Boulder, Colo., USA: The Geological Society of America regularly publishes articles online ahead of print. GSA Bulletin topics studied this month include the nature and dynamics of China and Tibet, including the Central Asian Orogenic Belt, the North China Craton, the Lhasa terrane, and the Sichuan Basin. You can find these articles at https://bulletin.geoscienceworld.org/content/early/recent .
Kinematic evolution of the central Andean retroarc thrust belt in northwestern Argentina and implications for coupling between shortening and crustal thickening
Susana Henriquez; Peter G. DeCelles; Barbara Carrapa; Amanda N. Hughes
Abstract: The Andes are the culmination of shortening and crustal thickening that commenced during Late Cretaceous time. First-order questions regarding the tectonic evolution of the central Andes include the magnitude and timing of shortening, and controls on the along-strike variability in observed styles of shortening and deformation. Along-strike differences in the time of surface uplift have spawned two contrasting hypotheses: (1) uplift is related to dynamic and isostatic processes accompanying lithospheric removal and is decoupled in space and time from crustal thickening and shortening, and (2) uplift is directly coupled with shortening and crustal thickening. Although considerable work has been done in Bolivia to address these hypotheses, work in northern Argentina has not yet produced a trans-orogenic balanced structural cross section from which the total amount and kinematic history of shortening can be evaluated. To help understand the evolution of the thrust belt in northernmost Argentina, we present a regional, retrodeformable cross section at 23°−24°S across the Puna and Eastern Cordillera. New apatite fission-track thermochronological data integrated with other geochronological, sedimentological, and structural data constrain incremental retrodeformation of the cross section between ca. 45 and 6.5 Ma. Regional shortening was facilitated by at least 12 major thrust systems, linked to a regional mid-crustal décollement. Deformation generally propagated eastward through time and involved two major episodes of eastward advance of the orogenic front, separated by periods of internal out-of-sequence shortening and kinematic stagnation of the orogenic front. A new minimum estimate of ∼271 km of total shortening from the Cordillera de Domeyko to the eastern orogenic front explains crustal thickening at northern Puna latitudes. Together with previously published paleoaltimetry data, our new structural and thermochronologic data indicate that regional uplift in the northern Argentine Puna and Eastern Cordillera was synchronous with, and thus directly linked to, crustal shortening and thickening.
View article: https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36231.1/613418/Kinematic-evolution-of-the-central-Andean-retroarc
(U-Th)/He chronology: Part 1. Data, uncertainty, and reporting
R.M. Flowers; P.K. Zeitler; M. Danišík; P.W. Reiners; C. Gautheron ...
Abstract: The field of (U-Th)/He geochronology and thermochronology has grown enormously over the past ∼25 years. The tool is applicable across much of geologic time, new (U-Th)/He chronometers are under continuous development, and the method is used in a diverse array of studies. Consequently, the technique has a rapidly expanding user base, and new labs are being established worldwide. This presents both opportunities and challenges. Currently there are no universally agreed-upon protocols for reporting measured (U-Th)/He data or data derivatives. Nor are there standardized practices for reporting He diffusion kinetic, 4He/3 He, or continuous ramped heating data. Approaches for reporting uncertainties associated with all types of data also vary widely. Here, we address these issues. We review the fundamentals of the methods, the types of materials that can be dated, how data are acquired, the process and choices associated with data reduction, and make recommendations for data and uncertainty reporting. We advocate that both the primary measured and derived data be reported, along with statements of assumptions, appropriate references, and clear descriptions of the methods used to compute derived data from measured values. The adoption of more comprehensive and uniform approaches to data and uncertainty reporting will enable data to be re-reduced in the future with different interpretative contexts and data reduction methods, and will facilitate inter-comparison of data sets generated by different laboratories. Together, this will enhance the value, cross-disciplinary use, reliability, and ongoing development of (U-Th)/He chronology.
View article: https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36266.1/613176/U-Th-He-chronology-Part-1-Data-uncertainty-and
(U-Th)/He chronology: Part 2. Considerations for evaluating, integrating, and interpreting conventional individual aliquot data
R.M. Flowers; R.A. Ketcham; E. Enkelmann; C. Gautheron; P.W. Reiners ...
