Marine Geophysics and Sea-Floor Spreading in thePacific-Antarctic Area: A Review

DENNIS E. HAYES and WALTER C. PITMAN III

Lamont-Doherty Geological Observatoryof Columbia University

Introduction

The marine geophysical data collected in thePacific Ocean south of 35°S. have proven extremelyvaluable in furthering our understanding of manyfundamental geological problems. Foremost amongthem are such issues as continental drift, the originand evolution of the ocean basins, the relationshipof earthquakes to tectonic provinces and processes,and the distribution and age of sediments. Whilethere is by no means complete agreement on theanswers to these questions, there has been a broadconvergence of opinions toward the general conceptsof sea-floor spreading and plate tectonics. Theseprocesses can account for many previously perplexingobservations.

One purpose of this review is to point out somesignificant results of the analysis of the marine geo-physical data collected in the Pacific-Antarctic andadjacent ocean areas as they apply to sea-floorspreading. The bulk of these data has been collectedon board the National Science Foundation's antarc-tic research vessel Eltanin. Significant supplementarydata have been collected by the Lamont-DohertyGeological Observatory vessels Verna and Robert D.Conrad, by vessels of the U.S. Naval OceanographicOffice, and by others. Fig. 1 is an index map of aportion of the southern oceans showing selectedtracks along which two or more types of geophysicaldata have been collected. Programs of simultaneousand continuous precision echo sounding, seismic pro-filing, gravity measurements, and total-field magneticmeasurements were conducted along many of thetracks shown. Large portions of these data have beenanalyzed and the results published (e.g., Pitmanet al., 1968; Ewing et al., 1969; Herron, in press;Heirtzler et al., 1969; Hayes et al., 1969).

Precision Depth Recording

The morphology of the ocean floor is mapped by

*Lamont Doherty Geological Observatory ContributionNo. 1513.

means of a precision narrow-beam, high-frequency(generally about 12-17 kHz) echo sounder. Except inareas of extremely rough topography, the depth canbe determined and recorded continuously with anaccuracy of about 1 fathom (1.83 m). The floor ofthe ocean is dominated by a prominent submarinemountain range often referred to as the Mid-OceanicRidge because of its approximate median position inthe oceans. This ridge system, present in all theoceans of the world, is a nearly continuous featureencircling the globe for some 40,000 miles. The ridgesrise 2,000-2,500 m above the depth of the adjacentocean basins, which makes them comparable in sizeto the most spectacular continental mountain ranges(Figs. 1 and 2).

In contrast to these broad mountain ranges, certainareas of the oceans are characterized by extremelylong, narrow trenches or depths (see Fig. 2). Thesetrenches are for the most part situated along theborders of the Pacific Ocean; however, isolatedtrenches are also found in the Caribbean, SouthAtlantic, and Indian Oceans.

Seismic Profiling

Seismic profiling is undertaken routinely to meas-ure the thickness of the sedimentary layer that coversthe floor of the ocean. This layer commonly rangesfrom near 0 to about 2,000 m in thickness. Theseismic profiler is a special type of echo sounderthat utilizes low-frequency sound waves. In the past,explosive charges were used to create sound wavesfor seismic profiling, but these have since been re-placed by "air guns." An explosion is simulated bythe sudden release of highly compressed air from achamber towed behind the ship. The air gun pro-duces acoustic waves of much lower frequency(-100 Hz) than the common echo sounder(-12 kHz). The low-frequency waves penetrate wellinto the sediments and are reflected back to the sur-face from horizons where contrasts in sedimentdensity and seismic-wave velocity are encountered.These contrasts represent changes in rock type. Thequantity measured is the time that it takes for the

70 ANTARCTIC JOURNAL

Figure 1. Index map of the SouthPacific and southwest Indian Oceans,showing geophysical tracks of Eltanin,Verna, and Robert D. Conrad. Boldlines and numbers indicate sectionsand figure numbers where profiles areshown. The axis of the mid-oceanicridge system is shown schematicallyby heavy double lines. Offsets of theridge (fracture zones) are indicated bydashed lines. Locations of deep-sea

trenches are given by dots.

energy pulse to travel from the surface, into thesediments to a reflecting horizon, and back to theship again. By making these measurements on aclosely spaced basis, a continuous profile of the up-permost layers of the ocean floor is obtained (Fig. 3).

