This volume deals with the geology of the northeastern part of the Pacific ocean and the adjacent parts of the North American continental margin. The areal coverage is from the margin west to the Hawaiian Seamount chain, and south to the equator. The volume comprises several major sections containing related chapters: regional bathymetry and tectonics (Chapters 2 to 4 and Plates 1 and 3); Spreading ridge systems (Chapters 5 to 11 and Plate 4); Hawaii-Emperor and other seamount chains (Chapters 12,13, and Plate 5); sediments of the northeast Pacific (Chapters 14 to 19, and Plate 2); and Pacific continental margins of North America (Chapters 20 to 28, and Plate 6). In the paragraphs below are a few introductory remarks about each chapter, which may help readers to find their way.

Historians consider Ptolemy to be the founder of cartography. Part of the early effort to define lands and seas in Ptolemy’s time and for many centuries thereafter was devoted to the charting of coast lines. It was not until 1584, though, that the first maritime atlas was published: Der Spieghel der Zeevaert by Lucas Janszoon Waghenaer (Tooley and others, 1968). For the first time, printed charts showing soundings, sandbanks, landmarks, and coastal profiles became available to navigators, and from these charts an awareness evolved that beneath the sea water lay a surface of uneven relief. For two more centuries, depth soundings were taken with hand-held or winched hemp lines, each observation requiring several hours to make. Toward the end of the nineteenth century, that technology was only slightly improved by the use of galvanized steel wire mounted on reels. And yet, with information gathered by those primitive means, Maury was able to publish in 1854 a bathymetric map of the North Atlantic Basin, the first chart ever of an entire ocean basin with contour lines drawn every 1,000 fathoms (Schlee, 1973). H.M.S. Challenger, in 1870, opened the Pacific Ocean to scientific inquiry, and the first chart of that immense basin was published by John Murray (Murray and Lee, 1909). One cannot help admiring the intuition of these men who, with the help of very few observations, established charts that are still correct in their major outline.

The ideas of sea-floor spreading and plate tectonics were just exploding upon the oceanographic community when I (T.A.) was a young graduate student at the Scripps Institution of Oceanography in the late 1960s. This was an exciting time; it seemed that the whole world lay waiting for reinterpretation. Recognizing the power of the Vine-Matthews hypothesis, Bill Menard had asked his draftswoman, Isabel Taylor, to plot all of the magnetic anomaly profiles so far collected by ships traversing the Northeast Pacific. The resulting map and its updates occupied a central spot on Bill’s huge work table for many years. Although I was not officially working with Bill, I was almost irresistibly drawn to his laboratory and to this map whenever I had a spare moment. For some years before, Bill had been mapping and writing about the great North Pacific fracture zones. With the addition of the magnetic isochrons, the sea-floor spreading story of the region unfolded before our eyes. He was like a child in a candy store; I was in heaven. It was on this chart that we identified and mapped out the isochron patterns for the region and, using their mapped geometry, we were able to explore many sea-floor spreading and plate- tectonic phenomena (e.g., Menard and Atwater, 1968, 1969; Atwater and Menard, 1970; Atwater, 1970; Menard, 1978). However, the compilation of the magnetic profiles, itself, was never published. Over the years since, many additional profiles have been measured, and various detailed surveys and partial compilations have been published. On Plates 3A and 3B we have collected and presented these, along with many of the original profiles, and superimposed them upon our updated interpretations. We present these tectonic maps in honor of H. W. Menard and in memory of his tremendous, contagious joy in science.

In the present configuration of tectonic plates, the northeast Pacific region is dominated by the huge Pacific Plate. Along its eastern edge, the Pacific Plate presently interacts with two medium-sized oceanic plates, the Juan de Fuca and Cocos Plates, and a few related small platelets: the Yakutat, Explorer, South Gorda, and Rivera Plates (Fig. 1). All of these occupy relatively small regions along the edge of the Pacific Plate, interacting as well with the North American Plate along its western rim. The most complex modern plate boundaries in the northeast Pacific region occur where the eastern edge of the Pacific Plate abuts directly against North America. These are primarily strike- slip boundaries with subordinate amounts of extension: the Queen Charlotte-Fairweather fault system, including the oblique motion of the Yakutat block, and the San Andreas fault system, including the oblique extension in the Gulf of California. The diffuse nature of the earthquake zones around these features, depicted in Figure 1, shows that plate boundaries within the continental lithosphere are not as narrow and simple as those within the oceanic plates. The broad zones of activity in the Great Basin show the existence of at least two diffuse inland zones of deformation, as well.

