Tectonic Period Research Articles

Tectonic controls on styles of sediment accumulation in the Late Cretaceous Morley Coal Measures of Ohai Coalfield, New Zealand

The tectonic environment of a sedimentary basin can have a very significant effect on the style of sediment deposition in that basin. Effects of tectonic environment can be hard to separate from those of purely sedimentary processes. However, in the Morley Coal Measures of the Ohai Coalfield, tectonic effects are very marked. The development of the basin through two differing tectonic regimes, first extensional, then oblique slip, resulted in the character of sediment deposited varying considerably within the coalfield. In particular, the sediments differ between two sub-basins, the Ohai and Mossbank, of which the Ohai Sub-basin was oriented more favourably for extension in the tectonic setting. The other major effect of tectonism on the Morley Coal Measures was in creating alternating environments of clastic deposition with peat deposition during the Late Cretaceous, seen now as vertically alternating units of sandstone/mudstone with coal/carbonaceous mudstone. It is conjectured that these alternating units were the result of periods of active tectonism and erosion with transport of clastic sediments followed by more quiescent times when peat deposition occurred, relatively unaffected by clastic sediments.

Temporal relationships of lode gold mineralization to accretion, magmatism, metamorphism and deformation — Archean to present: a review: R. Kerrich & K. F. Cassidy, Ore Geology Reviews, 9(4), 1994, pp 263–310

Abstract Late Archean (ca. 2.7 Ga) lode gold mineralization in the Superior Province of Canada and Yilgarn Craton of Australia was late kinematic, and syn to late peak metamorphic, and intimately related in space and time to accretionary tectonic processes. Gold mineralization in the Superior Province was coeval with late-kinematic diachronous accretion of allochthonous subprovinces from north to south over ca. 2710 to 2670 Ma. Timing of gold genesis is constrained by field relationships, and rigorous lower ages of cross-cutting dikes. Gold vein formation is an integral part of the thermal and rheological history of deposit host rocks, such that, in general, deposits in greenschist facies rocks have greenschist facies assemblages and occur in brittle-ductile structures, whereas counterparts in amphibolite facies rocks feature amphibolite facies alteration assemblages and generally occur in ductile structures. Gold mineralization in the southeastern Yilgarn Craton is also temporally related to craton-scale compressional events. There is abundant evidence for thermal, structural and hydrothermal disturbance post orogeny in the Yilgarn Craton, with most ArAr, RbSr and reset UPb zircon ages corresponding to known periods of tectonism in adjacent terranes. In addition, gold-related sulphides typically have reset model Pb ages recorded by secondary isochrons. Accordingly, anomalously young UPb ages on rutile and titanite and ArAr ages on hydrothermal silicates in deposits that post-date peak metamorphism by 100 to 240 m.y. are interpreted to reflect multiple secondary hydrothermal events that also locally remobilized primary ore metals. These variably young, reset ages are also recorded in lithological units of undisputed primary age, including VMS deposits, syn-tectonic granitoids and komatiites. These variably young ages provide an exact parallel for reset secondary ages in gold deposits. Mineralization interpreted to have formed 100 to 240 m.y. after terrane accretion based on scattered ages from non-robust isotopic systems or minerals is not compatible with the above relationships, but rather reflects intermittent secondary reactivation of the primary ore hosting structures. The southern Superior Province has experienced thermal, structural and hydrothermal disturbances since termination of S-type magmatism at ca. 2645–2611 Ma, reflected in KAr, RbSr and SmNd ages 100 to 240 m.y. younger than UPb zircon ages on the same lithological units, and reversed apparent blocking sequences. Eight different models, with distinct time connotations, exist for lode gold deposits in the late Archean terranes. The TTG model linking mineralization in the southern Abitibi Subprovince to synvolcanic tonalite-trondhjemite granodiorite batholiths (2695–2685 Ma) can be ruled out on geochronological grounds, as the deposits are most closely associated in space and time to late-kinematic, calc-alkaline and shoshonitic rocks (2685–2670 Ma). Two “late gold” models based on the non-concordant young ages, involving either delayed thermal rebound or young events in Kapuskasing-like mid-crust can be reinterpreted as late kinematic primary mineralization with multistage isotopic resetting. Both young gold models are based on geochronology alone, and ignore tectonic, structural and metamorphic relationships that provide evidence of deposits forming in thermal and rheological equilibrium with their host terranes. Five models (“early gold” models) for lode gold deposits in late-Archean terranes, namely granulization, cratonization, two gold-shoshonite schemes and the crustal continuum model, explicitly relate mineralization to processes resulting from terrane accretion. All five are consistent with rigorously constrained ages of some gold deposits, including primary mineralization at Val d'Or as given by hydrothermal zircons. Lode gold deposits in the Proterozoic Trans-Hudson orogenic belt (ca. 2.0 to 1.7 Ga) also closely follow terrane collision and metamorphism. Similarly, lode gold deposits in late Paleozoic orogenic belts of Appalachia and the Lachlan fold belt of southeastern Australia show temporal relationships of mineralization to collisional tectonics, metamorphism, magmatism and deformation. The temporal relationships of mineralization to collisional tectonics is also repeated in the Mesozoic-Cenozoic Cordilleran belt of the North American continent and the Cenozoic Alpine-Himalayan chain. Giant lode gold metallogenic provinces formed at three times in Earth history, in the late-Archean (2.7-2.6 Ga), late Paleozoic (450–340 Ma) and Mesozoic-Cenozoic. These times correspond to accretionary tectonic assembly of a supercontinent, in external orogens. Minor gold deposits also formed in space and time with collisional tectonics in internal orogenic belts of the supercontinent cycle. Consequently, the local temporal relationships of lode gold mineralization to collisional tectonics and post-peak metamorphism is also part of the secular development of a larger global geodynamic cycle.

