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We redescribed the~0.5-km gabbro section drilled in Hole 735B at the Ocean Drilling Program Gulf Coast Repository. Included in this work was a redivision and clarification of the location and nature of the major lithologic boundaries and a division of the major units into subunits. In all, we found 495 distinct lithologic intervals in the core. Most of the section consists of a single olivine gabbro body having only minor cryptic variations, which we think represents a small intrusion. At the top of the section, the olivine gabbro is intercalated with a medium-to coarse-grained gabbronorite, which we postulate was intruded by the olivine gabbro. The base of the olivine gabbro has been intruded by troctolites and troctolitic gabbros, which may be the precursors of a major troctolite intrusive body immediately below the base of the hole. This section is variously crosscut by small microgabbro bodies, which are the products of crystallization and wall-rock reaction of small magma bodies that migrated through the olivine gabbro prior to complete solidification. Overall, the plutonic section drilled in Hole 735B is unlike those found at layered intrusions as it lacks evidence for extensive magmatic sedimentation. Rather, it appears to represent a plutonic basement composed of small, relatively short-lived, rapidly crystallized intrusions. This is consistent with the ephemeral volcanism and low rates of magma supply postulated for very slow-spreading ocean ridges. This whole section underwent "syntectonic differentiation": a process in which deformation and compaction of a rigid, partially molten gabbro drove intercumulus melt out of the olivine gabbro into ductile shear zones. Chemical exchange, precipitation of oxides, and trapping of the migrating melt at the end of deformation altered the gabbro in the shear zones to ferrogabbro. These oxide-rich horizons have the potential to be major shallow-dipping seismic reflectors. The largest such zone is 103 m thick and consists of foliated disseminated oxide olivine and oxide olivine gabbros of lithologic Units III and IV. The last igneous event was back-intrusion of trondhjemite veins that formed either by fractional crystallization from the interstitial melt and/or by wall rock anatexis of intruded amphibolites. Alteration and relatively rapid cooling of the gabbro body occurred by penetration and circulation of seawater into the plutonic section caused by thermal contraction and cracking under tensile stress, much as envisaged by Lister (1970). Initially, this circulation was greatly enhanced tectonically by the tensile component provided by lithospheric necking and the formation of brittle-ductile faults beneath the median valley. This circulation was sufficiently pervasive to alter about 25% of all the matrix pyroxene in the body, mostly to amphibole, in the amphibolite facies. Alteration was heaviest in the vicinity of the brittle-ductile faults, where formation of crack networks, cataclasis, and granulation were ongoing processes continuously creating porosity and p...

Extensional accommodation zones, or tilt-block domain boundaries, facilitate reversals in the dominant tilt direction of fault blocks and possibly inversions in the dip of regional detachment systems in rifted continental crust. The amount and direction of movement of the footwall (lower plate) and hanging wall (upper plate) of the detachment terrane dictate the deformational style along accommodation zones. Various models of extension can potentially be evaluated by defining modes of deformation along accommodation zones.A 40-km-long, east-west-trending, middle Miocene accommodation zone bisects the central Black Mountains, northwestern Arizona, and southern Eldorado Mountains, southern Nevada. The Black and Eldorado Mountains lie within the northern Colorado River extensional corridor, a 50-to 100-km-wide region of severely extended crust. The generally sublinear, 5-to 10-km-wide accommodation zone separates more than 5,000 km 2 of east-tilted fault blocks to the north from 25,000 km 2 of dominantly west-tilted fault blocks to the south. The zone may also mark the join between regionally extensive, oppositely dipping detachment systems.Transversely oriented segments (i.e., perpendicular to strike of tilted blocks) of the accommodation zone in the upper-plate rocks correspond to areas of intermeshing conjugate normal faults. East-and west-dipping normal faults dominate the west-and east-tilted domains, respectively, whereas east-and west-dipping faults are equally common in the axial part of the zone. Some of the major normal faults in the west-and east-tilted domains terminate in drag folds within the axial part of the zone. Fault-block tilting on either side of the accommodation zone commonly exceeds 60°. Tilting decreases progressively toward the axis of the zone, where transversely oriented, obliqueslip normal faults accommodate scissors-like torsional offset between gently tilted (10 to 35°) individual fault blocks of opposing polarity. Concomitant with the decrease in tilting, fault spacing decreases, and average fault dip increases. Fault blocks within the zone were periodically tilted in opposite directions during the same episode of extension.Minor amounts of open to tight folding characterize along-strike segments (i.e., parallel to strike of tilted blocks) of the accommodation zone. •Present address: Structural development of a major extensional accommodation zone in the Basin and Range Province, northwestern Arizona and southern Nevada; Implications for kinematic models of continental extension, in Wernicke, B. P., ed., Basin and Range extensional tectonics near the latitude of Las Vegas, Nevada: Boulder, Colorado, Geological Society of America Memoir 176. J. E. Faulds and OthersThe lack of strike-slip faulting along transversely oriented segments and only minor amounts of compression on along-strike segments of the accommodation zone indicate little relative movement between opposing tilt-block domains. The transversely oriented, oblique-slip normal faults in the zone facilitated torsional offset, a...

