Читать книгу Continental Rifted Margins 2 - Gwenn Peron-Pinvidic - Страница 18

The diversity of existing current models developed at the WIM shows that we still fundamentally do not know how the continental crust thins to zero, exposing the Earth’s mantle to the surface during rifting, leading to eventual continental breakup. The models presented in the previous section (Figure 1.7) are all potentially viable but imply fundamental differences in the mechanisms of thinning of the crust. The differences between these models can be summarized in three major, yet unanswered, questions, emerging from our current knowledge of the structures of the Galicia Margin, and more globally related to the development and evolution of magma-poor rifted margins.

Figure 1.7. Models proposed to explain fault development and the evolution of the West Iberian Margin, highlighting different predictions in terms of the age distribution of synrift sequences, the different role of detachments and different distributions of prerift/top basement

CONTINUATION OF CAPTION FOR FIGURE 1.7.– Although based on observations made at the West Iberian Margin, all of these models have been proposed as explanations for commonly-observed characteristics of magma-poor rifted margins worldwide, including the extension discrepancy. For each model, a simplified interpretation of part of a Galicia 3D is presented, flowline illustrating the different possible resulting stratigraphic geometries of deposition of the syn-tectonic sequences. The same dogleg line through the Galicia 3D volume is shown in each case. Model M1: all faults active over the same time period, before the crust was thinned by late displacement of the lower crust beneath a detachment. M1 predicts widespread prerift sediments (light blue), the same timing of fault onset and abandonment across the margin, and see rapid deepening in the postrift succession. Model M2: Polyphase cross-cutting faults, with extension focusing towards the line of eventual breakup. We would expect limited occurrence of prerift sediments (most top basement being the footwall of earlier faults, showing considerable exhumation), similar ages for the onset of faulting across the margin but very different timing of the cessation of faulting, and complex distribution of synfaulting sequences with a larger age range than predicted by sequential faulting. Model M3: sequential faulting predicts widespread prerift sediments, a pronounced age progression in both the onset and ending of active faulting across the margin (Unit B), and thus also in the oldest post-faulting sequences (Unit C). Block numbering from Ranero and Pérez-Gussinyé (2010); fault and synrift unit numbering from Lymer et al. (2019).