The study of tectonics in later part of the 19th century was dominated by the work of Eduard Suess, one of the more remarkable individuals in the history of geology (see my notes). Scientifically the main issue facing tectonics well into the 20th century was the importance of horizontal (lateral shortening) versus vertical (isostacy) tectonic drivers. Other important questions included the role of contraction, the permanance of oceans and continents, and the rigidity of the crust. These selections focus on Suess’ ideas about contraction and lateral shortening, and the reactions to them (more on this story over the next few classes). First, here is a quick overview of the geology of the Alps which may be useful.

Harris sect. 25

Greene (1982) chapters 6-9 

These chapters cover the geological side of the development of tectonics (we will be looking at the geophysical side next time). Chapters 6 and 7 deal with the synthesis developed by Suess in his short 1875 and lengthy (3 vol.) 1878-1901 books. The central role of the Alps in European thought is clear – as you probably know, this is a complex of mountain chains that were difficult to unravel. Chapter 8 disusses the recognition of large overthrusts in the Alps and the development of the idea of nappes. Finally, chapter 9 looks at some variations on Suess’ ideas, the most important is Haug’s effort to combine them with a version of geosynclines.

We will focus on selections to highlight some critical developments. (Of course, if you have time, all these chapters are worth reading in full.) In spots, the text gets rather detailed about specific Alpine localities – as  you might expect, the details are less important here than the insights and ideas.

• Ch. 6, p. 147-157 (middle): This section introduces Eduard Suess and his re-interpretation of the Alps (and ultimately other mountains) as the result of lateral motion of the chain. The contrasted with the predominent idea that mountains were due to primarily vertical movements. The underlying driver of cooling-driven contration was the same – as Greene points out: “the mechanism proposed was the cooling and contraction of the earth and the relative subsidence of adjacent segments of the crust.” (p. 150) – but the structural pattern was very different.
• Ch. 7: p. 173-178 (top): Shifting to the “Face of the Earth”, Greene summaries how Suess accomodated both horizontal and vertical movements within his overall scheme, capped off with an overview of the Alps (p. 176). After brief note that extends to other ranges, there is a surprising note (middle p. 177-178): the crust is generally subsiding – continents are just the remnants that have not yet sunk beneath the waves.
• Ch. 7, p. 185 (bottom) -187 (end): Suess developed a very influencial theory regarding marine transgessions and regressions that is summarized here. His integration sedimentation/stratigraphy with tectonics persisted well into the twentieth century, even after his tectonic ideas fell out of favor. One point that is in the skipped pages needs repeating: Suess “transformed the content of geological theory by challenging in the most forceful way the idea of continental permanence, arguing that the present arrangements were only transitory…” (p. 190).
• Ch. 8, p. 194 (middle)-203 (bottom): This chapter deals with the recognition of large thrust faults and nappe structures – be sure to look at the figures in this chapter! The first selection recounts the work of Heim in identifying the structural anomoly in the Glarus area as a great double fold (this was re-interpreted in time as single thrust and then a nappe – see Figure on p. 199). Note how his work unleashed a flurry of interest that led to the identification of these structures throughout the Alps and elsewhere (most notably the Northwest Highlands of Scotland). This section concludes with Bertrand’s suggestion of major deformational phases in Europe (p. 203)
• Ch. 8, p. 205 (middle)-216: This selection describes the development of the idea of nappes. (Nappes are essentially huge recumbent folds that were stretched laterally so that the lower limb passes into a thrust fault along which the nappes moved many tens of kilometers.) In some ways a natural extension of thrust faults, nappe theory was conceptually challenging due to the scale of the featues, the distance of travel, and the implied shortening of the upper crust. As you might guess, the acceptance of large thrusts and nappes reinforced Suess’ idea of lateral motion of the crust. The details of some of the various ideas (p. 206 bottom-p. 212 top) can be skimmed but look at diagrams. Then the text turns to some important insights and useful figures.  Unfortunately, as noted by Greene a little further on (p. 222), nappe workers “were nearly indifferent to the mechanical problems raised by the extent of the lateral movements they proposed.”
• Ch. 9: p. 226-234: As you might expect, others attempted to put these structural insights into a wider perspective within the framework developed by Suess. This section focuses on the model of Emile Haug because it was one of the popular tectonic theories of the early 20th century. It mixed Suess and and a variation on Dana’s geosynclines in a rather unique way – see the figure on p. 229. A critical point is that Haug strongly favored vertical movements (essentially continental oscillations), a theme that would become popular by the turn of the century, and a unique idea about the impermanence of oceans and continents. (We will see how this tied into the new field of geophysics as applied to the crust in our next class.)

To dos

• Suess
  • Consider the varied tectonic ideas before Suess – for example, those of Elie de Beaumont, Von Buch, Lyell, Sedgwick, Agassiz, Murchison, (all of whom died in the early 1870s). How easy was it to apply their ideas in a general way (i.e., beyond the area that inspired them)? How well did they apply to the complex elongate mountains systems (Alps, Appalachians, Andes, Himalaya, Pyrenees, Urals, Zagros, etc.) that dominate discussions of tectonics?
  • Can you explain the mechanics of Suess theory?
  • How did it explain the variations in Alpine structural patterns?
  • How did his sea level model work? (This was one of the first models for eustacy.)
• Nappes
  • The recognition of overthrusts and nappes was a major change in geological thinking. Why was this?
  • Why was this difficult? (Recall Escher’s comment on the Glarus structure – p. 195)
  • What were the implications for structural patterns and crustal behavior?
• Haug
  • How did this theory work?
  • Some important conceptual points:
    • “Mobile belts”: what are they?
    • What explained transgressions and regressions?
    • How did this theory compare to Dana’s geosyncline theory, and Suess’ mobilist interpretation?
    • That much of the oceans were underlain by former continental areas that subsequently foundered (I suspect that this was once inspiration for some of the 20th century land bridge ideas). What does this idea suggest about continental versus oceanic crust? (I.e., different or similar?)

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