Читать книгу Continental Rifted Margins 2 - Gwenn Peron-Pinvidic - Страница 21

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, necking, mantle upwelling, mantle exhumation, melt production and serpentinization? The key here is determining the rate of rifting, a natural product of determining the detailed rift history.

These key questions are global in scope, apply to the breakup of all continents and center around the fourth dimension: time. Despite the recent progress of the scientific community in understanding the processes of development of rifted margins, mostly favored by international cooperation (e.g. the “International Ocean Drilling Program”, see also Peron-Pinvidic et al. (2019)), the timing of the key steps at rifted margins remains mostly unconstrained. Observations from seismic data have revealed the geometry of rifted margins, but not the absolute timing and rate of the key steps of rifting, thus the ages of the syn-tectonic units remain interpretive (Figure 1.7). Onshore, observations from analogues of margins cannot fully constrain the processes of rifting because they are discontinuous and overprinted by later, often strong, deformations. Other approaches, such as numerical models, are dependent on time input parameters, for example the velocity of plate divergence, that are often too poorly constrained. The time aspect of the geological events at rifted margins can only be addressed by scientific drilling. Given the amount of previous work, the presence of a 3D dataset and the key role the GM has played in framing these questions, the logical place to carry out such an experiment would be the Galicia Margin.