Extensional Episode Research Articles

Synsedimentary Deformation Characterization of Niamey Sandstones in the Tondibia Area (Man Shield Northeastern Margin, Western Niger, Region of Niamey)

Niamey sandstones belong to a group of formations of Neoproterozoic-age, located on the Man shield northeastern margin. They sporadically outcrop along the Niger river valley. These geological formations, which occupy a central position in relation to the Taoudenni basin (further north) and the Voltas basin (further south), share similarities with the formations of the aforementioned basins. The research objective is to determine the synsedimentary deformation that has affected these Niamey sandstones in the Tondibia area. The methodological approach used focuses, firstly, on field measurements of synsedimentary deformation structures, and secondly, on projecting these measurements into the Win-Tenseur program in order to calculate stress tensors (s1, s2, s3). Synsedimentary deformations appear during the early stages of lithification, i.e. when the sediment is still loose and contains a high percentage of water. The analysis of these deformations is of great interest for the tectonic-sedimentary analysis of basin deposits. Deformation analysis reveals that the synsedimentary deformation phase affecting the Niamey sandstones is characterized by a NNW-SSE to NNE-SSW direction of elongation. This phase of deformation is marked in the field by normal faults with an average orientation of N80°. This extensive episode is concomitant with the extension of the Neoproterozoic Ocean (870 to 800 Ma).

Early Triassic high-K granitoids and enclaves of the Daheba pluton, West Qinling (China): Implications for relative contributions of crust and mantle

Early Triassic granitoids are widespread in the northwestern West Qinling Orogen, China, but their petrogenesis and geodynamic implications remain unclear. In this study, we integrated new field and petrological observations, mineralogical compositions, zircon U-Pb dating, Hf isotopic and whole-rock geochemical analyses for the early Triassic granitic pluton in the Daheba area to determine its magma source and geodynamic scenario. The pluton is composed of granodiorite and syenogranite and carries microgranular enclaves (MEs) which formed at ca. 253–249 Ma. The granitoids show wide SiO2 contents of 63.73–77.49 wt% (av. 69.89), high K2O contents of 3.4–5.4 wt% (av. 4.1) and moderate Mg# of 16–51 (av. 36), belonging to high-K, calc-alkaline I-type granites. These rocks have high radiogenic but uniform Hf isotopic compositions with 176Hf/177Hf and ɛHf(t) of 0.282511–0.282658 and −3.85 – +1.25, respectively. The MEs hosed within the granite are characterized by high Mg# of 35–58 (av. 46) and variable εHf(t) of −4.64 – +9.35, which likely represent a hybridized melt derived from a slab-modified mantle and lower crust. In combination with coeval magmatic rocks in the western Gonghe-East Kunlun area, we propose a genetic model where mafic magmas derived from an enriched mantle and underplated beneath the overlying lower crust are considered to have produced the high-K felsic magma. Further, the hybridized melt ascended to shallower crustal levels to generate a series of rocks ranging from dioritic MEs to granodiorite to syenogranite. Mass balances modeling suggests that the generation of these rocks involved 36 % of the lower crustal-derived melt and 64 % of the SCLM (R2 = 0.9). Our new data, in tandem with published results suggest that the Daheba pluton formed during the subduction stage of the Paleo-Tethyan Ocean and that a local extensional episode occurred at 253–249 Ma in the western Gonghe area.

Two episodes of extension induced by slab rollback and root foundering in the Eastern Tianshan, southwestern Altaids: Insights from granites and intermediate dykes

The widespread Late Paleozoic-Early Mesozoic magmatism in the Eastern Tianshan provides critical insights into geodynamic evolution of the southern Altaids. In this study, we conduct an integrated study of field geology, petrology, U-Pb zircon geochronology and major-trace elemental and Sr–Nd–Hf isotopic geochemistry on the Late Carboniferous monzogranite and diorite dyke and the Late Triassic gabbroic diorite dyke in the central part of the Eastern Tianshan. Geochronological data indicate that the monzogranite, diorite dyke and gabbroic diorite dyke formed at ∼318 Ma, ∼317 Ma and ∼235 Ma, respectively. The Late Carboniferous monzogranite shows transitional characteristics between highly fractionated and A-type granites, with relatively high zircon saturation temperatures (800–836 °C). It was derived by partial melting of juvenile infracrustal rocks, followed by plagioclase-dominated fractional crystallization. The Late Carboniferous diorite dyke has high Al2O3 contents (mostly >17 wt%) and Ba/La ratios, indicating an origin from a hydrous lithospheric mantle source that had been metasomatized by slab-derived fluids. The Late Triassic gabbroic diorite dyke, with relatively high Mg# values (57–58) and Th contents (3.28–3.46 ppm), originated from a less hydrous lithospheric mantle source containing significant sediment-derived components. Our new results, combined with previous studies, reveal an end-Early Carboniferous to middle Late Carboniferous magmatic flare-up in the Dananhu arc. The magmatic flare-up occurred in a long E-W trending banded zone parallel to the strike of the arc and was accompanied by significant normal arc magmatism, which widely migrated to the south of the Dananhu arc in comparison to prior magmatism. These together with coeval extension-related volcanic, sedimentary, structural and metamorphic events in the Dananhu and adjacent areas were attributed to southward rollback of the subducted Kanguer oceanic slab. The long-time arc magmatism and regional crustal thickening before the Late Triassic led to the formation of an arclogite-bearing root beneath the Dananhu arc. The Late Triassic extension-related magmatism in the Dananhu arc, which was synchronous with or slightly younger than regional thickened crust-derived magmatism, was linked to the local root foundering of the long-lived Dananhu arc at the late stage of orogenesis. The episodic occurrence of geodynamic processes such as slab rollback, arc root foundering and ridge subduction/slab tearing, which were accompanied by massive input of juvenile materials into continental crust, contributed to multi-stage significant crustal growth in southern Altaids.

