Rheological Stratification Research Articles

Effects of rheological stratification and elasticity of lithosphere on subduction initiation

The breaking and bending of rigid and elastic lithosphere was probably essential for the initiation of plate subduction, although how this occurred is still poorly understood. Here we test effects of rheological stratification and elasticity of lithosphere on subduction initiation, which are possibly resulting from thermal cracking and seawater penetration into the lithosphere. In addition to the strong influence of water on rheological behavior, the material rigidity is also sensitive to the development of crack and fluid saturation. Numerical modeling indicates that water-weakening and a low-rigidity lithosphere are essential for the initiation of plate subduction, and such conditions are likely to have arisen on the early Earth due to extensive thermal contraction of the planet. Our results indicate that the formation of thermal cracks and penetration of seawater play an important role on subduction initiation, and are likely to have operated on planets other than Earth. However, if the ocean is disappeared, fluid penetration is likely to cease and plate tectonics would have stopped due to increasing the strength and rigidity of the lithosphere.

Sustained indentation in 2-D models of continental collision involving whole mantle subduction

SUMMARY Continental collision zones form at convergent plate boundaries after the negatively buoyant oceanic lithosphere subducts entirely into the Earth’s mantle. Consequently, orogenesis commences, and the colliding continents are sutured together. During the collision, plate convergence and motion of the sutured boundary towards the overriding plate are manifest in its deformation, as is the case for the long-term (∼50 Ma) and nearly constant convergence rate at the India–Eurasia collisional zone that hosts the Himalaya. However, despite the long history of modelling subduction-collision systems, it remains unclear what drives this convergence, especially in models where subduction is driven solely by buoyancy forces. This paper presents dynamic self-consistent buoyancy-driven 2-D whole-mantle scale numerical models of subduction-and-collision processes to explore variations in density and rheological stratification of the colliding continent and overriding plate (OP) viscosity (a proxy for OP strength) that facilitate post-collisional convergence and collisional boundary migration. In models with a moderately buoyant indenting continent, the collisional boundary advance is comparatively low (0.1–0.6 cm yr–1), and convergence is driven by the dense continental lithospheric mantle that continues to subduct as it decouples from its deforming crust. Conversely, models with a highly buoyant indenting continent show sustained indentation at 0.5–1.5 cm yr–1 until the slab detaches. Furthermore, models with a weaker OP and lower backarc viscosity show an enhanced propensity for indentation by a positively buoyant continent. These models additionally highlight the role of whole mantle flow induced by the sinking of the detached slab in the lower mantle as it sustains slow convergence at an average rate of 0.36 cm yr–1 for ∼25 Myr after break-off as well as prevents the residual slab from educting. In previous buoyancy-driven partial mantle depth models such eduction does generally occur, given that free-sinking of the detached slab in the mantle is not modelled. Although these findings widen the understanding of the long-term convergence of indenting continents, the lower post-collisional advance rates (0.3–1.5 cm yr–1) compared to India’s approximate 1000–2000 km of northward indentation during the last 50 Myr attest to the need for 3-D models.

THE DISTINQUISHING FEATURES OF THE FAULTS IN THE PLATFORM COVER: RESULTS OF THE APPLICATION OF TECTONOPHYSICAL APPROACH TO THE STUDY OF THE TAMBEY HYDROCARBON DEPOSIT (YAMAL PENINSULAR)

