Tectonic Breccia Research Articles

The Brevard Zone in South Carolina and Adjacent Areas: An Alleghanian Orogen-Scale Dextral Shear Zone Reactivated as a Thrust Fault

The displacement history of the Brevard zone, a linear, orogen-scale structure in the metamorphic core of the southern Appalachians, has been controversial. Geological and seismic data indicate that the Brevard zone and the Taconic/Acadian metamorphic rocks in which it resides are contained in a huge crystalline thrust sheet that overlies relatively unmetamorphosed platform sedimentary rocks along a major Alleghanian thrust fault. Along a 100 km segment in South Carolina and adjacent states, the Brevard zone contains three separate structural features, all of which dip moderately SE: (1) the narrow Rosman fault, which contain tectonic breccia and slices of the buried platform sedimentary rocks; (2) the Brevard mylonite zone, which contains mylonitic rocks that extend about 3 km on either side of the Rosman fault and exhibits shear sense criteria which consistently indicate dextral strike-slip; (3) the Brevard fault zone, SE of the Rosman fault, which contains stratified metamorphic rocks that are repeated...

Deformation and remobilization in some Western Australian nickel ores

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.

Structurally incorporated and water extractable chlorine in the Boettstein granite (N. Switzerland)

Samples from fresh to strongly hydrothermally altered granite have been analysed for water-, HNO3-extractable and total chlorine. From the total of 115 ppm Cl, 60 ppm are contained in fluid inclusions, 1–2 ppm in apatite and 50 ppm in biotite. It is obvious that the high salinity (1,330 mg/l) of the deep ground water in this granite can be obtained by extraction from a fraction of chlorine originating from broken inclusions in tectonic breccias in fault zones.

Cuspate ore-wall rock interfaces, piercement structures and the localization of some sulfide ores in deformed sulfide deposits

Studies on three deformed massive sulfide deposits (Trojan nickel deposit, Zimbabwe; Matchless copper deposit, Namibia (South-West Africa); Broken Hill silver-lead-zinc deposit, Australia) show that the interfaces between massive sulfide rock and silicate wall rock are characterized by irregularities and projections of sulfide rock into the wall rock. The structures range from simple cusps, in which ore layering remains parallel to the ore-wall rock interface and to wall-rock layering, to veins which occupy brittle fractures in the wall rocks. In size, the features range from minor projections less than 1 m across to major piercement structures which may exceed 50 m in length. The structures can usually be related to specific phases of rock deformation. In most cases, the contained sulfide rock is a tectonic breccia with a foliation parallel to margins of the structure.The cusp and related structures reflect competency contrast during rock deformation. Simple cusps develop in a similar manner to certain types of mullions. However, where the competent wall rock is relatively brittle, ductile sulfides take advantage of brittle fractures and other zones of weakness to penetrate into the wall rock and develop discordant vein structures. The mechanism by which the sulfides move is plastic flow, although there is some evidence for the activity of a fluid phase.The process has economic significance in the generation of locally thickened parts of ore-bodies.

An intrusive-like tectonic breccia in the archipelago of southwestern Finland

EDELMAN, N.; BERGMAN, L. and LINDBERG, B., 1986: An intrusive-like tectonic breccia in the archipelago of southwestern Finland. Bull. Geol. Soc. Finland 58, Part 1, 235—240. On an islet in the archipelago of southwestern Finland with metamorphic rocks belonging to the Svecofennian zone it is possible to follow in exposure for a distance of some metres the transition from a layered paragneiss with amphibolite bands to a breccia with amphibolitic fragments in a strongly recrystallized gneiss with tonalitic composition. The ultrametamorphic parts of the gneiss lack schistosity. The absence of microcline in these shows that no metasomatic granitization has taken place. The occurrence shows an initial stage of the process which has developed the banded series, a strongly deformed gneiss-amphibolite mixture which forms a zone, about 170 kilometres long, through the archipelago.