The boundary between the Pacific and Juan de Fuca Plates in the northeast Pacific Ocean is marked by a series of spreading centers (Fig. 1) and their connecting fracture zones (transform faults). The longest (490 km) of these, the Juan de Fuca Ridge, is bounded on the south by the Blanco Fracture Zone and on the north by the Sovanco Fracture Zone. Despite its relatively small size compared to other mid-ocean ridges, the Juan de Fuca has played a historic role in the development of plate tectonics, and is still one of the most intensively studied spreading centers in the world. The Juan de Fuca Plate, lying east of the ridge system, forms an actively convergent margin with the North American Plate. The Juan de Fuca spreading center is composed of a series of at least six ridge segments, 50 to 150 km long, which although generally spreading at a total opening rate of 6 cm/yr, display a remarkable diversity of ridge-axis morphology.

The East Pacific Rise was discovered during the famous Challenger expedition in 1875 on its passage from Tahiti to Valparaiso (Murray, 1895), and was investigated by marine geologists in 1904 aboard the RV Albatross (Agassiz, 1906) and in 1928 aboard the RV Carnegie (Soule, 1944). The original soundings were laborious, with hemp ropes used to dredge and sound. Later, wire rope was used, and the major advance to echo sounders was made aboard Carnegie , although these early measurements were often inaccurate (Menard, 1964). Referred to as the “Albatross Plateau” and the “Easter Island Ridge” during the early years of exploration (e.g., Ewing and Heezen, 1956; Menard, 1960), the East Pacific Rise (EPR) acquired its present name during the mid-1950s (H. W. Menard, 1960, personal communication, 1985). During the 1950s and 1960s, evidence was gathered that indicated that the EPR was a major bathymetric structure extending from the Gulf of California at least as far south as the Eltanin Fracture Zone in the Pacific, and that it was tectonically and volcanically active (Menard, 1960). Heat-flow measurements showed that the flux of heat was considerably higher than the average of the ocean basins (Von Herzen, 1959; Von Herzen and Uyeda, 1963); fresh, young pillow basalts were dredged from the EPR axis (Engel and Engel, 1964); seismic activity was found to be discontinuous but high (Gutenberg and Richter, 1954); and enormous fracture zone escarpments were mapped that intersected its axis at a high angle (Menard, 1955,1964). The greatest enigma at the time was the highly linear magnetic anomaly pattern measured by Mason and Raff (1961), which was later used to show that the EPR was part of a major spreading center system of mid-ocean ridges (Vine and Matthews, 1963).

The regional maps of the northern Pacific Basin by Menard and Dietz (1952) established that offsets in the California margin, first documented by Murray (1939) and Shepard and Emery (1941), could be traced westward as bands of grossly irregular topography for more than 2,000 km. Typically these swaths of disturbed terrain separated regions of different depths and were made up of troughs, ridges, escarpments, and seamounts. This distinctive terrain was called a fracture zone, and continued reconnaissance mapping efforts by Menard and colleagues identified a family of subparallel fracture zones that could be traced as lineaments for thousands of kilometers (Menard, 1955; Menard and Fisher, 1958). The straightness of these fractures, their continuity along strike, and the fact that they separated terrain of contrasting depths as well as distinctive and correlatable north-south-trending magnetic anomalies (Mason, 1958; Vacquier and others, 1961) suggested that fracture zones were the product of faulting, with the dominant displacement having been horizontal. The recognition of a world-girdling and seismically active Mid-Oceanic Ridge system established that this feature was the major morphotectonic element of the ocean basins (Ewing and Heezen, 1956; Heezen, 1957). Models of ridge formation proposed that the ridge axis was the site of extension, stretching, and volcanism (Heezen, 1960; Menard, 1960; Hess, 1962). The long, straight scars of the fracture zones that frequently offset the axis of the Mid-Oceanic Ridge were thought to be large strike-slip faults that broke the ridge into large blocks with differential motion taking place along the length of the fault. Wilson (1965) proposed that major faults, orogenic belts, trenches, and the Mid-Oceanic Ridge were not isolated features but are part of a continuous network of mobile belts that encircle the Earth. In this model, the portion of a fracture zone that links the extensional regions of the two offset ridge axes was called a transform fault. The key prediction of the model was that the relative motion along the transform would be opposite to that predicted if the ridge axis was offset along a transcurrent fault. The kinematic character of fracture zones was established when first-motion studies of earthquakes along several ridge-transform-ridge plate boundaries were shown to have motions in agreement with Wilson’s model (Sykes, 1967).

