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1) How long does it take for ocean lithosphere that formed in the hot (> 1000 oC) MOR environment to cool to an equilibrium state and sink to its maximum depth below sea level? One answer only. approx. 5 m.y. approx. 50 m.y. approx. 100 m.y. approx. 150 m.y. 2) Which of One answer only. the following will subduct most readily? common feature? oceanic plateau old cold oceanic crust continental crust young hot oceanic crust 3) Where would One answer only. east Pacific Indian ocean Atlantic ocean west Pacific you find marginal basins are a 4) Which of One answer only. the following best describes a marginal basin? a small ocean basin surrounded by continents a large ocean basin adjacent to a continental arc system a large ocean basin separated from a continent by an island arc a small ocean basin between a continent and an island arc 5) Which of the following was formed by old back arc spreading or represents normal ocean crust that has been trapped behind a recently developed oceanic island arc? One answer only. inactive marginal basin with high heat flow inactive marginal basin with normal heat flow active marginal basin with high heat flow 6) In the diagram below, what does the ? represent? Type your answer in the textarea below. 7) Geophysical evidence suggests that there is a low-Q zone behind the arc. This is compatible with which of the following? One answer only. extension in the back arc region magnetic anomalies in the back arc basin melting in the back arc region sediments deposited in the back arc region 8) Which of the following models for back arc spreading involves; roll-back of the slab and regional extensional stresses in the lithosphere? One answer only. passive diapirism active diapirism stepwise migration convection driven 9) Which of the following models for back arc spreading involves; snapping off of the subducting slab, and initiation of a new subduction zone oceanwards? One answer only. passive diapirism active diapirism stepwise migration convection driven 10) The angle of dip of the subducting slab appears to govern whether or not extension occurs in the back arc region. Which of the following is more likely to promote extensional conditions in the back arc region? One answer only. steep dip shallow dip Structural Geology GEOL 3512 Fall 1998 Mid-term review sheet Here are some highlights of the things that we've covered so far. Any of this material, plus anything from the covered text chapters (2,3,4,14,15,16, and 18) and from your class notes, can and may be on the exam. Whole Earth and Plate Tectonics We started off talking about the structure of the whole Earth, which means how many and how thick are its layers and how we obtain this view of the Earth. Seismology is the main tool by which this view (or model) is obtained. Seismic discontinuities reveal a mantle that has layer boundaries at ~400 and 670 km; the origin of these discontinuities is probably related to a phase change that occurs in Olivine, from Spinel to Perovskite. Both P and S waves take a jump at these boundaries, the deeper of which is related (it is thought) to the structure of the convecting mantle. The mantle convects as either a two-layer system or as one layer; there is no consensus. Seismic images (ie catscan-like images of the distribution of seismic velocities) of the mantle show that subducting plates reach at least as deep as 670 km and in places perhaps deeper. Some plates seem to bottom out at this level,which implies that convection is limited to the upper mantle. There is some evidence, however, that plates can puncture the 670 level and move into the lower mantle. It has been suggested (and as far as I know it seems to a popular idea) that the mantle probably convects as a two-layer system most of the time, but that there are times when a large turn-over occurs and the entire mantle is involved. What sort of arguments might you assemble that would suggest a solution to this issue? (Perhaps geochemical.) This will not be on the exam! Lower seismic boundaries include the mantle-core boundary and the inner core boundary. These are also phase changes; the mantle is literally freezing from the core (and releasing huge amounts of heat as it does so). The inner core is solid. How do we know this? The upper mantle, above ~400 km, is the asthenosphere, the upper part of which contains a small fraction (~8%) of melt (the so-called low-velocity zone). This is the shear boundary across which the rigid lithosphere moves as a plate - the very plate of plate tectonics. The lithosphere is divided too into layers: upper mantle, lower crust, upper crust. There are essentially two types of lithosphere: oceanic and continental. You should know the approximate composition and dimensions of each of these. A hypsometric plot of Earth elevations reveal the two types of lithosphere very nicely. Could you sketch such a hypsometric curve? Be familiar with the internal structure of the two types of lithosphere, particularly at the scale of the crust. What is the name given to the boundary between the crust and the upper mantle? The lithosphere is broken into about 8 large plates and a number of smaller ones. You should know the geography of these plates; where the boundaries are, what types of boundaries these are, and roughly how the plates are moving with respect to the hot spot refrence frame (fig 14.12). Remember that transform faults fall on small circles about the Euler pole and spreading rates are a sine function of the distance to the Euler pole. Earthquakes reveal the subsurface geometry of subducting plates (or slabs as they are often called), and show that the configuration of the slab can be quite variable. These seismic zones are called Wadati-Benioff zones. We have covered the basics of plate tectonics kinematics. You should know what an Euler pole is (both conceptually and specifically - what parameters define one) and how to obtain one for any pair of plates given a plate "model" (such as the one you were given in lab). We have covered the basics of plate boundary development and the stability and evolution of triple junctions. Make sure you review these items. And remember that these exercises provide very powerful tools in getting a first-order understanding of the geological evolution of a region. Extensional Tectonics Extension of the lithosphere may occur by several means: by a whole-scale pure shear (in which extension of the whole column occurs, the lower crust and upper mantle homogeneously) or by various asymmetric means (in which extension of the upper crust is laterally offset from the lower crust or upper mantle; extension may also be relatively discrete rather than homogeneous). Normal faults may be planar or listric, the latter is more commonly shown. Faults in the upper crust are