Download Reheating of old oceanic lithosphere: Deductions from observations

EPSL
EISEVIER
Earth and Planetary
Science Letters I39 (I 996) 9 I - IO4
Reheating of old oceanic lithosphere: Deductions from
observations
Seiichi Nagihara a**,Clive R.B. Lister bV’,John G. Sclater
’
a Depurtment of Geosciences, University of Houston, Houston, 7x 77204-5503, USA
b School of Oceanography,
’ Scripps lnsrirurion of Oceanography,
University of Wushington,
University of California,
Received
Seurtle, WA 98195-7940, USA
Sun Diego, 9500 Gitmun Drive, La Jollu, CA 92093-02 15. USA
I9 June 1995; accepted 5 January
I996
Abstract
Deep, wide oceanic basins are the only regions of old seafloor where depth is truly representative
of thermal isostasy.
When the depths of these basins are corrected for the effect of sediment accumulation, and variation in crustal thickness, the
principal non-thermal factors have been eliminated. We collect the most precise and reliable values of heat flow for the same
basins, from multi-penetration measurements with in situ thermal conductivity, or deep sea drilling thermal gradients backed
up by surface surveys. The 9 data points that result from this selection process have heen plotted on a depth versus heat flow
graph and compared to published thermal models of lithosphere. When considered without regard to age, all the points fall at
greater depths than predicted by the ‘plate’ models with constant temperature lower boundaries, and remarkably close to
boundary-layer cooling with parameters determined from the pre-80 Ma depth and heat flow history of the ocean floor. They
are differentiated by their heat flows not much by their depths and the order they plot in along the heat flow axis is random
with respect to crustal age. Modeling of discrete reheating events shows that near boundary layer conditions are
re-established after about 40 Myr, but corresponding to a younger-than-real
age. The data therefore favor discrete reheating
events rather than a continuously hot basal boundary, as implicitly assumed by the plate model. Lithospheric reheating
appears to start only on ocean floor > 100 Ma. The data alone cannot discriminate between a few discrete reheating events
due to convective peel-off at the base of the lithosphere or one or more catastrophic events. However, the distribution of
points from the Blake-Bahama
basin is more consistent with distal reheating associated with the Bermuda hotspot than local
convective peel-off.
Keywords:
Blake-Bahama
Basin; hot spots; oceanic crust; lithosphere;
1. Introduction
Few direct observables are available to deduce the
thermal structure of the oceanic lithosphere. Heat
flow through the seafloor is the most obvious one.
Seafloor depth, in part, reflects the average tempera-
* E-mail: [email protected]
’ Clive Lister died in August 1995.
thermal history
ture between the seafloor and the depth of compensation. We infer that the lithosphere cools with time,
because seafloor deepens and heat flow decreases
away from mid-oceanic ridges. The initial decay
patterns of the heat flow and bathymetry can be
explained theoretically if the lithosphere acts as the
thermal boundary layer of mantle convection [l-4].
The global and regional compilations
of
bathymetry show that the general trend of seafloor
0012-821X/96/$12.00
8 I996 Elsevier Science B.V. All rights reserved
PII SO0 i2-82 I X(96)000
1O-6
92
S. Nugihuru er ul./Earlh
and Planemy
deepening ceases at - 80 Ma. At the same time,
topographic variance increases drastically [5,6]. One
of the reasons for this phenomenon is the high
population of seamounts and oceanic plateaus on old
seafloor: these are known to have thicker crust than
the deep ocean basins. This is most obvious in the
western Pacific. Another reason is the increase in
sediment thickness. This is prevalent in many parts
of the Indian and Atlantic oceans, where passive
continental margins abut the old seafloor. Therefore,
the large topographic variance in old seafloor is not
necessarily associated with the thermal structure of
the lithosphere. The cooling process is better reflected in data from flat, deep basins which are wide
enough to be in thermo-isostatic equilibrium. Such
basins are also an environment where highly accurate heat flow measurements are possible.
The bathymetric data from basins away from
seamounts still show the flattening at old ages [7].
Also, heat flow, even if measured with high accuracy, is still significantly greater than expected by
the boundary layer cooling theory [8-IO]. There
seems no strong correlation between the ages of old
seafloor and the two observables. The average
‘flattening’ of seafloor indicates that some mechanisms have provided additional heat to the old lithosphere. In this study, we provide observational constraints on the reheating mechanism, by analyzing
highly accurate depth and heat flow data from the
old basins in the western North Pacific and the
northwestern Atlantic. It is essential that the seafloor
depth data adequately represent the thermo-isostatic
elevation of the lithospheric column. Thus, it has
been standard practice to correct the seafloor depth
for the loading effect of the sediments [7,1 l-131. We
also apply a similar correction to account for regional variation in crustal thickness. Then, unlike
many previous studies, we plot depth against heat
flow, because, as we explain later, this is a more
effective way to infer the temperature distribution
within the lithospheric column than the conventional
depth versus age, or heat flow versus age, plots. We
examine how the data points of individual basins
deviate from the geotherms predicted by various
thermal models and attempt to distinguish between
the reheating mechanisms needed to flatten the age
dependence of depth and heat flow.
Science Lerrers 139 (1996) 91-104
2. Data presentation: conventional depth versus
age and heat flow versus age plots
2.1. Data sources
The data used here need to be accurate enough to
show the subtle variation in the thermal regime of
old lithosphere. Table 1 [8-241 lists the data from
the individual sites. So far, the western North Pacific
and the northwestern Atlantic are the only places
where a substantial number of such high accuracy
heat flow measurements are available (Fig. 1). For
the former, Von Herzen et al. [25] and Lister et al.
