Geology Reference
In-Depth Information
Fig. 2.7
Reconstruction
of the North Atlantic
region in the early
Burdigalian (
20 Ma).
The continental lithosphere
is shown in
grey
. Thinned
continental areas are shown
in
light brown
. Regions in
black
are LIPs. Present day
coastlines are shown for
reference. Late Cretaceous
oceanic crust is shown by
green
colours, Eocene crust
is shown by
yellow
to
orange
colours, Oligocene
oceanic crust is shown in
red. RR
Reykjanes Ridge,
RT
Rockall Trough,
NB
Norway Basin,
JM
Jan
Mayen microcontinent,
AR
Aegir Ridge
the North Atlantic. Figure
2.7
illustrates a plate
reconstruction of the North Atlantic region at
20 Ma, shortly after a westward ridge jump
that determined the extinction of the Aegir Ridge
and cessation of extension in the Norway Basin.
The newly formed spreading segment rifted the
Greenland margin, determining the separation of
a continental fragment: the Jan Mayen microplate
(Jung and Vogt
1997
).
Other extinct plate boundaries formed as a
consequence of cessation of divergent motion
between the conjugate plates, not because of a
reorganization of the boundary. In this instance,
a direct causal relation with a nearby onset of
spreading is missing, although the final result is
the same: a remnant oceanic basin and an extinct
ridge testifying the former existence of divergent
plate motions. Important examples of remnant
oceanic basins associated with ridge extinction
are the Jurassic Ligurian Basin in the western
Mediterranean (Schettino and Turco
2011
), the
Labrador Basin (Roest and Srivastava
1989
), the
South China Sea (Briais et al.
1993
), the Somali
Basin (e.g., Coffin and Rabinowitz
1987
), the
Gulf of Mexico (Ross and Scotese
1988
), the
Amerasian Basin (Rowley and Lottes
1988
), and
the Tasman Sea (Gaina et al.
1998
).
The second kind of oceanic plate boundaries
is represented by the trench zones (or subduction
zones). These are convergent boundaries, where
oceanic lithosphere bends and sinks into the as-
thenosphere. The structural, stratigraphic, and
petrologic features associated with trenches and
island arcs have been described extensively in the
geologic literature (e.g., Frisch et al.
2011
). Here
we shall consider only some aspects that are sig-
nificant for plate kinematics. The geometry of a
subduction zone is that of a small circle arc, both
if we consider the subducting lithosphere as a
flexible-inextensible spherical shell (Frank
1968
)
or as a body that can be extended or shortened
during the passive sinking in the mantle (e.g.,
