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Mobility and immobility of mid-ocean ridges and their implications to mantle dynamics D.C.P. Masalu Institute of Marine Sciences, University of Dar es Salaam, P.O. Box 668, Zanzibar, Tanzania ABSTRACT In the past two decades, the mobility of mid-ocean ridges relative to the mantle (absolute migration) has been correlated with major observable features, such as spreading asymmetry and asymmetry in the abundance of seamounts. The mobility of mid-ocean ridges is also thought to be an importantfactorthat influences the diversity of ridge-crest basalts. However, this mobility has not yet been defined and mapped.The absolute migration ofglobal mid-ocean ridges since 85 Ma has been computed and mapped. Global mid-ocean ridges have migrated extensively at varying velocities during that period. Presently, the fast-migrating ridges are the PacificAntarctic, Central Indian Ridge, Southeast Indian Ridge, Juan de Fuca, Pacific-Nazca, Antarctic-Nazca, and the Australia-Antarctic ridges, migrating at velocities of between 3.3 and 5.5 cm/yr. The slow-migrating ridges are the Mid-Atlantic and the Southwest Indian ridges, migrating at velocities between 0.3 and 2.0 cm/yr. Comparison of these results with mantle tomography resultsshowthat the slow-migrating ridges have deeper depth of origin than the fast-migrating ones, suggesting a correlation between the absolute migration velocity and the depth of origin of ridges. Furthermore, the Southwest Indian ridge appears to be tapping the same portion of mantle as did the Central Indian ridge. These results have important thermo-chemical implications, such as variations in the extent of melting and mineralogical composition ofthe mantle beneath different ridges, which may influence mantle dynamics. INTRODUCTION Mid-ocean ridges are important features of the earth for several reasons: in plate tectonics they are boundaries of plates and play a key role in driving them; and in geophysics they are locations along which new seafloor is created. Absolute migration of mid-ocean ridges have beencorrelated with major observable features ofthe ridges. For example Stein et al. (1977) correlated spreading asymmetry with migration rate, and Davis and Karsten (1986) explained asymmetry in seamountabundance byabsolute 77 ridge migration. Ridge migration rate is also thought to be an important factor that influences the diversity of ridge-crest lavas and the compositional uniformity of ridgecrest basalts (Davis and Karsten, 1986). Although absolute migration of mid-ocean ridges has been linked with many features, its influence to mantle dynamics has not yet been investigated. In this paper, therefore, the absolute migration of global mid-ocean ridges since 85 Ma is computed and its possible influence to mantle dynamics investigated. methods Maqnetic lineaBons fbrni concurrendy with new seafloor on mid-ocean ridges when passively Magnetic domains/miner seafloor. Fifteen ridoe geomagnetic field, are selected (Figure 1), sides. Their corresponding magnetic lineation segmenS'^K^'^ gg |via to Present are reconstructed by rotatinn ^ ^ their former positions (Masalu and Tamaki 1994^^'^®^ magnetic lineations bacK w the models of absolute moti Clague (1985). The absolute m-g computed. j^iuller and Smith (1993) and n./ "^'"9 velocities for each respective ridae ndge are then .magnetic lineations were obtained from aCD-rqm Locations of identified ^g ,.^gations (Cande etal., 1989). Ages were f data of locations ofglobal magn were ass.gn^^ 70°N 60°N 30°N 30°S eo-s ' ^ 70°S 120'W 60°W 78 to identified magnetic lineations based on a recent geomagnetic polarity time scale for the Late Cretaceous and Cenozoic time (Cande and Kent, 1992). The assumption in the methodology is that plates have remained rigid over the past 85 million years. This Is certainly not completely correct because both inter- and intra-plate deformations are known to exist. Errors may also arise from incorrectly identified magnetic lineations. There are also inherent errors from the use of models of absolute plate motions. RESULTS Migration of global mid-ocean ridges Regardless of the possibility of the errors described above, the results present a first overview of global motion of mid-ocean ridges. The results obtained indicate that global mid-ocean ridges have migrated extensively in the past 83 million years (Table 1and Figure 2). All ridges appear to be migrating: the East Pacific Rise (EPR) appears to have rotated clockwise for about 50° since 83Ma (Figure 2i). The South Mid Atlantic Ridge (SMAR) shows small lateral migration but indicates extensive longitudinal migration toward north (Figure 2ii). Similarly, the northern EPR and the Juan de Fuca ridge segments appear to have migrated northerly at least since about 