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; the setting and process evolution of the central Newfoundland gold district; observations from Kīlauea Iki lava lake, Hawaii; . You can find these articles at https://bulletin.geoscienceworld.org/content/early/recent .
Impacts of late Miocene normal faulting on Yarlung Tsangpo River evolution, southeastern Tibet
Tianyi Shen; Guocan Wang; Peter van der Beek; Matthias Bernet; Yue Chen ...
Abstract: The Jiacha Gorge in southeastern Tibet is the second-largest deeply incised gorge of the Yarlung-Tsangpo River, after the Tsangpo Gorge. A late Cenozoic N-S−trending normal fault, the Woka Rift fault, coincides with the western limit of the gorge. However, the relationship between the formation of the gorge, drainage evolution, and rift activity remains unclear. Analysis of the river long profile suggests that the Jiacha Gorge developed as a ∼45-km-wide knickzone, rather than a local knickpoint. Projection of tributary stream profiles indicate significantly deeper incision in the gorge than in the downstream area, suggesting different controls on incision. Thermochronological data collected along two age-elevation profiles in the Woka Rift footwall and the Jiacha Gorge record rapid cooling at ca. 12−10 Ma followed by moderate cooling between ca. 10 and 7 Ma, with additional accelerated cooling after ca. 5 Ma in the gorge. We interpret late Miocene (ca. 12−10 Ma) rapid cooling to reflect the onset of east-west extension and normal faulting along the Woka Rift, whereas the Pliocene (post−ca. 5 Ma) accelerated cooling is inferred to be driven by river incision in the Jiacha Gorge. Geomorphic and sedimentologic observations suggest diversion of the Yarlung-Tsangpo River through the Jiacha Gorge, from an earlier more southerly course, after the onset of rifting. Therefore, we suggest that normal faulting of the Woka Rift anchored the Jiacha Gorge knickzone on its shoulder, while footwall uplift and drainage diversion led to enhanced incision, forming the steepest channel upstream of the Tangpo Gorge along the Yarlung-Tsangpo River.
View article: https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36210.1/612828/Impacts-of-late-Miocene-normal-faulting-on-Yarlung
Subduction initiation of the western Paleo-Asian Ocean linked to global tectonic reorganization: Insights from Cambrian island-arc magmatism within the West Junggar, NW China
Yunying Zhang; Min Sun; Jiyuan Yin; Chao Yuan; Zhen Sun ...
Abstract: The subduction initiation associated with the beginning of accretionary orogens has been thought to be related to global plate reorganization. To characterize the initial subduction within the western Central Asian Orogenic Belt, this integrated study focuses on Cambrian tholeiitic to calc-alkaline plutons in the Barleik-Mayile-Saleinuohai area of West Junggar, NW China. Zircon U-Pb results of felsic plutons reveal a wide range (511−488 Ma) of ages with older ages up to 514−511 Ma. The felsic rocks exhibit variable SiO2 (53.0−77.4 wt%) and K2O (0.05−2.24 wt%) contents and can be classified as diorite, granodiorite, trondhjemite, and tonalite. On the basis of their low TiO2 (0.12−0.71 wt%) contents and characteristic trace element trends as well as high zircon εHf(t) (+10.5 to +14.5) and mantle-like zircon δ18O (5.0 ± 0.48‰ to 5.4 ± 0.43‰, two standard deviations) values, we interpret that the Cambrian felsic rocks have diverse origins, involving differentiation of arc basalts and partial melting of subducted oceanic crust, arc mafic crust, and metasomatized mantle wedge. The Saleinuohai gabbroic pluton shows zircon δ18O ratios from 4.2 to 4.7‰, which are lower than those of igneous zircons in equilibrium with mantle and thus reflect modification of their mantle source by hydrothermal fluids with seawater-like oxygen isotopes at high temperature. Combined with regional data, we propose that the West Junggar arc represents the extending of the Boshchekul-Chingiz arc in the Early Cambrian, defining a long (>1000 km) E-W−trending subduction zone. The earliest island-arc tholeiitic felsic plutons in the West Junggar took place at ca. 514−511 Ma, which, coupled with other early subduction records (e.g., 530 Ma SSZ-type Kopu-relisay ophiolites) in the western Paleo-Asian Ocean, indicates that initial stages of subduction of the western Paleo-Asian Ocean probably occurred in the Early Cambrian. The simultaneity between the initial subduction of the western Paleo-Asian Ocean, Gondwana assembly, and Laurasia breakup suggests a causal link between the three, collectively correlated to a global plate adjustment event.
View article: https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36304.1/612799/Subduction-initiation-of-the-western-Paleo-Asian
A composite Llandovery δ13Ccarb record from the Michigan Basin, USA
Mohammed Al-Musawi; Stephen E. Kaczmarek; William B. Harrison, III; Peter J. Voice; Jeffrey J. Kuglitsch ...
