Seismic velocities within the sedimentary succession of the Canada Basin and southern Alpha-Mendeleev Ridge, Arctic Ocean: evidence for accelerated porosity reduction?

Abstract

S U M M A R YThe Canada Basin and the southern Alpha-Mendeleev ridge complex underlie a significant proportion of the Arctic Ocean, but the geology of this undrilled and mostly ice-covered frontier is poorly known. New information is encoded in seismic wide-angle reflections and refractions recorded with expendable sonobuoys between 2007 and 2011. Velocity-depth samples within the sedimentary succession are extracted from published analyses for 142 of these records obtained at irregularly spaced stations across an area of 1.9E + 06 km 2 . The samples are modelled at regional, subregional and station-specific scales using an exponential function of inverse velocity versus depth with regionally representative parameters determined through numerical regression. With this approach, smooth, non-oscillatory velocity-depth profiles can be generated for any desired location in the study area, even where the measurement density is low. Practical application is demonstrated with a map of sedimentary thickness, derived from seismic reflection horizons interpreted in the time domain and depth converted using the velocity-depth profiles for each seismic trace. A thickness of 12-13 km is present beneath both the upper Mackenzie fan and the middle slope off of Alaska, but the sedimentary prism thins more gradually outboard of the latter region. Mapping of the observed-to-predicted velocities reveals coherent geospatial trends associated with five subregions: the Mackenzie fan; the continental slopes beyond the Mackenzie fan; the abyssal plain; the southwestern Canada Basin; and, the Alpha-Mendeleev magnetic domain. Comparison of the subregional velocity-depth models with published borehole data, and interpretation of the station-specific best-fitting model parameters, suggests that sandstone is not a predominant lithology in any of the five subregions. However, the bulk sand-to-shale ratio likely increases towards the Mackenzie fan, and the model for this subregion compares favourably with borehole data for Miocene turbidites in the eastern Gulf of Mexico. The station-specific results also indicate that Quaternary sediments coarsen towards the Beaufort-Mackenzie and Banks Island margins in a manner that is consistent with the variable history of Laurentide Ice Sheet advance documented for these margins. Lithological factors do not fully account for the elevated velocity-depth trends that are associated with the southwestern Canada Basin and the Alpha-Mendeleev magnetic domain. Accelerated porosity reduction due to elevated palaeo-heat flow is inferred for these regions, which may be related to the underlying crustal types or possibly volcanic

Significance of Northeast‐Trending Features in Canada Basin, Arctic Ocean

“…Crustal velocity mapping (Chian et al, ) and sediment thickness data (Shimeld et al, ) also demonstrate that the polygon of oceanic crust is bisected by the Canada Basin Gravity Low anomaly (CBGL, Figure ). The CBGL is associated with a roughly 45 km wide valley with high‐relief basement blocks along its axis (Shimeld et al, ).…”

“…In the deep water of Canada Basin, velocities mapped from sonobuoys using seismic refraction forward modeling (Chian et al, 2016) indicate that oceanic crust, characterized by oceanic layer 3 velocities of 6.7-7.1 km/s, is restricted to the central portion of the basin (white squares, Figure 1). Transitional crust, characterized by higher velocities of 7.2-7.6 km/s, in general surrounds the polygon of oceanic crust, but varies considerably in width, >350 km in the wide southeastern portion of the basin beneath the Mackenzie Fan north of the Mackenzie River (MR, Figure 1) to <40 km in a narrow zone offshore northern Alaska (blue diamonds, Figure 1).…”

“…This paper characterizes these features within and on the edges of Canada Basin, as well as features on land in Alaska and Canada. Following a description of the data sets used, we synthesize our observations with the crustal results of Chian et al (2016) and finally propose that the most likely explanation for this trend is an inherited subparallel fabric indicative of widespread strike slip, or transtensional, tectonics in the early formation of the basin. We propose a two-phase model of opening in which development of these strike-slip features in the latest Jurassic to Early Cretaceous (phase 1) preceded seafloor spreading and rotation in the Early Cretaceous (phase 2).…”

“…Thin solid black lines show seismic reflection data locations. Sonobuoy results from Chian et al (2016) use symbols shown in the Explanation except for small black dots (HALIP) not interpreted as part of Chian et al (2016). Symbols correspond to shading for: OC, oceanic crust in white; TC, transitional crust in dark gray; CC, continental crust in pink.…”

“…The crustal velocity mapping presented in Chian et al (2016) provides new constraints for integrating existing geological and geophysical data. In attempting such an integration, we emphasize the existence of northeast-trending features in the basin.…”