Crustal structure beneath Orphan Basin and implications for nonvolcanic continental rifting

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. B6, PAGES 10,923-10,940, JUNE 10, 2001 Crustal structure beneath Orphan Basin and implications for nonvolcanic continental rifting D. Chian Geological Survey of Canada (Atlantic), Dartmouth, Nova Scotia, Canada I.D. Reid Danish Lithosphere Centre, Copenhagen, Denmark H. R. Jackson Geological Survey of Canada (Atlantic), Dartmouth, Nova Scotia, Canada Abstract. A wide-angle seismic profile was acquired across the northeast Newfoundland margin, seaward to Orphan Knoll. The profile is complemented by existing multichannel reflection data, two online deep wells, and other deep seismic reflection and refraction lines. Modeling of data from 15 ocean bottom seismometers provides the seismic velocity structure for a significant portion of the -400-km-wide stretched continental crust on this margin. Prerift metasedimentary rocks have a topographic relief of up to 5 km, with a velocity of -5.2 km/s increasing to 5.9 km/s. Unaltered crystalline crust below the metasedimentary section has a velocity range of km/s. Most of the lower crust landward of Orphan Knoll has a typical continentalower crustal velocity ( km/s), and the lower crust shallows significantly beneath Orphan Knoll. Our model shows no evidence for the 5-km-thick, 7.35 km/s layer modeled from data of an earlier experiment and interpreted as magmatic underplating but instead indicates that thinned continental crust extends seaward for 360 km without underplating, implying nonvolcanic rifting for the NE Newfoundland margin. We suggesthat continental stretching persisted from Ma to Chron 34N time (--101 Ma) when the final breakup between Canada and western Europe took place, leaving a 400-kmwide zone of thinned continental crust underneath the shelf and deep water area of the Orphan Basin. On the landward side of the basin, gravity modeling indicates a zone of very thin (6-8 km) crust, possibly a failed rift center formed as a result of the northward progression of nonvolcanic rifting between Canada and Europe. A possible sub-moho reflector suggests generally uniform stretching of the crust and upper mantle, and the absence of volcanics and underplating is in contrasto the observations on related rift basins of the Grand Banks. 1. Introduction The rifted continental margin northeast of Newfoundland is characterized by a wide (400 km) zone of thinned and extended continental crust, known as Orphan Basin (Figure 1). A wide-angle seismic profile was shot here, in order to determine the crustal structure across the basin and to investigate the processes involved in its formation. The experiment was designed to constrain the two-dimensional (2- D) crustal velocity structure beneath a preexisting deep multichannel reflection (MCS) line [Keen et al., 1987]. The primary specific objective was to resolve any major lateral variation in the crustal structure and thickness, which might have significant implications for the processes of rifting. In particular, earlier work had suggested the existence of a high- 'Now at Geological Institute, University of Copenhagen, Copenhagen, Denmark. Copyright 2001 by the American Geophysical Union. Paper number 2000JB /0227/01/2000JB ,923 velocity lower crustal layer, indicative of magmatic underplating [Keen and Barrett, 1981] and a characteristic feature of nearby volcanic margins [Holbrook and Keleman, 1993; Korenga et al., 2000], and we hoped to be able to delimit any such layer. The Orphan Basin is the site of the largest gravity anomaly between the Scotian margin and the Labrador Sea [Keen and Dehler, 1997], and the source for this anomaly was sought, as well as an indication of the extent to which the rugged basementopogaphy is reflected in Moho structure, with corresponding implications for the distribution of crustal extension across the basin. It was also hoped that the velocity structure would provide new geological constraints on the structure and evolution of the basin. For example, the seismic velocity of petroleum basement should indicate whether this is true crystalline basement or metasedimentary in nature, and the thickness of any such basement layer could be determined. This paper is concerned primarily with the continental crust beneath the basin itself; a full discussion of data acquired Farther seaward across the ocean-continentransition will be presented in a future paper. To the north, Orphan Basin is bounded by the Charlie Gibbs Fracture Zone and its landward extension, the Dover

2 10,924 CHIAN ET AL.' CRUSTAL STRUCTURE BENEATH ORPHAN BASIN 55 ø 50 ø 45 ø, ",,, 3-4,,,,,, ' 't... Refraction Lines } '" ' :... ":'"'""'"' ::'::*'... 2'" 50 ø 70 ß, ::.,.,.[::::..:,.,,, :... :... ::½.:::::. ::.::..½::... '".'.....:;..::5::::..:::- ::. "::.r:'.[.::,::-:.}.-..' :;..:.[... : 48 ø ß ::.:.....::,.:-'..:% :,.,,..,:,:.::......:.::::..:... ::::::::::::::::::::::::::: :;.::.... :'::::::::::?.:...: '., :,: '...:....A ',,... ::: :.: :.,. ::: <' Grand Banks. ':... :::.:.:.c..'..'-....',* ' ' :-.: *.::-.. : Figure 1. Detailed map of study area with an inset map of the North Atlantic in loweright. Shown on the inset are the nonvolcanic margins referred to in the text: FC, Flemish Cap; GAL, Galicia; GB, Grand Banks; GS, Goban Spur; IB, Iberia Abyssal Plain; LAB, Labrador, SWG; Southwest Greenland; FZ, fracture zone; OB, Orphan Basin; OK, Orphan Knoll; PB, Porcupine Bank. Also labeled are Newfoundland (NFLD) and magnetic anomalies M0 and 34. On the detailed map the contours (in m) show the depth to basement. Dashed lines are MCS reflections; solid lines are wideangle refractions. Line 84-3 is the MCS line coincident with refraction lines 86-6 and 86-8, altogether called the "main profile." Refraction line 77-1 is described by Keen and Barrett [1981]. Dotted lines are oldest magnetic anomalies. Stars are OBS. Open circles are deep wells. Avalon, Gander, and Dunnage are preexisting continental terranes [e.g., Marillier et al., 1984]. Fault [Haworth, 1977]. To the south lie the continental shelf areas of the Grand Banks and Flemish Cap, dissected by elongate rift basins. To the west are the continental terranes of the Appalachian Orogen, with tectonostratigraphic boundaries that place most of the Orphan Basin within the Avalon Zone [Keen et al., 1986; Marillier et al., 1994]. At the eastern edge of the basin lies the presumed continental fragment of Orphan Knoll, and beyond this is the continent-ocean transition [Keen et al., 1987] ( km wide up to Chron 34N). Final continental rifting in Orphan Basin occurred at about 105 Ma or later and was associated with a prolonged period of rifting between North America and Europe [Srivastava and Tapscott, 1986]. Seafloor spreading started seaward of Orphan Knoll at anomaly 34N (84 Ma, Santonian), or earlier at a Kanomaly ( Ma; a magnetic high identified only between CharlieGibbs Fracture Zone and Flemish Cap) [Srivastava et al., 1988a, 1998b]. Farther landward, the magnetic field over Orphan Basin shows low-amplitude anomalies that can be correlated with distinctive highs in the basement layer as seen on seismic reflection profiles, which we denote as petroleum basement. The term petroleum basement in this paper (Figure 2) includes Carboniferous strata and underlying Paleozoic formations [Parson et al., 1985]. It is the base horizon in the depth to basement maps created from seismic reflection data. We use this term to distinguish it from crystalline crusthat occurs at a deeper level. Recent studies of the rifted continental margins of eastern Canada have shown that they are primarily nonvolcanic or amagmatic margins [e.g., Louden and Chian, 1999]. These show no evidence of the seaward dipping reflector sequences or high-velocity lower crustal layers that are characteristic of large-scalextrusive and intrusive magmatism at volcanic rifted margins [e.g., Holbrook and Keleman, 1993]. Because of the absence of thick igneous crustal layers they are less elevated than volcanic margins [Ruppel, 1995]. In the case of amagmatic margins, there is limited melting during rifting and initial spreading, so that little or no igneous oceanicrust is present within an ocean-continent transition zone, which can be up to 200 km wide [Louden and Chian, 1999]. Extensive drilling and MCS/refraction studies suggesthat this is the case for the Iberian margin [Chian et al., 1999; Dean et al., 2000], and its western conjugate, the Newfoundland Basin east of the Grand Banks, also appears to be largely amagmatic

