Wrench Fault Research Articles

Tectonic evolution of a sequence of related late Permian transtensive coal-bearing sub-basins, Mongolia: A global wrench tectonics portrait

During the late Permian in Mongolia, inertia-driven transtensive reactivation of primordial fracture zones gave rise to the development of a sequence of related, but isolated, fault-bounded sub-basins; some of these became the locus of substantial peat accumulation that evolved into economically important coal deposits. The present study focuses on late Permian coal measures in two widely separated areas: Area 1: located in central Mongolia, developed along the southern margin of the Mongol-Transbaikalian Seaway. The late Permian coal sequence forms a c. 420 m thick middle part of a Permo-Triassic succession which spans c. 2,600 m. The V-shaped, fault-bounded NE oriented sub-basin evolved under transtensive conditions. The thick infill records a transition from shallow marine and humid coal forming depositional environments during the late Permian to relatively arid desolate terrestrial conditions during early Triassic times, considered here to mark the dramatic drainage of the Mongol-Transbaikalian Seaway across the Permo-Triassic boundary. Area 2: situated in southern Mongolia, is a NE oriented elongate sub-basin, bounded by two wrench faults, which formed under transtensive conditions. Thickness of the late Permian coal-bearing strata is c. 650 m. The sedimentary strata record a transition from a humid coal-bearing environment to predominantly marine conditions. Both study areas are located proximal to two controversial suture zones. However, the zones do not show the presumed shortening, major thrusting, regional metamorphism and given the complete absence of tuffs within the studied Permo-Triassic successions it could be argued that the sutures are not only cryptic but non-existent.

Failure mechanism by wrench faulting in Central Anatolia

The study area, where volcanism and tectonism are active at the same time, consists of the Cappadocia Volcanic Complex (CVC) in the middle, Tuzgölü on the west side, and Ecemiş fault systems on the east side. Remote sensing image processing method and derivation to aeromagnetic data were applied to identify the structural lineations that tectonically control the study area within the scope of the study. As a result of the lineation analysis performed in the west of the study area, 167 lineaments in the west, most of which are NW oriented, and 316 lineaments, most of which are NE oriented, were determined in the east. These findings are in agreement with the known fault orientations in the area. It is known that earthquakes greater than M > 4 occurred in these known faults during the neotectonics period, and they also have the potential to produce M > 6 earthquakes. In addition, the Tuzgölü and Ecemiş Faults' shearing effect relative to each other is an indication that a wrench tectonics mechanism has developed in the study area. With the remote sensing study and following the existing surface traces, two fault systems in opposite directions, the Tuzgölü Fault (−30°) dipping counterclockwise and the Ecemiş Fault, clockwise (60°) dipping, were determined. The orientation and locations of the faults could be determined by tilt transformation applied to the magnetic anomaly map of the study area. In particular, the effects of wrenching in the study area can be clearly observed in magnetic anomalies.

An Evolving Lithospheric‐Scale Wrench Fault System Along the Eastern End of the Altyn Tagh Fault: Kinematics and Quaternary Activity of the Heishan Fault System, Western China

AbstractThe Altyn Tagh fault (ATF) is a major intracontinental strike‐slip fault system that defines the northern margin of the Tibetan Plateau. The fault system loses its obvious surface expression north of the Qilian Shan fold‐and‐thrust belt, but may link eastward with multiple faults in the northern Hexi Corridor and southern Alxa block. To better understand the potential connectivity and displacement transfer between the ATF and the northern Hexi Corridor fault array, we carried out a multidisciplinary field and remote sensing‐based investigation of the actively deforming Heishan and Jinta'Nanshan region north of Jiayuguan City. We document Quaternary sinistral strike‐slip motion on the Heishan fault (HF) system and active sinistral transpression within a 70‐km‐long, E‐W deforming belt east of the Heishan, which is characterized by multiple left‐stepping, en‐échelon Quaternary folds and linking faults. Cosmogenic10Be dating of displaced terrace surfaces yields a vertical slip rate of ∼0.2 mm/a for the northern Heishan thrust and a sinistral strike‐slip rate of 0.6 ± 0.2 mm/a for the HF. Within the uplifted core of the Heishan massif, the HF contains ductilely deformed Paleozoic basement lithologies with dextral‐sense shear fabrics that indicate that the modern HF has reactivated and inverted an older shear zone. A previously published magnetotelluric profile across the HF system suggests that it roots into a steep, deeply penetrating fault that is unlinked to the Qilian Shan thrust wedge. Instead, we suggest that the active sinistral deformation belt of the Heishan‐Jinta'Nanshan represents the eastward‐evolving, upper crustal expression of the modern ATF system.

