Low-angle Faults Research Articles

Structural analysis and evolution of large Venusian coronae: Insights from low-angle faults at coronae rims

We analyzed topography, fracture patterns, and faults of the asymmetric Atahensik Corona (700 × 900 km diameter), formerly known as Latona Corona, and their surrounding troughs using Magellan SAR imagery, and compare the results with the smaller, ovoid Didilia (400 × 450 km diameter) and Pavlova coronae (550 × 650 km diameter) to get insights on corona formation on Venus. Atahensik contains a high density of radial, oblique, and concentric fractures, the latter are inferred to be the youngest fractures. A high density of concentric fractures particularly occurs along the outer rise and indicates elastic downward bending of this part of the lithosphere in the later stage of corona formation. Along the steep inner slopes of Atahensik's arcuate troughs, large-scale faults are exposed that dip gently towards the corona center and crosscut all fractures. We propose that these low-angle faults were initially formed as thrust planes but subsequently became reactivated as low-angle normal faults, thereby exposing parts of their fault surfaces. Such faults have been identified not only along the arcuate troughs of Atahensik but also occur in Dali Chasma, northwest of Atahensik Corona.A phenomenological formation model of large coronae is presented: corona initiation starts with radial fracturing, which is caused by the dike emplacement and thermal uplift of the corona center due to the rise of a hot asthenospheric mantle plume. Uplift and lateral plume spreading steepen the outer rim of the uplift and cause intense radial fracturing of a central volcanic edifice and the corona's outer rim. This intermediate stage is preserved in several less-evolved coronae such as Didilia and Pavlova Coronae. The fractured ridge thrusts outward onto an intact and cooler lithosphere along strongly localized thrust planes. The overthrusted, cooler lithosphere is elastically bent downward and forms arcuate troughs and associated outer rises with numerous concentric fractures along their crest line. The fractured ridge annulus of the corona is supported by the intact and thickened lithosphere surrounding the corona. The present morphology of Atahensik Corona indicates subsequent subsidence in its central part due to declining plume activity and reduced thermal buoyancy. Reactivation of the thrusts as low-angle normal faults results from the subsidence of the corona interior, a gravitational instability of the elevated corona annulus, and a lack of shortening. The evolutionary sequence derived on the basis of structural data is in agreement with geodynamic models on corona formation involving a bending lithosphere at the plume margin.

A Numerical Study of Fault Reactivation Mechanisms in CO2 Storage.

Due to extensive industrialization and ongoing fossil fuel consumption, CO2 emissions have significantly contributed to climate change and rising greenhouse gas levels. The collection and storage of CO2 in subsurface geological formations have been proposed as a feasible alternative. The well-documented In Salah storage site is the subject of the chosen case study. For the numerical analysis, a two-dimensional finite element simulation of fault reactivation processes was conducted in the context of the CO2 storage. The goal of this research is to conduct a mechanistic numerical analysis of a typical CO2 storage condition. The selected analysis domain is 4 km (width) × 2 km (depth) and includes all important domains and formations (overburden, main caprock, lower caprock, and underburden) of the In Salah site. The simulation results indicate that the influence of fault reactivation under 32 MPa of base injection pressure results in peak vertical deformation of 0.044 m in the caprock and a vertical displacement magnitude of 0.015-0.020 m at the surface level (Z = 0 m). The derived vertical deformation findings at the surface level are in agreement with the data obtained from the in situ InSAR monitoring system in 2009. The effects of the changes in the fault dip angle, key caprock mechanical parameters, and in situ stress ratio on the displacement profile are evaluated within the parametric study. In comparison to the benchmark numerical run, the scenario ratio of k = 0.5 led to a significant reduction in the displacement. The simulation in which the fault dip angle was 30° produced a more pessimistic result with a larger displacement field. This could be an indication of heightened fault reactivation risks associated with low-angle faults in storage sites with strong horizontal stress regimes due to the combined effect of increased shear stress and reduced inherent frictional resistance on the fault plane. Considering that a vertical fault dip angle (90°) and an additional three 20 m long vertical fractures above the reservoir produced similar vertical displacement observed with the fault dipping at a 60° angle, this indicated that the vertical faults in the vicinity of the storage site pose limited safety risks to the integrity of the sealing rock.

