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1.
The thermal evolution of Corsica as recorded by zircon fission-tracks   总被引：1，自引：0，他引：1  
New zircon fission-track (ZFT) ages from Corsica record multiple thermal events that can be tied to the structural evolution of the western Mediterranean region. The Corsican zircons have a wide scatter of ZFT grain ages (243–14 Ma), which together define several age domains. Western Corsica consists largely of stable Hercynian basement characterized by ZFT ages in the range 161–114 Ma. We interpret these ages (Late Jurassic–Early Cretaceous) as the product of a long-lived Tethyan thermal event related to continental rifting and subsequent drifting during the separation of the European and African plates and the formation of the Liguro–Piemontese ocean basin. In contrast to Hercynian Corsica, Alpine Corsica (northeast Corsica) experienced widespread deformation and metamorphism in Late Cretaceous(?)–Tertiary time. Dated samples from Alpine Corsica range in age from 112 to 19 Ma and all are reset or partially reset by one or more Alpine thermal events. The youngest ZFT grain ages are from the northernmost Alpine Corsica and define an age population at  24 Ma that indicates cooling after Tertiary thermal events associated with the Alpine metamorphism and the opening of the Liguro–Provençal basin. A less well-defined ZFT age population at  72 Ma is present in both Alpine Corsica and Hercynian basement rocks. The thermal history of these rocks is not clear. One interpretation is that the ZFT population at  72 Ma reflects resetting during a Late Cretaceous event broadly synchronous with the early Alpine metamorphism. Another interpretation is that this peak is related to variable fission-track annealing and partial resetting during the Tertiary Alpine metamorphic event across central to north-eastern Corsica. This partial age resetting supports the presence of a fossil ZFT partial annealing zone and limits the peak temperature in this area below 300 °C, for both the affected pre-Alpine and Alpine units.  相似文献   

2.
The crystalline terrane of the Tongbai–Dabie region, central China, comprising the Earth's largest ultrahigh-pressure (UHP) exposure was formed during Triassic collision between the Sino–Korean and Yangtze cratons. New apatite fission-track (AFT) data presented here from the UHP terrane, extends over a significantly greater area than reported in previous studies, and includes the (eastern) Dabie, the Hong'an (northwestern Dabie) and Tongbai regions. The new data yield ages ranging from 44 ± 3 to 142 ± 36 Ma and mean track lengths between 10 and 14.4 μm. Thermal history models based on the AFT data taken together with published 40Ar/39Ar, K–Ar, apatite and zircon (U–Th)/He and U–Pb data, exhibit a three-stage cooling pattern that is similar across the study region, commencing with an Early Cretaceous rapid cooling event, followed by a period of relative thermal stability during which rocks remained at temperatures within the AFT partial annealing zone (60–110 °C) and ending with a possible renewed phase of accelerated cooling during Pliocene to Recent time. The first cooling phase followed large-scale transtensional deformation between 140 and 110 Ma and is related to Early Cretaceous eastward tectonic escape and Pacific back arc extension. Between this phase and the subsequent slow cooling phase, a transition period from 120 to 80 Ma (to 70 to 45 Ma along the Tan–Lu fault) was characterised by a relatively low cooling rate (3–5 °C/Ma). This transition is likely related to a tectonic response associated with the mid-Cretaceous subduction of the Izanagi–Pacific plate as well as lithospheric extension and thinning in eastern Asia. The present regional AFT age pattern is therefore basically controlled by the Early Cretaceous rapid cooling event, but finally shaped through active Cenozoic faulting. Following the transition phase the subsequent slow cooling phase pattern implies a net reduction in horizontal compressional stress corresponding to increased extension rates along the continental margin due to the decrease in plate convergence. Modelling of the AFT data suggests a possible Pliocene–Recent cooling episode, which may be supported by increased rates of sedimentation observed in adjacent basins. This cooling phase may be interpreted as a response to the far-field effects of the frontal India–Eurasia collision to the west. Approximate estimates suggest that the total amount of post 120 Ma denudation across the UHP orogen ranged from 2.4 to 13.2 km for different tectonic blocks and ranged from 0.8 to 9.7 km during the Cretaceous to between 1.7 and 3.8 km during the Cenozoic.  相似文献   

