Geology: Plate Tectonics, Rifting of Continents, and Mountain Building - Prof. S. Bart, Study notes of Geology

Download Geology: Plate Tectonics, Rifting of Continents, and Mountain Building - Prof. S. Bart and more Study notes Geology in PDF only on Docsity! Chapter 9 Fall ‘11 I. Three kinds of plate boundaries: 1. Spreading zones a. If a spreading zone grows through continental crust, continents break apart (a new sea is made in the area where the landmasses have separated) 2. Seduction zones a. At a subduction zone, the subduction of oceanic crust causes volcanoes and earthquakes. b. The subduction process may ultimately consume all ocean crust and join two continental landmasses c. Suture- (a mountain forms along this) major fault zone through an orogen or mountain range. Sutures separate terranes, tectonic units that have different plate tectonic, metamorphic and paleogeographic histories. The term is borrowed from surgery where it describes the sewing together of two pieces of tissue. 3. Transform Faults a. The enormous strike-slip faults offsetting spreading zones, sometimes intersect the margins of continents II. The Rifting of Continents A. Hot spots give rise three continental rifting:  Modern example of active continental rifting: o Involves East Africa and the seas around it o Rift arms are: the Red Sea, Gulf of Aden and the East African Rift o All three at grabens at different stages  Graben-is a depressed block of land bordered by parallel faults. Grabenis German for ditch. Graben is used for both the singular and plural o Hot Spot- At the center of the arms, land is often elevated, forming a dome o The African Rift may split Africa in the future.  Ancient triple junctions. o The red Sea provides a modern equivalent, but the following are relics of triple junctions:  Many three-armed junctions formed between the US, South Africa, and Africa (when this mega-continent broke-up the Atlantic Ocean was formed)  Failed Rift- for some reason, often only two arms of the rift remain active and the third dies out and forms a graben.  Examples: o Mississippi River o Amazon Valley o Niger River  Many rivers flow through these valleys which are actually grabens of failed rifts B. Rift Valleys form when continental breakup begins  Rift Valleys- product of faulting o As extension breaks continental crust, the break is not clean, but occurs several fault blocks o Fault-Block Basin- Blocks decrease rapidly and accumulate sediments  Ex: Lakes, such as Lake Tanganyika in Africa rift Valley, form in the depression o Rift Basins are associated with:  Mafic volcanoes that have rose up from the mantle along the faults  Mafic dikes and sills  Flood Basalt  Felsic volcanism from melting the surrounding continental crust o The Red sea has many of these features (like volcanism, block faulting, and more) C. Rifting Creates Passive Margins  Passive Margins- when margins are far enough from the spreading center, they become tectonically inactive o Passive margins accumulate sediment along shallow shelves  Active margins- tectonic deformation zones with igneous activity III. Bending and Folding Rocks  When rocks are affected by stress (compression, etc.) they can o Break o Change Shape o A central part of igneous rocks (volcanoes and plutons) o These are surrounded by a metamorphic belt (metamorphosed by heat from the plutons). These metamorphic rocks are often severely folded. o Fold and thrust belt- rocks can be folded and faulted and distant from the heat source. Deformation is due to convergence. The rocks further away from the heat source will suffer more brittle deformation (faulting) and less folding . o Thrust sheets- large slices of crust break, as a result of the converging plates E. The weight of a mountain belt creates a foreland basin  Foreland basin- is a broad depression of the lithosphere inland of the continent’s thickened leading edge from the volcanic emplacements and fold/thrust belts o The downwraping of lithosphere beneath actively forming mountain chains extends inland of the fold and the thrust belt. o Foreland basin- the elongated depression o Foreland basin characteristics:  Axis is parallel to the mountain chain  Basins are usually formed rapidly  They can be very deep, often are flooded by the sea  What types of sediments are found in a foreland basin? o Flysch- deep-water deposits of shale and turbidites  The basin floods rapidly, so the first type of sediments found are deep-water muds, such as shales  Turbidites accumulate when the slope from the foot of the mountain is steep enough to send turbidity flows out of the basin o Molasse- The basin becomes filled with non-marine sediments (alluvial fans, floodplains, riverbeds, etc.) . Sediment eroding from the adjacent mountain may eventually fill the deep basin  Molasse is often called Clastic wedge because of its shape that pinches out of the mountain’s flank  What type of depositional transition characterized a foreland basin? o The sequence shows a transition between  Deep-water sediments first to non-marine sediments  What happens once the orogenic activity stops? o Igneous activity and folding/thrusting STOP o Molasse deposition comes to an end as erosion levels the mountainous terrain. o An eroded core of rocks once deeply buried is exposed at the surface. F. The Andes exemplify mountains building without continental collision • Due to collision/subduction between the Pacific oceanic crust and the continental plate bearing South America today. • Enormous volume of igneous rocks have been added since the Mesozoic • This process is continuing to build up today (part of the Pacific ring of fire). • The mountain belt is also being raised isostatically • The mountain chain is spreading to the East as the source of the magma is moving at lower angle in this direction, releasing magma further inland. • See slide 32 for an explanation. • The depth at which the subducted plate melts and releases magma is more or less constant. • If the angle of subduction is high, the magna is released closer to the trench . • But the magma is released further inland if the subduction angle is reduced. • Change in angle of subduction is believed to be linked to change in the rate of plate movement. • About 10 Ma ago (Miocene), a foreland basin connected to the Atlantic was located along the Andes. • A connection to the Atlantic existed along the axis of a failed rift. • The Amazon River occupies the axis of this partly filled ancient seaway. • Some Atlantic marine animals that occupied the Amazon Valley evolved to live in freshwater environments. • The animals include fresh-water species of dolphins, stingrays and manatees that descended from marine ancestors. G. The Pyrenees exemplify mountain building by continental collision  Pyrenees Mountains separate France from Spain.  They were formed when Iberia collided with Eurasia o Iberia was originally part of Eurasia but was split by a MOR. o Then, a change in plate movement created subduction which reattached Iberia. o Ophiolites in northern Pyrenees mark the suture o Foreland basins received flysch then molasse on both sides of the Pyrenees suture. V. Tectonics of Continental Interiors Plate margins vs. Plate interiors… Here, we review the deformation of Earth’s crust far inland, away from the continental margins o Causes: o Poorly understoond  Features:  Structural Basin- small movement downward of the continental crust produces circular r oval depression of stratified rock  Structural dome- small movement upward (uplift) of the continental crust produces circular or oval uplift of stratified rock  NOTE: both basins and domes can result from compressive forces.  Erosion leads to circular pattern o In a dome, the oldest beds lie in the center. o In a basin, the youngest beds lie in the center Examples of domes and basins: 1. Black hills of South Dakota • Its an oblong dome • It lies about 250km (150mi) from the Rockies • It was formed at the same time as the Rockies. • Erosion has now exposed some of very old strata in the center. 2. Map of the main domes and basins of North America  Mountain chains such as the Cordilleran and Appalachian typically depress adjacent continental crust to produce a foreland basin, and as seen on this map, the next geographic zone is often found to be uplifted/depressed as domes and basins. 3. The Michigan basin • It is a structural basin • It accumulated 4 km of sediment since its creation • May have been associated with thermal weakening of the crust.