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1.
Compressional wave velocities (VP) at above-solidus temperatures and at 1 GPa were obtained for a granite and amphibolite, which are considered to be major constituents of the continental crust. The temperature variation of velocities showed that the VP values of granite decreased with rising temperature, but substantially increased beyond the melting temperature (850–900 °C). Such an increase may be caused by the α–β transition of quartz. The velocities of amphibolite decreased linearly with increasing temperature and dropped sharply at temperatures above the solidus (700 °C), indicating that partial melting of amphibolite acts to significantly lower the seismic velocities.  相似文献   

2.
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3.
The evolutionary history of the Pacific Ocean is reconstructed back to 60 m.y. B.P. based on the Hawaiian Island chain and Emperor seamounts, which join at an elbow to form the Hawaiian hotspot trace on the Pacific plate. This trace can be interpreted as a series of two rotations of the Pacific plate about the Hawaiian hotspot, presently located beneath Hawaii. Utilizing a pair of rotation poles in accordance with previously proposed models, the evolution can be described by the following:
1. (1) a rotation of 0.8°/m.y. about the Emperor pole of 17°N and 107°W from 60 to 42 m.y. B.P., and
2. (2) a rotation of 0.89°/m.y. about the Hawaiian pole of 69°N and 68°W from 42 m.y. B.P. to present.
This model fits several constraints:
1. (1) the hotspot trace must continually pass through the hotspot,
2. (2) the elbow must reach the hotspot at 42 m.y. B.P,
3. (3) transform faults must lie on observed fracture zones,
4. (4) the first contact between the North American and Pacific plates must occur at about 30 m.y. B.P., and
5. (5) the motion between the North American and Pacific plates has been right-lateral from 30 m.y. B.P. to present.
The overall fit of the digitized data of the hotspot trace to this model shows that it is possible to use the given poles for finite rotations and that the Kula-Pacific ridge is just now subducting.  相似文献   

4.
We have measured P- and S-wave velocities on two amphibolite and two gneiss samples from the Kola superdeep borehole as a function of pressure (up to 600 MPa) and temperature (up to 600 °C). The velocity measurements include compressional (Vp) and shear wave velocities (Vs1, Vs2) propagating in three orthogonal directions which were in general not parallel to inherent rock symmetry axes or planes. The measurements are accompanied by 3D-velocities calculations based on lattice preferred orientation (LPO) obtained by TOF (Time Of Flight) neutron diffraction analysis which allows the investigation of bulk volumes up to several cubic centimetres due to the high penetration depth of neutrons. The LPO-based numerical velocity calculations give important information on the different contribution of the various rock-forming minerals to bulk elastic anisotropy and on the relations of seismic anisotropy, shear wave splitting, and shear wave polarization to the structural reference frame (foliation and lineation). Comparison with measured velocities obtained for the three propagation directions that were not in accordance with the structural frame of the rocks (foliation and lineation) demonstrate that for shear waves propagating through anisotropic rocks the vibration directions are as important as the propagation directions. The study demonstrates that proper measurement of shear wave splitting by means of two orthogonal polarized sending and receiving shear wave transducers is only possible when their propagation and polarization directions are parallel and normal to foliation and lineation, respectively.  相似文献   

5.
Simultaneous measurements of compressional and shear wave velocities, Vp and Vs, in acidic and basic igneous rocks and volcanic glasses, were made up to 900°C and at 10–20 kbar.The effects of pressure and temperature on Vp and Vs in glasses and glassy rocks change at about 600°C, presumably the glass transition temperature. These effects are directly related to the silica content in the samples. and for obsidian are negative at room temperature and 245°C, but are positive at 655°C. The velocity—pressure relations for obsidian display an obvious hysteresis phenomena. for basalt glass is slightly negative, but is positive for usual substances at room temperature, and for obsidian and glassy andesite are positive up to about 600°C but are negative above that temperature. However, for basalt glass as well as other crystalline rocks, and are negative at all temperatures. Glass once heated above the glass transition temperature Tg under pressure P1 retains the memory of pressure P1 after it is cooled down below Tg and while subjected to another pressure P2. An abrupt shift of the velocities correlating to pressure P2 occurs when the glass is again heated to Tg. Vp−T and Vs−T relations for obsidian, glassy andesite, and basalt glass clearly exhibit this pressure memory.  相似文献   

