Axial Trend Research Articles

Seismic intensity investigation of the 2001 M=6.0 Yajiang-Kangding earthquake and discussion on its background of seismogenic structures

An M=6.0 earthquake occurred on February 23, 2001 in the western Sichuan Province, China. The macro seismic epicenter situated in the high mountain-narrow valley region between Yajiang and Kangding counties. According to field investigation in the region, the intensity of epicentral area reached VIII and the areas with intensity VIII, VII and VI are 180 km2, 1 472 km2 and 3 998 km2, respectively. The isoseismals are generally in elliptic shape with major axis trending near N-S direction. The earthquake destroyed many buildings and produced some phenomena of ground failure and mountainous disasters in the area with intensity VIII. This event may be resulted from long-term activities of the Litang fault and Yunongxi fault, two main faults in the western Sichuan. The movements between the main faults made the crust stress adjusted and concentrated, and finally the earthquake on a secondary fault in the block released a quite large energy.

Magnetic fabric study across the Ailao Shan–Red River shear zone

The anisotropy of magnetic susceptibility (AMS) of 351 specimens from 51 sites across the Ailao Shan–Red River shear zone (ASRR) was measured to determine its magnetic fabric. Rocks range westward from core schistose gneiss, through low-grade schist, to Triassic sediment. Magnetic ellipticity analysis shows that 41 of 51 sites have an oblate compressional fabric and the other 10 sites have a prolate fabric. P J value drops by 22.4% in the low-grade schist and by 27.4% in the Triassic sediment on average with respect to the gneiss, suggesting a rapid decrease of deformational intensity. The directions of principal susceptibilities are closely related to the deformation of the Ailao Shan–Red River shear zone. The susceptibility plane always coincides with the schistosity or cleavage plane. Most of the maximum susceptibility axes trend NW–SE. In the shear zone, the maximum susceptibility axes ( K max) are parallel to the lineation within the foliation plane. With increasing distance from the shear zone, there is a trend that they become parallel to the down-dip of reverse faults or cleavage. This indicates changes in deformation mode, inside and outside the shear zone. Within the shear zone, horizontal movement is dominant. Outside, shortening prevails. The overall minimum magnetic axes align NE–SW with subhorizontal to low dip angles, suggesting that the dominant shortening is NE–SW directed. Caution should be exercised when AMS is used to determine shear sense in strong shear zones because the angle between the minimum susceptibility axis ( K min) and pole of foliation is small, and also because the attitude of foliation varies from place to place. They result in unreliable or even wrong shear sense. Another important result is the axial ratio of magnetic susceptibility ellipsoid along the study section. With these data, it is possible to establish an axial ratio relationship between the finite strain ellipsoid and magnetic susceptibility ellipsoid for quantitative calculation of offset.

The state of stress in the limb of the Split Mountain anticline, Utah: constraints placed by transected joints

Transected joints (i.e. systematic joints that strike at an angle to the present fold axis trend) occur on the flanks of Split Mountain, a Laramide anticline near the eastern end of the Uinta Mountains, Utah. The common orientation on both flanks for these WNW-striking joints is inconsistent with joints driven by a syn-folding stretch normal to the direction of highest curvature. A smaller dispersion of the poles to these transected joints occurs when they are rotated with bedding to their ‘pre-fold’ orientation. This dispersion of poles is inconsistent with a post-fold genesis in a regional stress field but permits the possibility that these WNW joints propagated as a systematic set prior to Laramide folding. A pre-fold interpretation is substantiated by a regional WNW-striking joint set within Cretaceous and older rocks in the surrounding Piceance, Uinta, and southeastern Sand Wash basins. During tilting accompanying the upfolding of Split Mountain, most joints of this WNW-striking regional set remain locked without slipping under a shear stress. Fracture toughness and frictional strength are two rock properties that serve to lock a joint until a critical resolved shear stress is achieved. A gravity load caused down-dip slip on some joints that were tilted to a dip of about 62°. This suggests that a local principal stress remained roughly vertical during bedding rotation. Assuming fracture strength and friction prevented slip on most joints during tilting, the ratio of least horizontal, Sh, to vertical stress, Sv, at the critical tilt angle was approximately 0.55.

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.

