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

We present results from a combined multichannel seismic reflection (MCS) an d wide-angle onshore/offshore seismic experiment conducted in 1996 across t he 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 ray-tracing scheme based on the graph method and the local ray-bending refinement to efficien tly obtain an accurate forward solution, and we employ smoothing and option al damping constraints to regularize an iterative inversion. We invert 2318 Pg and 2078 PmP travel times to construct a compressional velocity model f or the 350-km-long transect, and a long-wavelength structure with strong la teral heterogeneity is recovered, including (1) similar to 30-km-thick, und eformed continental crust with a velocity of 6.0 to 7.0 km/s near the landw ard end, (2) 30- to 15-km-thick igneous crust within a 150-km-wide continen t-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 h igh-velocity 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 h igher than 7.0 km/s, reaching a maximum of 7.2 to 7.5 km/s at the Moho. A n onlinear Monte Carlo uncertainty analysis is performed to estimate the a po steriori model variance, showing that most velocity and depth nodes are wel l 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 crusta l composition, is surprisingly constant (similar to 7.0 km/s) along the tra nsect. On the basis of a mantle melting model incorporating the effect of a ctive mantle upwelling, this velocity-thickness relationship is used to con strain the mantle melting process during the breakup of Greenland and Europ e. Our result is consistent with a nearly constant mantle potential tempera ture of 1270-1340 degrees C throughout the rifting but with a rapid transit ion in the style of mantle upwelling, from vigorous active upwelling during the initial rifting phase to passive upwelling in the later phase.