Tectonophysics, 22 (1974) 301-310 0 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

TECTONICS OF MID-OCEAN RIDGES

C.G.A. HARRISON

University of Miami, Rosenstiel School of Marine and Atmospheric Science, Miami, Fla. (U.S.A.)

(Accepted for publication January 15, 1974)

ABSTRACT

Harrison, C.G.A., 1974. Tectonics of mid-ocean ridges. Tectonophysics, 22: 301-310.

Various simple models for the emplacement of new material at the mid-oceanic ridge are discussed. Ridges with median valleys and ones without such valleys are considered.

The emplacement of both extrusive and intrusive material is taken into account. For each situation a simple pattern of tectonic behavior is suggested which will allow a steady state. It is shown that reverse faulting must occur if median valleys are in a steady state. It is also shown that where extrusive material is present this reverse faulting produces a smearing out of the boundary between zones of oppositely polarized material. The extent of smearing is dependent directly on the total thickness of extrusive material and on the cotangent of the angle of faulting.

INTRODUCTION

The problem of the formation of the median valley of the mid-oceanic ridge system has not received the attention it deserves. Since this is the posi- tion where the basalt which causes the linear magnetic anomalies is emplaced, an understanding of the tectonics of the mid-oceanic ridge can give us some insight into the mechanisms of sea-floor spreading on a local scale, and can also tell us something about the reliability of the magnetic tape recorder in reproducing the reversals of the earth’s magnetic field.

Median valleys are not ubiquitous. Most of the East Pacific Rise is without a median valley, whereas the Atlantic and Indian Ocean Ridges are character- ized by such valleys (Anderson and Noltimier, 1973).

Deffeyes (1970) and Anderson and Noltimier (1973) have presented models of the median valley where a median valley is formed when the increase in strain occurs over a shorter distance from the center of the ridge than does the increase in the supply of basalt. Anderson and Noltimier (1973) have also shown that when the increase in strain occurs over a greater distance than the increase in basalt supply, then a horst structure is formed. Deffeyes (1970) recognized that to have a median valley as a steady-state function of time, there has to be reverse faulting occurring at the edges of the median valley.

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This fact was also recognized by Osmaston (1971), who presented an inge- nious method of producing this reverse faulting, based on a concept of Sleep (1969). This concept is that rising lavas will solidify at a depth below the hydrostatic head, because of contact with the cold walls of the conduit. Con- tinued occurrence of this will lead to an upward force on a portion of the oceanic crust wider than the median valley, and when this force becomes larger than the strength of the rock, reverse faulting will take place.

The models presented here are very simplified over that proposed by Osmaston, but emphasize the occurrence of reverse faulting necessary for a steady-state median valley, and also show how much movement apart is ac- companied by dyke injection, and how much by faulting. These models also do not attempt to explain why median valleys are formed in some cases and not in others. This variation is accepted as an observed fact, and the patterns of tectonics deduced from this variation are discussed.

THE MODELS

First let us assume that the median valley is a steady-state feature. Let us suppose also that emplacement of basaltic material takes place entirely as dykes which reach the sea floor in the median valley (Matthews and Bath, 1967; Harrison, 1968). It is then necessary, in order to keep the median valley at the same width, that there be a combination of normal faulting and reverse faulting. Fig. 1 shows schematically a slice of crustal material being slid upwards, to take its place at the boundary of the median valley, with normal faulting on one side and reverse faulting on the other. In reality, the two types of faulting might not take place at the same time, but might be accomplished by first_normal faulting on one side, followed by reverse fault- ing on the other side and so on until the slice had completed its upward move- ment and was emplaced at the edge of the median valley.

