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Tectonophysics
Elsevier Publishing Company, Amsterdam - Printed in The Netherlands
RECENT CRUSTAL MOVEMENTS: TECHNIQUES AND ACHIEVEMENTS
H.W. WELLMAN
Department of Geology, Victoria University of Wellington, Wellington (New Zealand]
(Received October 15, 1971)
ABSTRACT
Wellman, H.W., 1972. Recent crustal movements: techniques and achievements. In: A.R. Ritsema (Editor), The UpperMantle. Tectonophysics, 13 (l-4): 373-392.
A fuller knowledge of the nature and rate of recent crustal movements would do much to relate geomorphology to geology to the benefit of both. The study of recent crustal movements is a diverse one, and extends from structural geology through geomorphology to geodetics. The three fields differ in their techniques, but the all-important difference between them is the interval of time they cover. Tectonic processes are cyclic, with periodicities that are not well known, and cannot be known unless all three fields are covered. The critical periodicity for large-scale mountain building is probably from 0.5 to 10 million years. The period is largely that of geomorphology but the geomorphological part of recent crustal movements is the part least well quantitized. This is partly because the necessary surveys are too time-consuming to appeal to most geologists and too trivial to appeal to most geodesists. Too often geomorphic studies are largely descriptive, and rarely are estimates given of errors in time and distance.
An attempt is made in the following account to cover all fields, to relate errors of observations to probable movements, and to give an account of the achievements considered most significant.
INTRODUCTION
I have been asked to speak on Recent Crustal Movements of the earth’s crust, and in
doing so to take into account those advances that have taken place during the last seven
years - the duration of the Upper Mantle Project. My most important task is to assess the
new problems raised and to indicate the best direction for future projects. I have also
been asked to analyse and evaluate new interpretations.
An immediate conflict comes to mind. In itself the study of recent crustal movements
on land is conservative, geodesists knowing full well that some five years have to pass
before useful re-observations can be made. A conservative attitude has thus to be
contrasted with the exciting oceanic discoveries that have been made during these last
seven years. I will thus attempt to evaluate the oceanic discoveries. and then revert to
recent crustal movements on land.
Essentially the oceanic discoveries depend on a well-established sequence of magnetic
reversals that show up as a symmetrical pattern at a few places. The symmetry is critical
and contrasts with the irregularity on land. It is hard to explain away, and is most simply
explained by continuous intrusion along the axes of the oceanic rises. If this is true, then
tnore is now known about relative horizontal movements from below the oceans than on
land.
One of the main advantages of the “Recent Crustal Movement” approach to the study
of tectonics is the concept of rate - the attempt to estimate annual rates and to give
some idea of their accuracy. In general the time factor is the most inaccurate factor for
determining movement rates from geological and topographrc data. Ages have to be
obtained for observed features, and to be useful they need to be accurate to at least 50%.
This is satisfied for many ages from 500 m.y. to 20 m.y. and for many from 30 (thousand
years) to the present day. But a serious gap exists between 20 m.y. and 30 thousand
years. The gap is likely to be partly closed in the next few years from work on magnetic
reversals at the old end and by new dating techniques at the young end of the time range.
Deformation is certainly periodic and not continuous, but the length of the long-term
periodicities is not well known, and they may well be different at different places. In New
Zealand and Japan average rates for the last 100 thousand years are an order of
magnitude greater than the average rates for the last 20 m.y., indicating a long term
period of about 2 m.y. If this is true generally, then the critical information lies within
the 20 m.y.-30 thousand years dating gap.
Geometr), for recent crustal movements
The structural data of geology is so complex that geologists use graphical and intuitive
rather than mathematical solutions. For example, the shape of folded strata can be shown
by structural contours which are far from simple geometrically and are not infrequently
made even more complex by the discontinuities commonly known as faults.
On the other hand, simplicity appears to reign in the oceans. There, relative movement
is assumed to be horizontal and thus, the earth being round, relative movement can be
simply expressed by rotation about an axis.
Analysis of recent crustal movements is impossible without a geometrical model or
models. Two are used, but within different time scales, and both, simple as they are, are
too complex for full solutions to be obtained with the available data. The first and long
term one is the rigid block model, in which the relative movement of adjacent blocks is
determined by the theory of screws. Movement is a combination of rotation and
translation, and six parameters are required to define it. The second is that of
homogeneous deformation in which the crust is assumed to be deformed so that straight
lines remain straight. Again six parameters are required for full definition.
In most cases instrumental techniques determine the way in which recent crustal
movements are observed: almost all techniques are controlled by the direction of gravity.
Theodolites measure vertical and horizontal angles, and’in levelling, measurement is solely
RECENTCRUSTALMOVEMENTS 315
in the direction of gravity. Gravity is equally important in geology, the basic assumption
for structural interpretation being the approximate horizontality of strata when they
were deposited.
In geodetic as contrasted with topographic and geological observations the critical
information is not the time between observations, but the accuracy of the observations
themselves, and because the accuracy of observations has generally increased with time,
the accuracy of the first observation is generally the more important. Consequently for
geodetic surveys the first requirement is to assess the validity of the differences that have
been observed. All geodetic surveys contain internal checks, which give some idea of
order of accuracy, but systematic errors are less easily assessed, and external checks
inspire most confidence in the validity of geodetic results.
Three checks are possible: (1) agreement with topographic or geological observations;
(2) the results having a simple geometrical pattern; and (3) agreement in time: it being
assumed that the rate of movement is uniform and a third survey being consistent with
the first and second.
The first check is satisfied in Finland and Sweden where uplift is in the right sense and
of about the same amount as would’be expected from the heights of dated marine shore
line features; and in New Zealand and California where dextral movement is the same
sense as that inferred from topographic and geological observations. The second check is
satisfied in Finland, where the uplift isobases are almost straight and evenly spaced, and
in California where some re-surveys show a vector pattern indicating homogeneous strain.
