Earthquake mechanisms of the Adriatic Sea and Western Greece: implications for the oceanic subduction-continental collision transition

We present 21 focal solutions (magnitude > 5.5) reliably computed by body-wave modelling for the western Hellenic arc from Yugoslavia to the southern Peloponnese. Mechanisms located within the Aegean show normal faulting, the T-axis trending N-S in the centre and parallel to the active boundary in the external part. Mechanisms associated with the Keffalinia fault are consistent with dextral strike-slip motion. Reverse mechanisms located along the active boundary are remarkably consistent and do not depend on the nature of the active boundary (continental collision or oceanic subduction). The consistency in azimuth of the slip vectors and of the GPS velocity relative to Africa, all along the active boundary, suggests that the deformation is related to the same motion. The discrepancy between seismic-energy release and the amount of shortening confirms that the continental collision is achieved by seismic slip on faults but the oceanic subduction is partially aseismic. The northward decrease in velocity between continental collision and oceanic subduction suggests the continental collision to be a recent evolution of the active subduction.     

Neogene supradetachment basin development on Crete (Greece) during exhumation of the South Aegean core complex

Tertiary extension in the Aegean region has led to extensional detachment faulting, along which metamorphic core complexes were exhumed, among which is the Early to Middle Miocene South Aegean core complex. This paper focuses on the supradetachment basin developed during the final stages of exhumation of the South Aegean core complex along the Cretan detachment, plus the Late Miocene to Pliocene basin development and palaeogeography associated with the southward motion of Crete during the opening of the Aegean arc. For the latter purpose, the sedimentary and palaeobathymetric evolutions of a large number of Middle Miocene to Late Pliocene sequences exposed on Crete, Gavdos and Koufonisi were studied. The supradetachment basin development of Crete is characterised by a break‐up of the hanging wall of the Cretan detachment into extensional klippen and subsequent migration of laterally coexisting sedimentary systems, and finally the deformation of the exhumed core complex by processes related to the opening of the Aegean arc. Hence, three main tectonic phases are recognised: (1) Early to Middle Miocene N–S extension formed during the Cretan detachment, exhumed in the South Aegean core complex. The Cretan detachment remained active until 11–10 Ma, based on the oldest sediments that unconformably overlie the metamorphic rocks. Successions older than 11–10 Ma unconformably overlie only the hanging wall of the Cretan detachment, and do not contain fragments of the footwall rocks; they therefore predate the oldest exposure of the metamorphic rocks of the footwall. The hanging wall rocks and Middle Miocene sediments form isolated blocks on top of the exhumed metamorphic rocks, which are interpreted as extensional klippen. (2) From approximately 10 Ma onward, southward migration of the area that presently covers Crete was accompanied by E–W extension, and the opening of the Sea of Crete to the north. Contemporaneously, large folds with WNW–ESE striking, NNE dipping axial planes developed, possibly in response to sinistral transpression. (3) During the Pliocene, Crete emerged and tilted to the NNW, probably as a result of left‐lateral transpression in the Hellenic fore‐arc, induced by the collision with the African promontory.     

Active tectonics of the Alpine—Himalayan belt: the Aegean Sea and surrounding regions

Summary. New fault plane solutions, Landsat photographs, and seismic refraction records show that rapid extension is now taking place in the northern and eastern parts of the Aegean sea region. The southern part of the Aegean has also been deformed by normal faulting but is now relatively inactive. In northwestern Greece and Albania there is a band of thrusting near the western coasts adjacent to a band of normal faulting further east. The pre-Miocene geology of the islands in the Aegean closely resembles that of Greece and Turkey, yet seismic refraction shows that the crust is now only about 30 km thick beneath the southern part of the sea, compared with nearly 50 km beneath Greece and western Turkey. These observations suggest that the Aegean has been stretched by a factor of two since the Miocene. This stretching can account for the high heat flow. The sinking slab produced by subduction along the Hellenic Arc may maintain the motions, though the geometry and widespread nature of the normal faulting is not easily explained. The motions in northwestern Greece and Albania cannot be driven in the same way because no slab exists in the area. They may be maintained by blobs of cold mantle detaching from the lower half of the lithosphere, produced by a thermal instability when the lithosphere is thickened by thrusting. Hence generation and destruction of the lower part of the lithosphere may occur beneath deforming continental crust without the production of any oceanic crust.     