COMPRESSIONAL CONTROL ON SEDIMENT AND FACIES DISTRIBUTION SW NAPPAMERRI SYNCLINE AND ADJACENT MURTEREE HIGH, COOPER BASIN

The northeast-trending Nappamerri Syncline and its flanking high to the southeast, the Murteree-Nappacoongee (NM) Trend, show structural development throughout the deposition of the Permo-Triassic Cooper Basin sequences. Pre-existing topography, such as around Moomba, influenced early depositional patterns within the area. Rates of sedimentation were influenced by the rate of uplift of source areas around the basin, largely as a consequence of buckling. Periods of active tectonism resulted in non-deposition and stripping of sediments from uplifted blocks while deposition continued in deeper areas. Hangingwall blocks, pushed up along reverse faults, strongly influenced the areas of sedimentation and the facies being deposited. This is particularly evident along the NM Trend. Phases of uplift were immediately followed by increased sedimentation where fluvial deposition dominated. The effect of displacement along the northwest-trending basement lineaments was to subdivide the northeast trends into compartments which contain similar facies within each time slice. Adjacent compartments may display different facies and different tectonic histories. An inversion episode, particularly evident in the Big Lake/Moomba area, resulted in a reversal of the depocentre and was related to phases of Sakmarian compression.Facies distribution and sandstone percentage maps of the chronostratigraphic units of the Patchawarra Formation suggest that a northeast-trending major channel system entered this part of basin along the NW edge of the Murteree area.

Temporal relationships of lode gold mineralization to accretion, magmatism, metamorphism and deformation — Archean to present: A review

Late Archean (ca. 2.7 Ga) lode gold mineralization in the Superior Province of Canada and Yilgarn Craton of Australia was late kinematic, and syn to late peak metamorphic, and intimately related in space and time to accretionary tectonic processes. Gold mineralization in the Superior Province was coeval with late-kinematic diachronous accretion of allochthonous subprovinces from north to south over ca. 2710 to 2670 Ma. Timing of gold genesis is constrained by field relationships, and rigorous lower ages of cross-cutting dikes. Gold vein formation is an integral part of the thermal and rheological history of deposit host rocks, such that, in general, deposits in greenschist facies rocks have greenschist facies assemblages and occur in brittle-ductile structures, whereas counterparts in amphibolite facies rocks feature amphibolite facies alteration assemblages and generally occur in ductile structures. Gold mineralization in the southeastern Yilgarn Craton is also temporally related to craton-scale compressional events. There is abundant evidence for thermal, structural and hydrothermal disturbance post orogeny in the Yilgarn Craton, with most ArAr, RbSr and reset UPb zircon ages corresponding to known periods of tectonism in adjacent terranes. In addition, gold-related sulphides typically have reset model Pb ages recorded by secondary isochrons. Accordingly, anomalously young UPb ages on rutile and titanite and ArAr ages on hydrothermal silicates in deposits that post-date peak metamorphism by 100 to 240 m.y. are interpreted to reflect multiple secondary hydrothermal events that also locally remobilized primary ore metals. These variably young, reset ages are also recorded in lithological units of undisputed primary age, including VMS deposits, syn-tectonic granitoids and komatiites. These variably young ages provide an exact parallel for reset secondary ages in gold deposits. Mineralization interpreted to have formed 100 to 240 m.y. after terrane accretion based on scattered ages from non-robust isotopic systems or minerals is not compatible with the above relationships, but rather reflects intermittent secondary reactivation of the primary ore hosting structures. The southern Superior Province has experienced thermal, structural and hydrothermal disturbances since termination of S-type magmatism at ca. 2645–2611 Ma, reflected in KAr, RbSr and SmNd ages 100 to 240 m.y. younger than UPb zircon ages on the same lithological units, and reversed apparent blocking sequences. Eight different models, with distinct time connotations, exist for lode gold deposits in the late Archean terranes. The TTG model linking mineralization in the southern Abitibi Subprovince to synvolcanic tonalite-trondhjemite granodiorite batholiths (2695–2685 Ma) can be ruled out on geochronological grounds, as the deposits are most closely associated in space and time to late-kinematic, calc-alkaline and shoshonitic rocks (2685–2670 Ma). Two “late gold” models based on the non-concordant young ages, involving either delayed thermal rebound or young events in Kapuskasing-like mid-crust can be reinterpreted as late kinematic primary mineralization with multistage isotopic resetting. Both young gold models are based on geochronology alone, and ignore tectonic, structural and metamorphic relationships that provide evidence of deposits forming in thermal and rheological equilibrium with their host terranes. Five models (“early gold” models) for lode gold deposits in late-Archean terranes, namely granulization, cratonization, two gold-shoshonite schemes and the crustal continuum model, explicitly relate mineralization to processes resulting from terrane accretion. All five are consistent with rigorously constrained ages of some gold deposits, including primary mineralization at Val d'Or as given by hydrothermal zircons. Lode gold deposits in the Proterozoic Trans-Hudson orogenic belt (ca. 2.0 to 1.7 Ga) also closely follow terrane collision and metamorphism. Similarly, lode gold deposits in late Paleozoic orogenic belts of Appalachia and the Lachlan fold belt of southeastern Australia show temporal relationships of mineralization to collisional tectonics, metamorphism, magmatism and deformation. The temporal relationships of mineralization to collisional tectonics is also repeated in the Mesozoic-Cenozoic Cordilleran belt of the North American continent and the Cenozoic Alpine-Himalayan chain. Giant lode gold metallogenic provinces formed at three times in Earth history, in the late-Archean (2.7-2.6 Ga), late Paleozoic (450–340 Ma) and Mesozoic-Cenozoic. These times correspond to accretionary tectonic assembly of a supercontinent, in external orogens. Minor gold deposits also formed in space and time with collisional tectonics in internal orogenic belts of the supercontinent cycle. Consequently, the local temporal relationships of lode gold mineralization to collisional tectonics and post-peak metamorphism is also part of the secular development of a larger global geodynamic cycle.

