Effective Elastic Thickness Of Lithosphere Research Articles

Study on effective elastic thickness of lithosphere and its tectonic significance in surrounding area of Ordos Block

Based on EIGEN6C4 gravity field model data and ETOPO1 topographic data, we adopt the correlation analysis method based on Fan wavelet to calculate and analyze the distribution characteristics of effective elastic thickness (Te) and load ratio (F) of lithosphere in surrounding of Ordos block. On this basis, the relationship between the lithosphere Te and geological structure, seismic distribution, terrestrial heat flow and crustal deformation is analyzed. The results show that the lithosphere Te in the surrounding area of the Ordos block is obviously different among the tectonic blocks. The lithosphere Te value is low in the south of Qinling orogenic belt and the east of Fen-Wei graben basin. The maximum lithosphere Te value is distributed in the northwest and northeast margin of Ordos block. The lithosphere Te value is negatively correlated with the load ratio F. Earthquakes mainly occur in areas with low lithosphere Te value. Small lithosphere Te value indicates low strength of lithosphere. In such areas, the ability to resist crustal deformation is weak, and it is easy to generate stress accumulation and breed earthquakes. The area with high terrestrial heat flow value corresponds to low lithosphere Te value, showing a negative correlation. However, this negative correlation does not exist in the northwest margin of Ordos Block and Alashan border area, and the eastern margin of Ordos Lvliangshan area. Haiyuan-Liupanshan area has strong tectonic activity, frequent earthquakes and low lithosphere Te value. The internal strain accumulation of Ordos block is small, and the lithosphere Te value is high. The strain rate of the northern Shanxi fault depression zone is large, but the lithosphere Te value is also large, it is speculated that the terrestrial heat flow is the main factor affecting the lithosphere Te value.

Total isostatic response to the complete unloading of the Greenland and Antarctic Ice Sheets

The land surface beneath the Greenland and Antarctic Ice Sheets is isostatically suppressed by the mass of the overlying ice. Accurate computation of the land elevation in the absence of ice is important when considering, for example, regional geodynamics, geomorphology, and ice sheet behaviour. Here, we use contemporary compilations of ice thickness and lithospheric effective elastic thickness to calculate the fully re-equilibrated isostatic response of the solid Earth to the complete removal of the Greenland and Antarctic Ice Sheets. We use an elastic plate flexure model to compute the isostatic response to the unloading of the modern ice sheet loads, and a self-gravitating viscoelastic Earth model to make an adjustment for the remaining isostatic disequilibrium driven by ice mass loss since the Last Glacial Maximum. Feedbacks arising from water loading in areas situated below sea level after ice sheet removal are also taken into account. In addition, we quantify the uncertainties in the total isostatic response associated with a range of elastic and viscoelastic Earth properties. We find that the maximum change in bed elevation following full re-equilibration occurs over the centre of the landmasses and is +783 m in Greenland and +936 m in Antarctica. By contrast, areas around the ice margins experience up to 123 m of lowering due to a combination of sea level rise, peripheral bulge collapse, and water loading. The computed isostatic response fields are openly accessible and have a number of applications for studying regional geodynamics, landscape evolution, cryosphere dynamics, and relative sea level change.

Lithospheric flexural isostasy background of the 2017 Ms7.0 Jiuzhaigou earthquake and its implications for material extrusion in the northeastern Bayan Har block

