- Email: [email protected]
The evolution of the Lau Ridge, Fiji Islands
EARTH AND PLANETARY SCIENCE LETTERS 8 (1970) 258-260. NORTH-HOLLAND PUBLISHING COMPANY
THE EVOLUTION OF THE LAU RIDGE, FIJI ISLANDS J.S.MILSOM
Department of Geophysics, Imperial College of Science end Technology, London, S. W. 7, England Received 24 February 1970
The Lau Ridge lies about 150 miles west of the Tonga Ridge and is sub-parallel to it . It may be an example of a "dead" island arc, activity having transferred to the Tonga-Kermadec region after an Upper Miocene change in the .pattern of sea floor spreading in the Pacific . As a consequence of this change the Fiji Plateau has become detached from the main Pacific lithosplieric plate . The continental-type Fiji Islands are seen as having evolved inside a sharp bend in the former arc .
During the last few years a large amount of data has been gathered in oceanic areas supporting the dual hypotheses of sea floor spreading and continental drift. Much of this work has been conveniently summarised by Heirtzler et al . [ 1 ] . They suggest that the North Pacific and South Pacific lithospheric plates moved as separate units until about 10 my ago, after which time tile spreading direction in the North Pacific Filtered by about 1.0° . Studies of Pacific Basin sedimentation [2] appear to confirm the existence of a late Miocene change in spreading pattern . Carey's Tethyan Shear Zone [3j , originally postulated on the basis of regional geological mapping; in New Guinea, may be a :onsequence of the attendant pre-Pliocene stress situation in the equatorial Pacific; much of the structural complexity of the Melanesian area could be due to such a change in spreading direction . Changes in configuration and orientation of the midocean ridges associated with changes in spreading direction have been discussed by Mer:ard and Atwater [4] . The possibility of similar changes occurring at oceanic trenches has received less attention, possibly be ,--ause of the apparent ability of these features to accommodate to movements from a variety of directions; failure patterns must differ widely, for instance, at different points along the Aleutian Trench . However the Tonga-Kermadec trench is at present accurately normal to the direction of motion of the com-
bined North and South Pacific plates [5] and it seems unlikely that in this case trench orientation and spreading direction are independent . The fracture zones and magnetic anomalies associated with the movement of the North and South Pacific plates away from the East Pacific Rise have not been traced into the western Pacific . However, unless there existed an as yet unidentified sink area in the central Pacific, one or other of these spreading regimes must have been dominant in pre-Pliocene times in the Melanesian and Tonga-Kermadec regions. If it was the South Pacific regime the present alignment of the Tonga-Kermadec trench could be very old, as has been suggested by Tarling [61, but the structural complexity of Melanesia remains unexplained . If the North Pacific regime was dominant we might expect to find a second, fossil, island arc sub-parallel to the Tonga arc and made up of older rocks. Also we would not expect to find Miocene or older andesites or shallow water sediments : n the Tonga Islands, nor in Samoa, which appears to have evolved around stress features due to tile very sharp change in strike of the Tonga trench at its northern end. Within limits this is the pattern observed . The Lau Ridge, west of Tonga, is roughly normal to the direction of the North Pacific fracture,- zones (regarded as small circles on the globe) and the islands on the ridge are built up of older (pre-Upper Tertiary) vol-
THE EVOLUTION Oh THE LAU RIDGE, FIJI ISLANDS
Etl1ce
is
259
KEY land Water depth less than t km Water depth more than 6 km Contour depth in km -L-
Q
Samoa
0 % Futuna
New Negrides 1 t:
40
Ni ua Fo'ou
e
óa
.01 0 v
s
0
u
Fig . 1 . 1 he Fiji area, South-west Pacific . Bathymetry after Menard et al . 1161 .
canics and reef limestone [7] . Dickinson [8] considers the Lau Ridge as paralleling the Tonga Ridge and indicative of a regional pattern of dextral strikeslip . However, the parallelism is only approximate and the present day seismicities of the two ridges are quite different ; the only earthquakes in the Lau region are very deep shocks associated with the TongaKermadec Benioff zone [9] . Insofar as is known tile Lau Ridge has the properties to be expected of a "dead" island arc. Tarling [6] has made estimates of the ages of most of the formationâ of 1"estern Samoa, dating the oldest at about 2 .6 my . The volcanic arc in Tonga also consists of very recent rocks, but Eocene limestones are known on islands of the coralline chain and older volcanics and tnetamorphics oil Eua, on its eastern hank [101 . The Eocene limestones are not reef but deeper water facies, and so do not conflict with theories re- .
