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Lineation, Symmetry, and Internal Movement in Monoclinic Tectonite Fabrics

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Lineation, Symmetry, and Internal Movement in Monoclinic Tectonite Fabrics BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA VOL. es. pp. 1-18.7 FIGS. JANUARY 1957 LINEATION, SYMMETRY, AND INTERNAL MOVEMENT IN MONOCLINIC TECTONITE FABRICS BY FRANCIS J. TURNER ABSTRACT The current controversy regarding kinematic interpretation of lineation in schists hinges on the significance of symmetry in tectonite fabrics. Interpretation of such fabrics by Sander and Schmidt is based on an assumption that symmetry of fabric reflects sym- metry of internal movements accompanying deformation. In identifying b lineation, which is the symmetry axis of certain monoclinic fabrics, as the principal direction of movements concerned in the evolution of those fabrics, some recent writers have neg- lected or rejected Sander's postulate regarding symmetry. This course seems unjusti- fied for two reasons: (1) Those who identify regional lineation with movement direction have done so for Precambrian or Paleozoic rocks whose metamorphic and deformational history is ambiguous and often highly complex; the "direction of movement" in such rocks has not been established independently of fabric evidence. (2) Since it was pro- posed 30 years ago, the symmetry concept of Sander and Schmidt has become strength- ened by evidence accruing from studies on fabrics of experimentally deformed metallic aggregates and ceramic bodies. Additional supporting evidence now comes from fabrics of experimentally deformed marble. In deformed marble cylinders symmetry of fabric is identical with symmetry of movement as inferred from measurable strain. The pattern of strain is controlled by the orientation of the cylinder (a mechanically anisotropic aggregate of grains) in relation to applied force. Even in highly deformed material (e.g., where elongation exceeds 500 per cent) the influence of the original anisotropy on sym- metry of experimentally induced movement and strain is obvious in the final fabric. CONTENTS TEXT Page Marble 12 Page General statement 12 Acknowledgments 2 Symmetry of stress 12 Kinematic significance of symmetry in an- Symmetry of strain 12 alysis of tectonite fabrics 2 Symmetry of fabric..'.'.'.'.'.'..'.'.'.'.'.'.'.'.'.'.'.'. 13 Symmetry and lineation in monoclinic move- Conclusion 16 ments and fabrics 3 References cited' '.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'. 16 Critical examination of attempts to correlate b lineation with a of the movement plan 4 ILLUSTRATIONS General statement 4 Movement in regional deformation 4 Figure Page Critical review of field evidence as to move- 1. Symmetry in orientation diagrams for ment directions 5 quartz and mica 7 Critical review of evidence as to movement 2. Shear planes in quartzite pebbles and en- on a small scale 6 closing matrix 9 Evidence of minor folds 6 3. Relation of pebble elongation of lineation Evidence of preferred orientation of quartz in metaconglomerate 9 and mica in schist and in quartzite 6 4. Stress patterns in experimental deformation. 11 Evidence of shape of deformed pebbles 8 5. Symmetry of compressional strain 13 Evidence of internal fabric of deformed 6. Ideal orientation diagrams for c axes of pebbles 8 calcite in marble 14 Evidence of preferred orientation of 7. Orientation diagrams for c axes of, calcite in pebbles 9 experimentally deformed marble 15 Concluding statement 10 Symmetry of experimental deformed fabrics. .. 11 TABLE General statement 11 Metallic aggregates 11 Table Pase Clay aggregates 12 1. Values of strain in deformed marble cylinders. 13 1 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/68/1/1/3441575/i0016-7606-68-1-1.pdf by guest on 30 September 2021 F. J. TURNER—MONOCLINIC TECTONITE FABRICS ACKNOWLEDGMENTS the nature of which can be demonstrated only by mapping in the field. Our knowledge of the fabric of experimentally Kinematic and dynamic interpretation of deformed rocks is based almost entirely on tectonite fabrics is conspicuous in the publica- specimens of marble, dolomite rock, granite, tions of the Innsbruck school. This aspect of and dunite deformed at 20-500° C and 3000- structural petrology necessarily rests on a less 5000 atmospheres by Professor D. T. Griggs secure foundation than the descriptive phase. and associates in the Institute of Geophysics, It contains much that is speculative; and I University of California, Los Angeles. Both the would now reject as unwarranted many pub- experimental program of Professor Griggs and lished conclusions, reached in Austria and else- subsequent petrofabric analysis of the deformed where, with regard to hypothetical processes of material by the writer and associates have been grain orienting. This applies especially to inter- generously