Abstract Group velocities for a period range of 6–60 s for the fundamental mode of the Rayleigh wave passing across the Himalaya–Karakoram–Tibet orogen are used to delineate the structure of the upper lithosphere using the data from 35 broadband seismic stations. 2D tomography velocity maps of group velocities were obtained at grids of 1° separation. Redefined local dispersion curves are inverted non-linearly to obtain 1D velocity models and to construct a 3D image of the S-wave structure down to a depth of 90 km. The Moho discontinuity is correlated with c. 4.0 km s −1 S-wave velocity. The results depict a NE-dipping trend of the Moho depth from c. 40 km beneath the frontal part of the Himalaya to up to c. 70–80 km beneath the collision zone before shallowing substantially to c. 40 km beneath the Tarim Basin. The study also reveals thick deposits of sediments in the Indo-Gangetic plains and the Tarim Basin. A broad low-velocity zone at mid-crustal depth in the western Tibetan Plateau, the Karakoram region and the surface-collision part of the India–Eurasia tectonic plates is interpreted as the effect of partial melting and/or the presence of aqueous fluid. The high velocities in the southern deeper part indicate that the lower crust and uppermost mantle of the Indian Plate are dense and cold.

Abstract The first comprehensive geological and geophysical surveys of the Brazilian continental margin during the 1970s recognized the crust in the SE Brazilian basins as ‘anomalous’ but models for the opening of the South Atlantic proposed at that time invoked a very narrow continent–ocean transition. Nevertheless, such studies established the presence of a thick sedimentary prism, including an extensive salt layer under the São Paulo Plateau. The earliest reconstructions for the South Atlantic invoked a seaward shift of the spreading axis to account for the asymmetric widths of the salt layer between the Brazilian margin and its conjugate in offshore Africa. Although our understanding of continent–ocean transition has progressed since then, direct seismic imaging at crustal scale has only been possible recently through long offset (10 km), deep recording (18 s), pre-stack depth migrated (PSDM) to 40 km, seismic-reflection data. These data allow us to generally image the Moho from under thick continental crust (>30 km) to thin oceanic crust ( c. 5 km). Although the nature of the transitional crust is still contested, these seismic data allow for constraints on various models for continent–ocean transition. Future integrated studies utilizing PSDM and refraction-seismic data will further refine these models.

Abstract The Canadian Arctic margin, from the Alaska/ Canada boundary north-northeastward almost 1,000 km to the north of Banks Island, represents one of the largest sedimentary wedges in the world. With primary input from the Mackenzie River, the margin appears to have all the necessary components of a “world-class” petroleum province: possibilities of structural and stratigraphic traps, multiple potential source rocks, an abundance of potential reservoirs and seals, and timely migration resulting in almost 50 hydrocarbon accumulations discovered to date. However, lack of a large enough discovery to warrant commercial production has resulted in exploration being limited only to the shallow parts of the Mackenzie River Delta (water depths <40 m). Interpretation of recent, long-offset (9 km), deep (18-sec recording), prestack-depth migrated (PSDM to 40 km) data has resulted in extending the petroleum potential to deeper waters and to areas away from the delta. Industry has recognized this potential by acquiring leases beyond the shallow waters, but the full potential of the area will only be realized by new exploratory drilling.

Abstract The US Chukchi Shelf is a proven petroleum province similar to the prolific hydrocarbon-bearing region in the adjacent North Slope of Alaska. This shelf is part of the Arctic Alaska plate that was presumably connected to the Canadian Arctic Islands until the opening of the Canada Basin. A new data set (3130 km) of deep 2D seismic reflection profiles, acquired in 2006 and depth processed to 40 km, provides excellent images of the drillable ( c . 8 km depth) stratigraphy as well as of deeper structures. The data also permit regional mapping of the tops of crystalline basement and the Moho discontinuity. The area has a tectonic history of multiple phases of rifting: a pre-Late Devonian phase that culminated in inversion and thrusting; a Late Devonian–Mississippian phase that accommodated Ellesmerian sedimentary deposits as a sag sequence; and a Jurassic to Early Cretaceous phase that is contemporaneous with the opening of the Canada Basin. Hanna Trough is recognized as a Palaeozoic basin with a multistage history while the North Chukchi Basin formed in early Cretaceous as an extensional basin. Only five wells have been drilled in the area; hence numerous opportunities for further exploration exist in the Chukchi Shelf.