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EPSL ELSEVIER Earth and Planetary Science Letter5 15 I (1997) 1,X.‘-254
Variations in heat flow across the ocean-continent transitioln in the Iberia abyssal plain
Keith E. Louden a,v, Jean-Claude Sibuet “.I, Francois Harmegnies ’
Received 3 January 1997: accepted 14 July 19Y7
Abstract
New heat flow observations have been made in the Iberia abyssal plain off the Galicia margin along the transeat of Ocean Drilling Program Leg 149 drill sites. in order to investigate the nature of this unusually wide and deep contir+ent-ocean transition region. Our results indicate the presence of three separate zones. Average values of 47.5 & 3 mW m-’ in the
westernmost zone III agree with predictions of standard oceanic lithospheric models for its estimated age of I;?6 Ma. In contrast, the heat flow within zone II is 5-15 mW m-’ higher than predicted. assuming that the mantle heat flow remains constant across the basin. This region of high values is coincident with the location of a major intra-crustal’ “S”-type
reflector east of ODP Site 900. and the anomaly is consistent with the presence of 2-3 km of primarily upper continental crust above the reflector, with concentrations of radiogenic components similar to those from granodiorite samplles dredged off Galicia Bank. It is not, however, consistent with the low values of heat production measured on gabbroic sanhples from
its western end at ODP Site 900. In zone I, detailed measurements across the tilted fault block south of ODP Site 901 show consistent variations which closely match predictions due to the effects of basement structure and sediment deposition. There is no evidence for variations due to vertical convective transport along the dipping basement fault block. Once corrected for these variations. measurements in zone 1 yield average values that agree quite well with previous measurements across Calicia Bank. indicating no systematic landward increase in heat flow with decreasing amounts of continental, extension. 0 1997 Elsevier Science B.V.
h’c~~\~)rr/\: heat now: continental margin: Iberian Peninsula: abyssal plains: Ocean Drilling Program
1. Introduction
Variations in heat flow across rifted continental margins are produced by the extension of the crust and lithosphere in two basic ways. First. the thinning
Corresponding author. Fax: + I-902-494-3877: e-mail:
’ Fax: (33) 98-22-45-39: c-mail: [email protected]
of the lithosphere during active rifting of ahe margin produces a seaward increase in heat flow from low
values representative of the thick. old continental lithosphere to high values representative of thin. young oceanic lithosphere. Second, the thinning of the crust produces a seaward decrease in heat flow from high values representative of the granitic upper continental crust to low values representaive of the basaltic oceanic crust.
0012-X21X/47/$17.00 8 1997 Elsevier Science B.V. All right:. reserved
Pi1 so0 12-82 I X(97)00 136.’
234 K.E. Lou&n et al. / Earth cd Pianetnrv Science Letters 151 (19‘971 233-254
Analysis of these variations may help under cer-
tain circumstances to constrain mechanisms of conti- nental extension. The variation due to lithospheric thinning is by far the larger of the two, but it decays
exponentially with age following a characteristic time constant of - 63 m.y. [l]. The variation due to crustal thinning remains constant over characteristic
formation times for ocean basins (i.e., 200 Ma), but
its magnitude depends on the concentration of radio- genie components in the original continental crust. In
general, this concentration decreases with the age of
the continental crust due to erosion of the more radiogenic upper crust [2]. In principle. these ex-
pected patterns allow us to measure both processes independently by observations across margins in spe-
cific geologic settings. For example, to constrain lithospheric extension we would study a margin that has been created by the recent (i.e., < 30 Ma1 rifting of an Archean continent; while to constrain crustal
extension we would investigate a margin formed by
older (i.e.. > 120 Ma) rifting of a Paleozoic or
younger continent. Most heat flow studies of margins have concen-
trated on the study of lithospheric thinning across young margins [3,4]. However, there are relatively
few young margins available for study and hy- drothermal activity within the adjacent young oceanic
crust produces a large scatter in the offshore observa- tions. In contrast, very few heat flow observations have been attempted across the more numerous old margins, perhaps partly because of the severe envi-
ronmental difficulties that such regions pose. For example, the largest variation in crustal structure for
many old margins occurs beneath shallow continen- tal shelves with thick sedimentary basins. In this circumstance, very deep borehole measurements are required in order to adequately resolve perturbations
in the underlying geothermal flux caused by large seasonal variations in bottom water temperature and by the deposition of thick sediment.
An alternative is to study old margins which have only a thin sediment cover. A number of thinly
sedimented margins exist off NW Europe (Fig. 11.
