Tectonic Breccia Research Articles

A genetic model for mineralization of lower Windsor (Visean) carbonate rocks of Nova Scotia, Canada

A genetic model for mineralization of lower Windsor (Visean) carbonate rocks of Nova Scotia, Canada

Breccias in the lower part of the Mississippian Windsor Group and their relation to Zn-Pb mineralization; a summary

Base metal occurrences in the Mississippian Windsor Group (Nova Scotia, Canada) are commonly associated with carbonate breccias in the Macumber Formation. Based on sedimentologic, structural, and diagenetic data, three types of temporally distinct breccias are recognized near the top of the Macumber Formation: early synsedimentary, synburial tectonic, and late, postburial karstic. Early breccias originated as rotational slides on a Visean deep-water slope and host mineralization such as the Walton deposit (Ba-Cu-Pb-Zn-Ag). Later tectonic breccias of Westphalian age are a manifestation of the Ainslie detachment, a regional flat-lying extensional fault, and host the Jubilee deposit (Zn-Pb). Finally, an irregularly distributed postburial, karstic solution-collapse barren breccia is designated as the Pembroke breccia. The recognition of three temporally and genetically distinct breccias is significant for comprehension of the breccia-hosted mineralization in the Mississippian section in eastern Canada. The synsedimentary and tectonic breccias are pre- to synore, respectively, and both have played a key role in localizing mineral deposition through enhanced permeability. The late karstic breccia was not involved in the mineralization and, where present, subaerial exposure leading to the formation of this breccia has removed significant portions of the orebodies.

Ext≐nsion post-orogénique et zones de cisaillement. Étude d'une brèche tectonique située le long d'un niveau de décollement à Tinos (Cyclades, Grèce)

We describe a tectonic breccia which formed during extension at the base of the seismogenic upper crust. This breccia belongs to a large low dipping shear zone active from the ductile to the brittle crust. The main characteristics of this breccia are a high density of fracture planes whose dips are very variable, as well as a strong fluid activity as shown by spectacular talc, carbonate and quartz precipitations. The characteristics of the breccia are in agreement with geophysical data concerning the base of the brittle crust, which suggest the existence of a shallow dipping plane where microseisms and important fluid circulations concentrate.

Cooling history of granulite samples from the ocean–continent transition of the Galicia margin: implications for rifting

Zircon and apatite fission track ages were obtained on two granulite samples that were recovered from the sea floor in the ocean–continent transition area of the Galicia margin (North Atlantic) using the French submersible Nautile. Zircon ages indicate that the rocks cooled through about 250°C in Carboniferous to Early Permian time (307 ± 42 Ma and 287±35 Ma). Hence, the granulites do not represent the prerift lower crust but were in an upper crustal position long before rifting started. Apatites yielded Early Cretaceous ages (126 ± 6.7 Myr and 129 ± 13.4 Myr), indicating cooling through 90 ± 30°C coeval with the main rifting phase that preceded continental breakup. We assume that the granulite samples originate from a tectonic breccia cropping out near one of the sample locations. This breccia formed along a synrift detachment accommodating continental breakup and final exhumation of the Galicia margin’s peridotite ridge.

