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Lithospheric strength variations in Mainland China: Tectonic implications
No full textWe present a new thermal and strength model for the lithosphere of Mainland China. To this purpose, we integrate a thermal model for the crust, using a 3-D steady state heat conduction equation, with estimates for the upper mantle thermal structure, obtained by inverting a S wave tomography model. With this new thermal model and assigning to the lithospheric layers a "soft" and "hard" rheology, respectively, we estimate integrated strength of the lithosphere. In the Ordos and the Sichuan basins, characterized by intermediate temperatures, strength is primarily concentrated in the crust, when the rheology is soft, and in both the crust and upper mantle, when the rheology is hard. In turn, the Tibetan Plateau and the Tarim basin have a weak and strong lithosphere mainly on account of their high and low temperatures, respectively. A comparison of temperatures, strength, and effective viscosity variations with earthquakes distribution and their seismic energy released indicates that both the deep part of the crust and the upper mantle of the Tibetan Plateau are weak and prone to flow toward adjacent areas. The high strength of some of the tectonic domains surrounding Tibet (Tarim, Ordos, and Sichuan basins) favors the flow toward the weak western part of South China block
Contrasts of seismic velocity, density and strength across the Moho
No full textWe provide an overviewof contrast of elastomechanical parameters across the Moho, basically contrasts in seismicwave
velocities, density and yield strength. These can be regarded as dynamic and quasi-static endmembers
of elastomechanical parameters of the crust–mantle transition. With respect to practical applications they are
closely related because seismic investigations play the role of exploring the earth structure and physical state
forming the boundary conditions for determining density and rheological models. We address both average
global Moho models and petrophysical and structural factors that cause deviations from the global averages,
such as variations in Moho depth, temperature and rock composition, fabric (anisotropy) and macro-scale heterogeneity.
Besides principle considerations these factors are put in a regional context in order to demonstrate
howthey are related to past and present tectonic processes. Seismic velocity and density contrasts are found between
0 and 25% whereas strength or viscosity contrastsmay be orders of magnitude higher, but may vanish as
well or showa discontinuity of higher order only. Especially, oceanic and old cratonic crust and near-Mohomantlemay
appear as a rheological unit although they are seismically well distinguishable. In terms of seismic velocities
the Moho may “vanish” mainly under the influence of serpentinization or eclogitization. Originally defined
as a first-order discontinuity in seismic velocity, the Moho has turned out to be an interface across which other
seismic properties, such as seismic velocity gradients, anisotropy and heterogeneity scale parameters, can
change strongly, too. However, knowledge of these parameters, as well as their relation to the rheology of the
crust–mantle transition, is still restricted to local or regional examples so no global conclusions can be drawn
How rigid is Europe's lithosphere?
No full textThe integrated strength distribution and variations of the effective elastic plate thickness (Te) have been estimated for the European lithosphère based on thermal and rheological data for the crust and upper mantle. The new results show a significant spatial variability demonstrating that both 'jelly sandwich' and 'crème brÛlée' models might be valid depending on lithospheric physical conditions. In most of Europe crustal strength provides a relatively large contribution (50%) to the lithospheric strength. Western Europe appears mostly characterized by mechanically decoupled lithospheric layers, low strength and Te < 30 km. The contribution of the mechanically strong mantle to Te is low in most parts of western Europe. No clear relationship between Te and thermal age is found in the continent: the values for the tectonic provinces older than 85 Ma are significantly smaller than theoretically expected for their age and crustal thickness, whereas the opposite is true for the younger provinces
Upper plate deformation as marker for the Northern STEP fault of the Ionian slab (Tyrrhenian Sea, central Mediterranean)
Full text linkThe Eastern Tyrrhenian margin (ETM), the active boundary of the Tyrrhenian Sea backarc basin, is the key for understanding the geodynamics of the central Mediterranean. Numerous seismic tomography studies have been carried out in this region, proposing different reconstructions of the lower subducting plate and cause of the slab-break-off existing beneath the Southern Apennines. However, the area and mode of the recent deformation of the Tyrrhenian Sea are still not fully defined and understood. In this study, we combine the analysis of a recent seismic tomography model and geological data, in order to understand the relationship between the subducting lower plate and the tectonic evolution of the sedimentary basins formed on the upper plate.With this aim, we interpreted a large data set of seismic reflection profiles and several well logs. The results consist in 2D and 3D geological models of the basins, sedimentary infill, and fault networks. Taking into account the geological data of the ETM and those of the adjacent inner flank of the Apennines, we observe: (i) a system of linked sedimentary basins developed on a narrow deformation belt bounded by transform fault zones; (ii) a polyphase rifting within the upper plate; (iii) an abrupt change of the direction of extension (~. 90°), from NE-oriented in the Lower Pleistocene to SE-oriented in the Middle Pleistocene. Since these ETM features are not the typical expressions of the current backarc extensional models, we propose a link between the evolution of upper plate and the onset and development of a STEP (Subduction-Transform-Edge-Propagator) fault along the northern margin of the Ionian slab
