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. 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. Sional bends generates gaps where deep basins known as pull-apart basins form. San Andreas Fault in Carrizo Plain, California, marking the transform plate boundary between North America and Pacific plates (USGS) Dilational bends often initiate with their long axes perpendicular to the compressional bends, but large These are often associated with flower structures, containing a strike-slip fault at depth, and folds and thrusts near the surface. Compressional bends form at high angles to the principal compressive stress and at about 30-45° from the main strike-slip zone. Many ramp valleys started as pull-apart basins, and became ramp valleys when the fault geometries changed.Ī distinctive suite of structures that form in predictable orientations characterizes transform plate margins. In a few places along compressional bends, two thrust-faulted mountain ranges may converge, forming a rapidly subsiding basin between the faults. 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. 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. Examples of compressional (or restraining) bends include the Transverse Ranges along the San Andreas fault and Mount McKinley along the Denali fault in Alaska. Sion of the extra volume of crust compressed into the bend in the fault. Movement along fault segments with exten. 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.Įxtensional segments of transform boundaries form at left steps in left-slipping (left lateral) faults and at right steps in right-slipping (right lateral) 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. They never occur as a single fault, but rather as a set of subparallel faults. Transform faults in continents show strike-slip offsets during earthquakes and are high angle faults with dips greater than 70°. 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.
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