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Ductile-brittle transition structures in the basal shear zone of a rhyolite lava flow, eastern Australia
ELSEVIER
Journal of Volcanologyand GeothermalResearch72 (19%) 217-223
Ductile-brittle
transition structures iil the basal shear zone of a rhyolite lava flow, eastern Australia John V. Smith *
Centre for Coastal Management, Southern Cross Uniuersity, P.O. Box 157, Lismore, N.S.W. 2480, Australia
Received 16 October 1995;accepted21 February 1996
Abstract Structuraland textural evidence indicates that the basal breccia of the Tertiary Minyon Falls Rhyolite of Tweed Volcano, eastern Australia, formed mainly by fragmentation within the basal shear zone of the lava flow. Cooling at the flow base led to lava behaviour passing through the ductile-brittle transition while subjected to progressive deformation. The intact lavas of the basal zone record intense ductile shearing in multiple folds, rotated phenocrysts, strong alignment of crystallites and micro-folding of these crystal alignments. Minor brittle structures, including faults on fold limbs and microfaults, occur in the intact lava immediately above the basal breccia. These structures represent incipient brecciation and complete brecciation has occurred by intense faulting of previously intact lava. Brittle deformation migrated upward into overlying intact lava as cooling occurred. This process is distinctly different from overriding and incorporation of surface breccias which is commonly invoked as the primary process of basal breccia formation.
1. Introduction
Breccias within the basal zone of a lava flow are most commonly interpreted as surface breccias which have been overridden at the front of a flow (e.g., Fink, 1980, 1983; Hausback, 1987; Stasiuk et al., 1993) or incorporated internally through crevasses (Sparks et al., 1993). However, intact basal lavas can brecciate during flow if cooling causes lava to change from ductile to brittle behaviour. The conditions of temperature, pressure and strain rate under which such a transition occurs has been studied experimentally in sedimentary and metamorphic rocks (e.g., Paterson, 1958; Heard, 1960). Field evidence sug-
l
E-mail: [email protected].
gests a similar transition occurs during simultaneous cooling and flow of volcanic rocks; for example, Dadd (1992) suggested that lenticular breccias in the basal zone of rhyolite lavas of the Comerong Volcanics in southeastern Australia and the Ngongotaha lava dome, New Zealand may have formed from banded lavas by stretching and mechanical brecciation within the basal zone due to high shear strains. A transition from ductile to brittle processes is indicated by structures such as folds truncated on one or both limbs as have been described in rhyolite flows (Christiansen and Lipman, 1966). The operation of both external and internal processes in forming basal breccias has been recognised by Manley and Fink (19871, Bonnichsen and Kauffman (1987) and Laznicka (1988, p. 469). Differentiating between overridden breccias and breccias formed by fragmentation of rocks within the basal
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zone is difficult as both processes produce a deformed breccia and in both cases heat can modify the breccia fabric. Mayo (1944) recognised remelting of overridden breccias and suggested that where sufficient heat is available these breccias could be stretched out into the appearance of banded lavas. The complex mechanical and thermal processes occurring in the basal zone are concealed by overlying lavas and can only be directly studied in ancient flows dissected by erosion. If internal brecciation has occurred in the basal zone it should be possible to recognise evidence at the boundary between the intact and brecciated lava in the basal zone. In the basal zone of the Minyon Falls Rhyolite (Smith and Houston, 1994, 1995) structural relationships indicate that breccia was formed by ductilebrittle transition within the basal shear zone of the lava flow. Ductile structures in the intact lava including folds, foliation banding and crystal alignment will be described and compared to similar structures within fragments of the breccia. The distribution of brittle structures, principally faults, supports propagation of these structures upward into the intact lava.
Fig. 1. Location map of the Minyon Falls Rhyolite within the Nimbin Rhyolite. Stereographs show orientation of banding in the lavas of the basal obsidian at selected locations. Inferred vent is marked V.
