Subduction zones: An introduction to comparative subductology

133

T~~fo~op~ysies, 8 1 (1982) 133- 159 Elsevier Scientific

~blishing

Company,

SUBDUCTION ZONES SUBDUCTOLOGY **

SEIYA

Amsterdam-Punted

in The Netherlands

AN INTRODUCTION TO COMPARATIVE

UYEDA

Depurtment oj Geophysics, Texus A&M tJnioersi
July 9, 1981)

ABSTRACT

Uyeda,

S., 1982. Subduction

Geodynamics

zones: an introduction

Final symposium.

The subduction model consequence earthquakes model.

of sea-floor

But, other

features

are difficult

of the two basically

Many

such as extensional

different

Implications DSDP

active margin

large-scale subduction,

thermal

drilling.

at trenches

In discussing

and stress regimes

but of some additional

with the motion accretion

one Chilean-type

the other

-

Mariana-type

of major

and erosion

factors

plates.

Finally,

at subduction

the possible involving

or less as a logical

as thrust-type

explained

-

causing

causing

problems,

with special reference

great

energy,

importance

stress

stress regime. such as vertical

to the results of recent

causes of the two modes, it is suggested

much greater

and arc

the existence

a compressional

a tensional

arc areas may not be the direct

the possible

inter-plate

by the subduction

may be to recognize

for some tectonic

are discussed,

in the back

more

and high heat flow in back arc regions

to solve these problems

modes of subduction:

accretion

In: A.L. Hales (Editor),

such

zones, are readily

of the two types of the mode of subduction and sediment

was deduced

in these systems,

spreading

An approach

regime in the arc and back arc regions; movement

arc systems features

along Wadati-Benioff

to explain.

subductology.

8 1: 133- 159.

for trench-arc-back spreading.

and deep earthquakes

volcanism

to comparative

Tectonophysics,

products

such as mantle of various

that the of simple

flow associated

types of collision,

zones is emphasized.

It has long been suspected that various tectonic phenomena associated with the trench- arc- back arc systems have a common basic cause, namely the down-thrusting of mantle convection currents (Fisher, 188 I ; Holmes, 1929; Griggs, 1939; and many others). Later, with the advent of plate tectonics, this idea became more definite and the process is now called subduction. The concept of subduction in the framework of plate tectonics was introduced more or less as a logical consequence of sea-floor spreading to keep the surface area * On leave from the Earthquake ** Texas A&M Geodynamics

OectO-1951/82/oooO-oooO/$O2.75

Research Research

Institute,

Program

University

Cont~bution

@I982 Elsevier Scientific

of Tokyo,

Japan

No. 23.

Publishing

Company

I 13.

134

of the earth constant. Although deep sea trenches are obviously the most plausible candidates for the sites of plate consumption, whether or not the subduction model can explain

the various

arc systems important

is not a priori

problems

margins

geological

and

present

and geophysical

obvious.

In the present

related to the tectonics a possible

features

of the trench-arc-back

paper

I review

of trench-arc-back

approach

to solve

some of the

arc systems or active

these

problems.

It will

be

suggested that both of the major back arc features, namely back arc extension and high heat flow, and the major mountain belts may not be the simple results of subduction itself. In addition to subduction, the former may require sea-floor spreading require

in the back arc induced

by the motions

the process of collision/accretion

GENERAL

FEATURES

OF SUBDUCTION

of major plates and the latter may

of buoyant

features.

PLATE BOUNDARIES

In order to review the general characteristics of the trench-arc-back arc systems, an example is taken from the east-west cross section at 40’N of northeast Honshu, Japan in Fig. 1. As seen in Fig. lB, the crustal structure of the Sea of Japan, a back arc basin, is oceanic. It is now a widely held view that the Sea of Japan was formed by the seaward migration of the Japanese islands relative to the Asiatic continent during some relatively young but disputed geological period. Another notable feature in Fig. IB is the ~om~ously low Pn velocity (7.5 km/set) of the uppermost mantle under the main arc of Japan. It is suspected that the asthenosphere or the low-velocity layer is raised under the Japanese arc almost up to the Mohodiscontinuity. This is in harmony with yet another equally remarkable observation shown in Fig. lE, that mantle earthquakes are distinctly absent in the uppermost mantle wedge under the arc. Both facts appear to indicate anomalously high temperature in the uppermost mantle. The boundary between the outer seismogenic mantle and the aseismic mantle in Fig. 1E is called the aseismic front (Yoshii, 1975). The

aseismic

front

is located

slightly

more

oceanward

than

the volcanic

front

(Fig. 2). Heat flow distribution shown in Fig. 1D (Uyeda and Horai, 1964; Yasui et al., 1968) is also in harmony with the above observations because heat flow is low on the Pacific side and high on the continent

side of the Japanese

arc and in the back

arc basin. As can be seen in the figure, the exact position of the transition between the low and high heat flow zones can not be defined by the presently available data with sufficient accuracy to be compared with the locations of the volcanic and aseismic fronts. If the formation of such a back arc basin as the Sea of Japan is due to sea-floor spreading as assumed in this paper, the heat flow in the Japan Basin suggests that its age would be 30-25 m.y. (Anderson, 1980). Altough not clearly shown in Fig. lE, it has been shown that the lithosphere under the Sea of Japan is as thin as 30 km (Abe and Ranamori, 1970). This indicates that the upper mantle under the back arc basin represents

a really large-scale

thermal

anomaly.

