14 - Continental Transform Faults 1

Continental transforms are analogous to oceanic transforms.
- Defined as conservative plate boundaries where lithosphere is neither created nor destroyed.
- Characterized by strike-slip motion resulting in lateral displacement of rocks or geological features along the strike of the fault zone.
- Potential vertical displacement can occur in transpressive or transtensional environments.
- Strike-slip faulting can occur at various scales within virtually any tectonic setting, but the term 'transform faults' specifically refers to plate boundaries.

Major strike-slip faults are often accompanied by linear fault scarps.
Scarps and Troughs result from:
- Differential erosion of juxtaposed materials.
- Erosion of fault gouge.
- Lateral offsets of surface features.
Terminology:
- Dextral: lateral movement to the right.
- Sinistral: lateral movement to the left.
Age and magnitude of offsets serve as indicators for determining slip rates.

Most large strike-slip and transform faults comprise repeating linear and en echelon segments.
- En echelon: refers to segments that are approximately parallel but oriented at an angle to the primary fault direction.
Bend Structures:
- Releasing bends and restraining bends characterize the behavior of faults.
- Releasing bends:
- Both sides of the fault diverge, characterized by normal faults, pull-apart basins, subsidence, and deposition.
- Restraining bends:
- Both sides of the fault converge, characterized by thrust faults, folds, and topographic uplifts (push-ups).

Faults in an array that closely parallel shear-related plate motions are prominent.
- Tend to grow longer and assume near-vertical dips.
- Exhibit large offsets.
Other faults oriented at angles may rotate further out of alignment, develop shallower dips, and greater components of dip-slip movement.
The various splays of a strike-slip fault zone may converge at depth, leading to:
- Negative Flower Structures: upward-branching faults with normal offsets located beneath a synformal surface depression.
- Positive Flower Structures: upward-branching faults with reverse offsets beneath an antiformal surface culmination.

At increasing depths, strike-slip fault structures are expressed as broadening zones of deformation below the brittle-ductile transition.
Faulting can extend beyond crustal features.
- Seismic studies indicate that the Mohorovičić discontinuity (Moho) can vary in depth by several kilometers across continental transform faults.
- Crustal Behavior: At lower crustal depths, strain becomes difficult to localize.
- Steep shear zones spanning 50- to 100-km wide are observed in the upper mantle, exemplified by the Alpine Fault in New Zealand.
- Exhumed ultramafic mylonites in peridotite massifs indicate shear deformation at those levels.

Strong, cool continental lithosphere tends to exhibit transforms with narrow zones of deformation at the surface, such as:
- Dead Sea Transform: deformation localized into a 20–40-km wide zone.
In contrast, weak continental lithosphere exhibits transforms with diffuse zones of deformation, as observed in the southern part of the San Andreas Fault, which spans hundreds to thousands of kilometers wide.

Strain localization can trigger several mechanisms that enhance crustal weakening, reducing the work needed for ongoing deformation.
These include:
- Positive Feedback Mechanism: Linking increased pore fluid pressure with lower differential stress required for fault slip, significant in transpressional zones.
- Thermal Advection: In transpressional zones where deep crustal rocks are exhumed, rapid exhumation can lead to increased heat in the shallow crust if the rate of exhumation exceeds the diffusion rate of heat.

The San Andreas Fault system is structurally complex, displaying different seismic behaviors in its segments.
Three identified segments:
- Northern Segment: Runs from Hollister to the Mendocino triple junction and includes significant offshore sections. Known for the 1906 San Francisco earthquake with a magnitude of approximately Mw 7.8 and ~3 m right-lateral offset, resulting in over 3,000 fatalities and $400 million in damage.
- Central Segment: Stretches from Parkfield to Hollister. Exhibits infrequent significant earthquakes, with slight aseismic creep allowing infinitesimally small offsets that accommodate strain without major seismic events.
- Southern Segment: Extends from south of Parkfield towards the southern Riviera triple junction between Baja California and Manzanillo, Mexico. This segment is seismically active with the potential for high-magnitude earthquakes greater than Mw 8.0 in the near future.

A timeline of significant seismic events recorded along the San Andreas Fault including:
- Various major earthquakes from 1812 onwards, with highlighted events in 1906, 1989 (Loma Prieta), and several others leading to extensive damage and fatalities.

Observations of sag pond deposits near Pallett Creek, ~30 km northwest of Los Angeles, inform about historical seismic activity through paleoseismology.
Techniques:
- Excavation of trenches into sag pond deposits to analyze offsets in organic layers using radiocarbon (14C) dating methods.
- Example: Layer ruptured between 1470 A.D. and 1225 A.D.
- Calculated mean recurrence intervals of earthquakes:
- Mean recurrence interval: 105 years (range between 31–165 years).
- Mean slip: 3.2 meters (range between 0.7–7 meters), as reported by Weldon et al. (2004).

Two predictive models:
- Slip-Predictable Model: Predicts the amount of slip in the next event based on the length of the interseismic period.
- Time-Predictable Model: Predicts the time remaining until the next event based on the offset of the last earthquake. Both models can be represented on a graph where points should align on a designated line if the models hold true, illustrating the correlation between time, magnitude, and seismic behavior.