One fact regarding the distribution of the sedi-ments that particularly bears on the discussion hereis that, near the crests of the mid-oceanic ridges, theblanket of sediment is generally thin or entirely ab-sent. As one proceeds laterally toward either sidefrom the crest, the thickness of the sediments in-creases towards some maximum in the basins (Fig.4). Many factors can influence the total accumula-tion of sediments at a particular place, but timeis perhaps the most fundamental element. In general,the older the area, the more potential it has foraccumulating thick layers of sediment.

Magnetic Measurements

Another type of geophysical measurements madeaboard ocean-going vessels—and also from aircraft—is that of magnetic-field strength. These measure-ments are obtained by means of a special sensor,towed several hundred feet behind the ship to mini-mize the effects of the ship's metal hull. The primarypurpose of these measurements is to determine thedistribution of magnetically polarized rocks near theearth's surface. If rocks are magnetized in the di-rection of the earth's field, they will enhance thestrength of the magnetic field at that point. If theyare magnetized in the opposite direction, they willreduce it. When the total field strength is comparedwith the theoretical field strength—assuming nomagnetic bodies were present in the near-surfacelayers—the difference gives a measure of the so-calledmagnetic anomalies. Areas of positive anomaly andnegative anomaly in general are not randomly dis-tributed; rather, there are areas of extensive positiveand negative magnetic stripes that occupy a large

Figure 2. Topographic profiles across the eastern South Pacific near45 0 S. (see Fig. 1 for location). The upper and lower profiles

represent one continuous profile (1 fathom= 1.83 m).

BuriedChile Trench

NWI

SE

2400 6

50 N.M. ELT4N/N28-- Figure 3. Seismic reflection profile of the continental margin ofsouthern Chile (see Fig. 1). Note the presence of a sediment-filled trench. The gently sloping reflectors were probably onceflat-lying, and their dip indicates subsequent downwarping of thecrust at the buried trench. Adapted from Hayes and Ewing (in

press).

part of the ocean basin (Fig. 4). These stripesindicate that there are long, linear pieces of crustalrock that appear to be uniformly magnetized withone polarity adjacent to other long, linear crustalblocks magnetized with the opposite polarity.

May-June 1970 71

22- Is-

\ \ \

1000m contour

14tp

Vidal Kilo, North AtIantc

500 0 500

SWVerna 22, South AtlanticNE

I.IA&4AAAAAIIIALI:

400 0 400

nOsw Conrad 8, Indian NE

kL

8IA.i.500 0 500

NW E toni n 19, South POCIOC

500 0 50€

W Conrad 10, Verna 24, Equatorial Pacific

-

500 0 500Kilometers

Figure 4. Graphs of sediment thickness vs. distance from the crustof the mid-oceanic ridges for five areas of the world ocean (fromEwing and Ewing, 1967). The location of Eltanin Cruise 19 profile

is given in Fig. 1.

Figure 5. Magnetic lineation stripes over the ReykjanesRidge south of Island. Black stripes are positive anom-

alies (from Heirtzler et al., 1966).

Gravity

Measurement of the strength of the earth's gravi-tational field constitutes still another phase of marinEgeophysical observations. These measurements ardifficult to make aboard ship, and the method didnot become widely used until about 1961. The gra.vimeter is a sophisticated spring balance that is highi)damped to minimize the effects of the ship's accelera-tions. The instrument is placed on a gyrostabilizedplatform near the center of the ship, so that th€roll and pitch of the ship will not affect the measure.ments. An electrical signal proportional to the motiorof the spring balance is selectively filtered to isolatthe residual accelerations due to ship's motion frorr,small changes in the earth's gravitational field. It ithis latter quantity that is of interest: from it, onEcan deduce the distribution of density within thcearth's crust and upper mantle.

