Читать книгу Continental Rifted Margins 2 - Gwenn Peron-Pinvidic - Страница 13

At the DGM (Figure 1.1, section ISE1 on Figure 1.2 and Figure 1.3), the hyper-extended crust is made of tilted and faulted crustal blocks that decrease abruptly in size west of ODP Site 639 (Figure 1.3), underlined by the S reflector (de Charpal et al. 1978; Boillot and Winterer 1988). Mantle rocks forming the partially buried “Peridotite Ridge” (see ridges and ODP Site 637 on Figure 1.1) have been drilled at ODP Site 637, 25 km to the west of Galinaute dredging Site 11 (Boillot et al. 1988) that last sampled continental fault block.

The S detachment fault: S is thought to be both a detachment fault and a tectonic CMB (Figure 1.3), separating the overlying tilted blocks and the underlying partially serpentinized mantle (e.g. Reston 1996). The normal faults that bound the crustal blocks appear to root onto S. The distribution of serpentinization below S obtained from tomographic models revealed that the degree of mantle hydration increases with the slip of the overlying fault, resulting in more serpentinization around where the roots of the faults reach S (Bayrakci et al. 2016). Observed in 3D, the surface of the S exhibits corrugations, whose orientation change from E-W to ESE-WNW oceanwards, remaining approximately perpendicular to the strikes of the faults (Schuba et al. 2018; Lymer et al. 2019). The corrugations confirm the nature of S as a detachment surface, and their change of orientation suggests that the direction of extension changed during rifting, remaining parallel to the corrugations. The corrugations on the block-bounding fault planes align and match with the corrugations observed down-dip on S, suggesting the block-bounding faults and S represented a single slip surface when the faults were active and the corrugations formed. S is therefore a composite surface made of juxtaposed root segments of block-bounding faults (Lymer et al. 2019). The partitioned nature of S is further supported by its pronounced distortion visible on the depth sections, where S meets the crust-mantle boundary, and also where some block-bounding faults distort and cut across the S (F4.0 and F5.2 on Figure 1.3).

DGM 3D fault network development: at the DGM, Creswell (2018) and Lymer et al. (2019) have described a series of fault sets (Figure 1.6), interpreted as having been made of faults that were active simultaneously. Each set is defined by (1) the neighboring faults with similar strikes that are roughly orthogonal to the underlying corrugations, (2) the geometrical linkage and/or overlap of faults along strike and (3) the complementarity of the heaves within each set: as the heave on one fault decreases laterally, the heaves on an overlapping fault increase to accommodate regional extension (Figure 1.6c, d, e, f). Each set is bounded eastward by a fault that offsets S and continues oceanwards as S. Both the limited along strike length of each fault and the complementarity of heaves between linked faults indicate that the faults within each set were active simultaneously.

Following these criteria, within the most proximal fault set 3/4 (Figure 1.6) the main faults (F3.0, F3.1, F3.2 and F4.0) probably developed with limited lateral extent to accommodate the same regional extension, and continued to slip over the same time period as their heaves are complementary, with a steady if slightly northward decreasing heave sum, a result of the general northward propagation of rifting (Whitmarsh and Miles 1995). As extension increased, the faults became hard-linked as they physically merged laterally, until the faults within the set deactivated and a new set of faults developed oceanward (Figure 1.6b). Similar observations also apply to the oceanward fault sets 5 and 6 (Figure 1.6): F5.1 cuts across the S to the east but is continuous with S to the west, marking the start of fault set 5. Faults within set 5 (F5.1, 5.2, 5.3, 5.4) merge directly and have complementary heaves. Fault F6.4 marks the eastern boundary of the most distal fault set 6, trending NNE-SSW parallel to the underlying corrugations, within which the heaves of F6.0, F6.1, F6.4, are complementary again, and the sum of the heaves remains approximately constant across the volume.

Figure 1.4. Structure of the Southern Iberia Abyssal Plain (source: adapted from Whitmarsh et al. 1998 and Mohn et al. 2015)

CONTINUATION OF CAPTION FOR FIGURE 1.4.– a) Summary of drilling results from ODP Legs 149 and 173 using a similar nomenclature to that defined at the Deep Galicia Margin (Figure 1.3). Thickness of the stratigraphic units are not shown as ODP drills only partially sampled each unit. b) Interpreted seismic reflection profile (LG12+TGS, Krawczyk et al. 1996 and Sutra and Manatscha, 2012; Sonne 16, Whitmarsh, et al. 1998) across the Southern Iberia Abyssal Plain, with location of ODP sites.

The faults marking the eastern boundary of each set systematically cut across the S reflector to the east and are continuous with S to the west (Figures 1.3 and 1.6), consistent with an oceanwards migration of the faulting, as each new set cuts across the set immediately landwards. Conversely, the lack of any distortion of S where intersected by other faults within each set (e.g. F5.1, F5.3, F5.4) is interpreted as showing that these faults were active over the same time interval, so that S was simultaneously active beneath these faults.