Transcript GEOL 451
GEOL 451-2010 Geology of North America Review of some Lithotectonic Principles Updated January 2011 University of Regina GEOL 451-2011 R. Macdonald, Instructor Coverage in this presentation Uniqueness and interactive nature of the Earth system Basic Earth Structure Lithotectonic entities Largely from Condie p.14 onwards An Approach to Earth Processes 1. PETROCENTRIC Processes concerning only rocks of the earth’s crust and mantle, e.g. sedimentation, metamorphism, even diagenesis But rocks react with the biosphere, oceans and atmosphere Climate factor, asteroids, flood, tsunamis, etc Earth physiology - Jim Lovelock’s GAIA hypothesis The earth system maintains itself through positive feedbacks 2. TIME-CENTRIC Geologists tend to think in very long periods of time But some earth processes can occur very rapidly A return to CATASTROPHISM? Uniqueness of the Earth and interaction of the Earth Elements Need to consider the entire Earth system: earth-ocean-atmosphere Earth physiology: James Lovelock’s Gaia Hypothesis Feedback loops (+) and (-) Recycling lithosphere Knowledge explosion of the past 15 or so years Tuzo Wilson (1968): Data collecting Hypotheses (transient) New unifying theories The whole earth system Crustal recycling Crustal evolution Thermal history ET impacts Metamorphism Life history Life Crust Tectonism and tectonic history Mantle hotspots Earth cooling Oceans Atmosphere Solar radiation Climate Magmatism Earth’s axial tilt Earth’s core-mantle Magnetic fields Weathering 1 Fundamental Earth Structure 1. Rigid lithosphere rests on weaker asthenosphere 2. Lithosphere is fragmented into segments and plates in relative motion which continually change shape and size What are some of the major lithotectonic features of the Earth? Intraplate - Continental Cratons: Shields and Platforms Precambrian Shields Relatively stable older cratons, generally Precambrian and without a cover of Phanerozoic rocks. Continental platforms Relatively stable older cratons overlain by oval shaped Phanerozoic sedimentary, shallow water, ssts, lsts, shales, deltaic and fluvial, commonly not much more than 1000 m thick Intraplate - Continental Buried Precambrian Shields, cored with older cratons aka Platforms Relatively stable older cratons, generally Precambrian but with a cover of Phanerozoic rocks. Intraplate - Continental (Intra)cratonic basins, aka ENSIALIC basins (2) Deep, sometimes formed over failed rifts Other causes (see Kent) Epicontinental seas, some evaporites (e.g. Prairie Evaporite) Examples:Williston, Hudson Bay and Michigan basins, Amadeus and Carpentaria basins of Australia, Paris Basin, Parana Basin, Chad basin. Sedimentary and volcanic loading produces crustal densification on cratons and continental platforms. Interior sag basins Diverse origins, extension, thermal effects, higher density of underlying crust Typically have the longest timeframe Intraplate - Continental (Intra)cratonic basins, aka ENSIALIC basins Intraplate - Continental Inland-sea basins Major I style, typically dormant Overlie continental crust, connected intermittently to open seas, or cut off with extensive saline de[posits e.g. Black Sea Caspian Sea Gulf of Mexico Regional Crustal Subsidence due to local sediment loading Example: Gulf of Mexico and Mississippi River Sediments delivered by major river systems eventually deposit a nonnegligible load on the crust, resulting in some subsidence. This provides accommodation (space) for further sediment loading. (positive feedback). NOTE: Some reinforcement by petroleum extraction Basin Formation Due to sags produced in the crust by diverse mechanisms: Magma depletion Isostatic compensation: melting of ice caps Deep crustal/mantle underthrusting Magma accession: emplacement of higher temperature melts in the crust Basement block movements by a variety of causes Load deepening etc. Some basin subsidence mechanisms Robert Macdonald: Why do Continents Break Up? The Earth's interior is hot. The heat comes from the heat of formation of the Earth that has not yet dissipated and heat generated by of decay Largely recognized today as formed over Mantle unstable isotopes distributed through the hotspots/plumes mantle and crust. While the lithosphere May be a sign of incipient plate movements, marking cools primarily by conduction, thebeginning mantle of continental break-up cools by convection. Most of theconvective Why do continents break-up? heat from the mantle is dissipated at the midocean ridges and through cooling seafloor. Beneath large continents, however, heat builds up in the mantle. This excess heat should weaken the continental lithosphere making it easier to rift. Intraplate - Continental Continental Rifts the Robert