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.

Bedrock fracture systems facilitate weathering, allowing fresh mineral surfaces to interact with corrosive waters and biota from Earth's surface, while simultaneously promoting drainage of chemically equilibrated fluids. We show that topographic perturbations to regional stress fields explain bedrock fracture distributions, as revealed by seismic velocity and electrical resistivity surveys from three landscapes. The base of the fracture-rich zone mirrors surface topography where the ratio of horizontal compressive tectonic stresses to near-surface gravitational stresses is relatively large, and it parallels the surface topography where the ratio is relatively small. Three-dimensional stress calculations predict these results, suggesting that tectonic stresses interact with topography to influence bedrock disaggregation, groundwater flow, chemical weathering, and the depth of the "critical zone" in which many biogeochemical processes occur.

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rav. Science 255. 165 11 992). 5. D.'s. Fisher, M. P. A. ~j s h e r , '~. A. Huse, Phys. Rev. 5 4 3 , 130 (1991). 6. G. Blatter, M. V. Feiqel'man, V. B. Geshkenbein, A. I. Larkin, V. M. ~inokur, Rev. Mod. M. Lelovic, P. Kr~shnaraj, N. G. Eror, U. Balachandran, ibid. 242, 246 (1 995). 11. Q. Li, H. J. Wiesmann, M. Suenaga, L. Motow~dlo, P. Haldar, Appl. Phys. Lett. 66, 637 (1 995). 12. P. Majewski, Adv. Mater. 6, 593 (1 994). 13. The problem of thermally activated flux motion is less severe In the HTSC YBa, Cu, O, (YBCO) and hence it offers better intrinsic behav~or at high temperatures and magnetic fields. The processing strategies developqd for BSCCO fail to yield viable YBCO wires as a result of poor intergranular current flow. Recent work suggests, however, that good alignment between grains can be achieved in thick films deposited on nickel tapes by Ion beam depos~tion [X. D. Wu et a/., Appl. Phys. Lett. 67, 2397 (1 99511. The commercial viabil~ty of th~s strategy remains to be demonstrated. 14. D. R. Nelson and V. M. Vinokur, Phys. Rev. Lett. 68, 2398 (1 9 9 2 ) ; , Phys. Rev. 5 48,13060 (1 993). 15. T. Hwa, P. Le Doussal, D. R. Nelson, V. M. Vinokur, Phys. Rev. Lett. 71, 3545 (1 993). 16. L. C~vale et a/., ibid. 67, 648 (1991); M. Konczykowski et a/., Phys. Rev. 5 44, 7167 (1991); R. C. 995). 25. A recent report of carbon nanotube-BSCCO composites (24) showed some evidence of J, improvement; however, the J, value of both the reference and nanorod-conta~ning sample in this report were lower than the good-quality BSCCO samples reported previously and in the present study. It is thus difficult to conclude that there is an improvement in behav~or upon adding nanotubes. In addition, th~s report and our own stud~es show that few nanotubes survive the synthesis process, leaving in doubt their ab~lity to create well-defined columnar defects in the HTSCs. 982).32. Nanorod-HTSC composites have also been successfully prepared with TI Ba, Ca, Cu, O, and T12Ba2Ca,Cu,010 materials. Preliminary measurements show that there are significant enhancements in J, for these composites (P. Yang and C. M. Lieber, unpublished results). . 34. The actual density of columnar defects that can pin flux lines may be larger than that corresponding to the dens~ty of MgO nanorods; that is, lattice strains associated w~th nanorod-BSCCO interfaces can lead to dislocations and other correlated defects that exhibit columnarlike pinning behavior. 35. The density of nanorods oriented close to the c axis was about 1 x 101° cm-'; a s~m~lar dens~ty was determined for nanorods oriented In the ab plane. Although this density is sign~ficantly lower than that obtained by heavy-ion and proton irradiation, we have not tr~ed to maximize the dens~ty of MgO nanorods and also believe that the density of correlated defects is probably significantly higher than that of nanorods (34). 36. C. P. Bean, Rev. Mod. Phys. 36, 31 (1964). 37. An inverse dependence of J, on defect size was also reported previously for Y,BaCuO, inclus~ons of 1 to 10 p m in diameter in YBC...