Megascale geologic processes, such as earthquakes, tsunamis, volcanic eruptions, floods, and meteoritic impacts have occurred intermittently throughout geologic time, and perhaps on several planets. Unlike other catastrophes discussed in this volume, a unique process is unfolding on Earth, one in which humans may be the driving agent of megadisasters. Although local effects on population clusters may have been catastrophic in the past, human societies have never been interconnected globally at the scale that currently exists. We review some megascale processes and their effects in the past, and compare present conditions and possible outcomes. We then propose that human behavior itself is having effects on the planet that are comparable to, or greater than, these natural disasters. Yet, unlike geologic processes, human behavior is potentially under our control. Because the effects of our behavior threaten the stability, or perhaps even existence, of a civilized society, we call for the creation of a body to institute coherent global, credible, scientifically based action that is sensitive to political, economic, religious, and cultural values. The goal would be to institute aggressive monitoring, identify and understand trends, predict their consequences, and suggest and evaluate alternative actions to attempt to rescue ourselves and our ecosystems from catastrophe. We provide a template modeled after several existing national and international bodies.

Abstract Nearly continuous successions of late Proterozoic through Upper Devonian rocks are widespread in the western United States between the structural fronts of the Sevier and Sonoma orogens (Plates 2-1 to 2-6, 3-1 to 3-4). West of the Sonoma orogenic front in Washington, Oregon, northe Rn and southe Rn California, western Idaho, and western Nevada, rocks of this age are limited to many small areas of exposures, most of which are shown on Plate 3-5. East of the Sevier orogenic front, some large areas on the cratonic platform contain only partial stratigraphic records because of eithe R nondeposition or erosion. The western limit of mapped carbonate-shelf rocks can be obtained from only a few tectonic windows because those rocks disappear beneath thrust plates of western facies rocks of the same age or of younger and older rocks that moved eastward as much as 200 km or more during the Antler and later orogenies. Figure 1 shows Devonian and older paleotectonic features, major post-Devonian faults, and locations of the 12 generalized stratigraphic columns shown in Figure 2.

Abstract This volume is about the evolution of the North America Plate. It also briefly discusses the evolution of the Pacific Ocean and the Caribbean as they relate to North America. We will briefly sketch the present outlines of the North America Plate, review the plate-tectonic development of North America in a global context, and offer an overview of the major tectonic (Fig. 1) and geomorphic elements of our continent. Earthquakes are primary indicators of present plate boundaries and of plate motions that are related to these boundaries. The outline of the North America Plate is shown clearly on Figure 2, which is based only on the distribution of earthquakes occurring from 1977 to 1987. Note also the good definition of the Caribbean and Cocos Plates. In the Arctic regions (Fig. 3), earthquakes clearly outline the plate boundary along the mid-ocean ridge up to the north coast of Siberia. However, from there southward, across northeastern Siberia, the margin of the North America Plate is quite diffuse. Substantial intraplate earthquakes (Fig. 2) have occurred particularly within the United States. The 1811 to 1812 New Madrid (Missouri) and the 1866 Charleston (South Carolina) earthquakes exceeded magnitude 7 and occurred in areas that are still active today (Seeber and Armbruster, 1988; Hinze and Braile, 1988). New England is another area of intraplate seismic activity. According to Zoback and others (1986), the midplate stress of the North America Plate is compressive, with a maximum horizontal principal stress oriented northeast to east northeast. The stress field extends from the Rocky Mountain front to within 230 km of the Mid-Atlantic Ridge.

Abstract The recently developed Gravity Anomaly Map of North America is the product of a 12-year multinational effort to compile, critically edit, and merge gravity anomaly data on a continental and global scale (Gravity Anomaly Map Committee, 1987). This color-pixel map is printed on four quadrant sheets at a scale of 1:5,000,000 and includes a fifth sheet showing a color index map and data references. This map is the first at such a global scale to include several hundreds of thousands of precise bits of surface data of Canada, the United States, Mexico, and Central America, as well as other high-quality surface data from neighboring continental and oceanic areas. A 1:20,000,000 version of the map is shown on Plate 1A. The prospect of producing a gravity anomaly map of North America was formally advanced in 1975 by way of a cooperative agreement between the U.S. Geological Survey and the Society of Exploration Geophysicists. Before these cooperators linked to specialists of Canada, Mexico, and other countries, they agreed that an updated version of the gravity anomaly map of the United States should first be developed. In 1983, following publication of the U.S. map, the Society of Exploration Geophysicists and the U.S. Geological Survey concluded that the planned Centennial Map Series of the Decade of North American Geology program would be an excellent medium for publication of the gravity anomaly map of North America as well as the magnetic anomaly map of North America. The cooperating groups subsequently coordinated with the Geological Society of America, and the map was published in 1988.

