Rheology Of Lithosphere Research Articles

Seismic structure and rheology of the crust under mainland China

The crust and upper mantle in mainland China were relatively densely probed with wide-angle seismic profiling since 1958, and the data have provided constraints on the amalgamation and lithosphere deformation of the continent. Based on the collection and digitization of crustal P-wave velocity models along related wide-angle seismic profiles, we construct several crustal transects across major tectonic units in mainland China. In our study, we analyzed the seismic activity, and seismic energy releases during 1970 and 2010 along them. We present seismogenic layer distribution and calculate the yield stress envelopes of the lithosphere along the transects, yielding a better understanding of the lithosphere rheology strength beneath mainland China. Our results demonstrate that the crustal thicknesses of different tectonic provinces are distinctively different in mainland China. The average crustal thickness is greater than 65km beneath the Tibetan Plateau, about 35km beneath South China, and about 36–38km beneath North China and Northeastern China. For the basins, the thickness is ~55km beneath Qaidam, ~50km beneath Tarim, ~40km beneath Sichuan and ~35km beneath Songliao. Our study also shows that the average seismic P-wave velocity is usually slower than the global average, equivalent with a more felsic composition of crust beneath the four tectonic blocks of mainland China resulting from the complex process of lithospheric evolution during Triassic and Cenozoic continent–continent and Mesozoic ocean–continent collisions. We identify characteristically different patterns of seismic activity distribution in different tectonic blocks, with bi-, or even tri-peak distribution of seismic concentration in South Tibet, which may suggest that crustal architecture and composition exert important control role in lithosphere deformation. The calculated yield stress envelopes of lithosphere in mainland China can be divided into three groups. The results indicate that the lithosphere rheology structure can be described by jelly sandwich model in eastern China, and crème brulee models with weak and strong lower crust corresponding to lithosphere beneath the western China and Kunlun orogenic belts, respectively. The spatial distribution of lithospheric rheology structure may provide important constraints on understanding of intra- or inter-plate deformation mechanism, and more studies are needed to further understand the tectonic process(es) accompanying different lithosphere rheology structures.

Thermal State and Strength of the Lithosphere Beneath the Chinese Mainland

Abstract:The temperature distributions of the lithosphere underneath the mainland of China were estimated by applying local isostatic equilibrium‐constrained geothermal calculations. Maps of the lateral temperature variation at depths of 40, 70, and 100 km are presented for the whole Chinese continent, with the thermal thickness of the lithosphere is calculated. Lithospheric roots of 160–200 km thickness underlie Tarim and the Upper Yangtze platform, but are absent beneath the entire Sino‐Korean platform. In general, the Tibetan plateau and fold belts to the north have warm but thick lithospheres, whereas thinner thermal lithospheres have been identified in northern Tibet and central Tian Shan around Issyk‐Kul Lake. The warm and soft lithosphere in the Tibetan plateau and Tian Shan are caused by uniform north‐south shortening, which may represent a snapshot of the early stage of convective thinning of the convergent lithosphere. However, the lithospheric thinning beneath northeastern China might be related to volatile infiltration by dehydration of the deeply subducting Pacific slab during the Cenozoic. Dry and wet upper mantle rheology display “jelly sandwich” and “crème brûlée” pictures, respectively, demonstrating the mechanical behaviour of the Chinese lithosphere outside the Tibetan plateau. Considering a more geologically evident wet‐mantle rheology, the “crème brûlée” model can approximate the lithospheric rheology for the most earthquake‐prone regions on the Chinese mainland.