The study was aimed to identify the features of the formation and regularities of manifestation of faults in the platform environment applying the tectonophysical approach to the study of the structure of the Tambey hydrocarbon deposit (northern Yamal), largest in the West Siberia. Such research is important in the oil and gas industry at the present stage of transition from the exploitation of declining unique and large deposits to exploration and exploitation of deposits of complex structure. The tectonophysical approach was applied consistently in three levels of research. Initial consideration was given to regular trends in the structure of the platform cover in the context of general tectonophysical ideas of disjunctive faults, their inner structure and formation features. Then, the identification of a network of large fault zones has been done at the regional level for the northern Yamal on the basis of the lineament analysis of the relief and optical modeling, three main stages of its formation have been reconstructed, and there have been identified the features of the state of stress, among other factors determining the Tambey deposit contours in three areas – western and northern Tambey and Tassyi. Finally, based on tectonophysical interpretation of 3D seismic attribute analysis data and elastoplastic modeling experiment results, for the northern Tambey area at the local level there were identified the faults zones, the features of their structures in rheologically stratified unit, and the paragenetic relationship with the regional-level structures. The study has shown that the structure of the sedimentary cover, whose formation is tectonically influenced by the adjacent mobile belts, is zone-block. It reflects the zone-block structure of the basement, though, in contrast, is not represented by narrow main-fault planes (1 st -order faults). The blocks in the cover contact along rather wide zones, the inner structure of which corresponds to the early stages of faulting and is represented by a dense network of the 2 nd -order fractures and faults. The fault zones are characterized by an inhomogeneous – segment – structure which is determined by an initially irregular development of deformations and complicated by rheological stratification of the sedimentary cover. Fault segments in relatively brittle rocks (sandstones) are composed of long faults whereas in more ductile (clayey) varieties these are wide parts of concentration of small faults and fractures. A style of the zone-block structure and the types of dynamic environments of its formation might be specific in different regions. The application of tectonophysical approach to the analysis of the geological-geophysical information, obtained for certain deposits, will make it possible to identify the structural conditions for hydrocarbon accumulation and migration in the sedimentary cover which is essential to choose an effective method of deposit exploitation.

Influence of magma-poor versus magma-rich passive margins on subduction initiation

We present a new numerical modelling study of subduction initiation at (hyper-extended) magma-poor and magma-rich continental passive margins. In particular, we test how the structure and rheological stratification of these two end-member types control the formation and thermo-mechanical evolution of subduction zones. The serpentinization of mantle lithosphere in a magma-poor continental rifted margin leads to rheological decoupling at the base of the continental crust, which induces shear localization during subsequent shortening. Under these conditions, a shear zone propagates into the mantle lithosphere and leads to subduction initiation at the transition between ocean and passive margin. In contrast, a magma-rich rifted continental margin is rheologically coupled, which creates a buttressing effect and transfer of deformation into the oceanic domain during shortening, where the subduction zone initiates. These results are quantitatively compared and in agreement with the geological record of subduction initiation in the Alps and Dinarides – Hellenides, where the relics of end-member types of continental rifted margins of the same Adriatic micro-continent bordering the Alpine Tethys and Neotethys oceans, respectively, are observed.

Pressure–temperature–time constraints on gneiss dome formation in an intracontinental orogen

AbstractThe Entia Gneiss Complex represents the mid‐crustal core of the intracontinental Alice Springs Orogen. Located in the Harts Range, central Australia, it is characterized by the development of a domal structure including two sub‐domes separated by a steeply dipping median high‐strain zone. Dominantly, orthogneiss basement crops out through a structurally overlying cover sequence represented by the Harts Range Group and records evidence of hydration, recrystallization, and partial melting of precursor Paleoproterozoic granulite facies assemblages at amphibolite facies conditions. The structurally lowest parts of the Harts Range Group were metamorphosed to peak conditions of 10.5 kbar and ~880°C during rift‐related magmatism at 480–460 Ma, immediately prior to the onset of the Alice Springs Orogeny at 450–300 Ma. By contrast, the underlying Entia Gneiss Complex records widespread metamorphism and wet melting at upper amphibolite facies conditions. Phase equilibrium modelling and in situ LA–ICP–MS geochronology of rare, low‐variance kyanite–garnet‐bearing metapelites indicate that maximum P–T conditions of ~9 kbar and ~680°C were reached between c. 360–330 Ma, demonstrating a distinctive metamorphic and temporal evolution relative to the overlying Harts Range Group that can be linked to rheological stratification. Based on the integration of our results with existing monazite, zircon, and titanite geochronology, we interpret doming of the Entia Gneiss Complex to have involved compressive ascent of rheologically weakened crust below a region of extending upper crust near the termination of the Alice Springs Orogeny. In this way, the Harts Range Group represents a cooled, locally extending thrust sheet over ductile basement quasi‐concurrent with doming. Texturally‐late sillimanite combined with increasing Y content in monazite indicates high‐temperature, kyanite‐grade metamorphism was closely followed by decompression (3–4 kbar drop from peak conditions) and rapid cooling below 600°C during emplacement of the Entia Gneiss Complex at shallower crustal levels. The findings of this study highlight the feedback between hydration, retrogression, rheological weakening, and strain accommodation, thus allowing better evaluation of the thermomechanical history of gneiss dome formation within a narrow (<80 km wide) preconditioned intracontinental corridor.Highlights First modern P–T–t framework for gneiss dome formation in the Entia Gneiss Complex, central Australia. The onset of metamorphism, partial melting and ductile flow at c. 360 Ma was catalysed by hydration of granulite facies basement during rift inversion. Rapid exhumation of partially molten crust from depths of >20 km is marked by fluid‐mediated resetting of monazite down to c. 310 Ma. Rheological stratification enabled and accelerated exhumation of the highest‐grade corridor of a narrow intracontinental orogen.

