Basal Unit Research Articles

A few remarks on the Triassic paleogeography and geodynamic link between the Dinarides an the Serbo-Macedonian unit

A recently recycled tectonic hypothesis in which the Serbo-Macedonian Unit rifted off during Ladinian from the Dinarides is tested. The Triassic rifting, according to this concept, led to the opening and the formation of a northwestern branch of Neotethys (=Vardar Ocean). Similar concept explains the developments within the Inner Hellenide allochoton (Serbo-Macedonian Unit and Pelagonides). The testing of early Alpine configuration in Serbia shows, however, that the two crystalline basement units have significant paleogeographic variances. The study moreover includes the differences in Paleozoic metamorphic grades between two inliers, their exhumation times, etc. The investigated pre-Triassic paleogeographic and geodynamic variances unambiguously contradict any connection before and during Triassic (Ladinian). In this regard, by using the available field data, and the juxtaposed Late Paleozoic Variscan tectonometamorphic differences, the paper underscores the large differences in the (pre)Triassic plate configurations and paleogeography. The synthesis displays the two separate positions of these microplates situated along with the length of this segment of the Triassic Eurasian continental margin.

Between the supercontinents: Mesoproterozoic Deer Trail Group, an intermediate age unit between the Mesoproterozoic Belt–Purcell Supergroup and the Neoproterozoic Windermere Supergroup in northeastern Washington, USA

Mesoproterozoic and Neoproterozoic basins in western North America record the evolving position of the Laurentian craton within two supercontinents during their growth and dismemberment: Columbia (Nuna) and Rodinia. The western-most exposures of the Columbia rift-related Belt–Purcell Supergroup are preserved in northeastern Washington, structurally overlain by the Deer Trail Group and depositionally overlying the Neoproterozoic Windermere Supergroup. It has been disputed whether the Deer Trail Group is correlative with the Belt–Purcell Supergroup, or younger. To help resolve the uncertain correlation of these units and their bearing on supercontinent evolution, we characterized the detrital zircon age populations of units from the Deer Trail Group, the Windermere Supergroup, and the Belt–Purcell Supergroup in northeastern Washington. These data show that the western part of the Columbia supercontinent (now located in Australia and eastern Antarctica) remained attached to western Laurentia and continued to supply 1600–1500 Ma detrital zircon grains to the Belt–Purcell Supergroup until after ca. 1391 Ma. The Deer Trail Group is younger than the Belt–Purcell strata, with the basal unit younger than ca. 1362 Ma and a middle unit younger than ca. 1300 Ma. The Deer Trail Group has a pre-Grenville-age provenance from the southwestern USA and possibly east Antarctica. The Buffalo Hump Formation is younger than the Deer Trail Group, with Grenville-age (ca. 1112 Ma) detrital zircon grains and a detrital zircon signature like that of the overlying Neoproterozoic Windermere Supergroup. We interpret the Deer Trail Group to have been deposited during the rift-demise of supercontinent Columbia and before the Grenville-age assembly of the supercontinent Rodinia.

Emplacement of massive deposits by sheet flow

AbstractTurbidity current and coastal storm deposits are commonly characterized by a basal sandy massive (structureless) unit overlying an erosional surface and underlying a parallel or cross‐laminated unit. Similar sequences have been recently identified in fluvial settings as well. Notwithstanding field, laboratory and numerical studies, the mechanisms for emplacement of these massive basal units are still under debate. It is well accepted that the sequence considered here can be deposited by waning‐energy flows, and that the parallel‐laminated units are deposited under transport conditions corresponding to upper plane bed at the dune–antidune transition. Thus, transport conditions that are more intense than those at the dune–antidune transition should deposit massive units. This study presents experimental, open‐channel flow results showing that sandy massive units can be the result of gradual deposition from a thick bedload layer of colliding grains called sheet flow layer. When this layer forms with relatively coarse sand, the non‐dimensional bed shear stress associated with skin friction, the Shields number, is larger than a threshold value approximately equal to 0·4. For values of the Shields number smaller than 0·4 the sheet flow layer disappeared, sediment was transported by a standard bedload layer one or two grain diameters thick, and the bed configuration was characterized by downstream migrating antidunes and washed out dunes. Parallel laminae were found in deposits emplaced with standard bedload transport demonstrating that the same dilute flow can gradually deposit the basal and the parallel‐laminated unit in presence of traction at the depositional boundary. Further, the experiments suggested that two different types of upper plane bed conditions can be defined, one associated with standard bedload transport at the dune–antidune transition, and the other associated with bedload transport in sheet flow mode at the transition between upstream and downstream migrating antidunes.

