Subduction-Transform Edge Propagator Research Articles

Modern Approaches for Historical Seismograms: Moment Tensor Inversion of the 1947 Squillace Basin, South Italy, Earthquake

Abstract The scientific community has become increasingly aware of the importance of preserving and recovering historical seismic data, also because of their possible use in combination with modern techniques of analysis. Seismograms coming from the analog recording era cover more than 100 yr of seismic activity and may have a great relevance, especially for seismic risk evaluations in regions struck by destructive events in the past centuries but characterized by minor activity in the last decades. In this study we used analog seismograms to investigate an earthquake of presumed magnitude 5.7 that occurred in 1947 in central Calabria, south Italy, a high-seismic risk region framed in a complex geodynamic setting led by northwest-trending Nubia–Eurasia convergence and southeastward Ionian slab rollback. According to seismic catalogs, the 1947 is the only M > 5.5 earthquake instrumentally recorded in an area where the presence of the lateral edge of the Ionian slab has been suggested and an intense debate is still open concerning possible existence, and proper location, of a subduction-transform edge propagator (STEP) fault zone. To study this earthquake, we selected 15 medium- to long-period analog seismograms with related instrumental parameters, and we proceeded with vectorization process and proper waveform corrections. A technique specifically developed for time-domain moment tensor computation through waveform inversion of analog seismograms has been applied to the digitized recordings. The moment tensor solution estimated for the 1947 earthquake indicates strike-slip mechanism, focal depth of 28 km and Mw 5.1. The obtained hypocentral depth and left-lateral kinematics on about west-northwest–east-southeast-oriented fault fit well with the local seismotectonic framework and are compatible with STEP fault activity in central Calabria, furnishing a new seismological constraint to the debate concerning slab edge kinematics. Moreover, the presented analysis is useful for sharing with the scientific community new data and methodological issues related to historical seismogram management.

Oblique rifting triggered by slab tearing: the case of the Alboran rifted margin in the eastern Betics

Abstract. The tectonic evolution of highly oblique continental margins that result from extension above lithospheric subduction–transform edge propagator (STEP) faults is poorly understood. Here, we investigate the case of the Alboran margin in the eastern Betics characterized by crustal thinning of 15–10 km, oblique to the direction of slab retreat. The current deformation patterns indicate that oblique rifting is underway. However, it is unclear whether these conditions are those that prevailed during the formation of the metamorphic domes and intramontane basins. We review the temporal and spatial evolution of Neogene sedimentary basins and brittle deformation in the eastern Betics and exploit offshore seismic reflection lines to propose a crustal-scale section across the oblique margin. The history of sediment infill and rates of subsidence combined with the analyses of fault slip data confirm that brittle extension oriented from north 20∘ E to E–W occurred during an interval spanning from the Serravallian–early Tortonian to the late Tortonian (14–8 Ma). This extension is associated with both normal and strike-slip regimes and the evolution of the strike-slip fault zones flanking the metamorphic domes. The transtensional model forms a coherent scheme linking the ductile deformation associated with metamorphic domes and the formation of E–W- and NW–SE- or NNW–SSE-directed sedimentary basins in the brittle upper crust during the Tortonian. The oblique extension, which is closely associated with STEP faulting, occurred during the regional convergence between Africa and Iberia since the Miocene. Only recently, around 8 Ma, has slab detachment started to migrate westward, leading to tectonic inversion in the eastern Betics. Such a type of narrow oblique-rifted margin associated with transform-like plate boundaries is not unique but is expected to be hardly preserved in the geological record due to the transient nature of retreating subduction systems.

