T.H. Bell

Centre for Addiction and Mental Health

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Kenneth A. Hickey

University of British Columbia

A. Forde

James Cook University

Nicholas Hayward

University of Western Australia

Scott E. Johnson

Beth Israel Deaconess Medical Center

A.P. Ham

James Cook University

M. J. Rubenach

James Cook University

C. Fay

James Cook University

Ioan V. Sanislav

James Cook University

Hyeong Soo Kim

Korea University

J. Wang

James Cook University

Cooperative Institutions

James Cook University

University of Leeds

Geological Survey of Western Australia

Australian National University

Imperial College London

University of Manchester

Centre National de la Recherche Scientifique

Mineral Resources

Instituto Andaluz de Ciencias de la Tierra

Universidad de Granada

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Papers

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Deformation partitioning and porphyroblast rotation in meta‐morphic rocks: a radical reinterpretation

Journal of Metamorphic Geology (1985)

T.H. Bell

Abstract Most porphyroblasts never rotate during ductile deformation, provided they do not internally deform during subsequent events, with the exception of relatively uncommon but spectacular examples of spiralling garnets. Instead, the surrounding foliation rotates and reactivates due to partitioning of the deformation around the porphyroblast. Consequently, porphyroblasts commonly preserve the orientation of early foliations and stretching lineations within strain shadows or inclusion trails, even where these structures have been rotated or obliterated in the matrix due to subsequent deformation. These relationships can be readily used to help develop an understanding of the processes of foliation development and they demonstrate the prominent role of reactivation of old foliations during subsequent deformation. They can also be used to determine the deformation history, as porphyroblasts only rotate when the deformation cannot partition and involves progressive shearing with no combined bulk shortening component.

Lineation

Foliation (geology)

Shearing (physics)

Strain partitioning

10.1111/j.1525-1314.1985.tb00309.x

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Citations (247)

Porphyroblast nucleation, growth and dissolution in regional metamorphic rocks as a function of deformation partitioning during foliation development

Journal of Metamorphic Geology (1986)

T.H. Bell M. J. Rubenach Peter D. Fleming

Abstract In regional metamorphic rocks, the partitioning of deformation into progressive shearing and progressive shortening components results in strain and strain‐rate gradients across the boundaries between the partitioned zones. These generate dislocation density gradients and hence chemical potential gradients that drive dissolution and solution transfer. Phyllosilicates and graphite are well adapted to accommodating progressive shearing without necessarily building up large dislocation density gradients within a grain, because of their uniquely layered crystal structure. However, most silicates and oxides cannot accommodate strain transitions within grains without associated dislocation density gradients, and hence are susceptible to dissolution and solution transfer. As a consequence, zones of progressive shearing become zones of dissolution of most minerals, and of concentration of phyllosilicates and graphite. Exceptions are mylonites, where strain‐rates are commonly high enough for plastic deformation to dominate over diffusion rates and therefore over dissolution and solution transfer. Porphyroblastic minerals cannot nucleate and grow in zones of active progressive shearing, as they would be dissolved by the effects of shearing strain on their boundaries. However, they can nucleate and grow in zones of progressive shortening and this is aided by the propensity for microfracturing in these zones, which allows rapid access of fluids carrying the material presumed to be necessary for nucleation and growth. Zones of progessive shortening also have a number of characteristics that help to lower the activation energy barrier for nucleation, this includes a build up of stored strain‐energy relative to zones of progressive shearing, in which dissolution is occuring. Porphyroblast growth is generally syndeformational, and previously accepted criteria for static growth are not valid when the role of deformation partitioning is taken into account. Porphyroblasts in a contact aureole do not grow statically either, as microfracturing, associated with emplacement, allows access of fluids in a fashion that is similar to microfracturing in zones of progressive shortening. The criteria used for porphyroblast timing can be readily accommodated in terms of deformation partitioning, reactivation of deforming foliations, and a general lack of rotation of porphyroblasts, with the spectacular exception of genuinely spiralling garnet porphyroblasts.

