Pressure dependence and anisotropy of P-wave velocities in ultrahigh-pressure metamorphic rocks from the Dabie–Sulu orogenic belt (China): Implications for seismic properties of subducted slabs and origin of mantle reflections

The compressional wave velocities (Vp), pressure derivatives (Vp′) and anisotropy (A) of three types of eclogites and country rocks from the Dabie–Sulu ultrahigh-pressure (UHP) metamorphic belt, China, have been measured under confining pressures up to 800 MPa. Type-1 eclogites, which are coarse-grained and subjected to almost no retrograde metamorphism, experienced recovery-accommodated dislocation creep at peak metamorphic conditions (in the diamond stability field). Type-2 eclogites are fine-grained reworked Type-1 materials that experienced recrystallization-accommodated dislocation creep under quartz/coesite boundary conditions during the early stage of exhumation. Type-3 eclogites are retrogressed samples that were overprinted by significant amphibolite facies metamorphism during a late stage of exhumation within the crust. Type-1 eclogites are richer in Al₂O₃ and MgO but poorer in SiO₂ and Na₂O than Type-2 and Type-3 eclogites. Anisotropy of Type-1 and Type-2 eclogites is generally low (<4%) because volumetrically important garnet is elastically quasi-isotropic, while Type-3 eclogites can exhibit high anisotropy (>10%) due to the presence of strongly anisotropic retrograde minerals such as amphibole, plagioclase and mica. The transition of the pressure dependence of velocity from the poroelastic to elastic regimes occurs at a critical pressure (P_c), which depends mainly on the density and distribution of microcracks and in turn on the exhumation history of rocks. The Vp–pressure relationship can be expressed by Vp=a(lnP)²+blnP+c (P≤P_c) and Vp=V₀+DP (P≥P_c), where P is the confining pressure, a and b are constants describing the closure of microcracks below P_c, c is the velocity when P is equal to one (MPa), V₀ is the projected velocity of a crack-free sample at room pressure, and D is the intrinsic pressure derivative above P_c. When data are curve-fit, pressure derivatives and anisotropy as functions of pressure are determined. The average Vp of the eclogites in the linear regime is 8.42+1.41×10⁻⁴P for Type-1, 7.80+1.58×10⁻⁴P for Type-2, and 7.33+2.04×10⁻⁴P for Type-3, where Vp is in km/s and P in MPa. The decrease in V₀ and increase in D from Type-1 to Type-3 eclogites are attributed to a decrease in garnet content and an increase in retrograde minerals. The NE–SW trending, NW-dipping, slab-like high Vp anomaly (8.72 km/s at a depth of 71 km) which extends from the Moho to at least 110 km beneath the Dabie–Sulu region, can be interpreted as the remnant of a subducted slab which is dominated by Type-1 eclogites and has frozen in the upper mantle since about 200–220 Ma. Such relic crustal materials, subducted and preserved as eclogite layers intercalated with felsic gneiss, garnet–jadeite quartzite, marble and serpentinized peridotite, could be responsible for regionally observed seismic reflectors in the upper mantle.     

Algal pigments in sediments as a measure of eutrophication in the Baltic environment

A method of quantitative comparison of eutrophication of an area is proposed for the Baltic Sea, based on pigment content in sediments. The pigments concerned were chlorins a (i.e. chlorophylls a, b and selected chlorophyll a derivatives) and chlorophylls c. The analyses were performed on 300 samples from different layers of recent (0–10 cm) sediments, collected from about 50 stations, at different sites of the southern Baltic, including the estuaries of the two largest Polish rivers, in different seasons between 1992 and 2001, before and after the great flood of July 1997. The results are related to sampling site, sediment layer and hydrological conditions and also to organic carbon and E_h in sediments, oxygen and salinity in near-bottom waters. Depending on different chlorin a content in 0–1 and 0–10 cm layers, the sampling sites are classified into one of three groups: 1. Szczecin Lagoon and the Deep of Gda sk stations (permanently eutrophic, chl a in 0–1 cm >40 nmol/g, Σchlns a in 0–1 in 0–10 cm layer, Σchlns a_A in Σchlns a=55–65%), 2. Open sea stations (mesotrophic/oligotrophic, chl a in 0–1 cm <10 nmol/g, Σchlns a in 0–1 in 0–10 cm layer, Σchlns a_A in Σchlns a 50%; and 3. Coastal stations (periodically eutrophic, chl a in 0–1 cm 10–40 nmol/g, Σchlns a_A in Σchlns a 40%). The correlation coefficient between chlorophyll a and chlorophylls b and c indicates the classes of algae, which could be the main source of organic matter in the sediments. A high correlation with chlorophylls c is a marker of diatoms; a high correlation with chlorophyll b is a marker of green algae; and low correlation both with chlorophylls b and c—indicates a high blue–green algae input.     

