Viscosity Of Lower Crust Research Articles

Coseismic and postseismic deformation of the 2008 Wenchuan Earthquake from InSAR

On 12 May 2008,an M_w7.9 earthquake occurred on the Longmenshan fault system at the eastern margin of the Qinghai-Tihet Plateau near Wenchuan City.Here we obtained the ionospheric perturbations in Japanese ALOS/PALSAR images covering the epicenter of Wenchuan earthquake from the corresponding azimuth offsets,and determined highly precise and continuous InSAR surface displacements using those that are not strongly affected by ionospheric perturbations.Combing with the GPS coseismic deformation,a joint model considering both coseismic and postseismic phases is constructed to study the coseismic slip distribution of the Wenchuan earthquake and the viscous structure of the Longmenshan region.The best-fitting slip model suggests that the earthquake is an event with very complex fault rupture.The slips of Hongkou,Yuejiashan,Beichuan,and Hanwang segments are dominated by the thrust movement and the Qingchuan segment experiences a dominant right-lateral strike slip.The major slip occurred mainly at the depth of less than 10 km,with a maximum slip of 10.7 m on the northeastern Hongkou segment.The determined geodetic moment is approximately 9.28 × 10~(20) N ? m(M_w7.91),which is consistent with the results of seismology.The mid to lower crustal viscosity derived from the joint model in a Maxwell half-space is 2×10~(18) Pa ? s,which places a strong low bound on viscosity in the Longmenshan region.Postseismic displacements time series covering a longer time span might have the potential to improve this constraint.

The effects of rheological decoupling on slab deformation in the Earth’s upper mantle

Processes within subduction zones have a major influence on the plate dynamics and mantle convection. Subduction is controlled by a combination of many parameters and there is no simple global relationship between the resulting slab geometry and deformation and any specific subduction parameter. In the present work we perform a parametric study of slab dynamics in a two-dimensional model with composite rheology including diffusion creep, dislocation creep and stress limiter or Peierls creep. The mechanical decoupling of the subducting and overriding plates is facilitated by a low viscosity crust. We are particularly interested in the effect of the contact of subducting and overriding plates on the plate geometry in the upper mantle. We also study the influence of the surface boundary condition and of the rheological description (yield stress of stress-limiting rheology, additional viscosity contrast at 660-km discontinuity). Our results demonstrate that the slab morphology and deformation in the upper mantle and the transition zone is sensitive not only to the slab strength, but also to the decoupling mechanism at the contact of the subducting and overriding plates. Weak crust with a viscosity of 1020 Pa s effectively decouples the subducting and overriding plates and produces reasonable slab morphologies. The geometry of the slab in the upper mantle is strongly influenced by the initial geometry of the contact between the subducting and overriding plates. Further, a step-wise viscosity increase by about an order of magnitude at 660 km depth is necessary to limit the plate velocities to a reasonable value around 5 cm/yr.

New insights on the Messina 1908 seismic source from post-seismic sea level change

The identification of a source model for the catastrophic 1908 December 28 Messina earthquake (M w = 7.2) has been the subject of many papers in the last decades. Several authors proposed different models on the basis of seismological, macroseismic and geodetic data sets; among these models, remarkable differences exist with regard to almost all parameters. We selected a subset of six models among those most cited in literature and used them to model the post-seismic sea level variation recorded at the tide gauge station of Messina (until 1923), to attempt an independent discrimination among them. For each model, we assumed a simple rheological structure and carried out a direct-search inversion of upper crust thickness and lower crust viscosity to fit the post-seismic sea level signal. This approach enabled us to identify a class of fault geometries which is consistent with the post-seismic signal at the Messina tide gauge and with the known structural and rheological features of the Messina strait.

