Palaeozoic oceanic crust preserved beneath the eastern Mediterranean

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1 Palaeozoic oceanic crust preserved beneath the eastern Mediterranean Roi Granot Magnetic Anomaly Data Total field Three components 75 m 75 m Total field 250 m Overhauser Three-axial fluxgate Overhauser Supplementary Figure 1. Experimental setup for the MEDMAG project. The first and third towed sensors were total field magnetometers (SeaSpy TM ) that were operated in a longitudinal gradiometer mode. A three-axial fluxgate magnetometer (Magson TM ) was mounted between the two total field sensors. Assuming that the magnetic source has a white power spectrum, the wavenumber band where the spectra are decaying with a constant slope constrains the typical frequencies of crustal anomalies. The observed lineated magnetic crustal anomalies from the eastern Mediterranean could therefore be limited to the wavelengths band between 20 and 300 km (Supplementary Fig. 2). A bandpass filter was applied to the total and component anomalies to remove wavelengths shorter than 20 km and longer than 300 km that cannot be attributed to crustal sources. Besides the non-crustal anomalies, this filter removes the high frequency scatter of NATURE GEOSCIENCE 1

2 the magnetic components (Supplementary Fig. 4) that formed mostly due to the dynamic motion of the towed vector sensor and that cannot be accounted for by the tilt meters (which assume static motion). The power spectral densities of the unfiltered total field anomalies, computed separately for the Herodotus Basin (red, Supplementary Fig. 2) and Levant Basin (blue, Supplementary Fig. 2), provide estimations of the height of the sensor above the magnetic source layer 1-3. Although this approach often overestimates the true depth of the magnetic source layer, these results hint that the source layer resides between 16 and 20 km below the sea surface in the Herodotus Basin (i.e., between 13 to 17 km below the seafloor) and between 12 and 14 km below the sea surface in the Levant Basin (i.e., between 10 and 12 km below the seafloor). Supplementary Figure 2. Stacks of power spectra densities computed from the unfiltered total field magnetic profiles. Power spectra from the Herodotus and Levant Basins are plotted in red and blue, respectively. Shaded areas represent variance. Black lines show the slopes expected for magnetic sources of different depths. The wavenumber band where the spectra display a constant decay (Band A of ref. 2, gray shaded area above) is confined between wavelengths of 20 km to around 300 km and is most likely due to crustal sources. 2 NATURE GEOSCIENCE

3 SUPPLEMENTARY INFORMATION Long lineated marine magnetic anomalies that straddle passive margins and found within the transition zones between the continental and oceanic crust (e.g., in the Iberia-Newfoundland margins) have been associated with major magmatic events that occurred during the rifting stage, prior to the final breakup and establishment of seafloor spreading 4. Using seismic and drill-hole data, these authors associated the anomalies with magmatic underplating that resulted in thickening of the crust. Interestingly, these variations in crustal structure and the magnetic anomalies ( J anomalies) associated with them are, for the most part, accompanied by gravity anomalies (Supplementary Fig. 3). The Herodotus Basin (unlike the Levant Basin, Fig. 4 and Supplementary Fig. 7) does not show pronounced gravity anomalies, and therefore, I conclude that the lineated magnetic anomalies of the Herodotus Basin result from spatial variations in crustal magnetization (i.e., that the crust is oceanic) J J Vertical gravity gradient (Eötvös) Supplementary Figure 3. Vertical gravity gradient map of the Iberia margin, North Atlantic. White lines indicate the location of the J magnetic anomaly 4. NATURE GEOSCIENCE 3

4 Supplementary Figure 4. Full vector magnetic profiles. Unfiltered horizontal and vertical magnetic components (bottom two rows) are represented with black lines. Red lines show the filtered anomalies. The second row displays the observed filtered horizontal components (solid lines) and the π/2 phase-shifted filtered vertical components (dashed lines). The top row shows the three-dimensional index. White-filled stars projected on the map mark the locations of transition between the coherent (west of the stars) and non-coherent (east of the stars) appearance of the horizontal and shifted vertical components. Black circles indicate the location of every hundredth km. 4 NATURE GEOSCIENCE

5 SUPPLEMENTARY INFORMATION To obtain a crustal age estimation for the anomalies found in the Herodotus Basin, I used the shapes of the anomalies, which depend, among other known variables, on the palaeoreconstruction of the African Plate. Two independent steps were taken: the first was quantification of the shape (skewness) of the observed anomalies and the second comprised the construction of the reference, time-dependent curve that show the expected skewness angles throughout the investigated period. The observed, stacked total field anomalies were de-skewed in one-degree intervals and compared, using a coherence analysis (inset in Fig. 3a), to a simple magnetic anomaly model computed at the pole (i.e., zero skewness). Best coherency was achieved when the shape of the de-skewed stack achieved a roughly symmetrical shape and the shoulders of the anomalies were at the same height (i.e., at skewness angle of 160º). Uncertainty in the determination of skewness is based on the 95 percent confidence bound of the coherence curve (confidence is 0.04) suggesting ± 10º uncertainty in the determination of the skewness angle (Supplementary Fig. 5). This uncertainty bound is rather conservative when compared with those of previous works 5,6, in which skewness angles were determined visually and assigned uncertainties of 4º to 7º. NATURE GEOSCIENCE 5

