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Physical modelling of deformation in the Tasman Orogenic Zone
Tectonophysics 375 (2003) 37 – 47 www.elsevier.com/locate/tecto
Physical modelling of deformation in the Tasman Orogenic Zone Myra Keep * Tectonics Special Research Centre, School of Earth and Geographical Sciences, University of Western Australia, 35 Stirling Highway, Nedlands, WA 6009, Australia Received 13 May 2002; received in revised form 19 March 2003; accepted 5 June 2003
Abstract Structural vergence within the Western Subprovince of the Lachlan Fold Belt is towards the hinterland rather than the foreland, in contrast to many well-known orogenic belts. High angle-reverse faults and upright folds verge eastwards, away from the Australian craton, towards the inferred centre of orogenic and magmatic activity. We designed a series of analogue models to test the anomalous vergence in the western Lachlan Fold Belt, particularly the interaction of a stable Australian craton with Tasman Line geometry, interacting with weaker oceanic or transitional lithospheric material. We found consistently that vergence direction in the models was towards the hinterland, not the foreland, as in the western Lachlan Fold Belt, irrespective of the way the model was deformed. Strength gradients between the oceanic and cratonic lithosphere control the deformation patterns. An important result of the models is that they demonstrate that fold belts with different vergences can be generated without the requirement of subducting oceanic lithosphere. D 2003 Elsevier B.V. All rights reserved. Keywords: Oceanic lithosphere; Deformation; Craton
1. Introduction The Tasman Orogenic Zone of eastern Australia comprises Neoproterozoic to Triassic age orogenic sequences that form part of an extensive Gondwanan Palaeozoic orogenic system (e.g. Li and Powell, 1993; Foster et al., 1999). The Tasmanide sequences young from west to east (e.g. Scheibner and Basden, 1996; Foster and Gray, 2000), and are proposed to represent the Cambrian to Early Carboniferous stepwise continental accretion of oceanic or basinal material (e.g. * Tel.: +61-8-9380-7198; fax: +61-8-9380-1037. E-mail address: [email protected] (M. Keep). 0040-1951/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2003.06.002
Foster et al., 1999; Foster and Gray, 2000). Three recognised north-trending tectonic realms that comprise the Tasmanides include the westernmost Kanmantoo (Delamerian) Fold Belt, the easterly-adjacent Lachlan –Thompson Fold Belt and the easternmost New England Fold Belt (e.g. Powell et al., 1990; Foster et al., 1999; VandenBerg, 1999; Foster and Gray, 2000; Cayley et al., 2002) (Fig. 1). The tectonic histories of these three belts include episodic shortening against the Precambrian Australian craton in the Middle Cambrian to Ordovician, the Mid-Devonian to early Carboniferous and the Permo-Triassic (e.g. Powell et al., 1990; Foster et al., 1999; VandenBerg, 1999; Foster and Gray, 2000; Cayley et al., 2002).
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Fig. 1. (a) Schematic location map of the Kanmantoo (Delamerian), Lachlan – Thompson and New England fold belts within the Tasman Orogenic Zone. The geometry of the Tasman Line used in the models is indicated in thick black lines (solid and dashed); box indicates area covered in (b). (b) Location map of the Lachlan Fold Belt showing the locations of the Western, Central and Eastern subprovinces. Modified from Foster and Gray (2000).
The Tasman Line (Fig. 1) separates the Tasman Orogenic Zone from Palaeo-Mesoproterozoic rocks of the Australian craton to the west, and represents the principal suture in eastern Australia (Scheibner and Basden, 1996). The location of the Tasman Line varies according to different authors (e.g. compare Veevers, 1984 with Glen, 1992); however, the lineament shows up clearly on residual Bouguer gravity maps (Murray et al., 1989), and results of the recent SKIPPY project (van der Hilst et al., 1998) show a clear change in shear wave velocity at approximately 140 E, which coincides with the proposed location of the suture. The Delamerian Orogen and eastern parts of the Lachlan Fold Belt (Fig. 1) display structural conformity with the salients and embayments along the Tasman Line (Foster and Gray, 2000).