3 CHIAN ET AL.' CRUSTAL STRUCTURE BENEATH ORPHAN BASIN 10,925 Blue Well and Blue wells in the western part of the section. Basement highs and lows are a distinctive feature of the seismic profile.... No Samples In some cases the highs show internal reflectors [Grant and McAlpine, 1990]. The highly dissected petroleum basement Conglomerate drops abruptly beyond the continental shelf and then rises gradually seaward toward Orphan Knoll. The topography is generally interpreted as being due to high-angle normal faulting associated with crustal extension [Keen et al., 1987], :. x, Claystone 4 but recurring vertical displacement (i.e. inversion tectonics), without relation to extension, has also been suggested [Grant, 1987]. Beyond Orphan Knoll, near the seaward end of the profile, a zone of landward dipping reflectors was mapped Their limited lateral extent, as well as opposite dip direction, Claysto.he contrasts with the seaward dipping reflectors typical of volcanic margins, but they are comparable to reflectors seen on the nonvolcanic rifted margin of the Grand Banks [Keen and De Voogd, 1988]. Within the continental crust a Limestone subhorizontal decollement zone at midcrustal depths was interpreted [Keen et al., 1987]. However, primary seismic reflections at these depths are severely obscured by water lo Sandstone layer multiples, and we will show that there is no evidence for Figure 2. Comparison of stratigraphic column of the Blue well and the new velocity model. Multichannel channel seismic line drawing is overlain on the model. Velocity labels such a decollement zone in our refraction data. Three wells [Laughton et al., 1972; Koning et al., 1988] that have lithostratigraphic, biostratigraphic, and sonic logs are available to constrain rock types, ages, and velocities for are in km/s. Lowercase letters (a-e) are the interfaces used in our wide-angle modeling along the MCS line 84-3 (Figure 1). the wide-angle reflection/refraction modeling. C33N and The Bonavista C-99 well, landward of the profile, was drilled C34N are magnetic chrons. Triangles indicate OBS positions. Petroleum basement is the Paleozoic formation at the base of in 329 m water depth and reached a total depth of 3779 m. the well and the horizon used for basement in the sedimentary The oldest rocks that it penetrated are Coniacian to Santonian thickness map. in age (83-88 Ma). The Blue H-28 well situated in 1486 m of water reached a total depth of 6103 m and provides significant control for interpreting the seismic reflection data. [Todd and Reid, 1989; Reid, 1994]. The eastern margin of Flemish Cap, just south of Orphan Basin, shows no indication of seaward dipping reflectors associated with a volcanic margin [Keen and de Voogd, 1988], and the results of Reid Four principal lithological and biostratigraphic units were encountered (Figure 2). The Tertiary section is 3500 m thick. The Eocene/Paleocene boundary is associated with a change in velocity, but the transition to the Cretaceous is not marked and Keen [1990a] are consistent with faulted and by a significant difference in velocities. The boundary serpentinized mantle at a nonvolcanic margin. The conjugate between the Cretaceous and the Paleozoic is a major margin off Goban Spur is also nonvolcanic [Horsefield et al., discontinuity in both stratigraphy and velocity. The Paleozoic 1993]. To the north, the conjugate margins of Labrador unit consists of 720 m of limestones, sandstones and shales, [Chian et al., 1995a, 1995b] and southwest Greenland [Chian and Louden, 1994] appear to be amagmatic. deposited in a shallow marine environment and dated as Mississippian and corresponding to petroleum basement. The deep MCS profile indicates another reflector beneath this 2. Existing Data layer, that we interpret as early Paleozoic sedimentary rocks. The Deep Sea Drilling Project (DSDP) Site 111 drilled The area offshore Newfoundland has been extensively Orphan Knoll (Figure 1). This site (-40 km off our seismic surveyed by geophysical methods, including drilling, seismic profiling, and a dense grid of gravity/magnetic surveys. Figure 1 shows a depth-to-basement map (petroleum basement) of line) was drilled to a depth of 250 m below the seabed. Coarse sandstone and shales were cored at the base of the well [Laughton et al., 1972]. They were determined to be the region around Orphan Basin, compiled using seismic Jurassic in age, with nearby source beds of Late Paleozoic reflection data, primarily from hydrocarbon exploration, with control information from wells [Tucholke and Fry, 1985; Oakey and Stark, 1995]. It clearly shows that Orphan Basin age. None of the three wells penetrated volcanics. The data from the wells demonstrate that the basin is underlain by rocks older than the age of seafloor spreading, consistent with is characterized by numerous north to northeastrending its being foundered continental crust. linear highs and lows in acoustic or petroleum basement. The rugged topography associated with the basin stops abruptly to the east of Orphan Knoll, at the continent-ocean transition. Most pertinento this study, the deep MCS line 84-3 [Keen et al., 1987] extends from the unstretched crust of the continental shelf off Newfoundland, across Orphan Basin, An earlier seismic refraction profile across Orphan Basin [Keen and Barrett, 1981] showed continental crust with a thickness of km (Figure 1). Two crustal layers with velocities of 6.1 and 7.4 km/s were interpreted. The latter velocity is consistent with magmatic underplating, and the evidence for it will be reviewed in section 4. Deep reflection and almosto the first seafloor spreading magnetic anomaly line 84-2, intersects the landward end of line 84-3 [Keen et (Chron 34N). Sedimentary units were identified on the basis al., 1986]. This line also connects refraction lines 91-1 of their seismic character and correlated using the Bonavista [Chian et al., 1998] and 91-2 [Marillier et al., 1994], which

