Structural Observations in Central Alpine Region of New Zealand
[Received by the Editor, November 4, 1956.]
Strata, ranging in metamorphic grade from sandstones and argillites of Chlorite 1 sub-zone to garnet-biotite schists, are strongly folded, generally with axes trending N 30° E. On the east edge of the field isoclinal folds in less metamorphosed rocks dip steeply north-west; on the west side of the field folds in the more highly metamorphosed rocks are more steeply inclined to south-east.
Towards the east, in the less metamorphosed rocks, cleavage is parallel to bedding or hardly distinguishable. On proceeding west, into more metamorphosed rocks, axial plane schistosity (cleavage) becomes very marked On steeply dipping limbs, where schistosity and bedding planes are nearly parallel, patterns of drag folds indicate positions of major fold axes; but in other places, especially on crests of major folds, strata, are extremely corrugated and the plane (S2) schistosity surfaces cut across the bedding schistosity. Lineations mask the intersections of stratification and schistosity.
Boundaries of metamorphic zones cut obliquely across the trend of schistosity planes. Whilst metamorphic grade may tend to increase in older rocks, it seems likely that beds of same horizon occur in different metamorphic zones.
Certain metamorphic rocks are less structurally deformed than others of lower grade. Metamorphism and some folding probably preceded a later phase of metamorphism connected with formation of schistosity and corrugations.
Locally the strike of strata swings considerably from the general trend, and such swings are probably connected with considerable folds of very steep plunge to northwest: these folds do not necessarily result from later superimposed folding.
The observations collected in this paper, which is intended to give a general account as a foundation for further detailed studies, result from field work during the summers of 1954, 1955, and 1956. As all three writers must interrupt work in this field for at least two years it seems opportune to publish these notes now.
For the present study rocks have been collected over the whole field, and examined, but petrographic information is quoted only for the purpose of confirming and checking the field observations.
One of us (B. M. G.) has made a more detailed study of the Fox and Franz Josef Glaciers, including work on petrography and structural petrology. Most of the results of this more detailed work are reserved for later publication. Dr. Brian Mason is at present making a comprehensive petrographic study of the metamorphic rocks of the Southern Alps, and this description of structure should in part complement his work.
In a reconnaissance survey of a mountainous region where clear stratification can be seen in views, observation of dips and strikes made at a distance can be of considerable value, even if localities are inaccessible; the possibilities of error can be limited by views from many different angles, by a careful study of ground and air photographs, and by comparing the structure viewed at a distance with that nearby. Data collected at a distance or from photographs have been marked on the map by broken lines to distinguish them from the data of rocks actually handled.
It is noteworthy that many of the structures, especially those showing departures from the regional trends, were observed independently by the writers, who worked together for a short time only. The observations were found to tally on comparison.
The map of the Mount Cook Alpine Regions on the small scale of 1:100,000 (Lands & Survey Department, 1953) was employed as topographic base for much of the work. Other useful topographic information has been obtained from the following sources: a report accompanied by a map of the Franz Josef Glacier (Bell, Greville, and Cockayne, 1910), a map of the Fox and Franz Josef Glaciers drawn in 1911 and published by the Canterbury Progress League, a remarkably fine perspective drawing by Broadbent (Lands and Survey Department, 1929), and numerous accounts scattered through the literature of mountaineering. Among the latter is a particularly useful pocket guide to the region east of the Main Divide, by Hewitt and Davidson, 1953. Runs of overlapping oblique air photographs, kindly lent by the Royal New Zealand Air Force, have been examined stereoscopically and have provided a certain amount of photogrammetric data, only a little of which is shown, however, on the map.
An allowance from the University Research Grants helped to defray the expenses of field work. We are indebted to many of the local people who know the mountains, especially to the guides on both sides of the divide, and also to those who helped as climbing companions, Mr. A. Packard and Mr. A. M. Hopgood. One of us (A. R. L.) is particularly grateful to Dr. Brian Mason for his advice and genial company during a journey of introduction to this exhilarating region.
An account of earlier geological work, excluding glaciological data, is included in a preliminary paper (Lillie and Mason, 1955) which sketches the approximate boundaries of metamorphic zones ranging from Chl. 1 to Garnet (the latter, perhaps of doubtful zonal value) and notes briefly some outstanding structural features. The present paper contains more structural observations, mostly confirming but partly modifying very general statements of the earlier paper. Some of the information of the early paper is repeated here in order to make the modifications clear. Over large tracts the strike of the strata, both metamorphosed and little altered, remains generally remarkably constant, varying from 030° to 045°, commonly with steep dips between 60° and 90° and less commonly with dips between 45° and 60°. Nevertheless there are very considerable departures from this general trend and from the angles of dip cited. Generally when the angle of dip is less than 45° the strikes of strata swing and, in places, depart markedly from the regional trend. On the east side of the field the dip is generally north-west. On the west side of the field the direction of dip is more variable.