Abstract: The (U-Th)/He dating technique is an essential tool in Earth science research with diverse thermochronologic, geochronologic, and detrital applications. It is now used in a wide range of tectonic, structural, petrological, sedimentary, geomorphic, volcanological, and planetary studies. While in some circumstances the interpretation of (U-Th)/He data is relatively straightforward, in other cases it is less so. In some geologic contexts, individual analyses of the same mineral from a single sample are expected to yield dates that differ well beyond their analytical uncertainty owing to variable He diffusion kinetics. Although much potential exists to exploit this phenomenon to decipher more detailed thermal history information, distinguishing interpretable intra-sample data variation caused by kinetic differences between crystals from uninterpretable overdispersion caused by other factors can be challenging. Nor is it always simple to determine under what circumstances it is appropriate to integrate multiple individual analyses using a summary statistic such as a mean sample date or to decide on the best approach for incorporating data into the interpretive process of thermal history modeling. Here we offer some suggestions for evaluating data, attempt to summarize the current state of thinking on the statistical characterization of data sets, and describe the practical choices (e.g., model structure, path complexity, data input, weighting of different geologic and chronologic information) that must be made when setting up thermal history models. We emphasize that there are no hard and fast rules in any of these realms, which continue to be an important focus of improvement and community discussion, and no single interpretational and modeling philosophy should be forced on data sets. The guiding principle behind all suggestions made here is for transparency in reporting the steps and assumptions associated with evaluating, integrating, and interpreting data, which will promote the continued development of (U-Th)/He chronology.
View article: https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36268.1/613175/U-Th-He-chronology-Part-2-Considerations-for
Locating Lhasa terrane in the Rodinia and Gondwana supercontinents: A key piece of the reconstruction puzzle
Xiu-Zheng Zhang; Qiang Wang; Wei Dan; Derek Wyman
Abstract: The debate over global continental reorganization from Rodinia to Gondwana likely stems from some key Neoproterozoic units being overlooked in previous reconstructions. Here we provide a self-consistent set of evidence, based on magmatism and metamorphism, that the Lhasa terrane is the “lost” part of NW India and retains crucial archives, including 760−730 Ma ophiolitic-arc magmatic rocks and evidence of a distinctive metamorphic event at ca. 660 Ma. Hence, the Lhasa terrane together with Madagascar and Seychelles formed in a late Neoproterozoic juvenile arc system laying either along the periphery of rifting Rodinia or outboard of the supercontinent. Subsequently, these terranes made up the northern East African Orogen (EAO) during the initial assembly of Gondwana. As a result of lateral-escape tectonics after collision and dextral strike-slip along the northern margin of Gondwana, the Lhasa terrane escaped from the EAO and had migrated to the northern margin of Australia by the Middle Cambrian.
View article: https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36152.1/613137/Locating-Lhasa-terrane-in-the-Rodinia-and-Gondwana
Two-stage exhumation of deeply subducted continental crust: Insight from zircon, titanite, and apatite petrochronology, Sulu belt of eastern China
Songjie Wang; Michael Brown; Lu Wang; Tim E. Johnson; Hugo K.H. Olierook ...