Vp/Vs ratio along the Vøring Margin, NE Atlantic, derived from OBS data: implications on lithology and stress field

A total of 13 regional Ocean Bottom Seismograph (OBS) profiles with an accumulated length of 2207 km acquired on the Vøring Margin, NE Atlantic have been travel time modelled with regards to S-waves. The V_p/V_s ratios are found to decrease with depth through the Tertiary layers, which is attributed to increased compaction and consolidation of the rocks. The V_p/V_s ratio in the intra-Campanian to mid-Campanian layer (1.75–1.8) in the central Vøring Basin is significantly lower than for the layers above and beneath, suggesting higher sand/shale ratio. This layer was confirmed by drilling to represent a layer of sandstone. This mid-Cretaceous ‘anomaly’ is also present in the northern Vøring Basin, as well as on the southern Lofoten Margin further north. The V_p/V_s ratio in the extrusive rocks on the Vøring Plateau is estimated to be 1.85, conformable with mafic (basaltic) rocks. Landward of the continent/ocean transition (COT), the V_p/V_s ratio in the layer beneath the volcanics is estimated to be 1.67–1.75. These low values suggest that this layer represents sedimentary rocks, and that the sand/shale ratio might be relatively high here. The V_p/V_s ratio in the crystalline basement is estimated to be 1.67–1.75 in the basin and on the landward part of the Vøring Plateau, indicating the presence of granitic/granodioritic continental crust. In the lower crust, the V_p/V_s ratio in the basin decreases uniformly from southwest to northeast, from 1.85–1.9 to 1.68–1.73, suggesting a gradual change from mafic (gabbroic) to felsic (granodioritic) lower crust. Significant (3–5%) azimuthal S-wave anisotropy is observed for several sedimentary layers, as well as in the lower crust. All these observations can be explained by invoking the presence of liquid-filled microcracks aligned vertically along the direction of the present day maximum compressive stress (NW–SE).     

The morphological dynamics of intertidal megaripples in the Mawddach Estuary, North Wales, and the implications for palaeoflow reconstructions

The morphology and migration rate of tidal bedforms are important because of their use in interpretation of modern and ancient sediment transport regimes. Tidal flow, megaripple morphology and migration were studied in the mesotidal Mawddach Estuary, North Wales, to examine the veracity of published flow-bedform relationships, quantify spatial variations in migration and assess consequences for palaeoflow reconstruction. Two transects were surveyed along a megarippled intertidal shoal (mean grain size 280 μm) for a period of 22 semi-diurnal tidal cycles. A vertical array of current meters recorded tidal current profiles at the centre of one of the transects. Flood tidal currents dominate at Fegla Fach shoal, with peak velocities over 1 m s^?1 at spring tides, and 0.5 m s ^?1 at neaps, and bed sediment transport was also flood-dominated. Over the lunar cycle, the morphology of the megaripples on the survey lines was divisible into three phases: 1 the neap mode-consisting of near-moribund two-dimensional (2-D) flood-orientated megaripples of wavelength c. 6 m and height c. 0.2 m; 2 a transitional mode-where, on rising tidal ranges, scour pits formed and developed into 3-D megaripples which underwent net migration with the flood tide; 3 the spring mode-consisting of 3-D megaripples of wavelength c. 4 m and height c. 0.2 m. Despite complete re-orientation by the ebb tide, these were recognizable from one low water survey to the next, and net migration was c. 1 m per tide with the flood tide. We infer the presence of the equilibrium ‘spring tidal form’ occurring as flood-orientated megaripples during the flood tide. The data support previously reported separation of 2-D and 3-D megaripples at a depth to grain size ratio of 8000, and at a depth-mean velocity of the dominant tide (U_maxdom) of 0.75-0.8 m s^?1. A migration threshold exists at U_maxdom of c. 0.53-0.57 m s^?1. Measures of migration which might be used on preserved sections have been applied to the data. These measures systematically overestimated bedform migration at most stages of the lunar cycle (by <25% at spring tides and <140% in the post-spring transition period), but were accurate when the megaripples had developed into their 'spring tidal form’. There is significant variation of migration rates within the survey populations. We conclude that whilst the occurrence of megaripple cross-sets may be used as a palaeoflow indicator, and sedimentary structures associated with 2-D to 3-D transitions may also be indicative of palaeoflows, there are likely to be significant uncertainties involved in using tidal bundles as an indication of sediment transport rates.     

