There are currently contrasting views on the way strain is distributed within the lithosphere during rifting and the formation of passive continental margins, with direct implications for the subsidence and heat flow histories of the overlying sedimentary basins, and potentially also for the timing and degree of source rock maturity in these systems.
According to some authors, the asymmetry observed between most conjugate margin pairs (e.g. West Iberia-Newfoundland, East Coast USA-NW Africa, NE Brazil-West Africa and the Southern Australia-Antarctica) results from the activity of low-angle normal faults (detachments), which shift the region of pervasive upper crustal thinning and normal faulting (lower plate) from that of intense lower crust and mantle lithosphere thinning (upper plate; see Rosenbaum et al., 2008 and references therein). A paradoxical observation, nevertheless, is that in most margins the extension measured from normal fault throws appears to be much smaller than that inferred from subsidence and gravity modelling, thus implying ubiquitous upper-plate rift margin settings (the “Upper Plate Paradox”; Driscoll & Karner, 1998; Davis & Kusznir, 2004).
Pervasive depth-dependent stretching (DDS) is also implied in dynamic models of rifting to explain features such as the deposition of salt over extremely thinned crust (e.g. off western Angola) and the exhumation of continental mantle prior to breakup in magma-poor margins (e.g. the West Iberia Margin; Lavier & Manatschal, 2006; Huismans & Beaumont, 2011). In contrast, results from a recently published kinematic rift model suggest that the crustal structure and subsidence along most passive continental margins can be explained assuming an essentially depth-uniform strain distribution through time (Crosby et al., 2011). Alternative models have also been put forward to explain the apparent deficit of extension in the brittle upper crust, namely by Reston (2005) and Ranero & Perez-Gussinyé (2010), who argue that the amount of extension accommodated in normal faults may have been largely underestimated in earlier studies.
The figures below illustrate the results from two simple experiments in actively explored rift settings: (Figure 1) the North Sea; and (Figure 2) the Angola passive continental margin. The pseudo wells were built from published seismic data and assume a simplified stratigraphy, where the thin black layers correspond to the location of two hypothetical source rocks in each setting. For simplicity all models assume a constant temperature at the base lithosphere of 13300C, and the source rocks use a Type II, marine shale kerogen facies, with an initial TOC of 5% wt and HI of 500 mg/g TOC. The impact of varying the rift model assumptions is then evaluated in terms of the SR’s maturity.
The North Sea comprises a series of rift basins that formed over a sequence of extensional pulses between the Permian-Triassic and Early Cretaceous, interspersed with periods of thermal quiescence, volcanic activity and doming (Ziegler and Cloetingh, 2003). The Pseudo-well in this experiment was built from a NW-SE seismic constrained transect redrawn from Bell et al. (2014), at a location where the inferred total stretching factor (β) is 2; i.e. the crust, or the whole lithosphere, have been stretched to half their initial thickness during rifting (if the rift is assumed instantaneous; McKenzie, 1978). For the purposes of the experiment it is assumed that all extensional deformation took place during the Late Jurassic (160-150 Ma), except in the last scenario, where most thinning occurs during an earlier rift stage, in the Permian (260-250 Ma), in agreement with the published profile (see Bell et al., 2014 and references therein).
The models show that changing the amount of lithosphere thinning within a reasonable range (black lines), imposing significant differential stretching between crust and mantle, has some impact on the timing/degree of maturity of the deeper, pre-rift SR. This results from differences in the post-rift thermal structure of the basin combined with rapid sediment burial. However, a similar effect is obtained by varying the steady state thickness of the lithosphere by only ±10 km, often beyond realistic model constraints, and a greater impact is even predicted when distributing the extensional deformation over several rift events (or varying the duration of rifting). The maturity of the shallower SR is independent of the rift model, although some differences are noticed for variations in the thickness of the steady state lithosphere.
The Angola (deep) passive continental margin formed due to intense stretching during the Early Cretaceous (mostly Berriasian-Aptian) followed by a transition period of thick salt deposition (Aptian) and continental break-up (e.g. Teisserenc & Villemin 1990). The transect shown above is redrawn from Lentini et al. (2010), based on deep seismic reflection and refraction data. At the location of the Pseudo-well the present day thickness of the crust is 8 km, measured between the base of the sediments and the Moho. For the experiment it is assumed that all extensional deformation took place during the Early Cretaceous (145-135 Ma) and that the initial crustal thickness is 32 km (i.e. βcrust = 4).
In the margins, where the lithosphere stretches to infinity prior to break-up, depth dependent stretching (DDS) may have a greater impact on the distribution of heat during and following rifting, and thus in the maturity of SR’s. In the experiment above this is observed when varying the amount of stretching in the mantle (βmantle) between a factor of 3 and 4. For higher stretching factors, in this particular setting, the increase in heat flow converges asymptotically. The models also show, however, that similar magnitude effects, or even more pronounced, are produced when varying the thickness of the lithosphere and/or the duration of the rifting events. As in the case of the North Sea experiment the maturity of the shallower SR is independent of the rift model.
In summary, the experiments discussed here show that the implications of assuming conceptually different rift models for the timing and degree of source rock maturity in these settings may be of the same order of magnitude, and thus indistinguishable, from those inherent to the uncertainty in the parameterization of the rift model, such as the thickness of the underlying lithosphere and the age and duration of the rift events. Moreover, it is likely that the maturity of most syn- and post-rift source rocks does not depend significantly on the rift model, but mostly on the rate of post-rift burial. As good practice, these effects should be tested in order to identify the key sensitivities of the basin model, at least within a first order approximation.
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