The Genesis of Mineralising Brines in the South West Massif Central, France

Geological setting. The base-metal, fluorite and barite deposits of the SW Massif Central are hosted by EW-trending vertical fractures that transect Lower Cambrian to Lower Carboniferous nappe sequences in the Rouergue, Albigeois, northwestern Montagne Noire and Monts de Lacaune regions of the Hercynian Massif. To the north, base-metal and fluorite veins outcrop in a Cambro-Ordovician series of pelites, quartzites and metabasites. South of the Albigeois, base-metal veins outcrop in a weakly metamorphosed sequence of Cambrian black shales, sandstones ans volcanoclastics. In the eastern Monts de Lacaune, karst-vein-hosted barite deposits occur in dolomites and limestone of Devonian age and in the adjacent Triassic clastics of the Causses Basin. Mineral paragenesis and microthermometric data. Base-metal fluorite mineralisation in the SW Massif Central is associated with a number of generations of gangue quartz that are distinct in terms of their texture and minerals associations. They may be regional in extent and occur in a variety of different lithologies. Four major stages of quartz deposition have been identified. 1. Quartz I typically takes the form of a silicified tectonic breccia and is known regionally as 'B.T.H.' (br~che tectonique hypersilicifi6e). Sheared fragments of local wall-rock are cemented first by chalcedony and then by successive generations of zoned prismatic quartz giving rise to charasteristic cockscomb textures. B.T.H. may occur independently and is found as prominent EW-trending 'dykes' throughout the region. Where B.T.H. occurs in association with Pb-Zn-F-Ba mineralisation it typically forms the wall-rock to the deposit. Primary fluid inclusions have aqueous two-phase fills with salinities between 20 and 24 wt.% NaC1 equiv, and homogenisation temperatures of 85 to 130~ First melting temperatures indicate that CaC12 is also present (-52~ 2. Quartz la is referred to as lamellar quartz and consists of interlocking, randomly orientated lamellae of quartz. Lamellar quartz may occur in association with silicified breccias though relationship between the two are ambiguous. Lead-Zinc sulphides may fill angular vugs between lamellae. Quartz l a occurs in the Rouergue and Monts de Lacaune regions of the Massif Central but is not present in the Albigeois and NW Montagne Noire. Fluid inclusions are similarly CaCl2-bearing liquidvapour fills where temperature of final ice melting indicate salinities of approximately 26 wt.% NaC1 equiv. Homogenisation temperatures range from 185 to 213~ 3. Quartz 2 occurs in association with sulphide cockade ores in the NW Montagne Noire and Rouergue regions. Local host-rock, silicified breccia and lamellar quartz are brecciated and cemented by siderite and sphalerite. Coarse prismatic quartz fills terminal vugs within the sphalerite matrix. Primary inclusions show aqueous two-phase fills with salinities that vary between 20 and 22 wt.% NaC1 equiv. (including CaCI2) and homogenisation temperatures range of 117 to 166~ 4. Quartz 3 occurs in association with massive blue fluorite veins in the Albigeois, as drusy quartz fills of terminal rugs. Fluorite stage mineralisation, is seen to cross-cut sulphide ores in the deposit of Peyrebrune in the NW Montagne Noire. Fluid inclusions in quartz associated with the fluorite are CaCl2-bearing liquid-vapour inclusions with salinities of 20-26 wt.% NaC1 equiv, and homogenisation temperatures range of 125-170~ Although pressures prevailing during mineralisation are presently unknown, Munoz et al., (1994) find good correlation between homogenisation tempera-

Ultramafic and gabbroic exposures at the Mid-Atlantic Ridge: geological mapping in the 15°N region

The outcrops of mantle-derived ultramafic rocks in the 15°N region are the most extensive yet reported for the Mid-Atlantic Ridge. North of the Fifteen Twenty fracture zone, these outcrops form a belt at least 20 km long along the west wall of the axial valley and also crop out on the east axial valley wall. Ultramafic rocks also crop out extensively south of the Fifteen Twenty fracture zone. Based on dredging and on a morphological analysis of the bathymetric map, we propose that ultramafic outcrops may be common in the crust formed between 14°30′N and 15°50′N during at least the past 2.4 m.y. Moderately dipping fault planes and large expanses of tectonic breccia have been observed during dives on the ultramafic outcrops. Diving observations also show that the ultramafic rocks are capped, in stratigraphic contact, by a thin layer of basalt. This suggests that these rocks were tectonically emplaced at the axial seafloor, or very close to it, then uplifted in the footwall of the faults that bound the axial valley. The occurrence of ultramafic rocks on both walls of the axial valley may be due to frequent changes of faulting polarity in the axial region: instead of one master shear zone, there would be a complex array of cross-cutting conjugate faults and shear zones that could jump inward in the axial domain as spreading proceeds.

The significance of various garnet types in surficial materials in southeastern Gaspé Peninsula, Quebec

