Proterozoic Basement Research Articles

Influence of Triassic and Jurassic arc magmatism in Lower Jurassic Sierra de Santa Rosa Formation, Northwestern México: Constraints from geochemistry and U-Pb geochronology

The Permo-Triassic and Lower Jurassic sedimentary succession of the El Antimonio Group in northern Sonora, Mexico is inferred to be deposited in a forearc basin and its age has been constrained by fossil assemblages and detrital zircon geochronology. In this work, petrography, geochemistry and detrital zircon U-Pb geochronology were undertaken on the Sierra de Santa Rosa Formation, the younger unit of the El Antimonio Group in the locality of the Sierra del Álamo, to constrain its tectonic setting, age, source area paleoweathering, and provenance. Sandstone of Sierra de Santa Rosa Formation is petrographically classified as arkose. The elemental ratios, REE patterns and Eu anomaly, as well as bivariate and ternary plots suggest igneous felsic sources subjected to weak to moderated chemical weathering under arid climatic conditions for detritus of the Sierra de Santa Rosa Formation. Detrital zircon U-Pb geochronology of seven sandstone samples of this unit record main populations of Proterozoic age (58% of the total grains) with age peaks at 1.68. 1.39 and 1.14 Ga and Triassic age (31% of total grains) with age peak at 219 Ma, and subordinate groups of Paleozoic, Jurassic and Archean ages. Main sources for the zircon grains are the regional nearby igneous-metamorphic Proterozoic basement rocks, recycled detritus from the Proterozoic and Paleozoic sedimentary cover, and the Permo-Triassic and Jurassic continental magmatic arcs of southwestern North America. The detrital zircon U-Pb geochronology of the dated samples allows to establish Sinemurian to early Toarcian maximum depositional ages for the Sierra de Santa Rosa Formation in the locality of the Sierra del Álamo.

Petrogenesis and Ni–Cu–(PGE) prospectivity of the Mount Ayliff Complex in the Karoo Igneous Province: new insights from the Ingeli and Horseshoe lobes

The Mount Ayliff Complex comprises five cognate mafic–ultramafic bodies emplaced along the southern margin of the Kaapvaal Craton. This study examines the Ingeli and Horseshoe lobes and assesses their magmatic sulfide prospectivity with respect to the Insizwa lobe, which hosts massive sulfides at its Waterfall Gorge occurrence. The Ingeli lobe consists of 465 m of olivine–chromite cumulates overlain by 340 m of (olivine-)gabbronorite, while the Horseshoe lobe consists of 45 m of olivine gabbronorite overlain by 5 m of gabbro. The Ingeli lobe possesses sparsely disseminated sulfides at its mafic–ultramafic transition, whereas disseminated sulfides are present throughout the Horseshoe lobe. A petrogenetic model is proposed, where magma accumulated and fractionated nickeliferous olivine and chromite in an upper-crustal staging chamber hosted by Proterozoic basement rocks. Magma then ascended and deposited olivine–chromite cumulates at the level of the complex. Prolonged magma flux in the staging chamber facilitated the assimilation of basement rocks, triggering sulfide saturation and the crystallization of Ni-poor olivine. Contaminated magma then ascended and interacted with pre-existing cumulates, depositing sulfide melt that may have backflowed as magmatic activity waned. Basaltic magma then flowed over the ultramafic cumulates, depositing disseminated sulfides whilst undergoing closed-system fractionation. Basal depressions and underlying feeder structures are the most prospective locations for magmatic sulfide mineralization.

The real McCoy: A record of deep‐water basin deposition in southwestern North America during the Cretaceous

AbstractThe McCoy Mountains Formation (McMF) in southern California–Arizona preserves an anomalously thick record of sedimentation during the Mesozoic at a critical time when western North America experienced contrasting tectonic events related to intracontinental rifting along the Mexican Border rift system and consolidation of the North American Cordilleran system. The spaciotemporal interactions among these events and the development of the McCoy basin challenge our understanding of the evolution of the southern extent of North America. At its type locality in the McCoy Mountains, the McMF consists of ~ 7 km of low‐grade metasedimentary rocks, originally interpreted as meandering fluvial to alluvial‐fan deposits. Uncertainty in the initial timing of sedimentation in the McCoy basin has resulted in multiple tectonic models. We measured ~ 7160 m of detailed stratigraphy and present new sedimentological and detrital zircon results showing that the McCoy basin was occupied by deep‐water turbidite systems. These systems deposited an upward‐coarsening succession of fine‐ to coarse‐grained detritus during the Cretaceous (ca. 137–70 Ma). Provenance data indicate that the McCoy basin received sediment from Proterozoic basement rocks and metamorphosed Palaeozoic to early Mesozoic sedimentary units. These source rocks are equivalent to the stratigraphy found in the Grand Canyon and Colorado Plateau regions and were likely shed from the southward‐advancing Maria fold‐thrust belt and possibly the southern Sevier belt in southern Nevada and California. These results, combined with subsidence curves typical of foreland basins, favour deposition within a subaqueous flexural foreland basin system. The presence of a Cretaceous foreland basin this far southwest challenges previously proposed models and suggests that the contractional tectonic regime associated with the North American Cordillera extended into the southwestern most United States during the Early–Late Cretaceous.