An explosion of research during the past decade has been dedicated to discovering, describing, and understanding the extent and importance of submarine hydrothermal processes, and much of the important field work on this topic has been carried out on and near eastern Pacific spreading centers. This chapter reviews the empirical data (field observations, measurements, and sample analyses) and some of the prevailing interpretations and hypotheses acquired from 15 years of study of hydrothermal processes along and adjacent to the Galapagos Rift and East Pacific Rise (EPR) spreading center systems. Hydrothermal processes elsewhere in the eastern Pacific, i.e., in the Gulf of California and along the Gorda-Juan de Fuca-Explorer Ridge spreading centers in the northeast Pacific, are discussed in separate chapters by Lonsdale and by Johnson and Holmes (this volume).

The petrologic and geochemical characteristics of the East Pacific Rise and other actively spreading ridges provide important direct evidence concerning the composition of the Earth’s upper mantle because mid-ocean ridge basalts are thought to represent partial melts of upper mantle material rising beneath mid-ocean ridges. Since the characteristics of the upper mantle can be used to constrain models for the accretion and differentiation history of the Earth as well as the nature of physical and chemical processes taking place in the mantle, MORB (midocean ridge basalt) and other oceanic basalts play an important role in constraining models of mantle convection, continent formation, and related questions. This chapter describes the petrologic and geochemical characteristics of lavas from the axes of East Pacific spreading ridges, primarily the East Pacific Rise (EPR) between the Tamayo Fracture Zone and the equator, and the Galapagos spreading center (GSC). The Juan de Fuca Ridge and the Gulf of California are considered in other chapters of this book (Johnson and Holmes, this volume; Lonsdale, this volume). Figure 1 shows locations of the principal sampling areas along the EPR and the GSC and Figure 2 shows the distribution of dredge hauls in the eastern Pacific. Until very recently, the spreading centers of the eastern Pacific were very poorly sampled compared to the Mid-Atlantic Ridge. The Juan de Fuca and Gorda Ridges are reasonably well sampled (Melson and others, 1976; Delaney and others, 1982; Davis and Clague, 1987; Dixon and Clague, 1986). The Gulf of California is mostly covered with sediment, but basalt has been recovered by drilling (Saunders and others, 1982; Lonsdale, this volume). The data base for the GSC includes analyses from Melson and others (1976), Schilling and others (1976), Fornari and others (1983), Christie and Sinton (1981), White and others (1987), and references therein. For the EPR, detailed studies have been conducted at 23°N near the Tamayo fracture zone (Bender and others, 1984), 21°N (Moore and others, 1977; Juteau and others, 1980; Hawkins and Melchior, 1980), 12° to 13°N (Hekinian and others, 1983; Batiza and Vanko, 1984; Macdougall and Lugmair, 1986), and 8° to 9°N (Batiza and others, 1977; Batiza and Johnson, 1980; Natland and Melson, 1980; Morel and Hekinian, 1980). Many of these papers include analyses of volcanic rocks from the EPR axis, as do Engel and Engel (1964), Engel and others (1965), Bonatti (1967), Kay and others (1970), and Sun and others (1979). Recent expeditions have greatly increased the number of dredge hauls of the EPR axis at 10° to 12°N (Thompson and others, 1985), 5°30′ to 14°30′N (Langmuir and others, 1986) and overlapping spreading centers (OSCs) at 12°54′N, 9°03′N, 5°30′N, and 3°57′N (Natland and others, 1986), but these studies are not yet fully complete.

Abstract Propagating rifts are extensional plate boundaries that progressively break through rigid lithosphere. If the rifting advances to the sea-floor spreading stage, propagating sea-floor spreading centers follow behind, gradually extending through the rifted lithosphere. The combination of sea-floor spreading and propagation produces a characteristic V-shaped wedge of lithosphere formed at the propagating spreading center, with progressively younger and longer isochrons abutting the pseudofaults that bound this wedge. Figure 1 shows several variations of this geometry. Although conceptually simple, the propagating rift hypothesis has important implications for both large-scale and fine-scale plate tectonic evolution. It explains: (1) the existence of several classes of structures that are oblique to both relative and absolute plate motion and that previously seemed incompatible with plate tectonic theory; (2) why some continental margins are not parallel to sea-floor isochrons and why some continental drift reconstructions are inaccurate; and (3) the large-scale reorganization of some sea-floor spreading systems, including both the onset and termination of many fracture zones as well as the formation of some transient microplates. The hypothesis provides a mechanistic explanation for the way in which many (if not all) spreading center jumps occur and why they occur in systematic patterns, how spreading centers reorient when the direction of sea-floor spreading changes, and the origin of large areas of sea floor with high petrologic diversity, including the major abyssal ferrobasalt provinces.

