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We investigate the main crustal structures involved in the collision zone of the eastern d’Entrecasteaus Zone with the central Nem Hebrides island arc, using primarily gravity data and supporting seismic-refraction and magnetic data. Over the arc, gravity anomalies trend north-south and include a comples western gravity high, a median gravity low, and an eastern gravity high. Large variations in the crustal thickness of the arc are deducible from the velocity structure of the upper crust and from two-dimensional gravity calculations. Beneath the median gravity low, the 12- to 13-km-thick crust includes 4 to 5 km of strata within the North Aoba basin. In contrast, the western and eastern gravity highs are underlain by 20-km-thick crust that characterizes the islands of the Western and Eastern Belts. The thin basement of the basin may be either a piece of old trapped oceanic crust or attenuated island-arc crust. Because of the thin crust, collision-induced fractures developed that facilitated formation of the large Ambrym and Aoba volcanoes. Along the Western Belt, gravity and magnetic anomalies outline faults and old buried volcanoes. Large gravity and magnetic anomalies located near the north coast of Espiritu Santo Island may indicate a fault zone intruded by igneous rocks. Small sedimentary basins, such as the klalakula and Banks basins, are evidenced in the gravity data by local depressions in the western gravity high. Across the Eastern Belt, an asymmetric thick crustal root, together with other geologic and geophysical evidence, supports the model of incipient backarc thrusting along hfaewo Island. On the downgoing plate, the eastern d’Entrecasteaux Zone is flanked by two east-west-trending gravity lows related to the North Loyalty and West Santo Basins. The d’Entrecasteaux Zone itself has a gravity pattern that includes a southern gravity high (over the South d’Entrecasteaux Chain), a central low (over the Central d’Entrecasteaux Basin), and a northern high (over North d‘Entrecasteaux Ridge). Seismic-refraction and gravity data show that the crust of this submarine mountain zone does not differ greatly in thickness and structure from the surrounding oceanic crust. The South d’Entrecasteau chain is probably composed of volcanic edifices built upon oceanic crust, whereas North d’Entrecasteaux Ridge appears to have a thicker and shallower, high-velocity crust. The uppermost 5 km of oceanic crust vary greatly in the yelocity and density of rocks between the d'Entrecasteaux Zone and the adjacent basins. These variations may be responsible for some tectonic effects of the collision, such as complex structure in the accretionary wedge of the forearc, uplifting of the islands, and flexure of the sedimentary basins.