Impact of Decelerating India‐Asia Convergence on the Crustal Flow Kinematics in Tibet: An Insight From Scaled Laboratory Modeling

The factors controlling the spatiotemporally varying deformation patterns in Tibet, a prolonged period (∼50 to 19 ± 3 Ma) of NNE-SSW shortening, accompanied by eastward flow and orogen-parallel extension in a later stage (19 ± 3 Ma to present-day), are still poorly constrained. Using viscous models, we performed scaled laboratory experiments with steady and unsteady state collision kinematics to address this issue. Our model Tibet under steady-state collision, irrespective of high (5.5 cm/yr) or low (3.5 cm/yr) indentation rates fails to produce the present-day crustal velocity fields and the deformation patterns, reported from GPS observations. An unsteady-state collision with decelerating convergence rates (5.5–3.5 cm/yr) is found to be a necessary condition for the initiation of eastward flow and ESE-WNW extensional deformations. The model results also suggest that the mechanical resistance offered by the rigid Tarim block resulted in crustal uplift at faster rates in western Tibet, setting a west to east topographic gradient, existing till present-day. This topographic gradient eventually polarized the gravity-controlled flow in the east direction when the convergence velocity decelerated to ∼3.5 cm/yr at around 19 ± 3 Ma. Our model shows the present-day eastward flow in central Tibet follows nearly a Poiseuille type velocity profile, bounded by the Himalaya in the south and the Tarim basin in northern Tibet. This flow kinematics allows us to explain the preferential locations of crustal-scale dextral and sinistral faults in southern and northern Tibet, respectively. Finally, the present-day model crustal-flow velocity, strain-rates, and topographic variations are validated with GPS and geological field data

Back arc extension and collision : an experimental approach of the tectonics of Asia

International audienceThe deformation of the eastern Asian lithosphere during the first part of the India-Asia collision was dominated by subduction-related extension interacting with far effects of the collision. In order to investigate the role of large-scale extension in collision tectonics, we performed analogue experiments of indentation with a model of lithosphere subjected to extension. We used a three-layer rheological model of continental lithosphere resting upon an asthenosphere of low viscosity and strained along its southern boundary by a rigid indenter progressing northward. The lithosphere model was scaled to be gravitationally unstable and to spread under its ownweight, so that extension occurred in thewhole model. The eastern boundarywas free or weakly confined and always allowed eastward spreading of the model. We studied the pattern of deformation for different boundary conditions. The experimental pattern of deformation includes a thickened zone in front of the indenter, a major northeast-trending left-lateral shear zone starting from the northwest corner of the indenter, antithetic north-south right-lateral shear zones more or less developed to the east of the indenter, and a purely extensional domain in the southeastern part of the model. In this domain, graben opening is driven by gravitational spreading, whereas it is driven by gravitational spreading and indentation in the northeastern part where grabens opened along strike-slip faults. The results are compared with the Oligo- Miocene deformation pattern of Asia consecutive to the collision of India. Our experiments bring a physical basis to models which favour distributed deformation within a slowly extruded wide region extending from the Baikal Rift to the Okhotsk Sea and to southeast Asia and Indonesia. In this large domain, the opening of backarc basins (Japan Sea, Okinawa Trough, South China Sea) and continental grabens (North China grabens) have been associated with approximately north-south-trending right-lateral strike-slip faults, which accommodated the northward penetration of India into Eurasia

Lithospheric deformations in Central Asia, derived from gravity data

Central Asia is characterized by a number of spectacular tectonic units such as the Tibetan Plateau, the Tien Shan intra-continental mountain belt and the Altyn Tagh strike-slip fault. Deformation of the lithosphere of these units is not well understood. In this dissertation, an attempt is made to answer the following tectonic questions: To what extent do the tectonic units formed in the upper crust in the course of the convergence between India and Eurasia weaken the underlying lithosphere? How is the Tibetan Plateau gravita-tionally compensated along its northern boundary? Do the Altyn Tagh and/or West Kunlun faults persist as vertical strike-slips throughout the crust and mantle lithosphere, thus representing a fundamental plate boundary? Which dynamic processes in the crust and mantle of the Tien Shan are responsible for intra-continental mountain building? What characterizes the local stress field in the Tien Shan and what causes the deformation of these mountains? To answer these questions, the elastic plate theory is used to interpret gravity and topography data, and in turn to derive specific characteristics of the lithospheric structure in Central Asia. Theoretical deflection of the elastic plate or plates were calculated using the 3D and 2D finite-difference methods. Variable-rigidity elastic plates are subjected to vertical and horizontal loads, shear forces, and terminal bending moments. Surface topo-graphic data from the Topo30 dataset is used as vertical surface loading to calculate the flexure of the elastic lithosphere...thesi