3.
The Tan–Lu Fault Zone (TLFZ) extends in a NNE–SSW direction for more than 2000 km in Eastern China. It has been considered either as a major sinistral strike-slip fault, as a suture zone or as a normal fault. We have conducted a structural analysis of the southern segment of this fault zone (STLFZ) in the Anhui Province. The ages (Triassic to Palaeocene) of the formations affected by the faults have been re-appraised taking into account recent stratigraphical studies to better constraint the ages of the successive stages of the kinematics of the STLFZ. Subsequently, the kinematics of the faults is presented in terms of strain/stress fields by inversion of the striated fault set data. Finally, the data are discussed in the light of the results obtained by previous workers.We propose the following history of the STLFZ kinematics during the Mesozoic. At the time of collision, a  NNE orientated Tan–Lu margin probably connected two margins located north of the Dabie and Sulu collision belts. During the Middle–Late Triassic, the SCB has been obliquely subducted below the NCB along this margin which has acted as a compressional transfer zone between the Dabie and Sulu continental subduction zones. The STLFZ has been initiated during the Early Jurassic and has acted as a sinistral transform fault during the Jurassic, following which the NCB/SCB collision stopped. A  NW-trending extension related to metamorphic domes was active during the basal Early Cretaceous ( 135–130 Ma); it has been followed by a NW–SE compression and a NE–SW tension during the middle–late Early Cretaceous ( 127 to  105 Ma, possibly  95 Ma); at that time the TLFZ was a sinistral transcurrent fault within the eastern part of the Asian continent. During the Late Cretaceous–Palaeocene, the STLFZ was a normal fault zone under a WNW–ESE tension.  相似文献   

4.
Fission-track (FT) thermochronologic analysis was performed on zircon separates from rocks in and around the Nojima fault, which was activated during the 1995 Kobe earthquake. Samples were collected from the University Group 500 m (UG-500) borehole and nearby outcrops. FT lengths in zircons from localities > 25 m away from the fault plane as well as one 0.1 m away from the fault in the footwall are characterized by concordant mean values of  10–11 μm and unimodal distributions with negative skewness, which showed no signs of appreciable reduction in FT length. In contrast, those adjacent (< 3 m) to the fault at depths on the hanging wall side showed significantly reduced mean track lengths of  6–8 μm and distributions having a peak around 6–7 μm with rather positive skewness. The former pattern is interpreted to reflect cooling through the zircon partial annealing zone (ZPAZ), without later, partial thermal overprints. The latter indicates substantial track shortening due probably to secondary heating by a thermal event(s) that locally perturbed the geothermal structure. Modeled zircon FT length and age data of partially annealed samples from the UG-500 borehole revealed a cooling episode in the ZPAZ that started at  4 Ma within  3 m from the fault plane, whereas those from the Geological Survey of Japan 750 m borehole record cooling started at  31–38 Ma within  25 m from the fault. On the basis of one-dimensional heat conduction modeling as well as the consistency between the degree of FT annealing and the degree of deformation/alteration of borehole rocks, these cooling ages in both boreholes are interpreted as consequences of ancient thermal overprints by heat transfer or dispersion via fluids in the fault zone. Together with the zircon FT data of a pseudotachylyte layer recently analyzed, it is suggested that the present Nojima fault system was reactivated in the Middle Quaternary from an ancient fault initiated at  56 Ma at mid-crustal depths. Also shown is a temporal/spatial variation in terms of the thermal anomalies recorded in the fault rocks, implying heterogeneity of hot fluid flows in the fault zone.  相似文献   

