Paul F. Williams

The partitioning of deformation in flowing rock masses

Abstract Most geological structures owe their development to heterogeneous and/or non-steady flow. Flow partitioning refers to the division of the instantaneous velocity field into components related to translation, strain and rotation. Flow partitioning during the development of a geological structure usually changes with time, and different histories of flow are involved at different points in the developing structure. Heterogeneity leads to the development of flow domains with different characteristics, notably the degree of non-coaxiality of deformation, and different partitionings of the rotational component of flow into shear-induced vorticity and spin. During non-steady flow there is often continual repartitioning of vorticity between the spin and shear-induced components. Spin is an important aspect of flow leading to the development of structure. This paper discusses the partitioning of deformation in a flowing rock mass. Coaxial flow tends to occur in isotropic materials in regions near weakly constrained boundaries. Non-coaxial flow is favoured by suitably oriented material anisotropy and/or the presence of well-defined boundary constraints involving parallel relative displacements. A developing anisotropy (e.g., a crystallographic fabric in a shear zone) can induce a change from coaxial to non-coaxial flow. Other effects are related to the scale of approximation, for example bulk shear-induced vorticity can be locally converted to spin. Flow in the crust and mantle of the earth produces geological structures on all scales and involves many different types of rheological response. The factors that affect flow partitioning (e.g., spin versus non-coaxial deformation) are therefore of interest.

Transpression (or transtension) zones of triclinic symmetry: natural example and theoretical modelling

Abstract We describe a natural shear zone with triclinic symmetry, present a general model for triclinic shear zones based on natural examples, and investigate the kinematics and strain geometry within such zones. In the Roper Lake shear zone in the Canadian Appalachians, the orientation of a stretching lineation is oriented approximately down-dip near the shear zone boundary and becomes gradually shallower towards the centre. The structures in the central portion of the shear zone exhibit approximately monoclinic symmetry where the poles to both the S- and C-surfaces, the stretching lineation on the S-surfaces and the striations on the C-surfaces all plot in a great circle girdle. However, the lineations from the marginal portion do not plot in the same girdle, and the bulk symmetry of the shear zone is triclinic. Theoretical modelling shows that the observed strain geometry can be interpreted by an oblique transpression with a larger ratio of simple shear to pure shear in the centre of the shear zone than in the margin. The latter suggests a higher degree of localization of the zone boundary-parallel movement component relative to the boundary-normal compression component. We emphasize that, as the imposed boundary displacements for most natural shear zones lie between dip-slip and strike-slip, their movement pictures are generally triclinic; monoclinic shear zones are special end members. Structural data that exhibit monoclinic symmetry do not necessarily mean that they resulted from a monoclinic movement picture; the present modelling demonstrates that a triclinic movement picture with a high ratio of boundary-parallel movement to boundary-normal movement can result in apparent monoclinic structural geometry. The results of the modelling also show that the simple statement made for simple shear zones that stretching lineations will align with, and therefore indicate, the shear direction cannot be extrapolated to three-dimensional transpressional (or transtensional) shear zones.

Kinematics of crystal growth in syntectonic fibrous veins

Abstract Detailed observations of a set of fibrous antitaxial calcite veins in a slate reveal that some of the calcite fibres do not connect material markers on both sides of the vein and can therefore not have tracked the full opening trajectory during vein growth. This calls for a better understanding of the mechanisms of fibre formation and reliable criteria to test the tracking hypothesis. Based on surface roughness characteristics of the vein wall we develop a simple model for shape-growth of crystals in a crack-seal environment which can account for both tracking and non-tracking behaviours, and propose a set of ‘tracking criteria’ for antitaxial veins. Finally we discuss ways by which the model can be tested against natural examples.

High-strain zones: a unified model

Abstract Where a shear zone is bounded by undeformed wall rocks and there is no volume change, the deformation path within the zone on the bulk scale approximates a simple shear. Where the zone boundaries are stretched and the shear direction between the two boundaries is parallel to one of the principal stretching directions of the boundaries, the deformation path within the zone is not a simple shear but still has monoclinic symmetry. Both simple shear and general monoclinic deformation paths should be regarded as special cases in nature, because generally not only are the zone boundaries stretched but also the shear is oblique to the principal stretching directions of the boundaries. In this general case, the deformation path is triclinic. We establish a model which includes this general case also taking into consideration volume change. Five independent parameters are required to characterize the rate of deformation of the flow; four are sufficient to characterize the flow kinematics. The model is generally triclinic, and orthorhombic and monoclinic deformations are subgroups. By varying the values of the characterizing parameters, the various types of high-strain zones can be represented. The kinematics of flow within such zones and the accumulated finite deformation geometry are investigated. Many field observations such as, for example, lineations in transcurrent shear zones that vary between vertical and horizontal, cleavage transected folds, and the absence of sheath folds in some shear zones where folds have been significantly rotated, can sometimes be better interpreted in the context of our general model.

Multiply deformed terrains—problems of correlation

Abstract The best tools available for carrying out a structural analysis in a complex area are overprinting, style and patterns of orientation. Unfortunately, all have limitations which are reviewed briefly and discussed. In areas of continuous, unfaulted outcrop the deformational history can be established purely on the basis of overprinting, but even in such areas there can be problems since not all overprinting relationships are unambiguous. Most fold interference patterns are reliable. Overprinting relationships based on foliations, however, are commonly unreliable, and various examples are cited and discussed. Even in areas where the sequence of deformational events can be established, their temporal significance is unclear, since the methods applied only determine local relative timing, and absolute relationships must vary in space and time across an orogen.

