Tectonics Spring 2016 General Questions
The following questions result from a general course on Earth tectonics. Sketches are essential to most of the answers. For each question, provide references for the key arguments and of course, for the figures. Include the key workers who have made important contributions.
(1) Whole Earth
(a) Draw a cross section of Earth and locate the boundaries of the continental crust, ocean crust, lithosphere, asthenosphere, and core.
(b) Show a P-wave seismic profile through the sketch, and define the major discontinuities and gradients, particular the mantle discontinuities.
(c) Define the adiabatic gradient, geothermal gradient, and mantle solidus. Use these curves to illustrate the general behavior of mantle plumes, particularly those that originate near the core-mantle boundary.
2 Ocean Crust and Ridges
(a) Draw a cross-section of ocean crust showing the major seismic domains and a geologic interpretation for these domains. Discuss the difference(s) between the seismic or geophysical MOHO and the petrological MOHO.
(b) What is the classical "infinite onion" model for the generation of ocean crust. How have modern studies modified this picture (For example, what is the AMC reflector and LVV, how big is it and what does it imply)?
(c) Contrast the physical structure of fast ocean ridges and slow ocean ridges. Give an example of each. Discuss along-strike variations along a ridge system.
(d) Show and the Free-air and Bouguer gravity character of mid-ocean ridges. Discuss the implications of each.
Ocean Crust and Ridges (cont.)
(e) Why does mantle melt at the ridges? How do we classify basalts? What is a tholeiite? What is MORB? Provide a petrologic model (history) for the production of MORB. (i.e. melting at what pressure? fractional crystallization??)
(f) Contrast N-MORBS and P-MORBS. What is M. Wilson's explanation for the difference?
(g) Discuss metamorphism of ocean crust. Where does it occur and what is its character? Where do we find polymetamorphic rocks in the ocean crust?
3 Heat Flow
(a) Discuss the relationship between heat flow and age of the ocean crust. Give some values and typical scatter. Discuss the relationship between the depth of oceans and age. Where does the model break down. Why do heat flow vs. age models seem insufficient in young regions of ocean crust?
(b) Contrast the two basic thermal models for the ocean lithosphere-mantle system: (a) thermal boundary layer model and (b) plate model (Sclater et al., Figure 3).
4a Subduction Zones and Arcs
(a) Draw a complete, idealized cross-section of a subduction zone. Include and label: bulge, trench, accretionary prism (subduction complex), forearc, arc, marginal basin (back-arc basin), remnant arc etc. Show vectors of motion for particles of material in each part of the section (upper plate and lower plate). Write a detailed caption explaining your figure.
(b) Show (and explain) a free air gravity profile across the sketch and explain the major characteristics.
(c) Show a few isotherms on the sketch and briefly summarize the thermal structure of the sketch (You can make a second sketch if you like).
4b Subduction Zones and Arcs
(d) Why to mantle rocks melt in arcs? Briefly discuss two alternative models for the origin of felsic rocks in arcs.
(e) Discuss the spatial and temporal variation in composition of arc magmas (volcanics) i.e. basalt-andesite-rhyolite etc...
(f) Sketch and discuss the structural development and evolution of accretionary prisms. Incorporate critical wedge theory in your answer. What is eclogite and how can it be present at the surface in some accretionary prisms.
(g) Discuss metamorphism in the general subduction zone-arc setting. Contrast the two basic settings and styles of metamorphism. What are paired metamorphic belts and what is their interpreted origin. Why have questions been asked recently about the simple interpretation of some paired metamorphic belts?
5 Contrasting Arcs and Ridges
(a) Contrast the gravity structure (profile) of arcs and ridges. Include the concept of "compensation" for topographic features.
(b) Contrast the melting process of arcs and ridges. Include cause of melting, depth of melting, evolution of melts, etc.
(c) What geochemical characteristics make arc melts distinctive from ridge melts? (Rewiew Wilson).
(6) Continental Crust and Collisional Orogenesis
(a) Draw a cross-section of continental crust. Include a "to-scale" geothermal gradient and pressure gradient. Draw (on the section) a strength vs. depth profile for continental crust. Discuss the controls on strength in the upper crust, middle crust, and lower crust. Define the middle crust strength beam (stress riser) and mantle strength beam (stress riser), and discuss the reasons for their existence and the relative strengths of each.
(b) Define nappes and summarizing their role in collisional orogenesis. Is there a different role or setting for thrust nappes and fold nappes? If possible, construct a sketch to illustrate your statements.
(c) Discuss the concept of delamination. Why, where, and when might it occur? Contrast the two main settings of styles for delamination processes (1. associated with collisional orogenesis, and 2. in "stable" or established crustal regions). What types of magmas and magmatism might we expect to be associated with delamination processes.
(d) Contrast deformation and metamorphism (particularly the history of each) in the foreland and the hinterland of collisional orogens.
(7) Continental Extension
(a) Discuss the interpreted difference between "active" and "passive" rifts. What characteristics would you expect to be associated with each. Could a single rift have both characteristics?
(b) From the literature, what is an aulocogen and why are they associated with rifted continental margins?
(c) Compare and contrast the three common views of the nature of the lower crusts in regions of continental extension? (i.e. how does the lower crust extend to balance the upper crust...)
(d) What is a metamorphic core complex? Draw a cross section of a metamorphic core complex; label the parts; and discuss the history.
(e) From metamorphic core complex theory, what are the two basic types of passive margins. What are the expected characteristics of each?
(9a) Earth History: Discuss the (three) alternative models for the timing of accumulation of continental crust through time. Which model do you favor and Why?
(9b) General: What, in your opinion, wil be the hot areas of tectonic research over the next decade?