bluuebunny/crossref_metadata_2025 huggingface dataset:
Geochemical and isotopic variation along the Mid-Atlantic Ridge axis from 79°N to 0°N
Authors: Schilling Jean-Guy
Published: April 2015

The first hint of the existence of the Mid and how gradually its topographic definition and latitudinal extent were made known in the 1850 to 1900 period. In the summary volumes of the “Challenger Reports,” the MAR is shown to be continuous from 50°N southward throughout the South Atlantic. The widest part of the Mid-Atlantic Ridge, between 30°N and 50°N, was referred to as the Dolphin Plateau. The narrow portion south of 30°N was named the Connecting Plateau, as it was thought to connect the Dolphin Plateau to the Challenger Plateau running down the center of the South Atlantic; but Murray and Peake (1904) pointed out that this connection is broken for a short interval at the Equator, because of deep soundings found there (evidently a first hint of the Equatorial Fracture Zones). The name of the Mid-Atlantic Ridge seems to first appear in Murray and Hjort’s (1912) classic book, The Depths of the Ocean, and was probably coined in the 1904–1912 period. the

Atlantic Ridge occurred in the period of 1849-1853 after soundings were made by US Navy Ships under the direction of USN Lieut. M. F. Maury. Congress had authorized the effort to assist Maury in his study of winds and currents and apparently also in the preparation of laying the first Trans-Atlantic cable. It was first called the Telegraphic Plateau. Murray and Peake (1904) gave a detailed account and colorful anecdotes surrounding the initial discovery of the Mid-Atlantic Ridge, Only in the 1950s was it discovered that
The geology of North Atlantic transform plate boundaries and their aseismic extensions
Authors: Fox Paul J. , Gallo David G.
Published: April 2015

By the late 1960s the morphotectonic character (Menard and Dietz, 1952; Menard, 1955, Heezen and others, 1964a,b) and kinematic significance (Wilson, 1965; Sykes, 1963, 1965, 1967; Stover, 1966; McKenzie and Parker, 1967; Morgan, 1968; LePichon, 1968) of fracture zones had been established and it was generally accepted that fracture zones are comprised of two principal tectonic elements; a seismically active zone of strike-slip tectonism linking two offset ridge axes called the transform fault or ridge-transform-ridge (RTR) plate boundary, and two aseismic limbs, continuous with each end of the transform fault, that represent the fossil trace of the transform. With this basic tectonic framework developed, investigators turned their attention to elucidating the details about how RTR plate boundaries behave in time and space and how this fundamental plate boundary conditions the accretion of oceanic lithosphere. There are important kinematic and geometric variables that are likely to condition and govern the structural and petrologic processes operating along a RTR plate boundary. These variables will be reviewed before we summarize the salient properties of the slowly-slipping RTR plate boundaries found in the North Atlantic.

Kinematic considerations of the plate tectonic hypothesis mandate that strike-slip motion must occur along an oceanic transform fault at a rate equal to the rate of plate separation of the associated ridge segments. The total opening rates of the ridge segments comprising the mid-oceanic ridge system vary from less than 1.7 cm/yr (Arctic Ridge) to approximately 18 cm/yr southern
The present plate boundary configuration
Authors: Vogt Peter R.
Published: April 2015

The discovery of an elongated geological feature is soon followed by studies of how the feature varies along its length. This is true of the Mid-Oceanic Ridge (MOR), the longest feature on our planet, although there has been far more work done on transverse than on longitudinal variations. Since the MOR accreting plate boundary (the zero-age isochron) and older isochrons are offset by many transform faults, a precise longitudinal profile must be assembled from numerous detached segments. In practice such profiles are generally constructed from transverse crossings. By contrast, long continuous transverse profiles can be obtained (e.g., Klitgord and Schouten, this volume) by following flowlines of plate motion, keeping a constant distance from adjacent fracture zones (FZ). Early ignorance of the existence of fracture zones led to misinterpretation of transverse bathymetric profiles across the Mid-Atlantic Ridge (MAR), e.g., in terms of spurious elongated terraces (the “Upper,” “Middle,” and “Lower Step” of Heezen and others, 1959). Even the later TransAtlantic Geotraverse (TAG) profiles (Rona, 1980) were collected without knowledge of the late Cretaceous/early Tertiary FZ bend, and consequently cross fracture zones at low angles and jump from one flowline to another.

The first longitudinal profile of the MAR (both North and South Atlantic) showed regional variations in minimum crestal depth (Stocks and Wï¿½st, 1935). Later, Heezen and others (1959) plotted rift valley depth, west and east rift mountain heights, and other bathymetric parameters from 14°N to 55°N. Heirtzler and LePichon (1965) examined the along-strike amplitude variation of the central
Geoid undulations mapped by spaceborne radar altimetry
Authors: Vogt P. R.
Published: April 2015

Were the ocean and atmosphere at rest, the sea surface would describe a surface of constant gravitational potential. This equipotential surface is called the geoid. Removal of a simple reference surface (spheroid or ellipsoid) leaves geoid anomalies, or undulations of various wavelengths. Geoid undulations range in amplitude from many tens of meters for the broad high in the Azores-Iceland area (Fig. 1) down to a few centimeters for the local effect of a small seamount or minor fracture valley. The positive density anomaly (mass excess) represented by a seamount acts as a small “moon” on the ocean floor by pulling the sea towards itself, producing a bump (undulation) on the ocean surface. Conversely a sea-floor valley produces a dimple on the ocean surface.