Seismic anisotropy to investigate lithospheric-scale tectonic structures and mantle dynamics in southern Italy

Subduction zones may be characterised by deep-seated tectonic structures whose effects propagate to the upper plate through faulting and magmatism. The overall geodynamic framework, as well as the roots of the many active faults affecting such regions, can be investigated by the study of the upper mantle anisotropic patterns, through the analysis of core-transiting teleseismic phases. Here, we discuss the results of XKS waves splitting observed in the central Mediterranean, particularly in southern Italy, which is characterised by the Adriatic-Ionian subduction system. Azimuths of polarisation of the fast wave (fast directions) were found to be generally trench-parallel, as an effect of the subducting slab, albeit a change to a perpendicular direction, in central Italy and Sicily, suggests discontinuities in the structure of the slab itself. However, while in central Italy a gradual rotation of fast directions points to a toroidal upper mantle flow through a tear in the Apenninic slab, in central-eastern Sicily, the splitting parameters show an abrupt change that matches well with the main crustal tectonic structures. There, the rapid trench migration, taking place at the transition between the subduction and continental collision domains, produced a rather complex Subduction Transform Edge Propagator fault system. The sharp variation in the pattern of the upper mantle anisotropy marks the main element of such a fault system and suggests its primary role in the segmentation process of the collisional margin. Our findings further show that the study of seismic anisotropy may be fundamental in investigating whether tectonic structures only involve the crust or extend down to the upper mantle.

Late Mesozoic Tectono-stratigraphic evolution of the Hekimhan Basin and the environs (central eastern Anatolia): implications for the eastern Taurides and Gürün Curl

ABSTRACT The east-west trending Taurides form a curved area in central eastern Anatolia known as the Gürün Curl. In order to understand the origin of the Gürün Curl and Tauride evolution in general, the results of a new field study of this region have been synthesized together with previously published data. We suggest that the geodynamic evolution of the area began with the likely presence of a Tethys Ocean transform fault. This fault separated the Taurides into the Akdere Sector in the west and the Munzur Sector in the east in the Late Cretaceous. During the late Santonian–early Campanian, ophiolites obducted onto the Munzur Sector, while platform sediments continued to accumulate in the Akdere Sector. This was followed by the development of an Andean-type arc-type magmatism (the Baskil Arc) during the early–middle Campanian in the Munzur Sector. Continued closure of the Tethys led to the collision of the Bitlis Massif in the south of the Munzur Sector in the Campanian. This, in turn, resulted in continental subduction and slab roll-back that was controlled by a Subduction Transform Edge Propagator (STEP) Fault that lay on the original transform fault between the Akdere and Munzur sectors. Because the subducted slab was free at its western corner, the western edge rolled back faster than in the east, leading to an asymmetrical extensional regime on the upper plate that created the late Campanian Hekimhan Basin. While these geodynamic events were taking place in the Munzur Sector, the Akdere Sector was in a platform setting. During the Palaeocene, the Late Mesozoic units of the Akdere Sector began to overthrust on the Hekimhan Basin and the ophiolites. Following the Palaeocene, all these tectonostratigraphic units were covered by Eocene sediments around the Gürün Curl of which the modern appearance was completed by the Miocene to Recent movements along the strike-slip faults.

Miocene to Quaternary geodynamic evolution of the southern section of the Burdur-Fethiye Shear Zone and its offshore continuation, eastern Mediterranean

The main interrelated structures of the eastern Mediterranean are shaped by the complex tectonic regimes including active subduction, uplift, and progressive counterclockwise rotation. The Hellenic Arc, the Anaximander Mountains, the Western Taurides, the extensional western Anatolian graben, the Burdur-Fethiye Shear Zone and the Rhodes Basin are these structures located in one of the most tectonically active regions. The Burdur-Fethiye Shear Zone is a 75- to 90- km wide and 300-km-long transtensional left-lateral shear zone developed during the formation of the Aegean back-arc extensional system and the thrusting of Western Taurides. In this study, fault kinematic analysis, and DEM and earthquake data characterize the tectonic controls of the southern section of the Burdur-Fethiye Shear Zone. The high-quality multibeam bathymetry data, their processing and interpretation with the onshore structures and an integrated interpretation with multichannel seismic profiles allowed the geodynamic evolution of the Burdur-Fethiye Shear Zone, Rhodes Basin and environs to be evaluated. Despite the heated debates regarding the presence of the zone, the latest offshore data reveals again that the continuation of the subduction transform edge propagator (STEP) fault between the Hellenic and Cyprus arcs comes into play as the Burdur-Fethiye Shear Zone on land.

