May 10, 1971 - on the coast of Ellsworth Land in Antarctica. (72°S, 90°E). .... E Ellsworth Land, Antarctica .... Isacks, B., J. Oliver, and L. R. Sykes, Seisinology.
VOL. 76, NO. 14
JOURNAL OF GEOPHYSICALRESEARCH
MAY 10, 1971
Contrasting Geometric Adjustment Styles of Drifting Continents andSpreading SeaFloors K•WN
S. Ro•o•o
Department of Geological Sciences University of Illinois, Chicago 60680
The geometry of movingoceanic lithospheric platesis modeled easilyby combining plate subduction anddestruction with an application of Euler'stheoremto the Euclidean sphere
of the earth'ssurface.However,continentalmasses mustrespondto drift motionin a differ-
entmanner because theycannot sinkintothedenser mantle.A theoretical geometric analysis, testedby datafromtheeastern Pacificmargin, is presented to illustrate theuniqueresponse to drift of largecontinental masses. If theyarelargeenough to comprise a significant fraction of the earth'scircumference, an alignment of continental masses shouldundergo extension until the frontierbetweenthem and the oceanoverwhichthey encroach approximates a globalgreatcircle.This extension shouldentail secondary sea-floor spreading transverse to primaryspreading directions,localizedwherecontinentalmassesalong the frontier are narrowand thereforeweakest,and ductilestretchingand fracturingof the narrowcon-
tinentalspans. Continued sea-floor spreading andcontinental driftbeyond thestageat which the continent-ocean frontierapproximates greatcircleconfiguration presents a newproblem. The maximizedfrontierlengthmustnow adjustto a decreased area of encroached ocean.
Adjacent continents mustnowapproach eachotheralong thefrontier direction, andformerly
stretchedand disruptedcontinentalspansmustbe 'bent' to accountfor the excess frontier length.Recentreconstructions of post-Paleozoic continental drift indicatethat the American
andAntarctic continents andintervennig Caribbean andScotia seas andisland arcsmayhave
undergone such a history.
The hypothesisof sea-floorspreadingpostulated independentlyby Holmes [1945] and Hess [1962], which a recent series of significant papers [Morgan, 1968; Le Pichon, 1968; Heirtzler e• al., 1968; Isacks e• al., 1968] developed into the 'new global tectonics,' attributes continentaldrift to the movementof rigid lithospheric plates composedof oceanic crust and upper mantle. These plates are thought to be generated at oceanic ridges from rising asthenospheric mantle material. Continental drift is envisioned as being caused when spreadingbeginsunderneathcontinentalmasses that are rifted apart and rafted away from spreading centers. If the distal portion of a moving plate is oceanic,it can underthrust an opposingplate that may or may not carry continental crust. Such an underthrustingplate is believed to be destroyedby reassimilationinto the mantle.
The
site of this destruction is
marked at the surface by a trench, which separatesthe downgoinglithosphere from an Copyright ¸
197I by the American GeophysicalUnion. 3272
islandarc or an Andean-type continental margin.
Continental masses are generally treatedas essentially passive and rigidunless they are raftedto a trench by a downgoing plate.Only in this circumstance do they manifesttheir individuality because theycannotsinkintothe
denser mantle. Thuscontinents borne byoppos-
ingplateseventually collideto formlinearzones
of thickened sialiccrust(compressional mountain beltsof alpinetype).
Oneofthemostintellectually appealing characteristics of platetectonictheoryis the elegantlysimplegeometry of plategeneration and propagation, i.e.,Euler'stheorem asit applies to a Euclideansphere:any relativemotionof two platesat the earth'ssurfacecan be resolved
intoa rotationabouta globalaxis,andall relative velocities betweenthe two platesmustlie along small latitudinal circlesabout that axis. Spatial geometricproblemsdo not arise from
thecontinued production andmigration of lithosphere because theplatesdepartfromtheearth's surfaceand are destroyed at depth.
