Search

We used S-to-p converted waves from over a thousand seismic stations of the permanent Chinese National Seismic Network to study the large scale structure of the mantle lithosphere beneath the cratons in China. To avoid possible sidelobes of the Moho caused by the deconvolution in the S-receiver function method, we skipped the deconvolution and used the SV onset time as reference time instead of the time of the deconvolution spike. With this new method the lithosphere-asthenosphere boundary (LAB) is observed near 80 km depth below both the North and the South China cratons. However, at the north-eastern margin of the Tibetan Plateau the LAB is observed at about 160 km depth, smoothly shallowing towards the west and abruptly ending at the western end of the North China Craton. This structure is not visible in traditional S-receiver function data because it is overwhelmed by Moho sidelobes. There is no indication of a deeper (near 200 km) cratonic lithosphere-asthenosphere boundary in the other parts of the China cratons as it is observed in other cratons. We hypothesise that at the north-eastern margin of the Tibetan Plateau the original thickening of the lithosphere towards the craton is preserved whereas in most parts of the North and South China Cratons the lower part of the lithosphere is removed by some mechanism. The Moho is well observed. It deepens from 30 to 40 km at the sea shores in the east to 60–70 km below eastern Tibetan Plateau.

Continental collision between the Indian and Eurasian plates in the past ~55 ± 5 Ma has led to the development of the Tibetan Plateau, where nearly doubled crustal thickness indicates intense lithospheric thickening during the collision. On the other hand, geological evidence indicates significant lateral extrusion of Tibetan materials around the edges of the plateau, especially around the Eastern Himalaya Syntaxis. Two end-member models, “pure shear thickening” and “tectonic escape”, emphasize respectively the vertical and lateral deformation of Tibetan lithosphere during the collision. However, their competing roles in accommodating the Tibetan continental collision remain debated. Here, we developed a series of 3-D high-resolution numerical models to systematically study strain partitioning between crustal thickening and lateral extrusion during the collision. The model focuses on eastern Tibetan Plateau, where most of the escaping tectonics occurred. The model results indicate that the amount of crustal thickening is generally several times higher than that of lateral extrusion in accommodating the lithosphere shortening. The partitioning between crustal thickening and lateral extrusion depends on the rheological contrast between the Tibetan and neighboring blocks, whereas the relic suture zones within the Tibetan lithosphere do not control the strain partitioning but affect the morphology. The southward motion of the Indochina block, resulting from extension caused by trench retreating, has major impacts on the development of large-scale lateral extrusion tectonics. The vertical variation of lithospheric rheology leads to faster motion of the crust than the mantle lithosphere in the extruding block, hence mechanical decoupling between the extruding crust and lithospheric mantle.

Stepovers and bends along strike-slip faults are where push-up ranges and pull-apart basins are formed. They are also commonly where fault ruptures terminate. Field study and analogue models suggest that the configuration of faults plays a key role in crustal deformation around bends and stepovers, but the related mechanics of stress perturbation, strain partitioning, and fault evolution remains poorly understood. Here we present results of systematical mechanical models of stress changes and strain partitioning around simple stepovers and bends, using three-dimensional viscoelasto-plastic finite element code. Our model predicts elevated deviatoric stress around all stepovers and bends, with higher stresses around the restraining ones. Narrow stepovers localize strain between the fault gaps to form connecting faults, whereas wide stepovers localize strain on the tips of fault segments so the stepovers may evolve into subparallel faults. We explored how various configurations of stepovers and bends change the stress field and strain distribution, and show that these results can help explain some key differences between the pull-apart basins in the Dead Sea Trough and Death Valley, and the push-up ranges along the San Andreas Fault.