Breccias related to explosive volcanism

Explosive breccia pipes were formed in the phreatomagmatic eruption of Taal volcano 60 km south of Manila, Philippines in September 1976. They were photographed in eruption which consisted of a series of small explosions occurring during the peak of activity at 10 sec intervals. These four breccia pipes formed on the collapse fault marking the margin of the much more intense eruption of 28 September 1965 when a diatreme 800 m in diameter formed. Renewed eruption in October 1977 on one of the breccia pipes blasted out the sealed plug and breccia fragments in all stages of alteration were included in the tephra. They ranged from barely agglomerated to strongly cemented through argillized and pyritized to totally replaced by silica and pyrite, fragments indistinguishable from those found in many mining districts. The violence of the explosion initiating a phreatomagmatic eruptive sequence results in strong crackling of the walls of the pipe and the crakle zone may become mineralized (cf. Braden, Chile). Usually the crater collapses on ring faults after eruption, forming a marginal tectonic breccia which can be mineralized (cf. Balatoc, Philippines.) The small craters with steep inner walls and tapering outer slopes which form during phreatomagmatic eruptions, sometimes containing lakes, are called maars and they are the surficial expression of an explosive breccia pipe or diatreme. Superficially similar craters are formed over kimberlite pipes. Hydrothermal explosion craters are somewhat similar. Collapse breccia pipes can form in the same sequence with explosive pipes. When one rising plug encounters ground water and explodes, fluostatic pressure on any other cupolas rising from the same magma drops rapidly, resulting in withdrawal of magma and collapse of walls and roof (Perry 1961). “Fluidization” is not thought to be of any significance in formation of breccia pipes (Wolfe 1980) contrary to Reynolds (1954) and many authors who have cited that work. An explosive breccia pipe is like an open window resulting in rapid depression of the isotherms of a pluton. A concentrated brine front can build up in minutes after an explosion. The very rapid cooling may explain why many explosive breccia pipes are altered by silica and pyrite only, the system being cooled before metallic solutions have time to replace breccia matrix or fragments.

Mineralized veins and breccias of the Cripple Creek District, Colorado

The Cripple Creek district has yielded nearly 21 million troy ounces of gold since its discovery in 1891. The orebodies occur as narrow veins within Precambrian and Tertiary rocks and as bulk tonnage deposits within tectonic and hydrothermal breccias.The district is localized within and adjacent to a 27.9- to 29.3 + or - 0.7-m.y.-old nested diatreme-intrusive complex. Two magmas, phonolitic and alkali basaltic in composition, generated volcanic flows, subvolcanic intrusions, and phreatomagmatic breccias. Magma mixing is suggested by intermediate composition latite-phonolite and syenite. Subsidence of the diatreme complex rocks is indicated by (1) a thick fluvial-lacustrine sedimentary sequence in the eastern subbasin, (2) the presence of carbonaceous debris, ripple-laminated rocks, and dessication cracks in sedimentary rocks at depths more than 300 m below the present surface, (3) by the fracture systems near the diatreme subbasin margins that reflect basement rock influence, and (4) by flat-dipping veins near intrusive bodies or small breccia bodies (e.g., the Cresson diatreme).The vein deposits as exemplified by those of the Ajax mine cut Precambrian crystalline rocks and Tertiary rocks of the diatreme complex. Within the Precambrian rocks the veins are radial to the margins of the diatreme system and are sheeted zones with rock dissolution and open-space fillings. Where the veins cut the Cripple Creek breccia, they are an irregular anastomosing fracture zone. The major veins exhibit remarkable vertical continuity, extending to more than 1,000 m below the present surface. Vein-related hydrothermal alteration occurs in a narrow selvage that extends outward no more than five times the vein width. Secondary K-feldspar, dolomite, roscoelite, and pyrite occur within an inner zone adjacent to the veins, whereas an outer zone contains sericite, montmorillonite, magnetite, minor secondary K-feldspar, and pyrite. There is no expansion of the alteration zones in the upper level mine exposures.Five stages of minerals are recognized in the Ajax mine veins: (1) quartz-fiuorite-adularia-pyrite-(dolomite-marcasite), (2) base metals-quartz-pyrite, (3) quartz-fluorite-pyrite-hematite-rutile, (4) quartz-pyrite-rutile-calaverite-acanthite, and (5) quartz-fluorite-dolomite. The proportions of each stage vary within and between veins, but the ore mineralogy is consistent throughout the vertical extent of the developed vein systems. Horizontally, gold values ranged between 0.5 and 1.0 oz Au per short ton.Fluid inclusion analyses have documented the presence of early stage 1 saline fluids (33- >40 equiv. wt % NaCl) with the higher salinities found in the upper 300 m of the Ajax mine levels; the fluids were boiling and contained CO 2 . Stage 2 and 3 fluid inclusions exhibit progressively lower homogenization temperatures, and salinities are markedly lower (0-8.3 equiv. wt % NaCl). The telluride ore was deposited from weakly boiling, dilute fluids (1.4-3.5 equiv. wt % NaCl) with temperatures below 150 degrees C.The bulk tonnage deposits, as exemplified by the Globe Hill area, consist of mineralized tectonic and hydrothermal breccias cutting pyroxene-bearing alkali trachyte. Four structural events occurred at Globe Hill: (I) emplacement of hydrothermal breccia bodies along a north-west-trending 1,800- by 700-m zone; (II) intersecting tectonic adjustments along steep variable-strike zones on the western margin of the stage 1 breccias; (III) intrusive breccia emplacement at the major stage II fault intersection; and (IV) hydrothermal brecciation centered to the immediate east of the Globe Hill pit and characterized by a matrix consisting of anhydrite, carbonate, fluorite, pyrite, and base metal sulfides.Two hydrothermal events generated gold-silver mineralization and associated wall-rock alteration in the bulk tonnage deposits. The precious and base metals occur with alteration products in breccia clasts or in matrix minerals within the hydrothermal and tectonic breccias. The fluids responsible for alteration-mineralization were boiling as indicated by wide ranges of filling temperatures in fluid inclusions of the same mineral grain, extensive development of explosion texture in quartz and celestite, and large variations of liquid/vapor ratios in fluid inclusions within individual crystal growth zones. Temperatures were below 200 degrees C as indicated by minimum filling values. Capping of boiling shallow hydrothermal fluids appears to have been enhanced by the alkali trachyte porphyry intrusion at Globe Hill, which acted as a permeability barrier to upward-migrating fluids. Vapor-dominated fluids developed over-pressuring, leading to hydrothermal brecciation and low-grade gold deposits. On the other hand, the vein systems in the Cripple Creek district formed along structures open to the surface; hence, hydrothermal brecciation did not occur.

