Project 2: Seismic imaging of the crust and uppermost mantle offshore northern Nova Scotia
Dalhousie University Project
Funded by the Natural Sciences and Engineering Research Council of Canada and Offshore Energy Research Association
When continental lithosphere rifts apart and new ocean basins form, a transition region is created between the old, thick continental crust and young, thin oceanic crust. Studying the structure of such continent-ocean boundaries provides direct observational constraints on the nature of rifting processes. Such studies also have direct commercial importance, since the investigated structures play a critical role in the formation of hydrocarbon deposits during subsidence and burial of the sediment. Two primary classes of margins, volcanic and non-volcanic, have been recognized based on the amount of volcanism that occurs during rifting. The Nova Scotia margin is located in a unique position bounding volcanic margins to its south and non-volcanic margins to its north. Previous studies of the structure of this margin during the MARIPROBE Program (1999-2004) resulted in three long combined MCS reflection and wide-angle refraction OBS transects that crossed the margin in three locations (Figure 1; SMART profiles). These profiles show significant variations in rifting style along the margin: (i) a volcanic-type margin in the southwest; (ii) a non-volcanic margin in the centre, with a sharp boundary between highly faulted continental crustal blocks and oceanic crust; and (iii) an extremely amagmatic margin in the northeast, with exposed mantle within the continent-ocean crustal transition and very thin oceanic crust seaward. These zones of transitional crystalline crust appear to coincide with northward transitions within overlying sedimentary structures, from salt-free to salt bodies in the southwest and from autochthonous salt diapirs in the centre to allochthonous salt tongues in the northeast. Understanding of the regional structure of the margin is further enabled by recent long-streamer industry MCS profiles, collected in 2003 by the ION/GXT Corp (Figure 1; NovaSPAN profiles). These profiles make it possible to greatly augment previous knowledge of the crustal transitions along the margin by conducting additional coincident wide-angle refraction profiles in a similar manner to previous studies and then jointly interpreting deep reflection and refraction images.
To better understand the structure of the extremely amagmatic margin in the northeast, Offshore Energy Research Association (OERA; formerly known as OETR) of Nova Scotia funded collection and basic data analysis of a dense OBS margin-normal profile. The OETR Profile was collected by Geopro in the Fall of 2009 and consisted of 100 OBS deployments along NovaSPAN MCS profile 2000 and exteding more than 100 km further seaward (Figure 1; OETR/Geopro Profile). The OETR Project was followed in the Fall of 2010 by a margin-parallel OBS profile named OCTOPUS (Ocean-Continent Transition Offshore Profiling Using Seismics). This profile (Figure 1; OCTOPUS Profile) is about 200 km long, consisted of 20 OBS (Figure 2), was coincident with NovaSPAN Profile 5100, and crossed OETR and SMART-1 MCS/OBS profiles. The OCTOPUS profile examines the transition from non-volcanic to extreme amagmatic rifting, associated with a major change in crustal reflectivity recorded on GXT Nova Span Line 5100 from SP 10000 to SP 160000. The profile crosses the northeastern half of the major structure, as outlined by the change in the basement reflectivity, which we believe represents an important crustal transition.
The coincident OBS and MCS data complement each other and allow iterative processing of the two datasets until the maximum possible information about the subsurface is extracted. We received a 2 year grant from OERA in 2015 to produce first arrival tomographic models and first and secondary arrival layered models of the OETR and OCTOPUS OBS profiles. The obtained information was used to detail with remarkable precision the ocean-continent transition and formation of early ocean crust. We have submitted our first manuscript (Lau et al. J. Geophys. Res.) and will soon submit the second. Dense OBS surveys, like he OETR Profile, contain sufficient information to run waveform tomography and prestack depth migration of wide-angle reflections. Both waveform tomography and prestack depth migration can result in images revealing structural details at depth otherwise not possible to capture. Because of the success achieved during the first two years of funding, OERA decided to provide at least one additional year of funding. We are now focusing on full waveform inversion of the OETR-2009 profile. At the same time we are working on obtaining data for the MIRROR-1 conjugate proife located on the Morocco margin. The goal is to collaorate with our collegues at Iframer in France to produce full waveform sections of the two conjugate profiles.