Upper crustal seismic structure of the Mazury complex and Mazowsze massif within East European Craton in NE Poland

The large-scale seismic experiment POLONAISE '97 (POlish Lithospheric ONsets—An International Seismic Experiment) was carried out in May 1997 in Poland, Lithuania, and Germany. Its main purpose was to investigate the structure of the crust and the uppermost mantle in the region of the Trans European Suture Zone (TESZ) that lies between the East European Craton (EEC) and the Palaeozoic Platform. This paper covers the interpretation of seismic data along the NW–SE-trending, 180-km-long profile P5 located on the EEC. The recordings were of a high quality with seismic energy clearly visible along the whole profile. We have not found waves refracted below the upper crust in first arrivals. In the NW part of the profile, we have delineated a high-velocity body with the P-wave velocity in the range of 6.5–6.75 km/s in the upper crust. It corresponds to the K trzyn anorthosite massif within the Mazury complex. The Mazowsze massif is rather uniformly characterized by P-wave velocities 5.9–6.05 and 6.2–6.35 km/s in two layers, respectively. Sufficient S-wave data were available to estimate the V_p/V_s ratio (as well as the Poisson ratio), being 1.80 (0.277) in the high-velocity body and 1.67 (0.220) in the upper crust.Apart from the 2-D model along the profile, results of 3-D modelling in the area of the P5 profile are presented. Using off-line recordings, we got P-wave velocity field up to 8 km/s below the P5 profile at the depth of about 40 km as well as horizontal extent of the high-velocity body.     

Seismic velocities of granulite-facies xenoliths from Central Ireland: Implications for lower crustal composition and anisotropy

V_p and V_s values have been measured experimentally and calculated for granulite-facies lower crustal xenoliths from central Ireland close to the Caledonian Iapetus suture zone. The xenoliths are predominantly foliated and lineated metapelitic (garnet–sillimanite–K-feldspar) granulites. Their metapelitic composition is unusual compared with the mostly mafic composition of lower crustal xenoliths world-wide. Based on thermobarometry, the metapelitic xenoliths were entrained from depths of c. 20–25 ± 3.5 km and rare mafic granulites from depths of 31–33 ± 3.4 km. The xenoliths were emplaced during Lower Carboniferous volcanism and are considered to represent samples of the present day lower crust.V_p values for the metapelitic granulites range between 6.26 and 7.99 km s^{− 1} with a mean value of 7.09 ± 0.4 km s^{− 1}. Psammite and granitic orthogneiss samples have calculated V_p values of 6.51 and 6.23 km s^{− 1}, respectively. V_s values for the metapelites are between 3.86 and 4.34 km s^{− 1}, with a mean value of 4.1 ± 0.15 km s^{− 1}. The psammite and orthogneiss have calculated V_s values of 3.95 and 3.97 km s^{− 1}, respectively.The measured seismic velocities correlate with density and with modal mineralogy, especially the high content of sillimanite and garnet. V_p anisotropy is between 0.15% and 13.97%, and a clear compositional control is evident, mainly in relation to sillimanite abundance. Overall V_s anisotropy ranges from 1% to 11%. Poisson's ratio (σ) lies between 0.25 and 0.35 for the metapelitic granulites, mainly reflecting a high V_p value due to abundant sillimanite in the sample with the highest σ. Anisotropy is probably a function of deformation associated with the closure of the Iapetus ocean in the Silurian as well as later extension in the Devonian. The orientation of the bulk strain ellipsoid in the lower crust is difficult to constrain, but lineation is likely to be NE–SW, given the strike-slip nature of the late Caledonian and subsequent Acadian deformation.When corrected for present-day lower crustal temperature, the experimentally determined V_p values correspond well with velocities from the ICSSP, COOLE I and VARNET seismic refraction lines. Near the xenolith localities, the COOLE I line displays two lower crustal layers with in situ V_p values of 6.85–6.9 and 6.9–8.0 km s^{− 1}, respectively. The upper (lower velocity) layer corresponds well with the metapelitic granulite xenoliths while the lower (higher velocity) layer matches that of the basic granulite xenoliths, though their metamorphic pressures suggest derivation from depths corresponding to the present-day upper mantle.     