Abstract: A new high-resolution, composite δ13Ccarb curve for the Michigan Basin, USA, was constructed using two stratigraphically overlapping subsurface cores that span the complete Llandovery interval. The Llandovery succession of the Michigan Basin measures between 140 m and 265 m thick and is represented by three lithostratigraphic groups: the Cataract (Manitoulin formation and Cabot Head Shale), the Burnt Bluff (Lime Island, Byron, and Hendricks formations), and the Manistique (Schoolcraft and Cordell formations). The composite δ13Ccarb curve exhibits five positive carbon isotope excursions (CIEs). Conodont and 87Sr/86Sr data suggest that the CIEs correlate with the Hirnantian, Early Aeronian, Late Aeronian, Valgu, and Ireviken global excursions. The new composite δ13Ccarb curve from the Michigan Basin represents the most stratigraphically complete, single-basin Llandovery record worldwide. The composite δ13Ccarb curve permits the ages of three Llandovery lithostratigraphic groups to be chronostratigraphically determined within ca. 1 Ma resolution. The composite δ13Ccarb curve also implies that the Ordovician−Silurian boundary in the Michigan Basin is located at the top of the Manitoulin formation and not at the base of the Manitoulin formation, which is consistent with recent findings from Manitoulin Island, Ontario. From the regional perspective, the high-resolution composite δ13 Ccarb curve provides a valuable tool for delineating the depositional history of the Michigan Basin during the Llandovery. From a global perspective, the new composite δ13Ccarb curve sheds light on the dynamics of the global carbon cycle, particularly during the Rhuddanian, which is rarely preserved in other carbonate basins worldwide.
View article: https://pubs.geoscienceworld.org/gsa/gsabulletin/article/doi/10.1130/B36317.1/612800/A-composite-Llandovery-13Ccarb-record-from-the
Arc-continent collision during culmination of Proto-Tethyan Ocean closure in the Central Qilian belt, NE Tibetan Plateau
Zhen Yan; Changlei Fu; Jonathan C. Aitchison; Manlan Niu; Solomon Buckman ...
Abstract: Accurate lithostratigraphy framework of the Central Qilian belt is key to understand subduction-collision of the Proto-Tethyan Ocean in the NE Tibetan Plateau. The Dongchagou and the Moshigou formations share similar detrital zircon age populations with youngest age peak at ca. 1.15−1.21 Ga, which is consistent with the granitoid gneiss of the Liujiatai Formation. In combination with rock assemblages, metamorphic facies, and associated oldest granitoid plutons, we revised Precambrian lithostratigraphy of the Central Qilian belt as the >1.2 Ga Huangyuan Group and the ca. 1200−930 Ma Huangzhong Group in this study. The former is the same as the Liujiatai Formation, whereas the later includes the Dongchagou and Moshigou formations. Siliciclastic turbidites of the Qingshipo Formation gradually pass upward into carbonates of the Huashishan Group, we therefore ascribe them to the Huashishan Group. This group unconformably overlies the Precambrian basement of the Central Qilian belt and consists of continental slope to coastal plain deposits with a bi-direction of the S- and N-ward paleocurrent. These sediments contain abundant metamorphic, magmatic, and minor ophiolitic detritus with main age populations of ca. 910 Ma, ca. 487 Ma, and ca. 447 Ma detrital zircons, probably derived from the Central and South Qilian belts. The basement of the Huangzhong Group experienced 440−430 Ma continent-continent collision-related metamorphism and deformation. These results and regional geology demonstrate that the Huashishan Group was formed by arc-continent collision deposition during culmination of the Proto-Tethyan Ocean closure in the Qilian orogenic belt, NE Tibetan Plateau.
