Late Cenozoic and Quaternary Extension and Volcanism

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Late Cenozoic and Quaternary Extension and
Volcanism
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Late Laramide Effects

• Convergence to the west compresses and thickens
  crust in Arizona.
• Magmatism may also have heated the crust.
• Early Tertiary (40-50 Ma) steepening of
  subduction angle relaxes compression.
• Mid-Tertiary (Oligocene) resurgence in magmatism.

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Late Cenozoic Extension

• Between 30-20 Ma, crust began to stretch.
• Extension of up to 100.
• Brittle faulting in upper crust
• Ductile stretching in lower crust

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Late Cenozoic Magmatism

• Oligocene-Miocene calderas, plutons, and dikes
  (30-15 Ma)
• Related to breakup of subducted plate (?)
• Calc-alkalic chemistry, crustal melting, westward
  migration over time

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Late Cenozoic Magmatism

• Miocene-Quaternary scattered volcanism (15
  Ma-present)
• Basaltic or bi-modal
• (basaltic and rhyolitic)
• Mantle melting, rapid rise,
• little interaction with crust

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Late Cenozoic Magmatism

• Post-Miocene basaltic or bi-modal volcanism in
  AZ, NM, and Mexico

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Late Cenozoic Magmatism
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Late Cenozoic Extension

• Crustal extension began 30-20 Ma
• Two phases with different characteristics
• First phase - listric (curved) faults with low
  angle detachment at depth
• Second phase - high angle, linear, forming
  grabens or half grabens

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Causes of Extension

• No general agreement
• Active cause rifting caused by asthenospheric
  upwelling
• Passive cause
• Overthickening of crust during Laramide, gave
  crust potential energy
• Compression stops, crust relaxes and extends
• Magmatism may have heated crust, making it more
  ductile

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Causes of Extension

• Passive cause
• Formation of San Andreas transform boundary to
  the west
• Changes in extension style, magmatism suggest
  control by San Andreas
• Composite causes
• Active in mid-Tertiary
• Passive in late-Tertiary to Quaternary

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Low Angle Detachment Faulting

• Fault angle decreases below surface, faults
  truncate into regional detachment fault.
• Ductile deformation at depth
• Mylonitization (microbrecciation, metamorphism,
  recrystallized and oriented grains)

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Low Angle Detachment Faulting

• Faulting oriented E-NE to W-SW
• Fault blocks rotated, up to 15 km of isostatcic
  uplift
• Eroded sediments fill basins between fault blocks

Rotated fault blocks
Basin fill
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Low Angle Detachment Faulting

• Detachment, uplift and erosion bring basement
  rocks to the surface
• Metamorphic core complex exposed, brittle and
  ductile mylonitization

Metamorphic core complex
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Metamorphic Core Complexes

• In SE AZ, core complex exposed in Santa Catalina
  and Pinaleno Mountain ranges.

Tucson Mountains tilted NE, displaced 20-30 km
to the west
Metamorphic core complexes
Tucson Mtns caldera
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High Angle Normal Faulting

• Post 15 Ma, style of extension abruptly changes
• High angle normal faults, striking E-SE to W-NW
• Form grabens or ½ grabens, tilting lt15o
• Modern landscape reflects this style
• Individual basins/ranges 10-30 km wide, 50-150 km
  long

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Tucson Basin Cross Section

• Faulting starts in the center and moves outward,
  sediments several km thick

Post 15 Ma fill
Pre-Tertiary bedrock
Mid-Tertiary seds./volcanics
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Basin and Range Evolution

• Mid-Tertiary detachment faults cut by high angle
  normal faults
• Fault block mountains uplifted
• Hydrologically closed basins fill with sediments

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Basin and Range Evolution

• Faulting slows, mountains eroding and being
  buried, basins still closed
• Bedrock pediments form, erosion of some basin
  sediments, basin hydrologically connect