Certain problems of the structure and evolution of transition zones between continents and oceans

79

~~,~/of?oph,l~rc.\, 10s ( 1984) 79- 102 Elsevier Science Publishers B.V.. Amsterdam

- Prmted in The Netherland

CERTAIN PROBLEMS OF THE STRUCTURE AND EVOLUTION OF TRANSITION ZONES BETWEEN CONTINENTS AND OCEANS

ABSTRACT

Beloussov.

V.V..

continents Structure.

1984.

Certain

and oceans. Dynamics

In:

problems SM.

H.K.

Gupta

and evolution

of transition

and S. Balakrishna

(Editors).

transition

from two other commonly

between

I.irhosphere:

zone. referred

IO as the C‘olumbian

crust into intermediate

changes are the basification mantle. and its substilution

transItion

zone. 12

known types of these zones. The subsidence of the Earth’s crust.

typical of all transition zones. IS shown IO be connected (by geophysical properties) of continental

lone>

and F.volurion. Tecmuophvsrc~s. 105: 79-102.

A type of continental-oceanic distinguished

of the structure

Naqvi.

IO the transformation

crust and later into oceanic. The most likely mechanisms of buch

of continental

crust. its foundering.

block by block. into the heated upper

by new oceanic crust. The evolution of transition zones of the Pacific type is

largely influenced by deep faults. which reach down IO the level of undepleicd volatile products rise IO the surface which results in the formation

manile. From this level. the

of talc-alkali

magmas on Island arcs.

The Benioff zones are deep faults. whose inclinations are dependent on the density contrasts in the upper mantle on either side of the Benioff zones. The denser mantle flows beneath the mantle of lower density. This phenomenon

is depicted by plate tectonics as subduction.

On the whole. the evolution of transition zones gives rise IO the growth of the oceans at the expense of the continents.

though oceanic crust becomes thicker by addition

andesite. in the transition

THE

“COLUMBIAN”

of volcanogenic

layers composed of

zones (type IWO) of the Pacific type at island arcs.

TRANSITION

ZONE

To the commonly known two types of transition zones, i.e. the Atlantic. or passive, and the Pacific, or active, the author finds sufficient reasons to add a third type. which he calls the Columbian type. This type of transition zone is developed along the Pacific margin of North America, between California and Alaska. The peculiar features of its structure define it as an intermediate type of transition zone. A young folded zone and a mountain range stretch along the continent’s margin, which identifies this zone as an example of the Pacific type. This is further emphasized by the fact that this zone lies on the continuation of the South American oo40-1951/x4/.$03.00

,I: 1984 Elsrvirr

Science Publishers B.V

and Central

American

margins

which without

the other hand. the North American have the deep-water active

margins.

margins

marginal

marginal

plateau marginal

The enumerated between grounds

plateaus

plateaus

make the Columbian

type.

is actually

doubt

between

belong

to the Pacific type, On

California

and Alaska

the Benioff zone, nor the type of seismicity

features

of the Atlantic

submarine typical

trench.

These

margin

This

similarity

in the C’olumbian

transition

is supported

expected

zone similar

zone. For example.

contradictory

transition

peculiarities

in

to the

by the presence

01

the C‘alifornian

verb much like the Blake and Voring plateau

of the Atlantic

does not

which are

zones.

in the structure

of the Pacific coast

California and Alaska are so specific that we believe them to be sufficient to consider this region as an example of a separate type of transition zone.

TRANSITION ZONES OF THE

Transition

zones

ATLANTIC‘

TYl’t

of the Atlantic

type

are superimposed

continents, being in the state of old or young between the continent and the ocean truncates tures with an unconformity. The most important

problem

in transtion

on the edge

of the

platforms. Moreover. the boundary the pre-Mesozoic continental struc-

zones of this type is that of the stability

of the continent-ocean boundary, i.e. the line dividing the continental and the oceanic crusts. Was it always in the place, where it is now found? As is known from geophysical data, the change of continental crust to oceanic usually occurs on continental slope at the point where the ocean’s depth reaches 2 or 3 km. But thickness of the continental crust does not remain constant as it approaches line, it becomes thinner. For example, on the Atlantic margin of North America

the the that the

thickness of the continental crust decreases from 33 km to 16 km from land towards the ocean. Also. a layer appears with high seismic velocities (7.1-7.4 km/s) appears at the base of the crust. These changes of the basement

of the continental

take place in a direction

crust, from the coastal

outer parts of the shelf and on to the continental

parallel

plain,

to deepening

to the inner

slope. As a rule, this deepening

and is

not gradual. but step-like. the steps being divided by vertical faults (Fig. 1). Geological data imply that this deepening is the result of the process of crustal subsidence,

which increases

from the continent

to the ocean. This is supported

by

the large thickness of shallow-water and continental sediments on the shelf which may reach more than 10 km on its outer edge. The history of transition zones of this type can be divided into three stages: continental, lagoon, and marine. The first stage normally appears in the rift regime, in which the crust is divided by steep faults into graben and horsts. the graben accommodating the first continental sediments accompanied by basalt effusions and intrusions of dolerite dykes. The second stage is a period of evaporite accumulation. During the third stage. the layers of sediments are formed on the shelf, and these lie unconformably on the deformed and metamorphosed basement. The rift stage in the

81

Ne

V

X++~tantee a,o-a.2

:Yv.

