Transform Boundaries and Plate Tectonics

Introduction to Transform Boundaries

• Third type of plate boundary.
• Plates slide past each other horizontally.
• Little vertical motion.
• Example: San Andreas Fault (Pacific Plate and North American Plate).

Characteristics of Transform Boundaries

• Often have bends, not straight lines.
• Create strike-slip faults.
• Two scenarios with bending:
  1. Compression at Bends:
     • Plates converge slightly, causing uplift (similar to convergent boundary).
     • Formation of mountain ranges (e.g., San Gabriel Mountains).
  2. Tensional Stress at Bends:
     • Plates pull apart, stretching the crust.
     • Formation of basins or low areas (e.g., Salton Sea).

Case Study: San Andreas Fault

• Southern Part:
  • Bend causes tensional stress leading to a pull-apart basin (Salton Sea).
• Northern Part (Big Bend):
  • Causes compressional stress and uplift (San Gabriel Mountains).

Plate Boundary Interactions on the West Coast of North America

• Mendocino Triple Junction:
  • Pacific Plate, North American Plate, Juan de Fuca Plate converge.
  • End of San Andreas Fault.
• Convergent Boundary:
  • Juan de Fuca Plate subducts beneath North America.
  • Formation of Cascade Range.
• Divergent Boundary:
  • Located in the Sea of Cortez.
  • Formation of new oceanic crust; Baja California pulling away from Mexico.

Differences Between Plate Boundaries

• Convergent Boundaries:

  • Directly responsible for volcanism (hydration melting).
  • Destructive (recycle crust).

• Divergent Boundaries:

  • Can cause volcanism (decompression melting at ridges).
  • Constructive (form new crust).

• Transform Boundaries:

  • Conservative (no creation or destruction of crust).
  • Not directly related to volcanism.

Summary

• Transform boundaries are critical in understanding plate movements and their effects on the earth's surface.
• The west coast of North America is a tectonically active region with all three types of plate boundaries present.