View article: https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36328.1/612653/Arc-continent-collision-during-culmination-of
Melt surges, flow differentiation, and remobilization of crystal-rich mushes in response to unloading: Observations from Kīlauea Iki lava lake, Hawaii
Rosalind T. Helz
Abstract: Field and laboratory studies of the 1959 Kīlauea Iki lava lake have provided insight into differentiation processes in mafic magma chambers. This paper explores how partially molten basaltic mushes responded to unloading as a consequence of drilling. Most holes drilled from 1967 to 1979 terminated in a melt-rich internal differentiate with a sharp crust-melt interface. These interfaces were not stable, so the boreholes were backfilled by melt-rich (<5% crystal) ooze. This process, with melt ascent rates of 1.3−4.2 m/s, occurred within minutes of intersecting the bodies, mimicking volcanic eruptions, albeit on a small scale. One borehole (KI79-1), which did not encounter such a discontinuity, was backfilled over a period of 16 days by upward flow of crystal-rich mushes rather than melt-rich ooze. The first interval of ooze recovered had undergone extensive internal differentiation. Its most conspicuous feature was production of melt-rich layers by lateral migration of interstitial melt from the wallrock into the rising crystal-rich mush. In addition, two smaller-scale processes occurred within the rising mush: segregation of melt into discrete blebs within the rising mush column and aggregation of groundmass crystals into crystal-rich clumps formed adjacent to coarser olivine crystals. The upper parts of the ooze are enriched in melt relative to deeper samples, which suggests that the melt blebs rose relative to their olivine-rich matrix. Similar melt blebs and crystal-rich clumps are observed in naturally occurring diapiric bodies within the lava lake. These processes appear to be intrinsic to the upwelling of narrow cylindrical mush bodies whether constrained within a borehole (like the oozes) or unconstrained (as were the diapirs in the lava lake). The most striking behavior observed during repeated reentry of KI79-1 was a sharp change in rheology during the second and third re-entries of the borehole. The shift in behavior observed was that the oozes rose up the borehole, with ascent rates of 1.0−1.7 m/s, which are comparable to the rates of the crystal-poor oozes from melt-rich internal differentiates. These oozes contain more melt than the original core at equivalent depths, presumably because melt moved relative to crystals down the pressure gradient created by the open borehole. Groundmass textures in these inflated mushes show erosion of crystal outlines, especially of grain-to-grain contacts between different phases, so that the tenuous crystalline network observed in the original core samples was replaced by rounded crystals in continuous melt at crystallinities of 55−65 vol%. The transition from stable coherent mush to inflatable mush occurred at 25−28 vol% melt. This behavior appears similar to certain types of reactive transport observed in other studies.
View article: https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36248.1/612593/Melt-surges-flow-differentiation-and
Tectonic burial of sedimentary rocks drives the building of juvenile crust of magmatic arc
Sheng-Kai Qin; Ze-Ming Zhang; Richard M. Palin; Hui-Xia Ding; Xin Dong ...
Abstract: Continental arcs grow primarily by addition of mantle-derived magmas, thus forming juvenile crust, although geophysical evidence, alongside field investigation of exhumed terranes, show that supracrustal rocks are common components of the lower portions of continental arcs. The mechanisms by which metasedimentary rocks are transported to the deep arc crusts and their contributions to the juvenile arc crusts are ambiguous. Here, we conduct a systematic petrological, geochronological, and geochemical study of pelitic migmatites within Late Cretaceous meta-gabbros from the lower crust of the eastern Gangdese arc, southern Tibet. Our results show that the pelitic migmatites were derived from the Late Carboniferous sedimentary rocks of the upper arc crust, have significantly enriched Sr-Nd-Hf-O isotopic compositions, and underwent Late Cretaceous (95−80 Ma) high-pressure granulite-facies metamorphism and partial melting at ∼850 °C and 15 kbar to generate voluminous granitic melts. This indicates that the eastern Gangdese arc underwent significant crustal thickening and chemical differentiation during the final stages of subduction of the Neo-Tethys. We suggest that the metasedimentary rocks were transported into the lower crust of the Gangdese arc by underthrusting of arc crust and accretion of mantle-derived magma, and the deeply buried supracrustal rocks altered the petrological constitution and chemical compositions of juvenile lower arc crust, and assimilated the mantle- and juvenile crust-derived melts. We propose that tectonic burial of sedimentary rocks is a key mechanism driving the building and reworking of juvenile crust of magmatic arcs throughout most of geological time.
View article: https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36271.1/612594/Tectonic-burial-of-sedimentary-rocks-drives-the
Large-scale, flat-lying mafic intrusions in the Baltican crust and their influence on basement deformation during the Caledonian orogeny
Rodolphe Lescoutre; Bjarne Almqvist; Hemin Koyi; Théo Berthet; Peter Hedin ...
Abstract: The Fennoscandian Shield in central Sweden displays a complex structural and compositional architecture that is mainly related to the Proterozoic history of the Baltica paleocontinent. In its western parts, the Precambrian basement is covered by the allochthonous rocks of the Caledonide orogen, and direct information about the underlying crust is restricted to a few unevenly distributed basement windows in western Sweden and Norway. In this study, we use preliminary results from the second borehole of the Collisional Orogeny in the Scandinavian Caledonides project (COSC-2), new gravity data, forward gravity, and magnetic modeling and interpretation of seismic reflection profiles to assess the 3-D architecture of the basement. Our results reveal a wide (∼100 km) and dense network of mainly flat-lying and saucer-shaped dolerites intruding the volcanic and granitic upper crustal rocks of the Transscandinavian Igneous Belt. Similar intrusion geometries related to 1.2 Ga dolerites can be recognized in the Fennoscandian Shield. We discuss that the formation of these sill complexes occurred in a lithologically and structurally heterogeneous crust during transtension, which is in disagreement with the current understanding of sill emplacement that involves crustal shortening, layering, or anisotropy of the host rock. Our seismic interpretation and the structural observations from the COSC-2 drilling show that part of the Caledonian-related basement deformation was localized along the margins of the dolerite sheets. We propose that the dolerite intrusion geometry, akin to a flat-ramp geometry, guided the basement deformation during the Caledonian orogeny.
