Strike-slip Zones Research Articles

A self-similar dynamic rupture model based on the simplified wave-rupture analogy

SUMMARYThe investigation of stresses, faults, structure and seismic hazards requires a good understanding and mapping of earthquake rupture and slip. Constraining the finite source of earthquakes from seismic and geodetic waveforms is challenging because the directional effects of the rupture itself are small and dynamic numerical solutions often include a large number of free parameters. The computational effort is large and therefore difficult to use in an exploratory forward modelling or inversion approach. Here, we use a simplified self-similar fracture model with only a few parameters, where the propagation of the fracture front is decoupled from the calculation of the slip. The approximative method is flexible and computationally efficient. We discuss the strengths and limitations of the model with real-case examples of well-studied earthquakes. These include the Mw 8.3 2015 Illapel, Chile, megathrust earthquake at the plate interface of a subduction zone and examples of continental intraplate strike-slip earthquakes like the Mw 7.1 2016 Kumamoto, Japan, multisegment variable slip event or the Mw 7.5 2018 Palu, Indonesia, supershear earthquake. Despite the simplicity of the model, a large number of observational features ranging from different rupture-front isochrones and slip distributions to directional waveform effects or high slip patches are easy to model. The temporal evolution of slip rate and rise time are derived from the incremental growth of the rupture and the stress drop without imposing other constraints. The new model is fast and implemented in the open-source Python seismology toolbox Pyrocko, ready to study the physics of rupture and to be used in finite source inversions.

Coseismic Surface Ruptures of the 2003 Chuya Earthquake (Gorny Altai): Slip Geometry and Spatial Patterns

A 48 km long zone of surface deformation produced by the Ms = 7.3 intracontinental earthquake of 2003 in Gorny Altai is studied in its five segments between the Aktru and Irbistu rivers, where ruptures show the greatest offsets and distinct structural patterns. A total of 554 coseismic ruptures of five slip geometry types are analyzed in terms of length, orientation, and relative percentage. The rupture patterns are discussed with reference to previously published evidence and compared with other strike-slip zones worldwide.

Eurekan faults on northern Ellesmere Island, Arctic Canada: from Cenozoic strike-slip tectonics to recent seismicity

The Eurekan deformation is a partially contractional Cenozoic tectonic event that affected large parts of the Arctic region. In the study area on northern Ellesmere Island, major NE-SW trending strike-slip faults occur, which are related to the Eurekan deformation. The outcrop data show that left-lateral strike-slip kinematics slightly dominate, but also right-lateral kinematics were documented. Cross-cutting relationships of the individual faults give evidence for multiple fault reactivations within major strike-slip zones. The reconstructed paleostress fields show two phases. The first phase started with a N-S compression and shifted over a NNE-SSW compression into a NNW-SSE compression. The second phase was a WNW-ESE compression. The paleostress field evolution reflects the movements of Greenland. During the Eurekan phase 1, Greenland moved northward and during Eurekan phase 2 it moved to the WNW. These motions likely controlled the stress field on northern Ellesmere Island. From the paleostress field analyses and the orientation of the strike-slip faults in the study area, it can be derived that the Eurekan phase 1 deformation is characterized by left-lateral strike-slip faults, whereas most-likely during Eurekan phase 2 the majority of right-lateral strike-slip faults formed. The paleostress field analysis implies that many Eurekan faults are reactivated Ellesmerian faults. Recent seismic events indicate ongoing tectonic activity at some of the major strike-slip faults. This sheds new light on the geodynamics of northern Ellesmere Island, which was mechanically coupled to the Greenland plate, and implies that under the recent stress field, earthquakes at strike-slip faults are still possible and some of these faults were active in at least three phases over the last 350 Myr.

