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Illite age analysis (IAA), the method of comparing radiometric ages of successive size fractions with varying percentages of detrital illite, has been successfully applied to several rock types, including fault gouge, shales and argillaceous limestones. IAA results are presented for fi ve fault rocks, including four clay gouges and one cataclasite, from the exhumed Southern Appalachian foreland fold-thrust belt (eastern United States). Determining detrital versus authigenic illite is now an established procedure, utilizing X-ray analysis to quantify illite polytypes. Less established, however, is how to interpret 40 Ar/ 39 Ar ages as a function of diagenetic grade. Both a total gas age, incorporating the recoiled argon fraction after irradiation, and a retention age (omitting the recoiled fraction) are obtained for a sample. We relate their respective use to diagenetic grade and, specifi cally, the crystallite thickness of illite. When measured crystallite sizes are equal to or smaller than that calculated from Ar recoil we use total gas ages (here ~5 nm), while for crystallites greater than ~10 nm we use retention ages. The four clay gouge ages are all the same within error and much younger than that of the cataclasite. We conclude that a major period of frontal Appalachian fault activity occurred during the early Permian (Cisuralian, 276–280 Ma) and that these foreland faults were active simultaneously as part of an internally deforming, regional thrust wedge.

On Kodiak Island, Alaska, decimeterthick black fault rocks are at the core of foliated cataclasites that are tens of meters thick. The cataclasites belong to melange zones that are regarded as paleodecollements active at 12–14 km depth and 230–260 °C. Each black layer is mappable for tens of meters along strike. The black fault rocks feature a complex layering made at microscale by alternation of granular and crystalline micro textures, both composed of micronscale subrounded quartz and plagioclase in an ultrafi ne, phyllosilicate-rich matrix. In the crystalline microlayers, tabular zoned micro lites of plagioclase make up much of the matrix. No such feldspars have been found in the cataclasite. We interpret these crystalline microlayers as pseudotachylytes. The granular microlayers show higher grainsize variability, crushed microlites, and textures typical of fl uidization and granular fl ow deformation. Crosscutting relationships between granular and crystalline microlayers include fl ow and intrusion structures and mutual brittle truncation. This suggests that each decimeters-thick composite black fault rock layer records multiple pulses of seismic slip. In each pulse, ultracomminuted fl uidized material and friction melt formed and deformed together in a ductile fashion. Brittle truncation by another pulse occurred after solidifi cation of the friction melt and the fl uidized rock. X-ray powder diffraction (XRPD) and X-ray fl uorescence (XRF) analyses show that black fault rocks have similar mineral composition and chemical content as the cataclasites. The observed systematic chemical differences cannot be explained by bulk or preferential melting of any of the cataclasite components. The presence of an open, fl uidinfi ltrated system with later alteration of black fault rocks is suggested. The geochemical results indicate that these subductionrelated pseudotachylytes differ from those typically described in crystalline rocks and other tectonic settings.

Remnants of a rift-related, low-angle detachment system are spectacularly exposed in the continental Err nappe and occur discontinuously in the underlying oceanic Platta nappe in southeast Switzerland. Rocks along the detachment fault exhibit systematic changes in mineralogy, fabrics, and structures that are related to deformation and hydration of granite to form cataclasite and subsequent gouge. Structural, mineralogical, and geochemical data indicate a continuous evolution of fluid flow and mass transfer along the detachment fault. Mass-balance calculations together with stable isotope data suggest (1) an altered seawater source and local interaction with mantle rocks; (2) limited fluid-rock interaction and mass transfer during initial cataclasis at deeper crustal levels and temperatures of 150 to 200 °C; (3) increased fluid flux and mass transfer at slightly lower temperatures (100–150 °C) during gouge formation at shallower crustal levels; and (4) strongly localized fluid-rock interaction at low temperatures (65–80 °C) near the sea floor at a final stage of detachment faulting. We propose a model in which seawater-dominated fluids penetrated and locally interacted with mantle rocks before they were caught and channelized along large-scale detachment faults. Flow direction is upward along the detachment from higher to lower temperatures, suggesting that the driving force of fluid flow was thermal fluid convection associated with mantle exhumation. Our study suggests that this large-scale detachment fault is a good analog for extensional structures along present-day, nonvolcanic, rifted margins. Such structures along ocean-continent transitions are important in controlling fluid pathways and mass transfer between mantle and continental crust.

