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Contemporary movements and tectonics on Canada's west coast: A discussion
Tectcmophysies, 86 ( 1982) 3 19-34 Elsevier Scientific
Publishing
1
319
Company,
Amsterdam-Printed
CONTEMPORARY MOVEMENTS COAST: A DISCUSSION*
ROBIN
in The Netherlands
AND TEXTONICS
ON CANADA’S
WEST
P. RIDDIHOUGH
Pacific Geoscience Cenrre, Earth Physics Branch, Departmeni of Energy, Mines and Resources, P.O. Box 6000, Sidney, B.C. V8L 4B.2 (Canada) (Received
18, 198 I ; revised version accepted
October
January
26, 1982)
ABSTRACT
Riddihough,
R.P., 1982. Contemporary
movements
and tectonics
on Canada’s
west coast:
a discussion.
Teclonophysics, 86: 319-341. Evidence
from published
there is a consistent inner coast (l-2
and Victoria.
north
by considerable
Although pattern
Vancouver
topography
margin.
is essentially
that the vertical
on a scale of hundreds
of thousands
indicates
and subsidence Hecate
Strait,
that
on the Georgia
State but is interrupted
to the
Alaska. vertical movements
contemporary
seen opposite
Comparison similar
tectonic
the Queen
with movements
rates are much
that discontinuous
in the region over the last 10,000
movements
observed
too great
are not compatible
with the
in origin. There is, however,
Charlotte
to that seen in association
movement
may be soluble by assuming
runs through
into Washington
from this source and are most probably the movements
data in British Columbia
or “hinge-line”
southwards
recovery has dominated
Island convergent
The paradox
continues
and trend of the observed
between
show that the pattern
relevelling
uplift on the outer coast (2 mm,+)
uplift in southeastern
glacio-isostatic
to be expected
and geodetic
The zero uplift contour
This pattern
the distribution
clear distinction
tidal records
of contemporary
mm/yr).
Strait
years,
pattern
transform
margin
no
and the
at other active plate margins
with subduction to explain
and convergence.
observed
lateral shifts of the movement
geology pattern
and occur
of years.
The geological record is characterized by continual reminders that vertical movements are a fundamental part of the geological process. The discoveries that marine organisms can now be observed as fossils many thousands of metres above sea-level provoked some of the fiercest debates in the history of earth science. The advent of plate tectonic theory in the last two decades has given a new impetus to investigations of the details of such movements. If contemporary plate tectonic processes are
* Contribution
from the Earth Physics Branch
0040-1951/82/oooO-O/SO2.75
No. 977
0 1982 Elsevier Scientific
Publishing
Company
indeed
represenrative
oi’ the processes
in home way they must produce that are observed In recent movements
that have occurred
in the geologic past. then
both the scale and style of the vertical
years,
a number
of studies
have
in regions near plate boundaries.
been
Among
initiated
ipetit _
map and
plate boundaries
of Canada’s
i i-
west coast
to determine
the most well studied
Plate
Fig. 1. Location
rnovrments
in the record.
-.
these
areas are
321
California, Alaska, Japan and New Zealand, areas where plate boundaries are exposed at the surface or where seismicity and presumed convergence rates are high. From inte~retation of the Juan de Fuca ridge system (Vine and Wilson, 1963, through establishment of a plate tectonic framework (Atwater, 1970; Chase et al., 1975), to its detailed application (Riddihough and Hyndman, 1976; Milne et al., 1978), the western Canadian continental margin is established as representing two styles of contemporary plate tectonic interaction (for a recent review, see Keen and Hyndman, 1979). North of Queen Charlotte Sound there is dominant transform interaction between the Pacific and America plates; south of this point, interaction along the margin is principally convergence and subduction between the oceanic Juan de Fuca plate and the continental America plate (Fig. 1). From both geology (Tiffin et al., 1972) and plate geometry (Riddihough, 1977) the triple junction which marks the boundary between these two regimes has remained near northern Vancouver Island for the last 10 Ma. Calculations of present subduction rates along the convergent margin vary between 4 and 2 cm/yr (Riddihough, 1977); transform rates along the Queen Charlotte margin are between 5 and 6cm/yr (Minster and Jordan, 1978). If this plate tectonic pattern is valid and can be applied now and at least over the last few thousand years, it seems reasonable to expect that the regional pattern of contemporary vertical movements should relate to it in some coherent manner. To examine this expectation, the present paper reviews the observed pattern of vertical movements, discusses the rate. of glacio-isostatic rebound in the region, examines vertical movements observed in similar tectonic environments elsewhere and finally speculates on the relation between these movements and the observations of the geological record. PRESENT-DAY VERTICAL MOVEMENTS
Sea-level and tidal records
Tidal stations on the west coast have been operating regularly since approximately 1920 (Wigen and Stephenson, 1980) (for locations see Fig. 2 and Table I). De Jong and Siebenhuener (1972) used both linear and quadratic regression on annual means from seven stations in an attempt to determine trends of secular change. They concluded that land movement existed and that there was uplift at Victoria and subsidence at Vancouver, Point Atkinson and Prince Rupert which was probably tectonic in origin. Vanicek and Nagy (1980), as part of their broad study of vertical movements in Canada, calculated linear trends from monthly mean values of sixteen west coast tidal stations. Wigen and Stephenson (1980) argued that simple linear regression through values at different time periods, unless data sets are long, can give very different trends. They proposed that differences between stations in similar hydrodynamic regimes
TABLE Linear
I sea-level
use monthly errors
trends
for II corrected
to nearest mm/yr, see original
for west coast stations
means, Wigen and Stephenson from published
in mm/yr
with standard
I are from annual
values: Stephenson.
means.
per>. comm..
error.
Vanicek
and Nagy
pairs. (Standard
1981). Plotted
values (Fig. 3) are
and selected as having the smallest SE less than 1.5. For details of analysis
references.
Station
Station
locations
and
Nagy (1980)
Wigen and Stephenson
Wigen and I
(1980) I.
Alert Bay Bamfield
-- 1.1*
0.7
Inlet
3.
Betla Bella
4.
Campbell
-0.1
s.
Fulford
h.
Little River
Harbour
0.15:? 1.52
2.8
.~ 3.57 2 1.22
e 0.8
-~ 0.15 -t 0.65
0
-6.46
7.
New Westminster
x.
Point Atkinson
Y.
Port Alberni
-- 1.9% 0.8
10.
Port Hardy
-2.5-r
II.
Prince Rupert
12.
Queen Charlotte
13. 14.
Plotted II
- 1.95 .t 0.64
2. 2.0
-3.12
Stephenson
Value
(1980)
2.93 c 2.81
River
periods etc..
are shown in Fig. 2.
Vanicek
,7.
values
II from difference
r-2.15
-2 2.36 * 1.79 2.32 +-0.73 -0.65-‘-
1.10
i-2 -1
0.25 t-o.44 ~ 2.42 i I .0X
0 -2
-68125.4 i.2+
0.2 2.7
0.12.-0.48
0.46 + 0.44
1 -2
-4.75+
1.48
~ 1.44t0.84
-1
- 1.89:~ 1.48
-1.19*1.09
-I
- 2.96 CL2.22
-0.57?0.61
-1
0.75
0.2
1.25i- 0.43
- 1.9%
1.4
Sooke
-1.82
2.1
Steveston
-2.11:
5.9
IS.
Tofino
- 1.6’
0.3
16.
Tswassen
2.8%
I 7.
Vancouver
0.1 _” 0.2
0.64 2 0.42
0
IN.
Victoria
0.6’
0.27 kO.40
0
City
- 1.25 to.47
fl
-2
3.4 0.2
would give more reliable indications of crustal movement and demonstrated that the differencing technique involved significantly less variability than the analysis of single station records. The results of both Wigen and Stephenson (1980) and Vanicek and Nagy (1980) are shown in Table I. Although differencing clearly reduces the “variability” (as used by Wigen and Stephenson, 1980) of the data set, trend values from this technique do not always have a significantly lower standard error than trends from long period single station analysis. The linear trends with the lowest standard errors, eliminating any stations with standard error > 1.5 mm/yr, are listed to the nearest mm/yr in column 4 of Table I and plotted on Fig. 3. While this selection represents a gross simplification of the data, it does not misrepresent them and shows that in general terms the inner coast is subsiding relative to mean sea-level, Vancouver Island and the outer coast is rising, and that zero level runs approximately through Victoria and the Strait of Georgia.
323
Fig. 2. Locations
Fig. 3. Vertical Alaska
of tidal stations
movements
listed in Table I
of land relative
from Hicks and Shofnos
to sea-level in mm/yr
(I 965). in Washington
from tidal stations
from Ando and Balms
(I 979).
of Table I. Data in
324
Levelling Relevelling in a study
on Vancouver
Island
of the 1946, magnitude
was examined 7.3 earthquake.
uplift in the centre of the island (l-2 and
1946) followed
by uplift
mm/yr).
(c. 1 mm/yr)
( 1978)
by Rogers and Hasegawa They
suggested
and subsidence on the east coast,
that apparent
(10 cm between south
1930
of Campbell
River (4 in Fig. 2) were probably significant. However, modelling of the coseismic deformation of their preferred earthquake mechanism did not match this deformation. Another solution for the same event based on re-triangulation surveys (Slawson and Savage, 1979) may be compatible with the relevelling data (H. Dragert. pers. commun., 198 1). Levelling data in the Fraser Valley around and east of Vancouver (I 7 in Fig. 2) were examined by Mathews et al., (1970). They observed that all bench marks had apparently moved downward son, 8 in Fig. 2). Discounting alluvium
or aquifer
mm/yr. Levelling
“‘
withdrawal.
data were included
they concluded
order
surface
of vertical
that depression
in the work of Vanicek
rates were up to 1.5
and Nagy (1980.
