OC EANOLOG ICA ACTA, 1981, N' SP
Subsidence and tectonics
of Atlantic-type
•
continental margms
E:al-----
Atlamic con tinental margin
Bac ks lripping sludies
TeclOn Îc subs idcm.:e
Strctc hin g
Marge conlinentale at la nti que
Suppression de l'effet de s séd imCllIs
Suhs idcncc tcctonique
lÔ,iremenl
A. B. Walls. M. S. S'ee kler
Lamont- Dohert y Geologiea1 Observiltory of Columbia University. Pal isaJes . New York
10964 , USA.
ABST RA C T
The sed ime nts whic h acc umulalc :11 :.t continental ma rg in following ini tial rifti ng fepresen! a
load on the lithosphcrc whic h should sag due 10 ils wcight. Bac kstripping sludies in which
se dime nt. as wcl! us waler , loads are removed fro m a m;lrg in Ju ring differenl geological
inlc rva ls of lime ha ve now becn ca rric d out ~l! both re lative]y young and o ld cont inental
margins. These s lUdies s how that Il numbc r of fa ctors affect the s ubside nce and tectonics of
margins whic h includc cuslasy, Ihe flexural s trength of the bascme nt, com paction. and
palaeob'lt hymetry. These fa ctors cannot. howe ver, accoun t for al! the obscrvcd fcatures of
the s ubs idence and tcctonies of margins and ot her proeesscs mu st be involved . T he mos t likely
o lher conlri bulor 10 the observed s ubs ide ncc is thermal contraction, fo!1owing stretching of
t he m~\rgi n lit the time of initial rifting. Simple s tretchi ng models appear to bc able 10 expla in
the exponential c ha rac ter of the lec tonic s ubs idence of margins (il though a numbc r of
problems still remain . The most important of these a rc the rela ti ve proportion of s yn and
pre·rif! to pos t.rif' sediments a nd the amount of crustal th inning ac ross the margin.
Ocemzol. A c /a. 1981 . Procccdi ngs 26'h Internat io na l G.:ologica!
continental ma rgi ns symposium. Paris . J ul y 7·1 7, 1980. 143·153.
RÉSU MÉ
Consres~.
Geology of
Tect oniq ue et subside nce de s ma rges conti nentales de type atlant ique .
L'accum ula tion des sédiments post·rift s ur une marge continentale repré sente une charge sur
la lithosphè re. qu i dev rait fléchir so us cc poids. De!), ét udes visant 11 s uppri mcr l·cffel de c harge
due à J"eau e t a ux sédiments ont é té menées tant s ur les mnrges nnc i enne~ que sur les marges
re lativeme nt jeune!),.
Ces études montre nt qu ' un certa in no mbre de fa c le urs jo uent s ur la tectonique et la
s ubs idence des marges. parmi lesquels l ·e u ~til s ic , 1;1 f lexure du substratum , la compact ion ct la
pilléob.1th ymé trie. Ces fa cleurs ne peuvent explique r néanmoi ns !"ensemble de ~ ca ractéristi·
q ues de la subsidence des marges continentales . pour lesquelles il fa ut recherc her d 'ilutres
causes. Parm i cellcs·ci, la plus probi.ble es t une contraction thermique aprh un é tirement de la
marge a u moment du rifting.
Des modèles d ·ét irement s imples expliquent le carac tère ex ponentiel de la subsidelll;:e
tec to nique des ma rges. bien qu ï ls laissent un certa in nombre de prO blèmes non réso lus. le plus
important é tan t la proportio n re hlli ve de sédimen ts anté· ct synrift l'Hf n .pport à I,:CUX dé posés
après le rifting c t le Inux d'amÎncisse ment à travers la marge.
O ct'{lI! O/. A t M . 1981. Actes 26< Congrès Internationa l de Géologie. c o lloq uc Géologie des
ma rges continentales . Par ÎS. 7· 17 juil. 1980. 143·153.
143
A. B. WATIS, M. S. STECKLER
Watt s (197K), Walls and Steckler (1979) and Ke e n (1979),
using commerôal wells oH eastern North America. showed
the subsidence could he explained in terms of a s imple
cooling plate model (Parsons, Scll:lter. 19771.
Recently. Mc Kenzie (1978) proposed a modc! in which the
subsidence is caused by cooling of the lilhosphere following
uniform extension and thinning al the lime of rifting.
Christie and Sclater (1980) have shown this model nllly
expIa in the post Mid-Cretaceous s ubsidence of the North
sea b<lsin and Ro yden and Keen (in press) have l:Ipplied Ihis
modeJ to weil dala off Labradur and castern North Ameriça .
INTROD UCTION
The continenta l margins of Atlantk-t)'pe whÎch border the
Atlantic. Indian and ArclÎC oceans are characlerÎ:a:d by
s ubstantial thic knesses of seaward dipping sedimen ts.
These sediments range in age from Triassic tO Recent for
the margin off North America and Arrica (Schlee et lIl . .
1976; Sheridan, 1976; Von Rad. Arthur. 1979) to Late
Tertiary tO Recent for the margins of the Red Sea and
Western Meditermnean (Lowell el al .. 1975; Biju-Du val
el al .. 1974). The rclath'cly old murgins off North Amcrica
and Africa, which are c h;lracterized by thicknesses in
excess of > 1C) km , appear to overlie a faulted b.""lsemenl.
The relatively yo ung m:lTgins of the Red Sea and Western
Mediterranean, on the Olher hand. arc characterized by
thinner sediments « J km) whic h overlie a faultcd eonlinental basement.
Dietz (1963) s ugges ted that the loading of sed iments on the
continental slope and rise would produce a regional downwarp or flellure of the adjacent continental crust. Gunn
(1944). Walcotl (1972), Cochran (1973). and Watts and Ryan
(1976) have q uantitatively s hown that subslllfltiai thiçknesses of shallow and deep water sediments may accumulate at
a margin simpl y as a result of sedimentary loading. They
used ~imple flexural models with the lithos phere responding
tu loads liS an elastic plate uverlying a weak fluid.
Biostraligraphic data from deep commerc ial boreholes in
the outer continenlal shelf off eaStern North America
(Scholie, 1977 : Gradstein el al., 1975; Jansa. Wade . 1975).
Northwest Africa (Von Rad, Arthur. 1979) and the WeSte r n
Medi!erranean (Cravlllle el Ill .. 1974) s how that much of
these sediments were deposited in çOnlinental or ne ritic 10
inncr shclf marine environments. While larse thicknesses of
relati vely s haJJow water sediments cannot be produced by
sedimentary 10adÎng alone. ail hypo t heses for the origin of
the s ubs idence of continent;11 margins arc in agreement that
sedimentllry loadins must çon tribule tu a major portion of
thc s ubsidence.