Abstract: The rates and mechanisms by which deeply subducted continental crust was exhumed back to the surface are not well understood, but can be better characterized using multimineral petrochronology. Here, we combine zircon, titanite, and apatite U-Pb ages from leucogranite and phengite gneiss with a pressure−temperature (P−T) path from eclogite to provide robust quantitative constraints on cooling and exhumation of the Sulu belt, a large ultrahigh-pressure metamorphic terrane in eastern China. The leucogranite, which formed during exhumation, is enriched in light rare earth elements (REE) relative to heavy REE and in large ion lithophile elements relative to high field strength elements, similar to hydrous crustal melts. Whole-rock Sr-Nd isotope compositions indicate that the leucogranite was not directly derived from the host phengite gneiss, but was more likely sourced from deeper in the exhuming crust. For the gneiss, mantles on inherited zircon yield an age of 230 ± 2 Ma and a temperature of 802 ± 36 °C based on a minimum pressure of 2.9 GPa, which records the minimum timing and P−T of initial decompression. Overgrowths on inherited zircon from the leucogranite constrain crystallization to 224 ± 1 Ma, coeval with the growth of zircon rims in the gneiss, at a temperature of 764 ± 42 °C and a pressure within the quartz-eclogite facies. Titanite and apatite define single populations with lower concordia intercept ages of 222 ± 3 Ma and 198 ± 7 Ma, at temperatures of 720 ± 30 °C and ∼450 ± 100 °C, respectively, recording the timing of passage through the quartz-eclogite to the amphibolite facies and then the transition to the upper greenschist facies. Although the data yield a nearly constant cooling rate of 10.9+4.5−3.6 °C/m.y., exhumation was completed in two stages. The first stage from coesite-eclogite facies to ∼1.2 GPa, corresponding to the depth of the Moho, occurred at a rate of 7.5 +5.8−2.6 km/m.y. Thereafter, exhumation into the mid-crust occurred at a much slower rate of 0.87+0.86 −0.71 km/m.y. The first stage of faster exhumation was accompanied by migration of leucogranite melt along foliation in the gneiss, which would have decreased the average density and weakened the crust, enhancing the rate of return flow.
View article: https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36309.1/613138/Two-stage-exhumation-of-deeply-subducted
Contrasting mechanisms and timescales of subduction and exhumation as recorded by Paleoproterozoic and late Paleozoic high-pressure granulites in the North China Craton
Shan-Shan Li; Richard M. Palin; M. Santosh
Abstract: The North China Craton records multiple metamorphic events related to supercontinent assembly during the Paleoproterozoic, forming Columbia, and again during the late Paleozoic, forming Pangea. Here we show that the Paleoproterozoic high-pressure granulites (HPGs) formed from enriched mid-ocean ridge basalt protoliths and record a clockwise pressure-temperature-time (P−T−t) path with prograde metamorphism at 7.8−10.0 kbar and 780−820 °C, peak granulite-facies metamorphism at 12−12.3 kbar and ∼860−880 °C, and retrograde metamorphism at 8.7−9.1 kbar and 850−855 °C. Subduction initiated prior to 1.90 Ga, with final collision and orogeny at 1.88 Ga, followed by post-collision/exhumation at 1.80 Ga, defining a prolonged exhumation period (∼90 m.y.) that occurred at a slow velocity of ∼0.16 ± 0.08 mm/y. Late Paleozoic HPGs are normal mid-ocean ridge basalt type and record a near clockwise P−T−t path, with peak/post-peak amphibolite-facies metamorphism at 11.0−12.5 kbar and 860−890 °C, isothermal decompression to 7.2−7.5 kbar and 810−820 °C, and retrogression to 5.5−7.2 kbar and 805−850 °C. Subduction initiated earlier than ca. 340 Ma, exhumation and uplift initiated at 335−309 Ma and continued to 297−287 Ma. The exhumation was short-lived (∼50 m.y.) and relatively fast (0.38 ± 0.14 mm/y). When compared to granulite-facies metamorphism documented in many Paleoproterozoic HPGs, late Paleozoic HPGs appear to commonly form with an initial period of steep subduction leading to eclogite-facies metamorphism, with subsequent exhumation to middle/lower levels of the crust. Our results further reveal that the exhumation velocity for supercontinent collision was facilitated and duration shortened through time, and that the exhumation mechanism might have been controlled by subduction angle, compression pressure, and temperature.
