Showing papers in "Annual Review of Earth and Planetary Sciences in 1989"

Strontium Isotopes in Seawater through Time

Abstract: where pis the p-particle, v is an antineutrino, and Q represents the decay energy (0.275 MeV). The recommended decay constant for 87Rb is 1.42 x 10-11 yr-I (Steiger & Jager 1977), and its half-life T is therefore 48.8 Gyr. On geological time scales, the isotopes 84, 86, and 88 are stable, in contrast to 87Sr, and their abundance ratios are therefore invariant. The present-day quantity of isotope 87 (87Srp) depends on its initial stock (87Sr 0) and the amount of radiogenic Sr generated from decay of 87Rb over time t: 87Srp = 87Sro+87Rb (eAt_I).

Geochemistry and Dynamics of the Yellowstone National Park Hydrothermal System

Abstract: The magmatic-hydrothermal system at Yellowstone National Park is unique among presently active systems in regard to its unparalleled geysers, tectonic environment, and magnitude of caldera-forming volcanic activity. However, over geologic time, systems like Yellowstone have been common and appear to have been responsible for the formation of many important basc-metal and precious-mctal ore deposits. For these reasons, Yellow­ stone has received a great amount of scientific attention. Geologic inves­ tigations in Yellowstone began with the Hague expedition in the late 1 800s, and the results of the first chcmical analyses of thermal waters from the Park's many hot springs and geysers were published in 1 888 by Gooch & Whitfield ( 1 888). Since then, numerous investigators have studied the hydrothermal system and its geologic environmcnt, and data collectcd over a span of a hundred years are available for comparison and interpretation.

Pressure solution during diagenesis

Abstract: Pressure solution is a durable problem in geology. It has engendered continuing arguments on its mechanism, thermodynamic basis, and geo­ logical significance. Recently, it has drawn renewed attention both for its important role in the diagenesis of sedimentary rocks and for its relation to rock deformation. Pressure solution is defined as a process by which grains dissolve at intergranular or intercrystalline contacts. This process is presumably related to the higher solubility under nonhydrostatic stress at the contacts than at free grain surfaces. The process often, although not always, is accompanied by reprecipitation at adjacent free grain surfaces. Nevertheless, it is not justifiable to treat pressure solution as a recrys­ tallization process. Two common types of pressure solution are recognized: intergranular pressure solution and stylolitization. The former, occurring at individual grain contacts, was first described by Sorby (1863) based on experiments and theoretical predictions by Thompson ( l862a,b). The latter produces "a surface or contact that is marked by an irregular and interlocking penetration of the two sidcs, and is supposedly formcd diagenctically by differential vertical movement under pressure, accompanied by solution" (Bates & Jackson 1980). Here we use the term stylolite for a surface that involves an aggregate of grains (the "aggregate stylolite" of Park & Schot 1968), originating exclusively by dissolution under nonhydrostatic stress.

The Nature of Deep-Focus Earthquakes

Abstract: Wadati (1928) first proved convincingly that some earthquakes occur at depths well beneath the Earth's crust. He essentially proposed the nomenclature used today by the International Seismological Centre (ISC), i.e. earthquakes with focal depths exceeding 300 km are deep earthquakes, and those with depths between 70 and 300 km are intermediate earth­ quakes. However, when there is little reason to distinguish between the two groups, both types are called simply deep earthquakes. To avoid confusion, in this paper we refer to those with focal depths exceeding 300 km as very deep earthquakes, and those with focal depths between 70 and 300 km as intermediate earthquakes. Deep earthquakes are of interest for at least four reasons. First, they are exceedingly common. Between 1964 and 1986 they constituted no less than 22% of all earthquakes having mb greater than 5 . 0 in the ISC catalog. Second, they most often occur in association with deep ocean trenches and volcanic island arcs in subduction zones. One of the great achievements of twentieth century geophysics was the recognition that their occurrence in inclined planar groups, or Wadati-Benioff zones, apparently delineates the cold down going cores of convection cells in the uppermost mantle. Third, seismologists use body waves of deep earthquakes dispro­ portionately more often than those of shallow earthquakes to inves­ tigate core, mantle, and crustal structure. This is because (a) deep earth­ quakes often possess relatively more impulsive sources, (b) their body phases traverse the heterogeneous uppermost mantle only once on the ray path from hypocenter to station, and (c) surface waves do not contaminate these body phase arrivals on seismograms. Finally, deep earthquakes are

Magnetofossils, the Magnetization of Sediments, and the Evolution of Magnetite Biomineralization

Abstract: Magnetite (Fe_3O_4) is one of the most stable carriers of natural remanent magnetization (NRM) in sedimentary rocks, and paleomagnetic studies of magnetite-bearing sediments, such as deep-sea cores and pelagic limestones, have provided a detailed calibration between the biostratigraphic and magnetic polarity time scales. Despite this important role, there is as yet a very poor understanding of how ultrafine-grained (< 0.1 µm) magnetite is formed, transported, and preserved in marine environments. A major conceptual advance in our understanding of these processes is the recent discovery that biogenic magnetite, formed by magnetotactic bacteria and/or other magnetite-precipitating organisms, is responsible for much of the stable magnetic remanence in many marine sediments and sedimentary rocks. Since these magnetite particles are of biogenic origin, they are termed properly magnetofossils (Kirschvink & Chang 1984).