thought to sole into detachment faults that then transfer extension of the lower crust to another location. Faults are often considered passive or inert features where the hanging-wall slides down the fault and does all the deforming. But in fact, the footwalls of normal faults do a lot of deforming, too. Hence, the margins of rift or extensional zones are often lifted to form imposing flanks. You should be familiar with the schematic cross-section of a Steerhead basin. Why do early draping sediments tend to be salts? Why do later sediments tend to onlap the adjacent rift flanks? Whare are good examples of Steerhead basins? Metamorphic core complexes (MCCs) are rather different beasts (perhaps?) than typical rift basins. They are most commonly found along the western side of the Basin and Range province in the USA, but they are also found within the core complex of mountain chains such as the Canadian Rockies. Review the basic elements of MCCs. What is an accommodation zone? Where have old rifts tended to initiate? Convergent Tectonics We covered this topic by starting out with a cross-section of a archetype convergent zone, starting with the downgoing plate that is flexed at the outer bulge, the trench (and those turgid turbidites), the accretionary prism (and its thrusts and melange character), the forearc basin, the volcanic arc, and the back-arc basin. Remember that the character of the back-arc basin (of more perhaps?) is determined by the relative motion across the convergent plate boundary. The subducting plate may be rolling back (falling away), which will initiate back-arc spreading. But back-arcs may also be convergent (e.g., there is a fold-and-thrust belt along the back-arc position of the Andes), which suggests that the downgoing plate may be moving hard into the upper plate. Convergence of two continental plates inevitably gives rise to high mountains and significant extensive deformation. The classic continental convergent belt is the Himalyan belt. The Himalayas are what used to be the Indian subcontinent that is now emerging from the depths of subduction. Although continental material is too light to be subducted to great depths, still it is underthrust to depths of tens of km before being transferred to the hanging-wall and brought back up. Remember that what brings it back is a combination of erosion and the fact that thrusting has propagated to the foreland. The Ganges River currently occupies the modern foreland of the Himalayas, and its alluvial basin is the detritus of the Himalayas, the foreland basin. These sediments will eventually be incorporated into the next hanging-wall and become the source for future foreland basin sediments. The Himalayas and all the deformation to the north is often viewed (and understood) in terms of the collision with Tibet of a rigid indentor (India). This model explains the variety of structures that extend will into Asia, including the large strike-slip faults, seafloor spreading in the South China Seas, etc. But remember that there are problems and limitations on these models that have to do with boundary conditions. Be familiar with these issues. Strike-Slip Tectonics Here we covered the basic development of strike-slip faults, particularly as they evolve from buried and discontinuous to mature systems that are relatively planar. Deformation above the nascent, buried strike-slip faults is often characterized by en echelon strike-slip faults that strike obliquely to the main fault. These eventually join as displacement on the main fault increases. Generally the fault becomes smoother over time. Strike-slip fault zones often contain reverse and normal faults that accommodate the shearing the of the zone. There are some classic strike-slip faults in the world that you should know about (at least where they are and why they are): San Andreas fault, Anatolian fault, Semangko (sp?) fault, etc. What is a flower structure? Stress Make sure you know these things: The definition of stress Units of stress Meaning of mean and deviatoric stress Lithostatic stress and how to calculate it Stresses may be derived from body forces or from surface forces. When we consider stress at a point and in a state of equilibrium, stresses form a tensor that has 9 independent components. Be familiar with the symmetry and reason for the symmetry of the stress tensor. We covered the derivation and use of Mohr's circle for stress. We also went over the meaning of isostasy and the means to calculate the expected height of mountains. What important factor(s) determined the height of mountains and what important assumptions did we have to make to figure this out? Strain Make sure you know these things: Definition of deformation as the sum of strain, rigid-body rotation, translation, and volume change. Units and the various measures of strain. Concept of the strain ellipse, coaxial deformation, pure shear, simple shear, general deformation, etc. Plate Tectonics 2 Plate Structure •The various plate margins leave their mark on the plates they define. This helps geologists piece together both the present and past geology of the regions. •Plates bordering a subduction zone have specific features depending on whether both plates are oceanic or one is a continental plate. Features in the first case include a trench, a melange of piled sediments (a zone of high-pressure, low-temperature metamorphism), a volcanic island arc (Japan is one of these), and a back-arc basin (the Sea of Japan). In the second case, the back-arc basin is absent and the island arc is replaced by a volcanic mountain range (such as the Cascades). Ocean-ocean plate convergent boundary. Structure of a continent-ocean convergent boundary. Continent-continent collision. •Plates bordering a mature zone of extension (a divergent boundary) are commonly oceanic plates. The structure includes the mid-ocean ridge, which is a line of mountains with a rift valley in the middle where magma extrudes to the surface. The crust slopes away from the ridge, decreasing in height and increasing in age with distance from the ridge. Along the ridge are transform faults. These are the most predictable (and likely least dangerous) volcanoes in the world. Mid-ocean ridge divergent boundary showing transform faults. •Plates at a new zone of extension can also be continental plates. Upwelling of magma underneath a continental plate first causes a bulge, then a rift zone with fault blocks. Once the ridge sinks below sea level, the zone is mature and a mid-ocean ridge results. Two stages of an opening rift. The top image shows the second stage of the rift zone, while the bottom image shows the next stage, in which the rift has sunk below sea level. •Plates defining a transform fault can be of any type. Typically the features on either plate consist of a main fault and a network of