Seismic swarms in the Pollino seismic gap: Positive fault inversion within a popup structure

Seismic swarms frequently occur along continental fault systems and their relation with large earthquakes is often contradictory. Such a case is documented in the Pollino mountain range of southern Italy, a decoupling zone where the belt-normal stretching drastically rotates accommodating the differential SE-retreat of the Ionian slab. The paucity of historical large earthquakes has led to hypothesize the presence of a seismic gap. A long-lasting seismic swarm that climaxed with a ML = 5.2 earthquake in October 2012 was therefore thought as a possible signal of an impending large earthquake filling the gap. Seismicity data collected during a 4-years long monitoring are a powerful microscope to look through the seismic swarm. In this study, we present accurate relocations for 2385 earthquakes and high-resolution Vp and Vp/Vs models of the fault system. Seismicity occurred on two separate normal faults that were formerly part of a thrusts and back-thrusts system, originally formed as a pop-up at restraining bends of the Pollino fault, a wrench fault system that inverted the original left lateral sense of slip accommodating a differential motion induced by the southward retreat of the Ionian slab.

GPS and InSAR derived evidences of intra‐basin stress and strike‐slip tectonics in the vicinity of 2001 (M7.7) earthquake, Kachchh, western India

The current geodetic investigation is based on nearly a decade of continuous Global Positioning System (GPS) data towards the western part of the Indian Plate (2009–2019). This research focused on the Kachchh Rift Basin (KRB), the seismically most active intra‐plate region of the Indian Plate, which has seen three ≥ M 7.0 earthquakes in the last two centuries. The GPS stations on the northern and southern margins show SSW and NNE directed motion, indicating that the KRB is currently under the influence of regional compressive stress on both flanks and the same is reflected in the form of earthquake activity in the center portion. Furthermore, the existence of intra‐basin stress, in addition to regional stress, increases strain accumulation and results in the creation of a Principal Deformation Zone (PDZ) in the central part of the KRB. The average annual deformation in the middle of KRB remains 1.0 ± 0.5 mm/year, while sites near the South Wagad Fault (SWF) experience 1.2 mm/year of fault parallel motion. The dominant fault normal motion along the flanks with a maximum fault parallel motion in the middle intimates the existence of strike‐slip tectonics in the central Kachchh. The geometric relationship of strike‐slip faults in a compressive margin and strain accumulation in pre‐existing faults (PDZ) is responsible for current strike‐slip and thrust motion in the Kachchh, including the 2001 Bhuj earthquake (M 7.7). Our geodetic model is well‐corroborated with the geological observations of the wrench fault model of strike‐slip faults. PS‐InSAR results in the northern part of the region between Khadir and Bela Islands show ≤6.0 mm of annual LOS displacement. GPS results indicate ≈ 1.3 to 1.4 ± 0.5 mm/year of fault parallel motion along the transverse Ekal Amrapar Fault (EAF). The non‐significant variation in GRACE‐derived TWS values (from 2009 to 2018) rules out non‐tectonic deformation in the vicinity of EAF and thus points to tectonic activity as the cause of the derived deformation in the region.

Early Cretaceous provenance-depositional systems and volcano-sedimentary responses to strike-slip faulting in southern Jilin and eastern Liaoning provinces, NE China: a case study of the Tonghua Basin

