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Spreading center jumps and sub-axial asthenosphere flow near the Galapagos hotspot
Tectonophysics, 37 (1977) 41-52 @ Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
41
SPREADING CENTER JUMPS AND SUB-AXIAL ASTHENOSPHERE FLOW NEAR THE GALAPAGOS HOTSPOT *
RICHARD
HEY and PETER
VOGT **
Hawaii Institute of Geophysics, University of I-law&i, Honolulu, Hawaii 96822 ~Ha~ui~~ U.S. Naval Oceanographic Office, Chesapeake Beach, Maryland 20732 (U.S.A.) (Received June 17, 1976)
ABSTRACT Hey, R. and Vogt, P., 1977. Spreading center jumps and sub-axial asthenosphere flow near the Galapagos hotspot. In: S. Uyeda (editor), Subduction Zones, Mid-Ocean Ridges, Oceanic Trenches and Geodynamics. Tectonophysics, 37 (l-3): 41-52. We have identified a sequence of small rise-axis jumps on the Cocos-Nazca spreading center between 93O and 95.5’W. The locus of jumps has migrated 150 km west along the rise axis, away from the Galapagos Islands, during the last three million years, at an average rate of SO mm/year. The linear increase in jump distance during this sequence of jumps has resulted in a change in regional azimuth of the rise axis from about 085’ to 095O. We visualize this sequence of jumps as a new rift propagating through the Cocos plate, forming a new Cocos-Nazca spreading center. The region affected by these rise jumps appears to correlate with an area of exceptionally high-amplitude magnetic anomalies. The high-amplitude region seems to result from Fe-T&rich (FeTi) basalts of high remanent magnetization. We speculate that the development of the new accretion axis and concomitant rise jumps are related to the flow of FeTi basalt-producing asthenosphere away from the Galapagos hotspot. The snout of anomalous asthenosphere has remained nearly stationary, with respect to the Galapagos hotspot, during the last 3 m.y. A northwestward component of flow, reflecting the southward position of the plume center with respect to the spreading axis, might explain why the new spreading center is developing along a more northwesterly azimuth. The rise jumps have resulted in the sort of pattern of asymmetric accretion which is required to substantiate the hotspot hypothesis for the origin of the Cocos and Carnegie ridges. Several other puzzling platetectonic phenomena may be explained by the propagating rift model developed here.
One of the basic tenets of the rigid-plate hypothesis as originally formulated is that mid-ocean rises spread symmetrically. This approximation, although not demanded by the rigid-plate hypothesis, is constantly assumed * Hawaii Institute of Geophysics Contribution No. 784. ** Now with the Naval Research Laboratory, Washington, D,C, 20375.
42
_____
___ _. ______ _ _______
-
43
3 yyhp
- 92”
w
930
w
3 I‘YbP
?
-
Model 27 mm/yr 4: N
3: N
PdN
,- 940 w lo N
B Fig. 1. A. Schematic location map modified from Hey et al. (1976). B. Observed magnetic anomalies plotted perpendicular to track, positive (reversed polarity) toward east. Isochrons according to time scale of Talwani et al. (1971). Synthetic model parameters same as in Fig. 3.
in the literature, and has proved to be so successful throughout the world ocean that exceptions to the rule are of great interest (e.g. Weissel and Hayes, 1971). In the Galapagos region, Hey et al. (1976) have shown that an asymmetric accretion model is necessary to resolve the geometric objections to the hotspot hypothesis as to the origin of the Cocos and Carnegie aseismic ridges. They have found evidence in the magnetic anomalies for such asymmetric accretion, which is caused at least in part by small discrete jumps of segments of the rise axis. Their interpretation is supported by recent work by Anderson et al. (1976). A detailed dredging program reported by Vogt et al. (1975) has shown that the exceptionally strong magnetic anomalies, more or less symmetrically distributed about the Galapagos Islands (Vogt and Johnson, 1973; Anderson et al., 1975), are correlated with FeTienriched basalts of high remanent magnetization (Byerly et al., 1976; Vogt and De Boer, in preparation). Vogt et al. (1975) proposed a speculative model in which the partial melts producing these FeTi basalts have flowed eastward and westward from a center of upwelling presently under the western Galapagos Islands. The flow is channeled under and along the rise axis, at rates on the order of several centimeters per year. Progressive fractional crystallization and mixing of these melts complicate the geochemical consequences of this model. Vogt et al. futher speculated that the spread of the FeTi zone was associated with riseaxis jumps. In this paper we investigate in detail one sequence of small riseaxis jumps between 93” and 96” W along the Cocos-Nazca spreading center.
44 OBSERVATIONS
A detailed geophysical survey was conducted in 1974 to chart the extent of the Galapagos high-amplitude magnetic anomaly zone, and to correlate this information with other geological and geophysical parameters. The profiles between 92” and 96”W from this and earlier cruises are shown in Figs. 1 and 2. Under the assumption of symmetric accretion the length of a rise-rise transform fault (not extending into a triple junction) does not change with time (Wilson, 1965). Thus if our magnetic anomaly correlations in Fig. 1 are correct, accretion cannot have been symmetric on this segment of the spreading center. Note that whereas the 3 m.y.b.p. isochrons are continuous, the rise axes are presently offset by 25 km. This observation can be explained either by a jump of the rise axis or by asymmetric spreading; in either case the result has been asymmetric accretion and the birth of a riserise transform fault within the last 3 m.y. Figure 2 shows the anomalies created during the last 4 m.y. along one segment of the Cocos-Nazca spreading center. Note the change in anomaly trend which occurs along the rise axis. Another conspicuous along-strike change is the dramatic westward drop in anomaly amplitude along the rise (compare, for example, the amplitudes of the axial anomalies on the profiles at 94” and 96” W) which appears to be related to the change in anomaly
96O W Fig. 2. Observed magnetic anomalies Isochrons at 1 m.y. intervals
plotted
perpendicular
to track,
out to 4 m.y.b.p.