The distribution patterns of four types of garnet in surficial materials of southeastern Gaspé Peninsula are interpreted in terms of the glacial history of the area, the lithostratigraphy of the bedrock, and mineral occurrences. Each type is derived from a distinct source, and all have undergone at least some southeastward glacial dispersion. Type 1 garnets, anhedral Ca-garnet fragments, are from the McGerrigle granite metamorphic aureole and were brought into the region by the same ice flow that caused the well-documented southward dispersal of granite debris. Their distribution within the study area depicts the southern part of this important dispersal train with an improved degree of precision. Type 2 garnets, Ca-garnet crystals, seem to be related to skarns within the boundaries of the study area; some patterns cannot be linked to any known source, thus providing challenging exploration opportunities. Type 3 garnets, colored pyralspite garnet crystals, are related to a tectonic breccia within the Maquereau–Mictaw contact zone. Type 4 garnets, Grenville-type anhedral garnets, were not brought into the region by Laurentide ice, as one might suspect, but rather seem to be related to specific horizons within the Silurian Lower Chaleurs Group. On the basis of a palinspastic reconstruction of southern Gaspé Peninsula and a plate tectonic model for the Paleozoic of eastern North America, we propose that these garnets were derived from the erosion of a metamorphic Grenvillian terrane, which presently forms the Northwestern Highlands Zone of Cape Breton Island. The distribution data for this garnet type also lends support to the concept put forth in a previous study whereby a local Late Wisconsinan ice mass penetrated some 5–6 km into the region from the southeast near Saint-Godefroi.

Lithostratigraphical correlation of the Neoproterozoic Roan Supergroup from Shaba (Zaire) and Zambia, in the central African copper-cobalt metallogenic province

New data on the lower Katangan sequences in Shaba (Zaire) and Zambia, collected during the 1989 and 1990 UNESCO-sponsored Geotraverses, reveal an important development on friction breccias throughout the Zambian Copperbelt, which still remains poorly documented, and shows that the Zairean and Zambian facies of the Roan Supergroup can be correlated in detail. As in Zaire, the deformation of Katangan terranes during the Lufilian orogeny produced important friction breccias in Zambia. Such breccias occur mostly between the upper part of the Lower Roan Supergroup and the Mwashya Group (R-4): above the shale with grit (RL3) at Konkola and Mindola, or within the Upper Roan Dolomite at Chambishi South, Muliashi and Nchanga. At Mufulira, a typical fragment of Shaba Mines Group was observed within a major heterogeneous tectonic breccia. This situation is similar to that reported at Kipapila (Kimpe) and Lubembe in Zaire, both located on the same tectonic trend as Mufulira. However, a continuous stratigraphical succession can be observed in Zambia from the basal unconformity to the Mwashya Group. Strong lithological similarities were found, formation by formation, between the Roan sequences of Zambia and Zaire. In particular, the complete Mines Group of Zaire (R-2) and the units from the RL6 to the RL4 in Zambia were deposited under comparable conditions of sedimentation and show a similar and correlatable evolution of lithologies. Furthermore, the overlying Dipeta Group (R-3) of Zaire and the RL3, RU2/RU1 of Zambia, are equally comparable. Above the Upper Roan Dolomite, Lower Mwashya dolomitic rocks, identical with the ones of Shaba, have been noted to occur in Zambia in stratigraphical continuity with the typical black shales of the Upper Mwashya. The correlation between the coarse clastics of the Zambian footwall (RL7) and the red dolomitic argillites and sandstones of the Zairean R.A.T. (Roches Argillo Talqueuses: R-1) remains uncertain. However these two sequences show some similarities suggesting a lateral facies change from high-energy siliciclastic sedimentation in Zambia, to quieter, less clastic and more carbonate rich sedimentation in Zaire. In agreement with the proposed lithostratigraphical correlation, volcanic and pyroclastic rocks, occurring both in Zaire and Zambia in the Lower Mwashya, testify to a major period of igneous activity in the region. Intrusive rocks found in the Zambian Roan Group and in the Zairean Dipeta Group can probably be attributed to the same episode of magmatism. Finally it can be shown that several copper-cobalt orebodies are found at the same lithostratigraphical position in Zambia and Zaire: the Zambian ore shale corresponds to the classical Shaba orebodies at the base of the Mines Group (R-2), the Nchanga upper orebody to the lower R-2.3 mineralization and the Zambian RL3 anomalous copper occurrences to those of the R-3.1.2 Dipeta unit.