135

A

*n ._

1

......

20

B CRUSTAL

STRUCTURE

t.1

C GRAVITY

-lOO-

ANOMALY

D

E

Fig. 1. A cross-section

of northeast

for sea and land areas, respectively; waves (Adapted

from Yoshii,

Japan

at 40’N.

In Fig. IC, Free-air

and Bouguer

in Fig. IE, V and Q mean, the velocity

1979a; Utsu,

1971; Hasegawa

anomalies

and quality

are shown

factor of seismic

et al., 1978).

The shallow earthquakes in the uppermost right comer of the wedge in Fig. IE are mainly of thrust-type and include almost all the great earthquakes. These earthquakes are interpreted as caused by the relative motion of the overlying plate and the subducting slab of the oceanic lithospherewhich has been delineated by the

i

.b

Fig. 2. Positions epicenters

Meteorological and

of Quatemary Agency

the “aseismic

triangles

volcanoes

and earthquakes

with depth

of all the events of which focal depths were determined during period

front”

are presently

respectively,

1%7- 1977. Dotted (Yoshii,

1979b).

\

of 40-60

to be in 40-60

km. Black dots show km range by the Japan

and dashed curves indicate Open

symbols

the “volcanic

are Quatemary

volcanoes

front” and

active ones.

anomalously high V and high Q tongue (Utsu, 1971) as shown in Fig. 1E. The source mechanism of the shallow events right under the arc and its western extension, including the central part of the Sea of Japan (IL Shimazaki, personal communication), is known to be ch~acte~stic~y thrust type, t~tif~ng that the crust of the arc

137

and the back arc of Japanese subduction zone is now under a horizontally compressional tectonic stress. Very shallow earthquakes oceanward of the trench axis (not shown in Fig. 1E) are of normal fault type and are commonly interpreted as caused by the downward bending of the oceanic plate before subducting. This agrees with the observations of numerous normal faults on the oceanward wall of the trench (Ludwig et al., 1973; Hilde and Sharman, 1978; Honza et al., 1978). Some of the normal fault type earthquakes, such as Sanriku earthquake, 1933, are of great magnitude and attributed to the tensile fracture penetrating through the whole thickness of the subducting slab (Kanamori, 1971). The origin of the tensile stress for these great normal fault type events is suspected to be the gravitational pull of the long-extended tongue of the slab. One of the important recent discoveries concerning the Wadati-Benioff zone under Japan (Wadati, 1935) is its double-layered structure as shown in Fig. 1E (Hasegawa et al., 1978). The source mechanism in the upper layer is characteristically down-dip compression and that in the lower layer is down-dip tension; unbending of the down-bent slab (Engdahl and Scholz, 1977), sagging of the plate (Yoshii, 1979a) and thermal stress within it (Goto and Hamaguchi, 1978) are the examples of the suggested causes of the double-layered Wadati-Benioff zone. Such a double-layered Wadati-Benioff zone has been found for some other arcs also: e.g. Central Japan (Tsumura, 1973), Kurile Arc (Veith, 1977) and Mariana Arc (Samowitz and Forsyth, 1981). Now, the question is, “can we explain all these features by the subduction of a cold slab of oceanic lithosphere ?’ Apparently it is not an easy matter (Uyeda, 1977). There are two main difficulties, which undoubtedly are closely inter-related; namely how to explain the extensional tectonics behind convergent plate boundaries and how to explain the anomalously hot upper mantle under the arc and back arc regions where a cold slab is subducted. By intuition alone, both appear paradoxical. In order to overcome these difficulties, various thermo-mechanical models have been proposed, calling for such mechanisms as frictional heating followed by a diapiric rise of an enormous amount of magma from the shear zone between the slab and the mantle wedge (e.g. Hasebe et al., 1970; Oxburgh and Turcotte, 1970; Karig, 1971a) or a secondary convection cell induced in the m,antle wedge (e.g. Holmes, 1965; McKenzie, 1969; Sleep and Toksoz, 1973; Hsui and Toksoz, 1979). It may be important to note, however, that the amount of energy to produce the enormous thermal anomaly under young back arc basins is orders of magnitude greater that the potential energy available from subduction of high density slab (Artyushkov, 1981) ruling out, at least, the models in which the latter energy is assumed to be the main source of the thermal anomaly. It may, however, be neither necessary nor appropriate to explain all the features by a single model. Although the Japanese arc is probably one of the typical subduction zones, all the subduction zones are by no means the same. Moreover, the features also change with time. For instance, arcs such as Mariana and Tonga have

138

the actively Peru-Chile hand,

spreading

had actively

subduction situation

Mariana

Trough

arc has no back arc basin. spreading

is an important

Lau

Basin

back arc basins

behind

them,

and Kuriles,

SUBDUCTOLOGY”

while

the

on the other

in the past but not now. Obviously,

factor but not the only agent that engineers

in the trench-arc-back

“COMPARATIVE

and

The arcs of Japan

the complex

arc systems.