Kinematic-structural modeling of hybrid fold-thrust belt systems: Insights from the Southern Patagonian Andes

This paper focuses on the analysis of the Southern Patagonian fold-thrust belt at ca. 50° SL, employing a kinematic-structural approach that makes it possible to define its step-by-step structural evolution. Seismic interpretation is combined with outcrop structural data to provide the basis for an integrated evolutionary model including a Jurassic extensional stage, followed by contraction events since the Late Cretaceous. Our interpretation and results reveal a hybrid fold-thrust belt system, with a high decoupling between basement structures and the sedimentary cover. The basement is deformed by tectonic inversion of the Jurassic rift fault system and duplex stacking geometries, while the sedimentary cover is folded by the action of predominantly west-vergent low-angle faults. The Jurassic rifting model identifies listric faults with detachment depths ranging from −6000 to −6500 m.a.s.l. and 11 %–14 % magnitude extension. Shortening calculations yield values of less than 6 %. Growth strata and thickness variations in Cretaceous units suggest that part of the shortening occurred during the Coniacian/Santonian. Overall, this research contributes to the knowledge of fold-thrust belts, highlighting the importance of doing a comprehensive structural modeling including previous deformational stages, and describing steps to provide reliable measurements of deformation.

The seismogenic structure of March 2021 Tyrnavos (central Greece) doublet (Mw 6.3 andMw 6.0), constrained by aftershock locations and geodetic data

SUMMARYThe Northern Thessaly Basin in central Greece ranks amongst the most well pronounced extensional (graben) basins in the backarc Aegean Sea region, with well-mapped faults having an ∼E–W orientation, compatible with the ongoing predominant ∼N–S extension. The southern margin of the basin is bounded by major faults associated with strong (M6 to M7) earthquakes, whereas along its northern margin, strong events are more scarce, in the documented catalogues. Along this northern margin, a weak, albeit persisting foreshock activity, culminated within 3 d, to an Mw 6.3 earthquake on 3 March 2021 associated with a 15-km-long NE dipping fault segment. It was followed the next day, by the second Mw 6.0 main shock associated with a 13-km-long NE dipping fault segment and 9 d later by an Mw 5.5 earthquake associated with an 8-km-long SW dipping fault segment, with its aligned epicentres, showcasing the cascade type activation of adjacent fault segments. The sequence, evolved to be very productive, with aftershocks extending ∼50 km along a ∼NW–SE trending narrow seismic zone. All events indicate pure normal faulting, with an NNE–SSW oriented extensional axis, oblique to our previous consensus of the prevalence of ∼N–S extension. This observation documents that inherited fault fabric can be reactivated within the modern tectonic stress field. We use high-quality seismological data, alongside Interferometric Synthetic Aperture Radar (InSAR) methodology and Global Navigation Satellite System (GNSS) data, to study the temporal and spatial evolution of the sequence, and to provide inferred kinematic models that describe the complexity of the seismic process, in terms of heterogeneous slip distribution, activated fault planes, fault geometry and displacement field. Cross-sections show that the activity defines the crustal seismogenic layer at depths between 5 and 10 km, associated with low-angle fault segments dipping to the NE. Other faults, both antithetic and secondary ones, appear active and accommodated aftershocks clusters. Using our preferred finite fault source model, we calculated the changes of Coulomb failure stress on the neighbouring faults.