1981) in
,,,..... :...i
“‘Uyy _,-...i
Fig. 4. Fourth Tidal stations
relative to the bench mark at Caulfield (Point Atkinthose that were clearly connected with compaction of
movement
used are shown as dots, segments
velocity
of relevelling
in mm/yr
from Vanicek
shown as dotted
lines.
and Nagy
(1980).
325
which they used a combination of tidal data and sections of relevelled geodetic data to construct a map of vertical velocity. Their fourth-order surface for British Columbia (their zone 2) is shown in Fig. 4 together with the relevelled segments and the tide gauges that they used. The standard deviation for most of this surface except near the Queen Charlotte Islands is OS-l.5 mm/yr. In the latter area it varies from 1 to 4 mm/yr. The features shown in the tidal data of Fig. 3 are reproduced with uplift along the margin, zero level near the inner coast and subsidence over the Coast Mountains, Two other areas of uplift occur inland near 55” N, 125” W and 52” N, 118OW. In commenting on the validity of a 4th order surface through the data, Vanicek and Nagy (1981) suggest that some of the “sagging and peaking” merely reflects holes in the data. While this may be true for the eastern and southern edges of their zone 2 map, the qualitative correspondence of the surface with the tidal data (i.e. Fig. 3) seems reassuring. Comparison
with Washington
State and Alaska
Ando and Balazs (1979) reviewed the available vertical movement data from tidal data and relevelling lines in the northern part of Was~ngton (Holdahl and Hardy, 1979). Their contours are included in Fig. 3 and show a pattern which is continuous with that in southern British Columbia. Zero change contours match closely and an area of rising land to the west corresponds with the Olympic Mountains and the Coast Range. To the east, Puget Sound is sinking at up to 1 mm/yr. The same pattern, in terms of eastwards tilt, has been commented upon by Adams and Reilinger (1980) as extending southwards along the Oregon Coast Range almost to the California border. Work on marine terraces (Adams et al., 1980; Adams, pers. commun. 1981) allows vectors of these tilts to be estimated and shows that they are in general perpendicular to the coastline. In Alaska, studies from tide gauge data by Hicks (1972, 1978) and Hicks and Shofnos (1965) show that land movement is predominantly upward. Although some very short period tidal observations were used to construct a network of stations, longer periods of observations at Sitka, Ketchikan, Juneau and Skagway provide a foundation for their contoured map of vertical uplift included in Fig. 3. Values at these stations are + 3 mm/yr, 0 mm, + 13 mm/yr and + 17 mm/yr respectively; values at their temporary stations reach an estimated + 39 mm/yr near Glacier Bay. The regular pattern of landward tilting seen along the British Columbia margin may be continuous as far north as 56” N but seems to come to an abrupt change near that point. The extreme values seen north of this may be related either to tectonics or glacial uplift and are discussed in a later section. GLACIO-ISOSTATIC
EFFECTS
A realistic assessment of any pattern of recent vertical movements in Canada is impossible without an examination of the role of glacio-isostatic rebound (for a
broad
review of the problem
rebound
the most vaiuable Cathles,
sources of information
1980). Modelling
ters including viscosity)
earth
structure
and the magnitude.
are available
see Lambert
asthenosphere. and timing
by Walcott
Canadian
1979). This is one of
(e.g. Walcott,
has to take into account
extent
as stressed
(Fennoscandia,
Vanicek,
of glacial loading
for the earth’s rheology
(lithosphere, spatial
and
to the removal
of the response
glacial load. Unfortunately, data
in Canada,
of the Earth’s surface in response
values
1980:
several paramefor rigidity
of the applied
and
and removed
(1980), even in areas where good
Shield),
there
is no agreement
as to
which earth model provides the most realistic response. Attempts to predict or assess responses in other areas from a presumed glacial history are consequently very uncertain. Nevertheless, some general qualitative conclusions seem to be valid. Surrounding Walcott, collapses
the main
area of ice loading
1970). As the load is removed in place or migrates towards
is a region
of peripheral
“bulge”
(e.g.
and the crust beneath it rises, this “forebulge” the retreating load. The combination of these
processes and the global effects of the redistribution of water and loading on the geoid produce a series of zones (Clark et al., 1978) concentric to the main ice sheets in which particular The general estimated
patterns
distribution
(e.g. Prest,
of land and sea-level movements
seem to occur.
and timing of the North American
1974; Peltier and Andrews,
ice complex
1976) with a maximum
has been extent
and
thickness about 18,000 yrs before present (ka B.P.). The major effects in southwestern British Columbia of its removal (complete for all but residual ice caps by 6 ka B.P.) are suggested to be those of Zone I, Zone II or the transition between them (Clark et al , 1978, figs. 5 and 15). Zone I is characterized by emergence, Zone II by continual submergence because of the collapsing forebulge and the I/II transition zone near the ice margin
by a more complex
followed by submergence. Crustal response in southwestern
British
by the fact that the region was affected the Cordilleran
response Columbia
the I8 ka B.P. “Laurentide”
initial
may be further
by what is regarded
ice Sheet, part of which advanced
(15 ka B.P.) than
involving
as a separate
emergence complicated ice system,
over the region substantially
later
peak. This ice sheet reached
as far
south as Olympia (47’N) in Puget Sound (Thorson, 1980) and across Vancouver Island to its western coast (Fig. 5) (Prest, 1974; Clague et al., 1980, 1981). Retreat was rapid as the Vancouver
area was again clear by I3 ka B.P. Vertical
movements
along the British Columbia margin may thus be a compound effect of long wavelength responses to the retreat of the major North American ice complex plus the local effects of the removal of the Cordilleran ice sheet. The timing of the crustal response and the length of time for total relaxation is another variable about which there is still apparent disagreement. Walcott (1980) noted that depending upon the earth model used, the relaxation time could be proportional or inversely proportional to the dimensions or wavelengths of the depressions produced by loading. In practice, he suggests that relaxation times seem to vary with position with respect to the ice sheet; relaxation times near the edge of
327
the Laurentide
uplift being shorter
with > 5000 yrs). The relaxation
than those near the centre (2000 yrs as compared
time is also clearly related
to the rate of removal
of
the ice load (Clark et al., 1978). All studies
Fig. 5. Glacial
of the British
retreat in western
numbered in thousands tians in southern
British
Columbia
Canada
region
have stressed
from Prest (1974).
Contours
of years before present. Solid areas-present
Columbia see Clague (1981).
that
show
(residual)
relaxation
times
ice margin positions,
ice cover. For modifica-
from tilts and sea-level Clague,
1975: Fulton
al.. 1981). The apparently particularly
curves appear
and Walcott.
to be very rapid (e.g.. Mathews
1975: Andrews
rapid response
rapid local load removal.
and Retherford.
may be a function
of crustal
of the short wavelength
1976). Mathews
et al. (1970) and Clague (1975) concluded
structure.
of the C‘ordilleran
sheet or of the fact that the area was close to the ice margin 5000 yrs were minor and that glacio-isostatic adjustment Nevertheless, as re-stressed by Clague et al. (1981),
et al.. 1970;
197X: (‘Iague et
(Peltier
of a ice
and Andreus,
that changes
in the last
was now virtually complete. it cannot automatically be
assumed that any small vertical movements seen today, or over the last few thousand years, are entirely free of any glacio-isostatic component. It is. however, reasonable to conclude geographical general
that any such component pattern which is either
shape of the Cordilleran
QUATERNARY
SEA-LEVEL
is by now greatly diminished and exhibits a uniform over a broad area or related to the
ice load.
CURVES
Clague (1975) and Clague et al. (1981) have compiled a series of sea-level curves for the last 13,000 yrs measured at various sites along the west coast from Puget Sound to Juneau, Alaska (Figs. 6 and 7). All the curves except those on the outer coast, show a steep emergent trend after deglaciation reaching close to present sea-level between 8 and 11 ka B.P.. Those on the northern inner coast (Prince Rupert, Juneau) may not have dropped significantly below present sea-level, whilst the remainder (Comox-Nanaimo (18 in Fig. 2), Bella Bella/Bella
(E. Vancouver Island), Fraser Coola) did and have been
Lowland, followed
Victoria by slow
submergence. In all these cases, there has been little movement over the last 5000 yrs: no more than 1 mm/yr emergence for Juneau and 0.7 mm/yr submergence in the Fraser Lowland (average rates calculated from the graphs of Fig. 7). By contrast, on the outer coast there has been steady emergence since 4 ka B.P. at Tofino (1 mm/yr) (5 in Fig. 2) and 8 ka B.P. on the Queen Charlotte Islands (2 mm/yr). In the Queen Charlotte Islands this was preceded by submergence; at Tofino, limited data
suggest
that
there
could
have been
(Clague et al., 1981). Compared with the characteristic
emergence
back
curves of the post-glacial
to at least
13 ka B.P.
zones of Clark et al.
(1978), the sites can be characterized as in Fig. 6. In general, the northern inner coast sites correspond with Zone I and the southern inner coast sites with the transition between
Zones I and II. In terms of the glacial history of Fig. 5, this distribution can 1981) to correspond well with the position of these sites be argued (e.g. Clague et al., beneath or close to the margin of the Cordilleran ice sheet. The sites on the outer coast, however, do not correspond to either Zone1 or I/II or to the postglacial response of the next outer zone, Zone II-collapsing forebulge submergence. It seems likely that here at least, there may be a superposition of a tectonic trend on the glacio-isostatic response. The contrast of the Queen Charlotte Islands pattern
329
ZONE II
ZONE III
Fig. 6. Glacio-isostatic
recovery
response
zones from Clark
location
and characterization
from Quatemary
ZONE I/II
sea-level curves: (i) theoretical
et al. (1978); (“) u curves of glacio-isostatic
ZONE
I
curves for glacio-isostatic
for three west coast sites from Clague
response
from comparisons
(1975);
(iii)
of (i) and (ii), see discussion
in text.
+100
I
VICTORIAGULF ISLANDS
2
+50
e fi
0
QUEEN CHARLOTTE
15
10
5
Ka B.i?
