Steep Shear Zones Research Articles

Thermo-tectonic evolution of the Neoarchaean Southern Marginal Zone of the Limpopo granulite Complex (South Africa)

Abstract Combined geophysical, structural geological, metamorphic, geochronological, and stable isotope information is employed to elucidate the Neoarchaean thermo-tectonic evolution of the Southern Marginal Zone (SMZ) within the Limpopo Complex (South Africa) during the Limpopo orogeny (2.72 to 2.62 Ga). The complex evolutionary history of the SMZ was controlled by an allochthonous SMZ granulite nappe that was extruded from a rising granulite diapir through a process of mid-crustal heterogeneous channel flow. This granulite nappe with its embedded structures (steeply plunging reclined folds and steep shear zones) was formed during emplacement of the diapir to mid-crustal level (6 kbar, 20 km depth) from where it was thrust south-westwards along the Hout River shear zone (HRSZ) sole thrust against the Kaapvaal Craton (KVC) at 2.72 to 2.69 Ga. Evidence for the thermo-tectonic interaction of the granulite nappe with the KVC includes (1) thrust complexes (referred to as hot-iron zones) that are developed at the frontal ramp sections of the HRSZ juxtaposed against the granite-greenstone belts of the KVC, and (2) strike-slip shear deformation associated with the lateral ramp section of the HRSZ, which developed against the KVC devoid of greenstone belts. The emplacement of the post-tectonic Matok granitic pluton at ~2.68 Ga into the SMZ signified the end of the thermo-tectonic event that established the regional fold- and shear deformational framework of the granulite facies SMZ. Post-Matok secondary shear zones reflect evidence for HRSZ-linked tectonism that continued intermittently to 2.65 to 2.62 Ga. Low H2O-activity fluids (H2O activity of 0.1 to 0.3) released from devolatilisation of underthrust greenstone material passively infiltrated and interacted with the overlying cooling granulites. This established a retrograde anthophyllite-in isograd at ~6 kbar and ~620°C that subdivides the SMZ into a northern granulite domain and a southern retrograde hydrated granulite domain. Simultaneously, gold-bearing fluids focused into these minor shear zones established shear zone-hosted orogenic gold mineralisation at 2.65 to 2.62 Ga. Emplacement of the post-tectonic Palmietfontein granite at ~2.46 Ga and associated sub-volcanic granitic dykes into both the retrograde hydrated granulite domain and the granulite domain signifies the end of all thermo-tectonic activity in the SMZ. A Palaeoproterozoic thermal overprint at ~2.1 Ga is recorded by Rb-Sr biotite and phlogopite ages derived from various rocks from the SMZ and adjacent KVC. This thermal event is not associated with deformation and did not result in the formation of new mineral assemblages. Integrated data presented and discussed in this paper contradict the interpretation of age and petrological data utilised to support alternative models for the evolution of the SMZ, including a proposed ~2.1 Ga Palaeoproterozoic polymetamorphic amphibolite-grade thermo-tectonic event.

Early Neoproterozoic Deformation Kinematics in the Chottanagpur Gneiss Complex (Eastern India): Evidence from the Curvilinear Hundru Falls Shear ZoneAnalysis

Abstract Curvilinear steep shear zones originate in different tectonic environments. In the Chottanagpur Gneiss Complex (CGC), the steeply dipping, left-lateral and transpressive Early Neoproterozoic Hundru Falls Shear Zone (HFSZ) with predominantly north-down kinematics comprises two domains, e.g., an arcuate NW-striking (in the west) to W-striking (in the east) domain with gently plunging stretching lineation that curves into a W-striking straight-walled domain with down-dip lineation. The basement-piercing HFSZ truncates a carapace of flat-lying amphibolite facies paraschist and granitoid mylonites, and recumbently folded anatectic gneisses. The carapace—inferred to be a midcrustal regional-scale low-angle detachment zone—structurally overlies an older basement of Early Mesoproterozoic anatectic gneisses intruded by Mid-Mesoproterozoic/Early Neoproterozoic granitoids unaffected by the Early Neoproterozoic extensional tectonics. The mean kinematic vorticity values in the steep HFSZ-hosted granitoids computed using the porphyroclast aspect ratio method are 0.74–0.83 and 0.51–0.65 in domains with shallow and steep lineations, respectively. The granitoid mylonites show a chessboard subgrain microstructure, but lack evidence for suprasolidus deformation. The timing relationship between the two domains is unclear. If the two HFSZ domains were contemporaneous, the domain of steep lineations with greater coaxial strain relative to the curvilinear domain formed due to strain partitioning induced by variations in mineralogy and/or temperature of the cooling granitoid plutons. Alternately, the domain of gently plunging lineations in the HFSZ was a distinct shear zone that curved into a subsequent straight-walled shear zone with steeply plunging lineation due to a northward shift in the convergence direction during deformation contemporaneous with the Early Neoproterozoic accretion of the CGC and the Singhbhum Craton.