Three major low-angle normal faults in the eastern Lake Mead area, Nevada and Arizona, are parts of a 55-km-long, regional detachment fault that may form the principal breakaway for extensional allochthons at this latitude. This fault, herein named the South Virgin–White Hills detachment, comprises the Lakeside Mine, Salt Spring, and the Cyclopic Mine faults, and it extends from the South Virgin Mountains in southeastern Nevada to the central White Hills in northwestern Arizona. The previously unstudied central segment, the Salt Spring fault, is a 50-m-thick, gently west-dipping cataclastic fault zone that separates Proterozoic crystalline rocks in its footwall from northeast-dipping Tertiary volcanic and sedimentary rocks, including footwall-derived megabreccia, in its hanging wall. The 40 Ar/ 39 Ar and K/Ar dates on volcanic rocks in the upper plate constrain the principal phase of extension to between 15.2 and 14.6 Ma, although lesser extension continued to about 11 Ma. Lower-plate crystalline rocks retrograded from granulite to greenschist facies and regional steep foliations were tilted to low dips as a result of Tertiary extension. All three segments of the regional detachment record top-to-the-west displacement and were active principally during the time interval between 16 and 14 Ma. Displacement on the South Virgin–White Hills detachment decreases from a maximum at the Gold Butte block in the north to a minimum at the Cyclopic Mine in the south. This along-strike, southward decrease in extensional strain along the South Virgin–White Hills detachment is accompanied by a change in fault and footwall rock type from mylonite along the Lakeside Mine fault (northern segment), to chlorite cataclasite along the Salt Spring fault (central segment), to unconsolidated fault breccia along the Cyclopic Mine fault (southern segment). Differences in footwall and fault rocks reflect decreasing exhumation of footwall rocks due to decreased extension from north to south.

The Bench Canyon shear zone is a 20-km-long, moderately to steeply dipping zone of ductile to brittle strain that cuts Cretaceous plutonic and volcanogenic rocks located in the central Sierra Nevada, California. This zone is one of several Cretaceous-age ductile shear zones that record fluctuating strain fields associated with the tectonic/magmatic evolution of the central Sierra Nevadan magmatic arc. As determined by field relations, fabric analysis, and U-Pb and 40 Ar/ 39 Ar geochronology, the Bench Canyon shear zone underwent an episodic deformation history involving both contractional and extensional strain over a period of ∼17 m.y. or longer. Deformation is divided into early (ca. 101?–95 Ma), main (ca. 95–90 Ma), and late (ca. 90–78 Ma) phases. Temperature during main-phase deformation followed a retrograde T - t path of ∼600–300 °C after emplacement of multiple plutons. Main-phase deformation involved ductile thrust movement in both the 95 ± 1 Ma Red Devil Lake pluton and Lower to mid-Cretaceous volcanic rocks, where thrusting was facilitated by heat and fluids associated with plutonism. Fingerlike sills associated with the Red Devil Lake pluton were emplaced syntectonically with respect to main-phase deformation, but cooled rapidly, undergoing heterogeneous strain more typical of “pre-tectonic” emplacement. The ca. 90 Ma Mount Givens pluton cuts the shear zone fabric in the wall rock, limiting the bulk of deformation to have occurred prior to that time. Domainal and diachronous late-phase reactivation(s) along the zone involved weak, fluid-enhanced ductile (∼500 > T > 300 °C) deformation, and generation of abundant cataclasite and pseudotachylyte. The long-lived, episodic and diachronous deformation history of the Bench Canyon shear zone illustrates the complexities to be expected within shear zones during continental magmatic arc evolution.

