LECTURE 7

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Published on October 3, 2007

Author: Janelle

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Slide1:  Evolution of the Earth Seventh Edition Prothero • Dott Chapter 7 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Mountain Building and Drifting Continents Fig. 7.1:  Fig. 7.1 Ocean Drilling Vessel JOIDES Resolution. This scientific drilling ship is equippped to drill 5 miles below the ocean surface. To date, it has drilled over 500 wells worldwide. Together with its earlier sistership, the Glomar Challenger, they have drilled over 1500 wells. Mountain Building and Drifting Continents: Major Concepts:  Mountain Building and Drifting Continents: Major Concepts By end of 1857 (“Heroic Age”) models of Earth all assumed that continents were fixed in place. Attempts were made to explain mountains by crustal cooling and contraction of Earth. However, even then evidence existed that the continents had moved. By 1950, submarine data had beg;un to show astonishing sea floor features. This eventually lead to “sea floor spreading” and “plate tectonics”, the crowning jewels of geologic thought. Fig. 7.2:  Fig. 7.2 Early (1840) depiction of a cross-section across the Applachian Mountains. Section is across Pennsylvania from northwest (top) to southeast (bottom) Fig. 7.3:  Fig. 7.3 Early theories of mountain building based largely on geologic observations in the U.S. Appalachian Mountains. A. Hall(1857) postulated that sedimentation depressed the crust at continental margins. B. Dana (1873) postulated that the crust underwent bending as the interior cooled and shrank. Fig. 7.4:  Fig. 7.4 Two different interpretations of the structure of the Jura Mountains (Switzerland). Top - upper rocks slip along “basement” along a flat shear surface. (“Thin-skinned” tectonics.) Bottom - Basement rocks are involved in folding and faulting along steep (thrust) faults. (“Thick-skinned” tectonics) Fig. 7.5:  Fig. 7.5 Early conception of continental drift (Arthur Holmes, 1910) showing formation of Cenozoic mountains. Fig. 7.6:  Fig. 7.6 Early attempt to fit continents together by “cutting” present-day globe and rotating and squeezing islands and peninsulas. (Baker, 1912, Michigan Academy of Sciences) Fig. 7.7:  Fig. 7.7 Alfred Wegener’s famous reconstruction showing three stages of continental drift. (Wegener, 1929) Fig. 7.8:  Fig. 7.8 Wegener’s reconstruction of Permian continents and paleoclimate zones based on rock assemblages (glacial striations) and paleoclimate data (salt deposits, fossil ferns, etc.). Fig. 7.9:  Fig. 7.9 Holmes convection current mechanism for continental drift. (A. Holmes, 1928, Physical Geology) Fig. 7.10:  Fig. 7.10 A. Example of remnant magnetism with respect to present field for two rocks of different ages. B. Restoration of Cambrian magnetic field after correcting for post-Cambrian folding. Slide13:  Figure 4.7 How do we reconstruct paleo continental positions?:  How do we reconstruct paleo continental positions? Early pioneers had to use geology (glacial striations, salt deposits), paleontology (fossils) and geometry But all this changed when paleomagnetism was discovered and used to recreate positions of continents in the past. Fig. 7.11:  Fig. 7.11 Schematic representation of components of fossil magnetism. The declination angle provides information on paleolongitude. The inclination angle provides information on paleslatitude. With paleo-lat-long coordinates and the age of the rock, its position on the Earth can be plotted. Fig. 7.12:  Fig. 7.12 Relationship of earth’s magnetic field to remanent magnetism in rocks. I. Present II. Present discordant (measured) positions III. Restored positions (paleogeographic locations) Slide17:  Figure 2.8 Slide18:  Figure 2.9 Slide19:  Figure 2.10 Fig. 7.13:  Fig. 7.13 Different positions of North America relative to the equator from Cambrian time to present. Note progressive counterclockwise rotation and northward drift throughout time. Fig. 7.14:  Fig. 7.14 Restoration of continental positions of longitude in the past using paleomagnetic data. The declination angle of a sample points toward the paleomagnetic pole. Continental positions must be adjusted until ancient pole positions for two continents coincide. Having outline for original continental margins helps. Fig. 7.18:  Fig. 7.18 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. Nature of Sea Floor:  Nature of Sea Floor Slide24:  Prior to the 1960’s most geologists considered the ocean floors to be generally featureless plains, the oceanic crust to be very old and topographically featureless. It was also assumed to be fixed in place. By 1970, all this had changed. Fig. 7.19:  Fig. 7.19 Model for sea-floor spreading showing expansion of ocean ridges (divergent) and arc-trench (convergent) systems. Three lithospheric plates are shown moving over the weak low-velocity zone of the upper mantle. 