Zircon and cassiterite geochronology of Sn-polymetallic pegmatite from the Xianghualing ore field, South China: Implications for multi-stage magmatic-hydrothermal events

The Nanling metallogenic belt (South China) is one of the largest W-Sn polymetallic provinces in the world. The ore bodies are mainly hosted in Triassic-Jurassic granitoids which formed during a complex history of magmatic evolution. Among them, the Laiziling pluton, which forms the northern part of the Xianghualing district, is an example of Sn-polymetallic deposits that underwent multiple Mesozoic episodes. In this study, the first in-situ U-Pb age dating of zircon and cassiterite minerals, trace element, and Lu-Hf isotopic data are reported for Laiziling NYF-type pegmatites. Four types of pegmatites are identified: stock-like pegmatite, vein-type pegmatite, peripheral vein-type pegmatite, and cap/sill-like pegmatite. Zircon LA-ICP-MS U-Pb geochronology of pegmatites yields an age of ~150 Ma, which is consistent with the age of the associated mineralized granitic pluton. Results of cassiterite age dating indicate that the W-Sn mineralization took place at ca. 150 Ma coeval with pegmatite emplacement. Combined with structure and trace element composition, two young ages (i.e., ~130 and 90 Ma) can be identified from pegmatite veins, suggesting that the studied zircon crystals experienced two distinct periods of magmatic-hydrothermal evolution during the Cretaceous (related to Late Yanshanian magmatism). In addition, some zircon grains are obviously enriched in LREE, which is associated with hydrothermal metasomatism. Compared to the narrow range of magmatic zircons, a wide range of inherited zircons (Proterozoic to Paleozoic) are observed in the studied Late Jurassic pegmatite (Pg1 and Pg2b), suggesting that they were incorporated from pre-existing basement rocks during magma ascent and/or emplacement. Furthermore, U-Pb data for 14 disseminated cassiterite grains yield a 206Pb/238U age of 221.5 ± 4.2 Ma, documenting evidence of cassiterite inheritance during melt ascension from country rocks or representing the initial stage of Sn enrichment. The εHf values of magmatic zircons show negative εHf values from −4.4 to −16.6 and their crustal model ages (TDM2) vary between 1328 Ma to 2200 Ma, suggesting the parent magma of the Laiziling pegmatites were derived from partial melting of older crustal components (Proterozoic basement in the Cathaysian Block of South China) and the mixing of mantle materials. This study proposes that multi-stage magmatic-hydrothermal events may have occurred not only in the Xianghualing area but also in the whole Nanling Range, and provides fresh insights into the formation processes of rare metal-bearing pegmatites in South China.

Structural interpretation of the basement beneath the Southern Desert of Iraq based on aeromagnetic data

The Southern Desert (SD) of Iraq is located in the southernmost region of Iraq and is tectonically located on the stable part (proximal domain) of the Arabian Platform. The area is characterized by a considerable thickness of nonmagnetic Phanerozoic sediments overlying a reworked Proterozoic basement. Due to the intense nature of the karst near-surface geology, the basement has neither been seismically imaged nor drilled. In such situations, potential field (gravity and magnetic) exploration methods can often help to identify and map the deep structure of the basement. Here, the aeromagnetic field data, which cover all of the SD, are used to determine the structure and approximate depth of the basement. We use the combined results of the tilt derivative and phase preserving dynamic range compression (PPDRC) methods to qualitatively delineate the main basement structures and then use the source parameter imaging (SPI), tilt-depth (TD), and finite SPI (FSPI) methods to determine the basement depths. The qualitative interpretation of the tilt derivative and PPDRC methods identifies three north–south to northeast–southwest-trending linear negative anomalies that could represent extensional grabens in the basement surface. These grabens divide the basement into three blocks: the northwest block, the central block, and the southeast block. The magnetic anomalies over the basement blocks suggest that the northwest and central blocks are cut by a set of north–south to north-northwest–south-southeast and northeast–southwest faults. Depth estimation methods over the uplifted blocks have minimum depths between 4 and 5 km, whereas, over the graben, the depths range from 7 km to in excess of 12 km. The FSPI method, unlike the SPI and TD methods that use an infinite depth source body, gives depths generally deeper by up to 1.1 km if the assumed Curie-point depth is 21 km. A more realistic Curie-point depth of 32 km is used in the final interpretation model. These inferred basement blocks, grabens, and subbasin structures agree in general with the regional structures associated with the Arabian Peninsula and could provide an important framework for developing future hydrocarbon exploration strategies of the SD.