[lo] report high quality multi-penetration heat flow
measurements. However, some of their surveys are
located in areas of unknown sediment/crustal thickness or of tectonic complexity. These problems limit
the number of high accuracy sites to 3 in the western
North Pacific. We add 3 Deep Sea Drilling Project
(DSDP) and Ocean Drilling Program (ODP) sites
with well constrained basement depths. A review by
Hyndman et al. [26] rates the downhole temperature
measurements from sites 576 and 578 [ 181 as ‘excellent’. In addition, there are several probe measurements around these sites [ 171 that yield heat flow
values consistent with the downhole measurements.
Site 801C also provides a high quality temperature
log that was obtained during the re-entry 2.5 yr after
the hole was drilled [21].
In the northwestern Atlantic, there have not been
very many high accuracy heat flow measurements in
tectonically simple, deep basins [8,9,22]. Some of
them do not have the information on the local crustal
thickness or are located over the Bermuda Rise,
which is a remnant of hotspot volcanism. All but one
of the usable sites lie in the Blake-Bahama Basin
(Fig. lb).
We believe that sediment deposition and radiogenie heat production within the sediments have
negligible effects on the geothermal fields of these
sites. Even at the Atlantic sites, where the sediment
cover is relatively thick (l-l.5 km in the BlakeBahama basin and - 3 km in the Sohm abyssal
plain), the previous studies [8,9] concluded that the
heat flow values measured on the seafloor adequately represent the flow associated with the lithosphere.
93
S, Nagihara ef al./ Earth and Planetary Science Letters 139 (1996) 91-104
2.2. Thenno-isostatic depth
dense than the mantle. Thus, if the crustal thickness
of a basin deviates significantly from the global
average, the isostatic compensation of this mass
anomaly affects the height of the lithospheric column
(i.e., the seafloor depth). For example, a basin with a
thin crust would be deeper than the average. Assuming Airy-type isostasy, one can estimate the magnitude of the deviation in height:
We first show the conventional depth versus age
and heat flow versus age plots (Fig. 2). The dimensions of the ovals are the uncertainties of the individual data points. Shades and hatches within the ovals
correspond to different crustal thicknesses, and these
are based on the seismic velocity structures from
refraction measurements collected within several tens
of kilometers. Here we have corrected the depth data
for sediment load only, as most previous studies
have done (e.g., [7]). We have used the correction
method of Crough [ 121, which is based on the empirical relationship between the weight of the sediment
column and the two way travel-times (TWT) measured on seismic reflection records. This correction
(0.6 km/s of TWT) is accurate to lo%, provided
that the basement is correctly identified on the seismic records. Following Detrick et al. [22], we use a
conservative estimate for the magnitude of the error:
- 0.1 km per 1 s TWT. Because we deal with data
only from wide, flat basins, this one-dimensional
isostatic correction should be adequate.
In the depth versus age plot, we also show how
crustal thickness varies by more than 3 km between
the sites (Fig. 2a). The crust is 500-600 kg/m3 less
Table 1
Reliable heat flow, sediment
lithospheres
thickness
and crustal thickness
Age
(Ma)
A
B
C
D*
E*
F*
G
H
I
I
K
L
M
N
Central Marlana basin
Namu basin
Central Pacific basin
Mercator basin
Cipangu basin
Figafetta basin
Blake-Bahama basin
Blake-Bahama basin
Blake-Bahama basin
Blake-Bahama basin
Blake-Bahama basin
Blake-Bahama basin
Blake-Bahama basin
Sohm abyssal plain
* Thermal gradient based on downhole
I221.
170
165
146
138
145
166
152
1.50
140
115
140
147
152
163
temperature
Ah
=
_
4
Pi -
PC)
where AC is the excess crustal thickness, pC is the
average crustal density, p, is the mantle density, and
p, is the water density. A 1 km difference in crustal
thickness creates approximately 180 m of change in
initial depth if we choose the average crustal density
to be 2800 kg/m3 [27] and the mantle density to be
3300 kg/m3.
We note that the results of seismic velocity modeling often depend on the processing technique used
and model assumptions, as pointed out by White et
al. [28]. For example, we have two different seismic
data sets available in the Cipangu Basin. One uses
only the first arrivals of refractions collected by a
sonobuoy [ 171 and the other [ 191 combines the
data from deep old basins of the northwest
Water
DeOtll
Sediment
Thick.
5.96
5.18
6.12
6.22
6.02
5.67
na**
na**
na**
0.41141
1.08f’el
n.21to1
--0.41’7l
O.61”1
0.521201
na**
na**
na**
5.51
5.38
4.55
4.8
4.9
0.261231
1.01*1
1.8181
1.41*1
2.7191
measurements.
(1)
PI - Pw
’
l
Altantic
Normal&d
Depth
(km)
6.2
5.8
6.2
6.5
6.4
6.0
6.01=1
5.81=1
5.71”)
5.8
6.0
5.6
5.6
6.5
Thick.
(km)
Depth
(km)
7.31’51
8.11’61
?
3.61’71
5.11’91
6.81201
8.41231
8.41231
6.3
6.0
8.61231
6.1
6.2
87or241
?
Chtly the depths corrected
5.9
6.1
6.0
6.3
6.1
and west Pacific oceanic
Heat
Flow
(mW/m2)
46.61101
44.21101
49.51’01
57.Olu31
45.01’81
52.012tl
50.7lUl
5 1.8lul
Sl.or~J
47.3181
49.dal
47.dal
49.dsl
53.1191
H.F.
