South Atlantic hot spot-plume systems: 1. Distribution of volcanism in time and space

Earth and Planetary Science Letters, 113 (1992) 343-364 Elsevier Science Publishers B.V., Amsterdam

343

[cc]

South Atlantic hot spot-plume systems: 1. Distribution of volcanism in time and space J o h n M. O ' C o n n o r a,1 a n d A n t o n P. le R o e x b a College of Oceanography, Oregon State University, Corvallis, OR 97331-5503, USA b Department of Geology, University of Cape Town, Rondebosch 7700, South Africa Received August 5, 1991; revision accepted August 11, 1992

ABSTRACT New Ar-Ar dating of rocks dredged from seamounts and ridges distributed along the St. Helena and Gough volcanic chains suggests that these features were formed by, respectively, the activity of the St. Helena and Walvis hot spot-plume systems. The St. Helena and Walvis (referred to elsewhere as Tristan) hot spots probably consist of broad zones of diffuse volcanism (i.e. oceanic islands, seamounts, and small ridges), at least 500 km in diameter. It remains unclear as to whether one or several narrow plume(s) is (are) upwelling to form these broad zones of hot spot volcanism, which results from decompression melting across parts of the broad, impacted 'mushroom head' of the plume(s). The very slow velocity of the African plate, in association with the westward flow of St. Helena and Walvis plume material to the South Atlantic spreading-axis, are likely to be important factors in the development of these broad fields of mid-plate volcanism. On a more localized scale, lithospher!c structure (e.g. fracture zones) probably controls the locations of sites of hot spot volcanism. The distributions of Ar-Ar ages along the St. Helena and Gough chains, in conjunction with the proposition of their having been formed by broad hot spots, have been incorporated into a reconstruction of African plate motion over hot spot-plume systems since the opening of the South Atlantic. Estimates of the velocity of the African plate suggest that the African plate might have slowed significantly between ~ 31 and 0 Ma. The South Atlantic spreading axis migrated westward away from the (fixed?) St. Helena and Walvis hot spot-plume systems, this migration beginning between ~ 80 and 70 Ma. This led to a transition from an on-spreading-axis to a mid-plate constructional setting along the St. Helena Chain and the Walvis Ridge.

1. Introduction T h e St. H e l e n a C h a i n is a b r o a d b a n d of scattered s e a m o u n t s a n d volcanic ridges extending f r o m the island of St. H e l e n a towards the A f r i c a n coast (Fig. la). M o r g a n [3,4] a t t r i b u t e d the f o r m a t i o n of this c h a i n to the activity of a hot spot which is p r e s e n t l y active b e n e a t h the island of St. H e l e n a a n d which is located ~ 800 k m to the east of the M i d - A t l a n t i c spreading-axis o n m a g n e t i c a n o m a l y 10 o c e a n i c crust ( ~ 30 M a according to the time scale of [5]). K - A r ages [6,7] indicate that s u b a e r i a l activity o n St. H e l e n a oc-

Correspondence to: J.M. O'Connor, Christian-Albrechts-Universit~it, Geologisch-Pal~intologischesInstitut und Museum, Olshausenstrasse 40, W-2300 Kiel, Germany

c u r r e d b e t w e e n 14 a n d 7 Ma. T h e most r e c e n t K - A r d a t i n g of subaerial lavas from this > 4000 m high volcanic s t r u c t u r e suggests, however, that it was c o n s t r u c t e d primarily b e t w e e n 9 a n d 7 M a [8]. T h e isotopic geochemical c o m p o s i t i o n of St. H e l e n a subaerial lavas is characterized by a high r a d i o g e n i c lead c o m p o n e n t (i.e. H I M U ) , which has b e c o m e progressively m o r e p r o n o u n c e d since 9 Ma, p r o b a b l y as a result of the d e c r e a s i n g i n f l u e n c e of d e p l e t e d m a n t l e m a t e r i a l in association with dying volcanism [8]. G e o c h e m i c a l studies of basalts d r e d g e d from the s p r e a d i n g axis to the west of St. H e l e n a suggest that there is m i g r a t i o n of m a t e r i a l from the St. H e l e n a p l u m e to the s p r e a d i n g axis [9,10]. T h e p r e s e n c e of a St. H e l e n a - l i k e isotopic signature in these basalts strongly supports a m o d e l of sub-lithospheric flow of p l u m e m a t e r i a l from a

0012-821X/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

344

J.M. O’CONNOR AND A.P. LE ROEX

currently active St. Helena plume [9]. Helium isotope geochemistry of these spreading-axis basalts has also been interpreted in terms of such a model [ill. High-resolution seismic tomography has recently confirmed the transport of plume material away from the St. Helena hot spotplume system to the South Atlantic spreading axis [121. This tomographic picture shows that the low-velocity anomaly beneath the spreading axis at the latitude of St. Helena is elongated and

deepens towards the east to a depth of about 200 km; the island of St. Helena lies above the far eastern end of this anomaly [12, fig. 71. A 2000 km long line of ocean islands and continental intrusive centers (the Cameroon Line) forms an apparent extension of the St. Helena Chain into the Gulf of Guinea and onto the African continent (Fig. la>. This lineament does not become older with increasing proximity to the African coast [e.g. 131. Our study provides the

10N

15W

low

SW

0

5E

10E

15 E

Fig. 1. (a) Bathymetry of the St. Helena Seamount Chain, after [l]. Fracture zones and spreading axes after [Z]. Dredge and island samples analyzed in this study are shown with x’s. Cameroon Line volcanism is shown in black. The 4000 m isobath approximately outlines the features, and the coastline of Africa is marked. Mercator projection. (b) Bathymetty of the Walvis Ridge, after [I]. The 4000 m isobath approximately outlines the ridges and seamounts. Mercator projection. (c) Bathymetry of the Pernambuco and Bahia volcanic lineaments after [l]. Fracture zones and spreading axes are after [2]. 4oAr-39Ar ages shown are after O’Connor [unpublished data]. The bathymetry contour is again primarily the 4000 m isobath. Mercator projection.

S. A T L A N T I C

HOT SPOT-PLUMES:

DISTRIBUTION

OF VOLCANISM

first evidence that the C a m e r o o n Line does not extend seaward of the most southwestern of the C a m e r o o n Line islands. Located to the south of the St. Helena Chain is the Walvis Ridge (Fig. lb). A r - A r dating [14] has established that, between 7 ° and 8°E and the African coast, the Walvis Ridge is ~ 30 Ma, and that it becomes progressively older towards the African coast. The G o u g h Chain (i.e. Gough Island, McNish and R S A seamounts), connects the 30 Ma section of the Walvis Ridge with the presently active Walvis hot spot located in the Tristan da Cunha and G o u g h Island region, 500 km to the east of the South Atlantic spreading axis. K-Ar ages for subaerial volcanism on Gough Island range from 2.5 + 0.5 Ma to ~ 0.1 Ma [15]. A p p a r e n t K-Ar ages for Tristan da Cunha range between 0.01 + 0.02 to 0.21 + 0.01 [16]. Inaccessible and Nightingale are two smaller islands which lie to the southwest of Tristan da Cunha. The maximum age of Inaccesible Island, previously estimated at 6 Ma Miller in [17], has now been

345

IN TIME AND SPACE

revised to about 1 Ma [18]. The maximum age of Nightingale Island was previously determined by conventional K-Ar as being 18 Ma Miller [17]. As the general erosional state of Inaccessible Island is intermediate between that of Tristan and Nightingale, it can be assumed that this age determination must also be revised downward [18]. We report here the results of an Ar-Ar study of the first volcanic rocks recovered from the St. Helena (Fig. la) and Gough (Fig. lb) Seamount chains. In addition, ages for samples from V e m a and 7°East seamounts, located ~ 550 and 700 km respectively to the south of the Walvis Ridge, and ~ 500 km to the east of the C34 (84 Ma) seafloor isochron (Fig. lb), are also reported. As more information about the age and geographical distribution of volcanism along African plate hot s p o t - p l u m e - g e n e r a t e d chains becomes available, revisions to models of African plate motion over hot spots become necessary [e.g. 4,14,19]. H e r e we propose a new rotation model for the African plate in order to incorporate all available Ar-Ar

/

(b)

15S

20 S

25S :,:<.::.:...::.:.-.: ::::::::::::::::::::::::::::

[

o~: ...:iiii: ,

.a

Vema

30 S

35S

Chron 34 (84 Ma) 7 East

Crawford

40S " RSA Gough McNish

20W

15W

10W

5W

0 5E Fig. 1 (continued).

