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Earthquake swarms are observed worldwide, especially in connection with fluid movement and volcanism. Two regions are compared
by numerical investigations using the finite element method: The Vogtland/NW-Bohemia area situated at the border between Germany
and the Czech Republic and the Magadi region in the Kenya Rift. For the Vogt-land area a high-precision three-dimensional
gravity model was developed. That modelling shows an interaction between geometries of geological structures and geodynamic
processes and yields strong indications that a magmatic system at the crust mantle boundary is much more probable than an
upwelling mantle as a source of the earthquake swarms. The geodynamic models for the two regions under investigation take
into account the regional stress field and thermal stresses as well as creep and plasticity with a porous elastic rheology.
The investigations are fo-cussed on the interaction between pore pressure variations, temperature changes, fluid movements,
stress accumulation and deformations. It is suspected that these processes play an essential role in the generation of earthquake
swarms. An essential result of the modelling is that the existence of the regional stress field alone neither explains the
occurrence of the earthquake swarms in the Vogtland area nor in the Magadi area. Temperature changes and periodic pore pressure
variations in the earth’s crust are most important for the geodynamic processes, although they are weighed differently in
each focal area.
Apatite fission-track (AFT) thermochronological modeling as a diagnostic tool for periods of stability (peneplanation) and tectonic activity (orogeny) has been broadly used in tectonic studies of Central Asia in recent years. We discuss more than 100 AFT ages of samples from the Kyrgyz Tien Shan and Altai and compare them with AFT data from northern Kazakhstan. Geological, geomorphological, and AFT data indicate intense activity in the Late Cenozoic Eurasian continental interior. The impact from the India-Eurasia collision on the northern Tien Shan, Altai, and northern Kazakhstan regions showed up at 11, 5, and 3 Ma, respectively, as a result of stress propagation into the continent, with the ensuing reactivation and mountain growth. We hypothesize that a distant effect of the Late Cenozoic India-Eurasia collision was to rejuvenate Paleozoic fault zones and to deform the Mesozoic sedimentary cover north of the collision front as far as the West Siberian Plate. The reactivation facilitated formation of tectonic oil and gas traps. The activity in northern Central Asia under the effect of the Indian indentation into Eurasia appears to continue and may evolve to include uplift of southern West Siberian plate with uplift.
The National Geospatial-Intelligence Agency (NGA) of the USA has embarked upon the development of a new Earth Gravitational
Model (EGM), to support future realizations of NGA’s World Geodetic System. Current plans call for the development of the
new EGM (EGM05) by the end of 2005. The new model will be complete to degree and order 2160, and aims at a ±15 cm global Root
Mean Square (RMS) geoid undulation error requirement. The new model will combine optimally the gravitational information that
is extracted from dedicated geopotential mapping satellite missions (CHAMP, GRACE), with the information contained within
a global gravity anomaly database of 5′×5′ resolution. This paper describes the development of a Preliminary Gravitational
Model (PGM2004 A). We developed PGM2004A by combining the GRACE-only model GGM02S, with a 5′×5′ global gravity anomaly database
compiled by NGA. PGM2004A is complete to degree and order 2160, and is accompanied by propagated error maps at 5′×5′ resolution,
accounting for the entire bandwidth of the model (from degree 2 to degree 2160), for various model-derived gravimetric quantities (Δg, N, ξ, η). We have evaluated PGM2004A through comparisons with independent data including GPS/Leveling data, astronomic deflections of the vertical over the conterminous US (CONUS), and altimeter
data from TOPEX. The results of these comparisons indicate that the goal set for EGM05 is well within reach. We summarize
in this paper our current status and technical accomplishments, and discuss briefly our next steps towards the development
of EGM05.
Records of deep-focus Hindu Kush earthquakes in the depth ranges 70–110 and 190–230 km made by 45 digital and analogue seismic
stations were analyzed to study the attenuation field of short period seismic waves in the lithosphere of central Tien Shan.
