Download Research Note How Genuine is the Circum

Research Note
How Genuine is the Circum-Pacific Belt?
R. W. Girdler
(Received 1964 February 19)
Over the years, it has become customary to think of the Pacific Ocean as being
surrounded by an almost continuous belt of seismicity and volcanism--“the fiery
ring of the Pacific”. This has led to an impression of uniformity in the structure
and tectonics of the lands surrounding and immediately bordering the Pacific
Ocean. For example, Benioff (1958, 1962), Aki (1960) and others have been interested in the analysis of fault plane movements and find that there is a systematic
clockwise movement of the land masses around the Pacific Basin. Such conclusions
are often difficult to reconcile with modern ideas of mantle convection and continental drift and often seem to conflict with the obvious differences which exist
in surface geology and tectonics (see for example, the geological studies of Allen
(1962). T h e purpose of this note is to try to throw a fresh light on the structure
of the Pacific Basin and its surrounds.
T h e ideas presented here originate from geophysical studies of rift regions such
as the Red Sea (Girdler 1958, Drake & Girdler 1964,) and the Gulf of California
(Harrison & Mathur 1964). These two regions have many similarities and their
origin seems to be due, in part, to extension of the Earth’s crust. Both form part
of the world rift system which is continuous for thousands of kilometres (Ewing &
Heezen 1956, Menard 1959) and which is usually found near the centres of the
oceans but occasionally intersects the land masses in such regions as Afro- Arabia
and western North America.
One explanation for the origin of the world rift system is that it is a consequence
of regions of rising, diverging convection currents in the mantle. Earlier, I
tried to show that this is consistent with mantle convection patterns used to explain
continental drift (Runcorn 1962, Girdler 1963). T h e rifts are associated with
shallow seismicity and high heat-flow and are usually thought to be due to crustal
tension. It was therefore reasonable to use the rifts to locate regions of possible
rising convection.
More recently, I have been trying to find some quantitative or semi-quantitative criteria for locating regions of possible descending convection currents.
Several workers such as Vening Meinesz have supposed that the island arcs are
due to descending convection currents. These are regions of deep and/or intermediate focus earthquakes and I therefore tried plotting the distribution of the
epicentres of these earthquakes.
For the Pacific Ocean, two charts (following Gutenburg & Richter 1954) have
been prepared :
(I) the distribution of deep and intermediate focus earthquakes
(2) the distribution of shallow earthquakes only (shallow earthquakes, of course,
also occur in the regions of deep and intermediate earthquakes).
5
537
538
R. W.Girdler
These are shown in Figures I and 2 respectively. When the earthquakes are
plotted in this way, it is seen that the “circum-Pacific belt’’ no longer exists as a
continuous belt of similar seismicity. Figure I shows that deep and intermediate
focus earthquakes are absent from the North American coast between Alaska and
Central America. They are also absent in South Island New Zealand and the
whole of the southern Pacific south of 40’.
The region of shallow seismicity (Fig. 2) mainly follows a distinct belt from
South Island New Zealand across the southern and south-eastern Pacific to join
with the Gulf of California and the west coast of the United States. A branch runs
to southern Chile.
These two very different types of distribution can of course, be associated with
diverse types of geological structure and tectonics. T h e deep and intermediate
seismicity is associated with the island arc and ocean trench structures of the
Aleutians, Japan, the Philippines, New Guinea, ’ronga and Kermadec and the
intense fold mountains of the Andes.
FIG.1.-Distribution
of deep focus earthquakes
focus earthquakes. (0)
(v)and intermediate
How genuine is the circum-Pacific belt?
539
I n contrast, the shallow seismicity is associated with the transcurrent faults of
New Zealand and California and the oceanic rifts and rises of the South Pacific,
Easter Islands and East Pacific. As stated earlier, these are thought to be regions
of crustal tension. So far, no rift valley has been found along the East Pacific Rise
but it is associated with shallow seismicity and Menard (1960) considers the Rise
is due to a rising convection current. T h e same rising convection current may
explain the Gulf of California and the Basin and Range structure of the western
United States. Gravity work by G. Thompson (1959) in the latter area suggests
that this is a region of extension and somewhat similar to East Africa. In late
Mesozoic times, this was a geosynclinal area and this seems strong evidence for a
change in the mantle convection pattern in the last 70 million years. I t is difficult
to explain such a change over in structural pattern in any other way.
I t is seen, therefore, that the Pacific Ocean is bordered by both regions of
crustal compression (deep and intermediate focus earthquakes) and crustal tension
(shallow earthquakes). It seems, erroneous, for example, to compare the tectonics
of California with those of South Amcrica for the former is in continuity with the
FIG.2.-Regions where only shallow earthquakes (0)occur. The heavy
line represents the approximate position of the 2000 m contour.
R. W. Girdler
540
East Pacific Rise, a tensional feature and the latter is a region of compression.
Transcurrent faults are, of course, compatible with both but care must be taken
to interpret them in relation to the two types of regions rather than in relation to
the so called “circum-Pacific belt”.
Department of Physics,
University of Newcastle upon Tyne.
I 964 January.
References
Alri, K., 1960. J . Geophys. Res. 65, 2405.
Allen, C. R., 1962. J . Geophys. Res. 67, 4795.
Benioff, H., 1958. Puhl. Domin. Ohserv., 20, No. 2 , 395.
Benioff, H., 1962. Continental Drift (Ed. S. K. Runcorn) Chapter 4, Academic
Press, London.
Drake, C. L. & Girdler, R. W., Geophys. J. 8, 473.
Ewing, W. M. & Heezen, B. C., 1956. Antarctic I.G. Y . Monograph No. I , 75.
Girdler, R. W., 1958. Quart. J . Geol. Sac. Lond. 114,79.
Girdler, R. W., 1963. Nature, Lond. 198, 1037.
Gutenburg, B. & Richter, C. F., 1954. Seismicity of the Earth, Princeton University Press.
Harrison, J. C. & Mathur, S. P., Gravity anomalies in the Gulf of Califovnia, in
press.
Menard, H. W., 1959. Experimentia, 16,205.
Menard, H. W., 1960. Science, 132, 1737.
Runcorn, S. K., 1962. Nature, Lond. 193,311.
Thompson, G.A,, 1959. J . Geophys. Res. 64,217.