45
trend. Scattered profiles across this change in trend could have been interpreted as resulting from a zig-zag rise axis such as that found by Johnson and Vogt (1973) in the North Atlantic. Here we will propose an alternative explanation. The axial anomaly can be easily traced west from the top of Fig. 2 to 95.1”W, where the anomaly width begins to decrease. The narrowing axial anomaly can be traced farther west, to 95.35”W, where the anomaly is much narrower than demanded by simple accretion at 30 mm/year (half-rate) throughout Brunhes time. Three additional profiles between 95.1” and 95.42”W, which are too closely spaced to be presented here, strongly support this correlation (see Vogt et al., in preparation). Similarly, the axial anomaly can be identified at 2.3” N, 96” W and followed east to 95.65” W. Note that on this segment of the axis the anomaly amplitude is much lower than on the segment to the east. Only a portion of the axial anomaly can be traced east to 95.35” W, and it has a low amplitude. Farther north on the same profile (which is parallel to the spreading direction) another part of the axial anomaly, with relatively high amplitude, is observed. These observations suggest to us that a new spreading center is breaking through the old anomaly pattern. Investigation of the older anomalies supports this interpretation and indicates that the high-amplitude magnetic anomalies are associated with the new spreading center. The 93” W profile matches the reversal time scale for an average spreading half-rate of about 27 mm/year (Fig. 1). On the Cocos plate the Kaena and Mammoth events (3 m.y.b.p., see Fig. 3) within the Gauss normal epoch are centered at about 3.2” N on this profile (Fig. 2). At 94” W these anomalies are too narrow; at 94.15” W they are yet narrower; and at 94.4”W they have disappeared. The Gauss normal epoch here is thus far too narrow. In contrast, on the Nazca plate the Kaena and Mammoth events and the Gauss normal epoch have normal width at 93” W; they are too wide on the profiles at 94” and 94.15”W; the Kaena and Mammoth events are twice their normal width at 94.4” W, at the same longitude where they disappear on the Cocos plate. Farther west, at 94.95” W, the Kaena and Mammoth events and the Gauss normal epoch have normal widths (there is a slight decrease in anomaly width from east to west because the recent Cocos-Nazca relative rotation pole is located near 2”N, 126” W, Hey et al., 1976). The amplitudes of these anomalies are much less than they were farther east (near 93” W in Fig 2, for example). However, on this profile part of lower Matuyama reversed epoch seems to be missing on the Cocos plate, whereas this anomaly on the Nazca plate is clearly wider than on the reference profile at 93” W. Farther west, at 95.1” W, part of the upper Matuyama reversed epoch seems to be missing on the Cocos plate whereas on the Nazca plate this anomaly is too wide. (Note that the width of the high-amplitude anomaly zone is also decreasing to the west.) This pattern of ever younger crust missing on the Cocos plate and repeated
46
on the Nazca plate can be followed from east to west along the CocosNazca spreading center as far as the 95.35” W profile, where we observe part of the axial anomaly with very high amplitude at 2.65”N, and part with relatively low amplitude at 2.3”N, as previously discussed. INTERPRETATION
We interpret the observed pattern of magnetic anomalies here to be the result of a sequence of rise-axis jumps which began about 3 m.y.b.p. near 94”W and reached 95.35”W about 0.3 m.y.b.p. This sequence was monotonic; all axis jumps were to the north, and thereby produced only leftlateral offsets in the spreading axis. Figure 3 shows our interpretation of the 94.15”W profile. This was one of the first and smallest jumps in this particular sequence (the next profile farther east may show a slightly older jump). The 94.15” W jump is revealed by the unequal widths of the Kaena and Mammoth events on the observed profile; these anomalies are too narrow on the Cocos plate and too wide on the Nazca plate. To model this profile we have therefore removed a small piece of crust from the Cocos plate and added it to the Nazca plate, corresponding to a discrete 5 km northward shift of the rise axis at 2.9 m.y.b.p. That portion of the anomaly sequence caused by this segment of crust is shown as a dotted line. The shapes of the surrounding anomalies on this S
N IOOOnT I
Observed
Synthetic aawmlnq at 2.9
v-&
Synthetic I
1-i
M
/
maqmtic anomalies,94.15”
lwqnotic a jump mybp
magnetic
W
onmnaller wlwlated cd 5 kltometwr north
anom(~IIes
wlthwt
jump
I
B
/
M
Fig. 3. Correlation of synthetic anomalies with the observed profile at 94.15’W, assuming a 5 km northward jump of the rise axis at 2.9 m.y.b.p. Pattern expected without jump is shown for comparison. Postulated jump would have shifted portion of block model under bar, corresponding to dotted line portion of anomalies, from Cocos to Nazca plate. Synthetic anomalies were generated using the Talwani et al. (1971) time scale and a basement layer 2.8 km deep and 1.7 km thick; topographic effects are assumed negligible as the basement is quite smooth. Effective susceptibility (ratio of the remanent magnetism intensity to the strength of the present magnetic field) is 0.012. Model spreading halfrate is 30 mm/year from 0 to 0.89 m.y.b.p. and 27 mm/year from 0.89 to 4 m.y.b.p. B, M, G are Brunhes normal, Matuyama reversed and Gauss normal epochs. h and m are Kaena and Mammoth events in Gauss normal.
47
OOOnT
I
Observed
magnetic
anomalies.
95.35O W
Synthetic assuming
magnetic anomalles calculated o Jump of 27 kilometers north
m
+ r$-&.