Aeromagnetic signature of an exhumed double dome system in the SW Grenville Province (Canada)

AbstractAnalysis of aeromagnetic data in the Grenville Province reveals the presence of two regional‐scale unmapped structural domes (the Morin and Mékinac‐Taureau domes) with an oval‐shaped magnetic pattern bounded by regional‐scale shear zones and a geometry that is similar to that produced in crustal flow models under extension, which predict two upright domes of foliation (double dome) separated by a steep shear zone. The Mékinac‐Taureau dome, a metamorphic core complex, and the Morin dome may have been exhumed by channel flow. Exhumation occurred by a combination of thrust, normal‐sense and wrench shear zones. The preservation of paragneisses in the Morin dome suggests that it underwent a lesser degree of exhumation than did the Mékinac‐Taureau dome. This study shows how the integration of local field information with magnetic data in a regional tectonic setting can reveal and delineate concealed gneiss domes and highlights a role for strike‐slip tectonics in the creation of regional structures involving the exhumation of deep crust.

How is strain localized in a meta-granitoid, mid-crustal basement section? Spatial distribution of deformation in the central Aar massif (Switzerland)

This study investigates strain distribution in granitoid rocks formerly in the middle crust in the Central Aar massif, Switzerland and places the deformation behavior in the tectonic framework of the Alpine orogeny. Strain is heterogeneously distributed in terms of strain partitioning forming several hundreds of closely spaced shear zones (SZ) (>80 SZ/km with SZ thicknesses <10 cm; about 10 SZ/km with SZ thicknesses of 0.5–10 m) separating 3D bodies of low to moderate background strain. Both the degree of background-strain intensity as well as the number of shear zones increases from granitic to granodioritic host rocks and is controlled by primary variations in the mica content between 10 and 15 vol% (granodiorite) and <8 vol% (granite). Shear zones evolved from ductile shearing in granodiorites, whereas they often nucleated from fractures in the stronger granites.The majority of the steep shear zones preferentially accommodated upward motion by the southern block leading to an increase in peak metamorphic conditions from 250 °C in the North to 450 °C in the South of the Aar massif. The shear zones initiated at about 18–20 km depths during a stage of crustal thickening (Handegg phase). Subsequent deformation reactivated some shear zones with a gradual transition from reverse dip-slip over oblique-slip to strike-slip shear zones under local transpressional conditions (Oberaar phase).

The influence of partial melting and melt migration on the rheology of the continental crust

The presence of melt during deformation produces a drastic change in the rheological behavior of the continental crust; rock strength is decreased even for melt fractions as low as ∼7%. At pressure/temperature conditions typical of the middle to lower crust, melt-bearing systems may play a critical role in the process of strain localization and in the overall strength of the continental lithosphere. In this contribution we focus on the role and dynamics of melt flow in two different mid-crustal settings formed during the Brasiliano orogeny: (i) a large-scale anatectic layer in an orthogonal collision belt, represented by the Carlos Chagas anatexite in southeastern Brazil, and (ii) a strike-slip setting, in which the Espinho Branco anatexite in the Patos shear zone (northeast Brazil) serves as an analogue. Both settings, located in eastern Brazil, are part of the Neoproterozoic tectonics that resulted in widespread partial melting, shear zone development and the exhumation of middle to lower crustal layers. These layers consist of compositionally heterogeneous anatexites, with variable former melt fractions and leucosome structures. The leucosomes usually form thick interconnected networks of magma that reflect a high melt content (>30%) during deformation. From a comparison of previous work based on detailed petrostructural and AMS studies of the anatexites exposed in these areas, we discuss the rheological implications caused by the accumulation of a large volume of melt “trapped” in mid-crustal levels, and by the efficient melt extraction along steep shear zones. Our analyses suggest that rocks undergoing partial melting along shear settings exhibit layers with contrasting competence, implying successive periods of weakening and strengthening. In contrast, regions where a large amount of magma accumulates lack clear evidence of competence contrast between layers, indicating that they experienced only one major stage of dramatic strength drop. This comparative analysis also suggests that the middle part of both belts contained large volumes of migmatites, attesting that the orogenic root was partially molten and encompassed more than 30% of granitic melt at the time of deformation.

Evidence for a long‐lived accommodation/transfer zone beneath the Snake River Plain: A possible influence on Neogene magmatism?

