Extensional Stress Regime Research Articles

The Dauki fault at the Shillong plateau-bengal basin boundary in northeastern India: 2D finite element modeling

The Dauki fault is a major fault along the southern boundary of the Shillong plateau that may be a source of destructive seismic hazards for the adjoining areas, including northeastern Bangladesh. In this study, we simulated the present stress distribution and the ongoing convergence deformation within the study area using numerical simulation. This simulation attempts to determine the proper dip angle of the Dauki fault from a variety of differently established models but with the same realistic boundary conditions. The simulated stress field helped to predict the type of tectonic environment and the faulting patterns of the study area. The best-fit model can account for the compressive stress regime in the study area, except in the uppermost part of the Shillong plateau, the Bengal basin, and Assam areas where extensional stress regimes exists. In the model, the elements for which Mohr-Coulomb’s failure occurs correspond with the observed earthquake data that refer to the Dauki fault as a north-dipping thrust fault.

Mapping the Dyke Swarms of the Neoproterozoic Basement in Southwestern Jordan Using Remote Sensing and GIS Techniques

The current study documents for the first time the existence of four suites of dykes intruded the Neoproterozoic crustal rocks in southwestern Jordan. The oldest suite strike E-W followed by suites strike NE-SW, N-S and NW-SE which is the youngest one. Remote sensing and GIS techniques were used for mapping the dykes observed at the satellite image of the study area. Landsat Thematic Mapper color composite scenes allowed mapping of most the dyke swarms in the study area. Digital image processing technique was performed to map the dyke swarms in the study area. The emplacement of the dykes is consistent with extensional continental tectonic setting during the Precambrian time and before the deposition of the Cambrian sediments. The study gives an evidence for an anticlockwise rotation of the extensional stress regimes that caused these suites of dykes. Remote sensing and GIS technique used here could thus guide the interpretation and provide exact and accurate dyke mapping tools.

Tectonic development of the southern Karasu Valley, Turkey: successive structural events during basin formation

Abstract New fault data are presented for the Karasu Valley, southern Turkey. Field measurements concentrate on the Eocene–Miocene ( c. 48–7 Ma) sediments exposed on the south-western rift margin, in order to investigate the early development of this basin. Fault data show two trends in orientation NW–SE and NE–SW with a subordinate north–south trend. Stress inversions combined with field relationships indicate at least three phases of faulting. Firstly, an extensional event characterized by NE–SW and NW–SE normal faults, which are interpreted to have formed owing to flexural uplift in the forebulge region to the Bitlis–Zagros collisional front prior to the Middle Miocene. Secondly, north–south normal faults invert to give a stress ratio [ R =(σ 2 – σ 3 /σ 1 – σ 3 )] indicative of an extensional stress regime, transitional to strike-slip faulting. The final stress phase (Pliocene–Recent) is of strike-slip faulting and east–west-trending normal faulting. This stress regime is interpreted as the result of the propagation of the Dead Sea Fault or East Anatolian Fault. Previous models of rift formation have invoked either transpressional or transtensional origins for the area; the new data presented here indicate that the southernmost Karasu Valley developed through extension followed by transtension. Supplementary material: The fault data (including location, orientation, kinematic information) used in this study are available at www.geolsoc.org.uk/SUP18532 .

Anatomy of a microearthquake sequence on an active normal fault

The analysis of similar earthquakes, such as events in a seismic sequence, is an effective tool with which to monitor and study source processes and to understand the mechanical and dynamic states of active fault systems. We are observing seismicity that is primarily concentrated in very limited regions along the 1980 Irpinia earthquake fault zone in Southern Italy, which is a complex system characterised by extensional stress regime. These zones of weakness produce repeated earthquakes and swarm-like microearthquake sequences, which are concentrated in a few specific zones of the fault system. In this study, we focused on a sequence that occurred along the main fault segment of the 1980 Irpinia earthquake to understand its characteristics and its relation to the loading-unloading mechanisms of the fault system.