Isotopic evolution of Mauna Loa and the chemical structure of the Hawaiian plume

New He isotopic data from the HSDP pilot hole core, lava accumulation rate models, and data from the literature are used to develop a 200,000 year isotopic record for the lava erupted from the Mauna Loa volcano. This record, coupled with an analogous record from Mauna Kea from the Hawaii Scientific Drilling Project (HSDP) pilot hole project and other literature data from the GEOROC database, are used to construct a “map” of lava isotopic compositions for the island of Hawaii. The isotopic map is converted to a map of the He and Nd isotopic compositions of melts from the mantle plume, which can be compared with a published melt supply map derived from geodynamic modeling. The resulting map of the plume indicates that values of helium 3He/4He > 20 Ra are confined to the core of the plume (radius ≈ 20–25 km) and correspond to potential temperatures >1565°C, suggesting the He isotopic signal is derived from deep in the mantle. The 3He/4He map has closed contours down to 10 Ra; the contours are teardrop‐shaped and elongated in the general direction of plate motion. The closed contours indicate that most of the plume He signal is lost during the early stages of melting, which is consistent with helium behaving as a strongly incompatible element (KHe ≤ 0.001). The εNd contours (and by inference the contours for Sr, Pb, Hf, and Os) do not all close on the scale of the island of Hawaii but instead partially follow material flow lines within the plume beneath the lithosphere. The plume signal for Nd extends circa 100 km in the direction of plate motion, which is consistent with the moderately incompatible behavior of Nd (KNd ≈ 0.02). Downstream from the plume core epicenter, plume Nd occurs with asthenospheric He; this could be mistaken for an additional plume component, whereas it may be only a manifestation of differing incompatibility. Data from Mauna Loa suggest the presence of a low‐3He/4He plume component that has low εNd and high 87Sr/86Sr. The plume map suggests that this component may be a blob (circa 20 km scale), located between Mauna Loa and Hualalai and separated from the main plume core by a zone of more asthenosphere‐like material. The HSDP data preclude a proposed model where this material represents a ring of entrained material from the lower mantle. The orientation of the elongation of contours on the plume He and Nd isotope maps (∼N45°W) does not match the modern plate motion as measured by GPS (N65°W) nor does it match the trend of the ridge axis between Maui and Loihi (N30°W). The geochemical evidence, as well as the locations and growth histories of the Hawaiian volcanoes, suggest that the plume, as well as the Pacific plate, has been moving at a velocity of several centimeters per year over the past 1 to 2 million years.

Self-potential chenges associated with volcanic activity: Short-term signals associated with March 9, 1998 eruption on La Fournaise volcano (Reunion Island)

After six years of quietness La Fournaise volcano entered into activity on March 9, 1998. Fissures opened gradually downwards on the northern flank of the cone. Two cones, Kapor and Krafft built, from which lava poured until September 1998. Several other vents opened during this eruption. Mappings, surveys, and continuous recordings of the Self-Potential have been performed on the volcano for twenty years. SP mappings disclose the variability of large scale SP anomalies due to the modification of the hydrothermal system over some ten years. Most of the eruptions take place along a Main Fracture Zone (MFZ) in which ground water flows prevail. SP measurements have also regularly been made on the northern flank of the cone, on a west-east profile crossing the MFZ. Between 1981 and 1992 an enlargement and a shift of the MFZ to the east are evidenced. In particular, the eastern fissural axis trending N35°E could be related to the possible collapse of the east flank of the volcano. After a decrease between 1992 and 1997, the SP anomaly was enhanced again by the 1998 eruption. Short scale, about 250 m wide, 750 mV amplitude anomalies were superimposed on a large scale one, 2500 m wide, and about 250 mV in amplitude. For several years, continuous stations have been measuring the electric field along two directions, with a 20 s sampling, in order to record the genesis of SP signals associated with the volcanic activity. Oscillations belonging to the ULF band were evidenced several days before the 1988 eruption, some of them at 9 km from the summit. Their amplitude reached several tens mV/km. These oscillations sometimes present a phase lag from one station to another; they progressively shift towards the location of the future effusive vents. The polarisation of the oscillations is similar to the polarisation of longer SP variations (1 h period or more) and are correlated with the structural anisotropy. Finally, during the last hours preceding the effusive activity, huge SP signals, up to a few Volts/km, appeared at the stations located on the MFZ, and especially on the branch where the magma migrated. We interpret these SP signals as due to electrokinetic effects generated by fluid flow in cracks opened by the stress field changes.