AbstractGeochronologic data compiled from 12 metamorphic core complexes and their flanking regions outline important differences in tectonic and magmatic histories north and south of the Snake River Plain‐Yellowstone Province (SRP‐Y). Magmatism, crustal flow, metamorphism, and extensional exhumation of core complexes north of the SRP occurred mostly between 55 and 42 Ma as compared to 42–25 Ma south of the SRP, with final exhumation of the southern complexes occurring only during younger Miocene (20–0 Ma) Basin and Range faulting. These significant differences in the timing of events suggest that the now lava‐covered SRP, which is at a high angle to Cordilleran trends, may have at times operated as a steep shear or transfer zone accommodating difference in strain to the north and south. Following previous suggestions, we infer that this proposed accommodation or transfer zone developed above an important lithospheric boundary localized above a tear in the subducting slab (shallower slab angle to the south) used to explain both the locus of Late Cretaceous‐Paleocene magmatism and the different ages and mechanisms of slab reconfiguration and removal north and south of the SRP during the Cenozoic. The details of these different histories help outline the complex evolution of this zone and also suggest that this zone of lithospheric weakness may have subsequently focused Miocene SRP‐Y hot spot magmatism.

Crustal growth during island arc accretion and transcurrent deformation, Natal Metamorphic Province, South Africa: New isotopic constraints

The Natal Metamorphic Province consists of, from north to south, the Tugela, Mzumbe, and Margate Terranes. These were accreted to the southeastern margin of the Kaapvaal Craton in the late Mesoproterozoic, and followed by intrusion of a large suite of A-type granitoid bodies. New U–Pb data from zircon, titanite, and monazite further constrains the temporal framework of these geological events.The Tugela and Mzumbe Terranes record protracted magmatism in an island arc complex from ∼1200Ma to 1160Ma, followed by the accretion of these terranes onto the southern margin of the Kaapvaal Craton at ∼1150Ma. Arc magmatism in the Margate Terrane continued until ∼1120Ma and was followed by extension and bimodal volcanism immediately prior to accretion to the Kaapvaal/Mzumbe continental margin at ∼1090Ma. This accretion was accompanied by high-pressure and high-temperature metamorphism, juxtaposition of the Mzumbe and Margate Terranes along the Melville Thrust, and the formation of a number of syntectonic intrusive units derived from melting of the pre-existing arc crust. After accretion, extensional collapse is evidenced by the intrusion of mafic/ultramafic and alkaline intermediate magmatic suites at ∼1085Ma, resulting from mafic underplating and/or lower crustal delamination. Nd and Hf isotopic data imply the magmatic rocks of the Natal Metamorphic Province were derived from relatively juvenile continental crust, initially generated by island arc magmatism and subsequently reworked during the accretion event(s). The combined Kaapvaal-NMP region (the southern margin of the enlarged Kalahari Craton) then experienced extensive sinistral transcurrent deformation centred along a series of discrete steep shear zones that are found from the Kaapvaal cratonic margin to the southernmost portion of the Natal Metamorphic Province. This deformation is accompanied by low-pressure, (ultra)high-temperature metamorphism, isobaric cooling, and intrusion of the voluminous A-type Oribi Gorge Suite porphyritic granites and charnockites throughout the Mzumbe and Margate Terranes.

The Rwenzori Mountains, a Palaeoproterozoic crustal shear belt crossing the Albertine rift system

This contribution discusses the development of the Palaeoproterozoic Buganda-Toro belt in the Rwenzori Mountains and its influence on the western part of the East African Rift System in Uganda. The Buganda-Toro belt is composed of several thick-skinned nappes consisting of Archaean Gneisses and Palaeoproterozoic cover units that are thrusted northwards. The high Rwenzori Mountains are located in the frontal unit of this belt with retrograde greenschist facies gneisses towards the north, which are unconformably overlain by metasediments and amphibolites. Towards the south, the metasediments are overthrust by the next migmatitic gneiss unit that belongs to a crustal-scale nappe. The southwards dipping metasedimentary and volcanic sequence in the high Rwenzori Mountains shows an inverse metamorphic grade with greenschist facies conditions in the north and amphibolite facies conditions in the south. Early D1 deformation structures are overgrown by cordierite, which in turn grows into D2 deformation, representing the major northwards directed thrusting event. We argue that the inverse metamorphic gradient develops because higher grade rocks are exhumed in the footwall of a crustal-scale nappe, whereas the exhumation decreases towards the north away from the nappe leading to a decrease in metamorphic grade. The D2 deformation event is followed by a D3 E-W compression, a D4 with the development of steep shear zones with a NNE-SSW and SSE-NNW trend including the large Nyamwamba shear followed by a local D5 retrograde event and D6 brittle reverse faulting. The Palaeoproterozoic Buganda-Toro belt is relatively stiff and crosses the NNE-SSW running rift system exactly at the node where the highest peaks of the Rwenzori Mountains are situated and where the Lake George rift terminates towards the north. Orientation of brittle and ductile fabrics show some similarities indicating that the cross-cutting Buganda-Toro belt influenced rift propagation and brittle fault development within the Rwenzori Mountains and that this stiff belt may form part of the reason why the Rwenzori Mountains are relatively high within the rift.