Structural development of Neogene basins in western Greece

Abstract An account is given of the structural setting of the various Neogene sedimentary basins of western Greece. Compressional basins are attributable to foreland loading by the Alpine fold and thrust belt of the Outer Hellenides, and to active subduction in the adjacent western Hellenic arc. Late extensional basins are related to N-S crustal extension in the Aegean marginal basin and, in western Greece, are superimposed on the earlier compressional structures. The local seismicity provides evidence that the main E-W-trending basin-bounding faults of the extensional basins form a linked system that includes NW-SE- and NE-SW-trending transfer zones of transtension. The transfer zones are themselves the sites of small extensional basins.     

Roll-back controlled vertical movements of outer-arc basins of the Hellenic subduction zone (Crete, Greece)

Crustal thickening north of the Hellenic subduction zone continued in the most external zones (e.g. Crete) probably until the late middle Miocene. The following period of predominant extension has been related by various workers to a number of causes such as: (1) trench retreat (roll back) driven by the pull of the African slab and (2) gravitational body forces associated with the thickened crust, both in combination with NNE motion of the African plate combined with westward extrusion of the Anatolian block along the North Anatolian Fault. To verify these hypotheses an inventory of fault orientations and fault-block kinematics was carried out for central and eastern Crete and adjoining offshore areas by combining satellite imagery, digital terrain models, and structural, seismic, sedimentary and stratigraphical field data, all set up in a GIS. The GIS data set enabled easy visualization and combination of data, which resulted in a relatively objective analysis. The geological results are discussed in the light of a numerical model that investigated the intraplate stresses resulting from the above mentioned forces. Our tectonostratigraphic results for the late Neogene of central and eastern Crete show three episodes of basin extension following a period of approximately N–S compression. In the earliest Tortonian, N130E- to N100E-trending normal faults developed, resulting in a roughly planar, arc-parallel fault system aligning strongly asymmetric half-grabens. The early Tortonian to early Messinian period was characterized by an orthogonal fault system of N100E and N020E faults resulting in rectangular grabens and half-grabens. From the late Tortonian to early Pleistocene, deformation occurred along a pattern of closely spaced, left-lateral oblique N075E faults, orientated parallel to the south Cretan trenches. Deformation phases younger than early Pleistocene are dominated by normal to oblique faulting along WSW–ENE (N050E) faults and dextral, oblique motions along NNW–SSE (N160E) faults. Many faults that were generated during previous deformational episodes appear to be reactivated in later periods. Our tectonostratigraphy points to a three step anticlockwise rotation of active fault systems since the late middle Miocene compressional phase. We suggest here that the rotation is associated with a reorganization of the stress field going from SSW–NNE tension in the early late Miocene to NE–SW left-lateral shear in the Quaternary. The rotation is likely to be a response to arc-normal pull forces combined with a progressive increase of the curvature of the arc. During the Pliocene to Recent period, the SSW-ward retreat of the arc and trench system relative to the African plate was accomplished by transform motions in the eastern (Levantine) segment of the Hellenic Arc, resulting in, respectively, NNW–SSE and NE–SW left-lateral shear on Crete.     

Formation and fragmentation of a late Miocene supradetachment basin in central Crete: implications for exhumation mechanisms of high‐pressure rocks in the Aegean forearc