Fig. 7. Quaternary
sea-level
curves
tidal trends from Table I. Shaded
from Clague
(I 975) and Clague et al. ( I98 I ) compared * I mm/yr.
cones are tidal trends
to projected
0
and its relation both Andrews Apart inner
forebulge
from very rapid
collapse
curves
relaxation
in other parts
that, by comparison,
is of small amplitude.
or tectonic
movement
was stressed
by
(1978) and Clague et al. (198 I ). times,
coast at least, are thus consistent
deglaciation implies
to either
and Retherford
the Quaternary
with the pattern.
sea-level
curves
of the
style and magnitudes
of the globe. To a first order of magnitude
any continuous.
systematic
tectonic
It also allows that the submergence
of this
trend at these sites
of the last 5000 yrs along
the southern inner coast and the possible steady sea-level or slightly emergent trend along the northern inner coast could be a residual glacio-isostatic response. The question of whether or not this pattern is a glacial response must be addressed by examining
the broader
OBSERVED VERTICAL
picture. MOVEMENTS
AND DEGLACIATION
Although at first sight the parallelism of topography, vertical movement patterns (Figs. 3 and 4) and deglaciation (Fig. 5) along the inner coast provide a confusing background from which to isolate a distinction between glacial response possible tectonic movements, a number of features suggest that the present movements are predominantly tectonic:
and day
(1) If the vertical movement pattern of Fig. 4 is accepted, then the area of thickest late Pleistocene ice corresponds closely with the area of present downward movement. Further, the rate of subsidence seems to increase landwards from the inner coast. In contrast, the curves of Clark et al. (1978) and Fig. 6, suggest that residual isostatic subsidence should decrease towards the position of the main ice load (i.e. from Zone the load.
I/II
to Zone I) and be replaced
by upward movement
(Zone I) beneath
(2) The pattern of eastward-tilting approximately perpendicular to the coast continues at similar amplitudes and wavelength into Washington (Fig. 3) and further southwards (Adams and Reilinger, 1980). This is well outside the main glaciated area and transverse to the principal east-west termination of the overall ice-sheet ( 13- 15 ka B.P. contours in Fig. 5). Fulton and Walcott (1975) noted that the post-glacial tilting
in the Merritt
area (50’ N, 121’ W) was consistently
northwards
towards
the
main ice load. (3) The Puget Sound area of downward movement again coincides closely in position and orientation with the Puget Sound ice lobe detailed by Thorson (1980) where Zone I recovery might more reasonably be expected. (4) As noted in the previous section, Quaternary sea-level movements on the outer coast cannot be matched in standard isostatic response curves and probably include a tectonic component. It seems unlikely, given the distances involved, that such tectonic movements would be entirely restricted to this narrow zone.
331
SUPERIMPOSITION
OF TECTONIC MOVEMENTS AND ISOSTATIC RECOVERY
If present-day vertical movements (from tidal and levelling data) are accepted as predominantly tectonic in origin, it is reasonable to assume that their time scale is considerably
longer
The observed effects
and
complete,
than
the most recent
Quaternary
sea-level
On Fig. 7, the assumed
of glacial
should
therefore
curves
for the last few thousand
should correspond
episode
years,
when
with the projected tectonic
trends
glacial
tectonic
(* 1 mm/yr)
loading
and recovery.
be a sum of the two recovery
was virtually
trends. are superimposed
on the
Quaternary sea-level curves of Clague (1975) and Clague et al. (1981). Excepting for E. Vancouver Island and Juneau, the curves do correspond with the projected tidal trends at least since 5 ka B.P. Removal
of the trends from the curves should result in
“corrected” glacial response curves in accordance with the patterns discussed earlier. It is clear that with the two exceptions noted above, this is true. For the Fraser Lowland,
Bella/Bella
Coola
and Prince
Rupert
the correction
might
remove
the
period of low sea-level characteristic of Zone I/II and put these stations into Zone I. A corrected Queen Charlotte Islands curve would correspond with Zone II as discussed earlier and the curve for Tofino might resemble a Zone I/II response. However, these changes do not substantially contradict the observation that in general the observed curves are close to the expected pattern of glacial recovery and that the tectonic trends are in fact relatively small. The two exceptions of E. Vancouver Island and Juneau are of considerable significance. The curves imply that the present vertical movements observed at these points cannot have been continuous over the last few thousand years and must be a transient deformation pattern. In the case of E. Vancouver Island (Comox-Nanaimo), the proximity have
to the 1946 magnitude
occurred
deformation
in central
Island
(Milne
et al., 1978) suggest
may be part of a seismic cycle. Such a relation
study of geological, ute towards
7.3 event and the other large earthquakes
Vancouver
tidal and geodetic
an understanding
levelling
of the seismicity
information
indicates
which that
the
that a close
may be able to contrib-
in this area.
Clark (1977) calculated that up to 35% of the emergence of the last 50 years in southeast Alaska was attributable to elastic rebound following the rapid retreat of ice in Glacier Bay, an idea first suggested by Hicks and Shofnos the Quaternary sea-level curve for Juneau in Figs. 6 and 7 confirms that this retreat was not a continuation of the main and is thus an isolated short period event. Although historical
(1965). Nevertheless (from Clague, 1975) deglaciation episode seismicity in the area
is low (Milne et al., 1978), Juneau lies close to the Chatham Strait and Denali Fault systems and an origin associated with local strain seems to be equally plausibe.
VERTICAL
MOVEMENTS
Subduction
TECTONIC’
ENVIRONMENTS
and convergence
The pattern in Japan.
IN SIMILAR
of vertical
As discussed
underthrusting post-seismic.
movements by Scholz
in a subduction (1972).
zone has been closely studied
movements
can be related
to seismic.
events in a series of phases: inter-seismic, pre-seismic, co-seismic, An initial depression of the material above the thrust is followed by
elastic rebound during the dislocation of the earthquake The integration of these phases is long-term depression and elevation of the overlying plate.
(Savage and Hastie, of the underthrusting
1966). plate
Thatcher and Rundle (1979) proposed a more complex model than that of Scholz (1972) in which an elastic plate (lithosphere) overlay a visco-elastic half-space (asthenosphere). The underthrust cycle (with continuous aseismic slip at depth) produced similar depression lithosphere thickness inland both
the Shikoku
and uplift, with further depression beginning about one from the fault outcrop. Thatcher and Rundle examined
and South
Kanto
districts
of Japan
where underthrusting
Phillipine plate beneath the Eurasian plate occurs and concluded complications of each area, such a model was satisfactory. Scholz and Kato (1978) also considered
the South
Kanto
of the
that give the local
district
where highly
oblique subduction occurs at an estimated 3 cm/yr along the Sangami trough (Seno, 1978). They concluded that the subduction mechanism was far from simple and occurred on a series of dipping thrust faults; during the interseismic period aseismic slip occurred
below
15 km depth
while strain
accumulated
in the upper.
locked,
portions of the system. Nevertheless, as is shown in Fig. 8, three elevation profiles perpendicular to the subduction zone (Atami to Tokyo, east and west Bozo Peninsula) show integrated elevation changes (including and accumulating the co-, post- and inter-seismic
deformations)
further “inland”. In the Shikoku
area,
which involve deformation
uplift
nearest
is associated
the trench
with
4cm/yr
and subsidence perpendicular
subduction at the Nankai trough (Seno, 1978). Again the same pattern of leading edge uplift with depression further inland is evident (Fig. 8). Thatcher and Rundle (1979) related their model to a thrust fault outcrop 90 km inshore of the trench axis, noting that this was a common feature of other solutions. Ando used the Nankai trough example as their principal comparison
and Balazs (1979) with Washington
State. Another area in Japan, opposite the Japan Trench near 36” N, was reviewed by Abe (1977). Here the coast and the eastern end of the levelling line is nearly 200 km from the trench (convergence rate 10 cm/yr: Minster and Jordan, 1978) and the integrated movement seems to be entirely subsidence (Fig. 8, Pt. Shioya). Kato (1979) showed that a similar pattern occurred in the Tohuku district between 38’ and 40” N (Fig. 8, Tohuku-Sendai, Kamaishi) and concluded that while it was
333
TOHUKU (SENDAl) TOHUKU (KAMAISHI+
I
JAPAN TRENCH
..
:“-c-/ I
PT. SHIOYA
EASTERN ALEUTIANS WASHINGTON STATE
--,
____
I CENTRAL VANCOUVER ISLAND
I I
Okm
I
!
I
I
I
200 100 DISTANCE FROM TRENCH AXIS
I
I
300
Pig. 8. Net vertical velocity curves relative to distance from trench axis for various subduction zones. For Japanese examples, movement was integrated over complete earthquake cycles and divided by time. For Alaska (eastern Aleutians), the curve of Plafker (1970) was divided by his estimated 800-yr return period. Andes (Pert-Chile) is observed movement in metres for the 1960 Chilean earthquake, scale shown on right (Planter, 1972). Sections for central Vancouver island and Washington State are from Fig. 3 and Ando and Balazs ( 1969). Japanese sources are referred in the text.
qualitatively consistent with subduction at the trench (300 km distant) modelling was quantitatively improved by the introduction of localized sinking of the subducted slab. Similar observations of continuing subsidence with co-seismic subsidence during the 1973 event were discussed for eastern Hokkaido (175 km from the Japan trench axis) by Fujita et al. (1975). A characteristic of these areas is that the do~n~t ~mponent of the net deformation is the deformation produced during the co-seismic phase. in South Kanto, for instance (Scholz and Kato, 1978), the maximum co-seismic displacement is ten times that occuring in the inter and post-seismic phases. The principal
difference
in pattern
(e.g., Ando and Balazs, 1979. fig. 3) is that the “inland” closer to the trench than the final integrated area ot
co-seismic depression subsidence. The
similarity
begins
between
long-term
and
Plafker (1972) in his study of the vertical and 1960 Chilean
earthquake.
co-seismic
deformation
movements
He concluded
produced
that despite
was noted
by
by the 1964 Alaskan
some pre-tectonic
submer-
gence, the co-seismic areas of uplift in Alaska corresponded with the areas of net Holocene emergence and that the co-seismic upward movements could therefore be regarded
as a pulse
in a long continuing
trend
Co-seismic deformation profiles from southeast included in Fig. 8 (Plafker, 1972).
of movement
of the same style.