Rheological stratification in impure rock salt during long-term creep: morphology, microstructure, and numerical models of multilayer folds in the Ocnele Mari salt mine, Romania

Abstract. At laboratory timescales, rock salt samples with different composition and microstructure show variance in steady-state creep rates, but it is not known if and how this variance is manifested at low strain rates and corresponding deviatoric stresses. Here, we aim to quantify this from the analysis of multilayer folds that developed in rock salt over geological timescale in the Ocnele Mari salt mine in Romania. The formation is composed of over 90 % of halite, while distinct multiscale layering is caused by variation in the fraction of impurities. Regional tectonics and mine-scale fold structure are consistent with deformation in a shear zone after strong shearing in a regional detachment, forming over 10 m scale chevron folds of a tectonically sheared sedimentary layering, with smaller folds developing on different scales in the hinges. Fold patterns at various scales clearly indicate that during folding, the sequence was mechanically stratified. The dark layers contain more impurities and are characterised by a more regular layer thickness compared to the bright layers and are thus inferred to have higher viscosities. Optical microscopy of gamma-decorated samples shows a strong shape-preferred orientation of halite grains parallel to the foliation, which is reoriented parallel to the axial plane of the folds studied. Microstructures indicate dislocation creep, together with extensive fluid-assisted recrystallisation and strong evidence for solution–precipitation creep. This provides support for linear (Newtonian) viscous rheology as a dominating deformation mechanism during the folding. Deviatoric stress during folding was lower than during shearing in the detachment at around 1 MPa. We investigate fold development on various scales in a representative multilayer package using finite-element numerical models, constrain the relative layer thicknesses in a selected outcrop, and design a numerical model. We explore the effect of different Newtonian viscosity ratios between the layers on the evolving folds on different scales. By comparing the field data and numerical results, we estimate that the effective viscosity ratio between the layers was larger than 10 and up to 20. Additionally, we demonstrate that the considerable variation of the layer thicknesses is not a crucial factor to develop folds on different scales. Instead, unequal distribution of the thin layers, which organise themselves into effectively single layers with variable thickness, can control deformation on various scales. Our results show that impurities can significantly change the viscosity of rock salt deforming at low deviatoric stress and introduce anisotropic viscosity, even in relatively pure layered rock.