10E

15E

45 S 20 E

346

J.M. O'CONNOR AND A.P. LE ROF_.X

'/Icl J

J t

5s

,

"

;~ %" D-1{~78 Ma)

._._.___

f

~

15 S

f

J 40 W

35 W

10S

30 W

25 W

20 W

15 W

20 S 10 W

Fig. 1 (continued).

age data from the Walvis Ridge and Gough Chain, the St. Helena Chain, and the track of the Reunion hot spot-plume system on the African plate. An additional aspect of the evolution of hot spot-plume volcanism in the South Atlantic is the fact that the Cretaceous phase of activity was closely interlinked with the South Atlantic spreading axis. In an earlier study of the evolution of the Walvis Ridge-Rio Grande Rise system it was shown that the South Atlantic spreading axis migrated westward, away from the mantie-fixed Tristan hot spot beginning at about 70 Ma, resulting in a transition from mid- to intraplate volcanism [14]. We discuss the evolution of the St. Helena hot spot-plume system in the context of such a South Atlantic-wide migration of the South Atlantic spreading axis away from fixed hot spot-plume systems. 2. Sample sites Rock samples dated by the 4°mr-39Ar incremental heating method were dredged from three

St. Helena seamounts-minor volcanic ridges (Fig. la), RSA and McNish seamounts (Fig. lb), and the submarine flanks of the islands of Gough and Tristan da Cunha (Fig. lb). Samples from 7°East and Vema seamounts (Fig. lb), located to the south of the Walvis Ridge, are also reported here as part of a continuing study of South Atlantic seamounts. Sample descriptions, coordinates and ages are in Table 1. 4°mr-39Ar ages of single seamounts from each of the Bahia and Pernambuco chains [O'Connor et al., unpublished data, discussed in [20]] (Fig. lc) are also included in this discussion. 3. Radiometric dating technique and results T h e 4°mr-39Ar method is based on the generation of 39Ar from 39K by the irradiation of Kbearing rock samples with neutrons in a nuclear reactor. The principles and application of this dating technique have been presented previously by [21,22,231.

S. ATLANTIC HOT SPOT-PLUMES: DISTRIBUTION OF VOLCANISM IN TIME AND SPACE

Whole rock samples (0.5-1 mm chips) were dated by the incremental heating 4°mr-39Aytechnique at Oregon State University. Samples selected for age dating were crushed following removal of any obvious vesicles and surface alteration. The 0.5-1.0 mm rock fragments were washed ultrasonically in distilled H 2 0 and dried. About 1 g of each sample was then encapsulated in an evacuated quartz vial. Five vials were arranged around a sixth vial, containing a hornblende flux monitor of known age [520.4 Ma, Mmhb-1, 24], which measured the efficiency of

347

conversion of 39K to 39my, expressed as the J-factor. Three such groups of vials were arranged vertically in an aluminum tube, which was then placed in the T R I G A Reactor at Oregon State University and irradiated for 10 h at the 1 MW power level. Samples and standards were placed individually in an outgassed Mo crucible, which was then heated in a high-vacuum extraction line. The standards were fused in a single heating step, while the basalts were heated incrementally in a series of five to seven steps (each of 30 min

TABLE 1 Locations and brief description of dredged rock samples from the St. Helena and the G o u g h seamount chains Samples

Location

St. Helena Seamount AC-02 2°19'5"S; 4°46'4"W AC-02A AC-02B AC-02C AC-02E AC-02G AC-02H AC-02K AC-02I St. Helena Seamount AC-05 4°17'4"S; 4 ° 2 8 ' 7 " E AC-05A AC-05B St. Helena Seamount AC-06 8°25'6"S; 1°33'0"E AC-06

Depth ~ 1980-2000 m

Age

Description

80-82 Ma highly altered volcanic rock moderately altered trachyte highly altered volcanic rock moderately altered basalt moderately altered ol + pl basalt moderately altered trachyte highly altered volcanic rock moderately altered volcanic rock

~ 1768 m

(78 Ma) moderately altered ol basalt highly altered volcanic rock (alkali basalt?)

~ 1340 m

52 Ma slightly altered trachyte

Cameroon Line (Tinhosa Granda Island) l°21'l"N; 7°16'6"E TG-A-01

3.4 + 1.4 Ma moderately altered volcanic rock

Tristan da Cunha AG51-21-1

~ 2000 m

< 1 Ma

basanite

Gough Island AG51-3-6

~ 2000 m

< 1 Ma

basalt

McNish Seamount AG51-7-1 40°10'S; 8°33'W

3000 m

8 Ma

RSA Seamount AG51-12-3

31°38'S; 8°20'E

3000 m

15 Ma

mugearite

V e m a Seamount AG51-9-1

39028'S; 6°13'W

3000 m

18 Ma

trachyte

91 Ma

mugearite

7° East Seamount AG51-11-10 37°S; 7°E

trachyte

348

J.M. O'CONNOR AND A.P. LE ROEX

duration) and then melted during the final step. The isotopic composition of argon (4°Ar, 39mr, 3TAr and 36mr) released from each individual heat-

ing or fusion step was measured immediately by means of an AEI MS-10S mass spectrometer, after active gases had been removed by cooling

TABLE 2 Argon isotopic data for whole rock samples, St. Helena Seamounts Increment

4°Ar//36Ar

4°mr/39Ar

3TAr*/4°Ar

% Radiogenic 4°Ar

%39Ar of total

Age + 1 ~r × 106 yrs

Sample AC-D-O2E, J = 0.00395 1 354.3 2 411.9 3 1208.8 4 990.1 5 530.0 6 437.1 7 747.8 Tot. Fusion 1243.4

69.98 37.69 14.79 16.04 23.07 28.89 15.51 14.48

0.0212 0.0216 0.0337 0.0408 0.0902 0.0843 0.0338 0.1348

16.8 28.4 75.8 70.4 44.9 33.0 63.1 77.2

3.7 5.3 27.6 25.8 8.1 7.0 22.5

80.6 + 74.8 + 78.2 + 78.8 + 72.5 + 66.9 + 68.7 + 78.1 +

6.2 1.6 0.5 0.5 0.7 1.3 0.8 0.6

Sample AC-D-O2B, J = 0.00395 1 759.3 2 1183.7 3 8701.4 4 2792.9 5 641.5 6 529.5 7 321.7 Tot. Fusion 1927.3

28.31 17.63 11.97 13.13 20.95 25.22 107.76 14.03

0.0070 0.0092 0.0104 0.0108 0.0136 0.0197 0.0151 0.0215

61.1 75.1 96.6 89.5 54.0 44.3 8.3 84.8

2.7 3.7 38.6 37.4 6.0 4.0 7.7

119.3 + 91.9 + 80.6 + 81.9 + 78.9 + 78.0 + 62.3 + 82.8 +

1.5 1.7 0.4 0.4 1.9 1.2 1.4 0.5

Sample AC-D-O2H, J = 0.00395 1 866.0 2 4219.0 3 6056.7 4 1570.2 5 597.3 6 584.0 7 340.1 Tot. Fusion 3823.0

23.14 11.93 12.1 14.07 21.76 22.42 74.94 12.61

0.0223 0.0164 0.0110 0.0121 0.0179 0.0237 0.0155 0.0450

66.0 93.1 95.2 81.2 50.6 49.6 13.2 92.6

3.0 9.1 45.4 16.6 6.5 10.1 9.3

105.7 + 0.9 77.5 + 0.6 80.3 + 0.4 79.6 + 0.8 76.9 + 3.4 77.5 5:0.5 69.4 + 1.4 81.4 + 0.4

Sample AC-D-O5A, J = 0.00395 1 373.0 2 434.4 3 1382.6 4 3392.7 5 1352.6 6 947.3 7 424.5 Tot. Fusion 335.5

34.48 30.14 13.94 12.18 14.16 15.77 33.69 75.16

0.0121 0.0140 0.0280 0.0330 0.0458 0.0520 0.1306 0.0199

20.8 32.1 78.7 91.5 78.5 69.2 31.4 12.1

1.6 3.1 10.6 33.7 9.3 18.2 23.6

50.5 + 3.8 67.6 + 2.5 76.7 + 0.5 77.8 + 0.4 77.6 + 0.6 76.1 + 0.5 74.1 + 0.6 63.5 + 1.2

Sample AC-D-06, J = 0.00439 1 573.4 2 721.3 3 8037.8 4 10214.8 5 4446.9 6 2098.3 7 1957.4 Tot. Fusion 346.9

19.83 10.78 6.89 7.02 7.2 7.56 7.79 43.5

0.0282 0.0383 0.0482 0.0381 0.0288 0.0590 0.1121 0.0245

48.7 59.3 96.6 97.3 93.5 86.3 85.7 15.0

0.4 0.9 15.3 26.6 14.0 12.2 30.6

74.9 + 8.0 49.9 + 4.1 52.0 + 0.2 53.3 + 0.4 52.6 + 0.4 51.0 + 0.5 52.1 +0.3 51.0 + 0.9