The Tian Shan mountain ranges in central Asia are one of the largest and most active intracontinental orogenic belts in the world. Its Cenozoic reactivation and deformation manifest the far-field impact of the Indian-Eurasian continental collision. How the collision-induced crustal shortening is accomondated in the Tian Shan region, however, remains debated. Some of the shortening is clearly accommodated by the piedmont fold-and-thrust belts; but the role of deformation in the interior of the orogen, and the overall pattern of crustal deformation in the Tian Shan region, differs in previous studies. We have used NeoKinema, a kinematic finite-element computer code, to analyze the long-term average rates of strain and its partitioning in the Tian Shan region. The model is constrained by updated kinematic data sets, including fault traces, geological fault slip rates, GPS site velocities, principal stress directions, and kinematic boundary conditions. Our results indicate that, in addition to shortening in the piedmont fold-and-thrust belts, significant shortening and strike-slip faulting have occurred in the interior of the Tian Shan orogen. The intra-orogen strain is concentrated north of the Nalati Fault, around the intramontane basins. The overall pattern of crustal deformation in the Tian Shan region is pure shear shortening, facilitated by NEE-trending sinistral and NW-NWW trending dextral strike-slip faults that cut across the mountain ranges. We also calculated the long-term potential of seismicity in the region and compared it with earthquake records.

Long records often show large earthquakes occurring in supercycles, sequences of temporal clusters of seismicity, cumulative displacement, and cumulative strain release separated by less active intervals. Supercycles and associated earthquake clusters are only partly characterized via the traditionally used aperiodicity, which measures the extent that a sequence differs from perfectly periodic. Supercycles are not well described by commonly used models of earthquake recurrence. In the Poisson model, the probability of a large earthquake is constant with time, so the fault has no memory. In a seismic cycle/renewal model, the probability is quasi-periodic, dropping to zero after a large earthquake, then increasing with time, so the probability of a large earthquake depends only on the time since the past one, and the fault has only “short-term memory.” We describe supercycles with a Long-Term Fault Memory (LTFM) model, where the probability of a large earthquake reflects the accumulated strain rather than elapsed time. The probability increases with accumulated strain (and time) until an earthquake happens, after which it decreases, but not necessarily to zero. Hence, the probability of an earthquake can depend on the earthquake history over multiple prior cycles. We use LTFM to simulate paleoseismic records from plate boundaries and intraplate areas. Simulations suggest that over timescales corresponding to the duration of paleoseismic records, the distribution of earthquake recurrence times can appear strongly periodic, weakly periodic, Poissonian, or bursty. Thus, a given paleoseismic window may not capture long-term trends in seismicity. This effect is significant for earthquake hazard assessment because whether an earthquake history is assumed to contain clusters can be more important than the probability density function chosen to describe the recurrence times. In such cases, probability estimates of the next earthquake will depend crucially on whether the cluster is treated as ongoing or over.

The Solonker suture zone is one of the most important tectonic boundaries in the southeastern part of the Central Asian Orogenic Belt (CAOB). An ~ 630 km-long reflection seismic profile across this suture was recently completed by the Chinese SinoProbe Project. The processed seismic data show clear crustal structures and provide new constraints on the tectonic and crustal evolution models. The Moho is delineated as a relatively flat boundary between a strongly reflective lower crust and a transparent mantle at a depth of ~ 40–45 km (~ 14.5 s two-way travel time), which is in agreement with the refraction data recorded along the same profile. In a broad view, the profile images an orogen that appears bivergent with, and approximately centered on, the Solonker suture zone. The southern portion of this profile is dominated by a crustal-scale, cratonward propagating fold-and-thrust system that formed during the late Permian and Triassic through collision and subsequent convergence in a post-collisional stage. The major thrust faults are truncated by Mesozoic granitoid plutons in the upper crust and by the Moho at the base of the crust. This geometry suggests that the Moho was formed after the thrusting event. The northern portion of the profile, although partially obliterated by post-collisional magmatic bodies, shows major south-dipping folding and thrusting. Bands of layered reflectors immediately overlying the Moho are interpreted as basaltic sills derived from the mantle. Episodic mafic underplating may have occurred in this region, giving rise to post-collisional magmatic events and renewal of the Moho. A few mantle reflectors are also visible. The overall geometry of these mantle reflectors supports the tectonic models that the southern orogen (Manchurides) experienced south-directed subduction and the northern orogen (Altaids) underwent north-directed subduction prior to collision along the Solonker suture zone.