ABSTRACT Understanding how the sedimentary responses adapt to the structural styles of fault basins in the strike-slip fault systems is one of the key goals of basin analysis. Variability of volcanic and epiclastic provenance-depositional systems and tectonic controls of the Lower Cretaceous Hengtongshan Formation in the Tonghua Basin are crucial to determine the evolution of the fault basins and wrench tectonics in southern Jilin and eastern Liaoning provinces (SJELP), NE China. Study of volcanism-induced sedimentation, composition plot, and depositional systems elucidates the sequence stratigraphy and volcano-sedimentary responses to wrench tectonics based on cores, outcrop sections, and seismic profiles of the Tonghua Basin. Two types of provenance-depositional systems (volcanic and epiclastic) were reconstituted using eight lithofacies and seven lithofacies associations. Of the lithofacies, six pyroclastic lithofacies and two epiclastic lithofacies were identified based on compositions, textures, and sedimentary structures. The presence of these lithofacies in different vertical successions was described as different lithofacies associations to interpret depositional processes and sedimentary environments. Depositional-system maps of the three sequences in the Hengtongshan Formation and analysis of structural framework indicate that volcanism and sedimentation were controlled by wrench faulting. Statistical analysis of rock compositions reveals that the deposition of the Hengtongshan Formation shows mixed contribution from both the volcanic and granitic fragments. Triangular compositional plots of detrital composition suggest a diverse provenance by straddling the litho-feldspatho-quartzose (lFQ), feldspatho-litho-quartzose (fLQ), feldspatho-quartzo-lithic (fQL), and quartzo-feldspatho-lithic (qFL) fields, indicating that the sources were derived from uplifted basement blocks and volcanic eruptions in a rift basin related to strike-slip faulting. The Tonghua Basin developed as a wrench-fault basin on the east side of Dunhua–Mishan Fault systems (DMF). This model for the Tonghua Basin can be used to understand the Liuhe Basin in the DMF and other Early Cretaceous basins in adjacent areas.

Polyphase Permo-Carboniferous magmatism adjacent to the Intra-Sudetic Fault: constraints from U–Pb SHRIMP zircon study of felsic subvolcanic intrusions in the Intra-Sudetic Basin, SW Poland

The SHRIMP U–Pb dating of zircons from felsic subvolcanic rocks in the Carboniferous formations of the Intra-Sudetic Basin in SW Poland verifies previous views on the timing of Late Palaeozoic magmatic activity in this area and constrains the links between regional tectonics and intra-basinal volcanism in the mid-European Variscides. Two main stages of magmatism are identified: the Late Carboniferous late orogenic stage, and the Early Permian post-orogenic stage. The Carboniferous late orogenic subvolcanic to volcanic activity was contemporaneous with the formation of the Karkonosze granite pluton to the west and comprised episodic emplacement of predominantly felsic laccoliths and sills within the basin fill during the Westphalian–Stephanian in two sub-stages at ca. 313–310 Ma and 306–305 Ma. Local folding of the Carboniferous succession peaked at ca. 303–300 Ma adjacent to the Intra-Sudetic Fault, in relation to late-stage sinistral movements along this regional wrench fault. The emplacement of minor subvolcanic intrusions at ca. 293 Ma completed the activity in the northern Intra-Sudetic Basin; this Permian post-orogenic volcanism, however, developed fully in areas further south. The predominance of subvolcanic intrusions over extrusions in Carboniferous can be linked to the trapping of rising magmas within the several kilometres thick sedimentary basin fill. Zircon xenocrysts in the Carboniferous subvolcanic rocks indicate that older crustal lithologies, possibly metamagmatic and metasedimentary rocks of dominantly Neoproterozoic–Palaeozoic ages, were involved in petrogenesis as magma sources and/or contaminants. The crustal influence on magma formation was stronger during the earlier magmatic events.

Methods and Case Studies of Ultra‐deep Fault Seal Evaluation

AbstractFault seals are significant for petroleum exploration and production. This study summarizes the fault sealing impacting factors, including lithological juxtaposition, mud smearing, fault rocks and the fault plane stress states, as well as evaluation methods like Allan maps and Shale Gouge Ratio (SGR). The seal evaluation for a wrench fault focuses on its particular structural features. The evaluation methods were applied to the Jinma–Yazihe structure and the Shunbei oilfield. The source rock is the Xujiahe Formation of the Upper Triassic, the reservoirs and caprocks being of the Shaximiao Formation of the Lower Jurassic. The fault sealing evaluations in major faults proved the reservoir formation processes in the wells Jinfo 1 (JF1) and Chuanya 609 (CY–609), based on the editions of the Allan map showing lithological juxtaposition, the calculation of SGR showing mud smear and analyses of fault stress states. The analyses of stress states were also applied to Shunbei 5 strike‐slip fault in the Shunbei area in Tarim Basin. The various sections along the fault were of different mechanical properties, such as compression and extension. Petroleum exploration has demonstrated that the extensional sections are more favorable for oil accumulation than the compressional sections. These evolutionary methods and other understandings will help in analyses of deep fault sealing.

