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1.
Although subducting slabs undergo a bending deformation that resists tectonic plate motions, the magnitude of this resistance is not known because of poor constraints on slab strength. However, because slab bending slows the relatively rapid motions of oceanic plates, observed plate motions constrain the importance of bending. We estimated the slab pull force and the bending resistance globally for 207 subduction zone transects using new measurements of the bending curvature determined from slab seismicity. Predicting plate motions using a global mantle flow model, we constrain the viscosity of the bending slab to be at most ~ 300 times more viscous than the upper mantle; stronger slabs are intolerably slowed by the bending deformation. Weaker slabs, however, cannot transmit a pull force sufficient to explain rapid trenchward plate motions unless slabs stretch faster than seismically observed rates of ~ 10− 15 s− 1. The constrained bending viscosity (~ 2 × 1023 Pa s) is larger than previous estimates that yielded similar or larger bending resistance (here ~ 25% of forces). This apparent discrepancy occurs because slabs bend more gently than previously thought, with an average radius of curvature of 390 km that permits subduction of strong slabs. This gentle bending may ultimately permit plate tectonics on Earth.  相似文献   

2.
Water is a dominant component of volcanic clouds and has fundamental control on very fine particle deposition. Particle size characteristics of distal tephra-fall (100s km from source volcano) have a higher proportion of very fine particles compared to predictions based on single particle settling rates. In this study, sedimentological analyses of fallout from for the 18 August and 16–17 September 1992 eruptions of Crater Peak, Alaska, are combined with satellite observations, and cloud trajectory and microphysics modeling to investigate meteorological influences on particle sedimentation. Total grain size distributions of tephra fallout were reconstructed for both Crater Peak eruptions and indicate a predominance of fine particles < 125 μm. Polymodal analysis of the deposits has identified a particle subpopulation with mode ~ 15–18 μm involved in particle aggregation. Accounting for the magmatic water source only, calculated ice water content of the 3.7 hour old September 1992 Spurr cloud was ~ 4.5 × 10− 2 g m− 3 (based on an estimated cloud thickness of ~ 1000 m from trajectory modeling). Hydrometeor formation on particles in the volcanic cloud and subsequent sublimation may induce a cloud base instability that leads to rapid bulk (en masse) sedimentation of very fine particles through a mammatus-like mechanism.  相似文献   

3.
Changes in oceanic O–Sr isotopic compositions and global cooling beginning in the Eocene are considered to have been caused by the uplift of the Tibetan Plateau. The specific timing and uplift mechanism, however, have long been subjects of debate. We investigated the Duogecuoren lavas of the central-western Qiangtang Block, which form the largest outcrops among Cenozoic lavas in northern-central Tibet and have widely been considered as shoshonitic. Our study demonstrates, however, that most of these lavas are high-K calc-alkaline andesites, dacites and rhyolites. Moreover, they are characterized by high Sr (367–2472 ppm) and Al2O3 (14.55–16.86 wt.%) and low Y (3.05–16.9 ppm) and Yb (0.31–1.48 ppm) contents and high La/Yb (27–100) and Sr/Y (48–240) ratios, similar to adakitic rocks derived by partial melting of an eclogitic source. They can be further classified as either peraluminous and metaluminous subtypes. The peraluminous rocks have relatively high SiO2 (> 66 wt.%) contents, and low MgO (< 1.0 wt.%), Cr (4.94–23.3 ppm) and Ni (2.33–17.0 ppm) contents and Mg# (20–50) values, while the metaluminous rocks exhibit relatively low SiO2 (55–69 wt.%) contents, and high MgO (1.41–6.34), Cr (25.7–383 ppm), Ni (14.13–183 ppm) and Mg# (46–69) values, similar to magnesian andesites. 40Ar/39Ar and SHRIMP zircon U–Pb dating reveal that both peraluminous and metaluminous adakitic rocks erupted in the Eocene (46–38 Ma). Paleocene–Early Miocene thrust faults and associated syn-contractional basin deposits in the Qiangtang Block suggest that this region was undergoing crustal shortening within a continent during the Eocene. The low εNd (− 2.81 to − 6.91) and high 87Sr/86Sr (0.7057–0.7097), Th (11.2–32.3 ppm) and Th/La (0.23–0.88) values in the Duogecuoren adakitic rocks further indicate that they were not derived by partial melting of subducted oceanic crust. Taking into account tectonic and geophysical data and the compositions of xenoliths in Cenozoic lava in northern-central Tibet, we suggest that the peraluminous adakitic rocks were most probably derived by partial melting of subducted sediment-dominated continent of the Songpan-Ganzi Block along the Jinsha suture to the north at a relatively shallow position (the hornblende + garnet stability field), but the metaluminous adakitic rocks likely originated from the interaction between peraluminous adakitic melts generated at greater depths (the garnet + rutile stability field) and mantle. Because the Duogecuoren adakitic rocks must have originated from a garnet-bearing (namely, eclogite facies) source, Eocene continental subduction along the Jinsha suture caused the thickening of the Qiangtang crust. Given that crustal thickening generally equates with elevation, the uplift of the Central Tibetan Plateau probably began as early as 45–38 Ma, which provides important evidence for tectonically driven models of oceanic O–Sr isotope evolution during global cooling and Asian continental aridification beginning in the Eocene.  相似文献   

