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Thematic Kinematic-Structural Projects

Ongoing thematic structural projects are being undertaken in collaboration with Prof. David Gray (Melbourne University), and Prof. Cees Passchier (Mainz University). Ongoing structural studies are based on datsets from the Damara Orogen in Namibia and ongoing fieldwork on shear zones in the Gawler Craton, Olary Province and Adelaidean Fold Belt in South Australia. Projects still in development include:

• Developing the use of boudin trains as qualitative kinematic vorticity (flow regime) indicators.

• Classification of problems associated with the application of shear sense indicators.

• Pseudo-shear zones

• Mechanisms of kinkband formation

• Field characteristics of collapsed veins

 

Below are links to summaries of outcomes from previous thematic structural studies:

• Boudinage Classification

• Boudinage Shear Sense

• Antithetic Shear Sense in Shear Zones

• Field Based Strain Mapping Methodology

• Geometry of folded shear sense indicators

• Mega-scale kink folds

• Mega-scale sheath folds

• Tabberaberan Orogeny in Tasmania

• Deformation history west Tasmania


[1] END-MEMBER BOUDIN CLASSIFICATION AND MODIFIED BOUDIN STRUCTURES.

Ben Goscombe1, Cees Passchier2, Martin Hand1, David Gray3
1School of Earth and Environmental Sciences, Adelaide University, South Australia, 5005, Australia.
2School of Earth Sciences, University of Mainz, Germany.
3School of Earth Sciences, University of Melbourne, Parkville, 3010, Victoria.

Also see Journal of Structural Geology 26, 739-763 (2004).

In monoclinic shear zones, there are only three ways a layer can be boudinaged, leading to three kinematic classes of boudinage. These are (1) symmetrically without slip on the inter-boudin surface (no-slip boudinage), and two classes with asymmetrical slip on the inter-boudin surface: slip being either (2) synthetic (S-slip boudinage) or (3) antithetic (A-slip boudinage) with respect to bulk shear sense. In S-slip boudinage, the boudins rotate antithetically, and in antithetic slip boudinage they rotate synthetically with respect to shear sense.

We have investigated the geometry of 2100 natural boudins from a wide variety of geological contexts worldwide. Five end-member boudin block geometries that are easily distinguished in the field encompass the entire range of natural boudins. These five end-member boudin block geometries are characterized and named drawn, torn, domino, gash and shearband boudins. Groups of these are shown to operate almost exclusively by only one kinematic class; drawn and torn boudins extend by no-slip, domino and gash boudins form by A-slip and shearband boudins develop by S-slip boudinage. In addition to boudin block geometry, full classification must also consider boudin train obliquity with respect to the fabric attractor. Boudin trains lying at a low angle (<10º) to the fabric attractor are classified as foliation-parallel boudin trains and those exceeding 10°, are classified as foliation-oblique boudin trains. All foliation-oblique boudin trains operate by S-slip boudinage, regardless of end-member boudin block geometry. Material layeredness of the boudinaged rock must also be considered and has been classified into object boudinage, single-layer boudinage, multiple-layer boudinage and foliation boudinage of a foliated rock devoid of, or irrespective of, layers of differing competence.

Analysis of modified (or complex) boudin structures recognizes two types: (1) Sequential boudin structures experienced continued extension (i.e. progressive congruent structures) and show sequential development of two or more different end-member boudin block geometry components during progressive boudinage. (2) Reworked boudins were modified by subsequent deformational episodes and are polyphase non-congruent structures (folded, sheared and shortened types). Correct classification of boudins and recognition of their modification is the crucial first stage of interpretation of natural boudin structures, necessary to employing them as indicators of shear sense, flow regime and extension axes.

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[2] Boudin Trains as a Kinematic Tool Kit

Ben Goscombea, Cees Passchierb
aDepartment of Geology and Geophysics, Adelaide University, Adelaide, S.A. 5005, Australia. bInstitut fuer Geowissenschaften, Johannes Gutenberg Universitaet, Becherweg 21, Mainz, Germany.

