Normal Fault System Research Articles

Seismic swarms and diffuse fracturing within Triassic evaporites fed by deep degassing along the low‐angle Alto Tiberina normal fault (central Apennines, Italy)

AbstractWe present high‐resolution elastic models and relocated seismicity of a very active segment of the Apennines normal faulting system, computed via transdimensional local earthquake tomography (trans‐D LET). Trans‐D LET, a fully nonlinear approach to seismic tomography, robustly constrains high‐velocity anomalies and inversions of P wave velocity, i.e., decreases of VP with depth, without introducing bias due to, e.g., a starting model, and giving the possibility to investigate the relation between fault structure, seismicity, and fluids. Changes in seismicity rate and recurring seismic swarms are frequent in the Apennines extensional belt. Deep fluids, upwelling from the delaminating continental lithosphere, are thought to be responsible for seismicity clustering in the upper crust and lubrication of normal faults during swarms and large earthquakes. We focus on the tectonic role played by the Alto Tiberina low‐angle normal fault (ATF), finding displacements across the fault consistent with long‐term accommodation of deformation. Our results show that recent seismic swarms affecting the area occur within a 3 km thick, high VP/VS, densely cracked, and overpressurized evaporitic layer, composed of dolostones and anhydrites. A persistent low VP, low VP/VS volume, present on top of and along the ATF low‐angle detachment, traces the location of mantle‐derived CO2, the upward flux of which contributes to cracking within the evaporitic layer.

Estimating friction in normal fault systems of the Basin and Range province and examining its geological context

Abstract The life cycle of a fault following initiation is governed in part by the reshear criterion, of which rock surface friction is the critical factor limiting the dip of a fault at its death. Using structural restorations where the initial and final dips of faults can be ascertained, the coefficient of rock surface friction is calculated for well-characterized extended locales ( n =20) in the Basin and Range province, many with multiple fault generations ( n =34). The calculated values exhibit a considerably wider range (0.19–1.33) than previously reported. The amount of tilting associated with each fault generation is compared with eight characteristics (mean slip magnitude, tilting per unit of slip, fault spacing, percentage extension, absence or presence and composition of magmatism, duration of extension, timing of extension and strain rate). No statistically strong correlation was found with any of the examined characteristics, although tentative linkages were noted with percentage extension, strain rate and mean slip magnitude from weighted regression analysis. These results are consistent with normal faults behaving as non-linear systems, with friction being an emergent property.

Analysis of gravity-driven normal faults using a 3D seismic reflection dataset from the present-day shelf-edge break of the Otway Basin, Australia

The growth, interaction and controls of gravity-driven normal faults is somewhat understudied. Using three-dimensional (3D) and two-dimensional (2D) seismic reflection data, located at the present-day shelf-edge break and into the deepwater province of the Otway Basin, southern Australia, we aim to temporally and spatially constrain the development of a normal fault system and determine the controls on growth. The Otway Basin is a Late Jurassic to Cenozoic age, rift-to-passive margin basin. The seismic reflection data images a gravity-driven fault array, consisting of ten fault segments, striking NW-SE (128-308), located within Upper Cretaceous clastic sedimentary rock. We analyse the growth of a gravity-driven hard-linked fault assemblage interacting with basement normal faults. Our analysis shows that the fault assemblage is linked to major basement faults and displays Turonian-Santonian nucleation, continued growth until the latest-Maastrichtian and a maximum throw of 1.74 km. High variability of throw along-strike and down-dip of the fault assemblage indicates growth via lateral and vertical segment linkage. We interpret that the spatial and temporal evolution of the fault assemblage is the result of rifting basement fault control during Upper Cretaceous resumed crustal extension in the Otway Basin. The control of the rifting basement faults on these gravity-driven normal faults has implications towards the growth and petroleum prospectivity of gravity-driven normal faults on passive margins such as the Niger Delta and Gulf of Mexico, but also towards gravity-driven normal faults developed in supra-salt sedimentary rock in rift basins, such as the North Sea and Suez Rift.