The dynamic characteristics studied include the ratio of peak amplitudes in S and P waves (S/P) and the ratio of the S-wave maximum to the coda level in the range t = 400 ± 5 s, where t is the lapse time (S/c400) for 1.25 Hz. Comparatively high values of S/P are shown to prevail in most of the area, corresponding to lower S-wave attenuation. Upon this background is a band of high and intermediate attenuation in the west of the area extending along
the Talas-Fergana fault in the south and afterwards turning north-northeast. The rupture areas of the two largest (M ≥ 7.0)
earthquakes which have occurred in Tien Shan during the last 25 years are confined to this band. Abnormally high values of
S/c400 were obtained for stations situated in the rupture zone of the August 19, 1992, magnitude 7.3 Suusamyr earthquake and
around it. For two of the stations we found considerable time variations in the coda envelope before the earthquake. The effective
Q was derived from compressional and shear wave data for the entire area, as well as for the band of high attenuation. Comparison
with previous data shows that the attenuation field in the area has changed appreciably during 20–25 years, which can only
be due to a rearrangement of the fluid field in the crust and uppermost mantle. It is hypothesized that a large earthquake
is very likely to occur in the northern part of the attenuating band.
A 3-D velocity model of the Tien Shan crust and upper mantle is constructed through the inversion of the receiver functions
of P and S waves together with teleseismic traveltime anomalies at nearly 40 local seismic stations. It is found that in the vast central
region, where no strong earthquakes have been known over the past century, the S wave velocity at depths of 10–35 km is lower than in adjacent regions by up to 10%. These data are evidence for mechanical
weakness of the crust preventing the accumulation of elastic energy. Apparently, the lower velocity and the weakness of the
crust are due to the presence of water. The weakness of the crust is one of the possible reasons for the strain localization
responsible for the formation of the present Tien Shan but can also be due in part to the young orogenesis. The crustal thickness
is largest (about 60 km) in the Tarim-Tien Shan junction zone. The crust-mantle boundary in this region descends by a jump
as a result of an increase in the lower crust thickness. This is probably due to the underthrusting of the Tien Shan by the
Tarim lithosphere. This causes the mechanically weak lower crust of the Tarim to delaminate and accumulate in nearly the same
way as an accretionary prism during the subduction of oceanic lithosphere. In the upper mantle, the analysis has revealed
a low velocity anomaly, apparently related to basaltic outflows of the Upper Cretaceous-Early Paleogene. The Cenozoic Bachu
uplift in the northern Tarim depression is also associated with the low velocity anomaly. The Naryn depression is characterized
by a high velocity in the upper mantle and can be interpreted as a fragment of an ancient platform.
On the base of the GPS-measured velocity field referring to the recent crust movements over sizable terrestrial areas (Central Tien Shan), the strain rate tensor is evaluated as the tensor components are governed by space gradients of the velocity field. The areas of the extreme values of the strain rate tensor components are shown to coincide with the highest seismic activity areas. Also shown is the fact that, in the direction of the crust surface layer compression, the deep layer electric conductivity reaches its maximum. A simplest explanation of this phenomenon is proposed.
Multivariate kernel density estimation provides information about structure in data. Feature significance is a technique for deciding whether features-such as local extrema-are statistically significant. This paper proposes a framework for feature significance in d-dimensional data which combines kernel density derivative estimators and hypothesis tests for modal regions. For the gradient and curvature estimators distributional properties are given, and pointwise test statistics are derived. The hypothesis tests extend the two-dimensional feature significance ideas of Godtliebsen et al. [Godtliebsen, F., Marron, J.S., Chaudhuri, P., 2002. Significance in scale space for bivariate density estimation. Journal of Computational and Graphical Statistics 11, 1-21]. The theoretical framework is complemented by novel visualization for three-dimensional data. Applications to real data sets show that tests based on the kernel curvature estimators perform well in identifying modal regions. These results can be enhanced by corresponding tests with kernel gradient estimators.
The GSHAP Regional Centre in Moscow, UIPE, has coordinated the seismic hazard mapping for the whole territory of the former U.S.S.R. and border regions. A five-year program was conducted to assemble for the whole area, subdivided in five overlapping blocks, the unified seismic catalogue with uniform magnitude, the strong motion databank and the seismic zones model (lineament-domain-source), which form the basis of a newly developed deterministic-probabilistic computation of seismic hazard assessment. The work was conducted in close cooperation with border regions and GSHAP regional centers. The hazard was originally computed in terms of expected MSK intensity and then transformed into expected peak ground acceleration with 10% exceedance probability in 50 years.
