NW-SE Trending Research Articles

Petrology and crustal evolution of the Tartarugal Grande Granulitic Complex - Northeastern Amazonian Craton

The Tartarugal Grande Complex (TGC) is an association of high-grade metamorphic rocks with Archean and Paleoproterozoic protoliths, located at the border of the Archean Amapá and Rhyacian Lourenço domains, in southeastern Guiana Shield, northeast of the Amazonian Craton. It consists of felsic granulites, and aluminous leucogneisses and rare mafic granulites. Both granulites and gneisses are affected by thrust and transcurrent shear zones along NW-SE trend. Migmatization with quartz-feldspathic neosomes, with or without orthopyroxene, and garnet-rich neosomes are present in felsic granulites and leucogneisses, respectively. The gneisses have high silica contents and are peraluminous due to the presence of biotite, garnet, and cordierite, indicating the derivation from metasedimentary sequences. The felsic granulites have medium to high silica content and geochemical signature of calc-alkaline magmatic arc series. The mafic granulites are small bodies embedded in the felsic granulites and leucogneisses. They display geochemical affinities with tholeiitic series and represent ancient diabase dykes. The metamorphic paragenesis in the granulite facies is represented by Pl-antiperhtite + Qtz + Mc-mesoperthite + Opx + Cpx + Bt ± Hbl (enderbitic granulite), Pl-antiperhtite + Qtz + Mc-mesoperthite + Opx ± Bt (charnoenderbitic granulite); Mc-mesoperthite + Qtz + Pl + Opx + Bt (charnockitic granulite), Pl (An60)+Opx + Cpx ± Hbl ± Grt (mafic granulite), and Qtz + Mc + Pl ± Bt ± Grt ± Crd (leucogneiss). The presence of orthopyroxene, clinopyroxene and hornblende in equilibrium in the granulites allows estimating temperatures at 800° ± 20 °C. In addition, non-extensive anatexis under anhydrous conditions produced charnockitic neosomes in the granulites and garnet-bearing leucogranite neossomes in the paragneisses. The presence of cordierite in leucogneisses indicates low-medium pressure conditions around 6–7 kbar. The partial or total replacement of pyroxenes by hornblende and/or biotite, garnet by biotite, clinopyroxene by titanite, and myrmekite intergrowths, indicate retrometamorphism and cooling during exhumation to higher crustal levels. The U–Pb LA-ICP-MS dating on zircon from a charnoenderbitic granulite yielded an age of 2082 ± 5 Ma for the crystallization of the igneous protolith of the granulites. Metamorphic zircons from an enderbitic granulite yielded a U–Pb LA-ICP-MS age of 2045 ± 14 Ma, which corresponds to the high-grade metamorphic episode that has affected the region at the end of the Rhyacian. The age of the protoliths is not constrained but 207Pb/206Pb dates at 2.57–2.70 Ga indicated inherited crystals and/or xenocrystals from Archean source. In agreement with previous works, we conclude that the TGC consists of calcium-alkaline magmatic rocks and pelitic sedimentary sequences formed during the development of a Rhyacian magmatic arc at the edge of the Archean Amapá Block. This was followed by a thermo-tectonic event at the end of Rhyacian, around 2.05 Ga, under high-grade conditions reaching the granulite facies, with restricted anatexis. Records of the tectonic exhumation of these terranes were defined by partial disequilibrium of some mineral phases in amphibolite facies. This Late-Rhyacian thermo-tectonic event is contemporaneous to the ultra-high temperature metamorphism (2.09–2.03 Ga) of the Bakhuis Granulite Belt in Suriname. The thermal differences between these two high-grade metamorphic terrenes may be related to the different crustal levels where they settled. Late-Rhyacian asthenospheric upwelling by crustal stretching during oblique continental collision seems to be the more plausible scenario as previously proposed, even if a slab break-off scenario cannot be ruled out.