The third check has been rarely used because of the long time needed. The repeated
geodetic observations on the San Andreas fault which indicate continuing strike-slip
movement in the same sense and at about the same rate is the only well-known example.
Sudden movemet? ts
A main distinction that has to be made is between sudden and gradual movements. All
sudden movements are directly associated with earthquakes. Gradual movements may be
isostatic or may precede or follow earthquakes. The higher the degree of observational
accuracy the better can gradual movement be determined. The position is entirely
different for sudden movements, the only important factor being the ratio of the sudden
movement to the accuracy by which the movement can be determined. Three cases have
to be considered for both gradual and sudden movements: faulting; tilting; and uplift,
either positive or negative.
The degree of accuracy by which fault movements can be determined depends on the
topographic and cultural conditions of the region. For instance in New Zealand almost all
reports of active faulting are of roads, railways, or other cultural features. As an example,
two active faults were found after the Inangahua earthquake of 1968 ~~ one displaced a
road and the other a railway. Dense forest covered most of the area, and doubtless most
of the faulting took place within the forest, but it was impossible to find, and is thus
unrecorded. Environmental factors thus determine the discovery of active faulting.
376 ti.w. Wl!LL~IAN
After a severe earthquake landslides are far more conspicuous in mountainous country
than are active faults, and it is not surprising if non-geologists should fail to recognise the
importance of a fault with a displacement of 3 m when confronted with a region in which
ten per cent of the mountain surfaces have slid into the valleys. Active faulting is thus
only recorded by critical observers.
For natural features under the best conditions, such as those at Meckering, Western
Australia (Gordon, 1971) vertical displacement can be determined with an accuracy of
about 0.1 m. Because of the absence of natural reference lines, horizontal displacement is
less easily measured than vertical, and may well go unrecorded. General accuracy is
probably no better then I m. An extra hazard is that most faulting takes place along old
faults, and in the absence of datable artificial features, care has to be taken not to
confuse the results of earlier with those of the latest fault movement. Confusion of this
kind took place after the central New Zealand earthquake of 1848, when trenches that
extend along the active faults on hill slopes and are probably many thousand years old,
were considered to have just formed.
The measurement of uplift and tilting after sudden movement depends on the pre-
existence of some reference surface, either cultural or natural. Existing level surveys
define the most commonly used cultural reference surface. In New Zealand we have been
extremely lucky in this respect, and have been able to determine uplift and tilting for
three areas of earthquake uplift through the accident of railway and other level surveys.
The sea, lakes and rivers produce natural reference surfaces. For coastlines a few
simple observations made prior to earthquake uplift would do much to increase the
accuracy to which the earthquake uplift could be determined. For instance in New
Zealand and doubtless elsewhere the heights of beach ridges and other growing shore lines
features can range from less than 1 m to up to 10 m above sea level. If the heights were
determined now, the accuracy to which future sudden coastal uplifts could be determined
would be increased considerably. The height of analogous biological features, such as the
barnacle line, would be equally useful on rocky coasts.
Sudden uplifts must have taken place in the past, but they are rarely recorded with
certainty. Uplifted beach ridges along the eastern side of the North Island of New Zealand
(Wellman, 1971) and possibly those in Alaska probably record “fossil” sudden uplifts.
They are younger than the 6.5 thousand-year-old highest Holocene shore line feature and
at most places there are six or seven ridges, indicating that sudden uplift at any particular
locality probably took place at intervals of about a thousand years.
If the shore lines could be correlated from place to place it would be possible to get
some idea of the extent of each individual uplift, and thus to judge the magnitude and
frequency of major earthquakes, but this still has to be done.
RECENTCRUSTALMOVEMENTS 377
UPLIFTANDDRIFT
Uplift
The use of mareographs (tide gauges) is the classical method for recording uplift, either
positive or negative. As the simplest hypothesis it is assumed that the level of the sea has
changed synchronously over the whole world. It is then necessary to know for any
particular interval of time: the change in the level of the sea (Lennon, 1965); and the
relative change between land and sea at the mareograph station. If mareographs were
evenly and sufficiently closely spaced over the earth’s surface it would be possible to
estimate the change in the level of the sea by taking an average of all values. Spacing is
erratic and the best that can probably be done is to accept the modal as being the true
value of change. From modal values it is generally considered that for the last 30 years
the level of the sea has been rising at about 0.8 mm/year. There is some evidence from the
mareograph at Amsterdam (Waalewijn, 1966) - that with the longest record - and from the
spacing of shore-line features in isostatically rising areas that the present-day sea level rise
is part of a 400 year cycle with an amplitude of about 1 m.
In order to provide useful information, mareographs need to be under the control of a
permanent and trustworthy organisation that makes periodical checks to ensure reliability
of observations, and by periodical relevel surveys ensure that the observations are related
to firm ground and not, for example, to a point that may be slowly subsiding because of
compaction.
Neglecting sudden movements, which are already dealt with, the best that can be
hoped for is to obtain significant results after an interval of some five to ten years.
Mareographs provide uplift information only for the coast, but inland uplift values can be
obtained from a precise level net that is tied to mareograph stations, provided that the net
is resurveyed at suitable intervals. Direct control by a geodetic organisation that is
interested in recent crustal movements is thus the only way in which mareographs are
likely to be reliably serviced for a sufficient length of time to be tectonically useful.
At most places mareograph records extend back for less than 100 years. For this
interval of time it is generally assumed that uplift is linear, and that the non-linear results
are climatic, and due to variations in atmospheric pressure, wind, and tides. The non-
linear differences can be roughly halved by a trial and error relation against suitably sited
atmospheric pressure observations. The most direct way of reducing the size of the non-
linear elements within a small area is by the differences method; differences however
indicate tilt and not uplift and are so discussed.
