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1.
High temperature creep of single crystal gadolinium gallium garnet (GGG) was studied in the temperature range of 1723–1853 K (0.86–0.94 Tm, Tm: melting temperature) and strain rate from 9 · 10?7 s?1 to 2 · 10?5 s?1. The compression tests were made along the 〈100〉 and 〈111〉 orientations. We have performed both constant strain-rate and stress-dip tests. For the 〈100〉 orientation, deformation occurs via the 〈111〉 slip systems. For the 〈111〉 orientation, both the 〈100〉 {010} and the 〈111〉 slip systems can be activated. GGG garnet is very strong under these conditions: σ/μ=(1–3)×10?3 (σ: creep strength, μ: shear modulus). The creep behavior is characterized by a power law with stress exponent n=2.9–3.3 and high activation energies E*=612–743 kJ/mol (E*～45×RTm, at zero stress which decrease with the increase of stress). Stress-dip tests suggest a small internal stress (σi/σ～0.62; σi: internal stress, σ: applied stress) compared to other materials. These results suggest that the high creep strength of GGG is mainly due to difficulty of dislocation glide rather than dislocation climb.  相似文献   

2.
In an extensional shear zone in the Talea Ori, Crete, quartz veins occur in high-pressure low-temperature metamorphic sediments at sites of dilation along shear band boundaries, kink band boundaries and boudin necks. Bent elongate grains grown epitactically from the host rock with abundant fluid inclusion trails parallel to the vein wall indicate vein formation by crack-seal increments during dissolutionprecipitation creep of the host rock. The presence of sutured high-angle grain boundaries and subgrains shows that temperatures were sufficiently high for recovery and strain-induced grain boundary migration, i.e. higher than 300 -350℃, close to peak metamorphic conditions. The generally low amount of strain accumulated by dislocation creep in quartz of the host rock and most veins indicates low bulk stress conditions of a few tens of MPa on a long term. The time scale of stress-loading to cause cyclic cracking and sealing is assumed to be lower than the Maxwell relaxation time of the metasediments undergoing dissolution-precipitation creep at high strain rates(10-10 s-1 to 10-9 s-1), which is on the order of hundred years. In contrast, some veins discordant or concordant to the foliation show heterogeneous quartz microstructures with micro-shear zones, sub-basal deformation lamellae, shortwavelength undulatory extinction and recrystallized grains restricted to high strain zones. These microstructures indicate dislocation glide-controlled crystal-plastic deformation(low-temperature plasticity) at transient high stresses of a few hundred MPa with subsequent recovery and strain-induced grain boundary migration at relaxing stresses and temperatures of at least 300 -350℃. High differential stresses in rocks at greenschist-facies conditions that relieve stress by creep on the long term, requires fast stress-loading rates, presumably by seismic activity in the overlying upper crust. The time scale for stress loading is controlled by the duration of the slip event along a fault, i.e. a few seconds to minutes.This study demonstrates that microstructures can distinguish between deformation at internal low stress-loading rates(to tens of MPa on a time scale of hundred years) and high(coseismic) stress-loading rates to a few hundred MPa on a time scale of minutes.  相似文献   

3.
Deformation experiments on olivine aggregates were performed under hydrous conditions using a deformation-DIA apparatus combined with synchrotron in situ X-ray observations at pressures of 1.5–9.8 GPa, temperatures of 1223–1800 K, and strain rates ranging from 0.8 × 10?5 to 7.5 × 10?5 s?1. The pressure and strain rate dependencies of the plasticity of hydrous olivine may be described by an activation volume of 17 ± 6 cm3 mol?1 and a stress exponent of 3.2 ± 0.6 at temperatures of 1323–1423 K. A comparison between previous data sets and our results at a normalized temperature and a strain rate showed that the creep strength of hydrous olivine deformed at 1323–1423 K is much weaker than that for the dislocation creep of water-saturated olivine and is similar to that for diffusional creep and dislocation-accommodated grain boundary sliding, while dislocation microstructures showing the [001] slip or the [001](100) slip system were developed. At temperatures of 1633–1800 K, a much stronger pressure effect on creep strength was observed for olivine with an activation volume of 27 ± 7 cm3 mol?1 assuming a stress exponent of 3.5, water fugacity exponent of 1.2, and activation energy of 520 kJ mol?1 (i.e., power-law dislocation creep of hydrous olivine). Because of the weak pressure dependence of the rheology of hydrous olivine at lower temperatures, water weakening of olivine could be effective in the deeper and colder part of Earth’s upper mantle.  相似文献   