2. Geological setting The Minyon Falls Rhyolite is a large subaerial rhyolite dome which forms the southernmost extent of the Nimbin Rhyolite complex of Tertiary rhyolite lavas (Crook and McGarity, 1955; Smith and Houston, 1995) which erupted onto the southern flank of the basaltic Tweed Volcano (Stevens et al., 1989). The dome is in excess of 100 m thick and gravitation forces influenced the geometry of folds in lava bands (Smith and Houston, 1994). The thickness of the dome is also expected to have influenced the formation of the basal shear zone of the lava. The basal shear zone comprises brecciated and intact banded obsidian whereas the main mass of the flow is characterised by banded crystalline rhyolite with spherulitic devitrification textures. Detailed description of basal zone structures were obtained from three main localities: the base of Minyon Falls within 200 m of the inferred vent (Site A); Fox Road quarry 2 km south of the inferred vent (Site B); and Nightcap Range Road quarry 4 km southwest of the inferred vent (Site C) (Fig. Il.
Based on thickening of the obsidian carapace and topographic constraints Crook and McGarity (1955) and Smith and Houston (1995) considered the pre-
Table I Major element composition of a sampIe of Minyon Falls Rhyolite @hlggan, 1974) SiO, TiO, A’,03 Fe,O, Fe0 MnO MgO CaO Na,O ‘GO HzO+ H,OPZOS Total
74.55 0.15 13.12 0.82 0.63 0.02 0.10 0.43 2.84 5.78 0.77 0.12 0.03 99.36
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sent extent of the lava to approximate the original dimensions of the flow except for erosion of the eastern part of the flow and erosion of canyons in the southern part.
219
An analysis of major elements of a sample from the Minyon Falls Rhyolite by Duggan (1974) is shown in Table 1. Duggan (1974) estimated an extrusion temperature between 920 and 1040°C.
Fig. 2. (a) Basal obsidian at the base of Minyon Falls between lower pyroclastic deposits and upperrhyolite lavas (the vertical white bar is 3 m long and marks the basal obsidian, Site A). (b) Multiply folded banded lavas with axial plane cleavage detined by alignment of crystallites(field of view 4 cm, Site A). (c) Microfoldingarounda phenocrystshowing sense of shear flow (field of view 2 mm, Site B). (d) Crenulationof banding due to intense ductile flowage (field of view 1 mm, Site C).
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3. Basal zone fabric
Near the vent (Site A) the wide-spaced jointing of the rhyolite is distinct from the close-spaced jointing of the basal obsidian and the pyroclastic deposits
below (Fig. 2a). The basal shear zone is characterised by a 3 m thickness of obsidian with a thin basal breccia (0.5 m) overlain by intact obsidian lava (2.5 m) with banding at a low angle to the base of the flow with multiple isoclinal recumbent folds
Fig. 3. (a) L.ocalised faulting in intact banded lava isolating a fold binge (field of view 40 cm, Site C). (b) Field view of strongly developed brecciation (field of view 30 cm, Site C). (c) Microscopic view of strongly developed brecciation with distinct fragment margins, within the top 1 m of basal breccia (field of view 2 cm, Site C). (d) Microscopic view of strongly developed bre-cciation with indistinct fragment margins, 2 m below the top of the basal breccia (field of view 2 cm, Site C).