Contrasting strike-slip motions on thrust and normal faults: Implications for space-geodetic monitoring of surface deformation

Recent global positioning system (GPS) records of surface deformation caused by earthquakes on intracontinental dip-slip faults revealed in unprecedented detail a significant strike-slip component near the fault tips that is markedly different for thrust and normal faults. In the hanging wall of the thrust fault ruptured during the A.D. 2003 Chengkung (Taiwan) earthquake, a divergent displacement pattern was recorded. In contrast, a convergent slip pattern was observed in the hanging wall of the normal fault that produced the 2009 L’Aquila (Italy) earthquake. Such convergent slip patterns are also evident in field records of cumulative fault slip from central Italy, which underlines the coseismic origin of cumulative displacement patterns. Here we use three-dimensional numerical modeling to demonstrate that the observed fault-parallel motions are a characteristic feature of the coseismic slip pattern on normal and thrust faults. Modeled slip vectors converge toward the center of normal faults, whereas they diverge for thrust faults; this causes contrasting fault-parallel displacements at the model surface. Our model also predicts divergent movements in normal fault footwalls, which were recorded for the first time during the L’Aquila earthquake. During the postseismic phase, viscous flow in the lower crust induces fault-parallel surface displacements, which have the same direction as the coseismic displacements but are distributed over a larger area that extends far beyond the fault tips. Therefore, detecting this signal requires GPS stations in the prolongation of the fault strike. Postseismic velocities vary over several orders of magnitude depending on the lower crustal viscosity and may reach tens of millimeters per year for low viscosities. Our study establishes the link between coseismic and cumulative slip patterns on normal and thrust faults and emphasizes that understanding fault-parallel slip components and associated surface displacements is essential for inferring regional deformation patterns from space-geodetic and fault-slip data.

Delamination vs. break‐off: the fate of continental collision

AbstractThe fate of a convergent continental margin is investigated. We perform a set of 2D numerical models to study how and why continental collision can evolve in different scenarios. Since the rheology of continental lithosphere has a major control on the dynamics of subduction, we explore a range of different lithosphere and lower crust viscosity values to understand their sensitivity on the possible scenarios. We find that with a rheologically layered crust both delamination and break‐off are feasible. We identify three modes: (1) slab detachment, in which the lithospheric mantle and the crust are strongly coupled, subduction slows down and the slab eventually breaks; (2) delamination of the lithospheric mantle that separates from the crust and continue to subduct and (3) an intermediate mode where the lithospheric mantle and the crust remain partially coupled, resulting in an initial stage of delamination followed by the slow down and cessation of subduction.

Three-dimensional mechanical modeling of the GPS velocity field around the northeastern Tibetan plateau and surrounding regions

The northeastern corner of the Tibetan plateau is a complex tectonic region with different fault mechanisms from left-lateral thrusting of the Qilian Shan, left-lateral strike slipping of the Haiyuan fault along the plateau edge, and right-lateral extension across the Alashan and the Ordos out of the northeastern Tibetan plateau. Here, we use 3D finite element models incorporating fault as Coulomb-type friction zone to investigate mechanical relation between crustal rheology and long-term deformation of the main active fault systems. Models are constrained with GPS velocity field and available geological slip rates. Crustal rheology is simplified as an elastobrittle upper part, underlying with viscoelastic crust. We test models with fault frictions (μ) from 0.4 to 0.02 on different fault systems, and mean viscosities (η) of the lower crust from 1019Pa.s to 1021Pa.s in the Tibetan plateau and 1021Pa.s to 1023Pa.s out of the Tibetan plateau. A common feature from the numerical experiments is that the Haiyuan fault reflects a low fault friction (μ<0.1–0.08). The predicted low fault friction associated with the mean viscosities of the lower crust of ~1019Pa.s in the Tibetan plateau and ~1021Pa.s out of the Tibetan plateau can fit the geological slip rates well for the active faults. This suggests that slip partitioning around northeastern boundary of the Tibetan plateau is related mechanically to the low fault friction. Numerical experiments also show that after strain rates are absorbed by the main fault systems, the rest are strongly affected by crustal rheology. Finally, even the fault friction decreases to ~0.05, the mean viscosities of lower crust attain to 1021Pa.s and 1023Pa.s in and out of the Tibetan plateau, the northeastern Tibetan plateau and the Alashan still deforms diffusively, except that the Ordos behaves more like a rigid block.

