140
120
~ 100
~
~ ~
80e: ~
20
0
7.3 MM/YR ...
tendency for the east flank to spread faster than the "est during the last several million years. This effect appears real, despite the difficulties involved in measuring spreading rates to the necessary accuracy over a 51m-.' spreading ridge. It also appears that over longer periods, tens of millions of years, the asyrrmetry has averaged out
to produce a general symmetry.
Asymmetric spreading occurs at many locations in the Pacific. Kli tgord ~ al. [1975] describe symmetric spreading on the Cocos-
Pacific boundary at 210 N. Rea [1977) shm.,s slight asymmetry at 9-12°S, on the East Pacific rise; 80 mm/yr to the "est and 77 mm/yr to the east. Herron's [1971) t,w profiles further south (19°S, 28°S) show the Nazca plate to be spreading faster than the Pacific plate over the past five million years. At 3loS Rea [1977) finds average east and west flank rates of 86 and 77 mm/yr, respectively, since 2.41 m.y. ago. The Costa Rican rift at 83°\, spreads faster to the south (Nazca) side than to the north [Klitgord et aI. , 1975]. Further wes t (86 OW) the Galapagos ridge shows the reverse direction out to Jaramillo time [Ditgord ~ aI., 1975]. lIey [1977] and Hey ~ a1. [1977] also find the north flank spreading faster near the Galapagos (99°W-93°1-l), and the south flank faster at 83°\,-84°11 on the Costa Rican rift. The Chile ridge sho"s symmetric spreading [Klitgord ~ al. , 1973).
The profiles of Holnar ~~. [1975) in the South Pacific shm; the Antarctic side (out to anomaly 2 or 2 ') to be spreading faster in almost all cases over a range from 110°\, to 180°\, and 35°S to 65°S. Some of these profiles are shown in Figure 2.6. {Neasuring only out to 2 or 2' avoids confusion with ridge jumps which appear on the
50 5b
EL 24 1050
2
OC.7008 100°
7
EL 20 1050
9
CONRAD 12-12 lOGo
1 7
~
EL 23 112·
20
Figure 2.6. Nagnetic anomalies and bathymetry across the East Pacific Rise, shm.;ing asymmetric spreading with the east (right) side fast. Ridge jumps appear as "missing" and "extra" anomalies, fracture zones as dashed lines.
same profiles.) This asymmetry was noted by Herron [1971]. Falconer [1972] reports symmetric spreading at 6loS, l6l oE on the India-
Antarctica boundary. South of Australia, Heissel and Hayes [1971] show one profile ',ith the Indian side fast, and another with the reverse out to seventeen mil lion years. (Prior to seventeen million years
both profiles had the north side fast.) Further west, at 400S, 80°\\, SchUch and Patriat [1971] (profile GAl-6, anomaly 1-2) shot's Antarctic
spreading faster than India.
It appears, then, that asymmetric spreading is a common worldwide phenomenon. Frequently the direction of asymmetry over the last
several million years is the same for substantial distances along the ridge (for example, the mid-Atlantic and Pacific-Antarctic ridges).
ANALYSIS
The majority of these measurements are consistent with the pre- diction of this model: the flank trailing the migrating ridge spreads faster. Only a small fraction of the data show asymmetries opposite those predicted. Frequently the spreading is symmetric, but this model predicts a preferred direction of asymmetric spreading rather than its certain occurrence.
The most convincing evidence for a fundamental process of
asymmetric spreading is the data [Dickson et a1.,
- -
1968; Van Andel and Heath, 1970; Loomis and Horgan, 1973] in the South Atlantic and inthe South Pacific [nolnar et a1., 1975; Herron, 1971]. The consistency of the direction of asymmetry over thousands of kilometers argues
against its being a purely local phenomenon.
On the other hand, the magnitude of the asymmetry varies
considerably. t-leasurements (central anomaly to 2 or 2' , or any more sophisticated scher.1e) on the South Pacific profiles yield asyrmnetries varying bet"een 5 and 25 percent. Some of this variation may be due to noise in the data, but it still appears that the magnitude of the asymmetry is being controlled by local effects. Hany processes at the ridge, unrelated to ridge migration, can influence the pattern of spreading. These include structural control of intrusion patterns, the shape and cooling history of magma chambers, and local irregularities in mantle flow.
The full complexity of the process of asyrmnetric spreading can be seen in detailed studies of the FAl'IOUS area [Macdonald, 1977; Hacdonald and Luyendyk, 1977]. Two adjacent ridge segments approximately
fifty ki lometers apart have very different asymmetries. The
direction and magnitude of asymmetry changes quite rapidly (in less than .15 m.y.). The spreading is qui te oblique (17°) . At least in this area, asymmetric spreading occurs on a very fine scale, with no
evidence of discrete ridge jumps of more than several hundred meters. Clearly no simple model of the dynamics of a ridge can adequately describe these local effects. Thus it ",ould be surprising to find a simple relation between migration velocity and asymmetry. It would not be surprising to find occasional sites where local effects counteract or even reverse the effects of ridge migration.
Another possible problem is the assumption that the ridge is migrating much faster than any motion in the underlying mantle. If
this assumption is violated then the migration vector "ill not predict