Introduction

The localization of strain into narrow zones during extension is achieved by processes that lead to a mechanical weakening of the lithosphere. Lithospheric weakening may be accomplished by the elevation of geotherms during lithospheric stretching, heating by intrusions, interactions between the lithosphere and the astheno-sphere, and or by various mechanisms that control the behavior of faults and shear zones during deformation. Working against these strain softening mechanisms are processes...

Ductile deformation

The mechanisms of ductile flow in crystalline solids have been deduced from studies of metals, which have the advantage that they flow easily at low temperatures and pressures. In general, where the temperature of a material is less than about half its melting temperature (Tm in Kelvin), materials react to low stresses by flowing slowly, or creeping, in the solid state. At high temperatures and pressures, the strength and flow of silicate minerals that characterize the crust (Tullis, 2002) and...

Ophiolites

The study of oceanic lithosphere has been aided by investigations of characteristic rock sequences on land known as ophiolites (literally snake rock, referring to the similarity of the color and texture to snakeskin see Nicolas, 1989, for a full treatment of this topic). Ophiolites usually occur in collisional orogens (Section 10.4), and their association of deep-sea sediments, basalts, gabbros, and ultramafic rocks suggests that they originated as oceanic lithosphere and were subsequently...

Volcanic margins

Rifted volcanic margins are defined by the occurrence of the following three components Large Igneous Provinces (Section 7.4.1) composed of thick flood basalts and silicic volcanic sequences, high velocity (Vp > 7 km s-1) lower crust in the continent-ocean transition zone, and thick sequences of volcanic and sedimentary strata that give rise to seaward-dipping reflectors on seismic reflection profiles (Mutter et al., 1982). The majority of rifted continental margins appear to be volcanic,...

Earlier supercontinents

The origin of the first supercontinent and when it may have formed are highly speculative. Bleeker (2003) observed that there are about 35 Archean cratons today (Plate 11.1a between pp. 244 and 245) and that most display rifted margins, indicating that they fragmented from larger landmasses. Several possible scenarios have been envisioned for the global distribution of the cratons during the transition from Late Archean to Early Proterozoic times (Fig. 11.21). These possibilities include a...

Seismic tomography

Much important information on the three-dimensional structure of the mantle has been supplied by seismic tomography (Section 2.1.8). Convection is driven by lateral differences in temperature and density. These variables affect seismic velocity, which typically decreases with decreasing density and increasing temperature (Dziewonski & Anderson, 1984). By mapping velocities in the mantle it is possible to infer the differences in temperature and density that are a consequence of convection....

Paleocumaidlogy

The distribution of climatic regions on the Earth is controlled by a complex interaction of many phenomena, including solar flux (i.e. latitude), wind directions, ocean currents, elevation, and topographic barriers (Sections 13.1.2, 13.1.3). The majority of these phenomena are only poorly known in the geologic record. On a broad scale, however, latitude is the major controlling factor of climate and, ignoring small micro-climatic regions dependent on rare combinations of other phenomena, it...

Straininduced weakening

Although differences in the effective elastic thickness and flexural strength of the lithosphere (Section 7.6.4) may explain variations in the length of border faults and the width of rift basins, they have been much less successful at explaining another major source of variability in rifts the degree of strain localization in faults and shear zones. In some settings normal faulting is widely distributed across large areas where many faults accommodate a relatively small percentage of the total...

The reconstruction of continents around the Atlantic

The first mathematical reassembly of continents based solely on geometric criteria was performed by Bullard et al. (1965), who fitted together the continents on either side of the Atlantic (Fig. 3.2). This was accomplished by sequentially fitting pairs of continents after determining their best fitting poles of rotation by the procedure described in Section 3.2.2. The only rotation involving parts of the same landmass is that of the Iberian peninsular with respect to the rest of Europe. This is...

The reconstruction of Gondwana

Geometric evidence alone has also been used in the reconstruction of the southern continents that make up Figure 3.3 Reconstruction of the Central American region within the Bullard et al. fit of the continents around the Atlantic (Fig. 3.2). C, location of pre-Mesozoic portions of Cuba (redrawn from White, 1980, with permission from Nature 283,823-6. Copyright 1980 Macmillan Publishers Ltd). Figure 3.3 Reconstruction of the Central American region within the Bullard et al. fit of the...

Seismicity plate motions and subduction geometry

The general pattern of seismicity in the Andes is in accord with the eastward subduction of the Nazca plate beneath South America (Molnar & Chen, 1982). Geodetic data suggest that convergence velocities with respect to South America are 66-74 mm a-1 at the trench (Norabuena et al., 1998 Angermann et al., 1999 Sella et al., 2002). These rates are slower than the 77-80 mm a-1 predicted by the NUVEL-1A model of plate motions (Section 5.8) and appear to reflect a deceleration from a peak of some...

Oceanic layer

Layer 2 is variable in its thickness, in the range 1.02.5 km. Its seismic velocity is similarly variable in the range 3.4-6.2 km s-1. This range is attributable to either consolidated sediments or extrusive igneous material. Direct sampling and dredging of the sediment-free crests of ocean ridges, and the necessity of a highly magnetic lithology at this level (Section 4.2), overwhelmingly prove an igneous origin. The basalts recovered are olivine tholeiites containing calcic plagioclase, and...

Feasibility of mantle convection

In order to gain insight into the feasibility and nature of convection within a spherical, rotating Earth, it is convenient to assume that the mantle approximates a Newtonian fluid. Although this assumption may be erroneous, it does allow simple calculations to be made on the convective process. The condition for the commencement of thermal convection is controlled by the magnitude of the dimen-sionless Rayleigh number (R), which is defined as the ratio of the driving buoyancy forces to the...