Middle Jurassic to Early Cretaceous orogenesis in the Klamath Mountains Province (Northern California–southern Oregon, USA) occurred by tectonic switching: Insights from detrital zircon U-Pb geochronology of the Condrey Mountain schist

The Klamath Mountains Province of Northern California and southern Oregon, USA, consists of generally east-dipping terranes assembled via Paleozoic to Mesozoic subduction along the western margin of North America. The Klamath Mountains Province more than doubled in mass from Middle Jurassic to Early Cretaceous time, due to alternating episodes of extension (e.g., rifting and formation of the Josephine ophiolite) and shortening (e.g., Siskiyou and Nevadan events). However, the tectonic mechanisms driving this profound Mesozoic growth of the Klamath Mountains Province are poorly understood. In this paper, we show that formation of the Condrey Mountain schist (CMS) of the central Klamath Mountains Province spanned this critical time period and use the archive contained within the CMS as a key to deciphering the Mesozoic tectonics of the Klamath Mountains Province. Igneous samples from the outer CMS subunit yield U-Pb zircon ages of ca. 175–170 Ma, which reflect volcanic protolith eruptive timing. One detrital sample from the same subunit contains abundant (~54% of zircon grains analyzed) Middle Jurassic ages with Paleozoic and Proterozoic grains comprising the remainder and yields a maximum depositional age (MDA) of ca. 170 Ma. These ages, in the context of lithologic and thermochronologic relations, suggest that outer CMS protoliths accumulated in an outboard rift basin and subsequently underthrust the Klamath Mountains Province during the Late Jurassic Nevadan orogeny. Five samples of the chiefly metasedimentary inner CMS yield MDAs ranging from 160 Ma to 130 Ma, with younger ages corresponding to deeper structural levels. Such inverted age zonation is common in subduction complexes and, considering existing K-Ar ages, suggests that the inner CMS was assembled by progressive underplating over a >10 m.y. timespan. Despite this age zonation, age spectra derived from structurally shallow and deep portions of the inner CMS closely overlap those derived from the oldest section of the Franciscan subduction complex (South Fork Mountain schist). These relations suggest that the inner CMS is a composite of South Fork Mountain schist slices that were sequentially underplated beneath the Klamath Mountains Province. The age, inboard position, and structural position (i.e., the CMS resides directly beneath Jurassic arc assemblages with no intervening mantle) of the CMS suggest that these rocks were emplaced during one or more previously unrecognized episodes of shallow-angle subduction restricted to the Klamath Mountains Province. Furthermore, emplacement of the deepest portions of the CMS corresponds with the ca. 136 Ma termination of magmatism in the Klamath Mountains Province, which we relate to the disruption of asthenospheric flow during slab shallowing. The timing of shallow-angle subduction shortly precedes that of the westward translation of the Klamath Mountains Province relative to correlative rocks in the northern Sierra Nevada Range, which suggests that subduction dynamics were responsible for relocating the Klamath Mountains Province from the arc to the forearc. In aggregate, the above relations require at least three distinct phases of extension and/or rifting, each followed by an episode of shallow-angle underthrusting. The dynamic upper-plate deformation envisioned here is best interpreted in the context of tectonic switching, whereby slab steepening and trench retreat alternate with slab shallowing due to recurrent subduction of buoyant oceanic features.