4.
The Cape Verde Islands are located on a mid-plate topographic swell and are thought to have formed above a deep mantle plume. Wide-angle seismic data have been used to determine the crustal and uppermost mantle structure along a ~ 440 km long transect of the archipelago. Modelling shows that ‘normal’ oceanic crust, ~ 7 km in thickness, exists between the islands and is gently flexed due to volcano loading. There is no direct evidence for high density bodies in the lower crust or for an anomalously low density upper mantle. The observed flexure and free-air gravity anomaly can be explained by volcano loading of a plate with an effective elastic thickness of 30 km and a load and infill density of 2600 kg m− 3. The origin of the Cape Verde swell is poorly understood. An elastic thickness of 30 km is expected for the ~ 125 Ma old oceanic lithosphere beneath the islands, suggesting that the observed height of the swell and the elevated heat flow cannot be attributed to thermal reheating of the lithosphere. The lack of evidence for high densities and velocities in the lower crust and low densities and velocities in the upper mantle, suggests that neither a crustal underplate or a depleted swell root are the cause of the shallower than expected bathymetry and that, instead, the swell is supported by dynamic uplift associated with the underlying plume.  相似文献   

5.
We report an 39Ar–40Ar age determination of a whole rock sample of the olivine-rich, martian meteorite Northwest Africa (NWA) 2737. Those extractions releasing 0–48% of the 39Ar define an 39Ar–40Ar isochron age of 160–190 Ma, when evaluated in various ways. Higher temperature extractions show increasing ages that eventually exceed the reported Sm–Nd age of 1.42 Ga. At least part of this excess 40Ar may have been shock implanted from the martian atmosphere. We considered two possible interpretations of the Ar–Ar isochron age, utilizing the measured Ar diffusion characteristics of NWA 2737 and a thermal model, which relates Ar diffusion to the size of a cooling object after shock heating. One interpretation, that 40Ar was only partially degassed by an impact event ~ 11 Ma ago (the CRE age), appears possible only if NWA 2737 was shock-heated to temperatures > 600 °C and was ejected from Mars as an object a few 10 s of cm in diameter. The second interpretation, which we prefer, is that NWA experienced an earlier, more intense shock event, which left it residing in a warm ejecta layer, and a less intense event ~ 11 Ma ago, which ejected it into space. Our evaluation would require NWA 2737 to have been heated by this first event to a temperature of ~ 300–500 °C and buried in ejecta to a depth of ~ 1–20 m. These conclusions are compared to model constraints on meteorite ejection from Mars reported in the literature. The second, Mars-ejection impact ~ 11 Ma ago probably heated NWA 2737 to no more than ~ 400 °C. NWA 2737 demonstrates that some martian meteorites probably experienced shock heating in events that did not eject them into space.  相似文献   

6.
Collapsed calderas are the structural surface expression of the largest volcanic eruptions on Earth and may reach diameters of tens of kilometres while erupting volumes larger than 1000 km3. Remnants of collapse calderas can be found along the South American volcanic arc and are thought to be inactive. However, this study shows that systems of such dimension may become active in a relatively short period of time without attracting much attention. Using satellite-based InSAR data, a 45 km wide elongated area of ground deformation was observed in the Lazufre volcanic region (Chile), where no deformation was detected 10 years ago. The deformation signal shows an uplift of up to ~ 3 cm yr− 1 during 2003–2006, affecting an area of about 1100 km2, comparable in size to super-volcanoes such as Yellowstone or Long Valley. This deformation signal can be explained by an inflating magma body at about 10 km depth, expanding and propagating laterally at a velocity of up to 4 km per year. Although it is not clear whether this intrusion will lead to an eruption, its dimensions and the rapid deformation rate insinuate that a potentially large volcanic system is forming.  相似文献   