Rift brittle deformation of SE-Brazilian continental margin: Kinematic analysis of onshore structures relative to the transfer and accommodation zones of southern Campos Basin

High-resolution drone-supported mapping and traditional field work were used to refine the hierarchy and kinematics of rift-related faults in the basement rocks and Early Cretaceous mafic dikes onshore of the Campos Basin, SE-Brazil. Two sets of structures were identified. The most significant fault set is NE-SW oriented with predominantly normal displacement. At mesoscale, this fault set is arranged in a rhombic pattern, interpreted here as a breached relay ramp system. The rhombic pattern is a penetrative fabric from the thin-section to regional scale. The second-order set of structures is an E-W/ESE-WNW system of normal faults with sinistral component. These E-W structures are oriented parallel with regional intrabasinal transfer zones associated with the earliest stages of Campos Basin's rift system. The crosscutting relationship between the two fault sets and tholeiitic dikes implies that the NE-SW fault set is the older feature, but remained active until the final stages of rifting in this region as the second-order fault set is older than the tholeiitic dikes. Paleostresses estimated from fault slip inversion method indicated that extension was originally NW-SE, with formation of the E-W transfer, followed by ESE-WNW oblique opening associated with a relay ramp system and related accommodation zones.

The Amatrice 2016 seismic sequence: a preliminary look at the mainshock and aftershocks distribution

<p><em>We relocated the aftershocks of the M<sub>W</sub> 6.0 Amatrice 2016 mainshock by inverting with a non-linear probabilitstic method P- and S-arrival time readings produced and released in near realtime by the analyst seismologists of IGNV on duty in the seismic monitoring room. Earthquakes distribution shows the activation of a normal fault system with a main SW-dipping fault extending from Amatrice to NW of Accumoli village for a total length of 40 km. On the northern portion of the main fault hanging-wall volume, the structure become more complex activating an antithetic fault below the Norcia basin. It is worth nothing that below 8-9 km of depth, the whole fault system has an almost continuous sub-horizontal layer interested by an intense seismic activity, about 2 km</em> thick.</p>

Segmentation pattern and structural complexities in seismogenic extensional settings: The North Matese Fault System (Central Italy)

We investigated the northern slope of the Matese Mts. (Molise, Central Italy) with the aim of characterizing the N- to NE-dipping active normal fault system in the Bojano basin, a sector of primary importance from a seismic hazard perspective. We collected field data to define the geometry and segmentation pattern of two sub-systems (Patalecchia-Colle di Mezzo and Bojano–Campochiaro). New evidence of late Quaternary faulting was obtained by exploiting well log interpretations. Kinematic analysis revealed the interaction of pre-Quaternary inherited (mainly E-W-striking) and newly formed (NW-SE-striking) normal faults. Slip accommodation through linkage was clearly noted in the case of the Patalecchia-Colle di Mezzo sub-system. Detailed topographic profiles across the active fault segments provided post-LGM (15 ± 3 kyr) slip rates up to ∼2 mm/yr which agree with the high deformation rates based on different approaches in the literature. Finally, the instrumental seismicity analysis constrained the bottom of the seismogenic layer to depths of 13–14 km, and the gathered information allowed us to reconstruct the North Matese seismogenic source. Its 3D geometry and dimensions agree with both the dimension-magnitude relationships and macroseismic information available for the 1805 earthquake (Mw 6.6), the main historical earthquake to have struck the Bojano basin.

Rolling open Earth’s deepest forearc basin

Abstract The Weber Deep—a 7.2-km-deep forearc basin within the tightly curved Banda arc of eastern Indonesia—is the deepest point of the Earth’s oceans not within a trench. Several models have been proposed to explain the tectonic evolution of the Banda arc in the context of the ongoing (ca. 23 Ma–present) Australia–Southeast Asia collision, but no model explicitly accounts for how the Weber Deep achieved its anomalous depth. Here we propose that the Weber Deep formed by forearc extension driven by eastward subduction rollback. Substantial lithospheric extension in the upper plate was accommodated by a major, previously unidentified, low-angle normal fault system we name the “Banda detachment.” High-resolution bathymetry data reveal that the Banda detachment is exposed underwater over much of its 120 km down-dip and 450 km lateral extent, having produced the largest bathymetric expression of any fault discernable in the world’s oceans. The Banda arc is a modern analogue for highly extended terranes preserved in the many regions that may similarly have “rolled open” behind migrating subduction zones.

