Download The Rocky Mountain Region

Cascadia
Subduction
Zone
The Rocky Mountain Region
– Steep hills that are foothills of the Rocky Mountains in the
northern corner of the state
Overview of the Geologic History of
Washington State
1.5 Billion years-750 MY
• Belt Supergroup– failed rift
– Thick (70,000 ‘) sed seq
– Lifeless plain clay-siltsand
• Windermere 750 MY
– Continental break-up
west sid e of Belt Group
dirfts west, Windermere
sed rx deposited on the
wedge
The oldest rocks in Washington State are part of
the 1 billion year old Belt Supergroup
The North American
Continental Coastline
was located
approximately 30 miles
west of the the
present day border of
Idaho and Washington
Pangea Breakup- 175 Ma
• Continental rifting
• Atlantic basin sends
NAm to the west
• Subduction zone set up
on west coast
The continued addition of material to the North American
Continent was through the process of Plate Tectonics
• Plate tectonics conveyed
small volcanic islands
and fragments of other
continents called
terranes onto the North
American Continent and
accreted them in the
area of the existing
coastline.
Orogenies and Mobile Belts
• Terranes originally were:
– Island arcs (e.g., Japan)
– Seamounts (e.g., Hawaii)
– Fragments of continents etc.
• Accreted to western margin of North
America as the continent moved west and
over-rode Pacific Ocean, starting with the
Mesozoic Sevier orogeny.
The first terrane to arrive was the Okanogan Micro-continent at
190 to 160 million years ago; formed from the collision of former
islands Cache Creek, Quesnellia, and Stikinia.
• This micro-continent
crushed the coastal plain
that had been building for
600 million years (800 to
200 myBP) between the
North American Continent
and the Okanogan creating
an area of displaced marine
sedimentary materials
called the Kootenay Arc
The next major addition occurred ~90 to 100 million
years ago with the arrival of the North Cascade Terranes
• The North Cascade
Terranes are believed to
be made up of a group of
6 different islands.
• Each individual terrane
can be distinguished by its
unique rock type.
• These terranes were
covered later by volcanic
materials and complicated
their interpretation.
The Insular Terrane arrived behind the North Cascade
Terranes at about 100 million years ago
• This terrane is
believed to have
been two large
pieces that make up
the basement rocks
under the San Juan
Islands and the Blue
Mountains
The last event to add to the Washington coastline was approximately
25 million years ago is called the Crescent Terrane located in the areas
of the Puget Lowlands, Olympic Peninsula and the Willapa Hills
• The Crescent Terrane is believed to
be a portion of ocean crust that was
stranded between an extinct trench
and the present day trench. Some
how more buoyant rocks were
carried underneath the continental
crust, then escaped pushing the
overlying rocks as much as 2 miles
above the ocean crust
Miocene
Now
Juan de Fuca plate and Cocos plate are the
remnants of the larger Farallon plate now
almost completely subducted.
Perhaps 25% of western N.
America consist of accreted
exotic terranes:
 they differ completely in
their Mesozoic/Paleozoic
fossil genera, stratigraphy,
structural trends,
paleomagnetic properties
from the surrounding
rocks
 >200 recognized
 Range in size from 100s of
km wide/long to a few km
wide/long
Baja BC” hypothesis:
Much of the western margin of British Columbia (including Vancouver Island) was moved
approximately 3000 km northward, from a position close to the California – Mexico border,
some time between 50 and 90 million years ago.
Inferred movement of “Baja BC”: 2000-3000 km of displacement in 1540 million years ?!
Subduction Produces Two Parallel Mountain
Ranges in the Pacific Northwest
Parks and Plates, ©2005 Robert J. Lillie
Olympic National Park, Washington
Steady-State
Subduction
Parks and Plates
©2005 Robert J. Lillie
Olympic National Park, Washington
Steady-State
Subduction
Parks and Plates
©2005 Robert J. Lillie
Olympic National Park, Washington
Steady-State
Subduction
Parks and Plates
©2005 Robert J. Lillie
E
E’
Parks and Plates
©2005 Robert J. Lillie
Filling of the
Columbia
Embayment
Westward roll-back of
subduction
Fills Columbia Embayment
Stabilized near present
location for ~ 40 Ma
Cascade range:
Uplift affects climate on east
side
Volcanoes supply ash to
winds and area
The Columbia River Basalts
Basalts yield
excellent soils in
Hawaii and other
tropical areas
where rainfall is
high and the
climate is warm
Our soils in WW are
not derived from
the underlying
basalts-an unusual
feature
The Columbia River Basalts
Table 15-1. Major Flood Basalt Provinces
Name
Volume
5
3
Age
Locality
CRB
(1.7x10 km )
Miocene
NW US
Keeweenawan
(4x105 km3)
Precambrian
Superior area
Deccan
(106 km3)
Cret.-Eocene
India
Parana
(area > 106 km2 ) early Cret.
Karroo
(2x106 km3?)
Brazil
early Jurassic S. Africa
The Columbia River Basalts
The Columbia River Basalts
Aerial extent of the N2 Grande Ronde flow unit (approximately 21 flows).
The Columbia River Basalts
Location of the exposed feeder dikes
red) and vents (blue V's) of the
southeastern portion of the
Columbia River Basalts.
CRBs probably result from SRPYellowstone hot-spot- difficult to
explain northward deflection
WW
The last significant volcanic events to shape the
Washington landscape were the birth of the Cascade
Volcanoes between 1 million and 75,000 years ago
• Creation of the Cascade
Volcanoes was a direct result
of plate tectonics and the
subduction of the Fallaron
Plate under the western coast
of North America.
• The volcanoes were created in
a progressively northern
direction beginning in
Northern California and now
extending into Canada.
SUBDUCTION ZONE EARTHQUAKES
PACIFIC NORTHWEST
2/28/2001 M6.8 Nisqually earthquake
SUBDUCTION ZONE EARTHQUAKE
PACIFIC NORTHWEST
2001 Nisqually earthquake
M6.8 Normal fault
Earthquake hypocenter 52.40
km depth under Tacoma
Because it was so deep caused
remarkably little
damage at the surface.
Seismicity in the Pacific Northwest
Washington Geology
4. Cascades continental volcanic arc begins (30
million years ago to present)
5. Columbia River basalts erupt (17-6 million
years ago, mostly around 15 Ma)
6. Cordilleran glaciation (2 million years –
11,000 years ago and again?)
7. Lake Missoula floods (13,000 years ago to
present)
Pleistocene ice sheets and glaciers that occurred from 2
million to 500 years ago
• The area we are in was
carved by the glaciers and
the sharp features on the
high peaks are called aretes,
horns, cirques and hanging
valleys.
• Many of the lakes were
created by the glaciers and
glacial outwash called till and
loess blanket the region.
The Great Floods
~ 12,000 years ago