The crustal and lithospheric thicknesses of the Philippine Sea as compared to the Pacific

Earth and Planetary Science Letters, 50 (1980) 275-288

275

Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands [51

THE CRUSTAL AND LITHOSPHERIC THICKNESSES OF THE PHILIPPINE SEA AS COMPARED TO THE PACIFIC KEITH E. LOUDEN

Department o f Geodesy and Geophysics, Madingley Rise, Madingley Road, Cambridge [England) Received April 14, 1980 Revised version received June 3, 1980

Results from 12 new two-ship seismic refraction profiles in the Philippine Sea detail regions of crustal thickness significantly less than average for the Pacific. A comparison of layer 3 and mantle intercept times shows that layer 3 in the West Philippine basin is 1-2 km thinner than for similarly aged crust in the Pacific. In the Parece Vela basin layer 3 is on average 0.5 km thinner than its Pacific counterpart but varies considerably across the basin. Layer 2 parameters are also quite variable between profiles but its thicknesses are in tile mean 0.5 -1.0 km less in the West Philippine basin than for either the Parece Vela basin or for any of the 7 Pacific age groups. In the northeastern sector of the West Philippine basin layer 2 and 3 are both particularly thin which results in a total crustal thickness of as little as 3-4 kin. Pacific and Philippine depth versus age data from DSDP holes are corrected for these variations in crustal thicknesses. The resultant compensated mantle depths can only be fitted by theoretical conductive cooling curves which are depressed for the Philippine basins by an additional 1 km from those that would match Pacific depths. Given such an offset, Philippine Sea depth and heat flow values are consistent with thermal models in which the lithosphere may remain thinner than it is in the Pacific, but still must reach a minimum thickness of at least 50-75 kin.

1. Introduction The Philippine Sea consists o f a series o f deepwater basins and shallow ridges, and is surrounded by an almost unbroken belt o f deep-sea trenches and island arcs which separates it from adjacent seas and major sources o f terrigenous sediments. A variety of conflicting models have been proposed to explain these features. It was not until the late 1950's that the earliest models of basin genesis by continental subsidence [ 1 ] were finally disproven by seismic refraction measurements [2], which indicated a crustal structure with similar velocities and mantle depth to that o f the major oceans. Subsequent studies have corroborated the oceanic nature of the Philippine Sea [ 3 - 5 ] as well as of other similarly deep, marginal basins in the western Pacific (e.g. [ 6 - 9 ] ) and form the basis for more recent orogenic models. These current theories stress two possibilities. One is that older oceanic crust is trapped by the initiation

or relative motion of a subducting plate boundary; the Other is that new crust is formed in an extensional regime behind the island arc, which may be caused by the subduction o f the oceanic plate. The former has been used to explain the origins of the older West Philippine basin [10], and the latter for the younger Parece Vela basin, Shikoku basin and Mariana trough [11]. Paleontological dates and remnament magnetic inclinations o f Deep Sea Drilling Project (DSDP) cores, heat flow measurements, and a limited discontinuous set o f magnetic anomaly identifications have helped to define histories for these basins which are consistent with plate tectonic models derived from the study o f other oceans. At the same time they also emphasize some remaining discrepancies. The West Philippine basin formed by sea-floor spreading during the Eocene, symmetric about the Central Basin fault, south o f the magnetic equator and in a nearly northsouth direction [ 12,13]. Spreading stopped around