Subduction related faults and sedimentary basins: The Western Ionian Sea case

The Pliocene-Quaternary evolution of the western margin of the Calabrian Arc has been investigated by analysing the configuration and distribution of local sedimentary basins and their relationship with shallow and deep tectonic structures. Research focussed on the evolution of the more recent offshore sedimentary basins located to the east of Sicily, along the Alfeo Fault System, using multichannel seismic lines, high-resolution bathymetry and newly acquired single-channel seismic profiles. The chosen study area encompasses a crustal sector where external thrusts of the Calabrian Arc branch into the Malta Escarpment, and this zone has been interpreted as a Subduction Transform Edge Propagator (STEP) Fault. The Western Ionian Sea is bounded by regional faults that controlled the deposition of the Pliocene-Quaternary sequences and define the present-day bathymetry. In this work we identified four main seismic sequences and three main unconformities in the basins, to better understand the geometry and the depositional setting of the basins. In addition, lateral correlation and 3D reconstruction of the sedimentary basins allows us to establish the direct relationship between the Alfeo Fault System activity and the geometry, the thickness, the evolution and filling phases of the sedimentary basins in this area. Results presented show how the Alfeo Fault drove the development of separate basin depocenters in the early stages of tectonic activity, resulting in a single elongated basin in the latest phases. The inferred fault kinematics, the analysis of displacements along strike and sediment thickening suggest a link with the slab geometry, as occurs in other subduction systems that develop subduction related faults. The results and the method applied in this study can also be applied for the interpretation of similar systems with similar geological settings in other areas of the world.

Active tectonics and kinematics of Fethiye-Göcek Bay, SW Turkey: Insight about the eastern edge of Pliny-Strabo Trenches

Pliny-Strabo Trench (PST)is a Subduction Transform Edge Propagator (STEP) fault developed at the northern edge of the African oceanic lithosphere connecting the Aegean and Cyprian trenches in the eastern Mediterranean. Recent studies have demonstrated that the PST terminates close to Turkish Border and links to the Cyprian trench east of Rhodes and south of Fethiye-Göcek Bay. However, it is also claimed that the PST extends into SW Anatolia along a sinistral transtensional shear zone, so-called Fethiye-Burdur Fault Zone (FBFZ) implying that a lithospheric tear in the downgoing plate extends into the over-riding plate, although, this is kinematically almost impossible since full mechanical coupling between down going and the over-riding plate is necessary. To test this hypothesis and understand the kinematics of the transition zone between PST and Cyprian trench we have conducted a rigorous paleostress inversion study combined with interpretation of 2D seismic data of which 228 km total length obtained from Fethiye-Göcek Bay.The seismic reflection data are used for the interpretation and delineation of off-shore faults and to determine their recent activity. The geometry and kinematics of the exposed on-land faults are determined by analyzing 13969 fault slip data obtained from 211 sites distributed evenly throughout in an area encompassing the bay in an area extending 70 km in E-W and 50 km in N–S that cover the whole area where the Pliny-Strabo STEP fault would emerge on-land.Results of analyses indicated that most of the faults in the study area are developed under multi-directional extension, except for some NE-SW-striking faults, which have dextral strike-slip components contrary to proposed sinistral nature of Fethiye-Burdur Fault Zone. Although there are numerous normal faults some which have been active until recently, however, there is almost no NE-SW striking sinistral strike-slip fault in the region to justify the presence of Fethiye-Burdur Shear Zone. Additionally, there are a number of earthquakes with dominantly strike-slip moment tensor solutions in the deeper part of the bay while they are in normal character in the on-shore areas suggesting that the Pliny-Strabo Trench stops very close to the shoreline and it does not propagate into SW Anatolia. Therefore, existence and alleged characteristics of the Fethiye-Burdur Fault Zone could not be verified.