The Kanmantoo Orogen (Delamerian), immediately adjacent to the Tasman Line, comprises Late Precambrian to mid-Cambrian quartzose clastics deposited in turbiditic facies. Craton-verging thrusts emplace the Cambrian Kanmantoo Group over Neoproterozoic Adelaidean metamorphosed sequences (Foster and Gray, 2000; Jenkins and Sandiford, 1992; Flottman et al., 1994). Various authors propose that exhumation of the Delamerian Orogen provided the source for turbidites within the Cambro-Ordovician sequences of the adjacent Lachlan Fold Belt (Foster and Gray, 2000; Turner et al., 1996; Foster et al., 1998). The easterly-adjacent Lachlan Fold Belt (which merges to form the Thompson Fold Belt to the north) (Fig. 1), contains three subprovinces, the Western, Central and Eastern subprovinces (Fig. 1). Turbidite
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successions comprising quartz-rich sandstones and black shales dominate the Western and Central subprovinces (e.g. Gray, 1997; Foster and Gray, 2000), whilst the Eastern Subprovince contains mafic volcanic, volcaniclastic and carbonate rock in addition to quartz-rich turbidites and black shales (VandenBerg and Stewart, 1992; Foster and Gray, 2000). The Western Subprovince of the Lachlan Fold Belt is dominated by east-vergent thrust belts (away from the craton), in contrast to the westerly vergence in the immediately adjacent Delamerian Orogen to the west. Some authors suggest thin-skinned deformation, with the steep upper crustal thrusts within the Western Subprovince flattening with depth into a common detachment (Fergusson et al., 1986; Glen and VandenBerg, 1987; Gray et al., 1991; Leven et al., 1992; Gray, 1997). Several authors (e.g. Glen and VandenBerg, 1987; Fergusson et al., 1986; Glen, 1992) further suggest that the interpreted steep faults exposed at the surface of the western Tasman Fold Belt represent the listric upturned edges of giant thrust faults, hundreds of kilometres long, which detach the turbiditic fold-belt sediments from their underlying basement (Fig. 2). In the steep high-angle reverse fault slices, normal upright sedimentary contacts occur between the Late Cambrian to earliest Ordovician basal turbidites and the underlying pillow basalts of
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the ocean floor basement (e.g. Glen, 1992; Gray, 1997; Anderson et al., 1998). The hinterland vergence of upright folds and highangle reverse faults within the Western Subprovince, that is, towards the inferred centre of orogenic and magmatic activity and away from the stable craton, contrasts with the normal behaviour in orogenic belts. Thrusts and nappes, especially in the latter stages of orogenic activity, commonly verge away from the internal parts of an orogen towards the external zones or cratons (e.g. Moores and Twiss, 1995; Price and Hatcher, 1983; Hatcher and Williams, 1986), and so the Lachlan Orogen, especially to the west, is unlike classic orogenic systems (Gray and Foster, 1998). There are currently two main groups of theories for the tectonic evolution of the Lachlan Orogenic Belt. Some authors argue that subduction zones control the geometry and vergence of the deformation sequence (e.g. Crook, 1980; Gray and Foster, 1998), whilst other authors support a view that the deformation reflects the influence of Proterozoic basement blocks with episodic deformation in an intraplate tectonic setting, driven by outboard convergence (e.g. Fergusson and Coney, 1992; Vandenberg et al., 2000; Cayley et al., 2002). We designed a series of analogue models specific to the Western Subprovince of the Tasman Fold Belt to test whether Tasman-type structures, especially the
Fig. 2. Cross section of the Southern Lachlan Fold Belt, redrawn from Anderson et al. (1998). Note eastward vergence of the main thrust slices.
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ogy against another, as plate convergence must be the ultimate driving force (VandenBerg, 1999). By necessity all the models involve shortening from a source that could be considered as outboard. Our model geometries do not have additional complexities such as Precambrian or Delamerian crust in the substrate beneath the turbidites (e.g. Scheibner and Basden, 1996; Cayley et al., 2002).
2. Physical modelling We attempted to recreate the basement structures of the Tasman Fold Belt using physical modelling tech-
Fig. 3. Strength profiles of two-, three- and four-layer model lithosphere (dashed), compared to experimentally determined profiles from flow laws (solid lines). BC = brittle crust; DC = ductile crust; BM = brittle mantle; DM = ductile mantle. Modified from Davy and Cobbold (1991), and Keep (2000).
hinterland vergence direction, were reproducible under experimental conditions. The models involved an analogue cratonic block with the geometry of the Tasman Line shortening against weaker material representing oceanic or transitional lithosphere. We did not specifically try to design subduction or intraplate deformation models, we simply shortened one rheol-
Fig. 4. (a) Map view of the model, showing relative locations of cratonic versus oceanic/transitional lithosphere. The outline of the craton mimics the shape of the Tasman Line. (b) Schematic cross section showing the contrast between a two- and three-layer lithosphere. (c) Schematic section showing the contrast between a two- and four-layer lithosphere.
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niques, scaled for both gravitational body forces and surface forces. We modified the techniques of Davy and Cobbold (1991), and Pubellier and Cobbold (1996), which use experimentally determined flow laws to calculate the strength profile of the lithosphere, deforming steadily under simple conditions of uniform strain rate and thermal equilibrium. The lithosphere, modelled as a simplified segmented profile including layers representing brittle crust, brittle mantle, ductile crust and ductile mantle (Kirby, 1983; Ranalli and Murphy, 1987) (Fig. 3), is recreated using analogue materials that are weak enough to flow under their own weight. Sugar solutions with densities of approximately 1.4 g cm 3 and viscosities of around 102 Pa s, act as Newtonian fluids and mimic the behaviour of the asthenosphere. Silicon putties with viscosities of 104 Pa s and densities of approximately 1.2 gm cm 3 exhibit both Newtonian and non-Newtonian behaviour dependent on strain rates, and at room temperature have been used as analogues for the ductile lower mantle and/or the ductile lower crust. For the brittle upper crust we use dry, cohesionless sand, with an internal angle of friction of 31j to 40j, which exhibits Mohr – Coulomb behaviour; mixtures of sand and castor sugar are also used where lower density brittle analogues are required. Assumptions made in choosing appropriate materials include: (1) that material
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properties of each layer do not vary with depth; (2) that brittle layers have Mohr – coulomb behaviour; and (3) that viscosity is constant throughout each ductile layer. These assumptions are generally accepted as being geologically reasonable (e.g. Davy and Cobbold, 1991; Burg, 1994). Materials, layered sequentially in a tank of honey, equilibrate at room temperature prior to deformation. Deformation is achieved through a single advancing perspex plate, attached to a stepper motor, moving at a rate of approximately 2 cm h 1 (1.5 10 6). Models are photographed throughout deformation by time-lapse photography, using both conventional and digital cameras, and time-lapse videos. Models of the Tasman Fold Belt, constructed from the base up through addition of new brittle and/or ductile layers, include a four-layer thick construction representing old, cold cratonic lithosphere, a threelayer construction representing younger, warmer continental lithosphere, and a two-layer construction representing oceanic lithosphere (Fig. 4). The models tested various configurations of ‘‘oceanic’’ lithosphere deforming against an ‘‘Australian craton’’ of Tasman Line geometry, during simple unidirectional compression (Fig. 5). The ‘‘cratonic’’ lithosphere comprised either a three- or four-layer construction, to test whether the thickness of the craton affected the final struc-
Fig. 5. Summary of model types. a and b show four-layer/two-layer collision, with oceanic crust driven towards the craton and vice versa. c and d show the same configurations, except with a shorter length of oceanic lithosphere. e and f show the configurations of two-layer/three-layer collision, with oceanic crust driven towards the craton and vice versa.
