S339 Understanding the ContinentsSection 1 - Continental Drift and Plate Tectonics Modern continents have drifted apart over geological time, as revealed by matching margins and similar geological features that occur on continents now separated by major oceans. How far some continents have drifted can be gauged from distinctive climatic, and therefore latitudinal, indicators. Rocks containing minerals that preserve remanent magnetization stemming from the Earth's magnetic field when they formed can be used as paleolatitude indicators. Changes of magnetic latitude through time confirm that all continents have drifted. Plotting paleolatitudes and directions of remanent magnetization for different periods in the geological evolution of continents produces apparent polar wander paths. Assuming that the Earth's magnetic poles do not wander far relative to its poles of rotation, such plots for pairs of continents give clues to periods when they were united or drifting independently. Alternating normal and reversed poloraities of the Earth's magnetic field have produced strip-like magnetic anomalies. Such patterns are roughly symmetrical across oceanic ridges. The anomalies reflect past reversals of the Earth's magnetic field, and can be dated both in continental volcanic rocks and those dredged from the ocean floor. The time-scale of oceanic magnetic reversals reveals an increasing age away from active ocean ridges. This is the key to sea-floor spreading that is inseparable from and provides a mechanism for continental drift. Moreover, varying widths of the age-stripes provide an accurate means of judging the rates at which plates move relative to one another. The parallelism of chains of Pacific islands and seamounts, formed by passage of the Pacific Plate over a series of hot spots, strongly suggests that the hot spots have remained stationary. If that applies to all hot spots, they form the points of reference for estimating true (absolute) plate movements. There are three principal plate boundary types: constructive or divergent plate boundaries, destructive or convergent plate boundaries, and conservative plate boundaries. Forces acting on plates involve gravitational effects that induce motion, and various resistive forces that oppose it. A balance between the two sets of opposed forces restricts plate motions to constant speeds. The downward pull exerted by subduction of cold dense oceanic lithosphere is very large compared to other plate forces. However, ridge-slide force is sufficiently large to drive plates that have no connection to subduction zones. Increased elevation due to the thermal effects of hot spots can reach levels where outward gravitational forces exceed resistive forces. That induces extentional stresses and increased likelihood of the plate failing. Constructive plate boundaries are charaterized by a magmatically active spreading axis, comprising several spreading centres offset by transform faults. Earthquakes due to extensional faulting characterize spreading centres, while those along active parts of transofrm faults show strike-slip motions. Ophiolites are sements of oceanic lithosphere, build up by igneous rocks sourced by partial melting of mangle materials, toegether with deep-water sediments that acculumated after mafic-ultramafic lithosphere formed. They are emplaced onto continental crust by tectonic activity. At destructive plate boundaries, oceanic lithosphere is subducted into the mantle. Continual subduction accounts for the absence of oceanic lithosphere older than 170 Ma. Subduction zones are characterized by deep ocean trenches and by zones of earthquake foci related to the position of a slab of subducting oceanic crust. The dipping seismic zones are called Wadat-Benioff zones. Closure of oceans, accretion of iland arcs formed in them and the eventual collision of continents that they once separated, result from the subduction of spreading axes, when addition of new matieral to plates halts. Such accretion of once widely separated terraces adds to the volume of continental lithosphere. Overriding plates scrape materials from the upper part of subducting lithosphere, to create accretionary prisms at their leading edges. Such prisms grow trenchwards with the accretion of progressively younger material. Tectonic processes beneath overriding plates may stuff part of the subducted slab beneath evolving arcs, thereby imparting increased surface elevation of the arc. Such tectonic underplating, together with magmatic additions, can therefore impart extensional stresses to the developing arc, which can result in failure of the crust. Extensional forces above subduction zones manifest themselves by inducing back-arc spreading. Where oceanic lithosphere occurs in the overriding plate, this involves creation of new lithosphere through magmatism at a fixed depth in relation to the subduction zone. Overriding continental lithosphere undergoes extension, exemplified by the Basin and Range Province of western North America. Conservative plate margins involve plates moving horizontally relative to one another, requiring faults with dominant strike-slip motions. Where island arcs or continental fragments become associated with such margins, they may eventually meet the margins of larger continental plates, thereby becoming components of laterally accreted complexes of terranes. Terranes are fragments of far-travelled lithosphere, separated from others by