Transform Fault Zone Research Articles

On the Segmentation of the Cephalonia–Lefkada Transform Fault Zone (Greece) from an InSAR Multi-Mode Dataset of the Lefkada 2015 Sequence

We use Interferometric Synthetic Aperture Radar (InSAR) to study the Cephalonia–Lefkada Transform Fault Zone (CTF) in the Ionian Sea. The CTF separates continental subduction to the north from oceanic subduction to the south, along the Hellenic Subduction Zone. We exploit a rich multi-modal radar dataset of the most recent major earthquake in the region, the 17 November 2015 Mw 6.4 event, and present new surface displacement results that offer additional constraints on the fault segmentation of the area. Based on this dataset, and by exploiting available information of earthquake relocation, we propose a new rupture process for the 2015 sequence, complementary to those published already. Our modelling includes an additional southern fault segment, oblique to the segment related with the mainshock, which indicates that the CTF structure is more complex than previously believed.

Cephalonia-lefkas Transform Fault Zone (CLTFZ) Complexity: Insights from 2015 Lefkas Earthquake Sequence

In order to define a better model for the Cephalonia-Lefkas Transform Fault Zone the sequence of 2015 Lefkas earthquake was examined. On 17 November 2015 (07:10 GMT) a major earthquake (Mw=6.4) occurred on the central-western part of Lefkas island. Several destructive events were located in the past in this fault zone, so an extensive seismotectonic study is feasible for that area. Manual analysis was performed using a custom velocity model that was determined for that purpose, applying the average travel-time residuals and location uncertainties errors minimization method. Several clusters belonging to the aftershock sequence were identified, whereas three are directly related to the causative fault, covering an area of about 25 km. The central one, which includes the mainshock, comprises of only a few aftershocks. The northern, within which the majority of aftershocks are located, lies in the central part of Lefkas island and the southern occurred close to the SW edge of the island. In addition, offshore clusters with distinct characteristics have been identified to the south, between Lefkas and Cephalonia islands. The temporal evolution of the aftershock sequence indicates that no migration was observed, given that after the occurrence of the mainshock the entire epicentral area was activated. Focal mechanisms of the Seismological Laboratory of the University of Athens showed dextral strike-slip faulting for both mainshock and major aftershocks of the sequence. Taking into account the spatial distribution of the aftershocks, supported by the tectonic and geomorphological settings of the region, a deformation pattern, consisting of the Cephalonia-Lefkas and Ithaca-Lefkas major fault zones which converge in the area of Vassiliki bay is proposed. The appearance of the southernmost clusters was interpreted by the positive Coulomb stress changes transfer due to major earthquake Mw=6.4.

Structure and evolution of Belomorian-Severodvinsk shear zone in the Late Proterozoic and Phanerozoic, East-European Platform

Tectonics, morphological features and development stages of Belomorian-Severodvinsk shear zone (north-western part) found in the East European platform are considered. We traced the shear zone (length ≈1000 km) from NW to SE from the Baltic shield to the Russian plate sedimentary cover. It inherited Paleoproterozoic structures of Belomorian-Lapland mobile belt and Riphean grabens of the White Sea rift system. Belomorian-Severodvinsk zone was represented in the modern structure by a system of neotectonic grabens limited by normal and normal–strike sleep faults and segmented by the transform fault zones. We came to conclusion that the shear zone experienced multiple repeated activation in different dynamic conditions in the Riphean–Phanerozoic. Cyclic alternation of riftogenic trans-tension and compression or transpression conditions in the sequence stages of its development was noted. We defined three cycles of transtensive-transpressive transformations of Belomorian-Severodvinsk shear zone in the Riphean and the Early Vendian. At least four times shear zone suffered changes of deformation mode and directions of shear displacement in the Phanerozoic. The postglacial neotectonic deformations in the Belomorian-Severodvinsk shear zone revealed under the Kola block horizontal movement to the S–E and subsequent counterclockwise rotation.