Thermo-hydro-mechanical modeling of carbon dioxide injection for enhanced gas-recovery (CO2-EGR): a benchmarking study for code comparison

The objective of this paper was to investigate the THM-coupled responses of the storage formation and caprock, induced by gas production, CO2-EGR (enhanced gas recovery), and CO2-storage. A generic 3D planer model (20,000 × 3,000 × 100 m, consisting of 1,200 m overburden, 100 m caprock, 200 m gas reservoir, and 1,500 m base rock) is adopted for the simulation process using the integrated code TOUGH2/EOS7C-FLAC3D and the multi-purpose simulator OpenGeoSys. Both simulators agree that the CO2-EGR phase under a balanced injection rate (31,500 tons/year) will cause almost no change in the reservoir pressure. The gas recovery rate increases 1.4 % in the 5-year CO2-EGR phase, and a better EGR effect could be achieved by increasing the distance between injection and production wells (e.g., 5.83 % for 5 km distance, instead of 1.2 km in this study). Under the considered conditions there is no evidence of plastic deformation and both reservoir and caprock behave elastically at all operation stages. The stress path could be predicted analytically and the results show that the isotropic and extensional stress regime will switch to the compressional stress regime, when the pore pressure rises to a specific level. Both simulators agree regarding modification of the reservoir stress state. With further CO2-injection tension failure in reservoir could occur, but shear failure will never happen under these conditions. Using TOUGH-FLAC, a scenario case is also analyzed with the assumption that the reservoir is naturally fractured. The specific analysis shows that the maximal storage pressure is 13.6 MPa which is determined by the penetration criterion of the caprock.

Intrusive mechanism of the 2008–2009 Mt. Etna eruption: Constraints by tomographic images and stress tensor analysis

Since January 2008, several geophysical parameters have evidenced a recharging phase at Mt. Etna volcano culminating with an effusive eruption that began on May 13, 2008. Seismic activity recorded at Mt. Etna from January 2007 to May 2008 was analyzed in order to provide seismological constraints to the volcano dynamics leading to the eruption. A total of 336 selected earthquakes, with ML≥1.5, were used as data source for this study. Specifically, we calculated 3D velocity and attenuation tomography, including a 3D relocation of the events, and we computed 53 selected fault plane solutions (FPSs) that were used for stress tensor inversion. The most important result obtained from the joint analysis of VP, VP/VS and P-wave attenuation is an anomalous zone with normal to high VP (values between 3.5 and 4.5km/s) and low VP/VS (values≤1.64), which partially overlaps with a low QP (values≤50) volume located along a NS trending channel beneath the central crater. This can be interpreted as a shallow volume characterized by high temperature where the magma is located with the presence of supercritical fluids. The analysis of seismic stress tensor evidenced an extensional regime in the depth range 3–13km with a vertically oriented σ1. This finding may suggest an extensional stress regime, probably related to the kinematic response of the volcanic edifice to both a deep magmatic intrusion and a condition of decreased regional compressive stress facilitated by sliding processes of the eastern flank of the volcano.

Slip preference on pre-existing faults: a guide tool for the separation of heterogeneous fault-slip data in extensional stress regimes

Synthetic fault-slip data have been considered in the present paper, in order to examine through a simple graphical manner the validity and use of the widely mentioned and applied criteria such as the slip preference, slip tendency, kinematic (P and T) axes, transport orientation and strain compatibility. The examination and description concern extensional stress regimes whose greatest principal stress axis (σ1) always remains in vertical position as in Andersonian stress states. In particular, radial extension (RE), radial–pure extension (RE–PE), pure extension (PE), pure extension–transtension (PE–TRN) and transtension (TRN) are examined with the aid of the Win-Tensor stress inversion software. In all of these extensional stress regimes only extensional faults can be activated. The lower dip angle of the reactivated faults is about 40° assuming that the coefficient of friction is no smaller than 0.6. The increase of the stress ratio and/or the fault dip angle up to 70° results in the increase of the slip deviation from the normal activation. Based on the present examination of the slip preference and slip tendency in different extensional stress regimes, a new simple and practical method is proposed herein in order to separate originally heterogeneous fault-slip data into homogeneous fault groups, by which different extensional stress regimes could be determined. The application of the method on the already published fault-slip data of Lemnos Island supports its validity since over 90% the resulted fault groups and stress regimes coincide to the already published ones.