5.
Late- to post-magmatic deformation in slightly diachronous contiguous intrusions of the north-western Adamello batholith (Southern Alps, Italy) is recorded as, from oldest to youngest: (i) joints, (ii) solid-state ductile shear zones, (iii) faults associated with epidote-K-feldspar veins and (iv) zeolite veins and faults. Structures (ii) to (iv) are localized on the pervasive precursory network of joints (i), which developed during the earliest stages of pluton cooling. High temperature ( 500 °C), ductile overprinting of joints produced lineations, defined by aligned biotite and hornblende, on the joint surfaces and highly localized mylonites. The main phase of faulting, producing cataclasites and pseudotachylytes, occurred at  250 °C and was associated with extensive fluid infiltration. Cataclasites and pseudotachylytes are clustered along different E–W-striking dextral strike-slip fault zones correlated with the activity of the Tonale fault, a major tectonic structure that bounds the Adamello batholith to the north. Ductile deformation and cataclastic/veining episodes occurred at P = 0.25–0.3 GPa during rapid cooling of the batholith to the ambient temperatures ( 250 °C) that preceded the exhumation of the batholith. Timing of the sequence of deformation can be constrained by 39Ar–40Ar ages of  30 Ma on pseudotachylytes and various existing mineral ages. In the whole composite Adamello batholith, multiple magma pulses were intruded over the time span 42–30 Ma and each intrusive body shows the same ductile-to-brittle structural sequence localized on the early joint sets. This deformation sequence of the Adamello might be typical of intrusions undergoing cooling at depths close to the brittle–ductile transition.  相似文献   

6.
Integration of on-land and offshore geomorphological and structural investigations coupled to extensive radiometric dating of co-seismically uplifted Holocene beaches allows characterization of the geometry, kinematics and seismotectonics of the Scilla Fault, which borders the eastern side of the Messina Strait in Calabria, Southern Italy. This region has been struck by destructive historical earthquakes, but knowledge of geologically-based source parameters for active faults is relatively poor, particularly for those running mostly offshore, as the Scilla Fault does. The  30 km-long normal fault may be divided into three segments of  10 km individual length, with the central and southern segments split in at least two strands. The central and northern segments are submerged, and in this area marine geophysical data indicate a youthful morphology and locally evidence for active faulting. The on-land strand of the western segment displaces marine terraces of the last interglacial (124 to 83 ka), but seismic reflection profiles suggest a full Quaternary activity. Structural data collected on bedrock faults exposed along the on-land segment provide evidence for normal slip and  NW-SE extension, which is consistent with focal mechanisms of large earthquakes and GPS velocity fields in the region. Detailed mapping of raised Holocene marine deposits exposed at the coastline straddling of the northern and central segments supplies evidence for two co-seismic displacements at  1.9 and  3.5 ka, and a possible previous event at  5 ka. Co-seismic displacements show a consistent site value and pattern of along-strike variation, suggestive of characteristic-type behaviour for the fault. The  1.5–2.0 m average co-seismic slips during these events document Me  6.9–7.0 earthquakes with  1.6–1.7 ka recurrence time. Because hanging-wall subsidence cannot be included into slip magnitude computation, these slips reflect footwall uplift, and represent minimum average estimates. The palaeoseismological record based on the palaeo-shorelines suggests that the last rupture on the Scilla Fault during the February 6, 1783 Mw = 5.9–6.3 earthquake was at the expected time but it may have not entirely released the loaded stress since the last great event at  1.9 ka. Comparison of the estimated co-seismic extension rate based on the Holocene shoreline record with available GPS velocities indicates that the Scilla Fault accounts for at least  15–20% of the contemporary geodetic extension across the Messina Strait.  相似文献   

7.
In the Gawler Craton, the completeness of cover concealing the crystalline basement in the region of the giant Olympic Dam Cu–Au deposit has impeded any sufficient understanding of the crustal architecture and tectonic setting of its IOCG mineral-system. To circumvent this problem, deep seismic reflection data were recently acquired from  250 line-km of two intersecting traverses, centered on the Olympic Dam deposit. The data were recorded to 18 s TWT ( 55 km). The crust consists of Neoproterozoic cover, in places more than 5 km thick, over crystalline basement with the Moho at depths of 13–14 s TWT ( 40–42 km). The Olympic Dam deposit lies on the boundary between two distinct pieces of crust, one interpreted as the Archean–Paleoproterozoic core to the craton, the other as a Meso–Neoproterozoic mobile belt. The host to the deposit, a member of the  1590 Ma Hiltaba Suite of granites, is situated above a zone of reduced impedance contrast in the lower crust, which we interpret to be source-region for its  1000 °C magma. The crystalline basement is dominated by thrusts. This contrasts with widely held models for the tectonic setting of Olympic Dam, which predict extension associated with heat from the mantle producing the high temperatures required to generate the Hiltaba Suite granites implicated in mineralization. We use the seismic data to test four hypotheses for this heat-source: mantle underplating, a mantle-plume, lithospheric extension, and radioactive heating in the lower crust. We reject the first three hypotheses. The data cannot be used to reject or confirm the fourth hypothesis.  相似文献   