1

26 28

30 Will 0 Fig. 1. Diagrammatic

I ioo

I 200

structural

I 300

cross-section

400

,v+ 500

across the Grand

1 600

, 700

800

Banks (from Sheridan,

Km 1974).

northern part of the Atlantic lasted from the Permian to the middle of the Jurassic, the lagoon stage was during the Late Jurassic, and the condition of an open sea began in the Early Cretaceous (in the Barremian or Aptian). In the south of the Atlantic the riftogenesis was active in the Triassic, Jurassic and during the larger part of the Early Cretaceous; the evaporites were formed in the Aptian, and the great marine transgression started in the Late Cretaceous (see, e.g., Kent, 1974; Renard and Mascle, 1974; Sheridan, 1974; Campos et al., 1974; Kent, 1976; and many others). The marginal subma~ne plateaus, typical of Atlantic transition zones, are also the result of crustal subsidence. The change of sediments in the vertical direction indicates that such plateaus were initially a part of the shallow-water shelf. For example, the Blake plateau off the Florida coast remained shallow till the end of the Cretaceous and only later did it subside to the depth of l-2 km (Sheridan, 1974). It is interesting that ~gh-velocity layers are well defined in the deep structure of marginal plateaus. For example, according to seismic data, a large massif of dense rocks lies at a depth of 10 km on the Voring plateau off the coast of Norway (Fig. 2). It is assumed to be built up of intruded basic and ultrabasic mantle rocks (Hinz,

S NORWEGIAN

SEA

VORING

PLATEAN

N

SHELF

1972). It seems natural to suggest that the subsidence of the plateau was caused by the intrusion of these heavy rocks. According to Sheridan (1969) the bottom of the Bahamas

basin adjoining

the Blake plateau has subsided since the Early Cretaceoua also subsided by the same amount, but on the subsidence was compensated by sedimentation until the end of the If the layer with velocities of 7.2-7.4 km/s now observed under the

by 5 km. The crust on the Blake plateau

that plateau Cretaceous.

basin and the plateau. already existed at that time. then it would have been at a depth of not less than 7 km, which would have upset the isostasy. Thus, this layer probably

appeared

later. i.e. during

The logical conclusion the Atlantic anomalously

is that the history

of transition

Lanes of

type seems to be connected to a change in crustal structure in which dense rocks (according to geophysical parameters) appear. This makes

this kind of crust to a certain oceanic

subsidence.

of this reasoning

crust.

It is then

extent

reasonable

intermediate to suggest

between that

the

typical

continental

universal

continental

crust towards

the ocean is also a manifestation

progressive

“oceanization”

of the crust from the point of view geophysical

thinning

of the same process

and of of

character-

istics. In this connection it is interesting to note that the salt deposited during the lagoon stage is found not only on the shelf but also beyond the continental slope at oceanic depths where the crust has oceanic structure. For example, in the Gulf of Mexico, salt lies 8 km below sea level (3.5 km deep) and geophysical research (Martin and Case, 1975) suggests that the consolidated part of the crust is oceanic in character. In the region of the Canary Islands the salt-bearing Triassic is found under the water layer 4.5 km thick and on the crust of oceanic structure (Storevedt et al., 1978). Consequently. during the lagoon stage the shallows were much farther from the coast than they are now. There is little doubt that the shallow basin was composed of continental crust. This was later replaced by oceanic crust. thus. the

83

deep-water

depression

was partly

formed

by oceanization

of the retreating

edge of

the continent. Recently,

a discussion

started

occur along all transition and the belts of linear

magnetic

mid-ocean ridge). In the light of the spreading formed during occur.

about

anomalies

periods

of magnetic

type (between

which are confined

model it was initially

a period of geological

Different

the nature

zones of the Atlantic

supposed

time in which magnetic

of geological

time,

quiet zones which the transition

zones

to the slopes of the that these zones were

field inversions

did not

from the Late Carhoniferous

to the

middle of the Late Cretaceous, were indicated as such quiescent epochs. But if everything is determined by magnetic geochronology. then the magnetic quiet zone should however,

have

as its oceanward

is not universal,

boundary

for example

a definite

linear

in the Northern

anomaly.

Atlantic

This

rule,

off the coast

of

America, the last anomaly is the 31st whereas, at the Voring plateau off the opposite coast, the last is the 24th anomaly (Fig. 3). The last anomaly off the coast of Southern Australia is the 21st anomaly (Fig. 4). There are two opinions in the literature on this subject: either the crust beneath magnetic quiet zones was always “special”

(i.e. neither

continental

nor oceanic)

or, being

formerly

continental.

it

somehow acquired the geophysical features of the oceanic crust (e.g.. Rabinowitz, 1974; Talwani and Eldho~m, 1973; K&rig and Talwani. 1977). The theory of mobilism

and “piate

Fig. 3. Linear magnetic anomalies in North

tectonics”

Atlantic

represent the quiet magnetic zones; circles-continental

(after

usually

attributes

the transforma-

map by Pitman et al., 1974).

crust.

Hatched

areas

l/J _5

13 -i

-IL i

I

5---

4- -- l3- l-

.

5c

4 0

tion of the continental crust on the Atlantic margins to tension, which in the upper layers occurs along “listric” faults and in the deeper layers by plastic flow. Factual geological data, however. reveal not listric faults but almost vertical ones (see Fig. 1). The presence of these vertical faults does not comply with the idea of considerable tension in the crust. The faults are deep-seated; at least some of them penetrate to the upper mantle. which also rouses doubts about the existence of plastic flow in the crust at depth. The analogues structures gradual

should

with some of the inner not be discarded.

transition

basin of the Caspian

to “basaltic”

seas and

The thinning

crust

are observed,

even

geological

past,

inner

for instance.

Sea and in the Black Sea (Aksenovich

et al.. 1972). In the recent

with

of the continental

continental crust and its

in the southern

et al., 1962: Goncharov

there were massifs

of uplifted

land of

undoubted continental crust in place of these basins. An example of an inner continental structure is the Peri-Caspian depression (Zhuravlev, 1972; Nevolin. 1978). In the Riphean. there were continental conditions in place of this depression and, the crust was continental in character. Later. associated with the process of subsidence. more than 20 km of sediments accumulated. Beneath them the continental crust, 40 km thick on the outer contour of the depression. gradually thins to 30-26 km near the center of the depression and changes in composition. The rocks with “ basalt” seismic velocities become persistently dominant. In the most subsided part of the depression the consolidated

85

crust underlying “basalt”

the sediments

(granulite-basite?)

is only 6-9

material,

km thick and is composed

whereas the granite-gneiss

structure

should

be regarded

crustal warping

is beyond

doubt.