A case study of compression to escape tectonic transition: Tectonic evolution of the Nallıhan Wedge and comparison with the Tercan Wedge (Eastern Mediterranean, Turkey)

The Nallıhan and Çayırhan regions in western Anatolia host the stratigraphically and tectonically important features of Nallıhan Wedge. The stratigraphic relationships and structural architecture of the wedge are not well understood, which are essential for better understanding of the timing and tectonic evolution of western Anatolia. Today, the Nallıhan Wedge is bounded by the North Anatolian Fault and the Thrace-Eskişehir Fault in the northwest and southwest, respectively. The existence of Dağküplü Mélange within the wedge represents the closure of the İzmir-Ankara Ocean. The occurrence of the Lower Jurassic shelf, slope, reef and deep sea successions transitioning upward into the Cretaceous shelf and slope successions represent the opening and closing cycles of the Intra-Pontide Ocean. The tight and asymmetric folds (steep in the west, gentle in the east) within this stratigraphic sequence and the development of the Eocene units reflect the closure of the Intra-Pontide and İzmir-Ankara oceans. The geometric structure and evolution of the Nallıhan Wedge is directly related to the final continental collision. W-E and SW-NE striking large-scale oblique thrusts also exhibit notable strike-slip components within the study area. The principal direction of the fold axes are W-E and WNW-ESE oriented for the Eocene and Neogene units. During the Eocene, the depositional environment of the study area was a ramp basin mainly dominated by a braided river, when a portion of the Intra-Pontide Ocean was situated on the northern side of the study area. The compressional tectonic regime continued to the end of the Oligocene, which is followed by a new tectonic regime during the Miocene, when the triangular shape region between the Thrace-Eskişehir Fault and the North Anatolian Fault developed as an intermontane basin. The evolution of this basin can be compared with the Tercan Wedge, bordered by the North Eastern Anatolian Fault and the North Anatolian Fault.In the Nallıhan and Çayırhan regions the N-S-trending compressional regime started during the Upper Cretaceous and continued until the final continental collisional during the Upper Eocene. The compressional structures in the area became reactivated during the post-collisional escape tectonic phase in western Anatolia, leading to the development of the prominent Isparta Angle during the Miocene. During this period, the Thrace-Eskişehir Fault and Burdur-Fethiye Shear Zone also formed as intracontinental transform fault zones in western Anatolia. The Nallıhan Wedge started to form in a wedge-shaped area between the Thrace-Eskişehir Fault and the zone of strike-slip faults with thrust component that later developed into the North Anatolian Fault. Thus, Eocene and pre-Eocene structures obtain strike-slip component by an inversion during the Miocene. During and post Pliocene, the Thrace-Eskişehir Fault lost its tectonic activity as the North Anatolian Fault propagated westward into the region previously controlled by the Thrace-Eskişehir Fault. During this interval the Nallıhan Wedge entered a new stage of deformation, which is also observed today.

Rift‐Parallel Strike‐Slip Faulting Near the Iceland Plate Boundary Zone: Implications for Propagating Rifts

AbstractStrongly lineated terrain outside of Iceland's active plate boundary zones is created by faults and dikes aligned with the rift zones where they formed, similar to the spreading fabric defined by abyssal hills generated at mid‐ocean ridge spreading centers. As expected, rift‐parallel normal faults and fissures dominate in the active rift zones, but in older crust to the east and west, faults with strike‐slip and oblique‐slip displacements dominate. Some areas have widespread, small‐scale, strike‐slip, and oblique‐slip faults, while others have more widely spaced, larger, strike‐slip fault zones. In most cases, the strike‐slip and oblique‐slip faults strike subparallel to nearby older dikes and normal faults assumed to indicate the orientation of the rift zones where they formed. Strike‐slip displacements overprinting normal faults and along dike margins suggest reactivation of spreading‐related zones of weakness. More complicated fault geometries and kinematics occur near the oblique rifts and the major transform fault zones. The sense of movement on the strike‐slip and oblique‐slip faults is broadly systematic with respect to the active Northern and Eastern Rift Zones supporting the interpretation that they are the result of crustal block rotations on either side of rift zones that propagate to the north and south away from the center of the Iceland hot spot. Similar fault kinematics may occur along mid‐ocean ridges and other magmatic rifts where rift propagation occurs on a range of scales.

Marine Geophysical Investigation of the Chain Fracture Zone in the Equatorial Atlantic From the PI-LAB Experiment.