lesser faults. •In some areas plate motions can raft small fragments of crust tremendous distances. Eventually, any fragment that is not consumed by subduction will be added to a larger continental mass (accreted). These fragments are called terranes, and they are geologically transient. Their ultimate fate is to be accreted to a larger continental mass. Portions of accreted terrane on the Pacific plate. •Examples of this type of tectonics include the islands of Taiwan and the Philippines. Other fragments form when they are sliced off the margin of a large continent; this is the case with the western side of the San Andreas Fault. What Drives Plate Tectonics? •Although the driving motion of plate tectonics is not known for certain, scientists believe the mechanism is related to thermal convection in the Earth's mantle. •Thermal convection is a process in which hot material within the mantle rises toward the surface due to buoyancy (it is hotter and therefore less dense). As it rises, it cools and spreads. When it cools enough, it begins to sink. •This convection forms thermal cells that act as conveyor belts, moving the plates along. Divergent plates form at the rising portions of these cells, while subduction zones occur where the cells sink. •Cells may be confined to the aesthenosphere, may involve the entire mantle, or may be more complex, with thermal plumes rising from the core-mantle boundary. Upper mantle convection cells. Full-mantle convection cells. •Another force that might be at work is called slab-pull or ridge-push. •In slab-pull, subducting slabs are pulled down because of their greater density. They take the rest of the plate along with it. In ridge-push, the upwelling magma pushes the extending crustal plates out of the way as it extrudes. •It appears that all three of these forces are at work in plate motion, but the contribution of each is currently unclear. Turning Back the Clock: Past Plate Motions Determining past plate positions •Past plate positions must be reconstructed from the evidence that remains from those time periods. •Magnetic anomalies along the mid-ocean ridges (discussed earlier) can be reconstructed —essentially pushing the plates back together — to trace past plate motion. However, this method only works so far, as the plates are not only created but destroyed (at subduction zones), thus erasing previous magnetic stripes. •Continental fit can also be used to piece together previous continental (and thus plate) positions. Regions where the continents do not fit together perfectly can often be explained through subsequent erosion or other activity (the formation of deltas and deepsea fans, creation of volcanic rocks at hot spots and so on). •Portions of poor fit can also be the result of erosion of regions that were deposited from other plates (sedimentary basins and such). •Paleomagnetism also works in volcanic regions away from the mid-ocean ridge, and can be very useful when occurring on the lighter, more long-lived continental crust. Putting it together •Evidence suggests that the mechanism or mechanisms that drive plate tectonics have been operating for at least 2 billion years and maybe longer. •The cycle of supercontinent formation and breakup is known as the Wilson Cycle. •1 billion years ago — The supercontinent Rodinia broke up along western and eastern N. America. •450 million years ago — N. America collided with Europe. •250 million years ago — Pangaea formed when N. America and Europe collided with Gondwanaland (S. America, Africa, India, Antarctica, Australia). The Earth as scientists have reconstructed it 250 million years ago. •180 million years ago — The beginning of the current state of the continents. A complicated series of events occurred, in which N. America and Africa separated, and Africa and India began their collision with Eurasia. •20 million years ago — the remains of the Tethys Sea (small gaps between Eurasia and Africa) closed. •6 million years ago — Mediterranean Sea was cut off from the Atlantic. •What's next — Assuming, as we must, that the plates continue their current speed and direction of movement, the next 50 million years will see Australia crush the islands of Indonesia in its move towards Asia, the disappearance of the Mediterranean and the opening of a new ocean at the East African Rift Zone. California will continue its move towards the Aleutian Islands. Australia moves north in this projection of the state of the continents 110 million years from now. Resources •Subduction zones are regions where hot fluids are percolating through igneous and metamorphic rock. These are ideal conditions for the deposition of heavy precious metals such as gold and silver. •Sedimentary resources like gypsum and salt are most likely deposited in geologically quiet regions such as the continental interiors. parental moment: Gordon-Michael Scallion has predicted that three large earthquakes, each one larger than the one that preceded it would strike the LA area. The third of these great quakes, which Scallion says will measure about 8.3 on the Richter scale (+/- .5), will initiate the breaking up of America. According to Scallion, the first two of these predicted LA quakes have already taken place: A 6+ on April 22, 1992 and the 7.5 Landers Earthquake which took place on June 28, 1992. Scallion is predicting the third one will occur before December of 1995. When the third earthquake occurs, a three stage land fracture and break-up of the entire western portion of America will begin. According to Scallion, the cause of this massive break-up is a very large magma bubble that is pushing up beneath the United States. This magma bubble is caused by a build up of ice at the poles. The ice build-up at the poles is causing an instability in the Earth's rotation which, in turn, is creating instability in the earth's magma and core. A third fracture will occur. This time vast sections of Utah, Nevada, Arizona, and Colorado will be swallowed up by the ocean. Phoenix, now a coastal city, will eventually emerge as a major seaport city. A very large seaport city will also be established in the Nebraska region. Denver and Sedona will become coastal regions. Lecture 12: Sedimentary Basins Concept of an area of sediment acccumulation or basin, e.g. sediment thickness contours in Himalaya. Concept of a Basin Three dimensional architecture of basin fill. Affected by spatial and temporal pattern of tectonic subsidence: o Lithospheric deformation process. o Three basic causes of subsidence: Loading and flexure (like an elastic plate). Thermal and density changes - isostasy. Faulting - isostasy. Sea level changes. Sediment supply rates and source position (drainage basin outlets). Basin Classification: Plate Tectonic Position 1. Extensional: o Half graben/graben - Rift Basin, e.g. Basin and Range, U.S.A. o Mature oceanic speading - e.g. Atlantic margin (Passive Margin). o Syn-rift, post-rift megasequences. 