In this case, as in the previous

between

tension

in the crust to produce

continental

massifs,

because

“opening”

and its association

with

one, there is no need

of the basalt

the surrounding

of

layer is missing. This

kind of crustal to invoke

as secondary

entirely

structures

oceanic restrain

bottom this pro-

cess. It is more likely that the transformation to oceanic crust (as indicated by geophysical parameters) takes place with minimal strain, in situ. The possible mechanism of such transformation shall be discussed later on. TRANSITION

Transition

ZONES OF THE PACIFIC TYPE

zones

of the Pacific

type extend

along

the Mesozoic

and Cenozoic

mobile belts which experienced long polycyclic orthogeosynclinal development and which are now in the erogenic regime. Transition zones are superimposed on mobile belts,

and

this is the principal

difference

between

these zones

and

Atlantic type, which are superimposed on a platform. The complete “set” of modern structures of the Pacific transition deep-water

trench, an island arc, and a marginal

those

of the

zones includes

sea. This list is conditional,

a

because

not all island belts are arcs and the Andes, though belonging to the transition zone of this type, are not an island arc, but a mountain range without a marginal sea behind them. Island arcs are divided into two types: the first and the second (Beloussov and Rudich, 1961). Island arcs of the first type (Japan, the Philippines, New Guinea, etc.) experienced polycyclic orthogeosynclinal development and have the normal continental domination

crust. From the Neogene they passed to the erogenic of uplifts, block dislocations, seismicity. surface volcanism

basalt and andesite composition. Island arcs of the second type usually

have no exposed

basement

regime with of andesitewhich under-

went any polycyclic orthogeosynclinal development. Most of them are located on oceanic crust, though it is thicker than the crust of the open ocean. They are partly underlain

by the continental

crust as well, for instance,

the northern

and southern

ends of the Kuril arc. The post-Late Cretaceous history of arcs of the second type is better known than their earlier history (Tuezov, 1975; Sergeiev, 1976; Rodnikov, 1979; Suchev, 1979; and many others). It started with crustal subsidence and violent magmatism, which indicates the important role of faulting. At first tholeiitic magmas were dominant, but then they were supplemented by talc-alkali magmas. Since the Miocene or later, uplift and active surface andesite-basalt and andesite volcanism developed (Frolova et al., 1977, 1978). The arcs of the second type have no folding of general crumpling, instead all dislocations have a block character and are connected by vertical faults. The slopes of deep-water trenches are also step-like and only locally are there more complicated deformations formed by gravity. The arcs of

Sf!

87

the second type also lack manifestations of regional metamorphism and granitic anatexis. Marginal seas are located partly on continental but mostly on oceanic crust. From deep-water drilling data it is found that the seas have different ages (Initial Reports DSDP, 1970-1976; Falvey, 1971; Case, 19’75: Fox and Heezen, 1975; Initial Core Descriptions, 1979; Rodnikov, 1979; etc.). The oldest sea is the Caribbean where the Turonian sediments are deposited on the oceanic crust of its basins. The Tasmanian Sea appeared between the Cretaceous and Paleocene. The other seas have younger crust ranging in age from the Eocene to the Pliocene. As seen in Fig. 5, where there are several marginal seas between the continents and the open ocean. the younger seas are farther from the continent. There is a tendency to consider an island arc as a modern geosyncline, however, their present-day state is not that of a geosyncline, but that of the erogenic regime, analogous to its intracontinental manifestations. The preceding state, as already mentioned, was accomplished by island arcs of the first type during the polycyclic orthogeosynclinal development with all its manifestations, which was identical to the development of geosynclines within continents. The arcs of the second-type, however, lack the folding of general crumpling, regional metamorphism and granitisation, and do not show any effects of an orthogeosynclinal regime. We believe, the zones of similar development on continents are those which were classed as “parageosynclines with volcanism” by Beloussov (1978). In both places the faults were a very important factor in the history of the development of the structures and the latter are “near-fault” parageosynclines. The deep-water trenches are very young judging by the age and thinness of the sediments. The trenches were formed after the Oligocene (during the erogenic epoch) simultaneously with the rise of island arcs. They may regarded as foredeeps in relation to island arcs. The step-like structure of their slopes shows the same block tectonics typical of all island arcs in the erogenic stage. Marginal seas are peculiar formations. The time of their appearance does not coincide exactly with the development of island arcs, since most of the seas already existed before the island arcs of the second type rose up and acquired their present-day features. The other seas were formed either simultaneously with, or after, the rise of the arcs. The marginal seas which lie on continental crust, show a decrease in the thickness of the crust as the depth of the sea increases, this is similar to transition zones of the

Fig. 5. Scheme of structure

of the West Pacific transition

and island arcs of the first type; 2 = marginal type on the continental ridges

in marginal

7 = Jurassic: I-7 = Miocene:

seas;

crust:

4 = island

6 = trenches:

8 = Early Cretaceous; 13 = Pliocene:

zone (compiled

seas on the continental

arcs of the second 7- 14 = marginal

Y = Late Cretaceous

14 = Quaternary.

by V.V. Beloussov).

I = continents

crust; 3 = island arcs of the second

type on the oceanic

seas on the oceanic and Paleocene;

crust: crust

10 = Eocene;

5 = submarine of various

age:

1 I = Oligocene;

xx Atlantic type. The continental crust is wedged out and is substituted by oceanic crust at sea depths of 2 -3 km. Moreover, there is a lot of evidence which indicates that the history of marginal of crustal

seas. on both continental

subsidence.

seas on continental sediments

Where drilling

and oceanic

or dredging

crust. the surface of the basement

Yamamoto

uplift

New Zealand

in the Sea of Japan.

and

the basement

in the

is found to be eroded and the

begin with shallow water or even continental

the case on the Queensland,

crust, is on the whole one

has reached deposits.

