406
Chapter 8 Subsidence
a
150
100
50
0 Ma
,.L---L----JL-....I---..L..----J"-r-.L---..L..::-::-'::-=-'::-::~_=_'"-.... -L----L_;_;::'==:7=:_:_'_-'___1 B.P.
JURASSIC
CRETACEOUS
TERTIARY
~-~~~~--~----~~~~~-----L-----------------~0 . 2
SEDIMENT
SURFACE
~.w;9)'T.~ 0
PALEO-WATER DEPTH
........ _
THERMO- TECTONIC
2
- -.... /
SUBSIDENCE
------- -------::c 3
ICl.
UJ
o 4
b
::c
ICl.
W
o
5
6
2
(OPENING OF ATLANTIC)
VOLCANIC
EVENT
VIKING GRABEN,
NORTHERN
NORTH SEA
(TOTAL SUBSIDENCE
OF SEDIMENTFILLED BASIN)
10
100 40
20 m/ Ma
This amount of stretching is not sufficient, however, to
predict the actual amount and history of subsidence observed on many passive continental margins. Therefore it
has been proposed that an additional thinning of the lower
part ofthe Iithosphere takes place (Fig. 8.15a,b), which is
caused by heat transfer from the mantle to the Iithosphere
but does not involve extension (Chenet et al. 1983). Hence,
the apparent stretching factor, ß, of the lower layer of the
crust becomes greater than that of the upper layer (twolayer model). This thermal effect mayaIso lead to an upward shift of the 450°C isotherm and thus to a thinning of
the effective elastic thickness of the lithosphere adjacent to
the rift basin (Fig. 8.15c). This in turn will reduce the flexural rigidity of the Iithosphere and therefore promote
FAR AWAY FROM
RIFT BASIN
CENTRAL
3 NORTH SEA
NEAR RIFT ZONE
5
6
7
B
WITHIN RIFT
BASIN
9
km
3
4
5
6
Fig. 8.13. a Total and
thermo-teetonic subsidenee on US Atlantie
eontinental margin,
affeeted by long-term
sea level ehanges and
variations in bathymetry, induding periods of emergenee.
Volcanism has initiated a seeond eooling
eyde. (WeIl C.O.S.T.
B-2, after Greenlee et
a1. 1985). b Total subsidenee determined
from wells in eentral
and marginal parts of
Paris basin (Coehran
1983b) and general
trend of subsidenee in
North Sea basin (eurve
for Viking graben from
Beaeh et a1. 1987;
trend for eentral North
Sea from Sdater and
Christie 1980). See
text for explanation
downward bending and the formation of a sediment wedge
adjacent to the rift basin.
The following examples of passive continental margins take into account these processes in the development of the rift basin, as weIl as thermal thinning of
the effective elastic thickness of the adjacent lithosphere. First, some subsidence curves for single sites
are described, before we proceed to differential subsidence along transects across continental margins.
Chapter 8 Subsidence
a
150
100
50
0 Ma
,.L---L----JL-....I---..L..----J"-r-.L---..L..::-::-'::-=-'::-::~_=_'"-.... -L----L_;_;::'==:7=:_:_'_-'___1 B.P.
JURASSIC
CRETACEOUS
TERTIARY
~-~~~~--~----~~~~~-----L-----------------~0 . 2
SEDIMENT
SURFACE
~.w;9)'T.~ 0
PALEO-WATER DEPTH
........ _
THERMO- TECTONIC
2
- -.... /
SUBSIDENCE
------- -------::c 3
ICl.
UJ
o 4
b
::c
ICl.
W
o
5
6
2
(OPENING OF ATLANTIC)
VOLCANIC
EVENT
VIKING GRABEN,
NORTHERN
NORTH SEA
(TOTAL SUBSIDENCE
OF SEDIMENTFILLED BASIN)
10
100 40
20 m/ Ma
This amount of stretching is not sufficient, however, to
predict the actual amount and history of subsidence observed on many passive continental margins. Therefore it
has been proposed that an additional thinning of the lower
part ofthe Iithosphere takes place (Fig. 8.15a,b), which is
caused by heat transfer from the mantle to the Iithosphere
but does not involve extension (Chenet et al. 1983). Hence,
the apparent stretching factor, ß, of the lower layer of the
crust becomes greater than that of the upper layer (twolayer model). This thermal effect mayaIso lead to an upward shift of the 450°C isotherm and thus to a thinning of
the effective elastic thickness of the lithosphere adjacent to
the rift basin (Fig. 8.15c). This in turn will reduce the flexural rigidity of the Iithosphere and therefore promote
FAR AWAY FROM
RIFT BASIN
CENTRAL
3 NORTH SEA
NEAR RIFT ZONE
5
6
7
B
WITHIN RIFT
BASIN
9
km
3
4
5
6
Fig. 8.13. a Total and
thermo-teetonic subsidenee on US Atlantie
eontinental margin,
affeeted by long-term
sea level ehanges and
variations in bathymetry, induding periods of emergenee.
Volcanism has initiated a seeond eooling
eyde. (WeIl C.O.S.T.
B-2, after Greenlee et
a1. 1985). b Total subsidenee determined
from wells in eentral
and marginal parts of
Paris basin (Coehran
1983b) and general
trend of subsidenee in
North Sea basin (eurve
for Viking graben from
Beaeh et a1. 1987;
trend for eentral North
Sea from Sdater and
Christie 1980). See
text for explanation
downward bending and the formation of a sediment wedge
adjacent to the rift basin.
The following examples of passive continental margins take into account these processes in the development of the rift basin, as weIl as thermal thinning of
the effective elastic thickness of the adjacent lithosphere. First, some subsidence curves for single sites
are described, before we proceed to differential subsidence along transects across continental margins.