Rheology of the mantle and tectonics of the oceanic lithospheric plates

The modern concepts of the rheology of viscous mantle and brittle lithosphere, as well as the results of the numerical experiments on the processes in a heated layer with a viscosity dependent on pressure, temperature, and shear stress, are reviewed. These dependences are inferred from the laboratory studies of olivine and measurements of postglacial rebound (glacial isostatic adjustment) and geoid anomalies. The numerical solution of classical conservation equations for mass, heat, and momentum shows that thermal convection with a highly viscous rigid lithosphere develops in the layer with the parameters of the mantle with the considered rheology under a temperature difference of 3500 K, without any special additional conditions due to the self-organization of the material. If the viscosity parameters of the lithosphere correspond to dry olivine, the lithosphere remains monolithic (unbroken). At a lower strength (probably due to the effects of water), the lithosphere splits into a set of separate rigid plates divided by the ridges and subduction zones. The plates submerge into the mantle, and their material is involved in the convective circulation. The results of the numerical experiment may serve as direct empirical evidence to validate the basic concepts of the theory of plate tectonics; these experiments also reveal some new features of the mantle convection. The probable structure of the flows in the upper and lower mantle (including the asthenosphere), which shows the primary role of the lithospheric plates, is demonstrated for the first time.

Formation and upwelling of mantle diapirs through the cratonic lithosphere: Numerical thermomechanical modeling

This paper reports the results of the numerical modeling of gravitationally instable processes in the lithospheric mantle of ancient cratons. The gravitational instability is considered as a result of melting at the lithosphere base owing to its local heating by anomalous mantle. Modeling was based on a finite element method in 2D formulation and took into account the geological structure and thermomechanical parameters of the lithosphere of the Siberian platform. Numerical results revealed the main tendencies in the mantle diapirisim of the mafic and ultramafic magma ascending through the “cold” high-viscosity lithosphere. It was shown that the shape of diapiric magmatic bodies is controlled by realistic visco-elastic-plastic rheology of lithosphere. The ascent of diapir in lithosphere was modeled for diverse regimes differing in duration, temperature field, and upwelling depth. It was concluded that the ascent of melt through lithosphere to the crust-mantle boundary is mainly controlled by rheology, and conditions of oscillatory diapirism with recurrent magma replenishments were modeled. Modeling results may shed light on some features related to the trap magmatism of the Siberian igneous province. The duration and rate of magma upwelling as well as the parameters of periodical magma upwelling were estimated and attempt was made to explain the high-velocity seismic anomalies that were recorded in the subcrustal regions of the Siberian platform.

Constraints on fault and lithosphere rheology from the coseismic slip and postseismic afterslip of the 2006 Mw7.0 Mozambique earthquake

The 2006 Mw7.0 Mozambique (Machaze) normal‐faulting earthquake ruptured an unusually steeply dipping fault plane (∼75°). The amount of slip in the earthquake decreased from depths of ∼10 km toward the surface, and this shallow slip deficit was at least partly recovered by postseismic afterslip on the shallow part of the fault plane. An adjacent normal fault segment slipped postseismically (and possibly also co‐seismically) at shallow depths with a large strike‐slip component, in response to the stresses generated by slip on the main earthquake fault plane. Our observations suggest that the fault zone behaves in a stick‐slip manner in the crystalline basement, and that where it cuts the sedimentary layer the coseismic rupture was partially arrested and there was significant postseismic creep. We discuss the effects of such behavior on the large‐scale tectonics of continental regions, and on the assessment of seismic hazard on similar fault systems. The steep dip of the fault suggests the re‐activation of a preexisting structure with a coefficient of friction at least ∼25–45% lower than that on optimally oriented planes, and analysis of the deformation following an aftershock indicates that the value of the parameter ‘a’ that describes the rate‐dependence of fault friction lies in the range 1 × 10−3–2 × 10−2. The lack of long‐wavelength postseismic relaxation suggests viscosities in the ductile lithosphere of greater than ∼2 × 1019 Pa s, and an examination of the tectonic geomorphology in the region identifies ways in which similar fault systems can be identified before they rupture in future earthquakes.

