On Judge Daly Promontory, faults cut through a lower Paleozoic sedimentary succession and also affect small Paleocene occurrences. Structural analyses suggest that displacements were caused by a sinistral transpressive regime under north-northwest - south-southeast to northwest - southeast compression, which led to the formation of major faults and fault zones displaying sinistral, oblique, or reverse displacements. The deformational history is interpreted to have taken place in two stages: sinistral pull-apart graben formation during the Paleocene, followed by compression in the Eocene, and involved reactivation of former sinistral strike-slip faults. In a plate-tectonic scenario these two stages of deformation suggest that displacements along the Wegener Fault were related to 1) the sinistral displacement of the North American plate with respect to Greenland, and 2) the northward drift of Greenland as a result of combined seafloor spreading in the North Atlantic and Labrador Sea. The amount of left-lateral movement along the Wegener Fault does not support the existence of a plate boundary in Nares Strait.
Abstract Two Brasiliano schist belts (Porongos belt and São Gabriel block) of contrasting geotectonic setting exhibiting a distinct structural evolution are exposed in the southern Brazilian shield east of the Rio de la Plata craton. Trace-element and isotope data (Sm-Nd, Rb-Sr) demonstrate that the São Gabriel block and Porongos belt represent two distinct tectonostratigraphic blocks, with the former consisting of Neoproterozoic juvenile rocks, and the latter being characterized by intense reworking of old continental crust. The São Gabriel block in the west consists of Neoproterozoic relics of two Brasiliano magmatic arcs, an intra-oceanic arc, and a younger continental arc or active continental margin. The Porongos belt located on the Encantadas block, in contrast, formed in a passive-margin setting on thinned continental crust in an extensional or transtensional regime. Basin formation was accompanied by volcanism due to partial melting of the stretched continental crust. Comparison of the structural evolution and age data provide constraints on the timing of tectonic juxtaposition of the two terranes. Plate tectonic evolution started with opening of an oceanic basin to the east of the Rio de la Plata craton since at least 0.9 Ga, possibly earlier. First subduction occurred at about 0.87 Ga, leading to accretion of an intra-oceanic island arc to the passive margin of the Rio de la Plata craton. An active continental margin developed above a west-directed subduction zone beneath the block consisting of the Rio de la Plata craton and the attached island arc. At the same time, the Porongos basin formed on stretched continental crust of the Encantadas passive margin. The final stage of the São Gabriel event represents the collision of the Rio de la Plata craton with the Encantadas block between about 700 to 660 Ma. Progressive shortening resulted in SE-directed brittle thrust faulting in the São Gabriel block, and left-lateral ductile shear at the Dorsal de Canguçu Shear Zone (DCSZ) led to DB4 NW-vergent folding and thrusting in the Porongos belt. Activity at the DCSZ ceased at about 620 Ma, whereas deformation became localized in transcurrent shear zones accompanied by synkinematic granite intrusions in the Dom Feliciano belt farther to the east. Post-tectonic granites of 600-580 Ma are widespread in both western and eastern areas. However, shearing continued in localized fault zones lasting at least until 540 Ma as recorded by fault-related granites.
Between the Lake Hazen Fault Zone and Nares Strait on northeast Ellesmere Island, a several kilometre thick succession of deep-water and shelf deposits of the early Paleozoic Franklinian Basin was affected by Late Devonian to Early Carboniferous Ellesmerian deformation (D1). Dominant structures were long anticlines and synclines, and detachment faults above a major décollement at the base of the Franklinian Basin. In the Early Tertiary, sinistral strike-slip tectonics (D2) affected the Franklinian deposits and Paleocene basins parallel to Nares Strait. In the Eocene, all structures, as well as the D2-strike-slip faults, were affected or reactivated by Eurekan deformation (D3). Crustal shortening took place along the Ellesmerian décollement and was dominated by thrusting. On the surface, the Eurekan structures are characterized by major, hundreds-of-kilometres long, distinct thrust zones with subordinate, thrust-related fold structures (F3), and are interpreted as large-scale splay thrusts ramping up from the basal décollement. In the areas between those thrust zones, already folded during the Ellesmerian deformation, no evidence for Eurekan overprint was found.
To mitigate the uncertainties in assessing the geohazards and rock conditions that affect the nuclear, mining (including hydrocarbon extraction) and civil engineering activities in South Africa, the authors are working to improve the data coverage concerning the present day stress field. In principle, this implies constraining the principal compressive stresses (σ1>σ2>σ3) or at least the maximum horizontal compressive stress (σH) because knowledge of these parameters may determine the reactivation potential of known faults, or the behaviour of large excavations and wells. By contrast, much of the subcontinent is under-represented in the World Stress Map database. For this reason we have taken a number of steps, firstly by installing a compact Trillium seismic sensor at Stofkloof (Namaqualand; adjacent to the Vaalputs low and intermediate level radioactive waste disposal facility) and 1-sec sensors at Aggeneys and Koffiemeul (Bushmanland). All stations are equipped with Reftek data loggers and powered by solar panels. The data from these stations will be integrated with data from the national network to obtain focal mechanism solutions for seismic events in the Northern Cape southernmost Namibia region (also known as the Grootvloer cluster). These neotectonic stress tensors are then combined with σH parameters obtained from calliper logs of off-shore wells and from the geometry of joints, faults and sheared fractures in palaeosols (Bushmanland), soils and calcrete (NW Free State) and aeolianites (southern Cape). We also include underground rock engineering phenomenological observations and measurements, and data in the public domain. Our data consistently indicate a NNW-SSE oriented σH (Wegener Stress Anomaly or WSA) that prevails across most of central, southern and western South Africa, Namibia up to the Ruacana hydroelectric power plant at the Angola border. However, in the Congo basin, a few earthquake focal mechanisms suggest rotation of the regional σH to an E-W direction. Geological units affected by the WSA include the Cretaceous oceanic lithosphere (Walvis Ridge), the southern Angola-Kasai craton, the offshore Outeniqua and Orange Basins, the Cape Fold Belt, the Namaqualand metamorphic complex, and the Archaean Craton up to the Witwatersrand basin and the Witbank coal field. In contrast, σH azimuths in the NE-SW quadrants seem prevalent in E Mpumalanga, N Natal, and northern Limpopo. Whereas the origin of these latter stress azimuths are probably linked to the propagation of the E African Rift System, the strike-slip to transpressional character of much of the WSA remains unexplained. Similarly puzzling are a 3-fold increase in seismic events (proxy for strain rate) over the past 20 years in the Grootvloer cluster, and evidence that the WSA is the last of at least 7 successive tectonic regimes to leave their brittle imprints along the SE Atlantic seaboard since the break-up of W Gondwana.
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