Transform Fault System Research Articles

Opening of the North Atlantic Ocean and the rise of Scandinavian mountains

Large-scale topography is usually associated with tectonic plate boundaries, but the Scandinavian mountains (Scandes) are located far from any active tectonic setting, and their origin is unknown. We demonstrate that the Precambrian lithospheric structure of Fennoscandia controlled both the Cenozoic ocean opening and mountain rise in the North Atlantic region. Our new seismic receiver function analysis reveals a block of thick continental crust formed by Proterozoic crustal stacking. We propose that this block created a wide continental shelf bounded by two transform fault systems during continental breakup. This geometry resulted in the formation of lithospheric steps at the continental margin on both sides of the stacked crustal structure, coinciding with the highest Southern and Northern Scandes. We propose that edge-driven convection at these steps caused the mountain rise. This study presents a general model for the formation of high elevation behind passive margins.

Shallow Thermal Anomalies and Their Role in the Breakup Evolution Along the Conjugate Margins of the Fram Strait (Svalbard and Eastern North Greenland), Indicated by Low‐Temperature Thermochronology

AbstractWe investigated highly mature sedimentary rocks exposed along both sides of the Fram Strait in the northern North Atlantic using apatite fission track and (U‐Th)/He thermochronology to obtain information on the thermal imprint of rifting and continental breakup processes along a sheared margin. Our data showed that the conjugate margins experienced several heating episodes, which we explain as resulting from heat transfer along segments of the De Geer Fracture Zone, a large continental transform system which connected magmatic centers north and south of the Fram Strait. Heating occurred prior to and during the Eurekan intraplate orogeny, which occupied the position of the present‐day Fram Strait during the Eocene. Heat transfer may have caused or contributed to lithospheric weak zones, which focussed deformation during intraplate orogeny. Movements along the transform fault system continued during the Oligocene, after the end of the Eurekan Orogeny, causing further structural weakening of pre‐existing fault zones. These were exploited during the final continental breakup leading to the opening of the Fram Strait. No unambiguous thermal signature associated with this latest stage of breakup was detected. Our data underline recent studies on the importance of structural inheritance and continental transform faults for the prolonged and complex processes of continental rifting and breakup.

Spatial Variations in the Degree of Upper‐Mantle Depletion in a Mid‐Ocean Ridge–Transform Fault System

AbstractPartial melting beneath a mid‐ocean ridge creates a chemically depleted layer in the uppermost mantle. This chemical depletion lowers the density of the lithosphere compared with the unmelted mantle. Furthermore, dehydration associated with depletion leads to an increase in mantle viscosity that may affect the structure and dynamics of the oceanic plate. Previous studies have mainly considered the formation of this depleted upper‐mantle layer in a two‐dimensional mid‐ocean ridge setting, leaving the dynamics of mid‐ocean ridge–transform fault systems largely unexplored. In this study, spatial variations in the degree of depletion in the uppermost mantle are predicted for a mid‐ocean ridge–transform fault system using a three‐dimensional thermomechanical model. The degree of depletion generally increases with increasing half‐spreading rate. Less depletion is predicted beneath the transform fault and fracture zone compared with the surrounding mantle. Lateral differences in the degree of depletion in the ridge‐parallel direction are reduced when plastic yielding is considered. The degree of variation in the predicted depletion is related to the transform fault length especially at a low spreading rate, thereby suggesting that the large scatter in observed abyssal peridotite compositions with slow spreading rates could be partly attributed to the length of the fault.