Error
0.5
0.9
1.1
3
2
2
2.5
1.3
0.8
2.1
1.0
1.5
1.7
2.6
for sediment load are given in the source
S. Nagiharu
94
a
130”E
14O’E
ef ai. / Earth
and
150’E
Piunefary
160’E
Science
Lerters
170-E
I39
(1996191-104
180’E
170-w
40’N
40-N
30-N
30-N
20-N
20-N
10-N
130’E
140’E
80-W
150”E
160-E
70-w
170”E
180-E
17O’W
60-W
Fig. 1. (a) Location of the sites with high quaiity heat flow and depth measurements in the old ocean basins of the western North Pacific.
Bathymetric contour interval is 2000 m with the 6OCM m contour emphasized. (b) Location of the sites with high quality heat ffow and depth
measurements
in the old ocean basins of the northwestern
Atlantic. Bathymetric contour intental is ICOO m with the 5000 m contour
emphasized.
‘15
S. Nagihara et cd./ Earth and Planetary Scienc@ Lerfers 139 (1996) 91-104
sonobuoy refraction data with multi-channel reflection information. The latter gives a crustal thickness
of 5.1 km, which is more than 1 km greater than the
former result. For this case, we use the latter.
Using Eq. Cl), we have corrected the depth data
(already corrected for sediment load) by normalizing
them to the average oceanic crustal thickness of 7
km [27,28]. We add another f0.1 km error in the
crustal depth correction to account for the uncertainty of the seismic velocity structure. In Fig. 3a,
we show the depth versus age plot after the full
correction. Some of the sites in Fig. 2 have been
dropped because no seismic measurements with
Moho refractions are available nearby.
a
CnlstalThldolsss:
c
0
fj@j
Bee
1
S<C<B
m
c-c5
c: unkrlow n
80
100
120
140
160
lti
TIME (My)
b
80
103
120
140
180
TIME {ixy)
Fig. 2. (a) Basement depth (corrected for sediment load) plotted against crustal age for the high quality sites in the western North Pacific and
the northwestern Atlantic. The sites are identified by letter (Table 1). Cmstal thicknesses of the basins are indicated by hatching and
shading. (b) Heal flow data at the same sites plotted against cmstal age. Solid lines represent the predicted depth and heat flow for the
boundary layer model (BL) and the plate model of Parsons and Sclater (71 (P.S). The hatched line gives the predictions for the thin, hot,
plate model of Stein and Stein [29] (GDHl). The ovals represent the uncertainties m age, depth, and heat flows
S. Nagihara er al./ Earrh and Plunetary Science Lerrers 139 (1996) 91-104
96
a
so
120
140
160
180
TIME (My)
b
TIME (My)
Fig. 3. (a) Basement depth (normalized to 7 km crustal thickness)
plotted against crustal age for the sites with known crustal thickness in the western North Pacific and the northwestern Atlantic.
(b) Heat flow data at the same sites plotted against cmstal age.
Notations are the same as in Fig. 2.
2.3. Observations
In Figs. 2 and 3, the plots are compared with
predictions by three theoretical models: the boundary
layer (BL) and the so-called ‘thick plate model’
(PSI, both which are based on the parameters determined by Parsons and Sclater [7], and the ‘thin and
hot plate model’ (GDHl) by Stein and Stein [29].
The plate model assumes a horizontal isotherm at a
fixed depth while the boundary layer model has no
bottom boundary. Thus, the plate model predicts that
the two observables approach constant values as the
lithosphere nears thermal equilibrium at great ages.
The difference between the ‘thick plate’ (PS) and
‘thin plate’ (GDHl) is the depth and the temperature
of this bottom boundary.
The heat flow and depth data vary significantly
between the sites, but neither of them correlates
systematically with the crustal age as predicted by
any of the thermal models (Fig. 3). All the points are
well above the boundary layer curve. The crustal
depth correction reduces the scatter in the depth
versus age plot, but it is still impossible to fit a
unique, algebraic relationship to the data without
ignoring the range of uncertainties. The heat flow
plot has similar scatter. GDHI treats the variation as
error, and represents the average of the heat flow and
bathymetry compiled globally [29], while PS was
selectively [7l fitted to bathymetric data from flat
basins. The difference in the two approaches is
clearly shown in Fig. 3a. GDHl deviates upward,
because its data source includes seamounts and
oceanic plateaus. However, this model shows a good
overall fit to the heat flow data (Fig. 3b) because
heat flow does not seem to vary systematically between basins and surrounding shallower regions of
old ocean floor [ 101. The relative elevation of these
is not due to heating of the deep lithosphere, so
GDHI fits data that do not reflect thermo-isostatic
depths. PS is based on data from flat ocean basins
and thus fits the depth plot better on average, but it
fails to account for the elevated level of the average
heat flow.
3. Analysis in the depth-heat flow domain
We have so far demonstrated that neither depth
nor heat flow of old seafloor has a unique, algebraic
relationship with lithospheric age. Note that we have
chosen highly accurate data from flat seafloors, which
are, with little doubt, thermo-isostatically compensated. In addition, these data have been fully corrected for the minor non-thermo-isostatic effects.
This suggests that no one can ‘improve’ the lithospheric models by searching for a different set of
parameters that would somehow better ‘fit’ the two
observables as function of age. We need a new
approach.
S. Nagihara et al./Earth
TEMPERATURE (-C)
500
1000
1500
50
2l
3
s
3
100
150
Fig. 4. Geotherms of the boundary
(PS), and thin, hot plate (CDHI)
5.65 km.
