Response of a marine ice sheet to changes at the grounding line

A numerical model was designed to study the stability of a marine ice sheet, and used to do some basic experiments. The ice-shelf/ice-sheet interaction enters through the flow law in which the longitudinal stress is also taken into account. Instead of applying the model to some (measured) profile and showing that this is unstable (as is common practice in other studies), an attempt is made to simulate a whole cycle of growth and retreat of a marine ice sheet, although none of the model sheets is particularly sensitive to changes in environmental conditions. The question as to what might happen to the West Antarctic Ice Sheet in the near future when a climatic warming can be expecied as a result of the CO₂ effect, seems to be open for discussion again. From the results presented in this paper one can infer that a collapse, caused by increased melting on the ice shelves, is not very likely.     

Paleomagnetism of the Cambro-Ordovician McClure Mountain alkalic complex,Colorado

The Middle to Late Cambrian loop in the North American apparent polar wander path (APWP) has been variously attributed to tectonic rotations, remagnetizations and primary magnetizations. Although no primary thermal remanent magnetizations or primary detrital remanent magnetizations have as yet been demonstrated, the temporally self-consistent nature of the loop has been used as an argument for primary magnetizations. We have studied535 ± 5Ma nepheline syenites and syenites of the McClure Mountain alkalic complex, as well as495 ± 10Ma red trachyte dikes which intruded the complex, in an effort to find a primary TRM. Because Zijderveld analysis yielded consistent results for only one trachyte dike, remagnetization great-circle analysis was employed, giving a pole for the trachyte dikes at the tip of the loop (43°N, 114°E), while the syenites and nepheline synenites gave a pole at the base of the loop (18°N, 142°E). The magnetic carrier in the trachytes is hematite which apparently formed during a pervasive hydrothermal alteration. KAr whole rock dating of the trachytes suggests a Pennsylvanian age for the alteration, and thus a late Paleozoic remagnetization of the trachytes. Thus, the low-latitude Cambrian pole is confirmed, but we find no evidence in this study to support the primary nature of the Cambrian APWP.     

Mica beards in three slates: Morphology,formation and bearing on the strain history

Beards are intergrowths of mica, chlorite and quartz or feldspar on detrital grains in weakly metamorphosed sedimentary rocks. The three samples studied contain beards of mica and quartz (or feldspar). Their internal morphology shows a variation in mica orientation related to the external foliation (matrix). Formation occurred during foliation development with a syntaxial growth history. In three dimensions, the beards have oblate shapes, so that they are present in all planes cut parallel to the shortening direction. This indicates flattening strain conditions around the detrital grains during the beard formation.Bulk inhomogeneous shortening, resulting in an anastomosing foliation, can explain the local extension within the plane containing the fold axis and the shortening direction. Concurrently, an approximately constant lenght in the direction of the fold axis is maintained on a regional scale (plane strain). Volume loss probably plays an important role during the formation history.

---

Zusammenfassung Glimmerbärte in schwach metamorphen Sedimenten sind syntektonische Verwachsungen von Glimmer, Chlorit, Quarz oder Feldspat auf detritischen Körnern. Die drei hier untersuchten Proben enthalten Bärte aus Hellglimmer und Quarz (oder Feldspat). Die Glimmerorientierung zeigt Variationen in Relation zum umgebenden Schieferungsgefüge (Matrix). Die Bildung der Bärte fand mit syntaxialem Wachstum während der Schieferung statt. Dreidimensional haben die Bärte eine abgeplattete Gestalt, so daß sie in allen Schnittlagen parallel zur Verkürzungsrichtung zu beobachten sind. Dies weist auf plättende Verformung um detritische Körner während der Bildung der Bärte hin.Bulk inhomogeneous shortening führt zu einem linsigen Schieferungsgefüge und kann bei gleichzeitiger Annahme einer nicht volumenkonstanten Deformation als Erklärung für die im Korngefüge beobachtete Dehnung parallel zur Faltenachse herangezogen werden. Im regionalen Maßstab wird aber eine konstante Länge in Richtung der Faltenachse und damit ebene Deformation angenommen.

---

Résumé Les barbes de mica sont des intercroissances de quartz (ou feldspath) et de mica sur des grains détritiques dans les roches sédimentaires peu métamorphisées. Elles ont été étudiées dans trois échantillons. Leur morphologie interne montre une variation dans l'orientation du mica en relation avec la schistosité externe (matrice). Leur formation s'est produite pendant le développement de la schistosité, avec une histoire de croissance «syntaxiale». Les barbes ont une forme oblongue en trois dimensions, de sorte qu'elles sont visibles dans toutes les sections parallèles à la direction de raccourcissement. Cela indique, pendant la formation des barbes, l'existence d'un aplatissement autour des grains détritiques.Le raccourcissement non homogène, qui entraîne une foliation anastomosée peut être invoqué, conjointement à une déformation avec perte de volume, pour expliquer l'allongement que présente localement la microstructure parallèlement à l'axe des plis. A l'échelle régionale, toutefois, on admet la constance de la longuer selon la direction des axes et, partant, le caractère plan de la déformation.

---

, (Glimmerbärte). Glimmerbärte ( ). — . Glimmerbärte . Glimmerbärte , , . Bulk inhomogeneous shortening , , , . .

     

Trace element zoning in pyroxenes from ODP Hole 735B gabbros: diffusive exchange or synkinematic crystal fractionation?

