Ancient Shear Zone Research Articles

New geochronological and geochemical constraints on the Banke, Dutsen Wai, Guraka, and Zuku alkaline granite complexes from the northcentral Nigerian younger granite province

ABSTRACT An integrated study was carried out on four anorogenic syenite-granite complexes (i.e. Banke, Dutsen Wai, Guraka, and Zuku) from the Nigerian Younger Granite (NaYG) province, in a bid to constrain their magma sources, rare metal mineralization potentials, and tectonic implications. These four complexes primarily consist of quartz syenite, granite porphyry, arfvedsonite aegirine granite, and biotite granite. The studied rocks are chemically characterized by high alkalis, HFSEs, Ga/Al ratios, and zircon saturation temperature (776–1127°C, average = 932°C), typically of A1-type granites. The zircon εHf(t) values of the arfvedsonite aegirine granite (−6.0 to −0.79) are higher than those of the granite porphyry (−9.57 to −7.67) and biotite granite (−9.49 to −5.86) suggesting moderate crustal contamination of mantle-derived melts for the origin of the peralkaline granites. Generation of the biotite granite likely involved inputs of both crustal- and mantle-derived melts and is evidenced by the presence of inherited Pan-African (600–610 Ma) zircons and low whole rock Nb/U and Nb/Ta ratios. Furthermore, input of crustal material likely increased the metal budget of the residual melt and enhanced Nb, Sn, and sulphide mineralization in the Banke granite porphyry and biotite granite and Dutsen Wai biotite granite. Zircon U-Pb dating results constrain the emplacement ages to early Mesozoic era, from the Banke granite porphyry of 180 ± 1 Ma, Dutsen Wai biotite granite of 177 ± 3 Ma, Guraka arfvedsonite aegirine granite of 227 ± 2 Ma and Zuku arfvedsonite granite of 186 ± 2 Ma. These age data indicate a local E-W age migration pattern for the alkaline magma emplacement in northcentral NaYG province. Integrated with other geological evidence, we favour a formation model for the anorogenic NaYG province likely driven by the reactivation of ancient shear zones and structures.

Stress in recrystallized quartz by electron backscatter diffraction mapping

AbstractThe long‐term state of stress at middle and lower crustal depths can be estimated through the study of the microstructure of exhumed rocks from active and/or ancient shear zones. Constitutive equations for deformation mechanisms in experimentally deformed rocks relate differential stress to the size of recrystallized grains. Cross et al. (2017, https://doi.org/10.1002/2017GL073836) take advantage of electron backscatter diffraction mapping to systematically separate new recrystallized grains from host grains on the basis of the measurable lattice distorsion within the grains. They produce the first calibrated piezometer for quartz with this technique, reproducing within error a previous calibration based on optical microscopy.

Crustal velocity structure associated with the eastern Tennessee seismic zone: Vp and Vs images based upon local earthquake tomography

AbstractWe present three‐dimensional P and S wave velocity models for the active eastern Tennessee seismic zone (ETSZ) using arrival time data from more than 1000 local earthquakes. A nonlinear tomography method is used that involves sequential inversion for model and hypocenter parameters. We image several velocity anomalies that persist through most of the inversion volume. Some anomalies support the presence of known features such as an ancient rift zone in northern Tennessee. Other anomalies reveal the presence of basement features that can be correlated with regional gravity and magnetic anomalies. We image a narrow, NE‐SW trending, steeply dipping zone of low velocities that extends to a depth of at least 24 km and is associated with the vertical projection of the prominent New York‐Alabama magnetic lineament. The low‐velocity zone may have an apparent dip to the SE at depths exceeding 15 km. The low‐velocity zone is interpreted as a major basement fault juxtaposing Granite‐Rhyolite basement to the NW from Grenville southern Appalachian basement to the SE. Relocated hypocenters align in near‐vertical segments suggesting reactivation of a distributed zone of deformation associated with a major strike‐slip fault. We suggest that the ETSZ represents reactivation of an ancient shear zone established during formation of the super continent Rodinia.

