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The majority of sediment acoustics research has focused to date on sediments with sand‐sized particles or smaller; measurements for sediments containing cobbles (6–26 cm) are rare. This paper presents the first measurements (to the authors’ knowledge) over a wide frequency range of compressional‐wave velocity and bulk density for a sediment with cobbles. The in situ velocity from inversion from wide‐angle reflection‐coefficient data at 0.4–2 kHz for cobbles suspended in sand is found here to be 1800 m/s with 95% credibility interval bounds of [1787–1828] m/s and for cobbles suspended in clay 1532 [1528–1536] m/s. Measured core velocities at 50 kHz and 200 kHz are lower for each sediment due to multiple scattering. The in situ bulk density for cobbles suspended in sand is 2.25 [2.21–2.28] g/cm3 and for cobbles suspended in clay 1.83 [1.78–1.87] g/cm3. Though cobbles in the upper few metres of sediment may be considered unusual on the mid‐shelf at mid‐latitudes, they appear to be present over tens of square kilometres on the Malta Plateau in a several metre thick layer starting at about 1 m below seafloor. In fact, geologic process considerations suggest that cobbles may be generally more common in mid to outer shelf environments than the paucity of measurements would suggest. Increased interest in the Arctic continental shelf environment, where pebbles and cobbles are expected to be prevalent, provides an additional motivation for this work.

Geophysics is a field of earth sciences that uses the methods of physics to investigate the complex physical properties of the Earth and the natural processes that have determined and continue to govern its evolution. ‘What is geophysics?’ explains how geophysical investigations cover a wide range of research fields—including planetary gravitational and magnetic fields and seismology—extending from surface changes that can be observed from Earth-orbiting satellites to complex behaviour in the Earth’s deep interior. The timescale of processes occurring in the Earth also has a very broad range, from earthquakes lasting a few seconds to the motions of tectonic plates that take place over tens of millions of years.

Hundreds of thousands of earthquakes occur worldwide each year, but most of them go unnoticed. Only a few are very destructive. Most earthquakes have a tectonic origin and happen in well-defined, relatively narrow seismic zones. ‘Seismicity—the restless Earth’ first describes the elastic rebound model that explains how an earthquake occurs. There are two measures of the size of an earthquake: its magnitude and intensity. Magnitude is a measure of the energy released by the earthquake classified by the Richter scale, while intensity is a qualitative measure based on observed effects using a twelve-part scale. Maps of the locations of earthquake epicentres show that these are concentrated in narrow seismically active zones. Earthquake monitoring and prediction are discussed.

The Earth’s internal heat is its greatest source of energy. It powers global geological processes such as plate tectonics and the generation of the geomagnetic field. ‘The Earth’s heat’ explains that the internal heat arises from two sources: the decay of radioactive isotopes in crustal rocks and the mantle, and primordial heat left over from the planet’s fiery formation. The internal heat has to find its way out of the Earth. The three basic forms of heat transfer are radiation, conduction, and convection. Heat is also transferred in compositional and phase transitions. Heat transport by conduction is most important in solid regions of the Earth, while thermal convection occurs in the viscoelastic mantle and molten outer core.

‘Gravity and the figure of the Earth’ discusses the measurement of gravity and its variation at the Earth’s surface and with depth. Gravity is about 0.5 per cent stronger at the poles than at the equator and it first increases with depth until the core–mantle boundary and then sinks to zero at the Earth’s centre. Using satellites to carry out geodetic and gravimetric observations has revolutionized geodesy, creating a powerful geophysical tool for observing and measuring dynamic processes on the Earth. The various measurement techniques employed fall in two categories: precise location of a position on the Earth (such as GPS) and accurate determination of the geoid and gravitational field. Bouguer and free-air gravity anomalies and isostasy are explained.

Seismology is the most powerful geophysical tool for understanding the structure of the Earth. It is concerned with how the Earth vibrates. Physically, seismic behaviour depends on the relationship between stress and strain in the Earth. ‘Seismology and the Earth’s internal structure’ explains compressional and shear elastic deformation and the four types of seismic waves caused by earthquakes: P-waves and S-waves that travel through the body of the Earth, and Rayleigh and Love waves that spread out at and near the Earth’s surface. It describes the reflection, refraction, and diffraction of body waves and how their observation and measurement by seismometers can be used to understand the internal structure of core, mantle, and crust.

