Hotspots in the Southern Oceans — an absolute frame of reference for motion of the Gondwana continents

Tectonophysics, 74 (1981) 29-42 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

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HOTSPOTS IN THE SOUTHERN OCEANS - AN ABSOLUTE FRAME OF REFERENCE FOR MOTION OF THE GONDWANA CONTINENTS

R.A. DUNCAN * Geology Department,

University

of Tasmania, Hobart,

Tasmania 7005 (Australia)

(Received October 24, 1980)

ABSTRACT Duncan, R.A., 1981. Hotspots in the southern oceans - An absolute frame of reference for motion of the Gondwana continents. In: S.C. Solomon, R. Van der Voo and M.A. Chinnery (Editors), Quantitative Methods of Assessing Plate Motions. Tectonophysics, 74: 29-42. The geometry and geochronology of aseismic ridges and oceanic islands in the southern oceans provide a good test of the proposition that hotspots remain fixed over long periods of time; that is, motion of an order of magnitude less than the relative motion between plate pairs. In most cases it is concluded that inter-hotspot movement cannot be discerned for the period 100 m.y. to Present and that widely distributed hotspots in the Atlantic and Indian Oceans provide a frame of reference for plate motions following the disintegration of Gondwanaland, which is independent of paleomagnetism. This frame of reference is “absolute” in that it gives the motion of the lithosphere with respect to the mantle (= hotspots). The absolute motion model indicates that Africa and Antarctica are now moving only very slowly, that there has been significant relative movement between East and West Antarctica since the Cretaceous, and prescribes the relative motion between the Somali and African plates.

INTRODUCTION

Linear volcanism in the ocean basins has attracted considerable attention in recent years. Part of the interest lies in the possibility that such lines of oceanic islands and seamounts may record the passage of the lithosphere over fixed volcanic sources - in this paper called hotspots. If these hotspots, whatever their origin, are active and fixed with respect to one another over geologically significant periods of time then they would constitute a frame of reference for directly and precisely measuring plate motions. This reference frame would be independent of paleomagnetic estimates of plate motions

* On leave from the School of Oceanography, Oregon State University, Corvallis, Oregon 97331 (U.S.A.). 0040-1951/81/0000~000/$02.50

0 1981 Elsevier Scientific Publishing Company

30 and would not have the latter’s ambiguity in lon~tud~al movement. Additionally, evidence that hotspots do not move with respect to one another would imply that they manifest a very stable pattern of mantle convection. Previous estimates of inter-hotspot motion have varied depending on the method employed and on the period of time considered. After determining the best-fitting set of relative motions between plates and constraining the Pacific plate to follow the Hawaii Islands volcanic lineament, Minster et al. (1974) found that there has been no me~urable motion among presently active hotspots for the last 10 m.y. Burke et al. (1973), Molnar and Atwater (1973) and Molnar and Francheteau (1975), however, concluded inter-hotspot movement of at least 15 mmfyr over the last 38 to 120 m.y. More recently Morgan (1980) estimated that hotspots move less than 5 mm&r with respect to one another. Critical to these studies (and to the approach considered in this paper) is the choice of ancient positions of hotspots and of the relative motions between plates. Since these data are constantly being improved it is worth re-equating the hotspot question. In this paper postulated hotspot paths occurring mainly as aseismic ridges and island chains in the ocean basins and spanning the last 100 m.y. of lithospheric motion are compared with those predicted by adding plate-pair relative motions. It is concluded that hotspots in the southern oceans are fixed with respect to one another for this period (that is, less than 5 mm/yr relative motion). It follows that the geometry and distribution of ages along volcanic lineaments in the southern oceans provide a record of the disintegration of Gondwanaland, Several predictions for as yet poorly known relative motions emerge.