P - V - T equation of state of stishovite to the mantle transition zone conditions

In-situ synchrotron X-ray diffraction experiments were conducted using the SPEED-1500 multi-anvil press of SPring-8 on stishovite SiO₂ and pressure-volume-temperature data were collected at up to 22.5 GPa and 1,073 K, which corresponds to the pressure conditions of the base of the mantle transition zone. The analysis of room-temperature data yielded V₀=46.56(1) ų, K_{T 0}=296(5) GPa and K _T =4.2(4), and these properties were consistent with the subsequent thermal equation of state (EOS) analyses. A fit of the present data to high-temperature Birch-Murnaghan EOS yielded (K_T /T) _P =–0.046(5) GPa K^–1 and = a + bT with values of a =1.26(11)×10^–5 K^–1 and b =1.29(17)×10^–8 K^–2. A fit to the thermal pressure EOS gives ₀=1.62(9)×10^–5 K^–1, ( K _T / T) _V =–0.027(4) GPa K^–1 and (²P /T ²) _V =27(5)×10^–7 GPa K^–2. The lattice dynamical approach by Mie-Grüneisen-Debye EOS yielded ₀=1.33(6), q =6.1(8) and ₀=1160(120) K. The strong volume dependence of the thermal pressure of stishovite was revealed by the analysis of present data, which was not detectable by the previous high-temperature data at lower pressures, and this yields ( K _T / T) _V 0 and q 1. The analyses for the fictive volume for a and c axes show that relative stiffness of c axis to a axis is similar both on compression and thermal expansion. Present EOS enables the accurate estimate of density of SiO₂ in the deep mantle conditions.     

An experimental investigation of hydroxyl solubility in jadeite and Na-rich clinopyroxenes

The solubility and incorporation mechanisms of water in synthetic, water-saturated jadeite and Na-rich clinopyroxenes have been experimentally investigated. Infrared spectra for water-saturated jadeite synthesised from 2.0 to 10 GPa show two prominent sharp peaks at 3,373 and 3,613 cm^–1 together with several weaker features in the OH-stretching region, indicating that there are at least 5 distinct modes of hydrogen incorporation in the structure. Water solubility in pure jadeite reaches a maximum of about 450 ppm by weight at 2 GPa and slowly decreases with increasing pressure to about 100 ppm at 10 GPa. Solubility can be described by the function c_OH=A f_H2O^0.5 exp (–PV_Solid/RT), where c_OH is water solubility in ppm H₂O by weight, A is 7.144 ppm/bar^0.5, f_H2O is water fugacity, and V_Solid=8.019 cm³/mol is the volume change of the clinopyroxene upon incorporation of OH. Jadeite provides a good model for understanding hydrogen incorporation mechanisms in more complex omphacite compositions. Assignment of absorption bands in IR spectra verifies the importance of cation vacancies on the M2 site in providing mechanisms for hydrogen incorporation. However, results also suggest that substitution of lower valency cations onto the M1 site may also be important. Solid solution of jadeite with diopside and in particular, with Ca-Eskola component leads to a drastic increase of water solubility, and the bulk composition has a more important effect on the capacity of omphacite to store water than pressure and temperature. Omphacite is expected to be the major carrier of water in a subducted eclogite after the breakdown of hydrous minerals.Editorial responsibility: W. Schreyer     

Crustal transect from the North Atlantic Knipovich Ridge to the Svalbard Margin west of Hornsund