8.
Coaly source rocks are sufficiently different from marine and lacustrine source rocks in their organic matter characteristics to warrant separate guidelines for their assessment using Rock-Eval pyrolysis. The rank threshold for oil generation is indicated by the increase in BI (S1/TOC) at Rank(Sr)9–10 (Tmax 420–430 °C, Ro 0.55–0.6%), and the threshold for oil expulsion is indicated by the peak in QI ([S1+S2]/TOC) at Rank(Sr)11–12.5 (Tmax 430–440 °C, Ro 0.65–0.85%). The pronounced rank-related increase in HI (S2/TOC) prior to oil expulsion renders the use of immature samples inappropriate for source rock characterisation. A more realistic indication of the petroleum generative potential and oil expulsion efficiency of coaly source rocks can be gained from samples near the onset of expulsion. Alternatively, effective HI′ values (i.e. HIs near the onset of expulsion) can be estimated by translating the measured HIs of immature samples along the maturation pathway defined by the New Zealand (or other defined) Coal Band. Coaly source rocks comprise a continuum of coaly lithologies, including coals, shaly coals and coaly mudstones. Determination of the total genetic potential of coaly source rock sequences is best made using lithology-based samples near the onset of expulsion.  相似文献   

9.
Knowledge of the Cretaceous–Tertiary history of upper crustal shortening and magmatism in Tibet is fundamental to placing constraints on when and how the Tibetan plateau formed. In the Lhasa terrane of southern Tibet, the widely exposed angular unconformity beneath uppermost Cretaceous–lower Tertiary volcanic-bearing strata of the Linzizong Formation provides an excellent geologic and time marker to distinguish between deformation that occurred before vs. during the Indo-Asian collision. In the Linzhou area, located  30 km north of the city of Lhasa, a > 3-km-thick section of the Linzizong Formation lies unconformably on Cretaceous and older rocks that were shortened by both northward- and southward-verging structures during the Late Cretaceous. The Linzizong Formation dips northward in the footwall of a north-dipping thrust system that involves Triassic–Jurassic strata and a granite intrusion in the hanging wall. U–Pb zircon geochronologic studies show that the Linzizong Formation ranges in age from 69 Ma to at least 47 Ma and that the hanging wall granite intrusion crystallized at  52 Ma, coeval with dike emplacement into footwall Cretaceous strata. 40Ar/39Ar thermochronologic studies suggest slow cooling of the granite between 49 and 42 Ma, followed by an episode of accelerated cooling to upper crustal levels beginning at  42 Ma. The onset of rapid cooling was coeval with the cessation of voluminous arc magmatism in southern Tibet and is interpreted be a consequence of either (1) Tertiary thrusting in this region or (2) regional rock uplift and erosion following removal of overthickened Gangdese arc lower crust and upper mantle or break-off of the Neo-Tethyan oceanic slab.  相似文献   

10.
The great Indian Ocean earthquake of December 26, 2004 caused significant vertical changes in its rupture zone. About 800 km of the rupture is along the Andaman and Nicobar Islands, which forms the outer arc ridge of the subduction zone. Coseismic deformation along the exposed land could be observed as uplift/subsidence. Here we analyze the morphological features along the coast of the Andaman and Nicobar Islands, in an effort to reconstruct the past tectonics, taking cues from the coseismic effects. We obtained radiocarbon dates from coastal terraces of the island belt and used them to compute uplift rates, which vary from 1.33 mm yr− 1 in the Little Andaman to 2.80 mm yr− 1 in South Andaman and 2.45 mm yr− 1 in the North Andaman. Our radiocarbon dates converge on  600 yr and  1000 yr old coastal uplifts, which we attribute to the level changes due to two major previous subduction earthquakes in the region.  相似文献   

