Pull-apart basins

Abstract

Pull-apart basins are located along strike-slip faults and transform zones and are developed when the master fault bends or oversteps extensionally. While the master faults are parallel or subparallel, the secondary basin forms with faults perpendicular or diagonal to the master faults giving the basin its characteristic shape. The extension which develops within the pull-apart basins depends on the local crustal rheology. Stress fields suggest that the mean shear stress decreases inside the basin and the maximum stress increases at fault ends. The anatomies and evolutions of pull-apart basins are closely related to their geometric aspects. Their shapes, scales, and angular characteristics are useful to discuss their structures. Pull-apart basins are generally shown as rhomb-shaped depressions. But the shape of the basin varies widely, from spindle to lazy Z or S to other extreme examples. In an ideal rhomb- shaped pull-apart basin, while the acute angle between the master faults and basin-bounding diagonal faults is generally a value between 30° and 35 °, the 2D scale characteristics of pull-apart basins represent a mean aspect ratio of 3:1, and 3D characteristics represent a quantitative relationship between length, width, and depth. These values are suggested for basins shown in ideal geometries, and they can vary in a wide range due to their spatial and temporal positions in an evolving environment. The changing and intersecting tectonic regimes and local crustal, lithological, or climatic properties can also influence basin geometry. In contrast to their small-sized and short-lived characters, pull-apart basins have great thicknesses of basin-fill deposits. Higher sedimentation rates, displaced fan-source relationships and skewed fans, and migrated depocenters are some sedimentary aspects of pull-apart basins that differentiated this type of basin from the other tectonic depressions.