The crustal structure along a 312 km transect, stretching from the axial mountains of the North Atlantic Knipovich Ridge to the continental shelf of Svalbard, has been obtained using seismic reflection data and wide angle OBS data. The resulting seismic V_p and V_s models are further constrained by a 2-D-gravity model. The principal objective of this study is to describe and resolve the physical and compositional properties of the crust in order to understand the processes and creation of oceanic crust in this extremely slow-spreading counterpart of the North Atlantic Ridge Systems. V_p is estimated to be 3.50–6.05 km/s for the upper oceanic crust (oceanic layer 2), with a marked increase away from the ridge. The measured V_p of 6.55–6.95 km/s for oceanic layer 3A and 7.10–7.25 km/s for layer 3B, both with a V_p/V_s ratio of 1.81, except for slightly higher values at the ridge axis, does not allow a clear distinction between gabbro and mantle-derived peridotite (10–40% serpentized). The thickness of the oceanic crust varies a lot along the transect from the minimum of 5.6 km to a maximum of 8.1 km. The mean thickness of 6.7 km for the oceanic crust is well above the average thickness for slow-spreading ridges (<10 mm/year half-spreading rate). The areas of increased thickness could be explained by large magma production-rates found in the zones of axial highs at the ridge axis, which also have generated the off-axial highs adjacent the ridge. We suggest that these axial and off-axial highs along the ridge control the lithological composition of the oceanic crust. This approach suggests normal gabbroic oceanic crust to be found in the areas bound by the active magma segments (the axial and off-axial highs) and mantle-derived peridotite outside these zone.     

Solid bitumen reflectance and Rock-Eval Tmax as maturation indices: an example from the “Nordegg Member”, Western Canada Sedimentary Basin

Marine, organic-rich rock units commonly contain little for vitrinite reflectance (VR₀) measurement, the most commoly used method of assessing thermal maturity. This is true of the Lower Jurassic “Nordegg Member”, a type I/II, sulphur-rich source rock from the Western Canada Sedimentary Basin. This study examines the advantages and pitfalls associated with the use of Rock-Eval T_max and solid bitumen reflectance (BR₀) to determined maturity in the “Nordegg”. Vitrinite reflectance data from Cretaceous coals and known coalification gradients in the study area are used to extrapolate VR₀ values for the “Nordegg”.T_max increases non-linearly with respect to both BR₀ and extrapolated VR₀ values. A sharp increase in the reflectaance of both solid bitumen and vitrinite occurs between T_max 440–450°C, and is coincident with a pronounced decrease in Hydrogen Index values and the loss of solid bitumen and telalginite fluorescence over the same narrow T_max interval. This T_max range is interpreted as the main zone of hydrocarbon generation in the “Nordegg”, and corresponds to extrapolated VR₀ values of 0.55–0.85%. The moderate to high sulphur contents in the kerogen played a significant role in determining the boundaries of the “Nordegg” oil window.A linear relationship between BR₀ and extrapolated VR₀, as proposed elsewhere, is not true for the “Nordegg”. BR₀ increases with respect to extrapolated VR₀ according to Jacob's (1985) formula (VR₀=0.618×(BR₀)+0.40) up to VR₀≈0.72% (BR₀≈0.52%). Beyond this point, BR₀ increases sharply relative to extrapolated VR₀, according to the relatioship VR₀ = 0.277 × (BR₀) + 0.57 (R² = 0.91). The break in the BR₀−VR₀ curve at 0.72%VR₀ is thought to signifiy the peak of hydrocarbon generation and represents a previously unrecognized coalification jump in the solid bitumen analogous to the first coalification jump of liptinites.     

Compressional behaviour of CaNiSi2O6 clinopyroxene: bulk modulus systematic and cation type in clinopyroxenes

A single-crystal of composition CaNiSi₂O₆ (space group C2/c) was investigated at high pressure up to about 7.8 GPa by X-ray diffraction. The unit-cell parameters were measured at 18 different pressures. The P-V data were fitted by a third-order Birch-Murnaghan equation of state V₀=435.21(1) ų, K ₀=117.6(3) GPa and K^=6.4(1). The linear axial compressibilities _a, _b, _c and a sin are 2.14(1), 3.00(1), 2.43(1) and 1.63(1) × 10–3 GPa^–1. Comparing the compressibility data with other CaM1Si₂O₆ pyroxenes we suggest that the empirical K × V = constant relationships are followed in C2/c pyroxenes only if the same valence electron character is shared.     