0012-821X/80/0000-0000/$02.25 © 1980 Elsevier Scientific Publishing Company

276 40 Ma and resumed in an east-west direction to form the Parece Vela and Shikoku basins to the east during the early Miocene [14,15,55] or late Oligocene [24, 56]. A hiatus separated that period from the recent crustal extension in the Mariana trough, which started during the early Pliocene [ 14,17,57] or late Miocene [21 ] and probably is still active [ 16]. These dates suggest that the mean depths of the Philippine Sea basins are considerably greater than found for similar ages in the major oceans, while the heat flow remains nearly normal. Sclater [18] suggested that this may be explained by reducing the plate thickness. A thinner plate would have shorter characteristic diffusion times and would more quickly reach an equilibrium state of approximately 2.2. HFU and 4.6 kin. This simple model cannot, however, account for the 6 km depths and normal heat flow values for Eocene ages in the West Philippine basin using acceptable values for thermal constants. A possible resolution to this problem is suggested by a re-examination of previous refraction studies. There is some evidence that the crustal structure of the Philippine Sea may be thinner than normal when compared to average Pacific structure, particularly for seismic layer 3 [19]. Such a thin crust would serve to compensate for its greater water depths by raising the level of the higher density mantle without affecting the conductive cooling of the lithosphere. In this paper we present 12 new refraction profiles in the Parece Vela and West Philippine basins. A comparison of intercept times suggests that the West Philippine basin's structure has anomalously thin layer 2 and 3 while the Parece Vela basin shows more normal thicknesses. These radically different structures suggest major differences between the orogenic processes that formed the basins but cannot by themselves explain the discrepant depths. Conductive cooling curves can fit the topographic data only when depressed by an additional constant offset of approximately 1 kin. The depths of some other marginal basins may necessitate similarly large offsets [20], but the cause of the positive density anomalies needed to produce such an effect remains uncertain. 2. New results from expedition INDOPAC During July 1976 on Leg 5 of expedition INDOPAC, the R/V "Thomas Washington" of Scripps

Institute of Oceanography and the R/V "Chiu Lien" of the National Taiwan University conducted 26 twoship refraction profiles in the Philippine Sea. These profiles sample deep basin crustal structure in the active Mariana and passive Parece Vela and West Philippine basins and are located in Fig. 1 along with the deep basin stations of Murauchi et al. [3] and Ludwig et al. [4]. Results from the Mariana trough are discussed elsewhere [21 ], and in this paper we will concentrate on the 12 profiles in the Parece Vela (stations 7 - 9 , 12 and 17) and West Philippine (stations 18--21 and 24 26) basins which recorded welldefined mantle arrivals. Figs. 2 and 3 show the travel-time plots of these stations. Record sections used in picking the arrival times are given in Bibee et al. [22] and not here, as their signal amplitudes cannot be compared from shot to shot. Groups of arrivals assumed to come from a discrete refracting layer are least squares fit to linear segments for our subsequent slope-intercept analysis. Second arrivals have not been used because of ambiguities both in their observation and association with a particular ray path. Times, slopes and layer thicknesses are computed to two decimal places but only the first is significant. Uncertainty increases when only a few arrivals are observed for a given layer, as is often true for the upper transitional basement (stations 7, 12, 17, 24 and 26). Travel times are corrected for source and receiver depths and for variations in water depth from the mean. Depth corrections assume horizontal subsurface layers, and have small effect except in a few instances of high relief over sections of profiles 12 and 19. The sediment layer is not removed and variation in its thickness can cause some scatter in arrival times. • From the slopes and intercepts of Figs. 2 and 3, plane layer thicknesses can be calculated using the relationship [23] : rt--| Trt = ~9 G hi-(o; 2 /=1

/)-2)1/2n

(1)

where rn is the intercept of the nth layer and hy, v] are the thickness and velocity of the/th layer. Sediment of velocity 2.0 km/s is assumed to cover any high-velocity basement. An additional assumption results from the fact that many profiles do not record arrivals from a transitional layer 2 (4.0 ~< v ~< 6.0 kin/

277

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13

54.11 15°N

o.o

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125°E

1:_-_-55°E

145°E

Fig. 1. The Philippine Sea with uncorrected thousand fathom depth contours from Chase and Menard [53] and locations of selected Deep Sea Drilling Sites and seismic refraction stations. Profile identifications beginning with "m" are from Murauchi et al. [3]: "LS" is from Ludwig et al. [4]; "CR19" is from Gaskell et al. [2]; "TAS 5-2" is from Henry et al. [5].

s) overlying the oceanic layer 3 (6.4 ~< v ~< 7.2 kin/s). This is a p r o b l e m c o m m o n to m o s t surface receptions o f refracted arrivals in ocean basins: the high-freq u e n c y reflected water wave masks all but a short interval o f first arrivals from layer 2 [23]. The situa-

tion worsens if layer 2 is thin or if the water d e p t h is large, as it m a y be in the West Philippine basin, or if it is covered by thick sediments as in the eastern Parece Vela basin. We are therefore not certain w h e t h e r our f r e q u e n t failure to observe typical layer 2

278 Parece

Vela

Basin 7

8

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Fig. 2. Plots of travel time and water depth vs. distance for INDOPAC seismic profiles in the Parece Vela basin taken from the record sections of Bibee et al. [22]. The reducing velocity is 7.0 km/s. Dashed lines are arrival times of the reflected water wave. Times are corrected for variations in water depths but not in sediment thicknesses. Apparent velocities and intercept times are given for least squares fits of straight-line segments to the arrival times.