Sheets of salt and ductile shale advancing beyond the thrust front of the Gibraltar Arc (Iberian-Moroccan Atlantic continen- tal margin) triggered downslope movements of huge allochth- onous masses. These allochthons represent the Cadiz Nappe, which detached from the Gibraltar Arc along low-angle normal faults and migrated downslope from the Iberian and Moroccan continental margins towards the Atlantic Ocean. Extensional tectonics initiated upslope salt withdrawal and downslope diapirism during large-scale westward mass wasting from the shelf and upper slope. Low-angle salt and shale detachments bound by lateral ramps link extensional structures in the shelf to folding, thrusting and sheets of salt and shale in the Gulf of Cadiz. From backstripping analyses carried out on the depo- centres of the growth-fault-related basins on the shelf, we infer two episodes of rapid subsidence related to extensional collapses; these were from Late Tortonian to Late Messinian (200-400 m Myr )1 ) and from Early Pliocene to Late Pliocene (100-150 m Myr )1 ). The extensional events that induced salt movements also affected basement deformation and were, probably, associated with the westward advance of frontal thrusts of the Gibraltar Arc as a result of the convergence between Africa and Eurasia. The complexities of salt and ⁄ or shale tectonics in the Gulf of Cadiz result from a combination of the deformations seen at convergent and passive continental margins.

In the Ebro Delta, NE Spain, small- to large-scale synsedimentary deformation has been observed by means of high- and low-resolution seismics correlated with a network of wells. Lithoseismic units involved are middle–upper Pleistocene clays and gravels and Holocene deltaic sequence. Gassy sediments and large river-floor pockmarks are identified from seismic profiles along the Ebro Delta. Gas-charged sediments are identified by zones of acoustic turbidity and other acoustic anomalies on seismic profiles. Most of the gas is believed to be biogenic in origin, resulting from the decay of organic matter contained within rapidly developing shelf margin deltas. A group of large symmetric river-floor pockmarks also occurs along the inner delta plain, both on the river-floor surface and buried in the sediment column. Listric fault geometry is a typical scope-shaped plane with a steep upper surface (40–60°) that passes downward into a horizontal shear plane. The shear plane appears seismically as chaotic and convoluted high-amplitude reflectors and is correlated with well data showing layers that include gravel, sand and clays. Associated structural features include reverse drag of reflectors, antithetic faults, and fluid escapes (methane/water). The growth faulting along the Ebro Delta includes a break-away and thin, near-surface layer created by overstepping of a sandy Holocene delta slope. The over-steeping lies down-dip from growth faults along the shelf-break. The overall growth faults province appears to represent a deeper-seated general movement of the delta mass involving differential loading, overpressure of fluids, diapiric movements, and extensional tectonics that cause listric faults and associated structures at different scales. The recent growth faults are regularly spaced, with the distance between successive faults controlled by the depth to the detachment layer. Three main fault systems have been identified: A first or primary system spaced 5.2–10.5 km above a detachment layer at 55–95 m deep; a second system spaced 3.5–6 km; and a third system spaced 1.5–2.5 km above a detachment layer at 25–30 m. The slope angle calculated by empirical equations ranges between 0.5° and 1.0° for the three systems.

Hydrates are near-surface phenomena that occur when temperatures and pressures are appropriate. Among such stable continental margins is the East Coast of the United States that, with its ubiquitous, leisurely, uniform deposition and relatively minor deformation, there can be associated hydrates. The geologic uniformity and long-term stability provide opportunities for stress homogenization with few perturbations, creating zones of weakness. With widely spaced sampling adequate to define a regional model, uncertainty can be reasonably calculated. Along unstable continental margins such as the northern Gulf of Mexico that contain such dynamic elements as salt and/or shale, as well as highly variable and massive deposition rates, there can be deformed stress fields that result from compression, lateral motions and variations thereof. Locally, any stress may be principal for an indeterminate time. With such variability, no detailed regional model can be developed. Risk analysis is possible only when a 3D model at sufficient resolution is available so that reasonable integrations can be performed and posed questions can be answered. To derive such a 3D model requires that all impacting processes, their intensities and periodicities, and spatial distributions be catalogued and understood. Thus, the “final” 3D model will be of variable resolution. To know at what resolution data need to be collected will be determined by the specific processes being mapped in a given locality. To apply an equitable risk analysis along unstable passive continental margins could be premature, given the uncertainty of all the operative processes. A conceptually valid 3D model of processes is not available, nor is an understanding of intensities and periodicities. Synergistic relationships can only be guessed. The principles of risk analysis are formulated; data and methodologies for use on unstable margins are not.

The 146.5 m-thick Upper Triassic limestone section at Pizzo Mondello in the Sicani Mountains of western Sicily is characterized by high quality of exposure, accessibility, and stratigraphic continuity. Magnetostratigraphic results delineate 12 normal and reverse polarity magnetozones, labelled successively from the base upwards as PM1n, PM1r, PM6n, PM6r. The Carnian/Norian boundary, based on conodont biostratigraphy, falls somewhere in the PM3n to PM5n interval which corresponds to the E14n to E16n magnetozone interval in the Newark reference sequence of polarity reversals. Comparison of magnetobiostratigraphic data from the Newark basin, Pizzo Mondello and other Late Triassic marine sections available from the literature suggests the existence of a reduction in sedimentation rate in the Tethyan marine domain at around the Carnian/ Norian boundary. Although the Newark and the expanded Pizzo Mondello sections correlate well with each other, correlation with the condensed Kavur Tepe and Scheiblkogel sections is unsatisfactory. A re-interpretation of the Kavur Tepe results suggests that the section is younger than its previous correlation with the Newark section, and that it was deposited in the northern instead of the southern hemisphere. Most of the condensed Tethyan marine sections are seen to be highly discontinuous, as evidenced by concantenated conodont total range zones. q 2001 Elsevier Science B.V. All rights reserved.

