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OCR for page 31
II
PLATE- B O UN D ARY
TECTON ICS
OCR for page 32
OCR for page 33
INTRO D UCTI ON
Complexities of
Modern ant! Ancient
Subcluction Systems
WARREN lIAMILTON
U.S. Geological Survey
Until the advent of plate-tectonic concepts in the late
1960's, geologists and geophysicists studying the compo-
nents of ancient erogenic and magmatic terranes on the
continents had no actualistic frarneworlc within which to
comprehend the origins and interrelationships of those
components. We now see that modern analogs for many
features of the ancient terranes form primarily in conti-
nental-margin arcs and island arcs, above oceanic litho-
spheric plates sliding beneath continental plates or other
oceanic plates. These active temnes can be understood in
a plate-tectonic framework, and many relationships ex-
plained on a genetic basis. Ancient tenants are being
increasingly comprehended in teens of analogy with the
active ones.
The first decade of plate-tectonic explanations of conti-
nental geology has been enormously fruitful. Increas-
ingly, however, our early explanations appear overly sim-
plistic; we have seen the broad relationships but have
missed many ofthe detailed ones because we have visual-
ized plate interactions as much less complex than has
been the actual case. Correspondingly, recent studies of
33
active continental margins and island arcs show them to
undergo rapid and complex changes in response to fast-
evolving plate interactions. Too little communication
takes place in either direction I,etueen the scientists pri-
marily studying the modern marine systems and their d~y-
land counterparts studying ancient systems. Some pur-
portedly plate-tectonic explanations published recently
for ancient systems are mere ad hoc conjectures, incom-
patible with the known features of active systems. Con-
~ersely, the active-margin students could infer much
about their domains by analogy with deeply eroded
equivalent tracts exposed on land.
A cast amount of work is required to increase over ~~n-
derstanding of the products and processes of plate tec-
tonics. Much more integration of data already obtained is
possible. More data need to be gathered in both field and
laboratory, with emphasis on the petrological. structural.
and geophysical criteria for identifying, distinguishing,
and characterizing the component products of plate inter-
actions. on paleontologic and radiometric criteria Or
defining their ages, and on paleobiogeographic, paleocli-
rnatic, and paleomagnetic studies to constrain their global
wanderings.
OCR for page 34
34
The brief summary here of complexities of modern arc
systems is adapted primarily from my study (Hamilton,
1979) of the onshore and offshore tectonics of the Indone-
sian, western Melanes~an, and southern Philippine re-
gions. The discussion of ancient arcs in western North
America Is derived mostly from another report (Hamilton,
19781. The reader is referred to those works for elabora-
tion and documentation of the concepts summarized here
and for references to the work, by hundreds of other geo-
scientists. from which those concepts are derived.
ACTIVE CONTINENTAL.UARGINS
Where an oceanic lithospheric plate is subducted beneath
a continental plate, a tectonic and magmatic system like
WARREN HAMILTON
that of modern Sumatra or the Andes is developed. \tost
early continental plate-tectonic papers (including my
owns, and unfortunately also many current papers. as-
sumed erroneously that in such settings the oceanic plate
tips abruptly down at a trench to an inclined trajectory,
along which the plate dips directly into the mantle.
Rather it is now obvious that the usual case is that exem-
plif~ed by Sumatra ~ Figure 3. 1). The subducting plate dips
very gently beneath the continental slope, landward of a
trench with gently sloping sides, for a distance of 100 km
or more; only beyond this distance does the plate roll
toward the steeper dip of the inclined Benioff zone as
deduced from mantle earthquakes. The continental slope
typically rises gently to an outer-arc ridge, landward of
which is an outer-arc basin. Along some active margins,
the basin is full of sedimentary straw and is expressed
i>JDIA.N JAVA TER RC C)UT=R-ARC BASIN SULFA . RA
Cc =~ TR E.NC~ RIDGE _
Pre-subduct i on s trata, _
beneath teas i n sedi meets, ~ ~
A Water I .04 1 ie on ocean; c crust ~ O
/ ~ ~
a __
=` ~ .~eiange wedge 2.2 2.4 \~
Edge of trench is ~~ lower continental crust 2.9
1 i mi t of downbow i rig
by `me I ange wedge
~ Water . ._
-
-
O 50 ~
-
._
-
c,
P.hase-transi tion z~
5 US D L'C IliNG P 4~ T ~ \\ 0V ERRIDING PLA T E
~ Mantle 3.4
-_
100 ~
.ASTHE~C)SPHERE
Hor i zon ta I and ver t i ca i s ca i e
Horizontal and vertical scale
0 50 KM
1 1 ! 1 ~ 1
Sof t trench and
pelagic sediments
SC raped c>Ff a t
toe of wedge 7
Gravitational flow
i rub r i ca tes me I ange wedge
- .~
—Wedge s tands h i ghes t "here
me I ange b u t t res sed age i ns t
leading edge of i i thosphere
p T a te
G rev i ta t i one I f I ow
deforms outer teas; n S trata
\= _
in
E
o
~ 20
. _
30~ l
40 -
\
-
Subduc t i nc ocean i c p 1 ate
d rags and th i ckens me I ange wedge
FIGURE 3.1 Section through the subduction system of the Java Trench, off southern Sumatra The Cretaceous system of California
presumably was similar. The configuration of the melange wedge and the structure of the outer-arc basin are constrained by reflection
profiling, and the shape of the subdue tiny plate is (onstrained primanly by the location of the inclined Benioff zone of mantle
earthQualces. L'pper diagram: major components; densities in g/cm3. Lower diagram: mechanism of deformation of accretionary wedge
`>t melange and imbricated materials.
OCR for page 35
Complexities of Modern and Ancient Subduction Systems
bathymetucallY as ~ subhonzontal continental shelf.
seismic-reflection profiles and some drilling show that
between the continental-slope seafloor and the top of the
subducting oceanic plate there is commonly an accre-
tionary wedge fondled largely of sediments scraped off the
undersliding plate and imbricated ant] converted to
melange along shear structures that dip moderately land-
`~vard, sharply discordant to the gentle dip of the oceanic
plate beneath. Large ;~`retionaIy wedges develop where
voluminous elastic sediments are conveyed into the s~b-
duction system. Such sediments are deposited mostly
longitudinally relative to the trench, as turbidites in the
trench or as abyssal fans.
Some recent papers have interpreted voluminous com-
plexes of melange and sheared rocks in ancient sr~bduc-
tion settings to be conned primarily by submarine slump-
ing rather than by shearing within the ;ac~cretionary wedge
and its tectonic basement. Such conjecture receives no
support from studies of modem trench systems, within
which large submarine slides are uncommon.