Ore-related breccias in volcanoplutonic arcs

An overview of breccias related to a variety of base metal, precious metal, and lithophile element deposits in volcanoplutonic arcs permits definition of six possible mechanisms for subsurface brecciation.1. Release of magmatic-hydrothermal fluids from high-level hydrous magma chambers during second boiling and subsequent decompression generates a spectrum of breccia types in which fragments may suffer collapse and/or ascent. Single or multiple intrusion-related breccia pipes and pre- and intermineral breccias in porphyry copper deposits provide widespread examples.2. Magmatic heating and expansion of meteoric pore fluids may lead to brecciation, commonly of late or postmineral age and including pebble dikes, in porphyry-type and related deposits. Magmatic heating of rocks saturated with seawater may cause submarine hydrothermal eruptions late in the emplacement histories of Kuroko-type massive sulfide deposits; many of the resultant breccias underwent limited sedimentary transport. Overpressuring of heated fluids beneath semipermeable, partly self-sealed cap rocks may lead to brecciation and subaerial hydrothermal eruptions in shallow epithermal precious metal settings; magmatic heating or tectonic disturbance may have triggered brecciation.3. Interaction of cool ground waters with subsurface magma can generate phreatomagmatic explosions. Postmineral phreatomagmatic diatremes associated with porphyry systems and premineral diatremes with epithermal precious (+ or - base) metal deposits were generated in this manner; these attained the palcosurface to produce pyroclastic base surge and fall deposits that accumulated as tuff rings around maar craters.4. Magmatic-hydrothermal brecciation may lead to disruption of rocks through to the paleosurface, decompression, and fragmentation and eruption of the top part of an underlying magma chamber. Pre- and postmineral magmatic diatremes of this sort are inferred to accompany a few porphyry-type and other base and precious metal systems; they were manifested at the palcosurface by accumulations of pyroclastic fall and flow deposits.5. Breccias may result from mechanical disruption of wall rocks during subsurface movement of magma. Any intrusion-related deposit may include such intrusion breccias.6. Tectonic breccias resulting from fault displacement may accompany any type of ore deposit.A continuum exists between many of these breccia types and it is difficult to identify unique criteria for their unambiguous distinction.