3273
Unfortunately, because plate theory treats the continentsas passive,and becauseof the ease with which most plate motion can be modeled by a combination of Euler's theorem and plate destruction,attention hasbeenfocused mainly on the movementsof oceaniclithosphere. Many 'new global tectonicians' have become overly casualabout the role, behavior,and deformation of the continentsundergoingdrift. This report is virtually in completeharmony with the fundamental assumptionsand data of plate tectonic theory. However, an attempt is made here to extend the 'new global tectonics' by examining an additional, important global aspect. The principal thesis of this report, doubtlesslycontroversial,is that drifting continental massesmust also obey the geometry that governsthe movementsof large areal segments over a sphere; further, that continents cannot resolve areal geometric problems by sinkingbeneath the surfaceand thus must re-
the theoremto an analysisof plate production and propagation in the North Pacific, demonstrating that Wilson's [1965] ridge-ridge transform faults and their associated fracture constitute
small circles that
zones
describe the rela-
tive displacements betweenplates. The now classicseriesof paperspublishedin 1968 by Morgan, Le Pichon,Heirtzler et al. and Isacks et al. applied the same geometry to demonstrateconvincinglythat sea-floorspreading rates calculated from magnetic anomalies parallel to ridge crestsvary directly with distance from plate rotational poles. From these reports, it can be concludedthat: (1) each ridge-crestsegmentbetweenconsecutivetrans-
forms is a segmentof great circle passing throughthe spreading poles(Figure1); (2) an
spondto migration in a manner that differs fundamentallyfrom that of oceaniclithosphere. If driftingcontinentalmasses are largeenough in area to approach global magnitude,their deformativeadjustmentsto the requirements of sphericalgeometryshouldremainevidentat the earth's surface. The eastern margin of the Pacific Ocean will be examined in this light. A
corollaryto this thesisis that where a continentalmasscomprises a significantportionof a tectonic plate, the dynamic adjustments required of it by the earth's sphericalconfiguration may preventit from passivelyobeyingthe advective surface motion of the lithosphere on
which it rides.If this corollaryis valid, drifting continentsmay even influenceand alter the
geometryof the lithospherebeing produced behind them. The Atlantic Oceanwill be viewed
briefly in this contextduringdevelopment of the principalargument.It is hopedthat these analyses will generatefurther reexamination of the behavior of continentsduring drift and sea-floorspreading. LIMITATIONS OF EULER'S THEOREM
The rulesof sphericalgeometryasthey apply to continental drift were first demonstrated
when Bullard el al. [1965] applied Euler's theoremin reconstructing the predrift fit of the continents around the Atlantic
Ocean. Subse-
Fig. 1. Sea-floor spreading and Euler's Theorem. Transform faults are necessary to reconcile the constant width of an individual magnetic stripe segment with the requirement of increased spreading rates with decreasedspreading latitude. Consecutive
transforms
are drawn
much
farther
apart than is typical in nature. Modified from quently,McKenzieand Parlce•'[1967] extended Heirtzler, 1968.
KELVIN S. RODOLFO
327-! individual
m:1gnetic
anomaly
stripe
between
consecuti,·e transforms or fracture zones has a constant width;
and
identical
within
stripes
two chronologically
(3) a
single
lithospheric
which
trend
respectively
eastward
into
the
Tethys belt between Europe and Africa, and westward into the Caribbean
region between
North and South America.
plate but bounded by different transforms or
Ball and Harrison [1970J suggest that plate
fractures ha\"e different widths because angular
size (and, presumably, operations of Eulerian
rotational velocities of spreading must be iden
spreading about a single pole) are limited to
tical, but spreading velocities must vary with
the magnitude of typical continental blocks.
latitude from the spreading poles. Transform
One possible explanation for the segmentation
faults
thus
appear
to
be
mechanisms
that
and discordant
movement
of the American
reconcile conditions 2 and 3 and sen·e as zones
Western Atlantic lithosphere is simply that the
of
history of the Atlantic opening is not a single
geometric
adjustment.