Falkland

The succession

For example,

plateaus.

of sediments

and

this is on the

in the marginal

seas on the oceanic crust reveals gradual subsidence and the sediments on submarine uplifts in these seas in many cases show that these uplifts were initially above sea level. Such is the history of the submarine Kyushu Palau in the Philippine Sea. This case is therefore similar to the transition Lanes of the Atlantic type mentioned previously and we can assume that the thinning of continental crust and its transition to oceanic crust is historically associated with its subsidence. Geophysical data indicate that the crust continental into typically oceanic This evidence.

assumption

is also

An example

in this process slowly transforms through the intermediate crust.

supported

of such structural

by other features

structural

and

is the truncation

from

typically

paleogeographic of continental

structures of the island arcs of the first type by oceanic crust. A spectacular example is seen along the southern coast of the Hokkaido Island. The Miocene conglomerates on the Great

Kuril

They contain

pebbles

Islands

could

be mentioned

of gneisses and granites

among

the paleogeographic

and were apparently

brought

data. from the

South Okhotsk basin which is now underlain by oceanic crust (Sergeiev. 1976). The same kind of conglomerates are known on the Aleutian Islands where they provide evidence of an earlier proximity of land to continental source of coarse Paleogene deposits on the Kii Peninsula Philippine California Its average

crust ((‘oats, 1956). The in Japan is located in the

Sea (Tokuoka, 1967). The coarseness of Franconian deposits in Southern increases towards the ocean, where their source should have been located. mineral

composition

should

be close to granodiorites

judging

by the

composition of sandstones of this formation (Blake et al., 1974). Modern plate tectonics also consider marginal seas to be of secondary origin, but associate it with “diffuse spreading” which is accompanied by the formation of new oceanic crust (Karig, 197 I ). The possibility of this origin

of marginal

seas should

have

been

tested

by

reconstruction of the situation before spreading. Strangely enough, these tests were not made. But if marginal seas did not exist earlier, but appeared later due to spreading. then the material of all the island arcs of the Pacific Ocean should have been initially concentrated near the Asiatic and Australian continents. Moreover, the band of this material should be extremely tortuous in shape, following the rough produce the regular edge of the continental crust. How could “diffuse spreading” arcs and in some cases even the straight chains of islands from such a tortuous band? For example. the straight line of the archipelago of the Tonga and Kermadek

89

Islands

appeared

periods

of time in the Tasmanian,

process

be considered

was displaced,

island

as the result of the summary likely?

because

spreading,

arcs of the second

at different

South Fiji, North Fiji and Lau Seas. Could such a

Besides that, the problem

in most cases marginal type. Spreading

crust were still in existence

which occurred

arises as to what material

seas appeared

before

the uplift

should occur, when the depressions

zones

of the

Benioff zones are a specific feature in the structure

Pacific

of the

at the place of future island arcs. Should we believe then

that the spreading provoked the movement of such depressions? Everything points to the formation of marginal seas by an oceanization not by spreading. The seismofocal

of

type.

Plate

tectonics

associates

their

process,

of marginal

appearance

with

the

subduction of oceanic lithosphere. No doubt, the deep faults forming the Benioff zones are closely associated with deep-water trenches, or rather, with the zone of contrast at the conjunction of the latter with the rising island arc. Consequently, the time of their formation neotectonic especially existence

erogenic drilling

is apparently epoch.

the same as the age of that conjunction,

As regards

data on the structure

of subduction

processes

subduction,

in nature

(Huene.

et al., 1979; Muzylev, 1980; and others). As examples of such data we may mention on the bottom

of the deep-water

a lot of geophysical

of deepwater

trenches

trenches

evidence

1972; Murdmaa,

the undisturbed

which lie horizontally

i.e. the data and

against

the

1978; Vasiliev

nature

of sediments

against

inner slope

of the trench without the slightest deformation. This is evidence of tension, universally found on the slopes of the trench. No evidence of compression deformations is seen, except that easily attributed to gravitational sliding of loose sediments. Of special importance is the fact that in all known cases the inner slope of trenches is composed not of oceanic sediments “accreted” during the process of subduction, but by sediments whose source is the adjoining island arc. The transition along the strike of deep-water trenches into foredeeps on the continental crust is also an important factor.

Such a transition

is observed

in the Java trench

strike, i.e. in Burma and near the Timor the Tonga-Kermadek

trench

Island

off the coasts

in both directions

and it is also found of New

Zealand

along its

on the strike of

where

this trench

transforms into the Hikuranga depression on the continental crust. The Puerto Rico trench in Venezuela has a direct connection with the Orinoco continental foredeep. Plate tectonics duction of oceanic Ringwood (1968)

associates

the formation

of talc-alkali

magmatism

with the sub-

lithosphere. The pattern of this process, suggested by Green is too complicated and unreliable in many points. This

and was

repeatedly noted (Wyllie, 1973; Genschaft, 1977; Frolova et al., 1977; etc.). The most important circumstances, refuting the pattern of Green and Ringwood (1968), are the impossibility for water to penetrate to depths exceeding 100 km and the thermal regime. The assumption that the heating of the mantle is induced by friction on the surface of the sinking slab of lithosphere has no physical justification, on the contrary, the sinking cold lithosphere will cool the surrounding mantle (Artyushkov, 1979).

The light

problem of

mantle. are

data

c>n the

Tholeiitic

basaits

in v&tiles

poor

from

the origin

of

new

already

Lrtnihov.

and

the depleted (Sobole~

There

material

climinatit>n

c;ii~mi

by

the

during

the process

between

uneven.

basalt

with rising

from

Amctng other

numerous

rocks

pressure

25 khar

1975).

We

later

shall

The

whv

of

in this

the 5 km isobath

zone

the xhelf does not subside shelf

are described

deepens

very

km from

hut rises

(Kulm

The

structure

example,

this

origin

and Fowler. refers

platform. plateau

to the history

of the Transverse

of the C’oast Range though

Ridge.

almost

it is a typical but

1974:

of the continental

inside

are also found

rift,

and

these

nearrr

are

to the melts

of water at a and Boettcher.

faults

on

srismol‘ocal

above.

We

the

Pacific

Henioff

/~nrs.