The Sierra Nevada de Santa Marta (Colombia) and Nevado de Famatina (Argentina): The effects of tectonic syntaxis on the topography of the Andes

The Sierra Nevada de Santa Marta (Santa Marta) in Colombia and Nevado del Famatina (Famatina) in Argentina are two isolated peaks within the Andes of South America, having elevations of about 5,500 and 6,000 m, respectively. Both massifs are bounded by major wrench faults of opposite kinematics but comparable magnitude in their lateral displacement. Despite the volumes of research available, little has been said in the literature about the tectonics surrounding these particular systems. In this context, digital terrain elevation (DEM), regional geology, satellite imagery and geophysical data were analysed to illustrate how a network of transcurrent faults uplifted the peaks. We propose a model to account for topography by the opposed movement of faults operating in two compartments: internal (acute) and external (obtuse). Fault motion caused uplift of the internal compartments: a) the Santa Marta tectonic syntaxis is generated by the sinistral Santa Marta-Bucaramanga Fault on the west and the dextral Oca-Pilar Fault to the north whilst, b) the Famatina tectonic syntaxis is caused by the Valle Fértil sinistral fault on the western side and the dextral Tucumán lineament to the north. Elevation maps documenting the lateral displacement of modern alluvial fans provide evidence of the existence and kinematics of these faults. Active since the Neogene, the transcurrent faults resulted in displacements in the order of 25–60 km, with velocities of approximately 1–2 cm/year. Seismic records suggest that the syntaxis occurred after reactivation of oblique discontinuities in the basement due to the convergence of the Nazca, Caribbean, and South American plates. This tectonic framework led to the formation of a remarkable positive relief and the development of internal positive compartments subject to constrictive forces. Simultaneously, external distensive and negative compartments controlled the syntectonic deposition of thick sedimentary sequences. The significant topographic difference between the internal positive compartments and their external negative counterparts triggered high rates of erosion, transport, and ultimately sedimentation. Regional subsurface surveys indicate strong anomalies of the underlying bedrock influenced by tectonic syntaxis.

A STRUCTURAL MODEL FOR NORTH SUMATRA

The geological evolution of North Sumatra has been controlled by strike slip tectonics troughout most of the Tertiary. The area experienced extensional and transtensional regimers during the Paleogene, rift basins opened up between major wrench faults, and rates of subsidence and sedimentation were rapid.

STRUCTURAL CHARACTERISTICS OF THE KALLIVOLAMPI FLUORITE-SULPHIDE OCCURRENCE, VEDLOZERO-SEGOZERO GREENSTONE BELT, KARELIA

The exposed zone of the Kallivolampi fluorite-sulphide occurrence, located at the tectonic contact of heterogeneous Lopian (Neoarchean) and Sariolian (Paleoproterozoic) rocks, clearly displays structural differences in shears of various ages. The numerous parageneses of subplanar structural elements (PSSE) were detected and identified using tectonophysical methods. Their comparison indicates that they formed in post-Sariolian time. The multi-phase pattern of the shear and signs of a multiple activation of most of the structural elements were revealed. The most probable deformation regimes of SPSE formation were recognized: a dominantly faulting and wrench fault regime in an early, pre-ore stage and a dominantly shear and wrench-fault regime in a late ore and probably post-ore stage. Considering the presence of noble-metal minerals among the ore minerals, this ore occurrence can be compared with gold bodies. Many PSSEs in the zones of the Kallivolampi ore occurrence and the Post-Jatulian Pedrolampi gold deposit studied earlier, spaced far apart, were shown to be fairly similar and even fully identical and to have formed in the same deformation regimes. The general formation sequence of shears differing in spatial orientation was as follows: structures with the predominant north-western and north-north-western strike of displacement planes were the first to form; these were followed by structures with the dominantly north-north-eastern strike of general displacement planes. The similarity, evolution sequence, and deformation conditions of the structures suggest that they were produced by one Paleoproterozoic, Post-Jatulian ore-generating system. Further tectonophysical and geochronological studies are needed to find out whether other gold bodies with varying characteristics revealed in the Vedlozero-Segozero greenstone belt and its flanks belong to this system.

