Geology Reference
In-Depth Information
South of the 45th parallel at the latitude of Salem the
fold gradually inverts to a broad basin or syncline. The
southern part of the Coast Range synclinorium extends
all the way to the limit of the physiographic province
where the axis swings southwest near its extremity in
the Klamaths.
Because the Coast Range has been severely
flexed and bent by folding, rocks in the province are
cracked and broken by a network of criss-crossing
faults and joints. When viewed from the air, the major
faults follow three patterns. One group runs in an
east-west direction, one in a northeast by southwest
direction, and one in a northwest by southeast direc-
tion. Across the continental slope and into the shelf
most of the faults are steeply angled thrust faults where
the overhead or hanging wall has gone up and over the
bottom or footwall. This kind of fault, typical of crustal
shortening or telescoping, is produced as the oceanic
Juan de Fuca plate is thrust eastward beneath the
North American continental plate. Thrust faults along
the coast, continental shelf, and slope reflect the
pressure on rocks caught between these two large-scale
plates. Within the Coast Range itself, most faults,
known as normal faults, are nearly vertical or dip
steeply to the east or west. These faults ordinarily have
their overlying part or hanging wall dropping with
respect to the underlying footwall. This kind of rock
failure represents crustal extension or stretching and
reflects the tension placed on rocks by folding and
flexing. Examples of such enormous structures are the
Kings Valley, Corvallis, Gales Creek, and Portland
Hills faults. Displacement on the Corvallis fault is as
much as several miles. A third type of fault is exempli-
fied by the north-south Fulmar fault midway across the
Oregon continental shelf. This fault is a shear in the
crust where the east side moves south and the west side
moves north much like the San Andreas fault in
California. Called a strike-slip fault, this large-scale
feature may have had as much as 120 miles of move-
ment in middle and late Eocene time.
Earthquakes
Oregon's recorded history of earthquakes is
minimal and within the written records, which go back
only 175 years, no significant quakes have been noted.
However, evidence of catastrophic Holocene seismic
activity has come to light as recently as 1987 with
studies of buried coastal lowland swamps and bogs as
well as offshore turbidity flows in Oregon and Wash-
ington. Along the coast the land dropped abruptly from
2 to 8 feet a number of times over the past 7,000 years.
The subsidance of sections of the land below intertidal
muds must have occurred very quickly as many of the
trees in the swamp deposits are still preserved in an