4 10,926 CHIAN ET AL.: CRUSTAL STRUCTURE BENEATH ORPHAN BASIN were modeled using three crustal layers with average was by a combination of satellite navigation, Loran-C, and velocities of 6.3, 6.5, and -6.8 km/s, each with low gradients. dead reckoning. Absolute positions are estimated to be The crustal thickness was measured to be 40 km along line 91-2, while only -32 km along line Seismic velocities of <7.0 km/s are typical of the entire Newfoundland accurate within 100 m, with relative error <10 m between nearby shots. An additional time shift of up to 50 ms was required on some OBS, possibly due to uncertainties tape continental crust. skew and clock calibration. Final adjustment of OBS positions was made using water wave arrivals. For modeling, 3. Data Acquisition and Processing In 1986, a 400-km-long wide-angle seismic reflection/ refraction profile was shot in two segments (86-6 and 86-8) along an existing multichannel seismic (MCS) reflection line 84-3 (Figure 2) [Keen et al., 1987] and across the continental margin northeast of Newfoundland. The wide-angle profile was shorter than planned; owing to loss of ocean bottom seismometers, instruments were not deployed along the landward end of the reflection profile. The seismic source was OBS positions were projected onto MCS line 84/3. A bandpass filter of 3-10 Hz was applied to the data, together with, where necessary, a mix over approximately seven traces along a coherency-defined optimal velocity [Chian and Louden, 1992]. Arrival times were picked from the hydrophone and (occasionally) geophone channels. Unfiltered data were sometimes used for picking water waves and shallow sedimentary refractions, where substantial higher-frequency energy is present. Coherency mixing was important for picking weak phases, particularly from lower crust and an array of 6 similar 16.4 L air guns (6x1000 in3), deployed 20 m below sea surface and fired at intervals of either 60 s or 90 s to produce a shot spacing of m. This nontuned air gun array produces a signal that is reverberatory, but adequate in energy and frequency content for a wide-angle crustal study such as described here. The ocean bottom seismometer (OBS) instrumentation is described by Heffier and Barrett [1979] and Heffier [1984]. Analogue seismic signals from a hydrophone and 4.5-Hz vertical and horizontal geophones were recorded on cassette tapes, which were later replayed and digitised in the laboratory to a fixed sampling rate of 100 samples per second, with timing information corrected for clock calibrations and tape skew. Positioning mantle. In this paper, we model the wide-angle data from 15 OBS and shots along line 86-6/8 across Orphan Basin and along two short cross profiles near the Orphan Knoll (lines 3 and 4). The OBS positions are numbered from 1 to 15 followed by the OBS instrument label (e.g., 2H). Coincident MCS data, well data (Bonavista C-99, and Blue H-28; Figure2), and the new wide-angle data were used to determine the layering of the thick sedimentary section, which includes a layer of early Paleozoic deposits with severe (up to 5 km) relief. To constrain the crustal structure of the preexisting continent, we use MCS line 84-2 [Keen et al., 1987], together with N-S Model: LINE 3 SE N-$ Model: LINE 4 NW Center of O han Knoll n I. Tie Figure 3. Record section of (a) strike line 3 and (c) strike line 4 in Orphan Knoll region. Data were averaged in a mixing window of 0.7 km (or approximately 7 traces) at an optimal slope. Note that insets are reduced at a different velocity to emphasize the Pn phase. (b) and (d) Modeling results. Lines 3 and 4 are tied at their southern end.

5 CHIAN ET AL.: CRUSTAL STRUCTURE BENEATH ORPHAN BAS1N 10,927 refraction lines 91-1 [Chian et al., 1998] and 91-2 [Marillier et al., 1994] (Figure 1). The MCS data discussed here provide coverage of the complete extended continental crust, but there is a gap in the wide-angle data (Figure 1). The most seaward part of line 86-6 and cross-profiles 1 and 2 are within the transition to oceanic crust and will be presented elsewhere. 4. Modeling The two-dimensional ray-tracing algorithm of Zelt and Smith [1992] was used for modeling the wide-angle data. Two-way travel times of sedimentary and petroleum basement reflectors from coincident MCS reflection data along the main profile, and velocity from well logs, provided additional constraints on the model. Since the main refraction line is shorter than the coincident MCS line, the leftmost point of the model is set at -220 km in order to place the first OBS at 0 km. Model points are projected to follow the MCS line which is slightly curved (Figure 1). The model is constructed with 11 boundaries, each of which contains up to 138 boundary nodes to cover a distance of 560 km. These boundaries form 10 layers, each of which is specified by up to 35 velocity nodes for the top and bottom of the layer. Model parameters are linearly interpolated between adjacent nodes during raytracing and display. Distances in the velocity model follow the navigation of the MCS line. All OBSs were deployed along the MCS line, except OBS 6, which is slightly offline and therefore projected to the nearest location on the model. We use P2, P3, Pn to label refractions from petroleum basement (i.e., Paleozoic metasediment and upper crust), lower crust, and upper mantle. PmP represents wide-angle reflections from the Moho, and Pm2P a submantle reflector. Refractions from sedimentary layers are denoted by Pa-e. To simplify labeling in figures, we will omit the letter P in most cases. We define layer b (for example) as lying between boundaries b and c, and similarly for other layers. Error analyses are per/brmed by perturbing model parameters until the computed arrival times no longer provide an adequate fit to the data on one or more OBS sections. In general, the error bounds from several OBS are smaller than or equal to that for a single OBS section. For example, the velocity uncertainty in layer 2 is sometimes found to be 0.15 km/s for a single OBS, but when several OBS are considered, it becomes 0.10 km/s. Amplitude modeling was not deemed appropriate here because of its sensitivity to complexities (topographical and structural, 2-D or 3-D) which are severe in the Orphan basin. However, 2-D travel time modeling is still a valid approximation since out-of-plane 3-D effects are relatively small [Zelt and Zelt, 1998] N-S Strike Lines 3 and 4 We first discuss the two refraction sections along strike of the margin, which provide important constraints for modeling the complicated Orphan Knoll region along the primary line. These two cross profiles (Figure 1) were designed to provide increased structural resolution across Orphan Knoll (OK), which is a bathymetric high interpreted as a continental fragment. Modeling of these profiles is constrained by the information simultaneously recorded on single channel reflection profiles (not shown) and the depth-to-basement map (Figure 1). Modeling of arrivals from basement shows, as expected from the bathymetric variation, significant lateral structural change from north to south. The northern half of line 4 (Figures 3c and 3d) exhibits a very early P3 arrival, modeled by a significantly shallower lower crust beneath the center of OK. Here, the velocity steadily increases from km/s near seafloor to 6.4 km/s at the Moho, while the depth of the Moho is undefined from our seismic data. However, south of OK along the line, a well defined PmP phase puts the Moho at 19 km, on top of which an additional km/s layer generates a clear P3 phase. A concave P2 phase is also clearly observed, modeled by a thick layer of km/s velocity. The upper mantle velocity is modeled as 8.0_+0.15 km/s on the basis of a clear Pn phase (see inset of Figure 3c). Line 3 lies seaward of OK. Its southern end ties with line 4. Similar to line 4, velocity increasing from--3.7 km/s downward to 6.4 km/s at the Moho is observed. This rather low crustal velocity is required to model a strong P2 phase, bundled with a possible PmP. A clear Pn (see inset of Figure 3a) indicates an upper mantle velocity of 8.0_+0.15 km/s. There is a possible sub-moho reflected phase that would imply a reflector m2 at --24 km Sedimentary Layers The MCS data and upper part of the velocity model along the main profile are shown in Figure 4. The (<11 km thick) sedimentary column is divided into five layers, (boundaries a- e from seafloor down in Figures 2 and 4). This division is sufficient to model the wide-angle data, although more layers can be distinguished on the MCS record. All OBS recorded sedimentary phases (see Figures 5-11). Layer "a" includes the shallowest sediment, with a velocity of km/s indicated from the Blue and Bonavista sonic logs. Boundary "c" matches the top of the Late Eocene sedimentary strata [Keen et al., 1987] and can be continuously traced and correlated with the Bonavista and Blue wells. Boundary "d" follows a relatively strong and continuous reflector, not related to a major stratigraphic boundary but introduced for convenience of modeling. Near the bottom of the Blue well, layer "d" includes -500 m of Paleocene sandstones overlain by an upper/lower Cretaceous unconformity (Figure 2). The lowest boundary e in the sedimentary sequence follows a lower Cretaceous/Paleozoic unconformity [Keen et al., 1987] landward for 30 km. Seaward of this point, the Cretaceous sedimentary layer becomes very thin (<500 m) and pinches out (probably at 70 km), and beyond this point we set "e" to follow the top of the Paleozoic sedimentary sequence deposited before the start of rifting. This is the pre Mesozoic basement layer as defined in hydrocarbon exploration [e.g., Koning et al., 1988]. The main crustal layer, however, is below this and was not drilled by the Blue well (Figure 2). Clear refractions from sedimentary layers are observed on nearly all OBS (Figure 5). Phases Pb and Pc are linked by their similar velocities. Pe is observed as a separate phase when the layer e is relatively thick at and near the OBS (e.g., Figure 6), or is connected to Pd when there is little velocity contrast at boundary e (e.g., Figure 8). Perturbation of sedimentary velocities in the model suggests a maximum error of 0.1 km/s for layers b-d and 0.2 km/s for layer e. The modeled sedimentary velocities are consistent with the previous velocity analyses from coincident MCS data and with the average sonic log velocities from the two deep wells (Blue and Bonavista).