Recent seismicity detection increase in the Santorini volcanic island complex

Abstract. Santorini is the most active volcanic complex in the South Aegean Volcanic Arc. To improve the seismological network detectability of the seismicity in this region, the Institute of Geodynamics of the National Observatory of Athens (NOA) recently installed 4 portable seismological stations supplementary to the 3 permanent stations operating in the region. The addition of these stations has significantly improved the detectability and reporting of the local seismic activity in the NOA instrumental seismicity catalogue. In this study we analyze quantitatively the seismicity of the Santorini volcanic complex. The results indicate a recent significant reporting increase mainly for events of small magnitude and an increase in the seismicity rate by more than 100%. The mapping of the statistical significance of the rate change with the z-value method reveals that the rate increase exists primarily in the active fault zone perpendicular to the extensional tectonic stress regime that characterizes this region. The spatial distribution of the b-value around the volcanic complex indicates a low b-value distribution parallel to the extensional stress field, while the b-value cross section of the volcanic complex indicates relatively high b-values under the caldera and a significant b-value decrease with depth. These results are found to be in general agreement with the results from other volcanic regions and they encourage further investigations concerning the seismic and volcanic hazard and risk estimates for the Santorini volcanic complex using the NOA earthquake catalogue.

Late Pleistocene folding above the Mail Arrouy Thrust, North-Western Pyrenees (France)

The Western Pyrenees have experienced several major earthquakes in the last 400years. Herein, we investigate the ongoing tectonic activity of the Arudy area affected by one of the largest earthquakes (M=5.1; 1980) registered in the instrumental seismicity catalogue in the Western Pyrenees. Folding of alluvial terraces younger than 17+/−3ky above the Mail Arrouy Thrust (MAT), Attest from a Late Pleistocene tectonic activity. This deformation resulted in a shallow fold with a wavelength of 2800m and amplitude of 7 to 8m. The MAT is rooted above a former normal fault of the Cretaceous Iberian margin, which is a potential seismogenic source.Such observations suggest that the northern flank of the Western Pyrenees is still subjected to shortening in response to a compressional stress regime. This compression is consistent with the African-Eurasian plate kinematics and the Arudy 1980 earthquake. Nonetheless, recent seismological data suggest the presence of an extensional stress regime in the range. This extension is indicated by some seismic events mostly observed in the Pyrenean high chain. Our hypothesis is that these extension-driven events could be due to a local stress-field induced by the elevation of the range. On the contrary, the compression, associated with the regional stress regime, could prevail in the outermost domain of the range.

How lithospheric subduction changes along the Calabrian Arc in southern Italy: geophysical evidences

Recent tomographic investigations performed down to ~300 km depth in the Calabrian Arc region gave insight in favor of the hypothesis that the Ionian subducting slab is continuous in depth beneath the central part of the Arc, while detachment of the deep portion of the subducting structure may have already taken place beneath the edges of the Arc itself. In the present study, we perform new geophysical analyses to further explore the structure of the subduction system and the structure and kinematics of the crustal units in the study area for a more comprehensive view of the local geodynamic scenario. Local earthquake tomography that we address to the exploration of the upper 40 km in the whole region of southern Italy furnishes P-wave velocity domains, suggesting southeast-ward long-term drifting of the southern Tyrrhenian unit with an advancement front matching well with the segment of Calabrian Arc where the subducting slab was found continuous and trench retreat can be presumed to have been active in the most recent times. This scenario of retreating subduction trench inducing drifting of the lithospheric unit overriding the subducting slab is further supported by the analysis of gravity anomalies, allowing us to better constrain the transitional zones between different subduction modes (continuous vs. detached slab) along the Arc. Also, the relocation of recent crustal seismicity, associated with geostructural data taken from the literature, provides evidence for NW-trending seismogenic structures in northeastern Sicily and northern Calabria that we interpret as Subduction-Transform Edge Propagator (STEP) faults guiding the southeast-ward drifting process of the southern Tyrrhenian unit. Crustal earthquake relocations show also seismolineaments in southern Calabria corresponding to the NE-trending longitudinal structures of the Arc where the great shallow earthquakes of 28 December 1908, and 5 and 7 February 1783 occurred. Seismicity and the extensional stress regime detected in these structures find also reasonable location in the proposed scenario, being interpretable in terms of shallow response of the central segment of the Arc to slab rollback and trench retreat.