The Hypothesis of Sea-Floor Spreading

The concepts of sea-floor spreading and plattectonics have been reviewed recently by Heirtzlei(1968), Menard (1969), and many others, and on])the highlights of these important hypotheses will bnoted here.

The idea that the sea floor is spreading laterall)with time, that it is breaking aparf at the mid.oceanic ridges, and that new material from belowis filling this "void" was formally proposed by Hes(1962) and by Dietz (1961). Earlier ideas of continental drift (e.g., Carey, 1958), which had fallerout of vogue, gained renewed interest. The primar)objection to the continental-drift theory had beerthe absence of a plausible mechanism by which thEcontinents could move about through the rigid crustof the earth. The new concept, that the sea flooiwas breaking in the center and moving laterallyproposed that the continents were not movingthrough the rigid crust of the earth but along witlit. The entire crust was moving over the surface othe earth as large, rigid plates or slabs, and thcontinents were simply protuberances and werintegral parts of these rigid plates. These ideas still dicnot gain much prominence, until Vine and Matthew,(1963) suggested an important corollary to the seafloor spreading hypothesis. They contended that anew material came up where the mid-oceanic ridgesplit apart, the material cooled and acquired a mag.netization in the direction of the earth's magnetifield. Since the earth's magnetic field is known tchave reversed its polarity many times throughout geologic time (Cox et al., 1967), there should be alter.nate positively and negatively polarized rockdistributed symmetrically about the mid-oceani

72 ANTARCTIC JOURNAl

ridges. Vine and Matthews then proceeded to testthis hypothesis by proposing a model of alternatelypolarized magnetic blocks (see Fig. 6). If the seafloor had been separating at a uniform rate, and ifthe chronology of the earth's magnetic field wereknown, one could compute the theoretical magneticanomalies due to the alternately polarized modelblocks and compare them with the observed anom-alies.

The Hypothesis Tested

The data collected along several transects of thePacific-Antarctic Ridge proved invaluable in theearly tests of the sea-floor spreading hypothesis andthe Vine and Matthews corollary. Pitman andHeirtzler (1966) presented and analyzed severalbathymetric and magnetic-anomaly profiles for theSouth Pacific and pointed out the following facts:

1) Magnetic-anomaly lineations were present andthey were parallel to the ridge axis;

2) The anomaly lineations were bilaterally sym-metric about the ridge axis;

3) A magnetic model consistent with that pro-posed by Vine and Matthews (1963) couldaccount for the observed anomalies near theridge crest;

4) A similar model was applicable to other ridgesin different parts of the world.

Fig. 7, adapted from Pitman and Heirtzler (1966),illustrates points 2 and 3 above. The magnetic-polar-ity chronology is known with confidence only back toabout 3'/2 million years ago (Doell et al., 1966), dueprimarily to the limitations of radiometric datingtechniques. If, however, one assumes a constant rateof spreading and generates a model of magneticblocks which are in agreement with the observedmagnetic anomalies, a geomagnetic time scale canbe extrapolated well into the geologic past (Vine,1966; Heirtzler et al., 1968). The South AtlanticOcean was selected by Lamont-Doherty workers asthe area where the extrapolation was most likely tobe valid, and a geomagnetic time scale was adoptedon the basis of the observed magnetic anomalies thereand the known geomagnetic time scale (Heirtzleret al., 1968). This hypothetical time scale has gainedvery strong support from the recent JOIDES deep-sea drilling operations in the South Atlantic Ocean(Maxwell, 1969). At the South Atlantic drillingsites, the ages of the basal sediments (sedimentslocated just above the basalt crust) were found tobe in close agreement with those predicted fromobserved magnetic anomalies and the extrapolatedgeomagnetic time scale.

Workers at Lamont-Doherty (e.g., Heirtzler et al.,1968) and elsewhere have since proposed that a

1000-4, I^ 1000

0A

11

C

3.541)'DGEOMAGNETICTIME SCALE

Figure 6. Schema of ridge growth, magnetic blocks, and theoreti-cal magnetic anomalies and their relationship to the known geo-magnetic time scale. Black areas represent positive polarity epochsand positively polarized magnetic blocks. Note the symmetry ofthe model about the ridge axis and the corresponding symmetryin the anomaly pattern. The to; ographic relief generally does not

affect the anomaly pattern significantly.

