Stepovers that migrate with respect to affected deposits: field characteristics and speculation on some details of their evolution

Traditionally, geologists have viewed strike-slip stepover regions as progressively increasing in structural relief with increasing slip along the principal displacement zones (PDZs). In contrast, some stepover regions may migrate along the strike of the PDZs with respect to deposits affected by them, leaving a ‘wake’ of formerly affected deposits trailing the active stepover region. Such stepovers generate comparatively little structural relief at any given location. For restraining bends of this type, little exhumation and erosion takes place at any given location. Another characteristic of migrating stepovers is local tectonic inversion that may migrate along the strike of the PDZs. This is most easily observed for migrating releasing bends where the wake is composed of former pull-apart basin deposits that have been subject to shortening and uplift. This type of basin inversion occurs along the San Andreas Fault, wherein the wake is affected by regional transpression. Some migrating stepovers may evolve by propagation of the PDZ on one side of the stepover, and shut-off of the PDZ on the other side. Possible examples of migrating stepovers are present along the northern San Andreas fault system at scales from metres (sag ponds and pressure ridges) to tens of kilometres (large basins and transpressional uplifts). Migrating stepovers and ‘traditional’ stepovers may be end members of stepover evolutionary types, and the ratio of wake length to the amount of slip along the PDZs during stepover development measures the ‘migrating stepover component’ of a given stepover. For a ‘pure’ migrating type, the wake length may be equal to or greater than the PDZ cumulative slip during the time of stepover evolution, whereas for a ‘pure’ traditional type, there would be no wake. Bends and stepovers occur along all strike-slip systems (e.g. Crowell 1974a, b; Christie-Blick & Biddle 1985). To aid discussion one can define stepover terms as follows: (1) main strike-slip faults or bounding faults also known as principal displacement zones or PDZs, and (2) transverse or relay structures that accommodate the transfer of slip between the PDZs on either side of the stepover region (Fig. 1). Geological features related to stepovers and bends have received considerable attention from researchers (e.g. Crowell 1974b; Mann et al. 1983; Christie-Blick & Biddle 1985; Westaway 1995; Dooley & McClay 1997). Studies of the evolution of stepover features have traditionally considered stepovers as features that progressively increase in structural relief with increasing slip on the PDZs connected to them (e.g. Mann et al. 1983; Dooley & McClay 1997; Dooley et al. 1999; McClay & Bonora 2001); such stepovers will be referred to herein as ‘traditional’ stepovers. More recently, Wakabayashi et al. (2004) presented field evidence for a type of stepover that appears to have migrated with respect to the affected deposits rather than increased in structural relief with greater slip accommodation; such stepovers will be referred to herein as ‘migrating’ stepovers. In this paper, I will review some field examples presented by Wakabayashi et al. (2004) and present an updated discussion speculating on the evolution of such structures. The new material presented in this paper, compared with Wakabayashi et al. (2004) includes the following: 1. A discussion of a full spectrum of hypothetical migrating stepover types with different migration alternatives and evaluation of different reference frames (whereas the earlier paper discussed only one type of migrating stepover). 2. The earlier paper considered migrating stepovers as a counter-example to ‘traditional’ stepovers, whereas herein a unifying model is proposed with migrating stepovers and ‘traditional’ stepovers as end members of stepover evolution types. 3. More detailed maps are provided for the field examples, and additional diagrams are provided to aid in visualization of the various stepover models proposed. From: CUNNINGHAM, W. D. & MANN, P. (eds) Tectonics of Strike-Slip Restraining and Releasing Bends. Geological Society, London, Special Publications, 290, 169–188. DOI: 10.1144/SP290.4 0305-8719/07/$15.00 # The Geological Society of London 2007. Field examples of migrating stepovers from the San Andreas fault system General field characteristics Field observations along the San Andreas fault system of coastal California suggest that some types of stepovers or bends migrate with respect to formerly affected deposits (Wakabayashi et al. 2004). Some field observations suggesting migrating stepovers are as follows (shown schematically on Fig. 2): 1. Structural relief of a stepover region is much smaller than expected for the estimated amount of slip accommodated by the structure during its lifetime. For releasing stopovers, this means a smaller basin than expected, and for restraining bends this means much less uplift and exhumation than expected. Note that the amount of expected slip through the stepover region is not necessarily the total amount of slip on the PDZ, because a stepover may form much later than the fault (the Olema Creek Formation example presented below may be an example). 2. Tectonic inversion has occurred out-of-phase with known regional tectonic changes. 3. Former basinal deposits are found along the strike of a PDZ adjacent to an active pull-apart basin (or, more generally, an active transtensional basin), forming a ‘wake’ (by analogy to the wake of a ship) that appears to mark the earlier presence of a pullapart environment. 4. For some stepovers, propagation of PDZs has occurred, and some also exhibit progressive along-strike dying out of activity on a PDZ. The San Andreas fault system The dextral San Andreas fault system (SAFS) of coastal California accommodates 75–80% (38– 40 mm/a) of present Pacific–North American plate motion (e.g. Argus & Gordon 1991), and 70% (540–590 km) of the dextral displacement that has accumulated across the plate boundary over the last 18 Ma (Atwater & Stock 1998). Regional fault-normal convergence across the northern San Andreas system (NSAFS) occurs at less than 10% of the dextral slip rate (Argus & Gordon 1991, 2001). Although this regional convergence contributes to some of the shortening seen along the NSAFS, the most prominent transpressional features are largely driven by local restraining bends or stepovers along strike-slip faults (e.g. Aydin & Page 1984; Burgmann et al. 1994; Unruh & Sawyer 1995). Subduction, associated with the development of the Franciscan Complex, occurred along the western margin of North America prior to the initial interaction between an East Pacific Rise and the subduction zone at about 28 Ma, but the SAFS did not become established until about main strike slip faults main strike slip faults transverse structure b o u n d i n g f a u l t ( P D Z ) b o u n d i n g f a u l t ( P D Z ) b o u n d i n g f a u l t ( P D Z ) b o u n d i n g f a u l t ( P D Z ) transverse structure transverse structure transverse structure uplifted region pull-apart basin (a) (b) Fig. 1. Simple classification of some structures associated with stepovers or bends along strike-slip faults. An idealized releasing stepover (a), and restraining stepover (b), are shown. active step-over/bend region (a) (b) “Wake”: area no longer in active step-over region. Affected by step-over tectonics in past. Black lines: active structures Grey lines: inactive structures less exhumation than expected for amount of PDZ slip estimated to have passed through restraining step-over. smaller basin (depth of sediments, and map dimensions) than expected for amount of PDZ slip predicted to have passed through releasing step-over local inversion of basin deposits adjacent to and along strike from active basin region affected by earlier transpressional deformation; no longer active Fig. 2. Diagram illustrating some of the features associated with migrating stepovers described in the text. J. WAKABAYASHI 170