6 10,928 CHIAN ET AL.: CRUSTAL STRUCTURE BENEATH ORPHAN BASIN 2H 5B 6D 7R 8P.h Orphan 'F-- Knoll B lo BLUE H-28 DSDP 111 Old Ref.mction Line 77-i LINE 4 LINE 3 lo Distance (kin) Figure 4. (a) Migrated MCS reflection section coherency mixed and converted to depth using the velocity model derived from the wide-angle modeling in Figure 4b. Triangles at the seafloor indicate OBS positions. (b) Sediment/basement velocity model that fits all data (MCS, wide-angle, and sonic logs). Velocities are contoured at 0.1 km/s interval except at layer boundaries (thick lines). A label on a contour shows the velocity value for that contour, while label not on a contour indicates velocity value for that model point. L, Paleocene; UC, Upper Cretaceous; Z, Paleozoic. Letters a-e along the depth axis refer to sedimentary layers used during wide-angle modeling Acoustic Basement The ridge-basin structure synonymous with petroleum basement [Koning et al., 1988], together with another deeper layer, overlies a layer modeled with a typical crustal velocity and gradient. The ridges are defined on the MCS data by a strong reflector with high relief (up to 5 km offset in depth). The relief has a lateral scale of km and gradually shallows seaward until exposed at the seafloor at km distance (Figure 4). Velocities in the ridges range from 5.2 to 5.9 km/s and have a high vertical gradient, best defined by those OBS lying on top of ridges (e.g., OBS 4 Figure 9). Rays diving through the flanks of the ridges generate a clear P2, and result in well constrained (+_0.1 km/s) velocity estimates. Away from the ridges, the arrival pattern is less regular, making it more difficult to model the phase, so that the velocity uncertainty here may be up to +_0.2 km/s. Velocity at the top of the ridges is consistently in the range km/s at distances <250 km, with a high vertical -1 gradient of 0.1 s. Such velocity and gradient observed are not typical of crystalline continental crust (which should have velocities of >5.7 km/s) but are consistent with older Paleozoic metasedimentary or carbonate rocks. Although it is possible for fractured and faulted crustal rocks to have velocities in this range, regional geology indicates the existence of a Paleozoic section, and internal reflectors are consistent with a sedimentary section (Figure 5). Deformed lower Paleozoic metasedimentary rocks are present on the Grand Banks and northeast Newfoundland shelf, where they are overlain locally by upper Paleozoic units [Bell and Howie, 1990] similar in age to the rocks at the base of the Blue well. The exact boundary between metasedimentary strata and crystalline crust is not associated with a large velocity contrast and therefore not well determined from our data. Modeling suggests, however, that it is best placed near the roots of the ridges (dotted line in Figure 4b), coinciding with velocity contours km/s. This layer division is consistent with that used landward on line 91-2 (Figure 1) [Marillier et al., 1994]. There is little if any velocity discontinuity across this

7 CHIAN ET AL.' CRUSTAL STRUCTURE BENEATH ORPHAN BASIN 10,929 boundary. The shallowest ridge, at--175 km, is clearly imaged near the seafloor in the MCS data, and its velocity of 5.5 km/s -1 and gradient of 0.1 s are well constrained. Farther seaward, the MCS and wide-angle data on the main profile show rather complex structure beneath Orphan Knoll Crystalline Crust and Moho The metasediment-crust boundary is not drilled by the Blue well but is interpreted to follow contours km/s. Refraction phases from these two layers (L2a and L2b; Figures 6 and 7) are connected and labeled as P2a and P2b, OBS 2' Hydrophone 4 lo o 1 o Figure 5. (a) Observed wide-angle data from the filtered hydrophone channel of OBS 2. Phases b-d are refracted P waves from layers b-d, respectively. Phases 2 and 3 are refracted P waves from layers 2 and 3. The b and c are connected due to similarity of velocity across layers b and c. (b) Migrated depth section of MCS profile around the Blue well, overlain with ray paths for OBS 2. Eocene strata are in layer c, mid-cretaceous in layer d, Upper Paleozoic rocks (petroleum basement)[koning et al., 1988] in layer e, and META (Lower Paleozoic metasedimentary) rocks in layer 2. MID-CRET UNCON, mid-cretaceous unconformity. Dipping reflectors in Upper Paleozoic petroleum basement are visible. 8O

8 10,930 CHIAN ET AL.' CRUSTAL STRUCTURE BENEATH ORPHAN BASIN OBS 5 o BLUE Old Refraction ' lo n '0...'... ' Figure 6. (a) Record section of OBS 5, overlain with computed travel time curves. (b) The final velocity model overlain with computed ray path. Computed rays for the water and sediment layers are removed for clarity: a-e, sedimentary arrivals; 2a and 2b, refractions from layer L2; 3, refraction from layer L3; m, calculated (but not observed) PmP phase from the Moho; m2, a wide-angle phase from a sub-moho reflector at 31 km depth. respectively. A distinct lower crust is seen on the OBS record sections, producing wide-angle reflections and refractions that are both labeled as P3. The wide-angle reflections from the top of the lower crust are not easily separable from the refracted phase (e.g., Figure 6). In some cases, P3 is observed to merge with P2 (e.g., Figures 9 and 10), implying a less strong midcrustal velocity discontinuity. Modeling of refraction phases from ridges/acoustic basement in the Orphan Knoll region is possible using results intracrustal reflectors are seen in both the MCS and wideangle data. The reflection Moho is not seen seaward of Orphan Knoll, as on other continent-ocean transitions at nonvolcanic margins in the North Atlantic [Louden and Chian, 1999]. In this transitional area the Moho depth shown in the model of Figures 10 and 11 is consistent with gravity modeling. Modeling of all of the P3 and PmP phases gives a consistent model, in which the lower crust has a velocity of from coincident MCS data and lines 3 and 4. In addition to km/s with an error bound of _+0.2 km/s. This is the phases P2 and P3, a few extra intracrustal wide-angle reflections can be recognized in the Orphan Knoll region (R in OBS 12, Figure 10), consistent with the presence of significantly different from the 1-D model interpreted from an older refraction survey in the same area (at -150 km) by Keen and Barrett [ 1981 ]. Their model included a 5-km-thick lower abundant reflectors in the MCS data. At 330 km distance, crustal layer with a high velocity of 7.35 km/s, interpreted as MCS data show a shallowing intracrustal reflector, modeled by P3 of OBS 15 (Figure 11). Moho depth is constrained by the Prop phase observed on OBS 1-4, OBS 6-13, and line 4. We estimate an error bound of _+1 km for the Moho depth in the profile range 0-95 and km. The error estimates are degraded to _+2 km at km due to the lack of clear PmP on OBS 5 and OBS 6, and at the seaward end of Orphan Knoll due to increased structural complexity. The Moho shallows rapidly near the seaward end of Orphan Knoll. Several groups of strong underplated melt. Only one OBS on this older line (at-150 km on our model; Figure 9) showed evidence for this underplated layer. To explore this discrepancy, we have reinterpreted Keen and Barrett's [1981] data, using the method of Zelt and Smith [ 1992]. The depth to basement map in Figure 1 is used to construct a new 2-D model. We find that a km/s layer can indeed fit the modeling of a strong secondary phase (Figure 12a). However, ray tracing of the same model without a km/s layer such as in Figure 12d can also fit the observed data (Figure 12c), where the