West EastMM400 200 0 200 400KM

Gammas00

10 00

OBSERVED1000

10 00

MODELoo: [ 1000

,FLTANIN 19

KMKM

'iii"JaJlIIJ.,:

Figure 7. Symmetric magnetic anomalies in the South Pacific incomparison with theoretical anomalies from model studies (modi-fied from Pitman and Heirtzler, 1966). The top anomaly curve isthe mirror image of the observed anomaly curve (middle) to allowfor evaluation of symmetry. The observed topography (black) andthe model of magnetic blocks are shown near the bottom. See

Fig. 1 for location.

May-June 1970 73

65150135120105 9075W

70 L-165 Li 7060

65180165150135120105907530

/ '

:: p'ENX

PLEISNX

60

60—130

40 Figure 8. lsochrons of the maximumage of the oceanic crust as geologicepochs. These epoch boundaries were

50 deduced from magnetic anomalies anda proposed geomagnetic time scale(Heirtzler et al., 1968). Discontinuitiesin age occur across fracture zones

60(shown as dashed lines). The figureis taken from Pitman (1969) and Her-

ron and Hayes (1969).

/EPICENTER

RIDGE --

O/7-TRANSFORMFAULTT

T RA N SC URRN IFAULT

Figure 9. Schema of ridge offsets and earthquakes. A) Hypotheti-cal mode of development now generally rejected on the basis ofearthquake distribution and mechanism studies (Sykes, 1967).B) Initial offset of ridge axis and corresponding earthquake dis-tribution. The area between the offset ridge axes is a transformfault. Note that the direction of relative motion in B is oppositeto that of A2. Studies by Sykes (1967) demonstrated that presentmotion along ridge offsets is consistent with B, the transform fault.

similar pattern of magnetic anomalies is observedin all oceans, and that this pattern represents theunique character of the earth's magnetic-polarityhistory during the last 80 m.y. By identifying char-acteristic anomalies within the pattern, a maximumage was assigned for the underlying igneous rock,and a map of isochrons—the lines of equal age—was proposed for large parts of the ocean basins (Fig.8).

The Mid-Ocean Ridge system, although contin-uous in the overall sense, is composed of a seriesof discrete segments, which are offset by as muchas several hundred miles in many instances. Theexistence of these offset ridge segments has beendifficult to explain. The lines along which theoffsets occur have commonly been referred to as"fracture zones." Only that portion of a fracturezone lying between the offset ridge axes is character-ized by seismic activity (see Fig. 9B) ; it has beenreferred to by Wilson (1965) as a transform fault,a special type of strike-slip fault. On opposite sidesof these fracture zones there is a discontinuity inage and in morphology or in average depths of the

74

EL 23000 3130 29 28 27

ommasJ N_Vv_^^

-I -- .-A-

0160 200 300 400 500KM

Figure 10. Bathymetric and magnetic-anomaly profiles from thecrossing of a fracture zone associated with the Pacific-AntarcticRidge. Note that the magnetic-anomaly pattern is repeated (anom-alies numbered 27, 28, 29), indicating that a fracture zone wascrossed. Along this profile, the location of the fracture zone is alsocoincident with a major change in morphology. The offset ofmagnetic anomalies is about 400 km. Taken from Pitman of at,

1968.

ocean (see Fig. 10). In fact, the fracture zone shouldrepresent a discontinuity in almost every observablegeophysical parameter.