Ot “1
o.3
mybp
_PqfJ-L Synthetic
magnetic
anomalies
without
jump
-
N
S
Fig. 4. Correlation of synthetic anomalies with the observed profile at 95.35OW, assuming a 2’7 km northward jump of the rise axis at 0.3 m.y.b.p. Symbols and model parameters same as Fig. 3, but with amplitude of synthetic anomalies 75% of those in Fig. 3, further demonstrating the decrease in anomaly amplitude to the east. Model spreading half-rates are 29 mm/year (O-O.89 m.y.b.p.) and 24 mm/year (0.89-4 m.y.b.p.). A is the spreading axis.
model profile are changed slightly by the new edge effects resulting from the changed geometry of normal and reverse polarity crust. For comparison we show the theoretical anomaly sequence expected if accretion had been symmetric. Figure 4 shows our interpretation of the 95.35”W profile. We think this is perhaps the most recent and largest jump in this sequence. The jump occurred at 0.3 m.y.b.p., within the Brunhes normal epoch. (The next profile farther west may show a slightly younger and larger jump, but we are uncertain of this interpretation.) As in Fig. 3, the dotted portion of the synthetic profile results from crust transferred from the Cocos to the Nazca plate by a 27 km northward axis jump at 0.3 m.y.b.p. If our interpretation of these anomalies is correct, two parts of the axial anomaly are now separated by older, reversely polarized crust. This pattern could only have been produced by a discrete shift of
30
0 . l
.
0
.
.
96OW Distance
95ow along dse axis
94DW
r-
I
96OW Distance
95”W along rise axis
94”W
Fig. 5. Time of jump plotted against distance along axis for seven profiles containing postulated jumps. Closed circles are confident and open circles less certain interpretations. Slope of line is 50 mm/year. Fig. 6. Distance of jump vs. distance along axis. Symbols same as Fig. 5.
DISCUSSION
Our physical model, independent of frames of reference, is that a new rift is gradually propagating through the Cocos plate, i.e. a new Cocos-Nazca accreting plate boundary is being born as the old one is becoming extinct. At any time the farthest westward extension of this new rift is connected to the preexisting rise axis by a transform fault; as the rift breaks farther, therefore, young extinct fracture zones will be added to the Nazca plate just as extinct rise segments are. Shih and Molnar (1975) have postulated a similar model and sequence of rise jumps between 35 and 20 m.y.b.p. along the Juan de Fuca (Pacific-Farallon) spreading center. Unfortunately, their conclusions (with which we generally concur) are necessarily limited by the subduction of the eastern half of the spreading record. Considering our model of the Cocos-Nazca spreading center in the frame of reference of the triple junction leads to some interesting consequences. If the spreading-center jump sequence should migrate west relative to the Cocos-Nazca rise axis faster than the triple junction, it would eventually reach the triple junction. At that time the triple junction would shift abruptly, isolating an old rise-axis segment, and triple junction, on the Nazca plate. Perhaps the most interesting frame of reference in which to consider these observations is that of the Galapagos hotspot. The analyses of Hey et al. (1976) and Hey (1975) strongly suggest that the Cocos and Carnegie aseismic ridges were formed by plate motions over a Galapagos hotspot. They
49
have further shown that the geometric objections (Sclater and Klitgord, 1973) to the hotspot hypothesis as an explanation for these aseismic ridges can be resolved by the sort of asymmetric accretion discussed here. According to the model of Hey et al. (1976), during the last 3 m.y. the Cocos and Nazca plates both have had eastward components of motion of about 50 mm/year relative to the Galapagos hotspot. (The identical east-west motion of the two plates is demanded by the north-south trend of transform faults along the rise axis). Relative to a frame of reference fixed with respect to the active Cocos-Nazca rise axis, which of course has the same eastward component as the plates, this sequence of rise jumps has migrated 150 km west down the axis at an average rate of 50 mm/year during the last 3 m.y. Therefore the point in space at which these jumps occur has not moved east or west appreciably relative to the hotspot during the last 3 m.y. The same is true of the boundary between high and normal magnetic-anomaly amplitudes (Vogt and De Boer, in preparation). Vogt and Johnson (1975) suggested that many transform faults have affected longitudinal flow of sublithospheric partial melts along various rise axes; they speculated that the 95.5”W transform fault may have partly blocked the flow of FeTi basalt-producing partial melts away from the Galapagos. A modified explanation for this flow ending at a transform fault is that the rise jumps are associated with the advance of these partial melts; in this case the propagating rift must always be bounded by a transform fault. The flow of melts against an existing transform fault may somehow impede the eastward migration of the fault over the asthenosphere, as described by our relative motion model. The relative westward push of the FeTiproducing melts would perhaps tend to heat, weaken and erode the lithosphere across the transform fault, and thus cause the observed breaking and concomitant rise axis jumps (see fig. 23 of Vogt and Johnson, 1975). After a new section of rise axis develops, the FeTi-producing partial melts rapidly occupy the melt zone and then are dammed once again, building up until the next “break-through”. If these speculations are correct, the sequence of axis jumps is caused by the eastward motion of the spreading axis across the westward flowing partial melts from the Galapagos hotspot. Schilling et al. (1976) have shown that ambiguities exist in the geochemical origin of the observed magnetic-anomaly amplitude variations. Our interpretation is therefore speculative, although plausible. Whatever the mechanism, it seems that the influence of the hotspot on active plate-boundary configurations may extend to more than 500 km from the locus of active hotspot volcanism. CONCLUSIONS
We have identified a along the Cocos-Nazca km westward along the as resulting from a new
