Measurements of the extension required for crustal 1 breakup on the magma-poor Iberia-Newfoundland 2 conjugate margins 3 4 Júlia Gómez-Romeu 1,* , Nick Kusznir 1,2 , Alan Roberts 2 & Gianreto Manatschal 3 5 1 Department of Earth, Ocean and Ecological Sciences, University of Liverpool, Liverpool, UK 6 2 Badley Geoscience Ltd, Spilsby, UK 7 3 Institut de Physique du Globe de Strasbourg-EOST, Université de Strasbourg-CNRS, France 8 * Currently: M&U sas, Saint-Égrève, France 9 10 Abstract 11 Rifted margins have been extensively studied, however there are still many important 12 questions related to their formation processes. We use the Iberia-Newfoundland conjugate 13 rifted margins as a natural laboratory to investigate how much extension is required to 14 produce crustal rupture and separation. To achieve our aim we use; (i) gravity anomaly 15 inversion to measure continental crustal thinning, (ii) subsidence analysis to measure 16 continental lithosphere thinning and (iii) fault heave summation from seismic observations to 17 obtain brittle continental crust extension. Integration of thinning from gravity anomaly 18 inversion and subsidence analysis is used to determine continental crust extension and 19 continental lithosphere extension respectively. These measurements have been made between 20 the proximal continental crust and the distal Limit of Contiguous Continental Crust (LCCC) 21 on the northern (SCREECH1 and ISE01) and the southern (SCREECH2 and TGS/LG12) 22 conjugate seismic profiles. For the Iberia-Newfoundland conjugate rifted margins, extension 23 values determined from the integration of crustal and lithosphere thinning are similar and 24 suggest that on average 172 km of extension is required to produce crustal breakup. In 25 contrast, measured extension from fault heave summation is on average 128 km which 26 indicates an apparent extension discrepancy at the scale of the whole conjugate margin 27 system when observed brittle continental crust extension is compared with lithosphere and 28 crust extension. 29