6.
Two samples of Pliocene lignites from the Ptolemais basin of Greece, one from the upper and one from the lower lignite seams, were heated and dried in air at 50°C intervals from 50 to 1200°C. The two lignite samples initially contained the same minerals, namely calcite, dolomite, quartz, kaolinite, illite, pyrite and gypsum, but in different proportions. The lignite sample from the upper lignite seam is rich in Fe2O3, CaO and SO3, while that from the lower lignite seam is rich in SiO2 and Al2O3.Hematite, periclase, melilites, calcium ferrite and brownmillerite are constituents of the 1200°C lignite ash from both samples. The heating conditions and the chemistry of the samples lowered the formation temperatures of brownmillerite, which appeared in both samples at 950°C. In the Fe2O3, CaO- and SO3-rich sample, magnesioferrite is present from 850 to 1100°C and hematite appears at 300°C. In the SiO2- and Al2O3-rich sample, magnesioferrite was not detected at any temperature and hematite appeared at 600°C.Anhydrite, which normally decomposes in air at 1638°C, is the main constituent at 1150°C, on heating the lignite sample that was rich in Fe2O3, CaO and SO3. Anhydrite diminishes at 1200°C. In the SiO2- and Al2O3-rich lignite sample, anhydrite is main constituent at 1100°C, but diminishes considerably at 1150°C and decomposes at 1200°C.  相似文献   

7.
About 30 samples representing major lithologies of Sulu ultrahigh-pressure (UHP) metamorphic rocks were collected from surface exposures and exploration wells, and compressional (Vp) and shear wave (Vs) velocities and their directional dependence (anisotropy) were determined over a range of constant confining pressures up to 600 MPa and temperatures ranging from 20 to 600 °C. Samples range in composition from acidic to ultramafic. P- and S-wave velocities measured at 600 MPa vary from 5.08 to 8.64 km/s and 2.34 to 4.93 km/s, respectively. Densities are in the range from 2.60 to 3.68 g/cm3. To make a direct tie between seismic measurements (refraction and reflection) and subsurface lithologies, the experimental velocity data (corresponding to shallow depths) were used to calculate velocity profiles for the different lithologies and profiles of reflection coefficients at possible lithologic interfaces across the projected 5000-m Chinese Continental Scientific Drilling Program (CCSD) crustal segment. Comparison of calculated in situ velocities with respective intrinsic velocities suggests that the in situ velocities at shallow depths are lowered by an increased abundance of open microcracks. The strongly reflective zone beneath the Donghai drill site can be explained by the impedance contrasts between the different lithologies. Contacts between eclogite/peridotite and felsic rocks (gt-gneiss, granitic gneiss), in particular, may give rise to strong seismic reflections. In addition, shear-induced (lattice preferred orientation (LPO)-related) seismic anisotropy can increase reflectivity. For the explanation of the high velocity bodies (>6.4 km/s) around 1000 m and below 3200-m depth, large proportions of eclogite/peridotite (about 40 and 30 vol.%, respectively) are needed.  相似文献   

8.
The thermal stability of Paleozoic oil in eastern Tarim Basin, NW China was investigated through laboratory kinetic simulation experiments. Laboratory cracking of a selected marine oil sample from Ordovician strata in well LG-1 of Tarim Basin was performed by confined, dry pyrolysis system at T = 300–650 °C, P = 50 MPa. Results indicated the oil required higher temperature for cracking. At laboratory heating rates, oil cracking started at 390–400 °C and the laboratory cracking was completed at around 650 °C. At geological heating rates, the onset temperature is about 148–162 °C for cracking start and was completed at 245–276 °C. The oil-cracking history was recovered using the acquired kinetic parameters and the geothermal history of TD-2, and the threshold temperature for oil cracking under geological conditions was calculated. The oil cracking started at 165 °C (Ro = 1.45%) and stopped in early Devonian (390 Ma), and the oil-cracking rates in the strata of -O1 reached 60–70% at the end of Silurian. The calculated oil generation and oil cracking windows overlapped to some extent and were completed rapidly. The possible geological controls for the occurrence of residual oil reservoirs in Eastern Tarim basin have been discussed, including the high stability of the Paleozoic oil in Tarim Basin, the fast heating rate and longer duration time for oil cracking, the slight biodegradation in later uplift, the good preservation of the paleo-reservoirs and the moderate structural adjustment, which were critical for the exploration of residual oil and gases in this area.  相似文献   

