T.H. Bell

Foliation development — The contribution, geometry and significance of progressive, bulk, inhomogeneous shortening

Abstract The geometric consequences of modelling deformation histories involving pure shear, simple shear, inhomogeneous simple shear and bulk inhomogeneous shortening are reviewed. It is shown that zones of rock affected by these deformation histories have similar discontinuities on most of their boundaries with unaffected rock. The “space problem” of progressive pure shear and bulk inhomogeneous shortening can only be resolved if the maximum finite elongation plunges down the dip of any associated foliation plane. However, many fold belts and a number of mylonite zones possess this property and consequently bulk shortening deformation histories must be considered in such cases. The strain field in these models does not account for the inherently heterogeneous nature of deformation on all scales and they generally cannot accommodate the anastomosing character of foliations as it reflects variation in strain. Hence a model is required that provides a solution to the boundary discontinuities associated with bulk shortening and which can accomodate the heterogeneous and anastomosing nature of strain in rocks. An unusual geometry called a “millipede microstructure” preserved in crenulated and crenulation cleavages and also in inclusion trails in plagioclase porphyroblasts is described in Bell and Rubenach (1980). This structure represents a specific deformation history which involved bulk inhomogeneous shortening. A number of constant-area (volume) models of the possible strain in this structure were developed that accommodate the effects of change and/or redistribution of volume, variation in strain and a heterogeneous distribution of inhomogeneity. They also provide a solution to the boundary discontinuities of progressive bulk inhomogeneous shortening. Some of the implications of these models are as follows: 1. (1) Mylonite zones can form by progressive, bulk, inhomogeneous shortening with or without a large non-coaxial component. 2. (2) Foliations formed by progressive, bulk, inhomogeneous shortening must anastomose if the foliation is to remain planar on the average and that this reflects strain variation, both in degree and orientation. 3. (3) Metamorphic differentiation associated with crenulation cleavage development may be controlled directly by strain, strain rate and local strain history variation and only indirectly by stress. 4. (4) Foliations develop on the local scale parallel or nearly parallel to the XY plane of the strain ellipsoid whether they be slaty or crenulation cleavages, schistosity, gneissosity or mylonitic foliations. However on the bulk scale of a fold limb they will commonly lie at a low angle to the XY plane of the strain ellipse. 5. (5) The microstructure of individual mineral species may provide criteria for distinguishing strain history, and individual minerals in favoured orientations for slip control subsequent development of heterogeneity. 6. (6) Geometric criteria exist on various scales which allow differentiation between bulk shortening and shear strain histories. 7. (7) Many arcuate fold belts can be readily explaned by non-plane strain during their development.

Progressive deformation and reorientation of fold axes in a ductile mylonite zone: the woodroffe thrust

Abstract The structural geometry of a mylonite zone (the Woodroffe thrust) and the country rock in its immediate vicinity is described. Mylonitic schistosity formed axial planar to folds in country rock foliation and contains a mineral elongation lineation which is constant in orientation. However, the fold axes (and associated intersection lineation) spread in orientation within the mylonitic schistosity but with a strong maximum parallel to the mineral elongation lineation. It is demonstrated that the fold axes formed initially at approximately 90° to mineral elongation but rotated with increase in strain towards it. Where this phenomenon was homogeneous on a macroscopic scale, rotation of large blocks of country rock across zones of mylonitization accompanied reorientation of fold axes within the mylonite. The controversy of progressive simple versus progressive pure shear for mylonite zones is discussed in the light of recent fabric and other evidence. It is concluded that the inhomogeneous forms of both progressive pure shear and progressive simple shear played a part and that the former dominated initially but gradually gave way to the latter until brittle rupture with large simple-shear displacements on a zone lubricated by the formation of pseudotachylite, brought granulite over amphibolite facies rocks.

