Just an ordinary

1
The Mesozoic Batholith of western North America
1) Establishment of continuous arc (Farallon
plate subduction) 2) Episodically active
magmatism (probable delamination) 3) Eastward
migration of the arc (probable slab
flattening, see next page too)
90-120 Ma (initial slab flattening)
130-200 Ma (early Farallon subduction)
200 80 Ma
Just an ordinary volcanic arc
Two magmatic flare-ups 160-150 Ma
100-85 Ma
The classical cross section California transect
at 100-200 Ma
Nevadan
2
Age (Ma)
Transition from the intense Sevier to the very
intense Laramide Plateau subduction
Subduct twin of Shatsky Rise beneath southern
California lift slab and rip away the
base of the lithosphere.
flareup
Large flood basalt flows occurred 145 Ma on the
Pacific-Farallon spreading center. The half that
went with the Pacific plate is the Shatsky Rise.
By plate reconstruction, we infer that the
half on the Farallon plate subducted beneath
southern California between 85-65 Ma. This
would have made the Farallon slab buoyant and is
thought to have caused slab flattening against
the base of North America during the Laramide.
Sevier
Laramide
Early Sevier
Late Sevier
Laramide
3
The underplate of the Pelona-type schist
beneath southern California
The track of the oceanic plateau
Based on arc magmatism shutdown, the slab is
inferred to flatten to the north and south of the
plateau.
Q. Was cause of Laramide uplifts gravitational
collapse of Sevier welt or of the Farallon
slab on the base of the Colorado Plateau? A.
Direction of Colorado Plateau motion indicates it
was basal drag so too the in-line relation
between the Colorado Plateau and the
Pelona-type schist
First cooling
Tectonic (and magmatic) effects of plateau
subduction 1. Rocky Mountain uplifts (NE
transport of Colorado Plateau in plateau
track) 2. Slab flattening N and S of plateau
track, more Rocky Mountain uplifts 3. Removal of
southern California basal crust and mantle
lithosphere 4. Underplate of subduction complex
(Pelona-Orocopia schist) 5. Magmatic quiescence
of the Sierra Nevada
Second cooling
4
Shape of the lithosphere beneath southwestern U.S.
So, what did the flat slab do to North America?
Across track of ocean plateau
Along track of ocean plateau
Considering flat-slab subduction, what processes
are possible?
All I can think of is lithospheric hydration
under cool conditions
at 0.5 Ga
5
Slab removal The ignimbrite flareup and the
metamorphic core complexes
Removal of Farallon plate from beneath North
America caused the ignimbrite flareup, as
asthenosphere replaced the Farallon slab and made
contact with the hydrated base of North
America. Western U.S. underwent a rapid
transition from being essentially amagmatic to
being very magmatic. Magmatism propagated across
western U.S. in two sweeps (see figure at right
numbers give age of initial magmatism in m.y.).
This is information on how the Farallon slab was
removed. Magmatism apparently weakened the
lithosphere, allowing the thick Sevier crust to
collapse gravitationally and creating the
metamorphic core complexes (far right fig)
The northern sweep is associated with accretion
of Siletzia (green in figures). Bottom row of
figures shows how accretion of Siletzia caused a
tear in the subducted Farallon slab, which
resulted in opening a window in this slab. This
started the ignimbrite flareup and started the
end of the Laramide. The southern magmatic sweep
is discussed on another page.
47
6
Why the area now occupied by the Basin and Range
may have been so magmatically active (compared to
areas to the east)
The southern magmatic sweep and window, and the
Mendocino triple junction
Southern magmatic sweep related to the slab-free
area south of the Mendocino triple junction,where
no slab is subducted (it is a transform margin,
see figure at bottom right). Colored dots show
where volcanic rocks occur of age 21 Ma.
What the slab may have been doing beneath western
U.S.
(a) Shows possible slab shape at 40 Ma. South of
Mendocino fracture zone (mfz) the young, hot and
relatively thin slab is shown dripping off. The
Farallon slab subducts normally beneath Siletzia
(gray), with a tear between it and the flat slab
that extends beneath the Rocky Mtns. The central
portion of slab is shown tacoing so that the
edges pull toward the center, creating the two
propagating magmatic sweeps. (b) Two alternative
means for slab removal. Top shows the taco
illustrated in (a). Bottom shows a north-to-south
sideways rollback or peeling away.
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Present
Yakama fth
Siletzia
P
NW BR
NA
BR
JdF
Mz arc terrain
motion
wrt NA
str-slip
normal
thrust
k
sn
SAF
ecsz
P
NA
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Jon Spencer
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Magmatism along the old lithospheric margin
Pc N.A.
Accreted N.A.
Rapid development
gt Starting with Steens (SE Oregon) gtS-to-N
propagation along Pc margin gtLargest with
Columbia River Eruptions (where buoyant flow
would be trapped)
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Putting PNW magmatism into a plume context
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II.
Plume hits craton, dribbles over edge
1
3
III.
Subducting slab drags plume NE plume ruptures
slab below NE Oregon
2
(B Jordan)
CRB
I.
Plume hits in SW Oregon, flattens rapidly along
craton edge
(Geist Richards)
(Vic Camp)
plume
Note CRB flood basalt is NOT attributed directly
to plume head impact
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Columbia River Basalt Eruptions Timing
Considerations
History of eruption
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