View article: https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36380.1/612999/Contrasting-mechanisms-and-timescales-of
Early Paleozoic subduction fingerprints of the Paleo-Asian ocean in easternmost Central Asian Orogenic Belt (CAOB): Identification of the oldest Alaskan-type complex in the CAOB
Meng-Meng Cui; Ben-Xun Su; Jing Wang; Yong Wu; Anton Kutyrev
Abstract: The early-stage subduction records of the Paleo-Asian ocean are poorly preserved in the eastern segment of the Central Asian Orogenic Belt (CAOB), which hinders constraints on the evolution of the whole CAOB. This study presents new age data and zircon Hf-O isotopes as well as bulk-rock geochemistry of the Wuxing mafic-ultramafic complex in the Xingkai massif in northeastern China, which has been identified as Alaskan-type complex with aspects of field occurrence, petrological and mineral assemblages, and mineral chemistry in our recently published work. The results indicate that the complex formed mainly between 517 Ma and 510 Ma with a lithological sequence of Sanying clinopyroxenite (517 Ma), then Sanying gabbro (514 Ma), and Erying hornblendite (513 Ma), and finally Erying hornblende clinopyroxenite (510 Ma). The lithological formation sequence is consistent with the intrusive relations between lithological phases and their irrelevant major element compositions and variable trace element patterns of the bulk rocks. The εHf(t) values of zircon in two samples in this study vary from from +4.45 to +7.61 and from −11.8 to +4.42, respectively and tend to be more depleted with age. These features suggest that the Wuxing complex was a product of long-term arc magmatism and experienced significant ancient crustal assimilation in early-stage magmas and negligible contamination in later ones. The presence of 1222 Ma and 706 Ma inherited zircon grains implies existence of Proterozoic basement in the Xingkai massif and its continental arc setting in Cambrian. The Wuxing complex is the oldest Alaskan-type complex found so far in the entire CAOB and is a good witness of the Paleo-Asian oceanic subduction in the easternmost CAOB. The older age of the Wuxing complex compared to the regional Hongqiling intrusion is also compatible with its Alaskan-type nature and platinum-group element mineralization, which are distinct to the Permian-Triassic Ni-Cu sulfide deposit-hosting mafic-ultramafic intrusions in the southern CAOB.
View article: https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36307.1/613000/Early-Paleozoic-subduction-fingerprints-of-the
New Late Cretaceous paleomagnetic results from the Lhasa terrane and their implications for the suturing of India and Eurasia and the closure of the Neo-Tethys Ocean
Yabo Tong; Zhenyu Yang; Junling Pei; Jianfeng Li; Shuchen Jin ...
Abstract: The forms of the margins of the Lhasa terrane and the Tethyan Himalaya prior to the collision of India and Eurasia as constrained by paleomagnetism are ambiguous due to the disordered Cretaceous paleomagnetic data from the central Lhasa terrane and the counterclockwise rotation of the Indian plate during the Cretaceous. This ambiguity has induced controversy over the processes of suturing of India and Eurasia and the closure of the Neo-Tethys Ocean. We obtained a set of high-quality Late Cretaceous paleomagnetic data from the central Lhasa terrane, which, integrated with reliable Cretaceous and Paleogene paleomagnetic data sets from the other parts of the Lhasa terrane and Tethyan Himalaya, confirmed that the southern margin of the Lhasa terrane and the northern margin of the Tethyan Himalaya were originally oriented ∼317° and ∼326°, respectively, prior to the collision of India and Eurasia. The margins of the Lhasa terrane and Tethyan Himalaya were almost consistent with the original straight fold axes of Cretaceous strata in the southern part of the Lhasa terrane, which were oriented 332.5° ± 8.5°, indicating that the subduction of the Neo-Tethys Ocean beneath Eurasia and the movement of the Tethyan Himalaya consistently maintained a stabilized direction of 62.5° ± 8.5° during the Late Cretaceous. The different kinematic characteristics of the Indian plate and Tethyan Himalaya and the overlap of the margins of the Tethyan Himalaya and Lhasa terrane during 59.0−56.0 Ma indicate that the Tethyan Himalaya was already rifted from the Indian plate prior to 62.5−59.2 Ma, and then it quasi-parallelly collided with the Lhasa terrane during 59.0−56.0 Ma, quasi-synchronously closing the Neo-Tethys Ocean.
View article: https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36310.1/613001/New-Late-Cretaceous-paleomagnetic-results-from-the
Mechanisms of strain localization and nucleation of earthquake faulting by grain-scale processes at the middle crustal level
Chunru Hou; Junlai Liu; Yuanyuan Zheng; Yanqi Sun; Tieying Zhang ...