The Basal Unit of the Verkhoyansk Sedimentary Complex, Northern Kharaulakh: Lithostratigraphy, Biostratigraphy, and Deposition Environments

Abstract —The origin and biostratigraphic constraints of the Krestyakh conglomerate remain among most controversial issues in the Late Paleozoic history of the Northern Kharaulakh basin. The Krestyakh conglomerate is a sequence of coarse sand to pebble-size sediments at the base of the Late Paleozoic Verkhoyansk clastic complex. According to geological, lithological, and sedimentation data, the Krestyakh conglomerate in the Atyrdakh Formation is composed of debrites i.e., deposits carried by debris flows that fill submarine canyons. The Verkhoyansk clastic deposition began at the middle Visean stage of the Early Carboniferous.

Sedimentological and stratigraphic signature of the Plio-Pleistocene tectonic events in the Southern Apennines, Italy: The Calvello-Anzi Basin case study

Abstract The Plio-Pleistocene Calvello-Anzi Basin (CAB) represents a synorogenic shallow marine to alluvial succession deposited during the late phase of the Apennine Orogenic event. While the stratigraphy and sedimentary cycles are well known, the precise role and timing of the tectonic phases affecting this area are unclear. In this paper we report the major findings from a study aimed at unraveling the signatures of the Plio-Quaternary tectonic events affecting the CAB and this portion of the Southern Apennines by using a field integrated approach characterized by facies analysis, structural surveying, and basin architecture. The CAB is located in the axial portion of the Southern Apennine chain and has a roughly E-W orientation which is at a high angle with respect the main NW-SE striking regional tectonic structures controlling the foreland-directed migration of the compression. In the study area, an approximately 1,200-meter-thick package of Zanclean (early-middle Pliocene) to Santernian (middle Pleistocene) shelf, shallow marine, and alluvial deposits accumulated during the late stages of the Apennine orogenic evolution. Within the CAB deposits we recognize two main depositional motifs which roughly correspond to previously established regional cycles that are genetically linked to two corresponding tectonic phases of the Southern Apennine chain. Our results reveal how these two main tectonic phases played a primary role in controlling the depositional architecture, facies, depocenter distribution, and the basin shape throughout the CAB's lifespan. In the proposed model, during the first depositional stage (Zanclean), the CAB formed as a wedge-top basin, controlled by the subsurface propagation of NW-SE-trending thrusts related to the main NE-SW directed shortening of the Apennines. This stage is recorded in the basal units of the CAB infill by progressive unconformities, the development of syn-tectonic shallow marine sedimentary wedges along a high gradient tectonically controlled depositional surface, and a marked SE migration of the main depocenter. During the second depositional stage (upper Piacenzian to Santernian), the CAB was controlled by a series of roughly E-W-trending normal faults which crosscut the preexisting compressional structural fabric. Syndepositional activity along these faults played a direct role in creating an E-W oriented tectonic depression and controlling the accommodation space for the upper portion of the CAB infill. This second stage is marked by two points of evidence. The first point is revealed by a southwestward migration of the CAB depocenter driven by ~90-degree rotation from a roughly NW-SE elongated pattern to roughly elongated W-E transverse to the Pliocene regional structural fabric. This depocenter drift also coincides with an abrupt change in depositional regime from shallow marine to continental sedimentation. The second and most important point of evidence is the presence of several basin-bounding extensional fault zones which cut the lower Pliocene basin-fill units. We relate these features to the middle upper Pleistocene transition to along-strike extension, widely reported in the Southern Apennines, which is related to the overall extensional regime that is acting on the chain also at present. Data from this study provide additional insight into the evolution of the Southern Apennine chain, the formation of the Plio-Pleistocene CAB, and the debated role of Quaternary tectonics on the basin's development during the post-collision orogenic phases. Given the proximity of the study area with several oil fields and considering that hydrocarbon traps in Southern Apennines are Plio-Quaternary in age, the constraints provided on the timing and the style of the regional deformation may improve the definitions of hydrocarbon traps in the buried Apulian platform.

