Geophys. J . Int. (1990) 100, 521-526

RESEARCH NOTE

Similarity of new palaeomagnetic data from the Santa Rosa Mountains with those from Steens Mountain gives wide regional evidence for a two-stage process of geomagnetic field reversal

Neil Roberts' and Mike Fuller2 'Geomagnetkm Laboratory, Department of Earth Sciences, University of Liverpool, PO Box 147, Liverpool, L69 3BX, U K 'Department of Geological Sciences, University of California, Santa Barbara, CA 93106, USA

Accepted 1989 September 14. Received 1989 August 4; in original form 1987 June 29

SUMMARY New palaeomagnetic data of much improved resolution are presented for the geomagnetic field reversal recorded by lavas of the Santa Rosa Mountains in Nevada, USA; five of six intermediate field directions have (yg5 values of less than 5.5". The variation in the direction of the palaeomagnetic field recorded by the Santa Rosa lavas is of striking similarity to that recorded by a sequence of lavas of similar age at Steens Mountain, Oregon, 200km away. The data from the Santa Rosa lavas represent an important field test to substantiate the detailed recording of a two-stage geomagnetic field reversal in the middle Miocene at Steens Mountain (Watkins 1969; Mankinen et al. 1985.)

Key words: Geomagnetic field reversal, palaeomagnetism.

INTRODUCTION

Larson, Watson & Jennings (1971) made the first palaeomagnetic investigation of the Miocene lavas of the Santa Rosa Mountains, northern Nevada, USA and reported that the lavas record a transition of the Earth's magnetic field from reversed to normal polarity. The authors noted that thin basaltic flows in the Santa Rosa lavas contain labradorite phenocrysts up to 5 cm across. This is in common with the nearby laveas of Steens Mountain, Oregon from which Watkins (1969) presented an extraor- dinarily detailed record of a field reversal [most recently dated at 15.5 f 0.3 Ma by Steiger & Jager (1977)l. The lavas of Poker Jim Ridge in the same range as Steens Mountain, and for which Goldstein, Strangway & Larson (1969) report the recording of a reversed to normal polarity transition, also contain similar phenocrysts. It is possible that the very same reversal has been recorded in all three lava sequences. Radiometric dating is too imprecise, with respect to the average time interval between reversals, to be able to confirm that this is the case.

Larson et af. (1971) compared Watkins' (1969) palaeo- magnetic record from Steens Mountain with their own from the Santa Rosa Mountains. They considered the records to be of sufficient similarity to be able to conclude that the same reversal has been recorded in the two lava sequences situated almost 200 km apart. However, Larson's record

from the Santa Rosa lavas is poorly resolved. Larson et af. (1971) only report cu,,:values for six of 11 intermediate field directions; the values are T , 12", 12", 2W, 26" and 31".

Mankinen et al. (1985) have now made a detailed reinvestigation of the lavas at Steens Mountain. The new Steens Mountain data are better resolved than those of Watkins (1%9). Mankinen et al. (1985) suggest that the Steens Mountain reversal occurred in two stages; an initial switching of the field direction from reversed to normal polarity followed by a large counter-clockwise looping of the field vector.

With the aim of clarifymg the degree of similarity between the Steens Mountain and Santa Rosa reversal recordings, a renewed palaeomagnetic investigation has been made of the lavas of the Santa Rosa Mountains.

METHOD

The new collection of the Santa Rosa lavas was made in the same road and hill sections as those described by Larson et af. (1971). Ten lavas were sampled in the road section and 25 in the hill section. Larson et al. (1971) collected hand samples. Each data point that they presented represents the mean palaeofield direction obtained from a group of consecutive samples that yielded similar directions. In the new study, oriented drill cores, an average of five per lava, have been taken in both the road and hill sections. The

521

522 N. Roberts and M . Fuller

Table 1. Declination (DECL) and inclination (INCL) values obtained for lavas in the Santa Rosa road section. N specimens were measured for each lava though, as is described in the text, the values of ag, and k relate to 2N directions. The latitude (PLAT) and longitude (PLONG) of the Virtual Geomagnetic Pole (VGP) position are also presented for each lava, with associated uncertainty limits (DP, DM). The lava coded 1 is the oldest, and that coded 10 the youngest.

CODE N DECL INCL (us, K P U T PLONG DP DM WURITY

1. 3 2. 3 3. 3 4. 2 5. 3 6 . 6 7. 3 3. 3 9. 3

10. 3

102.0 100.0

183.9 102.6 172.0 107.2 3 . 9 29.0

30.8 52.9

-55.4 1.1

-55.5 0.4 -58.6 0.5 -43.3 0.9 -3 .3 7.2 -13.3 15.9 55.3 3.1 76.5 5.4 54.6 2.3 53.7 1.6

3846.2 25000.0 16666.7 10000 .o

00.2 0.4

450.7 154.0 019.7

1051.9

mean palaeofield direction has been determined for each lava, separately.