A general coincidence of bedding planes with plane schistosity surfaces in many of the metamorphic rocks was also noted in the preliminary paper. Although this exact of near correspondence has been confirmed over a very large area, it is now clear that in many places, particularly in rocks of biotite and higher metamorphic grade, bedding planes and schistosity planes do not coincide. Two planes of schistosity are then developed; S1, parallel to the bedding planes, and S2, cutting them and following the axial planes of microfolds: in most places strikes of both are the same or nearly so, even when dips are different, but in other rocks there is no near correspondence of either. Such rocks show considerable complexity of major and finer structures though the trend and dip of schistosity planes maintain this regularity. Since nearly all the lineations are formed by intersection of bedding planes with schistosity planes, always steeply dipping, the trend of lineations remains fairly regular.
In the rocks of lowest grade, the greywackes and argillites east of the main divide, fracture cleavage and traces of cleavage occur locally, but they are not conspicuous. In rocks of Chl. 2 and higher grade west of the main divide cleavage is not always distinct because it commonly follows the bedding planes, but in places it becomes very prominent and then follows the axial planes of folds. Similar distinct plane
surfaces can be followed into the biotite and biotite-garnet schists, and these S2 surfaces are absolutely analogous to the cleavage of lower grade rocks. In the highest grade rocks of the garnet zone the S2 surface may become indistinct through crumpling and other S surfaces may be developed.
In this account, following traditional practice, the term cleavage is employed for rocks of lower grade of metamorphism and the term schistosity is applied for the S2 surfaces in the schists proper. The S1 surfaces are referred to generally as planes of stratification or bedding planes.
The boundaries of metamorphic zones have not been drawn on the map, but the most easterly appearances of the index minerals biotite and garnet, are marked B and G to indicate where some modification is required.
Summary of Lithologic Features
The outstanding features of most rocks, including the most metamorphosed rocks of the central Alps is the remarkable clarity of bedding structures. In this respect the rocks of the Chlorite 1 and Chlorite 2 zones differ from many of the other greywackes and subschists grouped cartographically by the Survey in the comprehensive group of “9A-undifferentiated Jurassic-Triassic-Permian.” The greywackes near Auckland City, for example, and in many parts of the Ruahines show much less distinct bedding and appear especially to be more deformed, probably as a result of superimposed folding. (Brothers, 1956.)
In the Central Alps these bedding structures are best seen in the least metamorphosed strata consisting of thick (10ft to 30ft) bands of medium- and coarse-grained sandstones interbedded with abundant laminae, dominantly formed of argillites and very fine sandstones. These latter are the “ribboned” sequences. The prevalence of these ribboned strata is stressed, since in most other parts of New Zealand the passage of schists into massive greywackes has been emphasized, and in most regions of New Zealand where metamorphic rocks have been accurately described in detail bedding seems to have been either indeterminable or largely ignored. Furthermore, the banding so clearly seen in the little metamorphosed rocks is also very distinct in nearly all the rocks of high metamorphic grade traced in the Central Alps (easly Survey geologists—e.g., Cox, 1877, commented on this clear stratification). On the whole, the chief effect of metamorphic segregation has been to accentuate this banding, except in rocks of very high grade.
Ribboned sequences can be most easily seen in the ascent of the ridge between the Hooker Hut and the Copland Valley. The strata there consist dominantly of sandstones, mostly fine-grained with medium and coarse-grained sandstones of subordinate volume. Interlaminated with the light-coloured finest sandstones and siltstones are argillites or slates of dark blue-grey colour; the alternating laminae range from a fraction of a millimetre to one or two millimetres in thickness.
The medium-grained sandstones tend to make thicker bands, some 10ft thick, and the coarser sandstones appear as bands generally reaching some 30ft thick. The medium-grained sandstones and some of the finer sandstone bands show in places graded bedding, though this is not as a rule very distinct. At one place in the upper part of the ridge this grading suggests that the beds “young” to the east* and are therefore inverted, but on all lower parts of the ridge graded bedding appears to “young” to the west. A fold, presumably a syncline, obscurely seen from a distance high on the west wall of the Hooker is consistent with this pattern. Much more clearly defined, and probably more reliable than the graded bedding, are abundant minute current-bedding structures in the fine sandstones on the slopes east of this inferred axis, and these all give a consistent picture of younger beds to the west. Convolute bedding, always on a fine scale, is seen in places.
[Footnote] * The terms east, west, etc., are here used only in a rough sense.