Unlike

other

transltion

1 km

zone

of the

San Andrea

which

cuts across

Great

in :I strange zone).

that

the

hones.

foraminifera.

transition

structure. The

by

add

the continent:

the

the Late

since

et aI.. 1970).

of this

angles.

might

by the henthonic

and amplitude

folded

in the folded

arcs

of island

data. andcsite

deep

that

in the composition

(Mysrn

became shallower

part

it is located

a young

magma~

is generated

cf

the coast.

Dehtinger

L\ unique

at right

the

of alkali

WC heiicvc

as it recedes from

uloul\i,

thus. judging

off the coasta of the State of Oregon

Miocene

It is

of a large amounts

mhy

all

to the surface.

element

1200°C‘

At

to several ele-

layer along deep faults.

features

hounder\

km.

II’l’i

/one

lies about 2000

the

the laker

that the “plumes”

type. Water

it ih

rare-earth

be asat)ciated.

of about

the\1 have

this

and light

As hho\vn by experimental

and a temperature

hut

material

fhc 70)

ol

material

reach down

of talc-alkali

out 1971:

the initial

about

faults

(an tmportant

c,n gk\ 011 to discussion

features

oceanic bottom

source

the undepieted

%ONk 01: I HI-. (‘OI.IIMBIN

basic

&ep

melted

the densit,

is helou.

of

They

(tlart.

Therefore,

Iaver of the mantle

zone\ of the Pacific

and

depths.

of the mantle

hyIn>gen

were

paradoxical.

from

mantle

should

the

c>t’ the undcplrted

and radioactive

layer

thev

it seems

depth

of the manthz in the presence

;~rc inclined

‘I’K.ANS1l~toN

alkalis.

of its oxidation.

f‘JOm ultrabasic

margins

from

in the transition

of 15

4nct:

is the primary

rises

as the result

c3se.

the undepleted

product\.

of the deep interior)

hurface

any

that

at the indicated the

the

in

components

that

undcpleted

in

layers oct’tin~.

of 60 km or more.

first

aspect

of the seas and

Rich as garnet.

he at

another

undepleted

mobile

than :Zt

regiona of the oceans.

the same ulld~p~~t~~~ layer ;I> ~,ell.

In

the deep undepletzd in certain

iess

1979).

to

v&tiles.

ment\ can be cxtractud

is

of basalt:,

assumed

and

indicate5 and

that at depth5

above. whilst

is

of kilometers.

actually

light

of hea\\ mineral\

stavs

layers

~r~~b~~bil~t~ it is very hundreds

probahlv

.lordan.

of melting

mantle

these

which

of’ the mantle 1977:

acyutrcs

on the hnttom

lacking

is evidence

and Soholcc.

of depleted

aikaiis. mantlc

magma

of’ depleted

arc ~~(~nliil~rlt

depleted

1977).

of talc-alkali

di~tributi~~n

The

Basin place

flood

is mysterious. Fault.

the folded zone

is also a problem for

a rift

hasahs

zone and not on the platform.

of

For or the

{i.e.

not

in that on

the

the Columbian The

andesites

of

91

the Cascade Range presents

are also a problem.

The origin

of the Fransiscan

formation

many riddles as well.

We shall resume

this discussion

later on and try to provide

answers

to some of

these problems. CONDITIONS

FOR OCEANIZATION

As already inferred, the tranformation very important factor in the development Pacific types. Let us note history.

that

the problem

Arkhangelsky

described

(1941)

the oceanization

of the continental crust into oceanic of transition zones of the Atlantic

of oceanization

was one

mechanism

of continental

of the first in terms

crust

to suggest

of crustal

has a long

the idea.

basification.

and

when

heavy basic magma intrudes into the crust and outflows on its surface. Later, various points of view were suggested as regards the oceanization Besides Arkhangelsky’s

(1941) concept.

zation

seems interesting

in this process

Artjushkov, 1979; etc.). In his earlier possible: basification and eclogitization

the idea about the important (Packham

and

works. the author (Beloussov, 1968).

Fulvey,

he the

process.

role of eclogiti-

1971;

thought

is a and

Bott. 1976:

both

processes

There are data, however, which throw doubt on the mechanism of eclogitization. There is an obvious contradiction between the necessary rate of reaction and the thermodynamic conditions in the crust. The kinetics of eclogitization were studied by Ito and Kennedy (1971) and also by Sobolev (1978). Considering the age of marginat seas in The Western Pacific. the period of the reaction should necessarily be about lo’-10’ yrs. The results of the studies indicate that this rate can be reached if the temperature is not lower than 800°C. But at that temperature the reaction would require the pressure to be not less than 20 kbar, which is impossible in continental crust. The temperature of the reaction might be lowered to 500°C under the pressure

of 10 kbar, which can occur in the lower layers of the continental

but these conditions is known is evidence same

would make the reaction

that in the marginal of higher

contradiction

crust, figure. It

seas of the Pacific the heat flow is above normal.

temperatures is true

last 10” yrs.. an unacceptable

in the crust,

of the Mediterranean

which prevent

eclogitization.

Sea, to which

the concept

eclogitization has been lately appfied (Artyushkov et al., 1979). Consequently, basification is the favoured mechanism of oceanization.

This The of

Metamor-

phic processes are auxiliary; they extend the zone of granulite facies of metamorphism in the interior of the crust, thus increasing its density. The intrusion into the crust and the outflow on the surface of basic and ultrabasic mantle rocks are the most important processes in basification. We should emphasize here that data have become available recently which suggest that the roIe of ultrabasic intrusions in the structure of the continental crust is greater than previously assumed (Vasiliev, 1978). The basic and ultrabasic magmas penetrate into the

crust along vertical faults and then cover large areas in the form of sills. By this process, large blocks of the crust become isolated between the vertical and horizontal intrusions,

and hot magma completely

experience dense.

intensive

Obviously, melting. reduced under

metanlorphism

surrounds in which

them. These blocks are heated and they lose water

and

become

this process can take place only if the upper layers of the mantle

more are

The temperature there should rise to 1300”.-1400°C. and the density is accordingly. For example, it has been calculated for a similar situation the axis of the midocean

melted mantle

is 3.05 g/cm’

ridge,

that

the density

at a depth of 20 km (Bottinga

of the heated

and partly

and Steinmetz.