Thick- and thin-skinned contractional styles and the tectonic evolution of the northern Sangre de Cristo Mountains, Colorado, USA

The Sangre de Cristo Mountains of southern Colorado and northern New Mexico, USA, contain an unusual combination of thick- and thin-skinned contractional structures involving both basement and cover rocks in the Laramide Rocky Mountain foreland. These structures are truncated by down-faulted extensional basins to the east and west. Together with synorogenic sediments, these structures preserve a record of the rise of the Ancestral Rocky Mountains, the Laramide orogeny, and Rio Grande rifting. Laramide structures within the mountains provide clues to processes that link the three events and to necessary conditions for thin-skinned and thick-skinned contractional structures to form together in continental interiors. To examine the full variety of structural styles, a portion of the northern Sangre de Cristo fold-and-thrust belt in Colorado was described and interpreted using geologic maps and structural cross-sections. Stratigraphic relations of the Ancestral Rocky Mountain highlands and basin fill were reconstructed from existing maps. These relations allow identification of faults inherited from the Ancestral Rocky Mountains, differentiation of thrust sheets, and in some cases, estimation of the magnitude of displacement. To examine relations between Laramide thrusts and Rio Grande rifting, kinematic data were collected from a thrust fault adjacent to rift faults. Three thrust fault styles were recognized: thin-skinned basement, thin- skinned cover rocks, and thick-skinned basement. Thin-skinned thrusts arising from a hinterland beneath the present San Luis Valley carried sheets of Proterozoic basement rocks northeast over a Laramide foreland. These basement thrusts are interpreted to be faults of the Ancestral Rocky Mountains that reactivated during the Laramide orogeny. The Laramide foreland consists of thin-skinned thrusts and folds in sedimentary cover rocks as young as 49 Ma. Both thin-skinned thrusts in basement and cover rocks are bounded by thick-skinned basement thrusts that moved intermittently throughout the Laramide orogeny. We infer that thin-skinned thrusts form in continental interiors where deformation is focused in weak strata of thick basin fill and in fluid-reaction weakened preexisting faults in basement rocks. Both conditions are met in the Sangre de Cristo Mountains. Basement thrusts adjacent to the San Luis Valley contain evidence of plastic contractional microstructures overprinted by extensional microstructures that may record the transition from Laramide contraction to Rio Grande extension of the crust.

Geophysical insights into basement faults influencing basin evolution: A case study from Tennant Creek Block, North Australian Craton

Geophysical interpretation and modelling of the poorly exposed Proterozoic basement rocks of the Tennant Creek Block in the North Australian Craton elucidated the distribution, location, connectivity, and overprinting relationships of regional faults. The prominent structural grain within the basement comprises west-northwest-striking faults that commonly bound geophysically defined domains of the Warramunga Formation and felsic plutons of the Tennant Creek Supersuite. Constrained by seismic reflection data, forward models of gravity and magnetic data identified several faults as major structures that link into a shallow south-dipping structure at approximately 20 km depth. The modelling also reveals a 20 km wide and 14 km southward deepening half-graben bounded by a northeast dipping normal fault. The half-graben forms a sub-basin filled with the Ooradidgee Group at the top of the basement stratigraphy. The west-northwest-trending structural grain of the basement is not reflected in the Tomkinson Creek Group in the overlying younger basin. The younger basin has dominant north-oriented faults and basement highs. This suggests basement structures had limited influence in controlling the evolution of younger basins in this region. Our approach has wider relevance, addressing similar challenges encountered in other cratonic areas globally, such as India and Africa, where sedimentary cover obscures critical aspects of basement architecture and tectonic evolution. This underscores the broader significance of our methodology in advancing the understanding of Precambrian terranes.

Petrology of the Cretaceous Riacho do Cordeiro dike swarm: Unraveling the giant low-Ti tholeiitic plumbing system of the Equatorial Atlantic Magmatic Province