Lithostratigraphy of the Neoproterozoic Shaba-type (Zaire) Roan Supergroup and metallogenesis of associated stratiform mineralization

The Neoproterozoic Roan Supergroup of the Shaba (Zaire) metallogenic subprovince was largely affected by Pan-African Lufilian orogenic events, which generated important tectonic breccias. This paper reviews the sedimentological, lithostratigraphical and metallogenic information related to these terranes, with special reference to recent data, mainly from the Kambove district. The Shaba-type Roan sedimentary succession is subdivided from bottom to top into four Groups: R.A.T. (Roches Argilo Talqueuses), Mines, Dipeta and Mwashya. The succession is characterized by continental, tidal, lagoonal and reef lithofacies and contains several transgressive and regressive sequences. Two major cycles are recognized, each starting with continental or sub-continental deposition, followed by carbonated platform-type sediments (first cycle: R.A.T. + Mines; second cycle: Dipeta + Mwashya). These cycles translate as being a general transgression resulting from progressive widening and deepening of a rift-like basin. A major igneous event is clearly identified in the Lower Mwashya Formation (R-4.1), in the form of pyroclastic flows evolving to volcaniclastic strata in the central part of the Shaba arc (as at Shituru/Likasi and Kambove). The major metallic mineralizations of the Roan Supergroup occur as stratiform CuCo deposits, mainly located in the Mines Group (R-2). One occurrence is related to the R-2.1 and R-2.2.1 units (the classic lower and upper orebodies); one lenticular orebody is related to the R-2.3 Formation (Kambove Dolomite). Data from the Kambove district show characteristic vertical and lateral primary sulphide zonations similar to those reported in the Zambian-type deposits. The sulphide paragenesis confirms a ‘syn-diagenetic’ model of mineralization. Significant diagenetic enrichment has resulted from the variable and successive crystallizations of ore minerals, related to both the chemical-biological changes and the residual porosity of the host sediments. The origin of the metals remains speculative. Hypotheses of the leaching of the R.A.T. Group sediments and of the pre-Katangan basement are examined.

Early deformation of ultrahelvetic mélanges in the helvetic nappes (Western Swiss Alps)

The Ultrahelvetic mélanges represent an important clue for understanding the Tertiary Alpine orogeny, particularly in the external Alps of western Switzerland. The remarkable architecture of the Upper and Lower Ultrahelvetic units is classically explained by an important phase of gravity sliding and orogenic sliding accompanied by the formation of chaotic complexes and olistostromes (wildflysch), a phenomenon called ‘diverticulation’. A structural analysis reveals evidence for early phases of deformation, despite strong overprinting by late Alpine deformation. These early fabrics are observed in foliated pebbly-mudstone (a tectonic breccia, part of the mélange matrix), in disrupted flysch sequences and in dark shales. They attest to brecciation related to a complex pattern of veining, with several generations of calcite cement and several stages of failure during lithification, and they show the important role of fluids during the deformation. They present numerous analogies with the fabrics described in association with thrusts in accretionary prisms. Considering the tectonic fabrics and the relations among the infra-Prealpine mélanges, a tectonic model to explain the emplacement of the Ultrahelvetic units is proposed in which the décollements developed in three weak levels. The early fabrics evolved within a basal thrust formed in a final stage of the Valaisan subduction.

Late Diagenetic Dolomitization of Lower Ordovician, Upper Knox Carbonates: A Record of the Hydrodynamic Evolution of the Southern Appalachian Basin

Late diagenetic dolomitization of the Lower Ordovician, Upper Knox Group in the southern Appalachian basin was closely associated with widespread secondary porosity development, hydrocarbon migration, and local Mississippi Valley-type mineralization. Regionally extensive (^sim70,000 km2), late diagenetic dolomites consist of replacement dolomites and zoned dolomite cements. Late diagenetic replacement dolomites comprise 15 to 50% of all Knox matrix dolomites. The ^dgr18O (-11.9 to -5.3^pmil), ^dgr13C (-3.8 to +0.9^pmil), and 87Sr/86Sr (0.70895 to 0.70918) values of late diagenetic replacement dolomites overlap with those of the first zone of dolomite cements (zone 2), early replacement dolomites, and Lower Ordovician arine calcites, reflecting rock buffering of initial dolomitizing fluids and extensive neomorphism of replacement dolomites by subsequent late diagenetic fluids. Nonporous to sucrosic, late diagenetic dolomites have porosities (1 to 16%) and permeabilities (0 to 1030 md) significantly greater than those of early diagenetic replacement dolomites and host limestones ( 165°C), saline (13 to 22 wt.% NaCl equivalent) basinal brines that underwent extensive fluid-rock interaction with clastics. Precipitation temperatures of late diagenetic dolomites estimated from fluid inclusion homogenization temperatures and systematic trends in ^dgr18O va ues record a regionally developed, prograde-to-retrograde thermal history. Knox late diagenetic dolomites are interpreted to record the spatial and temporal evolution of large-scale fluid flow systems that developed in response to different burial and tectonic stages of the southern Appalachian basin. The occurrence of zoned dolomite cements in tectonic fractures and breccias, and their close association with noncarbonate diagenetic minerals of Pennsylvanian to Early Permian ages, suggest that most Knox late diagenetic dolomites record deep subsurface (2 to >5 km) fluid migration in response to late Paleozoic Alleghenian tectonism (330 to 265 Ma). Late diagenetic matrix dolomites served as long-lived conduits that focused and channeled diagenetic fluids in the deep subsurface. The occurrence of bitumen in secondary porosity within late diagenetic dolomite indicates that they likely were the most viable reservoirs during hydrocarbon migration in the late Paleozoic.

