Geology of the Telescope Hill area, Gorge plateau, South Westland.
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Abstract:
The geology of an area around Telescope Hill, directly inland from Big Bay, southernmost Westland, has been studied. Structurally the area is dominated by an internally faulted, north to northeasterly trending belt of Permian to latest Jurassic rocks. North of Telescope Hill the belt runs approximately parallel to the Alpine Fault, the main recent trace of which trends 046°.
Humboldt Group rocks are exposed in the eastern part of the area and comprise serpentinised peridotites of the Red Mountain Ultramafites and the overlying Livingstone Subgroup which consists dominantly of altered basic intrusions overlain by metavolcanic extrusives or metasediments. Together the Red Mountain Ultramafites and the Livingstone Subgroup comprise a portion of the Permian Dun Mountain ophiolite belt. Some evidence is presented for evolution at a slow spreading ridge.
Directly west of the Livingstone Subgroup and separated from it by a major shear zone, the Stethoscope Fault, lie rocks of the Maitai Group. These rocks constitute a sequence which is dominantly volcanogenic, extensively faulted along bedding planes, and folded into a tightly appressed, nearly isoclinal syncline (Key Summit Regional Syncline) with an overturned east limb. The basal Maitai rocks are inferred to have been faulted out along the Stethoscope Fault. The units recognised in this sequence are, from the base: Little Ben Sandstone, Greville Formation, Waiua Formation and Stephens Formation. An environment of deposition on the flanks of a generally active volcanic arc during Mid to Late Permian, and possibly ranging into Triassic times is envisaged. The Maitai sediments are inferred to be dominantly redeposited.
West of the Maitai Group, sediments correlated with the late Jurassic Barrier Formation are exposed as a fault bounded sliver. The bounding faults are termed the East and West Hollyford Faults, Medium to coarse grained sandstones from the Barrier Formation have been point counted and are classified as either lithic arkoses or feldspathic litharenites. The Barrier rocks contain significantly more quartz than the other youngest known Murihuku Supergroup rocks, from southwest Auckland. Possible reasons for this variation are considered.
West of the Barrier Formation lie rocks of the Eglinton Subgroup and MacKay Intrusives. These rocks are bounded to the west by the Alpine Fault. The Eglinton Subgroup consists of acidic and basic volcanics, and volcanogenic sediments. Both Plato and Largs terrane rocks are provisionally recognised within this sequence, The Eglinton Subgroup is inferred to represent the remnants of a Permian volcanic arc. Six plutonic bodies mapped within the Eglinton Subgroup are correlated with the MacKay Intrusives. These crystalline rocks appear similar to the Mistake Diorite of the MacKay Intrusives, thought to have intruded the Eglinton rocks in Triassic times.
Soon after evolution at a spreading ridge, the Humboldt Group rocks were subjected to low P : T metamorphism, reaching a maximum grade of amphibolite facies. This early metamorphism was overprinted by a regional metamorphic event which probably coincided with the late Jurassic-early Cretaceous Rangitata Orogeny. Regional metamorphism, up to pumpellyite- tremolite/actinolite grade has affected the Humboldt Group, Maitai Group, Barrier Formation, Eglinton Subgroup and MacKay Intrusives. High P : T lawsonite-albite-chlorite facies metamorphism occurred in the axial portion of the Key Summit Regional Syncline. A northward increase in the metamorphic grade of the Barrier Formation rocks is noted, An example of localised frictional metamorphism within the Barrier Formation is tentatively suggested; however, a case is presented against extensive metamorphism due to shear heating on the Alpine Fault.
The bending of the mapped units from an approximately north-south trend south of Telescope Hill to a northeasterly trend north of Telescope Hill is consistent with dextral drag along the Indo-Australian/ Pacific plate boundary. At least two major subvertical axes of folding are recognised.
Faulting is the most conspicuous mode of deformation in the area. Faults separate all major mapping units and generally run parallel to regional structural trends.
Offsets on the Stethoscope Fault are examined and experimental duplication is attempted. The stress field suggested experimentally is inconsistent with that predicted by plate tectonics. Possible reasons for the observed offsets being incompatible with plate tectonic predictions are proposed.
Fault rock development along the Alpine Fault and the Stethoscope Fault is discussed. A minimum dip of 35° SE is suggested for the Alpine Fault at depth, despite the apparent rotations of fault rock attitudes during uplift.
Shear stress magnitudes on the Alpine Fault have been estimated using the known critical resolved shear stress for the mechanical twinning of clinopyroxene, and the sizes of dynamically recrystallised grains of quartz and calcite. These independent methods consistently indicate extremely high shear stresses on the Alpine Fault. A maximum shear stress of 240 MPa is suggested.
Physiographically the most striking feature of the area is the extent of recent deformation. Several very large landslides and slumps occur in the area. A large rotational slump is dated at approximately 200 yr. b.p. on the basis of the estimated ages of tilted beech trees, Recent undissected fault scarps and traces abound in the area. Unusual ridge top depressions occurring along Earthquake Ridge are interpreted as fault traces.
Quartz grains ranging from silt to pebble in size have been collected from a peat bog on Earthquake Ridge, The quartz grains range from angular to well rounded and include all intermediate stages, The varying degrees of roundness of the quartz grains in this environment are attributed primarily to the mechanism of solution rounding. It is suggested that humic acids in the peat bog significantly increase the solubility of Si, SEM photomicrographs are consistent with this postulate.
The general area contains much evidence of recent mountain glaciation including cirques, aretes, cirque lakes, screes and roches moutonnees.
A microseismic survey indicates that during 8-16th January 1982, seismicity was low in the Telescope Hill area, South Westland.