Abstract Failed rifts are inactive mid-ocean ridge segments abandoned during processes that change the spreading geometry of lithospheric plates (Table 1). Morphologically, they resemble active ridge crests, and it seems in some cases that their failure or abandonment was closely coupled to the propagation of a neighboring active rift. In such cases, the propagating rift advances into old lithosphere formerly created by the dying rift, while the dying or abandoned rift ceases to function as a spreading center. A transient and complex transform-type plate boundary forms between the advancing rift and the dying one, as discussed by Hey and others (1986). For propagating/retreating rifts (PRRs) (discussed fully in Hey and others, this volume) and migrating overlapping spreading centers (OSCs), also called nontransform offsets, (Rea, 1978; Lonsdale, 1983; MacDonald and Fox, 1983; Macdonald, this volume; Fox and Gallo, this volume), the distance separating the coupled propagating and failing rifts is usually less than or about 50 km. It is possible that this relatively small distance between the advancing and retreating rifts leads to close coupling of ridge behavior and ease in establishing a transform-type boundary between them. If even smaller rift offsets such as small non-overlapping offsets (SNOOs) (Batiza and Margolis, 1986) and deviations from axial linearity (DevALs) (Langmuir and others, 1986) migrate, it would be expected that close coupling of advancing and dying rifts and the ease of creating a transient plate boundary between them would be correspondingly enhanced due to the close proximity (<3 km) of the two rifts and the hot, thin, weak lithosphere between them

Abstract Intraplate volcanism within the Pacific Plate not generated at spreading plate margins is most obvious in Hawaii and the Hawaiian-Emperor volcanic chain. This chain forms a global relief feature of the first order. This chapter consists of five separate sections that summarize the volcanism and geology of Hawaii and the Hawaiian-Emperor chain. Less obvious but probably greater in overall volume are other seamounts and seamount chains scattered across the northern and eastern Pacific basin. Some of these appear to owe their origin to intraplate volcanism, but many probably formed at mid-ocean ridges. Batiza (this volume, Chapter 13) discusses these other, largely submarine, volcanoes. The Island of Hawaii lies at the southeastern end of the Hawaiian-Emperor volcanic chain—a dogleg ridge, largely submarine, stretching nearly 6,000 km across the north Pacific Ocean basin. From Hawaii the chain extends northwestward along the Hawaiian Ridge to a major bend beyond Kure Atoll. North of the bend the chain continues in a northward direction as the submarine ridge of the Emperor Seamounts. Volcanoes are active at the southeast end of the chain and become progressively older to the northwest, reaching ages of 75 to 80 million years at the north end of the Emperor Seamounts. Most of this volcanic chain, with an estimated area of 1,200,000 km 2 , lies beneath the ocean. Only the Hawaiian Islands and a few atolls of the Hawaiian Ridge, totaling some 16,878 km 2 , rise above the sea (Plate 5).

Abstract The part of the Pacific covered in this volume contains a wide variety of constructional volcanic features: volcanic rises (Hess Rise), volcanic ridges (Cocos Ridge), large seamount chains (Line Islands, Pratt-Welker chain, the Fieberling Chain, and the Musician seamounts), small chains like those west of the Juan de Fuca Ridge, guyots, and many seamounts that are not members of linear chains but instead are distributed in patchy clusters or small groups or are isolated. In the eastern Pacific, unlike the western and south Pacific, volcanic topography is not dominated by large linear island and seamount chains (Menard, 1964). Instead, the most abundant seamounts of the eastern Pacific are clustered and isolated volcanoes. The purpose of this chapter is to discuss the characteristics, distribution, and origin of the volcanoes and groups of volcanoes in the eastern Pacific exclusive of the Hawaii-Emperor chain, which is discussed else-where (Clague, this volume). Some of the volcanoes and linear groups of the eastern Pacific are probably of hotspot origin; most probably are not. The origin of many of these not-hotspot volcanoes is probably linked to mid-ocean ridge volcanism, but others may originate on old lithosphere far from active ridge crests. Oceanic volcanoes of non-hotspot character are almost certainly of several types, inasmuch as there are many tectonic environments in which a favorable combination of magma availability and plumbing systems can lead to the formation of volcanoes. These conditions may prevail for a variety of reasons in both near–ridge crest and off-ridge locations. The study of seamount volcanism thus contributes directly to solution of a variety of problems concerning the characteristics, origin, and evolution of oceanic lithosphere.