Shallow marine basaltic volcanism of the Machadodorp Member (Silverton Formation, Pretoria Group), Transvaal Basin, South Africa — An example of Paleoproterozoic explosive intraplate volcanic activity in an epeiric embayment

The early Paleoproterozoic Machadodorp Member of the Silverton Formation (Pretoria Group, Transvaal Supergroup), South Africa, is an example of ancient submarine explosive volcanism in an intra-cratonic setting under incipient rifting conditions. The Silverton Formation consists of three different members, which include, in ascending order, the Boven Member, the Machadodorp Member and the Lydenburg Member. The formation is primarily characterized by the muddy sediments of the Boven and Lydenburg members, which formed in a relatively shallow submarine environment, predominantly as sub-storm wave-base pelagic deposits, within an epeiric embayment on the Kaapvaal Craton. The subaqueous Machadodorp Member volcanic activity is mostly distributed in the eastern part of the basin. So far, the volcanism of the Machadodorp Member has not been studied in detail. Therefore, a detailed facies analysis of its volcanism was initiated, allowing constraints on its volcanic evolution.The Machadodorp Member consists of a basal unit of basaltic sheet lava, followed by a fining-upward succession of volcaniclastic rocks, ranging from breccias, tuff-breccias and tuffs, rich in fluidal-shaped clasts, to laminated tuffs that are related to subaqueous mass flows, and a final sheet lava that characterized the end of volcanic activity in the section studied.The Machadodorp Member rocks are tholeiitic basalts in composition, with all samples plotting in the low-TiO2 suite. Fractional crystallization of clinopyroxene, olivine and plagioclase was the predominant evolutionary process as illustrated by mineral chemistry and major element geochemical modeling. AFC (Assimilation and Fractional Crystallization) processes could not account for all the trace element variations, nor for the arc-like signatures observed on the primitive mantle normalized plots. Variable ancient mantle enrichment beneath this area is a more plausible cause for the trace elements variations observed in the Machadodorp rocks. Trace element modeling indicates that the parental composition of these rocks was a variably enriched spinel lherzolite source. Such enrichment must have occurred before 3 Ga during episodes of arc collision and subduction.

2D spatial analysis of the natural fractures in the organic-rich Qusaiba Shale outcrop, NW Saudi Arabia

Natural fracture is the main control on production from unconventional shale reservoirs as it serves as a drainage conduit and promotes the efficiency of the hydraulic fracturing technique. The Qusaiba Shale (QS), particularly its basal unit was recently identified as a potential unconventional shale gas reservoir in Saudi Arabia. This study presents a comprehensive and systematic description as well as statistical analysis of the natural fractures that cut through the QS outcrop in the Tabuk Basin, NW Saudi Arabia. These analyses were used to identify the factors that control the development of a fracture system in this organic-rich Shale. Three main approaches including areal sampling, scanline and topology analyses were utilized to collect fracture measurements. A total of 495 fracture traces were measured. The fracture length ranges between 7 cm and 8.2 m. The measured fracture traces are classified into four groups; subvertical to vertical, high-angle, low angle and bedding to subparallel fracture traces. Using twenty-one scan lines, the total number of fracture traces that cut through are 374, with average fracture intensity ranging from 0.9 to 3.7 fracture/meter. Four main fracture sets trending E-W, NE-SW, NW-SE and NNW-SSE were identified. The measured fractures vary from cemented (59%), partially filled (38%) to open-mode (3%). The cement in fractures is either gypsum or calcite. Applying the topology method analysis, the total number of nodes is 811, where the isolated nodes (I-type) account for 522, the termination nodes (Y-type) accounts for 222, and the cross-cutting nodes (X-type) are only 83. Microfractures are sharp with high-angle, closely spaced, open-mode fractures with lithology-induced terminations. The small vertical to sub-vertical fracture traces are terminated against the more resistant silty laminae. Pyrite shows positive correlation with the linear fracture density. The high amount of pyrite in association with high quartz and subordinate feldspars contents in the studied interval might have enhanced the brittleness of the lithologies and facilitated fracture propagation. Tectonically, two main fracture types exist in the studied section; shear and extensional fractures. The strike of the shear fractures in the study area is NE-SW and NW-SE directions. The open-mode, vertical long fractures cutting through the entire outcrop indicate an extension stress regime, which has already shaped the Tabuk Basin with multiple NW-SE rift structures.