GEOCHRONOLOGY OF DISCRETE STRUCTURAL-METAMORPHIC EVENTS IN A MULTIPLY DEFORMED PRECAMBRIAN TERRAIN

Abstract Rb—Sr total-rock and mineral studies are combined with detailed structural-metamorphic work to investigate polymetamorphism within the Georgetown Inlier of north Queensland, Australia. A sequence of five major, temporally discrete, regional structural events is established. The earliest events, which were marked by tight folding and prograde metamorphism, occurred at 1570 ± 20 and 1469 + 20 m.y. Subsequent deformations were less intense, and generally associated with retrogressive metamorphism. The third is documented by Rb—Sr total-rock isochrons at 967 ± 28 m.y., and the fourth by mineral and total-rock studies at about 400 m.y. The last deformation is inferred to have occurred at about 300 m.y., a time of extensive igneous activity in the east of the area. The isotopic data indicate that Sr isotopic homogenisation is approached on the scale of a cubic metre during amphibolite-facies regional metamorphism and deformation. Consequently, the Rb—Sr isochron age of a metamorphic unit is highly dependent on its deformation-metamorphic history, and in particular its last significant metamorphism; a structural-metamorphic event which forms a prominent schistosity appears to be an important requisite. The overall success of the method demonstrates its potential in unravelling the geological history of other polymetamorphic terrains.

A rationalized and unified shorthand terminology for lineations and fold axes in tectonites

Abstract The description of structures in poly-deformed terrains necessitates a logical and concise shorthand terminology. This paper delineates such a terminology. For the nth deformation the terminology becomes D n , S n , F x n , L x n where the superscript represents the S -surface affected by D n .

Crenulation cleavage development—evidence for progressive bulk inhomogeneous shortening from “millipede” microstructures in the robertson river metamorphics

Abstract Unusual “millipede” microstructures in plagioclase porphyroblasts and crenulated cleavage developed in the Robertson River Metamorphics synchronous with the second deformation. Their geometry is such that a heterogeneous simple shear deformation path alone or combined with volume change is precluded. They appear to have formed by progressive bulk inhomogeneous shortening (i.e. a general deformation history in which the boundaries of the deformed zone came closer together), which may have been combined with some volume redistribution and/or change.

Do smoothly curving, spiral-shaped inclusion trails signify porphyroblast rotation?

Smoothly curving, spiral-shaped inclusion trails may form in porphyroblasts during the overprinting of near-orthogonal foliations in schists that are relatively quartz rich and, in general, graphite poor. The smooth curvature results from the combined effects of (1) the protection from progressive shearing provided by projecting crystal faces and (2) greater competency and generally wider zones of progressive shearing in these rock types than in ones that are less quartz rich and more graphtic. Therefore, lack of truncations or abrupt changes in compositional zoning profiles and the presence of smoothly curving, spiral-shaped inclusion trails do not necessarily signify that the porphyroblast containing them rotated as it grew. This has considerable significance for shear-sense determinations in deformed and metamorphosed rocks.

Distribution of pre-folding linear indicators of movement direction around the Spring Hill Synform, Vermont: significance for mechanism of folding in this portion of the Appalachians

Abstract Three distinctly oriented sets of pre-folding, and one set of syn-folding, axes of curved inclusion trails are preserved in garnet porphyroblasts in 50 samples around the doubly plunging Spring Hill Synform in southeast Vermont. Over one third of the samples contain consistent changes in the trend of these axes from the core to rim. Since the core grew before the rim this enabled the relative timing of each set of axes to be determined, from the oldest to the youngest, as NE-SW, E-W, NNW-SSE and NNE-SSW. The youngest trend is parallel to the axial plane of the regional folds. Only those samples with the latter trend have their inclusion trails connected continuously to the matrix foliation. The three pre-folding sets of axes have the same orientation on both limbs. This consistency in orientation has significant implications for the processes operating during folding, and three mechanisms are presented that could potentially explain it. These involve the classic card deck model of shear folding, De Sitters model of clay bricks shortening as they shear past one another, and the progressive bulk inhomogeneous shortening model. The relative merits of each of these models are discussed.