Spatiotemporal variations of the S-wave attenuation field in the Earth's crust of the Tien Shan region were studied from the records of underground nuclear explosions at the Semipalatinsk nuclear test site. It is shown that anomalously strong changes in the attenuation field structure are observed for the paths that cross or pass near the source zones of strong earthquakes (M ≥ 6.8). We suggest that this phenomenon is related to the ascent of mantle-derived fluids (first of all, water) into the Earth's crust before strong earthquakes and their subsequent lateral migration from the source zones.
We have determined slip rates on the most active reverse faults, reconstructed an extensive preorogenic erosion surface, constructed local and regional cross sections, and dated syntectonic Tertiary sedimentary rocks by magnetostratigraphy along a north-south transect that spans the Kyrgyz portion of the west-central Tien Shan. The cumulative Late Quaternary shortening rate along this transect is 10 mm/yr. The trabsect consists of five major fault zones, and the most active faults lie in the interior of the range. Using geometric models developed in other regions of basement-involved determination, we estimate shortening during the Late Cenozoic at 35-80 km. Apparent simultaneous onset of sedimentary basins (at least 3 major basins) about 12 Ma BP is interpreted to mark the onset of the current orogeny. Given the current shortening rate of about 10 mm/yr, measured across active faults and by GPS, we infer that the rate increased with time. We assumed accelerated shortening and have shown that it has always been of similar style, dominated by north-south shortening across east-west trending basement-involved reverse faults. Deformations were localized in five zones, which border the largest and deepest Tertiary basins, show the greatest structural relief, and contain the currently most active faults.
The 1911 Ms =8.2 Kemin (Kebin) earthquake in the northern Tien Shan (Kazakhstan, Kyrgyzstan) formed a complex system of surface ruptures nearly 190 km long and numerous landslides and rock avalanches up to tens of millions of cubic meters in volume. Judging from their distribution, six fault segments of the Kemin-Chilik and the Aksu fault zones with different strikes, dips, and kinematics were activated. The Kemin earthquake was one of the strongest events of a sequence of seismic catastrophes that affected the Kungei and Trans-iii-Alatau mountain ranges between 1887 and 1938. The effects of the Kemin earthquake are well documented in a monograph published soon after the event by K. I. Bogdanovich. In the framework of the European INCO·COPERNICUS program, the surface ruptures, landslides, and rockslides associated with this earthquake have been reexamined in detail. In addition, the large-scale tectonic setting of the Kemin-Chilik and Aksu fault zones has been re-evaluated, and their segments have been identified and described. The whole system forms a sinistral transpressional structure, which controls the formation of the mountain ranges between the Issyk-Kul' depression and the Kazakhstan block. The surface ruptures of the 1911 earthquake can presently be observed in the field over a total length of nearly 100 km and :generally reactivate longer-term cumulative paleoseismic fault scarps. The presence of well-expressed paleoseismic fault scarps and several tremendous ancient landslides in the Chon-Kemin, Chon-Aksu, and Aksu valleys can be considered as evidence for strong prehistoric earthquakes. Active fault, landslides, 1911 Kemin earthquake, Tien Shan, Kyrgyzstan
The large scale structure of the earth is caused by geodynamic processes which are explained using energetic, kinematic and dynamic descriptions. While “geodynamic processes” are understood to include a large variety of processes and the term is used by earth scientists quite loosely, the methods of their description involve well defined fields. Energetic descriptions are involved with distribution of energy in our planet, typically expressed in terms of heat and temperature. Kinematic descriptions describe movements using velocities, strains and strain rates and Dynamic descriptions indicate how stresses and forces behave.
To obtain an image of the deep structure of the Tien Shan in central Asia, we invert P and S receiver functions jointly for almost 40 local broad-band seismograph stations. The inversion is performed using a simulated annealing technique. The combined inversion is an improvement on earlier studies, where P and S receiver functions were inverted separately. Using this approach, we deal with structural imaging problems that are usually investigated with teleseismic body wave and surface wave tomography techniques. We demonstrate that the uppermost mantle in the north of the central Tien Shan is composed of a high-velocity lid a few tens of kilometers thick above a pronounced low-velocity zone. The crustal structure in this region provides evidence of magmatic underplating. These features are likely related to a small plume that is manifested by basaltic eruptions of Cretaceous–Paleogene age. The low-velocity layer is also found in a southeast trending corridor, which may correspond to the Bachu uplift in the Tarim basin. Crustal thickness beneath the orogen varies from about 45 to about 70 km. The smallest values, most likely inherited from the pre-orogenic era, are found in a neighborhood of the Talas–Fergana fault. Similar values are characteristic of the Kazakh shield in the north and the Tarim basin in the south. The largest values are found beneath the bounding ranges. We infer that uplift of the central Tien Shan is unlikely to be caused by crustal shortening alone.
