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Volume 83, 1955-56
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The Structure and Petrography of Greywackes Near Auckland, New Zealand

R. N. Brothers

[Received by the Editor, January 27, 1955.]

Abstract

Structural analyses of deformed greywackes and argillites at Tawharanui Peninsula and Bream Tail show that these basement locks have been affected by at least two periods of folding. The oldest and dominant structural direction is identified from axes of minor folds and intersections of shear planes. In successions of thick sandstones near Auckland, this axis is horizontal and undeformed with a north-and-south trend, but in areas of argillite a younger oblique axis of folding directed W.S.W.E.N.E has modified the earlier structures. The sandstones and argillites have undergone low grade regional metamorphism and contain quantities of such crystalloblastic minerals as albite, quartz, sphene, epidote, prehnite, stilpnomelane, chlorite, sericitic mica and occasionally calcite and hornblende. The rocks are compared with subzone Chlorite 1 schists of Otago.

Introduction

The basement rocks of North Auckland Peninsula are mainly indurated greywacke sandstones and argillites of Permian-Triassic age. On the eastern side of the peninsula the greywackes form high coastal ranges and are found in many of the off-shore islands, but to the west the surface of the undermass declines rapidly beneath a thickening cover of Cretaceous and Tertiary rocks.

Wherever they are seen the basement rocks offer ample evidence of deformation, such as shear planes and a multitude of joints in the hard and strong sandstones, and in the highly contorted nature of the argillites However, the apparent discordance of strike lines even over small areas, the inhomogenetty of fold axes, and the absence of a simple regional structure pattern are indications of a complicated tectonic history. These difficulties were encountered by Ferrar (1925; 1934) when mapping large areas of greywacke in Whangarer-Bay of Islands and Dargaville-Rodney Subdivisions. As a result Ferrar was able only to infer a regional fold axis based on observations of the northwestward prolongation of the peninsula and a similar alignment of certain greywacke ranges. Nevertheless, axes trending approximately east-west had been described by Bell and Clarke (1909) from Whangaroa Subdivision and Battey (1950) later found that a structural axis in this direction was dominant in basement rocks at Doubtless Bay.

In the present study a stereographic method of structural analysis has been attempted along the lines of Sander's work (1934; 1950) in Austria. The results seem promising, for not only are two main axial directions revealed in the greywackes, but also plunging fold structures are shown to be important. In the present case syntheses have been made at only two localities, 20 miles apart, near Auckland, and more analyses are required from a larger area before any worthwhile conclusions can be expounded on a regional basis. All projections have been made on the lower hemisphere of an equal area net.

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Petrographic descriptions of the North Auckland greywackes generally are lacking in the literature so that little information is available regarding composition, texture, replacement products and vein minerals throughout the district. The rocks described here have been altered under conditions of low temperature and high shearing stress to such a degree that comparisons can be made with the Chlorite 1 subzone of regional metamorphism in Otago.

Structure

In North Auckland a rough distinction can be made in the field between areas of massive greywacke sandstones and areas composed mainly of inter-bedded sandstones and argillites. The massive sandstones are the most competent beds and where they reach several thousand feet in thickness, as at Waiheke Island (Halcrow, 1953), it is unusual to find any megascopic effects of deformation other than joint systems. On the other hand, the relatively incompetent successions of thinly bedded argillites and sandstones have been sensitive to fold movements, and they provide ample evidence of the pattern of deformation. Beds of this type are found on the east coast of North Auckland at Tawharanui Peninsula, two miles north of Kawau Island, and at Bream Tail, 12 miles south of Whangarei Heads (Fig. 1). Analyses of structure discussed below have been made at these two localities.

Three planar structural elements, developed as a result of deformation, have been recognized in the interbedded sandstones and argillites:

(a)

schistosity parallel to bedding planes (S1).
(b)

shear planes (S2) at an angle to the bedding.
(c)

joints.

In addition, the northwest-southwest trend of one warped tectonic axis in these rocks can be measured directly from folds involving S1. A second fold axis with trend approximately 070° has been determined from stereographic projections of bedding planes (S1).

Bedding schistosity (S1).

The development of schistosity in the greywacke rocks was conditioned in the first instance by the presence of interbedded sandstones and argillites, usually in half-inch to 3-inch layers (Fig. 2). Movement along the interfaces between the coarse and fine grained beds, and along bedding surfaces in the argillites, has been the most immediate effect of deformation upon these rocks. The argillites typically were adjusted to stress conditions by slip movements along numerous, close spaced S1 planes, and in this fashion they attained a high degree of plasticity. However, each rigid intercalated sandstone band yielded not by flowage, but by a system of tension fractures parallel to the tectonic axis. Where movement along S1 in the enclosing argillite layers produced stretching effects exceeded the ultimate strength of the sandstone, the latter broke into segments. A form of plastic flow thus set up in these rocks, and operating by bedding plane slip, has developed a highly characteristic type of tectonite. The sandstone layers are seen in all stages of boudinage, with individual boudins up to several feet in diameter elongated parallel to the axis of folding and slickensided in directions normal to this axis. The boudins are strung out in the plane of S1 and are separated by the darker fissile argillites. Due to the highly deformed nature of the rocks it is difficult to determine if such features as boudinage and

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Fig. 2.—Hand specimen of argillite sheared parallel to S1 and containing numerous boudins of greywacke sandstone. Broken lines indicate the traces of megascopic S2 planes Thin-section 3935 Tawharanui Peninsula.
Fig. 3.—β diagram for S2 planes at Tawharanui Peninsula, contours at 22%. 14%. 8% and 4% per 1% area X = extreme positions of fold axes (A1) involving S1 and measured in the field The pole of the younger told axis A2 is shown.