9.
The geometry and thermal history of fractures have been determined at 59 stations from Reykjavik to Hvalfjördur in southwestern Iceland. The data provide information on crustal stress regimes in the vicinity of mid-ocean ridges.Two major, generalized fracture orientations are present
1. (1) a northeast system, trend 010°–030°, except on Akranes where the orientation is 040°–060°
2. (2) a broad east—west system containing one or more sets with strike between 070°–130°.
Thermal history of the host rock and fractures was determined from secondary minerals in vugs and fractures. The thermal history indicates that the northeast fracture set opened while the area was within the relatively hot axial zone of active volcanism and rifting. Some of the east—west trending fractures also opened at this time but many formed later, after the area had begun to cool and drift from the active zone.The northeast fracture set is essentially parallel to the trend of dikes and normal faults in southwestern Iceland. They have been interpreted as extension fractures (resulting in about 0.4% maximum extension) forming generally from the same stress field associated with normal faulting and dike injection in the active zone. Fracturing in an east-west direction (estimated 0.1% maximum extension), mainly near the edge and outside the active zone, indicates a reorientation of this stress field. The dominant mechanism related to the origin of the east—west fractures may be thermoelastic stresses arising from axial and basal accretion and cooling of lithospheric plates.Both fracture systems are inferred to have formed, in the Griffiths idealization, under nearly biaxial effective compressive loading on the order of 200 bar. The discrepancy between this value and the kilobar-order strengths of short-time laboratory tests reflects such factors as high temperature stress corrosion and fatigue. Fracture propagation is assumed to have been stable, but governed primarily by lateral load-diminishing mechanisms rather than by progressive loading. These relaxation mechanisms may have been episodic (northeast-system fissure swarm activity) or steady-state (thermoelastic contraction) in time.  相似文献   

10.
Eighty-two palaeomagnetic samples of calcareous and jaspilitic grainstones (iron-formation or ‘taconite’) and chert carbonate were collected from the 1.88-Ga Gunflint Formation at 22 sites in the Thunder Bay area, Ontario. Twenty clasts of Gunflint taconite also were sampled from the basal conglomerate of the overlying Mesoproterozoic Sibley Group. Anisotropy of magnetic susceptibility measurements indicate the Gunflint Formation in the sampling area has not experienced regional dynamic metamorphism. Analyses by variable-field translation balance and X-ray diffraction show that the predominant magnetic mineral is hematite but a small amount of magnetite also is present in some samples. Altogether, 213 Gunflint specimens and 59 Sibley conglomerate specimens were subjected to stepwise thermal demagnetisation and 74 Gunflint specimens to stepwise alternating-frequency demagnetisation. The following components were isolated for the taconites:
• Gunflint magnetite: normal declination D=293.4°, inclination I=30.8°, α95=7.2°, n=21; reverse D=86.7°, I=–54.6°, α95=5.8°, n=29.
• Gunflint hematite: normal D=243.6°, I=23.6°, α95=6.0°, n=11; reverse D=70.3°, I=–51.4°, α95=3.2°, n=79.
• Sibley clasts magnetite: normal D=282.7°, I=33.4°, α95=7.6°, n=20.
• Sibley clasts hematite: normal D=254.5°, I=56.2°, α95=8.4°, n=13; reverse D=110.6°, I=–55.7°, α95=8.3°, n=11.
None of these sets passed the reversal test, with the normal component generally being the shallower. Fold tests were negative or inconclusive and the conglomerate test also was negative. Chert carbonate at one other site appears to have acquired a remanence carried by magnetite (D=97.3°, I=−78.2°, α95=6.3°, n=18) prior to folding related to Keweenawan (1.1 Ga) Logan diabase sill emplacement. Most of the components we identified match components for Keweenawan sills, volcanic rocks, intrusions and baked contact rocks in the Thunder Bay area, indicating that Keweenawan magmatism caused widespread chemical remagnetisation of the Proterozoic country rock in our sampling area. Although others have argued that asymmetry was a feature of the Keweenawan geomagnetic field, the declinations of our Gunflint and Sibley hematite and magnetite components are different, suggesting that the components were acquired at significantly different times. We conclude that the reversal asymmetry shown by our Gunflint and Sibley data may best be ascribed to apparent polar wander during Keweenawan times.  相似文献   