Morphotectonic evolution of the Quaternary landforms in the Yangui River basin in the Indo-Myanmar Range

The present study is an attempt to gain insight of the geodynamic implications on the evolution of Quaternary landforms in the Yangui River basin in the Indo-Myanmar Range (IMR), taking a multi-disciplinary approach. Originating at an elevation of ~2568 m asl, the Yangui River flows almost parallel to the regional strike of the IMR and drains into the Imphal valley at Yaripok and exhibits varied valley floor morphology. The valley floors are narrowest in the transverse valley sections. The fluvial landforms are represented by fill terraces, strath terraces and paleochannels. The chronologically constrained ages from fluvial stratigraphy suggests that the oldest phase of aggradation in the area began much before 8 ka. A total of 1164 lineaments were mapped, most trending NE-SW, which is slightly oblique to the regional strike of the IMR. East of the study area, the most prominent lineaments are those trending NE-SW. Lineament analysis suggests that the NNE-SSW trending lineaments are the first generation; followed by the ENE-SWS, while the youngest are the NW-SE trending lineaments. Quaternary landforms related to tectonic activity are observed in the Ringui area, where the morphology suggests transpressional tectonics. Cross-cutting relationships amongst faults suggest multiple phases of tectonic activity; linear valleys are observed along the fault planes. The cross conjugate faults and NW-SW lineaments of the area confirm tectonic activity in the area, which corroborates well with the seismogenic potential and oblique geometry of the Churachandpur-Mao Fault (CMF). Morphometric data also suggest ongoing tectonic activity in the area.

Application of the improved parabola-based method in delineating lineaments of subsurface structures: A case study

Identification of lateral boundaries for the subsurface structures is a popular topic in many applications of geoscience. For this purpose, many edge detection and enhancement methods have been introduced to determine the lineaments of subsurface structures. In this study, the improved parabola-based method has been applied to reveal the lineaments of subsurface structures of Nam Dinh province. The obtained results have shown that there are NW–SE trending structures in this region and most of the lineaments are in the depth range of 2 km to 8 km. These results provide a new perspective to help us better understand the structural framework and tectonic settings of the study area.

Structure and tectonics of a Late Jurassic, arcuate fold belt in the Ban Don Group, Southern Vietnam

The Ban Don Group (BDG) of south-central Vietnam and NE Cambodia significantly constrains the Jurassic tectonic history of the SE Indochina plate margin. The Early to Middle Jurassic BDG comprises internally conformable marine shales and sandstones with a high volcaniclastic content, bounded below and above by major unconformities, deposited in a NW-SE trending basin. A contractional fold belt developed in the basin during the latest Middle to Late Jurassic. Folds are upright, planar, doubly plunging, and lack systematic vergence. Thrust faults are seldom observed. The Ban Don Fold Belt (BDFB) is 350 km long and 240 km wide. It forms an arcuate salient towards the foreland to the NW with nearly a 90° change in fold trend orientation and an estimated average shortening of 37%. The change in fold orientation follows sedimentary facies boundaries, which parallel the depositional basin margins. The NW-trending BDG basin axis forms a line of symmetry for the facies boundaries and fold trends. An analog for the upright fold-dominated, low-taper wedge of the BDFB is the Parry Islands Fold Belt (salt detachment). However, the absence of salt in the BDFB suggests a very weak detachment was facilitated by high fluid pressures in the basal shales. Deposition and deformation of the BDG plus recently published regional data indicate the well documented Late Jurassic and Cretaceous Andean-type margin of Southeast Asia also existed during the Early and Middle Jurassic. The BDG reflects back-arc deposition with subsequent deformation driven by changes in subduction angle and/or subduction obliquity during the Jurassic. Alternatively, the arcuate geometry of the BDFB is also compatible with the collision of a small indenter.

The Carrara Marble: geology, geomechanics and quarrying

The industrial exploitation of the Carrara Marble dates back to I century B.C. with the Romans. After a black out in the Early Mediaeval age due to barbarian invasions, since XIII century the Carrara Marble has becoming more and more used for ornamental purposes and appreciated and requested worldwide. Geologically the Carrara Marble derives from a Tertiary polyphasic tectono-metamorphic deformation of an Early Jurassic epicontinental carbonate platform. That got rise to the diverse merceological types of the Carrara Marble, this and two neo-tectonic sub-vertical fracture systems, NNE-SSW and NW-SE trending respectively and grouped into bands irregularly spaced, control the geo-mining setting of the Carrara Marble. For centuries the Carrara Marble has been excavated only following the feeling the quarrymen had with the rock-mass, but the new cultivation technologies with diamond-wire cutting machines and saw-cutting chains, with big frontal-loaders and excavators, have greatly increased the production and require for a robust design based on reliable geomechanical data. Underground quarrying is increasing because technical and environmental concerns and asks for robust design and in situ stress determinations.