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(Fig. 2b). The b ase of the flow undulates and the banding maintains near-parallelism to the base. Crystall&s are strongly aligned such that the obsidian splits along a mineral cleavage which is axial-planar to the folds. Midway between the vent and the flow periphery (Site B) the basal shear zone is characterised by a thickness of obsidian exceeding 3 m with banding dipping 22” toward the east-northeast (Fig. 1). The base of the flow undulates by at least 10 m. The obsidian is banded and shows evidence of ductile flow, such as rotated phenocrysts (Fig. 2c) but the bands are discontinuous as a result of abundant microscopic faults cutting the banding at a low angle. Near the southern periphery of the flow (Site C) basal obsidian is in excess of 20 m thick (Fig. 1). Detailed study has been made of an exposure of the basal shear zone in an obsidian quarry used for road construction. The quarry face shows a transition from intact obsidian lava with fine, gently dipping banding, down to highly brecciated lava at the base, with the transition from one lava type to the other over an interval of about 0.5 m. The intact and brecciated lavas at Site C will be described in detail below. 3.1. Basal intact banded lava The banding in the lava dips north at 15” consistent with the site being the base of a southward flowing lava. The lavas show abundant evidence of extreme ductile deformation of the lava during flow. Thinning of the bands is evidenced by greater width of the bands where they contain ridged inclusions such as crystals. Ductile shear flow is evidenced by microfolds adjacent to crystals similar to those described in other obsidian lavas (Vernon, 1987). Meso-scale folds were observed in the intact lava. These recumbent folds had mainly E-W-trending hingelines and N-dipping hinge surfaces supporting a N-S flow direction. Some bands were observed to contain a micro-crenulation (Fig. 2d), in which the foliation defined by the alignment of crystallites had been multiply deformed. Some faults were observed in the intact lava, particularly in association with folds (Fig. 3a). Typically, the faults show truncation of banding on one
221
side of the fault only, the other side of the fault having banding parallel to the fault plane. Faults are clearly seen where banding is truncated, but the faults tend to become parallel to banding and thus more difficult to recognise. 3.2. Basal breccia The breccia contains a strong alignment of elon-
gated fragments (Fig. 3a and b) parallel to the banding foliation in the intact lava above. Fragments range in size from 15 cm down to microscopic fragments which are indistinguishable from the glassy matrix in the field (Fig. 3c and d). In profile sections (N-S), fragments are of rhomboidal shape typically with one pair of faces parallel to banding and one pair of faces oblique to banding. In cross section (E-W), the fragments are equant in shape indicating a prolate or “cigar-like” morphology in three dimensions. Within the top metre of the breccia, fragments are angular to subrounded and have distinct margins (Fig. 3c) whereas, two metres below the top of the breccia most fragments are rounded and the boundary between fragments and matrix is indistinct (Fig. 3d). The fabric of the lower breccias indicates some ductile deformation possibly associated with minor remelting. The fragments show internal ductile textures similar to those in the intact lava above. Orientation of the banding within clasts is highly variable (Fig. 3d) although in most cases the banding is approximately parallel to the long axis of the elongated fragments.
Fig. 4. Section through breccia (Site C, north to the right) showing fragment-bounding and discontinuous faults. Note the bending of the bands at the termination of the fault, upper right (field of view 8 cm).
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The disorientation of banding probably occurred by the tilting of fragments as they moved along fault planes in the developing breccia. The initiation of faults on fold limbs, as observed in the intact lava, may also contribute to the disorientation of banded fragments. Where displacement has been small it can be seen that fragment boundaries have formed by faulting. In some fragments in which fault terminations are found, the banding is seen to be bent in advance of the fault (Fig. 4). Thes e ob servations indicate that faulting is the main mechanism of brecciation in these rocks.
4. Discussion The basal shear zone of the Minyon Falls Rhyolite, like other silicic lava flows, consists of basal breccia and low-angle banded lava (Fig. 5). This zone accommodates most of the shearing, while the main mass of lava is translated above. The top of the basal shear zone migrates upward into the overlying lava as the flow advances (Manley and Fink, 1987). This upward propagation of the basal shear zone implies that the base becomes cooler and more viscous and so shear flow progressively transfers to hotter and less viscous lava above. The interface between breccia and intact lava apparently migrates in a similar way. When the viscosity becomes so high that brittle behaviour is induced, breccia forms and faults continue to propagate upward into the intact lava (Fig. 6).
a
Fig. 6. Schematic representation of the formation of a basal breccia by ductile-brittle transition of basal lavas during progressive deformation.