Pre Mesozoic reconstructions

Paleogeographic maps for the Mesozoic and Cenozoic can be computed by the fitting together of continental margins or oceanic lineations of the same age on either side of an ocean ridge (Chapters 3, 4). The location of the paleopoles can be determined from paleomagnetic measurements (Section 3.6) and so the only unknown in these reconstructions is the zero meridian of longitude. These combined techniques cannot be used for reconstructions prior to the Mesozoic because in situ oceanic crust is...

B

Fig. 6.3 (a) Axial relief and (b) seismic crustal thickness as a function of full spreading rate at mid-ocean ridge crests. A ridge classification scheme is shown by the heavy black straight lines which indicate the spreading rate ranges for ultraslow, slow, fast and two intermediate classes (modified from Dick et al., 2003, with permission from Nature 426, 405-12. Copyright 2003 Macmillan Publishers Ltd). 20 40 60 80 100 120 Full spreading rate (mm yr-1) Fig. 6.3 (a) Axial relief and (b)...

Terrestrial Heat Flow

The study of thermal processes within the Earth is somewhat speculative because the interpretation of the distribution of heat sources and the mechanisms of heat transfer are based on measurements made at or near the surface. Such a study is important, however, as the process of heat escape from the Earth's interior is the direct or indirect cause of most tectonic and igneous activity. The vast majority of the heat affecting the Earth's surface comes from the Sun, which accounts for some 99.98...

Ridge jumps and transform fault offsets

The different offsets observed across the Murray Fracture Zone from magnetic lineations (Fig. 4.1) are thought to be due to a change in location of the ridge crest to the south of the fracture zone about 40 Ma ago. The change in offset of anomalies of the same age implies a ridge jump of approximately 500 km to the east (Harrison & Sclater, 1972). Similar but better documented ridge jumps, which also greatly reduce the offset of crust of the same age on either side of a fracture zone, occur...

True Polar Wander

In Section 3.6 it was demonstrated that paleomagnetic techniques can be used to construct apparent polar wandering paths which track the motions of plates with respect to the magnetic north pole and hence, using an axial geocentric dipole model, the spin axis of the Earth. In Section 5.5 it was suggested that hotspots are nearly Figure 5.11 Predicted hotspot track assuming that the Iceland hotspot is fixed relative to the other Indo-Atlantic hotspots of Fig. 5.9. Position of hotspot at 10 Ma...

Deposits of sedimentary basins

The majority of fossil fuels are found within sedimentary basins whose formation can be related directly or indirectly to plate motions. In addition to the sedimentary environment, quite stringent conditions are necessary for the development and preservation of these resources. There are four principal criteria which must be met for the development of petroleum and gas, hereinafter referred to as hydrocarbons layers rich in organic matter within the sedimentary succession a source of heat...

Apparent polar wander curves

Paleomagnetic data can be displayed in two ways. One way is to image what is believed to be the true situation, that is, plot the continent in a succession of positions according to the ages of the sampling sites (Fig. 3.13a). This form of display requires the assumption of the paleolongitudes of the sites. The other way is to regard the continent as remaining at a fixed position and plot the apparent positions of the poles for various times to provide an apparent polar wander (APW) path (Fig....

Gravity Anomalies Of Subduction Zones

Figure 9.4 shows a free air gravity anomaly profile across the Aleutian arc that is typical of most subduction zones. The flexural bulge of the downgoing lithosphere to seaward of the trench is marked by a positive gravity anomaly of about 500 g.u. (Talwani & Watts, 1974). The trench and accretionary prism are typified by a large negative anomaly of some 2000 g.u. amplitude which results from the displacement of crustal materials by sea water and low density sediments. Conversely, the island...

The Core

The core, a spheroid with a mean radius of 3480 km, occurs at a depth of 2891 km and occupies the center of the Earth. The core-mantle boundary (Gutenberg discontinuity) generates strong seismic reflections and thus probably represents a compositional interface. The outer core, at a depth of2891-5150 km, does not transmit S waves and so must be fluid. This is confirmed by the generation of the geomagnetic field in this region by dynamic processes and by the long period variations observed in...

Arccontinent Collision

Tectonic Collision Timor

Orogenic belts that result from the collision between an island arc and a continent typically are smaller than those that form by continent-continent collision (Dewey & Bird, 1970). Arc-continent collision also tends to be relatively short-lived because it usually represents an intermediate step during the closure of a contracting ocean basin. Active examples of this type of orogen occur in Taiwan (Huang C.-Y. et al., 2000, 2006), Papua New Guinea (Wallace et al., 2004), and the Timor-Banda...

Margins

Ghana Margin Basins

Where a transform fault develops during continental rifting the continental margin is defined by the transform fault and is termed a transform continental margin. The history of such a margin, first considered by Scrutton (1979), reflects its initial contact with its continental counterpart on the adjacent plate and subsequent contact with oceanic lithosphere and an ocean ridge as the separation proceeds. These margins differ from rifted or passive margins (Section 7.7) by a narrow (< 30 km)...

Info

Fig. 6.18 Model for the construction of oceanic crust at a slow-spreading ridge. Transient magma bodies rise to the brittle-ductile transition within the crust and shoulder aside and depress older plutons. Part of the magma body erupts through a fissure to produce a volcano or hummocky lava flow on the sea floor and the remainder solidifies to form part of the main crustal layer (redrawn from Smith & Cann, 1993, with permission from Nature 365,707-15. Copyright 1993 Macmillan Publishers...

The convection process

The nature of convective flow in the mantle is problematic. Analytical solution is difficult because of the complex rheological structure, including the presence of a transition zone (Section 2.8.5), the presence of heat sources within the convecting layer as well as beneath it, the influence of an overlying rigid lithosphere on the pattern of convection, and the fact that the convecting layer has the form of a spherical shell. However, as a result of advances in numerical simulations and...

