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The Okinawa Trough, lying to the east of China, is a back arc basin formed by extension within continental lithosphere behind the Ryukyu trench‐arc system. Middle to late Miocene uplift, associated with normal faulting of the initially adjacent Ryukyu nonvolcanic arc and the Taiwan‐Sinzi folded belt, corresponds to the first rifting phase. The timing of rifting is supported by the presence of marine sediments of corresponding age drilled in the northern Okinawa Trough. The rifting occurred after a major early Miocene change in the motion of the Philippine plate with respect to Eurasia and ceased during the Pliocene. A second rifting phase started about 2 m.y. ago, at the Plio‐Pleistocene boundary and has continued until the present time. It has proceeded to a more advanced stage in the middle and southern Okinawa Trough than it has farther north. Detailed bathymetric (Sea Beam), seismic reflection, and magnetics data collected during the POP 1 cruise of the R/V Jean Charcot reveal the principal features of the extensional processes. The back arc spreading phase started very recently in the southern and middle Okinawa Trough, as exemplified by several en échelon and, in some cases, overlapping active, central graben oriented N70°E–N80°E. Some of these depressions are intruded by volcanic ridges of fresh back arc basalt with associated large magnetic anomalies. Transform faults between these en échelon active rifts are not obvious. We suggest that the major part of the southern Okinawa Trough is underlain by a thinned continental crust and that except for the system of en échelon rifts of the southern Okinawa Trough, the back arc basin oceanic domain is limited to a width of a few tens of kilometers or less in the axial portion of the trough. The system of axial back arc volcanic ridges that occur in the rifts ends at the latitude of Okinawa Island whereas active volcanoes in the Ryukyu arc occur only north of Okinawa Island. We refer to this transition between active arc and back arc volcanism as the volcanic arc‐rift migration phenomenon (VAMP). Globally, back arc volcanism propagated from the southern Okinawa Trough to the Okinawa VAMP area. Rifting continues to occur in the northern Okinawa Trough but is not yet accompanied by associated volcanism. The Okinawa VAMP area is characterized by a series of parallel basaltic ridges oriented N75°E with associated linear magnetic anomalies characteristic of dyke intrusions. We suggest that the formation of the back arc oceanic domain took place along the axial back arc extensional zone trending N75°E and that this zone presently ends at the southern extremity of the active volcanic chain. The initial phase of formation of back arc basin oceanic crust is non‐steady state and is characterized by the lack of a developed fracture zone pattern. The termination of the VAMP area in the direction of the volcanic zone of the arc is consistent with the suggestion of Molnar and Atwater that the volcanic arc is a fundamental line of weakness which determines where initial back arc...

A seismic refraction transect across the Galicia Bank continental margin shows that the original continental crust thins westward from 17 to 2 km immediately east of a margin‐parallel peridotite ridge (PR). Immediately west of the PR, oceanic crust is only 2.5–3.5 km thick, but farther west (oceanward) it thickens to 7 km. The PR caps a ∼60‐km‐wide lens‐shaped serpentinized peridotite body underlying both thinned continental and thin oceanic crust. When superimposed on a reflection time version of the velocity model, the S reflector is clearly intracrustal at its east end. Westward, S cuts down to lower crustal levels, eventually coinciding with the top of the serpentinized peridotite lens (original crust‐mantle boundary). These observations render almost impossible the seafloor exposure of the PR by S acting as a top‐to‐the‐west detachment fault. Numerical models of melting and borehole subsidence information constrain our rifting model. The easternmost continental crust experienced a total stretching factor of 4.3 (most likely in two stages); it probably occurred over ∼25 m.y., with the highest rate of stretching at the beginning of the main earlier rift phase (Valanginian; 141–135 Ma). The 3 (4.7) km thick continental crust (depending on whether serpentinized peridotite is assigned to crust or mantle), which may include melt products, requires stretching factors of more than 11 (7) and a rift duration of more than 25 (13) m.y. The thin oceanic crust immediately west of the PR is explained by conductive cooling of the mantle during the long prebreakup stretching phase, which temporarily caused reduced melting immediately after breakup.

The stretching model of McKenzie is applied to the formation of passive continental margins, assuming local isostatic equilibrium. We present the quantitative implications of the model; we then discuss its fit to the IPOD data on the Armorican and Galicia continental margins of the northeast Atlantic. The amount of brittle stretching observed in the upper 8 km of the prestretched continental crust reaches a maximum value of about 3. This large amount of thinning is comparable to the thinning of the whole continental crust observed by seismic refraction measurements and required by the model for the whole lithosphere. This agreement suggests that the simple stretching model is a good first approximation to the actual physical process .of formation of the margin. It is thus possible to compute simply the thermal evolution of the margin •iiid to discuss its petrological consequences. It is also possible to obtain a quantitative reconstruction of the edge of the continent prior to breakup. Finally, the large slope of the base of the lithosphere during the formation of the margin results in a force similar but opposite to the 'ridgepush' force acting on accreting plate boundaries. Pc=P,ol-TTa ( pt=pm 1---•-Ta---•-Ta Thus h, {(Pm --p½o)(hc/ht)[1 -(a/2) T.(hc/ht)] -pm(Ol/2) Ta} pm(1 --olTa) --Pw The expression for Zi is linear in 1 -(1/fi). The condition for subsidence to occur is that the numerator be positive, which expresses that the lithosphere has a positive buoyancy h___c > pm(Ot/2)Ta (jOm --Pco)[1 --(a/2)Ta(hc/hJ] Note that we have neglected the effect of compressibility on density, as most authors have done [e.g., McKenzie, 1978a]. First, this effect is quite smaller than the temperature effect. Second, as we really are interested in the variation of density between two columns where the distribution of pressure is the same and only the distribution of temeprature differs, it is justitled to neglect the pressure effect in a first approximation. 3708 LE PICHON AND SIBUET: PASSIVE MARGINS 3709 50 ø 45 ø 40o 35 15 ø 10 ø 5 ø 0 o

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