The formation of the Early Cretaceous Equatorial Atlantic Magmatic Province (EQUAMP), northeastern Brazil, is related to the fragmentation of the West Gondwana supercontinent that culminated in the separation of South America and Africa, during the Lower Cretaceous. The EQUAMP is characterized by its intrusive nature, where mafic dike swarms intrude the Proterozoic basement of the Borborema Province. The other components of this province are represented by sill complexes hosted between the Paleozoic sediments in the Parnaíba Basin, described as the Sardinha Formation. The magmatism related to the EQUAMP is tholeiitic, with high-Ti, and low-Ti magmas compositions that with similar features that are observed in other Mesozoic igneous provinces of Gondwana. The Riacho do Cordeiro dike swarm (RCo) represents the southernmost manifestation of the EQUAMP, consisting of a series of igneous bodies aligned along the N45E trend, arranged in an en echelon pattern. This structure extends for about 350 km, cutting the Precambrian basement of the southern segment of the Borborema Province. The dikes have a composition similar to tholeiitic basalts and basaltic andesites, but they are distinguished from the other components of the EQUAMP by their exclusively low-Ti content (TiO2 = 1.7–1.2 wt%). Magmatic differentiation is the main petrogenetic mechanism of the low-Ti magmas, typified by the systematic fractionation of major elements with respect to the variation of MgO (6.4–2.4% wt%). Trace (including rare earth) element patterns show enrichment in incompatible elements and negative anomalies in NbTa, a pattern recognized in continental basalts derived from (or contaminated with) lithospheric mantle. The Sr (87Sr/86Sr(i) 0.7072 to 0.7091) and Pb (206Pb/204Pb 18.360 to 18.785) isotopic signatures, combined with negative values for εNd(i) (−4.8 to −6.1) and TDM model ages >1.2 Ga, confirm the involvement of reservoir(s) in the upper mantle possibly modified by ancient subduction processes. The first reported 40Ar/39Ar ages for the low-Ti tholeiites of the RCo indicate ages from 134.53 ± 3.10 to 132.19 ± 1.28 Ma. Geochemically, the low-Ti tholeiites of the RCo are equivalent to low-Ti dikes that constitute part of the Rio Ceará Mirim dike swarm, the main component of the EQUAMP. Besides that, in a scenario external to this province, the same compositional correspondence is found within the Cretaceous dikes of the Vitória-Colatina swarm, southeastern Brazil, and the low-Ti type lavas of the Paraná-Etendeka Province. This compositional and geochronological connection between the tholeiitic magmatism located north and south of the São Francisco craton supports a possible genetic connection between these two Early Cretaceous LIPs and serve as base to future investigations that deal with what would have been the (global) mechanisms that led to the rifting of the Atlantic, from the southern segment to the equatorial margin.

Mantle-like to low oxygen isotopes in zircon from the mid-Cretaceous high-silica granites reveal unweathered basement recycling along the present coastal area of SE China

Zircon oxygen isotope compositions can provide potential constraints on the origin of granitic magmas. In this work, we report new data of zircon UPb dating, trace elements and HfO isotopes for two types of mid-Cretaceous high-silica granites from Zhoushan archipelago in Zhejiang Province, SE China. The in-situ zircon data are carefully screened for radiation damage and post-magmatic alteration. The analyzed magmatic zircon grains indicate that the Zhoushan calc-alkaline granites (ZCAG; 101–92 Ma) have mantle-like δ18O values of 5.29–5.88 ‰, in contrast to the Zhoushan peralkaline granites (ZPAG; 91–89 Ma) with relatively low δ18O of 4.42–4.6 ‰. The slight decreases in δ18O and εHf(t) from zircon center to periphery of both groups suggest that they may have consistent oxygen isotopic compositions in their parental magma sources and have gone through assimilations to hydrothermally altered wall rocks in the shallow crust. The mantle-like δ18O values in zircon are widespread in the Cretaceous felsic rocks in the present coastal area of SE China, which, in combination Hf isotopes, indicates the possible existence of a buried unweathered Proterozoic basement in the eastern part of the Cathaysia Block. This work highlights the role of deep crustal magma sources, assimilation and post-magmatic radioactive damage on the formation of mantle-like to low δ18O signals in zircons, and has fundamental implications for understanding the formation of widespread mid-Cretaceous felsic rocks along the present coastal area of SE China.

Linking carbonate-hosted Zn[sbnd]Pb deposit to deep mantle activity: Evidence from in situ U[sbnd]Pb geochronology of calcite from the world-class Huayuan Zn[sbnd]Pb ore field in South China

Carbonate-hosted ZnPb deposits are a major source of zinc, lead, and some critical raw metal resources worldwide (e.g., germanium and gallium). However, the timing and geodynamics setting of their mineralization remain enigmatic, leading to ongoing debates about their origins. To shed light on these issues, we utilized LA-ICP-MS in situ UPb dating of ore-related calcites to determine the timing and geodynamic setting of mineralization events in the world-class Huayuan carbonate-hosted ZnPb ore field in South China. The petrographic and cathodoluminescence features, fluid inclusion characteristics, and the Pb isotope data indicate that the tested calcites were formed from the same ore-forming fluids as the intergrown sphalerite and were not overprinted by possible later hydrothermal fluids. Our results show that the calcite UPb ages of 510–495 Ma refute the previously proposed Mississippi Valley-type (MVT) model, which suggested that the younger Caledonian orogeny (460–420 Ma) played a significant role in the genesis of the Huayuan ZnPb ore field. Instead, our findings link the formation of this ore field to crustal extension and high heat flux resulting from mantle upwelling events in northeast Gondwana during the Cambrian. The findings indicate that deep mantle processes can trigger shallow crustal fluid circulation to create a giant carbonate-hosted ZnPb ore field in a long-dormant rift.We present a genetic model proposing that the extensive carbonate platform of the Qingxudong Formation was likely a “brine factory” that supplied the brines to the Proterozoic basement through reactivated syn-rift faults. The high heat flux from upwelling mantle not only heated these brines to high temperatures but also caused early maturation of organic matter in the lower Cambrian Niutitang black shales to generate hydrocarbons. The hydrocarbons migrated into the limestone of the Qingxudong Formation. Meanwhile, the hot brines leached Zn and Pb from the Proterozoic basement rock. These ZnPb bearing brines ascended to the Qingxudong Formation where they mixed with the hydrocarbons. These hydrocarbons acted as reducing agents for ZnPb sulfide precipitation, forming the Huayuan giant ZnPb ore field. Furthermore, UPb dating results of syn-mineralization calcite, together with the relationships between the ores and host rocks, indicate that ZnPb mineralization occurred over timespans during or shortly after host-rock deposition and continuing during lithification for several million years in this ore field. The similar prolonged duration of mineralization may be relatively prevalent in other carbonate-hosted ZnPb deposits/ore fields. We therefore recommend that reliable age dating for all mineralization stages is necessary to establish genetic models for carbonate-hosted ZnPb deposits.