Cenozoic Dextral Shearing Along the Arusan Sector of the Great Kavir–Doruneh Fault System (Central Iran)

AbstractThe structural analysis of large intracontinental wrench faults is fundamental for deciphering the long‐term evolution of continental crust in complex areas in terms of their geodynamic evolution and large‐scale crustal block displacements. In this contribution, we demonstrate a pre‐Miocene dextral activity of the present‐day left‐lateral Great Kavir ‐ Doruneh Fault System (GKDFS, Central Iran), one of the major intracontinental active strike‐slip faults extending from the Afghan border to the Nain region between Central Iran and the Sanandaj‐Sirjan Zone. We document important dextral shearing recorded along a segment of the GKDFS, the Arusan Fault System (AFS), located east of Jandaq, close to the present‐day active trace of the GKDFS. The AFS include several ENE‐WSW striking strands exposed for a length of more than 50 km, which couple pre‐Cretaceous ophiolites and metamorphic basement units with the Cretaceous succession of the Khur basin. The fault shows transpressional structures consistent with a dextral shear including thrusts and en échelon folds affecting the Cretaceous carbonate units. Paleostress reconstruction based on mesoscopic fault analysis and related folds geometry allowed to establish vorticity parameters indicating that deformation occurred close to a total simple shear regime with a calculated Wk between 0.9 and 1. The enormous Meso‐Cenozoic dextral displacements occurred along the AFS and along the entire GKDFS are attested by the up to several hundreds of kilometers offset of the Paleotethys suture zone, from NE Iran to the western border of Central Iran.

Structural analysis of the Casper Mountain fault zone and area, Wyoming surrounding area, Wyoming: Implications for Laramide kinematics and structural inheritance across the Wyoming Province

Casper Mountain is an E–W trending anticlinal structure that is bound on the north by the oblique-slip Casper Mountain fault. The fault is postulated to reflect preexisting Precambrian structure/fabrics that were reactivated and/or guided deformation during the Laramide orogeny. A structural analysis of the fault zone and surrounding area was conducted to confirm this hypothesis, and to garner insight into both Precambrian origins and Laramide kinematics. Surface and subsurface data for structural analysis was collected and synthesized from numerous published sources along the proposed deformation corridor that roughly coincides with the Oregon Trail structural belt of central Wyoming. The Casper Mountain fault zone is characterized by an E–W rectilinear zone of en échelon, steeply inclined faults. The Casper Mountain fault strikes E–W with smaller faults in the zone striking N65°E. Folds trend to the WNW and are left-stepping. Foliations in Precambrian rocks of Casper Mountain are oriented subparallel to the Casper Mountain fault. The North Granite Mountains fault zone is located due west of Casper Mountain and is similarly oriented E–W with associated faults striking NE, NW/SE, and ENE/WSW, off the dominant master fault. Curvilinear, left-stepping, en échelon folds trend to the northwest and are truncated on the south by the North Granite Mountains fault. Faults in basement rocks of the Popo Agie Primitive Area of the central Wind River Mountains are characterized by moderate to high-angle faults striking E–W, NNW, and NE that coincide with mapped surface lineaments and fabric data. Fabric data suggest that Laramide deformation along the Casper Mountain fault was guided by Precambrian anisotropies. Surface and subsurface mapping of the fault zone and the deformation corridor to the west indicate that the Casper Mountain and North Granite Mountains faults are part of a basement-rooted system (wrench fault) that likely extends westward into the Popo Agie Primitive Area. Here, the steeply inclined (75–90°) proposed master fault is exposed within a WNW-striking corridor of faults that sinistrally offset steeply dipping, NE-striking Proterozoic diabase dikes. The dikes likely intruded older faults that are antithetic to the WNW-striking faults. Other faults strike to the NNW and have shallower dips of 45–65°. These three directions of anisotropy (WNW, NE, and NNW) are proposed to have formed from SW–NE-directed subduction along a long-lived, Neoarchean, active continental margin.