83Ma (Chron 34) to the Present (Figure 21). The SMAR runs between several hotspots located relatively close to it on both sides which probably played a role in limiting its lateral migration (Uyeda and Miyashiro, 1974). Contrary to theSMAR, the Central Mid Atlantic Ridge (CMAR) between the equator and latitude 40° N, and the Northern Mid Atlantic Ridge (NMAR) north of the 40° N latitude, has been migrating northwesterly since 83Ma to Present (Figure 2ii). This suggests the existence of active deformation between the SMAR and the CMAR at least since 83Ma. The present results may offer one explanation of the origin of tectonic Table 1. Migration of global mid-ocean ridges Identifier Ridge Distance Time to Present migrated (km) (millionyears) JUAN Juan de Fuca 3900 PacAnt Pacific-Antarctic 2400 PacNazca Pacific-Nazca Antarctic-Nazca 200 12 1010 20 South Mid-Atlantic Ridge Central Mid-Atlantic Ridge North Mid-Atlantic Ridge 1300 83 1200 83 1550 Southeast Indian Ridge Central Indian Ridge Southwest Indian Ridge Australia-Antarctic 3900 83 83 AntNaz SMAR CMAR NMAR SEIR QR SWIR AustAntar 79 83 73 3100 1250 83 1400 43 64 180° 60° 240' 210' 270° 300° •K, 60° % .'''•v. V >17 JUAN- 30° 30° Vil' 2« ^27 • 30 •'~t. ^33 ^34 • PacNazca -30° -30° PacAnl iiiii. I AntNaz r PacAnt. -60° - -60° -^.32 180° 210° 270° 300' Figure 2 (i) gure 2. i^igration of mid-ocean ridges for some of the selected ridge segments. Solid thick ne segments are reconstructed zero-age paleopositions ofselected ridge segments at a Chron age indicated by the adjacent numbers. Text with a pointing arrow is ridge identifier as in Table . UHabove) Pacific ridges (Juan de Fuca and South Pacific ridges), (ii) (top, opposite page) rrip" cc?o (SMAR, CMAR and NMAR), (iii) (bottom, opposite page) Indian Ocean ridges on fh r Notesame that locality the CIRatand SWIR times (line connecting solid squares tne figure, and inset)AustAntar). sampled the different 80 —i T- / ;s\,S-5 60" - NMAR -H 60" 330° 340° 350° B -10° C=:!5j CMAR 1 1- 30° - / w •20° 1, 340^ 350" -40* — iivi V ,1'', SMAR r' -50° • I 30° - ff* 300° ^ 330' 340° Figure 2 (ii) 30° T^, S. (2^ "•'C 0° a V 5 I •30° - - vV' ^ -SEIR J^' " SWIR—♦! » AustAniar \ -60' f-Niv. r 30° 60° 90° Figure 2 (iii) 81 120° 150° compl^ty ,n tt,e equatonal Mantic r^ion, an area characterised by adense pattern of mosuy m^ium to laqe offeet fracture zones as well as aseries of unusual ridges and troughs (Mueller and Smith, 1993). These investigators have also suggested that ^the nNorth and1Atlanta region recorded the migration of evidence the plate from bounda^ between South Amerian plates. Based mainly on fracture zone trends, It has been proposed that the boundary was located in the equatorial Atlantic Cretaceous and Late Tertiary (Roest, 1987; Roest and Collette, 1986) periods. paleopositions of fte Central Indian Ridge (QR) between Chron 29 and Chron 25 »iat coincide with those of the Southwestern Indian Ridge (SWIR) between Chron 20 and Present (Figure 2iii). This implies that the SWIR may be tapping the same portion of the mantle as did the CIR between Chron 29 and 24. Absolute ridge migration velocities and mantle tomography The absolute migration velocities of all ridges investigated In this study are shown in Figure 3. The ridges divide into two groups: the slow-migrating ridge group includina the CMAR, SMAR, NMAR and the SWIR ridges, and the fast migrating ridge group including all other remaining ridges. Presently, slow-migrating ridges are migrating at velocities between 0.3-2.0cm/yr whereas fast-migrating ridges are migrating at velocities between 3.3-5.5cm/yr. The results were compared with seismic velocity anomalies beneath global midocean. Su et al. (1992) shows that seismic velocity anomalies associated with mid12 —AustAntar —JUAN — NMAR —CIR -CMAR -r O 8 - SEIR —SMAR —SWIR - -PacAnt —PacNazca —AntNaz o O i_r : . t"" : ^ __ n •ii 1 ,'-l •-"I" -i-l .I-n •L-J ^ -- , 1 r • .-3—: • r- iliniliii.i liii Age (Ma) Figure 3. Variation of migration velocities of global mid-ocean ridges. Text identifiers as in Table 1 82 - -- - ^ ocean ridges appear as continuous features to 300km deptti. For the NIMAR SI^IAR, Pacific-Antarctic ridge (PacAnt), SWIR, and the Carlsberg ridges, seismic velocities remain slower than normal, on average down to 400l<m depth, and this situation persists to 600l<m depth. Except for the PacAnt, the 'deep-rooted' ridges are the slow-migrating ridges (Figure 3). As for the PacAnt, though failing in the same group with fast-migrating ridges, it is the slowest ridge in the group. These results suggest the existence of a correlation between absolute migration velocities and the depth of origin of mid-ocean ridges, whereby fast-migrating ridges have shallow depth of origin and slow-migrating ones have deeper depth of origin. Intuitively, this