Geology and geochemistry of a 800 m section through young upper oceanic crust in the North Fiji Basin (Southwest Pacific)

We report the results of geological and structural observations made during one dive of the French submersible Nautile during the starmer Cruise in 1989 in the vicinity of the 16°40'S triple junction in the North Fiji Basin. This dive provided a spectacular, 800 m continuous section of oceanic crust exposed along the northern wall of a 3000 m deep basin at the eastern branch of the triple junction. A number of volcanic facies have been encountered including massive and pillowed lavas, sheeted dikes and spectacular columnar-jointed massive lavas. Pillowed lavas have been observed only at the base and at the top of the section. The ratio between massive and pillowed basalts is high suggesting that the crust in this area grew mostly during a stage of high eruptive rate. Dikes have been observed in the middle part of the section. They probably do not represent the top of a classical “dike complex” but an isolated set of sheeted dikes, possibly the feeders of overlying flows. The abundance of vertical tectonic breccias observed during the first part of the dive confirms that the basal part of the wall can be regarded as a major tectonic boundary along which occurred significant vertical and strike-slip motions. Micropaleontological data from sedimentary rocks collected during the dive bring new constraints to the evolution of the triple junction. The oceanic crust in the surveyed area is at least as old as 1.9-1.3 Ma, based on the age of sedimentary rocks collected in talus at the base of the wall and observed at the summit of the section. The collected basalts include (1) N-type MORBs, (2) E-type MORBs and (3) BABBs. This emphasizes the heterogeneous nature of the mantle beneath the central NFB. Some portions of the depleted mantle underlying the central NFB triple junction area have recorded contamination from melts or fluids from an ancient subduction zone (probably the New Hebrides during the opening of the NFB), whereas adjacent mantle areas show the influence from an alkali-enriched source. Such alkali-enriched characters are reported from recent basalts of the northern NFB (South-Pandora ridge-Rotuma island).

The influence of orientation, temperature and frequency on fatigue crack growth: Henderson, M. B. and Martin, J. W. Proc. Conf. Superalloys 1992, Champion, Pennsylvannia,USA, 20–24 Sept. 1992 (The Minerals, Metals & Materials Society, 420 Commonwealth Dr, Warrendale, PA 15086, USA) 1992 pp 707–716

Volcanic-associated nickel deposits in the Archaean greenstone belts of Western Australia show a variety of features related to post-depositional deformation and medium- to high-grade regional metamorphism. Deformation and accompanying remobilization have resulted in changes to the gross structure, shape and orientation of ore shoots, and have led to the development of separate offset ores, tectonic breccia ores, and irregular sulphide stringers. Physical remobilization of ore has been mainly on a mesoscopic scale (up to 150 m) and largely by mechanical processes, but with some fluid-assisted and stress-induced chemical remobilization of sulphur and possibly copper. Most of the present ore fabrics and textures are the result of combined deformation, metamorphically induced phase changes and recrystallization. Massive and matrix ores show well-developed tectonite fabrics defined by mineralogical layering, preferred orientation of pyrrhotite and alignment of silicate and spinel inclusions. Layering in the ores is probably the result of both strain-controlled exsolution from high-temperature phases and mechanical segregation of different minerals within polyminerallic aggregates during deformation. Observational and experimental data suggest that sulphide deformation was dominantly by dislocation-type mechanisms (kinking and twinning) at medium temperatures (300–550°C), with diffusional mechanisms becoming important at high temperature (500°C). Dynamic recrystallization during deformation probably accounts for the annealed state of individual mineral grains within most ores, except those deformed at a late stage.