1979). It

is natural, therefore, that in places where basification occurred in recent geological times and is probably still active. as in the young marginal seas of the Pacific and in the Mediterranean Sea, the heat flows are higher and. according to seismic data. the density of the upper layers is less and the seismic velocities lower. But in the much older

marginal

sea, the Caribbean,

and also on the margins

of the Atlantic

where the process apparently ceased long ago and the mantle has cooled, heat flow and the seismic velocity at the top of the mantle are normal. Lower density

in the upper layers of the mantle,

and a higher density

type.

both the

of the crust

after consolidation of the mantle rocks which intruded into the crust and overflowed onto its surface, might cause density inversion at the boundary between the crust and mantle. This fact is crucial. There is no need for the average density of the whole crust to become greater than the density of the heated upper layers of the mantle immediately. It is sufficient that the density, let US say. 3.1 g/cm-‘. appears at the base of the crust in certain blocks isolated by magma intrusions. These blocks will break off and sink into the mantle and their place will be filled by rising mantle material. The whole process will move gradually upwards through calculations, made by Turcotte et al. (1977) confirm this possibility

the crust. The and show that

heated mantle can rise to the uppermost horizons of the continental crust (i.e. 3.25 km beneath the hydrogeoid level) provided there are open vertical channels (Turcotte et al.. 1977). It should be emphasized.

that in accordance

with our scheme

the new oceanic

crust does not appear due to the basification of the continental crust in situ, but as the result of its substitution by mantle melts. whilst the crustal blocks gradually break

off from the base of the crust and sink into the mantle because they get heavier. This process is accompanied by extraction of water. When the heavy blocks of the crust have sunk to more than 60 km in the mantle, which would expedite their further sinking. they might experience eclogitization. They would

“drop”

to the bottom

of the asthenosphere

and

form a heavy

layer

there. This concept finds support in some of the results of gravity measurements. Artemiev (1975) has shown that regional positive anomalies, typical of transition zones of the Pacific type. are caused by heavy massifs deposited at the depth of

93

several hundreds of kilometers. This heavy layer could have been formed by an accumulation of high density blocks of continental crust after basification. There are similar heavy massifs under the western part of the Mediterranean Sea and under the Aegean Sea. Certain circumstances are also favourable to the basification process. Among them are low viscosity. which allows basaltic magma to spread more easily between layers within the crust, and the previous uplift and regional erosion, which eliminated a considerable part of the most acid and light uppermost layers of the crust over a large area. The later condition exists on platforms and median massifs. Moreover. according to seismic data the platforms are unlike the erogenic zones in that a greater role is played by the dense granulite-basite layer in the crustal section. Therefore. the conditions in the crust of the platforms and median massifs are favourable for basification. The platforms and median massifs are especially prone to basification due to the fact that in these structural regions the deep faults have been healed. whereas the mantle is apparently depleted (i.e. devoid of volatile elements). As a result of this, the mantle can universally and regularly be heated over vast areas without explosions, avoiding the appearance of large inhomogeneities and the untimely loss of heat to the surface. The heat is preserved in the interior and is used in the transformation of the crust. CONDITIONS

IN THE DEEP INTERIOR

AND

DEVELOPMENT

OF TRANSITION

ZONES

Finally. we shall discuss how the suggestions made might be applied to transition zones of different types. In the history of the transition zones of the Atlantic type. rifting is considered to be the initial stage. As commonly known, rifting is connected with the heating of the upper mantle. Both the temperature and the heat flow were obviously higher in these zones at that initial stage. Afterwards cooling set in and the heat flow became normal. In this article we shall not analyze the entire history of the oceans, nor repeat the arguments of the author as regards the ideas of mobilism (Beloussov, 1968. 1975). Assuming that the continents did not move during the formation of the Atlantic. Indian and Arctic Oceans, we have to admit that rifting initially involved the entire area of these oceans. At that time. a lens of heated mantle existed under the whole area of each of the oceans. Rifting initiated the conditions favourable for penetration of the heated mantle material into the crust and consequently, for its basification. As evidenced by the geological situations in which these oceans developed, rifting and later basification, succeeded the regimes of the platform and median massifs. The lens of heated mantle material gradualiy cooled with time. Under the Atlantic Ocean the cooling began from the edges. Vobanic activity attenuated gradually from the margins towards the axis of the ocean and was substituted by

normal

sedimentation.

sediments

ridge is associated the crust

with

a rift.

basification,

continental

crust

whereas

The

believes

stage of their

within

the Mesozoic

framed

arcs of the first

zones.

long

strike.

with faults.

that

neotectonic Why

of their

zones

densitv

were

median

massifs massifs

The

median

which

massifs

were

i.c. on all the

type.

developed

Most

of the

as “nearfault

where

geosynclinea

they attenuate

and the large thickness resisted

extent

of talc-alkali

We

ha\z

magmas

their the

mentioned

on Island

mantle

occurred

interior

along

and experienced

of the undepleted Inflow,

As

often

of the crust.

basification

at the edges.

arcs

along deep

at the last. erogenic

was universally

lf

and on

Benioff

contrast.

high at this

velocity should

flow

tith

over

arcs

coincide

On the side under

lower

density

the denser

tilt

is secondary

on either

This

of

side of the

is indicated

the first

with

with is

upper

such

in density.

its

mantle.

This

the

contrast

and beyond).

of lower

material

then the denser

and. accordingly.

during

sea the heat flow

trench

mantle

that

typt:

by the

the entire

a temperature

the marginal

the deep-water

is connected

caused by the difference

the mantle

vertical.

the island

of the mantle

that this

mantlc

arcs of the second type during

always

side (beneath

heat flow

the viscosity

may suggest

of the upper

were initially

zones

that higher

We

densities

on the island

than it is on the other

under

in regions

inclined?