Microearthquake characteristics and crustal VP/VS structure at the Mid‐Atlantic Ridge, 35°N

We report on the results of a microearthquake experiment conducted at the along‐axis bathymetric high of the slow spreading ridge segment near 35°N on the Mid‐Atlantic Ridge (MAR). Of a total of 255 microearthquakes recorded during the 43‐day experiment, 31 were located near the Oceanographer transform at the northern end of the segment, 79 occurred at the nontransform offset at the southern end of the segment, and 145 were at the segment center. At the segment center, earthquake epicenters lay within the median valley inner floor and formed a ∼12‐km‐long trend paralleling a steep scarp bounding the western wall of the inner valley; focal depths were 3–4 km below the seafloor. Most (80%) of the focal mechanisms for 32 segment center earthquakes are consistent with normal faulting on faults approximately parallel to the axial trend. From a joint inversion for hypocenters and P and S wave velocity structures, we determined a horizontally averaged VP/VS ratio that decreases from 2.9 in the shallowmost 300 m to 1.7 at 2‐km depth, and we interpret this decrease as indicating a decreasing contribution of thin cracks to fracture porosity with depth. The maximum depth of seismicity, 4 km, is anomalously shallow compared with other MAR segments at which microearthquake experiments have been carried out. Cross‐axis relief is also anomalously low for this segment's center, and on the basis of this and other MAR microearthquake experiments, there appears to be a correlation between cross‐axis relief and maximum depth of seismicity. From the correlation of cross‐axis relief and inferred crustal thickness we suggest a relationship between thick crust, high crustal temperatures, and low cross‐axis relief, in qualitative agreement with thermomechanical models for the depth of the axial valley.

Fluorescence spectroscopy of aromatic species produced in rich premixed ethylene flames

The fluorescence spectra of the condensed species (CS) collected in the soot inception region of a rich premixed laminar ethylene/oxygen flame have been measured by excitation in the UV at 266 and 355 nm excitation wavelength. The contribution of the most abundant polycyclic aromatic hydrocarbons (PAH) to the CS fluorescence has been evaluated in order to attribute the CS fluorescence at different emission wavelengths to specific aromatic structures. The fluorescence peaks detected in the UV region of the CS fluorescence spectrum was found to be mainly due to a typical PAH like fluorene, that is, the most fluorescent one among the PAH analyzed in the CS by chromatographic analysis. The CS exhibited the larger emission in the visible where the PAH contribution has been shown to be negligible and other fluorescing aromatic species, not identified by chromatographic analysis of the CS, have to be considered responsible for the visible fluorescence. Laser induced fluorescence (LIF) flame measurements excited at 266 nm and detected at two selected wavelengths (310 and 410 nm) have been performed along the flame axis and compared with the CS fluorescence intensity. The LIF and CS fluorescence signals show quite similar axial trends demonstrating that the LIF signals are related to CS fluorescence. In particular, the LIF fluorescence signals detected in the UV could be attributed to the PAH fluorescence whereas the unidentified species contained in the CS can be followed by LIF detection in the visible region.

The morphoneotectonic structure of Parnis Mt. (Attica, Greece)

The morphoneotectonic study of the Parais' mountain showed that the higher planation surfaces lay at the central area of the eastern Parais, while the rest of them lay lower towards the north, south and west, giving the impression of a mega-anticlinic structure, in accordance to the east-west trending water divide of the mountain. In addition, the open folds of the L. Cretaceous carbonates with axes trending E-W, appear to have folded the older isoclinal or closed folds whose axes plunge northwards or southwards respectively. The contour-map of the contact between the L. Cretaceous carbonates and the underlying formations (originally an unconformity, but now only observed as a thrust surface) reveals that this contact is curved in a mega-anticline shape, with an axis trending approximately E-W, and plunging westwards. Thus, we conclude that Parais Mt. is a complicated morphoneotectonic structure due to a long term active brittle-ductile deformation, expressed as a mega-anticline of an E-W axis with active fault zones with respective strike.