42 M. Keep / Tectonophysics 375 (2003) 37–47 Fig. 6. Aerial view of analogue models showing the results of collision between a two-layer (blue) and four-layer (yellow) lithosphere. White lines represent a surface grid to track deformation; pink lines represent thrusts, with barbs on the upper plates. a and b show early (20% shortening) and late (40% shortening) stages of collision with ocean driven towards the craton. c and d show the same model except that the length of the oceanic lithosphere is shorter. Note the reduced number of thrusts in this model. e and f show the results of a two-layer/four-layer collision with the craton driven towards the oceanic lithosphere. In all cases, the majority of the thrusts verge away from the craton. Note in e and f that cake sprinkles were distributed on the surface of the model, in order to test the ability of pattern recognition software in determining displacement paths. The presence of the sprinkles had no effect on the final results, as they were simply passive surface markers. Shortening in all photographs is from left to right.
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tures, and both the three- and four-layer models were run once with oceanic material (two-layer) being pushed against the craton, and again with the craton being pushed against the oceanic lithosphere, to test the effects of convergence directions (Figs. 6 and 7). A fifth model tested a two-layer/four-layer collision (indicating the two-layer lithosphere was pushed against the four-layer lithosphere), but with a shorter length of colliding oceanic crust (Fig. 6), to test for the
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effects on deformation of the volume of oceanic material subjected to shortening. Material parameters follow those of Keep (2000), and all models included a free edge, to allow the model to extrude during deformation (Jones et al., 1997). One potential drawback of the putties used in these experiments is their inability, due to their viscosity, to physically break, and so the situation of one sheet of putty sinking beneath another (i.e. subduction) cannot
Fig. 7. Aerial view of analogue models showing the results of collision between a two-layer (blue) and three-layer (yellow) lithosphere. White lines represent a surface grid, as in Fig. 6, and pink lines represent thrusts, with barbs on the upper plate. a and b show early and late (20% and 40% shortening) of deformation with the oceanic lithosphere pushed towards the craton. c and d show the same experiment with the craton pushed towards the oceanic lithosphere. Shortening in all photographs is from left to right.
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be achieved. Rather, the shortening generated by convergence is accommodated by a significant thickening of the putty, and the development of deep troughs within the model, some of which trap surface sand and take it down to depth.
3. Results In all models, deformation initiated in the weaker, two-layer, ‘‘oceanic crust’’ of the model. Initial structures (Fig. 6a– c) included hinterland-facing scarps, which developed into steep hinterland-verging thrust sheets and upright folds with continued deformation (Fig. 6d –f). In the model with the shorter length of oceanic lithosphere (Fig. 6b, e), there are fewer contractional structures, but almost all verge towards the hinterland. Curvilinear trends in the developing structures (Fig. 7) mimicked the predetermined geometric outline of the Tasman Line, imposed on the model. This is especially clear in Fig. 6e, during the late stages of deformation with a short oceanic lithosphere. As the oceanic material deformed, packages of sand on the surface of the model were incorporated into the cores of developing structures, and taken down to depth in the models. Commonly foreland- and hinterland-verging thrust packages grew towards each other, producing large troughs in the oceanic lithosphere (Fig. 6f). Troughs initiated as low areas between advancing thrust fronts, which eventually were destroyed by continued convergence. During shortening, the locations of the troughs did not change, although with continued shortening the size of the troughs was reduced. Some of these trough areas remained active throughout the deformation sequence, and are preserved in the final model. The main results show that during deformation vergence sense in the deforming oceanic lithosphere is commonly away from the craton, reproducible and independent of whether oceanic lithosphere is pushed towards cratonic, or vice versa (Fig. 6). Identical experiments using a three-layer craton instead of a four-layer craton (Fig. 7) produce similar results. Early structures initiate in the oceanic (thinner) material, verging predominantly towards the hinterland, and the curvilinear structures mimic the imposed shape of the Tasman Line. In these models, foreland-verging thrusts did initiate close to the advancing perspex sheet, and thus may be considered as
edge effects in the model. In both sets of models, vergence direction appears to be controlled solely by the relative strength and mean density differences between the two types of lithosphere. In both the threeand four-layer lithosphere constructions the strength contrasts with the two-layer lithosphere were enough to influence the vergence direction. Similar structures appeared regardless of the relative strength of the craton. We note that we did not find large strike-slip or oblique slip faults nucleated from the promontories in the cratonic lithosphere during the modelling.
