The hotspot-generated Louisville Ridge is a 4000-km chain of seamounts (typically 2-2.5 km high and 10-40 km in diameter) and an underlying crustal swell (1.5 km high and 100+ km wide) trending NNW across the southwestern Pacific. The northwest end of the Ridge collides with the north trending Tonga Trench (26øS) which, just north of that point, is exceptionally deep (10.8 km) and lacks both a turbidite wedge and a bordering accretionary complex. The collision zone is moving rapidly southward. Multichannel seismic reflection data in the collision zoneshow a west dipping subsurface platform 2-3 km beneath the lower western trench slope, which is interpreted as the flat summit of a subducted guyot, Motuku, of the Louisville chain. Projected eastward, the summit plain passes 1-2 km above the trench floor. Dredging of the nearby inner trench wall recovered uppermost Cretaceous (Maestdchtian) oceanic pelagic sediments interpreted to be fragments of the sedimentary mantle of a subducted Louisville seamount. The principal effects of hotspot-ridge collision with a sediment-starved trench are (1) the impacting seamounts are subducted rather than accreted, and (2) although some seamount rocks are temporarily accreted, the inner trench wall is tectonically eroded arcward at rates possibly as high as 50 km/m.y. Accelerated tectonic erosion is related to (1) fracturing, shearing and general weakening of arc substrate rocks as they are lifted by the swell, penetrated by impacting seamounts, and left to collapse as the ridge moves away, (2) a more effective removal of weakened rock in underthrusting grabens which are larger in the crustal swell,(3) a possible elevation of the subduction decollement to account for the removal of as much as 30,000 km 3 of material from a 400 km sector of the trench in 1 million years, and (4) a reduction in supply of arc-derived debris resulting from the gap in arc volcanism accompanying subduction of the ridge. "Normal" tectonic erosion in the Tonga Trench is apparently minor, and we conclude that the bulk of the -37,000 km 3 of material which fills subducting grabens each million years is arc-derived volcanic and pelagic sediment. Dupont, J., Morphologie et structures superficielles de l'arc insulaire des Tonga-Kermadec, in Contribution a l'EtudeGeodynamique du Sud-Ouest Pacifique, vol. 147, pp. 263-282, Office de la Recherche Scientifique et Technique Outre-Mer, Paris, 1982. Dupont, J., and R.H. Herzer, Effect of subduction of the Louisville Ridge on the structure and morphology of the Tonga Origins of nonvolcanic seamounts in a forearc environment, in Seamounts, Islands and 93, 3078-3104, 1988. McCann, W.R., and R.E. Haberman, Morphologic and geologic effects of the subduction of bathymetric highs, Pure Appl. Geophys., in press, 1989. McCarthy, J., and D.W. Scholl, Mechanisms of subduction accretion along the central Aleutian Trench, Geol. Soc. Leg 91 Scientific Party, Tectonic evolution of the southwestern tropical Pacific basin (abstract), Eos Trans AGU, 64, 315, 1983. of Leg 91 basalts and...

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Geophysical and regional geologic data provide evidence that parts of the oceanic crust in the abyssal basins of the Bering Sea have been created or altered by crustal extension and back‐arc spreading. These processes have occurred during and since early Eocene time when the Aleutian Ridge developed and isolated oceanic crust within parts of the Bering Sea. The crust in the Aleutian Basin, previously noted as presumably Early Cretaceous in age (M1–M13 anomalies), is still uncertain. Some crust may be younger. Vitus arch, a buried 100‐ to 200‐km‐wide extensionally deformed zone with linear basement structures and geophysical anomalies, crosses the entire west central Aleutian Basin. We suggest that the arch and the inferred fracture zones in the Aleutian Basin are early Cenozoic structures related to the early entrapment history of the Bering Sea. These structures lie on trend with known early Cenozoic structures near the Bowers‐Shirshov‐Aleutian ridge junction and on the Beringian continental margin (with possible continuation into Alaska); the structures may have coeval and cogenetic(?) histories for early Cenozoic and possibly younger times. Cenozoic deformation within parts of the Bering Sea region is principally extensional, although the total amount of extension is not known. As examples, the Komandorsky basin formed by back‐arc seafloor spreading, the Aleutian Ridge has been extensively sheared, and extensional block faulting is common. Sedimentary basins of the Bering shelf have formed by extension associated with wrench faulting. The Cenozoic deformation throughout the Bering Sea region probably results from the interaction of major lithospheric plates and associated regional strike‐slip faults. We present models for the Bering Sea over the past 55 m.y. that show oceanic plate entrapment, back‐arc faulting and spreading along Vitus arch, breakup of the oceanic crust in the Aleutian Basin at fracture zones, and back‐arc spreading in Bowers Basin.

Nondestructive images of refractive-index variation within a type I fiber Bragg grating have been recorded by the differential interference contrast imaging technique. The images reveal detailed structure within the fiber core that is consistent with the formation of Talbot planes in the diffraction pattern behind the phase mask that had been used to fabricate the grating.

From a global point of view, chronic haematogenous osteomyelitis in children remains a major cause of musculoskeletal morbidity. We have reviewed the literature with the aim of estimating the scale of the problem and summarising the existing research, including that from our institution. We have highlighted areas where well-conducted research might improve our understanding of this condition and its treatment.