We present a new lithostratigraphy and chronology for the Miocene on central Crete, in the Aegean forearc. Continuous sedimentation started at ～10.8 Ma in the E–W trending fluvio‐lacustrine Viannos Basin, formed on the hangingwall of the Cretan detachment, which separates high‐pressure (HP) metamorphic rocks from very low‐grade rocks in its hangingwall. Olistostromes including olistoliths deposited shortly before the Viannos Basin submerged into the marine Skinias Basin between 10.4 and 10.3 Ma testifies to significant nearby uplift. Uplift of the Skinias Basin between 9.7 and 9.6 Ma, followed by fragmentation along N–S and E–W striking normal faults, marks the onset of E–W arc‐parallel stretching superimposed on N–S regional Aegean extension. This process continued between 9.6 and 7.36 Ma, as manifested by tilting and subsidence of fault blocks with subsidence events centred at 9.6, 8.8, and 8.2 Ma. Wholesale subsidence of Crete occurred from 7.36 Ma until ～5 Ma, followed by Pliocene uplift and emergence. Subsidence of the Viannos Basin from 10.8 to 10.4 Ma was governed by motion along the Cretan detachment. Regional uplift at ～10.4 Ma, followed by the first reworking of HP rocks (10.4–10.3 Ma) is related to the opening and subsequent isostatic uplift of extensional windows exposing HP rocks. Activity of the Cretan detachment ceased sometime between formation of extensional windows around 10.4 Ma, and high‐angle normal faulting cross‐cutting the detachment at 9.6 Ma. The bulk of exhumation of the Cretan HP‐LT metamorphic rocks occurred between 24 and 12 Ma, before basin subsidence, and was associated with extreme thinning of the hangingwall (by factor ～10), in line with earlier inferences that the Cretan detachment can only explain a minor part of total exhumation. Previously proposed models of buyoant rise of the Cretan HP rocks along the subducting African slab provide an explanation for extension without basin subsidence.     

A comparative study of neotectonic basins across the Hellenic arc: the Messiniakos, Argolikos, Saronikos and Southern Evoikos Gulfs

Abstract Detailed single-channel continuous seismic reflection profiling data from four gulfs as well as onshore neotectonic investigations have allowed the study of the neotectonic structure of the Hellenic arc along a complete transverse section from its external area in the trench to the internal back-arc area.
Messiniakos Gulf is an asymmetric NW-SE structure with considerable tilt towards the NE. It is the direct continuation of the continental slope from the trench to the island arc (Peloponnesus, Crete, Dodekannese). Argolikos Gulf is an almost symmetric NW-SE graben occupying the northern edge of the Cretan back-arc basin. Saronikos Gulf is a multi-complex structure of a NW-SE graben in the SW (Epidaurus Basin) and alternation of E-W horsts and grabens in the North. Its neotectonic evolution is characterized by the Plio-Quaternary volcanic arc activity. Southern Evoikos Gulf is a relatively shallow neotectonic graben in the back-arc area at the northern prolongation of the Cycladic Platform.
Each of the above neotectonic basins has its own characteristics which are probably due to their geodynamic position in the Hellenic arc. In general, there is a decrease in the neotectonic deformation, the sediment thickness and the sedimentation rates from SW to NE, going from the periphery to the core of the arc.     

Rates of active deformation in the Aegean Sea and surrounding regions

Abstract Average strain rates are calculated from earthquakes in the period 1908-81 that occurred in the Aegean Sea extensional region, and in the convergent zone associated with the Hellenic Trench. In spite of large uncertainties resulting from the use of an M_S : M_o relationship, seismic N-S extensional rates in the Aegean are in the region 20–60 mm yr^-1 whereas seismic shortening rates in the Hellenic Trench are less than about 15 mm yr^-1. This is surprising because Africa and Eurasia are known to be converging, not separating. This apparent anomaly is caused by most of the convergence in the Hellenic Trench occurring aseismically. By contrast, the seismic extensional rates in the Aegean agree quite well with those expected from other arguments. The present day extensional rates are sufficiently high for McKenzie's instantaneous stretching model to be applicable. There is some evidence that these high extensional rates have operated throughout the last 5 Myr.     