Isosta tic considerations suggeS! thut the maximum
thickncss of sediments which can accumulatc at any time
d uring margin evolulio n is about 21/2 times the available
water depth. If flexure b important. the maximum sed iment
thickness will be less. but the ~ubsidellce will occ ur o\er a
larger region due to the strenglh of the lithosphere. Thus the
effect of scdiment loading at il margin is tu amplify and
o therwise modif y the subsidence due tO other causes.
A uscful appruach 10 the problem of the subsidence of
con tinental margÎns. therefore , is to quantitatively accoun t
for the effccts of sedimentar y loading durins marsin evolution (WlI H ~. Rylln. 1976: Steckler. WlittS. 1978; Keen ,
1979). ln this ma nner, thlll part of the subsidence which is
no t caused by the weight of the sediment s can be isolated.
so Ihm any similarities in the structural fealUres of margins
thal are rnasked by sed iments can be seen.
Sleep (1971 ) showed that the subsidence of the US coaStal
pluin. when corrected fo r the effects of sed iment londins.
could be fit by an ellponential decay with il time constant of
50 MY Watts and Ryan (1976) and Steckler and Wutts
(1978) referred to the sub!iidence corrected for sediment
lond ins as the tectonic subsidence. The y s howed that the
tectunic subsidence wns similar in form to that of a
mid-ocean ridge.
A number of slUdies hl:lve now cornpared the teclonic
s ubsidence at a margin 10 the subsidence ca1culated based
on simple cooling models for the lithosphere. Steckler and
ln order to quantitatively evaluate and remove the effects of
sediment load ing ;It a margin two procedures shouJd be
followed. Firs t, biostratigTliphic data should be used to
reconstruct the sedimenta ry section during evolution of the
margin. Second. the sedimenlary section should bc "backstripped" and the s ubsidence of the margin not caused by the
weight of the sediments obtained. The information required
10 reconSl r ucl the sed imenta r y sect ion through time consists
of sediment thicknesses correcled for the effects of
compact ion. water depth of sediment deposilion (paJcobathymetry). and Ihe long-term eustalie com ponent of
sea-Icvel (Van Hinte . 1978: Steckler. WaIlS, 1978). '"Backstripping" (Watts, Ryan. 1976) is the removal of seùimcnlary and waler loads through lime and requires info rmlllion
bath on the densities within the sediment column and the
long·term (> 1(f' years) mechaniclii behavior of the lithosphere.
The purpose of this paper is to briefly review the contribution of sediment and water loading to the s ubs idence of
Atlantic-type continental margins. We will use biost ratigraphic data. mainJy from the margin off eastern North
America. to illustrate how factors s uch as seùiment compaction, pa[eob;lihymetry, sea- Ievel changes and the mechanical response of the bascmcnt contribute to the s ubs idence.
The overall objective of thc paper is 10 better understand
the origin of the tectonic s ubsidence of Atlantic-type
continental margins.
SEDIMENTS
The devclopment of seism ic technique~ which use the
multi-channel array along with commercial ;lnd DSD PtlPOD
drilling have greatly improved knowledge of the sedimentary s tructure of Atlan tic-type cont Înental margins. There
have now been numerous geophysical studies of these
margins although only a few have a llempted to integrate
both the seÎsmic and weil data (fo r exampJe. Schlee et (lI..
1976: Von Rad. Arthur. 1979).
Figure 1 (1 s ummarizes the sedimentary s tructure of the
continental margin off e:ISlern North America based on
available seismic and weil data. Each profile of thc marsin
(Fig. 1 h) shows that the coastal plain sediments gTlLdually
increase in thic kness from the fall line to il hÎnge zone
(Jansa. Wade, 1975; Watts, Steckler, 1979; Austin etai ..
1980) whcre the depth tO continental oosement rllpidly
increases . The nature of Ihe basement seawtlrd of the hinge
zone i5 obscured by the large th ickness of sedimenh.
Acoustic basemenl , which has been identified as the upper
surface of oceanic layer 2, occurs a l distances as far as 100
to 300 km seawarJ of the hinge zone ( Fig. 1 h).
Stuùies of biostratigraphic data from commercilll well s off
easlern North Amerkll suggests a large increase in the
amount of crustal thi nning occurs across the hinge 10ne
144
SUBSIDENCE AND TECTONICS OF ATLANTI C-TYPE MARGINS
l
v '"
7, , , ,
r, ,
Figu re 1 a
Lonltio/I map of profiles uff eas/en! NOr/ir America.
The position uf tire faillille is based 0/1 Kilr~ (1969) Utr
lallli and latr$(j atrd Wade (1975) (jnd Aus/in e t al.
(1980) of#hare. Tire Im;atioll of lire crest of tire EcUl
COIlS / Ma!;/ltlic Allamal)' is from Rabillm<"it z (1974) mrd
G. Kam er (pers. comm.) alld mag/lelk anumal)' M -15
(155 m .)'. H. P.) i.~ from Schoutell alld Kii/gord (/977).
Tire posi/lm. 01 tire Irin ze W il l' is baud on Mailer and
Applin (1971). l allsa alld Wode- (1975). Dillun el al.
(1979) a nd Ails/in ct al. (1980). BUllrymetr)' is Irom
Uc/mpi (/971 ). T ire Ire(lI'Y litles lacQte lire geolilgic
pmfile)' ill Figure lb.
\
,.,'~',-
"i
........
U"t
- . .._-
IIC"""
~.
.........
@-
~""L ../-
LOC' !ION
M'~
---_.-'- i·
H_I_
•
1
J
.",'
';'
•.J
.... c _
• • •
•
(Wlitt s, S teckle r, 1979). Gravit y and geoid modelting 1Iiso
confirm that at this locatio n both the crust and thermal
IÎthos phere thin from typical continental values 10 values
more typical of oce a nic c rust (Watts, Steckler. 1979).
Unfortunll1eJy, onJy limited seismic rcfraction data is pre·
sentI y available off eas tern North AmeriC:1 10 test these
models. The evidence tha t is a vaiJablc suggests the hinge
zo ne is a major thermal and mec hani cal boundary a nd that
il S location ( Fig. 1 a) ~l long the margin has played an
impo rtant role in its teeto ni \: e vol ution .