The Chain Fracture Zone is a 300‐km‐long transform fault that offsets the Mid‐Atlantic Ridge. We analyzed new multibeam bathymetry, backscatter, gravity, and magnetic data with 100% multibeam bathymetric data over the active transform valley and adjacent spreading segments as part of the Passive Imaging of the Lithosphere Asthenosphere Boundary (PI‐LAB) Experiment. Analyses of these data sets allow us to determine the history and mode of crustal formation and the tectonic evolution of the transform system and adjacent ridges over the past 20 Myr. We model the total field magnetic anomaly to determine the age of the crust along the northern ridge segment to better establish the timing of the variations in the seafloor fabric and the tectonic‐magmatic history of the region. Within the active transform fault zone, we observe four distinct positive flower structures with several en échelon fault scarps visible in the backscatter data. We find up to −10 mGal residual Mantle Bouguer Anomaly in the region of the largest positive flower structure within the transform zone suggesting crustal thickening relative to the crustal thinning typically observed in fracture zones in the Atlantic. The extensional/compressional features observed in the Chain Transform are less pronounced than those observed further north in the Vema, St. Paul, and Romanche and may be due to local ridge segment adjustments.

Active fault system across the oceanic lithosphere of the Mozambique Channel: Implications for the Nubia–Somalia southern plate boundary

Seismic reflection and multibeam echosounder data were acquired in the Mozambique Channel in 2014 and 2015 during the PTOLEMEE, PAMELA-MOZ02 and -MOZ04 marine surveys aboard the RV Atalante and Pourquoi Pas? These data revealed that an active fault system is deforming the oceanic lithosphere of the Mozambique Basin which has developed during Jurassic-Cretaceous times. The correlation between the fault system and the arrangement of earthquake epicenters suggests that this tectonically active zone directly connects northward with the southern part of the eastern branch of the East African Rift System which corresponds to the seismically active graben system bounding the northern part of the Davie ridge. The fault zone extends southwestward of the Mozambique Ridge along the same trend as the Agulhas-Falkland transform fault zone. The general organization of the fault zone shows the characteristics of an extensional system north of the Mozambique Channel (north of the Europa Island) and a right-lateral transtensional system with coeval normal faults and strike-slip faults south-west of Europa. This tectonic activity is associated with volcanic activity since at least Late Miocene times. Our findings emphasize that the eastern branch of East African Rift System is extending largely toward the south, not only in continental domains but also through the oceanic lithosphere of the Mozambique basin. This fault zone is participating to the complex plate boundary between the main African continent (Nubia Plate) and Madagascar (Somalia Plate).

The ARGONET (Greece) Seismic Observatory: An Accelerometric Vertical Array and Its Data

The ARGOstoli NETwork (ARGONET) consists of a vertical seismic array and a close by (440 m) free‐field station on bedrock, located in Cephalonia, western Greece, in the vicinity of the Cephalonia transform fault zone (CTFZ) that is characterized by a high seismic activity, one of the highest in Europe. It is intended to investigate the effects of local surface geology on the incoming seismic wavefield, with emphasis on strong ground motion (nonlinearity). The vertical array is composed of accelerometer sensors placed on the ground surface at 5.6‐, 15.5‐, 40.1‐, and 83.4‐m depth. The P‐ and S‐wave velocity ranges from surface to bottom bedrock between 600 and 2700m/s and between 130 and 700m/s⁠, respectively. The ARGONET started its operation in July 2015. As of 31 December 2017, this array, which operates in continuous mode with Global Positioning System (GPS) absolute time synchronization, has recorded high‐quality recordings for 478 local and regional earthquakes (from ML 1.8 to Mw 6.4 and hypocentral distance from 7 to 180 km, with peak ground acceleration (PGA) from 0.83 to 169cm/s2⁠). Event dataset along with its metadata are open. The ARGONET data are expected to form the basis for further research on the effects of complex surface geology on ground motion and validation of 1D/2D/3D simulation methods, as well as for studying nonlinear seismic wave propagation phenomena.