View article: https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36202.1/612534/Large-scale-flat-lying-mafic-intrusions-in-the
Late Mesozoic intracontinental deformation and magmatism in the Chinese Tianshan and adjacent areas, Central Asia
Fujun Wang; Meng Luo; Zhiyuan He; Rongfeng Ge; Yuanyuan Cao ...
Abstract: The Tianshan Range−Junggar Basin−Kalamaili Range system represents the southwestern Central Asian Orogenic Belt and is a natural laboratory for studying intracontinental deformation processes. Its current topography is a product of the far-field effects of the Cenozoic India-Asia collision. However, the Mesozoic topographic and tectonic evolution of the Tianshan and Kalamaili Ranges and their impacts on the Junggar Basin remain enigmatic due to the scarcity of data. Here, we present a comprehensive synthesis of sedimentological and geochronological data on these ranges and adjacent basins to reconstruct the intracontinental evolution from the Early Jurassic to the Early Cretaceous. Based on field observations and seismic profile analysis, we identified several unconformities within the late Mesozoic strata in the Tianshan Range and the Junggar Basin. Detrital zircon U-Pb dating results for Lower Jurassic to Lower Cretaceous sandstones of the eastern and southern Junggar Basin, with published paleocurrent data, reveal a complex intracontinental topographic evolution. Moreover, tuffaceous gravels and tuff samples yielded weighted mean zircon 206Pb/238U ages of 156.5 ± 3.2 Ma and 156.3 ± 2.2 Ma, respectively, which indicates the presence of contemporary magmatic activity. The deformation and magmatism mentioned above were possibly related to multi-plate convergence in East Asia during the late Mesozoic. This study provides new insights into the late Mesozoic tectonic-magmatic evolution of the Tianshan Range and its adjacent areas.
View article: https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36318.1/612535/Late-Mesozoic-intracontinental-deformation-and
Provenance and metamorphic records of forearc sediments from the Heilongjiang Complex, NE China: Implications for subduction erosion associated with the Mudanjiang Ocean
Jiahao Jing; Hao Yang; Wenchun Ge; Zheng Ji; Yanlong Zhang ...
Abstract: Subduction erosion is widely thought to play a significant role in crustal recycling at modern convergent plate margins. However, identifying subduction erosion in fossil accretionary margins is difficult due to multiple episodes of tectonic superimposition and an absence of direct geophysical evidence. By assessing provenance and metamorphic records in this study, we are the first to document the record of subduction erosion in Late Triassic to Middle Jurassic meta-sedimentary rocks from the Luobei Heilongjiang Complex, NE China. Most detrital zircons from four meta-sedimentary samples have typical core-rim textures. Approximately 90% of the detrital zircon cores yielded Phanerozoic ages with εHf (t) values of +12.8 to −17.6. Geochronological and isotopic comparisons indicate that the meta-sedimentary rocks have a provenance in the eastern Songnen Block and were deposited in a forearc basin. The youngest ages of detrital zircon cores span from 229 to 176 Ma, while the metamorphic ages obtained from detrital zircon rims range from 209 to 185 Ma. Combined with the 186−165 Ma phengite 40Ar-39Ar ages of the meta-sedimentary rocks, this suggests that their protoliths were deposited during the Late Triassic−Middle Jurassic. Mineral inclusions define peak blueschist-facies metamorphism at pressure-temperature conditions of 0.9−1.0 GPa and 359−365 °C; this indicates that forearc sediment from the overriding plate was abraded into the subduction channel and underwent high-pressure metamorphism. Metamorphic ages (209−165 Ma) obtained from phengite and detrital zircon rims suggest that a period of subduction erosion occurred during slab subduction. These results reveal rapid progression from early forearc sedimentation (229−165 Ma) to subduction erosion (209−165 Ma) during westward subduction of the Mudanjiang Ocean.
View article: https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36218.1/612536/Provenance-and-metamorphic-records-of-forearc
Two magma fractionation paths for continental crust growth: Insights from the adakite-like and normal-arc granites in the Ailaoshan fold belt (SW Yunnan, China)
Jian Xu; Xiao-Ping Xia; Qiang Wang; Christopher J. Spencer; Chun-Kit Lai ...