9 CHIAN ET AL.' CRUSTAL STRUCTURE BENEATH ORPHAN BASIN 10,931 same phase is treated as a wide-angle reflection from an m2 reflector at 26 km depth. Unfortunately, only five to eight traces recorded this relatively strong secondary phase in this older data set, and modeling is also complicated by the probable strong lateral variation of structure. In view of the more detailed resolution of the present study, with 11 OBS recording the phase P3 with a velocity of km/s, we conclude that it is more probable that the Orphan Basin area does not have a high-velocity lower crustalayer that can be attributed to magmatic underplating. Throughout the main line, the Moho derived from our refraction data is -8 km shallower than the interpretation from coincident MCS reflection data by Keen et al. [1987]. However, strong multiples and the structural complexity make interpretation of the reflection Moho uncertain, in contrast to the good control provided by the much clearer wide-angle reflections from the Moho in our data Upper Mantle The refraction phase from the upper mantle, Pn, is modeled along strike lines 3 and 4 to be 8.0_+0.15 km/s. Along the main line, Pn is observed only at offsets of km from OBS 8 (Figure 7) and is modeled by a mantle layer with 7.9_+0.2 km/s velocity. Our data show evidence for the presence of a reflector, m2, km below the Moho. A wide-angle phase Pm2P, modeled as a reflection from m2, appears as a secondary but continuous arrival, generally >l s later than P3 (e.g., OBS 6) and Prop (e.g., OBS 12). The apparent velocity of Pm2P is greater than ProP, and it is therefore distinguishable from multiples of Prop (e.g., OBS 5 and 12). It is a later arrival than Pn, and we cannot model it as a Pn phase on any OBS. Line 3 (Figure 3) records both Pn and Pm2P, where the latter is affected by reverberations of P3 but is still recognisable. A Pm2P phase is seen on OBS 1, 4-6, and 12 and possibly on OBS 7 and 9. Two possible interpretations for these Pm2P events are a high-velocity crustal layer as modeled by Keen and Barrett [1981], or a sub-moho event (Figure 13). However, the phase P3, observed on 11 OBS, determines the lower crustal velocity of km/s with an error of-0.20 Moho event could be due to a thin high-velocity layer or to a velocity discontinuity in the mantle [Chian et al., 1999]. Our data do not constrain the velocity below the m2 reflector to discriminate between these possibilities. However, this Pm2P phase is similar to a sub-moho phase observed on the nearby shelf (lines 91-1a and 91-lb) [Chian et al., 1998] as well as on land beneath Newfoundland (line 91-3) [Hughes et al., UU- O85 8 Old Refraction E 10 ( Figure 7. (a) Record section of OBS 8, overlain with computed travel time curves. (b) The final velocity model overlain with computed ray path. Computed rays for the water and sediment layers are removed for clarity. Prop phase is clearly observed. (b) 200

10 10,932 CHIAN ET AL.' CRUSTAL STRUCTURE BENEATH ORPHAN BASIN OBS 1 rn rn2 6 - m? \ OBS 7 ', : m 6 v 3! o -: BLUE Old Refraction! ' 5 lo L3 Moho :. _ i i i i i ' I '1 i "'1... i i i i 1' Figure 8. (top) Travel time picks of OBS I and 7 overlain with computed travel time curves. Ticks of the time axis are 1 s apart. Vertical size of each pick indicates the uncertainty in time. (bottom) Final velocity model, overlain with computed ray path. Velocities are in km/s. Velocity uncertainties are estimated for individual layers, and depth uncertainties are for layer boundaries. 1994], where it is modeled as a reflector km below the Moho. In addition to this strong reflector a few shallower and less continuous sub-moho reflectors are also observed in the Avalon terrain [Chian et al., 1998]. A sub-moho refraction was the principal result of a 600-km-long wide-angle refraction experiment across the Maritime Appalachians [Dehler et al., 1996]. It was modeled by a stepwise velocity increase from 8.1 to 8.5 km/s. Our reflector m2 is similar, but at a depth of km, in contrast with km under the Newfoundland and Maritime Appalachians. The shallower depth here is consistent with the position of our line on the highly stretched continental margin. Our data are consistent with the observation that a sub-moho layer may be a typical feature under the northern Appalachians and Caledonides of northern Britain [Barton, 1992]. velocity nodes of the velocity model Each block is assumed to have a constant density, calculated from its average velocity (v) by linear interpolation between control points given in Table 1. The control points in Table 1 for the sedimentary section are obtained from sonic logs of the deep well Blue H-28, while the densities p for crust are obtained from the empirica! relationship: 4p= v v v' _1 lv [Ludwig et al., 1970]. A maximum density of 3.30 x10 : kg/m j is assigned to the upper mantle. Table 1. Velocity-Density Relationship Used for Constructing Density Model Velocity, km/s 3 't Density, 10 kg/m 5. Gravity Modeling Gravity modeling (Figure 13) provides a valuable check on the seismic results. With the MCS data, it also allows the model to be extended landward of the Orphan Basin and beyond the present wide-angle survey. The gravity data are extracted from a dense regional data grid [Oakey and Stark, 1995]. To model the gravity data, the final velocity model is divided into small blocks, each bordered by boundary or

