Crustal structure of the southeast Greenland margin from joint refraction and reflection seismic tomography

Abstract

Abstract. We present results from a combined multichannel seismic reflection (MCS) and wideangle onshore/offshore seismic experiment conducted in 1996 across the southeast Greenland continental margin. A new seismic tomographic method is developed to jointly invert refraction and reflection travel times for a two-dimensional velocity structure. We employ a hybrid raytracing scheme based on the graph method and the local ray-bending refinement to efficiently obtain an accurate forward solution, and we employ smoothing and optional damping constraints to regularize an iterative inversion. We invert 2318 Pg and 2078 PmP travel times to construct a compressional velocity model for the 350-km-long transect, and a long-wavelength structure with strong lateral heterogeneity is recovered, including (1) -30-km-thick, undeformed continental crust with a velocity of 6.0 to 7.0 km/s near the landward end, (2) 30-to 15-km-thick igneous crust within a 150-km-wide continent-ocean transition zone, and (3) 15-to 9-km-thick oceanic crust toward the seaward end. The thickness of the igneous upper crust characterized by a highvelocity gradient also varies from 6 km within the transition zone to -3 km seaward. The bottom half of the lower crust generally has a velocity higher than 7.0 km/s, reaching a maximum of 7.2 to 7.5 km/s at the Moho. A nonlinear Monte Carlo uncertainty analysis is performed to estimate the a posteriori model variance, showing that most velocity and depth nodes are well determined with one standard deviation of 0.05-0.10 km/s and 0.25-1.5 km, respectively. Despite significant variation in crustal thickness, the mean velocity of the igneous crust, which serves as a proxy for the bulk crustal composition, is surprisingly constant (-7.0 km/s) along the transect. On the basis of a mantle melting model incorporating the effect of active mantle upwelling, this velocitythickness relationship is used to constrain the mantle melting process during the breakup of Greenland and Europe. Our result is consistent with a nearly constant mantle potential temperature of 1270-1340øC throughout the rifting but with a rapid transition in the style of mantle upwelling, from vigorous active upwelling during the initial rifting phase to passive upwelling in the later phase.

An integrated geophysical study of Vestbakken Volcanic Province, western Barents Sea continental margin, and adjacent oceanic crust

“…In first arrival seismic tomography (Korenaga et al 2000;Zelt and Barton 1998) it is not necessary to specify the refracting layer for a given pick. The travel time residuals between picked and calculated first arrivals are used together with the partial derivatives of travel times with respect to model parameters to create the inverse problem, which is usually solved using the least squares method.…”

“…We used the Tomo2d code (Korenaga et al 2000) to run first arrival tomography on our picks. Before we could run the code we had to remove all secondary arrivals from our original picks.…”

Resolving the fine-scale velocity structure of continental hyperextension at the Deep Galicia Margin using full-waveform inversion

“…This model was developed using OBS data collected from the Galicia-3D seismic experiment, supplemented with data from the ISE-1 seismic profile (Sawyer et al 1997;Zelt et al 2003), and inverted using the "TOMO2D" travel-time inversion code of Korenaga et al (2000). The final TOMO2D model has an overall travel-time misfit of 53 ms, and a chi-squared value of 0.97.…”

Variations in seismic velocity distribution along the Ryukyu (Nansei-Shoto) Trench subduction zone at the northwestern end of the Philippine Sea plate

“…Constructing V p models, we used information about the shallow sedimentary structures and depths of the subducting PHS plate interface deduced from MCS survey. As there were ample ray paths in V p models shallower then 10 km, we obtained upper crust V p models by means of a tomo2d tomographic inversion (Korenaga et al 2000) for the observed travel times of first arrivals. Misfits of the tomographic inversion were less than 50 ms for nearly all V p models.…”