Distributed earthquake focal mechanisms in the Aegean Sea

Nearly 2,000 earthquake focal mechanisms in the Aegean Sea and the surroundings for the period 1912- 2006, for 1.5 <M<7.5, and depths from 0 to 170 km, indicate a uniform distribution and smooth variation in orientation over wide regions, even for the very small magnitude earthquakes. ~ 60% of the focal mechanisms show normal faulting, that mainly strikes ~E-W. However, a zone ofN-S normal faulting runs the backbone of Albanides-Hellenides. Low-angle thrust and reverse faulting is confined in western Greece (Adria-Eurasia convergence) and along the Hellenic trench (Africa-Eurasia). In the central Aegean Sea the effect of the propagating tip of the North Anatolian Fault into the Aegean Sea is pronounced and strike-slip motions are widely distributed. Shearing does not cross central Greece. Strike-slip motions reappear in the Cephalonia-Lefkada Transform Fault zone and in western Péloponnèse, which shows very complex tectonics, with different types of faulting being oriented favourably and operating under the present stress-field. Moreover, in western Péloponnèse the sense of the observed shearing is not yet clear, whether it is dextral or sinistral, and this lack of data has significant implications for the orientation of the earthquake slip vectors compared to the GPS obtained velocity vectors.

3D numerical modelling of the Wilson cycle: structural inheritance of alternating subduction polarity

Abstract Alternating subduction polarity along suture zones has been documented in several orogenic systems. Yet the mechanisms leading to this geometric inversion and the subsequent interplay between the contra-dipping slabs have been little studied. To explore such mechanisms, 3D numerical modelling of the Wilson cycle was conducted from continental rifting, breakup and oceanic spreading to convergence and self-consistent subduction initiation. In the resulting models, near-ridge subduction initiating with the formation of contra-dipping slab segments is an intrinsically 3D process controlled by earlier convergence-induced ridge swelling. The width of the slab segments is delimited by transform faults inherited from the rifting and ocean floor spreading stages. The models show that the number of contra-dipping slab segments depends mainly on the size of the oceanic basin, the asymmetry of the ridge and variations in kinematic inversion from divergence to convergence. Convergence velocity has been identified as a second-order parameter. The geometry of the linking zone between contra-dipping slab segments varies between two end-members governed by the lateral coupling between the adjacent slab segments: (1) coupled slabs generate wide, arcuate linking zones holding two-sided subduction; and (2) decoupled slabs generate narrow transform fault zones against which one-sided, contra-dipping slabs abut.

The 17 November 2015 Mw 6.4 Lefkas (Ionian Sea, Western Greece) Earthquake: Impact on Environment and Buildings

On Tuesday, November 17, 2015 at 07:10:07 (UTC) a strong earthquake struck Lefkas Island (Ionian Sea, Western Greece) with magnitude Mw 6.4, depth of about 6 km and epicenter located 20 km southwest of Lefkas town. It was felt in Lefkas Island and the surrounding region and caused the death of two people, the injury of eight others, many earthquake environmental effects (EEE) and damage to buildings and infrastructures. Secondary EEE were observed in western Lefkas and classified as ground cracks, slope movements and liquefaction phenomena. Primary effects directly linked to surface expression of seismogenic source were not detected in the field. The maximum intensity VIIIESI 2007 was assigned to large-volume slope movements along western coastal Lefkas. Damage to buildings was mainly observed in villages located in Dragano-Athani graben arranged almost parallel to the northern segment of the Cephalonia Transform Fault Zone (Lefkas segment). Among structures constructed with no seismic provisions, the stone masonry buildings and monumental structures suffered most damage, while the traditional buildings of the area with dual structural system performed relatively well and suffered minor damage. Reinforced-concrete buildings were affected not so much by the earthquake itself but by the generation of secondary EEE. The maximum seismic intensities VIIIEMS-98 were assigned to villages located in Dragano-Athani graben due to very heavy structural damage observed on masonry buildings mainly attributed to the combination of the recorded high PGA values, the poor antiseismic design and construction of buildings and the geological and tectonic structure of the affected area.

