Figure F3 shows a prestack time migration of the seaward portion of line 2MCS, and Figure F4 shows the rest of the lines in the SCREECH transect 2 survey. Prior to migration and stacking, processing consisted only of muting bad traces and applying a minimum-phase bandpass filter that limited the frequency content to 10–100 Hz. For all lines except lines 104 and 305, smoothed interval-velocity sections were generated from stacking velocities picked on semblance plots and were used to apply Kirchhoff migrations to prestack data. Kirchhoff migration of CMP gathers yielded migrated gathers, which were then stacked to produce the final prestack time-migrated section. Lines 104 and 305 have a different processing flow. Line 104 is a poststack, water-velocity, frequency-wavenumber migration, and line 305 is an unmigrated stack.
Time migrations of the SCREECH transect 2 survey reveal several first-order seismic characteristics of the crust. Three distinct crustal zones are readily identified (Fig. F3). Unambiguous continental crust extends seaward to at least CMP ~220000 on line 2MCS. At this location, a continental block capped by what may be faulted prerift sediments is observed both on line 2MCS and on line 301, which crosses line 2MCS at this location (Figs. F3, F4Q). Seaward of CMP 220000, apparently featureless basement, capped by a sequence of very bright reflections including the U reflection, continues seaward for ~70 km. This interval constitutes the transitional crust in this part of the Newfoundland Basin. It is unclear where the top of basement lies in most of this crustal domain. However, toward the seaward end of this zone, some basement topography gradually becomes apparent and then gives way to higher-amplitude relief at CMP ~230000 (near magnetic Anomaly M3). High basement relief (>1 km) continues seaward to the end of the SCREECH transect 2 survey.
Basement in the 70-km-wide transitional zone from CMP 220000 to 230000 appears nearly featureless in MCS sections (Figs. F3, F4A, F4E, F4P, F4R, F4S, F4T, F4U). The top of basement usually cannot be identified, and intracrustal reflections are not observed in most of this domain. A few hints of the seismic character of this crust can be observed on lines 209 and 303 (Fig. F4P, F4S), where some basement topography can be identified. The apparent lack of reflectivity of the transitional crust might indicate that it is homogeneous, that its impedance is little different from the overlying deep lithologic section, or that there is low signal penetration through the U reflection and other bright reflections in the lowermost lithologic section.
Basement topography seaward of magnetic Anomaly M3 on line 2MCS forms a series of margin-parallel ridges; a contoured plot of basement topography (in two-way traveltime) is shown in Figure F5. The ridge topography is most apparent where lines 204a, 204b, and 206 (Fig. F4J, F4K, F4M) cross line 2MCS (Fig. F3) and along lines 107 and 109 (Fig. F4D, F4F); in this area, three distinct high-amplitude ridges are observed. One of these ridges rises above the seafloor, as is seen on line 205 (Fig. F4L). The reflective character of the ridges is consistent between lines; the uppermost basement is highly reflective, whereas the deeper crust appears relatively transparent.
Magnetic anomalies were calculated by subtracting the International Geomagnetic Reference Field (2000–2005) for epoch 2000.0 from the total magnetic intensity measured on the ship (Mandea et al., 2000). Total magnetic intensity readings were taken from a towed Geometrics G-886 marine magnetometer at 12-s intervals. All values where the magnetometer stopped working (and the measured total magnetic intensity equaled zero) were removed, and the remaining data were smoothed using a boxcar filter with a length of 1 km. Figure F6 shows ship-track magnetic data collected on margin-normal lines plotted on top of an image of gridded magnetic data compiled by Verhoef et al. (1996). The shipboard magnetic data are also plotted in blue above each of the seismic sections in Figures F3 and F4. Missing data in the figures indicate long periods of time when the magnetometer was not working.
Magnetic anomalies in the transitional crust along line 2MCS display comparatively low amplitudes, as observed in previous magnetic profiles collected in this region (e.g., Srivastava et al., 2000; Verhoef et al., 1996). Average magnetic-anomaly amplitudes are typically <100–150 nT. The "J-anomaly," which is located at the seaward edge of the transitional crust, has been identified on both the Newfoundland and Iberia margins and was formed between Anomalies M0 and M2 (Rabinowitz et al., 1978; Russell and Whitmarsh, 2003; Tucholke and Ludwig, 1982). Although this is a high-amplitude anomaly in the southern Newfoundland Basin, its amplitude in the area of the SCREECH transect 2 survey is similar to anomaly amplitudes in the transitional crust. The locations of magnetic Anomalies M3 and M0 that were previously identified by Srivastava et al. (2000) are labeled on Figures F3 and F4 by arrows that indicate the center of an idealized block of constant polarity. These anomalies are also labeled as dashed lines on Figure F6.
Prior to calculating gravity anomalies, Eötvös corrections were applied to raw gravity readings to correct for ship course and speed. The free-air anomaly was calculated by subtracting the theoretical gravity from the corrected measured gravity. Theoretical gravity was derived by the 1980 Gravity Formula (Moritz, 1988). All measurements where the gravimeter stopped working or was not level (and the reading equaled zero) were removed, and the remaining data were smoothed using a boxcar filter with a length of 1 km. The free-air gravity data are plotted in red above each seismic section in Figures F3 and F4. Figure F7 shows ship-track free-air gravity data along margin-normal lines plotted on top of gridded Geosat free-air gravity data (e.g., Douglas and Cheney, 1990). Missing data indicate long periods of time when the gravimeter was not working.
The seaward portion of the SCREECH transect 2 survey lies in a region of consistently positive free-air anomalies. In the SCREECH survey, as in prior studies (Verhoef et al., 1987), little variation in free-air anomaly values is observed.