2023 Earthquake Doublet in Türkiye Reveals the Complexities of the East Anatolian Fault Zone: Insights from Aftershock Patterns and Moment Tensor Solutions

Abstract The East Anatolian Fault Zone (EAFZ) is a 700-km-long left-lateral transform fault system along the boundary between the Anatolian and Arabian plates. In the interseismic period, the eastern segments of the EAFZ display relatively uniform seismic activity, whereas the western segments exhibit seismic gaps, localized clusters, and extensive diffuse zones. Hence, our understanding of the geometry and kinematics of the western and northern segments remain limited. The occurrences of the 6 February 2023 Mw 7.8 Kahramanmaraş on the main branch and Mw 7.6 Elbistan earthquakes on the northern branch have led to complex aftershock activity shedding light on the nature of these relatively silent segments. In this study, to better understand the complexities of the fault, we constructed a comprehensive catalog of ∼32,000 earthquakes that occurred between 6 February 2023 and 30 March 2023, using a deep-neural-network-based picker. In addition, 170 earthquake source mechanisms with Mw 3.5+ were obtained from regional moment tensor inversion. The spatial distribution of the aftershocks shows that most of the activity clusters around the fault bends and major depressions. Previously unmapped and inactive secondary faults of varying lengths are identified within these geometrical complexities. The new seismological observations provide compelling evidence of extension along the Karasu valley, compression occurring along the Erkenek segment, reactivation of basin faults near the Narlı fault zone and the persistent shallow seismic creep of the Pütürge segment. The analysis of seismicity and earthquake source mechanisms along the northern branch reveals the structures of previously inactive faults, both near the extensional Göksun bend in the west and the compressional Nurhak fault complex in the east. In summary, we illustrate the intricacies of previously quiet segments of the EAFZ and aim to gain a deeper understanding of how secondary faults and geometrical discontinuities along the EAFZ played a role in shaping the 2023 Türkiye doublet earthquakes.

Sensitivity of gravity anomalies to mantle rheology at mid-ocean ridge – transform fault systems

Marine gravity data can provide information on the distribution of mass anomalies in the oceanic crust and upper mantle. Computing corresponding gravity anomalies, especially so-called ‘residual’ gravity anomalies that directly reflect variations in the crustal structure, relies on gravity corrections of both seafloor relief and lithospheric thermal structure. The lithospheric thermal gravity correction involves either a plate cooling approximation or a mantle flow model with the latter typically done using simplified assumptions on mantle rheology. However, a detailed study of how differing rheological models affect the computed gravity anomalies is still missing. Here, we systematically examine the differences in residual mantle Bouguer anomalies (RMBA) caused by differing assumptions on mantle rheology for 16 mid-ocean ridge – transform fault systems. Our calculations show that isoviscous models tend to underpredict RMBA values within the transform deformation zone and overpredict them in the far field at older plate ages, when compared to plate cooling and nonlinear viscoplastic models. This discrepancy stems from isoviscous models failing to capture plate-like deformation, as well as their inability to resolve brittle failure and the associated strain localization that leads to warm upwelling beneath the transform fault. By exploring a wide parameter range, we find that the importance of mantle rheology scales with plate tectonic parameters at the mid-ocean ridge – transform fault system such as transform age offset, spreading rate, and transform fault length. These findings suggest that gravity thermal corrections at the intrinsically three-dimensional ridge – transform systems should employ mantle flow models that resolve plate-like deformation and brittle failure.

Machine Learning‐Based New Earthquake Catalog Illuminates On‐Fault and Off‐Fault Seismicity Patterns at the Discovery Transform Fault, East Pacific Rise

AbstractOceanic transform faults connect spreading centers and are imprinted with previous tectonic events. However, their tectonic interactions are not well understood due to limited observations. The Discovery transform fault system at 4°S, East Pacific Rise (EPR), represents a young transform system, offering a unique opportunity to study the interplay between faulting and other tectonic events at an early phases of an oceanic transform system. Discovery regularly hosts M5–6 characteristic earthquakes, and the seafloor north of Discovery includes a 35 km‐long rift zone that records a complex history of rifting, faulting and volcanism, suggesting that the transform faults likely interact with regional tectonic activity. We apply a machine‐learning enabled workflow to locate 21,391 earthquakes recorded during a 1‐year ocean bottom seismometer experiment in 2008. Our results indicate that seismicity on the western Discovery fault is separated into seven patches with distinct aseismic and seismic slip modes. Additionally, we observe a patch of off‐fault seismicity near where seafloor abyssal hills intersect the rift zone. This seismicity may have been caused by varying opening rates as spreading rate decreases from north to south in the rift zone. Our findings suggest that the Discovery system is still evolving, and that system equilibrium has not been reached between rifting and faulting. These results reflect the complex yet rarely observed interactions between fault slip, plate rotation, and rifting which are likely ubiquitous at oceanic transform systems.

