Geology of Bald Hill, northern Southland.

Borns, David J.

Bald Hill lies 155 km south of the Alpine Fault within the Dun Mountain Ophiolite Belt and the Livingstone Fault System. At least three subparallel fault zones of the Livingstone Fault System cross the Bald Hill area. The pre-Quaternary rocks of Bald Hill are divided into five major litho-tectonic units (west to east): a Western Zone consisting of a volcanic-plutonic complex which is depositionally overlain by a coarse breccia-conglomerate and tectonically underlain by a broken body of serpentinized harzburgite and gabbro; a sequence of tectonic melanges; a body of textural zone I to IIB semischist of West Bald Hill; a unit of textural zone IIIA to IV schist of East Bald Hill; another unit of textural zone IIA to IIB semischist of East Bald Hill which is of Caples Group affinity.
The term, Bald Hill Complex, is proposed for the tectonically juxtaposed or mixed units of the Humboldt Group(Livingstone Volcanics and Red Mountain Ultramafics), Maitai Group, and Caples Group bounded on the west by the coarse breccia-conglomerate of the Western Zone and bounded on the east by the Mavora Lakes Fault Zone, the most eastern segment of the Livingstone Fault System.
The tectonic melanges are divided on the basis of block-type facies: ultramafite and gabbro block-type melange; ophiolite or lava-dolerite-gabbro(minor sediment and amphibolite) block-type melange; sediment or mudstone-greywacke(minor limestone) block-type melange; multiple blocktype melange. The block-type melanges form subparallel sheet-like bodies. Melange matrices consist of cataclasite, sheared serpentinite and argillite. Some melanges are polykinematic resulting from the reworking of pre-existing melanges. Replacement of plagioclase by prehnite, prehnite veining, and disruption of tectonic blocks increase west to east across ii the main sequence of melanges which occur between the Western Zone and the semischists of West Bald Hill, This large body of melanges dips 50° 0 to 70 east, and components preserving stratigraphic sequences are overturned, tops to the west.
Metamorphism of the Bald Hill units is characterized by high temperature, low pressure epidote-amphibolite and greenschist facies assemblages, high pressure-low temperature lawsonite-albite facies assemblages with the sporadic conversion of strained calcite to aragonite, and intermediate pressure prehnite-pumpellyite assemblages. Mineral assemblages in metabasic rocks contain hornblende-actinolite pairs and albites with a complete range of anorthite contents from 1 to 9 molecular percent. The appearance of lawsonite at Bald Hill is largely compositionally controlled and is related to the reaction between interstitial clays and calcite to produce lawsonite and muscovite in metasediments but is not related to the hydration of the anorthite component of plagioclase.
The coarse breccia-conglomerate of the Western Zone suggests an environment for the unit similar to the inward (landward) wall of an ocean trench such as the south wall of the Puerto Rico Trench. Evidence is as yet inconclusive on whether the ophiolite units of the Bald Hill are· of island-arc or mid-ocean ridge origin, but the metabasic units of Bald Hill may represent both environments.
The tectonic and metamorphic history of the Bald Hill area spans a long time range from at least the Lower Permian through the Lower Cretaceous culmination of the Rangitata metamorphic event and subsequent uplift.

vi, 187 leaves : illus., 3 fold. maps in pocket ; 30 cm.

1975Borns

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