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not always consistent with that at other boreholes. There-
fore, as heat-flow cannot vary significantly in the past and
as burial/erosion is also questionable, alternative
explanations to increase sediment temperatures in the
past are needed. Hydrothermal fluids or volcanism are
obviously efficient mechanisms that could contribute to
anomalous maturation (Makhous and Galushkin 2003 ),
but they should be long enough that the kinetic transfor-
mation are possible (few Ma)! In this paper, we tested an
intermediate scenario including advection of heat at the
base of the crust (magma trapping) and diffusion of this
heat within the crust and basin. This hypothetical scenario
produce a maturation profile that can match all boreholes
data without violating thermal observations (present day
heat-flow, xenoliths P/T and time, lithosphere thickness).
Obviously, additional geological constraints are needed
to infer scenarios for the maturation of organic matter in a
cold cratonic lithosphere.
as the mantle cools at the same time as it deforms. This
reduces the degree of subsidence during the stretching phase.
The strain rate
ʵ
is related to the thinning factor
ʲ
by:
ʲ ¼
exp
ðÞ
ʵʔ
t
ð
Þ
12
:
4
The subsidence of the CB constrains the thinning of the crust
to a maximum of about 1.4. If we assume that extension ranges
from 700 to 630 Ma, the strain rate is of the order of 10 16 s 1 .
Lithosphere Physical Properties
The thermal conductivity of the mantle includes a lattice
component
ʻ I , which depends on temperature and pressure,
and a radiative component
ʻ r :
r
298
T abs
2410 5
ʻ I ¼
4
:
13
1
þ
0
:
032 3
:
ð
z
z crust
Þ þ
1
:
14
Acknowledgements Marteen De Witt, Robert Harris, Jean Braun and
Nicky White are thanked for their constructive remarks and reviews.
This is IPGP contribution # 3394.
ð
12
:
5
Þ
6810 10 T abs
ʻ r ¼
3
ð
12
6
Þ
:
:
These expressions are derived from a compilation of
literature data (Jaupart and Mareschal 1999 ).
The thermal conductivity of the crust also depends on the
temperature and the thermal conductivity
Appendix 1: Thermal Model
ʻ 0 measured in
laboratory conditions (Durham et al. 1987 ):
The thermal evolution is described by the 1D heat equation:
618
2
T abs þ ʻ 0
355
6
T abs
:
:
z ʻ
½
ð z T
z
t
þ
Az
ðÞ ¼ ρ
t
cz
ðÞ
t
d t T
ð 12 : 3 Þ
;
;
;
ʻ ¼
2
264
0
3025
ð
12
7
Þ
:
:
:
where
ʻ
( z , t ) is the thermal conductivity at depth z and
time t,
c ( z , t ) is the specific heat per volume unit and A ( z , t )
is the heat production. This equation is solved numerically
by a finite differences method initially developed by
Lucazeau and Le Douaran (Lucazeau and Le Douaran
1985 ). We use a Lagrangian frame, which allows to describe
more easily the sedimentation (by adding nodes), the erosion
(by removing nodes) or compaction and extension (by
changing the thickness of a cell). The initial conditions
assume thermal equilibrium within a 200 km thick litho-
sphere, with a 40 km thick crust and a 10 km enriched
radiogenic upper crust. The upper and and lower boundary
conditions are fixed temperatures (20 and 1,350 C). This
corresponds to a surface heat-flow of 43 mW m 2 and a
mantle heat-flow of 16 mW m 2
ρ
Isostasy
We assume Airy-type isostasy, with a constant weight per
surface unit at a reference level Z ref in the asthenosphere.
The initial state provides the reference weight M 0 assuming
that elevation corresponds to the sea-level:
M 0 ¼ ρ c Z
c
ð
1
ʱ
Tz
ðÞ
Þ
dz
0
ð
Þ
þ ρ m Z
12
:
8
L
ð
1
ʱ
Tz
ðÞ
Þ
dz
þð
Z ref
L
Þρ asth
c
(i.e.
the upper limit
ˁ m the
density of the crust and mantle in laboratory conditions, T L
the temperature at the base of the lithosphere and
ʱ
ˁ c and
assumed for the north American craton).
The time-dependant equation is solved implicitly, which
gives less precision but more stability. In a first stage, we
consider the rifting of the lithosphere. Modelled strain rates
are typically low and therefore temperatures do not increase
significantly due to the upward advection of mantle material,
where
is the coefficient of thermal expansion,
ρ asth ¼
ρ m (1
T L ) the density of asthenosphere.
At each time, the weight M1 is calculated to estimate the
subsidence S:
ʱ
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