Early Eocene Arctic volcanism from carbonate-metasomatized mantle

Melilitite, nephelinite, basanite, and alkali basalt, along with phonolite differentiates, form the Freemans Cove Complex (FCC) in the south-eastern extremity of Bathurst Island (Nunavut, Canada). New 40Ar/39Ar chronology indicates their emplacement between ~ 56 and ~ 54 million years ago within a localized extensional structure. Melilitites and nephelinites, along with phonolite differentiates, likely relate to the beginning and end phases of extension, whereas alkali basalts were emplaced during a main extensional episode at ~ 55 Ma. The melilitites, nephelinites, and alkali basalts show no strong evidence for significant assimilation of crust, in contrast to some phonolites. Partial melting occurred within both the garnet- and spinel-facies mantle and sampled sources with He, O, Nd, Hf, and Os isotope characteristics indicative of peridotite with two distinct components. The first, expressed in higher degree partial melts, represents a relatively depleted component (“A”; 3He/4He ~ 8 RA, εNdi ~ + 3 εHfi ~ + 7, γOsi ~ 0). The second was an enriched component (“B” 3He/4He < 3 RA, εNdi < – 1 εHfi < + 3, γOsi > + 70) sampled by the lowest degree partial melts and represents carbonate-metasomatized peridotite. Magmatism in the FCC shows that rifting extended from the Labrador Sea to Bathurst Island and reached a zenith at ~ 55 Ma, during the Eurekan orogeny. The incompatible trace-element abundances and isotopic signatures of FCC rocks indicate melt generation occurred at the base of relatively thin lithosphere at the margin of a thick craton, with no mantle plume influence. FCC melt compositions are distinct from other continental rift magmatic provinces worldwide, and their metasomatized mantle source was plausibly formed synchronously with emplacement of Cretaceous kimberlites. The FCC illustrates that the range of isotopic compositions preserved in continental rift magmas are likely to be dominated by temporal changes in the extent of partial melting, as well as by the timing and degree of metasomatism recorded in the underlying continental lithosphere.

Mammalian faunal change of the Miocene Dove Spring Formation, Mojave region, southern California, USA, in relation to tectonic history

Tectonic processes drive the evolution of basins through local and regional changes in topographic relief, which have long-term effects on mammalian richness and distribution. Mammals respond to the resulting changes in landscape and climate through evolution, shifts in geographic range, and by altering their community composition. Here, we evaluate the relationship between tectonic episodes and the diversification history of fossil mammals in the Miocene Dove Spring Formation (12.5−8.5 Ma) of southern California, USA. This formation contains a rich fossil record of mammals and other vertebrates as well as structural and sedimentological evidence for tectonic episodes of basin extension, rotation, and translation. We used several methods to compare the fossil record to the tectonic history of the Dove Spring Formation. We updated the formation’s geochronology to incorporate current radiometric dating standards and measured additional stratigraphic sections to refine the temporal resolution of large mammal (&amp;gt;1 kg) fossil localities to 200-kyr (or shorter) intervals. Observed species richness over time follows the same trend as the number of localities and specimens, suggesting that richness reflects sampling intensity. Estimates of stratigraphic ranges with 80% confidence intervals were used to conduct per capita diversification analysis and a likelihood approach to changes in faunal composition from one time interval to the next. While edge effects influence time bins at the beginning and end of the study interval, we found changes in diversification rates and faunal composition that are not solely linked to preservation. Several rare species appear at 10.5 Ma and persist through the top of the formation despite variable preservation rates. Changes in faunal composition at 12.1 Ma and 10.5 Ma are not associated with elevated preservation rates, which indicates that some faunal changes are not primarily driven by sampling effort. The lower portion of the formation is characterized by high origination rates and long residence times. The upper portion has high per capita extinction rates that increased in magnitude as basin rotation and translation progressed from 10.5 Ma. The greatest change in faunal composition coincided with basin rotation and translation that interrupted a long-running extensional period. Tectonics played key roles in the diversity of mammals by determining fossil productivity and shaping the landscapes that they inhabited.

Ultra-crystalline pyroclastic deposits and rhyolitic lavas controlled by crystal mushes: insights from the Acoculco Caldera Complex, México

AbstractThe Acoculco Caldera Complex (ACC), located in eastern Mexico, began its activity during the Pleistocene ~ 2.7 Ma. One of the most relevant and largest rhyolitic eruption in the complex, the Piedras Encimadas Ignimbrite (PEI), occurred during the late post-caldera phase at ~ 1.2 Ma. This ignimbrite is unique with respect to the other caldera products and other contemporaneous ignimbrites in the Trans-Mexican Volcanic Belt (TMVB) because of its ultra-high crystallinity and the absence of pumice fragments. The PEI is made almost entirely of crystals where the main constituents are k-feldspars and silica polymorphs that range from ≤ 5 µm to tens of centimeters in size. XRD on bulk rock, geochemical modeling, FTIR, Raman, and EPMA analyses were carried out in all mineral phases to assess the origin and the causes of high crystallinity within the PEI. We interpret the high crystallinity on the basis of magmatic crystallization of a magma body that was remobilized and altered by post-depositional hydrothermal alteration processes. We suggest that ACC rhyolites are geochemically influenced by at least one crystal mush established during the Pleistocene. We suggest that the PEI could be the result of an erupted crystal mush (melt + crystals), or a cumulate, or an ancient and crystallized reservoir generated after the first ACC collapse due to intrusion or underplating of mafic hot magmas. Extensional episodes within the ACC facilitated the ascent of mafic magmas. This interaction increased the liquid fraction of the mush through partial melting/crystal dissolution, generating a drop in density and viscosity in the mush, thus triggering eruption. The PEI provides evidences for an association between the geochemically-diverse ACC rhyolites with the complex interaction between mafic transitional alkaline magmas and a crustal mush system, promoted by continuous changes in the stress field during the Pleistocene.