Fabric evolution at basement–cover interfaces in a fold-and-thrust belt and implications for décollement tectonics (Autochthon, Lower Allochthon, central Scandinavian Caledonides)

Microstructural and magnetic investigations (anisotropy of magnetic susceptibility, AMS) on sections across basement–cover interfaces (BCI) revealed a complex evolution in the crystalline basement rocks beneath and in the basal units of the Caledonian fold-and-thrust belt: (1) Pre-Caledonian mylonitic fabrics in basement granite relate to steep shear zones. (2) Palaeoweathering formed smectite and illite at the expense of feldspar and mica. Secondary Fe-bearing clay minerals and the intensity of the chemical weathering control the bulk susceptibility. Changing susceptibility and AMS relate to a (time) sequence from primary magnetite to secondary paramagnetic clay to pyrite and ferrimagnetic pyrrhotite. (3) Burial compaction with BCI-parallel fabrics. (4) Caledonian cleavage, overprinted by decollement zones with S–C–C′ fabrics. Decollement cataclasis overprinted pre-existing magnetic fabrics and produced horizontal magnetic lineations and subhorizontal foliations defined by the S–C–C′ fabrics. Clay mineral enrichment, together with subsequent, BCI-parallel compaction fabrics, decreased the shear strength in the basement rocks beneath the BCI. Detachments initiated at such low-strength zones and produced allochthonous units with their footwall within crystalline basement rocks, an observation of general importance for orogenic fold-and-thrust belts.

Hot lithosphere at Mount Isa: implications for Proterozoic tectonics and mineralisation

Conductive heat transfer in the lithosphere at Mount Isa has been modelled based on published data on heat flow, thermal rock properties, and lithosphere architecture. Modelling results suggest that the high heat flow at Mount Isa is due to a combination of a thicker than normal crust and high concentrations of radioactive elements. During the last major tectonic activity in the Proterozoic, higher concentration of parent isotopes and the additional sedimentary overburden have increased lithosphere temperatures further. These results confirm that heat-producing elements play a key role in the thermal history of the Mount Isa Inlier and in other Australian Proterozoic terrains. The elevated lithosphere temperatures have significant implications for understanding Proterozoic tectonics and mineralisation, in particular the potential for shallow crustal melting, thermal weakening of the lithosphere, and for the evolution of deformation-related permeability. In general, high temperature gradients in continental crust cause a tectonic style dominated by pervasive viscous flow in partially molten crust, which in Mount Isa is evident from the formation of metamorphic core complexes during extension, and the absence of alpine-type nappes during contraction. In contrast, steep shear zones and fabrics formed in the upper crust during shortening of the rift complex in the Mid-Proterozoic Isan Orogeny. It is suggested that these steep structures formed where lateral strength contrasts existed in the lithosphere, caused by the differences in burial depths between cooler, tectonically denuded basement, and hotter basement covered by sedimentary basins. Increased permeability along steeply dipping faults and shear zones enables fluids to flow at higher rates across steep gradients of pressure, temperature and chemical concentration between magmatic, metamorphic and surface reservoirs. It is concluded that the link between thermal and mechanical processes in hot continental lithosphere greatly enhances the likelihood of hydrothermal mineralisation.

Age and tectonic framework of structurally controlled Palaeoproterozoic gold mineralization in the Häme belt of southern Finland