Interannual variability of gps heights and environmental parameters over europe and the mediterranean area

Vertical deformations of the Earth’s surface result from a host of geophysical and geological processes. Identification and assessment of the induced signals is key to addressing outstanding scientific questions, such as those related to the role played by the changing climate on height variations. This study, focused on the European and Mediterranean area, analyzed the GPS height time series of 114 well-distributed stations with the aim of identifying spatially coherent signals likely related to variations of environmental parameters, such as atmospheric surface pressure (SP) and terrestrial water storage (TWS). Linear trends and seasonality were removed from all the time series before applying the principal component analysis (PCA) to identify the main patterns of the space/time interannual variability. Coherent height variations on timescales of about 5 and 10 years were identified by the first and second mode, respectively. They were explained by invoking loading of the crust. Single-value decomposition (SVD) was used to study the coupled interannual space/time variability between the variable pairs GPS height–SP and GPS height–TWS. A decadal timescale was identified that related height and TWS variations. Features common to the height series and to those of a few climate indices—namely, the Arctic Oscillation (AO), the North Atlantic Oscillation (NAO), the East Atlantic (EA), and the multivariate El Niño Southern Oscillation (ENSO) index (MEI)—were also investigated. We found significant correlations only with the MEI. The first height PCA mode of variability, showing a nearly 5-year fluctuation, was anticorrelated (– 0.23) with MEI. The second mode, characterized by a decadal fluctuation, was well correlated (+0.58) with MEI; the spatial distribution of the correlation revealed, for Europe and the Mediterranean area, height decrease till 2015, followed by increase, while Scandinavian and Baltic countries showed the opposite behavior

Loading Deformation On Various Timescales Using Gps And Grace Measurements

Thesis (Ph.D.) University of Alaska Fairbanks, 2012Tidal, seasonal and long-term surface mass movements cause the earth to deform and the gravity field to change. Current geodetic satellites, GPS and GRACE, accurately measure these geophysical signals. I examine the effect on GPS solutions of using inconsistent reference frames to model ocean tidal loading (OTL). For seasonal loading, I choose two study areas, Nepal Himalaya and southern Alaska, and compare GPS-measured and GRACE-modeled seasonal hydrological ground loading deformation. Globally distributed stations are employed to compare GPS coordinate solutions with OTL corrections computed in different reference frames: center of mass of the solid Earth (CE), and center of mass of the Earth system (CM). A strong spectral peak at a period of ~14 days appears when inconsistent OTL models are applied along with smaller peaks at ~annual and ~semi-annual periods. Users of orbit/clock products must ensure to use OTL coefficients computed in the same frame as the OTL coefficients used by the analysis centers; otherwise, systematic errors will be introduced into position solutions. Continuous GPS measurements of seasonal deformation in Nepal Himalaya are compared with load model predictions derived from GRACE observations. The GPS seasonal height variation and GRACE-modeled seasonal vertical displacement due to the changing hydrologic load exhibit consistent results, for both amplitude and phase. GRACE indicates a long-term mass loss in the Himalaya region, which leads to crustal uplift since the earth behaves as an elastic body. We model this effect and remove it from GPS observed vertical rates. Then most GPS vertical rates can be explained by interseismic strain from the Main Himalayan Thrust. In southern Alaska, vertical seasonal loading deformation observed by GPS stations and modeled displacements due to seasonal hydrological loading inferred from GRACE are highly correlated. The effects of atmosphere and non-tidal ocean loading are important. Adding the AOD1B de-aliasing model to the GRACE solutions improves the correlation between these two geodetic measurements, because the displacements due to these loads are present in the GPS data. Weak correlations are found for some stations located in areas where the magnitude of the load changes over a short distance, due to GRACE's limited spatial resolution

Intracontinental Neotectonics: Case Studies from the Tian Shan Orogen and Kuqa Fold-Thrust Belt