These margins were fomred in the E. Cretaceous by rifting of Paleozoic continental crust with generally high concentrations of radiogenic heat production. Previous studies were made by Foucher and Sibuet [5] on the Armorican margin and by Louden et al. [6] across Goban Spur and Galicia Bank. The latter
study represents the most extensive measurements to
date across old rifted margins. Heat flow across Goban Spur increases systematically with crustal thickness. as determined by seismic measurements of
crustal structure [7], in general agreement with a pure shear rifting model. Results across Galicia Bank,
however, trend in the opposite direction, in disagree- ment with previous suggestions that it represents a simple shear upper plate margin [8]. One particularly high value suggested in addition, that advective wa- ter circulation might play an important role in re-
gions with basement fault scarps that reach or come close to the seafloor. However, the number of mea-
surements available from these widely-spaced sta- tions were too limited to adequately describe these
small-scale variations. Thus it was uncertain, without further study, how representative the observations
were of broad-scale variations across the entire mar-
gin. This paper extends this investigation by present-
ing new determinations of geothermal heat flow along a transect of stations across the Iberia abyssal
plain, immediately to the south of Galicia Bank. Our results are consistent with the previous measure- ments on Galicia Bank as well as with borehole
measurements from ODP Leg 149 [9]. These results suggest that values remain roughly constant across the basin from ocean to thinned continental crust,
except possibly for one region which has anoma- lously high values. Detailed measurements across a
continental fault block show close similarity to varia- tions predicted from the basement relief and sedi-
ment distribution. No additional perturbations from advective fluid flow are required to explain the variations. The region of high values is coincident
Fig. I. Heat flow values from the northwestern Atlantic Ocean and adjacent regions of western Europe. after Louden et al. [61. Box indicate
location of detailed study area in the Iberia abyssal plain (Fig. 2): letters and shaded areas denote general regions of Paleozoic basement
adjacent to the Mesozoic margins. after Montadert et al. [36]. Abbreviations are: CM = Celtic margin: AM = Armorican margin;
ES = Estremadura Spur: MTR = Madeira-Tore Rise; TAP = Tagus abyssal plain.
K.E. Louden et al. / Earth and Planetay Science Letten 151 (I9971 233-254 2.75
.-. . . ,:I’ .,.n A-, \ 0” c I
41° N
40°N
--Ok
-
5
4
‘_
.__
._^
.^
_^._
. 14
'W
13”
12”
1 1”
1O
”W
Fig.
2.
Det
aile
d st
udy
regi
on
in t
he
Iber
ia
abys
sal
plai
n in
clud
ing
loca
tions
of
Fl
uiga
l he
at
flow
st
atio
ns
PF4-
PF48
an
d K
F1 -
KF1
2.
DSD
P L
eg
47(2
) Si
te
398,
O
DP
Leg
14
9
Site
s 89
7-90
I,
mul
ticha
nnel
se
ism
ic
refl
ectio
n pr
otile
s L
G-
12 [
151
and
Flui
gal
IO [
26],
and
seis
mic
re
frac
tion
prof
iies
(cir
cled
nu
mbe
rs
l-4)
[ 1
9).
VD
GS
= V
asco
da
G
ama
Seam
ount
: V
S =
Vig
o Se
amou
nt;
PS =
Por
t0
Seam
ount
. S
l-S3
ar
e lo
catio
ns
of
sono
buoy
s us
ed
for
dete
rmin
atio
n of
sed
imen
t ve
loci
ties
[19]
. PR
(th
ick
gray
lin
es)
are
loca
tions
of p
erid
otite
ri
dges
fr
om
Bes
her
et a
l. [ 1
61.
Bat
hym
etry
(i
n km
) w
ith
cont
ours
ev
ery
200
m.
Fille
d sy
mbo
ls
(ide
ntif
ied
in t
he
lege
nd)
indi
cate
su
cces
sful
st
atio
ns;
open
sy
mbo
ls,
unsu
cces
sful
.
K.E. Louder! et ul. / Earth and Planeta? Science Letter.7 ISI f IYY7I 233-254
: :
a :
8 :
a :
H 0
$
3
2.17
Tab
le
1 Fl
uiga
l ‘9
3 he
at
flow
st
atio
ns
ID
Lat
itude
N
L
ongi
tude
W
D
epth
D
ist.
Pen.
T
ilt
NF
G
N,
k Q
C
L Q
c B
WT
Y)
U
0 (‘
) (m
) (k
m)
(m)
(‘?
(mK
Z
K
(W
m-’
$r
nwi
(mW
Z
W
(mW
(“
C)
mm
’)
m-‘
) K
- ‘)
K
- ‘)
m
-‘1
rn-‘
) m
M2)
IWO1
40
40
.97
- 10
56
.26
4746
20
1.1
KFo
2 40
41
.07
- 10
55
.92
4135
20
1.6
KF0
3 40
40
.65
- 11
3.
22
4740
19
1.3
KF0
4 40
41
.65
-II
17.6
8 48
47
171.
0
KF0
5 40
40
.99
- 11
28
.16
4984
15
6.3
KF0
6 40
40
.93
- I
I 36
.89
5065
14
4.0
KFo
7 40
41
.12
-11
56.4
8 51
94
116.
5
KFO
8 40
41
.79
-12
8.45
53
03
99.7
KF0
9 40
41
.07
- 12
16
.65
5313
84
.8
KFI
O
41
0.63
-1
3 12
.12
5362
3.
0 K
FI I
40
53
.68
- 12
41
.32
5339
48
.0
KF1
2 40
41
.00
-10
45.0
0 46
75
216.
9
PF04
40
40
.46
- 11
3.