Spatiotemporal distribution of the seismicity along the Mid‐Atlantic Ridge north of the Azores from hydroacoustic data: Insights into seismogenic processes in a ridge–hot spot context

The seismicity of the North Atlantic was monitored from May 2002 to September 2003 by the ‘SIRENA array’ of autonomous hydrophones. The hydroacoustic signals provide a unique data set documenting numerous low‐magnitude earthquakes along the section of the Mid‐Atlantic Ridge (MAR) located in a ridge‐hot spot interaction context. During the experiment, 1696 events were detected along the MAR axis between 40°N and 51°N, with a magnitude of completeness level ofmb≈ 2.4. Inside the array, location errors are in the order of 2 km, and errors in the origin time are less than 1 s. From this catalog, 15 clusters were detected. The distribution of source level (SL) versus time within each cluster is used to discriminate clusters occurring in a tectonic context from those attributed to non‐tectonic (i.e. volcanic or hydrothermal) processes. The location of tectonic and non‐tectonic sequences correlates well with regions with positive and negative Mantle Bouguer Anomalies (MBAs), indicating the presence of thinner/colder and thicker/warmer crust respectively. At the scale of the entire array, both the complete and declustered catalogs derived from the hydroacoustic signals show an increase of the seismicity rate from the Azores up to 43°30′N suggesting a diminishing influence of the Azores hot spot on the ridge‐axis temperature, and well correlated with a similar increase in the along‐axis MBAs. The comparison of the MAR seismicity with the Residual MBA (RMBA) at different scales leads us to think that the low‐magnitude seismicity rates are directly related to along‐axis variations in lithosphere rheology and temperatures.

Lithospheric thermal structure and rheology of the eastern China

Temperature distributions of the lithosphere beneath the eastern China were estimated by local isostasy equilibrium constrained geothermal calculation in this study. Maps of the lateral temperature variation at depths of 50 and 100km as well as Moho boundary are presented for the eastern China continent, and the 600°C and 1100°C isotherm depth maps are also given. Meanwhile, the thermal thickness of lithosphere is calculated as the depth of 1350°C isotherm. The rheological strength of lithosphere is calculated based on a four-layer model, which consists of the upper, lower, lowermost crust layers and a wet lithospheric mantle layer. The results show that, the lithosphere of 160–180km thick exists under Sichuan basin in the western part of Yangtze Craton, but keel is absent beneath the entire North China Craton. It is notable that the lithospheric thickness is only 130–140km beneath Ordos basin in the western part of North China Craton; accordingly, there is no lithospheric keel beneath Ordos basin. These geothermal modeling results are in good agreement with those of seismic tomography and other geothermal studies. The temperatures in the lower crust of the North China Craton estimated are in range of 500–600°C. Consequently, the composition of lower crust of the North China Craton should be more mafic than that of previous estimation based on much cooler geotherms. The spatial variation of lithospheric rheology in the eastern China is influenced by local geotherms and crustal compositions. The “crème brûlée” model, which is represent by a strong crust portion but a weak lithospheric mantle portion in vertical strength profile, approximates the lithospheric rheological layering for the regions of the western half of North China Craton and the northern part of NE China, which has thicker crust or moderate-to-high geotherm. On the other hand, the “jelly sandwich” model demonstrates the rheological layering of the western part of the Yangtze Craton and the eastern half of the North China Craton, which has low-to-moderate geotherm and/or thinner crust. In general, thermal age of the tectonic province controls the present-day thermal structure and rheology of the lithosphere of the eastern China.

Spatial distribution of seismic layer, crustal thickness, and Vp/Vs ratio in the Permian Emeishan Mantle Plume region

Seismological studies of lithospheric structure and rheology can provide important information regarding the lithosphere's interaction with the mantle plume and its successive deformation characterization. The Emeishan Large Igneous Province (ELIP) in eastern Tibet was probably produced by a Late Permian Emeishan mantle plume and experienced tectonically driven modifications during Mesozoic–Cenozoic, such as the eastward subduction of the Indian Ocean plate and roughly north-southward tectonic escape or middle crustal flow. The crustal responses to the Emeishan mantle plume and its modification from successive tectonic activities are still unclear. Here, we present the lithosphere rheology structure derived from seismic activity and the spatial distribution of seismic energy release, which records the lithospheric deformation from the Late Permian mantle plume activity and the Mesozoic–Cenozoic modifications. In addition, we estimate the crustal thickness and the average crustal Vp/Vs ratio from wide-angle seismic profiling and receiver function imaging. Our results demonstrate that the seismogenic layer thins away from the proposed center of the Emeishan mantle plume. The layer is approximately 24km in depth beneath the center of the Emeishan mantle plume and approximately 10km in depth beneath the margin of the plume, with corresponding crustal thinning and spatial variations of the average crustal Vp/Vs ratio. Distinctive patterns among crustal thickness, seismogenic layer and the average crustal Vp/Vs ratio are observed both east and west of the Xiao Jiang Fault (XJF). These remarkable features are interpreted to result from the modification of the Late Permian mantle plume, probably by tectonic escape in the west of the XJF and by a north-southward middle crustal flow in the east of the XJF.