The strategy of this test will follow that of a more ambitious paper by Morgan (1980) which examines Atlantic and Indian Ocean hotspots from 200 m.y. to the Present. Specifically, the motion of a single plate over hotspots underlying it is determined from the geometry and dated localities along the hotspot paths. This will be called the &so&e motion of that plate. Next, the relative motion between this starting plate and neighbo~g plates (known from seafloor spreading magnetic anomalies and transform fault directions) is added to obtain the absohte motion for those neighboring plates. These inferred absolute motions predict the volcanic lineaments that would be left by fixed hotspots underlying those plates. Assuming that the plate-plate relative motions are known accurately, comparison of such predicted with observed hotspots paths gives a measure of the amount of interhotspot movement. This analysis differs from Morgan (1980) in the following respects. In this paper greater emphasis is placed on the expression of hotspots underlying oceanic lithosphere as being more accurate determinations of hotspot positions than continental volcanic occurrences. Consequently, the period con-

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Pig. 1. Africa - hotspot motion since 100 m.y. B.P. Presently active hotspots are large open circles of lo diameter (= 110 km). Bathymetric contours are 4 and 2 km depth and continental volcanic provinces are bounded by dashed lines. A best-fitting series of rotation poles constructed from the geometry of the hotspot paths produces the solid line segments, connected by bubbles of 20 m.y. age increments. Hotspot paths and reported geochronology (m.y.) are: A = Canaries (McDougall and Schminke, 1977); B = Cape Verde; C = Ascension; D = St. Helena (Baker, 1973); E = Tristan da Cunha (Baker, 1973; Supko et al., 1977; Bolli et al., 1978; Siedner and Miller, 1968); F = Vema (Krijner, 1973); G = Bouvet (Dingle and Gentle, 1972; Duncan et al., 1978; Allsopp and Barrett, 1975); H = Prince Edward (Simpson et al., 1974; Andriamirado, 1971); J = Comores; K = Afar; L = Jebel Mara; M = Mt. Cameroon; N = Tibesti; P = Ahaggar.

32

sidered is from 100 m.y. to the Present rather than the 200 m.y. period examined by Morgan (1980). Because a larger set of age determinations is now available, more attention is given to matching measured ages with those predicted and the absolute motion for the starting plate, Africa, is better fitted in this study. There is still room for improvement, however, with additional geochronology. Finally, this paper attempts to tie the hotspots underlying the Pacific plate to those recorded on the Gondwanan plates and this predicts intra-Antarctic plate motion during the late Cretaceous--early Tertiary. Our starting point is the African plate. This plate is endowed with a large number of proposed hotspots, many of which have been active since the Cretaceous (and record the entire history of the opening of the South Atlantic Ocean). Africa is also surrounded by spreading ridges, which facilitates direct addition of relative motions of neighboring plates. Figure 1 illustrates presently active African hotspots, hotspot paths with chronology where known and predicted lineaments based on one set of absolute motion poles of rotation. Bathymetric contours are for 4 and 2 km depth and the large circles of present hotspot activity are lo across (110 km). The St. Helena, Tristan da Cunha, Bouvet and Prince Edward Island hotspot paths are the longest continually active hotspots. They have produced subparallel lineaments which can be used to define a best-fitting (visually) series of rotations for African plate-hotspot movement (Table I). The hotspot paths predicted from these rotations are shown in Fig. 1 as solid lines connected by bubbles of 20 m.y. increments from 100 m.y. to the Present, describing northeasterly African motion of variable velocity. There are some departures from the actual hotspot lineaments (e.g., the Bouvet path where an offset of -200 km at 60 m.y. z 3.3 mm/yr is noted) but the overall fit is good and known ages agree well with those predicted. Thus hotspots underlying the African plate show virtually no inter-hotspot motion (<5 mm. yr) from the middle Cretaceous to the Present. Those hotspots which began more recently exhibit shorter lineaments which are consistent with the determined African absolute motion (with the exception of the Comores path which is discussed below). Table II lists the relative motions used to extend the African absolute motion to other plates. These are based on marine magnetic anomalies and transform fault directions and are the most recent studies available. All magnetic anomaly ages follow the La Brecque et al. (1977) time scale. Table I presents the motion of Africa over its hotspots and the calculated absolute motion for plates surrounding the southern oceans. Figure 2 illustrates proposed South American absolute motion, derived by adding the relative motion between the South American and African plates to the absolute motion of Africa. Three lineaments can be examined for possible inter-hotspot movement. From north to south they are those resulting from the Fernando de Noronha, Martin Vas and Tristan da Cunha hotspots. Again predicted ages are shown in 20 m.y. increments. The actual hot-

