Thermal evolution of onshore West Iberia: A better understanding of the ages of breakup and rift-to-drift in the Iberia-Newfoundland Rift

Highlights

• Apatite FT data were acquired for the Variscan basement of western Iberia.

• Two main cooling events are recorded at ca. 150 Ma and ca. 110 Ma.

• 150 Ma cooling is interpreted as uplift of the rift shoulder (Variscan basement).

• Whole lithosphere failure (breakup) is at the origin of the 150 Ma major uplift.

• The 110 Ma event is interpreted as the onset of oceanic spreading (rift-to-drift).

Abstract

The age of breakup which formed the Central-North Atlantic has been debated for many decades and is still subject to debate: from ca. 150 Ma to 110 Ma. To address this issue, we have carried out a thermochronological study of the eastern margin of the rifted Iberia-Newfoundland sector. New apatite fission-track (AFT) data acquired on samples from the footwall (Variscan basement) and one sample (Triassic of the Lusitanian Basin) from the hanging wall of the principal normal fault bounding the basin. Thermal history of western Iberia can be then reconstructed since ca. 250 Ma. Fission-track ages of Variscan granitoids (whose crystallization age is >275 Ma) range between 191 ± 8 Ma and 75 ± 5 Ma, indicating that significant thermal events affected the study area during that period. Thermal inversion supports two main cooling events that we attribute to a major uplift and denudation of the Variscan basement, consistent with widespread basement-derived siliciclastic rocks of similar ages: Late Jurassic/Early Cretaceous (ca. 150–145 Ma) and late Early Cretaceous (ca. 110 Ma). From the sedimentary record in the Lusitanian Basin and the new AFT data, we deduce that: (1) temperatures >70–110 °C affected west Iberia between ca. 190 and 150 Ma, which can be explained partly by a subsiding basement in both foot and hanging walls; (2) the main cooling event at ca. 150–145 Ma is interpreted as major rift flank uplift; (3) the ca. 110 Ma cooling event may be linked to the final evolution of the margin and onset of oceanic spreading. We infer that the AFT main cooling event at ca. 150–145 Ma reflects the break of the elastic core of the lithosphere (whole lithosphere failure = breakup) with significant rift shoulder uplift (end of Rift 1), which was followed by hyper-extension and mantle exhumation (Rift 2) and finally, by oceanic spreading (rift-to-drift) at ca. 110 Ma (onset of Rift 3).

Introduction

Breakup of the continental lithosphere (rift) and sea-floor spreading (drift) are two significant phases of the supercontinent cycle (Allen and Allen, 2005). Timing is crucial but may still be subject to debate in some key areas. The Newfoundland-Iberia rift system has been considered the archetype of a magma-poor rift (Boillot et al., 1980, Boillot et al., 1995; Manatschal and Bernoulli, 1999; Whitmarsh et al., 2001; Reston, 2009; Pereira et al., 2017) and its age of rift-to-drift stage is subject to debate as there is still no agreement on the position and the ages of the magnetic anomalies. This is partially due to the existence of a diffuse continent ocean transition due to ultra-slow opening (Dean et al., 2000). There are three main views on its timing: Late Jurassic (160 Ma-145 Ma) to Early Cretaceous (> 128 Ma) (e.g. Mauffret et al., 1989; Hiscott et al., 1990; Whitmarsh and Miles, 1995; Srivastava et al., 2000; Wilson et al., 2001; Russell and Whitmarsh, 2003; Tucholke et al., 2007), Early Cretaceous (ca. 112 Ma) (e.g. Driscoll et al., 1995), and spread over time (145–128 Ma and 112 Ma) (e.g. Péron-Pinvidic et al., 2007; Bronner et al., 2011). A strong argument has been put forward by Srivastava et al. (2000) in favour of an Upper Jurassic rift-to-drift stage in the south (rifting was diachronous along the strike, i.e. becoming younger as you move from south to north), because the oldest magnetic anomaly that has been found west of Iberia and in the Grand Banks of Canada is M20, i.e. ca. 147 Ma. Subsidence has been analysed to estimate the successive rift stages in western Iberia but has not brought decisive answers (e.g., Stapel et al., 1996; Rasmussen et al., 1998; Leinfelder and Wilson, 1998; Alves et al., 2002, Alves et al., 2009; Alves and Cunha, 2018). Three rift phases have been recorded: Rift 1 (during the Late Triassic >202 Ma), Rift 2 (during most of the Jurassic, ca. 200 to 150 Ma) and Rift 3(at the end of the Jurassic, especially in the transition from the Jurassic to the Cretaceous, ca. 145 Ma). Additional data is required to clarify this issue.

If the concept of rift-to-drift is clear and well established as the moment of first creation of basaltic oceanic crust, the concept of breakup is unclear, and is often assumed to be equivalent to rift-to-drift. Breakup may also be the stage when one continental lithosphere becomes two separate plates, followed by hyper-extension and an intervening transition zone made up of exhumed mantle and intrusive gabbro and mafic dykes (Soares et al., 2012). To resolve this ambiguity, in this study we have used the mechanical definition of lithosphere (elastic in time scales of Ma; e.g. Burov and Diament, 1995) to consider that a lithospheric plate is broken (breakup moment) when its elastic core vanishes, i.e. its elastic thickness becomes zero. This corresponds to the concept of “whole lithosphere failure” (WLF) (e.g. Kusznir and Park, 1982; see also Marques and Podladchikov, 2009 and Fig. S1 in the supplementary material). Following this definition, we should see the effects of WLF on the vertical movements of rift shoulders (Beaumont et al., 1982; Weissel and Karner, 1989), encompassing the whole basin. Weissel and Karner (1989) suggested that rift flank uplift during extension may result from mechanical unloading of the lithosphere and a consequent isostatic rebound. This mechanism is preferred here as an alternative to explanations for rift flank uplift involving thermal processes and magmatic thickening of the crust, because we are studying a magma-poor rift.

The aim of this paper is to understand the behaviour of the rift shoulder, thus allowing us to deduce the age of WLF from rift shoulder uplift. We investigated this uplift, using low-temperature thermochronology coupled with the onshore geological records. AFT thermochronology has long been used to detect upper crustal rock uplift and cooling, in particular on passive margins (e.g., Gallagher and Brown, 1997).