Geoscience Reference
In-Depth Information
Figure 10.26.
The effect
of depth migration on the
deep reflections of Fig.
10.25(a). The dipping
reflectors steepen and
migrate up-dip. OIF, Outer
Isles Fault; and FT,
Flannan Thrust. (From
McGeary and Warner
(1985).)
is visible between 30 and 80 km is the Outer Isles Fault (see also Fig. 4.57).
Two sets of clear, strong reflections originate within the mantle: the first, a
100-km-long sub-horizontal reflector at two-way time 13-15 s, and the second,
the dipping Flannan Thrust reflector which extends from 7 s down to at least
27 and possibly 30 s two-way time (the recording time of the survey was 30 s).
Figure 10.26 shows the effect of depth migration (see Section 4.5.4) on these deep
reflectors: the dipping reflectors steepen and migrate up-dip. The most plausible
explanation for the Flannan Thrust reflector is that it is a fossil subduction zone
dating from the Caledonian orogeny when the Iapetus Ocean closed. If so, it is
a 400-Ma-old thrust, though it could have been reactivated by the later Meso-
zoic extension in the region. Nevertheless, such strong reflections, which clearly
originate from within the lower part of the lithosphere, show that the lower litho-
sphere can be structurally complex and can support localized strains over a long
time.
The western part of North America has been a continent-ocean boundary
for some 700-800 Ma (Fig. 3.30). Evidence from the series of accretionary
complexes and magmatic arcs that make up the over-600-km-wide northern
Cordilleran orogen show that it has been a convergent boundary since the Devo-
nian. Since the early Jurassic the North American plate has moved over a series of
oceanic plates and has accreted subduction-zone terranes and intra-oceanic arcs.
Magmatic arcs have then been emplaced on and in these accreted terranes. Over
this time the west coast of North America may have consumed a region wider
than the present-day Pacific Ocean. Extensive programmes of deep-seismic-
reflection profiling and associated geological and geophysical work have imaged
details of the structure of the crust and uppermost mantle along this margin (see
http://www.litho.ucalgary.ca/atlas/index1.html
for
the
Canadian
Lithoprobe
data).
The core of northern and western North America is made up of a series
of Archaean cratons that were assembled during the Proterozoic (Fig. 10.27).
The
Trans-Hudson orogen
,now about 500 km wide and exposed in Canada, is
what remains of a major Himalayan-scale continental collision. It is part of the
1.75-1.85-Ga collision zone that extends northwards from the central U.S.A.
through Canada and then eastwards across Hudson Bay to southern Greenland
and on into northern Scandinavia and Russia. The orogen consists of four distinct