4. Discussion Analogue models of deformation in the Tasman Fold Belt show vergence of deformed oceanic lithospheric material verging away from the craton, in contrast to the vergence direction commonly seen in younger orogenic belts (e.g. Price and Hatcher, 1983; Hatcher and Williams, 1986). In the models, the oceanic lithosphere is weak, and the strength gradient between the cratonic and the oceanic lithosphere drives deformation away from the craton. The main results show that during deformation vergence sense in the deforming oceanic lithosphere is commonly away from the craton, as in the western Tasman Fold Belt, and is reproducible and independent of whether oceanic lithosphere is pushed towards cratonic, or vice versa (Figs. 6 and 7). The craton acted as a rigid bulldozer, shortening oceanic material ahead of itself, and broadly mimicked the early stages of structural evolution of the Tasman Fold Belt. Vergence direction seems to be controlled solely by the relative strength and mean density differences between the two types of lithosphere. The thickening and folding of the putty represents a thick-skinned deformation rather than subduction. The inability of the silicon putty to subduct could be considered a disadvantage to the models in that they cannot accurately mimic known present-day tectonic processes. However, the models do show that fold belts with different vergences can be generated without the requirement of subducting oceanic lithosphere. O’Halloran and Rey (1999) indicated using numerical modelling that the mafic substrate in the basal turbidites must be thickening during deformation of the overlying substrate, otherwise the system would be isostatically
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out of balance. These authors therefore argue that oceanic crust was not consumed during subduction, a conclusion in agreement with the results of the analogue models. Similarly, Cayley et al. (2002) also support an intra-oceanic setting rather than subduction, although in their view deformation is thin-skinned, not thick-skinned, as suggested by the modelling. The forces that caused shortening with the Lachlan Fold Belt have been the source of some discussion (e.g. Crawford and Keays, 1978; Foster and Gray, 2000; VandenBerg et al., 2000). Constraints of the modelling technique mean that other than to shorten the model overall, subtle differences between tectonic stimuli for deformation cannot be modelled. In these simple experiments, we selected unidirectional shortening as the means for initiating deformation. Aspects of the Tasman Fold Belt deformation correspond well to deformation in the models. Firstly, major thrusts in the deformation sequence that emplace Cambrian greenstones and sedimentary rocks above Ordovician sequences verge away from the craton (Fig. 2) (Glen, 1992; Anderson et al., 1998). Vergence directions in the analogue models are driven by density contrasts in the modelling materials, which may be analogous to gravitational contrasts in the fold belts. The transition from cratonic to oceanic crust would have generated a gravitational potential during deformation, driving the thrust transport direction away from the craton. Interestingly, Collins and Vernon (1991) noted that the depth to the Moho within the Kanmantoo (Delamerian) Fold Belt is shallower than for the western Lachlan Fold Belt. This contrast between the two adjacent deformational provinces may also have influenced vergence direction. The main difference between the Tasman Fold Belt and the analogue models is that in the latter, the entire brittle crust deforms, without any internal detachments. This limitation derives from the use of a single layer to represent brittle oceanic crust. If a master detachment exists in the Tasman Fold Belt (e.g. Fergusson et al., 1986; Glen and VandenBerg, 1987; Gray et al., 1991; Leven et al., 1992; Gray, 1997), it would separate the upper crustal section from the lower crust – upper mantle section (Fig. 2). However, major shortening structures may cut the granulites (Glen, 1992; Anderson et al., 1998), involving deformation of the entire upper crustal section, and increasing the similarities with the analogue models.
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These analogue models serve to indicate that the vergence directions with the Tasman Fold Belt can be mimicked by analogue models deforming under certain conditions with unidirectional shortening. Implications for deformation in the Tasman Fold Belt include that: (1) Fold belts with different vergences can be generated without the requirement of subducting oceanic lithosphere. (2) Vergence direction during episodic shortening relates directly to strength and gravitational gradients between the craton and the impinging material. (3) The direction and amount of convergence do not affect the vergence directions. (4) Hinterland-verging structures may be produced at considerable distances from the collisional front. (5) The internal parts of the craton remain relatively undeformed. Acknowledgements Chris Powell first suggested the idea of modelling Tasmanide structures in 1998. This paper is dedicated to him. He never did get around to writing up his ideas, but I hope I have come close to conveying what he wanted. Detailed and thoughtful reviews and subsequent comments from Bill Collins and John Miller helped to fill the holes in the story, especially in regard to the geology and tectonics of the Tasmanides, and were much appreciated. My thanks also to Keith Sircombe, whose patience and good will allowed me the time to complete the revisions. References Anderson, J.A.C., Price, R.C., Fleming, P.D., 1998. Structural analysis of metasedimentary enclaves: implications for tectonic evolution and granite petrogenesis in the southern Lachlan Fold Belt, Australia. Geology 26, 119 – 122. Burg, J.-P., 1994. Shortening of analogue models of the continental lithosphere: new hypotheses for the formation of the Tibetan Plateau. Tectonics 13, 475 – 483. Cayley, R.A., Taylor, D.R., Vandenberg, A.H.M., Moore, D.H., 2002. Proterozoic – early Palaeozoic rocks and the tyennan orogeny in central Victoria: the Selwyn block and its tectonic implications. Australian Journal of Earth Sciences 49, 225 – 254.