The gravity field is the normal component of the gradient of the potential. Thus, gravity anomalies (Rabinowitz and Jung, this volume) are a measure of the spacing between the geoid and neighboring potential surfaces. The slope of the geoid is described by the Vertical Deflection (V.D.), a small angle (rarely more than 0.5′ arc) between the normal to the reference ellipsoid (a function of latitude only) and the normal to the geoid (the local astronomic vertical). Both geoid and V.D. can be calculated from gravity data (Rice, 1952; Brammer, 1979). Using a gyro- stabilized telescope, von Arx (1966) measured a V.D. of 60 to 90 (±12) arc sec over the Puerto Rico Trench. This pioneering work in direct measurement of V.D. was supplanted by
Magnetic anomalies and crustal magnetization
Authors: Vogt Peter R.
Published: April 2015

The study of geomagnetism is, after geodesy, the oldest of the geophysical sciences and, with the advent of plate tectonics, perhaps the most important. European mariners were using compasses, probably of the “floating lodestone” type, by no later than the treatise by Alexander Neckam in the year 1187. The GSA centennial thus also marks eight centuries of geomagnetic measurement. When Columbus measured magnetic declination (variation) at several points on his 1492 voyage, he made the first “geophysical transect” across the Atlantic Ocean, some 3½ centuries before the first bathymetric profile was drawn. Of course, Columbus was actually prospecting the earth’s core, not the oceanic crust. Geomagnetic field strength was first measured by Gauss in 1832, but continuous, accurate measurement suitable for mapping magnetic anomalies of geologic origin was only achieved a century later. However, the first magnetic chart detailed enough to resolve magnetic anomalies of geologic origin in the western Atlantic (Bermuda; Cole, 1908) depicted anomalies of declination, not intensity. Cole referred to the anomalies merely as “sources of considerable local disturbance.” Declination anomalies measured already in 1786 in Icelandic harbors by P. Lövenörn (published in 1799) were correctly attributed to magnetic effects of iron-rich minerals in nearby basalts (Kristjánsson, 1982).

Continuous magnetic field strength measurements began after the end of World War II, which had stimulated the development of airborne magnetometers to detect submarines. In 1948 a U.S. Navy self-orienting fluxgate magnetometer was converted to a “towed fish” at Lamont Observatory (Heezen and others, 1953). That year the
Petrologic and geochemical evolution of pre-1 Ma western North Atlantic lithosphere
Authors: Bryan W. B. , Frey F. A.
Published: April 2015

The Mid-Atlantic Ridge in the central North Atlantic was the source of the first deep-sea basement rocks to be studied in detail (Nicholls and others, 1964; Muir and Tilley, 1966; Miyashiro and others, 1969). The historical background and compositional details of basalts recovered from the modern Mid-Atlantic Ridge are discussed by Schilling and by Melson and O’Heara in other chapters in this volume. Some of the first recoveries of Old oceanic basement rocks were obtained by drilling in the western North Atlantic (Ayuso and others, 1976; Bryan and others, 1977). In this paper we summarize (1) the characteristics of pre-1 Ma lithosphere in the western North Atlantic and (2) inferences about its geologic evolution during the past 200 m.y. Such a review is subject to some serious limitations. Very large areas of the North Atlantic remain unsampled by drilling, and many crustal rocks that have been recovered may be too altered to yield definitive information about their magmatic geochemistry. Also, the deepest crustal penetrations in the North Atlantic have failed to reach the base of layer 2, which is composed almost entirely of extrusive basalts. The nature of the deeper levels of layer 2 and of the (presumably) gabbroic layer 3 must be inferred from samples recovered from a few holes apparently drilled in anomalously thin layer 2, from dredged samples recovered in fracture zones, or by extrapolation of observations at site 504 which was drilled into the deeper levels of layer 2 near the
Mid-plate stress, deformation, and seismicity
Authors: Zoback Mary Lou , Nishenko Stuart P. , Richardson Randall M. , Hasegawa Henry S. , Zoback Mark D.
Published: April 2015

The state of stress in the lithosphere is the product of a variety of forces that act on the plates. The interior region of a plate may be stressed by large-scale plate-driving forces, local density contrasts in'the plate, regional thermal anomalies, the residual effects of past tectonic events, or load redistribution caused by glaciation, volcanism, or erosion/sedimentation. Investigation of the tectonic in-situ stress field can enhance our understanding of mid-plate deformation and may also elucidate the relative importance of the various forces acting on the plates.

To this end we have assembled deformation, seismicity, and stress data for the mid-plate region of the North American plate. Our area of study covers continental North America and the oceanic portions of the North American plate, excluding zones of deformation along the plate boundaries. Excluded are the Mid- Atlantic Ridge and the shear and subduction boundaries associated with the Caribbean plate (covered by other chapters in this volume) and the entire western Cordillera of Canada and the United States (from the eastern Rocky Mountains front to the Pacific coast). The state of stress in the broad, intensely deformed western part of the North American plate is considered to be influenced primarily by local processes related to Pacific-North American plate interaction.