Mantle rheology and the scaling of bending dissipation in plate tectonics

[1] Plate tectonics on Earth involves the bending deformation of plates at subduction zones, and because plates are generally considered to be stiff owning to the rheology of mantle minerals, the role of energy dissipation by plate bending in the global energy balance has been frequently debated in the recent literature. Here we consider how bending dissipation should scale with slab parameters such as dip angle, plate age, the radius of curvature, and plate velocity by systematically exploring the parameter space with instantaneous Stokes flow calculations. We derive the scaling of bending dissipation for a range of mantle viscosity functions, including pseudoplastic rheology with olivine flow laws. Our results indicate that, as we move away from the isoviscous case, the scaling gradually deviates from what has commonly been assumed in previous studies, most notably for the radius exponent, which exhibits more than threefold reduction and even a sign reversal in some cases. These modifications in scaling exponents originate in the complication of the deformation field caused by viscosity variations within the bending plate. Approximating the lithospheric rheology by a single effective viscosity in the dynamical models of subduction has been a common practice, but we suggest that such approximation may limit the geological relevance of modeling studies, in particular when estimating the significance of bending dissipation.

High-resolution imaging of the deep anisotropic structure of the San Andreas Fault system beneath southern California

SUMMARY Geological and geodetic records show that motion is distributed over a broad and complex system of faults in southern California, with many large earthquakes occurring off the main San Andreas fault. This unusually complex plate boundary geometry is imposed by the big bend of the San Andreas Fault and the dynamics of the underlying lithosphere, but lithosphere rheology and distribution of applied forces are still poorly constrained. Geodynamic modelling is thus very uncertain, and distribution of lithospheric deformations highly controversial. Here, exploiting the property that deformation orients olivine crystals, we show that deep lithospheric deformations can be mapped directly with a new imaging approach of seismic anisotropy, with a resolution fine enough for detailed geodynamic interpretation. This method relies entirely on finite-frequency effects in the splitting of SKS waves. We build an extensive data set of more than 3400 SKS splitting measurements from the analysis of SKS waves recorded by all the broad-band stations in southern California. Resolution tests demonstrate that, using this data set, we are able to resolve anisotropic structures smaller than the size of the first Fresnel zone of SKS waves. The good vertical resolution in our tomographic images results from the short inter-station spacing in southern California. In our 3-D anisotropy model, we find no evidence for localized lithospheric shear deformation beneath the San Andreas Fault. Instead, the lithospheric plate boundary is localized along a broad shear zone beneath the East California Shear Zone. Therefore, surface and deep deformation patterns are poorly correlated and most likely decoupled. Active N–S convergence in the Transverse Ranges results in strongly coherent E–W alignment of olivine fast axes in the shallow lithosphere. At the top of the asthenosphere, the observed low level of anisotropy suggests weak lithospheric basal tractions and thus a strong decoupling between the lithosphere and the asthenosphere.