Figu re 1 bshows. however. thll t there a re large variations in
the ove rail distribution of sedi men ts off eas tern North
America . Off New York a nd Florida the shelf brellk in s lope
extends seaward of the hinge wne while off Ha lifax a nd
Clipe Hatteras it is s lightl y land ward of the hinge zo ne. The
larger d is tance to the shelf break off New York muy bc due
in part to the s uppl y of te rr igeneous and bioclas tic sediments to the margin in the Earl y Ju rassic. prior to the
de position of Horizon ~ (Tuchol ke. Mountain. 1979). In
fa cl. the she lf break off New York a p p~ars to have
exlcndcd cven f urt he r sc award than at present .md may
have becn e roded by 20-30 km by counter·eurrents which
beg:tn in the rn iddle Tert iary (G row el al.. 1977).
These considerationS o f the sed imentary structure off
eastern North Ame rica s uggest tha l whiJe the ovcra[[
tcctonic evolution of thc mllrgin is con tro[[ed by the hinge
zo ne. the pos ition of the s he!f hreak and the wid th of the
s he lf depe nd On the a vailab ility of sediment s and s urfiçj;11
geologic proccsses. T hus, thesc e ffects s hould be isolated
and removed if t he the rmal and mechanical properties of the
margi n. and how they va r y across li m;lrgin. ;Ire to bc be tte r
understood.
0
o~
0
h '
0
..
.
.'
.
"
Ja.'"
tec tonic evolution. The stratigraphy of the s he!f region
records the ver tical movenlent ~ which dominate the evolution of the margin . The sediment thick nesses observed al fi
ma rgin a rc 11 consequence of several fa ctors, incl uding
com pact ion. sea-Ievel c hanges and Ihe nature of the response of the basement to sedime nt and wa ter Joads. In o rde r
to obtain t he tectonic s ubsidence a t a ma rg in , it is necessary
to acco unt for thesc facto rs.
The procedures whic h should bc fo llowed have becn
discussed previous ly in the geohistory analysis of Va n H illte
(1978) and t he "backslripping'" a pproach of Sted ler and
Wa tts (1 978). AvailabJe bios tratigraph ic and sc is mic reflection profile data a re used to reconstruct the stratigraphy of
the margin fo r diffe rent inter vals of geologic time. I n
reconstr ucting the sedimcnHtTy thie kncss the effects of
compactÎo n, sea-Icvel changes and va riatÎons În waler
de pths o f depos ition should be included. T he n. the sediment
and water loads are " backs tr ippcd" fo r va rio us intcrvals of
gcologieal time using diffcrent mo<lcls for the n:s ponse of
the baseme nt to these loads.
Thc gcncnrl cquation for "bll\:k:.trip ping" can bc wril1cn
y = tf! ·I S·(P.. - P.) - .6..~
( P..
P.)
(p_
P~
Il. )
l + w.+ .6.s~
(1)
where
y
depth of baseme nl without the effc c ts of !lo cdimcnt
a nd wa te r loads ;
sediment thick ness corrected for compaction :
S'
P. = men n densilY of sed ime nts;
mean dens il Y of ma ntle ;
P.
p", '" mell n dens it y of wate r :
.6. SI. = sea JcveJ relllti ve to the prese nt day ;
bascment respOnse function .
SU BS IDE NCE
The sed iments whic h l\cc umulale a t fi COntinental margin
d udng and af ter rift ing prov ide the bcst record of its
•
145
A. B. WATTS, M. S. STECKLER
-
•
~.
"
Fiyure lb
Summur)' /felJioKic cron stctiolls tllolIg Ihe COlllilltll/a1 margin "ff
,vorlll Amuietl aligned (llollg Ihe hillge wlle (heo~y doshed lille).
Tlle Sfrllligraph)' of th e/tllld boreholes are from Bro ....11 et al. ( 11)71).
The Slruligraph)' of Ille ....el/s offshore from NO "a Seot;a ore frrml
Williams (1975). Ihe CaST B·1 ..... ell fm", Sc:/wlle (1977), Il,e COST
GE·' "'ell fro", AmUIlI Ulld BeboUl (/978) (l/Id DSDP site 391 f,o/ll
Scfelllific Part)' (1976). The so/id /illts ill Ihe four secliolls indicute
promill elll seismic ref/eClOrs illdenlifitd lm nearb)' mu/ti·dwlIlII!I
seismic profiles (GiL-en. 1977; Oro ..... el :.1.. 1979; GroM'. Markl .
1977; Buffleret al .. 1979; Dillol! et al .. 1979; Sheriduli. 1'J76). Tlle
a,ro", luheled ECM refers fO Ihe pmi/ iOlI of Ihe Easl C"/.ISI
MugllelÎ(' All o/llllly (lIId BSA 10 lire Blake Spu r A lloma/)' .
..
,
~D'
,.
,. ,............."'"
response of the bllsemenl to the wc igh t of sed imen tary infiU
and cha nges in sea-lt:vcl. while the last term represents Ihe
water depth relative 10 the present day sen level. The last
term is thcrefo re that part of Ihe tectonie subsidence which
has not been filled in and loaded by ~edÎme n l.'>.
Ir the basement rcsponds to the sediment and water load in li
manner simiJar 10 a thin elastiç plate ovcrlying a weak flu id
then
(2)
.p = c ~. (Cos J..x ... Sin }"xl .
whe re
,
I(p.
,='4 \ .
ET!
p,) g
40
D = J2(J
0')'
(3)
CompaclÎon correcl ion
and
o
T.
flexuraJ parame 1er :
flexu ral rigid ity :
clas tic thickness ;
E
Young ... ModuJus;
0'
Poisson's ratio;
average gravity.
Î.
g
If. however. the basement
res pond~
As the weight of overburden increases sediments will expcl
porc fluids ,lnd compact. Thus the pre ~en l day thicknesses
o f older strata al a margin do not refleel Ihe actual amo unt
of subsidence durins those time Înter\'ab.
The mensure of the amol/n t of eOmpilclÎon in l>ed imeOls is
the porosÎty, which is defined li ... Ihe ra l io of the pore
volume to the IOtal vol ume . l n orde r to correc t for compaclion it is t he refore necessary to know how the porosity
varies wil h dept h during the evolution of the margin.
Lilhologic log~ (so nie. densi ty, porosity) only provide
information. however. 011 the prcl>ent day variation of
porosity wi! h dep t h.