Late Jurassic Ductile Shearing and Tectonic Breccias in the Eastern Great Basin of Nevada and Utah

The Paleozoic sedimentary prism was dismembered in the Late Jurassic by shearing initiated at stratigraphic positions in the following list: (1) base of Meade Peak Shale, (2) base of Summit Springs Evaporites, (3) top of contact Chert Member of the Rib Hill Formation, (4) base of Unit G of the Ely Formation, (5) base of the Ely Limestone, (6) base of the Diamond Peak Sandstone, (7) base of Chainman Shale, (8) base of the Joana Limestone, (9) base of the Pilot Formation, (10) base of Lower Guilmette Limestone, (11) in siltstone at the top of Ely Springs (Hanson Creek) Formation, (12) top and base of the Eureka Quartzite, (13) base of the Dunderberg Shale, and (14) base of Pioche Shale and top of Prospect Mountain Quartzite. The largest are regional decollectments, e.g., Summit Springs, Chainman, Blue Eagle, Silver Island, and Snake Range. The lesser shears vary in position and extent with lithofacies distribution. Shales at all levels behaved ductilely. Carbonates locally behaved ductilely up to the base of the Pilot Formation, but more commonly below the Dunderberg Shale. Brittle deformation in the carbonates produced megatectonic breccias below the shears. These regional breccias became reservoirs for the pore fluids and clay-heldmore » fluids expelled from primary source rocks and reservoirs by the dynamic pressures. Igneous plutons injected from 150-30 Ma crosscut and dilated the shear zones, creating hydrothermal systems within the tectonic reservoirs.« less

Phosphorites of Rocinha Mine; Patos de Minas (Minas Gerais, Brazil); genesis and evolution of a middle Proterozoic deposit tectonized by the Brasiliano Orogeny

In central Brazil (the states of Minas Gerais and Goias) the Brasiliano orogeny ([asymp]600 Ma) delineates the Sao Francisco craton and, to the west, the Brasilia fold belt. Rocks of the fold belt are thrust to the east and display increasing deformation and metamorphism westward.Almost all the phosphate deposits are found in the Brasilia fold belt, either along its boundary with the craton or in external structural units showing polycyclic deformation and greenschist metamorphism. They occur in the metasedimentary rocks of the middle Proterozoic Paranoa Group and in the upper Proterozoic Sao Francisco Group and its equivalents. Middle Proterozoic phosphorites are economically the more important. Most are spatially and temporally associated with a stromatolite reef barrier. However, the Rocinha-Patos de Minas deposit, the only presently mined, is located south of the reef barrier. It has developed on a shallow to deep ramp between a wide internal basin with reduced oxygen conditions and limited detritus and a more oxygenated marine environment to the west.In the Rocinha mine, phosphate forms grains, angular fragments, lenses, or centimeterthick beds in schists and carbonate or glauconitic schists. The P 2 O 5 content of phosphatic rocks ranges between 10 and 15 percent. Annual production is around 200,000 tons of ore containing 24 percent P 2 O 5 . The mine area comprises four tectonic slices made up of schists, quartzoschists, and phosphatic schists, separated by subhorizontal shear planes. These are defined by tectonic breccias in which secondary apatite has recrystallized. The mine is exposed in a 50- to 75-m phosphatic sequence which is repeated in a lower slice about 100 m below mine level.The phosphatic ore is a homogeneous crypto- to microcrystalline apatite in fairly continuous centimeter-thick beds which were stretched, fragmented, and boudinaged during Brasiliano isoclinal folding and subsequent subhorizontal shearing.X-ray diffraction, IR spectrometry, and chemical analysis indicate that the metasedimentary phosphate is a weakly carbonated fiuorapatite with low F and CO 2 contents. The influence of the Brasiliano orogenesis is marked by partial substitution of F by OH. Secondary prismatic apatite, kaolinite coatings, and wavellite sheaflike structures are products of weathering and are found in mine sections and bore holes to a depth of 10 to 15 m. Secondary apatites are enriched in CO 2 compared to primary metasedimentary apatites.

