Quaternary E-W Extension Uplifts Kythira Island and Segments the Hellenic Arc

Several crustal and lithospheric mechanisms lead to deformation and vertical motion of the upper plate during subduction, but their relative contribution is often enigmatic. Multiple areas of the Hellenic Forearc have been uplifting since Plio-Quaternary times, yet spatiotemporal characteristics and sources of this uplift are poorly resolved. The remarkable geology and geomorphology of Kythira Island, in the southwestern Hellenic forearc, allow for a detailed tectonic reconstruction since the Late Miocene. We present a morphotectonic map of the island, together with new biostratigraphic dating and detailed analyses of active fault strikes and marine terraces. We find that the Tortonian-Pliocene stratigraphy in Kythira records ∼100 m of subsidence, and a wide coastal rasa marks the ∼2.8-2.4 Ma maximum transgression. Subsequent marine regression of ∼300-400 m and minor E-W tilt are recorded in ∼12 marine terrace levels for which we estimate uplift rates of ∼0.2-0.4 mm/yr. Guided by simple landscape evolution models, we interpret the coastal morphology as the result of initial stability or of slow, gradual sea-level drop since ∼2.8-2.4 Ma, followed by faster uplift since ∼1.5-0.7 Ma. Our findings on- and offshore suggest that E-W extension is the dominant mode of regional active upper crustal deformation, and N-S normal faults accommodate most, if not all of the uplift on Kythira. We interpret the initiation of E-W extension as the result of a change in plate boundary conditions, in response to either propagation of the North Anatolian Fault, incipient collision with the African plate, mantle dynamics or a combination thereof.

The 27 September 2021 Earthquake in Central Crete (Greece)—Detailed Analysis of the Earthquake Sequence and Indications for Contemporary Arc-Parallel Extension to the Hellenic Arc

The Arkalochori village in central Crete was hit by a large earthquake (Mw = 6.0) on 27 September 2021, causing casualties, injuries, and severe damage to the infrastructure. Due to the absence of apparent surface rupture and the initial focal mechanism solution of the seismic event, we initiated complementary, multi-disciplinary research by combining seismological and remote sensing data processing, followed by extensive field validation. Detailed geological mapping, fault surface measuring accompanied with tectonic analysis, fault photorealistic model creation by unmanned aerial system data processing, post-seismic surface deformation analysis by DInSAR image interpretation coupled with accurately relocated epicenters recorded by locally established seismographs have been carried out. The combination of the results obtained from these techniques led to the determination of the contemporary tectonic stress regime that caused the earthquake in central Crete, which was found compatible with extensional processes parallel to the Hellenic arc.

Late Quaternary Marine Terraces and Tectonic Uplift Rates of the Broader Neapolis Area (SE Peloponnese, Greece)

Marine terraces are geomorphic markers largely used to estimate past sea-level positions and surface deformation rates in studies focused on climate and tectonic processes worldwide. This paper aims to investigate the role of tectonic processes in the late Quaternary evolution of the coastal landscape of the broader Neapolis area by assessing long-term vertical deformation rates. To document and estimate coastal uplift, marine terraces are used in conjunction with Optically Stimulated Luminescence (OSL) dating and correlation to late Quaternary eustatic sea-level variations. The study area is located in SE Peloponnese in a tectonically active region. Geodynamic processes in the area are related to the active subduction of the African lithosphere beneath the Eurasian plate. A series of 10 well preserved uplifted marine terraces with inner edges ranging in elevation from 8 ± 2 m to 192 ± 2 m above m.s.l. have been documented, indicating a significant coastal uplift of the study area. Marine terraces have been identified and mapped using topographic maps (at a scale of 1:5000), aerial photographs, and a 2 m resolution Digital Elevation Model (DEM), supported by extensive field observations. OSL dating of selected samples from two of the terraces allowed us to correlate them with late Pleistocene Marine Isotope Stage (MIS) sea-level highstands and to estimate the long-term uplift rate. Based on the findings of the above approach, a long-term uplift rate of 0.36 ± 0.11 mm a−1 over the last 401 ± 10 ka has been suggested for the study area. The spatially uniform uplift of the broader Neapolis area is driven by the active subduction of the African lithosphere beneath the Eurasian plate since the study area is situated very close (~90 km) to the active margin of the Hellenic subduction zone.