We applied a 2-dimensional, non-spectral technique to investigate the spatial variations of the lithospheric effective elastic thickness (Te) in the eastern Bayan Har block and its adjacent areas. A program was designed to calculate the Moho flexure induced by topography loading, as well as the Bouguer gravity anomalies caused by that Moho flexure. The Te values were determined by minimizing the RMS differences between the observed and calculated Bouguer gravity anomalies. The results suggested that Te varies significantly from eastern Tibet to the Sichuan Basin. The eastern Bayan Har block and northern Sichuan-Yunnan block have low Te values (0 < Te < 20 km), indicating an easily deformable lithosphere. The Sichuan Basin has high Te values (40 km < Te < 100 km) acts as a rigid block that resists the eastward extrusion of plateau materials. The moderate Te values (30 km < Te < 40 km) found under the Longmen Shan fault belt indicate that the lithosphere of the Sichuan Basin plays an important role in supporting the Longmen Shan topography. The extremely low Te (Te < 10 km) observed in the seismogenic zone of the Ms7.0 Jiuzhaigou earthquake indicated that the topography is compensated locally, which is significantly different from the Longmen Shan topography. A banded region of relatively low Te values (< 40 km) stretching from the northeastern Bayan Har block to the southwestern Ordos block sketches out an escape channel for plateau materials, and challenges the existing of crustal flow on the north side of the Sichuan Basin.

The lithospheric effective elastic thickness and its tectonic implications in the Ordos Block and adjacent areas

The Ordos Block, located at the western edge of the North China Block, is a stable rigid block. It is characterized by inactive tectonic activity and rare earthquakes, while the south‐west margin of this block and adjacent areas present the opposite features. There is still considerable ambiguity with regard to the lithospheric mechanical strength in these areas. The effective elastic thickness (Te), widely used for comprehending the mechanism of cratonic deformation and lithospheric dynamics, is a proxy of the lithospheric strength. This study provided detailed two‐dimensional lithospheric Te spatial variations in the Ordos and adjacent areas by using coherence analysis of the topography and Bouguer anomaly. The result showed that the higher Te values in the Ordos Block varied from 80 to 100 km and implied that it is capable of strong rigidity. The lower Te values were distributed over the Qinling Block (20–45 km), the Qilian Block and Haiyuan arcuate tectonic region (20–40 km), as well as the Baryan‐Har Block (15–30 km). By comparing the Te values with the geophysical results, we believe that the higher Te value in the Ordos Block was supported not only by the crust, but also by the upper mantle. In contrast, the mechanical strength of the south‐west margin, especially the Baryan‐Har Block and western Qinling Block, was mainly borne by the brittle upper crust, but the tectonic activity and brittle failure likely resulted in a lower Te. Furthermore, we found that a large number of shallow earthquakes were located above low‐velocity and low‐resistivity layers, most of which probably occurred in the brittle upper crust with small mechanical strength.

Estimating the effective elastic thickness of the Arctic lithosphere using the wavelet coherence method: Tectonic implications

Currently, knowledge of structure and tectonic evolution of the Arctic lithosphere remains limited. The effective elastic thickness of the lithosphere (Te) is a parameter that describes the lithospheric integrated strength and reflects its thermal and rheological properties, which helps to reveal intraplate tectonic process. In this study, we present a high-resolution Te model in the Arctic region (north of 67°N), which is highly heterogeneous and strongly correlated with various regional tectonic elements. To verify the recovered Te, its pattern is compared to those obtained in previous studies that used different methods and to available heat flow measurements and seismic velocity model and shows good agreement. High Te values are found in the Greenland shield, North American craton and Siberian craton, which are compatible with results from previous studies of high seismic velocity and low heat flow, suggesting that cold, thick lithosphere remains tectonically undisturbed. Low Te values occur in the Amerasia Basin, Baffin Bay, Eurasia Basin and North Atlantic Ocean where young oceanic lithosphere is created. Moreover, the areas affected by regional volcanism in the Siberian Traps and HALIP are dominated by reductions in Te, which we attribute to thermal rejuvenation triggered by the intrusion of magma. Weak lithosphere also dominates in tectonically active orogens as well, including the Alaska-Chukotka, Novaya Zemlya and Caledonides, and is related to the stress released during collisional deformation. We find that a few deep sedimentary basins are characterized by intermediate Te, which implies mechanical support of the surface sediment loads in a non-isostatic state. Our new Te model provides potentially significant insights into various tectonic and geodynamic problems of the Arctic lithosphere.