The discontinuity in depth across a fracture zoneis dependent primarily on three factors: the distanceof the observation point from the axis of the ridge,the amount of offset between ridge crests, and theregional and local relief of the ridge. As one proceedsvery far away from the ridge crests, the morphologicexpression of the fracture zones usually becomes quitesubtle or totally indiscernible. It is at these localitiesthat the discontinuities in other observed parameters(e.g., magnetic anomalies) must be employed to rec-ognize the presence of fracture zones. If these dis-continuities are successfully identified, one can mapthe surface trace of the fracture zones over verylong distances. In so doing, the relative motion ofadjacent crustal plates at the time the fracture zoneswere active is determined. If one can map therelative motion of these plates continuously through -out geologic time, one can work backwards fromthe present to determine the relative positions ofthe continents at any time (assuming that the

ANTARCTIC JOURNAL

29 28 2726 25

travelled along passively with the crustal plates). Thereconstruction of Gondwanaland by this approach isin general agreement with reconstructions that havebeen proposed on the basis of the fit of continentaloutlines and on the basis of geologic and structuralcontinuities and similarities across the oceans.

As mentioned previously, seismic profiling showsthat there is very little sediment present near thecrests of the mid-oceanic ridges. The thickness ofthe sediment veneer generally increases with in-creasing distance from the ridge crest; however, therate of sediment deposition is affected by bottomcurrents, levels of biologic productivity, and depthsof the ocean floor. If all of these factors remainedconstant, the thickness of sediment at any point onthe floor of the ocean should be directly propor-tional to the age of crustal rocks there.

The basic concepts of sea-floor spreading requirethat the crests of the mid-oceanic ridges be theyoungest part of the crust beneath the ocean floor.The theory thus predicts the near absence of sedi-ments on the crests of the ridges, as those portionsare not old enough to have acquired a significantamount of sediment. Likevise, the general increasein the thickness of sediments in the directions awayfrom the ridge crests is conformant with theory (seeFig. 4).

The gravity observations, while extremely valuableor other studies, have added little direct insightnto the nature of the sea-floor spreading and plate-ectonic processes. The oceanic trenches, presumedites of crustal underthrusting, are characterized byarge, negative gravity anomalies. These anomaliesre not simply the result of narrow topographicrenches: other relatively low-density crustal mate-rials must be present near the trenches, but theisposition and nature of the low-density materials

S

a subject of controversy. One view is that low-ensity sedimentary material has been "scraped off"he plunging crust at the trench axis and piled ontohe landward flanks of the trenches. However, this

process alone cannot explain the observed gravityanomalies, and other major crustal readjustmentsmust be introduced to do so. Gravity anomaliesobserved over the mid-oceanic ridges and the densitydistributions deduced from them (Taiwani et al.,1965) place strong constraints on the nature of themechanism giving rise to the sea-floor spreadingprocess.

In addition to many important marine geophysicalobservations, earthquake seismology has contributedsignificantly to theories regarding the evolution ofthe ocean floor. Earthquakes are not randomly dis-tributed over the surface of the earth, but are con-centrated in narrow zones. The major portion ofseismic activity centers around the circum-Pacific,near the deep-sea trenches. A large number of earth-

quakes also occur along young mountain belts, forexample in southern Europe and central Asia. Otherearthquakes are concentrated very near the crestalportions of the mid-oceanic ridges and along theoffsets between these ridges. It has only been withthe advent of refined seismic instrumentation, theincreased density of seismic stations throughout theworld, and the use of electronic computers that theprecise location of these earthquakes became ob-tainable (Sykes, 1963). It was found that many ofthe earthquake zones thus defined were even moresharply delineated than had been expected. Also,most earthquakes appear to be systematically dis-tributed with respect to depth. Earthquakes associ-ated with the mid-oceanic ridges are generally clas-sified as shallow—that is, they occur within the upper30 km of the earth's crust. Nearly all of the inter-mediate and deep earthquakes (those from 30 kmto 700 km) are distributed in and around themid-oceanic trenches. Shallow earthquakes are alsoassociated with the trenches. The circum-Pacific earth-quakes define sloping planes which dip beneath thecontinents or island arcs bordering the deep oceanictrenches.

