Transform boundaries on the continents include the San Andreas fault in California, the North Anatolian fault in Turkey, the Alpine fault in New Zealand, and, by some definitions, the Altyn Tagh and Red River faults in Asia. Transform faults in continents show strike-slip offsets during earthquakes and are high angle faults with dips greater than 70°. They never occur as a single fault, but rather as a set of subparallel faults. The faults are typically subparallel because they form along theoretical slip lines (along small circles about the pole of rotation), but the structural grain of the rocks interferes with this prediction. The differences between theoretical and actual fault orientations leads to the formation of segments that have pure strike-slip motions and segments with compressional and extensional components of motion.
Extensional segments of transform boundaries form at left steps in left-slipping (left lateral) faults and at right steps in right-slipping (right lateral) faults. Movement along fault segments with exten-
sion of the extra volume of crust compressed into the bend in the fault. Examples of compressional (or restraining) bends include the Transverse Ranges along the San Andreas fault and Mount McKinley along the Denali fault in Alaska. Many of the faults that form along compressional bends have low-angle dips toward the main strike-slip fault but progressively steeper dips toward the center of the main fault. This forms a distinctive geometry known as a flower or palm tree structure, with a vertical strike-slip fault in the center and branches of mixed thrust/ strike-slip faults branching off the main fault.
In a few places along compressional bends, two thrust-faulted mountain ranges may converge, forming a rapidly subsiding basin between the faults. These basins are known as ramp valleys. Many ramp valleys started as pull-apart basins, and became ramp valleys when the fault geometries changed.
A distinctive suite of structures that form in predictable orientations characterizes transform plate margins. Compressional bends form at high angles to the principal compressive stress and at about 30-45° from the main strike-slip zone. These are often associated with flower structures, containing a strike-slip fault at depth, and folds and thrusts near the surface. Dilational bends often initiate with their long axes perpendicular to the compressional bends, but large
San Andreas Fault in Carrizo Plain, California, marking the transform plate boundary between North America and Pacific plates (USGS)
sional bends generates gaps where deep basins known as pull-apart basins form. The planet presently has about 60 active pull-apart basins, including locations like the Salton trough along the San Andreas Fault and the Dead Sea along the Dead Sea transform. Pull-apart basins tend to form with an initially sigmoid form, but as movement on the fault continues, the basin becomes very elongate parallel to the bounding faults. In some cases the basin may extend so much that oceanic crust is generated in the center of the pull-apart, such as along the Cayman trough in the Caribbean. Pull-apart basins have stratigraphic and sedimentologic characteristics similar to rifts, including rapid lateral facies variations, basin-marginal fanglomerate and conglomerate deposits, interior lake basins, and local bimodal volcanic rocks. They are typically deformed soon after they form, however, with folds and faults typical of strike-slip regime deformation.
Compressional bends form at right bends in left lateral faults, and left bends in right lateral faults. These areas are characterized by mountain ranges and thrust-faulted terrain that uplift and aid ero-
Orientation of structures in transform margins, including strike-slip faults, normal faults, and thrust faults amounts of extension may lead to the long axis being parallel to the main fault zone. Folds, often arranged in en echelon or a stepped manner, typically form at about 45° from the main fault zone, with the fold axes developed perpendicular to the main compres-sive stress. The sense of obliquity of many of these structures can be used to infer the sense of shear along the main transform faults.
strike-slip faults along transform margins often develop from a series of en echelon fractures that initially develop in the rock. As the strain builds up, the fractures are cut by new sets of fractures known as Riedel fractures, in new orientations. Eventually, after several sets of oblique fractures have cut the rock, the main strike-slip fault finds the weakest part of the newly fractured rock to propagate through, forming the main fault.
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