Exi�tence
of
this
mechanism made theoretically permissible the
event.
assumption
[lsacks et al.., H)68; Morgan, 1968; 1968; Molnar and Sykes, 1969J that
geomagnetic data that the separation of North
a single plate consisting of both American con
tion of South America from Africa. A comple
Le Pichon,
[1968J
Heirtzler
America
from
Europe
has
concluded
preceded
the
from
separa
tinents and the western Atlantic can move as
mentary explanation exists: a single continental
a unit.
mass composed of the Americas constitutes such
More detailed work, however, has indicated
a
large
proportion
of
the
globe's
area
and
that individual plates may be limited to smaller
meridional circumference that it cannot migrate
dimensions. A series of paper
over the sphere without being areally deformed.
Smith, 1968;
Ball
[Funnell and and HalTi
et al.., 1969; Bal.l
son, 1970J has shown the Atlantic Ocean to be composed of three latitudinal pairs of plates separated by
two east-west
oriented
shaped zones (Figure
2).
and
characterize
sinistral
shear
wedge
North-south extension the
wedges,
If the continental mass undergoing distortion remains coupled to the lithosphere on which it rides, the lithosphere must also be distorted. Such a deformation of
oceanic lithosphere
would have to take two forms. First, the direc tion of sea-floor spreading at the ridge behind the continent could undergo adjustment in the manner
proposed
by
Menard
and
Atwater
[1968J. Second, oceanic lithosphere already in existence would have to be either augmented or destroyed along directions transverse to the primary spreading direction. The north-south extension and shear of the Atlantic wedges in Figure 2 would have served this function. The
foregoing
analysis
would
remain idle
speculation unless it is demonstrated that con tinental masses do undergo deformation solely as a geometric consequence of their migrations over the earth sphere. The following analysis provides such a demonstration. Some of the terminology used to describe areal relationships (e.g., empire, country, frontier)
were inspired
by Blackett's [1967J exposition of t.he famous topologic 'four-color problem.' MODIFICATIONS IN AREAL GEOMETRY OF CONTINENTS AND OCEANS DURING Fig. 2. Atlant.ic sea fioor, composed of three latitudinal pairs of lithospheric p lates between which north-south extension and transcurrent faulting have occurred in two zones (stippled areas). Modified from Funnell and Smith, 1968.
CONTINENTAL DRIFT General. statement.
The following qualified
assumptions, all in conformit�· with 'new global tectonic' dogma, are made:
DRIFTING CONTINENTS,SPREADING SEAFLOORS
3275
sively of all the continentsand covered40% of the globe.The other empire, coveringthe 2. Once differentiated out of the mantle, remaining 60% of the earth's surface, was continental material remains at or close to the wholly oceaniclithosphereand, extendingthe earth's surface.It may be recycledpalingeneti- terminologyof Dietz and Holden [1970b], can 1. Oceanic lithosphere is continually producedat ridgesand destroyedat trenches.
cally, but continentalmasses,even when fragmented by drift, occupy essentiallypermanent percentagesof global surface. 3.
The
continents
at the end of the Paleo-
zoic era were a single landmass.Modern seafloor spreading data support this assumption over the concept of separate Laurasian and Gondwanic supercontinents[Heirtzler, 1968; Dietz and Holden, 1970a, b]. If future data revive the idea of two separatelandmasses the argument will not be affectedsignificantly. 4.
Pre-Triassic
continental
crust
has not
diminishedappreciablyin areaup to the present (as a consequenceof assumption2). At any moment in time since Pangaeabegan to break up, its fragmentshave coveredapproximately 40% of the earth's surface.This figure includes all oceanicareas of continental shelf and slope shoaler than 2000 meters (from the data of Menard and Smith [1966]). Someminor palinspastic areal lossmay have resulted from thickening of sialic crust at trenchesand from continental collisionsalong the Alpine-Himalayan belt.