See Journal of Structural Geology 25, 575-589 (2003)

We have compiled a comprehensive dataset of 1800 boudin structures from a wide variety of geological provinces worldwide. This dataset has permitted the construction of a robust geometric classification scheme in which all natural boudin structures can be sub-divided into; shearband, domino, sygmoidal, torn and drawn morphological groups. Shearband boudins are asymmetric with inter-boudin surfaces at low angles (<30º) to the enveloping surface of the boudin train and with curved, sigma-shaped boudin blocks, typically with tapering wings. Domino boudins are also asymmetric, have angular blocky shaped boudins with inter-boudin surfaces at high angles to the enveloping surface (>50º) and in contrast to shearband boudins, often with dilation across the inter-boudin surface. Sygmoidal boudins are very similar to domino boudins, differing only in having inter-boudin surfaces with sygmoidal trace (either of curved tension-gash-type or angular forked-type). Both torn and drawn boudins are symmetric; Torn boudins have angular, blocky shapes with inter-boudin faces at high angles to the enveloping surface of the boudin train. Drawn boudins have lenticular, ellipsoidal and tapering boudin shapes with appearances suggestive of been ductilely drawn.

Recognition of these morphological groups has permitted the testing of correlations between boudin geometry and numerous kinematic parameters. For example:

(1) By comparison with associated stretching lineations (mineral and mineral aggregate lineations), it was found that the extension axis associated with boudin structures (orthogonal to the long axis or neck line of the boudin and contained within the layer enveloping surface) was in all cases sub-parallel to the stretching lineation. Thus boudin structures can be employed to accurately indicate the orientation of the principal axis of the strain ellipsoid (X-axis) in terranes otherwise devoid of stretching lineations, such as in low-grade, low-strain or fine-grained terranes.

(2) There are three kinematic classes by which a layer can be boudinaged; symmetrically without slip on the inter-boudin surface called no slip boudinage (NSB), and asymmetrically with slip on the inter-boudin surface that is either synthetic or antithetic with respect to bulk shear sense. Antithetic slip boudinage (ASB) and synthetic slip boudinage (SSB) have mirror-image symmetries, thus we have investigated the geometry of boudins developed by ASB or SSB as indicated by other independent shear sense indicators. We found that the geometry of asymmetric boudins, in boudin trains parallel to the fabric attractor, were indeed different depending on whether they formed by ASB or SSB. We refer to these two geometric classes as "shearband-type" (formed by SSB in 100% of cases) and "domino-type" (formed by ASB in 98% of cases), which have been quantitatively defined and readily recognised in the field and thus can be employed as shear sense indicators. In contrast, we found that in all cases where the boudin train is oblique to the fabric attractor, bulk shear sense is synthetic with "forward-rotation" of the boudin train towards the fabric attractor and that all asymmetric boudin geometries (both shearband- and domino-types) formed by SSB.

(3) Considering only two aspects of boudinage; geometric class and obliquity of the boudin train with respect to the fabric attractor, we found that the suite of boudins developed in different terranes, or deformational episodes, that experienced different flow regime (ie. pure shear, simple shear, transpressional general shear, transtensional general shear), are markedly distinct in many cases. It was found that symmetric boudin trains form in both coaxial and non-coaxial progressive deformation where the X-Y plane of the strain ellipsoid remains coincident with the enveloping surface of the boudin train throughout deformation. domino boudins have been produced experimentally in coaxial progressive deformation while both shearband and domino boudins have been formed experimentally in simple shear and noted in transpressional terranes. Oblique boudin trains with shearband boudin geometry have been modelled in simple shear and noted in transpressional terranes. However, oblique boudin trains with domino boudin geometry have not been found in transpressional terranes but are developed in pure shear regimes, implying that domino boudins form by either coaxial progressive deformation or by non-coaxial progressive deformation only where the boudinaged layer is approximately coplanar with the fabric attractor. Not-with-standing the problems of applying such findings in the field (ie. evolution in the flow regime with progressive deformation, and how to interpret a suite containing different boudin-types developed in close proximity), boudin geometry in combination with boudin train obliquity can in some cases indicate flow regime.