349

S. ATLANTIC HOT SPOT-PLUMES: DISTRIBUTION OF VOLCANISM IN TIME AND SPACE

TABLE 2A Argon isotopic data for whole rock samples, other seamounts and islands Increment

4°mr/36mr

4°Ar//39Ar

37At * / / 4 ° A r

% Radiogenic 40Ar

%39Ar

A g e _ + 1 o-

of total

× 106 yrs

Tristan da C u n h a A G 5 1 - 2 - 1 , J = 0.00242

1 2 3 4 5 6

306.04 321.93 317.12 303.71 305.68 299.55

113.472 38.967 16.978 10.621 3.844 7.031

0.0037 0.0143 0.0445 0.0623 0.2225 0.3754

3.5 8.3 7.1 3.1 4.9 4.2

12.6 13.3 16.2 12.7 11.1 34.2

0.0031 0.1033 0.4408 0.8243 1.1255 1.1924

0.0 13.6 15.9 0.0 0.0 7.7

0.02 0.48 4.7 11.1 15.29 68.4

0.1140 0.1399 0.2889 0.3716 0.2234 1.3509

51.7 73.5 63.5 48.1 30.6 35.6

8.1 21.6 30.2 16.2 8.2 15.9

7.8 8.1 7.8 7.7 7.3 8.1

+ + + + + _+

1.3 0.5 0.1 0.3 1.5 0.2

0.0135 0.0137 0.0117 0.0101 0.0229 0.3292

13.1 58.2 92.0 87.7 75.3 65.9

2.3 8.3 37.5 27.21 9.7 15.1

11.1 16.2 15.5 14.9 15.4 14.2

+ + + + _ +

2.1 0.4 0.1 0.1 0.3 0.4

0.0044 0.0055 0.0066 0.0389 0.0435 0.0294 0.0459

17.7 32.6 32.1 84.2 92.9 86.3 86.9

0.6 3.7 10.2 11.0 20.4 15.3 38.7

64.0 94.7 109.6 88.6 90.0 87.5 88.1

+ _+ _+ + + + +

7.5 9.2 1.7 0.6 0.5 0.7 0.7

0.0089 0.0134 0.0151 0.0129 0.0118 0.0125 0.0180 0.0447

30.6 72.3 95.7 96.8 97.0 96.5 94.1 92.4

0.3 1.3 17.1 15.0 17.8 11.5 17.6 19.5

19.2 17.7 18.6 19.0 19.2 19.2 19.0 18.6

+ + + + + _+ _+ +

6.1 0.5 0.1 0.1 0.2 0.2 0.2 0.2

17.1 14.1 5.3 1.5 0.8 1.3

-+ -+ + + -+ +

1.6 0.7 0.6 0.5 0.4 0.1

G o u g h Island A G 5 1 - 3 - 6 , J = 0.002161

1 2 3 4 5 6

158.53 339.27 248.37 267.37 272.02 291.59

116.758 6.326 1.537 1.010 0.941 1.952

- 443 _ - 373 3.4 + 9.6 - 0.95 + 0.6 -0.19+ 0.28 - 1.8 + 0.2 0.58 + 0.07

M c N i s h S e a m o u n t A G 5 1 - 7 - 1 , J = 0.0027

1 2 3 4 5 6

603.59 1082.93 766.8 541.48 416.18 394.98

3.093 2.271 2.556 3.266 4.879 4.664

V e m a S e a m o u n t A G - 5 1 - 1 2 - 3 , J = 0.0027

1 2 3 4 5 6

339.91 706.24 3730.15 2427.41 1193.07 809.75

17.454 5.740 3.469 3.506 4.210 4.423

7 ° E a s t S e a m o u n t A G 5 1 - 1 1 - 1 0 , J = 0.00222

1 2 3 4 5 6 7

358.77 438.17 435.18 1834.47 3984.36 2131.72 2227.83

92.060 74.453 87.766 26.914 24.795 25.907 25.907

R S A S e a m o u n t A G 5 1 - 9 - 1 , J = 0.00242

1 2 3 4 5 6 7 8

425.54 1067.85 6885.03 9184.64 10112.14 8444.71 4985.43 3769.16

14.469 5.621 4.468 4.522 4.547 4.591 4.646 4.644

Here, A = 5.53 × 1 0 - l ° / y r . Ratios in table have not been corrected for neutron interferences. Correction factors: (36Ar//37Ar) Ca = 0.000264; (39Ar/37Ar) Ca = 0.000673; (4°Ar/39Ar) K = 0.0006. * Corrected for decay since neutron irradiation (A 3 7 A t / = 1.975 × 10-2/day).

15.3 -+0.1

89.6 -+0.5

18.8 +0.1

* Recalculated total fusion ages

AG51-12-3

Vema Seamount

AG51-11-10

7° East Seamount

AG51-9-1

RSA Seamount

AG51-7-1

8.1 _+0.05

0.58 -+0.07

Gough Island AG51-3-6

McNish Seamount

1.3 5:0.1

52.3 _+0.3

77.5 +0.4

78.5 5:0.4 81.6 _+0.4 80.2 _+0.4

Age (m.y.)_+ lo"

2, 3, 4, 5, 6

4, 5, 6

2, 3, 4, 5, 6, 7,8

all

6

4, 5, 6

3, 4, 5, 6, 7

3, 4, 5, 6

2, 3, 4 3, 4, 5, 6 3, 4, 5, 6

Steps used

Plateau calculation

AG51-2-1

Tristan da Cunha

AC-D-06

St. Helena Seamount 6

AC-D-05A

St. Helena Seamount 5

AC-D-02E AC-D-02B AC-D-02H

St. Helena Seamount 2

Sample

Age calculations from argon isotopic data

TABLE 2B

97.8

46.8

99.7

100

68.4

57.9

98.7

71.8

58.7 86.0 87.7

15.2 _+0.3

90.4 _+1.5

18.7 _+0.2

8.1 _+0.8

0.64 -+0.3

52.2 + 0.7

77.6 + 0.6

79.0 _+0.9 80.1 + 1.3 78.8 -+ 1.0

Age (m.y.)_+ 1~ % of total 39mE

Age (m.y.)-+ l~r

263.1-+43.1

265.9-+51.6

292.1_+ 5.6

301.8 _+ 3.3

292 +27

288 +10

15.2 _+0.3

91.2 +1.1

18.8 +0.2

8.2 +0.07

0.65 _+0.3

52.9 _+0.6

78.2 _+0.5

2, 3, 4, 5,6

4, 5 , 6

Total fusion

63.5 _+1.2

292.4-+

301.8_+

5.3

6.4

310.2+ 25.3

15.1 + 0 . 1 "