Evolution of a Cold Intra‐Transform Ridge Segment Through Oceanic Core Complex Splitting and Mantle Exhumation, St. Paul Transform System, Equatorial Atlantic

AbstractAccretionary processes at mid‐ocean ridge segments with low magma input have seldom been investigated over the long term. The evolution of such magma‐starved segments over time is still largely unknown. We present a study on the structure and evolution of the southernmost intra‐transform ridge segment of the St. Paul Transform Fault System in the Equatorial Mid‐Atlantic Ridge, based on new bathymetry, gravity, and rock sampling data. We show that this area evolves differently from previously described tectonics along ridge segments of similar spreading rate. On the flanks of the axial ridge segment, we observe a succession of structures exhumed by detachment faulting, evolving from east‐facing, long‐lived, corrugated oceanic core complexes (∼6 Ma ago), to short‐lived detachment faults exposing lower crust and mantle rocks and facing alternatively east and west in the more recent part of the segment. The oldest detachment faults have been repeatedly split and partially transferred to the opposite flank through the formation of new detachments into the footwall. The terminations of three old, east‐facing detachments are observed on the east flank of the segment. The westward relocations of the plate boundary appear to compensate for the asymmetry of accretion through detachment faulting, overall creating the same amount of lithosphere on both flanks of the ridge. We interpret the observed changes in the time of the accretionary processes to reflect a decrease of the melt supply over the last ∼6 Myr.

Seismotectonics of the Blanco Transform Fault System, Northeast Pacific: Evidence for an Immature Plate Boundary

AbstractAt the Blanco transform fault system (BTFS) off Oregon, 138 local earthquakes and 84 double‐couple focal mechanisms from ocean‐bottom‐seismometer recordings jointly discussed with bathymetric features reveal a highly segmented transform system without any prominent fracture zone traces longer than 100 km. In the west, seismicity is focused at deep troughs (i.e., the West and East Blanco, and Surveyor Depressions). In the east, the BTFS lacks a characteristic transform valley and instead developed the Blanco Ridge, which is the most seismically active feature, showing strike‐slip and dip‐slip faulting. Sandwiched between the two main segments of the BTFS is the Cascadia Depression, representing a short intra‐transform spreading segment. Seismic slip vectors reveal that stresses at the eastern BTFS are roughly in line with plate motion. In contrast, stresses to the west are clockwise skewed, indicating ongoing reorganization of the OTF system. As we observed no prominent fracture zones at the BTFS, plate tectonic reconstructions suggest that the BTFS developed from non‐transform offsets rather than pre‐existing transform faults during a series of ridge propagation events. Our observations suggest that the BTFS can be divided into two oceanic transform systems. The eastern BTFS is suggested to be a mature transform plate boundary since ∼0.6 Ma. In contrast, the western BTFS is an immature transform system, which is still evolving to accommodate far‐field stress change. The BTFS acts as a natural laboratory to yield processes governing the development of oceanic transform faults.

Imaging the magmatic plumbing of the Clear Lake Volcanic Field using 3-D gravity inversions