Controls of lithospheric structure and plate kinematics on rift architecture and evolution: An experimental modeling of the Baikal rift

Analog models investigate the evolution and architecture of the Baikal rift in relation to the rheology of the extending lithosphere and rift kinematics. The models focus on the development of the narrow, deep, and asymmetric basins composing Lake Baikal and reproduce the extension between the strong Siberian craton and the weaker Sayan‐Baikal belt. Model results suggest that the presence of a near‐vertical weak suture separating the cratonic keel from the mobile belt represents the more convenient rheological configuration leading to a narrow rift characterized by prominent vertical motions and deep depressions. These depressions are typically asymmetric, and model results suggest that this asymmetry is a consequence of lateral variations in lithospheric rheology, which is in turn related to both the variation in thickness of the strong mantle and, more importantly, the variation in the brittle‐ductile transition depth between the craton and the belt. A significant shallowing of the brittle‐ductile transition in the crust passing from the craton to the belt is required to fit the asymmetric architecture of the Baikal basins, with a master fault on the craton side and a monocline with no significant faulting on the belt side. Analysis of the model deformation pattern suggests that the overall architecture of the basins hosting Lake Baikal is best fitted for a N140°E directed extension, similar to the current GPS‐derived motion and compatible with the stress field inferred on the basis of fault and focal mechanism data. This kinematics (along with the shape of the Siberian craton) exerted the major control on the plan view fault architecture and its along‐axis variations.

Long‐term growth and subsidence of Ascension Island: Constraints on the rheology of young oceanic lithosphere

The dating of material from deep boreholes drilled in volcanic ocean islands allows constraints to be placed on their growth and long‐term subsidence rates. We dated lavas from a 3 km geothermal borehole at Ascension Island by the laser‐heating 40Ar/39Ar technique. The samples yield ages of up to 3.4 Ma and volcanic growth rates of ∼0.4 km/Myr. The transition from submarine to subaerial eruption occurs at ∼710 m below present sea level and 2.5 Ma. Since 2.5 Ma, there has been ∼430–500 m of subsidence over and above the expected ∼190–260 m due to lithospheric cooling. Plausible elastic thicknesses and growth histories would generate a maximum elastic subsidence since 2.5 Ma of ∼200 m. We infer that the subsidence includes a component of viscous relaxation resulting from rapid loading prior to 2.5 Ma, and place constraints on the timescale of this relaxation, and hence the viscosity of the underlying lithosphere.

Ice age True Polar Wander in a compressible and non-hydrostatic Earth

SUMMARY Issues related to long timescale instability in the Earth's rotation, named True Polar Wander (TPW), have continuously been debated, after the pioneering works of the sixties. We show ice age TPW results from a newly developed compressible model, based on the numerical integration in the radial variable of the momentum and Poisson equations and on the contour integration in the Laplace domain which allows us to deal with the non-modal contribution from continuous radial rheological variations. We thus fully exploit the long term behaviour of the Earth's rotation and we quantify the effects of the compressible rheology, compared to the widely used incompressible one. We discuss the so-called ‘traditional approach’ to the Earth's rotation developed during the eighties and nineties, both for ice age and mantle convection TPW and we explain within this approach the sensitivity of TWP predictions to the elastic and viscoelastic rheologies of the lithosphere. We agree on the necessity to include the effects of the non-hydrostatic bulge from mantle convection to obtain realistic ice age TPW rates in the lower mantle viscosity range [1021, 1022] Pa s, as first indicated by Mitrovica et al. Their analysis represents a first attempt to couple the effects on TPW from mantle convection and glacial forcing, by including the non-hydrostatic bulge due to mantle convection but not the other time-dependent driving terms. This partial coupling freezes in space the non-hydrostatic contribution due to mantle convection, thus damping the present-day ice age TPW and forcing the axis of instantaneous rotation to come back to its initial position when ice ages started as discussed in Mitrovica et al. We also describe a peculiar behavior of the new ice age TPW predictions exhibiting a dampened pendulum motion, with the axis of instantaneous rotation overcrossing the position it had before ice ages started. We argue that a viscoelastic rather than elastic lithosphere should be adopted in the modelling of TPW although, on the time of ice ages, it is difficult to disentangle the effects of lithospheric rheology and of mantle convection. We discuss the implication of self-consistent convection calculations of the non-hydrostatic contribution and its impact on the long term Earth's rotation stability during ice ages. The ice age TPW cannot account for more than 70 per cent of the observed one, at least for lower mantle viscosities lower than 1022 Pa s: mantle convection must therefore contribute to the observed TPW.