The 3-D shallow velocity structure and sedimentary structure of 2017 Ms6.6 Jinghe earthquake source area derived from dense array observations of ambient noise

To understand the shallow structure and complex sedimentary environment of the 2017 Jinghe Ms6.6 earthquake focal area, we used one mouth of continuous sesimic data from a dense seismic array of 208 short period stations around the earthquake focal area, and applied the ambient noise tomography (ANT) method to image the three-dimensional Shear wave velocity structure at the depth less than 4 km. The shear wave velocity shown clear lateral variations and vertical variations from the surface to the deeper regions and has a tight correlation with surface geological and tectonic features in the study area. Obvious low-velocity anomalies has been presented throughout most of the Jinghe depression, whereas the Borokonu Mountains presented high-velocity anomalies. The thickness of the Cenozoic sedimentary basement in the study area is approximately 1–4 km, and the distribution of thickness is highly uneven. The crystalline basement in the study area has strong bending deformation, and the non-uniform Cenozoic sediments are related to the strong bending deformation of the crystalline basement. The Kusongmuchik piedmont fault is a high-angle thrust fault cutting through the base. There are also many medium low-angle faults, which do not penetrate the surface, which has indicated that they are in a concealed state at present. The results have provided a shallow high-resolution 3D velocity model that can be used in the simulation of strong ground motion and for evaluating potential seismic hazards.

Proterozoic fold and thrust belt imaged on reflection seismic in Arabian plate basement

Major structures are imaged on 2D reflection seismic data within the Precambrian basement of the western Rub’ Al-Khali of Saudi Arabia for the first time. In this area the base-Cambrian Angudan Unconformity that defines top basement is at a depth of 1–2 km with a gentle easterly dip. Large antiformal structures are clear on 2D reflection seismic within the basement. The antiforms are separated by zones that are interpreted as low-angle faults. No basal detachment is seen and the faults pass below the base of reflection seismic at a depth of approximately 12 km. Individual folds have dimensions in the order of 20 km wavelength and 2 km amplitude and occur between 3 and 10 km depth. The structural style is interpreted as compressional by comparison with thrust systems and accretionary prisms elsewhere. Regional studies indicate that basement is comprised of a series of north-south trending oceanic Proterozoic terranes that were brought together in the Ediacaran East African Orogeny. Based on current regional tectonic maps the newly-imaged fold belt is on the east margin of the Khida-Abas terrane. Depth conversion shows the thrusts dip at approximately 10° north in the line of section. A novel combination of cross-section to structural trend obliquity estimates combined with matched filtering of potential field data indicate the fold and thrust belt trends 015°-195° with ESE vergence. This compressional belt may be some internal structure of the Khida-Abas terrane, or may be a non-outcropping accretionary prism on the margin of that terrane, or may even represent a set of amalgamated microterranes that do not outcrop along trend. Either way, these observations represent a major, previously undescribed element of this sector of the East African Orogen.

Terrane and core complex architecture of the Otago Schist in the Dunstan and Cairnmuir Mountains, New Zealand, from U-Pb and (U-Th)/He zircon dating

ABSTRACT We report ten new zircon (U-Th)/He ages from psammitic greyschists collected in a southwest-northeast profile in the Otago Schist across the Dunstan and Cairnmuir Mountains, New Zealand. Six of the samples have accompanying new detrital zircon U-Pb ages. In this part of the Otago Schist, a broad antiformal core of garnet-biotite-albite zone schist defines a range-scale footwall block to the gently northeast-dipping Thomsons Gorge Fault and the gently southwest-dipping Cromwell Gorge faults. Hanging wall blocks above the faults comprise lower-grade chlorite zone schists. Nine schist samples from footwall and hanging wall blocks close to the low-angle faults exhibit a weighted mean (U-Th)/He age of 82 ± 3 Ma. We interpret that both low-angle fault systems were active prior to c. 82 Ma and confirm that they were extensional. We interpret the antiformal schist core as the late Early Cretaceous (c. < 122–109 Ma) ductile extensional lower plate of a metamorphic core complex. The lower plate (footwall) core was juxtaposed against the upper plate (hanging wall) schists by the low-angle normal decollement faults. Detrital zircon U-Pb ages indicate Carboniferous, Permo-Triassic, Jurassic and Cretaceous maximum depositional ages of schist protoliths which demand more complex terrane and metamorphic interpretations for the Otago Schist than previously recognised.

