Lecture4

Mountain Building and the Origin of the Continents: 

Mountain Building and the Origin of the Continents Chapter 10

Goals: 

Goals Understand the origin of the continents When were the continents formed? How were the continents formed? Why did it happen the way that it did? Understand the difference between continental and oceanic crust and lithosphere Understand why continents are high and oceans are low Understand the role of mountain building in construction of the continents Recognize interactions between tectonics and climate in mountain building Know three main types of mountain building

How Continents are Built: 

How Continents are Built

Cratons: 

Cratons A craton is crust that hasn’t been deformed in 1 Ga. Low-geothermal gradient; cool, strong and stable crust. Two cratonic provinces. Shields: Outcrops of Pre-C igneous and metamorphic rocks. Platforms: Shields covered by layers of relatively undeformed Phanerozoic sedimentary rocks. Usually surrounded by highly deformed Phanerozoic orogenic terranes

Shields: 

Shields Intensively deformed old (> 1 Ga) metamorphic and igneous rocks. Record of voluminous silicic magma production ca. 3.5-2.5 Ga Form central core, or foundation, of modern continents

Shield Formation: 

Shield Formation Archean Tectonics Higher geothermal gradients (known from high-T magmatic rocks) Rapid plate motions Large volumes of silicic melt production (from “fertile” mantle) Granitic magmas form silicic “microcontinents” embedded in tectonic plates composed mainly of mafic & ultramafic magmatic rocks (archean “oceanic” crust) Microcontinents collide and are welded together (~3.3-1.0 Ga) Shields composed of Granite/Gneiss complexes (microcontinent fragments) Greenstone belts (archean oceanic crust trapped in suture)

Cratonic Platforms: 

Cratonic Platforms Sedimentary rocks covering Pre-Cambrian basement. Deposited in shallow seas (shield continental margins) Some in lacustrine (lake) or swamp environments Exhibit large domes and basins. From vertical crustal adjustment. Created by stresses transmitted to interior from an active margin.

Economic Importance: 

Economic Importance Shields: high temperatures of archean magma genesis yields unusually high concentrations of econimically important metals Nickel Magnesium Platinum, gold, silver Platforms: Basins and domes provide depositional centers and structures suitable for concentrating energy resources Oil Gas Uranium

How Continents Grow: Mobile Belts, Accreted Terranes, and Orogeny: 

How Continents Grow: Mobile Belts, Accreted Terranes, and Orogeny

Mountains: 

Mountains Occur in elongate linear belts, typically at the edges of the cratons. Mountains are constructed by tectonic plate interactions in a process called orogenesis. Geologists call mountain belts orogens, even if eroded flat

Mountains: 

Mountains provide vivid evidence of tectonic activity. They embody… Uplift. Deformation. Metamorphism. Exhumation (by faulting & erosion) Mountains

Mountains: 

Mountains Mountain building involves… Structural deformation. Jointing. Faulting. Folding. Partial melting. Foliation. Metamorphism. Glaciation. Erosion. Sedimentation. Constructive processes build mountains up; destructive processes tear them back down again.

Orogenesis and Rock Genesis: 

Orogenesis and Rock Genesis Orogenic events create many kinds of rocks. Igneous rocks – Intrusive and extrusive. Subduction related volcanic arc. Rift related decompressional melting. Metamorphic rocks – Regional and contact. Igneous intrusion. Deep burial. Horizontal compression.

Orogenesis and Rock Genesis: 

Orogenesis and Rock Genesis Orogenic events create many kinds of rocks. Sedimentary rocks – Weathering and erosion. Erosional debris is shed to adjacent regions. Sediments accumulate in basins created by crustal flexure. Sediments can preserve evidence of mountains eroded away.

Erosional Sculpting: 

Erosional Sculpting Mountainous terrain is often steep and jagged due to erosion by water and ice. Mountain heights reflect the balance between… Uplift. Erosion. Rock structures can effect erosion. Resistant layers form cliffs. Easily eroded rocks form slopes.