11.
The Yidun Arc is a Triassic volcanic arc located between the Songpan Garzê Fold Belt and the Qiangtang Block, southwest China. To constrain the age of a number of the major granitic plutons from the Yidun Arc, laser ablation ICP-MS U/Pb analysis of zircon was conducted. Hafnium isotope data was also acquired through laser-ablation multicollector ICPMS analysis of zircon, with the aim of gaining insight into the age and nature of the source region of the plutons. Three age groups have been identified from seven granite samples: Early–Middle Triassic ( 245 to 229 Ma), Late Triassic ( 219 to 216 Ma) and Cretaceous ( 105 to 95 Ma). Hafnium analysis shows the Triassic granites to have negative and variable εHf values and Mesoproterozoic ( 1.6 Ga) depleted-mantle model ages, which is interpreted to reflect derivation from an isotopically heterogeneous, largely crustal source. The Cretaceous granite shows higher and less variable εHf values and slightly younger model ages ( 1.3 Ga), and is interpreted to be derived from melting of a more homogeneous crustal source. A depleted-mantle model age of  1.5 Ga is calculated from the pooled Triassic and Cretaceous samples. The source region for these magmas may be tentatively correlated with Mesoproterozoic material of the Yangtze Craton, which has been suggested to underlie the Yidun Arc; however, further work is necessary to demonstrate this suggestion.  相似文献   

12.
A laminated sequence (core BAP96-CP 24°38.12′N, 110°33.24′W; 390 m depth) from the Alfonso Basin in Bay of La Paz, southern Gulf of California, contains a record of paleoceanographic and paleoclimatic changes of the past 7900 yr. Radiolarian assemblages and magnetic susceptibility are used as proxies of oceanographic and climatic variability. The records provide a regional scenario of the middle and late Holocene, suggesting two major climatic regimes and several millennial-scale events. Conditions relatively warmer and drier than today occurred from 7700 to 2500 cal yr BP, promoting the intensification of evaporation processes and the prevalence of the Gulf of California water in the Basin. These conditions correlate with strong droughts in the middle Holocene of North America and with minimal incursion of tropical waters into the Gulf of California. Proxies indicate a warm scenario and the dominance of the Equatorial Surface Water in the Alfonso Basin from 2400 to 700 cal yr BP, suggesting the intensification of ENSO cycles. A climatic signal between 1038 and 963 cal yr BP may be correlated with global signal of the “Medieval Warm Period.” Several cooling events are recognized at 5730, 3360, 2700, 1280 and 820 cal yr BP and are associated with intensification of northwest winds leading to upwellings and enhanced productivity in the Basin.  相似文献   

13.
High velocity (1 m/s) friction experiments on bituminous coal gouge display several earthquake-related phenomena, including devolatilization by frictional heating, gas pressurization, and slip weakening. Stage I is characterized by sample shortening and reduction in the coefficient of friction (μ) from  1 to 0.6. Stage II is characterized by high frequency ( 5 Hz) oscillations in stress and strain records and by gas emissions. Stage III is marked by rapid weakening (μ  0.1 to 0.35) and sample shortening, together with continued gas emissions. Stage IV produces stable stress records and continued weakness (μ  0.2), but without gas emission. Stage I shortening is due to compaction of the gouge and the weakening is attributed to mechanical or thermal effects. Stage II behavior is interpreted as due to coal gasification and fluctuations in fluid pressure, resulting in high frequency stick-slip type behavior. Dramatic reduction in shear stress in stage III is attributed to gas pressurization by pore collapse and corresponds to a frictional instability, analogous to nucleation of an earthquake. Microstructural observations indicate the deformation was brittle during stages I and II but ductile during stages III and IV. Time dependent finite element frictional heat models indicate the center of the samples became hot ( 900 °C) during stage II, whereas the edge of samples remained relatively cold (< 300 °C). Vitrinite reflectance of coal samples shows an increase in reflectance from  0.5 to  0.8% over the displacement interval 20–40 m (20–40 s), indicating that the reflectance responds to frictional heating on a short time scale. The energy expended per unit area in these low stress, large displacement experiments is similar to that of higher stress ( 50 MPa), short displacement ( 1 m) earthquakes ( 107 J/m2).  相似文献   