A second example of a median valley condition involves basaltic pxtrusives produced in the median valley (Watkins and Richardson, 1971,1972; Harrison, 1972). If a steady state exists, then in addition to the faulting of slices de- scribed in the previous paragraph, there must also be downward faulting of the central block, to keep the bottom of the median valley at a constant height. This is illustrated in Fig. 2. Downfaulting of this central block along normal faults is accompanied by horizontal movement, and it is possible to calculate how much movement is accompanied by dyke emplacement, and how much is associated with this faulting. If the depth of the median valley is 1 km and the total thickness of the crust away from the median valley is 5 km, then if a proportion, p, of crustal material is extruded, the ratio of the thickness of the lava flow to the dyke in Fig. 2 is given by:

6 z/6x = 4p/w (l-p)

where w is the width of the median valley. Faulting will produce a movement

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A- C D

A B.C C’ D' D.E F 0

I

I’ m

Fig. 1. Case 1 where a steady-state median valley exists with no extrusive flows. (a) shows the original condition, (b) shows the condition after a certain amount of crustal material has been intruded at the center. In order to maintain the median valley at a constant width, slices CC’ and DD’ have to be moved upwards, as shown in (c). The shaded region shows the intruded material. Dashed lines show faults.

apart of 262 cot 0, and therefore, the proportion of spreading accompanied by faulting is given by:

P = 262 cot e/(&X + 262 cot e )

The value of 19 can be estimated from fault-plane solutions of earthquakes occurring on mid-oceanic ridges away from fracture zones. The four earth- quakes caused by normal faulting analyzed by Sykes (1967) and Tobin and Sykes (1968) had an average dip of the steepest fault plane of 60”. For sim- plicity, we take 0 = 63”26’ for which cot 0 = 0.5. Then we have:

P = &?/(8x + 62) = 4p/[(l--p) w + 4p]

This is illustrated in Fig. 3, for various values of p. It can be seen that with a typical median valley width of lo-20 km and with moderate proportions of extrusive material, there is a significant proportion of spreading accom- panied by faulting. For angles of faulting higher than 8 = 63” 26’, the propor- tion of spreading accompanying faulting will decrease.

Bodvarsson and Walker (1964) have suggested that in Iceland, each dyke results in a lava flow which has approximately the same area of cross-section as the dyke, which is regarded as going down to a depth of 30 km. When we consider the cross-sectional area of the dyke above 5 km, this proportion would change so that each dyke would produce six times its area of cross- section of lava flow. This shows the great importance in understanding in which way the vertical build-up of lava flows can be transformed mto the horizontal movement classically associated with sea-floor spreading.

It is also possible to calculate the total thickness of lava flows accumulated on either side of the median valley. It is probable that the magnetic anomalies observed at the surface are largely caused by lava flows, because their more rapid cooling compared to dykes leads to greater intensity of magnetization.

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I

\ -t?- ’ 0

-“-

Fig. 2. Case 2 where a steady-state median valley exists with extrusive flows. The shaded material in (b) and (c) shows the new intrusive and extrusive material. Since the median valley is wider in (b) than in (a), two slices have to move upwards, as shown by the arrows in (b). Also, since the floor is now higher in (b) than in (a), the floor of the vplley has to sink downwards along normal faults. The final configuration is shown in (c). Dashed lines show faults.

1 0 IO 20 30 40 WIDTH OF MEDIAW VALLEY IN km

Jam

- 0.09

- 0.09

. O.lS

- 0.2

. 0.23

- 0.5

-I

Fig. 3. This shows the variation of the proportion, P, of spreading accompanied by fault- ing as a function of the width of the median valley. The values of p by the side of each line show the proportion of extrusive material produced above the floor of the original

median valley. The dashed lines are for case 2 (frig. 2), which is a steady-state median valley. The full lines are for case 3 (Fii. 4), which is a non-steady-state median valley.

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--d-

Fig. 4. Case 3, where the median valley is not in a steady state. It becomes filled with extrusives, as in (b), and then a new median valley forms by downfaulting along normal faults, as in (c). The dashed lines are faults.

The material within the median valley is spreading apart solely due to the injection of dykes. The number of lava flows of thickness 6z accumulated at any point as this point moves from the center to the edge of the median valley is thus:

w/6x

The thickness is:

w&z/6x = 4p/(l-p) km

Therefore, for a thickness of 1.5 km, representing layer 2, the value of p becomes 3/11. Since some estimates of the magnetization layer 2A are con- siderably less than 1.5 km (Talwani et al., 1971), it appears that a rather small percentage of crustal material is actually extruded; these results are in contrast to those obtained from Iceland.