Joint estimate of the co-seismic 2011 Tohoku earthquake fault slip and post-seismic viscoelastic relaxation by GRACE data inversion

SUMMARY Satellite-derived gravity data offer a novel perspective for understanding the physics of megathrust earthquakes at subduction zones. Nonetheless, their temporal resolution and observational errors make it difficult to discern the different phases of the seismic cycle, as the elastostatic deformation (co-seismic) and the stress relaxation by viscous flow (post-seismic). To overcome these difficulties and to take advantage of the physical constraints on the temporal evolution and on the spatial pattern of the earthquake-induced gravity disturbances, we have jointly estimated the fault slip of the 2011 Tohoku earthquake and the rheological stratification by means of a Bayesian inversion of GRACE data time series and within the framework of spherically symmetric self-gravitating compressible viscoelastic Earth models. This approach, in addition to improve the exploitation of satellite-derived gravity data, allows us (i) to constrain the fault slip taking advantage of information from both the co- and post-seismic signatures and (ii) to investigate the trade-off between the fault slip and the shallow rheological stratification. In this respect, it can be used to improve the modelling of crustal displacements from GPS data, even if their higher accuracy and temporal resolution allow to discriminate well the co-seismic signature from the others.

Deformation of intrasalt competent layers in different modes of salt tectonics

Abstract. Layered evaporite sequences (LESs) comprise interbedded weak layers (halite and, commonly, bittern salts) and strong layers (anhydrite and usually non-evaporite rocks such as carbonates and siliciclastics). This results in a strong rheological stratification, with a range of effective viscosity up to a factor of 105. We focus here on the deformation of competent intrasalt beds in different endmember modes of salt tectonics, even though combinations are common in nature, using a combination of conceptual, numerical, and analog models, and seismic data. In bedding-parallel extension, boudinage of the strong layers forms ruptured stringers, within a halite matrix, that become more isolated with increasing strain. In bedding-parallel shortening, competent layers tend to maintain coherency while forming harmonic, disharmonic, and polyharmonic folds, with the rheological stratification leading to buckling and fold growth by bedding-parallel shear. In differential loading, extension and the resultant stringers dominate beneath suprasalt depocenters, while folded competent beds characterize salt pillows. Finally, in passive diapirs, stringers generated by intrasalt extension are rotated to near vertical and encased in complex folds during upward flow of salt. In all cases, strong layers are progressively removed from areas of salt thinning and increasingly disrupted and folded in areas of salt growth as deformation intensifies. The varying styles of intrasalt deformation impact seismic imaging of LES and associated interpretations. Ruptured stringers are often visible where they have low dips, as in slightly extended salt layers or beneath depocenters, but are poorly imaged in passive diapirs due to steep dips. In contrast, areas of slightly to moderately shortened salt typically have well-imaged, mostly continuous intrasalt reflectors, although seismic coherency decreases as deformation intensifies. Similarly, wells are most likely to penetrate strong layers in contractional structures and salt pillows, less likely in extended salt because they might drill between stringers, and unlikely in tall passive diapirs because the stringers are near vertical. Thus, both seismic and well data may be interpreted to suggest that diapirs and other areas of more intense intrasalt deformation are more halite rich than is actually the case.

Rheological stratification and spatial variations in the effective elastic thickness of the lithosphere underneath the central Anatolian region, Turkey

In this study, the regional components of global model EGM08 Bouguer anomalies obtained by low pass filtering were inverted to map the geometries of Moho and Lithosphere-Asthenosphere Boundary (LAB) of the central Anatolian region. It was determined the Moho and LAB depths in the region to be 35.8–41.2 km and 67–91 km, respectively. The results from rheological modeling indicate mechanical decoupling of the crust and uppermost lithospheric mantle in eastern part and coupling in the western part of the study area. We also compare the rheological stratification with the focal depth distribution of earthquakes to examine the possible discrepancies between the brittle-ductile transition zone and the maximum depths of earthquakes along the selected profiles. The spatial variations of effective elastic thicknesses (EET) of the lithosphere have been estimated from the strength of the crust and lithospheric mantle by implying deformation gradient at Moho and LAB. The EET values vary in the range of 19–24.3 km. Although the EET values are relatively high in the eastern part of the region, lower EET values are directly underlain by thinned lithosphere of northwestern and southwestern part of central Anatolian region. We also analyze the crustal rheologies obtained from the lithospheric strength by delineating the pattern of crustal seismic activities.