An AI approach to automated magnetic formation mapping beneath cover

SummaryMost regional scale magnetic maps are dominated by the magnetic characteristics of steeply dipping basement units truncated by an unconformity surface. It is easy to demonstrate that 80 to 90% of each total field magnetic anomaly is contributed by this intersecting surface. We approach this problem by mapping the boundaries between contrasting magnetic units along each line in the magnetic survey using the full precision of the line data and 3D information from the magnetic gradient tensor. Additionally, we derive the azimuth of each boundary, depth to the unconformity and magnetic properties of the anomalous units. The segments are overlain on any image such as existing geological maps, satellite imagery, gravity or magnetic imagery to provide a new geological interpretation concept. This method provides a new way to interpret new and old magnetic surveys.Eigenvector analysis of the magnetic tensor and normalised source strength (NSS) are combined with an artificial intelligence (AI) approach to estimate the basement properties. The method is applied to full tensor magnetic survey data or a grid of the total magnetic intensity data is processed using FFT transformations to derive the magnetic gradient tensor. These data are used as input to the pre-trained AI process for calculation of depth, width, azimuth, magnetic susceptibility and magnetisation direction. The rock properties and depth information can be used for 3D visualisation of the unconformity and 2D mapping of the magnetic lithology of the unconformity surface.

Tectono-sedimentary evolution of a fossil ocean-continent transition: Tasna nappe, central Alps (SE Switzerland)

AbstractMagma-poor ocean-continent transitions at distal rifted margins record complex stratigraphic interactions engendered by extreme crustal thinning and mantle exhumation. The Tasna ocean-continent transition, exposed in the Middle Penninic Tasna nappe in eastern Switzerland, is so far the only known example where the lateral transition from continental crust to exhumed serpentinized mantle lithosphere is exposed and not overprinted by later Alpine deformation. This paper presents sedimentological, structural, and petrographical observations and detrital zircon provenance data to document: (1) the processes controlling continental hyperextension and mantle exhumation; and (2) the facies, depositional systems, sediment sources, delivery pathways, and depositional stacking patterns associated with magma-poor ocean-continent transitions. Our results show that the basement units of the Tasna ocean-continent transition are composed of prerift upper and lower crust and subcontinental mantle rocks juxtaposed as part of the continental crustal thinning process. The absence of pervasive, synrift deformation in the lower-crustal rocks indicates that the thinning was likely achieved by deformation along localized shear zones before being exhumed at the seafloor by brittle, late extensional detachment faulting and not by any form of lower-crustal flow.The age of the first sediments deposited on the continental crust and exhumed mantle, the so-called Tonschiefer Formation, is considered to be Late Jurassic. A key observation is that the restored morpho-tectonic and sedimentary evolution of the Tasna ocean-continent transition shows the intercalation of downdip, transported platform-derived sediments and along-axis–derived siliciclastic sediments originating from the recycling of prerift sediments, local basement, and/or extra-Alpine sources.

Origin of Au-Rich Carbonate-Hosted Replacement Deposits of the Kassandra Mining District, Northern Greece: Evidence for Late Oligocene, Structurally Controlled, and Zoned Hydrothermal Systems

Abstract The Au-rich polymetallic massive sulfide orebodies of the Kassandra mining district belong to the intrusion-related carbonate-hosted replacement deposit class. Marble lenses contained within the Stratoni fault zone host the Madem Lakkos and Mavres Petres deposits at the eastern end of the fault system, where paragenetically early skarn and massive sulfide are spatially associated with late Oligocene aplitic and porphyritic dikes. Skarn transitions into predominant massive and banded replacement sulfide bodies, which are overprinted by a younger assemblage of boulangerite-bearing, quartz-rich sulfide and late quartz-rhodochrosite vein breccias. The latter style of mineralization is most abundant at the Piavitsa prospect at the western end of the exposed fault system. The sulfide orebodies at the Olympias deposit are hosted by marble in association with the Kassandra fault, where textural and mineralogical similarities to the sulfide bodies within the Stratoni fault zone suggest a genetic relationship. Estimated trapping temperatures and pressures based on fluid inclusion data indicate that carbonate replacement mineralization took place at depths less than about 5.9 km. Carbon and oxygen isotope patterns in carbonate from the Stratoni fault zone support isotopic exchange principally through fluid–wall-rock interaction, whereas decarbonation and fluid-rock exchange reactions were important at the Olympias deposit. Carbonate minerals associated with skarn and replacement sulfide throughout the district have isotopic compositions that are consistent with formation from a hydrothermal fluid of magmatic origin. Lower homogenization temperatures and salinities in the younger quartz-rich sulfide assemblage and quartz-rhodochrosite vein breccias, together with low δ18O values of gangue carbonate, suggest dilution of a primary magmatic fluid with meteoric water late in the evolution of the hydrothermal system in both the Olympias area and the Stratoni fault zone. The replacement sulfide orebodies in the district likely inherited their uniform Pb isotope composition from a late Oligocene igneous source and the isotopically heterogeneous metamorphic basement units. Metal distribution patterns at the scale of the Stratoni fault zone show diminishing Cu concentration with decreasing Pb/Zn and Ag/Au ratios from Madem Lakkos to Mavres Petres and the Piavitsa prospect in the west. The sulfide orebodies at the Olympias deposit exhibit elevated Cu values in the east with increasing Pb/Zn and Ag/Au ratios down-plunge to the south-southwest. Metal concentration and ratios support zoning related to temperature and solubility changes with increasing distance from a probable magmatic source. Structural and igneous relationships, together with fluid inclusion microthermometric and carbon-oxygen isotope data and metal distribution patterns, are supportive of a zoned hydrothermal system that exceeded 12 km along the Stratoni fault zone, sourced by an igneous intrusion to the southeast of the Madem Lakkos deposit. The Olympias replacement sulfide orebodies, associated with the Kassandra fault, resulted from a local hydrothermal system that was likely derived from a concealed igneous intrusion to the east of the deposit.