Extensional tectonic regimes in the Aegean basins during the Cenozoic

Abstract Kinematics of faults in the Northern Aegean show three extensional tectonic regimes the tensional directions of which trend (1) WNW-ESE, (2) NE-SW and (3) N-S. These were active during the Upper Miocene, Pliocene-Lower Pleistocene and Mid Pleistocene-Present day, respectively. The main characteristics of the stress patterns (1) and (2) on the overall Aegean is tentatively explained by variations of the horizontal lithospheric stress value σ_zz due to the slab push and of the vertical lithospheric stress value σ_zz due to mass heterogeneities. During the Mid Pleistocene-Present, due to the slab push, tectonics were compressional along the arc boundary: σ_zz was σ1. In the Aegean basins, tectonics were extensional, c_2Z was σ1 as a consequence of the thickness of the continental crust and, possibly of an updoming asthenosphere; thus σ_zz became σ2, allowing tension σ3 to be orthogonal to the compression along the arc, i.e. to be roughly parallel to the arc trend. During the Pliocene-Lower Pleistocene, the extensional regime was distinctly different. The tensional directions were roughly radial to the arc. It is suggested that σ_zz was weakly compressional, or eventually tensional, due a seaward migration of the slab so that σ_zz became σ3. In the Northern Aegean, the stress pattern has been also controlled by the westward push of the Anatolian landmass. During the Mid Pleistocene-Present day, this was typically extensional (al was vertical) and the right lateral strike-slip motion on the North Anatolian Fault transformed into a N-S-stretching, E-W-shortening of the Northern Aegean. Dextral strike-slip motions along the North Aegean Trough fault zone were possible on NE-SW-striking faults. During the Pliocene-Lower Pleistocene, normal fault components were higher; however, because the angle between the NE-SW trend of the tensional axis and the strike of the fault zone was acute, dextral strike-slip components were possible on all the faults striking NE-SW to E-W. A clockwise 15^o rotation of Limnos with respect to Samothraki, Thraki and Thassos, suggested by structural data, was probably associated with these dextral motions. The WNW-ESE trending tension during the Upper Miocene indicates that the dextral North Anatolian Fault had not yet merged into the North Aegean Trough fault zone at that time. We propose that the formation of Aegean basins during the Cenozoic was related to the activity of two major Hellenic arcs. The ‘Pelagonian-Pindic Arc’ resulted in the formation of the subsident Aegean basins of Middle Eocene-Lower Miocene age and of the older Northern Aegean orogenic volcanism. The ‘Aegean Arc’ resulted in the formation of the subsident Aegean basins of Middle Miocene to Present day age and of the Southern Aegean orogenic volcanism. Were these arcs associated with a unique subduction zone or with two such zones ? In the first case, the slab is no more than 16 Myr old, in the second it may be as old as 45–50 Myr. The answer depends on the accuracy of the seismic tomography profiles.     

Late Jurassic subsidence and passive margin evolution in the Vulcan Sub-basin, north-west Australia: constraints from basin modelling

The Vulcan Sub-basin, located in the Timor Sea, north-west Australia, developed during the Late Jurassic extension which ultimately led to Gondwanan plate breakup and the development of the present-day passive continental margin. This paper describes the evolution of upper crustal extension and the development of Late Jurassic depocentres in this subbasin, via the use of forward modelling techniques. The results suggest that a lateral variation in structural style exists. The south of the basin is characterized by relatively large, discrete normal faults which have generated deep sub-basins, whereas more distributed, small-scale faulting further north reflects a collapse of the early basin margin, with the development of a broader, 'sagged' basin geometry. By combining forward and reverse modelling techniques, the degree of associated lithosphere stretching can be quantified. Upper crustal faulting, which represents up to 10% extension, is not balanced by extension in the deeper, ductile lithosphere; the magnitude of this deeper extension is evidenced by the amount of post-Valanginian thermal subsidence. Reverse modelling shows that the lithosphere stretching
factor has a magnitude of up to β=1.55 in the southern Vulcan Sub-basin, decreasing to β=1.2 in the northern Vulcan Sub-basin. It is proposed that during plate breakup, deformation in the Vulcan Sub-basin consisted of depth-dependent lithosphere extension. This additional component of lower crustal and lithosphere stretching is considered to reflect long-wavelength partitioning of strain associated with continental breakup, which may have extended 300–500 km landward of the continent–ocean boundary.     

