Integrating Seismological Studies of Crustal Structure

1. 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

2. 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.

3. 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.

4. 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.

5. Integrating Tomography and Receiver Functions • Our aim was to correlate specific horizons from rf’s with velocity contrasts in the tomography. This allowed us to extend the horizons over an evenly-spaced grid of data • points, using the tomography.

6. Extending Horizons Through Tomography

7. 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.

8. 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.

9. 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.

10. 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

11. 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.

12. 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.

13. 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.

14. 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.

15. Relative Velocity Grid: Shallow Layer Fixed End • Dashed line separates our area of interest from area influenced by Great Valley tectonics.

16. 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.

17. Relative Velocity Grid: Mid-Crust Layer • Dashed line separates our area of interest from area influenced by Great Valley tectonics.

18. Relative Velocity Grid: Mid-Crust Layer • Vectors indicate direction and magnitude of velocity

19. Relative Velocity Grid: Deep Layer • Dashed line separates our area of interest from area influenced by Great Valley tectonics.

20. 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.

21. 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

22. NW-SE Profile, Line 2 • Sense of motion reversed east of vertical shear

23. 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

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

25. 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),

26. 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