A sedimentological approach to P-A relationships for tidal inlet systems: an example from Yuehu Inlet, Shandong Peninsula, China

Power-law relationship between tidal prism (P) and the cross-sectional area of the entrance channel (A) is applicable to assess the equilibrium conditions of a tidal inlet system. The classic method of determining P-A relationships proposed by O’Brien depends on datasets from multi-tidal inlet systems, which has shown some limitations and is unable to assess equilibrium of a single tidal inlet. This paper focuses on establishing a new P-A relationship for a single tidal inlet. Our experimental result shows that in order to maintain the status, power n should be > 1, implying that the inlet width will narrow and current speed within the entrance will increase as tidal prism becomes smaller. A possible explanation for power n <,1.0, as many researchers argued before, is that the influence of tidal prism has been exaggerated. Meanwhile, the magnitude of coefficient C is dependent on many factors such as longshore drift, freshwater discharge, etc, resulting in a wide range of variation of C. It should be pointed out that P-A relationship given by the sediment dynamical approach is still a representative of average status for tidal inlets in equilibrium. As tide, wave, freshwater discharge and tidal inlet morphology change with time, actual P-A relationships will fluctuate also. The problems that need to be solved when applying sediment dynamic methods to P-A relationships include the cross-sectional distribution pattern of tidal current speeds in the entrance channel, the relationship between the tidal current and the tidal water level at the entrance, and the calculation of the ratio of width to depth. This paper will establish a sediment dynamical approach of P-A relationship for a single tidal inlet. The results are tested for P-A relationships of Yuehu Inlet, a small inlet-lagoon system located in Shandong Peninsula, China. __________ Translated from Oceanologia Et Limnologia Sinica, 2005, 36(3): 269–276 [译自: 海洋与湖沼]     

The use of assumed displacement fields in finite-element analysis to calculate the gravity effect of crustal deformation

An expression is derived which relates a displacement field u defined on a continuum to the change in gravity (Δg_D) brought about by deformation:

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where GG is the gravitational constant; ρ_R is the pre-deformation density function; V_R is the volume occupied by the continuum before deformation; T_D is a transformation which represents the one-to-one mapping of V_R onto the volume V_A occupied by the continuum after deformation: A₁ = R₁ + U₁(R₁, R₂, R₃)T_D: A₂ = R₂ + U₂ (R₁, R₂, R₃)A₃ = R₃ + U₃ ( R₁, R₂, R₃) where A₁, a₂, A₃, are the Cartesian coordinates of a point in V_A; R₁, R₂, R₃ are the Cartesian coordinates of a point in V_R; U₁, U₂, U₃, are the components of u; and α is the distance from the field point (x, y, z) to a general point in V_A or V_R. The derivation is based on the principle of conservation of mass as stated in continuum mechanics. The formulation of the finite-element procedure for problems in linear elasticity typically involves the selection of an assumed function for u within each element. Unknown constants which characterize the function are determined in terms of nodal displacements. The integral in the first equation above may therefore be evaluated for each element, and the results summed to obtain Δg_D. Convergence to the exact solution for Δg_D depends on the convergence of the finite-element solution for u. The procedure is demonstrated for axisymmetric, torsion-free deformation of a solid of revolution in which the elements are tori of triangular cross-section. These results may be useful in the analysis of gravity change associated with tectonic deformation of the earth's crust.     

3-D crustal structure of the extensional Granada Basin in the convergent boundary between the Eurasian and African plates

Three-dimensional P and S wave velocity models of the crust under the Granada Basin in Southern Spain are obtained with a spatial resolution of 5 km in the horizontal direction and 2 to 4 km in depth. We used a total of 15407 P and 13704 S wave high-quality arrival times from 2889 local earthquakes recorded by both permanent seismic networks and portable stations deployed in the area. The computed P and S wave velocities were used to obtain three-dimensional distributions of Poisson's ratio (σ) and the porosity parameter (V_p×V_s). The 3-D velocity images show strong lateral heterogeneities in the region. Significant velocity variations up to ±7% in P and S velocities are revealed in the crust below the Granada Basin. At shallow depth, high-velocity anomalies are generally associated with Mesozoic basement, while the low-velocity anomalies are related to the neogene sedimentary rocks. The south–southeastern part of the Granada Basin exhibits high σ values in the shallowest layers, which may be associated with saturated and unconsolidated sediments. In the same area, V_p×V_s is high outside the basin, indicating low porosity of the mesozoic basement. A low-velocity zone at 18-km depth is found and interpreted as a weak–ductile crust transition that is related to the cut-off depth of the seismic activity. In the lower crust, at 34-km depth, a clear slow V_p and V_s anomalous zone may indicate variations in lithology and/or with the rigidity of the lower crust rocks.     