11 ß CHIAN ET AL.' CRUSTAL STRUCTURE BENEATH ORPHAN BASIN 10,933 In the area controlled by the velocity modeling of seismic data (i.e., km) the observed and computed gravity are in reasonable agreement without adjustment of the model. Several short-wavelength (-50 km) gravity highs closely match the basement highs, suggesting that the gravity anomaly is correlated with the relief in this area. However, farther landward under the shelf of the Orphan Basin, no wide-angle data exist between -120 km and -10 kin, where the thickest (up to 11 km) sedimentary section exists. OBS 1 and 2 provide the first constraints on the shape of Moho from the km range, where it deepens from about 18 to 23 kin. In the region with no wide-angle coverage the MCS data show high-amplitude reflectors succeeded at depth by a zone without reflectors that we interpret as the crust-mantle transition. A general observation from deep crustal reflection profiles is that the crust is more reflective than the mantle for seismic frequencies between l0 and 80 Hz [Cook, 2000]. Therefore in this region we rely on gravity and MCS data to control the Moho depth, assuming no major change in the nature of the crust. The gravity anomaly increases significantly in this area, around -120 to -20 kin, indicating a significantly shallowed Moho at-16 km depth. The modeled Moho is consistent with a group of abundant and discontinuous reflectors, under which no reflectors can be observed. Alternatively, the gravity could be modeled by a denser lower crustal layer, but this is not consistent with existing seismic data and would conflict with our interpretation that no magmatic underplating exists in the region. The gravity model is consistent with the MCS profile (Figure 4) that shows a thick pile of postrift sedimentary section without a prograding pattern. The gravity modeling has also been used in determining the Moho depth seaward of the Orphan Knoll, where the Moho is not convincingly determined from refraction data. 6. Discussion We present a 560x40 km velocity model (Figure 13) showing the detailed seismic properties of the stretched continental crust and upper mantle across the Orphan Basin. Data used for this result include coincident deep MCS reflection (84-2 and 84-3) and wide-angle OBS (86-3, 86-4, 86-6/8, and 8-7-3) profiles, gravity, and well (Blue, Bonavista, and DSDP 111) information. At the base of the Blue well, an Upper Paleozoic section 700 m thick identified as petroleum basement was drilled. This layer is clearly distinguished from a deeper reflector observed across the entire margin by both the MCS and wide-.., m2 OBS rn BLUE Old Refracti()n 5" L3 Moho 25-8,0 30- m Figure 9. (top) Travel time picks of OBS 4 and 6. S, phase for an S wave. See caption of Figure. 8 for other explanations.

12 10,934 CHIAN ET AL.: CRUSTAL STRUCTURE BENEATH ORPHAN BASIN ß OBS 12 3 R 2' " de 25 3O rn Figure 10. (top) Travel time picks of OBS 9 and 12. R, intracrustal wide-angle reflection. See caption of Figure 8 for other explanations. angle data. The subsurface highs on both of these layers were and Mooney, 1995], although the faulted and probable highly previously mapped as basement ridges (Figure 1) [Tucholke eroded nature of the highs does not rule out a crystalline and Fry, 1985; Oakey and Stark, 1995]. After the first major landward high, near the Bonavista well, the layers drop to a composition. Acoustic basement with standard velocities of -6.1 km/s associated with crystalline crust is observed below depth of-11 km. Seaward, the petroleum basement highs the ridges (Figure 13), consistent with the km/s clearly shallow on the depth section (Figure 13); the highs are deeper than 6 km at the Blue well and penetrate the seafloor velocity structure of unstretched Avalon continental crust [Marillier et al., 1994]. Avalon crystalline crust has a highly west of Orphan Knoll, with a seaward merging of sedimentary magneti character, and the ridges are only slightly magnetic, layers. Internal reflections, regional geology, velocity consistent with our interpretation as mostly Paleozoic measurements, and lithostratigraphic log from the Blue well indicate that these highs in the MCS profile are pre-mesozoic rocks, called petroleum basement. Limestone dated as Late Ordovician was recovered from a bedrock pinnacle on Orphan Knoll and carbonates are widely distributed on the continental margin of southern Labrador [Bell and Howie, 1990]. The velocity ( km/s) at the top of the ridges is consistent with a limestone or metasedimentary rock [Christensen, 1982], rather than crystalline continental crust metasedimentary rocks. The complexity of the highs, and their unconformable relationship, suggests that initial faulting and extension of the basin may have begun in the Paleozoic. That the ridges are composed primarily of metasedimentary rocks does not have major implications for the extensional models of Keen and Dehler [1997], beyond a slightly different depth to the Moho. The reduced topography of the crystalline crust, as given by the 6.1-km/s contour (Figures 2 and 13), might suggesthat the degree of block faulting during crustal (which typically has velocities <5.7 km/s) [cf. Christensen extension diminishes with depth. Our work confirms that

13 ß CHIAN ET AL.: CRUSTAL STRUCTURE BENEATH ORPHAN BASIN 10,935 stretched continental crust extends for-410 km, as far as Orphan Knoll (OK). This is evident from the following: (1) the 700 m upper Paleozoic section in sedimentary layer "e" as drilled by the Blue well appears to extend seaward to OK, as indicated by Late Ordovician limestones dredged from a bedrock pinnacle [Legault, 1982] (Figure 4); (2) velocity contours in the basement highs and the crust exhibit consistent lateral continuity and continental velocities as far as OK; (3) the subbasement highs seaward of OK show different patterns compared to that landward of OK, and (4) the m2 reflector that is a feature of the northern Appalachians upper mantle appears to extend across the basin. Keen et al. [1987] also proposed that the continent-ocean transition occurs just seaward of OK. Beneath Orphan Knoll, the lower crustal layer shallows, so that a velocity of 6.5 km/s is reached at 10 km depth. The crust here has a velocity -0.2 km/s higher than that to the landward (Figure 13). On the seaward side of OK is a thick sedimentary sequence, probably Paleozoic, with a velocity (3.9 km/s and up) and vertical gradient similar to those of the Paleozoic sedimentary rocks at the Blue well and at the other metasedimentary troughs. This is consistent with results from the DSDP 111 site (in the center of OK) where eroded, shallow water, late Paleozoic carbonates were dredged [Laughton et al., 1972]. Numerous reflectors exist within this Paleozoic sedimentary section (Figures 4 and 13), indicative of extensive continental stretching before the final continental breakup between NE Newfoundland and its conjugate (Porcupine Bank) off western Europe. Seaward of OK, the basement structure is associated by large amplitude magnetic anomalies (Figure 13). Srivastava et al. [1988a, 1998b] concluded from regional mapping and modeling of magnetic anomalies that the earliest seafloor spreading northeast of Newfoundland occurred at Chron 34N (84 Ma) or possibly earlier at a K anomaly (-105 Ma). Along our main profile, the K anomaly is associated with a basement high (at -330 km), with velocity >6.6 km/s. As with other nonvolcanic margins, such as the Labrador margin, [Chian et al., 1995a, 1995b]; Flemish Cap margin [Todd and Reid, 1989], Grand Banks margin [Reid, 1994], the transition to oceanic crust is not a clear-cut boundary but is a broad area with rather complicated structure, possibly associated with OBS 15 m? ":' ',-q OBS m. / "' 3 LINE 4 LINE 3 o 5 lo E o Figure 11. (top) Travel time picks of OBS 11 and 15. R, intracrustal wide-angle reflection; question mark, uncertain phase; m2', multiple of Pm2P phase. See caption of Figure 8 for other explanations.

