Abstract The interaction of tectonic plates has been used to predict the direction of tectonic stresses well away from plate boundaries. These studies have used individual stress indicators to infer trajectories of tectonic stress (maximum horizontal compression or MHC; e.g., Nakamura and others, 1977, 1980; Nakamura and Uyeda, 1980; Zoback and Zoback, 1980). The indicators are often derived from earthquake focal mechanisms (slip vectors and/or pressure, tension axes). Taking such data from a wide variety of mechanisms allows one to infer the orientation of the causative stress field (e.g., Angelier, 1979; Gephart and Forsyth, 1984) despite the fact that individual mechanisms are solely geometrical strain indicators that describe the orientation of the fault on which the earthquake occurred (McKenzie, 1969). Differences of up to 45° have been observed between the in situ stress indicators (borehole hydrofractures and breakouts) and the geometrical strain indicators (faults, earthquake focal mechanisms; Mount and Suppe, 1987; Zoback and others, 1987). With these preconditions in mind, we infer the directions for maximum horizontal compressive stress (MHC) from individual indicators for Alaska and its surroundings. Several studies have used stress and strain indicators to infer stress directions across the Alaska region. Nakamura and others (1977, 1980) used volcanic stress and geologic fault strain indicators, and Davies (1983), Biswas and others (1986), and Estabrook and others (1988) used earthquake focal mechanism strain indicators to infer stress trajectories across interior Alaska. Nakamura and others (1977, 1980), Nakamura and Uyeda (1980), and Biswas and others (1986) have proposed a transition from

Abstract We present a tectonic interpretation of the stress-orientation data for Mexico and western Central America shown on the Decade of North American Geology (DNAG) continental-scale stress map of North America (Zoback and others, 1991). Geographically, the study area comprises Mexico, northern and western Guatemala, and Belize (Fig. 1). The plate-tectonic setting of the region is the southernmost part of the North American Plate and a part of the Pacific Plate in the form of the Baja California Peninsula (Fig. 1). The stress-orientation data are displayed in their neotectonic framework (Figs. 3 to 6). They are listed for some of the acquisition techniques (Tables 1 and 2); additionally the entire dataset will be deposited with the U.S. National Geophysical Data Center in Boulder, Colorado. In the text we follow geographic regions and review, for each region, first the state of active deformation and then the state of stress. Our stress-orientation maps contain, to our knowledge, all available intraplate measurements as well as interplate measurements in the transtensional Gulf of California and transpressional Motagua-Polochic plate boundary zones between the North American and Pacific, and the North American and Caribbean Plates, respectively (Fig. 1). However, we did not compile focal mechanisms of seismic events located in the Wadati-Benioff zone between the North American, and the Cocos and Rivera Plates at the Pacific margin of Mexico and Guatemala (Fig. 1). For this subduction zone, the stress distribution is presently much better resolved (e.g., Lefevre and McNally, 1985) than for the area analyzed in this

Abstract The objective in creating a Geothermal Map of North America (Blackwell and Steele, 1991) was to show as accurately and completely as possible the state of knowledge of the geothermal field of the continent in all its variations. As a consequence, the types of information shown are combined in a way that is different from existing maps. The only other continent-wide map representations of aspects of temperature, geothermal gradient, or heat-flow data are the AAPG/USGS Subsurface Temperature Map of North America (1976b) and the Geothermal Gradient Map of North America (1976a) both at a 1:5,000,000 scale. Geothermal gradient maps of the United States have been prepared by Kron and Stix (1982) and Nathenson and Guffanti (1988). However, because both temperature and interval gradient can be calculated if the heat flow and lithology are known, the basic quantity that is needed is heat flow. Therefore, these gradient maps have limited usefulness outside the range of direct observation. Numerous countrywide heat-flow maps have been described for North America. However, the knowledge of the thermal regime of North America has increased many fold in the last 10 to 15 years because of academic studies and because of the exploitation of geothermal energy and associated resource studies. Thus, it seemed to be an appropriate undertaking at this time to compile a continent-wide representation of the thermal field using this old and new knowledge. The preparation and publication of other continent-wide maps on a new digital base at the 1:5,000,000 scale is also part of