Magnitude distribution and clustering properties of the 3-D seismicity in Central Apennines (Italy)

SUMMARYIn this paper we deal with statistical features of earthquakes, seeking possible correlations between the Gutenberg–Richter magnitude distribution and the short-term clustering in an area of the Central Apennines, Italy, where significant seismicity with earthquakes exceeding magnitude 6.0 has been repeatedly observed from 1990 to the present. For this purpose, a recently developed version of the ETAS model, incorporating a 3-D spatial triggering kernel, has been adopted. Our analysis has been carried out representing the b-value and the probability of independence of events on six vertical cross-sections suitably related to the seismic structures that are considered responsible of the seismicity observed in the study area. The results of the statistical analysis of the seismicity in the study area have shown a clear distinction between the western normal low-angle fault system, characterized by eastward dip and the eastern normal fault systems, with westward dip. In the former (Etrurian Fault System; EFS) we found seismicity with a high b-value and high probability of independence, that is a scarce capacity of producing clusters and strong aftershock sequences. The eastern fault systems of our study area are distinguishable in two main distinct systems, which generated two strong seismic sequences in 1997 and 2016–2017. In the former (Colfiorito) sequence the seismicity showed a very low b-value and a modest probability of independence, while in the latter (Central Italy) sequence the b-value was significantly higher and the probability of independence had extremely low values (manifesting a high level of clustering). The much higher b-value of the EFS than the other extensional sources could be caused by its peculiar seismotectonic role of discontinuity at the base of the normal active faulting, and its reduced capacity of accumulating stress. This circumstance may be interpreted by a difference in the rheological properties of these fault systems, possibly also in relation to their present status in the earthquake cycle and the presence of strong aftershock sequences.

Fluid-rock interaction control on fault reactivation: A review of the Montmell-Vallès Fault System, central Catalan Coastal Ranges (NE Iberia)

Structural inheritance is a key factor controlling the tectonic evolution of the central Catalan Coastal Ranges. Up to two periods of tectonic inversion (one positive during the Paleogene and the other negative during the Neogene) affected a previously well-developed Mesozoic extensional basin system and characterized the Cenozoic evolution of the area. In this scenario, tectonic fault inversion is often observed along the Montmell-Vallès Fault System. Fault reactivation shows differences along strike from NE to SW and appears decoupled from surface to depth due to its kinked-planar geometry and the change of fault dip from >60° to 30° in depth. The ability of the Mesozoic faults to be reactivated appears also influenced by changes in the mechanical properties of the inherited fault zone. Whereas the deeper and less dipping panels of the major faults are reactivated in the entire zone (contractional during the Paleogene and extensional during the late Oligocene-Neogene), the upper and highly dipping parts of the faults only show local reactivations. The observations indicate that fault dip, the indirect role of hosting lithologies (granites and siliciclastic metasediments versus carbonate rocks) on fault rocks, the indirect role of mineral precipitation and cementation product of fluid circulation, and the direct role of the mechanical properties of the resulting fault rocks (gouge versus cemented breccias) significantly control the fault reactivation. Upper crust low angle fault segments are easily reactivated during contractional deformation but not during the extensional one. Conversely, the segments with a high-angle dip are more easily reactivated during the extensional deformation but not during the contractional one. In the study area, this resulted in the formation of footwall short-cuts developed during the Paleogene compression and extensional short-cuts occurred during the Neogene extension. On the other hand, reactivation is effective in areas where granites and siliciclastic metasediments characterize the host-rock, and non-cohesive fault gouge forms the pre-existent fault core. Instead, fault reactivation appears restricted or even prevented where the host-rock includes thick carbonate successions, and the pre-existent damage zone is formed by highly cemented and cohesive breccias.

