Sinistral

Key Takeaways

• The Anderson fault classification system, which relates fault type (normal, reverse, and strike-slip) to the orientation of the three principal stress axes, predicts that sinistral and dextral strike-slip faults form when the maximum compressive stress (sigma-1) and minimum compressive stress (sigma-3) are approximately horizontal and the intermediate stress (sigma-2) is approximately vertical, with the fault plane forming at approximately 30 degrees to sigma-1 and the sense of movement (sinistral vs. dextral) determined by the orientation of sigma-1 relative to the fault: for a vertical north-south fault with sigma-1 oriented northeast-southwest, the northwest block moves to the left relative to an observer on the southeast block (sinistral motion on the north-south fault), while for sigma-1 oriented northwest-southeast the motion would be dextral; field identification of sinistral vs. dextral motion on an exposed fault plane uses kinematic indicators including slickenlines (the striations scratched on the fault surface by the relative motion of the two blocks), Riedel shears (subsidiary fractures at consistent angles to the main fault plane), and the offset of geological markers, all of which record the direction and sense of net displacement accumulated over the fault's history.
• Pull-apart basins (releasing bends) form along sinistral fault systems where the fault trace bends or steps in a right-stepping configuration (where the fault strand is offset to the right of the original trace), creating a local extensional zone between the overlapping fault segments where the crust is pulled apart and subsides: at a right-step along a sinistral fault, the blocks on either side of the releasing bend move to the left relative to each other but diverge locally at the step, creating a rhomboidal basin (the pull-apart basin) that subsides rapidly, accumulates thick sediment, and may develop volcanic or hydrothermal activity at its base; pull-apart basins along the Dead Sea transform fault (which has sinistral motion accommodating the northward motion of Arabia relative to Africa along the left-lateral transform plate boundary from the Red Sea spreading center to the Anatolian collision zone) are among the most studied examples, including the Dead Sea itself (a pull-apart basin at a right step in the sinistral Dead Sea Fault), the Sea of Galilee, and the Gulf of Aqaba; in petroleum exploration, pull-apart basins along strike-slip fault systems can host oil-prone lacustrine or marine source rocks in their deep depocenters and structural traps in the inverted anticlines along their margins.
• Transpressional flower structures form along sinistral fault systems where the fault trace bends or steps in a left-stepping configuration (where the fault strand steps to the left), creating a local compressional zone where the two blocks converge and the crust shortens, producing a positive flower structure (a pop-up) in which reverse faults splay upward from the strike-slip master fault in a flower-like pattern, with the central block uplifted relative to the surrounding blocks: positive flower structures along sinistral fault zones can create structural traps in the uplifted central block, with the main strike-slip fault itself serving as a potential lateral seal along the flanks of the structure; negative flower structures (extensional or pull-apart geometries) produce the opposite pattern, with normal faults splaying downward from the master fault and the central block subsiding; distinguishing positive from negative flower structures on seismic reflection profiles is done by the vergence of the subsidiary faults (reverse faults dipping toward the master fault in positive flower, normal faults dipping away from the master fault in negative flower) and the relative uplift or subsidence of the central block relative to the surrounding country rock.
• Sinistral offset of geological markers provides one of the most reliable measurements of cumulative fault displacement in map-scale structural geology, using the offset of distinctive geological contacts (stratigraphic horizons, intrusive contacts, metamorphic isograds, or stream courses) across a fault to measure how far the two sides have moved relative to each other since the marker was continuous: the San Andreas Fault in California, a dextral (right-lateral) fault, has offset Cretaceous granitic plutons by up to 315 kilometers in central California (the "big bend" segment) and has offset recent stream courses by amounts ranging from a few meters (recent earthquakes) to several kilometers (accumulated Quaternary slip); sinistral fault examples include the North Anatolian Fault of Turkey (which has accommodated 85 kilometers of sinistral offset since the Miocene as the Anatolian plate moves westward relative to Eurasia) and the Levant Fault System (Dead Sea transform), with offsets of 105 kilometers measured from offset Cretaceous river systems and plutonic bodies; in the petroleum subsurface, sinistral offset of reservoir formations between wells provides a direct measurement of fault displacement that can be used to constrain the lateral seal integrity of the fault if the offset creates juxtaposition of the reservoir against a sealing shale on the opposite side of the fault.
• Seismic recognition of sinistral fault systems requires attention to both the horizontal displacement geometry (which is often not directly visible on vertical seismic sections but appears as en-echelon patterns of subsidiary faults on seismic attribute maps) and the flower structure geometry in vertical sections (which provides the best single-view diagnostic of strike-slip fault systems in the subsurface): coherence or similarity attributes computed from 3D seismic data image the fault traces in plan view as continuous or en-echelon linear features that can be traced across the seismic volume, with the direction of offset of cut geological markers providing the kinematic indicator (sinistral vs. dextral) for each fault segment; in 2D seismic profiles perpendicular to the fault strike, a strike-slip flower structure appears as a positive or negative flower of diverging fault traces, distinguished from thrust sheets (which also produce convergent fault patterns in cross-section) by the absence of consistent vergence direction and the presence of both reverse and normal sense motions on different branches of the flower; interpretation of sinistral fault systems in 3D seismic volumes is one of the more technically challenging tasks in structural seismic interpretation because the horizontal slip is largely invisible in any single vertical seismic section.