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[3] Antithetic shear sense due to differential strain rate domains in shear zones

Martin Hand, Ben Goscombe
Department of Geology and Geophysics, Adelaide University, Adelaide, SA 5005.

Unpublished conference abstract (2001)

In the past 15-20 years, the widespread use of kinematic indicators, combined with evidence such as metamorphic and stratigraphic offsets has greatly increased the understanding of the bulk flow paths within orogenic belts. Generally speaking, shear sense criteria are applied in a local reference frame, ie “top to the north, or, that side goes that way”, and from this, extrapolations are made to a larger scale by assuming that the local kinematic environment is reflective of the larger one. However in many instances not all the kinematic indicators point to a single bulk shear sense, and there may be a significant percentage of reliable indicators that oppose the bulk flow direction. Does this mean that the large picture was also characterised by a complex kinematic evolution, or that other factors can lead to the formation of opposing shear sense domains?

Within a shear zone(s) there a several scenarios in which opposing shear sense may be generated.

(1) Near-pervasive reactivation of a shear zone may preserve sub-domains with fabrics developed during earlier shearing events, possibly with opposing shear sense.
(2) Minimal reactivation of a shear zone may result in localised shear zones of opposing shear sense, within and sub-parallel to the earliest phase of shearing.
(3) The rate of flow may be heterogeneously partitioned across shear zone, even during the one deformational event, resulting in sub-parallel domains with different strain rate (Sk). The gradient of differential flow experienced by adjacent domains of sufficiently different strain rate (∆Sk either negative or positive), can potentially result in marginal sub-domains with shear sense couples that oppose the bulk flow of the shear zone.
The reactivation scenarios (1) and (2) are generally widely accepted, and are usually the only interpretative scenarios considered to explain mixed synthetic/antithetic suites of shear sense observations. In contrast, the differential strain rate scenario (3) is rarely considered as a mechanism to produce opposing shear sense domains. In shear zones with mixed shear sense populations, recognition of differential strain rate scenarios versus reactivation scenarios may be critical to correct tectonic interpretations. For example, where shear zones with mixed shear sense populations are simplistically and intuitively interpreted to imply alternating thrusting and extensional movements, complex tectonic models must be developed, such as "squeeze box orogen" (e.g. Bell & Johnston, 1992). However, such interpretations and the resultant models may be wrong if the shear zones investigated had a simpler history, and mixed shear sense populations were the result of flow along domains of differential strain rate.
The scenario outlined is effectively an extrusion model, in which material is “forced” along the shear zone within a high-strain rate channel. In small-scale examples, strain compatibilities would have to be accommodated by the generation of additional structures, or mass transfer out of the channel during partial melting or some other fluid-related dissolution process. If local channelised flow can occur within shear zones, it should give rise to spatially organised domains of opposing shear sense that track out the channel boundaries. Furthermore, the differing kinematic domains should have exactly the same metamorphic history, which is unlikely to be the case if the contrasting domains are the result of reactivation. Compared to the number of studies that have used kinematic indicators for terrain analysis there have been relatively few studies that have systematically documented the spatial distribution of shear sense indicators within individual shear zones (e.g. Steinhart, 1991).
Whether effective channelised flow can occur on the small scale is not clear, however on the larger scale channelised flow has been proposed for a number of settings that include both compressional and extensional systems (e.g. Gans, 1987). In the Himalayas, extrusion has been proposed to explain the exhumation of the high-grade Greater Himalayan Sequence (GHS) and the existence of the apparently synchronous Main Central Thrust, (which underlies the GHS), and the South Tibetan Detachment Zone, which is a normal fault system that separates the GHS from the overlying low-grade Tibetan sedimentary series (e.g. Burell et al., 1992). At this scale, strain compatibilities associated with the existence of a high-strain rate channel could be balanced by erosion of material from the channel. Himalayan-style orogens present a daunting challenge for structural geologists attempting to interpret the kinematic record. Although these systems are clearly convergent, there is abundant evidence for normal fault movements parallel to the shortening direction (e.g. Burrell et al., 1992). An intriguing question is to what extent normal faults such as the South Tibetan detachment and faults like it are true active extensional structures in the sense that they extend the orogenic line length. An alternative scenario is that they are passive-style structures in which the footwall is forced out (extruded) from under the hanging wall. In the latter case, both the hanging wall and the footwall may move in the same direction, only at different rates. The detachment at the upper boundary of the extruding channel would develop kinematically as a normal fault, but may not result in extension of the orogenic line length (Figure 2), thereby being a contractional normal fault. With faults such as the South Tibetan Detachment Zone, it is not obvious how much of the displacement is due to true extension, verses differentially fast footwall extrusion. However in either case, the boundary conditions to the Himalayan collision are relatively well understood. In older orogenic systems we do not have this luxury, and deciphering the kinematic record on the orogenic scale must be built up from shear sense observations on the local scale. Consequently, it is imperative that we know what these shear sense observations actually mean, and as illustrated here, must also consider the possibility of differential strain rate, in addition to multiple shear episode scenarios, in interpreting mixed shear sense populations.