Newly identified active faults in the Pollino seismic gap, southern Italy, and their seismotectonic significance

The following is a geological study of a Quaternary and active normal fault-system, which crops out in the Pollino area, a seismogenic sector of the Southern Apennines, Italy.From 2010 to 2014, this area was affected by long lasting seismic activity characterized by three major events which occurred in May 2012 (Mw 4.3), in October 2012 (Mw 5.2) and in June 2014 (Mw 4.0). The integration of structural-geological data with morpho-structural and remote sensing analyses, led to define the geometry, the kinematics, the cross-cutting relationships and the slip rates of the inferred active fault segments within and near the epicentral area.We reconstructed an asymmetric extensional pattern characterized by low-angle, E and NNE-dipping faults, and by antithetic, high-angle, SW- to WSW-dipping faults. The geometry of the faults at depth was constrained using high-resolution hypocenter distributions. The overall system fits well with the deformation field obtained from focal mechanisms and geodetic data.Comparing the fault pattern with the time-space evolution of the Pollino seismic activity, we identified the seismogenic sources in two, near-parallel, WSW-dipping faults, whose seismogenic potential were assessed.The peculiar perpendicular-to-fault-strike evolution of the seismic activity, is discussed in the frame of the reconstructed seismotectonic model.

Meteoric fluid infiltration in crustal-scale normal fault systems as indicated by δ18O and δ2H geochemistry and40Ar/39Ar dating of neoformed clays in brittle fault rocks

Both the sources and pathways of fluid circulation are key factors to understanding the evolution of low-angle normal fault (LANF) systems and the distribution of mineral deposits in the upper crust. In recent years, several reports have shown the presence of meteoric waters in mylonitic LANF systems at mid-crustal conditions. However, a mechanism for meteoric water infiltration to these mid-crustal depths is not well understood. Here we report paired δ 18 O and δ 2 H isotopic values from dated, neoformed clays in fault gouge in major detachments of the southwest United States. These isotopic values demonstrate that brittle fault rocks formed from exchange with pristine to weakly evolved meteoric waters at multiple depths along the detachment. 40 Ar/ 39 Ar dating of these same neoformed clays constrains the Pliocene ages of fault-gouge formation in the Death Valley area. The infiltration of ancient meteoric fluids to multiple depths in LANFs indicates that crustal-scale normal fault systems are highly permeable on geologic timescales and that they are conduits for efficient, coupled flow of surface fluids to depths of the brittle-plastic transition.

Linking the distribution of microbial deposits from the Great Salt Lake (Utah, USA) to tectonic and climatic processes

Abstract. The Great Salt Lake is a modern hypersaline lake, in which an extended modern and ancient microbial sedimentary system has developed. Detailed mapping based on aerial images and field observations can be used to identify non-random distribution patterns of microbial deposits, such as paleoshorelines associated with extensive polygons or fault-parallel alignments. Although it has been inferred that climatic changes controlling the lake level fluctuations explain the distribution of paleoshorelines and polygons, straight microbial deposit alignments may underline a normal fault system parallel to the Wasatch Front. This study is based on observations over a decimetre to kilometre spatial range, resulting in an integrated conceptual model for the controls on the distribution of the microbial deposits. The morphology, size and distribution of these deposits result mainly from environmental changes (i.e. seasonal to long-term water level fluctuations, particular geomorphological heritage, fault-induced processes, groundwater seepage) and have the potential to bring further insights into the reconstruction of paleoenvironments and paleoclimatic changes through time. New radiocarbon ages obtained on each microbial macrofabric described in this study improve the chronological framework and question the lake level variations that are commonly assumed.