Natural fractures and their relationships to structure, stress, and permeability in the Raton Basin

Fractures in Cretaceous and early Tertiary strata record several deformation events that were imposed on the formations that fill the Raton Basin in Colorado and New Mexico. A regional, generally WNW-ESE striking extension-fracture set is present across much of the basin, but fracturing also includes both dip-slip and strike-slip conjugate shear-fracture sets as well as irregular deformation-band shear fractures. In some areas of the basin, the extension fractures are dynamically-compatible with associated conjugate shear fractures, both recording a maximum compressive stress that was horizontal, trending predominantly WNW-ESE. Fracture strikes vary from NW-SE to ENE-WSW but are approximately normal to the front of the Laramide thrust-fault system that forms the western edge of the basin, implying that fracturing was the result of a horizontal compressive stress anisotropy created by indentation of the thrust system into the basin margin. Fracture anomalies occur over local structures including a N-S basement wrench-fault system that connects two large anticlines within the basin, the Tercio and Vermejo Park anticlines, where N-S strike-slip offset along the basement wrench faults caused folding and fracturing in the overlying strata. The Laramide stress system in the basin changed from thrust-related WNW-ESE horizontal compression to the present-day N-S maximum horizontal compressive stress in mid-Tertiary time as the thrust system became inactive and was replaced by regional E-W extension. No new fracture sets were formed by the re-oriented stress system, although stress-release fractures normal to the regional set formed in outcrops as overlying strata were eroded. Fracture datasets were derived from three sources during this study: outcrops, image logs, and cores. Each source provides a somewhat different perspective on the fractures that enhance permeability in Raton Basin reservoirs. Taken together, the three datasets provide the basis for a relatively complete conceptual model of the Raton fracture system. Fracture-controlled permeability anisotropy will be greatest in the WNW-ESE direction, parallel to the strike of the dominant set of Laramide-age natural fractures, but hydraulic stimulation fractures will propagate N-S, across the strike of those fractures under the influence of the present-day stress system. The apertures of the WNW-ESE fractures will be susceptible to closure under that stress system since the maximum horizontal compressive stress is approximately normal to fracture strike.

Longest and still longer: The Messejana-Plasencia dyke and its links with later Alpine deformation belt in Iberia

The Messejana-Plasencia dyke (End-Triassic), NE-SW oriented, is the longest simple structure in Iberia (~500 km) and constitutes a first-order rheological discontinuity that crosses the entire crust. During the Alpine deformation, this discontinuity nucleated the Messejana-Plasencia left-lateral strike-slip fault and its related strike-slip deformation belt, consisting of a series of wide coalescent stepovers and small basins developed on the footwalls. Both structures can be followed from offshore of SW mainland Portugal to the central sector of the Spanish-Portuguese Central System. Using tectonic, geophysical, magnetic, and gravimetric analyses, our study demonstrated that it extends NE more than ~100 km below the continental sediments of the Duero Cenozoic Basin, until the Iberian Chain. We used a tectonostratigraphic analysis to determine the age of the wrench fault movement and that of the Spanish-Portuguese thrusts. In this range, the largest displacement during the Cenozoic occurred in the southern thrust of the Spanish sector of the Central System over the Madrid Cenozoic Basin, and NE-SW trending. This thrust displaces the Moho more than 10 km and probably joins the Messejana-Plasencia strike-slip fault at depth. The deduced age movement for both is Oligocene-Lower-Middle Miocene, related to the intraplate stresses transmitted from the Pyrenean orogen towards its foreland. A strain partitioning process was then deduced for the simultaneous movement of both main Alpine faults in Central Iberia.

Active negative flower structure at the western edge of the Tunisian Atlas: Morphotectonic evidence from the case study of Borj Edouane-El Gara Quaternary basin

