Gernigon mafic bodies : implications for microcontinent formation Redistribution of the lithosphere deformation by the emplacement of underplated Journal of the Geological Society service Email alerting

Sedimentary basin migration and microcontinent formation could be the result of deformation redistribution, in which the emplacement of an underplated mafic body (UPMB) may play an important role. In this study, a 2D finite-element model is used to examine the redistribution of the deformation that can be triggered by a UPMB. It is shown that three modes of deformation redistribution can exist: (1) a shiftcompleted mode where the deformation is completely redistributed into the newly weakened region; (2) a transition mode where the deformation is redistributed, but extension is accommodated by thinning in both regions; (3) a shift-failed mode where the deformation is not redistributed. Whether or not the deformation is redistributed depends on the configuration of the UPMB and on the initial rheological heterogeneity in the initially deformed region. However, it becomes difficult for any UPMB to initiate the redistribution of the deformation once the stretching factor in the first deformed region exceeds a critical value. The microcontinent formations observed in the Indian Ocean, in the Norwegian–Greenland Sea, in the Red Sea– Afar region and in the western Mediterranean are particularly discussed in relation to the transition mode, providing some important geodynamic implications. In a system where forces are applied uniformly, a deformation is distributed into a particular region that has rheologically less strength. In such rheology-controlled deformation localization, which is different from that controlled by a locally applied force, the deformation could be redistributed into a different region if an additional weak zone is formed. This may be a fundamental concept that explains some geological phenomena including sedimentary basin migration and microcontinent formation. Strengthening of the lithosphere in the extensional process is caused by thermal relaxation and replacement of weaker crust with stronger mantle, which has usually been considered as being responsible for causing successive deformation to migrate to adjacent regions (e.g. Braun 1992; Negredo et al. 1995; van Wijk & Cloetingh 2002; Corti et al. 2003b; Yamasaki & Gernigon 2009). However, the cessation, or at least a slow rate, of lithospheric thinning is required to obtain a significant strength contrast between extended and non-extended regions; otherwise the deformation is hardly redistributed. The condition of the mechanism is, therefore, so tightly limited that the strengthening does not work effectively when the thinning rate is fast enough to result in a large stretching factor of the lithosphere or in the onset of continental break-up. Instead, the emplacement of underplated mafic bodies (UPMBs) can be an effective mechanism for deformation redistribution. Indeed, previous studies (Corti et al. 2002, 2003a, 2004; Yamasaki & Gernigon 2009) showed that the UPMB has a significant potential to bring about localization of the deformation, depending on its configuration (e.g. width, thickness and temperature) and on the thermal structure of the lithosphere into which the UPMB is emplaced. However, even though the UPMB ought to be formed by the magmatic activity that is controlled by the asthenosphere dynamics, the direct and intrinsic behaviour of the asthenosphere has not been well prescribed quantitatively and is still rather vague. Therefore, the nature of the UPMB’s emplacement is uncertain; nevertheless, such an accidental thermal event, brought about by the asthenosphere dynamics, is a plausible controlling factor in the redistribution of deformation (e.g. Müller et al. 2001; Yamada & Nakada 2006; Mittelstaedt et al. 2008). In this study, we examine the redistribution of deformation induced by the emplacement of a UPMB during extension. It is known that the deformation can be redistributed successfully if an additional weak zone is artificially introduced in some region. However, it still remains to evaluate the conditions required for the redistribution to occur. Mittelstaedt et al. (2008) has examined mid-ocean ridge jumps by means of a hotspot magma source, in which an abnormal heat source beneath a hot oceanic plate, as well as the magmatic intrusion in the plate, is imposed in a system where the sea floor is spreading at a constant rate. The present study, contrarily, describes the behaviour of the deformation redistribution on the condition that the UPMBrelated weak zone is formed in the continental lithosphere before the initially deformed region develops into sea-floor spreading. It is shown that there exist three modes of deformation redistribution, depending on the configuration of the UPMB’s emplacement. The critical stretching factor in the first deformed region, below which the deformation localization can be redistributed into a different region where the UPMB is emplaced, is also evaluated. Then, based on the numerical model behaviour, we consider the implications for the microcontinent formations observed in the Indian Ocean, the Norwegian–Greenland Sea, the Afar–Red Sea region and the western Mediterranean Sea (see Fig. 1), including whether these natural example are suitable for the application of the model.