58
4740
19
0.8
PFO
6 40
40
.58
- I
I 4.
53
4817
18
9.5
PF09
40
40
.43
- 11
6.
08
4817
18
7.3
PFIO
40
40
.41
-I1
7.18
48
04
185.
8
PFI
I 40
40
.35
-11
8.86
48
27
183.
4
PF12
40
40
.36
- I 1
10
.15
48.2
2 18
1.6
PF13
40
40
.26
- I I
I
I .4?
48
19
179.
8 PF
l4
40
40.4
9 -1
1 12
.64
4813
17
8. I
PF
15
40
40.3
I
-11
14.5
2 48
3 I
175.
5
PFlh
40
40
.48
-11
15.5
9 48
38
174.
0 PF
17
40
40.5
6 -
1 I
16.8
7 48
65
172.
2
PFl9
40
40
.90
- I1
31
.89
5014
15
1.0
PF2
1 40
41
.01
-I1
35.8
9 50
42
145.
4
PF22
40
40
.97
- 11
37
.93
5065
14
2.6
PF23
40
40
.87
-II
38.3
5 50
70
112.
0
1.5
4.3
4.0
8.8
10
29.8
3.
7
12
55.7
1.
8
11
57.6
1.
1
6 1.
085
0.08
0
14
1.07
5 0.
069
B
1.05
0 0.
050
26
1.01
3 0.
052
29
1.05
3 0.
065
32
1.07
2 0.
071
20
1.04
9 0.
033
32.0
6.
0 -3
.2
33.1
(2
.82)
58.5
4.
7 21
.0
46.2
2.
54
58.3
4.
1 5.
1 55
.4
2.55
8.8
ICI
67.2
3.
8 72
.0
8.8
2.9
3 32
.6
0.8
d I .
050
0.05
0 34
.2
2.5
5.1
9 43
.2
2.3
25
1.04
0
3.4
5 9
56.2
0.
6 7
1.04
3
2.8
6 6
53.5
0.
6 7
1.15
8
3.4
7 9
44.5
1.
1 7
1.06
2
3.3
14
8 35
.9
1.9
7 1.
066
3.4
8 9
37.2
0.
6 7
1.06
9
3.4
10
9 38
.1
1.1
7 1.
064
3.4
9 9
38.1
1.
0 7
1.05
2
3.4
II
8 40
.4
0.9
5 1.
064
3.3
16
9 44
.0
0.9
6 1.
029
3.4
8 9
49.0
1.
1 7
1.02
5
3.4
7 9
48.5
1.
3 7
1.01
1
3.8
40
9 51
.2
0.8
9 1.
111
5.0
4 9
52.6
1.
6 8
1.14
2
0.07
5
0.06
8
0.05
9
0.14
7
0.06
4
0.07
5
0.03
6
0.18
1
0.05
9
0.03
3
0.02
6
0.07
2
0.16
8
44.9
5.
6
58.6
4.
4
62.0
3.
9
47.3
7.
7
38.3
4.
3
39.8
3.
4
40.5
2.
5
40.1
4.
4
43.0
8.
3
45.3
3.
5
50.2
2.
7
49.0
2.
6
56.9
4.
6
60.1
IO
.7
4.3
68.9
(2
.84)
2.59
2.60
-0.3
34
.3
2.60
2.60
-1.1
45
.4
2.53
33.0
39
.3
2.37
3.1
60.0
2.
37
-4.6
49
.4
2.36
-4.9
40
.1
2.38
-3.2
41
.0
2.38
-2.3
41
.5
2.38
-1.8
40
.8
2.37
-1.6
43
.1
2.37
0.3
45.1
2.
37
4.2
48.1
2.
37
6.1
36.0
2.
37
-0.7
57
.3
2.55
6.9
55.9
2.
57
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240 K.E. Lmden et al. / Errrth and Planefary Science Letters 151 (19971233-254
with the location of a major intra-crustal “S”-type reflector, indicating the possible existence of upper continental crust within this region.
2. Location and geological background
The Iberia abyssal plain is located west of the central Iberian peninsula, south of Galicia Bank and north of the Tagus abyssal plain (Fig. 1). It was formed by Mesozoic rifting between the L. Protero-
zoic to E. Paleozoic basements of western Iberia and the eastern Grand Banks (Newfoundland), in a seg-
ment south of the Biscay and north of the Azores
triple junctions [IO]. Rifting extended over three
main phases between the L. Triassic and E. Creta- ceous, culminating in final separation and formation
of oceanic crust by the late Aptian. In the Iberia abyssal plain, Whitmarsh and Miles [I l] and Whit- marsh et al. [ 121 have dated the onset of oceanic
crustal formation at Chron M-3 in the early Bar- remian (126 Ma) from the identification of deep-
towed magnetic anomalies. The region of our survey is located in deep water
(4000-5400 m) immediately west of Port0 Seamount and south of Vigo and Vasco da Gama seamounts,
which mark the steep southern boundary of Galicia Bank (Fig. 2). A number of deep multichannel seis-
mic profiles have been taken across the basin which, along with a series of boreholes drilled during Leg 149 of the Ocean Drilling Program [I 31 and Leg 47(2) of the IPOD Program [ 141, constrain the sedi- ment and crustal structures. The LG- 12 profile, shown in Fig. 3, crosses three borehole sites [15]. At
the landward end of the transect, pre-rift sediment, sampled at Sites 901 and 398, is inferred to overlie rifted continental basement. At the seaward end, serpentinized peridotite was sampled at Sites 897
and 899. The outermost serpentinite ridge has been predicted by Beslier et al. [I61 to extend northward toward the peridotite ridge, sampled at ODP Site 637, that marks the ocean-continent boundary west of Galicia Bank.