91
and Planetary Science Letters 139 (1996) 91-104
layer cooling (BL), thick plate
models at a seafloor depth of
As we have mentioned before, the thermo-isostatic depth of seafloor should be a measure of the
average temperature between the surface and the
compensation depth. The heat flow gives the boundary flux directly and, if there are no great variations
in conductivity, the boundary thermal gradient. The
ratio between these two observables should reflect
the temperature distribution within the lithospheric
column. Thus, the question that arises is whether the
current lithospheric models can be distinguished by
such a ratio and what kind of accuracy is required of
the measurements.
Let us first consider the heat flow predicted by the
models when the seafloor has a fixed depth. Since
GDHI, based on averages for all seafloor, never
reaches the depth of the deep basins examined in this
paper (see Fig. 3a), we can approach most closely
only by taking the equilibrium values: these are
effectively reached for crustal ages greater than 160
Myr. The equilibrium state of a plate model features
a linear temperature gradient between the fixed basal
temperature and the surface (Fig. 4). The values for
GDHl are 5.65 km depth and 48 mW/m* of heat
flow 1291. The PS lithosphere reaches this depth at
91 Myr, when the heat flow is 52 mW/m*. Note
that the basal temperature and the thermal plate
thickness are different for these models to fit their
respective data sets. The BL lithosphere reaches 5.65
km depth at only 81 Myr and the heat flow, using the
same parameters as PS, is 55 mW/m*. Geotherms
for all these cases are shown in Fig. 4.
The whole range of heat flow difference is only 7
mW/m* or 14%, while early heat flow measurements have a precision of 5% at best, and possible
local systematic error (due to different sediment
structures) of at least another 5%. Only multi-station
or drill hole heat flow of extremely high accuracy
can distinguish between these values, and therefore
between the different thermal distributions. The accuracy required in the depth determination can be
estimated by multiplying 4-7 mW/m* with the
gradient of depth/heat flow for the BL model at
5.65 km: 280-490 m. If the errors are as large as we
have estimated: 100 m for crustal thickness correction and 100 m/set of TWT, there may well be
E
5.5
z
2p
6.0
6.5
70
60
40
HEAT FL&
3
(mWhn2)
Fig. 5. The depth, for stations with known crustal thickness
normalized to 7 km and corrected for sediment load, versus heat
flow for high quality sites in the western North Pacific and the
northwestern
Atlantic. The widths of the ovals represent the
uncertainties in depth and heat flow. The cooling paths predicted
for the boundary layer cooling (BL) and the thick plate (PS)
models are shown as solid lines. The BL predictions with thermal
conductivity and thermal expansivity both varied by + 5% in the
same direction and are also shown as thin solid lines. ntese
represent the outer limits of changes compatible with data from
young seafloor. The path for the thin, hot plate model (GDHI) is
shown as a dashed line. ‘Ihe circles on the paths mark the age in
20 Myr intervals. Each oval represents a single site. Ages of the
sites are shown instead of the letter identifications, but the unique
numbers can be found in Table 1 or Table 2.
S. Nagihara et al./ Earth and Planetary Science Lerrers I39 (1996) 91-104
98
difficulty in distinguishing between the PS and BL
geotherms, but not between those of BL and GDHI.
We present the reduced data as a plot of depth,
increasing downward, against heat flow, decreasing
to the right (Fig. 5). The three models produce
curves extending downward to the right; the paths
have been calculated directly from the depth versus
age and heat flow versus age equations previously
published [3,7,29]. The two plate models trend taward their equilibrium points, but the boundary-layer
curve would continue off the plot at great ages, if it
were not for reheating. Age is the parameter along
the curves, in the absence of sudden reheating.
The models prescribe a range of long-term stable
a
I
BOUNDARY
LAYER
solutions that are thermo-isostatic, but are based on a
restricted parameter set compatible with the depthage and heat flow-age relations in the OS-80 Myr
age range. Ignoring the age parameter, the basins
studied here fall near or into the zone of allowable
thermo-isostatic solutions. All but two of the ellipses
intersect the BL curve and the remaining two, the
Nauru and Mercator basins, miss by only a short
distance, while they all fall well below the GDHI
curve. Therefore, thermal isostasy seems to hold in
all of these basins, even without proposing alteration
of any of the parameters. This also means that the
basins are, on average, in isostatic balance with
younger ocean floors as well.
b
HOT PLUME REHEATING
I
1
I
I
I
I
I
I
I
C
PLATE
I
d
SMALL-SCALE
CONVECTION
I
CONVECTIVE
PEELING
I
I
I
’
I
FlsItmlng In bmymauy
I
I
I
I
I
I
I
I
I
Fig, 6. Schematic diagrams showing the mechanisms of: (a) the boundary layer cooling model; (b) the boundary layer moving over a hot
ascending thermal plume; (c) the plate model; (d) very small-scale convection (after 1321); and (e) the lower portion of the boundary layer
peeling off, forming a cold descending blob that is passively replaced by the hot surrounding asthenosphere (after [36n, T, is the initial
temperature of the lithosphere. Note the relative change in bathymetry (thin line) associated with the cooling and basal reheating.
S. Nugiharu et al./Eurth
4. Lithospheric
and Planetmy
reheating
The boundary-layer curve seems to go through the
overall data set very well in the depth-heat flow
domain. This again demonstrates that the basins are
indeed thenno-isostatic. However, the crustal ages
do not agree with the thermal ages, and the points do
not align in age order. This again stresses the point
that no pure age-dependent process can explain the
data. The best one could do is to recognize that the
points are in a fairly tight group, and modify the
plate model so that the early age range collapse falls
as close to the center of the group in depth ( * in Fig.
5), as GDHI already does in heat flow. However, the
fit to younger lithosphere would not be as good, and
there would remain no explanation of the difference
between basins that is mostly in heat flow: they
could all be assigned the same depth of 6.2 km,
except for site G of the Blake-Bahama basin (152,
Fig. 5).