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Fig. 5a—Lower hemisphere projection of the interred position of told axis A1 and A2 at the onset of folding about A2 Axis A1 is restored to a horizontal position, allows on the great circle normal to A indicate the theoretical path for folding of A1, poles about A2
Fig. 5b—Axis A1 unrolled about A2 A1 for Tawharanui occupies a north-and-south position but for Bream Tail trends roughly east-west
Fig. 6—Saussuritized and sheared detrital feldspar in greywacke sandstone, the matrix is wholly reconstituted to a fine grained mesh of chlorite, prehnite, quart and sericitic mica Crossed nicols Thin-section 3933, near Goat Island Leigh
Fig. 7—Quartz-preh [ unclear: ] te-albite vein in greywacke sandstone Crossed nicols Thin-section 3934, Kawan Island
Fig. 8—S2 shear planes in argillite, bedding planes (S1) run left and right Ordinary light Thin-section 3917. Tawharanui Peninsula
Fig. 9—High angle quartz veins in argillite broken and folded by renewed movement on S1 Ordinary light Thin-section 3913. Thin-section Peninsula

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Fig. 1 Areas of Paleozoic & Mesozoic Rocks in North Auckland

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shear planes are related to pre-existing sedimentary slump structures. However, the strong regional orientation of the boudins and slip surfaces, together with their consistent alignment with an axis of major folding, indicates formation during a period of compressive deformation.

Megascopic quartz segregations are not abundant along the S1 surfaces, although sweating out of silica in the finer grained rocks was probably commenced at a very early stage of deformation. Quartz veins are commonly present in the sandstone beds and boudins where they occupy tensional fractures and line small shear surfaces, thus forming a rough B-lineation.

High angle shear planes (S2).

Well defined shear planes (S2) which cut across the bedding schistosity (S1) at angles between 5° and 70° can be identified in hand specimens and thin sections (Figs. 2, 8). In all cases S1 is an older structure than S2. but in places renewed movement along S1 has annealed and blurred the outlines of the high angle shear planes. The angle between the bedding schistosity and S2 most commonly lies between 40° and 60° and for this reason S2 is marked in the field by abrupt changes of lithology where sandstone beds have been brought side by side with argillites, or where the limbs of small folds have been sliced offsetting identifiable horizons Unlike the bedding schistosity, S2 is not confined to the argillites. These shear planes cut across both lithologic types, and so far as observations have been made they do not suffer any change in orientation in passing from fine to coarse grained rocks in thinly bedded successions.

Although S2 planes appear to be diversely oriented in the field, equal area projections (Fig. 3) show that lines formed by intersection of these planes are parallel to a warped tectonic axis trending northwest-southeast; this axis can be identified in the field from folds involving S1. Similarly, fine lineations developed on the quartz-chlorite-epidote sheets lining S2 surfaces lie in a zone normal to the fold axis in the same manner as slickenside lineations on the sandstone boudins.

Joints.

Regularly spaced cleavages with consistent dip and with no evidence of movement on their surfaces have been recorded as joints. At Tawharanui and Bream Tail major sets of joints which intersect S2, but show neither displacement nor distortion, must be regarded as features developed at a very late stage in deformation. The majority are (Okl) planes oriented approximately at right angles to the warped northwest-southeast fold axis. Jointing is a much more common feature in the massive competent sandstones than in the argillites.

The multitude of joints that cleave the sandstones into irregular facetted blocks are characteristic of greywackes in North Auckland. In the field it is almost always possible to recognize one major joint system at a particular locality and to relate this rationally to a fold axis, but the regional picture for all joint trends is less clear. Since it can be shown that the greywackes have undergone at least two periods of effective deformation about axes not co-axial, this maze of joint systems must be the product of at least two different stress patterns. However, an older set of buckled or folded joints cannot be found in outcrops. The absence of joint distortion is probably related to the manner in which the competent sandstones have yielded during deformation; that is, by well spaced surfaces of shear and broad flexures rather than by tight folds.

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Structural Analysis

In interbedded successions of argillites and sandstones, even where the latter may reach 6ft to 8ft in thickness, several clear distinctions may be made regarding the adjustment of the sandstones on the one hand and the argillites on the other, to compressive deformation.

Sandstones. Lack S1, but cleaved by joints and major S2 planes; boudinage usually developed with boudins elongated parallel to the structural axis; tension cracks parallel to B filled with quartz; small scale folding absent.

Argillites. Possess well developed S1 and S2 cleavages; joints usually not present; quartz veins less common than in sandstone; tensional partings absent; small scale folds often evident.

At Tawharanui and Bream Tail the beds strike approximately east-west and S1, the most distinct planar element in the rocks, forms open folds about an axis plunging to west-southwest. However, in the same beds such axial structures as boudins, small tight folds in S1, and the imperfect B-lineation formed by planar intersections of quartz veins to S1, and S1 to S2, are clearly related to an axis trending roughly northwest-southeast. Furthermore, this axis coincides with lines of intersection of all S2 planes, and in addition it is the normal to a zone containing the lineations on these planes. At Bream Tail the axis plunges northwest at up to 50°, but at Tawharanui the plunge is reversed to 35° southeast.