The relationship between alkaline magmatism, lithospheric extension and slab window formation along continental destructive plate margins

Abstract Two distinct groups of Late Cenozoic (15-<0.1 Ma), OIB-like basalts are recognized along the Antarctic Peninsula. Small volumes of undersaturated basanites, tephrites, alkali and olivine basalts and ‘within-plate’ tholeiites, which post-date the cessation of subduction as a result of ridge crest-trench interactions by 40 to <10 Ma, are scattered along much of the southern part of the peninsula. At James Ross Island, in the north of the peninsula, alkali basalts, hawaiites and rare mugearites were erupted synchronously with subduction at the South Shetland Islands trench to the west. Both groups of basalts are remarkably similar in terms of their geochemical and isotopic characteristics, although they apparently owe their origin to two distinct combinations of tectonic processes. The south Antarctic Peninsula basalts are causally related to the cessation of subduction, the formation of slab windows and upwelling and decompressional melting of sub-slab asthenosphere. Correlated trace element-isotope systematics demonstrate that the slab window-related basalts exhibit little evidence for interaction with subduction-enriched mantle or continental lithosphere and must have been derived from sub-slab mantle that had not recently been affected by subduction; LILE/HFSE ratios (e.g. Th/Ta 1.0–2.5, Rb/Nb, 0.25–1.5, Ba/Nb 2.5–7.0) and Sr- and Nd-isotope ratios ( 87 Sr/ 86 Sr 0.7027–0.7035, 143 Nd/ 144 Nd 0.51286–0.51296) are well within the range for ocean island basalts. The slab window-related basalts underwent rapid uprise from the mantle to the crust accompanied by limited fractional crystallization. The syn-subduction alkalic basalts of James Ross Island were generated during slab roll-back, probably related to the formation of the extensive slab window to the south. The James Ross Island Volcanic Group (JRIVG) exhibits evidence of polybaric fractionation including ponding at the base of the crust. Again, there is little evidence for interaction of these asthenospheric magmas with subduction-enriched mantle or continental lithosphere, although in detail, there are recognizable differences between the JRIVG and the slab windowrelated basalts. Melting at James Ross Island was facilitated by slab roll-back and associated lateral and vertical asthenospheric migration into the locus of a pre-existing area of attenuated lithosphere (‘thinspot’). This phase of extensional tectonism was probably originally initiated during the Late Jurassic, but other periods of extension may have taken place in latest Cretaceous times. Neither of these suites of basalts were generated as a result of significant lithospheric extension and passive asthenospheric upwelling on a regional scale. In addition, there is no evidence for the existence of a mantle hotspot beneath the region. The commonly held assumption that alkaline volcanism along consuming plate margins results from periods of significant inter-arc extensional tectonism is, therefore, not necessarily valid. The unique tectono-magmatic regime resulting from the formation of slab windows is probably the only setting in which small degree melts of MORB source asthenosphere not associated with a plume are generated and preserved in the geological record.

