Narrow Wedges Research Articles

Structural variability at the active continental marginoff southernmost Chile

Newly collected seismic reflection data offshore Southernmost Chile, between 54°Sand 57°S, reveal the complex tectonic setting of this active continental margin. This region is stillpoorly known, because frequent bad weather conditions among these latitudes have prevented theacquisition of geological and geophysical data.Three main tectonic domains are clearly imaged south of the strait of Magellan: 1.(1) The oceanic area of the Antarctic plate, where a 2 km thick and largely undeformedsedimentary section, rests on the oceanic basalts;2.(2) The subduction complex, formed by a relatively narrow accretionary wedge ofhighly deformed sediments and a thick forearc basin;3.(3) The seaward dipping continental backstop formed partly by the PatagonianBatholith and partly by the Paleo-Mesozoic subduction complex.The accretionary complex is related to the subduction of the Antarctic plate beneath Scotiaplate which resumed after the collision between the Chile Trench and the Chile Ridge (10 14 Ma).The structural style of the subduction complex, such as structural vergence, width of theaccretionary wedge, taper angle and deformation in the forearc basin, varies along the margin.Large taper values are related to narrow wedges and seaward vergent structures. Low tapersoccur where deformation at the toe of the accretionary complex is spread over wide areas and isrelated both to landward and seaward vergent thrust faults. The parameters which control thesestructural variations are: 1.• thickness of the offscraped sedimentary section (variations in the depth of the decollement level);2.• presence of overpressured fluids (the BSR is extraordinarily continuous where thelandward vergence structures are well developed);3.• configuration of the continental margin after the consumption of the Chilean midoceanic ridge (14 Ma) and its related phase of tectonic erosion.4.presence of strike slip faults belonging to the South America-Scotia plate boundarysystem which could bound crustal blocks with different mechanical behaviour (i.e. Strait ofMagellan-Lago Fagnano andBeagle channel faults)

Sequence Stratigraphic Setting of the Priob Field Within the Neocomian Prograding Complex of the West Siberian Basin, Russia: ABSTRACT

The Neocomian strata of the West Siberian Basin are a prograding complex unique in its aerial extent, structure and hydrocarbon reserves, such as Priob Field in the Ob River area. Interpretation of the basin history is based on well data and seismic reflection profile. As many as 45 transgressive-regressive depositional sequences, called cyclites, prograde into the deep-water basin formed during Late Jurassic and Early Cretaceous. Each transgressive-regressive sequence is defined by a coarsening-upward cycle of shale, siltstone and sandstone, and is considered a chronostratigraphic subdivision of the prograding complex. Each sequence boundary is regionally correlatable on seismic reflection profiles, and is identified on well logs by sharp contacts between regressive sandstones below and thin transgressive shales above. Subordinate progradational wedges are locally developed within these sequences and contain major hydrocarbon reserves. These depositional wedges map as lens-shaped packages downlapping the outermost shelf (Priob zone) or as narrow progradational wedges downlapping the foreset reflections of the slope clinoforms immediately beyond the shelf break (Sugmut zone). Regressive facies of the shelf consist of thin but wide spread sandstones that also contain major hydrocarbon reserves. Pervasive sediment starvation during the Late Neocomian resulted in deposition of thin regionally extensive shales that provide top-sealmore » to the Neocomian hydrocarbon system. At the Priob field, a deep erosional incision has been mapped at the AS11 shelf-edge. Sands transported through this incised valley were deposited as a prograding wedge along the shoreline, forming the reservoir facies for the Priob hydrocarbon accumulation. Stratigraphic aspects of the Priob trap include top and lateral shale seals and subtle regional structural tilt. Types of stratigraphic traps are discussed and the possibility of predicting additional such traps are analyzed.« less

An analysis of crack pattern in clay soil: its density and orientation

SUMMARYThe structure of much clay soil is angular blocky, and the pore space consists of a three‐dimensional network of narrow, approximately planar cracks. This paper presents a means of describing such a network in terms of its density and orientation.Undisturbed samples are impregnated with resin containing a fluorescent dye. Plane sections of known azimuth and elevation are cut through them and photographed under ultra‐violet light. The photographs are projected onto a screen and the spacings between successive cracks along linear probes are measured in several directions. The sets of distances obtained are then analysed.The method was applied to subsoil of the Windsor series. The distribution of inter‐crack distances appeared to be exponential but with a dearth of distances less than about 1 mm. The crack pattern was judged to be the outcome of a Poisson process, but one in which some of the very narrow wedges of soil had collapsed. The crack network was perceptibly anisotropic, and was modelled as two superimposed patterns, one isotropic and the other a set of approximately parallel cracks. The orientation of the parallel component was estimated from the sections. The cracks were found to be aligned parallel to the contour of the land and to dip into the hill slope at angles ranging from about 25° to 75° to the horizontal. The intensities in the soil of the two components were estimated as 0.18 mm−1 for the isotropic component and 0.2 mm−1 normal to the parallel component, equivalent to 0.1 mm−1 for a random direction.

Compositions of Glasses from St. Paul's Peridotite Partially Melted at 20 Kilobars

St. Paul's peridotite with 5.7% H₂O (considered to be derived from the upper mantle) was partially-melted at 20 kb in and Pt capsules with run durations up to 17 hours. Run products were analyzed for Fe, Mg, Ca, and Al using an electron microprobe. FeO loss from sample to capsule wall increased with temperature and run duration and was as much as 90% in Pt capsules at 1,250°C and 4 hours, and 73% at 1,150°C and 17 hours in capsules. Iron loss was lower in larger Pt capsules with a higher sample/platinum ratio. The amount of liquid produced in runs increased as a function of run duration, confirming the nonequilibrium condition. It was difficult to measure the composition of glass in narrow seams and wedges between mineral boundaries, and the problem was compounded by the presence of numerous minute olivine grains from the mylonite starting material and of quench amphibole in the glass. Glass compositions were estimated from analyses of glassy areas and quench amphiboles, and the compositional trend between 1,100°C and 1,250°C was plotted on variation diagrams. Expressed in terms of Al₂O₃ and CaO only, the glass analyses are similar to andesite and tholeiite. Our estimated glass compositions are compared with published data for measured glasses from partially melted peridotites of similar composition. In terms of the four elements that we analyzed the compositions of glasses vary considerably from one set of experimental conditions to another. We conclude that this is due in part to the experimental problems outlined above. The best results that can be achieved represent a compromise between runs long enough to ensure reaction and short enough to minimize the changing composition of the reacting system through iron loss.

The adaptive significance of the flexible shell of the tortoise Malacochersus tornieri

Early reports suggest that the soft, flexibly-shelled tortoise Malacochersus tornieri escapes from predators into narrow rock crevices and wedges itself by inflating its body with air and using its legs as struts. Direct records of intrapulmonary pressure, heart rate, and fore- and hindlimb activity while animals were pulled from an artificial crevice (150 trials) showed no significant or sustained increase in intrapulmonary pressure associated with the wedging action. Disturbed tortoises dug in the foreclaws and rotated the forelimbs outward, wedging the body into place. The soft shell presumably evolved in response to the advantages for entering and fitting into narrow rock crevices and for improved locomotor energetics.