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Lecture 8b: Introduction to California Geology


It takes years to assemble such observation into large-scale ... of the Rocky Mountains) and emplacement of Pelona and related schists under Southern California ... – PowerPoint PPT presentation

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Title: Lecture 8b: Introduction to California Geology

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,
  • 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

Owens Valley
Long Valley
Sierra Nevada
Mojave Desert
You are Here
Long Valley
Owens Valley
You are Here
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
  • 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

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
  • 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

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.

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
  • 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.

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.
  • The situation like Japan today, where subduction
    is occurring on both sides of the arc, with the
    Pacific plate and the Japan Sea both subducting
    under Japan. If not for back-arc spreading in
    the Japan Sea, Japan would eventually be accreted
    to Asia.

400-250 Ma Japanese-type Offshore subduction
  • In the late Devonian, this offshore arc ran up
    against the North American margin in the Antler

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.
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.
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

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
  • 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.

250-50 Ma Andean-type subduction
  • This new slab was a relatively low angle slab,
    becoming 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 behind the arc (the Sevier fold and
    thrust belt).
  • 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).

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

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.

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
  • 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.

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.

Topographic Expression of Extension in Western
Basin and Range
Owens Valley
Rio Grande Rift
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.

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 an offset drainage.

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
  • 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.

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).

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.

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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