Latent heat effects of the major mantle phase transitions on low-angle subduction     

Jeroen van Hunen  Arie P van den Berg  Nico J Vlaar 《Earth and Planetary Science Letters》2001,190(3-4):125-135

Very low to zero shallow dip angles are observed at several moderately young subduction zones with an active trenchward moving overriding plate. We have investigated the effects of latent heat for this situation, where mantle material is pushed through the major mantle phase transitions during shallow low-angle subduction below the overriding plate. The significance of the buoyancy forces, arising from the latent heat effects, on the dynamics of the shallowly subducting slab is examined by numerical modeling. When a 32-Ma-old slab is overridden with 2.5 cm/yr by a continent, flat subduction occurs with a 4–5 cm/yr convergence rate. When latent heat is included in the model, forced downwellings cause a thermal anomaly and consequently thermal and phase buoyancy forces. Under these circumstances, the flat slab segment subducts horizontally about 350 km further and for about 11 Ma longer than in the case without latent heat, before it breaks through the 400-km phase transition. The style of subduction strongly depends on the mantle rheology: increasing the mantle viscosity by one order of magnitude can change the style of subduction from steep to shallow. Similarly, an overriding velocity of less than 1 cm/yr leads to steep subduction, which gradually changes to flat subduction when increasing the overriding velocity. However, these model parameters do not change the aforementioned effect of the latent heat, provided that low-angle subduction occurs. In all models latent heat resulted in a substantial increase of the flat slab length by 300–400 km. Varying the olivine–spinel transition Clapeyron slope γ from 1 to 6 MPa/K reveals a roughly linear relation between γ and the horizontal length of the slab. Based on these results, we conclude that buoyancy forces due to latent heat of phase transitions play an important role in low-angle subduction below an overriding plate.  相似文献   

Fluids in granulites of the Southern Marginal Zone of the Limpopo Belt, South Africa     

R. van den Berg  J. Huizenga 《Contributions to Mineralogy and Petrology》2001,141(5):529-545

The Southern Marginal Zone of the Limpopo Belt in South Africa is characterised by a granulite and retrograde hydrated granulite terrane. The Southern Marginal Zone is, therefore, perfectly suitable to study fluids during and after granulite facies metamorphism by means of fluid inclusions and equilibrium calculations. Isolated and clustered high-salinity aqueous and CO₂(-CH₄) fluid inclusions within quartz inclusions in garnet in metapelites demonstrate that these immiscible low H₂O activity fluids were present under peak metamorphic conditions (800-850 °C, 7.5-8.5 kbar). The absence of widespread high-temperature metasomatic alteration indicates that the brine fluid was probably only locally present in small quantities. Thermocalc calculations demonstrate that the peak metamorphic mineral assemblage in mafic granulites was in equilibrium with a fluid with a low H₂O activity (0.2-0.3). The absence of water in CO₂-rich fluid inclusions is due to either observation difficulties or selective water leakage. The density of CO₂ inclusions in trails suggests a retrograde P-T path dominated by decompression at T<600 °C. Re-evaluation of previously published data demonstrates that retrograde hydration of the granulites at 600 °C occurred in the presence of H₂O and CO₂-rich fluids under P-T conditions of 5-6 kbar and ~600 °C. The different compositions of the hydrating fluid suggest more than one fluid source.  相似文献   

Storm deposits and storm-generated coarse carbonate breccias on a pelagic outer shelf (South-East Basin, France)   总被引：2，自引：0，他引：2  

Michel Seguret  Alexis Moussine-Pouchkine  Guilherme Raja Gabaglia  & Frederic Bouchette 《Sedimentology》2001,48(2):231-254

