Post-glacial Sea Level Change and Isostatic Rebound along North America's Pacific Northwest Coast
1. Post-glacial Sea Level Change and Isostatic
Rebound in Conjunction with other
Processes on the Northwestern Coast of
North America
Processes affecting relative sea level (RSL)
compared to today
Brief look at six areas along the Pacific NW
coast of North America as well as the
processes that have affected them the most
especially since the last glacial maximum
(LGM)
Conclusion and discussion
2.
3. Processes -
Eustacy
Eustacy – a change in global sea level due to
(in this case) glacial melt water entering the
oceans.
Release of freshwater buildup from, for
example, lakes Agassiz and Missoula
Last glacial maximum ~21,000 ka BP
4. Processes -
Steric Effects
Steric Effects – Caused by the thermal
expansion or contraction of water in the
oceans.
Determined to be within the range of
uncertainty in most data ranges and most likely
a very minor factor since the last glacial
maximum.
5. Processes -
Interseismic and Coseismic
Crustal Deformation – Between earthquakes
there is crustal shortening and uplift towards
the continent and subsidence towards the sea.
After an earthquake the opposite occurs.
Much of the Cascadia subduction zone area is
experiencing uplift especially near the coast
now.
6. Processes -
Isostatic Depression and Forebulge Development
Isostatic Depression – The pushing down of
the crust due to the overlying ice sheet's
weight. This eventually returns to a point of
equilibrium after the ice is removed i.e.
Isostatic Rebound. Can cause a crustal
forebuldge.
7. Processes -
Sedimentation
Sedimentation – Increased (or decreased)
sedimentation from a variety of sources
including glaciers but also rivers, human
activity and tsunamis and change the amount
of sediments in the oceans causing changes in
the relative sea level over time.
8. Processes -
Combination
Many of these effects can be found working
together or against each other at the same time
They can also work in succession e.g. isostatic
depression followed by a rise in sea level
followed by isostatic rebound with overlap very
possible.
Depending on the combination, length, and
strength of effects they could create a variety of
scenarios
9.
10. Southern Cascadia
Unglaciated Cascadia subduction zone from N.
California to Olympia, WA (furthest extent of
Cordilleran Ice Sheet)
Free of fjords and islands that you see in the
Northern Cascadia zone
Many sandy beaches backed by cliffs
Sedimentation rate not high enough for sandy
beaches, likely caused by rising sea level near
the end of the Pleistocene (ended 11.7 ka BP)
11. Southern Cascadia
Most directly affected by eustatic sea level
since the LGM (last glacial maximum – 21 ka
BP) much less affect from isostatic depression
During the LGM RSL (relative sea level) was -
120m below today's level
16.5 ka BP at -75m due to depressed land
being flooded. Back to -100m at 13 ka BP due
to isostatic rebound. From this time RSL
continued to increase to present day levels
12. Southern Cascadia
N. California and S. Oregon
affected by tectonic uplift
creating an emergent coast
Evidence of repeated
coseismic subsidence of
1m +/- 0.5m over the last
4,000 years.
Generally thought to be
much less of a factor in
RSL rise and fall in the area
in comparison to eustatic
sea level changes
13.
14. Northern Cascadia
From Olympia, WA to the northern part of
Vancouver Island
Glaciated Cascadia Subduction zone
Fjords found that extend up to 150km inland
Coastal region characterized by plutonic rocks
that are resistant to erosion and have steep
slopes
15. Northern Cascadia
RSL (Relative sea level) for this area is diverse
and largely depended upon the amount of ice
and the amount of time it spent in an area
Many areas had a RSL +50-180m due to
isostatic depression
From around 11 ka BP (though this varies
depending on location) RSL in many areas
would have been lower than today due to
isostatic rebound being faster than eustatic sea
level rise
16. Northern Cascadia
Generally areas that were
deglaciated first were also
flooded first
Coseismic subsidence,
though a factor, generally
was 0.5-2m per Great
Cascadia Subduction
Earthquake which is
considered minor in
comparison to isostatic
depression and rebound as
well as eustatic sea level
rise.
