The chemistry and origin of melange megablocks at Bald Hill, Southland.
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Bald Hill lies at the junction of the north-south trending Livingstone and the northeast-southwest trending Moonlight Fault systems. It comprises the the width of the Dun Mountain ophiolite belt in this area which has been proposed to represent the suture between the Permian to Triassic Maitai terrane to the west and the inferred Permian to early Triassic Caples terrane to the east. Two subparallel strands of the Livingstone Fault system cross Bald Hill separating three north-south striking coherant units of dominantly semischistose and schistose metasediments. These units are informally called from west to east: the West Bald Hill semischist megablock, the East Bald Hill green schist megablock, and the East Bald Hill semischist. Detailed mapping shows the two western sedimentary units are megablocks within the Dun Mountain ophiolite belt, therefore, the term oromictite is proposed to describe the melange in the Bald Hill area. Discrete mappable packets of different melange types would be termed terramictites or megamictites depending on component block size. Two more coherent sedimentary megablocks to the north and south at West Bum and Lincoln Hill respectively show the oromictite extends beyond Bald Hill.
Metamorphic grade in all three blocks at Bald Hill has reached lawsonite-albite-chlorite facies, however, the presence of authigeilic blue amphibole in the green schist megablock shows this block has undergone higher metamorphic pressures than the other units at Bald Hill.
The West Bald Hill is a deep-water marine sequence up to 1.75km thick of dominantly structurless black argillites and siltstones with intercalated greywackes indicative of distal turbidite depositional mechanisms. Tholeiitic pillow lavas and a hypabyssal sill, constitute the only mafics observed. Minor cherty red argillites, green phyllites, and green schist are also present. The only fossils observed were abundant sponge spicules which are not suitable for dating purposes. The East Bald Hill green schist megablock is composed of up to 2.3km of dominantly sedimentary green and grey schists. Pebbly conglomerates and minor volcanic rocks are still recognizable. The East Bald Hill semischist is a sequence of unknown thickness consisting of structurless greywackes, siltstones, and argillites.
Tholeiitic basalts from the West Bald Hill megablock and the West Burn semischist are characterized by high abundances of P, Zr, Ti, and Y. Bulk rock and component clinopyroxene chemistry discriminations indicate these basalts most likely originated from a depleted MORB-like source in an ocean floor paleotectonic environment. The high Zr, Ti, and Y contents suggest this environment was perhaps a fast-spreading ridge or possibly a transform fault zone.
Multiple discriminant analysis studies using both major and trace elements of the petrographically defined Maitai, Caples, and Torlesse terranes show these units can be easily discriminated, but a subdivision of the Caples terrane formations show that the Upper Peak, Bold Peak, and Momus Sandstone formations are chemically very difficult to distinguish. These investigations indicate that the Bald Hill, Lincoln Hill, and West Burn sediments are dominantly compatible with a Caples-like source, but there is a large proportion of West Bald Hill megablock and West Burn semischist sediments suggesting both Torlesse- and Maitai-like sources. The West Bald Hill megablock in a sequence under 1.75km thick reflects the entire geochemical variety of the Caples terrane. The green schist megablock sediments reflect an even more basic source than the basic components of the Maitai or Caples terrane. The P, Zr, Ti, and Y abundances in the green schist megablock sediments suggest a MORB-like source. The East Bald Hill semischist and Lincoln Hill megablock possibly represent adjacent sequences of the flanking Caples terrane which have been tectonically incorporated into the Dun Mountain oromictite.
A possible reconstruction modified after MacKinnon (1983) and Korsch and Wellman (in press) is proposed for the origin and emplacement of the Bald Hill area megablocks and the resulting implications this has on the formation of the Maitai, Caples, and Torlesse terranes. The Caples and Maitai were being depositied during Permian-Triassic times in a forarc basin and associated trench respectively, with a common oceanic island-arc source. This island-arc was laterally separate from an active continental margin (Antarctic Peninsula?) along a common trench-transform plate boundary. Voluminous sedimentation from a volcano-plutonic arc at the active continental margin was forming the Torlesse with sediment prograding outboard of the trench and onto an obliquely converging plate. Oblique subduction at the shared Caples-Torlesse trench resulted in some of the Torlesse-type sediments being rafted into the Caples depositional site. This could account for the oceanic- island and continental-type sedimentary characteristics of the Caples terrane. Longitudinal transport of sediments along the common trench is also likely. A transform fault system linking the Caples-Maitai trench to that of the Torlesse seems likely as a result of oblique subduction. The West and East Bald Hill megablocks and the West Burn semischists might represent sediment traps in such a trench-transform zone which was distal to both the Maitai-Caples and Torlesse sites and received sediment being transported along the trench. Sedimentation in these fault basins would have been slow but coeval with that of the Caples in order to produce an abbreviated thickness of sediments showing a similar geochemical variety as the Caples terrane. It is possible some Torlesse-type sediments may also be represented.
A major transform faulting event, possibly in the Cretaceous, might have resulted in the emplacement of the megablocks into the ophiolite melange and the juxtoposition of the Torlesse against the Caples terrane. Tectonic mixing and thinning of the ophiolite are likely to occur during such an event. In this scheme, the Livingstone Fault and the Haast Schist may represent paleotransform fault zones.