The Quaternary Clear Lake Volcanic Field (CLVF) in the Northern California Coast Range is the youngest of a string of northward-younging volcanic centers in the state. The CLVF is located within the broad San Andreas Transform Fault System and has been active intermittently for ∼2 million years. Heat beneath the CLVF supports The Geysers, one of the largest producing geothermal fields in the world.Previous geophysical studies proposed the existence of a magma reservoir beneath Mount Hannah, which is northeast of The Geysers, near the geographic center of the CLVF. The lateral extent, depth, and presence of melt within this reservoir are poorly constrained, as is the relationship between this body and the broader magmatic plumbing of the CLVF. To gain a clearer and more comprehensive picture of the CLVF magma source region, a gravity dataset was compiled and the first 3-D gravity inversions of the CLVF were completed.Field and synthetic model inversions from the current study both indicate that the gravity low roughly centered on Mount Hannah is not accurately explained by a 5–7 km thick lens of Mesozoic Great Valley Sequence (ρ=2.58g/cm3) as proposed by Stanley et al. (1998). The observed gravity low is more accurately described by one or more silicic, partial melt bodies between The Geysers and Mount Hannah. Although our inversions cannot constrain the exact depth and geometry of these bodies, the recovered models indicate the existence of a partial melt zone between 6 and 13 km depth.The prolonged eruption history of the CLVF, coupled with the compositional variation of erupted rocks over time and space, is consistent with the existence of several, potentially ephemeral, melt-bearing bodies as opposed to one large melt body. Given the density and location of the recovered anomaly, rhyolite-MELTS thermodynamic modeling suggests the existence of 10–30% rhyodacitic melt within the proposed silicic magma reservoir at about 700 °C and 8 km depth (210 MPa). Independent petrologic, geochemical, and seismic evidence indicates that this silicic partial melt zone is underlain by basaltic melt in the lower to middle crust (13 to 21 km depth), which is fed by a mantle source.Eruptions in the past ∼8.5–13.5 thousand years; high regional heat flow; 3He enrichment of hydrothermal fluids; and our modeling, which suggests the presence of a mid-crustal, silicic partial melt zone, point to a still-active CLVF. The relatively low estimates of partial melt (10–30%) predicted by thermodynamic modeling indicates that an injection of new magma into the imaged partial melt zone is needed to generate sufficient melt to incite future eruptions. Despite the low percent melt estimates within the proposed silicic partial melt zone the potential for future volcanic eruption remains. Due to the proximity of the CLVF to cities surrounding Clear Lake and the densely populated San Francisco Bay Area, continued research and monitoring of the volcanic field are warranted. The geophysical and petrologic modeling presented here improves our understanding of the CLVF magma plumbing system and allows us to better characterize its associated volcanic hazards.

Disparate crustal thicknesses beneath oceanic transform faults and adjacent fracture zones revealed by gravity anomalies

AbstractPlate tectonics describes oceanic transform faults as conservative strike-slip boundaries, where lithosphere is neither created nor destroyed. Therefore, seafloor accreted at ridge-transform intersections should follow a similar subsidence trend with age as lithosphere that forms away from ridge-transform intersections. Yet, recent compilations of high-resolution bathymetry show that the seafloor is significantly deeper along transform faults than at the adjacent fracture zones. We present residual mantle Bouguer anomalies, a proxy for crustal thickness, for 11 transform fault systems across the full range of spreading rates. Our results indicate that the crust is thinner in the transform deformation zone than in either the adjacent fracture zones or the inside corner regions. Consequently, oceanic transform faulting appears not only to thin the transform valley crust but also leads to a secondary phase of magmatic addition at the transition to the passive fracture zones. These observations challenge the concept of transform faults being conservative plate boundaries.