241.8+ 31.0 85.4 + 0 . 6 *

18.9 -+0.1 *

7.9 +0.2 *

0.25+0.1 *

5.6 _+0.3 *

268 _+ 21.7 51.0 _+0.9

278 _+ 4.5

290 + 6.3 78.1 +0.6 294 + 16.1 82.8 _+0.5 284.3+ 5.6 81.4 _+0.4

Intercept

2, 3, 4, 5, 6, 321 +331 7, 8

2, 3, 4, 5,6

4, 5, 6

3, 4, 5, 6

3, 4, 5,6

2, 3,4 3, 4, 5,6 2,3,4,5,6

Steps used

Inverse calculation

290 + 6.2 79.1 +0.9 319 +31.4 81.6 +0.7 298.2-+20 80.4 -+0.5

Intercept

2, 3, 4, 5, 6 298.6 + 26.3

4,5,6

2,3,4,5,6, 7, 8

2,3,4,5,6

4, 5, 6

3, 4, 5, 6,7

3,4,5,6

2, 3,4 3, 4, 5,6 2,3,4,5,6

Steps used

Isochron calculation

S. A T L A N T I C H O T S P O T - P L U M E S : D I S T R I B U T I O N O F VOLCANISM IN T I M E A N D SPACE

Ti-TiO 2 getters. The apparent ages of the individual heating steps were calculated from the measured 4°Ar* (i.e. radiogenic) to 39mr ratios, after corrections for all interfering nuclear reactions had been applied using the equation of [25]. Hence an age-temperature spectrum was obtained for each sample, based on the 4°mr-39Ar compositions of the gas fractions released incxementally, from low to high temperature sites. In the case of the irradiated St. Helena Seamount samples approximately one quarter of the contents of each vial was fused in a single heating step, from which a total fusion age was determined. Argon isotopic data and apparent ages are reported in Table 2. The most common age-temperature spectrum observed in highly altered oceanic basalts is of a middle temperature plateau bounded by older ages at the low temperature steps, and by younger ages at the high temperature steps (e.g. Fig. 3a). Turner and Cadogan [26] first proposed that such an 'inverse staircase' age-temperature profile might result from neutron-capture recoil of 39Ar from K-rich to K-poor sites within fine-grained basalts, during irradiation. As the low-temperature sites are K-rich, recoil effects may lead to the transfer of 39Ar (but not 4°Ar*) from low temperature (i.e., alteration minerals and groundmass) to high-temperature sites (e.g., feldspar and pyroxene), resulting in a descending agetemperature release pattern. Significant amounts of 39mr may be lost from slightly altered volcanic rocks during the irradiation process [27,28,29]. Thus, in conjunction with the loss of 40Ar., due to the effects of seawater alteration, 39Ar may also be lost from, or relocated within, a multiphase sample. In cases where 39Ar recoil is evident, the best age estimate for a sample comes from summing the gas composition of all the heating steps into a 'recalculated' 4°mr-39Artotal fusion age [30]. Weighted mean plateau ages were calculated for samples yielding an age-temperature plateau, primarily for middle temperature heating steps. These middle temperature sites are considered to be least disturbed by the loss or addition of 39Ar resulting from recoil effects, as discussed above. The slope formed by the correlation of the 4oAr/ 36Ar and 39Ar//36Ar ratios for selected heating steps yielded an 40Ar.//39mr ratio, from which an

351

isochron age was determined for most samples. Ideally, the 4°Ar/36Ar intercept of such isochrons should reflect the composition of the rock at the time of crystallization, i.e., 295.5, consisting of only atmospheric argon without any contribution from potassium decay. Isochron slopes and intercepts were calculated using the least squares fitting technique of York [31], which allows for correlated errors in both 4°mr/36Arand 39Ar/36Ar isotopic ratios. Errors involved in measuring Ar ratios and J-factors (a typical error of 0.5% was assigned) and in making corrections for interfering nuclear reactions were combined to yield a standard deviation for each heating age. Agetemperature spectra and isochron plots are shown in Fig. 2 and 3. A conventional K-Ar age of 3.8 _+ 1.4 Ma was determined for a subaerial sample from the island of Tinhosa Grande (Fig. la), confirming that it belongs to the Cameroon Line; the large uncertainty assigned to this apparent age reflects the very low percentage of radiogenic 4°Ar released when this sample was fused. 4. Discussion of the 4°Ar=39At data 4.1 St. Helena Seamount Chain

AC-D-2E (St. Helena Seamount 2) produced a reasonable plateau age of 78.5 _+0.4 Ma (Fig. 2a), which is supported by an isochron age of 79.0 + 0.9 Ma (intercept of 290 + 6.0) (Fig. 2b), an inverse isochron of 79.1 _+ 0.9 Ma (Fig. 2c) (intercept of 290 _+ 6.3), and a total fusion age of 78.1 _+0.6 Ma. An apparent age of 79 Ma is indicated for this sample. AC-D-2B (St. Helena Seamount 2) produced a viable plateau age of 81.6 + 0.4 Ma (Fig. 2d), which is supported by an isochron age of 80.1 _ 1.3 Ma (Fig. 2e) (intercept of 319 _+31), an inverse isochron of 81.6 _+0.7 Ma (Fig. 2f) (intercept of 294 _+ 16), and a total fusion age of 82.8 _+ 0.5 Ma. An apparent age of between 82 and 83 Ma is indicated for this sample. AC-D-2H (St. Helena Seamount 2) also produced a good plateau age of 80.2"_+ 0.4 Ma (Fig. 2g), which is supported by an isochron age of 78.8 _+ 1.0 Ma (Fig. 2h) (intercept of 298 _+20), an inverse isochron age of 80.4 -+ 0.5 Ma (Fig. 2I) (intercept of 284 -+ 5.6), and a total fusion age of

352

J.M. O ' C O N N O R A N D A.P. L E R O E X

81.4 ___0.4 Ma. The best estimate of the age of this sample is between 79 and 81 Ma. The best overall age estimate for Seamount 2 is between 80 and 82 Ma, AC-D-5A (St. Helena Seamount 5) produced a good plateau age of 77.5 5:0.4 Ma (Fig. 2j), which is supported by an isochron age of 77.6 5:0.6 Ma (Fig. 2k) (intercept of 288 + 10), an inverse

I00 95 90 ~ 80

1400

. . . . . . . . . . / i

(=)AC-D-O2E I

~

,~ 1000 79.0+0.9 M a /

; ~0.0020

e

E

•

•

:

(c)AC-D-O2Et

. . . . .

10000;

(d) AC-D-O2B

120 A 110 m Iv I® o

200

.

i

....

"

,

, , 6000t

~

80.1+1.3 M /

"°°I Y 2%.V or7,.6,s.

50

.

.

.

.

.

.

.

.

.

,

1

"~ f ~

80.2+0.4Ma

=1

5000"J (h)AC'D'O2H

f

~

,

/

0.09

O

O

N

.

.

o.oo®

. . . . .

70 60

1000~ 0]

.

. . . ' ~ . 3 .

0.00 0.02 0.04 0.06 0.08 0.10

3gAr/4~OAr 0.0040. . . . . . . . . . .

f

7 ~

(i) AC'D'O2H

o.oo2o

40004

~ 30001 2000.4

. . . . . . . . . ~0 20 30 40 50 60 70 80 90 100 Cumulative% ~Ar released

0.07

,° \ , .

, , , , • ....... .

0.05

7Ma .

3tAr/ 3SAr

(g)AC-O-O2H .

0.03

0.0010

100 200 300 400 500 600 700 800

3g

Cumulative% Ar released 120 1101

\t

0.01

3~r/40Ar

O

t.t

7O

t

0.0030

. ,J

2

I

0.0040. . . . . . . . . . .

8000

81.6 _+0.4 Ma

5

:1

.~ °o

. . . . . .

~

OOOLO t 79,-+o9..

0 10 20 30 40 50 60 70 80 90 100 3SAr/~Ar

Cumulative% 3gAr released

50

•

78.5 +0.4 Ma

55~ 5O ~...... I 0 1"0 20 30 40 50 60 70 80 90 100

~

0.0035 . . . . 0.0030 1

(b)AC-O-O2E

70

2

isochron age of 78.2 + 0.5 Ma (Fig. 21) (intercept of 278 + 5), and a total fusion age of 63.5 + 1.2 Ma. The apparent age of this site is 78 Ma. AC-D-06 (St. Helena Seamount 6) produced an excellent plateau age of 52.3 + 0.3 Ma (Fig. 2m), which is supported by an isochron age of 52.2 + 0.7 Ma (Fig. 2n) (intercept is 292 5: 27), an inverse isochron age of 52.9 + 0.6 Ma (Fig. 20)

°.°°1°1 100 200 300 400 500 600

3gArI 36Ar

"-(.

0.00 0.02 0.04 0.06 0.08 0.10

39Ar/4°Ar

Fig. 2. Age-temperature spectra and 40AT//36Ay v e r s u s 39Ar//36AI"a n d 36Al'//4°mr v e r s u s 3 9 A l ' / / 4 ° m r correlation diagrams for basaltic samples from the St. Helena Seamount Chain. Age bands in spectra plots are the measured heating step ages + 2 tr. Errors on plateau ages are + 1 ~r. The numbered points on the correlation diagrams correspond to the individual heating steps. Solid boxes in correlation diagrams indicate heating steps used in isochron calculations.

353

S, A T L A N T I C H O T S P O T - P L U M E S : D I S T R I B U T I O N O F VOLCANI SM IN T I M E A N D SPACE

•

,1@o

•

•

,

•

•

•

•

•

4000; . . . . . . . . . . . . . . .

(j) AC-D-0SA

9oi It

77.5 + 0 . 4 Ma

q

]

0.0030

3500 / (k) AC-D-SA

'

'

r~

3000

, z

1'

4

(I) AC-D-OSA

00020

2500t

. _ .

7,.°

"~ 20001

P

z501

el <

50

5 10 20 30 40 50 60 70 80 90 100 Cumulative %

0 oi!

. . . . . .

~

0.0005 o.oooo

40 80 120 160 200 240 280

0.02 003 0.04 0.05 0.06 0.07 008 009

39AtI 3~r

Ar released

•

1~ i

'

. . . . . . . . . . . . .

0

39

0

(m) AC'O'06i

•

1000012000 (n)

• • • • • A C - D I / 4

3 ~ r l *0At

i

i

0.0030

i

(o) AC-D-06 010025

g 6°1 ;= ; 50L

l

60®

0.0020

.J. ~-~ 6000

o.oo15

I 52.3+0.3 Ma

52.2+0.7

30

':I

"

20

I °°°

0 10 20 30 40 50 60 70 80 90 100 Cumulative %

39

Ma

-'~ o.oolo 0.0005

2000

0.0000

400

800

1200 1600 200O

"A,, ~A,

A t released

0.04

0.06

0.08

0.10

0.12

0.14

016

~gAr/'%

Fig. 2 (continued).