Available data snuggest that an outer-arc basin. like that
of Figure 3.1, den elops only where the leading edge ofthe
continental plate consists of a strip of oceanic angst, either
formed in contact w ith the continental crust by a previous
spreading event or sutured to the continent following pre-
~ious subduction. The basin is formed by the raising of
the leading edge of this strip, not by depression fit the
center. The raising is presumably a result of the stoning
beneath the strip of accretionary melange. Where the ac-
cretionary wedge is buttressed directly against ~ leading
edge of cont~riental crust, no basin is present.
The active volcanism of the Sumatra system, like that of
most s~bd~ction systems, is concentrated in a belt above
that part of the seismic Benioff zone that is about 125 km
deep. The volcanic rocks are of silicic and intel~`lediate
compositions and, in part, fonn calderas developed by
Option from large, shallow magma chambers. It is rea-
sonable to assume that granitic batholiths moist now be
forming beneath the `~,lcanic belt. The magmatic belt is
Imposed on ~ regional anticline consisting of pre~ol-
canic roclcs bulged broadly upward; apparently the angst
has been thickened by ~olr~mino~s magmatic intrusions.
BE HAN I()R O F MODERN ISLAND ARCS
IsIand arcs do not sit stably fin tenderly ing mantle for an
era, constantly Naming inc`'ming manic lith`~phere,
although many c`'ntinenta1 geologists `~me them to be-
ha`e Ail. Rather, arcs cl~aracteristicall~ migrate relative to
the plates behind them; they reverse their s`~bdr~ction
~~>larit`, fir g<, deacl; they c`'llicle with each <'ther and ~ ith
c`,ntinents; and tines are del`,rrned by strike-sli~' tilt and
`'r`~clinal holds Tale flits <'t these ~'r`~s care e.Y-
ceeclinglv c`>mple.x aggregates an(1 s~1~er~siti`'nx `'t airy
materials. Oceanic crust ~lisal'pear.s It
OCR for page 36
36
New Guinea; west of that intersection, subduction south-
west beneath New Guinea is very rapid, whereas east of
the intersection subduction is slow. The slow-subduction
Mussau arc system trends southward to a trench~ench-
trench triple junction against the curving New Ireland-
Manus arc, which continues past the junction to another
trench~ench-trench junction off the north coast of
central New Guinea. Northern New Guinea records the
collision dttring Pliocene time of a south-facing island arc
(itself likely a composite) with a continental margin,
mostly stable since the Jurassic ricing away of another
continental mass. The collision progressed eastward with
time, and the east, or unblocked, part of the arc advanced
past the end of the New Guinea continent to form the
Papuan Peninsula. Subduction, previously northward
beneath the island arc, reversed to southward beneath the
continent, as enlarged by the addition of the arc and un-
derlying lithosphere, after the collision; only subse-
quently did subduction reverse beneath the peninsular
extension. Strike-slip faulting, orocl~nal folding, and other
collisions have been additional complications.
Large accretionary wedges of melange and imbricated
rocks are developed in front of only those island arcs along
which voluminous sediments are available on the ocean
floor. Abyssal-fan and longitudinal-trench sedimentation
can, however, extend several thousand kilometers from a
major continental source. Immature oceanic island arcs,
represented above the surface, if at all, only by small
volcanic islands, lack such sediment supply; these arcs
face steep-sided trenches, at which the subducting oce-
anic plate tips sharply downward and dips steeply
beneath the arc, without any broad melange wedge and
underlying gently dipping oceanic plate. Ophiolitic
melange~pelagic sediments and mafic oceanic crustal
rocks, in a matrix of serpentinite disrupted and hydrated
from oceanic mantle rocks~re known in many erogenic
assemblages that are now parts of continents, where they
are associated with magmatic assemblages analogous to
modern island arcs. These ophiolitic melanges may
mostly represent the small accretionary wedges fanned
along oceanic island arcs, and contest win melanges,
dominated by te~Tigenous elastic sediments that fanned
along active continental margins.
.Uodern arcs provide no support for the various hy-
potheses of back-arc abduction, or flake tectonics. Where
slabs of oceanic crust and mantle have ridden onto other
rock assemblages and are exposed in young arc terranes,
dips and transport directions are as required for those
slabs to be the fronts of overdying plates.
^\lECHANISM OF SUBDUCTION
WARREN HA.MILTON
tory. Neither mechanism of subduction can explain the
geometry of many modern systems. The Banda Antill~c
and Scotia arcs have subducting trenches that are C-
shaped in plan and Benioff zones, hence subducted
plates, that are spoon-shaped. Opposed arcs are in the
process of colliding in the Slolucca Sea and Solomon Sea
regions, over subducted plates that have an inverted V
configuration in section.
Such geometry suggests that the subduction process
below a depth of 100 or 150 km may consist primarily of
the vertical sinking of the downgoing plate and that the
apparent dip of a subducting plate is a Unction of the rate
of advance of the plate hingeline over the mantle beneath
the subducting plate. This rate would be equal to the
convergence rate only where the subducting plate is fixed
with regard to the mantle beneath it and where no mar-
ginal sea spreading occurs behind the advancing arc. The
sinking velocity of the subducted lithosphere must be a
function of the density contrast between subducting slab
and surrounding mantle and, hence, must increase, at an
exponentially decreasing rate, with age ofthat slab. Given
a constant rate of hinge advance, the dip of a subducted
slab should be a function of its age.
.
PRE-CRETACEOUS ISLAND-ARC AND
MELANGE ASSEMBLAGES OF THE
WE STE RN U N ITE D S TATE S
The Cretaceous and early Tertiary development of the
western conterminous United States was dominated by
subduction of large plates of oceanic lithosphere directly
beneath the continent. Similar activity of Andean type
occurred during parts of Triassic and Jurassic time also,
but much ofthe subduction recorded dunng those periods
occurred beneath oceanic island arcs. The arcs consist
dominantly of submarine basaltic and andesitic lavas,
breccias, and tufts; their intrusive and volcaniclastic
equivalents; carbonate strata; and diverse accretionary-
wedge materials including ophiolitic melanges. These
assemblages resemble those that characterize modem
oceanic arcs. These arcs were added to the west edge of
the continent when intervening oceanic lithosphere was
wholly subducted beneath either the continent or arcs
advancing toward it. Complexities ire these ancient, ac-
creted arc tenanes are still poorly understood but are
numerous enough to indicate that complex histories. com-
parable with those of modern Indonesia, .~Ielanesia, and
the Philippines, are recorded. We must be dealing with
the products of consumption of dozens of large and small
lithosphenc plates and with complex, jumbled aggregates
of arc components that were in part pre-assembled far
from their final resting place along the continental mar-
gin.
