About Transforms

Where lithospheric plates do not come together in convergent zones or move apart in divergent zones, they move past each other, rubbing along their edges. These places are called transform zones or, more commonly, transforms.

Each of these three types of plate boundary has its own particular type of fault, or crack along which motion occurs. Transforms are strike-slip faults, in which there is no vertical movement, only horizontal. (Convergent zones are thrust or reverse faults, and divergent zones are normal faults. Here's more about these fault types.)

_{The Mid-Atlantic Ridge consists of spreading segments running north-south, transforms connecting them, and fracture zones extending beyond the transforms. Bathymetric map from NGDC.}

Notice that between the spreading segments, the sides of the transform are rubbing together, but as soon as the seafloor spreads beyond the overlap, the two sides stop rubbing and travel abreast. And the result is a split in the crust, called a fracture zone, that extends across the seafloor far beyond the small transform that created it.

The other type of transform occurs mostly on land. The San Andreas fault of California is a prime example; others are the North Anatolian fault of northern Turkey, the Alpine fault crossing New Zealand, the Dead Sea rift in the Middle East, the Queen Charlotte Islands fault off western Canada and the Magellanes-Fagnano fault system of southernmost South America.

These continental transforms are more complex than their short oceanic counterparts. The forces affecting them include a degree of compression or extension across them, creating dynamics called transpression and transtension respectively. This is why coastal California, basically a transform tectonic regime, also has many mountainous welts and downdropped valleys due to these extra forces. Movements across the fault are up to 10 percent as much as the pure transform motion.

Because of the thickness of the continental lithosphere and its variety of rocks, transforms on continents are not simple cracks but wide zones of deformation. The San Andreas fault itself is just one thread in a 100-kilometer-wide skein of faults making up the San Andreas fault zone. The dangerous Hayward fault takes up a share of the total transform motion, for instance. In fact the Walker Lane belt, far inland beyond the Sierra Nevada, takes up a small amount too.

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