Earthquake processes of the Himalayan collision zone in eastern Nepal and the southern Tibetan Plateau

Focal mechanisms determined from moment tensor inversion and first motion polarities of the Himalayan Nepal Tibet Seismic Experiment (HIMNT) coupled with previously published solutions show the Himalayan continental collision zone near eastern Nepal is deforming by a variety of styles of deformation. These styles include strike-slip, thrust and normal faulting in the upper and lower crust, but mostly strike-slip faulting near or below the crust–mantle boundary (Moho). One normal faulting earthquake from this experiment accommodates east–west extension beneath the Main Himalayan Thrust of the Lesser Himalaya while three upper crustal normal events on the southern Tibetan Plateau are consistent with east–west extension of the Tibetan crust. Strike-slip earthquakes near the Himalayan Moho at depths >60 km also absorb this continental collision. Shallow plunging P -axes and shallow plunging EW trending T -axes, proxies for the predominant strain orientations, show active shearing at focal depths ∼60–90 km beneath the High Himalaya and southern Tibetan Plateau. Beneath the southern Tibetan Plateau the plunge of the P -axes shift from vertical in the upper crust to mostly horizontal near the crust–mantle boundary, indicating that body forces may play larger role at shallower depths than at deeper depths where plate boundary forces may dominate.     

The gravity field of a dipping plate in Greece

Summary. The surface gravity anomalies for a three-dimensional density model of a dipping lithospheric plate under the Aegean island arc and the Aegean Sea in Greece in the eastern Mediterranean Sea have been calculated. Such a dipping plate has been suggested in geophysical investigations. The calculated gravity maximum over the dipping plate is located in the area of the largest observed Bouguer anomalies inside the island arc. It is suggested that this should be taken into account in studies of crustal structure and gravity in the area.     

A catalogue of seismicity in Greece and adjacent areas

Summary. A new earthquake catalogue for Greece has been formed to cover the instrumental period 1901–78, in particular 605 earthquakes for the period 1917–63 inclusive are relocated using first arrival data from the International Seismological Summary. These relocations incorporate macro-seismically and other well-controlled master events into an ensuing joint epicentre determination technique. The largest annual average shift is 165 km for 78 earthquakes during the decade after 1917, decreasing to 17 km for the later years 1957–63. Magnitudes are redetermined mainly using readings from the Swedish network and Uppsala since as early as 1908. Catalogue completeness exists for magnitudes around 5.5 for at least the last 60 years, but magnitudes of about 4.7 are completely reported only during the most recent 15 years.
The new epicentral positions lead to a better delineation of the seismic zones than has previously been achieved. The majority of shallow earthquakes are contained in a belt parallel to the Hellenic Arc which extends north into Albania, in the south-east the epicentral locations give a more diffuse extension of this zone into the west coast of Turkey. Depths of intermediate earthquakes along the Hellenic Arc tend to relocate to shallower depths, in the south-west part of Crete no earthquake focus deeper than 100 km is found, although in the south-eastern section of the arc a tendency to increased depths is observed. A second zone starts at Leukas Island in the west and extends through central Greece to near Volos on the east coast, where it divides into two branches, which are less well defined, but which eventually join the seismicity of western Turkey. A third zone follows the Saronikos and Corinth gulfs.     

Flexural uplift of the Stara Planina range, central Bulgaria

The Stara Planina is an E–W-trending range within the Balkan belt in central Bulgaria. This topographically high mountain range was the site of Mesozoic through early Cenozoic thrusting and convergence, and its high topography is generally thought to have resulted from crustal shortening associated with those events. However, uplift of this belt appears to be much younger than the age of thrusting and correlates instead with the age of Pliocene–Quaternary normal faulting along the southern side of the range. Flexural modelling indicates the morphology of the range is consistent with flexural uplift of footwall rocks during Pliocene–Quaternary displacement on S-dipping normal faults bounding the south side of the mountains, provided that the effective elastic plate thickness of 12  km under the Moesian platform is reduced to about 3  km under the Stara Planina. This small value of elastic plate thickness under the Stara Planina is similar to values observed in the Basin and Range Province of the western United States, and suggests that weakening of the lithosphere is due to heating of the lithosphere during extension, perhaps to the point that large-scale flow of material is possible within the lower crust. Because weakening is observed to affect the Moesian lithosphere for ≈10  km beyond (north of) the surface expression of extension, this study suggests that processes within the uppermost mantle, such as convection, play an active role in the extension process. The results of this study also suggest that much of the topographic relief in thrust belts where convergence is accompanied by coeval extension in the upper plate (or 'back arc'), such as in the Apennines, may be a flexural response to unloading during normal faulting, rather than a direct response to crustal shortening in the thrust belt.     