Fold uplift and synkinematic stratal architectures in a region of active transtensional tectonics and volcanism, eastern Tyrrhenian Sea

The detailed stratigraphic analysis of synkinematic stratal architectures permits the calculation of fold uplift rates and the reconstruction of fold kinematics. This approach was applied to active folds in the Bay of Pozzuoli, eastern Tyrrhenian Sea. This area features late Quaternary volcanism, shallow magmatic intrusions, and transtensional tectonics. Seismostratigraphic analysis of synkinematic strata is constrained by a physical correlation to an already established stratigraphic succession in the neighboring Campi Flegrei and by tephra layers in a gravity core located in Pozzuoli Bay. A stratigraphic resolution in thousands of years was established. Structural and stratigraphic analyses yield quantitative data on the timing of fold inception, kinematics, and the amount and rates of uplift. The folds extend over ∼2 km, and display a curved axial pattern and wedge-shaped packages of synkinematic strata. The rate of fold uplift ranges from 1 to 20 mm/yr. The structures feature limb rotation kinematics and decreasing uplift rates. The uplift rates and geometry of the Pozzuoli Bay folds suggest that they are of tectonic origin and are a product of detachment folding. Their limited longitudinal extent and arcuate axial trends are compatible with an east-trending left transtensional shear zone that was active along the eastern Tyrrhenian Sea during the late Quaternary. A relative sea-level curve was calculated for an anticline culminating in the city of Pozzuoli. This relative sea-level curve highlights the relationship between the deformation timing and the genesis of related abrupt changes in sedimentary facies and erosional surfaces.

ABSTRACT: Slump axial trends and palaeoslope orientation within the submarine fan environment: the value of image logs

ABSTRACT: Slump axial trends and palaeoslope orientation within the submarine fan environment: the value of image logs

サハリン（樺太）島の地質と地形 サハリン最北部の地質構造からみた北米・ユーラシアプレート収束境界のテクトニクス

The region from Sakhalin in Russia to the eastern margin of the Japan Sea has been regarded as a convergent plate boundary zone between the Eurasia and the North America Plates because large earthquakes and active crustal movements are prominent in this zone. These activities along the eastern margin of the Japan Sea to southern Sakhalin are almost consistent with the expected relative plate motion deduced from the Euler pole which is estimated from the magnetic anomaly lineations in the Atlantic Ocean, but the modern tectonic aspect in northern Sakhalin is inconsistent; e.g., 1995 Neftegorsk Earthquake is one of the typical events.As a result of a structural and tectonic study in northern Sakhalin, the NE-SW compressive tectonic feature since the Late Miocene was clarified. In the northernmost area of the Schmidt Peninsula, the early Cretaceous ophiolite thrusts upon the Late Cretaceous sedimentary rocks. The ophiolite has an overturned sequence: serpentinites, gabbros, basaltic rocks, and hemipelagic sediments in descending order. The Cretaceous sediments form a map-scale and NW vergent synclinorium. All the deformation structures in outcrop scale, such as micro-folds, minor reverse and normal faults, axial plane cleavages and bedding slips, are consistent with a large scale folding. These structural relationships suggest only one event of deformation, which appears to be linked to overthrusting of the ophiolitic rocks.A structural investigation of the southeastern part of the Schmidt Peninsula revealed that the Middle Miocene sediments composed of siltstone and sandstone are folded with wavelength of several tens to several hundred meters. Their fold axes trend in the NW direction, which indicates that deformation under the NE compression is the same as the deformation in the northernmost area. The thickness of sedimentary layers are constant everywhere in folding. This fact indicates that the timing of the deformation is after sedimentation, that is, after the Middle Miocene. This deformation event is consistent with modern activity in northern Sakhalin. Therefore, the modern tectonic framework might have started in the Late Miocene time.A plate tectonic model indicates that the modern relative motion between the Eurasia and the North America Plate started at about 11 m. y. ago. The Late Miocene onset of the modern tectonic framework in northern Sakhalin occurred at almost the same time. The most reliable model to explain the discrepancy of sense of movement from the Eurasia-North America retative motion, may be the “extrusion” of the Okhotsk Block toward the Pacific Ocean. The dextral extrusion boundary in northern Sakhalin may be traced along theeastern coast of Sakhalin to the northern edge of the Kuril Basin, where active seismicityhas been observed although a detailed study of focal mechanisms and other tectonic aspects is needed in the future.

