1. Ice and Quartz Mosaics in Regions of Minimum and Maximum Shearing Stress

Within recent years the writer has been studying Scottish psammitic granulites (metamorphosed sandstones) of the Moine Series, which H. H. ReadReference Read ¹³ had found to exhibit a “normal” quartz-crystallization in parts of east-central Sutherland, and to become progressively “abnormal” westwards, as they approach the outcrop of the Moine thrust-plane, along which the Moines have been thrust westwards over other rocks for at least ten miles. The “abnormality” was thus produced by conditions of marked shearing stress in the vicinity of Scotland’s major thrust-plane, which has a gentle inclination eastwards.

In “normal” Moines the quartz is tessellate, that is to say it formsFootnote * a mosaic of more or Iess equidimensional crystals with relatively smooth mutual junctions. In “abnormal” Moines the quartz appears as a highly sutured, interlocking, inequigranular mosaic of crystals that show pronounced optical anomalies due to strain.Footnote † The average grain-size of the inequigranular sutured and strain-shadowed mosaic gets smaller on close approach to the Moine thrust; that is to say, grain-size decreases with increasing shearing stresses.

To the writer there seem to be striking resemblances between (a) these two contrasted styles of quartz-crystallization known to be due to differences in shearing stress (Figs. 1 and 3, p. 567), and (b) two contrasted styles of ice-crystallization illustrated in glacier publications of Seligman and Perutz, and of others, viz. the tessellate (equidimensional and unsutured) mosaics of firn and of the upper layers of ice in the névé region; and the sutured inequigranular ice crystals of the main body of the glacier (Figs. 2 and 4, p. 567; see also Ahlmann and Droessler, Fig. 2, p. 270Reference Ahlmann, Droessler and . ¹ ).Footnote ‡

Fig. 1

Tessellate quartz-mosaic of psammitic granulite, east of Beinnnan Losgann, Ardnamurchan. Mosaic largely quartz, with sporadic alkali feldspar and mica. Nicols crossed. × 32. Compare Fig. 2

Photomicrograph by W. Fisher

Fig. 2

Tessellate mosaic of new ice (6 years old) at 23 metres, in a crevasse in the Mönchfirn, near the head of the Aletsch Glacier. Dark blebs with high relief represent enclosed air spaces. Nicols crossed. × 4.1. Compare Fig. 1

Photomicrograph by M. F. Perutz

Fig. 3

Inequigranular, sutured and strain-shadowed quartz-mosaic of psammitic granulite, east of Mallaigmore, North Morar. Mosaic largely quartz, with subordinate alkali feldspar, mica and epidote. Nicols crossed. × 32. Compare Fig. 4

Photomicrograph by W. Fisher

Fig. 4

Outlines of insquigranular sutured ice crystals in tunnel in snout of Upper Grindelwald Glacier. Reduced (scale on photograph). Compare Fig. 3

Photograph of pencil rubbings by G. Seligman

Tessellate ice characterizes a region at the head of a glacier where shearing stress is at a minimum, and crystallization goes on under conditions of compression due to the weight of overlying fim and snow (Perutz and Seligman, p. 335, 340 and Plate 17Reference Perutz and Seligman ¹¹ ; Seligman, Plates 24, 25Reference Seligman ¹⁴ ).

Highly inequigranular sutured ice crystals characterize the ice of the glacier tongue (the main body of the glacier); and abnormally small sutured crystals have been observed along shear-planes in glaciers, and in the Grindelwald ice tunnel situated not far downstream from an ice fall. Seligman has consequently made the tentative suggestion that abnormally small ice crystals (which his illustrations show to be sutured) are formed as the result of the operation of abnormally great shearing stresses.