Grenvillian evolution of the Qaidam block and its position in Rodinia constrained by U–Pb–Hf composition of detrital zircons from the Altyn Tagh, Northern Tibet

The U–Pb ages and Hf isotopic composition of detrital zircons provide important constraints on basin evolution and paleogeography. The Qaidam block, a continental fragment along the northeastern margin of the Tibetan Plateau, has been suggested to have been either in an internal or a peripheral position in Rodinia. We present a systematic study of late Mesoproterozoic to Ediacaran sedimentary sequences from the Altyn Tagh belt, a part of the Qaidam block displaced by the Altyn Tagh fault, in an effort to establish its position within Rodinia. The (meta-)sedimentary rocks of the Altyn Complex of South Altyn can be correlated with the Taxidaban Group, the part of the basement(meta-)sedimentarysequence of Central Altyn, on the basis of lithological and geochronological similarities. Both of these were deposited in the late Mesoproterozoic to early Neoproterozoic. The early Neoproterozoic Suoerkuli Group of Central Altyn and its coeval magmatism formed in an extensional setting. This extensional episode facilitated thinning of the lithosphere during the late Tonian–Ediacaran when the sedimentary protoliths of the Hongliuquan Complex were deposited. Detrital zircon grains from late Mesoproterozoic to Ediacaran sedimentary sequences in the Altyn Tagh orogenic belt show a wide distribution of Meso- and Paleoproterozoic ages between 1800 and 1000 Ma, which match well with contemporaneous supracrustal sequences in northeastern Laurentia. In combination with the similarity in late Mesoproterozoic to Ediacaran tectono-thermal events, we propose the Qaidam block was located along the northeast Laurentian margin during the late Meso- to Neoproterozoic assembly of Rodinia.

The Missing Link in the Genesis of the Lower Paleozoic Copper Deposits of the Anti-Atlas (Morocco): The Late Triassic Central Atlantic Magmatic Province Event

Copper mineralization in the Lower Paleozoic sedimentary cover of the Anti-Atlas (Morocco) is continually being revised not only to improve its mining capacity, but also to determine its origin, which remains a matter of debate. As evidenced by the various models proposed, the related research is fragmented, localized, and confusing. The origin of the Anti-Atlas Lower Paleozoic copper mineralization is shared between synergistic and epigenetic processes or a superposition of the two processes. Based on new tectono-magmatic data and a reinterpretation of the ore structural arrangement, we propose a link between the last concentration of copper deposits and the Late Triassic–Early Liassic CAMP (Central Atlantic Magmatic Province) tectono-thermal event, as evidenced by the significant concentration of copper mineralization in the three NE–SW corridors affected by extensional faults, some of which are filled with dolerite CAMP magma. The heat flow generated by the mafic dykes within these reactivated corridors causes mineralized fluids to up well into the sedimentary layers, depositing material rich in juvenile or leached copper, or even a mixture of the two. In some cases, these fluids are trapped by fracture systems that accompany passive folds initiated on normal faults. In other cases, these fluids can infiltrate bedding planes, and even karst caves, formed during carbonate exhumation. Notably, extensive NE–SW faults systematically cover the early Hercynian structures, suggesting that they belong to a post-Hercynian extensional episode. During the Late Triassic, the global fragmentation of the Pangaea supercontinent was manifested by the stretching of the continental crust at the margin of northwest Africa, with the simultaneous opening of the Central Atlantic Ocean and emplacement of CAMP magmatism. This last and often overlooked tectonothermal event must be considered in the remobilization and reconcentration of copper mineralization and other mineralization in Morocco.

Neogene subsidence rates of the southern Peri Pannonian realm (1D basin modeling): Constraints on the extensional geodynamic drivers of the asymmetric Toplica basin (central-southern Serbia)

With the objective to quantify the Neogene extensional geodynamic drivers of the Peri Pannonian Realm s.l., a series of 1D basin models were carried out by integrating the available geological and geochemical data extracted from the landlocked Toplica Basin (central-southern Serbia; Serbo-Macedonian Unit). The modeling was conducted on four wells located immediately south of Jastrebac Mt., in the area of the Toplica Basin. The investigated southernmost point of the Peri Pannonian Ream s.l. (areas surrounding the basin, to the south of rivers Danube and Sava) underwent intense tectonically-driven subsidence episodes, which lasted from the Lower Miocene up to the Pliocene. The restored Neogene burial history (subsidence rates), paleotemperature, and calibrated paleo-heat fluxes (or changes in the ancient crustal or basal heat flow) indicated a two-staged crustal stretching of the wider Toplica area. The combined tectonic, i.e., thermal subsidence, resulted in a very high sedimentation rate, having an average ca. 300–400 m/Ma (Čučale unit). The intense tectonically-driven subsidence is consistent with the involvement of extensional processes, affecting the crustal and lithospheric layers of the retreating Adria/Dinarides, amplified by a distant Carpathian rollback stage. The regional extension led to the asymmetric opening and the formation of the Toplica Lower Miocene mesosaline to the freshwater lacustrine system. Higher participation of mixed microbiologically reworked/terrestrial organic matter incorporated in oil-prone Type II kerogen mainly characterized the investigated lacustrine sedimentary sequence. The latest Oligocene–earliest Neogene subsidence onset was characterized by elevated temperatures, affecting the early development of the oil window (ca. at 16 Ma). The 1D models also showed that the Lower Čučale unit achieved ca. 50% transformation ratio for the Type II organic matter. The two intra-Neogene extensional episodes suggested the existence of the Early Neogene “syn-rift” stage, followed by a thermal sagging. The Neogene continuous thinning of the overriding Serbo-Macedonian continental crust contributed to the “negative inversion” of the former pre-Neogene nappes. Therefore, Neogene tectonic reactivation induced contemporaneous subsidence, further allowing the extensional rise of the Jastrebac core complex.