Geochemical characteristics of basaltic glasses from the amar and famous axial valleys, Mid-Atlantic Ridge (36°–37°N): Petrogenetic implications

The axial valley of the Mid-Atlantic Ridge from 36° to 37°N was intensively sampled by submersible during the famous and amar projects. Our research focussed on the compositional and isotopic characteristics of basaltic glasses from the amar valley and the narrowgate region of the famous valley. These basaltic glasses are characterized by: (1) major element abundance trends that are consistent with control by multiphase fractionation (olivine, plagioclase and clinopyroxene) and magma mixing, (2) near isotopic homogeneity δ 18O= 5.2to6.4 , 87Sr/ 86Sr= 0.70288to0.70299 and 206Pb/ 204Pb= 18.57to18.84 , and (3) a wide range of incompatible element abundance ratios; e.g., within the amar valley chondrite-normalized La/Sm ranges from 0.7 to 1.5 and La/Yb from 0.6 to 1.6. These ratios increase with decreasing MgO content. Because of the limited variations in isotopic ratios of Sr, Nd and Pb, it is plausible that these compositional variations reflect post-melting magmatic processes. However, it is not possible to explain the correlated variation in MgO content and incompatible element abundance ratios, such as La/Sm and Zr/Nb, by fractional crystallization or more complex processes such as boundary layer fractionation. These geochemical trends can be explained by mixing of parental magmas that formed by very different extents of melting. In particular, the factor of three variation in Ce content in samples with ∼ 2.1% Na 2O and 8% MgO requires a component derived by < 1% melting. If the large variations in abundance ratios of incompatible elements reflect the melting process, a large, long-lived magma chamber was not present during eruption of these amar lavas. The geological characteristics of the amar valley and the compositions of amar lavas are consistent with episodic volcanism; i.e., periods of magma eruption were followed by extensive periods of tectonism with little or no magmatism.

Metamorphic evolution of exhumed middle to lower crustal rocks in the Mojave Extensional Belt, southern California, USA

The Waterman Metamorphic Complex of the central Mojave Desert was exposed as a consequence of early Miocene detachment‐dominated extension. However, it has evidence consistent with a more extensive geological history that involves collision of a crustal fragment(s), tectonic thickening by overthrusting and two periods of extension. The metamorphic complex contains granitoid intrusives and felsic mylonitic gneisses as well as polymetamorphic rocks that include marble, calc‐silicate, quartzite. mafic granulite, pyribolite, amphibolite, migmatite and biotite schist. The latter group of rocks was affected by an initial series of high‐grade metamorphic events (M1 and M2) and a localized lower grade overprint (M3). The initial metamorphism (M1) can be separated into two stages along its high‐grade P–T path: M1a, a granulite facies metamorphism at 800–850° C and 7.5–9 kbar and Mlb, an upper amphibolite facies overprint at 750–800° C and 10–12 kbar. M1a developed mineral assemblages and textures consistent with granulite facies conditions at a reduced activity of H2O and is associated with intense ductile deformation (D1) and minor local partial melting. M1b overprinted the granulite assemblages with a series of hydrous phases under conditions of increasing pressure and H2O activity and is accompanied by little or no deformation. M2 developed at lower pressures and temperatures (650–750° C, 4.5–5.5 kbar) and is distinguished by a second local overprint of hydrous phases that reflects an input of aqueous fluids probably associated with the intrusion of a series of granitic dykes and veins. Effects of M3 are confined to the Mitchel detachment zone, an anastomosing early Miocene detachment fault, and are characterized by local ductile/brittle deformation (D2) of the pre‐existing high‐grade rocks and granitoid intrusives and by the production of mylonites and mylonitic gneisses under greenschist facies conditions (300–350° C, 3–5 kbar). The initial overprint (M1a) represents metamorphism, devolatilization and minor partial melting of supracrustal rocks under granulite facies conditions as a consequence of tectonic and, possibly, magmatic thickening. The increasing pressure transition of M1a to M1b reflects a period of continued compressional tectonism, thrusting and influx of H2O, in part, locally related to crystallization of partial melts. The near isothermal decompression between M1b and M2 probably represents a pre‐112‐Ma extensional episode that may have been the result of a decompressional readjustment of a thickened crust. Following the initial extensional event, the metamorphic complex remained at depths of 10–17 km for at least 90 Ma until it was uplifted following Miocene extension. M3 develops locally in response to this second extensional period resulting from the early Miocene detachment faulting.

Periodicity in the accretion process on the Southeast Indian Ridge at 27°40′S

Image processing of a Seabeam bathymetric grid on the Southeast Indian Ridge (SEIR) at 27°40′S has been used to analyse the morphotectonics of the ridge flanks. Steep topographic slopes indicating inward-facing or outward-facing faults and those representing tilted blocks were automatically separated. An autocorrelation process allows an estimation of the periodicity of the fault spacing. The outward-facing faults first appear at 16–18 km from the axial valley, are more common on bathymetric highs, and correspond to tilted block zones in adjacent deeper areas. The tilting of the blocks is ~ 4°. The transformation of the rift valley into the smooth undulating relief of the flanks is accomplished by these outward-facing faults and tilted blocks. Inward-facing faults appear either as large faults with major throws or as a series of smaller faults with minor throws. The minor faults are mainly localized on bathymetric highs and are densely organized. The major faults, continuous and symmetrically spaced on each side of the axis zone, are organized with a periodicity of ~ 8.2 km. These faults seem to be old inner walls which have been moved outwards periodically during the spreading process. They alternate with positive magnetic lineaments reflecting old neovolcanic zones, linked to intense magmatic periods. Considering a mean half-spreading rate of 31 km/m.y. during the past million years, this would mean the development of a major fault each ~ 0.26 m.y. We propose a model for the evolution of the SEIR at 27°40′S controlled by a periodic succession of magmatic and tectonic episodes: 1. (1) faulting and volcanism are concentrated in the axial zone above the melt reservoir where the crust is weakened (magmatic period) — outside this zone the external relief is frozen-in, supported by the finite strength of the plate; 2. (2) the axial valley is enlarged, fills up with lavas and bulges due to thermal effects; 3. (3) at the end of the magmatic period, this bulged zone collapses by thermal subsidence along lines of weakness such as the faults forming the inner walls; 4. (4) when the melt reservoir is empty (tectonic period), the processes are no longer concentrated and active faulting extends over tens of kilometres from the axis — the lithosphere is thinned, a new axial valley is created, the flanks are uplifted, and outward-facing faults and block tilting are induced.