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. The 1000 C contour illustrates the contrast between hot upper mantle beneath ridges and cooler region beneath the arcs. Fig. 7.20:  Fig. 7.20 Gemini spacecraft photo of Gulf of Aden and southern Red Sea Fig. 7.21:  Fig. 7.21 Comparison of motion on transform and transcurrent (strike-slip) faults. Red lines are spreading ridge axes. A and B show two different stages for each case. Upper: Plates are spreading away from ridge axis and the transform fault connects two offset segments of that axis. Segments of adjacent moving oceanic crust slide past one another along the transform while spreading occurs. Lower: Sea-floor spreading ceased before A and then a transcurrent fault cut the ridge. Between times A and B the dead ridge was offset in the direction shown by the arrows in the opposite sense of displacement from that of the transform fault. Fig. 7.22:  Fig. 7.22 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. Rates of spreading are indicated by lenghs of arrows and increase from rotation pole to (rotation) equator. Fig. 7.23:  Fig. 7.23 Fig. 7.24:  Fig. 7.24 Fig. 7.25:  Fig. 7.25 Fig. 7.26:  Fig. 7.26 Paleomagnetism helps date age of oceanic crust. Fig. 7.27:  Fig. 7.27 Earthquake epicenters 1961-1967 Note how epicenters outline plate boundaries. Arrows indicate direction of horizontal motion during earthquakes. Fig. 7.28:  Fig. 7.28 Major Lithospheric Plates Plates as defined by seismicity (previous slide). Arrows show direction of plate motion and confirm hypothesis of sea-floor spreading by showing divergence (extension) away from ocean ridges and convergence (compression) toward volcanic arc-trench (subduction) zones. Fig. 7.30:  Fig. 7.30 Possible Driving Mechanisms for Plate Tectonics 1. Ocean ridge push 2. Gravity sliding (down slope of an ocean ridge) 3. Gravitational pull on a cold plate (down a subduction zone) 4. Carried on convection cell. Fig. 7.31:  Fig. 7.31 Types of Plate Interactions Fig. 7.32:  Fig. 7.32 The Six Major Types of Sedimentary Basins The six major types of sedimentary basins are shown in their plate-tectonic settings. The major physical cause or causes of subsidence for each case are shown below the diagram. Some examples are indicated in top. Michigan Basin E. Africa Nevada Offshore Calif. Indonesia E. Coast NA Fig. 7.33:  Fig. 7.33 Stages in the Development of a Passive Margin How did we get from B to C? Fig. 7.34:  Fig. 7.34 Detailed Cross-section of a Passive Margin Atlantic Margin Triassic rift valley sediments Jurassic salt Cretaceous & Cenozoic sediments What is the relative age of the basalt? Fig. 7.35:  Fig. 7.35 Regional Crustal Subsidence due to local sediment loading Example: Gulf of Mexico and Mississippi River Sediments delivered by major river systems eventually deposit a non-negligible load on the crust, resulting in slight deformation (subsidence) and opens accomodation space for further sediment loading. (positive feedback). Fig. 7.36:  Fig. 7.36 Formation of an intercratonic basin and a foreland basin Formation of an intercratonic basin and a foreland basin and an intervening arch by “thrust loading” e.g thrust faulting a package of rock onto a continental margin. Fig. 7.37:  Fig. 7.37 Conversion of a passive margin to a convergent margin A classic passive margin (A) can be converted into an active convergent margin by collision with an arc (B). Thrust loading and erosion of mountains produce a foreland basin (FB). Cessation of tectonic activity and deep erosion can produce a new passive margin (C). GO TO “SEAFLOOR SPREADING” SLIDES:  GO TO “SEAFLOOR SPREADING” SLIDES

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