Alaska and southern
Peru- Chile are
The differences between the vertical deformation along the southern Columbia and Washington margin and the deformation at other subduction (Fig. 8) may exist for a number of reasons. to affect the pattern are rate and direction
Amongst the variables which seem likely of convergence, age and thickness of the
converging lithospheres and the dip of the intervening clear that in a general sense the deformation is similar. the co-seismic
or complete
cycle deformation
British zones
thrust plane. However, it is Further, it is similar to either
of other margins
rather
than to any of
the intervening phases. This observation was used by Ando and Balazs (1979) to argue that in the absence of any major thrust earthquakes during the period of tidal and levelling observations, the deformation in Washington is most likely to be that of a complete Transform
cycle occurring
aseismically.
motion
Vertical movements in areas of highly oblique convergence form interaction are apparently much less systematic and Zealand mm/yr (Walcott,
(Lensen, 1975), vertical movements along but diminish eastwards to -0.3 mm within 1978) between
approximately
the
Pacific
and
Indian
or dominantly transpredictable. In New
the Alpine Fault reach + 10 120 km. Motion on this fault plates
is lo”-15’
oblique
at
4 cm/yr.
Along the San Andreas Fault, the Palmdale Bulge was proposed as a large area (100 X 300 km) of uplift which rose up to 300 mm (20 mm/yr) between 1959 and 1974. Since 1974, parts of the area have subsided up to 200 mm and a considerable debate is in progress as to whether or not these measurements are real or at least partially affected by levelling errors and miscalibrations (e.g. Mark et al., 1981; Rundle and McNutt, 1981; Strange, 198 1). Vertical movements near the Sangami Trough (Fig. 8) have been discussed in the previous section. However, interaction along the through between the PhiIlipine and Eurasian plates is highly oblique (< 10’) at 3 cm/yr. As can be seen in Fig. 8, uplift of up to 20 mm/yr falls to negative values within 30 km of the trench axis. Along the northern part of the Queen Charlotte-Fairweather Fault zone, vertical
335
movements from tidal me~urements are pr~ominantly upward and reach very high values (Fig. 3 and earlier section’s Hicks and Shofnos, 1965). Studies of movement on the Fait-weather Fault can be interpreted as showing of the order of 6 mm/yr vertical movement over the last 1000 years in one locality (Page, 1969). Regionally the terrain to the northeast of the fault is estimated (Page, 1969) to have risen an average of 2-3 mm/yr since the early Pleistocene. However, as discussed earlier in connection with movements at Juneau, the role of deglaciation in producing uplift in this area may be difficult to evaluate. The recent vertical movement (1 mm/yr) of the Queen Charlotte Islands is thus compatible with a position close to a transform fault or a highly oblique convergent plate boundary. With the closest additional information at Prince Rupert (120 km distant), it could also be compatible with a normal subduction situation. The fact that the Pacific-America vector of Minster and Jordan (1978) is 15” oblique to the southern Queen Charlotte Fault has led to recent speculation that oblique subduction is in fact occurring along this margin (Hyndman and Ellis, 1981; Perez and Jacob, 1980). The observed vertical movements do not provide any useful distinction. Hot spot
Bevier et al, (1979) suggested that an east-west volcanic belt in southern British Columbia, appro~mately along 52” N, could be the trace of a hot spot fixed in the mantle over which the North American plate has moved westwards during the last 25 Ma. The idea has been further discussed by Rogers (1981) who points out that an area of seismicity occurs beyond the eastern end of this belt (near 52” N, 118’ W) where the hot spot might be expected to occur today. The 3th order surface of Vanicek and Nagy (1980) (Fig. 4) forms a closed area of upward movement (7 mm/yr) at the same position. By comparison, doming of 3-5 mm/yr over an area of 8000 km2 may be associated with the Yellowstone continental “hot spot” (Reilinger et al., 1977). CONCLUSIONS AND DISCUSSION
Contemporary vertical movements along the British Columbia margin are real and can be summarised as upward movement (c. 2 mm/yr) along the outer coast and submergence (l-2 mm/y@ along the inner coast. A similar pattern continues southwards into Was~ngton and Oregon. It seems unlikely that this pattern is due to glacio-isostatic re-adjustments and it conforms to patterns seen elsewhere along a converging or subducting margin. An apparently similar pattern continues northwards beyond the triple junction position on the margin into the area which is expected to be predominantly transform in character. Although this continuation seems to conflict with the contrast in tectonic character, it is not untypical of other
areas of transform or very ohiique subduction. Inland coincide with a recently proposed hot spot position. Thus tectonic
far the pattern environments.
increasingly Firstly,
common similar
of movements However,
paradoxes
is consistent
as in many in levelling
rates of apparent
vertical
others,
an apparent
with those it exhibits
occur in the Maritimes
Nagy. 1981), movements explanation.
(Vanicek,
for which
there
in other.
may
similar,
what are becoming
studies. movement
occur in areas which are not
tectonically active and certainly do not exhibit similar Canada. for example, contemporary vertical movements parently
doming
levels of seismicity. of several mmjyr
1976) and Newfoundland is no simple
tectonic
(Vanicek
In apand
or geophysical
Secondly, the values of vertical movement are far too great to satisfy the integrated geological observations over a long period. For example. up to 6 km thickness of lithified but unmetamo~hosed Eocene-Oligocene (c. 30 Ma B.P.) marine sedimentary rocks outcrop at elevations of up to 1 km along the northern flank of the Olympic Peninsula (MacLeod et al., 1977). Plate convergence at this subduction zone can be shown to have been similar to or greater than that today for at least the last 20 Ma (Atwater
and Molnar.
1973: Riddihough,
1977; Coney,
1978).
If vertical movements had been continuous present elevation could have been achieved
at their present rate (2--3 mm/yr), the in less than 3 Ma. A similar dilemma in
the
by Adams
Oregon
Coast
Range
was
discussed
and
Reilinger
(1980)
who
concluded that tilt rates had been steady for the last 0.25 Ma but that all the tilting observed, even that of the oldest Eocene strata. could have been produced in the last 7 Ma. Two solutions for the paradoxes have been proposed (e.g., Adams and Reilinger, 1980). neither entirely satisfactory. The first is that much levelling data is wrong and obtained with a systematic bias so that even repeat surveys continue to give the same results. This is under active investigation for land surveys (e.g. Brown and Reilinger, 1980; Rundle and McNutt. 1981) and may have some validity. It may be more difficult to apply to tidal data (e.g. Kumar and Soler, 1980). The second solution is that movements are oscillatory, episodic or discontinuous with periods of tens to hundreds of thousands of years. A key objection to this from the present area is that the observed movement pattern is apparently characteristic of all other subduction zones that have been measured. Model studies of such environments (Savage and Hastie, 1966; Thatcher and Rundle, 1979; Scholz and Kate, 1978) fit remarkably well. The coincidence that all subduction zones are simultaneously going through the same deformation mode seems untenable. A variant to the second solution that retains this deformation mode at ail times but ensures that the long term integrated deformation is not impossibly large, seems to be that of moving the deformation laterally. In the case of a subduction zone, shifting of the deformation pattern over a zone 100-200 km wide will suffice to superimpose deformations so that the integrated vertical movements are no greater
337
Fig. 9. Vertical imbricate
movement
patterns
thrust system showing
in amplitude
than
from underthrusting.
broadened
a single
event.
Effect of superimposition
uplift and subsidence
In Fig.
9,
pattern
five episodes
of five events from an
with similar average amplitudes.
of dip-slip
deformation
of
the general shape envisaged by Savage and Hastie (1966) are superimposed with fault outcrops spread over a range of 200 km. The final, integrated, region of “uplift”
is “noisy”
but
has an average
elevation
of less than
the uplift
of an
individual event. The depression is similar or slightly larger. An investigation of Quaternary deformation in Japan by Ota and Yoshikawa (1979) shows that a very similar mechanism has been in effect along the Pacific coast. The hinge lines between co-seismic uplift and co-seismic depression (equivalent to the zero contour in Fig. 3) moved discontinuously landwards during the Pleistocene,
the present
will move landward may. occur at intervals Geological considerable. Nankai Plafker
pattern
another
being set up only 0.5 Ma B.P. They estimate
50 km in the next
1 Ma and suggest
that it
that such moves
of this order.
evidence for past imbricate and multiple The co-seismic fault modelled by Thatcher
thrust and and Rundle
fault zones is (1979) for the
Trough was 90 km inshore of the trench. The Patton Bay Fault discussed by (1972) is 150 km from the axis of the Aleutian trench but was clearly
involved in the 1964 co-seismic deformation. Episodic shifting of transform motion from one fault to another within the fault zone is well documented historically in California and has probably recently occured on part of the Fairweather fault system. As applied to the Juan de Fuca plate subduction zone, aseismic slip (as by Ando and Balazs, 1979) on the contemporary dislocation plane may continuous from 0.25 Ma (Adams and Reilinger, 1980). Previous to principal slip may have occurred on other fault planes. (Such planes
envisaged have been that, the could be
essentially
separate
at depth, as in Fig. 9, but convex upwards
into a low angle imbricate surface. Transference
of the slip from one dislocation
that at any one time (e.g. the present), large but that the integrated than the projected
so that they converge
thrust zone near the trench and are not identifiable plane to another
the vertical
sum over geological
movements time would
at the
would ensure
would
appear
too
he considerably
less
effects of any single episode.
ACKNOWLEDGEMENTS
I am indebted for an extremely sea-level
to Fred Stephenson for discussions on tidal data and John Clague valuable review and current information on west coast Quaternary
curves.
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Related
Can. Inst. Surveying,
to the Redefinition Ottawa,
of North
Ont., pp. 105-124.