In the field area, the Santa Rosa lavas were measured to be dipping at 17" in a direction 65" east of north. It has been assumed that the lavas were extruded onto level ground, and correction for their present attitude was made during the measurement of magnetization directions in the laboratory.

Each specimen studied was subjected to alternating field (AF) demagnetization. The AF was incremented in steps of 5 or 10mT up to a maximum of 200mT. Viscous magnetizations were usually readily removed by 50 mT. The approach used for defining palaeomagnetic field directions was that described by Dagley & Ade-Hall (1970). A consecutive pair of directions was taken from the AF demagnetization measurements of each of N specimens from the lava in question, and the mean palaeofield direction

-03.0 -81.5 -06.1

-73.3 -60.7 -17.3 59.4 62.0 65.1 48.3

220.7 1.1 1.5 192.3 0.4 0.6 191.7 0.6 0.0

234.3 0.7 1.1 263.1 5.0 0.5 325.9 8.3 16.2 330.7 3.2 4.5 268.9 9.3 10.0

337.7 2.3 3.3 324.0 1.5 2.2

REVERSH)

REVERSED

ReyERsED

REVERSED

REVERSED -UTE

NORMAL

NORMAL

NORMAL NORMAL

calculated. This procedure was repeated for all possible combinations of pairs of directions to find that combination which gave the smallest value for abS. Each of the palaeofield directions presented in Tables 1 and 2 is the mean of the 2N directions which gave the least scatter. Correspondingly the values of a95 and k refer to 2N directions for each lava.

To determine only palaeofield directions from the Santa Rosa lavas it would have been sufficient to demagnetize specimens to, at most, 100mT. The reason for the demagnetization of specimens in fields up to 200 mT lies in the fact that this represents the first stage in Shaw's method of palaeointensity determination (Shaw 1974). Palaeointen- sity values have in fact been obtained from specimens representing several lavas. However, these data are not straightforward and will not be presented until they can be compared with those from the application of the Thelliers'

Table 2. Directions of magnetization obtained for the Santa Rosa hill section lavas.: Column headings are the same as in Table 1. Lava 11 is the oldest and 35 the youngest.

K P U T PLONC DP DM POUkITY

11. 2 130.3 -77.9 040 15OOO.O -56.3 34.4 1.3 1.4 REVHISED 12. 2 149.4 -70.0 2.1 1075.0 -66.4 14.0 3.1 3.7 REvEIlsED 13. 2 177.2 -72.5 0.5 3'3000.0 -73.9 57.0 0.0 0.9 REVEIISED

CODE N DECL INCL (us,

14. 2 212.1 -73.0 2.9 1000.0 -63.9 101.4 4.6 5.2 REvHIsH) 15. 2 179.0 -69.0 1.3 5000.0 -70.1 61.0 1.9 2.2 lW%%DSED 16. 2 208.2 -59.4 1.2 6000.0 -60.0 147.1 1.3 1.0 REVERSED 17. 2 192.7 -57.7 3.0 588.2 -79.7 168.1 4.1 5.6 REVERSED