Some properties of the development of the perturbed zone and shock preceding a coronal mass ejection

SOHO/LASCO C2 and C3 data have been used to carry out a detailed study of the perturbed zone and shock that form as a coronal mass ejection (CME) moves away from the Sun, as a result of its interaction with the ambient solar wind. The event of January 4, 2002 is used as an example. The perturbed zone is most extensive along the direction of propagation of the CME, decreases away from this direction, and reaches its minimum values perpendicular to this direction. The mass of the perturbed zone is ≥0.1 of the total mass of the CME. The condition for the formation of a shock preceding the CME (in the direction of propagation of the CME) is V − V _SW > V _A , where V, V _SW , and V _A are the CME, solar wind, and Alfvén velocities, respectively. Perpendicular to the CME axis, at distances of ≈4–6R _⊙ fromthe center of the Sun, the condition for the formation of shock is V/2 > V _A .     

The role of tidal,wave and river currents in the evolution of mixed‐energy deltas: Example from the Lajas Formation (Argentina)

Many modern deltas show complex morphologies and architectures related to the interplay of river, wave and tidal currents. However, methods for extracting the signature of the individual processes from the stratigraphic architecture are poorly developed. Through an analysis of facies, palaeocurrents and stratigraphic stacking patterns in the Jurassic Lajas Formation, this paper: (i) separates the signals of wave, tide and river currents; (ii) illustrates the result of strong tidal reworking in the distal reaches of deltaic systems; and (iii) discusses the implications of this reworking for the evolution of mixed‐energy systems and their reservoir heterogeneities. The Lajas Formation, a sand‐rich, shallow‐marine, mixed‐energy deltaic system in the Neuquén Basin of Argentina, previously defined as a tide‐dominated system, presents an exceptional example of process variability at different scales. Tidal signals are predominantly located in the delta front, the subaqueous platform and the distributary channel deposits. Tidal currents vigorously reworked the delta front during transgressions, producing intensely cross‐stratified, sheet‐like, sandstone units. In the subaqueous platform, described for the first time in an ancient outcrop example, the tidal reworking was confined within subtidal channels. The intensive tidal reworking in the distal reaches of the regressive delta front could not have been predicted from knowledge of the coeval proximal reaches of the regressive delta front. The wave signals occur mainly in the shelf or shoreface deposits. The fluvial signals increase in abundance proximally but are always mixed with the other processes. The Lajas system is an unusual clean‐water (i.e. very little mud is present in the system), sand‐rich deltaic system, very different from the majority of mud‐rich, modern tide‐influenced examples. The sand‐rich character is a combination of source proximity, syndepositional tectonic activity and strong tidal‐current reworking, which produced amalgamated sandstone bodies in the delta‐front area, and a final stratigraphic record very different from the simple coarsening‐upward trends of river‐dominated and wave‐dominated delta fronts.     

Contact metamorphism and metasomatism at a dolerite-limestone contact in the Gebel Yelleq area, Northern Sinai, Egypt

Summary At the northeastern flank of Gebel Yelleq, northern Sinai, pure limestones of Upper Cretaceous age were subjected to a thermal overprint, caused by a c. 80m thick Tertiary olivine dolerite sill. Metasomatic supply of Si, Al, Fe, Mg and Ti was greater to the c. 7m wide upper than to the c. 25m wide lower thermal aureole. The greater width of the lower aureole is possibly due to a longer duration of the thermal overprint at this contact. Mineral assemblages in both aureoles are (from the contact outward):(i) clinopyroxene + garnet ± wollastonite + calcite(ii) garnet ± wollastonite + calcite;(iii) wollastonite + calcite.In places, late stage xenoblasts of apophyllite and witherite overgrow these assemblages. Garnets are grandites to melanites with Grs_56–86Adr_14–42Sch_0–2Sps_0–0.2Prp₀ in the lower, and Grs_29–94Adr_5–64Sch_0–12Sps_0–0.2Prp_0–1.7 in the upper aureole. Close to the upper contact, clinopyroxene is virtually pure diopside with X _Mg = Mg/(Mg + Fe²⁺) = 0.97–1.0, whereas clinopyroxenes farther away from the upper contact and in the lower aureole have X _Mg-values of 0.49 and 0.53, respectively.The minimum temperatures reached during contact metamorphism in the upper and lower aureole are defined by the lower stability limit of wollastonite. The temperatures are inferred with a calculated T-X(CO₂) projection in the system CMASCH and are estimated at c. 290 °C and 380 °C for X(CO₂) values of 0.05 and 0.25, respectively. A pressure of roughly 100 bar is estimated for the lower dolerite-limestone contact. As indicated by one-dimensional thermal modelling, a maximum temperature of 695 °C was attained at this contact, assuming a magma temperature of 1150 °C. Further modelling results indicate (i) wollastonite, which occurs first 13 m away from the lower contact, formed at a maximum temperature of c. 575 °C, (ii) there, wollastonite formation lasted for approximately 170 years and, (iii) at the outer rim of the lower aureole, the maximum temperature reached was 480 °C, and temperatures sufficient for wollastonite formation lasted for about 140 years.     