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M. Keep / Tectonophysics 375 (2003) 37–47 VandenBerg, A.H.M., Willman, C.E., Maher, S., Simons, B.A., Cayley, R.A., Taylor, D.H., Morand, V.J., Moore, D.H., Radojkovic, A., 2000. The Tasman Fold Belt system in Victoria. Geological Survey of Victoria Special Publication. van der Hilst, R.D., Kennett, B.L.N., Shibutani, T., 1998. Upper mantle structure beneath Australia from portable array deploy-
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Physical modelling of deformation in the Tasman Orogenic Zone Myra Keep * Tectonics Special Research Centre, School of Earth and Geographical Sciences, University of Western Australia, 35 Stirling Highway, Nedlands, WA 6009, Australia Received 13 May 2002; received in revised form 19 March 2003; accepted 5 June 2003
Abstract Structural vergence within the Western Subprovince of the Lachlan Fold Belt is towards the hinterland rather than the foreland, in contrast to many well-known orogenic belts. High angle-reverse faults and upright folds verge eastwards, away from the Australian craton, towards the inferred centre of orogenic and magmatic activity. We designed a series of analogue models to test the anomalous vergence in the western Lachlan Fold Belt, particularly the interaction of a stable Australian craton with Tasman Line geometry, interacting with weaker oceanic or transitional lithospheric material. We found consistently that vergence direction in the models was towards the hinterland, not the foreland, as in the western Lachlan Fold Belt, irrespective of the way the model was deformed. Strength gradients between the oceanic and cratonic lithosphere control the deformation patterns. An important result of the models is that they demonstrate that fold belts with different vergences can be generated without the requirement of subducting oceanic lithosphere. D 2003 Elsevier B.V. All rights reserved. Keywords: Oceanic lithosphere; Deformation; Craton
1. Introduction The Tasman Orogenic Zone of eastern Australia comprises Neoproterozoic to Triassic age orogenic sequences that form part of an extensive Gondwanan Palaeozoic orogenic system (e.g. Li and Powell, 1993; Foster et al., 1999). The Tasmanide sequences young from west to east (e.g. Scheibner and Basden, 1996; Foster and Gray, 2000), and are proposed to represent the Cambrian to Early Carboniferous stepwise continental accretion of oceanic or basinal material (e.g. * Tel.: +61-8-9380-7198; fax: +61-8-9380-1037. E-mail address: [email protected] (M. Keep). 0040-1951/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2003.06.002
Foster et al., 1999; Foster and Gray, 2000). Three recognised north-trending tectonic realms that comprise the Tasmanides include the westernmost Kanmantoo (Delamerian) Fold Belt, the easterly-adjacent Lachlan –Thompson Fold Belt and the easternmost New England Fold Belt (e.g. Powell et al., 1990; Foster et al., 1999; VandenBerg, 1999; Foster and Gray, 2000; Cayley et al., 2002) (Fig. 1). The tectonic histories of these three belts include episodic shortening against the Precambrian Australian craton in the Middle Cambrian to Ordovician, the Mid-Devonian to early Carboniferous and the Permo-Triassic (e.g. Powell et al., 1990; Foster et al., 1999; VandenBerg, 1999; Foster and Gray, 2000; Cayley et al., 2002).
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Fig. 1. (a) Schematic location map of the Kanmantoo (Delamerian), Lachlan – Thompson and New England fold belts within the Tasman Orogenic Zone. The geometry of the Tasman Line used in the models is indicated in thick black lines (solid and dashed); box indicates area covered in (b). (b) Location map of the Lachlan Fold Belt showing the locations of the Western, Central and Eastern subprovinces. Modified from Foster and Gray (2000).
The Tasman Line (Fig. 1) separates the Tasman Orogenic Zone from Palaeo-Mesoproterozoic rocks of the Australian craton to the west, and represents the principal suture in eastern Australia (Scheibner and Basden, 1996). The location of the Tasman Line varies according to different authors (e.g. compare Veevers, 1984 with Glen, 1992); however, the lineament shows up clearly on residual Bouguer gravity maps (Murray et al., 1989), and results of the recent SKIPPY project (van der Hilst et al., 1998) show a clear change in shear wave velocity at approximately 140 E, which coincides with the proposed location of the suture. The Delamerian Orogen and eastern parts of the Lachlan Fold Belt (Fig. 1) display structural conformity with the salients and embayments along the Tasman Line (Foster and Gray, 2000).
