Late Nineteenth Century Tectonics

Problem of the Alps

• Nature of the range
  • Scale, structural complexity
  • Mostly compressive features, secondary extensional
  • Variable stratigraphy east-to-west
  • Degree of alteration/metamorphism
• Models proposed through 1860s all seem inadequate!
  • Von Buch: Craters of Elevation
    • Not explain linear geometry of range, dominance of compression
    • Volcanic activity observed inadequate
  • Elie de Beaumont: Contraction-driven “pop up” ranges
    • Pentangle system not believable by 1870
    • Not explain detailed structural pattern (dominance of compressional features)
  • Lyell: Igneous intrusions and withdraws
    • Not fit the linear geometry
    • Stresses vertical uplift, not explain compressional structures (folds)
  • Herschel: Loading and deep crustal lateral flow
    • Not work since no adjacent ocean
    • More of an uplift than folding
  • Hall: Geosyncline
    • Developed for continental margins
    • Not include an explanation for uplift
  • Dana: Geosynclines and continent-ocean stress
    • Developed for continental margins
    • Needs oceanic crust to exert pressure for deformation

Suess

• Basic idea builds off the idea of contracting crust
• Stress released by lateral shortening that stacks up crust (folds and thrust faults)
• Accommodates local extension and vertical movement (sinking as cooling occurs) and igneous activity
• Grand vision: Alps results of northward-directed thrust faulting (and nappe emplacement when these were recognized)
• Suggests that global sea levels (ecstasy) are related to tectonics
  • Sea floor subsides due to contraction
  • Sea level falls so world-wide regression
  • Erosion increases and infills margins of ocean basins
  • Transgression of seas back onto continents
  • Until next contraction reactivates the cycle
• Integrated structural features and crustal dynamics and sedimentation!

Nappes

• Heim identified a huge “double fold” near Glarus, Switzerland in 1878
  • Suggested considerable shortening of upper crust and deformation below
  • Interpreted as a double-sided, symmetrical structure
  • Reinterpreted as a nappe later (with push from south)
• Nappes
  • Largely worked out in 1890s
  • Enormous structures that developed from overturned folds
  • Lower limb stretched out and detached as nappe slides over underlying units
  • Scale of transport direction and degree of shortening hard to picture
  • Infer extreme shortening (>200 km across Alps), deformation at depth
  • Finally explained the overall structure of Alps!

Huag

• Tectonic theory that attempted to combine Suess’ vision with geosynclines
• Model
  • Earth covered by broad continental areas separated by relatively narrow “mobile belts” that are analogous to geosynclines
  • As earth contracts, the mobile belts are squeezed and deformed
  • At a relatively recent stage, continued contraction accommodated by collapse of some of the continents to make modern ocean basins (MTH: I assume that the idea was that the mobile belts were squeezed enough that they could no longer absorb the contractive stress.)

Finally

• This is about as far as geology got in regards to tectonics by the end of the 19th century
• Geophysics would challenge the basic assumption of contraction as the driver for most of these theories – as we will see