Competing Effects of Mountain Uplift and Landslide Erosion Over Earthquake Cycles

AbstractLarge earthquakes can construct mountainous topography by inducing rock uplift but also erode mountains by causing landslides. Observations following the 2008 Wenchuan earthquake show that landslide volumes in some cases match seismically induced uplift, raising questions about how the actions of individual earthquakes accumulate to build topography. Here we model the two‐dimensional surface displacement field generated over a full earthquake cycle accounting for coseismic deformation, postseismic relaxation, landslide erosion, and erosion‐induced isostatic compensation. We explore the related volume balance across different seismotectonic and topographic conditions and revisit the Wenchuan case in this context. The ratio (Ω) between landslide erosion and uplift is most sensitive to parameters determining landslide volumes (particularly earthquake magnitude Mw, seismic energy source depth, and failure susceptibility, as well as the seismological factor responsible for triggering landslides), and is moderately sensitive to the effective elastic thickness of lithosphere, Te. For a specified magnitude, more erosive events (higher Ω) tend to occur at shallower depth, in thicker‐Te lithosphere, and in steeper, more landslide‐prone landscapes. For given landscape and seismotectonic conditions, the volumes of both landslides and uplift to first order positively scale with Mw and seismic moment Mo. However, higher Mw earthquakes generate lower landslide and uplift volumes per unit Mo, suggesting lower efficiency in the use of seismic energy to drive topographic change. With our model, we calculate the long‐term average seismic volume balance for the eastern Tibetan region and find that the net topographic effect of earthquakes in this region tends to be constructive rather than erosive. Overall, destructive events are rare when considering processes over the full earthquake cycle, although they are more likely if only considering the coseismic volume budget (as was the case for the 2008 Wenchuan earthquake where landsliding substantially offset coseismic uplift). Irrespective of the net budget, our results suggest that the erosive power of earthquakes plays an important role in mountain belt evolution, including by influencing structures and spatial patterns of deformation, for example affecting the wavelength of topography.

The effect of dynamic topography and gravity on lithospheric effective elastic thickness estimation: a case study

Lithospheric effective elastic thickness (T-e), a proxy for plIate strength, is helpful for the understanding of subduction characteristics. Affected by curvature, faulting and magma activity, lithospheric strength near trenches should be weakened but some regional inversion studies have shown much higher T-e values along some trenches than in their surroundings. In order to improve T-e-estimation accuracy, here we discuss the long-wavelength effect of dynamic topography and gravity on T-e estimation by taking the Izu-Bonin-Mariana (IBM) Trench as a case study area. We estimate the long-wavelength influence of the density and negative buoyancy of the subducting slab on observed gravity anomalies and seafloor topography. The residual topography and gravity are used to map T-e using the fan-wavelet coherence method. Maps of T-e, both with and without the effects of dynamic topography and slab gravity anomaly, contain a band of high-T-e values along the IBM Trench, though these values and their errors are lower when slab effects are accounted for. Nevertheless, tests show that the T-e map is relatively insensitive to the choice of slab-density modelling method, even though the dynamic topography and slab-induced gravity anomaly vary considerably when the slab density is modelled by different methods. The continued presence of a high-T-e band along the trench after application of dynamic corrections shows that, before using 2-D inversion methods to estimate T-e variations in subduction zones, there are other factors that should be considered besides the slab dynamic effects on the overriding plate.

Three-dimensional admittance analysis of lithospheric elastic thickness over the Louisville Ridge

Using bathymetry and altimetric gravity anomalies, a 1° × 1° lithospheric effective elastic thickness (T e) model over the Louisville Ridge and its adjacent regions is calculated using the moving window admittance technique. For comparison, three bathymetry models are used: general bathymetric charts of the oceans, SIO V15.1, and BAT_VGG. The results show that BAT_VGG is more suitable for calculating T e than the other two models. T e along the Louisville Ridge was re-evaluated. The southeast of the ridge has a medium T e of 10–20 km, while T e increases dramatically seaward of the Tonga-Kermadec trench as a result of the collision of the Pacific and Indo-Australian plates.