Episodic magmatism in the Lianhuashan tectonic belt: Implications for late Mesozoic crustal reworking in SE South China

The Lianhuashan tectonic belt preserves abundant evidence of episodic magmatism and deformation that is crucial for understanding the late Mesozoic crustal reworking in SE South China. Here, we report zircon U-Pb ages, geochemical, and Sr−Nd−Hf isotopic data for representative granitoids and volcanic rocks to clarify the episodic magmatism in the Lianhuashan tectonic belt and its association with regional tectonic evolution. Our results define three episodes of magmatism. The Late Jurassic (160−150 Ma) magmatic rocks show high CaO/Na2O and molar CaO/(MgO + FeOt) (CMF) ratios and enriched Sr−Nd−Hf isotopes that are consistent with high-temperature melting of psammitic sources. The Early Cretaceous (144−131 Ma) magmatic rocks display variable CaO/Na2O ratios and zircon εHf(t) values (−12.0 to −2.5), which implies binary mixing of melts from pelitic and basaltic sources. The Late Cretaceous (ca. 94 Ma) magmatic rocks show high A/CNK and low CaO/Na2O and CMF ratios, which are characteristics of pelite-derived melts. They have zircon Hf isotopes (−4.4 to −1.9) that are slightly higher than those of the Early Cretaceous rocks, which is consistent with partial melting of diverse sources with more involvement of juvenile crust. These magmatic records, coupled with regional structural observations, suggest that the Lianhuashan tectonic belt underwent alternating crustal shortening and extension during the Late Jurassic to Cretaceous time. The Late Jurassic magmatism possibly marks the initiation of crustal extension following Middle Jurassic retro-arc crustal shortening related to the advancing Paleo-Pacific subduction. In the Cretaceous, the two magmatic flare-ups were separated by a magmatic lull associated with crustal shortening. The magmatic flare-ups occurred coevally with two stages of regional extension, as evidenced by widespread generations of basins and extensional domes. The time consistency implies that the magmatism was associated with or facilitated crustal extension that occurred at a regional scale and may have resulted from slab rollback during subduction retreat. We suggest that the magmatism and extension provide essential evidence of the crustal reworking in SE South China governed by the Paleo-Pacific subduction. Changes in the slab dynamics (steepening and shallowing) may control the flare-up, lull, and resumption of magmatism, as well as alternating episodes of crustal extension and shortening.

Structural analysis and measured extension in fault complexes along the Lofoten‐Vesterålen margin, offshore Norway, in the context of crustal‐scale rifting towards breakup in NE Atlantic

AbstractLate Cretaceous–Palaeocene continental extension within the Lofoten‐Vesterålen margin is investigated by integrating 2D‐3D seismic and potential field datasets, together with updated crustal transects. Most of that deformation is recorded by two low‐angle detachment structures named West Røst High Fault Complex (WRHFC) and North Utrøst Ridge Fault Complex (NURFC) located at the southern and central‐northern portions of the studied area, respectively. Multiple extensional episodes of various intensities were mapped as different fault stages, including one Albian‐Cenomanian phase, four early Late Cretaceous phases, three Late Cretaceous to latest Cretaceous–Palaeocene phases, and one Palaeocene phase. The WRHFC is narrower in extent and has accommodated a relatively greater amount of localized extension (ca. 18–19 km), whereas the NURFC occupies a wider area with widespread extension intensity (ca. 6–8 km). In comparison, the total across‐margin average extension within the southern, central, and northern portions of the Lofoten‐Vesterålen and NE Greenland conjugate margins are ca. 192, 221, and 266 km, respectively. Such results indicate an apparent extension discrepancy between derived extension from measured fault‐block geometries within the fault complexes and the whole conjugate margin system, with only ca. 11% and 13% of the extension seen on the studied seismic profiles on the WRHFC and NURFC, respectively. The corrected maximum extension for purely sub‐seismic resolution faulting on both the WRHFC and NURFC fault complexes is ca. 164 and 46 km, respectively. Finally, both WRHFC and NURFC structures provide key evidence for a ductile mode of deformation towards breakup that is expressed through shear zones‐rift topography interactions with overlaying listric/detachment faults. These features reflect the resulting multiphase tectonic evolution across the asymmetric Lofoten‐Vesterålen and NE Greenland conjugate margins, and the obliquity in the breakup axis location along them. The study outcomes are pertinent and applicable to understand the breakup evolution of the northern NE Atlantic and its vicinity.