Shearing (physics)

10.1111/j.1525-1314.1986.tb00337.x

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Citations (245)

Microstructure of mylonites and their descriptive terminology

Lithos (1973)

T.H. Bell M. A. Etheridge

Mylonite

Recrystallization (geology)

Brittleness

10.1016/0024-4937(73)90052-2

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Citations (213)

Spiral and staircase inclusion trail axes within garnet and staurolite porphyroblasts from schists of the Bolton Syncline, Connecticut: timing of porphyroblast growth and the effects of fold development

Journal of Metamorphic Geology (1997)

T.H. Bell Kenneth A. Hickey J. Wang

In the Littleton Formation, garnet porphyroblasts preserve three generations of growth that occurred before formation of the Bolton Syncline. Inclusion trails of foliations overgrown by these porphyroblasts are always truncated by the matrix foliation suggesting that garnet growth predated the matrix foliation. In contrast, many staurolite porphyroblasts grew synchronously with formation of the Bolton Syncline. However, local rim overgrowths of the matrix foliation suggest that some staurolite porphyroblasts continued to grow after development of the fold during younger crenulation producing deformations. The axes of curvature or intersection of foliations defined by inclusion trails inside the garnet porphyroblasts lie oblique to the axial plane of the Bolton Syncline but do not change orientation across it. This suggests the garnets were not rotated during the subsequent deformation associated with fold development or during even younger crenulation events. Three samples also contain a different set of axes defined by curvature of inclusion trails in the cores of garnet porphyroblasts suggesting a protracted history of garnet growth. Foliation intersection axes in staurolite porphyroblasts are consistently orientated close to the trend of the axial plane of the Bolton Syncline on both limbs of the fold. In contrast, axes defined by curvature or intersection of foliations in the rims of staurolite porphyroblasts in two samples exhibit a different trend. This phase of staurolite growth is associated with a crenulation producing deformation that postdated formation of the Bolton Syncline. Measurement of foliation intersection axes defined by inclusion trails in both garnet and staurolite porphyroblasts has enabled the timing of growth relative to one another and to the development of the Bolton Syncline to be distinguished in rocks where other approaches have not been successful. Consistent orientation of foliation intersection axes across a range of younger structures suggests that the porphyroblasts did not rotate relative to geographical coordinates during subsequent ductile deformation. Foliation intersection axes in porphyroblasts are thus useful for correlating phases of porphyroblastic growth in this region.

Syncline

Crenulation

Staurolite

Foliation (geology)

10.1111/j.1525-1314.1997.00032.x

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Citations (21)

Rotation of relatively large rigid objects during ductile deformation: Well established fact or intuitive prejudice?

Australian Journal of Earth Sciences (1990)

T.H. Bell Scott E. Johnson

Geologists have generally accepted the intuitively appealing concept that rigid objects, which are larger than matrix grains, rotate relative to geographic co‐ordinates during non‐coaxial ductile deformation. This has been reinforced by experiments in which rigid body rotation occurs in continuous media deformed under bulk simple shear. Widespread acceptance of this concept, and its application to rock deformation, incorrectly implies that it has been thoroughly and successfully tested in real rocks. When data obtained from deformed rocks suggest that relatively large rigid objects do not rotate, they have generally been rationalized as products of coaxial deformation and considered rarities. This degree of rationalization is no longer acceptable in the light of accumulating data, as well as more recent modelling which suggests that non‐rotation of relatively large rigid objects during ductile deformation is the more common situation, and that very special circumstances are required for such objects to rotate. If these objects do not rotate relative to geographic co‐ordinates during non‐coaxial ductile deformation, they provide a unique tool for unravelling the kinematic history of orogenic belts.

Coaxial

10.1080/08120099008727943

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Citations (19)

Prolonged Acadian Orogenesis: Revelations from FIA Controlled Monazite Dating of Foliations in Porphyroblasts and Matrix

Eleventh Annual V. M. Goldschmidt Conference (2001)

T.H. Bell P. J. Welch

Matrix (chemical analysis)

Source

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Citations (19)

Geochronology of discrete structural-metamorphic events in a multiply deformed precambrian terrain

Tectonophysics (1979)

L. P. Black T.H. Bell M. J. Rubenach I. W. Withnall

Isochron dating

Geochronology

Isograd

Dalradian

Radiometric dating

10.1016/0040-1951(79)90114-8

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Citations (103)

Dissolution, solution transfer, diffusion versus fluid flow and volume loss during deformation/metamorphism

Journal of Metamorphic Geology (1989)