Structural Study of the Southern Perth Basin by Geophysical Methods

Seismic, gravity and geothermal data were interpreted to examine the structure and the evolution of the southern Perth Basin. Seismic data show that the principal fault trend is north-south and that strike-slip faulting is of major importance in the formation of the basin. Gravity modelling shows that: (a) crustal thickness of the basin is approximately 30 km; (b) greater crustal thickness beneath the basin than on either side implies isostatic imbalance; (c) the Darling and Busselton Faults are steeply dipping faults which extend into the lower crust and control the structure of the southern Perth Basin. Analysis of these data shows that the evolution of the basin has had three major periods of tectonism which reactivated major faults: (a) a right-lateral strike-slip motion along the Darling Fault in the Late Permian-Early Triassic; (b) a phase of tectonism with associated left-lateral motion along the Dunsborough Fault in the Jurassic; (c) the separation of Australia from India in the Early Cretaceous which produced a predominantly tensional, and some oblique-transcurrent, style of faulting.

Reservoir Development in Brahaney Northwest and Patricia Fields, Northern Midland Basin, Texas: ABSTRACT

Porous pay zones in the Silurian-Devonian section of the northern Midland basin, Texas, vary stratigraphically, as well as structurally, with their locations beneath the pre-Woodford unconformity. These variations are related to at least two major periods of widespread pre-Woodford tectonism and erosion. A major unconformable surface, present at the top of the Lower Silurian Fusselman formation, was later modified by tectonism and erosion immediately preceding deposition of the Woodford Shale. In places where the Fusselman subcrops beneath the Woodford, its capacity for reservoir development is dependent upon the severity of geologic events that affected the formation during the two major tectonic/erosional events. Where the Wristen and Thirtyone formations subcrop, their reservoir capacity depends upon the extent of structuring and subsequent erosion immediately prior to Woodford deposition. Two pre-Woodford oil fields in the northern Midland basin illustrate geologic complexity that bears on the successful application of subsurface mapping in defining potential pay zones. Brahaney Northwest field in Yoakum County is productive from fractured, coarse crystalline Silurian-Devonian dolomites on subtle, fault-bounded structures. These structures, defined seismically on the base of the Woodford, do not reveal the more complex structural and stratigraphic variations within the reservoir itself. Patricia field in Dawson County ismore » productive from Fusselman carbonates where upper Fusselman structural and topographic relief coincides with post-Woodford faulting and low-relief anticlinal closure as defined at the base of the Woodford.« less

Granite provinces and basement terranes in the Lachlan Fold Belt, southeastern Australia

Many granites have compositional features that directly reflect the composition of their source rocks. Since most granites come from the deeper parts of the Earth's crust, their study provides information about the nature of parts of that deep crust. Granites and related volcanic rocks are abundant and widely distributed in the Palaeozoic Lachlan Fold Belt of southeastern Australia. These granites show patterns of regional variation in which sharp discontinuities occur between provinces which internally are of a rather constant character. Such a discontinuity has long been recognized at the I‐S line and the extent of that line can now be defined more fully. Breaks of this type are thought to correspond to sharp changes in the composition of the deep crust that correspond to unexposed or basement terranes. Nine such basement terranes can be recognized in the Lachlan Fold Belt. The character of these basement terranes appears to be different from that of the terranes recognized in the Mesozoic‐Cainozoic Cordilleran fold belt, in which the plates accreted during the period of tectonism reflected in the exposed surface rocks. In the Lachlan Fold Belt, it is postulated that fragments of continental crust, or microplates, were assembled in the Late Proterozoic or Early Palaeozoic to form the substrate of the presently exposed Palaeozoic sedimentary rocks; the compositional features of these fragments were later redistributed vertically by magmatic processes. The identification of basement terranes of this type shows that models which involve the lateral growth of the Lachlan Fold Belt during the Palaeozoic, in a manner analogous to the accretion of younger belts, are untenable. These basement terranes have implications for mineral exploration because the content of heavy metals can vary from one to another and this would ultimately affect the probability of concentrating these metals to form a mineral deposit.