14.
New mapping in the northern part of the Paleozoic Acatlán Complex (Patlanoaya area) records several ductile shear zones and brittle faults with normal kinematics (previously thought to be thrusts). These movement zones separate a variety of units that pass structurally upwards from: (i) blueschist-eclogitic metamorphic rocks (Piaxtla Suite) and mylonitic megacrystic granites (Columpio del Diablo granite ≡ Ordovician granites elsewhere in the complex); (ii) a gently E-dipping, listric, normal shear zone with top to the east kinematic indicators that formed under upper greenschist to lower amphibolite conditions; (iii) the Middle–Late Ordovician Las Minas quartzite (upper greenschist facies psammites with minor interbedded pelites intruded by mafic dikes and a leucogranite dike from the Columpio del Diablo granite) unconformably overlain by the Otate meta-arenite (lower greenschist facies psammites and pelites): roughly temporal equivalents are the Middle–Late Ordovician Mal Paso and Ojo de Agua units (interbedded metasandstone and slate, and metapelite and mafic minor intrusions, respectively) — some of these units are intruded by the massive, 461 ± 2 Ma, Palo Liso megacrystic granite: decussate, contact metamorphic muscovite yielded a 40Ar/39Ar plateau age of 440 ± 4 Ma; (iv) a steeply-moderately, E-dipping normal fault; (v) latest Devonian–Middle Permian sedimentary rocks (Patlanoaya Group: here elevated from formation status). The upward decrease in metamorphic grade is paralleled by a decrease in the number of penetrative fabrics, which varies from (i) three in the Piaxtla Suite, through (ii) two in the Las Minas unit (E-trending sheath folds deformed by NE-trending, subhorizontal folds with top to the southeast asymmetry, both associated with a solution cleavage), (iii) one in the Otate, Mal Paso, and Ojo de Agua units (steeply SE-dipping, NE–SW plunging, open-close folds), to (iv) none in the Patlanoaya Group. 40Ar/39Ar analyses of muscovite from the earliest cleavage in the Las Minas unit yielded a plateau age of 347 ± 3 Ma and show low temperature ages of  260 Ma. Post-dating all of these structures and the Patlanoaya Group are NE-plunging, subvertical folds and kink bands. An E–W, vertical normal fault juxtaposes the low-grade rocks against the Anacahuite amphibolite that is cut by megacrystic granite sheets, both of which were deformed by two penetrative fabrics. Amphibole from this unit has yielded a 40Ar/39Ar plateau age of 299 ± 6 Ma, which records cooling through  490 °C and is probably related to a Permo-Carboniferous reheating event during exhumation. The extensional deformation is inferred to have started in the latest Devonian ( 360 Ma) during deposition of the basal Patlanoaya Group, lasting through the rapid exhumation of the Piaxtla Suite at  350–340 Ma synchronous with cleavage development in the Las Minas unit, deposition of the Patlanoaya Group with active fault-related exhumation suggested by Mississippian and Early Permian conglomerates ( 340 and 300 Ma, respectively), and continuing at least into the Middle Permian (≡ 260 Ma muscovite ages). The continuity of Mid-Continent Mississippian fauna from the USA to southern Mexico suggests that this extensional deformation occurred on the western margin of Pangea after closure of the Rheic Ocean.  相似文献   

15.
Fission-track (FT) analysis using apatite and zircon was performed on samples from two fracture zones (FZ) at the depths of 1140 and 1310 m within the 1838 m borehole core penetrating the Ryoke Granitic Rocks in the Nojima Fault at Nojima-Hirabayashi, Awaji Island, Japan, drilled just after the 1995 Hyogo-ken Nanbu earthquake. Clear discordance in apatite and zircon FT age was found for two samples located at  2 m below the central part of each FZs where the presence of pseudotachylyte and/or fault gouge would predict the largest amount of slip. Asymmetric distribution was identified by discordant ages with respect to the central part of FZs. These very local discordant ages in the fault reflect thermal anomalies caused by secondary heating with an inferred maximum temperature in the region between apatite and zircon closure temperatures at a time post-48 Ma. As a source of the secondary heating, heat transfer or dispersion via geothermal fluids caused presumably the observed similarity in asymmetric distribution of discordant FT ages at two different FZs. Other samples yield concordant FT zircon and apatite ages and these indicate rapid cooling within the bounds of two closure temperatures of these minerals at  60 Ma of the Ryoke Granitic Rocks.  相似文献   

