Tectonics of the West Iberia continental margin from seismic reflection data
Henning, Alison Teagan
Sawyer, Dale S.
Doctor of Philosophy
Continental rifting is a fundamental component of the plate tectonic cycle. The West Iberia passive margin is a classic example of a nonvolcanic rifted margin. The West Iberia margin contains an enigmatic north-south ridge of serpentinized peridotite located within the ocean-continent transition. Interpretation of multichannel seismic data and tectonic subsidence analyses suggests that the ridge is located within a broad zone of exhumed mantle that has been serpentinized. This implies that seafloor spreading does not immediately follow continental breakup. Where the peridotite ridge is well-developed, it parallels a deeply-penetrating, west-dipping normal fault. Hydrodynamic circulation drove seawater down this fault close to the beginning of seafloor spreading and caused a concentration of serpentinization at its base. This water-driven process of formation accounts for the variability of the ridge along strike. Prestack depth migration of a 340 km long seismic reflection profile across the margin served as the basis for stratigraphic interpretation. The proximal margin displays horsts and grabens, with 1--2 km thick synrift deposits from Early Cretaceous (Valanginian) rifting. The deep water margin displays rotated blocks with distinct internal seismic patterns. These blocks formed during the final Cretaceous rifting event that led to continental breakup, and contain earlier Valanginian synrift deposits as part of the rotated blocks, thus providing evidence of two separate rifting stages along this profile. A large mantle-penetrating fault located seaward of the last rotated block of continental crust provides evidence for a third and final Cretaceous rifting event that may have been coeval with early seafloor spreading. Two independent estimates of crustal thickness along this line indicate stretching factors of 50% on the proximal margin (corresponding to a continental crustal thickness of ∼16 km), increasing to 100% in the deep water. Plate tectonics is one of the most important concepts that Earth Scientists can convey to the public. While this concept is often presented at the middle school level in Texas, the appropriateness of teaching plate tectonics to middle school students has been questioned. We present data that suggest that middle school students grasp the abstract concepts of plate tectonics as well as college students.