In the transition region between ODP Sites 901 and 898, there have been differing tectonic interpre- tations of the basement and upper crustal structures. In one model, the region is underlain by highly stretched and detached continental crust, although
the geometry of the detachment and the relative
amounts of upper vs. lower continental crust are still in dispute [17]. In two other models, the region is underlain either by thin oceanic crust or by unroofed
upper mantle which have been tectonized during formation at very low spreading rates [ 181. Gabbroic basement, formed at the time of rifting, was sampled at Site 900. However, the almost complete absence
of both basaltic and continental upper crustal rocks at any of the drill sites gives us no conclusive evidence with which to confirm either of the two
models. Earlier refraction profiles (l-4 in Fig. 2) indicate
thin crust with velocity signatures across the basin
atypical of either oceanic or continental type, under-
lain by low velocities in the upper mantle probably due to a broad region of serpentinization [ 191. Com- pared to the neighboring margins of Galicia Bank and the Tagus abyssal plain, the width of the transi- tion region between clearly continental and oceanic
crust is unusually wide in the Iberia basin (i.e., N 100 km) [IO]. In this respect, the margin is similar to the SW Greenland margin which shows many of
the same features [20,21], except for the possible absence of major serpentinite ridges. It is the nature of this transition region which we intended to study
by use of heat flow measurements.
3. Heat flow observations
Marine heat flow observations were made in July 1993 along the transect of ODP drill sites during the
Fluigal cruise of the N/O “Le Suroit”. Two sepa- rate probes were used. The Dalhousie multiple- penetration probe measured temperatures using a
violin-bow thermistor string with 9 sensors over a total length of 4 or 6m [22]. In-situ conductivities
were determined using a heat pulse technique [23]. The Ifremer single-penetration probe measured tem- peratures using a maximum of 12 thermistors out- rigged onto a piston corer. Conductivities for these
sites were measured in the core once the temperature of the sediment had re-equilibrated on the ship, using a needle probe technique [24] with a typical spacing of _ 20 cm. Values were corrected to seafloor tem- perature and pressure using equations from Ratcliffe [25]. Values of thermal gradient (G), mean conduc-
K.E. Louden et al. / Earth and Planetag Science Letters 151 C 1097) 233-254 211
tivity (k), and heat flow <Q) with their associated
standard deviations are reported in Table 1 for all successful stations, together with water depth. bot-
tom water temperature (BWT), geographic position, depth of maximum penetration, tilt and topographic correction factor CC,). Measurements are also in-
cluded from ODP Sites 897. 898, and 900 as reana- lyzed by Louden and Mareschal [9].
Stations were positioned along the transect of
ODP Leg 149 boreholes (Fig. 21, along MCS line
LG-12 [ 151 on the eastern half and along a near bottom seismic profile Fluigal 10 [26] on the western half. The two profiles join near ODP Site 898. Significant operational difficulties were experienced
in penetrating turbidite layers over the western half of the profile. These difficulties had been experi-
enced only occasionally during measurements west
of Galicia Bank [6]. The turbidites probably origi-
0
Temperature (“C)
5 10 15 20
40°45’N, 12 14’ W
IOOO--
2000--
z s 3000 - -
9 z & 2
4000--
5000 - -
i-
20 July ‘93 1500 U J
? .‘_.~_.
;, .w,( Salinity
s c
. -3 4
_
6000.1 34.5 35 35.5 36 3
Salinity (4/m)
.5
Fig. 4. Temperature and salinity vs. depth (in dbar water pressure) from CTD measurements at 40”45’N, 12”14’W. The local maxima at
- 1000 m is caused by outflow of warm saline water from the Mediterranean. Variations in this depth over time would credte significant
bottom water temperature variations for seafloor depths < 2500 m: while potential temperatures for depths > 4000 m (see insert) show very
stable conditions.
242
O-
1.5-
z 5 8 n
3.0-
4.5-
O-
1.5-
e
5
r! 3.0-
4.5-
E
H
O-
1.5-
3.0-
K.E. Louden et al./ Earth and Planetav Science Letters 151 llYY7) 233-254
PF04 PF06 PF09 PFlO PFil PF12 PF13 PF14 PF15 PFIS PF17
Temperature (mK) 1 I , 0 150 300
PFl9 PF21 PF24 PF25 PF26 PF27 PF28 PF29 PF30 PF31
PF39 PF40 PF41 PF42 PF43 PF44A PF44B PF45 PF46 PF47
K.E. Louden et al. /Earth and Planetap Science Lrtters 151 i 1997) 233-254 743
nated on the steep southern margin of Galicia Bank
to the north of our profile and flowed down canyons into the deep basin [27.].