After Precambrian rifting and the formation of a new
ocean in the direction that is now west, the western mar-
gin of North America during early and middle Paleozoic
time trended from west-central Idaho to east~entral Cali-
The inclined descent of subducted lithospheric plates,
down to depths of at least 700 km, is commonly visualized
as representing either the injection of a slab down an
inclined trajectory that is fixed with regard to the mantle
or the gravitational sliding of a slab down such a trajec-
OCR for page 37
Complexities of.~fc~dern ctnd.Ancient Subduction Systems
Doria. This margin was stable tectonically and on it de-
~eloped an oceanward-thickening wedge of continental
shelf strata. In Late Devonian and Early Mississippian
time, creep-water strata from the west, with some of their
underlying oceanic crust, were shoved onto the shallow-
water shelf strata, and both were pushed eastward above
the continental basement in the Nevada sector (Figure
3.2). This event may record the collision of an island arc
with the continent; but the arc itself did not remain at-
tached to the continent. tor another period of stable-
margin sedimentation followed, with deep water again to
the west. Presumably the polarity of subduction reversed
beneath the collided arc, which migrated away from the
continent, opening an ocean basin behind it. Another
similar collision occurred against Nevada near the middle
of Triassic time. Again, dee~water sedimentary rocks and
oceanic basement were driven eastward upon shallow-
water strata, and both moved eastward over the conti-
nental crust; but this time, the causative island arc re-
mained attached to the continent. The Permian and
Triassic marine faunas of this island arc differ only moder-
ately from those of mainland North America, so the pre-
collision wandering of the arc need not have been at vast
distances from the continent. The collision was followed
by a reversal of subduction polarity, and Upper Triassic
arc magmatic rocks were erupted through the old conti-
nental crust, above the subduction system dipping east-
ward from the Pacific edge of the added island arc.
Still greater motions of lithospheric plates are required
by juxtapositions of other arc and melange assemblages
that yield fossil or paleomagnetic evidence for distant
sites of origin. One belt of such assemblages occurs in the
tectonic-accretion terrane from the southern Sierra Ne-
~ada of Califomia to the Yukon Ternto~y in Canada; it is
characterized by shallow-water Upper Permian lime-
stones bearing tropical Asian fossils. Particularly distinc-
tive among these fossils are fusulinids aIgal-grazing,
calcareous-test protozoa~of the verbeekinid family.
These are widespread in correlative deposits of paleo-
tropical Eurasia yet are completely unknown in paleo-
tropical mainland North America. (The equator of the
time trended through Texas, in the direction now north-
eastward, so appropriate habitats were available; but the
American fusulinid assemblages are quite different from
the Eurasian ones.) These fossils occur in limestone
blocks in melanges, in reef limestones capping atolls, in
the scraped-off complexes along the paleomargin of North
America, and in part between island-arc belts. Subduction
of more than 10,000 km of oceanic lithosphere beneath
North America during Late Triassic and Jurassic time, and
beneath island arcs that were themselves added to North
America in Jurassic time by subduction of intervening
oceans, is required. Oceanward of this tract in British
Columbia and southern Alaska is still another tract of
isIand-arc and related rocks, characterized by southem-
hemisphere paleomagnetic latitudes. The Cretaceous and
early Tertiary pattern of relatively simple subduction of
oceanic lithosphere beneath the continent, with minimal
.~_
,~ ~
involvement of island arcs. followed the addition of these
various terranes to North America.
C R ETAC E O ~ S AN' D C E NO ZO I C
CALI FORN {A
SU BDUCTION CONFIGURATION
Both the geology of Califomia and the seafloor-spreading
history of the northeast Pacific Ocean require that during
Cretaceous and early Tertiary time oceanic lithosphere
was being subducted beneath Califomia. The Cretaceus
components of most of Califomia fit the model of Figure
3.1, when palinspastic restoration is made to reverse the
disruption and offset by late Cenozoic strike-slip faults
and other structures. In the west, forming most of the
Coast Ranges (Figure 3.~, is the accretionary u edge, the
Franciscan complex of melange and imbricated rocks-
mostly of abyssal uppermost Jurassic, Cretaceous, and
lower Tertiary elastic sediments but including fragments
of oceanic crust and mantle and of seamounts, and masses
of high-pressure, low-temperature metamorphic rocks.
Shear structures in the chaotic Franciscan complex typi-
cally dip steeply to moderately eastward, yet gravity stud-
ies indicate that the oceanic lithosphere beneath the
wedge dips only gently eastward. Along the east side of
the Coast Ranges, coeval outer-arc-basin strata, the Great
Valley facies, lie with depositional contact upon oceanic
crust and upper mantle, which in turn is in thrust contact
with the tectonically underlying Franciscan accretiona~y
wedge. Lower Great Valley strata were deposited at
abyssal depth on oceanic cmst, a narrow strip of which
remained attached to the continent while Cretaceous sub-
duction proceeded. The strata lap eastward across the
oceanic crustal strip and in the subsurface butt against
what was in Cretaceous time the moderately sloping edge
of continental crust. The leading edge ofthe oceanic strip
was rotated upward concurrent with Cretaceous subduc-
tion.
Farther inland, the SielTa Nevada batholith, granitic
rocks mostly of Cretaceous age, followed synchronously
with the formation of the Franciscan accret~onary wedge
and with the development of the Great Valley outer-arc
basin. The batholith represents the subjacent equivalent
of a volcanic arc such as that of modern Sumatra. Batho-
lithic rocks of any one age display a cross-strike increase
in the ratio of potassium to silicon. characteristic of action e
volcanic terranes foxed above subducting plates. and
the distribution of the rocks defining the gradient indi-
cates moderately steep subduction dips compatible with
the Franciscan and Great Valley geometry in a system like
that illustrated in Figure 3.1.
The magmatic record of Cretaceous and early Tertiary
time In the western states indicates that the dip of the
subducting slab was moderately steep and relatively con-
stant during Early and early Late Cretaceous time but that
the dip was markedly gentler during late Late Cretaceous
OCR for page 38
38
WARREN H~ILTO!