Petrology and tectonic evolution of the Kiskunhalas-NE fractured hydrocarbon reservoir, South Hungary

The Kiskunhalas-NE (KIHA-NE) fractured hydrocarbon reservoir is part of the structurally rather complex crystalline basement of the Great Hungarian Plain. In the course of petrologic and thermometric examinations various rock types of the investigated area have been classified and characterized. There are four basic lithological units in the area. In the lowest structural position orthogneiss is common, which according to its petrographic features is assumed to be identical to the orthogneiss body of the adjacent Janoshalma (JH) basement high (metamorphic peak temperature T < 580 °C according to Zachar and M. Toth 2004). The next rock unit upward is the highly mylonitized variety of the orthogneiss with textural features suggesting deformation in an extensional stress regime. In the higher section of the mylonite zone graphitic gneiss mylonite is characteristic, with a peak metamorphic T of 410±45 °C. The lithology in the shallowest position of the area is a graphitic carbonate phyllite, with a T of 375 ± 15 °C. Estimation of the deformation temperature for both mylonitic rocks results in approximately Tdef ~ 455 °C. All data together suggest that between the top (graphitic carbonate phyllite) and the bottom (orthogneiss) of the ideal rock column there is about 200 °C peak metamorphic temperature deviation. The two extreme metamorphic blocks probably became juxtaposed along an extensional fault zone in the basement at approximately 15 km depth.

The Italian present-day stress map

SUMMARY In this paper, we present a significant update of the Italian present-day stress data compilation not only to improve the knowledge on the tectonic setting of the region or to constrain future geodynamic models, but also to understand the mechanics of processes linked to faulting and earthquakes. In this paper, we have analysed, revised and collected new contemporary stress data from borehole breakouts and we have assembled earthquake and fault data. In total, 206 new quality-ranked entries complete the definition of the horizontal stress orientation and tectonic regime in some areas, and bring new information mainly in Sicily and along the Apenninic belt. Now the global Italian data set consists of 715 data points, including 499 of A–Cquality,representinganincreaseof37percentcomparedtothepreviouscompilation.The alignment of horizontal stresses measured in some regions, closely matches the ∼N–S firstorder stress field orientation of ongoing relative crustal motions between Eurasia and Africa plates. The Apenninic belt shows a diffuse extensional stress regime indicating a ∼NE–SW direction of extension, that we interpret as related to a second-order stress field. The horizontal stress rotations observed in peculiar areas reflect a complex interaction between first-order stress field and local effects revealing the importance of the tectonic structure orientations. In particular, in Sicily the new data delineate a more complete tectonic picture evidencing adjacent areas characterized by distinct stress regime: northern offshore of Sicily and in the Hyblean plateau the alignment of horizontal stresses is consistent with the crustal motions, whereas different directions have been observed along the belt and foredeep.

Impact of CO2geological sequestration on the nucleation of earthquakes

[1] Can CO2 storage cause earthquakes? What is the maximum possible earthquake magnitude resulting from CO2 injection? Here, as a theoretical case study we investigate these questions using coupled hydromechanical modeling with multiphase flow and seismological variables for quantifying earthquake magnitude and energy. Our simulations consider transient fluid flow and stress coupling, and the evolution of fault properties. We simulate CO2 injection into a reservoir-caprock system bounded by a subvertical normal fault subjected to different extensional stress regimes and over a range of initial fault permeability values. For our assumed system and injection rate, the simulation results show that sudden stress drop and fault slip primarily initiated along the fault portion intersecting the storage reservoir after a few months of injection when a sufficiently high reservoir pressure has been reached. The size of the rupture area, and consequently, the earthquake magnitude and energy, depends on initial horizontal-to-vertical stress ratio and fault permeability, which strongly influences the size of the pressurized area, and subsequent stress variations.