Batopilas mining district, Chihuahua

The Batopilas mining district is one of the few mining districts where the major ore mineral is native silver. The native silver occurs as fissure vein deposits in granodiorite, fine-grained diorite, and dacite. Orebodies are irregularly shaped pods of massive native silver in calcite and may contain up to 75 wt percent silver. The veins formed along north- and northeast-trending fractures and faults.The geologic history of the district is summarized as follows: intrusion of the Pastrana dacite, Dolores microquartz diorite, and Tahonas granodiorite, sequentially, into preexisting Mesozoic sediments and/or igneous rocks. Some of these intrusive events may have vented to the surface. After the intrusions were eroded, the San Jose, Arenal, and Casas Coloradas flow breccias were deposited. Between each episode of flow breccia deposition, there occurred periods of tectonism and erosion. After a fourth major episode of tectonism and erosion, the massive ignimbrites of the upper volcanic series represented by the Yerbanis Formation were deposited. Contemporaneous with Yerbanis deposition was the intrusion of rhyolite dikes. Numerous andesitc dikes of uncertain age are also present.The native silver orebodies are generally less than 30 m in length with the width varying from a few centimeters to 4.6 m and averaging 0.8 m. The ore-grade mineralization terminates abruptly in both vertical and horizontal directions. The orebodies are connected vertically and along strike by up to 90 m of unmineralized fractures or vein stringers of calcite, sometimes less than 3 mm wide.Flexures in vein orientation and/or cross fractures are associated with some orebodies, whereas other orebodies have no apparent structural control. Most orebodies are hosted by Pastrana dacite and Tahonas granodiorite.Three types of wall-rock alteration are found with the silver mineralization: (1) chlorite-actinolite alteration, (2) silicification, and (3) pyritization. Chlorite-actinolite alteration is often associated with gray and green argillization. The host rocks of many, but not all, orebodies have experienced chlorite-actinolite alteration and pyritization. Silicification occurs in dikelike masses within the Dolores and Pastrana units. These silicified zones are generally less than 9 m wide. Pyritization is found in the intrusions. Minor quartz-pyrite veins are found throughout.Gangue in the Batopiias ores is calcite and is associated with small amounts of sphalerite, galena, and laumontite. Cobalt-nickel arsenides and ruby silvers have been reported, but their presence is not confirmed by this investigation. Pyrite is abundant in the host rocks but is absent from the ores.Two kilometers southwest of the village of Batopilas is an area of approximately 5 km 2 devoid of vegetation, the hillsides and tops have a thick gossan (Corralitos). The underlying Tahonas granodiorite exhibits widespread sericitic alteration, and extensive to almost complete quartz veining and replacement are present. Chalcanthite stalagtites form on many overhangs along the arroyos. This area is interpreted as the Tahonas porphyry copper deposit.A well-developed pattern of mineralogic zonation exists in the district. This zonation is concentric around the Tahonas porphyry copper deposit. Preliminary fluid inclusion studies document homogenization temperatures (T h ) between 217 degrees and 434 degrees C, and salinities over 20 equiv wt percent NaCl for quartz from the Tahonas porphyry copper deposit. Studies on quartz from outlying regions give homogenization temperature ranges of 224 degrees to 351 degrees C, and salinities of from 9 to 18 equiv wt percent NaCl. Studies of the later generation of calcite, silver, and minor sulfides give (T h ) values between 137 degrees and 257 degrees C for calcite and 170 degrees to 196 degrees C for sphalerite. Salinities in calcite are concentrated between 7.6 and 13.2 equiv wt percent NaCl. The estimated 300 million oz (9.3 million kg) of silver produced to date probably was deposited from circulating connate waters adjacent to the Tahonas porphyry copper mineralizing system.

On the metamorphic history of an Archaean granitoid greenstone terrane, East Pilbara, Western Australia, using the 40Ar/ 39Ar age spectrum technique

Age spectrum analyses of blue-green hornblendes from amphibolites from the Western Shaw Belt, East Pilbara, Western Australia, indicate an age of at least 3200 Ma for early regional metamorphism. Ages on hornblende and muscovite from the narrow contact zone with the adjacent Yule Batholith probably date updoming of the granitoid gneiss terranes at 2950 Ma. Hornblendes from within the Shaw Batholith and from a contact zone of a post-tectonic granitoid yield ages of 2840–2900 Ma, indicating either prolonged high temperatures within the granitoid gneiss terranes or a separate thermal pulse associated with the intrusion of post-tectonic granitoids. The preservation of very old hornblendes in a narrow greenstone belt surrounded by massive granitoid gneiss domes indicates that remarkable contrasts in metamorphic geotherms existed over short distances during the Late Archaean, suggesting that updoming occurred during a period of rapid tectonism.

Cold-climate, fluvial to paralic coal-forming environments in the Permian Collinsville Coal Measures, Bowen Basin, Australia

The Middle Permian Collinsville Coal Measures of the northern Bowen Basin illustrate a range of cold to cold-temperate, coal-forming environments. Cold climate is indicated by Glossopteris flora in the coal measures, and by restricted marine fauna dominated by brachiopods and bryozoa in correlative marine sequences of the Back Creek Group which contains also abundant lonestones (dropstones). Sedimentation was characterised by an overall transgression, interrupted by local fluvial and coastal progradation in a shallow, epicontinental sea during a relatively quiescent tectonic period. Six sedimentary environments are represented: fluvial, fluvio-paralic, barrier-strandplain, back-barrier, tidal flat and open marine. The basal coal formed from peat of swamps of abandoned areas of gravelly braided streams, and is massive, dull, and with high ash (20%), low sulphur (1%) contents. Overlying coals developed from peats formed in fluvio-paralic and paralic environments, and thicker seams are generally brighter, with low to moderate ash (8–17%) and moderate to high total sulphur (1–6%) contents. Seams associated with fluvial influence show splits and high ash yield, while seams associated with coastal deposits show high sulphur levels (up to 21%). In contrast to reported models of coal-forming environments, no clearly defined deltaic or inter-distributary bay-fill sequences were identified in the area studied. Rather, vast freshwater wetlands backed low-gradient, progradational coasts locally having bars and barriers. The barriers were not prerequisites for substantial peat accumulation, although may have locally assisted peatland development by raising the profile of coastal equilibrium.

