Magmatism at the west Iberia non-volcanic rifted continental margin: evidence from analyses of magnetic anomalies

Abstract

SUMMARY We discuss the magmatic development of the west Iberia non‐volcanic rifted continental margin in the North Atlantic Ocean. So‐called ‘non‐volcanic’ rifted continental margins are characterized by a lack of syn‐rift magmatism and are considered to be largely amagmatic. However, this is clearly an oversimplification since seafloor spreading itself is a magmatic process and it is implausible that seafloor spreading begins instantaneously. We concentrate our attention on the recently described zone of exhumed continental mantle (ZECM) to investigate what magmatic processes accompanied the breakup of the continental lithosphere and the subsequent formation of the ZECM leading to the onset of seafloor spreading. We use magnetic anomalies supplemented by the interpretations of multichannel seismic reflection profiles and wide‐angle seismic experiments presented elsewhere. Forward and inverse modelling of a sea‐surface magnetic anomaly chart and of surface and deep‐towed magnetometer profiles shows that anomalies within the ZECM differ in trend, amplitude and source type from those in the adjacent oceanic crust and thinned continental crust. The ZECM anomalies appear to be caused by elongated source bodies within 8 km of the top of the acoustic basement aligned parallel to the margin. We interpret such bodies as syn‐extensional intrusions that increase in volume oceanward. They eventually merge in the vicinity of a margin‐parallel, basement peridotite ridge to give rise to a continuous crust that records reversals in the Earth's magnetic field from the time of anomaly M4(N)–M5(R), i.e. to mark the onset of seafloor spreading. We find no evidence for anomalies formed by seafloor spreading, at either slow or ultraslow rates, before M5(R) (128 Ma).

Magnetic signature of large exhumed mantle domains of the Southwest Indian Ridge – results from a deep-tow geophysical survey over 0 to 11 Ma old seafloor

“…Therefore, in view of the low magnetization of the young (< 11 Ma) serpentinized rock at the SWIR, it is unlikely that strong magnetic anomalies could be related solely to serpentinization; this would be even truer at > 100 Ma old OCTs. Instead, it supports the hypotheses that (1) intrusive or extrusive material is required (Bronner et al, 2011;Russell and Whitmarsh, 2003) to account for a significant magnetic signal in the exhumed mantle domains of OCTs and that (2) the interpretation of this signal as resulting from seafloor spreading is precluded in the absence of a homogeneous and well-established upper oceanic crust. Consequently, the kinematic reconstructions of magma-poor passive margins using weak anomalies identified over exhumed mantle domains need to be treated with caution.…”

“…The aim is to better understand the complexity of the marine magnetic anomalies observed above the serpentinized mantle rocks exhumed at mid-oceanic ridges . Finally we discuss the implications of our findings for the understanding of exhumed mantle domains at OCTs of magma-poor rifted margins and the origin and significance of broad zones of chaotic magnetic patterns are discussed (Russell and Whitmarsh, 2003;Sibuet et al, 2007;Bronner et al, 2011;Tucholke and Sibuet, 2012;Bronner et al, 2012).…”

Geophysical characterisation of the ocean–continent transition at magma-poor rifted margins

“…Anomalies typically have low amplitude and are only weakly linear, so their identification is difficult. In these circumstances, the acquisition of magnetic data near the seafloor, where amplitudes are much enhanced, can be valuable [12]. The existence of these anomalies does not require the presence of a highly magnetised upper oceanic crust formed by seafloor spreading, and can alternatively be explained by magmatic intrusion at depth or simply by the presence of magnetite in serpentinites [53].…”

“…The presence of these anomalies indicates the presence of rocks with higher magnetisation than the adjacent continental crust, but there exists a range of views on whether the process that generates them can be called "seafloor spreading" [11][12][13]. Many of the characteristics of the OCT are shared by some regions of oceanic crust formed at very slow spreading rates.…”

Aspects of marine geoscience: a review and thoughts on potential for observing active processes and progress through collaboration between the ocean sciences

“…Various forms of the latter have been proposed to explain, for example, the distribution of lithologies and geophysical characteristics of the Iberian-Newfoundland conjugate margin pair [154][155][156][157][158], the Rockall Plateau-Hatton Bank conjugate margin to Greenland [159] and the Amazon conjugate margin to Liberia [160]. In terms of UK efforts, the Iberian margin is arguably the best-characterized continental margin, with seismic, magnetic, gravity and scientific drilling data [161][162][163][164][165][166][167][168][169][170].…”