Magnetic and Gravity Signatures of the Rifted Margin

The margin along the Atlantic coast of the U.S. is classified as a volcanic margin based on seismic interpretations of thick layers of igneous material emplaced at the continent-ocean transition. A strong linear magnetic anomaly, the East Coast Magnetic Anomaly (ECMA), is coincident with the region of volcanism and in particular with sequences of seaward dipping reflectors (SDRs) believed to be rift-related volcanic flows overlying basement. Off Nova Scotia, the anomaly changes character, becoming disjointed and lower in amplitude until it fades to the northeast into the regional background level. SDRs are observed off southwestern Nova Scotia where the magnetic anomaly is strong, but are not seen on two deep seismic multichannel profiles located further east.

Seaward-dipping reflectors larger image [GIF, 253.4 kb, 600 X 576, notice]

There is no clear link between the ECMA and the present shelf geometry. Along much of the U.S. Atlantic margin, the ECMA is roughly coincident with the edge of the continental shelf, but veers seaward off Nova Scotia. From a position about 100 km seaward of the shelf break along the southwest part of the Scotian margin, the ECMA approaches the shelf edge near Sable Island and reduces rapidly in amplitude to the northeast as it moves into the region of thickest sedimentary basin fill. The magnetic signature is lost near the western edge of the Laurentian Channel. The reduction in ECMA amplitude off Nova Scotia may be due to a decrease in the volume of source material (extrusive volcanic layers).

Magnetic Map larger image [JPEG, 734.0 kb, 2196 X 1775, notice]

The free-air gravity signature is dominated by a positive anomaly that coincides with the edge of the continental shelf. The anomaly follows the shelf break quite closely and does not match the seaward shift in position off Nova Scotia shown by the ECMA. There does not appear to be a direct correlation between the gravity signature and the seaward-dipping volcanic layers that are interpreted to be the source of the ECMA.

Gravity Map larger image [JPEG, 625.8 kb, 2196 X 1769, notice]

New Magnetic Maps

A new compilation of magnetic anomaly data has been prepared through rigorous re-processing and re-levelling of all existing marine trackline data and aeromagnetic surveys for the Atlantic Canada region. This compilation has been gridded at 500 m intervals to prepare new maps showing the Atlantic region (1:3000000 scale) and 3 sub-areas at 1:1500000 scale. These maps were released in March 2004 as GSC Open Files 1813 (Atlantic region), 1814 (Sotian Margin), 1815 (Gulf of St. Lawrence) and 1816 (Grand Banks of Newfoundland).