The Torlesse Complex in the southeastern Tararua Range, New Zealand, consists of two distinctive lithologic associations: a sedimentary association (95% of outcrop), consisting of sandstone, mudstone, calcareous siltstone, conglomerate, and olistostrome; and a volcanic association (5% of outcrop) which consists of basalt, red and green argillite, chert, and limestone. Rocks of the sedimentary association were deposited by turbidity currents and debris flows in a deep marine environment, as part of a mid-Quter submarine fan. Detrital framework modes and whole rock geochemistry indicate that the sediments were eroded from an active, continental-margin magmatic arc that was heavily dissected and shedding sediment of mainly plutonic and metamorphic origin.Rocks of the volcanic association, although volumetrically insignificant, are widely distributed but occur mainly in the western half of the area. Both tholeiitic and alkalic basalts occur, and were erupted in an oceanic environment, both in mid-ocean ridge and intraplate settings. Sedimentary rocks of the volcanic association were also deposited in these environments. Where observed, contacts between the two associations are always faulted, implying postdepositional juxtaposition of the two associations, which is considered to have occurred during the development of a subduction-related accretionary wedge.
Radiolarians have been successfully extracted from cherts from two localities within the Torlesse Complex of the eastern Tararua Range. Several genera have been identified and these indicate a Middle Jurassic -Early Cretaceous age for the chert from near the Tauherenikau River, and a Late Jurassic -Early Cretaceous age for the chert from the eastern Manawatu Gorge. The radiolarians define the minimum age of oceanic crust upon which the clastic submarine fan-type greywacke and argillite, which make up most of the Torlesse C.omplex in the eastern Tararua Range, was deposited. The boundary between fossil zones 4 (Monotisi and 5 (Late Jurassic -Early Cretaceous) occurs In the central part of the Tararua Range and not to the east of the range, as previously thought.
Deformation of the Late Jurassic -Early Cretaceous Torlesse Complex in the southeastern Tararua Range is characterised by development of melange, several generations of folds, faults at both a low angle and high angle to bedding, shear foliation, and cleavage. The region has undergone the following deformational sequence: (1) Development of at least two fold generations; (2) Fragmentation and disruption by faulting. Faults at a low angle and high angle to bedding have disrupted the sequence, in places producing chaotically disrupted units (melange). The Tauherinikau Melange represents a probable along-strike northern continuation of the Esk Head Melange from the South Island; (3) Post-melange folding; (4) Holocene faulting. Overall, the deformation is consistent with accretion at a convergent plate margin, followed by the present strike-slip dominant regime.
<p>Basement rocks within the southeastern Tararua Range belong to two associations: a sedimentary association (greywacke, argillite, calcareous siltstone, conglomerate and olistostrome) and a volcanogenic association (metabasite, chert, red argillite and limestone). Rocks of the sedimentary association are more abundant and have been deposited by turbidity currents and debris flows in a deep water, marine environment. Three turbidite and two intercalated non-turbidite lithofacies are recognized. Sedimentological data suggest that the sediment was deposited in a submarine fan system (mid-fan environment), probably in a trench. The alternating greywacke-argillite beds have detrital compositions which are essentially quartzo-feldspathic. Framework mode and geochemical analyses indicate that the sediment was derived from an active continental margin that was shedding detritus of mainly acid-volcanic and metamorphic origin. Rocks of the volcanogenic association, although volumetrically minor, are widely distributed. Geochemical analyses of metabasites suggest that they were erupted in an oceanic environment, both at a mid-ocean ridge and an intra-plate setting. The presence of radiolaria skeletons in red argillite and chert indicates a hemiplagic depositional environment for these rocks. Rocks of the volcanogenic association often have conformable contacts. These rocks have a related depositional environment and represent seafloor material. Where observed, contacts between rocks of the two associations are always faulted. Deformation in the field area is characterized by development of the following types of structures: several generations of folds, faults at both a low angle and high angle to bedding, shear foliation and melange. The region has undergone the following deformational events, outlined from oldest to youngest: 1) folding with at least two fold generations present. 2) fragmentation and disruption of the beds by faults. Low-angle to bedding faults and high-angle to bedding faults have disrupted the bedding. Where these structures have occurred to a great extent, a chaotically disrupted unit, melange, has formed. 3) post-melange folding. 4) recent faulting related to the present strike-slip regime in New Zealand. Rocks have undergone prehnite-pumpellyite facies metamorphism. The rock types, their field relationships and the deformation that the area has undergone is consistent with accretion at a convergent plate margin. Radiolaria were extracted from two red chert samples. In the study the radiolaria define a Middle Jurassic age, which indicates that the sediments in the southeastern Tararua Range must be of Middle Jurassic in age or younger (possibly Cretaceous). A similar sample from the Manawatu Gorge to the north of the study area contained radiolaria of Late Jurassic-Early Cretaceous age. Sediments in both areas therefore belong to fossil zone 5 (Late Jurassic-Early Cretaceous) of MacKinnon (1983).</p>
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