The

in a heat flow

flow

they

seas.

seas at an

above. the median

intracnntinental

of the Earth’s

development.

reflected

sufficient

in marginal

arcs of the second tvpe were formed.

T-lie most powerful

these faults

stage.

vice versa.

island

the deep layer

of dislocations

erogenic

obvious,

the

same basification.

orthogeosynclinea.

z.ones.

the appearance

of the different

In all priobability.

block character

higher

The

of the arcs of the second

are fault

to some

because the activity

are the Benioff

period

of

stage.

and is the result fault.

com-

remains

in the marginal

baaification.

of deep faults

from

zones.

the

is manifested

indicated

(1961 ). the

zones

we associate of matter

i.e. the Benioff

stage. possibly

for

of deep material

bv collapsing

the inflow

As

represented

of deep faults

and the

partiallv

previously

partly

Puscharovsky

the zones

of the zones. tvpe.

representation

favourable

where the normal

orthogeosynclines it only

tectonic

however.

to the inflow

of

to tht:

underwent

active.

was present

geosyncline.

which

type.

ago by

into

Due

crust

type and in some parts

parageosynclines”. transform

In

-C‘enozoic

of the second

shown

fragmentation

of the Atlantic

are structures

by mobile

still

corresponds

basins

z.ones of the P~~crfic~!,‘pe is much more complicated

zones

historv.

margins

oceanic

(11 hasification

is possiblv

that continental

earlier

and platforms

process

The

under

state of the whole oce;rn. The

on the Atlantic

ridges

is caused bv considerable

author

arcs

the

and the

where the mid-ocean

and a lens of heated material

of the earlier

basification.

in the transition

as that in the marginal

vSolcanism

crust

in mid-ocean

situation

and this

Recent

and unaccomplished

oceanic crust

thev arc‘ to ocean’s axis.

ridge is a relict

of the continental

interrupted pletelv

the age of the consolidated

the nearer

of the mid-ocean

vvedging out

The

Therefore.

on it are younger

density.

can flow mantle

the mantle

is the reason

why

with

the tilt

is

It is and with

should lower of tht:

95

Benioff zone is always inclined

towards warmer

This mechanism supplies an

mantle.

for the reversed tilt of the Benioff zone near the Solomon Islands and the New Hebrides: the mantle of the younger North Fiji basin north of these islands was formed in the Miocene and is therefore still hot, and less dense, than the mantle explanation

south

of the Coral Sea formed

in the Eocene.

Gainanov

suggested the same as the cause of the inclined Some inclination

twenty

years

of draining

ago

Lustikh

channels

(1961)

for mantle

position wrote

(1978) and Sychev (1979) of the Benioff zone.

about

differentiation.

the

importance

of the

The tilted position

of the

channels was actually responsible for differentiation in the underlying rocks. In this case this means the process of leaching of rocks immediately below the Benioff zone by hydrogen and other volatiles rising along the zone (Luts, 1980). This also accounts for the presence of the layer with high seismic velocities and high Q coefficients under the Benioff zone. Though subduction of oceanic lithosphere does not occur, the flow of denser mantle under the mantle with lower density within a certain zone does cause some subsidence of the denser mantle and some uplift of the mantle with lower density. These relative displacements cause the seismicity associated with the Benioff zone and also induce the formation of a deep-water trench at the place of contact of differently

heated

tectonospheres.

The

almost

complete

absence

of the inclined

Benioff zones on continents can be attributed to the high viscosity of the mantle under the platforms, which prevents the flow of this mantle under the mantle of the neighbouring folded zone. But in places where the mantle is hotter, the inclined zones also appear between

on the continents.

the Pamirs

Shan was sufficiently

For example,

and Tien

Shan appeared

warmed

and its viscosity

the tilted deep seismogenic

because

the mantle

under

fault

the Tien

was reduced.

The island arcs of the second type have neither regional metamorphism, nor granite anatexis. There is no folding but there are block dislocations. These peculiar features are apparently the reason for the great permeability of the crust concentrated along deep faults. In consequence, deep flows of volatiles, alkalis and magma easily pass through surface.

the crust and considerable

In the course of this process,

cal interaction

with the surrounding

these substances

quantities

of them burst to the

have no deep physico-chemi-

rocks of the crust. But the close physico-chemi-

cal interaction of deep products with the crust is vitally important for regional metamorphism, granitization, and the formation of deep diapirs, which serve as the basis for folding of general crumpling. There is a similarity

in this respect between

the arcs of the second

type and the

middle part of the Andes. During the entire Mesozoic-Cenozoic cycle of development, the Peruvian-Chilean Andes were an area of powerful talc-alkali magmatism without a trace of regional metamorphism, granitization and folding of general crumpling. There are only block dislocations in this part of the Andes (Lomize, 1975; Puscharovsky et al., 1975). Greater penetrability along deep faults was actually the cause of a specific evolution of this zone, whereas, the relation of the middle part

97

of the Andes to the orthogeosynclines in the northern and southern parts of the same ridge is similar to the relation between the island arcs of the second type and the orthogeosynclinal folded zones of the arcs of the first type. Figure 6 shows a scheme representing the author’s view of the deep conditions in transition zones of the Pacific type during their development. The conditions for the evolution of the transition zone of the Columbian type are very peculiar. They are connected with reciprocal superposition of orthogeosynclinal and erogenic processes on the one hand, and with rifting on the other. In the Mesozoic and the beginning of the Cenozoic, the Cordilleras of North America developed as an orthogeosyncline. In the Neogene, the geosynclinal regime changed to the erogenic, but at the same time the rift regime appeared. Rifting is connected with the midocean ridge, one part of which stretches near the shore and another part under the continent. The East Pacific Rise “enters” the Gulf of California and its continuation, according to earthquake epicenters, is traced on land under the San Andreas Fault, the Great Basin, and the Columbian Plateau. North of the Mendotine Cape, the mid-ocean ridge is again on the ocean bottom. It is divided into the Gorda, Juan de Fuka and Explorer submarine ridges, and these are separated by transverse fractures. Orthogeosynclinal and erogenic development is associated with heating of undepleted mantle. In the first case this is indicated by the processes of regional metamorphism and granite anatexis, and in the second, by talc-alkali magmas. The undepleted mantle apparently lies at a shallow depth under the orthogeosyncline and the erogenic zone. Contrary to this, the depleted mantle is favourable for rifting and basification. Consequently, rifting and basification can lead to orthogeosynclinal and erogenic development in places where mantIe becomes depleted. If in a certain zone the mantle is in some parts depleted, and in others is still essentially undepleted, then heating of the inhomogeneous mantle may result in orthogeosynclinal and erogenic regimes, and rifting and basification occurring concurrently. This is what is observed in the transition zone of the Columbian type. Rifting and basification penetrated locally into the Cordilieras erogenic zone in various forms which depend on the degree of rifting or basification “accomplished”. For example, in the Great Basin they caused tension and the formation of horsts and grabens, and on the Columbian Plateau, the effusion of flood basalts. We may therefore attribute these manifestations to definite degrees of exhaustion of the mantle material, A wide swell-like uplift of the crust appeared in connection with the mid-ocean