Short‐time postseismic deformation of 2001 Ms8.1 Kunlun (China) earthquake

AbstractAfter the Ms 8.1 Kunlun earthquake, postseismic displacements were resolved with GPS. The observed data showed that there were significant differences between the deformations on the south and the north sides of the Kunlun fault. In addition, it is shown that the direction of deformation on the north side shifted from westward to eastward after a short‐time adjustment. Such GPS data are used as the constraints in finite element method (FEM) modeling of postseismic deformation. Using 3D poro‐viscoelastic model, we show that the horizontal inhomogeneous viscosities in lower crust can play an important role in the geodynamic behavior. The best‐fitting viscosities of lower crust are about 5.0×1017 Pa s, 9.0×1018 Pa s in Qiangtang block and Qaidam basin, respectively. The simulation suggests that the postseismic deformation of 2001 Kunlun Ms8.1 earthquake probably be caused by both the viscoelastic relaxation and poroelastic rebound. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Studying the viscosity of lower crust of Qinghai-Tibet Plateau according to post-seismic deformation

The viscosity of lower crust of Qinghai-Tibet Plateau on earth should be determined. It has become a predominant problem in quantitative research on geodynamics. Its order of magnitude will have a great influence on the results of quantitative modeling. To obtain the viscosity of lower crust of Qinghai-Tibet Plateau, this parameter was calculated by three methods. The first is based on the estimation on the temperature state of Qinghai-Tibet Plateau in the deep part, and the viscosity of lower crust of northern Plateau was recomputed with strain rate derived from rheology law and GPS observation. Effective viscosity of middle crust in Kunlun region is between 1020 and 1022 Pa·s, and that of lower crust is between 1019 and 1021 Pa·s; the second is based on three kinds of rheological models used to fit the post-seismic deformation recorded by cross-over fault GPS sites set after M s8.1 Kunlun earthquake in 2001. The viscosity of lower crust obtained by this method is of 1017 Pa·s order of magnitude. However, higher viscosity is required to fit the data of south fault better, and the lower one is required to fit the data of north fault better. The viscosity of lower crust, which was obtained by fitting the cross-over fault post-seismic deformation after M s7.6 Luhuo earthquake in 1973, is of 1019 Pa·s order of magnitude. Non-linear relationship between effective viscosity and strain rate is ignored in the former research of effective viscosity. This research shows the difference of effective viscosity obtained from laboratory experiment, and shorter and longer time post-seismic deformation after large earthquakes can be explained in phase.

Numerical Simulation and Discussion on the Mechanism of Postseismic Deformation After Kunlun Ms8.1 Earthquake

AbstractOn November 14th 2001 an earthquake of magnitude Ms8.1 occurred at the Qinghai‐Xizang Plateau in China. The deformation observed by GPS showed that there were many differences between the deformations of south and north sides of the Kunlun fault. And after a short span of about three months, the deformations at all GPS stations were eastward. In this paper, the postseismic deformation observed by GPS is used as constraints and we analyze the possible mechanism of postseismic deformation after Kunlun earthquake. Firstly, this paper constructs the virtual work equation and theoretically analyzes the effects of heterogeneous, viscoelastic, poroelastic characters on the postseismic deformation. Then this paper uses grid‐search procedure to determine the viscosity for different models. The best‐fitting viscosity of lower crust is 5.0 × 1017 Pa·s and 9.0 × 1018 Pa·s for the south and north side of Kunlun fault, respectively. The difference of viscosities brought on the special postseismic deformation. And this difference is not only the result of long‐term geological action but also one of the crucial factors of modern geodynamic environment. On the other hand, the simulation shows that the postseismic deformation after 2001 Kunlun Ms8.1 earthquake is caused not only by the viscoelastic relaxation but also by poroelastic relaxation. So, it is necessary that the viscoelastic relaxation and poroelastic rebound should be combinedly considered when analyzing poseismic deformation.

