Certain general conclusions seem to be warranted on the evidence of the accumulated observations despite their incomplete nature.
(1) A zone of remarkably well banded rocks, belonging to the biotite-garnet grade and containing abundant needles of amphibole (on south wall of Fox Glacier), shows absence of distinct mineral alignment along with excellently preserved simple rectangular boudinage structure. In this zone of Plattenung it would seem that the rocks attained their metamorphic condition with resulting foliation parallel to bedding (S1) without much accompanying internal movement. To admit such conditions for this zone is to admit that similar effects were also likely for at least the early (but most pronounced) stages of mineral reorganisation throughout most of the region; and thus all the rocks may have reached approximately their present metamorphic grade without notable small-scale tectonic deformation (cf. Bucher, 1953, p. 292).
(2) Some of the larger structures traced in the sedimentary banding (S1) appear to have no direct relation to the dominant and well-marked more regular schistosity planes (S2) and to be of earlier origin than those planes (Fox Range, Mascarin Glacier, La Perouse). The formation of major tectonic units as defined by bedding planes was probably earlier than the finer-scale deformations associated with shearing movement. (See paragraph 4.)
It is likely that the earliest tectonic deformations were of a large-scale order, producing rather flexuous structures with nevertheless a general trend approximately parallel to the dominant present trend of the strata. The local swings in strike of strata are not necessarily evidence of superimposed foldings of different regional trend before the production of schistosity. The latter interpretation introduces as many difficulties indeed as it resolves, and is not a necessary explanation in this field. It seems likely that the swings are evidence of cross-buckles formed at localities where distinct changes in intensity of regional stresses occurred, producing a change in local stress patterns. In the upper Tasman, at La Perouse, and at Sefton big swings in the regional strike of strata occur. These swings can be regarded as flexures playing a role rather similar to that of Blätter.
The swings in strikes at Sefton and the swings on the flanks of the Sealy anticline imply a tectonic sag west of Sefton, and it is likely that the folds of the Tasman west wall either die out or plunge markedly to the south-west.
(3) The later tectonic trends were probably determined by the earlier dominant fold trend of the strata, and presumably the regional stress pattern did not change radically. This is suggested by the remarkably simple relations of bedding planes to dominant planar schistosity (S1) over a large part of the field.
(4) The next stage in tectonic evolution was probably further folding, accompanied by much shear along the axial planes of isoclinal folds and therefore parallel, or nearly parallel, to the limbs. Because in the rocks of lower metamorphic grade this shearing largely followed the even bedding planes of isoclinal folds no distinct schistosity plane is observed in them.
In rocks of somewhat higher metamorphic grade (Chl. 2 and higher grades) distinct cleavage or schistosity planes (S2) may be discerned only where the attitude of schistosity diverges from that of strata, which happens in many places west of Big Mac—but above all near the crests of the microfolds most abundantly developed in the finer grained rocks. As Fourmarier (1953), says in a discussion of structures in metamorphic rocks, in bands with a relatively steep dip subjected to sub-vertical forces microfolds develop easily. In this field many of the microfolds show a consistent drag pattern which suggests the presence of major fold axes within certain tracts; where arches of major structures have been seen at the Fox and Franz Josef Glaciers, the drag patterns on limbs point consistently towards or away from these.
The microfolds are closely related to the schistosity planes, and their production has been part of the secondary metamorphic process, that associated with conspicuous shear. In places, for example, where the strata show a general strike much interrupted by crenulations the latter show no regular drag pattern. The axes of microfolds, or even corrugations of 20ft to 30ft amplitude, are “stamped” across the general strike of the strata, roughly at right angles to it in places on the Fox Range; and this is particularly so on the flatter crest of the anticlinal fold.
In the schists of biotite and neighbouring grades, whilst most of the flaky metamorphic minerals are aligned along the traces of bedding planes (S1), there is also distinct alignment of large biotite flakes parallel to the axial-plane schistosity (S2), particularly where intersecting the crests of microfolds.
Since, however, the divergence between planes of bedding and schistosity is commonly slight, the two S surfaces are usually distinct only on microfolds. In this respect it should be noted that although the dip of a steep limb is sometimes hardly distinguishable from that of the schist plane, the drag folds cause the surface of stratification to cut markedly across the plane of schistosity in a long limb; this can be seen when the bedding plane is followed for a considerable distance upwards.
Even in rocks of the chlorite grades the thinning of metamorphosed sandstone bands on the flanks of the microfolds leads to the development of lensoid quartz segregations which locally are crumpled in ptygmatic style (Lillie and Mason, 1955, Fig. 2). Such rocks even although not of high grade may present a very “gneissic” aspect.
Abundant thin veins of quartz apparently fed from such metamorphosed siliceous beds tend to be concentrated along the axial planes of drag folds. (These remarks do not necessarily aply to thicker quartz veins with adularia mentioned briefly earlier.)