10 the Joad locally t hen
1.. = 0 and $ '= 1. Equation 2 shows thal relatively wide
load ... will be loeally supported while narrow load~ arc
$upported by Ihe strength of the base ment. The two term ... in
bracket~ in equ,liion (1) thcrefore represent the effect (If the
146
SUBSIDENCE AND TE CTONICS OF ATLANTIC-TYPE MARGINS
The aclual procedures whic h s ho uld be used 10 corre ct for
compaclion are com plicated since Ihe proce sses by which
sedimentS compact are not full Y unde rs lood . The compaclion of sediments (for example . vo n Engelhardt. 1977) is nol
o nly a mecha nical proce ss which dcpends on the dep th o f
burial o f the sedi men ts. Chemical processes of minerai
solution and recr ys lall iUllio n phl y a n increasingly larger role
wi th d epl h. These proce sses. like t he mechanical process,
grad ually Ciluse a reduction in the pore volume o f sedi·
menls.
Figure 2 b shows the total sediment accum ulation in t he
weil a lo ng with the sediment thickness corre c ted for
compac lio n us ing each of the porosit y ve rs us de pth profiles .
The ve rtical ax is in Fig ure 2 b indicates either the corre cted
deplh to sedimentary horizo ns in Ihc weil or the d e plh to
baseme nt Ihrough lime. Figure 2 s ho ws thm the s ize o f the
compac l io n correction is s ubslantial. reaching nea r ly 1 km
for sed iments w hich fo rmed 140 MY B. P.
The calculations o f Ihe compaction correc tions in Figure 2
assume . however. Ihe porosity remained constant bc:neath
the base o f Ihe weil and the porosity ve rsus depth profile is
independe nt of lithology. The lack of knowledge of Ihe
porosity between the base of the weil and t he basement
severely hampers the acc uracy o f the compac tion correc·
tio n. Sed iments suc h as sands to nes and s hales compilc t with
inc re as ing dep lh in a s imilar wa y but because of a numbe r o f
fac to rs, including the depositio nal history and th ickness of
Ihe overburden , th is compact ion may vary. We believe,
however. t hat the form of the s ubsid ence curves shown in
Figure 2 would no t he al te red s ignificantly if a different
porosity ve rs us depth profile had been used for each
lit hology in Ihe weiL
S ieckler and Wa Us (1978) assumed Ihe simple model of a
constant porosity ve rsus de pt h profile through geologÎç
time. In o rder to compute Ihe thic kn css o f sediments a l any
lime in the pas i they ren,oved the sediments yo un ger tha n
the a ge considered and allowed the re nHlining sediments to
deçom pacl by s liding them up the porosil Y profile. The
sed imenl thickness correcled for compaction is thcn given
by
(4)
whcre
••
.',
T,
poros it y of the inter val prio r to s liding up Ihe
pro fil e:
porosit y a rter sl iding up the profile:
.. in terval thickness.
Paleobalhytmlry
Sclater a nd Chrisl ie (in press) used a s imilar approach to
S teckler a nd Watts (1978) but a llowed each lithology to slide
up its own porosity ve rsus d e pth cur ve. T hey fi tted expo·
nential curves 10 the availa ble data for each lithology and
used these cur ves 10 correct fot compactio n in the North sea
basin. There is much o bservationill evidence . in faCI ,
s uggesti ng there is an exponential decrease in porosilY with
d epth fo r sandslones and chalks. Plots of porosity ve rs us
depth c ur ves for sh,tles. however . s how considerable
se'lIIer.
We illustrate in F igure 2 the effect of using different
porosity vers us depth profiles in the e,rlcuhllion of sed iment
thic knesses for data obtained in the COST 1l ·2 weil off New
York (Sm ith el (II .. 1976). Figure 2 li shows the bcst fining
estimate for porosity ve rsus de pt h prome (b) a long with
upper (a) and lower bounds (c) for the profile and
--
.)
.'
-
• •
.. ....
.. .. t~ . . ........... ~ ..
, ,-. _,
'. -'-''''
,-
Fiaure 2
a) Porosily as a function of d~pth in tl!~ COST 8 ·2 ...~II. Clln·~ b is a
bUI fil fIl Iht ()bun,~d dala (Rhodfhamft. 1977) /J"d cun'fS /J o1tdc
/Jr~ UfJfJU and tOIl'U bounds o/Iht fII"g~ : b) ffftcl of fhf comp/J~·.
lion correclio .. . Tht obstn'td stditlltll / IICCtmw/llIioll alld d~p l " /Q
J lrm/R'llp lric
horiuJ/ls a/lU corru l!lrg for comp(lclimr .
Alltm(l/il't/y. cun'U /J. ba/rd c Shll'" tlrt ~·(lku /ll(td d"/Ilh 10
bastllltnt tI/rOllgil limt for ~acl! porosil)' profilt.
147
The lectonic s ubsidence o f a margin provides a depression
in w h ic h sediments infill. The sed iments d o not necessarily.
ho wever. fi l! the de press io n to se:l·level. The waler dep th
which remains, re lative 10 present day sea level. is therefore . lin important IXlrl of the tec to nic s ubside nce o f the
margin. Unfortu natcly. eslimates of the water dep l h
through l ime (palaeobalh ymetry) a re difficult to oblain and
constitute a major uncerlainty in obtaining the teclOnic
subsidence of a margin .
Although the distribution of some faunal assemblages seem
to he relaled d irectly to depth (for examp!e. Van H inlc.
1978) the availability of di rect wHter deplh indicators is
limited . Estimtltes a re made either by d irect compa r isons to
p resent day occurrences o f ccrta in species or assemblage s.
o r by quantillitively d c te rmining the rel;rti ve ab unda nce o f.
for exumple. benthon id planklonic foram s and radiolar ians
or OSlracods. In ge neral. eSlimales of water dep lhs are mos t
accu rate in regions whe re recent faunal assemblages are
weil known (for example, Gulf Co..1S t. US. Eastern Med irerranea n) and in sediments fo rmed in shallow-water environ·
ments (neritic andlor s helf facie .!». ESlimates are less
prccise in o lder sediment.s and in sediments formed in
deep·wa ler enviro nmenlS.
Figure J summarize s eSl imates of the palaeobalhymelry for
s ix wells off eastern North Americ.. . The two mosl
land ward wells. Moh" wk 11-93 "nd Naskapi N·JO. a re associn ted wilh the s hallowes t eslimatcs of paleobathymclry and
t he three most seaward wells. Oneid a 0-25. Sable Is land C·
67 ,lOd COST B·2 wells arc associated with the IlITgeSI
eSlimates. E .. rl y in Ihe his tor y of the margin . during the
mOSI rap id s ubs idence. Ihe s uppl y o f sediment s seems 10
have been adequale to kecp the water deplhs shallow . Later
in t he hislory o f the mMgin. however . hia tuses and large
v:. riat ions in water depths OCCUT. These differe nces probably a r ise bccause, as the subsidence slows. Ihe poSition of
t he s horeline becomes se ns itive 10 t he m te o f c hange in
sea-Ie vel w hic h cal' res ull in large c ha nges in wate r dep lhs
(Pit mnn. 1978).