Evidence for dehydration-modulated small-scale convection in the oceanic upper mantle from seafloor bathymetry and Rayleigh wave phase velocity

It is well established that the evolution of oceanic lithosphere departs from the predictions of the half-space cooling model (HSC). Understanding the magnitude, spatial distribution and physical origin of this departure is fundamental to our understanding of plate tectonics and the Earth's thermal evolution. Here we analyze Rayleigh wave phase velocities and seafloor topography separately along spreading-history trajectories, which track each piece of seafloor since its formation at the mid-ocean ridge, in the Pacific and the Atlantic basins. We identify the age along each trajectory at which HSC fails to satisfy the observations. Bathymetry and seismic velocity yield consistent results on whether or not HSC gives a statistically better fit to the observations than the plate cooling model at 80% of the locations we examine. Locations of HSC failure identified by velocity but not topography overlap with SS precursor detections in the Pacific, suggesting partial melting at these places. For the locations where the two data sets agree, in the Atlantic the probability of HSC failure is correlated with the axial depth of the present-day ridge. Along trajectories for which the present-day ridge is shallow (deep), HSC failure occurs later (earlier). A similar dependence in the Pacific is difficult to observe since both ridge depth and HSC failure age vary little. We show that the patterns of HSC failure are best explained by small-scale convection in the oceanic upper mantle for which the onset time is governed by rheological stratification created by partial melting at the ridge.

The impact of crustal rheology on natural seismicity: Campi Flegrei caldera case study

We analyze the crustal rheology beneath the active resurgent Campi Flegrei caldera (CFc) in Southern Italy by modelling the 3D brittle-ductile (B/D) transition, based on available thermal, geological and geophysical data. Firstly, the thermal field in the conductive physical regime is modeled using a finite element method; based on an optimization tool, this method is applied to evaluate the location and dimensions of the deep thermal source beneath the caldera. A horizontally-extended thermal anomaly located at about 5000 m depth below sea level is identified beneath Pozzuoli Bay, a part of the CFc. The same isotherm is located at a depth of 20,000 m beyond the caldera. This indicates a higher horizontal temperature gradient in the caldera with respect to the surrounding area. Next, we utilize this thermal model to image the 3D rheological stratification of the shallow crust below the caldera with two different values of strain rates. Within the caldera, the B/D transitions with ɛ equal to 10−12 s−1 and 10−8 s−1 are located at 3000 m and 5000 m depths, respectively. Outside the caldera, the transition is very deep (15,000–20,000 m), seemingly uninfluenced by the thermal state of the CFc volcanism. Finally, we compare these results with the spatial distribution of earthquake hypocenters, Benioff strain release and b-value distribution to investigate the relationship between crustal rheology and seismicity characteristics. Our analysis reveals that the image of the B/D transition is in agreement with the distribution of earthquake hypocenters, constraining the potential seismogenic volume of the region. Our study demonstrates that knowledge of the rheological state of a volcanic system is an important element to interpret its dynamic, forecast future activity and improve evaluation of the associated seismic hazard.

The 21 August 2017 Ischia (Italy) Earthquake Source Model Inferred From Seismological, GPS, and DInSAR Measurements

AbstractThe causative source of the first damaging earthquake instrumentally recorded in the Island of Ischia, occurred on 21 August 2017, has been studied through a multiparametric geophysical approach. In order to investigate the source geometry and kinematics we exploit seismological, Global Positioning System, and Sentinel‐1 and COSMO‐SkyMed differential interferometric synthetic aperture radar coseismic measurements. Our results indicate that the retrieved solutions from the geodetic data modeling and the seismological data are plausible; in particular, the best fit solution consists of an E‐W striking, south dipping normal fault, with its center located at a depth of 800 m. Moreover, the retrieved causative fault is consistent with the rheological stratification of the crust in this zone. This study allows us to improve the knowledge of the volcano‐tectonic processes occurring on the Island, which is crucial for a better assessment of the seismic risk in the area.

