Integrating Seismological Studies of Crustal Structure

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Integrating Seismological Studies of Crustal
Structure

• Investigating the Northern California Coastal
  Ranges to Construct a Regional 3D Strain Model

Gavin P. Hayes1, Kevin P. Furlong1, S. Schwartz2,
C. Hall2, C. Ammon1
1.Department of Geosciences, Penn State
University 2. Earth Sciences Department,
University of California Santa Cruz
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• Regional Overview
• Area of study in Northern California, between San
  Francisco Bay and the Mendocino triple junction.
• Data available from 5 local seismic stations 3
  permanent and 2 temporary.
• Research aims to test models of thickening and
  thinning associated with the northward migration
  of the triple junction.

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• The Mendocino Crustal Conveyor (MCC)
• Furlong and Govers (Geology, 1999)
• As the triple junction migrates north, upwelling
  asthenosphere fills the slab gap and accretes
  to both plates.
• This coupling pulls North America into itself,
  causing crustal thickening, and thinning further
  south.

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Available Data

• Regional Tomography model (Villasenor et al,
  1998) gives a smoothed velocity profile over the
  whole area, consistent with Mendocino Seismic
  Experiment (Beaudoin et al., JGR 1998)

• Receiver Functions from local stations (Hall,
  2003
• Hayes, 2002) give more specific velocity models.

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Integrating Tomography and Receiver Functions

• Our aim was to correlate specific horizons from
  rfs with velocity contrasts in the tomography.
  This allowed us to extend the horizons over an
  evenly-spaced grid of data
• points, using the tomography.

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Extending Horizons Through Tomography
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Layer Depth Shallow Horizon (12km)

• Little variation in depth of surface layer.
• Slightly thicker swath correlates to Central Belt
  of the Franciscan Complex.

• Dashed line separates our area of interest from
  area influenced by Great Valley tectonics.

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Layer Depth Mid-Crustal Horizon (25km)

• Structure of deeper layers cuts across the grain
  of the Franciscan.
• More variation in thickness apparent in deeper
  layers.

• Dashed line separates our area of interest from
  area influenced by Great Valley tectonics.

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Layer Depth Moho (32km)

• Structure of deep layer roughly follows
  mid-crustal layer

• Dashed line separates our area of interest from
  area influenced by Great Valley tectonics.

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Mapping Horizons into Strain Rates

• Vertical Strain from simple change in length
• / original length relationship
• Calculations made in plate motion direction, as
  thickening assumed a result of crustal conveyor
  processes.
• Horizontal strain calculated using a Conservation
  of Area assumption

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Horizontal strain-rate shallow layer (0-12km)

• Strain Rate in direction of PAC/NA Plate Motion
• Strain Rate of
• 1Myr -1 3.2x10 -14 s-1

• Dashed line separates our area of interest from
  area influenced by Great Valley tectonics.

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Horizontal strain-rate Mid-Crust layer (12-25km)

• Strain Rate in direction of PAC/NA Plate Motion
• Strain Rate of
• 1Myr -1 3.2x10 -14 s-1

• Dashed line separates our area of interest from
  area influenced by Great Valley tectonics.

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Horizontal strain-rate Deep layer (25-32km)

• Strain Rate in direction of PAC/NA Plate Motion
• Strain Rate of
• 1Myr -1 3.2x10 -14 s-1

• Dashed line separates our area of interest from
  area influenced by Great Valley tectonics.

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Converting Strain to Relative Velocity

• Horizontal strain rate accumulated over the
  distance between grid nodes gives a
  point-to-point relative velocity.
• All velocities are relative to a pinned
  north-west end of the grid.

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Relative Velocity Grid Shallow Layer
Fixed End

• Dashed line separates our area of interest from
  area influenced by Great Valley tectonics.

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Relative Velocity Grid Shallow Layer

• Vectors indicate direction and magnitude of
  velocity

• Opposite sense of motion in the west indicates a
  vertical shear at this depth.

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Relative Velocity Grid Mid-Crust Layer

• Dashed line separates our area of interest from
  area influenced by Great Valley tectonics.

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Relative Velocity Grid Mid-Crust Layer

• Vectors indicate direction and magnitude of
  velocity

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Relative Velocity Grid Deep Layer

• Dashed line separates our area of interest from
  area influenced by Great Valley tectonics.

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Relative Velocity Grid Deep Layer

• Vectors indicate direction and magnitude of
  velocity

• Again, opposite sense of motion in the west
  indicates a vertical shear at this depth. Here,
  higher velocities indicate a more developed
  shear.

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NW-SE Profile, Line 1

• Cross sections through velocity grid identify
  areas of horizontal shear.
• Thickening and thinning is predominantly
  localized to the mid-lower crust area

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NW-SE Profile, Line 2

• Sense of motion reversed east of vertical shear

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NW-SE Profile, Line 3

• Thickening/thinning pattern more developed
  further inland, where the MCC dominates
• Shallow layer shows little-to-no thickening or
  thinning

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Shear Zone Implications

• Combining information from grids and profiles
  identify several key areas of vertical and
  horizontal shear.

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Shear Zone Implications

• Combining information from grids and profiles
  identify several key areas of vertical and
  horizontal shear.
• These can be interpreted as shear zones and
  mid-crustal detachments.
• Western horizontal shear zones may correlate with
  Bay-area mid-crustal reflectors (BASIX, Brocher
  et al., Science 1994),

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Regional Interpretation

• Location of vertical shears correlate well with
  northern extensions of Hayward Fault
• Western horizontal shear may indicate a link
  between these faults and the San Andreas Fault
  further west
• Eastern horizontal detachments indicate a
  decoupling of shallow and deep crust
• This effect may mask the geodetic signature of
  the thickening and thinning at depth