Plagioclase preferred orientation in layered mylonites: Evaluation of flow laws for the lower crust

We evaluate the applicability of plagioclase and gabbro flow laws by comparing predicted and observed deformation mechanisms in gabbroic shear zones. Gabbros and layered gabbro mylonites were collected from the Southwest Indian Ridge (SWIR), Ocean Drilling Program Hole 735B. Deformation temperatures are constrained by two‐pyroxene thermometry, stress is estimated from grain size, and deformation mechanisms are analyzed by microstructure and the presence or absence of a lattice preferred orientation (LPO). Our analyses indicate that mylonite layers deformed at a strain rate in the range of 10−12 to 10−11 s−1, while coarse‐grained gabbro deformed at a strain rate of approximately 10−14 to 10−13 s−1. Plagioclase in pure plagioclase mylonite layers exhibit strong LPOs indicating that they deformed by dislocation creep. Plagioclase grain size in mixed plagioclase‐pyroxene mylonite layers is finer than in pure plagioclase layers and depends on the size and proportion of pyroxenes. Progressive mixing of pyroxene and plagioclase within gabbro mylonite layers is accompanied by weakening of the LPO, indicating that phase mixing promotes a transition to diffusion creep processes that involve grain boundary sliding. Our results indicate that experimental flow laws are accurate at geologic strain rates, although the strain rate for diffusion creep of fine‐grained gabbro may be underestimated. At the conditions estimated for the SWIR crust, our calculations suggest that strain localization leads to a factor of 2–4 decrease in lower crustal viscosity. Away from shear zones, the viscosity of lower gabbroic crust is predicted to be similar to that of dry upper mantle.

New constraints on the thermal and volatile evolution of Mars

The thermal and volatile evolution of Mars has not been studied from the perspective of consistency with the preservation of the Martian global dichotomy and with estimates of the elastic thickness over time. We use three thermal evolution models for Mars: (1) stagnant lid, (2) early plate tectonics followed by stagnant lid, and (3) mantle overturn, to calculate the amount of relaxation of the dichotomy boundary and elastic thickness values for Noachian- and Hesperian-aged terrains. To explore a wide range of parameters, we evaluate two different initial mantle temperatures, and wet and dry rheologies. Our model results show that the relative water content of the crust has an effect roughly equal to 500 K variations of initial mantle temperature. For all three thermal models, a lower crust viscosity of 10 20–10 21 Pa s during the first 0.1 Ga after formation of dichotomy would allow for the preservation of the long-wavelength topography of Mars and fitting of the elastic thickness. This viscosity range implies either wet, cold (∼1500 K) lower crust, or dry, hot (∼2000 K) lower crust in Noachian. Additional constraints are necessary to distinguish between the individual thermal models. For the stagnant lid model, neither the cold, wet crust nor the hot, dry crust agree with timing and amount of the crustal production [Hauck II., S.A., Phillips, R.J., 2002. Thermal and crustal evolution of Mars. J. Geophys. Res. 107, 5052]. Moreover, drying of the crust is required for this model in order to match the admittance elastic thickness at the Hesperian/Amazonian boundary implying remelting of the crust. The hot, dry crust in the early plate tectonics model limits a plate tectonic epoch to only 100–200 Myr and implies dry mantle, which is in disagreement with water found in meteorites. The cold, wet crustal rheology implies the formation of crust during the plate tectonics regime because of the low crustal production during the stagnant lid regime. For mantle overturn, the temperature required for wet crust does not fit the original mantle profile while the dry crust does; however, in order to explain the initially hot thermal profile the crust must have been emplaced very fast. Generally, dry crustal rheology does not fit low elastic values in the Hesperian and implies either that rheology may differ between the southern and northern hemispheres: wet in northern hemisphere and dry in southern hemisphere, or that local weakening occurred. Wet crustal rheology fits well all elastic data except S. Hellas rim, which may be anomalous. Mantle rheology is unconstrained by our modeling and can be either dry or wet.