This dissertation focuses on the kinematic properties of intracontinental deformation during short geologic time scales. Using three case studies this work characterizes active deformation at varying spatial scales within the continental interior of Eurasia. The far-field effect of the Cenozoic Indo-Asia collision is the driving force controlling deformation within the Eurasian continental interior. Active deformation across the intracontinental Tian Shan range challenges the plate tectonic model that proposes crustal deformation is concentrated along plate boundaries. This work further constrains the active kinematics of intracontinental motion that is necessary to understand the dynamics of the Eurasian intracontinental system. The two standing hypothesis that explain intracontinental deformation include the (A) discrete or (B) diffuse deformation models. While the discrete model suggests deformation occurs primarily along major faults between crustal blocks, the diffuse model suggests motion is distributed throughout the continental interior. On a smaller scale, I examine active deformation of sub-aerial salt bodies. Ephemeral subaerial salt exposure during the evolution of a salt structure can greatly impact the subsequent development and deformation of its tectonic setting. InSAR time series analysis and inspection of individual interferograms confirm that the majority of the salt bodies in western Kuqa are active, with significant InSAR observable displacements at 3 of 4 structures studied in the region. Decoupling between surface salt motion and climatic conditions suggests that the regional tectonic regime controls surface salt displacement rates. Lastly, on a more local scale, this work examines the characteristics of anthropogenic deformation. Unnatural, rapid rates of subsidence and/or uplift have extreme hazard potential because it may lead to infrastructure damage and increased flood potential. Surface subsidence resulting from hydrocarbon extraction has been widely observed across the globe. However, the occurrence of surface uplift caused by fluid injection has only recently been noted and is less documented. An unusual surface displacement distribution at the Dawanqi oil field in the Kuqa fold-thrust belt of northwestern China suggests that fluid extraction may not only cause widespread, irreversible subsidence but also facilitate local uplift

Dome Formation During Crustal Extension in the Himalaya: Kinematic and Pressure-Temperature-Time-Deformation Constraints on Extensional Exhumation Along the Southern Margin of the Tibetan Plateau

The Himalaya and Tibetan Plateau were built by a combination of south-directed thrusting, north-directed extension, and generally east-west-directed extension within the Himalaya and Tibetan Plateau all to accommodate convergence between the Indian and Eurasian plates that began in the Eocene. Normal-sense shear zones that accommodate roughly east-west-directed extension across the southern margin of the Himalaya have exhumed young metamorphic domes across the Himalayan front. These metamorphic domes contain high-grade metamorphic rocks bound by normal-sense shear zones. The purpose of this study is to evaluate how these normal-sense shear zones develop and how they exhume metamorphic domes, which remains poorly understood and is critical for understanding the processes that accommodate extensional exhumation in this convergent setting. Two examples of metamorphic domes that were exhumed by east-west-directed normal-sense shear zones in the Himalaya are the Ama Drime Massif, southern Tibet, and the Leo Pargil dome, northwest India-Tibet. The Ama Drime Massif is a 30 km-wide north-south-striking structure that thins toward the north. It is located ~50 km northeast of Mount Everest and is bound by the Nyönno Ri detachment on the eastern flank and the Ama Drime detachment on the western flank. The Leo Pargil dome, ~950 km west of Ama Drime, is a 20 km-wide, northeast-southwest-striking structure composed of high-grade metamorphic rocks and leucogranite. It is bound on the east by the Qusum detachment and on the west by the Leo Pargil shear zone. Field mapping and sample collection were combined with kinematic, microstructural, thermobarometric, and geochronologic methods to constrain the metamorphic conditions, the kinematics of deformation during shearing, the amount of exhumation, and the timing of metamorphism and shear zone initiation. These data demonstrate the exhumation on these normal-sense shear zones in the Himalaya are controlled by an interplay between various processes including a regional kinematic setting that favored extension which led to strain partitioning, fault reactivation, decompression-driven melting, and the development of these deeply-rooted extensional systems

Controls on the distribution of landslides triggered by the 2008 Wenchuan earthquake, Sichuan Province, China

Landsliding is the dominant mass wasting process in upland areas where the rate of river incision is higher than that of rock weathering of hillslopes. Although progressive erosional processes can provide sufficient conditions for slope failure, the majority of landslides are induced by earthquakes, rainstorms or a combination of these two. Landslides are also one of the most destructive geological processes, being the primary cause of damage and fatalities associated with severe storms and earthquakes in mountainous regions. On 12th May 2008 the magnitude 7.9 Wenchuan earthquake occurred in the Longmen Shan mountain range, on the northwest margin of the Sichuan Basin. Landsliding contributed greatly to the high death toll of over 70,000 and widespread infrastructural damage produced by the earthquake. The event offers an opportunity to both broaden the global database of seismically induced landslides and study the processes involved in earthquake-triggered landsliding, for a large continental thrust event with complex faulting mechanisms and diverse geophysical conditions. To achieve this, the following investigation builds upon recent advances in landslide remote sensing, to develop automated detection algorithms through which landslides can be accurately mapped using a range of satellite data. Using these techniques, a first order, regional landslide inventory map of slope failures triggered by the Wenchuan earthquake is produced, over an area of 12,000km2 along the main rupture zone. The production of this dataset demonstrates the application of automated classification techniques for the rapid generation of landslide data, for both geomorphological research and hazard management applications. The data is used to examine the interaction of fault rupture dynamics, topography and geology on landslide failure location, and identify key characteristics of the landslide distribution. Findings of the study demonstrate high levels of landslide occurrence along the entire mapped length of the rupture zone, and an exponential decay in landslide density with distance from the co-seismic surface ruptures. This is superimposed over a marked hanging wall effect, along with clear geological and topographic controls on landslide occurrence. Through generalised linear modelling, peak ground acceleration attenuation patterns, hillslope gradient, relief, local elevation and geology are identified as core controls on the location of landslides. The results of this research shed light on some increasingly recognised though poorly understood characteristics of seismically induced landslide distributions. The dataset produced contributes to the limited global database of earthquake-triggered landslide inventories, as well producing a widely applicable resource for further study of the Wenchuan earthquake and post-seismic landscape evolution