The Aegean region: deep structures and seismological properties

3-D P -wave velocity anomaly patterns, the geometry of the Hellenic Wadati-Benioff zone and of the seismically active fracture zones in the overlying continental wedge, and the depth distribution of seismogenic properties (seismic activity, energy and b value) for the Aegean region are examined in this study. The western and eastern parts of the area studied differ significantly in terms of the depth distribution of the considered seismological parameters and the velocity structures. The results indicate different physical conditions in the western and eastern parts of the region in question. Data for the surface heat flow and the evolution of the volcanism in the Aegean region since the early Eocene are employed to interpret the results of the present study. Possible geodynamic processes at the plate boundaries between Africa and Eurasia are discussed.     

Neogene tectonics and basin fill patterns in the Hellenic outer-arc (Crete, Greece)

Six time-slice reconstructions in the form of palaeogeographical maps show the large-scale tectonic and sedimentary evolution of the Hellenic outer-arc basins in central and eastern Crete for the middle and late Miocene. The reconstructions are based on extensive field mapping and a detailed chronostratigraphy. Latest compressional features related to subduction and associated crustal thickening are poorly dated and assigned a middle Miocene age. These are possibly contemporaneous with widespread occurrence of breccia deposits all over Crete. The precise date for the onset of extension, possibly controlled by the roll-back of the subsiding African lithosphere, remains at this point a discussion. We present circumstantial evidence to place the beginning of the roll back in the middle Miocene, during the accumulation of an arc-parallel, westward-draining fluvial complex. The continental succession is transgressed steadily until it is interrupted by an important tectonic event at the boundary of the middle and late Miocene (normally seen as the onset of slab roll-back). In the earliest late Miocene a few large-sized fault blocks along arc-parallel normal faults subsided rapidly causing a deepening of the half-graben basins up to approximately 900 m. About 1 Myr later, a new N020E and N100E fault system developed fragmenting the existing half-grabens into orthogonal horst and graben structure. The development of the new fault system caused original continental regions to subside and original deep basins to emerge, which is not easy to reconcile with roll back controlled extensional processes alone. Underplating and inherited basement structure may have played here an additional role, although evidence for firm conclusions is lacking. In late Miocene times (late Tortonian, ≈7.2 Ma), the extensional outer arc basins become deformed by N075E-orientated strike-slip. The new tectonic regime begins with strong uplift along existing N100E fault zones, which developed about E–W-striking topographical highs (e.g. Central Iraklion Ridge and Anatoli anticline) in about 0.4 Myr. The strong uplift is contemporaneous with abundant landsliding observed along an important N075E fault zone crossing eastern Crete and with renewed volcanic activity of the arc. The origin of the ridges may be due to active folding related to the sinistral slip.     

Geodetically determined strain across the southern end of the Tonga-Kermadec-Hikurangi subduction zone

Summary. A method of simultaneous reduction is presented for determining strain rates from multiple triangulation surveys where common triangulation stations have been used, but the angles of the old survey have not necessarily been reobserved. This method is applied to triangulation in the northern South Island, at the southern end of the Tonga-Kermadec-Hikurangi subduction zone. From a profile of shear strain across the Indian-Pacific plate boundary, the displacement of the Indian plate relative to the Pacific is calculated to be 54 ± 9 mm yr⁻¹ at an azimuth of 84°± 10°, in remarkable agreement with the motion predicted by global plate tectonic models. Most of this motion occurs within a 150 km wide zone bounded on the east by the Hikurangi Trough. Within this zone the motion is partitioned: near the Hikurangi Trough no slip is occurring at the upper surface of the subducting Pacific plate (the subduction thrust) and motion is predominantly thrusting normal to the trough axis: to the west is a region of predominantly dextral strike slip faulting. This pattern is consistent with Fitch's model of oblique subduction. To the south of the profile, a change is observed in the azimuth of the faulting along a line which marks the southern extent of the subduction slab, indicating the end of the partitioned motion.     

