摘要：Plate tectonic modellers often rely on the identification of “break-up”markers to reconstruct the early stages of continental separation. Along theIberian-Newfoundland margin, so-called break-up markers includeinterpretations of old magnetic anomalies from the M series, as well as the“J anomaly”. These have been used as the basis for plate tectonicreconstructions are based on the concept that these anomalies pinpoint thelocation of first oceanic lithosphere. However, uncertainties in thelocation and interpretation of break-up markers, as well as the difficultyin dating them precisely, has led to plate models that differ in both thetiming and relative palaeo-positions of Iberia and Newfoundland duringseparation.We use newly available seismic data from the Southern Newfoundland Basin(SNB) to assess the suitability of commonly used break-up markers along theNewfoundland margin for plate kinematic reconstructions. Our data show thatbasement associated with the younger M-series magnetic anomalies iscomprised of exhumed mantle and magmatic additions and most likelyrepresents transitional domains and not true oceanic lithosphere. Becauserifting propagated northward, we argue that M-series anomaly identificationsfurther north, although in a region not imaged by our seismic, are alsounlikely to be diagnostic of true oceanic crust beneath the SNB. Similarly,our data also allow us to show that the high amplitude of the J Anomaly isassociated with a zone of exhumed mantle punctuated by significant volcanicadditions and at times characterized by interbedded volcanics andsediments. Magmatic activity in the SNB at a time coinciding with M4 (128 Ma) and the presence of SDR packages onlapping onto a basement faultsuggest that, at this time, plate divergence was still being accommodated bytectonic faulting.We illustrate the differences in the relative positions of Iberia andNewfoundland across published plate reconstructions and discuss how theseare a direct consequence of the uncertainties introduced into the modellingprocedure by the use of extended continental margin data (dubious magneticanomaly identifications, break-up unconformity interpretations). We concludethat a different approach is needed for constraining plate kinematics of theIberian plate pre-M0 times. Downloadandlinks Article (PDF, 32354 KB) How to cite Back to top top How to cite. Causer, A., Pérez-Díaz, L., Adam, J., and Eagles, G.: Uncertainties in break-up markers along the Iberia–Newfoundland margins illustrated by new seismic data, Solid Earth, 11, 397–417, https://doi.org/10.5194/se-11-397-2020, 2020. 1 Introduction Back to toptop Over the past decade, plate tectonic modellers working on divergent settingshave focused their efforts on better-constraining the early stages ofcontinental separation, partly driven by the oil and gas industry's move tomore distal and deeper exploration targets(Péron-Pinvidic and Manatschal, 2009; Skogseid, 2010; Nirrengarten et al., 2017; Sandoval et al., 2019). As of today,bridging the gap between the onshore and offshore geological evolution ofrifted continental margins still presents a challenge, due to the difficultyin unequivocally interpreting the complex geology of extended continentalmargins (Alves and Cunha, 2018; Keen et al., 2018).When studying divergent settings, the onset of seafloor spreading is oftenbased on so-called “break-up markers” that originate in tectonicinterpretations made along the extended continental margins. Identified andmapped from geophysical data, these features include depositionalunconformities (e.g. Pereira and Alves, 2011; Soares et al., 2012;Decarlis et al.,2015), packages of landward-dipping reflectors (e.g.Keen and de Voogd, 1988), and seismic amplitude changes inthe top-of-basement surface (e.g. Tucholke et al., 2007),interpreted as marking the change from continental to oceanic crust. Theseinterpretations are utilized as the basis for many computer-generated platereconstructions, which are in turn highly susceptible to uncertaintiesassociated with the interpretation and mapping of said break-up markers. Arecent global census and detailed analysis of these markers highlighted thevery large average locational (167 km) and temporal (>5 Myr)uncertainties associated with defining them (Eagles et al., 2015).Uncertainties of this kind, and their impact on tectonic reconstructions,have been illustrated by, for example, the alternative scenarios proposed inthe literature for the movements of the Iberian plate between the LateJurassic to Early Cretaceous (Srivastava et al., 