The accuracy (i.e., probable error) to which uplift can be determined from
mareographs, assuming linear uplift, a hundred year operating period, and an accuracy of
annual means of 20 mm, is about 0.2 mm/year.
Two factors make it difficult or impossible to relate the relatively short-term
movements recorded by mareographs with long-term mountain building movements. In
regions subject to sudden earthquake movements, the gradual vertical movements
378 H.W.WELLMAh
between earthquake uplifts cannot be at the same rate and may well be in a different direction from the overall long-term movement. It would thus be wrong to assume that a
particular locality, for example Wellington, New Zealand, is tectonically stable because uplift is not shown by its mareograph when uplift is known to have occurred during an
earthquake and is recorded by uplifted Holocene and Pleistocene shore-line features. The
other problem is that of compaction associated with changing ground water conditions. Subsidence of several metres has taken place in a few tens of years at several places (Meade, 1971) and significant subsidence may be taking place at the many places where mareographs are not sited on bedrock. Periodic level connections with bedrock stations is thus essential.
Mareographs may have importance for earthquake prediction, anomalous changes that reach amplitudes of 200 mm and begin some years prior to major earthquakes having been reported by Yamaguti (1971).
Uplift determined from shore-line features
Continuous shore-line features can be looked upon as being old, and somewhat inaccurate, level surveys, that only need to be relevelled in order to provide useful information on uplift. They more commonly provide information on positive than on negative uplift. At many places uplift is self-evident, and often information on uplift rates can be obtained with little effort relative to that required in geodetic levelling surveys.
The basic requirements for shore-line features are much the same as those for mareographs. In order to determine the average rate of uplift since the formation of a specific shore-line feature it is necessary to know: (1) the age of the feature; (2) the height at which the feature formed above mean sea level; (3) its present height above mean sea level; and (4) the height of mean sea level relative to that of the present day at the time when the feature formed.
Age can be determined by direct or by indirect dating. Direct dating methods are archaeological and radiometric. The radio-carbon is the most used radiometric method, but is useful for the last 20,000 years or so only.
Indirect dating can be used for those prominent and continuous shore-line features that were formed during specific events, provided that the particular specific event can be dated. The most easily dated specific event is a time interval during which there was no relative vertical movement between land and sea.
For instance, if the sea is rising at 2 mm/year, then prominent shore-line features are unlikely to form unless the land is rising at about the same rate. The rate of rise of the sea decreased from about 20 mm/year to almost zero between six and seven thousand years ago, and the level of the sea has not changed much since. Hence there is a praminent feature, conveniently termed the highest Holocene shore line, on most coasts that were being uplifted at a rate of 2-20 mm/year. If the average rate of uplift is assumed uniform, then provided that the eustatic sea level curve is known, the rate of uplift and the approximate age of the shore line can be determined.
RECENTCRUSTALMOVEMENTS 379
The rise of the sea was due to the melting of inland ice at the onset of the present
interglacial period. Analogous shore-line features formed during the onset of earlier
interglacials, but being older, they are less easily correlated, and being beyond the range
of radio-carbon are far less easily dated.
The only direct way of determining the height at which a particular shore-line feature
formed is to estimate the height at which it would form today. The main controlling
factors are the tidal range, storm wave intensity, and as an extra complication, the coast
line changes that have taken place since the old feature formed.
Accuracy depends on local conditions, and in general the more exposed the site and
the greater the tidal range the lower is the accuracy of the estimate. Estimates depend on
local conditions, and it is essential that those who record heights of elevated shore-line
features should estimate a height of formation. Unless this is done, and done clearly, a
reported height above mean sea level can be interpreted as meaning the amount of uplift.
It is better to have an approximate estimate than none, for increased accuracy in one
measurement does nothing to compensate for lack of information on another that is
equally important.
The determination of the height of a shore-line feature above mean sea level is merely
one of levelling. The most suitable method is determined by the degree of accuracy
required. Most Holocene shore-line features have irregularities of at least 0.2 m, and older
ones are even more irregular. Where the distance to the sea is no more than one kilometre,
the visible sea horizon, provided its depression is allowed for, provides a convenient line
for quick levelling (Wellman, 1971).
The determination of the level of the sea at the time when a particular shore-line
feature formed depends on having an acceptable curve showing the changes in eustatic sea
level that includes the time concerned. The construction of such a curve is a complex
problem that becomes increasingly difficult with increasing distance back in time.
As with mareographs the best estimate of the height of the sea at any time in the past
is given by modal values. Such values depend on sampling being unbiassed, and not for
example being concentrated on areas of uplift. The distribution of samples is reasonably
satisfactory for the present interglacial, the level of the sea being known to about 3 m or
better for the last six thousand years. However, for earlier interglacials most studies have
been made in areas with uplifted shore line features, and for this reason it had been
assumed until recently that the level of the sea had been decreasing for the last 300
thousand years or so. Better sampling has shown this to be probably untrue.
Most of the curves that have been constructed to date are based on single samples from
different localities, and the ages depend solely on assuming the radiometric dates to be
correct. The accuracy of the present curve for the last 10,000 years (Holocene) could be
greatly increased at relatively little cost by drilling holes at suitable localities to provide a
series of samples in sequence (Sir&, 197 1) so as to provide a check on the accuracy of
the radiometric dates.
For uplifted shore-line features less than 7,000 years old, accuracy of inferred uplift
rates is largely determined by uncertainty of the height of formation of the feature where
uplift is small, and uncertainty of age of feature where the uplift rate is large. The first case is the more important and on most coasts makes it impossible to determine positive uplift rates of less than 0.5 mm~year, For uplift rates of 1 mm or more per year the accuracy is about 0.2 mm/year where the age of the uplifted feature is reasonably well kIlOWll.