Variations of thick-skinned deformation along tumuxiuke thrust in Bachu uplift of Tarim Basin, northwestern China

The curved Tumuxiuke thrust is the northeastern boundary of the Bachu uplift in the Tarim Basin. Based on the interpretation of 2D seismic reflection sections, thick-skinned deformation along the Tumuxiuke structural belt is characterized by monoclinal fault-propagation folds that alternate with push-up structures. The localization and patterns of thick-skinned deformation were controlled by pre-existing normal faults. The forward trishear modeling process further revealed the existence of short-cut thrusts branched from pre-existing normal faults and tested their effect on folding deformation. Monoclines decapitated by low-angle short-cut thrusts coexist with anticlines decapitated by high-angle inverted faults due to the selective inversion of pre-existing normal fault segments. Furthermore, the fault propagation to fault slip ratio (P/S) and the combined effect of multiple propagating thrusts determine the variations in the fold amplification rate and the growth-stratal architecture. The coeval activity of the western Kunlun orogen and the Tumuxiuke thrust in the Cenozoic indicates the far-field transmission of orogenic stress into the distal foreland. Under the approximately N–S-directed compressive stress, inverted structures in nearly W-E-striking segments exhibit monoclinal fault-propagation folds controlled by forethrusts; for the oblique NW-SE-striking segments, the transpressional deformation was partitioned into the frontal contractional deformation zone and the backlimb strike-slip zone.

Seismotectonics of Vlora-Elbasani-Dibra Transversal Fault Zone (Albania): A Review

The Vlora-Elbasani-Dibra transversal fault zone crosses from south-west to northeast the Albanides collision zone starting from Vlora area to the Dumre diapir and continuing from Elbasani Quaternary graben to Dibra area, within the internal Albanides extensional zone. The Vlora-Elbasani-Dibra seismogenic fault zone is expressed by an alignment of important historical and instrumental seismicity with 15 Ms > 6 earthquakes. The Vlora-Berati-Elbasani segment of this fault zone during the year 1851 has been activated along all is length. The geological and neotectonic investigations and the focal mechanism solutions of earthquakes that occurred along the Vlora-Elbasani-Dibra fault zone prove its division into two distinct fault zones: a) the Mali i Kanalit Block - Dumre diapir strike-slip fault zone NNE trending in transpression, and b) the Elbasan-Dibra normal fault zone NE trending in transtension. The Mali i Kanalit Block-Dumre diapir strike-slip zone NNE trending in transpression offsets dextrally and to SW the folds and thrusts of the Çika, Kurveleshi and Berati anticlinal belts of Ionian zone. The Neogene (mainly since Middle Miocene) offset of the Çika, Kurveleshi and Berati anticlinal belts from generally NW-SE trending to near N-S one has led to thrust’s steepening and turning into high-angle strike-slip faults due to NE-SW oblique compression from the Albanides collision zone. The Othoni Island-Dhermi strike-slip fault offsets the dextrally dragged Devolli (to the east of Dumre diapir), Osumi (near to Berati town), and Vjosa (west of Shkoza village) rivers’ valleys formed from the Middle Pleistocene (180,000 years) to historic times. The Othoni Island-Dhermi dextral strike-slip fault, that reaches the maximum velocity of greater than 5 mm/year to the south of it, is the only leading fault that shaped the Mali i Kanalit Block-Dumre diapir dextral strike-slip fault zone. Focal mechanism solutions of earthquakes that occurred on the Mali i Kanalit Block - Dumre diapir dextral strike-slip zone show their generation from strike-slip faults in transpressional motions. Focal mechanism solutions of earthquakes that occurred on the Elbasan-Dibra were generated from normal faults with dextral strike-slip movements.