Active deformation of the continents

Abstract: Whereas relatively narrow bands of deformation define oceanic plate boundaries, in continental situations deformation is shown to be distributed over much broader zones. The complex and heterogeneous nature of boundary regions between major plates is outlined with reference to the Mediterranean and the Himalayas. Relatively aseismic regions have traditionally been defined as microplates but although they are presently acting as rigid blocks undergoing slow deformation, it is argued that this need not always have been the case, and that deformation may simply have moved elsewhere. Broad zones of deformation surrounding rigid blocks are highlighted as fundamental to the analysis and interpretation of continental tectonics. Detailed theoretical consideration is given to the kinematics of distributed formation and the dynamics of continental deformation. -A.J.Barker

Mechanics of Faulting

Abstract: Brittle tectonics may be considered on two timescales, in which earthquakes are the short-timescale phenomena and faulting is the long timescale process. Faults grow and develop by the cumulative action of earthquakes, and the faults therefore contain the history of past seismicity. In this chapter we discuss the mechanics of faults, which are treated as quasi-static shear cracks with friction. We begin with a discussion of the elementary theory of faulting, followed by a more modern treatment of the formation and growth of faults and a description of the rocks and structures formed by faulting. Here we rely more heavily on geological observations than elsewhere. We summarize with a discussion of the strength and rheology of faults, finishing with the topic of heterogeneity and its role in faulting, which continues a subtheme to be found throughout this book.

Metamorphic fluids in the deep crust

Abstract: Most of the inferences about fluid action in the deep crust are drawn from field, petrographic, and geochemical studies in the granulite facies terrains. Deductions on the nature of fluids are made from major and minor element distributions and isotopic patterns in metamorphic rocks, calculations of fluid-mineral equilibria based on observed assemblages, fluid inclusions, and field evidence of fluid pathways. Discussion of these features has focused attention on several major problems concerning the timing, flow mechanisms, and origin of the fluids. A key issue concerns episodicity versus secular activity of fluids. There are strong arguments that most fluid action in the deep crust is linked to thermal/deformational events, including magmatism and regional meta­ morphism. Discrete radiometric ages of high-grade terrains often define relatively short periods of crystallization or recrystallization. The late Archean granulite facies metamorphism of southern India is an example, where U-Pb systems of zircons (Buhl et al 1983), whole-rock U-Pb data (Peucat et al 1 987), and Sm-Nd systems of whole-rocks and minerals (Bernard-Griffiths et al 1 987) all show a major high-temperature recrys­ tallization event, inferred by several workers to have involved pervasive fluid action, centered closely around 2.5 Ga. Another geochemical indi­ cation of episodic fluids comes from stable isotope studies of progressive metamorphic sequences, as in the Damara orogen, Namibia, where whole­ rock b 180 increases slightly at isograds representing arrested dehydration events but is monotonous between isograds (Hoernes & Hoffer 1 985).

Raman Spectroscopy in Mineralogy and Geochemistry

Abstract: An introduction to the theory and principles of Raman spectroscopy is given. The technique provides a useful and highly versatile probe of solids, liquids and gases. Important applications in mineralogy and geochemistry are described. These include studies of silicate glass and melt structure volatile dissolution mechanisms in aluminosilicate melts, phase transitions in solids, speciation in aqueous fluids and micro-Raman analysis of fluid inclusions. In Earth Sciences, Raman scattering is most often used as a vibrational spectroscopy, but the potential of other related laser spectroscopies are also outlined. -A.J.Barker

Alpine-Himalayan Blueschists

Abstract: Blueschists gained a special tectonic significance after the advent of plate tectonics as marking the location of former subduction zones. Eskola (1939) proposed a high-pressure/low-temperature origin for the blueschists based on the greater density and hydrous nature of their minerals compared with the minerals of the neighboring greenschist and amphibolite facies. During the 1940s and 1950s a high-pressure/low temperature (HP/LT) origin for the blueschists was upheld in Europe (e.g. Brouwer Egeler 1952, de Roever 1955), whereas a metasomatic origin was generally favored among geologists studying the North American Franciscan complex (e.g. Taliaferro 1943, Turner 1948, Brothers 1954). During the 1960s an HP/LT metamorphic origin for the blueschists became the predominant view (e.g. Coleman & Lee, 1963, Ernst 1965), although a satisfactory tectonic mechanism for creating the abnormal conditions required for blueschist generation was still missing. This mechanism was provided eventually by plate tectonics; Hamilton (1969) ascribed the origin of the Franciscan blueschists to processes in subduction zones, where the cold oceanic lithosphere is carried down into the hot asthenosphere. The very low heat conductivity of rocks is responsible for the generation of HP/LT conditions in the downgoing slab. The southern part of the Cordilleran orogen in North America, where the Franciscan complex is located, is a noncollisional orogen, and the Franciscan blueschists represent rocks carried down to the subduction zone and subsequently uplifted and accreted into the active continental margin. Thus, the protoliths of the blueschists are either basic volcanic rocks and pelagic sediments representing the upper levels of the downgoing oceanic crust or voluminous graywackes carried down into the trench by turbidity currents. By contrast, the Alpine-Himalayan orogenic system is