The large-scale physical process of the Indo-Asian continental collision and the formation of the Himalayan–Tibetan Plateau have been simulated in various viscous thin-sheet models, but the thin-sheet simplification also kept some important issues from being fully explored. Among these issues are the role of strike-slip fault zones in facilitating large-scale lateral translation of lithospheric blocks (the escaping tectonics) during the collision, and the speculated lateral flow of the ductile middle–lower crust under the Tibetan Plateau. Here we present a fully three-dimensional finite element model to simulate the Indo-Asian continental collision. The model includes major boundary faults to simulate the escaping tectonics, and vertically variable rheological structures to model lower crustal flow. The collisional process is constrained by the history of the Indo-Eurasian plate convergence and the present crustal thickness and topography of the Tibetan Plateau. Our results indicate that the restrictive boundaries of the Tibetan Plateau, including the rigid Tarim and South China blocks, largely control the spatiotemporal patterns of crustal deformation in the collision zone. As the Indian indenter moves toward the Tarim block, higher strain rates and topography developed in the western part of the collision zone than in the eastern part, causing the northward migration of the deformation front to gradually change to eastward migration in the past 10–20 Myr, broadly consistent with the initiation of widespread E–W extension in the Plateau. These restrictive boundary blocks also force the crustal and mantle materials in the collision zone to flow coherently, hence providing an alternative explanation for the apparently vertically coherent deformation in Tibet. Assuming that the crust weakens as it thickens, our model predicts the lateral expansion of the Tibetan Plateau, an important feature of the Tibetan tectonics that is missing in previous models with constant rheology.

Studies of many large strike-slip earthquakes have shown that the seismicity rate increases in quadrants in which the Coulomb stress is increased by the earthquake rupture. This mechanism explains the concentration of aftershocks near the tips of the fault rupture, but cannot explain the aftershocks on the sides of the rupture zone, where the aftershocks are commonly abundant. We suggest that these aftershocks may be induced by variations of fault-zone properties not included in the previous models. In a finite-element model that treats the rupture plane as a zone with heterogeneous physical properties, as suggested by field observations and laboratory experiments, the Coulomb-stress changes triggered by the mainshock no longer uniformly decrease on the sides of the rupture zone, but show complex patterns of negative and positive changes. These results may explain some of the aftershocks on the sides of the mainshock rupture.

The 2011 Tohoku earthquake is another striking example – after the 2008 Wenchuan and 2010 Haiti earthquakes – of highly destructive earthquakes that occurred in areas predicted by earthquake hazard maps to be relatively safe. Here, we examine what went wrong for Tohoku, and how this failure illustrates limitations of earthquake hazard mapping. We use examples from several seismic regions to show that earthquake occurrence is typically more complicated than the models on which hazard maps are based, and that the available history of seismicity is almost always too short to reliably establish the spatiotemporal pattern of large earthquake occurrence. As a result, key aspects of hazard maps often depend on poorly constrained parameters, whose values are chosen based on the mapmakers' preconceptions. When these are incorrect, maps do poorly. This situation will improve at best slowly, owing to our limited understanding of earthquake processes. However, because hazard mapping has become widely accepted and used to make major decisions, we suggest two changes to improve current practices. First, the uncertainties in hazard map predictions should be assessed and clearly communicated to potential users. Recognizing the uncertainties would enable users to decide how much credence to place in the maps and make them more useful in formulating cost-effective hazard mitigation policies. Second, hazard maps should undergo rigorous and objective testing to compare their predictions to those of null hypotheses, including ones based on uniform regional seismicity or hazard. Such testing, which is common and useful in similar fields, will show how well maps actually work and hopefully help produce measurable improvements. There are likely, however, limits on how well hazard maps can ever be made because of the intrinsic variability of earthquake processes.

