Field evidence, map compilation, geochemistry, geochronology, and potential fi eld data document six intervals of Cretaceous magmatism in the central Sverdrup Basin. These are: (1) Hauterivian (ca. 130 Ma) volcaniclastic deposition in the lower Isachsen Formation; (2) 126.6 ± 1.2 Ma (U-Pb zircon) gabbroic intrusion; (3) 120.8 ± 0.8 Ma (U-Pb baddeleyite) diabasic intrusion; (4) 105.40 ± 0.22 Ma (U-Pb detrital zircon) pyroclastic deposition at the top of the Invincible Point Member, Christopher Formation; (5) upper Albian (ca. 103 Ma) pillow and hydroclastic breccia in the upper Christopher Formation; and (6) uppermost Albian (ca. 101 Ma) volcanic breccia and scoria in the Hassel Formation.Whole-rock geochemical data show that these magmatic rocks are similar to previously documented High Arctic large igneous province tholeiitic basalts, but analyses of fresh glass in tuffs reveal evolved ferroandesite to dacite compositions not recorded in whole-rock data. Approximate ages of saucer-shaped sills inferred from the relationship of sill width to depth of emplacement suggest at least three intervals of sill emplacement between 130 and 120 Ma. The new data show that volcanism in the Sverdrup Basin was of greater spatial extent, and that magmatism occurred more frequently, than was previously recognized. Comparison of the new central Sverdrup Basin data and interpretations with other data sets from the Sverdrup Basin, Svalbard, and Franz Josef Land suggests that High Arctic large igneous province magmatism occurred over a more extended period of time in the central Sverdrup Basin than in other regions.
A reconstruction of the Tintina fault is applied to regional geophysical and topographic data, facilitating the definition of west trending lineaments within the lower crust and/or mantle lithosphere, oblique to the NW trending structure of the Cordilleran terranes. The lineaments, which exhibit a range of geophysical and geological signatures, are interpreted to be related to the Liard transfer zone, continuous to the Denali fault, that divided lower and upper plates during late Proterozoic-Cambrian rifting of the Laurentian margin. Three-dimensional gravity models show a density increase in the lower crust and mantle lithosphere to the north. The transfer zone also divides bimodal mantle xenolith suites to the south from unimodal suites to the north. These conclusions suggest that extended North American basement, related to Laurentian margin rifting that would have brought mantle lithosphere rocks to a shallow depth, continuously underlies a thin carapace of accreted terranes in western Yukon and eastern Alaska. The interpreted continuity of North American basement reaffirms that if oroclinal bending of the Intermontane terranes occurred, then it was prior to its emplacement upon the rifted basement. Examination of the spatial relationships between mineral occurrences and postaccretionary, Cretaceous lithospheric lineaments, from their manifestation in geophysical, geological, and topographic data, suggest that the late Proterozoic lineaments influenced Mesozoic mineralization through influence on the development of the shallow crustal structure, intrusion, and exhumation and erosion.
