U–Pb and Rb–Sr geochronology of the Wedgeport granitoid pluton, southwestern Nova Scotia

Zircons from biotite monzogranite of the Wedgeport Pluton, intrusive into deformed metasediments of the Cambrian(?) Goldenville Formation in the southwestern Meguma Terrane of Nova Scotia, yield concordant U–Pb ages of 316 ± 5 Ma. This is interpreted as the time of intrusion and crystallization. Within the error limits, the 323 ± 12 Ma Rb–Sr whole-rock isochron age is identical and gives an initial 87Sr/86Sr ratio of 0.7137 ± 0.0056. Rb–Sr analyses of mineral separates of biotite, potassium feldspar, and quartz–plagioclase from several samples yield subparallel, internal isochrons with an average age of 257 ± 8 Ma. Initial ratios of the internal isochrons range from 0.716 to 0.759. A slow-cooling model for these latter data is discarded because the mineral data fall on straight lines. Instead, a reheating event related to plutonism ca. 257 Ma ago, which was sufficient to cause local grain-to-grain migration and reequilibration of strontium and rubidium but not large-scale redistribution, is invoked. This reheating is also inferred to be responsible for the hydrothermal alteration and Sn–U mineralization concentrated along the northwestern margin of the pluton. A dextral northeast–southeast shear zone cutting the pluton is also inferred to be ca. 257 Ma old. It may be related to the last stages of westward obduction of the Meguma Terrane.These results provide a clear example of Permo-Carboniferous plutonism in the southwestern Meguma Terrane and suggest a similar interpretation may apply to other anomalously young ages recorded in this area. In light of these results, the Permo-Carboniferous age of the large East Kemptville tin deposit and its location in a dextral shear zone suggest that the association of younger plutonism and shear zones may be a significant factor for economic mineralization.

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Large slow rock-slope deformations affecting hydropower facilities

10.5194/egusphere-egu2020-8288 ◽

2020 ◽

Author(s):

Margherita C. Spreafico ◽

Federico Agliardi ◽

Matteo Andreozzi ◽

Alessandro Cossa ◽

Giovanni B. Crosta

Keyword(s):

Shear Zone ◽

Urban Areas ◽

Large Scale ◽

Rock Slope ◽

Shear Zones ◽

Seasonal Effects ◽

Evolutionary Stage ◽

3D Analysis ◽

Future Evolution

Large-scale creeping landslides are widespread in alpine areas. Associated long-term, slow deformations threaten urban settlement, railways, main roads and hydropower facilities, on which our society is strictly dependent. Over the next decades, the continuous growing of the global population, the resulting increase in the urbanization (also closer to hazard-prone areas), and the climate change (e.g. melting of alpine glaciers) will increase these interactions and the related risk. Nevertheless, assessing the vulnerability of different types of elements at risk to this kind of hazard is not obvious, especially when hydropower structures (including dams, tunnels, penstocks, etc.) are involved. Large rockslides complexity often results in a variety of different evolutionary trends, making their forecasting and risk reduction a challenge. While catastrophic collapse can cause huge instantaneous damages, slow movements along long periods may lead to progressive damage of structures and infrastructures. In the alpine and pre-alpine areas of Lombardia (Central Italian Alps), slow rock-slope deformations affect an area of 750 km2, threatening more than 10 km2 of urban areas and about 100 km of penstocks or tunnels related to hydropower facilities. Here we focus on the Mt. Palino slope (Valmalenco, Italian Central Alps), that is affected by a complex, apparently long-lived DSGSD (Deep seated Gravitational Slope Deformation) with a relief exceeding 1000 m. The slope hosts hydropower facilities and a tourist resort. In order to recognize dominant processes and their possible evolution (internal deformation, low-rate steady activity, progressive behaviour, seasonal effects) for better risk assessment and mitigation, we investigated the volume and depth of displaced rock mass and the possible localization of deformations along a basal shear zone.&#160; Geomechanical and geomorphological surveys, seismic tomography, deep borehole logs and monitoring data (borehole instrumentation, precise levelling, topographic and GB-InSAR) allowed recognizing different sectors with different evolutionary stage and activity degree. The DSGSD which affect the entire Mt. Palino was probably active before the last LGM (Last Glacial Maximum), while only the northern slope sector is now considered as active. We recognized multiple nested phenomena faster than the main mass, identified as large rockslides. They are suspended over the valley floor and may evolve into fast rock avalanches. One of them is located in correspondence with the hydropower penstock, causing differential deformation to the structure. Borehole evidence of localization along cataclastic shear zones was found, motivating a petrographic geomechanical characterization of both rock masses and shear zone samples. Integrated 3D analysis of different information permitted to reconstruct displacement patterns, long-term mechanisms and the controlling factors of possible future evolution.&#160;

