Trending Shear Zones Research Articles

Cretaceous deformation, magmatism, and metamorphism in the Lower Benue Trough, Nigeria

A microtectonic analysis carried out in the Abakaliki Anticlinorium has revealed various types of deformation of different ages. Early soft-sediment deformation (normal faulting, slumping) occurred during the Albian–Pre-Santonian period and is related to active N50°E trending shear zones. A compressional tectonic phase of Santonian age was responsible for the shortening of the sedimentary cover and resulted in tight folding and axial plane cleavage. The thermal history of the Lower Benue Trough is characterized by two major episodes. A first phase (Albian) is related to the emplacement of volcanic and subvolcanic rocks. These rocks have alkaline affinities, with some characteristics of continental tholeiites. Contact metamorphism has occurred around the intrusions with the formation of andalusite in the shales. This thermal phase has been dated by the K-Ar method, giving an age of 104 Ma corresponding to the emplacement of the Wanakande syenite and associated basic sills. A second thermal phase occurred during the Santonian tectonic event resulting in low-grade metamorphism characterized by a higher illite crystallinity values, and by a paragenesis including quartz, chlorite, calcite, white mica, and epidote. A K-Ar age of 81 Ma obtained on igneous and sedimentary rocks in the Lower Benue Trough corresponds to the regional metamorphic phase contemporaneous with the Santonian tectonic episode.

Hercynian-thrust related shear zones and deformation of the Varied Group on the contact of granulites/Southern Moldanubian, Bohemian Massif/

The three deformation phases inferred from the detailed structural analysis of the Cesky Krumlov Varied Group record the Hercynian development of the Southern Moldanubian of the Bohemian Massif. The deformation is related to the NW-SE thrusting of the large crustal units including granulites. The formation of the NS and NW-SE trending shear zones is connected with the thrust movement. The structural development begins with F1 isoclinal fold formation, that could originate in unmetamorphosed sediments. In the final stage, they were strongly flattened and B1 boudinage developed in the rocks. Aplite dykes and migmatitization of paragneisses occurred at the same time the rocks were metamorphosed. The D2 deformational phase was produced by the simple shear deformation of the unit and folds of various styles around rigid inclusions and the strike-slip shear zone near the boundary of the granulite and the Varied Group were formed. The F1 and F2 folds are parallel with the stretching and mineral lineation indicating a NS to NW-SE direction of the thrusting. The youngest deformation is characterized by spectacular boudinage and by folding of the vertically oriented planes.

Anomalous oceanic lithosphere formed in a leaky transform fault: evidence from the Western Limassol Forest Complex, Cyprus

Detailed studies in the Western Limassol Forest Complex, Cyprus have revealed plutonic and structural histories that differ markedly from the Penrose type ophiolite of the Troodos Massif to the north. A tectonized harzburgite of upper mantle origin acts as host to multiple mafic and ultramafic intrusions, forming a plutonic core complex. Mafic and picrite dykes cut and are cut by E–W trending shear zones, demonstrating syn-tectonic magmatism. A retrogressive metamorphic and progressive cooling history of the intrusive suite indicates uplift of the host, mantle harzburgite while in a sea-floor setting. It is suggested here that the geological evolution of the Western Limassol Forest Complex indicates formation of anomalous oceanic lithosphere in the active domain of an Upper Cretaceous, leaky transform fault.

One orogenic belt or two? A structural reinterpretation supported by Landsat data products of the Precambrian metamorphics of Singhbhum, eastern India

Recent structural studies, supported by an analysis of digitally enhanced Landsat data products, attempt at bringing in certain significant changes in the two contrasting concepts about the stratigraphic and tectonic classification of the highly deformed and metamorphosed Precambrian rocks of Singhbhum, eastern India. Both the concepts introduced large-scale horizontal displacements of the rocks in a N-S direction along a prominent E-W trending shear zone. An earlier concept professed that the rocks in the north and south of the shear zone, in spite of their apparent structural and metamorphic contrasts, belong to one broad stratigraphic division (“Iron Ore Series”). A later concept grouped the rocks into two different stratigraphic and orogenic belts: the belt north of the shear zone with an E-W strike (“Singhbhum Group”) being younger than the belt in the south striking NNE-SSW (“Iron Ore Group”). The present structural and remote sensing studies reveal that: (1) the shear zone does not demarcate two different stratigraphic and orogenic belts; and (2) it does not involve any large-scale N-S displacements; rathe, it indicates severe ENE-WSW subhorizontal sinistral shearing in western Singhbhum that brought about marked structural contrasts within the same stratigraphic belt.