Detachments and normal faulting in the Marche fold-and-thrust belt (central Apennines, Italy): inferences on fluid migration paths

In the outermost domains of the central Apennines fold-and-thrust belt, the structural architecture of the late Miocene–early Pliocene contractional edifice was controlled by competence contrasts in the Calcareous–Marly sequences of Mesozoic–Tertiary age, and by a different state of lithification of the rock units at the onset of deformation. Field data on relative chronology of outcrop-scale structures (cleavage, veins, faults, folds) are presented for the three largest thrust-ramp anticlines of the Marche fold-and-thrust belt: Monte Gorzano, Acquasanta and Montagna dei Fiori-Montagnone. The data show that the timing and geometry of deformation structures differ for: (1) the lower Calcareous interval of late Triassic–early Cretaceous age (LCI) bounded on top by the intermediate detachment (ID) of the Fucoidi Marls; (2) the upper Calcareous–Marly interval (UCMI) of late Cretaceous–Oligocene age; (3) the uppermost detachment zone (UDZ) of lower–middle Miocene age; (4) the topmost Messinian Flysch sequence (FS). In the UDZ early episodes of deformation are manifested by compaction of a poorly lithified sequence followed by pervasive development of layer-parallel pressure-solution cleavage. Reverse faults ramp obliquely across the stratigraphic sequence, and are coated by multiple overgrowths of calcite fibers. These structures are deformed by large, eastward-verging asymmetric folds with N–S axial trends, and are cut by late generations of reverse faults. Normal faults started to develop in the fold backlimbs during the final stages of shortening, in middle–late Pliocene times. These early normal faults were reactivated during episodes of late Pliocene–Pleistocene extensional downfaulting, and are now superposed on the compressional edifice. The UDZ is interpreted to have temporarily sealed the upward escape of fluids during the initial episodes of shortening. Pervasive interlayer flow in the poorly lithified sequence was responsible for development of broken beds and scaly fabrics, similar to those observed in accretionary prisms. Only in the latest stages of deformation did propagation of discrete faults provide an interconnected pathway for fluid migration, until the final offset of the UDZ. The structural relationships suggest that fluids trapped within the fold cores and sealed by the UDZ were finally driven upwards due to progressive disruption of the thrust belt by late normal faults of late Pliocene to Pleistocene and Holocene age. Large-scale fluid migration along structurally-controlled pathways was enhanced by the strong components of uplift consequent to the final stages of deformation in the Marche fold-and-thrust belt, and was eventually associated with episodes of normal seismic faulting.

Tertiary Tsondab Sandstone Formation: preliminary bedform reconstruction and comparison to modern Namib Sand Sea dunes

The Tertiary Tsondab Sandstone Formation, which underlies much of the present Namib Sand Sea, is a key element in understanding the Cenozoic evolution of the Namib Desert. Outcrops of the aeolian facies of the Tsondab Sandstone at Elim and Diep Rivier consist of two sequences of bioturbated cross-strata separated by likely formation-scale surfaces of stabilisation. Cross-strata consist of scalloped sets about 200 m in width and separated by southeast dipping bounding surfaces. Internally, sets contain reactivation surfaces of probable seasonal origin. The north to south-southeast dipping foresets define crescent shapes with a trough axis trending northeast. Although additional data are needed to define the Tsondab bedform, the outcrop data is best satisfied in computer simulations by north trending, east migrating main bedforms, which had relatively large and slow-moving dunes superimposed upon their eastern flanks and migrated to the north. Foresets dipping to the south to south-southwest at Elim suggest that superimposed dunes also occurred on the western flanks of the main bedform and migrated to the south, but that their record was largely lost with net eastward migration of the main bedform. This preliminary Tsondab model shares attributes such as trend, scale of cross-strata, and presence of scalloped sets with reactivation surfaces with computer models of the modern linear dunes in which large-scale sinuosity migrates alongcrest to the north. Differences emerge in the overall set architecture and the orientation of cross-strata and bounding surfaces, as well as the degree of vegetation that must have characterised Tsondab dunes.

Stratigraphic architecture and evolution of the early Paleoproterozoic McGrath Trough, Western Australia