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Stratigraphy and depositional setting of the Silurian-Devonian Rockville Notch Group, Meguma terrane, Nova Scotia, Canada

Atlantic Geology ◽

10.4138/atlgeol.2017.015 ◽

2017 ◽

Vol 53 ◽

pp. 337-365 ◽

Cited By ~ 4

Author(s):

Chris E. White ◽

Sandra M. Barr

Keyword(s):

Nova Scotia ◽

Volcanic Rocks ◽

Rock Formation ◽

Metasedimentary Rocks ◽

South Mountain Batholith ◽

Metavolcanic Rocks ◽

Northwestern Margin ◽

Meguma Terrane ◽

Two Stages ◽

Depositional Setting

The Silurian–Devonian Rockville Notch Group occurs in five separate areas along the northwestern margin of the Meguma terrane of southern Nova Scotia. In each area, the lowermost unit of the group is the White Rock Formation, which unconformably overlies the Lower Ordovician Halifax Group. Early Silurian U–Pb (zircon) dates from metavolcanic rocks in the White Rock Formation indicate that the unconformity represents a depositional gap of about 25 Ma. The U–Pb ages are consistent with early Silurian (Llandovery) trace fossils and sparse shelly faunas in metasedimentary rocks interlayered with the metavolcanic rocks. The metasedimentary rocks locally contain phosphatic ironstone and Mn-rich beds, and are overlain by mainly metasiltstone with abundant quartzite and metaconglomerate lenses. Some of the latter were previously interpreted to be Ordovician tillite. The White Rock Formation is conformably overlain by the slate- and metasiltstone-dominated Kentville Formation, which contains Upper Wenlock to Pridoli graptolites and microfossils. The overlying Torbrook Formation consists of metalimestone, metasandstone and metasiltstone, interbedded with phosphatic ironstone and minor mafic metatuff, and contains Pridoli to early Emsian fossils. It is in part laterally equivalent to the New Canaan Formation in the Wolfville area, which is dominated by slate, pillowed mafic metavolcanic rocks and fossiliferous metalimestone. Volcanic rocks in the Rockville Notch Group are alkalic and formed in a within-plate setting, probably related to extension as the Meguma terrane rifted from Gondwana. This process may have occurred in two stages, Early Silurian and Early Devonian, separated by a hiatus in volcanic activity. Stratigraphic differences suggest that the Meguma terrane was not adjacent to Avalonia before emplacement of the South Mountain Batholith.

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U–Pb (zircon) ages and provenance of the White Rock Formation of the Rockville Notch Group, Meguma terrane, Nova Scotia, Canada: evidence for the “Sardian gap” and West African origin

Canadian Journal of Earth Sciences ◽

10.1139/cjes-2017-0196 ◽

2018 ◽

Vol 55 (6) ◽

pp. 589-603 ◽

Cited By ~ 8

Author(s):

Chris E. White ◽

Sandra M. Barr ◽

Ulf Linnemann

Keyword(s):