Understanding Regional Structural Framework of Borena and the Surrounding Regions in the Southern Main Ethiopia Rift (SMER), Ethiopia

This study integrates remote sensing data via Digital Elevation Model (DEM) extracted from SRTM and satellite gravity data to detect surface and subsurface geologic structural features of Borena and the surrounding regions in the Southern Main Ethiopia Rift (SMER). Shading from different azimuth with 45-degree interval and tilt angle method were used to map lineaments and detect edges of anomalies from Digital elevation model and gravity data, respectively. The surface geology of the study area comprises alluvial and elluvial sediments deposited during of Holocene age, pre-rift and rift volcanics (basaltic rocks) and Precambrian banded gneiss. The Ririba rift and Mega rift systems make the main structural fabric of the area. About 793 lineaments from Digital elevation model data and, 441 from gravity data were outlined and their orientations were statistically analyzed using rose diagrams. These aretrending dominantly to NW–SE and N–S directions. Lineament depth map obtained from gravity data analysis suggests that the depth ranges from 300 m to more than 7 km below the ground with most of the lineaments are at shallow depth. The results obtained agrees with the main tectonic features found in the study area such as the N–S oriented Ririba rift and Chewbahir Rift and the NW–SE trending Mega fault zones.The integration of remote sensing data viz Digital elevation model and satellite gravity data has facilitated delineating structural features in studying the regional geological makeup of large area.

Cenozoic temporal variation of crustal thickness in the Urumieh-Dokhtar and Alborz magmatic belts, Iran

We present regional variations of whole-rock Sr/Y and (La/Yb)N ratios of magmatic rocks along the Cenozoic Urumieh-Dokhtar and Alborz magmatic belts, Iran. Both the magmatic belts are located at the north of the main Zagros Neo-Tethyan suture. The Urumieh-Dokhtar magmatic belt (UDMB), which trends NW-SE for 1000 km across Iran, was characterized by the intensive volcanism and plutonism, and defined the magmatic front (MF) of the Zagros orogenic belt. The Alborz magmatic belt (AMB) is situated to the north, and characterized by less intense magmatic activity. The Alborz magmatic belt was formed behind it in the rear-arc (RA) domain. A striking feature of the both magmatic belts is the transition from normal calc-alkaline arc magmatism during the Eocene–Oligocene to adakite-like calc-alkaline magmatism during the Middle to Late Miocene–Pliocene. The late-Cenozoic magmatism of the UDMB and AMB shows higher Sr/Y and (La/Yb)N. However, it should be noted that crustal thickening event is intensive in the UDMB than AMB during Late Cenozoic. Using the composition of the Lale-Zar zircons from the SE UDMB we determined the oxygen fugacity (fO2) during zircon crystallization to be between FMQ (fayalite–magnetite–quartz buffer) -0.69 to +2.41, whereas those of the Hashroud-Teckmdash-Gormolla zircons from NW AMB range from −1.22 to +5.99. The fO2 estimates suggest relatively more oxidized conditions for the Late Cenozoic igneous rocks of the AMB. Compiled data from the UDMB and AMB intrusions show an increase in average zircon crystallization temperatures with decreasing age. These outcomes have been interpreted in terms of variation of the crustal thickness, from 30 to 35 km during Eocene-Oligocene to 40–55 km during the middle-late Miocene. We propose the increase in crustal thickness is associated with the collision between the Arabian plate and Iran and subsequent convergence during the middle-late Miocene.

Shallow Shear-Wave Velocity Structure beneath the West Lake Area in Hangzhou, China, from Ambient-Noise Tomography

Urban geophysical exploration plays an important role in the sustainable development of and the mitigation of geological hazards in metropolitan areas. However, it is not suitable to implement active seismic methods in densely populated urban areas. The rapidly developing ambient-noise tomography (ANT) method is a promising technique for imaging the near-surface seismic velocity structure. We selected the West Lake area of the city of Hangzhou as a case study to probe the shallow subsurface shear-wave velocity (Vs) structure using ANT. We conducted seismic interferometry on the ambient-noise data recorded by 28 seismograph stations during a time period of 17 days. Fundamental-mode Rayleigh-wave group- and phase-velocity dispersion data were measured from cross-correlation functions and then inverted for a 3D Vs model of the uppermost 1 km that covers an area of about 7 km × 8 km. The tomographic results reveal two prominent anomalies, with high velocities in the southwest and low velocities in the northeast. The fast anomaly corresponds to the presence of limestone and sandstone, whereas the slow anomaly is due to the relatively low-velocity rhyolite and volcanic tuff in the area. The boundary between the two anomalies lies to the NE of an NW–SE trending fault, indicating that the fault dips toward the NE. In addition, the pronounced low-velocity anomalies appear under the Baoshi mountain, likely due to the thick rhyolite and volcanic tuff beneath the extinct volcano. Our results correlate well with regional geological features and suggest that ANT could be a promising technique for facilitating the exploration of urban underground space.

