Continental fold-thrust belts are often modeled as wedges of deformed rock that structurally overlie and are detached from underlying basement rocks (e.g.
The Irish sector of the Rockall Trough is the largest of the sedimentary basins offshore Ireland and is virtually unexplored. This study establishes the first regional structural and stratigraphic framework of the Mesozoic and Tertiary fill of the Irish sector of the Trough with the aim of assessing its hydrocarbon potential. Major structural features of the Irish Rockall Trough and the adjacent part of the Irish Continental Shelf have been mapped using a combination of 9000 km of confidential and released regional seismic lines, satellite gravity data and a compilation of marine and aeromagnetic data. A sequence stratigraphic framework for the Mesozoic and Tertiary has been established using the seismic data tied to ten wells in adjacent basins (Erris and Slyne troughs and Porcupine Basin) and to one well in the southern part of the UK Rockall Trough. A major advance was the recognition of new Mesozoic perched basins and tilted fault blocks on both margins of the Trough in areas previously interpreted as shallow basement. A further 12000 km of seismic data acquired in the summer of 1996 indicate that a considerable thickness of sediment is present in these basins containing a fill analogous to the Slyne and Erris troughs. The configuration of the perched basins suggests that a major linked rift system, coincident with the site of the present-day Rockall Trough, existed during the early Mesozoic. Within the Trough, a number of sub-basins have been identified, characterized by syn-rift wedges of interpreted Jurassic to Early Cretaceous age.
Growing orogenic wedges cool rocks during exhumation of thrust hanging walls and heat them during burial of footwalls, leaving behind a resilient thermal record of earlier deformation in fold-thrust belts. In order to investigate early burial of deformed strata within the retroarc Idaho-Montana fold-thrust belt, we use Raman spectroscopy of carbonaceous material to construct a maximum temperature profile that constrains the thicknesses of eroded rocks structurally above the Lemhi arch, a pre-thrusting basement high. In the eastern portion of the study area, a sharp maximum temperature change of ~120°C occurs across the Johnson thrust, signifying that regional burial and heating predated late-stage faulting. West of here, cumulative exhumation is irregular, varying by up to 5 km over large (~75 km) wavelength folds; however, maximum temperatures in this same region are consistently ~200°C higher than correlative stratigraphic units in the adjacent foreland. The pre-thrusting, low-relief unconformity above the Lemhi arch, which served as the early décollement to the fold-thrust belt, was everywhere buried to at least ~6.5 km depth, which is ~1.5-5.0 km deeper than can be explained by stratigraphic burial. We hypothesize that between ~145 and 80 Ma, a combination of Cretaceous deposition and folding and thrusting at higher structural levels buried the décollement of the Medicine Lodge-McKenzie thrust system to this depth. These results suggest that the early orogenic wedge had exceptionally low taper. We propose that thin strata over the low-relief Lemhi arch limited the availability of potential décollements, which restricted the maximum surface slope that could be constructed in a thin-skinned system. Subsequent growth of the orogenic wedge required activation of a much deeper décollement and a switch to a thick-skinned structural style, promoting a shift from burial to exhumation of the former décollement and the underlying Lemhi arch. This suggests that the growth of an orogenic wedge is dependent on the thicknesses of the preexisting strata and the availability of potential décollements, with sedimentation and burial heating potentially playing a key role.
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