Previous models of fault-propagation folding used kink-band geometries to approximate folding in front of propagating thrusts. However, kink-band kinematics cannot replicate the curved fold surfaces and complex strain patterns innatural and experimental fault-propagation folds, which also occur in front of steeper reverse and normal faults. Fault-propagation fold hinges tighten and converge downward, forming a triangular zone of penetrative deformation focused on the tip of the propagating fault. The downward convergence of deformation in fault-propagation folds can be modeled as triangular shear zones. Trishear, here defined as distributed, strain-compatible shear in a triangular (in profile) shear zone, provides an alternate kinematic model for fault-propagation folds. Trishear is analogous to simple shear in a tabular shear zone except that area balance in a triangular shear zone requires curved displacement oblique to the fault slip direction. Incremental computer models of trishear folding can replicate many geometric features of fault-propagation folds, including variably curved fold hinges, downward-tightening fold surfaces, heterogeneous strains, and multiple fault-propagation trajectories.
Three Twentieth Century earthquakes that Richter assigned M>=8 and two of comparable magnitude in the Nineteenth Century imply rapid deformation within the Tien Shan. Seismic moment tensors of major earthquakes in this century suggest an average shortening rate of 7 (+/-2) mm yr-1 across the Tien Shan. In the western part, however, where three of the five largest earthquakes occurred, the calculated rate is consistent with the value of ~20 mm yr-1 measured using GPS in that area by Abdrakhmatov et al. As Avouac et al. suggested, the high rate in the western part apparently is a manifestation of counter-clockwise rotation of the Tarim Basin relative to Eurasia about an axis near the east end of the Tien Shan.
The study of several swarm earthquake regions led us to the assumption that swarm quakes mostly occur in combination with fluid migration and often in the neighbourhood of volcanoes. Observations of the strong carbon dioxide flux and investigations of Weinlich et al. (1999) about recent volcanism in the Vogtland/ Western Bohemia are the linking part to global observations of swarm earthquakes. This idea and the theoretical modelling of Yamashita (1999) were the starting point to develop a model using the finite-element-method with the software package ABAQUS concerning stress field, pore pressure, temperature and deformation in the Vogtland swarm quake area. The model is intended to reveal the interaction between fluid migration, temperature changes and deformation in this area. Its horizontal dimensions are 55 km×60 km and 30 km in depth. The main geological units include the Marianske Lazne Fault Zone and the Eger Rift. We focus on that part of the Vogtland/Western Bohemia region where these two fault zones cross. The model takes into consideration the stress field, Mohr–Coulomb failure, thermal stresses as well as creep and poro-elastoplastic rheologies. The results show that the rate of deformation and the stress accumulation caused only by the regional stress field is not high enough to generate the swarm earthquakes. But periodic change of pore pressure (which includes fluid migration) in combination with temperature changes has a severe influence on the rate of deformation and is in the dimension to cause swarm quakes.
ABSTRACT This paper introduces a method for the evaluation of the seismic risk at the site of an engineering project. The results are in terms of a ground,motion parameter (such as peak acceleration) versus average,return period. The method,incorporates the influence of all potential sources of earthquakes and the average activity rates assigned to them. Arbitrary geographical relationships between,the site and po- tential point, line, or areal sources can be modeled with computational ease. In the range of interest, the derived distributions of maximum annual ground motions are in the form of Type I or Type II extreme value distributions, if the more com- monly assumed magnitude,distribution and attenuation laws are used.
Restorable cross sections of foreland basement uplifts must contain faults whose curvatures are consistent with the relative slip and tilt between adjacent basement blocks. Cylindrical fault surfaces can explain the uniform dip of strata on the back side of foreland uplifts; local zones of shortening and extension occur in hinge zones above transitions in fault curvature. High-curvature fault splays form fault wedges of basement which ease the transition from thrust and reverse faulting of basement blocks to folding in the sedimentary cover. 20 references, 4 figures.