Bearing in mind that hotspots are known to exist
[30] and affect the thermal structure of lithosphere
[31], a reasonable explanation for the discrepancy
between the crustal and thermal ages is some type of
reheating of the lithosphere. In order to infer further
the mechanisms of reheating, let us first review
briefly the possible alternatives previously proposed.
Many researchers believe in so-called ‘small-scale
convection’ as the reheating mechanism of old lithosphere (e.g., 132,331). In this mechanism, because the
lithosphere cools from the top and the isotherms are
being depressed downward, an instability occurs at
the lithosphere-asthenosphere boundary at a certain
age and the material there starts convecting on its
own (Fig. 6). Numerical models show various forms
of such convection. One extreme case is a small
vigorous convection confined to the shallow asthenosphere [34] (Fig. 6d). Such convection could maintain a virtually horizontal isotherm at a constant
depth, and thus can be well represented by the plate
model 1321. At the other end of the spectrum is
large-scale convection, which involves the whole
upper mantle, with the bottom of the lithosphere
peeling off and sinking as a big, cold blob [35,36]
(Fig. 6e).
Another type of reheating mechanism occurs as
the lithosphere passes over an ascending mantle
plume [22,37,38] (Fig. 6b). There are, again, a wide
Science Letters 139 (1996191-104
99
range of possibilities. The upper 60-70 km of the
lithosphere may be cold enough to behave as a
‘mechanical boundary layer’ impervious to anything
but the very localized penetration of melt (e.g., [321X
On the other hand, it may be extremely susceptible
to cracking and dike propagation [39]. Everything
between the bottom of the mechanical boundary
layer and the thermally defined base of the lithosphere (always much deeper than the depth of rigidity, although dependent on model) could be replaced
by extra-hot plume material [31,401 (Fig. 6b). The
process could be confined to the vicinity of the
hot-spot trace, or it could extend far into the basins
on either side (and mimic Fig. 6e). There are also
large areas of laterally widespread volcanism that
may or may not be associated with hotspot volcanism as we understand it: examples are site A (Central
Mariana basin) in the early Cretaceous and sites B
(Nauru basin) and F (Pigafetta basin) in the late
Cretaceous [1.5,20,41]. Common to all these mechanisms is that the reheating associated with hot spots
is sudden, drastic and randomly distributed in space
and time. This is where they differ from passive
convection models.
5. Thermal resetting
Injection of extra heat into the lithospheric column affects the depth-heat flow path no matter how
it is done. The question is whether the reheated
lithosphere could return to the original cooling path
of the boundary layer within a reasonably short time.
If it does, it must be at an age younger than its actual
age. In addition, could such information be used to
distinguish the reheating mechanism? Such questions
may be answered by a relatively simple mathematical model. Applying Occam’s Razor, the least arbitrary new parameters to add to the model are a
variable age and variable extent of reheating. Previous researchers [25,42] have already proposed models that describe the diffusive decay of heat introduced instantaneously within the lithospheric column. The initial uplift is proportional to the amount
of heat injected and the subsequent subsidence is
proportional to the heat flow through the seafloor
[4,32]. We can ignore dynamically induced bathymetric changes because we are not modeling the
present or very recent reheating by hotspots.
100
S. Nagihara et aI./Earth
and Planetary Science Letters 139 (1996) 91-104
We have conducted a number of such tests using
these models and learned that arbitrary reheatings
can decay back to the BL curve on the depth-heat
flow plot in a relatively short time and that the
apparent rejuvenation is dependent mostly on the
amount of heat injected and secondarily on the depth
of injection or the age at which the event occurred.
Here we show one such example. This particular
model assumes that a portion of the cooled lithosphere, below the depth L, simply peels off and is
replaced by asthenosphere at T, (similar to Fig. 6b
and e). The mathematical treatment has been fully
described in the literature [42]. The amount of heat
injection is a function of the depth of the peel-off,
and so the apparent rejuvenation varies dramatically:
from nearly 90 Myr at L = 20 km to only 35 Myr
for L = 80 km (Fig. 7). A feature of this mode of
depth-dependent reheating is the relatively precise
return of the model lithosphere to the BL curve. A
choice of temperature other than T, for the replaced
material would alter the picture.
6. Apparent rejuvenation
of the nine basins
If the error ellipses of the basins with full sets of
reliable data in Fig. 5 are projected on to the BL
curve, one can obtain a thermal age range applicable
to each basin. The results of this are shown in Table
2 and Fig. 8, where the amount of rejuvenation is
plotted against crustal age. The scatter does not
6
’
7
120
100
40
HEAT P%Vtt (tnW&
Fig. 7. Solid curves are the thermal cooling paths in the depth-heat
flow domain for a model reheated at 100 My to four different
depths of 20, 40, 60, and 80 km, based on Von Hetzen et al. 1421.
The cooling path for the boundary layer model is shown as a
dashed curve, and successive age points after the reheating are
identified by symbols with a 20 Myr interval.
allow for a high degree of statistical confidence, but
there is a definite trend to more rejuvenation at
greater crustal age, and a cut-off at about 100 Ma.
We have not studied younger seafloor, but the relatively good linearity between depth and the square
root of age out to 80 Ma [3] suggests that there are
no large disturbances to boundary-layer cooling prior
to the apparent cut-off.