may be explained as follows: mantle from the same vertical locality beneath a ridge have more time for passive upwelling for slow-migrating ridges and less time for fast-migrating ridges. Slow-migrating ridges allow the development of stable and deep-rooted mantle convection cells beneath them, whereas fast-migrating ones probably cause some disturbances to mantle convection cells beneath them, and thus allow only the development ofshallow-rooted convection cells. DISCUSSION The observations that the SWIR may be tapping the same portion of the mantle as did the CIR, and the correlation between absolute migration velocity and the depth of origin of mid-ocean ridges has far-reaching thermal and chemical implications. One of the most important dynamic processes in the earth's interior is thermal convection in the mantle. Currently, two methods are used to study temperature variations in the upper mantle: the study of major-element chemistry of basalts erupted at mid-ocean ridgeswhich is directly influenced bythe temperature ofthe mantle beneath (e.g. Klein and Langmuir, 1987), and seismic velocity studies (e.g. Su et al., 1992). Klein and Langmuir (1987,1989) studied the chemical systematicsof global midocean ridges and proposed a model to explain the observed chemical systematics of global MORB (Mid-Ocean Ridge Basalts) byvariations amongdifferent melting columns. According to their model, a hotter parcel ofthe mantle (from deeper depths) intersects the solidus at greater depths and produces a taller melting column, leading to greater mean pressures and extent ofmelting, whereas a cooler parcel ofmantle (from shallow depth) intersects the solidus at shallower depths and produces a shorter melting column, leading to lower mean pressures and extent of melting. Their model can becombined directly with the results of this study whereby fast-migrating ridges correspond to ridges with cooler sources of parcels of mantle, and slow-migrating ridges correspond to ridges with hotter sources of parcels of mantle. The combined result predicts that the fast-migrating ridges (e.g. theSEIR) will have deeper water depths and their basalts will have relatively high Nag,, and SiO^ with low Fe^p and CaO/AiPj, and that slow migrating ridges (e.g. the IMMAR) will beshallow in water depth and their basalts would have relatively low Nag j, and SiO^, but relatively high Feg ^and CaO/AiPj. Additionally, 83 these results suggest that the mantle beneath fast-migrating ridges has undergone smaller extents ofmelting (melt extraction) whereas thatbeneath slow-migrating ridges has undergone greater extents ofmelting (e.g. Figure 4 ofKlein and l_angmuir, 1989) which may influence mantle convection. Furtherstudy is needed from different branches ofgeosciences to investigate whether the 'deeper origin' ridges (Su et al., 1992) have any peculiar characteristics from other ridges. The observation that the SWIR may be tapping the same portion of the mantle as did the CIR, suggests that different portions of the mantle have undergone different phases/history ofrecycling, and thus offers one possible mechanism oforigin of lateral heterogeneities In the mantle. Astudy which will involve entire ridges may reveal more of such observations (Masalu and Tamaki, 1994). CONCLUSIONS The absolute migration of global mid-ocean ridges for the past 85 million years and its potential role in mantle dynamics have been investigated. Absolute migration velocities and depth of origin of ridges appear to correlate, whereby slow-migrating ridges (migrating at 0.3-2.0cm/yr) have deeper depth of origin than fast-migrating ones (migrating at 3.3-5.5cm/yr). The SWIR appears to be tapping the same portion of mantle as did the CIR, a phenomenon that may explain lateral mantle heterogeneity. Active deformation which appears to be taking place between the SMAR and CI^ar at least since 85 Ma is one possible explanation of the origin of tectonic complexity in the equatorial Atlantic. ACKNOWLEDGEMENTS Ithank Dr K. Tamaki who is my mentor at the Ocean Research Institute (ORI), University of Tokyo, Dr S. Uyeda and Dr W. W. Sager both from Texas A&M University, for their constructive critical reviews of another paper which is still under preparation from which the main ideas for the present paper originated. This work was partly done when I was a JSPS Fellow at ORI. REFERENCES Cande, S.C. &Kent, D.V. 1992. Anew geomagnetic polarity time scale for the Late Cretaceous and Cenozoic. Journal of Geophysical Research 97\ 13917-13951. Cande, S.C., LaBrecque, J.L, Larson, R.L., Pitman III, W.C., Golovchenko, X. & Haxby, w F 1989. Magnetic Lineations oftfie World's Ocean Basins. American Association of Petroleurri Geologists. Davis, E.E. &Karsten, J.L. 1986. 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