The third case is one in which the median valley is not a steady-state phenomenon. It gets filled up with extrusive and intrusive material and then is produced again by normal faulting. The process is illustrated in Fig. 4. If the proportion of material produced above A A ’ is p, then when the valley has been completely filled up, we have:

(w + d + 1/2)/4d =p/(l -p) for cot 8 = l/2

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Fig. 5. Case 5, where no median valley exists but extrusives require normal faulting to maintain a steady state.

This gives :

d = (w + l/2) (1 --p)/(5p - 1)

Faulting occurs, which produces a movement apart of 2 cot B or 1 if cot 0 = l/2. Therefore, the proportion of spreading due to faulting is given by:

P = l/(1 + d) = (5p - l)/f(5p - 1) + fw + l/2) (1 --p)]

This is also shown in Fig. 3. Again we can see the importance of faulting on movement or spreading.

Aumento (1972) has s ted that there may be a period&Sty in processes occurring at the Mid-Atlantic Ridge at 45”N, a stage of emplacement of lopoliths being followed by a quiescent stage when differentiation takes place, to be finally followed by a phase of major fracturing and faulting. This model of Aumento might find its counterpart in this model of periodic production and filling in of the median valley.

The last two cases to consider are ones in which no median valley is present. If the ridge is smooth, these must be steady-state systems. In case four, where no lava flows are produced, a simple dyke i-njection model is accompanied by movement involve no f*&mg. fn case five, where lava flows are prtiuced on the surface, the central part mu& be ~o~faul~ in order to avoid a build- ing up of the ridge crest (Pig. 5). In this case, again, movement apart aucom- panies the normal faulting. A r;ditculation of the relative amount of movement accompanied by faulting can he done in the same way as before:

W&z/56X = p/(1 -p)

where w is now the distance to which eaoh lava flows on either side of the central position. The pro-portion of spreadhrg accompanied by faulting is given by:

P = 262 cot 0 /(Sx + 2sz cot 0) = sz/(ax + 6z)

Fig. 6. A series of six dykes and lava flows produced at the axis of a ridge. The shading decreases with increasing age. The most recent lava flow has not yet been downfaulted, but the position of the fault is marked with a dashed line at an angle 0 to the horizontal. If all these six dykes and lava flows were extruded in a normal field and the ones before extruded in a reversed field, then the line at $J to the horizontal shows the approximate boundary between normal and reversed material.

for cot 0 = 0.5,

:. P = 5p/[5p + w (1 -p)]

The thickness of lava flows in this model can also be calculated. This is just:

T = w6z/6x = 5p/(l -p)

and for a figure of 0.5 km, p = l/11. It is also possible to calculate the shape of the boundary between normally

and reversely magnetized material. This boundary slopes downwards towards the spreading center from the surface at an angle $ (Fig. 6) where:

tan @ = Sz/(Sx + 262 cot 0) =&z/(&x + 62) = T/(T + w)

The boundary between normal and reversed magnetization has a horizontal extent of T cot # = T + w. For T = 0.5 km and w = 2 km, the figure is then 2 l/2 km. For a fault angle of 30” (as suggested by Atwater and Mudie, 1968), the boundary becomes 2T cot 30” + w or 3.7 km. Although this model gives a much simpler relationship for the width of the boundary zone between oppo- site polarities than do the models of Atwater and Mudie (1973), it does show that this width is dependent directly on the total thickness of flow material, because of the accompanying faulting.

DISCUSSION

We have shown above the importance of faulting in the development of median valleys, and in essentially transforming the vertical build up of the ridge crest by lava flows into the horizontal motions of sea-floor spreading. Other people have discussed the importance of faulting in producing hori- zontal movements, such as Cann (1968). He shows how a set of conjugate normal faults can produce extension of the crust and at the same time expose

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deep-seated layers of the crust. But his scheme is not able to explain the pres- ence of a median valley as a time-independent feature, Piper and Gibson (1972) have also discussed the tectonics and accretion of new material at ridge crests, but again, their model does not explain how a steady-state me- dian valley could exist. However, the simple models discussed in this paper could be developed further using their patterns of stress produced by magma chambers under pressure.