17.
18. Northern British Columbia
Northern Vancouver Island to the Alaska/
Canada border
Includes the mainland and inner coastal islands
Characterized by the Queen Charlotte strike-
slip fault as opposed to subduction
Steep slopes, deep fjords and mainly granitic
rocks but other types are present as well
Coast is currently glaciated
19. Northern British Columbia
Experienced much more isostatic depression
and rebound than Haida Gwaii due to much
thicker ice on the mainland
Some areas, such as Kitimat, B.C. (~100km
from the coast) found evidence of a +200 RSL
due to isostatic depression followed by flooding,
rebound and eustatic sea level rise
RSL likely dipped below modern levels slightly
before returning to what we have today
20.
21.
22. Outer Islands North Coast
Include the area around the Alexander
Archipelago, Haida Gwaii, Hecate Strait
(between Haida Gwaii and the mainland), and
Cook Bank
Characterized by isostatic forces and crustal
forebulge as well as eustatic sea level rise
As with Northern British Columbia, is unaffected
by the subduction that takes place in the
Cascadia regions
23. Outer Islands North Coast
Haida Gwaii as well as some places in the Alexander
Archipelago may have had areas free of ice although
there most certainly were glaciers in these areas
The ice might have converged with ice from the
mainland but was significantly less in size and scope
Crustal Forebulge under Haida Gwaii caused a lower
RSL of -32m (17-15.5 ka BP), -68m in the northern
Hecate Strait (11.2-10.6 ka BP), -118m in central
Hecate Strait and -135m in the Cook Bank area (15.4-
14.1 ka BP)
From around 12-10 ka BP Haida Gwaii’s RSL rapidly
adjusted to become close to what it is today
24.
25.
26. Southeast Alaska Mainland
Includes the mainland and coastal islands of
southeast Alaska
High, steep mountains, deep fjords, and large
glaciers many of which are thinning up to 10m
annually
16-14 ka BP glaciers in this area started to
retreat
Much less data has been gathered about this
area in comparison to B.C.
Fairweather Fault is a strike-slip fault
27. Southeast Alaska Mainland
Much of the area was +50-230m RSL due to
isostatic depression prior to 15 ka BP
Much of the area rebounded and was much
more similar to what we see as sea level today
by around 8 ka BP
During the Little Ice Age (LIA) +2-6m RSL
Still rebounding from the LIA
An estimated 6-8m of rebound is still to happen
in the Glacier Bay area due to ice lost
28.
29.
30. South Central Alaska
Yakutat Bay to Cook Inlet near Anchorage
Steep, heavily glaciated mountains
High wave energy and a largely exposed
coastline
Much less data has been gathered about this
area in comparison to B.C.
Although not definitively proven, many believe
that the ice in this area would have extended in
to the continental shelf at LGM
31. South Central Alaska
The tectonic setting is very complex and not
well understood with regards to RSL
The ice reached it's maximum extent and
started to regress 2-3 ka before those further
south
~ +30-56m or less RSL due to isostatic
depression
Large earthquakes due to subduction can
cause uplift and subsidence e.g. Alaska 1964
an earthquake caused 2.4m subsidence
32.
33. Conclusions
The NW Pacific Coast of North America is complex
and has a diverse set of processes that differentiate
the areas
Many factors can affect RSL along the coastline
Processes can work together to produce more
extreme differences in RSL
On the other hand, if they work against each other the
differences may appear relatively small even though
much is occurring
The amount of variables involved and the difficulty of
quantifying them especially in areas with limited data
sets is a continuing challenge for this area of study
34. References
Shugar, D.H., Walker, I.J., Lian, O.B., Eamer, J.B.R., Neudorf, C., McLaren, D. Fedje, D. “Post-glacial sea-level change
along the Pacific coast of North America”. 2014. Elsevier Ltd. Web.
http://www.sciencedirect.com.berlioz.brandonu.ca/science/article/pii/S0277379114002030
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