Tectonic geomorphology of the Yatağan Fault (Muğla, SW Turkey): implications for quantifying vertical slip rates along active normal faults

South Western Anatolia is dominated by E-W and NW-SE trending active faults. The dip-slip Yatağan Fault is one of these active structures that trends in a NW direction for ~30 km. To assess the relative tectonic activity of the Yatağan Fault, two geomorphic segments were defined along the fault: the FS-1 (northern segment) and the FS-2 (southern segment). The vertical slip rate pattern of the fault was analyzed using steepness indexes, chi (χ) plots, and log-log slope area graphs. Results of the analyses indicate that the steepness of the streams draining the footwall reveal increasingly higher values downstream along the fault. All of the main basins contain at least one slope-break knickpoint associated with tectonic uplift. Facet morphology-based investigations using empirical methods along faceted spurs of the Yatağan Fault indicate vertical slip rates of 0.16 ± 0.05 mm/year and 0.3 ± 0.05 mm/year for the FS-1 and the FS-2, according to relationship of facet slope angle (Rsa). Additionally, using the facet basal height relationship (Rbh) we calculated slip rates of 0.24 mm/year and 0.36 mm/year for the FS-1 and the FS-2 segments, respectively. Mountain front sinuosity analysis yields values of 1.34 and 1.2, while the ratio of valley-floor width to valley height gives values of 0.64 and 0.24 for the FS-1 and the FS-2 respectively, indicating typical active mountain front where the uplift rates are ? 0.5 mm/year. Hypsometric analysis suggest a transition from mature to older stage for catchments along the Yatağan Fault. Comprehensive interpretation of the results from morphometric analysis, vertical slip rate calculations, and data based on field observations suggest preponderance of tectonic activity over erosional process along the Yatağan Fault. Our analyses reveal that the rate of the tectonic activity gradually increases from the FS-1 to the FS-2 along the fault.

Geophysical interpretation for delineating groundwater and subsurface structure in the East of Cairo City, Egypt

This study aimed to delineate groundwater and the structural trends in the East of Cairo City. For this aim, we carried out different geophysical approaches such as gravity, magnetic, and electric methods. For the gravity and magnetic data analysis, we used filtering, Euler deconvolution, and radially power spectrum techniques. In addition, 3-D modeling was performed on the magnetic data only. The radial power spectrum technique shows that the depths of the surface anomalies corresponding to the short wavelength are 0.48 and 0.5 km. While, the depths of the surface anomalies corresponding to the long wavelength are ranging from 2.74 to 2.33 km from gravity and magnetic interpretation, respectively. The Euler deconvolution revealed that the investigation area was under the effect of several fault trends; a major NW-SE trend and NE-SW and NW-SE as minor trends for gravity data. While the fault trends are in N-S and NE-SW directions for RTP magnetic data. The 3-D modeling results show that the basement depth was fluctuating between 1.186 to 1.657 Km. The principal tectonic deformations of the investigating area are towards the E-W, N-S, and NE-SW trends. The gravity and magnetic techniques in this study revealed that different faults dissect the area of trends NE-SW matching the Gulf of Aqaba direction, NW-SE matching the Gulf of Suez, N-S matching the River Nile system, and E-W matching the Mediterranean tectonics. The geo-electrical method was used to investigate groundwater presence. Regarding the seismicity, the area can be considered moderate to highly active. Most of the area faults are active except in the area's southern part. So, this part can be considered the most suitable and safe part for construction.

