Facies analysis of the Trypali carbonate unit (Upper Triassic) in central-western Crete (Greece): an evaporite formation transformed into solution-collapse breccias

The Trypali carbonate unit (Upper Triassic), which crops out mainly in central‐western Crete, occurs between the parautochthonous series (Plattenkalk or Talea Ori‐Ida series, e.g. metamorphic Ionian series) and the Tripolis nappe (comprising the Tripolis carbonate series and including a basal Phyllite–Quartzite unit). It consists of interbedded dolomitic layers, represented principally by algally laminated peloidal mudstones, foraminiferal, peloidal and ooidal grainstones, as well as by fine‐grained detrital carbonate layers, in which coarse baroque dolomite crystals and dolomite nodules are dispersed. Baroque dolomite is present as pseudomorphs after evaporite crystals (nodules and rosettes), which grew penecontemporaneously by displacement and/or replacement of the host sediments (sabkha diagenesis). However, portions of the evaporites show evidence of resedimentation. Pre‐existing evaporites predominantly consisted of skeletal halite crystals that formed from fragmentation of pyramidal‐shaped hoppers, as well as of anhydrite nodules and rosettes (salt crusts). All microfacies are characteristic of peritidal depositional environments, such as sabkhas, tidal flats, shallow hypersaline lagoons, tidal bars and/or tidal channels. Along most horizons, the Trypali unit is strongly brecciated. These breccias are of solution‐collapse origin, forming after the removal of evaporite beds. Evaporite‐related diagenetic fabrics show that there was extensive dissolution and replacement of pre‐existing evaporites, which resulted in solution‐collapse of the carbonate beds. Evaporite replacement fabrics, including calcitized and silicified evaporite crystals, are present in cements in the carbonate breccias. Brecciation was a multistage process; it started in the Triassic, but was most active in the Tertiary, in association with uplift and ground‐water flow (telogenetic alteration). During late diagenesis, in zones of intense evaporite leaching and brecciation, solution‐collapse breccias were transformed to rauhwackes. The Trypali carbonate breccias (Trypali unit) are lithologically and texturally similar to the Triassic solution‐collapse breccias of the Ionian zone (continental Greece). The evaporites probably represent a major diapiric injection along the base of the parautochthonous series (metamorphic Ionian series) and also along the overthrust surface separating the parautochthonous series from the Tripolis nappe (Phyllite–Quartzite and Tripolis series). The injected evaporites were subsequently transformed into solution‐collapse breccias.     

Topography as a major factor in the development of arcuate thrust belts: insights from sandbox experiments

We have used sandbox experiments to investigate and to illustrate the effects of topography upon the development of arcuate thrust belts. In experiments where a sand pack shortened and thickened in front of an advancing rectilinear piston, the geometry of the developing thrust wedge was highly sensitive to variations in surface topography. In the absence of erosion and sedimentation, the surface slope tended to become uniform, as predicted by the theory of critical taper. Under these conditions, the wedge propagated by sequential accretion of new thrust slices. In contrast, where erosion or sedimentation caused the topographic profile to become irregular, thrusts developed out of sequence. For example, erosion throughout a hinterland caused underlying thrusts to remain active and inhibited the development of new thrusts in the foreland. Where initial topography was irregular in plan view, accreting thrusts tended to be arcuate. They were convex towards the foreland, around an initially high area; concave towards the foreland, around an initially low area. Initial plateaux tended to behave rigidly, while arcuate thrust slices accreted to them. Thrust motions were radial with respect to each plateau. Within transfer zones to each side, fault blocks rotated about vertical axes and thrust motions were oblique-slip. At late stages of deformation, the surface slope of the thrust wedge tended towards a uniform value. Initial mountains of conical shape (representing volcanoes) also escaped deformation, except at depth, where they detached. Arcuate thrust slices accreted to front and back. Where a developing thrust wedge was subject to local incision, accreting thrust slices dipped towards surrounding areas of high topography, forming Vs across valleys.Arcuate structural patterns are to be found around the three highest plateaux on Earth (Tibet, Pamirs and Altiplano) and around the Tromen volcanic ridge in the Neuquén Basin of northern Patagonia. We infer that these areas behaved in quasi-rigid fashion, protected as they were by their high topography.     

Numerical Analysis of Space–Time Finite Element Formulations for Miscible Displacements

A finite element formulation is proposed to approximate a nonlinear system of partial differential equations, composed by an elliptic subsystem for the pressure–velocity and a transport equation (convection–diffusion) for the concentration, which models the incompressible miscible displacement of one fluid by another in a rigid porous media. The pressure is approximated by the classical Galerkin method and the velocity is calculated by a post-processing technique. Then, the concentration is obtained by a Galerkin/least-squares space–time (GLS/ST) finite element method. A numerical analysis is developed for the concentration approximation. Then, stability, convergence and numerical results are presented confirming the a priori error estimates.     

Towards New Swedish Recommendations for Cautious Perimeter Blasting

A Swedish table of blast damage depths has, in one form or other, been in use since the late-1970s. Its history and the underlying theory are described. New experimental and theoretical findings that point out a number of shortcomings in the table and the underlying theory are presented and discussed. A revised version of the blast damage table was recently introduced in conjunction with new but incomplete recommendations for cautious perimeter blasting. The new table leaves the difficult task of taking into account factory like decoupling, water in the borehole, the rock properties, type of initiation, charge length and the actual bit diameter to the user. This paper discusses different ways of doing this, based on the experimental findings and a recent formula for the prediction of the lengths of radial cracks behind the half-casts. The material presented in this paper is meant to supplement and extend the new Swedish recommendations for cautious perimeter blasting of tunnels, shafts, pits and road cuts.     