Another conclusion of this paper is that if a steady-state median valley is to exist, then there must be reverse faulting occurring at the ridge crest. This has not been found in the first-motion studies of Sykes (1967) or Tobin and Sykes (1968). There has been a reverse fault discovered by first-motion studies (Udias and Lopez Arroyo, 1972), but this was several hundred kilometers from the axis of the Mid-Atlantic Ridge and so is not relevant. The absence of reverse faulting from the first-motion studies of earthquakes could mean that the median valley is not a steady-state feature. This might seem a satis- factory solution to the problem until we realize that if the median valley is not a steady-state solution, we would expect at least occasionally to find no median valley at the Mid-Atlantic Ridge crest. If the walls of the median valley spread apart so that they were several hundred kilometers apart, we would probably not recognize them as being a median valley, and so with spreading rates of several cm/year, renewal of the median valley by normal faulting would have to occur at least as often as once every ten million years in the Atlantic for a median valley to appear at every crossing of the ridge. It seems unlikely that a process which has been going on for over 100 million years could have kept in phase over the whole length of the Mid-Atlantic Ridge over a time involving ten cycles of renewal. Therefore, we must con- clude that reverse faulting does occur but that it is unaccompanied by earth- quakes big enough to allow first-motion studies to be made. This second alternative is quite possible as the crust is probably in tension at the position of the median valley (Deffeyes, 1970; Anderson and Noltimier, 1973). Tension usually produces normal faulting, but in this case, the reverse fault- ing is caused by the crustal material trying to get to its level of isostatic com- pensation. It is essentially being lifted up, and this process may decouple the material involved in the movement from the material below, thus inhibiting the production of earthquakes.

Comparisons may be made between features on the mid-oceanic ridges, and places on the continents where extension and crustal spreading is taking place. One such continental area is between Goubet el Kharab and Lake Asal in Territoire franqais des Afars et des Issas (formerly called French Somali- land). Here the Indian Ocean Ridge runs into the Gulf of Tadjoura and then curves towards the northwest and runs into the main part of the Afar Depres- sion. The area between Goubet el Kharab and Lake Asal is a site of recent vulcanism (lava flows only a few hundred years old exist in this region) and there is much evidence of extensional faulting. However, most of the surficial expressions of faulting in the central region show more or less vertical faults.

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Open vertical fissures with no throw are also prevalent. It appears that in such cases, there would not be great earthquake activity, as the movement is taking place along fault planes whose sides are only loosely coupled to each other. It is therefore possible that much of the vertical movement in extensional areas such as the mid-oceanic ridges may take place without significant earth- quake activity, thus explaining the absence of first-motion studies suggesting reverse faulting.

Deffeyes (1970) has produced an ingenious model of the mid-oceanic ridge median valley, and also came to the conclusion that if the median valley were in a steady-state situation there would be a necessity for reverse faulting, in order that the width of the median valley should remain the same. It is diffi- cult to see how viscous decay (Cramer, 1970) could remove some of the relief of the median valley, as suggested by Deffeyes; the surface rock at least would have to undergo relative movement, whatever the underlying cause of this movement might happen to be. It should also be pointed out that DeffeJpes’s model of the median valley goes no further than the models presented here, in actually explaining why there would be a median valley. His model depends on Gaussian functions for the strain rate, and for the rate of thickening of the crust, as functions of distance from the axis of the ridge. If the standard devia- tion of the strain rate is less than the standard deviation for the crustal supply rate, then, as he rightly points out, there has to be a thickening of the crust away from the axis. But this thickening could take place just as well by the lowering of the bottom surface of the crust away from the axis, as by a raising of the upper surface. In fact, he has both processes occurring in one of his dia- grams. What is necessary is to compute theoretical gravity profiles across such models as have been produced by Deffeyes (1970) in order to see whether the presence of a median valley is necessary to compensate for the very shallow Moho, with account being taken of the thermal conditions of the crust and mantle.