Late Cretaceous‐Early Paleogene Extensional Ancestry of the Harcuvar and Buckskin‐Rawhide Metamorphic Core Complexes, Western Arizona

AbstractMetamorphic core complexes in the western North American Cordillera are commonly interpreted as the result of a single phase of large‐magnitude extension during the middle to late Cenozoic. We present evidence that mylonitic shear zones in the Harcuvar and Buckskin‐Rawhide core complexes in west‐central Arizona also accommodated an earlier phase of extension during the Late Cretaceous to early Paleocene. Microstructural data indicate substantial top‐NE mylonitization occurred at amphibolite‐facies, and 40Ar/39Ar thermochronology documents post‐tectonic footwall cooling to &lt;500°C by the Paleocene to mid‐Eocene. Amphibolite‐facies mylonites are spatially associated with voluminous and variably deformed footwall leucogranites that were emplaced from ca. 74–64 Ma, and a late kinematic ca. 63 Ma dike indicates this phase of mylonitization had waned by the early Paleocene. Reconstruction of the footwall architecture indicates that this latest Cretaceous—early Paleocene deformation occurred within a NE‐dipping extensional shear zone. The leucogranites were likely the result of crustal melting due to orogenic thickening, consistent with a model whereby crustal heating triggered gravitational collapse of overthickened crust. Other tectonic processes, such as the Laramide underplating of Orocopia Schist or mantle delamination, may have also contributed to this episode of orogenic extension. Miocene large‐magnitude extension was superimposed on this older shear zone and had similar kinematics, suggesting that the location and geometry of Miocene extension was strongly influenced by tectonic inheritance. We speculate that other Cordilleran core complexes also experienced a more complex and polyphase extensional history than previously recognized, but in many cases the evidence may be obscured by later Miocene overprinting.

Multi-stage metallogeny in the southwestern part of South China, and paleotectonic and climatic implications: A high precision geochronologic study

The South China Block (SCB) is among the large-scale W-Sn mineralized regions of the globe. The Laojunshan W-Sn-dominant ore area (LOA) in the western part of the SCB preserves the records of the tectonic history of the Tethys realm extending through North Vietnam, and Yangtze to Cathaysia blocks, with coeval formation of giant metallic deposits. The prolonged tectonic activities and their control on the genesis and spatio-temporal distribution of giant metallic deposits in the LOA provide a window for a holistic understanding of the tectono-metallogenesis of the SCB. In this study, we present results from a multi-chronologic study to determine the timing of formation of the cassiterite-wolframite-scheelite mineralization. The results suggest three distinct tectono-metallogenic periods in the LOA during the geodynamic evolution of the surrounding tectonic units. The opening of the Proto-Tethys Ocean between the Yangtze-Indochina blocks and the westward Paleo-Pacific subduction beneath the Cathaysia block (420–380 Ma) jointly contributed to the Silurian to early Devonian intracontinental orogeny in the middle of the SCB that involved top-to-the-north thrusting along NE-striking shear zones. This event generated the Dulong-Song Chay granitoids, together with the formation of Xinzhai Sn deposit related to sheared mylonitic granites (ca. 419 Ma) and pegmatites (ca. 389 Ma), which include the early-stage Sn-sulfide skarn (ca. 418 Ma) and the late-stage Sn-bearing schist (ca. 389 Ma). During the Late Permian to Late Triassic (260–220 Ma), with the closure of the Proto-Tethys oceans in the west and ongoing Paleo-Pacific westward subduction in the east, the SCB and Indochina Block (IB) were amalgamated which also marks the time of formation of the Nanwenhe scheelite skarn deposit. The subducted Paleo-Tethys oceanic crust was likely entrained by the nearby rising Emeishan mantle plume (270–259 Ma), which formed the Maguan diabase (ca. 260 Ma) that shows significantly older Re-Os model age of ca. 268 Ma, suggesting that the Nanwenhe mineralization is potentially derived from ca. 260 Ma source. Furthermore, the intraplate shortening induced thin skinned crustal deformation and low grade metamorphism (ca. 230 Ma), with the main stage of scheelite-Sn-Mo mineralization (229.9, 229.8 and 219 Ma) and contemporary formation of the pegmatite (230.7 Ma). The Late Cretaceous involved two episodes of alternate extension and shortening, driven by the subduction polarity change from northwestward subduction of the Okhotomorsk block to northward subduction of the NeoTethys seafloor. The evolution of the LOA consists of the NW–SE transpression ending ca. 100 Ma, the WNW–ESE extension in the earlier episode lasting from 100 Ma to 86 Ma, the WNW–ESE transpression beginning at ca. 85 Ma and the NS extension in the later episode during the latest Cretaceous, which produced the extension-related three periods of Laojunshan granitic magmatism and coeval Sn-W mineralization, with ages in the range of 90–89 Ma, 87–85 Ma and 83–79 Ma. We also evaluate the implications of magmatic-metamorphic-metallogenic degassing on the regional paleoclimatic history.

