Transform Fault

A transform fault is a strike-slip fault that forms an active boundary between two tectonic plates. Unlike normal faults, where plates pull apart, or thrust faults, where one plate rides over another, a transform fault moves the plates sideways past each other along a horizontal surface. The name comes from the fact that the fault "transforms" the motion between two spreading ridge segments or other tectonic features. The real movement is always opposite to what you see when you look at the offset of the ridge crest on either side of the fault, which is one of the more counterintuitive ideas in plate tectonics.

Key Takeaways

Transform faults are strike-slip faults that form active plate boundaries. The plates slide laterally past each other. There is no significant vertical component, unlike at subduction zones or mid-ocean ridges.
Most transform faults connect two segments of a mid-ocean spreading ridge. The spreading pushes seafloor outward from both ridge segments, and the transform connects them. Between the ridges, the rock on either side of the fault moves in opposite directions. Outside the ridge segments, the offset rock moves in the same direction and the fault is inactive (called a fracture zone).
The apparent offset of the ridge crest looks like the fault moved one way when the actual motion is the other way. This was first explained by geophysicist J. Tuzo Wilson in 1965, and the recognition of transform faults was one of the key breakthroughs that confirmed plate tectonics as a theory.
The largest continental transform faults include the San Andreas Fault in California (Pacific and North American plates, roughly 45 mm per year right-lateral slip), the North Anatolian Fault in Turkey, and the Queen Charlotte Fault system along the British Columbia and Alaska coast.
In petroleum geology, transform fault systems create pull-apart basins at releasing bends and structural highs at restraining bends, both of which can trap hydrocarbons. Several productive oil fields in California, the Gulf of California, and the Middle East's Dead Sea region sit in basins related to transform tectonics.

What Is a Transform Fault?

Take a long orange and slice it in half lengthwise. Now push the two halves past each other sideways. That lateral sliding, where neither half goes up or down relative to the other but both move horizontally, is what a transform fault does. The plates are enormous, the movement is centimetres per year, and the fault zone can be hundreds of kilometres long, but the basic motion is exactly that sideways slide.

At a mid-ocean ridge, the seafloor spreads outward from the ridge crest in both directions. Where two ridge segments are offset from each other, a transform fault connects them. The ridge segments keep producing new seafloor, the transform fault accommodates the difference in position, and the seafloor that has already moved past the transform continues outward as a fossil fracture zone, no longer active.

J. Tuzo Wilson was the first to work out the geometry in 1965. Before Wilson, geologists looking at offset ridge crests assumed the apparent displacement represented actual fault movement. Wilson showed that the real motion is opposite to the apparent offset, and that the active part of the fault is only between the two ridge segments. The rest is a scar that no longer moves. This insight predicted earthquakes should only occur between the ridge segments, which is exactly what seismic records show.

Fast Facts

The San Andreas Fault system moves at roughly 45 to 50 millimetres per year, accumulating elastic strain between large earthquakes. The 1906 San Francisco earthquake ruptured about 470 kilometres of the northern section in roughly 45 seconds. The fault has been locked, loaded, and slowly slipping for tens of millions of years. The total offset along the fault since it became active is estimated at around 315 kilometres. The rocks of Los Angeles were originally formed in roughly the same location as rocks now found near San Francisco.

Transform Faults and Oil Traps

Petroleum geologists pay close attention to transform fault systems because of what happens at bends in the fault trace. A straight strike-slip fault just moves rock sideways. But when the fault curves or steps, the geometry creates either compression or extension locally.

At a restraining bend, where the fault curves so the two plates squeeze against each other, the crust buckles upward into a push-up ridge. This can create structural traps if the right source rock, reservoir, and seal conditions are present. The Transverse Ranges of California, sitting at a major restraining bend on the San Andreas system, host several oil-producing structures.

At a releasing bend, the fault steps so the two plates pull apart locally, dropping a block down to form a pull-apart basin. Pull-apart basins can be deep, rapidly subsiding, and rich in organic sediment. The Salton Trough at the south end of the San Andreas system, the Gulf of California (where Pemex operates), and the Dead Sea basin (where Israeli and Jordanian exploration has tested reservoirs) are all pull-apart basins on transform fault systems.

The Queen Charlotte Fault system off British Columbia and Alaska has been mapped as a major transform boundary. Exploration along this margin has identified potential hydrocarbon traps in associated basins, though the seismicity of active transform zones creates geohazard considerations for any offshore infrastructure.

A transform fault is sometimes called a transform plate boundary or a transcurrent fault (though transcurrent faults are technically a broader category that includes some non-plate-boundary strike-slip faults). A fossil transform fault beyond the ridge segments is called a fracture zone. Related terms include strike-slip fault (a fault on which movement is primarily horizontal and parallel to the fault strike; transform faults are a specific subset that form plate boundaries), plate tectonics (the framework that describes the Earth's lithosphere as a set of rigid plates moving relative to each other; transform faults are one of three types of plate boundaries, along with divergent and convergent boundaries), pull-apart basin (an extensional basin that forms at a releasing step or bend in a strike-slip fault system; associated with fast subsidence, deep water, and potentially high organic matter preservation), fracture zone (the inactive extension of a transform fault beyond the active spreading ridge segments; an aseismic linear feature on the seafloor that records the past positions of transform fault activity), and wrench tectonics (the complex deformation that occurs in the rock adjacent to a strike-slip or transform fault, including flower structures, en echelon folds, and associated compressional and extensional features).

How a 50-Million-Year-Old Transform Fault Became an Oil Trap in California

In the Cuyama Valley of southern California, a Miocene-age oil field produces from a structural trap formed at a restraining step in an ancestral strike-slip fault system. The field was discovered in the 1940s by geologists mapping surface anticlines. The anticlines formed as horizontal compression at the restraining bend buckled the sedimentary section upward, creating a four-way structural closure.

The Miocene source rock, deposited in a restricted marine basin that sat in an extensional pull-apart geometry on the same fault system further south, generated oil over tens of millions of years. The oil migrated northward along permeable sandstone beds until it reached the structural high at the restraining bend and filled the trap.

The irony is that the same fault motion that created the source basin (extension at the releasing end) also created the structural trap (compression at the restraining end) further along the same fault system. Transform faults do not just destroy structure; at the right geometry, they build the architecture that concentrates oil where geologists can find it.