3.1. Bottom water temperatures
Bottom water temperatures (BWTS) were deter-
mined at each site from an average of absolute temperatures recorded just prior to penetration of the probe. or in cases of partial penetration from those thermistors which remained above bottom. The mean
value from all sites (but neglecting less accurate values given in parentheses) of 2.39 + 0.02”C
(Dalhousiel and 2.57 + 0.02”C (Ifremerl is uniform over the basin and is similar to the value of 2.55 + 0.03”C (Ifremer) measured in deep water west of
Galicia Bank [6]. Most of the scatter between values and the difference in mean values between instru- ments are due to errors in calibration of the thermis- tors to absolute temperature.
In addition, profiles of temperature and salinity were made with a CTD probe at two locations along
transect. Both profiles were identical and the one at 40”45’N. 12”14’W is shown in Fig. 4. A local tem-
perature and salinity maximum, lying at a depth of
1000 + 300m. is caused by outflow of warm, saline Mediterranean water. Seasonal variation in the depth of this anomaly would cause significant bottom tem- perature variations (BTV) for water depths <:
2500 m, as observed on our earlier sediment tempera- ture data on Galicia Bank [6]. However for water
depths > 4400m, the CTD data indicate very stable conditions with temperature increasing adiabatically.
The bottom temperature of 256°C agrees well with our probe measurements.
3.2. Thermal gradient und corzductiuity
Background sediment temperatures vs. depth rela- tive to the uppermost thermistor penetrating the sedi- ment are plotted in Fig. 5 (Dalhousie probe) and Fig.
7 (Ifremer probe) with least-squares fits to the mean
linear temperature gradient for each station, Back- ground temperatures were determined to within - 2
mK relative accuracy by extrapolation of each sen- sor’s sediment temperatures relative to its own BWT, as a function of F( CY,T 1 (Dalhousie probe) or I /f (Ifremer probe). during post-penetration periods of 2-8 min. This removes the effect of frictional heat-
ing during penetration. Some scattered departures from a linear gradient
are observed at a number of stations and we assume
that they represent anomalous thermal behavior due to sediment disturbance during penetration of the
probe. These aberrant temperatures (indicated by dotted lines) were not used in the calculadion of the
linear gradient. At some stations, with very shallow penetrations over the western half of the profile,
gradients could not be determined (i.e., PF47. KFOS. KF09); at some other stations with larger than nor-
mal scatter, the gradient is probably not very reliable (i.e.. KFO3. KFlO, PF45). In general, stations be-
tween ODP Sites 901 and 898 are the most reliable; while between ODP Sites 898 and 897 the turbidites were impossible to penetrate. On the western end of-
the profile and despite a considerable effort, penetra-
tions were generally limited to < I .5 m and some stations had very high tilts (i.e., PF39, PF4il. PF43A).
Measurements of thermal conductivity are shown along with harmonic mean values at each station in Fig. 6 for the Dalhousie probe and in Fig. 7 for the
Ifremer stations. Weighted mean conductivities are 1.13f0.17 W mm’ Km’ (Dalhousiel and 1.05&
0.05 W mm ’ K. ’ (Ifremerl. These values, are higher than the mean conductivity of 0.88 + 0.07 W no ’ K-’ that was previously measured west of Galicia
Bank [6]. In addition, the conductivities im the Iberia basin have greater scatter, particularly in the upper-
most sediment. These differences are probably re- lated to the increased presence of turbidite layers in the Iberia abyssal plain.
Fig. 5. Sediment temperatures vs. depth relative to the uppermost thermistor penetrating the sediment for Dalhousie pogo iprobe stations
PFO&PF47. Solid lines give minimum least-squares tits to the linear thermal gradients as reported in Table 1. Large deviations from
linearity are shown by dashed lines. Values indicated by crosses are not used in the least-squares estimation.
244 K.E. Louden et al./Earth and Planetary Science Letters 151 (I997) 233-254
PFlO PFl I PF13 PF14
t ,’
,,,’ ,,I’
i’ 4’ . l
PF28
PF12 .
*
.
, * . c
0
1.5
z
:
P
o 3.0
4.5
0
1.5
E
5 %
Q 3.0
4.5
Conductivity (WImK )
6 0%
PF79
Iii0
PF21 PF26 PF29
e
PFi5 PF17
l
.,
PF39
.
* ..-..
.
.
*
I
E
PF30 PF31 PF43 PF46 PF47
o-
2 r 1.5 - z d
3.0 -
Fig. 6. In-situ thermal conductivity vs. sediment penetration depth for stations PFOkPF47. Values are plotted relative to the harmonic mean
for each station (shown by vertical solid lines). Dashed lines connect some adjacent points for clarity. Mean conductivities reported in Table
1 are calculated from data shown by solid circles: deviant data values (crosses) are not used.