quiring the Tonga arc to be a post-Miocene structure. Eua is not well known, but seems to be anomalous in a number of ways. It may leave originated as a sea mount or oceanic island in the former Pacific segment east of the Lau Group and could have received small amounts of volcanogenic sediments from eruptions in those islands. The definite identification of old non-oceanic lavas on Eua would probably make the reconstruction here outlined untenable, but it has, as a working hypothesis, two interesting corollaries . Firstly, the origin of the continental-type Fiji Islands is seen ill a Clew tight. Cafey [o] termed the ricbridesFiji-Tonga structure the Fiji Orocline, implying that tl,.ese features were once collinear. I agree with Dickinson [8] that the available evidence favours their formation in very much their present position . The prototype of the Fijian complex is perhaps seen today in Futuna and Niua Fo'ou, which lie behind the bend
260
J .S .MILSOM
of the Tonga Trench and are structurally distinct from the Tonga Grouv~ . The main islands of Fiji may have evolved, over a vary much longer period of time, in a similar bend of a Lau Trench . The apparently abnrpt cessation of vulcanism-in the main islands at the close of the Tertiary and also certain similarities between the geology of Puerto Rico [131, near the bend in the Antilles are, and of Fiji, are suggestive of such a history . Secondly the anomalous position of the Fiji Plateau is explained ; Le Pichon [51 was unable to include this area in any of his major rigid plates and was forced to treat it as an independent block . Prior to the Pleistocene this block could have formed part of the North Pacific plate, which was probably being absorbed at a trench on the eastern side of the New Hebrides. The change in spreading pattern resulted in shearing of the Pacific along the north side of the Fiji Plateau, which is now a small detached section of the lithosphere undergoing complex deformation by shear and thrust forces from the Australasian and Pacific plates . The complexity of these forces is indicated by the pattern of shallow earthquakes [ 1 11 and by the very variable and generally high }heat flow [121 The Soiomon Islands and the New Hebrides are possibly both in part relicts of arc-trench systems invulved in the absorption of the former North Pacific plate . 'Tlte main movements now appear to be, in the New Hebrides . absorption of the Australasian Plate [41, and, in the Solomons, similar absorption and
also shear of the Pacific past the island chain . The slight thrust component involved in the latter movement has probably contributed to the sigmoidal form noted by Coleman [ 151 in the Solomons' oceanic province . References [1] 121
[31 [41 151 16] [71 181 [91 [ 101 111 I 1121 [ 13] [141 1151 1 161
J .R .Heirtzler, G .O .Dickson, E .M .flerron, W .C.Pitman and X .Le Pichon, J . Geophys . Res. 73 (1968) 2119 . J .Ewing, M.Ewing, T.Aitken and W .J .Ludwig, in : The Crust and Upper Mantle of the Pacific Area, Am . Geophys . Un. Monograph 12 (1968) 141 . S .W.Carcy, in : Continental Drift, a Symposium, Univ . of Tasmania, Hobart (1958) 177 . H .W .Menard and T.Atwater, Nature 219 (1968) 463 . X .i.e Pichon, J .Geophys . Res. '73 (1968) 3661 . D .H .Tarling, Geophys . J . Roy . Astron . Soc . 1 0 (1966) . W .C .White and O .N .Warin, Bur . Min . Res. Aust . Bull . 6 9 (1964) . W .R .Dickinson, Tectonophysics 4 (1967) 543 . B .L .lsacks, J .Oliver and L .R .Sykes, Bull . Geol . Soc . Am . 8 0 (1969) 1443 . J .C .Schofeld, N .Z . J . Geol . Geophys . 1 0 (1967) 1424 . L .R .Sykes, B .L .1sacks and J .Oliver, Bull . Seism . Soc . Am . 5 9 (1969) 1093 . W .Sclater and H .W .Menard, Narurc 216 (1967) 991 . P .H .Mattson, Bull . Geol . Soc . Am . 7 1 (1960) 319 . C .Blot and R .Priam, Bull . Voir, 26 (1963) 167 . P .J .Coleman, Nature 211 (1966) 1249 . Il .W . MenardJ .F .('hasc, D .F .Karig, D .A .Ncwhouse and I L .Taylor, Bathymetry of the tiouthwest Pacific, Prelh iinary Map, Scripps Institute of Oceanography, University of California, San Diego (1969) .