Numerous techniques and stress indicators provide information on the principal tectonic stresses in the crust and, for the oceans, the mantle lithosphere. Compilations of available data show that stress orientations inferred from different methods or techniques quite often agree within the accuracy
Seismic structure of the ocean crust
Authors: Purdy G. M. , Ewing John
Published: April 2015

Several notable papers have been published recently that review our knowledge of the seismic structure of oceanic crust (e.g. Christensen and Salisbury, 1975; Tréhu and others, 1976; Kennett, 1977; Lewis, 1978; Ewing and Houtz, 1979; Spudich and Orcutt, 1980a; Houtz, 1980; White, 1984). To reduce duplication of those previous efforts, this paper will emphasize evaluation of the resolving power of existing seismic data in the Western Atlantic Ocean and will compare this evaluation with the scale of local and regional variations in crustal structure predicted by reasonable geological models. We shall not focus attention on the relationship between the seismic and petrological structure of the crust and will use the terms Layer 2, Layer 3, and Moho only in their original seismic sense. Layer 2 is a region of high velocity gradients (perhaps as high as 3s−1 at its upper surface), with velocities in the range 3 to 6.5 km/s that extends from the seafloor to depths of 2–3 km. (We neglect sediments in this chapter, restricting ourselves to the igneous crustal column.) Layer 3 is typically about 4 km in thickness, has velocities of 6.7 to 7.2 km/s, and is characterized by low velocity gradients, generally less than 0.1 s−1. The Mohorovicic discontinuity (Moho) is actually a transition zone varying in thickness from a few hundred meters (or less) to 1–2 km and is almost always the most prominent seismic characteristic of the ocean crust. Velocities increase by about 1 km/s
Plate kinematics of the North Atlantic
Authors: Srivastava S. P. , Tapscott C. R.
Published: April 2015

The problem of determining the past relative positions of continents now situated around the North Atlantic has been a challenge to geologists and geophysicists for many years. Yet a solution acceptable to all does not exist. Since the acceptance of the idea of continental drift, a number of reconstructions has been published. The earlier reconstructions, prior to the birth of the plate tectonic theory, were done qualitatively in order to match morphological features of the continental margins (Choubert, 1935) or geological boundaries on land. The first quantitative fit of the North Atlantic was published by Bullard and others (1965) who described the motion between continents or plates as angular rotation about a set of poles. Since then a number of papers have been published showing reassembly of continents during various stages of evolution of the North Atlantic using magnetic anomalies in the oceanic regions as isochrons or plate boundaries and fracture zones as the direction of motion between plates. The most notable of these are by Pitman and Talwani (1972), Laughton (1971, 1972), Williams (1975), LePichon and others (1977), Sclater and others (1977), Kristoffersen and Talwani (1977) , Talwani and Eldholm (1977), Kristoffersen (1978), Srivastava (1978, 1985), Olivet and others (1981), Unternehr (1982), Nunns (1983), and Vink (1982, 1984).

Most of these reconstructions are based on a limited amount of geophysical information from one or more regions of the North Atlantic available at the time of publication. Thus different criteria and assumptions had to be made
Plate kinematics during the last 20 m.y. and the problem of “present” motions
Authors: Vogt P. R.
Published: April 2015

While increasingly precise plate reconstructions are being made at intervals of the order 10 m.y. for times corresponding to well-defined magnetic lineations (Klitgord and Schouten, this volume; Srivastava and Tapscott, this volume), much less is known about possible irregularities in plate motions over shorter time scales. Obviously the most recent geologic past offers the best hope for detecting any “kinematic fine structure.” This paper discusses the last 10–20 m.y., with special emphasis on the “present,” which, of course, is not an instant but involves a finite averaging interval, whether several years for geodetic or earthquake measurements, or several million years for a measurement based on magnetic anomalies. The chapter begins with Wegener’s attempts to measure continental drift, reviews the geodetic measurements on Iceland where the MAR (Mid-Atlantic Ridge) plate boundary is exposed, and discusses the latest attempts to measure plate motions over the “historical” present (several years time scale). The latter field is just being launched (see Anderle, this volume), and whatever is written today will soon be outdated. The main part of this paper is devoted to the question of relative and absolute plate motions over the “geological” present (m.y. time scale) resolved by magnetic lineations, transform faults, and hotspot traces. There is very little information about variability of plate motion over time scales intermediate between the “historical” and “geological” present. A case is made for significant geologically “short-period” (a few m.y.) global fluctuations in rates of plate motions over the last 10–20 m.y. The latest episode began
Surficial sedimentary processes revealed by echo-character mapping in the western North Atlantic Ocean
Authors: Laine E. P. , Damuth J. E. , Jacobi Robert
Published: April 2015