7.
Hidden beneath the ~ 2 km thick low-velocity volcaniclastics on the western margin of the Central Volcanic Region, North Island, New Zealand, are two structures that represent the early history of volcanic activity in a continental back-arc. These ~ 20 × 20 km structures, at Tokoroa and Mangakino, form an adjacent gravity high and low, respectively. Interpretations from seismic refraction arrivals and gravity modelling indicate the − 65 mgal Mangakino residual gravity anomaly can be modelled, in part, by two low-density bodies that reach depths of ~ 6.5 km, whereas the Tokoroa gravity anomaly is due to a higher density rock coming, at most, to within ~ 650 m of the surface. The Mangakino anomaly is interpreted to be due to the remnants of magma chambers that fed large ignimbrite eruptions from about 1.2 Ma. An andesite volcano or complex volcanic structure is the preferred interpretation for the Tokoroa gravity high. The size of the putative volcanic structure is comparable to the presently active Tongariro Volcanic Complex in the centre of North Island.  相似文献   

8.
PUFF and HAZMAP, two tephra dispersal models developed for volcanic hazard mitigation, are used to simulate the climatic 1991 eruption of Mt. Pinatubo. PUFF simulations indicate that the majority of ash was advected away from the source at the level of the tropopause (~ 17 km). Several eruptive pulses injected ash and SO2 gas to higher altitudes (~ 25 km), but these pulses represent only a small fraction (~ 1%) of the total erupted material released during the simulation. Comparison with TOMS images of the SO2 cloud after 71 and 93 h indicate that the SO2 gas originated at an altitude of ~ 25 km near the source and descended to an altitude of ~ 22 km as the cloud moved across the Indian Ocean. HAZMAP simulations indicate that the Pinatubo tephra fall deposit in the South China Sea was formed by an eruption cloud with the majority of the ash concentrated at a height of 16–18 km. Results of this study demonstrate that the largest concentration of distal ash was transported at a level significantly below the maximum eruption column height (~ 40 km) and at a level below the calculated height of neutral buoyancy (~ 25 km). Simulations showed that distal ash transport was dominated by atmospheric circulation patterns near the regional tropopause. In contrast, the movement of the SO2 cloud occurred at higher levels, along slightly different trajectories, and may have resulted from gas/particle segregations that took place during intrusion of the Pinatubo umbrella cloud as it moved away from source.  相似文献   

9.
In 2001, a small H4 chondrite, Frontier Mountain (FRO) 01149, was found on a glacially eroded surface near the top of Frontier Mountain, Antarctica, about 600 m above the present ice level. The metal and sulphides are almost completely oxidized due to terrestrial weathering. We used a chemical leaching procedure to remove weathering products, which contained atmospheric 10Be and 36Cl in a ratio similar to that found in Antarctic ice. The FRO 01149 meteorite has a terrestrial age of 3.0 ± 0.3 Myr based on the concentrations of the cosmogenic radionuclides 10Be, 26Al and 36Cl. This age implies that FRO 01149 is the oldest stony meteorite (fossil meteorites excluded) discovered on Earth. The noble gas cosmic ray exposure age of FRO 01149 is ~ 30 Myr. The meteorite thus belongs to the 33 Myr exposure age peak of H-chondrites.The bedrock surface on which FRO 01149 was found has wet-based glacial erosional features recording a former high-stand of the East Antarctic ice sheet. This ice sheet evidently overrode the highest peaks (> 2800 m a.s.l.) of the inland sector of the Transantarctic Mountains in northern Victoria Land. We argue that FRO 01149 was a local fall and that its survival on a glacially eroded bedrock surface constrains the age of the last overriding event to be older than ~ 3 Myr. The concentrations of in-situ produced cosmogenic 10Be, 26Al and 21Ne in a glacially eroded bedrock sample taken from near the summit of Frontier Mountain yield a surface exposure age of 4.4 Myr and indicate that the bedrock was covered by several meters of snow. The exposure age is also consistent with bedrock exposure ages of other summit plateaus in northern Victoria Land.  相似文献   