Abstract Figure 1 portrays sediment thickness on the ocean floor in the Northeast Pacific. The very generalized contours show the total thickness of sediments, in kilometers, from the sea floor to the top of basaltic basement. Except for detailed maps of small areas, sediment thickness maps necessarily must generalize and average local variations in thickness. The abyssal hill topography created during accretion of normal oceanic crust generally has local relief on the order of 100 m over distances of 10 to 20 km, and sediments do not accumulate evenly over this initial relief, even in regions of purely pelagic sedimentation (Fig. 2). In areas of pronounced basement relief— for example, along fracture zones and around seamounts and submarine ridges—local thickness variations can assume extreme values. The contours in Figure 1 are meant to depict typical values; no attempt is made to show small-scale local variations. Over most of the map area, the thickness depicted is to the top of normal oceanic basaltic crust, but in some areas, strongly reflective chert layers and midplate volcanic rocks mask the oceanic basement on reflection records, and the thickness shown is what is discernible on seismic profiles.

The northeast Pacific between the equator and 60°N includes equatorial, subtropical, temperate, and subarctic waters (Sverdrup and others, 1946). Within the equatorial Pacific, upwelling is induced by winds and by the interaction of the Equatorial Countercurrent with both the North and South Equatorial Currents. Surface waters are enriched in nutrients, forming the highly fertile equatorial high productivity zone (Wyrtki, 1966; Theyer and others, this volume). The dominant oceanographic feature of the temperate to subarctic northeastern Pacific is the Subarctic Front, which separates fertile subarctic waters from warmer (>20°C in the summer), less fertile waters of the subtropical gyre. The Subarctic Front is maintained at about 40°N between the eastward-flowing West Wind Drift and North Pacific Current (Dodimead and others, 1963). As the West Wind Drift approaches the North American coast, it is deflected southward to become the California Current (Dodimead and others, 1963). To the north, the Aleutian Current is deflected northward into the Gulf of Alaska to form the counterclockwise Alaska Gyre (Dodimead and others, 1963). The California Current dominates the eastern margin of the northeast Pacific, where it transports cool, fertile waters as far south as the tip of Baja California (23°N) (Sverdrup and others, 1946). Within the fertile regions of the northeast Pacific, sediment is enriched by the carbonate and siliceous skeletons of foraminifers, nannofossils, diatoms, and radiolarians. Mixed carbonate and siliceous biogenic oozes are found beneath the equatorial high productivity zone, where the Calcite Compensation Depth (CCD) is depressed (van Andel and others, 1975; Theyer and others, this volume).

Much of the abyssal North Pacific is blanketed by very slowly accumulating, fine-grained pelagic clay that is dominantly eolian in origin (Fig. 1). The clays are unfossiliferous, except for microscopic fish debris, and are enriched in hydrogenous sedimentary components such as manganese nodules and zeolites (Goldberg and Arrhenius, 1958; El Wakeel and Riley, 1961; Horn and others, 1970). The general character of the pelagic clay province has been known since the Challenger Expedition of 1872 to 1876 (Tizzard and others, 1885) when it was first sampled. Subsequent studies have shown that the region extends across the deep eastern and central North Pacific basin and that a second area of pelagic clay deposition in the western North Pacificlies west of the Hawaiian and Emperor Seamount chains. Because the two regions form a single sedimentary province controlled by similar sedimentary processes, the entire region will be considered in this review.

Abstract The hydrogenous fraction of sediment has been defined by Goldberg (1963) as “composed of those solids which inorganically precipitate from sea water.” As it is often difficult to distinguish between the authigenic fraction of hydrogenous origin and a second authigenic fraction formed during early sediment diagenesis (i.e., diagenesis in the upper few 10s of centimeters of sediment), we have expanded the discussion to include the solidsthat precipitate from pore water of the surface sediment even when such reactions involve the diagenetic alteration of a particulate precursor rather than merely the precipitation of dissolved species. Evidence for these reactions has come from the analysis of pore waters (Gieskes and others, 1975) as well as of the solid products. However, interpretations have often been hampered by inherent problems. For example, pore water of rapidly accumulating shallow-water sediment exhibits strong elemental variations, suggesting dynamic reactions with the enclosing solid phases (Sayles and others, 1973). Unfortunately, the reactive solid products commonly constitute only a minor fraction of the total sediment, their composition and even their presence being masked by a largely nonreactive detrital component (Price, 1976). Within the slowly accumulating sediment of the pelagic environment, the case is reversed. The hydrogenous component can represent a major fraction of the bulk sediment, but reactions are slow enough that elemental gradients within the pore water are diminished by diffusion. Despite these problems, enormous progress has been made in recent years, owing to a vast improvement in analytical techniques.

