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 interior.

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 activity.
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 clastic wedge.
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 left!

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).

To Lecture Notes.

Last modified: 17 January 2027