  • Bell T.H. & Johnston,S.E. 1992. Journal of Metamorphic Geology, 10, 99-124.
    • Burell and 7 others,1992. Geological Society of America, Special Paper 269.
  • Gans, P.B., 1987. Tectonics, 6, 1-12.
  • Grujic, G., and 6 others. Tectonophysics, 260, 21-43.
  • Steinhart, C.C. 1991. Australian Journal of Earth Sciences, 38, 139-150.

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[4] Method For Documenting Distribution of Bulk Strain and Strain Type

Goscombe

See Gondwana Research Focus Paper, in press (2007)

Qualitative Estimates of Strain Intensity:

A qualitative scale (0 to 10) of bulk strain based on foliation intensity (Table 1) has been developed as a simple field-based system for mapping stain and enabling a more continuous pattern of strain distribution. This qualitative foliation intensity index (QFII) has been modified after the method of Goscombe et al., (2006) to the comprehensive scheme outlined in Table (1). High strain intensity (QFII ≥ 7) is defined by the proportion of grain refinement, at moderate strain (3 ≤ QFII ≤ 7) by the foliation intensity and at low strain (QFII ≤ 3) by the spacing of cleavage planes.
Foliation intensity is simply a qualitative assessment of the ratio; grain shape (or aspect ratio) over spacing, of the planar elements in the rock such as mica, amphibole and flattened mineral grains. Grain refinement is the proportion (%) of the rock that has been recrystallised to sub-grains and new mineral grains: >50% refinement in protomylonite, >70% in mylonite and >90% in ultramylonite (Passchier and Trouw, 2005). Grain refinement typically also involves grainsize reduction, which has not been quantified in this study. Nevertheless, reductions from 10 cm K-feldspar megacrysts to sub-grains <<0.01 mm are typical in ultramylonite within the TPMZ and PMZ.
The QFII scale is subjectively assigned and fabric development is dependent on a multitude of variables, such as metamorphic grade, rock type, deformation mechanism, strain rate, grainsize, mica proportion and rheology. An attempt has been made to account for metamorphic grade and different rock types, making the scheme more widely applicable (see field card in dowloads). Fabric intensity typically under-estimates bulk strain because (1) rock fabrics do not record the full accumulative strain history and (2) fabrics can be lost due to post-deformation annealing. Nevertheless, the QFII method is useful in documenting and illustrating relative strain distribution and strain gradients with a level of internal consistency (Goscombe et al., 2006).
Semi-Quantitative Bulk Strain Calculations:

Absolute strain determinations are notoriously difficult because all strain gauges have, to varying degrees, assumptions and rheological aspects of the method result in either or both under- or over-estimation, as well as numerous geological constraints, not least the incomplete record of accumulative strain in the rocks. Because of these limitations we have treated the bulk strain calculations as (1) semi-quantitative, (2) typically minimum estimates and (3) not absolute values that can be directly compared. We have used the results to illustrate relative strain intensity variation and have grouped results to facilitate comparison between like methods only.