Integrated analysis of magnetic, gravity and multichannel seismic reflection data along a transect southeast of Sri Lanka, Bay of Bengal: New constraints

Integrated analysis of high quality magnetic, gravity, bathymetry and multichannel seismic reflection data along a 1300 km long transect SK101 between the DSDP218/IODP1455 and ODP718 sites reveals the structure and evolutionary history of the underlying sedimentary load and basement. The smooth southward deepening bathymetry is pierced by several topographic highs of varying dimensions. Gravity does not mimic the bathymetry in general, and is as low as −75 mgal over a deep basement region. Magnetic anomalies depict significant variation along the transect, with chron A34 identified at its either end.Multichannel seismic reflection data reveal sediment thickness up to 3 s two-way travel time (TWT), but minimal (<0.1 s TWT) over protruding basement highs. Intense folding/faulting of sediment and basement producing low amplitude, long wavelength topographic highs is observed towards south along the transect. Flower structures seen within the sedimentary load suggest intermittent strike-slip faulting. The basement highs lying at the southern end of NW-SE trending fracture zones may have formed as the spreading center rotated clockwise during the Middle Cretaceous period. An arcuate gravity low is due to the presence of a deep and old basement, overlain by thick sediments. A prominent step in the basement coinciding with an abrupt change in magnetic anomaly pattern probably indicates the concurrent presence of oceanic crust of different ages, as a consequence of the Middle Cretaceous major plate reorganization.

Seismotectonic analysis of the 2017 moiyabana earthquake (MW 6.5; Botswana), insights from field investigations, aftershock and InSAR studies

The April 3, 2017 MW 6.5, Moiyabana (Botswana) earthquake occurred in the continental interior of the Nubian plate and in a seismogenic region previously considered as stable. Our objective is to combine several approaches (field and remote sensing investigations) in order to adopt a multidisciplinary strategy so as to enhance our understanding of earthquake occurrence in intraplate southern Africa. We analyse the mainshock and aftershocks sequence based on a local seismic network and local seismotectonic characteristics. The earthquake rupture geometry is constrained with more than 900 aftershocks recorded over a period of three months and from the InSAR analysis of Sentinel-1 images (ascending orbit). The mainshock (25.134 E, 22.565 S; depth 22 ± 3 km) was followed by more than 500 events of magnitude M ≥ 0.8 recorded in April 2017 including the largest aftershock (MW 4.6 on the April 5, 2017). Focal mechanism solutions of the mainshock and aftershocks display predominance of NW-SE trending and NE dipping normal faulting. Stress inversion of the focal mechanisms produced results that are compatible with a NE-SW extension under normal faulting regime. The InSAR study shows fringes (a pair of ascending images 2017-03-30 and 2017-04-11) with two lobes with 3.86 cm–5.15 cm coseismic slip on a NW-SE elongated and 40-km-long surface deformation consistent with the mainshock location and normal faulting mechanism. The modelling of surface deformation provides the earthquake rupture dimension at depth with ~50 cm maximum slip on a fault plane striking 315°, dipping 45°, −80° rake and with Mo 3.68 × 1018 Nm. Although the seismic strain rate is of low level, the occurrence of the 2017 Moiyabana earthquake, followed by an aftershock sequence in the central Limpopo Mobile Belt classifies the intraplate region as an active plate interior.

A seismic swarm at the Bransfield Rift, Antarctica

The Brasfield rift and South Shetland trench are part of the active margin of the Antarctic continent. From the records of a seismic station at the King George Island installed by the Observatorio Geofísico del Uruguay, 3186 seismic events during August 26 and September 11, 2020 were retrieved with an average of 187 earthquakes per day and a maximum of 359 on August 30. 217 events with a local magnitude of ≥4 were well recognized and located by a single-station method. The events show that an earthquake swarm close to the South Shetland Islands and at a short hypocentral distance (~25 km) to the station occurred in August of 2020. The earthquakes in the swarm are near Orca seamount and the listric normal faults, with a ENE-WSW trend of the Bransfield Basin, suggesting their volcano and volcanotectonic origin. In addition to the swarm, several earthquakes in the same time period related to the Artigas Fault with NW-SE trend are also located, which indicates the reactivation of this structure. From all the earthquakes, a relatively high b-value (1.22) is estimated, which might suggest variable resistance for stress accumulation in the Bransfield rift.

Structural architecture of the Middle Niger Basin, Nigeria based on aeromagnetic data: An example of a non-volcanic basin

The Middle Niger Basin is located within the west-central half of Nigeria, as a NW-SE trending Campano-Maastrichtian depo-centre which extends from the southern end of the Sokoto Basin west of Kainji reservoir southwestwards to the convergence of the Benue and Niger Rivers at Lokoja. The aeromagnetic anomaly data of the Middle Niger Basin was interpreted to characterize the structural architecture and depth to the magnetic basement of the basin. This is to expand the current knowledge of the region. The analysis of the data has been facilitated by the application of derivative and source parameter imaging techniques. The results from the application of total gradient to the aeromagnetic data provided here signify a rift origin of the basin and NW-SE trending fault systems in the surrounding basement complex terrain. The absence of magnetic highs on the first vertical derivative of the reduced-to-pole aeromagnetic data reveals lack of volcanic rocks within the sedimentary layers of the basin. Additionally, the consistent NW-SE trending source parameter imaging depth solutions within the basin confirm the internal geometry and NW-SE orientation of the basin with sediments not more than 1100 m thick.