American
West Coast. Vertical
In: Proc. 2nd Int. Geodetic
Networks
Publishing
1
319
Company,
Amsterdam-Printed
CONTEMPORARY MOVEMENTS COAST: A DISCUSSION*
ROBIN
in The Netherlands
AND TEXTONICS
ON CANADA’S
WEST
P. RIDDIHOUGH
Pacific Geoscience Cenrre, Earth Physics Branch, Departmeni of Energy, Mines and Resources, P.O. Box 6000, Sidney, B.C. V8L 4B.2 (Canada) (Received
18, 198 I ; revised version accepted
October
January
26, 1982)
ABSTRACT
Riddihough,
R.P., 1982. Contemporary
movements
and tectonics
on Canada’s
west coast:
a discussion.
Teclonophysics, 86: 319-341. Evidence
from published
there is a consistent inner coast (l-2
and Victoria.
north
by considerable
Although pattern
Vancouver
topography
margin.
is essentially
that the vertical
on a scale of hundreds
of thousands
indicates
and subsidence Hecate
Strait,
that
on the Georgia
State but is interrupted
to the
Alaska. vertical movements
contemporary
seen opposite
Comparison similar
tectonic
the Queen
with movements
rates are much
that discontinuous
in the region over the last 10,000
movements
observed
too great
are not compatible
with the
in origin. There is, however,
Charlotte
to that seen in association
movement
may be soluble by assuming
runs through
into Washington
from this source and are most probably the movements
data in British Columbia
or “hinge-line”
southwards
recovery has dominated
Island convergent
The paradox
continues
and trend of the observed
between
show that the pattern
relevelling
uplift on the outer coast (2 mm,+)
uplift in southeastern
glacio-isostatic
to be expected
and geodetic
The zero uplift contour
This pattern
the distribution
clear distinction
tidal records
of contemporary
mm/yr).
Strait
years,
pattern
transform
margin
no
and the
at other active plate margins
with subduction to explain
and convergence.
observed
lateral shifts of the movement
geology pattern
and occur
of years.
The geological record is characterized by continual reminders that vertical movements are a fundamental part of the geological process. The discoveries that marine organisms can now be observed as fossils many thousands of metres above sea-level provoked some of the fiercest debates in the history of earth science. The advent of plate tectonic theory in the last two decades has given a new impetus to investigations of the details of such movements. If contemporary plate tectonic processes are
* Contribution
from the Earth Physics Branch
0040-1951/82/oooO-O/SO2.75
No. 977
0 1982 Elsevier Scientific
Publishing
Company
indeed
represenrative
oi’ the processes
in home way they must produce that are observed In recent movements
that have occurred
in the geologic past. then
both the scale and style of the vertical
years,
a number
of studies
have
in regions near plate boundaries.
been
Among
initiated
ipetit _
map and
plate boundaries
of Canada’s
i i-
west coast
to determine
the most well studied
Plate
Fig. 1. Location
rnovrments
in the record.
-.
these
areas are
321
California, Alaska, Japan and New Zealand, areas where plate boundaries are exposed at the surface or where seismicity and presumed convergence rates are high. From inte~retation of the Juan de Fuca ridge system (Vine and Wilson, 1963, through establishment of a plate tectonic framework (Atwater, 1970; Chase et al., 1975), to its detailed application (Riddihough and Hyndman, 1976; Milne et al., 1978), the western Canadian continental margin is established as representing two styles of contemporary plate tectonic interaction (for a recent review, see Keen and Hyndman, 1979). North of Queen Charlotte Sound there is dominant transform interaction between the Pacific and America plates; south of this point, interaction along the margin is principally convergence and subduction between the oceanic Juan de Fuca plate and the continental America plate (Fig. 1). From both geology (Tiffin et al., 1972) and plate geometry (Riddihough, 1977) the triple junction which marks the boundary between these two regimes has remained near northern Vancouver Island for the last 10 Ma. Calculations of present subduction rates along the convergent margin vary between 4 and 2 cm/yr (Riddihough, 1977); transform rates along the Queen Charlotte margin are between 5 and 6cm/yr (Minster and Jordan, 1978). If this plate tectonic pattern is valid and can be applied now and at least over the last few thousand years, it seems reasonable to expect that the regional pattern of contemporary vertical movements should relate to it in some coherent manner. To examine this expectation, the present paper reviews the observed pattern of vertical movements, discusses the rate. of glacio-isostatic rebound in the region, examines vertical movements observed in similar tectonic environments elsewhere and finally speculates on the relation between these movements and the observations of the geological record. PRESENT-DAY VERTICAL MOVEMENTS
Sea-level and tidal records
Tidal stations on the west coast have been operating regularly since approximately 1920 (Wigen and Stephenson, 1980) (for locations see Fig. 2 and Table I). De Jong and Siebenhuener (1972) used both linear and quadratic regression on annual means from seven stations in an attempt to determine trends of secular change. They concluded that land movement existed and that there was uplift at Victoria and subsidence at Vancouver, Point Atkinson and Prince Rupert which was probably tectonic in origin. Vanicek and Nagy (1980), as part of their broad study of vertical movements in Canada, calculated linear trends from monthly mean values of sixteen west coast tidal stations. Wigen and Stephenson (1980) argued that simple linear regression through values at different time periods, unless data sets are long, can give very different trends. They proposed that differences between stations in similar hydrodynamic regimes
TABLE Linear
I sea-level
use monthly errors
trends
for II corrected
to nearest mm/yr, see original
for west coast stations
means, Wigen and Stephenson from published
in mm/yr
with standard
I are from annual
values: Stephenson.
means.
per>. comm..
error.
Vanicek
and Nagy
pairs. (Standard
1981). Plotted
values (Fig. 3) are
and selected as having the smallest SE less than 1.5. For details of analysis
references.
Station
Station
locations
and
Nagy (1980)
Wigen and Stephenson
Wigen and I
(1980) I.
Alert Bay Bamfield
-- 1.1*
0.7
Inlet
3.
Betla Bella
4.
Campbell
-0.1
s.
Fulford
h.
Little River
Harbour
0.15:? 1.52
2.8
.~ 3.57 2 1.22
e 0.8
-~ 0.15 -t 0.65
0
-6.46
7.
New Westminster
x.
Point Atkinson
Y.
Port Alberni
-- 1.9% 0.8
10.
Port Hardy
-2.5-r
II.
Prince Rupert
12.
Queen Charlotte
13. 14.
Plotted II
- 1.95 .t 0.64
2. 2.0
-3.12
Stephenson
Value
(1980)
2.93 c 2.81
River
periods etc..
are shown in Fig. 2.
Vanicek
,7.
values
II from difference
r-2.15
-2 2.36 * 1.79 2.32 +-0.73 -0.65-‘-
1.10
i-2 -1
0.25 t-o.44 ~ 2.42 i I .0X
0 -2
-68125.4 i.2+
0.2 2.7
0.12.-0.48
0.46 + 0.44
1 -2
-4.75+
1.48
~ 1.44t0.84
-1
- 1.89:~ 1.48
-1.19*1.09
-I
- 2.96 CL2.22
-0.57?0.61
-1
0.75
0.2
1.25i- 0.43
- 1.9%
1.4
Sooke
-1.82
2.1
Steveston
-2.11:
5.9
IS.
Tofino
- 1.6’
0.3
16.
Tswassen
2.8%
I 7.
Vancouver
0.1 _” 0.2
0.64 2 0.42
0
IN.
Victoria
0.6’
0.27 kO.40
0
City
- 1.25 to.47
fl
-2
3.4 0.2
would give more reliable indications of crustal movement and demonstrated that the differencing technique involved significantly less variability than the analysis of single station records. The results of both Wigen and Stephenson (1980) and Vanicek and Nagy (1980) are shown in Table I. Although differencing clearly reduces the “variability” (as used by Wigen and Stephenson, 1980) of the data set, trend values from this technique do not always have a significantly lower standard error than trends from long period single station analysis. The linear trends with the lowest standard errors, eliminating any stations with standard error > 1.5 mm/yr, are listed to the nearest mm/yr in column 4 of Table I and plotted on Fig. 3. While this selection represents a gross simplification of the data, it does not misrepresent them and shows that in general terms the inner coast is subsiding relative to mean sea-level, Vancouver Island and the outer coast is rising, and that zero level runs approximately through Victoria and the Strait of Georgia.
323
Fig. 2. Locations
Fig. 3. Vertical Alaska
of tidal stations
movements
listed in Table I
of land relative
from Hicks and Shofnos
to sea-level in mm/yr
(I 965). in Washington
from tidal stations
from Ando and Balms
(I 979).
of Table I. Data in
324
Levelling Relevelling in a study
on Vancouver
Island
of the 1946, magnitude
was examined 7.3 earthquake.
uplift in the centre of the island (l-2 and
1946) followed
by uplift
mm/yr).
(c. 1 mm/yr)
( 1978)
by Rogers and Hasegawa They
suggested
and subsidence on the east coast,
that apparent
(10 cm between south
1930
of Campbell
River (4 in Fig. 2) were probably significant. However, modelling of the coseismic deformation of their preferred earthquake mechanism did not match this deformation. Another solution for the same event based on re-triangulation surveys (Slawson and Savage, 1979) may be compatible with the relevelling data (H. Dragert. pers. commun., 198 1). Levelling data in the Fraser Valley around and east of Vancouver (I 7 in Fig. 2) were examined by Mathews et al., (1970). They observed that all bench marks had apparently moved downward son, 8 in Fig. 2). Discounting alluvium
or aquifer
mm/yr. Levelling
“‘
withdrawal.
data were included
they concluded
order
surface
of vertical
that depression
in the work of Vanicek
rates were up to 1.5
and Nagy (1980.
1981) in
,,,..... :...i
“‘Uyy _,-...i
Fig. 4. Fourth Tidal stations
relative to the bench mark at Caulfield (Point Atkinthose that were clearly connected with compaction of
movement
used are shown as dots, segments
velocity
of relevelling
in mm/yr
from Vanicek
shown as dotted
lines.
and Nagy
(1980).