Volcanic activity and crust-mantle exposure in the ultrafast Garrett transform fault near 13°28′S in the Pacific

The 130 km long Garrett transform fault, which offsets two ultrafast (14.5 cm/yr) EPR spreading segments, is the site of volcanic and tectonic activity. Evidence for recent volcanic events was observed during several dives on oblique ridges (3100–3500 m high) trending 040° (Alpha, Beta and Gamma), and on small constructional mounds located in the central transform basin and eastern valley at a depth of 3500–5000 m. The constructional features show a fissured terrain, suggesting a component of lithospheric opening. The strike-slip motion of the transform is inferred at several locations between the two EPR-transform intersection zones, from the presence of abundant sediment-free talus with crushed rock debris, fresh scarps, and well-developed striations on outcrops of serpentinized peridotites and metamorphosed volcanic breccia. The width of the active tectonic zone varies between 200 and 3000 m in the explored area near the eastern and western EPR-transform intersection zones, along the E-W trending Median Ridge walls in the eastern and western transform troughs of the central basin and at several sites on the south-facing wall of the transform domain first escarpment. The presence of recent volcanic activity implies that the Garrett transform behaves as a leaky transform fault. The most common volcanics, consisting of plagioclase and olivine phyric lava erupted in the transform, are compositionally more depleted [ K/Ti< 0.08 ] and less evolved (Mg#60–70) than the basalts encountered on the southern EPR segments [Mg#40–66, and K/Ti= 0.05–0.25 ]. This suggests that different magmatic conditions are operating in the transform domain. The transform strike-slip motion is responsible for the emplacement of a sliver of serpentinized peridotite located in an area about 2 km wide and 10 km long along the Median Ridge at the contact with outcrops of metamorphosed basaltic breccia. The ultramafics consist of residual harzburgites, often impregnated with irregular lenses and dykes of basaltic liquids (plagioclase-wehrlite and gabbro), and are found associated with troctolites, dunite and gabbros. The emplacement of the ultramafics and their gabbroic inclusions has resulted in the presence of metamorphosed tectonic breccia forming part of the Median Ridge in the central transform basin.

Regional framework and tectonics of the Yarlung Zangbo suture in southern Tibet

A synthesis of regional framework and tectonic characteristics of the Yarlung-Zangbo Suture in Southern Tibet has been attempted. The Yarlung-Zangbo Suture lies between the Tethys Himalayan zone to the south and the Lhasa Terrane to the north. In the northern part of the Tethys Himalaya, a belt of Neogene granites and older (Cambrian and Pre-Cambrian) orthogneiss is located. The Xigaze ophiolite, the ophiolite tectonic breccia and the Xigaze Group are the principal tectono-stratigraphic units of the Yarlung-Zangbo Suture. The Xigaze Group represents the remanant of the Neotethyan oceanic crust of Jurassic-Cretaceous age, deposited in a fore-arc basin. Magmatism of the Gangdise plutonic-volcanic arc began around 90 Ma and paroxysmal activity took place between 50–60 Ma. The magmatic arc owes its origin to northward Tethyan subduction beneath the Lhasa block. In Lhasa Terrane, the Precambrian Nyainqentanglha group crystallines are succeeded by Carboniferous to Cretaceous sequence of sedimentaries and volcanics. The Yarlung-Zangbo Suture contains crustal fragments of the Neotethys ocean that originated through rifting in Triassic and opening of the ocean basin in the Jurassic-Cretaceous times. The suture was closed as a result of the collision between the Indian continent and the Lhasa block at 45 Ma.

The Lapstone structural complex, New South Wales

The Lapstone Structural Complex forms the most prominent topographic feature in the Sydney region. The complex consists of a number of related folds and faults, trending generally north‐south, which together form a large south‐plunging structure between Kurrajong Heights and Lapstone. The east‐facing escarpment of the Blue Mountains, formerly called the Lapstone Monocline, varies in its character, being sometimes a single monocline, sometimes a double monocline and sometimes a normal or high‐angle reverse fault. Faulting west of Kurrajong and at Glenbrook is part of a series of overlapping en echelon faults, west‐side down, and sometimes overturned, rather than a single fault. This fault system forms the west side of the complex. Significant minor structures associated with major features include thrusts, minor folds, joint systems, tectonic breccias, sedimentary injections and igneous dykes. Many of the minor structures show a marked parallelism with the major structures. The main period of deformation forming the complex is believed to have taken place in the Early Tertiary, but the overall structure has a long and complex history. Field evidence suggests that sinistral strike‐slip faulting played a part in the deformation, particularly of the near‐surface rocks. Basement block faulting was also significant, producing the Cumberland Basin and associated structures when the main Lapstone structures were formed. Basement structural control is believed to consist of the northerly extension of the western edge of the Eden‐Comerong‐Yalwal Rift intersected by elements of the east‐trending Lachlan Lineament.

