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 E N E 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

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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.

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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

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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.

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L A R G E S C A L E S T R U C T U R E S

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.

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P H A S E 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

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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. There are also some rare shortening faults. In three-dimensional exposures, the faulting appears to have subdivided the rocks into irregular polygonal blocks.

Laminated veins (pre phase I folding): Groups of many subparallel quartz-prehnite veins are common. These swarms can consist of dozens of veins. In some groups, veinlets are only a few mm thick, and spaced a few mm to a cm apart; in others 10 to 15cm thick veins have spacings of the same order of magnitude. The lamination is caused by alternations of prehnite-rich and prehnite-poor vein portions and by vein-parallel tectonic discontinuities. The vein swarms are often subparallel or at low angle to bedding, however zones at higher angles to bedding also exist.

Crenulated veins: Crenulated veins (strongly ptygmatic-like) are often associated with the laminated veins described above and are always oriented obliquely or perpendicular to them. The oblique veins often show asymmetrical crenulations which change vergence with change in attitude of the vein.

Cleavage: Some argillites display a distinct cleavage oblique to bedding. In addition, the crenulation of the veins described above is due to intersection with a cleavage. In thin section, the grains of fine sandstones are surrounded by beard-like anastomosing seams of argillaceous material. This indicates that the rocks are well on their way to developing a slaty cleavage. We have not analysed the geometry of these cleavages systematically, but we have found cleavages both compatible with phase I and phase II folds.

Melanges: Highly disrupted melange or broken formation zones are not as common as in other areas of Waipapa terrane basement (Sporli 1978, Sporli et al. 1989). Occasional 10 to 50cm thick layers consisting of lenses of sandstone

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and vein material embedded in sheared argillite may represent equivalent structures.

Mesoscopic phase I folds: Highly asymmetrical, almost isoclinal folds affect the laminated veins and can also be seen along bedding contacts. Some of the folds form conjugate couples.

Post-laminated vein faulting (postdates phase I): Shortening faults were formed first and seem to be less common than later steeply dipping extensional faults, which are often quite closely spaced (10-30cm). In some instances, a progression from low angle to high angle shortening faults can be seen.

Simple white quartz veins: There are several phases of these veins. Some of them follow fault planes which disrupt the laminated veins (see above).

En echelon quartz veins (associated with phase II folding?): En echelon veins are 30-40cm long and are often sigmoidal, due to partially ductile deformation. Some indicate extension at right angles, others extension parallel to the dominant NW-trending phase II fold axis.

Quartz-chlorite fibre veins and faults: These are analysed further below. Some of the veins have breccia textures. Opening has taken place by the crack-seal mechanism (see Sporli & Anderson 1980).

Late faults: These faults display either sharp clean fault planes or contain fault gouge (crushed rock powder). The major fault zone along the east coast of Tiritiri belongs to this phase. A few of these faults, mostly extensional in nature and striking N N W or N-S, postdate deposition of the Waitemata Group.

Jointing in the basement rocks: The great majority of the joints predates deposition of the basal conglomerates of the Kawau Subgroup, i.e. they record a pre-Miocene period of uplift.

S T R I A T I O N A N A L Y S I S

Fibre striations on mesoscopic fault surfaces indicate incremental movements during brittle deformation. On Tiritiri, we have measured a large number of such striations, which mostly consist of quartz embedded in a chlorite matrix. If accretionary steps are present in the fault surface, the fibres not only give the orientation of the slip vector but also the sense of movement (Sporli & Anderson 1980 and Figs. 4, 5, this paper).

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A first step in an analysis of striations is construction of an m-axis for each fault, that is a line at right angle to the striation, in the plane of the fault (Fig. 4). The advantage of these lines is that there is no basic assumption about the parameters necessary for their construction and that they can be obtained whether or not a sense of slip on the striation is available. It has been demonstrated that m-axes, being the pole (perpendicular) of the "movement plane" and also being representative of an intermediate stress axis, are useful in linking seemingly complex patterns of striations (Angelier 1984).

The m-axis plot from Tiritiri Island illustrates several interesting points (Fig. 4):

Fig. 4. Upper left: Lower hemisphere plot of 273m-axes to quartz-chlorite fibre striations on Tiritiri Matangi. Block diagram below the net describes the construction of the m-axis. Note that the striation and the pole to the fault plane define the movement plane (M-plane) to which the m-axis is the pole. Right hand part of the figure: same data, contoured. Numbers on periphery of the net indicate maximum values for densities. Main trend of bedding is after equal area net in Fig. 1. Note gently west dipping and steeply east dipping great circle patterns. For further discussion see text.

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® N O R M A L FAULT TENSION (§)THRUST COMPRESSION © STRIKE SLIP COMPRESSION

Fig. 5. Results of analysis of quartz-chlorite fibre striations with steps indicating sense of movement, sorted into the three types of faulting (see diagrams of ideal conjugate fault couples below lower hemisphere equal area nets). Origin of fibre steps in an extensional jog is illustrated to the left of the thrust model. Steps are also shown in the models. Note that the 30° angle is assumed after work elsewhere (Sporli & Anderson, 1980). Large arrows with solid heads show tectonically relevant tension or compression axes (= principal stress axes). Main tectonic directions are shown with crosses and at the periphery of the equal area nets. N-S trend is taken from Fig. 4.