AND TWO MODES OF SUBDUCTION

Uyeda and Kanamori (1979) classified the subduction zones as shown in Table 1. The criteria for this classification were related to the nature of the back arc regions. Namely,

the arcs were first grouped

into continental

and island arcs. The former, by

definition, have no back arc basins. The island arcs, then, were grouped into those having inactive back arc basins and active back arc basins, each of which were further divided into subgroups according to the possible origin of the back arc basin formation. Among these groups, the continental arcs and island arcs with actively spreading back arc basins are considered to be the two end-members and the other groups are interpreted as intermediate between, or as hybrids of these end-members. The former end-member was called the Chilean-type and the latter the Mariana-type. Japan arc, for instance, is now a Chilean-type subduction zone, because the Sea of Japan is not actively spreading, but it was most probably a Mariana-type when the Sea of Japan was being formed. Uyeda

and Kanamori

(1979) demonstrated

that:

(1) Present-day stress in the back arc areas, deduced from source mechanisms of intraplate earthquakes within the landward plates, is indeed compressional for the Chilean-type arcs and tensional for the Mariana-type arcs. (2) Although

interplate

thrust-type

earthquakes

occur at every subduction

plate

boundary, truly great earthquakes with the new moment-based magnitude (Kanamori, 1977a) significantly greater than 8 occur exclusively in the Chilean-type arcs as can be clearly seen in Fig. 3. These observations

led these authors

to conclude

that the two types of subduction

zones represent two basically different modes of subduction process. The term mode, here, represents the strength of mechanical coupling between the subducting slab and the upper landward plate. In the Chilean-type mode, two plates are closely coupled whereas in the Mariana-type they are virtually decoupled. A number of trench-arc-back arc features, which vary from one arc to another, appear to be explained in terms of the difference in the mode of subduction defined above as indicated in Fig. 4 (see Uyeda and Kanamori, 1979; Uyeda, 1979; 1981). Although it is not intended to re-iterate on these features in this paper, some salient ones will be explained in the following in the light of recent information, notably that from the active margin drilling of IPOD (von Huene and Uyeda, 1981).

Back-arc

inactive

Back-arc

inactivated

spreading

Back-arc

Leaky transform

trapped

Back-arc

Tensional

Tensional

(or neutral)

with shear

Compressive

(or neutral)

Island arc

(with back-arc

basin)

Compressive

back arc basins)

(or neutral)

arc (without

Compressive

Continental

I

AK

Stress regime

of arcs

Classification

Classification

TABLE I

examples

Parece

Andaman

Lau Basin Sea

Trough Scotia Sea,

Mariana

Bering Sea

Vela Basins

Shikoku,

Kuril, Japan

Alaska

Peru-Chile

Typical

30

zones (Kanamori,

1978).

earthquakes

M, ? 8.0

1904 - 1976

Fig. 3. Great

a-

-4o-

0

90

I20

1904 to lY76. Conventional

rokochi-Qkl,1968 17.9i

Peru,1966 17.5)

Pius Alaska, I?58 :7.91

from

6Q

‘,

magnitudes

-120

-40

and new magnitudes

Azores, 1975 i7.91

arc in parentheses

Peru, 1970 17.8) Peru, 1974 (7.6)

Kuriie,1969 (7.8.

:

I*

‘---1946(8.‘?)

150

I.-,

are in brackets.

