Australian Journal of Earth Sciences (2002) 49, 407–421 INTRODUCTION The Nambucca Block is one of several fault-bound blocks that make up the eastern part of the southern New England Fold Belt (Figure 1). It contains Lower Permian mass-flow deposits, turbidites and approximately 2000 m of massive and pillowed basalt deposited in a rift basin (Leitch 1988). K–Ar, structural and metamorphic studies suggest that this rift basin sequence was multiply deformed and regionally metamorphosed under sub-greenschist to greenschist facies conditions at ca 255–250 Ma (Leitch 1975, 1978; Leitch & McDougall 1979; Fukui 1991; Offler & Brime 1994). An enigmatic feature of the Nambucca Block is that the structures within it trend east–west in contrast to the meridional trends recorded elsewhere in the Palaeozoic rocks of the accretion complex of the southern New England Fold Belt and rocks in other Early Permian rift basins. This has been a subject of great controversy and has led to several models being proposed for the formation of the Nambucca Block, the structures within it and the associated Texas – Coffs Harbour megafold (Figure 1). However, there is substantial disagreement among authors as to when the Texas – Coffs Harbour megafold formed and whether it was involved in the deformation of the sediments in the Nambucca Block. Further, there is a lack of consensus on the original location of the Nambucca Block. Some authors maintain that this basin developed in its current position (Roberts et al. 1993), whereas others believe that it formed part of a much larger, meridionally trending basin, the Barnard Basin (Leitch 1988), before Structural fabric evidence for indentation tectonics in the Nambucca Block, southern New England Fold Belt, New South Wales A. J. JOHNSTON, 1 * R. OFFLER 1† AND S. LIU 2 1 Discipline of Geology, School of Environmental and Life Sciences, University of Newcastle, NSW 2308, Australia. 2 Geoscience Australia, GPO Box 378, Canberra, ACT 2601, Australia. Structural studies of Lower Permian sequences exposed on wave-cut platforms within the Nambucca Block, indicate that one to two ductile and two to three brittle – ductile/brittle events are recorded in the lower grade (sub-greenschist facies) rocks; evidence for four, possibly five, ductile and at least three brittle – ductile/brittle events occurs in the higher grade (greenschist facies) rocks. Veins formed prior to the second ductile event are present in some outcrops. Further, the studies reveal a change in fold style from west-southwest-trending, open, south-southeast-verging, inclined folds (F 0 1 ) at Grassy Head in the south, to east-northeast-trending, recumbent, isoclinal folds (F 0 1 ; F 0 2 ) at Nambucca Heads to the north, suggesting that strain increases towards the Coffs Harbour Block. A solution cleavage formed during D1 in the lower grade rocks and cleavages defined by neocrystalline white mica developed during D1 and D2 in the higher grade rocks. South- to south-southwest-directed tectonic transport and north–south shortening operated during these earlier events. Subsequently, north-northeast-trending, open, upright F 2 3 folds and inclined, northwest-verging, northeast-trending F 2 4 folds developed with poorly to moderately developed axial planar, crenulation cleavage (S3 and S4) formed by solution transfer processes. These folds formed heterogeneously in S2 throughout the higher grade areas. Later northeast–southwest shortening resulted in the formation of en échelon vein arrays and kink bands in both the lower and higher grade rocks. Shortening changed to east- northeast–west-southwest during later north-northeast to northeast, dextral, strike-slip faulting and then to approximately northwest–southeast during the formation of east-southeast to southeast-trending, strike-slip faults. Cessation of faulting occurred prior to the emplacement of Triassic (229 Ma) granitoids. On a regional scale, S1 trends east–west and dips moderately to the north in areas unaffected by later events. S2 has a similar trend to S1 in less-deformed areas, but is refolded about east–west axes during D3. S3 is folded about east–west axes in the highest grade, multiply deformed central part of the Nambucca Block. The deformation and regional metamorphism in the Nambucca Block is believed to be the result of indenter tectonics, whereby south-directed movement of the Coffs Harbour Block during oroclinal bending, sequentially produced the east–west-trending structures. The effects of the Coffs Harbour Block were greatest during D1 and D2. KEY WORDS: Nambucca Block, New England Fold Belt, Permian, structural analysis, tectonics. *Present address: Geological Survey of New South Wales, Orange, NSW 2800, Australia. † Corresponding author: robin.offler@newcastle.edu.au