Transform Fault Zone Research Articles

Tidal triggering of seismic swarm associated with hydrothermal circulation at Blanco ridge transform fault zone, Northeast Pacific

Seismicity associated with hydrothermal systems (e.g., submarine volcanoes, mid-oceanic ridges, oceanic transform faults, etc.) share a complex relationship with the tidal forcing and induced fluid flow process under different tectonic settings. The hydrothermal circulation drives the deformation at the brittle-ductile transition zone within a permeable brittle crust. Although the tidal loading amplitudes are too small to generate a brittle deformation, the incremental pressure exerted by the tidal loading can modulate the flow of hydrothermal fluid circulation and trigger the critically stressed faults or fracture zones. We present a compelling case of tidal modulation in seismicity along the Blanco Ridge Transform Fault Zone (BRTFZ), in the northeast Pacific. The strong diurnal and fortnightly periodicity has been observed in the deeper seismic swarm (7–15 km), whereas the shallow seismic swarm (0–7 km) does not exhibit any such tidal periodicity. The dominance of diurnal and fortnightly periodicity in the deeper seismic swarm is explained by the high amplitude tidal cycles providing additional stress on the fluid circulation at the crust-mantle boundary. Moreover, our robust statistical correlation of seismicity with tidal stress and resonance destabilization model under rate-and-state friction formalism suggests that the fault segments are conditionally unstable and more sensitive to periodic tidal stress perturbation.

Tectonics and seismicity of the Gulf of Aden: Contributions of edge detection filters applied to potential field data

Several studies have applied edge detection filters based on potential field derivatives, ratios and statistical-based tools to evaluate structures and provide interpretations. These edge filters provide different responses that may lead to false interpretations, making the selection of an effective filter that can provide optimum response difficult for users. Evaluating the reliability of the edge filters and avoiding false interpretations of the geologic structures requires simultaneous evaluation of filters applied to the same set of theoretical models. By using 2.5D models of gravity and magnetic synthetic data, we evaluate the performance of 21 edge detection filters based on different scenarios. The outcome of the various tests shows that filters of the third class give best results, with reference to the second and first class, and present good to moderate resolution in gravity and magnetic models. This comparison provides a means of inferring the tectono-structural styles, having generated multiple results from different filters and screening for artefacts (i.e., false impressions). Thereafter, we applied the edge enhancement filters to the magnetic and free-air gravity data of Aden Rift. Boundaries corresponding to seafloor spreading, divergent plate and transform faults were identified. The boundaries correlate well with the source locations of earthquakes at shallow depths. In addition to the dominant extensional tectonic regime, we suggest that the local fault system influenced by the Afar plume provides insights to the complex interaction between magmatic, sedimentary, and tectonic forces, shaping the geology of the Arabian-Nubian triple junction and contributes to the dynamics of the Gulf of Aden. The occurrence of earthquake activities along normal and transform fault zones which correlates with identified structural boundaries resulting from the application of edge detection filters provides a novel application to the study of rifting processes by multiple observation techniques.

The Protracted Evolution of a Plate Boundary: Eastern Cuba Block and Old Bahamas Channel

AbstractThe Eastern Cuban block has experienced a complex tectonic history characterized by plate interactions, resulting in a diverse array of geological features observable in the offshore sedimentary record. We investigate the tectonic evolution of offshore Eastern Cuba, specifically in the Old Bahamas Channel and its surrounding areas, by integrating multi‐channel seismic (MCS) reflection and published geological data. Our analysis employs stratigraphic frameworks and MCS data to assess deformation and key geological events in the region. We highlight the complex tectonic history of the Eastern Cuban block, marked by significant geodynamic events, such as rifting, the subduction of the oceanic Proto‐Caribbean plate, and syn‐orogenic and post‐orogenic phases. The seismic units observed in the majority of the study area reveal the early evolution of the Northern Proto‐Caribbean margin, subsequently impacted by the Cuban orogeny and the reactivation of the Cuban Transform Fault zone corresponding to a former plate boundary. We propose estimated ages for the seismic sequences, correlating them with available well data from neighboring regions. This study offers valuable insights into the tectonic history and geological evolution of offshore Eastern Cuba, contributing to a more comprehensive understanding of the region's geodynamic development.