Transient rheology of the upper mantle beneath central Alaska inferred from the crustal velocity field following the 2002 Denali earthquake

The M7.9 2002 Denali earthquake, Alaska, is one of the largest strike‐slip earthquakes ever recorded. The postseismic GPS velocity field around the 300‐km‐long rupture is characterized by very rapid horizontal velocity up to ∼300 mm/yr for the first 0.1 years and slower but still elevated horizontal velocity up to ∼100 mm/yr for the succeeding 1.5 years. I find that the spatial and temporal pattern of the displacement field may be explained by a transient mantle rheology. Representing the regional upper mantle as a Burghers body, I infer steady state and transient viscosities of η1 = 2.8 × 1018 Pa s and η2 = 1.0 × 1017 Pa s, respectively, corresponding to material relaxation times of 1.3 and 0.05 years. The lower crustal viscosity is poorly constrained by the considered horizontal velocity field, and the quoted mantle viscosities assume a steady state lower crust viscosity that is 7η1. Systematic bias in predicted versus observed velocity vectors with respect to a fixed North America during the first 3–6 months following the earthquake is reduced when all velocity vectors are referred to a fixed site. This suggests that the post‐Denali GPS time series for the first 1.63 years are shaped by a combination of a common mode noise source during the first 3–6 months plus viscoelastic relaxation controlled by a transient mantle rheology.

Possible orogeny-parallel lower crustal flow and thickening in the Central Andes

The relatively low elevation and thick crust in the Altiplano, in comparison to the higher elevation, but thinner crust in the Puna plateau, together with geophysical data, suggests that isostatic equillibrium is achieved by cooler and denser lithospheric mantle in the Altiplano. Excess density in the Altiplano mantle could create differential horizontal stress in the order of 25 MPa between both lithospheric columns. Numerical models accounting for pressure and temperature-dependent rheology show that such stress can induce horizontal ductile flow in the lower crust, from the Puna towards the Altiplano. With a minimum viscosity of 1019 Pa s, this flow reaches 1 cm/year, displacing more than 50 km of material within 5 Ma. If the lower crust viscosity is smaller, the amount of orogeny-parallel lower crustal flow can be even greater. Such a mechanism of channel flow may explain that different amounts of crustal material have been accommodated by shortening in the Altiplano and in the Puna. Because of the strength of the elastic-brittle upper crust, this channel flow does not necessitate large amounts of surface deformation (except vertical uplift), making it difficult to detect from the geology.

One My scale subsidence of carbonate sedimentary bodies and the viscosity of the lower crust

The possibility of flow of the lower crust under the load produced by carbonate sedimentary accumulations is investigated through the example of the Paris basin during the Middle Jurassic (i.e. Bathonian). Depositional geometries, water depths and sedimentary environments have been estimated and correlated for 164 sites spread over a surface of 380 per 220 km for three successive periods lasting each less than 0.8 My. A signal of relative vertical displacement has been extracted from water-depth and sedimentary thickness. Data have then been interpolated to produce maps of velocity of vertical displacement, sedimentation rate, water depth, and water-depth variation between two periods. The maps show that the western part of the basin is affected by faults which are independent of the sedimentation. The eastern part of the basin consists of a shallow carbonate platform affected by diffuse subsidence patches which are positively correlated with sedimentation. The patches of diffuse subsidence are elliptic in shape, with a radius of 20 km, a mean thickness of 40 m, and a maximum elevation of 10 m above the surrounding sea bottom. We suggest that the load of these patches induces the flow of the lower crust. The topography produced by these patches would be maintained at steady state by in situ carbonate production/accumulation combined with the lower crust flow. This model is tested by estimating the viscosity and the thickness of the flowing crust necessary to meet the geometric and kinematic conditions obtained from the patches. This model is valid for a lower crust viscosity of around 10 21 Pa s. This value is higher by one order of magnitude than those inferred from other contexts but can be explained by the influence on viscosity of the load and temperature for a non-newtonian rock rheology.

When faults communicate: Viscoelastic coupling and earthquake clustering in a simple two‐fault system

3‐D finite element models of a simplified northern and southern San Andreas‐type fault system are presented with the goal of better understanding how great earthquakes (M ≥ 7.5) on one major segment of a fault can affect the earthquake cycle on another colinear fault segment separated from the first by an asesimically creeping segment. We find that the earthquake cycles of the two seismogenic fault segments become coupled as the lower crustal viscosity and/or the fault separation distance are decreased. Further, models with a 10%–30% difference in relative fault breaking strengths exhibit a bi‐modal distribution of repeat times for each fault, resulting in earthquakes that appear clustered in time.

