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[1] Results from the 1994 Aleutian Seismic experiment delineate basic oceanic arc crustal architecture, constrain magmatic flux rates and bulk arc composition, and address questions of continental crustal genesis. Here we present results from a transect across protocontinental crust of the westernmost Alaska Peninsula (line A3) and compare this structure to a purely oceanic arc transect farther west. Arc crustal structure is similar along these two transects. Magmatic accretion occurs at the top and bottom of preexisting oceanic crust as a 5-to 10-km-thick upper crustal carapace of low velocity (2-5.8 km s À1 ) volcaniclastics, flows and small plutons, and a mafic lower crustal underplate ($7.0 km s À1 ) of variable thickness, for a maximum arc crust thickness of $25-30 km. Lateral lower crustal velocity gradients and high velocities (>7.5 km s À1 ) beneath the forearc suggest dominantly vertical lower crustal accretion above a focused melt source and a forearc underlain by little magmatic crust but rather partially intruded and/or serpentinized mantle. The ratio of upper to lower crustal volume is $1, and the total arc crust volume implies a magmatic flux of $67 km 3 km À1 m.y. À1 , more than twice previous estimates for this arc and global productivity. The crust is thinner and more mafic than continental crust, and it lacks a massive tonalitic upper crust characteristic of the continents. An interpreted accumulation of upper crustal carapace material at midcrustal depths on line A3 has a velocity of $6.4 km s À1 , suggesting an intermediate composition. Accretionary complex terranes consisting of accumulations of this type material would thus have bulk compositions similar to continental crust.

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BIRPS deep seismic refection data collected on the western United Kingdom continental shelf show the existence of a wide variety of sedimentary basins, most of which originated during Palaeozoic-Mesozoic crustal extension. Symmetrical interior-fracture basins are numerous, but more complex basins are also common and show the importance of fault reactivation and the influence of pre-existing structures on basin development. All of the major basement-penetrating faults are interpreted as having been reactivated. They do not visibly cut through the entire crust and into the upper mantle. The crystalline crust thins dramatically beneath the basins indicating local crustal extensions of up to 60%, although regional extension is less than 30%.The typical BIRPS profile shows a highly reflective lower crust sandwiched between an unreflective upper crust and upper mantle. This pattern of reflectivity appears to be characteristic of deep seismic data collected from within extended regions. The relatively small amount of extension which has affected this region suggests that the highly reflective lower crust is more likely to be due to lithological variation formed by mafic igneous intrusion and underplating during crustal extension than to-extensional ductile fabrics within the lower crust. =SWAT profiles 2 to 7 and 9. Stipple pattern marks the extent of the post-orogenic basins crossed by the profiles: CB =Colonsay basin; CBB = Cardigan Bay basin; CISB = Central Irish Sea basin; ECB = Eastern Channel basin; FCB = Firth of Clyde basin; IHB =Inner Hebrides basin; LIB = Loch Indaal basin; MB = Malin basin; McB = Minch basin; NCB = North Channel basin; NCSB = North Celtic Sea basin; NLB = North Lewis basin; OHB = Outer Hebrides basin; PeB = Peel basin; PB = Portpatrick basin; PBB = Plymouth Bay basin; RT =Rathlin Trough; SB = Solway basin; SCSB = South Celtic Sea basin; SGB =St George's Channel basin; SHB = Sea of Hebrides basin; ST = Stanton Trough; WAB =Western Approaches basin; WCB =Western Channel basin; WOB=Western Orkneys basin. CP=Carnsore Point. Major faults shown: BFS =Bala fault system;

Seismic refl ection data show that the eastern Aleutian Arc is characterized by refl ectors that extend continuously from the lower arc crust to >50 km depth, which is considerably deeper than the crustal thickness of 27-35 km previously inferred from coincident wide-angle seismic surveys. Because the upper mantle is commonly homogeneous, and therefore nonrefl ective, relative to the overlying crust, we interpret these refl ectors to be gabbro, garnet gabbro, and pyroxenite intrusions within two 50-km-wide roots that represent a >25-km-thick heterogeneous transition from mafi c lower crustal rocks to ultramafi c mantle rocks. We suggest that the refl ectivity is linked to repeated differentiation and intrusion of mantle-derived melts into the subarc lithosphere, and that the depth of these roots shows that fractionation of arc crust can extend well below the seismically determined Moho. Because these deep roots are not evident beneath the central Aleutian Arc, either the roots form sporadically, perhaps as a consequence of an elevated magmatic supply, or such roots ultimately founder into the underlying mantle due to their relatively high mass density.