Macdonald: Why do Continents Break Up? The Earth's interior is hot. The heat comes from the heat of formation of the Earth that has not yet dissipated and heat generated by decay of unstable isotopes Earth’s distributed through the interior contains formational and isotope-generated heat mantleandLithospheric crust. crust cools by conduction, but the While the lithosphere Mantle cools by convection dissipated at MORS and ocean floors cools primarily by conduction, the mantle large continents heat builds up in the Mantle, weakening Beneath cools by convection. Crust Most of thethe convective heat from mantle is the Relatively higher membrane stress in equatorial regions due to dissipated at the higher amount of earth curvature midocean ridges and through cooling Trench rollback at subduction zones seafloor. Beneath large Hotspots/plumes (randomly formed) continents, however, heat builds up in the mantle. This excess heat should weaken the continental lithosphere making it easier to rift. Intraplate - Continental Some causes of continental rifts Intraplate - Continental The East African rift system showing the Afar Triangle as a triple-junction at the intersection of the Red Sea, Aden and East African rifts. Possibly the expression of a mantle plume. Diverging rifts starts a new round of continental drifting and ultimately “creates” new ocean floor. Dots indicate young volcanoes. Intraplate - Continental The East African rift system showing the Afar Triangle as a triple-junction at the intersection of the Red Sea, Aden and East African rifts. Possibly the expression of a mantle plume. Diverging rifts starts a new round of continental drifting and ultimately “creates” new ocean floor. Dots indicate young volcanoes. But not so simple Intraplate - Continental 1. 2. 3. 4. 5. 6. Initial doming and normal faulting. As lower crust & lithosphere thins by ductile shear, heat flow increases and normal faulting occurs in the brittle upper crust. Increased heat flow produces bimodal (basaltic and rhyolitic) volcanism Subsiding rift basins collect infill sediments . If rifting continues the crust/lithosphere thins to zero and seafloor spreading is initiated Sediments on continental passive margins drape drape over normal faulted basement After the initial thinning, margins continue to subside for tens of millions of years by continued cooling and loading subsidence Intraplate - Continental RRR Triple Junctions and Aulocogens If rifting stops before complete continental breakup, the failed rift or aulocogen infills with sediments and be buried in the subsurface, perhaps to be reexposed by some later episode of erosion or be discovered by seismic exploration. Aulocogens are commonly associated with continental breakup. Continental rifts seem to start as a number of rift-rift-rift triple junctions. Two of the rift arms become a new ocean basin and the third becomes a failed rift, although it may still be active as a continental rift system. The East African rift (EAR) appears to be a modern example, as ti is the failing arm from the triple junction including the Red Sea and Gulf of Aden. See also Basin and Range Half grabens East African Rift Transcurrent rifting Intraplate - Continental Rift-related igneous activity: bimodal volcanic signature distinctive trace element geochemistry continental rift basalts are enriched in alkalis (K, Ba, Rb), and incompatible elements, LIL. deep mantle-plume contribution mantle fluids and metasomatism. lithospheric mantle contribution Other features: distinctive trace element geochemistry with sediment traps, accommodation space arkoses, immature sediments half grabens fault driven sedimentation: alluvial fans and debris flows Along-strike changes = segmentation and depocentres every rift basin is unique Intraplate - Continental The failed third arm (called an aulocogen) is a topographic low. Many major rivers in the world flow down aulocogens e.g. Amazon, Mississippi, Niger, St. Lawrence, Rhine, and parts of the Nile Intraplate – Oceanic Crust Oceanic plateaux Ocean basins - sag basins pelagic clays, oozes, turbidites Volcanic islands/ seamounts/guyots Produced by Mantle plume hotspots - long-lived structures fixed within the mantle. Lithospheric plates move over them, typically in a datable track. e.g. Hawaii, Yellowstone, Galapagos Intraplate Oceanic Mantle plume hotspot tracks Ages in million years Intraplate Oceanic Intraplate Oceanic Long lived global hotspots Divergent - Continental Proto-oceanic troughs Red Sea <5 Ma oceanic crust in centre, thick salt deposits due to ocean cut off Passive margins Continental rises and terraces (prisms/wedges, continental crust thinned, transitory and oceanic crust, can include pelagic turbidite. May