Boudinage Structures Research Articles

Boudinage in Glacier Ice — Some Examples

Boudinage structures have only rarely been reported in glacier ice, yet they seem to be widespread in Swiss glaciers. They form in debris-free, strongly foliated ice by the stretching, necking and rupture of layers or groups of layers, when the principal compressive strain axis lies at a high angle to the layering. Two main types of boudinage are distinguished. The first results from the difference in competence between fine-grained and coarse-grained ice, and indicates that the former is more resistant to flow than the latter. The second occurs in more equigranular ice which shows a strong planar anisotropy; associated with the necking of such ice is the development of shear planes, along which the layers are displaced. As in deformed rocks, it is not possible to determine the directions of the finite principal strain axes from the boudinage structures alone. Although the boudins described here all occur in longitudinal foliation, it is suggested that they are likely to form in other situations also.

Variation in Degree of Hornblende Grain Alignment within two Boudinage Structures

The degree of preferred orientation of hornblende in two boudinage structures is analyzed by means of an X-ray texture goniometer. Relative values of reflected intensity for {110} are plotted on the stereographic net, and contoured at convenient intervals. As in conventional fabric diagrams for hornblende, {110} define a great-circle girdle whose breadth reveals the density of unimodal grouping of [001].The density of grouping of [001] varies throughout both boudinage structures. It decreases with increasing extension in structure A, and seems to have a similar trend in structure B, where the determined variation in degree of grain alignment is of doubtful statistical significance.The inverse relationship between the degree of preferred orientation and the magnitude of extension may be attributed to rotation of the principal directions of finite strain during syntectonic crystallization. Alternatively, reorientation of strained grains by annealing recrystallization may have produced the inverse trend.

Hornblende lineations in Trout Lake area, Lac la Ronge map sheet, Saskatchewan

Most rocks exposed in the Trout Lake Area are migmatitic gneisses severely deformed during the Hudsonian orogeny (± 1700 m.y. ago). Hornblende-bearing layers or lenses, mostly well lineated, are found throughout the entire area. The attitude(s) of the hornblende lineation (aligned prismatic grains) can be measured in every outcrop.Within cylindrical buckling folds and boudinage structures, involving hornblende-bearing rocks, the orientation of hornblende lineations is related to local directions of megascopic finite strain parallel to layering, for the main period of tectonic flow. Because the local strain ellipsoid is commonly oblique to layering, the strain ellipse parallel to layering is generally a non-principal section. The direction of its longest diameter is called M, regardless of whether it is a direction of extension, shortening or no longitudinal strain.Throughout all medium-scale folds and boudinage structures investigated, hornblende lineations are parallel to M. This relationship is considered as general, and hornblende lineations are employed as indicators of M within large-scale folds. (Other structural elements are used in distinguishing between finite extension and shortening parallel to M.) Finally, the strain pattern of the Trout Lake Area (west half) is analyzed by means of a contour map for the lineation plunge (non-statistical "plunge isogonic map" after D. Elliot 1965).The areal pattern of M contains a diagonal belt across which the plunge values tend to change "discontinuously." Because this narrow belt coincides roughly with a major branch of the Birch Rapids – Wepusko Bay fault zone, the angular "discontinuity" in the lineation map can be attributed to "late" rotational faulting.

CERTAIN CHARACTERISTICS OF THE MECHANISM OF THE DEVELOPMENT OF BOUDINAGE STRUCTURES IN QUARTZ-DIOPSIDE ROCKS IN THE SOUTHWEST OF THE LAKE BAYKAL' TERRITORY

Microscopic studies and microscopic analysis of boudinage structures in quartz-diopside rocks interbedded with marbles, quartzite, and others, with particular attention to marble boudines in quartzite, bring out the intricacies of the mechanisms involved in the process initiated by oriented pressures at right angles to the bedding of the rock. In the given case study, the evolution of lensshaped marble boudines in the matrix of quartzite, into blocky ones, some of which give origin again to lenes, is explainable by a rigid deformation of marble concurrent with a plastic deformation of quartzite, accompanied by a re-working of marble by quartzite, against the background of a complex network of re-distributed tensions in the boudinized rock horizons. V.P. Sokoloff

The Ore Knob massive sulfide copper deposit, North Carolina; an example of recrystallized ore

Massive and disseminated pyrrhotite-chalcopyrite-pyrite ore replaced a fault zone in Precambrian Carolina Gneiss of Keith. Closely following sulfide deposition, vein sulfides and silicates in the wall rock were recrystallized under a rising temperature gradient to coarse-grained unoriented aggregates that contain late pyrite porphyroblasts in pyrrhotite. Boudinage, dilation, and flow structures are common.

