🌎Plate Tectonics Unit 6 – Transform Boundaries and Fault Systems

Key Concepts and Definitions

• Transform boundaries occur where two tectonic plates slide past each other horizontally without creating or destroying lithosphere
• Faults are fractures in Earth's crust where rocks on either side of the crack have moved past each other
• Strike-slip faults involve lateral movement, where the blocks on either side of the fault move sideways past each other (San Andreas Fault)
• Dip-slip faults involve vertical movement, where one block moves up or down relative to the other
  • Normal faults occur when the hanging wall moves down relative to the footwall, often associated with extensional stress and rifting (Basin and Range Province)
  • Reverse faults occur when the hanging wall moves up relative to the footwall, often associated with compressional stress and mountain building (Himalayan Mountains)
• Shear stress is the force applied parallel to the surface of an object, causing deformation and displacement along the fault plane
• Strain is the change in shape or size of a rock body due to applied stress, which can be elastic (recoverable) or plastic (permanent)

Types of Transform Boundaries

• Conservative plate boundaries involve no creation or destruction of crust, only horizontal motion
• Oceanic-oceanic transform boundaries occur where two oceanic plates slide past each other, often associated with mid-ocean ridges (East Pacific Rise)
• Oceanic-continental transform boundaries occur where an oceanic plate and a continental plate slide past each other (San Andreas Fault)
• Continental-continental transform boundaries occur where two continental plates slide past each other (North Anatolian Fault)
• Leaky transform boundaries exhibit some volcanic activity and minor amounts of divergence or convergence in addition to the dominant strike-slip motion
• Transpressional and transtensional boundaries involve a combination of strike-slip and compressional or extensional motion, respectively

Fault Systems and Structures

• Fault zones are regions where multiple faults and fractures are concentrated, often with complex geometries and interactions (San Andreas Fault Zone)
• Flower structures are vertical arrangements of faults that resemble a flower in cross-section, often associated with strike-slip motion and transpression or transtension
  • Positive flower structures exhibit an upward-branching geometry and are associated with transpressional settings
  • Negative flower structures exhibit a downward-branching geometry and are associated with transtensional settings
• Pull-apart basins are depressions that form between two overstepping strike-slip faults, often filled with sediment and exhibiting subsidence (Dead Sea)
• Pressure ridges are linear topographic highs that form along strike-slip faults due to localized compression and uplift
• Fault scarps are steep slopes or cliffs formed by the differential movement of rocks on either side of a fault, particularly in dip-slip settings

Plate Motion and Stress

• Plate motion is driven by convection currents in the mantle, with hot, buoyant material rising and cooler, denser material sinking
• Transform boundaries accommodate lateral motion between plates, allowing them to move past each other without significant convergence or divergence
• Shear stress accumulates along transform boundaries as plates move in opposite directions, leading to elastic strain in the rocks
• Fault creep is the slow, gradual movement along a fault that occurs without generating significant earthquakes, helping to release built-up stress
• Stick-slip behavior involves the sudden release of accumulated stress in the form of earthquakes, followed by periods of little or no motion
• Strain partitioning occurs when the overall motion between plates is accommodated by different types of faults or structures, each taking up a portion of the total deformation

Geological Features and Formations

• Transform valleys are linear depressions that form along transform boundaries due to the lateral motion and erosion of the fault zone (Cayman Trough)
• Shutter ridges are elongated topographic highs that form when strike-slip motion offsets and juxtaposes different rock types or structures
• Sag ponds are small, enclosed depressions that form along strike-slip faults due to localized subsidence and sediment accumulation
• Offset streams and rivers exhibit abrupt changes in course where they cross an active strike-slip fault, with the displaced segments indicating the sense of motion (Wallace Creek)
• Fault gouge is a fine-grained, clay-rich material that forms along fault planes due to the grinding and crushing of rocks during fault motion
• Slickensides are polished, striated surfaces that form on fault planes due to the abrasion and friction of rocks sliding past each other

Seismic Activity and Earthquakes

• Transform boundaries are often associated with high levels of seismic activity due to the accumulation and release of stress along fault planes
• Earthquakes at transform boundaries typically have shallow focal depths (<70 km) and exhibit strike-slip focal mechanisms
• Seismic waves generated by earthquakes at transform boundaries can cause ground shaking, liquefaction, and landslides in nearby areas
• The magnitude of earthquakes at transform boundaries depends on factors such as the length of the fault rupture, the amount of slip, and the rock properties
  • The moment magnitude scale ($M_w$) is used to measure the energy released by an earthquake, based on the seismic moment ($M_0$): $M_w = \frac{2}{3} \log_{10} M_0 - 10.7$
• Earthquake swarms are clusters of small to moderate earthquakes that occur over a relatively short period, often associated with transform boundaries and fluid migration
• Seismic gaps are segments of transform boundaries that have not experienced significant earthquakes in recent history, indicating a higher potential for future events

Real-World Examples

• The San Andreas Fault is a right-lateral strike-slip fault that forms the boundary between the Pacific and North American plates in California, capable of producing large earthquakes (1906 San Francisco earthquake, $M_w$ 7.8)
• The North Anatolian Fault is a major right-lateral strike-slip fault in Turkey that has generated several destructive earthquakes in recent history (1999 İzmit earthquake, $M_w$ 7.6)
• The Alpine Fault is a transform boundary that runs along the west coast of New Zealand's South Island, accommodating the relative motion between the Pacific and Australian plates
• The Dead Sea Transform is a left-lateral strike-slip fault that separates the Arabian and African plates, forming the Dead Sea pull-apart basin
• The Marlborough Fault System in New Zealand consists of several right-lateral strike-slip faults that transfer motion from the Alpine Fault to the Hikurangi subduction zone
• The Queen Charlotte Fault is a major transform boundary off the coast of British Columbia, Canada, separating the Pacific and North American plates

Impacts on Earth's Surface

• Transform boundaries play a crucial role in shaping Earth's surface by accommodating the lateral motion between tectonic plates
• Strike-slip motion along transform boundaries can offset and juxtapose different rock types and structures, creating unique geological features and landscapes
• Erosion and sedimentation processes can be influenced by the presence of transform boundaries, with fault zones acting as conduits for fluid flow and sediment transport
• Transform boundaries can affect the distribution and evolution of natural resources, such as hydrocarbons and mineral deposits, by controlling the geometry and connectivity of sedimentary basins
• The seismic activity associated with transform boundaries poses significant hazards to human populations and infrastructure, requiring careful assessment and mitigation strategies
  • Seismic hazard maps and building codes are developed to identify high-risk areas and guide construction practices in regions near transform boundaries
• The study of transform boundaries and fault systems contributes to our understanding of plate tectonics, earthquake mechanics, and the long-term evolution of Earth's lithosphere