14 10,936 CHIAN ET AL.: CRUSTAL STRUCTURE BENEATH ORPHAN BASIN Old Refraction Model New Model E 10 v Figure 12. (a) Datum-corrected record section of OBS 7-8-3, overlain with computed travel time curves from 2-D ray tracing of (b) a high velocity model that is compatible with 1-D modeling of Keen and Barrett [1981]. (c) Same record section as in Figure 12a, overlain with computed travel time curves from 2-D ray tracing of (d) our preferred model. Note that the travel times and distances of each trace was corrected to the seafloor datum and therefore the seafloor in the 2-D models in Figures12c and 12d is at 0 km depth. serpentinized peridotite ridges of the kind sampled [Sawyer et al., 1994; ODP Leg 173 Shipboard Scientific Party, 1998; Whitmarsh et al., 1998] and seismically modeled [Chian et al., 1999] on the Iberia margin. It is also possible that such serpentinized peridotites may be present under the OK in the zone of higher lower crustal velocity seaward of the last basement ridge. The ocean-continent transition (OCT) zone starts t¾om here, extending seaward for-200 km to Chron 34N. The details of such transition seaward of OK, and its controlling seismic data, are rather complex and will be presented in a future paper. An important conclusion of this work is that no highvelocity (>7.2 km/s) layer exists above the mantle northeast of Newfoundland. This is in contrast to the interpretation of Keen and Barrett [ 1981 ], who included in their modeling of a secondary wide-angle phase with a 5-km-thick, 7.35 km/s layer (Figure 12), interpreted as magmatic underplating. The proposed 7.35-km/s layer was based on a limited number of arrivals, whereas our more extensive data and modeling are not compatible with such a layer. Furthermore, a velocity contrast of km/s would produce a strong reflection that is not observed on the coincident MCS and wide-angle data. We cannot, of course, rule out the presence of such a layer where there are no wide-angle data, particularly in the zone of thinnest crust between -120 and -20 km. However, on the underplated margins of east coast of the United States a linear magnetic anomaly parallel to the margin is observed, which is not the case here. In general, the absence of such a layer is consistent with the observations that (1) the deep Blue and Bonavista wells are devoid of any volcanic rocks; (2) the magnetic anomalies along track (Figure 13) closely follow basement highs without any apparent contribution from an additional magnetic source such as magmatic underplating or extrusives; (3) both the Labrador margin to the north [Chian et al., 1995] and the Flemish Cap and Grand Banks margins to the south [Reid and Keen, 1990a, 1990b; Todd and Reid, 1989; Reid, 1994] are not associated with any magmatic underplating; and (4) the water depth is deeper than observed on volcanic margins [Ruppel, 1995]. Therefore our new data strongly suggest a nonvolcanic rifting scenario for northeast Newfoundland. No Canadian margins rifted during the Mesozoic in the North Atlantic show evidence for volcanic rifting [Louden and Chian, 1999], except for the southern Nova Scotia margin, which is the northern limit of seaward dipping reflectors and the East Coast Magnetic Anomaly [Keen and Potter, 1995]. Therefore it is likely that nearly all the Canadian margins were rifted nonvolcanically, despite the volcanic rifting events that were responsible for all the U.S. east coast margins and in southern Greenland [e.g., Holbrook and Keleman, 1993; Korenaga et al., 2000]. This observation is clearly important for geodynamic modeling of the Atlantic area. The most apparent feature of the gravity anomaly map in the study area is a gravity high (up to 93 mga! compared to an average of 0 mgal seaward) forming a -100-km-wide band along the outer part of the shelf. As modeled in section 5, this gravity high is produced by mantle material elevated by -7 km compared to its seaward level, together with a thick wedge of postrift sedimentary rocks. The presence of the gravity high indicates that the sediment wedge is not isostatically compensated but is partially supported by lithospheric rigidity. Subsidence analysis [Keen et al., 1987] at the Bonavista well, which bottomed at a petroleum basement high in this zone, indicates rapid subsidence from at least 130 to 110 Ma. This is consistent with the major pulse of extensional tectonics beginning in mid-jurassic (160 Ma) in the Jeanne d'arc Basin 100 km south of Orphan Basin. If we assume that major extensional events affect large segments of the margin [Tankard and Welsink, 1989], then this suggests a timing for the thinning of the continental crust. This zone may be a failed rifting center generated by continental stretching, beginning in the mid-jurassic and

15 ß CHIAN ET AL.' CRUSTAL STRUCTURE BENEATH ORPHAN BASIN 10,937 ending when the final Canada-Europe rift occurred -200 km reconstructions show Flemish Cap (Figure 1) rotating and eastward of this center in late Cretaceous. Certainly, the thin stretching Orphan Basin in response to the motions of the crust here implies a high degree of local extension. A similar Iberian plate. Orphan Basin also experiences extension due to failed rift center, or zone of highly thinned crust, is not found the separation of the North American and European plates. in the conjugate margin at Porcupine Bank [Masson and The additional rifting forces could induce the asymmetry in Miles, 1986; Srivastavand Verhoef, 1992], according to the the conjugate margins and create the failed rift. The existence refraction data of Makris et al. [1988]. Immediately to the of a failed rift west of the final rift may explain why the south, on the Goban Spur margin, on the basis of seismic nonvolcanic margins of western Europe are sediment-starved reflection and refraction measurements [Peddy et al., 1989; compared to their Canadian counterparts. This failed rift Horsefield et al., 1993] the Moho changes depth gradually center occurred at the edge of the reconstructed rift zone, from km. On this margin, a small positive-negative consistent with the model of Bassi [1995], in which rifting anomaly pair occurs at the onset of thinning, in contrast to the takes place at one side due to strain localization caused by large positive anomaly at the edge of Orphan Basin. Keen and cooling and hardening of the central rift zone. The failure of Dehler [1997] examined the extensional style of rifted this earliest rift center may be related to the mechanism of margins in the North Atlantic by comparingravity anomalies nonvolcanic continental rifting or to the change in the where good seismic reflection control is available. Their direction of opening of the North Atlantic at Chron 34 time model of the Orphan Basin requires more substantial crustal [Srivastava and Tapscott, 1986]. thinning than that required in the Goban Spur transect. In It is interesting to compare our results with a recent study their plate kinematic solutions for the evolution of the of the southeast margin of Greenland [Korenaga et al., 2000]. sedimentary basins in the central north Atlantic, Srivastava This profile is just south of the Iceland plume track and is and Verhoef[1992] make the observation that Orphan Basin therefore representative of a highly volcanic continental doubles its size and question how this was accomplished. The margin. It is immediately clear that there is no deep margin failed rift provides an appropriate mechanism. Their plate basin in this case; instead, continental crust gives way to a ':",; 34N 0, "', ß -200 LINE 4 MCS 84-2 BLUE Old Refraction! LINE BONAVISTA 5; ' 3,0{ 5.3! Failed Crust 3o 40 ' -220 \ :... 7' Vertical Exageration: 8:1 I I I... i I I I I Distance (kin) Figure 13. Final velocity model across the entire stretched continental crust, overlain with MCS line drawing. Observed (crosses) and computed (shaded line) gravity anomalies are shown on top along with observed magnetic profile. OCT, ocean-continent transition zone; Serp, serpentinized peridotite.