4.
This paper discusses in detail the deformation textures, glide system, petrofabrics and olivine dislocation microstructures of mantle peridotites at Yushigou in the North Qilian Mountains, northwestern China. The peridotites have undergone high-pressure, high-temperature and low-strain rate plastic flow deformation. According to the dynamic recrystallized-grain size of olivine and the average spacing between the dislocation walls as well as the chemical composition of enstatite, the authors calculated the rheological parameters of the ancient upper mantle in the study area as follows: temperatures 1025-1093℃; pressures 3043-4278 MPa; depths 95-132 km; deviatoric stress 28-32 MPa; strain rates 0.2×10-14-2.13×10-14s-1 and equivalent viscosities 0.45×1020-4.65×1020 Pa · s. These parameters suggest that the position where plastic flow took place was correspondent to the low-velocity zone beneath the oceanic lithosphere and that oceanization characterized by middle-velocity (1-3 cm/a) sea-floor spreadi  相似文献   

5.
Interdiffusion coefficients of Al + Al vs. Mg + Si in the gehlenite–åkermanite system of melilite were determined by coupled annealing of synthesized end-member single crystals. The observed diffusion coefficients for a couple-annealed sample vary for about 2 orders of magnitude, showing strong dependence on the gehlenite–åkermanite composition: diffusion coefficient observed at 1350 °C, for example, is 3 × 10?13 cm2 s?1 at 5 mol% åkermanite composition (Ak5), increases to 2 × 10?11 cm2 s?1 at Ak80, and then decreases to 1 × 10?12 cm2 s?1 at Ak95. The diffusion coefficient–temperature relation indicates high activation energy of diffusion of about 420 kJ mol?1 for gehlenite-rich melilite. The observed diffusion coefficient–composition relation may be explained by a combination of (1) the diffusion coefficient–melting temperature relation (Flynn's rule) and (2) the feasibility of local charge compensation, which can possibly be maintained more easily in the intermediate chemical composition. The high activation energy value for gehlenitic melilite appears to correspond to the complex diffusion mechanism. The observed highly variable diffusion coefficients suggest that gehlenite–åkermanite zoning in the melilite crystals in Ca, Al-rich inclusions in the carbonaceous meteorites may provide a sensitive indicator for the thermal history of the inclusions.  相似文献   

6.
Eclogite plays an important role in mantle convection and geodynamics in subduction zones. An improved understanding of processes in the deeper levels of subduction zones and collision belts requires information on eclogite rheology. However, the deformation processes and associated fabrics in eclogite are not well understood. Incompatible views of deformation mechanism have been proposed for both garnet and omphacite. We present here deformation behaviour of eclogite at temperatures of 1027–1427 °C, confining pressures of 2.5–3.5 GPa, and strain rates of 1 × 10?5 s?1 to 5 × 10?4 s?1. We obtained a power‐law creep for the high temperature and pressure deformation of a ‘dry’ eclogite (50 vol.% garnet, 40% omphacite and 10% quartz) with A = 103.3 ± 1.0, n = 3.5 ± 0.4, ΔE =403 ± 30 KJ mol?1 and ΔV = 27.2 cm3 mol?1. The two principal minerals of eclogite have greatly different strengths. Progressive increase of garnet results in a smooth increase in strength. Analysis by electron back‐scattered diffraction shows that: (1) garnet displays pole figures with near random distributions of misorientation angle under both dry and wet conditions; (2) omphacite shows pronounced lattice preferred orientations (LPOs), suggesting a dominant dislocation creep mechanism. Further investigation into the water effects on eclogite show: (3) water content does not influence the style of omphacite fabric but increases slightly the fabric strength; (4) grain boundary processes dominate the deformation of garnet under high water fugacity or high shear‐strain conditions, yielding a random LPO similar to that of non‐deforming garnet, despite the strong shape preferred orientation (SPO) observed. {110} [001] slip may dominate the deformation of rutile. Quartz displays complicated and inconsistent LPOs in eclogite. These results are remarkably similar to observations from deformed eclogites in nature.  相似文献   

