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Application to the Mid-Norwegian Continental Margin Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021 Isostasy as a tool to validate interpretations of regional geophysical datasets – application to the mid-Norwegian continental margin SOFIE GRADMANN1*, CLAUDIA HAASE1 &JO¨ RG EBBING1,2 1Geological Survey of Norway, Leiv Eirikssons vei 39, 7040 Trondheim, Norway 2Present address: Department of Geosciences, Kiel University, 24118 Kiel, Germany *Correspondence: sofi[email protected] Abstract: Isostasy is a well understood concept, yet rarely applied to its full capacity in regional interpretations of crustal structures. In this study, we utilize a recent density model for the entire NE Atlantic, based on refraction seismic data and gravity inversion, to calculate isostatically balanced bathymetry along the mid-Norwegian margin. Since gravity and isostatically balanced elevation are independent observables but both depend on the underlying density model, consis- tencies and discrepancies point towards model deficits, erroneously interpreted or poorly understood areas. Four areas of large isostatic residuals are identified. Along the outer Vøring Margin, a mass deficit points to more extensive high-density bodies or a shallower Moho than currently mapped. Farther seaward, along the Vøring Marginal High, a mass excess indicates inaccurate mapping of the continent–ocean boundary and surrounding structures. A number of eclogitic bodies along the proximal mid-Norwegian margin have been described in recent publications and their presence is now also confirmed by isostatic calculations. Major elevation and gravity residuals along the transition between the Vøring and Møre margins signify that the structure of this region is poorly understood and modifications to the mapped continent–ocean transition may be required. Gold Open Access: This article is published under the terms of the CC-BY 3.0 license. The concept of isostasy has been applied to several Continental margins may be especially well regional studies worldwide, helping to estimate lith- suited for isostatic studies since the crust is thinned ospheric strength and explain the shape and origin and its deeper structure and Moho are more easily of topography (e.g. Watts 2001; Kaban et al. imaged by seismic methods than, for example, 2003). Local studies, however, which investigate those of cratonic lithosphere. Seismic mapping is the crustal structure in more detail by employing especially extensive (and accessible) along refraction and deep reflection seismic data as well hydrocarbon-bearing continental margins such as as gravity modelling, often ignore the additional the mid-Norwegian one. Furthermore, thinned insight that could be gained from isostatic consider- crust and lithosphere are generally weaker, implying ations. Positive exceptions to this are, for example, that lateral density variations and Moho undulations Watts et al. (1985), Zeyen & Ferna`ndez (1994), will have a larger expression in the bathymetry or Mat¸enco et al. (1997), Hirsch et al. (2009) and basin shape. Thermal weakening during rifting and Fullea et al. (2010), where isostasy has entered the breakup as well as rigidification during subsequent model building and interpretation. Methods for cooling additionally affect the flexure of the margin. joint analysis and inversion of gravity and isostasy The enormous amount of geophysical and geo- have, for example, been developed by Zeyen & Fer- logical data that exists for the region of the NE na`ndez (1994), Stewart et al. (2000), Braitenberg Atlantic has been gathered, analysed and compiled et al. (2004) and Salem et al. (2014). in recent years into a comprehensive tectonostrati- Only a few of the studies involving isostasy graphic atlas (the NAG-TEC Atlas, Hopper et al. cover the mid-Norwegian margin (Kimbell et al. 2014). Based on these data, a new seismic-derived 2004; Ebbing 2007) and we show in this paper that Moho grid has been compiled (Funck et al. 2016) simple isostatic considerations can also improve the and subsequently further improved using gravity understanding of the crustal structure. inversion (Haase et al. 2016). The latter study also From:Pe´ron-Pinvidic, G., Hopper, J. R., Stoker, M. S., Gaina, C., Doornenbal, J. C., Funck,T.