Major and trace element profiles of clinopyroxene grains in oceanic gabbros from ODP Hole 735B have been investigated by a combined in situ analytical study with ion probe, and electron microprobe. In contrast to the homogeneous major element compositions, trace elements (REE, Y, Cr, Sr, and Zr) show continuous core to rim zoning profiles. The observed trace element systematics in clinopyroxene cannot be explained by a simple diffusive exchange between melts and gabbros along grain boundaries. A simultaneous modification of the melt composition is required to generate the zoning, although Rayleigh fractional crystallization modelling could mimic the general shape of the profiles. Simultaneous metasomatism between the cumulate crystal and the porous melt during crystal accumulation is the most likely process to explain the zoning. Deformation during solidification of the crystal mush could have caused squeezing out of the incompatible element enriched residual melts (interstitial liquid). Migration of the melt along grain boundaries might carry these melt out of the system. This process named as synkinematic differentiation or differentiation by deformation (Natland and Dick in J Volcanol Geotherm Res 110(3–4):191–233, 2001) may act as an important magma evolution mechanism in the oceanic crust, at least at slow-spreading ridges.     

Stratigraphic filtering and source penetration depth

Seismic exploration underneath highly heterogeneous layers such as basalt flows is possible by lowering the principal source frequency. Unfortunately this also reduces resolution. Wave‐localization theory is a multiple scattering theory that can be used to study stratigraphic filtering in chaotic lithologies. It predicts the apparent attenuation due to scattering of a plane wave traversing a layer with high velocity fluctuations. It can therefore predict the optimum principle source frequency in the trade‐off between loss of resolution and increased penetration depth. We show how this can be done with the help of a few statistical parameters derived from a well‐log analysis; namely, the average background velocity, the expected standard deviation in the velocity fluctuations, the typical scale length of the heterogeneities and the thickness of the basalt layer. In the likely situation that no local well logs exist, a multitude of scenarios can easily be examined at low cost.     

Micromorphological Analysis of Coastal Sediments from Willapa Bay, Washington, USA: A Technique for Analysing Inferred Tsunami Deposits

Tsunami deposits are provisionally distinguished in the field on the basis of anomalous sand horizons, fining-up and fining-landward, coupled with organic-rich, fragmented `backwash' sediments. In this paper, micromorphological features of a sediment sequence previously interpreted as being of tsunami origin are described. These characteristics are shown to be consistent with the macro-scale features used elsewhere, but show additional details not seen in standard stratigraphies, including possible evidence for individual waves, possibly wave-magnitude progression, organic fragment alignment and intraclast microstructures. Although replication and more complete studies are needed, this analysis confirms the identification of a tsunami in Willapa Bay in ca.1700 AD, while demonstrating a widely applicable technique for confirming or refuting possible tsunami deposits.     

Approaches for modeling longitudinal dispersion in pore-networks

The purpose of this study is to quantify the dispersivity in the longitudinal direction by upscaling pore scale mixing over a network domain and to verify the dispersivity with that obtained through the more rigorous upscaling technique, the Brownian particle tracking model (BPTM). We model a porous medium with a network of pore-units that are comprised of pore bodies and bonds of finite volume. Such a pore-unit is assumed to be a mixing cell with the steady state flow condition for a single fluid. Dispersivity can be obtained by solving the mixing cell model (MCM) for the concentration in each pore-unit and by averaging the concentrations for a large number of pore units (as a function of time and space). A minimal size of network that ascertains an asymptotic value of dispersivity was determined and verified with large size pore networks. This numerically computed dispersivity is compared with the results from the BPTM for the same porous medium and flow conditions. We show that the dispersivity obtained from the MCM is equally reliable for the heterogeneous pore-networks and can be estimated as a function of pore size heterogeneity. For homogeneous networks with the MCM, the iteration time step plays an important role. On the other hand, for networks with the BPTM, the assumption of intra-bond velocity profile affects the results.     

The 3D shear experiment over the Natih field in Oman: the effect of fracture-filling fluids on shear propagation

This is the final paper in a series on the 3D multicomponent seismic experiment in Oman. In this experiment a 3D data set was acquired using three-component geophones and with three source orientations. The data set will subsequently be referred to as the Natih 9C3D data set. We present, for the first time, evidence demonstrating that shear waves are sensitive to fluid type in fractured media. Two observations are examined from the Natih 9C3D data where regions of gas are characterized by slow shear-wave velocities. One is that the shear-wave splitting map of the Natih reservoir exhibits much larger splitting values over the gas cap on the reservoir. This increase in splitting results from a decrease in the slow shear-wave velocity which senses both the fractures and the fracture-filling fluid. Using a new effective-medium model, it was possible to generate a splitting map for the reservoir that is corrected for this fluid effect. Secondly, an anomaly was encountered on the shear-wave data directly above the reservoir. The thick Fiqa shale overburden exhibits a low shear-wave velocity anomaly that is accompanied by higher shear reflectivity and lower frequency content. No such effects are evident in the conventional P-wave data. This feature is interpreted as a gas chimney above the reservoir, a conclusion supported by both effective-medium modelling and the geology.
With this new effective-medium model, we show that introduction of gas into vertically fractured rock appears to decrease the velocity of shear waves (S2), polarized perpendicular to the fracture orientation, whilst leaving the vertical compressional-wave velocity largely unaffected. This conclusion has direct implications for seismic methods in exploration, appraisal and development of fractured reservoirs and suggests that here we should be utilizing S-wave data, as well as the conventional P-wave data, as a direct hydrocarbon indicator.     

Contamination of a well in a uniform background flow

In this paper we describe the transport of pollution in groundwater in the neighbourhood of a well in a uniform background flow. We compute the rate at which contaminated particles reach the well as a function of the place of the source of pollution. The motion of a particle in a dispersive flow is seen as a random walk process. The Fokker-Planck equation for the random motion of a particle is transformed using the complex potential for the advective flow field. The resulting equation is solved asymptotically after a stretching transformation. Finally, the analytical solution is compared with results from Monte Carlo simulations with the random walk model. The method can be extended to arbitrary flow fields. Then by a numerical coordinate transformation the analytical results can still be employed.