Comparative structural analysis of inverted structures in the San Bernardo fold belt (Golfo San Jorge basin, Argentina): Inversion controls and tecto-sedimentary context of the Chubut Group

The Golfo San Jorge Basin is one of the most important hydrocarbon-bearing basins of Argentina. Its position in the distal Patagonian Broken Foreland confers a particular structural architecture. Specifically, the western margin was affected by positive tectonic inversion, with higher degrees of inversion at the San Bernardo Fold Belt (SBFB). This paper synthesizes the comparative structural analysis of outcropped and buried inverted structures in the SBFB. The study areas are regionally distributed, and the geometric characterizations integrate field structural data, topographic trends, cutting records, borehole logs, and 2D-3D seismic data. The geometries of the studied inversion folds are poorly vergent and can be classified as asymmetric anticlines or pop-up growth folds. The architecture of the inverted fault system shows: along-strike variability in reverse displacement, inherited extensional growth-strata, selective inversion, oblique inversion, and heterogeneous deformation. Pre-inversion major controls include: 1) an extensional network linked to the fabric of the pre-rift basement, 2) a non-coaxial extensional phase during the early Upper Cretaceous times, and 3) a basin-scale paleohigh of crystalline rocks located to the east of the SBFB. The kinematic analysis evaluates the timing of the tectonic inversion phase, which is actually under discussion. Our results are consistent with a tecto-sedimentary context dominated by normal fault reactivations during the deposition of the basal units of the Chubut Group (Barremian-Albian), and a reverse-reactivation during the record of the Laguna Palacios Formation (late Upper Cretaceous). The geometric-kinematic characterization of the inverted structures at the SBFB contradicts many of the evidence that supports recently proposed geodynamic models for the Cretaceous evolution of Southern Patagonia.

Origin and provenance of 2 Ma–2 Ga zircons ejected by phreatomagmatic eruptions of Pliocene basalts in southern Slovakia

U–Pb ages of zircons recovered from Pliocene pyroclastic deposits in northern part of the Cenozoic intra-Carpathian back-arc basin (Pannonian Basin) span the interval from Pliocene (2.2 Ma) to Paleoproterozoic (Orosirian–Rhyacian, 1850–2115 Ma). The scattered U–Pb ages reflect eruption ages of the host basaltic volcanic centres, two episodes of post-Eocene magmatic crustal growth, and the possible tectonic affiliation, provenance and age of the subjacent basement or the sedimentary basin detritus sampled by the basaltic magma. The youngest zircons define the maximum ages of phreatomagmatic eruptions during the Late Miocene–Pliocene extension. These zircons are distinguished from older zircons by Zr/Hf (40–90) and Th/U ratios (0.5–4.5) as well as super-chondritic εHf(t) values ranging from + 7 to + 14, indicating mantle-derived parental magmas. The locally increased Th/U ratios (up to 8) accompanied by Zr/Hf > 60 are diagnostic of evolved phonolite parental melt. Hence, the youngest zircons can be interpreted as antecrysts, originating from evolved melts cogenetic with the host alkali basalts. In contrast, older zircons represent xenocrysts scavenged by the uprising basalt from surrounding rocks. Subordinate Eocene–Early Oligocene (29–38 Ma) sub-group of zircon xenocrysts is coincidental with the magmatism and volcanism along the Periadriatic lineament and the middle-Hungarian zone. The Early Miocene (18 Ma) cluster is coeval with the deposition of the Bükk Mountains felsic ignimbrite correlated with the onset of the back-arc extension that triggered Miocene sedimentation within the Pannonian Basin. The Eocene–Early Oligocene zircons have been likely scavenged from pyroclastic and ash-fall deposits of the Palaeogene retroarc basin subjacent to the Miocene basin infilling. Sub-chondritic εHf(t) values between − 2.5 and − 8 in the Eocene–Early Miocene zircons attest their crystallization from subduction-related felsic-to-intermediate melts containing large amounts of recycled crustal material. Palaeozoic–Proterozoic zircons create a heterogeneous population with variable trace element abundances and εHf(t) values. The determined age clusters are reminiscent of some basement units cropping out recently in Central Western Carpathians. Zircon Hf isotope data indicate recycling of up to 3.4 Ga old mafic crust and also the presence of 2 Ga old juvenile mafic crust. These units had either underlain the northern part of the Pannonian Basin during Pliocene or had been exposed during the deposition of Miocene clastic sediments. The absence of Mesoproterozoic, Grenvillian zircons (0.9–1.8 Ga) in the pre-Cenozoic population of zircon xenocrysts is provisionally interpreted as indicating the evolution of the zircon source area within the west-African Craton.

