting was initiated by an extrusion mechanism due to the interaction of far-field compressional stress on a mechanically heterogeneous crust, with the southwards-pointing wedge of the Siberian Craton acting as a passive indentor. The Cenozoic time-space evolution of the stress field is believed to reflect the increasing influence of locally generated buoyancy extensional stresses associated with density anomalies of the lithosphere, on intraplate stresses generated by the India-Eurasia convergence and the West-Pacific subduction.

A finite-element model of the stress field in strike-slip basins: implications for the Permian tectonics of the Southern Alps (Italy)

A set of finite-element models have been used to simulate pull-apart basin formation related to a releasing overstep along two right strike-slip faults. The modelling results have shown the stress field at a releasing fault overstep with respect to strike-slip, transtensional and transpressional boundary constraints. Based on the assumption that normal faults strike parallel to the maximum horizontal stress we have predicted the orientation of potential normal faults within the overstep zone. Linear elastic rheology and plane stress conditions were assumed, and four kinds of models with different overlap/separation ratios (0, 0.5, 1 and 2) between the strike-slip faults were analysed. Displacement boundary conditions were applied to the upper left-hand and lower right-hand corners of the models in order to simulate a right strike-slip movement; outward and inward displacement components were applied respectively to reproduce transtensional and transpressional conditions. All the models mostly show an extensional stress field (i.e. with at least one extensional principal stress axis). Large zones inside the overstep are affected by general extension (both principal axes are extensional), and their size increases with both an increase in the overlap/separation ratio and with changes from transpressional to transtensional conditions. The areas of maximum extension are located near the ends of the fault segments inside the overlap area. It is fairly probable that the genesis of the extensional faults that bound the pull-apart basins observed throughout the world occurs in these maximum extensional areas, and that the extensional faults are parallel to the maximum compression stress axis trajectories. The angle between the potential normal faults and the strike-slip faults is strongly dependent on the overall boundary conditions (i.e. pure strike-slip, transtension and transpression) of the model. In contrast, the elastic rock properties and the overlap/separation ratio of the faults have a minor control. The angle between the normal and strike-slip faults can vary from a few degrees in transtensional conditions to 100°–120° in transpressional conditions. Inside the overlap zone an area of subsidence occurs, the axis of which is coincident with the junction of the strike-slip fault tips, where it is also at a maximum (i.e. in the same zones of maximum extension). A comparison of the modelling results with the structural setting of the Collio and Pramollo Permo-Carboniferous pull-apart basins (Southern Alps, Italy) indicates that the Collio basin developed under strike-slip conditions, while the Pramollo basin was formed under transtensional conditions.

The effect of focal depth error on moment tensor inversion

In the determination of focal mechanism and rupture process of earthquake sources by using moment tensor inversion technique, it is difficult to guarantee the focal depth used in calculating the Green’s functions (theoretical focal depth) is exactly equal to the real focal depth. The difference between the theoretical and real focal depths, i. e., the focal depth error, will affect the moment tensor inversion to some extent. Using synthetic seismograms, the effect of the focal depth error on moment tensor inversion for three basic types of faults is discussed systematically. For the normal and thrust fault, the focal depth error mainly affects the explosive (EP) component and the compensated linear vector dipole (CLVD) component. In the case that the theoretical focal depth is greater than the real focal depth, the focal depth error causes a false positive EP component and a false negative CLVD component for the normal fault. However, it produces a false negative EP component and a false positive CLVD component for the thrust fault. The absolute values of the false EP and CLVD components for both normal fault and thrust fault cases increase with increasing focal depth error. In the case that the theoretical focal depth is smaller than the real focal depth, the focal depth error causes a false negative EP component and a false positive CLVD component for the normal fault. However, it produces a false positive EP component and a false negative CLVD component for the thrust fault. Similarly, the absolute values of the false EP and CLVD components for both normal fault and thrust fault cases increase with increasing focal depth error. For a pure strike-slip fault the focal depth error mainly affects the shape of source the function, unlike for the normal and thrust faults. The source time functions have artificially extended tails when either the theoretical focal depth is greater or smaller than the real focal depth. The numerical experiments show that the focal depth error less than 20 km has no significant effect on the overall focal mechanism of the earthquake. In addition, the effect of the focal depth error on the inversion result is slighter in case that the theoretical focal depth is greater than the real focal depth than in the case that the theoretical focal depth is smaller than the real focal depth.

The kinematic paradox of the San Andreas Fault

ABSTRACTThe Pacific‐North America plate boundary along the San Andreas fault system is notoriously a right‐lateral transpressive margin where both almost pure thrust and strike‐slip tectonics take place. The Pacific plate travels WNW, forming an angle of about 25° with the boundary. Since the Pacific is moving WNW faster than North America, right lateral transtension should result along the San Andreas system. North America, in turn, travels westward obliquely to the boundary and a left‐lateral transpressive component would be expected along the same margin. Therefore, the right‐lateral transpression of the San Andreas system can be partitioned into (i) a sinistral transpression along the southwestern margin of the North America plate obliquely overriding (ii) a faster right lateral transtension occurring along the transfer margin of the Pacific plate between the East Pacific rise in the California Gulf and the Gorda ridge to the north‐west. This is due to the oblique trend of the Pacific and North America plate margins with respect to their motion in a absolute reference frame.The geodynamics of California is marked by a unique setting in which there is a special subduction where, in contrast with classic subduction zones, the footwall of the subduction plane is obliquely diverging from the hanging wall in an E‐W section, while it is converging at slower rates in a NE‐SW direction. The extensional E‐W component is absorbed into the Basin and Range rifting, whereas the compressive NE‐SW component is mainly expressed in the Coast Ranges and California offshore. The compression perpendicular to the San Andreas is then not intrinsic in the strike‐slip movement, but it is rather an independent tectonic factor. Therefore, the San Andreas system cannot be considered as an archetype of a pure strike slip fault.