Transfer zones in Mediterranean back-arc regions and tear faults

Slab tearing induces localized deformations in the overriding plates of subduction zones and transfer zones accommodating differential retreat in back-arc regions. Because the space available for retreating slabs is limited in the Mediterranean realm, slab tearing during retreat has been a major ingredient of the evolution of this region since the end of the Eocene. The association of detailed seismic tomographic models and extensive field observations makes the Mediterranean an ideal natural laboratory to study these transfer zones. We review in this paper the various structures in back-arc regions differential retreat from the Alboran Sea to the Aegean-Anatolian region and discuss them with the help of 3D numerical models to better understand the partitioning of deformation between high-angle and low-angle faults, as well as the 3-D kinematics of deformation in the middle and lower crusts. Simple, archetypal, crustal-scale strike-slip faults are in fact rare in these contexts above slab tears. Transfer zones are in general instead wide deformation zones, from several tens to several hundred kilometers. A partitioning of deformation is observed between the upper and the lower crust with low-angle extensional shear zones at depth and complex association of transtensional basins at the surface. In the Western Mediterranean, between the Gulf of Lion and the Valencia basin, transtensional strike-slip faults are associated with syn-rift basins and lower crustal domes elongated in the direction of retreat (a-type domes), associated with massive magmatic intrusions in the lower crust and volcanism at the surface. On the northern side of the Alboran Sea, wide E-W trending strike-slip zones in the brittle field show partitioned thrusting and strike-slip faulting in the external zones of the Betics, and E-W trending metamorphic core complexes in the internal zones, parallel to the main retreat direction with a transition in time from ductile to brittle deformation. On the opposite, the southern margin of the Alboran Sea shows short en-échelon strike-slip faults. Deep structures are not known there. In the Aegean-Anatolian region, two main tear faults with different degrees of maturity are observed. Western Anatolia (Menderes Massif) and the Eastern Aegean Sea evolved above a major left-lateral tear in the Hellenic slab. In the crust, the differential retreat was accommodated mostly by low-angle shear zones with a constant direction of stretching and the formation of a-type high-temperature domes exhumed from the middle and lower crust. These low-angle shear zones evolve through time from ductile to brittle. On the opposite side of the Aegean region, the Corinth and Volos Rift as well as the Kephalonia fault offshore, accommodate the formation of a dextral tear fault. Here, only the brittle crust can be observed, but seismological data suggest low-angle shear zones at depth below the rifts. We discuss the rare occurrence of pure strike-slip faults in these contexts and propose that the high heat flow above the retreating slabs and more especially above slab tears favors a ductile behavior with distributed deformation of the crust and the formation of low-angle shear zones and high-temperature domes. While retreat proceeds, aided by tears, true strike-slip fault system may localize and propagate toward the retreating trench, ultimately leading to the formation of new plate boundary, as shown by the example of the North Anatolian Fault.

Tectonic evolution of strike-slip zones on continental margins and their impact on the development of submarine landslides (Storegga Slide, northeast Atlantic)

AbstractSubmarine landslides have affected the mid-Norwegian margin since the Last Glacial Maximum. However, the role of tectonic movements, and most especially fault reactivation, in generating landslides offshore Norway is largely unconstrained. This study uses high-quality three-dimensional seismic and borehole data to understand how landslide development is controlled by faults propagating within the uplifted south Modgunn arch. Variance and structural maps above the south Modgunn arch show that: (1) local scarps of recurrent landslides were formed close to the largest faults, and mainly above strike-slip faults; (2) distinct periods of fault generation were associated with tectonic events, such as the breakup of the northeast Atlantic Ocean, and those events forming the south Modgunn arch; and (3) important fluid-flow features coincide with faults and sill intrusions. In total, 177 faults were analyzed to demonstrate that fault throw values vary from 10 ms to 115 ms two-way traveltime (8 m to 92 m). We propose that the long-term activity of faults in the study area has contributed to fluid migration, weakened post-breakup strata, and controlled the development of submarine slope instability. In particular, strike-slip faults coincide with the locations of several Quaternary landslide scars near the modern seafloor. Similar processes to those documented in Norway may explain the onset of large-scale landslides on other continental margins.