5. More importantly, if the leading edge of a lithospheric plate is continental, the length of this edge cannot be shortened.However, it may be lengthenedor bent if subjectedto the proper stressand if the width and thicknessof the continental mass is sufficiently small to permit plastic deformation. 6. The earth's 40,000-km meridional circumferencehas remained unchangedsince the breakup of Pangaea.Although the ensuingdiscussion embodies some of the
features
of a
'topologic'argument for an expandingearth presented by Meservey [1969], the present treatment does not involve global expansion. The expanding earth concept also has received insufficientattention from global tectonictans but is not germane to the present argument.
Predri/t setting. The earth's surfaceat the end of the Paleozoic era consistedof two sepa-
be termed the Panthalassicempire.
The total lengthof frontierbetweenthe two empiresis not known.In the ideal case(Figure 3), if Pangaeahad the form of a perfect spherical segment, the frontierwouldhavebeen a small circle equivalentto an 11.5ø parallel, with a perimeter of about 39,200 km, or some 800 km smaller than the global circumference. From any reasonableapproximationof Pangaea (Figure 4a), it can be safelyassumedthat the imperialfrontier was muchmore irregularand longer. Drift prior to 'great circle'stage. During the first stageof continentaldrift, whichcommenced about 200 m.y. ago, the Pangaean empire was fragmentedinto individualcontinents,and newly emergent 'countries'of post-Paleozoicsea floor between the fragments were incorporated into the Pangaeanempire. Areal growth of the Pangaeanempire must have been accomplishedat the expenseof the Panthalassicempire. Extending the ideal case of Figure 3a, the imperial frontier must have become larger until it attained a maximum, equivalent to the 40,000-km global circumference. In reality, the original departures of the frontier from the ideal configurationmust have persisted and may even have increased. No tectonic or geometric mechanism exists that would have forced an originally irregular frontier into a more ideal configuration. Thus, the 800-kin difference between the idealized Pangaeanperimeter and the global circumferencerepresentsa minimum required increasein frontier. An inefficientapproach of the frontier toward its maximum may well have resulted in more than 40,000 km of frontier. After spreadingand drift had operatedfor a sufficient length of time, the Pangaean and Panthalassicempireseventually each covered half of the globe,and the imperial frontier was at its maximum (Figure 3b). It must be emphasizedhere that the entire frontier couldnot have acted physicallyand temporally as a unit because stresses cannot be transmitted along
rate areal 'empires' characterizedby differing crustal massesof such magnitude. Thus some crust. The Pangaean empire consistedexclu- segmentsof frontier may have been still
............ .......
L/.././ / l
.... 4,..•'•-.•o,ooo •m
•'•.;-•!'•-::•: .?-:::..' ..:.. ?•..:: . •' e•• :-','*';L•,' '" ' '..",-•'.• --•'•
b
Ao= X 106km 2=2•Rh ß 204 . 6371-5100 ..o • bo: 5100 km
/
•o =arcsl n---•¾•T .... ••.go
•/• /-
l_
ro= Rcosq• ø : 62 43 km
Po: 2•,(6243) --39,200 km •)p: 19ø pp=37,840km
• '
..--
,•-": :
..:.:..-•: ,,•,,•
ß•'•'-.'*•?•":ø ....'•-*-•
70ø
/,// / /'•'• ••••......•• ""'"'"'•-'""'"• • ••
:.L,-'
-'%*-•'it-.*'*:: ...:..,-..?•-?'
•
.. q•*-.%.-:•%..'. •....
?•'-"':•"" '
•- •oo•.,•......._-......-.,:.• _.•._•....•
•/•.
d
•:• •t• ""•
•(•:N '•• '-...... ----.'•:
C.
'.•:i::.
ß:i.:'!'. ..:•!i•,.,. •.•;,,. ,-'"'."' '. ' ' i:i:i:?" .:,.-o.,,..-.-.. ß o ::..:..:.'.
...%:-2..'? •,-'-" ::.:.:3' ::..?,, *.,,-.V" --.-•.",•"'."• .... ' :'::' •: ß'-•
.'
ii.