Early formed regional antiforms and synforms that fold younger matrix schistosities: their effect on sites of mineral growth

In the metamorphic cores of many orogenic belts, large macroscopic folds in compositional layering also appear to fold one or more pervasive matrix foliations. The latter geometry suggests the folds formed relatively late in the tectonic history, after foliation development. However, microstructural analysis of four examples of such folds suggests this is not the case. The folds formed relatively early in the orogenic history and are the end product of multiple, near orthogonal, overprinting bulk shortening events. Once large macroscopic folds initiate, they may tighten further during successive periods of sub-parallel shortening, folding or reactivation of foliations that develop during intervening periods of near orthogonal shortening. Reactivation of the compositional layering defining the fold limbs causes foliation to be rotated into parallelism with the limbs. Multiple periods of porphyroblast growth accompanied the multiple phases of deformation that postdated the initial development of these folds. Some of these phases of deformation were attended by the development of large numbers of same asymmetry spiral-shaped inclusion trails in porphyroblasts on one limb of the fold and not the other, or larger numbers of opposite asymmetry spirals on the other limb, or similar numbers of the same asymmetry spirals on both limbs. Significantly, the largest disparity in numbers from limb to limb occurred for the first of these cases. For all four regional folds examined, the structural relationships that accompanied these large disparities were identical. In each case the shear sense operating on steeply dipping foliations was opposite to that required to originally develop the fold. Reactivation of the folded compositional layering was not possible for this shear sense. This favoured the development of sites of approximately coaxial shortening early during the deformation history, enhancing microfracture and promoting the growth of porphyroblasts on this limb in comparision to the other. These distributions of inclusion trail geometries from limb to limb cannot be explained by porphyroblast rotation, or folding of pre-existing rotated porphyroblasts within a shear zone, but can be explained by development of the inclusion trails synchronous with successive sub-vertical and sub-horizontal foliations.

THE DEVELOPMENT OF SLATY CLEAVAGE ACROSS THE NACKARA ARC OF THE ADELAIDE GEOSYNCLINE

Abstract The macro-, meso- and micro-structures associated with various intensities of slaty cleavage development across a slate belt are described. The deformation history of this slate belt was such that initially flexural slip folds were produced. However, as the deformation continued folding by flexural slip gave way to folding by inhomogeneous flattening. The slaty cleavage was produced during the latter stage of the deformation and formed slightly oblique to the axial planes of macroscopic folds due to a noncoaxial strain path. The slaty cleavage is defined mainly by preferred orientations of (001) of mica and chlorite. In rocks where slaty cleavage is more intensely developed it is also defined by alignment of the long and intermediate axes of ellipsoidal quartz, feldspar and calcite grains. These grain shapes are a product of pressure solution with dissolution and growth in directions respectively normal and parallel to the slaty cleavage. The mica preferred orientation is dominantly due to crystallization of mica with (001) parallel or at a low angle to the slaty cleavage direction. This involved pressure dissolution of bedding oriented mica with nucleation and growth of slaty cleavage mica on sites commonly dissociated from relic mica, for example other silicate grain boundaries. However, a small but important contribution from rotation of bedding micas towards the slaty cleavage direction accompanied the inhomogeneous flattening.

The progressive development of a macroscopic upright fold pair during five near-orthogonal foliation-producing events: complex microstructures versus a simple macrostructure

Abstract A macroscopic anticline in the Adelaide Fold Belt, with a profile similar to first deformation folds with an axial-plane slaty cleavage elsewhere in the orogen, folds bedding, schistosity, and a differentiated crenulation cleavage. All these structures are readily visible in outcrop and a young crenulation cleavage has locally developed parallel to the axial plane of the fold. Yet foliations preserved as inclusion trails, in andalusite, garnet and staurolite porphyroblasts, reveal a history of five foliations that formed successively in steep, shallow, steep, shallow and finally steep orientations without producing a refold geometry in bedding at the macroscopic scale. The predominantly dextral (looking north) asymmetry of curvature of overprinting foliations on both limbs of this fold tightly constrains any model for its development. The anticline formed as one hinge of an open fold couplet with a sub-vertical axial plane slaty cleavage during the first deformation. It was progressively tightened during each subsequent event, without producing a refold geometry in bedding. Steep axial-plane foliations were overprinted by shallow differentiated crenulated cleavages that transected the fold couplet. These shallow foliations were then folded during the next deformation, producing another steeply dipping foliation, as the fold couplet tightened. Thus simple macroscopic folds, in orogens with a multideformational history, may appear to have formed as late structures because they fold differentiated crenulation cleavages and a bedding-parallel schistosity, but actually formed very early during orogenesis and were then tightened by a succession of near-orthogonal foliation-producing deformations. Such deformation histories provide a simple explanation for the lack of large-scale refold geometries in many fold belts with a complex history of deformation. Small-scale structures are not always a direct reflection of larger-scale geometries in fold belts.