Mantle roots of major Precambrian shear zones inferred from structure of the Great Slave Lake shear zone, northwest Canada

Preserved, ancient orogens provide important perspective on modern continental convergent zones. Relatively deep levels of these systems are exposed at the surface, and these crystalline rocks promote the recording of seismic signals from great depths. In this study, multiazimuthal teleseismic observations reveal gently dipping discontinuities and wedge-shaped structures beneath the Great Slave Lake shear zone, Canada, which formed the Paleoproterozoic convergent boundary between the Archean Slave and Rae cratons during assembly of the Nuna supercontinent. This ancient shear zone experienced two distinct tectonic phases. An early convergence phase involved intracontinental underthrusting of the Slave craton beneath the Rae margin with little evidence of subduction. A later phase was characterized by brittle strike-slip faulting. Receiver functions exhibit discontinuities that imply the latter faults to be listric in the crust. A high-velocity layer below the Moho may mark eclogitic residues of melts that formed “calc-alkaline” granitoids within the crust of the shear zone, without invoking subduction-induced melting within the mantle. Uppermost mantle discontinuities and xenolith suites from nearby kimberlite pipes together defi ne lithological and structural geometries interpreted as two phases of wedging by the central Slave craton lithosphere, associated with either Slave craton construction or early Slave-Rae convergence. The new teleseismic data contain no direct or indirect evidence of faulting along near-vertical suturing structures within the crust or the mantle. The observed wedge-shaped cratonic margins imply that lithospheric blocks grew primarily by lateral accretion of similar blocks, with the strongest layer at 100–150 km depths. In the study area, the diamondbearing Slave lithospheric mantle wedge underlies the entire shear zone at depths where diamonds are stable, and data indicate that the Slave craton’s lithospheric mantle was intact and strong at these depths as early as 2.0–1.9 Ga, when this tectonic boundary was active.

Scale-dependent influence of pre-existing basement shear zones on rift faulting: a case study from NE Brazil

Rifting of continental crust initiates faults that are commonly influenced by pre-existing structures. We document newly identified faults cutting Precambrian units in the interior of the NE Brazilian margin to assess the effects of structural inheritance on both rift geometry and fault architecture. Stratigraphic and structural data indicate that the faults were active in the main phase of rifting of Gondwana. The influence of pre-existing structures on the Mesozoic rift faulting is scale dependent. Regionally, the faults trend parallel to subvertical, crustal-scale Brasiliano ( c . 750–540 Ma) shear zones. Mylonitic foliations and broadly distributed low strain in the lower crust indicated by shear-wave splitting controlled the overall orientation and kinematics of the rift faults. However, outcrop observations of the faults show that at scales up to hundreds of metres, mylonitic foliations have little influence on fault architectures. Faults cross-cut shear zones and do not commonly utilize foliation planes as shear fractures. Instead, slip zones and fractures have a range of orientations that form acute angles to the local foliation orientation. This observation explains the range of focal mechanisms associated with seismicity that coincides with ancient shear zones in intra-continental areas.

Exumação tectônica e reativação de paleolineamentos no Arco de Ponta Grossa: termocronologia por traços de fissão em apatitas

The South American Platform in southeastern Brazil is characterized by numerous dikes, shear zones, faults and fractures originated or even reactivated by several times between Upper Cretaceous and Paleogene/Neogene, and caused block tilting and normal faulting. Apatite fission-track ages (obtained in samples collected from Parana Basin, dikes, Precambrian basement and alkaline rocks in Ponta Grossa Arch) in association with geological evidences, allowed the cooling history reconstruction since approximately 60 Ma, in the central axis of Ponta Grossa Arch. Transtensional tectonic movements occurred between Paleogene and Neogene were the responsible for the reactivation of ancient shear zones and origin of southern portion of the Brazilian Southeastern Continental Rifts, represented in the study area by Curitiba basin, Guaraquecaba, Sete Barras and Cananeia Grabens, and Pariquera-Acu and Alexandra formations. In the Paleogene-Neogene limit, trantensional events related to the rearrangement of South American Platform deformed the sedimentary sections in the rift basins.

Jaibaras trough: an important element in the early tectonic evolution of the Parnaı́ba interior sag basin, Northern Brazil

