The northeastern edge of North America is a "textbook-case" passive
margin, formed during a Mesozoic (0.2 Ga) rifting event that created
the Atlantic Ocean, and with very little subsequent tectonic activity.
Because of the simple tectonic setting, we would expect a progressive
change of lithospheric properties from the center of the continent
towards the margin. Any mantle deformation subsequent to rifting would
be controlled by the motion of the plate relative to the asthenosphere,
and by the shape of the lithospheric mantle keel beneath the continent.
The overall pattern of seismic properties should reflect the progressive
thinning of the keel, and be roughly parallel to the continent-ocean boundary.

These expectations are to some degree borne out.  Cratonic North America
is systematically faster than the global average, and the Northeastern
North American margin is systematically slower. However, the overall pattern
of seismic velocity change is not margin-parallel. Instead, it
consists of a set of smaller-scale (<400 km) but large amplitude (6%)
heterogeneities, many of which are elongate and which trend
sub-perpendicular the margin. Timing of surface waves from sources on
the Mid-Atlantic Ridge confirms continuity of the most prominent of
these slow anomalies that cut across the continent-ocean boundary, and
extends into the western North Atlantic offshore New England.  In the
vicinity of this anomaly Rayleigh wave phase velocities are characterized
by a pronounced (10%) azimuthal dependence, with a lateral gradient
(direction of velocity increase) pointing towards the southeast (from
the ocean to continent). The surface wave data indicate that an as yet
unidentified post-rifting process has substantially modified both the
continental and oceanic lithosphere.

Patterns of seismic anisotropy indicators in northeastern North America
suggest mantle fabric consistent with absolute plate motion of the NA
plate, but also require additional contributions from horizons with
different orientations of fabric. A noteworthy property of seismic
anisotropy in the region is the uniformity of the regional pattern. It
displays no obvious correlation with any of the seismic velocity
anomalies, including the major feature cutting across the margin. Also,
seismic anisotropy can not be the cause of the cos(azimuth) variation
in Rayleigh wave velocity. Anisotropy should yield either a
cos(2azimuth) or a cos(4azimuth) variation in wave speeds, and
orientation of symmetry axes in various depth horizons of anisotropy
are not compatible with the direction of the above mentioned lateral
gradient.

Properties of the upper mantle beneath the Atlantic coast
and immediately offshore do not conform to expectations from a basic
plate tectonic scenario. The origin of seismic velocity heterogeneity,
mechanisms of fabric formation, and the relative location of various
features briefly mentioned above are all largely unknown. Lack of
offshore observations appears to be the most significant limiting
factor for the prospects of resolving these issues, and for advancing our
understanding of this "simple" region.