Kinematic evolution of a fold-and-thrust belt developed during basin inversion: the Mesozoic Maestrat basin, E Iberian Chain

AbstractThe Maestrat basin was one of the most subsident basins of the Mesozoic Iberian Rift system, developed by a normal fault system which divided it into sub-basins. Its Cenozoic inversion generated the N-verging Portalrubio–Vandellòs fold-and-thrust belt in its northern margin, detached in the Triassic evaporites. In the hinterland, a 40 km wide uplifted area, in the N–S direction, developed, bounded to the N by the E–W-trending, N-verging Calders monocline. This monocline is interpreted as a fault-bend fold over the ramp to flat transition of the E–W-trending, N-verging Maestrat Basement Thrust, and also indicates the transition from a thick-skinned (S) to a thin-skinned (N) style of deformation. This paper presents a kinematic evolutionary model for the northern margin of the basin and a reconstruction of the Maestrat Basement Thrust geometry, generated by the inversion of the Mesozoic normal fault system. It contains a low-dip ramp (9°) extended southwards more than 40 km, attaining a depth of 7.5 km. As this thrust reached the Mesozoic cover to the foreland, it propagated across the Middle Muschelkalk evaporitic detachment, generating a nearly horizontal thrust which transported northwards the supra-salt cover, and the normal fault segments within it, for c. 11–13 km. The displacement of the basement in the hanging-wall of the low-dip basement ramp generated the 40 km wide uplifted area, while the superficial shortening was accumulated in the northern margin of the basin – which contains the thinnest Mesozoic cover – developing the Portalrubio–Vandellòs fold-and-thrust belt.

Paleocene Pacific Plate reorganization mirrored in formation of the Suvarov Trough, Manihiki Plateau

AbstractThe Suvarov Trough is a graben structure that deviates from the Danger Islands Troughs within the Manihiki Plateau, a Large Igneous Province (LIP) located in the Central Pacific. New high‐resolution seismic reflection data provide evidence that the graben formed in two phases during the Paleocene (65–45 Ma). In a first phase extension occurred in southwestward direction, pulling apart the northern part of the Suvarov Trough and a parallel trending unnamed trough. In a second phase a change of extensional force direction occurred from southwest to west‐northwest, forming the southern part of the Suvarov Trough that extends onto the High Plateau. The formation of the Suvarov Trough is accompanied by a series of normal fault systems that apparently formed simultaneously. Comparing the seismic results to existing Pacific paleo strain reconstructions, the timing of increased strain and local deformation direction fits well to our findings. We thus suggest that the multiple strike directions of the Suvarov Trough represent an extensional structure that was caused by the major, stepwise Pacific Plate reorganization during the Paleocene.

The Neogene-Quaternary geodynamic evolution of the central Calabrian Arc: A case study from the western Catanzaro Trough basin

The Catanzaro Trough is a Neogene-Quaternary basin developed in the central Calabrian Arc, between the Serre and the Sila Massifs, and filled by up to 2000m of continental to marine deposits. It extends from the Sant’Eufemia Basin (SE Tyrrhenian Sea), offshore, to the Catanzaro Basin, onshore. Here, onshore structural data have been integrated with structural features interpreted using marine geophysical data to infer the main tectonic processes that have controlled the geodynamic evolution of the western portion of the Catanzaro Trough, since Upper Miocene to present.The data show a complex tectonostratigraphic architecture of the basin, which is mainly controlled by the activity of NW–SE and NE–SW trending fault systems. In particular, during late Miocene, the NW-SE oriented faults system was characterized by left lateral kinematics. The same structural regime produces secondary fault systems represented by E-W and NE-SW oriented faults. The ca. E-W lineaments show extensional kinematics, which may have played an important role during the opening of the WNW–ESE paleo-strait; whereas the NE-SW oriented system represents the conjugate faults of the NW-SE oriented structural system, showing a right lateral component of motion. During the Piacenzian-Lower Pleistocene, structural field and geophysical data show a switch from left-lateral to right-lateral kinematics of the NW-SE oriented faults, due to a change of the stress field. This new structural regime influenced the kinematics of the NE-SW faults system, which registered left lateral movement. Since Middle Pleistocene, the study area experienced an extensional phase, WNW-ESE oriented, controlled mainly by NE-SW and, subordinately, N-S oriented normal faults. This type of faulting splits obliquely the western Catanzaro Trough, producing up-faulted and down-faulted blocks, arranged as graben-type system (i.e Lamezia Basin).The multidisciplinary approach adopted, allowed us to constrain the structural setting of the central Calabria segment. The joined onshore with offshore structural data analysis allowed us to image a more faithful geodynamic evolution of the Calabrian Arc, included in the wider geodynamic framework of the Mediterranean regionMoreover, our results show the close correlation between the NE-SW and N-S normal fault systems and evidence of deformed Quaternary deposits. These findings are relevant to seismic hazard understanding in an area which is historically considered at the highest risk of earthquake and tsunami and where are present important infrastructures and cities.