Strike-Slip Earthquakes at the Northern Edge of the Calabrian Arc Subduction Zone

AbstractWe analyzed earthquakes of a swarm started in October 2019 in the Tyrrhenian Sea, at the northern border of the Calabrian arc subduction zone. The swarm is located in the same area where a subduction-transform edge propagator (STEP) shear- zone -oriented east–west is recognized from ocean floor morphology and submarine volcanoes. We computed focal mechanism, relative location, stress drop, corner frequency, and source directivity of the mainshock Mw 4.4 and of some aftershocks in the local magnitude range 2.3–3.7. Results indicate clearly that the mainshock occurred on a northwest–southeast-oriented fault, with right-lateral strike-slip motion, and it was characterized by a strong directivity of the rupture propagation from northwest to southeast. On the contrary, most of aftershocks were located on another strike-slip fault oriented northeast–southwest and had left-lateral kinematics. The kinematic features of these earthquakes indicate a strain field with the P-axis oriented north–south and the T-axis oriented east–west. Fault directions and stress field are in good agreement with the theoretical fracture model of shear zones associated with a STEP.

Geodynamic Modeling of Edge-Delamination Driven by Subduction-Transform Edge Propagator Faults: The Westernmost Mediterranean Margin (Central Betic Orogen) Case Study

Lithospheric tearing at slab edges is common in scenarios where retreating slabs face continental margins. Such tearing is often accommodated via subvertical STEP (SubductionTransform Edge Propagator) faults that cut across the entire lithosphere and can result in sharp lateral thermal and rheological variations across the juxtaposed lithospheres. This setting favors the occurrence of continental delamination, i.e., the detachment between the crust and the lithospheric mantle. In order to evaluate this hypothesis, we have chosen a wellstudied natural example recently imaged with unprecedented seismic resolution: the STEP fault under the central Betic orogen, at the northern edge of the Gibraltar Arc subduction system (westernmost Mediterranean Sea). The Gibraltar Arc subduction is the result of the fast westward roll-back of the Alboran slab and it is in its last evolutionary stage, where the oceanic lithosphere has been fully consumed and the continental lithosphere attached to it collides with the overriding plate. In this study we investigate by means of thermo-mechanical modeling the conditions for, and consequences of, delamination post-dating slab tearing in the central Betics. We consider a setup based on a STEP fault separating the orogenic Betic lithosphere and the adjacent thinned lithosphere of the overriding Alboran domain. Our model analysis indicates that delamination is very sensitive to the initial thermal and rheological conditions, transitioning from a stable to a very unstable and rapidly evolving regime. We find two clearly differentiated regimes according to the time at which the process becomes unstable: fast and slow delamination. Although the final state reached in both the fast and slow regimes is similar, the dynamic surface topography evolution is dramatically different. We suggest that given a weak enough Iberian lower crust the delaminating lithospheric mantle peels off the crust and adopts a geometry consistent with the imaged southward dipping Iberian lithosphere in the central Betics. The lack of spatial correspondence between the highest topography and the thickest crust, as well as the observed pattern of uplift/ subsidence are properly reproduced by a model where relatively fast delamination (Reference Model) develops after slab tearing.