T.H. Bell C. Cuff

ABSTRACT Dissolution and solution transfer during deformation/metamorphism are controlled by the partitioning of deformation into progressive shearing and shortening components. Progressive shearing is readily accommodated by slip on the planar crystal structure of phyllosilicates and graphite without accumulating dislocation density gradients across grain boundaries. Progressive shortening is accommodated by the cores of most other minerals (including sulphides). These minerals develop strain, and hence dislocation density gradients, on their rims due to progressive shearing along grain boundaries. These gradients are particularly large when the mineral abuts phyllosilicate or graphite. The resulting chemical potential gradients between the core and rim drive dissolution, causing removal of the highly strained grain margins. Removal of dissolved material by solution transfer is aided by the geometry of shearing of phyllosilicates and graphite around other grains in an active anastomosing foliation. Interlayers and interfaces on boundaries lying at a low angle to the direction of shearing, and oriented relative to the sense of shear such that they can open, gape by small amounts. Water present in these interlayer spaces becomes destructured, considerably enhancing diffusion rates along the foliation. Penetrative volume loss, especially in deforming/metamorphosing pelitic rocks, is large at all metamorphic grades, increasing and becoming more penetrative with depth to at least the transition into granulite and eclogite facies. Transference of material by fluid flow from deep to high levels in the earth's crust is precluded because thousands to tens of thousands of rock volumes of fluid are required, necessitating continual recirculation of fluid from shallow to deep crustal levels in one large or several small sets of cells, unless some extremely large‐scale form of fluid channelling is possible. Reassessment of diffusion mechanisms, and hence rates, during deformation and pervasive foliation generation in large volumes of rock where fluid channeling cannot provide enough fluid, indicates that diffusion can proceed with sufficient rapidity that massive recirculation of fluid is no longer required. The amount of fluid can be reduced sufficiently to allow large volume losses by a one‐way flow of fluid to the earth's surface, in deforming/metamorphosing environments where the fluid pressure equals or exceeds the hydrostatic pressure. Deformation partitioning‐controlled dissolution progressively changes the bulk chemistry of a rock containing phyllosilicates or graphite during deformation/metamorphism because matrix minerals, other than phyllosilicates and graphite, are preferentially removed. The large size of porphyroblasts, if present, tends to preserve them from dissolution. Hence, the bulk chemistry operative during subsequent porphyroblast growth can have changed considerably from that operative when the first porphyroblasts grew, in rocks in which bedding is still well preserved.

Shearing (physics)

10.1111/j.1525-1314.1989.tb00607.x

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Citations (112)

The role of thrusting in the structural development of the Mount Isa Mine and its relevance to exploration in the surrounding region

Economic Geology (1991)

T.H. Bell

Resolution and prediction of the structure underground at Mount Isa from the surface geology is now possible using evidence for north to south movement on large-scale thrusts during the first deformation at 1610 Ma. This evidence for the direction of movement during thrusting is preserved (1) macroscopically within three successive thrust imbricates as the north-south alignment of respective truncations of older stratigraphic units by the angular unconformity at the base of the Mount Isa Group; (2) macroscopically and mesoscopically as a zone of vertical north-south-striking foliation with a horizontal stretching lineation that formed on a sinistral transform fault during Dalong the Wonga-Duchess belt; (3) mesoscop- ically as a regionally distributed north-south-oriented stretching lineation (LI) that is locally well preserved on S0, surfaces in the hinges of folds with north-south axial planes that formed during the second regional deformation; and (4) microscopically as a horizontal, north-south- aligned mineral elongation lineation defined by inclusion trails preserved in porphyroblasts that grew early during the second deformation adjacent to the Wonga-Duchess belt. The swing in strike of a large-scale 70o-west-dipping lateral ramp on the western margin of the large-scale Kokkalukkanurker duplex from north-south (parallel to the direction of movement during thrusting) to north-northwest-south-southeast caused a portion of the Meerenurker roof thrust lying on this lateral ramp to be overturned for at least 50 km near Mount Isa. This overturning resulted from tilting the 70o-west-dipping lateral ramp through another 70 o as it rode obliquely (north-south) over the north-northwest-south-southeast- striking portion of the same lateral ramp on the next imbricate to the south. This generated the gross structural relationships seen in three dimensions at Mount Isa whereby the greenstone basement that truncates Mount Isa Group rocks below the mine overlies them to the west. The second deformation involved intense east-west shortening and formed macroscopic folds that conveniently expose varying levels of the thrust pile generated during the first deformation. In higher grade rocks west of Mount Isa and to either side of the Wonga belt, axial plane schistosity development and/or reactivation of bedding during this deformation was quite intense and tended to obliterate and/or rotate foliations and lineations formed during the first deformation. Only in the hinges of the folds formed during the second deformation, where the deformation was less intense or silicification had occurred, are earlier lineations preserved relatively unaffected by this event. The geology of the Mount Isa Valley and the rocks to the west of the mine is controlled by a series of north-south-striking high-angle reverse faults that slice through the Meerenurker thrust as it crosses the above-mentioned lateral ramp on the western edge of the large-scale duplex. Two of the faults, Mount Gordon and Mount Isa, dip steeply west with similar dis- placements of 4 to 6 km west side up. The other two faults, Holly and Tastoka, dip steeply east, with the latter fault having a more significant displacement of 2.5 to 3 km east side up that greatly affects the geology on the eastern side of the Mount Isa Valley. The effects of these faults on the geometry produced by thrusting during the first deformation and folding during the second allow accurate prediction of the third dimension anywhere in the Mount Isa Valley and the region immediately to the west. Consequently, they can be used to predict the location of structurally controlled sites of potential copper mineralization similar to those found in the Mount Isa mine where mineralization formed during the third deformation.