ACKNOWLEDGEMENTS

I thank M.M. Ball and E. Bonatti for helpful comments, and G. Ma~nelli, the leader of the expedition to the Afar Depression in which I participated. This research was supported by the Office of Naval Research contract No. NOOOl4-67-A-0201-0013, and NSF grants GA-26263 and GA-25710. Con- tribution from the University of Miami, Rosenstiel School of Marine and Atmospheric Science, Miami, Florida 33149.

REFERENCES

Anderson, R.N. and Noltimier, H.C., 1973. A model for the horst and graben structure of mid-ocean ridge crests based upon spreading velocity and basalt delivery to the oceanic crust. Geophys. J. R. Astron. Sot., 34: 137.

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Atwater, T. and Mudie, J.D., 1973. Detailed near-bottom geophysical study of the Gorda Rise, J. Geophys. Res., 78: 8665.

Aumento, F., 1972. The oceanic crust of the Mid-Atlantic Ridge at 45” N. Publ. Earth Physics Branch, Dep. Energy, Mines Resources, 42: 49.

Bodvarsson, G., and Walker, G.P.L., 1964. Crustal drift in Iceland, Geophys. J. R. Astron. Sot., 8: 285.

Cann, J.R., 1968. Geological processes at mid-ocean ridge crests. Geophys. J. R. Astron. sot., 15: 331.

Cramer, C.H., 1970. Viscosity of the Atlantic Ocean bottom. Science, 167: 1123. Deffeyes, K.S., 1970. The axial valley: a steady-state feature of the terrain. In: H. Johnson

and B.L. Smith (Editors), The Megatectonics of Continents and Oceans. Rutgers Uni- versity, New Brunswick, N.J., p. 194.

Harrison, C.G.A., 1968. Formation of magnetic anomaly patterns by dyke injection. J. Geo- phys. Res., 73: 2137.

Harrison, C.G.A., 1972. Comments on a paper by Watkins and Richardson: “Intrusives, extrusives and linear magnetic anomalies”. Geophys. J. R. Astron. Sot., 28: 187.

Matthews, D.H. and Bath, J., 1967. Formation of magnetic anomaly pattern of Mid-At- lantic Ridge. Geophys. J. R. Astron. Sot., 13: 349.

Mudie, J.D. and Atwater, T., 1968. Block faulting on the Gorda Rise. Science, 159: 729. Osmaston, M.F., 1971. Genesis of ocean ridge median valleys and continental rift valleys.

Tectonophysics, 11: 387. Piper, J.D.A. and Gibson, IL., 1972. Stress control of processes at extensional plate

margins. Nature, Phys. Sci., 238: 83. Sleep, N.H., 1969. Sensitivity of heat flow and gravity to the mechanism of sea-floor

spreading. J. Geophys. Res., 74: 542. Sykes, L.R., 1967. Mechanism of earthquakes and nature of faulting on the mid-ocean

ridges. J. Geophys. Res., 72: 2131. Talwani, M., Windisch, C.C. and Langseth, M.G., Jr., 1971. Reykjanes Ridge Crest: a

detailed geophysical study. J. Geophys. Res., 76: 473. Tobin, D.G. and Sykes, L.R., 1968. Seismicity and tectonics of the Northwest Pacific

Ocean. J. Geophys. Res., 73: 3821. Udias, A. and Lopez Arroyo, A., 1972. Plate tectonics and the Azores-Gibraltar region.

Nature, 237: 67. Watkins, N.D. and Richardson, A., 1971. Intrusive% extrusives and linear magnetic

anomalies. Geophys. J. R. Astron. Sot., 23: 1. Watkins, N.D. and Richardson, A., 1972. Reply to a paper by C.G.A. Harrison “Comments

on a paper by Watkins and Richardson ‘Intrusives, extrusives and linear magnetic anom- alies’ “. Geophys. J. R. Astron. Sot., 28: 191.