On field evidence the northwest-southeast fold line (A1) is taken as an older structure that has been warped across a younger superimposed east-west axis (A2). Further data on these axial trends has been assembled by statistical analyses of β axes constructed from projections of S1 and S2. This method has been used successfully by Sander (1950) in Austria and by Weiss (1954) in California to determine the presence of repeated foldings about axes which differ in trend and plunge. The following quotation from Weiss (1954, p. 14) is a clear account of the method for plotting β axes “One result of Sander's work that is of importance in any area, however, is the recognition of β axes. These are determined for individual small areas in the following way. The great circles corresponding to measured foliation planes in each small selected area are plotted upon an equal area projection (see Fig. 4a). The intersections of every possible pair of foliation planes in each area are then transferred as points to a second diagram which is contoured in the normal way for density of distribution of points. Any pronounced statistical maximum that appears in this diagram is termed a β-maximum, and represents the axis of the flexural folding which has affected foliation throughout the area. If a single homogeneous deformation has occurred, there should be one β-maximum coinciding with the B-maximum (the statistical maximum of B-lineation poles within the same area). Similarly, if the poles of the foliation planes (π-poles) are plotted, then they should fall upon a great circle of the projection with β as its pole. The preparation of β diagrams is thus a test for homogeneity of folding, and for the number of deformations recorded in the attitude of the foliation.”

The projection of S1 foliation planes for Tawharanui Peninsula (Fig. 4b) shows a marked spread of β axes in a broad girdle pattern with a distinct double maximum. Two fold axes, A1 and A2, have been measured in the field, but in projection the younger east-west axis A2 dominates the structure. S1 planes have thus been rotated from a former position related to A1, with apparent northwest-southeast strike, to their present attitudes. Incompleteness of reorientation about A2 is indicated by the double maximum on the 12% and 8%

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Fig. 4a.-Projection of foliation planes to determine a β-maximum, and of poles to foliation planes to determine the π-circle. After Weiss (1954).

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Fig. 4b.-β diagram for S1 planes at Tawharanui Peninsula, calculated on 480 β points Contours at 12%, 8%, 4%, 2% and 1% per 1% area. The extreme positions of poles of A1 axes measured in the field are shown for Bream Tail and Tawhaianui.

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Fig. 4c.-β diagram for S1 planes at Bream Tail, calculated on 100 β points. Contours at 15%, 10%, 6%, and 2% per 1% area. The extreme positions of poles of A1 axes measured in the field are shown for Bream Tail and Tawharanui.

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Fig. 4d.-β diagram for S1 on data combined from Tawharanui and Bream Tail, calculated on 300 β points; contours at 16%, 8%, 4%, 2% and 1% per 1% area. Axis A2 plunges to 250° at 40°. The extreme positions of poles of A1 axes measured in the field are shown for Bream Tail and Tawharanui.

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contours of Fig. 5, and by the spread of β points along the 1% contour at the limits of the girdle pattern. At Tawaharanui the beds dip mainly to the south, and elongation of β contours in this direction emphasizes the presence of relict attitudes developed by A1, which here plunges southeast.

A similar spread of β axes is present in the projection of S1 for the Bream Tail rocks (Fig. 4c). A strong girdle pattern is again evident with a double β-maximum. The interbedded argillites and sandstones dip mainly north-west at Bream Tail, and the plunge of A1 is in the same direction. The strongest β-maximum at 15% does not differ greatly in position from the pole of A1, suggesting that as folds parallel to A1 were warped downwards to the north about A2 there was only gentle flexuring of S1. However, dis-orientation of the original A1 structure was strong enough for the inhomogeneity of the second fold axis to be recorded as an elongated spread of β axes along a girdle pattern.

A combination of the data from Tawharanui and Bream Tail, plotted as a projection of β axes for S1, is given in Fig. 4d A strong maximum at the 16% and 8% contours gives the plunge of A2 as 40° to 250°. Attitudes of S1 impressed by the non-coaxial fold direction A1, and not completely re-oriented, cause the contours of 4%, 2% and 1% to follow a well defined girdle.

A study of axial and planar structures in the beds of both localities indicates that A1 was the line of most intense folding. The development of S1 foliation, the regular intersection of S2 shear planes, the alignment of tension fractures and boudins in the sandstones and a rough lineation parallel to tight folds are features that were imprinted on the rocks during deformation about axis A1. The monoelmic symmetry of these structures, with the fold axis as the pole to the symmetry plane, is typically that of B-tectonite Later flexural folding about A2 by comparison was weak in intensity, although reorganization of the fold pattern in the interbedded argillites and sandstones was sufficient to allow recognition of the trend and plunge of this younger axis. In addition, there are sparsely distributed major planes of shear in the argillites which show alignment to A2 and are approximately normal to S2 in orientation; also finely developed slickensides on some as joints related to A1 show that these older cleavages became planes of minor adjustment during folding about A2.