Paleomagnetic directions have been determined for a new collection of Early Oligocene and Late Miocene‐Pliocene Epiligurian clastic sediments from the frontal portions of the northern Apennines. These results are combined with Cenozoic data from the literature to evaluate whether rotations of units in this region are related to the OligoMiocene Corsica‐Sardinia rotation and/or to younger phases of deformation of the Apennine chain. When Corsica/Sardinia moved counterclockwise oV the coast of France, the Ligurian units located at its front were presumably pushed eastward and rotated counterclockwise above a main boundary thrust onto the Adria/Africa margin. We propose that about 24 out of a total of 52° of rotation observed in the Epiligurian units can be related to the Oligo-Miocene motion of the Corsica‐Sardinia block, in partial agreement with previous conclusions, and the remaining 28° to the Pliocene tectonic phase at the Apennine chain front, which may have (re)activated thrust planes in the Adria/Africa succession below the Ligurian wedge. © 2000 Elsevier Science B.V. All rights reserved.

In the framework of a structural-and-formational interpretation (SFI) approach to seismic data processing and geological interpretation, specific software/technology tools were created to facilitate sequence stratigraphy analysis and reservoir characterization. The most important tools are two types of time–frequency representation of seismic data: the first is spectrum-time analysis (STAN), which presents a seismic trace as a series of very narrow frequency band traces, while the second tool converts an initial seismic section into a set of sections ranked with preselected frequency bands, narrower than those of the initial section, but wider than those related to STAN traces. Jointly, these two representations reflect both general trends and local temporal and spatial variations of seismic data frequency content. The use of these tools, developed in Russia in the 1980s, facilitates detection of sedimentation cycles and their depositional environments, identification of hierarchies within faulting patterns, and delineation of geological anomalies on seismic sections. This, in turn, provides reliable starting points for palaeotectonic restoration and basin modelling. In many regions, these tools have helped to clarify obscure formation structures under study and to estimate the hydrocarbon potential of these formations.

The northern Gulf of Mexico's passive margin is interrelated and synergistic with both regional and local inputs. Regional inputs may be thought of as deterministic and may include eustatic sea-level oscillations, sediment deposition from erosion of uplifted areas, climate above and below sea level, compression and dewatering of sediments once deposited, and related tectonics. The synergistic sum of these deterministic processes impacts the occurrence of natural gas and their hydrate phases. Along the foot of the continental slope, compression is transmitted to the sediment beneath and down-dip. Hydrates occur when suitable cold temperatures and high pressures are present. In water depths of ∼4 km, the hydrate stability zone (HSZ) may be up to 1 km thick. A prograding slope imparts regional pressures. Local pressures may be sufficient to catalyze the gas-to-hydrate phase change. Further compression may cause the zone to heat and/or to generate fractures, promoting hydrate disassociation and/or escape. The impact of a salt/shale lateral wedge or diapir (or both) entering the zone can be observed in several ways. Increased salinity raises the gaseous phase, thereby reducing hydrate existence. Increased heat also augments gaseousness. Note that shales dissipate heat and that the top of a salt diapir can be cool, even though overall salt is a heat conductor relative to sediments. Migrating salt/shale can promote fracturing, where the size and rate of migration determine the dimensions and frequency of fractures. As gases enterfractions, gas may expand and possibly block further gas entrance. Local sediment shattering and thermal anomalies could obviate the gas expansion effect. As the Louann Salt, which is located over the slope-sediment wedge, advances basinward, the amount of compression rises. Thus, temperatures and pressures change and the pattern of gases/hydrates are dynamic. Specific local predictions are subtle and may be regarded as probabilistic in model development. Equal subtlety is experienced along the ocean/land contact. Hydrate existence is determined by gas chemistry, whether methane or ethane is present. As the sea level oscillates, temperatures and pressures also oscillate. Landward, the HSZ limit during sea-level highs is disassociated, and a new, more basinward HSZ is established. Specific landward HSZ limit is determined by the gas chemistry.