The 12 May 2008 Wenchuan earthquake (Mw 7.9) ruptured ∼ 300 km of the Longmen Shan fault, claiming ∼ 90,000 lives and devastating many cities in the Sichuan province, China. The coseismic stress changes due to the Wenchuan earthquake have been studied in kinematic models using the inferred coseismic fault slips, but the cause of the fault slips, the impact of other large earthquakes, and the mechanical interactions between the faults in eastern Tibet are uncertain. Here we explore these issues using a three-dimensional viscoelastoplastic dynamic model that calculates the regional stresses from tectonic and topographic loading, and simulates earthquakes and their stress perturbations. Our calculated coseismic changes of the Coulomb stress associated with the Great Wenchuan earthquake are similar to those in previous models. However, we show that the cumulative Coulomb stress changes, hence the implied earthquake risks, are significantly different when previous large earthquakes in the region are included in the model. Particularly, we show that in spite of stress increase from the Wenchuan earthquake, the southeastern segments of the Xianshuihe fault stay in a stress shadow because of the stress release by six M ≥ 6.9 events in this part of the Xianshuihe fault since 1893. We also found that interseismic locking on the Xianshuihe fault can increase the loading rate on the Longmen Shan fault by up to ∼ 50 Pa/year.

The 12 May 2008 Wenchuan earthquake (Ms 8.0) occurred on the Longmen Shan fault zone, where slip rates are low and earthquakes are infrequent in comparison with other major fault zones in the Songpan-Ganze region, eastern Tibet. We have investigated the evolution of strain energy along major faults in this region by comparing the accumulation and release of seismic moment. First, we calculated the slip rates on the Longmen Shan and other major faults in the region using a three-dimensional regional-scale block model, constrained by the latest GPS data. On the Longmen Shan fault, the predicted right-lateral and dip-slip rates are respectively 1.7 ± 0.8 mm/year and 1.2 ± 1.0 mm/year along the southwestern segments, and 1.4 ± 1.1 mm/year and 3.3 ± 1.3 mm/year on the northeastern segments. These slip rates are one order of magnitude lower than those on the Xianshuihe fault and other faults in the region. Second, using the earthquake catalog, we estimated the scalar moment released on major faults in the Songpan-Ganze region between 1879 and 2007. The released seismic moment was ~ 63% of the regional scalar moment accumulated during this period. The moment deficits were found mainly on the western Xianshuihe and eastern Kunlun faults. The eastern Xianshuihe fault has been a focus of studies because of the high slip rates and frequent earthquakes, but a sequence of major quakes there since 1893 has reduced the moment deficit to 3.68 × 1019 N m, barely enough for a Mw 7.0 event. It takes the Longmen Shan fault more than 1000 years to accumulate the seismic moment (1.15 × 1021 N m) released during the 2008 Great Wenchuan earthquake, hence we conclude that a repeat of great earthquakes on the ruptured segment of the Longmen Shan fault is unlikely in the next few hundred years, but the unruptured southwestern segment of the Longmen Shan fault is capable of producing a Mw 7.7 earthquake in the next 50 years.

The ∼ 2000 km crustal shortening from the Indo-Asian collision in the past 40–70 million years has been accommodated mainly by crustal thickening in the Himalayan–Tibetan Plateau and lateral extrusion of blocks of Asian continents. However, the spatiotemporal evolution of crustal thickening and lateral extrusion, hence the far-field impact of the Indo-Asian collision on Asian tectonics, remains controversial. Here we present a 3D viscous flow model that simulates the partitioning of crustal material between thickening and lateral extrusion during the Indo-Asian collision. The model assumes conservation of crustal mass, and is constrained by the history of the Indo-Asian plate convergence and by the present-day topography of the Himalayan–Tibetan Plateau. The results show that much of the early collision was absorbed by crustal thickening within the Himalayan–Tibetan plateau. However, lateral extrusion of crustal material has gradually become dominant in the past ∼ 10–20 Myr, indicating increasing influence of the Indo-Asian collision on Asian tectonics.