Variations of the lithospheric strength and elastic thickness in North America
No full textWe evaluate the effect of temperature variations on strength and effective elastic thickness (Te) of the lithosphere of the North American (NA) continent. To this purpose, we use two thermal models that are corrected for compositional variations and anelasticity effects in the upper mantle. These thermal models are obtained from a joint inversion of gravity data and two recent seismic tomography models (NA07 and SL2013sv). The crustal rheology was defined using NACr14, the most recent NA crustal model. This model specifies seismic velocities and thickness for a three-layer model of the crystalline crust. Strength in the lithosphere and in the crust has similar distributions, indicating that local geotherms play a dominant role in determining strength rather than crustal composition. A pronounced contrast is present in strength between cratonic and off-cratonic regions. Lithospheric strength in the off-cratonic regions is prevalently localized within the crust and Te shows low values (150 km). In contrast to previous results, our models indicate that Phanerozoic regions located close to the edge of the cratons, as the Appalachians, are characterized by low strength. We also find that locally weak zones exist within the cratons (e.g., beneath the intracratonic Illinois Basin and Midcontinent rift). Seismic tomography models NA07 and SL2013sv differ mainly in some peripheral parts of the cratons, as the Proterozoic Canadian Platform, the Grenville, and the western part of the Yavapai-Mazatzal province, where the integrated strength for the model NA07 is 10 times larger than in model SL2013sv due to a temperature difference (>200°C) in the uppermost mantle. The differences in Te between the two models are less pronounced. In both models, Proterozoic regions reactivated by Meso-Cenozoic tectonics (e.g., Rocky Mountains and the Mississippi Embayment) are characterized by a weak lithosphere due to the absence of the mechanically strong part of the mantle lithospheric layer. Intraplate earthquakes are distributed along the edges of the cratons, demonstrating that tectonic stress accumulates there, while the cores of the cratons remain undeformed. In both models, intraplate earthquakes occur in weak lithosphere (∼0.5 × 1013 Pa s, Te ∼ 15 km) or near the edges of strong cratonic blocks, characterized by pronounced contrasts of strength and Te. Key Points: We estimate strength and Te distribution of the North American continent Thermal more than compositional changes induce main strength and Te variations Seismicity is concentrated where large contrast of strength is estimate
From stretching to mantle exhumation in a triangular backarc basin (Vavilov basin, Tyrrhenian Sea, Western Mediterranean)
Full text linkIn this study, we describe the mode of extension of the Vavilov, a fossil backarc basin, triangle-shaped (approximately 240. km-wide and 200. km-long), located between Sardinia margin to the west and Campania margin to the east. We combine the analysis of recent geophysical and geological data, in order to investigate the relationship between the crustal/sedimentary structure and the tectonic evolution of both apex and bathyal parts of the basin. With this aim, we interpret a large data set of multichannel seismic reflection profiles and several well logs. We observe that the apex basin corresponds to a sediment-balanced basin, with a sedimentary infill recording the episodes of basin evolution. In contrast, the distal basin corresponds to an underfilled basin, characterized by localized volcanic activity and a thin sedimentary succession that covers the exhumed mantle. The basin architecture reveals the occurrence of rift and supradetachment basins in the Vavilov rift zone. We find that the rifting of the Vavilov triangular basin was synchronous from the apex to distal regions around a single Euler pole located in Latium, between 5.1 and 1.8. Ma. The kinematic evolution of the Vavilov basin occurred in two stages: initial pure shear mode (5.1-4.0. Ma) that produced high-angle normal faults and syn-sedimentary wedges, followed by simple shear mode (4.0-1.8. Ma) that caused supradetachment basins filled by a Transgressive-Regressive succession that documents high subsidence rates (1.22. mm/y) in the apex region. The final stage of extension in the distal region led to: (i) complete embrittlement of the crust; (ii) direct continuation of crustal faults to upper mantle depth; (iii) serpentinization and mantle exhumation. Based on constraints on the present-day crustal structure of the Vavilov basin, we obtain a stretching value (β. =3.5) and extension rates (3. cm/y) in the bathyal zone analogous to those reported for magma-poor rifted margins
Global strength and elastic thickness of the lithosphere
No full textThestrengthand effective elasticthickness (Te) ofthelithosphere control its response to tectonic and surface processes. Here, we present the first globalstrengthand effective elasticthickness maps, which are determined using physical properties from recent crustal and lithospheric models. Pronounced strength contrasts exist between old cratons and areas affected by Tertiary volcanism, which mostly coincide with the boundaries of seimogenic zones. Lithospheric strength is primarily controlled by the crust in young (Phanerozoic) geological provinces characterized by low Te (~ 25 km), high topography (> 1000 m) and active seismicity. In contrast, the old (Achaean and Proterozoic) cratons ofthe continental plates show strength primarily in the lithospheric mantle, high Te (over 100 km), low topography (< 1000 m) and very low seismicit
Global model for the lithospheric strength and effective elastic thickness