20. 3 21. 3 22. 7 23- 3 24. 3 26. 3 27. 3 20. 4 29. 3 P. 3 31. 2 32. 1

33. 3 34. 2 35. 3

106.0 4 8 . 5 1.7 100.4 -53.7 3.6 116.0 -17.0 4.2

7.7 63.5 0.9 148.0 52.7 16.1 95.0 44.9 5.5 48.1 -6.5 1.6 42.4 -5.5 1.0

43.4 -3.1 2.0 1.5 41.2 0.7

17.7 57.0 4.9 22.3 58.4 0.0

19.1 56.7 0.9 10.3 59.6 2.5

349.2 54.0 7.3

1612.9 344.8 92.5

5555 * 6 10.2

147.1 1666.7 3101.0 1136.4

60.7 352.9

99999.0 5000.0

1 W . 3 84.5

-76.5 -79.9 -25.1 83.5 -9.6 13.0

27.3 31.2 31.6 71.0 76.1 72.9 74.6 76.1 70.0

216.2 198.4 321.2 299.6 268.5 309.1

5.6 10.5 0.7

57.9 340.5 333.9 343.2 332.1 115.3

1.4 2.2

2.2 4.3 1.1 1.4

15.4 22.3 4.4 7.0 0.0 1.6 0.5 1.0

1.0 2.0 6.4 10.6

5.3 7.2 0.0 0.0

1.0 1.4

3.5 5.0

2.9 3.8 7.2 10.3

REvEIlsED ReyERsED

1NT-IATE

NORMAL

INTWWEOIATE INTWWEOUTE

INTwMpluTE INTERMEDIATE

INTWWEDIATE NORMAL

NORMAL

NORMAL

NORMAL

NORMAL

NOFINAL

Geomagnetic field reversal 523

19 and 20. The lowermost group of lavas, i.e. 11-19, proved difficult to sample in that they tended to break up in the drill bit. At most, only two intact cores were obtained from each. However, results from lavas 20-35 are the data appropriate for direct comparison with the road section lavas. The AF demagnetization behaviour of specimens from the hill section lavas was very similar to that of specimens from the road section lavas. Results obtained from the hill section lavas are presented in Table 2. The data suggest that the lavas record a two-stage process of field reversal.

The first stage of the reversal, recorded by lavas 20-23 and shown in Fig. 3(a), is a switching of the field from reversed to normal polarity similar to that recorded by lavas of the road section. Lava 22 records an intermediate palaeofield direction and this appears to correlate with that of lava 6 of the road section.

The second stage of the reversal is recorded by lavas 24-30, except lava 25; the specimens from this lava proved so unstable as not to give any acceptable result. The data define a large counter-clockwise looping of the palaeofield vector (see Fig. 3b) with five lavas recording intermediate field directions. Four of these are well resolved with cu,, values of less than 5.5".

method of palaeointensity determination (Thellier & Thellier 1959) to other specimens from the same lavas. For the present, the high-field demagnetization data serve to increase the confidence which can be placed in the palaeofield direction results.

RESULTS 1 Road section

Results for the 10 lavas of the Santa Rosa road section are presented in Table 1 and plotted in Fig. 1. The lava coded 1 is the oldest and that coded 10 the youngest. Exposure of the lavas is not, however, continuous and perhaps a further 10 lavas remain unsampled through not outcropping beside the road.

0

I - I \

180

B3gure 1. Stereoplot of the declination and inclination values of the road section lavas. In this and all further stereoplots closed (open) symbols refer to positive (negative) inclinations.

Lavas 1-5 in the road section record a palaeofield of reversed polarity, while lavas 7-10 record a palaeofield of normal polarity. Figs 2(a) and (b) are typical Zijderveld plots (Zijderveld 1967) for specimens of reversed and normal polarity respectively. Only lava 6 records an intermediate field direction but this is not well defined (a9, = 15.9"). The magnetizations of specimens from lava 6 are much weaker, and therefore not so readily measured as those of specimens from lavas of reversed or normal polarity; presumably this is due to the lava having been extruded in the reduced palaeointensity of a reversing field. Fig. 2(c) is a Zijderveld plot for one of the six specimens measured for the intermediate lava 6.

2 Hill section

Twenty-five lavas, coded 11-35, were sampled in the hill section. The exposure of these lavas was generally more continuous than was the case in the road section. There was, however, an appreciable gap in the exposure between lavas

DISCUSSION

The lavas of the Santa Rosa Formation apparently record a two-stage process of reversal for the Earth's magnetic field. This record is considerably simpler than was presented for the Santa Rosa lavas by Larson er al. (1971). An initial switching of the field from reversed to normal polarity is followed by a large counter-clockwise looping of the palaeofield direction. Larson et al. (1971) had suggested the Occurrence of loopings at various stages throughout the reversal.

The new Santa Ro?a record cannot be described as detailed. There are not sufficient pre- and post-transitional data for study to be made of the form of the palaeosecular variation. Nevertheless, there is a sufficient number of well-resolved palaeofield directions to permit useful comparison with the record from the Steens Mountain lavas. Upon comparison, it is found that both the Santa Rosa and Steens Mountain records describe a reversal of the Earth's magnetic field that apparently occurred in two stages.

The initial switching of the field from reversed to normal polarity, recorded by the Santa Rosa lavas, contains just one poorly defined intermediate field direction. However, this direction is broadly consistent between the road and hill sections sampled in the Santa Rosa Mountains, and lies in the same geographic quadrant as the directional path presented for the Steens Mountain lavas by Mankinen et al. (1985), redrawn in Fig. 4(a).