I1 __

-60

-40

Black areas are rupture

0

f

142

CASE HISTORY FOR THE NORTHEAST JAPAN ARC

The depth of trenches are systematically greater for the Mariana-type subduction zones than the Chilean-type ones. Therefore, if there is a change in the mode of subduction at a trench over a period of time, there will also be a corresponding change in its depth. This possibility may be tested for a trench where a change in the mode in the past is suspected. Figure5 shows the time change of the water depth of the Sanriku Deep Marine Terrace as deduced from benthic foraminifera obtained from Site 438/439 of DSDP Leg 57 (von Huene et al., 1980; Keller, 1981). This figure indicates that the site (see also Fig. 6) has subsided from sea-level to about 3000m depth since the early Miocene. This agrees with the view that the subduction at northeast Honshu started in early Neogene or late Paleogene (Uyeda and Miyashiro, 1974), whatever the physical cause of the subsidence to.forrn a trench may be (Langseth et al., 1981). An interesting point to be noted in Fig. 5 for our present discussion is the change in the trend from subsidence to uplift in the Pliocene. This change, possibly starting in Upper Miocene, may be interpreted as related to the corresponding change in the mode of subduction from Mariana-type to Chilean-type. Altough there are still some problems with regard to the exact timing of events, the above interpretation of the change in subduction mode will be substantiated in the following discussion. Stronger coupling between the two plates is expected to work favorably for scraping off the trench sediments or even oceanic crust on subduction and weaker coupling may allow easier subduction of sediments down into the mantle. Therefore, if there is an accretionary prism of oceanic sediments and crustal fragments at all, the Chilean-type margins are the more likely place to find it. Of course, the sedimentary structures in the fore arc depends much on the amount of sediment supply. The fore arc of Sumatra, for instance, receives an enormous amount of material from the Bengal Fan, and being Chilean-type, develops impressive fore arc structures (Karig et al., 1979). In spite of the name “Chilean” type, the accretionary prism does not seem to be particularly well developed at much of the Peru-Chile Trench, probably because of small supply of sediments from inland (Kulm et al., 1977), although a well developed accretion complex seems to exist in northern Peru and the southern Chile Trench where a large supply of sediment is available. The lack of an accretionary prism in the Andean margin may be related to the possible occurrence of tectonic erosion. Among various forms of tectonic erosion, abrasive erosion of the landward plate may be more likely to take place when the coupling of the plates are stronger. Hence, accretion on the slope and abrasion from below may be competing processes at typically Chilean-type subduction zones. Interplay of these competing processes may lead to an extremely complex situation in a long term development of the Chilean-type margin (Hussong et al.. 1976; Kulm et al., 1977). Moreover, the horst and graben structure of the subducting plate, that seems better developed in Ma~ana-tie zones, will also help subduction of sediments (Hilde and Sharman, 1978).

143

\ Sedimenr

\ \ \ Bosement

\ \

Fig. 5. A diagram o$ sediment thickness above the unconformity and depth of depositional environments for site 438 and 439 (von Huene et al., 1980).

Active margin drilling of IPOD has provided much new information relevant to the problem of sediment accretion also. In the landward wall of the Mariana Trench, for instance, no sign of accretion of ocean derived materials was found (Hussong, Uyeda et al., 1978). Most interesting is the finding that the thick sedimentary wedge

Fig. 6. Japan Trench drill site locations for Legs 56 and 57 of the Deep Sea Drilling Project (von Huene, Nasu et al., 1978).

144

of the landward wall of the Japan Trench is composed exclusively of terrigenous and hem&pelagic materials at least to the depth of hole penetration and the room assigned for possible accretion is curiously small (von Huene, Nasu et al., 1978) as illustrated in Fig. 6. To the present author, these results appear to be explained in the same way as the subsidence-uplift history mentioned above: the mode of subduction at the Japan Trench was Mariana-type during most of the Miocene period and it became Chilean-type only recently (probably only several m.y. ago), so that the effective time for possible sediment accretion has been very short. This supposition that the mode of subduction changed in the Japan Trench several m.y. ago fits quite well with the change in the tectonic stress of the Japanese area derived from independent approach as shown in Figs. 7A and 7B (Nakamura and Uyeda, 1980). These figures indicate the trajectories of the maximum horizontal stress, unmax, in northeast Japan for the present-day and for the period 21-7 m.y.B.P.: the directions of uHmaxhave been deduced from various lines of evidence including the directions of dikes, faults, folds, alignment of volcanoes and stress axes of earthquake source mechanisms. The regional tectonic stress today (Fig. 7A) is compressional in the direction parallel to that of the plate convergence as expected for the Chilean-type subduction. The tectonic stress for the 21-7 m.y. period (Fig. 7B) was clearly tensional in the back arc region of northeast Japan, suggesting that the mode of subduction at that time was Marina-type. It is, therefore, conceivable that at least part of the Sea of Japan was actively spreading during the above period. Vertical crustal movements during the late Cenozoic time in Japan have also been investigated. Matsuda et al., (1967) showed that since early Miocene, the western zone of northeast Japan has subsided several thousand meters, as shown in Fig. 8A. This zone of subsidence is characterized by an intensive volcanism known as the “Green-tuff volcanism”, which was essentially a submarine volcanic episode under presumably an extensional stress regime. On the other hand, the vertical movement in Japan during the Quaternary is characterized by an upheaval (Res. Group for Quat. Tect. Map, 1968) ,as shown in Fig. SB. Recently, Sugi et al. (1982) have estimated the vertical movements of northeast Japan since ca 17 m.y.B.P. in more detail. Their results as summarized in Fig. 9 clearly indicate that a rapid subsidence during the 17-10 m.y. period was followed by a decline of subsidence and then, an accelerated Quaternary uplift. The general feature is remarkably similar to that of Fig. 5 and the change in the stress regime (Fig. 7), although the exact timing is still problematic. Putting these lines of information together, the Neogene tectonics of northeast Japan may be described by a dominantly Mariana-type subduction which was changed to Chilean-type several million years ago. __.._~_...._ Fig. 8.A. Vertical

displacement

B Vertical displacement km.