120°
l ~ ~ ~ ~ volcanic ~ coLuMl~la PLATeAi}
. )- ~ (~} ~ ~ ~ arc ,,:._ ~ ''
I'd ~ 1
(_ `~_~c~~ ~ ,,, ~ ~ /
Us ! 'I
9 .r-~,o.~ == // ~~:
An eland of . cane
c / ~ ,; ~~Devor'ion to To,. .~7orc
~ i= ''''a
~ ~ . ~ ~ . _ . .
I ~ ~ ~ ~ / ITS.
· 1 At- it_
. ~ J Carboniterous- ~ ~ ~ ~~~
( Jura.ssic ore, , ~ Earty \ |
oph~olite, ond ~ , Pa's02sic \
: f \me'~nge /f me~coge
\ mater~ole, ~ /
~ \ J idt' :: ,_ : :? -. -.
Jurassic ana Eari, Crctaccous:, ~ `~; ~~ ''~43
pl u tons in nor t h east Oregon . ~_ ;' `',: k1 ~_'
postdcte most subduction ~ ~t i
accretion \'(
>"~-"~
~Al :2`.. .::.
NEOGENE BASALT
Fills th~nned and r~fted crust I
t ^~\~ ?~;~
!~.ndecino~ ~ ic,';
40° ~ ~.i~, ~ ~ ~ ~ ~ ~ nm~Cr9tm~rt.t ~ ~ -
tit ~ c ~
, . ~,
, §'
.'~k
~, ~
`^, I
's / PROv'NCE
1
l
~i~.\,\ ~
0tOut 173 of wl~th I
of tasin-rOnge proYinCe i
repreSents Neogene
crustal extens~on,
northwesteard re~ct~ve
tO croton
P~poroz imCt.
rt~ngel~ne De,~een /j
P~160201C She1~/
and crotCnic ~ j
\ st~atc ~
_ ~ `/ i
~Scn Gregorio fcult ~r,,cssic o, ~urc,s,c r,~t ot old continental morg'n
I ~ angul ~ aC b~ you n ger p I u rons
200 1< M
0 f00
FIGI:RE 3.2 Selected tectonic elements ot the west-central United States. The present dis~iblltion of these elements was atTected
greatly by disruption during middle and late Cenozo~c time.
OCR for page 39
Complexities of .~odern arid Ancient Subduction Sy.stem.s
time, perhaps as the resmelt of an increased convergence
rate between the .Nrorth American and subducting plates.
Nlagmatic-arc patterns changed complexly during early
Tertiary time, indicating corresponding variation in sub-
duction dips. In Oligocene time, for example, the sub-
ducting slab was apparently segmented: the southern seg-
ment decreased in dip northward Prom moderately gentle
to very gentle. there was an abrupt change across a line
trending northeast Tom southern Califomia to northern
Colorado, and the northern segment decreased in dip
northward from moderately gentle to moderately steep
(see Chapter 14). .~ch ofthe Tertiary complexity may be
due to the subduction of oceanic lithosphere of varying
age and density as the East Pacific Rise. offset along trans-
~nn faults, approached the continental margin and
young lithosphere of variable temperatures was being
subducted. Conversely, the spreading history and geom-
etm of eastern Pacific plates, since wholly s~bd~cted
beneath North America. can perhaps be inferred from the
magmatic-arc migrations.
In the Klamath Contains region of northwest Califor-
nia and southwest Oregon, the Franciscan melange
wedge lies directly beneath crystal assemblages that were
part `~t the continental crust in Cretaceus time (Figure
3.~). No `~ter-arc-basin strata lie to the east. Here, the
leading edge of the continental plate was ~~..'ed of conti-
nental cutest; the narrow strip of attached oceanic crust is
missing, and no o~ter-arc basin developed.
LATE CENOZO}C DISRUPTIO.N
During Cretaceus and early Tertiary time, the Pacific
margin ot Califomia was ~ relatively simple system of
parallel tectonic and magmatic belts, controlled by the
igoro~s s~bd~ction then occurring beneath all of Califior-
nia The subduction was more rapid than the spreading of
the East Pacific Rise offsl?ore to the west, so the rise
missed progressively closer to the trench despite the ac-
ti~e spreading. The oceanic plate east of the rise was
mov ing northeastward relative to North .\merica, whereas
the plate west of the rise was moving northwestward.
Then the rise itself hit the s~bd~ction system oboist 30
million years (m.~.) ago. the western oceanic plate came
into direct contact with ~ segment of the continental mar-
Kin. and strike-slip m<,tion resmelted. The strike-slip
bo~ndarv lengthened faith time. and its north limit mi-
gr;Ated northwestward, as spreading ridge and trench col-
lided along progressively more of the c<,ntine~ta1 margin.
Tile manic lithosphere. `` hich had liven Updated
shortly before the transition. `~as ·~ng and hilt. sit the
change from .~bd~cti`'n to strike-slip regimes Erred
`~hen the continental margin `~`I'; l~nderl.lin b~ sort. ~ eak
mantle.
The results `~t these changes and c`,n~liti<,ns elan be seen
in s`~them California and north est \~.YiC() ( Figure 3.3).
Tale Cretace`~s m`~gmatic ``rc (S it rra .N ex `~1~. Peninsr~l.~r
Ranges and Baja C.~lit`~rni`~ t'~th`,lith~ii. `~ter-~`rc basin.
39
and Franciscan melange wedge are all present blat their
distribution is scrambled. The simplest and ingest
major disruptive structure is the San Andreas Fault. which
is now the dominant break along which the western
Pacific plate is moving northwest past the continent,
carrying Baja California and coastal California along. The
San Andreas offset of a well-documented 300 km and the
concurrent opening of the Gulf of California hare oc-
curred within the past 5 m.y.
Lotion patterns during the transition period. between
30 m.y. and 5 m.y. ago, from subduction to strike-slip
regimes were much more complex. In the southern Coast
Ranges (northwest part of Figure 3.3), strike-slip faults
underwent pre-San Andreas offsets of at least 200 km; yet
these faults probably do not cut the Transverse Ranges,
w hich trend across them in the south. In the Coast
Ranges, the westward progression from magmatic arc to
`'uter-arc basin to Franciscan melange wedge is present,
variably truncated by Cenozoic faults; but then more
outer-arc-basin strata were present beyond the Francis-
can, in the Santa Ynez and Santa Monica Mountains of the
western Transverse Ranges. The complete progression is
present also in the Vizcaino Peninsula sector of Baja Cali-
tomia. Along southern California south of the Transverse
Ranges, however, the sequence is partly repeated: off-
shore in the San Nicolas Island~anta Rosa Island sector,
outer-arc basin strata are again present, out of place to the
west of Franciscan; and Franciscan is present west of
these strata' apparently in proper succession. One expla-
nation possible for some of these and other relationships
is that a western sliver of outer-arc basin and Franciscan
terraces slid northwestward from Baja California. along a
right-lateral strike-slip fault, around the Franciscan ter-
rane now enclosed to the east; the leading edge of the
sliver impinged on the continent, and the rest rolled
clockwise past it, forming the western Transverse Ranges
by rotation. The required rotation and northward transla-
tion are consistent with the paleomagnetic orientations of
lower Miocene basalts in the Santa Monica .\Iountains
and islands to the west.