We used the arrival times of local earthquakes and quarry blasts recorded by the Kyrgyzstan Broadband Network (KNET) to obtain three-dimensional (3-D) P and S wave velocity models of the upper crust beneath an actively deforming mountain front and its associated foreland in the Kyrgyz Tien Shan. The continuous velocity models, described by cubic B spline interpolation of the squared slowness over a regular 3-D grid, were computed by simultaneous inversion of hypocenter and medium parameters. Exact ray tracing was done in the smooth 3-D medium by shooting rays from the sources to the stations by an analytical perturbation method based on the paraxial ray theory. The deduced large, sparse, linear system was solved using the damped, iterative, least squares algorithm LSQR. The stability and resolution of the result was qualitatively tested by two synthetic tests: the spike test and the checkerboard resolution test. We found that the models are well resolved up to a depth of ~27km for most parts of our image domain. The P and S wave velocity models are consistent with each other and provide evidence for marked heterogeneity in the upper crustal structure beneath the northern Tien Shan. At shallower depths (
An increase in the use of computers in structural geology now encourages practical investigation of several topics which are of considerable importance to the explorationist. Computer-based cross section construction and analysis is one such application. Algorithms based on the geometry of flexural slip or flow deformation styles permit rapid construction restoration, and balancing of geological cross sections, which in turn allow evaluation of multiple working hypotheses in a time frame previously unattainable. These same techniques also simplify the application of several analytical methods which have tended to be restricted to structural geologists: predicted finite and incremental strain patterns within folds can be utilized in studies of porosity and permeability variation; the detailed geometry of fold can be evaluated and modified using local balancing methods when constraints provided by well, seismic, and surface data leave room for differing interpretations; and subsurface fault trajectories can be quickly and accurately predicted from knowledge or near-surface fold geometry. These and other methods discussed in the text permit the non-specialist to apply complex structural concepts of exploration in a practical and timely manner.
The problem of maintaining uniform magnitude scales is important in seismology. It is shown that the surface wave magnitudes measured in the Soviet Union (MLH) are in good agreement with those from Pasadena, Berkeley and Palisades, and are considered to be a good continuation of M as used by Gutenberg and Richter in their catalogue. Body wave magnitudes, however, show a systematic difference between USCGS and Russian determinations, the Coast and Geodetic Survey values being significantly lower. Several possible reasons for this are examined and it is concluded that the regular contribution of certain stations (BMO, UBO, TFO and WMO) which consistently report lower magnitudes than most stations is a major cause of the discrepancy.
This paper presents uniformly calculated surface-wave magnitudes of over 400 earthquakes in the European area and adjacent regions associated with strong-motion recordings. Seismic moments and body-wave magnitudes are estimated also, so that they may be used in the derivation of attenuation laws. Using a more complete set of data the attenuation law derived for western North American earthquakes is modified for surface-wave magnitudes.
The inference of fault geometry from suprajacent fold shape relies on consistent and verified forward models of fault-cored folds, e.g. suites of models with differing fault boundary conditions demonstrate the range of possible folding. Results of kinematic (fault-parallel flow) and mechanical (boundary element method) models are compared to ascertain differences in the way the two methods simulate flexure associated with slip along flat-ramp-flat geometry. These differences are assessed by systematically altering fault parameters in each model and observing subsequent changes in the suprajacent fold shapes. Differences between the kinematic and mechanical fault-fold relationships highlight the differences between the methods. Additionally, a laboratory fold is simulated to determine which method might best predict fault parameters from fold shape. Although kinematic folds do not fully capture the three-dimensional nature of geologic folds, mechanical models have non-unique fold-fault relationships. Predicting fault geometry from fold shape is best accomplished by a combination of the two methods.
In this paper we present magnitude (Ms) – magnitude(mb) and magnitude-intensity relationships which areconsidered the most adequate in the Ibero-Maghrebianregion. This work is based on selected samples ofrecently revised events with magnitude mb assigned bythe Instituto Geográfico Nacional (I.G.N.) and Msassigned by I.S.C and N.E.I.C., and isoseismal mapsfrom 142 events. Using these data, we have obtainedone magnitude (Ms) – magnitude (mb) relationship, twomagnitude (mb and Ms) assignment relationships viaepicentral intensity (I0), and ten magnitude (mb andMs) assignments relationships via macroseismicinformation: four using Ambraseys' methodology (1985)and six using the isoseismal area of degree III, IV and VI.