Located at the Tunisian Atlas area, the Quaternary basin of Borj Edouane-El Gara could serve as a good example for close relation between the continental infill and the neotectonic morphostructural pattern. This Quaternary basin is superimposed on pre-Quaternary basement rocks in the form of an active negative flower structure in a divergent wrench fault zone. The neotectonic evolution of the region has been guided by the plate convergence system. During the Middle-Late Pleistocene, subsidence took locally place in a transtensional deformation zone essentially controlled by strike-slip faults. Closely linked to N–S synthetic and NW-SE antithetic faults, the internal structure of this basin is controlled by a rotational scissor-like kinematics. These faults sets represent inherited structures with normal and lateral components producing a downward rotation of the northern part and an upward rotation of the southern one of the basin. Paleostress analysis shows a progressive change from transtensional in the North to transpressional and compressional deformation toward the Southern corner of the basin where the main synthetic and antithetic faults overlap. During the Holocene, the reactivation of the inherited faulting system has caused the landscape rejuvenation, the basin chopping and the present drainage network architecture. The rates of active tectonics in the Borj Edouane-El Gara Basin has been interpreted through a detailed morphotectonic study of the fault-generated mountain fronts and fluvial systems.

Ongoing Inversion of a Passive Margin: Spatial Variability of Strain Markers Along the Algerian Margin and Basin (Mediterranean Sea) and Seismotectonic Implications

Subduction initiation is an important but still poorly documented process on Earth. Here, we document one of a few cases of ongoing transition between passive and active continental margins by identifying the geometrical and structural signatures that witness the tectonic inversion of the Algerian continental margin and the deep oceanic domain, located at the northern edge of the slow-rate, diffuse plate boundary between Africa and Eurasia. We have analyzed and tied 7900 km of deep seismic reflection post-stacked data over an area of ∼1200 km long and ∼120 km wide. The two-way traveltime lines were converted into depth sections in order to reconstruct and map realistic geometries of seismic horizons and faults from the seafloor down to the acoustic basement. Along the whole length of this young transitional domain, we identify a clear margin segmentation and significant changes in the tectonic signature at the margin toe and in the deep basement. While the central margin depicts a typical thick- and thin-skinned tectonic style with frontal propagation of crustal thrust ramps, the central-eastern margin (Jijel segment) reveals a higher strain focusing at the margin toe together with the largest flexural response of the oceanic lithosphere. Conversely, strain at the margin toe is limited in the western margin but displays a clear buckling of the oceanic crust up to the Spanish margin. We interpret these contrasting, segmented behavior as resulting from inherited heterogeneities in (1) the geometry of the Algerian continental margin from West to East (wrench faulting in the west, stretched margin elsewhere) and (2) the Miocene thermal state related to the diachronous opening of the Algerian basin and to the magmatic imprint of the Tethyan slab tearing at deep crustal levels. The narrow oceanic lithosphere of the Western Algerian basin is assumed to favor buckling against flexure. From the dimension and continuity of the main south-dipping blind thrusts identified at the margin toe, we reassess seismic hazards by defining potential lengths for ruptures zones leading to potential magnitudes up to 8.0 off the central and eastern Algerian margins.

Wrench faulting and trap breaching: A case study of the Kizler North Field, Lyon County, Kansas, USA

The Kizler North Field in northwest Lyon County, Kansas, is a producing field with structures associated with both uplift of the Ancestral Rockies (Pennsylvanian to early Permian) and reactivation of structures along the Proterozoic midcontinent rift system (MRS), which contributed to the current complex and poorly understood play mechanisms. The Lower Paleozoic dolomitic Simpson Group, Viola Limestone, and “Hunton Group” are the reservoir units within the field. These units have significant vuggy porosity, which is excellent for field potential; however, in places, the reservoir is inhibited by high water saturation. The seismic data show that two late-stage wrench fault events reactivated existing faults. The observed wrench faults exhibit secondary P, R’, and R Riedel shears, which likely resulted from Central Kansas uplift-MRS wrenching. The latest stage event breached reservoir caprock units during post-Mississippian to pre-Desmoinesian time and allowed for hydrocarbon migration out of the reservoirs. Future exploration models of the Kizler North and analog fields should be based on four play concepts: 1) four-way closure with wrench-fault-related traps, 2) structural highs in the Simpson Group and Viola Limestone, 3) thick “Hunton Group,” and 4) presence of a wrench fault adjacent to the well location that generates subtle closure but not directly beneath it, which causes migration out of reservoirs. In settings where complex structural styles are overprinted, particular attention should be paid to the timing of events that may cause breaches of seals in some structures but not others. Mapping the precise location and vertical throw of the reactivated wrench faults using high-resolution seismic data can help reduce the drilling risk in analog systems.