The low-angle breakaway system for the Northern Snake Range décollement in the Schell Creek and Duck Creek Ranges, eastern Nevada, USA: Implications for displacement magnitude

AbstractDocumenting the kinematics of detachment faults can provide fundamental insights into the ways in which the lithosphere evolves during high-magnitude extension. Although it has been investigated for 70 yr, the displacement magnitude on the Northern Snake Range décollement in eastern Nevada remains vigorously debated, with published estimates ranging between &amp;lt;10 and 60 km. To provide constraints on displacement on the Northern Snake Range décollement, we present retrodeformed cross sections across the west-adjacent Schell Creek and Duck Creek Ranges, which expose a system of low-angle faults that have previously been mapped as thrust faults. We reinterpret this fault system as the extensional Schell Creek Range detachment system, which is a stacked series of top-down-to-the-ESE brittle normal faults with 5°–10° stratigraphic cutoff angles that carry 0.1–0.5-km-thick sheets that are up to 8–13 km long. The western portion of the Schell Creek Range detachment system accomplished ~5 km of structural attenuation and is folded across an antiformal culmination that progressively grew during extension. Restoration using an Eocene unconformity as a paleohorizontal marker indicates that faults of the Schell Creek Range detachment system were active at ~5°–10°E dips. The Schell Creek Range detachment system accommodated 36 km of displacement via repeated excision, which is bracketed between ca. 36.5 and 26.1 Ma by published geochronology. Based on their spatial proximity, compatible displacement sense, overlapping deformation timing, and the similar stratigraphic levels to which these faults root, we propose that the Schell Creek Range detachment system represents the western breakaway system for the Northern Snake Range décollement. Debates over the pre-extensional geometry of the Northern Snake Range décollement hinder an accurate cumulative extension estimate, but our reconstruction shows that the Schell Creek Range detachment system fed at least 36 km of displacement eastward into the Northern Snake Range décollement.

Oligocene magmatic accretion and transtensional tectonics in the Central Basin Fault rift of the West Philippine back-arc basin: New insights from high-resolution multichannel seismic data

High-resolution multichannel seismic transects that span from oceanic crust domains, and the Central Basin Fault rift to the Kyushu-Palau Ridge are utilized to investigate the dynamic mechanism of the Oligocene tectonic evolution in the West Philippine Basin based on an analysis of sismostratigraphic sequences and the geometries of acoustic substrates. In the oceanic domains, the physiography is characterized by ridges and troughs, and the pelagic stratigraphic succession with transparent facies covers the irregular top of the basement. Relatively homogeneous intracrustal reflections probably correspond to an area of abundant magma supply during the major spreading period. However, a widespread discrete, layered and high-amplitude intracrustal reflector is present in the Central Basin Fault rift that can be interpreted as an area of weaker and unstable magma supply after the major spreading period. The first group of reflectors shows typical low-angle fault shapes; the second and third groups of layered intracrustal reflectors are characterized by spatially limited high-amplitude seismic reflections. Inclined and mounded features of the intracrustal reflectors are interpreted as the result of tectonic processes, suggesting the presence of later magmatic accretion and transtensional tectonics within the acoustic substrate. Additionally, in the Kyushu-Palau Ridge, the anticline that appears within the stratigraphic infills of the basins serves as evidence of transtensional faulting. Overall, we propose that the Oligocene tectonic evolution of the West Philippine Basin occurred during a later transcurrent period due to Oligocene magmatic accretion followed by transtensional tectonics. The changes in the magmatic supply rate and tectonic processes of the Central Basin Fault rift may have originated from the compressive process within the Kyushu-Palau Ridge.

Northward migration of the Javanese volcanic arc along thrust faults

East Java is characterised by a complex interaction of volcanic and tectonic processes and it is marked by isolated eruptive centres scattered across the back-arc sedimentary basins. In 2006 a large sediment hosted geothermal system named Lusi, pierced the Kendeng basin in East Java and since then it continues in a relentless eruption of mud breccia. To investigate the spatial and structural relationships between the volcanic arc and the back-arc domains, we perform a local earthquake tomography. The inversion of regional earthquakes recorded by our seismic network (for about two years) shows sharp Vp and Vp/Vs transitions. We observe a marked reduction of P-wave velocities and a high Vp/Vs ratio in the back-arc basins. Our study highlights a clear connection between the plumbing system of the volcanic arc and the northern sedimentary province. We propose a conceptual model suggesting that magmas and hydrothermal fluids may migrate from the middle to the upper crust into the sedimentary basins capitalising on existing thrust faults. Such low angle faults, promoted by the compressional regime of the region, link the magmatic domain to the northern sedimentary provinces. This process may represent the early phase of volcanic arc migration when magma-derived fluids are focused into fractured and permeable geological structures. Our conceptual model would not only help to understand the occurrence of the abundant mantle-derived fluids sampled across the back-arc, but it is also consistent with the occurrence of isolated magmatic and hybrid systems piercing across sedimentary environments in the back-arc of Java.