Concentric Structures and Hydrothermal Venting in the Western Desert, Egypt

Large-scale concentric structures are enigmatic geological features observed on the Earth surface and on other planetary bodies. Their formation has been attributed to several processes. Here we describe for the first time the results of mapping and characterization of ∼100 large concentric circular structures found in Early Cenomanian argillaceous strata of the Bahariya depression (Egyptian Western Desert). The geological processes that lead to the formation of these features have remained so far elusive. We investigate the concentric structures with a multidisciplinary approach combining field observations, statistical analysis, soil-flux gas measurements and laboratory analyses of rock samples. The whole depression is dissected by the ∼90 km long N60°E-striking dextral strike-slip Bahariya fault. Effusive lavas and shallow intrusions crop out in the down faulted blocks. The mapped circular structures increase in number approaching the fault zone. These features are up to 10 m high and 625 m wide, have a morphology similar in shape to impact craters with steeper external flanks and a gently dipping internal subsided zone. Halite-cemented brecciated sediments from different geological units have been sampled in the central part of the concentric circular structures implying a subsurface mechanism involved in their formation. Petrography analyses revealed also the presence of high- and low-temperature minerals (e.g., Ba-K-feldspars and ferroaluminoceladonite) suggesting former phases of hydrothermal circulation. Soil-gas flux profiles (CO₂ and CH₄) reveal a modest CO₂ increase when crossing the central part of the circular structures inferring enhanced permeability. Field and laboratory data are consistent with a scenario envisaging a diffused and vigorous hydrothermal venting. The proposed scenario includes multiple phases where several geological elements and processes interact. The Bahariya fault, which activity initiated during the Late Cretaceous, provided pathways for Miocene magma ascent toward the surface and for the development of a network of subsurface intrusions in the organic-rich sedimentary rocks of the Bahariya Formation. The interaction of the igneous intrusions with carbon-rich sedimentary deposits produced overpressured fluids, causing the formation of sparse hydrothermal vents at the surface. The elongation of the main axis of the vents and the deformed structures located within the strike-slip zone suggest that faulting controlled the emplacement and the final shape of some of the hydrothermal vents. We speculate that this system may represent a palaeo sediment-hosted hydrothermal system and could be related to the opening of the Red Sea.

Spontaneous generation of ductile shear zones by thermal softening: Localization criterion, 1D to 3D modelling and application to the lithosphere

The generation of ductile shear zones is essential for the formation of tectonic plate boundaries, such as subduction or strike-slip zones. However, the primary mechanism of ductile strain localization is still contentious. We study here the spontaneous generation of ductile shear zones by thermal softening using thermo-mechanical numerical simulations for linear and power-law viscous flow in one-dimension (1D), 2D and 3D. All models are velocity-driven. The 1D model exhibits bulk simple shear whereas the 2D and 3D models exhibit bulk pure shear. The initial conditions include a small temperature perturbation in otherwise homogeneous material. We use a series of 1D simulations to determine a new analytical formula which predicts the temperature evolution inside the shear zone. This temperature prediction requires knowledge of only the boundary velocity, flow law and thermal parameters, but no a priori information about the shear zone itself, such as thickness, stress and strain rate. The prediction is valid for 1D, 2D and 3D shear zones in bulk pure and simple shear. The results show that shear heating dominates over conductive cooling if the relative temperature increase is >50°C. The temperature variation induced by the shear zone is nearly one order of magnitude wider than the corresponding finite strain variation so that no significant temperature variation occurs between shear zone and wall rock. Applying typical flow laws for lithospheric rocks shows that shear zone generation by thermal softening occurs for typical plate tectonic velocities of few cm.yr−1 or strain rates between 10−16 and 10−14 s−1. Shear stresses larger than 200 MPa can already cause strain localization. The results indicate that thermal softening is a feasible mechanism for spontaneous ductile shear zone generation in the lithosphere and may be one of the primary mechanisms of lithospheric strain localization.

Strain localization by shear heating and the development of lithospheric shear zones

Analogue and numerical models show that strong or weak domains in a deforming ductile material cause stress concentrations that may promote strain localization. Such domains commonly occur in the lithosphere through variations in composition or mineral fabric. Here we use a 2D plane-stress, non-Newtonian, viscous model to explore how strain localization develops from an initial isolated weak inclusion. We use a temperature-dependent rheological law for which the material weakens as a result of work done by shear converted to heat. The progress of strain localization follows a power-law growth that is strongly non-linear and may be regarded as an instability. Although this localization mechanism is ultimately limited by thermal diffusion, this parametrization permits a robust criterion for the conditions in which localized shear zones can form within the lithosphere. Shear zones in the lower crust are typically depicted as the downward continuation of faults. We argue that the depth-extent of narrow shear zones within the lithosphere is limited by the stability criteria that we infer from 2D numerical experiments. When applied to the rheological laws for common lithospheric minerals, the combination of temperature and stress-dependence provides a direct means of predicting the depth below which the localization instability does not occur. For an olivine based rheology, the maximum depth at which rapid localization is expected is in the range of ~20 to 60 km, depending on heat flow, strain-rate and water fugacity. We apply our calculations to two major continental strike-slip zones, the San Andreas Fault and North Anatolian Fault, and compare our predicted maximum localization depths with published seismological images. Strain localization in the lower crust requires a dry rheology comparable to plagioclase. Observations that imply localized strain in the uppermost mantle beneath these fault zones are consistent with the localization criteria and the rheological properties of dry olivine.