Experimental Investigation on the Effects of the Fixed Boundaries in Channelized Dry Granular Flows

The dynamics of granular mixtures, involved in several geophysical phenomena like rock avalanches and debris flows, is far from being completely understood. Several features of their motion, such as non-local and boundary effects, still represent open problems. An extensive experimental study on free-surface channelized granular flows is here presented, where the effects of the fixed boundaries are systematically investigated. The entire experimental data set is obtained by using a homogenous acetal-polymeric granular material and three different basal surfaces, allowing different kinematic boundary conditions. Velocity profiles at both the sidewall and the free surface are obtained by using high-speed cameras and the open-source particle image velocimetry code, PIVlab. Significantly, different sidewall velocity profiles are observed by varying the basal roughness and the flow depth. Owing to sidewall friction and non-local effects, such profiles exhibit a clear rheological stratification for high enough flow depths and they can be well described by recurring to composite functions, variously formed of linear, Bagnold and exponential scalings. Moreover, it has been discovered that transitions from one velocity profile to another are also possible on the same basal surface by merely varying the flow depth. This shape transition is due partly to the sidewall resistances, the basal boundary condition and, in particular, the occurrence/inhibition of basal grain rolling. In most of the experiments, the normal-to-bed velocity profiles and the velocity measurements at the free surface strongly suggest the occurrence of a secondary circulating flow, made possible by a chiefly collisional regime beneath the free surface.

Stability analysis of bilayer polymer fiber spinning process

The linear stability of a bilayer fiber spinning process is analyzed for axisymmetric disturbances under isothermal conditions. The co-extruded fiber consists of two concentric layers of rheologically stratified polymeric fluids. While the core fluid is an entangled polymer melt described using the eXtended Pom-Pom (XPP) model, the sheath layer is a low molecular weight unentangled polymer modeled as an Upper Convected Maxwell (UCM) fluid. The analysis indicates that, for fast extensional flows, such an arrangement enhances the critical draw ratio of the process over that found from the spinning of XPP fluid alone, thus delaying the onset of draw resonance and stabilizing the system. Further, the range of flow Deborah number for stable spinning is found to be much broader than that for the UCM fluid alone. For low to moderately elastic flows, the stability behavior is governed mainly by the rheology of the more-elastic core layer (XPP fluid), whereas for the highly elastic flows, the stability behavior is dominated by the less-elastic sheath layer (UCM fluid). The sensitivity of the stability diagram with respect to various spinning flow parameters, like the relative fraction of core and sheath layers, the rheological stratification of two fluids, and the spine-line dimensions, has also been examined in order to identify the region of enhanced stability such that the draw resonance is suppressed.

The implications of gas slug ascent in a stratified magma for acoustic and ground deformation source mechanisms in Strombolian eruptions

The interpretation of geophysical measurements at active volcanoes is vital for hazard assessment and for understanding fundamental processes such as magma degassing. For Strombolian activity, interpretations are currently underpinned by first-order fluid dynamic models which give relatively straightforward relationships between geophysical signals and gas and magma flow. However, recent petrological and high-speed video evidence has indicated the importance of rheological stratification within the conduit and, here, we show that under these conditions, the straightforward relationships break down. Using laboratory analogue experiments to represent a rheologically-stratified conduit we characterise the distinct variations in the shear stress exerted on the upper sections of the flow tube and in the gas pressures measured above the liquid surface, during different degassing flow configurations. These signals, generated by varying styles of gas ascent, expansion and burst, can reflect field infrasonic measurements and ground motion proximal to a vent. The shear stress signals exhibit timescales and trends in qualitative agreement with the near-vent inflation–deflation cycles identified at Stromboli. Therefore, shear stress along the uppermost conduit may represent a plausible source of near-vent tilt, and conduit shear contributions should be considered in the interpretation of ground deformation, which is usually attributed to pressure sources only. The same range of flow processes can produce different experimental infrasonic waveforms, even for similar masses of gas escape. The experimental data resembled infrasonic waveforms acquired from different vents at Stromboli associated with different eruptive styles. Accurate interpretation of near-vent ground deformation, infrasonic signal and eruptive style therefore requires detailed understanding of: a) spatiotemporal magma rheology in the shallow conduit, and b) shallow conduit geometry, as well as bubble overpressure and volume.