The development of high-resolution precision depth recorders (PDR) in the 1950s (Luskin and others, 1954; Knott and Hersey, 1956) not only permitted continuous and accurate measurement of ocean depths, but also provided a new tool for the study of deep-sea sedimentation processes. Changes in acoustic reflectivity and microtopography of the seafloor as observed on these early 12-kHz PDR records were found in many cases to be caused by erosional/depositional processes (Heezen and others, 1959). One of the first studies helped to confirm the existence of the Western Boundary Undercurrent (WBUC), a thermohaline, contour-following bottom current which is important in shaping the continental rise off eastern North America (Heezen and others, 1966; see also McCave and Tucholke, this volume). As part of this study, Hollister (1967) constructed an “echo character” map of the continental rise off Nova Scotia by classifying and mapping the regional changes of acoustic reflectivity observed on 12-kHz echograms. This map suggested that certain types of echoes (e.g. hyperbolae and prolonged echoes) were apparently reflected from sand waves or other depositional bedforms created by the WBUC. Regionally, these echo types were thought to be aligned parallel to bathymetric contours beneath the WBUC axis (Heezen and others, 1966; Hollister, 1967; Hollister and Heezen, 1972). Although subsequent and more detailed studies using much better quality 3.5-kHz echograms have shown that the regional pattern of echo character along the Nova Scotian margin is primarily downslope rather than along slope, this pioneering work by Hollister
A Jurassic to recent chronology
Authors: Kent Dennis V. , Gradstein Felix M.
Published: April 2015

We present an integrated geomagnetic polarity and geologic time scale for the Jurassic to Recent interval, encompassing the age range of the modern ocean floor. The time scale is based on the most recent bio-, magneto-, and radiochronologic data available.

The biostratigraphic bases for Jurassic, Cretaceous, and Cenozoic time-scales are discussed extensively elsewhere (e.g., Gradstein, this volume; Van Hinte, 1976b; Hardenbol and Berggren, 1978; Berggren and Van Couvering, 1974; Van Couvering and Berggren, 1977). Emphasis is placed here on magnetochronology and its integration with biochronology in the derivation of an internally consistent geologic time scale. The binary signal of normal and reversed geomagnetic polarity has little intrinsic absolute time value (ordinal scale), but it can be used to measure time according to its radiochronologic calibration (cardinal scale). The standard magnetic reversal sequence has a correlatable, characteristic pattern and is demonstrated to be continuous from numerous marine magnetic anomaly profiles from the world ocean. The reversal sequence is recorded by lateral accretion in sea-floor spreading and vertical accumulation in sedimentary or lava sections, allowing independent checks on the completeness and relative spacing of the reversal sequence as well as the opportunity to apply an assortment of geochronologic data for calibration. Although different phenomena and assumptions are invoked in their derivation, both magneto- and bio- chronologic time estimates involve indirect assessment according to calculated rates of sea-floor spreading, sedimentation, and biotic evolution. These extend the application of the relatively few reliable radiometric dates available, so that a continuous
Oceanic particles and pelagic sedimentation in the western North Atlantic Ocean
Authors: Honjo Susumu
Published: April 2015

Deep-sea sediment is formed by the accumulation of oceanic particles originally generated elsewhere. A large part of oceanic sedimentation is due to biological activity in the oceanic surface layers. Some lithogenic particles, however, are transported long distances from interiors and edges of continents into the pelagic environment. Transportation pathways of oceanic particles usually are complex, involving biological, chemical, and physical processes. The time necessary to complete such pathways varies from instantaneous to hundreds of years. Not all oceanic particles settle as permanent members of the deep-sea sediment. Biogenic detritus in particular can be dissolved, decomposed, or changed in its properties. Refractory particles increase their relative proportions in pelagic sediments as a result of such alteration through time.

There are two major kinds of marine particles in oceans, lithogenic and biogenic. Lithogenic particles are mostly layered or framework aluminosilicate minerals such as clay, quartz, and plagioclase. Only fine particles—clay and silt-size—are transported to the pelagic ocean floor in quantity, except for some large ice-rafted boulders. Lithogenic particles are refractory and therefore become permanent constituents of the pelagic marine sediment without significant chemical alteration. They are transported from American, European, and African landmasses to the open North Atlantic Ocean by processes such as river outflow, coastline erosion, and glacial erosion. Significant amounts of fine refractory particles are transported from the continental shelves and slopes of both sides of the North Atlantic by advective processes to the interior of the ocean basins (e.g., McCave and Tucholke, this vol.). In boreal and Arctic latitudes,
Mass wasting in the western North Atlantic
Authors: Embley Robert M. , Jacobi Robert
Published: April 2015

The balance between processes of slope degradation and slope aggradation on the ocean floor is constantly shifting. Along “active” continental margins, tectonic effects can provide large amounts of sediments to submarine slopes and they also can trigger frequent large slope displacements. Along “passive” continental margins such as that of eastern North America, the aggradation and degradation processes probably operate at a slower rate; they are stimulated primarily by changes in sediment supply caused by climatic changes or by occasional intraplate earthquakes.