sequence of rise-axis jumps between 93” and 96” W spreading center. The locus of jumps migrated 150 axis during the last 3 m.y. We interpret this sequence rift breaking through the Cocos plate; as a result of
50
the systematic axis jumps the eastward moving Cocos-Nazca spreading center is changing its azimuth from about 085” to about 095” as it passes 95.5” W. If short segments of the rise axis jumped together, our data imply that the average segment length was less than 25 km, and in some cases as short as 15 km (Figs. 2,5 and 6). They cuuld, of course, have been much shorter; our profiles are not spaced closely enough to place lower bounds on the lengths. The pattern of recent rise jumps appears to be intimately related to the pattern of exce~tion~ly strong magnetic anomalies which have been correlated with dredged FeTi basalts. We interpret the pattern of rise jumps to be related to the westward advance of asthenospheric partial melts flowing away from a center of upwelling - the Galapagos hotspot - presently located under the western Galapagos Islands. The successive axis jumps all lie near the boundary between high and normal anomaly amplitudes (Fig. 2). If these obse~ations are related, there should be no more sequences of young axis jumps beyond the western limit of the FeTi province at about 95.5”W. We expect that this model may explain many of the triple-junction shifts which have been postulated in the past, particularly those in the Galapagos area (Hey, 1975). In particular, a triple-junction jump resulting from a propagating rift intersecting the old triple junction may explain some of the confusion over the present nature of the Pacific~ocos-Nazca triple junction (K. Klitgord, personal communication, 1975) and why some rough bathymetry and strong magnetic anomalies occur (I). Rea, personal communication, 1973) outside the recently formed portion of the southern rough/smooth boundary found near the triple junction by Hey et al. (1972). It is likely that the change in rise azimuth associated with this jump sequence, and with other jumps identified east of the Galapagos Islands (Hey, 1975; Anderson et al., 1976) which may also have been progressive, probably accounts for the confusion regarding the recent spreading direction in this area. One interesting implication of the propagating rift model is that overall fracture-bode trends based solely on rna~~tic-Mornay offsets may be seriously in error. This is because progressively younger transform faults bound a new rift as it propagates. Thus a series of en-echelon fracture zones, each individual fracture zone parallel to the spreading direction, is frozen into progressively younger crust. Ongoing sea-floor spreading thus produces an apparent overall trend of offset anomalies different from the true spreading direction. This me~h~ism may be responsible for the seemingly incompatible fault trends in the Juan de Fuca area (see for example Vine, 1968). A similar process may explain the complexities in plate configuration near Easter Island, where small independent plates have previously been postulated (Elerron, 1972; Anderson et al., 1974). A propagating rift model may also explain the observation (e.g. Vogt et al., 1970; Hayes and Rabinowitz, 1975) that the boundary of the magnetic smooth zone along the east coast of North America is almost, but not quite, an isochron. Under this model the boundary could not be an isochron, and
51
the extent to which its trend diverged from the oldest isochrons would be a measure of the velocity of oropagation of the new rift. The asymmetric location of the Cocos-Nazca spreading center between the Cocos and Carnegie ridges may also reflect systematic southward jumps of the spreading center (Hey, 1975; Anderson et al., 1976). Such jumps, similar to those documented in this paper, would result in the net asymmetric accretion required by the hotspot explanation for the origin of the Cocos and Carnegie ridges. ACKNOWLEDGMENTS
We thank Don Hussong, Ralph Moberly, George Sutton and George Woollard for comments, and Steve Dang, Carol Yasui, Kathy Lee and Elsie Wipperman for assistance. Supported by the International Decade of Ocean Exploration, National Science Foundation Grant ID 071-04207-A04. R.H. particularly appreciates travel support which enabled the original presentation of this material at the 1975 Grenoble IUGG, where many of these ideas were put into perspective.
REFERENCES Anderson, R.N., Forsyth, D.W., Molnar, P. and Mammerickx, J., 1974. Fault-plane solutions of earthquakes on the Nazca plate boundaries and the Easter Island plate. Earth Planet. Sci. Lett., 24: 188-202. Anderson, R.N., Clague, D.A., Klitgord, K.D., Marshall, M. and Nishimori, R.K., 1975. Magnetic and petrologic variations along the Galapagos spreading center and their relation to the Galapagos melting anomaly. Geol. Sot. Am. Bull., 86: 683-694. Anderson, R.N., Moore, G., Schilt, S., Cardwell, R., Trehu, A. and Vacquier, V., 1976. Heat flow near a fossil ridge on the north flank of the Galapagos spreading center. J. Geophys. Res., 81: 1828-1838. Byerly, G.R., Melson, W.G. and Vogt, P.R., 1976. Rhyodacites, andesites, ferro-basalts and ocean tholeiites from the Galapagos spreading center. Earth Planet. Sci. Lett., 30 : 215-221. Hayes, D.E. and Rabinowitz, P.D., 1975. Mesozoic magnetic lineations and magnetic quiet zone of the eastern North Atlantic. Trans. Am. Geophys. Union, 56: 450. Herron, E.M., 1972. Two small crustal plates in the South Pacific near Easter Island. Nature Phys. Sci., 240: 35-37. Hey, R.N., 1975. Tectonic Evolution of the Cocos-Nazca Rise. Ph.D. thesis, Princeton Univ., 169 pp. Hey, R.N., Deffeyes, K.S., Johnson, G.L. and Lowrie, A., 1972. The Galapagos triple junction and plate motions in the East Pacific. Nature, 237 : 20-22. Hey, R.N., Johnson, G.L. and Lowrie, A., 1976. Recent plate motions in the Galapagos area. Geol. Sot. Am. Bull., in press. Johnson, G.L. and Vogt, P.R., 1973. The Mid-Atlantic ridge from 47’ to 51’N. Geol. Sot. Am. Bull., 84: 3443-3462. Schilling, J.-G., Anderson, R.N. and Vogt, P., 1976. Rare earth, Fe and Ti variations along the Galapagos spreading centre and their relationship to the Galapagos mantle plume. Nature. 261: 108-113.
