Mountain Building “Tectonic Forces at Work”

Terminology Mountain belts - chains thousands of km’s in length. Mountain ranges - group of closely spaced mountains or parallel ridges. Craton - portion of continent tectonically stable for long period of time. Shield - exposed area of craton.

Mountain Belts Characterized by: Layered sedimentary rocks commonly >10 km thick. Variety of folds and faults indicating intense deformation. Most sedimentary rocks in mountain belts are of marine origin. Wide zone of great thickness of volcanically derived rock.

Characteristics of Mountain Belts Fold and Thrust belts Crustal shortening and thickening Igneous (plutonic) and metamorphic rocks Found in core of mountain range Normal (extensional) faulting Associated with older portions of mountain belts that undergo uplift and extension.

Fold and Thrust Belts Characterized by large thrust faults stack one upon another. Thrust faults - low angle reverse faults Beneath lowermost fault (detachment fault) rock has remained in place. Folds and thrust faults indicate intense compressive forces leading to tremendous crustal shortening and thickening.

Evolution of Mountain Belts Accumulation stage Orogenic stage Uplift and block faulting stage

Accumulation Stage Accumulation of thick (several km) sedimentary and volcanic rocks. Most accumulation occurs in marine environment. Passive continental margin Thick sequence of shale, limestone and clean sandstones Active continental margin Volcanic rocks Graywackes (immature, “dirty” sandstones)

Orogenic stage Orogeny - stage of intense deformation, usually accompanied by metamorphism and igneous activity. Reverse (compressive) faulting is widespread.

Ocean-Continent Convergence Accretionary wedge - forms where layers of marine sediment are folded and faulted as they are scraped-off during subduction. Fold and thrust belts form on craton side of mountain belt. Thrust faulting due to crustal shortening caused by convergence. Central part can become too high resulting in gravitational collapse and spreading by normal faulting.

Arc-Continent Convergence Island arc is too buoyant to be subducted. Convergence may cause seafloor to break away from the arc and create new site of subduction. The arc becomes “welded” to the continent to form an accreted terrane.

Continent-Continent Convergence Thick sequences of sedimentary rocks on both continental margins become intensely faulted and folded. Fold and thrust fault belt develops. Underplating - when two continents collide, one may be forced beneath the other doubling the thickness of continental crust in the collision zone. Normal faulting occurs in the central portion of the belt due to gravitational collapse.

Uplift and Block-faulting stage After the plate convergence ends, crustal thickening leads to uplift due to buoyancy forces and isostactic adjustment. Erosion can lead to further uplift due to isostatic adjustment. Normal (extensional) faulting is common due to uplift.

Delamination Idea that gravitationally unstable lower portion of thickened lithosphere beneath mountain belt may break off and sink. The portion of lithosphere that sinks is then replaced by hot asthenosphere. Model explains block (extensional)-faulting, thin crust and young volcanic activity in regions like Basin and Range of southwestern United States.

Growth of Continents Continents may grow by igneous activity and accumulation of terranes. Terranes Regions where there is geological continuity. Geology in one terrane is markedly different than in adjacent terrane. Terrane boundaries are generally faults. Suspect terranes - may not have formed at present site but were accreted during collisional event.