Abstract In 1872, a 61-m sail-powered corvette, the H.M.S. Challenger, under the scientific direction of C. W. Thomson, left Portsmouth, England, on a monumental natural-history survey of the world's ocean basins. Returning in 1876, this ambitious exploration laid the foundation for modern marine geology and oceanography. Despite the immense success of this expedition, it was not until after World War II that marine sciences began their true evolution. The equatorial Pacific—particularly its central portion—played a pivotal role in this rapid post-war scientific development. Indeed, from the late 1940s until today, numerous research programs followed the lead of the Challenger into this region, including five legs of the Deep Sea Drilling Project (DSDP). Their cumulative results have demonstrated that central Pacific sediments are sensitive recorders of the interplay among tectonism, climate, oceanic circulation, and biological productivity. It is not surprising then that this oceanic region, probably more so than any other, has been instrumental in establishing global Cenozoic stratigraphic schemes, in shaping the understanding of paleoceanographic events, and in the development of tectonic models for plate motion. The evolution of our knowledge of the equatorial Pacific can be divided into three phases, with the H.M.S. Challenger expedition representing the first. Although not exclusively concerned with this region, the classic Challenger reports by Brady (1884) on foraminifers, by Haeckel (1887) on radiolarians, and particularly that by Murray and Renard (1891) on deep-sea sediments, unquestionably formed the basis of the unprecedented scientific growth that paleoceanographic research was to experience during the second phase, which spanned the late 1940s through mid-1970s.

Abstract Between the Murray and Mendocino Fracture Zones off the central California margin, the bulk of the continental rise is formed by two major submarine fan deposits, the Monterey Fan to the south and the Delgada Fan to the north (Menard, 1955, 1960) (Fig. 1 and Plate IB). In between these large canyon-fan systems, several small submarine canyons, which dissect the continental slope between Monterey Bay and Pt. Arena, have formed a sloping ramp of sediment without pronounced fan shapes. South of the Monterey Fan, the much smaller Arguello Fan (just south of Fig. 1) is the only other distinctly fan-shaped turbidite system on the deep-sea floor (deposited on oceanic crust) between central California and the southernmost part of the Baja California peninsula. Data in this chapter are largely drawn from recent review papers on the Monterey and Delgada Fans included in the COMFAN (Committee on Fans) compilation and comparison of turbidite depositional systems (Bouma and others, 1985). Little new data are available since the COMFAN summaries, and although the extensive GLORIA (Geologic Long Range Inclined ASDIC [side-scan sonar]) coverage over both these fans will shed light on morphologic features not adequately mapped to date (Field and others, 1984; McCulloch and others, 1984), these data will not be available in time for inclusion in this chapter. Herein, we focus on depositional processes and summarize earlier work on fan structure and history.

Abstract The eastern and western continental margins of the Gulf of Alaska are simple plate boundaries separating the Pacific and North American Plates. On the east is the Queen Charlotte–Fairweather transform fault system, along which the Pacific Plate moves northwest with respect to the North American Plate. On the west is the Aleutian Trench, along which the Pacific Plate converges against the North American Plate and produces a subduction zone (Fig. 1). Between the transform and convergent plate boundaries is a tectonically complicated area including the Yakutat Terrane (Jones and others, 1982), which migrated northwest on the Pacific Plate until it collided with the North American Plate. This collision occurred where the transform and thrust fault boundaries meet to form a 90° change in the trend of the plate boundary (Fig. 1). The general transform and convergent plate motion pattern has existed during the Cenozoic history of the Gulf, but variation in rate of plate motion, collision of features on the plate with North America, and subduction of oceanic crust of different age and physical properties have produced variations in the tectonic history. The tectonics produced by the subduction of thousands of kilometers of ocean crust shaped the geologic record of the convergent margin, but only the rudiments of that record are known because of the reconnaissance level of geological knowledge of the submerged margins around the Gulf of Alaska.