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Compressional and Shearwave Velocity Structure of The GEOPHYSICAL RESEARCH LETTERS, VOL. 40, 1–7, doi:10.1002/grl.50511, 2013 Compressional and shear-wave velocity structure of the continent-ocean transition zone at the eastern Grand Banks, Newfoundland Drew R. Eddy,1 Harm J. A. Van Avendonk,1 and Donna J. Shillington2 Received 25 February 2013; revised 25 April 2013; accepted 25 April 2013. [1] The seismic structure of the continent-ocean transition respect to the locus of breakup often results in asymmetry at (COT) at magma-poor rifted margins can explain conjugate magma-poor margins [Manatschal, 2004; Lavier geological processes leading to continental breakup. At the and Manatschal, 2006]. In the footwall of a detachment, small Newfoundland-Iberia rift, compressional seismic velocity amounts of lower crust and, more extensively, uppermost mantle (Vp) is interpreted with multichannel seismic reflections rocks are exhumed to form the COT. At the conjugate rift flank, and drilling results to document continental crustal stretching continental crust of the hanging wall is often highly thinned and and thinning, exhumation of the mantle, and incipient stretched above the upper mantle. Strain localization in weak- seafloor-spreading. However, Vp cannot uniquely constrain ened mantle rocks eventually leads to complete lithospheric COT geology. We present an updated 2-D model for Vp and breakup and a gradual increase in melt supply from the upwell- a new shear-wave velocity model (Vs) for SCREECH Line 2 ing asthenosphere [Lavier and Manatschal, 2006]. on the Newfoundland margin using multichannel seismic [3] Continental rifting may be accompanied by small reflections and coincident ocean-bottom seismometer amounts of magmatism even if the lithospheric mantle is rela- refraction data. In shallow COT basement we find Vp / Vs tively cold or significantly depleted during rifting [Müntener ratios average 1.77, which is normally too high for upper and Manatschal, 2006]. Although rifting of thick continental continental crust and too low for serpentinized mantle. This lithosphere often produces ample magma by decompression observation can be explained by stretching of a maficmiddle melting, only limited amounts of extrusive volcanism may and/or lower continental crust into the COT. We further reach the surface if the extension rate is very low [Lizarralde support the presence of hydrated mantle peridotites at depth et al., 2004]. Syn-rift melts can therefore be trapped below during rifting. Citation: Eddy, D. R., H. J. A. Van Avendonk, and thick COT lithosphere, so it is possible that not all magmatism D. J. Shillington (2013), Compressional and shear-wave velocity is accounted for in an ocean-bottom seismometer (OBS) structure of the continent-ocean transition zone at the eastern Grand refraction study of a rifted margin [Bronner et al., 2011]. A bet- Banks, Newfoundland, Geophys. Res. Lett., 40, doi:10.1002/ ter understanding of the melting history of nominally magma- grl.50511. poor margins is important, because even a small amount of melt introduced in the lithosphere during rifting can alter the style of deformation [Kaczmarek and Müntener, 2010]. 1. Introduction [4] The Newfoundland-Iberia conjugate margins are a prime example of mature magma-poor rift systems [Tucholke et al., [2] Slow rifting at magma-poor margins often occurs with- 2007]. A large wealth of data from drilling expeditions and out the weakening effects of magmatic diking. Continental b > marine geophysical studies here document brittle extension of crust may be therefore stretched by a large factor ( 5), continental crust, exhumation and serpentinization of continen- and rocks from the lower continental crust and mantle may tal mantle, and a slow onset of seafloor-spreading. The distal be brought to shallow depths via detachment faults fi Iberian margin is interpreted as almost entirely exhumed conti- [Whitmarsh et al., 2001]. During the nal phases of rifting, nental mantle with a wide zone of compressional seismic brittle faulting and hydrothermal circulation can lead to velocities (Vp) between 7.0 and 7.5 km/s [Whitmarsh et al., serpentinization of mantle peridotites, weakening the litho- 2001]. This high-velocity zone in the Iberian COT is capped spheric mantle [Manatschal, 2004; Reston and McDermott, by lower velocities (V =4–5 km/s) interpreted as mantle rock 2011]. Before complete breakup of the lithosphere and the p fl that was pervasively serpentinized after exhumation [Dean onset of normal sea oor-spreading, a ~100 km wide zone of et al., 2000]. A similar evolution has been proposed for the continental mantle may thus be exhumed to the surface in conjugate margin in the Newfoundland Basin [Sibuet et al., continent-ocean transition zones (COT) [Whitmarsh et al., 2007], although seismic images from the SCREECH project 2001]. The polarity of lithospheric detachment faults with (Studies of Continental Rifting and Extension on the Eastern Canadian Shelf; Figure 1) suggest that crust in the COT is Additional supporting information may be found in the online version significantly different. Multichannel seismic (MCS) reflections of this article. 