Devonian–Carboniferous extension and Eurekan inversion along an inherited WNW–ESE-striking fault system in Billefjorden, Svalbard

Background The Billefjorden area in central Spitsbergen hosts thick Lower–lowermost Upper Devonian, late–post-Caledonian collapse deposits presumably deformed during the Late Devonian Svalbardian Orogeny. These rocks are juxtaposed against Proterozoic basement rocks along the Billefjorden Fault Zone and are overlain by uppermost Devonian–early Permian deposits of the Billefjorden Trough, a N–S-trending Carboniferous rift basin bounded by the Billefjorden Fault Zone. Methods We interpreted seismic reflection (also depth-converted), bathymetric, and exploration well data. Results The data show abundant Early Devonian, WNW–ESE-striking (oblique-slip) normal faults segmenting the Billefjorden Trough, and a gradual decrease in tectonic activity from the Early Devonian (collapse phase) to early Permian (post-rift phase). Early Devonian–Middle Pennsylvanian WNW–ESE-striking faults were mildly reactivated and overprinted and accommodated strain partitioning and decoupling in the early Cenozoic. This resulted in intense deformation of Lower Devonian sedimentary rocks and in the formation of bedding-parallel décollements, e.g., between the Lower Devonian Wood Bay and the uppermost Pennsylvanian–lowermost Permian Wordiekammen formations. This suggests that intense deformation within Devonian rocks in Dickson Land can be explained by Eurekan deformation alone. Eurekan deformation also resulted in the formation of WNW–ESE- and N–S- to NNE–SSW-trending, kilometer-wide, open folds such as the Petuniabukta Syncline, and in inversion and/or overprinting of Early Devonian to Early Pennsylvanian normal faults by sinistral-reverse Eurekan thrusts. WNW–ESE-striking faults merge at depth with similarly trending and dipping ductile shear zone fabrics in Proterozoic basement rocks, which likely formed during the Timanian Orogeny. Conclusions A NNE-dipping shear zone, which is part of a large system of Timanian thrusts in the Barents Sea, controlled the formation of WNW–ESE-striking Devonian–Mississippian normal faults and syn-tectonic sedimentary rocks in Billefjorden. Eurekan strain partitioning and decoupling suggest that the Svalbardian Orogeny did not occur in Svalbard.

Crustal structure and seismic anisotropy of rift basins in Somaliland

Rift margins provide insights into the processes governing the rupture of the continental lithosphere and the subsequence formation of sedimentary basins. The Proterozoic basement underlying Somaliland has been affected by multiple rifting; however, the crustal structure of these rifted basins remains unknown. This study utilized teleseismic receiver function analysis, Bayesian inversion, common conversion point imaging and 2D forward gravity modelling to examine the crust and upper mantle of Somaliland. The results indicate 36.8–38.2 km of crust in southern Somaliland, while the central and northern regions feature thinned crust (~ 21 km) with 5–6 km thick sediments. The joint analysis of radial and transverse components of receiver functions and shear wave splitting revealed fast axis directions trending to 50–56° in the upper mantle, indicating that azimuthal anisotropy is oriented in the regional Africa-Arabia plate motion. Such orientation may have resulted from lattice preferred orientation of olivine from the asthenospheric flow. Additionally, the fast polarization of the crust in central Somaliland is oriented at − 15°, indicating fossil deformation in the thinned crust related to the NW–SE trending Late Jurassic rift event. Further, the fast polarization for stations near the Gulf of Aden is oriented at 75–80°, suggesting crustal deformation associated with the Oligocene rift event. The crustal anisotropy at southern Somaliland revealed fast polarization oriented at − 85°, indicating a preserved far-field response of the WNW-ESE trending Late Cretaceous rift event. Overall, the study provides for the first-time insight into the rift-related extensional strain fabric in the crust and upper mantle anisotropy induced by asthenospheric flow in Somaliland.