A number of gold occurrences in the Palaeoproterozoic Svecofennian domain of southern Finland show a clear structural control on mineralization and are classified as orogenic gold. Samples of pre-, syn- and late- to post-mineralization rocks taken from two gold prospects in the Somero-Tammela gold zone of the Häme belt have been studied to gain absolute ages of mineralization by using SIMS U–Pb of zircon and titanite, and Ar–Ar single grain step heating of hornblende and biotite. Gold mineralization in the Somero-Tammela gold zone is closely related to quartz veins and controlled by WSW–ENE to SW–NE trending shear zones and faults formed during prolonged dextral oblique contraction in the late stages of Svecofennian orogenic evolution. Zircons from a felsic volcanic host rock give an age of 1881 ± 3 Ma interpreted as the crystallization age of the rock and providing a maximum age for mineralization. Inherited zircons in a gold-bearing quartz vein with the youngest zircon rim of 1832 ± 15 Ma give a maximum age of quartz vein emplacement. A 1786 ± 7 Ma mean 207Pb/ 206Pb age of a pegmatitic quartz-feldspar dyke providing a minimum age for mineralization is interpreted as reflecting lead loss, whereas an 1815 ± 4 Ma zircon age is probably closer to the crystallization age of the dyke. This age overlaps with 1805.4 ± 5.4 Ma 39Ar– 40Ar age of hornblende from a quartz vein. 1.78 and 1.76 Ga 39Ar– 40Ar biotite cooling ages mark the minimum age for mineralization. In summary, the age of mineralization can be estimated at ca. 1.82–1.79 Ga, which is in good agreement with the age of dextral shearing in southern Finland. The controlling structures, WSW–ENE to SW–NE dextral shear zones and NW–SE oriented faults, formed during approximately WNW–ESE oriented sub-horizontal shortening during crossing ductile–brittle boundary conditions. The steep shear zones and faults and the intersection lines of the two sets impart a directional permeability allowing channelling of the mineralizing fluids in these structures. This was supported by prolonged dextral oblique contraction enabling ± continuous or episodic slip along the faults and shear zones. The late Svecofennian tectonic episode could be described as an extension–contraction cycle in a convergent plate margin setting above a retreating subduction zone. Oblique extension (1.86–1.83 Ga) caused by slab rollback was followed by a rapid switch to oblique contraction at ca. 1.82 Ga induced by arrival of positively buoyant crustal fragments (e.g. oceanic ridge, oceanic plateau or continental sliver) at the subduction zone. The contractional period characterized by dextral transpression provided a favourable setting for orogenic gold mineralization. The termination of the Svecofennian orogenic evolution in Finland after 1.79 Ga might be caused by lock-up of the subduction zone and slab break-off. The striking NW–SE alignment of orogenic-type gold occurrences in the Svecofennian domain of southern Finland suggests that the late Svecofennian 1.82–1.79 Ga gold mineralizing event may be regionally more widespread and of greater importance than hitherto expected.

Structural setting of Neoproterozoic mineralization, Asmara district, Eritrea

Late Neoproterozoic collision between East and West Gondwana concentrated transpressional deformation in the juvenile crust of the Nubian Shield in Eritrea along at least two steep, curvilinear crustal-scale belts, the Augaro-Adobha Belt (AAB) and the Asmara-Nakfa Belt (ANB). Volcanosedimentary rocks dominantly metamorphosed at greenschist-facies conditions characterize the belts. Each of these belts comprises a complex network of syn-metamorphic shear-fold structures. Steep strike-slip shear zones and accompanying vertical to steeply plunging folds dominated the latest phase of deformation. Quartz vein-hosted gold ± sulphide type and volcanic-hosted massive sulphide type deposits and occurrences are either deformed or hosted by these steep shear zones and folds. The deposits are broadly grouped into three major mineral districts, Asmara, Augaro and Bisha. The Asmara district, the main focus of this study, is located where the southern part of the Asmara-Nakfa Belt changes in strike from NNE–SSW to NNW–SSE. Combined field, micro-structural, and magnetic fabric studies are conducted in the sheared host rocks of a series of the mineral deposits and/or occurrences of the Asmara mineral district. These combined studies revealed that the Asmara area was subjected to a transpressional deformation accommodated in a complex and curved flower structure. Both the quartz vein and massive sulphide types of deposits are sheared, folded and generally spatially associated. The ore-bearing quartz veins are often concentrated along dilatant-extensional en-echelon fracture arrays in reverse and normal sense shear zones, and they either cut through or structurally overlie, the massive sulphide deposits. The massive sulphides that formed at the same time as the Neoproterozoic volcanosedimentary rocks were later deformed and metamorphosed with them. This study, along with previous investigations, further implies that the Asmara area represents an intra-arc, palaeo-oceanic trough or basin located over a west-northwestward dipping subduction zone that subsequently underwent transpression. The transpressional belts track the general locations of such oceanic basins into which ore-bearing fluids that resulted in various phases of vein type deposits were channeled. This study can help to locate new prospects and develop existing ore deposits and/or occurrences in Neoproterozoic Eritrea and elsewhere in areas of similar structural setting.

