The geophysical data used in this work were primarily seismic reflection data. The seismic reflection data were acquired in 2013 and 2014 within the Rockall Basin, offshore Ireland, and were provided by the Department of Communications, Climate Action & Environment of Ireland. The seismic reflection data in the Orphan Basin, offshore Newfoundland, were acquired in 2001 and were provided by TGS-NOPEC Geophysical Company (TGS).
A total of five seismic lines, NL1, NL2, NL3, IR1, and IR2, were chosen for the seismic interpretation and reconstructions (line locations shown in Figure 1
). The main seismic line used in the West Orphan Basin, NL1, lies across strike of the rifted margin, whereas the main seismic line in the Rockall Basin, IR1, lies along strike of the rifting axis. To compensate for the variability in the observed faulting trends and styles across the two basins, additional intersecting seismic lines, NL2 and IR2, were incorporated into the interpretation and analysis of this study. The addition of these intersecting seismic lines allows for a more direct comparison between the basins. The main seismic line in the East Orphan Basin (NL3) is a composite line that spans across the rift axis.
5.2. Seismic Interpretation
Building on the well correlations and methodology of [43
] in the Orphan Basin, seismic sequences in both basins were defined based on syn-rift and post-rift characteristics. As syn-rift sequences are accumulations of sedimentary rocks deposited during rifting, these sequences are generally heavily faulted and deformed as well as displaying characteristic growth fault geometries. Post-rift sequences are sedimentary rocks that accumulated after rifting has ceased. Generally, post-rift layers are more laterally continuous than syn-rift deposits.
Within the Rockall and the East and West Orphan basins, five seismic sequences were identified and interpreted (Figure 3
). These sequences are: acoustic basement, Jurassic sedimentary rocks (only in the Orphan Basin), Lower Cretaceous sedimentary rocks, Upper Cretaceous sedimentary rocks, and Cenozoic sedimentary rocks. Each sequence was identified and mapped in two-way travel time (TWT) along the interpreted seismic lines. The interpretations of lines NL1 and IR1 are provided in Figure 4
and Figure 5
Due to the lack of deep well control in the Rockall Basin (as demonstrated in Figure 1
C) and extensive Paleogene igneous intrusions [48
] (as shown in Figure 5
C), the seismic interpretation in the Rockall Basin was correlated with previously published interpretations [37
]. Seismic interpretations were also correlated with the Porcupine Basin through regional seismic lines and jump correlation. The lack of deep well control in the Orphan Basin (where drilling has been focused on basement highs) led to seismic interpretations being checked against previously published interpretations [21
Magnetic Chron A34, associated with the onset of seafloor spreading as interpreted by [16
], was used to position the boundary between transitional crust and oceanic crust. As the focus of this study is on the acoustic basement continental crust and the associated syn-rift and post-rift sedimentary rocks, the oceanic crust was excluded from the analysis. Thus, the seismic sequences and faults located on the oceanward side of Chron A34 along line IR1 were excluded from the basin reconstruction (Figure 1
C and Figure 5
The base of the Cenozoic sequence in both the Rockall and Orphan basins corresponds to an unconformity resulting from a temporary hiatus in deposition of post-rift sedimentary rocks. The top of the Cenozoic sequence is the first positive, high amplitude event observed and therefore allows for excellent seismic correlation and a high level of confidence in the pick. Due to the deeper focus of this study, the Cenozoic sequence was not subdivided into multiple sub-sequences. Numerous seismic characteristics occur within this undivided sequence (Figure 3
The Upper Cretaceous sequence represents post-rift sedimentation, as the Rockall and Orphan basins transitioned to thermal subsidence following Lower Cretaceous rifting [21
]. The continuously high amplitude reflection associated with the top of the Upper Cretaceous horizon allowed it to be picked with high confidence. This sequence contains numerous parallel, laterally continuous reflectors in both the Rockall and Orphan basins (Figure 3
The Lower Cretaceous sequence is a syn-rift sedimentary sequence within the Orphan and the Rockall basins. This sequence is generally continuous and is only interrupted by the intrusion of the younger Paleogene sills in the Rockall Basin [5
] (Figure 5
C). Interpretation of deep strata in the Rockall Basin was hindered by the presence of these sills. In the Rockall Basin, the Lower Cretaceous sequence is characterized by relatively high amplitude continuous to discontinuous reflections (Figure 3
). In the Orphan Basin, the top of the Lower Cretaceous sequence is marked by a high amplitude reflector that is continuous throughout the entire basin. The seismic character of the sequence is chaotic with minimal internal coherency (Figure 3
The Jurassic sequence is also a syn-rift sedimentary sequence, with highly variable seismic characteristics. In the Orphan Basin, a package of high amplitude, chaotic reflectors is typically associated with this sedimentary sequence. These reflectors are not parallel and have minimal lateral continuity (Figure 3
). Due to the prevalence of the large igneous sills within the Rockall Basin and the lack of deep coherent reflections in the seismic data, no Jurassic or Triassic sedimentary rocks could be reliably interpreted therein.