supported by grants from the Pen- pretation of preferred orientation patterns—for rose Bequest of The Geological Society of example those of quartz—in terms of hypo- America, the Office of Naval Research, U. S. thetical mechanisms of crystal gliding unsup- Department of the Navy, and the National ported by experimental data. Nevertheless, Science Foundation. To the John Simon Gug- what is perhaps the greatest achievement of genheim Memorial Foundation I am indebted Sander and his former coworker Schmidt lies for the opportunity to see at first hand the within the controversial field of interpretation. Moine and Dalradian schists of the Scottish From the outset, when he published his classic Highlands, and to discuss with Professor Sander paper (Sander, 1911) "On the relation between and general subject of this paper. I wish to rock fabric and movement of component parts", thank Mrs. E. B. Knopf for critical reading of Sander has favored kinematic rather than the manuscript, and Miss B. R. Dixon who dynamic interpretation of rock fabrics. To cor- drafted the figures. relate fabric with internal movements is less doubtful than is more tenuous correlation with KINEMATIC SIGNIFICANCE OF SYMMETRY IN forces responsible for such movements. ANALYSIS OF TECTONITE FABRICS Symmetry has emerged as the basic criterion for correlating fabric with movement (Cf. Bruno Sander's retirement from the chair of Schmidt, 1926; Sander, 1930, p. 53-73, 145-147; mineralogy at the University of Innsbruck 1948, p. 66-83; Knopf and Ingerson, 1938, p. marks the close of an era in the development of 42-62). On the basis of experience drawn from modern structural petrology (Gefiigekunde). such fields as metallurgy and hydraulics it was Largely through the work of the Innsbruck assumed that the symmetry of a tectonite fabric school over more than 4 decades the descriptive reflects the symmetry of movement responsible side of structural petrology has been placed on a for the evolution of the fabric. Analogies were firm basis. The statistical methods of petrofabric drawn with familiar examples of preferred analysis have been applied with equal success orientation of moving objects—wheat stalks to microscopic features (notably those relating bent in the wind, logs in a flowing stream, sand to preferred orientation of mineral grains) and in dunes, birds in flight. Today the symmetry to megascopic elements in the fabric of deformed rule stands as a well-tested postulate supported rocks (bedding, foliation, lineation, jointing). by an imposing body of experimental evidence Common patterns of preferred orientation of quartz, mica, calcite, and other minerals meta- drawn from such diverse fields as hydrody- morphically deformed are now well known. namics, sedimentation, metallurgy, and ce- Petrogenically they are comparable in im- ramics. Also significant is the persistence of portance to the accumulated data relating to fabric symmetry throughout successive phases mineral facies. Also well established by Sander of metamorphism. Although crystallization and coworkers is the uniformity of symmetry commonly outlasts deformation in meta- commonly displayed, within a given mass of de- morphism, it seldom obliterates, and indeed formed rocks, by fabric elements ranging from often emphasizes symmetry of fabric imprinted microscopic characters to large-scale features by deformation. In using symmetry to interpret Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/68/1/1/3441575/i0016-7606-68-1-1.pdf by guest on 30 September 2021 KINEMATIC SIGNIFICANCE OF SYMMETRY rock fabrics two points should be borne in pended in the flowing stream assume a state of mind: the over-all plan of movement deduced preferred orientation with their longer dimen- from fabric criteria applies only to the field sions parallel either to a or to b. Moreover this within which the fabric has been investigated (a state tends to be perserved in deposited sedi- thin section, hand specimen, single outcrop, or ment after flow has ceased. The pattern that map area); and the validity of the symmetry develops depends upon such factors as particle rule is independent of assumptions and in- dimensions, density of particles and of fluid, ferences (commonly highly speculative and un- viscosity, and rate of flow; but its symmetry in- warranted) regarding the mechanisms of rock variably is monoclinic. Orientation of sand deformation or of grain orienting. dunes, regular patterns of minor ripples on their surfaces, and the laminated fabric and sorting SYMMETRY AND LINEATION IN MONOCLINIC of the dune sand have in common a monoclinic MOVEMENTS AND FABRICS symmetry that faithfully reflects symmetry of observed wind movement. Monoclinic symmetry of movement is illus- In fabrics of metamorphically deformed rocks trated by flow of water under gravity in a flat- monoclinic symmetry likewise is
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