THE EVOLUTION OF THE LAU RIDGE, FIJI ISLANDS J.S.MILSOM
Department of Geophysics, Imperial College of Science end Technology, London, S. W. 7, England Received 24 February 1970
The Lau Ridge lies about 150 miles west of the Tonga Ridge and is sub-parallel to it . It may be an example of a "dead" island arc, activity having transferred to the Tonga-Kermadec region after an Upper Miocene change in the .pattern of sea floor spreading in the Pacific . As a consequence of this change the Fiji Plateau has become detached from the main Pacific lithosplieric plate . The continental-type Fiji Islands are seen as having evolved inside a sharp bend in the former arc .
During the last few years a large amount of data has been gathered in oceanic areas supporting the dual hypotheses of sea floor spreading and continental drift. Much of this work has been conveniently summarised by Heirtzler et al . [ 1 ] . They suggest that the North Pacific and South Pacific lithospheric plates moved as separate units until about 10 my ago, after which time tile spreading direction in the North Pacific Filtered by about 1.0° . Studies of Pacific Basin sedimentation [2] appear to confirm the existence of a late Miocene change in spreading pattern . Carey's Tethyan Shear Zone [3j , originally postulated on the basis of regional geological mapping; in New Guinea, may be a :onsequence of the attendant pre-Pliocene stress situation in the equatorial Pacific; much of the structural complexity of the Melanesian area could be due to such a change in spreading direction . Changes in configuration and orientation of the midocean ridges associated with changes in spreading direction have been discussed by Mer:ard and Atwater [4] . The possibility of similar changes occurring at oceanic trenches has received less attention, possibly be ,--ause of the apparent ability of these features to accommodate to movements from a variety of directions; failure patterns must differ widely, for instance, at different points along the Aleutian Trench . However the Tonga-Kermadec trench is at present accurately normal to the direction of motion of the com-
bined North and South Pacific plates [5] and it seems unlikely that in this case trench orientation and spreading direction are independent . The fracture zones and magnetic anomalies associated with the movement of the North and South Pacific plates away from the East Pacific Rise have not been traced into the western Pacific . However, unless there existed an as yet unidentified sink area in the central Pacific, one or other of these spreading regimes must have been dominant in pre-Pliocene times in the Melanesian and Tonga-Kermadec regions. If it was the South Pacific regime the present alignment of the Tonga-Kermadec trench could be very old, as has been suggested by Tarling [61, but the structural complexity of Melanesia remains unexplained . If the North Pacific regime was dominant we might expect to find a second, fossil, island arc sub-parallel to the Tonga arc and made up of older rocks. Also we would not expect to find Miocene or older andesites or shallow water sediments : n the Tonga Islands, nor in Samoa, which appears to have evolved around stress features due to tile very sharp change in strike of the Tonga trench at its northern end. Within limits this is the pattern observed . The Lau Ridge, west of Tonga, is roughly normal to the direction of the North Pacific fracture,- zones (regarded as small circles on the globe) and the islands on the ridge are built up of older (pre-Upper Tertiary) vol-
THE EVOLUTION Oh THE LAU RIDGE, FIJI ISLANDS
Etl1ce
is
259
KEY land Water depth less than t km Water depth more than 6 km Contour depth in km -L-
Q
Samoa
0 % Futuna
New Negrides 1 t:
40
Ni ua Fo'ou
e
óa
.01 0 v
s
0
u
Fig . 1 . 1 he Fiji area, South-west Pacific . Bathymetry after Menard et al . 1161 .
canics and reef limestone [7] . Dickinson [8] considers the Lau Ridge as paralleling the Tonga Ridge and indicative of a regional pattern of dextral strikeslip . However, the parallelism is only approximate and the present day seismicities of the two ridges are quite different ; the only earthquakes in the Lau region are very deep shocks associated with the TongaKermadec Benioff zone [9] . Insofar as is known tile Lau Ridge has the properties to be expected of a "dead" island arc. Tarling [6] has made estimates of the ages of most of the formationâ of 1"estern Samoa, dating the oldest at about 2 .6 my . The volcanic arc in Tonga also consists of very recent rocks, but Eocene limestones are known on islands of the coralline chain and older volcanics and tnetamorphics oil Eua, on its eastern hank [101 . The Eocene limestones are not reef but deeper water facies, and so do not conflict with theories re- .