K.E. louden et al./Earth and Planeta? Science Letters 151 (1997) 233-254
: : : : : j j \
246 K.E. Louden et al./ Earth and Planetary Science Letters I51 (1997) 233-254
0
1.5
3.0
+z g 4.5 E
$
6.0
7.5
9.0
0
1.5
g- 3.0
z z $ 4.5
6.0
7.5
KFOWPF21
A
KFOYPFO4 KFO4/PFl7
An
I Dal PF
A lfremer KF
Temperature (mK) I , 0 150 300
KFOWPF21 KFOjvPFO4 KFO4/PFi7
I Dal PF
- lfremer KF
Conductivitv (W/m-K)
6 0.?5 750
K.E. Louden et al./ Earth and Planetay Science Letters 151 (1997) 233-254 141
3.3. Comparison of heat ,$‘ow measurements between
instruments
Heat flow values in Table 1 are calculated using
the product of the mean gradients (G) and harmonic mean conductivities (li.1. Standard deviations are cal-
culated from ho = Ga, + ka,, where ac and Us are the standard deviations of the gradient (G) and
conductivity (k), respectively. Because of limitations in the sampling of the conductivity and temperature values. calculations of heat flow over smaller inter-
vals are not particularly useful. However. because our measurement techniques for both gradient and conductivity differed between probe types, we wished to compare raw observations for adjacent stations oi
the two probes. In Fig. 8, temperature and conductiv- ity values are shown for three such station pairs:
KF06/PF21 near ODP Site 900. KF03/PF04 near Site 901, and KF04/PF17 on the fault block (FB)
within the region of disputed crustal type (Fig. 3). Results for stations KF03 and PF04, which are
situated within 0.5 km of each other, are the same and yield identical uncorrected heat flow values. For
stations KF06 and PF2 1. temperatures agree within the same depth interval, although the gradient for KF06 is 28% higher than for PF21 because of higher (although scattered) temperatures at greater depths. The conductivities both show large variations, al- though the mean for KF06 is 6% lower than for
PF2 I. Thus, the heat flow. although higher by 20% for KF06 than PF21, agrees to within the estimated
uncertainty. We also note that the heat flow for adjacent ODP Site 900 falls between, and its uncer-
tainty overlaps, the two other values (Table 1). For stations KF04 and PF17, the conductivities are in close agreement but the gradient and heat flow for
KFO4 are 19% higher than for PF17. Although these two stations are positioned within 1.2 km of each other, they sample slightly different structures on top
of the basement fault block FB shown in Fig. 3. Thus. it is possible that the heat flow in this region has a significant westward increase.
4. Interpretation
Seismic profiles along our heat flow transect show considerable variation in basement topography and therefore also in sediment thickness. These variations can perturb the surface heat tlow in two ways. First.
focussing and dispersal of heat flow is produced by
contrasts between the low-conductivity post-rift sedi-
ment and higher-conductivity basement. Second, higher rates of sedimentation within the basement lows can reduce the heat flow at the surface if they persist over extended intervals. In order to assess
these environmental effects, we need first to calcu-
late the basement topography from the seismic travel times. using a velocity-depth structure within the sediments.
In Fig. 9 we show velocity information from two
sources. Direct measurement on borehole samples [13] indicates a linear increase with depth (d. in
mbsf), I’, = I‘,, + m * d, where L‘~, = 1500 m s- and
m = 0.9 5-I over the sampled interval of O-700 m.
Below this depth, sediment velocity is constrained by
analysis of wide-angle retlections from three sonobuoys located in Fig. 2 [ 191. Results are plotted as values of two-way travel time vs. depth and
compared with observed borehole basempnt depths and travel times. A linear increase in velokity results in depths,
d = - ( r,,/m) [ I - exp( mil~/2)].
where AT is the two-way travel time. The curve predicted by the measured values of m and I’(, can fit the seismic data but are not constrained for AT >
1.8 s. where a continued linear increase in velocity beyond a value of N 3.5 km s-’ is unlike~ly. A more
conservative estimate of basement depths over this interval is given by the dashed line. assuming that velocity remains constant at 13, = 3.5 km 6- ’
Using these two velocity functions. a basement profile is shown in Fig. 10 along the heat flow transect, as determined by profiles LG- 138 to the east of ODP Site 898 and Fluigal IO to the west. A few
Fig. 8. Comparisons of sediment temperatures (upper) and conductivities (lower) vs. sediment depth between neighboring pair6 of Dalhousie
pogo probe and Ifremer core stations. Temperatures are plotted relative to the uppermost thermistor for each station, and relitive positions
are horizontally adjusted to minimize misfit between station pairs. Conductivities vs. depth are plotted relative to the harmonlc mean for all
values at each pair of stations, excluding those indicated by crosses and dashed lines.