The vicinity of the Pioneer Hut is also convenient for study of the sedimentational features. Although sandstones of coarser and medium grade predominate, ribboned sequences also appear. Immediately east of the hut large sedimentary slump folds are clearly seen (Photo 1); and they indicate ancient downslumping very roughly from east to west. Good graded bedding was observed at several places on the ridge west of the hut, and all evidence indicates that the strata have not been inverted. The slump folds are apparently distinct from the numerous sharply-pointed drag folds of tectonic origin which also, by their pattern, suggest that the strata are not inverted (infra). At Marcel Col graded bedding again indicates westward younging in north-westward-dipping beds, and is consistent with a pattern of regular drag folds “pointing” south-east.*
The alterations of coarse sandstone and fine sandstone-argillite sequences occur all over the field and as yet no good marker beds have been noticed. In certain places massive bands of coarse sandstones dominate the sequences. Typical of such are the sandstones forming the steeper western flanks of Malte Brun, similar in lithology to sandstones outcropping on the ridge above the De la Beche Hut and on the Sealy Range at Mount Ollivier. These sandstones, rich in coarse quartz grains, and with siliceous cement, weather to give orange and light-red hues conspicuous at a distance. The descriptions of these as “red rocks” given by climbers tend to be misleading, in that they suggest hematitic red argillites. Probably a contributing cause of the red colour is local abundance of lichens.
The recurrence of the same lithologic types at different horizons and over a large area will delay the construction of any coherent stratigraphic sequence till marker beds have been identified by highly detailed mapping. Scattered observations of graded bedding, current bedding, and slump phenomena have been made and noted on the map; only in one case do we get sedimentational evidence of inversion, but the visible overturned folds clearly indicate that the strata in unvisited localities are upside down. It is evident that many isoclinal folds, some doubtless undetected, must lead to much repetition of the strata. One cannot therefore aver that the metamorphic rocks are either of the same age as, or of distinctly different age from the metamorphosed strata, but the ribboning of pelitic schists allow one to infer that most of them were of remarkably similar lithology. In one respect, nevertheless, the metamorphic strata differ, and that is in the local concentrations of thick bands of richly epidotic chlorite schists (for example on Paschendale Ridge), and of highly amphibolitic schists interstratified in thin bands with very quartzose biotite schists; these occur on the south wall of the Fox Glacier, on the south wall of the Franz Josef, and in the Copland Valley. They would suggest a pre-metamorphic material of highly tuffaceous origin, or even altered lava flows: rocks of this type have not so far been detected between the main divide and Malte Brun, with the exception of an altered volcanic rock on the Sealy Range (infra.). Possibly the higher-grade schists are of earlier age, including originally flow rocks or tuffs, but the structural evidence is insufficient to confirm this view.
Bearing in mind this paucity of any clear marker bands, the structural description that follows is necessarily a notation of major and minor structural features without stratigraphic sequence over large distances. Most of the folds in this field are over-turned, with steep limbs. In this account all upfolds are described as anticlines and all downfolds as synclines, since the regional pattern of structure suggests that major folds are most unlikely to be inverted so as to make synclines appear as “antiforms”.
Region East of Main Divide
Most of the rocks of the Chlorite 1 grade, particularly east of the main divide, do not show any prominent S surfaces distinct from bedding planes, although frac-
[Footnote] * By the term “pointing” as used here we mean the direction in which major anticlinal axes are believed, on the evidence of drag folds, to lie.
ture cleavage does exist locally in many of these rocks and near faults a schistose structure may appear.
Probably as a result of shear along bedding planes, the finer-grained rocks tend to have marked flaggy fracture and are slates, as noted by the geologists of the early Survey. Several overturned folds have been clearly observed in the rocks east of the main divide, and doubtless many more exist.
East Wall of Tasman Glacier from West Slopes of Malte Brun to the Junction with Murchison Glacier
The strata strike regularly on a general bearing of 040° and dip very steeply north-west on the west slopes of Malte Brun, but on the flanks of peaks south of Malte Brun dip in the opposite direction. These opposing steep dips appear to be measured on the same flexuous limb which in places is vertical. All the summit ridges have not been examined, but there is evidently some complexity of structure, as is clearly shown by Waterhouse's (1955) photograph of a fold on Mt. Häckel, north-east of Malte Brun (but his description is puzzling).
On the western slopes of Malte Brun, as well as on its summit and on neighbouring summits, sandstones of orange and red weathering hues are more abundant than usual; they appear to be mostly rather coarse-grained and highly siliceous. In interbedded siltstones good examples of grading as well as truncated fine current bedding and slump bedding show that the strata on all the lower western slopes of Malte Brun are not inverted. Ribboned sequences near the col leading to Beetham Glacier include glossy slates with fine siltstones which yield abundant problematica superficially resembling dendroid graptolites. (A 59; numbers refer to a small collection of Alpine rocks in the Department of Geology, Auckland University College.) Sinuous worm (?) tracks occur in argillite on Aiguilles Rouges and resemble tracks found in morainic blocks in the Hooker Glacier (A 88). A fine conglomerate of very angular fragments also occupies that summit.