No full textGlobal distribution of the strength and effective elastic thickness (Te) of the lithosphere are estimated using physical parameters from recent crustal and lithospheric models. For the Te estimation we apply a new approach, which provides a possibility to take into account variations of Young modulus (E) within the lithosphere. In view of the large uncertainties affecting strength estimates, we evaluate global strength and Te distributions for possible end-member ‘hard’ (HRM) and a ‘soft’ (SRM) rheology models of the continental crust. Temperature within the lithosphere has been estimated using a recent tomography model of Ritsema et al. (2011), which has much higher horizontal resolution than previous global models. Most of the strength is localized in the crust for the HRM and in the mantle for the SRM. These results contribute to the long debates on applicability of the “crème brulée” or “jelly-sandwich” model for the lithosphere structure. Changing from the SRM to HRM turns most of the continental areas from the totally decoupled mode to the fully coupled mode of the lithospheric layers. However, in the areas characterized by a high thermal regime and thick crust, the layers remain decoupled even for the HRM. At the same time, for the inner part of the cratons the lithospheric layers are coupled in both models. Therefore, rheological variations lead to large changes in the integrated strength and Te distribution in the regions characterized by intermediate thermal conditions. In these areas temperature uncertainties have a greater effect, since this parameter principally determines rheological behavior. Comparison of the Te estimates for both models with those determined from the flexural loading and spectral analysis shows that the ‘hard’ rheology is likely applicable for cratonic areas, whereas the ‘soft’ rheology is more representative for young orogens
Marsili and Cefalù basins: The evolution of a rift system in the southern Tyrrhenian Sea (Central Mediterranean)
Full text linkThe Marsili Basin (in the southern Tyrrhenian Sea), whose mode of extension is still a controversial issue, is the youngest bathyal basin of the Central Mediterranean. A thin sedimentary cover in the basin permits to image basement fabric and structure by swath mapping and seismic reflection data. We investigate the crustal structure of the southern Tyrrhenian Sea, extending from the bathyal Marsili Basin to the adjacent Sicily continental margin. Interpretation of seismic reflection profiles (calibrated by well and dredge data) and crustal cross-sections were used to identify the stratigraphic infill, structural pattern and large-scale crustal features of the region. We recognized three basins in the southern Tyrrhenian Sea: (1) the Termini basin that is an overfilled sedimentary basin on the continental shelf area; (2) the Cefalù basin, located on the continental slope area and filled by thick deep water turbidite deposits; (3) the distal Marsili basin, filled by hemipelagic and thin distal turbidite deposits. The sequence stratigraphy interpretation permitted us to recognize fourth-order depositional sequences and the stratigraphic signature of the rift stages. An important increase in the sedimentary supply from the continental shelf to the bathyal basin occurred approximately over the last 0.5. Ma and is related to the uplift of the coastal area. The stratigraphic constrains indicate a Lower Pleistocene age for the opening of the southern Tyrrhenian Sea basin. The structural map reveals a complex fault pattern, in which coeval normal faults disclose a triangular basin from the Marsili bathyal basin to the Sicily continental margin, associated to an Euler pole in the northern Sicily. Tacking in account the faults pattern that developed in the whole Southern Tyrrhenian Sea during the Lower Pleistocene, we reconstructed two opposite triangular basins separated by a perpendicular rift. For analogy with the contiguous Vavilov basin, we propose that the extension in the Marsili basin reached the mantle exhumation stage
Applying local Green’s functions to study the influence of the crustal structure on hydrological loading displacements
No full textThe influence of the elastic Earth properties on seasonal or shorter periodic surface deformations due to atmospheric surface pressure and terrestrial water storage variations is usually modeled by applying a local half-space model or an one dimensional spherical Earth model like PREM from which a unique set of elastic load Love numbers, or alternatively, elastic Green's functions are derived. The first model is valid only if load and observer almost coincide, the second model considers only the response of an average Earth structure. However, for surface loads with horizontal scales less than 2500 km2, as for instance, for strong localized hydrological signals associated with heavy precipitation events and river floods, the Earth elastic response becomes very sensitive to inhomogeneities in the Earth crustal structure. We derive a set of local Green's functions defined globally on a 1° × 1° grid for the 3-layer crustal structure TEA12. Local Green's functions show standard deviations of ±12% in the vertical and ±21% in the horizontal directions for distances in the range from 0.1° to 0.5°. By means of Green's function scatter plots, we analyze the dependence of the load response to various crustal rocks and layer thicknesses. The application of local Green's functions instead of a mean global Green's function introduces a variability of 0.5–1.0 mm into the hydrological loading displacements, both in vertical and in horizontal directions. Maximum changes due to the local crustal structures are from −25% to +26% in the vertical and −91% to +55% in the horizontal displacements. In addition, the horizontal displacement can change its direction significantly. The lateral deviations in surface deformation due to local crustal elastic properties are found to be much larger than the differences between various commonly used one-dimensional Earth models
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