Basement rocks of the Blue Ridge anticlinorium in central and northern Virginia were involved in two temporally distinct deformation episodes. The older event is characterized by a locally pervasive low-strain fabric and high-strain shear zones. A preferred grain shape orientation in recovered quartz, feldspar, and biotite, combined with a poorly developed symmetric quartz c -axis pattern, defines the low-strain fabric. Kinematic indicators are typically symmetric, but where asymmetric, they consistently indicate top down-to-the-southeast extensional motion. Discrete shear zones are within and close to Late Proterozoic plutons; kinematic evidence fron these zones records large noncoaxial strains also with a down-to-the-southeast extensional shear sense. Feldspar microstructures range from cataclastic fractures to complete recovery, suggesting that deformation occurred at greenschist to amphibolite grade. Both low-strain foliations and high-strain shear zones are cut by Paleozoic thrust-related tectonites. On the basis of kinematic evidence and spatial association with Late Proterozoic plutons, cross cutting relations with thrust-related structures, and regional geologic relations, we suggest that extensional deformation is related to Late Proterozoic rifting of the Laurentian margin. The kinematic histories of the Rockfish Valley fault zone and related shear zones appear to be relatively simple and consistent with Paleozoic west- to northwest-directed thrusting. The Rockfish Valley fault zone does not separate distinctly different basement terranes, and thus its displacement is thought to be small. Mylonitic rocks from this fault zone display crystal plastic deformation in quartz and cataclastic deformation in feldspars, under lower to middle greenschist-facies conditions. Solution transfer processes were an important deformation mechanism in these tectonites. Thrust- related mylonitic rocks are overprinted by related cataclasite and semibrittle shear bands, suggestive of a transition toward more brittle deformation as the thrust system cut higher in the crust. The Rockfish Valley fault zone appears to predate the formation of the South Mountain cleavage in the Valley and Ridge province, but it may be part of a lengthy, progressive, thrust-related deformation.

The obliqu-reverse Dent Fault, northwest England, throws Carboniferous limestone units in the footwall against mudston-dominated lower Paleozoic rocks in the hanging wall. The fault zone cuts the kilometer-wide steep limb of a precursory forced monocline. However, individual fault strands comprise centimeter-scale cataclasite cores fringed in the footwall carbonates by damage zones, some meters to tens of meters wide, composed of random-fabric dilation breccias. Breccia texture and microstructure, revealed by stained thin sections and peels, imply rapid coseismic fragmentation and then interseismic resealing by void-filling cements. The cements varied in composition through time from calcite to dolomite and then to ferroan calcite. Pervasive dolomitization of the protolith is common in the breccia zones. A key observation is that each volume of dilation breccia shows only limited refracture. This tendency to singl-phase brecciation suggests that cementation caused reseal-hardening of breccia with respect to intact protolith. Breccia thickness and refracture are greatest at jogs in the Dent Fault, but breccia distribution suggests that damage also accumulated in fault walls and at propagating fault tips. Dilation breccias are a common but poorly documented product of brittle deformation of limestone. Their reseal histories can provide valuable general clues to fault zone evolution.

Minimum slip rates calculated for plate-vector-parallel slickenside trends in cataclasite on the sole of the Alpine fault at Gaunt Creek, Westland, New Zealand, range from 18 to 24 mm/yr. Between half and two-thirds of the total relative motion between the Pacific and Australian plates is being accommodated by movement on a single structure, the Alpine fault. During the past 14 ka, the leading edge of the Alpine fault has changed from a moderately southeast-dipping, oblique reverse fault to a shallowly dipping thrust. The hanging wall (Pacific plate) is composed of a gradational sequence from basal gouge, through pseudotachylite-bearing cataclasite, to progressively more coherent schist-derived mylonite, which has been faulted against subhorizontally bedded, fluvio-glacial gravel in the footwall (Australian plate). During uplift the hanging-wall sequence has been internally sheared and imbricated, producing duplex structures, and retrogressively veined and altered by pervasive hydrothermal fluid flow. Erosion of the exhumed fault zone produced angular, cataclasite- and mylonite- derived, talus-fan breccias, building a west-dipping apron beneath the fault scarp. Wood fragments from near the base off the talus breccias have been 14 C dated at 12,650 ± 90 yr B.P. Progressive tectonic shortening resulted in 180 m of overthrusting of a schist-derived nappe across an irregular talus fan surface composed of its own erosional debris. The structural history of the Alpine fault at Gaunt Creek illustrates the importance of the interaction between fault-induced topography and erosion, and the control these processes exert on the continued tectonic, geometric, and geomorphic evolution of the fault zone.

A glass-coated half-meter-size boulder was sampled by the Apollo 17 crew at station 8 near the foot of the Sculptured Hills. The rock proved to be a coarse-grained (0.5-cm) plagioclase-orthopyroxene cumulate, and the samples are the only true norites returned from the lunar surface. Photographs of the boulder showed it to contain at least nine structural surfaces and four glass veins. Orientation and inspection of three of the returned samples resulted in the identification of six surfaces and one vein. One of the structural surfaces visible in the boulder was identified as primary cumulus planar lamination, which was folded through an angle of at least 35° between two oriented samples, whereas fracture sets representing the other surfaces were coincident. The boulder is believed to be a sample of the deeper highlands or submare lunar crust, derived from a depth of 8 to 30 km and somewhat shock-metamorphosed during at least two excavation events. The chemical composition of the norites, when determined, should be of special interest in view of the large amount of literature concerning glass, cataclasite, hornfels, and “basalt” of noritic composition returned by other Apollo missions. However, the cumulus texture of the boulder precludes its being representative of any magmatic liquid composition, suggests that the lunar crust is heterogeneously layered, and that plagioclase sank, not floated, in magmatic liquids that formed the lunar crust.