The top of the acoustic basement structure was interpreted beneath the last laterally coherent seismic event observed in both the Rockall and Orphan basins (Figure 3
). This horizon separates the pre-rift sedimentary rocks and basement from the syn-rift sedimentary rocks that were deposited during rift episodes [4
]. Due to the lack of additional constraints, such as well data or prominent crustal features, the interpretation of the top of the basement horizon is poorly constrained in the Rockall Basin.
5.3. 2D Modelling
Following seismic interpretation (seismic lines NL1 and IR1 shown in Figure 4
and Figure 5
, respectively), the interpretations were imported into the software package MOVETM
, by Petroleum Experts Ltd. and Midland Valley, to carry out 2D structural reconstructions. First, the seismic interpretations were converted from time to depth using the velocities in Table 1
. Next, the software was used to decompact each sedimentary layer and restore thermal subsidence while back stripping/removing successive layers of sedimentary rock.
Decompaction involves compensating for the weight of overlying sedimentary sequences by restoring porosity as each overlying layer is removed. This was undertaken using the 2D Decompaction module
and requires knowledge of compaction curves with depth. The default compaction curves, based on the work by [50
] using North Sea data, were used in this study. Basin-specific and lithology-specific parameters used in this study for the decompaction and thermal subsidence restoration are listed in Table 2
, Table 3
, and Table 4
. As each layer was removed, a local Airy isostasy assumption was used to calculate the isostatic response.
Based on the Moho proxy provided by [15
], a crust-mantle boundary was added to the interpretation. The influence of the crust-mantle boundary did not contribute to the restoration of the continental crust across the conjugate margin. However, the crust-mantle boundary (or Moho) was moved passively throughout the reconstruction, so that a pre-rift crustal thickness could be obtained after the restoration was complete.
Thinning of the lithosphere is eventually accompanied by thermal subsidence once the thermal buoyancy forces reduce through thermal diffusion. This thermal subsidence must be considered for each post-rift layer, prior to decompaction, to accurately restore each basin. Note that erosion cannot be accurately constrained and was not accounted for in this work, which represents an important limitation of our results. Other limitations include lack of constraints on paleobathymetry, sea level change, and sampled lithological densities.
The parameters used for the 2D Thermal Subsidence module
are shown in Table 2
, Table 3
and Table 4
. The average amount of post-rift and syn-rift sedimentary rock was calculated from the combination of the Cenozoic and Upper Cretaceous sedimentary sequences (post-rift sequences), and the Lower Cretaceous and, where applicable, the Jurassic sedimentary sequences (syn-rift sequences), respectively.
One of the parameters needed to calculate thermal subsidence is the whole crustal beta factor (β), which represents the ratio of final crustal thickness to the original crustal thickness. For the purpose of this study, a single average cumulative stretching factor was used for the restoration throughout geological time due to lack of constraints on the incremental variation of β through time. Estimates of cumulative β across the basins can be obtained by using modern crustal thicknesses derived from constrained gravity inversion [15
] and assumptions about the original crustal thickness prior to rifting. Tests were performed for line NL1 (results not shown) to determine how the choice of β affected the subsidence calculation and the overall restoration, specifically whether a constant β for the whole basin region or a variable β should be used. These tests revealed only minor differences between the restored 2D geological models. Consequently, based on β value maps derived by [15
], a constant β value of 2.0 was chosen as regionally representative and used across the Newfoundland-Ireland conjugate margin pair. The same value was used by [21
After the decompaction and the thermal subsidence calculations have restored each interpreted seismic line down to a horizon that has been heavily faulted due to rifting, fault modeling was performed using the 2D Move-on-Fault module
also from MOVETM
. A shear angle of ±60° was used, with the polarity depending on the dip direction of the fault [52
summarizes each of the stages of the decompaction, thermal subsidence restoration, and fault restoration for lines NL1 (left column) and IR1 (right column). The topmost profiles (Figure 6
A,B) show the effect of the depth conversion. The reconstruction progressed through the Cenozoic sedimentary layer decompaction and thermal subsidence restoration (Figure 6
C), Upper Cretaceous sedimentary layer decompaction and thermal subsidence restoration (Figure 6
D), Lower Cretaceous sedimentary layer decompaction (Figure 6
E), Jurassic sedimentary layer decompaction, fault restoration and unfolding (Figure 6
F) and acoustic basement rebound, fault restoration and unfolding (Figure 6
G). Note that for the Cenozoic subsidence calculation, a paleo-water depth must be assumed. For this study, the paleo-water depth determined by [43
] was used.
Prior to the fault restorations, following the interpretations of [21
] and the definition of the brittle-ductile transition zone [53
], a crustal boundary between the upper and lower crust was interpreted along all five seismic lines in the Rockall Basin and the East and West Orphan basins based on the maximum depth of interpreted faulting (Figure 6
F). Following fault restoration, the interpreted models were assumed to represent a pre-rift state.
The amount of extension observed from faulting in the Rockall and West Orphan basins was measured in MOVETM
based on the distance from the edge of the fully restored upper crustal section to the original extent of the seismic line. The distance measured represents the amount of extension, based solely on observed upper crustal brittle faulting for each basin. For comparison, the poles of rotation from [22
] were used in GPlates to estimate the amount of expected extension in each basin by measuring the change in length of the basins along the seismic line as the model changed through time (details below).