quiring the Tonga arc to be a post-Miocene structure. Eua is not well known, but seems to be anomalous in a number of ways. It may leave originated as a sea mount or oceanic island in the former Pacific segment east of the Lau Group and could have received small amounts of volcanogenic sediments from eruptions in those islands. The definite identification of old non-oceanic lavas on Eua would probably make the reconstruction here outlined untenable, but it has, as a working hypothesis, two interesting corollaries . Firstly, the origin of the continental-type Fiji Islands is seen ill a Clew tight. Cafey [o] termed the ricbridesFiji-Tonga structure the Fiji Orocline, implying that tl,.ese features were once collinear. I agree with Dickinson [8] that the available evidence favours their formation in very much their present position . The prototype of the Fijian complex is perhaps seen today in Futuna and Niua Fo'ou, which lie behind the bend
260
J .S .MILSOM
of the Tonga Trench and are structurally distinct from the Tonga Grouv~ . The main islands of Fiji may have evolved, over a vary much longer period of time, in a similar bend of a Lau Trench . The apparently abnrpt cessation of vulcanism-in the main islands at the close of the Tertiary and also certain similarities between the geology of Puerto Rico [131, near the bend in the Antilles are, and of Fiji, are suggestive of such a history . Secondly the anomalous position of the Fiji Plateau is explained ; Le Pichon [51 was unable to include this area in any of his major rigid plates and was forced to treat it as an independent block . Prior to the Pleistocene this block could have formed part of the North Pacific plate, which was probably being absorbed at a trench on the eastern side of the New Hebrides. The change in spreading pattern resulted in shearing of the Pacific along the north side of the Fiji Plateau, which is now a small detached section of the lithosphere undergoing complex deformation by shear and thrust forces from the Australasian and Pacific plates . The complexity of these forces is indicated by the pattern of shallow earthquakes [ 1 11 and by the very variable and generally high }heat flow [121 The Soiomon Islands and the New Hebrides are possibly both in part relicts of arc-trench systems invulved in the absorption of the former North Pacific plate . 'Tlte main movements now appear to be, in the New Hebrides . absorption of the Australasian Plate [41, and, in the Solomons, similar absorption and
also shear of the Pacific past the island chain . The slight thrust component involved in the latter movement has probably contributed to the sigmoidal form noted by Coleman [ 151 in the Solomons' oceanic province . References [1] 121
[31 [41 151 16] [71 181 [91 [ 101 111 I 1121 [ 13] [141 1151 1 161
J .R .Heirtzler, G .O .Dickson, E .M .flerron, W .C.Pitman and X .Le Pichon, J . Geophys . Res. 73 (1968) 2119 . J .Ewing, M.Ewing, T.Aitken and W .J .Ludwig, in : The Crust and Upper Mantle of the Pacific Area, Am . Geophys . Un. Monograph 12 (1968) 141 . S .W.Carcy, in : Continental Drift, a Symposium, Univ . of Tasmania, Hobart (1958) 177 . H .W .Menard and T.Atwater, Nature 219 (1968) 463 . X .i.e Pichon, J .Geophys . Res. '73 (1968) 3661 . D .H .Tarling, Geophys . J . Roy . Astron . Soc . 1 0 (1966) . W .C .White and O .N .Warin, Bur . Min . Res. Aust . Bull . 6 9 (1964) . W .R .Dickinson, Tectonophysics 4 (1967) 543 . B .L .lsacks, J .Oliver and L .R .Sykes, Bull . Geol . Soc . Am . 8 0 (1969) 1443 . J .C .Schofeld, N .Z . J . Geol . Geophys . 1 0 (1967) 1424 . L .R .Sykes, B .L .1sacks and J .Oliver, Bull . Seism . Soc . Am . 5 9 (1969) 1093 . W .Sclater and H .W .Menard, Narurc 216 (1967) 991 . P .H .Mattson, Bull . Geol . Soc . Am . 7 1 (1960) 319 . C .Blot and R .Priam, Bull . Voir, 26 (1963) 167 . P .J .Coleman, Nature 211 (1966) 1249 . Il .W . MenardJ .F .('hasc, D .F .Karig, D .A .Ncwhouse and I L .Taylor, Bathymetry of the tiouthwest Pacific, Prelh iinary Map, Scripps Institute of Oceanography, University of California, San Diego (1969) .