Three multichannel seismic reflection profiles were collected on the riffed continental margin southeast of Nova Scotia, eastem Canada. The profiles cross the East Coast Magnetic Anomaly (ECMA), which parallels much of the margin of eastem North America south of the Grand Banks and which is usually associated with the transition from continental to oceanic crust. Studies to the south of the work reported here suggest that the ECMA may be related to the emplacement of large thicknesses of late riff stage or early driff stage igneous material which is characterized by seaward dipping reflections in basement and a high-velocity lower crustal layer. The seismic data show that seaward dipping reflections (SDR) continue northward into the study area and support the correlation between the SDR unit and the presence of a well-developed ECMA. Magnetic modellng confirms this association, although it does not rule out an additional contribution to the magnetic anomaly from an edge effect or suture. Just north of the study area the ECMA diminishes and is no longer well developed. The SDR unit also terminates and it is not observed over most of the Nova Scotian margin. If our understanding of the origin of these features is correct then their disappearance marks a transition from a volcanic margin in the south to a nonvolcanic margin in the north. The association of the transition with significant changes in the preriff fabric of the adjacent continental crust, in the trend of synriff extensional structures, and in the width of the zone of thinned continental crust below the margins must be clues to the deeper processes controlling the amount of volcanism produced. We suggest that these clues are consistent with small-scale convection as a mechanism for delivering large melt volumes to crustal depths during riffing.

A deep marine seismic reflection profile was obtained across the Mesozoic rifted continental margin off Nova Scotia, eastern Canada. This profile crosses the Seotian Basin, one of the deepest basins on the margin of eastern North America, and it complements other deep crustal seismic data on this margin. The seismic data have been interpreted in conjunction with gravity anomaly and subsidence data. They show significant thinning of the continental crust over a zone about 200 km wide. The mode of extensional deformation is probably a combination of pure and simple shear; there is evidence for simple shear in the crust. The continent‐ocean boundary lies near the seaward edge of synrift salt below the continental rise. A 100‐km‐wide zone of very thin (approximately 9 km or less) continental or transitional crust extends seaward from the outer shelf to this boundary. Reflectivity of the oceanic crust adjacent to the margin shows evidence of progressive igneous construction, perhaps modified by extensional faulting. This margin is nonvolcanic, and the transition to the volcanic margin off the eastern United States occurs about 500 km southwest of the seismic line. The width of the zone of crustal extension is much greater on this nonvolcanic margin segment than it is on the volcanic margin to the south. It seems likely that the prerift fabric of the continental lithosphere controls this width. A narrow rift may be prone to vigorous asthenospheric convection and therefore to more voluminous volcanism. However, significantly narrower zones of crustal extension occur on other nonvolcanic margins, so factors in addition to rift width, such as the rate of rifting, may also be important.

New high-resolution magnetic data have been acquired along the coast of western Cape Breton Island near Cheticamp, Nova Scotia, in a transition zone between exposed, elevated basement of the Cape Breton highlands and the adjacent Carboniferous sediment-filled Magdalen Basin. These data were collected to provide continuity between the mapped onshore geology and the geophysical-based interpretations of offshore structure. Separation of the geologic component of the data from the effects of diurnal and other variations in the Earth's magnetic field was made difficult by recording problems at the nearby base recording station. Careful correlation of the fragmented station signal with records from a nearby permanent magnetic observatory enabled a reasonable diurnal signal to be synthesized and applied successfully to the data. Additional processing and filtering helped to enhance small anomalies in the data. Several low-amplitude, fairly linear magnetic anomalies are visible in the reduced anomaly data, generally trending north to northwest away from the coastline. Small-amplitude lineations in the offshore at Cheticamp are associated with folded, tilted, or faulted strata imaged on coincident seismic reflection data and are interpreted as representing juxtaposed units of Carboniferous strata. Other small anomalies appear to represent shallow contacts between intrusive or metasedimentary rocks visible in outcrops near the coast. A stronger, coast-parallel anomaly that extends across the study area from a regional magnetic high in the north is coincident with an offset in basement rocks or deeper strata beneath Carboniferous basin fill. This anomaly may mark part of the faulted transition zone between the elevated highlands of northwestern Cape Breton Island and the Magdalen Basin depocentre.

Research was carried out in the Bras d'Or Lakes, Nova Scotia under the aegis of Project X-29 in NRCan's Geoscience for Ocean Management Program, with additional funding from the Climate Change Impacts and Adaptation Program,. The goal was to provide the scientific information that would help resolve management problems arising from the expected impact of accelerated sea-level rise on the coasts of the Bras dOr Lakes. The specific objectives of the work were: Define the recent, present and future trends of water-level increase in the Bras dOr Lakes. Knowing the recent (last 5000 yr) trend would allow us to understand how rising water levels triggered changes to coastal environments in the lakes. Knowing the modern trend would help us to understand coastal changes over the past 100 years. Some idea of future water levels would be a prerequisite to assessing how the modern coasts will change. Map the modern coastal environments. Assess future impacts on the range of coastal environments, particularly those environments that we suspected to be most sensitive and hence vulnerable. Transfer of information on coastal vulnerability to sea-level rise to stakeholders in GIS formats suitable for their systems. The results of the work are summarized as follows: 1) The lakes were fresh until ca. 6,350 calendar years ago, when rising sea level crossed the -25 m sill and connected them with the ocean. The rate of sea-level rise at the start of inundation was 79 cm/century, and has declined throughout the past 6000 years. Coastal landforms such as spits, barrier beaches, and cuspate forelands were submerged when exposed to the high rate of relative sea-level rise. Submerged shores are visible on multibeam sonar imagery, mainly in the southern lakes, where sediment supplies were abundant. Submerged river networks occur in St. Patricks Channel and Denys Basin. 2) The trend of modern sea-level rise in the region is 36.7 cm/century. Assuming the median increase predicted by the International Panel on Climate Change (2001) (48 cm/century from 1990-2001) and assuming it is distributed equally around the globe, then sea level in the Bras dOr Lakes will increase by 75 cm over the period 1990-2100 AD. The rate of increase will be 60 cm/century by 2030 AD, 99 cm/century by 2080 AD, and 115 cm/century by 2100 AD. 3) Total shoreline length is 1272 km, or 14.4 % of the Nova Scotian coastline (8811 km). Shorelines are grouped into eleven classes: three types of rock shore, seven types of nonrock shore (unconsolidated), and artificial shores. Coastal barriers make up 12 % of the shoreline. 39 % of these barriers are building and established, 44 % are in the breakdown and collapse phase, 13 % are in transition, and 4 % are artificially constrained. 4) We group the shoreline types into three sensitivity classes, depending on the likelihood that changes will be triggered by sea-level rise. 18.8 % have high sensitivity, 73.9 % have moderate sensitivity, and 7.3 % have low sensitivity. The most sensitive shoreline types are unconsolidated cliffs, coastal barriers, and artificial shores. 5) Coastal barriers will continue to change in their natural cycles of growth and decay over the coming century, but at higher rates. There will be an increasing tendency for complete submergence of coastal barriers by 2030 AD, and a strong likelihood of submergence by 2045 AD. We predict accelerated unconsolidated cliff erosion and increasing effort and expense to maintain coastal defenses, particularly those on barrier beaches that would otherwise migrate or submerge. 6) The recommended response to these future changes is to allow them to take place with as little interference as possible, that is, to allow the coast to respond in a natural way as it did in the past. Having a natural coast will ultimately benefit the region more than having a coastline constrained by coastal protection structures.

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