Uppermost Jurassic limestones of the South‐East Basin (France) are organized into four facies associations that were deposited in four distinct zones: (1) peritidal lagoonal limestones; (2) bioclastic and reefal limestones; (3) pelagic lime mudstones; (4) lime mudstones/calcarenites/coarse breccias. Calcarenite deposits of zone 4 exhibit sedimentary structures that are diagnostic of deposition under wave‐induced combined flow. In subzone 4a, both vertical and lateral transitions from lime mudstone/calcarenite to breccia indicate in situ brecciation under wave‐cyclic loading. Breccias were produced by heterogeneous liquefaction of material previously deposited on the sea floor. Deposits in subzone 4a record relatively long periods (>400 kyr) of sedimentation below wave base, alternating with periods of deposition under wave‐induced currents and periods of in situ deformation. In this zone, storm waves were attenuated by wave–sediment interaction, and wave energy was absorbed by the deformation of soft sediment. With reference to present‐day wave attenuation, water depths in this zone ranged between 50 and 80 m. Landwards of the attenuation zone, in zone 3, storm waves were reduced to fair‐weather wave heights. Storm wave base was not horizontal and became shallower landwards. As a consequence, water depth and wave energy were not linearly related. On a small area of the seaward edge of subzone 4a, cobbles were removed by traction currents and redeposited in subzone 4b. There, they formed a 100‐m‐thick wedge, which prograded over 3 km and was built up by the stacking of 5‐ to 20‐m‐thick cross‐stratified sets of coarse breccia. This wedge records the transport and redeposition of cobbles by a high‐velocity unidirectional component of a combined flow. The increase in flow velocity in a restricted area is proposed to result from flow concentration in a channel‐like structure of the downwelling in the gulf formed by the basin. In more distal subzone 4c, the hydrodynamic effect of wave‐induced currents was quasi‐permanent, and brecciation by wave–sediment interaction occurred only episodically. This indicates that, seawards of the attenuation zone, hydrodynamic storm wave base was deeper than mechanical storm wave base. Uppermost Jurassic carbonates were deposited and soft‐sediment deformed on a hurricane‐dominated ramp of very gentle slope and characterized by a zone of storm wave degeneration, located seawards of a zone of sedimentation below wave base.  相似文献   

Lucinid clam influence on the biogeochemistry of the seagrass<Emphasis Type="Italic">Thalassia testudinum</Emphasis> sediments     

Laura K. Reynolds  Peter Berg  Joseph C. Zieman 《Estuaries and Coasts》2007,30(3):482-490

Lucinid bivalves dominate the infauna of tropical seagrass sediments. While the effect of seagrass on lucinids has been studied, the reverse effect has largely been ignored. Lucinids can alter porewater chemistry (i.e., increase porewater nutrients by suspension feeding and decrease porewater sulfides by oxygen introduction and bacterial oxidation), which can potentially change seagrass productivity and growth morphology. To observe correlations between porewater chemistry and lucinid presence, a field survey and laboratory microcosm experiment were conducted. Survey sampling sites with lucinids had significantly lower sulfide and higher ammonium concentrations than sampling sites without lucinids. There was no difference in phosphate concentration among sampling sites. Both lucinid species used in the microcosm experiment (Ctena orbiculata andLucinesca nassula) significantly lowered sulfide concentrations in the sediment porewater. Microcosm and field survey results were incorporated into a sulfide budget. In seagrass sediments, lucinids remove 2–16% of the total sulfide produced. Sulfide is a major stressor to both plants and animals in Florida Bay sediments; this removal may be important to maintaining seagrass productivity and health. Oxygen introduction into sediments byC. orbiculata was estimated in a dye experiment.C. orbiculata were added to small tubes containing sieved mud and incubated in a bath of seawater with a Rhodamine WT. Rhodamine WT accumulation in the sediment was measured. A first order estimate showed that oxygen introduction can account for less than 5% ofC. orbiculata sulfide removal.  相似文献   

Thermochronology and exhumation rates of granitic intrusions at Mesa Central,Mexico     

Angel F. Nieto-Samaniego  María de Jesús Paulina Olmos-Moya  Gilles Levresse  Susana A. Alaniz-Alvarez  Fanis Abdullin  Alexis del Pilar-Martínez 《International Geology Review》2020,62(3):311-319