7.
Timing of highly stable millisecond pulsars provides the possibility of independently verifying terrestrial time scales on intervals longer than a year. An ensemble pulsar time scale is constructed based on pulsar timing data obtained on the 64-m Parkes telescope (Australia) in 1995–2010. Optimal Wiener filters were applied to enhance the accuracy of the ensemble time scale. The run of the time-scale difference PTens?TT(BIPM2011) does not exceed 0.8 ± 0.4 μs over the entire studied time interval. The fractional instability of the difference PTens?TT(BIPM2011) over 15 years is σ z = (0.6 ± 1.6) × 10?15, which corresponds to an upper limit for the energy density of the gravitational-wave background Ω g h2 ~ 10?10 and variations in the gravitational potential ~10?15 Hz at the frequency 2 × 10?9 Hz.  相似文献   

8.
The Anita Peridotite, in southwestern New Zealand, is a ∼1 × 20 km ultramafic massif that was rapidly extruded from beneath a Cretaceous arc within the 4 km wide mylonitic Anita Shear Zone. The peridotitic body contains a spectacular array of textures that preserve evidence for changing temperature, stress, and deformation mechanisms during the exhumation process. Olivine and orthopyroxene microstructures and lattice-preferred orientations (LPO) record a three-phase deformation history. Dislocation glide on the C- and E-type slip systems is recorded by coarse pre-mylonitised olivine grains, and occurred under hydrous conditions at T ∼650 °C, stress ∼200–700 MPa and strain rate ∼10−15 s−1, probably within hydrated sub-arc mantle lithosphere. Rare protomylonite pods record deformation by dislocation creep in porphyroclasts and dislocation-accommodated grain boundary sliding in the matrix on {0kl}[100] in olivine and (100)[001] in orthopyroxene, under conditions of T ∼730–770 °C, stress ∼52–700 MPa and strain rate ∼10−15 s−1. The massif, however, is dominated by mylonite and ultramylonite that wrap the protomylonite pods, comprising mostly fine-grained olivine neoblasts that lack internal distortions and have uniform LPOs. These textures indicate deformation occurred by grain-size sensitive (GSS) creep at T ∼650 °C, stress ∼69–137 MPa and strain rate ∼10−15 s−1, and thus during conditions of cooling and decreasing stress. GSS creep became more dominant with time, as the proportion of randomly-oriented neoblasts increased and formed interlinked networks that accommodated much of the strain. Grain boundary pinning allowed GSS creep to be maintained in polyphase regions, following mixing of olivine and orthopyroxene, which may have occurred by grain boundary transport in a fluid phase during a “creep cavitation” process. The results indicate that the Anita Peridotite recrystallised and underwent rheological weakening at a constant strain rate, with strain distributed across the entire section. This widespread deformation caused rapid exhumation of the peridotite from the lithospheric mantle into the overlying arc crust. The massif therefore records multiple overprinting phases of deformation under mantle and crustal conditions associated with the rapid exhumation of a large orogenic peridotite.  相似文献   

9.
Na2MgSiO4 crystals prepared hydrothermally at 700° C and 3,000 atm are related to carnegieite with SG Pmn21, a=7.015(2), b=10.968(2), and c=5.260(1). Na conductivity in Na2MgSiO4 is 3.0×10?5 (ohm-cm)?1 at 300° C but can be raised to 1.1×10?3 (ohm-cm)?1 by creating Na vacancies in the composition Na1.9Mg0.9Al0.1O4. Na4Mg2Si3O10 is also a cristobalite-related carnegieite with the orthorhombic cell a=10.584(7), b=14.328(7), and c=5.233(5). The Na conductivity of Na4Mg2Si3O10 is 4.8×10?3 (ohm-cm)?1 at 300° C.  相似文献   