All the basins, with the possible exception of the
Nauru basin (age 165 Ma), have re-approached the
BL curve on Fig. 5. The Nauru basin is known to
Table 2
Crustal age, thermal age and amount of thermal rejuvenation for the sites with known crustal thickness
Area
Age
(MaI
Thermal
4%
(Ma)
A
CentraJ Mariana basin
170
103-107
B
Nauru basin
Mercator basin
Cipangu basin
Pigafetta basin
Blake-Babama basin
Blake-Bahama basin
Blake-Bahama basin
Blake-Bahama basin
165
105- 115
138
69-80
103-118
80-97
87-100
83-90
98-l 10
112-120
D
E
F
‘G
H
J
K
145
144
152
150
115
140
Amount of Comments
Rejuvenation
(MY)
63-67
50-60
58-69
27-42
69-86
52-65
60-67
5-17
20-28
Early Cmtaceous off-axis volcanism?
Volcanic flows -70 Ma
Late Ct&ieeous off-axis volcanism
Near Bermuda swell
Near Bermuda swell
S. Nagihara et al./ Earth and Planetary Science Letters 139 tl996191-104
b
5
(:
u
,‘OE
/ IA
._
20 I
I
oj.,.,.,.,.,.,,,.,,
0
20
40
60
I
80
/ J
. Q
100
CRUSTALAGE
120
140
160
180
200
(Ma)
Fig. 8. Plot of the amount of lhermal rejuvenation, expressed as an
age change, as a function of crustal age, for the nine data points of
Table 2. If one assumed the variations were ‘noise’ and not real,
then the dashed line is the best-fit line of slope 1. It is equivalent
to proposing a thin, hot, pIale model.
have intercalated basalts and limestones in its crust
[ 161, so that the estimate of crustal depth correction
could easily be too small. If they all have effectively
re-established thermal equilibrium, then our model
calculations (e.g., Fig. 7) show that at least 40 Myr
has elapsed since the last reheating episode.
Thermal analysis of these basins cannot establish
the time or age of the reheating, or discriminate
between several separate events and one large event.
The high age group of basins cluster around 70 Myr
of rejuvenation. If that was accomplished by a single
event, then a large proportion of the lithosphere must
have been reheated or replaced during it. For example, for the peel-off model of Fig. 7 and 100 Ma
lithosphere needs to lose the entire region below
about 50 km depth to show 70 Myr of rejuvenation.
At 100 Ma, the characteristic length of cooling diffusion is also about 50 km, so the level of the split that
permits peel-off is where ambient temperature was
about OS,.
7. Discussion
There are basically two views one could take
about the data plotted in Fig. 8. One is that there are
101
two groups of rejuvenation points: a low group at
around 25 Myr, and a high group, around 70 Myr
(amount of rejuvenation). This could imply that there
tend to be two events in the life of the oldest
lithosphere, one rejuvenating by 25 Myr and the next
by 40 Myr. The physical mechanism that could
accomplish this is quasi-deterministic convective
peel-off, as suggested by Parsons and McKenzie [32]
and Robinson and Parsons [36]. An alternative, that
would have the same apparent effect, is a violent
volcanic event that applied only to the oldest lithosphere, such as the widespread volcanism postulated
by Larson [43], and a less serious event that applied
to the younger basins. The difficulty here is that four
areas of the Blake-Bahama basin G, H, J, and K
(Fig. lb) spread into both groups. The ones that
suffered the largest rejuvenation are those closest to
the Bermuda rise hotspot: within 600 km, while the
least reheated are 900 km away. The advantage of
the hotspot rejuvenation hypothesis (e.g., [37]) is that
the arbitrary amounts of reheating can be ascribed to
distance from the hotspot. The apparently very large
reheating near the hotspot can be explained by the
emplacement of extra-hot mantle beneath the lithosphere [31,40,44], or the melt-parking and diffusive
reheating of the full thickness below a tensile zone
where magma moves by dyking [45],
Another way to look at the data in Fig. 8 is to say
that, subject to scatter from an unspecified cause, the
basins tend to fall near the dashed line, with rejuvenation proportional to age. In other words, lithosphere older than 100 Ma simply retains that thermal
age. On the original data diagram on Fig. 5, this is
equivalent to collapsing all the points onto the star at
100 Ma on the BL curve, or, if one does not like the
disagreement in depth, to a similar point on a BL
curve with slightly adjusted parameters. This would
ascribe the substantial heat flow variations between
the basins to instrumental measurement error, measurement error due to seafloor environment, or unspecified variations in key parameters, such as mantle temperature or conductivity. The first two possibilities have been studied exhaustively by Davis et
al. [8], Lister et al. f46] and Nagihara and Lister [47],
and rejected. The last is subject to Occam’s Razor: if
each piece of seafloor is given its own parameters,
then any model can be fitted, but without any gain in
understanding.
102
S. Nugihara et al. /Earth
ad
Planetary
There is lateral variation in upper mantle temperature most probably associated with the convection.
The effect of the variation is most significant at
spreading centers. Depending of the potential temperature of the upwelling asthenosphere, the volume
of the crustal rock created could vary [48,49]. This
affects the crustal thickness and, hence, the base
level from which the subsidence of the seafloor is
measured [50]. However, we have already removed
this effect by normalizing the depths of sites to a
constant crustal thickness. In older lithosphere, the
convection does not influence the seafloor depth
much because the low viscosity asthenosphere mechanically separates the two systems fairly well,
except at hotspots [36]. Lateral temperature variation
at depth may affect heat flow also, but such perturbation within the lithosphere probably averages out
during the long life of the old lithosphere.
Science Letters 139 (1996) 91- IO4
The Atlantic data, from the Blake-Bahama basin,
are more consistent with the influence of the Bermuda
rise hotspot than with convective peel-off; however
there are only four sample sites.