The North American continental margin has long been a “type area” for studies of submarine sedimentary features and the processes that formed them, including canyon incision (Veatch and Smith, 1939), turbidity currents (Heezen and Ewing, 1952), and contour currents (Heezen and others, 1966). Although slumping has always been considered an important process on the upper continental slope and in canyon heads, only within the past few years has it been recognized that slumps and slides are ubiquitous on the continental slope and upper rise and that mass- wasting products are an important component of the deep-water Pleistocene sedimentary sequence (Embley and Jacobi, 1977; Knebel and Carson, 1979; Embley, 1980). It also has only recently been appreciated how important such factors as jointing and biological erosion are to failure of consolidated sediments on steep slopes in the deep ocean. This chapter describes our present state of knowledge of mass-wasting processes in the provinces of the North American margin and basin.With increasing oil exploration in deeper water,
The crest of the Mid-Atlantic Ridge
Authors: Macdonald Ken C.
Published: April 2015

By 1854, there were enough deep ocean soundings to allow M. F. Maury to draw the first bathymetric chart of the North Atlantic Ocean in 1855. He defined the great shoaling “middle ground” of the Atlantic basin, later to be known as the Mid-Atlantic Ridge (MAR). It is interesting to note that during the Challenger expedition (1872–1876), Sir Wyville Thomson predicted, on the basis of water temperatures alone, that the MAR is a nearly continuous topographic barrier dividing the Atlantic Ocean. In the 1950s, continuous echogram profiles revealed the presence of a deep median valley along the axis of the MAR (Heezen and others, 1959; Hill 1960). Seismicity (Gutenburg and Richter, 1954) and the occurrence of youthful basaltic lavas on the MAR (Shand, 1949) indicated that the median valley was a geologically active rift zone. Heezen (1960) first suggested that extension across the rift valley might be responsible for accretion of basaltic oceanic crust and the drift of the continents, but his contention that this rifting resulted from expansion of the earth was incorrect. High heat flow measured on the MAR (Bullard and Day, 1961) and large axial magnetic anomalies (Heezen and others, 1959) were added to seismicity and recent volcanism as enigmas associated with the rift valley.

In his seafloor spreading hypothesis Hess (1962) elegantly explained these puzzling observations in a unified model, and the bilateral symmetry of magnetic anomalies found on other midocean ridges (Vine and Matthews, 1963; Vine, 1966)
Paleogene biofacies of the western North Atlantic Ocean
Authors: Premoli-Silva Isabella , Boersma Anne
Published: April 2015

This chapter presents an overview of the distributional patterns of the major calcareous and siliceous microfossil groups of the Paleogene in the western North Atlantic Ocean. From these patterns the evolution of open marine biofacies through space and time can be reconstructed. At first sight, this would seem to be an easy task, considering the large number of Deep Sea Drilling Project (DSDP) and commercial drill holes available, as well as the numerous piston cores which have retrieved pre-Quatemary sediments from the North Atlantic Basin in the last 25 years. However, two principal problems complicate detailed biofacies reconstructions. The first, and most frustrating, is the incompleteness of the Paleogene Stratigraphie record from the North Atlantic. Existing deep-sea cores are complicated by extensive drilling gaps, large sedimentary hiatuses, and/or the absence of age- diagnostic fossils. The second problem is the difficulty of intercorrelation of the many different Paleogene biostratigraphic zonations and time-scales.

Since about 1969, biostratigraphic schemes based on the major calcareous and siliceous fossil groups have been extensively refined or newly constructed. Because different biostratigraphic schemes and taxonomies have been used by different researchers in earlier studies of North Atlantic cores, the majority of Paleogene calcareous DSDP sections and many of the other boreholes from the western North Atlantic have been restudied and rezoned during preparation of this chapter. The sections have all been recalibrated according to the time scale and zonation shown on Plate 1. As a result, much of the early Paleogene can now be accurately correlated and
Paleogeographic and paleobathymetric evolution of the North Atlantic Ocean
Authors: Tucholke Brian E. , McCoy Floyd W.
Published: April 2015

The principles of plate tectonics developed over the past 25 years provide a conceptual and predictive framework that allows relatively detailed reconstruction of the geologic history of ocean basins. Excluding extra-terrestrial effects, tectonics of the earth’s crust is the primary mechanism controlling the evolution of the geologic record (Fig. 1). Secondary mechanisms related to tectonics through direct or indirect pathways create complicating and less easily predicted signals in the oceanic sedimentary record. In this paper we use plate-tectonic concepts to reconstruct the morphology and geologic conditions of the North Atlantic Ocean basin in twelve time slices from the Late Triassic (Atlantic closure) to the present. The specific time slices selected represent a balance between depiction of major tectonic/oceanographic “events” and portrayal of important, longer-term geologic episodes with relatively stable circulation and depositional patterns. Within each time-slice we have superimposed the major tectonic patterns, seamounts, oceanic circulation, and sedimentary lithofacies. While we have made this compilation as complete and coherent as possible within the time available, we point out that the synthesis is still a limited effort compared to the vast literature on the geology of the North Atlantic. Furthermore, because we have made the compilations at a small scale, many generalizations are necessary. Despite such deficiencies, these “first-generation” maps provide a good summary of our present knowledge of the geologic conditions in each time-slice.