Nova Scotia ◽

Detrital Zircon ◽

Rock Formation ◽

Metasedimentary Rocks ◽

Metavolcanic Rocks ◽

Northwestern Margin ◽

Lower Palaeozoic ◽

Volcanic Pile ◽

Meguma Terrane ◽

Age Range

The White Rock Formation is the lowermost formation of the Rockville Notch Group, an assemblage of Silurian–Devonian rocks preserved in five areas along the northwestern margin of the Meguma terrane of Nova Scotia. The formation consists mainly of mafic and felsic metavolcanic rocks, interlayered with and overlain by marine metasedimentary rocks. Felsic metatuff has now been dated from four locations near both the bottom and top of the volcanic pile and yielded a narrow age range (with errors) of about 446–434 Ma. These dates confirm a 30 Ma hiatus after deposition of the Early Ordovician Hellgate Formation in the underlying Halifax Group. This hiatus is coeval with the “Sardian gap” in the Lower Palaeozoic of peri-Gondwanan Europe. The metavolcanic–metasedimentary assemblage is overlain by mainly metasiltstone with abundant quartzite and metaconglomerate lenses; some of the latter were previously interpreted to be Ordovician tillite, an interpretation no longer viable. New detrital zircon data from metasedimentary samples indicate that the major sediment sources for the White Rock Formation have ages of ca. 670–550 and ca. 2050 Ma, similar to ages from the underlying Goldenville and Halifax groups. A smaller population of Mesoproterozoic zircon grains indicates that the Meguma terrane interacted with a terrane composed mainly of Mesoproterozoic crust during the Silurian and Devonian. The occurrence of the “Sardian gap” and the detrital zircon record constrain the palaeoposition of the Meguma terrane to have been close to Cadomia and West Africa in the Early Cambrian to Early Silurian.

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The Borborema Strike-Slip Shear Zone System (NE Brazil): Large-Scale Intracontinental Strain Localization in a Heterogeneous Plate

Lithosphere ◽

10.2113/2021/6407232 ◽

2021 ◽

Vol 2021 (Special 6) ◽

Author(s):

Sergio P. Neves ◽

Andréa Tommasi ◽

Alain Vauchez ◽

Thais Andressa Carrino

Keyword(s):

Shear Zone ◽

Large Scale ◽

Shear Zones ◽

Strike Slip ◽

Northeastern Brazil ◽

Main Stage ◽

Plate Boundaries ◽

Deformation Localization ◽

Continental Scale ◽

Dextral Shear

Abstract Large-scale strike-slip faults are fundamental tectonic elements of the continental lithosphere. They constitute plate boundaries (continental transforms), separate terranes with contrasting geological histories within accretionary orogens, or accommodate heterogeneous deformation in intracontinental settings. In ancient orogens, where deeper levels of the crust are exposed, these faults are expressed as shear zones materialized by up to tens of km-wide mylonitic belts. The Borborema shear zone system in northeastern Brazil is one of the largest and best-exposed intracontinental strike-slip shear zone systems in the world, cropping out over 250,000 km2. Here, we review its main geophysical, structural, petrologic, and geochronologic characteristics and discuss the factors controlling its development. This complex continental scale shear zone system is composed of a set of NE- to NNE-trending dextral shear zones from which there are two major E-trending dextral shear zones with horse-tail terminations into the transpressional belt branch, as well as several smaller E-trending dextral and NE-trending dextral and sinistral shear zones. The major shear zones are marked by extensive linear or curvilinear magnetic gradients, implying their continuation at depth. The major shear zones are materialized by migmatite to amphibolite-facies mylonites, but the entire system shows evidence of late deformation at lower temperatures. The system developed during the late stages of the Neoproterozoic Brasiliano (Pan-African) orogeny (mainly from 590 to 560 Ma), postdating by more than 20 Ma the main stage of contractional deformation. Localization of strike-slip shearing in this intraplate setting was controlled by rheological contrasts between blocks with distinct Paleoproterozoic histories, the presence of preorogenic Neoproterozoic rifts, the craton geometry, and zones of enhanced magmatic activity, highlighting the importance of rheological heterogeneity in controlling shear zone nucleation and evolution.

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Geochronological evidence for multiple tectono-thermal overprinting events in the East Kemptville muscovite–topaz leucogranite, Yarmouth County, Nova Scotia, Canada

Canadian Journal of Earth Sciences ◽

10.1139/e91-020 ◽

1991 ◽

Vol 28 (2) ◽

pp. 209-224 ◽

Cited By ~ 6

Author(s):

D. J. Kontak ◽

R. F. Cormier

Keyword(s):