Fluid-bearing alkaline carbonate melts as the medium for the formation of diamonds in the Earth''s mantle: an experimental study

We have experimentally studied the formation of diamonds in alkaline carbonate–carbon and carbonate–fluid–carbon systems at 5.7–7.0 GPa and 1150–1700 °C, using a split-sphere multi-anvil apparatus (BARS). The starting carbonate and fluid-generating materials were placed into Pt and Au ampoules. The main specific feature of the studied systems is a long period of induction, which precedes the nucleation and growth of diamonds. The period of induction considerably increases with decreasing P and T, but decreases when adding a C–O–H fluid to the system. In the range of P and T corresponding to the formation of diamonds in nature, this period lasts for tens of hours. The reactivity of the studied systems with respect to the diamond nucleation and growth decreases in this sequence: Na₂CO₃–H₂C₂O₄·2H₂O–C>K₂CO₃–H₂C₂O₄·2H₂O–C>>Na₂CO₃–C>K₂CO₃–C. The diamond morphology is independent of P and T, and is mainly governed by the composition of the crystallization medium. The stable growth form is a cubo-octahedron in the Na₂CO₃ melt, and an octahedron in the K₂CO₃ melt. Regardless of the composition of the carbonate melt, only octahedral diamond crystals formed in the presence of the C–O–H fluid. The growth rates of diamond varied in the range from 1.7 μm/h at 1420 °C to 0.1–0.01 μm/h at 1150 °C, and were used to estimate, for the first time, the possible duration of the crystallization of natural diamonds. From the analysis of the experimental results and the petrological evidence for the formation of diamonds in nature, we suggest that fluid-bearing alkaline carbonate melts are, most likely, the medium for the nucleation and growth of diamonds in the Earth's upper mantle.     

Hydraulic Characteristics of Rough Fractures in Linear Flow under Normal and Shear Load

Summary A hydro-mechanical testing system, which is capable of measuring both the flow rates and the normal and shear displacement of a rock fracture, was built to investigate the hydraulic behaviour of rough tension fractures. Laboratory hydraulic tests in linear flow were conducted on rough rock fractures, artificially created using a splitter under various normal and shear loading. Prior to the tests, aperture distributions were determined by measuring the topography of upper and lower fracture surfaces using a laser profilometer. Experimental variograms of the initial aperture distributions were classified into four groups of geostatistical model, though the overall experimental variograms could be well fitted to the exponential model. The permeability of the rough rock fractures decayed exponentially with respect to the normal stress increase up to 5 MPa. Hydraulic behaviours during monotonic shear loading were significantly affected by the dilation occurring until the shear stress reached the peak strength. With the further dilation, the permeability of the rough fracture specimens increased more. However, beyond shear displacement of about 7 to 8 mm, permeability gradually reached a maximum threshold value. The combined effects of both asperity degradation and gouge production, which prohibited the subsequent enlargement of mean fracture aperture, mainly caused this phenomenon. Permeability changes during cyclic shear loading showed somewhat irregular variations, especially after the first shear loading cycle, due to the complex interaction from asperity degradations and production of gouge materials. The relation between hydraulic and mechanical apertures was analyzed to investigate the valid range of mechanical apertures to be applied to the cubic law. Received June 12, 2001; accepted February 26, 2002 Published online September 2, 2002     

Equilibrium and Disequilibrium Trace Element Partitioning in Hydrous Eclogites (Trescolmen, Central Alps)

Despite the widespread presence of hydrous phases in subduction-related systems, experimental D^Min/Fluid trace element valuesfor many hydrous phases are lacking. To fill this gap, we presenta set of D^{Min/Clinopyroxene} values (where Min indicates amphibole,zoisite, phengite, paragonite or apatite) derived from equilibriumparageneses of eclogites from Trescolmen (Central Alps, Switzerland).These data can be combined with experimental data for D^{Clinopyroxene/Fluid},to estimate D^Min/Fluid values for the hydrous phases, thus circumventingexperimental problems with the direct determination of suchvalues. We analysed Li, Be, B, Sr, Y, Zr, Nb, Ba, Ce, Nd, Sm,Pb, Th and U in coexisting phases by laser ablation microprobeinductively coupled plasma mass spectrometry. Many of the valuesare extremely low; for example, Nb, Ba, Ce, Th and U are inthe lower ppb range in clinopyroxene. Attainment of equilibriumwas evaluated by textural, and major and trace element characteristics.Non-equilibrated assemblages are common in most eclogite localities,including Trescolmen, and using such samples would lead to thederivation of erroneous values for equilibrium partitioning.However, four of the 10 studied eclogites from Trescolmen havinghomogeneous clinopyroxene compositions and preferred orientationof high-pressure phases yielded consistent D^{Min/Clinopyroxene}values in all four samples (where Min indicates amphibole, phengite,paragonite, apatite), and hence were studied in detail. Thelow abundances in some phases result from strong preferentialincorporation of trace elements into other minor phases. Fromthe investigated hydrous phases (amphibole, zoisite, clinozoisite,phengite, paragonite, apatite and talc), zoisite was found tobe the most important carrier of Sr, light rare earth elements,Pb, Th and U, whereas phengite hosts Ba and is, along with clinopyroxeneand paragonite, an important phase for B. However, because oftheir low modal abundance in eclogite-facies rocks, phengiteand paragonite do not control the B whole-rock budget. We inferthat estimated D^{Min/Clinopyroxene} values from equilibrium assemblagescan be used as a good approximation for partition coefficientsunder the given PT conditions (