be caused by densification by metamorphism e.g. Eastern N. America seaboard. Stable EA coast Divergent - Continental Detailed Cross-section of a Passive Margin Atlantic Margin What is the relative age of the basalt? Jurassic salt Cretaceous & Cenozoic sediments Triassic rift valley sediments Divergent - Oceanic MORs (Mid-oceanic rifts) Divergent - Oceanic Oceanic Crustal Age revealed against passive margins Convergent - Intraoceanic Oceanic volcanic arcs with intra-arc basins Deep sea trenches – arc-trench gaps (containing fore-arc basins) – active volcanic (island arc) arc – back-arc Convergent - Intraoceanic Two oceanic slabs converge; one subducts The subducted slab produces melting in the overlying mantle wedge Magma Is less dense than overlying crust / lithosphere and rises as volcanoes. If the volcanoes emerge as islands, a volcanic island arc (or archipelago) is formed e.g. Japan, Aleutian islands, Tonga islands Oceanic Back-Arc Basins 1. 2. 3. 4. Back-arc basins (or retro-arc basins) are submarine basins associated with island arcs and subduction zones Found at some convergent plate boundaries, presently concentrated in the Western Pacific Ocean Most result from tensional forces caused by oceanic trench rollback rollback and the collapse of the edge of the continent Back-arc basins were not predicted by plate tectonic theory, but are consistent with the dominant model for how Earth loses heat Ocean ic Back-Arc Basins Convergent - Continental Continent:Continent with subduction North-south profile across the eastern Alps. Subsurface profile from seismic reflection data. After Adrian Pfiffner Common when two continents collide and the buoyant continental lithosphere does not subduct Any original trenches are eliminated Collision then thickens the crust, along the suture separating the original continents Crustal thickening then responds isostatically, producing a large mass of buoyant continental crust e.g. Himalayas, Alps, Appalachians Convergent Continental Continent:Continent with subduction Example from the Himalayas Part of Africa breaks away ca. 50 Ma ago Travelled to the north at ca. 10 cm/annum Is subducted under continental Asia, cause it to rise in elevation Plate movements continue today, so Hilary had it a few centimetres easier to climb Everest than today’s climbers Cause of the Indonesian tsunami Convergent - Continental Convergent - Continental Head 0n with obduction Convergent - Continental Obduction styles Convergent – Continental Margin Products: Deep sea trenches Trench slope subduction basins Accretionary complexes Mélange Foreland arcs Fore-arc basins Intra arc basins Back-arc basins Foreland fold-thrust belts Crustal melting occurs above the descending slab producing batholithic rocks surmounted by volcanic. Sediments are derived mainly from the arc and are siliclastic Sediments are subducted or scraped off into the accretionary complexes e.g. Sunda, Aleutian, Peru-Chili, and Japan. Convergent – Continental Margin Vertical sequence: Volcanic arc Crust (sub-arc lithosphere TTG) 2. Upper Mantle wedge 1. Subducting slab Convergent – Continental Margin Convergent – Continental Margin Convergent – Continental Margin and Oceanic Transcurrent (strike –slip & transform) Transtensional Transpressional Transrotational Intracontinental wedge basins Transcurrent (strike –slip & transform) Transform faults Most transforms are prominent linear breaks associated with midocean ridge segments. Known as fracture zones these occur between offsets in the spreading ridge. Fracture zones are a geometrical necessity due to the fact that seafloor genesis occurs on a SPHERE. Suspect terranes This term applies to a terranes which have been brought in from a long distances, exotic in nature to the terranes they now abut. With accurate age-dating and other methods of establishing provenance it may be possible where the suspect terranes come from, and how far they have travelled Analysis of such terranes is the main basis for constructing paleo maps Plate Tectonic Mechanisms No one mechanism accounts for all major facets of plate tectonics Convective flow in the plastic 2,900 km-thick mantle is the best option Other mechanisms generate forces that contribute to plate motion. Slab-pull on cold plate in subduction zone Ocean ridge-push Gravitational sliding on oceanic ridges The Six Major Types of Sedimentary Basin (with examples) Indonesia Nevada E. Africa Offshore Calif. Michigan Basin E. Coast NA Six major types of sedimentary basins are shown in their platetectonic settings. The major physical cause or causes of subsidence for each case are shown below on above the diagram. Seismicity related to Subduction A scheme relating igneous rocks to plate tectonics