The Kanmantoo Orogen (Delamerian), immediately adjacent to the Tasman Line, comprises Late Precambrian to mid-Cambrian quartzose clastics deposited in turbiditic facies. Craton-verging thrusts emplace the Cambrian Kanmantoo Group over Neoproterozoic Adelaidean metamorphosed sequences (Foster and Gray, 2000; Jenkins and Sandiford, 1992; Flottman et al., 1994). Various authors propose that exhumation of the Delamerian Orogen provided the source for turbidites within the Cambro-Ordovician sequences of the adjacent Lachlan Fold Belt (Foster and Gray, 2000; Turner et al., 1996; Foster et al., 1998). The easterly-adjacent Lachlan Fold Belt (which merges to form the Thompson Fold Belt to the north) (Fig. 1), contains three subprovinces, the Western, Central and Eastern subprovinces (Fig. 1). Turbidite
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successions comprising quartz-rich sandstones and black shales dominate the Western and Central subprovinces (e.g. Gray, 1997; Foster and Gray, 2000), whilst the Eastern Subprovince contains mafic volcanic, volcaniclastic and carbonate rock in addition to quartz-rich turbidites and black shales (VandenBerg and Stewart, 1992; Foster and Gray, 2000). The Western Subprovince of the Lachlan Fold Belt is dominated by east-vergent thrust belts (away from the craton), in contrast to the westerly vergence in the immediately adjacent Delamerian Orogen to the west. Some authors suggest thin-skinned deformation, with the steep upper crustal thrusts within the Western Subprovince flattening with depth into a common detachment (Fergusson et al., 1986; Glen and VandenBerg, 1987; Gray et al., 1991; Leven et al., 1992; Gray, 1997). Several authors (e.g. Glen and VandenBerg, 1987; Fergusson et al., 1986; Glen, 1992) further suggest that the interpreted steep faults exposed at the surface of the western Tasman Fold Belt represent the listric upturned edges of giant thrust faults, hundreds of kilometres long, which detach the turbiditic fold-belt sediments from their underlying basement (Fig. 2). In the steep high-angle reverse fault slices, normal upright sedimentary contacts occur between the Late Cambrian to earliest Ordovician basal turbidites and the underlying pillow basalts of
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the ocean floor basement (e.g. Glen, 1992; Gray, 1997; Anderson et al., 1998). The hinterland vergence of upright folds and highangle reverse faults within the Western Subprovince, that is, towards the inferred centre of orogenic and magmatic activity and away from the stable craton, contrasts with the normal behaviour in orogenic belts. Thrusts and nappes, especially in the latter stages of orogenic activity, commonly verge away from the internal parts of an orogen towards the external zones or cratons (e.g. Moores and Twiss, 1995; Price and Hatcher, 1983; Hatcher and Williams, 1986), and so the Lachlan Orogen, especially to the west, is unlike classic orogenic systems (Gray and Foster, 1998). There are currently two main groups of theories for the tectonic evolution of the Lachlan Orogenic Belt. Some authors argue that subduction zones control the geometry and vergence of the deformation sequence (e.g. Crook, 1980; Gray and Foster, 1998), whilst other authors support a view that the deformation reflects the influence of Proterozoic basement blocks with episodic deformation in an intraplate tectonic setting, driven by outboard convergence (e.g. Fergusson and Coney, 1992; Vandenberg et al., 2000; Cayley et al., 2002). We designed a series of analogue models specific to the Western Subprovince of the Tasman Fold Belt to test whether Tasman-type structures, especially the
Fig. 2. Cross section of the Southern Lachlan Fold Belt, redrawn from Anderson et al. (1998). Note eastward vergence of the main thrust slices.
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ogy against another, as plate convergence must be the ultimate driving force (VandenBerg, 1999). By necessity all the models involve shortening from a source that could be considered as outboard. Our model geometries do not have additional complexities such as Precambrian or Delamerian crust in the substrate beneath the turbidites (e.g. Scheibner and Basden, 1996; Cayley et al., 2002).
2. Physical modelling We attempted to recreate the basement structures of the Tasman Fold Belt using physical modelling tech-
Fig. 3. Strength profiles of two-, three- and four-layer model lithosphere (dashed), compared to experimentally determined profiles from flow laws (solid lines). BC = brittle crust; DC = ductile crust; BM = brittle mantle; DM = ductile mantle. Modified from Davy and Cobbold (1991), and Keep (2000).
hinterland vergence direction, were reproducible under experimental conditions. The models involved an analogue cratonic block with the geometry of the Tasman Line shortening against weaker material representing oceanic or transitional lithosphere. We did not specifically try to design subduction or intraplate deformation models, we simply shortened one rheol-
Fig. 4. (a) Map view of the model, showing relative locations of cratonic versus oceanic/transitional lithosphere. The outline of the craton mimics the shape of the Tasman Line. (b) Schematic cross section showing the contrast between a two- and three-layer lithosphere. (c) Schematic section showing the contrast between a two- and four-layer lithosphere.
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niques, scaled for both gravitational body forces and surface forces. We modified the techniques of Davy and Cobbold (1991), and Pubellier and Cobbold (1996), which use experimentally determined flow laws to calculate the strength profile of the lithosphere, deforming steadily under simple conditions of uniform strain rate and thermal equilibrium. The lithosphere, modelled as a simplified segmented profile including layers representing brittle crust, brittle mantle, ductile crust and ductile mantle (Kirby, 1983; Ranalli and Murphy, 1987) (Fig. 3), is recreated using analogue materials that are weak enough to flow under their own weight. Sugar solutions with densities of approximately 1.4 g cm 3 and viscosities of around 102 Pa s, act as Newtonian fluids and mimic the behaviour of the asthenosphere. Silicon putties with viscosities of 104 Pa s and densities of approximately 1.2 gm cm 3 exhibit both Newtonian and non-Newtonian behaviour dependent on strain rates, and at room temperature have been used as analogues for the ductile lower mantle and/or the ductile lower crust. For the brittle upper crust we use dry, cohesionless sand, with an internal angle of friction of 31j to 40j, which exhibits Mohr – Coulomb behaviour; mixtures of sand and castor sugar are also used where lower density brittle analogues are required. Assumptions made in choosing appropriate materials include: (1) that material
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properties of each layer do not vary with depth; (2) that brittle layers have Mohr – coulomb behaviour; and (3) that viscosity is constant throughout each ductile layer. These assumptions are generally accepted as being geologically reasonable (e.g. Davy and Cobbold, 1991; Burg, 1994). Materials, layered sequentially in a tank of honey, equilibrate at room temperature prior to deformation. Deformation is achieved through a single advancing perspex plate, attached to a stepper motor, moving at a rate of approximately 2 cm h 1 (1.5 10 6). Models are photographed throughout deformation by time-lapse photography, using both conventional and digital cameras, and time-lapse videos. Models of the Tasman Fold Belt, constructed from the base up through addition of new brittle and/or ductile layers, include a four-layer thick construction representing old, cold cratonic lithosphere, a threelayer construction representing younger, warmer continental lithosphere, and a two-layer construction representing oceanic lithosphere (Fig. 4). The models tested various configurations of ‘‘oceanic’’ lithosphere deforming against an ‘‘Australian craton’’ of Tasman Line geometry, during simple unidirectional compression (Fig. 5). The ‘‘cratonic’’ lithosphere comprised either a three- or four-layer construction, to test whether the thickness of the craton affected the final struc-
Fig. 5. Summary of model types. a and b show four-layer/two-layer collision, with oceanic crust driven towards the craton and vice versa. c and d show the same configurations, except with a shorter length of oceanic lithosphere. e and f show the configurations of two-layer/three-layer collision, with oceanic crust driven towards the craton and vice versa.