Silurian to Early Devonian tectonic evolution of the southeastern Qilian Orogen inferred from zircon U–Pb dating and geochemistry of intrusive rocks

Early to mid-Paleozoic tectonothermal events in the Qilian Orogen developed during the closure of the Proto-Tethys Ocean and the convergence of microcontinents in the periphery of East Gondwana. In this paper, we present geochronological, geochemical and Sr–Nd-Hf isotopic data for the granitoid rocks and mafic dykes in southeastern Qilian Orogen. The Liwan (440 Ma), Shixia (434 Ma) and Huchuan (429 Ma) granitoid rocks have metaluminous to weakly peraluminous features, with whole-rock ε Nd ( t ) values of 1.1 to 1.6, −4.2 to −4.4 and −2.6 to 1.1, and zircon ε Hf ( t ) values of 0 to 8.9, −6.6 to 1.6 and −4.6 to 2.2, respectively. Geochemical data suggest that the Liwan, Shixia and Huchuan granitoids are derived from partial melting of the Proterozoic basement with addition of juvenile material, felsic crustal basement and mafic crustal material, respectively. The Zhangjiayuan mafic dykes (403 Ma) are high-K to shoshonitic with whole-rock ε Nd ( t ) (−1.1 to −0.9) and zircon ε Hf ( t ) values (−0.9 to 13.3), derived from an enriched lithospheric mantle. We suggest that these Silurian–Early Devonian intrusive rocks were formed via complex events involving arc-related subduction followed by slab-tearing to post-collisional processes during the Silurian and subsequent lithospheric extension in the Early Devonian. Supplementary material: Tables giving zircon U–Pb isotopic data, whole-rock major and trace element results, whole-rock Sr–Nd isotopic data and zircon in situ Lu–Hf isotopic data are available at https://doi.org/10.6084/m9.figshare.c.6693575

Devonian-Carboniferous extension and Eurekan inversion along an inherited WNW-ESE-striking fault system in Billefjorden, Svalbard.

Background: The Billefjorden area in central Spitsbergen hosts thick Lower-lowermost Upper Devonian, late-post-Caledonian collapse deposits presumably deformed during the Late Devonian Svalbardian Orogeny. These rocks are juxtaposed against Proterozoic basement rocks along the Billefjorden Fault Zone and are overlain by uppermost Devonian-early Permian deposits of the Billefjorden Trough, a N-S-trending Carboniferous rift basin bounded by the Billefjorden Fault Zone. Methods: We interpreted seismic reflection (also depth-converted), bathymetric, and exploration well data. Results: The data show abundant Early Devonian, WNW-ESE-striking (oblique-slip) normal faults segmenting the Billefjorden Trough, and a gradual decrease in tectonic activity from the Early Devonian (collapse phase) to early Permian (post-rift phase). Early Devonian-Middle Pennsylvanian WNW-ESE-striking faults were mildly reactivated and overprinted and accommodated strain partitioning and decoupling in the early Cenozoic. This resulted in intense deformation of Lower Devonian sedimentary rocks and in the formation of bedding-parallel décollements, e.g., between the Lower Devonian Wood Bay and the uppermost Pennsylvanian-lowermost Permian Wordiekammen formations. This suggests that intense deformation within Devonian rocks in Dickson Land can be explained by Eurekan deformation alone. Eurekan deformation also resulted in the formation of WNW-ESE- and N-S- to NNE-SSW-trending, kilometer-wide, open folds such as the Petuniabukta Syncline, and in inversion and/or overprinting of Early Devonian to Early Pennsylvanian normal faults by sinistral-reverse Eurekan thrusts. WNW-ESE-striking faults merge at depth with similarly trending and dipping ductile shear zone fabrics in Proterozoic basement rocks, which likely formed during the Timanian Orogeny. Conclusions: A NNE-dipping shear zone, which is part of a large system of Timanian thrusts in the Barents Sea, controlled the formation of WNW-ESE-striking Devonian-Mississippian normal faults and syn-tectonic sedimentary rocks in Billefjorden. Eurekan strain partitioning and decoupling suggest that the Svalbardian Orogeny did not occur in Svalbard.

Aeromagnetic and structural characterization of dyke swarms in southeast Brazil: Evidence for Cenozoic reactivation of the Guapiara lineament, Ponta Grossa Arch

The Guapiara Lineament located on the northern edge of the Ponta Grossa Arch, Southeastern São Paulo state, Brazil, it is an important morphostructural feature defined by dykes oriented in NW-SE direction. The main objective of this work is applying new techniques for enhancing magnetic anomalies of the Guapiara Lineament and other structures, and using aeromagnetic data as support for the analysis of structure reactivation. Aerogeophysical techniques included qualitative and semiquantitative approaches (Euler Deconvolution and Radial Average Spectrum), we used the vertical and horizontal gradients, as well as deep magnetic sources and shallow magnetic sources filters, for an in-depth magnetic-structural characterization that was further correlated with the outcropping geology. We have also applied upward continuation (2000m) in the anomalous magnetic field map which was the basis for applying the Total Horizontal Gradient of the Analytical Signal (TDY), Analytic Signal Amplitude (ASA), Tilt angle (TDR), Total Horizontal Derivative of the Tilt Angle (TDR-THDR) and Tilt Angle of the Horizontal Gradient (TAHG) to improve our interpretations of the regional framework. The magnetic trends of the Guapiara Lineament are superimposed on the NE-SW (e.g.: Cubatão Lineament) and E-W (e.g.: São Sebastião Lineament) anomalies, which are considered structural inheritances of the Proterozoic basement. We report a set of reactivated N40–70W and E-W strike-slip faults showing the same orientation as the mapped magnetic lineaments. Moreover, they seem to be associated to deformational features in the dyke swarm, which was interpreted as evidence of Cenozoic tectonic reactivation.