Superimposed characteristics and genetic mechanism of strike-slip faults in the Bohai Sea, China

Abstract Based on the 3D seismic structure interpretation of Bohai Sea, combined with physical modeling of structure, structural style analysis and apatite fission track simulation, the structural characteristics and genetic mechanism of the Cenozoic strike-slip faults in Bohai Sea were investigated. The results show that Tanlu strike-slip fault experienced three stages of strike-slip activities in the Cenozoic, and the transition from left-lateral strike to right-lateral strike-slip was completed at the end of the fourth member of the Shahejie Formation. The strike-slip faults in the Bohai Sea have the characteristics of multi-stage and multi-strength stress superposition. According to the superimposed forms of different strengths, different properties and different ratios, they can be divided into three major genetic types, extension and strike-slip superimposition, extension and extrusion superimposition, extrusion and strike-slip superimposition, and fifteen typical structure patterns. Affected by multiple changes in the direction and rate of subduction of the Cenozoic Pacific plate, the difference between the Cenozoic extension and the strike-slip in the Bohai Sea area leads to the diversity of the fault system and the zoning of the depression structure. According to superimposition features of faults, the Bohai Sea area can be divided into the Liaoxi S-type weak strike-slip zone, Liaodong braided strong strike-slip zone, Boxi conjugated medium strike-slip zone, Bodong brush structure medium strike-slip zone and Bonan parallel strong strike-slip zone. These zones differ in oil and gas accumulation features.

ГЛУБИННАЯ ГЕОДИНАМИКА И ОСОБЕННОСТИ МЕТАЛЛОГЕНИИ КУНАШИР-УРУПСКОГО ЗВЕНА КУРИЛЬСКОЙ ОСТРОВОДУЖНОЙ СИСТЕМЫ

The southern part of the Kuril island-arc system (from Kunashir to Urup islands) differs from the northern part (from Simushir to Paramushir islands) in its spatial arrangement relative to the subducting Pacific megaplate’s motion vector, greater number of volcanoes, and endogenic mineralization diversity (Au, Ag, PGE, Re, IN, Sn, Cu, Zn, Pb and others). The location of deep northwest-trending zones of strike-slip faulting on the opposite oceanic slope of the Kuril Trench is another peculiarity of the Southern Kurils. Significant extent (up to 1000 km), great depth (more than 100 km) and permanent (J-Q) activity of the strike-slip faults allowed thermal-fluid flows from the undersubduction asthenosphere to penetrate through the seismic focal zone, above the mantle wedge and oversubduction asthenosphere and reach subcontinental lithosphere. The upwelling of the flows was accompanied by metasomatic processes, formation of initial magma reservoirs and both the transitional and peripheral reservoirs in the Kuril island-arc crust, as well as by the evolution of volcanic activity and ore genesis. The synthesis of the Southern Kurils’ metallogeny and geodynamics data allows us to recognize the islands as a natural phenomenon intended for study of geological problems and mineral resources exploitation.