velocities means that it does not exist or that it is not properly measured. Thus we consider the two extreme cases in which the well-determined delay to layer 3 is caused either solely by a sediment layer or solely by a transitional crust. Tables 1 and 2 summarize these possible horizontal layer thicknesses and velocities which are all consistent with the travel times of Fig. 2. We do not use our reversed or split pairs to solve for dipping layers. Ambiguities in observed upper crustal structure between certain paired profiles ( 7 - 9 , 2 4 - 2 6 ) would necessitate further assumptions that would qualify any solution more complicated than those given in Tables 1 and 2, while others ( 1 8 - 2 1 ) show little systematic variation in apparent velocities. From these first-order results we can make some general statements about the crustal structure. In the

Parece Vela basin station 7 records arrivals from layer 2 and these indicate a transitional basement, 2.0 km thick with velocity 5.4 km/s, under thick, 1200 m sediment. The sediments appear thicker in the east which is consistent with normal incidence and wide angle reflection records that show a thick 1 - 2 km apron of volcanoclastic sediments west of the South Honshu ridge (e.g. [11,19,24]). A sharp, linear trough which runs north from the intersection of the Yap and Mariana trenches splits the basin in half and truncates these sediments [24]. Seismic velocities within the uppermost sediment layer have been directly measured on DSDP cores at site 53, which is adjacent to stations 7 9 (Fig. 1) and penetrated 200 m of sediment above the probable basement. Well log velocities averaged 2.0 km/s and core samples from the thick (153 m) volcanic ash ranged from 1.50

279 West Philippine Basin 18

19

20

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to 2.33 k m / s [25]. Deeper units o f limestone and t u f f have velocities o f 3 . 1 9 - 4 . 0 7 k m / s , and these basal sediments although probably very thin [61] and intermixed with volcanic rocks m a y account for the 4.42 k m / s velocity measured at station 9. Alternatively this m a y sample the true upper part o f the transitional basement. L o w velocities ( 4 . 2 - 4 . 9 kin/s)

are also observed on profiles 12 and 17 in the western sector of the Parece Vela basin and on many, more densely sampled disposable sonobuoy results from upper crustal layers across the basin [24]. A transitional crust was not directly observed at station 8 but its layer 3 intercept time is similar to that o f station 7. Thus its upper crustal layers could have

TABLE 1 ttorizontal layer solutions to INDOPAC refraction profiles - Parece Vela basin Station No.

Eat. (°N)

Long. (°E)

Water h (km)

Sediment u (kin/s)/ h (kin)

Layer 2 v (km/s)/ h (km)

Layer 3 v (km/s)/ h (km)

Mantle o (km/s)

7 8 8' 9 12 17

18°07 ' 18o07 '

141°44 ' 141°43 '

4.59 4.60

5.37/2.05

18o11 ' 18o03 ' 17°59 '

141o06 ' 138000 ' 136o58 '

4.61 4.81 4.73

2.0"/1.19 2.0"/1.45 2.0"/1.00" 2.0*/0.27 2.0*/0.23 2.0*/0.27

7.39/3.89 7.31/4.47 7.31/4.05 6.60/2.14 6.50/4.95 6.50/5.04

8.39 8.57 8.57 8.04 8.84 9.31

5.4"/1.73 4.42/2.58 4.85/1.03 4.19/1.62

Stations are located in Fig. 1 and described by the travel-time plots of Fig. 2. Alternate solutions are indicated by primes and assumed velocities by asterisks. The position of each station is that of the recording ship.

280

TABLE 2 tlorizontal layer solutions to INDOPAC refraction profiles

WestPhillippine basin

Station No.