ARCHAEAN GOLD MINERALISATION DURING POST-OROGENIC EXTENSION IN THE NEW CONSORT GOLD MINE, BARBERTON GREENSTONE BELT, SOUTH AFRICA

The area around New Consort Gold Mine (NCGM) was complexly deformed during at least five distinct events named D1NC to D5NC. The D1NC to D3NC events involve progressive shearing and folding linked to the 3.25 to 3.23 Ga accretionary history of the Barberton greenstone belt (D1NC and D2NC), and to the subsequent ~3.1 Ga emplacement and doming of surrounding batholiths (D3NC). During D3NC the area experienced intense strain partitioning with the development of a network of shear zones that envelop structural domains characterised by locally unique deformation histories. Around NCGM, D3NC events involved early shearing (D3aNC) along the Consort Bar followed by two episodes of 600 m scale folding (D3bNC and D3cNC), resulting in a complex fold interference pattern. Pegmatites intruded during D3NC. The critical structures controlling gold mineralisation are D4NC extensional, 10 to 100 m scale, brittle-ductile shear fractures associated with kink bands and crenulation folds that formed concomitantly with gold mineralisation after all the D3NC structures had fully developed, marking a clearly separate event. This network of fractures is distributed along 100 to 200 m wide west-northwesterly, north-northwesterly and east-northeasterly trending corridors. The distribution of high-grade ore zones was controlled by the intersection orientation of D4NC fractures and suitable host lithologies, mainly the silicified hinge zones of D1NC folds and the siliceous Consort Bar. Because these host lithologies were complexly folded in D1NC-D3bNC-D3cNC fold interference patterns, the 3-D distribution of ore zones is highly discontinuous and complex. D5NC structures represent late reverse faults of unknown age, well developed in the nearby Albion Mine. Kinematic analyses and stress inversion using Bingham tensor solutions and a reduced moment tensorn method, of mineralised D4NC fractures along old gold workings at NCGM; consistently indicate a vertical σ1, and a horizontal northwesterly to southeasterly directed σ3, within an extensional stress regime. Results for the nearby Clutha, Albion and Woodstock mines are similar. This study suggests that gold mineralisation in the NCGM area can be linked to an extensional event that may have developed separately from the accretionary events shaping the craton, and may have coincided with deposition of the Dominion Group.

Slumping and mass transport deposition in the Nankai fore arc: Evidence from IODP drilling and 3‐D reflection seismic data

Multiple lines of evidence exist for a range of sediment mass movement processes within the shallow megasplay fault zone (MSFZ) area and the adjacent slope basin in the outer fore arc of the Nankai subduction zone, Japan. Diagnostic features observed in three‐dimensional reflection seismic data and in cores of the Integrated Ocean Drilling Program (IODP) document a multifarious mass movement history spanning ∼2.87 million years. Various modes and scales of sediment remobilization can be related to the different morphotectonic settings in which they occurred. From this evidence, we decipher the tectonic control on slumping and mass transport deposition in the Nankai fore arc. Three periods of intensified mass wasting coincided with pulses of enhanced activity on the splay fault: (1) an initial phase of juvenile out‐of‐sequence thrusting ∼1.95 to 1.7 Ma, (2) a reactivation phase between ∼1.55 and 1.24 Ma, and (3) at about 1 Ma, during a phase of uplift of the fore‐arc high and motion along the MSFZ. We suggest that slope oversteepening, extensional stress regimes, and lateral transmission of fluid overpressures may have preconditioned the slope sediments to fail. Individual mass‐wasting events may have been triggered by dynamic loading from earthquake waves and/or transient pulses of pore pressure along the splay fault. Overall, our results provide insights into the complicated interplay between tectonic and submarine mass movement processes. We demonstrate that detailed knowledge about the spatial and temporal distribution of submarine mass movements can be integrated into a holistic reconstruction of tectonostratigraphic evolution of accretionary margins.