[1] The state of compaction and fluid pressure in the Barbados accretionary wedge near its toe, at Ocean Drilling Program Site 949, were investigated by modeling travel times of seismic waves from ocean bottom shots to a borehole geophone array. The model, constrained by a three-dimensional seismic survey and well logs, shows (1) a velocity gradient of about 1-1.25 s À1 in the uppermost 180-230 m of the wedge; (2) a zone of variable, but no net change in, velocity between 230 and 350 m depth; (3) a low-velocity zone 40-50 m thick just above the décollement at 391 m; and (4) a displacement of the low-velocity zone by thrust faults. Pore fluid pressure sections derived from P wave velocity show that the upper half of the wedge is normally pressured while the lower half is overpressured. The $160 m thick, underconsolidated basal zone shows anisotropy, which increases downward. The lowest 40-50 m has velocity varying (1) azimuthally (3%), being fastest in the direction of plate convergence, and (2) in the vertical plane (2-5%), horizontal faster than vertical. After correction for the effect of anisotropy in the derivation of effective stress from seismic velocity the calculated pore fluid pressure ratio l does not exceed 0.9 and in the lowest 40-50 m of the basal zone, is between 0.71 and 0.82, with l* [(fluid pressure À hydrostatic)/(lithostatic pressure À hydrostatic)] between 0.5 and 0.65, in accordance with in situ measurements of fluid pressure in the décollement zone beneath. These indicate that the accretionary wedge is stronger and less overpressured than was previously supposed.INDEX TERMS: 3025 Marine Geology and Geophysics: Marine seismics (0935); 0935 Exploration Geophysics: Seismic methods (3025); 3022 Marine Geology and Geophysics: Marine sediments-processes and transport; 5114 Physical Properties of Rocks: Permeability and porosity; 8150 Tectonophysics: Plate boundary-general (3040); KEYWORDS: subduction zone accretionary complex, seismic velocity, anisotropy, porosity, pore fluid pressure Citation: Hayward, N., G. K. Westbrook, and S. Peacock, Seismic velocity, anisotropy, and fluid pressure in the Barbados accretionary wedge from an offset vertical seismic profile with seabed sources,
In 2008, a Vibroseis seismic reflection survey was acquired by Geoscience BC across the eastern part of the volcanic-covered Nechako basin in central British Columbia, where Cretaceous sedimentary rocks have been exhumed along a NNW trend. Good signal penetration through the volcanic cover is indicated by lower crustal reflections at 8–12 s, which were recorded by the entire seismic survey. Comparison of the 2008 seismic survey with data from a previous survey indicates that the lack of reflectivity in the earlier surveys is generally representative of the subsurface geology. The seismic data show that ∼1700 and ∼2900 m thick sub-basins are present at the northern and southern ends of this trend, but the intervening Cretaceous rocks are discontinuous and relatively thin. The creation of a passive-roof duplex by Campanian or later low-angle thrusting is inferred within the thickest Cretaceous strata, but elsewhere faulting is likely related to Eocene extension or transtension. Seismic reflections are also recorded from folded volcanic stratigraphy, the base of the surface volcanic rocks, an underlying volcaniclastic stratigraphy, and intrusions projecting into a Quaternary volcanic cone. Seismic interpretation is complemented by coincident audiofrequency magnetotelluric surveys, from which faulting is inferred at offsets in a regional conductor. No regionally extensive stratigraphy can be identified within the seismic data, and the central Nechako basin appears to be a complex network of small, deformed sub-basins, rather than a single large basin.
A new approach to the 3D inversion and interpretation of gravity data is applied to the crustal architecture of the northern Cordillera of Canada and Alaska. The technique models the distribution and depth extent of rocks with systematic density contrasts, such as sedimentary or intrusive rocks. In the northern Cordillera, the geometry of low-density zones, primarily associated with middle to Late Cretaceous granitic intrusions, and Neoproterozoic and Cretaceous sedimentary rocks, is defined. Variation in the depth extent of these zones delimits a surface, interpreted herein as a regional décollement syntectonic with, or postdating, middle Cretaceous intrusions, but predating and displaced~430 km by the Eocene-aged Tintina fault. The décollement trends N35°E and shallows from a depth of~15-20 km beneath Selwyn basin tõ 11 km beneath the Mackenzie Mountains. Here surface faults echo the décollement geometry, linked to fold and thrust belt development between the middle Cretaceous and Paleocene. The décollement continues southward for an unknown distance into northern British Columbia, but the Liard line represents a structural discontinuity between the fold and thrust belts of the Mackenzie Mountains and northern Rocky Mountains. The décollement's northwestern extent is broadly defined by surface faults in the northern Ogilvie Mountains. However, prior to Tintina fault displacement, the décollement was likely connected to a fold and thrust belt and related décollement in east-central Alaska. Estimates of post-middle Cretaceous exhumation suggest regional tectonic modification of the décollement, possibly including the presently active detachment inferred for the weak lower crust.HAYWARD 307
