The Anatoki fault and structure of the adjacent Buller and Takaka terrane rocks, northwest Nelson, New Zealand

• Main Author: Jongens, Richard
• Language: en
• Published: University of Canterbury. Geology 2010
• Online Access: http://hdl.handle.net/10092/4758

The Early Paleozoic rocks exposed in the northwest of the South Island, New Zealand, can be grouped into two terranes. An Ordovician deep water passive margin assemblage makes up the Buller terrane. A Cambrian arc assemblage and an Ordovician to Silurian mainly shallow water passive margin assemblage make up the Takaka terrane. The two terranes are cut by Late Paleozoic and Early Cretaceous plutonic rocks. The terrane boundary is the Anatoki Fault, which is a north-south striking, east dipping structure that steepens southwards. This thesis is a study of the movement history of the Anatoki Fault and a study of structures and microstructures in the flanking lithologies of the Buller and Takaka terranes. The study area is divided into seven areal domains, each containing structural evidence that can be related on an inter-domainal scale. In the study area, three major deformation events can be recognised in both the Buller and Takaka terrane rocks. Each consecutive deformation event recognised in one of the terranes has structures that can be matched in style, orientation, and timing of development with that in the adjacent terrane. D, is represented by upright to overturned towards the west, N-S trending, large scale folds with an axial planar slaty cleavage. D, structures are observed in all domains and relate to other N-S trending structures seen in both terranes. Based principally on constraints outside the study area, D, is thought to be Early-Middle Devonian in age. A well-developed foliation, which obliterates D, structures in close to the Anatoki Fault, represents D2• The foliation expresses a zone of non-coaxial ductile deformation observed in the north of the study area. Rb-Sr geochronology, and the relationship between the foliation and the adjacent ~lllMa Mt Olympus Pluton, suggest D2 formed in the Early Cretaceous (soon after ~ IllMa). D3 is represented mainly by mesoscale folds with an axial planar crenulation cleavage. D3 structures crenulate and refold both D, and D2 structures and are mid-Cretaceous in age. Two deformations are recognised in the Balloon Melange of the Takaka terrane. The first deformation is responsible for the melanging when the protolith was in an unlithified state. This deformation occurred in the Late Cambrian. The second deformation is expressed by a slaty fabric when the melange was lithified and is thought to be related to D,. The contact between the Balloon Melange and other units in the Takaka terrane is thought to be tectonic but locally intrusive. Most structures in Northwest Nelson can be correlated with D" D2, and D3 of the study area. However, some N-S trending folds in the Cambrian arc assemblage are thought to be older than D, and are here thought to relate to pre-D, Late Cambrian deformation recognised in the Balloon Melange. The Anatoki Fault records a complex history of both ductile and brittle movement, a history which differs in various segments of the fault. Thus, tectonites from central segments of the study area record ductile east over west reverse-slip associated with D" and represent an early stage in its movement history; D2 tectonites from the northernmost studied segment record Early Cretaceous ductile dextral-slip reactivation; tectonites from the southernmost segment records ductilelbrittle dextral normal-slip reactivation that postdates the intrusion of the ~137Ma Crow Granite. The post-137Ma movement, and brittle/ductile movement of unknown age recorded in the remaining segments, may be related to D2• The Anatoki Fault has also undergone Late Cenozoic brittle reactivation. A plate tectonic model is developed to explain the earlier structures of the study area and the general structure of Northwest Nelson. It is proposed that an accretionary wedge/fore-arc setting related to a Cambrian island arc subduction zone provides the environment in which pre-D, structures formed, and that in the Early-Middle Devonian, the two terranes amalgamated as a result of a convergent strike-slip plate boundary. Much of the Takaka terrane was truncated by the strike-slip movement which dominated the early stages of Devonian deformation whereas convergence related thin-skinned tectonics dominated the latter stages of deformation, and resulted in formation of the D, structures. In this model, the Central Belt of the Takaka terrane, which consists primarily of the Cambrian arc assemblage, is interpreted as an uplifted tectonic wedge in which the Anatoki Fault is a back-thrust. D2 and D3 relate to an extremely active and changing tectonic period of New Zealand in the Early to mid-Cretaceous. Twinning in ankerite porphyroblasts in D2 tectonites was examined to investigate the twin laws and their usefulness as a paleostress indicator. Results show that the twin laws are the same as those in dolomite and that twinning occurs at temperatures above ~250°C. Paleostress axes derived from the analysis provide evidence of microscale strain partitioning in which the porphyroblasts have accommodated a pure shear component. The c-axes preferred orientations in dynamically recrystallised calcite tectonites, from the Anatoki Fault zone, most closely resemble those produced in the grain-boundary migration regime of experimental studies. As such, the preferred orientations cannot be used to indicate shear-sense.