1. The dominant mode of movement is dip slip, as most of the m-axes are inclined (plunge) less than 45° , i.e. there are relatively few steep fault planes with low angle striations (which would correspond to steeply plunging m-axes).

2. Maximum concentrations of m-axes are oriented roughly north-south (in the 355° to 020° sector). This orientation differs by 30° to 55° from the major structural N W trend (see Fig. 4).

3. There is, however, a minor northwest plunging concentration of m-axes in the direction of the major structural NW-trend.

4. The main N-S concentration of m-axes is split into four submaxima, which may indicate that the deformation is not plane strain but has significant 3-D components (orthorhombic strain of Krantz 1988).

5. M-axes tend to spread out in two great circle patterns representing a) a vertical or very steeply east dipping plane and b) a plane gently dipping to the west. The second part of the striation analysis involves taking account of the sense

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of movement on those striated surfaces on which this could be determined. This allows determination of the "stress axes" (compression = major principal stress, m-axis = intermediate principal stress, tension = minor principal stress) of the system of faults.

For the purpose of this paper, we have not performed a full three-dimensional analysis of the fault population (e.g. Angelier, 1984; Marrett and Almendinger 1990). Instead, we have constructed compression and tension axes assuming an angle of 30° between compression axis and each striation, based on previous work at Whitford (Sporli & Anderson 1980).

In Figure 5 we have grouped the resulting axes according to the three major types of faults (normal, reverse (thrust) and strike slip). For dominantly normal faults, the tension direction is the tectonically most important axis, for thrust and strike slip the direction of compression is most important (see diagrams at the bottom of Fig. 5). A l l of these axes should be near horizontal.

Normal tension axes: These are mainly clustered around a direction at right angle to the N-S maximum m-axis concentration, with a slight tendency to swing towards the structural NW-SE trend, but there are no tension axes exactly parallel or perpendicular to the NW-trend. A few normal tension axes lie along the N-S direction.

Thrust compression axes: These are spread mainly between the 020° and the 130° directions, i.e. there is distinct thrusting perpendicular (NW trend + 90° in Fig. 5) to the main NW-SE direction. The swing towards the 130° direction indicates a regime of E-W thrusting, directly conflicting with the E-W tension axes described under normal faulting above. Since these two regimes are incompatible with each other, they must represent two different phases of faulting. There are also a few N-S trending thrust compression axes.

Strike slip compression axes: The two main clusters of compression axes are oriented N-S and NE-SW, the latter being at right angle to the principal NW trend. Minor groups of axes occur at right angles to the N-S direction and parallel to the NW trend. There are therefore two pairs of orthogonal and incompatible strike slip compression directions, N-S/E-W and NW-SE/NE-SW.

Discussion of striation analysis: It is possible to have normal faults, thrusts and strike slip faults all acting under one single regime of orthorhombic or 3D strain. The relations can be symbolised in the Harding diagram (Harding 1974, and Fig. 6, this paper) which follows the principle that thrust and strike slip compressions must be parallel to each other and that the tension direction of the normal faults coincide with the tension direction of the strike slip faults. This is also known

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C = compression

as the principle of coincidence of the principal horizontal stresses (Lensen 1975), because the axes of largest stresses in the horizontal plane (compressions of strike slip and thrust faults and the intermediate stress (m-axis) of the normal faults) all

are in the same direction. Applying this to the present

example, it is obvious that the tension axes trending E-W in Fig. 5A, and the thrust and strike slip compression axes trending N-S in Fig. 5B and C respectively form a Harding pattern and therefore are all compatible (Fig. 7A). An analogous pattern can be assembled around the of E-W compressions (Fig. 5B) and indicates an interchange of the compression and tension axes at some stage (Fig. 7B).

Another group faults which are compatible with each other have compressions (dominantly thrusting) at right angle (NE-SW) to the main NW trend (Fig. 5B and C) but there is no significant normal fault tension in the NW-SE direction

strike slip fault

thrust fault normal

fault

HARDING DIAGRAM

Fig. 6. Explanation of how a Harding diagram is constructed. The ideal conjugate fault couples shown in the upper three diagrams are all compatible with one single deformational system because their stress axes all coincide with one of three mutually perpendicular lines and in each case the largest principal stress in the horizontal plane (principal horizontal stresses = PHS) has the same orientation (note that C>m>T). The resulting Harding diagram at the bottom shows the strikes (horizontal map direction) of the faults involved in the pattern.