1990,2000; Sibuet and Collette, 1991; Sibuet et al., 2007; Greiner and Neugebauer, 2013; Barnett-Moore et al., 2016). Rotational polesderived from interpretations of the location of the continent–ocean boundary(COB), for example, have often resulted in overlaps of known continentalcrust along the Iberia–Africa plate boundary (e.g.Srivastava and Verhoef, 1992). Such overlaps are notpresent in kinematic models built on the basis of magnetic anomalies, whichassume Iberia moves together with Africa for much of this time period(e.g. Sibuet et al., 2012). They are greatly reducedin so-called “deformable” plate models that account for continental margindeformation during continental break-up (Ady andWhittaker, 2018; Müller et al.,2019; Peace et al., 2019). Becausethese models undo stretching deformation, the large uncertainty in COBlocation estimates reduces to a much smaller uncertainty envelope ofpalinspastically restored equivalents (Eagles et al., 2015). This reductionis unlikely to be useful because, as those authors note, the restoration isachieved using rotations about a stage pole that is determined using anarbitrary choice of post-stretching COB estimate and whose formalstatistical uncertainty is of a similar size to, or larger than, therestored envelope. Hence, the shape of the pre-stretching COB estimate issensitive to post-stretching COB estimates to an extent that is likely to belarger than the uncertainty that the palinspastic reconstruction techniquegenerates for it.The West Iberia and Newfoundland margins are considered by many as thetype example for magma-poor passive rifted margins(Boillotet al., 1995; Whitmarsh and Wallace, 2001;Reston, 2007; Tucholke and Sibuet, 2007;Péron-Pinvidic and Manatschal, 2009). Thecontinental margins are the result of Late Triassic to Early Cretaceousrifting and separation of the North American and Eurasian plates. This pairof conjugate margins has been the focus of more than 40 years of intenseresearch, including extensive geophysical surveying and drilling campaignsas part of the Ocean Drilling Programme (ODP) and Deep Sea Drilling Project(DSDP) (e.g. Whitmarsh andSawyer, 1996; Wilson et al.,1996). Research has revealed the margins' tectonic asymmetry and the gradualproximal-to-distal transition from regions of highly extended continentalcrust to zones of exhumed mantle locally intruded by pre- or post-break-upmagmatic intrusions. Despite this, the detailed plate kinematics, the age ofdistinct rift episodes, the timing of the final break-up, and the significance ofpre-existing structures and lithological heterogeneity are still heavilydebated. The difficulty in identifying, mapping, and dating the COB alongthis pair of conjugate margins is evident in the wide range of candidateCOBs suggested in the literature (Fig. 1) (i.e. Eagles et al., 2015 andrefs. therein). The age of final break-up and formation of first oceaniccrust is particularly uncertain. Drilling results and break-up unconformityidentifications date the onset of seafloor spreading at the Aptian–Albiantransition (113 Ma) (Tucholke andSibuet, 2007; Boillot et al., 1988). This issignificantly younger than the age of the oldest isochrons interpreted frommagnetic reversal anomalies (M20-145 Ma to M0-120 Ma) offshore of Iberia(Srivastava et al., 2000)(Fig. 1). The discrepancy means that the interpretation of these anomalies interms of M-series isochrons is disputed. Although interpreted by somestudies as markers of the first oceanic lithosphere (e.g.Vissers and Meijer, 2012; Sibuet et al., 2004), others have shownthat they may instead be associated with igneous bodies located within zonesof exhumed mantle (e.g. Sibuet et al.,2007, 2012).Figure 1(a) Study area showing the location of structural and tectonicfeatures significant to our study. Red envelopes mark the extent of the COTZas compiled by Eagles et al. (2015). Double black line: mid-ocean ridge;red dashed lines: fracture zone traces. Orange lines: seismic profilespresented and discussed in this study. Background image isSmith and Sandwell (1997) gridded satellite altimetry. (b, c) Location of M-seriesmagnetic isochron picks (Srivastava et al., 2000) and ODP legs 210 (Tucholkeet al., 2007), 103 (Boillot et al., 1987), and 173(Whitmarsh et al., 1988) drill sites. FC – Flemish Cap; FP – Flemish Pass; GB – Galicia Bank; GBA – Grand Banks; GIB – Galicia Interior Basin; IAP – IberianAbyssal Plain; IB – Iberia; MTR – Madeira Tore Rise; NFL – Newfoundland; NNB – Northern Newfoundland Basin; SNB – Southern Newfoundland Basin; TAP – TagusAbyssal Plain.