There are two ways in which it is possible to unroll the present attitude of the beds about axis A2, and thus cancel out the effect of that axis, in order to obtain the original trend of A1. The more thorough method involves rotation of all structures about A2 on a projection net. The directions and amount of correction would be about 50° northwards for the Bream Tail beds and about 35° southwards for Tawharanui assuming A1 was originally a horizontal axis, or nearly so. However, the second method is more quickly applied and it points out an anomaly in the data already considered. An apparently original and undeformed orientation for A1 can be determined from structures in the massive sandstones that outcrop in Hauraki Gulf at Waiheke Island and the Noisies Islands. These rocks usually are not bedded, but in places it is possible to measure the attitude of interbedded siltstones. For example, at Waiheke Island (Halcrow, 1953) a horizontal fold axis corresponding to A1 has an average trend of 007°. At the Noisies Islands the same axis consistently trends 350° in beds standing nearly vertical. As pointed out earlier, these thick beds of competent sandstone seem to have suffered only slight flexural deformation when compared with the less resistant argillite and sandstone successions. The structures preserved in the

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massive greywackes are those related to the earlier more active axis of folding A1. If the original orientation of A1 at Tawharanui and Bream Tail is taken as 350° and horizontal, the data given in Fig. 5a can be assembled. In Fig. 5a axis A1 is restored to a horizontal position directed 350° and A2 is plotted as plunging to 250° at 40°, giving an inferred structural picture at the onset of folding about A2. The intersection of A1 and A2 is almost orthogonal (84° and 96°) so that the effect of downwarping A1 to the south (Tawharanui) and north (Bream Tail) could be expected to produce a symmetrical picture of folding in the projection. That is, poles of A1 should rotate eastwards and downwards on a great circle, the pole of which approximates to A2. The great circle is shown as a solid line in Fig. 5a, with the correct direction of rotation of A1 indicated by arrows. It can be seen that at Tawharanui folding of A1 about A2 agrees quite closely with the theoretically determined path of the great circle normal to A2. However, further north at Bream Tail the warped positions of A1 do not conform with this scheme of folding, and they are significantly offset to the west by about 50°.

This anomaly is more clearly shown by restoring A1 to the horizontal by actual rotation about A2 on a projection net (Fig. 5b). The southward plunging axes at Tawharanui occupy positions which almost coincide with axes measured by Halcrow (1953) in simply deformed sandstones at Waiheke Island. However, when the Bream Tail axes are brought to the geographic horizon, or as close as possible, there is no accordance with any structure so far recorded in the greywackes; but it is perhaps significant that the axial trend is remarkably similar to the alignment of a W.N.W-E. S. E. anticlinorium recorded by Bell and Clarke (1909) in Whangaroa Subdivision.

Discussion on Structure

Evidence for the main structural lines in the North Auckland basement rocks has been reviewed by Ferrar (1925) and Bartrum and Turner (1928). In the Whangarei-Bay of Islands Subdivision Ferrar (1925. p. 18) found that “the directions of observed strike are discordant,” but from the alignment of Takon and Huriuki greywacke ranges in a northwesterly direction he thought it “reasonable to suppose that the Mesozoic earth-movements were in the nature of folds along northwest to southeast axes, the folds bending over eastward toward a submerged area.” In addition Ferrar recognized that another system of folding in these rocks was indicated by the E. N. E.-W.S.W antielinorium described by Bell and Clarke (1909) in the Waipapa Series of Whangaroa Subdivision. Bartrum and Turner (1928) agreed with Ferrar that the age of folding about the N.W. axis was probably Early Cretaceous and cite as evidence from North Cape a well developed foliation in gneissic gabbros.

In several places the structure of the greywackes has been shown to have no relation to the northwest extension of North Auckland Peninsula. For example, Marshall (1908, 1911) has recorded a north to northeast trend in the north part of the Bay of Islands, and Bell and Clarke (1909) have described two large anticlinal forms with axes directed respectively E.N.E.-W.S.W. and W. N.W.-Es.E. More recently Battey (1950) has referred to violent folding about east-and-west axes in the basement rocks at Doubtless Bay.

In the structural analyses made at Tawharanui and Bream Tail the age of the east-west folds has been fixed as post-dating an earlier apparent N -S axial direction. From the following table this sequence of events can be seen to fit the

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general pattern of deformation that other writers have described in North Auckland. Where an author has given no definite age for folding about an axial trend the approximate time is indicated in parentheses; unless otherwise stated the directions given are those of fold axes.

Author Area. Early Cretaceous Up. Cret.-Low Tert. Upper Tertiary
Bartrum and Turner (1928) North Cape N.W -S.E. Nw.-S.E. (fracturesystem)
Battey (1950) Doubtless Bay E.-W. N.W.-S.E. (post Cretaceous compression axis)
Bell and Claike (1909) Whangaioa Subdivision W.N.W.-E.S.E. and E.N.E.-W.S.W (post-Waipapa)
Healy (1949) North Auckland E.-W. (post-Waipapa strata)
Mason (1953) Hokianga E.-W. N.W.-S.E.
Ferrar (1925) Whangarei-Bay of Islands N. W.-S.E.
Ferrar (1934) Daigaville-Rodney N. W.-S.E. [ unclear: ] N.S [ unclear: ] E.-W.
Arhdge (1954) North Kaipara E.W. N.W.-S.E.
Turner and Baitrum (1929) Takapuna-Silverdale N. W.-S. E. N. E.-S.W. (fracture system)
Brothers (1954) Waitakere Hills N.W.-S.E.
Halcrow (1953) Waiheke Island [ unclear: ] (post-Trias-Jura)
Bartrum (1921) Great Barnier N. W.-S.E. (post-Trias-Juia)
Fleming (1953) North Auckland N. W.-S.E. ( [ unclear: ] -Tertiary)