10.
Strain rate during testing, uniaxial or triaxial, has important influence on the measured mechanical properties of rocks. Uniaxial compression tests were performed at nine pre-specified static-to-quasistatic strain rates (ranging from 1 × 10?5 to 1 × 10?1 s?1) on coarse crystal marble. The aim is to gain deep insight into the influence of strain rate on characteristic stresses, deformation properties and conversion of strain energy of such rock. It is found that the strain rate of 5 × 10?3 s?1 is the threshold to delineate the failure modes the tested coarse marble behaves in. At a strain rate less than this threshold, single-plane shear and conjugate X-shaped shear are the main failure modes, while beyond this threshold, extensile and splitting failures are dominant. The stress for crack initiation, the critical stress for dilation, the peak stress, and Young’s modulus are all found to increase with strain rate, with an exception that the above stresses and modulus appear relatively low compared to the strain rate in the range of between 1 × 10?4 and 5 × 10?3 s?1. The pre-peak absorbed strain energy, damage strain energy and elastic strain energy are found to increase with strain rate. In addition, the elastic strain energy stored before peak point favors brittle failure of the specimen, as the more stored elastic energy in the specimen, the stronger the fragmenting.  相似文献   

11.
The diffusivities of network-forming cations (Si4+, Al3+, Ge4+ and Ga3+) in melts of the jadeitic composition NaAl(Si, Ge)2O6 and Na(Al, Ga)Si2O6 have been measured at pressures between 6 and 20 kbar at 1400°C. The rates of interdiffusion of Si4+-Ge4+ and Al3+-Ge3+ increase with increasing pressure at constant temperature. The results are consistent with the ion-dynamics computer simulations of Jadeite melt by Angellet al. (1982, 1983). The coefficient measured for the Si4+-Ge4+ interdiffusion is between 8 × 10?10 and 2.5 × 10?8cm2sec at 6 kbar, depending on the composition of the melt, whereas at 20 kbar it is between 7 × 10?9 and 2 × 10?7cm2sec. The effect of pressure is greater for more Si-rich compositions (i.e., closer to NaAlSi2O6 composition). The coefficient measured for the Al3+-Ga3+ inter- diffusion is between 9 × 10?10 and 3 × 10?9 cm2/sec at 6 kbar and between 3 × 10?9 and 1 × 10?8cm2sec at 20 kbar. The rate of increase in diffusivity with pressure of Al3+-Ga3+ (a factor of 3–4) is smaller than that of Si4+-Ge4+ (a factor of 7–17).The Si4+-Ge4+ interdiffusion in melts of Na2O · 4(Si, Ge)O2 composition has also been measured at 8 and 15 kbar for comparison. The effect of pressure on the diffusivity in this melt is significantly smaller than that for the jadeitic melts. The increase in diffusivity of the network-forming cations in jadeitic melts with increasing pressure may be related to the decrease in viscosity of the same melt. The present results, as well as the ion-dynamics simulations, suggest that the homogenization of partial melts and mixing of magmas would be more efficient at greater depths.  相似文献   

12.
To estimate the water saving potential of an irrigation area and create a scientific water saving plan, the irrigation water use efficiency and water productivity of the Hulanhe irrigation area for 2007–2014 were calculated, and the water saving potentials of different water saving plans were determined from the perspectives of engineering and crop water saving. The results showed that the evapotranspiration calculated from the surface energy balance algorithm for land model (SEBAL) agreed well with the measured results. The irrigation water use efficiency in the Hulanhe irrigation area was positively correlated with precipitation of irrigated land and was negatively correlated with the net irrigation water volume. The engineering water saving potential ranges for periods of 5, 8, 11, and 15 years were (1.702?×?108, 5.103?×?108) m3, (1.783?×?108, 5.184?×?108) m3, (1.865?×?108, 5.266?×?108) m3, and (2.301?×?108, 5.702?×?108) m3, respectively, and the water saving potential increased year over year. Low amounts of precipitation of irrigated land corresponded with small amounts of net irrigation water and greater water saving potential. Based on the cumulative frequency of the water productivity calculated for the Hulanhe irrigation area from 2007 to 2014, the target water productivity for short (50% of the multi-year average cumulative water productivity) and long (70% of the multi-year average cumulative water productivity) terms were 1.03 kg/m3 and 1.22 kg/m3, respectively, and the cumulative crop water saving potentials for short and long terms were 1.18?×?108 and 2.74?×?108 m3, respectively. These results provided a theoretical reference for creating water saving plans for irrigation areas.  相似文献   

