Tectonic implications of exposure of lower continental crust beneath the Iberia Abyssal Plain, Northeast Atlantic Ocean: Geophysical evidence

Abstract

Abstract. We present new seismic data from a basement high beneath the southern Iberia Abyssal Plain that appears, from the latest Ocean Drilling Program (ODP) results and indirectly from various seismic observations, to consist of lower continental crustal material. This is a unique opportunity to use seismic profiles and other geophysical evidence to investigate the tectonic process which led to the exhumation of these rocks. We infer that the lower crust was exhumed at the seafloor and then uplifted toward the end of rifting. Our results lead to the following new observations: low-angle detachment faults, previously reported along the east-west Lusigal-12 profile across the southern Iberia Abyssal Plain, penetrate both the 3-10 km thick continental crust and, unusually, the uppermost mantle. Processing of new multichannel seismic reflection profiles across the basement high on which ODP Sites 900, 1067, and 1068 are located reveals that the high is bounded by a previously unsuspected steep, landward dipping normal fault on its east flank.Basement cores from the above ODP sites also reveal that the high is not capped by the previously predicted early syn-rift sediment but by rocks from the lower continental crust. These unexpected observations are incorporated into a new tectonic model of the development of this part of the west Iberia margin which is also consistent with other geophysical observations. A novel feature of this model is the proposal that the seaward edge of the continental crust is thinned to 3-6 km by a currently poorly understood process. The model implies three stages of deformation: (1) lithospheric extension, principally by symmetric pure shear, which leads to a-10 km thick crust in which the lower crust is largely absent over the axial zone and the crust-mantle boundary forms a shear zone, (2) further thinning and then dissection of the most distal continental crust by seaward dipping, low-angle normal faults, (3) inception of a high-angle, landward dipping normal fault that offsets the tectonized crust-mantle boundary and uplifts the lower crust to the crest of the basement high on which ODP Sites 900, 1067, and 1068 were drilled.

Lithospheric extension from rifting to continental breakup at magma-poor margins: rheology, serpentinisation and symmetry

“…One possibility is that symmetric extension evolves into more asymmetric extension with time (Sibuet 1992;Whitmarsh et al 2000). In this paper we argue that the observations are consistent with such a transition from symmetric to asymmetric rifting during late-stage extension and that this transition follows the embrittlement of the crust and the serpentinisation of the uppermost mantle.…”

Control of rheological stratification on rifting geometry: a symmetric model resolving the upper plate paradox

“…Figure 1 shows a cross-section through one of the best investigated margins, the Cretaceous Galicia Margin west of Portugal. The Galicia Margin and the adjacent Iberian Abyssal Plain Margin have been the subject of intense research including all of the above mentioned techniques, and often serve as reference margins in studies on magmapoor rifting (ODP legs 103, 149, and 173, Boillot et al 1980, 1995Mauffret and Mondatert 1987;Hoffmann and Reston 1992;Reston et al 1996;Pickup et al 1996;Whitmarsh et al 2000;Pérez-Gussinyé et al 2006). Researchers working on ancient margins now preserved in the Alps often compare their results with findings from the Galicia Margin (Manatschal and Bernoulli 1999;Whitmarsh et al 2001;Manatschal 2004).…”

Surface wave imaging of the weakly extended Malawi Rift from ambient-noise and teleseismic Rayleigh waves from onshore and lake-bottom seismometers

“…However, many ancient and active rift systems display evidence of volumetrically limited magmatic activity, for example, the Western Branch of the East African Rift System (EARS ;Furman 2007) and the Newfoundland-Iberia rifted margins (e.g. Whitmarsh et al 2000;Van Avendonk et al 2009). To shed light on extensional processes in an immature rift, including the controls of magmatism on extension, the SEGMeNT (Study of Extension and maGmatism in Malawi aNd Tanzania) experiment acquired a suite of data sets including GPS, active-and passivesource seismic, geochemical and magnetotelluric data across the Malawi Rift (Shillington et al 2016), where volcanism is restricted to the northern axial termination.…”