Abstract: Geochemical similarities between the continental crust and arc magmas have led to the inference that subduction zones may be the primary sites of crustal growth. Thus, it is necessary to unravel the petrogenetic mechanism(s) of granitoid generation in subduction-related settings to understand crustal growth through magmatic differentiation processes. In this study, we focused on granitoid generation in oceanic-continental subduction zones. We analyzed the whole-rock geochemistry and Sr-Nd isotopes, together with zircon U-Pb-Hf-O isotopes, of the newly identified Middle Triassic granitoids in the Ailaoshan high-grade metamorphic complex (Yunnan, SW China). All the studied granite samples were characterized by large ion lithophile element (e.g., Rb, Sr, and Ba) enrichments and high field strength element (e.g., Nb, Ta, and Ti) depletions, similar to arc-type rocks. They also showed a range of whole-rock Sr-Nd, (87Sr/86Sr)i = 0.7020−0.7048, εNd (t) = +0.6 to +4.2, and zircon Hf-O, εHf(t) = +10.3 to +18.1, δ 18Ozircon = 5.09‰−6.65‰, isotope compositions, which overlap with those of previously reported coeval (ca. 237−235 Ma) hornblende diorite and granodiorite, the formation of which was interpreted to have originated from a mantle wedge metasomatized by a sediment-derived melt. Furthermore, the fractionation trends of some of the granitic samples and diorite-granodiorite suite overlap. They can be divided into two geochemical groups: Group 1 has intermediate to high SiO2 (66.9−73.8 wt%) and K2O (3.40−5.42 wt%) and low MgO (0.19−1.09 wt%) contents and shows depletion in heavy rare earth elements (HREEs; e.g., Yb and Y), resulting in adakite-like high Sr/Y (61−183) and La/Yb (47−90) ratios. Group 1 shows positive SiO2 versus Sr/Y and La/Yb correlations and negative SiO2 versus HREE and Y correlations, implying fractionation of a garnet-bearing assemblage. The negative correlations between SiO2 and εNd( t) and Nb/La reveal a crustal assimilation trend. Group 2 has relatively high SiO2 (72.6−76.5 wt%) and low K2O (1.93−3.82 wt%) and MgO (0.05−0.83 wt%) contents and shows depletion in middle REEs (MREEs; e.g., Gd and Dy) with low Sr/Y (1−10) and La/Yb (4−11) ratios. Group 2 granites show negative Gd/Yb versus SiO 2 correlation, which indicates significant fractionation of an amphibole-bearing assemblage. Our results suggest that both group 1 and 2 granites were formed in a subduction setting from a common mantle-derived parental dioritic magma, but they experienced two distinct fractionation processes. While group 1 granites were likely formed by crustal assimilation and high-pressure (lower-crustal) garnet-dominated fractionation, group 2 granites were generated through low-pressure (middle-/upper-crustal) amphibole-/plagioclase-dominated fractionation. We suggest that these two fractionation trends are critical to crustal growth and the development of a more fractionated (felsic) upper crust.
View article: https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36230.1/612537/Two-magma-fractionation-paths-for-continental
A shallow-water depositional interpretation for the upper Miocene Chagres Formation (Caribbean coast of Panama)
Elena Stiles; Camilo Montes; Carlos Jaramillo; Murray K. Gingras
Abstract: The upper Miocene Chagres Formation, cropping out along the Caribbean coast of Panama, has been previously interpreted as the record of the last deep-water connection between the Caribbean and Pacific oceans based on bodyfossil paleodepth estimates. We test this interpretation by presenting an integrated ichnological and sedimentological analysis on eight coastal localities of the Chagres Formation (Toro Limestone and Chagres Sandstone members). The Toro Limestone Member accumulated under fully marine conditions in a mixed carbonate and siliciclastic marine shelf above storm wave base. Fluctuating high-moderate energy conditions modulated by a combination of tides and high energy currents likely led to the sparse bioturbation of Toro Limestone sediments, corresponding to an archetypalCruziana ichnofacies assemblage. Bioturbation, dominated by Thalassinoides and Ophiomorpha isp., increases in the overlying Chagres Sandstone Member and corresponds to a proximal expression of the Cruziana ichnofacies. The Chagres Sandstone Member was deposited within the lower shoreface-upper offshore transition zone in a shelf with low sedimentation rates under combined storm, wave, and tidal influence, and includes fossils and sediments of continental origin suggesting proximity to a river mouth. Our results suggest that the Chagres Formation accumulated in shallow waters, contrasting with a previously proposed deep-water accumulation, and are consistent with deposition on a tectonically stable platform of the northern Canal Basin at the apex of the Panama orocline.
View article: https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36291.1/612538/A-shallow-water-depositional-interpretation-for
Episodic massive release of methane during the mid-Cretaceous greenhouse
Biao Chang; Junhua Huang; Thomas J. Algeo; Richard D. Pancost; Xiaoqiao Wan ...
