A digital tectonic activity map of the earth
Preliminary Interpretations of the DTAM and GTA The purpose of this
paper is primarily to simply present the maps, their data sources,
and the methods used to compile them. However, a few major questions
are immediately suggested by even a simple inspection of the maps,
including the following.
Can the Earth's crust be
realistically described in terms of a finite number of plates?
A fundamental tenet of plate tectonic theory is that most
tectonic activity is the result of interaction among rigid and
relatively inactive lithospheric plates, 12 in the widely used
NUVEL-1 model (Stein, 1993). The NUVEL-1 model has been tested
repeatedly by space geodesy, and in general has been found to
predict site motions in direction and magnitude reasonably well.
Significant exceptions have been found even in classic plate
boundaries, such as the South America/Pacific plate area, where the
Arequipa site has been found to share in the eastward motion of the
Pacific plate (Robaudo and Harrison, 1993). However, these anomalies
are explainable in terms of plate theory.
Viewing the Earth
as a whole, on the other hand, we see clearly that large areas,
especially on continents, can not be assigned to discrete plates.
The diffuse nature of continental plate boundaries has been noted by
many authors, such as Stein (1993). Nevertheless, the broad zone of
intense seismic activity in, for example, south central Asia
emphasizes the artificiality of plate maps showing the Eurasian
plate as extending from Iceland to Indonesia. The DTAM is thus
presented as a much more realistic view of global tectonism.
Are old "inactive" orogenic belts truly inactive?
Two classic folded mountain belts, the Appalachians and the
Urals, are generally believed to have been formed by continental
collisions in the late Paleozoic, after which they have been
essentially inactive except for vertical movements. The World Stress
Map appears to confirm such interpretations, in that as pointed out
by Zoback (1992) "Residual stresses from past orogenic events do not
appear to contribute in any substantial way to the modern stress
field." However, the Appalachians are shown as active on the basis
of seismicity. This seismicity was explained by Sykes (1978) as
resulting from movement on the landward extension of oceanic
fracture zones. although Appalachian seismicity appears to parallel
the major Paleozoic structural trends, both fold axes and thrust
faults. Seismicity is not as well defined in the Urals, but Russian
geodetic and geological evidence (Trifunov, 1983) shows clearly that
they are still (or again?) undergoing faulting and uplift, although
utterly isolated from any possible transverse fracture zone.
Much older fold belts, such as the Proterozoic Labrador
Trough, appear truly inactive. However, the DTAM suggests that fold
belts may remain active much longer than previously realized. This
possibility should be tested by GPS surveys and further in situ
stress measurements of the sort used for the World Stress Map.
What causes seismic activity in the Alpine fold belt?
The intense seismicity of the western Tethyan fold belt, or
Alpine belt, is dramatically shown on the epicenter map. This
seismicity is generally explained in geology texts as the result of
continental or plate collisions, specifically between the African
and Eurasian plates. However, simple examination of the DTAM and GTA
suggests that both such plates are rotating in the same general
direction, counter-clockwise away from the Mid-Atlantic Ridge. Such
as interpretation can be justifiably described as simplistic, but
all numerical plate models, such as NUVEL-1, show the African and
Eurasian plates as the slowest-moving of all major plates. The
inverse relation between plate velocity and relative continental
crust area was first noticed by Minster et al. (1974). The African
plate has in fact been described by Burke and Wilson (1972) as fixed
over the mantle for the past 25 million years on the basis of the
absence of hot spot trails.
The situation is thus something
of a paradox: one of the most seismically active areas on the planet
is located between two plates that are hardly moving. Can this be
caused by "residual stresses from past orogenic events" as described
by Zoback (1992) in relation to the World Stress Map? Or is another
mechanism at work, such as "extensional collapse of orogens" (Dewey,
1988)? The DTM provides no obvious answer to these questions, but it
at least highlights the existence of a tectonic anomaly.
Do space geodesy results actually show plate motions?
The VLBI station velocities shown in
(Fig. 6) indicate motion of the Pacific Plate
agreeing in direction and magnitude not only with NUVEL-1 but with
several other well-known lines of evidence such as focal mechanisms
and hot-spot trails. However, an apparent inconsistency has been
noticed between two sets of space geodesy measurements in western
Europe. When plotted on the basis of NUVEL-1, with the Pacific plate
held stationary, the station vectors appear to trend uniformly to
the northeast
(Fig. 6) (Robbins et al., 1993) . However, a
similar compilation, using NUVEL-1 but with the North American plate
held stationary, shows the European vectors trending to the
southeast (Ryan et al., 1993), nearly orthogonal to the vectors
relative to a fixed Pacific Plate. The World Stress Map, and the
DTAM, show clearly that actual site motions , or at least strain
vectors, in western Europe should be in this direction, i.e., to the
southeast, presumably as the result of ridge push from the northern
Mid-Atlantic Ridge (Zoback, 1992; Richardson, 1992). The implication
of this inconsistency is that space geodesy results do not
necessarily show plate motion relative to the sub-lithospheric
mantle, i.e. "absolute" motions in the terminology of Minster and
Jordan (1978), and that caution should be used in interpreting them
as actual plate motions.
Summary The combination
of vastly increased knowledge of the Earth, from space-related and
conventional studies, with new data-handling techniques, has
produced a new view of the Earth's tectonic and volcanic activity.
This new view is generally compatible with plate tectonic theory,
but is a much more complex picture than the one familiar to all
geology students for the last two decades. It is also a more
realistic one, intended to show actual tectonically active features,
subject to scale and cartographic limitations, rather than the
idealized ones of plate maps.
These maps should be of value
not only in geologic education, but in natural hazard programs,
geophysical exploration, and perhaps in political problems such as
enforcement of the Comprehensive Test Ban Treaty. Their greatest
scientific value, however, may be in raising awareness of tectonic
questions not answered by now-conventional plate tectonic theory.