Gravity-Slide Thrusting and Folded Faults in Western Arbuckle Mountains, Oklahoma: ABSTRACT

End_Page 1318------------------------------One or more major gravity-slide thrusts have been documented in the Eola, Southeast Hoover, and Southwest Davis oil fields, and in the western Arbuckle Mountains, Garvin and Murray Counties, Oklahoma. The gravity-slide area initially covered portions of at least nine townships; it was more than 30 mi (50 km) long and 5-6 mi (8-10 km) wide. It involved a stratigraphic sequence greater than 5,000 ft (1,500 m), extending from the lower Springer Formation into the upper portion of the Arbuckle Limestone. The major slides moved to the northeast and northwest, probably in the Middle Pennsylvanian. Slides and faults were subsequently isoclinally folded in the Late Pennsylvanian. The tensional updip segment of the major folded slide fault now coincides with the trace of the Washita Valley fau t. The compressional end of the slide coincides with the Reagan fault in the east and the frontal Eola fault in the west. In the Lake Classen area the latest folding has turned all formations involved in the slide--and the associated faults--to a near-vertical position. Thus, the slide is exposed in a profile view on the south limb of the overturned Washita Valley syncline. On the north normal limb of the Washita Valley syncline, the slide is exposed in plan view, with the Dougherty anticline and related folds representing compressional folding at the toe of the slide. Several tectonic breccias near the top of the Kindblade Formation of the Arbuckle Group probably mark the orogenic event. End_of_Article - Last_Page 1319------------

Geochemical characteristics of sedimentation and metamorphism of the Dongchuan copper deposit, Yunnan Province, China

The Dongchuan copper deposit lies in the Kunyang Group of the Late Precambrian Sinian System in Yunnan Province. The deposit is stratiform and is restricted to transitional beds of argillaceous and arenaceous dolomites which lie between purple beds of slate and dolomite below and red dolomite above, and to algal beds at the base of the red dolomites. The copper ore minerals are bornite and chalcopyrite with minor chalcocite, covellite and enargite. The major stratiform sulphide minerals show a vertical zonation. From the top downwards they appear in sequence chalcocite, bornite, chalcopyrite and pyrite. Silver, germanium and gold occur in useful quantities. The gangue minerals include dolomite, calcite, quartz, and minor pyrite, chlorite and sericite. Most of the Sinian sedimentary rocks in the area are enriched in copper to levels well above the Clark value, particularly the barren transitional beds which contain six times the Clark value. Germanium has been detected in the transitional beds, but is only enriched in chalcopyrite and enargite veins within the hanging wall dolomite. Silver is concentrated in copper minerals, and there is a positive correlation between copper and silver contents. The transitional beds have much lower Fe 3+/Fe 2+ ratios than the adjacent beds, indicating a relatively reducing environment in the transitional beds. Nearby Precambrian mafic intrusive rocks might be considered to be a source of copper for the ore deposit. Bi, Sb, Be, Ag, Ge and Mo all occur in significant quantities in the ore, but not in the igneous rocks. Conversely the Ni, V and Ti are enriched in the igneous rocks, but not in the ore. This suggests that these mafic rocks were not the source of copper. Sulphur isotope studies suggest that the sulphur in the copper sulphides was derived mainly by the biological reduction of marine sulphate. Although most of the copper appears to be of sedimentary origin, there was significant remobilisation during subsequent folding and metamorphism. Veins of quartz, calcite, dolomite and sulphide formed in tensional fractures. Copper occurs in all these veins, but the highest contents are in the sulphide veins which are up to ten times as rich as the stratiform deposit. The relative depletion of the ore beds adjacent to copper-rich veins suggest that the metamorphic transport of copper was on a local scale, for distances varying from several cm to more than 200 m. The mineralised veins occur mainly in the hanging wall and less commonly in the footwall or in tectonic breccia zones. There are two types of mineralised vein: chalcocite and haematite; and enargite with minor chalcopyrite and bornite. The enrichment of germanium in the enargite veins is also a result of metamorphism. In the veins silver is concentrated in bornite and chalcopyrite, whereas gold is concentrated in enargite. The Dongchuan copper deposit is considered to be of sedimentary—metamorphic origin. Copper was first precipitated during the deposition of marginal marine sedimentary rocks and was later mobilised and locally reconcentrated during metamorphism.