TABLE I Plate motions relative to the hotapot reference frame Plate *

Time (m-y.)

Pole of rotation Lat. (+‘N)

Long. (+‘E)

Rotation ** (“) 4.6 9.5 12.8 26.0 31.5

AF AF AF AF AF

21-o 36-O 55-O 80-O 100-O

61.0 42.5 38.0 30.7 27.0

-45.0 -25.0 -30.0 -58.6 -45.0

SA SA SA SA SA

21-O 36-O 55-O 80-O 100-O

51.6 75.9 86.6 52.0 58.7

-30.1 -112.7 -51.5 66.2 32.0

AN AN AN AN AN

21-O 36-O 55-O 80-O 1oQ-Q

79.5 62.6 64.8 62.9 74.5

144.7 8.6 71.2 -106.7 -38.1

3.6 6.1 6.3 12.2 10.3

AU AU AU AU AU AU AU AU AU

9-O 21-O 27-O 33-o 36-O 42-O 55-O 80-O 100-O

22.0 28.7 26.5 23.5 22.4 20.6 20.0 29.1 24.7

38.0 35.3 30.9 30.0 29.3 31.8 32.7 18.4 23.2

7.1 13.9 18.5 22.7 24.9 27.6 35.2 32.0 35.8

IN IN IN IN IN IN

21-O 36-O 55-9 64-O 80-O 100-O

27.6 22.5 8.3 11.4 8.6 10.1

35.8 29.4 28.7 16.3 6.8 4.5

13.1 24.9 40.4 52.7 75.1 82.3

WA WA WA WA WA

21-O 42-O 55-O 80-O 100-O

22.8 31.8 22.0 53.0 44.9

-66.2 -114.0 92.4 97.2 86.9

-3.1 -5.2 15.3 27.7 29.0

PC PC PC PC PC PC

9-O 21-O 36-O 55-o 80-Q 100-0

63.9 65.0 71.6 54.6 47.2 48.3

-Q2.3 -81.2 -88.2 -87.2 -84.6 -91.3

-3.4 -5.2 -11.7 -19.5 -22.4

-10.2 -17.7 -31.3 -40.2 -53.8 -57.8

* AF = Africa; SA = South America; AN = Antarctica; AU = Australia; IN = India; WA = West Antarctica; PC = Pacific. ** Positive if counter-clockwise when reviewed from outside the earth.

Fig. 2. South American absolute motion is determined by adding South America-Africa relative motion to the African absolute motion illustrated (from Fig. 1). Predicted hotspot paths are solid line segments joined by bubbles of 20 m.y. age increments. Hotspot paths and reported geochronology (m.y.) are: A = Fernando de Noronha (A. Roche, pens. commun., 1979); B = Martin Vas (Macintyre, 1978); C = Tristan da Cunha (Maxwell et al., 1970; Supko et al., 1977).

spot paths and known ages are well matched by the predicted trends, implying fixed hotspots. The 80-100 m.y. segment of the predicted Tristan da Cunha path is offset from the Rio Grande Rise and may require some adjustment of the African absolute motion for this period. Antarctic plate absolute motion is calculated from African absolute motion and Antarctica-Africa relative motion (Tables I and II). The result is illustrated in Figure 3 along with Indian plate absolute motion, determined by adding Ind~-~~ctica relative motion to the Antarctic absolute motion. Such modelling predicts that the Antarctic plate is barely moving over the