Metamorphism of the Kangâmiut dykes and the metamorphic and structural evolution of the southern Nagssugtoqidian boundary in the Itivdleq – Ikertôq region, West Greenland

The structural evolution of the Nagssugtoqidian mobile belt is characterised by reworking of pre-existing Archaean rocks. At the southern Nagssugtoqidian boundary swarms of basic dykes, the Kangamiut dykes, intruded after the earliest Nagssugtoqidian movements. These dykes aet as time markers separating tectono-metamorphic events and record degree and extent of post-dyke metamorphism and deformation. The structural and metamorphic evolution at the southern Nagssugtoqidian boundary ean be summarised as follows. The rocks now exposed were stabilised in granulite and amphibolite facies in Archaean times. Uplift brought the rocks under low amphibolite facies conditions. Subsequent deformation (Nag. 1) caused the retrogression to low amphibolite facies and transformed the rocks into strongly schistose rocks along duetile, transcurrent, E-W trending shear zones. The Nag. 1 deformation was followed by intrusion of the mainly NE trending Kangamiut dykes. Later Nagssugtoqidian deformation (Nag. 2) affected country rocks with Nag. 1 fabrics as well as dykes and probably also Archaean gneisses unaffected by Nag. 1 deformation. In areas with intense Nag. 2 movements the deformation was characterised by duetile overthrusting towards the SSE along a linear zone striking ENE. New Nag. 2 fabrics were imposed on the country gneisses and dykes were transformed into strongly deformed amphibolites. The duetile overthrusting brought granulite facies rocks into juxtaposition with amphibolite facies rocks so that across the areas affected by Nag. 2 deformation a prograde metamorphic sequence with facies boundaries parallel to the overthrusting zone was established.

Geology, geochemistry, and mineralisation of the Endeavour Inlet antimony–gold prospect Marlborough Sounds, New Zealand

Geological, geochemical, and ore microscopy studies were undertaken on the Endeavour Inlet antimony–gold prospect, Marlborough Sounds, New Zealand. The occurrence lies along the southern tracts of a NNW–trending shear zone. It is characterised by a series of quartz veins of variable strike length and thickness. Stibnite occurs as sporadic lenticular bodies within the quartz vein system. Other ore minerals are arsenopyrite, pyrite, marcasite, and trace amounts of gold. The deep topographic relief of the area has made it possible to conduct the present investigation on key outcrops over a vertical range of approximately 400 m, Mineral zoning is well developed: stibnite is the predominant ore mineral vertically above arsenopyrite, pyrite, and gold. It is concluded that the mineralisation derives from a hydrothermal system motivated by connate waters and developed during the same metamorphic event that formed rocks of chlorite Subzone II rank (Marlborough Schists). The quartzo–feldspathic and mafic metavolcan...

Fluid Inclusion Assemblages of the Stratiform Broken Hill Ore Deposit, New South Wales, Australia

Minerals of the Broken Hill lode in New South Wales, Australia, have a rich assemblage of fluid inclusions. Most typically they contain a high-salinity aqueous liquid and a high-density carbon dioxide and methane gas. Northerly trending shear zones in the area are characterized by a distinct assemblage of inclusions. The inclusions provide a record of the sequence of metamorphic fluids since the period of high-grade metamorphism. Inclusions from the period of ore formation have been eliminated by repeated deformation and recrystallization.

Fluid Inclusion Assemblages of the Stratiform Broken Hill Ore Deposit, New South Wales, Australia

Minerals of the Broken Hill lode in New South Wales, Australia, have a rich assemblage of fluid inclusions. Most typically they contain a high-salinity aqueous liquid and a high-density carbon dioxide and methane gas. Northerly trending shear zones in the area are characterized by a distinct assemblage of inclusions. The inclusions provide a record of the sequence of metamorphic fluids since the period of high-grade metamorphism. Inclusions from the period of ore formation have been eliminated by repeated deformation and recrystallization.

