Abstract The Tinajitas and Peñas Blancas Limestones are thin bioclastic carbonate units of Eocene age in the foreland fold and thrust belt of eastern Venezuela. They represent the only limestone deposits in the otherwise siliciclastic Cenozoic succession of the eastern Venezuelan shelf. The geometry of the carbonate units suggests a coast-parallel, bar-like form; the particle population (larger benthic forams, glauconite, and algal balls) indicates shallow-marine provenance; and there is a regional unconformity with major lacuna landward from the location of the limestone units. We interpret these carbonates as the capping deposit of a coast-parallel ridge on the continental shelf that rose and sank in the Eocene. We propose that this transient ridge was an elastic forebulge that migrated landward across the shelf and craton hinge of the South American continent because of a brief episode of Eocene subduction of northern South American lithosphere. Such subduction may have been caused by closure between the North and South American plates. The bulge had an amplitude of at least 800 mand migrated landward at least 50–100 km. This distance suggests the minimum closure between the plates at the subduction zone. Supporting evidence comes from facies suites of the passive-margin wedge, now juxtaposed in the thrust and fold belt. The most seaward facies suite contains no evident Eocene lacuna or limestone; these facies were at too great a depth for the passing bulge to affect sedimentation. In the shallower intermediate facies, which contains the Tinajitas–Peñas Blancas Limestones, the passing ridge brought the seabed to depths of carbonate production, but not to shoal. In the most landward facies of the passive-margin wedge and on the platform, the ridge caused emergence and erosion, producing the Eocene unconformity of large coast-parallel extent. The resubsidence of the margin in the Oligocene is attributed to passage of a foreland basin, which trailed the forebulge.

Abstract The evolution of the North American continent and adjacent ocean basins in the central Cordillera of the western United States in Phanerozoic time was governed by three sequential tectonic regimes. The first included the creation of a passive margin during the latest Proterozoic to Early Cambrian (Stewart, 1976) and the removal of an unknown amount of sialic crust from the western margin of the continent. The second regime maintained a passive continental margin of western North America from Middle Cambrian to Triassic time, but permitted collisions of outboard terranes with the sialic margin in Mississippian and Permian-Triassic time (Speed, 1982; Dickinson and others, 1983). Since Triassic time, western North America, adjacent oceanic plates, and intervening microplates and other tectonic packets have existed in a regime of active margin tectonics (Hamilton, 1969; Coney and others, 1980; Saleeby, 1983; Saleeby and Busby-Spera, 1992) driven mainly by eastward subduction of oceanic lithosphere. This third and currently operating regime has been marked by diverse phenomena including subduction of oceanic lithosphere below the continent and phases of highly oblique convergence and suture-zone or intra-arc spreading, ridge-trench collision, growth of a continental arc, major foreland contraction and extension, and the accretion of displaced terranes to the sialic edge. Corridors C1 and C2 of the Ocean-Continent Transect Program traverse all essential elements of the transition from Pacific plate oceanic crust to cratonal North America that have resulted from these three tectonic regimes. Figure 1 shows the locations of the corridors in relation to the major tectonic elements

We present a study of older rocks exposed on Grenada and the Grenadine Islands of the southern Lesser Antilles arc platform (SLAAP) with a view toward understanding the structural and stratigraphic evolution of the platform that led to the modern (and Neogene) magmatic arc. The modern SLAAP is an east-tilted half horst that probably developed early in Miocene time by uplift along a zone of north-striking normal faults relative to both sea level and the crust of the adjacent Grenada Basin. The uplift was completed before Neogene magmas erupted at the surface of the SLAAP beginning at about 12 Ma. Twenty-six units of older rocks, defined as those that preceded local Neogene magmatism, have been identified by us and previous workers in the SLAAP. Our contributions are new stratigraphic and structural analyses and microfossil and radiometric dating. The age range of older rocks is early middle Eocene to middle Miocene. The most stratigraphic information comes from Carriacou, which contains newly defined formations of Eocene and Oligocene age that are thrust above markedly different late Eocene facies. The thrust is covered by Miocene, possibly late Oligocene, strata below which there is a lacuna possibly as long as 10 m.y. The oldest rocks of the SLAAP are the middle Eocene Mayreau Basalt. Earlier claims of a Cretaceous age of rocks on Union Island are in error; we show them to be Eocene. Assuming they were all deposited contiguously, the older rocks units can be grouped and interpreted as follows: I, middle Eocene pillow basalt of spreading origin and pelagic cover; II, late middle Eocene–middle Miocene deep basinal sediments comprising arc-volcanigenic turbidite and hemipelagite, together with minor intrusive basalt; III, early(?) and middle Miocene local carbonate platform. Group I basalts are correlated with crust of the Grenada Basin, and the Eocene and Oligocene sediments of I and II are equivalent to the deep strata of the Grenada Basin. No magmatic arc rocks of Paleogene age are recognized in the SLAAP. Older rocks of the SLAAP are deformed by folding, thrusting, and foliation development, and by Neogene normal faulting and intrusion. The principal deformation was a horizontal contraction of northerly bearing in late Oligocene and (or) early Miocene time before the uplift of the half horst. A later horizontal contraction of westerly bearing occurred in the middle Miocene after the horst had developed. The northerly contraction in the SLAAP is interpreted to have arisen by accretion of Grenada Basin cover and slices of shallow basement during a brief episode of subduction of the oceanic crust of the Grenada Basin relatively southward below the (?) Tobago terrane. The accretionary prism of the SLAAP is inferred to have been continuous with the accretionary belt of the Southern Grenada Basin Deformation Front that extends west-southwest to a point west of Margarita. A Paleogene magmatic arc was the source of copious volcanigenic and carbonate clastic sediment to the SLAAP basin in late Middle Eocene and Oligocene times. This arc was not in the SLAAP and has not been directly located. We infer its locus is south of the Grenada Basin, striking west-southwest from Grenada; its original orientation is uncertain, but scanty paleomagnetic data provide no evidence for large rotation. The Neogene magmatic arc of the southern Lesser Antilles is not a clone of the Paleogene arc. It developed transverse to the Paleogene arc and SLAAP accretionary prism, presumably by a major reconfiguration within the Caribbean-American plate boundary zone. The large change in arc trend is thought to be due to collision between the Paleogene arc system and continental South America.