Paleogeographic reconstructions based on paleomagnetism

Reconstructions of the relative positions of the main continental areas at various times in the past 200 Ma are best achieved using the very detailed information on the evolution of the present ocean basins provided by the linear oceanic magnetic anomalies Figure 3.14 Apparent polar wander paths for North America (solid circles and solid line) and Europe (open circles and dashed line) (a) with North America and Europe in their present positions, and (b) after closing the Atlantic ocean. Ages...

Velocity Structure Of The Earth

Knowledge of the internal layering of the Earth has been largely derived using the techniques of earthquake seismology. The shallower layers have been studied using local arrays of recorders, while the deeper layers have been investigated using global networks to detect seismic signals that have traversed the interior of the Earth. The continental crust was discovered by Andrija MohoroviCiC from studies of the seismic waves generated by the Croatia earthquake of 1909 (Fig. 2.14). Within a range...

Structure of accretionary orogens

Accretionary Orogens

One of the most fully investigated belts of accreted terranes is the Cordillera of western North America (Fig. 10.32). The distribution of terranes in this region forms a zone some 500 km wide that makes up about 30 of the continent (Coney et al., 1980). Most of the terranes in the Cordillera accreted onto the margin of ancestral North America during Mesozoic times (Coney, 1989). Some also experienced lateral translations along strike-slip faults. This latter process of dispersal, where...

Ocean Trenches

Oceanic trenches are the direct manifestation of under-thrusting oceanic lithosphere, and are developed on the oceanward side of both the island arcs and Andean-type orogens that form above subduction zones (Fig. 9.1). They represent the largest linear depressed features of the Earth's surface, and are remarkable for their depth and continuity. The Peru-Chile Trench is 4500 km long and reaches depths of 2-4 km below the surrounding ocean floor so that its base is 7-8 km below sea level. The...

Present Day Triple Junctions

Only six types of triple junction are present during the current phase of plate tectonics. These are RRR (e.g. the junction of East Pacific Rise and Galapagos Rift Zone), TTT (central Japan), TTF (junction of PeruChile Trench and West Chile Rise), FFR (possibly at the junction of Owen Fracture Zone and Carlsberg ridge), FFT (junction of San Andreas Fault and Mendocino Fracture Zone), and RTF (mouth of Gulf of California). The evolution of the San Andreas Fault illustrates the importance of the...

Accretionary Prisms

Where present, an accretionary prism forms on the inner wall of an ocean trench. The internal structure and construction of these features have been deduced from seismic reflection profiles and drilling at active subduction zones, and by the study of ancient subduction complexes now exposed on land. Accretionary prisms develop where trench-fill turbidites (flysch), and some pelagic sediments, are scraped off the descending oceanic plate by the leading edge of the overriding plate, to which they...

The D layer

It has long been recognized that the greatest contrasts in physical properties and chemical composition within the Earth occur at the core-mantle boundary and that this is almost certainly the location of a thermo-chemi-cal boundary layer (Section 2.8.6). Initially, seismologists were unable to detect any layering in the lower mantle and referred to it as Layer D (Bullen, 1949). Subsequently it was realized that a layer at the base of the mantle, perhaps 2-300 km thick, has distinctive, if...

Proterozoic plate tectonics

Early tectonic models of Proterozoic lithosphere envisaged that the Archean cratons were subdivided by mobile belts in which deformation was wholly ensialic, with no rock associations that could be equated with ancient ocean basins. These interpretations where Pro-terozoic orogenies occurred far from continental margins have since fallen out of favor. Most studies now indicate that Proterozoic orogens (Fig. 11.12) evolved along the margins of lithospheric plates by processes that were similar...

M

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Profile Figure 10.8 (a) Trench topography for 15 profiles of the Andes between 3 N and 56 S showing dip of the Benioffzone at 1 1 scale (left). Bathymetric profiles at right show a vertical exaggeration of 10. (b) Viscous model of trench topography showing mesh grid and boundary conditions (B.C.). Dynamic equilibrium of the trench is controlled by competing buoyancy (top-right) and tectonic (lower-right) forces. Velocity fields represented with arrows are for...

Mantle drag mechanism

Mantle drag was the first driving mechanism to be proposed, and envisages plate motion in response to the viscous drag exerted on the base of the lithosphere by the lateral motion of the top of mantle convection cells in the asthenosphere (Fig. 12.9a). The convection cells would consequently rise beneath oceanic ridges and descend beneath trenches, being largely absent beneath continental regions. This mechanism predicts that the oceanic lithosphere would be in a state of tension at the ocean...

Distribution Of Earthquakes

Volcanic Mineral Distribution Worldwide

Plate tectonic theory predicts that the majority of the Earth's tectonic activity takes place at the margins of plates. It follows, then, that the location of earthquake epicenters can be used to define plate boundaries. Figure 5.2 shows the global distribution of the epicenters of large magnitude earthquakes for the period 1961-67 (Barazangi & Dorman, 1969). Although in terms of most geologic processes this represents only a very short period of observation, the relatively rapid motions...

Basin inversion

Many sedimentary basins record a reversal in the sense of motion on dip-slip faults at different stages in their evolution. This reversal is known as inversion. At present there is no universal definition of the process. However, the most common type refers to the compressional reactivation of pre-existing normal faults in sedimentary basins and passive margins that originally formed by extension or transtension (Turner & Williams, 2004). Fault reactivation changes the architecture of the...