West Wall of Upper Tasman from the De la Beche Hut to Ranfurly Glacier
The general strike of strata seems to follow the wall of the glacier—i.e., roughly trending between 010° and 030° with westward dip of approximately 70°. About one mile from the De la Beche Hut a pronounced buckle (see Figure on Plate 11) interrupts this regular dip and one vertical limb of the buckle strikes approximately north-west. (See later discussion of regional structure.) A similar buckle reappears on the south-east flank of Glacier Peak and may represent the continuation of the same fold in higher strata.
The rocks on the south end of the De la Beche ridge consist of massive, hard, slightly greenish-grey siliceous sandstones, pink on weathered surface. Thin ribboned sequences contain bands of blue-grey slate with pyrite cubes. Fine current bedding in siltstones seen at several places indicates younging to west. A well-marked set of joint strikes between 300° and 325° and dips north-east at 60° to 90°. Another set strikes on average to 045° and dips south-east at an average of 20°.
Upper Tasman Glacier Beyond Malte Brun and the Ranfurly Glacier
In the upper Tasman the strike of strata swings considerably to assure locally a north-west trend. This has been established clearly on the ground as well as from air photographs of the main divide, though detail has not been collected. Great folds occur, with axes plunging very steeply. An anticline appears on the northern slope of the Darwin-Annan ridge, about three miles north-north-east of Malte Brun, and plunges about 60° to the north-west. It may be the continuation of the fold observed by Waterhouse (1955), for a steeply plunging anticline as viewed on a slope from a distance can have the appearance of a syncline. There would appear to be in this ground great cross flexures which, like the smaller buckle already seen south-west of the Ranfurly Glacier, either complicate the general trend between 030° and 045°, visible elsewhere in the region, or completely replace this trend.
West Wall of Lower Tasman Glacier
Views along this mighty wall, consisting of many subsidiary ridges separated by glaciers that are series of ice falls, reveal a consistent general strike of 025° to 030° with a steep north-west dip (Photo 2).
Bands of sheared black argillite, followed by eye from ridge to ridge between the Rudolf Glacier and the east wall of Anzac Peak, probably represent an approximate horizon sketched on the map. Even if one has, by mistake, “jumped the succession” in aligning these outcrops, it is unlikely that the regional strike traced can err by more than (say) 10°. On this wall several overturned folds have been observed independently by the writers.
Below Glacier Peak a cross flexure appears with one vertical limb striking roughly north-west. On Haidinger Ridge, in descending order, are an overturned anticline, a syncline and anticline, all with north-westward dipping strata. These folds have already been sketched by Park (1910, p. 66). On the Haast ridge a syncline and an underlying anticline occur some distance below the Haast Hut, and on the Anzac wall at least one anticline and a complex of strata appear, probably folded, but obscured by land slides and creep.
Below Nazomi and above Pibrac a prominent anticline (Photo 3), already noted by Park (1910, p. 64), occurs, and below it in the Ball Glacier* one can infer a syncline or shear zone to separate it from a lower anticline with succeeding syncline, both very clearly visible from the ridge above the Ball Hut (Photo 4). Exact correlations of these structures will require very detailed study, but tentatively the axis of the Pibrac anticline is regarded as continuing towards that of the upper Haidinger anticline.
West of these folds and tectonically higher is a great syncline on the Zurbriggen ridge.
East Wall of Hooker Glacier
The Zurbriggen syncline is likely to be continued to join a mighty syncline on the west wall of Mount Cook immediately north of the south-west ridge of Nazomi. This is probably the fold that was noted by Haast (1871, p. 23); it can be clearly seen from the west wall of the Hooker Glacier (Photos 5, 6). An anticline, less clearly defined, runs through the ridge of Nazomi and may continue east of the Zurbriggen syncline. The Nazomi anticline can just be perceived on the ridge east of the Hooker Glacier, but the other folds seen on the Ball Glacier have not been traced. Farther south, on the east wall of the Lower Hooker Glacier, the strike of strata swings considerably through an east-west into an approximately north-westward trend on going south. The ridge from Mt. Wakefield to the Ball Glacier requires detailed examination, for it seems necessary to postulate a syncline, not overturned, to explain a distinct difference in strike along the west slopes of Mt. Wakefield.
The visible height of one limb from the trough of the Cook syncline gives a rough estimate of minimum amplitude of the fold of the order of 5,000ft. The half wavelength is probably in the order of 3,000ft. The adjacent syncline and anticline in the Ball Glacier must have amplitude and wave-length of about one quarter of these dimensions. The folds on the west wall of the Tasman Glacier are sketched very diagrammatically on section d.