Other styles of crystal disruption affected the western
states further inland. Distributed crustal extension pro-
d~ced the Basin and Range province (see Chapters 8 and
9) of blocks bounded by downward-flattening nodal
tilts. The amount of total late Cenozoic extension is in
dispute; my interpretation, more extreme than most. is
that bet`` een 30 and o0 percent of the present width of the
turbine represents extension. The NetWare of the transition
from brittle fracturing and block rotations ot the tipper
crust to hypothetical uniform extension in the deep. hid-
den loner angst is also in dispute. Steadies in progress of
deeply eroded basin-range terranes in southeast Calitor-
nia and southern Arizona snuggest that the level of detor-
mati`,n beneath the horizontal bases of rotated fault blocks
is characterized by giant bodies or mullions, hating
lengths <~l as milch as several tens of kilometers. and thus
b` delonnation that is partly ductile and partly brittle. In
OCR for page 40
40
3so
30
WARREN H~!~{ILTON
COAST RANGES 120°
"S~ ~~\ ElASliV- RANGE
115°
~)PROV 1 N C£ I
\aclo: 5: !
~ N \~: He \ ~
\ ~ W~ ~: i
~ I,
-
l
1
_~. ~` -_ ~ ~ __
\ ~00%~ a~e SON ~
~ ~'t ;o~<,~ '^ ^~\ \ G
:1\(:
~ ~.\ 6~A \
` .:
\
FIG~'RE 3.3 Selected tectonic elements of the southwestem t;nited States and northwestern \1exico.
OCR for page 41
Cvmplexitie.s of Modern and Patient Subduct:<'n System.s
the northwest states. Mom northwest Nevada and south-
en, Idaho northwestward' continental crust older than
middle Tertiary was thinned by extension in some areas
and completely rifled in others, the volumes has ing been
filled subsequently mostly by middle and upper Ceno-
zoic basalts (Figure 3.21. Other aspects of Cenozoic dis-
ruption are discussed in Chapter 8.
~1
REFERE!.'CES
Hamilton. lo. (1978). Mesozoic tectonics of the western ~ noted
States. in Pacific Cubist Pale<~raphy Symp`'s2um A.
Pacific Section. Society yists and
Mineralogists. pp. 33~70
Hamilton At. t19~9). Tectonics of the Indonesian region C.S.
Geol. Surr. Proof. Paper 10~-8, .~34.o pp.
OCR for page 52
52
FIGURE 5.2 Diagrammatic exam-
ples of outer highs on divergent conti-
nental margins.
very irregular. For example, areas with extensive volt
canics, such as the Faeroe Island area (Talwani and
Eldholm, 1977) and eastern Greenland (Hailer, 1971) con-
trast with areas having virtually no extrusive volcanics in
the area of the outer high, such as the west Africa exam-
ples discussed later in this chapter. The distribution of
volcanics in presently rifts is similarly irregular. For
example, the Rhine Graben has two large volcanic
centers, Vogelsberg and Kaisers~hl, separated by areas of
limited volcanic activity (Rhine Graben Research Group
for Explosion Seismology, 1974~.
In general, divergent margins have a characteristic pro-
gression of sediment types that vary as an area progres-
sively undergoes rifting and then driRing. Rifts that form
in areas of thick continental crust, such as cratonic shields
and areas that have undergone an erogenic event, are in
most cases first filled with nonmarine sediments that
grade upward to shallow marine sediments that com-
monly include evapontes, as shown in the West Africa
example discussed later in this chapter. RiRs that form In
areas of thinned continental crust, such as older inactive
rids, commonly contain marine or interbedcled marine
and coastal deposits. Figure 5.3 Tom offshore east Green-
land (Hailer, 1971) is an example of divergent margin
evolution in which the crust has been thinned by earlier
riding.
Post-pull-apart or driR sediments are usually marine,
except where large prograding delta complexes are pres-
ent. In many areas, the marine sediments of the drift
phase are prograding carbonate banks and reefs. Marine
temgenous elastics commonly overlie the carbonates or
extend throughout the entire driR phase, where car-
bonates are absent.
GEOPHYSICAL EVIDENCE FOR OUTER HIGHS
Outer highs are best recognized on reflection seismic data
by looking for the relatively high area in between the
MARTIN A- SCHUEPBACH and PETER R. VAIL
_ - _
~ - 83 U4TE Rl~ SEDIMENTS - — _ ~
il ~NLAPPNG 0N10 DRIFT SEDIU0iig—
~ __==
, " n ~ Elton u=u I ~ OCEANIC
. ' CRUST
_ /
\ '\
~ . /
! 0~ 8~
-
SALT\
7D ~ 1 ~ seems
ted 'I
LATE RlfT SEDIIUENTS / , I,
_: ~6 ONTO oUrER ~ 5
~ ()—it —ORFT SEDIMENTS MINES
___ ~ ~ TWO WAY O
;Alri /~ /oUT~8 HIGH _ ~ _
/, /, / ' / - , ' t, Il CRUST'
/ / \ / \, i\ `// ~ \ _ ~ 1 - ,,
seismic expression of oceanic crust and seaward onlap of
the reflectors over the predrift unconformity. The reflec-
tor at the top of the outer high may be continuous or highly
faulted. In addition, reflectors and faults may or may not
be observable below the reflector at the top of the outer
high. The diagrams shown in Figure 5.2 are patterned
after seismic examples and thus illustrate typical reflec-
tion patterns of outer highs on divergent margins.
An example ofthe seismic expression of oceanic crust is
shown in Figure 5.4. Note the high amplitude with nu-
merous diffractions that characterize this reflection. In
this example, no good reflectors occur below it. This is the
common case. In other examples, however, dipping re-
flectors occur below the top of the oceanic crust. Areas
where these reflectors occur ale commonly paleotopo-
graphic highs, which may indicate local thickening of
oceanic crust by volcanic extrusives.
