Читать книгу Continental Rifted Margins 2 - Gwenn Peron-Pinvidic - Страница 20

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.

In Model M3, the detachment is a composite structure that comprises the root segments of successive steep faults which rotated to low angle before being abandoned and a new fault formed in the hanging wall of the former fault, propagating the detachment oceanwards and resulting in an apparently continuous sub-horizontal surface.

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.