According to the obtained results it could be concluded that historical magnitude assignment with lesser uncertainties are those obtained via macroseismic information using magnitude-intensity relationships with Ambraseys' methodology (1985). The magnitude-isoseismal area assignment relationships have, in most cases, great differences depending on the degree of the isoseismal area used. Magnitude assignments via epicentral intensity have the highest uncertainties. Geographic regionalization of the relationshipshas been studied but the highest correlations and statistical significance are obtained when we fit all the Ibero-Maghrebian region data.
Finally we have used the results obtained in this workto assign magnitude to some important historicalearthquakes in the Ibero-Maghrebian region: the 1755Lisbon earthquake, the 1680 Málaga earthquake, the1829 Torrevieja earthquake and the 1884 Arenas del Reyearthquake. According to our relationships andmethodology we have assigned an Ms value of 9.3 ±0.6 to the 1755 Lisbon earthquake (its mb magnitudecannot be estimated due to the saturation of the mbscale), an mb value of 6.3 ±0.4 and an Ms valueof 6.9 ± 0.6 to the 1829 Torrevieja earthquake, anmb value of 6.2 ± 0.4 and an Ms value of 6.4 ±0.6 to the 1680 Málaga earthquake and an mb valueof 6.1 ± 0.4 and an Ms value of 6.5 ± 0.6 tothe 1884 Arenas del Rey earthquake.
THE Tien Shan-a high, seismically active intracontinental mountain belt, 1,000-2,000 km north of the Himalaya-has grown as a result of India's collision with Asia(1). The crustal shortening (similar to 200 +/- 50 km; refs 2, 3) and thickening that gave rise to the Tien Shan accommodates only a small fraction of India's total penetration into Asia (2,000-3,000 km), and the temporal relationship of deformation in this belt to the India-Asia collision remains unclear. Here we report geodetic measurements of the Tien Shan, using the Global Positioning System (GPS), that indicate that the current crustal shortening rate is nearly half of India's convergence rate with Eurasia in this area(4). We infer a total shortening rate for the Tien Shan of similar to 20 mm yr(-1), which is approximately twice that inferred previously from the extrapolation of slip rates in the Holocene(3) and earthquake-induced displacements during this century(5), suggesting that the rate of mountain building in this region has accelerated severalfold since the onset of collision similar to 50-55 Myr ago(6,7). If, as we argue, the current shortening rate can be extrapolated to geological timescales, then our results suggest that most of the Tien Shan has been constructed during the past 10 Myr, perhaps in response to an increased horizontal force following an abrupt rise of the Tibetan plateau(8,9).
We have studied geometries and rates of late Cenozoic thrust faulting and folding along the northern piedmont of the Tien Shan mountain belt, West of Urumqi, where the M=8.3 Manas earthquake occurred on December 23, 1906. The northern range of the Tien Shan, rising above 5000 m, overthrusts a flexural foredeep, filled with up to 11,000 m of sediment, of the Dzungarian basement. Our fieldwork reveals that the active thrust reaches the surface 30 km north of the range front, within a 200-km-long zone of Neogene-Quaternary anticlines. Fault scarps are clearest across inset terraces within narrow valleys incised through the anticlines by large rivers flowing down from the range. In all the valleys, the scarps offset vertically the highest terrace surface by the same amount (10.2+/-0.7 m). Inferring an early Holocene age (10+/-2 kyr) for this terrace, which is continuous with the largest recent fans of the piedmont, yields a rate of vertical throw of 1.0+/-0.3 mm/yr on the main active thrust at the surface. A quantitative morphological analysis of the degradation of terrace edges that are offset by the thrust corroborates such a rate and yields a mass diffusivity of 5.5+/-2.5 m2/kyr. A rather fresh surface scarp, 0.8+/-0.15 m high, that is unlikely to result from shallow earthquakes with 6 < M < 7 in the last 230 years, is visible at the extremities of the main fold zone. We associate this scarp with the 1906 Manas earthquake and infer that a structure comprising a deep basement ramp under the range, gently dipping flats in the foreland, and shallow ramps responsible for the formation of the active, fault propagation anticlines could have been activated by that earthquake. If so, the return period of a 1906 type event would be 850+/-380 years. The