Orogenic Collapse: 

Orogenic Collapse The Himalayas are the maximum height possible. There is an upper limit to mountain heights. Why? Erosion accelerates with height. Weight of high mountains overwhelms rock strength. Deep, hot rocks eventually flow out from beneath mountains. The mountains then collapse downward like soft cheese. Uplift, erosion and collapse exhume deep crustal rocks.

Causes of Orogenesis: 

Causes of Orogenesis Mountain building is driven by plate tectonics. Convergent plate boundaries. Continental collisions. Rifting. Orogenic phases may last several hundred Ma. Ancient mountains are deeply dissected by erosion.

Causes of Orogenesis: 

Causes of Orogenesis Convergent tectonic boundaries create mountains. Subduction-related volcanic arcs grow on overriding plate. Accretionary prisms (off-scraped sediment) grow upward. Compression stacks thrust faults on the far side of mountain belt.

Causes of Orogenesis: 

Causes of Orogenesis Island fragments of continental lithosphere may be carried into trenches but they won’t subduct. These blocks are added to the overriding plate. These exotic terranes have geologic histories unlike surrounding rocks.

Causes of Orogenesis: 

Causes of Orogenesis Continental collisions. Oceanic lithosphere can completely subduct. This closes the pre-existing ocean basin. Brings two blocks of continental crust together. Buoyant continental crust will not subduct. Instead, subduction is extinguished.

Causes of Orogenesis: 

Causes of Orogenesis Continent – continent collision… Creates a broad welt of crustal thickening. Thickening due to thrust faulting and flow folding. Center of belt consists of high-grade metamorphic rocks. Fold and thrust belts extend outward on either side. The resulting high mountains may eventually collapse.

Causes of Orogenesis: 

Causes of Orogenesis Continental rifting. Continental crust is uplifted in rift settings. Thinned crust is less heavy; mantle responds isostatically. Decompressional melting adds asthenospheric magma. Increased heat flow from magma expands and uplifts rocks. Rifting creates linear fault block mountains and linear basins.

Case Study - Appalachians: 

Case Study - Appalachians A classic example of a complex orogenic belt. The Appalachians formed by 3 separate orogenic events. Preserves a complete Wilson cycle (the opening and closing of an ocean basin). The Appalachians today are the eroded remnants of the former mountains.

Appalachians: 

Appalachians A giant orogenic belt existed before the Appalachians. The Grenville orogeny (1.1 Ga), formed a supercontinent. By 600 Ma, much of this orogenic belt had eroded away.

Appalachians: 

Appalachians The Grenville orogenic belt rifted apart 600 Ma. This formed a new ocean (the proto-Atlantic). Eastern North America developed as a passive margin. A thick pile of sediments accumulated along this margin. An east-dipping subduction zone built up an island arc.

Appalachians: 

Appalachians Subduction carried the margin into the island arc. The collision resulted in the Taconic orogeny 420 Ma. A doubly-dipping subduction zone developed. Exotic blocks of continental crust were carried in. These blocks were added to the margin during the Acadian orogeny 370 Ma.

Appalachians: 

Appalachians East-dipping subduction continued to close the ocean. Africa collided with North America 270 Ma during the Alleghenian orogeny. Created a huge fold and thrust belt and mountain range. Assembled the supercontinent of Pangea.

Appalachians: 

Appalachians Pangea began to rift apart 180 Ma. Faulting and stretching thinned the lithosphere. Rifting led to development of a divergent margin. Sea-floor spreading created the Atlantic ocean.

Modern Orogenesis: 

Modern Orogenesis Modern instrumentation can measure mountain growth. Global positioning systems (GPS) measure rates of… Horizontal compression. Vertical uplift.

Isostasy: 

Isostasy High mountains are supported by thickened lithosphere. Thickening is caused by collisional orogenesis. Average continental crust – 35 to 40 km thick. Beneath orogenic belts – 50 to 70 km thick. This thickened crust helps buoy the mountains upward.