The Kefalonia Transform Fault: A STEP fault in the making

Vertical edges along subducted slabs have been recognized in the majority of subduction zones. Surprisingly, slab edges evolved into Subduction-Transform-Edge-Propagator (STEP) faults in only a few regions; the conditions under which STEPs form are special. It is relevant to constrain the conditions that facilitate STEP fault initiation because they leave a clear geological footprint in the overriding plate, whereas vertical tears generally do not. We therefore study a candidate region for STEP fault initiation in the western Hellenic Subduction Zone.We investigate the structure and seismicity of the shallow western Hellenic Subduction Zone using a recent full-waveform inversion model which both captures details of crustal and upper-mantle structure, yielding constraints in the depth interval from 10 to 200 km where lithosphere-mantle interactions have tectonic expressions. The western end of the Hellenic Subduction Zone is fragmented near the Kefalonia Transform Fault. We identify a separate Epirus lithospheric fragment that is roughly vertical below the southern Albanides. We also identify a new and major contrast within the lithospheric mantle of the Ionian ocean basin, which aligns with a gradient in free-air anomalies.In the overriding plate, the Kefalonia Transform Fault zone accommodates right lateral strike-slip deformation. We interpret this fault zone as a proto-STEP fault that formed simultaneously with Pliocene fragmentation of the Epirus fragment.Comparing the recent evolution of the NW Hellenic slab edge with currently active STEPs indicates that along-trench variations in convergence velocity are a prerequisite for STEP fault initiation. Such velocity variations may result from subduction of continental crust along part of the trench. Resistance to sea ward tear propagation by a mechanically strong subducting plate may prevent variations in convergence velocity to occur, and thus STEP fault initiation. The amount of time over which a velocity contrast persists will also be relevant for STEP fault initiation. Mechanical coupling between upper- and lower-plate, and the deformability of the upper-plate appear to also play a role in the initiation of the STEP fault once a slab is fragmented.

Lithosphere tearing along STEP faults and synkinematic formation of lherzolite and wehrlite in the shallow subcontinental mantle

Abstract. Subduction-transform edge propagator (STEP) faults are the locus of continual lithospheric tearing at slab edges, resulting in sharp changes in the lithospheric and crustal thickness and triggering lateral and/or near-vertical mantle flow. However, the mechanisms at the lithospheric mantle scale are still poorly understood. Here, we present the microstructural study of olivine-rich lherzolite, harzburgite and wehrlite mantle xenoliths from the Oran volcanic field (Tell Atlas, northwest Algeria). This alkali volcanic field occurs along a major STEP fault responsible for the Miocene westward slab retreat in the westernmost Mediterranean. Mantle xenoliths provide a unique opportunity to investigate the microstructures in the mantle section of a STEP fault system. The microstructures of mantle xenoliths show a variable grain size ranging from coarse granular to fine-grained equigranular textures uncorrelated with lithology. The major element composition of the mantle peridotites provides temperature estimates in a wide range (790–1165 ∘C) but in general, the coarse-grained and fine-grained peridotites suggest deeper and shallower provenance depth, respectively. Olivine grain size in the fine-grained peridotites depends on the size and volume fraction of the pyroxene grains, which is consistent with pinning of olivine grain growth by pyroxenes as second-phase particles. In the coarse-grained peridotites, well-developed olivine crystal-preferred orientation (CPO) is characterized by orthorhombic and [100]-fiber symmetries, and orthopyroxene has a coherent CPO with that of olivine, suggesting their coeval deformation by dislocation creep at high temperature. In the fine-grained microstructures, along with the weakening of the fabric strength, olivine CPO symmetry exhibits a shift towards [010] fiber and the [010] and [001] axes of orthopyroxene are generally distributed subparallel to those of olivine. These data are consistent with deformation of olivine in the presence of low amounts of melts and the precipitation of orthopyroxenes from a melt phase. The bulk CPO of clinopyroxene mimics that of orthopyroxene via a topotaxial relationship of the two pyroxenes. This observation points to a melt-related origin of most clinopyroxenes in the Oran mantle xenoliths. The textural and geochemical record of the peridotites are consistent with interaction of a refractory harzburgite protolith with a high-Mg no. melt at depth (resulting in the formation of coarse-grained clinopyroxene-rich lherzolite and wehrlite) and with a low-Mg no. evolved melt in the shallow subcontinental lithospheric mantle (forming fine-grained harzburgite). We propose that pervasive melt–peridotite reaction – promoted by lateral and/or near-vertical mantle flow associated with lithospheric tearing – resulted in the synkinematic crystallization of secondary lherzolite and wehrlite and had a key effect on grain size reduction during the operation of the Tell–Rif STEP fault. Melt–rock reaction and secondary formation of lherzolite and wehrlite may be widespread in other STEP fault systems worldwide.