of Japan

-__- _-

since eariy Miocene (Matsuda

of Japan during Quaternary

(Research

Group

et al., 1967). Unit in km. for Quat. Tect. Map, 196X). Unit in

r i Fig. ?.A. oHmax trajectory

map for the present-day

angle thrust events. Dots: active volcanoes.

northeast

Japa~~. Arrows

Bars: dikes. Trajectory

to be u, than where it is judged to be u2 (Nakamura and Uyeda, 1980). B. 0 t+,_ trajectory map for a part of northeast Japan for Miocene (ca. 21-7 is the southern

portion

of the area shown in Fig. 7A. Dotted

lines show that it is not certain

if aHmax is ut or 9.

indicate

lines are darker

trajectories

slip vectors

far low

where uHma* is judged

m.y.B.P.).

The area covered

show that cHmax is a2 and parallel

Short bars: dikes. {Nakamura

and Uyeda,

1980),

146

WESTERN

COASTS

DEWA

HILLS

6U

BACKBONE

INTRA

m

-MOUNTAIN BASINS

loo0

RANGE

KITAKAMI

MOUNTAINS

_.,,,_p’;;:, Ma

Ma

i

__,, ___-•----

v/

INTRA

-MOUTAIN

Fig. 9. Vertical movement

BASI

BB CC’

NS

since 17 m.y.B.P. of northeast Japan (Sugi et al., 1982)

CASES OF THE MARIANA,

MEXICO AND GUATEMALA

TRENCHES

Figure 10 is the schematic representation of some of the drilling results obtained from the four IPOD Trench transects, i.e. Japan, Mariana, Mexico off Oaxaca and Guatemala Transects. The case for the Japan Trench, discussed in the last section, is included in Fig. 1OA as a guide to other cases. As mentioned already, no ocean derived material was recovered from the Mariana (Fig. 10B) fore arc drilling and the vertical tectonics in the Mariana fore arc has been that of dominant subsidence ever since the initiation of the present subduction in probably Eocene time (Hussong, Uyeda et al., 1978).

147

This, in our view, is the typical case for the Mariana-type subduction. Detailed history of the Philippine Sea, however, indicates that the spreading of back arc basins has been episodic; i.e. 32-15 m.y. for the Pareee Vela Basin and 6-O m.y. for the Mariana Trough (Kroenke et al., 1978; Hussong, Uyeda et al., 1978). Therefore, it may be inferred that during the pause of spreading, namely in 15-6 m.y. period, the mode of subduction could have been more of the Chilean-type. More detailed investigation on the vertical tectonics in future may well reveal such a history as shematically shown in Fig. 10B by a broken line. From the fore arc of the Mexican Trench off Oaxaca (Fig. lOC), landw~d-~pp~g reflectors observed in seismic profiling were found to be the progressively seaward accretion of the trench sediments (Moore, Watkins et al., 1979). (Even in this case, most or all of the truly oceanic components are considered to be consumed.) Vertical movements across the transect have been studied by McMillen and Bachman (1982) 4381439 Japan

Mariana

Trench

MexicoTrench

*A*

Guatemala

Cross

Trench

section

Trench

Depth

VS.

time

Fig. 10. Simplified representation of the results of IPOD drilling at four active margins. Left-hand figures are cross sections and right-hand figures are the estimated time variations of depth. (Compited from van Huene and Uyeda, 1981; McMill~ and Bachman, 1982).