Retracted Article: A Spiritual View to the Huge Earthquake in Türkiye

The shaking of the earth is a common phenomenon that has been experienced by humans from the earliest of times.[1] The Dead Sea Transform fault system, also sometimes referred to as the Dead Sea Rift, is a series of faults that run from the Maras Triple Junction (a junction with the East Anatolian Fault in southeastern Türkiye) to the Northern end of the Red Sea Rift (just offshore of the Southern tip of the Sinai Peninsula).[2] Throughout history, many large earthquakes have occurred in the Dead Sea Transform fault system. In the 1800s before Christ, the earthquake, which is stated in the Quran[3] [4] as punishment for the people of Lut (Lot)[5] for their denial and immorality, probably took place in the Dead Sea Rift region.[6] Maghreb (Northwest African region), Egypt, Antakya (Antioch), Damascus (Dimaşk), Mosul, Homs (Hims), and whole Al-Jazira region were affected by the earthquake that occurred on November 24, 847 with an estimated magnitude of 7.0. In that earthquake, 20,000 people in Antakya and 50,000 people in Mosul died.[6] Lastly, on February 6, 2023 at 04:17 a.m. Türkiye time, a massive earthquake struck Southern Türkiye and Syria with a magnitude of Mw 7.7. An unusually strong Mw 7.6 aftershock as well as hundreds of smaller aftershocks occurred at 01:24 p.m. Türkiye time after the mainshock. Very heavy damage and tens of thousands of deaths occurred. Herein, we discussed earthquake from the Islamic perspective to attract attention to the spiritual dimension of the earthquake.

GIS-BASED HIGH-RESOLUTION GEOLOGICAL MAP (SCALE 1:50,000): A NEW WINDOW INTO STRUCTURAL DOMAINS OF THE QUETTA AND SURROUNDING AREAS

The Quetta and surrounding areas are part of the collision zone between the Indian and Eurasian plates, named as Kirthar and Sulaiman Fold-Thrust belts. The collision is accommodated by folding, thrusting and the Nushki-Chaman Transform Fault System. Detailed high-resolution (scale 1:50,000) mapping and structural analyses were carried out using modern remote-sensing techniques of the ArcGIS to understand mutual relationships of the structural patterns and geometries, and the regional and local stress patterns in the study area. Fieldwork was carried out to acquire the stratigraphic, structural and geomorphological data, using topographic maps and satellite images as base maps in order to plot additional information and further incorporate them in the GIS-based map. Balanced structural cross-sections were also prepared along the selected lines using ArcGIS techniques. Based on new mapping, the understudy area has been subdivided into five distinct structural domains. These domains are classified as; Domain I: broad syncline intervened by a narrow anticline; Domain II: upright folds and thrusts; Domain III: tight, over-turned thrust zone; Domain IV: flysch and molasse successions of Paleocene-Holocene age; and Domain V: suture belt (ophiolites) and associated mélanges and sediments.

Development of the Whitehorse trough as a strike-slip basin during Early to Middle Jurassic arc-continent collision in the Canadian Cordillera

Abstract The Whitehorse trough is a synorogenic basin in the northern Cordillera that resulted from arc-collision processes along the northwestern margin of North America, but its filling history and tectonic significance remain uncertain. New detrital zircon U-Pb-Hf isotope analyses of 12 rock samples, including six basal sandstones that sit unconformably on Triassic rocks of Stikinia, were combined with published detrital zircon and fossil data to establish the depositional ages of synorogenic Laberge Group strata in Yukon and test proposed links between Intermontane terrane exhumation and basin-filling events. Laberge Group strata yielded 205–170 Ma and 390–252 Ma detrital zircon populations that indicate derivation from local Late Triassic to Middle Jurassic arc and syncollisional plutons and metamorphosed Paleozoic basement rocks of the Stikinia and Yukon-Tanana terranes. Basal sandstone units have Early Jurassic depositional ages that show the Whitehorse trough filled during early Sinemurian, late Sinemurian to Pliensbachian, and Toarcian subsidence events. Late Triassic to Early Jurassic detrital zircon grains confirm that syncollisional plutons near the northern trough were exhumed at 0.5–7.5 mm/yr and replicate their excursion to subchondritic Hf isotope compositions as a result of increasing crustal contributions from Rhaetian to Sinemurian time. The new detrital zircon data, combined with recent constraints for Triassic–Jurassic metamorphism and magmatism in Yukon, require modification of published forearc to syncollisional basin models for the Whitehorse trough. We reinterpret Jurassic subsidence patterns and architecture of the Whitehorse trough to reflect sinistral transtension within a transform fault system that resulted from the reorganization of subduction after end-on arc collision.