Characteristics and genesis of the Zhongnan Fault Zone in the South China Sea oceanic basin: Insights from an integrated analysis of multibeam bathymetric and 2D multichannel seismic data

The Zhongnan Fault Zone (ZFZ) is a large-scale tectonic belt in the South China Sea (SCS) oceanic basin and plays an important role in the formation and evolution of the basin. Despite numerous studies over the past few decades, debates persist regarding its location, orientation, nature, time of activity, and genesis. In this study, we evaluate our findings on the characteristics and origin of the fault zone through an integrated analysis of multibeam bathymetric and 2D multichannel seismic data. Our results reveal the ZFZ as a fault zone approximately 400 km long and 50–90 km wide, situated between the east subbasin (ESB) and southwest subbasin (SWSB) of the SCS. The fault zone is oriented N8°W, roughly perpendicular to and laterally displacing the relict spreading center and related spreading lineaments. Bounded by discontinuous linear seamounts, the fault zone comprises two V-shaped subparallel pull-apart basins and a separating basement high. Steeply dipping (>60°) normal basement-involved faults bound these pull-apart basins, forming typical negative flower structures. Numerous northeast-oriented en-echelon linear bathymetric highs within the fault zone are identified as secondary antithetic shears (P′ shears). These shears are characterized by their orientation relative to the principal displacement zones defined along the pull-apart basins. The ZFZ exhibits differences from the adjacent subbasins in water depth, basement burial, stratal thickness, and seismic stratigraphic characteristics. Five seismic sequences (S1–S5 upward) in the ZFZ and nearby ESB and SWSB are defined, dating to the Early Miocene synspreading (S1) and the Middle Miocene to Recent postspreading (S2–S5) stages, respectively. The difficulty in correlating seismic facies in sequences S1–S3, compared with the comparable seismic facies in sequences S4–S5 between the ZFZ and adjacent subbasins, suggests a horizontal displacement during the synspreading and early postspreading stages. We propose that the ZFZ functioned as a right-lateral transform fault zone during the synspreading period (the Early Miocene, approximately 24–16 Ma) of the SWSB and transitioned into a left-lateral strike-slip fault zone during the successive early postspreading period (the Middle-Late Miocene, approximately 16–5.3 Ma).

Magmatism controls global oceanic transform fault topography

Oceanic transform faults play an essential role in plate tectonics. Yet to date, there is no unifying explanation for the global trend in broad-scale transform fault topography, ranging from deep valleys to shallow topographic highs. Using three-dimensional numerical models, we find that spreading-rate dependent magmatism within the transform domain exerts a first-order control on the observed spectrum of transform fault depths. Low-rate magmatism results in deep transform valleys caused by transform-parallel tectonic stretching; intermediate-rate magmatism fully accommodates far-field stretching, but strike-slip motion induces across-transform tension, producing transform strength dependent shallow valleys; high-rate magmatism produces elevated transform zones due to local compression. Our models also address the observation that fracture zones are consistently shallower than their adjacent transform fault zones. These results suggest that plate motion change is not a necessary condition for reproducing oceanic transform topography and that oceanic transform faults are not simple conservative strike-slip plate boundaries.

Overture for the Mandara and Vasuki Plates

Models of past plate motions in the Indian Ocean help map the supercontinent Gondwana and investigate how mantle plumes influence plate tectonics. Reducing confidence in this, however, the range of available models all produce large pre-94 Ma movements, in various kinematic senses, between India and Madagascar. There is no observational evidence for any of these motions, suggesting along with their diversity that they are artefacts stemming from contrasting resolutions of techniques used for reconstructing India and Madagascar to Antarctica. A higher resolution approach to India–Antarctica reconstruction concentrates on geophysical records of relative plate motion azimuths. Applying its results regionally eliminates Indo-Malagasy motions before 94 Ma, and prompts new hypotheses of two small tectonic plates. The early Cretaceous Mandara plate, in the Enderby Basin off East Antarctica, may have initiated and rotated at a mid-ocean ridge that was supplied by excess melt from the Kerguelen plume. The late Cretaceous Vasuki plate may have conveyed Sri Lanka southwards across the western Bay of Bengal. The 85°E and Comorin ridges may have formed at active transform fault zones along Vasuki’s margins that were supplied with excess melt from the Crozet and Marion plumes. The model confidently implies the presence of 500,000 km2 of continental crust beneath the Kerguelen Plateau, places Sri Lanka 1000 km further east within Gondwana than previous reconstructions, and casts doubt on the existence of plate kinematic signals that have previously been attributed to the arrival and spread of the Marion plume beneath India and Madagascar at ~105 Ma.