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n o r m a l t h r u s t s t r i k e s l i p f a u l t

Fig. 7. Harding diagrams indicating groupings of fault movements, based on fibre striation data from Tiritiri Matangi in Fig. 5. Note that the left hand diagrams represent the major movements, and the right hand diagram minor movements only. The upper pair are aligned on the N-S trend, the lower pair on the on the NW-SE trend. Converging solid head arrows represent tectonically significant compression, diverging arrows tectonically significant tension

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(Fig. 5A) associated with them. This regime of NE-SW compression (Fig. 7C) is similar to that seen at Whitford (Sporli & Anderson 1980). A minor regime of NW-SE compression (Fig. 7D) again indicates an interchange of the compression and tension axes.

It therefore appears that there were two groupings of faults. One consists of faults with a horizontal stress axis aligned subparallel to the dominant NW-SE trend (Fig. 7C and D). In the other, the horizontal stress axes are oblique to the NW trend (Fig. 7A and B). It is a yet unsolved problem whether these two groupings are totally independent or are part of one single, but relatively complex, regime. The coherent pattern of m-axes, consisting of two almost orthogonal planes intersecting along the N-S direction (Fig. 4, see spread of axes along the periphery and across the middle of the net) may indicate the latter.

DISCUSSION

The structures found in the basement greywackes of Tiritiri Matangi Island fit in very well with those seen in nearby areas (e.g. Sporli, 1978; Sporli et al. 1989). They are compatible with deformation in the accretionary prism active along the margin of the Gondwana super-continent in the Mesozoic.

In such a situation, sediments deposited on the ocean floor and transported towards the trench on the subducting plate would experience initial deformation as they approach the prism (Fig. 8, right hand part of diagram). If the plate movement vector is oblique to the margin, any folds formed at this stage may also be oblique (initial folds in Fig. 8). The sediments are then detached from the subducting plate and incorporated into the accretionary prism by thrust imbrication. As new thrust sheets are pushed under from the right, those already present are tilted to successively steeper dips (Fig. 8, middle part of the accretionary prism).The prism is thickened by this process, its base experiences metamorphism and becomes decoupled from the subducting slab. Because of this decoupling, only the component of the plate movement vector at right angle to the margin will be transmitted to this region, leading to de-activation of the initial oblique folds, formation of new folds with horizontal axes parallel to the continental margin and fault patterns with compression perpendicular to the margin, as shown in Harding diagram C of Fig. 8. Such a partitioning of the plate motion leaves residual strike slip motion not accounted for, which is taken up further back, in the most consolidated part of the accretionary prism. This leads to formation of steeply plunging folds, which in our case have a dextral asymmetry. Fault patterns in this region are represented by a Harding diagram with compression oblique to the margin (A in Fig. 8). After cessation of subduction, the sediments, now transformed into greywackes, are uplifted and

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Frontal Ridge Accretionary prism Strike Slip Trench - parallel Phase III Folding (decoupled) Oblique

Fig. 8. Schematic diagram showing tectonic situation of the accretionary prism in which the greywackes of Tiritiri Matangi were deformed in the Late Mesozoic. Modified after Lewis (1980). Sediments are transferred from the subducting to the overriding (Gondwana) plate by offscraping. Initial oblique folding is first replaced by margin-parallel folding (compression perpendicular to margin) and then by steeply plunging dextral folds (compression oblique to the margin) towards the back of the accretionary prism. Harding diagrams above the model (for explanation see Figs. 6 and 7) show associated patterns of faulting which caused the quartz-chlorite fibre striations. The lettering of the diagrams corresponds to that in Fig. 7.

exposed by erosion. Both large scale (Fig. 3) and mesoscopic structures seen on Tiritiri Matangi

fit into such a scheme. They range from early soft sediment and weak rock deformation to folding under conditions of regional metamorphism, followed by final very brittle faulting at high levels in the crust. Very early formed swarms of veins at low angle to bedding may record the initial horizontal shortening and dewatering of the sedimentary pile.

Dips of bedding are steeper than in other areas of Waipapa terrane rocks, where the average dip is about 50° (Sporli 1978). These steep dips cannot be related to the proximity of the western edge of the upfaulted greywacke block (Fig. IB), because at other localities along this boundary (Whitford, Motutapu) average dips are not as steep. Instead the steep dips may be due to the

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"backfolding" mentioned in the description of the large scale folds earlier in this paper.

The groupings of quartz-chlorite fibre striation faults (Fig. 7) may either record different regimes intermittently active at the same place or permanent regimes specific to different zones in the accretionary prism (as suggested in Fig. 8). For each of the regimes, interchange of compression and tension axes has occurred (Fig. 7).This replacement of compression by tension in the same direction may be due to a "stick-slip" mechanism involving alternating episodes of shortening and relaxation of the accretionary prism.

ACKNOWLEDGMENTS

Most of the field work for this paper was done by L M M in the summer of 1982/83 under supervision of KBS, who did the structural analysis and wrote the paper. We would like to thank Department of Conservation, Auckland Regional Council and University of Auckland Zoology Department staff for their logistic help and support with accommodation. Bruce Hayward and an unnamed referee made very helpful comments on the manuscript. Louise Cotterall is thanked for draughting of the figures. Rosemary Bunker helped with typing of the manuscript.

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