52 Sclater, J.G. and Klitgord, K.D., 1973. A detailed heat flow, topographic, and magnetic survey across the Galapagos spreading center at 86’W. J. Geophys. Res., 78 : 69516975. Shih, J. and Molnar, P., 1975. Analysis and implications of the sequence of ridge jumps that eliminated the Surveyor transform fault. J. Geophys. Res., 80: 4815-4822. Talwani, M., Windisch, C.C. and Langseth, M.G., 1971. Reykjanes ridge crest: a detailed geophysical study. J. Geophys. Res., 76: 473-517. Vine, F.J., 1968. Magnetic anomalies associated with mid-ocean ridges. In: R.A. Phinney (Editor), The History of the Earth’s Crust. Princeton University Press, Princeton, N.J., pp. 73-89. Vogt, P.R. and Johnson, G.L., 1973. Magnetic telechemistry of oceanic crust? Nature, 241: 189-191. Vogt, P.R. and Johnson, G.L., 1975. Transform faults and longitudinal flow below the mid-oceanic ridge. J. Geophys. Res., 80: 1399-1428. Vogt, P.R., Anderson, C.N., Bracey, D.R. and Schneider, E.D., 1970. North Atlantic magnetic smooth zones. J. Geophys. Res., 75: 3955-3968. Vogt, P.R., Hey, R.N., Byerly, G., De Boer, J. and Trehu, A., 1975. Magnetic telechemistry, sub-axial flow, and the Galapagos hotspot: new observations. Trans. Am. Geophys. Union, 56: 445. Weissel, J.K. and Hayes, D.E., 1971. Asymmetric sea-floor spreading south of Australia. Nature, 231: 518-522. Wilson, J.T., 1965. A new class of faults and their bearing on continental drift. Nature, 207: 343-347.
41
SPREADING CENTER JUMPS AND SUB-AXIAL ASTHENOSPHERE FLOW NEAR THE GALAPAGOS HOTSPOT *
RICHARD
HEY and PETER
VOGT **
Hawaii Institute of Geophysics, University of I-law&i, Honolulu, Hawaii 96822 ~Ha~ui~~ U.S. Naval Oceanographic Office, Chesapeake Beach, Maryland 20732 (U.S.A.) (Received June 17, 1976)
ABSTRACT Hey, R. and Vogt, P., 1977. Spreading center jumps and sub-axial asthenosphere flow near the Galapagos hotspot. In: S. Uyeda (editor), Subduction Zones, Mid-Ocean Ridges, Oceanic Trenches and Geodynamics. Tectonophysics, 37 (l-3): 41-52. We have identified a sequence of small rise-axis jumps on the Cocos-Nazca spreading center between 93O and 95.5’W. The locus of jumps has migrated 150 km west along the rise axis, away from the Galapagos Islands, during the last three million years, at an average rate of SO mm/year. The linear increase in jump distance during this sequence of jumps has resulted in a change in regional azimuth of the rise axis from about 085’ to 095O. We visualize this sequence of jumps as a new rift propagating through the Cocos plate, forming a new Cocos-Nazca spreading center. The region affected by these rise jumps appears to correlate with an area of exceptionally high-amplitude magnetic anomalies. The high-amplitude region seems to result from Fe-T&rich (FeTi) basalts of high remanent magnetization. We speculate that the development of the new accretion axis and concomitant rise jumps are related to the flow of FeTi basalt-producing asthenosphere away from the Galapagos hotspot. The snout of anomalous asthenosphere has remained nearly stationary, with respect to the Galapagos hotspot, during the last 3 m.y. A northwestward component of flow, reflecting the southward position of the plume center with respect to the spreading axis, might explain why the new spreading center is developing along a more northwesterly azimuth. The rise jumps have resulted in the sort of pattern of asymmetric accretion which is required to substantiate the hotspot hypothesis for the origin of the Cocos and Carnegie ridges. Several other puzzling platetectonic phenomena may be explained by the propagating rift model developed here.
One of the basic tenets of the rigid-plate hypothesis as originally formulated is that mid-ocean rises spread symmetrically. This approximation, although not demanded by the rigid-plate hypothesis, is constantly assumed * Hawaii Institute of Geophysics Contribution No. 784. ** Now with the Naval Research Laboratory, Washington, D,C, 20375.
42
_____
___ _. ______ _ _______
-
43
3 yyhp
- 92”
w
930
w
3 I‘YbP
?
-
Model 27 mm/yr 4: N
3: N
PdN
,- 940 w lo N
B Fig. 1. A. Schematic location map modified from Hey et al. (1976). B. Observed magnetic anomalies plotted perpendicular to track, positive (reversed polarity) toward east. Isochrons according to time scale of Talwani et al. (1971). Synthetic model parameters same as in Fig. 3.
in the literature, and has proved to be so successful throughout the world ocean that exceptions to the rule are of great interest (e.g. Weissel and Hayes, 1971). In the Galapagos region, Hey et al. (1976) have shown that an asymmetric accretion model is necessary to resolve the geometric objections to the hotspot hypothesis as to the origin of the Cocos and Carnegie aseismic ridges. They have found evidence in the magnetic anomalies for such asymmetric accretion, which is caused at least in part by small discrete jumps of segments of the rise axis. Their interpretation is supported by recent work by Anderson et al. (1976). A detailed dredging program reported by Vogt et al. (1975) has shown that the exceptionally strong magnetic anomalies, more or less symmetrically distributed about the Galapagos Islands (Vogt and Johnson, 1973; Anderson et al., 1975), are correlated with FeTienriched basalts of high remanent magnetization (Byerly et al., 1976; Vogt and De Boer, in preparation). Vogt et al. (1975) proposed a speculative model in which the partial melts producing these FeTi basalts have flowed eastward and westward from a center of upwelling presently under the western Galapagos Islands. The flow is channeled under and along the rise axis, at rates on the order of several centimeters per year. Progressive fractional crystallization and mixing of these melts complicate the geochemical consequences of this model. Vogt et al. futher speculated that the spread of the FeTi zone was associated with riseaxis jumps. In this paper we investigate in detail one sequence of small riseaxis jumps between 93” and 96” W along the Cocos-Nazca spreading center.