CONSTRUCTION OF DISCUSSION FOR POSTPONEMENT OF 2024 ELECTIONS IN TVONE'S “DUA SISI” PROGRAM (ENTMAN FRAMING ANALYSIS)

The purpose of this research was to analyze the news framing scheme for the issue of postponing the 2024 Election, and to reveal news framing scheme for the issue of postponing the 2024 Election on TVOne's "Dua Sisi" program, the More Gaduh episode of President's Term Extension. This study used a qualitative approach with the method of media framing analysis, namely the Robert N Etman framing analysis model. The data collection technique was carried out by analyzing documents related to the discourse on the postponement of the 2024 Election and dividing it into four elements, namely defining problem, (diagnosing causes) estimating the source of the problem, (make moral judgment) moral decision, and (treatment recommendation) emphasizing resolution. The results of this research indicated that TVOne's “Dua Sisi” program tends to defend the president. Defining the problem regarding the postponement of the 2024 election is against the constitution, the estimated source of the problem of postponing the election heated up after being raised by three political parties supporting Jokowi, moral decisions should be subject to and obey the constitution, emphasizing the solution, namely that everyone has the right to voice their opinion as a democratic country. The construction of news about the postponement of the 2024 election can be seen from the topics of discussion, the arrangement of questions or narratives, the selection of sources and the display of visual images showing a tendency to construct news.

Zircon dating of the basalt and felsic dyke in Haveri, SW Finland

The E-MORB type Haveri basalt differs from the volcanic arc type rocks in the Tampere belt showing no subduction signature. It is considered to have formed in a marginal basin prior to the volcanic arc. We present here zircon U-Pb dating on two samples. The basalt and the felsic dyke yielded 207Pb/206Pb ages of 1902 ± 5 Ma and 1891 ± 2 Ma, respectively, interpreted as crystallisation ages. The basalt also contains older 1.98 Ga grains while the felsic dyke contains older 1.92 Ga, 1.94 Ga, 1.98 Ga and 2.0 Ga grains, which are inferred as inherited. The age dating of E-MORB type basalts can be used to identify the extensional episodes of the accretionary Svecofennian orogeny.

Early cretaceous topographic evolution associated with the collapse of the Mongol-Okhotsk orogen in Western Transbaikalia: an integrated analysis

ABSTRACT The Early Cretaceous topographic evolution of Transbaikalia was largely governed by the tectonic evolution of the Mongol-Okhotsk orogen. The collapse of the Mongol-Okhotsk orogen triggered the formation of metamorphic core complexes and associated extensional basins, widespread throughout Transbaikalia, North Mongolia, and North China. Numerous lithofacies and biostratigraphic studies have been carried out from the sedimentary deposits of the Transbaikalia basins. However, the absence of absolute ages for the sedimentary series, as well as sediment source-to-sink analysis do not allow to accurately characterize the regional topographic evolution. We focused our study on the Gusinoozersk Basin of Western Transbaikalia, where extensive sedimentary sections of Lower Cretaceous deposits have been preserved. We provide new U/Pb (LA-ICP-MS) data on detrital zircons from sedimentary series and 40Ar/39Ar data on intruding rocks. We review the paleontological data to clarify the age of the paleogeographic events associated with the collapse of the Mongol-Okhotsk orogen, as well as to correct the age of faunal complexes in Western Transbaikalia. Our geochronological results show that the formation of the Cretaceous basins of Transbaikalia began around 136–130 Ma, accompanying the main episode of extension associated with the exhumation of the metamorphic core complexes. The lowest coarse-clastic formation characterizes the rapid subsidence and the predominance of proximal sediment sources. Distal provinces also made a contribution to sedimentation indicating the rise of a positive topography characterizing the exhumation of the metamorphic core complexes. Overlying fine-grained formations indicate a significant smoothing of the topography, suggesting that from middle Aptian, Western Transbaikalia developed in a relatively calm tectonic regime. We also show that the basins of Transbaikalia were formed both in conjunction with the exhumation of metamorphic cores complexes and reactivated structural sutures. Revised data on dinosaur fauna and palynology, together with the dating of host deposits, provide insights on the Early Cretaceous paleoenvironmental evolution.

