Transform and Transcurrent Faults

Evolution of a classical transcurrent or strike slip fault.
Initially a linear feature crosses the future fault.  This could be a stream, a road, or a linear geologic feature like a dyke.
After each earthquake, the displacement along the fault increases, in this case in a left lateral sense.

 

 

J. Tuzo Wilson hypothesized that the transform faults on the sea floor would work very differently.
The ridge segments are in green, the transforms in yellow, and the fracture zones in orange.
At anomaly 3 time, both sides of the ridge experience spreading.  Despite the fact that the patterns match left lateral features as seen in the top figure, the motion is opposite in sense.
At anomaly 2 time, the anomaly 3 crust in no longer within the transform fault region, and that crust is no longer moving horizontally compared to the crust south of the fracture zone.  The crust on the two sides of the fracture zone will be different ages, and hence depths, and the only relative motion will be different subsidence rates.
At anomaly 1 time, the anomaly 2 age crust will be out of the transform zone.
Note that at the fracture zone, the ages, depths, and magnetic anomaly present will all be different across the fracture zone.

 

The earthquake focal mechanisms verified Wilson's prediction.

A.  The traditional transcurrent strike slip fault has a focal mechanism that is left lateral.

B.  The transform fault has a right lateral focal mechanism.  The ridge was never in fact continuous, but developed from the start with the transform offsets.

Because we know the fault traces in both cases should be east-west, we can select that focal plane for the fault plane.

 


Last revision 10/6/2016