42 M. Keep / Tectonophysics 375 (2003) 37–47 Fig. 6. Aerial view of analogue models showing the results of collision between a two-layer (blue) and four-layer (yellow) lithosphere. White lines represent a surface grid to track deformation; pink lines represent thrusts, with barbs on the upper plates. a and b show early (20% shortening) and late (40% shortening) stages of collision with ocean driven towards the craton. c and d show the same model except that the length of the oceanic lithosphere is shorter. Note the reduced number of thrusts in this model. e and f show the results of a two-layer/four-layer collision with the craton driven towards the oceanic lithosphere. In all cases, the majority of the thrusts verge away from the craton. Note in e and f that cake sprinkles were distributed on the surface of the model, in order to test the ability of pattern recognition software in determining displacement paths. The presence of the sprinkles had no effect on the final results, as they were simply passive surface markers. Shortening in all photographs is from left to right.
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tures, and both the three- and four-layer models were run once with oceanic material (two-layer) being pushed against the craton, and again with the craton being pushed against the oceanic lithosphere, to test the effects of convergence directions (Figs. 6 and 7). A fifth model tested a two-layer/four-layer collision (indicating the two-layer lithosphere was pushed against the four-layer lithosphere), but with a shorter length of colliding oceanic crust (Fig. 6), to test for the
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effects on deformation of the volume of oceanic material subjected to shortening. Material parameters follow those of Keep (2000), and all models included a free edge, to allow the model to extrude during deformation (Jones et al., 1997). One potential drawback of the putties used in these experiments is their inability, due to their viscosity, to physically break, and so the situation of one sheet of putty sinking beneath another (i.e. subduction) cannot
Fig. 7. Aerial view of analogue models showing the results of collision between a two-layer (blue) and three-layer (yellow) lithosphere. White lines represent a surface grid, as in Fig. 6, and pink lines represent thrusts, with barbs on the upper plate. a and b show early and late (20% and 40% shortening) of deformation with the oceanic lithosphere pushed towards the craton. c and d show the same experiment with the craton pushed towards the oceanic lithosphere. Shortening in all photographs is from left to right.
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be achieved. Rather, the shortening generated by convergence is accommodated by a significant thickening of the putty, and the development of deep troughs within the model, some of which trap surface sand and take it down to depth.
3. Results In all models, deformation initiated in the weaker, two-layer, ‘‘oceanic crust’’ of the model. Initial structures (Fig. 6a– c) included hinterland-facing scarps, which developed into steep hinterland-verging thrust sheets and upright folds with continued deformation (Fig. 6d –f). In the model with the shorter length of oceanic lithosphere (Fig. 6b, e), there are fewer contractional structures, but almost all verge towards the hinterland. Curvilinear trends in the developing structures (Fig. 7) mimicked the predetermined geometric outline of the Tasman Line, imposed on the model. This is especially clear in Fig. 6e, during the late stages of deformation with a short oceanic lithosphere. As the oceanic material deformed, packages of sand on the surface of the model were incorporated into the cores of developing structures, and taken down to depth in the models. Commonly foreland- and hinterland-verging thrust packages grew towards each other, producing large troughs in the oceanic lithosphere (Fig. 6f). Troughs initiated as low areas between advancing thrust fronts, which eventually were destroyed by continued convergence. During shortening, the locations of the troughs did not change, although with continued shortening the size of the troughs was reduced. Some of these trough areas remained active throughout the deformation sequence, and are preserved in the final model. The main results show that during deformation vergence sense in the deforming oceanic lithosphere is commonly away from the craton, reproducible and independent of whether oceanic lithosphere is pushed towards cratonic, or vice versa (Fig. 6). Identical experiments using a three-layer craton instead of a four-layer craton (Fig. 7) produce similar results. Early structures initiate in the oceanic (thinner) material, verging predominantly towards the hinterland, and the curvilinear structures mimic the imposed shape of the Tasman Line. In these models, foreland-verging thrusts did initiate close to the advancing perspex sheet, and thus may be considered as
edge effects in the model. In both sets of models, vergence direction appears to be controlled solely by the relative strength and mean density differences between the two types of lithosphere. In both the threeand four-layer lithosphere constructions the strength contrasts with the two-layer lithosphere were enough to influence the vergence direction. Similar structures appeared regardless of the relative strength of the craton. We note that we did not find large strike-slip or oblique slip faults nucleated from the promontories in the cratonic lithosphere during the modelling.