Hydrothermal dolomitization of Jurassic-Cretaceous limestones in the southern Alps (Italy): Relation to tectonics and volcanism

Dolomitization has affected up to 750m of the Jurassic and Cretaceous pelagic carbonate sequence of the southern continental margin of the Alpine Tethys; the sequence crops out in the southern Alps of Italy (Monti Lessini). Late Paleocene to Miocene extrusion of basaltic tuffs, breccias, and lavas was contemporaneous with the dolomitization was was associated with extensive tectonism in an ancient back-arc basin. More than 200 samples were analyzed by X-ray diffraction, cathodoluminescence, scanning electron microscopy, stable isotope ratios (carbon, oxygen, strontium), and clay mineralogy. The dolomite contains 40% to 50% MgCO{sub 3}. In thin sections, the crystal size distribution is unimodal (about 100 {mu}m), possibly indicating a single nucleation for the main crystallization phase. The {delta}{sup 13}C of the dolomite is not appreciably different from the undolomitized pelagic limestone (+1.0{per thousand} to +2.0{per thousand} Peedee belemnite (PDB)). The {delta}{sup 18}O variation (-5.0{per thousand} to -13.0{per thousand} PDB) is due to temperature variation in the system. The {sup 87}Sr/{sup 86}Sr ratio in the dolomite (0.70839-0.70867) is consistent with the ratio in late Oligocene-Miocene marine water. Clay minerals in limestone and dolomite differ in the presence of neoformed Mg-chlorite, indicating a maximum temperature of about 150C for dolomitization. The dolomite is suggested more » to have a hydrothermal origin. The heat flow associated with the volcanism allowed marine water to penetrate the system and circulate in convective cells through the tectonic breccias, locally dolomitizing the limestone. « less

Hydrothermal brecciation in the Jemez fault zone, Valles Caldera, New Mexico: Results from continental Scientific Drilling Program core hole VC‐1

An unusual breccia sequence penetrated in the lower 30 m of Continental Scientific Drilling Program core hole VC‐1 (total depth 856 m) records a complex hydrothermal history culminating in hydraulic rock rupture and associated alteration at the edge of the Quaternary Valles caldera. The breccias, both tectonic and hydrothermal in origin, were formed in the Jemez fault zone, near the intersection of this major regional structure with the caldera's ring‐fracture margin. Tectonic breccias in the sequence are contorted, crushed, and sheared. Coexisting hydrothermal breccias lack such frictional textures but display matrix flow foliation and prominent clast rounding, features characteristic of fluidization. These hydrothermal breccias were intensely altered, during at least five major stages, to quartz‐illite‐phengite‐pyrite aggregates; traces of molybdenite occur locally. This assemblage indicates interaction with hydrothermal fluid at temperatures in excess of 200°C. The extrapolated present maximum temperature of 184°C in the breccia zone therefore represents considerable cooling since these phases were formed. Fluid inclusions in the breccias also preserve evidence of the prior passage of hotter fluids. The inclusions are principally two phase, liquid rich, secondary in origin, and concentrated in hydrothermal quartz. Older, high‐salinity inclusions, unrelated to brecciation, homogenize in the temperature range 189°–246°C. Younger inclusions, in part of interbreccia origin, are low‐salinity and homogenize in the temperature range 230°–283°C; locally coexisting liquid‐ and vapor‐rich inclusions document periodic boiling of the dilute fluids. These fluid‐inclusion data, along with the probable age of the hydrothermal breccias (&lt;1.5 Ma), the assumed depth at which they developed (about 515 m), and the contemporaneous state of stress (extensional) can be combined to model hydrothermal brecciation at the VC‐1 site. The minimum fluid pressure (Pfr) required to hydrofracture these rocks was probably about 7.5 MPa (0.0146 MPa/m). A boiling point versus depth curve based on these Pfr values graphically defines the physical conditions prevailing when the breccias were formed. When fluid pressure at the assumed depth of brecciation exceeded that curve, in response to rapid release of confining pressure possibly augmented by renewed heating, flashing hydrothermal fluid fractured the enclosing rock. Large overpressures, most likely induced by sudden seismic cracking of a hydrothermally sealed portion of the Jemez fault zone, led to local fluidization of the resulting breccias. Late quartz veining, hydrothermal alteration, and molybdenite mineralization were probably produced by the fluids responsible for brecciation.