10.
Mercury deposition histories have been scarcely documented in the southern hemisphere. A sediment core was collected from the ecologically important estuarine floodplain of the Berg River (South Africa). We establish the concentration of Hg in this 210Pb-dated sediment core at <50 ng g−1 HgT throughout the core, but with 1.3 ng g−1 methylmercury in surface sediments. The 210Pb dating of the core provides a first record of mercury deposition to the site and reveals the onset of enhanced mercury deposition in 1970. The ratio of methylmercury to total mercury is relatively high in these sediments when compared to other wetlands.  相似文献   

11.
A series of experiments and petrographic analyses have been run to determine the pre-eruption phase equilibria and ascent dynamics of dacitic lavas composing Black Butte, a dome complex on the flank of Mount Shasta, California. Major and trace element analyses indicate that the Black Butte magma shared a common parent with contemporaneously erupted magmas at the Shasta summit. The Black Butte lava phenocryst phase assemblage (20 v.%) consists of amphibole, plagioclase (core An77.5), and Fe–Ti oxides in a fine-grained (< 0.5 mm) groundmass of plagioclase, pyroxene, Fe–Ti oxides, amphibole, and cristobalite. The phenocryst assemblage and crystal compositions are reproduced experimentally between 890 °C and 910 °C, ≥ 300 MPa, XH2O = 1, and oxygen fugacity = NNO + 1. This study has quantified the extent of three crystallization processes occurring in the Black Butte dacite that can be used to discern ascent processes. Magma ascent rate was quantified using the widths of amphibole breakdown rims in natural samples, using an experimental calibration of rim development in a similar magma at relevant conditions. The majority of rims are 34 ± 10 μm thick, suggesting a time-integrated magma ascent rate of 0.004–0.006 m/s among all four dome lobes. This is comparable to values for effusive samples from the 1980 Mount St. Helens eruption and slightly faster than those estimated at Montserrat. A gap between the compositions of plagioclase phenocryst cores and microlites suggests that while phenocryst growth was continuous throughout ascent, microlite formation did not occur until significantly into ascent. The duration of crystallization is estimated using the magma reservoir depth and ascent rate, as determined from phase equilibria and amphibole rim widths, respectively. Textural analysis of the natural plagioclase crystals yields maximum growth rates of plagioclase phenocryst rims and groundmass microlites of 8.7 × 10− 8 and 2.5 × 10− 8 mm/s, respectively. These rates are comparable to values determined from time-sequenced samples of dacite erupted effusively from Mount St. Helens during 1980 and indicate that syneruptive crystallization processes were important during the Black Butte eruptive cycle.  相似文献   

12.
Thermal effects related to burial and hydrothermal alteration leads to chemical remanent magnetization (CRM). We present an experimental study of CRM production by heating claystones at 95 °C. A vertical magnetic field of 2 mT was applied to the claystones during heating and the evolution of the remanence during heating in air is monitored intermittently for up to four months. Solid fragments (9 to 26 g) of claystones are included in a Teflon holder that is placed in the oven under a controlled atmosphere. Newly formed grains acquire a CRM and a thermoviscous magnetization (TVRM), both being parallel to the applied magnetic field. CRM is related to the amount of newly formed grains that pass the critical volume during the reaction. To measure the acquired remanence, the claystones are first cooled in a zero magnetic field and then measured using a 2G SQUID magnetometer.In the frame of the research programme on the feasibility of radioactive waste disposal in a deep geological formation, we investigate the magnetic transformation of Mont Terri Lower Dogger claystones (Switzerland) due to thermal imprinting at 95 °C. We simulate the dehydration that occurs in the walls of galleries after excavation when interstitial water evaporates and rehydration when the galleries are refilled allowing water to move towards dehydrated zones. During dehydration, the remanence gains one order of magnitude at the beginning of the experiment and then it follows a linear rate of 0.23 ± 0.07 mA m− 1/day between 3 and 14 days. The magnetic susceptibility increases by a few percent. The increase of the remanence and of the magnetic susceptibility stops after 15 days. Mass monitoring indicates that interstitial water evaporates when remanence and magnetic susceptibility stabilizes. During rehydration, the remanence increases again whilst magnetic susceptibility drops by a few percent. After 20 days, the remanence during rehydration follows a rate of 0.42 ± 0.15 mA m− 1/day. By contrast, when rehydration takes place later, after 66 days, the rate is much lower (0.09 ± 0.04 mA m− 1/day). Low temperature investigation of magnetic properties indicates an initial magnetic assemblage of magnetite and pyrrhotite. Newly formed magnetite and hematite carry the remanence. We propose that magnetite is formed at the expense of pyrite. Hematite results from the progressive oxidation of newly formed magnetite. Our results suggest the possibility that any claystones that pass the oil window can be remagnetized due to the unique action of temperature.  相似文献   