Abstract: Methane-derived carbonates (MDCs) are common along modern and ancient continental margins, and the majority of such formations are associated with seafloor cold seeps. Here, we document petrographic, rare earth element + yttrium (REE+Y), carbonate clumped isotope temperature (T Δ47), and carbon-isotopic evidence from a shale succession in southern Tibet spanning a ∼28 m.y. interval (ca. 113−85 Ma) that coincided with the mid-Cretaceous greenhouse event. At least nine limestone nodule-bearing horizons exhibit seep-associated sedimentary structures, including carbonate fabrics (e.g., micritic crusts, crystal fans, and botryoidal textures) and 13C-depleted isotopic compositions (δ 13Ccarb < ‒32.3‰), which are indicative of methane-derived carbon sources. Along with sedimentary evidence, the patterns of TΔ47−δ13Ccarb−δ18O carb support precipitation of these MDCs over a large temperature range. The REE+Y compositions and europium (Eu) anomalies indicate that the release of methane was associated with hydrothermal fluids. Methane may have been derived from both thermogenic and biogenic sources based on the inorganic carbon isotopic signatures of the carbonate. These nodular carbonate horizons document multiple episodes of seafloor methane release during the mid-Cretaceous and represent exceptionally long-lived, active methane seepage. Massive methane release events may have played a role in generating the greenhouse climate of the mid-Cretaceous.
View article: https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36169.1/612491/Episodic-massive-release-of-methane-during-the-mid
Middle-lower crustal flow in response to the India-Eurasia collision: Structural evidence from the southern Chong Shan belt within the Sundaland block, southeastern Tibetan Plateau
Xiaoyu Chen; Junlai Liu; Yinchuan Qi; Xiaoxi Bao; Chengyang Ling
Abstract: In this contribution, we present new structural, microstructural, fabrics, and geochronological data from the southern Chong Shan complex, one of the metamorphic complexes in the southeastern Tibetan Plateau that were sheared and exhumed during the India-Eurasia convergence. The NW-SE−striking complex is comprised of a central high-grade metamorphic zone (Unit I) flanked by two low-grade metamorphic zones (units II and III) on the northeastern and southwestern sides, respectively. High-grade metamorphic rocks (e.g., amphibolites, sillimanite-mica schists) of up to amphibolite facies, of the Proterozoic Chong Shan group and granitic intrusions of Permo-Triassic to Cenozoic in age in Unit I are characterized by high-temperature deformation. Units II (i.e., the Wuliangshan group) and III (i.e., the Lancang group) on both sides of the high-grade Unit I consist of metamorphic rocks of low greenschist facies (e.g., phyllites) with low-temperature deformation. The high- and low-grade units possess consistent kinematics, i.e., northwestward motion of the core rocks relative to the two limbs, and they are separated by large scale shear discontinuities. Thereby, the high- and low-grade units are kinematically linked but mechanically decoupled. Zircon laser ablation−inductively coupled plasma−mass spectrometry U-Pb dating of syn-shearing granitic dikes reveals that ductile shearing occurred from 29 to 19 Ma. Structural analysis reveals that these units constitute an A-type dome that has long axis parallel to the stretching lineations and fold axes of outcrop-scale A-type folds. It is shown that three stages of deformation contributed to the formation of the southern Chong Shan dome, during which subhorizontal shearing were in connection with regional doming. The events occurred as the consequence of middle to lower flow that led to lateral crustal flow and vertical exhumation of crustal masses. Therefore, the lateral crustal flow was not only limited along the boundary high strain zones of the Sundaland block, but distributed within the southeastern Tibetan Plateau. We would argue that the tectonic extrusion of the Sundaland block occurred through ductile crustal flow of a viscous middle and lower crust in the plate interior combined with concurring channel flow along the block margins.
View article: https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36244.1/612434/Middle-lower-crustal-flow-in-response-to-the-India
Latest Silurian syntectonic sedimentation and magmatism and Early Devonian orogenic gold mineralization, central Newfoundland Appalachians, Canada: Setting, structure, lithogeochemistry, and high-precision U-Pb geochronology
I.W. Honsberger; W. Bleeker; S.L. Kamo; H.A.I. Sandeman; D.T.W. Evans ...
Abstract: The eastern Dunnage Zone of the central Newfoundland Appalachians hosts Paleozoic orogenic gold mineralization along a northeast-trending, crustal-scale fault corridor that extends for more than 200 km. This orogenic gold system is characterized by polyphase, structurally controlled, quartz vein systems that cut Neoproterozoic granitoid rocks and unconformably overlying syntectonic, polymict conglomerate and associated transitional to calc-alkaline bimodal igneous rocks. High-precision chemical abrasion−isotope dilution−thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb geochronology confirms a widespread, latest Silurian magmatic pulse (422−420 Ma) that is attributed to a transient phase of lithospheric extension resulting from asthenospheric and crustal melting related to slab break-off. Syntectonic conglomerate was deposited as a basal unit during extension-related uplift and erosion that lasted until ca. 418 Ma in north-central Newfoundland. Orogenic gold mineralization associated with syntectonic sedimentation and magmatism is hosted within third-order shear vein systems that form offshoots within a triangle zone−like structural corridor between southeast- and northwest-dipping, second-order fault splays and shear zones. The southeast-dipping fault system formed during northwest migration of the Acadian thrust front, whereas the northwest-dipping faults and shear zones are Salinic structures that were reactivated in the Early Devonian. Primary hydrothermal rutile in the orogenic gold-mineralized quartz veins produced ages of ca. 410 Ma. These ages are consistent with quartz vein emplacement and orogenic gold mineralization as a result of hydrothermal fluid-pressure cycling related to far-field compression and thermal perturbations during the Early Devonian Acadian orogenic cycle. The setting and process evolution of the central Newfoundland gold district are remarkably similar to that of world-class orogenic gold systems of the Canadian Shield.