(intercept of 268 + 22) and a fusion age of 51.0 + 0.9 Ma. The apparent crystallization age of this sample from St. Helena Seamount 5 is 52.2 Ma.

4.2 Gough Lineament AG51-2-1 (Tristan da Cunha) produced an equivocal plateau age of 1.3 + 0.1 Ma (Fig. 3a). This apparent age is not supported by an isochron age of 0.64 ___0.3 Ma (Fig. 3b) (intercept of 301.8 ___3.3) and a concordant inverse correlation age of 0.65 + 0.3 Ma (Fig. 3c) (intercept of 301.8 + 6.4). We consider the recalculated total fusion age of 5.6 + 0.3 Ma as being unreliable in view of the discrepancy between, and with, plateau and isochron ages. The best age estimate for this sample ranges between 0.65 and 1.3 Ma. AG51-3-6 (Gough Island) produced a plateau age of 0.58 + 0.07 Ma (Fig. 3d) based on the fusion heating step. The earlier heating steps produced meaningless negative ages, which can be attributed to the low yield of radiogenic Ar in this very young sample. Straight lines could there-

fore not be fitted to the scattered heating steps for calculating isochron and inverse correlation ages (Figs. 3e and 3f). The recalculated total fusion age is 0.25 + 0.12 Ma. The best estimate of the age of this sample from Gough island is 0.6 Ma. AG51-7-1 (McNish Seamount) produced a convincing plateau age of 8.1 + 0.05 Ma (Fig. 3g). This age estimate is reinforced by an isochron age of 8.1 + 0.9 Ma (Fig. 3h) (intercept of 292.1 + 5.6), a concordant inverse correlation age of 8.2 + 0.07 Ma (Fig. 3i) (intercept of 292.4 + 5.3), and a recalculated total fusion age of 7.9 + 0.2 Ma. An apparent age of 8.1 Ma can be confidently reported for this sample. AG51-9-1 (RSA Seamount) produced a very reliable plateau age of 18.8 + 0.1 Ma (Fig. 3j), which is supported by an isochron age of 18.7 + 0.2 Ma (Fig. 3k) (intercept of 265.9 + 51.6), an inverse correlation age of 18.8 ___0.2 Ma (Fig. 31) (intercept of 321 + 331), and a recalculated total fusion age of 18.9 + 0.1 Ma. 18.8 Ma is a reliable apparent age for this seamount sample.

354

J.M. O'CONNOR AND A.P. LE ROEX

0.6 Ma to be erroneously young. The best apparent age of this seamount sample is between 90 and 91 Ma. AG51-12-3 (Vema Seamount) produced a convincing plateau age of 15.3 ___0.1 Ma (Fig. 3p), which is supported by an isochron age of 15.2 + 0.3 Ma (Fig. 3q) (intercept of 298.6 + 26.3), an inverse correlation age of 15.2 + 0.3 Ma (Fig. 3r)

4.3 7°East and Vema seamounts

AG51-11-10 (7°East Seamount) produced a plateau age of 89.6 + 0.5 Ma (Fig. 3m), which is supported by an isochron age of 90.4 + 1.5 Ma (Fig. 3n) (263.1 + 43) and an inverse correlation age of 91.2 + 1.1 Ma (Fig. 30) (241.8 + 31). We consider a recalculated total fusion age of 85.4 + 30 28 26 24

370

(a) AGSl-2-1 Tristan de Cunha

i

0.6

|

{b) AG5.1-2-1 . Tristan oa uunna

350 i

~'22

20

a

16 14

~

~. 10

330,

:t

4 2 0

1.3 + 0.1 Ma

250 0

I

I

I

'

,

m

m

=

0.58 + 0.07 Ma

(

~0 ~t

Cumulative%

20. 18.

r~ 2

v12., Q

rl

0.003 4

100 1~=0 260 250 300 350 400

200

m

m

l

|

m

|

|

i~ (.h) AG51-7-2

~

i

~

|

!

~ 2

|

-

56

8.1 + 0.09 Ma

2,

Cumulative%

39

Ar released

200

.

.

4 an 5

0.2

0.4

0.6

0.8

1.0

1.2

0.0030 "

.

.

.

.

.

.

.

.

(i) AG51-7-2

5 0.0025 '15

McN,sh Seamount

16~,,.4 "%'

°.°°,,1

400

10 20 30 40 50 60 70 80 90 100

n

0.0

I

300,

6,

.

39Ar/ 40Ar

$00, m

.

7 []

2

0.002, . . . . . . . . . . .

0.o02o

~==

|

AG51:3-6 ~ f)ough Island

3 []

39Ar/ 3~r

8.1 + 0.05 Ma

-~t0'~8,

|

i

0.004 t

D00,

b

. . . .

0.005 t

4

50

0.0033

0.006 ~ 1

S rt

[]

Ar released

16.

(•e)

6

39

{g.) AGSl-7-2 McNish Seamount

~" 14. =E

0.(~032

200'

lOO

o l b 20 3"o 4o ~o 8o ~o 80 ~ol00

"

39At/ 40Ar

0.007 ;

250'

-5

-

. . .~ .

0.0031'

AGSl:3-6. ough Ismna

300,

15o!

-

Trfstan da Cunha

I

o.0

1~ 2~ 3b 4b sb ~ ~ ~

350'

-3, -4,

•

_--

4OO

~

•

(e) AGSl-2-1

o.t

A

~2

|

39Ar/ 36Ar

do)AG51:3-6 ugh Ismna

3

•

0.64 + 0.3 Ma

Ar released

4

I

4

39

Cumulative%

-

5

270.

1() L~) 30 40 50 60 70 80 90 100

•

6 .

290, m

-

0.4

3

[]

310. 1

I

5

2

"

,~ 0.5 ~

' 100 ' '2~ 0 ' ~ o ' 39Ar/ 3~r

~ 0 ' 500 '

-,. o.oo,o1.<.

211;21...... 0.2

0.3

0.4

.'>.. 0.6

0.5

3~r/4~r

Fig. 3. Age-temperature spectra a n d 4°mT//36AT v e r s u s 39Ar/36AT a n d 36Ar/4°mT v e r s u s 39AT//4°AT correlation diagrams for whole rock samples from Gough Chain, Tristan da Cunha and Vema and 7°East Seamounts. Other details are the same as in Fig. 2.

S. A T L A N T I C

40

HOT

•

'

SPOT-PLUMES:

•

|

|

35 !

DISTRIBUTION

2000 1000

"

" " " • AGSl-9-1 RS'A Seamount (|7

A ¢= 30. 25.

18.8

OF VOLCANISM

I

IN TIME

" " " ' (k) AG51-9-1

AND

355

SPACE

•

'

"

0.0045'

" " 5_/

+ 0.1Ma

2o.

o

iii 40°0 3OO0

~

20oo 5

•

~

18.7 ±0.2

/

. . . .

i

\

t

.... \

\,

o.oo151 ,8,o=..

Me

ooo,o1

-%

1000

0

,

,

,

•

•

•

'•

•

0

•

0 10 20 30 40 50 60 70 80 90 100 39 Cumulative % Ar r e l e a s e d

120 .

.

110

""1

.

.

.

.

-L

100

r

89.6

±

. . . (m) AG51-11-10 7 East Seamount J 0.S Ma 1

•

•

500

•

,

.

,

1O00 1500

,

2000

•

, = | . . . . . . .

4500 4OOO 350O

90

m i-

80

25130,

~ n

70

2OO0,

,n<.

60,

,

0.0040

' . . . . . . . . .

7 East

5A

0.0030 ~

0 10 20 30 40 50 60 70 80 90 100 39 Cumulative% Ar r e l e a s e d |

25

|

.

.

.

.

.

,

A20

"3

I

•

•

"

"

(o)AGS1-11-10 7 East Seamounl

n \ 11n 91.2

+ 1.1 Ma

~"

90.4

+ 1.5 Ma

~¢¢ 0.0015, 0.0010. 0.0050,

.

.

.

.

.

.

4

~

5

0.0000

o "~ "*~"~ "~ "1&'1~'1,~"l&"1~'~

0.015

0.005

0.025

'

35oo.[ /

"

.

.

.

.

.

.

.

0,0040

.

(.q) AG51-12-3 vema seamount

3°°°1

0.035

0.045

39Ar1 40Ar

39Ar/ 3~r

4oo0l

r~

I

-~ 0.0020 J e~

•

~p) AGS1-12-3 e m a Seamount

I

0"00251

1000. 500

40

I

0.0035

1500,

50,

0.04 0.08 0.12 0 . 1 6 0.20 39Ar/ 40Ar

30oo1

i!