From the field evidence and the literature on North Auckland geology there appears to be little doubt that a fold direction now oriented northwest-southeast is the dommant structural element. The structural analysis presented here suggests that the compression axis A1 originally was aligned closer to north-and-south; in areas of softer rocks, folds associated with A1 were warped across a younger axis (A2) to form plunging structures directed northwest-southeast. Turner and Bartrum (1929) believed that a recurrence of folding along this N.W. -S. E. line in the Upper. Tertiary was the last orogenic impulse. Similarly, Mason (1953, p. 368) has shown that “in the Hokianga area, Lower Miocene rocks have been folded about a north-west axis, whilst Upper Cretaceous rocks in addition to this north-west folding show evidence of a roughly east-and-west folding.” It is difficult to estimate the extent to which late Tertiary northwest folding affected the greywacke basement rocks. In general, beds of the Lower Miocene Waitemata Group, the youngest Tertiary marine sediments in North Auckland, are only gently flexed and in most places are characterised by low dips. On the other hand Mason (1953, p. 368) believes that late Tertiary northwest folding is “the logical explanation for the trend of the North Auckland Peninsula.” In the interpretation of data from Tawharanui and Bream Tail the anomalous orientation of the originally north-south fold axis A1 at Bream Tail (Fig. 5a) could be explained perhaps as the effect of localised renewed folding along a N.W.-S.E. structural line in the Upper. Tertiary. However, the writer holds the opinion that during the orogeny which post-dated the Lower Miocene, fold movements

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were not sufficient in intensity to produce contortions in the greywackes, and that flexural folding was subordinate to faulting along old structure lines. It is possible that disorientation of the A1 axis at Bream Tail is the result of transcurrent fault movements, evidence for which at present is obscure in the greywackes.

In summary, the fold axes described in this paper from within the basement rocks are tentatively related to the regional structure as follows:

(a)

Axis A1 is equivalent in direction, but not necessarily in age, to the Early Cretaceous northwest-southeast fold axis recognized by Ferrar (1925), Bartrum and Turner (1928) and Macpherson (1946). When this warped axis is unrolled its original trend appears to have been north-and-south rather than northwest-southeast.
(b)

Axis A2 is equivalent in direction, but not necessarily in age, to the Upper Cretaceous or Lower Tertiary east-west line of folding recorded by Bell and Clarke (1909), Battey (1950) and Mason (1953).

(c)Disorientation of the warped axis A1 at Bream Tail may be the result of renewed Upper Tertiary folding along the northwest-southeast line, or an effect of fault-block dislocation.

Addendum

The structural synthesis given above is based upon field measurements and analyses of projections of dip surfaces, small-scale fold axes and shear planes. As far as possible speculation and extrapolation have not coloured any of the arguments advanced. However, there are several features of the structure that do not merge with the deduced tectonic history and in certain respects that history must be regarded as unsatisfactory.

Briefly, this history is:

(1)

Severe folding about a north-to-south axis (A1).
(2)

Later folding about an east-to-west axis (A2).

On the evidence presented in this paper two corollaries must be appended:

(a)

The east-to-west axis (A2) had a primary plunge of 40° to 250°.
(b)

The apparently “anomalous “position of A1 at Bieam Tail must be explained.

The antipathetic association in the same tectonic region of a steeply plunging axis (A2) with an axis (A1) that is mainly horizontal is unusual. In this respect Sander (1934) has distinguished between areas with steep fold axes and those with horizontal or slightly inclined axes, and he has emphasized that this distinction is fundamental in the study of regional tectonics. If it is presumed that A2 was originally a flat axis, with bearing 250°, then the two unusual corollaries are eliminated and A1 at Bieam Tail occupies the correct position after warping across a horizontal A2 axis The validity of this assumption could be tested by constructing further β diagrams for other areas in North Auckland; of A2 was originally a horizontal axis of deformation then no poles of A1 should be found in the north-east quadrant of the projections. Under those circumstances the present plunge of A2, at 40° to 250°, must be due to a renewal of folding along A1. post-dating A2.

Petrography

The main feature in the mineralogy of the North Auckland greywackes is the large degree of alteration that has taken place under conditions of low grade regional metamorphism. The environment of reconstitution has been determined by a combination of high shearing stress and low temperature.

In terms of total reconstitution the incompetent fine grained argillites have progressed much further than the rigid sandstones, but the latter have nevertheless suffered sufficient micro-brecciation and re-crystallization as to warrant comparison with the Chlorite I sub-zone of metamorphism in the Otago schist mass. The mineralogy described here is based on thin-sections cut from greywacke sandstones and argillites at Bream Tail, Tawharanui Peninsula, Kawau

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Island, Waiheke Island and the Hunua Ranges. Numbers given in parentheses refer to slides in the petrology collection at Auckland University College.

Sandstones.

In hand specimens the sandstone boudins contain few obvious shear planes, but under the microscope the effects of stress are more apparent. Small offsets in crystals and short chlorite-lined dislocations are common, the most marked effects being seen in distorted twin lamellae of detrital feldspar (3933; Fig. 6). Quartz and feldspar grains often show undulatory extinction to some degree. Chlorite, white mica and recrystallized quartz fill solution embayments and fractures in the clastic fragments. The calcic feldspar (andesine to labradorite) is usually obscured by dense aggregates of saussuritization products (e.g., 3932). Detrital flakes of brown biotite are mainly replaced by chlorite, and similarly small pools of wispy chlorite probably were derived from the alteration of ferro-magnesian minerals. Simple rounded crystals of green epidote with strong birefringence and high relief stand out in thin-section.

The matrix of the sandstones invariably is recrystallized to a fine-grained mesh containing chlorite, quartz, albite, prehnite, epidote, clinozoisite, sphene, ilmenite and rarely calcite or flakes of stilpnomelane (3840). Epidote and clinozoisite are granular in form, but in a few large aggregates, probably from an original detrital source, the calcium rich variety forms a rim on ferriferous epidote. Prehnite similarly is found in irregular granular patches (e.g., 3921, 3922, 3924) where it has crystallized from the matrix. On the other hand, small pools of albite (1508, 1611) contain many euhedral crystals twinned on the albite law, allowing the composition to be fixed between An0 and An4.