If uplift is considered to have been uniform for the last 100 thousand years or so then uplifted features belonging to the last and to earlier interglacials are particularly important for determ~ing upiift rates less than 0.5 mm/year. The accuracy of inferred uplift rates is Iimited by uncertainties as to the height of sea level during the inter~a~i~s and by unce~aint~es as to the age of the features. The first problem is being solved by better sampling, and the second by new methods of radiometric dating that cover the
50-300 thousand year time range, and average rates are likely to be known for many more coasts in the next few years.
Uplift is used for movements in the direction of the eart!h’s gravitational field. There are two directions at right angles, and drift is used for movement in either or both of these directions. The earth’s rotation makes it convenient to resolve drift into changes in
latitude and changes in longitude. Only changes have to be considered, consequently irregularities in the geoid are unimportant and it is sufficient to consider how changes in latitude or longitude at a particular point can be determined with an accuracy comparable with the changes that are likely to have taken place. Observational difficulties that are caused by changes in the short-term position of the earth’s rotational pole, and by short- term changes in the rate of the earth% rotation can be resofved by well timed observations.
As for other recent crustal movement observations, accuracy has increased with time, and the accuracy of the determined rate of change depends on the accuracy of the first observation and on the time between ovulations. So far the observations that have been made are not accurate enough to show movements in either Xatitude or longitude (Markowitz, 1966). For the last ten years or so prismatic astrolabe observations with an accuracy of about I m have been made. They provide a maximum accuracy of about 100 mm/year. The expected maximum rate of movement is about 50 mm/year, and provided that the required observations have been made, significant results may be obtainable in the next 20 years. However, new techniques with a ~onsid~ab~y higher accuracy such as laser shots to arti&iaI. satellites ur to the m&n v&l probably produce the first signiR&nt results. According to Aardoom (1971) the required accuracy may be obtained by the use of “Very long base line interferometry” using two or more galactic radio sources.
Complete coverage of recent erus%l movements derrtands &at some mention be made of the changes in palaeodimate recorded in the geological column by fossils or by special
RECENT CRUSTAL MOVEMENTS 381
deposits. Fossil plants, used as an indication of temperature are the most important for
recent crustal movement studies. Most of the proved temperature changes are due merely
to temperature oscillations related to successive glaciations in the Late Cenozoic. They
have had a range of up to 7”C, which at latitude 45” represents an altitude change of
about 1,500 m or a latitude change of about 10”. Surfaces of equal temperature have a
complex shape but in general they slope down from the equator and extend below sea
level when traced towards the poles. Hence a proved temperature change may be
explained as being due to a change in latitude, for instance, the fossil plants in Antarctica,
or as being due to a change in altitude, for instance, the fossil plants in Tertiary deposits
in the Andes.
The more the fossil plants differ from those growing today the more difficult it
becomes to infer climate from them, and it is only for the last 25 m.y. that resemblance
to present-day plants is close enough to make estimates of temperature differences
reliable to better than a few degrees centigrade.
DIFFERENTIAL MOVEMENT
At many places differential movement of the earth’s crust takes place along zones that
are sufficiently narrow to be treated as planes; the rocks between the planes can then be
considered to be rigid and the relative movement analysed fairly simply (Barazangi and
Dorman, 1969). Two kinds of movement have to be considered: spreading by dyke
intrusion, and faulting. The first is relatively simple. Most dykes are vertical, and for
most, the rocks on each side appear to have moved apart in a direction at right angles to
the strike of the dyke. Ocean-floor spreading is considered to be this kind of displacement.
The second type is more complex; a fault can have any attitude from horizontal to
vertical and the movement on the fault plane can have any direction. For convenience of
description only two kinds are considered: dip-slip and strike-slip, but most faults are
partly of one kind and partly of the other. The first kind can be either normal or reverse,
the normal including an extension of the earth’s crust, and the reverse a contraction. The
second kind can be either dextral or sinistral. Knowledge of the nature of faults is basic to
an understanding of the tectonics of a region. Unfortunately, full information is rare
except for a few faults that have been active during the last few thousand years. Two
requirements are essential: knowledge of the dip of the fault plane, and the recognition of
a reference line that once extended across the fault. Faults are generally bounded by
crushed rock and only rarely can the dip of a fault plane be established with certainty.
Moreover it cannot be assumed that the dip of a fault plane is the same at depth as it is at
the ground surface; for instance, at many places along the higher parts of the Southern
Alps of New Zealand the dip of the Alpine fault increases from 0 to 60” when traced
along valley sides towards and under the Alps. The difference between normal and reverse
faulting is one of direction of dip relative to the side relatively upthrown, and unless the
direction of dip is known, it is impossible to distinguish between reverse and normal
faults, and thus impossible to determine if the crust is expanding or contracting.
The general absence of reference lines makes it impossible to determine the true direction of displacement for most faults, and ali that can be determined is the component of displacement normal to bedding planes. To be extensive enough to define a fault for any distance a bedding plane has to be nea~hori~o~ta~~ consequently the dip-&p component of faulting is the only component known from stratigraphy. A better idea of the relative importance of strike-slip to dip-slip faulting is obtained from th.e nature of
ground breakage during historical earthquakes (Pavoni, 1971) and to a lesser degree from displaced topographic features as seen in air photographs. For earthquake displacements account has to be taken not only of the amount of ~sp~acement but afso of the length over which the faulting has taken place. When this is done the strike-slip shows as being equally if not more important than the dip-slip component, notable examples of strike- slip displacement being the earthquake movement on the San Andreas fault of California and that on the Anatolia fault of Turkey. The nature of the fault displacement of topographic features is much more eassiIy appreciated when the features are viewed from a distance than when seen on the ground. The stereoscopic examination of verticaf air photographs is thus the best way of studying the direction of displacement for active
faults. There are two further problems. Faults are rotated with the rocks that contain them,
and the dip of a fault may change s~~i~eant~y with time. The other is the difficulty for most faults of obtaining any idea of the change in their rate of displacement with time. All that is generally known is the total displacement from some time in the past until1 the present day. Displacement is unlikely to have been uniform in time, but for convenience of description it is generally assumed that the total displacement took place during a more or less hypothetical orogeny.
