VIDEO
CZ Subduction on NW Coast
End Mz (Cretaceous) beginning of Cz (Tertiary) (65mybp)
4 large plates
Farallon, Kula, Pacific and N. American
Kula Plate was derived from Breaking of Farallon plate
Plate configuration Beginning CZ
4 plates
NA
Kula
Farallon
Pacific
Plate interactions are important to NW because causes many different volcanic episodes throughout Tertiary
Kula plate moves north
at fast rate. Subducts totally
under
Farallon plate subducted under N. American plate with remnant being the Juan de Fuca, the Gorda and the Explorer plate
Plate interaction….what does this mean
Subduction
Angle of Subduction
Rate of Subduction
Angle plates are colliding
Interaction of colliding plates
Causes rotation
Extension
Compression
Interaction of plates creates geological structures and features
Basin and Range
VERY LARGE AREA 300,000 mi2 (8%
Extends from
Series of N-S trending mountains and valleys
Created
by tensional stresses…What kind of faults do we get from tensional stresses?
Structure created by stretching the crust
Back arc spreading
Due to extension and rotation from plate interactions
Mantle plume
Magma pushes up on overriding crust extending it
Old subducted slab
Broken slab from old subduction zone pushing upward on crust
Probably caused by a combination of these
Area has been extended by 50-100% of original
1000 miles to the north and south
300-600 miles from east to west
Results of Stretching
Series of Mountains and Valleys form
Valleys at VERY high elevations
3000-5000 ft
Rain Shadow deserts
Internal drainage
Extension created many volcanoes from the middle of the Miocene to present
Why does extension create volcanism?
Composition of volcanoes primarily basalt with some intermediate
Not the same composition as the CRB’s
Internal drainage to basins
During
the
As the Climate changed lakes dried out and created
Salt lakes (Bonneville, Albert, Borax)
Playas
Many basins filled with sediment from adjacent mountains (5’000-10,000’ thick)
General history of B &R
Lt Mesozoic: Compression due to accretion of terranes. Created many thrust and reverse faults. Also created mountain
ranges.
Probable some areas in NW are underlain by Paleozoic accreted terranes.
Early Cenozoic: Opposing plate interactions caused shearing across ID and NV. Wide spread normal faulting
2nd phase of extension in Miocene (17 mybp) due to back arc spreading and change of plate motion
“Tears” develop in N Amer. Plate that run NW-SE across B&R…Back arc spreading continues as does volcanism
SE corner of State
Forms during Miocene and Pliocene in two phases
20-10 mybp:
Opened Nevada-Oregon rift
Fractures in
Steens Moutain Basalt
Silicic volcanism (rhyolitic):
McDermitt Caldera
Diameter of 22 miles
Possible tie in to Yellowstone Hotspot
Last 10 my: Create fault block topography
2 Trends for mountain rangesà NW & N-NE
Creates V shaped basins which open at north
Some Silicic volcanism due to a changing subduction zone or hot spot
Back arc spreading in Miocene possibly related to subduction slab “breaking” and leaving remnant slab under continent]
Possible hotspot causing extension
Get a combination of basaltic and silicic volcanism
Basalt from extension
Silicic due to influence of magma coming through crust
Fault Blocked Mountains and Basins
Basic Stratigraphy
Oldest Rocks possibly date to accreted terranes (Pz-Mz)
Most Mz rocks covered with younger volcanics
Volcanics are mix of basalt and silica rich depending on what was happening tectonically.
Granite Pluton emplaced during Nevadian Orogeny (Jurassic)
Pike Creek formation
Miocene in Age
1200 ft thick
Silica flows of Rhyolite and dacite…Banded rhyolite
Miocene in age
Mix of andesite and basalt
Steens Basalt
5000’ thick
Ash Flow Tuffs
Devine Canyon: 9 mybp
Prater Creek: 8.4 mybp
Rattlesnake Ash: 6.4 mybp
Also found in Western Blue mtns
All of these derived from the Harney basin/caldera
Topping all of these is the Harney formation:
Sediments and conglomerate deposits
Wright’s point basalt
Large Fault Block (elevation 9800 ft)
Fault extends 90 miles from
Lower Part of mountain:
Rocks date 21mybp (volcanic)
Upper Part of mountain:
Steens Basalt: 12mybp
Mountain is VERY glaciated
Pleistocene created pluvial lakes due to high rain fall
Features in the area
Mining
Uranium, Mercury
Diamond craters