(5) That later shearing affected the axial-plane quartz veins is indicated by less pronounced drag folds seen in the veins themselves; these are much less abundant than those affecting the bedding. Thin, tapering vertical and plane quartz veins, with trend aligned at an angle of some 12° to the other veins, presumably represent traces of planes of late tensional release.
(6) From Marcel Col to the west end of the Chancellor Ridge the S2 surface presents a flat, well-defined aspect. This S2 planar schistosity can be traced in places
to the extreme western limit of the field, but becomes gradually less distinct, particularly after passing the zone of comparatively undeformed metamorphic strata mentioned in (1). The rocks beyond this zone—e.g., west of the end of the Franz Josef Glacier, begin to assume a more corrugated appearance, the surface inferred to be the S2 surface becoming undulous and irregular, especially in the more westerly exposures; the rocks are commonly distinctly rodded, and all bear good lineations and are of distinctly gneissic aspect. The traces of highly contorted relict bedding seem in these rocks to maintain their identity most distinctly, but even these become faint in some rocks which show evidence of milling and twisting.
Evidently these rocks have suffered intense deformation, the stresses involved including shearing stresses, which latter, however, have not been sustained in successive periods, the rocks eventually tending to react rather by complex flow than by shear. Only petrofabric analysis can elucidate a sequence of deformation in these rocks.
(7) Lineations in the schistosity planes are most distinct where they mark the alignment of the arches of microfolds. They show their most consistent angle and trend (approximately south-south-west) of plunge where the limbs are steep and the drag pattern is regular.
In such places—e.g., on the Chancellor Ridge, at the Franz Josef and Fox Glaciers (except near Cone Rock and opposite it on the north bank), they might be interpreted as giving the direction of plunge of larger folds because the schistosity planes follow their axial planes. If they are so interpreted this would imply that the general plunge is steep, commonly exceeding 20° and reaching 45°, and it would also imply that despite considerable plunge the degree of metamorphism decreases only slightly along the trend of the plunge.
On the crests of broader folds the angle of plunge of lineations is obviously quite unreliable as an indication of amount of plunge of the major structures. Furthermore, the fairly regular trend of the lineations is largely due to the very steep dip of the schistosity planes, so that constancy of or variations in amount of plunge in one direction are more significant than their approximate regularity of trend.
Since in some parts of the field it seems likely that larger folds were formed earlier than the microfolds associated with schistosity, and that the trends of the former are not necessarily coincident with that of schistosity, the lineations must be regarded as of limited value for detecting larger structures without very detailed accompanying study; they may merely record a component of the dip.
The forms of major folds at the Fox and Franz Josef Glaciers suggest that the axes in the Franz Josef locality are likely to plunge under those at the Fox Glacier. On the whole, over most of the field the lineations show a general south-south-west plunge, which is probably consistent with a general plunge of major axes at variable inclinations in the north-west of the area, and with a likely south-west dip of strata in the south-west part. The lineations at the end of the Fox Glacier are directed in the opposite direction and do not fit this scheme.
(8) The metamorphic state is here considered only in relation to structure. From the available evidence we arrive at the following tentative conclusions:
(a) The boundaries of metamorphic zones are not strictly parallel to the planes of schistosity, but run across them and across the axial planes of the larger folds where these are defined by the schistosity planes.
(b) Simple relations between stratigraphic depth and grade of metamorphism have not been established. One of the writers (A. R. L) formerly considered that with a decrease in the amplitude of folds east of the Almer Syncline such correspondence might be imagined. The structural evidence now available suggests rather that chlorite schists may be of the same horizon as biotite and biotite-garnet schists. Although at first sight the grade of metamorphism on the west edge of the field seems on the whole to increase towards the anticlinal crests, it can be argued that
on the whole there is progressive increase westwards which is really independent of structural axes. On the other hand, west of the Almer syncline interposition of strike faults leads to successive upthrows of western blocks in relation to eastern blocks, which may locally pass unperceived, so that one does tend to descend in the succession on going west. More detailed work is required to clear up this obscurity, and it will have to be supported by detailed petrography. According to Wellman, Grindley and Munden (1952, p. 217), it can be shown that the metamorphic grade is simply related to original stratigraphic depth in the less metamorphosed rocks, and they assume a similar relationship in the more metamorphosed rocks.
(c) Without close collection for petrographic study the chlorite zone can be only rather vaguely divided into three mappable units: “unmetamorphosed” greywackes, mostly east of the main divide but crossing it; more metamorphosed chlorite schists (or sub-schists); and most metamorphosed chlorite schists without biotite. These are only very roughly taken to be the equivalents of the Chl. 1, 2, and 3 subzones of Hutton and Turner (1936). We have misgivings concerning a correlation of subzones with the presence of certain macrostructural features—e.g., axial plane cleavage which can be traced into planar schistosity in the biotite schists. Any more precise distinctions within the chlorite zone should be based on petrographic criteria, especially distinctions that are not closely allied to structure. We find it difficult to establish on the map a well defined Chl. 4 sub-zone corresponding to that of Hutton and Turner, although locally developed rocks corresponding with it can be found.
