Geology Reference
In-Depth Information
Fig. 2.19
Major Mesozoic
and Cenozoic tectonic
elements of Africa.
MOR
Morocco,
NWA
northwest
Africa,
CAF
central Africa,
NEA
northeast Africa,
NUB
Nubia,
SOM
Somalia,
TUN
Tunisia. Numbers
refer to the temporal range
of independent motion
as exotic terranes, thrust sheets, microplates, sliv-
ers, continents, blocks of oceanic crust, etc. Usu-
ally the boundaries of these tectonic elements are
traced using specialized GIS software, such as
GlobalMapper™, but some plate tectonic mod-
elling tools allow to draw directly the shape of
the crustal blocks as
spherical polygons
(Schet-
tino
1998
,
1999a
). If we normalize the Earth's
radius to unity, then a spherical polygon repre-
senting a tectonic element is a cyclic ordered
sequence of
n
unit vectors,
P
Df
r
1
,
r
2
:::,
r
n
g
,
placed along its boundaries. The greater is the
number of polygon vertices,
n
, the higher is the
detail through which we graphically represent a
crustal block. In general, the definition of the
tectonic elements is a digitizing procedure, which
could also require employing specialized hard-
ware (digitizers) and thematic base maps, such as
gravity or magnetic anomaly maps, topography,
bathymetry, geologic maps, structural maps, etc.
In any case, the tectonic elements have two time
attributes that specify the temporal range of activ-
ity along their boundaries. These attributes define
the time interval of existence of the crustal blocks
as independent kinematic entities. For example,
Mesozoic plate reconstructions involving Africa
require the specification of at least six tectonic
elements to account for the deformation of this
continent. They are: Morocco, Tunisia, northwest
Africa, northeast Africa, central Africa, and Nu-
bia (Fig.
2.19
).
Geological field studies performed in Mo-
rocco suggest that this block moved with respect
to northwest Africa only between
230 Ma (late
Ladinian) and
185 Ma (Pliensbachian), during
the formation of the Atlas Rift (e.g., Laville
and Piqué
1991
), whereas for the rest of the
Mesozoic it remained fixed to northwest Africa
(Schettino and Turco
2009
). Similarly, north-
west Africa moved with respect to central Africa
only between
120 Ma (Aptian) and
80 Ma
(Campanian), during an episode of extension that
produced the Benue Trough, a failed arm of the
northward propagating South Atlantic rift (e.g.,
Fairhead
1988
). Therefore, it is always neces-
sary, when defining a set of tectonic elements,
to start from reliable field data, having strong
geochronologic constraints, and use only faults
that were active during the time interval under
consideration.