Stress-state evolution of the brittle upper crust during compressional tectonic inversion as defined by successive quartz vein types (High-Ardenne slate belt, Germany)

Abstract: In the frontal part of the Rhenohercynian fold-and-thrust belt (High-Ardenne slate belt, Germany), two successive types of quartz veins, oriented normal and parallel to bedding respectively, are interpreted to reflect the early Variscan compressional tectonic inversion of the Ardenne–Eifel sedimentary basin. Fracturing and sealing occurred in Lower Devonian siliciclastic multilayers under very low-grade metamorphic conditions in a brittle upper crust. A geometrical and microthermometric analysis of these veins has helped to constrain the kinematic and pressure–temperature conditions of both vein types, allowing the reconstruction of the stress-state evolution in a basin during tectonic inversion. It is demonstrated that bedding-normal extension veins, which developed under low differential stresses and repeatedly opened and sealed (crack-seal) under near-lithostatic fluid pressures, reflect the latest stage of an extensional stress regime. Bedding-parallel veins, which developed at differential stresses that were still low enough to allow the formation of extension veins, cross-cut the bedding-normal veins and preceded the regional fold and cleavage development. These veins show a pronounced bedding-parallel fabric, reflecting bedding-normal uplift and bedding-parallel shearing under lithostatic to supra-lithostatic fluid pressures during the early stages of a compressional stress regime. This kinematic history corroborates that fluid overpressures are easy to maintain during compressional tectonic inversion at the onset of orogeny.

Spatial variation of the crustal stress field along the Ryukyu‐Taiwan‐Luzon convergent boundary

We applied an improved stress inversion method to a comprehensive data set of earthquake focal mechanisms to depict the pattern of crustal stress along the western convergent boundary of the Philippine Sea plate. Our results indicate that the crustal stress along the Ryukyu fore arc is segmented with boundaries at or near the places of seamount subduction, including the Tokara channel. An extensional stress regime is observed along the entire Ryukyu back arc, implying that back‐arc rifting may have extended northward to Kyushu. A triangular area near the southernmost terminus of the Ryukyu arc is characterized by a unique stress signature. The eastern boundary of this Ryukyu‐Taiwan Stress Transition coincides with the 123°E meridian where the Gagua ridge intercepts the Ryukyu trench; whereas its western boundary agrees remarkably well with the border between the postcollision and waning‐collision domains in northern Taiwan. The Taiwan collision zone is dominated by compression that rotates locally according to the structural configuration of the Lukang Magnetization High (LMH), suggesting that the LMH may be critical in controlling the local stress distribution. The stress signature of the Luzon arc–Taiwan collision reaches as far south as 19.5°N. The tectonic stress along the Manila trench–Luzon fore arc is dominated by a complex regime of extension that cannot be explained by simple plate bending or in‐slab membrane stress. Since this extensional regime is observed only south of ∼22°N, it probably marks the northern limit of the contemporary boundary between the subduction along the Manila trench and the collision in Taiwan.

Focal mechanisms of micro-earthquakes in the Dobrá Voda seismoactive area in the Malé Karpaty Mts. (Little Carpathians), Slovakia

We have analyzed 44 micro-earthquakes with magnitudes between 1.2 and 3.4, which occurred in the Dobrá Voda area, Slovakia, in the period 2001–2009. The epicentres of the micro-earthquakes form a cluster elongated in the ENE–WSW direction. This direction coincides with the orientation of the main fault systems in the area: Dobrá Voda and Brezová faults. The depths of the hypocentres vary from 1 km to 14 km. Three different methods were used to calculate the focal mechanisms: (a) a method using the polarities of Pg and Pn waves, (b) the P-wave amplitude inversion of moment tensors, and (c) the waveform inversion of moment tensors. The majority of the analyzed micro-earthquakes have a left-lateral strike-slip focal mechanism with weak normal or reverse components. The full moment tensors comprise significant non-double-couple (non-DC) components. The non-DC components are partly numerical errors of the inversion but might be also of a physical origin. The most accurate values of the non-DC components are obtained from the P-wave amplitude inversion. For this inversion, the isotropic component (ISO) and the compensated linear vector dipole component (CLVD) are mostly positive and well correlated. This might indicate tensile faulting. Adopting the model of tensile faulting, we estimated the mean ratio of P to S wave velocities in the focal area from the values of ISO and CLVD, v P / v S = 1.5–1.6. The three different datasets of the focal mechanisms have been inverted for the present-day tectonic stress in the Dobrá Voda area. The slip shear stress component criterion was applied in the stress inversion. The results of the three inversions are well-consistent and point to a high reliability and good accuracy of the inverted stress. The orientations of the principal stresses are (azimuth/plunge): σ 1 = 210–220°/5–25°, σ 2 = 70–105°/55–75°, and σ 3 = 305–315°/15–25°, and the shape ratio is R = 0.45–0.60. The azimuth is measured clockwise from the north and the plunge downwards from the horizontal plane. The retrieved maximum compression lies along the belt of the Malé Karpaty Mts. The local tectonic stress reflects complex tectonic conditions in the area. The presence of tensile faulting might point to an extensional stress regime in the area.