ABSTRACT

The Mesa Central of Mexico (MC) is an elevated plateau located 2000 m above sea level in central Mexico, where intrusions outcrop that register the history of exhumation-erosion occurring during the Late Cretaceous-Paleogene. The tectonic history of the region records formation of the Late Cretaceous-Paleogene ‘Mexican orogen’; this was followed by extension of the entire region and several plutons were then exhumed. The age and magnitude of the crustal uplift and erosion occurring during exhumation has not been addressed to date. Therefore, this study reports the crystallization and cooling ages of two plutons, the Tesorera Granodiorite and the Comanja Granite, and estimates their emplacement depths. Based on these data, the exhumation age of the Tesorera Granodiorite is estimated to be between ~73 Ma and ~63 Ma at an exhumation rate of ~528 m/m. y. and that of the Comanja Granite is 52 Ma and 48 Ma at an exhumation rate of ~2500 m/m. y. Exhumation-erosion event of the Tesorera Granodiorite was located on the trace of the San Luis-Tepehuanes Fault System and that of the Comanja Granite on the a trace of the El Bajío Fault System. Furthermore, the high exhumation rate in the Comanja Granite suggests that gravitational collapse played an important role during exhumation.  相似文献   

Relative stress variations as determined by b-values from earthquakes in circum-pacific subduction zones     

Jerry A. Carter  Eduard Berg 《Tectonophysics》1981,76(3-4)

Assuming a relation of “b” to stress state, the possibility of globe-wide stress variation and transmission was investigated. The NOAA earthquake data file served to determine the temporal change in “b” of log N = a − bM from 1963 to 1975.Periods of six to eight years are observed in the b-values (stress pattern) for most circum-Pacific areas (South America, Tonga, Kermadec, New Hebrides, Kamchatka and Eastern Aleutians).In the Kurils, fore- and aftershock sequences of large earthquakes seem to mask any characteristic global pattern that might exist. These sequences exhibit low b-values (high stress) through the time of foreshocks and early stages of aftershocks, followed by rapid increase in b-values (decrease in stress).Use of a worldwide earthquake data file clearly yields less resolution of the temporal “b” variation than the use of local network studies published by other authors.Incidental to the study, 1124 earthquakes of the NOAA data file yield the M_s − m_b relations: M_s = 1.16m_b − 0.835 for 4.5 m_b 6 and: log₁₀M_s = 0.1432m_b − 0.0629 form_b > 6 with correlative coefficients of 0.994 and 0.992 respectively.  相似文献   

The rotation of Italy: Preliminary palaeomagnetic data from the Umbrian sequence,Northern Apennines     

C.T. Klootwijk  J. van den Berg 《Earth and Planetary Science Letters》1975,25(3):263-273

A preliminary collection of 43 palaeomagnetic samples (10 sites) from the miogeosynclinal and supposedly autochthonous Umbrian sequence in the Northern Apennines, Italy, was analysed by means of alternating magnetic fields and thermal demagnetization studies. The older group of samples, taken from the upper part of the Calcari Diasprini (Malm), the Fucoid Marls (Albian/Cenomanian) and from the basal part of the Scaglia Bianca (Early Late Cretaceous), all showed normal polarity directions and resulted in a mean site direction:D = 290.5°,I = +51.5°,α₉₅ = 11°,k = 74,N = 4.The younger group of samples, taken throughout the Scaglia Rossa sequence (Latest Cretaceous/Middle Eocene) showed normal and reversed polarity directions. In contrast to the older group, the magnetic analysis of these samples resulted in a considerably less dense grouping of site mean directions. This presumably is due to inaccuracies introduced with the very large bedding tilt corrections that had to be applied to the samples of some sites. A tentative mean site direction for these Scaglia Rossa samples was computed as:D = 351°,I = +52.5°,α₉₅ = 23.5°,k = 11.5,N = 5.Despite the low precision of the Scaglia Rossa result, the significant deviation between this Latest Cretaceous/Early Tertiary direction and the Late Jurassic/Early Late Cretaceous direction indicates a counterclockwise rotation of more than forty degrees. This rotation can be dated as Late Cretaceous.How far these data from the Northern Apennines apply to other parts of the Italian Peninsula has yet to be established. The timing of this rotation is not at variance with the data from other parts of Mediterranean Europe (Southern Alps, Iberian Peninsula) and from Africa. However, taking into account the preliminary nature of the results, the amount of rotation of the Northern Apennines seems to surpass the rotation angle which is deduced from the palaeomagnetic data for Africa.  相似文献   