Occurrence and development of folding related to normal faulting within a mechanically heterogeneous sedimentary sequence: a case study from Inner Moray Firth, UK

Abstract Folds associated with normal faults are potential hydrocarbon traps and may impact the connectivity of faulted reservoirs. Well-calibrated seismic reflection data that image a normal fault system from the Inner Moray Firth basin, offshore Scotland, show that folding was preferentially localized within the mechanically incompetent Lower–Middle Jurassic pre-rift interval, comprising interbedded shales and sandstones, and within Upper Jurassic syn-rift shales. Upward propagation of fault tips was initially inhibited by these weak lithologies, generating fault propagation folds with amplitudes of c. 50 m. Folds were also generated, or amplified, by translation of the hanging wall over curved, convex-upward fault planes. These fault bends resulted from vertical fault segmentation and linkage within mechanically incompetent layers. The relative contributions of fault propagation and fault-bend folding to the final fold amplitude may vary significantly along the strike of a single fault array. In areas where opposite-dipping, conjugate normal faults intersect, the displacement maxima are skewed upwards towards the base of the syn-rift sequence (i.e. the free surface at the time of fault initiation) and significant fault propagation folding did not occur. These observations can be explained by high compressive stresses generated in the vicinity of conjugate fault intersections, which result in asymmetric displacement distributions, skewed towards the upper tip, with high throw gradients enhancing upward fault propagation. Our observations suggest that mechanical interaction between faults, in addition to mechanical stratigraphy, is a key influence on the occurrence of normal fault-related folding, and controls kinematic parameters such as fault propagation/slip ratios and displacement rates.

Deep electrical resistivity tomography along the tectonically active Middle Aterno Valley (2009 L'Aquila earthquake area, central Italy)

Three 2-D Deep Electrical Resistivity Tomography (ERT) transects, up to 6.36 km long, were obtained across the Paganica-San Demetrio Basin, bounded by the 2009 L'Aquila Mw 6.1 normal-faulting earthquake causative fault (central Italy). The investigations allowed defining for the first time the shallow subsurface basin structure. The resistivity images, and their geological interpretation, show a dissected Mesozoic-Tertiary substratum buried under continental infill of mainly Quaternary age due to the long-term activity of the Paganica-San Demetrio normal faults system (PSDFS), ruling the most recent deformational phase. Our results indicate that the basin bottom deepens up to 600 m moving to the south, with the continental infill largely exceeding the known thickness of the Quaternary sequence. The causes of this increasing thickness can be: (1) the onset of the continental deposition in the southern sector took place before the Quaternary, (2) there was an early stage of the basin development driven by different fault systems that produced a depocentre in the southern sector not related to the present-day basin shape, or (3) the fault system slip rate in the southern sector was faster than in the northern sector. We were able to gain sights into the long-term PSDFS behaviour and evolution, by comparing throw rates at different timescales and discriminating the splays that lead deformation. Some fault splays exhibit large cumulative throws (>300 m) in coincidence with large displacement of the continental deposits sequence (>100 m), thus testifying a general persistence in time of their activity as leading splays of the fault system. We evaluate the long-term (3–2.5 Myr) cumulative and Quaternary throw rates of most of the leading splays to be 0.08–0.17 mm yr−1, indicating a substantial stability of the faults activity. Among them, an individual leading fault splay extends from Paganica to San Demetrio ne’ Vestini as a result of a post-Early Pleistocene linkage of two smaller splays. This 15 km long fault splay can explain the Holocene surface ruptures observed to be larger than those occurred during the 2009 L'Aquila earthquake, such as revealed by palaeoseismological investigations. Finally, the architecture of the basin at depth suggests that the PSDFS can also rupture a longer structure at the surface, allowing earthquakes larger than M 6.5, besides rupturing only small sections, as it occurred in 2009.