11.
Aggregates composed of olivine and magnesiowüstite have been deformed to large strains at high pressure and temperature to investigate stress and strain partitioning, phase segregation and possible localization of deformation in a polyphase material. Samples with 20 vol.% of natural olivine and 80 vol.% of (Mg0.7Fe0.3)O were synthesized and deformed in a gas-medium torsion apparatus at temperatures of 1127 °C and 1250 °C, a confining pressure of 300 MPa and constant angular displacement rates equivalent to constant shear strain rates of 1–3.3 × 10− 4 s− 1. The samples deformed homogeneously to total shear strains of up to γ  15. During constant strain rate measurements the flow stress remained approximately stable at 1250 °C while it progressively decreased after the initial yield stress at the lower temperature. Mechanical data, microstructures and textures indicate that both phases were deforming in the dislocation creep regime. The weaker component, magnesiowüstite, controlled the rheological behavior of the bulk material and accommodated most of the strain. Deformation and dynamic recrystallization lead to grain refinement and to textures that were not previously observed in pure magnesiowüstite and may have developed due to the presence of the second phase. At 1127 °C, olivine grains behaved as semi-rigid inclusions rotating in a viscous matrix. At 1250 °C, some olivine grains remained largely undeformed while deformation and recrystallization of other grains oriented for a-slip on (010) resulted in a weak foliation and a texture typical for pure dry olivine aggregates. Both a-slip and c-slip on (010) were activated in olivine even though the nominal stresses were up to 2 orders of magnitude lower than those needed to activate these slip systems in pure olivine at the same conditions.  相似文献   

12.
The 3-D P- and S-wave velocity models of the upper crust beneath Southwest Iberia are determined by inverting arrival time data from local earthquakes using a seismic tomo~raphy method. We used a total of 3085 P- and 2780 S-wave high quality arrival times from 886 local earthquakes recorded by a per- manent seismic network, which is operated by the Institute of Meteorology （IM）, Lisbon, Portugal. The computed P- and S-wave velocities are used to determine the 3-D distributions of Vp/Vs ratio. The 3-D velocity and Vp/Vs ratio images display clear lateral heterogeneities in the study area. Significant veloc- ity variations up to ~6% are revealed in the upper crust beneath Southwest lberia, At 4 km depth, both P- and S-wave velocity take average to high values relative to the initial velocity model, while at 12 km, low P-wave velocities are clearly visible along the coast and in the southern parts. High S-wave velocities at 12 km depth are imaged in the central parts, and average values along the coast; although some scattered patches of low and high S-wave velocities are also revealed. The Vp/Vs rztio is generally high at depths of 4 and 12 km along the coastal parts with some regions of high Vp/Vs ratio in the north at 4 km depth, and low Vp/Vs ratio in the central southern parts at a depth of 12 km, The imaged low velocity and high Vp/Vs ratios are related to the thick saturated and unconsolidated sediments covering the region; whereas the high velocity regions are generally associated with the Mesozoic basement rocks.  相似文献   