The 2011–2014 Pollino Seismic Swarm: Complex Fault Systems Imaged by 1D Refined Location and Shear Wave Splitting Analysis at the Apennines–Calabrian Arc Boundary

In the years between 2011 and 2014, at the edge between the Apennines collapsing chain and the subducting Calabrian arc, intense seismic swarms occurred in the Pollino mountain belt. In this key region, <2.5 mm/yr of NE-trending extension is accommodated on an intricate network of normal faults, having almost the same direction as the mountain belt. The long-lasting seismic release consisted of different swarm episodes, where the strongest event coinciding with a ML 5.0 shock occurred in October 2012. This latter comes after a ML four nucleated in May 2012 and followed by aseismic slip episodes. In this study, we present accurate relocations for ∼6,000 earthquakes and shear-wave splitting analysis for ∼22,600 event-station pairs. The seismicity distribution delineates two main clusters around the major shocks: in the north-western area, where the ML 5.0 occurred, the hypocenters are localized in a ball-shaped volume of seismicity without defining any planar distribution, whilst in the eastern area, where the ML 4.3 nucleates, the hypocenters define several faults of a complex system of thrusts and back-thrusts. This different behavior is also imaged by the anisotropic parameters results: a strong variability of fast directions is observed in the western sector, while stable orientations are visible in the eastern cluster. This tectonic system possibly formed as a positive flower structure but as of today, it accommodates stress on normal faults. The deep structure imaged by refined locations is overall consistent with the complex fault system recently mapped at the surface and with patterns of crustal anisotropy depicting fractures alignment at depth. The possible reactivation of inherited structures supports the important role of the Pollino fault as a composite wrench fault system along which, in the lower Pleistocene, the southward retreat of the ionian slab was accommodated; in this contest, the inversion of the faults kinematics indicates a probable southward shift of the slab edge. This interpretation may help to comprehend the physical mechanisms behind the seismic swarms of the region and defining the seismic hazard of the Pollino range: nowadays a region of high seismic hazard although no strong earthquakes are present in the historical record.

Seismic evidence for crustal architecture and stratigraphy of the Limpopo Corridor: New insights into the evolution of the sheared margin offshore southern Mozambique

The Gondwana breakup along the Mozambique continental margin caused the formation of extensional and/or sheared margins. One of these sheared margins offshore southern Mozambique, the Limpopo Corridor (LC), is a key area for understanding the evolution of the southward movement of Eastern Gondwana with respect to Western Gondwana. However, crustal architecture and stratigraphy in this area have been inadequately studied, mainly due to the lack of deep seismic data.In this study, we present interpretations of new multichannel seismic reflection profiles oriented west to east across the LC and Mozambique Fracture Zone (MFZ). Our results show that the basement of the LC is characterized by dipping and alternating strong and weak reflectors, revealing a thick Volcano-Sedimentary Sequence (VSS). To the west of the LC, a marginal basement high zone extends from north to south, consisting of an eroded and fractured VSS. Besides, the acoustic basement is characterized by an eastward dipping or flat VSS in the LC. Sediments overlying the top of the VSS reveal an onlap geometry against the basement high. Five major stratigraphic units (U1-U5) with well-layered sedimentary sequences are mainly constrained from the western boundary of the basement high to the eastern MFZ, and dated to Neocomian, Middle Cretaceous, Late Cretaceous, Paleogene, and Neogene times respectively.We propose that the VSS in the acoustic basement likely formed in pre-Neocomian time. An uplift event occurred continuously from the Oxfordian-Kimmeridgian to Middle Cretaceous, resulting in a basement high that represents the western limit of the LC. Between this limit and the MFZ, the LC experienced the intracontinental rifting, wrench faulting, and discrepant subsidence associated with the southward strike-slip movement of East Gondwana along the MFZ. The earliest sediments overlying the fractured and deformed VSS consist of a wedge-shaped unit (U1) that deposited along the entire LC during the active transform stage. When the spreading centre passed the adjacent segment of the LC in the Mozambique Basin, the depocentre rapidly shifted to the basin, and the LC entered a passive transform margin stage. We propose that the LC results from three successive stages: (1) intracontinental rifting stage with substantial uplift and erosion, (2) active transform margin stage with strike-slip movement along the MFZ, and (3) passive transform margin with a different sedimentary setting.