325
which they used a combination of tidal data and sections of relevelled geodetic data to construct a map of vertical velocity. Their fourth-order surface for British Columbia (their zone 2) is shown in Fig. 4 together with the relevelled segments and the tide gauges that they used. The standard deviation for most of this surface except near the Queen Charlotte Islands is OS-l.5 mm/yr. In the latter area it varies from 1 to 4 mm/yr. The features shown in the tidal data of Fig. 3 are reproduced with uplift along the margin, zero level near the inner coast and subsidence over the Coast Mountains, Two other areas of uplift occur inland near 55” N, 125” W and 52” N, 118OW. In commenting on the validity of a 4th order surface through the data, Vanicek and Nagy (1981) suggest that some of the “sagging and peaking” merely reflects holes in the data. While this may be true for the eastern and southern edges of their zone 2 map, the qualitative correspondence of the surface with the tidal data (i.e. Fig. 3) seems reassuring. Comparison
with Washington
State and Alaska
Ando and Balazs (1979) reviewed the available vertical movement data from tidal data and relevelling lines in the northern part of Was~ngton (Holdahl and Hardy, 1979). Their contours are included in Fig. 3 and show a pattern which is continuous with that in southern British Columbia. Zero change contours match closely and an area of rising land to the west corresponds with the Olympic Mountains and the Coast Range. To the east, Puget Sound is sinking at up to 1 mm/yr. The same pattern, in terms of eastwards tilt, has been commented upon by Adams and Reilinger (1980) as extending southwards along the Oregon Coast Range almost to the California border. Work on marine terraces (Adams et al., 1980; Adams, pers. commun. 1981) allows vectors of these tilts to be estimated and shows that they are in general perpendicular to the coastline. In Alaska, studies from tide gauge data by Hicks (1972, 1978) and Hicks and Shofnos (1965) show that land movement is predominantly upward. Although some very short period tidal observations were used to construct a network of stations, longer periods of observations at Sitka, Ketchikan, Juneau and Skagway provide a foundation for their contoured map of vertical uplift included in Fig. 3. Values at these stations are + 3 mm/yr, 0 mm, + 13 mm/yr and + 17 mm/yr respectively; values at their temporary stations reach an estimated + 39 mm/yr near Glacier Bay. The regular pattern of landward tilting seen along the British Columbia margin may be continuous as far north as 56” N but seems to come to an abrupt change near that point. The extreme values seen north of this may be related either to tectonics or glacial uplift and are discussed in a later section. GLACIO-ISOSTATIC
EFFECTS
A realistic assessment of any pattern of recent vertical movements in Canada is impossible without an examination of the role of glacio-isostatic rebound (for a
broad
review of the problem
rebound
the most vaiuable Cathles,
sources of information
1980). Modelling
ters including viscosity)
earth
structure
and the magnitude.
are available
see Lambert
asthenosphere. and timing
by Walcott
Canadian
1979). This is one of
(e.g. Walcott,
has to take into account
extent
as stressed
(Fennoscandia,
Vanicek,
of glacial loading
for the earth’s rheology
(lithosphere, spatial
and
to the removal
of the response
glacial load. Unfortunately, data
in Canada,
of the Earth’s surface in response
values
1980:
several paramefor rigidity
of the applied
and
and removed
(1980), even in areas where good
Shield),
there
is no agreement
as to
which earth model provides the most realistic response. Attempts to predict or assess responses in other areas from a presumed glacial history are consequently very uncertain. Nevertheless, some general qualitative conclusions seem to be valid. Surrounding Walcott, collapses
the main
area of ice loading
1970). As the load is removed in place or migrates towards
is a region
of peripheral
“bulge”
(e.g.
and the crust beneath it rises, this “forebulge” the retreating load. The combination of these
processes and the global effects of the redistribution of water and loading on the geoid produce a series of zones (Clark et al., 1978) concentric to the main ice sheets in which particular The general estimated
patterns
distribution
(e.g. Prest,
of land and sea-level movements
seem to occur.
and timing of the North American
1974; Peltier and Andrews,
ice complex
1976) with a maximum
has been extent
and
thickness about 18,000 yrs before present (ka B.P.). The major effects in southwestern British Columbia of its removal (complete for all but residual ice caps by 6 ka B.P.) are suggested to be those of Zone I, Zone II or the transition between them (Clark et al , 1978, figs. 5 and 15). Zone I is characterized by emergence, Zone II by continual submergence because of the collapsing forebulge and the I/II transition zone near the ice margin
by a more complex
followed by submergence. Crustal response in southwestern
British
by the fact that the region was affected the Cordilleran
response Columbia
the I8 ka B.P. “Laurentide”
initial
may be further
by what is regarded
ice Sheet, part of which advanced
(15 ka B.P.) than
involving
as a separate
emergence complicated ice system,
over the region substantially
later
peak. This ice sheet reached
as far
south as Olympia (47’N) in Puget Sound (Thorson, 1980) and across Vancouver Island to its western coast (Fig. 5) (Prest, 1974; Clague et al., 1980, 1981). Retreat was rapid as the Vancouver
area was again clear by I3 ka B.P. Vertical
movements
along the British Columbia margin may thus be a compound effect of long wavelength responses to the retreat of the major North American ice complex plus the local effects of the removal of the Cordilleran ice sheet. The timing of the crustal response and the length of time for total relaxation is another variable about which there is still apparent disagreement. Walcott (1980) noted that depending upon the earth model used, the relaxation time could be proportional or inversely proportional to the dimensions or wavelengths of the depressions produced by loading. In practice, he suggests that relaxation times seem to vary with position with respect to the ice sheet; relaxation times near the edge of
327
the Laurentide
uplift being shorter
with > 5000 yrs). The relaxation
than those near the centre (2000 yrs as compared
time is also clearly related
to the rate of removal
of
the ice load (Clark et al., 1978). All studies
Fig. 5. Glacial
of the British
retreat in western
numbered in thousands tians in southern
British
Columbia
Canada
region
have stressed
from Prest (1974).
Contours
of years before present. Solid areas-present
Columbia see Clague (1981).
that
show
(residual)
relaxation
times
ice margin positions,
ice cover. For modifica-
from tilts and sea-level Clague,
1975: Fulton
al.. 1981). The apparently particularly
curves appear
and Walcott.
to be very rapid (e.g.. Mathews
1975: Andrews
rapid response
rapid local load removal.
and Retherford.
may be a function
of crustal
of the short wavelength
1976). Mathews
et al. (1970) and Clague (1975) concluded
structure.
of the C‘ordilleran
sheet or of the fact that the area was close to the ice margin 5000 yrs were minor and that glacio-isostatic adjustment Nevertheless, as re-stressed by Clague et al. (1981),
et al.. 1970;
197X: (‘Iague et
(Peltier
of a ice
and Andreus,
that changes
in the last
was now virtually complete. it cannot automatically be
assumed that any small vertical movements seen today, or over the last few thousand years, are entirely free of any glacio-isostatic component. It is. however, reasonable to conclude geographical general
that any such component pattern which is either
shape of the Cordilleran
QUATERNARY
SEA-LEVEL
is by now greatly diminished and exhibits a uniform over a broad area or related to the
ice load.
CURVES
Clague (1975) and Clague et al. (1981) have compiled a series of sea-level curves for the last 13,000 yrs measured at various sites along the west coast from Puget Sound to Juneau, Alaska (Figs. 6 and 7). All the curves except those on the outer coast, show a steep emergent trend after deglaciation reaching close to present sea-level between 8 and 11 ka B.P.. Those on the northern inner coast (Prince Rupert, Juneau) may not have dropped significantly below present sea-level, whilst the remainder (Comox-Nanaimo (18 in Fig. 2), Bella Bella/Bella
(E. Vancouver Island), Fraser Coola) did and have been
Lowland, followed
Victoria by slow
submergence. In all these cases, there has been little movement over the last 5000 yrs: no more than 1 mm/yr emergence for Juneau and 0.7 mm/yr submergence in the Fraser Lowland (average rates calculated from the graphs of Fig. 7). By contrast, on the outer coast there has been steady emergence since 4 ka B.P. at Tofino (1 mm/yr) (5 in Fig. 2) and 8 ka B.P. on the Queen Charlotte Islands (2 mm/yr). In the Queen Charlotte Islands this was preceded by submergence; at Tofino, limited data
suggest
that
there
could
have been
(Clague et al., 1981). Compared with the characteristic
emergence
back
curves of the post-glacial
to at least
13 ka B.P.
zones of Clark et al.
(1978), the sites can be characterized as in Fig. 6. In general, the northern inner coast sites correspond with Zone I and the southern inner coast sites with the transition between
Zones I and II. In terms of the glacial history of Fig. 5, this distribution can 1981) to correspond well with the position of these sites be argued (e.g. Clague et al., beneath or close to the margin of the Cordilleran ice sheet. The sites on the outer coast, however, do not correspond to either Zone1 or I/II or to the postglacial response of the next outer zone, Zone II-collapsing forebulge submergence. It seems likely that here at least, there may be a superposition of a tectonic trend on the glacio-isostatic response. The contrast of the Queen Charlotte Islands pattern
329
ZONE II
ZONE III
Fig. 6. Glacio-isostatic
recovery
response
zones from Clark
location
and characterization
from Quatemary
ZONE I/II
sea-level curves: (i) theoretical
et al. (1978); (“) u curves of glacio-isostatic
ZONE
I
curves for glacio-isostatic
for three west coast sites from Clague
response
from comparisons
(1975);
(iii)
of (i) and (ii), see discussion
in text.
+100
I
VICTORIAGULF ISLANDS
2
+50
e fi
0
QUEEN CHARLOTTE
15
10
5
Ka B.i?