Palaeocurrents, provenance and stratigraphic analysis of the upper part of the Mount Bruce Supergroup and the Wyloo Group in the southwestern part of the Hamersley Province of the Pilbara region of Western Australia have been used to define an early Paleoproterozoic foreland basin, the McGrath Trough, in front of the northwards-advancing Ophthalmian fold belt. The foreland basin succession commences with the Boolgeeda Iron Formation and includes the overlying Turee Creek and lower Wyloo Groups. The basin geometry is asymmetric, thinning northwards away from the Ophthalmian fold belt, and the trough axis trends east-southeast parallel to the Ophthalmian folds. Multiple intraformational unconformities in the upper Turee Creek Group record the advance of the fold belt into the axis of the McGrath Trough in the Hardey syncline area. East-southeast-trending folds and steep southward-dipping cleavage post-date deposition of the Beasley River Quartzite, a shallow-marine to fluvial sandstone that records an interval of tectonic quiescence during basin evolution. Deposition of the Beasley River Quartzite was followed by renewed uplift of the southern hinterland, resulting in continued deposition of fluvial siliciclastics into the foreland basin, which was eventually filled with the eruption of the Cheela Springs Basalt. The age of the McGrath Trough is constrained broadly between ∼2.45 Ga, the age of the underlying Woongarra Rhyolite, and ∼2.2 Ga, the age of the Cheela Springs Basalt at the top of the succession. The sedimentary fill of the McGrath Trough was derived mainly from the south, and becomes texturally and compositionally more mature upward, reflecting its recycled orogen provenance. Two main sources are identified: a dominant cratonic source interpreted to be similar to the granitoid-greenstone basement of the Pilbara Craton, and a volumetrically lesser source from banded iron formation (BIF), and volcanic and sedimentary rocks of the Mount Bruce Supergroup. Felsic volcanic detritus, likely to have been derived from the underlying Woongarra Rhyolite, is most common in the lower Turee Creek Group and becomes increasingly less common upsection, although it is reworked from underlying formations. Chert and BIF clasts appear first in the upper Turee Creek Group, and are important components of the most proximal alluvial fan to braided stream deposits in the upper Turee Creek and lower Wyloo Groups. Grains of microplaty haematite, identical in appearance to microplaty haematite in the giant Hamersley iron-ore deposits, appear in conglomeratic units from the base of the Beasley River Quartzite upwards. These fluvial sediments were derived from the south and record erosion of ore-grade deposits from the uplifted Ophthalmia fold belt during lower Wyloo Group times.

Extension in the southern Ryukyu arc (Japan): Link with oblique subduction and back arc rifting

Compilation of earthquake focal mechanisms, morphological analysis, and examination of mesofractures help to clarify the Cenozoic and recent stress history in the southern Ryukyu arc between the Ryukyu trench and the southern Okinawa Trough. (1) An older, pre‐Miocene, intermediate‐type stress field (vertical stress component σv = σ2) is attested by conjugate strike‐slip faults and shear joints in Eocene rocks of Ishigaki Island. The σ1 axis trends N90°E to N110°E in present‐day coordinates. (2) Normal faults and extensional joints in Mio‐Pliocene strata of Iriomote and Yonaguni Islands and fault scarps in Pleistocene strata at Yonaguni allow us to reconstruct a Miocene to Pleistocene stress field of tensional type (σv = σ1). The σ3 axis trends about NW–SE. The effects of this stress field can be traced back in Eocene rocks of Ishigaki Island. (3) Evidence for the youngest, Holocene to present‐day stress field comes from the analysis of the Harvard Moment Tensor Catalog (1976–1997) for shallow earthquakes, Quaternary fault scarps visible on the two islands of Miyako and Yonaguni, and extensional joints in Holocene rocks of Yonaguni. This stress field is tensional. Two directions of extension are inferred: a regional, arc‐perpendicular (N–S) one and a local, arc‐parallel (N60°E) one in the easternmost part of the area near Miyako. The record of Neogene to Quaternary tensional stress fields demonstrates that the extension in the Okinawa Trough can be traced until the arc region. Arc‐oblique to arc‐perpendicular stretching can be directly correlated with oceanward rifting in the back arc basin, while the arc‐parallel stretching reflects the increase of the arc curvature. The predominance of extensional features, the absence of shortening or transcurrent deformation structures, and a weak interplate coupling do not support the forearc sliver model for the present‐day southern Ryukyu arc.

Macquarie Island mapping reveals three tectonic phases

The first detailed mapping of Macquarie Island, perhaps the world's quintessential ophiolite has revealed three tectonic phases—extension, transtension, and transpression. Nearly all of the crust and most deformational structures formed during the first phase (D1), which involved extension along the paleo‐Macquarie seafloor spreading axis. Magmatism, block tilting, and large‐scale differential block uplift were synchronous, resulting in juxtaposition of disparate rock associations characteristic of different crustal levels. As D1 waned, the second phase (D2) developed. Involving transtension with a north‐south extension axis, it produced minor late‐stage dolerite dikes and seafloor volcanic centers.The third phase (D3), still in progresses dextral transpression. Its initiation along the Australia‐Pacific plate boundary terminated igneous activity and extension on the island.The D3 shortening axis trends east‐west to northeast‐southwest.