The Cobequid–Chedabucto fault system of the Canadian Appalachians is a major anastomosing fault system over 300 km in length. It separates the Meguma Terrane of southern Nova Scotia from the Avalon Terrane to the north. These terranes are distinct tectonic and lithological entities in the Appalachian Orogen. Two areas at either end of this fault system have been examined in detail to determine the sense and history of offset along it. Both areas are situated on major component fault zones of the system, and both exhibit structures due to early intense ductile shearing that are overprinted by semi-brittle to brittle structures caused by later faulting. Along the eastern Chedabucto fault zone (area A), ductile structures were examined. This area is characterized by the progressive development of S–C textures and shear bands, rotated syntectonic porphyroblasts, and asymmetric minor folds, features indicative of and caused by ductile shearing. Along the western Cobequid fault zone (area B), semi-brittle and brittle structures were studied. A distinctive asymmetric geometrical package of faults, self-similar at a variety of scales, is developed throughout this part of the fault system. Ductile and brittle displacement sense (kinematic) indicators at both sites indicate a protracted history of dextral strike-slip movement. No evidence was observed for major sinistral movement.

The Proterozoic metamorphic belt of northern and central New Mexico contains rocks of two distinctly different metamorphic grades, locally lying in direct contact. A large region exhibits coexisting kyanite, andalusite, and sillimanite. The triple-point assemblages resulted from peak metamorphic conditions falling near 500°C, 4 kb, and they occur across 75,000 km 2 in 14 separate mountain ranges. Triple-point metamorphic grade was attained after north-verging folding and ductile thrusting but before southverging mylonitic shearing. Prograde and retrograde P-T paths were nearly isobaric.However, two mountain ranges preserve a different metamorphic history. Granitic and migmatitic gneiss in the Santa Fe Range formed at 650° to 750°C. Sillimanite-Kfeldspar gneiss in the northern Taos Range records peak conditions as high as 700° to 800°C, 9 to 11 kbar. In the latter area, high-grade metamorphism was followed by south-verging shearing, southeast-verging folding, and decompression, on a retrograde P-T path that passed through 500°C, 4 kb. Deformation produced a thick mylonitic shear zone that appears in scattered outcrops across the entire mountain range. The shear zone separates gneiss from structurally overlying kyanite-andalusite-sillimanite quartzite. Mineral textures indicate that the shearing occurred during and after the thermal peak in the gneissic rocks, and during the thermal peak in the overlying triplepoint rocks.We suggest that the kyanite-andalusite-sillimanite rocks are separated from the rocks of higher metamorphic grade by a synmetamorphic ductile detachment fault represented by the thick mylonitic shear zone. The detachment separated an actively extending lower crust from an upper crust that was not being thinned. We infer that the detachment extends in the subsurface beneath the entire belt where rocks show kyaniteandalusite-sillimanite metamorphic grade. Lower-crustal extension, with heat carried upward toward the detachment, seems to have caused the triple-point metamorphism.