The Mineral Mountains intrusive complex records the interaction since Oligocene time between magmatic, structural, and hydrothermal processes. These processes continue to the present, as exemplified by the Roosevelt Hot Springs geothermal system, which is structurally controlled and driven by a pluton emplaced during the Pleistocene. The intrusive rocks of the complex show changes in composition from an early calc-alkaline suite (Oligocene) to a slightly alkaline main intrusive sequence (Miocene) that makes up most of the range. At about 9 Ma, the intrusive activity was associated with low-angle faulting. The oldest volcanic rocks are dated at ∼7 Ma. Volcanic activity continued in the region until ∼0.5 Ma and may be active still. Examples of the youngest volcanism include a sequence of rhyolite flows, domes, and pyroclastic deposits that were erupted along the crest of the range between 0.8 and 0.5 Ma. Minor basaltic flow and pyroclastic eruptions also were localized in the northern part of the range. A major low-angle detachment fault, containing as much as 200 m of cataclasite, is preserved in the southern Mineral Mountains. This fault separates lithologies of the Mineral Mountains intrusive complex from an overlying Paleozoic sedimentary sequence. We propose that this zone developed at the brittle-ductile transition and has been exposed because of the extreme uplift of the range. The range also preserves a series of listric faults which may sole into a major low-angle fault at depth. High-angle normal faults strike both north and east. Both sets have important controls on the flow of fluid in the Roosevelt Hot Springs geothermal field. The east-west structures are presently seismically active. Hydrothermal activity has been associated with all intrusive events in the Mineral Mountains. Base-metal mineralization accompanied the older events, whereas precious-metal mineralization accompanied the younger. We propose an evolution of the intrusive complex, involving the interaction of magmetic, structural, and hydrothermal components. From oldest to youngest, each phase of activity reflects emplacement of magma into progressively higher levels of the crust. Uplift reflected by this progression has been quite rapid at times, as shown by fission track dating. As a consequence, the Mineral Mountains constitute a profound structural high in the region. This extreme uplift probably was caused by diapiric rise of this area under the influence of repeated magmatic intrusion.

Near Parkfield in the southern Diablo Range, the smooth summit of Table Mountain, a narrow ridge 12 miles long, is capped by a subhorizontal sheet of weakly foliate serpentinite breccia composed of massive serpentinized-peridotite blocks set in a slickensided matrix of crushed serpentine. The breccia was extruded by plastic flowage from longitudinal fault fissures now occupied by steeply dipping serpentinite breccia dikes that lie near the crest of a complex anticline whose core is Jurassic-Cretaceous(?) Franciscan tectonic breccia made of exotic blocks set in a cataclasite matrix. The fissure feeders probably tapped a long-dormant mass of Franciscan serpentinite that was mobilized by fault disruption and high lateral stresses during the Pliocene-Pleistocene orogenesis of the Diablo Range. Upon reaching the surface, the breccia buried an uneven ridge crest of hills and saddles beneath a gently arched summit carapace. The breccia then moved downslope in earthflow fashion to form narrow, sloping ramps that filled draws between resistant spurs on the flanks of the ridge. On the lower slopes, the extrusion again coalesced to form an evenly sloping piedmont plate through which rejuvenated streams have cut steep gorges. Extensive landslides with prominent headwall scarps have scarred eroded edges of the extrusive sheet, and a degradational sequence of Quaternary pediment gravel surfaces at the base of Table Mountain also postdates the extrusion. The piedmont plate rests discordantly across a steeply dipping anticline limb of marine Cretaceous Panoche Group and a largely marine Tertiary sequence that includes Pliocene-Pleistocene(?) continental gravels. The Tertiary beds rest unconformably on the Panoche Group, but the Panoche Group lies upon the underlying Franciscan tectonic breccia along a folded thrust. The thrust emplacement followed widespread tectonic brecciation of the Franciscan, but preceded deposition of Eocene sandstones containing Franciscan detritus. The folded thrust is cut locally by younger, steep reverse and strike-slip faults that offset Tertiary strata.