Injury Patterns and Medical Evacuation of Patients in Chifeng Tornado in China, August 11, 2017.

To study injury patterns and medical rescue operations related to tornadoes that occur in rural areas, this study investigated the data pertaining to the 2017 Chifeng tornado (China). Medical records of 52 tornado-related patients were investigated. Data were collected from 3 local hospitals that received all the tornado victims. A total of 148 injuries were diagnosed. Tornado-related injuries were mainly caused by collapsed houses (51.9%) and direct physical trauma caused by the tornado (38.5%). Most injuries occurred outdoors (63.5%). The head (20.3%) and thorax (14.8%) were most 2 frequent anatomical injury sites. Soft-tissue injuries (43.9%) and contusions and lacerations (37.3%) were the 2 most common injury types. On evaluating the Abbreviated Injury Scale scores, a score of 1 was the most common (66.2%), and a score of 6 was not recorded. A trailing phenomenon in the distribution of time to admission among the victims of a particular tornado in China was observed. The delivery is timely compared with nondisaster situation. There was a statistically significant difference of injury causes between outdoor and indoor patients. Helmets should be used by potential tornado victims. Basement units capable of functioning as shelters should be built in villages.

Aeroradiospectrometry in the spatial formation characterization of Ogun State, south-western, Nigeria

In this study, aeroradiospectrometric data sets of Ogun State, Latitudes (6.2° to 7.8° N) and Longitudes (3.0° to 5.0° E), Nigeria were analyzed for lithological characterization. The data used comprised of Total count (TC), Uranium (U), Potassium (K) and Thorium (Th). The major objectives of this study are to characterize and identify the anomalies associated with various Interpreted Radiometric Lithological (IRL) units with respect to the three radioelements as well as Total count. The lithologies of the study area were described by three formations (basement, weathered basement and sediment), from which forty- seven IRL units were deduced. The study made use of the geological map in conjunction with the petrographic map as a tool to examine the three formation groups. The Interpreted Radiometric Lithologic (IRL) units in each formation group were subjected to the inferential test to examine the homogeneity in the total count of various rock units by adopting T-test and F-test. Multivariate statistical analysis was also conducted on the radioactive data to ascertain the radioactive variable that was associated with the same bedrock source. The results obtained showed that the anomalous responses of radiometric parameters to bedrock units were observed to be more prevalent in the Basement (B) and Weathered Basement group (WB). Majority of the radioelements deposition of 67.5%, 48.6% and 48.5% for potassium, thorium, and uranium respectively; were found in sediment (S) formation. This may be attributed to geomorphic activity such as weathering, leaching, and intrusion. Inferential test results showed that up to 26 and 32 pairs in the basement while 3 and 5 pairs in weathered basement were found to be F-test and T-test statistically and aeroradiometrically homogeneous at 5% significant level. Of the 13 pairs identified for the two tests in sediment formation, only 5 pairs were found to be uniquely homogenous. The results also revealed that most of the formation groups have their uranium and thorium values above the world average. Multivariate analysis revealed thorium and uranium as the major constituent of the total count. This study has therefore successfully characterized and identified the formations associated with anomalous response in bedrock units of the study area.