Abstract The first heat-flow data in Canada were published in 1951 (Misener and others). This was twelve years after the earliest publication of completely measured heat flow (Bullard, 1939; Benfield, 1939), a period that included the Second World War. Canada was the third country, after the United Kingdom and the United States, to produce heat-flow data, and was followed by Australia, Poland, South Africa, and Iceland. At present, heat-flow values have been published or submitted for publication from 298 sites in Canada, including the continental shelves. A list of these data is to be found in Jessop (1989). A “site” may include several boreholes if they are close together, the limiting distance being about 10 km. However, this definition cannot be adhered to strictly. A line of boreholes spaced at less than 10 km cannot be reconciled with this definition, and closely spaced measurements that reflect different geothermal settings cannot provide useful averages. In general, the concept of “site” is intended to permit the averaging of closely spaced measurements, for example, in several boreholes at a mining property. The criteria for combination are: (1) the geological setting is uniform, (2) the results are similar, and (3) the quality of the data is sufficiently low to indicate combination into one result. Some of the continental data have been acquired by oceanic probe techniques in shallow lakes. A summary of all continental data by style of site and type of measurement is given in Table 1. Table 2 shows mean and distribution of

Abstract The rugged Canadian Cordillera has been and is being created by various dynamic processes that are dominated by past and present plate-tectonic motions and interactions. A summary of the tectonic models proposed for the Cordillera, together with the geologic record and geophysical data that support them, is given by Gabrielse and Yorath (DNAG volume: Geology of Canada Series: Cordilleran Orogen, Canada, in preparation). In this active area the heat flux is determined by various regimes and processes, and the reduced heat flow is a diagnostic indicator of the state of the lithosphere. In general, the heat flux is decreased by subducting plates, increased by uplift and erosion, and increased and decreased, respectively, by exothermic and endothermic reactions occurring within the crust. It is quite variable over small areas where crustal intrusions are advectively cooling. Relatively high heat flux occurs in regions that are undergoing crustal extension. The magnitude of these variations can be used to evaluate the rates at which these processes are occurring and/or to infer other details concerning the complex evolution of this region. Current plate interactions along the western margin of the Canadian Cordillera (Riddihough and Hyndman, in preparation) are indicated in Figure 1. South of the triple junction, the Juan de Fuca plate is subducting under southern Vancouver Island and the coastal mainland; the smaller Explorer plate broke away and has now ceased to subduct. North of the triple junction, there is right-lateral transform movement along the Queen Charlotte fault with a small amount of oblique

Abstract Over the past two decades a great many heat-flow measurements and estimates have been made in the southwestern United States and neighboring areas of northern Chihuahua, Mexico. Although these data have enhanced our understanding of the geothermal regime in the southwestern United States, many questions have developed while analyzing the data. In this chapter we present some of the conclusions that can be made from the heat-flow data in the study areas. In addition, we discuss some of the important unanswered questions concerning the geothermal regime in various geologic provinces of the southwestern United States and northern Chihuahua, Mexico. We consider below the heat-flow data in the various geologic provinces of the study area, and when appropriate, interrelate observations from different geologic regions. Articles by Roy and others (1968), Lachenbruch and Sass (1977), and Blackwell (1978) review principles and concepts concerning heat flow in the western United States. The Colorado Plateau is a roughly circular region, about 700 km in diameter, that encompasses large areas in the four-corner states of Arizona, Colorado, New Mexico, and Utah (Fig. 1); it is a relatively stable region within the tectonically and volcanically active western United States. The region has been uplifted 1,000 to 1,500 m since Eocene time, but in general has maintained more lithologic and structural continuity than other areas of the western United States (King, 1959; Eardley, 1962). In keeping with its relatively stable tectonic setting, the heat flow in the Colorado Plateau is intermediate between higher values characteristic of neighboring