Devonian-Mississippian faulting controlled by WNW-ESE-striking structural grain in Proterozoic basement rocks in Billefjorden, central Spitsbergen

In Billefjorden, central Spitsbergen, Devonian collapse and Carboniferous rift-related sedimentary strata were deposited unconformably over Proterozoic basement rocks displaying well developed N-S-trending Caledonian grain. Caledonian structures and fabrics are thought to have controlled the location and trend of subsequent Devonian and Carboniferous basin-bounding faults like the Billefjorden fault zone and Lemströmfjellet–Løvehovden fault. However, fieldwork and interpretation of aerial photographs in Proterozoic basement rocks reveal the existence of steep, abundant, WNW-ESE-striking brittle faults that are sub-orthogonal to known major Caledonian and post-Caledonian structures in Billefjorden, but that do not extend into adjacent-overlying, rift-related, Pennsylvanian rocks of the Gipsdalen Group. Structural analysis of field data and aerial photographs suggest that WNW-ESE-striking faults in basement rocks in Billefjorden formed as (sinistral) strike-slip and normal faults during Devonian-Mississippian extension in agreement with previously inferred models of sinistral transtension. The abundance of these faults suggest that their formation was controlled by analogously trending, preexisting structural grain (planar anisotropies) at depth, and their pronounced WNW-ESE strike suggest that the strike of preexisting anisotropies were comparable to recently identified, crustal-scale, WNW-ESE-striking Timanian thrust systems in Svalbard and the northern Barents Sea.

Spatial variations of effective elastic thickness of Eastern Arabia: Implications for lithospheric weakening in the Oman-UAE mountain belt

The continental lithosphere of the Arabian Plate has evolved through a long and complex tectonic history and comprises major geological features such as an ophiolite complex, Cenozoic basalt extrusion, and Proterozoic basement outcrops. In eastern Arabia, an NW-SE to N-S oriented orogenic belt is present in Oman and the United Arab Emirates (UAE). The mountain belt formed due to the Late Cretaceous obduction of the Semail ophiolite, where a thick slab of oceanic crust and upper mantle was emplaced on the rifted Arabian continental margin. However, the mechanical strength of the lithosphere over the entire region is poorly known. In this study, we employ Bouguer gravity anomaly and topographic data, along with data on crustal properties obtained from teleseismic analyses, to assess the spatial variations in the effective elastic thickness (Te) of the eastern Arabian Plate. The joint inversion of coherence and admittance yielded Te values ranging from 10 to 50 km and an initial subsurface-to-surface load ratio (F) of 0.1 to 0.8. Te values in the southern part of eastern Arabia are low (< 20 km), while the northeastern regions of Saudi Arabia, the UAE, and Qatar exhibit intermediate to high Te values ranging from 30 to 45 km. The Oman-UAE mountain belt is associated with an intermediate Te of 30 km in the northern segment and a low Te of 20 km in the southern segment. The reduction in Te observed along the strike of the mountain belt suggests weakening of the lithosphere during the obduction of the Semail ophiolite. Te correlates well with both the seismic thickness of the lithosphere (Ts) and the decrease in shear-wave velocity at a depth of 150 km. The seismogenic thickness in eastern Arabia, however, is about 15 km, which is significantly less than Te and shows that both the crust and upper mantle contribute to the strength of the continental lithosphere. The Ts/Te ratio ranges from 1 to 9, with a maximum peak at 3–5, which is similar to the results observed for the oceanic lithosphere.

Разломная тектоника Ухтинской складки Тиманской гряды

he article deals with a model of the fault tectonics of the Ukhta fold in the Timan Ridge, created on the basis of reinterpretation of seismic data. By the results of constructions along the top of the Upper Proterozoic basement, the majority of the faults have a northwestern direction and extend both continuously and fragmentarily throughout the entire structure. Only north-west-directed faults are clearly traced over the surface of the Devonian deposits. The overlay of the obtained scheme onto the map of the anomalous magnetic field indicates that the identified faults are normally missed in the magnetic field when surveying at a scale of 1:50 000. For the detail site location of the faults and the continuous research of the geodynamic situation, a complex of field geophysical works is required to be conducted in the future.