Tectonic Setting of Happy Springs and Near-by Structures in Sweetwater Uplift Area, Central Wyoming: ABSTRACT

The Happy Springs, Crooks Gap, Kirk, and Sheep Creek oil fields are situated along the Green Mountain-Crooks Mountain trend in southeastern Fremont County, Wyoming. Structurally, the fields lie within the southwestern part of the Wind River basin, near the western end of the so-called Sweetwater arch. The main structural features of the area were formed by Laramide compressive forces and consist of (1) several easterly trending shear zones, and (2) a series of northwest-trending folds and associated thrust faults. The folds and shear faults formed concomitantly in response to Laramide compressive forces. The shear faults divide the area into several elongate blocks and developed to accommodate different deformational patterns within the blocks. The Happy Springs and adjacent fields consist of tightly folded, thrust-faulted, northwest-trending anticlines formed en echelon on the south side of an easterly trending shear zone. These folds represent crustal shortening in the block lying south of the shear zone. The features in which crustal shortening is manifested in the block north of the shear zone are located about 10 miles west of the Happy Springs area in the large Mormon Trail anticline and thrust fault. The shear zone is a line of release that developed between the two blocks when they responded differentially to compressive forces. The main accumulation of oil in the Happy Springs and Kirk fields is in the deformed strata along the foot-wall of the shear zone. End_of_Article - Last_Page 793------------

The mother lode mine in British Columbia: R. H. Allen. Eng. Min. J., lxxxviii, 1101

Copper–gold–bismuth–tellurium mineralization in the Stanos area, Chalkidiki Peninsula, Greece, occurs in the Proterozoic- to Silurian-aged Serbomacedonian Massif, which tectonically borders the Mesozoic Circum-Rhodope metamorphic belt to the west and crystalline rocks of the Rhodope Massif to the east. This area contains the Paliomylos, Chalkoma, and Karambogia prospects, which are spatially related to regional NW–SE trending shear zones and hosted by marble, amphibolite gneiss, metagabbro, and various muscovite–biotite–chlorite–actinolite–feldspar–quartz schists of the Silurian Vertiskos Unit. Metallic minerals occur as disseminated to massive aggregates along foliation planes and in boudinaged quartz veins. Iron-bearing sulfides (pyrite, arsenopyrite, and pyrrhotite) formed prior to a copper-bearing stage that contains chalcopyrite along with galena, sphalerite, molybdenite, and various minerals in the system Bi–Cu–Pb–Au–Ag–Te. Fluid inclusion homogenization temperatures of primary aqueous liquid–vapor inclusions in stage I quartz veins range from 170.1 °C to 349.6 °C (peak at ~ 230 °C), with salinities of 4.5 to 13.1 wt.% NaCl equiv. Calculated isochores intersect P–T conditions associated with the upper greenschist facies caused by local overpressures during late-stage tectonic movement along the shear zone in the Eocene, which produced stretching and unroofing of rocks in the region. Values of δ34S for sulfides in the Stanos shear zone range from 2.42 to 10.19‰ and suggest a magmatic sulfur source with a partially reduced seawater contribution. For fluids in equilibrium with quartz, δ18O at 480 °C varies from 5.76 to 9.21‰ but does not allow for a distinction between a metamorphic and a magmatic fluid.A 187Re–187Os isochron of 19.2 ± 2.1 Ma for pyrite in the Paliomylos prospect overlaps ages obtained previously from intrusive rocks spatially-related to the Skouries porphyry Cu–Au, the Asimotrypes Au, and the intrusion-related Palea Kavala Bi–Te–Pb–Sb ± Au deposits in northern Greece, as well as alteration minerals in the carbonate-replacement Madem Lakkos Pb–Zn deposit. Ore-forming components of deposits in the Stanos area were likely derived from magmatic rocks at shallow depth that intruded an extensional shear environment at ~ 19 Ma.