A simplified minimum estimate of strain ratio (R=X/Z) and Flynn ratio indicating strain type (k), are calculated from the aspect ratio of stretched conglomerate clasts and augen, assuming an initial sphere (Ramsay, 1967). Shear strain is calculated from the inter-marginal hinge angle and aspect ratio of the cross sectional ellipse in sheath folds, assuming simple shear (Lacassin and Mattauer, 1985). A maximum estimate of shear strain is calculated from the degree of rotation of planar elements, such as tensional veins producing flanking folds, and assuming an original orientation orthogonal to layering (Ramsay and Graham, 1970). Accurate strain ratios are calculated from matrix extension indicated by pressure fringes, such as quartz fibres or partial melt around pyrite or garnet porphyroblasts that are assumed to be undeformed (Hobbs et al., 1983). Layer extension in drawn- and shearband-boudin trains is used as a simple minimum estimate of strain assuming no deformation of the boudin blocks (Ramsay, 1967; Goscombe et al., 2004a). Accurate strain ratios are calculated from the acute angle between schistosity and C-plane in composite S-C foliations, assuming simple shear (Ramsay and Graham, 1970).
Crucial simplifications and assumptions about rheology and flow regime are compared and their potential effect on results evaluated. All methods assume plane strain and end-member flow regimes such as simple or pure shear that do not necessarily reflect natural systems. Sheath folds, rotated planes and C-S fabrics are moderately reliable methods that cannot under-estimate strain, but later flattening of these structures or initial obliquity of the rotated plane, can result in over-estimation. Pressure fringe methods are considered the most accurate because these fringes record true extension of the matrix, inaccuracies arising only if clasts are resorbed (over-estimate) or stretched (under-estimate). Boudin train extension typically under-estimates bulk strain because the assumption that all strain is partitioned into the matrix is rarely realised; most boudins are deformed to some degree, especially in high-grade ductile conditions (Goscombe et al., 2004a; Mandal et al., 2007). In most scenarios the simplified method used to calculate strain from stretched clasts and augen, both under- and over-estimate strain. Clasts and augen are typically more competent than the matrix and do not partition strain giving an under-estimate and augen can further under-estimate strain due to recrystallization and mortar development (Schmid and Podladchikov, 2004). Trends towards over-estimation occur because the original clast shape and orientation have not been accounted for in this simplified method.
  • Goscombe, B.D., Passchier, C.W., Hand, M., 2004a. Boudinage classification: end-member boudin types and modified boudin structures. Journal of Structural Geology 26, 739-763.
  • Hobbs, B.E, Means, W.D., Williams, P.F., 1983. An outline of structural geology. John Wiley, New York, 571 pp.
  • Lacassin, R., Mattauer, M., 1985. Kilometre-scale sheath fold at Mattmark and implications for transport direction in the Alps. Nature 315, 739-742.
  • Mandal, N., Dhar, R., Misra, S., Chakraborty, C., 2007. Use of boudinaged rigid objects as a strain gauge: insights from analogue and numerical models Journal of Structural Geology (in press).
  • Passchier, C.W., Trouw, R.A.J., 2005. Microtectonics, 2nd edition. Springer, pp. 366.
  • Ramsay J.G., 1967. Folding and fracturing of rocks. Mc Graw and Hill, New York.
  • Ramsay J.G., Graham R.H., 1970. Strain variation in shear belts. Canadian Journal of Earth Sciences 7, 786-813.
  • Schmid, D.W., Podladchikov, Y.Y., 2004. Are isolated stable rigid clasts in shear zones equivalent to voids? Tectonophysics 384, 233-242.

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[5] The geometry of folded tectonic shear sense indicators

Ben Goscombe1, Rudolph Trouw2
1Namibian, Geological Survev, P.O. Box 2168, Windhoek, Namibia
2Departamento de Geologio, I. GEO, URFRJ, Ilha do Fundao, CEP: 21910-900, Rio de Janeiro, Brazil.