Sealy Range (Sections a, b, c, in Plate?)
Between Mt. Ollivier and Waihi Pass coarse siliceous sandstones dominate a sequence which contains subordinate argillaceous beds. About half a mile at 200° from Ollivier, siliceous white tubes (2 cm long) represent organic traces (A 68)
[Footnote] * The Ball Glacier has now retreated so that it no longer joins the Caroline Glacier which feeds the Tasman Glacier.
Photo 7.—Air photo from a point about 2 miles north-west of Mount Mitchell.
Photo 8.—Drag folds at Marcel Col.
The structure of the Sealy Range is obviously continued into that of the lower wall of the main divide between Scissors and Sefton, but its connection with the Mount Cook Range remains puzzling.
The strikes of strata on both walls of the Mueller Glacier are very flexuous but make a consistent picture that can be related to the western dip slopes of an anticline whose crest, trending north, forms the ridge from Mt. Ollivier to Waihi Pass and beyond. On the east flanks of this anticline and immediately adjoining the crest the strata steepen rapidly to become vertical, and they are slightly overturned below the Mueller hut.
Running along the east face of the Sealy ridge a shear zone separates this anticline from the lower slopes. The sequence at the shear zone is:
Massive siliceous sandstone with thin layers of phyllite and quartz veins.
Purple sheared rocks with green streaks.
Hard green rock in lenses 1ft to 2ft thick.
Sheared hard siliceous sandstone.
Sub-schists with lineation down dip. Extending as far east as Sealy Tarn, these rocks are more metamorphosed than the rocks west of the shear zone and those below the lake.
These beds dip west or west-south-west at some 60°. The purple and green rocks, 60ft to 100ft thick, are highly altered and sheared volcanics containing phenocrysts of albite, and appear to be albite-chlorite-epidote schists (A 63, 65). It is not clear whether they represent an original flow or a sill. The strike of the rocks between the Mueller Hut and Sealy Tarn is between 330° and 360°, but farther south the trend of shear zone is generally meridional. The zone has been traced as far as the upper part of Black Birch Creek, where the green and purple rocks, dipping very steeply west, must be over 300ft thick.
From Sebastopol it can be seen that the north-westward dipping strata on the east wall of Sealy are upturned sharply, and the eastern limb of the Ollivier anticline is not visible. Presumably the shear zone continues beyond Sealy; east of its likely position, and south of the Hoophorn stream, from west to east, a broad well-defined overturned syncline, a sharp anticline and another less clearly-defined syncline occur, all with limbs dipping west. These folds can be expected to project northward, but their arches have not there been distinguished. The Ollivier anticlinal axis is also likely to be prolonged into the wall of Footstool, judging by the pattern of strikes along the Mueller Glacier. South of Sebastopol the approximate meridional strike of beds stand out regularly and clearly over a long distance.
A marked scarp on the east face of Sebastopol must be controlled by a fault, for it coincides with a line along which the strike changes abruptly. East of the line, strikes oriented west and roughly north-west are probably related to those on the north-west side of Wakefield. Traces of indeterminable fossils have been observed in blue-black argillite (A 60) beside the water-pipe on the north bank of Sawyer Stream near the Unwin Hut.
Region West of Main Divide
(See Photo 7 for a view of localities.)
Fox Glacier from Marcel Col to Chancellor Ridge
On the walls of the Fox Glacier enough detail has been collected to present a general picture of structure, which can best best be described in terms of a traverse and views of neighbouring ridges along a route from Marcel Col on the Main Divide to the end of the glacier.
At the col the strata strike on a bearing of approximately 045° and dip north-west at 65° and, although of only Chlorite 1 grade, show good drag folds pointing south-east. Graded bedding indicates north-westward “younging”. The drag folds suggest that at least a major anticline is interposed north-west of the Zurbriggen syncline if the latter is prolonged east of the Col (Photo 8).
The same strike and dip of strata are to be seen on the surrounding walls of Haast and Lendenfeld and seem to occur in Torres and Tasman. Across the Fox Glacier on its north walls a similar strike appears to hold from Mallory westwards and was confirmed in place at Newton Pass and Mt. Halcombe.
The strike at the Pioneer Hut, where no cleavage distinct from the bedding planes appears, is again on a bearing of 045°, but the dip locally steepens. A few drag folds on the ridge immediately west of the hut “point” consistently “southeast”, with axes approximately horizontal, for about a mile west of the hut, but they then deviate from this regularity and show vertical axes of drag folds and lineations on cleavages. Folds of minor amplitude occur (see section d, in Plate 11) at this point. Abundant thin quartz veins aligned along vertical joints strike 290°, and along some of these westward heaves of 2ft in nearly vertical beds indicate dextral fault displacements. Other joints strike 340° and have low dips of approximately 20° to east-north-east. Aligned down the bedding planes on Pioneer Ridge are many slickensides, often bearing thin recrystallized quartz layers. A hackly fracture across these slickensides (or lineations) suggests very late south-eastward overthrust.