Nova Scotia ◽

North America ◽

Low Temperature ◽

Thermal Activity ◽

Geochronological Data ◽

Step Heating ◽

Meguma Terrane ◽

Tin Deposit ◽

Incremental Step ◽

Geochronological Evidence

The East Kemptville muscovite–topaz leucogranite, located in Yarmouth County, Nova Scotia, Canada, is host rock to the only producing primary tin deposit in North America (56 Mt, 0.165% Sn). Previous geochronological studies include (i) Rb–Sr whole-rock analyses of the quartz–topaz greisens, which indicated a date of 337 ± 5 Ma, and (ii) 40Ar/39Ar analyses of greisen muscovite, which indicated apparent plateau dates of ca. 300 Ma. However, the pervasive development of deformational fabrics at East Kemptville suggests that both the Rb–Sr whole-rock and 40Ar/39Ar muscovite ages are at best minimum estimates for the inferred time of mineralization. In the present study, Rb–Sr whole-rock and mineral (muscovite, plagioclase, K-feldspar) analyses and 40Ar/39Ar incremental-step heating of a muscovite separate indicate the following: (i) diffusion of Sr on the whole-rock scale terminated at 344 ± 5 Ma (11 point isochron date), coincident with closure of muscovite to intracrystalline diffusion of Ar (apparent plateau date of 338 ± 2 Ma) and (ii) internal reequilibration of Sr among muscovite, feldspar, and whole rock varied considerably such that Rb–Sr whole rock – muscovite pairs give dates of 361–311 Ma (mean = 330 Ma, n = 7), whereas whole rock – plagioclase – K-feldspar give dates of 276–240 Ma (mean = 254 Ma, n = 7). This younger thermal event is reflected in apparent dates of 269–286 Ma for the low-temperature steps of the 40Ar/39Ar muscovite age spectrum.Collectively the data indicate that the East Kemptville area either cooled slowly over a protracted period of time (ca. 100 Ma) or experienced episodic tectono-thermal activity at ca. 344, 330, and 254 Ma. Examination of previously published geochronological data for the southern Meguma Terrane indicates that these aforementioned ages broadly coincide with earlier documented magmatic or tectono-thermal events (e.g., intrusion of Wedgeport Pluton at 315 Ma). Inferences by some workers of mid-Carboniferous magmatism in the Meguma Terrane are, however, not supported by the present study.

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Petrology, age, and tectonic setting of the White Rock Formation, Meguma terrane, Nova Scotia: evidence for Silurian continental rifting

Canadian Journal of Earth Sciences ◽

10.1139/e01-074 ◽

2002 ◽

Vol 39 (2) ◽

pp. 259-277 ◽

Cited By ~ 24

Author(s):

Lisa A MacDonald ◽

Sandra M Barr ◽

Chris E White ◽

John WF Ketchum

Keyword(s):

Nova Scotia ◽

Shear Zone ◽

Tectonic Setting ◽

Volcanic Rocks ◽

Granitic Rocks ◽

Chemical Characteristics ◽

Rock Formation ◽

Meguma Terrane ◽

Felsic Volcanic ◽

Felsic Volcanic Rocks

The White Rock Formation in the Yarmouth area of the Meguma terrane of southern Nova Scotia consists mainly of mafic tuffaceous rocks with less abundant mafic flows, epiclastic and clastic sedimentary rocks, and minor intermediate and felsic crystal tuff. It is divided into seven map units that appear to young from west to east, inconsistent with a previously assumed synclinal structure. The White Rock Formation is flanked on both northwest and southeast by mainly the Cambrian to Lower Ordovician Halifax Formation; the western contact is interpreted to be a sheared disconformity, whereas the eastern contact appears to be a major brittle fault and shear zone that juxtaposes different crustal levels. The granitic Brenton Pluton forms a faulted lens within the eastern shear zone. A felsic tuff from the upper part of the White Rock Formation yielded a UPb zircon age of 438+32 Ma, identical within error to published ages for the Brenton Pluton and felsic volcanic rocks near the base of the White Rock Formation in the Torbrook area of western Nova Scotia. The chemical characteristics of the mafic volcanic rocks and associated mafic intrusions consistently indicate alkalic affinity and a continental within-plate setting. The felsic volcanic rocks and Brenton Pluton have chemical characteristics of within-plate anorogenic granitic rocks, and the pluton is interpreted to be comagmatic with the felsic volcanic rocks. The igneous activity may have occurred in response to extension as the Meguma terrane rifted away from Gondwana in the latest Ordovician to Early Silurian. Epsilon Nd values are similar to those in voluminous Devonian plutonic rocks of the Meguma terrane, and the magmas appear to have been derived from similar sources.