13.
Abstract The Lesnaya Group is part of a thick, poorly dated turbidite assemblage that sits in the footwall of a regionally extensive collision zone in which the Cretaceous–Paleocene Olutorsky island arc terrane was obducted onto continental margin basin strata. Nannoplankton from 18 samples from the upper part of the Lesnaya Group yield Paleocene through Middle Eocene assemblages. Detrital zircons from nine sandstone samples have a young population of fission-track ages that range from 43.7 ± 3.4 to 55.5 ± 3.5 Ma (uppermost Paleocene to Middle Eocene). The deformed footwall rocks of the Lesnaya Group and the overlying thrusts of the Olutorsky arc terrane, are unconformably overlain by neoautochthonous deposits which are Lutetian (lower Middle Eocene) and younger. Together, these new data indicate that thrusting, which is inferred to have been driven by collision of the Cretaceous–Paleocene island arc with north-eastern Asia, took place in the mid-Lutetian, at about 45 Ma.  相似文献   

14.
Here we present the first species-specific study of boron isotopes in the epibenthic foraminifer species Cibicidoides wuellerstorfi. Coretop samples from a water depth profile from 1000 to 4500 m on the northern flank of the Walvis Ridge are 4.4‰ lower than the values expected, based on calculations of the δ11Bborate of ambient seawater. Similar values for this foraminifer species are presented from ODP site 668B at the Sierra Leone Rise, in the equatorial Atlantic. The consistency between data of the same species suggests the offsets are primary, rather than diagenetic. Glacial C. wuellerstorfi from ODP 668B and Walvis Ridge have boron isotope compositions only slightly different to interglacial samples, that is no larger than + 0.10 pH units, or + 23 µmol kg− 1 in [CO32−] above the reconstructed glacial lysocline, and − 0.07 pH units, or − 14 µmol kg− 1 in [CO32−] below. We use these results to suggest that glacial deep water pH in the Atlantic was similar to interglacial pH. The new data resolve the inconsistency between the previously reported high bottom water pH and the lack of significant carbonate preservation of the glacial deep ocean.  相似文献   

15.
The middle Miocene Valdearenas–Muduex section in the internally-drained, continental Madrid Basin (central Spain) is dated bio-magnetostratigraphically between 15.2 Ma and 11.5 Ma. The section contains two formation-scale, sedimentary sequences, that both consist of a siliciclastic lower part and a calcareous upper part. Siliciclastic sedimentation took place in distal floodplain and fluvial environments, while limestones resulted from carbonate precipitation in calcic soil profiles and in ephemeral lacustrine water bodies. Spectral analysis of the L* colour time series points to the influence of the ～ 405-kyr and 0.97-Myr eccentricity cycles, while the bases of the two calcareous intervals correlate to successive minima of the 2.4-Myr eccentricity cycle. The 405-kyr cycle lags maximum eccentricity, whereas the 0.97 and 2.4-Myr cycles lag minimum eccentricity, each by approximately a quarter of a cycle. No obliquity forcing is detected. The observed orbital configuration of 2.4-Myr minima at the base of limestone-dominated intervals is similar to a previously documented Late Miocene shift in the Teruel Basin of northeast Spain. Our results indicate that long-period eccentricity climate forcing may well be a significant player on long, tectonic time scales in continental basin fill.  相似文献   