Shallow vs. Deep Subsurface Structures of Central Luconia Province, Offshore Malaysia Reveal by Aeromagnetic, Airborne Gravity and Seismic Data

Across the Luconia continental shelf, the nature and structures of the crust are lacking geological understanding and precise characterization. Newly acquired, aeromagnetic, and airborne gravity data were used to assess deep and shallow sub-surface signals within the Central Luconia Province, off the coast of Sarawak, offshore Malaysia. Regional aeromagnetic anomalies appear to primarily reflect deep crustal features while depth (Z) tensors of airborne gravity anomalies evidence shallow subsurface structures. Strike directions of the interpreted structural trend on aeromagnetic and airborne gravity anomalies maps are measured and plotted into rose diagrams to distinguish the structural orientations for all datasets. Signature patterns extracted from the depth profiles were correlated with parallel seismic lines and nearest exploration wells and coincide well with the top of carbonate for Cycle IV/V and structures seen within the Cycle I and II sediments. The orientation of faults/lineaments at shallower depth is dominated by a NW-SE orientation, similar with the faults extracted from two recently published structural maps. Deeper subsurface sections yielded E-W to NWW-SEE dominant directions which were never presented in the published literature. The E-W oriented anomalies are postulated to represent the remnants of the accretion between the Luconia crustal block and southern boundary of the Palawan block. The NW-SE trend follows the same direction as prominent faults in the region. The insight into shallow and deep subsurface structures in Central Luconia Province imaged through airborne gravity and aeromagnetic data should provide guidelines and complementary information for regional structural studies for this area, particularly in combination with detailed seismic interpretation. Further evaluation on the response of Air-FTG® gravity and aeromagnetic could lead to the zonation of potential basement highs and hydrocarbon prospects in this area.

Provenance and Climatic Implications of the Middle-Pleistocene Loess Deposit, Southern China

The middle-Pleistocene Xiashu loess deposit in the lower reaches of the Yangtze River (LRYR), southern China, may yield evidence with significant climatic and environmental significance. However, its provenance remains controversial. In this study, grain size analysis, and quartz grain surface microtextural and morphological observations, enabled comparison of the provenance of loess in the LRYR with those on the Loess Plateau in northern China. The results show that the grain size characteristics of the Xiashu loess do not follow the coarse-fine NW-SE trend in northern China, and the surface microtextures of the quartz grains in the Xiashu loess are distinctly different from those on the Loess Plateau in northern China, indicating that the loess deposits in the two regions have different provenances. The Gobi Desert in inland northwestern China are not considered as the primary provenance of the Xiashu loess. Instead, the adjacent floodplains in the LRYR, the alluvial plains of the Huai River and the Yellow River to the north during glacial periods are suggested as the dominant source materials for the Xiashu loess. Under the background of middle-Pleistocene climate transition (MPT), regional aridity and a strengthened east Asian winter monsoon (driven by global cooling and the rapid uplift of the Tibetan Plateau) were considered as the primary drivers for the development of the Xiashu loess. The sustained eolian loess accumulation represent a regional response of the northern subtropical area of southern China to the MPT global cooling event at around 0.9 Ma.

Petrographic Studies of Rocks around Arum and Its Environs, North Central Nigeria

A detailed geological mapping of the area around Arum and environs part of Kurra sheet 189 SW was carried out on the scale of 1: 12, 500. Geologic field mapping and petrographic study (both megascopic and microscopic) were the methodology used. The geologic mapping of the area identified four rock units which are; granite, porphyritic granite, granitic gneiss and Porphyroblastic gneiss. These rock types were distributed such that the granite at the north-eastern part covered about 25%, the north –western portion was occupied by the porphyritic granite which occupied the largest portion of about 30% of the area. The third rock unit is the granitic gneiss which covered only about 20%. The fourth (last) and the oldest rock unit is the Porphyroblastic gneiss covering about 25% of the total area at the south-eastern corner. Megascopic and microscopic study revealed that the rocks in the area comprised of minerals such as; quartz, biotite, muscovite, microcline, feldspar, hornblende, garnet, etc. Structures that were clearly evident in the area included fault, foliation, joints, and veins. Structural analysis showed that their rose diagrams proved a NW-SE, NNE-SSW and NE-SW trends to be dominant.