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Large-scale interseismic deformation along the Altyn Tagh Fault determined from Sentinel-1 InSAR

10.5194/egusphere-egu21-10278 ◽

2021 ◽

Author(s):

Lin Shen ◽

Andy Hooper ◽

John Elliott ◽

Tim Wright

Keyword(s):

Velocity Field ◽

Tibetan Plateau ◽

Shear Zone ◽

Large Scale ◽

Slip Rate ◽

Shear Zones ◽

Elastic Half Space ◽

Strike Slip ◽

Space Model ◽

Altyn Tagh

The 1600 km-long Altyn Tagh Fault (ATF) is a major intra-continental strike-slip fault along the Northern Tibetan Plateau, the slip rate of which has significant implications for our understanding of the present-day tectonic processes of the Tibetan Plateau region. We present an interseismic velocity field along ~1500 km length of the fault, derived from Sentinel-1 interferograms spanning the period between late 2014 and 2019. It is the first time such a large-scale analysis has been carried out for this fault with Interferometric Synthetic Aperture Radar (InSAR).Using a modified elastic half-space model, we find significant strain accumulation along the 1500 km length of the ATF, at a relatively fast rate of ~10 mm/yr and quite localised along the fault. The results indicate an eastward decrease of the slip rate along the fault from 11.6 &#177; 1.0 mm/yr to 7.5 &#177; 1.2 mm/yr over the western portion to the central portion, whereas it increases again to 11.1 &#177; 1.1 mm/yr over the eastern portion. Furthermore, the results suggest that no significant creeping occurs along the fault.We find a high slip rate of 11.5 &#177; 1.0 mm/yr along the south-western segment of the ATF, a region not typically covered by previous studies, is transferred to the structurally linked left-lateral strike-slip Longmu-Gozha Co Fault. It demonstrates that the generation of the NS-trending normal faulting events in this region, such as the 2008 Mw 7.2 Yutian earthquake, is ascribed to the EW-trending extensional stress at the Ashikule step-over zone between the two left-lateral faults. We also find a high surface shear strain rate greater than 0.4 &#956;strain/yr in this region, which could be caused by the stress loading effects of the recent seismic activities.To investigate the pattern of strain localisation along the ATF, we fit a shear zone model to the derived long-term InSAR velocity field. Inverting for shear zone width reveals two broad shear zones along the ATF, where the strain is distributed over multiple strands rather than concentrated on a single narrow strand. The broad shear zones explain the high estimates of the locking depth found when using the elastic half-space model and also off-fault seismic activity on the strands away from the ATF in these areas. The results also show a relatively wider shear zone from the central portion eastward, where the ATF breaks into three parallel strands.&#160;This study suggests that a slip deficit of around 1 m has been accumulated along the ATF over the last century, and indicates that the fault is capable of rupturing with the potential for a magnitude 7.5 or larger earthquake.

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Effective rheology of a two-phase subduction shear zone calculated by numerical simple shear experiments

10.5194/egusphere-egu2020-11189 ◽

2020 ◽

Author(s):

Paraskevi Io Ioannidi ◽

Laetitia Le Pourhiet ◽

Onno Oncken ◽

Philippe Agard ◽

Samuel Angiboust

Keyword(s):

Shear Zone ◽

Simple Shear ◽

Large Scale ◽

Physical Nature ◽

Shear Zones ◽

Dislocation Creep ◽

Rheological Parameters ◽

Two Phase ◽

Phase Medium ◽

Modelling Studies

The physical nature and the rheology of a subduction shear zone play an important role in the deformation and the degree of locking along its interface with the upper plate. Inspired from exhumed subduction shear zones that exhibit block-in-matrix characteristics (m&#233;langes), we create synthetic models with different proportions of strong clasts within a weak matrix and compare them to natural m&#233;lange outcrops. Using 2D Finite Element visco-plastic numerical simulations and simple shear kinematic conditions, we determine the effective rheological parameters of such a two-phase medium, comprising blocks of basalt embedded within a wet quartzitic matrix. We treat our models and their structures as scale-independent and self-similar and upscale published field geometries to km-scale models, compatible with large-scale far-field observations. Exhumed subduction m&#233;langes suggest that deformation is mainly taken up by dissolution-precipitation creep. However, such flow laws are neither well-established yet experimentally nor of ample use in numerical modelling studies. In order to make our results comparable to and usable by numerical studies, we assume dislocation creep as the governing flow law for both basalt and wet quartz and by using different pressures, temperatures and strain rates we provide effective rheological estimates for a natural subduction interface. Our results suggest that the block-in-matrix ratio affects deformation and strain localization, with the effective dislocation creep parameters varying between the values of the strong and the weak phase, in cases where deformation of both materials is purely viscous. As the contribution of brittle deformation of the strong blocks increases, however, the value of the stress exponent, n, can exceed that of the purely strong phase.

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