Subduction Transform Edge Propagator Fault 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.

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.

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.

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.

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.

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.

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.

Comment on “The Fethiye–Burdur Fault Zone: A component of upper plate extension of the subduction transform edge propagator fault linking Hellenic and Cyprus Arcs, Eastern Mediterranean. Tectonophysics 635, 80–99” by J. Hall, A.E. Aksu, İ. Elitez, C. Yaltırak, G. Çiftçi

• NE-trending Pliny–Strabo trench has envisaged being continued into Anatolia as strike-slip fault.

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.

Reply to the comment by M.C. Alçiçek on “The Fethiye–Burdur Fault Zone: A component of upper plate extension of the subduction transform edge propagator fault linking Hellenic and Cyprus Arcs, Eastern Mediterranean,” Tectonophysics, 635, 80–99, by J. Hall, A.E. Aksu, İ. Elitez, C. Yaltirak and G. Çifçi

• This confirms our previous suggestions that the Fethiye-Burdur Fault Zone is a 75-90 km-wide, left-lateral, fault zone. • This is based on GPS vectors, earthquake first motions, magnetotelluric data and detailed mapping including fault kinematics.

Composition and evolution of the Ancestral South Sandwich Arc: Implications for the flow of deep ocean water and mantle through the Drake Passage Gateway

The Ancestral South Sandwich Arc (ASSA) has a short life-span of c. 20m.y. (early Oligocene to middle–late Miocene) before slab retreat and subsequent ‘resurrection’ as the active South Sandwich Island Arc (SSIA). The ASSA is, however, significant because it straddled the eastern margin of the Drake Passage Gateway where it formed a potential barrier to deep ocean water and mantle flow from the Pacific to Atlantic. The ASSA may be divided into three parts, from north to south: the Central Scotia Sea (CSS), the Discovery segment, and the Jane segment. Published age data coupled with new geochemical data (major elements, trace elements, Hf–Nd–Sr–Pb isotopes) from the three ASSA segments place constraints on models for the evolution of the arc and hence gateway development. The CSS segment has two known periods of activity. The older, Oligocene, period produced basic–acidic, mostly calc-alkaline rocks, best explained in terms of subduction initiation volcanism of Andean-type (no slab rollback). The younger, middle–late Miocene period produced basic–acidic, high-K calc-alkaline rocks (lavas and pyroclastic rocks with abundant volcanigenic sediments) which, despite being erupted on oceanic crust, have continental arc characteristics best explained in terms of a large, hot subduction flux most typical of a syn- or post-collision arc setting. Early–middle Miocene volcanism in the Discovery and Jane arc segments is geochemically quite different, being typically tholeiitic and compositionally similar to many lavas from the active South Sandwich Island Arc front. There is indirect evidence for Western Pacific-type (slab rollback) subduction initiation in the southern part of the ASSA and for the back-arc basins (the Jane and Scan Basins) to have been active at the time of arc volcanism. Models for the death of the ASSA in the south following a series of ridge–trench collisions are not positively supported by any geochemical evidence of hot subduction, but cessation of subduction by approach of progressively more buoyant oceanic lithosphere is consistent with both geochemistry and geodynamics. In terms of deep ocean water flow the early stages of spreading at the East Scotia Ridge (starting at 17–15Ma) may have been important in breaking up the ASSA barrier while the subsequent establishment of a STEP (Subduction-Transform Edge Propagator) fault east of the South Georgia microcontinent (<11Ma) led to formation of the South Georgia Passage used by the Antarctic Circumpolar Current today. In terms of mantle flow, the subduction zone and arc root likely acted as a barrier to mantle flow in the CSS arc segment such that the ASSA itself became the Pacific–South Atlantic mantle domain boundary. This was not the case in the Discovery and Jane arc segments, however, because the northward flow of the South Atlantic mantle behind the southern part of the ASSA gave an Atlantic provenance to the whole southern ASSA.