By measuring the direction and nature of the firstearthquake motion at recording stations all over theearth, one can often determine the probable orienta-tion and direction of the faulting that released theenergy of a particular earthquake. As previouslymentioned, the mid-oceanic ridges are offset, and theearthquakes occur, along the crests and along theoffsets of the crests. If the ridge crests were oncecontiguous and were subsequently displaced along thepresent offsets, and if the recent earthquakes repre-sent the continued activity along these zones, oneshould be able to determine if the nature of the in-ferred faulting is consistent with the sense of offsetof the mid-oceanic ridges (Fig. 9A).

Studies by Sykes (1967) indicated that the observedsense of earthquake first motions were opposite to thatinferred from the apparent offset of the mid-oceanicridges, but consistent with that of a transform fault(Fig. 9B). In fact, Sykes' study was undertaken totest the plausibility of the new type of fault—the"transform fault"—proposed by Wilson (1965). Thetransform fault is an integral part of the present con-cepts of sea-floor spreading and global tectonics.

Several important observations on a global scalemust be explained in any hypothesis about the evolu-tion of the ocean basins, viz:

1) The presence of mid-oceanic ridges with off-sets; 2) the presence of oceanic trenches; 3) the dis-tribution (amount and age) of sediments over theocean floor; 4) the similarity of continental outlines;5) the apparent geologic fit of the continents andother indications of polar wandering and/or conti-nental drift; 6) the presence of long, linear magnetic

May-June 1970 75

stripes over the ocean floor that are sub-parallel tothe mid-oceanic ridges; 7) the striking symmetry ob-served in the magnetic lineation pattern; and 8) thedistribution and nature of earthquakes associated withdeep-sea trenches, oceanic ridges, and transformfaults.

Sea-floor spreading and plate tectonics are by farthe most satisfactory hypotheses available to explainthe above observations. Probably the most importantquestion facing us is that of the mechanism for sea-floor spreading. What is it that causes rigid crustalplates to split apart and give rise to the mid-oceanicridges? Heat is by far the largest known source ofenergy available, and the most popular view is thatsome type of interior convective motion is responsiblefor moving the plates about the surface of the earth.It was thought earlier that there was a direct corres-pondence between the positions of the mid-oceanicridges and trenches and the configuration of the un-derlying hypothetical convection cells. However, inview of the highly complex surface distribution ofthe ridges and trenches, this now seems improbable.If convection is the basic mechanism, it seems morelikely that the geometry of the underlying convectivecells may be simple, but that the convective patterngives rise to a much more complicated expression ofcrustal-plate boundaries. The observed earthquakezones define the boundaries of the crustal plates andattest to the interaction of major crustal elementsalong these zones (Isacks et al., 1968). The preciselocation of earthquake epicenters provides extremelyvaluable information for the high-latitude regions,which are inaccessible to many oceanographic re-search vessels. The Antarctic Continent, for instance,is almost entirely encircled by earthquake epicenters(Fig. 11). However, there is little or no evidence ofearthquakes on the Antarctic Continent itself, im-plying an absence of major active tectonic processesthere, and indicating that the crustal plates surround-ing the Antarctic are presently moving away fromthe antarctic plate. The distribution of magneticanomalies in a broad latitudinal zone about the Ant-arctic strongly suggests that this has been the situa-tion for a significant length of time (of the order of35-40 m.y.). The effective migration of spreadingcenters away from the antarctic plate and also fromthe area between the Pacific-Antarctic Ridge and theChile Ridge (Herron and Hayes, 1969) is a majorfactor in the conclusion that there is no one-to-onecorrespondence between the geometry of hypotheticalconvection cells and plate boundaries (e.g., Isacks etal., 1968; Le Pichon, 1968; Hayes and Ewing, inpress).

The major fracture zone traces in the southwesternIndian Ocean are mutually consistent as defined byoffset magnetic lineations, morphology (unpublishedEltanin data), and earthquake epicenters (l3arazangi

0.