,•, ,,,
... / 'i:i::'.•
"-., "T. ' .....:,.'.-:!•!:i:'
.'.'
'-: -!..•. ;;,•
ß //
Fig.3. Idealized area] geometry of driftingj_
6oø•
............... i!ii::i::iiiii::::i::i•i::ili:,:...'" ':':*•::::i•i::::.
continents and the Pangaean-Pantha]•ssie fron-
tier. In •, Pangaea approximates a spherical segment of one base, the small circle. po "'• 39,200
',
km, calculated as shown. Inb,new se• floor has "• .... '"'"'"'"'"•!'"""•o/i• rafted the continents to the maximum imperial frontier p• ----40,000 km. Continents have moved apart along frontier by transversesea-floorspreading (stippled zones). Narrow continental spans at transverse spreading zones have been attenuated and fragmented. In c, spreading has brought frontier to the present Pacific margin p•. Continents have moved closer to each other along frontier; stretched and fragmented continental spans have looped behind the continents.
Fig. 4. Aitoff projections' of post-Paleozoic breakup and drift. of Pangaea according to Dietz and Holden [1970a, b]. In a, great circle distance A-B is •--13,100 km; in b, •--14,900 km; in c, --•16,600; in d, •--16,100km; and at present in e, --•14,160 km. These v•lues 'are plotted in Figure 6. Heavy meridians in a indicate an arbitrary great circle.
DRIFTIN(• CONTINENTS,SPREADING SEA FLOORS
proaching the maxi•num while others already beyond it were responding to new geometric requirements. The frontier could have been increased by
sea-floor spreading transverse to the primary spreading direction and frontier. The Atlantic wedge-shaped zones of Figure 2 could have served this purpose.
Continentsundergoingthis motionwouldhave moved farther from each other along the frontier direction. Such movement is indicated by
the Pangaean reconstructionof Dietz and Holden [1970a, b]. The relative distancesof two arbitrarily chosen points along the AntarcticNorth American portion of the frontier were measured by standard spherical trigonometry. Point A is located at the present head of the Gulf of Alasks (60øN, 142øE) and point B lies on the coast of Ellsworth
Land
in Antarctica
(72øS, 90øE). An original great circle distance of about 13,100 km 200 m.y. ago is indicated. After 20 m.y. of drift, the distanceis shownto have increasedby 1800 km (Figure 4b). A maximum distanceof 16,600 km, representing an increaseof 3500 km, is shown for the end of the Jurassic,135 m.y. ago (Figure 4c). The increasein frontier representedby these figures as well as by the idealized geometry would have been accomplishedmost easily in localities
where
continental
masses were either
3277
Hamilton [1966], summarizingreports of a number of workers,has concludedthat narrow isthmian connections between the Americas and between South America and Antarctics existed
during Mesozoictime. Islands of both the Carib-
beanandScotiaarcs(Figure5) includeTriassic and Jurassicrocksnow areally divorcedfrom their original provenanteson the continents. Dalziel [1970] reports that the northern and southern limbs of the Scotia arc inchide Paleo-
zoic and possibly Precambrian rocks as well. From the initial conceptionof continentaldrift [Wegener, 1924] both arcs have been interpreted as disruptedspans betweenthe conti-
nents.Carey [1958] gavethem as examples of oroclinotaths(orogenicbelts that have under-
gonesubstantial bendingand stretching). Dri]t subsequent to 'greatcircle'stage. After
theimperialfrontierhadattaineditsmaximum, andthe composite Pangaean empireandPanthalassieempireeachcovered halfthe earth,a new geometric problem arose. The stretched frontier
nowbeganto enclose a continually decreasing Panthalassic area.