0.00

2500

39Ar/ 36Ar

5000,

.

•

•

0

~

m

,

°00201

a.

E .=

,

oo = ~ 1 ~ 000301

--

al

c

'

oo=o1\

0.003¢ ~ ~

25001

i

l

0.0035

/3

~ , /

m

i

2

" " ' 'i ,(.r) AGS1-12-3 vema Seamount

~

0.0025

0.3

Ma

o.002(:] 15.3

1o,

+ 0.1 Ma

1

1°°°L

~

0

200

15004

o. Q. 5

~< o.oo15

.......

s7

15.2

+0.3

Ma

0.001C 0.0005 0.000(

10 20 30 40 50 60 70 80 90 100 39 Cumulative% Ar r e l e a s e d

400

600

39Ar/ 3~r Fig.

800

1O00

2 a

.

.

o~o o.o, ~ =

.

.

.

S

.

~

.

3

.

o.12o.,~ o~o o~, o ~ 0.32

39A,/ 40Ar

3 (continued).

(intercept of 310.2 + 25), and a recalculated total fusion age of 15.1 + 0.1 Ma. An apparent age of 15.2 Ma can reliably be assigned to this seamount sample. 5. Discussion

The apparent Ar-Ar ages for St. Helena and Gough seamount and ridge samples (this study

and [14]) become older with increasing distance from the islands of St. Helena and Gough (Fig. 4). It would appear, therefore, that these volcanic chains formed as the result of African plate migration over 'fixed' hot spot-plume systems. The assumption that mantle plumes/hot spots are not moving with respect to the earths' lithosphere, i.e. are 'fixed' in the mantle, has lead to the development of models for reconstructing plate

356

J.M. O’CONNOR

AND

A.P. LE ROEX

1ON

(4

25 S

30s rla

15OM 35 s

Tristan dc

40s Goug

45 s 2ow

15w

low

5w

0

SE

10E

15E

Fig. 4. (a) The modeled traces of African motion over hot spots (Table 3) are shown by the heavy lines connecting the southwestern ends of the St. Helena Seamount Chain and the Walvis Ridge to the African coast. The (0) symbols distributed along these lines represent the progress of the African plate at intervals of 10 Ma. Basement ages for the Walvis Ridge are from [14]. The DSDP 363 fossil age-range is from [33]. Dashed hot spot tracks and (0) symbols illustrate the earlier model of [14]. The misfit between the modeled track of the St. Helena hot spot and the St. Helena Chain evident in this earlier model illustrates the problem of fitting hot spot tracks to the Walvis Ridge/Gough and St. Helena chains without incorporating the assumption of broad hot spots (shown as the large circles). See Fig. 5 for calculation of rotation angles used in this model. Mercator projection.

S. ATLANTIC

HOT

SPOT-PLUMES:

DISTRIBUTION

OF VOLCANISM

0

I

lb)

5s

10 s

15s

20s

25s SOE

55 E

60E

65 E

Fig. 4. (b) The predicted direction of Africa motion over the Reunion hot spot (rotation parameters in Table 3) is illustrated by the bold line with (0) symbols, which represent the progress of the African plate at intervals of 10 Ma. The dashed hot spot tracks and (0) symbols similarly illustrate the

earlier model [14]. 4oAr-39Ar ages shown for the Mascarene Plateau are from [34]. The K-Ar age range for the islands of Mauritius is from [35]. Mercator projection.

TABLE 3 Finite reconstruction

poles for African plate motion

Age

Latitude

(Ma)

0-z)

Longitude (“El

9 19 31 40 52 62 80 120

38.0 38.0 38.0 38.0 38.0 38.0 38.0 37.0

-61.0 -61.1 -61.1 -61.1 -61.1 -61.1 -61.1 -65.0

Angle *

Rate C/m.y.)

- 1.0

-0.11 -0.17 - 0.23 - 0.27 - 0.25 - 0.26 - 0.23 -0.31

-2.7 -5.5 -7.9 - 10.9 - 13.5 - 17.75 - 30.0

* Negative angles indicate counterclockwise rotation.

IN TIME

AND

SPACE

357

motions on the basis of the orientations of such volcanic trails, and the distribution in time (i.e. age) and space (i.e distance from proposed hot spot) of crystallization ages of rocks dredged and drilled from these features [e.g. 321. A limitation of dredge sampling is that it recovers samples only from the outer, usually youngest, surface of a seamount or ridge. Thus, as in the case of most oceanic islands, the older periods of seamount formation have not yet been sampled, so our understanding of the spatial distribution of hot spot volcanism is necessarily two dimensional. We have revised African plate motion on the basis of the distribution of dated seamounts along the Gough Chain (this study), Walvis Ridge [14], St. Helena Chain (this study), and the Mascarene Plateau [34] (formed in the Indian Ocean by the Reunion hot spot for about the past 40 Ma). Two rotation poles (Table 3) reconstruct satisfactorily the direction of African plate migration over the possibly greater-than-500 km diameter zones of St. Helena and Walvis hot spot volcanism, in agreement with that recorded by the NE-SW orientations of the respective volcanic traces (Fig. 4). The assumption has been made that the ‘centers’ of these hot spots are located close to Kutzov Seamounts and Crawford Seamounts respectively [36], the centers of the proposed > 500 km diameter zones of volcanism associated with the St. Helena and Walvis hot spot-plumes (discussed further in the following section). This model results in the center of the hot spot track following the region of highest elevation along the Walvis Ridge. The northeastern (i.e. older) section of the Walvis Ridge (between u 275, YE and the African coast) trends more to the north than the southwestern (i.e. younger part), suggesting a change in the location of the Africa Euler pole from 37”N, 65”E to 38”N, 61”E at approximately 80 Ma. 5.1 Broad zones of hot spot volcanism The Island of St. Helena, Bonaparte and Kutzov seamounts, and an unnamed seamount cluster, are located at the southwestern end of the St. Helena Chain (Fig. la). The islands of Tristan da Cunha (including the nearby islands of Inaccessible and Nightingale), Crawford Seamount, and Gough Island (Fig. lb) mark the southwestern

358

e n d o f t h e W a l v i s R i d g e (Fig. l b ) . A s d i s c u s s e d previously, T r i s t a n , G o u g h a n d I n a c c e s s i b l e a p p e a r to have f o r m e d w i t h i n t h e p a s t 2.5 to 0 M a . Thus, it is p o s s i b l e to s p e c u l a t e t h a t t h e m o s t recent known lithospheric surface expression of t h e Walvis h o t s p o t - p l u m e is t h a t o f a b r o a d z o n e o f diffuse v o l c a n i s m ( o c e a n i c islands, seam o u n t s a n d small ridges) t h a t is at least 500 k m in d i a m e t e r . I n t h e c a s e o f t h e St. H e l e n a h o t spot t h e s i t u a t i o n is less c l e a r d u e to a l a c k o f available age control. K - A r age d a t a for t h e isl a n d o f St. H e l e n a points, however, to t h e b u l k o f t h e s u b a e r i a l p o r t i o n o f t h e i s l a n d as having formed between 9 and 7 Ma. T h e s u g g e s t i o n o f b r o a d St. H e l e n a a n d Walvis z o n e s o f h o t s p o t v o l c a n i s m is c o m p a t i b l e with a m o d e l d e v e l o p e d to e x p l a i n a n o m a l i e s in t h e g e o i d a n d h e a t flow, a n d t h e b r o a d t o p o g r a p h i c swell, across t h e C a p e V e r d e I s l a n d s [37]. T h i s m o d e l envisages a n a r r o w ~ 150 k m d i a m e t e r p l u m e o f h o t m a n t l e rising u p w a r d a n d m e l t i n g p a r t i a l l y to f o r m i s l a n d lavas o v e r a z o n e t> 300 k m in d i a m e t e r , with m o s t p l u m e m a t e r i a l b e i n g d e f l e c t e d by t h e overlying p l a t e a n d s p r e a d i n g o u t r a d i a l l y to f o r m a m u s h r o o m - s h a p e d h e a d (in terms of isotherms) of anomalously hot mantle 1500 k m across [37,38]. T h e v o l u m e a n d c o m p o s i tion o f m e l t g e n e r a t e d by a d i a b a t i c d e c o m p r e s sion over such a h o t s p o t is c o n t r o l l e d by t h e temperature structure of the thermal plume and its i n t e r a c t i o n with t h e overlying l i t h o s p h e r e [39].