Late cenozoic stress field in the Cameli Basin, SW Turkey

A history of deformation has been determined for the Cameli Basin located in the western part of the major Fethiye–Burdur Fault Zone, interpreted as the on-land continuation of Pliny–Strabo fault system (e.g., eastern boundary of the Hellenic Arc). Inversion of fault slip vectors affecting Mio-Pliocene to Quaternary formations in the Cameli Basin, in the southwestern segment of the transtensional Fethiye–Burdur Fault Zone, yields two different normal faulting stress regimes characterized by a roughly orthogonal set of extensional axes; a NW–SE (N129 ± 19 °E) σ 3 axis and a NE–SW (N50 ± 16 °E) σ 3 axis. The orientation of fault sets is predominantly around the NE–SW direction in the major Fethiye–Burdur Fault Zone, making the extension NW–SE. The mean R values are 0.74 and 0.69 for both extensions indicating a triaxial stress state, which is clearly different from radial extension and from transitional to strike-slip stress state. The NW–SE extension is probably responsible for the formation of the Cameli Basin during Mio-Pliocene time. In contrast, the inversion of focal mechanism solutions of shallow earthquakes occurring within the Cameli basin identifies a present-day, predominantly extensional stress regime, characterized by an approximately N–S (N184 °E) σ 3 axis, which has an R value of 0.637 indicating a triaxial stress state. The nearly orthogonal extensional stress regimes seem to have been acting contemporaneously with each other at different intensities from the Mio-Pliocene onwards in SW Turkey. This may be attributed to the geodynamic effects related to the subduction of the African plate beneath Anatolia, diffusing along Hellenic and Cyprus arcs and in the west-southwestward extrusion of Anatolia. Our hypothesis is that the cause of the early NW–SE extension is the slab-pull force due to the subduction process along the Cyprus arc, considered to be dominant up to Plio-Quaternary. The later NE–SW to present-day ∼ N–S extension, dominant since the Plio-Quaternary, is related to the combined forces of the Anatolian extrusion and the subduction process along the Hellenic arc.

Eruptive versus non-eruptive behaviour of large calderas: the example of Campi Flegrei caldera (southern Italy)

Caldera eruptions are among the most hazardous of natural phenomena. Many calderas around the world are active and are characterised by recurrent uplift and subsidence periods due to the dynamics of their magma reservoirs. These periods of unrest are, in some cases, accompanied by eruptions. At Campi Flegrei caldera (CFc), which is an area characterised by very high volcanic risk, the recurrence of this behaviour has stimulated the study of the rock rheology around the magma chamber, in order to estimate the likelihood of an eruption. This study considers different scenarios of shallow crustal behaviour, taking into account the earlier models of CFc ground deformation and caldera eruptions, and including recent geophysical investigations of the area. A semi-quantitative evaluation of the different factors that lead to magma storage or to its eruption (such as magma chamber size, wall-rock viscosity, temperature, and regional tectonic strain rate) is reported here for elastic and viscoelastic conditions. Considering the large magmatic sources of the CFc ignimbrite eruptions (400–2,000 km3) and a wall-rock viscosity between 1018 and 1020 Pa s, the conditions for eruptive failure are difficult to attain. Smaller source dimensions (a few cubic kilometres) promote the condition for fracture (eruption) rather than for the flow of wall rock. We also analyse the influence of the regional extensional stress regime on magma storage and eruptions, and the thermal stress as a possible source of caldera uplift. The present study also emphasises the difficulty of distinguishing eruption and non-eruption scenarios at CFc, since an unambiguous model that accounts for the rock rheology, magma-source dimensions and locations and regional stress field influences is still lacking.