13.
Shale, as a kind of brittle rock, often exhibits different nonlinear stress-strain behavior, failure and time-dependent behavior under different strain rates. To capture these features, this work conducted triaxial compression tests under axial strain rates ranging from 5×10?6 s?1 to 1×10?3 s?1. The results show that both elastic modulus and peak strength have a positive correlation relationship with strain rates. These strain rate-dependent mechanical behaviors of shale are originated from damage growth, which is described by a damage parameter. When axial strain is the same, the damage parameter is positively correlated with strain rate. When strain rate is the same, with an increase of axial strain, the damage parameter decreases firstly from an initial value (about 0.1 to 0.2), soon reaches its minimum (about 0.1), and then increases to an asymptotic value of 0.8. Based on the experimental results, taking yield stress as the cut-off point and considering damage variable evolution, a new measure of micro-mechanical strength is proposed. Based on the Lemaitre’s equivalent strain assumption and the new measure of micro-mechanical strength, a statistical strain-rate dependent damage constitutive model for shale that couples physically meaningful model parameters was established. Numerical back-calculations of these triaxial compression tests results demonstrate the ability of the model to reproduce the primary features of the strain rate dependent mechanical behavior of shale.  相似文献   

14.
The stress conditions of the ductile-to-brittle regime have been assessed along the Asuke Shear Zone (ASZ), which strikes NE–SW in the Cretaceous Ryoke granite terrain in SW Japan. Along the ASZ, pseudotachylyte and mylonitized pseudotachylyte are locally developed together with cataclasite. The simultaneous operation of dislocation creep and grain-size-sensitive creep, as indicated by the coexistence of the Z-maximum and relatively random c-axis lattice preferred orientations as well as the sizes of dynamically recrystallized quartz grains (6.40–7.79 μm) in the mylonitized pseudotachylyte, suggest differential stresses of 110–130 MPa at ∼300 °C. The e-twin morphology, twinning ratio, and distribution of the glide direction on the e-twin plane of the twinned calcite in the amygdules of the pseudotachylyte suggest the stress conditions of the σ1 and σ3 axes trend 228° and 320° and plunge 55° and 1°, respectively, and indicate differential stresses of 40–80 MPa at 150–200 °C. Based on kinematic indicators in the fault rocks, the stress conditions estimated from calcite twins, and the cooling history of the granitic protolith, the ASZ is inferred to have been activated under a stress state that caused sinistral normal movements before and after pseudotachylyte formation at 70–50 Ma.  相似文献   

15.
We have investigated the effect of undercooling and deformation on the evolution of the texture and the crystallization kinetics of remelted basaltic material from Stromboli (pumice from the March 15, 2007 paroxysmal eruption) and Etna (1992 lava flow). Isothermal crystallization experiments were conducted at different degrees of undercooling and different applied strain rate (T = 1,157–1,187 °C and $ \dot{\gamma }_{i} $ γ · i  = 4.26 s?1 for Stromboli; T = 1,131–1,182 °C and $ \dot{\gamma }_{i} $ γ · i  = 0.53 s?1 for Etna). Melt viscosity increased due to the decrease in temperature and the increase in crystal content. The mineralogical assemblage comprises Sp + Plg (dominant) ± Cpx with an overall crystal fraction (?) between 0.06 and 0.27, increasing with undercooling and flow conditions. Both degree of undercooling and deformation rate deeply affect the kinetics of the crystallization process. Plagioclase nucleation incubation time strongly decreases with increasing ΔT and flow, while slow diffusion-limited growth characterizes low ΔT—low deformation rate experiments. Both Stromboli (high strain rate) and Etna (low strain rate) plagioclase growth rates (G) display relative small variations with Stromboli showing higher values (4.8 ± 1.9 × 10?9 m s?1) compared to Etna (2.1 ± 1.6 × 10?9 m s?1). Plagioclase average nucleation rates J continuously increase with undercooling from 1.4 × 106 to 6.7 × 106 m?3 s?1 for Stromboli and from 3.6 × 104 to 4.0 × 106 m?3 s?1 for Etna. The extremely low value of 3.6 × 104 m?3 s?1 recorded at the lowest undercooling experiment for Etna (ΔT = 20 °C) indicates that the crystallization process is growth-dominated and that possible effects of textural coarsening occur. G values obtained in this paper are generally one or two orders of magnitude higher compared to those obtained in the literature for equivalent undercooling conditions. Stirring of the melt, simulating magma flow or convective conditions, facilitates nucleation and growth of crystals via mechanical transportation of matter, resulting in the higher J and G observed. Any modeling pertaining to magma dynamics in the conduit (e.g., ascent rate) and lava flow emplacement (e.g., flow rate, pāhoehoe–‘a‘ā transition) should therefore take the effects of dynamic crystallization into account.  相似文献   