148

as summarized in Fig. 1OC. Upheaval during the last ca 10 m.y. at deeper sites (488, 492,491) started with a rapid rise and became slower at a later stage, corresponding to the initial uplift at the trench toe and slower uplift on the slope. The former process is a local one of imbrication and the latter may be of more regional origin as it is approximately equal to the uplift rate on the continental crust (sites 489, 493). I consider these latter movements to reflect, at least partially, the vertical tectonics of interest here. The curves for the shallower sites show a very rapid (1 km/m.y.) subsidence in the early Miocene. It may be inferred that this early subsidence corresponds to the onset of the present phase of subduction, roughly synchronous to the age of the trans-Mexican volcanic belt, and the later uplift corresponds to accretion and vertical tectonics inherent to the Chilean-type subduction. Whether or not the earliest several m.y. mark the Mariana-type subduction is not certain. Lastly, in Fig. IOD, the case of the Guatemala Trench (von Huene, Aubouin et al., 1980) revealed practically no sediment accretion, and the benthic foraminifera data indicated dominant subsidence of the trench slope since early Miocene, probably attesting to the occurrence of the Mariana-type subduction throughout the above period. The above reasoning also agrees with the dominantly extensional tectonics in the Mariana and Guatemala fore arcs (Hussong, Uyeda et al., 1978; von Huene, Aubouin et al., 1980) and contractional tectonics in the modem sediments of the Japanese fore arc (von Huene, Nasu et al., 19’78) and in the Mexican fore arc (Moore, Watkins et al., 1979). It would be very pertinent to ask why the two relatively closely spaced parts of the Middle American Trench show such different trench tectonics, or the contrasting modes of subduction. A possible answer to this question will be given later after a discussion of the possible causes of the two modes of subduction is presented. POSSIBLE CAUSES OF THE TWO MODES OF SUBDUCTION MIDDLE AMERICAN TRENCH

AND THE ENIGMA OF THE

Here, one is concerned with the coupling between the buoyant landward plate, and the sinking oceanic plate at subduction zones (Uyeda and Kanamori, 1979). Three posibilities have been suggested: (1) The difference in the strength of coupling is ascribed to the difference in the nature of the contact zone between the two plates. More specifically, Kanamori (1977b) suggested that the different modes of subduction represent different stages of an evolutionary process: the subduction starts with low-angle thrusting of the Chilean-type mode and as the process goes on, the coupling gradually weakens and finally the mode becomes the Mariana-type. (2) The age of the subducting plate controls the mode of subduction, because the older the plate, the colder and heavier it is and, hence, the more likely it is to sink with greater speed and less coupling with the landward plate, giving rise to the

149

Mariana-type mode. Conversely, when the subducting plate is younger, the Chileantype mode will result (Molar and Atwater, 1978). (3) The motion of the landward plate relative to the trench line controls the mode (Chase, 1978; Uyeda and Kanamori, 1979). Namely, if the landward plate tends to go away from the trench line, as with the Philippine Sea plate, the Mariana-type mode of subduction occurs, whereas when the landward plate advances toward the trench, as with the South American plate, the mode is Chilean-type. Although the trench line in general should move, when the slab of the oceanic plate sinks to a great depth in the mantle, it may be, so to speak, anchored to the mantle for its lateral movements. Since the motions in the deep mantle are probably much slower than the plate motions, we may then be able to regard the position of the trench line as virtually stationary. Then, the relative motion of the upper plate and the trench line can be approbated by the absolute velocity of the upper plate alone. Figure 11 shows the absolute velocity vectors of the c&urn-Pacific upper plates taken from Minster et al. (1974) and Fitch (1972). It can be seen in the figure that the possibility (3) is well supported. The above three possibilities all seem to contain some truth in them. Recently, Dewey (1980) combining the possibilities (2) and (3), developed a general description of the problem. Possibility (1) appears partly inherent to the subduction process and

I

’

f

’

t180”

+150

tlzo”

+90

’

’

’

’

I-

I+90

Fig.

L

-120”

-90

-30”

-80

0’

1

'-I

-40

-WI

3.5

‘2.6

4 !? 7

y.2.i

j

-40’

+120”

T150”

1tl80’

ttrill -150”

-120”

-90’

-60”

-30’

0’

Il. Absolute velocity vectors of upper plates around the Pacific (after Minster et al., 1974 and Fitch,

1972). ‘l’be number attached to arrows are velocities in cm/yr.

(Uyeda,

1979).

150

may explain some cyclic nature in the arc evolution (Kobayashi and Isezaki, 1976; Niitusuma, 1978). If, however the process in possibility (1) is the sole agent, one would have to prove that similar cycles have taken place at every subduction zone, including the Peru-Chile zone. Possibility (2) is physically sound and well supported by the present-day data to a first approximation. Without some other factors, however, it may be difficult to explain the episodic nature of back arc spreading or the time-variation of the mode of subduction from one type to another. Possibility (3) is in a way free from this difficulty, because the major role is assigned to the absolute velocity of the landward plate. In the case of the South American continent, its absolute velocity has probably been steady, at least in its sense, during the opening of the Atlantic Ocean, and if so, the mode of subduction in the western South American coat may have been essentially of Chilean-type throughout the period. On the other hand, the absolute velocity of the Eurasian plate at its eastern margin (Fig. ll), which is relevant to the discussion of the western Pacific subduction zones, is very smaIl at present (Minster et al., 1974). Therefore, it seems quite possible that the velocity had a component away from the trench lines in relatively recent geologic past. From these considerations, a simple solution to the enigma of the Middle American Trench, introduced in the previous section, may be found as shown in Fig. 12. The key point is the position of the Mexican and Guatemala Transects relative to the Caribbean plate. At the Mexican Trench, the landward plate is a part of the westward moving North American plate, whereas at the Guatemala Trench, it is the Caribbean plate of which present day absolute motion is not significantly different

Fig. 12 Simplified tectonic system of the Middle America. NA =North American plate; CAR=Caribbean G=Leg 67 area; GT=Grenada

plate; CO=Cocos Trough; MF=Motagua

plate; NAZ=Nazca Fault.

