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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. 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