The Iceland Plate Boundary Zone: Propagating Rifts, Migrating Transforms, and Rift‐Parallel Strike‐Slip Faults

AbstractUnlike most of the Mid‐Atlantic Ridge, the North America/Eurasia plate boundary in Iceland lies above sea level where magmatic and tectonic processes can be directly investigated in subaerial exposures. Accordingly, geologic processes in Iceland have long been recognized as possible analogs for seafloor spreading in the submerged parts of the mid‐ocean ridge system. Combining existing and new data from across Iceland provides an integrated view of this active, mostly subaerial plate boundary. The broad Iceland plate boundary zone includes segmented rift zones linked by transform fault zones. Rift propagation and transform fault migration away from the Iceland hotspot rearrange the plate boundary configuration resulting in widespread deformation of older crust and reactivation of spreading‐related structures. Rift propagation results in block rotations that are accommodated by widespread, rift‐parallel, strike‐slip faulting. The geometry and kinematics of faulting in Iceland may have implications for spreading processes elsewhere on the mid‐ocean ridge system where rift propagation and transform migration occur.

The Mw6.5 17 November 2015 Lefkada (Greece) Earthquake: Structural Interpretation by Means of the Aftershock Analysis

The 2015 Mw6.5 Lefkada main shock occurred at the south western part of Lefkada Island (Greece), less than 2 years after the occurrence of a doublet along the western part of the nearby Kefalonia Island, Paliki peninsula (on 25/01/2014, with Mw6.1 and 03/02/2014 with Mw6.0) and 12 years after the 2003 Mw6.2 main shock that struck the northwestern part of Lefkada Island. The four failed dextral strike slip fault segments belong to the Kefalonia transform fault zone (KTFZ), the major active boundary that bounds from the west the area of central Ionian Islands, namely Lefkada and Kefalonia. It is associated with several known historical earthquakes and is considered the most hazardous area in the Greek territory. The KTFZ fault segments are characterized by high slip rates (of the order of tens of millimeters per year), with maximum earthquake magnitudes up to 6.7 for Lefkada and 7.2 for Kefalonia fault zone, respectively. The double difference location technique was employed for relocating the aftershocks revealing a seismogenic layer extending from 3 to 16 km depth and multiple activation on well-defined fault planes, with strikes that differ than the main rupture and dips either to east or to west. This implies that strain energy was not solely released on a main fault only, but on secondary and adjacent fault segments as well. The reliable definition of their geometry forms the basis for the structural interpretation of the local fault network. The aftershock spatial distribution indicates three main clusters of the seismic activity, along with activation of smaller faults to an extent of more than 50 km. A northeasterly striking cluster is observed to the north of the main shock epicenter, with a remarkable aftershock density. The central cluster is less dense than the previous one with an epicentral alignment in full accordance with the strike provided by the main shock centroid moment tensor solution, and is considered as the main rupture with a length of 17 km. The third cluster, encompassing a large number of aftershocks, is located in the offshore area between Lefkada and Kefalonia Islands with a NE–SW epicentral alignment, alike the first cluster. The northeast–southwest striking secondary faults positioned obliquely and in continuation of the main fault segment, reveal that the KTFZ is being deformed in a complex tectonic setting. The presence of faults with this geometry implies strain partitioning and sheds light to new components necessary to be taken into account in the seismic hazard assessment. Stress transfer models of the M ≥6.0 main shocks were investigated and the calculated static stress changes may well explain their sequential occurrence. Static stress changes due to the 2015 coseismic slip were also calculated with the main objective of exploring the aftershock occurrence pattern and it was found as the driving mechanism that triggered the vast majority of the off-fault aftershocks.

Crustal accretion of thick mafic crust in Iceland: implications for volcanic rifted margins

Rifting near hotspots results in mantle melting to create thick mafic igneous crust at volcanic rifted margins (VRMs). This mafic crust is transitional between rifted continental crust with mafic intrusions landward and oceanic crust into which it grades seaward. Seismic velocities, crustal drilling, and exhumed margins show that the upper crust in these areas is composed of basaltic lava erupted in subaerial to submarine conditions intruded by downward increasing proportions of dikes and sparse gabbroic intrusions. The lower crust of these regions is not exposed but is inferred from seismic velocities (Vp > 6.5 km/sec) and petrological constraints to be gabbroic to ultramafic in composition. Limited access to crustal sections generated along VRMs have raised questions regarding the composition and structure of this transitional crust and how it evolves during the early stages of rifting and subsequent seafloor spreading. Active processes in Iceland provide a glimpse of subaerial spreading with the creation of a thick (40–25 km) mafic igneous crust that may be analogous to the transitional crust of VRMs. Segmented rift zones that propagate away from the Iceland hotspot, migrating transform fault zones, and rift-parallel strike-slip faults create a complex plate boundary zone in the upper, brittle crust. These structures may be decoupled from underlying lower crustal gabbroic rocks that are capable of along-axis flow that smooths-out crustal thickness variations. Similar processes may be characteristic of the early history of VRMs and volcanic hotspot ridges related to rifting and seafloor spreading proximal to hotspots.