Abstract The North American Continent-Ocean Transects Program is a study of the tectonics and Phanerozoic evolution of the transitional region of the North American hemisphere between its bordering ocean basins and the craton or longstable continental interior. Corridor CI transects the North American Cordillera from the active Mendocino triple junction off the northern California coast, to the North American craton in Wyoming. The North American Cordillera in this transect displays examples of terranes presently being displaced, ancient displaced terranes and deformed North America. The primary aims of this explanatory pamphlet are: 1) to guide and assist the reader in the use of the graphic display; 2) to provide documentation on the sources of data and manner of construction for the graphic display; and 3) to supplement the graphic display with supporting discussions on crustal structure, tectonostratigraphic units, major structures and tectonic evolution. An in-depth synthesis of the CI display will be integrated with that of the C2 display (central California offshore to Colorado Plateau) in the North America Transects Synthesis Volume to be published as part of the DNAG (Decade of North American Geology) series. In order to give the reader a reasonable geological context from which to explore the graphic display and explanatory pamphlet, the first section of the pamphlet will provide an overview of the major tectonic features displayed. From west to east Corridor C-l (Fig. 1) crosses a number of major tectonic features that record the growth of the North American continental margin throughout the Phanerozoic, as well

Abstract DNAG Transect C-1. Part of GSA's DNAG Continent-Ocean Transect Series, this transect contains all or most of the following: free-air gravity and magnetic anomaly profiles, heat flow measurements, geologic cross section with no vertical exaggeration, multi-channel seismic reflection profiles, tectonic kindred cross section with vertical exaggeration, geologic map, stratigraphic diagram, and an index map. All transects are on a scale of 1:500,000.

Abstract The theme of these explanatory notes is the westward growth of the North American continent in the region transected by the strip maps and cross-sections on the accompanying displays. The focus is on continent-ocean interactions and on the transition from the Precambrian sialic basement of North America to accreted terranes during Mesozoic and Cenozoic time. The first section describes the active plate boundaries along which the Juan de Fuca plate is being created and consumed. Next is a brief review of major tectonic events during which diverse terranes were accreted to the margin of North America.

Abstract DNAG Transect B-3. Part of GSA's DNAG Continent-Ocean Transect Series, this transect contains all or most of the following: free-air gravity and magnetic anomaly profiles, heat flow measurements, geologic cross section with no vertical exaggeration, multi-channel seismic reflection profiles, tectonic kindred cross section with vertical exaggeration, geologic map, stratigraphic diagram, and an index map. All transects are on a scale of 1:500,000.

Abstract DNAG Transect B-1. Part of GSA’s DNAG Continent-Ocean Transect Series, this transect contains all or most of the following: free-air gravity and magnetic anomaly profiles, heat flow measurements, geologic cross section with no vertical exaggeration, multi-channel seismic reflection profiles, tectonic kindred cross section with vertical exaggeration, geologic map, stratigraphic diagram, and an index map. All transects are on a scale of 1:500,000.

Abstract The A-2 Kodiak to Kuskokwim Transect crosses the seismically active eastern Aleutian Trench in the Gulf of Alaska, traverses the marine and insular forearc region that includes southern Kodiak Island and Shelikof Strai t, traverses the Alaska Peninsula which includes the modern Aleutian Are, drops into the Bristol Bay and lowland area of the Bering Sea, and comes ashore in the southwestern Kuskokwim Mountains. Thus the A-2 transect crosses an ocean to continent transition from the oceanic crust of the north Pacific Basin to the interior cordillera of Alaska. This transition is made across a series of fossil arc-trench systems. We located the transect to take advantage of a concentration of marine geophysical data, a local network of seismometers to record earthquakes accurately, recent geologic studies on the Alaska Peninsula and Kuskokwim Mountains, and a tectonic style that generally represents the eastern Aleutian arc-trench system. We are identified on the graphic display with our individual contributions. In the graphic display we have shown the basic data allowed by both the small scale and the transect's group format which emphasizes intra-transect comparison. Most of the data we synthesize here appears in greater detail elsewhere and the other publications are referenced. A similar but shorter multi-authored cross section located north of this transect was published as part of the previous U.S. Geodynamics Committee transect series (von Huene et a L, 1979a); this transect differs from the previous one in being longer and in encompassing more recent studies. Two marine geophysical surveys were