90 W 90 E

\\

/80

Figure 11. Polar projection of earthquake epi-centers from 900 S. to 30° S. The data, plotted byDorman and Laverty, are the same as those pre-

sented in Barazangi and Dorman (1969).

and Dorman, 1969). These data collectively providevery strong independent evidence in favor of therelationship of Antarctica and Australia as contiguousparts of a larger landmass (Gondwanaland) prior toearly-middle Cenozoic time. It is interesting to notethat all popular reconstructions of Gondwanaland(e.g., Hurley, 1968) leave segments of the antarcticcoast open. The exact positions of these open coastalsegments are not well determined, but one may heisomewhere near the present longitudes of 40°E.-170°E. The ocean basin opposite this area should rep-resent an old ocean—existing prior to the breakupof Gondwanaland; thus, very old oceanic crust shouldbe found on this oceanic periphery of Antarctica.

Acknowledgements: The results summarized herwere taken from the works of many scientists. Thereferences cited are not intended to be an exhaustivelist, but to be representative of many possible sources.The extensive bibliographies of the selected referenceinclude the major works drawn on for this brief re-view. The research has been supported by the Na-tional Science Foundation and the Office of NavalResearch.

References

Barazangi, M. and J . Dorman. 1969. World seismicity mapscompiled from ESSA, Coast and Geodetic Survey, epi-center data, 1961-1967. Seismological Society of America.Bulletin, 59: 369.

Carey, S. W. 1958. A tectonic approach to continental drift.In: Continental Drift, a Symposium (S. W. Carey, Edi-tor), p. 177-355.

Cox, A., G. B. Dalrymple, and R. R. Doell. 1967. Reversalsof the earth's magnetic field. Scientific American, 216: 44.

Dietz, R. S. 1961. Continent and ocean 'basin evolution byspreading of the sea floor. Nature, 190: 854.

Doell, R. R., G. B. Dalrymple, and A. Cox. 1966. Geomag-netic polarity epochs: Sierra Nevada data, 3. Journal ofGeophysical Research, 71: 531.

Ewing, J . and M. Ewing. 1967. Sediment distribution of themid-ocean ridges with respect to spreading of the sea-floor. Science, 156: 1590.

76 ANTARCTIC JOURNAL

Ewing, M., R. Houtz, and J . Ewing. 1969. South Pacificsediment distribution. Journal of Geophysical Research,74: 2477.ayes, D. E. and M. Ewing. In press. Pacific boundarystructure. In: The Seas, vol. VI (A. Maxwell, Editor).New York, Wiley, Interscience.

iayes, D. E., J . R. Heirtzler, E. M. Herron, and W. C.Pitman III. 1969. Preliminary Report of Volume 21,U.S.N.S. Eltanin Cruises 22-27, Jan. 1966-Feb. 1967,Part A: Navigation; Part B: Bathymetric and Geomag-netic Measurements. Lamont-Doherty Geological Observa-tory. Technical Report No. 2-CU-2-69.

Heirtzler, J. R., G. 0. Dickson, E. M. Herron, W. C. Pit-man III, and X. Le Pichon. 1968. Marine magneticanomalies, geomagnetic field reversals, and motions of theocean floor and continents. Journal of Geophysical Re-search, 73: 2119.

Fleirtzler, J . R. 1968. Sea-floor spreading. Scientific Ameri-can, 219: 60.

Heirtzler, J . R., D. E. Hayes, E. M. Herron, and W. C.Pitman III. 1969. Preliminary Report of Volume 20,U.S.N.S. Eltanin Cruises 16-21, Jan. 1965-Jan. 1966,Part A: Navigation; Part B: Bathymetric and Geomag-netic Measurements. Lamont-Doherty Geological Observa-tory. Technical Report No. 3-CU-3-69.

J-Ieirtzler, J . R., X. Le Pichon, and J . G. Baron. 1966. Mag-netic anomalies over the Reykjanes Ridge. Deep-Sea Re-search, 13: 427.

Herron, E. M. In press. Crustal plates and sea floor spread-ing in the Southwest Pacific. Antarctic Research Series.

Herron, E. M. and D. E. Hayes. 1969. A geophysical studyof the Chile Ridge. Earth and Planetary Science Letters,6: 77.

Hess, H. H. 1962. History of the ocean basins. In: Petro-logic Studies, p. 599-620. New York, Geological Societyof America.

