Euler pole description of relative plate motion NORTH AMERICAPACIFIC

Euler pole description of relative plate motion NORTH AMERICAPACIFIC Trace (a portion of) the plate boundary on tracing paper

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

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

Euler pole Transform faults Length of transform faults decreases closer to pole, implying slower slip rate (on transforms) and slower opening rate (on ridge segments). Euler pole description of relative plate motion

v v v max δ Ω E P P Euler pole description of relative plate motion 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

r = R sin δ R δ δ Ω E P v v v max δ Ω E P P P r = R sin δ 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 Note that the linear velocity is constant along each small circle (in this case, latitude lines) Calculating linear velocity from rotation rate Horizontal view (into equator) Vertical view (along pole)

Visualizing RPM as small circles around the Euler pole

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.

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

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

Relationship between RPM and fault slip rates Plate motion: º Fault segment: strike 352º A B C Fault-parallel slip (strike-slip) Fault-perpendicular slip (opening/closing) 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

Measuring plate motion obliquity Average strike of plate boundary

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

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

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