Sur la serie metamorphique de Velez-Malaga (province de Malaga, Espagne)

Abstract Four major petrographic facies are distinguished along the Malaga-Motril road between Torre de Mar and Nerja (Spain)--sericite schist, mica schist, gneiss, and lenticular gneiss facies. The mechanical forces which caused the fold and boudinage structures affecting the feldspathic mica schists and the mica schists with andalusite occurred contemporaneously with, if not before, the albitization of the Velez series. The sericite schists have been correlated with certain Triassic series, leading to the proposal of a post-Triassic age for the metamorphism of the Velez series. Two phases of crystallization are recognized in the petrogenesis of the Velez-Malaga crystalline schists: transformation of an older sedimentary series by intense metamorphism, and albitization of the resultant metamorphic series related to major sodic metasomatism.

Natural and Experimental Boudinage and Pinch-and-Swell Structures

Examples of boudinage and pinch-and-swell structures are described and the mechanisms of formation discussed. One-dimensional plastic compression and compensating one- or two-dimensional expansion of layered cakes of putty, plasticene, and cheese result in structures strikingly similar to the natural boudinage and pinch-and-swell structure. Studies of boudinage and related structures give information about relative plasticity and relative strength of rocks. The commonness of boudinage structures in gneiss-crystalline schist regions shows that one-dimensional compression and compensating elongation in other dimensions is a very common mode of deforming crystalline rocks. The tension which this kind of plastic flowage creates in competent rock layers not only produces boudinage, or pinch-and-swell structure, but is also important in localizing the growth of minerals during metasomatism. Mathematical analysis of the boudinage evolution shows that field measurements combined with data for tensile strength of ...

PETROGRAPHY, STRUCTURES, AND PETROFABRICS OF THE PINCKNEYVILLE QUARTZ DIORITE, ALABAMA

The Pinckneyville quartz diorite complex underlies an area in eastern Alabama extending from the Coosa River in northwest Elmore County northeast through Coosa and Tallapoosa counties into Clay County. Dark-gray, coarse-grained quartz diorite gneiss constitutes the major part of the complex, but there are smaller amounts of granodiorite and granite gneiss. Hornblende-biotite gneisses also occur and are considered to represent metamorphosed basic sills and dikes intruded before the quartz diorite complex. Schist septa lie within the complex and are usually parallel to the regional strike. The sequence of intrusion as interpreted from inclusions and petrographic data is (1) quartz diorite, (2) granodiorite, (3) granite. Small but persistent amounts of allanite and epidote are characteristic of the rocks of the complex. The texture is subporphyritic and shows evidence of protoclastic deformation. A northeast-southwest trend of inclusions, foliation, lineation, and contacts dominates. Primary flow structures are shown in the orientation of inclusions and micas. Secondary structures were superposed on the original flow structures shortly after intrusion and prior to complete consolidation. A prominent secondary structure is a lineation in b. Stretching in b is indicated by boudinage structures, the lineation, and small displacements of dikes. The intensity of the planar structures in the gneiss grades from strong to indistinct or lacking from east to west. Structures in schist inclusions indicate that some folding had taken place prior to intrusion. Thin pegmatite, granodiorite, and granite dikes are common. Most of them are essentially parallel to the regional strike. The petrographic boundary between gneiss and schist is always sharp. On a large scale, the gneiss structures are essentially parallel to the contacts, but slight to marked discordant relations occur at many localities. The schist structures usually parallel the contacts. Petrofabric analysis confirms the structural field observations. The mica fabrics are ac -girdles and are most complete where the foliation is weakest. Microscopic s -planes do not always coincide with the observed foliation. The quartz fabrics are also ac -girdles but are not so well developed as the mica girdles. Quartz orientations cannot always be related to known s -planes. It is concluded that the Pinckneyville quartz diorite complex represents a syntectonic intrusion—that is, that intrusion took place while external or regional forces were still active. The shape of the intrusion was controlled largely by the wall-rock structures which had developed earlier. The rocks show both igneous and metamorphic characteristics because the intrusion took place under regional metamorphic conditions.