Oceanic Transform Faults and Fracture Zones

I. Knowledge of these areas comes from:

• Bathymetric Measurements

• Geophysical Measurements

• Dredge Sampling

• Surveys by Submersibles (least likely method)

• Exposures on Land

Images taken from NOAA site. Show the Pacifc Ocean and highlight the South Pacific.

II. Physiography and Structure of Transform Fault Zones

• Transform Fault Zones are Strike-Slip Zones, which range in width from 1 to many kilometers wide. Three possible situations can arise in which a transform fault will appear. See Below.

1. They are not a planar feature. Their structure is that of a steep-sided, narrow deep basin.Fault scarps form the sides of the basin and are most apparent where there is a large difference in age across the basin. The fault scarp is a direct result of the age difference. Young, hot lithosphere floats higher than older, more cooler lithosphere.
2. A good example is the Tamayo fracture zone, a ridge-ridge transform fault in the Gulf of California. This zone connects with segments of the East Pacific Rise. It's width ranges from 1 to 30 km's, and its younger side is higher than its older side.
3. Rock types taken from transform fracture zones by dredging include: -serpentinites;mylonitized and foliated ultramafic and mafic plutonic rocks and scattered olivine-rich, low silica volcanic rocks. There are also diapir-like structures found by seismic reflection profiles which are hypothesized to be serpentinite diapirs.
4. Topography is dependent on the spreading rate and the age of lithosphere on either side of the fault. See Figure Below.

• Slow-spreading ridge- pronounced valley, usually <1 km wide. Major difference in thickness of lithosphere across the transform fault zone, due to the age discontinuity. this difference tends to prevent the migration of the fault zone, keeping it narrow.
• Intermediate rate-spreading ridge- wider fault zone than a slow ridge. Alternating basins and ridges mark areas of intrusion and extension. This type of fault may have more than one zone of active slip. Lithospheric thickness across the fault is not as pronounced as in the slow-spreading ridge, since the ages are not as different. If the fault is segmented, lithospheric thickness may vary in increments across the fault zone.
• Fastspreading ridge- have wide zones of complex faulting with widths from 10's to over 100 km's. Short spreading centers may develop within the sones; they develop at an angle to the direction of spreading (not exactly perpendicular). There is little contrast in lithospheric thickness across the zone since the age contrast is also small, therefore, little restraint on the precise location of the fault.

Image taken from USGS site. Various transform faults off the western coast of N. America.

 

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