Cretaceous thermal evolution of the closing Neo-Tethyan realm revealed by multi-method petrochronology

A Cretaceous paleo-accretionary wedge, the Ashin Complex, now exposed along the Zagros suture zone in southern Iran, exhibits mafic, metasedimentary and ultramafic lithologies. Field, geochemical and petrological observations point to an anomalous high-temperature event that gave rise to the formation of peritectic (trondhjemitic) melts associated with restitic garnet-bearing amphibolites. Lu-Hf isotopic dating of centimetre-sized garnet in amphibolite-facies metasediments yielded a crystallization age of 113.10 ± 0.36 Ma, possibly representing the age of prograde to near-peak metamorphic conditions. SHRIMP U-Th-Pb zircon dating from trondhjemitic leucosomes yields crystallization ages of 104 ± 1 Ma, interpreted as the age of the temperature peak, which occurred in the upper amphibolite-facies (c. 650–680 °C at 1.1–1.3 GPa), according to thermodynamic modelling and Ti-in-zircon thermometry. Rutile crystals from two leucosomes yield Zr-in-rutile temperatures in the range of 580–640 °C and a LA-ICP-MS U-Pb age range from 85 to 112 Ma, interpreted as a consequence of partial re-equilibration during incipient cooling. A late static recrystallization event is indicated by the presence of sodic-calcic clinopyroxene, sodic amphibole, Si-rich phengite, titanite overgrowths after rutile and lawsonite within former leucosomes and late fractures. This mineral assemblage is a typical blueschist-facies (high pressure-low temperature) paragenesis and is interpreted as reflecting long-term isobaric cooling that occurred until the end of the Cretaceous as a consequence of increasing slab thermal age. This first report of a melting event in the Zagros paleo-accretionary wedge reveals the presence of a transient, abnormally high thermal gradient of c. 18 °C/km that occurred at c. 105–113 Ma. We speculate that this could be explained by the subduction of a thermal anomaly such as a seamount chain, a transform fault system or, more likely, a spreading ridge under the southern Iranian margin. Indeed, paleogeographic reconstructions of the Tethyan realm suggest the entrance of the Northern Tethyan basin ridge into the subduction zone shortly after 120 Ma.

Late Triassic to Jurassic Magmatic and Tectonic Evolution of the Intermontane Terranes in Yukon, Northern Canadian Cordillera: Transition From Arc to Syn‐Collisional Magmatism and Post‐Collisional Lithospheric Delamination

AbstractEnd‐on arc collision and onset of the northern Cordilleran orogen is recorded in Late Triassic to Jurassic plutons in the Intermontane terranes of Yukon, and in development of the synorogenic Whitehorse trough (WT). A synthesis of the extensive data set for these plutons supports interpretation of the magmatic and tectonic evolution of the northern Intermontane terranes. Late Triassic juvenile plutons that locally intrude the Yukon‐Tanana terrane represent the northern extension of arc magmatism within Stikinia. Early Jurassic plutons that intrude Stikinia and Yukon‐Tanana terranes were emplaced during crustal thickening (200–195 Ma) and subsequent exhumation (190–178 Ma). The syn‐collisional magmatism migrated to the south and shows increasing crustal contributions with time. This style of magmatism in Yukon contrasts with coeval, juvenile arc magmatism in British Columbia (Hazelton Group), that records southward arc migration in the Early Jurassic. Exhumation and subsidence of the WT in the north were probably linked to the retreating Hazelton arc by a sinistral transform. East of WT, Early Jurassic plutons intruded into Yukon‐Tanana record continued arc magmatism in Quesnellia. Middle Jurassic plutons were intruded after final enclosure of the Cache Creek terrane and imbrication of the Intermontane terranes. The post‐collisional plutons have juvenile isotopic compositions that, together with stratigraphic evidence of surface uplift, are interpreted to record asthenospheric upwelling and lithospheric delamination. A revised tectonic model proposes that entrapment of the Cache Creek terrane was the result of Hazelton slab rollback and development of a sinistral transform fault system linked to the collision zone to the north.