See Journal of Structural Geology 21, 123- 127 (1999)

The geometry of refolding of early stretching lineations and associated sense of shear indicators is often not fully considered and can potentially lead to grossly incorrect tectonic interpretations of orogens and orogenic events. Sense of shear is not inverted or 'flipped' in all cases of refolding by folds with closure angle <90º. The geometry of inversion of 'flipping' of shear sense is determined entirely by the angle (X) between the stretching lineation and the latter fold axes. Folding around an axis orthogonal to the stretching lineation (X = 90º) does not result in shear sense inversion. This is a special case; if there is any degree of obliquity (X < 90º) between fold axis and stretching lineation, there is a change in the orientation of the stretching lineation on alternate limbs and if X < 45º the sense of shear is inverted in alternate limbs.

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[6] Multi-scale kinking in northeast Tasmania: crustal shortening at shallow crustal levels

BEN D. GOSCOMBE, R. H. FINDLAY, M. P. MCCLENAGHAN and J. EVERARD
Tasmanian Department of Mines, P.O. Box 56, Rosny Park 7018, Tasmania, Australia.

See Journal of Structural Geology 16, 10770-1092 (1994)

Analysis of S1 cleavage trends in the Ordovician-Devonian Mathinna Group of northeast Tasmania has identified mcga-scale (up to 9 km axial surface spacing) kink-bands and kink folds. These formed after the regional steep slaty cleavage and are contemporaneous with but post-date small-scale (cm-scale) vertical kinkbands. Small-scale sinistral kink-bands strike approximately 032º and predominate in number over the approximately E-W-striking dextral kink-bands. The geometry of the small-scale kink-bands indicates a 166º ± 10º-trending principal shortening during kinking. Kink-bands overprint minerals in the contact aureole of the Middle to Upper Devonian granite-granodior-ite Scottsdale Batholith and pre-datc deposition of the Parmeener Supergroup of Upper Carboniferous age. Megakinking in NE Tasmania is correlated with megakinking in the Lachlan Fold Belt (New South Wales), which occurred under the same bulk shortening direction and also during the Middle Carboniferous.
Prior to kinking, the regional slaty cleavage in the Mathinna Group was near vertical and trended 157º, which is subparallel to the principal shortening direction during kinking (166º ± 10º). Because of this close alignment and the presence of a pre-existing planar anisostropy throughout the region, megakinks developed despite only 4-5% bulk shortening. Kinking in NE Tasmania occurred at 120 ± 45 MPa (approximately 4.2 km depth). The brittle and 'instantaneous' nature of kinking is consistent with deformation at such shallow crustal levels. Megakinking may be a typical expression of crustal tectonics of minor bulk shortening at shallow crustal levels, only in the case of regions with a pre-existing steep planar structural grain subparallel to the principal shortening direction.

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[7] Intense non-coaxial shear and the development of mega-scale sheath folds
in the Arunta Block, Central Australia

B. GOSCOMBE
Department of Geology, Melbourne University, Parkville, Victoria 3052, Australia
See Journal of Structural Geology 13, 299-318 (1991)

The Strangways Orogenic Belt of 1800 Ma granulites in the central Arunta Block was completely reworked at granulite-facies grades during the Middle Proterozoic (1400-1500 Ma). The bulk strain regime of the initial episodes of this reworking is presented. Ductile reworking involved non-coaxial deformation on a regional scale and produced map-scale sheath folds. High bulk shear strains are inferred on microscopic and macroscopic scales and the spatial variation in bulk shear strain throughout the region is discussed. The high bulk shear strains were accommodated by east-over-west ductile ramping and fold repetition, resulting in easterly inclined structures, crustal shortening and crustal over-thickening.

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[8] Deformation of the Zeehan Tillite and re-evaluation of the Tabberabberan Orogeny in Tasmania