The Jaibaras Trough (JT) is a late Precambrian to Early Paleozoic basin that is related to the subsequent formation of the Paleozoic–Mesozoic Parnaı́ba intracratonic basin in northern Brazil. Based on available geological and geophysical data, we propose that the JT is a prototypical rift basin formed by brittle reactivation processes along ancient shear zones associated with the NE–SW Transbrasiliano Lineament of the Brazilian–Panafrican Orogeny. The JT evolution is partially constrained by the stratigraphic sequences outcropping at the border of the basin or drilled by exploratory boreholes. The preserved sedimentary column records, in a broad sense, the sedimentary and tectonic controls on the early subsidence in the Parnaiba Basin. These sequences are constituted by basal fault-scarp-related paraconglomerates followed laterally and vertically by fluvial–lacustrine sandstones interbedded with reddish shale. The JT geological evolution is also constrained by petrology of four magmatic events (Coreaú Dike Swarm, Mucambo and Meruoca plutons and mafic Parapuı́ Suite) related to continental rifting. The Late Precambrian–Early Paleozoic age of JT is perfectly compatible with continental fragmentation during the processes of lithospheric stretching that affected western Gondwana at that time. The contemporaneous development of a magmatic-sedimentary system, dated as Cambrian to Ordovician and confined to the rift trend, raised the thermal gradient and produced a mantle softening discontinuity, giving rise to a broad uplifted region. This might explain the close temporal and spatial association between the Transbrasiliano Lineament and the Early Paleozoic depositional axis of the Parnaı́ba Basin. An integrated interpretation of the JT and the early tectonostratigraphic behaviour of the Parnaı́ba Basin indicates that the JT might constitute a precursory stage that was slightly inverted before the initiation of sag deposition in the intracratonic basin.

Growth of plastic shear zone and its duration inferred from theoretical consideration and observation of an ancient shear zone in the granitic crust

A new model for growth of plastic shear zone is proposed based on the basis of a theory of fluid dynamics coupled with a rheological constitutive function, and is applied to a natural shear zone. Mylonite, ultramylonite and other ductile fault rocks are well known to deform in a plastic flow regime. The rheological behavior of these kinds of rocks has been well documented as a non-linear viscous body, which is empirically described as , where : strain rate, τ: shear stress, Q: activation energy, R: universal gas constant, T: absolute temperature, and A and n are constants. Strain rate- and temperature-dependent viscosity is obtained by differentiating the equation, and simplified by substituting n = 1. Then, substitution of the equation into a diffusion equation, , derives an equation δ = 4[t/p · A exp(−Q/RT)]1/2, where δ: thickness of active layer of viscous deformation, ν: kinematic viscosity, and ρ: density. The duration of creep deformation along the ancient plastic shear zone (thickness: 0.076 m) is estimated to be around 760 s, in a temperature range from 300 to 500°C. This estimation is rather good agreement with intermittent creep during inter-seismic period, than steady state creep or co-seismic slip.

Geologia e Geoquímica (ETR e Inclusões Fluidas) das Mineralizações (F, BA) da Parte Norte do Distrito Fluorítico de Santa Catrina, Sul do Brasil

This work deals on the fluorite deposits from the Grão Pará (GP) and Rio Bravo Alto (RBA) veins systems both situated at the northern part of Santa Catarina Fluorite District. The GP deposits are controlled by an ancient shear zone. Ore deposition is related to two tectonic phases: (1) dextral transtensive reactivation related to a NE/SW compression; deposition of green-purple massive or coarse banded fluorite; tectonic and hydraulic breccias assign successive transitions from hydrostatic to tectonic regimes; the ore was affected by latter silicification; (2) reactivation as normal fault, related to a extensional tectonic; deposition of purple and yellow banded fluorite, collapse breccias and cocardes. First phase fluorite was deposited at temperatures decreasing from 170º to 120ºC; silicification temperature is around 140ºC. Second phase fluorite was deposited at temperatures between 140º and 120ºC. The fluid inclusions salinity range from 0 to 2% eq. NaCl. The isotopic composition of water from primary fluid inclusions in the fluorite at RBA (δD ͌ -60‰ SMOW) is typical of meteoric water from interior continental region. The RBA fluorite has the same HREE signature as fluorite from the southern district deposits and regional granites. REE and fluid inclusion data reject a link with alkaline magmatism. Fluorine was leached from accessory fluorite of regional granites. The fluorite deposition was connected to a single hydrothermal event correlated to the earlier hydrothermal event described at the southern district.

New regional gravity and magnetic maps of the Levant

A new compilation of regional gravity and magnetic maps of the Levant, based on detailed local maps and data bases for various parts of the region, is presented. The most apparent feature of the Levant gravity map is the regional high associated with the Mediterranean basin. High values extend eastward on land to the Dead Sea rift (DSR), suggesting a relatively thin basic crust under northern Israel and Lebanon. The magnetic pattern offers additional evidence on the transform nature of the DSR; it also suggests that the Gulf of Suez is associated with an arcuate lineament which extends in a NNE direction, deep into the Mediterranean Sea, possibly up to the Cyprean Arc. This apparent lineament may indicate an ancient shear zone.