Acknowledgements
The main financial support for this research was
provided by the National Science Foundation grant
OCE91-03341. We thank D. McKenzie and C. Jaupart for their comments on an early version of this
manuscript. [PTI
References
ill D.L. Turcotte and E.R. Oxburgh, Finite amplitude convec-
8. Conclusions
When the depths of deep ocean basins have been
corrected to a nominal 7 km of crust with no sediment cover, they should be close to thermo-isostatic.
When precise heat flow data is available, thermal
isostasy can be tested on a depth versus heat flow
plot, and is found to apply within the limits of error.
The points fall closer to a boundary-layer cooling
curve than to either of the published plate models.
Their crustal ages are quite inconsistent with where
they plot on the curve, being always much younger.
We believe that the heat flow values by which the
data is spread out along the heat flow axis are more
precise than the indicated differences: that is, that the
variation is real.
The corrected depths are not sufficiently accurate
to separate basins by depth convincingly. The least
arbitrary explanation for the somewhat random thermal rejuvenation of these basins is that they suffered
reheating from the passage of a hotspot some distance away, or, in the case of the west Pacific, were
reheated by pervasive late Cretaceous volcanism of a
style different from present day hotspots. The data
alone cannot discriminate between a few discrete
reheatings due to convective peel-off at the base of
the lithosphere, or one or more catastrophic events.
tion cells and continental drift, J. Fluid Mech. 28, 29-42,
1967.
l21 R.L. Parker and D.W. Oldenburg. Thermal model of ocean
ridges, Nature Phys. Sci. 242, 137-139, 1973.
131 E.E. Davis and C.R.B. Lister, Fundamentals of ridge crest
topography, Earth Planet. Sci. Lett. 21, 405-413, 1974.
141 C.R.B. Lister, Estimators for heat flow and deep rock properties based on boundary layer theory, Tectonophysics 44,
157-171, 1977.
[51 J.G. Sclater and L. Wixon, The relationship between depth
and age and heat flow and age in the western North Atlantic,
in: The Western North Atlantic Region, P.R. Vogt and B.E.
Tucholke, eds., pp. 257-270, Geol. Sot. Am., Boulder, CO,
1986.
I61M. Renkin and J.G. Sclater, Depth and age in the North
Pacific, J. Geophys. Res. 93. 2919-2935, 1988.
171 B. Parsons and J.G. Sclater, An analysis of the variation of
ocean floor bathymetty and heat flow with age, J. Geophys.
Res. 82, 803-827, 1977.
ls1 E.E. Davis, C.R.B. Lister and J.G. Sclater, Towards determining the thermal state of old ocean lithosphere: heat flow
measurement from the Blake-Bahama outer ridge, northwestern Atlantic, Geophys. J. R. A&on. Sot. 78. 507-545,
1984.
191 K.E. Louden, D.O. Wallance and R.C. Courtney, Heat flow
and depth versus age for the Mesozoic northwest Atlantic
Ocean: results from the Sohm abyssal plain and implications
for the Bermuda Rise, Earth Planet. Sci. Lett. 83, 109-122.
1987.
[lOI C.R.B. Lister, J.G. Sclater, E.E. Davis, H. Villinger and S.
Nagihara, Heat flow maintained in ocean basins of great age:
investigations in the north-equatorial West Pacific, Geophys.
I. Int. 102, 603-630, 1990.
S. Nagihuru
et al./Earth
and Planetary
[I l] S. Le Douaran and B. Parsons, A note on the correction of
ocean floor depths for sediment loading, J. Geophys. Res. 87,
4115-4722.
1982.
1121 S.T. Crough, The correction for sediment loading on the
sea-floor, J. Geophys. Res. 88.6449-6454,
1983.
[ 131 H.P. Johnson and R.L. Carlson, Variation of sea floor depth
with age: a test of models based on drilling results, Geophys.
Res. Lett. 19, 197 1- 1974, 1992.
[14] L.J. Abrams, R.L. Larson, T.H. Shipley and Y. Lancelot, The
seismic stratigraphy
and sedimentary
history of the east
Mariana and Pigafetta basins of the western Pacfic, Proc.
ODP Sci. Results 129, 551-569,
1992.
[15] Y.P. Lancelot, R. Larson, A. Fisher, et al., Site 802, Proc.
ODP Init. Rep. 129, 171-243, 1990.
[16] L.K. Wipperman, R.L. Larson and D.M. Hussong, The geological and geophysical
setting near site 462, Init. Rep.
DSDP 61, 763-770,
1981.
[l7] R.D. Jacobi, D.E. Hayes and J.E. Damuth, High-resolution
seismic studies and site survey results near deep sea drilling
project sites 576 and 578, northwest Pacific, Init. Rep. DSDP
86, 23-49, 1985.
[ 181 K. Horai and R.P. Von Herzen, Measurement of heat flow on
leg 86 of the deep sea drilling project, Init. Rep. DSDP 86,
759-777,
1985.
[19] R. Houtz, C. Windisch and S. Murauchi, Changes in the
crust and upper mantle near the Japan-Bonin
trench, J.
Geophys. Res. 85, 267-274,
1980.
[20] Y.P. Lancelot, R. Larson, A. Fisher. et al.. Site 801, Proc.
ODP Init. Rep. 129, 91-170,
1990.
[21] R.L. Larson, A.T. Fisher, R.D. Jarrard, K. Becker and
0.L.l.S.S.
Party, High permeability
and layered Jurassic
oceanic crust in the western Pacific, Earth Planet. Sci. Lett.
119, 71-83, 1993.
[22] R.S. Derrick, R.P. Von Herzen, B. Parsons, D.T. Sandwell
and M. Dougherty, Heat flow observations on the Bermuda
rise and thermal models of mid-plate swells, J. Geophys.