View article: https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36083.1/612305/Latest-Silurian-syntectonic-sedimentation-and
Orbital forcing of late Early Devonian storm events on a mixed carbonate-siliciclastic shelf, Longmenshan area, Sichuan Province, China
Fengjie Li; Zheng’an Chen; James G Ogg; Zuoqiang Li; Xiaokang Ma ...
Abstract: The Yangmaba Formation of latest Early Devonian age in the Longmenshan area of Sichuan Province, China, is a shelfal facies that consists of four types of carbonate-siliciclastic deposits: clay-rich, siliciclastic sand-rich, carbonate-dominated, and hybrid mixed. Storm deposits vary in their thickness, composition, and abundance-per-meter within these mixed carbonate-siliciclastic deposits. Meter-scale statistics of the relative storm frequency (events-per-meter) and magnitude (bed thickness) were compiled in two coeval sections through each of the hybrid facies in the lower Yangmaba Formation, which spans approximately the entire conodont Polygnathus patulus Zone of the uppermost Emsian Stage. Low-pass filtering, spectral analysis, and Acycle software interpretation of these tempestite statistics yield 3.5 main oscillations in each section, with an average wavelength of ∼23 m. These long-wavelength trends are semi-coincident with interpreted long-term variations in sea level, where shallower depths allowed a greater influence by storms. Superimposed on the long-wavelength cycles are medium-wavelength cycles of 5.5−6.3 m. The estimated ∼1.5 m.y. time-span of this conodont zone of the lower Yangmaba Formation and the approximate 1:4 ratios of these wavelengths indicate that frequency and intensity of major storms and the recording of tempestites in the sedimentary record were modulated by ∼100- and 95-k.y.-short-eccentricity orbital-climate oscillations superimposed on a main 405-k.y.-long-eccentricity cycle. These eccentricity climate cycles governed storm intensity and regional sea level on this margin of the tropical Yangtze Platform of South China.
View article: https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36105.1/612261/Orbital-forcing-of-late-Early-Devonian-storm
Mercury enrichments during the Carnian Pluvial Event (Late Triassic) in South China
He Zhao; Stephen E. Grasby; Xiangdong Wang; Lei Zhang; Yongsheng Liu ...
Abstract: The Carnian Pluvial Event (CPE) was an interval marked by global climatic and environmental change, along with biotic turnover, which occurred during the early Late Triassic. Although the causes and consequences of this event remain unclear, one possible scenario is enhanced volcanism injecting greenhouse gases into the atmosphere, perturbing the global carbon cycle, and negatively impacting the global environment. However, there is an underlying challenge in showing a true cause-and-effect relationship between volcanism and the CPE, as both the sedimentary and volcanic records are difficult to date accurately enough to demonstrate temporal correspondence. However, mercury (Hg) can be used to fingerprint catastrophic volcanism in the sedimentary record. We examined two sections that record the CPE at Laishike and Wayao in Guizhou Province, southwest China, which display high Hg contents along with spikes of Hg/total organic content (TOC), Hg/Al, Hg/total sulfur (TS), and Hg/(Mo/Al) during the CPE that indicate a shift to excess Hg loading. These Hg anomalies are correlative with the global negative excursion in δ13C carb values at the CPE, which suggests that increased volcanism injected both massive amounts of Hg and isotopically light carbon into the atmosphere, and these were ultimately recorded in marine sediments. This interpretation is supported by slightly negative or near-zero Δ 199Hg values that are consistent with a volcanic Hg source. Our study supports the hypothesis that enhanced volcanism played a major role in the evolution of biota and the environment during the CPE.