J.M. O'CONNOR AND A.P. LE ROEX

A n e w g l o b a l S-wave velocity m o d e l [12] has r e v e a l e d t h e p r e s e n c e in t h e m a n t l e o f low-velocity a n o m a l i e s b e n e a t h sites o f i d e n t i f i e d h o t s p o t volcanism, such as t h e island o f St. H e l e n a a n d t h e T r i s t a n - G o u g h r e g i o n , at d e p t h s o f 1 0 0 - 2 0 0 k m d e e p e r t h a n t h e low-velocity a n o m a l i e s always p r e s e n t u n d e r s p r e a d i n g axes. T h e s e a n o m a l i e s imply a h o t t e r o r m o r e v o l a t i l e rich m a n t l e u n d e r h o t s p o t s t h a n u n d e r s p r e a d i n g ridges. T h e h o t s p o t - p l u m e m o d e l [12] d e v e l o p e d to e x p l a i n this seismic i m a g e o f p l u m e h e a d s is o f a p l u m e c o n d u i t o f r a d i u s ~ 1 0 0 - 2 0 0 k m rising f r o m t h e deep mantle, spreading out radially beneath the l i t h o s p h e r e a n d p r o d u c i n g a m u s h r o o m h e a d (also in t e r m s o f i s o t h e r m s ) a p p r o x i m a t e l y 1000-2000 k m in d i a m e t e r . T h e velocity a n o m a l i e s u n d e r t h e h o t spots m a y b e r e l a t e d to t h e t o p of such p l u m e s . T h e h e a d h e r e refers to t h e e l o n g a t e d t h e r m a l a n o m a l y t h a t always exists at t h e t o p o f t h e p l u m e s d u e to t h e h o r i z o n t a l o u t w a r d flow o f w a r m m a n t l e , in c o n t r a s t to t h e ' s t a r t i n g p l u m e ' o f G r i f f i t h s a n d C a m p b e l l [40]. I n t h e case o f t h e St. H e l e n a a n d Walvis p l u m e s t h e m u s h r o o m h e a d is e l o n g a t e d t o w a r d s t h e s p r e a d i n g axis, confirming the previously discussed plumes p r e a d i n g axis c o n n e c t i o n . P l u m e c o n d u i t s a r e p r e s u m e d to b e n a r r o w e r t h a n t h e p r e s e n t r e s o l u t i o n o f this, a n d o t h e r such m o d e l s , so it is n o t y e t p o s s i b l e to d e t e r m i n e the number of plume conduits supplying the p l u m e head(s), which in t u r n s u p p l y m e l t s to t h e

Fig. 5. (a) 4°mr-39Ar ages for rock samples dredged or drilled from the St. Helena (this study), Walvis Ridge [14] and Gough (this study), and Mascarene lineaments [34] are plotted versus the angle of rotation (about rotation poles in Table 3) required to move them from their present-day geographical coordinates to the postulated present-day centers of their associated hot spots (i.e. Kutzov Seamount, Crawford Seamount, and the island of Reunion, respectively). A regression line (the longer line) has been fitted to Walvis-Gough and St. Helena data, but excludes the 0 to ~ 2.5 + 0.5 Ma age range for the islands of Gough [15], the 0.01 + 0.02 to 0.21 + 0.01 Ma for Tristan da Cunha [16], and the ~ 9 to 7 Ma age range for the island of St. Helena. A regression line has also been fitted to the Mascarene Plateau age data, but excluding some data: the very young age range for the island of Reunion and the 0-7 Ma age range for Mauritius [35]. Mascarene Plateau ages form a distinct array, which suggests a much faster rate of migration of the African plate (for about the past 30 Ma) than do the Walvis-Gough Ar-Ar data. The fossil age-range for DSDP 363 is from Bolli et al. [33]. Bold horizontal lines through each age date indicate a + 2.25° variability in accurately locating the "centers" of hot spots, which is due to the broadness of the St. Helena and Walvis hot spots (see text for discussion). The line fitted through the Walvis-Gough data is taken here as being representative of the average rate of African plate motion over hot spots. We select rotation angles (Table 3) from along this line for selected time periods (open boxes) for which reliable age dates have been determined. The fact that this regression line does not intersect at the assumed zero age centers of the St. Helena and Walvis hot spots may indicate that either the African plate slowed down between 0 and 31 Ma, or that the "centers" of the present-day Walvis and St. Helena hot spots are located 1.5° further to the west than is suggested by the location of Crawford and Kutzov seamounts. This latter possibility is considered further in (b): This diagram differs from that shown in (a) in that the present-day centers of the Walvis and St. Helena hot spots are assumed to be 170 km (i.e. 1.5°) to the west of Crawford and Kutzov seamounts respectively. Other details are the same as in (a). The speed of the African plate over hot spots, based on this estimation, is consequently about 1.5° faster on average than estimated in (a).

S.

359

ATLANTIC HOT SPOT-PLUMES: DISTRIBUTION OF VOLCANISM IN TIME AND SPACE

clear as to whether one (or several?) narrow plume(s) are upwelling to form these broad zones of hot spot volcanism, resulting from decompres-

overlying broad zone of hot spot volcanism. In the case of both the Walvis and St. Helena zones of hot spot volcanism, it remains therefore un-

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360

sion melting across the broad 'mushroom head(s)' of the plume(s). Support for the existence of multiple mantle plumes is provided by [41], who attributed the existence of numerous small-wavelength elongated features visible in filtered geoid and topographic maps for the South Atlantic to the presence of magmatic traces far more numerous than can be explained in terms of the present list of hot spots. In a later section it is proposed that the Bahia and Pernambuco seamounts formed in the Cretaceous during the period in which the St. Helena hot spot-plume system was beneath or close to the South Atlantic spreading axis. We consider that the existence of multiple narrow plumes associated with the St. Helena hot spot is most compatible with this model.

5.2 Some further evidence for broad hot spots A series of multiple spreading-axis jumps and propagating rifts (over a period of 0-5 Ma) at the latitudes of the islands of St. Helena and Ascension has been explained by the presence of broad, 500 km diameter St. Helena and Ascension hot spots [42]. Although the center of this broad St. Helena plume lies ~ 400-600 km from the spreading axis, the flow of hot material away from the plume(s) responsible for the formation of the broad hot spot produces a large region of anomalously hot (and geochemically distinct) mantle [42]. The topography of the spreading axis at the latitude of Ascension Island comprises large volcanic ridges that are more rugged than at the latitude of St. Helena. This difference can be explained in terms of the spreading axis being located closer to the Ascension plume, resulting in it being supplied by hotter plume material than at the latitude of St. Helena [42].

5.3 The distribution of volcanism associated with broad hot spots Pacific plate motion over the Hawaiian hot spot-plume system generates immediate uplift and a large geoid anomaly at the 'upstream nose' of the Hawaiian chain [e.g. 37,43]. It has been suggested that the radial flow of mantle from the central plume is deflected downstream [43, fig. 4], in contrast to forming a mushroom-like plume as described earlier for the Cape Verdes. A possible

J.M. O ' C O N N O R A N D A.P. LE R O E X

explanation for such a difference between the Cape Verdes and the Hawaiian plume models is that the African plate is practically stationary with respect to the Cape Verdes plume. In the case of the Walvis and St. Helena plumes, a combination of faster plate velocity and flow of plume material to the spreading axis produces an upwelling plume head which is elongated westward as it rises towards the spreading axis. On a more local scale the spreading axis no doubt played an important role in the distribution of volcanism associated with the St. Helena and Walvis hot spots [e.g. 14]. Following the transition to mid-plate volcanism, lithospheric structure, particularly fracture zones, may well play a critical role in determining the location of hot spot volcanism. The Gough Chain and Tristan da Cunha, for example, lie along fracture zones [e.g. 44] (Fig. 1). In the case of the St. Helena hot spot, which is apparently a much weaker plumehot spot system than the Walvis system--at least in terms of amount of plume-head melt delivered to the lithospheric plate surface--lithospheric structure could well have played a very significant role in controlling the distribution of hot spot volcanism. In the case of the Reunion hot spot, Bonneville [45], following [46,4,47], suggests that volcanism on the island of Reunion (the oldest lavas on Reunion being 2.1 m.y. old [35]) is controlled by an underlying Paleocene spreading axis (A27). The weakened lithosphere under this fossil spreading axis provides an easier access route for Reunion plume melts. This model also suggests the existence of broad zones of hot spot volcanism generated by one or more plume conduits. Lithospheric control of hot spot volcanism has also been identified in the vicinity of the Pitcairn hot spot [48,49].

5.4 Estimation of plate rotation angles The velocity of the African plate since the South Atlantic opened has been determined from the distribution of radiometric ages along the St. Helena Chain and the Walvis Ridge-Gough Chain (and the Mascarene Plateau) (Fig. 5a). In this reconstruction the Walvis and St. Helena hot spots are presumed to be centered presently beneath a ~ 500 km diameter zone at the southwestern ends of the Walvis Ridge and the St.