Mylonitization is never well developed in the sandstones, even on a microscopic scale. Tension fractures filled with quartz, prehnite and albite are common (Fig. 7), and in some small boudins the original texture has been concealed by extensive silicification. In these veins prehnite forms crystals tabular parallel to [001] and elongated parallel to a crystallographic axis (= X vibration direction); 2Vz lies between 48° and 80° (3926, 3927, 3934). The quartz-filled fractures in some cases were the sites of later small scale shearing, so that the veins have been slickensided and their boundaries lined by bladed chlorite. In places veins are cut across by minor surfaces of shear, allowing the introduction of fresh tongues of quartzose material which occasionally transect larger crystals of prehnite. As a result, older crystals of quartz and albite in these deformed veins show anomalous polarisation developed by strain (3934). Thin seams containing fine grained epidote and oriented laths of hornblende occur rarely (e.g., 3933). The amphibole is undoubtedly a crystalloblastic mineral and has the following properties: pleochroism, dark brownish green to light brownish green; elongation negative; birefringence 0.020; 2Vx large; and extinction angle 18°–20°. The crystals reach a maximum size 0.12 mm by 0.03 mm with a perfect cleavage parallel to length.

Detrital quartz and feldspar are generally subequal in amount in the sandstones. By comparison the rock chips present vary in quantity from minor amounts to almost 90% of the total clastic content. These composite fragments are mainly greywacke sandstone, finer grained sediments usually chloritised or iron stained, and igneous flow rocks made up of small elongated feldspars with trachytic texture. Some of the lavas are undoubtedly alkaline, with sodic feldspars giving extinction angles perpendicular to a at 8°–17°. Other undetermined

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igneous rocks are largely replaced by prehnite, but appear to be andesitic in nature.

Argillites.

Detrital fragments in the argillites are finer grades of the clastic material forming the sandstones. Complete recrystallization of the matrix as a fine grained crystalloblastic aggregate of quartz, albite, chlorite, colourless micas and lime silicates is typical of the argillites. Blastoporphyritic kernels formed from broken sandstone layers stand out in thin-section with a coarser grain and much lower content of regenerated mica than the enclosing argillite. The bedding schistosity is nearly always emphasized by a strong alignment of mica flakes. Detrital feldspars are thoroughly saussuritised or sericitised and clastic quartz grains show solution effects.

The S2 planes of shear show incipient development even in the least deformed argillites (e.g., 3917, 3918), and in some slides cut perpendicular to the tectonic axis A1 traces formed by white mica flakes parallel to S2 are more prominent than those of S1. The manner in which the high angle planes of shear are brought into operation within the argillites is clearly demonstrated in thin-sections. Where the angle of S1 to S2 has remained below 35°, partial adjustment to shearing effects on S2 has been effected by slight rotation of the clastic fragments. Complete rotation of the quartz and feldspar particles does not seem to have taken place as their long axes are never consistently parallel to S2; moreover, chlorite swirls and pressure shadows of mica are absent. Differential solution, probably activated by shearing stress on S2, has been active around the periphery of each grain producing pits filled by small mosaics of recrystallized quartz. In many cases the clastic fragments have been reduced by solution to a rhombic shape, with the edges parallel to the traces of S2 (3918). A sympathetic orientation is shown along the length of elongated white mica flakes that have been developed by paratectonic crystallization close to the detrital grains. The grain pattern in thin section is remarkably similar, but on a smaller scale, to diagrams given by Phemister and Williamson (1954, pp. 7–8) in a discussion on quartz orientation in a comparable type of rock called by them a quartz-schist.

At angles greater than 35°, and up to a maximum of 70°, there has been considerable movement on S2 with rupture of the S1 bedding layers (Fig. 8). Although megascopic veins are not common in the argillites, thin-sections reveal many segregations of quartz which parallel S1 as elongated pods. In rocks where S2 planes have reached an angle greater than 35° to S1, and have become surfaces of dislocation, quartz veins transgressive to the bedding become plentiful. Invariably these veins lie along the high angle S2 planes (3915) and appear to be formed by crystallization from solutions using S2 as a means of egress from areas of high stress.

In the most intensely deformed argillites movements along S1 surfaces have intersected and disrupted many of the earlier formed quartz veins and reduced them by flexural slip to “schuppen” of microscopic size arranged en echelon (3917). Where the argillites reached this stage of plasticity with S1 and S2 operating freely and synchronously as movement planes, the development of small folds involving S1 and quartz-filled S2 planes can be seen in thin-section (Fig. 9).

There is often a strong colour contrast in hand specimens between the dark argillites and the included boudins of greywacke sandstone. Chemical tests for

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carbon in the argillites proved negative, but partial analysis of specimen 3935 by Mr. T. Wilson, Chemistry Department, revealed the presence of a high percentage of iron oxides.

  • SiO2 = 69.4%

  • R2O3 (mainly Fe2O3 = 23.3.

  • CaO = 2.2.

  • Ignition loss and moisture = 3.5.