6 θ Model 50 nt 0 Supplementary Figure 5. Representatives de-skewed total field anomaly stack (Fig. 3b). Phase shift angles (θ) are shown on the left. The simple model profile formed at the pole (i.e., zero skewness) is shown in red (bottom). Dimensionality of Magnetic Source Layer The vector components provide an estimate on the degree to which the magnetic source layer is three-dimensional. When the absolute difference between the horizontal component (F H ) and the π/2 phase shifted vertical component (F Z ) is larger than the level of error (i.e., three dimensional index 7, here calculated to be 40 nt), then the magnetic source layer is regarded as 6 NATURE GEOSCIENCE

7 SUPPLEMENTARY INFORMATION having a three dimensional shape. Several geological structures can potentially cause a magnetic signature of a three-dimensional body to the oceanic crust (e.g., seamount 8 and ridge propagator 9 ). However, a regional consistent change in the three-dimensional index indicates large-scale change in the characteristics of the crust, i.e., transition between oceanic and continental crust. The results from the eastern Mediterranean demonstrate that east of longitude 31ºE the horizontal and phase-shifted vertical components generally mismatched (Supplementary Fig. 4) whereas west of it, within the Herodotus Basin, they generally conform to each other (within the uncertainty), suggesting that the source of the anomalies in the Levant and Herodotus Basins comprises three- and two-dimensional magnetic bodies, respectively. Interestingly, strong total field anomalies in the Levant Basin (Supplementary Fig. 7a, up to 600 nt peak-to-trough) generally follow the main structures of the Levant Basin as seen by seismic reflection 10 and gravity data (Supplementary Fig. 7b). These anomalies cannot be explained by the typical magnetization of an oceanic crust buried under 10 km of sediments, which would create anomalies nearly one order of magnitude smaller (Fig. 3b), further confirming their continental origin. NATURE GEOSCIENCE 7

8 Eurasia Plate Zagros Collision 26 Arabia Plate Makran Subduction Supplementary Figure 6. Seismicity map of the transition zone between the Zagros collision and the Makran subduction zones. Red circles denote earthquake epicenters (details follow the epicenters shown in Fig. 4). The motion of the Arabia Plate relative to that of the Eurasia Plate 11 is shown with a blue line (33.7 mm/yr). 8 NATURE GEOSCIENCE

9 SUPPLEMENTARY INFORMATION a 31 b nt mgal 40 Supplementary Figure 7. Magnetic and Bouguer anomalies for the Levant Basin. a, Total field magnetic anomaly map of the Levant Basin. b, Residual gravity map (high-pass filtered (200 km)) of the Bouguer anomalies calculated based on satellite free-air gravity field (Version 2312). Bouguer correction was calculated using water density of 1035 kg m-3 and sediments density of 2000 kg m-3. Brown contours delineate the 1000 m and 1500 m contours surrounding the Eratosthenes Seamount. References 1. Horner-Johnson, B. C. & Gordon, R. G. Equatorial Pacific magnetic anomalies identified from vector aeromagnetic data. Geophys. J. Int. 155, (2003). 2. Parker, R. L. & O'Brien, M. S. Spectral analysis of vector magnetic field profiles. J. Geophys. Res. 102, (1997). 3. Spector, A. & Grant, F. S. Statistical models for interpreting aeromagnetic data. Geophysics 35, (1970). NATURE GEOSCIENCE 9

10 4. Bronner, A., Sauter, D., Manatschal, G., Péron-Pinvidic, G. & Munschy, M. Magmatic breakup as an explanation for magnetic anomalies at magma-poor rifted margins. Nature Geo. 4, (2011). 5. Cande, S. C. A palaeomagnetic pole from Late Cretaceous marine magnetic anomalies in the Pacific. Geophys. J. R. Astrom. Soc. 44, (1976). 6. Dyment, J., Cande, S. C. & Arkani-Hamed, J. Skewness of marine magnetic anomalies created between 85 and 40 Ma in the Indian Ocean. J. Geophys. Res. 99, (1994). 7. Korenaga, J. Comprehensive analysis of marine magnetic vector anomalies. J. Geophys. Res. 100, (1995). 8. Engels, M., Barckhausen, U. & Gee, J. S. A new towed marine vector magnetometer: methods and results from a Central Pacific cruise. Geophys. J. Int. 172, (2008). 9. Korenaga, J. & Hey, R. N. Recent dueling propagation history at the fastest spreading center, the East Pacific Rise, 26º-32ºS. J. Geophys. Res. 101, (1996). 10. Gardosh, M. A., Garfunkel, Z., Druckman, Y. & Buchbinder, E. in Evolution of the Levant Margin and Western Arabia Platform Since the Mesozoic (eds C. Homberg & M. Bachmann) 9-36 (Geol. Soc. Spec. Publ., 341, 2010). 11. Argus, D. F., Gordon, R. G. & DeMets, C. Geologically current motion of 56 plates relative to the no-net-rotation reference frame. Geochem. Geophys. Geosyst. 12, Q11001 (2011). 12. Sandwell, D. T., Müller, R. D., Smith, W. H. F., Garcia, E. & Francis, R. New global marine gravity model from CryoSat-2 and Jason-1 reveals buried tectonic structure. Science 346, (2014). 10 NATURE GEOSCIENCE