Joint local earthquake and teleseismic inversion for 3-D velocity and Q in New Zealand

We have developed a joint inversion of local earthquake and teleseismic data and applied it in New Zealand to obtain improved 3-D seismic velocity, Qp and Qs models. To accommodate sharper detail where there are dense local earthquake data, varied spatial resolution is enabled with linking of nodes into larger inversion elements where data are sparse. The teleseismic t∗ measurements are obtained by stacking and removing the average for the common source and path. The combined inversions used data from 126 teleseisms and 2365 local earthquakes that extend to 350 km depth. Vp results are similar to previous studies, but have enhanced detail near the slab provided by local seismicity. Sub-slab low velocity and low Q is imaged along the eastern North Island, which may represent asthenosphere with entrained water and melt. Along the western South Island, there is a high Vp, high Q feature distinct from the current slab, and interpreted to be a slab remnant from the period of Subduction-Transform Edge Propagator fault development and trench rollback. The axial South Island high Vp, high Q feature of the Hikurangi Plateau is imaged, and is bounded on the southeast by thinner lithosphere that may be related to the period of Great South Basin rifting.

Paleomagnetic evidence for upper plate response to a STEP fault, SW Anatolia

Pliny-Strabo Trench is a Subduction Transform-Edge Propagator (STEP) Fault developed on the northern edge of the subducted African Oceanic Lithosphere. It connects the Aegean and Cyprian trenches in the Eastern Mediterranean convergent system. Although, deep geometry of the STEP fault and associated slab tear in mantle are imaged, its shallow vertical and lateral continuation in the crust and impact on the over-riding plate are still unknown. Thus, we have studied SW Anatolia, the candidate site where this structure could propagate laterally and vertically, for its vertical axis rotations using paleomagnetic tools.In this study, more than 2000 paleomagnetic samples were collected and analysed from 86 different sites which were later classified into 11 separate geographic domains displaying similar tectonic characteristics. Moreover, available paleomagnetic data in the literature were parametrically resampled, analysed and combined with our data. In the region, there is a positive correlation between rotation amounts and sample ages supporting continuous deformation throughout the Neogene period. The spatial variations of results indicated that the study area can be divided into three main domains based on vertical axis rotations. From the south to the north these domains include SW Anatolian domain with ∼20° counter-clockwise rotation, Burdur–Dinar–Ulubey domain with ∼4° counter-clockwise rotation and northern areas characterized by clockwise rotations. The identified domains of counter-clockwise rotation are separated by a well-defined NW-SE striking Acıpayam Transfer Zone and there is no differential rotation in agreement with the presence of NE-SW striking shear zone in the region. Therefore, we concluded that the Pliny-Strabo STEP Fault have not propagated into the over-riding plate as a shear zone. This implies that there is no evidence to support the presence of alleged Fethiye–Burdur Fault Zone as suggested, and its existence is dubious.

Impacts of Hikurangi Plateau subduction on the origin and evolution of the Alpine Fault

ABSTRACTThe Alpine Fault has traditionally been interpreted as a plate boundary through continental lithosphere, with attendant widespread deformation. But recent three-dimensional imaging of deep crustal structure has shown that the habitat of the fault is dominated by strong, large igneous province (Hikurangi Plateau) lithosphere, producing a much narrower deformation zone. The fault formed at c. 23 Ma as a subduction–transform edge propagator (STEP fault). STEP fault propagation stopped at c.15 Ma, and motion on the fault renewed at c. 7 Ma, when the fault linked with the Marlborough fault system. If the 480 km offset of geological terranes across the fault occurred only during these two episodes, this implies an average strike-slip rate close to that in the Late Quaternary. The current pattern of convergence along the fault is dominated by the 16° mismatch between the strikes of the STEP fault and the converging plateau.