  • Undetermined = 1.6

In thin section 3935 is typical of most argillites, with a very fine grained skein of minerals developed parallel to S1. A few grains of detrital quartz and epidote are embedded in this mesh. Small crystalloblastic minerals are elongated along the bedding planes in fine sheaf-like clusters with a greenish-yellow to yellow-brown pleochroism; cleavage is imperfect and extinction is parallel, but lacks the mottled effect seen in biotite Although precise optical constants could not be measured the mineral has many features in common with the stilpnomelane group (Hutton, 1938). This tentative identification is supported by the unusually high content of Fe2O3 in the rock. Redistribution and concentration of Fe, Si and alkalies by metamorphic differentiation has been an important process during alteration of the greywacke rocks, and it is reflected by the numerous veins and crystalloblastic patches of quartz, albite, micaceous minerals and the lime silicates.

Discussion on Petrography

Partial metamorphism of the argillites and sandstones in the greywacke series has produced a type of semischist which is very similar to low grade rocks in the South Island chlorite schists. In hand specimens of the greywacke sandstones there is generally no sign of schistosity except in the case of some smaller boudins that have been finely sheared and silicified on a macroscopic scale Lack of foliation in the greywackes, the absence of augen structures derived from clastic grains, and the minor amount of granulation in thin-section are criteria for rocks in the Chlorite 1 subzone of regional metamorphism. In the literature many described examples of Chlorite 1 and lower grade Chlorite 2 rocks (e.g., Turner. 1935; Mackie, 1936; Hutton, 1940) leave no doubt that in the Auckland greywackes the degree of alteration has reached the intensity of the Chlorite 1 subzone. Apart from differences in the occurrence of pumpellyite, the Auckland rocks can be favourably compared with greywackes of the Waihemo Series in Central Otago, classed by Turner (in Williamson, 1939) as Chlorite 1 subzone schists.

In some instances the more complete recrystallization of the argillites to a stage where crystalloblastic materials form up to 80% of the rocks emphasizes the rapid reconstitution of the finer grained greywacke members. Turner (in Williamson, 1939, p. 36) has commented upon the advanced nature of metamorphic recrystallization in similar pelitic rocks interbedded with greywackes and semischists of the Kakanui Series.

The main differences between the greywacke series described here and low grade rocks on the margins of the Otago schists are related to the presence of interbedded argillites in the Auckland area. Turner (1935) and Hutton (1940) have outlined the succession of Chlorite subzones in Northwest Otago, where the parent rocks are mainly massive greywackes. Turner (1938) noted that pelites are relatively rare in the Central Otago schists and defined the subzones

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in terms of derivation from greywacke sandstones. In listing these divisions of the Chlorite Zone Turner (1938) placed most emphasis on textural criteria—i.e., the completeness of recrystallization and destruction of clastic texture, rather than index minerals, since the same assemblages of reconstituted minerals tend to occur throughout the Chlorite Zone. The two lowest subzones (Turner, 1938, p. 163) are defined in this way:

Subzone Chl. 1.

Greywackes retain their original clastic structure; schistosity has not developed; slight cataclasis is evident and an interstitial matrix of reconstituted minerals (epidote, chlorite, calcite, actinolite, sphene) has commenced to form; feldspars are converted to saussurite or are much sericitized.

Subzone Chl. 2.

Greywackes give place to semischists in which clastic structure has been partially obliterated, the grain-size reduced by shearing, and a definite schistosity has developed; chemical reconstitution is far advanced but not yet complete.

When these divisions are applied to the partially metamorphosed successions of interbedded greywacke sandstone and argillite in North Auckland the coarser rocks fall mainly into Chl. 1 subzone. On the other hand the pelitic members often have a texture characteristic of subzone Chl. 2, but a notable difference in the Auckland argillites is that cataclastic fragmentation is not important; differential solution and concomitant recrystallization associated with close-spaced planes of shear have modified the detrital fragments from the onset of deformation in these fine-grained beds. In this way metamorphic recrystallization of both the matrix and the clastic grains proceeded more rapidly than in the coarse greywackes and produced in some places alternating beds respectively Chl. 2 and Chl. 1 in grade. Mackie (1936, p. 130) encountered a similar difficulty when classifying regionally metamorphosed greywackes in North Otago and concluded that some allowance had to be made for the grain size of the parent rock.

Despite a careful search no undoubted grains of crystalloblastic pumpellyite have been identified in the Auckland greywackes except for rare pools in igneous rock fragments (3933). Granular minerals associated with the replacement of plagioclase feldspar or occurring as crystalloblastic constituents of the rock matrix invariably have the properties of calcium epidote and lack the characteristic pleochroism and birefringence of pumpellyite. However, prehnite is developed in the altered greywacke sandstones in larger quantities than has yet been recorded in the greywackes or their regionally metamorphosed equivalents. Hutton (1940, p. 29) recorded prehnite on only one occasion as a doubtful component in the saussuritic matrix of a green schistose rock interbedded with normal greywackes near Lake Wakatipu in Western Otago. Apart from the absence of pumpellyite, the mineral assemblages in altered greywackes at Hunua. Bream Tail and other North Auckland localities are the same as those of the lower Chlorite subzones in Otago; i.e., epidote, clinozoisite, sphene, chlorite, sericite, stilpnomelane, quartz, albite and rarely calcite. Prehnite typically occurs in sandstone members of the Auckland rocks where solutions rich in Ca and Si, strained from the highly sheared argillites during deformation, have crystallized in tension fractures. However, the presence of the lime silicate in abundance in some boudins as idiomorphic crystals and granular aggregates in the

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matrix, as well as in quartz veins, suggests that the environment within boudinage structures has been peculiarly suited to the development of prehnite. The physical conditions within a boudin that might promote crystallization of prehnite may be resolved as (a) reduced shearing stress, (b) local areas of lowered hydrostatic pressure where tension fractures were opened, and (c) possibly a lower temperature in the absence of active shear planes.