Magnetostratigraphic and biostratigraphic data are presented from the Anisian (Middle Triassic) Peri-Tethyan Edivetur section of northwestern Bulgaria. A dual-polarity component of magnetization carried by magnetite delineates a magnetic stratigraphy of mainly reversed polarity. Magnetozones are dated by means of foraminifer and conodont biostratigraphy. Data from Edivetur are compared with data from Middle Triassic Tethyan limestone sections with the aim of contributing to the completion of the Middle Triassic magnetic polarity time scale. We also propose that paleomagnetic data from Edivetur can be used as proxy data for the paleogeographic position of the Moesian platform. The Moesian platform was located at 21‐24°N along the southern margin of Europe. It was probably marginally separated, but not detached or rotated away from Europe by the North Dobrugea transtensional trough, which is interpreted as a back-arc basin resulting from the northward subduction of the Neo-Tethys ( Vardar) or Paleo-Tethys ocean. Paleomagnetic data from this study and other minor tectonic elements are used to generate a paleogeographic sketch map of the Pangea-bounded western Tethys and Peri-Tethys at Middle/early Late Triassic time. © 2000 Elsevier Science B.V. All rights reserved.

Calcareous deposits, including shell, hash, and oolites, have been recorded in Upper Pleistocene lowstand deposits seaward of major rivers such as the Mississippi and the Ganges. These calcareous sediments indicate a major reduction or cessation of terrigenous deposition during lowstand/glacial maximum. Extensive high-energy sediments, including coarse sands and gravels, can bracket thin, low-energy, fine-grained calcareous deposits within lowstand sequences. These various sequences may indicate up to three orders of magnitude of variations in deposition rates. Summarizing glacial lake outbursts/rapid stream rerouting suggests that during final deglaciation there could have been massive outflows at multiple decade to century intervals. Continental glacial oscillations throughout the entire glacial maxima could also have produced massive, short-term floods and deposition rates. Such swings in deposition rates must impact on geologic processes within the underlying sedimentary wedge. Processes impacted include (but are not limited to) compacting, faulting, dewatering, diapiric motion, diagenesis, and heat generation.

The magnetic properties of a sediment core from a high altitude lake in the Swiss Alps were compared with palynological and geochemical data to link climatic and mineral magnetic variations. According to pollen data, the sediments extend from the present to the Younger Dryas, i.e., they cover more than 10,000 years of environmental change in the Alps. The major change in magnetic properties corresponds to the climatic warming of the early Holocene. High-coercivity magnetic minerals that characterize the late-glacial period almost disappeared during the Holocene and the concentration of ferrimagnetic minerals increased sharply. The contribution of superparamagnetic grains also decreased in the Holocene sediments. Similar variations in SP content and coercivity, of smaller magnitude, are found in the Holocene and are interpreted to represent minor climatic variations. Comparison with the historical record of the last 1000 years confirms this interpretation. The magnetic mineralogy, the superparamagnetic contents, and the IRM intensity in the coarse-grained, late-glacial sediments are similar to those measured in the catchment bedrock. This indicates a detrital origin. The different properties and the higher concentration of magnetic minerals in the Holocene sediments are due to authigenic phases. Magnetic properties provide a high resolution record of climatic change. They are sensitive even to small variations that are not recorded in the pollen or LOI data. Magnetic parameters show fine-scale variation and constitute a valuable supplement to conventional climatic indicators. © 1999 Elsevier Science B.V. All rights reserved.

Although some empirical relationships have been established between the anisotropy of magnetic susceptibility (AMS) and the state of finite strain in studies from specific geologic areas, the mechanisms governing correlations are not well understood. A comparative study has been made in order to elucidate the AMS-strain relationship by investigating mineral preferred orientations and the responsible microstructural deformation processes. Detailed analyses are presented from low-grade metamorphic slates of a Palaeozoic sequence exposed in the Variscan of SW Sardinia. Rock magnetic experiments suggest that paramagnetic minerals dominate the AMS in these rocks. Excellent agreement was found between magnetic fabrics and lattice preferred orientations (001) of mica and chlorite, determined from X-ray texture goniometry measurements. Depending on the oblate or prolate ellipsoidal shape of anisotropy, the minimum principal axes of AMS and X-ray textures can coincide with the poles to slaty cleavage or the maximum principal axes can coincide with the bedding/cleavage intersection, respectively. The measured AMS and X-ray textures only partially reflect the finite strain determined from deformed micro-pebbles and reduction spots. Axial orientations of magnetic fabrics, mica and chlorite preferred orientations, and finite strain ellipsoids are generally in good agreement depending on development of oblate or prolate fabric ellipsoidal shapes. Linear correlations can be established for magnitudes of minimum and intermediate axes of the AMS and finite strain ellipsoids but shape comparisons are complex. Microstructural studies by scanning electron microscopy (SEM) indicate that the preferred orientation of the phyllosilicates is caused by different grain-scale deformation mechanisms, such as kinking, micro-folding and preferential growth. Thus, magnetic fabrics and mineral preferred orientations evidently result from different deformation mechanisms than the finite strain. They reflect the heterogeneous deformation due to micro-mechanical reorientation processes of mineral grains while finite strain reflects the homogeneous deformation accommodated cumulatively by the strain markers.