44 OBSERVATIONS
A detailed geophysical survey was conducted in 1974 to chart the extent of the Galapagos high-amplitude magnetic anomaly zone, and to correlate this information with other geological and geophysical parameters. The profiles between 92” and 96”W from this and earlier cruises are shown in Figs. 1 and 2. Under the assumption of symmetric accretion the length of a rise-rise transform fault (not extending into a triple junction) does not change with time (Wilson, 1965). Thus if our magnetic anomaly correlations in Fig. 1 are correct, accretion cannot have been symmetric on this segment of the spreading center. Note that whereas the 3 m.y.b.p. isochrons are continuous, the rise axes are presently offset by 25 km. This observation can be explained either by a jump of the rise axis or by asymmetric spreading; in either case the result has been asymmetric accretion and the birth of a riserise transform fault within the last 3 m.y. Figure 2 shows the anomalies created during the last 4 m.y. along one segment of the Cocos-Nazca spreading center. Note the change in anomaly trend which occurs along the rise axis. Another conspicuous along-strike change is the dramatic westward drop in anomaly amplitude along the rise (compare, for example, the amplitudes of the axial anomalies on the profiles at 94” and 96” W) which appears to be related to the change in anomaly
96O W Fig. 2. Observed magnetic anomalies Isochrons at 1 m.y. intervals
plotted
perpendicular
to track,
out to 4 m.y.b.p.
45
trend. Scattered profiles across this change in trend could have been interpreted as resulting from a zig-zag rise axis such as that found by Johnson and Vogt (1973) in the North Atlantic. Here we will propose an alternative explanation. The axial anomaly can be easily traced west from the top of Fig. 2 to 95.1”W, where the anomaly width begins to decrease. The narrowing axial anomaly can be traced farther west, to 95.35”W, where the anomaly is much narrower than demanded by simple accretion at 30 mm/year (half-rate) throughout Brunhes time. Three additional profiles between 95.1” and 95.42”W, which are too closely spaced to be presented here, strongly support this correlation (see Vogt et al., in preparation). Similarly, the axial anomaly can be identified at 2.3” N, 96” W and followed east to 95.65” W. Note that on this segment of the axis the anomaly amplitude is much lower than on the segment to the east. Only a portion of the axial anomaly can be traced east to 95.35” W, and it has a low amplitude. Farther north on the same profile (which is parallel to the spreading direction) another part of the axial anomaly, with relatively high amplitude, is observed. These observations suggest to us that a new spreading center is breaking through the old anomaly pattern. Investigation of the older anomalies supports this interpretation and indicates that the high-amplitude magnetic anomalies are associated with the new spreading center. The 93” W profile matches the reversal time scale for an average spreading half-rate of about 27 mm/year (Fig. 1). On the Cocos plate the Kaena and Mammoth events (3 m.y.b.p., see Fig. 3) within the Gauss normal epoch are centered at about 3.2” N on this profile (Fig. 2). At 94” W these anomalies are too narrow; at 94.15” W they are yet narrower; and at 94.4”W they have disappeared. The Gauss normal epoch here is thus far too narrow. In contrast, on the Nazca plate the Kaena and Mammoth events and the Gauss normal epoch have normal width at 93” W; they are too wide on the profiles at 94” and 94.15”W; the Kaena and Mammoth events are twice their normal width at 94.4” W, at the same longitude where they disappear on the Cocos plate. Farther west, at 94.95” W, the Kaena and Mammoth events and the Gauss normal epoch have normal widths (there is a slight decrease in anomaly width from east to west because the recent Cocos-Nazca relative rotation pole is located near 2”N, 126” W, Hey et al., 1976). The amplitudes of these anomalies are much less than they were farther east (near 93” W in Fig 2, for example). However, on this profile part of lower Matuyama reversed epoch seems to be missing on the Cocos plate, whereas this anomaly on the Nazca plate is clearly wider than on the reference profile at 93” W. Farther west, at 95.1” W, part of the upper Matuyama reversed epoch seems to be missing on the Cocos plate whereas on the Nazca plate this anomaly is too wide. (Note that the width of the high-amplitude anomaly zone is also decreasing to the west.) This pattern of ever younger crust missing on the Cocos plate and repeated
46
on the Nazca plate can be followed from east to west along the CocosNazca spreading center as far as the 95.35” W profile, where we observe part of the axial anomaly with very high amplitude at 2.65”N, and part with relatively low amplitude at 2.3”N, as previously discussed. INTERPRETATION
We interpret the observed pattern of magnetic anomalies here to be the result of a sequence of rise-axis jumps which began about 3 m.y.b.p. near 94”W and reached 95.35”W about 0.3 m.y.b.p. This sequence was monotonic; all axis jumps were to the north, and thereby produced only leftlateral offsets in the spreading axis. Figure 3 shows our interpretation of the 94.15”W profile. This was one of the first and smallest jumps in this particular sequence (the next profile farther east may show a slightly older jump). The 94.15” W jump is revealed by the unequal widths of the Kaena and Mammoth events on the observed profile; these anomalies are too narrow on the Cocos plate and too wide on the Nazca plate. To model this profile we have therefore removed a small piece of crust from the Cocos plate and added it to the Nazca plate, corresponding to a discrete 5 km northward shift of the rise axis at 2.9 m.y.b.p. That portion of the anomaly sequence caused by this segment of crust is shown as a dotted line. The shapes of the surrounding anomalies on this S
N IOOOnT I
Observed
Synthetic aawmlnq at 2.9
v-&
Synthetic I
1-i
M
/
maqmtic anomalies,94.15”
lwqnotic a jump mybp
magnetic
W
onmnaller wlwlated cd 5 kltometwr north
anom(~IIes
wlthwt
jump
I
B
/
M
Fig. 3. Correlation of synthetic anomalies with the observed profile at 94.15’W, assuming a 5 km northward jump of the rise axis at 2.9 m.y.b.p. Pattern expected without jump is shown for comparison. Postulated jump would have shifted portion of block model under bar, corresponding to dotted line portion of anomalies, from Cocos to Nazca plate. Synthetic anomalies were generated using the Talwani et al. (1971) time scale and a basement layer 2.8 km deep and 1.7 km thick; topographic effects are assumed negligible as the basement is quite smooth. Effective susceptibility (ratio of the remanent magnetism intensity to the strength of the present magnetic field) is 0.012. Model spreading halfrate is 30 mm/year from 0 to 0.89 m.y.b.p. and 27 mm/year from 0.89 to 4 m.y.b.p. B, M, G are Brunhes normal, Matuyama reversed and Gauss normal epochs. h and m are Kaena and Mammoth events in Gauss normal.