Fig. 7. Quaternary
sea-level
curves
tidal trends from Table I. Shaded
from Clague
(I 975) and Clague et al. ( I98 I ) compared * I mm/yr.
cones are tidal trends
to projected
0
and its relation both Andrews Apart inner
forebulge
from very rapid
collapse
curves
relaxation
in other parts
that, by comparison,
is of small amplitude.
or tectonic
movement
was stressed
by
(1978) and Clague et al. (198 I ). times,
coast at least, are thus consistent
deglaciation implies
to either
and Retherford
the Quaternary
with the pattern.
sea-level
curves
of the
style and magnitudes
of the globe. To a first order of magnitude
any continuous.
systematic
tectonic
It also allows that the submergence
of this
trend at these sites
of the last 5000 yrs along
the southern inner coast and the possible steady sea-level or slightly emergent trend along the northern inner coast could be a residual glacio-isostatic response. The question of whether or not this pattern is a glacial response must be addressed by examining
the broader
OBSERVED VERTICAL
picture. MOVEMENTS
AND DEGLACIATION
Although at first sight the parallelism of topography, vertical movement patterns (Figs. 3 and 4) and deglaciation (Fig. 5) along the inner coast provide a confusing background from which to isolate a distinction between glacial response possible tectonic movements, a number of features suggest that the present movements are predominantly tectonic:
and day
(1) If the vertical movement pattern of Fig. 4 is accepted, then the area of thickest late Pleistocene ice corresponds closely with the area of present downward movement. Further, the rate of subsidence seems to increase landwards from the inner coast. In contrast, the curves of Clark et al. (1978) and Fig. 6, suggest that residual isostatic subsidence should decrease towards the position of the main ice load (i.e. from Zone the load.
I/II
to Zone I) and be replaced
by upward movement
(Zone I) beneath
(2) The pattern of eastward-tilting approximately perpendicular to the coast continues at similar amplitudes and wavelength into Washington (Fig. 3) and further southwards (Adams and Reilinger, 1980). This is well outside the main glaciated area and transverse to the principal east-west termination of the overall ice-sheet ( 13- 15 ka B.P. contours in Fig. 5). Fulton and Walcott (1975) noted that the post-glacial tilting
in the Merritt
area (50’ N, 121’ W) was consistently
northwards
towards
the
main ice load. (3) The Puget Sound area of downward movement again coincides closely in position and orientation with the Puget Sound ice lobe detailed by Thorson (1980) where Zone I recovery might more reasonably be expected. (4) As noted in the previous section, Quaternary sea-level movements on the outer coast cannot be matched in standard isostatic response curves and probably include a tectonic component. It seems unlikely, given the distances involved, that such tectonic movements would be entirely restricted to this narrow zone.
331
SUPERIMPOSITION
OF TECTONIC MOVEMENTS AND ISOSTATIC RECOVERY
If present-day vertical movements (from tidal and levelling data) are accepted as predominantly tectonic in origin, it is reasonable to assume that their time scale is considerably
longer
The observed effects
and
complete,
than
the most recent
Quaternary
sea-level
On Fig. 7, the assumed
of glacial
should
therefore
curves
for the last few thousand
should correspond
episode
years,
when
with the projected tectonic
trends
glacial
tectonic
(* 1 mm/yr)
loading
and recovery.
be a sum of the two recovery
was virtually
trends. are superimposed
on the
Quaternary sea-level curves of Clague (1975) and Clague et al. (1981). Excepting for E. Vancouver Island and Juneau, the curves do correspond with the projected tidal trends at least since 5 ka B.P. Removal
of the trends from the curves should result in
“corrected” glacial response curves in accordance with the patterns discussed earlier. It is clear that with the two exceptions noted above, this is true. For the Fraser Lowland,
Bella/Bella
Coola
and Prince
Rupert
the correction
might
remove
the
period of low sea-level characteristic of Zone I/II and put these stations into Zone I. A corrected Queen Charlotte Islands curve would correspond with Zone II as discussed earlier and the curve for Tofino might resemble a Zone I/II response. However, these changes do not substantially contradict the observation that in general the observed curves are close to the expected pattern of glacial recovery and that the tectonic trends are in fact relatively small. The two exceptions of E. Vancouver Island and Juneau are of considerable significance. The curves imply that the present vertical movements observed at these points cannot have been continuous over the last few thousand years and must be a transient deformation pattern. In the case of E. Vancouver Island (Comox-Nanaimo), the proximity have
to the 1946 magnitude
occurred
deformation
in central
Island
(Milne
et al., 1978) suggest
may be part of a seismic cycle. Such a relation
study of geological, ute towards
7.3 event and the other large earthquakes
Vancouver
tidal and geodetic
an understanding
levelling
of the seismicity
information
indicates
which that
the
that a close
may be able to contrib-
in this area.
Clark (1977) calculated that up to 35% of the emergence of the last 50 years in southeast Alaska was attributable to elastic rebound following the rapid retreat of ice in Glacier Bay, an idea first suggested by Hicks and Shofnos the Quaternary sea-level curve for Juneau in Figs. 6 and 7 confirms that this retreat was not a continuation of the main and is thus an isolated short period event. Although historical
(1965). Nevertheless (from Clague, 1975) deglaciation episode seismicity in the area
is low (Milne et al., 1978), Juneau lies close to the Chatham Strait and Denali Fault systems and an origin associated with local strain seems to be equally plausibe.
VERTICAL
MOVEMENTS
Subduction
TECTONIC’
ENVIRONMENTS
and convergence
The pattern in Japan.
IN SIMILAR
of vertical
As discussed
underthrusting post-seismic.
movements by Scholz
in a subduction (1972).
zone has been closely studied
movements
can be related
to seismic.
events in a series of phases: inter-seismic, pre-seismic, co-seismic, An initial depression of the material above the thrust is followed by
elastic rebound during the dislocation of the earthquake The integration of these phases is long-term depression and elevation of the overlying plate.
(Savage and Hastie, of the underthrusting
1966). plate
Thatcher and Rundle (1979) proposed a more complex model than that of Scholz (1972) in which an elastic plate (lithosphere) overlay a visco-elastic half-space (asthenosphere). The underthrust cycle (with continuous aseismic slip at depth) produced similar depression lithosphere thickness inland both
the Shikoku
and uplift, with further depression beginning about one from the fault outcrop. Thatcher and Rundle examined
and South
Kanto
districts
of Japan
where underthrusting
Phillipine plate beneath the Eurasian plate occurs and concluded complications of each area, such a model was satisfactory. Scholz and Kato (1978) also considered
the South
Kanto
of the
that give the local
district
where highly
oblique subduction occurs at an estimated 3 cm/yr along the Sangami trough (Seno, 1978). They concluded that the subduction mechanism was far from simple and occurred on a series of dipping thrust faults; during the interseismic period aseismic slip occurred
below
15 km depth
while strain
accumulated
in the upper.
locked,
portions of the system. Nevertheless, as is shown in Fig. 8, three elevation profiles perpendicular to the subduction zone (Atami to Tokyo, east and west Bozo Peninsula) show integrated elevation changes (including and accumulating the co-, post- and inter-seismic
deformations)
further “inland”. In the Shikoku
area,
which involve deformation
uplift
nearest
is associated
the trench
with
4cm/yr
and subsidence perpendicular
subduction at the Nankai trough (Seno, 1978). Again the same pattern of leading edge uplift with depression further inland is evident (Fig. 8). Thatcher and Rundle (1979) related their model to a thrust fault outcrop 90 km inshore of the trench axis, noting that this was a common feature of other solutions. Ando used the Nankai trough example as their principal comparison
and Balazs (1979) with Washington
State. Another area in Japan, opposite the Japan Trench near 36” N, was reviewed by Abe (1977). Here the coast and the eastern end of the levelling line is nearly 200 km from the trench (convergence rate 10 cm/yr: Minster and Jordan, 1978) and the integrated movement seems to be entirely subsidence (Fig. 8, Pt. Shioya). Kato (1979) showed that a similar pattern occurred in the Tohuku district between 38’ and 40” N (Fig. 8, Tohuku-Sendai, Kamaishi) and concluded that while it was
333
TOHUKU (SENDAl) TOHUKU (KAMAISHI+
I
JAPAN TRENCH
..
:“-c-/ I
PT. SHIOYA
EASTERN ALEUTIANS WASHINGTON STATE
--,
____
I CENTRAL VANCOUVER ISLAND
I I
Okm
I
!
I
I
I
200 100 DISTANCE FROM TRENCH AXIS
I
I
300
Pig. 8. Net vertical velocity curves relative to distance from trench axis for various subduction zones. For Japanese examples, movement was integrated over complete earthquake cycles and divided by time. For Alaska (eastern Aleutians), the curve of Plafker (1970) was divided by his estimated 800-yr return period. Andes (Pert-Chile) is observed movement in metres for the 1960 Chilean earthquake, scale shown on right (Planter, 1972). Sections for central Vancouver island and Washington State are from Fig. 3 and Ando and Balazs ( 1969). Japanese sources are referred in the text.
qualitatively consistent with subduction at the trench (300 km distant) modelling was quantitatively improved by the introduction of localized sinking of the subducted slab. Similar observations of continuing subsidence with co-seismic subsidence during the 1973 event were discussed for eastern Hokkaido (175 km from the Japan trench axis) by Fujita et al. (1975). A characteristic of these areas is that the do~n~t ~mponent of the net deformation is the deformation produced during the co-seismic phase. in South Kanto, for instance (Scholz and Kato, 1978), the maximum co-seismic displacement is ten times that occuring in the inter and post-seismic phases. The principal
difference
in pattern
(e.g., Ando and Balazs, 1979. fig. 3) is that the “inland” closer to the trench than the final integrated area ot
co-seismic depression subsidence. The
similarity
begins
between
long-term
and
Plafker (1972) in his study of the vertical and 1960 Chilean
earthquake.
co-seismic
deformation
movements
He concluded
produced
that despite
was noted
by
by the 1964 Alaskan
some pre-tectonic
submer-
gence, the co-seismic areas of uplift in Alaska corresponded with the areas of net Holocene emergence and that the co-seismic upward movements could therefore be regarded
as a pulse
in a long continuing
trend
Co-seismic deformation profiles from southeast included in Fig. 8 (Plafker, 1972).
of movement
of the same style.