The Pacific Oceanrepresentswhat remainsof
Panthalassa. Its area is about 172 X 100km2, includingthe deeperoceanicportions of the SouthChina,Japan,Okhotsk,and Beringmarginalseas.Extendingthe idealcase(Figure3c), if the Pacific were configuredwith maximum
missingor narrowand thereforeweak.Tensional geometric economy,its perimeter would be a stresses indicated by these movements may small circleequivalentto a 19ø parallelwith a have fragmented the narrow continental crust length of 37,840 km, 2160 km less than the into islandsseparatedby the oceaniclithosphere producedby secondary(transverse)spreading. However, as Cullen [1969] has pointed out, tensional fracture of rock is often precededby some form of plastic deformation. Plastic lengtheningof narrow isthmian portionsof the continental frontier may also have contributed to the lengthening of the frontier. Southern Mexico
and Central
America
do not conform to
attempted pre-Atlantic reconstructionsof the continentalfits. Bullard e• al. [1965] terminate North America just south of the Tropic of Cancer without explanation. Dietz and Holden [1970a, b] fragment Yucatan and Honduras into their eratonic elements and arbitrarily rotate the eratons into the gap of the Gulf of Mexico. These difi%uItiesmay be due to plastic deformation of the original Pangaeanconfiguration of these narrow portions of the continents.
globalgreat circle.In actuality,irregularities in the Pacific margin result in a perimeter of about 41,300 km (Table 1; Figure 5, insert), invalidating Meservey's [1969] assertionthat the Pacificmargin was never large enoughto enclose half the earth'ssurface.In any case,the 2160-km value representsthe most reasonable value of the difference between the actual maximum and modern frontiers.
As the continents continued to drift beyond the maximum frontier stage, adjacent continents along the frontier must have moved closer toward one another. Such a motion along the frontier
sector from
North
America
to Ant-
arctic• is indicated by the reconstruction in Figure 4. Here it is seenthat points A and B, which had moved farther apart during pregreat circle drift, began to approach each other
in post-Jurassictime, from the maximumsepa-
3278
K•,•,wN
S. RO•)OT,FO
CL_•At•ION Ft•ACTUR•ZONeCUBA
HISPANIOLA P•J•'/•7'O /:t/CO CLIPPElO TONFt•ACTUt•' ZONe' VIRGIN
IS
!-LL•sS•R
ß..
,.j ANTILLES ß
.o
ß
FALKLAND IS.
".3 "d-x• S.SH[TLAUD•S .... • S OR•UE••S•EOR•A!
_
••0•
••111t111t11•1111•11• •
SSANDWICH ..... •o•
._
Fig. 5. The Caribbean and Scotia arcs, and the Pacific margin (insert). Letters denote the generalized Pacific perimeter of Meaervey [1969]. The perimeter segments of Table I correspondto this notation.
3279
DRIFTING CONTINENTS,SPREADING SEA FLOORS
ration of 16,600km to the modern distanceof 14,160 kin. In consequence of this motion, any transverse sea-floor spreading that marked the previous stagemust have diminishedand ceased.A more seriousgeometric problem is posed by the in-
folding, by short wrench translations along fractures or zones of weakness originally developed during the elongationstage; by large-
scaletranscurrentfaulting at the northern end and southern
boundaries
of the Caribbean
and
Scotia lithosphere; or by any combination of
elastic continentalportions of the frontier that the three mechanisms. Between the Americas a earlier had undergonestretching.Its inelasticity first stage of perimeter adjustment became required that the frontier length had to remain complete when formation of the isthmian link constant even as the Panthalassic area became during the Plioceneproduced a more linearly smaller. economicPacificmargin and strandedthe CaribThe most logical areas for the necessary bean plate in the Atlantic. adjustmentwould have been the zonesthat had RATES AND HISTORY OF FRONTIER DISTORTION formerly provided the required expansion.Two If it is assumed that the sea floor of the such zones,from the foregoinganalysis,would have been the new lithosphereof the Caribbean earth has spread at a constant rate since the and Scotiaseasbearingthe stretchedand frag- breakupof Pangaea,the historyof frontier dismented Caribbean and Scotia bridges. The tortion can be geometricallyapproximated. In frontier slack would have been taken up in a the ideal caseof Figure 3, constantspreading 'bending'of these insular-peninsular arcs as the would mean that the frontier was first enlarged continents moved