FOCAL MECHANISMS OF EARTHQUAKES IN THE SUBDUCTION ZONE OF THE WESTERN PACIFIC PLATE

Deformation features of the subducting Pacific lithospheric plate are considered according to the data on earthquake focal mechanisms. The territory includes the convergent boundaries between the Pacific Plate and the North American (in the Aleutian arc region), the Okhotsk, the Eurasian and the Philippine plates.It has been shown that the angle of subducting Pacific Plate in the Aleutian subduction zone affects the focal mechanisms of earthquakes that occurred in the upper, 35 km part of the oceanic plate in the zone of its bending. There occur normal-fault earthquakes at a steep-angle subduction and rare thrust earthquakes at a shallow-angle subduction. The azimuthal orientation of P-axes of the focal mechanism solutions in the upper (1–70 km) contact zone corresponds to the Pacific Plate displacement vector when the plate fragments are subducting west-northwestwards. There occurs a change in azimuthal orientation of the compression axes in the subducting plate at a depth of more than 70 km: the axes occupy different azimuthal sectors showing difference in the orientation of their slope, with the orientations of the T-axes become multidirectional.The calculation of seismotectonic deformations was carried out based on the data on focal mechanisms of 7768 earthquakes. It was revealed that the Exx and Ezz deformation fields are the most homogeneous at depths of 1–70 km. The pattern of seismotectonic deformations changes abruptly for deep parts of the subducting plate (105–200, 200–400, and 400–700 km), there are observed heterogeneous deformation fields Exx, Eyy and Еzz with alternating episodes of extension and shortening.There has been proposed the author’s scheme of the influence of the upper mantle convection structure on the geometry of the subducting plate (slab) as a potential catalyst for the processes responsible for the separation of seismic activity zones and the change of earthquake types with depth and in different parts of the extended subduction zone.

Seismic Reflection Profiles Reveal the Ore-Controlling Structures of Carlin-Style Gold Deposits in Lannigou Gold Fields, Southwestern Guizhou, China

AbstractThe Carlin-style Lannigou gold deposit is hosted in Triassic flysch in the Nanpanjiang basin in southwestern Guizhou Province, southern China. This study is the first to use seismic exploration data to elucidate the structural architecture of the Lannigou Carlin-style gold deposit. We use regional seismic reflection data to show that gold orebodies are controlled by faults that link with the regional Lannigou-Weihuai fault, and we use this new insight to determine the relationship between mineralization and inverted basin faults to constrain the structural controls of this gold system.A grid of seven seismic sections covering the Lannigou gold fields is combined with field geologic data to reveal two distinct structural patterns. Seismic data reveals that the inverted, E-dipping Lannigou-Weihuai fault is shallowly penetrating (less than 2 km) in the north and increasingly penetrates the pre-Devonian basement rocks to the south, where the fault can be imaged to depths greater than 5 to 7 km. Basement-penetrating faults link the metamorphic basement and overlying basin successions that include potential source rocks for oil generation, interpreted gas reservoirs, and gold deposits. Therefore, these deep-seated faults are important channels for the migration of ore fluids, especially for the transport of hydrocarbon gas, which may have served as an important chemical trap for gold mineralization.Seismic data also reveals the W-dipping Qiaoluo fault that bounds the inverted Qiaoluo half-graben. Fault crosscutting relationships reveal an extensional episode from the Late Paleozoic to the early Anisian age of the Middle Triassic epoch (i.e., the depositional age of the second member of the Middle Triassic Xuman Formation), which is overprinted by an episode of middle Anisian basin inversion during deposition of the third member of the Xuman Formation. This inversion occurred at ca. 248 to 246 Ma. Based on the ore-controlling constraints and previous geochronological data, we consider that the Lannigou gold deposit was formed in the Late Triassic to Early Jurassic during syndepositional inversion of existing basement-penetrating basinal faults in a foreland basin. The first-order faults in the above structural belts, such as the Lannigou-Weihuai fault and the blind Qiaoluo fault, are most favorable targets for further exploration of undiscovered gold orebodies. This study demonstrates that seismic reflection is a powerful tool to reveal deep structures at varying scales from mineral deposits to sedimentary basins.

Analogue modeling of the role of salt in the structuration of thin-skinned pull-apart basins: The case study of El Hamma basin, Central Tunisia

Pull-apart basins are structurally complex features that typically form in between two laterally offset strike-slip zones. The vast majority of analogue experiments on the formation of this type of basins have been conducted using basal discontinuities that transmitted from the bottom up strike-slip motion into the overlying sedimentary cover. Here, we present a series of models whose aim is to demonstrate that pull-apart basins above a viscous layer can be initiated not from the base up, but by the sole effect of lateral forces that allow two blocks to move laterally one past the other. This occurs independently from the basement, from which the brittle cover is fully decoupled by the viscous layer. This work was inspired by a natural example, the El Hamma Basin, located in Central Tunisia West of the city of Gabes and is bounded by two N100-110 trending major fault corridors, the Oum Ali- Fejij corridor in the North, and the Oglet Merteba-Matmata in the South. The area underwent multiple tectonic episodes of both extension and compression from Late Liassic to the Plio-Quaternary times. The region was subjected to regional NE-SW extension during the upper-cretaceous and major NW-SE compression during upper Miocene (Tortonian) and early quaternary (Early Pleistocene). The latter stage resulted in right-lateral strike slip along both corridors with faults forming a pull-apart basin that comprises two lateral mega-grabens separated by a central horst. Although one sub-circular diapir in the area (the El Melah diapir) crops out along the northern wrench zone, whether or not the mobile Triassic series extend farther South of it, beneath the pull-apart basin, is still debated.Through a series of analogue experiments using an innovative technique that generates two narrow, parallel strike-slip zones located within the sedimentary cover, rather than in the basement. Experimental results show remarkably similar geometry and evolution with those of the El Hamma basin. Moreover, from a regional perspective, our results suggest that the pinch out of the Triassic evaporites extends further South than previously thought and underlies at least part of the El Hamma basin.