&A´ rting,U.E. (eds) The NE Atlantic Region: A Reappraisal of Crustal Structure, Tectonostratigraphy and Magmatic Evolution. Geological Society, London, Special Publications, 447, https://doi.org/10.1144/SP447.13 # 2017 The Author(s). Published by The Geological Society of London. Publishing disclaimer: http://www.geolsoc.org.uk/pub_ethics Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021 S. GRADMANN ET AL. constructed a first-order density model over the formation in the early Eocene (c. 54 Ma, Talwani entire NE-Atlantic, which we utilize here to forward & Eldholm 1977; Skogseid & Eldholm 1987). calculate an isostatically consistent bathymetry that This volcanic activity was part of the North Atlantic reflects all internal loading of the model. The result- Igneous Province (Eldholm & Grue 1994; Meyer ing differences from the true bathymetry enable us et al. 2007) and is reflected along the present-day to detect consistencies and discrepancies between mid-Norwegian outer margin by extensive regions the datasets mapping the deep subsurface structure. with seaward-dipping reflectors and massive vol- Since gravity and isostatically balanced elevation canic flows. Magmatic activity also led to intrusions are independent observables but both depend on into the thinned continental crust and underplating the underlying density model, consistencies and dis- in the continent–ocean transition zone. Oceanic crepancies can point towards poorly understood or spreading along the Aegir and Mohn’s ridges led erroneously interpreted areas. The obtained eleva- to the complete separation of the Greenland and tion residuals are then discussed with respect to Eurasian plates. Continued, contemporaneous existing seismic profiles and structural interpreta- Cenozoic rifting in East Greenland and associated tions along the margin. By considering residuals breakup in late Oligocene (c. 25–24 Ma) led to of shorter wavelengths only, we focus on the crustal the formation of a second spreading axis and to structure, which is much better mapped and dis- the separation of the Jan Mayen microcontinent cussed than the underlying lithospheric mantle. from Greenland (Talwani & Eldholm 1977; Vogt Previous studies that investigated regional isostatic et al. 1980; Nunns 1982; Gudlaugsson et al. 1988; and gravity modelling in the NE Atlantic region Gaina et al. 2009; Gernigon et al. 2015). Today, (Kimbell et al. 2004; Reynisson et al. 2010) had the Kolbeinsey Ridge is the active spreading centre less complete datasets at hand and focussed on linking the Reykjanes Ridge in the south to the somewhat different questions. This study merely Mohn’s Ridge in the north. tests an existing subsurface model and does not A large number of seismic refraction profiles attempt to improve it. cover the mid-Norwegian margin but become more sparse to the north (Fig. 1). Some profiles are several decades old and large receiver spacing Geological history of the mid-Norwegian (10–20 km) often prevents the detailed mapping margin of smaller-scale features. Nevertheless, this exten- sive grid predominantly shaped our present-day The mid-Norwegian margin, like any passive rif- understanding of the deep margin structures. ted margin, consists of numerous rifted blocks The 300 km wide Møre Margin is characterized and interjacent sedimentary basins and sub-basins by a narrow platform and a rapid thinning of the (Fig. 1). The margin is subdivided into the three continental crust. The main area of crustal thinning major segments of the Møre, Vøring and Lofoten is confined to a narrow zone with a width of only a margins. The large shelf areas of the North Sea few tens of kilometres (Olafsson et al. 1992; Raum and Barents Sea extend to the south and north, 2000; Mjelde et al. 2008; Osmundsen & Ebbing respectively. The structure of the mid-Norwegian 2008; Kvarven et al. 2014; Nirrengarten et al. margin reflects a long period of extension following 2014). West of this major necking zone lies the the orogenic collapse of the Caledonian orogeny. nearly 200 km wide and long Cretaceous Møre Earliest extension during the post-orogenic collapse Basin, which reaches depths of more than 15 km. initiated in Devonian–Carboniferous times, form- This basin is underlain by highly thinned crust (in ing large detachment faults and basins that are places less than 10 km thick) and high-velocity, now partly exposed in onshore areas (e.g. Seranne high-density lower crustal bodies (LCBs) have & Seguret 1987; Osmundsen & Andersen 2001, been mapped both outbound and inbound of the and references therein) and possibly buried by youn- necking zone but not directly underneath it (Olafs- ger sediments in the offshore domain. Carbonifer- son et al. 1992; Kvarven et al. 2014; Nirrengarten ous to early Mesozoic extension created the early, et al. 2014). The outer limit of the margin compri- proximal rift basins (e.g. the Trøndelag Platform; ses the Møre Marginal High which is overlain by a Brekke 2000). Distal rift basins formed during Mid- series of volcanic flows. Whereas some authors dle Jurassic to Early/middle-Cretaceous (e.g. the interpret the basement of the Møre Marginal High Lofoten, Vøring and Møre basins; Brekke 2000). as thick continental crust (e.g. Reynisson et al. Late Cretaceous to Paleocene extension shaped the
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