Structural evolution of the Day Nui Con Voi metamorphic complex: Implications on the development of the Red River Shear Zone, Northern Vietnam

The Day Nui Con Voi (DNCV) metamorphic complex in North Vietnam is the southernmost high-grade metamorphic zone along the NW–SE trending Red River Shear Zone (RRSZ) in Indochina. The RRSZ was considered as a classical large-scale continental strike-slip fault that had played a significant role in the continental extrusion of Southeast Asia since the collision of India and Eurasia. Earlier ideas determined the RRSZ as a steep shear zone that penetrated the entire lithosphere. Both metamorphism and structures within rocks along the DNCV metamorphic complex have been previously thought to be formed syn-tectonically by left-lateral shearing of the RRSZ during the Oligocene–Miocene continental escape tectonics. However, our meso- and microstructural re-examination of this region shows that these metamorphic rocks were formed during earlier tectonic episodes unrelated to strike-slip shearing. High angle to near orthogonal overprinting fabrics indicated that this region recorded three episodes of ductile deformation followed by brittle faulting events with different intensity spanning from the Triassic to the Tertiary. D 1 is preserved as NW–SE striking upright folds under garnet grade regional metamorphism during the Triassic Indosinian orogeny as South China block amalgamated with the Indochina block. The large-scale horizontal D 2 folds with a dominant top to N–NW bottom to S–SW sense of shear, and sub-horizontal fold axial planes suggest that the DNCV metamorphic complex remained at midcrustal depths since the Indosinian orogeny. The youngest ductile deformation event, D 3, refolded D 2 recumbent folds into a dome, and uplifted the DNCV as lower-temperature fabrics, S 3, indicated. Steep mylonite zones with left-lateral kinematic indicators and brittle faulting were developed on both limbs of the dome along the steep Song Hong and Song Chay faults during left-lateral movement of the RRSZ. Our new spatial, temporal and kinematic correlations of metamorphic fabrics within the DNCV metamorphic complex support the suggestion that the RRSZ developed after regional metamorphism and remained purely a crustal fault. The complicated deformation history recorded within the DNCV metamorphic complex provides an alternative interpretation and suggests that crustal scale strike-slip faults (such as the RRSZ) need not root from the mantle.

Gold-bismuth-telluride-sulphide assemblages at the Viceroy Mine, Harare-Bindura-Shamva greenstone belt, Zimbabwe

AbstractGold mineralization at the Viceroy Mine is hosted in extensional veins in steep shear zones that transect metabasalts of the Archaean Arcturus Formation. The gold mineralization is generally made up of banded or massive quartz carrying abundant coarse arsenopyrite. However, most striking is a distinct suite of Au-Bi-Te-S minerals, namely joseite-A (Bi4TeS2), joseite-B (Bi4Te2S), hedleyite (Bi7Te3), ikunolite (Bi4S3), ‘protojoseite’ (Bi3TeS), an unnamed mineral (Bi6Te2S), bismuthinite (Bi2S3), native Bi, native gold, maldonite (Au2Bi), and jonassonite (AuBi5S4). The majority of the Bi-Te-S phases is characterized by Bi/(Se+Te) ratios of &gt;1. Accordingly, this assemblage formed at reduced conditions at relatively low fS2 and fTe2. Fluid-inclusion thermometry indicates depositional temperatures of the main stage of mineralization of up to 342°C, in the normal range of mesothermal, orogenic gold deposits worldwide. However, melting temperatures of Au-Bi-Te phases down to at least 235°C (assemblage (Au2Bi + Bi + Bi7Te3)) imply that the Au-Bi-Te phases have been present as liquids or melt droplets. Furthermore, the close association of native gold, native bismuth and other Bi-Te-S phases suggests that gold was scavenged from the hydrothermal fluids by Bi-Te-S liquids or melts. It is concluded that a liquid/melt-collecting mechanism was probably active at Viceroy Mine, where the distinct Au-Bi-Te-S assemblage either formed late as part of the main, arsenopyrite-dominated mineralization, or it represents a different mineralization event, related to rejuvenation of the shear system. In either case, some of the gold may have been extracted from pre-existing, gold-bearing arsenopyrite by Bi-Te-S melts, thus leading to an upgrade of the gold ores at Viceroy. The Au-Bi-Te-S assemblage represents an epithermal-style mineralization overprinted on an otherwise mesothermal (orogenic) gold mineralization.

A Study of the hydrogeological environment of the lishan landslide area using resistivity image profiling and borehole data

Resistivity Image Profiling (RIP) surveys was used to develop a lithological and hydrogeological model of the subsurface in the southeastern part of Lishan landslide area of central Taiwan. The bedrock consists of slate in the study area. Based on RIP and rock samples collected from boreholes results, three electrical strata are recognized: colluvium, the shear zone composed of shear gouges and shattered slate, and the undisturbed slate formation. The steep shear zone with resistivity ranging between 100 ~ 260 Ω-m, plays a crucial role in the local hydrogeological environment, because it forms a natural barrier which blocks and retains groundwater flowing down the slope. Groundwater will brim over the barrier when the water level is high. Thus the inclined groundwater table remains stable from long-term monitoring. It strongly indicates that the groundwater recharge is greater than that of discharge. Therefore, the shear zone can provide information about the optimum locations for draining the excess groundwater in-situ for slope stability consideration. The curved basal surface of the colluvium and the weathered slate can also be discerned from the resistivity variations and boreholes data. A series of circular patterns may associate with the main slope failure which migrated upwards from the lower slope.