Lat. (°N)

Long. (°El

Water h (kin)

Sediment v (kin/s)/ h (kin)

18 18' l9 19' 20 20' 21 21' 24 25 26

18° 03'

133049 '

5.89

2.0*/0.28

Layer 2 t) (kin/s)/ h (kin)

5.4 */1.38 18° 03'

133048 '

5.90

2.0*/0.42

18o03 '

133008 '

6.04

2.0*/0.36

18003 '

133o08 '

6.04

2.0*/0.35

21 °04' 2F'04' 21 ° 12'

126° 30' 126027 ' 125°41'

5.57 5.50 5.28

2.0*/0.42 2.0*/0.23 2.0*/0.31

5.4 * / 1 . 8 5 5.4 * / 1 . 6 5 5.4"/1.57 5.81/1.36 5.68/1.29

Layer 3 u (kin/s)/ h (kin)

Mantle v (kin/s)

6.28/3.49 6.28/2.93 6.69/3.01 6.69/2.43 6.54/2.69 6.54/2.11 6.57/2.90 6.57/2.37 6.98/5.42 5.98/4.17 7.02/2.48

7.97 7.97 8.03 8.03 8.11 8.11 7.98 7.98 8.68 8.22 7.72

Stations are located in Fig. 1 and described by the travel-time plots of l:ig. 2. Alternate solutions are indicated by primes and assumed velocities by asterisks. The position of each station is that of the recording ship.

similar velocities and thicknesses (solution 8' in Table 1). In the West Philippine basin stations 1 8 - 2 1 show a remarkably thin upper crust with no arrivals from layer 2. Sediment should be thicker to the west of the Palau-Kyushu ridge than to the east [14] and yet the delay time to layer 3 is much greater for station 17 than for stations 1 8 - 2 1 . Only if we assume a negligible sediment cover (solutions 18', 19' and 20' in Table 2) can an assumed layer 2 have similar thickness to that in the Parece Vela basin, and even then for station 18 it is still too thin. DSDP Sites 290 and 447, located near stations 1 8 - 2 1 , bottomed in volcanic conglomerate which overlies normal oceanic pillow basalts after 140 m (290A), 250 m (290) and 183 m (447) of ash and chalk layers. Sonic velocities in site 290 cores ranged from 1.6 km/s at 120 m to 1.9 km/s at 222 m, and increased to 3.1 km/s in the conglomerate [26]. Such a sediment thickness lies between the extreme solutions given in Table 2 and argues for an intermediate situation of both thin sediment and thin layer 2. This is also consistent with multichannel reflection profiles which show less than 0.5 second two-way travel times to basement at these sites which are just outside the draped sediments immediately to the west of the Palau-Kyushu ridge (D. Hayes, personal communication). Stations 2 4 - 2 6

are also situated above thin sediments just outside the eastern limit of the thick sedimentary apron sampled by DSDP Site 293 (545 m above a volcanic breccia). Layer 2 velocities are observed at stations 24 and 26, but of all the stations in the West Philippine basin only station 24 has arrivals consistent with both a thicker sedimentary cover and a normally thick layer 2 as observed in the Parece Vela basin. Results from layer 3 and mantle arrivals are more consistent within both basins. Thicknesses for layer 3 are less than average for normal oceans in the West Philippine basin, but are normal in the Parece Vela basin except for station 9 which has poorly controlled layer 3. Apparent layer 3 velocities are normal in both basins (6.86 -+ 0.45 km/s in the Parece Vela basin; 6.58 + 0.37 km/s in the West Philippine basin) but there is a large scatter. Station 25 has an unusually low layer 3 velocity (5.98 km/s) and station 7 unusually high (7.39 km/s). Apparent mantle velocities are higher in the Parece Vela basin (8.63 + 0.48 km/s) than in the West Philippine basin (8.10 + 0.30 km/s). Although higher mantle velocities in the Parece Vela basin may be biased by dip, especially for the very high velocities in the unreversed profiles 12 and 17, they are also consistent with the reversed profiles 8 - 9 and Murauchi et al.'s [3] reversed stations 2 3 - 2 5 .

281

3. A comparison of intercept times It is the layer 3 and mantle intercept times and apparent velocities which are most precisely determined by the travel-time data in Fig. 2. Once we use these values with equation (1) to compute layer 2 and 3 thicknesses, we begin to incorporate inaccuracies caused by the paucity of arrivals from layer 2 and assumptions of sediment thickness and velocity. For instance, note the effect that changing the upper layer parameters in the alternative solutions of Table 1 and 2 has on layer 3 thicknesses. The indeterminateness of upper layers also limits our use of reverse profiles to remove the scatter in apparent velocities caused by sloping layers. If we want to compare crustal thickness in the Philippine Sea to that in the Pacific, a more precise approach would be to contrast intercept times rather than computed thicknesses and velocities. Differences in intercept time could then be related by equation (1) to differences in layer thickness for any particular velocity-depth model. Trehu et al. [27] compiled mantle intercept times for stations in three major oceans to show that total thickness remains constant for crust older than 30 Ma. Scatter was reduced by using intercept times instead of thicknesses, particularly for data in the Atlantic. However this approach still leaves the uncertainties in correcting for the poorly constrained sedimentary parameters. An alternative approach is to present both mantle (rm) and layer 3 (r0) intercept times before removing the sediment layer; then compute: 6 T = 1-m -- 1-0

