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.

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[1] Seismic reflection and refraction data from the SE Greenland margin provide a detailed view of a volcanic rifted margin from Archean continental crust to near-toaverage oceanic crust over a spatial scale of 400 km. The SIGMA III transect, located $600 km south of the Greenland-Iceland Ridge and the presumed track of the Iceland hot spot, shows that the continent-ocean transition is abrupt and only a small amount of crustal thinning occurred prior to final breakup. Initially, 18.3 km thick crust accreted to the margin and the productivity decreased through time until a steady state ridge system was established that produced 8-10 km thick crust. Changes in the morphology of the basaltic extrusives provide evidence for vertical motions of the ridge system, which was close to sea level for at least 1 m.y. of subaerial spreading despite a reduction in productivity from 17 to 13.5 km thick crust over this time interval. This could be explained if a small component of active upwelling associated with thermal buoyancy from a modest thermal anomaly provided dynamic support to the rift system. The thermal anomaly must be exhaustible, consistent with recent suggestions that plume material was emplaced into a preexisting lithospheric thin spot as a thin sheet. Exhaustion of the thin sheet led to rapid subsidence of the spreading system and a change from subaerial, to shallow marine, and finally to deep marine extrusion in $2 m.y. is shown by the morphological changes. In addition, comparison to the conjugate Hatton Bank shows a clear asymmetry in the early accretion history of North Atlantic oceanic crust. Nearly double the volume of material was emplaced on the Greenland margin compared to Hatton Bank and may indicate east directed ridge migration during initial opening.

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[1] We used data from both permanent and temporary seismic networks on Iceland and Greenland to investigate the crustal thickness by partly reinterpreting earlier data (P receiver functions) and adding S receiver functions to better constrain the results. We obtained good conversions from the Moho and also crustal multiples in both Iceland and Greenland. The central ice covered part of Greenland has an average crustal thickness of 40 km, typical for a craton. At the edges of Greenland the crustal thickness decreases to 30-40 km. On the east coast of Greenland, where the track of the Iceland plume is thought to have affected the lithosphere, the crustal thickness is only 24-26 km. In contrast to previous studies, we find that the crustal thickness in the east and the northwest coastal regions of Iceland is more than 40 km, similar to beneath the active volcanic region. In the southwest region of Iceland and along the mid-ocean ridge, the crustal thickness is only 25 km or less. Also in contrast to earlier receiver function interpretations, which deduced a broad crust-mantle transition zone for Iceland, we find indications for a normal, sharper Moho beneath a number of sites.

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The combined Greenland‐Senja Fracture Zones (GSFZ) represent a first‐order plate tectonic feature in the North Atlantic Ocean. The GSFZ defines an abrupt change in the character of magnetic anomalies with well‐defined seafloor spreading anomalies in the Greenland and Norwegian basins to the south but ambiguous and weak magnetic anomalies in the Boreas Basin to the north. Substantial uncertainty exists concerning the plate tectonic evolution of the latter area, including the role of the East Greenland Ridge, which is situated along the Greenland Fracture Zone. In 2002, a combined ocean‐bottom seismometer and multichannel seismic (MCS) survey acquired two intersecting wide‐angle reflection and coincident MCS profiles across and along the East Greenland Ridge. We present the results of integrated reflection seismic interpretation, first‐arrival tomography, 2D kinematic raytracing, full‐wave amplitude modeling, and gravity modeling of the intersecting profiles. The results show that (1) the Greenland Basin is characterized by a normal oceanic crustal velocity structure, (2) the velocity structure of the East Greenland Ridge is of overall continental type, and (3) a major faulted basin province above highly extended continental crust exists to the NE of the ridge. The results further suggest that a zone of extremely thin and faulted continental crust above partially serpentinized mantle peridotite defines the NW edge of the East Greenland Ridge and the transition to the NE Greenland margin.