Structure and evolution of the Taieri-Wakatipu synform

Author:

Devlin, Sean.

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Abstract:

Analy.:sis of the nature of the Taieri-Wak:atipu Synform in the Eyre Mountains, central Otago, allows definition of the Taieri-Wakatipu Synform as a genuine post-metamorphic fold structure. The Taieri-Wakatipu Synform is defined by folding of a single penetrative S1 foliation into an open symmetrical upright synform with no associated repetition of textural zone or metamorphic facies. Similarities in the nature of the structure identified from detailed structural, petrological, and geochemical analysis in central and coastal Otago identify the Taieri-Wak:atipu Synform as a regionally extensive structure observable along the entire margin of the schist belt, from coastal Otago through central Otago and across the Alpine Fault into the Nelson region, consistently occupying the low grade margin of the Caples-derived schist with the hinge at the TZIIa-TZIIb boundary. The geometry of the structure means that the synform cannot develop from a simple postmetamorphic warp of flat lying foliation. Rather, it must result from folding of foliation which is in a non-parallel orientation to the regional metamorphic field gradient. Two models are proposed to account for the development of oblique foliation and metamorphic grade based on the existence of either widespread regional continuity or discontinuity at textural zone boundaries. (1) The development of shear zone imbrication would result in tectonic emplacement of higher grade rocks into the sequence inverting the metamorphic field gradient and developing oblique foliation and metamorphic grade by juxtaposing higher grade over lower. (2) Metamorphism within an imbricate thrust wedge during shortening would result in the formation of foliation oblique to textural and metamorphic boundaries due to the physical constraints on the formation of cleavage and metamorphic minerals. Both of these models then require uplift of the margin of the imbricate zone to expose the 1 observed sequence. The ~~~uenc~_illlplie~n___:asterly dipping structure " uplifted in the east. The only mechanism to facilitate uplift to the east would be exhumation ~~--- of the schist within the Otago anticlinorium. Based on the lateral continuity of the synform, and the nature of the metamorphic boundaries the latter model is favoured. However, either model would have similar implications for the evolution of the schist in that they both imply that the Taieri-Wakatipu synform is a consequence of exhumation related to uplift of the Otago Schist across a pre-existing structure formed during low-grade metamorphism.

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76 leaves : ill. (some col.), maps (2 folded in pocket) ; 30 cm.

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2002Devlin

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MULTIPOLYGON (((168.511185233856594 -45.341749315510071,168.50685721676021 -45.392070922266072,168.400086999515764 -45.387335697280392,168.404864752419002 -45.336798001837259,168.511185233856594 -45.341749315510071)),((168.584930476405958 -45.236023369942615,168.517656021007951 -45.268408999192545,168.437553700158418 -45.264158905886305,168.450070390836999 -45.227861611081018,168.559013752780601 -45.172853439098795,168.594739850915289 -45.174597054129343,168.584930476405958 -45.236023369942615)))

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http://download.otagogeology.org.nz/temp/Abstracts/2002Devlin.pdf

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Citation

Devlin, Sean., “Structure and evolution of the Taieri-Wakatipu synform,” Otago Geology Theses, accessed March 29, 2024, http://theses.otagogeology.org.nz/items/show/394.

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