The s nmll scale va r iations in paleoba lhyme try in F igure 3
proba bl y are due to local sed imcn t:.r y processcs. The hiat us
A. 8 . WATTS, M. S. STECKlER
...
..00;._
~ ~S
. .",
"~
WEIOA
,.~
SABLE
t-.,
I Sl A~O
~OS I
BZ
•
•
•
--'- ,-----"---"--"-. ~f:
---,
"""'"
mllgnitudes estimated from continenta l flooding (Wise.
1974: Bond, 1978) .
Figure 4 illustrales Ihe etfect of using a different sea-leve!
c ur ve in Ihe calc ulat ion of the tec lonic su b~id ence for t he
Oneida 0-25 weil off eastern North America . This figure
s ho ws that changes in sea-level can con tribu te in a major
way to the lec lOnic subs idence at the weil. For exa mp!e. if
the Pitman (1 978) curve b used the teClOnic subsidence
increascs linearJy with time . Howeve r. if the Wa tts and
Stec kler (1979) c ur ve is uscd the margin s ubsid e ~ exponential1y with timc .
~
•
COSI Gn
Figure: 3
Pa/(obmll)"II1f'l r)" eslim/lln fo r ofls/lOrle wclls off elUlcm North
AII1( rica ( Fig. 1). OUlU for Ille NOI'o Seolilln wflls are lram F. J.
Pou los (pers. comm.). Ihe COST n -twell islrom Smith et al. (1976)
alld rhe Ca S T GE-' ...cll is [rom Ama/O olld Beboul (1978). H
illdicu!e.'· Il l!Îal/l.'· III Ille slmlil/N,phic ,cco rd (wd S i"dicmes
sl/bilerial depoûlü", .
in the Sable Island C-67 weil du ring a time of deep water
dcposition may be due to sediment s bypassing the shelf and
bcing depos itcd on the rise. The shallow ing of the water
depths in the Miocene at the COST 8 -2 weil is due to a rap id
influx of sediments 10 the s helf while thc earlicr shallowing
in the Eoce ne /Oligocene may bc due. in pilrt, to c rosion of
thc s hc!f ed ge by Mid-Tertiary eounter-currenb (Grow
el ,,1 .. 1977 : S tec kler. Walts. 1978).
~'·'S
,., ''io
~-f: . _f'
-....
"i' '05'*
.... ,..
........."
~ ·O.,'
':..!::C!
Figure "'
[o r III" Ouf'idu 0-25 ...t'II /nued ou
dil[erelf/ MSlw1ed sea-IC I'el cun'cs Il''QUlfh lime. Tlle "pper C'/fI 't' is
Ihe ucwnic subsidenre william /1 sen-lcI,t'1 correeliol!. Till' middle
cu,,·c ussu"'f'.'· Ihe Vui! el al. (1977) ji, sl-orde, ~·e/l·le.·el cu,,·t' lI11d
Illt' Pilmall (1978) st'u-Ie"e/ ClIn·t'. The Im,'er cun'" h haSf'd 011 Ille
sta-/el'cl cu ,,·t 01 \V/I/IS alld Sleeliler (1979). Tilt' [or", uf rhe
If'~'/(JI!;C sl/hsidell ce depellds slrollRI)' "" Ihe ua -Iel'el cun'e IIscd. I[
Ihe CII"'e o[ Pirmul! (1978) Ü· uSf'J Ihe It'cf(lIIic: sl/h.iidellee iS/llillear
[mlclioll 01 rime bUI illhe clm't' of W a ll s iUld Sledler (/979) i5 Il.\"t'd
111t' $Uh fide ll ct' is /III i'XpoJ/tlllinJ f/lltClioll o/rimt'.
PlOI of tec/onk sub)·id .. ,,("(
Sea le \'cl
The variations in sea-Ievel which CX:l,:ur through time (for
examplc, Pitman. 197R) contribute in two main way~ to the
tectonic s ubsidence o f the margin. T he hcight of sea-Ievel is
the reference surface fo r pllleob(lthymetry and sediment
thic k ness estimales. Thu.. cha nge ~ in seH-level with respect
10 the present da y are required in order 10 provide li
reference surface for t he tectonic su bsidence. ln addition.
the cxcess watcr associa ted wit h a highsland in sea-Ievel
acb as a load on Ihe base me nI and dcprcsscs il. Steckler and
Wam (1978) and Walls and Sleckler (1979) modelJed this
dftct by ass uming the basement responlh 10 the water load
as an Air y-ty pe crus!. This is justified in the case of the Late
Cretaceous sea-Ievel high~tand becausc of ü s large areal
extent. However. near Ihe s horeline flexuml effects lire
important (Chappell. 1974) and should he taken Înto
accounl.
The magnitude of sea-level changes through tÎme i~ a
s ubject of controver~y a t the prese nt lime (Vail el ,,1., 1977 :
Pi lman, 1978 ; Watts, Sleckler, 1979; Bond. 1978). Pitman
(1978) es timatcd Ihat sen-level has fa llen by about
350 metcrs since the La te Cretaceous using changes in the
volume of mid-ocean rid ge erests through time. This es timate appears to agree with that of S leep (1971). ba~ed o n the
present ele vation of Crelaceous sed iments in ri teclo nically
undiSlUrbcd region of the continental interior. Watts and
S teckler (1979) e~tima t ed that sea-level has fallen by less
Ihan abo ut 200 ln s ince the Late Cretaceous using weil data
off eastern North America. Their method yjeld~ a minimum
e~tim"te. but their values arc in beucr agreement with the
!Jusement rcspOllse
The response of the base ment to sediment and waler load~
at a continental margin have bcen modelled eilher as
Air y- t ype or as flexurc of a thin elas tic plft,e overlying a
weak fJuid (G unn. 1944: Waleoll. 1972: Cochran. 1973 :
C happell, 1974 ; Walls. RYlln, 1976: Stecklcr. Wa n s, 1971'! ;
Watts. Steckler. 1979; Keen. 1979). Seismic reflection
profiling s uggests that aClive faul ti ng a ccompanies the carly
stages of rifting (Bœ uf. DouSI , 1975 : Ponte. A smu~. 1976:
Gi vcn . 1977: de C harpa] e/ al .• 1978) and that an Airy type
mode!. in which the sediment and wate r loads are locally
supported by the basement. i~ m O~1 applicable carly in t he
rifting history. The presence of genlly dipping postrift
sediments ,lnd a wide coastal plain. however . s ugge~t thll! a
flexure model i~ more applicable laler in margin evo!ution.
