GEOL 102 Historical Geology
Spring Semester 2012
Every Mountain Shall Be Exalted...: Orogenesis I
Every valley shall be exalted, and every mountain and hill shall be made low:
and the crooked shall be made straight, and the rough places plain -- Isaiah 40:4
Orogenesis = Mountain Building
In pre-plate tectonics Geology, different models about mountain formation in different
parts of the world. American geologists tended to think mountains formed at the edges of
continents, while Eurasian geologists favored ideas where mountains built up in the
Plate Tectonics gave a unified view of how mountains formed: it turns out both concepts
were correct, to a certain degree!
Beneath the lithosphere a mantle plume heats up the material above, forming a
divergent boundary. If underneath an ocean basin, forms a new mid-ocean ridge (go to 3
below); if underneath a continent, go to 1 below:
1. Continental material begins to split apart, forming a rift valley. With
interior drainage, very short distances of sediment transport: alluvial fans, arkoses,
playas, etc. Structurally many grabens and half-grabens. Lots of volcanic
2. As rift widens, volcanic activity drops on continent. The rift valley becomes a
new ocean basin. Now that the continent is draining into an ocean basin rather than
interior drainage, get typical stable margin deposition (fluviolacustrine -> coastal ->
continental shelf deposits).
3. Along the active mid-ocean ridge, new volcanic islands form and move away to become
guyots. If a hotspot exists, may develop an island chain.
4. At ocean-ocean subduction zones, generate volcanic island arcs, with a deep-
sea trench towards the subducted plate.
5. At ocean-continent subduction zones, generate volcanic arcs, with a deep-sea
trench towards the subducted plate.
6. In both cases, a subduction complex or accretionary wedge is generated: a
fold-and-thrust belt where sediments and oceanic rocks are folded & faulted &
metamorphosed into mélange.
7. Between accretionary wedge and arc is the forearc basin, where downwarping of
the crust produces deep water (and thus deep-water sediments accumulate)
8. Down the core of the volcanic arc is an igneous arc of plutons (or potential
plutons); surrounding this on either side is a metamorphic belt.
9. A fold-and-thrust belt mirroring the subduction-side fold-and-thrust belt
forms on the side away from the subduction. On the far side of the fold-and-thrust belt
(away from the subduction zone) is a backarc basin (on continents, a foreland
basin) formed by downwarping of crust.
10. The backarc/foreland basin is typically flooded by ocean water, drops very deep in
geologically short period of time. Deepwater shales mixed with greywacke turbidites (due
to common earthquakes) produce characteristic sedimentary units called flysch.
Flysch will often contain some volcanics from the arc.
11. In continental system, the foreland basin can typically be filled in geologically
quick period. Flysch is replaced with nonmarine units of coastal sediments,
fluviolacustrine beds, braided streams, up to arkoses & alluvial fans: together called
molasse. Will often be some volcanics from the arc present. Because of shape of
the sedimentary units added together, a thick package of molasse is sometimes called a
12. As long as subduction continues, arcs will be active. Island arcs can be pushed
into continental mass, and whole continental plates can collide and suture. Oceanic
crust and upper mantle rock gets pinched between continents, forming ophiolites;
also, the accretionary wedge and forearc basin sediments get lifted and metamorphosed on
top of ophiolite package. Mountains build up to great thickness: intense metamorphism,
but volcanic activity stops.
13. After suturing, tectonic activity slows. Erosion wears down mountain range, and
sediments are transported towards continent (filling up foreland basin) or out towards
ocean (as new stable margin). With isostatic rebound (balancing out mountain
height and mountain root), deeper and deeper igneous arc & metamorphic belt rocks are
exposed. When it isostatic balance is achieved, may be no actual mountain topography
Different parts of the world reflect different parts of the cycle above:
1. The Rift Valley of eastern Africa
2. The Red Sea
3. Various ocean basins (Hawaii for hotspots)
4. The Lesser Antilles of the Caribbean
5. The Andes
6. see above
7. see above
8. see above
9. The Sea of Japan is in part a foreland/backarc basin
10. see above
11. Because of the current oceanic lowstand, few good examples of molasse at present.
For much of the last 180 million years or so, the western interior of North America was
a clastic wedge.
12. The Himalayas and Andes.
13. The current Appalachians and Ouachitas.
Other types of tectonic activity in continental interiors, produced by stresses on
margins: structural basins (downwarping) or domes (upwarping).
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Last modified: 19 January 2012