Temporal and spatial variation of stress field in Taiwan from 1991 to 2007: Insights from comprehensive first motion focal mechanism catalog

The total amount of 4,761 focal mechanisms was determined based on P-wave first motion polarities from 1991 to 2007 in Taiwan region. This dataset offers us a good opportunity to examine temporal and spatial variability of the stress field. We find that the spatial variations of stress axes are mainly controlled by tectonic structures while the temporal changes are greatly influenced by the Chi-Chi earthquake. The orientation of the maximum horizontal compressive stress axes (S H) shows a general agreement with the direction of plate motion between a depth range of 0–30 km. The 20° anticlockwise rotation of S H from the Longitudinal Valley (LV) to western Taiwan is probably caused by the left-lateral motion on the Longitudinal Valley Fault (LVF) that has consumed part of the oblique motion of plate convergence. On the other hand, part of the oblique convergence is transferred into the Central Range and the Hsuehshan Range judging from counterclockwise rotation of S H from east to west and strike-slip faulting in the Hsuehshan Range. Most events with a depth greater than 30 km occur offshore eastern Taiwan and the azimuth of S H is close to E–W directed, different from NW–SE directed at shallow depths. This may infer the existence of the transition of lithosphere rheology in offshore eastern Taiwan. The trends of S H in the depth of 0–10 km are strongly affected by the coseismic stress change of the Chi-Chi earthquake. In the northern half of the Chi-Chi rupture area, the trends of S H rotate 30° clockwise and the stress ratio increased by a factor of six after the mainshock. The orientations of S H still differ by 30° in 2007 comparing to that in the period before the Chi-Chi earthquake. The variation of S H trend is more diverse in the southern half of the rupture area, showing 20° counterclockwise rotation immediately after the 1999 mainshock followed by a clockwise rotation. The trend of S H returns to the pre-seismic direction of 110° in 2001. These notable changes of S H before and after the Chi-Chi mainshock suggests that the magnitude of background stress in the rupture area is close to the coseismic stress drop. We also recognize a significant 10° counterclockwise rotation of S H in the entire Chi-Chi rupture area between 1991 and 1999 before the earthquake took place. However, to the south of the Chi-Chi rupture, the trends of S H remain little changed before and after the Chi-Chi earthquake.

Conductivity structure and rheological property of lithosphere in Southern Tibet inferred from super-broadband magnetotelluric sounding

To understand deep lithosphere structure beneath the Qinghai-Tibet Plateau more comprehensively and objectively and to explore important scientific issues, such as characteristics of plateau lithospheric deformation, state of strain, thermal structure, plate (or terrane) movement, and crust-mantle rheology, it is necessary to research the variation of crust-mantle electrical structure in the east-west direction in every geological unit. For this purpose, six super-broadband magnetotelluric (MT) sounding profiles have been completed by INDEPTH-MT Project in the Himalayas-Southern Tibet. Based on the imaging results from the six profiles, three-dimensional electrical conductivity structure of the crust and upper mantle has been analyzed for the research area. The result shows that the high-conductivity layers in the middle and lower crust exist widely in Southern Tibet, which extend discontinuously for more than 1000 km in the east-west direction and become thinner, shallower and more resistive toward the big turning of the Yarlung Zangbo River. The discussion on the rheology of lithosphere in Southern Tibet suggests that the mid-lower crust there is of high electrical conductivity, implying the existence of “partial-melt” and “hot fluid” in the thick crust of Tibet, which make the medium hot, soft, and plastic, or even able to flow. Combining the experimental result of petrophysics and the MT data, we estimate the melting percentage of the crustal material to be up to 5%–14%, which would reduce the viscosity of aplite in the crust to meet the flow condition; but for granite, it is likely not enough to cause such a change in rheology.