Wc iJIu~trate in Figu re 5 the response of Ihe basement to
sediment loading us ing both an Airy-type ,Ind flexure
modei. Figure 5 (1 is based on Ihe Airy mode! and 5 b is
based on the flexure mode! with T~ = 15 km. In both models
it is ass umed the tectonic sub:.idence (shown by a dashed
tinc in Fig. 5) increases steeply ~ea ward of the original
shore-line and rcaches a constant value of 5 km bem:ath Ihe
continental s he!f and slopc . The s hape of the resulting
148
SUBSIDENCE AND TECTONICS OF ATLANTIC-TYPE MARGINS
sed ime ntary basin for the Airy model exactly matc hes that
of the tectonie subsidence. In the flexure model , however,
the stre ngth of the basemc nt causes t he basin to e"tend over
a broadcr region. formi ng a coastal plain sequence evcn
though t hcre is no lectonic s ubside nce in the Tegio n.
CretaceousITe rtiary sedi me nls, forming a subcrop a l the
point X (Fig. 5). near the hinge zone . T his subc rop is
obser ved a long mos t of the ma rgin off cas te rn North
America (Fig_ 1 b )_ For ellample, neit her Ihe Island Beach
wett nor the CQST GE-' weil ( Prof iles IlB ' and 0 0 ' .
Fig. 1 h ) sampled J urassic and the Hatte ras Light-1 only
pcnetratcd a s mal1 thic kncss of Upper Jumssic before
reaching basement (Profile CC', Fig. 1 b: Brown el al ..
1972; Ama to. 8eboUl , 1978).
A furthe r c haraeteristic of fl exural models is an outer high
in t he s trllt igrap hy at the point labeled Y beneath Ihe s helf
ed ge (Fig. 5 b. cl. T his high is a consequence of the nature
of the flexu ral re sponse of the basemen t and the s he lf ed ge
and does not necessarily imply the existance of a basement
ridge beneath t he s hel f edge (for ellample , 8 ur k. 1967). For
a partic ula r s tratigrap hie horizon in the sedimentary sectio n,
however, it does suggest relative u plift compare d to adjacent regions. The location of the outer high is of s tratigraphie interes t. purticul;lrly since wide s helves (for example
off New Yo r k and Blake Plateau , Fig. l b ) appeliT 10 be
associa ted wit h buried reef compJelles a l or near the s helf
edge_
l n most current models for the e vo lulÎon of Atlantic-type
continental m<t rgi ns (Sleep.1971 : Fal ve y. 1974: Mc Kenzie ,
1978) the pos t-rift tectonic s ubsidence is caused by thermal
contraction foltowi ng healing and t hi nning of t he lithosphere at the lime of rift ing. Studies of the res ponse of Ihe
lilhos phere to s urface loud s. such as seamo unts and oceanic
islands. show Ihe flexural rigidit y of the lithos phere is a
strong functio n of lempera lu re (Watts, 1978). Loads formed
on yo ung lithos phe re are associ;l\ed wilh s malt values of the
fle x ural Tigid ity while louds fo r med on old lilhosphere a re
ussociated with highe r values_ T h us we would expec t th;11
t he fJex ura l rigid ity a l a margin wo uld increase wilh lime as
the basement cools.
A recent compilatio n of the results of flexure studies in the
oceans (Watts et al .. 1980) s hows tha t loads which rorm o n
or near a ridge c res t can be adequa tely ellplained by
T "" 5 km while loads fo rmcd off-ridge can be explained by
25 km. Figure 5 c shows a s imple mode! in whieh the
ti rst 213 of the sediments al a continental margin loads a
5 km thiek elastic plate and the remai ning 1/3 loads a 25 km
thiek plate. T he main effect of using a model in whic h the
baseme nl rigid ity inc reases with time is tha t the you nger
sediments overste p the older sed iments_ Th is is mos t eas ily
seen (Fig. 5 c) at the edges of the bas in and s hould be easily
recognized in observed seismic reflect io n profile s and weil
data at margins .
The proport io ns of .the sediments used in Figure 5 c correspond dosely to tha t o f Ihe Ju rassic and CretaceouslTerl iary
sec tions off eastern North America. T hus Figure 5 s uggests
the
J urassic
s hould
pi nc h
o ut
benealh
the
T:. .
.,
T hese fea lU res of_ the flexure model (Fig. 5 b, cl, ure
signific an! a nd s hould be ta ken inlO accoun t in deter mining
t he tectonic subsidence of Ihe ma rgin. Unfor tu na teJy , Ihere
has been little q uantitative modelting of flexure at margins
(Walcou. 1972; Cochran. 1973) and none of the backstri pping sl udies (Watts. Ryan. 1976 ;Slec kler. Watt s. 1978), fo r
example . have yet considered the effeet of the o uter high or
a time vary;ng respo nse of the baseme nt o n the tec lonic
su bside nce.
Finalty, in order to bac kstrip the sed iments it is necessary to
estimate the mean de nsity of the sed iments. P" (equation 1)
th ro ugh l ime. As the sed iments compact t hei r ave rage
dens ily will increase and hence their loading ability inc reases. T he mean density is therefo re mosl easily obtained
from
""'?'?'?'7'7))7,'7,,~"--_'
__'--,-,-;)-'-'-'-'-'-'--II~' -'
P. -
l
,
($ , !lw+( I -<tJ,)p,)1',
S*
(S)
whc re p. = sed iment grain dens it y and ~ i' TI a nd S· have
bee n pre vio usly defined .
.,
TECTONI C SUBSID ENCE
"
""''7
.....,
A number of au thors ha ve d iscussed the origin of the
s ubs idence of Atla ntic-type cont inenlal margi ns. Although
mOSI hypo theses appeal to one o r mo re mec h:ïnisms, t hey
may be di vided into three main groups .
. ..
N'
Tlrl'orl'ric ul cross-suriolls acfOSS a cVllri"" .. ro! IIImBi/! due ra
1) Su ess based models in whic h differential JO'ld ing produces seaward creep of lowe r c rustal roc ks and su bsidenc e on
t he s helf (Boit, 1973).