2. Compressional: o Foreland basin - flexural loading of the Earth's lithosphere. o Two types: Collisional, e.g. Himalaya. Back arc, e.g. Andes/Precordillera. o Piggy-back basin (thrust sheet top basin). 3. Strike-Slip Basins: o E.g. Dead Sea, Israel. 4. Passive Margins: o E.g. Atlantic Margin. 5. Subduction Related: o Oceanic trench, e.g. Marianas Trench. o Fore-arc basin, e.g Taiwan or Median Valley in Scotland. o Back-arc basin, e.g. Sea of Japan. 6. Cratonic "sag" Basins: o E.g. Chad Basin, Africa. 7. Abyssal Plains. 8. Predictive models of facies distributions: useful for subsurface exploration of oil or understanding dispersal of pollutants. Reading: Leeder, M.R. & Gawthorpe, R.L. (1987) Sedimentary models for extensional tiltblock/half-graben basins: Geological Society of London Special Publication, No. 28, pp. 139-152. Geomorphology from Space: Compendium of Satellite Images. (Short Loan in Library). Why the Philippine Sea Plate Moves as It Does Tetsuzo Seno Earthquake Research Institute, University of Tokyo J. Geol. Soc. Phil.55 105-117 2000 ABSTRACT The Philippine Sea plate (PH) is rotating relative to Eurasia (or to hot spots) around the pole south of Kamchatka central Kuril. I address a question why this pole position is produced by available driving forces of the PH. The ridge push and slab pull are the two major sources of driving forces for oceanic plates. The ridge push forces in the PH are generated by the age gradient within the plate; the West Philippine Basin is oldest and the Mariana Trough youngest. The torque calculated from the ridge push forces gives a pole of 70.1،N, 326.1،E in Greenland, which produces a westward motion of the PH. On the other hand, the slab pull, which is operated at the Kyushu - Ryukyu Trench and the Philippine Trench, produces a torque pole of 61.7،N, 257.9،E in northern Canada. Both of these poles are far from the pole of the PH motion relative to hot spots, implying some other driving forces are necessary. I propose here viscous drag forces beneath the Kyushu-north Ryukyu arc directing toward east are one of such driving forces. In the East China Sea west of Kyushu, an upwelling plume from the deep mantle has been suggested in previous studies. The lateral flow from this upwelling can drag the PH toward east. If a magnitude of this torque is comparable to those of the other driving torques, the desired torque pole south of Kamchatka-Kuril can be produced. Figure 1 The Philippine Sea (PH) - Eurasian (EU) plate Euler poles and the Euler pole of the PH with respect to hot spots (PH-HS). The PH is rotating clockwise around these poles looked from above. Sn77, Seno (1977); Sn99, Seno et al. (1993); WS, Wei and Seno (1998); Kt, Kotake et al. (1998). The PH-EU pole is derived from the sum of the PH-HS Euler vector of Seno et al. (1993) and the EU-HS Euler vector of HS2-NUVEL1 (Gripp and Gordon, 1990). Figure 2 The age distribution of the backarc basins in the Philippine Sea: the west Philippine Basin (See Seno [ 1988] for the references), Shikoku Basin (Okino et al., 1994), Parece Vela Basin (Okino et al., 1998), Mariana Trough (Hussong and Uyeda, 1981), and Bonin back-arc (Taylor et al., 1991). The bottom picture shows that from the difference of the age among these basins, the ridge push forces are generated at the boundaries between the basins. Figure 3 Distribution of the ridge push forces and the magnitude of the torque at each segment. The total torque pole (70.1،N, 326.1،E) is located in Greenland. Figure 4 Distribution of the slab pull forces and the magnitude of the torque at each segment. The total torque pole (61.7،N, 257.9،E) is located in central north Canada. Figure 5 The possible location of the pole of the additional torque required to produce the observed PH-HS pole, when it is added to the ridge push-slab pull torque. It should be located south of the PH and rotates the PH clockwise looked from above. Figure 6 A schematic illustration showing the mantle upwelling in the East China Sea west of Kyushu which is driving Kyushu and westernmost Honshu to the east by the viscous drag forces (Seno, 1999). The resulting compression in the forearc in Kyushu north Ryukyu might provide driving forces to the PH directing toward east. Figure 7 The location of the upwelling plume in the back-arc of Kyushu north Ryukyu. The resulting driving forces due to the flow from the upwelling are indicated by the arrow. The direction of these forces is conformable to the gap between the slab pullridge push torque pole and the PHHS pole, if they act as the driving forces of the PH. The collision forces at the Palau-Yap Trenches are also indicated by the arrow. They, however, do not help to reduce the gap. The Macolod Corridor is another location where a mantle upwelling is expected around the PH. Geology 304 Review (Review Homeworks 1 through 4!!) I. History of Plate Tectonics: Fixists versus Mobilists 1. List the Major Scientists First Proposing Continental Drift 2. Give the early problems with the theory of Continental Drift 3. Draw a time line of major advancements in the development of Plate Tectonic Theory and Continental Drift 4. State Wegner’s arguments for Continental Drift 5. State the arguments against Continental Drift II. Earth Structure: From the Crust to the Core A. Seismology Overview 1. Identify a P-wave and S-wave on a seismogram 2. Know the relation between wavelength, frequency, and velocity of a wave 3. Give the approximate velocities for P-waves and S-waves as a function of depth in the earth 4. Locate an earthquake using a 3 component seismogram 5. Describe the earthquake location procedure using at least three stations 5. Solve for the optimal focal mechanism given first motion polarity data 6. Describe the motion along a fault given a focal mechanism 7. Be able to draw a strike-slip, thrust, and normal focal mechanism 8. Define the term P and T axes related to earthquake focal mechanisms 9. Describe the relationship between P and T axes and stress changes due to an slip along a fault 10. Describe the procedure used in seismic travel time tomography (what are the data and what are model parameters) 11. Interpret a seismic tomography image 12. Describe how a refraction experiment is used in determining crustal structure B. Crustal and Lithospheric Mantle 1. List the differences in properties and composition between oceanic and continental crust 2. Give average values of oceanic and continental crustal thickness 3. Define the term Moho 4. Give the seismic velocity variations across the Moho 5. Define the terms mantle lid and lithospheric mantle 6. Define the Lithosphere-Asthenosphere Boundary 7. Define Airy and Pratt Isostasy 8. Be able to use the principle of Isostasy in solving for topography, density, and root thickness 9. Define Bouguer and Free Air Gravity anomalies 10. Define the Isostatic anomaly 11. Be able to prove that the Free Air is equal to the Isostatic Anomaly for long flat topographic features C. Mantle Transition Zone, Lower Mantle, and D" 1. Define the Mantle Transition Zone (depths and velocities) 2. Define of the Claperyon slope for a endothermic and exothermic phase transition 3. Describe the solid phase transitions at 410 and 660 discontinuities 4. State how temperature anomalies change the thickness of the mantle transition zone 5. List the predominant minerals in: Upper Mantle Mantle Transition Zone Lower Mantle 6. Define D” 7. Give the Seismic Properties of D” 8. State the difference between the core mantle boundary and D” D. 1. 