Microstructures in deformed sediments, Denbigh Moors, North Wales

In the Denbigh Moors area of North Wales some of the Silurian sedimentary rocks are intensely disrupted. Recently the British Geological Survey has confirmed that this is largely the result of extensive slumping of the sediments, which was part of a long sequence of deformation events ranging from what they called ‘penecontemporaneous movements’ and related ‘additional processes’ to at least three periods of regional tectonism. New knowledge of the microstructural behaviour of deforming sediments obtained from laboratory studies allows the prospect of assigning field structures to a position within this long scenario and a better understanding of the complex pre-lithification deformation processes. Experimental deformation of argillaceous sediments has shown that they deform, over a wide range of water content, by intense slip within narrow shear zones rather than by pervasive grain slip. Shear zones of strikingly similar appearance are common in parts of the Denbigh Moors succession, and, being demonstrably pre-diagenetic, can be used as indicators of pre-lithification disturbance. The shear zones give a polished, finely striated, and crenulated appearance to many of the exposure surfaces in the area. The diverse orientations of the zones reflect to some extent different locations within the sliding sedimentary masses. The style of the zones gives an indication of the water content of the sediment and hence the burial depth at the time of deformation. Some beds show features such as truncated upper contacts and are therefore thought to have been disturbed before lithification, yet the rocks lack shear zones. These are interpreted as having been deformed while very near surface, with a water content too high for the shear zone mechanism to have operated. The possibility remains to be explored that some of the shear zones in the area are the result of regional tectonism acting on unlithified sediments.

Geology and Regional Implications of Carbonatites in the Lemitar Mountains, Central New Mexico

More than 100 carbonatite dikes and veins intrude a complex Precambrian terrain in the Lemitar Mountains, New Mexico. These Ordovician carbonatites are not associated with any alkalic rocks or kimberlites, but they exhibit textures, mineralogy, chemistry, and alteration characteristic of carbonatites. Despite variations in texture, the Lemitar carbonatites can be grouped on the basis of mineralogy, chemistry, and texture as primary-magmatic and replacement silicocarbonatites and rodbergs. The Lemitar silicocarbonatites consist of more than 50% carbonate minerals and varying amounts of accessory minerals, and they are enriched in LREE. The rodbergs consist of more than 50% carbonate minerals, are stained red by hematite, and exhibit flat REE patterns. Carbonate and barite-fluorite-galena veins of uncertain origin are found in the carbonatites and country rocks. Periodic episodes of alkalic magmatism occurred in New Mexico and southern Colorado during the Proterozoic, late Paleozoic, and Tertiary. The Lemitar carbonatites are part of a regional episode of alkalic and carbonatite magmatism which affected New Mexico and southern Colorado during the late Paleozoic. Continental rifting may have occurred during the Precambrian and late Paleozoic in central New Mexico, although exact geographical limits of paleorifts cannot be determined because overlapping periods of tectonism may have affected the area since Paleozoic times.

Influence of tectonics and palaeoenvironment on late cretaceous clay sedimentation in the upper benue trough, Nigeria

AbstractThe Late Cretaceous sedimentary sequence in the Upper Benue Trough consists of seven formations. Outcrop samples from the type localities of these units were analysed by X‐ray diffraction for their day mineral content. The composition and distribution of the clay minerals are influenced by tectonics, relief and water depth. Three main depositional phases are recognizable on the basis of clay mineral assemblages, two of which are associated with tectonic movements. The first phase, characterized by the presence of chlorite, illite, kaolinite and mixed‐layer clays is common to Albian‐Cenomanian sediments. This phase is succeeded by mixed‐layer clays and smectite‐rich, Turonian‐Early Santonian sediments.The re‐occurrence of chlorite and illite in association with kaolinite and mixed‐layer clays in the Post‐Santonian sediments constitutes the third depositional phase.These clay mineral assemblages are indicative of two periods of tectonism (Albian and Late Santonian) and an intervening Turonian‐Early Santonian period of quiescence in the Upper Benue Trough.