13.
D. Arcay  M.-P. Doin  E. Tric  R. Bousquet   《Tectonophysics》2007,441(1-4):27-45
At continental subduction initiation, the continental crust buoyancy may induce, first, a convergence slowdown, and second, a compressive stress increase that could lead to the forearc lithosphere rupture. Both processes could influence the slab surface P–T conditions, favoring on one side crust partial melting or on the opposite the formation of ultra-high pressure/low temperature (UHP-LT) mineral. We quantify these two effects by performing numerical simulations of subduction. Water transfers are computed as a function of slab dehydration/overlying mantle hydration reactions, and a strength decrease is imposed for hydrated mantle rocks. The model starts with an old oceanic plate ( 100 Ma) subducting for 145.5 Myr with a 5 cm/yr convergence rate. The arc lithosphere is thermally thinned between 100 km and 310 km away from the trench, due to small-scale convection occuring in the water-saturated mantle wedge. We test the influence of convergence slowdown by carrying on subduction with a decreased convergence rate (≤ 2 cm/yr). Surprisingly, the subduction slowdown yields not only a strong slab warming at great depth (> 80 km), but also a significant cooling of the forearc lithosphere at shallower depth. The convergence slowdown increases the subducted crust temperature at 90 km depth to 705 ± 62 °C, depending on the convergence rate reduction, and might thus favor the oceanic crust partial melting in presence of water. For subduction velocities ≤ 1 cm/yr, slab breakoff is triggered 20–32 Myr after slowdown onset, due to a drastic slab thermal weakening in the vicinity of the interplate plane base. At last, the rupture of the weakened forearc is simulated by imposing in the thinnest part of the overlying lithosphere a dipping weakness plane. For convergence with rates ≥ 1 cm/yr, the thinned forearc first shortens, then starts subducting along the slab surface. The forearc lithosphere subduction stops the slab surface warming by hot asthenosphere corner flow, and decreases in a first stage the slab surface temperature to 630 ± 20 °C at 80 km depth, in agreement with P–T range inferred from natural records of UHP-LT metamorphism. The subducted crust temperature is further reduced to 405 ± 10 °C for the crust directly buried below the subducting forearc. Such a cold thermal state at great depth has never been sampled in collision zones, suggesting that forearc subduction might not be always required to explain UHP-LT metamorphsim.  相似文献   

14.
Finite-element folds of similar geometry   总被引：3，自引：0，他引：3  
Model folds of similar geometry have been produced by using the finite-element method and the constitutive relations of a layer of wet quartzite embedded in a marble matrix with an initially sinusoidal configuration and a 10° limb dip. The power law for steady-state flow of Yule Marble (Heard and Raleigh, 1972) is used for the matrix and our new law for Canyon Creek quartzite deformed in the presence of water is used for the layer. The equiv- alent viscosity of the wet quartzite is highly temperature-sensitive, giving rise to a strong temperature dependence of the quartzite: marble viscosity ratio which, at a strain rate of 10−14/sec, drops from 543 at 200° to 0.13 at 800°C. At 375°C (ηq/ηm = 10), concentric folds develop at all strains to 80% natural shortening and stress, finite strain and viscosity distributions are somewhat similar to those found previously. Raising the temperature to 550° C (ηq/ηm = 1), at any stage of prior amplification, causes the folds to flatten with increasing strain, accompanied by attenuation of limbs and thickening of hinges, leading to folds with similar geometries and isoclinal folds at extreme strains. The effects are more pronounced at higher temperatures and at 700° C (ηq/ηm = 0.3) limb attenuation is so severe as to give rise to unrealistic geometries. At temperatures below about 600° C (ηq/ηm = 2), similar folds do not form. It thus appears as if a viscosity contrast near unity is required to produce similar folds in rocks, under the conditions simulated and different temperature dependencies of viscosities of materials in layered sequences is one important means of reducing viscosity contrasts.  相似文献   

15.
Compressional (VP) and shear (VS) wave velocities and the dependent elastic constants have been determined by the pulse transmission technique to 6 kb confining pressure at room temperature and to 700° C at 6 kb confining pressure for eleven basalts from the Faeroe Islands. The Faeroe basalts investigated are tholeiitic, they clearly lie within the tholeiitic area, and display a pronounced trend of iron enrichment from rocks with an M/M + F ratio of 0.5 to rocks with an M/M + F ratio of about 0.25. The mean VP and VS for eleven specimens are 5.57 km/sec and 3.18 km/sec, respectively. Velocity—density relations for the basalts might be more appropriately described by non-linear solutions than by linear relations commonly used for basalts. In general, VP and VS remain unaffected by temperature up to 300° C. At higher temperature the changes in wave velocities are influenced by metamorphic processes and are, therefore, somewhat erratic. In zeolite-bearing specimens an abrupt velocity decrease around 350°C is observed, which correlates well with a drastic compaction of bulk volume. Additional experiments on cold-pressed zeolite powder clearly indicate that the sharp velocity decrease in the basalts is related to dehydration of zeolite minerals. Partial-melting processes, which occur within vesicules and pore-spaces at distinctly higher temperatures have no additional effect on wave velocity. Comparison with field data reveals that, without exception, the velocities at 0.5 kb confining pressure display the same range that has been commonly noted in refraction data for Layer 2. There are no significant differences in wave velocities and the pressure—temperature dependence in samples recovered from the upper, middle, and lower basalt series in the Faeroe Islands.  相似文献   