Horizontally deplacement geomasses in the continental rift geostruсtures (on the example of the Dnipro-Donets paleorift). Part 2. Structural parageneses of the reid deformation of the sedimentary cover

Formulation of the problem. The inversion stages of the structural-material evolution of the continental crust are characterized by regional scale of the deformation of volume tectonic flow of platform sedimentary rocks, which caused significant horizontally-healthy movements of tectonically activated geo-mass in the intra-plate graben-rift. The dynamic deformation of geomases manifests itself at the final stages of the geological development of such structures; therefore, it determines the main features of the systemic organization of the modern architecture of the basement and sedimentary cover complexes. Review of previous publications and studies. Secondary deformation linear slope, according to Patalakha E. (1979); Alekseev V. (1990) is considered as a set of local strike-slip zones, formed on spherically-located viscous faults, forming joint tectonic flows. The structural skeleton of tectonic flows, as the basis of the tectonic dislocation process, make offsets with the horizontally and rotational component movements. Horizontal landslides are typical disjunctive elements of the continental structures formed in the geodynamic conditions of the transtension (grabens, rifts), transpression (folded mobile belts, intra-plate activation zones), as well as the basic structural elements of the strike-slip tectonics of the sedimentary basin. The purpose of the article. The second part of the trilogy continues regional geotectonic studies of post-rift complications of the sedimentary cover structure of Dnipro-Donets Paleorift (DDP), covering the three main stages of the platform tectonic activation. Late Hercynian epoch was characterized by the formation of large linear anticlinal zones and salt-shafts against the background of the general syneclizal deflection of sedimentary basin, located within the paleorift. Cimmerian epoch of tectonic activity led to significant thrust deformations of the primary structural forms of the cover in the geodynamic setting of collision compression with intensification of the horizons of the sedimentary cover. The structural manifestation of strike-slip tectonics at Alpine epoch in sedimentary complexes are mainly horizontal displacements of geomases of blocks, lineaments, local tectonic elements and structures along dynamically interconnected coulisse of strike-slip domains. Methods. Structural kinematic analysis of transformations of the fracture systems of the Upper-Visean sedimentary complex under the influence of natural mechanism of reid tectonic flow of rocks. Results. The secondary structures of strike-slip deformation, structural kinematic parageneses, dislocations zones and subregional structural waves were identified in the structure of the platform cover, which are natural geological objects formed at the inversion stages of geological development of DDP. Scientific novelty and practical significance. The result of structural manifestation of volume tectonic flow in the platform complex was horizontal displacements of the original tectonic elements, blocks, segments, structures along dynamically-linked strike-slip domains. This is caused by the formation of secondary deformation structures of various scales, morphology and genesis, which comprise longitudinal structural waves of strips of post-rift deformations in the structure of the sedimentary cover of DDP.

Quantitative finite strain analysis of the quartz mylonites within the Three Pagodas shear zone, western Thailand

Abstract The NW–trending Three Pagodas shear zone exposes a high–grade metamorphic complex named Thabsila gneiss in the Kanchanaburi region, western Thailand. The quartz mylonites within this strike–slip zone were selected for strain analysis. 2–dimensional strain analysis indicates that the averaged strain ratio (Rs) for the lower greenschist facies increment of XZ– plane is Rs = 1.60–1.97 by using the Fry’s method. Kinematic vorticity analysis of the quartz mylonites in the shear zone showed that the mean kinematic vorticity number of this increment is Wk = 0.75–0.99 with an average at 0.90 ±0.07. The results implied that the quartz mylonites within the Three Pagodas shear zone have a dominant simple shear component of about 72% with a small pure shear component. A sinistral shear sense is indicated by kinematic indicators from macro– to micro–scale. We conclude that the Three Pagodas shear zone deformed in the process of sinstral shear–dominated transpression, which is similar to the Mae Ping shear zone in the north.

Editorial to the special Special Volume of Global and Planetary Change “Coupled Deep Earth and Surface Processes”

