Strike-slip reactivation of segmented normal faults: implications for basin structure and fluid flow

Abstract

Reverse reactivation of normal faults, also termed ‘inversion’, has been extensively studied, whereas little is known about the strike-slip reactivation of normal faults. At the same time, recognizing strike-slip reactivation of normal faults in sedimentary basins is critical, as it may alter and impact basin physiography, accommodation and sediment supply and dispersal. Motivated by this, we present a study of a reactivated normal fault zone in the Liassic limestones and shales of Somerset, UK, to elucidate the effects of strike-slip reactivation of normal faults, and the inherent deformation of relay zones that separate the original normal fault segments. The fault zone, initially extensional, exhibits a series of relay zones between right-stepping segments, with the steps between the segments having subsequently become contractional due to sinistral strike-slip movement. The relay zones have therefore been steepened and are cut by a series of connecting faults with reverse and strike-slip components. The studied fault zone, and comparison with larger-scale natural examples, lead us to conclude that the relays-turned-contractional-steps are associated with (i) complex fault and fracture networks that accommodate shortening, (ii) anomalously high numbers of fractures and faults, (iii) layer parallel slip and (iv) folding and uplift. Comparison with published statistics from global relay zones shows that whereas the reactivated relay zones feature aspect ratios similar to those of unreactivated relay zones, bed dips within reactivated relay zones are significantly steeper than unreactivated relay zones. Given the potential of reactivated relay zones to form areas of local uplift, they may affect basin structure and may also form potential traps for hydrocarbon or other fluids. The elevated faulting and fracturing, on the other hand, means reactivated relays are also likely loci for enhanced up-fault flow.

Tectonic Features of the Wufeng–Longmaxi Formation in the Mugan Area, Southwestern Sichuan Basin, China, and Implications for Shale Gas Preservation

“…However, the MS is a depositional area in a low strain state that favors shale gas cap rocks to be relatively intact. This model can well explain the differential shale gas preservation status of the five boreholes in the Mugan area, which is also consistent with the classical conclusion that the triangular wedges of the thrust faults are regarded as important oil and gas enrichment areas [73][74][75].…”

Geophysical Constraints on the Crustal Architecture of the Transtensional Warm Springs Valley Fault Zone, Northern Walker Lane, Western Nevada, USA

“…Detailed three‐dimensional seismic studies in the North Sea show that preexisting normal faults can be reactivated in a wide variety of slip senses when overprinted by shear and transtension (T. B. Phillips et al., 2018). Structural studies of long‐dead normal fault systems reactivated as strike‐slip faults document structural, rather than seismogenic, characteristics such as how normal fault relay zones appear to control basin geometry and fluid flow even after reactivation in strike slip (Barton et al., 1998; Rotevatn & Peacock, 2018). Analog laboratory models demonstrate that preexisting normal faults strongly influence the pattern of subsequent strike‐slip faulting (Richard & Krantz, 1991), but the manner in which crustal‐scale, former normal‐fault systems accommodate large strike‐slip earthquakes has not been fully explored.…”