Ocean Ridge Topography

Ocean ridges mark accretive, or constructive plate margins where new oceanic lithosphere is created. They represent the longest, linear uplifted features of the Earth's surface, and can be traced by a belt of shallow focus earthquakes that follows the crestal regions and transform faults between offset ridge crests (Fig. 5.2). The total length of the spreading margins on mid-ocean ridges is approximately 55,000 km. The total length of the active ridge-ridge transform faults is in excess of...

Volcanic And Plutonic Activity

Where subducting oceanic lithosphere reaches a depth of 65-130 km, volcanic and plutonic activity occurs, giving rise to an island arc or an Andean-type continental arc approximately 150-200 km from the trench axis (England et al., 2004). The thickness of arc crust reflects both the age of the system and the type of crust on which the arc forms. Relatively young island arcs, such as the 3-4 Ma active part of the Mariana volcanic arc, may be underlain by a crust of 20 km thickness or less. Thin...

The formation of Archean lithosphere

Komatiite Mantle

The distinctive composition and physical properties of the stiff, buoyant mantle roots beneath the cratons (Section 11.3.1) result from the chemical depletion and extraction of melts from the primitive mantle. These two processes lowered the density and increased the viscosity of the residue left over from partial melting and resulted in a keel that consists mostly of high-Mg olivine and high-Mg orthopyroxene (O'Reilly et al., 2001 Arndt et al., 2002). Both of these components are absent in...

General geology of the Himalaya and Tibetan Plateau

Main Boundary Thrust Himalaya

The Himalaya are composed of three large, imbricated thrust slices and related folds separated by four major fault systems (Figs 10.19, 10.20). These imbricated thrusts, which occupy a section about 250-350 km wide, appear to accommodate approximately one-third to one-half of the 2000 or more kilometers of post-col-lisional shortening between India and Eurasia (Besse & Courtillot, 1988 DeCelles et al., 1998). At the base of the stack the mostly buried Main Frontal Thrust lies along the...

Middle and lower continental crust

For a 40 km thick average global continental crust (Christensen & Mooney, 1995 Mooney et al., 1998), the middle crust is some 11 km thick and ranges in depth from 12 km, at the top, to 23 km at the bottom (Rudnick & Fountain, 1995 Gao et al., 1998). The average lower crust thus begins at 23 km depth and is 17 km thick. However, the depth and thickness of both middle and lower crust vary considerably from setting to setting. In tectonically active rifts and rifted margins, the middle and...

Buoyancy forces and lower crustal flow

In addition to crustal thinning and the compression of geotherms (Section 7.6.2), lithospheric stretching results in two types of buoyancy forces that influence strain localization during rifting. First, lateral variations in temperature, and therefore density, between areas inside and outside the rift create a thermal buoyancy force that adds to those promoting horizontal extension (Fig. 7.23). This positive reinforcement tends to enhance those aspects of lithospheric stretching (Section...

General characteristics of cratonic mantle lithosphere

A defining characteristic of the cratonic mantle lithosphere is a seismic velocity that is faster than normal subcontinental mantle to depths of at least 200 km and locally to depths of 250-300 km (Plate 11.1b,c between pp. 244 and 245). Many Proterozoic belts lack these fast velocity anomalies at similar depths. In addition, Archean cratons are characterized by the lowest surface heat flow of any region on Earth, with a heat flux that is lower than adjacent Proterozoic and Phanerozoic crust by...

Continental growth and craton stabilization

Many of the geologic features that comprise Protero-zoic belts (Section 11.4.1) indicate that the continental lithosphere achieved widespread tectonic stability during this Eon. Tectonic stability refers to the general resistance of the cratons to large-scale lithospheric recycling processes. The results of seismic and petrologic studies (Sections 11.3.1, 11.3.3) and numerical modeling (Lenardic et al., 2000 King, 2005) all suggest that compositional buoyancy and a highly viscous cratonic...

Metamorphism At Convergent Margins

Magmatism Accretionary Prism

As oceanic basalt is subducted at convergent margins, it undergoes a series of chemical reactions that both release water into the upper mantle wedge (Section 9.8) and increase the density of the subducting slab. These reactions involve specific metamorphic transformations that reflect the abnormally low geothermal gradients (10 C km-1) and the high pressures associated with the subduction zone environment (Section 9.5). Prior to its subduction, oceanic basalt may exhibit low pressure (< 0.6...

Expanding Earth Hypothesis

The expanding Earth hypothesis was first proposed in the 1920s and was subsequently adopted by several geologists as the mechanism behind the break-up of continents, the formation of continental rifts, and the presence of extensional features such as normal faults (Carey, 1976, 1988). Their proposal was that the continental lithosphere was originally continuous over the surface of an Earth of smaller radius and that, as the Earth expanded and its surface area increased, the continental...

Precambrian Heat Flow

One of the most important physical parameters to have varied throughout geologic time is heat flow. The majority of terrestrial heat production comes from the decay of radioactive isotopes dispersed throughout the core, mantle, and continental crust (Section 2.13). Heat flow in the past must have been considerably greater than at present due to the exponential decay rates of these isotopes (Fig. 11.1). For an Earth model with a K U ratio derived from measurements of crustal rocks, the heat flow...

Terrane analysis

Many orogens are composed of a collage of fault-bounded blocks that preserve geologic histories unrelated to those of adjacent blocks. These units are known as terrones and may range in size from a few hundreds to thousands of square kilometers. Terranes usually are classified into groups according to whether they are native or exotic to their adjacent continental cratons (e.g. Section 11.5.5). Exotic (or allochthonous) terranes are those that have moved relative to adjacent bodies and, in some...

Natural remanent magnetization

Rocks can acquire a natural remanent magnetization (NRM) in several ways. If the NRM forms at the same time as the rock it is referred to as primary if acquired during the subsequent history of the rock it is termed secondary. The primary remanence of igneous rocks is known as thermoremanent magnetization (TRM). It is acquired as the rock cools from its molten state to below the Curie temperature, which is realized after solidification. At this stage its ferromagnetic minerals pick up a...