Isacks, B., J . Oliver, and L. R. Sykes. 1968. Seismology andthe new global tectonics. Journal of Geophysical Research,73: 5855.

Le Pichon, X. 1968. Sea-floor spreading and continentaldrift. Journal of Geophysical Research, 73: 3661.

Maxwell, A. 1969. Recent deep sea drilling results from theSouth Atlantic. American Geophysical Union. Transac-tions, 50: 113.

Menard, W. H. 1969. The deep ocean floor. Scientific Amer-ican, 221: 126.

Pitman, W. C., III. 1969. Sea floor spreading. Science Jour-nal, 51.

Pitman, W. C., III and J . R. Heirtzler. 1966. Magneticanomalies over the Pacific-Antarctic Ridge. Science, 154:1164.

Pitman, W. C., III, E. M. Herron, and J. R. Heirtzler, 1968.Magnetic anomalies in the Pacific and sea floor spreading.Journal of Geophysical Research, 73: 2069.

Sykes, L. R. 1963. Seismicity of the South Pacific Ocean.Journal of Geophysical Research, 68: 5999.

Sykes, L. R. 1967. Mechanism of earthquakes and nature offaulting on the mid-oceanic ridges. Journal of Geophysi-cal Research, 72: 2131.

Talwani, M., X. Le Pichon, and M. Ewing. 1965. Crustalstructure of the mid-oceanic ridges: 2, computed modelfrom gravity and seismic refraction data. Journal of Geo-physical Research, 70: 341.

Vine, F. J . 1966. Spreading of the ocean floor: new evi-dence. Science, 154: 1405.

Vine, F. J . and D. H. Matthews. 1963. Magnetic anoma-lies over oceanic ridges. Nature, 199: 947.

Wilson, J . T. 1965. A new class of faults and their bearingon continental drift. Nature, 207: 343.

Late-Season Field Activities(February-March 1970)

By February, the field research of the 1969-1970summer had been virtually completed except in theAntarctic Peninsula area and the Weddell Sea. Otheractivities continued, however, as the station operationsphased into the winter programs. The operationalseason came to a close on April 4.

As reports of all research projects carried out dur-ing the 1969-1970 summer will be presented in theJuly-August issue of the Antarctic Journal, only cur-sory mention of them will be made in this summary.

McMurdo Station

A late-summer research program was conductedat McMurdo in February by Dr. Albert R. Towle ofStanford University for echinoderm materials to beused in studying physiological aspects of respiration.With the aid of the icebreaker USCGC Burton Is-land, Dr. Towle collected sea urchins and starfish offCape Royds; other starfish were caught near the sta-tion using mesh-wire traps operated from the shore.

U.S.S.R. exchange scientist Sergei MikhailovichMiagkov, who is to winter over at McMurdo Station,continued his geomorphological studies in the Trans-antarctic Mountains. Flights for visual and photo-graphic observations were made from the ThielMountains to Drygalski Glacier, and 30 days of fieldwork was accomplished in Taylor and Wright Valleys.A phototheodolite survey was made of Taylor, Rhone,Meserve, Wright Lower, and Sandy Glaciers to deter-mine their evolution in recent times.

The new USARP chalet, NSF's operations centerfor the summer research programs, was occupied onFebruary 15, and a new 62-man USARP quartersbuilding was occupied on February 25. The 188-manNavy wintering-over complement will also be housedunder one roof, as the barracks portion of the new,spacious 257-man personnel facility had been com-pleted by the end of summer.

By February 25, the last of this year's USARP sum-mer personnel, including Mr. Jerry W. Huffman,USARP Antarctica Representative, had left forChristchurch. The same day, the Commander, Ant-arctic Support Activities, Captain E. B. Rubey, Jr.,turned over command of Detachment Alpha, ASA,to Commander W. L. Frost, the McMurdo Stationleader for the Deep Freeze 70 winter. With the de-parture of Captain Rubey and Mr. Huffman on thelast LC-130 flight, McMurdo's winter commenced.

Winter research programs at McMurdo Station in-clude satellite tracking by the University of Texas,

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