American plate; SA =South plate; M=Legg

66 area;

I.51

from zero (Jordan, 1975). The absolute velocity vector for Middle America in Fig. 11 is that of the South American plate because the model AM-l of Minster et al. (1974) does not include the Caribbean plate. The absolute velocity vectors of the Caribbean plate given in Jordan (1975) and Minster and Jordan (1978) are directed more northerly and smaller in magni,ude (- lcm/yr), providing a component away from the Middle America Trench. Although the absolute motion of the Caribbean plate is uncertain, its eastward motion relative to the North American plate is undeniable from the nature of its northern transform boundaries and eastern subduction boundary at the Antilles arc. Thus, the landward plate at the Guatemala Trench is much more likely to give rise to the Mariana-type su~uction than at the Mexican Trench. In fact, the presence of diffuse zone of volcanism and graben structure south of Motagua fault system indicates that the stress regime in Guatemala is extensional (Plafker, 1976) and, therefore, the mode of subduction is Mariana-type. Probably the situation has been the same for some geologic past. This is consistent with the drilling result that showed subsidence and no sediment accretion since early Miocene. If, however, back arc spreading was taking place to generate the Grenada Through behind the Antilles Trench at some time in the past, the mode of subduction at the Antilles Trench then should have been Mariana-type. In that case, the motion of the Caribbean plate would have been westward relative to the Antilles Trench and, therefore, in turn, the subduction at the Guatemala Trench could have been Chilean-type. Therefore, to examine if the timing of the possible back arc spreading in the Grenada Trough far to the east is matched by the changes in the mode of subduction at the Guatemala Trench to Chilean-type would be an interesting test of the model presented here. It may be suspected that the Nicoya Ophiolite Complex in Costa Rica (Kuijpers, 1980) is extending under the fore arc zone of the Guatemala Trench (Seely, 1979). Such an abduction of ophiolite body might mark either the event of the initiation of subduction or strongly coupled subduction (the Chileantype). However, the present discussion is concerned mainly with the present phase of subduction. In order to develop a more .chomprehensiveargument on the tectonic evolution of the region, may more factors such as the truncation of ancient geologic structures either by tectonic erosion or lateral displacement will have to be taken into consideration. Aside from these possible complications during earlier tectonic phases, it seems that at least the modem differences between the Mexican and the Guatemala Trenches can be explained by the difference in the mode of subduction, the Mexican Trench being more of Chilean-type and the Guatemala Trench more of Mariana-type, and the different modes of subduction along these juxtaposed margins are probably due to the difference in the absolute motion of the corresponding landward plates. APPLICATION OF THE TWO MODES OF SUBDUCTION TO METALLOGENESIS ‘Ike nature of volcanic rocks is closely related to the nature of tectonic settings (e.g. Miyashiro, 1975). From the ~nsideration of the possible role of the tectonic

152

stress in controlling

the composition

of erupted

magma,

it may be expected

Chilean-type subduction zones have more abundance of talc-alkaline the Mariana-type subduction zones. Namely, it may be inferred compressive

that the

adesites than that under a

stress regime, magma is not allowed to reach the earth’s surface as freely

as under a tensional regime, so that it has more chance to become talc-alkaline andesite through processes such as fractionation and assimilation. This view is undoubtedly supported by observation on the end-member subduction zones, namely andesites are much more abundant in the Andes than in the Marianas. Somewhat related to the above is the possible role of the different modes of subduction on metallogenesis. Figures 13A and 13B show the world distributions of post-Mesozoic porphyry copper and massive sulphide deposits, respectively. The genesis of these copper ores is believed to be the result of volcanic activities at convergent plate boundaries. Figure 13A shows that the porphyry copper deposits are abundant in both the North and South American coastal regions, but are distinctly Ryukyu,

scarce in the western Pacific island arcs of Aleutians, I&riles, Japan, Mariana and Tonga. Instead, there are a number of massive sulphides

(mostly Kuroko-type) in Japan (Fig. 13B). The absence of porphyry copper deposits in the western Pacific island arcs has long been an enigma in economic geology. Uyeda and Nishiwaki (1980) pointed out that the uneven and complementary occurrences of porphyry copper and massive sulphide deposits can be explained by the difference in the stress field between the Chilean- and Mariana-type subduction zones: porphyry copper mineralization is favored by compressive tectonic stress of the Chilean-type subduction zones whereas massive sulphide mineralization is favored by submarine hydrothermal activities in the rift-like situations of the back arc regions of the Mariana-type subduction zones. It should be recalled that Japanese subduction was probably Mariana-type during most of Miocene when the massive sulphide deposits were formed in association with the submarine “Green-tuff’ volcanism in the back arc zone, which may have been an aborted back arc spreading center.

