Gwenn Peron-Pinvidic

Continental Rifted Margins 2


Скачать книгу

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).

       Question 1: how is the crust thinned from 20 km to zero km?

      Model M1 implies that all the brittle faulting is imaged (Figure 1.7); thus, top basement should, except where subject to erosion, be capped by prerift and early synrift rocks.

      Model M2 infers that top basement is locally the exhumed slip surface of an earlier fault (Figure 1.7), cut across and displaced by the more recent faults.

      In Model M3, the timing of the fault movement should systematically get younger oceanwards (Figure 1.7). Only minor extension discrepancy is excepted towards the deep margin (Ranero and Pérez-Gussinyé 2010), where the crust cut by later faults has locally been prethinned by movement on preceding faults.

       Question 2: what is the role of detachments in rifting to breakup and how do they develop?

      Large-offset, apparently low-angle normal faults, commonly referred to as detachment faults, accommodate crustal thinning in some areas of high factors of extension, such as at the DGM (S, Figure 1.3) and at the SIAP (H, Figure 1.4). The end-member models M1, M2 and M3 (Figure 1.7) imply different mechanisms of development of detachments.

      In Model M1, the detachment zone develops late in the rifting history and corresponds to the interface at the top of the mantle and the base of the upper crust, along which the lower crust has been laterally displaced by flow during rifting.

      In Model M2, the detachment consists of an early fault that cut through the mantle, allowing for serpentinization, which makes it possible for the detachment fault to remain active during all of the latest stages of rifting due to weakening of the underlying rocks. The top of the mantle thus acts as a detachment as extension focuses through subsequent faulting.

      Detachment faults have been imaged at a variety of margins and hyperextended basins (e.g. de Charpal et al. 1978; Reston et al. 2001), but are best known from the GM (e.g. Reston et al. 2007). There, 3D analysis (Lymer et al. 2019) suggests that S formed from a combination form of models M2 and M3, with an early fault that cut through the CMB and led to mantle serpentinization (M2), but evolved into a 3D rolling hinge onto which multiple faults rooted at once, once the crust had thinned to <10 km (variant of M3). Thus, sequential faulting may develop as one of the later phases of extension in a polyphase mode.

       Question 3: what is the evolution of the lithospheric rheology, strain distribution, melt production and serpentinization during rifting?

      In contrast to magma-rich margins, magma-poor margins such as the WIM display few and small magmatic bodies atop the well-defined faulted blocks. Several hypotheses have been advanced to explain the lack of magma at such margins: very slow rifting (Pérez-Gussinyé et al. 2006), pre-depletion of the asthenosphere following earlier melting events (Muentener and Manatschal 2006), lithospheric (as opposed to crustal) DDS in which crustal breakup occurred before lithospheric separation (Huismans and Beaumont 2011) and cool sub-lithospheric mantle (Reston and Phipps-Morgan 2004). A key to understanding the lack of melt is to constrain the timing and rate, both absolute and relative, of crustal rifting and of mantle thinning (Brune et al. 2014, 2017; Ros et al. 2017).

       To summarize the above section, the remaining key unknowns at the WIM are as follows:

       – How are the lithosphere and the crust thinned in space and time from 20 km to zero? This can be tested by addressing the extension discrepancy (Figure 1.5) and determining the detailed rift history of the WIM by sampling the age of the synrift in each half-graben to calibrate the existing excellent seismic database at this margin.

       – How and when do detachments develop and control the structural evolution of distal margins? Determining the detailed rift history will place the role of detachments into a well-constrained framework to test the current models of development of the WIM, and to determine the timing of any switch to migrating, detachment-controlled extension.

       – How does evolving lithospheric rheology control strain localization,