Structural Geology of the Greywackes of Tiritiri Matangi Island, by K.B
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Tane 35: 95 - 111 (1995) STRUCTURAL GEOLOGY OF THE GREYWACKES OF TIRITIRI MATANGI ISLAND K. B. Sporli and L. M. McAlister Department of Geology, University of Auckland, Private Bag 92019, Auckland SUMMARY Bedding in the Waipapa terrane greywacke basement on Tiritiri Matangi Island is mainly right way up, strikes NW-SE, dips steeply to the west and has been affected by three phases of folding: (I) approximately ENE trending asymmetrical, tight folds; (II) NW-SE trending, open, upright folds and (III) steeply plunging, dextral folds. Associated small scale structures include pre- phase I clastic dikes, extensional faults and swarms of parallel quartz-prehnite veins at low angle to bedding, post-phase I en echelon quartz veins, fibrous quartz-chlorite veins and striations, and very late gouge faults. Analysis of the quartz chlorite fibre striations reveals shortening across the dominant phase II NW-SE trending fold axis, combined with E-W extension by normal faulting oblique to this fold axis, indicating the presence of a dextral shear system, probably representing a vector oblique to the continental margin of Gondwana during Mesozoic subduction. INTRODUCTION Tiritiri Matangi Island is one of number of exposures at the western edge of a belt of uplifted greywacke basement along the eastern side of Auckland/Northland peninsula (Fig. 1), which include Whitford Quarry (Sporli and Anderson 1980) and Motutapu Island (Mayer 1968). The edge of the uplifted greywacke is formed by a fault or a fault zone (Milligan 1977, Anderson 1977 and Fig. IB, this paper). Faults of this type which can be traced offshore west of the North Island indicate that they were initially formed during separation of New Zealand from Gondwana in the Cretaceous (80 million years ago). However, there is evidence for subsequent reactivation (Sporli 1989a). To the west of the greywacke high lie down-faulted younger rocks, including the Cretaceous-Tertiary sedimentary rocks of the Northland Allochthon and the Miocene Waitemata Group. At Tiritiri Matangi, no western bounding fault can be seen directly in outcrop, but such a structure has been postulated under the channel between Tiritiri and the eastern tip of Whangaparaoa Peninsula to the west (Gregory 1966). Around Whangaparaoa, locally strongly folded and faulted 95 Fig. 1 A. Location map with exposures of greywacke basement (black) and major basement terranes of the North Island, modified after Sporli (1978); B. Detailed location map and greywacke exposures of the Auckland region. 96 Fig. 2. Geological map of Tiritiri Matangi Island. Upper right hand corner: Lower hemisphere equal area net showing poles (perpendiculars) to bedding planes in the Waipapa terrane "greywacke". The dots represent piercing points of lines going through the centre of the hemisphere. Steeply inclined lines are represented by points near the centre of the net, lines with shallow inclinations by points near the periphery. Note the main NW-SE trend of bedding strike derived from NE-SW cluster of poles. Partial great circles connect poles of bedding planes within a fold. Circles near the periphery indicate steeply plunging folds, the circle bisecting the sphere is a fold with a NW-SE trending horizontal axis. Waitemata Group turbidites (Gregory 1969, Sporli 1989b) are exposed. Greywacke basement at Tiritiri is overlain by sandstones and conglomerates of the Kawau Subgroup (Fig. 2) of the Waitemata Group (Ballance 1976, Hayward & Brook 1984, Rickettse^ al. 1989) on a rugged, pre-Waitemata-Group 97 erosion surface. Best exposures are in the bays at the northeastern tip of the island (Fig. 2). Kawau Subgroup is overlain by Waitemata Group turbidite sandstone/mudstone sequences indicating rapid deepening after initial shallow water sedimentation at the base of the Waitemata Group. No Kawau Subgroup is exposed on Whangaparaoa Peninsula to the west. It and the underlying greywacke have been down-faulted an unknown distance below present land surface. The basement rocks under the unconformity on Tiritiri are part of the Waipapa terrane, which was accreted onto the New Zealand sector of the Gondwana margin by early Cretaceous time, after a long period of subduction under this margin during the Mesozoic (Sporli 1978). The accretionary processes have caused the Waipapa terrane to be strongly imbricated, with formation of repeated thrust slices, each with ocean floor volcanics, cherts and green argillites at the base, overlain by terrigenous elastics ("greywackes"). The slices dip predominantly to the west, at moderate to steep angles. In the Auckland region, the basal sequences within the slices have yielded late Triassic and early Jurassic radiolaria, while the elastics are of late Jurassic age (Sporli et al. 1989). The rocks in the Auckland area have been metamorphosed to prehnite-pumpellyite grade (Black 1989). LITHOLOGIES The basement rocks of Tiritiri are dominated by