Abstract: The mechanism of strain localization is the key to our understanding of the transition from steady-state to unstable flow, and therefore of earthquake faulting in the middle crust. In this study, biotite grains in mylonitic gneisses along the Jinzhou detachment fault zone, Liaodong peninsula, northeast China, acted as a preexisting weak phase that had important influences on deformation of mid-crustal rocks. High phase strength contrasts between biotite and other mineral phases resulted in stress concentrations at the tips of biotite grains and induced semi-brittle deformation of neighboring quartz and feldspar grains. As a consequence, the biotite grains became interconnected to form zones of weakness, while basal plane slip and grain boundary sliding operated in biotite grains and fine-grained biotite-feldspar-quartz aggregates, respectively. The zones filled with biotite grains and fine-grained quartz-feldspar aggregates continued to propagate and coalesce during the deformation. These processes led to transition from load-bearing (i.e., coarse plagioclase grains) framework to interconnected weak phase (i.e., biotite grains and fine-grained feldspar aggregates) domination, that further led to the formation of initial strain localization zones (SLZs). With the propagation and linkage of the SLZs, high stress concentrations at the tips of the SLZs led to nucleation of rupture along the SLZs. As a consequence, there occurred an abrupt increase in strain rates that resulted in transition from stable to unstable slip within the SLZs. The processes were accompanied by occurrence of mid-crustal earthquake faulting and formation of pseudotachylites along the SLZs.
View article: https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36303.1/612977/Mechanisms-of-strain-localization-and-nucleation
High-resolution record of multiple organic carbon-isotope excursions in lacustrine deposits of Upper Sinemurian through Pliensbachian (Early Jurassic) from the Sichuan Basin, China
Marco Franceschi; Xin Jin; Zhiqiang Shi; Bin Chen; Nereo Preto ...
Abstract: The Sinemurian-Pliensbachian boundary event (ca. 193 Ma) is recorded as a global perturbation of the carbon cycle, as evidenced by a large negative carbon-isotope excursion recorded in many marine sedimentary successions. Whereas multiple lines of evidence testify that the Sinemurian-Pliensbachian boundary event was associated with environmental and climatic changes, sea-level oscillations, and biotic turnovers in marine settings, the record and effects of the event on continents are poorly known. In this paper, we report a high-resolution δ13C org record and palynological data from the Lower Jurassic lacustrine succession of the Sichuan Basin that allow a prominent 8‰ negative carbon-isotope excursion to be identified at the Sinemurian-Pliensbachian transition. We therefore interpret this perturbation as the expression of the Sinemurian-Pliensbachian boundary event in the Sichuan Basin, and we propose a correlation with the marine realm. Facies evolution illustrates that the Sinemurian-Pliensbachian boundary event coincided with a phase of expansion of the lacustrine systems. Palynological analyses indicate a sharp shift from arid to humid climate conditions coincident with the carbon-isotope perturbation that supports a scenario of lake expansion driven by increased rainfall. In contrast to observations in the Sichuan Basin, where deep lake conditions persisted across the Sinemurian-Pliensbachian boundary event, a global drop in the sea level is documented at the onset of the isotope perturbation. This suggests that eustatic oscillations due to increased continental water storage in lakes and aquifers in the context of a wetter climate phase may have been associated with the early stages of the Sinemurian-Pliensbachian boundary event.
View article: https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36235.1/612978/High-resolution-record-of-multiple-organic-carbon
New constraints on the timing and character of the Laramide Orogeny and associated gold mineralization in SE California, USA
Tarryn K. Cawood; Amy Moser; Ariel Borsook; Alan D. Rooney
Abstract: The timing of deformation and associated gold mineralization in SE California, USA, is contentious, partly due to the challenges involved with dating ductile deformation. We therefore combine modern geo- and thermochronology with field and microscopic observations to show that the Cargo Muchacho Mountains preserve evidence of northward thrusting in a kilometer-scale ductile shear zone during the Late Cretaceous Laramide Orogeny, accompanied by hydrothermal fluid flow, gold mineralization, and pegmatite emplacement. Penetrative strain was largely accommodated within the Jurassic metavolcaniclastic Tumco Formation, whereas intrusive Jurassic granitoids behaved as competent bodies. Quartz microstructures suggest deformation at ∼500 °C, which is consistent with fabrics defined by amphibolite facies minerals. The timing of thrusting is constrained by dynamically recrystallized titanite with a U-Pb age of 68 ± 1 Ma and late syn-kinematic pegmatites that yield U-Pb zircon ages of 65.0 ± 4.2−63.2 ± 4.8 Ma. Syn-kinematic fluid flow was focused into a lateral thrust ramp where the shear zone foliation was deflected around a relatively rigid pluton, creating zones rich in magnetite-quartz veins and epidote, and precipitating gold associated with pyrite and chalcopyrite. Dating of these sulfides via Re-Os yields an age of 64.7 ± 0.8 Ma, which confirms a Laramide age for the gold mineralization. Together, apatite from the pegmatites and a nearby Jurassic granite yields a U-Pb age of 60.4 ± 3.5 Ma, reflecting cooling to below 530−450 °C. Comparison with published studies suggests that thick-skinned deformation in the Cargo Muchacho Mountains was driven by flat-slab subduction of the conjugate Hess Plateau, which occurred several million years after and to the south of flat-slab subduction of the conjugate Shatsky Rise. This suggests that the conjugate Hess Plateau may have been subducted up to several hundred kilometers farther north than previously thought. Metamorphic devolatilization of underplated Orocopia Schist likely generated the gold-bearing hydrothermal fluids, and anatexis of the schist formed the peraluminous pegmatites, which highlights the importance of schist underplating and devolatilization along much of the Californian and Mexican cordillera.