The Fethiye–Burdur Fault Zone: A component of upper plate extension of the subduction transform edge propagator fault linking Hellenic and Cyprus Arcs, Eastern Mediterranean

The Hellenic and Cyprus Arcs, that mark the convergent boundary of the African and Aegean–Anatolian plates, are offset along a subduction transform edge propagator (‘STEP’) fault running NE–SW along the Pliny and Strabo Trenches. The continuation of the fault to the northeast through the Rhodes Basin and into SW Anatolia is assessed. Seismic reflection profiles show that the structural architecture of the northern sector of the Rhodes Basin includes a large crustal-scale fold–thrust belt which is overprinted by numerous faults with small extensional stratigraphic separations. A protracted episode of convergence in the Miocene resulted in the development of a prominent NE–SW-striking and NW-verging fold–thrust belt in the Rhodes Basin. The absence of evaporites in the Rhodes Basin and several seaward prograded vertically stacked Quaternary delta successions resting at 2500–3500m water depth collectively suggest that the Rhodes Basin must have remained above the depositional base of marine evaporite environment during the Messinian and that the region must have subsided very rapidly during the Pliocene-Quaternary. During the Pliocene-Quaternary, a NE–SW-trending belt developed across the Rhodes Basin: while the structural framework of this belt was characterised by reactivated thrusts in the central portion of the basin, a prominent zone of NE–SW-striking and NW- and SE-dipping faults with extensional separations developed in the northern portion of the basin. Two seismic profiles running parallel to the present-day coastline provide the much needed linkage between the Fethiye–Burdur Fault Zone onland and the reactivated thrusts in central Rhodes Basin, and show that the Pliocene-Quaternary zone of high-angle faults with extensional separations clearly link with the similarly trending and dipping strike–slip faults onland in the Eşen Valley, thus providing the continuity between the Pliny–Strabo Trenches in the southwest and the Fethiye–Burdur Fault Zone in the northeast. Mapping of many faults in parts of the Fethiye–Burdur Fault Zone shows evidence for sinistral strike–slip but total displacement across the fault zone is at maximum a few tens of kilometres. The STEP fault thus appears to have diminishing displacement associated with it as it propagates upwards into the upper plate from its originating tear in the subducting plate.

Origin and consequences of western Mediterranean subduction, rollback, and slab segmentation

AbstractThe western Mediterranean recorded subduction rollback, slab segmentation and separation. Here we address the questions of what caused Oligocene rollback initiation, and how its subsequent evolution split up an originally coherent fore arc into circum‐southwest Mediterranean segments. We kinematically reconstruct western Mediterranean geology from subduction initiation to present, using Atlantic plate reconstructions as boundary condition. We test possible reconstructions against remnants of subducted lithosphere imaged by seismic tomography. Transform motion between Africa and Iberia (including the Baleares) between ~120 and 85 Ma was followed by up to 150 km convergence until 30 Ma. Subduction likely initiated along the transform fault that accommodated pre‐85 Ma translation. By the ~30 Ma inception of rollback, up to 150 km of convergence had formed a small slab below the Baleares. Iberia was disconnected from Sardinia/Calabria through the North Balearic Transform Zone (NBTZ). Subduction below Sardinia/Calabria was slightly faster than below the Baleares, the difference being accommodated in the Pyrenees. A moving triple junction at the trench‐NBTZ intersection formed a subduction transform edge propagator fault between the Baleares and Calabria slab segments. Calabria rolled back eastward, whereas the Baleares slab underwent radial (SW‐S‐SE) rollback. After Kabylides‐Africa collision, the western slab segment retreated toward Gibraltar, here reconstructed as the maximum rollback end‐member model, and a Kabylides slab detached from Africa. Opening of a slab window below the NBTZ allowed asthenospheric rise to the base of the fore arc creating high‐temperature metamorphism. Western Mediterranean rollback commenced only after sufficient slab‐pull was created from 100 to 150 km of slow, forced subduction before ~30 Ma.

Subduction initiation along the inherited weakness zone at the edge of a slab: Insights from numerical models

SUMMARY A preexisting weakness zone in the lithosphere is required to initiate subduction. Here, we focus on a new type of weakness zone, a Subduction-Transform Edge Propagator (STEP) fault, which is inherited from a tear along the edge of a slab. Using coupled thermal–mechanical models, we show that STEP fault-perpendicular convergence results in a dipping shear zone in any tectonic setting. At a continental margin, this shear zone dips towards the continent, which is an excellent starting condition for ocean–continent subduction. If (far field) convergence persists, STEP faults become new subduction boundaries. The trench moves landward during the earliest stages of convergence. When slab pull becomes a dominant driving force, after ∼80 km convergence, trench roll-back commences. The initial geometry and mechanical properties of the sub-crustal STEP fault zone affect the results; subduction initiation is facilitated by a wide (∼100 km) and low-viscosity weakness zone. Incipient subduction is easier for young oceanic lithosphere due to its lower flexural rigidity and is insensitive to the far field convergence rate. As STEP faults are commonly associated with young oceanic lithosphere, subduction initiation is thus relatively easy along them. Of particular interest are continent–ocean margins where STEP faulting has occurred.

