Structural and geological studies in the Naryn and Atbashi depressions (Tien Shan) and geological interpretation of magnetotelluric data

The article presents the results of complex geological and geophysical studies in the Naryn depression and Atbashi depression in the Middle Tien Shan. They included the geological interpretation of new magnetotelluric data along the detailed profile crossing the key segment of the Tien Shan, and the study of the morphology and spatial position of the sedimentary cover and basement structures. The compilation of the results of structural-geological and geophysical studies makes it possible to create a 2D model of the upper-crust geological structure, consistent with the structure of the electrical conductivity to depths of about 10 km and to analyze the structural features of deeper horizons. Two types of structural patterns of the electric conductivity, corresponding to the sedimentary complexes of the cover and the folded-metamorphic complexes of the basement, have been identified. Sedimentary rock complexes in depressions have a high electrical conductivity and subhorizontal structure. The upper crust above the K2 density layer is characterized by an alternation of rocks volumes with contrasting conductivity, elongated vertically. The recorded structure of the field confirms the presence of steep zones of fluid permeability and fragmentation, noted earlier in seismic profiles and probably corresponding to the Paleozoic structures of fragmentation of the Earth’s crust, activated during Alpine orogeny. Comprehensive research allow to characterize the deformations of the Cenozoic sedimentary complex and the surface of the Paleozoic basement associated with the Alpine activation of the key segment of the Tien Shan.

Structural Inheritance Controls Strain Distribution During Early Continental Rifting, Rukwa Rift

Little is known about rift kinematics and strain distribution during the earliest phase of extension due to the deep burial of the pre-rift and earliest rift structures beneath younger, rift-related deposits. Yet, this exact phase of basin development ultimately sets the stage for the location of continental plate divergence and breakup. Here, we investigate the structure and strain distribution in the multiphase Late Paleozoic-Cenozoic magma-poor Rukwa Rift, East Africa during the earliest phase of extension. We utilize aeromagnetic data that image the Precambrian Chisi Shear Zone (CSZ) and bounding terranes, and interpretations of 2-D seismic reflection data to show that, during the earliest rift phase (Permo-Triassic ‘Karoo’): 1) the rift was defined by the Lupa border fault, which exploited colinear basement terrane boundaries, and a prominent intra-basinal fault cluster (329° ± 9.6) that trends parallel to and whose location was controlled by the CSZ (326°); 2) extensional strain in the NW section of the rift was accommodated by both the intra-basinal fault cluster and the border fault, where the intra-basinal faulting account for up to 64% of extension; in the SE where the CSZ is absent, strain is primarily focused on the Lupa Fault. Here, the early-rift strain is thus, not accommodated only by border the fault as suggested by existing magma-poor early-rift models; instead, strain focuses relatively quickly on a large border fault and intra-basinal fault clusters that follow pre-existing intra-basement structures; 3) two styles of early-rift strain localization are evident, in which strain is localized onto a narrow discrete zone of basement weakness in the form of a large rift fault (Style-1 localization), and onto a broader discrete zone of basement weakness in the form of a fault cluster (Style-2 localization). We argue that the CSZ and adjacent terrane boundaries represent zones of mechanical weakness that controlled the first-order strain distribution and rift development during the earliest phase of extension. The established early-rift structure, modulated by structural inheritance, then persisted through the subsequent rift phases. The results of our study, in a juvenile and relatively well-exposed and data-rich rift, are applicable to understanding the structural evolution of deeper, buried ancient rifts.

Thermotectonic events recorded by U-Pb geochronology and Zr-in-rutile thermometry of Ti oxides in basement rocks along the P2 fault, eastern Athabasca Basin, Saskatchewan, Canada