View article: https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36251.1/612979/New-constraints-on-the-timing-and-character-of-the
U-Pb dating of oil charge in superimposed basins: A case study from the Tarim Basin, NW China
Jiaxu Chen; Xiaowen Guo; Ze Tao; Zicheng Cao; Bin Wang ...
Abstract: Direct dating of oil charge in superimposed basins is essential to understanding the evolutionary histories of petroleum systems, especially in sedimentary basins with complicated tectonic evolution and thermal histories. Based on analyses of different phases of calcite veins and primary oil inclusions, episodes of oil charge were determined by laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) in situ U-Pb dating of calcite veins from an Ordovician reservoir within the Tahe Oilfield of the Tarim Basin, NW China. This basin has been subjected to multiple uplifts and erosions and repeated oil charges. The U-Pb dating results indicate that the first phase of oil charge occurred from 329.7 ± 1.6 Ma to 308.1 ± 4.1 Ma, and the second phase occurred from 249.3 ± 2.6 Ma to 220.5 ± 7.3 Ma. The timing of oil charge determined by fluid inclusion analysis alone can lead to great uncertainties due to the existence of multiple phases of oil charge and complex thermal evolution in superimposed basins. Our study demonstrates that U-Pb dating of calcite veins originating from the reservoirs offers a unique solution to determining the oil charge history, which avoids the multi-solution uncertainties in the timing of oil charge inferred from fluid inclusion analysis in superimposed basins.
View article: https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36324.1/612933/U-Pb-dating-of-oil-charge-in-superimposed-basins-A
Ridge subduction and episodes of crustal growth in accretionary belts: Evidence from late Paleozoic felsic igneous rocks in the southeastern Central Asian Orogenic Belt, Inner Mongolia, China
Jialiang Li; Jingao Liu; Di-Cheng Zhu; Bruce K. Nelson; Ruohan Gao
Abstract: The Central Asian Orogenic Belt (CAOB) is one of the largest accretionary orogens on Earth and preserves evidence that more than 50% of this orogen represents juvenile crustal growth over an extended period of some 750 million years from ca. 1000 Ma to ca. 250 Ma. However, the mechanism of crustal growth is controversial, as implied by a variety of proposed models ranging from contributions of mantle-derived basaltic underplating in a post-collisional extensional setting to subduction-related processes in an island arc setting. To distinguish among these models, we report petrological, geochemical, whole-rock Sr-Nd isotope and zircon U-Pb and Hf isotope analyses of late Paleozoic felsic igneous rocks from the northern Inner Mongolia region, southeastern CAOB. New zircon U-Pb analyses of three plutonic and extrusive magmatic suites yield Late Carboniferous to Early Permian ages of 319−279 Ma. The Xi Ujimqin granodiorites have low-K tholeiitic to calc-alkaline, metaluminous to weakly peraluminous compositions, and are magnesian I-type granitoids. These granitoids are also characterized by relatively high MgO and Sr contents, high Mg # values, mid-ocean ridge basalt (MORB)-like Nd-Hf isotope compositions and young Nd-Hf model ages of 600−298 Ma. These features indicate that the parental magmas of the Xi Ujimqin I-type granitoids originated from a depleted lithospheric mantle that had been metasomatized by fluids released from a subducting slab. By contrast, the Xilinhot alkali-feldspar granites and Dashizhai rhyolites display geochemical signatures of ferroan granites. These, together with their moderately depleted Nd-Hf isotope compositions and young Nd-Hf model ages of 809−277 Ma, suggest that they were produced by re-melting of juvenile lower crust via underplating of mantle-derived magmas. The coexistence of an association