Geometry of fold arrays in the Silesian-Cracovian region of southern Poland

Abstract A zone of deformation characterized by en echelon folds extends along the northeast margin of the Upper Silesian Coal basin in southern Poland. The zone is well exposed by mining in numerous collieries where the deformation involving Upper Carboniferous strata can be observed. Folds and associated faults form a structural pattern consistent with an origin in dextral convergent wrenching on both the principal deep-seated fractures and the subsidiary faults affecting crystalline basement beneath this part of the basin. The zone is cross-cut by sublatitudinal fold belts which, according to their geometrical parameters, seem to result from alternate transcurrent shearing along west-east oriented fractures in the basement, sinistral in one case and dextral in the other. Among folds caused by the concentrated operation of a force couple whose vectors lie in the horizontal plane, there are examples of bending folds (forced folds) in each array resulting from vertical components of movement of deep-seated fault blocks. The dimensions of the folds change with depth, and the axial trend and the inclination of axial surfaces differ with distance from the basement fault trace. The length of fold segments strongly depends on axes orientation. Widely distributed evidence of interlayer slip demonstrates a variable spatial distribution of tectonic-transport direction in the sedimentary sequence. The above-mentioned features and other geometrical attributes of the fold arrays enable the dynamics of the forcing structures during Variscan tectogenesis to be determined and illustrate a hierarchical structure of the basement boundary zones. Examples from the Silesian-Cracovian region suggest that bending folds (forced folds) can sometimes originate in conditions provided by a strike-slip regime.

Overthrust shear deformation of a foreland basin; structural studies south-east of the Pelvoux massif, SE France

The Late Tertiary Champsaur-Prapic fold zone lies to the southeast of the Pelvoux basement massif in the footwall of the Frontal Pennine Thrust of the western Alps. The 0.8–1.3 km thick Late Eocene-Oligocene foreland basin succession lies directly on crystalline basement and was deformed by WSW-directed shear below the overthrusting internal alpine units on the Frontal Pennine Thrust. This deformation decouples from the basement in a top to WSW-SW basal shear zone. Above, the turbidite sequence (Grès du Champsaur) is folded in chevron-like folds. A 12 km long continuous profile across the Champsaur fold zone documents variations in chevron fold shape, axial plane dips and zonal distribution of folding which indicate progressively higher shear strains towards the Frontal Pennine Thrust in the east. Chevron fold axes trend generally NNWSSE except close to the Pelvoux massif where fold axes curve to N-S possibly due to lateral drag. In the western part of the investigated area SW-directed thrusting on the Champsaur Thrust almost doubles the Tertiary stratigraphy. The Champsaur Thrust and chevron folding in its footwall deform the overlying internal thrust sheets. The Champsaur-Prapic fold zone is a particular example of overthrust shear deformation in the footwall of internal alpine nappes in which deformation is concentrated in diffuse ramp zones. To the north, late alpine deformation changes to WNW-directed partly post-dating the WSW-SW-directed deformation further south. Therefore, in the core of the external western Alpine are, two sub-perpendicular transport directions which barely overlap can be distinguished.

Deformation of the Kenai Peninsula, Alaska

Crustal deformation on the Kenai Peninsula in southern Alaska has been studied using data obtained from Global Positioning System (GPS) measurements in 1993 and 1995 and leveling observations in 1964, immediately after the Prince William Sound earthquake. This analysis shows that the Kenai Peninsula has experienced as much as ∼900 mm uplift during the past 3 decades and that the uplift forms an ∼125 km wide elongate dome with its major axis trending southwest to northeast following the trend of the major tectonic features of the region. The averaged uplift rate between 1964 and 1995 is as high as 30 mm yr−1, although the current uplift rate may be substantially lower. The GPS measurements cast further doubt on previously suspect tide gauge data for Nikiski, Alaska, which indicated rapid postseismic uplift at this site located in northwest Kenai Peninsula adjacent to Cook Inlet. Examination of the three‐dimensional GPS data indicates that the eastern Kenai Peninsula is currently undergoing significant SSE to NNW contraction in response to North American‐Pacific Plate convergence. The horizontal velocities are consistent with the predictions of an elastic half‐space model for the interseismic deformation. This result, taken in combination with the small changes in uplift between 1993 and 1995, suggests that most of the present deformation is due to steady plate convergence rather than transient postseismic rebound.