Transcript Lecture 6: Igneous classification, mid-ocean ridges

Lecture 8b: Introduction to California Geology
• In the field one makes small-scale observations of many
geological processes. It takes years to assemble such
observation into large-scale regional processes, but this lecture
sets the stage for understanding the importance of small-scale
features retroactively by introducing the big picture.
• Questions
– How can we use the framework of plate tectonics to make a
logical narrative of the geological history of a particular
continental margin, California?
– How do the features one sees in the field (mountains,
valleys, faults, volcanoes, glacial deposits) relate to the stuff
we have been discussing in lecture?
• Tools
– Your eyes
– Maps
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Long Valley
Owens Valley
Sierra Nevada
Mojave Desert
You are Here
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Long Valley
Owens
Valley
You are Here
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Continental Margin settings
• There are four
recognized types of
ocean-continent margin.
California has been all
four, at various times in
the past billion years.
• Atlantic-type: a passive
margin, not a plate boundary
• Andean-type: subduction
close to shore, arc volcanoes
built on continental basement
• Japanese-type: subduction
offshore, with a marginal sea
between the arc and the
mainland
• Californian-type: transform
fault, no subduction, no
spreading
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Synoptic history of Californian margin
Hence the major events
affecting the tectonic
evolution of California are:
– A rifting event in the
latest Pre-Cambrian;
– Two orogenies, the
Antler (Devonian)
and the Sonoma
(Permian-Triassic):
collisions of offshore
arcs with North
America;
– Initiation of continental margin subduction with trench in today’s western
Sierra foothills (Triassic-Jurassic);
– Interruption of subduction by Late Jurassic Nevadan orogeny (accretion of
another island arc terrane), initiation of mature Andean trench-gap-arc system
at Franciscan-Great Valley-Sierra location;
– Cenozoic subduction of Pacific-Farallon ridge leading to growing no-slab
zone, Laramide orogeny in Rocky Mountains, then initiation of basin-andrange extension and San Andreas transform.
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Before 700 Ma: Not a Margin
• In the Proterozoic, the west
coast of North America was
attached to some other
continent (East Antarctica?),
and had not previously been a
continental margin since at
least 2 Ga. Beginning around
800 Ma, this other continent
rifted away along an irregular
margin that truncated the old
age provinces and established
a stepped western boundary
of the continent.
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700-400 Ma: Atlantic-type Passive Margin
• Through mid-Devonian time, this remained a stable passive
continental margin (Atlantic type) and accumulated a passive
margin sequence (“miogeoclinal belt”) of clastic and carbonate
sediments up to several km thick.
– The miogeocline is the part of this system on the continental
crust, inboard of the continental slope.
– It tends to be preserved when the margin is activated and
rocks further out, on oceanic crust, are subducted and lost.
– These Proterozoic-Paleozoic sediments still make up much
of the exposed rock in the Western U.S.
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400-250 Ma: Japanese-type Offshore subduction
• The ocean offshore widened and aged until it became unstable
to subduction, which initiated sometime in the early Paleozoic
under an offshore arc, with a steadily closing marginal sea
attached to the North American plate.
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400-250 Ma: Japanese-type Offshore subduction
• In the late Devonian, this offshore arc ran up against the North
American margin in the Antler orogeny
The arc terrane ended up
accreted to the continent,
overlying the miogeocline
across the Roberts Mountain
Thrust Fault
The orogeny ends when the arc terrane is transferred
to the North American plate and a new subduction
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boundary is initiated offshore.
400-250 Ma: Japanese-type Offshore subduction
• After a couple of arc-polarity reversals, the same thing happened again,
more or less, in the Early Triassic Sonoma orogeny, bringing a new
sequence of oceanic rocks on top of the miogeocline and the Antler rocks.
The island arc so accreted forms
the basement for the western
Sierra Nevada batholith
A modern example of arc-polarity reversal can be
seen in the New Ireland-New Britain system in the
Western Pacific, resulting from the arrival of the
Ontong-Java plateau at the trench.
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250-50 Ma: Andean-type subduction
After Triassic time, the polarity
of subduction remained
“normal”; i.e., the North
American continent was the
upper plate.
– Thus a recognizable Andeantype continental margin arc
formed, built on basement of
Pre-Cambrian North America,
Paleozoic miogeocline, and the
arc terranes accreted in the
Antler and Sonoma events.
– The dip angle of the slab was
initially steep, with the arc
rather close to the trench and
minimal deformation deep in
the continental interior
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250-50 Ma: Andean-type subduction
In the late Jurassic, another island
arc terrane, riding on the
subducting Farallon Plate, collided
with the North American continent
in the Nevadan orogeny.
– This time the system responded by
stepping subduction out beyond the
new terrane to establish the
subduction system that lasted
through the Cretaceous and
generated the still clearly
recognizable sequences of
Franciscan trench, Great Valley
forearc, and Sierran arc
– The Sierra Nevada batholith is the
deep assemblage of plutonic rocks
that formed under the arc
volcanoes.
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250-50 Ma: Andean-type subduction
• A relatively low angle slab became more flat with time, causing a wide arc that
propagated inland, farther from the trench (remember the volcanic front is always
~100 km above the Benioff zone, so a flatter slab causes more inland volcanism),
and caused extensive compressional deformation in the continental interior.
