Deep structure of the Porcupine Basin from wide-angle seismic data

The Porcupine Basin, part of the frontier petroleum exploration province west of Ireland, has an extended history that commenced prior to the opening of the North Atlantic Ocean. Lithospheric stretching factors have previously been estimated to increase from <2 in the north to >6 in the south of the basin. Thus, it is an ideal location to study the processes leading to hyper-extension on continental margins. The Porcupine Median Ridge (PMR) is located in the south of the basin and has been alternatively interpreted as a volcanic feature, a serpentinite mud diapir or a tilted block of continental crust. Each of these interpretations has different implications for the thermal history of the basin. We present results from travel-time tomographic modelling of two approximately 300 km-long wide-angle seismic profiles across the northern and southern parts of the basin. Our results show: (1) the geometry of the crust, with maximum crustal stretching factors of up to 6 and 10 along the northern and southern profiles, respectively; (2) asymmetry of the basin structures, suggesting some simple shear during extension; (3) low velocities beneath the Moho that could represent either partially serpentinized mantle or mafic under-plating; and (4) a possible igneous composition of the PMR.

Structure profonde du mont Ross d'après la réfraction sismique (îles Kerguelen, océan Indien austral)

Mont Ross is the main volcanic feature of the Kerguelen Archipelago (terres Australes et Antarctiques françaises). This newly formed volcano buildup over 2 Ma provides us with an outstanding model of volcanism occurring on an intraplate structure already aged 40 Ma. Mont Ross is the subaerial part of a plutonic complex located in Galliéni Peninsula. From seismic refraction studies, P-wave velocities within the upper crust range downward from 5.35 km/s at sea level to 6.60 km/s at a depth of 11 km. These are definitely higher than those encountered within surrounding basalts known as plateau basalts. These high velocities reveal, at first glance, an origin and composition of the basement of Mont Ross far distinct from those of tholeiitic or transitional lava flows generated near spreading centres. By comparison with plutonic ring complexes, it is reasonable to state that monzonite and syenite are the basic materials of the basement. Seismic velocities (6.85 to 7.30–7.35 km/s) and related Poisson ratio (σ = 0.30) within lower crust are consistent with gabbros as prominent material. The thickness of the lower crust below Mont Ross (6–7 km) is roughly the same as that below the archipelago. Gabbros are exposed around several plutonic ring complexes spread over the archipelago. The transition to mantle might be modelled by a 2 km thick transition zone, with high velocity gradient, already noticed below the archipelago. Velocities of 7.30–7.35 km/s at the base of the crust below Mont Ross do not preclude contamination of the lower crust by mantle material. Both gravity and seismic data substantiate the occurrence of high density (velocity) within the upper crust below Mont Ross. Isostatic compensation of Mont Ross is rather achieved by a flexural deflection of the lithosphere than by an Airy-type model. The structures of Mont Ross and Hawaiian volcanoes bear analogies likely related to their intraplate genesis.

Bracketing ages for the formation of the Ring Creek lava flow, Mount Garibaldi volcanic field, southwestern British Columbia

The Ring Creek lava flow, extending from Opal Cone, generally is considered to be the youngest volcanic feature in the southern section of the Mount Garibaldi volcanic field, southwestern British Columbia. An in situ stump dated 10 650 ± 70 BP (Beta 43865) obtained from a raised delta partially overridden by the lava flow indicates that the lava was extruded after ca. 10.7 ka. At the Skookum Creek and Mamquam River confluence, sediments eroded from the Ring Creek lava flow form an alluvial fan that contains charcoal dated 9360 ± 160 BP (Beta 38914), which indicates that the extrusion probably occurred before ca. 9.3 ka. Thus these radiocarbon ages bracket the age of the Ring Creek lava flow and indicate that volcanic activity in the southern section of the Mount Garibaldi volcanic field has been dormant since at least ca. 9.3 ka.