Expanding the speciation of terrestrial molybdenum: Discovery of polekhovskyite, MoNiP2, and insights into the sources of Mo-phosphides in the Dead Sea Transform area

Abstract Polekhovskyite, MoNiP2, is the first terrestrial Mo phosphide, a phosphorus-rich homolog of meteoritic monipite, MoNiP. The mineral represents a novel phosphide type of terrestrial Mo speciation. It was discovered among phosphide assemblages in pyrometamorphic rocks of the Hatrurim Formation (the Mottled Zone) in Israel, the area confined to the Dead Sea Transform fault system. Polekhovskyite occurs in the altered diopside microbreccia, as micrometer-sized euhedral crystals intimately intergrown with murashkoite, FeP and transjordanite, Ni2P, in association with Si-rich fluorapatite, hematite, and magnetite. In reflected light, the mineral has a bluish-gray color with no observable bireflectance and anisotropy. Chemical composition (electron microprobe, wt%): Mo 44.10, Ni 22.73, Fe 4.60, P 29.02, total 100.45, which corresponds to the empirical formula Mo0.99(Ni0.83Fe0.18)1.01P2.01 and leads to the calculated density of 6.626 g/cm. Polekhovskyite is hexagonal, space group P63/mmc, a = 3.330(1), c = 11.227(4) Å, V = 107.82(8) Å3, and Z = 2. The crystal structure has been solved and refined to R1 = 0.0431 based on 50 unique observed reflections. The occurrence of Mo-bearing phosphides at the Dead Sea Transform area is a regional-scale phenomenon, with the localities tracked across both Israel and Jordan sides of the Dead Sea. The possible sources of Mo required for the formation of Mo-bearing phosphides are herein reviewed; they are likely related to the processes of formation of the Dead Sea Transform fault system. The problem of anthropogenic contamination of geological samples with Mo and Ni is also discussed in the paper in the context of the general aspects of discrimination between natural and technogenic ultra-reduced phases.

Asymmetry of faults and stress patterns within the Dead Sea basin as displayed by seismological analysis

The Dead Sea pull-apart basin (DSB), which is located within the Dead Sea Transform fault system, displays tectonic asymmetry between its eastern and western longitudinal zones. We investigate the seismological and mechanical signature of this asymmetry by the analyzing the hypocenter distribution and focal-mechanisms of 114 Mw = 1.5–5.2 earthquakes recorded from 1985 till 2012. The analysis indicates that the seismicity along the western longitudinal zone is deeper than the eastern one. Focal mechanism analysis indicates that about 50% of solutions are strike-slip, compatible with the plate motions along the Dead Sea transform. Comparison between the two longitudinal zones of the basin shows that the focal mechanisms in the eastern DSB are dominated by strike-slip faulting shallower than 12 km depth, whereas those in the western DSB are dominated by oblique faulting below 12 km depth. The b value of the Gutenberg-Richter magnitude distributions also show difference between the two zones with ~0.9 in the west and ~0.7 in the east zone.We develop stress-inversion analysis to identify the fault planes of the focal mechanisms by using friction-dependent selection process. The horizontal maximum compression (σHmax) trends NNW-SSE, with increasing value of the vertical stress component along the western part of the basin, corresponds to the oblique faulting in this zone. The optimal friction coefficient determined by the stress-inversion for the fault planes is μ ~ 0.5. Our analysis emphasizes the significant contribution of frictional dependent stress-inversion as an effective tool in seismotectonic analysis.

Segmentation of the Apenninic Margin of the Tyrrhenian Back‐Arc Basin Forced by the Subduction of an Inherited Transform System