16.
4He accumulated in fluids is a well established geochemical tracer used to study crustal fluid dynamics. Direct fluid samples are not always collectable; therefore, a method to extract rare gases from matrix fluids of whole rocks by diffusion has been adapted. Helium was measured on matrix fluids extracted from sandstones and mudstones recovered during the San Andreas Fault Observatory at Depth (SAFOD) drilling in California, USA. Samples were typically collected as subcores or from drillcore fragments. Helium concentration and isotope ratios were measured 4?C6 times on each sample, and indicate a bulk 4He diffusion coefficient of 3.5?±?1.3?×?10?C8 cm2?s?C1 at 21°C, compared to previously published diffusion coefficients of 1.2?×?10?C18 cm2?s?C1 (21°C) to 3.0?×?10?C15 cm2?s?C1 (150°C) in the sands and clays. Correcting the diffusion coefficient of 4Hewater for matrix porosity (??3%) and tortuosity (??6?C13) produces effective diffusion coefficients of 1?×?10?C8 cm2?s?C1 (21°C) and 1?×?10?C7 (120°C), effectively isolating pore fluid 4He from the 4He contained in the rock matrix. Model calculations indicate that <6% of helium initially dissolved in pore fluids was lost during the sampling process. Complete and quantitative extraction of the pore fluids provide minimum in situ porosity values for sandstones 2.8?±?0.4% (SD, n?=?4) and mudstones 3.1?±?0.8% (SD, n?=?4).  相似文献   

17.
Radon is a radioactive hazardous and ubiquitous gas. It has been recognized to be one of the major contributors to natural radiation even causing lung cancer if present at enhanced levels. There are large variations in data available in the literature for radium content and radon exhalation rates of various materials. It is a well-documented fact that radon exhalation from the ground surface depends upon a number of parameters such as soil grain size, soil porosity and radium content. For this purpose, in this study the so-called can technique has been used to measure radium content and exhalation rates of radon in soil samples collected from different places of Aligarh, Etah and Mathura districts of Uttar Pradesh??a province in northern India. These districts lie within the subtropical region of the Indo-Gangetic plains. The values of effective radium content are found to vary from 8.11 to 112.83?Bq?kg?1 with a mean value of 33.21?Bq?kg?1 and a standard deviation of 28.15. The values of mass exhalation rates of radon vary from 0.76?×?10?6 to 15.80?×?10?6?Bq?kg?1?day?1 with a mean value of 4.21?×?10?6?Bq?kg?1?day?1, while the surface exhalation rates vary from 1.97?×?10?5 to 41.03?×?10?5?Bq?m?2?day?1 with a mean value of 10.93?×?10?5?Bq?m?2?day?1.  相似文献   

18.
The thermal expansion of anhydrite, CaSO4, has been measured from 22° to 1,000° C by X-ray diffraction, using the Guinier-Lenné heating powder camera. The heating patterns were calibrated with Guinier-Hägg patterns at 25° C, using quartz as internal standard. Heating experiments were run on natural anhydrite (Bancroft, Ontario), which at room temperature has lattice constants in close agreement with those of synthetic material. The orthorhombic unit cell at 22° C (space group Amma) has a=7.003 (1) Å, b=6.996 (2) Å and c=6.242 (1) Å, V=305.9 (2) Å3. At room temperature, the thermal expansion coefficients α and β (α in °C?1×104, β in °C?2×108) are for a, 0.10, ?0.69; for b, 0.08, 0.19; for c, 0.18, 1.60; for V, 0.37, 1.14. Second-order coefficients provide an excellent fit over the whole range to 1,000° C.  相似文献   