4. Discussion Analogue models of deformation in the Tasman Fold Belt show vergence of deformed oceanic lithospheric material verging away from the craton, in contrast to the vergence direction commonly seen in younger orogenic belts (e.g. Price and Hatcher, 1983; Hatcher and Williams, 1986). In the models, the oceanic lithosphere is weak, and the strength gradient between the cratonic and the oceanic lithosphere drives deformation away from the craton. The main results show that during deformation vergence sense in the deforming oceanic lithosphere is commonly away from the craton, as in the western Tasman Fold Belt, and is reproducible and independent of whether oceanic lithosphere is pushed towards cratonic, or vice versa (Figs. 6 and 7). The craton acted as a rigid bulldozer, shortening oceanic material ahead of itself, and broadly mimicked the early stages of structural evolution of the Tasman Fold Belt. Vergence direction seems to be controlled solely by the relative strength and mean density differences between the two types of lithosphere. The thickening and folding of the putty represents a thick-skinned deformation rather than subduction. The inability of the silicon putty to subduct could be considered a disadvantage to the models in that they cannot accurately mimic known present-day tectonic processes. However, the models do show that fold belts with different vergences can be generated without the requirement of subducting oceanic lithosphere. O’Halloran and Rey (1999) indicated using numerical modelling that the mafic substrate in the basal turbidites must be thickening during deformation of the overlying substrate, otherwise the system would be isostatically
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out of balance. These authors therefore argue that oceanic crust was not consumed during subduction, a conclusion in agreement with the results of the analogue models. Similarly, Cayley et al. (2002) also support an intra-oceanic setting rather than subduction, although in their view deformation is thin-skinned, not thick-skinned, as suggested by the modelling. The forces that caused shortening with the Lachlan Fold Belt have been the source of some discussion (e.g. Crawford and Keays, 1978; Foster and Gray, 2000; VandenBerg et al., 2000). Constraints of the modelling technique mean that other than to shorten the model overall, subtle differences between tectonic stimuli for deformation cannot be modelled. In these simple experiments, we selected unidirectional shortening as the means for initiating deformation. Aspects of the Tasman Fold Belt deformation correspond well to deformation in the models. Firstly, major thrusts in the deformation sequence that emplace Cambrian greenstones and sedimentary rocks above Ordovician sequences verge away from the craton (Fig. 2) (Glen, 1992; Anderson et al., 1998). Vergence directions in the analogue models are driven by density contrasts in the modelling materials, which may be analogous to gravitational contrasts in the fold belts. The transition from cratonic to oceanic crust would have generated a gravitational potential during deformation, driving the thrust transport direction away from the craton. Interestingly, Collins and Vernon (1991) noted that the depth to the Moho within the Kanmantoo (Delamerian) Fold Belt is shallower than for the western Lachlan Fold Belt. This contrast between the two adjacent deformational provinces may also have influenced vergence direction. The main difference between the Tasman Fold Belt and the analogue models is that in the latter, the entire brittle crust deforms, without any internal detachments. This limitation derives from the use of a single layer to represent brittle oceanic crust. If a master detachment exists in the Tasman Fold Belt (e.g. Fergusson et al., 1986; Glen and VandenBerg, 1987; Gray et al., 1991; Leven et al., 1992; Gray, 1997), it would separate the upper crustal section from the lower crust – upper mantle section (Fig. 2). However, major shortening structures may cut the granulites (Glen, 1992; Anderson et al., 1998), involving deformation of the entire upper crustal section, and increasing the similarities with the analogue models.
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These analogue models serve to indicate that the vergence directions with the Tasman Fold Belt can be mimicked by analogue models deforming under certain conditions with unidirectional shortening. Implications for deformation in the Tasman Fold Belt include that: (1) Fold belts with different vergences can be generated without the requirement of subducting oceanic lithosphere. (2) Vergence direction during episodic shortening relates directly to strength and gravitational gradients between the craton and the impinging material. (3) The direction and amount of convergence do not affect the vergence directions. (4) Hinterland-verging structures may be produced at considerable distances from the collisional front. (5) The internal parts of the craton remain relatively undeformed. Acknowledgements Chris Powell first suggested the idea of modelling Tasmanide structures in 1998. This paper is dedicated to him. He never did get around to writing up his ideas, but I hope I have come close to conveying what he wanted. Detailed and thoughtful reviews and subsequent comments from Bill Collins and John Miller helped to fill the holes in the story, especially in regard to the geology and tectonics of the Tasmanides, and were much appreciated. My thanks also to Keith Sircombe, whose patience and good will allowed me the time to complete the revisions. References Anderson, J.A.C., Price, R.C., Fleming, P.D., 1998. Structural analysis of metasedimentary enclaves: implications for tectonic evolution and granite petrogenesis in the southern Lachlan Fold Belt, Australia. Geology 26, 119 – 122. Burg, J.-P., 1994. Shortening of analogue models of the continental lithosphere: new hypotheses for the formation of the Tibetan Plateau. Tectonics 13, 475 – 483. Cayley, R.A., Taylor, D.R., Vandenberg, A.H.M., Moore, D.H., 2002. Proterozoic – early Palaeozoic rocks and the tyennan orogeny in central Victoria: the Selwyn block and its tectonic implications. Australian Journal of Earth Sciences 49, 225 – 254.
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