Sapanca Gölü Yakınında Tek Bir Sismik İstasyonda Tekrarlayan Deprem Gözlemleri

The North Anatolian Fault Zone (NAFZ), which forms the plate boundary between Anatolian Plate and Eurasian Plate, is one of the most active transform fault zones in the world. Following two consecutive magnitude M>7 earthquakes in 1999, an intensified monitoring of western portion of NAF is commenced. Dense networks of onshore/offshore seismic, acoustic, geodetic sensors and surface creep and strain sensors were installed. A single seismic sensor among these, which is located at the midpoint of 1999 ruptures, near Lake Sapanca, exhibits some unusual seismic activity. On a fault segment where creep is known to be present, a series of minor seismic events was observed with identical locations and a recurrence time of three years. These events are quite short in duration and highly similar in their waveforms. Using a single station approach, their angle of incidence and back azimuth were found to coincide with the location of two M2.3 and M2.1 events. At this stage, it is not clear whether these events reflect fault creep at seismogenic depth. Nevertheless, these initial observations emphasize the necessity of monitoring this segment more densely, where recurrent minor earthquakes are likely to be observed.

Lateral changes of crustal extension and passive margin type along the Brazilian southeastern margin

For the last half century, studies based on an increasing, diverse data set, have focused on passive continental margin evolution, as a result of a sequence of tectonic processes that occur in crustal scale and in the geological time. Where extensive subsurface data exists, two distinct endmembers of continental margin architectures were first described along specific margins of the North Atlantic: 1) wide continent-ocean transitions or hyper-extended; 2) narrow continent-ocean transitions or hypo-extended. The lateral transition between these endmember margins, which can occur laterally in short distances, is still not fully understood. The same two endmember margin types are observed across passive margins around the world and notably along the South Atlantic margin. In an area known for controversial interpretations about the crustal nature and the limits of thinned continental crust, this investigation integrates crustal thickness, seismic interpretation, and facies analyses across ∼150,000 km of seismic data along the Brazilian southeastern margin. This study has implications for those investigating the crustal geometry variations for basin analysis and results impact hydrocarbon assessments for the southeastern margin of Brazil. We resume continental margin analyses, in a critical moment as hydrocarbon exploration advances from the continental slope to ultra-deep waters. Results indicate a marked change from a narrow, hypo-extended, sub-aerial, Iceland-like, plume-related volcanic crust in the Pelotas Basin, to a hyper-extended, Iberia-like, magma-rich crust in the Santos Basin, which is separated by a pronounced oceanic transform-fault zone. Dextral movement along this NW-oriented fault zone accommodated differential extensional rates between the hypo- and the hyper-extended margins. Lateral variations in magmatic content within these margin types are interpreted as result from the interaction with mantellic plumes. Margin architecture can locally be affected by pre-existing fault zones and be later modified by oceanic transform faults. The interactions between crustal extensional rates, crustal rheology, mantle underplating, and volcanic material exhumed through mantle-derived plumes, are the key controls for the evolution of continental margins. Tectonic framework classification proposed in this study presents an alternative, original model for continental passive margins evolution.

Homogenizing instrumental earthquake catalogs – a case study around the Dead Sea Transform Fault Zone

The creation of a homogenized earthquake catalog is a fundamental step in seismic hazard analysis. The homogenization procedure, however, is complex and requires a good understanding of the heterogeneities among the available bulletins. Common events within the bulletins have to be identified and assigned with the most suitable origin time and location solution, while all the events have to be harmonized into a single magnitude scale. This process entails several decision variables that are usually defined using qualitative measures or expert opinion, without a clear exploration of the associated uncertainties. To address this issue, we present an automated and data-driven workflow that defines spatio-temporal margins within which duplicate events fall and converts the various reported magnitudes into a common scale. Special attention has been paid to the fitted functional form and the validity range of the derived magnitude conversion relations. The proposed methodology has been successfully applied to a wide region around the Dead Sea Transform Fault Zone (27N-36N, 31E-39E), with input data from various sources such as the International Seismological Centre and the Geophysical Institute of Israel. The produced public catalog contains more than 5500 events, between 1900 and 2017, with moment magnitude Mw above 3. The MATLAB/Python scripts used in this study are also available.

