Lecture 8b: Introduction to California Geology

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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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Owens Valley
Long 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 todays 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-and-range 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 boundary is initiated offshore.
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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 Andean-type 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, K2O, less 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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lt50 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 strike-slip 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 strike-slip 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 ring-shaped 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 Cut along Highway 395.

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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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