ABSTRACT We describe and interpret a system of well-preserved normal and reverse faults in the Kayo Formation of the Miocene Shimanto belt, an exhumed accretionary complex exposed on Okinawa Island. The normal and reverse fault systems both strike NE-SW, suggesting systematic horizontal stress variations between compression and extension. Temperature and pressure conditions for the normal and reverse fault systems were estimated from the densities of water in fluid inclusions in the veins along the faults, and previously reported maximum paleotemperature based on values of vitrinite reflectance and illite crystallinity. The fluid inclusion analyses yielded similar estimates for water density in both normal and reverse fault systems. The minimum geothermal gradient was constrained to a narrow range of 40–50 °C/km. These results suggest that the normal and reverse fault systems developed at a similar depth within the seismogenic zone. This can be interpreted as a change between horizontal compression and horizontal extension occurring at a maximum depth of 3.8–7.5 km below the seafloor, assuming lithostatic fluid pressure. This 90° rotation of the principal stress could be controlled by the seismic cycle, as exemplified by the rotation of stresses that occurred after the Tohoku-Oki earthquake.

Soft-sediment deformation structures crop out in the Lower Cretaceous succession of the Gubbio anticline in the Umbria-Marche Apennines of Italy. The deformation interval is ~13 m thick and occurs between the upper Hauterivian–lower Aptian Maiolica Formation and the Aptian Marne a Fucoidi Formation. It can be observed along the anticline for a distance of 12 km. Different types of deformation structures are distributed in several outcrops, with detachment extensional structures prevailing in the southeast sector. Imbricated slides, slump structures, and chaotic layers are distributed vertically and longitudinally in the middle and/or lower part of the deformed sediments. In the northwest sector of the anticline, compressional duplex structures can be considered the lower section of a large sediment failure. Geometrical and kinematic analysis of the fold axis trends and sliding surfaces have led to infer a single, large gravitational event possibly Albian in age. The synsedimentary deformation could be activated by several internal trigger mechanisms induced by external regional tectonic events such as earthquakes. An orthogonal system of calcite veins crossing the limestone layers represents the primary pathway for fluid-driven breaching of joint seals. These fluids can be related to the significant increase in the total organic carbon in the Hauterivian–Aptian layer of the Maiolica and Marne a Fucoidi Formations. This suggests the possibility that the limestone layer, sandwiched and sealed between clay of the organic-rich black shales, could have favored a pore pressure increase approaching lithostatic stress. With a thin overburden, lithostatic stress is more easily reached at low hydrostatic pressure. This slump sheet occurrence suggests the existence of a local paleoslope dipping toward the north-northwest, where the mass involved in the deformation is distributed over an estimated area of 60 km 2 for a volume of 0.8 km 3 of displaced sediments. The restoration and rotation of the slump fold hinges to the Early Cretaceous direction, in line with available paleomagnetic data, have shown that the strike of the slope corresponds to the main trend of the oldest Jurassic extensional lineaments and is linked to transform faults of the westernmost Tethys rifting systems.

Fluid inclusion microthermometric data from five temporally controlled vein sets in the southeastern Piedmont province record changes in fluid composition and deformation conditions during regional exhumation and cooling related to postorogenic Mesozoic rifting. In general, compositions of postmetamorphic fluids are remarkably consistent across the southeastern Piedmont, indicating regional fracture connectivity during fluid-trapping events between the end of the Alleghanian collisional orogeny and the opening of the Atlantic Ocean. Late Paleozoic P 1 and P 2 quartz veins associated with post-Alleghanian stress relaxation contain metamorphic CO 2 -H 2 O-NaCl fluids that were trapped under fluctuating lithostatic (125–240 MPa) to hydrostatic pressures (20–100 MPa) at burial depths of up to 9.3 km. Maximum depths are similar to emplacement depths of post-kinematic plutons, suggesting a period of rapid isobaric cooling. Fluids trapped in veins in the Modoc fault zone give similar trapping pressures, but significantly higher temperatures, suggesting fluid migration from depth along the zone. Triassic TR 1 quartz veins related to early failed rifting record regional decompression as fracturing above the brittle-to-ductile transition allowed regional pore-fluid pressure to drop to hydrostatic levels. During this time, extensive fluid mobility occurred due to opening of regional fractures associated with initiation of rifting and tapping of deep, hot, saline fluid reservoirs. Calcite in TR 2 veins, which are related to the development of Triassic basins, and later TR 3 veins related to initial Jurassic dike emplacement host common H 2 O inclusions. These inclusions are evidence for complete opening of the hydrologic system and for convective circulation of meteoric water, which resulted in dilution of “in situ” fluids, and ultimately to a pure-H 2 O system. These fluids continued to be trapped in vein minerals through the Early Cretaceous.