Material transport in the crustal mill

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Material transport in the crustal mill

• Mihai Ducea

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functions

• The melting function
• (the extraction function)
• The chemistry function
• The structural or architectural style, the
  conceptual framework

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An arc constrained by chemistry
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The real thing.
Andronicos et al., 2003
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Major factors

• The architectural style of batholiths
• The added complexity of arc continent collision
• Pluton emplacement and metamorphism
• Strength of the arc crust
• Thrust tectonics, vs gravity currents
• Shear zones in arcs
• The surface connection

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• Every tectonic setting has its universal features
  that makes it recognizable in the geologic
  record
• There are particularities that make every example
  unique in detail
• What is global and what is local in an arc
  environment?

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Arcs

• Have a recognizable chemical signature
• Have a unique structural style compared to other
  convergent regions
• Have a thermal identity

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The meaning of geologic contacts
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Hamiltonian view - ca. 1967 and continuing
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Klepeis et al., 2003
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Dinkey Creek pluton and surrounding (central
Sierra)
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Any structures in the upper crust? Where do
plutons level off?
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The deeper crust
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Take home points

• Lower crust - hot, gt800 C, over tens of My
  dehydration melting omnipresent emplacement of
  basalt, relatively continuous
• Upper crust - batches of melt escape periodically
  and are isotopically equilibrated- come from the
  lower crust

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Strength of the lower crust

• Very weak rocks at the deepest levels of exposure
  with intermediate compos
• E.g. tonalitic orthogneisses

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Plagioclase rules
Kidder and ducea, 2006 epsl
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consequence

• Lower crustal arc rocks flow
• Ortho and paragneisses horizontally layered at
  30 km
• Not true for 50 -60 km deep

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Thermal structure in the upper mid crust

• Inverted Pt gradients are common higher P rocks
  over lower P
• Require reverse faulting

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Typical thrust belt?
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Different sense of migration, steep, carry melt
sills or young magmatic rocks metaseds,
difficult to map Migrate at mm/yr rates
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Panels of plutonic suites of progressively
younger ages in one direction Not all have to
be separated by shear zones (ductile faults)
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Butler et al., 2001
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Migrating (kinematic) arcs
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Arc-continent collisions

• Most arcs record terrane accretion
• Soft collisions while an arc is active

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Intracrustal indentors

• Zones of localized thickening of the crust via
  subduction of upper crust into the
  astenospheric lower crust
• Only if crust is hot, perfect for terrane
  collisions

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Consequences

• A series of reverse ductile faults that get
  embrittled at shallower levels
• A mechanism for evacuating mid to deep crustal
  rocks, magmas and others
• Generate a leaky orogenic core, a vertical
  shear zone in its late stages

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Static arcs

• Different emplacement mechanism
• Large plutons of g-dioritic composition
• Flare up on the continental interior side of arcs
  late in the magmatic history
• Only in areas of very thick crust
• Extracted from depths gt 40 km
• Pond as sills and/or cross cut

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Static arcs
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Extension probably plays a role in facilitating
ascent Buoyant large volume bodies Significant
topography, crustal wealth The larger the body -
more buoyancy A critical size for eruption vs.
ponding in the upper crust
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Rapid In-ruption in the upper to mid crust
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A relevant example

• PRB, so Cal and Baja
• Migrating arc early to mid K
• Late K static arc
• Leaky core, called the Fan Structure because it
  is doubly vergent

Schmidt and Paterson, 2001
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The surface connection

• What kind of magmatism is expected to correspond
  to these structures at the surface?
• Try to frame this in a modern context.

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TEM image of Western Cordillera, Altiplano, and
part of the Eastern Cordillera
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Andesitic volcanoes vs. large silicic eruptions

• Volcanic arcs - linearly distributed in the upper
  plate at any given timethey are the surface
  equivalents of the tonalite sill etc
• Rhyolite/ignimbrite centers are scattered the
  equivalent of the Eocene plutons

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Central Gneiss

• Is the metamorphic complex that resulted from the
  collapse of the BC-plano
• The layered sills are the APVC equivalents

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We can make a significant contribution to
understanding the dynamics of arcs with this
study.