Signature of cryptic sedimentary basin inversion revealed by shale compaction data in the Irish Sea, western British Isles

Abstract

Inversion is one of the most common modes of intraplate deformation and a primary mechanism of sedimentary basin exhumation. The key criterion for identification of basin inversion is the contractional reactivation of extensional faults. Reverse displacement associated with inversion is focused along the upper segments of faults, so the evidence for inversion may be removed in deeply exhumed basins. We believe this is the case in the Irish Sea, western British Isles. Previous authors have attributed the severe exhumation of this basin to plume‐related uplift and have ruled out inversion as a cause of uplift. We present a method for identifying tectonic inversion in a basin where exhumation has removed evidence of reverse displacement on the upper segments of faults. We utilize compaction‐based mapping of erosion patterns recorded by anomalously high sonic velocities in Upper Triassic shales. We show that the amount of section removed during exhumation in the Irish Sea varies between 1.3 and 3.3 km and that exhumation patterns are markedly heterogeneous. Neither the heterogeneity of exhumation nor its focusing over hanging wall blocks of major faults is consistent with solely plume‐related uplift, and we believe that our results record cryptic basin inversion that is superimposed on epeirogenic exhumation (some of which may be plume related). The direct evidence for this inversion has been removed because erosion has proceeded beneath the null lines of reactivated faults, and we propose erosion of reactivated fault segments as one of the major reasons why shortening is frequently underestimated in inverted basins.

Quantifying Cretaceous–Cenozoic exhumation in the Otway Basin, southeastern Australia, using sonic transit time data: Implications for conventional and unconventional hydrocarbon prospectivity

“…This is broadly consistent with thermal history data, indicating that this well is presently close to maximum burial. Similar short-length scale variations in exhumation magnitude have been reported from compaction studies in inverted basins in the British Isles (Hillis et al, 2008;Holford et al, 2009).…”

Quantifying Neogene plate-boundary controlled uplift and deformation of the southern Australian margin

“…We note that it is likely that a large proportion of the shortening strain in the Otway Basin is accommodated by slip on normal faults that have been reactivated under a compressional stress regime. It is difficult to quantify shortening on inverted faults since their fault heaves are apparent (Holford et al 2009b). …”

“…Aseismic volumetric strains in the upper crust could be accommodated by small sub-seismogenic structures (i.e. small faults, joints, cleavage) and/or ductile deformation mechanisms such as folding (Holford et al 2009b). This may be important in parts of the Otway Basin stratigraphy dominated by lesscompetent fine-grained rocks such as the shales and mudstones of the Lower Cretaceous Eumeralla Formation.…”

“…Another possible mechanism whereby aseismic shortening strain may be accommodated is horizontal compaction of porous sedimentary rocks (Turner & Williams 2004;Holford et al 2009b). The eastern Otway Basin has likely been in a reverse faulting regime since the onset of exhumation in the Late Miocene -Early Pliocene (Nelson et al 2006); S Hmax has therefore been the maximum principal effective stress reducing porosity (Giles et al 1998).…”

Cenozoic post-breakup compressional deformation and exhumation of the southern Australian margin

“…The Wangerrip megasequence, which is bound by the intra-Maastrichtian and intra-Lutetian unconformities clearly thins towards the Morum structural high in the northwest of the basin, and underlying reflections clearly define this structure as a faulted anticline with a wavelength of ~20 km, and a classic inversion structure (cf. Williams et al, 1989;Holford et al, 2009b). Additional constraints on the formation of this structure are provided by thermochronological (AFTA, VR and apatite (U-Th)/He) and stratigraphic data from the Morum-1 well (Duddy et al, 2003).…”