(2)

which should depend primarily on layer 3 and remain relatively independent of sediment or layer 2 thicknesses. Using the standard Raitt [28] velocity model (sediment, 2.0 kin/s; layer 2, 5.0 km/s; layer 3, 6.7 km/s; mantle, 8.2 kin/s) with equations (1) and (2), 61- increases by 1.5% and 18% of the increase in ro for increases of 100% in sediment and layer 2 thicknesses respectively. In Figs. 5a, 5b and 6 we compare 1-o and 61- from the Pacific and Philippine seas. Data from the Philippine Sea is located in Fig. 1 and is divided into groups of values from the Parece Vela and West Philippine basins. Pacific data, located in Fig. 4, is the same set used by Woollard [29] and Trehu et al.

[27], and is split into seven age provinces ( 0 - 9 , 9 2 1 , 2 1 - 3 6 , 3 6 - 6 3 , 6 3 - 1 0 0 , 1 0 0 - 1 3 5 and 135190 m.y.B.P.) from the magnetic anomaly identifications as mapped by Pitman et al. [30] for the Cainozoic and Hilde [31] for the Mesozoic. Data from the Bering Sea and the Caroline basin are treated separately;stations on anomalous features such as the Shatsky rise, Hawaiian buldge, or those near oceanic trenches are deleted. The effect of sediment thickness appears in the skewed distribution of ro shown in Fig. 5a. The wide spread of high values comes primarily from stations close to the eastern continental edge, where we might expect a greater terrigenous or turbidite sediment accumulation. Four high values come from the oldest age provinces in regions of less than normal water depth (3.86-4.93 km) for their age, and may be influenced by one of the many volcanic ridges in that area. These deletions leave a very small sample population in the oldest group and led Trehu et al. [27] to consider the more numerous results from undisturbed Mesozoic crust in the Atlantic. This is not a problem for our comparison of the much younger Philippine Sea, which spans an age range densely sampled by Pacific refraction stations. The scatter in 6r (Fig. 5b) is much less than for ro (Fig. 5a) and confirms our previous belief that 6r values should not contain the effects of sedimentary variations. Most stations with large ro values have normal 6r, and only a few discrepant values remain. High values in the Bering Sea come from stations close to and probably are affected by the Aleutian trench. Other stations with large 61- are geographically scattered, comprise only 4% of the total number, and are deleted from the statistics of Fig. 6. To be safe we have also deleted all 5r values from stations with high To .

Fig. 6 shows that 6r values for young Pacific crust increase until 30 m.y.B.P. This must be caused primarily by the increasing thickness of layer 3, as velocities are independent of age [32] and span a range that at most could only explain a 0.1 second increase in intercept time. Layer 2 can remain essentially constant, with the possible increase in ro caused by sediment accumulation. Previous analyses of Pacific data come to essentially the same conclusion [33,34]. Intercept time averages from the West Philippine

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60°,( Fig. 4. Location of seismic refraction stations in the Pacific Ocean and Bering Sea. For tile Pacific, data sources are the same as listed in Woollard [ 29] ; for the Bering Sea (B) they come from Shor [6 ] ; Shor and Fornari [ 361 and Ludwig et al. [ 35 ] ; for the Caroline basin (C) from Hussong [54[. Stations within the Hawaiian swell (H) are excluded in subsequent analysis. Those located by filled circles have high r0 and those with half-filled circles have high fir in Fig. 4. Isochlons in Ma from the magnetic anomaly identifications of Pitman et al. [30] and Hilde [ 31 ] divide the Pacific into 7 age provinces. and 6 r and differences in layer thicknesses ( ~ h l , Ah2, basin lie outside the range of Pacific values. It has low 3 r which appears more consistent with averages for Pacific crust 1 0 - 3 0 Ma younger. The Parece Vela basin has a higher 6 r average and although it is also less t h a n the Pacific values for its age this difference is n o t significant. The two Philippine basins have even. more widely differing r0: less t h a n Pacific averages in the West Philippine basin and greater in the Parece V e i l basin. Given similar velocity structures, e q u a t i o n (1) yields a linear relationship b e t w e e n differences in ro