Shallow sedimentation of Natal shelf and coastal erosion implications, NE Brazil

High-resolution seismic stratigraphic analysis to reconstruct the depositional history of the inner shelf Holocene successions was performed on the northeastern Brazilian, adjacent to Natal City. Boomer seismic data was collected between the coastline up to ~ 15-m water depth. Three seismic units and three seismic surfaces were observed with regional significance and low lateral variability. They represent the transgressive to highstand deposits lying on the Pleistocene/Holocene boundary surface S1—older, regional, and the irregular unconformity at ~ 35 m, forming a semi-enclosed environment near the coast. The basal unit (U1) is related to a moderate/high-energy environment and grade upward to units (U2 and U3) of lower energy conditions and thin thickness that decrease seaward with large through near coast on the west. Above this surface, chaotic and parallel internal configurations represent unit U1 that limited at the top by S2. The ravinement surface S2 at ~ 18 m reveals wave-tide influence in a very shallow inner shelf, which controlled the accommodation space and sediment transport. Unit U2 fills the scours with onlapping terminations over S2. The shallowest surface (S3) occurs at ~ 12 m below modern sea level probably associated with the maximum flooding surface underlying the U3 with dominant aggradation character in parallel/wavy configuration. U1 represents a shallow marine environment with a greater sediment supply controlled mainly by the water depth during the initial transgressive shelf inundation. The lower sediment accumulation of U2 and U3 indicates a decreasing of sediment supply, higher sediment transport or higher rates of erosion in shallow marine environments.

Geochemical characteristics of a pre-Middle Jurassic oceanic crust fragment from the Central Pontides in northern Turkey: Geodynamic implications on intra-oceanic subduction initiation

The Central Pontides (northern Turkey) is one of the key localities to understand the geodynamic evolution of the Palaeo- and Neotethyan oceans. It consists of the pre-Jurassic basement units, the Early Jurassic and the Early Cretaceous accretionary complexes, the widespread Middle Jurassic continental arc magmatics and the Late Jurassic to Tertiary cover units. The Early Cretaceous accretionary complex is represented by the Central Pontide Structural Complex and includes the Middle Jurassic oceanic units, which were metamorphosed during the Early Cretaceous. Apart from these oceanic units, a few metaophiolite and serpentinite fragments have been recognized within the basement units, which may represent the remnants of an older ocean. The pre-Middle Jurassic Devrekani Metaophiolite is the largest oceanic fragment and tectonically intercalated within/between the Devrekani Metamorphics and the Çangaldağ Metamorphic Complex. It is mainly composed of harzburgites, dunites with chromite veins and metagabbros, and cut by metabasaltic andesites and metadacites. Petrographically, the gabbro consists mainly of plagioclase and clinopyroxene, and displays phaneritic/porphyritic texture. In contrast, the metabasaltic andesite includes plagioclase and mica phenocrysts within a fine-grained groundmass. Also, the metadacite is composed predominantly of quartz, plagioclase, and mica minerals. Two different magmatic groups belonging to completely different tectono-magmatic settings have been geochemically determined based on the immobile trace element systematics. The metadacites and metabasaltic andesites are akin to continental arc magmatics and characterized by negative Nb and Ta anomalies and depleted HFSE relative to Th and La contents. However, the metagabbro samples display the geochemical signatures of boninitic rocks and characterized by highly depletion in HFSEs and REEs relative to N-MORB. The Devrekani Metaophiolite in the Central Pontides may represent another remnant of pre-Middle Jurassic oceanic crust generation and can be north-eastward continuation of the Permian-aged Almacık complex and the Boğazköy Metaophiolite fragment in the western Sakarya Composite Terrane. It may have been cut by intrusions of the extensive Middle Jurassic continental arc magmatism after its imbrication within the basement unit. The presence of pre-Middle Jurassic oceanic units may indicate that the Paleozoic ocean may have survived as the Jurassic Intra-Pontide Ocean between the Scythian Platform and Sakarya Composite Terrane during the Mesozoic time. Thus, the Intra-Pontide Suture may normally include the Palaeozoic and Mesozoic remnants of the long-lived northward subducting Tethyan ocean.

Seismic analysis of polygonal fault systems in the Great South Basin, New Zealand

Abstract Polygonal fault systems (PFS) consisting of networks of layer-bound normal faults are analyzed for Eocene strata in the Great South Basin, New Zealand. An advanced fault enhancement and skeletonization method is applied to delineate faults using 3D seismic data from the Great South Basin. The process sharpens structural and stratigraphic discontinuities and smears the incoherent noise on coherence to obtain a skeletonized fault probability volume which can be directly used to extract and map fault geometries. The characteristics of the polygonal faults in cross-section, planform, and 3D space were studied. The faults dip approximately 50°, with displacements on a single fault ranging from tens of meters to a hundred meters. The fault planform patterns are polygonal but also related to the regional horizontal stress anisotropy caused by slope changes of the basal unit. The patterns vary from linear for high slopes, rectangular for moderate slopes and polygonal for low slopes. Two preferred orientations may reflect the superposition of deep Cretaceous trends on the polygonal system in the Eocene strata. A local concentric pattern with outward dipping normal faults is related to passive or active draping above a circular plutonic-volcanic structure. The genetic mechanism of the polygonal fault systems is interpreted to be related to volume loss and shear failure related to Opal A/CT transition within the sediments.

