Euler pole description of relative plate motion

1. Euler pole description of relative plate motion PACIFIC NORTH AMERICA

2. Euler pole description of relative plate motion PACIFIC NORTH AMERICA Trace (a portion of) the plate boundary on tracing paper

3. Euler pole description of relative plate motion Spin tracing paper clockwise around the thumbtack, representing Pacific Plate motion relative to North America PACIFIC NORTH AMERICA

4. Euler pole description of relative plate motion The (small) rotation represents millions of years of tectonic motion. PACIFIC NORTH AMERICA

5. Euler pole description of relative plate motion Ω E δ v P vmax v P • Suppose a divergent plate boundary lies along a line of longitude (a great circle) • Euler pole (E) = North pole • Relative plate motion is along lines of latitude (a small circle) • The rate of rotation about the Euler pole gives rise to variable linear velocities, calculated at points P. • The linear velocities are maximized along the equator (90º away from E) and smoothly decrease toward the north and south poles. Euler pole (E) Calculation point (P) Active spreading boundary Crust created since spreading began

6. Ω E δ v P vmax v P Calculating linear velocity from rotation rate Ω δ Horizontal view (into equator) E r = R sin δ P δ R r = R sin δ P Ω v = Ωr = Ω R sin δ v = linear velocity (mm/yr) Ω = Euler rotation rate (º/yr) R = Earth radius (mm) δ = angular distance between E and P (º) E = Euler pole, P = calculation point Vertical view (along pole) Note that the linear velocity is constant along each small circle (in this case, latitude lines)

7. Visualizing RPM as small circles around the Euler pole

8. In the Gulf of California, greater distance from Euler pole = faster relative plate motion Relative motion means that one plate moves with respect to the other Baja moves NW relative to North America; North America moves SE relative to Baja.

9. The relative nature of relative plate motion Baja Microplate moves NW relative to North America Approximate outline of Baja Microplate

10. The relative nature of relative plate motion North America moves SE relative to Baja Microplate Approximate outline of Baja Microplate

11. Relationship between RPM and fault slip rates B Fault segment: strike 352º Fault-perpendicular slip (opening/closing) A Fault-parallel slip (strike-slip) C Plate motion: 44 mm/yr @ 313º A point on the Baja Microplate immediately adjacent to the fault segment (plate boundary) moves relative to North America as described by the RPM vector

12. Measuring plate motion obliquity 1 Average strike of plate boundary 2 3

13. 1 2 3 Measuring plate motion obliquity Average strike of plate boundary α ( Local direction of RPM

14. Relating other faults to oblique rifting Deformation along rift-parallel strike-slip faults Deformation along rift-parallel strike-slip faults Deformation along normal faults perpendicular to RPM Deformation along normal faults perpendicular to RPM Dorsey and Umhoefer, 2012

15. Structural geology of the Gulf of California Low-angle detachment faults Off-axis normal-dextral faults; no detachment faults Dorsey and Umhoefer, 2012