Active displacement field in the Suez–Sinai area: the role of postseismic deformation

By means of a spherical viscoelastic model of co- and postseismic deformation we compute the postseismic displacement associated with the November, 1995, M w=7.2, Aqaba (Jordan) earthquake. We compare our results with the observational data obtained by a GPS campaign performed in a wide portion of Sinai between November 1997 and May 1998. Though the original purpose of the campaign was to explain the tectonic features of the region, the displacement values deduced from the GPS observations are hardly reconcilable with any of the proposed tectonic models of Sinai. Our hypothesis is that the detected deformation field could be due more to the postseismic relaxation following the Aqaba earthquake rather than to tectonic movements. Our results show that both GPS observational data and the postseismic simulations predict a compressional regime in the Gulf of Suez though the contraction detected by GPS is generally larger. The best agreement with the data is obtained for an asthenospheric and lower crust viscosity of 10 18 Pa s and an absence of aseismic afterslip on the fault plane. We conclude that the contribution coming from the postseismic viscoelastic relaxation of the ductile shallow layers following the 1995 Aqaba earthquake is likely to play an important role in determining the present deformation field in Sinai. The transient postseismic relaxation seems to play a more important role than the secular tectonic deformation.

Finite-element modelling of the structure and evolution of the South Kenya Rift, East Africa

A finite-element (FE) model was used in this study to investigate the intraplate rifting process in south Kenya, East Africa. The rifting model shows that the important factors influencing the amount of shoulder uplift and rift subsidence include the horizontal deviatoric extensional stresses, the viscosity of the lower crust, and the dimension and density contrast of the low-velocity upper-mantle anomaly. Thus, for instance, a reduced lower-crust viscosity as well as an increased extensional far-field stress favours the subsidence of the rift basin. In turn, for a reduced lower-crust viscosity, the magnitude of the applied extensional stress should be reduced in order to end up with the same topography of the rifted area as can be obtained with a more viscous lower crust combined with a relatively high extensional far-field stress. In addition, it also became obvious that the uplift of the crust-mantle boundary beneath the rift depends predominantly on the size and density contrast of the upper-mantle anomaly. An increased density contrast between the upper-mantle anomaly and normal mantle results in increased buoyancy forces thus producing an increased uplift of the crust-mantle boundary. In turn, in order to avoid too large values for the surface topography, a reduced lower-crust viscosity in the order of η = 10 21 Pa s is required. Models without an upper-mantle low-density anomaly have resulted in crustal thickening beneath the rift and not in crustal thinning. In accordance with the geological field evidence, we have modelled the evolution of the South Kenya Rift in two stages. In the first stage, from 16 to 10 Ma ago, a broad regional uplift of a few hundred metres produced by a relatively small but broad low-density upper-mantle anomaly was accompanied by an insignificant amount of faulting. In the second stage, from 10 Ma ago to the present day, significant faulting accompanied by the buoyancy effects of a larger, low-density, low-velocity upper-mantle anomaly concentrated more beneath the rift itself have quite successfully reproduced the rift subsidence, shoulder topography and crust-mantle boundary uplift observed at the present day.

Finite element modeling of lower crustal flow: A model for crustal thickness variations

Small‐scale convection beneath continental lithosphere is likely to initiate viscous flow in the ductile lower crust. Positive lithospheric bending will develop above upwelling mantle flow leading to a lateral squeezing out of crustal material from elevated areas which results in significant crustal thickness variations in its final stage. This process was studied with a finite element approach for viscous material for a number of different viscosity contrasts between lower crust and lithospheric mantle. Significant Moho undulations of the order of at least 5 km within a time period of about 50 m.y. can only be reached if the lower crustal viscosity is less than 1021 Pa s or if the lithospheric mantle viscosity is less than 1024 Pa s. The maximum topography is a function of the viscosity contrast and is generally of the order of a few hundreds of meters with a relaxation time between 5 and 103 m.y., strongly dependent on the crustal viscosity. In conclusion, this process might be important to explain crustal thinning and thickening of the order of about 10 km in areas of enhanced thermal gradient.