47
OOOnT
I
Observed
magnetic
anomalies.
95.35O W
Synthetic assuming
magnetic anomalles calculated o Jump of 27 kilometers north
m
+ r$-&.
Ot “1
o.3
mybp
_PqfJ-L Synthetic
magnetic
anomalies
without
jump
-
N
S
Fig. 4. Correlation of synthetic anomalies with the observed profile at 95.35OW, assuming a 2’7 km northward jump of the rise axis at 0.3 m.y.b.p. Symbols and model parameters same as Fig. 3, but with amplitude of synthetic anomalies 75% of those in Fig. 3, further demonstrating the decrease in anomaly amplitude to the east. Model spreading half-rates are 29 mm/year (O-O.89 m.y.b.p.) and 24 mm/year (0.89-4 m.y.b.p.). A is the spreading axis.
model profile are changed slightly by the new edge effects resulting from the changed geometry of normal and reverse polarity crust. For comparison we show the theoretical anomaly sequence expected if accretion had been symmetric. Figure 4 shows our interpretation of the 95.35”W profile. We think this is perhaps the most recent and largest jump in this sequence. The jump occurred at 0.3 m.y.b.p., within the Brunhes normal epoch. (The next profile farther west may show a slightly younger and larger jump, but we are uncertain of this interpretation.) As in Fig. 3, the dotted portion of the synthetic profile results from crust transferred from the Cocos to the Nazca plate by a 27 km northward axis jump at 0.3 m.y.b.p. If our interpretation of these anomalies is correct, two parts of the axial anomaly are now separated by older, reversely polarized crust. This pattern could only have been produced by a discrete shift of
30
0 . l
.
0
.
.
96OW Distance
95ow along dse axis
94DW
r-
I
96OW Distance
95”W along rise axis
94”W
Fig. 5. Time of jump plotted against distance along axis for seven profiles containing postulated jumps. Closed circles are confident and open circles less certain interpretations. Slope of line is 50 mm/year. Fig. 6. Distance of jump vs. distance along axis. Symbols same as Fig. 5.
DISCUSSION
Our physical model, independent of frames of reference, is that a new rift is gradually propagating through the Cocos plate, i.e. a new Cocos-Nazca accreting plate boundary is being born as the old one is becoming extinct. At any time the farthest westward extension of this new rift is connected to the preexisting rise axis by a transform fault; as the rift breaks farther, therefore, young extinct fracture zones will be added to the Nazca plate just as extinct rise segments are. Shih and Molnar (1975) have postulated a similar model and sequence of rise jumps between 35 and 20 m.y.b.p. along the Juan de Fuca (Pacific-Farallon) spreading center. Unfortunately, their conclusions (with which we generally concur) are necessarily limited by the subduction of the eastern half of the spreading record. Considering our model of the Cocos-Nazca spreading center in the frame of reference of the triple junction leads to some interesting consequences. If the spreading-center jump sequence should migrate west relative to the Cocos-Nazca rise axis faster than the triple junction, it would eventually reach the triple junction. At that time the triple junction would shift abruptly, isolating an old rise-axis segment, and triple junction, on the Nazca plate. Perhaps the most interesting frame of reference in which to consider these observations is that of the Galapagos hotspot. The analyses of Hey et al. (1976) and Hey (1975) strongly suggest that the Cocos and Carnegie aseismic ridges were formed by plate motions over a Galapagos hotspot. They
49
have further shown that the geometric objections (Sclater and Klitgord, 1973) to the hotspot hypothesis as an explanation for these aseismic ridges can be resolved by the sort of asymmetric accretion discussed here. According to the model of Hey et al. (1976), during the last 3 m.y. the Cocos and Nazca plates both have had eastward components of motion of about 50 mm/year relative to the Galapagos hotspot. (The identical east-west motion of the two plates is demanded by the north-south trend of transform faults along the rise axis). Relative to a frame of reference fixed with respect to the active Cocos-Nazca rise axis, which of course has the same eastward component as the plates, this sequence of rise jumps has migrated 150 km west down the axis at an average rate of 50 mm/year during the last 3 m.y. Therefore the point in space at which these jumps occur has not moved east or west appreciably relative to the hotspot during the last 3 m.y. The same is true of the boundary between high and normal magnetic-anomaly amplitudes (Vogt and De Boer, in preparation). Vogt and Johnson (1975) suggested that many transform faults have affected longitudinal flow of sublithospheric partial melts along various rise axes; they speculated that the 95.5”W transform fault may have partly blocked the flow of FeTi basalt-producing partial melts away from the Galapagos. A modified explanation for this flow ending at a transform fault is that the rise jumps are associated with the advance of these partial melts; in this case the propagating rift must always be bounded by a transform fault. The flow of melts against an existing transform fault may somehow impede the eastward migration of the fault over the asthenosphere, as described by our relative motion model. The relative westward push of the FeTiproducing melts would perhaps tend to heat, weaken and erode the lithosphere across the transform fault, and thus cause the observed breaking and concomitant rise axis jumps (see fig. 23 of Vogt and Johnson, 1975). After a new section of rise axis develops, the FeTi-producing partial melts rapidly occupy the melt zone and then are dammed once again, building up until the next “break-through”. If these speculations are correct, the sequence of axis jumps is caused by the eastward motion of the spreading axis across the westward flowing partial melts from the Galapagos hotspot. Schilling et al. (1976) have shown that ambiguities exist in the geochemical origin of the observed magnetic-anomaly amplitude variations. Our interpretation is therefore speculative, although plausible. Whatever the mechanism, it seems that the influence of the hotspot on active plate-boundary configurations may extend to more than 500 km from the locus of active hotspot volcanism. CONCLUSIONS