16.
Glide systems of hematite single crystals in deformation experiments   总被引：1，自引：0，他引：1  
The critical resolved shear stresses (CRSSs) of hematite crystals were determined in compression tests for r-twinning, c-twinning and {a}<m>-slip in the temperature range 25 °C to 400 °C, at 400 MPa confining pressure, and a strain rate of 10− 5 s− 1 by Hennig-Michaeli, Ch., Siemes, H., 1982. Experimental deformation of hematile crstals betwen 25 °C and 400 °C at 400 MPa confining pressure. In: Schreyer, W. (Ed.) High Pressure Research in Geoscience, Schweizerbart'sche Verlagsbuchhandlung, Stuttgart, p. 133–150. In the present contribution newly performed experiments on hematite single crystals at temperatures up to 800 °C at strain rates of 10− 5 s− 1 and 300 MPa confining pressure extends the knowledge about the CRSS of twin and slip modes. Optical observations, neutron diffraction goniometry, SEM forescatter electron images and electron backscatter diffraction are applied in order to identify the glide modes. Both twinning systems and {a}<m>-slip were confirmed by these methods. Besides the known glide systems the existence of the (c)<a>-slip system could be stated. Mechanical data establish that the CRSS of r-twinning decreases from 140 MPa at 25 °C to  5 MPa at 800 °C and for {a}<m>-slip from > 560 MPa at 25 °C to  40 MPa at 700 °C. At room temperature the CRSS for c-twinning is around 90 MPa and at 600 °C  60 MPa. The data indicate that the CRSSs above 200 °C seem to be between the values for r-twinning and {a}<m>-slip. For (c)<a>-slip only the CRSS at 600 °C could be evaluated to  60 MPa. Exact values are difficult to determine because other glide systems are always simultaneously activated.  相似文献   

17.
This investigation presents and interprets fluid inclusion data from different lithological units of the Cu skarn deposits at Mazraeh, north of Ahar, Azarbaijan, NW Iran. The results provide an assessment of the P–T conditions and mineral–fluid evolution and suggest new exploration parameters. Five types of inclusions are recognized from quartz and garnet. The temperature of homogenization of Type I inclusions with daughter minerals halite and sylvite ranges from 312° to 470 °C with total salinity of 52 to 63 wt.% NaCl equiv.; Type II and III inclusions with halite have homogenization temperatures of 230° to 520 °C and salinity of 31 to 50 wt.% NaCl equiv. The salinity of Types IV and V biphase (liquid + vapor) inclusions, based on their final ice melting temperature, varies between 10.2 to 20.8 wt.% NaCl equiv. Th vs. salinity plots of inclusions show that the salinity of the fluids correlates positively with temperature. The inclusions formed at low pressure. Changes in the temperature and salinity of the fluids can be reconstructed from the inclusions. Highly saline, high-temperature fluids were most abundant during the main chalcopyrite ore-forming phase in the skarn and mineralized quartz veins. Low-salinity aqueous fluids were abundant in barren veins, in which there is no evidence for early hot high-salinity brine, and might have resulted from late-stage dilution and mixing of hydrothermal fluids with meteoric water. Based on petrographic features and fluid-inclusion data, early-stage magnetite deposition is related to boiling of fluid at temperatures of about 500 °C. At a later stage, boiling at temperatures of around 320° to 400 °C favored the deposition of sulfides and Fe mobility was decreased at these lower temperatures. The following inclusion characteristics may be used as exploration parameters in the Mazraeh area. (i) Presence of high-temperature, salt-bearing inclusions, with Th between 300 and 500 °C; (ii) High-salinity fluid inclusions; and (iii) Inclusions showing evidence of boiling of the fluid. In addition, the presence of magnetite is an important exploration parameter.  相似文献   