Sur l'origine des faciès de cargneules et de brèches rencontrées dans une coupe du Trias de la Vésubie, Alpes-Maritimes, France

A logged section of mostly Mushelkalk limestones (Middle Trias) in the Vesubie Valley is described. Cargneule a yellow-brown dedo-lomite, is superimposed on Triassic and possible tectonic breccias. In one locality cargneule appears to have formed along recent tree roots. Comparison with cargneules of the Alpine internal zones suggests that many external zone cargneules also formed by dédolomitisation as a product of recent weathering of gypsum and dolomite.

Fracturing and Brecciation Along the Max Meadows Thrust, Southwestern Virginia: ABSTRACT

Fracturing is an important mechanism of porosity development in deformed hydrocarbon provinces such as the Eastern Overthrust belt, but the sizes and shapes of fractured zones place critical constraints on exploration strategies. Fracturing and brecciation associated with the Max Meadows thrust, along which the Cambrian Rome Formation have been emplaced atop the younger Cambrian Elbrook and Conococheague Formations of the Pulaski thrust sheet, are controlled by lithology, proximity to the fault, and mesoscopic folding. Within the Max Meadows sheet, Rome carbonates are highly fractured and, in fold cores near the fault, brecciated. Rome mudstones and sandstones are tightly folded, and near the fault have developed both an incipient axial planar cleavage and a set of closel spaced fractures striking perpendicular to fold axes. In comparison, the wholly carbonate sequence of the Pulaski sheet had earlier been folded into a large syncline characterized by bedding-parallel shear in shaly and thin-bedded layers, flexural slip folding, and localized fracturing of thick layers. Thus breccia and fracture porosity zones in the study area are highly localized, of irregular geometry, and essentially restricted to the upper thrust sheet. Zones of tectonic breccia and fracture porosity are not attractive exploration targets, then, unless they occur as uniform and widespread broken zones in sedimentologically and mechanically homogeneous beds. End_of_Article - Last_Page 483------------

Gravity Slide Thrusting and Folded Faults in Western Arbuckle Mountains and Vicinity, Southern Oklahoma

Documentation for one or more major gravity slide thrusts and numerous lesser slides is recognized in the Eola, Southeast Hoover, and Southwest Davis oil fields, and in the Western Arbuckle Mountains, Garvin and Murray Counties, Oklahoma. The gravity slide area initially covered portions of at least nine townships; it was more than 30 mi (50 km) long and 5 to 6 mi (8 to 10 km) wide. It involved a stratigraphic sequence > 5,000 ft (1,500 m) extending from the lower Springer Formation into the upper portion of the Arbuckle Limestone. The major slides moved to the northeast and northwest, probably in Middle Pennsylvanian. The slides and the faults were subsequently isoclinally folded in Late Pennsylvanian. Small depositional structures within beds ranging from upper Arbuc le through the Viola Limestone indicate the area was seismically active during deposition and probably also was subject to gravity sliding in the early Paleozoic. The tensional updip segment of the major folded slide fault now coincides with the trace of the Washita Valley fault. The compressional end of the slide coincides with the Reagan fault in the east and the frontal Eola fault in the west. In the Lake Classen area the latest folding has turned all the formations involved in the slide, and the associated faults, to a near-vertical position. Thus, the slide is exposed in a profile view on the south limb of the overturned Washita Valley syncline. On the north normal limb of the Washita Valley syncline, the slide is exposed in plan view, with the Dougherty anticline and related fo ds representing compressional folding at the toe of the slide. The major slide fault and the Washita Valley fault now share the same trace, but separate movements are still recognizable, the slides having moved north, northeast, and northwest followed by thrusting and probably left-lateral displacement along the Washita Valley fault. Compressional folding and thrusting in the toe of the slide, and extensional faulting along the trailing edge are both recognized. Several tectonic breccias near the top of the Kindblade Formation of the Arbuckle Group probably mark the orogenic event. Tectonic breccias within the fault zone also are interpreted in several wells in the Eola field, and in the past have been identified as Eola conglomerate (Pontotoc). The Arbuckle Group, which had been beneath at least 5,000 ft (1,500 m) of younger strata, was exposed immediately following the slide, which explains the inclusion of boulders and cobbles of this group of formations as major constituents in the earliest known Middle Pennsylvanian conglomerates in this area. The interpretation also explains the absence of any record in the geologic column of the sequential erosion of strata overlying the Arbuckle Group.