Alaska and southern
Peru- Chile are
The differences between the vertical deformation along the southern Columbia and Washington margin and the deformation at other subduction (Fig. 8) may exist for a number of reasons. to affect the pattern are rate and direction
Amongst the variables which seem likely of convergence, age and thickness of the
converging lithospheres and the dip of the intervening clear that in a general sense the deformation is similar. the co-seismic
or complete
cycle deformation
British zones
thrust plane. However, it is Further, it is similar to either
of other margins
rather
than to any of
the intervening phases. This observation was used by Ando and Balazs (1979) to argue that in the absence of any major thrust earthquakes during the period of tidal and levelling observations, the deformation in Washington is most likely to be that of a complete Transform
cycle occurring
aseismically.
motion
Vertical movements in areas of highly oblique convergence form interaction are apparently much less systematic and Zealand mm/yr (Walcott,
(Lensen, 1975), vertical movements along but diminish eastwards to -0.3 mm within 1978) between
approximately
the
Pacific
and
Indian
or dominantly transpredictable. In New
the Alpine Fault reach + 10 120 km. Motion on this fault plates
is lo”-15’
oblique
at
4 cm/yr.
Along the San Andreas Fault, the Palmdale Bulge was proposed as a large area (100 X 300 km) of uplift which rose up to 300 mm (20 mm/yr) between 1959 and 1974. Since 1974, parts of the area have subsided up to 200 mm and a considerable debate is in progress as to whether or not these measurements are real or at least partially affected by levelling errors and miscalibrations (e.g. Mark et al., 1981; Rundle and McNutt, 1981; Strange, 198 1). Vertical movements near the Sangami Trough (Fig. 8) have been discussed in the previous section. However, interaction along the through between the PhiIlipine and Eurasian plates is highly oblique (< 10’) at 3 cm/yr. As can be seen in Fig. 8, uplift of up to 20 mm/yr falls to negative values within 30 km of the trench axis. Along the northern part of the Queen Charlotte-Fairweather Fault zone, vertical
335
movements from tidal me~urements are pr~ominantly upward and reach very high values (Fig. 3 and earlier section’s Hicks and Shofnos, 1965). Studies of movement on the Fait-weather Fault can be interpreted as showing of the order of 6 mm/yr vertical movement over the last 1000 years in one locality (Page, 1969). Regionally the terrain to the northeast of the fault is estimated (Page, 1969) to have risen an average of 2-3 mm/yr since the early Pleistocene. However, as discussed earlier in connection with movements at Juneau, the role of deglaciation in producing uplift in this area may be difficult to evaluate. The recent vertical movement (1 mm/yr) of the Queen Charlotte Islands is thus compatible with a position close to a transform fault or a highly oblique convergent plate boundary. With the closest additional information at Prince Rupert (120 km distant), it could also be compatible with a normal subduction situation. The fact that the Pacific-America vector of Minster and Jordan (1978) is 15” oblique to the southern Queen Charlotte Fault has led to recent speculation that oblique subduction is in fact occurring along this margin (Hyndman and Ellis, 1981; Perez and Jacob, 1980). The observed vertical movements do not provide any useful distinction. Hot spot
Bevier et al, (1979) suggested that an east-west volcanic belt in southern British Columbia, appro~mately along 52” N, could be the trace of a hot spot fixed in the mantle over which the North American plate has moved westwards during the last 25 Ma. The idea has been further discussed by Rogers (1981) who points out that an area of seismicity occurs beyond the eastern end of this belt (near 52” N, 118’ W) where the hot spot might be expected to occur today. The 3th order surface of Vanicek and Nagy (1980) (Fig. 4) forms a closed area of upward movement (7 mm/yr) at the same position. By comparison, doming of 3-5 mm/yr over an area of 8000 km2 may be associated with the Yellowstone continental “hot spot” (Reilinger et al., 1977). CONCLUSIONS AND DISCUSSION
Contemporary vertical movements along the British Columbia margin are real and can be summarised as upward movement (c. 2 mm/yr) along the outer coast and submergence (l-2 mm/y@ along the inner coast. A similar pattern continues southwards into Was~ngton and Oregon. It seems unlikely that this pattern is due to glacio-isostatic re-adjustments and it conforms to patterns seen elsewhere along a converging or subducting margin. An apparently similar pattern continues northwards beyond the triple junction position on the margin into the area which is expected to be predominantly transform in character. Although this continuation seems to conflict with the contrast in tectonic character, it is not untypical of other
areas of transform or very ohiique subduction. Inland coincide with a recently proposed hot spot position. Thus tectonic
far the pattern environments.
increasingly Firstly,
common similar
of movements However,
paradoxes
is consistent
as in many in levelling
rates of apparent
vertical
others,
an apparent
with those it exhibits
occur in the Maritimes
Nagy. 1981), movements explanation.
(Vanicek,
for which
there
in other.
may
similar,
what are becoming
studies. movement
occur in areas which are not
tectonically active and certainly do not exhibit similar Canada. for example, contemporary vertical movements parently
doming
levels of seismicity. of several mmjyr
1976) and Newfoundland is no simple
tectonic
(Vanicek
In apand
or geophysical
Secondly, the values of vertical movement are far too great to satisfy the integrated geological observations over a long period. For example. up to 6 km thickness of lithified but unmetamo~hosed Eocene-Oligocene (c. 30 Ma B.P.) marine sedimentary rocks outcrop at elevations of up to 1 km along the northern flank of the Olympic Peninsula (MacLeod et al., 1977). Plate convergence at this subduction zone can be shown to have been similar to or greater than that today for at least the last 20 Ma (Atwater
and Molnar.
1973: Riddihough,
1977; Coney,
1978).
If vertical movements had been continuous present elevation could have been achieved
at their present rate (2--3 mm/yr), the in less than 3 Ma. A similar dilemma in
the
by Adams
Oregon
Coast
Range
was
discussed
and
Reilinger
(1980)
who
concluded that tilt rates had been steady for the last 0.25 Ma but that all the tilting observed, even that of the oldest Eocene strata. could have been produced in the last 7 Ma. Two solutions for the paradoxes have been proposed (e.g., Adams and Reilinger, 1980). neither entirely satisfactory. The first is that much levelling data is wrong and obtained with a systematic bias so that even repeat surveys continue to give the same results. This is under active investigation for land surveys (e.g. Brown and Reilinger, 1980; Rundle and McNutt. 1981) and may have some validity. It may be more difficult to apply to tidal data (e.g. Kumar and Soler, 1980). The second solution is that movements are oscillatory, episodic or discontinuous with periods of tens to hundreds of thousands of years. A key objection to this from the present area is that the observed movement pattern is apparently characteristic of all other subduction zones that have been measured. Model studies of such environments (Savage and Hastie, 1966; Thatcher and Rundle, 1979; Scholz and Kate, 1978) fit remarkably well. The coincidence that all subduction zones are simultaneously going through the same deformation mode seems untenable. A variant to the second solution that retains this deformation mode at ail times but ensures that the long term integrated deformation is not impossibly large, seems to be that of moving the deformation laterally. In the case of a subduction zone, shifting of the deformation pattern over a zone 100-200 km wide will suffice to superimpose deformations so that the integrated vertical movements are no greater
337
Fig. 9. Vertical imbricate
movement
patterns
thrust system showing
in amplitude
than
from underthrusting.
broadened
a single
event.
Effect of superimposition
uplift and subsidence
In Fig.
9,
pattern
five episodes
of five events from an
with similar average amplitudes.
of dip-slip
deformation
of
the general shape envisaged by Savage and Hastie (1966) are superimposed with fault outcrops spread over a range of 200 km. The final, integrated, region of “uplift”
is “noisy”
but
has an average
elevation
of less than
the uplift
of an
individual event. The depression is similar or slightly larger. An investigation of Quaternary deformation in Japan by Ota and Yoshikawa (1979) shows that a very similar mechanism has been in effect along the Pacific coast. The hinge lines between co-seismic uplift and co-seismic depression (equivalent to the zero contour in Fig. 3) moved discontinuously landwards during the Pleistocene,
the present
will move landward may. occur at intervals Geological considerable. Nankai Plafker
pattern
another
being set up only 0.5 Ma B.P. They estimate
50 km in the next
1 Ma and suggest
that it
that such moves
of this order.
evidence for past imbricate and multiple The co-seismic fault modelled by Thatcher
thrust and and Rundle
fault zones is (1979) for the
Trough was 90 km inshore of the trench. The Patton Bay Fault discussed by (1972) is 150 km from the axis of the Aleutian trench but was clearly
involved in the 1964 co-seismic deformation. Episodic shifting of transform motion from one fault to another within the fault zone is well documented historically in California and has probably recently occured on part of the Fairweather fault system. As applied to the Juan de Fuca plate subduction zone, aseismic slip (as by Ando and Balazs, 1979) on the contemporary dislocation plane may continuous from 0.25 Ma (Adams and Reilinger, 1980). Previous to principal slip may have occurred on other fault planes. (Such planes
envisaged have been that, the could be
essentially
separate
at depth, as in Fig. 9, but convex upwards
into a low angle imbricate surface. Transference
of the slip from one dislocation
that at any one time (e.g. the present), large but that the integrated than the projected
so that they converge
thrust zone near the trench and are not identifiable plane to another
the vertical
sum over geological
movements time would
at the
would ensure
would
appear
too
he considerably
less
effects of any single episode.
ACKNOWLEDGEMENTS
I am indebted for an extremely sea-level
to Fred Stephenson for discussions on tidal data and John Clague valuable review and current information on west coast Quaternary
curves.
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to the Redefinition Ottawa,
of North
Ont., pp. 105-124.
American
West Coast. Vertical
In: Proc. 2nd Int. Geodetic
Networks