American Geology and Geosynclines

American Geology

• Practical and economic orientation on natural resources
• Infrastructure
  • State Surveys
    • Rather ephemeral due to state funding
    • Focus on resources
    • Same individuals participated in many state surveys
  • National Surveys
    • Western Surveys after Civil War
    • USGS founded 1878
  • Few universities with small staffs
    • First generation trained in Europe under Werner’s students
    • Distributed along East Coast
  • Some differences from Europe
    • No “gentleman scientists”
    • Strong emphasis on practical
    • Many areas were previously unexplored

Tectonics and Geosynclines

• Rodgers brothers
  • Highly influential maps and sections of folded Appalachians
• Dana (early version)
  • Idea
    • Cooling earth caused contraction
    • Initial cooling resulted in lower ocean basins
      • Ocean basins gently warped
    • Oceans pushed against margins of continents
      • Uplift of mountains along the edges of the continents
  • Observations
    • Trends in the Pacific: depths, cleavage lines
    • Different composition of ocean basins versus continent
  • Driven by cooling-driven contraction (in the European tradition)
  • Prior experience
    • Dana’s Manual Mineralogy
    • United State Exploring Expedition (Pacific)
• Hall
  • Idea
    • Erosion and deposition of sediments along the margin of the continent
    • Results in incremental subsidence of trough as sediments keep it full
    • Lower part of pile is stretched and undergoes extension – normal faults and igneous intrusions
    • Upper part of pile is compressed, resulting in erosion
  • Observations
    • Thickness variations across east-west transects
  • Driven by loading and sagging of crust (in the Herschel tradition)
  • Prior experience
    • Survey work in New York and Midwest
  • Dana’s comment that this is a fine theory for sediment accumulation but it leaves out how to make mountains.
• Dana (late version)
  • Combines elements of his earlier work with Hall
  • Idea
    • Cooling-driven contraction, greater in oceans
    • Deformation greatest at the ocean-continental boundary
    • Warping creates a pair of broad structure features
      • Geosyncline: trough for accumulating sediment
      • Geoanticline: adjacent uplifted area to supply sediment
    • Once geosyncline is stiff enough (which may take quite a long time), shift to more seaward belt
    • Leads to sequential addition of marginal belts around continental core
  • Observations
    • Uses all those noted above for early Dana and Hall
  • Driven by cooling-driven contraction
  • Some version of this would persist in the US into the 1960s (!)

Bigger picture

• Geosynclines were an important tectonic concept
  • Linked tectonics to sedimentation patterns
  • Worked reasonably well for North America
  • Failed in most European settings (Apennines might work)
• All these tectonic models reflected the influence of European ideas (contractions, loading) but moved into an American settings
• Easily overlooked (at first) is Dana’s recognition of the fundamental difference between oceans and continents. Implied a permanence to these features that was completely lacking in earlier tectonic ideas.