A STEP fault in Central Betics, associated with lateral lithospheric tearing at the northern edge of the Gibraltar arc subduction system

We study the crustal and lithospheric mantle structure under central Betics in the westernmost Mediterranean region by migrating P-receiver functions along a dense seismic profile (∼2 km interstation distance). The profile, North–South oriented, probes the crustal structure of different geological units, from the Alboran domain in the south with metamorphic rocks, through the External Zones with sedimentary rocks to the Variscan terrains of the Iberian Massif in the north. From north to south, the Moho depth increases from ∼30 km to ∼46 km underneath the Guadix basin, due to the underthrusting of the Iberian crust below the Alboran crust, and suddenly shallows to ∼30 km underneath the Internal Zones with a step of 17 km. This sharp Moho step correlates well with a lithospheric step of ∼40 km, where the thickness of the lithosphere changes abruptly from ∼100 km in the north to ∼50 km in the south. We interpret this sharp and prominent lithospheric step as the termination of the Iberian lithosphere caused by a near-vertical STEP (Subduction-Transform-Edge-Propagator) fault that continues towards the surface as a positive flower tectonic structure of crustal scale. This STEP fault is located at the northern edge of the narrow Westernmost Mediterranean subduction system facilitating the slab rollback motion towards the west. The sharp termination of the Iberian lithosphere occurs under the contact between the Alpujarride and the Nevado-Filabride complexes of the Alboran domain in an ENE-WSW right-lateral transpressive shear zone. The thickest crust and lithosphere do not correlate with the highest topography along the profile suggesting that this high topography is a combined effect of the positive flower structure, and the push up of the asthenosphere produced by the removal of the Iberian lithosphere.

Evidence of displacement-driven maturation along the San Cristobal Trough transform plate boundary

The San Cristobal Trough (SCT), formed by the tearing of the Australia plate as it subducts under the Pacific plate near the Solomon Islands, provides an opportunity to study the transform boundary development process. Recent seismicity (2013–2016) along the 280 km long SCT, known as a Subduction-Transform Edge Propagator (STEP) fault, highlights the tearing process and ongoing development of the plate boundary.The region's earthquakes reveal two key characteristics. First, earthquakes at the western terminus of the SCT, which we interpret to indicate the Australia plate tearing, display disparate fault geometries. These events demonstrate that plate tearing is accommodated via multiple intersecting planes rather than a single through-going fault. Second, the SCT hosts sequences of Mw ∼7 strike-slip earthquakes that migrate westward through a rapid succession of events. Sequences in 1993 and 2015 both began along the eastern SCT and propagated west, but neither progression ruptured into or nucleated a large earthquake within the region near the tear.Utilizing b-value and Coulomb Failure Stress analyses, we examine these along-strike variations in the SCT's seismicity. b-Values are highest along the youngest, western end of the SCT and decrease with increasing distance from the tear. This trend may reflect increasing strain localization with increasing displacement. Coulomb Failure Stress analyses indicate that the stress conditions were conducive to continued western propagation of the 1993 and 2015 sequences suggesting that the unruptured western SCT may have fault geometries or properties that inhibit continued rupture.Our results indicate a displacement-driven fault maturation process. The multi-plane Australia plate tearing likely creates a western SCT with diffuse strain accommodated along a network of disorganized faults. After ∼90 km of cumulative displacement (∼900,000 yr of plate motion), strain localizes and faults align, allowing the SCT to host large earthquakes.