Two multichannel seismic lines hum offshore West Af-
rica (A, Figure 5.5, and B. Figure 5.7) are presented to
illustrate the seismic expression of outer highs. Both lines
extend from the present-day shelfto near the continental-
oceanic crustal boundary. This paper only discusses the
interpretations in the vicinity of the outer highs. A de-
tailed discussion of seismic line A (Figures 5.5 and 5.6) is
presented in Vail et al. (in press). A dashed line indicates
the interpreted boundary between rift and drift reflectors'
and a heavy solid line indicates the top of the outer highs
On both seismic lines the interpreted presalt early-riD
reflectors are bordered on the seaward side by outer highs
consisting ofblocks bounded by steep faults. Seismic line
A (Figure 5.5) shows a relatively smooth surface at the top
of the outer high with underlying faults and reflectors
Seismic line B (Figure S.7) shows large tilted fault blocks
Late-rift salt reflectors onlap or pinch out against the outer
highs. Onlap patterns on seismic line A indicate that the
paleotopographic high point of the outer high was on its
eastern side and the top of the interpreted oceanic crust
actually was higher than the western portion of the outer
OCR for page 53
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OCR for page 54
54
.UARTIN A. SCHUEPBACH and PETER R. VAIL
Am_
~ _,
OCEANIC CRUSTS
, ,
__
!~
FIGURE 5.4 Example of the seismic expression of oceanic crust from the Blake-Bahama Basin (from Shipley asked Buffler, 1978,
courtesy American Association of Petroleum Geologists).
high in the early drifting phase. Unfortunately, seismic
line B did not extend across the continental~ceanic
crystal boundary, but the magnetic quiet zone, inter-
preted as oceanic crust, is known to exist just west of this
seismic line. The interpreted salt interval thins, but does
not completely pinch out at the western end of this line.
The outer high on seismic line A is bordered by what
we interpret to be oceanic crust. The boundary is placed
between reflection patterns similar to those characteris-
tics of oceanic crust and to the outer high.
Both seismic lines are controlled by drilling on the shelf
and by regional outcrop geology. Based on these data, line
A rift sediments include Triassic and Lower Jurassic
(Hettangian) red beds and Lower Jurassic carbonates and
anhydrites. Based on its stratigraphic position, we inter-
pret the thick salt on line A to be Early Sinemurian in age,
but it could be significantly older or younger. The rift
sediments are overlain by drift sediments consisting of a
Middle Jurassic to Cretaceous Berriasian prograding car-
bonate bank and a Cretaceous-Tertiary prograding elastic
wedge (see Vail et al., 19771.
Wells penetrating the shelfnear seismic line B indicate
that the rift sediments are Neocomian-Aptian elastics
overlain by late Aptian salt. DriR sediments are post-
Aptian Cretaceous carbonates and elastics and Tertiary
elastics.
No good evidence for volcanics is shown on these sec-
tions in the vicinity of the outer highs. However, on
seismic sections from other areas, the reflection paKem of
volcanics is common and quite variable. Volcanics exhibit
considerable variation in paleotopography at the top and/
or base of the unit as evidenced by onlap. Interval reflec-
tions within the unit commonly dip at a greater angle than
the top or base. In many cases these dipping reflections
are high amplitude and continuous for short distances. In
other examples, extrusives appear as lens-shaped lobes
with a relatively high-amplitude reflection at the top and
little to no reflections within the lobe. Interval velocities
it,
tend to be intermediate to high in the first example and
high in the second. We interpret the first example to be
largely interbedded volcanics and the second example to
be massive flows. Intrusives commonly appear on seismic
data as irregular high-amplitude reflectors, sometimes
crossing and sometimes subparallel to reflections from
stratal surfaces. Reflections originating Mom intnlsives
commonly occur sporadically in the vicinity of outer
highs.
Only limited magnetic and gravity data were available
over the outer highs described in this study. Where
magnetic data were available, the presence of magnetic
stripe anomalies correlates well with the typical seismic
expression of oceanic crust discussed earlier. However,
the steep magnetic gradients oRen associated with the
continental~ceanic crustal boundary commonly occur
well landward of the outer high. In areas where extensive
volcanics are present, the relation of the magnetic pat-
terns to the continental-oceanic crust boundary was vari-
able. In some cases, where volcanics were present, the
steep magnetic gradient was seaward of the outer high.
Gravity data commonly show positive isostatic gravity
anomalies extending along the interpreted outer edge of
the continental margin (Emery et al., 1970; Talwani and
Eldholm, 1972; Rabinowitz, 1972~. Figure 5.8 is an exam-
ple. A gravity low is commonly present landward of the
outer high' indicating a deep sedimentary basin. The as-
sumed shallow basement of the outer high causes a rather
strong gravity anomaly but not a large magnetic anomaly
(Talwani and Eldholm, 1972~.
SUMMARY OF EVOLUTION OF DIVERGENT
(PAS S IVE ) CONTINENTAL MARGIN S
If the formation of outer highs during the late rid stage is
as prevalent as we believe, it would add another phase to
the evolution of divergent (passive) continental margins.
OCR for page 55
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OCR for page 56
56
AGES
~=
TO RTONIAN
SER RAVALLlAN
BU RDIGALIAN
CHATTIAN
RUPELIAN
PR IA80 N IAN
8ARTONIAN
LUTETIAN
YPR ESIAN
THANETIAN
OAN IAN
MAASTRICHTlAN
CAMPANIAN
SANTONIAN
_
CONIACIAN
TU RONIAN
CENOMANIAN
ALBIAN
APTIAN
BAR REMIAN
HAUTER IVIAN
VALANGINIAN
z BER RIASIAN
_
PO R r LAN D IAN TITH O N -
, ~ ~,
KIMMERIOGIAN ~ JAI'
OXf O RDIAN
CAL LOVIAN
BATHONIAN
BAJOCIAN
AAUENIAN
TOARCIAN
PLI ENS8ACHIAN
Sl NEMU RIAN
HEUANGIAN
R HAETIAN
NORIAN
~ Il
. -=-—~ T. ~
. T~T2 T Te _
:T02.2 Td —
TO 1 Tc _
E2 2 T
TE3; Ta —
T92j 1: _
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~/'~ ~ K~ _ Q
K~ I Ka -
_
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FIGURE 5.6 Stratigraphic col~nn represented in seismic
line A (Figure 5.~).