~3): ~ h t = a ( v t , v2, v3) 2gz2 +b13 Aro

(3)

Ah3 ~ c(o2, v3, 04) ~xh2 + b34 2x~r where :

brim = 1/2(On 2 + Um2)'/2

(4)

The c o n s t a n t s a(vl, u2, v3) and c(v~, 03, v4) depend on more complicated ratios o f the indicated veloci-

283

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Fig. 5. Histograms of (a) layer 3 intercept times, to, and (b) layer 3 intercept time differences, 6r. The data are split into groups which include: the 7 age provinces of the Pacific (numbers in Ma); the Bering Sea (BS) and Kormandorsky basin (KB); and the West Philippine basin (WPB), Parece Vela basin (PVB), Shikoku basin (SB) and Caroline basin (CB). All stations with well-defined values located in Figs. 1 and 4 are used. Low values of r 0 are primarily caused by thin layer 2 or sediment; low 6r is primarily caused by thin layer 3. The distributions of r 0 values in the Pacific are skewed toward high values, which are shown in (a) in solid black. These values are derived from the filled circle stations located in Fig. 4. Values of 6r in (b) are more tightly grouped, with predominantly normal values shown in solid black from the same stations which have high to. The few remaining high 6r values in the Pacific and Bering Sea (shown by stippling) come from stations which are located by half-filled circles in Fig. 4. ties. F o r o u r s t a n d a r d velocities, a ~- c = 0 . 2 7 , b 13 = 1.05 k m / s a n d b34 = 5.81 k m / s . F r o m Fig. 5 averages, e q u a t i o n s (3) b e c o m e : z2xhl ( A h 3 ) = - 0 . 2 7 Z2xh2

+ 0 . 1 8 ( 1 . 1 0 ) WPB - 0 . 2 8 ( 0 . 4 6 ) PVB

T h e f o l l o w i n g c o n c l u s i o n s f o l l o w f r o m equa-

(5)

t i o n s (3) a n d (5)~ (a) T h e average d i s c r e p a n c y b e t w e e n Pacific a n d West Philippine b a s i n 6 r can o n l y b e e x p l a i n e d b y a layer 3 w h i c h is o n t h e o r d e r o f 1 k m t h i n n e r in t h e West P h i l i p p i n e basin a n d 0.5 k m t h i n n e r in t h e Parece Veta b a s i n . ( b ) ro in t h e West P h i l i p p i n e b a s i n c a n b e caused

284 I0 ===========================================================

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I 14

Fig. 6. Intercept time m e a n s (_+2 standard errors) versus age for Pacific (shaded boxes), West Philippine basin (WPB) and Parece Vela basin (PVB) data given in Fig. 5. For the Pacilic, stations with e x t r e m e values either o f r o or 6r are excluded. Variations in 6 r are caused by changes in the thickness of layer 3 ; variations in ro by changes either in sediment or layer 2 thicknesses.

either by sediments 180 m or layer 2 650 m thinner than for the Pacific. (c) ro in the Parece Vela basin is explained by sediments 280 m thicker than for the Pacific if layer, 2 remains the same. (d) The differences in ro between stations in the western Parece Vela and eastern West Philippine basins (Aro = 0.60 s) is caused either by layer 2 up to 2.2 km thinner in the West Philippine basin or sediments 650 m thicker in the Parece Vela basin. One model that is self-consistent with these results and known sediment distribution has: in the West Philippine basin, a normal sediment cover and layers 2 and 3, 0.65 and 1 km thinner than for the Pacific; in the Parece Vela basin, a thicker sediment cover, normal layer 2 and layer 3, 0.5 km thin. It is not clear whether similar evidence for thin crust exists for other marginal basins. There are often not enough results to form a consistent pattern, and it is also necessary to distinguish basins of similar tectonic origin. This can be attempted by dividing these into two groups: one with heat flow averages (>~2.0 HFU) similar to the West Philippine and Parece Vela basins, and the other whose heat flow is less (~<1.5 HFU). Within the higher heat flow group the