Crustal conductors along the SVEKA Profile in the Fennoscandian (Baltic) Shield, Finland

SUMMARY Electrical properties of the crust in the central Fennoscandian (Baltic) Shield are examined along the 435 km magnetotelluric profile SVEKA. A 2-D geoelectric model shows large variations in the crustal resistivity with the model resistivities ranging from 0.1 to 35 000 Ωm. The main geoelectrical features of the crust and upper mantle are: (1) a resistive Archaean lower crust (conductance ≤30 S). (2) A conductive Palaeoproterozoic lower crust beneath the Central Finland Granitoid Complex (170-900S). Free H2O-rich fluids trapped into the lower crust seem less plausible causes for enhanced conductivity as long retention times are required and no possible sources seem to be available to produce fluids into the dry lower crust. Most likely causes for permanent enhancement of lower crustal conductivity include the conductive fill of ductile shear zones, conductive graphite-bearing lamellae, conduction in mafic rocks perhaps via conduction in magnetite, or conductive solid remnants of an ancient shear zone between two crustal blocks. Thin graphitic grain-boundary films precipitated from CO2-rich fluids also seem less plausible. (3) A very resistive (10 000-35 000 Ωm) upper crust of the Palaeoproterozoic Central Finland Granitoid Complex and the Archaean Domain. (4) Narrow and elongated conductors (0.1-100 Ωm) beneath the Palaeoproterozoic Kainuu (KSB) and Tampere (TSB) Schist Belts are caused by graphite-bearing metasedimentary rocks buried deep into the crust in the collisions of crustal terranes during the formation of the Palaeoproterozoic Svecofennian crust. The 500 km long TSB conductor represents an internal Svecofennian terrane boundary whereas the KSB conductor is a part of a discontinuous Archaean-Proterozoic boundary conductor. (5) Resistive upper mantle. There are no indications of an upper mantle conducting layer within the uppermost 300 km in the central part of the Fennoscandian Shield.

Karoo igneous activity, and the early stages of the break-up of Gondwanaland

Abstract The widespread early Jurassic Karoo vulcanism of southern Africa appears to have been generated from a large-scale mantle plume originating in the sub-lithospheric mantle, but with a considerable additional contribution in some areas of material from the lithosphere. Subduction-related vulcanism may also play a significant role, but the relationships of the two types of vulcanism, if indeed a clear distinction does exist, are at present uncertain. Continental break-up probably took place in two stages, the first of which, in the early Jurassic, was not accompanied by true seafloor spreading. In this stage, the reactivation of an ancient shear zone is thought to have resulted in the movement of Antarctica north-eastwards relative to Africa (in the present-day African reference frame), with the creation of a zone of thinned continental crust, or a mixed zone of oceanic and continental crust, in Mozambique. The second stage took place 10–30 Ma later as Antarctica and Madagascar moved southwards relative to Africa with the formation of the oldest-known oceanic crust of the Indian Ocean.

Thermal evolution of the western Svalbard margin

The northern Norwegian-Greenland Sea opened up as the Knipovich Ridge propagated from the south into the ancient continental Spitsbergen Shear Zone. Heat flow data suggest that magma was first intruded at a latitude of ≈75° N around 60 m.y.b.p. By 40–50 m.y.b.p. oceanic crust was forming at a latitude of 78° N. At ≈12 m.y.b.p. the Hovgard Transform Fault was deactivated during a northwards propagation of the Knipovich Ridge. Spreading is now in its nascent stages along the Molloy Ridge within the trough of the Spitsbergen Fracture Zone. Spreading rates are slower in the north than the south. For the Knipovich Ridge at 78° N they range from 1.5–2.3 mm yr-1 on the eastern flank to 1.9–3.1 mm yr-1 on the western flank. At a latitude of 75° N spreading rates increase to 4.3–4.9 mm yr-1. Thermal profiles reveal regions of off-axial high heat flow. They are located at ages of 14 m.y. west and 13 m.y. east of the northern Knipovich Ridge, and at 36 m.y. on the eastern flank of the southern Knipovich Ridge. These may correspond to episodes of increased magmatic activity; which may be related to times of rapid north-wards rise axis propagation. The fact that the Norwegian-Greenland Sea is almost void of magnetic anomalies may be caused by the chaotic extrusion of basalts from a spreading center trapped within the confines of an ancient continental shear zone. The oblique impact of the propagating rift with the ancient shear zone may have created an unstable state of stress in the region. If so, extension took place preferentially to the northwest, while compression occurred to the southeast between the opening, leaking shear zone and the Svalbard margin. This caused faster spreading rates to the northwest than to the southeast.