major faults and displaying strongly contrasted tectonic, stratigraphical and biological histories. Mantle discontinuities occur at depths of 410 and 670 km, thought to be due to abrupt changes in density that result from mineralogical phase changes. The coincidence of the 670 km discontinuity with the deepest earthquakes associated with subduction could be due either to completely ductile behavious of descending slabs below that depth or to their ponding and complete resorption at or above that level. Two possibilities for deep mantle motion are (i) that it extends as a single convective system fo the core-mantle boundary, and (ii) that it has two components, one above 670km and another acting independently in the deep mantle. Seismic tomography produces 'images' of the mantle that display increased or decreased body-wave speeds relative to those generally expected at different depths. Cool and dense mantle transmits seismic waves at relatively high speeds, whereas warm, lower-density mantl transmits them at slower speeds. The mantl beneath hot spots can have various tomographic signatures. The hot spot beneath Iceland has warm low-density mantl extending no deeper than the 670km discontinuity. The hot spot associated with the Red Sea, on the other hand, has low speed anomalies extending as deep as the core-mantle boundary. So far, there is little evidence for the notion of narrow plumes beneath hot spots, but their origin beneath the lithospehre is confirmed. Destructive plate margins are characterized by narrow, high-speed seismic anomalies that extend to the core-mantl boundary, and are best interpreted as descending slabs of cool, dense oceanic lithosphere from active or extinct subduction zones. While complex and not yet fully resolved by seismic tomography, convective motion within the Earth seems to be a mantle-wide phenomenon. Section 2 - The engine's power source Relatively little heat is generated within oceanic lithosphere. Oceanic lithosphere is fairly uniform in composition. Half-space and plate models can be used to predict the relationship of oceanic heat flow and depth with age. The Global Depth and Heat flow plate model fits the data best for lithosphere older than 65Ma, probably because sealing at this age allows the lithosphere to lose heat mainly through conduction (which the model assumes). For ages younger than 65Ma, all models overestimate heat flow in oceanic lithosphere, suggesting that heat flow in these regions is a result of convection as well as conduction. There is some relationship between surface heat flow and age in continental regions, with greater heat flow in younger lithosphere. Continental heat flow data indicate that there are regions with similar characteristics. These are called heat flow provinces. About 60% of the observed continental heat flow is from the lower crust and mantle. This is termed reduced heat flor and it is constant within a heat flow province. The remaining 40% of the observed continental surface heat flow is generated by radiogenic isotopes within the upper crust. Over the period 0 to about 1500 Ma, 'heat of formation' heat flow decreases with age since last metamorphic or magmatic heating event. Background heat flow from the lower crust and mantle is constant for each heat flow province and for older ages (Paleozoic and Precambrian) is everywhere about 25mW mM-2. Old oceanic lithosphere and old continental lithosphere have about the same average surface heat flow. Heat flow is not uniformly distributed through the lithosphere. Radiogenic heat production falls off (probably exponentially) with depth in continental crust and it is much lower in oceanic lithosphere. Section 3 - Strengths and weaknesses in the lithosphere. In this Section, you have seen how the thickness of the crust and upper mantle and the lithosphere as a whole can vary with the temperature profile. The strength of the lithosphere depends on its thickness and temperature. The behaviour of the lithosphere when subjected to stress depends on the type of stress (extensional or compressional) and on the strain rate. Stresses acting on the lithosphere are not necessarily constant and they are not necessarily simple, in that they comprise components acting in different directions at different rates. It is the sum of these stresses and how they oppose one another within the lithosphere that are important. The main points summarized:
Section 4 - Plate tectonics revisited. Subducting plates do not simply fall down a 'slot' into the mantle. The subduction hinge rolls back oceanward away from the overriding plate. The subducting slab sinks more steeply than it dips. The mantle wedge above a subducting slab must be replenished with fresh mantle material in order to feed arc magmatism. Global tomography using nodes rather than blocks has much higher resolution than other tomographic methods. Using this method, a low-speed region can be resolved running from the core-mantle boundary to the Iceland hot spot. Mantle plumes may be deflected by mantle flow and so may the overlying hot spot, but at a very low rate compared with plate-tectonic speeds. Big plumes do not necessarily underlie highly productive hot spots. Hot-spot productivity depends on the plume feeder and the amount of entrained eclogite. Costa Rica is located on continental lithosphere which has evolved through plate tectonic processes from an immature island arc to its present setting. Back to S339 |