Abstract Transform-margin development around the Arctic Ocean is a predictable geometric outcome of multi-stage spreading of a small, confined ocean under radically changing plate vectors. Recognition of several transform-margin stages in the development of the Arctic Ocean enables predictions to be made regarding tectonic styles and petroleum systems. The De Geer margin, connecting the Eurasia Basin (the younger Arctic Ocean) and the NE Atlantic during the Cenozoic, is the best known example. It is dextral, multi-component, features transtension and transpression, is implicated in microcontinent release, and thus bears close comparison with the Equatorial Shear Zone. In the older Arctic Ocean, the Amerasia Basin, Early Cretaceous counterclockwise rotation around a pole in the Canadian Mackenzie Delta was accommodated by a terminal transform. We argue on geometric grounds that this dislocation may have occurred at the Canada Basin margin rather than along the more distal Lomonosov Ridge, and review evidence that elements of the old transform margin were detached by the Makarov–Podvodnikov opening and accommodated within the Alpha–Mendeleev Ridge. More controversial is the proposal of transform along the Laptev–East Siberian margin. We regard an element of transform motion as the best solution to accommodating Eurasia and Makarov–Podvodnikov Basin opening, and have incorporated it into a three-stage plate kinematic model for Cretaceous–Cenozoic Arctic Ocean opening, involving the Canada Basin rotational opening at 125–80 Ma, the Makarov–Povodnikov Basin opening at 80–60 Ma normal to the previous motion and a Eurasia Basin stage from 55 Ma to present. We suggest that all three opening phases were accompanied by transform motion, with the right-lateral sense being dominant. The limited data along the Laptev–East Siberian margin are consistent with transform-margin geometry and kinematic indicators, and these ideas will be tested as more data become available over less explored parts of the Arctic, such as the Laptev–East Siberia–Chukchi margin.

Abstract Restraining- and releasing bends with similar morphology and structure have been described by many previous studies of strike-slip faults in a variety of active and ancient tectonic settings. Despite the documentation of at least 49 restraining and 144 releasing bends along active and ancient strike-slip faults in the continents and oceans, there is no consensus on how these structural features are named and classified, or how their wide range of structures and morphologies are controlled by the distinctive strike-slip tectonic settings in which they form. In this overview, I have compiled published information on the strike-slip tectonic setting, size, basin and bend type, age, and models for active and ancient releasing and restraining bends. Examples of bends on strike-slip faults are compiled and illustrated from five distinctive active strike-slip settings: oceanic transforms separating oceanic crust and offsetting mid-oceanic spreading ridges; long and linear plate-boundary strike-slip fault systems separating two continental plates whose plate-boundary kinematics can be quantified for long distances along strike by a single pole of rotation (e.g. the San Andreas fault system of western North America); relatively shorter, more arcuate indent-linked strike-slip fault systems bounding escaping continental fragments in zones of continent–continent or arc–continent collision (e.g. the Anatolian plate); straight to arcuate trench-linked strike-slip fault systems bounding elongate fore-arc slivers generated in active and ancient fore-arc settings by oblique subduction (e.g. Sumatra); and cratonic strike-slip fault systems removed from active plate boundaries, formed on older crustal faults, but acting as ‘concentrators’ of intraplate stresses. By far the most common, predictable and best-studied settings for restraining and releasing bends occur in continental-boundary strike-slip fault systems, where arrays of two to eight en échelon pull-apart basins mark transtensional fault segments and single and sometimes multiple large restraining bends mark transpressional segments; fault areas of transtension versus transpression are determined by the intersection angles between small circles about the interplate pole of rotation and the trend of the strike-slip fault system. These longer and more continuous boundary strike-slip systems also exhibit a widespread pattern of ‘paired bends’ or ‘sidewall ripouts’, or adjacent zones of pull-aparts and restraining bends—that range in along-strike-scale from kilometres to hundreds of kilometres. En échelon arrays of pull-apart basins are also observed on active ‘leaky’ or transtensional oceanic transforms, but restraining bends are rarely observed. In indent-linked strike-slip settings, strike-slip fault traces bounding escaping continental fragments tend to be more arcuate, less-continuous, and more splayed – but paired bends are common. Trench-linked strike-slip fault patterns closely mimic the trends of the subduction zone; these strike-slip faults can vary from long and continuous to short and arcuate, depending on the trace of the adjacent subduction zone. Paired bends are also observed in this setting. Bends on active, cratonic strike-slip fault form isolated, seismically active structures that act as ‘stress concentrators’ for intraplate stress. Cratonic strike-slip faults are generally not associated with pull-apart basins, and therefore paired bends are not observed in this setting.ȃThe most likely geological models for the formation of releasing, restraining bends, and paired bends along boundary and trench-linked strike-slip faults include: progressive linkage of en échelon shears within a young evolving shear zone; this model is not applicable to older strike-slip fault traces that have accumulated significant, lateral fault offsets; formation of lenticular ‘sidewall ripout’ structures at scales ranging from outcrop to regional; ripouts are thought to form as a response to adherence or sticking along an adjacent and relatively straight strike-slip fault zone; this structural concept may help to explain the large number of paired bends embedded within strike-slip systems, sinusoidal curvature along the traces of many strike-slip faults, and the episodic nature of lateral shifts in the main strike-slip fault zone; interaction of propagating strike-slip faults with pre-existing crustal structures such as ancient rift basins. Propagation of new strike-slip faults and interaction with older structures may occur on plate boundary, indent-linked, and trench-linked strike-slip faults; and concentration of regional maximum compressive stress on pre-existing, basement fault trends in stable cratonic areas can produce active restraining-bend structures; periodic release of these bend-related stress concentrations is one of the leading causes of intraplate earthquakes within otherwise stable cratons.

In coastal southern California, west of the principal strand of the San Andreas fault system, structural features formed since the Cretaceous have primarily been attributed to transform motion along the San Andreas fault system (Fig. 1). Where extensional deformation is clearly present in coastal southern California, it has generally been attributed to wrench faulting associated with strike-slip motion of the San Andreas fault system (e.g., Wilcox and others, 1973; Crowell, 1974). Strike-slip versus extensional models for the evolution of coastal southern California had previously been discussed but widely dismissed in favor of transform tectonics by most workers. The continued refinement of the plate configurations for the Cenozoic and the re-evaluation of some of the structural features in coastal southern California suggest that there was indeed a distinct Miocene extensional event. This deformation occurred contemporaneously with the extensional events of the Colorado River region and other portions of the Basin-and-Range province of the western United States and Mexico. Recently, however, the presence of regional detachment fault systems have been verified by regional seismic reflection profiles processed by the U.S.G.S.