Abstract Geothermal resources within the Great Plains occur as strata-bound waters in regional aquifers. Generally, all aquifers in the Great Plains including and underlying the Inyan Kara and Newcastle Formations of the Dakota Group (Cretaceous) contain waters warm enough for development as geothermal resources (Table 1). Many of these aquifers occur throughout the Great Plains, but the formation names may differ across the province. The Dakota aquifers are the most extensive and the best mapped in the subsurface; thus, a contour map of temperatures on top of the Dakota Group provides a large-scale view of the minimum geothermal potential of the Great Plains. This temperature contour map is the principal contribution of this study to the geothermal resource map of North America (Blackwell and others, 1991). Data on subsurface temperatures and geothermal aquifers in the Great Plains have been published primarily in technical reports for the U.S. Department of Energy (Sorey and others, 1983). Primary sources for data are reports for North Dakota (Harris and others, 1982; Gosnold, 1984), Nebraska (Gosnold and Eversoll, 1982), Kansas (Blackwell and Steele, 1989), and South Dakota (Gosnold, 1987). Analyses of these data are found in geothermal literature that are primarily conference proceedings (Gosnold, 1984, 1987; Gosnold and Eversoll, 1981, 1982) and in reports of the U.S. Geological Survey (Sammel, 1979; Sorey and others, 1983). Downey (1986) synthesized data on the subsurface extent and hydrologic characteristics of several Paleozoic and Mesozoic aquifers in the northern Great Plains. Detailed information on depth, thickness, subsurface extent, and permeability

Abstract The objective of this chapter is to provide information on the magnitude of elevation changes and rates of deformation that are associated with the recovery of North America, Greenland, and Iceland from the late Quaternary ice loads (Fig. 1). These topics are addressed in detail in various chapters of The Quaternary Geology of Canada and Greenland (Fulton, ed., 1989). In particular, the various regional treatments contain much of the basic information on which this present chapter is based. Andrews and Peltier (1989) also provide a survey of both the observational data and glacial isostatic theory. Of the major glaciated areas of North America, only Alaska has a paucity of studies on relative sea-level movements during the last deglacial cycle from the region directly influenced by an ice load (see Mann, 1986). However, Clark (1977) has evaluated the effect of recent changes in glacier thickness on the sea-level history of southeast Alaska. Neotectonics associated with glacial unloading of the Earth's surface are a different set of processes than movements associated with normal faulting or with the movements of plate margins, although there may be connections between them (e.g., Clark, 1982; Quinlan, 1984; Adams, 1989a, b). Glacial isostatic recovery affects large areas (ca. 14 × 10 6 km 2 of North America) and is characterized most commonly by spatial and temporal coherence of the response (e.g., Peltier, 1976; Clark, 1980; Quinlan, 1985; Quinland and Beaumont, 1982). Glacial isostatic observations have been used on this predictive basis for several decades to constrain appropriate models of

Abstract Although there has been much interest in crustal stresses in Canada in previous decades, notably related to deep mining (e.g., Herget, 1980), it is only in the last decade that the rate of data collection has accelerated. Because the amount of the data began to exceed the space available in tables (e.g., Bell and Babcock, 1986; Franklin and Hungr, 1978; Hasegawa and Adams, 1981; Hasegawa and others, 1985), the Geological Survey of Canada began a computer data base of crustal stress data in 1985 (Adams, 1987). This was prompted largely by the need for Canadian input to the Decade of North American Geology (DNAG) Stress Map of North America and later to the International Lithosphere Program World Stress Map. The timing was fortuitous in that a great deal of oil-well breakout data was beginning to be published (e.g., Bell and Gough, 1981), and a renewed emphasis was being placed on earthquake focal mechanisms (e.g., Wahlstrom, 1987; Adams and others, 1988). In this chapter we discuss the sources of in situ stress information for Canada, the nature of the information provided by the various methods, the recognition and definition of stress provinces, regions with anomalous stress signatures, the effects of glaciation, and finally, possible causal mechanisms. All localities, including sedimentary basins mentioned in the text, are shown on Figure 1. We conclude that Canada east of the Cordillera is being compressed in a northeast-southwest direction in what is an extensive and relatively uniform stress province, which also includes the eastern and