Abstract Although the charting of anomalous variations in the Earth′s magnetic field to aid in the mapping of the Earth′s crust has been practiced for over a century, the development of aeromagnetic surveying technology has made it possible in recent decades to conduct surveys of extensive regions efficiently and precisely. These improvements in surveying instrumentation, procedures, and data processing made it possible to move from magnetic “anomaly hunting” to the preparation of regional total field contour maps. Aeromagnetic surveys have been conducted over limited areas for specific geologic objectives, with little attention paid to the possibility of compositing individual surveys into regional, small-scale maps for the study of continental-scale geologic features. However, interpretation of compilations of simplified near surface, aeromagnetic anomaly maps (e.g., MacLaren and Charbonneau, 1968), and high-level, broadly spaced profile surveys (e.g., Zietz and others, 1969; Sexton and others, 1982) has shown that small-scale, low-resolution magnetic anomaly maps of extensive regions can be very useful in mapping continental scale geologic features. Since the mid-1940s, airborne magnetic surveys have been conducted over vast regions of North America. The public availability of many of these surveys led to the preparation of national maps by Canada and the United States. The first 1:5,000,000 scale colored magnetic anomaly map of Canada (Morley and tohers, 1968) was prepared with hand-compilation techniques by the Geological Survey of Canada (GSC). Subsequent editions of the map have provided greater coverage and improved detail, leading to the fifth edition (Dods and others, 1987), which was produced by machine-compilation of digital data.

Abstract The seismic structure of the crust and upper mantle provides critical information regarding lithospheric composition and evolution. There are large variations in fundamental properties such as crustal thickness, crustal and upper-mantle velocity structure, and the depth to the lithosphere/asthenosphere boundary, which are interpretable in terms of processes that have formed and modified the lithosphere. Much of what we know about the seismic structure of the lithosphere has been accumulated over the past 30 years from seismic refraction profiles and surface-wave studies. Modern seismic refraction studies use denser arrays of seismic sources and recorders, improving the resolution and reliability of crustal models. Within the last 15 years, the deep-seismic reflection technique has been widely applied and has provided fundamental new insights into the structure and physical properties of the crust. Recently, seismic investigations have provided a fresh look at the properties of the Moho and the upper mantle where we may discover the “driving forces” of continental tectonics. In this discussion we define the lithosphere as the crust and the portion of the upper mantle above the seismic low-velocity layer (asthenosphere) that occurs at a depth of 60 to 200 km and is generally more evident in the shear-wave structure than in the compressional-wave structure. The low-velocity layer contrasts with the base of the crust (Moho), which is very pronounced in compressional-wave structure and is defined as the depth below which the seismic velocity (measured on a reversed seismic refraction profile) is greater than 7.6 km/s. Where the crust/mantle boundary has been examined in detail, it appears to consist of a laminated transition zone with a thickness of 2 to 5 km. The thickness of the Earth’s crust is highly variable; typical oceanic crust has a thickness of about 7 + 3 km (excluding the water column), and continental crust typically has a thickness of 25 to 50 km.

Abstract In creating Volume M, the Western North Atlantic region (Vogt and Tucholke, 1986a) for the Geology of North America series, we deemed it best from both oceanographic and plate-tectonic viewpoints to deal with the entire North Atlantic spreading system from the equator to the Arctic (Figs. 1 and 2), rather than limiting treatment to the western half of the ocean basin. Even so, the scope in some places had to be expanded. The Atlantic, like other ocean basins, did not evolve in isolation from global changes in tectonic regime, oceanic circulation, or climate patterns (Fig. 3). The development of plate-tectonic theory since the late 1960s clearly has emphasized the importance of these large-scale linkages. The present chapter continues this philosophy, summarizing the geology of the North Atlantic but noting linkages to areas outside this ocean basin. The synthesis is based largely on material presented in Volume M. The citation or lack of citation of Volume Μ references here, however, reflects only the thematic fabric of the present synthesis, not the scientific merit of the chapters. We refer the reader to original sources in Volume Μ for more complete treatment. We begin this chapter by noting ties between Volume Μ and several other Geology of North America volumes, and we continue with some “vital statistics” that describe three basic components of the Atlantic in space and time: igneous crust, sediments, and ocean waters. This is followed by a discussion of scales of spatial and temporal variability, with emphasis on the latter. The chapter concludes with a summary of some of the important advances that have occurred in the three years since Volume Μ was published.