Whole-earth wiring diagram summarizing the hierarchy and interaction of the most direct controls on sedimentation and erosion in the North Atlantic basins. Numerous less direct
Paleoceanography and evolution of the North Atlantic Ocean basin during the Jurassic
Authors: Jansa Lubomir F.
Published: April 2015

Jurassic paleoceanographic conditions in the Atlantic Ocean can be reconstructed from information derived from the preserved sediments and the biogeography of the fossil fauna, from reconstructed basin paleogeographies (shape, size, depth, and positions of landmasses and seaways), and from indications of paleotopography, paleobathymetry, and paleoclimate. An extensive literature exists about Mesozoic sediments and biota of the Teth- yan realm from Europe and parts of northwestern Africa. Such data, supplemented by my own field studies in central and southern Europe and Morocco, and by studies of oil exploratory wells on the eastern North American margin, were major sources of the Jurassic lithofacies data summarized in Plate 9. Additional data comes from Continental Offshore Stratigraphie Test (COST) wells drilled on the eastern North Atlantic U.S. shelf and summarized in U.S. Geological Survey reports. However, the most important information is derived from the Deep Sea Drilling Project (DSDP), which drilled numerous deep holes in the deep North Atlantic basin and its continental margins.

Greater insight into the early evolution of the North Atlantic basin was provided particularly by DSDP Leg 79, which drilled four holes on the deep-water Moroccan continental margin, at water depths of 3151 to 3992 m (Sites 554 to 547, Hinz and others, 1984; Plates 2, 9). These holes provide the only information about the Lower Jurassic in the central North Atlantic basin, and are used extensively in this chapter on the assumption that basin development was similar on both sides of the Atlantic. This assumption is supported
Cretaceous paleoceanography of the western North Atlantic Ocean
Authors: Arthur Michael A. , Dean Walter E.
Published: April 2015

In this paper we summarize available information on the Cretaceous lithostratigraphy and paleoceanography of the western North Atlantic. The data and some of our interpretations draw in large part on papers published in the Deep Sea Drilling Project (DSDP) volumes. We have attempted to cite relevant references when possible, but space limitations make it difficult to give proper credit to all sources; we apologize for any omissions.

Organic carbon (Corg) and carbonate (CaCO3) analyses were tabulated for each site from papers in the DSDP Initial Report volumes and other published works (e.g., Summerhayes,1981). Corg, CaCO3, and non-CaCO3 mass accumulation rates (MARS) were calculated using core by core averages of component percentages for the more continuously cored sites; core averages for wet bulk density and porosity (from DSDP data files); biostratigraphies of de Graciansky and others (1982), Roth and Bowdler (1981), and Cool (1982); and the time scales of the Decade of North American Geology (Palmer, 1983; Kent and Gradstein, this volume) or Harland and others (1982; see Plate 1).

Backtracked paleodepths for western North Atlantic DSDP Sites from Tucholke and Vogt (1979) with the revised stratigraphy of de Graciansky and others (1982) were used in plotting Corg and CaCO3 in Figures 2, 3,4 and 5 (see also Thierstein, 1979).

Backtracking curves of seafloor paleodepth versus age (Sclater and others, 1977; Tucholke and Vogt, 1979) for selected western North Atlantic DSDP sites. Average CaCO3 concentrations per core are shown by code number
Tertiary paleoceanography of the western North Atlantic Ocean
Authors: Tucholke Brian E. , Mountain Gregory S.
Published: April 2015

The Tertiary paleoceanography of the North Atlantic Ocean is known in greater detail than that of earlier periods because the upper part of the geologic record is more accessible to geologic sampling and geophysical observation. The Tertiary record also appears more complex, partially because of the greater data density. Nonetheless, there are large segments of this record about which we know relatively little. In particular, the Paleocene and Oligocene sedimentary records are not well represented on the shallow continental margin because erosion during sea-level low- stands removed much of the section. Equivalent records in the deep basin are poorly known because low sediment accumulation rates or erosion by abyssal currents resulted in preservation of abnormally thin sections. Many of these sections also are poorly sampled in existing Deep Sea Drilling Project (DSDP) drillsites.

In contrast to earlier periods, the Tertiary North Atlantic Ocean basin was strongly affected by the action of abyssal contour-following currents, particularly following Eocene time. The currents intermittently caused extensive seafloor erosion along the continental margins and they decorated the basin with large sediment drifts. The development and patterns of surface currents, and to some extent deep currents, are dealt with from a paleobiogeographic viewpoint by Berggren and Olsson (this volume). In this paper, we summarize the Tertiary paleoceanography based on other information from the geologic record, principally seismic Stratigraphic interpretations that are correlated with rock stratigraphy at DSDP and other boreholes. These data are interpreted within the framework of the plate-tectonic evolution of the North Atlantic
The juridical ocean basin
Authors: Knauss John A.
Published: April 2015

Since World War II, claims of national jurisdiction over areas of the ocean and its resources have been moving seaward in an irregular and often confusing way. The trend has been aptly described as “creeping jurisdiction.” A strong case can be made for the proposition that the fisheries and mineral resources of the ocean have been the primary driving force for these claims.

“Every nation is free to travel to every other nation and to trade with it,” wrote Hugo Grotius, often called the father of international law, in Mare Liberum published in 1609. He viewed the sea as res communis, a common possession for all and the private property of none. His views slowly gained international acceptance, in part because it was indeed in the general interest to encourage common use of the ocean for trade, and there was little in the ocean that could be subject to private property rights. Even fish were sufficiently abundant, given the crude methods of catching, so that whatever regulations were necessary could be con-trolled by local agreements.