Geologic setting of the Teutonic Bore massive sulfide deposit, Archean Yilgarn Block, Western Australia

The Teutonic Bore massive sulfide deposit occurs in a sequence of Archean mafic and felsic volcanic rocks in the northeastern Yilgarn Block, Western Australia. Felsic volcanic rocks form part of a regionally continuous linear belt consisting of locally emergent volcanic centers separated by interbedded aquagene tuffs and water-lain epiclastic sedimentary rocks. Mafic and felsic rocks in the area have been extensively intruded by gabbro and subsequently by high-level granitoids believed to be cogenetic with the rhyolitic rocks.Regionally the deposit lies in a linear zone of complex structure and lithology characterized by repeated periods of extensional tectonism. Rhyolite and cogenetic granitoids are related to earlier phases of extension, whereas subsequent activity led to the formation of a suite of saturated to undersaturated syenites and fault-bounded units of polymictic conglomerate derived largely from the rhyolite and high-level granitoids.The Teutonic Bore deposit consists of one relatively small, steeply dipping lens of predominantly banded massive sulfides (mainly pyrite, sphalerite, chalcopyrite, and galena) underlain by concordant and discordant stringer mineralization. The mineralization occurs within a sequence of pillowed basalt and tuffaceous sediment 0 to 150 m stratigraphically above rhyolitic volcanic and volcaniclastic rocks. Primary textures within the massive sulfides and underlying chert indicate depositional instability. Active growth faulting controlled the deposition of felsic epiclastic sedimentary rocks, subsequent basaltic rocks, and the development of the third-order basin which provided the site for deposition of massive sulfides. The faults probably provided the conduits for hydrothermal solutions; these fluids spread laterally and vertically prior to exhaling at the rock-seawater interface during a volcanic hiatus. Features within the local environment are compatible with regional extension. Regional extension resulted in bimodal magmatism, and the associated rifting formed basins suitable for the deposition of extensive epiclastic sedimentary rocks.The bimodal volcano-plutonic host at Teutonic is similar to many examples from the Archean of Canada, but the style of felsic volcanism, the carbonate-rich alteration, and the lead- and silver-enriched ore are most similar to deposits in the Sturgeon Lake area.

Late Permian to Cainozoic tectonics of the New England Orogen

Abstract The development of the New England Orogen, until the middle Permian, was divided into three tectonic periods by Harrington and Korsch (Aust. J. Earth Sci. 32, 163–179,1985). These tectonic periods were dominated firstly by W‐facing volcanic arc systems which in tectonic period (TP) 3 were replaced by the E‐facing Gympie arc system. In TP4 the upper marine beds and coal measures of the Sydney, Gunnedah and Bowen basins were deposited and were covered by Early Triassic strata. In the W there was little structural disturbance in TP4 except in the Gunnedah Basin near the Mooki Fault where the Lower Triassic is unconformable on the Permian coal measures, but in the E there was a fundamental change with the formation of the major Late Permian and Early Triassic TP4 magmatic arc in Zone B and in the Yarrol Orogen. During TP5, in the Middle and Late Triassic, the Stanthorpe Plutonic Suite was intruded and the Ipswich Basin was formed and filled with volcanics and coal measures. A deformation in the Late Triassic at the beginning of TP6 was significant throughout the orogen, causing the deformation of the Gunnedah, Bowen and Ipswich basins which are overlain unconformably by the Surat and Moreton basins. In TP7 in the Late Jurassic and Early Cretaceous, there was volcanism and deformation in the Maryborough Basin and granitoids were intruded. It is inferred that there was major deformation and plutonism in Zone C because the major Rangitata Orogeny occurred in New Zealand. In TP8 the Whitsunday belt of mid‐Cretaceous silicic volcanics and high‐level plutons was probably a major feature because volcanic debris is abundant in sediments of the same age in the Surat and Eromanga basins. In TP9, in the Late Cretaceous and Cainozoic, Zone C was removed from Australia when the Tasman and Coral seas opened, the eastern part of Australia developed into a cymatogen by vertical movements, and there were extensive eruptions of basaltic volcanoes and lava fields in a set of arcs. Seven sets of granitoid rocks occur in Zone B and the Yarrol Orogen and it is thought that they formed by the subduction of continental crust (C‐subduction).

Regional Appraisal of Hydrocarbon Potential of Trans-Pecos Texas: Methodology and Conclusions: ABSTRACT

Evaluation of large areas requires a different approach from that used to develop prospects. Regional cross-section networks generally ignore minor correlation problems. We examined and related all potentially significant parameters, and used reduced scales to subdue nonessentials and to cover a large area. Organization and planning must permit free interchange of ideas and close cooperation among those working on the project. Regional structural and stratigraphic analysis of Trans-Pecos Texas strongly suggests that widespread trap-destruction by faulting and erosion led to hydrocarbon leakage and induction of fresh water into prospective zones, which reduced the likelihood of accumulation and preservation of economic reserves. At least 4 major periods of tectonism, pervasive fresh water, mineralization, basic igneous intrusives and extrusives, high heat-flow regime, increasing percentage of carbon dioxide southwestward from the Delaware basin, and contemporaneous vertical movement suggest that the opportunity for major reserves is minimal. A review of 1,000 mi of CDP seismic data from 4 areas disclosed 65 structural leads or traps, of which 45 had significant tests. All these tests were failures; many gave indications of sizable amounts of fresh water. None of the areas have been completely condemned. However, only relatively small reserves can be anticipated for nearshore to continental Jurassic and Cretaceous sections in the Chihuahua Trough, Pennsylvanian and Permian reefs formed on a shelf environment or on the flanks of major uplifts, or in areas such as the Marfa basin. End_of_Article - Last_Page 249------------