Large Earthquakes Recurrence Time in the Kefalonia Transform Fault Zone (KTFZ), Greece: Results from a physics-based simulator approach

Large earthquakes mean recurrence time (Tr) on specific fault segments is one of the primary input parameters for developing long-term Earthquake Rupture Forecast (ERF) models in a specific time span considering either a time-independent or an elastic rebound motivated renewal assumption. An attempt is made to define Tr on the major fault segments comprised in Kefalonia Transform Fault Zone (KTFZ), which is an active boundary demarcating from the west the area of central Ionian Islands, namely Lefkada and Kefalonia, and is associated with remarkably high seismic activity. Frequent large (Mw ≥ 6.0) earthquakes are reported to have caused severe damage during the last six centuries. Although the number of large earthquakes (including both historical and instrumental) is satisfactory enough for regional hazard studies, their number become very limited when they are subdivided into subsets assigned to specific fault segments. Physics-based earthquake simulators are approaches to overcome recurrence intervals shortage, due to their ability to generate long lasting earthquake catalogs. The application of a physics-based simulatorn the KTFZ, is attemped upon a detailed fault network model and implemented multiple times and with a wide range of input parameters, aiming at the definition of the most representative simulated catalog in respect to the observed regional seismicity. The most representative simulated catalog is finally used for investigating the recurrence behavior of large (Mw ≥ 6.0) earthquakes and assessing whether the renewal model performs better that the Poisson model, after considering both individual and multiple ruptured segments scenarios.

Long-Term Recurrence Pattern and Stress Transfer along the Kefalonia Transform Fault Zone (KTFZ), Greece: Implications in Seismic Hazard Evaluation

An effort is exerted to investigate the recurrence pattern of large earthquakes (Mw ≥ 6.0) in the Kefalonia Transform Fault Zone (KTFZ), Greece, by considering the incorporation of the 74-year (1948–2022) evolving stress field. Four earthquake occurrence models—the Poisson, Poisson with the incorporation of the static stress changes (Poisson + ΔCFF), Brownian passage time (BPT) and Brownian passage time with the incorporation of the static stress changes (BPT + ΔCFF)—have been applied to estimate the occurrence probabilities of nearly characteristic earthquakes for the seven fault segments of the study area. The mean recurrence time, Tr, is estimated using the physics-based seismic moment rate conservation method. The results show large variability depending upon fault parameters. Incorporating the state of stress into Tr results in both advanced and delayed recurrence patterns. The occurrence probability estimates for the next 10, 20 and 30 years indicate that the fault segment most likely to be ruptured is the Paliki North fault segment in all models. Overall, the occurrence probabilities, combined with the state of stress along the fault segments, emphasize the high seismic moment rate of the study area. The application of time-dependent models (BPT, BPT + ΔCFF) resulted in significant increases or decreases in the associated seismic hazard.

Morphological evidence for the extension of the Zabargad Transform Fault Zone to the Saudi Arabian Red Sea margin

The fault locations and orientation of the Zabargad Transform Fault Zone, also called the Zabargad Fracture Zone (ZFZ), have previously only been delineated by satellite-based geophysical data, causing intense debate over the last few decades. Newly recognized geomorphological features identified in bathymetric and LiDAR data from the NE Red Sea margin present the first ground evidence for the northern extent of the ZFZ. The features are aligned over 84 km, starting from the Mabahiss Deep, near the spreading axis, and continuing to the shallow Saudi Arabian shelf along the northern termination of the Al Wajh carbonate platform. Analysis of the seafloor morphology reveals three geomorphic terrains: (1) a deeply incised canyon feeding into the Mabahiss Deep, which is characterized by dozens of amphitheatre-shaped scarps; (2) a 22 km wide head-scarp that follows the edge of the Al Wajh platform; and (3) multiple fault scars and graben-like structures on the shallow shelf. We interpret these morphological features as deformation indicators associated with the deformation processes in the ZFZ and postulate that they represent the northern end of the ZFZ. In addition, the fault zone delineates the NW margin of the Al Wajh carbonate platform and most likely continues to shape it. This paper gives new insights into the interaction between fracture zones and continental margins and their role in the morphogenesis of the seafloor.