Lake-floor morphologies may be significantly different from seafloor topographies of other basins, typically observed in passive or active continental margins. The bathymetry of large paleo-lakes is often overwritten by subsequent tectonic evolution, burial beneath thick overburden and inherent compaction effects. We study the evolution of such an initial underfilled, balance fill and finally overfilled large paleo-lake basin by the interpretation of 2D and 3D seismic data set corroborated with calibrating wells in the example of the Neogene Pannonian Basin of Central Europe. Lake Pannon persisted for about 7–8 Myr and was progressively filled by clastic material sourced by the surrounding mountain chains and transported by large rivers, such as the paleo-Danube and paleo-Tisza. We combined sedimentological observations with a backstripping methodology facilitated by well lithology and porosity data to gradually remove the sediment overburden. This approach has resulted in a morphological reconstruction of the former depositional surfaces with special focus on the prograding shelf-margin slopes. Our calculations show that the water depth of the lake was more than 1000 m in the deepest sub-basins of the Great Hungarian Plain of the Pannonian Basin. The significant compaction associated with lateral variations of Neogene sediment thicknesses has created non-tectonic normal fault offsets and folds. These features have important effects on fluid migration and hydrocarbon trapping. We furthermore compare the geometries and effects of such non-tectonic features with the activity of larger offset sinistral strike-slip zones using 3D seismic attributes.

Miocene basin opening in relation to the north-eastward tectonic extrusion of the ALCAPA Mega-Unit

AbstractThe opening and evolution of the Western Carpathians Miocene basins was closely related to the north-eastward tectonic extrusion of the ALCAPA Mega-Unit lithosphere caused by the final stage of collision of the Eastern Alpine–Western Carpathian orogenic system with the European Platform and Alpine convergence with the Adria plate. The roll back effect of the oceanic or thinned continental crust of the Magura–Krosno realms, subduction below the front of the Carpathians in the north-east, east and relative plate velocities led to gradual stretching of the overriding micro-plates (defined as the ALCAPA and Tisza Dacia Mega-Unit). Diverse movement trajectories of the ALCAPA crustal wedge individual segments (Eastern Alps, Western Carpathians, and Northern Pannonian domain) were accompanied by several counter-clockwise rotational phases. Beside the interpreted Early Miocene “en-block” counter-clockwise rotation, most of the rotations in the Central Western Carpathians were caused by “domino-effect tectonics” inside strike-slip zones and took part in the basin opening, which was in most cases followed by rapid subsidence.

Tectonic control on the genesis of magmas in the New Hebrides arc (Vanuatu)

We present here new bathymetric, petrological and geochemical whole rock, glass and mineral data from the submarine Epi volcano in the New Hebrides (Vanuatu) island arc. The structure has previously been interpreted to be part of a larger caldera structure but new bathymetric data reveal that the volcanic cones are aligned along shear zones controlled by the local tectonic stress field parallel to the recent direction of subduction. We aim to test if there is an interaction between local tectonics and magmatism and to what extent the compositions of island arc volcanoes may be influenced by their tectonic setting. Primitive submarine Epi lavas and those from the neighbouring Lopevi and Ambrym islands originate from a depleted mantle wedge modified by addition of subduction zone components. Incompatible element ratios sensitive to fluid input (e.g., Th/Nb, Ce/Yb) in the lavas are positively correlated with those more sensitive to mantle wedge depletion (e.g., Nb/Yb, Zr/Nb) amongst the arc volcanoes suggesting that fluids or melts from the subducting sediments have a stronger impact on the more depleted compositions of the mantle wedge. The whole rock, glass and mineral major and trace element compositions and the occurrence of exclusively normally zoned clinopyroxene and plagioclase crystals combined with the absence of inversely zoned crystals and water-bearing phases in both mafic and evolved lavas suggest that the erupted melt was relatively dry compared to other subduction zone melts and has experienced little disequilibrium modification by melt mixing or assimilation. Our data also imply that differentiation of amphibole is not required to explain the incompatible element patterns but may rather result from extensive clinopyroxene fractionation in agreement with petrographic observations. Thermobarometric calculations indicate that the melts fractionated continuously during ascent, contrasting with fractionation during stagnation in an established crustal magma reservoir. We interpret the occurrence of this fractional crystallisation end-member in a relatively thick island arc crust (~30 km thickness) to result from isolated and relatively rapid ascent of melts, most likely through a complex system of dykes and sills that developed due to the tectonic positioning of Epi in a complex tectonic zone between a compressional environment in the north and an extensional setting in the south. We can show that the alignment of the cones largely depends on the local tectonic stress field at Epi that is especially influenced by a large dextral strike-slip zone, indicating that structural features have a significant impact on the location and composition of volcanic edifices.