Earthquake hazard maps are produced to tell the public that some areas are more at risk than others, and to help governments prepare appropriately. To the extent a hazard map proves accurate, it helps society protect itself against earthquake damage. On the other hand, an inaccurate hazard map may lead to either inadequate preparation or suboptimal use of limited resources (Stein and Stein, in press). As we noted in our original paper, Stein et al. (2012), cited as SGL2012 below, the March 11, 2011 Tohoku, Japan, earthquake is a classic and unfortunate example of what can go wrong. Japan's national hazard map said (and still says) that the Tokai region (along the Pacific coast from Shizuoka to Nagoya) and the Tonankai and Nankai regions (farther to the west) had the highest hazard, with the hazard in Tohoku being considerably smaller. This indirectly led to a relatively low level of countermeasures in Tohoku generally and at the Fukushima Dai-ichi nuclear power plant in particular, which contributed to an unnecessarily large number of casualties and property damage, including the level-7 nuclear accident at Fukushima (Geller, 2011; Noggerath et al., 2011). The 2011 Tohoku earthquake is, alas, not an isolated example. As pointed out by SGL2012, the 2003 Algeria earthquake, 2004 Morocco earthquake, 2008 Wenchuan earthquake, and 2010 Haiti earthquake all also occurred in regions mapped as relatively low hazard. SGL2012 also cited work (Reyners, 2011) that showed that the February 22, 2011 Mw6.3 earthquake, which did considerable damage in Christchurch, New Zealand, produced much stronger ground motion than the hazard map predicted would occur in the

The Indo–Asian continental collision is known to have had a great impact on crustal deformation in south-central Asia, but its effects on the sublithospheric mantle remain uncertain. Studies of seismic anisotropy and volcanism have suggested that the collision may have driven significant lateral mantle flow under the Asian continent, similar to the observed lateral extrusion of Asian crustal blocks. Here we present supporting evidence from P-wave travel time seismic tomography and numerical modeling. The tomography shows continuous low-velocity asthenospheric mantle structures extending from the Tibetan plateau to eastern China, consistent with the notion of a collision-driven lateral mantle extrusion. Numerical simulations suggest that, at the presence of a low-viscosity asthenosphere, continued mass injection under the Indo–Asian collision zone over the past ∼50 My could have driven significant lateral extrusion of the asthenospheric mantle, leading to diffuse asthenospheric upwelling, rifting, and widespread Cenozoic volcanism in eastern China.

Following a protracted phase ( � 155–60 Ma) of crustal shortening and mountain building, widespread extension and magmatism have dominated the tectonic history of the North American Cordillera since early Tertiary. Although decades of intensive investigations have made North American Cordillera one of the best studied continental regions in the world, many fundamental issues, including the cause of metamorphic core complexes and basin-and-range extension and their relationship with the overlapping magmatism, remain controversial. Recent studies have emphasized the role of gravitational collapse in causing both extension and magmatism in the Cordillera, but the geodynamics of gravitational collapse are not well understood. Using simple thermal–rheological and thermomechanical modeling, we address the following questions. (1) Could gravitational collapse of the thickened Cordilleran crust have formed the metamorphic core complexes? (2) Did gravitational collapse induce the intensive mid-Tertiary volcanism in the Cordillera? (3) What caused basin-and-range extension and the associated basaltic volcanism? Our results show that, although a thickened crust at isostatic equilibrium is dynamically unstable and tends to collapse, major postorogenic extension happens only when the lithosphere is sufficiently weakened by thermal processes associated with orogenesis, including thermal relaxation, radioactive heating, shear heating, and mantle thermal perturbations. Ductile spreading within the lower crust plays a major role in postorogenic gravitational collapse. This mechanism can explain most metamorphic core complexes and the associated plutonism. However, it is unlikely to have induced major mantle upwelling required by the voluminous silicic eruption during the mid-Tertiary. The cause of the mid-Tertiary mantle upwelling remains speculative, but strong mantle thermal perturbations under the northern Basin and Range province may have persisted since mid-Miocene. Thermomechanical modeling shows that this mantle upwelling may have caused significant ductile deformation within the surrounding lithosphere, with lithospheric material being pushed away and downward from the upwelling asthenosphere. The loci of maximum lithospheric thinning are near the margins of the upwelling mantle and have migrated outward during basin-and-range extension. This process can explain some of the spatial–temporal evolution of extension and volcanism in the Cordillera since mid-Miocene. The resultant lithospheric structures are consistent with geophysical observations in the Basin and Range province. D 2001 Elsevier Science B.V. All rights reserved.