A 39.4 m long, 10 cm diameter vertical core was drilled through the Eocene/Oligocene boundary close to the Massignano quarry stratotype section near Ancona, Italy. Total recovery was about 95% and 73% of the core was intact. It was oriented by comparing the dip of bedding planes visible on the core surface with bedding measured in the nearby quarry outcrop. The core was correlated to the quarry section by linear regression of the depths of four biotite-rich layers in each section. The fissile condition of the Scaglia cinerea marls made sampling difficult, but 260 samples were obtained at an average separation of 12 cm in this and the underlying Scaglia variegata formation. Progressive alternating field and thermal demagnetization isolated two magnetization components in most samples. A stable component with unblocking temperatures of 300–540°C and coercivities over 20 mT is probably carried by magnetite. A stratigraphic plot of the ChRM directions shows well-defined normal and reversed magnetozones. The polarity sequence correlates well with chrons C12r to C16n-2. A few single-sample normal magnetozones within chron C16n.1-r have no corollary in the geomagnetic polarity timescale. The magnetozones corresponding to chrons C12r or C13r exhibit no short subchrons that might account for low-amplitude, short-wavelength magnetic anomalies reported in this part of the marine magnetic record.

The confining pressure exerted by the prograding Tertiary clastic wedge has caused extensive salt deformation in the Gulf of Mexico. The high rates of lateral salt motion from the Louisiana shelf break to the Sigsbee Escarpment are expected to have significant impact on the dynamic evolution of the surrounding sedimentary formations. The evolution of fluid pressure in supra- and subsalt sediments has been modeled in a series of pseudo-wells along a profile intersecting numerous salt sheets and structures offshore Louisiana. Based upon the estimated rates of salt movement, the timing can be determined for insertion/depletion of salt in the sediments as the salt has moved through, and by, sediments on its way basinward. An execution of a series of I-D fluid/flow compaction models has enabled estimation of the spatial variation in overpressure build-up with time; indicating that overpressure uncertainty is about 10–20% about a mean of about 40 atmospheres, with uncertainties on salt thickness and salt speed roughly comparable in importance in contributing to the uncertainty. The quantitative behaviors suggest that the speed of lateral salt insertion and the thickness of the salt are the main factors causing anomalous overpressure build-up. A quantification of the dynamic behaviors is significant when modeling the timing of potential trapping of hydrocarbons beneath salt sheets and of the subsalt overpressure to be expected in subsalt drilling.

Comparison of the paleomagnetic pole positions of late Paleozoic secondary magnetizations with a time-averaged reference apparent polar wander path for North America shows that sedimentary rocks within the central Appalachians were remagnetized in the later half of the Permian, between ca. 255 and 275 Ma. Ages of remagnetization do not vary for sites distributed along or across strike of the central Appalachian fold and thrust belt or across the Appalachian Plateau in New York, nor do they vary with stratigraphic position of the host rocks. However, the pattern of folding relative to remagnetization across the Valley and Ridge and Great Valley in Pennsylvania, Maryland, and West Virginia shows a distinct temporal variation. Individual fold tests yield postfolding magnetizations near the hinterland margin of the fold and thrust belt, synfolding magnetizations in the central part of the belt, and prefolding magnetizations near the foreland. Because of the similar age of the late Paleozoic magnetization, we conclude that rocks across the central Appalachians were remagnetized in a relatively short period of time as folding and thrusting propagated more slowly toward the foreland. We interpret this pattern to indicate that older folds near the hinterland grew prior to remagnetization, folds in the central region grew during remagnetization, and folds near the foreland grew after remagnetization. Rapid remagnetization relative to folding is also evident from the physical characteristics and magnetic properties of secondary hematite in remagnetized (synfolding) folds. In such folds, rocks were apparently remagnetized during an increment of fold growth, in which there was partial and possibly negligible fold-limb rotation. Collectively, these results may constrain the mechanism for basinal fluid mobilization during deformation. Current models for remagnetization involve the migration of chemically active brines into the foreland during tectonism. If realistic, our results bear on this model by suggesting that brine migration was not directly related to the successive emplacement of individual thrust sheets, as has been suggested for other fold and thrust belts.

In the Gulf of Mexico, uncertainties in salt sheet thickness, lateral salt speed, sub-sea floor depth of salt top, and lithology underlying salt all contribute to the uncertainty in assessing excess pressure development to be expected when drilling through a salt mass. In an example, the mean excess pressure is around 170 atm with + or -30atm uncertainty. The uncertainty in the estimate of excess pressure is shown to be dominated (58-65%) by uncertainties in salt sheet thickness, with salt speed and lithology of lesser importance, each contributing around 10-25% to the excess pressure uncertainty, depending on the case considered. The cumulative probability of obtaining an excess pressure greater than a specified magnitude at any depth is investigated, as is the volatility of estimates on mean pressure. The calculations provide information on which uncertainties-in salt thickness, salt speed or sub-salt lithology-need to be improved first in any attempt to narrow the range of uncertainty of predicted pressure.