Perimagmatische und apomagmatische Lagerstätten     

G. Berg 《International Journal of Earth Sciences》1924,15(2):143-143

Ohne Zusammenfassung  相似文献   

Substrate selection and seasonal variation in abundance and size composition of isopod Lirceus fontinalis in Ontario streams, Canada     

ZHAO Xiaoxia Michael G FOX David C. LASENBY Alexis C. ARMIT Dolly N. KOTHAWALA 《中国海洋湖沼学报》2007,25(2):215-220

The abundance and size composition of stream isopods Lirceus fontinalis were investigated from April 2001 to August 2002 in 3 streams in southern Ontario, Canada. Effects of current speed was released from the analysis by choosing slow flowing water. The effects of substrate, season and water depth on the abundance, distribution and size composition of the isopods were analyzed. It was found that substrate and season influence isopods the most. The rocky substrate with attached filamentous macro-algae contains an isopod abundance 7.05 times that of bare rock substrate and 14.6 times that of fine-sand and mud substrate. There was a large variation with respect to seasonality in both abundance and size composition of the isopods, with the highest abundance occurring in summer and the lowest in winter and spring; individual isopods also tend to be larger in the winter and spring. In all substrates, shallow areas tend to support higher densities of isopods than deeper areas.  相似文献   

Meteorites at Meridiani Planum provide evidence for significant amounts of surface and near‐surface water on early Mars     

Alberto G. FAIRÉN  James M. DOHM  Victor R. BAKER  Shane D. THOMPSON  William C. MAHANEY  Kenneth E. HERKENHOFF  J. Alexis P. RODRÍGUEZ  Alfonso F. DAVILA  Dirk SCHULZE‐MAKUCH  M. Ramy EL MAARRY  Esther R. UCEDA  Ricardo AMILS  Hirdy MIYAMOTO  Kyeong J. KIM  Robert C. ANDERSON  Christopher P. McKAY 《Meteoritics & planetary science》2011,46(12):1832-1841

Abstract– Six large iron meteorites have been discovered in the Meridiani Planum region of Mars by the Mars Exploration Rover Opportunity in a nearly 25 km‐long traverse. Herein, we review and synthesize the available data to propose that the discovery and characteristics of the six meteorites could be explained as the result of their impact into a soft and wet surface, sometime during the Noachian or the Hesperian, subsequently to be exposed at the Martian surface through differential erosion. As recorded by its sediments and chemical deposits, Meridiani has been interpreted to have undergone a watery past, including a shallow sea, a playa, an environment of fluctuating ground water, and/or an icy landscape. Meteorites could have been encased upon impact and/or subsequently buried, and kept underground for a long time, shielded from the atmosphere. The meteorites apparently underwent significant chemical weathering due to aqueous alteration, as indicated by cavernous features that suggest differential acidic corrosion removing less resistant material and softer inclusions. During the Amazonian, the almost complete disappearance of surface water and desiccation of the landscape, followed by induration of the sediments and subsequent differential erosion and degradation of Meridiani sediments, including at least 10–80 m of deflation in the last 3–3.5 Gy, would have exposed the buried meteorites. We conclude that the iron meteorites support the hypothesis that Mars once had a denser atmosphere and considerable amounts of water and/or water ice at and/or near the surface.  相似文献