2. 3. 4. 5. III. Inner Core and Outer Core List the properties of the Earth’s Outer Core List the Properites of the Earth’s Inner Core List the predominant elements in the Outer and Inner Core Give the seismic evidence for a Liquid Outer Core Describe the reason for the Geodynamo (Fluctuation of Earth’s Magnetic Field) Observations A. Framework of Plate Tectonics 1. Defining an Euler Poles 2. Know Euler’s Theorem for motion on a sphere 3. Know the necessary parameters (data) to determine an Euler pole 4. Know how to calculate the position of an Euler pole given two points defining an arc on a sphere 5. Know how to use the law of cosines and law of sines for a sphere 6. Define a great circle 7. Define a small circle 8. Describe a transform fault 9. Understand the relationship between the isochron velocity (Viso) and the true plate velocity 10. Define sea floor spreading 11. Know the Vine-Mathews Hypothesis 12. Describe what magnetic stripes are 13. Know how to estimate spreading rates from magnetic stripes 14. Describe why there are magnetic stripes on the sea floor 15. Define Isochron B. Plate Motions 1. Define the terms ferromagnetism and paramagnetism 2. Define the Curie temperature 3. Know the three primary types of remnant magnetization 4. Describe how paleomagnetically derived polar wander can be used to find relative plate motions 5. Define the Magnetic Declination and Inclination 6. Describe the Hot Spot reference frame 7. Describe how the Global Positioning System can be used to measure plate motion 8. Describe, briefly, SLR (satellite laser ranging), and VLBI (very long baseline interferometry) 9. Define the term triple junction 10. Know how to determine if a triple junction is stable (Hint: use vectors) 11. Give an example of an unstable triple junction 11. Know the naming convention used in defining triple junctions C. Seismicity Patterns 1. Define the term Wadati-Benioff zone 2. List and describe the three basic types of plate boundaries (ridge,trench, and transform) 3. List the regions where you expect to find deep seismicity 4. Describe seismicity patterns in a subduction zone, transform fault, and diffuse plate boundary 5. Describe the type of focal mechanisms you would expect see in a rift, subduction zone, spreading center, and plate boundary strike split fault D. Ocean Floor and Ocean Ridges 1. Describe fast, medium, and slow spreading mid-oceanic center/ridge and give examples of each 2. Describe gravity profiles over a mid-oceanic ridge and be able to sketch examples 3. Describe why trenches are the deepest parts of the world’s oceans 4. Describe the process of ocean floor subsidence and how it relates to the age of the oceanic lithosphere 5. Describe the Petrology of Mid Ocean Ridge Basalts (MORBs) IV. Kinematics 1. Know all stages of the Wilson Cycle 2. Describe each of the stages and be able to give examples A. 1. 2. 3. 4. 5. Transform Plate Boundaries Know two primary types of transform plate boundaries Be able to define restraining bend Be able to define pull-apart basin Know the meaning of dextral and sinistral Be able to list three major strike slip faults (San Andreas, Dead Sea Fault, Altyn Tagh, North Anatolian, Denali, etc.) B. 1. 2. 3. 4. 5. 6. 7. 6. 7. 8. Subduction Zones Describe an accretionary prism Define back-arc basin Describe the formation and origin of back-arc basins Be able to describe at least two hypothesis for back-arc spreading (induced convection and retreating subduction) Be able to list various factors influence the angle subduction Be able to list the properties of flat slab subduction (e.g. no volcanism, broad region of deformation, etc.) Know the typical free air and bouguer gravity signals across subduction zones Define active and passive margins Know the location and types of metamorphism occurring within a subduction zone. Be able to relate metamorphic bands/facies with parts of a subduction zone C. Continent Collisional Tectonics 1. Define escape or extrusion/escape tectonics (and know the scientists credit with developing these theories) 2. List the supporting evidence for escape tectonics 3. Know the properties of the upper mantle beneath the plateaus related to continent-continent collision 4. Be able to define delamination 5. Know the reasons the theories for the initiation of delamination 6. Describe slab breakoff 7. Be able to list and briefly describe the competing models for the development of the Tibetan Plateau 8. Define ophiolite, and continental accretion 9. Define decollment and nappe (thrust sheet) 10. Describe the difference between thin- and thick- skin tectonics 11. Know the crustal thickness values beneath Tibet (approximately) 12. Know the major boundaries within the Tibetan Plateau 13. Know the age of the Himalayan Orogen 14. Know the types of volcanism observed in continent-continent collisional belts D. Rifts 1. Be able to list and describe at three hypotheses for the origins of continental rifts 2. Describe the origin of high topography in continental rifts (doming and superswells) 3. Describe the asymmetry of continental rifts 4. Describe the origin of collision related rifts and give an example 5. Describe how a continental rift evolves in a young oceanic lithosphere (and give an example) 6. Describe the kind of igneous rocks you expect to find in continental rifts 7. Describe the type of crustal and upper mantle structure beneath rifts 8. Know the link between rifts and plumes 9. Describe what a metamorphic core complex is 10. Describe possible causes for continental splitting 11. Define aulacogen V. Driving Forces 1. Know the proposed driving forces of Plate Tectonics 2. Describe Ridge Push 3. Describe qualitatively how oceanic lithosphere is a function of age 4. Describe Slab Pull 5. Describe Basal Drag 6. Be able to able to list evidence for which force is principle force driving plate motions. Crashing Continents Benioff Zones XEarthquakes occur at shallow, intermediate and deep levels beneath subduction zones XThe earthquakes define a plane which begins at the trench and dips at about 45° beneath