Formation of the basal breccia of the Minyon Falls Rhyolite by ductile-brittle transition is supported by evidence that brecciation occurred by faulting rather than extensional cracking. The propagation of faults into the intact lava records the way the ductile-brittle transition was migrating upward into the flowing lava above. This implies that brecciation occurred mainly due to mechanical stresses rather than thermal stresses. This is illustrated by the presence of ductile bending at the termination of a fault (Fig. 4). Overridden surface breccias typically comprise equant fragments (e.g., Bonnichsen and Kauffman, 1987). Elongation of fragments would have to occur by ductile flattening during overriding. It is expected that such flattening would result in more irregularly shaped fragments and is not compatible with the through-going planar surfaces seen to be bounding fragments in this study.
A
S low-angle 3 banded lava 2 4
Fig. 5. Schematic representation of the elements of the basal shear zone of a silicic lava flow.
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5. Conclusions
of the Minyon Falls district, N.S.W. Proc. R.
SOC. N.S.W.,
89:
212-218.
Structures and textures of intact lavas of the basal shear zone of the Minyon Falls Rhyolite indicate accommodation of very large shear strains. The basal breccia developed as upward directed cooling changed lava behaviour from ductile to brittle while the basal zone was subjected to dynamic loading from overlying lavas. This interpretation is supported by fault-fragmentation which decreases in intensity upward into the intact lavas and by the similarity of textures in fragments and intact lavas. This ductilebrittle transition mechanism formed the entire basal breccia zone near the lava vent and the upper part of the of the basal breccia near the periphery of the flow. The lower parts of the peripheral basal breccia zone are not exposed and it is possible that the observed ductile-brittle transition breccias may overlie and grade into overridden crumble breccias.
Acknowledgements Brian Marshall of University of Technology, Sydney is thanked for helpful discussions on structural interpretations of volcanic rock fabrics. The paper benefited from reviews by Bill Bonnichsen and Curtis Manley.
References Bonnichsen, B. and Kauffman, D.F., 1987. Physical features of rhyolite lava flows in the Snake River Plain volcanic province, southwestern Idaho. In: J.H. Fink (Editor), The Emplacement of Silicic Domes and Lava Flows. Geol. Sot. Am., Spec. Pap., 212: 119-145. Christiansen, R.L. and Lipman, P.W., 1966. Emplacement and thermal history of a rhyolite flow near Fortymile Canyon, southern Nevada. Geol. Sot. Am. Bull., 77: 671-684. Crook, K.A.W. and McGarity, J.W., 1955. Volcanic stratigmphy
Dadd, K.A., 1992. Structures within large volume rhyolite lava flows of the Devonian Comerong Volcanics, southeastern Australia, and the Pleistocene Ngongotaha lava dome, New Zealand. J. Volcanol. Geotherm. Res., 54: 33-51. Duggan, M.B., 1974. The mineralogy and petrology of the soutlem portion of the Tweed shield volcano, northeastern New South Wales. Ph.D. Thesis, Univ. New England (unpubl.). Fink, J., 1980. Surface folding and viscosity of rhyolite flows. Geology, 8: 250-254. Fink, J., 1983. Structure and emplacement of a rhyolitic obsidian flow: Little Glass Mountain, Medicine Lake Highland, northem California. Geol. Sot. Am. Bull., 94: 362-380. Hausback, B.P., 1987. An extensive, hot, vapour-charged rhyodacite flow, Baja California, Mexico. In: J.H. Fink (Editor), The Emplacement of Silicic Domes and Lava Flows. Geol. Sot. Am., Spec. Pap., 212: 11l-l 18. Heard, H.C., 1960. Transition from