35

/

Fig. 3. Antarctic and Indian absolute motions result from adding Antarctica-Africa and India-Antarctica relative motions to African absolute motion illustrated (from Fig. 1). Solid line segments and bubbles are as in Fig. 1 and dashed lines connect old volcanic lineaments on the Indian plate with their proposed sources now underlying the Antarctic (F) and Somali (B) plates. Hotspot paths and reported geochronology are: A = Cornores; I3 = Reunion (McDougall and Chamalaun, 1969; Fisher et al., 19’74; Wellman and McEIhinny, 1970); C = Prince Edward (as in Fig. 1); D = Crozet; E = Ob Seamount; F = Kerguelen (Watkins et al., 1974; Lameyre et al., 1976; Duncan, 1978; ~cDoug~1 and McElhinny, 1970); G = Heard; H = Gaussberg; J = Mt. Erebus. The offset of predicted from observed paths for the Comores and Reunion hotspots can be used to determine SomaliAfrica relative motion.

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hotspot network. Geoc~onolo~ for Antarctic hotspot lineaments is sorely lacking but dated volcanism at Kerguelen Island (2’7 to 5 m.y., Watk~s et al., 1974; Lameyre et al., 1976) suggests that motion between the Antarctic plate and this hotspot has been slow enough to allow extensive volcanism at Kerguelen over a long period of time. The short predicted hotspot paths generally match the bathymetric highs. The Chagos-Laccadive and Ninetyeast Ridges are strikingly subparallel volcanic lineaments in the central Indian Basin. Because they reflect the rapid no~hw~d movement of India during the last 100 m.y, their geometry and geochronology will be particularly sensitive to inter-hotspot motion. Predicted hotspot paths are superimposed on actual trends in Fig. 3. The geometry of the Ninetyeast Ridge is well matched by the modelled path of the Kerguelen hotspot. Predicted ages also agree well with those documented along this lineament (McDougall and McElhinny, 1970; Duncan, 1978). There is a systematic offset to the west of the predicted path of the reunion hotspot from the Chagos-Laccadive Ridge. Predicted ages are good estimates of those few known along this trend. Taken at face value, the offset (-300 km/60 m.y. = 5 mm/yr) implies slow motion between Reunion and the previously examined hotspots. Small errors in the relative motion between India and Antarctica, however, could also be involved. The Comores and Reunion hotspots lie in the western Indian Basin and have produced the volcanic lineaments illustrated in Fig. 3. The hotspot paths predicted from African absolute motion are offset to the west. This is because these hotspots underlie the Somali plate and no motion across the East African Rift has been included in the modelling. In fact, sub~action of the predicted motion shown from the observed paths should give an estimate of the magnitude and timing of the relative motion between the African and Somali plates. This calculation awaits age determinations from the Comores Islands to northern Madagascar volcanic trend. Australian absolute motion (Table I) matches the Eastern Australian Cainozoic province geometry and geochronology (Wellman and McDougall, 1974) and the trend of the Tasmantid Guyots (Vogt and Conolly, 1971) as seen in Fig. 4. Also pictured is the hypothetic~ path left by a hotspot centered at the Balleny Islands which may have surfaced only recently. Pacific plate absolute motion can be calculated by adding Pacific-Antarctic relative motion to Antarctic absolute motion (Tables I and II). As shown in Fig. 4, the resulting Pacific motion agrees with the well studied, subparallel volcanic trends of the Pacific Basin from present-day activity to the distinctive bend in the Hawaiian-Emperor, Tuamotu-Line and Aus~al-M~~~/ Gilbert lineaments. However, instead of turning to the north to mirror the observed trends, the predicted paths continue far out to the west, contradicting the observed paths. Other studies have concluded that these Pacific Basin lineaments have a geometry and chronology consistent with formation by fixed hotspots (Morgan, 1972; Jarrard and Clague, 1977; McDougall and Duncan, 1980).