MARTIN A. SCHUEPBACH and PETER R. VAIL
This section summarizes the evolution of divergent conti-
nental margins as a basis for placing the formation of
the outer high as an evolut~onary phase in divergent
continental-margin development
A vast amount of literature has contributed to the un-
derstanding of clivergent margins. Just a few publications
are citecl here: Drake et al. (1959~; Belmont' et al. (1965~;
Burk (1968~; Emery et al. (1970~; Talwani asld Eldholm
( 1972, 1977~; Rabinowitz ( 1974~; Boeuf and Ooust ( 19751;
Exon and W~llcox (19781.
A summary of the evolution of divergent (passive) con-
tinental margins (Figure S.9) is presented by the IPOD
Committee on Passive Continental Margins (CulTay, in
press). In that paper, the evolution of divergent margins is
subdivided into the following phases: (1) variable degrees
of doming, (2) niting, and (3) driflting. Doming of graben
margins, such as occurs in the East African riR (Pilger and
Rosler, 1976) and the Rhine Graben (Illies, 1977) might
not always be present and in some cases may occur while
the riRing was taking place (Carey, 19581. RiRing occurs
as a single graben or as a complicated subparallel system
of grabens and horsts. The driRing is characterized by
the fonnation of oceanic crust, seafloor spreading, and
subsidence.
TECTONIC-SEDIMENTARY MODEL FOR
THE EVOLUTION OF OUTER HIGHS ON
DIVERGENT MARGINS
The tectonic-sedimentary model presented here (Figure
5.10) for the evolution of the outer high differs fiom the
Curray (in press) model for divergent margins by adding
a late-riR phase consisting of a longitudinal predriR upliR
within the rifle graben or graben complex. Our model
shows two phases of rifting and two phases of clriPcing.
The initial nflcing phase fonns a single graben in ~is
model [Figure 5.10(a)~. However, as discussed in the
previous section, the initial nPring may be an intricate
graben system. The initial grabening phase may be
caused by crustal thinning over a zone several times
wider than the graben itseIf (Rhine Graben Research
Group for Explosion Seismoiogy, 1974~. Figure 5.11, re-
produced £rom the Rhine Graben Research Group report,
shows a cross section of the Rhine Graben. This section is
an example of the relationship between early ri:°cing and
crus~l thinning.
Whether the sediments deposited during the early-riR
phase are manne or nonmarine depends primarily on ~e
thickness of the continental crust in which the nflc forms.
Where the crust is thick, the early rifle tends to be topo-
graphically high and nonmarine sediments tend to be
deposited. Where the crust is thinner, ~e level of the rifle
floor tends to be close to or below sea level, resulting in
marine or interbedded marine and nonmarine sediments.
Sea-level fluctuations as described in Vail et al. (1977)
and access to marine waters are important factors in con-
trolling the sediments deposited in these types of rifts.
OCR for page 57
:. 1 '
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: 1
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en
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OCR for page 58
58
5a
BURIED RIDGE
,~\
50
~INTOUR INTERVAL 20mgal Ad\\ ~
JO , . , , , I'\ \ , to
10° 15°
FIGURE 5.8 Isostatic gravity anomaly map (from Rabinowitz,
1972) of Me Angola Continental margin showing the outer high as
a positive anomaly.
Dunng the late-nRing phase [Figure 5.10(b)], a narrow
irregular longitudinal uplift fortes within the graben. In
cross-sectional view, the narrow uplifted strip has the am
pearance of a basement high. Also, during the late-nE
phase, the area of thinned continental crust on both sides
of the basement high continues to subside. Our data in-
dicate that the late-nh phase sedimentary sequences in
these two subsiding basins commonly consist of facies
varying from moderately dee~water sediments to evade
orites. In general, erosion removes the early-rifle-phase
nonmarine sediments Dom the uplifted median basement
high, and late-rifle-phase marine strata onlap it. Our geo-
physical studies indicate that the outer high is commonly
injected by basic intrusions and is in many cases covered
by basic extrusives. The upward movement ofthe median
basement high appears to us to be caused by ~ thermal-
MARTIN A. SCHUEPBACH and PETER R. VAIL
DON15116
R~6
~ -
~5~ 1 ~EDU~
l
l
|2,W'~ ~ ~ MAN
FIGURE 5.9 Passive margin model Tom Curray (in press).
]5D mechanic event Mat immediately precedes We fo~a-
tion of oceanic crust. The process of the therrnal-
mechanical event is not well understood and is an area
recommended for further study.
At the beginning of the drift phase [Figure 5.10(c)], the
outer high splits longitudinally into two outer highs sepa-
rated by oceanic crust The outer highs subside rapidly
from close to sea level to We presently depths following
the subsidence curves for thinned continental crust and
oceanic crust cliscussed by Parsons and Sclater (1977~.
Cooling ofthe mantle below the thinned continental crust
and the newly formed oceanic crust and sedimentary
C ~ ~
LEO -
PROGRADHG SWAMPS
OF DREG PHASE
i, ~ MARE ~M~
2- ~ / _ ', --, / `'' \ _ I,—' BROW ~ - Rem OF
MOW ~ _' ~ ~—~ 1__ _\ —_ ~U RR ~
I -.-1 SEDIMENTS OF EARLY
'' '---"aid-,-' ~ PRERH~M~
FIGURE 5.10 Tectonic sedimentary model showing the evolu-
tion of outer lights on divergent margins.
OCR for page 59
Evolution of Outer Highs on Divergent Continental Margins
w
FIGURE 5.11 Crustal cross section through Rhine Graben
(from Rhine Graben Research Group for Explosion Seismology,
1974). Thinning of the continental crust and mantle upwelling
occur in a zone several times wider than the width of the graben.
loading results in tilting and subsidence of Me continental
margin. In the proper climate, the rapid subsidence and
initially small Drainage areas for elastics provide a deposi-
tional site ideally suited for the development of Dick pro-
HYPOTHEllCAL DEPTH
CONTOUR UN~ \
MEDLAN
OCEANS m=1
rat
4.
+ 1
EAflLY RlR WAGE
ONMM.E SWIMS)
LATE B" n"E
MARIIIE SEDINIEIJ'S)
PROGRAD.16 MARIIIIE
SE0 - Em
FIGURE 5.12 Schematic evolution of divergent margin open-
ing in a pivotlike fashion. All stages of evolution, from initial
rifting to the final drift stage, may occur simultaneously along a
margin opening in a pivotlike fashion. The marine basins of the
late-rift stage may be restricted and completely separated From
the early-riPc and driR phase by the outer high and transform
faulting.
59
grading carbonate banks and reefs. As subsidence slows
down througl, time, the drainage area caused by subsi-
dence expands. This tends to cause tenigenous elastics to
build seaward, commonly covering the carbonate bank
margins.