Interactions between active faulting, volcanism, and sedimentary processes at an island arc: Insights from Les Saintes channel, Lesser Antilles arc

AbstractNew high‐resolution marine geophysical data allow to characterize a large normal fault system in the Lesser Antilles arc, and to investigate the interactions between active faulting, volcanism, sedimentary, and mass‐wasting processes. Les Saintes fault system is composed of several normal faults that form a 30 km wide half‐graben accommodating NE‐SW extension. It is bounded by the Roseau fault, responsible for the destructive Mw 6.3 21 November 2004 earthquake. The Roseau fault has been identified from the island of Basse‐Terre to Dominica. It is thus 40 km long, and it could generate Mw 7 earthquakes in the future. Several submarine volcanoes are also recognized. We show that the fault system initiated after the main volcanic construction and subsequently controls the emission of volcanic products. The system propagates southward through damage zones. At the tip of the damage zones, several volcanic cones were recently emplaced probably due to fissures opening in an area of stress increase. A two‐way interaction is observed between active faulting and sedimentary processes. The faults control the development of the main turbiditic system made of kilometer‐wide canyons, as well as the location of sediment ponding. In turn, erosion and sedimentation prevent scarp growth at the seafloor. Faulting also enhances mass‐wasting processes. Since its initiation, the fault system has consequently modified the morphologic evolution of the arc through perturbation of the sedimentary processes and localization of the more recent volcanic activity.

Kinematics of post-orogenic extension and exhumation of the Taku Schist, NE Peninsular Malaysia

Recent studies imply that the formation and evolution of many SE Asian basins was driven by extensional detachments or systems of low-angle normal faults that created significant crustal exhumation in their footwalls. In this context, the architecture of the Triassic Indosinian orogen presently exposed in Peninsular Malaysia is compatible with significant extension post-dating the orogenic event. In this study we performed a kinematic analysis based on fieldwork and microstructural observations in the Taku Schist, Kemahang granite and the surrounding Gua Musang sediments of northern Peninsular Malaysia in order to shed light on processes related to the build-up and subsequent demise of the Indosinian orogen. The first three phases of deformation were related to an overall period of E–W oriented contraction and burial metamorphism. These phases of deformation are characterized by isoclinal folding with flat lying axial plane cleavages (D1), asymmetrical folding, top-to-the-W–SW shearing (D2) and upright folding (D3). All are in general agreement with observations of the previously inferred Permo–Triassic Indosinian orogeny. During these times, the Taku Schist, a sequence of Paleozoic clastic sediments with mafic intercalations was metamorphosed to amphibolite facies. These rocks are most likely equivalent to the ones exposed in the Bentong–Raub suture zone. Structural relations suggest that the Triassic Kemahang pluton is syn-kinematic, which provides important constraints for the timing of these contractional events. We demonstrate that the overall shortening was followed by a hitherto undescribed extension in NW–SE direction resulting in the formation of a large-scale detachment, the Taku detachment, in northern Peninsular Malaysia. Extension probably reactivated the former subduction plane as a detachment and exhumed previously buried and metamorphosed rocks of similar lithological composition to the neighboring Bentong–Raub suture zone. Such a mechanism is similar to that observed in other regions, such as the Aegean, Apennines, Dinarides or the Betics–Rif system, where exhumation of (high-pressure) metamorphic rocks is largely controlled by detachments or low angle normal shear/fault systems.