S U M M A R YThe Tofino Basin is a sedimentary forearc basin that overlies the continental shelf of the Cascadia margin to the southwest of Vancouver Island. The basin, which contains up to ∼4 km of marine clastic sedimentary rocks, formed following accretion in the Early Eocene of the Crescent and Pacific Rim Terranes, and subsequent accretionary wedge basement. Subduction of the Juan de Fuca plate has since been the primary tectonic driving force in the development of the basin's structure.Investigations using coincident seismic reflection profiles, tomographic velocity models and recently reassessed biostratigraphic well data show that basement composition has largely controlled deformation of the overlying Tofino Basin sediments. Anticlinal folds overlying the accretionary wedge exhibit low P-wave velocities at the apex of the fold, which may be related to fracturing of older, more lithified sediments accompanied by fluid expulsion from the accretionary wedge. In contrast the velocity variation across folds over the Crescent Terrane mimics the fold geometry, and does not appear anomalous.A sub-basin (containing up to ∼3 km of Oligocene to Holocene sediment) has developed in the central part of the Tofino Basin at the boundary between the Crescent and Pacific Rim Terranes. Seismic interpretation suggests that deposition has increased more rapidly in the Late Miocene to Holocene. Subsidence within the sub-basin is likely to have been controlled by sediment loading, flexure and regional tectonic forces, localized by pre-existing zones of weakness such as the Tofino Fault. The development of the sub-basin may also have been influenced by the displacement landward of part of the lower forearc crust during subduction erosion. Diapiric structures along the axis of the sub-basin suggest that fluid expulsion into the Tofino Basin from the deeper accreted terranes is localized by the terrane-bounding fault. Further seaward, fluid expulsion from the accretionary wedge may be more pervasive. The seismic data demonstrate that subsidence of the sub-basin beneath the inner continental shelf has been occurring relatively late in the history of the Tofino Basin, and that the Cascadia accretionary complex cannot be viewed as growing gradually seaward with no inboard deformation.
The northeastern Stevenson Ridge map sheet (parts of NTS 115-J, I, P and O) is underlain by Paleozoic to Paleogene rocks that locally host Cu-Au porphyry and Au mineralization. The southwestern part of the area is dominated by the mid-Cretaceous Whitehorse plutonic suite which forms the backbone of the Dawson Range Mountains. The north side of the Dawson Range comprises rocks typical of the Yukon-Tanana terrane, including a belt of Permian Klondike schist (metavolcanic rocks), Permian Sulphur Creek plutonic suite and pre-Devonian Snowcap assemblage metasedimentary rocks. The northeast side of the area is dominated by the Mississippian Simpson Range plutonic suite that is intruded by the Triassic Pyroxene Mountain suite and early Jurassic Aishihik suite. The Mississippian to Jurassic rocks are thrust over the Snowcap assemblage along the post-Triassic Yukon River thrust. Late Cretaceous and younger faults occur throughout the area but have only modest offsets.
Geological investigation of the near-surface in the southeastern Nechako Basin is difficult. Shallow seismic reflection imaging is poor due in part to an extensive cover of Eocene and Neogene volcanic rocks. Outcrops of these volcanic rocks, and the primarily Cretaceous bedrock, are commonly obscured by Quaternary deposits and vegetation. Estimates of near-surface P-wave velocity are derived from the tomographic inversion of seismic first-arrivals, an effective tool when seismic imaging is poor. Tomographic model velocities are in agreement with sonic logs and laboratory samples, except for those from the Neogene Chilcotin Group. Cretaceous sedimentary rocks have velocities of ∼2800–4200 ms–1. The Eocene Endako and Ootsa Lake groups, which have velocities of ∼3000–4200 ms–1, are not distinguishable based on velocity. The velocity, the character (density, focus, and penetration depth) of rays, and ties with well and surface geology constrain the subsurface extent of the Endako Group adjacent to well b-82-C. The Chilcotin Group typically exhibits velocities (∼2400–3000 ms–1) lower than corresponding velocities from sonic logs (4500–5200 ms–1) and laboratory measurements (5000–5200 ms–1). These low model velocities may be due to the presence of high porosity, brecciated rocks near to the surface, in comparison with the other measurements that have focussed on lower porosity massive lavas. The lowest mean velocities, located to the southeast, are related to anomalously thick, high porosity, breccia-rich deposits of Chilcotin Group. This conclusion is consistent with the interpretation that the Chilcotin Group is thicker in paleo river valleys.
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