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Fig. 4. Pacific absolute motion is calculated by adding Pacific-Antarctica relative motion to Antarctic absolute motion (Fig. 3). The discrepancy between predicted and actual botspot paths prior to the age of the bend in the Pacific volcanic lineaments requires deformation within Antarctica. Solid lines with short ticks are the predicted Pacific hotspot paths in 20 m.y. increments considering a single Antarctic plate, while the solid lines connected by bubbles are paths predicted allowing separate East and West Antarctic plate motion. Hotspot paths and reported geochronology are : A = Eastern Australian Cainozoic Province (Wellman and McDougall, 1974); B = Tasmantid Guyots (Vogt and Conolly, 1971); C = Balleny; D = Mt. Erebus; E = Gaussberg; F = Macdonald; G = Easter; H = Hawaii (Dalrymple et al., 1974, 1977; Kono et al., 1978; Scholl et al., 1971); J = PrattWelker.

Consequently, either the Pacific hotspots (as a group) had been moving relative to African, Australian, Indian and Antarctic hotspots before the age of the Pacific bend and then suddenly locked into the hotspot network, or there has been some additional plate boundary between (eastern) Antarctica and the (northern) Pacific plate, active prior to the age of the Pacific bend.

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Fig. 5. The southern continents are reconstructed to their 80 m.y. positions using the fixed hotapot framework. To reconcile Pacific botspot paths with those in the Atlantic and Indian Oceans, Antarctica must be a group of separate plates which move asshown between 80 m.y. and 42 m.y. (the age of the Pacific “bend”): Marie Byrd Land (B) rifts northward away from East Antarctica (A). The Antarctic Peninsula (C) cannot overlap with the Falkland Plateau (dotted line) and is shown in one possible Cretaceous position.

Since no other large inter-hotspot motions have been implied from examination of the hotspot paths in the southern oceans, the second alternative seems more likely. The position of this late Cretaceous-early Tertiary plate boundary is unknown. Molnar et al, (1975) divided East and West Antarctica while an intra-Pacific plate (north and south) boundary is possible. This analysis cannot distinguish between these possibilities but will examine the relative motion between East and West Antarctica for which there is paleomagnetic and seafloor spreading evidence (Molnar et al., 1975; Herron and Tucholke, 1976; Cox and Gordon, 1978; Jurdy, 1979). Subtraction of the predicted Pacific absolute motion (assuming no deformation within An~tica) from the observed hneaments gives the relative motion between East and West Antarctica required to reconcile the Pacific

39

trends with those recorded in the Atlantic and Indian Oceans. Figure 5 illustrates the hotspot-reconstructed positions of the southern continents at 80 m.y. East and West Antarctica are separated by a possible plate boundary close to the western Transantarctic Mountains. West Antarctica itself is composed of at least two separate plates, Marie Byrd Land and the Antarctic Peninsula, as suggested by Stump (1973) and Herron and Tucholke (1976). The 80 m.y. position of the Antarctic Peninsula is problematic. If it remains attached to Marie Byrd Land an unacceptable overlap with the Falkland Plateau results. This portion of West Antarctica must then either fit into TABLE

II

Relative

motions

Plate pair

between

plates

Time (m.y.)

Ref.

Pole of rotation Lat. (+‘N)

Long.

(+OE)

Rotation

SA-AF SA-AF SA-AF

360 80-36 109-80

57.4 66.6 24.1

-37.5 -37.5 -15.7

13.6 20.5 22.4

AN-AF AN-AF

360 140-36

10.0 10.0

-41.0 -41.0

5.0 33.6

AU-AN AU-AN AU-AN AU-AN AU-AN AU-AN AU-AN AU-AN

90 209 27-20 33-27 36-33 42-36 55-42 100-55

IN-AN IN-AN IN-AN IN-AN IN-AN

36556480lOO-

WA-AN WA-AN WA-AN

420 BO- 0 100-80

PC-WA PC-WA PC-WA PC-WA PC-WA PC-WA PC-WA PC-WA

90 219 36-21 42-36 59-42 67-59 81-67 100-81

0 0 0 0 0

* Positive if first-named plate when viewed from outside the Ref: (1) = Sibuet and Mascle, (4) = Johnson et al., 1976; (5)