18.
Subduction zones with deep seismicity are believed to be associated with the descending branches of convective flows in the mantle and are subordinated to them. Therefore, the position of subduction zones can be considered as relatively fixed with respect to the steady-state system of convective flows. The lithospheric plate overhanging a subduction zone (as a rule of continental type) may:
1. (1) either move away from the subduction zone; or
2. (2) move onto it. In the first case extensional conditions originate behind the subduction zone and the new oceanic crust of back-arc basins forms. In the second case active Andean-type continental margins with thickening of the crust and lithosphere are observed.
Behind the majority of volcanic island-arcs, along the boundary with marginal-sea basins, independent shallow seismicity belts can be traced. They are parallel to the main seismicity belts coinciding with the Benioff zones. The seismicity belts frame island-arc microplates. Island-arc microplates are assumed to be a frame of reference to calculate relative movements of the consuming and overhanging plates. Using slip vector azimuths for shallow seismicity belts in the frontal parts of the Kurile, Japan, Izu-Bonin, Mariana and Tonga—Kermadec arcs, the position of the pole of rotation of the Pacific plate with respect to the western Pacific island-arc microplates was computed. Its coordinates are 66.1°N, 119.2°W. From the global closure of plate movements it has been determined that for the past 10 m.y. the Eurasian and Indian plates have been moving away from the Western Pacific island-arc system, both rotating clockwise, around poles at 31.1°N, 164.2°W and 1.3°S, 157.5°W, respectively. This provides for the opening of the back-arc basins. At the same time South America is moving onto the subduction zone at the rate of 4 cm/yr. Some “hot spots”, such as Hawaiian, Tibesti, and those of the South Atlantic, are moving relative to the island-arc system at a very low rate, viz. 0.5–0.7 cm/yr. Presumably, the western Pacific subduction zone and “hot spots” form a single frame of reference which can generally be used for the analysis of absolute motions.  相似文献   

19.
To investigate the strength of frictional sliding and stability of mafic lower crust, we conducted experiments on oven-dried gabbro gouge of 1 mm thick sandwiched between country rock pieces (with gouge inclined 35° to the sample axis) at slip rates of 1.22 × 10− 3 mm/s and 1.22 × 10− 4 mm/s and elevated temperatures up to 615 °C. Special attention has been paid to whether transition from velocity weakening to velocity strengthening occurs due to the elevation of temperature.Two series of experiments were conducted with normal stresses of 200 MPa and 300 MPa, respectively. For both normal stresses, the friction strengths are comparable at least up to 510 °C, with no significant weakening effect of increasing temperature. Comparison of our results with Byerlee's rule on a strike slip fault with a specific temperature profile in the Zhangbei region of North China shows that the strength given by experiments are around that given by Byerlee's rule and a little greater in the high temperature range.At 200 MPa normal stress, the steady-state rate dependence a − b shows only positive values, probably still in the “run-in” process where velocity strengthening is a common feature. With a normal stress of 300 MPa, the values of steady-state rate dependence decreases systematically with increasing temperature, and stick-slip occurred at 615 °C. Considering the limited displacement, limited normal stress applied and the effect of normal stress for the temperatures above 420 °C, it is inferred here that velocity weakening may be the typical behaviour at higher normal stress for temperature above 420 °C and at least up to 615 °C, which covers most of the temperature range in the lower crust of geologically stable continental interior. For a dry mafic lower crust in cool continental interiors where frictional sliding prevails over plastic flow, unstable slip nucleation may occur to generate earthquakes.  相似文献   

20.
Five Neotoma spp. (packrat) middens are analyzed from Sand Canyon Alcove, Dinosaur National Monument, Colorado. Plant remains in middens dated at approximately 9870, 9050, 8460, 3000, and 0 14C yr B.P. are used to estimate Holocene seasonal temperature and precipitation values based on modern plant tolerances published by Thompson et al. (1999a, 1999b). Early Holocene vegetation at the alcove shows a transition from a cool/mesic to a warmer, more xeric community between 9050 and 8460 14C yr B.P. Picea pungens, Pinus flexilis, and Juniperus communis exhibit an average minimum elevational displacement of 215 m. Picea pungens and Pinus flexilis are no longer found in the monument.Estimates based on modern plant parameters (Thompson et al., 1999a) suggest that average temperatures at 9870 14C yr B.P. may have been at least 1° to 3°C colder in January and no greater than 3° to 10°C colder in July than modern at this site. Precipitation during this time may have been at least 2 times modern in January and 2 to 3 times modern in July. Discrepancies in estimated temperature and precipitation tolerances between last occurrence and first occurrence taxa in the midden record suggest that midden assemblages may include persisting relict vegetation.  相似文献   

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