Crustal structure and fracture zone in the Central Basin of the South China Sea from wide angle seismic experiments using OBS

We present two E-W trending wide-angle seismic profiles (OBS2013-ZN, OBS2014-ZN), which cross the boundary (Zhongnan fault zone) between the east sub-basin and the southwest sub-basin of the South China Sea (SCS). We processed the data and used 2D ray-tracing to determine the oceanic crust thickness, velocity structures and Moho depth variations related to the fault zone. The simulated velocity models show that the oceanic basin of the SCS has a typical oceanic crust covered by a 1–2km thick sediment layer with a velocity of 2.0–3.5km/s. The crust has a thickness of 5–8km, of which the oceanic layer 2 is 1.8–3km thick, with velocity increasing downward from 4.3km/s to 6.4km/s, and the oceanic layer 3 is 3–5km thick, with velocity increasing downward from 6.4km/s–7.0km/s. The Moho depth in the oceanic basin is approximately 6–7km below seabed. The Moho discontinuity has a prominent upheaval zone with a low velocity of 7.6km/s, whose location corresponds to the low velocity zone in oceanic layer 2. Our results suggest the presence of a NW-SE-trending fracture zone (40–60km wide) rather than a major “Zhongnan fault” oriented N-S by connecting the upheaval portions of the Moho in the two profiles. The NW-SE orientation Zhongnan transform fault zone in our study area is consistent with the direction of opening of the South China Sea in the last stage of its evolution. This large transform fault zone connected and offset the spreading centers of the east and southwest sub-basin.

Upper plate deformation as marker for the Northern STEP fault of the Ionian slab (Tyrrhenian Sea, central Mediterranean)

The Eastern Tyrrhenian margin (ETM), the active boundary of the Tyrrhenian Sea backarc basin, is the key for understanding the geodynamics of the central Mediterranean. Numerous seismic tomography studies have been carried out in this region, proposing different reconstructions of the lower subducting plate and cause of the slab-break-off existing beneath the Southern Apennines. However, the area and mode of the recent deformation of the Tyrrhenian Sea are still not fully defined and understood. In this study, we combine the analysis of a recent seismic tomography model and geological data, in order to understand the relationship between the subducting lower plate and the tectonic evolution of the sedimentary basins formed on the upper plate.With this aim, we interpreted a large data set of seismic reflection profiles and several well logs. The results consist in 2D and 3D geological models of the basins, sedimentary infill, and fault networks. Taking into account the geological data of the ETM and those of the adjacent inner flank of the Apennines, we observe: (i) a system of linked sedimentary basins developed on a narrow deformation belt bounded by transform fault zones; (ii) a polyphase rifting within the upper plate; (iii) an abrupt change of the direction of extension (~90°), from NE-oriented in the Lower Pleistocene to SE-oriented in the Middle Pleistocene. Since these ETM features are not the typical expressions of the current backarc extensional models, we propose a link between the evolution of upper plate and the onset and development of a STEP (Subduction-Transform-Edge-Propagator) fault along the northern margin of the Ionian slab.

Slip model of the 17 November 2015 Mw = 6.5 Lefkada earthquake from the joint inversion of geodetic and seismic data

AbstractWe inferred the rupture process of the 2015 Lefkada earthquake by combining teleseismic and near‐field strong motion data, together with static and dynamic GPS displacements. The joint inversion of all data sets revealed a relatively complex slip pattern with a heterogeneous distribution. Slip is confined in the upper 10 km. Two principal asperities with peak slip amplitude of 2.35 m, which released 65% of the total seismic moment, were recovered southwest of the epicenter. Remarkably, the 2015 earthquake ruptured the part of the Cephalonia Transform Fault Zone that remained unbroken during the 2003 event, revealing a pattern of rupture segmentation along the fault zone. In this context the Lefkada segment can be divided into a northern and southern part. Preseismic deformation onshore southern Lefkada revealed that since the 2003 rupture, the northern part of the Lefkada segment continuously loaded the southern part, which eventually broke during the 2015 event.

