Abstract Paleoproterozoic terranes of the Man-Leo Shield in the southern part of the West African craton host one of the world’s largest gold provinces with an overall endowment >10,000 metric tons (t). Although gold deposition commenced by ca. 2170 Ma, most deposits formed later, either during the inversion and metamorphism of intraorogenic sedimentary basins between ca. 2110 and 2095 Ma, or during later transcurrent deformation and associated widespread high K plutonism following docking of Archean and Paleoproterozoic domains within the craton at ca. 2095 Ma. Deposits formed between ca. 2110 and 2095 Ma include those with free gold in quartz veins and refractory gold in arsenopyrite and/or pyrite, and are associated with halos of carbonate, sericite, chlorite, and albite alteration. Most are located in bends and intersections between shear zones, minor faults, folds, and entrained blocks of relatively reactive igneous rock. Conglomerate-hosted gold deposits of the Tarkwa district formed early in the 15-m.y.-long period. Gold deposits that formed subsequently between ca. 2095 and 2060 Ma have a wider variety of styles, geologic settings, and metal assemblages. District-scale albite, carbonate, and tourmaline alteration, hydrothermal breccias, and a close relationship to high K granitoids characterize some of these deposits, whereas others are more typical orogenic gold deposits that are similar to those formed earlier during the craton evolution.

Abstract In multichannel seismic reflection record sections, clear imaging is accomplished largely by processing. The data acquired across ocean trenches, where water is from 5 to 8 km deep, the sea floor is irregular, and the geology is complex, must be processed with procedures not commonly used on data from the adjacent continental shelf. We have experimented to develop better processing with data from the eastern Aleutian Trench. One record is used here to illustrate the improvement possible. It contains a complex faulted fold at the base of the slope, and processing is made difficult by the opposing dips of faults and by the changing vergence of folds (Figure 2.1). Careful velocity analysis and a special processing sequence are needed to image the velocity-sensitive areas of these data. In our experience, clear imaging of complex structures is assured only when each step in the processing sequence is accompanied by an appropriate level of velocity analysis. Here we show the difference in imaging a complex structure in the spectrum between first-level or production processing and research-level processing.

Abstract The seismic section across the Eastern Aleutian margin off southern Kodiak Island illustrates structure from the axis of the eastern Aleutian Trench to the edge of the Kodiak shelf. The seafloor morphology includes a flat trench axial area, a lower slope with two main steps, and a sharp topographic break marking the base of the steepened upper slope. The seismic section crosses a deep canyon in the upper slope, connected to one of the relict glacial troughs that cross the Kodiak Shelf (Hampton, 1983). The Kodiak margin is composed of the insular outcrops containing metamorphosed accretion complex of Upper Cretaceous to Eocene age, the Kodiak shelf with the Neogene Albatross basin behind a high at the edge of the shelf named Albatross bank, and the landward slope of the trench. Albatross basin is filled with upper Miocene to Recent sediment 5 km deep (Fisher and von Huene, 1980) and is floored by a subareal erosion surface across landward-tilted Eocene and Oligocene (?) strata. These strata were sampled northeast of the seismic record section at Middleton Island (Rau et al., 1977; Keller et al., 1984), on the seaward flank of Albatross basin (Herrera, 1978), and southwest of it near Sanak Island (Bruns et al., in press). Subsidence of the Miocene regional erosion surface began about 6 Ma and subsequently, about 2 Ma, Albatross bank was uplifted at least 3 km (Fisher and von Huene, 1980; von Huene et al., in press). Thus, the steep upper slope that descends from Albatross bank

Abstract The geologic history of Cenozoic sedimentary and volcanic rocks of the Oregon continental margin is interpreted as having involved episodic periods of underthrusting, transcurrent faulting, and extension between the oceanic and North American plates. The seismic sections across this margin illustrate that both compressional and extensional forces have molded the tectonic framework (Snavely et al., 1980). The Oregon Coast Range and inner shelf is floored by Paleocene to lower Eocene ridge basalt (see diagrammatic cross section), which is interpreted to represent eruptions in an elongate basin formed by rifting of the continental margin (Snavely, 1984). Oceanic islands and sea mounts were constructed on the basaltic ocean floor, and the volcanic flows and breccia that erupted from these centers intertongue with neritic to bathyal sedimentary rocks of early and middle Eocene age. Middle Eocene turbidite sandstone overlaps both the oceanic basalts and the pre-Tertiary rocks of the Klamath Mountains, suturing the Coast Range to North America about 50 Ma. Oblique convergence between the oceanic and the North American plates occurred during most of post-middle Eocene time, but periods of more head-on convergence occurred in the middle late Eocene and late middle Miocene. Sedimentation, punctuated by episodes of volcanism, was virtually continuous in the forearc basin, the axis of which lay along the present inner continental shelf. More than 7000 m of sedimentary and volcanic rocks were deposited in this basin (Snavely et al., 1980), which is located east of seismic lines 4 and 5 (see diagrammatic cross section). The principal structure

Abstract