Three-dimensional estimate of the lithospheric effective elastic thickness of the Line ridge

Using a new bathymetry grid formed with vertical gravity gradient anomalies and ship soundings (BAT_VGG), a 1°×1° lithospheric effective elastic thickness (Te) grid of the Line ridge was calculated with the moving window admittance technique. As a comparison, both the GEBCO_08 and SIO V15.1 bathymetry datasets were used to calculate Te as well. The results show that BAT_VGG is suitable for the calculation of lithospheric effective elastic thickness. The lithospheric effective elastic thickness of the Line ridge is shown to be low, in the range of 5.5–13km, with an average of 8km and a standard deviation of 1.3km. Using the plate cooling model as a reference, most of the effective elastic thicknesses are controlled by the 150–300°C isotherm. Seamounts are primarily present in two zones, with lithospheric ages of 20–35Ma and 40–60Ma, at the time of loading. Unlike the Hawaiian-Emperor chain, the lithospheric effective elastic thickness of the Line ridge does not change monotonously. The tectonic setting of the Line ridge is discussed in detail based on our Te results and the seamount ages collected from the literature. The results show that thermal and fracture activities must have played an important role in the origin and evolution of the ridge.

2D Strain Rate Forward Modeling

AbstractGiven the variation of two‐dimensional strain rate through space and time, the velocity field is calculated. This velocity field determines the evolving thermal structure of the lithosphere. The temperature structure constrains the changing density structure which controls the loading history. Finally, subsidence history is calculated by solving the flexural equation with the imposing loads. Based on the forward modeling, we analyze the effects of heat source on surface heat flow and subsidence and study how the lithospheric effective elastic thickness affects the geometry of rifted basins. The results indicate that heat source has significant effect on surface heat flow but has little impact on subsidence. The lithospheric effective elastic thickness controls the geometry of rifted basins. The forward modeling provides a good foundation for the inversion of 2‐D strain rate.

Crustal structure and rift flank uplift of the Adare Trough, Antarctica

The Adare Trough, located 100 km northeast of Cape Adare, Antarctica, represents the extinct third arm of a Tertiary spreading ridge between East and West Antarctica. It is characterized by pronounced asymmetric rift flanks elevated up to over 2 km above the trough's basement, accompanied by a large positive mantle Bouguer anomaly. On the basis of recently acquired seismic reflection and ship gravity data, we invert mantle Bouguer anomalies from the Adare Trough and obtain an unexpectedly large oceanic crustal thickness maximum of 9–10.5 km underneath the extinct ridge. A regional positive residual basement depth anomaly between 1 and 2.5 km in amplitude characterizes ocean crust from offshore Victoria Land to the Balleny Islands and north of Iselin Bank. The observations and models indicate that the mid/late Tertiary episode of slow spreading between East and West Antarctica was associated with a mantle thermal anomaly. The increasing crustal thickness toward the extinct ridge indicates that this thermal mantle anomaly may have increased in amplitude through time during the Adare spreading episode. This scenario is supported by a mantle convection model, which indicates the formation and strengthening of a major regional negative upper mantle density anomaly in the southwest Pacific in the last 50 million years. The total amount of post‐26 Ma extension associated with Adare Trough normal faulting was about 7.5 km, in anomalously thick oceanic crust with a lithospheric effective elastic thickness (EET) between 3.5 and 5 km. This corresponds to an age between 3 and 5 million years based on a thermal boundary layer model and supports a scenario in which the Adare Trough formed soon after spreading between East and West Antarctica ceased, confined to relatively weak lithosphere with anomalously thick oceanic crust. There is little evidence for major subsequent structural activity in the Adare trough area from the available seismic data, indicating that this part of the West Antarctic Rift system became largely inactive in the early Miocene, with the exception of minor structural reactivation which is visible in the seismic data as offsets up to end of the early Pliocene.

