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.

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<p>Basement rocks in the southern Tararua Range are part of the Torlesse Supergroup, possibly Late Triassic to Late Jurassic in age, and form two distinct associations. The sedimentarv association consists mainly of quartzo-feldspathic sandstone and argillite with minor olistostrome, calcareous siltstone and microsparite. The sandstone and argillite were deposited as turbidites in a mid- to outer- submarine fan environment. The sediment was derived from a heavily dissected active continental margin that was shedding sediment of mainly plutonic and metamorphic origin. The volcanic association consists mainly of metabasite and coloured argillite with minor chert and limestone. Geochemical data indicate that the metabasites were erupted in an oceanic intraplate environment. The nature of amygdules in amygdaloidal metabasites suggests eruption in less than 800m of water. Coloured argillites have two distinct origins, namely sediments formed by the degredation of basalt; and also pelagic material modified by metal-rich effluent either from hydrothermal systems associated with mid-ocean ridges or intraplate volcanism. The rocks of the volcanic association indicate formation in an environment similar to present day mid-ocean islands. Nowhere were rocks of the two associations observed to be conformable. Coupled with this, the nature of the two associations suggests that they were formed in separate environments. The following structural history is proposed: 1) Early veining; 2) Isoclinal folding and development of a NNE striking cleavage; 3) Faulting both at low and high angles to bedding, extreme amounts of which have resulted in mélange; 4) NE-SW trending close to open folds; 5) E-W trending open to gentle folds; 6) Recent faulting, predominantly NE trending strike-slip faults. The nature of the two associations and the deformational style and history supports an accretionary prism model for the development of the Torlesse Supergroup. Rocks of the southern Tararua Range show many similarities with, and probably represent a northward continuation of, the Esk Head Mélange of the South Island.</p>