Res. 91, 3701-3723,
1986.
[23] R. Mithal and J.C. Mutter, A low-velocity zone within the
layer 3 region of 118 Myr old oceanic crust in the western
North Altantic, Geophys. J. Int. 97, 275-294,
1989.
[24] E. Morris, et al., Seismic structure of oceanic crust in the
western North Atlantic, J. Geophys. Res. 98, 13,879- 13,909,
1993.
[25] R.P. Von Herzen, MJ. Cordery, R.S. Detrick and C. Fang,
Heat flow and the thermal origin of hot spot swells: the
Hawaiian Swells revisited, J. Geophys. Res. 94, 13,78313,800, 1989.
[26] R.D. Hyndman, M.G. Langseth and R.P. Von Herzen, Deep
Sea Drilling Project geothermal
measurements:
a review,
Rev. Geophys. Space Phys. 25, 1563-1582,
1987.
[27] W.J. Ludwig, J.E. Nafe and CL. Drake, Seismic refraction,
in: The Sea, A.E. Maxwell, ed., pp. 53-84, Wiley, New
York, NY, 1970.
[28] R.S. White, D. McKenzie and R.K. O’Nions, Oceanic crustal
thickness from seismic measurements and rare earth element
inversions, J. Geophys. Res. 97, 19.683-19.715,
1992.
Science Letters 139 11996) 91-104
103
[29] C.A. Stein and S. Stein, A model for the global variation in
oceanic depth and heat flow with lithospheric age, Nature
359, 123-129, 1992.
[30] W.J. Morgan, Convection plumes in the lower mantle, Nature 230, 42-44, 1971.
[3 I] R.C. Courtney and R.S. White, Anomalous heat flow and
geoid across Cape Verde Rise: evidence for geodynamic
support from a thermal plume in the mantle, Geophys. J. R.
Astron. Sot. 87, 815-868,
1986.
[32] B. Parsons and D.P. McKenzie, Mantle convection and thermal structure of the plates, J. Geophys. Res. 83, 4485-4496,
1978.
1331 A. Davaille and C. Jaupart, Onset of thermal convection in
fluids with temperature-dependent
viscosity: application to
[34]
(351
[36]
[37]
(381
[39]
[40]
[41]
1421
1431
1441
1451
[46]
the oceanic lithosphere, J. Geophys. Res. 99, l9,853- 19,866,
1994.
W.R. Buck and E.M. Paramentier, Convection beneath young
oceanic lithosphere: implications for thermal structure and
gravity, J. Geophys. Res. 91, 1961-1974,
1986.
G. Houseman and D.P. McKenzie, Numerical experiments
on the onset of convective instability in the Earth’s mantle,
Geophys. J. R. Astron. Sot. 68, 133-164, 1982.
E.M. Robinson and B. Parsons, Effect of a shallow lowviscosity zone on small-scale instabilities under the cooling
oceanic plates, J. Geophys. Res. 93, 3469-3479,
1988.
S.T. Crough, Thermal origin of mid-plate hot-spot swells,
Geophys. J. R. Astron. Sot. 55, 451-470.
1978.
R.S. Detrick, R.S. White, R.C. Courtney and R.P. Von
Herzen, Heat flow on midplate swells, in: CRC Handbook of
Seafloor Heat Flow, J.A. Wright and K.E. Louden. eds., pp.
l69- 190,CRC Press, Boca Raton, FL, 1989.
C.R.B. Lister, Differential
thermal stresses in the earth,
Geophys. J. R. Astron. Sot. 86, 319-330,
1986.
S. Watson and D. McKenzie, Melt generation by plumes: a
study of Hawaiian volcanism, J. Petrol. 32, 501-537,
1991.
R.L. Larson and S.O. Schlanger, Geological evolution of the
Nauru basin, and regional implications, Init. Rep. DSDP 61,
841-862,
1981.
R.P. Von Herzen, R.S. Detrick, S.T. Crough, D. Epp and U.
Fehn, Thermal origin of the Hawaiian swell: heat flow
evidence and thermal models, J. Geophys. Res. 87, 67116723, 1982.
R.L. Larson, The Mid-Cretaceous
superplume episode. Sci.
Am. 272, 82-86, 1995.
D.A. Yuen and L. Fieitout, Thinning of the lithosphere by
small-scale convective destabilization, Nature 3 13, l25- 128.
1985.
C.R.B. Lister, The upward migration
of self-convecting
magma bodies, in: Ophiolite Genesis and Evolution of the
Oceanic Lithosphere, T.J. Peters, ed.. Ministry of Petroleum
and Minerals, Sultanate of Oman, 1991.
C.R.B. Lister, Thermal leakage from beneath sedimentary
basins - an experimental test of the contribution of convective flow structures, Earth Planet. Sci. Lett. 99, 133- 140,
1990.
1471 S. Nagihara
and C.R.B. Lister, Accuracy
of marine heat flow
104
S. Nagihum
et ul./Eorth
und Plunetq
instrumentation: numerical studies on the effects of probe
construction and the data reduction scheme, Geophys. J. Int.
112, 161-177, 1993.
[48] E.M. Klein and C.H. Langmuir, Global correlations of
oceanic ridge basalt chemistry with axial depth and crustal
thickness. J. Geophys. Res. 92, 8089-8115, 1987.
Science Letters 139 (1996) 91-104
[49] D. McKenzie and M.J. Bickle, The volume and composition
of melt generated by extension of the lithosphere, J. Petrol.
29, 625-679. 1988.
[50] R. White and D. McKenzie, Magmatism at rift zones: the
generation of volcanic continental margins and flood basalt,
J. Geophys. Res. 94.7685-7729, 1989.