Is the Troodos ophiolite (Cyprus) a complete, transform fault–bounded Neotethyan ridge segment?

We report new paleomagnetic data from the sheeted dike complex of the Troodos ophiolite (Cyprus) that indicate that its northern limit is marked by a hitherto unrecognized oceanic transform fault system. The style, magnitude, and scale of upper crustal fault-block rotations in the northwestern Troodos region mirror those observed adjacent to the well-known Southern Troodos transform fault zone along the southern edge of the ophiolite. A pattern of increasing clockwise rotation toward the north, coupled with consistent original dike strikes and inclined net rotation axes across this region, is compatible with distributed deformation adjacent to a dextrally slipping transform system with a principal displacement zone just to the north of the exposed ophiolite. Combined with existing constraints on the spreading fabric, this implies segmentation of the Troodos ridge system on length scales of ∼40 km, and suggests that a coherent strip of Neotethyan lithosphere, representing a complete ridge segment bounded by transforms, has been uplifted to form the currently exposed Troodos ophiolite. Moreover, the inferred length scale of the ridge segment is consistent with formation at a slow-spreading rate during Tethyan seafloor spreading and with a supra-subduction zone environment, as indicated by geochemical constraints.

Precise relative locations for earthquakes in the northeast Pacific region

AbstractDouble‐difference methods applied to cross‐correlation measured Rayleigh wave time shifts are an effective tool to improve epicentroid locations and relative origin time shifts in remote regions. We apply these methods to seismicity offshore of southwestern Canada and the U.S. Pacific Northwest, occurring along the boundaries of the Pacific and Juan de Fuca (including the Explorer Plate and Gorda Block) Plates. The Blanco, Mendocino, Revere‐Dellwood, Nootka, and Sovanco fracture zones host the majority of this seismicity, largely consisting of strike‐slip earthquakes. The Explorer, Juan de Fuca, and Gorda spreading ridges join these fracture zones and host normal faulting earthquakes. Our results show that at least the moderate‐magnitude activity clusters along fault strike, supporting suggestions of large variations in seismic coupling along oceanic transform faults. Our improved relative locations corroborate earlier interpretations of the internal deformation in the Explorer and Gorda Plates. North of the Explorer Plate, improved locations support models that propose northern extension of the Revere‐Dellwood fault. Relocations also support interpretations that favor multiple parallel active faults along the Blanco Transform Fault Zone. Seismicity of the western half of the Blanco appears more scattered and less collinear than the eastern half, possibly related to fault maturity. We use azimuthal variations in the Rayleigh wave cross‐correlation amplitude to detect and model rupture directivity for a moderate size earthquake along the eastern Blanco Fault. The observations constrain the seismogenic zone geometry and suggest a relatively narrow seismogenic zone width of 2 to 4 km.

Upper mantle structure of the southern Arabian margin: Insights from teleseismic tomography

We image the lithospheric and upper asthenospheric structure beneath the central and eastern parts of the northern Gulf of Aden rifted passive continental margin with 59 broadband stations to evaluate the role of transform fault zones on the evolution of magma-poor continental margins. We used teleseismic tomography to compute a relative P wave velocity model in eastern Yemen and southern Oman down to 400 km depth. Our model shows low-velocity anomalies located in the vicinities of five major fracture zones and regions of recent volcanism. These low-velocity anomalies are likely caused by localized asthenospheric upwelling and partial melting, caused by small-scale convection promoted by gradients in the lithosphere-asthenosphere boundary topography near the fracture zones. In addition, low velocities underlie regions of elevated topography between major sedimentary basins. We suggest that locally buoyant mantle creates uplift and dynamic topography on the rift margin that affects the course of seasonal rivers and the sedimentation at the mouth of those rivers. Our new P wave velocity model suggests that the dynamic topography and recent volcanism in the central and eastern Gulf of Aden could be due to small-scale convection at the edge of the Arabian plate and/or in the vicinity of fracture zones.