the arc XThis dipping plane of earthquake foci is called the Benioff Zone XThe Benioff Zone follows the upper part of the descending oceanic plate XShallow earthquakes also occur through the arc 33-04 Crashing Continents Island Arcs XIsland arcs are of chains of volcanically active islands arranged in a curved arc XAn ocean trench occurs on the oceanwards side XIsland arcs first develop on oceanic crust XThe crustal thickness in an arc is intermediate between oceanic and continental XVolcanic activity begins abruptly at a Volcanic Front about 200 - 300 km in from the trench XThe volcanic front and trench are separated by an Arc-Trench gap with no volcanism 33-05 Crashing Continents Island Arc Volcanism XVolcanic rocks in island arcs are mostly of andesitic composition XThe magmas originate mostly by partial melting of subducted oceanic crust and overlying mantle XMelting begins when the slab descends to about 100 km depth, forming the volcanic front Partial melting of basaltic ocean crust Rising magmas Volcanic eruptions Crashing Continents Chemical Differentiation XMid-Ocean Ridge: Partial melting of Mantle basalt magma XSubduction Zone: Partial melting of Basalt crust andesite magma XMature Arcs: Partial melting of Andesite crust rhyolite magma XAll of this is an irreversible chemical differentiation of the mantle in several stages XContinental crust grows by accumulation of increasingly silica-rich rocks 33-06 Crashing Continents Ocean trench Sedimentation XUnconsolidated sediment from the ocean floor is scraped off the descending plate at the trench XSlices of the oceanic crust may be included as ophiolite belts XThese rocks form a complex rock mass called an Accretionary Wedge XThe Accretionary Wedge is buckled upwards as new material is pushed beneath its base XThe chaotic jumble of rocks in the Accretionary wedge is called a Tectonic Mélange 33-08 33-09 33-07 Accretionary Wedge Crashing Continents Metamorphic Rocks and Subduction XHigh Temp - Low Pressure Metamorphism • Occurs in the core of volcanic arcs • Abnormal heating of the crust • thermal effects of subduction-related magmatism XHigh Pressure-Low Temp Metamorphism • occurs in the accretionary wedge • cold rocks are dragged to great depths and then upthrust again 33-10 Granites Basin Analysis Basin Analysis Geology 355, Sedimentology ©Copyright, 2002, Ron Parker Geology 355, Sedimentology ©Copyright, 2002, Ron Parker December 11, 2002 December 11, 2002 Geology 355, Sedimentology ©, 2002, Ron Parker Basins Basins _Topographically low places where sedimentary materials accumulate. _Basins are characterized by accomodation space. _Accomodation is created by… _Eustatic sea-level rise _Subsidence _Topographically low places where sedimentary materials accumulate. _Basins are characterized by accomodation space. _Accomodation is created by… _Eustatic sea-level rise _Subsidence Geology 355, Sedimentology ©, 2002, Ron Parker Basin Analysis Basin Analysis _Basin analysis is the detailed investigation of the processes that _Form basins _Fill basins _Alter basins _Uplift (invert) basins _Destroy basins _Requires sedimentology, stratigraphy, hydrogeology, petroleum geology, seismology, geophysics, geochemistry, paleontology, etc. _Basin analysis is the detailed investigation of the processes that _Form basins _Fill basins _Alter basins _Uplift (invert) basins _Destroy basins _Requires sedimentology, stratigraphy, hydrogeology, petroleum geology, seismology, geophysics, geochemistry, paleontology, etc. Geology 355, Sedimentology ©, 2002, Ron Parker Types of Basins Types of Basins _Intracratonic _Rift related _Strike-Slip related _Collision / Subduction related _Intracratonic _Rift related _Strike-Slip related _Collision / Subduction related Geology 355, Sedimentology ©, 2002, Ron Parker Intracratonic Basins Intracratonic Basins _Intracratonic – Basins that form within continental crust _Intracratonic – Basins that form within continental crust Geology 355, Sedimentology ©, 2002, Ron Parker Rift Basins Rift Basins _Rifts – Divergent plate boundaries that eventually develop into spreading centers. _Rifts – Divergent plate boundaries that eventually develop into spreading centers. _Initial rift sediments are arkosic sandstones interbedded with basalts. _Rifts then flood and deposit a thick sequence of evaporites. _Then marine sedimentation takes over. _Initial rift sediments are arkosic sandstones interbedded with basalts. _Rifts then flood and deposit a thick sequence of evaporites. _Then marine sedimentation takes over. Basins and stratigraphic successions I. II. Basins & basin types A. Basin: a region of depressed crust, typically with greater thicknesses of sediment accumulation than surrounding regions. B. Basins form by tectonic processes that cause the crust to subside, and so to create large amounts of accommodation space. Rapid A-space growth translates into abundant and well-preserved organic matter within the sediment. C. The abundance of organic matter combined with the low grade burial metamorphism that sediments in thick accumulation experience translate into major oil and gas accumulations. D. Basins provide the most complete successions of the geological record. E. Tectonic setting for the majority of basins falls into four general categories with most associated in one way or another with a plate boundary: 1. Constructive (spreading) margins: This setting produces a genetic series of basins from localized rift-related basins through narrow oceans (Red Sea phase) to voluminous passive margin basins. 2. Destructive (convergent) margins: This setting produces basins associated with subduction zones (foredeep basins), intra-arc spreading basins, back-arc spreading basins (Sea of Japan, Great Basin), and back-arc thrust (foreland) basins. 3. Transform (strike-slip) margins: mostly localized pull-apart basins such as the Gulf of California and Dead Sea. Grades into rift-type and thrust-type tectonics where relative plate motion is not fully strike-slip (i.e., transtensional and transpressional regimes). 4. Intra-cratonic basins. Fairly mysterious, concentrically subsiding basins formed on continental crust well away from plate boundaries. The cause of subsidence is unknown, but is generally considered to involve some combination of densitydriven stress and in-plane stress. Constructive boundary basins. A. Rifting and passive margin formation. 1. Continental rifting begins with dome-formation. Produced by hot-spot volcanism (bimodal: dominantly basaltic plus some rhyolite from partial melting of the crust by basaltic magma and decompression melting). 2. Much extension by normal-faulting. Individual domes link up to form a more or less continuous series of rift valleys (grabens, as in the East African Rift zone). Extension results in thinning of the upper crust. The lower crust thins by ductile flow. 3. High heat flow produces thermal uplift; uplift leads to further thinning of the upper crust by erosion. 