Impact of basement thrust faults on low-angle normal faults and rift basin evolution: a case study in the Enping sag, Pearl River Basin

Abstract. Reactivation of pre-existing structures and their influence on subsequent rift evolution have been extensively analysed in previous research on rifts that experienced multiple phases of rifting, where pre-existing structures were deemed to affect nucleation, density, strike orientation, and displacement of newly formed normal faults during later rifting stages. However, previous studies paid less attention to the extensional structures superimposing onto an earlier compressional background, leading to a lack of understanding of, e.g. the reactivation and growth pattern of pre-existing thrust faults as low-angle normal faults and the impact of pre-existing thrust faults on newly formed high-angle faults and subsequent rift structures. This study investigating the spatial relationship between intra-basement thrust and rift-related faults in the Enping sag, in the northern South China Sea, indicates that the rift system is built on the previously deformed basement with pervasive thrusting structures and that the low-angle major fault of the study area results from reactivation of intra-basement thrust faults. It also implies that the reactivation mode of basement thrust faults is dependent on the overall strain distribution across rifts, the scale of basement thrust faults, and the strain shadow zone. In addition, reactivated basement thrust faults influence the nucleation, dip, and displacement of nearby new faults, causing them to nucleate at or merge into downwards it, which is representative of the coupled and decoupled growth models of reactivated thrust faults and nearby new faults. This work not only provides insights into the growth pattern of rift-related faults interacting with reactivated low-angle faults but also has broader implications for how basement thrust faults influence rift structures, normal fault evolution, and syn-rift stratigraphy.

Tidal Triggering of Microseismicity at the Equatorial Mid‐Atlantic Ridge, Inferred From the PI‐LAB Experiment

AbstractThe gravitational pulls from the moon and the sun result in tidal forces which influence both Earth's solid and water mass. These stresses are periodically added to the tectonic ones and may become sufficient for initiating rupture in fault systems critically close to failure. Previous research indicates correlations between increased seismicity rates and low tides for fast‐ and intermediate‐spreading mid‐ocean ridges in the Pacific Ocean. Here, we present a microseismicity data set (4,719 events) recorded by an ocean bottom seismometer deployment at the equatorial Mid‐Atlantic Ridge. We show that low, as well as decreasing ocean water level, result in relatively elevated seismicity rates at higher magnitudes (lower b‐values), translated into increased probabilities of stronger event occurrence at or towards low tides. Moreover, seismic bursts (enhanced activity rate clusters), occurring at rates well above the reference seismicity, are exclusively present during values of either high tidally induced extensional stresses or high extensional stress rates. Although the b‐value differences are not significant enough to be conclusive, the seismicity rate variations exhibit statistical significance, supporting the previous findings for tidal triggering at low tides within normal‐faulting regimes and extending the range of observations to slow‐spreading ridges. Observed triggering of slip on low angle faults at low tides is predicted by Coulomb stress modeling. The triggering of slip on high angle faults observed here, is not easily explained without another factor. It may be related to the presence of a shallow magma body beneath the ridge, as supported by previous seismic imaging in the region.