Kef Anticline--Box Folding in Tunisian Atlas, Clue to Regional Tectonic Style: ABSTRACT

Near Le Kef, Tunisia, Triassic gypsum and supratidal carbonates with subordinate terrigenous clastic rocks and metabasites (so-called ophites) form the highly deformed core of a northeast-southwest anticlinal structure. The competent enveloping strata, chiefly massive limestone and marlstone, range in age from Aptian-Albian to early Tertiary. The Triassic rocks are multiply deformed and so dismembered as to be best described as tectonic breccia. The Kef anticline, as well as numerous analogous features in the northern Tunisian Atlas, commonly has been interpreted as a diapiric structure emplaced into the younger cover rocks during the early deformation phases of the Alpine orogeny (i.e., Late Cretaceous to Paleogene) and partly reactivated during the Neogene (late Alpine) Our detailed mapping and structural analysis of the Kef structure suggest modifications of this model involving the following major elements: (1) local or regional unconformity between Triassic and Cretaceous strata, (2) regional decollement gliding in part localized within the Triassic evaporite-rich sequence, and (3) late-phase box folding of both the highly deformed Triassic assemblage and the more competent cover rocks. The unconformity between Triassic and Cretaceous rocks is preserved only locally, for late brittle faulting along the margins of the anticline and local bedding-plane thrusts between the competent cover and the incompetent core are common. However, where well-preserved, a distinctive bedded sedimentary carbonate breccia overlies the Triassic assemblage. This sedimentary breccia is in turn conformably overlain by Aptian-age rocks and, thereby, indicates major pre-Aptian uplift and erosion prior to the main phase of deformation in the Tunisian Atlas. The nature of this uplift is problematic, but pre-Aptian diapirism is a viable hypothesis. According to this model, diapirism was restricted to pre-Aptian time, whereas compressive buckling and associated lateral shortening are Neogene defo mation mechanisms superimposed on earlier halokinesis. End_of_Article - Last_Page 531------------

Subaqueous Gravity-Displacement Products: ABSTRACT

All sediments deposited on subaqueous slopes are affected by the tangential component of the earth's gravity. Among the kinds of gravity-displacement processes are subaqueous rock falls, slumps, and debris flows. Turbidity currents are likewise processes affected by the slope, but are not a subject of discussion here. Individual blocks of rock move down a slope from reefs and coastal cliffs; they result from bioerosion, the effects of storm, or ordinary gravity. Incoherent slumps generate a body of sediment so thoroughly mixed and churned that nearly all traces of stratification are obliterated. Debris-flow deposits containing large blocks are known as olistostromes. In places, such olistostromes have been mistaken for melanges, a mixture of huge blocks of diverse kind and provenance dispersed in pervasively sheared and fine-grained matrix which forms a special kind of tectonic breccia. Deep-water rubble of shallow-water carbonate rocks, usually angular, interstratified with dark deep-water marine shale is known as brecciola. Brecciolas accumulate at the toe of the slope. On the upper part of the s ope accumulate sediments resulting from coherent slumps which have moved down a slope with their strata still preserved. Deposits of slope-influenced sediments may also result from contour-following currents (contourites) which travel along the lower parts of slopes. Many examples of ancient gravity-displacement products have been reported from the rock record, among them the so-called melanges (in fact, olistostromes) of Turkey, slope-fan-basin-plain deposits of the Appalachians, and basinal deposits in the Delaware basin of west Texas. In the Taconic sequence of the Appalachians, as in many other hinge-line deposits, only the lower slope and base-of-slope portion of the early Paleozoic continental margin has been preserved. End_of_Article - Last_Page 453------------