• Caltech geologists and
geochemists (notably
Silver and Taylor) have
done much work to
document the age
progression of magmatic
activity in the Sierra
Nevada and Peninsular
Ranges batholiths and
the progressive
incorporation of more
continental source
materials (higher 18O,
87Sr/86Sr, K O, less
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mafic).
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~50 Ma: End of subduction
• In the Cenozoic, the slab became completely horizontal, probably due to
progressive decrease in age of the subducting Farallon Plate as the ridge approached
the trench. Results include end of calc-alkaline volcanism, major compressive
orogeny far inland (Laramide orogeny of the Rocky Mountains) and emplacement
of Pelona and related schists under Southern California
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~50 Ma: End of subduction
• The same process can be
seen today in Chile, where
a relatively flat region of
the slab defined by depth to
the seismic Benioff zone
correlates with a gap
between the Central and
Southern Volcanic Zones of
the Andes and extensive
deformation and K-rich
volcanism far inland in
Argentina.
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<50 Ma: Growth of San Andreas Transform Fault
• The flat-slab event was
probably related to
decreasing age and
increasing buoyancy of the
slab as the Pacific-Farallon
ridge approached the
continent
• This naturally leads to the
next tectonic arrangement,
as subduction of the
Pacific-Farallon ridge
leads to transform motion
between Pacific and N.
American plates between
the Mendocino and Rivera
triple junctions.
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Modern Californian Margin
• Subduction continues
north of Cape Mendocino
(the Cascade margin) and
south of the Rivera triple
junction (Mexican
Volcanic Belt).
• In between, some
combination of drag from
the Pacific plate, back-arc
type tension from the
cascades, and perhaps
thermal doming of the
North American itself have
led to large scale extension
of the Basin-and-Range
province, forming the
characteristic topography
of the Great Basin, and
perhaps also the Rio
Grande Rift.
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Topographic Expression of
Extension in Western U.S.
Basin and Range
Owens Valley
Rio Grande Rift
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Modern California: Basin and Range Extension
• Eastern California is the
westernmost edge of the
Basin and Range
extensional province.
(Owens Valley is a Basin;
the Sierra Nevada and
White-Inyo Mountains are
Ranges).
• Extension is
accommodated by normal
faults. When conjugate
normal faults form,
dipping in opposite
directions with the same
strike, the downdropped
block between is called a
graben.
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Modern California: Strike-Slip tectonics
• On the other hand, California is also a transform
plate boundary zone, which is accommodated be
a series of strike-slip faults.
• There is evidence of strikeslip motion across the surface
rupture of the 1872 Lone Pine
earthquake. This air-photo of
the San Andreas Fault shows
a somewhat clearer offset
drainage.
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Modern California: Transtension and transpression
• When you combine strikeslip motion with a component
of extension or a component
of compression, perhaps due
to bends in the faults or
motions oblique to the fault
directions, you create a
number of characteristic
topographic features.
• Death Valley, and parts of the
Owens Valley, are pull-apart
basins formed by combined
extension and shear.
• The Transverse Ranges, like
the San Gabriel Mountains,
are formed by compression
due to a big bend in the San
Andreas Fault.
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Modern California:
Post-arc magmatism
• Large-scale extension of
continental lithosphere leads to
upwelling of asthenospheric
mantle and basaltic volcanism.
• This plot shows the thinning of
mechanical and thermal
lithospheres and the growth
upwards of the partially molten
region as a function of stretching
factor b = (final area) / (initial
area). There is substantial
Miocene basaltic volcanism
through the Basin and Range
province.
• If the basalt is too dense to erupt, it underplates and heats the crust. Basalt
underplating due to finite extension leads eventually to crustal melting and
Rhyolite volcanism as at the Rio Grande Rift (Valles Caldera) or Owens Valley
(Long Valley Caldera). It takes time to conduct heat through the crust, so we
expect a delay between onset of extension (Miocene) and rhyolite activity
(Pleistocene).
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Modern California:
Post-arc magmatism
• Long Valley is a Caldera, an
elliptical hole formed by
collapse of the roof of a
magma chamber along a ringshaped normal fault. It formed
in a single catastrophic
eruption 760,000 years ago,
ejecting the Bishop Tuff.
• The Bishop Tuff is a volcanic ash deposit consisting
of airfall units (of individual ballistically emplaced
particles), and ignimbrites or pyroclastic flow
deposits (resulting from gravity-driven currents of
air and suspended particles). The figure at right
shows the stratigraphy of the Tuff units (Ig for
ignimbrite, F for fall), as exposed at the Big Pumice
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Cut along Highway 395.
Pleistocene California: Valley Glaciers
• There is considerable evidence of the action of Valley Glaciers that
descended from the High Sierra during the Pleistocene ice ages.
• Glaciers create a range of characteristic rock deposits and geomorphic
features. We will have more time to discuss such things later, but here is the
thumbnail version.
A deposit of unsorted,
unlaminated debris from
glacial outwash is called till.
Mounds of till pushed by
glaciers or dropped at
locations where the ablation
front of the glacier stalled
for a time are called
moraines:
• lateral moraine
• medial moraine
• terminal moraine
• recessional moraine
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