A chemically evolving or eroding arc source could also explain the oceanic and continental-like characteristics of the Caples terrane. This does not exclude a transform fault type model. The relative timing of deposition between the primitive volcanogenic Maitai and the more evolved Caples would be different.
Metamorphic grade in all three blocks at Bald Hill has reached lawsonite-albite-chlorite facies, however, the presence of authigeilic blue amphibole in the green schist megablock shows this block has undergone higher metamorphic pressures than the other units at Bald Hill.
The West Bald Hill is a deep-water marine sequence up to 1.75km thick of dominantly structurless black argillites and siltstones with intercalated greywackes indicative of distal turbidite depositional mechanisms. Tholeiitic pillow lavas and a hypabyssal sill, constitute the only mafics observed. Minor cherty red argillites, green phyllites, and green schist are also present. The only fossils observed were abundant sponge spicules which are not suitable for dating purposes. The East Bald Hill green schist megablock is composed of up to 2.3km of dominantly sedimentary green and grey schists. Pebbly conglomerates and minor volcanic rocks are still recognizable. The East Bald Hill semischist is a sequence of unknown thickness consisting of structurless greywackes, siltstones, and argillites.
Tholeiitic basalts from the West Bald Hill megablock and the West Burn semischist are characterized by high abundances of P, Zr, Ti, and Y. Bulk rock and component clinopyroxene chemistry discriminations indicate these basalts most likely originated from a depleted MORB-like source in an ocean floor paleotectonic environment. The high Zr, Ti, and Y contents suggest this environment was perhaps a fast-spreading ridge or possibly a transform fault zone.
Multiple discriminant analysis studies using both major and trace elements of the petrographically defined Maitai, Caples, and Torlesse terranes show these units can be easily discriminated, but a subdivision of the Caples terrane formations show that the Upper Peak, Bold Peak, and Momus Sandstone formations are chemically very difficult to distinguish. These investigations indicate that the Bald Hill, Lincoln Hill, and West Burn sediments are dominantly compatible with a Caples-like source, but there is a large proportion of West Bald Hill megablock and West Burn semischist sediments suggesting both Torlesse- and Maitai-like sources. The West Bald Hill megablock in a sequence under 1.75km thick reflects the entire geochemical variety of the Caples terrane. The green schist megablock sediments reflect an even more basic source than the basic components of the Maitai or Caples terrane. The P, Zr, Ti, and Y abundances in the green schist megablock sediments suggest a MORB-like source. The East Bald Hill semischist and Lincoln Hill megablock possibly represent adjacent sequences of the flanking Caples terrane which have been tectonically incorporated into the Dun Mountain oromictite.
A possible reconstruction modified after MacKinnon (1983) and Korsch and Wellman (in press) is proposed for the origin and emplacement of the Bald Hill area megablocks and the resulting implications this has on the formation of the Maitai, Caples, and Torlesse terranes. The Caples and Maitai were being depositied during Permian-Triassic times in a forarc basin and associated trench respectively, with a common oceanic island-arc source. This island-arc was laterally separate from an active continental margin (Antarctic Peninsula?) along a common trench-transform plate boundary. Voluminous sedimentation from a volcano-plutonic arc at the active continental margin was forming the Torlesse with sediment prograding outboard of the trench and onto an obliquely converging plate. Oblique subduction at the shared Caples-Torlesse trench resulted in some of the Torlesse-type sediments being rafted into the Caples depositional site. This could account for the oceanic- island and continental-type sedimentary characteristics of the Caples terrane. Longitudinal transport of sediments along the common trench is also likely. A transform fault system linking the Caples-Maitai trench to that of the Torlesse seems likely as a result of oblique subduction. The West and East Bald Hill megablocks and the West Burn semischists might represent sediment traps in such a trench-transform zone which was distal to both the Maitai-Caples and Torlesse sites and received sediment being transported along the trench. Sedimentation in these fault basins would have been slow but coeval with that of the Caples in order to produce an abbreviated thickness of sediments showing a similar geochemical variety as the Caples terrane. It is possible some Torlesse-type sediments may also be represented.
A major transform faulting event, possibly in the Cretaceous, might have resulted in the emplacement of the megablocks into the ophiolite melange and the juxtoposition of the Torlesse against the Caples terrane. Tectonic mixing and thinning of the ophiolite are likely to occur during such an event. In this scheme, the Livingstone Fault and the Haast Schist may represent paleotransform fault zones.
A chemically evolving or eroding arc source could also explain the oceanic and continental-like characteristics of the Caples terrane. This does not exclude a transform fault type model. The relative timing of deposition between the primitive volcanogenic Maitai and the more evolved Caples would be different.
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x. 140 p., ill., map folded in pocket., 30 cm.
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1986Ransbottom
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Ransbottom, LN, “The chemistry and origin of melange megablocks at Bald Hill, Southland.,” Otago Geology Theses, accessed March 23, 2025, https://theses.otagogeology.org.nz/items/show/203.