Full development of the features associated with riding
is in part dependent on the total extension across the rift.
A divergent margin that undergoes a pivotlilce opening
(Figure 5.12) will fully develop its characteristic features
sooner at large distances from the pivot point than at small
distances, because large extensions are achieved sooner
there. Thus, even though extension along tlie length of
the opening is synchronous, basin formation and filling
are diachronous. Also, the early-driR basin may be sub-
divided into subbasins by highs caused by transform fault-
ing. Subbasins thus formed can restrict the circulation
within the late-rift- and early-driR-phase basins, thus
causing conditions favorable to the deposition of black
shales and/or evapontes.
PROBLEMS AND RECOMMENDATIONS
The combination of regional geological studies and multi-
channel seismic profiles used in conjunction with other
geophysical data has provided the information for the
definition of some key problems related to the evolution
of outer highs on divergent continental margins. We rec-
ommend that a long-term program consisting of geo-
physical studies, emphasizing regional multichannel
seismic lines, and regional geological studies, including
outcrop, piston, and dredge samples, be undertaken to
locate optimum drill sites for the best possible confinna-
tion ofthe geophysical interpretations and understanding
of the geological problems. Such work is proposed for the
1980's by the JOIDES program (White, 1979~. The key geo-
logical problems that we have defined concerning the
outer highs on divergent continental margins are the
following:
1. The character, age, and location of the continental-
oceanic crusted boundary;
2. The existence, nature, size variations, and evolution
of median highs within grabens to outer highs on diver-
gent continental margins;
3. The nature and evolution of the overthickened
oceanic crust and volcanics commonly present in We area
adjacent to the continental-oceanic crustal boundary;
4. The type, age, and controls of late-riR-phase sedi-
mentation, particularly organic-nch shales and evap-
orites;
5. The relation of subsidence and heat flow to crustal
type and thickness;
6. The paleoenvironmental history of the sedimentary
section through the drift phase including age, paleontol-
ogy, hiatuses, unconformities, lithofacies, paleobathym-
etry depth, physical properties, and other significant
parameters.
OCR for page 60
60
Venfication of the model relies on extensive geophys-
ical surveys involving multichannel seismic, magnetic,
and gravity data and testing of optimum sites by drilling.
If the mode! is verified, it will better explain many of
the geological problems associated win divergent conti-
nental margins, including a better understanding of the
nature of the oceanic~ontinental crusted transition, asso-
ciatec3 deposition including salt and organic-nch shales,
and Me relation of volcanism to the onset of seafloor
spreading.
SUMMARY
Observation of a large number of geological and seismic
profiles across divergent continental margins has demon-
strated the widespread occurrence of structural highs that
were formed during the late-nflc phase of divergent mar-
gin evolution and are now the boundary between conti-
nental and oceanic crust. This feature originates as a
median high, which divides the rift zone into two subsid-
ing basins. This median high later evolves into an outer
high along doe newly forrneil condnental~ceanic bound-
ary during the drift phase. The outer high commonly ex-
tends the whole length of a divergent margin but can be
segmented by such factors as transform faults andJor vol-
canics. In many cases, the outer highs are complicated by
extensive intrusives and extrusive volcanics. Early-riP:
sediments are commonly nonmarine, but in areas of
Tinned continent crust they may be manne. Manne
sediments or volcanics are almost Sways deposited in the
late-nflc basins on opposite sides of the median high. Salt
and organic-rich shales are commonly deposited during
this phase.
This chapter has presented a mode! for the develop
ment of an outer high on clivergent continental margins
that relates the major structural episodes with the asso-.
ciated sedimentation. It consists of (1) early-nflc-phase
graben formation, ~a' late-nR-phase development of a
median high subdividing the graben complex into two
subsiding basinal areas, and (3) driflc-phase Connation of
oceanic crust and He outer high as a remnant of the
median high and subsidence.
ACKNOWLE OGM ENTS
The writers are indebted to E. B. Nelson of Esso Explora-
tion, Inc., anil I. Viellart of Esso aEP for Heir seismic
interpretations; to I. P. Verdier and R. I~ehmar~n of Esso
Production Researc~European Laboratones for their
paleontological contributions; to H. R. Gould of Esso Pro-
duction Research Company for his guidance; to C. G.
Zinkan, D. R. Seely, R. W. Murphy, I. K. Davidson, D. B.
Macurda, K. T. Biddle, R. C. Vierbuchen, R. P. George,
Ir., all of Exxon Production Research Company, who re-
viewed the manuscnpt; and to Esso Exploration, Inc., for
permission to publish He seismic lines.
MARTIN A. SCHUEPBACH and PETER R. vAIL
RE FE RE NC I: S
Belmonte, Y., P. Hirtz, and R. Wenger (1965). The salt basins of
the Gabon and He Congo (Brazzaville): a tentative paleogeo-
graphic interpretation, in Salt Basins Around Africa, Elsevier,
Amsterdam, pp. 5~74.
Boeuf, M. G., and H. Doust (1975). Stnlcture and development of
the southern margin of Australia, Aust. Petrol. Explor. Assoc. J.
15, 3~43.
Bulk, C. A. (1968). Buried ridges within continental margins,
N.Y. Acad. Sc'. Trans. al:), 135 160.
Carey, S. W. (1958). A tectonic approach to continental driR, in
Continental Drift, a Symposium, S. W. Carey, ea., Geology
Department, University of Tasmania, Hobart, Tasmania,
Australia, pp. 177~49.
Curray, J. R. (in press). The I~D P=g~me on Passive Conti-
nental Margins, in The Evolution of Passive Continental Mar-
gins in the Light of Becent Dnlling Results, Philosophical
Transactions of the Royal Society, London, England.
D'alce, C. L., M. Ewing, and G. H. Sutton (1959). Continental
mans and geosynclines: the east coast of North Amenca,
north of Cape HaKeras, in Physics and Chemistry of the Earth,
Vol. 3, L. H. Ahrens, ea., Pergamon, New Yorlc, pp. 11~158.
EmeIy, K O., E. Uchupi, J. D. Phillips, C. O. Bowin, E. T. Bunce,
and I. T. Knott (1970). Continental rise oReasten~ North Amer-
ica, Am. Assoc. Petrol. Geol. Bull. 54, 99~108.
Exon, N. F., and T. B. Wilicox (19~78). Geology and petroleum
potential of Exmouth Plateau and off western Australia, Am.
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Representative terms from entire chapter:
rhine graben
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