brittle fracture to ductile flow in Solenhofen limestone as a function of temperature, confming pressure. and interstitial fluid pressure. In: D.T. Griggs and J. Handin (Editors), Rock Deformation. Geol. Sot. Am. Mem., 79: 193-226. Laznicka, P., 1988. Breccias and Coarse Fragmentites: Petrology, Environments, Associations and Ores. (Developments in Economic Geology, 25.) Elsevier, Amsterdam. Manley, C.R. and Fink, J.H., 1987. Internal structures of rhyolite flows as revealed by research drilling. Geology, 15: 549-552. Mayo, E.B., 1944. Rhyolite near Big Pine California. Geol. Sot. Am. Bull., 55: 599-620. Paterson, M.S., 1958. Experimental deformation and faulting in Wombeyan marble. Geol. SIX. Am. Bull., 69: 465-476. Smith, J.V. and Houston, E.C., 1994. Folds produced by gravity spreading of a banded rhyolite lava flow. J. Volcanol. Geotherm. Res., 63: 89-94. Smith, J.V. and Houston, E.C., 1995. Structure of lava flows of the Nimbin Rhyolite, northeast New South Wales. Aust. J. Earth Sci., 42: 69-74. Sparks, R.S.J., Stasiuk, M.V., Gardeweg, M. and Swanson, D.A., 1993. Welded breccias in andesite lavas. J. Geol. Sot. London, 150: 897-902. Stasiuk, M.V., Jaupart, C. and Sparks, R.S.J., 1993. Influence of cooling on lava-flow dynamics. Geology, 21: 335-338. Stevens, N.C., Knutson, J., Ewart, A. and Duggan, M.B., 1989. Tweed Volcano. In: R.W. Johnson (Editor), Intraplate Volcanism. Cambridge Univ. Press, Cambridge, pp. 114- 115. Vernon, R.H., 1987. A microstructural indicator of shear sense in volcanic rocks and its relationship to porphyroblast rotation in metamorphic rocks. J. Geol., 95: 127-133.
Journal of Volcanologyand GeothermalResearch72 (19%) 217-223
Ductile-brittle
transition structures iil the basal shear zone of a rhyolite lava flow, eastern Australia John V. Smith *
Centre for Coastal Management, Southern Cross Uniuersity, P.O. Box 157, Lismore, N.S.W. 2480, Australia
Received 16 October 1995;accepted21 February 1996
Abstract Structuraland textural evidence indicates that the basal breccia of the Tertiary Minyon Falls Rhyolite of Tweed Volcano, eastern Australia, formed mainly by fragmentation within the basal shear zone of the lava flow. Cooling at the flow base led to lava behaviour passing through the ductile-brittle transition while subjected to progressive deformation. The intact lavas of the basal zone record intense ductile shearing in multiple folds, rotated phenocrysts, strong alignment of crystallites and micro-folding of these crystal alignments. Minor brittle structures, including faults on fold limbs and microfaults, occur in the intact lava immediately above the basal breccia. These structures represent incipient brecciation and complete brecciation has occurred by intense faulting of previously intact lava. Brittle deformation migrated upward into overlying intact lava as cooling occurred. This process is distinctly different from overriding and incorporation of surface breccias which is commonly invoked as the primary process of basal breccia formation.
1. Introduction
Breccias within the basal zone of a lava flow are most commonly interpreted as surface breccias which have been overridden at the front of a flow (e.g., Fink, 1980, 1983; Hausback, 1987; Stasiuk et al., 1993) or incorporated internally through crevasses (Sparks et al., 1993). However, intact basal lavas can brecciate during flow if cooling causes lava to change from ductile to brittle behaviour. The conditions of temperature, pressure and strain rate under which such a transition occurs has been studied experimentally in sedimentary and metamorphic rocks (e.g., Paterson, 1958; Heard, 1960). Field evidence sug-
l
E-mail: [email protected].
