The early metamorphic history of the Haast Schists and related rocks of New Zealand

Ductile structures in the greenschist facies rocks of the Alpine Schist that lie in the headwaters of the Whataroa, Callery, and Balfour valleys formed during Mesozoic deformation, similar to those in the Otago Schist to the south. A general westward increase in metamorphic grade is locally disrupted and repeated by several late metamorphic faults. The first appearance of biotite occurs at tectonic boundaries in the Callery and Balfour sections. Metagreywackes are juxtaposed against a greenschistmetapelite-metachert sequence by a synmetamorphic ductile high strain zone in the lower Balfour valley. A prominent quartz rodding lineation in the high strain zone defines a stretching direction that plunges about 30° southwest. Structures in the high strain zone are locally cut by quartz (±biotite) veins, which have been weakly deformed. Fluid inclusions in quartz veins indicate that the veins formed about 8-10 km deep and that uplift was initially nearly isothermal. Pervasive foliation that formed during Mesozoic deformation was deformed into upright, open, kilometrescale folds, with some foliation-parallel boudinage, associated with the Miocene inception of the Alpine Fault. The resultant north to northeast trending folds plunge gently southwards. Deformation associated with Alpine Fault movement was only minor in the Alpine Schist greenschist facies and resulted in passive differential uplift of deeper parts of the schist pile in the west.

Section: Upper Balfourmentioning

confidence: 83%

Section: Lower Balfour-craig Peakmentioning

confidence: 91%

Structures within greenschist facies Alpine Schist, central Southern Alps, New Zealand

Craw

Rattenbury

Johnstone

1994

“…The Haast Schist is a classic example of a moderate-pressure regional metamorphic belt related to a convergent continental margin (Hutton & Turner 1936;Mason 1962;Grindley 1963;Wood 1963;Bishop 1972;Cooper 1974;Brown 1974;Yardley 1982;Craw 1985;Mortimer 1993a;Grapes 1995). The Haast Schist embraces the Otago, Alpine, and Marlborough schists in the South Island ( Fig.…”

Section: Introductionmentioning

confidence: 99%

An episodic Cretaceous cooling model for the Otago‐Marlborough Schist, New Zealand, based on⁴⁰Ar/³⁹Ar white mica ages

Little¹,

Mortimer²,

McWilliams³

1999

New 40 Ar/ 39 Ar ages of 12 white mica samples from deep levels of the Otago and Marlborough Schists, together with previously published whole-rock K-Ar ages yield a nonlinear age-depth profile suggesting a fossil partial retention zone for argon. In contrast to earlier studies, we interpret (1) the peak of Otago Schist metamorphism to have occurred in the Middle Jurassic (170-180 Ma) rather than Early Jurassic; and (2) subsequent cooling to have been episodic, not slow and continuous. These data cannot be modelled uniquely but support a model where the schist was held at mid to lower crustal depths until c. 135 ± 5 Ma, after which it was rapidly unroofed at 0.6-1.0 mm/yr during regional crustal thickening along the eastern margin of New Zealand. We infer that there were also one or more younger periods of argon loss affecting deep levels of the Otago/ Marlborough Schists in a spatially heterogeneous way after c. 120 Ma. Late Cretaceous argon loss at <75-84 Ma coincided with seafloor spreading offshore of New Zealand and probably involved crustal reheating. Post-mid Cretaceous phases of argon loss may have coincided with periods of heat input into the crust, including metamorphism of the Alpine Schist at deep levels of the Haast Schist.

“…1A). The Otago Schist is a moderately high pressure metamorphic belt (Yardley 1982;Mortimer 2000), which formed by amalgamation of the Eastern Province Caples and Torlesse Terranes during the "Rangitata I" Orogeny in the Early-Middle Jurassic (Bradshaw et al 1981;Adams & Robinson 1993;Little et al 1999). The Otago Schist provides a view into the deeper part of the Torlesse accretionary wedge and forms a c. 150 km wide, two-sided arch.…”

Section: Geological Settingmentioning

confidence: 99%

Tectonic significance of Cretaceous bivergent extensional shear zones in the Torlesse accretionary wedge, central Otago Schist, New Zealand

Deckert

Ring

Mortimer

2002

We describe two shear zones in the Otago Schist of the Torlesse accretionary wedge, South Island, New Zealand: the north-dipping Rise-and-Shine Shear Zone (RSSZ) and the south-dipping Cromwell Gorge Shear Zone (CGSZ). Kinematic indicators (shear bands and asymmetric folds) indicate top-north movement for the RSSZ and topsouth transport for the CGSZ. Back rotation of the shear zones into their Late Cretaceous orientation and consideration of the relationship of the shear zones to arching of the Otago Schist show that both shear zones are extensional. Offset of textural zones suggests up to 15 km of dip-slip displacement on the RSSZ and probably a similar amount of slip for the CGSZ. We speculate that the goldmineralised RSSZ may be the western continuation of the late Mesozoic gold-bearing Hyde-Macraes Shear Zone in eastern Otago, forming a c. 100 km long extensional shear zone on the northern flank of the Otago Schist. Age constraints suggest that shear zone formation took place between 135 and 105 Ma. The shear zones aided the final exhumation of the deeper parts of the Otago Schist. We discuss whether normal shearing is related to syn-orogenic supercritical tapering of the Torlesse wedge, or due to postorogenic New Zealand-wide Albian rifting.