Emplacement of the Ronda peridotite, Sierra Bermeja, Spain

Metasedimentary rocks of the Blanca unit, previously reported as roof pendants overlying the Ronda peridotite (Betic Cordilleras, southwest Spain), have been reinterpreted on the basis of field evidence to underlie this ultramafic body. Textural analysis of Blanca rocks reveals that they have experienced first mechanical degradation and later static metamorphism. Blastomylonites several hundred metres below the base of the peridotite show fluxion banding overprinted by the metamorphic assemblage crd + bio + sil + ksp + pla + qtz. In the intervening layer, the mylonitic texture has been totally annealed to form a medium-grained, nonfoliated gneiss of the same mineralogy (hornfels of some authors). Lithic fragments in the gneiss show mineralogical and textural similarities with underlying blastomylonites and are interpreted as resistant blocks in a subsequently metamorphosed tectonic breccia or melange. The structural position of the peridotite together with the mylonitic history of the Blanca rocks is most satisfactorily explained by the tectonic emplacement of the peridotite over the Blanca unit along a major thrust fault. The distribution of cordierite-bearing gneiss in a layer directly beneath the ultramafic sheet suggests that the peridotite was sufficiently hot to cause metamorphism at a temperature between 700 and 800 °C in an aureole at its base. To account for pressures of about 3.5 kb indicated by the cordierite assemblages, the metamorphic complex that overlies the peridotite (Casares unit) is interpreted as cover riding passively on the ultramafic sheet during thrusting. The interpretation of Ronda peridotite as part of a thrust sheet suggests that it has participated in Alpine nappe tectonics in the Betic Cordilleras. The magnitude and direction of apparent tectonic displacement are similar to those of nappes in the western end of this foldbelt.

Metamorphic rocks of the Yap arc-trench system

The Yap trench-arc is a link between the Mariana and Philippine arcs; the latter are both loci of acive volcanism and seismicity but the Yap arc is formed of metamorphic rocks and has had few historic earthquakes. It does not appear to be an active subduction zone. The 8–9 km deep Yap trench has a steep west well, it has little or no sediment ponds on the trench slope. There is no modern volcanism and the few volcanic rocks present on Yap may be large fragments in a tectonic breccia. The main rock type on Yap is greenschist with textural evidence for intense penetrative deformation. The chemistry of these rocks, especially the Mg, Cr, Ni and Co abundances, suggest that the protolith was an ultramafic rock. Rocks from the trench wall east of Yap include amphibolite facies metabasite and metasediments, e.g. (1) Ca-Al-rich hornblende + labradorite + epidote schist; (2) calcite + diopside marble; (3) grossularite-andradite + diopside + vesuvianite calc-gneiss; and (4) diopside + calcite + epidote calc-gneiss. These units (1–4) are overlain by Yap ultramafic schists; a major tectonic break characterized by a polymictic breccia including clasts representative of Yap schists, metabasites, metasediment, pyroxenite and feldspathic gabbro separates the trench lithologies from the ultramafic schist. Other sites sampled in the trench wall have rocks typical of seamounts. It is proposed that a Tertiary(?) subduction zone was terminated when “un-subductible” seamounts blocked the trench. This led to overthrusting, from the west, of seafloor crust and upper mantle. The volcanic arc ceased to exist, some of its fragments are seen in the tectonic breccia. Termination of subduction may be caused by the blocking of subduction zones by large seamount-atoll chains or thickened oceanic crust such as the Caroline Ridge. In some cases this may lead to obduction of oceanic lithosphere.

Cretaceous palynomorphs from the Waihere Bay Group and Kahuitara Tuff, Chatham Islands, New Zealand

Abstract Palynomorphs from the Pitt Island and Waihere Bay Groups, Pitt Island, indicate an age range of ?Urutawan (Early Albian) to Teratan (Santonian) for the deltaic and shallow-water marine sequence. The Kahuitara Tuff is of Mangaotanean to Teratan (Coniacian-Santonian or Campanian) age; the Tupuangi Sandstone is of Ngaterian to Arowhanan (Cenomanian) age; the Headland Conglomerate, lacking in recognised angiosperm pollen, is of ?Urutawan to Motuan (Late Albian) age; a Motuan age is favoured. The first two formations contain angiosperm pollen. No fossils have previously been reported from the Headland Conglomerate, which could be a time correlative of tectonic breccias of the South Island of New Zealand. First records from New Zealand of stratigraphically important fossil palynomorphs are given. Probable reworking of palynomorphs and poor preservation negate more precise age deteminations, and first appearances, particularly of angiosperm pollen, are thus not reliable for age determinations.