Deformation of the Pacific/North America Plate Boundary at Queen Charlotte Fault: The Possible Role of Rheology

AbstractThe Pacific/North America (PA/NA) plate boundary between Vancouver Island and Alaska is similar to the PA/NA boundary in California in its kinematic history and the rate and azimuth of current relative motion, yet their deformation styles are distinct. The California plate boundary shows a broad zone of parallel strike slip and thrust faults and folds, whereas the 49‐mm/yr PA/NA relative plate motion in Canada and Alaska is centered on a single, narrow, continuous ~900‐km‐long fault, the Queen Charlotte Fault (QCF). Using gravity analysis, we propose that this plate boundary is centered on the continent/ocean boundary (COB), an unusual location for continental transform faults because plate boundaries typically localize within the continental lithosphere, which is weaker. Because the COB is a boundary between materials of contrasting elastic properties, once a fault is established there, it will probably remain stable. We propose that deformation progressively shifted to the COB in the wake of Yakutat terrane's northward motion along the margin. Minor convergence across the plate boundary is probably accommodated by fault reactivation on Pacific crust and by an eastward dipping QCF. Underthrusting of Pacific slab under Haida Gwaii occurs at convergence angles >14°–15° and may have been responsible for the emergence of the archipelago. The calculated slab entry dip (5°–8°) suggests that the slab probably does not extend into the asthenosphere. The PA/NA plate boundary at the QCF can serve as a structurally simple site to investigate the impact of rheology and composition on crustal deformation and the initiation of slab underthrusting.

Visualisation and analysis of shear-deformation bands in unconsolidated Pleistocene sand using ground-penetrating radar: Implications for paleoseismological studies

Deformation bands in unconsolidated sediments are of great value for paleoseismological studies in sedimentary archives. Using ground-penetrating radar (GPR), we investigated an array of shear-deformation bands that developed in unconsolidated Pleistocene glacifluvial Gilbert-type delta sediments. A dense grid (spacing 0.6 m) of GPR profiles was measured on top of a 20 m-long outcrop that exposes shear-deformation bands. Features in the radargrams could be directly tied to the exposure. The shear-deformation bands are partly represented by inclined reflectors and partly by the offset of reflections at delta clinoforms. 3-D interpretation of the 2-D radar sections shows that the bands have near-planar geometries that can be traced throughout the entire sediment volume. Thin sections of sediment samples show that the analysed shear-deformation bands have a denser grain packing than the host sediment. Thus they have a lower porosity and smaller pore sizes and therefore, in the vadose zone, the deformation bands have a higher water content due to enhanced capillary forces. This, together with the partially-developed weak calcite cementation and the distinct offset along the bands, are likely the main reasons for the clear and unambiguous expression of the shear-deformation bands in the radar survey. The study shows that deformation-band arrays can clearly be detected using GPR and quickly mapped over larger sediment volumes. With the 3-D analysis, it is further possible to derive the orientation and geometry of the bands. This allows correlation of the bands with the regional fault trend. Studying deformation bands in unconsolidated sediments with GPR is therefore a powerful approach in paleoseismological studies. Based on our data, we postulate that the outcrop is part of a dextral strike-slip zone that was reactivated by glacial isostatic adjustment.

Fault-related domes: Insights from sedimentary outcrops at the northern tip of the Gulf of Suez rift, Egypt

Domes may develop with various mechanisms in different types of rocks. Knowing the geometries and occurrences of domes is an important concern in classifying different types of domes. A number of fault-related domal structures are outcropped at the northern Gulf of Suez rift, Egypt and they are easily visualized on satellite imagery. Detailed surface geological field mapping is carried out at several localities to assess types and origins of these domes. Three main subdivisions of fault-related domes are suggested in the present study. These are: (1) domes formed in strike-slip zones (D-SZ), (2) domes formed by drag along normal fault arrays (D-DF) and (3) domes formed at transfer zones of normal faults (D-TZ). Structural analysis of the studied domes reveals that each dome type has its own geometric parameters, structural style and tectonic setting. The obtained results are without doubt interesting for the analysis of similar domes at the subsurface and other exposures worldwide.