From

these

considerations,

it can

be expected

that

present-day

active

metallogenesis of massive sulphide is taking place in the floor of the actively spreading back arc basin such as the Mariana Trough, where active hydrothermal circulation has recently been discovered (Hobart et al., 1979). With regard to the incompatibility of porphyry copper and massive sulphide deposits, Sillitoe (1980) has come to more or less the same conclusion but from a quite different standpoint. In Fig. 13A, it may be noticed that many porphyry copper deposits occur along the southwestern Pacific islands. from the Philippines to the Solomons. The mode of subduction in these areas does not appear to be characterized as typically Chileantype. Uyeda and Nishiwaki (1980), however, pointed out that these islands have undergone complex tectonic history, involving collisions of arcs and continental fragments and reversals of the polarity of subduction. Since the collision process invariably produces strong compressive stress, the occurrences of porphyry copper deposits in these islands (including New Guinea) fit the above stated premise. This

B

Ksrmodsc-Tonqo

Antorctrc

-

S”bd”c+ron

_--

A

wlc*r+oin Port

lWS

p,a+*

*sromz

wundory

*o**wo

*“+Phlde

cf--c

co:,irwJn

-

f+i*ps

d*p*tr+*

Plate

A”t.rC+iC

ox,s

A

__

ironstorm

-

Dtractlon

Present

r”Dmorlne

0, txJ+

Plots

pm+e motian w(i+*,

J”,cp

Fig. 13.A. Distribution of young porphyry copper deposits (Uyeda and Nisbiwaki, 1980). B. Distribution

of young massive sulphide deposits (Uyeda and Nishiwaki, 1980).

Fig. 14. Generalized thonous

cratonic

arrows=direction suspect

terranes

map of Cordilleran

basement.

Barbed

of major strike-slip

Suspect

Zinezeastern movements.

(after Coney et al., 1980).

Terranes.

Dashed

pattern=North

American

autoch-

Mesozoic

deformation.

Barbed

limit of Cordilleran Abbreviated

symbols

such as NS. Kv, En etc. arc the

final point is suggestive of the possible importance of collisions at convergent plate boundaries. IMPORTANCE OF COLLISION/ACCRETION

TECTONICS

If the process of subduction continues, as plate tectonics asserts, and the intervening oceanic crust is all subducted, collisions of continents or island arcs are inevitable. Moreover, the world’s oceans have numerous uplifted features of various sizes and origins, i.e. seamounts, islands, ridges, arcs, plateaus and micro-continents. These features will arrive at the subduction zones and collide with the landward plate before the final closure of oceans. In fact, numerous actual and past instances of such collisons have been documented and their importance in tectonics emphasized (e.g. Dewey and Bird, 1970; Vogt et al., 1976). Recently, the interest in collision/accretion have been remarkably heightened with the recognition that numerous terranes on the circum-Pacific continents may be the accreted features (e.g. Fujita, 1978; Kerr, 1980; Churkin and Trexler, 1980; Coney et al., 1980; Saito and Hashimoto, 1982; Fig. 14), and even might have come from a single hypothetical lost continent “Pacifica” (Nur and Ben-Avraham, 1977). We still do not know exactly what tectonic significance accretion may have, in addition to contributing to the areal increase of continents. One of the basic questions may be posed as “Can the Cordillera be formed by strai~t-fo~ard subduction (presumably of the Chilean -type) alone or it can be formed only when accretion of exotic terranes is involved?’ Sometime ago we (Matsuda and Uyeda, 197 1) proposed the “Pacific-type” orogeny as one of the basic forms of orogeny, contrasting to the collison type one. Now, the third type that might be called “Accretion-type” orogeny is emerging. In fact, this third type may be much more common and important than the others. The other basic question would be the relation of collision/accretion with tectonic erosion of various types which also has been suggested as an important process at continental margins (e.g. Karig, 1974; Hussong et al., 1976; Kulm et al., 1977; Murauchi and Ludwig, 1980). In the next years to come, evaluation of the relative roles of subduction collison, accretion and tectonic erosion at convergent plate boundaries would be one of the major objectives of geotectonics. CONCLUSIONS

Problems related to subduction zones and back arc basins were reviewed from the standpoint of “comparative subductology”, and the existence of two basic modes of subduction (i.e. Chilean-type and Mariana-type) was postulated. It was shown that the concept of the two modes of subduction zones may be viable in understanding various arc related phenomena in terms of the tectonic stress; especially the recent results of DSDP drilling at active margins. Finally, the possible importance of ~ollision/accretion and erosion in the process of subduction was emphasized.

156

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