If the presence of garnets, of any dimensions, is regarded as marking a distinct zone, the zone of biotite without garnet is considerably reduced in width. There appears, nevertheless, to be always a band of biotite schists without garnet. Garnets found in the more easterly parts of the field have refractive index of 1.795 and may be almandine-grossularite as described by Mason and Taylor (1955, p. 1064).
Garnets from the western side of the field have refractive index of 1.805. Perhaps none of the garnetiferous rocks belong to the almandine zone (cf. Mason and Taylor, p. 1069).
Especially on the western side of the field there are many great faults. It is not easy to detect all these, or to estimate their throw, and they have not been mapped in this reconnaissance study. (B. Gunn has made a more detailed study of the fault and joint patterns.) Faults parallel to strike of bedding west of the Almer syncline seem to throw the western blocks successively upwards in relation to the eastern blocks, and their total displacement over some distance may be very considerable.
To the east of the Almer syncline “a” lineations, essentially slickensides, mark the direction of dip on the bedding planes at the Pioneer Ridge. These surfaces are coated with greenish quartz crystals and their hackly pattern suggests a later south-eastward overpush in the eastern part of the field.
The formation of these faults is probably related closely to the overthrusting on the Alpine Fault during Tertiary and Pleistocene time, for although Pleistocene and Recent movement on the fault has been largely transcurrent, evidences of overthrusting have long been known in many places, and this movement has continued until Pleistocene time for Wellman (1955) has shown that the schists are locally thrust over morainic gravels.
(10) West of the Main Divide great joints (A C joints) strike on a bearing of 290° to 310° and are either vertical or have very steep dips to the north-north-east or south-south-west. They are especially distinct and open on the Fox Range and on the south edge of the Victoria Glacier, where they form great chasms, bound the edges of precipices, and determine the edges of huge blocks, thousands of cubic yards in volume, which are tilted toward the valleys. Many of these joints are faults, usually planes of dextral transcurrent as well as vertical movement. Only a few of the faults are marked on our map and they are not discussed in detail here. A great
fault runs along the face of Cone Rock and is accompanied by a breccia zone exposed in the west branch of Straight Creek, across which it runs to cut the north face of Sam Peak. It may well be continuous with a great gash which crosses the south wall of the Mascarin Glacier and which probably joins a fault in the wall of Teichelmann, where the strikes in vertical strata seem to swing through 90° in a short distance. This would appear to be a transcurrent fault. The faulting along these joints is probably closely related in origin to the transcurrent movement on the great Alpine Fault.
Joints dipping at lesser angles have been observed on many air photographs but have not been methodically measured. Those on the De la Beche ridge, striking north-east and dipping south-east at 20°, are characteristic of such joints, with low angles of dip, and they appear clearly on the western slopes of Malte Brun and also at the Balfour Glacier. The orientation of nearly all the great glaciated valleys west of the Main Divide appears to be related to the pattern of vertical joints, often faulted.
East of the Main Divide the joints are not at first so conspicuous, but they determine at least the secondary topographic features, such as the several buttresses on the east wall of Mount Cook and the trend of the subsidiary glaciers on the east flanks of the Tasman Valley. The directions of the principal glaciers east of the Main Divide are, however, parallel to the general strike of the strata. The curiously curved valley of the Mueller Glacier swings in places in sympathy with the strike before cutting across it at its junction with the Hooker Glacier.
(11) An outstanding feature of the regional structure is the fan-like disposition of folds which on the east side of the field are inclined to the north-west and on the west side of the field more steeply inclined to the east. The Almer syncline marks the line of separation between structures with axial planes tending to dip in opposite directions.
The steep south-eastward inclination on the west side seems to have been formed during a period of deep-seated deformation that was sufficient to produce intense crenulation of the metamorphic rocks, and it is likely to have been connected with some north-westward overthrust when the Alpine Fault may have been already active. This movement is likely to date back to Mesozoic time. There is no evidence to suggest that a low-angle thrust plane was formed, and the degree of shortening of the beds is not comparable to that described in many parts of the Alps of Europe, to which those of New Zealand have sometimes been compared by writers on tectonics The nappes of schist that overlie morainic gravels are probably, as Wellman (1955, p. 40) suggests, due in part to thrust and in part to vast landslides in late Pleistocene or Recent time, thus resulting from superficial movements of much later date than those forming the folds in the schists.
Following recent tendencies of thought, one might try to imagine the opposing dips as resulting from great gravity collapse of folded beds towards lower primitive relief features, so that beds on the east side of the field were rotated around; but the absence of clear cascade folding is unfavourable to this view, and so also are the drag patterns traced as far east as Marcel Col.