361

S. ATLANTIC H O T SPOT-PLUMES: DISTRIBUTION OF VOLCANISM IN TIME AND SPACE

is supported by independent estimates of current African plate speed over hot spots [50, fig. 1]. Pollitz [51] has also identified a decrease in African velocity over hot spots, during the period between 8 and 4 Ma, citing geological changes along a large part of the African plate boundary, including the Red Sea and Gulf of Aden spreading system, and the along the Alpine deformation zone. Figure 5b (and Table 3) illustrate how rotation angles vary when the Walvis and St. Helena hot spots are relocated 170 km to the west, as suggested by the negative intercept of the regression line through the St. Helena and Walvis-Gough Ar-Ar data in Fig. 5a. Although the most elevated point of the ~ 300 km long Crawford Seamount has been selected as the center of the Walvis hot spot, this seamount, however, extends ~ 200 km further to the west of this location in the form of a much less elevated lineament. Radiometric dating of Crawford Seamount and surveying in the vicinity of the St. Helena and Tristan-Gough regions is required in order to evaluate the models proposed here. An

Helena Chain, with the Reunion hot spot being centered beneath the island of Reunion. The Ar-Ar ages for the Mascarene plateau and the K-Ar ages for the islands of Mauritius and Reunion are substantially offset from the fitted line through the St. Helena, Walvis and Gough data. In the case of the Reunion hot spot there is, however, doubt as to the location of the center of hot spot volcanism due to the probability of lithospheric control being involved in the location of volcanism [e.g. 45]. A best fitting line through all available age constraints (exclusive of the 0-2.5 Ma age range for Tristan and Gough and the ~ 7-9 Ma range for St. Helena) produces a negative intercept of ~ 170 km. This can be interpreted as an indication that the centers of the Walvis and St. Helena hot spots could be ~ 170 km to the west of the locations postulated here. An alternative explanation is that African plate speed decreased between ~ 31 and 0 Ma (Fig. 5a and Table 3). The possibility that the African plate is moving more slowly at present than it did during earlier times I DI-A " C ~

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Fig. 6. Reconstructions of the relationship between the St. Helena and Waivis hot spots, their respective volcanic traces, and the South Atlantic spreading axis. Relative motion parameters for Africa/South America are from [52]. Magnetic anomaly picks are from [53], ( + ) and ( o ) symbols representing rotated African and South American anomaly picks, respectively. Bathymetry is from [1]. The 80 Ma age shown for the Rio Grande Rise is from O'Connor (Ar-Ar analysis, unpublished data 1989]. The large shaded circles represent the broad, diffuse zones of hot spot-plume volcanism. Mercator projection.

362

apparent difficulty with velocities defined on the basis of Fig. 5b is that African plate velocity over hot spots is significantly faster than estimated by independent estimates of current African plate velocity [e.g. 50]. 5.5 Comparison with earlier models

The hot spot tracks for the Tristan da Cunha and St. Helena hot spots computed from the rotation parameters of Morgan [32, fig. 2] follow routes that are more southerly than that presented here. This earlier model of Morgan, which was based on the very limited At-At data available at that time, proposes a much greater velocity for the African plate [32, table 2]. The more recent model by Fleitout [41, fig. ld] leads to a computed track which follows the the most visible northern part of the Walvis Ridge. However, the computed track for a St. Helena hot spot-plume is not shown in this figure, and when calculated does not match the St. Helena Chain well. The velocities for the African plate proposed in this reconstruction are somewhat faster than our model for the 40-0 Ma period, similar between 80 and 40 Ma, and once again faster between 120 and 80 Ma [41, table 1]. In conclusion, we consider that the sets of angular rotations defined in Figs. 5a and b are useful working models for reconstructing African plate motion over hot spots. Further surveying, sampling and subsequent Ar-Ar dating of seamount samples in the proposed regions of St. Helena and Walvis hot spot-plume volcanism are needed in order to further evaluate these models. 6. Transition to mid-plate hot spot-plume volcanism

Reconstructing the relationship between the South Atlantic spreading axis, the St. Helena hot spot-plume, and the St. Helena Chain indicates that St. Helena Seamount 6 (52 Ma) formed ~500 km to the east of the South Atlantic spreading axis, i.e., in a mid-plate setting (Fig. 6). At 52 Ma the Walvis Ridge was also forming in an intraplate setting. In contrast, St. Helena Seamounts 2 ( ~ 82 Ma) and 5 ( ~ 78 Ma) appear to have formed on, or close to, the South Atlantic spreading axis (Fig. 6). An initial history of on-axis

J.M. O ' C O N N O R A N D A.P. L E R O E X

St. Helena hot spot volcanism is supported by the intersection of the southeastern ends of Pernambuco and Bahia seamounts (South American plate) with the spreading axis at chron 34 time (84 Ma) (Fig. 6). These Brazilian seamount chains are probably analogous to the Rio Grande Rise (South American plate), which co-evolved with the Walvis Ridge [e.g. 14, fig. 6]. The simultaneous termination of Brazilian Seamount Chain and Rio Grande Rise construction strongly suggests that, beginning between 80 and 70 Ma, the spreading axis migrated westward away from both the St. Helena and Walvis hot spots. The poor coverage of seafloor magnetic anomalies identified in the northern South Atlantic, however, makes it more difficult to reconstruct the relationship between the spreading axis and the St. Helena hot spot than is the case between the spreading axis and the Walvis hot spot. A transitional period of overlapping on-axis and intraplate volcanism is suggested by a chron 30 (67 Ma) reconstruction (Fig. 6), as noted previously in the case of the Walvis Ridge-Rio Grande Rise system [14]. Evidence to support the association of the Brazilian Seamount Chains and the St. Helena hot spot-plume system is provided by the presence of a St. Helena-like radiogenic lead signal in samples dredged from the Bahia and Pernambuco seamounts [O'Connor et al., submitted, 1991]. A problem, however, with the reconstructions presented here is the fact that the Bahia Seamounts formed apparently to the south of the St. Helena hot spot (Fig. 6). If, however, multiple mantle plumes were/are associated with the broad St. Helena hot spot, this could be a plausible mechanism to explain the formation of the Bahia Seamount Chain by the St. Helena hot spot. 6.1 Vema and 7°East seamounts

The age of 15 Ma determined in this study for dredge samples from Vema Seamount is somewhat older than the 11.0 + 0.3 Ma conventional K-Ar age determined by McDougall and reported in [54]. The 7°East Seamount is about 76 m.y. older than Vema Seamount and is clearly related to a different mantle source. Vema Seamount clearly formed in a mid-plate tectonic setting, whereas 7°East Seamount formed far closer to

S. ATLANTIC HOT SPOT-PLUMES: DISTRIBUTION OF VOLCANISM IN TIME AND SPACE

the spreading axis. On the basis of the topographic information presently available, these seamounts do not appear to be members of more extensive chains. This can be interpreted as evidence supporting the suggestion by [41] that there are more mantle plumes in the South Atlantic than documented in the 'classical hot spot list'.

7. Conclusions (1) The St. Helena and Walvis (referred to elsewhere as Tristan) hot spots-plumes consist of broad zones of diffuse volcanism (oceanic islands, seamounts and small ridges) at least 500 km in diameter. In the case of both of these zones of hot spot volcanism one or several(?) narrow plume(s) are upwelling to form broad zones of hot spot volcanism, which result from decompression melting across the broad 'mushroom head(s)' of the plume(s). (2) A new reconstruction of African plate motion has been developed on the basis of the proposition of broad St. Helena and Walvis hot spots, in combination with the new Ar-Ar data reported here. This model suggests that African plate velocity might well have decreased significantly between ~ 31 and 0 Ma. (3) A simultaneous transition from on/ ne a r spreading axis to intraplate volcanism is shown to have occurred along the St. Helena Seamount Chain and the Walvis Ridge between ~ 80 and 70 Ma, this transition occurring in response to the westward migration of the South Atlantic spreading axis away from, respectively, the St. Helena and Walvis hot spots.

Acknowledgements Reviews by David Christie, Alice Gripp, an anonymous reviewer and Colin Devey were very helpful. Henry Fleming and Jorge Palma were responsible for the recovery of dredge samples from the St. Helena Chain. Samples from the Gough Seamount Chain and Vema and 7°East seamounts were recovered during cruise 51 of the S.A. Agulhas. Logistics and financial support were provided by the S.A. Department of Environmental Affairs, the University of Cape Town and the Foundation for Research Development. Funding for the use of the Ar-Ar dating facility at Oregon

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State University and partial support for JMO was provided by ONR grant N00014-87-K-0420 to R. Duncan.

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