On the lower slopes of Big Mac a divergence between bedding planes and distinct planes of cleavage is visible, the two striking approximately north-north-east, but whereas the strata dip west at 75°, the cleavage planes are vertical.
At the Chancellor ridge stratification and schistosity planes are distinct, with a consistent but slight divergence between the two. Small patches of biotite first appear about half-way along the ridge. The bedding planes strike very regularly on an average bearing of 030° and all dip east at 50° to 80°: the schistosity planes, striking generally to 020° and usually dipping west at 80° to 90°, are outlined commonly by abundant thin quartz veins varying in thickness from a fraction of an inch to one or two inches. Vertical gashes are infilled with thinner quartz veins which taper away from the other quartz veins on a bearing of 008° to 010°. The differenced between the strikes of these elements remains constant when the strike of bedding planes varies slightly from the average. Microfolds mostly of sharp angular and asymmetric outline are very abundant at the Chancellor Ridge, and are present indeed in most rocks of the Chl. 3 and 4 zones and particularly of the biotite zone. They are mostly of small amplitude, about 1ft or less, and rather shorter wave length, and have been interpreted as drag folds, as they all point consistently north-westward. The schistosity planes, now clearly seen to be axial-plane cleavages in relation to the microfolds, bear distinct lineations, and these, as nearly everywhere in the field, are bedding-plane traces, so that they are particularly marked near plunging microfolds.
The plunge of the lineations on the Chancellor ridge, on an average 20° on a bearing of 210° to 215°, often directly reflects the obliquity between strikes of axial-plane cleavage and bedding planes.
Similar plunges of lineation at the Franz Josef and Copland valleys may also reflect a slight obliquity not appreciated in the field. In a few places where plunging lineations were observed very closely the strikes of strata and planes of schistosity appeared, nevertheless, to coincide. A syncline occurs on the east of the ridge with consistent drag patterns on its limbs. Vertical joints striking 310° which abound on this ridge have determined the lines of great trenches and the edges of steep precipices.
Fox Glacier from Chancellor Ridge to Cone Rock
North, west, and south of the Chancellor Ridge the trend of the schistosity planes remains regular, and in some places they are more conspicuous than the planes of stratification. The structure of the latter becomes more complex, especially south-west and west of the Chancellor Ridge.
On the north wall of the Fox Glacier and opposite the mouth of Boyd Creek the strata, including green schists, are very corrugated with extremely variable general dip, in many places near the horizontal, and are cut markedly by the steep regularly-trending schistosity. These beds, often showing a general dip towards a synclinal depression, are, farther west, turned up sharply, giving place to approximately vertical strata that consist of garnet-biotite schists with bands of amphibole schist similar to those on the south wall of the glacier (infra.).
The structure of the south wall at the eastern and upper end of the Paschendale Ridge is similar to that of Chancellor Ridge, for the strata strike approximately 030° with a dip of roughly 70° to south-east and the planes of schistosity dip very steeply north-west with roughly the same strike. On the western end of this ridge, however, the structure becomes more complex. Paschendale Bluff is built in part of a formidable pile of huge unstable blocks which obscure the structure in places. Nevertheless it is clear that, despite abundant corrugations of all dimensions from inches to some twenty or thirty feet in amplitude, the general dip of the strata approaches the horizontal in several parts of this wall. The rocks consist of chlorite schists, chlorite-epidote schists, and chlorite-biotite schists, often rich in pyrite. In Boyd Creek and Straight Creek rocks of similar variable attitude reappear, but these give place farther west to more regularly and more steeply dipping beds with drag folds. At Straight Creek garnets appear in abundance in the biotite schists.
Text-fig. 1.—Boudinage in interbedded biotite-garnet-quartz schists and amphibolites near Cone Rock, Fox Glacier. From a field sketch of a loose slab.
Text-fig. 2.—Faults in vertical garnet-biotite schists near Cone Rock, Fox Glacier. From a field sketch.