Origin of kinkbands and shear-band cleavage in shear zones: an experimental study

Specimens of artificial KCl-mica schist have been deformed in simple shear with different initial orientations (10° intervals between 0° and 170°) of the mica schistosity (S1) relative to the imposed simple-shear plane. For most experiments the angular shear strain was 30°, the strain rate 4.8 × 10−4s−1 and the stress normal to the imposed shear plane 20 MPa. All experiments were conducted at room temperature. Deformation of the specimens is achieved largely by slip parallel to the S1 mica foliation. However, two new types of slip surfaces also develop, as either a single orientation set or as a pair of conjugate sets. These new slip surfaces are kinkbands and a shear-band cleavage. The type and number of sets of new slip surfaces which develop depends on the orientation of S1 relative to the shortening and extension fields of the instantaneous strain ellipsoid. Our experiments indicate that kinkbands or shear-band cleavage form in shear zones in which S1 is inclined to the margin of the zone. Which structure forms and whether it occurs as a single set of structures or as a conjugate pair depends on the initial orientation of S1. In situations where S1 is parallel to the margins of the zone neither structure develops if the deformation is a simple shear. The presence of kinkbands or shear-band cleavage in such zones, where S1, is parallel to the margins, is an indication that the zones were transtensional or transpressional, respectively. We conclude that shear-band cleavage is not only an excellent indicator of shear sense but also a qualitative indicator of the ratio of S1-normal shortening to S1-parallel shear. Conjugate shear bands indicate that S1-normal shortening was relatively strong and single shear-band sets indicate that S1-parallel shear was relatively strong.

Deformation path in high-strain zones, with reference to slip partitioning in transpressional plate-boundary regions

Abstract The current status of the kinematics and strain geometry of high-strain zone studies is briefly summarized. A general high-strain zone has a triclinic deformation path, and monoclinic shear zones are special end member cases. Fabrics observed in natural shear zones and theoretical considerations based on continuum mechanics are compatible with this conclusion. Non-steady deformation paths remain difficult to deal with, and may ultimately rely on a realistic mechanical treatment of high-strain zones which may be possible when our knowledge of the mechanical behavior of rocks under natural deformation conditions is improved. An examination of the phenomenon of slip partitioning in transpressional plate boundary regions shows that the bulk deformation path in the forearc area (trench-parallel high-strain zone) is generally triclinic. The Alpine Fault in the South Island of New Zealand provides an example of a currently active triclinic shear zone. The Southern Knee Lake shear zone of Manitoba, Canada, provides an Archean example of a triclinic shear zone.

Three-dimensional collisional structure of the Trans-Hudson Orogen, Canada

Abstract The three-dimensional structure of the eastern Trans-Hudson Orogen (THO), part of a Paleoproterozoic continent-continent collision zone in central North America, is revealed through a network of LITHOPROBE seismic reflection profiles. The seismic images are interpreted to delineate a series of stacked thrust sheets essentially confined to the crust. E-W profiles show strong, E-dipping reflections extending throughout the crust while N-S profiles record events outlining antiformal and synformal structures. This allows the identification of decollements that may have localized along pre-existing structures (e.g. possible basin-bounding extensional faults) and at major rheological boundaries (e.g. basement-cover contact, upper-middle crust transition). The present topographic surface displays oblique crustal sections with 10–15 km of structural relief, generated during post-collisional, intracontinental transpression of THO as a result of crustal-scale folding and faulting.

Integrated model for Meguma Group lode gold deposits, Nova Scotia, Canada

Meguma Group lode gold deposits represent concentrations of a variety of auriferous quartz veins in lower Paleozoic metaturbidites of the Meguma terrane of southern Nova Scotia. Our studies, incorporating extensive field (regional and detailed mapping) and laboratory (petrography, fluid inclusions, geochemistry, isotopes) studies of 30 of these deposits in the eastern Meguma terrane, suggest that mineralization occurred at ca. 370 Ma ( 40 Ar/ 39 Ar dating of hydrothermal vein minerals) during widespread incursion of mafic and felsic magmatism into the crust concurrent with subvertical shear-zone development, thus postdating regional Acadian deformation by some 30-40 m.y. Isotopic data (S, C, O, Sr) indicate that vein-forming fluids were not solely magmatic and were in part exotic to the Meguma Group. Thus, earlier magmatic and metamorphogenic-lateral-secretion models that considered either late-stage magmatic fluids or the enveloping Meguma Group metasedimentary rocks as the sole source of vein components are invalidated.

Sediment slump structures: a review of diagnostic criteria and application to an example from Newfoundland

Abstract In the study of the history of deformed sedimentary rocks it is important that the cause of the deformation and the timing of deformation relative to metamorphic and tectonic events be assessed. There is a continuous gradation between the deformation of freshly deposited sediments by gravitational forces and the deformation of well-lithified sediments by tectonic forces, so determining the degree to which gravity, tectonism and lithification influenced deformation can be very difficult. The characteristics that may be considered in determining the origin of deformational structures in sedimentary rocks can be divided into five categories: Ductile deformation structures (1) can provide definitive criteria for recognizing post-lithification deformation, but not pre-lithification deformation. This also holds for (2), brittle deformation structures and decollements. Overprinting of sediment reworking or remobilization structures (3) are the best criteria for recognizing pre-lithification structures. Fabrics (4) can be very useful but are as yet poorly understood, and there are misconceptions in the literature about what sort of fabrics are or are not found in unlithified sediments. Spatial relationships (5) may also tell us a great deal, but are most useful when taken in conjunction with other types of evidence. Complex fold patterns in sandstones on Farmer Head, north-central Newfoundland, provide a case study for criteria that may be used to determine the degree of lithification during deformation. Although these folds have been interpreted as slump folds by earlier workers, fabrics and spatial relationships point to a tectonic post-lithification origin for the structures, a conclusion which has significant implications for the interpretation of the regional geology.