The Santorini-Amorgos Shear Zone: Evidence for Dextral Transtension in the South Aegean Back-Arc Region, Greece

Bathymetric and seismic data provide insights into the geomorphological configuration, seismic stratigraphy, structure, and evolution of the area between Santorini, Amorgos, Astypalea, and Anafi islands. Santorini-Amorgos Shear Zone (SASZ) is a NE-SW striking feature that includes seven basins, two shallow ridges, and hosts the volcanic centers of Santorini and Kolumbo. The SASZ initiated in the Early Pliocene as a single, W-E oriented basin. A major reorganization of the geodynamic regime led to (i) reorientation of the older faults and initiation of NE-SW striking ones, (ii) disruption of the single basin and localized subsidence and uplift, (iii) creation of four basins out of the former single one (Anafi, Amorgos South, Amorgos North, and Kinairos basins), (iv) rifting of the northern and southern margins and creation of Anydros, Astypalea North, and Astypalea South basins, and (v) uplift of the ridges. Dextral shearing and oblique rifting are accommodated by NE-SW striking, dextral oblique to strike-slip faults and by roughly W-E striking, normal, transfer faults. It is suggested here that enhanced shearing in NE-SW direction and oblique rifting may be the dominant deformation mechanism in the South Aegean since Early Quaternary associated with the interaction of North Anatolian Fault with the slab roll-back.

Seismic structure, crustal architecture and tectonic evolution of the Anatolian-African Plate Boundary and the Cenozoic Orogenic Belts in the Eastern Mediterranean Region

The modern Anatolian–African plate boundary is represented by a north-dipping subduction zone that has been part of a broad domain of regional convergence between Eurasia and Afro–Arabia since the latest Mesozoic. A series of collisions between Gondwana-derived ribbon continents and trench-roll-back systems in the Tethyan realm produced nearly East–West-trending, subparallel mountain belts with high elevation and thick orogenic crust in this region. Ophiolite emplacement, terrane stacking, high‐P and Barrovian metamorphism, and crustal thickening occurred during the accretion of these microcontinents into the upper plates of Tethyan subduction roll-back systems during the Late Cretaceous–Early Eocene. Continued convergence and oceanic lithospheric subduction within the Tethyan realm were punctuated by slab breakoff events following the microcontinental accretion episodes. Slab breakoff resulted in asthenospheric upwelling and partial melting, which facilitated post-collisional magmatism along and across the suture zones. Resumed subduction and slab roll-back-induced upper plate extension triggered a tectonic collapse of the thermally weakened orogenic crust in Anatolia in the late Oligocene–Miocene. This extensional phase resulted in exhumation of high‐P rocks and medium- to lower-crustal material leading to the formation of metamorphic core complexes in the hinterland of the young collision zones. The geochemical character of the attendant magmatism has progressed from initial shoshonitic and high‐K calc‐alkaline to calc‐alkaline and alkaline affinities through time, as more asthenosphere-derived melts found their way to the surface with insignificant degrees of crustal contamination. The occurrence of discrete high-velocity bodies in the mantle beneath Anatolia, as deduced from lithospheric seismic velocity data, supports our Tethyan slab breakoff interpretations. Pn velocity and Sn attenuation tomography models indicate that the uppermost mantle is anomalously hot and thin, consistent with the existence of a shallow asthenosphere beneath the collapsing Anatolian orogenic belts and widespread volcanism in this region. The sharp, north-pointing cusp (Isparta Angle) between the Hellenic and Cyprus trenches along the modern Anatolian–African plate boundary corresponds to a subduction-transform edge propagator (STEP) fault, which is an artifact of a slab tear within the downgoing African lithosphere.

Subduction, convergence and the mode of backarc extension in the Mediterranean region

30-35 Ma ago a major change occurred in the Mediterranean region, from a regionally compressional subduction coeval with the formation of Alpine mountain belts, to extensional subduction and backarc rifting. Backarc extension was accompanied by gravitational spreading of the mountain belts formed before this Oligocene revolution. Syn-rift basins formed during this process above detachments and low-angle normal faults. Parameters that control the formation and the kinematics of such flat-lying detachments are still poorly understood. From the Aegean Sea to the Tyrrhenian Sea and the Alboran Sea, we have analysed onshore the deformation and P-T-t evolution of the ductile crust exhumed by extension, and the transition from ductile to brittle conditions as well as the relations between deep deformation and basin formation. We show that the sense of shear along crustal-scale detachments is toward the trench when subduction proceeds with little or no convergence (northern Tyrrhenian and Alboran after 20 Ma) and away from the trench in the case of true convergence (Aegean). We tentatively propose a scheme explaining how interactions between the subducting slab and the mantle control the basal shear below the upper plate and the geometry and distribution of detachments and associated sedimentary basins. We propose that ablative subduction below the Aegean is responsible for the observed kinematics on detachments (i.e. away from the trench). The example of the Betic Cordillera and the Rif orogen, where the directions of stretching were different in the lower and the upper crust and changed through time, is also discussed following this hypothesis.