Footwall uplift during normal faulting — implications for structural geometries in the North Sea

Abstract

In recent years, studies of major normal faults in actively extending regions (Aegean, Basin and Range) have documented the vertical motions associated with normal faulting. In addition to the expected subsidence of the hanging wall, it has been found that uplift of the footwall occurs during earthquakes on normal faults. This uplift has been modelled as an elastic effect, treating the fault as a dislocation in an elastic medium. Over geological time scales, however, creep processes below the upper crust act to restore isostatic equilibrium, and so a purely elastic model is not appropriate. Instead, a flexural model is used, in which the upper crust is treated as an elastic layer, overlying a “fluid” substratum. A single fault cutting such a layer gives rise to buoyancy forces that produce reverse drag in both footwall and hanging wall. The wavelength of the reverse drag depends principally on the effective elastic thickness of the lithosphere. The relative amplitude of the footwall and hanging wall vertical movements is dominated by the nature of the fill in the half-graben: loading by sediment pushes the whole structure down, whereas loading by water (sediment-starved) allows more footwall uplift. When a number of faults occur adjacent to one another, their reverse-drag fields overlap to produce the familiar tilted-block profile of normal-faulted terrains. If the major faults are evenly spaced, dips within the fault blocks will be relatively uniform and can be approximated by a “domino” model. Such a fault model can be readily incorporated into a lithospheric stretching model (which causes subsidence of the whole basin), to predict vertical motions relative to a fixed datum such as sea level. We apply these concepts to a variety of fault systems in the North Sea and the mid-Norway shelf. Normal faulting in the Late Jurassic created sediment-starved half-graben with significant bathymetric relief. The footwalls of major platform boundary faults are typically eroded, often showing post-rift sediments resting on basement (e.g., East Shetland Boundary Fault, Brae Fault). In these cases, no direct evidence remains of the amount of footwall rotation, so that the eroded crest might be assumed rigid and erroneously projected into the basin as a regional datum for “section balancing”. However, some boundary faults preserve a condensed footwall sequence, or contain intra-basement reflections, which demonstrate that flexural uplift of the footwall accompanied normal faulting. The Frøya Fault (Haltenbanken) is an example of these. Erosion of marginal footwalls was an important source of clastic material, and is, therefore, a major factor in the distribution of Upper Jurassic reservoir formations (e.g., Frøya Formation, Brae Formation, Claymore Sst. Member, Fulmar Sand Formation). Within the graben, arrays of fault terraces can be described by the domino model, leading to the conclusion that the extent of erosion on intra-basinal footwalls is largely a function of fault spacing. Thus, in the Brent Province, the Snorre fault block suffered severe erosion during the Late Jurassic extension, whereas on the Brent fault-block erosion was less marked, approximate fault spacings being 30 km and 15 km, respectively. Erosion of intra-basinal footwalls has also been a significant source of Late Jurassic sediment for reservoirs, e.g., the Magnus member and Munin member.