Mountain Building and Drifting Continents PDF

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Gad lecture over mountain building and drifting continents....

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Chapter 7 Mountain Building and Drifting Continents Champion of continental drift  Alfred wegner’s famous reconstruction showing three stages of continental drift Conceptual analog – lava lakes  Geologists can use modern day lava lakes as a way of visualizing how the plates move and interact w each other Thermal convection- mechanism of mountain building and continental drift  Allows magma to move the mantle/continental blocks o Continuous source of heat in magma from radioactive decay Fossil magnetism  Both inclination and declination angles can be determined in the lab by using sensitive magnetometer  With paleo-lat-long coordinates and the age of the rock, its position on the Earth can be plotted  Fossil magnets o We can infer approximate geographic paleopole and paleoequator o Steep inclination of fossil magnets indicates formation of the rock at high latitudes whereas low angle indicate a location near the equator According to peleomahentic evidence  Different positions of north America relative to the equator from Cambrian time to present  Note: progressive counterclockwise motion Prior to 1960s, most geologists considered the ocean floors to be generally featureless plains, the oceanic crust to be v old and topographically featureless. It was also assumed to be fixed in place Model for sea-floor spreaing  Expansion of ocean ridges (divergent) and arc-trench (convergent) systems  Magmas are produced in arcs by heating along the subduction zone  Deep earthquakes are concentrated in the relatively cool, brittle downgoing slab  Shallower earthquakes occur under the spreading ridges Geometry of spreading ridge axis, transform faults and subduction zones  Lithospheric plates (A,B,C,) move (rotate) around an imaginary pole  Transform faults are perpendicular to the spreading axis (parallel to imaginary lines if latitude around the rotation pole) Confirmation of sea-floor spreading  Parallel linear magnetic anomalies unique to the oceanic crust  Consistent pattern of anomalies associated w/ all ridges  The stripes are parallel to the ridge axis and have remarkable bilateral symmetry Magnetic polarity reversal time scale  Fossilized polarity episodes associated w/ reversal history

Chapter 7 Mountain Building and Drifting Continents 

The distant of each anomaly pair froma ridge axes is proportional to age

Paleomagentism helps date age of oceanic crust  Total rates of spreaing vary from 2 cm/year to more than 18cm/yr  Pacific is spreading 10 times faster than atlantic  None of the present oceanic crust can be old as the Paleozoic  Much of the westernpacific and some of northern atlantic crust is Mesozoic, but most of the rest of the present oceanic crust is Cenozoic which is confirmed by fossil evidence Tectonic activities: past and present  The alpine-himalayan belt of Eurasia o The tethyan region between godwanaland and Eurasia underwent close ad deformation, culminating in late Cenozoic time with the upheaval of the alps, Himalayas, and mountain ranges in asia minor o Parts of northern Africa collided with Europe to form the alops. India collided w asia to produce Himalayas o Arabia and iran collided w asia to produce the Taurus, zagros and other mountain belts of asia minor o SOME ACTIVITES STARTED LONG AGO AND ARE STILL ACTIVE TODAY Three types of plate boundaries  Each plate behaves as a rigid unit  Divergent plate boundary (constructive) each plate is moving away from a neighbor at an ocean ridge  Convergent plate boundaries (Destructive) each plate is moving toward another plate along volcanic arc and trench  Transform plate boundaries: where each plate is moving past another Isostasy  Thinner AND MORE DENSE UNITS (OCEANIC CRUST) STAND LOWER WHEREAS THICKER AND /OR LESS DENSE ONES (CONTINENTAL CRUST) STAND HIGHER  From isostacy o Subduction of oceanic material beneath continental material o When two plate with thick continental crust, collision result Subduction: most of the magma are andesite (not basaltic, the way they are in mid oceanic ridges)  Obduction: thick slices of the oceanic material have been shoved up over the lighter continental crust  Based on the relative age of the subducting plate and the rate of sea floor spreading o Older plate, more colder are denser and will subduct more easily than younger, warmer more buoyant ones o Faster rates of spreading, bring younger plates to convergent and result in faster subduction and the angle of subduction is less than with older plate Continental Collision

Chapter 7 Mountain Building and Drifting Continents  

Suture zone??? Intensely compressed and metamorphosed rocks define where two plates collided (also the former ocean) o Found on West coast of US; Appalachian Mountains There is collision between o Two continents (supercontinents) o Two island acrs o Arc and a continent o Oceanic ridges o Microcontinents might collide later w/ another continent or Arc

Sedimentary Basins: regions of the earth of long-term subsidence creating accommodation space for infilling by sediments  Types o Rift-related basins  The down-dropped basin formed during rifting because of stretching and thinning of the continental crust  EX: East African Rift  Subduction-related basinds  Trench basins o Downward flexure of the subducting and nonsubducting plates (sites of accretionary wedges)  Forearc basin o The area between the accretionary wedge and the magmatic arc, largely caused by the negative buoyancy of the subducting plate pulling down on the overlying continental crust SVoD  Foreland basin o Depression caused by the weight of a large mountain range pushing the adjacent crust below sea level o IN FRONT OF THE LAND (mountain belt) o EX: The Morrizon Formation  Is a clastic wedge composed of sahle, sandstone, rare conglomerates and volcanic ash  Volcanic ash deposits indicate volcanic eruptions primarily to the west Causes of subsidence  Subduction subsidence o Caused by the profound forcible depression of one lithosphere plate as it is subducted beneath another o Genererally result in the ormation of a long, narrow, deep topographic trench on the sea floor  Thermal or cooling subsidence o Cooling of the lithosphere plate as it moves away from hot, spreading ridge o More dense will sink

Chapter 7 Mountain Building and Drifting Continents 

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Crustal thinning subsidence o Thinning can occur by:  Phase change in the lower crust from normal to denser material  Stretching of crust beneath newly formed spreading center o Occur around hot spots Regional crustal subsidence due to local sediment loading o Sediments delivered by major river systems eventually deposited a non-negligible load on the crust, resulting in slight deformation (subsidence) and opens accommodation space for further sediment loading o EX: Gulf of Mexico and Mississippi River

Major types of sedimentary basins  Trench: fine sediments overlying ocean-floor basalt  Passive margins: quartz-rich sand and limestones passing seaward to mud Active versus passive margins  Active margins o Margins of the contients where it is involved in plate tectonic activity (subduction, transform, collision) o An excellent example is the west coast of South America. Earthquakes, volcanoes o Bc of the mountainous terrain, the continental shelf is narrow to non-existent, dropping off quickly into the depths of the subduction trench  Passive margins o Along the coastlines away from plate tectonic boundaries and generally consists of a gently sloping sheld, a slope and a rise o This leads to lots of low-relief flat land extending both directions form the beach, long river systems, and the accumulation of thick piles of sedimentary debris on the relatively wide continental shelves o South America and north America plates illustrate this difference...