(1) (1) (1) (2) (2)

-6.8 -5.5 -4.0 -3.1 -1.6 -3.0 -8.0

(3) (3) (3) (3) (3) (3) (3)

34.6 -151.1 20.4 15.6 10.5

-20.8 38.0 -51.8 -76.5 -82.0

(4) (4) (4) (4) (4)

64.1 26.7 no motion

-72.2 98.1

9.0 -24.6

(5) (5)

-68.7 -76.7 -77.3 -75.9 -63.5 -51.4 -66.3 no motion

79.7 123.2 146.2 -178.2 125.1 127.2 150.9

-9.3 -6.4 -12.3 -5.2 -9.0 -8.1 -16.7

(6) (‘3) (6) (6) (6) (6) (6)

9.7 19.2 16.1 2.1 5.0 -0.3 14.1 no motion 12.0 0.5 3.9 3.9 5.7

36.5 32.7 29.4 38.8 35.2 34.8 25.4

* (“)

moves clockwise with respect to the second (which is fixed) earth. 1977; (2) = Barron et al., 1978; (3) = Weissel et al., 1977; = this study; (6) = Molnar et al., 1975.

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the Cretaceous Atlantic Ocean south of the Falkland Plateau, or curl around the Pacific side of the southern Andes (South America) as illustrated and as proposed by Barron et al. (1978). Between 80 m.y. and 42 m.y. (the age of the Pacific lineaments’ bend, Clague and Dalrymple, 1976) the Marie Byrd Land portion of West Antarctica separated fromEast Antarctica, adding a northward component to Pacific plate motion which is seen in the orientation of the Emperor Seamounts. During this period East Antarctica rotated slowly as shown. By 42 m.y. the present configuration of Antarctica was nearly attained, followed by slow compression between East and West Antarctica (Table II). This analysis has not included reconstruction of the continental fragments in the New Zealand region. Identification of seafloor spreading anomalies by Molnar et al. (1975) required that East and West Antarctica be separate plates in the Late Cretaceous to eliminate overlap of the Lord Howe Rise and Champbell Plateau. Weissel et al. (1977) and Barron and Harrison (1979) re-interpreted the spreading history in this area and found no need to fragment Antarctica. The Pacific-Antarctica relative motions proposed by Barron and Harrison (1979) do not, however, predict the northerly trend (>42 m.y.) of the Pacific hotspot paths and East-West Antarctic relative motion or possibly an i&a-Pacific plate boundary is still required under the model of fixed hotspots. CONCLUSIONS

This study has noted that: (1) Hotspots underlying a single plate (such as Africa or the Pacific) produce lineaments which can be well described by a single set of rotations, the absolute motion of that plate. Such constructions indicate inter-hotspot motion less than 5 mm/yr over the last 100 m.y. (2) Using relative plate-pair motions, plate boundaries can be crossed and hotspots underlying all plates can be tested for fixity. Comparisons between predicted and actual lineaments are close matches, both geometrically and chronologically (where known). (3) Where significant discrepancies occur an explanation involving suspected relative motions can reconcile predicted and actual lineaments. In fact such corrections may be good estimates for poorly known relative motions, such as Africa-Somali motion and East-West Antarctica motion. ACKNOWLEDGEMENTS

The plate rotations and additions were calculated using computer programs written by Jason Morgan. I thank him also for a preprint describing modelling of hotspot trends in the Atlantic and Indian Oceans. Many of the conclusions of that paper are shared by the analysis presented here. C.G.A. Harrison, C.M. Emerick and D.M. Jurdy provided stimulating discussion. This work

41

was supported

by a Queen’s Fellowship

from the Australian

Government.

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