Proceeding farther west along the south wall one enters a sequence, at least 700ft thick, of rocks which, although dipping at a constant angle of 80° on a bearing of 130°, show no microfolds and no distinct traces of internal deformation. The bedding surfaces are plane and remarkably parallel and contrast sharply with those both east and west of the zone. At one place rectangular unsheared boudins are clearly seen, affecting a bed some two inches thick, and in an adjacent locality small parallel faults occur. These were probably formed as normal faults, and they show throws of only a few inches; they distinctly resemble faults formed during the process of sedimentation. (See Figs. 1 and 2.) Varying in texture from phyllitic to almost gneissic, this sequence, mostly of garnet-biotite-quartz schists, includes silky green amphibole schists, very coarse, dense black amphibolite, garnet-amphibole-quartz schists, with garnets ½in diameter, and quartzite containing minute laminae of fine white mica. The long amphibole needles do not show a marked alignment on the relict bedding planes. These undeformed beds are succeeded to the west by highly sheared garnet-biotite schists; these strike between 020° and 040° and dip steeply south-east at 75° and 90°, showing at Cone Rock drag folds, which are also to be found along the strike on the north wall of the Fox Glacier. The drag folds “point” north-west, and the plunge of the lineations on both sides of the glacier is now in a generally north-easterly direction, but is very variable between.
020° and 045°. The rocks are very crumpled and the dominant schist surface is no longer plane.
The Mascarin Glacier and the Fox Range
On ascending Boyd Creek one passes through a great thickness of corrugated chlorite-epidote schists and biotite-chlorite schists with variable strike and variable dip, generally to east-south-east. Higher on the ascent these give place to biotite-chlorite-quartz schists and to crenulated biotite schist at the end of the Mascarin Glacier, whose south wall, below the great ice-fall, show an extremely complex structure best seen on the adjacent Fox Range btween Crozet Peak and Sam Peak.
The chlorite and biotite schists outcropping on this ridge are extremely crenulated and contorted into zig-zag concertina-like folds of all amplitudes from an inch to thirty feet, the general strike of the beds being now variable but in many places tending to run parallel to the ridge and perpendicular to the schistosity. These strata are cut by schistosity planes, of fairly regular strike between 190° and 210°, with a westerly dip of 70°, which show very distinct lineations, with plunge varying from 20° to 45° to south-south-west, many of the lineations representing the axes of the crenulations. Very thin pencil and long rods appear, particularly where the crenulated beds are of fine grain and present a very shattered appearance (Photo 9). The finest beds are represented by paper-like laminae. The planes of schistosity are often filled by quartz veins with thin tapering offshoot veins striking 008°. The complex relations of relict stratification to schistosity (S2) already seen in part at Paschendale Bluff show up most clearly on this part of the Fox Range.
All the rock types seen on the Chancellor Ridge occur in this sequence also, as well as some bands of highly sheared chlorite schist. It is evident, however, that these rocks must be of different tectonic position, and this part of the range probably lies on the crest of a broad anticline that stretches from Chancellor Ridge to Straight Creek. Whatever the reason for the complexity, it is likely to be associated with those portions of the Balfour Ridge and La Perouse walls where the evidence of air photographs indicates that the general dip of strata may be roughly parallel to the strike of the planes of schistosity.
Franz Josef Glacier
In general the rocks at the Franz Josef Glacier are similar to those in the Fox valley and follow in the same sequence. West of the Almer Glacier a rather flexuous syncline shows drag folds on the steeper flanks, but near the crest are minor folds and abundant corrugations in biotite schists. Immediately west of the Defiance Hut the zone of biotite-garnet-quartz schists reappears, including interbedded bands of coarse amphibolite and silky amphibole schists with garnets, all similar to those found in the lower Fox Glacier, but showing more signs of deformation.
The garnetiferous biotite schists dip steeply south-east, and show abundant drag folds “pointing” consistently north-west and extending for one mile west of the hut; but farther west at the end of the Franz Josef, on the west wall, they point south-east in strata, probably overturned, that strike on a 045° bearing and dip south-east at 70° to 80° (Photo 10). The axis of an anticline, with slight north-westward over-push, has been crossed and the crest of a fold has been found on the east wall. West from Defiance Hut planar schistosity (S2), visible locally on microfolds as axial-plane cleavage, becomes gradually less distinct until at the end of the glacier the only outstanding schistosity is the bedding surface crumpled into microfolds. The lineations, which remain distinct either on schist surfaces or as corrugations, plunge at varying angles between 25° and 45°, always approximately south-west or south-south-west. West from the end of the glacier the rocks assume a very crumpled and contorted structure, often of milled appearance, presenting many minute thrusts intersecting on a clear lineation.
From the scattered observations collected along the Fox and Franz Josef Glaciers we have attempted to draw two composite sections (see Plate 11) based on the following assumptions: that the recorded dips and strikes are usually of value for a considerable distance along the strike; that drag folds, where showing a consistent pattern, may be related to major anticlinal axes lying in the direction of “pointing”; that the regular schistosity planes are likely to be parallel to axial planes of great folds.
The more hypothetical portions of the sections are dotted. One of the sections has been extended to the east side of the divide to include a schematic picture of the folds in unmetamorphosed rocks. The topography shown is that along the lines of section, and structures along the line of strike, on both sides of the lines of section, as well as the general profiles of the Fox and Franz Josef Glaciers, are projected on the planes of section.