On Iand the only ~sp~acem~nt rates that are re~onab~~ we11 known are fa~tjng displacements for the fast miihon years or less. For the Alpine fat& of New Zeatand the average rate of displacement can be estimated from displaced Late Quaternary terrace edges and from displaced river valleys of possible last Interglacial Age. For both features inaccuracy of age determines the inaccuracy of the estimated rate of movement. The displaced river terraces are mostly on branches of the Alpine fat& and not on the fault itself, At seven places a flight of river terraces is displaced, and at most places the displacement, both horizontal and vertical, is about proportional to the height of the displaced terrace above present river level. The terraces are cut in an aggradational surface, and at all places the surface appears to be the last one to have formed. The surface is thus Late Quaternary In age, but it cannot be dated directly and estimates of its age have ranged from 10 to 30 thousand years. A probable age is I5 ? 5 thousand years. The sum of the horizontal displacement on all the sub-parallel faults is about 300 m, and the total average annual rate of displacement is thus about 20 t 10 mm/year.
The displaced river valleys are confined to the Alpine fault itself, and provided the first evidence for dextral faulting. The valleys have been glaciated since they were displaced, and possibly came into existence because of strong do~~utting during the last Interglacial some 100 thousand years ago. The average dextrai displacement is about 1.3 km and the average annual rate thus about 13 mm/year, if the age is correct.
RECENTCRUSTALMOVEMENTS 383
Ocean-floor spreading
In the writer’s opinion the most significant data on relative crustal movement now
comes from the sea floor (Le Pichon, 1968). The symmetry of a few magnetic profiles,
and the way in which most profiles match, is most simply explained by substantially
continuous dyke intrusion along the crests of the oceanic rises. The profiles, dated by
matching them against dated magnetic reversals, indicate that at many places relative
movement has been substantially continuous for the last 10 m.y. The geometry used for
interpretation has application to the land as well as to the sea floor. All that is assumed is
that differential horizontal movement at any particular time is much greater along narrow
belts than elsewhere. The areas between the belts are thus considered to be essentially
rigid, and because the earth is round the relative movement across any belt is a rotation
about an axis. For any area that is small relative to the earth, the geometry is simply that
of sliding one piece of paper relative to another. Geologists concerned with major strike-
slip fault zones have done this for many years, but speaking for myself, doing it in a flat
earth way, and without appreciating that rotation as well as translation is possible.
The Eulerian geometry although complete in itself takes account only of horizontal
movement, and vertical movement - that of most general geological interest - is
disregarded. Rates of spreading range up to 50 mm/year and are thus more rapid than any
horizontal displacement recorded so far on land.
TILTANDYAW
Tilt is used for a change in slope and yaw for a change in azimuth. The two are analogous, the first being the change in a vertical, and the second the change in a
horizontal angle.‘In order to simplify comparisons both are given in radians, the unit used
being lo-’ rad, that is a movement of one millimetre in 1,000 kilometres.
Tilt
Of the kinds of observations made in recent crustal movement studies, tilt is the one
most easily observed. It is the same as strata1 dip, and like dip has a value and a direction.
In any other direction there is a tilt component that is less than the full tilt. For
determining tilt all that is required is the difference in the amount of uplift with distance,
the true amount of uplift being irrelevant. Tilt is thus known at many places where uplift
is uncertain, tilted strata being the best known example.
If the uplift (or the difference in uplift) that has taken place in any particular area for
any particular epoch of time is shown by isobases, that is by lines of equal uplift, then the
tilt can be inferred from the horizontal distance between the isobases, and the tilt
direction by an arrow normal to the isobases in the direction of decreasing uplift.
The accuracy of any particular tilt rate depends on the measurement of a time interval
and the measurement of an angle. As with other recent crustal movement rate
measurements. errors in the rate of tilting depend largely on the accuracy of the angle for geodetic measurements, and on the accuracy of the time int%rval for non-geodetic measurem~~~t~. The g%od%tic method of d%ter~~ning tilt is by making a level survey and then re~%ve~~~ng after an a~~~~r~ate time interv& As afready ~entjoned, the accuracy of level surveys has increased with time, consequently the most important accurrxcy is that of the first survey. Bendefy (1965) has mad% estimates of the accuracies of early level surveys. On the other hand, if the rate of tilt is considered constant and all level surveys were equally accurate, then the accuracy of tilt rate determinations would depend directly on the time between surveys. Two factors have thus to be considered: in~~%as~ng a~cur3cy with time; and time between surv%ys. Because of the way in w&h random errors partly balance out, the errors of a lev%lling survey are proportional to the square root of the distance and not directly to the distance itself. Consequently the greater the distance the less is the errors in the tilt rat%, and a particular distance has to be assumed in order to make ~o~~~so~s. ffthe distance is taken as 1,000 km, and th% time inferva~ betwe%n surveys 30 Y%ars, then the rn~~~~~ accuracy to which the averag% annual tilt rate is likely to have been determined in units of IO-’ rad is about 3,000 for 1850,200 for 1900, and 2 for 1970.