Abstract The tectonic stress field is both the cause and result of active geologic processes. At the largest scale, stresses in the earth's lithosphere arise from such processes as mantle convection and lithospheric density imbalances. The forces generated by these processes are ultimately responsible for driving the plates. Examples include the ridge-push force at spreading centers and the trench-pull force associated with negative buoyancy of subducting plates (utilizing the terms of Forsyth and Uyeda, 1975). A variety of important processes act at more regional scales. These include stresses generated by relative plate motion, magmatic and thermal processes, regional crust and lithospheric thickness and density variations, and topography (e.g., Jeffreys, 1976; Bott and Dean, 1972; Artyushkov, 1973; Fleitout and Froideveaux, 1983), and lithospheric flexure (e.g., McNutt, 1984). Improved knowledge of the tectonic stress field is intimately tied to improved understanding of the mechanisms that drive plate motion, the dynamics of faulting along both plate boundaries and in intraplate areas, and the overall mechanical, thermal, and rheological constraints on volcanism, mountain building, basin formation, and other active geologic processes. We have been accumulating data on the orientation and relative magnitude of the tectonic stress field for over a decade. The study described here is based upon integration of our previous analyses of regional stress patterns in the conterminous United States (Zoback and Zoback, 1980, 1981, 1989; Zoback and others, 1986, 1987) and the studies of Canada (Adams and Bell, this volume), Mexico and Central America (Suter, this volume), and Alaska (Eastabrook and Jacob

Abstract The three topics covered in this volume—seismicity, stress, and heat flow—are essential for an understanding of the neotectonics of North America. These topics are discussed in three groups of chapters that support associated map compilations (seismicity—Engdahl and Rinehart, 1988; stress—Zoback and Zoback, 1991; thermal aspects—Blackwell and others, 1991) prepared on a standardized continent-wide base to provide information on the neotectonic setting of North America. Each of the three topics in this volume was organized separately and internally edited. The seismicity chapters, edited by E. R. Engdahl, show general regional earthquake patterns and indicate the locations, styles, and relative frequencies of earthquakes from neotectonic faulting and fault-related seismogenic folding. The chapters on stress, edited by Mark D. Zoback, highlight regional differences in stress type and orientation, and are important for determining likely earthquake focal mechanisms and styles of neotectonic deformation. The chapters on geothermal aspects, edited by David D. Blackwell, provide an indication of neotectonic activity developed over much longer time frames, particularly for volcanic and geothermal phenomena. The original intent was to include a fourth section and an accompanying neotectonics (strict sense) map, the first compilation of North American faults, folds, and other neotectonic features. Problems of compiling and analyzing the overwhelmingly large accumulation of new paleoseismic data, particularly in the western United States, prevented completion of this task in time for inclusion in this volume, with the exception of this chapter, which emphasizes midplate neotectonics (Slemmons), and a chapter on glacial rebound (Andrews). Accordingly, in this chapter I highlight

Abstract A highlight of the Decade of North American Geology (DNAG) program is the publication of new continent-scale maps of North America. The map set includes a Geological Map, Gravity Anomaly Map, Magnetic Anomaly Map, Seismicity Map, Stress Map, Neotectonic Map, and Thermal Aspects Map. The maps are wall size (four 42-in x 55-in sheets per map), in color, and made on a common base map at a scale of 1:5,000,000. A GSA-sponsored workshop was held at the 1984 Seismological Society of America meeting in Anchorage, Alaska, to discuss construction of the Seismicity Map. Consensus was reached that the goal of the Seismicity Map would be to provide a useful resource on North American seismicity for seismologists, earth scientists at the postgraduate level, and industry, and to accurately portray North American seismotectonic features using the entire earthquake history from the early 1500s (pre-instrumental) through the modern period. It was agreed that construction of the map database would require the rationalization of hundreds of thousands of earthquake hypocenters from global, national, regional, and local catalogs, and from other source materials, and that because modern data are more useful than historic data for resolving seismotectonic features, a scheme of data selection and representation would have to be devised that could reveal details of the seismotectonic fabric of North America yet preserve a perspective of historical earthquake occurrence. The problem was further complicated by the highly variable monitoring capability of seismic networks and by the different types and rates of occurrence of earthquake activity