The initiation and evolution of the transpressional Straight River shear zone, central Fiordland, New Zealand

Structural data and U/Pb geochronology on zircon from central Fiordland, New Zealand show the role of pre-existing structural heterogeneities in the kinematic evolution of a newly discovered zone of transpression. The Straight River shear zone consists of steep zones of high strain that are superimposed onto older fabrics across a 10 × 80 km region. The older foliation formed during two periods of tectonism: contraction and magmatism of mostly Carboniferous (∼312–306 Ma) age and Early Cretaceous batholith emplacement ending by 113.4 ± 1.7 Ma followed by extension that ceased by 88.4 ± 1.2 Ma. The primary mechanism for the formation of steep shear zone foliations was the folding of these older fabrics. Conjugate crenulation cleavages associated with the folding record shortening at high angles to the shear zone boundaries. Fold axial surfaces and axial planar cleavages strike parallel to the shear zone with increasing strain as they progressively steepened to subvertical. In most areas, shear sense flips from oblique-sinistral (east-side-down component) to oblique-dextral (west-side-down) across zones of intermediate and high strain. High strain zones display subvertical mineral lineations, steep strike-slip faults and shear sense indicators that record strike-slip motion across the steep lineations. These patterns reflect triclinic transpression characterized by narrow zones of mostly strike-slip deformation and wide zones of mostly contraction. Zones of high strain align with offshore traces of late Tertiary strike-slip faults, suggesting that a previously undocumented component of late Tertiary shortening and strike-slip motion is accommodated within Fiordland.

Fluid chemistry of the Palaeoproterozoic Fäboliden hypozonal orogenic gold deposit, northern Sweden: Evidence from fluid inclusions

A new ore province, the Gold Line, southwest of the Skellefte District, northern Sweden, is currently under exploration. The largest known deposit in the Gold Line is the hypozonal Fäboliden orogenic gold deposit. The mineralization is hosted by arsenopyrite-bearing quartz veins, within a steep shear zone in amphibolite facies metagreywacke host rocks. Gold occur in fractures and as intergrowths in arsenopyrite-löllingite, and as free grains in the silicate matrix of the host rock. The hydrothermal mineral assemblage in the proximal alteration zone is diopside, calcic amphibole, biotite, and minor andalusite and tourmaline. Primary fluid inclusions in the Fäboliden quartz veins show a CO2-CH4 or a H2S (±CH4) composition (the latter recognized for the first time in a Swedish ore deposit). The primary fluid inclusions are associated with arsenopyrite-löllingite (+gold) and the CO2-CH4 fluid was also involved in precipitation of graphite. A prevalence of carbonic over aqueous fluid inclusions is characteristic for a number of hypozonal high-temperature orogenic gold deposits. The Fäboliden deposit, thus, shows fluid compositions similar to other hypozonal orogenic gold deposits. The proposed main mechanism for precipitation of gold from the fluids, is a mixing between H2S-rich and H2O?-CO2±CH4 fluids. Fluid inclusion data indicate arsenopyrite-löllingite and graphite deposition at a pressure condition of about 4 kbar. Graphite thermometry indicates maximum temperatures of 520-560°C for the hydrothermal alteration at Fäboliden, suggesting that at least the late stages of the mineralizing event took place shortly after peak-metamorphism in the area, i.e. at c. 1.80 Ga.

The evolution of an exposed mid-lower crustal attachment zone in Fiordland, New Zealand

