Old Man Range geology


McIntyre, Calan.


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Two lithological associations are mapped within a fifty square kilometre area of the Otago Schist, situated on the easternĀ· flank of the Old Man Range, Central . Otago, New Zealand. The eastern and western lithological associations each have a distinctive outcrop appearance, mineralogy and geochemistry, and have been juxtaposed by the north trending Old Man Fault.
Five structural generations are recognised within the schists of the Old Man Range area. The first structural generation involved total greenschist fades metamorphic reconstitution of a variety of sedimentary and volcanic protoliths, accompanied by transposition of bedding parallel to a penetrative schistosity.
The second structural generation was associated with the development of a macroscopic nappe within the western lithological association. Segregated and massive greyschists were thrust southward over platy greyschists along the highly strained lower limb of this nappe. Variation in deformation style, rotation of mesoscopic structures, and progressive fabric development are attributed to a finite strain gradient across the nappe.
The third structural generation is inferred to be associated with the northward thrusting of the eastern lithological association over the western lithological association. Subsequent faulting out of section by the Old Man Fault prevents detailed mapping of an inferred high strain zone separating the two lithological associations.
The fourth structural generation postdated metamorphism, and involved the formation of east trending vertical quartz vein systems with associated localised brittle deformation. Two quartz reefs containing economic concentrations of gold occur in the study area, both hosted in low displacement normal faults.
The fifth structural generation followed uplift of the metamorphic pile, and involved near surface brittle deformation of the schist. The Old Man Fault developed prior to the Miocene, most likely as a discrete normal fault, uplifting the western lithological association by at least two kilometres relative to the eastern lithological association. Following the subsequent peneplanation of the schist, and deposition of the Miocene Manuherikia Group, the Old Man Fault was reactivated as a c. one kilometre wide low displacement reverse shear zone. Uplift of the Old Man Range occurred dominantly by flexure of the preMiocene peneplain surface.
The eastern and western lithological associations have distinct major and trace element geochemical signatures. Geochemical discriminant diagrams show the eastern and western lithological associations to belong to the Caples and Torlesse terranes respectively. The association of platy greyschists and greenschists, underlying the D2 nappe within the western lithological association, is proposed to be a small exposure of the Aspiring terrane, exposed through uplift and erosion. Geochemical analyses of greenschists indicate they .were ~erive.d from tho;9tic basalts, erupted in a mid ocean ridge or intraplate / tectomc settmg.

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57 leaves : ill., maps ; 30 cm.


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McIntyre, Calan., “Old Man Range geology ,” Otago Geology Theses, accessed April 16, 2021, http://theses.otagogeology.org.nz/items/show/282.

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