Humboldt Group rocks are exposed in the eastern part of the area and comprise serpentinised peridotites of the Red Mountain Ultramafites and the overlying Livingstone Subgroup which consists dominantly of altered basic intrusions overlain by metavolcanic extrusives or metasediments. Together the Red Mountain Ultramafites and the Livingstone Subgroup comprise a portion of the Permian Dun Mountain ophiolite belt. Some evidence is presented for evolution at a slow spreading ridge.
Directly west of the Livingstone Subgroup and separated from it by a major shear zone, the Stethoscope Fault, lie rocks of the Maitai Group. These rocks constitute a sequence which is dominantly volcanogenic, extensively faulted along bedding planes, and folded into a tightly appressed, nearly isoclinal syncline (Key Summit Regional Syncline) with an overturned east limb. The basal Maitai rocks are inferred to have been faulted out along the Stethoscope Fault. The units recognised in this sequence are, from the base: Little Ben Sandstone, Greville Formation, Waiua Formation and Stephens Formation. An environment of deposition on the flanks of a generally active volcanic arc during Mid to Late Permian, and possibly ranging into Triassic times is envisaged. The Maitai sediments are inferred to be dominantly redeposited.
West of the Maitai Group, sediments correlated with the late Jurassic Barrier Formation are exposed as a fault bounded sliver. The bounding faults are termed the East and West Hollyford Faults, Medium to coarse grained sandstones from the Barrier Formation have been point counted and are classified as either lithic arkoses or feldspathic litharenites. The Barrier rocks contain significantly more quartz than the other youngest known Murihuku Supergroup rocks, from southwest Auckland. Possible reasons for this variation are considered.
West of the Barrier Formation lie rocks of the Eglinton Subgroup and MacKay Intrusives. These rocks are bounded to the west by the Alpine Fault. The Eglinton Subgroup consists of acidic and basic volcanics, and volcanogenic sediments. Both Plato and Largs terrane rocks are provisionally recognised within this sequence, The Eglinton Subgroup is inferred to represent the remnants of a Permian volcanic arc. Six plutonic bodies mapped within the Eglinton Subgroup are correlated with the MacKay Intrusives. These crystalline rocks appear similar to the Mistake Diorite of the MacKay Intrusives, thought to have intruded the Eglinton rocks in Triassic times.
Soon after evolution at a spreading ridge, the Humboldt Group rocks were subjected to low P : T metamorphism, reaching a maximum grade of amphibolite facies. This early metamorphism was overprinted by a regional metamorphic event which probably coincided with the late Jurassic-early Cretaceous Rangitata Orogeny. Regional metamorphism, up to pumpellyite- tremolite/actinolite grade has affected the Humboldt Group, Maitai Group, Barrier Formation, Eglinton Subgroup and MacKay Intrusives. High P : T lawsonite-albite-chlorite facies metamorphism occurred in the axial portion of the Key Summit Regional Syncline. A northward increase in the metamorphic grade of the Barrier Formation rocks is noted, An example of localised frictional metamorphism within the Barrier Formation is tentatively suggested; however, a case is presented against extensive metamorphism due to shear heating on the Alpine Fault.
The bending of the mapped units from an approximately north-south trend south of Telescope Hill to a northeasterly trend north of Telescope Hill is consistent with dextral drag along the Indo-Australian/ Pacific plate boundary. At least two major subvertical axes of folding are recognised.
Faulting is the most conspicuous mode of deformation in the area. Faults separate all major mapping units and generally run parallel to regional structural trends.
Offsets on the Stethoscope Fault are examined and experimental duplication is attempted. The stress field suggested experimentally is inconsistent with that predicted by plate tectonics. Possible reasons for the observed offsets being incompatible with plate tectonic predictions are proposed.
Fault rock development along the Alpine Fault and the Stethoscope Fault is discussed. A minimum dip of 35° SE is suggested for the Alpine Fault at depth, despite the apparent rotations of fault rock attitudes during uplift.
Shear stress magnitudes on the Alpine Fault have been estimated using the known critical resolved shear stress for the mechanical twinning of clinopyroxene, and the sizes of dynamically recrystallised grains of quartz and calcite. These independent methods consistently indicate extremely high shear stresses on the Alpine Fault. A maximum shear stress of 240 MPa is suggested.
Physiographically the most striking feature of the area is the extent of recent deformation. Several very large landslides and slumps occur in the area. A large rotational slump is dated at approximately 200 yr. b.p. on the basis of the estimated ages of tilted beech trees, Recent undissected fault scarps and traces abound in the area. Unusual ridge top depressions occurring along Earthquake Ridge are interpreted as fault traces.
Quartz grains ranging from silt to pebble in size have been collected from a peat bog on Earthquake Ridge, The quartz grains range from angular to well rounded and include all intermediate stages, The varying degrees of roundness of the quartz grains in this environment are attributed primarily to the mechanism of solution rounding. It is suggested that humic acids in the peat bog significantly increase the solubility of Si, SEM photomicrographs are consistent with this postulate.
The general area contains much evidence of recent mountain glaciation including cirques, aretes, cirque lakes, screes and roches moutonnees.
A microseismic survey indicates that during 8-16th January 1982, seismicity was low in the Telescope Hill area, South Westland.
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Thesis description:
157 leaves : ill., maps ; 30 cm.
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OU geology Identifier:
1982MacDonell
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Location (WKT, WGS84):
POLYGON ((168.276268789659298 -44.230015685994857,168.37160731551333 -44.239286116484628,168.365049796514256 -44.301543766057812,168.2676972944368 -44.295459107831576,168.276268789659298 -44.230015685994857))
Collection
Citation
MacDonell, Barry J., “Geology of the Telescope Hill area, Gorge plateau, South Westland.,” Otago Geology Theses, accessed May 15, 2025, https://theses.otagogeology.org.nz/items/show/158.