of boninite-adakite-high-Mg andesite-Nb-enriched basalt, and MORB to ocean island basalt-type mafic rocks in the northern Inner Mongolia region implies a dramatic change in composition from fluid-related calc-alkaline arc magmatism to melt-related and mantle-derived magmatism initiated by upwelling asthenosphere. We infer that ridge subduction may have occurred in this region during Late Carboniferous to Early Permian. Combined with previous studies, there are two ridge subduction events during the early and late Paleozoic in Inner Mongolia within the southeastern CAOB, which coincide with two large-volume magmatic flare-ups (300 ± 20 and 450 ± 20 Ma). Furthermore, our model calculations suggest that most of the juvenile crust in the southeastern CAOB was generated in a relatively short interval (∼20−40 m.y.) during each long-lived tectonic cycle (>140 m.y. spanning oceanic rifting, subduction, and collision) of magmatic activity. Ridge subduction may have played a significant role in the rhythmic growth of juvenile crust in the CAOB during the Phanerozoic. Repeated cycles of punctuated, rapid growth of juvenile crust associated with long-lived subduction systems represent a pattern that may be generalized to other Phanerozoic accretionary belts on Earth.
View article: https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35986.1/612907/Ridge-subduction-and-episodes-of-crustal-growth-in
3-D seismic chronostratigraphy of reefs and drifts in the Browse Basin, NW Australia
Sebastian Thronberens; Stefan Back; Julien Bourget; Tony Allan; Lars Reuning
Abstract: In its subsurface, the Browse Basin on the Australian Northwest Shelf accommodates one of the largest Miocene−Pliocene carbonate platform provinces on Earth. This study presents a comprehensive three-dimensional (3-D) documentation and investigation of the Neogene Browse Basin carbonates based on the analysis of 15 3-D seismic reflection volumes covering ca. 33,000 km2 and 22 industry boreholes. Eight basin-wide seismic marker horizons tied to Sr isotope and biostratigraphic data provide a robust 3-D chronostratigraphic framework that (1) newly documents the occurrence of Miocene to recent carbonate systems in vast inboard basin areas; (2) presents the 3-D distribution and dynamic development of carbonate platforms through time constrained by absolute ages; and (3) supports the detailed 3-D interpretation of Miocene to recent depositional processes and key stratigraphic controls. Around 18.6 Ma, the extent of all Browse Basin carbonate platforms was ca. 8600 km2, rising to >10,000 km2 between 15.6 Ma and 12 Ma. By ca. 9.7 Ma, the total platform extent had decreased to ca. 5700 km2, waning to <2800 km2 in the Pliocene. The observed reef demise is less abrupt than previously thought and comprises two steps, with the first in the late Miocene on the outer shelf and the second in the Pliocene on the middle and inner shelf. The Miocene outer-shelf platform demise coincided with (1) strong subsidence that outpaced sedimentation, (2) an increase in NW−SE-oriented tidal current activity, (3) the development of NNE−SSW-oriented bottom currents, and (4) the onset of drift sedimentation. The Pliocene demise of middle-shelf reefs coincided with (1) an increase of clastic sediment input from land and (2) stalling of the Indonesian Throughflow around 3.7 Ma. The basin-wide, 3-D seismic chronostratigraphic analysis presented supports a re-evaluation of the key controls for Miocene and Pliocene reef growth and decay on the NW Shelf and highlights the complexity of interacting global, regional, and local processes and peculiarities in carbonate platform development.
View article: https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36286.1/612885/3-D-seismic-chronostratigraphy-of-reefs-and-drifts
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