4. Continued extension produces oceanic crust between the newly formed, thinned continental margins. Eventually (over 107 years), sea floor spreading and thermal subsidence yields a distinct mid ocean ridge, open communication with the oceans (Red Sea phase) to form a permanent seaway. 5. Continued seafloor spreading carries the new continental margins away from the active tectonic zone. The much-extended crust cools and subsides over the next 108 yrs. as a passive continental margin ("passive margin," for short). B. Rift basins. 1. Smallish volumes of sediment (100’s to a few 1000 m of accommodation space) in localized, fault-bounded basins. 2. Dominantly terrestrial (subaerial and lacustrine) and arid as result of uplifted margins that cast rain-shadows into the basins as well as their small drainage capture area. 3. Early rift sediments typically heterogeneous with basement clasts and volcanics dominant. Grains with low physical and chemical maturity. Late rift sediments often include extensive evaporites from episodic oceanic invasions (reflects III. variable rates of subsidence and sea level rise and fall). Remobilized salt becomes important in passive margin history later (see below). C. Passive margin basins. 1. Lots of accommodation space (10,000 to 15,000 m or more) from combination of thermal and sediment load-driven subsidence. 2. As oceans widen, marine conditions come to dominate. Early phases often still arid and dominated by carbonate deposition and fringing reefs along the shelf edge (at low latitude sites) 3. With continued subsidence the fringing mountains diminish, drainage basin size grows, and clastic sediment supply rates increase. In humid regions or times of low sea level, deposition switches to the clastic off-lap suite with formation of the familiar shelf environments: typical association of deltas, beaches, and subtidal clastic shelf deposits grading outward to deep sea fans and contourites in continental rise settings. Prograding sediments build a wide continental shelf upon a voluminous accumulation of clastic or carbonate (or some mix of clastic and carbonate). 4. The clastic off-lap suite is well developed along the present Atlantic coast of North America, South America, Africa, and Europe. The southern passive margin of Laurentia (what is now the eastern margin of North America) was dominated in the Cambrian and Ordovician entirely by carbonates. Both are major source of petroleum. Destructive (convergent) margin basins. A. Foredeep Basins. 1. Basin forming mechanisms: dominantly a combination of load-related and density-related subsidence. Loading leads to flexure of the crust. Subsidence near the load is matched by a compensating (but smaller amplitude) up-bend at the periphery of the down-bend. 2. At destructive plate boundaries, dense, old oceanic crust subducts beneath continental crust or younger, less-dense oceanic crust, forming an elongate, deep oceanic trench. 3. Interaction between the down-going plate and the overriding plate generates thrust slices (slabs of rock and deformed sediment) sheared off of one or the other plate, which are thrust ocean-ward, overtop of the down going plate. 4. The subduction zone may also include abundant sediment supplied to the trench (especially in humid climates; ex. central Andean margin of South America). 5. Thrust and sediment load causes subsidence of the crust, deepening the basin and creating large amounts of additional accommodation space. This is a foredeep basin: a basin that form from a combination of trench (density driven) and load-driven subsidence in the down-going (lower) plate. It is located oceanward of the volcanic arc and is rimmed distally by an uplifted forebulge. Ex: north Australian shelf-Banda Arc collision. B. Foreland Basins. 1. Thickened crust of an island arc complex or continental crust at the margin of the upper plate leads to intense interaction between the down-going plate and the upper plate. This generates considerable additional thickening and deformation of the upper plate via thrust slices that stack up to form (together with the volcanoes) a tall, complex mountain system and its isostatic roots. Ex: Andes Mountains. 2. Thrusts migrate away from the collision (subduction) zone toward the "foreland." Thrust and sediment loads causes subsidence of the crust forming a deep flexural basin (8-10 km accommodation space) followed by an uplifted forebulge and a much smaller second order basin (a few 100’s of meters Aspace at most). Ex: Amazon Foreland Basin. C. Facies patterns. 1. Load-driven flexure is rapidly accommodated by subsidence/uplift. 2. 3. 4. 5. Rapid growth of accommodation space in the foreland or foredeep basin leads to trapping of sediments in the source area and strong transgression and sediment starvation as relative water depth increases over a period of 10 4 - 105 years. Rapid uplift on the foreland or peripheral bulge results in loss of accommodation space, but facies response varies greatly with local conditions the state of eustatic sea level. Possibilities range from subaerial exposure to prograding carbonate or siliciclastic facies. Unlikely to exhibit transgression. Basin fill shows strong progradation of sediments filling deep basin. Sediments grade upward and proximally from black shales to flysch (mixed shale and turbiditic litharenites), to near shore clastics (deltaic and shelf-like sediments) to alluvial plane deposits (fans & rivers, etc.). Early basin fill with abundant, well preserved organic material. As the system ages, the mountains erode and the sediments become less volcanic, more plutonic and metamorphic (granitic) source-rock dominated, and become more mature both physically and chemically. These late phase deposits are sometimes referred to as mollasse. GEOLOGICAL HISTORY OF THE CENTRAL MEDITERRANEAN: OUTLINE OF CENOZOIC EVENTS Amanda Kolker GEOLOGICAL HISTORY OF THE CENTRAL MEDITERRANEAN BASIN: AN OUTLINE OF CENOZOIC EVENTS Geologic features in the present-day Mediterranean essentially result from two major processes: the tectonic displacement caused by the subduction of the African plate underneath the Eurasian plate; and the progressive closure of the Mediterranean sea involving a series of submarine-insular sills. The development of the Mediterranean basin begins with the breakup of the supercontinent Pangea in the Mesozoic. During this time, sea-floor spreading triggered the development of the Atlantic ocean in the Triassic period, which separated the African and Eurasian plates from the North American plate. Sea-floor spreading in another geographical location caused the development of the Tethys ocean, separating the African plate from the Eurasian. In the late Cretaceous