In discussing the results of geodetic height resurveys a distinction has to be made between those surveys that indicate changes in the same sense as those indicated by geomorp~~ observa~~o~s~ and those that indicate changes for which there is no geomorphic evidence. K’he first are of two kinds ~~ isostatic, and tectonic. fsostatic changes have a periodicity of some IO4 year, are consequently not sufficiently long continued to form mountains, but are shown geomorphically by uplifted and tilted marine shore line features. The classic geodetic level survey in such a region is that of Finland (~~r~~~~~~ 1963), where the resarh are in a~bsta~~a~ agreement, but more detailed, th3n those obtiliaed from the shore-line features. Geodetic $eveI surveys in the USA, (Meade, 1971) and Canada (Innes and Weston, 1965), are also in substantial agreement with geomorphically determined isostatic uplift of northern North America.
Tectonic changes are defined here as those sufficiently long continued to form mountains or ~orr%a~n~~~ negative features, and probably have a ~er~o~c~ty of I@-- fOS year. Geodetic ~on~r~at~on for present uplift of the margin of the Europ%an Alps comes from France (Scott, 1932), Italy, and Switzerland, with tilt rates of about 17. lO_’ rad/year. Downward tilting towards the Gulf of Mexico indicated by geology and geomorphology is confirmed from the level surveys for 300 km to the west of New Orleans, which indicate a southerly tilt with a rate of from 40.10-’ radiyear (deader 1971).
Most geodetic relevelling differences ar% of the second kind and do not corrtjspond to any known geomorphological changes. They are consequently suspect to geologists unless they greatly exceed the probable errors of levelhng. Uplift and tilt values that at% considerably greater than expected errors have been reported from the U.S.S.R,, from Canada, and from the U.S.A. The U.S.A. example (I&&%, 1973) is pe&aps the best defined* The area of uplift is ~p~Tox~rnate1~ circular as defined by four level lines with a
RECENTCRLiSTAtMUVENENTS 385
radius of about 300 km, a centre near Atlanta, Georgia, and a tilt rate of about 20. 10e9
rad/year, The interval between surveys is about 40 years and the errors are about an order
of magnitude less than the inferred tilt rate.
ft seems probable that the tilt is real, and it is thus probable that such movements are
taking place in many parts of the world at present, and have taken place in the past. Their
periodicity as inferred from lack of geomorphic expression cannot be greater than about
lo4 year and as inferred from the surveys themselves is probably greater than 10’ year.
The amplitude of movement is thus probably of the order of a few metres. Movements of
this kind are not inconsistent with stratigraphic evidence from stable regions. A feature
that is generally accepted without comment, is that instead of strata having been
deposited slowly and evenly, deposition was in pulses, most time being represented by the
diastems between the strata. Widespread pulsations with a wavelength of several hundred
kiiometres, a period of about IO3 year, and an amplitude of a metre or so would possibly
explain the rhythmic sedimentation provided it was superposed on a long term downward
movement. Such n~oveme~ts are however merely noise when considered in terms of
tectonic and isostatic movements and if widespread, will make it possible to infer long
term trends from geodetic observations.
Tiltmeters have an accuracy of 10e9 rad/year or better, provide a continuous record,
and would thus appear to be the ideal instruments for recording tilt. A single instrument
records the tilting of a single point or a small area. Local stability problems are thus all
important, and except for the refativety large and rapid movements associated with
volcanic eruptions the results from t&meters have been disappointing. A foundation of
solid rock, such as granite, would at first thought appear to be the best. However “solid
rock” is no more monolithic than is a concrete dam, but is separated into blocks by
joints, and each bIock may have its own tilt rate and tilt direction (King, 1971).
It is thus possible that sites on soft unjointed rocks wilt provide better Long term
regional rates than will sites on hard jointed rocks. At the moment t&meters are probably
most useful for recording the relatively rapid earth movements that precede some
earthquakes and for those on active volcanoes.
Most active volcanoes are symmetrical topographic features that are subject to more or
less periodic eruptions, periodicity ranging from a few years for some small andesite
volcanoes such as White Island, New Zealand, to a thousand ar more years for large
rhyolitic ash eruptions such as those from Lake Taupo, New Zealand. For most volcanoes
knowledge is limited to their shape, to the chemical composition of their ash, and to the
times when they erupted. For some, the seismic noise they make between, before, and
during eruptions is also known. Crustal movement studies have added important new
386 k&w. WELLMAN
information. Most volcanoes rise so steeply towards their vents that normal levelling methods are impracticable. However being symmetrical features a few tiltmeters and a few distance measurements can provide useful information on the rate a volcano swells before and subsides during an eruption, and on the volume of the swelling. The best known work of this kind is from Kilauea, Hawaii (Fiske and Kinoshita, 1969).
Normal levelling methods have been used on White Island, which has an unusually accessible and flat crater floor. Level surveys have been repeated every three months or so for the last four years, during which time there have been two small eruptions. Tilting has been rapid with rates of up to 106 . 10m9 radlyear, a waveiength of 100 m, and an amplitude of some hundreds of mihimetres. The pattern of uplift is complex, without any simple pattern of upward or downward doming (Clark, 1971).
lXt from mareugraphs
As already mentioned the annual means of mareograph records are subject to large irregular fluctuations because of changes in atmospheric and tidal conditions. On many coasts the fluctuations are much the same even if the stations are many hundreds of kilometres apart. Consequently most of the fluctuations are eliminated by taking differences, one station being assumed fixed and the tilt or rather the tilt component relative to that station determined. Needtess to say, even better results can be obtained by taking the atmospheric variations into account. The degree of accuracy to which the rate of tilt can be determined under favourable conditions is shown by the work of Jakubovsky (1966) for the Baltic Sea. Reliable records extend back for 60 years for several stations. The changes are linear. Annual differences are accurate to about 30 mm, and the distance between stations about 300 km. The accuracy of the average annual tilt component is thus about 2. 10F9 rad/year, which compares favourably with the accuracy of geodetic level surveys. The results are, however, limited to the actual stations and are consequently most useful where the distance between the stations is small relative to the wavelength of uplift, as is the case in the Baltic Sea.