nature of the crustal structure seems quite varied. Some have normal layer 3 thicknesses (Japan and Kuril basins), while others may have thin layer 3 (Mariana trough, Kormandorsky, Shikoku and Caroline basins). It does appear that all basins with low heat flow have a thicker layer 3 (Celebes and eastern Bering Seas). The Bering Sea has a relatively large sampling of refraction measurements [6,35,36] and can be split into two provinces: the Kormandorsky basin with high heat flow and a thinner sediment cover, and the Aleutian and Bowers basins with lower heat flow and thicker sediments [37]. Values of 6r in Fig. 5b, although not significantly different between basins, are predominantly low in the Kormandorsky basin. If its one high value is discounted, the Kormandorsky basin has a mean 8r of 0.67 + 0.06 s, as opposed to 0.82 + 0.10 s for the eastern basins which is more representative of Pacific values from crust older than 20 Ma. Cooper et al. [37] suggest that such a thin layer 3 in the Kormandorsky basin may explain its +50 mGal free-air gravity anomaly. The Shikoku and Caroline basins, with heat flow and magnetic anomaly ages similar to those of the Parece Vela basin [ 15,38], also have 6 r values gene rally lower than for Pacificcrust of a similar age (Fig. 5b). The five results from the Caroline basin, with mean 8r of 0.60 +- 0.04 s, are particularly low and consistent. In the Atlantic there are only a few marginal basins of oceanic crust which may be tectonically related to nearby island arcs or dipping Benioff zones. Of these, the Scotia Sea is not well sampled and the Venezuelan basin has average layer 3 thicknesses [39] and low heat flow [40]. Only the Tyrrhenian and Balearic basins in the western Mediterranean have high heat flow [41] and some evidence from a limited data set of thin or anomalous layer 3 [42,43].

4. Depth vs. age and lithospheric thickness The mean, sediment-corrected depths of the Philippine Sea basin are all 5 0 0 - 1 0 0 0 m greater than Pacific Ocean depths at comparable ages [19]. Other marginal basins in the western Pacific have similar depth vs. age or depth vs. heat flow anomalies [18, 20], and in this respect are nearly unique among

285 in the young Mariana trough (25 to 50 mGal) and lower in the older, northern regions of the West Philippine basin (0 to - 2 5 reGal). Because of its large extent (~106 km 2) its density structure must therefore be close to isostatic equilibrium. Sclater et al. [19] have suggested that in such an equilibrium state, with higher-density mantle compensating for lower-density water, the thin crust in the Philippine Sea may explain part of its greater depths. To account for this effect, we compare compensated Moho depths in Fig. 7a for DSDP sites in the Pacific Ocean and Philippine Sea as a function of age. We use the mean intercept times of Fig. 6 to correct sea water depths for variations in crustal thickness. Pacific values are similar to those previously calculated by Yoshii [34] and Watanabe et al. [20]. The crustal structure of the Mariana trough is taken from Bibee et at. [21 ]. Theoretical curves are

ocean basins; in a world-wide study Cochran and Talwani [44] show only two other oceanic regions with such large negative depth anomalies. These depth anomalies do not have correspondingly large free-air gravity anomalies associated with them. The deep basin gravity field in the Philippine Sea [45] is generally positive (0 to 25 mGal) with higher values

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u_ F-Fig. 7. A. C o m p e n s a t e d mantle depths in the Pacific Ocean and Philippine Sea as a function of age. DSDP sedimentcorrected water depths and ages are taken from Parsons and Sclater [47] for the Pacific Ocean and Sclater et al. [19] and Scientific staff [ 5 5 - 5 7 ] for the Philippine Sea, and isostatically compensated for crustal thicknesses which are compatible with the intercept time data presented in Fig. 6. Ranges in d e p t h values allow for the possible range of intercept times. For the Philippine Sea circles refer tq DSDP sites in the Mariana trough, stars for the Shikoku basin, boxes for the Parece Vela basin and triangles for the West Philippine basin. Theoretical subsidence curves are given for theoretical models of a cooling half-space with variable sub-lithospheric heat flux Q (numbers in cal cm -2 s-1) given by Crough [46] using the thermal parameters of Parsons and Sclater [47 ]. Philippine Sea depths are compatible with models using higher Q than would fit the Pacific data, assuming a constant offset o f 1 km. B. Lithospheric thickness as a function o f age for the thermal models in A. Higher values o f Q, which are compatible with depth and heat flow values, would depress the rate of lithospheric thickening relative to the Pacific.