We have identified a along the Cocos-Nazca km westward along the as resulting from a new
sequence of rise-axis jumps between 93” and 96” W spreading center. The locus of jumps migrated 150 axis during the last 3 m.y. We interpret this sequence rift breaking through the Cocos plate; as a result of
50
the systematic axis jumps the eastward moving Cocos-Nazca spreading center is changing its azimuth from about 085” to about 095” as it passes 95.5” W. If short segments of the rise axis jumped together, our data imply that the average segment length was less than 25 km, and in some cases as short as 15 km (Figs. 2,5 and 6). They cuuld, of course, have been much shorter; our profiles are not spaced closely enough to place lower bounds on the lengths. The pattern of recent rise jumps appears to be intimately related to the pattern of exce~tion~ly strong magnetic anomalies which have been correlated with dredged FeTi basalts. We interpret the pattern of rise jumps to be related to the westward advance of asthenospheric partial melts flowing away from a center of upwelling - the Galapagos hotspot - presently located under the western Galapagos Islands. The successive axis jumps all lie near the boundary between high and normal anomaly amplitudes (Fig. 2). If these obse~ations are related, there should be no more sequences of young axis jumps beyond the western limit of the FeTi province at about 95.5”W. We expect that this model may explain many of the triple-junction shifts which have been postulated in the past, particularly those in the Galapagos area (Hey, 1975). In particular, a triple-junction jump resulting from a propagating rift intersecting the old triple junction may explain some of the confusion over the present nature of the Pacific~ocos-Nazca triple junction (K. Klitgord, personal communication, 1975) and why some rough bathymetry and strong magnetic anomalies occur (I). Rea, personal communication, 1973) outside the recently formed portion of the southern rough/smooth boundary found near the triple junction by Hey et al. (1972). It is likely that the change in rise azimuth associated with this jump sequence, and with other jumps identified east of the Galapagos Islands (Hey, 1975; Anderson et al., 1976) which may also have been progressive, probably accounts for the confusion regarding the recent spreading direction in this area. One interesting implication of the propagating rift model is that overall fracture-bode trends based solely on rna~~tic-Mornay offsets may be seriously in error. This is because progressively younger transform faults bound a new rift as it propagates. Thus a series of en-echelon fracture zones, each individual fracture zone parallel to the spreading direction, is frozen into progressively younger crust. Ongoing sea-floor spreading thus produces an apparent overall trend of offset anomalies different from the true spreading direction. This me~h~ism may be responsible for the seemingly incompatible fault trends in the Juan de Fuca area (see for example Vine, 1968). A similar process may explain the complexities in plate configuration near Easter Island, where small independent plates have previously been postulated (Elerron, 1972; Anderson et al., 1974). A propagating rift model may also explain the observation (e.g. Vogt et al., 1970; Hayes and Rabinowitz, 1975) that the boundary of the magnetic smooth zone along the east coast of North America is almost, but not quite, an isochron. Under this model the boundary could not be an isochron, and
51
the extent to which its trend diverged from the oldest isochrons would be a measure of the velocity of oropagation of the new rift. The asymmetric location of the Cocos-Nazca spreading center between the Cocos and Carnegie ridges may also reflect systematic southward jumps of the spreading center (Hey, 1975; Anderson et al., 1976). Such jumps, similar to those documented in this paper, would result in the net asymmetric accretion required by the hotspot explanation for the origin of the Cocos and Carnegie ridges. ACKNOWLEDGMENTS
We thank Don Hussong, Ralph Moberly, George Sutton and George Woollard for comments, and Steve Dang, Carol Yasui, Kathy Lee and Elsie Wipperman for assistance. Supported by the International Decade of Ocean Exploration, National Science Foundation Grant ID 071-04207-A04. R.H. particularly appreciates travel support which enabled the original presentation of this material at the 1975 Grenoble IUGG, where many of these ideas were put into perspective.
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52 Sclater, J.G. and Klitgord, K.D., 1973. A detailed heat flow, topographic, and magnetic survey across the Galapagos spreading center at 86’W. J. Geophys. Res., 78 : 69516975. Shih, J. and Molnar, P., 1975. Analysis and implications of the sequence of ridge jumps that eliminated the Surveyor transform fault. J. Geophys. Res., 80: 4815-4822. Talwani, M., Windisch, C.C. and Langseth, M.G., 1971. Reykjanes ridge crest: a detailed geophysical study. J. Geophys. Res., 76: 473-517. Vine, F.J., 1968. Magnetic anomalies associated with mid-ocean ridges. In: R.A. Phinney (Editor), The History of the Earth’s Crust. Princeton University Press, Princeton, N.J., pp. 73-89. Vogt, P.R. and Johnson, G.L., 1973. Magnetic telechemistry of oceanic crust? Nature, 241: 189-191. Vogt, P.R. and Johnson, G.L., 1975. Transform faults and longitudinal flow below the mid-oceanic ridge. J. Geophys. Res., 80: 1399-1428. Vogt, P.R., Anderson, C.N., Bracey, D.R. and Schneider, E.D., 1970. North Atlantic magnetic smooth zones. J. Geophys. Res., 75: 3955-3968. Vogt, P.R., Hey, R.N., Byerly, G., De Boer, J. and Trehu, A., 1975. Magnetic telechemistry, sub-axial flow, and the Galapagos hotspot: new observations. Trans. Am. Geophys. Union, 56: 445. Weissel, J.K. and Hayes, D.E., 1971. Asymmetric sea-floor spreading south of Australia. Nature, 231: 518-522. Wilson, J.T., 1965. A new class of faults and their bearing on continental drift. Nature, 207: 343-347.