Published statements by Seligman and Perutz regarding the genetic significance of the abnormally small ice crystals are at first sight incompatible. To clarify Perutz’s difference of approach to the problem it is necessary to summarize, in the first instance, his explanation of the general increase in size of ice crystals, near the surface of an active glacier, from bergschrund to snout (Perutz, p. 133–34Reference Perutz ¹⁰ ). He suggests as a working hypothesis that deformation and growth in glacier ice are interconnected in the following manner: “Crystals having the right orientation for yielding to stresses by glide along their basal planes would have a higher energy than others which cannot yield; the former would therefore have a tendency to grow at the expense of the Iatter by molecular exchange across the crystal boundaries. In glaciers the change in crystal orientation brought about by intra-crystalline gliding and the alterations in stress to which the ice is subjected in the course of its flow to the valley would continuously give rise to fresh energy-differences between neighbouring crystals, and to consequent crystal growth.” Perutz states also that (a) the ice crystals of the glacier tongue (i.e. of the main body of the glacier) are often of very complex shape and always closely interlocked; (b) it is not certain whether growth follows or accompanies deformation; and (c) it appears certain, as a result of glacier observations, and of laboratory experiments by Tammann and Dreyer, that increase in average crystal size is invariably to be observed after strain has taken place.

This hypothesis, in which the time-element (length of period of crystallization) seems to be important (cf. Seligman, p. 262Reference Seligman ¹⁶ ), appears to have no obvious direct applications to the problem of quartz-crystallization in metamorphic rocks. To illustrate this statement, let us suppose that shearing stress (up to a certain limiting value) acting on a psammitic sediment, has resulted in increase of the average size of quartz grains; such an effect would, in general, be impossible to detect because the original size of the quartz grains would be unknown.

Recession of the Glacier du Valtournanche

Upper photograph, August 1942

Lower photograph, August 1949

Photographs by Manfredo Vanm

Seligman (p. 264Reference Seligman ¹⁶ ) has however suggested that a similar process, combined with recrystallization due to abnormally great shearing stresses (connected with the presence of an ice fall) may account for abnormally small ice crystals that he found in relatively deep layers of ice in a tunnel in the snout of the Upper Grindelwald Glacier; these crystals are shown by his illustrations to be inequigranular and sutured (see Fig. 4, p. 567; also Seligman 1948, Fig. on p. 485Reference Seligman ¹⁵ ; and 1949, Fig. 3, p. 267Reference Seligman ¹⁶ ) Seligman’s tentative hypothesis is thus apparently inconsistent with Perutz’s view that increase of average crystal size is the invariable result of strain. The two views may perhaps be reconciled by taking into consideration the increase in crystal size that has been observed by Seligman on the upper surfaces of ice-tables carved out in tunnels in the Upper and Lower Grindelwald Glaciers; Seligman attributes the increase in grain-size to relief of pressure (Seligman, p. 380Reference Seligman ¹⁷ ). We have seen that the grain-size of quartz close to the Moine Thrust was reduced as the result of increasing shearing stress. In this case, of course, relief of pressure took place millions of years later, under conditions that made renewed growth of quartz crystals impossible. It would thus appear that Seligman’s ice hypothesis gains support from petrographic evidence.

Seligman thinks that actual rupture of ice crystals is unlikely to accompany his postulated recrystallization; even if he is right, it would seem that an effect similar to that of mechanical granulation accompanied by recrystallization, might well be produced by a combination of unusually pronounced translation-gliding and the development of shear-planes (cf. Turner, p. 252Reference Turner ¹⁸ : explanation of mechanical granulation and concomitant recrystallization of quartz).

It seems probable to the writer that, in psammitic granulites, sutured margins of quartz crystals that show marked undulose extinction due to strain may be accounted for by a somewhat similar mechanism. Although there is as yet no certainty regarding the number and orientation of glide-planes in quartz, undulose extinction has been definitely correlated by petrologists with translation-gliding (Turner, p. 227, 255–66Reference Turner ¹⁸ ). It seems reasonable to infer that all suturing of strain-shadowed quartz is due to variable localized pressures and shearing stresses connected with internal differential movement along crystal glide-planes; these physical controls, operating in conjunction with a pore-fluid, may be expected to lead to marginal irregular, highly localized and migratory, solution and recrudescence of crystal growth, involving perhaps, in addition, molecular transfer across crystal boundaries (cf. Erdmannsdörffer, p. 283–84Reference Erdinannsdörffer ⁶ ; Griggs, p.1003, 1009Reference Griggs ⁷ ; Seligman, p. 263Reference Seligman ¹⁶ ).