Mount

Relevance

10.2113/gsecongeo.86.8.1602

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Citations (36)

Behaviour of rigid objects during deformation and metamorphism: a test using schists from the Bolton syncline, Connecticut, USA

Journal of Metamorphic Geology (1999)

Kenneth A. Hickey T.H. Bell

The behaviour of spherical versus highly ellipsoidal rigid objects in folded rocks relative to one another or the Earth’s surface is of particular significance for metamorphic and structural geologists. Two common porphyroblastic minerals, garnet and staurolite, approximate spherical and highly ellipsoidal shapes respectively. The motion of both phases is analysed using the axes of inflexion or intersection of one or more foliations preserved as inclusion trails within them (we call these axes FIAs, for foliation inflexion/intersection axes). For staurolite, this motion can also be compared with the distribution of the long axes of the crystals. Schists from the regionally shallowly plunging Bolton syncline commonly contain garnet and staurolite porphyroblasts, whose FIAs have been measured in the same sample. Garnet porphyroblasts pre‐date this fold as they have inclusion trails truncated by all matrix foliations that trend parallel to the strike of the axial plane. However, they have remarkably consistent FIA trends from limb to limb. The FIAs trend 175° and lie 25°NNW from the 020° strike of the axial trace of the Bolton syncline. The plunge of these FIAs was determined for six samples and all lie within 30° of the horizontal. Eleven of these samples also contain staurolite porphyroblasts, which grew before, during and after formation of the Bolton syncline as they contain inclusion trails continuous with matrix foliations that strike parallel to the axial trace of this fold. The staurolite FIAs have an average trend of 035°, 15°NE from the 020° strike of the axial plane of this fold. The total amount of inclusion trail curvature in staurolite porphyroblasts, about the axis of relative rotation between staurolite and the matrix (i.e. the FIA), is greater than the angular spread of garnet FIAs. Although staurolite porphyroblasts have ellipsoidal shapes, their long axes exhibit no tendency to be preferentially aligned with respect to the main matrix foliation or to the trend of their FIA. This indicates that the axis of relative rotation, between porphyroblast and matrix (the FIA), was not parallel to the long axis of the crystals. It also suggests that the porphyroblasts were not preferentially rotated towards a single stretch direction during progressive deformation. Five overprinting crenulation cleavages are preserved in the matrix of rocks from the Bolton syncline and many of these result from deformation events that post‐date development of this fold. Staurolite porphyroblast growth occurred during the development of all of these deformations, most of which produced foliations. Staurolite has overgrown, and preserved as helicitic inclusions, crenulated and crenulation cleavages; i.e. some inclusion trail curvature pre‐dates porphyroblast growth. The deformations accompanying staurolite growth involved reversals in shear sense and changing kinematic reference frames. These relationships cannot all be explained by current models of rotation of either, or both, the garnet and staurolite porphyroblasts. In contrast, we suggest that the relationships are consistent with models of deformation paths that involve non‐rotation of porphyroblasts relative to some external reference frame. Further, we suggest there is no difference in the behaviour of spherical or ellipsoidal rigid objects during ductile deformation, and that neither garnet nor staurolite have rotated in schists from the Bolton syncline during the multiple deformation events that include and post‐date the development of this fold.

Syncline

Staurolite

Crenulation

Foliation (geology)

Flattening

10.1046/j.1525-1314.1999.00192.x

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Citations (61)