Paleogeology

Paleogeology, Paleoclimate, in relation to Evolution of Life on Earth

mountains

Mountains rise abruptly above the surrounding region, while mountain ranges comprise closely spaced mountains and mountain belts comprise several mountain ranges that run roughly parallel to each other (Cordillera)

By contrast, orogenic belts may include mountain belts or the eroded remnants of mountain belts. Orogenic belts are typically long, arcuate bands of crystalline rocks in terranes or blocks of deformed rocks separated by dipping thrust faults.

Mountain belts typically comprise multiple layers of sedimentary and volcanic rocks. These accumulations, which can be several kilometers in thickness, were mostly originally deposited in a marine environment. The clastic components of the sedimentary rocks are derived from weathing and transport (erosion) of the continental crust of nearby terrestrial landmasses. These sediments are deposited and lithified to form shales, limestones, and sandstones at the continental shelves, slopes, and rise.

At convergent boundaries, rising magma forms volcanic island arcs and plutonic emplacements. Island arcs typically form in line with the interplate junction, perpendicular to the direction of crustal movement.

Mountains arise through several mechanisms:
  • volcanism at divergent plate boundaries, convergent plate boundaries, or above hotspots (mantle plumes)
  • folding and/or faulting as a result of tectonic forces

  • The general model proposes that mountain building involves three stages:

  • accumulation of sediments, followed by
  • an orogenic episode of compressional rock deformation and crustal uplift, followed by
  • further crustal uplift caused by isostatic rebound and block-faulting.
  • Tectonic convergence can be arc-continent, ocean-continent, or continent-continent:

  • Arc-continent convergence involves collision of an island arc with a continental margin, in which the oceanic plate area between the arc and the continent is subducted into the asthenosphere, accreting the island arc volcanic rocks and sediments onto the continental margin.
  • Ocean-continent convergence involves accretion of marine sedimentary deposits onto the continental margin.
  • Continent-continent convergence occurs when an ocean basin closes, bringing two continental plates into collision and thrusting huge mountain systems upward.
  • Tectonic convergence folds and elevates the sedimentary strata from the original ocean basin, and creates faulting when the compressional forces exceed the ability of rocks to deform. The high compressional forces typically cause reverse and overthrust faulting.


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