Radial anisotropy in the crust and upper mantle beneath the Qinghai-Tibet Plateau and surrounding regions

Relative SV and SH wave speeds are generally attributed to radial seismic anisotropy which can be used as the indicator of crust/mantle deformation styles. Surface wave data were initially collected from events of magnitude Ms ⩾ 5.0 and shallow or moderate focal depth occurring between 1980 and 2002: 713 of them generated Rayleigh waves and 660 Love waves, which were recorded by 13 broadband digital stations in Eurasia and India. Up to 1525 source-station Rayleigh waveforms and 1464 Love wave trains were earlier analysed by multiple filtering to obtain Love- and Rayleigh wave group velocity curves in the broad period range 10–105 s. We have performed tomographic inversion to obtain period-dependent group velocity and further shear wave velocity at 2° × 2°-sized grid-cells of a mesh covering the model region, after averaging azimuthal effects. Horizontally and vertically varying shear-wave velocities are observed, but the models of isotropic seismic velocity in the crust and upper mantle cannot fit simultaneously the inverted group-velocity dispersion curves due to the discrepancy in the transmission velocities of Love and Rayleigh waves, whose likely origin is the existence of radial anisotropy in the continental crust and topmost mantle. The strength of radial anisotropy computed from the Love–Rayleigh discrepancy and its spatial extent beneath the Qinghai-Tibet Plateau are shown as maps of percentage anisotropy at various depths down to 170 km and cross-sections along five profiles of reference. Areas in which radial anisotropy is in excess of ∼ 6% are found in the crust and upper mantle underlying most of the plateau, and even up to 10% in some places. The strength and spatial configuration of radial anisotropy seem to indicate the existence of a regime of horizontal compressive forces in the frame of the convergent Himalayan–Tibetan orogen, the laterally variation of the lithospheric rheology and the differential movement as regards the compressive driving forces.

The equivalent elastic thickness ( Te), seismicity and the long-term rheology of continental lithosphere: Time to burn-out “crème brûlée”? : Insights from large-scale geodynamic modeling

Depending on the conditions and time scale, the lithosphere exhibits elastic, brittle-plastic or viscous-ductile properties. As suggested by rock mechanics experiments, a large part of the long-term lithospheric strength is supported in the ductile regime. Unfortunately, these data, validated for strain rates ∼ 10 − 6 s − 1 , small scales (few cm) and simplified conditions, cannot be univocally interpolated to geological time and spatial scales (strain rates ∼ 10 − 17 –10 − 13 s − 1 , 100–1000 km spatial scales, complex conditions) without additional parameterization. An adequate parameterization has to be based on “real-time” observations of large-scale deformation. Indeed, for the oceanic lithosphere, the Goetze and Evan's brittle-elastic-ductile yield strength envelopes derived from data of experimental rock mechanics were successfully validated by a number of geodynamic scale observations such as the observations of plate flexure and the associated T e (equivalent elastic thickness) estimates. Yet, for continents, the uncertainties of flexural models and of the other data sources are much stronger due to the complex structure and history of continental plates. For example, in one continental rheology model, dubbed “jelly sandwich”, the strength mainly resides in the crust and mantle, while in another, dubbed “crème-brûlée”, the mantle is weak and the strength is limited to the upper crust. These models have arisen because of conflicting results from distributed earthquake, elastic thickness ( T e) and rheology data. We address these problems by examining the plausibility of each rheological model from general physical considerations. We review the elastic thickness ( T e) estimates and their relationship to the seismogenic layer thickness ( T s) to show that these two quantities have no direct physical relation. We also show that some of small T e must be artifacts of inconsistent formulation of the mechanical problem in some Free-Air anomaly admittance models. We point out that this does not necessarily detract from the admittance method itself but refers to its incorrect application in the continental domain. We then explore, by analytical and numerical thermo-mechanical modeling, the implications of a weak and strong mantle for tectonic structural styles. We conclude that rheological models such as crème-brûlée, which invoke a weak lithosphere mantle, are generally incompatible with observations. The jelly sandwich is in better agreement and we believe provides a useful first-order explanation for the long-term support of the Earth's main surface features.