O

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Fig. 8. Heat flow as a function of age for Pacific Ocean and Philippine Sea data compared with the same theoretical curves as in Fig. 7. Data for the Pacific is from Sclater et al. [58] and Sclater and Crowe [59] ("reliable" data) and Sclater and Francheteau [60] (means o f all data); data for the Philippine Sea is from Sclater et al. [19] (triangles) and Watanabe et al. [20] and Anderson [16] (circles). Hydrothermal circulation reduces values in regions of rough topography. The m e a n heat flow o f the Philippine Sea is the same as "reliable" values from well-sedimented areas of the Pacific and Reykjanes ridge. This limits values o f Q to less than 1.0 HFU.

286 given for a cooling half-space model [46] with variable sub-lithospheric heat flux, Q, and thermal constants determined from Pacific depth and heat flow [47]. These curves show that the crust of the Philippine Sea is not thin enough to completely account for its greater water depths; to fit the Philippine data we must offset the theoretical curves by approximately 1 km from the Pacific data. Depending upon what constant offset we choose, the curves can fit the Philippine depths for a range of Q (<1.5 HFU) larger than would fit the Pacific depths. The upper limit to Q can alternatively be defined by the surface heat flow. In Fig. 8 we compare heat flow values between the Philippine Sea and Pacific Ocean. When only considering measurements from well-sedimented regions the results are much the same. Previous suggestions that marginal basins have high heat flow (e.g. [18]) were therefore an erroneous product of comparisons with Pacific values reduced by the effects of hydrothermal circulation. A more recent suggestion by Watanabe et al. [20] that there is a two-stage thermal relaxation in marginal basins is also not supported by data from the Philippine Sea. If there is a higher sub-lithospheric heat flux under the Philippine Sea it must be limited to a maximum of 1.0 HFU. From Fig. 7b this means that the thermal lithospheric tlffckness of the Parece Vela and West Philippine basins must lie in the range 4 0 - 6 0 km. This is larger than results from surface wave studies in the Philippine Sea indicate. Seekins and Teng [48] present Rayleigh wave dispersion curves for ray paths across the Philippine Sea which resemble those of oceanic crust younger than 10 Ma [49] and are consistent with independent observations of low Pn velocities under the West Philippine basin [50]. The West Philippine and Parece Vela basins are separated into two provinces: ridge and plate. Pure path inversion results in plate thicknesses of 85 and 25 km, respectively. Unfortunately tiffs division of tectonic provinces does not always conform to surface topographic expressions. Thus while the extremely low value of 25 km is inconsistent with the heat flow and depth verus age curves presented in this paper, an average structure with a lithospheric thickness of 5 0 - 6 0 km is compatible. Alternatively there may be an as yet unexplained discrepancy

between seismic and thermal plate thicknesses in the Philippine Sea. It is paradoxical that a younger and hotter thermal structure be associated with greater water depths; one normally observes the reverse. The nature of the depth offset in Fig. 7a which can reconcile such a discrepancy is not certain, but has been ascribed to a density anomaly of 0.01--0.05 g/cm 3 in the upper mantle [20,34]. Similar deep density anomalies have also been evoked to compensate for the thick sedimentary layers in the Aleutian basin [51 ]. In cases where the basin is underlain by a subducted slab, the depressed temperatures in the slab could cause a greater depth behind the island arc. For AT - 2 × 102°C, Po = 3.4 g/cm 3 and c~= 4 × I0-5/°C, then A p = p o u A T ~ 3 X 10 -2 g/cm 3 . This At) acting over a length L - 10 z km could compensate for an additional water depth of 1 kin. The greater depths over the entire Philippine Sea, in basins well clear of the limited region of active subduction, may be caused by the accumulated effects of former subduction during earlier periods of its history. That such density increases exist in these regions is suggested by ScS2ScS travel-time residuals reported by Sipkin and Jordan [52]. Their lower values for surface reflection points under marginal basins are more representative of older oceans or continents than of young ocean, and could be caused by the larger densities needed to suppress the depths of this marginal basin.

Acknowledgements This work was supported by the U.S. Office of Naval Research contract No. N00014-75-C-0291 and by the U.K. Natural Environment Research Council. The data were collected from the R/V "Thomas Washington" of Scripps Institution of Oceanography and the R/V "Chiu Lien" of National Taiwan University. L.D. Bibee, G.G. Shor, Jr. and D. McGowan contributed to the reduction of the data. George Woollard and Donald Hussong kindly supplied compilations of data for Pacific refraction stations.

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