Crustal and upper-mantle structure in the Eastern Mediterranean from the analysis of surface wave dispersion curves

The dispersive properties of surface waves are used to infer earth structure in the Eastern Mediterranean region. Using group velocity maps for Rayleigh and Love waves from 7 to 100 s, we invert for the best 1-D crust and upper-mantle structure at a regular series of points. Assembling the results produces a 3-D lithospheric model, along with corresponding maps of sediment and crustal thickness. A comparison of our results to other studies finds the uncertainties of the Moho estimates to be about 5 km. We find thick sediments beneath most of the Eastern Mediterranean basin, in the Hellenic subduction zone and the Cyprus arc. The Ionian Sea is more characteristic of oceanic crust than the rest of the Eastern Mediterranean region as demonstrated, in particular, by the crustal thickness. We also find significant crustal thinning in the Aegean Sea portion of the backarc, particularly towards the south. Notably slower S -wave velocities are found in the upper mantle, especially in the northern Red Sea and Dead Sea Rift, central Turkey, and along the subduction zone. The low velocities in the upper mantle that span from North Africa to Crete, in the Libyan Sea, might be an indication of serpentinized mantle from the subducting African lithosphere. We also find evidence of a strong reverse correlation between sediment and crustal thickness which, while previously demonstrated for extensional regions, also seems applicable for this convergence zone.     

Structural evolution of sheared margin basins: the role of strain partitioning. Sørvestsnaget Basin,Norwegian Barents Sea

Spatio‐temporal analysis of basins formed along sheared margins has received much less attention than those formed along orthogonally extended margins. Knowledge about the structural evolution of such basins is important for petroleum exploration but there has been a lack of studies that document these based on 3D seismic reflection data. In this study, we demonstrate how partitioning of strain during deformation of the central and southern part of the Sørvestsnaget Basin along the Senja Shear Margin, Norwegian Barents Sea, facilitated coeval shortening and extension. This is achieved through quantitative analysis of syn‐kinematic growth strata using 3D seismic data. Our results show that during Cenozoic extensional faulting, folds and thrusts developed coevally and orthogonal to sub‐orthogonal to normal faults. We attribute this strain partitioning to be a result of the right‐lateral oblique plate motions along the margin. Rotation of fold hinge‐lines and indications of hinge‐parallel extension indicate that the dominating deformation mechanism in the central and southern Sørvestsnaget Basin during opening along the Senja Shear Margin was transtensional. We also argue that interpretation of shortening structures attributed to inversion along the margin should consider that partitioning of strain may result in shortening structures that are coeval with extensional faults and not a result of a separate compressional phase.     

A Pliocene–Quaternary compressional basin in the Interandean Depression, Central Ecuador

The segment of the Interandean Depression of Ecuador between Ambato and Quito is characterized by an uppermost Pliocene–Quaternary basin, which is located between two N-S trending reverse basement faults: the Victoria Fault to the west, and the Pisayambo Fault to the east. The clear evidence of E-W shortening for the early Pleistocene (between 1.85 and 1.21 Ma) favours a compressional basin interpretation. The morphology (river deviations, landslides, folded and flexure structures) demonstrates continuous shortening during the late Quaternary. The late Pliocene-Quaternary shortening reached 3400 ± 600 m with a rate of 1.4 ± 0.3 mm yr⁻¹. The E-W shortening is kinematically consistent with the current right-lateral reverse motion along the NE-SW trending Pallatanga Fault. The Quito-Ambato zone appears to act as a N-S restraining bend in a system of large right-lateral strike-slip faults. The compressive deformation which affects the Interandean Depression during the Pliocene is apparently coeval to the beginning subduction of very young oceanic lithosphere north of the Gulf of Guayaquil. The relatively buoyant new crust may have significantly increased the mechanical coupling in the subduction zone from Pliocene to Present.