16 10,938 CHIAN ET AL.: CRUSTAL STRUCTURE BENEATH ORPHAN BASIN thick igneous sequence, with characteristic seaward dipping reflectors, that gradually thins to become anomalously thick (-10 km) oceanic crust. The structure is relatively continuous, with no evidence of the highly faulted basemen topography and corresponding mantle depth variations that are characteristic of the Orphan Basin. The igneous sequence has a 6-km upper crust with high-velocity gradient km/s in contrast to the more uniform -6.1 crust. We note also that the igneous sequence reaches velocities >7.2 km/s, which are absent beneath Orphan Basin. The deep basin seaward of Orphan Knoll has no counterpart off southeast Greenland, and the thick, shallow (3 km) oceanic crust contrasts sharply with the 5-km-deep basement seaward of Orphan margin. Clearly, the transition from continent to ocean is very different in the two cases, and we interpret the Orphan Basin and margin to be dominated by crustal extension and faulting in a nonvolcanic environment, whereas the dominant effect off southeast Greenland is addition of crustal material by extrusive and intrusive magmatism. The crustal extension in the Orphan Basin is clearly related to the rift basins that intersect the Grand Banks shelf to the south. Its greater extent and complexity could be due to the complexity of stretching forces and focusing of strain into different regions at different times, or to a slower rate of extension at some periods, allowing cooling and strengthening, and tranfer of extension to a new zone [Bassi, 1995]. In this regard, there are some observations from our work that may be of significance. First, if our interpretation of the m2 reflector is correct, and it is indeed correlated with a similar feature beneath the adjacent unstretched continent, then the depths of Moho and m2 imply that the crust and upper mantle have been stretched by approximately equal amounts. This might be expected from the margin reconstruction, which shows a wide zone of extended continental crust and a general similarity of crustal structure on both sides. We present no information regarding the extension of the lower lithosphere, but the structure appears consistent with a generally uniform stretching. We also note that the strong basementopography in the Orphan Basin does not appear to be reflected in the deeper structure, implying that extensional block faulting soles out within the crust and that most of the extension takes place through a pure shear rather than a simple shear mechanism. The crust that we interpret to be a failed rift is only 6-8 km thick and shows no indication of associated magmatism. It therefore confirms that it is possible to obtain a high degree of crustal thinning during rifting, without the formation of significant amounts of melt, as is observed at amagmatic margins. There are synrift volcanics and evidence of underplating in the Grand Banks rift basins, although the degree of extension is less [Reid and Keen, 1990a]. Whether this difference is due to preexisting thermal conditions, or to differences in extension rate or mode of extension, remains to be determined. 7. Conclusions Through combined modeling of wide-angle refraction, coincident MCS reflection, gravity, and well data, we have derived a 550x40 km velocity model (Figure 13) that contains accurate estimation of velocity and depth through the postrift and prerift sedimentary layers, crystalline crust, and uppermost mantle across the entire thinned continental crust of Orphan Basin. 1. The velocity characteristics of the up to 11-km-thick sedimentary sequence are controlled by a combination of MCS profiles, sonic logs, and OBS data. The upper prerift sedimentary section is drilled, containing a sequence of clastic and carbonate strata that are dated from the Cenozoic to Upper Paleozoic. The maximum error associated with the velocities in the Cenozoic to Cretaceous section is 0.1 km/s and the error for the Upper Paleozoic strata is 0.2 km/s. 2. Two prominent reflectors are identified for the prerift sedimentary sequences (previously all known as petroleum basement). The lower reflector is characterized by a few prominent high-relief (up to 5 km) ridges with Vp= km/s at a high gradient of 0.1 s-1. This means that the published depth-to-basement map (Figure l) in Orphan Basin represents the depth of this Paleozoic metasedimentary layer. 3. Crystalline basement lies at the base of these ridges. The continental crust is extended seaward for 410 km, as far as Orphan Knoll (OK). The seismic velocity of the stretched continental crust is km/s, similar to the Avalon crust on land. The ocean-continent transition zone starts from the Orphan IZmoll region seaward, extending for-200 Chron 34N. km to 4. No high-velocity (>7.2 km/s) lower crustal layer exists along our profile, in accordance with the lack of seaward dipping reflectors, volcanic rocks in any deep well, or any high-frequency magnetic anomalies. This is in contrast to the interpretation of Keen and Barrett [1981], whose refraction model includes a 5-km-thick, 7.35 km/s layer interpreted as magmatic underplating. A nonvolcanic rifting scenario applies to this margin, as well as to other Canadian rifted margins north of the southern Nova Scotia to the northern end of the Labrador Sea. 5. A 110-km-wide zone of excessive gravity high along the outer part of the shelf is contributed by elevated Moho depth (17 km, or-8 km shallower than seaward). We propose that this area was a failed rift center, which may explain why the nonvolcanic conjugate margins of western Europe are all sediment-starved compared to their sediment-rich Canadian counterparts. This failed rift center occurred at the edge of the reconstructed rift zone, consistent with the model of Bassi [1995], in which rifting takes place at one side due to strain localisation caused by cooling and hardening of the central rift zone. The failure of this earliest rift center may be related to the mechanism of nonvolcanic continental rifting or to the change in the direction of opening of the North Atlantic at Chron 34N time. 6. A possible sub-moho boundary indicates approximately equal thinning in the crust and in the upper mantle. The thinning appears to have been accomplished primarily by pure shear, at least below the upper crust. A high degree of crustal thinning took place without significant melting, in contrasto the rift basins on the Grand Banks to the south. Acknowledgment. GSC contribution References Barton, P.J., LISPB revisited: A new look under the caledonides of northern Britain, Geophys. J. Int., 110, , Bassi, G., Relative importance of strain-rate and rheology for the mode of continental extension, Geophys. J. Int., 122, , Bell, J.S., and R.D Howie, Paleozoic geology, in Geology of the Continental Margin of Eastern Canada, Geol. Can., no. 2, edited by M.J. Keen and G.L. Williams, p , Geol. Surv. of Can., Ottawa, Ont., 1990.

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18 10,940 CHIAN ET AL.: CRUSTAL STRUCTURE BENEATH ORPHAN BASIN kinematic solution, in Basins qf the Atlantic Seaboard.' Petroleum Geology, Sedimentary Basin Evolution, edited by J. Parnell, Geol. Soc. Spec. Publ., 62, , Srivastava, S.P., J. Verhoef, and R. Macnab, Results from a detailed areomagnetic survey across the northeast Newfoundland margin, part 1, spreading anomalies and relationship between magnetic anomalies and the ocean-continent boundary, Mar. Pet. Geol., 5, , 1988a. Srivastava, S.P., J. Verhoef, and R. Macnab, Results from a detailed aeromagnetic survey across the northeast Newfoundland margin, part 1, Early opening of the North Atlantic between the British Isles and Newfoundland Mar. Pet. Geol., 5, , 1988b. Srivastava, S.P., W.R. Roest, L.C. Kovacs, G. Oakey, S. Levesque, J. Verhoef, and R. Macnab, Motion of Iberia since the Late Jurassic: Results from the detailed measurements in the Newfoundland Basin, Tectonophysics, 184, , Tankard, A.J., and H.J. Welsink, Mesozoic extension and styles of basin formation in Atlantic Canada. in Extensional Tectonics and Stratigraphy of the North Atlantic Margin, edited by A. J. Tankard and H.R. Balkwill, AAPG Mere., 46, , Todd, B.J., and I. Reid, The continent-ocean boundary south of Flemish Cap: constraints from seismic refraction and gravity, Can. J. Earth Sci., 26, , Tucholke, B.E., and V.A. Fry, Basement structure and sediment distribution in the northwest Atlantic, AAPG Bull., 69, , Whitmarsh, R.B., et al., Proceedings of the Ocean Drilling Program, Scientt.'fic. Results, vol. 173, Ocean Drill. Program, College Station, Tex., Zelt, C.A., and R.B. Smith, Seismic travel-time inversion for 2-D crustal velocity structure, Geophys. J. Int., 108, 16-34, Zelt, C. A., and B. C. Zelt, Study of out-of-plane effects in the inversion of refraction/wide-angle reflection travel times, Tectonophysics, 286, , D. Chian, H.R. Jackson, Geological Survey of Canada, (Atlantic), Dartmouth, NS, Canada, B2Y 4A2 (jacksonr@agc.bio.ns.ca) I.D. Reid, Geological Institute, University of Copenhagen, ester Voldgade 10, DK-1350 Copenhagen K, Denmark (Received December 20, 1999; revised November 6, 2000; accepted November 16, 2000.)