gests a similar transition occurs during simultaneous cooling and flow of volcanic rocks; for example, Dadd (1992) suggested that lenticular breccias in the basal zone of rhyolite lavas of the Comerong Volcanics in southeastern Australia and the Ngongotaha lava dome, New Zealand may have formed from banded lavas by stretching and mechanical brecciation within the basal zone due to high shear strains. A transition from ductile to brittle processes is indicated by structures such as folds truncated on one or both limbs as have been described in rhyolite flows (Christiansen and Lipman, 1966). The operation of both external and internal processes in forming basal breccias has been recognised by Manley and Fink (19871, Bonnichsen and Kauffman (1987) and Laznicka (1988, p. 469). Differentiating between overridden breccias and breccias formed by fragmentation of rocks within the basal
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zone is difficult as both processes produce a deformed breccia and in both cases heat can modify the breccia fabric. Mayo (1944) recognised remelting of overridden breccias and suggested that where sufficient heat is available these breccias could be stretched out into the appearance of banded lavas. The complex mechanical and thermal processes occurring in the basal zone are concealed by overlying lavas and can only be directly studied in ancient flows dissected by erosion. If internal brecciation has occurred in the basal zone it should be possible to recognise evidence at the boundary between the intact and brecciated lava in the basal zone. In the basal zone of the Minyon Falls Rhyolite (Smith and Houston, 1994, 1995) structural relationships indicate that breccia was formed by ductilebrittle transition within the basal shear zone of the lava flow. Ductile structures in the intact lava including folds, foliation banding and crystal alignment will be described and compared to similar structures within fragments of the breccia. The distribution of brittle structures, principally faults, supports propagation of these structures upward into the intact lava.
Fig. 1. Location map of the Minyon Falls Rhyolite within the Nimbin Rhyolite. Stereographs show orientation of banding in the lavas of the basal obsidian at selected locations. Inferred vent is marked V.
2. Geological setting The Minyon Falls Rhyolite is a large subaerial rhyolite dome which forms the southernmost extent of the Nimbin Rhyolite complex of Tertiary rhyolite lavas (Crook and McGarity, 1955; Smith and Houston, 1995) which erupted onto the southern flank of the basaltic Tweed Volcano (Stevens et al., 1989). The dome is in excess of 100 m thick and gravitation forces influenced the geometry of folds in lava bands (Smith and Houston, 1994). The thickness of the dome is also expected to have influenced the formation of the basal shear zone of the lava. The basal shear zone comprises brecciated and intact banded obsidian whereas the main mass of the flow is characterised by banded crystalline rhyolite with spherulitic devitrification textures. Detailed description of basal zone structures were obtained from three main localities: the base of Minyon Falls within 200 m of the inferred vent (Site A); Fox Road quarry 2 km south of the inferred vent (Site B); and Nightcap Range Road quarry 4 km southwest of the inferred vent (Site C) (Fig. Il.
Based on thickening of the obsidian carapace and topographic constraints Crook and McGarity (1955) and Smith and Houston (1995) considered the pre-
Table I Major element composition of a sampIe of Minyon Falls Rhyolite @hlggan, 1974) SiO, TiO, A’,03 Fe,O, Fe0 MnO MgO CaO Na,O ‘GO HzO+ H,OPZOS Total
74.55 0.15 13.12 0.82 0.63 0.02 0.10 0.43 2.84 5.78 0.77 0.12 0.03 99.36
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of Volcanology and Geothermal Research 72 (1996) 217-223
sent extent of the lava to approximate the original dimensions of the flow except for erosion of the eastern part of the flow and erosion of canyons in the southern part.
219
An analysis of major elements of a sample from the Minyon Falls Rhyolite by Duggan (1974) is shown in Table 1. Duggan (1974) estimated an extrusion temperature between 920 and 1040°C.