AbstractThe Tyrrhenian back‐arc basin developed at the rear of the E‐ward migrating Apennine fold‐and‐thrust belt, with northward decreasing rollback of the subducting Adria slab leading to northward fading of back‐arc extension. The northern portion of the Tyrrhenian basin is made of thinned continental crust, whereas in the central/southern portion extension eventually evolved to oceanic crust production. In this framework, a long‐lasting debate concerns the existence of a >200 km long transform zone along the 41st parallel, which should separate the two portions of the Tyrrhenian basin. At its eastern termination, a branch of the presumed transform zone enters the Tyrrhenian margin of the Apennine belt and occurs as an accommodation zone made of a ribbon of extensional faults and related basins. This accommodation zone, which separates areas of mutually perpendicular extension directions, is here introduced, described, and named the Ponza‐Alife accommodation zone. Interpretation of seismic lines and new structural and stratigraphic data from this accommodation zone have been used to constrain the pre‐orogenic and syn‐orogenic architecture of the subducting plate and the Plio‐Quaternary back‐arc extensional stage. Our data indicate that the studied zone retraces a deep‐seated transform fault system located in the subducting plate and inherited from an Early Jurassic rifting episode, which caused the lateral juxtaposition of different rift domains in the subducting plate. We propose that during collision and trench retreat, this lateral juxtaposition has controlled differential retreat of the subducting plate across the studied zone, forcing the development of the Ponza‐Alife accommodation zone in the overlying back‐arc basin's margin.

Interseismic deformation in the Gulf of Aqaba from GPS measurements

SUMMARYAlthough the Dead Sea Transform (DST) fault system has been extensively studied in the past, little has been known about the present-day kinematics of its southernmost portion that is offshore in the Gulf of Aqaba. Here, we present a new GPS velocity field based on three surveys conducted between 2015 and 2019 at 30 campaign sites, complemented by 11 permanent stations operating near the gulf coast. Interseismic models of strain accumulation indicate a slip rate of $4.9^{+0.9}_{-0.6}~\mathrm{ mm}\,\mathrm{ yr}^{-1}$ and a locking depth of $6.8^{+3.5}_{-3.1}~\mathrm{ km}$ in the gulf’s northern region. Our results further indicate an apparent reduction of the locking depth from the inland portion of the DST towards its southern junction with the Red Sea rift. Our modelling results reveal a small systematic left-lateral residual motion that we postulate is caused by, at least in part, late post-seismic transient motion from the 1995 MW 7.2 Nuweiba earthquake. Estimates of the moment accumulation rate on the main faults in the gulf, other than the one that ruptured in 1995, suggest that they might be near the end of their current interseismic period, implying elevated seismic hazard in the gulf area.

Seismic and Geodetic Crustal Moment-Rates Comparison: New Insights on the Seismic Hazard of Egypt

A comparative analysis of geodetic versus seismic moment-rate estimations makes it possible to distinguish between seismic and aseismic deformation, define the style of deformation, and also to reveal potential seismic gaps. This analysis has been performed for Egypt where the present-day tectonics and seismicity result from the long-lasting interaction between the Nubian, Eurasian, and Arabian plates. The data used comprises all available geological and tectonic information, an updated Poissonian earthquake catalog (2200 B.C.–2020 A.D.) including historical and instrumental datasets, a focal-mechanism solutions catalog (1951–2019), and crustal geodetic strains from Global Navigation Satellite System (GNSS) data. The studied region was divided into ten (EG-01 to EG-10) crustal seismic sources based mainly on seismicity, focal mechanisms, and geodetic strain characteristics. The delimited seismic sources cover the Gulf of Aqaba–Dead Sea Transform Fault system, the Gulf of Suez–Red Sea Rift, besides some potential seismic active regions along the Nile River and its delta. For each seismic source, the estimation of seismic and geodetic moment-rates has been performed. Although the obtained results cannot be considered to be definitive, among the delimited sources, four of them (EG-05, EG-06, EG-08, and EG-10) are characterized by low seismic-geodetic moment-rate ratios (<20%), reflecting a prevailing aseismic behavior. Intermediate moment-rate ratios (from 20% to 60%) have been obtained in four additional zones (EG-01, EG-04, EG-07, and EG-09), evidencing how the seismicity accounts for a minor to a moderate fraction of the total deformational budget. In the other two sources (EG-02 and EG-03), high seismic-geodetic moment-rates ratios (>60%) have been observed, reflecting a fully seismic deformation.