Rheological implications of the thermal structure of the lithosphere in the convergence zone of the Eastern Carpathians

Four different thermo-kinematic models for the tectonic evolution of the convergence zone of the Eastern Carpathians are studied, in which processes like the pre-collisional subduction of an oceanic lithosphere, the underthrusting of the European continental margin in collision and the post-collisional thermal relaxation control the long term thermal structure of the study area. The thermal field obtained for each tectonic evolution has been used to evaluate the rheological structure along a NE–SW 500 km long lithospheric cross-section perpendicular to the Eastern Carpathians structures. The thermal structure and rheological behaviour of the lithosphere rocks are discussed in correlation with characteristics of the intermediate-depth seismicity of the active Vrancea region, placed at the southern end of the Eastern Carpathians. In front of the convergence fault and in the East European Platform a mechanical decoupling between crustal and mantle layers of the lithosphere could explain the absence of intermediate-depth earthquakes along the most part of the cross-section. The crust seems to be still coupled with the mantle in a narrow volume of the convergence area in the models including an oceanic subduction process. In this volume the mechanically strong lithosphere (MSL) reaches depths of about 220 km, close to the maximum depth of the seismic activity in the Vrancea region, and the lithosphere effective elastic thickness (EET) has values as high as 80–140 km. The peculiarities of the intermediate-depth seismic activity in the Vrancea area could be interpreted in terms of the rheological behaviour of the crustal and mantle material only if an episode of pre-collisional subduction of an oceanic lithosphere is assumed.

Decoupled flexure in the South Pyrenean Foreland

Complex subsidence patterns during the mid‐late Eocene of the easternmost South Pyrenean foreland are well explained by a decoupled flexure model in which upper crustal buckling is accommodated by ductile flow in the mid‐lower crust while, simultaneously and independently, the lower lithosphere flexes above a ductile asthenosphere. The resulting predicted subsidence values agree with observation to well within the estimated uncertainties. The resulting lithospheric effective elastic thickness (EET) estimate (10±3 km) is low but similar to values obtained in the South Pyrenean foreland by other authors. The separate EET obtained for the upper crust (1.5±0.8 km) also agrees with tentative values given by other workers.

Spatial variability of tidal gravity anomalies and its correlation with the effective elastic thickness of the lithosphere

Associations of the Earth tidal gravity response to physical properties of the lithosphere have been attempted at least for the last four decades. Although experimental data suggest this association, rigorous models have not yet been proposed. In this work, statistical tests are performed on the available World Gravity Earth Tides data set. Autocorrelation analysis shows that the M2 tidal gravity anomalies (TGAs) are significantly correlated up to a distance of about 500 km, with an approximately exponential correlation decay. The analysis of the latitudinal dependence of the anomalies shows that the anomaly variance, estimated inside of different latitude bands, follows a cos 4 ϕ curve within the ±45° latitude interval and defines the noise level for the M2 gravity anomaly data set. The regression analysis between M2 TGA and the lithosphere effective elastic thickness (EET) estimates shows that these quantities are significantly correlated, with a correlation coefficient of −0.82. The wide range of TGA and EET values, combined with a good global distribution of the data used in the regression analysis, makes the regression equation suitable to be used as a predictor for EET values in areas where M2 TGA data exist and meet the required quality criteria.

Tectonic pattern of South America inferred from tidal gravity anomalies

The knowledge of several physical parameters that characterize a tectonic unit is important to evaluate its dynamic evolution and, therefore, its role in the continental evolution. Several methodologies have been used to measure the elastic properties of tectonic units using: seismic waves, the level of coherence between the wavelength pattern of the topography and gravity fields, borehole deformation rate, mineral fabric, etc. In this study, the Earth's dynamic elastic response to gravity tides is applied for measuring the lithosphere effective elastic thickness. This methodology proved to be a good alternative to the conventional coherence method. It was applied over the continent, taking into account the poor gravity coverage in South America and the restricted area of some tectonic units. Results are presented as lateral distribution of the effective elastic thickness in South America.