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Status of the Geomagnetic Polarity Time Scale Ian MCDOUGALL

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Status of the Geomagnetic Polarity Time Scale Ian MCDOUGALL J. Geomag. Geoelectr., 37, 129-137, 1985 Status of the Geomagnetic Polarity Time Scale Ian MCDOUGALL ResearchSchool of Earth Sciences,A ustralianNational University, Canberra, A.C.T. 2601,Australia (ReceivedApril 2, 1984;Revised September 14, 1984) During the last four years the directly measured geomagnetic polarity time scale covering the last s Ma of geological time has not been changed significant- ly, but several short intervals of inverse polarity appear to have been authen- ticated from measurements on volcanic rocks. The polarity time scale extending to about the Middle Jurassic(～165Ma)is based upon interpretation of marine magnetic anomaly data. For the interval extending to ～80Ma ago several new time scales have been published in recent years, with calibration by means of the directly determined scale for the last 3.5Ma, and then by use of a number of calibration points relating individual magnetic anomalies to the numerical time scale, usually through biostratigraphy and less commonly by direct dating of volcanic sequences. Considerable improvements have been effected, but further changes will occur in the future. From about 80Ma ago to the limit of the marine magnetic anomaly data, calibration is far less well controlled. Only the broad pattern of geomagnetic field reversals is known from the Middle Jurassic to the Middle Paleozoic, with little information available for earlier times. 1. Introduction The geomagnetic polarity time scale has assumed great importance in a wide range of geological and geophysical studies, and thus its accurate calibration is of continuing interest and relevance. The polarity time scale is conveniently considered in terms of three overlap- ping time intervals, relating to how it is calibrated. For the time interval from the present back to about s Ma ago, the polarity time scale was developed by measurement of the age(using the K-Ar method)and magnetic polarity of subaerial volcanic rocks from diverse localities. This directly determined polarity time scale initially was limited to the last s Ma of geological time because of loss of resolution in the K-Ar age measurements with increasing age. MCDOUGALL(1979)and MANKINEN aril DALRYMPLE(1979)gave compreheriSiVe summaries of this part of the polarity time scale, and in the present review com- ments will be confined to summarizing new information that has become available since then. Reporter review paper for IAGA Division I, WG 5. 129 130 I. MCDoUCALL Historically the directly determined polarity time scale, developed mainly between 1963 and 1966, was of great significance in that it facilitated the quan- titative interpretation of the marine magnetic anomaly profiles observed across mid ocean ridges(VINE and MATTHEWS, 1963), leading to the rapid acceptance of the hypothesis of seafloor spreading(HESS, 1962), and to the development of plate tectonic models of earth behavior. A detailed account of this history recently was given by GLEN(1982). VINE(1966)grid HEIRTZLER et al.(1968)used the known polarity time scale for the Last 3.5Ma to obtain spreading rates at the mid ocean ridges; they then applied these rates to the uncalibrated parts of the marine magnetic anomaly patterns to derive predicted polarity time scales extending back to about 80Ma ago into the Cretaceous. This history of polarity of the geomagnetic field is recorded in quite remarkable fidelity in the marine magnetic anomaly record, which is now documented from the present back to Middle Jurassic times, about 165Ma ago. There has been considerable effort expended in recent years in improving the calibration of the polarity time scale based upon interpretation of marine magnetic anomaly data, and a brief sum- mary of this work will be given below. For times earlier than about 165Ma ago, the history of polarity of the geomagnetic field is rather poorly known, with little definitive information prior to the Middle Paleozoic. This part of the polarity time scale always will be less well defined, as it has to be pieced together from a combination of biostratigraphic, paleomagnetic and isotopic age data from stratigraphic sequences from different localities. We will not consider this earlier part of the polarity time scale further in this review; auseful brief summary as to its present status was given by HARLAND et al. (1982). 