period, these African and Eurasion plates began to converge, closing the Tethys ocean basin, and the remnants of this ancient ocean are now called the Mediterranean sea. There are three major geomorphical settings within the Mediterranean basin; areas with stable margin characteristics, areas with unstable convergent margin charactericstics, and areas with extensional margin (rifting) characteristics. Thus the Mediterranean basin is a location of an intercontinental interplate system; with compressional and extensional events occurring within close proximity. Geologists have yet to come to a consensus about which plates in addition to the African and Eurasian ones, if any, are involved in Mediterranean tectonics. Subsidence-related and other vertical displacements are also found in compressional and extensional areas. A few notable events occurred during the Cenozoic which affected the entire Mediterranean; the Messinian "salinity crisis", when the closing off of the Mediterranean-Atlantic seaway caused complete isolation of the Mediterranean and thus widespread evaporation; and then the Pliocene "revolution", when the channel opened back up, causing reestablishment of marine conditions; and the Quaternary "transgressive raised terraces," of controversial geological origin; among others. The Central portion of the Mediterranean basin exemplifies the juxtaposition of compressional and extensional tectonic activity in the area. The region bordered to the west by Sicily and to the east by Turkey's west coast (encompassing the Aegean, Ionian, and Adriatic seas) exhibit a particular set of features and will be the focus of my study. There were four major periods of extension in this area. The first one occurred in the Mid-Upper Jurassic; evidence of this phase is seen in the Strepanosa Trough and Ionian plain. A second one occurred in the Mid-Late Triassic, opening up the Ionian sea and the Eastern Mediterranean. A third extensional phase occurred in the Mid-Upper Cretaceous, as evidenced by the stretched features of the Sirte Rise, a monocline with normal faults and tilted blocks. The fourth one, occurring in the Mid-Upper Miocene through to the Quaternary period, affected many areas of the Central Mediterranean. This extensional phase is closely associated with compressive motions; it is part of the reason for a counter-clockwise rotation of the Southern Appennine area which begins in the upper Cretaceous. All four of these extensional phases are the cause of geologic features found in the area, such as volcanic activity and rift-related sedimentary processes. Due to such extension, the oceanic crusts of the Central Mediterranean are considerably thinned in some places. The Mediterranean Ridge or Outer Median Ridge is a sea-floor feature that marks the ustable (convergent) margin between two or more oceanic plates (geologists know that the African and Eurasian plates are involved, but which, if any, smaller plates are involved is a matter of debate). The first stages of the major collision between the North of the African plate and the South of the Eurasion plate is believed to have occurred in the lower-middle Miocene. This collision is also associated with the counter-clockwise rotation of the Appennine area, and both of these associations are exhibited in the Calabrian (Italy & Sicily) and Hellenic (Greece) orogenic arcs which are situated among both compressive and extensional dynamics. The ridge extends geographically from Sicily to Cyprus along a generally E/W strike. It is an extensive fold-fault system corresponding to recent uplift and folding of past abyssal plains. The features in the Adriatic sea are results of this duality of compression as well as extension, and also from deposition-related subsidence on a deeply foundered foreland (on the shelf). The Sicily Channel Rift area is an example of the Miocene-Quaternary extensional phase. The Adriatic sea itself is relatively shallow, and almost all of the ocean floor (a thick carbonitic platform underlain by continental crust) exhibits compressional deformation structures, except for the Ionian Abyssal Plain, which is thought to be underlain by Paleoceanic crust. The history of the Alpine orogeny, constituting the northwestern portion of the Adriatic, really begins in the Mesozoic as well, for the sedimenary strata which constitutes most of its orogenic elements weas laid down in the continental margins of the ancient Tethys ocean. The Alpine orogeny and the Calabrian arc orogeny are both results of convergent plate margin movement between Africa and Europe, and display some vertical uplift associated with the subsidence of Mediterranean sea-floor deposits during the Cenozoic. The Ionian sea perhaps experiences the major amounts of subsidence in the Central Mediterranean. The Ionian Abyssal Plain in this region is characterized by differentially sudbsiding areas but generally experiences more than adjacent regions, contributing greatly to the uplift associated with the Alpine orogeny and the Quaternary coastal blocks. The Hellenic trench (a thrust fault linked to the convergent activity in the Mediterranean ridge) began propagation in Miocene and continues today; it constitutes a major element of Ionian seafloor topography. The extensional features in the Ionian region are somewhat subdued, the dominant tectonic activity is convergent and/or related to vertical movement. The Aegean sea experiences considerable amounts of extensional features as well, related to the suduction of the African plate underneath the Hellenic Arc. Subsidence in the late Miocene also had a grand affect on the region, resulting in the fragmentation of an Aegean landmass from vertical displacement. Extension in the Hellenic arc area runs generally N/S, and crustal shortening forms an E/W insular platform. Here the oceanic crust is thinned to almost 1/2 its original thickness The counter-clockwise motion is further expressed in the area by transcurrent faulting in the Northern Aegean, beginning in the fourth extensional phase of the Mid-Upper Miocene. The outer regions of the Hellenic zones, by conrtast, exhibit compressive geology. All of the volcanic activity in the Central Mediterranean is related to one or more of these processes; subduction, back-arc extension, and/or other tectonic events throughout the Cenozoic. OTHER LINKS OF INTEREST: Volcanoes of Italy Eolian and Aeolian island arcs geothermal activity in Italian volcanoes BIBLIOGRAPHY & WEB SOURCES Image of tectonic map from web adress: http://geothermal.marin.org/map/tect_map.gif Stanley & Wezel, Geological Evolution of the Mediterranean Basin, SpringerVerlag,1985 Berckhemer & Hsu, ed; Alpine-Mediterranean Geodynamics, American Geophysical Union,1982 Higgins & Higgins, A Geological Companion to Greece and the Aegean, Cornell University Press, 1996