Methane hydrates are ice-like compounds consisting of natural gas (mainly methane) and water, whose crystal structure effectively compresses the methane: each cubic metre of hydrate can yield over 150 cu.m of methane. Hydrates “cement” sediments and impart considerable mechanical strength; they fill porosity and restrict permeability. Both biogenic and thermogenic methane have been recovered from hydrates. Hydrates occur in permafrost regions (including continental shelves), and are stable in ocean-floor sediments below water depths of about 400 m in the “Hydrate Stability Zone” (HSZ). This is a surface-parallel zone of thermodynamic equilibrium that extends down from the sediment surface to a depth determined by temperature, pressure and local heat flow. Methane and water are stable below the HSZ. Although the economic recovery of hydrates has taken place in Arctic regions, oceanic hydrates offer far greater potential as an energy resource. A variety of traps for methane gas can be formed by oceanic hydrates. In addition to the gas within the hydrates themselves, simple gas traps in closures beneath the HSZ in the vicinity of bathymetric highs, and complex traps involving both hydrate and structural/stratigraphic components, have been observed. It has been estimated that at least twice as much combustible carbon occurs associated with methane hydrates as in all other fossil fuels on Earth. The evaluation of methane in, and associated with, oceanic hydrates therefore constitutes a major energy exploration frontier.

The magnetostratigraphy has been established in two sections of middle and late Oxfordian age from the Iberian Range (Spain). The polarity sequences correlate well with the magnetozones in sections at Aguilon and Tosos [Juarez et al., 1994]. A composite magnetostratigraphic sequence has been derived for all the sections in the Iberian Range. It correlates with the Handschumacher et al. [1988] extension of the geomagnetic polarity time scale, and ties the oceanic magnetic anomaly sequence to the Oxfordian sedimentary sections.

The magnetic properties of the late Quaternary hemipelagic sediment from piston core BS77-15 taken in the Tyrrhenian Sea were investigated and the results compared with palaeoclimatic records. The magnetic mineralogy of the sediment was studied and the magnetic carriers were identified by their spectra of coercivity and unblocking temperature. Investigations of rock magnetic parameters such as the Natural Remanent Magnetization (NRM), the low-field magnetic susceptibility χ, the frequency-dependent susceptibility χfd, the acquisition of Anhysteretic Magnetization (ARM) and Isothermal Magnetization (IRM) and the Median Destructive Field (MDF) have established the downcore variations in terms of magnetic mineralogy, grain size and coercivity in the sediments. The rock magnetic parameters were compared with an alternative climatic record from the same core based on the faunal association of planktonic foraminifera. A significant correlation was found between faunal climatic record and χfd. Together with the magnetic properties of the sediment this suggests that a palaeoclimatic signal is recorded by a fine-grained low-coercivity fraction of the magnetic minerals.

Paleomagnetic and biostratigraphic data from the Al Azizia Formation of northwestern Libya, consisting of uppermost Middle Triassic/lowermost Upper Triassic limestones, shed new light on the latitudinal drift of Africa and Adria and related climatic changes. A characteristic component of magnetization carried by magnetite delineates paleomagnetic poles which are coincident with coeval poles from the Southern Alps. Data from this study and the literature are integrated, showing that relatively unrotated remnants of the Adria. margin like the Southern Alps (e.g., the Dolomites), Istria, Gargano, Apulia and Iblei moved in close conjunction with Africa since at least Permian times. A Permian-Cenozoic apparent polar wander (APW) curve for Africa/Adria is constructed. The paleolatitude trend for northern Libya calculated from this APW superposed to the zonal latitudinal bands of relative aridity and humidity typical of modern-day climate predicts that northern Libya drifted northwards from the equatorial belt to the and tropic during the Triassic, and crossed the humid subequatorial/arid subtropical boundary zone at Late Triassic times. This inference is fully supported by Permo-Triassic palynological and facies analysis from this study and the literature. We conclude that a zonal climate model coupled with paleomagnetically constrained paleogeographic reconstructions provides a powerful null hypothesis for understanding past climatic conditions. (C) 2001 Elsevier Science BN. All rights reserved.

