Gwenn Peron-Pinvidic

Continental Rifted Margins 2


Скачать книгу

until the mid-Cretaceous, when eventual northward propagating breakup led to diachronic opening of the North Atlantic (e.g. Srivastava et al. 1990; Tucholke et al. 2007). Late Triassic to Early Jurassic early rifting affected a wide zone (Tucholke et al. 2007), producing large intracontinental basins (Figure 1.1 – LP, PB; Wilson et al. 1989, pp. 341–361; Murillas et al. 1990). Subsequent Jurassic extension thinned the crust to ≥10 km and led to the development of the GIB (e.g. Pérez-Gussinyé et al. 2003). ODP Site 639 recovered shallow water Tithonian carbonates at the western edge of the GB (Boillot and Winterer 1988) (Figures 1.1 and 1.3), that could correspond on seismic sections to relatively thin, poorly reflective packages (unit A on Figure 1.3). At the Early Cretaceous, the DGM developed due to extreme crustal thinning and mantle exhumation processes (Boillot et al. 1987), with hyper-thinning of the crust from ~10 km at the western edge of the Galicia Bank to zero in areas of exhumed serpentinized mantle (Figure 1.3). Syn-tectonic sediments (unit B) recovered at the top of a fault block at ODP Sites 638 and 639 have been dated as Valanginian–Barremian, whereas possible syn-tectonic units sampled at more oceanward Site 640 are Barremian–Aptian in age (Applegate and Bergen 1988). The westernmost synrift sample has been obtained by submersible from the top of a crustal block outcropping at the seafloor (GAL-11, Figure 1.3), consisting of sandstones whose age was not readily dateable (Reston 2005). The faulted blocks above S are consequently capped by a deep-water clastic sequence (Boillot and Winterer 1988), rather than true prerift sediments, such as the shallow water carbonates recovered at ODP Site 639. In addition, the ages of the recovered synrift sediments indicate that the top of the synrift units becomes younger towards the distal DGM (Boillot and Winterer 1988; Figure 1.3, compare ages at Sites 641 and 640). The age of the synrift sediments across the Galicia Margin is therefore diachronous (Reston 2005), which is consistent with the oceanwards migration of the rifting, as suggested from 3D observations on the kinematic development of the S detachment (Lymer et al. 2019). ODP Leg 103 clearly constrained the age of the extensional faulting at only one fault block (Sites 638, 639 and 641), where the prerift, early synrift and late synrift sequences were recovered. The age distribution of rifting across the Galicia Margin is therefore uncertain due to the punctual availability of time constraints on the age of synrift sequences.

Schematic illustration of the summary of fault heaves analysis from the Galicia 3D volume.

      To summarize, in the SIAP, the Tithonian–early Berriasian sequence is tilted and part of the pre-faulting units (Figure 1.4). The late Berriasian–early Albian sequence either onlaps previous sequences/basement, or locally thickens towards the block bounding faults. The evidence thus points towards major rifting and crustal extension occurring over perhaps 10 Myr between the Tithonian and the Valanginian, followed by a phase of mantle unroofing over a further 10 Myr, before the onset of seafloor spreading during the Aptian–Barremian. As within the DGM, extension appears to have migrated oceanwards, but culminates in the unroofing of a far wider zone of mantle rocks. Although the overall time between the onset of rifting and the spreading of seafloor is similar at the two margin segments, approximately half of that time at the SIAP only involved the unroofing and subsequent extension of mantle rocks. Hence, rift duration leading to complete crustal separation may have only been about 10 Myr. Both the DGM and SIAP exhibit tilted fault blocks, detachment faulting, tectonic CMB, serpentinization beneath hyper-extended crust, and a wide zone of mantle exposed at the seafloor during rifting. At both segments of the margin, ODP results suggest that extension during the rifting may have migrated oceanwards. However, although the WIM has been extensively studied, the details of the timing of rifting remain largely unconstrained due to the fact that most scientific drillings have targeted basement highs. This strategy, while revolutionizing our understanding of the structures of rifted margins and introducing the concept of mantle unroofing, has not recovered the complete synrift sequences needed to determine the timing of motion on individual