Abstract Studies of convergent margins suggest that large subhorizontal shear zones in the lower crust help regulate how displacements are transferred horizontally and vertically through the lithosphere. We present structural data from the Fiordland belt of SW New Zealand that illustrate the progressive evolution of a 25 km thick section of exhumed, Early Cretaceous middle and lower crust. The data show that the mechanisms by which displacements were relayed through the crust during a 25 Ma cycle of arc-related magmatism, high-grade metamorphism and contraction changed repeatedly. During the period 126–120 Ma, a ≥10 km thick batholith composed of gabbroic-dioritic magma was emplaced into the lower crust. Melt-enhanced shear zones evolved at the upper and lower contacts of the batholith where magma and steep temperature gradients created strength contrasts. By ∼ 120 Ma, partial melting of mafic-intermediate lower crust resulted in the formation of high-pressure (14–16 kbar) migmatite and steep, regionally extensive vein networks up to 10 km below the batholith. Melt segregation and transfer through and out of the lower crust were aided by melt-induced fracture arrays and ductile deformation in shear zones. During the period 116–105 Ma, differential shortening of the crust produced a network of subhorizontal and subvertical shear zones at different crustal depths. Near-vertical shear zones up to 15 km wide formed at the deepest part of the section. These shear zones cut upwards across the entire lower crust to merge with a gently dipping upper amphibolite facies fold-and-thrust zone that formed in the middle crust. A 1 km thick, subhorizontal shear zone underlies this mid-crustal fold-and-thrust zone and physically connected shear zones that formed at different crustal depths. Our data suggest that deformation above and below this mid-lower crustal attachment zone was coupled kinematically and accommodated subhorizontal arc-normal displacements in the middle crust and oblique sinistral displacements on steep shear zones in the lower crust. The steep lower crustal shear zones also record components of subhorizontal arc-normal shortening and vertical thickening. These results strongly suggest that large, kinematically coupled networks of flat and steep shear zones separated the Fiordland crust into distinctive structural domains and relayed displacements vertically and horizontally through the lithosphere during Early Cretaceous oblique convergence.

Late Paleozoic remagnetization of Precambrian crystalline rocks along the Precambrian/Carboniferous nonconformity, Rocky Mountains: a relationship among deformation, remagnetization, and fluid migration

Paleomagnetic and rock magnetic data and petrographic observations demonstrate that Archean to late middle Proterozoic rocks (three localities in metamorphic rocks, two in granitic plutons) in parts of the Rocky Mountains, along and below a regional nonconformity with Carboniferous strata, have acquired a secondary magnetization. This magnetization is exclusively of reverse polarity and, based on its unique direction, was presumably acquired during the Permo-Carboniferous Reverse Superchron. At most sites, the remanence is carried exclusively by hematite, although it also resides in magnetite at some localities. Based on comparison between paleomagnetic poles derived from locality mean secondary magnetizations (e.g. Decl.=137.2°, Incl.=−18.2° α95=4.1°, k=221.4, N=7 of seven samples, Archean gneiss, northern Laramie Range; Decl.=148.7°, Incl.=−15.0°, α95=4.6°, k=77.0, N=14 of 23 samples, Sherman Granite, southern Laramie Range; Decl.=174.3°, Incl.=−18.9°, α95=6.6°, k=48.6, N=11 of 13 samples, metavolcanic rocks, Sandia Mountains) and late Paleozoic reference poles for North America, this secondary magnetization does not appear to have been acquired as a direct consequence of subareal erosion prior to deposition of younger sedimentary strata. At two localities, the age of the remanence is demonstrably younger than the oldest overlying strata. Locally, the nonconformity between basement rocks and late Paleozoic strata, as well as steep shear zones within the basement rocks, may have been efficient channelways for brines flushed out of basins created during Ancestral Rocky Mountains deformation. On a continental scale, migration of fluids and attending remagnetization is consistent with epeirogenic uplift of the Pangean supercontinent and a relative lowering of ground water levels. The fact that these basement rocks do not show evidence of subsequent remagnetization during latest Cretaceous to early Tertiary (Laramide) contraction in this area suggests that the characteristics of the late Paleozoic remagnetization phenomenon were unique in time.

Formation of Multiple Basins above Low Angle Detachments; A Centrifuge-Model Approach

Extended scaled centrifuge models, simulating lithospheric deformation, show that depocenters of sedimentary basins, formed above lithospheric detachment zones, migrate both in space and time. If associated with a low-angle mantle detachment, this migration results in formation of multiple basins from a single phase of model extension. Unlike the multiple detachment model proposed for formation of multiple basins or models invoking transtensional deformation, the current analogue models suggest that multiple basins may form due to continuous displacement along one single low-angle detachment during orthogonal rifting. Model results suggest that the geometry and location of sedimentary basins are strongly dependent on the coupling between the upper crust and mantle lithosphere by the ductile lower crust, and the dip of the prescribed “fault/shear zone” in the mantle lithosphere. Steeply dipping lithospheric detachment enhances coupling and hence formation of a single basin in the upper crust above the maximum asthenospheric upwelling in the centre of the rift. In contrast, low-angle detachment in the lithosphere promotes decoupling of the upper crust and any resulting basins from the underlying mantle lithosphere. The dip angle of the mantle detachment affects both location and geometry of the basin. A low-angle shear zone in the mantle lithosphere decouples the broad sag in the upper crust, whereas a steep shear zone promotes coupling and initiates narrow, but deep basins.