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

Results from two Inner Moray Firth wells

Wells 12/23-1 and 12/24-2 were selected because of their proximity to one another on adjacent margins of a major subbasinal extensional fault (Smith Bank Fault). Well 12/23-1 is located in the footwall and 12/24-2 is located in the hanging-wall section (Fig. 1). AFTA data in six samples from these two wells are summarized in Table 1. AFTA parameters in each well (fission-track ages and mean track lengths) are plotted as a function of depth and present temperature in Fig. 7, where the fission-track age data are contrasted with the variation of stratigraphic age through the section.

Qualitative examination of the AFTA parameters from these wells shows significant contrasts that suggest major differences in the thermal history of the sequences in these two wells. For instance, fission-track ages in samples from well 12/23-1 in Fig. 7 show a more rapid decrease with depth or temperature compared with results from well 12/24-2, and at depths around 2600 m the mean track length is shorter in well 12/23-1 than in well 12/24-2.

These differences are further emphasized in Fig. 8, which shows the pattern of fission-track age v. chlorine content in sample GC237-30 from well 12/23-1 and sample GC237-55 from well 12/24-2. Most common apatites are fluorine rich; however, they may contain appreciable amounts of chlorine. The amount of chlorine in the apatite lattice exerts an important compositional control on the degree of fission-track annealing. Apatites richer in fluorine are more easily annealed than those richer in chlorine. The result of this effect is that in a single sample, individual apatite grains may show a spread in the degree of annealing. This effect becomes more pronounced at temperatures ranging from 90 to 120 deg C and can therefore be useful when dealing with samples exposed to temperatures in this higher range (Green et al. 1996).

For each sample, the measured ages are contrasted with the pattern of age v. chlorine content predicted from the respective default thermal history for each sample. These results illustrate a key difference between the two samples, taken from present-day temperatures of 80 deg C and 90 deg C, respectively. Given that the default thermal histories for both samples involve residence at or above these temperatures for at least 65 Ma, representing the present-day sea-bed unconformity in each well, we might expect a slightly greater degree of annealing (age and length reduction) in sample GC237-55 (from 90 deg C in well 12/24-2) compared with sample GC237-30 (from 80 deg C in well 12/23-1). In fact, as the sea-bed unconformity in well 12/23-1 represents a longer interval than that in well 12/24-2, the predicted degrees of annealing are very close in the two samples despite the lower present-day temperature of GC237-30, as shown by the predicted lengths in Table 2. However, the measured data show a very different pattern. In sample GC237-30, all measured ages are much less than the values predicted from the default thermal history, showing that this sample has been hotter in the past. In contrast, measured ages in sample GC237-55 are either close to or greater than the values predicted from the default thermal history, suggesting that this sample has not been appreciably hotter than its present-day temperature at any time since deposition.