Download EVIDENCE TO THE CONTRARY (CHAPTER 6)

FOOT OF THE SLOPE
DETERMINED BY MEANS OF
EVIDENCE TO THE
CONTRARY
(CHAPTER 6)
Harald Brekke
Foot of the continental slope
• ”In the absence of evidence to the contrary,
the foot of the continental slope shall be
determined as the point of maximum
change of gradient at its base”
(Paragraph 4 (b))
• The Convention does not prescribe any specific
methods to define the foot of the slope when
evidence to the contrary is invoked
Application of evidence to the
contrary
• A provision designed to allow coastal States
to use the best geological and geophysical
evidence available to them to locate the foot
of the continental slope when the
geomorphological evidence given by the
maximum change in the gradient does not
or cannot locate reliably the foot of the
continental slope (See 6.3.1)
Application of evidence to the
contrary (Continued)
• The determination of the foot of the continental
slope is achieved as a general rule by means of the
point of maximum change in the gradient at its
base (See 6.1.1)
• The determination of the foot of the slope by
means of evidence to the contrary is regarded by
the CLCS as an exception to the general rule (See
6.1.2)
• If such evidence to the contrary is presented, the
CLCS will request that it be accompanied with the
results of applying the general rule (See 6.4.1)
The continental slope
• Some continental margins consist of three
elements – the shelf, the slope and the rise –
whereas others show no rise
• The continental slope forms a portion of the
continental margin and extends from the shelf
edge to the top of the rise, or to the top of the deep
ocean floor where no rise exists
(See 6.2.1)
Base of the continental slope
• The base of the slope is a zone where the lower
part of the slope merges into the top of the
continental rise or into the top of the deep ocean
floor, in the case where no rise exists (See 6.2.2)
• The foot and the base of the continental slope are
inseparable, and commonly lie close to the outer
edge of the continent, that is, near the place where
the crust changes from continental to oceanic (See
6.2.3)
Relevant cases
• Where the curvature of the seabed along the base
of the continental slope is constant, so that the
maximum change in gradient encompasses not
only a point, but also a region (See 6.3.2.)
• Where the seabed topography reveals a number of
local maxima in the change of the gradient at the
base of the continental slope, so that it is possible
that its maximum maximorum may not be
indicative of the location of its foot (See 6.3.3)
Continental margins
-plate tectonic categories
• Convergent continental margins
- at active plate boundaries
• Divergent (“rifted”) continental margins
- at inactive plate boundaries
• Transform (“sheared”) continental margins
- at both active and inactive plate boundaries
Convergent
continental margins
PORTRAIT
Fig 6.1.A,B og C om
konvergente
kontinentalmarginer
CLCS/11, Fig. 6.1
Eksempel på Accretionary convergent margin
Evt et kart over hele Stillehavet som viser alle
de tre typene, pluss at man får vist forskjellen
på Asiaplategresen med øybue-back-arcsystemer og Amerikaplategrnesen som er
rensket for slike.
Convergent
continental
margins
Divergent
continental
margins
Fig. 6.1.D og E
CLCS/11, Fig. 6.1
Divergent
continental margins
PORTRAIT
Divergent margins –
Atlantic margins
examples
Transform continental margin
CLCS/11, Fig. 6.1
Gulf of Alaska transform continental margin
Eks på sheared margin
Combination
of different
types of plate
boundaries
Foot of the continental slope
• Any point selected as the foot of the slope
must be located within the base of the
continental slope described and documented
as such (See 6.2.3)
• Any point selected as the foot of the slope
must be located inside the geological
continental margin (See 6.3.5)
Convergent continental margins
• The outer edge of the continental margin is
defined by either:
– The seaward edge of the accretionary prism
– The foot of the upper plate
– The foot of the inner trench wall
• Data required:
– Modern multi-channel seismic data
– Modern bathymetric data
(See 6.3.6, 6.3.7)
Rifted (non-volcanic) and sheared
continental margins
• The outer edge of the continental margin is
defined by the transition between continental and
oceanic crust
• The landward limit of this transition zone may be
considered as an equivalent to the foot of the slope
• Data required:
– Modern multi-channel reflection seismic data
– Modern refraction/wide angle reflection data
– Magnetic and gravimetric data
(See 6.3.8-10)
Rifted volcanic continental margins
• The outer edge of the continental margin is
defined by the transition zone, where the thickness
of the igneous continental crust decreases to
normal oceanic crust thickness, i.e. less than 15
kilometres
• The landward limit of this transition zone may be
considered as an equivalent to the foot of the slope
• Data required:
– Modern multi-channel reflection seismic data
– Modern refraction/wide angle reflection data
(See 6.3.11)