Exhumation with a twist: Paleomagnetic constraints on the evolution of the Menderes metamorphic core complex, western Turkey

Much remains to be understood about the links between regional vertical axis rotations, continental extension, and shortening. In western Turkey, Miocene vertical axis rotations have been reported that occur simultaneously with the extensional exhumation of the Menderes metamorphic core complex, which has been related to back‐arc extension in the eastern part of the Aegean back arc. In this paper we explore the spatial and temporal relationships between vertical axis rotations in southwestern Turkey and extensional unroofing of the Menderes Massif. To this end, we provide a large set of new paleomagnetic data from western Turkey, and integrate these with the regional structural evolution to test the causes and consequences of oroclinal bending in the Aegean region. The Lycian Nappes and Bey Dağları are shown to rotate ∼20° between 16 and 5 Ma, defining the eastern limb of the Aegean orocline. This occurred contemporaneously with the exhumation of the central Menderes Massif (along extensional detachments) and after the latest Oligocene to early Miocene exhumation of the northern and southern Menderes massifs. Exhumation of the latter two was not associated with vertical axis rotations. The lower Miocene volcanics in the region from Lesbos to Uşak, to the north of the central Menderes Massif underwent a small clockwise rotation, insignificant with respect to Eurasia. This shows that exhumation of the central Menderes Massif was associated with a vertical axis rotation difference between the northern and southern Menderes massifs of ∼25°–30°. This result is in excellent agreement with the angle defined by the trends of Büyük Menderes and Alaşehir detachments, as well as the angle defined by the regionally curving stretching lineation pattern across the central Menderes Massif. These structures define a pivot point (rotation pole) for the west Anatolian rotations. The rotation of the southern domain, including the southern Menderes Massif, the Lycian Nappes, and Bey Dağları, must have led to N–S contraction east of this pole. The eastern limit of the rotating domain is formed by the transpressional couple of the Aksu thrust and Kırkkavak Fault in the center of the Isparta angle. Previously reported clockwise rotations in the volcanic fields near Afyon may be the result of distributed N–S shortening east of the pivot point. The precise accommodation of this shortening history remains open for investigation. Late Oligocene to early Miocene extension in the eastern part of the Aegean back arc was NE–SW oriented and likely bounded by a discrete transform. This transform may be associated with an early evolution of the eastern Aegean subduction transform edge propagator fault. Oroclinal bending in the west Anatolian region is likely related to a reconnection of the eastern part of the Aegean orocline with the African northward moving plate in tandem with roll back in the Aegean back arc, comparable to a recently postulated scenario for western Greece.

Discrete Plio-Pleistocene phases of tilting and counterclockwise rotation in the southeastern Aegean arc (Rhodos, Greece): early Pliocene formation of the south Aegean left-lateral strike-slip system

The island of Rhodos represents an uplifted block in the largely submerged southeastern Aegean forearc. It has a complex history of subsidence, uplift and counterclockwise rotation during the Plio-Pleistocene, in response to the interplay between large-scale geodynamic processes. In this paper, we present a new chronostratigraphic framework for the continental Pliocene Apolakkia basin of southwestern Rhodos. We combine these time constraints with recently published chronostratigraphic data from the marine Plio-Pleistocene basins of northeastern Rhodos to reconstruct rotational and vertical motions. Our palaeomagnetic results identify two rotation phases for Rhodos: c . 10° (9 ± 6°) counterclockwise (ccw) rotation between 3.8 and 3.6 Ma, and c . 17 ± 6° ccw rotation since 0.8 Ma. Between these phases, Rhodos tilted to the SE, drowning the southeastern coast to a depth of 500–600 m between 2.5 and 1.8 Ma, then to the NW, which resulted in the re-emergence of the drowned relief between 1.5 and 1.1 Ma. We relate the rotations of Rhodos to incipient formation of the south Aegean sinistral strike-slip system and the foundering of the Rhodos basin. The previously shown absence of Messinian evaporites in the deep-marine Rhodos basin in combination with the 3.8 Ma onset of ccw rotation of Rhodos constrains the onset of the formation of the south Aegean strike-slip system between 5.3 and 3.8 Ma. The formation of this strike-slip system is probably related to the interplay of oblique collision between the southeastern Aegean region and the northward moving African plate, the westward motion of Anatolia, gravitational spreading of the overthickened Aegean lithosphere and the recently postulated southwestward retreat of the African subducted slab along a subduction-transform edge-propagator fault.