248 K.E. Louden et al. /Earth and Planetary Science Letters 151 (1997) 233-254
Two-Way Trave-time (s)
1 2
( w Velocity (m/s) 2000
“t , ‘+\ AI , Al
K.E. Louden et al. /Earth and Planetary Science Letters 151 (19971 233-254 149
typical sediment horizons are shown following Sibuet
et al. [26], Wilson et al. 1281, and Milkert et al. [29]. Horizon W4/5 corresponds to a L. Cretaceous boundary; horizon W3/4 to an Eocene unconformity
related to Pyrrenean compression; horizon S3 to the L. Eocene/E. Oligocene, possibly similar to W3/4; and horizon S2 to the E. Miocene Betic compression,
which is followed by higher sedimentation rates in the Plio-Pleistocene west of Site 897. The basement
lows west of Site 898 are not well resolved due to
the weaker seismic source used on the near bottom profile and may in places correspond to the west-
ward extension of horizon 4/5 rather than the true sediment-rock interface.
The two theoretical heat flow curves are calcu- lated using this basement structure for an assumed
constant basal heat flux of 48 mW rn-?. For the
effect of conductivity contrasts, we assume a con- stant basement conductivity k, = 2.5 W m-’ K-’ and sediment conductivity which increases with sedi-
ment depth, k, = 2.25 - exp( - 0.43d). This relation- ship gives values of 1.25 W m-’ K-’ at the seafloor and 1.48 at 600 mbsf which agree with measure-
ments on borehole samples [9]. The correction factor (C’,) is only very large near Site 901 where base- ment rises to just below the seafloor. An additional
correction factor for sedimentation (C,) is calculated using the relationship [30],
C,= I -(1_2X’)erfc(X) -(2/r)Xexp(-X’)
where
Ji x=c~----.
assuming a negligible radiogenic heat production. a constant rate of sedimentation CL:,) determined from the total sediment thickness, and a constant basement age of 126 Ma. This yields sedimentation rates be-
tween 0 and 25 m m.y.- ’ . As discussed by Louden and Wright [30], this simple relationship yields val-
ues in close agreement with the more complete
method of Hutchison [3 11, when determining L’, from the compacted (rather than decompacted) sediment
thicknesses. Comparison of our measured heat flow values to
these theoretical curves indicates three separate zones
across the basin (Fig. 10): (1) Zone I shows variations in heat fl@w across
the tilted fault block in the region of ODP Site 901 that agree well with the theoretical predictions, espe-
cially when including the reduction CC;> due to average compacted sediment deposition rakes. Thus, there is no evidence from our heat flow measure-
ments for fluid advection associated with the fault, even though the fault appears to be recently active as
evidenced by an abrupt (- 90 m high) seafloor scalp
(2) In zone 11, a region east of ODP Site 900, the heat flow measurements are - 10 mW mm-~? higher
than predicted. The transition to higher values is abrupt between sites PF17 and KF04 on the eastern
side and between PF24 and KF06 on the western
side. Higher than predicted heat flow is ~consistent for all three measurement techniques.
(3) Zone III is the wide western region where heat
flow values again fall within predictiorls. At the extreme western end the observations are ‘higher but
very scattered. probably due to disturbafice during penetration and a relatively small depth of penetra- tion.
In Fig. 1 I, we compare the average heat flow within the three zones and previous detdrminations
across Galicia Bank, both of which are corrected for C,. with predictions for pure-shear extension [32]. including contributions from radiogenic hdat produc- tion within the crust [33]. As discussed in Section 1. these calculations predict an increase of heat flow with decreasing amount of extension ( PI) for mar- gins older than - 50 Ma, assuming granitic-type
values of radiogenic heat production (q.g., H = 3 FW m-j) in the upper crust. For the Iberia basin.
Fig. 9. Relationships for conversion between seismic two-way travel time UWlT) and depth below seafloor). as discussed ih the text: (A)
wide-angle travel-time analysis of sonobuoys Sl-S3 (tilled circles) [19] and basement penetrating drill sites (symbols as identified): and (B)
velocity measurements on ODP Leg 119 cores [ 131.
80
70
h “E
2 60
&- B
LL
50
5 = 40
-I
. . . . . .
..-._
.....-
--
+
30 0
60
120
180
240
4;
/ I
I S
ite
897
899
898
900’
I
901
;’ ‘j
_ __
/
--4
- .I’
A
0 60
12
0
Dis
tanc
e (k
m)
180
240
K.E. Louden et al. /Earth and Planetam Science Letiers 151 C 19971 233-254 151
values of p have been determined by fits to the total
tectonic subsidence [28]. However, values for zones I and II may be underestimated, since they would suggest crustal thicknesses of 6-8.75 km, given an initial crustal thickness of 30-35 km [34]. These thicknesses are higher than estimates of 2-4 km based on the refraction models of Whitmarsh et al.
1191. This discrepancy is probably due the reduction
of mantle density by serpentinitization, which was not considered in determining the tectonic subsi- dence.