This paper evaluates thermotectonic models for metamorphism and leucogranite generation during the Proterozoic Trans-Hudson orogeny, as recorded in rocks exposed in the Black Hills, SD. Intrusion of the Harney Peak Granite and associated pegmatites at ∼1715 Ma occurred at the waning stages of regional deformation and staurolite-grade regional metamorphism. Published Consortium for Continental Reflection Profiling (COCORP) results indicate that Proterozoic sedimentary rocks were thrust over the Archean Wyoming province during the Trans-Hudson collision. Isotopic compositions of the Harney Peak Granite suggest that the exposed Proterozoic and Archean metasedimentary rocks in the Black Hills represent source rocks of the granites. Numerical simulations of the regional metamorphism and Harney Peak Granite generation, assuming crustal thickening by thrusting coupled with erosion, show the following: (1) Doubling of the crust with normal distribution of radioactive elements does not yield sufficiently high temperatures to cause anatexis anywhere in the crust or growth of garnet in the now exposed part of the crust; (2) a 35-km drop-off length for internal heat production can yield sufficient temperature for garnet growth at the current erosion level; it is, however, insufficient to produce staurolite, and melting can occur only in the deepest parts of the crust; (3) temperatures in crust with stable 70 km thickness for ∼40 Ma and 35 km drop-off length for heat production could become sufficient to produce staurolite at the current erosion level, and subsequent rapid denudation of the crust could potentially trigger decompression-melting of lower crustal rocks. Although this model could potentially explain the observed temporal relationship between regional metamorphism and leucogranite generation, it is inconsistent with melting of upper crustal Proterozoic source rocks that is indicated by isotopic compositions of the granites, with lack of evidence for rapid denudation of the Trans-Hudson orogen, and with confinement of the leucogranites to the deformed Proterozoic metapelitic rocks. Production of the Harney Peak Granite and its relationship to regional metamorphism of the country rocks are best explained by shear heating at the interface between the Wyoming province and overthrusted sedimentary rocks. We suggest that with reasonable rheologic properties of metapelites and rates of plate convergence, shear heating sufficiently perturbs locally the geotherms to cause anatexis in a deep shear zone system and growth of staurolite in the overlying crust. Modeling rheology of the lithologically stratified thickened crust, with granitic basement and metapelitic upper plate shows that the currently exposed part of the crust and the granite source region were ductile through much of the orogeny, which explains regional folding of the schists and predicts ductile shear zones in the granite source region. Because of the lithologic stratification, the granitic basement is likely to become significantly weaker during crustal thickening than the upper crust dominated by schists. A weak basement under a folded upper crust is likely to contribute to the observed relatively flat topography of high plateaus over thickened orogens.

Intrusion and underplating of mafic magmas tend to facilitate continental extension by thermatically weakening the lithosphere. However, adding rheologically hard mafic material to the crust also strengthens the lithosphere. We have investigated the time-dependent competing effects of thermal weakening and rheological hardening using a simple numerical model, and compared the results with the spatial and temporal developments of Tertiary tectonomagmatism in the North American Cordillera. The close temporal-spatial correlation between magmatism and formation of metamorphic core complexes in the Cordillera was consistent with model predictions when a relatively cold lithospheric mantle was assumed. In addition to thermally weakening the lithosphere, magma intrusion may have facilitated core-complex formation by reducing the effective viscosity of the lower crust, allowing crustal collapse to be decoupled from the mantle and to occur at relatively low stress levels. The complicated spatial and temporal patterns of tectonomagmatism in the Great Basin since the mid-Tertiary were predictable with intrusion and underplating of mafic magmas in conjunction with significant lithospheric thinning. Depending on the thermal structure of the lithosphere and the nature of mafic intrusion, the minimum lithospheric strength may lag the peak volcanism by a few million years; and the center of major volcanic fields may become relatively stronger than the surrounding areas because of rheological hardening.