Superposition of two kinematically distinct extensional phases in southern Death Valley: Implications for extensional tectonics

Abstract Geologic mapping in southern Death Valley, California, demonstrates Mesozoic contractional structures overprinted by two phases of Neogene extension and contemporaneous strike-slip deformation. The Mesozoic folding is most evident in the middle unit of the Noonday Formation, and these folds are cut by a complex array of Neogene faults. The oldest identified Neogene faults primarily displace Neoproterozoic units as young as the Johnnie Formation. However, in the northernmost portion of the map area, they displace rocks as young as the Stirling Quartzite. Such faults are seen in the northern Ibex Hills and consist of currently low- to moderate-angle, E-NE–dipping normal faults, which are folded about a SW-NE–trending axis. We interpret these low-angle faults as the product of an early, NE-SW extension related to kinematically similar deformation recognized to the south of the study area. The folding of the faults postdates at least some of the extension, indicating a component of syn-extensional shortening that is probably strike-slip related. Approximately EW-striking sinistral faults are mapped in the northern Saddlepeak Hills. However, these faults are kinematically incompatible with the folding of the low-angle faults, suggesting that folding is related to the younger, NW-SE extension seen in the Death Valley region. Other faults in the map area include NW- and NE-striking, high-angle normal faults that crosscut the currently low-angle faults. Also, a major N-S–striking, oblique-slip fault bounds the eastern flank of the Ibex Hills with slickenlines showing rakes of &amp;lt;30°, which together with the map pattern, suggests dextral-oblique movement along the east front of the range. The exact timing of the normal faulting in the map area is hampered by the lack of geochronology in the region. However, based on the map relationships, we find that the older extensional phase predates an angular unconformity between a volcanic and/or sedimentary succession assumed to be 12–14 Ma based on correlations to dated rocks in the Owlshead Mountains and overlying rock-avalanche deposits with associated sedimentary rocks that we correlate to deposits in the Amargosa Chaos to the north, dated at 11–10 Ma. The mechanism behind the folding of the northern Ibex Hills, including the low-angle faults, is not entirely clear. However, transcurrent systems have been proposed to explain extension-parallel folding in many extensional terranes, and the geometry of the Ibex Hills is consistent with these models. Collectively, the field data support an old hypothesis by Troxel et al. (1992) that an early period of SW-NE extension is prominent in the southern Death Valley region. The younger NW-SE extension has been well documented just to the north in the Black Mountains, but the potential role of this earlier extension is unknown given the complexity of the younger deformation. In any case, the recognition of earlier SW-NE extension in the up-dip position of the Black Mountains detachment system indicates important questions remain on how that system should be reconstructed. Collectively, our observations provide insight into the stratigraphy of the Ibex Pass basin and its relationship to the extensional history of the region. It also highlights the role of transcurrent deformation in an area that has transitioned from extension to transtension.

The March 2021 Damasi Earthquake Sequence, Central Greece: Reactivation Evidence across the Westward Propagating Tyrnavos Graben

On 3 March 2021, a strong shallow earthquake affected northern Thessaly, Greece, with an epicenter close to Damasi village causing significant destruction of many stone houses. In this contribution, we provide fieldwork observations, satellite radar interferometry, mapping of the active faults exposed in the epicentral area, liquefactions and coseismic surface ruptures, and preliminary geomorphological analyses of the epicentral area. The geomorphological analysis is based on air photographs, digital surface models analysis, Real-Time Kinematik (RTK) measurements with Global Navigation Satellite System (GNSS) receivers, and data from UAV flight campaigns. Although the seismotectonic setting of the area is complex and there is an apparent mismatch between field and interferometric data, the results of our investigations suggest that at least three fault segments were reactivated by the major shocks of the March seismic sequence. These tectonic structureslikely represent the westward propagation of the Tyrnavos Graben, where newly formed and inherited low-angle faults interplay in a complex manner.