Subparallel thrust and normal faulting in Albania and the roles of gravitational potential energy and rheology contrasts in mountain belts

The active tectonics of Albania and surrounding regions, on the eastern margin of the Adriatic Sea, is characterized by subparallel thrust and normal faulting which, we suggest, is likely to be related to gravitational potential energy contrasts between the low‐lying Adriatic Sea and the elevated mountainous areas inland. We calculate the magnitude of the force which the mountains and lowlands exert upon each other as a result of this potential energy contrast. It is likely that this force is largely supported by shear stresses on faults, and if so, the average stresses are less than ∼20 MPa. Alternatively, if the mountains are supported by stresses in the ductile part of the lithosphere, the stresses are likely to be ∼80–240 MPa in magnitude. The mountains of Albania are significantly lower than other ranges, such as the Peruvian Andes, which are thought to be extending in response to potential energy differences, and we discuss the relation between Albania and these other, higher, mountain belts from the perspective of differences in lithosphere rheology. We suggest that the lowlands of western Albania and the Adriatic Sea may have been weakened through time as a result of the deposition of large thicknesses of sediment, which lead to heating of the crystalline basement, a reduction in the potential energy contrast that could be supported by the lowlands, and so normal faulting in the mountains of eastern Albania.

Intraplate seismicity and related mantle hydration at the Nicaraguan trench outer rise

SUMMARY We examine micro-earthquake records from a dense temporary array of ocean bottom seismometers (OBS) and hydrophones that has been installed from September to November 2005 at the trench outer rise offshore Nicaragua. Approximately 1.5 locatable earthquakes per day within the array of 110 × 120 km show the high seismic activity in this region. Seismicity is restricted to the upper ∼15 km of the mantle and hence where temperatures reach 350–400 °C, which is smaller than values observed for large mantle intraplate events (650 °C). Determination of moment tensor solutions suggest a change of the stress region from tensional in the upper layers of the oceanic plate to compressional beneath. The neutral plane between both regimes is located at ∼6–9 km beneath Moho and thus very shallow. Fluids, which are thought to travel through the tensional fault system into the upper mantle, may not be able to penetrate any deeper. The earthquake catalogue, which seems to be complete for magnitudes above Mw= 1.6–1.8, suggests a strong change of the lithospheric rheology when approaching the trench. And b-factors, that is the ratio between small and large earthquakes increase significantly in the closest 20 km to the trench axis, implying that the crust and upper mantle is massively weakened and hence ruptures more frequently but under less release of stress. We explain this with a partly serpentinized upper mantle.

Numerical simulation of GPS observed clockwise rotation around the eastern Himalayan syntax in the Tibetan Plateau

From Global Position System (GPS) measurements, there is a clockwise rotation around the eastern Himalayan syntax in the Tibetan Plateau. This phenomenon is difficult to be interpreted by simple two-dimensional modeling from a geodynamic point of view. Because of the extremely thick crust and the lower crust with relatively high temperature in the Tibetan Plateau, the lithospheric rheology in Tibet and surrounding areas present a complex structure. In general, the tectonic structure of the Tibetan Plateau consists of brittle upper crust, ductile lower crust, high viscosity lithospheric upper mantle, and low viscosity asthenosphere, the same as the case in many other continental regions. However, the lower crust in the Tibetan Plateau is much more ductile with a lower viscosity than those of its surroundings at the same depth, and the effective viscosity is low along the collision fault zone. In this study, we construct a three-dimensional Maxwell visco-elastic model in spherical coordinate system, and simulate the deformation process of the Tibetan Plateau driven by a continuous push from the Indian plate. The results show that the existence of the soft lower crust under the plateau makes the entire plateau uplift as a whole, and the Himalayas and the eastern Himalayan syntax uplift faster. Since the lower crust of surrounding blocks is harder except in the southeastern corner where the high-temperature material is much softer and forms an exit channel for material transfer, after the whole plateau reaches a certain height, the lower crustal and upper mantle material begins to move eastward or southeastward and drag the upper crust to behave same way. Thus, from the macroscopic point of view, a relative rigid motion of the plateau with a clockwise rotation around the eastern Himalayan syntax is developed.