Interpreting Seismic Profiles in terms of Structure and Stratigraphy, an Example from Lower Indus Basin Pakistan

Structural delineation and Stratigraphic evaluation of proven geological basins are of prime importance for hydrocarbon exploration and exploitation. The current study aims to map the subsurface geology in a part of Lower Indus basin of Pakistan in terms of structure and stratigraphy using seismic data with some borehole information. In this paper, conventional integrated geophysical technique has been used to analyze the seismic data to characterize reservoir formations. To pursue the interpretation, 2D seismic data in SEG-Y format was used along with velocity and well logs information. Visual interpretation of seismic profiles is transformed into 2D and 3D (Time and Depth domain) contour maps, which is the representation of true subsurface geology. Three promising closures at shallow time have been identified on structural contour maps which are indicative of possible prospects. A closure, covering approximately 1 Sq. Km area over Eocene aged Sui Main Limestone and further two over Cretaceous aged Lower Goru formation covering areas of 500 and 400 Sq. km along shot point 260 and 380-390, respectively. Conjugate normal fault system among the stratigraphic layers existing in the study area can smoothly provide the hydrocarbon trapping mechanism. In order to further confirm the probable future prospects and support the study, modeling of P and S wave Acoustic Impedance have also been done. The advance integrated study of AVO modeling and numerical rock physics analysis may be helpful in providing further insights into current research.

Low shear velocity in a normal fault system imaged by ambient noise cross correlation: The case of the Irpinia fault zone, Southern Italy

AbstractWe extracted the Green's functions from cross correlation of ambient noise recorded at broadband stations located across the Apennine belt, Southern Italy. Continuous records at 26 seismic stations acquired for 3 years were analyzed. We found the emergence of surface waves in the whole range of the investigated distances (10–140 km) with energy confined in the frequency band 0.04–0.09 Hz. This phase reproduces Rayleigh waves generated by earthquakes in the same frequency range. Arrival time of Rayleigh waves was picked at all the couples of stations to obtain the average group velocity along the path connecting the two stations. The picks were inverted in separated frequency bands to get group velocity maps then used to obtain an S wave velocity model. Penetration depth of the model ranges between 12 and 25 km, depending on the velocity values and on the depth of the interfaces, here associated to strong velocity gradients. We found a low‐velocity anomaly in the region bounded by the two main faults that generated the 1980, M 6.9 Irpinia earthquake. A second anomaly was retrieved in the southeast part of the region and can be ascribed to a reminiscence of the Adria slab under the Apennine Chain.

Salt diapir reactivation and normal faulting in an oblique extensional system, Vulcan Sub-basin, NW Australia

Oblique extension and salt diapirism have their own distinct mechanisms that control the geometry and kinematics of structures. In this study, we document a geological phenomenon from the Vulcan Sub-basin in NW Australia that combines these processes: a salt diapir reactivated in an oblique extensional system. Detailed structural analysis of this natural example, with a focus on normal fault systems, allows characterization of the oblique extensional system and investigation of how the pre-existing structural fabrics and salt diapir control deformation, and interact with each other under oblique extension. After comparison with forward modelling results and using constraints from geological evidence, our fault strike analysis indicates that the Neogene flexural extension orientation in the oblique extensional system is around 347°, revealing a perpendicular relationship between extension direction and fault strikes from the deformation zone. The salt diapir, reactivated during Neogene extension, strongly influences local structure in the oblique extensional system by altering fault strikes, stepping patterns, deformation zone width and fault density, indicative of a ‘stress–strain concentration’ effect of the salt diapir owing to its extremely low strength. These results provide valuable insights into the understanding of oblique extensional systems, the geomechanical role of salt during extension, and the Neogene tectonic evolution of NW Australia. Supplementary material : Dominant seismic frequency analysis, uninterpreted seismic profiles, additional structural and isochron maps, a 3D view of fault systems, and original measurements of the fault system are available at http://doi.org/10.6084/m9.figshare.c.3148321 .