Insights into the Crustal Evolution and Tungsten Mineralization of the West Cathaysia Block: Constraints from the Inherited Zircons from the Mesozoic Dengfuxian and Paleozoic Tanghu Plutons, South China

The formation and evolution of the ancient continental crust are crucial issues in solid-earth geology which are commonly associated with global tectonic events and the formation of economically valuable magmatic-hydrothermal ore deposits. The Cathaysia Block, one of the ancient continents in Southeast Asia, can be subdivided into two parts: the West Cathaysia Block and the East Cathaysia Block. Unlike the East Cathaysia Block, no Precambrian rocks are exposed in the West Cathaysia Block, constraining further understanding of the formation and evolution of this block. In this study, a total of four hundred and thirty-three zircon U-Pb dating analyses and two hundred and eighteen Lu-Hf isotopic analyses on zircon grains from the Jurassic Dengfuxian granites and Ordovician Tanghu granites, Nanling Range, were carried out. LA-ICP-MS zircon U-Pb dating yields mean average 206Pb/238U ages of 152.6 ± 2.2 Ma (MSWD = 1.6) and 442.4 ± 1.7 Ma (MSWD = 3.8), which are regarded as the rock-forming age for the Jurassic Dengfuxian granites and Ordovician Tanghu granites, respectively. The 207Pb/206Pb ages of the inherited zircons from the Jurassic Dengfuxian granites and Ordovician Tanghu granites range from 522 Ma to 3395 Ma, hosting two major peaks at the 0.9–1.0 Ga and 2.4–2.5 Ga. In contrast to the East Cathaysia Block, the West Cathaysia Block lacks the age peak of 1.8–1.9 Ga, indicating that the West Cathaysia Block was not influenced by the assembly of the Columbia supercontinent in the Paleo-Proterozoic. In combination with the Lu-Hf isotopes, we proposed that the crust evolution of the West Cathaysia Block in Archean is dominated by juvenile crustal growth events, and dominated by the crustal reworking since the Proterozoic. The long duration of crustal reworking in the West Cathaysia Block resulted in the enrichment of lithophile elements (e.g., W, Sn, Nb, and Ta) in the crust of that region. Therefore, the Jurassic granites in the Nanling Range, which are mainly derived from the partial melting of Proterozoic basement rocks, became associated with large-scale tungsten polymetallic mineralization.

Tectonic Background of Carboniferous to Early Permian Sedimentary Rocks in the East Kunlun Orogen: Constraints from Geochemistry and Geochronology

The formation of the East Kunlun Orogen (EKO) was related to the tectonic evolution of the Proto-Tethys and Paleo-Tethys Oceans. However, how the Paleo-Tethys Ocean transited from the Proto-Tethys Ocean, and whether the Paleo-Tethys Ocean subducted northward beneath the East Kunlun–Qaidam Terrane in Carboniferous to Permian times, is still highly debated. Early Carboniferous Halaguole and Late Carboniferous to Early Permian Haoteluowa formations are extensively outcropped in the EKO, north Tibetan Plateau, and have thus recorded key information about the tectonic processes of the Paleo-Tethys Ocean that have implications for the reconstruction of the Northern Paleo-Tethys Ocean (Buqingshan Ocean). Siliciclastic rocks within these formations are collected for petrogeological, geochemical, and detrital zircon U–Pb dating research. Our results show that sandstones from Halaguole and Haoteluowa formations have an average total quartz–feldspar–lithic fragment ratio of Q67F12L21 and Q50F20L30, respectively, indicating relatively high compositional maturity. The geochemical results suggest that the average values of the Chemical Index of Alteration (CIA) are 57.83 and 64.66; together with their angular to subangular morphology, this indicates that their source rocks suffered from weak weathering and the sandstones are the result of proximal deposition. Geochemical features such as the low La/Th, TiO2, and Ni values suggest that the parental rocks in the provenance area are mainly acidic igneous rocks with minor intermediate igneous and old sedimentary components. The detrital zircon U–Pb age spectrum of these samples is dominated by age peaks at ~405–503 Ma and ~781–999 Ma, with subordinate age peaks at ~1610–2997 Ma and ~1002–1529 Ma, which show tectono–thermal events similar to those of the North Qimatag Belt (NQB), North Kunlun Terrane (NKT), and South Kunlun Terrane (SKT). These features suggest a contribution from the Early Paleozoic magmatic arc and Proterozoic basements in the NQB, NKT, and SKT to the Halaguole and Haoteluowa formations in these areas. In addition, the youngest zircon age of ~440 Ma from these sandstones is greater than the depositional age of Halaguole and Haoteluowa formations, which is a typical basin depositional feature in a passive continental margin. Geochemical tectonic discrimination diagrams, based on a major and trace element Ti/Zr–La/Sc plot, in combination with a detrital zircon age distribution pattern, all suggest a passive continental margin setting. Considering this together with the previous data, we argue that the Paleo-Tethys Ocean did not begin to subduct northward and that there was no oceanic subduction zone in the south EKO during Carboniferous to Early Permian times. Combining this information with that from previous studies suggests that the initial opening of the Paleo-Tethyan Ocean may have occurred before the Early Carboniferous time, and all the branches of the Paleo-Tethys Ocean constituted a complex ocean–continent configuration across parts of what is now Asia during the Early Carboniferous to Early Permian.