16.
Mineralizing fluids at the San Martín skarn show an evolution characterized by prograde and retrograde associations. The prograde mineral associations consist of (1) a massive garnet zone, (2) a tremolite ± garnet zone, and (3) a late association of quartz, sphalerite, calcite and fluorite lining the vugs in the garnet zone. The fluids of the prograde associations exhibit decreasing temperatures of homogenization (Th) and variable salinities. The fluids of the massive garnet zone have salinities of 36 wt.% NaCl equiv. and Th of 645 to 570 °C, corresponding to pressures of 1055 bar. At the tremolite ± garnet zone, Th range from 438 to 354 °C. In the late association at the endoskarn, the following evolution can be drawn: (a) salinities of 50 to 42 wt.% NaCl equiv., and Th of 455 to 346 °C in quartz, (b) salinities of 46 wt.% NaCl equiv., and Th of 415 to 410 °C in sphalerite, (c) salinities of 50 to 37 wt.% NaCl equiv., and Th of 479 to 310 °C in calcite, (d) salinities of 33 to 28 wt.% NaCl equiv. and of 24 to 22 wt.% KCl in fluorite, and (e) two types of fluids with salinities of 2 and 39 wt.% NaCl equiv. and Th 344 and 300 °C, respectively, in later saccharoidal quartz segregations. The retrograde mineral associations comprise pervasive propylitic alteration to carbonization, and mantos with sulfides. Fluids in epidote have salinities of 7.6 wt.% NaCl equiv. and Th of 287 to 252 °C, and in calcite have salinities of 9.2 to 1 wt.% NaCl equiv. and Th of 188 to 112 °C. Fluids in the sulfide assemblages in the mantos have salinities of 8 to 3 wt.% NaCl equiv. and Th 300 °C, with corresponding pressures of 94 bar. Fluids in late epithermal veins close to the intrusive body have salinities of 10 to 5 wt.% NaCl equiv. and Th of 275 to 200 °C, and distal veins show salinities of 2 to 1 wt.% NaCl equiv. and Th of 160 °C.  相似文献   

17.
M. Faccenda  G. Bressan  L. Burlini   《Tectonophysics》2007,445(3-4):210-226
The compressional and shear wave velocities have been measured at room temperature and pressure up to 450 MPa on 5 sedimentary rock samples, representative of the most common lithologies of the upper crust in the central Friuli area (northeastern Italy). At 400 MPa confining pressure the Triassic dolomitic rock shows the highest velocities (Vp  7 km/s, Vs  3.6 km/s), the Jurassic and Triassic limestones samples intermediate velocities (Vp  6.3 /s, Vs  3.5 km/s) and the Cenozoic and Paleozoic sandstones the lowest velocities (Vp  6.15 km/s, Vs  3.35 km/s). The Paleozoic sandstone sample is characterized by the strongest anisotropy (10%) and significant birefringence (0.2 km/s) is found only on the Cenozoic sandstone sample. We elaborated the synthetic profiles of seismic velocities, density, elastic parameters and reflection coefficient, related to 4 one-dimensional geological models extended up to 22 km depth. The synthetic profiles evidence high rheological contrasts between Triassic dolomitic rocks and the soft sandstones and the Jurassic limestones. The Vp profiles obtained from laboratory measurements match very well the in-situ Vp profile measured by sonic log for the limestones and dolomitic rocks, supporting our one-dimensional modelling of the calcareous-carbonatic stratigraphic series. The Vp and Vs values of the synthetic profiles are compared with the corresponding ones obtained from the 3-D tomographic inversion of local earthquakes. The laboratory Vp are generally higher than the tomographic ones with major discrepancies for the dolomitic lithology. The comparison with the depth location of seismicity reveals that the seismic energy is mainly released in correspondence of high-contrast rheological boundaries.  相似文献   