Extensional and compressional regime driven left-lateral shear in southwestern Anatolia (eastern Mediterranean): The Burdur-Fethiye Shear Zone

The tectonic framework of the eastern Mediterranean presented in this paper is based on an active subduction and small underwater hills/mountains on the oceanic crust moving toward the north. The Hellenic Arc, the Anaximander Mountains, the Rhodes and Finike basins, the compressional southern regions of the Western Taurides, and the extensional western Anatolian graben are the main interrelated tectonic structures that are shaped by the complex tectonic regimes. There are still heated debates regarding the structural properties and tectonic evolution of the southwestern Anatolia. GPS velocities and focal mechanisms of earthquakes demonstrate the absence of a single transform fault across the Burdur-Fethiye region; however, hundreds of small faults showing normal and left-lateral oblique slip indicate the presence of a regionally extensive shear zone in southwestern Turkey, which plays an important role in the eastern Mediterranean tectonics. The 300-km-long, 75–90-km-wide NE-SW-trending Burdur-Fethiye Shear Zone developed during the formation of Aegean back-arc extensional system and the thrusting of Western Taurides. Today, the left-lateral differential motion across the Burdur-Fethiye Shear Zone varies from 3 to 4mm/yr in the north to 8–10mm/yr in the south. This finding could be attributed to the fact that while the subduction of the African Plate is relatively fast beneath the western Anatolia at the Hellenic Trench, it is slow or locked beneath the Western Taurides. Therefore, the GPS vectors and their distributions on land indicate remarkable velocity differences and enable us to determine the left-lateral shear zone located between the extensional and compressional blocks. Furthermore, this active tectonic regime creates differences in topography. This study also demonstrates how deep structures, such as the continuation of the subduction transform edge propagator (STEP) fault between the Hellenic and Cyprus arcs in the continental area, can come into play as a shear zone on the brittle crust.

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.

Role Played by Strike-Slip Structures in the Development of Highly Curved Orogens: The Transcarpathian Fault System, South Carpathians

AbstractWe describe a previously unrecognized major dextral strike-slip fault system in the South Carpathians, hereafter referred to as the Transcarpathian fault system. The master fault has been active since the mid-Cretaceous and has a total offset of ∼150 km, of which only <35 km are post-Oligocene. The fault acted as a subduction-transform edge propagator (STEP) fault during the mid-Cretaceous subduction of the Ceahlau-Severin ocean system and separated an area to the north where the subduction system was accretionary (the East Carpathians) from an area to the south (the western half of the South Carpathians) where the subduction system was erosive. In the South Carpathians, the oceanic basin closed during the mid-Cretaceous after commencement of higher convergence rates and subduction erosion of the trench, leading to tectonic underplating and continental collision between the Dacia and Moesian microplates. The results bolster the idea that STEP-type strike-slip faults are critical in the development...

Investigating slab edge kinematics through seismological data: The northern boundary of the Ionian subduction system (south Italy)

An integrated analysis of seismotomography models, earthquake spatial distribution and focal mechanism solutions has been performed in the still poorly constrained region marking the northern boundary of the Ionian subducting slab (south Italy). In this area, research findings related to the continuous- vs detached-slab transition are diffused on a wide sector running from central Calabria to the southern Apennines tip and different locations of a subduction-transform edge propagator (STEP) have also been proposed in the recent literature. We present seismic data and analyses aimed to investigate the location of the northern edge of the in-depth continuous slab and the present-day kinematics of the concerned boundary region by also focusing on the possible clues of STEP fault activity. We used both standard linearized and non-linear probabilistic methods to locate, in a recently developed shallow-to-intermediate 3D velocity model, the seismicity that occurred between 1997 and 2012 at the Ionian slab northern boundary. Earthquake kinematics have also been investigated by integrating high-quality focal mechanisms selected from the literature with 70 waveform inversion solutions obtained in the present study.The results allow to identify in central Calabria the northern edge of the in-depth continuous slab and just north of it a ca. 100km-wide deformation zone reflecting lateral STEP migration in the slab edge area. The migration likely indicates an alternation of tear propagation along a slab edge and slab break-off progressively reducing the lateral extension of the subduction zone. In this framework, the recent seismicity seems also to suggest the possible location of an incipient shallow slab break-off beneath central Calabria.