At the Franz Josef Glacier an anticline with steep limbs separates two synclines, and at the Fox Glacier a portion of one anticline flanks to the west a similar disposition of two synclines on either side of a rather broad anticline. If the troughs of the two synclines at the Franz Josef are joined with those at the Fox Glacier the joining lines are almost exactly parallel and are parallel to the strike of the strata. It is inferred that the Callery syncline continues across the Fox Glacier immediately east of Cone Rock and that the Almer syncline in the upper Franz Josef is continued on the east side of the Chancellor Ridge.
Upper Copland Valley, Footstool, and Sefton
In the upper Copland Valley the general strike of bedding planes on a bearing of 015° to 030° holds, but, although the strike of schistosity, which becomes marked further west, remains constant and approximately the same, the strike of bedding planes swings between the Copland Pass and Footstool, and further west in the Copland Valley some divergence is also to be expected. Microfolds where examined show no regular pattern of drag, though their axes lie on the schist planes. Blocks of schist in the bed of Scott Creek show extremely crenulated bedding surfaces, with cleavage forming axial planes of crenulations; at Creamy Creek interbedded garnet-biotite-quartz schists and amphibole schists are much contorted, with crumpled surface, but strike generally 040° and are approximately vertical. On the arête of Sefton the strata show a general dip of 25° on a bearing of 235°, which is well-established from several photographs taken along different sight lines. West of Mt. Sefton a south-westerly dip also prevails for a considerable distance on the walls of Douglas Glacier and places to the north of it (evidence of ground photographs). Flexuous strikes can be followed into the Sealy Range and to Scissors, and there is evidently a structural “sag” south of Sefton.
On the east wall of the Banks Range, where the strata are very clear, a fault parallel to the bedding planes of the lower part of the wall may represent a dislocation along the flank of an overturned anticline (see Photo 11).
The Cook River and the La Perouse Glacier
The Cook River was ascended as far as La Perouse Glacier, and from views of bedding on steep walls compared with specimens on talus slopes it seemed reasonably certain that strikes and dips of bedding planes and schist surfaces, on a bearing of 025° to 030°, are coincident for some distance below the Glacier. This view is supported by lineations, approximately horizontal, to be seen on air photographs and ground photographs of Mt. Lyttle.
The data for La Perouse walls and the Balfour Ridge have been obtained from air photographs, using the methods of Crone for estimating location and height and measuring lineations on adjacent photographs. These methods are not very reliable where the angle between sight lines is small. It would seem that the trend of lineations is fairly regular, although the plunge varies considerably; and, if we accept
them as lying on schistosity planes, they indicate that the strike of these remains fairly constant, the bedding planes swinging and locally diverging very markedly from the north-east strike prevalent elsewhere. At a distance two of us have independently observed marked south-westerly dip of the strata on the wall of La Perouse. There is undoubted complexity of structure in this locality. Although the quantitative value of the data is uncertain, an examination of ground photographs suggests, however, that the schistosity remains constant in orientation. Complexity is likely to continue as far as the Fox Range.
Biotite schists outcrop as far east as the head of the Cook River (end of La Perouse Glacier) and garnet-amphibole schists have been collected a little west of that point, indicating that the metamorphic zonal boundaries of an earlier paper (Lillie and Mason, 1955) require modification.
Mineralized Quartz Veins
On the Fox Range west of Crozet Peak, at the Chancellor Ridge east of the Chancellor Hut, near the Almer Hut, and on the Gnome Pinnacle on the south-west ridge of Nazomi, quartz veins were found to contain pseudo-rhombo-hedral twin crystals, identified by Professor Coombs as adularia. Some pyrite, chalcopyrite, and galena have been detected in addition in some of these veins. They are thicker than most of the quartz veins in the field and do not appear to follow the schist planes. Alluvial gold formerly worked in the Callery River was presumably derived from such veins (already noted by Cox, 1877, p. 73) which are likely to be widespread.
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.
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—— 1956. Schistosité et forme des plis, Soc. géol. Belg. Annales, 79: B317–B364.
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Topographical Map of the Fox and Franz Josef Glaciers, Westland, New Zealand. Canterbury Progress League, 1911.
Part of New Zealand Alpine Regions, a perspective drawing by B. A. Broadbent, Lands & Survey Department, Wellington, 1929. (This fine diagram brings out clearly the strike of many of more prominent strata east of the Main Divide.)
Map of Mount Cook Alpine Regions, Scale 1:100,000. Lands & Survey Department, Wellington: Govt. Printer, 1953.
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Professor A. R. Lillie, University College, Auckland.
Mr. B. M. Gunn, M.Sc., Geology Department, University of Otago, Dunedin.
Mr. P. Robinson, A B., 16 Allen street, Hanover, New Hampshire, U.S.A.