1University of Texas Institute for Geophysics, Jackson School of [Shillington et al., 2006] demonstrate that the Newfoundland Geosciences, The University of Texas at Austin, Austin, Texas, USA. Basin lacks the faulted allochthonous crustal blocks that have 2Lamont-Doherty Earth Observatory, Columbia University, Palisades, been imaged off the Iberian margin [Krawczyk et al., 1996]. New York, USA. In addition, Vp in the shallow basement of the Newfoundland Corresponding author: D. R. Eddy, University of Texas Institute for Basin is often lower than 6.0 km/s [Lau et al., 2006; Van Geophysics, Jackson School of Geosciences, The University of Texas at Avendonk et al., 2006], which may indicate unroofed continen- Austin, Austin, TX 78754, USA. ([email protected]) tal crust or exhumed, highly serpentinized mantle peridotites. ©2013. American Geophysical Union. All Rights Reserved. [5] Interpretation of the structure and evolution of the 0094-8276/13/10.1002/grl.50511 COT is complicated by strong seismic reflections from a 1 EDDY ET AL.: SHEAR WAVES IN NEWFOUNDLAND COT layering near the top of crystalline basement that must be included explicitly in the seismic velocity model if we want to correctly model the travel times of later arrivals. We there- fore express travel-time residuals for compressional seismic arrivals dTp,i as a sum of perturbations dVp and perturbations in the depth of layer boundaries dz [Thurber, 1985; Van Avendonk et al., 2004]. If ray paths calculated in the reference model are good estimates of the true source- receiver paths, this relationship can be approximated with a linearization Z À1 X d ; d þ Γ ; d Tp i 2 Vpds i j zj (1) Vp layer j path i [7] The first term of (1) accounts for the contribution to dTp,i of seismic velocity perturbations dVp by integration over the ray path. The coefficient Γi,j represents the partial derivatives of travel-time dTp,i with respect to the depth of velocity boundary zj in the model [Stork and Clayton, 1991]. We use (1) to express the model constraints of wide-angle reflections and refractions in the OBS data of SCREECH Line 2 [Van Avendonk et al., 2006], and to incor- porate two-way travel times from coincident MCS data Figure 1. Bathymetric map of the Grand Banks of [Shillington et al., 2006; Péron-Pinvidic et al., 2010]. [8]Tofind a smooth seismic velocity model we add spa- Newfoundland overlain by 800 m contours, basin features, fi ODP sites 1276 and 1277 (stars), magnetic anomalies M0 tial rst- and second-derivative constraints on the structure and M3 (dashed lines) [Srivastava et al., 2000], and the of Vp and z to the system of equations (1), and solve them location of SCREECH seismic lines (solid lines) and OBS for a model perturbation in a least-squares inversion [Van 17 (inverted red triangle). The segment of SCREECH Line Avendonk et al., 2004]. This linear inversion step is followed 2 analyzed herein is outlined in white. Left lower inset by raytracing in the updated seismic velocity model to calcu- shows Grand Banks in relation to the northeastern seaboard late new ray paths and travel times. We iterate the raytracing of North America. and linearized inversion procedures until we obtain a smooth package of postrift diabase sills just above the Newfound- land basement at SCREECH Line 2 [Shillington et al., 2008; Péron-Pinvidic et al., 2010]. A large injection of mafic material likely accompanied off-axis volcanism fol- lowing the complete breakup of the Newfoundland-Iberia lithosphere and production of oceanic crust by the early Albian [Karner and Shillington, 2005; Tucholke et al., 2007]. We present an integrated analysis of travel-time con- straints from MCS reflection and compressional OBS refrac- tion and reflection data to better characterize the postrift sills and underlying basement on SCREECH Line 2. As a result, we are able to present a new analysis of shear waves that sample the Newfoundland COT basement, and we develop smooth seismic velocity models with regularized tomo- graphic inversions of P- and S-wave travel times. We use both Vp and Vp / Vs ratio models in our geological interpre- tations of the rifted margin, assessing our interpretations by plotting seismic velocities with depth at key SCREECH Line 2 intervals. We also use shear-wave data to test simple bulk Vp / Vs models for the flat COT basement. 2. Travel-Time Tomography [6] The travel times of reflected and refracted phases Figure 2. A portion of the receiver gather for OBS 17. See observed in marine seismic data can be inverted for a layered text for descriptions of compressional refractions and reflec- seismic velocity model with smoothly varying seismic ve- tions (P1–P4, R1–R3). Also labeled are arrivals of the direct locities [Van Avendonk et al., 2004, and references therein]. wave in water (Water), P4 multiples (Multiple), and a shear- Observation of a series of distinct seismic reflections and wave refraction (S).
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