The Alpine Fault

The Alpine Fault system in New Zealand (Fig. 8.2a) provides an example of a continental transform whose structure reflects a large component of fault-perpendicular shortening. Geophysical observations of the sea floor south of New Zealand suggest that contraction originated with changes in the relative motion between the Australian and Pacific plates between 11 and 6 Ma (Walcott, 1998 Cande & Stock, 2004). Prior to 11 Ma, relative plate motion resulted in mostly strike-slip movement on the...

Mechanisms of noncollisional orogenesis

Orogenesis at ocean-continent convergent margins initiates where two conditions are met (Dewey & Bird, 1970) (i) the upper continental plate is thrown into compression and (ii) the converging plates are sufficiently coupled to allow compressional stresses to be transmitted into the interior of the upper plate. Studies of subduction zones in general suggest that the stress regime in the overriding plate is influenced by the rate and age of subducting oceanic lithosphere (Uyeda & Kanamori,...

Lithospheric flexure

Border faults that bound asymmetric rift basins with uplifted flanks are among the most common features in continental rifts (Fig. 7.25). Some aspects of this characteristic morphology can be explained by the elastic response of the lithosphere to regional loads caused by normal faulting. Plate flexure (Section 2.11.4) describes how the lithosphere responds to long-term (> 105 years) geologic loads. By comparing the flexure in the vicinity of Figure 7.25 Generalized form of an asymmetric rift...

Rheological stratification of the lithosphere

In most quantitative models of continental rifting, the lithosphere is assumed to consist of multiple layers that are characterized by different rheologies. (Section 2.10.4). This vertical stratification agrees well with the results from both geophysical investigations of continental lithosphere and with the results of laboratory experiments that reveal the different behaviors of crust and mantle rocks over a range of physical conditions. In the upper part of the lithosphere strain is...

Surface velocity fields and seismicity

Since about 50 Ma, continued convergence between India and Eurasia at a slowed rate has caused India to penetrate some 2000 km into Asia (Dewey et al., 1989 Johnson, 2002). This motion created a zone of active deformation that stretches 3000 km north of the Himalayan mountain chain (Fig. 10.13). Global Positioning System (GPS) measurements show that India is moving to the northeast at a rate of some 35-38 mm a-1 relative to Siberia (Larson et al., 1999 Chen et al., 2000 Shen et al., 2000 Wang...

General geology of Proterozoic crust

Proterozoic belts display two groups of rocks that are distinguished on the basis of their metamorphic grade and deformation history. The first group consists of thick sequences of weakly deformed, unmetamor-phosed sedimentary and volcanic rocks that were deposited in large basins on top of Archean cratons. The second group is composed of highly deformed, high-grade metamorphic rocks that define large oro-genic belts. Both these groups contain distinctive suites of igneous rocks. The most...

The San Andreas Fault

The San Andreas Fault formed in Oligocene times (Atwater, 1970, 1989) when the Pacific-Farallon spreading ridge collided with the western margin of North Figure 8.11 P-wave velocity model of the crust and mantle below the Arava Fault within the southern segment of the Dead Sea Transform (image provided by M. Weber and modified from the DESERT Group, 2004, with permission from Blackwell Publishing). Profile location is shown in Fig. 8.3a. Vertical exaggeration is 2 1. Triangles indicate shot...

Measuring The Strength Of Transforms

Measures of the strength of continental transforms and large strike-slip faults provide a potentially useful means of testing models of continental rheology and evaluating the driving forces of continental deformation (Section 8.5.1). In many intraplate areas, the long-range (1000-5000 km) uniformity of stress orientations and their relative magnitudes inferred from measures of strain or displacement suggest that plate-driving forces provide the largest component of the total stress field...

Relative plate motions and surface velocity fields

In the southwestern USA, relative motion between the Pacific and North American plates occurs at a rate of about 48-50 mm a-1 (DeMets & Dixon, 1999 Sella et al., 2002). Geodetic and seismologic data suggest that up to 70 of this motion presently may be accommodated by dextral slip on the San Andreas Fault (Argus & Gordon, 2001). Out of a total of approximately 11001500 km of strike-slip motion, since the Oligocene (Stock & Molnar, 1988), only 300 and 450 km of right lateral slip have...

The Forces Acting On Plates

In order to understand the structural styles and tectonic development of plate margins and interiors, it is necessary to consider the nature and magnitude of all the forces that act on plates. Forsyth & Uyeda (1975) solved the inverse problem of determining the relative magnitude of plate forces from the observed motions and geometries of plates. Since the present velocities of plates appear to be constant, each plate must be in dynamic equilibrium, with the driving forces being balanced by...

Metamorphism Of Oceanic Crust

Many of the rocks sampled from the ocean basins show evidence of metamorphism, including abundant green-schist facies assemblages and alkali metasomatism In close proximity to such rocks, however, are found completely unaltered species. It is probable that this metamorphism is accomplished by the hydrothermal circulation of seawater within the oceanic crust. There is much evidence for the existence of such circulation, such as the presence of metalliferous deposits which probably formed by the...

Alongaxis Segmentation Of Oceanic Ridges

Many early investigations of ocean ridges were essentially two-dimensional in that they were based on quite widely spaced profiles oriented perpendicular to their strike. More recently swath-mapping systems have Fig. 6.11 Interpretive models of magma chambers beneath a fast (a) and slow (b) spreading ridge (modified from Sinton & Detrick, 1992, by permission of the American Geophysical Union. Copyright 1992 American Geophysical Union). Fig. 6.11 Interpretive models of magma chambers beneath...