2. Directly Determined Polarity Time Scale Since the reviews of MCDOUGALL(1979)and MANKINEN and DALRYMPLE (1979),no major changes have been suggested for the polarity time scale for the last s Ma, as determined from direct age and paleomagnetic measurements on subaerial lava flows. However, a development of significance is that there is an increasing amount of evidence from volcanic sequences for very short intervals(～0.01Ma)of opposite polarity to that expected in the particular time range. These discoveries provide support for earlier claims that at least some of the short intervals of opposite polarity found in sedimentary sequences are real, and indeed record inversions of the geomagnetic field. Within the Brunhes chron, there are at least two examples of this behavior that appear to be well established. The reverse polarity Laschamp and Olby lava flows in the Chaine des Puys, France indicate a subchron of short duration somewhere within the interval 30,000 to 50,000 years ago based upon extensive K-Ar, 40Ar/39Ar, thermoluminescence and 230Th/238U disequilibrium dating (HALL and YORK, 1978; CONDOMINES, 1978; GILLOT et al., 1979). Note, however, that HEELER and PETERSEN(1982)believe there is some evidence for self-reversal Status of the Geomagnetic Polarity Time Scale 131 effects, particularly in the Olby flow. Reverse polarity for welded tuff of age about 30,000 years at Shibutami in Japan(TANAKA and TACHIBANA,1981)may also record the same inversion of the geomagnetic field. Two reverse polarity lava flows in Idaho with estimated age of 0.46±0.05 Ma, based on K-Ar dating(CHAMPION et al.,1981), provide good evidence for the reality of the Emperor reversed polarity event(subchron)within the Brunhes normal chron. RYAN(1972)named this short reverse polarity interval on the basis of reverse polarity found in some deep sea cores at a level thought to be about O.45Ma old. There also is supporting evidence from the interpretation of marine magnetic anomaly profiles across the Galapagos spreading center(WILSON and HEY, 1981). MANKINEN et al. (1978)recorded evidence for an excursion toward normal polarity within the rhyolite of Adler Creek, California, dated at 1.12±0.02Ma, and named this the Cobb Mountain normal polarity event(subchron)within the Matuyama reversed chron, somewhat older than the Jaramillo subchron. This interpretation was reinforced by MANKINEN and GROMME(1982)who found two basalt flows of normal polarity and similar age in the Coso Range; California. They referred to other supporting data from volcanic sequences elsewhere, and provided an estimate of 1.10±0.02Ma for its age with a maximum duration in the order of 10,000 years, see also SUEISHI et al. (1979). Collectively these data from volcanic rocks are providing convincing evidence that very short intervals of inverse polarity are present in the geomagnetic field polarity spectrum, as predicted by Cox(1968). No doubt further examples will be documented in the future. 3. Polarity Time Scale from Marine Magnetic Anomaly Data It is now widely recognized that the marine magnetic anomaly data provide an excellent time-integrated history of polarity of the geomagnetic field. As noted above, this was realized at an early stage by VINE(1966)and HEIRTZLER et al. (1968), who derived polarity time scales from the marine magnetic anomaly record extending to about 80 Ma ago, based upon extrapolations from the directly calibrated polarity time scale for the last 3.5Ma. Although very large extrapola- tions were involved, over a factor of 20 times the length of the baseline, the time scale proposed by HEIRTZLER et al.(1968)remains within 10 percent of current estimates. The particular magnetic anomaly profile from the South Atlantic USed by HEIRTZLER et al.(1968)in their derivation of the polarity time scale clearly was an enlightened choice, and indicates that indeed the rate of seafloor spreading in this region must have been nearly constant over this 80Ma interval . Since that time, numerous attempts have been made to improve the polarity time scale based upon the marine magnetic anomaly record , especially during the last six years(LABRECQUE et al., 1977; NESS et al ., 1980; LOWRIE and ALVAREZ, 1981; HARLAND et al., 1982; BERGGREN et al ., 1984). BUt instead of looking for a particular marine magnetic anomaly profile from an oceanic region 132 I. MCDOUGALL thought to have undergone uniform spreading,a differentphilosophy has been adopted. Stacking techniqueshave enabled an overallmarine magnetic polarity record to be establishedusing data from many profiles.The approach normally used has been to establishone or more calibrationpoints between the marine magnetic record and the numerical time scale;apolarity time scale is then derived by linearinterpolation between the chosen calibrationpoints. Clearly if a sufficientnumber of accurate ties are established,then a precisepolarity time scale of high resolutionwill be obtained. As a by product, information as to uniformity or otherwise of seafloorspreading within various parts of the ocean basins can be derived. The major problem with this approach, and the reason why there is a proliferationof time scales,is that there are difficulties in providingprecise and accuratecalibration points for definitionof the polarity time scale. Unfortunately it has not usuallybeen possibleto measure the age of a par- titularmarine magnetic anomaly directly,because basaltsrecovered from oceanic crust by drillingalmost invariablyare unsuitablefor-isotopic dating. Nor can sediments immediately overlyinga particularsegment of oceanic crust be dated reliablyby isotopicmethods to provide a minimum age for the underlyingcrust containing a record of a particularmagnetic anomaly. Thus indirectmethods involving correlationsof various kinds, often through several steps,must be employed in derivingan age for a particularmagnetic anomaly.
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