Magnetic radial inversion for 3-D source geometry estimation

SUMMARY We present a method for inverting total-field anomaly data to estimate the geometry of a uniformly magnetized 3-D geological source in the subsurface. The method assumes the total-magnetization direction is known. We approximate the source by an ensemble of vertically juxtaposed right prisms, all of them with the same total-magnetization vector and depth extent. The horizontal cross-section of each prism is a polygon defined by a given number of equi-angularly spaced vertices from 0° to 360°, whose polygon vertices are described by polar coordinates with an origin defined by a horizontal location over the top of each prism. Because our method estimates the radii of each polygon vertex we refer to it as radial inversion. The position of these vertices, the horizontal location of each prism and the depth extent of all prisms are the parameters to be estimated by solving a constrained non-linear inverse problem of minimizing a goal function. We run successive inversions for a range of tentative total-magnetization intensities and depths to the top of the shallowest prism. The estimated models producing the lowest values of the goal function form the set of candidate solutions. To obtain stabilized solutions, we impose the zeroth- and first-order Tikhonov regularizations on the shape of the prisms. The method allows estimating the geometry of both vertical and inclined sources, with a constant direction of magnetization, by using the Tikhonov regularization. Tests with synthetic data show that the method can be of utility in estimating the shape of the magnetic source even in the presence of a strong regional field. Results obtained by inverting airborne total-field anomaly data over the Anitápolis alkaline–carbonatitic complex, in southern Brazil, suggest that the emplacement of the magnetic sources was controlled by NW–SE-trending faults at depth, in accordance with known structural features at the study area.

Magnetic characteristics of a part of Closepet Granite, Eastern Dharwar Craton

The exposed granite gneissic complex to the west of Cuddapah basin in Eastern Dharwar Craton (EDC) is studied to understand the magnetic character of various plutons in this region with more emphasis on a part of Closepet Granite (CG). The NW–SE trending plutons in this region show gradual decrease in magnetic intensities from southwest to northeast; CG in the southwest corner of the study area have high mafic content, and there onwards show a depletion in mantle enrichment towards NE. Parts of the CG namely, the main mass and northern intrusions separated by a gap zone are differentiated based on magnetic character deduced by processing the aeromagnetic data. Reduced to the pole (RTP) and its upward continued maps revealed NNW trending eastward convex anomaly highs inferring the magnetic nature of CG due to Porphyritic Monzogranite (PMG). Gap zone is identified to be structurally significant, bounded by deep ENE faults and inferred to be associated with PMG at relatively deeper levels. 2.5D modelling of magnetic anomaly profile substantiated our qualitative analysis of magnetic anomalies over CG, inferring magnetic and non-magnetic units of CG. Known shear zones running north–south are identified with a few inferred shears revealed from magnetic data interpretation within CG.

Liquefaction Phenomena Induced by the 26 November 2019, Mw = 6.4 Durrës (Albania) Earthquake and Liquefaction Susceptibility Assessment in the Affected Area

On 26 November 2019, an Mw = 6.4 earthquake struck the central-western part of Albania. Its impact comprises secondary earthquake environmental effects (EEE) and severe building damage within the Periadriatic and the Tirana Depressions. EEE comprised mainly liquefaction phenomena in coastal, riverine, and lagoonal sites of the earthquake-affected area. From the evaluation of all available earthquake-related data, it is concluded that liquefaction sites are not randomly distributed within the affected area but are structurally and lithologically controlled. The affected areas are distributed within NW–SE striking zones formed in graben-like syncline areas with NW–SE trending fold axes. These graben-like areas are bounded by NW–SE striking marginal thrust faults and are filled with geological formations of Neogene to Quaternary age. These NW–SE striking zones and structures coincide with the NW–SE striking seismogenic thrust fault of the November 2019 earthquake as it is derived from the provided fault plane solutions. An approach for liquefaction susceptibility assessment is applied based on geological and seismological data and on liquefaction inventory. From the comparison of the compiled liquefaction inventory and the susceptibility maps, it is concluded that the majority of the observed liquefaction has been generated in very high and high susceptible areas.