16.
Hydrothermal emission of mantle helium appears to be directly related to magma production rate, but other processes can generate methane and hydrogen on mid-ocean ridges. In an on-going effort to characterize these processes in the South Atlantic, the flux and distribution of these gases were investigated in the vicinity of a powerful black smoker recently discovered at 8°17.9' S, 13°30.4' W. The vent lies on the shoulder of an oblique offset in the Mid-Atlantic Ridge and discharges high concentrations of methane and hydrogen. Measurements during expeditions in 2004 and 2006 show that the ratio of CH4 to 3He in the neutrally buoyant plume is quite high, 4 × 108. The CTD stations were accompanied by velocity measurements with lowered acoustic Doppler current profilers (LADCP), and from these data we estimate the methane transport to have been 0.5 mol s− 1 in a WSW-trending plume that seems to develop during the ebb tidal phase. This transport is an order of magnitude greater than the source of CH4 calculated from its concentration in the vent fluid and the rise height of the plume. From this range of methane fluxes, the source of 3He is estimated to be between 0.14 and 1.2 nmol s− 1. In either case, the 3He source is significantly lower than expected from the spreading rate of the Mid-Atlantic Ridge. From the inventory of methane in the rift valley adjacent to the vent, it appears that the average specific rate of oxidation is 2.6 to 23 yr− 1, corresponding to a turnover time between 140 and 16 days. Vertical profiles of methane in the surrounding region often exhibited Gaussian-like distributions, and the variances appear to increase with distance from the vent. Using a Gaussian plume model, we obtained a range of vertical eddy diffusivities between 0.009 and 0.08 m2m2 s− 1. These high values may be due to tidally driven internal waves across the promontory on which the vent is located.  相似文献   

17.
Age of Seychelles–India break-up   总被引：1，自引：0，他引：1  
Many continental flood basalt provinces are spatially and temporally linked with continental break-up. Establishing the relative timing of the two events is a key step in determining their causal relationship. Here we investigate the example of the Deccan Traps and the separation of India and the Seychelles. Whilst there has been a growing consensus as to the age of the main phase of the Deccan emplacement (65.5 ± 1 Ma, chron 29r), the age of the rifting has remained unclear. We resolve this issue through detailed seafloor magnetic anomaly modeling (supported by wide-angle and reflection seismic results) of the north Seychelles and conjugate Laxmi Ridge/Gop Rift margins, and geochemistry and 40Ar/39Ar geochronology of rocks from the north Seychelles margin. We show that syn-rift volcanics offshore the Seychelles Islands in the form of seaward-dipping reflectors were most likely erupted during chron 28n, and the first organized seafloor spreading at the Carlsberg Ridge also initiated during this chron at 63.4 Ma. The severing of the Seychelles occurred by a south-eastward ridge propagation that was completed by the start of chron 27n (~ 62 Ma). A brief, pre-28r phase of seafloor spreading occurred in the Gop Rift, possibly as early as 31r–32n (~ 71 Ma). Initial extension at the margin therefore preceded or was contemporaneous with the Deccan emplacement, and separation of the Seychelles was achieved less than 3.5 Ma afterwards. This is the shortest time interval between flood basalt emplacement and break-up yet reported for any continental flood basalt-rifted margin pair. A contributing factor to the apparently short interval in the Deccan case may be that rifting occurred by a ridge jump into already thinned continental lithosphere. However, we conclude that external plate-boundary forces, rather than the impact of a mantle plume, were largely responsible for the rifting of the Seychelles from India.  相似文献   

18.
Makoto  Saito 《Island Arc》2008,17(2):242-260
Abstract   Detailed geologic examination of the Eocene accretionary complex (Hyuga Group) of the Shimanto terrane in southeastern Kyushu revealed that the oceanic plate was composed of Paleocene to Lower Eocene mudstone and siliceous mudstone, lower Middle Eocene red mudstone, and mid-Middle Eocene trench-fill turbidite with siltstone breccia, successively overlying the pre-Eocene oceanic plate. This oceanic plate sequence was overlain by Upper Eocene siltstone. Deposition of the lower Middle Eocene red mudstone was accompanied by basalt flows and it is interbedded with continental felsic tuff, which indicates that the basalt and red mudstone were deposited near the trench just before accretion. The Hyuga Group has very similar geological structure to that of the chert–clastic complexes found in the Jurassic accretionary complexes in Japan: that is, a decollement fault formed in the middle of an oceanic plate sequence, and an imbricate structure formed only in the upper part of the sequence. Thus, it appears that the Hyuga Group was formed by the same accretionary process as the Jurassic accretionary complexes. No accretion occurred before the Middle Eocene, and the rapid accretion of the Hyuga Group was commenced by the supply of coarse terrigenous sediments in the mid-Middle Eocene, when the direction of movement of the Pacific Plate changed. The pre-Eocene oceanic basement and lower Middle Eocene volcanic activity suggest that the oceanic plate partly preserved in the Hyuga Group was very similar to the northern part of the present West Philippine Sea Plate.  相似文献   