Fig. 6. Hypothetical the Philippine 2 = oceanic

scheme of the evolution

Islands

crust;

and

Ma&ma

3 = asthenosphere;

therm and uplift of the anomalously granitization; basification.

9 = andesites,

of deep environments

arc region 4 = depleted heated mantie;

andesite-basalts;

(compiled mantle;

in the West Pacific transition

by V.V. Beloussov). 5 = undepleted

mantle;

7= tholeiite basal&; 8 5 regional

IO = faults:

I1 = Benioff

zone in

I = continental

crust;

6 = continental

iso-

metamorphism

and

zone; I2 = zone of leaching

and

ridge. The uplift had an effect on the distribution North America: instead of a deep-water trench seafloor

towards

the ocean.

of depths along the Pacific coast of there is a gradual deepening of the

The same swell-like

uplift

affected

the shelf where. as

noted earlier. there are signs of shallowing. The absence

of the Benioff

zones

is explained

by the absence

of temperature

contrasts.

The heat flows are high and about

including Therefore.

the Cordilleras on land and the adjoining part of the ocean bottom. there are no conditions that would cause the appearance of mantle blocks

with different the creation

density

and. ah II consequence.

the same in the entire

there are no conditions

transition

zone.

fa\,ourable

to

of the Benioff zone.

The author

finds sufficient

evidence

to believe that the processes

of oceanization

play a leading role in the development of the three types of transition zones. In all cases the ocean spreads at the expense of land. ln the transition zones of the Atlantic type. rifting

is the first stage of oceanization.

and in all transition

zones the process

of oceanization involves the thinning of continental crust and the intrusion of heavy basic and ultrabasic magmas in the crust. The heating and metamorphism of crustal rocks are essential and cause an increase of density. All these processes are related to the intensive heating and melting of the upper mantle. After the mantle intrusions in the crust solidify, a density inversion occurs between the heavier crust and the partly melted upper mantle. This inversion causes the blocks of the crust to founder into the mantle, and the new crust of the oceanic type appears in its place. Transition zones of the Atlantic type are formed entirely in the environment the platform of heated

or the median

material

leads to the beginning

under

massifs. The process the region

of rifting.

starts with the formation

of the future

ocean

Later. the stage of continental

in. The lens cools from the periphery

and the present-day

classed

which initially

as relics of endogenous

the ocean. Transition

zones

of the Atlantic

activity

of the Pacific

type develop

type and

crust basification median

embraced

of

of a lens sets

ridges can be

the whole area ot

in a rnore complicated

situation.

They are superimposed not on the platform. but on a mobile zone of complicated structure and a geosynclinal history. Basification here involves median massifs. In their place there appear marginal seas divided from each other by orthogeosynclinal or specific near-fault parageosynclinal zones. In the Neogene both kinds of these zones entered the stage of an erogenic regime. lsland arcs of the first type appeared on the basis of orthogeosynclines, whereas island arcs of the second type were formed in place of near-fault parageosynclines. The talc-alkali magmatism typical of the erogenic regime of arcs. is an indicator of the connection between the surface and the layer of undepleted mantle. This connection is made through faults. including the Benioff zones. The tilt of the Benioff zones is secondary. It is

Y9

associated

with the contrast

side of the fault. Cooler therefore

mantle

and density

flows under

always tilted towards

of the mantle nying

in temperature warmer

heated

of mantle

mantle

blocks on either

and the Benioff

and less dense mantle.

Relative

blocks create the seismic regime of these zones, whereas

denser layer is subjected

the fault. The existence

to leaching

of the transition

by fluids (mainly

zone of the Columbian

hydrogen)

zone is

dislocations the accomparising along

type is proposed

for the

first time. This zone has developed along the Pacific margin of North America. A peculiar superposition of rifting and orogeny has occurred. The forms of this superposition

reflect the inhomogeneity

ing both undepleted In earlier excitation

and depleted

works,

the author

of the upper mantle

of the mantle

repeatedly

emphasized

and the penetrability

endogenous regimes. The understanding revised in the light of this research. promotes properties

composed

of blocks contain-

material the importance

of thermal

of the crust in the formation

of

of the types and the role of penetrability is Diffuse penetrability close to the surface.

the interaction of deep products with enclosing rocks and determines the of orthogeosynclinal development. Concentration along faults and open

penetrability in the crust almost eliminates the interaction of deep products with the enclosing rocks and determines parageosynclinal and erogenic evolution. This study reveals a principal difference between rifting on the one hand. and geosynclinal and erogenic regimes on the other. These regimes reflect different tendencies in crustal development. Rifting is associated with the basification of the crust but the geosynclinal and erogenic regimes are produced by the process of enrichment

of the crust

distribution revealed. tion.

by sialic material

of undepleted

and

The geochemical

depleted

aspects

of deep origin. layers

of endogenous

The importance

and blocks

in the mantle

of the is also

regimes deserve special investiga-

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