19.
The exhumation history and tectonic evolution of the Qilian Shan at the north‐eastern margin of the Tibetan Plateau has been widely debated. Here, we present apatite fission‐track (AFT) data for 12 Ordovician granodiorite samples along a vertical transect in the eastern Qilian Shan. These thermochronometry data indicate that the eastern Qilian Shan experienced a three‐stage cooling history, including: (i) rapid initial cooling in the late Cretaceous; (ii) a stage of quasi isothermal quiescence from ~ 80 to 24 Ma; and (iii) rapid subsequent cooling beginning in the early Miocene. The inferred cooling rates for the three stages are 6.8 ± 4.9 °C Ma?1, 0.6 ± 0.2 °C Ma?1 and 2.7 ± 0.9 °C Ma?1 respectively (±1 σ). Assuming a geothermal gradient of 25 °C km?1, the exhumation rates for the three stages are 0.27 ± 0.20 mm a?1, 0.017 ± 0.007 mm a?1 and 0.11 ± 0.04 mm a?1 respectively (±1 σ). We suggest that the late Cretaceous cooling records collision of the Lhasa block with the Eurasian continent and that the Miocene cooling represents uplift/exhumation of the Qilian Shan.  相似文献   

20.
Current deformation in Pribaikalia, Western and Central Mongolia, and Tuva has been studied from measured horizontal GPS velocities and respective computed strain and rotation rates using 1994–2007 data of the Baikal–Mongolian GPS triangulation network.The GPS velocity field shows two main trends: an NE trend within Jonggaria, the Mongolian Altay, and the Great Lakes Valley and an SE trend in the Hangayn and eastern Gobi Altay mountains, and in the Transbaikalian block of the Amur plate. The velocity magnitudes and vectors are consistent with an SE motion of the Amur plate at a rate of ～2 mm/year.The derived strain pattern includes domains of crustal contraction and extension recognized from the magnitudes of relative strains. Shortening predominates in the Gobi and Mongolian Altay and in the Khamar-Daban Range, where it is at ?2 = (19.2 ± 6.0)×10?9 yr?1 being directed northeastward. Extension domains exist in the Baikal rift and in the Busiyngol–West Hangayn area, where the crust is stretching along NW axes at ?1 = (22.2 ± 3.1) × 10–9 yr–1. The eastern Hangayn dome and the Gobi peneplain on its eastern border show low and unstable strain rates. In central and northern Mongolia (Orhon–Selenge basin), shortening and extension are at similar rates: ?2 = (15.4 ± 5.4)×10?9 yr?1 and ?1 = (18.1 ± 3.1)×10?9 yr?1. The strain pattern changes notably in the area of the Mogod earthquake of 1967.Most of rotation throughout Central Asia is clockwise at a low rate of about Ω = 6×10?9 deg·yr?1. High rates of clockwise rotation are observed in the Hangayn domain (18.1 ± 5.2)×10?9 deg·yr?1, in the Gobi Altay (10.4 ± 7.5)×10?9 deg·yr?1, and in the Orhon–Selenge domain (11.9 ± 5.2)×10?9 deg·yr?1. Counterclockwise rotation is restricted to several domains. One is in western Tuva and northwestern Great Lakes Valley of Mongolia (Ω = 3.7×10?9 deg·yr?1). Two more counterclockwise rotation regions occur on both flanks of the Baikal rift: along the craton edge and in basins of Transbaikalia on the rift eastern border, where rotation rates are as high as (13.0 ± 3.9)×10?9 deg·yr?1, while rotation within the Baikal basin does not exceed the measurement error. Another such domain extends from the eastern Hövsgöl area to the Hangayn northern foothills, with the counterclockwise rotation at a highest rate of (16.3 ± 2.8)×10?9 deg·yr?1.  相似文献   

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