Such transient recrystallization at constantly changing positions on mutual crystal boundaries is just what one would expect to produce the sutured, irregularly interlocking and inequigranalar quartz-mosaic of psammitic granulites (cf. Turner, p. 252Reference Turner ¹⁸ : on mylonites).

The fact that the existence of similar glide-planes in ice and quartz has not been established, does not appear to affect the general analogy; the important point is that in each case translation-gliding has been independently inferred. The writer is not aware whether undulose extinction characterizes the inequigranular sutured ice of the Upper Grindelwald tunnel; but even if it is not developed, this may well be due to the fact that shearing stress produces translation-gliding much more readily in ice than in quartz. Again, a general analogy between the behaviour of ice and quartz under shearing stress would not appear to be vitiated by the fact that the scale of differential flow is often much greater in a glacier than it is in rocks undergoing dynamic metamorphism. (See also addendum, p. 571).

2. Effects of Shearing Stress on the Orientation of the Principal Crystallographic Axes of Crystals of Ice and Quartz

Observations that appear to have petrological significance have been made on the orientation of the principal crystallographic axes of ice crystals in ice subjected to shearing stress. This evidence suggests that extended and more detailed work on ice may go far to settle the present geological controversy regarding the relationship between the direction of tectonic thrust and the orientation of quartz crystals in psammitic granulites (Anderson, p. 122Reference Anderson ² ; Phillips, p. 286Reference Phillips ¹² ). In this connection (Perutz and Seligman, p. 350–55Reference Perutz and Seligman ¹¹ Footnote *) it has been found that in glacier tongues there exists a general tendency for ice crystals to be arranged with their principal axes perpendicular to planes of shear. Moreover, a study has been made of the orientation of crystals of natural ice, in relation to direction of movement, in an ice-apron on the north wall of the Sphinx ridge, near the Jungfraujoch (Seligman, Figs. 2 and 7, p. 299, 314Reference Seligman ¹⁴ ). Here, in an ice-grotto pillar distorted by creep, most of the principal axes were found to be perpendicular to the plane of shear, and all principal axes were distributed in a plane normal to the plane of shear and normal to the direction of flow. These findings have so far, however, not been reconciled with the results of Bader’s study of crystal orientation in a block of artificial ice after it had been subjected experimentally to shearing. Prior to the experiment, the crystals of ice had a random orientation; after shearing, the grain-size had increased but suturing had not developed. According to Perutz and Seligman,Footnote † who have summarized Bader’s results, most of the principal axes were found, after the experiment, to have taken up positions perpendicular to the plane of shear, and all principal axes were distributed in a plane normal to the plane of shear and parallel to the direction of flow. If we draw an analogy between the behaviour of crystals of ice and quartz, the ice-grotto results appear to support E. M. Anderson’sReference Anderson ² view that the planes of “quartz girdles” in Scottish psammitic granulites are normal to the direction of movement (and to the lineation). On the other hand, the Bader-Haefeli experiment apparently supports F. C. Phillips’sReference Phillips ¹² view that the Scottish “quartz-girdle” planes, although normal to the lineation, are parallel to the direction of movement. A satisfactory explanation of the apparently contradictory results of Perutz and Bader may thus be of fundamental importance to tectonic research and to petrofabric studies.

It may be mentioned in passing that a study of glacier motion has recently been recommended to petrofabric workers as a guide to the definition and interpretation of lineations in rocks (Cloos, p. 22–25Reference Cloos ⁴ ).

In conclusion the writer would record, with gratitude, his indebtedness to Mr. Seligman for the active interest he has taken in this paper, and for his assistance in illustrating it, and to Sir Edward Bailey and Dr. E. M. Anderson for encouragement and helpful criticism.