6.2 Strike-slip motion and displacement

Strike-slip faults are where plates slide horizontally past each other. These faults can cause major earthquakes and shape landscapes. The San Andreas Fault in California is a famous example, showing how these faults impact geology and seismic activity.

Understanding strike-slip motion is crucial for plate tectonics. It explains how plates move relative to each other, creating unique landforms and influencing earthquake patterns. This knowledge helps us predict and prepare for seismic events.

Strike-Slip Motion and Transform Faults

Defining Strike-Slip Motion

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• Strike-slip motion involves horizontal movement of tectonic plates along a vertical fault plane
• Blocks on either side of the fault move parallel to the strike of the fault
• Motion is primarily horizontal, with minor vertical components due to local fault geometry variations
• Strike-slip faults accumulate strain over time, leading to sudden energy releases (earthquakes)

Transform Faults and Plate Boundaries

• Transform faults form plate boundaries, allowing plates to slide past each other horizontally
• Minimal convergence or divergence occurs along transform faults
• San Andreas Fault in California exemplifies a transform fault with strike-slip motion
• Strike-slip faults occur within plates (intraplate faults) or at plate boundaries (transform faults)
• GPS and geodetic techniques measure displacement rates along transform faults, providing insights into plate motion and seismic hazards

Lateral vs Vertical Displacement

Lateral Displacement Characteristics

• Lateral displacement involves horizontal movement of rock masses parallel to the fault plane
• Displacement amounts vary from centimeters to hundreds of kilometers over geological time
• Cumulative displacement measured using offset geological features (rivers, ridges, rock units)
• Lateral displacement typically dominates in transform fault systems

Vertical Displacement Features

• Vertical displacement occurs when one side of the fault moves upward or downward relative to the other
• Results from local complexities in fault geometry
• Generally smaller than lateral displacement in transform faults
• Creates significant topographic features over time (fault scarps, pressure ridges)

Left-Lateral vs Right-Lateral Motion

Left-Lateral (Sinistral) Motion

• Observer standing on one side of the fault sees the opposite side move to the left
• Also known as sinistral faults
• Examples include the Garlock Fault in California and the Alpine Fault in New Zealand

Right-Lateral (Dextral) Motion

• Observer standing on one side of the fault sees the opposite side move to the right
• Also called dextral faults
• San Andreas Fault exemplifies right-lateral motion (Pacific Plate moving northward relative to North American Plate)

Identifying Motion Direction

• Classification depends on relative motion of blocks on either side of the fault, not absolute motion in space
• Determined by observing offset features or analyzing slickenside striations on the fault plane
• Fault motion direction crucial for understanding regional tectonics and seismic hazard assessment

Geomorphic Features of Strike-Slip Displacement

Linear Landscape Features

• Offset streams and rivers indicate strike-slip faults, with water courses displaced laterally along the fault line
• Linear valleys form along strike-slip faults due to erosion of weakened fault zone
• En echelon faults create series of short, parallel, overlapping faults at an angle to the main strike-slip fault

Topographic Anomalies

• Shutter ridges form when topographic ridge portions displace along strike-slip fault, blocking or deflecting streams
• Pressure ridges develop where local compression occurs along fault bends, creating uplifted areas
• Fault scarps form along strike-slip faults with vertical components, creating step-like landscape features

Water-Related Features

• Sag ponds develop in depressions formed by local extension or compression along strike-slip fault bends
• Offset drainage patterns reveal fault displacement history
• Springs and seeps often occur along fault traces due to groundwater movement along fracture zones