South and East African fracture zones: a long lifespan since the breakup of Gondwana

Abstract Gondwana started to split up during the Early Jurassic ( c. 180 Ma) with the separation of Antarctica and Madagascar from Africa, followed by the separation of South America and Africa during the Early Cretaceous. Thanks to recent seismic profiles, the architecture of rifted margins and the transform fault zones, which developed as a result of the relative motion between tectonic plates, have been recently evidenced and studied along the whole eastern and southeastern Africa margins (i.e. in the Western Somali Basin, the Mozambique Basin, the Natal Basin and the Outeniqua Basin). But, the structure and overall kinematic evolution of the three major transform fault zones – such as the Agulhas, the Davie and the Limpopo fracture zones (FZ) – that together control the opening of major oceanic basins (Antarctic Ocean, Weddell Sea and Austral South Atlantic) remain poorly studied. The interpretation of an extensive regional multi-channel seismic dataset coupled with recent studies allows us to propose a detailed regional synthesis of the crustal domains and major structural elements of the rifted margins along the whole eastern and southeastern Africa. We provide new constraints on the structure and evolution of these three transform systems. Although our findings indicate common features in transform style (e.g. a right-lateral transform system, a wide sheared corridor), the deformation and magmatism along these systems appear quite different. In particular, our results show that the Davie and Agulhas transform faults postdate the development of the rift zone-controlling faults, whereas the Limpopo margin seems to be a simple intra-continental transform. Moreover, the Davie and Agulhas FZ recorded spectacular inversions during the transform stage, whereas transtensional deformation is developed along the Limpopo FZ. This different style of deformation may be explained by two main forcing parameters: (i) the far-field forces that may induce a rapid change of regional tectonic stress and (ii) the magmatic additions that modify mainly the crustal rheology. In the post-drift history, several reactivations of transform fault zones are recorded, implying that some transform margins are excellent recorders of large plate kinematic changes. Such reactivations can serve also as drains for magmatic fluids in the vicinity of hotspots emplacement.

Ultramafic-Hosted Ni-Cu-Co-(As) Mineralization from an Ancient Oceanic Transform Fault Zone in the Troodos Ophiolite, Cyprus: An Analogue for Ultramafic Sea-Floor Massive Sulfide Mineralization?

Abstract Accumulations of sulfide minerals that are enriched in Ni-Cu-Co-(As) occur as sea-floor massive sulfide (SMS) deposits associated with ultramafic rock types on the sea floor and in ophiolite terranes as Outokumpu-type mineralization. In this study we focus on similar mineralization at Lakxia tou Mavrou in the Limassol Forest Complex of Cyprus, which represents the on-land exposure of an oceanic transform fault zone preserved within the Troodos ophiolite. Mineralization here consists of massive lenses of pyrrhotite associated with veins of isocubanite, chalcopyrite, Co pentlandite, and chrome spinel hosted in serpentinized mantle peridotite. We reexamine the field context of mineral occurrences and use in situ mineral chemistry, element mapping, and sulfur isotope ratios (δ34S) to constrain metal sources and provide an updated paragenetic model for Lakxia tou Mavrou. Highly variable S/Se ratios (304–108,571), a depletion in platinum group elements relative to mantle values, and an average δ34S value of –3.7 ± 2.4‰ (1σ, n = 17) in sulfide minerals support a hybrid hydrothermal and magmatic origin for the mineralization. Metals at Lakxia tou Mavrou were sourced from both the serpentinization of peridotites and from crosscutting intrusions, with later intrusions into the already serpentinized mantle lithosphere host providing a heat source to drive prolonged hydrothermal circulation. The reexamination of the field context of mineralization shows that the Ni-Cu-Co-(As) mineralization at Lakxia tou Mavrou originally formed because of the fault-guided intrusion of hot primitive magma bodies into serpentinized shallow mantle lithosphere in the active domain of an ocean-floor transform fault zone. The mineralization was subsequently partially disrupted by structures related to emplacement of the Troodos ophiolite. We show that the relationship between serpentinization, magmatism, and hydrothermal circulation at Lakxia tou Mavrou can be used to understand the formation of ultramafic-hosted SMS deposits in transform fault and other ultramaficdominated slow- and ultraslow-spreading mid-ocean ridge settings.