Basement rocks below the Athabasca Basin, Saskatchewan, have been intensely altered through paleoweathering and multiple hydrothermal events, including the formation of world-class unconformity-type uranium deposits. Here, we demonstrate the utility of Ti-oxide thermochronology for identifying thermotectonic events in these altered rocks leading to uranium mineralization along basement structures. Rutile grains along the P2 fault, a major fault in the eastern Athabasca Basin, exhibit 207Pb/206Pb ages of ca. 1850−1700 Ma, with a weighted mean of 1757 ± 6 Ma (mean square of weighted deviation [MSWD] = 1.4, n = 116). The older ages (>1770 Ma) record regional metamorphism reaching a temperature of 875 °C during the Trans-Hudson orogeny. Pb diffusion modeling indicates that metamorphic rutile should exhibit cooling ages of 1760−1750 Ma. Rutile grains showing young ages, <1750 Ma, reflect isotopic resetting during regional asthenospheric upwelling between 1770 and 1730 Ma related to the emplacement of the Kivalliq igneous suite to the north. This thermotectonic event (temperature > 550 °C) promoted hydrothermal activity to produce silicified rocks, i.e., “quartzite,” along the P2 fault, which later focused mineralizing fluids for unconformity-type uranium deposits. The young rutile ages also indicate that the basement rocks remained hot until 1700 Ma, providing the maximum age for the deposition of the Athabasca sediments. Anatase yields a concordia age of 1569 ± 31 Ma (MSWD = 0.30, n = 5), which is within uncertainty of the oldest ages for uraninite of the McArthur River deposit. This age corresponds to the incursion of basinal fluids in the basement along the P2 fault during uranium mineralization.

Orogenic Segmentation and Its Role in Himalayan Mountain Building

The continental collision process has made a large contribution to continental growth and reconfiguration of cratons throughout Earth history. Many of the mountain belts present today are the product of continental collision such as the Appalachians, the Alps, the Cordillera, the Himalaya, the Zagros, and the Papuan Fold and Thrust Belt. Though collisional mountain belts are generally elongate and laterally continuous, close inspection reveals disruptions and variations in thrust geometry and kinematics along the strike of the range. These lateral variations typically coincide with cross structures and have been documented in thrust fault systems with a variety of geometries and kinematic interpretations. In the Himalaya, cross faults provide segment boundaries that, in some cases separate zones of differing thrust geometry and may even localize microseismicity or limit areas of active seismicity on adjacent thrust systems. By compiling data on structural segmentation along the length of the Himalayan range, we find lateral variations at all levels within the Himalaya. Along the Gish fault of the eastern Indian Himalaya, there is evidence in the foreland for changes in thrust-belt geometry across the fault. The Gish, the Ganga, and the Yamuna faults all mark boundaries of salients and recesses at the mountain front. The Benkar fault in the Greater Himalayan sequence of eastern Nepal exhibits a brittle-ductile style of deformation with fabric that crosscuts the older thrust-sense foliation. Microseismicity data from several regions in Nepal shows linear, northeast-striking clusters of epicenters sub-parallel to cross faults. The map pattern of aftershock data from the 2015 Nepal earthquakes has an abrupt northeast-trending termination on its eastern side suggesting the presence of a structure of that orientation that limited slip. The orientations of the recognized cross faults and seismic patterns also align with the extensional zones to the north on the Tibetan Plateau and the Indian basement structures to the south. Results from multiple studies are consistent with a link between cross faults and either of these structural trends to the north or south and suggest that cross faults may play a role in segmenting deformation style and seismic activity along the length of the Himalaya.

Subsurface Investigation of Basement Structures Using Electrical Resistivity Methods at Zainawa Village Kano, Nigeria

Electrical Resistivity Methods involving Schlumberger Vertical Electrical Sounding (VES) and Wenner Electrical Profiling (EP) were carried out to map the Geological features of the earth subsurface in Zainawa Area of Kano State, Nigeria. Five profiles were established; consist of six (6) VES points at each profile. GEOPULSE resistivity meter (SAS 300) was used for the data acquisition. The field data obtained have been analyzed using computer software (IPI2win) which gives an automatic interpretation of the apparent resistivity. A maximum of three geoelectric subsurface layers were delineated from the VES master curves. The geoelectric section beneath the study area was composed of top soil (clayey-sandy and sandy-lateritic), weathered layer, partly weathered (fractured basement) and fresh basement. The resistivity value for the topsoil layer varies from 20 Ωm to 600 Ωm with thickness ranging from 0.5 to 7.2 m. The weathered basement has resistivity values ranging from 15 Ωm to 593 Ωm and thickness of between 2.75 to 33.04 m. The fractured basement has resistivity values ranging from 201 Ωm to 835 Ωm and thickness of between 11 to 20.4 m. The fresh basement (bedrock) has resistivity values ranging from 1161 Ωm to 3115 Ωm with infinite depth. The depth to basement map was produced to give a good picture of the basement topography within the study area. The depth to basement ranges from 11 m around VES 01 to 85 m around VES 25 m. The map also reveals linear structures (VES 05, 21, 22 and VES 23) which trends in the NE-SW direction. These structures suggest a basement depression at these points. However, the depth from the topsoil to the bedrock surface varies between 2.5 to 37.75 m.