Average values of heat flow for zones I and III agree well with measurements for Galicia Bank that
show no increase with decreasing p. The average for zone III of 47.5 i 3 mW m-’ also agrees well with a value of h 42 mW m-l predicted for a
standard oceanic lithosphere of 126 Ma, although most measurements on old oceanic crust are higher (i.e., 50-55 mW m-’ ) [30]. Measurements in the vicinity of ODP Site 901 do not show any increase that might be expected due to an increased amount
of radiogenic heat production in the upper crust. Measurements in zone II suggest an increase of 5- 15 mW m-‘. but this increase is not compatible with
pure shear extension, since otherwise values of heat
production would need to be unacceptably high. However. if the observed 2-3 km crustal thickness above the “H” reflector (Fig. 3) consisted entirely
of upper continental crust (with values of heat pro- duction similar to values of 1.7 + 0.7 FW rnmi
measured on granodiorite dredge samples off Galicia
Bank [9]). it could explain most of the increase. At present there is no direct evidence for this effect. On
the contrary, metamorphosed gabbro samples drilled at ODP Site 900 are typical of oceanic or lower continental crust in containing very low values of radiogenic heat production (0.2 1 & 0.19 ~J-W m -’ 1
[91.
5. Conclusions
The new heat flow observations which we have made across the Iberia abyssal plain indicate the presence of three separate zones.
(1 f Measurements in the westernmost zone III agree with predictions of oceanic lithospheric models
for its estimated age of 126 Ma.
(2) The heat flow in zone II is 5-15 mW rn-’
higher than predicted. This anomaly is consistent with the presence of 2-3 km of primatily upper continental crust above an intercrustal reflector “H”
observed on the LG- 12 seismic reflection profile across this region, assuming values of nadiogenic heat production similar to ones measured on conti-
nental samples dredged off Galicia Bank. It is not,
however. consistent with the low value% of heat production measured on gabbroic samples ffrom ODP
Site 900.
(3) Detailed measurements across the tilted fault block at ODP Site 901 show consistent variations
which closely match predictions due to the effects of basement structure and sediment deposition on a
constant basal heat flow. There is no evidence for variations due to vertical convective transport along
the dipping basement fault block. (4) With the exception of the measurements in
zone II. average values agree quite well with previ-
ous measurements across Galicia Bank. indicating no systematic landward increase with decreasing amounts of continental extension.
It would be of considerable benefit if additional measurements were available in the mid-section of
zone II and in the eastern end of zone I. Indeed, such measurements were planned for a subsequent re- search cruise to the area [35]. but unfortunately they remained unfulfilled when transport of the1 heat flow equipment to the vessel met an untimely delay.
Fig. IO Station locations. corrected heat flow (+ I a) and topographic correction factor along the LG-I 2 and Fluigal IO refleation profiles. Stations indicated by open symbols are considered less reliable than those with filled symbols. Seismic profiles are joined at the location of
ODP Site 898. Several characteristic reflector boundaries are identified, as described in the text. Depths to basement are determined from the
reflection TWIT using the velocity relationships shown in Fig. 9. Correction factor C, is calculated for basement topography separating
high-conductivity rock from low-conductivity sediment. An additional correction factor C, is included to estimate the effe t due to the average rdte of sediment deposition. A generalized interpretation of basement type follows that of Sawyer et al. [37].
252 K.E. Louden et al. /Earth and Planetap Science titters I51 (19971 233-254
1 00 ___ _(AJ Pure-Sty?r Model: H=3 pW/rr? ; h= 15 km
I ___.----- 25
/./J- <--
90 - .A’ //’ +*..,-
_,.... -” Tiz __./
_,/ z 80 -
+------- /----* //
i%l a --F
-------------~- 50 70 -
60 -
50 -
In Beta 40 ’ 1 I 1 I
0 0.5 1 .o 1.5 2.0
90
80
60
iB1 Pure-Shear Model: Age=1 30 Ma; h=l5 km
Oeta ,. I _ _i_._..._i~ _... _J , .-~__-- .---
* 2 3 4 5 6 /
Fig. 11. (A) Variations in heat flow with stretching factor ( p) for a pure-shear model [32] including the effects of radiogenic heating 1331.
Separate curves are for separate lithospheric ages as indicated in millions of years. with expected values for the 126 Ma Iberia margin falling
within the shaded region. Assumed values of radiogenic heat production (H) and thickness (h) are 3 p.W m-’ and IS km, respectively.
After Louden et al. ]6]. (B) Corrected heat flow vs. stretching factor ( p ). Iberia basin values are averaged by geographical region 1, I1 and III shown in Fig. 10:
error bars represent & lo. Galicia Bank values [6] are recalculated as Tukey box plots [38] with median values (horizontal line within
shaded box), hinges (vertical bounds of shaded boxes) and whiskers (vertical lines) shown for 3 groupings of p. Theoretical curves for
pure-shear models are shown for various amount of radiogenic heat production (H in p,W m-’ ), as indicated. Lithospheric age is 130 Ma;
other parameters in the model are the same as used in (A).
K.E. Louden et al. / Earth and Plawtap Scierzcr Letters 1.51 f IYO7) 233-254 253
Acknowledgements
We thank the captain and crew of the N/O “Le
Suroit” for their dedicated help in conducting the measurements at sea often during difficult condi- tions. We also thank J. Scrutton and T. Duffet (Dalhousie) and J.-P. Le Formal (Ifremer) for techni-
cal assistance with the heat flow and CTD instru- mentation. Support for K.E.L. came from the Na-
tional Science and Engineering Research Council of Canada. [CL]
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