Message #175716

[Carolyn Silver]

Isostasy and Geosynclinal Theory

At the turn of the century, the geosynclinal theory as proposed by James Hall (1857) supplanted the contracting Earth theory as the dominant explanation for mountain building. Orogeny occurred as follows:

    * A fixed linear trough of sediments with deep water sediments and volcanic rocks (eugeosyncline) at the center and shallow water sedimentary rocks on the flanks develops.
    * When deformation occurs, it is symmetrical, with thrusting occurring away from the center onto flanking continental platforms. Eugeosynclinal rocks are the most highly metamorphosed and deformed. Miogeosynclinal rocks - more mildly deformed.
    * Dana (1873)
          o Stage 1: Crust initially solidified as granitic plateaus and basaltic basins
          o Compression due to contraction causes basaltic crust to act as lever against granitic crust.
          o Stage 2: Lateral pressure from basaltic crust produces bending and flexing of granitic crust which leads to erosion of highs and deposition in lows.
          o Stage 3: Process continues through combination of isostatic adjustment (causing uplift and subsidence) and lateral pressure (causing folding and mountain belt formation). Folding, melting occurred as sediment was depressed to great depth and heated.