indurated sandstone. On the west coast, beds up to several tens of metres thick predominate, giving the outcrops a massive appearance. More thinly bedded intercalations of sandstone and argillite are present along the northern part of the east coast. The thick sandstone horizons are either green or grey and vary from very fine-grained to very coarse, ranging up to conglomerate horizons with pebbles up to 1cm or so in diameter. Horizons of disrupted black argillite fragments (chipwacke horizons of Mayer 1968) are common. The fragments can be either rounded or angular. The coarsest layers contain dominantly angular fragments up to several tens of cm in diameter. Some of the fragment are contorted. The thinner sandstone beds are graded, displaying a coarse bottom section with flame structures and load casts (because of the later deformation, these are only seen in smooth exposures and never in three dimensions). Laminations, cross bedding and convolutions mark the top of the beds. Argillites are either dark grey, black or green. Individual beds range up to several tens of cm thickness. Bedding-parallel quartz-rich horizons up to 15cm thick may either be recrystallized chert horizons or part of the vein swarms described below. 98 LARGE SCALE STRUCTURES Attitude of bedding: Northwesterly strikes and subvertical dips predominate on Tiritiri, with a small bias towards very steep westerly dips (Fig. 2, equal area net). The small number of younging determinations from graded bedding and other sedimentary structures (flame structures, load casts, Bouma sequences) all indicate younging to the SW. Major swings in strike occur near the wharf on the west side, and localities A and C on the east side of the island (Fig. 2). An area with dominantly eastward dips is located at the northernmost point. Overturned strata in steeply east dipping sequences are present at localities B and C. Large scale folding: very tight to isoclinal folds are the earliest folds visible (Fig. 3). Refolding patterns of minor structures indicate that they may be due to more than one phase of deformation. Fold hinges plunge moderately to very steeply, indicating that they were formed at a high angle to the axis of the next phase of folding. In their original orientation, these folds probably had subhorizontal axes trending approximately ENE and axial planes with relatively low dips. Asymmetry (vergence) is not known. The next phase (phase II, Fig. 3) consists of subhorizontal open folds with axes trending NW-SE (mode 145°) which determine the overall trend of bedding on the island. Axial planes appear to be steep. Asymmetry is difficult to detect, but at locality C (Fig. 2), a vergence to the east is indicated. The persistence of westward-younging across the island confirms that these folds consist of long west-dipping limbs and short east-dipping limbs. The major swings in strike of the beds indicate a third phase of deformation (phase III Fig. 3), consisting of folds with steeply plunging axes, but of a more open style than the steeply plunging first phase folds. These folds are still more or less in their original orientation. A steeply plunging fold refolding horizontal folds at locality C (Fig. 2) has a dextral vergence. The existence of westward-younging, steeply overturned beds at locality B (Fig. 2) may indicate yet another phase of folding, consisting of gentle warps with subhorizontal fold axes and low angle axial planes. The exact timing of this event is not certain, but it could be similar to the late "backfolding" present in the greywackes of the Ruahine Range of the eastern North Island (Sporli & Bell 1976, location see Fig. 1, this paper). Large scale faulting: An E-W trending fault at the northern end of the island, and the major NW-trending Eastern Fault Zone (Fig. 2) are the most prominent structures. The latter is parallel to the fault postulated between Tiritiri Matangi and Whangaparaoa Peninsula and therefore may represent part of the same tectonic regime. 99 PHASE III Fig. 3. Schematic diagrams, not to scale, showing phases of folding derived from analysis of orientation of bedding on Tiritiri Matangi. Note that a medium size dextral phase III fold exists in at least one locality. MESOSCOPIC STRUCTURES These need to be analysed in more detail; however some features relevant to the structural development of the basement rocks are listed below. Structures are 100 described approximately in the order of their formation and are related to the fold phases recognised above. Clastic dikes (pre phase I folding): Dikes up to 10cm in width can be observed in the thick sandstones of the northern part of the west coast. They are steeply dipping and trend mainly east to northeast, at right angles to a phase of very open subhorizontal folds. Early disruption (pre phase I folding): Where bedding is relatively little disturbed by later structures, systems of faults causing extension in the bedding can be recognised. The faults consist of thin, sharply defined zones of dark grey to black argillitic material.