The Grotius view was generally unchallenged for more than three centuries, a few international fisheries and whaling agreements notwithstanding. The first significant challenge came in 1945 when President Truman declared U.S. jurisdiction “with respect to the natural resources of the subsoil and seabed of [its] continental shelf’ and implied that other coastal nations should do likewise. The reason, of course, was oil. Although “under water”ï¿½ oil wells had been drilled from piers or slant drilled
The Arctic Region
Authors: Grantz Arthur , Johnson Leonard , Sweeney J. F.
Published: April 2015

The synthesis of the geology of the North American plate in the Arctic Ocean (Fig. 1) will cover the area between the Arctic (Nansen) Mid-Ocean Ridge and its extension into the Siberian continental shelf on the Eurasian side, and the Arctic continental margins of Greenland, Canada, Alaska, and Chukotka on the Amerasian side (Fig. 2). This predominantly oceanic region consists of a complex of ridges and plateaus of diverse and, in part, enigmatic origin, several oceanic deeps, and thick continental-rise sedimentary prisms. The margins of the region consist of an active spreading center along the Nansen Ridge, an active fracture zone off northern Greenland, and older passive margins off Canada, Alaska, and Chukotka.

The bathymetry and geography of all the world’s ocean basins and their borderlands are now known— except for the Arctic. Although apparently detailed maps of this region have been published, the soundings supporting the bathymetry shown are sparse, resulting in contours that only grossly reflect the true bottom physiography. In the Arctic, geologic knowledge is so limited that suggestions about the nature of the Alpha-Mendeleev Ridge, for example, range from continental crust to fossil subduction zone or extinct spreading ridge. The hostile environment over most of the Arctic Ocean has thus far prevented a systematic study of this important subbasin of the world ocean. Despite numerous expeditions into its marginal regions and repeated attempts to visit and probe its interior, the deep Arctic Ocean remains a frontier for geologic exploration.

Knowledge of the Arctic is important for more than just solving Arctic problems, and its plate tectonic evolution is closely linked to the evolution of the Atlantic and Pacific basins as well as to the continents that almost surround it.
The western North Atlantic
Authors: Vogt Peter R. , Tucholke Brian E.
Published: April 2015

Although available space strongly limits the scope of this essay, we want it to serve variously as a prospectus of subject matter to be covered; an informal “minisynthesis” that geographically and topically delimits synthesis contents; a guide for authors; a definition of synthesis problems as well as scientific problems; a philosophical perspective; identification of relationships with other volumes; and finally, if we are successful, a stimulus to new research.
Continental platforms and basins of Canada
Authors: Stott D. F. , Aitken J. D.
Published: April 2015

This chapter is an extended annotated outline of the planned contents for the volume Continental Platforms and Basins of Canada that will form a part of the sixth edition of the Geology and Economic Minerals of Canada as well as part of the series on The Geology of North America that is being prepared in connection with the Decade of North American Geology. Sections preceded by arabic numbers will be chapters in the synthesis volume. Information on the Hudson Platform was provided by A. W. Norris, and information on the St. Lawrence platform was provided by B. V. Sanford. Their assistance is gratefully acknowledged.
The Eastern Pacific and Hawaii
Authors: Hussong D. M. , Winterer E. L. , Decker R. W.
Published: April 2015

This volume will synthesize and describe the geology of the eastern northern Pacific Ocean Basin, a particularly large, variable, and dynamic region that is far less studied than most continental areas. Interpretation of the geology and geophysics of oceanic regions has been remarkably useful for elucidating the geometry and history of large-scale tectonics, especially lithospheric plate tectonics. Because it is accomplished by remote sensing techniques that generalize sea-floor structures and features, the structural complexities that are observed onshore at road-cut scales are smoothed out in marine studies. Thus, in many ways the success of marine studies is that they observe the forest without the confusion of looking at each tree, while on land the geologist must try to interpret the forest by the more difficult task of generalizing the study of features with the relative size of the trees.

In the past few years the scales of onshore and offshore geologic studies have been converging. Continental geologists are using more geophysics and remote sensing, while marine geologists have tried to conduct higher-resolution studies.

This volume will strive to provide a comprehensive description of our present state-of-knowledge of marine geology. At the same time, we will emphasize two general objectives in our field, as follows:
The Precambrian
Authors: Hoffman P. F. , Card K. D. , Davidson A.
Published: April 2015

Phanerozoic tectonics have not affected the basic integrity of the North American continent. From time to time, sialic crustal elements have been rifted off, accreted onto, and slipped along the continent’s margins, but the vast continental interior has remained intact. Furthermore, the dimensions of the continent are little different now than they were in early Paleozoic time.