The structural framework of the Southeast Pyrenees led to two conflicting interpretations—thrust tectonics vs. wrench tectonics—to explain the geometry of this mountain range. In the present study palaeomagnetic data are presented in an attempt to resolve this conflict. The data reveal different magnetisation directions that indicate tectonic rotations about vertical axes. By means of a regionally homogeneous pattern of rotation, three tectonic units could be distinguished in the Southeast Pyrenees. The Internal Unit in the north reveals no rotation since the Permian. The External Unit to the south shows anticlockwise rotation of 25°, younger than the Early Oligocene. The Pedraforca Unit, placed on the External Unit, shows 57° clockwise rotation which can be assigned to the Neogene. The anticlockwise rotation of the External Unit can be explained by differential compression during the last phase of Pyrenean thrusting, whereas the clockwise rotation of the Pedraforca Unit can be interpreted by post-thrusting tectonics. The rotation pattern of the Southeast Pyrenees provides evidence for both Cretaceous to Paleogene N-S compression and Neogene right-lateral wrench tectonics.

The marine magnetic polarity record shows many small-scale anomalies that could be due to short normal polarity subchrons within chrons 12R and 13R. New examination of published magnetostratigraphic results from a section near Gubbio shows that suspected short chrons in this section are probably not real. The Eocene-Oligocene boundary in the stratotype section at Massignano lies just above a tentatively identified normal magnetozone that might correlate with a possible short normal subchron within chron 13R. In a new investigation of the weathered marlstones in this section, acquisition and thermal demagnetization of IRM reveal a magnetic mineralogy dominated by magnetite, with minor amounts of maghemite and hematite. Vector analysis of the natural remanent magnetization discloses a complex structure in some samples: at least two and up to four components were isolated. The magnetostratigraphy based on a magnetite component correlates well to the main features of the chron 13N–16N polarity sequence. No short polarity subchrons are found, although the resolution is inadequate to exclude the possibility that they exist. In particular, we did not find a short normal polarity magnetozone previously located close to the Eocene-Oligocene boundary. The results of this study favour the interpretation that low-amplitude magnetic anomalies represent geomagnetic intensity fluctuations rather than unresolved short polarity intervals.

Measurements of finite strain, the anisotropy of magnetic susceptibility, the magnetic carrier of the susceptibility, and fieldwork have been carried out in the Onaping Formation of the Sudbury Basin, Ontario. The magnetic fabric indicates that changes in the magnitude and type of magnetic susceptibility ellipsoids closely mirror the changing pattern of deformation from northwest to southeast across the Onaping Formation outcrops. A quantitative correlation has been established between finite strain measurements and the magnetic susceptibility results and this allows the number of available strain measurements to be extended by more than 500 samples. The finite strain results suggest a deformation path in which the Onaping Formation was first affected by NW-SE-directed, horizontal, layer-parallel shortening which produced an upright tectonic cleavage. Subsequently, components of overthrust, NW-directed simple shear were superimposed upon the already deformed rocks of the Onaping Formation. The result of removing the strain, according to the sequence of superimposed deformations revealed by the deformation path, suggests that the Sudbury Basin was nearly circular in shape prior to its deformation. Consequently, a major objection to the meteorite impact theory for origin of the Sudbury Structure is eliminated.

Paleomagnetic directions have been determined at 17 Cambrian and 28 Carboniferous limestone sites distributed along the Asturian arc of the Hercynian fold belt in northwestern Spain. The stable vectors are carried by hematite in all cases, and also by magnetite in some grey Carboniferous limestones. A secondary component of magnetization is usually removed well below 400°C, and the higher-temperature component is taken to be the characteristic direction. The structure at each site was carefully evaluated in order to make optimum local tectonic corrections. In addition the data must be corrected for rotations about a vertical axis to allow for a phase of radial folding superimposed on earlier structures. The corrected paleomagnetic declinations are found to vary systematically along the arc of the fold belt. Consequently, paleomagnetic data from the Asturian arc should not be included in compilations used for the construction of a Paleozoic apparent polar wander path. The paleomagnetic data allow us to distinguish between different tectonic models for the evolution of the Asturian arc. A two-stage model for the development of the present curvature is favored. Part of the curvature appears to be primary, preceding the Hercynian deformation. The first stage of the model involved rotations of thrust sheets during their emplacement, producing a more tightly curved arc than the original form. In a second stage, the development of radial folds further tightened the curvature of the arc. Both stages result in clockwise rotations in the north and anticlockwise rotations in the south.

Salt and related structures have played important roles in controlling hydrocarbon accumulations in the Gulf of Mexico. Using a two-dimensional fluid flow/compaction model, which allows for both conduction and convection of heat, an examination is given of the effects on thermal patterns of the combined influence of multiple salt features, including diapirs, pillows, sheets and wedges. The focusing and defocusing of heat due to the higher thermal conductivity of salt are accounted for in the modeling. The results show that there could be as much as a 30°C anomaly above multi-salt bodies due to the focusing of heat by salt, and as much as 50°C temperature contrast between internal salt positions and sediments external to the salt in the deep part of a section. The magnitude of the thermal anomaly depends on the size (or width) of the salt and on the depth of the rooted salt. The modeled results provide estimates of the influence of salt in expanding the oil generation window by approximately half of the salt thickness.