Relative plate motions and collisional history

The Himalayan-Tibetan orogen was created mainly by the collision between India and Eurasia over the past 70-50 Myr (Yin & Harrison, 2000). The orogen is part of the greater Himalayan-Alpine system, which extends from the Mediterranean Sea in the west to the Sumatra arc of Indonesia in the east over a distance of > 7000 km. This composite belt has evolved since the Paleozoic as the Tethyan oceans (e.g. Fig. 11.27) closed between two great converging landmasses Laurasia in the north and...

The Vine Matthews hypothesis

It is perhaps surprising to note that magnetic maps of the oceans showing magnetic lineations (Section 4.2) were available for several years before the true significance of the lineations was realized. The hypothesis of Vine & Matthews (1963) was of elegant simplicity and combined the notion of sea floor spreading (Section 4.1.4) with the phenomenon of geomagnetic field reversals (Section 4.1.3). The Vine-Matthews hypothesis explains the formation of magnetic lineations in the following way....

Mantle upwelling beneath rifts

The three-dimensional velocity structure of the upper mantle beneath rifts can be ascertained using teleseis-mic travel-time delays and seismic tomography. Davis & Slack (2002) modeled these types of data from beneath the Kenya Dome using two Gaussian surfaces that separate undulating layers of different velocities (Plate 7.2 between pp. 244 and 245). An upper layer (mesh surface) peaks at the Moho beneath the rift valley and has a velocity contrast of -6.8 relative to 8 km s-1 mantle. A...

Brittle deformation

Brittle fracture is believed to be caused by progressive failure along a network of micro- and meso-scale cracks. The cracks weaken rock by producing local high concentrations of tensile stress near their tips. The crack orientations relative to the applied stress determine the location and magnitude of local stress maxima. Fracturing occurs where the local stress maxima exceed the strength of the rock. This theory, known as the Griffith theory of fracture, works well under conditions of...

General Characteristics Of Narrow Rifts

Some of the best-studied examples of tectonically active, narrow intracontinental rifts occur in East Africa (Fig. 7.2). Southwest of the Afar triple junction, the Nubian and Somalian plates are moving apart at a rate of approximately 6-7 mm a-1 (Fernandes et al., 2004). This divergent plate motion results in exten-sional deformation that is localized into a series of discrete rift segments of variable age, including the Western Rift, the Eastern Rift, the Main Ethiopian Rift, and the Afar...

Plates And Plate Margins

The combination of the concept of transform faults with the hypothesis of sea floor spreading led to the construction of the theory of plate tectonics. In this theory the lithosphere is divided into an interlocking network of blocks termed plates. The boundaries of plates can take three forms (Isacks et al., 1968). 1 Ocean ridges (accretive or constructive plate margins) mark boundaries where plates are diverging. Magma and depleted mantle upwell between the separating plates, giving rise to...

Marine magnetic anomalies

Marine Magnetic Anomalies

Magnetic surveying is easily accomplished, and measurements have been carried out from survey vessels since the mid 1950s both on specific surveys and routinely on passage to the locations of other oceano-graphic investigations. A most significant magnetic anomaly map (Fig. 4.1) was constructed after detailed surveys off the western seaboard of North America (Mason & Raff, 1961 Raff & Mason, 1961). The magnetic field was shown to be anything but uniform, and revealed an unexpected pattern...

Modes of shortening in foreland foldthrust belts

A common characteristic of fold and thrust belts is the presence of one or more d collement (or detachment) surfaces that underlie shortened sequences of sedimentary and volcanic rock (Section 9.7). The geometry of these surfaces tends to conform to the shape of the sedimentary and volcanic sections in which they form. In most foreland basins sedimentary sequences thin toward the foreland, resulting in d collements that dip toward the hinterland (Figs 10.5b, 10.7). In thin-skinned thrust belts...

Finite Plate Motions

Euler Pole Plate Motion

The motions of the plates described in Section 5.3 are termed geologically instantaneous as they refer to movements averaged over a very short period of geologic time. Such rotations cannot, therefore, provide information on the paths followed by the plates in arriving at the point at which the instantaneous motion is measured. Although it is a basic tenet of plate tectonics that poles of rotation remain fixed for long periods of time, consideration of the relationships between plates forming...

Thermal Structure Of The Downgoing Slab

The strength and high negative buoyancy of subducting oceanic lithosphere and its capacity for sudden failure in the generation of earthquakes are consequences of its relatively low temperature with respect to normal mantle material at these depths. The subducting Figure 9.15 Summary of the distribution of down dip stresses in Benioff zones. Open circles, events with compressional axis parallel to dip of zone solid circles, events with tensional axis parallel to dip of zone crosses, neither P-...

Absolute Plate Motions

The relative motion between the major plates, averaged over the past few million years, can be determined with remarkable precision, as described in the preceding section. It would be of considerable interest, particularly in relation to the driving mechanism for plate motions, if the motion of plates, and indeed plate boundaries, across the face of the Earth could also be determined. If the motion of any one plate or plate boundary across the surface of the Earth is known, then the motion of...

Contracting Earth Hypothesis

In the 19th century it was believed that, since its formation, the Earth had been cooling due to heat loss by thermal conduction. Computations by Lord Kelvin on the rate of cooling of an initially molten Earth provided the first, erroneous, estimate of the age of the Earth of 100 Ma. As a corollary, it was suggested that the accompanying contraction of the Earth on cooling might provide a mechanism for mountain building. It was estimated that the circumference of the Earth had decreased by...