Effect of filtering on drift and anisotropy parameters determined by full correlation analysis

The effect of low-pass filtering on the various drift and anisotropy parameters obtained by full correlation analysis of spaced receiver records is studied theoretically, assuming Gaussian correlation functions. A decrease in the characteristic velocityV _cis shown with increasing filtering, unlessV ≫ V _v, whenV _cincreases with filtering. The ratioV _c/V however shows an increase as the decrease in the true drift velocityV is steeper thanV _c. An increase in the size of the ground pattern with increasing filtering is also shown, while the axial ratio for anisotropic patterns shows a decrease. The results are verified with experimental data recorded earlier at Tiruchirapalli.     

Crustal thickness and VP/VS variations in the Grenville orogen (Ontario, Canada) from analysis of teleseismic receiver functions

We have developed a simple semblance-weighted stacking technique to estimate crustal thickness and average V_P/V_S ratio using teleseismic receiver functions. We have applied our method to data from 32 broadband seismograph stations that cover a 700 × 400 km² region of the Grenville orogen, a 1.2–0.98 Ga Himalayan-scale collisional belt in eastern North America. Our seismograph network partly overlaps with Lithoprobe and other crustal refraction surveys. In 8 out of 9 cases where a crustal-refraction profile passes within 30 km of a seismograph station, the two independent crustal thickness estimates agree to within 7%. Our regional crustal-thickness model, constructed using both teleseismic and refraction observations, ranges between 34.0 and 52.4 km. Crustal-thickness trends show a strong correlation with geological belts, but do not correlate with surface topography and are far in excess of relief required to maintain local isostatic equilibrium. The thickest crust (52.4 ± 1.7 km) was found at a station located within the 1.1 Ga mid-continent (failed) rift. The Central Gneiss Belt, which contains rocks exhumed from deep levels of the crust, is characterized by V_P/V_S ranging from 1.78 to 1.85. In other parts of the Grenville orogen, V_P/V_S is found to be generally less than 1.80. The thinnest crust (34.5–37.0 km) occurs northeast of the 0.7 Ga Ottawa–Bonnechere graben and correlates with areas of high intraplate seismicity.     

EPR and ENDOR of the PO 4 2− radical in anhydrite,celestite and barite

Electron paramagnetic resonance (EPR) and eletron nuclear double resonance (ENDOR) low temperature measurements of natural anhydrite CaSO₄, celestite SrSO₄ and barite BaSO₄ have revealed the presence of PO ₄ ^2– radical. The principal values of the g tensor and the A tensor [MHz] of hyperfine interaction (HFI) are found to be g _xx =2.0124, g _yy =2.0159, g _zz =2.0098, A _xx =82.1, A _yy =81.4, A _zz =77.2 in CaSO4, g _xx =2.0250, g _yy =2.0070, g _zz =2.0131, A _xx =84.8, A _yy =82.7, A _zz = 90.5 in SrSO₄, g _xx =2.0302, g _yy =2.0079, g _zz =2.0135, A _xx =85.2, A _yy =82.3, A _zz =90.0 in BaSO₄. The principal axes A _xx , A _yy and A _zz are parallel to the crystallographic axes a, b, c in all three matrices. In anhydrite the principal g axes coincide with the A axes. In celestite and barite such coincidence is found to be only along the b axis (a low symmetry effect). The coherence effect of splitting of ENDOR lines by a strong microwave field has been detected. From the analysis of the relative intensities of the ENDOR lines of the PO ₄ ^2– radical in CaSO₄ the relation of probabilities of cross-relaxation processes W _x/W _xx =3.5 are evaluated.