small size of the scarp for an earthquake of this magnitude suggests that a large fraction of the slip at depth (almost-equal-to 2/3) is taken up by incremental folding near the surface. Comparable earthquakes might activate flat detachments and ramp anticlines at a distance from the front of other rising Quaternary ranges such as the San Gabriel mountains in California or the Mont Blanc-Aar massifs in the Alps. We estimate the finite Cenozoic shortening of the folded Dzungarian sediments to be of the order of 30 km and the Cenozoic shortening rate to have been 3+/-1.5 mm/yr. Assuming comparable shortening along the Tarim piedmont and minor additional active thrusting within the mountain belt, we infer the rate of shortening across the Tien Shan to be at least 6+/-3 nun/yr at the longitude of Manas (almost-equal-to 85.5-degrees-E). A total shortening of 125+/-30 km is estimated from crustal thickening, assuming local Airy isostatic equilibrium. Under the same assumption, serial N-S sections imply that Cenozoic shortening across the belt increases westwards to 203+/-50 km at the longitude of Kashgar (almost-equal-to 76-degrees-E), as reflected by the westward increase of the width of the belt. This strain gradient implies a clockwise rotation of Tarim relative to Dzungaria and Kazakhstan of 7+/-2.5-degrees around a pole located near the eastern extremity of the Tien Shan, west of Hami (almost-equal-to 96-degrees-E, 43.5-degrees-N), comparable to that revealed by paleomagnetism between Tarim and Dzungaria (8.6-degrees+/-8.7-degrees). A 6 mm/yr rate of shortening at the longitude of Manas would imply a rate of rotation of 0.45-degrees/m.y.and would be consistent with a shortening rate of 12 mm/yr north of Kashgar. Taking such values to be representative of Late Cenozoic rates would place the onset of reactivation of the Tien Shan by the India-Asia collision in the early to middle Miocene (16 +22/-9 m.y.), in accord with the existence of particularly thick late Neogene and Quaternary deposits. Such reactivation would thus have started much later than the collision, roughly at the time of the great mid-Miocene changes in tectonic regimes, denudation and sedimentation rates observed in southeast Asia, the Himalayas and the Bay of Bengal, and of the correlative rapid change in seawater Sr isotopic ratio (20 to 15 Ma). Like these other changes, the rise of the Tien Shan might be a distant consequence of the end of Indochina's escape.
Results from repeated measurements across a 90-sites global positioning system network in Central Asia provide direct evidence of current high rates of tectonic deformation far north of the India–Eurasia suture zone. The data help to quantify both the partitioning of deformation within the seismically active Tien Shan and Northern Pamirs, as well as the ongoing rotation of virtually undeformed blocks such as the Tarim with respect to stable Eurasia. The NNW to SSE shortening rates derived from the computations reach values of 23±3 mm yr−1 over the ranges studied, suggesting that regional deformation rates have increased considerably within the study area since the onset of Cenozoic shortening 20–25 Myr ago.
Strain within nappes and thrust sheets can be modelled using simple boundary conditions. An original rectangular prism of material resting on a thrust plane is considered to be subject to various pure and simple shear components operating parallel to the thrust. Deformation gradient tensors are determined for the models and solutions found for the principal strains.A two-dimensional model considers heterogeneous simple shear combined with a stretch (α) in the direction of shear. Simple shear zones arise as a special case where α = 1. Recumbent fold nappes, characteristic of gravitational collapse, can be modelled with α > 1, and thrust sheets with layer parallel shortening and folding with α < 1.Differential transport of nappes gives rise to additional wrench-type shears. Solutions for the principal strains in this type of model are found and used to interpret side-wall ramps and steep zones parallel to nappe movements. By including a stretch in the transport direction these differential transport models produce prolate strains (α > 1) and oblate strains (α < 1).Finally a brief consideration of bending strains produced in a sheet passing over a ramp indicates reversals in the shear direction during the strain history. These predict possible crenulation of early thrust fabrics at ramps.
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