B. D. Goscombe
See Division of Mines and Mineral Resources Tasmania - Report 1991/03

A strong penetrative vertical cleavage, trending NW-SE, is developed throughout the entire 4.5 kin long Section of outcropping Zeehan Tillite near Zeehan. This rock unit lies on an undulating angular unconformity that has been folded, in sympathy With bedding in the tillite, around an approximately NW-trending axis. Deformation of the tillite is here correlated with the latest folding event of the Tabberabberan Orogeny also of NW-trend (D4 after Seymour, 1980).
In addition to numerous Oonah Formation clasts, the Zeehan Tillite contains clasts of fossiliferous Silurian sandstone (Crotty Sandstone), keratophyre-tuff of the Dundas Group, and presumed Early Carboniferous Heemskirk Granite clasts (Spry, 1958). Furthermore, the tillite has been lithologically correlated with the Late Carboniferous Wynyard Tillite (Spry, 1958). Such age constraints on the Zeehan Tillite suggest that both its deformation and the last fold event of the Tabberabberan Orogeny (D4) are of post-Early Carboniferous age. Such an interpretation is not inconsistent with the previously accepted minimum age constraint of the Tabberabberan Orogeny based on the Eugenana cave deposits. This is because only one Tabberabberan Orogeny cleavage, the earlier N-Lrending D3 of Seymour (1980), is developed in the limestone blocks contained within the cave sediments.
Consequently the NW-trending episode of folding (D4), previously grouped with the Early-Middle Devonian Tabberabberan Orogeny, is here proposed to be a temporally distinct deformational event of post-Carboniferous age. Furthermore, D4 may also be tectonically distinct from the Tabberabberan Orogeny (deformations DI-D3, after Seymour, 1980). This is based on D4 having NE over SW transport, which is contrastingly opposed to the W over E transport during D3 (Williams and Turner, 1974).

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[9] Tectonothermal evolution of the northwest Zeehan Quadrangle and contact metamorphism of the Oonah Formation by the Heemskirk Granite

B. D. Goscombe
See Mineral Resources Tasmania REPORT 1993/11

The first deformational event (D1) recognised in the Precambrian Oonah Formation was the most pervasive and intense. D1 involved intense bedding-parallel foliation development and rare isoclinal folds; both accompanied the peak of regional metamorphism. The grade of regional metamorphism varies from sub-greenschist facies to greenschist facies in the very northwest corner of the Zeehan Quadrangle. Greenschist facies assemblages include chlorite-muscovite-quartz-calcite-plagioclase and plagioclase- tremolite- quartz- epidote. These possibly formed at typical greenschist facies temperatures of 300-400ºC, and phengite geobarometry suggests pressures of 5250-6450 bar. These highest grade rocks may be equivalents of the Arthur Lineament.
D1 tectonites experienced four phases of tight to open, inclined to upright folding (D2-D5) prior to deposition of Ordovician-Devonian sediments. The Oonah Formation in the most northwest portion of the Zeehan Quadrangle experienced approximately 18-22 km of uplift and denudation at some stage during the period between D1 and Ordovician sedimentation. This uplift is a major tectonic event and may be the result of isostatic rebound in response to crustal overthickening during D2-D5 folding and horizontal crustal shortening.
The Early Carboniferous (347-351 Ma) Heemskirk Granite intruded and contact metamorphosed the Oonah Formation. The thermal aureole is very narrow and contains a variety of metamorphic assemblages according to the bulk rock chemiistry. Such assemblages include andalusite- cordierite- biotite- muscovite- anthophyllite and garnet- chlorite- tourmaline- hornblende- anthophyllite- biotite in pelitic lithologies; and biotite- muscovite- tremolite- tourmaline ± plagioclase ± chlorite in quartzite lithologies. The hydrothermal aureole, defined by the presence of black tourmaline deposited in fractures and partially replacing pelitic layers in the Oonah Formation, is in part wider than the thermal aureole. The hydrothermal aureole is partially controlled by the distribution of pre-existing fractures/faults in the Oonah Formation.
Minerals from selected samples were analysed for geothermobarometry and equilibrium thermodynamics work. These analyses indicate that the maximum temperatures of the thermal pulse experienced by the Oonah Formation were approximately 450-535ºC. This is consistent with the assemblages developed. Such extremely low thermal aureole temperatures suggest that the Heemskirk Granite was a relatively cool granitic melt at the time of emplacement. None of the geobaraometers applied are considered reliable, but the preferred pressure during emplacement was 3.5-4 kbar (12-14 km). This pressure estimate indicates a significant uplift and denudation event subsequent to granite emplacement and prior to deposition of the Late Carboniferous Zeehan Tillite. This uplift may have accompanied south over north high-angle thrusting and asymmetrical west-trending folding (D6).

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