Abstract The eastern North American passive margin includes the Atlantic continental margin from the Bahamas to Baffin Bay. Formed by the rifting, breakup, and drift of North America away from Africa and Europe, the margin′s thick sedimentary cover of Mesozoic to Cenozoic age lying over the Coastal Plain, continental shelf, slope, and rise straddles three distinct basement types: continental, transitional, and oceanic. The emerged Coastal Plain, which is a seaward-thickening wedge of sediments 1 to 2 km thick, extends from Florida to Long Island (Fig. 1). It exists as a submerged wedge off Canada (Fig. 2). The continental crust under the inner continental shelf, Coastal Plain, and landward is marked by exposed and buried rift basins formed by asymmetric half grabens (Fig. 1). The continental crust consists of three grossly different ages and terranes: the Precambrian shield from Labrador northward (Fig. 2); the Paleozoic orogenic belt from Newfoundland to Georgia (Figs. 1 and 2); and the African Precambrian and Paleozoic Terranes of Florida (Fig. 1). The transitional crust under the outer continental shelves and marginal plateaus is deeply subsided and underlies major sedimentary basins. These deep sedimentary basins are: (1) South Florida Bahamas Basin, (2) Blake Plateau Basin, (3) Carolina Trough, (4) Baltimore Canyon Trough, (5) Georges Bank Basin (Fig. 1), (6) Scotian Shelf Basin, (7) Grand Bank Basins, and (8) Labrador Shelf Basins (Fig. 2). The boundary between the transitional crust and the true oceanic crust is marked by the linear East Coast magnetic anomaly north of Florida, and farther east by the Blake Spur magnetic anomaly in the Blake-Bahamas region (Fig. 1).

Abstract The northern Gulf of Mexico Basin, although one of the most intensely studied and explored regions in North America, is also one of the most structurally complex (Figs. 1 and 2). Cenozoic depocenters contain abundant growth faults of a variety of shapes, orientations, sizes, and complexities. In addition, salt domes, flows, and massifs combine to form a complex near-surface pattern that tends to mask the origins of many structures. Not surprisingly, a number of contrasting hypotheses have been proposed to explain the growth faults of this region, among them theories invoking shale diapirism, shale compaction, gravity gliding, salt diapirism, and salt flow. Clearly, the best way to understand the various origins of these features is to observe their structural underpinnings at depth; unfortunately, most of the large growth fault systems of the Texas and Louisiana shelf project below the bottoms of seismic lines of 6- or 7-sec record length. However, as will be discussed in this chapter, deep seismic data now available from the Louisiana slope greatly illuminate the spectacular structural development of this province. In addition, palinspastic reconstructions are useful for analyzing the structural development of these features, and for constraining hypotheses on their origins. Prior to discussing the Cenozoic tectonic development of the northern Gulf of Mexico—the main focus of this chapter—we will briefly review the pre-Cenozoic framework and basic Cenozoic depositional patterns of the Gulf of Mexico Basin, both of which influenced Cenozoic structural styles. The Gulf of Mexico Basin (Fig. 1) was initiated in the late Middle to early Late Jurassic as a result of crustal attenuation and sea-floor spreading associated with the breakup of the supercontinent Pangea.

Abstract The rediscovery of America by the Genovese Christopher Columbus and the conquest of Mexico by the Spaniards was followed by a more gradual, but equally relentless, occupation of North America by the French and the British. The last phase of this process involved the discovery and exploration of the western Cordillera of North America. Fur traders Anthony Henday (1754) and the La Verendrye brothers (1743) were the first non Indians to sight the western Cordillera, while James Cook (1778) and Vitus Bering (1728,1741) were the first to explore the coasts of the northern Pacific beyond a California that was already discovered and subdued by the Spanish. There followed many expeditions that were driven by the search for gold and the fur trade as well as political and missionary interests. Most spectacular perhaps was the first crossing of the Canadian Cordillera by the Scot, Alexander MacKenzie, who reached the Pacific near Bella Coola, British Columbia in 1793. In his footsteps, the expeditions of Simon Fraser (1807), David Thompson (1807–1812), and others led to the geographic reconnaissance of the Cordillera of the northwestern United States and Canada. Lewis and Clark (1804–1805), who traversed the northwestern U.S. Cordillera, were the first who, while obviously involved in a politically motivated expedition, had an important charge to also make scientific observations. Thus, the western Cordillera had first to be “discovered” through the arduous efforts of many early explorers before any significant geological studies could be undertaken. The first geological map of North America, published by Jean Etienne Guettard in 1752 in his “Mémoire dans lequel on compare le Canada à la Suisse,”