Ridgeridge transform faults

North Termination Mid Atlantic Ridge

Sykes (1967) determined focal mechanism solutions for earthquakes occurring in the vicinity of the fracture zones that offset the Mid-Atlantic Ridge to the left at equatorial latitudes (Fig. 4.17). Events along the ridge axis are consistent with normal faulting along north-south planes. Events along the fracture zones are much more common and the energy release is about a hundred times greater than along the ridge crest. Between the offset ridge segments events are of strike-slip type with one...

Variations In Subduction Zone Characteristics

The age and convergence rate of the subducting oceanic lithosphere affect not only the thermal structure of the downgoing slab, and the length of the seismic zone, but a number of other characteristics of subduction zones. It can be seen from Fig. 9.15 that, although the dip of the Benioff zone is often approximately 45 , as typically illustrated, there is a great variation in dips, from 90 beneath the Marianas to 10 beneath Peru. It appears that the dip is largely determined by a combination...

Driving Mechanism Of Plate Tectonics

Convection The Mantle Black And White

The energy available to drive plate motions is the heat generated in the core and mantle that is brought to the surface by convection in the mantle. It now remains to consider the manner in which this thermal energy is employed in driving the lithospheric plates. The proposal by Morgan (1971, 1972b) that plates are driven by the horizontal flow of material brought to the base of the lithosphere by hotspots was discounted initially (Chapple & Tullis, 1977), as the lateral flow would probably...

Lithospheric stretching

During horizontal extension, lithospheric stretching results in a vertical thinning of the crust and an increase in the geothermal gradient within the zone of thinning (McKenzie, 1978). These two changes in the physical properties of the extending zone affect lithospheric strength in contrasting ways. Crustal thinning or necking tends to strengthen the lithosphere because weak crustal material is replaced by strong mantle lithosphere as the latter moves upward in order to conserve mass. The...

Gondwana Pangea assembly and dispersal

Gondawana Mozambique Belt

The assembly of Gondwana began immediately following the break-up of Rodinia in Late Proterozoic times. According to the SWEAT hypothesis (Section 11.5.3) West Gondwana formed when many small ocean basins that surrounded the African and South American cratons closed during the opening of the proto-Pacific Ocean, creating the Pan-African orogens (Fig. 11.19b). Subsequent closure of the Mozambique Ocean resulted in the collision and amalgamation of West Gondwana with the blocks of East Gondwana....

Oceanic Fracture Zones

The Leaky Transform Zone

Transform faults in the oceans are well defined, in the absence of sedimentary cover, by fracture zones. These are long, linear, bathymetric depressions that normally follow arcs of small circles on the Earth's surface perpendicular to the offset ridge (Bonatti & Crane, 1984). The apparent relative simplicity of oceanic fracture zones is no doubt due in part to the fact that they are commonly studied from the sea surface several kilometers above the ocean floor. Direct observations of a...

Relative Plate Motions

Plate Tectonics Euler Vector

The present day motion of plates can now be measured using the techniques of space geodesy (Section 5.8). However, these techniques were only developed in the 1980s, and, ideally, measurements are required over a period of 10-20 years (Gordon & Stein, 1992). Prior to this relative plate motions, averaged over the past few million years, were determined using geologic and geophysical data. The motion of plates over the Earth's surface can be described by making use of Euler's theorem (Section...

Mechanisms of continental collision

Indentation Tectonics

Like all other major zones of continental deformation (e.g. Sections 7.6, 8.6, 10.2.5), the evolution of colli-sional orogens is governed by the balance among regional and local forces, the strength and rheology of the continental lithosphere, and by processes that change these parameters over time. To determine how interactions among these factors control the development of the Himalayan-Tibetan orogen, geoscientists have developed physical and analogue models of continental collision. This...

Structure Of Subduction Zones From Earthquakes

Tonga Ridge Subduction Attenuation

Subduction zones exhibit intense seismic activity. A large number of events occur on a plane that dips on average at an angle of about 45 away from the underthrusting oceanic plate (Fig. 9.5). The plane is known as a Benioff (or Benioff-Wadati) zone, after Figure 9.4 Gravity anomalies of an oceanic subduction zone (after Grow, 1973, with permission from the Geological Society of America). Figure 9.4 Gravity anomalies of an oceanic subduction zone (after Grow, 1973, with permission from the...

The Nature Of Convection In The Mantle

The evidence for convective flow in the mantle, from seismic tomography and studies of the regional elevation and subsidence of the Earth's surface, strongly suggests that there are two main driving forces for this convection. The negative buoyancy of cold subducting lithosphere would appear to determine the main sites of downwelling, and the positive buoyancy of hot, low viscosity material originating in the lowermost, D, layer of the mantle determines the upwellings. These two complementary...

Focal mechanism solutions of earthquakes

The seismic waves generated by earthquakes, when recorded at seismograph stations around the world, can be used to determine the nature of the faulting associated with the earthquake, to infer the orientation of the fault plane and to gain information on the state of stress of the lithosphere. The result of such an analysis is referred to as a focal mechanism solution or fault plane solution. The technique represents a very powerful method of analyzing movements of the lithosphere, in...

Lithosphere And Asthenosphere

Lithosphere Surface

It has long been recognized that for large-scale structures to attain isostatic equilibrium, the outermost shell of the Earth must be underlain by a weak layer Figure 2.35 Section from San Francisco, California to Lamar, Colorado based on seismic refraction data (redrawn from Pakiser, 1963, by permission of the American Geophysical Union. Copyright 1963 American Geophysical Union). Figure 2.35 Section from San Francisco, California to Lamar, Colorado based on seismic refraction data (redrawn...

General Characteristics Of Wide Rifts

Intermountain Seismic Belt

One of the most commonly cited examples of a wide intracontinental rift is the Basin and Range Province of western North America (Fig. 7.1). In this region, large extensional strains have accumulated across a zone ranging in width from 500 to 800 km (Fig. 7.8). In the central part of the province, some 250-300 km of horizontal extension measured at the surface has occurred since 16 Ma (Snow & Wernicke, 2000). In eastern Nevada and western Utah alone the amount of total horizontal surface...