Deformation associated with the movement of the Muddy Mountain overthrust in the Buffington window, southeastern Nevada

The Muddy Mountain overthrust, exposed in the Buffington window, southeastern Nevada, consists of a Paleozoic carbonate sheet thrust over Mesozoic Aztec Sandstone, with a molasse filling topographic lows. Evidence suggests that the thrust sheet moved across an erosional surface and that the molasse may have been a forethrust debris. A sharp contact with gouge marks the fault surface. The base of the overthrust sheet is a tectonic breccia containing injections of gouge that are rooted at the contact. Thrust-related changes in the underlying rocks related to proximity of the thrust plane include (1) increase in abundance of microfractures and decrease in grain size due to cataclastic deformation; (2) increase in intensity of macrofracturing parallel and at a low angle to the contact; (3) increase in degree of induration; (4) loss of well-defined bedding planes and color contrast within the Aztec Sandstone; and (5) slabs of dolomite sheared from the upper plate. Laboratory mechanical tests in conjunction with field observations suggest that the shear strength of the undeformed Aztec Sandstone was lower than the frictional strength of the sandstone sliding on quartz gouge. Therefore, cataclastic deformation within a 10- to 100-m-thick zone accompanied the initial advance of the thrust sheet. Following induration, which strengthened the cataclastic sandstone, slip was localized at the thrust contact. During this later stage the high permeability of the fractured upper plate and the Aztec Sandstone suggests that fluid communication with the surface at the leading edge of the thrust was rapid and, therefore, the advance of the thrust could not have been aided by high pore pressure.

New Views on Alpine Tethys Evolution Based on Joides Results: ABSTRACT

In the western central Atlantic, deep-sea drilling during Leg XI has revealed Late Jurassic to Cretaceous pelagic sediments that closely resemble Mesozoic sedimentary rocks exposed in the circum-Mediterranean orogens. This discovery is to be expected according to the concepts of plate tectonics because Alpine-type mountain ranges originate from the elimination and deformation of ocean basins and continental margins. In this context, the ophiolite belts of the central Mediterranean area are interpreted as remnants of oceanic crust and lithosphere which have been emplaced tectonically on the continental margins during Alpine orogeny. More specifically, close time correlations between the central Atlantic and Tethyan associations and palinspastic reconstructions suggest that the central Atlantic and the Alpine-Mediterranean Tethys had, during their early evolution, a parallel paleotectonic development and also were, at least from the Late Jurassic onward, connected directly to each other. In the Alpine-Mediterranean region Mesozoic pelagic facies are present in both true oceanic and continental-margin settings. In the latter, these facies overlie Bahamian-type platform carbonate rocks lying ultimately on a continental crust. The oldest, Triassic, pelagic sediments were deposited in basins resulting from early rifting. Ophiolites of possible Late Triassic age reported from Turkey suggest that a small ocean was formed in the eastern Mediterranean area at that time. During the Early Jurassic, the continental margins of the evolving Tethys were affected by block faulting and many of the former shallow-water sites were submerged; in these regions platform sedimentation was replaced by pelagic and turbiditic carbonate sedimentation and generally is analogous to the Cretaceous-to-recent geologic history in the Blake Plateau and the deeper troughs of the Bahamian region. Joides results and the magnetic-anomaly pattern suggest that this phase of block faulting 180 m.y. ago coincided with early rifting in the central Atlantic. The opening of this part of the Atlantic obviously implies transnational movements between Africa and Eurasia and, in fact, a more or less synchronous opening in the central Mediterranean Tethys is confirmed by radiometric data an the age of the oldest sediments. Transform movements are suggested by the local abundance of tectonic breccias in the oceanic realm (ophicalcites) and by indications of compressive movements as early as Late Jurassic. The oldest sediments recovered from the central Atlantic are greenish-gray and red, slightly nodular limestones of Late Jurassic age. These limestones are very similar to the Tethyan Ammonitico Rosso. However, the Ammonitico Rosso occurs in deeply submerged continental margin settings--different from what is observed generally; the first sediments deposited on the newly formed oceanic crust in the central Mediterranean were not pelagic limestones but radiolarites. As these radiolarites encroach locally on the deeply submerged continental margin, they may reflect a different current system resulting in a higher position of the calcite-compensation depth. From the Tithonian onward, white and gray chalky limestones and nannoplankton oozes were deposited in the western Atlantic and most of the deep parts of the Tethys. The transitional contact with overlying black clays indicates that compensation depth was reached in the truly oceanic realm during the middle Cretaceous. However, the occurrence of ophiolitic olistoliths in the Mediterranean association indicates the tectonic emplacement of large ophiolite nappes in the late Early Cretaceous and a diverging history of central Atlantic and Tethys from that time forward. A quantitative evaluation of the different plate movements in the Tethyan realm is still far from being reached, but comparisons of deformed and undeformed continental margins and ocean basins may help to provide some of the premises for such an attempt. End_of_Article - Last_Page 2215------------