Geodynamic factors in the formation of large gold-bearing provinces with Carlin-type deposits on continental margins in the North Pacific

<abstract> <p>Several similar indicators in Nevada (USA) and South Yakutia (Russia) gold-bearing provinces have been identified based on modern tectonic, geophysical and seismic tomography observations, followed by the analysis of the main geodynamic factors of the formation and distribution of large gold-bearing provinces in the North Pacific. One of the significant metallogenic peculiarities is a wide variety of formational and mineral deposits concentrated in the areas. Both provinces are situated at active margins surrounded by fold-thrust belts. In South Yakutia, a combination of sublatitudinal Baikal-Elkon-Ulkan and submeridional Seligdar-Verkhnetimpon gravity field gradient zones is recorded. In contrast, significant positive gravity anomalies of the Northern Nevada Rift and higher-order gradient zones are presented in Nevada. Large pluton and transform fault zones in both provinces support a conclusion about the fundamental role of geodynamic factors in developing ore-magmatic systems in the regions. Significant differences in the scale of the gold mineralization in the considered provinces are explained by the existence under the North American continent not only of the Mendocino transform fault zone but also of the Juan de Fuca paleo-spreading center. In contrast, the Inagli-Konder-Feklistov magmatic-metallogenic belt alone controls mineralization under the Asian continent.</p> </abstract>

A local earthquake tomography on the EAF shows dipping fault structure

The East Anatolian Fault Zone (EAFZ) is a left-lateral transform fault zone located between the Anatolian and Arabian plates. In this study, in order to image the upper crustal structure beneath the eastern segments of EAFZ, 3D seismic velocity variations are computed using local earthquake tomography. The initial catalog for the tomography process consists of 2200 well-located earthquakes recorded at 49 seismic stations around the study region between 2007 and 2020. 1D initial velocity model is constructed based on previous studies in the region. The maximum number of iterations and the velocity perturbations which sustain the linearity of the inversion are determined based on the detailed tests. Reliable zones of the final model are decided based on the Derivative Weighted Sum and Hit Count distribution. The resulting velocity model displays a clear velocity contrast across the surface trace of the EAFZ down to a depth of 12 km. While the Anatolian side of the fault displays higher velocities associated with the ophiolitic units in the region, the south of the fault zone is represented by lower velocities due to sedimentary deposits. The vertical cross-sections of tomographic models show a north dipping fault between Palu and Çelikhan. The complete earthquake catalog is relocated using the 3D velocity model. Together with the obtained velocity model, the relocated hypocenters indicate that the dip of the EAFZ is not uniform, the Palu segment dips to the north with an angle of ~80°, while the Pütürge and Erkenek segments dip to the north with a lower angle of ~60?70°.

Multisphere Tectonics of the Earth System

AbstractThe fundamental theoretical framework of the Multisphere Tectonics of the Earth System is as follows: (1) It intends to extend the geotectonic studies from the crustal and lithospheric tectonics to the multisphere tectonics of the Earth system as a whole. (2) The global dynamics driven by both the Earth system and the cosmic celestial system: solar energy, multispheric interactions of the Earth system and the combined effects of the motions of celestial bodies in the cosmos system are the driving forces of various geological processes. (3) The Continent‐Ocean transformation theory: the continent and ocean are two opposite yet unified geological units, which can be transformed into each other; neither continent nor ocean will survive forever; there is no one‐way development of continental accretion or ocean extinction; the simple theory of one‐way continental accretion is regarded as invalid. (4) The continental crust and mantle are characterized by multiple layers, with different layers liable to slide along the interfaces between them, but corroboration is needed that continents move as a whole or even drift freely. (5) The cyclic evolution theory: the development of Earth's tectonics is not a uniform change, but a spiral forward evolution, characterized by a combination of non‐uniform, non‐linear, gradual and catastrophic changes; different evolutionary stages (tectonic cycles) of Earth have distinctive global tectonic patterns and characteristics, one tectonic model should not be applied to different tectonic cycles or evolutionary stages. (6) The structure and evolution of Earth are asymmetric and heterogeneous, thus one tectonic model cannot be applied to different areas of the world. (7) The polycyclic evolution of the continental crust: the continental crust is formed by polycyclic tectonics and magmatism, rather than simply lateral or vertical accretion. (8) The role of deep faults: the deep fault zones cutting through different layers of the crust and mantle usually play important roles in tectonic evolution. For example, the present‐day mid‐ocean ridge fault zones, transform fault zones and Benioff zones outline the global tectonic framework. Different tectonic cycles and stages of Earth's evolution must have their own distinctive deep fault systems which control the global tectonic framework and evolutionary processes during different tectonic cycles and stages. Starting from the two mantle superplumes Jason (Pacific) and Tuzo (Africa), the study of the evolutionary process of the composition and structure of the crust and mantle during the great transformation and reorganization of the Meso‐Cenozoic tectonic framework in China and the other regions of Asia is a good demonstration of theory of Multisphere Tectonics of the Earth System.