18.
The late Quaternary paleoclimate of eastern Beringia has primarily been studied by drawing qualitative inferences from vegetation shifts. To quantitatively reconstruct summer temperatures, we analyzed lake sediments for fossil chironomids, and additionally we analyzed the sediments for fossil pollen and organic carbon content. A comparison with the δ18O record from Greenland indicates that the general climatic development of the region throughout the last glaciation–Holocene transition differed from that of the North Atlantic region. Between  17 and 15 ka, mean July air temperature was on average 5°C colder than modern, albeit a period of near-modern temperature at  16.5 ka. Total pollen accumulation rates ranged between  180 and 1200 grains cm− 2 yr− 1. At  15 ka, approximately coeval with the Bølling interstadial, temperatures again reached modern values. At  14 ka, nearly 1000 yr after warming began, Betula pollen percentages increased substantially and mark the transition to shrub-dominated pollen contributors. Chironomid-based inferences suggest no evidence of the Younger Dryas stade and only subtle evidence of an early Holocene thermal maximum, as temperatures from  15 ka to the late Holocene were relatively stable. The most recognizable climatic oscillation of the Holocene occurred from  4.5 to 2 ka.  相似文献   

19.
Detailed electron microscope and microstructural analysis of two ultrahigh temperature felsic granulites from Tonagh Island, Napier Complex, Antarctica show deformation microstructures produced at  1000 °C at 8–10 kbar. High temperature orthopyroxene (Al 7 wt.% and  11 wt.%), exhibits crystallographic preferred orientation (CPO) and frequent subgrain boundaries which point to dislocation creep as the dominating deformation mechanism within opx. Two different main slip systems are observed: in opx bands with exclusively opx grains containing subgrain boundaries with traces parallel to [010] and a strong coupling of low angle misorientations (2.5°–5°) with rotation axes parallel to [010] the dominating slip system is (100)[001]. Isolated opx grains and grain clusters of 2–5 grains embedded in a qtz–fsp matrix show an additional slip system of (010)[001]. The latter slip system is harder to activate. We suggest that differences in the activation of these slip systems is a result of higher differential stresses imposed onto the isolated opx grains and grain clusters. In contrast to opx, large qtz grains (up to 200 μm) show random crystallographic orientation. This together with their elongate and cuspate shape and the lack of systematic in the rotation axes associated with the subgrain boundaries is consistent with diffusion creep as the primary deformation mechanism in quartz.Our first time detailed microstructural observations of ultrahigh temperature and medium to high pressure granulites and their interpretation in terms of active deformation mechanisms give some insight into the type of rheology that can be expect at lower crustal conditions. If qtz is the mineral phase governing the rock rheology, Newtonian flow behaviour is expected and only low differential stress can be supported. However, if the stress supporting mineral phase is opx, the flow law resulting from dislocation creep will govern the rheology of the rock unit; hence, an exponential relationship between stress and strain rate is to be expected.  相似文献   

20.
Jun-Hong Zhao  Mei-Fu Zhou 《Lithos》2008,104(1-4):231-248
Numerous Neoproterozoic felsic and mafic–ultramafic intrusions occur in the Hannan region at the northern margin of the Yangtze Block. Among these, the Wudumen and Erliba plutons consist of granodiorites and have SHRIMP zircon U–Pb ages of  735 Ma. The rocks have high K2O (0.8–3.6 wt.%) and Na2O (4.4–6.4 wt.%) and low MgO (0.4–1.7 wt.%). They also have high Sr/Y (32–209) and (La/Yb)n ratios (4.4–38.6). Their εNd values range from − 0.41 to − 0.92 and zircon initial 176Hf/177Hf ratios from 0.282353 to 0.282581. These geochemical features are similar to those of adakitic rocks produced by partial melting of a thickened lower crust. Our new analytical results, combined with the occurrence of voluminous arc-related mafic–ultramafic intrusions emplaced before 740 Ma, lead us to propose that the crustal evolution in the northern margin of the Yangtze Block during Neoproterozoic involved: (1) rapid crustal growth and thickening by underplating of mafic magmas from the mantle which was modified by materials coming from the subducting oceanic slab from  1.0 to  0.74 Ga, and (2) partial melting of the thickened lower crust due to a thermal anomaly induced by upwelling of asthenosphere through an oceanic slab window, producing the  735 Ma adakitic Wudumen and Erliba plutons. Our model suggests that the crustal thickness was more than 50 km at the northern margin of the Yangtze Block at  735 Ma, and rule out the possibility of a mantle plume impact causing the > 735 Ma magmatism in the region.  相似文献   

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