19.
The large 100-kyr cycles evident in most late-Pleistocene (0–0.6 Ma) paleoclimatic records still lack a satisfactory explanation. Previous studies of the nature of the transition from the early Pleistocene (1.2–1.8 Ma) 41-kyr-dominated climate regime to the 100-kyr world have been based almost exclusively on benthic foraminiferal oxygen isotopic (δ18O) data. It is generally accepted that the late Pleistocene 100-kyr cycles represent a newly evolved sensitivity to eccentricity/precession, superimposed on an earlier, and largely constant, response to obliquity and precession forcing. However, orbitally-resolved Pleistocene sea surface temperature (SST) records from a variety of oceanic regions paint a rather different picture of the global climate transition across the mid-Pleistocene transition (MPT, 0.6–1.2 Ma). Reanalysis of these SST records shows that: (1) an early onset of strong 100-kyr-like cycles in two low-frequency bands (～ 120–145 kyr and ～ 60–80 kyr), derived from the bundling of two/three obliquity cycles into grand cycles (obliquity subharmonics), occurred in tropical SST records during the early Pleistocene, (2) these two early Pleistocene periods converge into the late-Pleistocene 100-kyr period in tropical SST records, (3) the dominance of 100-kyr SST power in the late Pleistocene coincides with a dramatic decline in the 41-kyr SST power, and (4) the correlation of timing of glacial terminations with eccentricity/precession variation could well extend back into the early Pleistocene. We demonstrate that most of these features also occur in δ18O records, but in a much more subtle manner. These features could be explained in two plausible ways: a shift in climate sensitivity from obliquity to eccentricity/precession (a modified version of the conventional view) or an increasingly nonlinear response to orbital obliquity across the MPT. However, our examination of the development of ～100-kyr cycles favors an obliquity bundling mechanism to form late Pleistocene 100-kyr cycles. We therefore suggest that the late Pleistocene 100-kyr climatic cycles are likely a nonlinear response to orbital obliquity, although the timing of late Pleistocene 100-kyr climatic cycles and their early forms appears to be paced by eccentricity/precession.  相似文献   

20.
Bengal Fan Miocene sediments were collected during International Ocean Discovery Program Expedition 354 and investigated using petrographic and detrital garnet chemistry analyses. The Miocene Siwalik Group, which is composed of sediments deposited in the Himalayan foreland basin, was also analyzed for comparison with the Bengal Fan data for the provenance change during the Miocene. Our petrographic analyses revealed that the Miocene sediments of the Bengal Fan and Siwalik Group consist predominantly of Higher Himalayan Crystalline (HHC)-derived detritus such as chloritoid, staurolite, sillimanite, and/or kyanite, which appear among the accessory minerals. The chemistry of the detrital garnet varies across the stratigraphy; most of the garnet is rich in almandine and poor in spessartine and pyrope. However, pyrope-rich garnet, which is considered to originate from the HHC core (granulite facies), was found in the lower to upper Miocene deposits. The deposition of HHC-derived detrital garnet began before the Middle Miocene (15 Ma) and before the Late Miocene (10–9 Ma) in the Siwalik Group. The Bengal Fan data, by contrast, indicated that pyrope-rich garnet appeared in the Early Miocene (17.3 Ma) and Late Miocene (8.5–6.5 Ma). We conclude that the Bengal Fan sediments record the erosion of the HHC zone since the Early Miocene that appears in the Siwalik sediments. Furthermore, we found that the HHC-derived inputs decreased from the late Middle Miocene (12 Ma) to the early Middle Miocene (10 Ma) in both the Nepal Himalaya foreland basin and the Bengal Fan. The disappearance of the HHC-derived detritus is probably the result of dilution by Lesser Himalayan detritus, which suggests that the Lesser Himalayan zone, which is composed of metamorphosed and unmetamorphosed sedimentary rocks, was uplifted.  相似文献   

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