Plume-induced continental rifting and break-up in ultra-slow extension context: Insights from 3D numerical modeling

Breaking the lithosphere in extension without exceeding the driving far-field forces available on Earth is a tough quantitative modeling problem. One can tear it apart by propagation of an existing oceanic basin or weakness zone or one must assist rifting with magmatic processes, which drop the effective stress and weakens locally the lithosphere. While previous 3D models have demonstrated that non-cylindrical plumes produce almost cylindrical rift structures in a lithosphere under slight far-field loading, our contribution goes one step further by producing models of complete continental break-up. We investigate in details how the rheological stratification of the continental lithosphere interacting with active mantle plume influences the geometry and dynamics of rifting to continental break-up in 3D. We find that, irrespective of the rheological stratification, a plume-induced rifting process always occurs in two stages: an early crustal rifting stage and a late lithospheric necking (breakup) stage. In case of a rheologically decoupled lithosphere, initial brittle deformation is concentrated in the upper crust and strongly localized due to compensating ductile flow of lower-crustal material (core complex extension mode). On the contrary, rheological coupling between upper crust and lithospheric mantle results in highly distributed brittle deformation in the crust above mantle plume head (wide rift mode). Both core complex-like and wide rifting are followed by an abrupt transition to narrow rift stage when a localized ascent of mantle plume material focuses high strain along faults zones breaking through the entire lithosphere. The Main Ethiopian Rift, the Basin and Range province, and the East Shetland Basin may be natural examples of regions that have passed through these two stages of extension. Across-strike and along-strike asymmetry of break-up patterns arising spontaneously within initially symmetrical and laterally homogenous environment seems to be an intrinsic characteristic of plume-induced rifting.

The role of thermo-rheological properties of the crust beneath Ischia Island (Southern Italy) in the modulation of the ground deformation pattern

In this paper we develop a model of the ground deformation behaviour occurred at Ischia Island (Southern Italy) in the 1992–2010 time period. The model is employed to investigate the forces and physical parameters of the crust controlling the subsidence of the Island. To this aim, we integrate and homogenize in a Finite Element (FE) environment a large amount of data derived from several and different observation techniques (i.e., geological, geophysical and remote sensing). In detail, the main steps of the multiphysics model are: (i) the generation of a 3D geological model of the crust beneath the Island by merging the available geological and geophysical information; (ii) the optimization of a 3D thermal model by exploiting the thermal measurements available in literature; (iii) the definition of the 3D Brittle/Ductile transition by using the temperature distribution of the crust and the physical information of the rocks; (iv) the optimization of the ground deformation velocity model (that takes into account the rheological stratification) by considering the spatial and temporal information detected via satellite multi-orbit C-Band SAR (Synthetic Aperture Radar) measurements acquired during the 1992–2010 time period. The achieved results allow investigating the physical process responsible for the observed ground deformation pattern. In particular, they reveal how the rheology modulates the spatial and temporal evolution of the long-term subsidence phenomenon, highlighting a coupling effect of the viscosities of the rocks and the gravitational loading of the volcano edifice. Moreover, the achieved results provide a very detailed and realistic velocity field image of the subsurface crust of the Ischia Island Volcano.

Crustal and upper mantle velocity model along the DOBRE-4 profile from North Dobruja to the central region of the Ukrainian Shield: 2. geotectonic interpretation

This part of the paper addresses the geotectonic interpretation of the velocity model obtained from the results of seismic studies under the DOBRE-4 project in Ukraine. The velocity field does not show distinct lateral changes from the Precambrian platform towards the younger tectonic structures in the southwest. Hence, based on the seismic data alone, it is not possible to recognize the tectonic units that are known on the surface. The Moho has an undulating pattern over an interval with a length of ~150 km. The amplitude of the undulations reaches 8 to 17 km. The similar wavelike behavior, although on a shorter spatial scale and lower amplitude, is also typical of the upper crust and upper mantle. The presence of several separate horizons in the folded crust revealed by the velocity model is consistent with the presence of the folded systems which have different extensions on the different depth levels in the Earth’s crust. This situation is believed to be typical of folding on the lithospheric scale and to reflect the rheological stratification of the crust. The DOBRE-4 velocity section of the crust and adjacent part of the mantle promotes a clearer view of the geodynamical model describing the formation of the southwestern part of East European Platform in the Early Precambrian from the plate’s tectonic standpoint.