Abstract This chapter provides a brief introduction to the geology of Mexico for members of the international geological community unfamiliar with the geology of the southern border region of the North American continent and adjacent parts of northern Central America. The main geologic features are presented by morphotectonic provinces, including the adjacent sea floor, accompanied by relevant references, primarily in English. Since most of the geological information is of reconnaissance nature, regional interpretations necessarily involve speculations and resulting controversial issues. These have been avoided, for their objective presentation would have exceeded the predetermined number of pages for this chapter. The first geological map of Mexico was prepared by the staff of Comisión Geológica Nacional, the starting organization of the Mexican Geological Survey, under the direction of Antonio del Castillo (1889a); the explanatory text constitutes the first geological synthesis of the country, the Bosquejo geológico de México , and was published with the participation and coordinating efforts of Aguilera (1896). The pioneer descriptions of the geology of the Yucatán Peninsula (Heilprin, 1891; Sapper, 1896) , Chiapas and Tabasco (Böse, 1905) and northern Central America (Sapper, 1899) allowed the presentation of a regional tectonic synthesis during the 8th International Geographical Congress in Washington, D.C. (Sapper, 1905). On the occasion of the 10th session of the IGC, held in Mexico in 1906, a set of 31 field trip guidebooks was edited, presenting the country’s geology in a very objective way. The so-called Bailey Willis geological map of North America was distributed to the participants, although it was published formally by the U.S. Geological Survey (Willis, 1912), while Aguilera (1906) presented an overview on the tectonics and volcanism in Mexico. The first description of the physiography of Mexico according to physiographic provinces is credited to Thayer (1916); the first book on the geology of Mexico, which was published in Berlin, is credited to Freudenberg (1921) .

Abstract This chapter highlights salient aspects of the geology of the northeast Pacific basin and its interactions with the rim of North America. We include all the Pacific from the equator northward to the Aleutians, and from the North American continental margin westward to longitude 165ŶE—an area about twice that of North America. The treatment is thus necessarily summary; for a more complete and balanced coverage, and for more complete lists of the works that have contributed to our knowledge of the northeast Pacific region, see Winterer and others (1989). There has been a veritable explosion of knowledge about the northeast Pacific in the 25 years since the appearance of Menard’s (1964) Marine Geology of the Pacific . Since then, the major features of the magnetic anomaly pattern have been mapped, and can be interpreted in the framework of plate tectonics; an extensive web of seismic reflection lines and cores from a network of about 150 deep-sea drill holes provides the data and samples for dating, correlating and interpreting the oceanic sedimentary cover, the constitution of the oceanic crust, and the tectonics of the continental margins. New swathmapping techniques and near-bottom observations and sampling using deep-towed instruments and manned submersibles have opened a window into the processes of crustal accretion and hydrothermal activity along active spreading centers, and modern instrumentation has provided a rich new data base for interpretation of active volcanism in Hawaii. To synthesize the new findings, we have organized this chapter into two topical sections and one geographic section;

Abstract The Caribbean area as defined here includes the Greater Antilles, Lesser Antilles, the northern boundary of South America, and Central America (Fig. 1); it spans approximately 7,800 km in an east-west direction and 3,900 km in a north-south direction. Although the geology of this region should be considered along with that of Mexico and the Gulf of Mexico, these areas form separate chapters in this book. This chapter is in part a condensation of numerous contributions prepared for the synthesis volume on the Caribbean region by Case and Dengo (1989). Further details are available in that book. Modern geological interest in the Caribbean has centered on its Cretaceous to Recent orogenic belts that resulted from plate interactions between North and South America. The Caribbean is the site of America’s most extensive Cretaceous and Cenozoic oceanic-continental tectonic zone and has (along with the Aleutians) its only real island arcs. It has the majority of the active volcanic centers of the New World and a major share of the destructive earthquakes. The goal of the Caribbean geologist is to reconstruct the history of a minor “plate” whose extensive internal deformation belies the strict application of this term. This “plate” has been broken and twisted within the jaws of three major plates (Farallon, North America, and South America), whose relative motions have changed dramatically from Jurassic to Recent time. However, the average motion among the three plates since the middle Cretaceous has been one of roughly east-west compression, and the aim of this paper is to place the geologic history in the context of these changing major plate motions.