Heterogeneous exhumation in the Inner Moray Firth, UK North Sea: Constraints from new AFTA and seismic data

Journal of the Geological Society, Nov 2002 by Argent, J D, Stewart, S A, Green, P F, Underhill, J R

Kinematics of basin exhumation

Here we synthesize the observations and interpretations presented so far into a tectonic model representing basin exhumation. Rather than choose a specific model straight away, we first define three end-member kinematic options, each model characterized by various combinations of regional exhumation (on a wavelength of hundreds of kilometres) and fault tectonics (Fig. 12). The effects of the end-member models are depicted in relation to an initial rift basin template (Fig. 12a) that is intended to represent the geometry of the Inner Moray Firth at the end of the Cretaceous. In the following discussion we choose a preferred model.

The first possibility shown in Fig. 12b has little or no contribution from intra-basinal faulting during regional uplift and can be termed epeirogenic exhumation (Hallam 1963; McKenzie 1984). Epeirogenic exhumation would lead to smooth, progressive variations in exhumation of flat, regional stratigraphic markers (Fig. 12b), as noted elsewhere on the Atlantic margins (e.g. Pazzaglia & Gardner 1994; Rohrman & van der Beek 1996). The spatial variations in the amount of exhumation suggested by previously published studies (Hillis et al. 1994) and confirmed by the investigation of the Smith Bank Fault presented here, suggest that local, differential exhumation has been important and we rule out epeirogenic exhumation as shown in Fig. 12b as the single mechanism responsible for basin exhumation of the Inner Moray Firth.

At the other extreme, reverse faulting could cause the pattern of exhumation and basin inversion. The differential exhumation of well 12/23-1 relative to well 12/24-2 could be accounted for if well 12/23-1 lay in an overthrust block. This defines a second kinematic possibility of basin-wide inversion controlled by reverse reactivation of extensional faults, with or without a background signature of epeirogenic exhumation, giving a heterogeneous picture of basin inversion (Fig. 12c) and perhaps better accounting for the rapid local variations in degree of exhumation suggested by various Inner Moray Firth studies. This type of heterogeneous inversion occurred during the Cretaceous and Tertiary on the south margin of the southern North Sea (Badley et al. 1989). However, the seismic data (Figs. 3 and 11) clearly show that the Smith Bank Fault is an extensional fault dipping to the SE and not a reverse fault dipping to the NW as required by the well 12/23-1 data. We rule out this option because of the incompatibility between the kinematics of this model and the data.

A third kinematic model combines a background of regional exhumation accompanied by extensional faulting (Fig. 12d). The displacement across extensional faults is usually partitioned c. 17:3 between hanging-wall subsidence and footwall exhumation relative to local datum points (Jackson et al. 1988: Gibson et al. 1989). However, if the entire area that is being extensionally faulted is simultaneously regionally exhumed, the net movement of hanging wall and footwall relative to regional datum points will be a function of both processes. Both sides of the extensional fault may become exhumed relative to regional datum points, in spite of extension across the fault (Fig. 12d). Downthrow of the hanging wall is essentially buffered by the regional exhumation. We propose that this third kinematic model accounts for the spatial variation in exhumation observed in the various lines of evidence from Inner Moray Firth presented in this paper. This model accounts for the curious phenomenon of wells in hanging-wall lows exhibiting conventional, nonexhumed thermal histories whereas adjacent wells on footwalls highs can show significant amounts of exhumation. The model reconciles documented regional uplift of the west margin North Sea basin (Rohrman & van der Beek 1996; Japsen 1997) and post-Jurassic basement-linked extensional fault reactivation, which although restricted to the Inner Moray Firth (Underhill 1991 a, 1998; Stewart 1996) has also been identified in the Bressay area of the East Shetland Basin (Underhill 2001). It should be noted that in adopting this model we do not suggest any genetic link between the regional exhumation and the extensional faulting in the Inner Moray Firth. The Cenozoic extensional faulting in the Inner Moray Firth basin demonstrated here may also have occurred onshore and contributed displacement to fault contacts in the deeply eroded basement of the Scottish Highlands (see Roberts & Holdsworth 1999 and Thomson et al. 1999).