2) Deep c r uSI;.1 me tamorph is rll in which su bside nce is
produced by a n increase in t he density of rocks in the lower
crust (Fal vey, 1974 ; S pohn. Neugebauer. 1978).
differl'ltI modeb of st:dime,,' !ouding. Th e aS!umed /l'c/onie sl/hsidenct ill toch model is delineufed b y fI,e duslrtd line alld re(Jd,l's a
m(Jximum v(J!l/e of .5 km belleoth th e shelf. a) Airy model ; b) Fle·
xI/rI' mQdtl ...ith eOllSIOIII el(Jslic thickn us : cl Fluure model ...ilh
varyin/( elastle thickn t5s. The so/id /b,u indit·u/l' Ih e positions of a
Iheorelical str(Jligraplric lroriZVII in ... lriel, 213 of tire lee /onie
s14bsidtllCt lies be/oK' ulld I/J ubow,_ Tlris co rruponds appfOxinrarely 10 lire POSitiOll of lire )urassiclC,elu ceo us burilldury 011 lire l'aM
('oust of Nor/h Ameriea_ The basemelll rt'spoltse for Ihe Ilrrel'
mode/s is diseus.ved ilt tire lUI .
3) Models in whic h subs idence is the result of thcrm:11
cOlltraction after thinning of the crus!. Th is thi nning cali
occur by u plift a nd eros ion (Sleep , 1971 ; Kinsman, 1975 :
T ureotle el al., 1977). e xte ns ion and nec ki ng of the cruSI
(Artemjev , Artyus hkov. 1971 : Mc Kenzîe , 1978) o r perva·
sive dike intrusion (Royden el al" 1980).
The ad van tage of the proced ures d iscussed in this paper
(sec also S tec kle r. Watts, 1978 and Van Hinle, 1978) is Ihat
' \ --- .
Figure'
"n >7nn;;
149
A. B. WAns, M. s . STECKLER
the effects of sediment and water loads and s urficial
geologic processes can be removed. a lJowing the tcctonic
s u bsidence to be isolated. It is the teetonie s ubsidence Ihal
any model fOf the origin of continental margins must be able
to explain.
We have previously shown that backslripping weil data
from offshore eastern North America reveals a leclonic
subs idence curve which exponcntiall y decreases wilh time.
The curve is smaller in amplitude and smoother tha n the
curve of sediment accumulation through time . Watts and
S teckler (1979)'showed that the shape and amplitude of the
tcctonic subsidence c ur ve varies across the margin. The
Oneida 0---25 weil (profile AA ' , Fig. 1 b). which is located
land ward of the hinge zone. is best fÎt by an exponential
deçay with a time constant of 110 m.y. and an amplitude of
2.5 km, while the COST 8-2 weil (profile OB ' , Fig.lh).
which is seaward of the hinge zone. is better explained bya
smaller time constant of 48 m.y. and a larger ,\mplilude of
5.3 km.
Watts and Steckler (1979) inlerpreted the lectonic subsidence at the COST 8-2 well in terms of a s imple thermal
model for the cooli ng lithosphere. They ussumed the total
thickness of sediments which aecumu k-lled afle r rifting at
the weil is 12 ,8 km . based on Illulli-channel seismic li ne 2
(Grow el al .. 1977). und thatlhe sediments beneath the wcll
were formed at or near sea-Ieve!. T hey used a cool ing plaIe
mode! and estimated the Ihermal parameters which Oesl
explained the tectonic subsidence <II the weil. The s ubsidence in their best fitling mode!. however. wa ~ associated
with a slope of 512 meterslMyll which exceeds thal of a
normal midocean ridge. One possibility thercfore is that not
ail the lectonic subsidence at the weil is of thernml origin.
For example. Ihere may have been an initial subsidence al
the margin duc to heating and thinning al the lime of rifting.
Mc Kenzie (1978) ha!> co n~ id ered (1 model in which the
lithos phere undergoes a pas sive exten~ion at the time of
rih ing. This extension C<luses necking or thinning of the
lithosphere , which subsequenlly cools and subsides with
time . Sincc isostatic compensation is a:.sumecJ bolh bcfore
and ,Irler extension there is an initial s ubs idence which
depends on the initial thickness of the assumed e rust.
Wc show the tectonic subsidence data fo r the COST B-2
weI! compared to IWO diffcrent the r mlll modcls in Figure 6.
T he sol id line is the cooling plate model of Parsons and
Scia ter (1977) using the best fitting parameters of Watt:. ,Ind
Steclder (1979). The dotted linc is the stretching mode! of
MeKenzie (1978) using 13 - 6. The main difference between
the models is in the nature of the initial :.u b:.icJence. For the
cooling plate model it is assumed the crUS! is initially at or
near sea-Jcvel but in the stretching model the CTUl>1 i:.
initiully 2.2 km bclow :.ea-Ievel. Unfortunately. there is
presently too litlle information on the age and environments
of deposition of the ellrliest sediment:. off eastern North
America to distillguish between these mode!:..
Thcse uncert(linties in the nature of the therm<ll model al the
COST B-2 weil eompli calc estima tes of the thermal history
(lI the margin. For example. Figure 7 ~hows the pa!aeOlempemlUres computed in the sediment:. for each thermal
mode! baloecJ on (\ simple model in which
T(t. z) "" T._o<. +
'
J.
Q(t) dz
-K--'
,
'1
"
lemperature in the :.edimenh
and depth .
;1 10 li
B·2
'~ ELL
'1 \
"
.'
"
j
Figure 6
Compuriso,r o[tlre u ('wllic subside,ree erm'" for lire COST B-2 ...,,1/
10 ca/calated pro[ilt's baud ull sim/llt Ihcmral /11odels. Tir e solid lille
is Ihe simple coolill/( plou 1I10dcl o[ Par~'u t1 s ulld Sc/Olt, (1977) ".illr
lire paramelas g;"e,r i/1 W/IIIS mrd Sleel;!u (1979). Tire deuh"d Ii/r f is
Ihe sire/clri"g mode! o[ hkKe/lûe (1978) ,dllr fJ Oc 6. ',r bo/Ir mod"'.~
Ille IlIwlllrickl1fSS of sedime/rl$ al Ille ".ellirus bUll assrmred 10 be
12.8 km. /11 lire eQ(}lùr~ plme model il is un>rmred Ihe /eclmr;e
$ubsicle",·t' is e"lir("/)" ,,1 tlramul orif(in. hr tire Slre/drillg model mrly
part of Ilrt l''CIO''ic subside/ret' is IIf tiraillai Ori8ill. III Ihis mod~l.
IIrere ;~ mr in;lial suhside'ree o[ 2.2 km UI lire lime o[ rl[lil1g.
o
'00
_0
_ .~,I ,
to
... r"" l
<lO
>4(1
2<10
.,
~
~"
Doo'.
~
•
~
•.A"
~.",.
