When there is a
bend in a strike-slip fault, movement along the fault causes
transtension or
transpression. This creates a basin where sediments can accumulate (also called the creation of
accomodation space.)
Original Bent Fault:
___________
/
/
_________/
Whether a tensional or compressional stress is created depends on the direction of movement on the fault. liha explains left and right handed faults above.
Basin created by transtension:
Viewed from above:
____________________
/ /
/ basin /
________/________/
The sense of motion here right handed.
It's pretty easy to imagine this from the side, just a big old hole in the ground. They are usually rectangular or rhombic when viewed from above. This is how the Dead Sea formed, as well as the Sea of Galilee, both lying on the same fault system. These can range in size from a few kilometers to tens of kilometers in length and width. They are often unusually deep, since the crust fractures brittly to about 10-15km to form the edges, and acts more ductile below these depths.
This is also called a pull-apart basin.
Basin created by transpression:
Viewed from above:
______________
_/
A _/ B
________/
The sense of motion here is left-handed.
Viewed from the side from point A to B:
__
__/ |
A____/ / _________B
/ basin______/
|______/
The earth on one side is pushed up on one side and down on the other. To get an idea of why this happens, push two fingers together end to end until they move. Chances are you ended up with one on top of another, with one bending slightly up and the other bending slightly down. Thats basically what happens here.
The subsidence caused by these mechanisms is usually fast, and allows several kilometers of sediments to accumulate in just a few million years (which is a lot compared to most other sedimentary basins.) The sediments are usually coarse grained, or physically immature, since they generally travel a short distance to these basins. In contrast to a regular lake-type sedimentation scheme, lateral changes in rock type (facies) are closely spaced.
Like the Dead Sea, these basins are often concentrated in salt or other solutes because they are generally isolated. In arid environments this means that the rate of evaporation is greater than the rate at which water is added, leading to the concentration.
The rock record in strike-slip fault zones shows that these basins open up and close frequently, every 10 million years or so, depending on the rate of movement on the fault.
Note: The diagrams are crude and are only meant to convey the basic idea. If geology were this simple it wouldn't really be a science. Most basins are hybrids of two or more of the several that exist.
Reference: Nichols, G., 2000. Sedimentology and Stratigraphy. Blackwell Science Ltd., London.