Lakes although useless for deterring uplift are as useful as the sea for tilt determinations. The best known examples are the great lakes of North America and the lakes of Finland. The changes for Lake Kallavesi in Finland (Siren, 1963) are approximately linear, annual differences are accurate to about 20 mm, the distances between the stations are about 50 km, and the records extend back for about 50 years. The accuracy of the average annual tilt component is thus about 8. 10W9 rad/year. For comparison the annual tilt rate for Finland is about 15 s 10P9 rad/year.
Tilt determined by continuous seismic reflections
Knowledge of Quaternary surfaces is no longer confined to elevated features and to those exposed in bore holes and shadow excavations, but is being extended seaward by the continuous seismic reflection technique. A vast new area is thus being opened up for
RECENTCRUSTALMOVEMENTS 387
study. As for most Quaternary surfaces the basic problem is that of dating. A dating
method that has been used is indirect, and similar to that used for dating the interglacial
shore line feature, but instead of using the high sea levels of the interglacials, the low
levels of the glacial periods are used. If the successive low sea levels were at a progressively
higher level, and sedimentation adequate to build up to wave base, and no tilting is taking
place, then a series of parallel unconformities would mark the low sea levels. Divergences
from parallelism of the unconformities are thus interpreted as being due to tilting, and
provided the ages of the low sea levels are known, the tilt rate can be estimated. Where
isobases are oblique to the coast results can be checked by comparing them with those
determined from the shore line features. For the south eastern part of the North lsland
of New Zealand it is now known (Lewis, 1971) that growing folds are not restricted to
the land area but extend seaward probably to the Hikurangi trench, the main difference
between the land and the sea being that for most of the land uplift is positive, and for
most of the sea negative, the zero uplift isobase being not far out from the coast at most
places.
Tilt determined from shore-line features
The optimum time interval is 30-50 years for levelling and for mareographs, earlier
observation in general being either non-existent or too inaccurate to be useful.
The next most generally useful time interval, that back to the highest Holocene shore
line, is much greater and some 6.5 thousand years. The required height-difference
accuracy to determine tilt rates if the rate is considered to have been uniform with time is
correspondingly less. The shore-line method has been used to determine tilt rates in
Alaska (Plafker and Rubin, 1967), Japan (Sugimura, 1967) North America and New
Zealand (Wellman, 1967) and is particularly useful where the uplift rate is greater than
about 0.5 mm/year.
The highest Holocene shore line is known under the name of “upper marine limit” in
regions of isostatic uplift. But in such regions there was an extremely large range in Early
Holocene uplift rates and a corresponding large range in its age. In addition, isostatic
uplift rates are not uniform with time but decrease exponentially with a half life of some
2,000 years. Average rates, except for intervals of a few hundred years, thus have little
meaning.
Strata1 tilt
If the age of all geological strata were known with the same degree of proportional
accuracy, and if the rate of tilt be assumed to have been uniform, then the accuracy to
which the annual rate of tilt can be determined would increase with increasing age of
strata. However annual tilt rates have not been uniform, and the tilted strata with the
most relevance for studies of recent crustal movements are those less than 20 m.y. old.
Unfortunately such strata lie within the dating gap mentioned earlier, and, except for the
388 fi.W. wk:LLMA’v
older, proportional age accuracy is poor. Dating by magnetic reversals is likely to improve the situation within the next few years, and it will then be possible in some areas to determine changes in the rate of tilt with time, and to get a much better idea of what actually happens during an “orogeny”.
Yaw
Yaw is used for change in azimuth with time. It is analogous to tilt except that it refers to the change in horizontal position and not that of altitude with distance. The essential requirement is that the azimuth of a line should be known at some time in the past and
should be re-observed at some later date. Information is theoretically possible from those special points on triangulation surveys where azimuth was observed, from astronomical observatories, from structures like the Py@ds of Egypt that were orientated as carefully as possible at the time, and from paleomagnetics. The differences in spreading rates along the axes of sea-floor spreading gives information on the differences in yaw rates for two adjoining plates. For points less than 60” from the poles of relative rotation for the two plates the difference in yaw rates is greater than half the relative rotation rate at the poles of rotation. Thus for many places yaw rates are of the same order of magnitude as relative rotation rates.
The differences reported from geodetic stations and observatories are less than the errors of observation which if assumed to be about two seconds of arc about 100 years ago indicates a rate of less than 100. lo-’ rad/year. According to Petrie the average azimuths for the four sides of the Great Pyramid of Egypt are 2.5,2.0,5.5 and 2.5 minutes of arc less than the four cardinal directions. If the differences are assumed to indicate the errors of observation at any time, then knowing that the Great Pyramid is about 4.5 thousand years old, then the rate of yaw in radians is not greater than 25. 10T9 rad/year. The rates inferred from sea-floor spreading are up to .!~0.10-~ rad/year and are thus slightly less than the best that have been determined on land.
Paleomgnetic yaw and latitude drift
Yaw and latitude drift can be inferred from rock paleomagnetism by making the following assumptions: (1) that the average position of the earth’s magnetic dipole over any particular sufficient interval of time in the past was always that of the pole of rotation; (2) that the paleomagnetic directions are actually those at the time when the rock cooled or was deposited; (3) that the shape, but not necessarily the size, of the earth’s magnetic field was always much the same as it is now; and (4) that the direction and amount that the rock samples have been tilted is known.
Paleomagnetic results are usually shown for convenience by plotting the inferred position of the north or south magnetic pole on a map, thus giving the impression that a pole was actually at a particular place at a particular time, but without taking into account the amount the continents have moved relative to each other since that time.
RECENTCRUSTALMOVEMENTS 389
Results are more realistically represented by a paleolatitude line through the sample site,
the angle between the line and an east-west line being the yaw, and the latitude
difference the drift in latitude. The drift in longitude cannot be determined.