Miocene structural evolution and exhumation of the Ximeng dome in Yunnan, southeastern Tibet: Implications for intraplate deformation during extrusion of the Sundaland block

The Ximeng complex is located in the eastern Shai Thai block, a part of the extruding Sundaland block in southeastern Tibet. The complex is mainly composed of Neoproterozoic medium-grade metamorphic rocks and Early Paleozoic granite in the core surrounded by Paleozoic low-grade metamorphic rocks as a cover. The two structural units are separated by a tectonic contact (TDC). Detailed structural analysis reveals that the structural framework of the Ximeng complex is a dome structure and the complex experienced a primary and progressive shearing during doming. The rheological stratification led to the differential behavior of rocks from different stratigraphic and crustal levels during shearing. Zircon U-Pb ages of the Laojiezi granites in the core range from 465.50±2.70 to 430.37±0.93Ma. The cooling path from 40Ar-39Ar and apatite fission track data indicates that the complex experienced two stages of Cenozoic cooling, i.e., an early rapid cooling and a subsequent slow cooling. The early rapid cooling and exhumation of the complex are attributed to doming and accompanied by middle to upper crustal flow from ca. 23 to 20Ma.Shearing, cooling and exhumation of the middle to lower crustal rocks are widely distributed along the boundary fault zones (i.e., the Ailao Shan Red River shear zone and the Sagaing shear zone) of the extruding Sundaland block and within the triangular area between them. The doming, ductile shearing and exhumation of the Ximeng complex highlight the importance of intraplate deformation coeval with Indian-Eurasian collision. Localized crustal flow within the Sundaland block is possibly an important process during the extrusion of the Sundaland block.

Evolution of Meso-Cenozoic lithospheric thermal–rheological structure in the Jiyang sub-basin, Bohai Bay Basin, eastern North China Craton

The Meso-Cenozoic lithospheric thermal–rheological structure and lithospheric strength evolution of the Jiyang sub-basin were modeled using thermal history, crustal structure, and rheological parameter data. Results indicate that the thermal–rheological structure of the Jiyang sub-basin has exhibited obvious rheological stratification and changes over time. During the Early Mesozoic, the uppermost portion of the upper crust, middle crust, and the top part of the upper mantle had a thick brittle layer. During the early Early Cretaceous, the top of the middle crust’s brittle layer thinned because of lithosphere thinning and temperature increase, and the uppermost portion of the upper mantle was almost occupied by a ductile layer. During the late Early Cretaceous, the brittle layer of the middle crust and the upper mantle changed to a ductile one. Then, the uppermost portion of the middle crust changed to a thin brittle layer in the late Cretaceous. During the early Paleogene, the thin brittle layer of the middle crust became even thinner and shallower under the condition of crustal extension. Currently, with the decrease in lithospheric temperature, the top of the upper crust, middle crust, and the uppermost portion of the upper mantle are of a brittle layer. The total lithospheric strength and the effective elastic thickness (T e) in Meso-Cenozoic indicate that the Jiyang sub-basin experienced two weakened stages: during the late Early Cretaceous and the early Paleogene. The total lithospheric strength (approximately 4–5 × 1013 N m−1) and T e (approximately 50–60 km) during the Early Mesozoic was larger than that after the Late Jurassic (2–7 × 1012 N m−1 and 19–39 km, respectively). The results also reflect the subduction, and rollback of Pacific plate is the geodynamic mechanism of the destruction of the eastern North China Craton.