Fig. 2. (a) Basal obsidian at the base of Minyon Falls between lower pyroclastic deposits and upperrhyolite lavas (the vertical white bar is 3 m long and marks the basal obsidian, Site A). (b) Multiply folded banded lavas with axial plane cleavage detined by alignment of crystallites(field of view 4 cm, Site A). (c) Microfoldingarounda phenocrystshowing sense of shear flow (field of view 2 mm, Site B). (d) Crenulationof banding due to intense ductile flowage (field of view 1 mm, Site C).
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3. Basal zone fabric
Near the vent (Site A) the wide-spaced jointing of the rhyolite is distinct from the close-spaced jointing of the basal obsidian and the pyroclastic deposits
below (Fig. 2a). The basal shear zone is characterised by a 3 m thickness of obsidian with a thin basal breccia (0.5 m) overlain by intact obsidian lava (2.5 m) with banding at a low angle to the base of the flow with multiple isoclinal recumbent folds
Fig. 3. (a) L.ocalised faulting in intact banded lava isolating a fold binge (field of view 40 cm, Site C). (b) Field view of strongly developed brecciation (field of view 30 cm, Site C). (c) Microscopic view of strongly developed brecciation with distinct fragment margins, within the top 1 m of basal breccia (field of view 2 cm, Site C). (d) Microscopic view of strongly developed bre-cciation with indistinct fragment margins, 2 m below the top of the basal breccia (field of view 2 cm, Site C).
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(Fig. 2b). The b ase of the flow undulates and the banding maintains near-parallelism to the base. Crystall&s are strongly aligned such that the obsidian splits along a mineral cleavage which is axial-planar to the folds. Midway between the vent and the flow periphery (Site B) the basal shear zone is characterised by a thickness of obsidian exceeding 3 m with banding dipping 22” toward the east-northeast (Fig. 1). The base of the flow undulates by at least 10 m. The obsidian is banded and shows evidence of ductile flow, such as rotated phenocrysts (Fig. 2c) but the bands are discontinuous as a result of abundant microscopic faults cutting the banding at a low angle. Near the southern periphery of the flow (Site C) basal obsidian is in excess of 20 m thick (Fig. 1). Detailed study has been made of an exposure of the basal shear zone in an obsidian quarry used for road construction. The quarry face shows a transition from intact obsidian lava with fine, gently dipping banding, down to highly brecciated lava at the base, with the transition from one lava type to the other over an interval of about 0.5 m. The intact and brecciated lavas at Site C will be described in detail below. 3.1. Basal intact banded lava The banding in the lava dips north at 15” consistent with the site being the base of a southward flowing lava. The lavas show abundant evidence of extreme ductile deformation of the lava during flow. Thinning of the bands is evidenced by greater width of the bands where they contain ridged inclusions such as crystals. Ductile shear flow is evidenced by microfolds adjacent to crystals similar to those described in other obsidian lavas (Vernon, 1987). Meso-scale folds were observed in the intact lava. These recumbent folds had mainly E-W-trending hingelines and N-dipping hinge surfaces supporting a N-S flow direction. Some bands were observed to contain a micro-crenulation (Fig. 2d), in which the foliation defined by the alignment of crystallites had been multiply deformed. Some faults were observed in the intact lava, particularly in association with folds (Fig. 3a). Typically, the faults show truncation of banding on one
221
side of the fault only, the other side of the fault having banding parallel to the fault plane. Faults are clearly seen where banding is truncated, but the faults tend to become parallel to banding and thus more difficult to recognise. 3.2. Basal breccia The breccia contains a strong alignment of elon-
gated fragments (Fig. 3a and b) parallel to the banding foliation in the intact lava above. Fragments range in size from 15 cm down to microscopic fragments which are indistinguishable from the glassy matrix in the field (Fig. 3c and d). In profile sections (N-S), fragments are of rhomboidal shape typically with one pair of faces parallel to banding and one pair of faces oblique to banding. In cross section (E-W), the fragments are equant in shape indicating a prolate or “cigar-like” morphology in three dimensions. Within the top metre of the breccia, fragments are angular to subrounded and have distinct margins (Fig. 3c) whereas, two metres below the top of the breccia most fragments are rounded and the boundary between fragments and matrix is indistinct (Fig. 3d). The fabric of the lower breccias indicates some ductile deformation possibly associated with minor remelting. The fragments show internal ductile textures similar to those in the intact lava above. Orientation of the banding within clasts is highly variable (Fig. 3d) although in most cases the banding is approximately parallel to the long axis of the elongated fragments.