""
Fillure 7
P<llaeotempaalures in tire s"dill1el1l~ ".hidr accal/rulmed al lire
COST B-2 "'1'11 lU '1 fU/ reriOlr of 11111" silru rl[lllIg. Tir" ump"'alllrl'S
ore based 0" l/re (·,,,,Ii,,): /11011' lI1'ldel (liPPU clln'f) u,rd lire
~'Irelclrlllf( model (10"'''' cur..,!) sltu ... /r br FI/:are 6. Pu ~ 3.J gll"lI1 '.
K, '" 5.0 X l{j-J culr e,olr-sec. mrd ollrer plmmre/eH (15 il1 "arw u!
arrd Sc/arer (1977).
K.
thermal cond uctivity of thc sediments:
Q(t) "" surface he:1I flux as a function of lime in the cooling
ba~emcnt.
The values of Q(t) for the cooling plate model were
eomputed using cquation (10) of Parsons and Sclater (1977)
<tod the thermal pllrameters determined by WaICs and
Sleckler (1979). For the ~Iretch in g model. equmion (7 ) of
McKenzie (1978) was used with J3 "" 6. Figure 7 s hows there
are large differences in the depth to the 100 and 200' C
isotherms for each model. Unfortunately, surface heat flow
mealourements and estima te:. or the geothermal gradients În
the weil C2.4<C/l00 melers. are 100 unccrtain 10 distinguish
between these thermal modcls.
(6)
wherc
T (1.ZF
cosr
'1"\\
function of time
'50
SUBSIDENCE AND TE CTONICS OF ATLANTIC-TYPE MAAGINS
Available free -air gravilY a no maly a nd geoid dala of Ihe
margin. ho we ver . a re consis te nt with the amplitude of the
tec tonie s ubs idence a t the COST B-2 weil. Simple isostatic
co nsidera tio ns irKIica te tha t t he corresponding amount o f
erustalthi nning thal has occurred a t the margin is give n by
T .. D.(p.. - P.)
(P.. - P.)
found thal Ihe subsidence was mos t r.t pid for the mosl
landwa rd weil . More recently . Stec kler a nd Wa tts (in press)
ha ve used a model in whic h both a venical a nd hori zo ntal
tra nsfer of heat is cons idered . They used a modified fo rm o f
Mc Ken zie 's (1978) stretching model in which differenl
a mo unls o f stretc hi ng occur ac ross the margin , The
amo unts of s iretching required in the ir models were in the
range ~ - 2.6 to 6 . whic h was larger tha n expecled from
presentthickness o f the pre-rift Qligocene sedimenls which
outcro p in Fra nc e a nd underlie the margin . They a llri buled
the large amounts of stretching 10 either a relativel y high
geothermal gradie nt at the time of rifling or to an a cti ve
hea ting of the margin.
(7)
where
Pc
mean dens ity of the c rus t ;
Du .. Πsymptote of the tectonic s ubs idence .
For e ithe r thermnl model in Figure 6 Do - 5.3 km so that the
amoun! of c rustal thinning is about 21 km . Thus. if the initial
c rustal th ic kn css off eas tern Nor th America is 30 to 35 km
these re s ullS indicate that a s ubs tantial amount of e r us tal
thinning has o ccurred beneath the weil. Figure 8 shows that
gravit y anomal y data o ve r the margin is consis tent with this
amounl of thinning. In add ition. geo id data can be adequately explained by this crusta l mode! a nd does nol require
any difference in the thickness o f the lithosphere beneath
the s helf a nd the I1djacenl o cean basin .
T he most direct e v idence fo r the nature o f the initial
subsidence o f continenta l margins. ho we ve r. has come fro m
s tudies o f biostra tigra phic d a ta from weil sedime nted young
ma rgins. Watts and Ryan (1976) sho wed (ha l the tec lo nic
s ubs idence o f the Gulf o f lion a nd Slmlinia margi n. whic h
fo rmed by the rotatio n of Corsica fro m Fra nce about
25 MY B . P .. is s imilar to thal o f a midocean rid ge. They
SUMMARY
This paper has re viewed some o f the fac tors a ffec ting the
s ubs idence and tecto nics o f Atla ntic-type continenta l ma rgins. We ha ve s hown tha! sediment a nd water looding ,
paleobathymetry and eUSlatic changes in sea-Ievel a nd
baseme nt response conlribute in a major way to the
s ubsidence of these margins. Although these factors do not
account for a il Ihe o bse rved s ubsiden ce the y amplify a nd
modif y Ihe s hape o f the tec to nic s ubs idence al a margin .
Bac kstripping techniques a re t herefo re required to qua nti ta·
lively account for these fac to rs and 10 isola le the lec lonic
subs ide nce .
Future s ludies in the ne xt dC Cltde s ho uld e mphas ize both o ld
margins , wilh their re la ti vely lo ng record of s ubsidence , and
younger mll rgins, which contain importa ni informatio n o n
the early history o f mll rgin d evelo pme nt . Bac kstripping
techniques are required in o rder 10 beUer understand the
form o f the lecton ic s ubsidence and how this s ubs idence
va ries ac ross a ma rgin . Howe ver . greater a ccura d e s will be
needed in appl ying these tec hniques . panic ularl y with
rel!ard 10 paleobltlh yme lry , compltction erfeclS and the
effec l of a base ment response wh ic h varies with time.
Studie s of biostra tigraph ic from deep wells on the cont inental s hel ve s, in combinlltion with seismic studies with large
,Iperture ana ys, appear to hold the mos t pro mise o f beller
unde rsta ndi ng the o rigin of the s ubs idence of continental
margins during the nC1l1 d ccnde.
Aeknowledl:.cments
FiG ure 8
Complfud 1:rtll·jty ond }.~ojd ~/lecf 01 a Jimple mode/ lo r fhel:Tuswl
slruCWrt! 01/ N~ .., YQrk com/la r~d /0 o"stn'~d {ru·air r rtn'ity
a,tOmaly t.utd CEOS-J altime/cr daw (fr",,, Watts. Steckler. 1979).
Th~ dOJhed \'en ;cal 1i11~ ;ndka t~s tht eslimale 01 the crus/al
Ihicknus IHl1 eo lh th e COST B-Z"'ell based (HI tire /htrmal models in
Figurt 6,
T his wo rk was supporled by Na tio nal Scie nce F o undat ion
gra nt OCE 77- H1647. We t ha nk R. $crullon a nd J . Cochran
fo r hclpful comme nt s o n the ma nusçript . La mo nt-Dohe rt y
Geological Obser va to ry Contri butio n N umbc r 2999 ,
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154