In the Santa Rosa Mountains the record of a second stage in the reversal has been found only in the more continuously outcropping lavas of the hill section. The Santa Rosa data shown in Fig. 3(b) can be compared with the data for the second stage of the Steens Mountain reversal which are redrawn in Fig. 4(b). All the Santa Rosa data points lie close to the path obtained from the Steens Mountain lavas. The Santa Rosa data cover an appreciable extent, but not all, of the counter-clockwise looping recorded at Steens

524 N. Roberrs and M. Fuller

1.00 , , , e.8ep R . 6 0

0 . 2 0

- 0 . w

-0.68 1 -8 .e

- ’

IP

DOWN .00 8 .18 0 . 4 0 0.60 B.80

Figure 2. Zijderveld plots for three representative specimens from the road section lavas. Open symbols refer to projections on the horizontal plane containing north, south, west (W) and east (E). Closed symbols refer to projections in the vertical plane which contains the directions UP, E, DOWN and W. (a) is for a specimen from the reversely magnetized lava 1 (AF demagnetization of this specimen was between 0 and 150 mT in increments of 10 mT), (b) is for a specimen from the normally magnetized lava 10 (AF demagnetization between 0 and 200 mT in increments of 10mT) and (c) is for a specimen from the intermediate lava 6 (AF demagnetization between 0 and 30mT in increments of 5 mT).

Mountain. Either the very same reversal has been recorded in two lava sequences separated by almost 200 km or two different reversals exhibit striking similarities.

Valet et al. (1985) have alternatively suggested that the Steens Mountain recording is of a reversal followed at some later date by an excursion. This invokes a more complicated scenario for the extrusion of the Santa Rosa and Steens Mountain lava sequences than does the suggestion that the ‘rebound’ was an inherent stage in the reversal process.

The hypothesis of a (at least occasional) two-stage process of geomagnetic field reversal is in accord with the findings of Hoffman (1986) following his palaeomagnetic study of Cenozoic lava sequences in New Zealand. Hoffman provides

evidence of the palaeomagnetic field occasionally taking a preferred intermediate direction, from which it sometimes goes on to completely reverse and on other occasions reverts back to the original polarity.

CONCLUSIONS

The original contention of Larson et af. (1971) that the Santa Rosa and Steens Mountain lavas record the same geomagnetic field reversal is corroborated by renewed palaeomagnetic study of the Santa Rosa formation. Further, it is concluded that the change from reversed to normal polarity occurred as a two-stage process.

Geomagnetic field reversal 525 0

180

Figure 3. Stereoplots of the declination and inclination values for (a) lavas 20-23 and (b) 23-39 of the hill section. Closed (open) symbols refer to positive (negative) inclinations.

n 0

270 90

180 180

Figure 4. Stereoplots of the declination and inclination values for (a) directional groups S44-S29 and (b) S29-Sl5 from Mankinen et al. (1985).

ACKNOWLEDGMENTS

The authors are grateful to Connie Hall, Robin Weeks, Heather Weeks, and Stan Cisowski for help with the fieldwork. This work was supported by the American National Science Foundation, and in the UK by the NERC.

REFERENCES

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Goldstein, M. A., Strangeway, D. W. & Larson, E. E., 1%9. Palaeomagnetism of an upper Miocene geomagnetic polarity transition in Oregon, Earth planet. Sci. Len., 7 , 231-239.

Hoffman, K. A., 1986. Transitional field behaviour from Southern Hemisphere lavas: evidence for two-stage reversals of the geodynamo, Nature, 32Q, 228-232.

Larson, E. E., Watson, D. E. & Jennings, W., 1971. Regional comparison of a Miocene geomagnetic transition in Oregon and Nevada, Earth planet. Sci. Len., 11, 391-400.

Mankinen, E. A., Prevot, M., GrommC, C. S. & Coe, R. S., 1985. The Steens Mountain geomagnetic polarity transition 1. Directional history, duration of episodes and rock magnetism, J. geophys. Res., 90, 10393-10416.

526 N. Roberts and M. Fuller

Shaw, J., 1974. A new method of determining the magnitude of the palaeomagnetic field: Application to five historic and five archaelogical samples, Geophys. J . R. ash: SOC., 39, 133-141.

Steiger, R. H. & Jager, E., 1977. Subcommission on geochronol- ogy: Convention on the use of decay constants in geo- and cosmochronology, Earth planet. Sci. Lett., 36, 359-362.

Theillier, E. & Thellier, O., 1959. Sur I’intensit6 du champ magnttique terrestre dans le passe historique et geologique, Ann. Geophys., 15,285-376.

Valet, J.-P., Laj, C. & Tucholka, P., 1985. Volcanic record of reversal, Nature, 316, 217-218.

Watkins, N. D., 1969. Non-dipole behaviour during an Upper Miocene geomagnetic polarity transition in Oregon, Geophys.

Zijderveld, J. D. A., 1967. AC demagnetisation of rocks: analysis of results, in Methods in Palaeomagnetkrn, pp. 256-266, Elsevier, New York.

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