Edgeforce mechanism

In this mechanism the oceanic lithosphere represents the top of the convection system, and the plates move in response to forces applied to their edges (Fig. 12.9b). The mechanism was first proposed by Orowan (1965) and Elsasser (1969, 1971) and is sometimes referred to as Orowan-Elsasser-type convection (Davies & Richards, 1992). Only a small percentage of the energy supplied from the mantle is available to drive the plates, but this fraction is adequate to power the present plate motions...

Changes in oceanic circulation and the Earths climate

Two of the most significant influences on the Earth's climate are the concentration of greenhouse gases in the atmosphere (Sections 5.7, 13.1.1), and the extent, distribution, and bottom topography of the oceans. The configuration of the ocean basins affects the transport of heat in the oceans, by surface currents and deep-water circulation, thereby affecting the temperature and moisture content of the atmosphere over oceanic areas. Surface currents are essentially wind driven, and, therefore,...

The Woodlark Rift

Abers Woodlark

The Woodlark Basin and adjacent Papuan Peninsula (Fig. 7.39a) record a continuum of active extensional processes that vary laterally from continental rifting in the west to sea floor spreading in the east. This example provides an important record of how sea floor spreading segments develop spatially during continental breakup and the formation of nonvolcanic margins. It also illustrates the type of lithospheric conditions that promote the development of metamorphic core complexes during...

Continental drift

3.2 Continental reconstructions 3.2.2 Geometric reconstructions of continents 3.2.3 The reconstruction of continents around the Atlantic 3.2.4 The reconstruction of Gondwana 3.6.6 Paleogeographic reconstructions 4 Sea floor spreading and transform faults 72 4.1 Sea floor spreading 73 4.1.3 Geomagnetic reversals 74 4.1.4 Sea floor spreading 77 4.1.7 Dating the ocean floor 84 4.2.3 Ridge jumps and transform 5.1 Plates and plate margins 92 5.2 Distribution of earthquakes 92 5.3 Relative plate...

Changes in sea level and seawater chemistry

The sedimentary record in continental areas is characterized by marine transgressions and regressions due to changes in sea level throughout geologic time. One of the highest sea level stands occurred in late Cretaceous time when, for example, the very pure marine limestone, Chalk, was deposited throughout much of northwest Europe. Major changes in sea level, of 100 m or more, are difficult to explain, except during ice ages, when large volumes of fresh water are locked up in land-bound ice...

The evolution of rifted margins

The evolution of rifted continental margins is governed by many of the same forces and processes that affect the formation of intracontinental rifts (Section 7.6). Thermal and crustal buoyancy forces, lithospheric flexure, rheological contrasts, and magmatism all may affect margin behavior during continental break-up, although the relative magnitudes and interactions among these factors differ from those of the pre-break-up rifting stage. Two sets of processes that are especially important...

Stability Of Triple Junctions

The stability of the boundaries between plates is dependent upon their relative velocity vectors. If a boundary is unstable it will exist only instantaneously and will immediately devolve into a stable configuration. Figure 5.19a shows an unstable boundary between two plates where plate X is underthrusting plate Y at bc in a northeasterly direction and plate Y is underthrust-ing plate X at ab in a southwesterly direction. The boundary is unstable because a trench can only consume in one...

Propagating Rifts And Microplates

The direction of spreading at an ocean ridge does not always remain constant over long periods of time, but may undergo several small changes. Menard amp Atwater 1968 proposed that spreading in the northeastern Pacific had changed direction five times on the basis of changes in the orientation of major transform faults Section 5.9 and magnetic anomaly patterns. Small changes in spreading direction have also been proposed as an explanation of the anomalous topography associated with oceanic...

Late Proterozoic supercontinent

Canada Proterozoic Reconstruction

Similarities between the Late Proterozoic geologic record in western Canada and eastern Australia Bell amp Jefferson, 1987 Young, 1992 and between the southwestern USA and East Antarctica suggest that these areas were juxtaposed during Late Proterozoic times Dalziel, 1991, 1995 Moores, 1991 Hoffman, 1991 Fig. 11.19a . This seemingly radical suggestion was referred to as the SWEAT South West US and East AnTarctica hypothesis. The widespread Grenville orogenic belts, that immediately pre-date the...

Geologic Evidence For Continental Drift

Palynology Baltic

The continental reconstructions discussed in Sections 3.2.3 and 3.2.4 are based solely on the geometric fit of continental shelf edges. If they represent the true ancient configurations of continents it should be possible to trace continuous geologic features from one continent to another across the fits. The matching of features requires the rifting of the supercontinent across the general trend of geologic features. This does not always occur as the location of the rift is often controlled by...

Pratts hypothesis

Pratt's hypothesis assumes a constant depth to the base of the outermost shell of the Earth, whose density varies according to the surface topography. Thus, mountain ranges would be underlain by relatively low density material and ocean basins by relatively high density material Fig. 2.29b . Equating the weights of columns of unit cross-section beneath a mountain range and a region of zero elevation gives If one substitutes appropriate densities for the crust, mantle, and sea water in these...

Magnetostratigraphy

Once the geomagnetic reversal timescale has been calibrated, oceanic magnetic anomalies may be used to date oceanic lithosphere. The method has been progressively refined so that it is now possible to deduce ages back to mid-Jurassic times with an accuracy of a few million years. The Vine-Matthews hypothesis explains the sequence of magnetic anomalies away from ocean ridges in terms of normal and reversed magnetizations of the oceanic crust acquired during polarity reversals of the geomagnetic...