Results are rarely accurate to better than IO-’ rad and are thus of little direct use in
studies of recent crustal movements. However, for rocks older than 10 million years or so
they are invaluable for showing the change in yaw and latitude with time.
CHANGEINSHAPEANDSIZEOFSURFACES
In order to obtain a general solution to problems of crustal deformation it is required
to know the changes in the size and shape of three mutually perpendicular surfaces over
any particular interval of time. In practice far less information is obtainable, the only
surface for which changes can be readily measured being that on which horizontal survey
points are situated, and which lie close to the earth’s surface. Even for this surface and
under the best conditions the information is extremely limited.
Two kinds of horizontal surveys are possible: (1) those in which angles are measured;
and (2) those in which distances are measured. Surveys in which distances are measured
have only been accurate enough to provide information on recent crustal movements for
the last ten years or less, and those in which angles have been measured and then re-
observed are thus at the moment the most important and will be considered first.
Accuracy in angular measurement has increased with time at a rate which makes the
optimum time of the first survey about thirty years ago. As with all geodetic surveys the
major problem is to decide if the observed differences are due to crustal movements or to
errors of observations. Two kinds of analysis are possible. On the one hand certain points
can be considered fixed and movement of other points can be given in terms of them. On
the other hand no points need to be considered fixed. instead the degree of distortion of
each geodetic triangle can be considered separately. Both methods have advantages and
disadvantages.
The fixed point method is by necessity somewhat arbitrary, and the points assumed
fixed by geodesists may not be the ones preferred fixed by geologists. If wrong points are
used, adjustments will swallow up tectonic movements. Two points, that is a line, are all
that are usually considered fixed. The triangulation is then adjusted accordingly and the
calculated co-ordinate differences plotted as vectors. Provided that no fault displacements
have taken place during the time interval between surveys, homogeneous strain is the
simplest expected movement. For the homogeneous situation the vectors will be parallel,
will progressively increase in length away from the fixed line, will reverse in direction on
crossing it, and can have any direction relative to the fixed line. Examples have been
shown for Alaska and California by Burford (1966). In choosing fixed points it is
generally assumed that no length change has taken place on lines parallel to a major active
fault. The fixed lines chosen are thus generally parallel and some distance back from a
major fault. Provided the fixed length assumption is true, and the azimuth of the fixed
line has not changed relative to other parallel lines further back from the fault, then the
fixed point method has the advantage of giving the direction of relative horizontal
displacement, a direction having importance in that it can be compared directly with that
inferred from sea-floor spreading data.
In the other method, strain is considered separately for each geodetic triangle, and is
conveniently shown by a line in the direction of greatest relative shortening and by a
value showing the amount a right angle facing in the line direction would have changed
during the survey time interval. No adjustments are required other than making the
angular differences in each triangle sum to zero. Knowing the angular errors from the zero
differences, it is relatively easy to estimate the tectonic significance of the angular
differences from the degree of consistency in the directions and values for the Iines in
each triangle. The main disadvantage of the method is that it does not give the relative
horizontal displacement as directly as does the fixed point method with ideal fixed
points. From the consistency shown by analysing the differences, the accuracy of angular
measurement now appears to be about 1 O-’ which is considerably less than for level
surveys. Accuracy is now limited by meteorological Factors.
So far only angular difference surveys have been considered. The introduction of
electromagnetic distance measuring instruments with an accuracy of lo-’ to 10m6 that
came into general use about 10 years ago has eliminated one degree of uncertainty from
horizontal geodetic surveys. In a few years, provided that the first surveys are made now,
it will be possible to determine the change in surface area and the true instead of the
relative arnount of shortening.
CONCLUSIONS
Crustal lnoveme~lts are poiycyclic, with ~rio~cities that range from 10-s to 10’ year,
some sixteen orders of magnitude. The range for recent crustal movement studies - 10e3
to lo7 year - is smaller but still so enormous as to dominate the division of what is
actually a single subject and make it necessary to discuss different periodicities separately.
The short time period from IO-” to 10’ year is the part important for earthquake
forecasting. Progress is slow but appreciable. Limited success wiIl raise problems that are
almost as alarming as those solved. Movements within the lo3 year range appear to have
been established by geodetic relevelling in many regions that have long-term stability and
represent a phenomenon previously unsuspected. Raised beach ridges and historical records
suggest that for any particular region, at least in some parts of the world, sudden
movements and major earthquakes have about the same lo3 year periodicity, and the two
kinds of movement may well be related. The IO4 year period, as represented by isostatic
uplift, is the only part where geodetic, mareograph, and geomorphic studies are in good
agreement, the main reason being the wide extent, the high past rate, and the continuity
of isostatic movement. The accuracy of geodetic levelling has been established in one such region, and isostatically rising regions would thus appear to be ideal for testing the
usefulness of other methods such as tiltmeters and gravity meters, for measuring tilt and
uptift. It would be particularly interesting to attempt to increase the accuracy of the lake
RECENT CRUSTAL MOVEMENTS 391
mareographs, so as to determine if tilting is actually uniform for periods of a few years,
and so encourage the use of lake mareographs in other regions.
A period of about lo6 year directly related to mountain building has been recognised
from New Zealand and Japan by comparing tilt rates determined from geomorphic
features with the tilt of strata. Dating difficulties, the limiting factors at the moment, are
likely to be partly resolved in the near future by new methods, and much information will
then be available from regions with elevated marine features and tilted Late Cenozoic strata.
The main advances have been in the oceans where spreading rates are now sufficiently
well known to provide a model for relative horizontal displacements covering the whole
world for at least the last 10 m.y. The major problem now is to find the critical places on
land where the inferred movements can be proved or disproved by geodetic, geomorphic,
and geological studies.
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