Prehnite may thus fill the roll of an “anti-stress” mineral in these low-grade regionally metamorphosed rocks. Additional evidence is found in the generally antipathetic relationship between prehnite and the epidote minerals, and in the degree of preferred orientation shown by prehnite in rare cases where the mineral has crystallized in sheared sandstone. No other metamorphic minerals in these rocks, apart from mica skeins, have shown a similar tendency for alignment.

Conclusions

The following conclusions may be used as a basis for further surveys of structure and metamorphic grade in the North Auckland greywackes.

(1) A dominant structural direction trending approximately N.-S. is preserved as a horizontal axis of folding (A1) in areas composed mainly of greywacke sandstone. Where argillites form an important part of the rock sequence this structural direction is indicated by the trend of axes of micro-folds and intersections of shear planes, but considerable warping about a younger oblique axis may have occurred.

(2) A subordinate younger structural direction is directed W.S.W.-E.N.E. Folds about this axis (A2) have been imprinted on the argillites, but apparently failed to modify the earlier N.-S. folds in areas of more rigid sandstone.

(3) The grade of metamorphism, although variable, is at least equivalent on textural criteria to subzone Chlorite 1 of the Otago schists. Chemical reconstitution has produced a mineral assemblage which is characteristic of the Chlorite zone of regional metamorphism—i.e., albite, quartz, chlorite, sericite, stilpnomelane, sphene and epidote. The occurrence of prelinite has probably been promoted by local fluctuations in temperature and pressure where compressive deformation has had a varied effect in the interbedded hard sandstones and soft argillites.

Plunging structures are located only within belts of the less competent argillites, and larger areas of resistant greywacke sandstone appear to have remained almost rigid after deformation about the dominant N.-S. tectonic axis. Intricate joint systems in the sandstones can be regarded either as an effect of unloading during post-orogenic relaxation of the rocks, or as the imprint of successive periods of compression related to nonsyngenetic axes. The complexity of joint patterns and their general absence in the argillites favours the latter hypothesis.

Some consideration has been given to the possibility that the contorted argillites at Tawharanui and Bream Tail lie within a broad zone of shear. Battey (1950) has discussed a system of close spaced tear faults which extend north-west-southeast from Doubtless Bay and possibly continue as far south as Whangarei Harbour to form part of an extensive plane of movement. An argument could be advanced that resolution of unilateral shearing stress within the

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greywacke series would produce movement planes located in softer argillites like those at Tawharanui, Bream Tail and Kawau Island. However, comparison of these rocks with similar beds affected by cataclastic metamorphism along shear zones reveals many differences in texture. For example, Fleming and Hutton (1949) found that schistose rocks from the east coast of Kapiti Island, near Wellington, are phyllonites developed by intense shearing and mechanical granulation of detrital fragments. The rocks therefore have a porphyroclastic structure in which cataclastic reduction of grain size is more important than a crystalloblastic growth texture of the type seen in the Auckland greywackes. In addition to textural evidence a most significant feature of the Auckland rocks is the presence of two sets of shear planes which intersect in fold axis A1 and which operated synchronously as a major mechanism of deformation in conjunction with flexural folding about this axis. The movement picture displayed by these planes of shear could not have developed if the argillites had been deformed by tectonic flow along a single movement horizon, and this evidence alone argues strongly against the existence of a shear zone.

Deformation by slip on two sets of planes, such as characterizes the argillites, is called “flattening” by Sander (1950) and the symmetry of the movements is figured by Turner and Verhoogen (1951, p. 521). Two interpretations can be placed upon the movement picture. The first interpretation, now largely discounted by Turner and Verhoogen (1951, p. 572-6), regards the two sets of shear planes “as the final imprint of a compressive force produced by load” (Sander, 1934, p. 42). Fold structures associated with the B-axis parallel to the intersection of the shear planes are proof against the operation of load metamorphism in the present case. The second view, offered as an alternative by Sander (1934) and supported by Turner and Verhoogen (1951), believes that deformation of this type is produced under conditions of compression where a plastic body is squeezed between the jaws of a vice formed by more rigid material. A similar comparison has been made in this paper to explain the complexity of folds and shear planes in the belts of softer argillites between rigid masses of competent greywacke sandstone.

In a recent detailed survey of altered Triassic sediments from Southland, Coombs (1954) has pointed out many important differences between these rocks and the adjacent Otago schists. In a similar fashion there is a strong contrast in tectonic pattern between Trias-Jura rocks on the southwest coast of Auckland Province and the greywackes described here. The fossiliferous west coast beds follow a simple structural line and have been folded into open anticlines and synclines with axes directed 330° (Purser, 1952) and thus roughly parallel to one of the tectonic trends recorded from Tawharanui, Bream Tail and elsewhere in North Auckland. However, in the Trias-Jura beds there are no tight minor folds, close spaced shear surfaces or tectonically formed boudinage structures and a younger oblique fold pattern is absent. Secondary mineralization has followed the type of hydrothermal alteration described by Coombs (1954), and the beds are much less indurated. The unified scheme of sedimentation and subsequent deformation proposed by Wellman (1952) for these distinct groups of rocks, his Hokonui facies and Alpine facies, therefore appears to be oversimplified, for it equates and co-ordinates two contrasted tectonic settings and two entirely different metamorphic environments.

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Acknowledgment

A grant in aid of field expenses from the University of New Zealand is deeply appreciated.

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Dr. R. N. Brothers

Geology Dept.
Auckland University College
Auckland

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