Paleomagnetic investigations of Jurassic, Cretaceous and Tertiary sites in the external Betic Cordillera indicate that, while the observed remanence vectors have inclinations which are conformable with the expected inclinations for stable Iberia, declinations deviate significantly. These results indicate that: (1) the Mesozoic limestones of the Penibetic have been rotated clockwise with respect to stable Iberia and, (2) there is a differential rotation between the Cretaceous sites (Dec.= 61° ± 11°) and the Jurassic sites (Dec.= 38° ± 8°) . The differential rotation of 23° between the late Jurassic and late Cretaceous is consistent with the Cretaceous anticlockwise rotation of the Iberian Peninsula with respect to Eurasia. It follows that during this time period this part of the Betic mobile belt waspart of the continental margin of the Iberian Peninsula. A study which was confined to a single unbroken stratigraphic section west of Ronda indicates that most of the anticlockwise rotation (20°) probably occurred between the Cenomanian and the Campanian. The anticlockwise rotation of these rocks must have been followed by a clockwise rotation of at least 60° to account for the observed clockwise declination anomalies.

The history of geomagnetic polarity reversals in the Cenozoic and Late Mesozoic is well known since the Late Jurassic (Oxfordian). A continuous record of polarity has been derived for this time interval from the interpretation of oceanic magnetic anomalies. Most of the polarity chrons in this oceanic record have been verified and dated in coordinated magnetostratigraphic and biostratigraphic studies. This has led to the generation of progressively refined and improved geomagnetic reversal time-scales that provide a framework for absolute dating of palaeontological zonations. By serving as a basis for statistical analysis of reversal frequency they provide information relevant to processes in the Earth's core. The rate of reversals since the Late Cretaceous shows a steady increase on which a cyclical variation appears to be superposed. A stochastic model for reversals predicts a Poisson distribution of polarity interval lengths. The polarity time scales contain many fewer short (± 50 kyr) polarity chrons than a Poisson distribution, and it has been suggested that a gamma renewal process with index greater than unity is a more appropriate statistical model. The statistical arguments give no convincing reason for abandoning the model and other, physical reasons must be sought to explain the incompleteness of the reversal record. The discovery and verification of short chrons in the oceanic record may best be investigated by deep-tow magnetometer surveys. The reversal history before the Late Jurassic is not well known. Magnetostratigraphy in coeval Early Jurassic sections has not given correlatable records and it has not been possible to compile a definitive polarity sequence. Evaluation of geomagnetic polarity history for the Early Mesozoic and the Palaeozoic will require unambiguous magnetostratigraphy in well-dated sections where verification of the polarity pattern is possible at the fossil zone or stage level.

This paper examines the origin of the 20-50-cm-thick set of whitish limestone beds found immediately underlying the Cretaceous-Tertiary (K-T) boundary in Umbrian sections. On the basis of isothermal remanent magnetization (IRM) acquisitions and thermal demagnetization experiments, it is argued that the white beds were deposited under the same conditions as the underlying pink beds and that the anomalously low IRM intensities found in the white beds resulted from the reduction of hematite in the originally pink beds followed by the removal of the Fe(2+) ions. The whitening of the beds is ascribed to the consequence of downward infiltration of reducing waters resulting from the large quantity of organic matter produced by the extinctions at the K-T boundary. The white interval below the K-T boundary is thus compatible with the hypothesis of impact-triggered mass extinction.

The common ferromagnetic minerals have distinctive, characteristic coercivities and thermomagnetic properties. The analysis of the acquisition curve of isothermal remanent magnetization (IRM) is a useful but often ambiguous diagnostic technique. For a more conclusive interpretation, IRM acquisition must be combined with subsequent thermal demagnetization of the IRM. A modification of this method is proposed as a more powerful analytical technique. Different coercivity fractions of IRM are remagnetized in successively smaller fields along three orthogonal directions. The thermal demagnetization of each orthogonal component of the composite IRM is then plotted separately. This method often gives a clearer interpretation of the ferromagnetic mineral content of a rock. Examples are described for limestone and sandstone samples.

This second edition of Fundamentals of Geophysics has been completely revised and updated, and is the ideal geophysics textbook for undergraduate students of geoscience with an introductory level of knowledge in physics and mathematics. It gives a comprehensive treatment of the fundamental principles of each major branch of geophysics, and presents geophysics within the wider context of plate tectonics, geodynamics and planetary science. Basic principles are explained with the aid of numerous figures and step-by-step mathematical treatments, and important geophysical results are illustrated with examples from the scientific literature. Text-boxes are used for auxiliary explanations and to handle topics of interest for more advanced students. This new edition also includes review questions at the end of each chapter to help assess the reader's understanding of the topics covered and quantitative exercises for more thorough evaluation. Solutions to the exercises and electronic copies of the figures are available at www.cambridge.org/9780521859028.