Gondwana fragments in the Eastern Alps: A travel story from U/Pb zircon data

We compare detrital U/Pb zircon age spectra of Carboniferous and Permian / Lower Triassic sedimentary rocks from different structural positions within the Austroalpine nappe pile with published ages of magmatic and metamorphic events in the Eastern Alps and the West Carpathians. Similarities between sink and possible sources are used to derive provenance of sediments and distinct frequency peaks in sink and source age pattern are used for paleogeographic plate tectonic reconstructions. From this, travel paths of Austroalpine and West Carpathian basement units are traced from the Late Neoproterozoic to the Jurassic. We place the ancestry of basement units on the northeastern Gondwana margin, next to Anatolia and the Iranian Luth-Tabas blocks. Late Cambrian rifting by retreat of the Cadomian Arc failed and continental slivers re-attached to Gondwana during a late Cambrian / early Ordovician orogenic event. In the Upper Ordovician crustal fragments of the Galatian superterrane rifted off Gondwana through retreat of the Rheic subduction. An Eo-Variscan orogenic event at ~390 Ma in the Austroalpine developed on the northern rim of Galatia, simultaneously with a passive margin evolution to the south of it. The climax of Variscan orogeny occurred already during a Meso-Variscan phase at ~350 Ma by double-sided subduction beneath Galatia fragments. The Neo-Variscan event at ~330 Ma was mild in eastern Austroalpine units. This orogenic phase was hot enough to deliver detrital white mica into adjacent basins but too cold to create significant volumes of magmatic or metamorphic zircon. Finally, the different zircon age spectra in today's adjacent Carboniferous to Lower Triassic sediments disprove original neighbourhood of basins. We propose lateral displacement of major Austroalpine and West-Carpathian units along transform faults transecting Apulia. The intracontinental transform system was released by opening of the Penninic Ocean and simultaneous closure of the Meliata Hallstatt Ocean as part of the Tethys.

Structure of the Cerro Prieto Pull-apart basin from joint inversion of gravity, magnetic and magnetotelluric data

The large transform fault system that separates the Pacific and the North American plates in the Gulf of California is conformed by a system of oblique rifts and large dextral transform faults. Besides their seismogenic importance, several of these structures are related to geothermal resources in the area. This system originates the Cerro Prieto Pull-apart basin (CPPAB), which is particularly important as it hosts the world's third largest geothermal field. It is remarkable that, despite intensive geological and geophysical exploration in the CPPAB, there is no published geophysical image of the basin that gives clear evidence of the underlying basement and the existence of an active pull-apart basin and whether it may be the source of heat of the reservoir. We posit that the joint inversion of multiple geophysical data can unveil the underlying CPPAB structure. We then applied the cross-gradient joint inversion to recently collected gravity, magnetic and magnetotelluric data on three intersecting profiles. From the joint models, we could clearly distinguish the major geometry of the basin and classify the basement units by their characteristic geophysical signatures, as well as propose the occurrence of various suites of intrusives. The interpreted models correlate well with surface geology and geophysical maps and depict a likely structure for the active pull apart basin in the area.

Compositional Constraints on the North Polar Cap of Mars from Gravity and Topography

AbstractThe polar ice caps are the largest reservoir of water ice on Mars. The north polar ice cap is composed of the ice‐rich north polar layered deposit (NPLD) and a lower‐lying, silicate‐rich basal unit (BU). Together, these units represent a record of Martian climate history throughout the Amazonian period. Here we place a bulk compositional constraint on the BU by modeling its gravity signature in both spatial and spectral domains using two independent approaches. We find the density of the BU to be kg/m3, suggesting that it may contain 55 + 25% water ice. We estimate that the BU contains ~1.5‐m global equivalent layer of water making it one of the largest reservoir of water‐ice on Mars. Our compositional constraint suggests that the north pole of Mars was not only a cold trap for ices but also an aeolian trap for silicates during the Amazonian period.