View article: https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36205.1/612262/Mercury-enrichments-during-the-Carnian-Pluvial
Timing of mass redeposition of sponge spicules from the peri-Tethyan shelf into the deep Carpathian Basin and their relation to mid-Cretaceous global sea level changes
Marta Bąk; Krzysztof Bąk; Zbigniew Górny
Abstract: Extensive Albian−Cenomanian sponge growth on the European peri-Tethyan shelves depended on sea level fluctuations that caused significant shoreline shifts and forced facies migration across shelves and partial redeposition of sponge spicules by gravitational currents into deep basins. Such phenomena played important roles in accumulation in the Carpathian Basins, a part of the western Tethys Ocean along the southern slopes of the European platform. Spiculitic deposits formed thick bodies in the mid-Cretaceous flysch of the Silesian nappe. This study reports new, detailed biostratigraphic and chemostratigraphic (δ13C org) data from a reference section in the Outer Carpathians that were used to date this paleoceanographic phenomenon. A high-resolution analysis of a continuous, 46.5-m-long sedimentary deep-water succession exposed in the Silesian nappe reveals that biogenic input of numerous sponge spicules occurred from latest Albian through middle Cenomanian time, controlled by third-order sea level oscillations. Chemostratigraphic data with biostratigraphic control mark carbon isotope excursions that are characteristic of the mid-Cretaceous (Albian−Cenomanian boundary interval containing oceanic anoxic event (OAE)1d, mid-Cenomanian event (MCE) Ia, MCE Ib, and MCE II) and allow precise determination of the beginning and termination of mass sponge spicule redeposition. The onset of redeposition corresponds to the top of OAE1d, which records a global regressive event (KAl8). The mass redeposition of biogenic material ended between MCE Ib and MCE II during the KCe3 eustatic event. The stratigraphic data indicate that mass redeposition of spiculitic deposits lasted ca. 4.5 Ma, with an average sedimentation rate of ∼5 mm kyr−1. Precisely determining the duration of mass redeposition of spicule-bearing material into the Silesian Basin enables their correlation with deposits in epicontinental basins.
View article: https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36178.1/612257/Timing-of-mass-redeposition-of-sponge-spicules
A Mississippi Valley−type Zn-Pb mineralizing system in South China constrained by in situ U-Pb dating of carbonates and barite and in situ S-Sr-Pb isotopes
Suo-Fei Xiong; Shao-Yong Jiang; Zuo-Hao Chen; Jian-Xin Zhao; Ying Ma ...
Abstract: The ages of Zn-Pb deposits are exceptionally challenging to determine owing to the lack of suitable mineral chronometers and techniques. Here we present the first result for in situ LA-ICP-MS U-Pb dating of carbonates and barite from a Mississippi Valley−type (MVT) Zn-Pb deposit in South China. Hydrothermal dolomite in close textural and paragenetic association with Zn-Pb sulfides, and calcite and barite cement from the breccia ores, yield ages of 473.4 ± 2.7 Ma and 368.7 ± 3.1 Ma, respectively. Together with new in situ S-Pb-Sr isotope values, these data reveal an epigenetic Zn-Pb mineralization history, agreeing well with a model involving basinal brine accumulation and MVT Zn-Pb sulfide precipitation. Because carbonate is a common mineral in Zn-Pb deposits worldwide, and other minerals in such deposits suitable for isotope dating are generally absent, in situ U-Pb dating of gangue carbonates opens a new window for better defining the ore genesis of this globally important Zn-Pb deposit type and for tracking hydrothermal fluid flow in sedimentary basins.
View article: https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36289.1/612258/A-Mississippi-Valley-type-Zn-Pb-mineralizing
Kinematics and 40Ar/39Ar geochronology of the Lincang-Inthanon tectonic belt: Implication for Cenozoic tectonic extrusion of SE Asia
Yang Wang; Yuejun Wang; Peizhen Zhang; Jinjiang Zhang; Bo Zhang ...
Abstract: The Lincang-Inthanon tectonic belt is a major tectonic boundary within the southeastern Tibetan Plateau and Indo-China Peninsula, which are typical examples of tectonic extrusion in SE Asia. The Lincang strain zone, Lancang-Gengma fault, and Inthanon metamorphic complex make up this nearly N−S-striking tectonic belt, which separates the Baoshan−Shan Thai and Simao-Indochina terranes. New petrographic, structural, and mica 40Ar/39Ar geochronological studies were conducted to reveal their deformation styles and constrain the timing of their tectonic evolution. W−E-directed compression related to the subduction of the Paleotethyan Ocean with subsequent continental collision and sinistral ductile shearing in the early Oligocene are recorded along the Lincang strain zone. The Lancang-Gengma fault zone switched from sinistral shearing to dextral motion in the late Cenozoic and shows a deformation history similar to that of the parallel Red River fault. The Inthanon metamorphic complex may have experienced crustal shortening in the early Cenozoic, followed by sinistral transtension in the early Miocene. The Lincang-Inthanon tectonic belt shows many lithological, tectonic evolutionary, and metamorphic similarities with the Gaoligong, Chongshan, and Ailaoshan−Red River shear zones. Therefore, the sinistral shearing along the Lincang-Inthanon tectonic belt and the Chongshan shear zone in the north, which may have initiated since the early Oligocene, played an important role in adjusting differential extrusion and rotation of the Baoshan−Shan Thai and Simao-Indochina terranes. Our results delineate the regional tectonic framework and provide insights into the characteristics and geodynamics of intracontinental deformation in the eastern India-Eurasia oblique convergence zone.
View article: https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36187.1/612259/Kinematics-and-40Ar-39Ar-geochronology-of-the
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