An 8‐Year‐Long Low‐Frequency Earthquake Catalog for Southern Cascadia

AbstractLow‐frequency earthquakes (LFEs) are small magnitude earthquakes with typical magnitude less than 2 and reduced amplitudes at frequencies greater than 10 Hz relative to ordinary small earthquakes. Their occurrence is often associated with tectonic tremor and slow‐slip events along the plate boundary in subduction zones and occasionally transform fault zones. They are usually grouped into families with all the earthquakes of a given family originating from the same small patch on the plate interface and recurring more or less episodically in a bursty manner. In this study, we extend the LFE catalog obtained by Plourde et al. (2015), https://doi.org/10.1002/2015gl064363 during an episode of high tremor activity in April 2008 to the 8‐year period 2004–2011. All of the tremors in the Boyarko et al. (2015), https://doi.org/10.1016/j.epsl.2015.06.026 catalog south of 42° North have associated LFE activity, but we have identified several smaller episodes of LFEs without associated tremor activity, and extend their catalog forward and backward by a total of about 3 years. As in northern Cascadia, the downdip LFE families have recurrence intervals several times smaller than the updip families. For the April 2008 Episodic Tremor and Slip event, the best recorded LFE families exhibit a strong tidal Coulomb stress sensitivity starting 1.5 days after the rupture front passes by each LFE family. The southernmost LFE family, which has been interpreted to be on the subduction plate boundary, near the updip limit of tremor, has a very short recurrence time. Also, these LFEs tend to occur during times when predicted tidal Coulomb stress is discouraging slip on the plate boundary. Both observations suggest that this LFE family may be on a different fault.

Early post-breakup kinematic adjustments of continental–oceanic transform fault zones: Cape Range, Coromandal and Romanche transform margin case study

Abstract A comparison of transform margins that started their evolution as continental transforms shows differences in their tectonic style, which can be attributed to the variable kinematic adjustments they underwent during the post-breakup continental-oceanic stage of their development. Three end-member examples are presented in detail. The Cape Range transform fault zone (Western Australia) retained its strike-slip character during its entire continental-oceanic stage, as documented by the transform-perpendicular system of spreading-related magnetic stripe anomalies. The Coromandal transform fault zone (Eastern India) adjusted its kinematics to a transtensional one during its continental-oceanic stage, as indicated by the transform-oblique system of magnetic stripe anomalies and extensional component of movement indicated by a narrow zone of crustal thinning. The Romanche transform fault zone (Equatorial Africa) adjusted its kinematics to transpressional, as documented by the changing geometries of magnetic stripe anomalies and transpressional folding during its continental-oceanic development stage. Based on the recognition of the aforementioned adjustments, we suggest a new categorization of transforms into (1) those that experience transpressional adjustment, (2) those that experience transtensional adjustment and (3) those that do not experience any adjustment during their continental-oceanic development stage.

Seismological and Ground Deformation Study of the Ionian Islands (W. Greece) during 2014–2018, a Period of Intense Seismic Activity

Seismicity in the Ionian Sea (W. Greece) is mainly generated along the Cephalonia–Lefkada Transform Fault Zone (CLTFZ) in the central Ionian, and on the northwestern termination of the Hellenic subduction margin in the south. Joint pre-, co- and post-seismic ground deformation and seismological analysis is performed at the broad Ionian area, aiming to homogeneously study the spatiotemporal evolution of the activity prior to and after the occurrence of strong (M > 6) earthquakes during the period of 2014–2018. The 2014 Cephalonia earthquakes (Mw6.1 and Mw5.9) were generated on a faulting system adjacent to CLTFZ, causing local ground deformation. The post-seismic sequence is coupled in space and time with the 2015 Lefkada earthquake (Mw6.4), which occurred on the Lefkada segment of the CLTFZ. Co-seismic displacement was recorded in the broader area. Seismicity was concentrated along the CLTFZ, while its temporal evolution lasted for several months. The 2018 Zakynthos earthquake (Mw6.7) caused regional deformation and alterations on the near-velocity field, with the seismicity rate remaining above background levels until the end of 2021. In the northern Ionian, convergence between the Apulian platform and the Hellenic foreland occurs, exhibiting low seismicity. Seismic hazard assessment revealed high PGA and PGV expected values in the central Ionian.