NASA geodynamics program: Bibliography

The Seventh Geodynamics Program report summarizes program activities and achievements during 1988 and 1989. Included is a 115 page bibliography of the publications associated with the NASA Geodynamics Program since its initiation in 1979

Modelling co- and post-seismic displacements revealed by InSAR, and their implications for fault behaviour

The ultimate goal of seismology is to estimate the timing, magnitude and potential spatial extent of future seismic events along pre-existing faults. Based on the rate-state friction law, several theoretical physical earthquake models have been proposed towards this goal. Tectonic loading rate and frictional properties of faults are required in these models. Modern geodetic observations, e.g. GPS and InSAR, have provided unprecedented near-field observations following large earthquakes. In theory, according to the frictional rate and state asperity earthquake model, velocity-weakening regions holding seismic motions on faults should be separated with velocity-strengthening regions within which faults slip only aseismically. However, early afterslip following the 2011 MW 9.1 Tohoku-Oki earthquake revealed from GPS measurements was largely overlaid on the historical rupture zones, which challenged the velocity weakening asperity model. Therefore, the performance of the laboratory based friction law in the natural events needs further investigation, and the factors that may affect the estimates of slip models through geodetic modelling should also be discussed systematically. In this thesis, several moderate-strong events were investigated in order to address this important issue. The best-fit co- and post-seismic slip models following the 2009 MW 6.3 Haixi, Qinghai thrust-slip earthquake determined by InSAR deformation time-series suggest that the maximum afterslip is concentrated in the same area as the coseismic slip model, which is similar to the patterns observed in the 2011 Japan earthquake. In this case, complex geometric asperity may play a vital role in the coseismic nucleation and postseismic faulting. The major early afterslip after the 2011 MW 7.1 Van mainshock, which was revealed by one COSMO-SkyMed postseismic interferogram, is found just above the coseismic slip pattern. In this event, a postseismic modelling that did not allow slip across the coseismic asperity was also tested, suggesting that the slip model without slip in the asperities can explain the postseismic observations as well as the afterslip model without constraints on slip in the asperities. In the 2011 MW 9.1 Tohoku-Oki earthquake, a joint inversion with the GRACE coseismic gravity changes and inland coseismic GPS observations was conducted to re-investigate the coseismic slip model of the mainshock. A comparison of slip models from these different datasets suggests that significant variations of slip models can be observed, particularly the locations of the maximum slips. The joint slip model shows that the maximum slip of ~42 m appears near the seafloor surface close to the Japan Trench. Meanwhile, the accumulative afterslip patterns (slip >2 m) determined in previous studies appear in spatial correlation with the Coulomb stress changes generated using the joint slip model. As a strike-slip faulting event, the 2011 MW 6.8 Yushu earthquake was also investigated through co- and post-seismic modelling with more SAR data than was used in previous study. Best slip models suggest that the major afterslip is concentrated in shallow parts of the faults and between the two major coseismic slip patterns, suggesting that the performance of the rate and state frictional asperity model is appropriate in this event. Other postseismic physical mechanisms, pore-elastic rebound and viscoelastic relaxation have also been examined, which cannot significantly affect the estimate of the shallow afterslip model in this study. It is believed that the shallow afterslip predominantly controlled the postseismic behaviour after the mainshock in this case. In comparison to another 21 earthquakes investigated using geodetic data from other studies, complementary spatial extents between co- and post-seismic slip models can be identified. The 2009 MW 6.3 Qinghai earthquake is an exceptional case, in which the faulting behaviours might be dominated by the fault structure (e.g. fault bending). In conclusion, the major contributions from this thesis include: 1) the friction law gives a first order fit in most of natural events examined in this thesis; 2) geometric asperities may play an important role in faulting during earthquake cycles; 3) significant uncertainties in co- and post-seismic slip models can appreciably bias the estimation of fault frictional properties; 4) new insights derived from each earthquake regarding their fault structures and complex faulting behaviours have been observed in this thesis; and (5) a novel package for geodetic earthquake modelling has been developed, which can handle multiple datasets including InSAR, GPS and land/space based gravity changes