closer to one another
in the
manner illustrated by Figure 3c. Bending and disruptionof the arcs during Tertiary time are indicated by field evidence [Hamilton, 1966; Dalziel, 1970]. Such an adjustment could have
and then contracted in a continuous series of
circlesdescribedby a linear changein g) with time. Consequently, the changein frontierlength wouldhavevariedwith the cosineof g),since
hence p• =•2wR cos• been accomplished through actual bendingby r, = R cos• plastic flow; in a manner analogousto shear with the subscriptn denotingthe generalcase. TABLE 1. Lengthsof the Segments of the AveragePacificPerimeterDepictedin the Insert of Figure 5 Segment
Point and Locality
Latitude
Longitude
A
Southeast Asia
14øN
108øE
B
North Australia
13øN
130øE
C
South Australia
38%
147øE
D
Victoria Land, Antarctica
71 øS
167øE
E
Ellsworth Land, Antarctica
73øS
82øW
F
Tierra del Fuego, South America
53øS
74øW
G
Southern
8øN
77øW
H
West-central
19øN
105øW
Isthmus
of Panama
Mexico
I
Western
Alaska
60øN
162øW
J
Eastern
Siberia
63øN
172øE
14øN
108øE
Segment AB
35OO
BC
38OO
CD
3200
DE
2400
EF
2900
FG
6900
GH
380O
HI
6100
IJ JA Southeast
Asia
Total averagePacific perimeter Meridional
circumference
of earth
Length,km
8OO 7900
41,300 40,000
KELVIN
3280
S. RODOLFO
The idealized frontier history is summarized by the solidline curvein Figure 6. The abscissa linearly equates• with time, as requiredby the foregoing assumption.Approximately 80% of the frontier elongationwouldhave beenaccomplishedby 160 m.y. ago, and the great circle stage would have been attained by 125 m.y. ago.Subsequent departurefrom the great circle would have been slow until about 70 m.y. ago but would have acceleratedalong the cosine curve to the present. Comparison of this idealized curve with the history of the American-Antarctic sectorof the
maximum, and the subsequenthistory of the frontier segmentcloselyfollowsthe idea]. This correspondenceappears too close to have occurred by happenstance. CONCLUSIONS
If the foregoingargumentis valid, the following points are indicated' 1. The large predrift departures of the American-Antarctic
sector from the idea] case
indicatea geometrically inefficientapproachof
an individual frontier segment toward great circleconfiguration. This excess elongationwould frontier is interesting.On Figure 6 the distances probably have been accompaniedby ine•betweenpointsA and B of Figure 4 havebeen ciency elsewherealong the Pacific perimeter. plotted with the maximum A-B distance ad2. Post-great circle frontier adjustments justed to the same ordinate level as the ideal could be accounted for by the Americanmaximum,for easy comparison.It can be seen that the Dietz and Holden reconstruction indi-
cates much more pre-great circle frontier elongationthan the ideal caserequiresfor the entire perimeter. Of much more interest is the
fact that the maximumlength of this frontier segmentoccurredvery closein time to the ideal
Antarctic sectoralone.However, becausestresses cannot be transmitted along the whole frontier,
adjustmentsare probably occurringelsewhere. Thus, the segmentbetweensoutheastAsia and Australia (A-B, Figure 5) is becomingshorter [Rodol/o, 1969, 1970]. This is at ]east partially offset by the post-Jurassicperimeter expansion between Australia
TIME,X 106YEARS BP 2OO
I00
and Antarctica
166
40-
circledeformationa] historyof theidealfrontier and the American-Antarctic
•'
39
P•o ///
-%4d
• • -15x
o
I
AB, 4eb
!
5Z
ofthesector hasbeen fairlyconstant since about 135m.y.ago. 4. Details of the history of sea-floorspread-