No such assertions can be made for the Precambrian of North America. The structure of the shield (Fig. 1) and buried basement of the interior platform (Fig. 2) is part of a continent-wide “tectonic collage” (Helwig, 1974) consisting of (1) five or more old cratonic elements having crystalline basement rocks of Archean age (3.8 to 2.5 b.y.) and remnants of Early Proterozoic (2.5 to 1.6 b.y.) cratonic cover: (2) anastomosing Early Proterozoic orogenic belts, mostly culminating between 1.9 and 1.8 b.y., that enclose and marginally affect the Archean cratons; (3) intracratonic igneous and sedimentary rocks, mairtly of Middle Proterozoic age (1.6 to 0.9 b.y.), that document consolidation of the various Archean cratons and Early Proterozoic orogens to form a vast composite craton here called “Proto-Laurentia;” and (4) the Grenville Orogen, culminating at about 1.1 b.y., which developed along the presumed southeast margin of Proto-Laurentia and may contain elements exotic to it. In addition, the bounding Phanerozoic orogens contain Middle and Upper Proterozoic rocks deposited marginal to Proto-Laurentia, as well as Precambrian rocks outboard of Phanerozoic sutures that may not belong to Precambrian North America at all.
Quaternary history of North America and adjacent oceans
Authors: Wright H. E. , Ruddiman W. F.
Published: April 2015

This Quaternary volume in the D-NAG series is to be topical rather than regional. The Quaternary is characterized by major environmental changes, and an understanding of the dynamic interactions of physical, chemical, and biological processes during the Quaternary can illuminate the operation of the natural world and the formation of the geologic record.

The book will be in two parts, the first of which will be concerned with the last deglaciation, approximately 18,000 to 6,000 years ago. For this interval, a wealth of information exists, closely controlled in chronology by radiocarbon dating. The approach here will be to focus on the response of natural processes to the climatic forcing that initiated the deglaciation, including (1) the relative sequencing of the various climatic responses and (2) the interrelations among these processes. The last deglaciation is the most accessible geologically and is representative of earlier major deglaciations, each of which brought to an end 100,000-year-long cycles of global climatic deterioration. Thus determining the speed, timing, and mechanism of the most recent deglaciation is fundamental to an understanding of the origin of these 100,000-year cycles that constitute the dominant rhythm of Quaternary climate.

The second part of the volume will treat the record prior to the last deglaciation—the Early and Middle Wisconsinan and previous glacial and interglacial cycles. The major discovery that many more climatic cycles are recorded in ocean cores than are thought to be evident in the terrestrial record and that the glacial intervals were consistently longer than the interglacials has added a new dimension to concepts of Quaternary climatic change.
The Cordilleran Orogen
Authors: Burchfiel B. C. , Lipman Peter W. , Eaton Gordon P. , Smith Robert B.
Published: April 2015

The Cordilleran sector within the conterminous United States extends from the offshore continental borderlands of the Pacific margin eastward as far as the Black Hills of South Dakota and the mountains of west Texas. It ranges from 800 to 1,600 km wide and is physiographically complex, consisting of high mountains, intervening lowlands, and plateaus that rise from the more gentle continental interior. The physiography of the Cordillera largely reflects the younger underlying structure, and the present orogenic belt has east and west limits coincident with its physiography.

As physiographically defined, however, (Fig. 1) the U.S. Cordillera thus includes diverse older sedimentary, igneous, and tectonic elements varying widely in age, geographic distribution, and geologic significance. Included are the margin of the Precambrian crystalline craton; an extensive cratonic sedimentary sequence developed along a passive margin in latest Precambrian and early Paleozoic time; a collage of terranes accreted to North America intermittently from middle Paleozoic time through the end of Mesozoic time; the igneous and tectonic record of successive plate-convergence events, probably beginning with Antler deformation in middle Paleozoic time and culminating in a fully developed Andean margin late in Mesozoic time; eastward Laramide migration of igneous and tectonic features related to plate convergence in early Cenozoic time; and gradual termination of subduction-related activity and inception of extensional and transform tectonic regimes later in Cenozoic time.

We have selected a time-slice organization for the syn-thesis of these diverse features. Ultimately, it is the grouping of events by their contemporaneity that permits an understanding of the kinematic and dynamic systems responsible for these events.
The Geologic Regions of Mexico
Authors: Ortega-Gutiérrez F. , Guerrero-García J. C.
Published: April 2015

The main objective of this volume will be to present the state of modern knowledge about the geology of the most important geologic regions of Mexico (Table 1). These regions were carefully selected on the basis of their geographic, chronological, and tectonic characteristics in order to enhance regional discussion and correlation with adjacent areas.

Mexico is a country endowed with an enormous variety of geological systems and mineral wealth. Doubtless, the geologic record of Mexico and its present tectonic activity will reveal many elements of basic importance to the resolution of some of the fundamental problems faced today by the science of geology.

In the recent geologic past, hundreds of thousands of cubic kilometers of volcanic material of felsic composition were “added” in a few million years to the Mexican continental crust. Where did this material come from? Was it extracted from the mantle, or from the fusion of older continental crust?

The answers to these sorts of questions could influence in a most definite way the presently debated theories explaining the petrologic evolution of the crust and mantle of the earth since the beginning of geologic history.

The study of recent tectonic activity in the Gulf of California and in the Acapulco Trench can be of primary importance to the understanding of those geologic processes that have shaped the lithosphere at least in Phanerozoic time, that is, ocean expansion and contraction.

This volume, as conceived, will deal with the geology of all Mexico: two billion years of geologic history written across two million square kilometers of exposed land.