Fig. 4. Section through breccia (Site C, north to the right) showing fragment-bounding and discontinuous faults. Note the bending of the bands at the termination of the fault, upper right (field of view 8 cm).
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The disorientation of banding probably occurred by the tilting of fragments as they moved along fault planes in the developing breccia. The initiation of faults on fold limbs, as observed in the intact lava, may also contribute to the disorientation of banded fragments. Where displacement has been small it can be seen that fragment boundaries have formed by faulting. In some fragments in which fault terminations are found, the banding is seen to be bent in advance of the fault (Fig. 4). Thes e ob servations indicate that faulting is the main mechanism of brecciation in these rocks.
4. Discussion The basal shear zone of the Minyon Falls Rhyolite, like other silicic lava flows, consists of basal breccia and low-angle banded lava (Fig. 5). This zone accommodates most of the shearing, while the main mass of lava is translated above. The top of the basal shear zone migrates upward into the overlying lava as the flow advances (Manley and Fink, 1987). This upward propagation of the basal shear zone implies that the base becomes cooler and more viscous and so shear flow progressively transfers to hotter and less viscous lava above. The interface between breccia and intact lava apparently migrates in a similar way. When the viscosity becomes so high that brittle behaviour is induced, breccia forms and faults continue to propagate upward into the intact lava (Fig. 6).
a
Fig. 6. Schematic representation of the formation of a basal breccia by ductile-brittle transition of basal lavas during progressive deformation.
Formation of the basal breccia of the Minyon Falls Rhyolite by ductile-brittle transition is supported by evidence that brecciation occurred by faulting rather than extensional cracking. The propagation of faults into the intact lava records the way the ductile-brittle transition was migrating upward into the flowing lava above. This implies that brecciation occurred mainly due to mechanical stresses rather than thermal stresses. This is illustrated by the presence of ductile bending at the termination of a fault (Fig. 4). Overridden surface breccias typically comprise equant fragments (e.g., Bonnichsen and Kauffman, 1987). Elongation of fragments would have to occur by ductile flattening during overriding. It is expected that such flattening would result in more irregularly shaped fragments and is not compatible with the through-going planar surfaces seen to be bounding fragments in this study.
A
S low-angle 3 banded lava 2 4
Fig. 5. Schematic representation of the elements of the basal shear zone of a silicic lava flow.
J.V. Smith/Journal
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of Volcanology and Geothemuzl Research 72 (1996) 217-223
5. Conclusions
of the Minyon Falls district, N.S.W. Proc. R.
SOC. N.S.W.,
89:
212-218.
Structures and textures of intact lavas of the basal shear zone of the Minyon Falls Rhyolite indicate accommodation of very large shear strains. The basal breccia developed as upward directed cooling changed lava behaviour from ductile to brittle while the basal zone was subjected to dynamic loading from overlying lavas. This interpretation is supported by fault-fragmentation which decreases in intensity upward into the intact lavas and by the similarity of textures in fragments and intact lavas. This ductilebrittle transition mechanism formed the entire basal breccia zone near the lava vent and the upper part of the of the basal breccia near the periphery of the flow. The lower parts of the peripheral basal breccia zone are not exposed and it is possible that the observed ductile-brittle transition breccias may overlie and grade into overridden crumble breccias.
Acknowledgements Brian Marshall of University of Technology, Sydney is thanked for helpful discussions on structural interpretations of volcanic rock fabrics. The paper benefited from reviews by Bill Bonnichsen and Curtis Manley.
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