Faunal Dynamics of Pleistocene Mammals

Abstract: Tee Age mammals, the dramatis personae of this review, can be divided into three groups-megafauna, microfauna, and humans-each of which plays a fundamentally different role. Surely the most conspicuous actors were megafauna, wide-ranging herd animals such as mammoths and horses; yet they were in a sense "fall guys," taking the brunt of the late Cenozoic extinctions not only in North America but also generally at high latitudes. The microfauna were not, as once supposed, "bit players," for in the course of the Pleistocene they, like the meek, inherited the Earth. It was an obscure group of late-blooming primates, Homo sapiens, that stole the show as they came out of Africa. They created an ecological drama that still awaits the final curtain. In the following pages we consider why this drama has played for the past several million years. The answer involves the three components of faunal dynamics: evolution, immigration, and extinction. Central to the discussion are the questions of whether or not Pleistocene extinctions

Achondrites and Igneous Processes on Asteroids

Abstract: The possible roles of partial melting and fractional crystallization in the formation of eucrite and ureilite achondrites are discussed, summarizing the results of recent petrological investigations and theoretical modeling efforts. Typical data are presented graphically, and it is found that there is as yet no consensus on the correct model of achondrite evolution, and hence no agreement on the chemistry of the parent bodies. Assuming that the ureilites formed as adcumulates, their properties support a role for fractionation in relatively small bodies, even though the magmatic composition of the ureilite parent bodies is different from that of terrestrial cumulates.

The Crustal Structure of Western Europe

Abstract: A thorough understanding of the tectonic processes that formed the con­ tinental lithosphere under changing geometric and physical conditions through time requires detailed knowledge of the structure of the crust. One of the best locations to study this fundamental problem is the northern and western part of Europe, because it is made up of a number of tectonic provinces ranging in succession from the oldest Precambrian areas of Fennoscandia to the currently active regions of the Mediterranean Sea. The geology of Europe, a region extending from the northern tip of Scandinavia (North Cape) southward to North Africa, encompasses the pre-2500 Ma Archean craton in the northernmost part of the Baltic (or Fennoscandian) shield; the Proterozoic, Paleozoic, and Cenozoic pro­ vinces of northern and central Europe that have been added on to this nucleus; and the active transition zone between the Eurasian and African plates in the Mediterranean area (cf. Figure 1). From a tectonic-geologic point of view, Europe can be divided into four main units that cover the whole time span of the history of the continental crust:

A Student's Garden of Anisotropy

Abstract: Anisotropy is seismology's most venerable skeleton in the closet. Of all elastic propagation problems, none eludes our intuition more than ani­ sotropy. Even the formidable problem of propagation in a three-dimen­ sional heterogeneous medium seems less intimidating because we have an intuitive sense (and some computational tools) for the behavior of classical rays and even of diffractions in isotropic media. This intuition is precious to us: It is a product of centuries of study and use of wave propagation and is for seismologists the most valuable heritage from mathematical physics. Many significant advances in seismology, especially the inference of gross Earth structure from travel-time analysis, depended on geometric ray theory in isotropic media. No comparable progress has been made in the study of anisotropy in the Earth except where simple travel-time differences could be predicted. The simplest of these observations is the azimuthal dependence of the apparent velocities of Pn refractions in the upper mantle under oceans (e.g. Raitt et al 1969, Bibee & Shor 1976) and under continents (e.g. Vetter & Minster 1981). The orientation of the Pn anisotropy in the oceanic upper mantle clearly indicates a tectonic origin. Using multicomponent seismic data, we can observe shear-wave birefrin­ gence in body-wave arrivals (Crampin 1984, 1985) and surface-wave propagation (Nataf et al 1984, Tanimoto & Anderson 1985, Lerner-Lam 1985), all of which are travel-time (phase) differences. Recent work in marine seismology gives convincing evidence for anisotropy in the basaltic upper oceanic crust and places some constraint on its variability with depth. Stephen (1985) used the time dependence of the polarization of shear-wave arrivals at a borehole receiver [Deep-Sea Drilling Project (DSDP) site 504B] in 6-Myr-old crust to show that oriented cracking in