Abstract. Small-scale modeling was performed to examine the effects of the superposition of two successive extensional phases from orthogonal to oblique (type I) and from oblique to orthogonal (type 2). In both the type 1 and type 2 models, faults produced during the first stage strongly control fault development during the second stage. In type 1 models, the oblique faults developed during the second oblique phase are confined within a first-phase graben, whereas in type 2 models the oblique faults, produced during the first phase, continue to develop during orthogonal extension and connect with each other to give sigmoidal fault blocks. Type 1 models are compared with the structural setting of the Ethiopian Rift; the evolution of the rift is related to a recent extensional event, whose principal direction of stretching trends at around 50 ø to preexisting major normal faults. Type 1 laboratory models are fairly comparable to the northem sector of the Ethiopian Rift, referred to here as MER. They account for both the development of the en echelon oblique faults of the Wonji Fault Belt and the sinistral shear gradient running parallel to the eastern border of the MER, which formed during an oblique rifting extension. The statistical analysis of the whole Ethiopian Rift fault pattern by reference to the experimental data allows the determination of a N100ø-N110 ø mean direction of stretching.

Back-arc extension in the Aegean, which was driven by slab rollback since 45 Ma, is described here for the first time in two stages. From Middle Eocene to Middle Miocene, deformation was localized leading to (i) the exhumation of high-pressure metamorphic rocks to crustal depths, (ii) the exhumation of high-temperature metamorphic rocks in core complexes, and (iii) the deposition of sedimentary basins. Since Middle Miocene, extension distributed over the whole Aegean domain controlled the deposition of onshore and offshore Neogene sedimentary basins. We reconstructed this two-stage evolution in 3D and four steps at Aegean scale by using available ages of metamorphic and sedimentary processes, geometry, and kinematics of ductile deformation, paleomagnetic data, and available tomographic models. The restoration model shows that the rate of trench retreat was around 0.6 cm/year during the first 30 My and then accelerated up to 3.2 cm/year during the last 15 My. The sharp transition observed in the mode of extension, localized versus distributed, in Middle Miocene correlates with the acceleration of trench retreat and is likely a consequence of the Hellenic slab tearing documented by mantle tomography. The development of large dextral northeast–southwest strike-slip faults, since Middle Miocene, is illustrated by the 450 km long fault zone, offshore from Myrthes to Ikaria and onshore from Izmir to Balikeshir, in Western Anatolia. Therefore, the interaction between the Hellenic trench retreat and the westward displacement of Anatolia started in Middle Miocene, almost 10 Ma before the propagation of the North Anatolian Fault in the North Aegean

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We discuss the opening mechanism of the Japan Sea in Miocene time using (1) tectonic and published paleomagnetic data along the eastern margin from the north of Hokkaido Island to Sado Island, (2) a mechanical model which is tested by small‐scale physical modeling, and (3) crustal structure and bathymetric features in the Japan Sea which constrain our kinematic model and preopening reconstructions. Our main conclusions are the following. The eastern margin of the Japan Sea was, as a whole, a dextral shear zone about 100 km wide. This conclusion is supported by the existence of a ductile dextral shear zone in Central Hokkaido (Hidaka Mountains) and associated brittle deformation in western Hokkaido and northeastern Honshu. The stress field during the opening (which ended about 12 Ma ago at the end of the middle Miocene) changes from right‐lateral transpression in the north to right‐lateral transtension in the south. The western margin, along the Korean peninsula, during the same period, also was an active dextral shear zone. Paleomagnetic results indicate that clockwise rotations occurred in the south during the opening and counterclockwise rotations in the north. We propose a model of right‐lateral pull‐apart deformation with clockwise rotations of rigid blocks in the southern transtensional domain and counterclockwise rotations in the transpressional one. Small‐scale physical models show that the clockwise rotation in transtension is possible provided that the eastern boundary (Pacific side) is free of stress. The opening stopped and compression subsequently began about 12 Ma ago. Finally, we show that the dextral shear, which is distributed over the whole Japan Sea area, is accommodated by N‐S trending right‐lateral faults and rotation of blocks located between these right‐lateral faults.

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