Structural and paleostress analysis within a fossil slow-spreading ridge: Tectonic processes involved during ocean expansion

This study aims to decipher the spatio-temporal chronology of tectonic processes leading to ocean widening at a slow-spreading oceanic ridge axis, involving mantle and gabbro exhumation and volcanism. Structural and petrographic studies of deformed lithologies were performed on peridotites, gabbros and basalts from the Chenaillet ophiolite (Alps, France). Inversion of fault-slip data reveals N030° and N060° σ3 paleostress trends. The dominant N030° extension, is consistent with the NNW-SSE direction of volcano feeder-dykes and ENE dipping low-angle normal faults crosscutting the mantle. The N060° extension accords with the mantle dome structure formation to the east. Low-angle faults intersect roots of volcanoes that thus belong to their hanging walls. Hydrothermalism is contemporaneous with or postdates low-angle faulting. A feeder-dyke major virgation northwards, synchronous with eruptions, suggests a dextral transform fault consistent with a N120° σ2. Oceanic expansion mechanisms are clarified. At the surface, magma eruption occurs along on-axis active high-angle normal faults, their footwalls enabling mantle exhumation. At depth, off-axis high-angle faults become low-angle faults as they spread at shallow level. With westward drift of the lithosphere, the uppermost levels of the ridge shift westward faster, such that volcanoes move to an off-axis position while their roots are cut by low-angle faults.

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.

The Mw = 5.6 Kanallaki Earthquake of 21 March 2020 in West Epirus, Greece: Reverse Fault Model from InSAR Data and Seismotectonic Implications for Apulia-Eurasia Collision

We identify the source of the Mw = 5.6 earthquake that hit west-central Epirus on 21 March 2020 00:49:52 UTC. We use Sentinel-1 synthetic aperture radar interferograms tied to one permanent Global Navigation Satellite System (GNSS) station (GARD). We model the source by inverting the INSAR displacement data. The inversion model suggests a shallow source on a low-angle fault (39°) dipping towards east with a centroid depth of 8.5 km. The seismic moment deduced from our model agrees with those of the published seismic moment tensors. This geometry is compatible with reverse-slip motion along the west-verging Margariti thrust fault that accommodates part of the convergence within the collision zone between Apulia and Eurasia. We also processed new GNSS data and estimate a total convergence rate between Apulia and Eurasia of 8.9 mm yr−1, of which the shortening of the crust between the Epirus coastal GNSS stations and station PAXO in the Ionian Sea (across the Ionian Thrust) is equivalent to ~50% of it or 4.6 mm yr−1. By back-slip modelling we found that a 60-km wide deformation zone takes up nearly most of the convergence between Apulia-Eurasia, trending N318°E. Its central axis runs along the southwest coast of Corfu, along the northeast coast of Paxoi, heading toward the northern extremity of the Lefkada island. The island of Paxoi appears kinematically as part of the Apulian plate.

The role of structural inheritance in the development of high-displacement crustal faults in the necking domain of rifted margins: The Klakk Fault Complex, Frøya High, offshore mid-Norway

The role of inherited structures during the development of normal faults in continental rifts and proximal domains of passive margins have been extensively studied. Few studies, however, have a focus on deciphering the role of inheritance in the development of high-displacement (>10 km), low-angle (<30°) normal faults in necking domains of passive margins. We integrated and interpreted potential field, 2D and 3D reflection seismic, and well data to study the role of structural inheritance in controlling the location and development of the southern part of the Klakk Fault Complex, offshore mid-Norway. The down-to-the-west Klakk Fault Complex is an N-S non-collinear fault complex that separates the Frøya High in its footwall from the Rås Basin in its hanging wall. The fault segments vary from low-angle planar to listric fault geometries in cross-section, with displacements of 17 km–34 km. These displacements led to syn-rift basement thinning of 12–14 km toward the west, which consequently, also affected the crustal wedge geometry of the necking domains. We identify three intra-acoustic-basement structures based on seismic facies and define their 3D geometry: (i) a bowl-shaped basin, (ii) a hyperbolic surface, and (iii) a domal structure. We discuss their origin and elucidate their role during later rifting. We conclude that pre-existing basement structures controlled the rift-related structures during the second rift phase (thinning), and affected the location, geometries, orientation and segmentation of the high-displacement low-angle faults in the necking domains. The results of this work offer new insights into the development of necking domains in areas where a thick continental crust (>25 km) is present during rifting.