A large-scale apparent polar wander occurred during the Jurassic interval, which is interpreted as true polar wander (TPW). As the motion is nearly orthogonal to the TPW axis, the North China Block (NCB) experienced the largest latitudinal and environmental changes and provides unique constraints on Jurassic TPW. However, due to the lack and uneven quality of paleomagnetic data, TPW records in North China are controversial. Here, we report a new paleomagnetic pole (80.8°N, 13.0°E, A95 = 7.4°) from the late Jurassic sills and dykes that intrude the Nandaling and Xiahuayuan formations in the NCB. The new pole places the NCB at 36.8° ± 7.4°N at 155 ± 3.4 Ma, using Beijing as the reference site. Combined with the reliable Jurassic poles, our study reveals a large, steady southward shift of 37.3° ± 7.2° for the NCB during the Middle and Late Jurassic, and reflects a component of TPW. The position of ∼155 Ma pole also supports significant TPW prior to ∼160 Ma and agrees with proposals attributing the diachronous 165–155 Ma aridification across the Eastern Asian blocks.
The widespread Cenozoic alkaline magmatism within and around the Tibetan Plateau offers a prime opportunity to probe the nature of the mantle at the depths where basalt magmas originate. The close temporal and spatial relationship between volcanism and regional strike-slip fault systems also helps better understand the geodynamics of outward growth of the plateau in response to the continued India-Asia convergence. We present a comprehensive study of the deeply sourced alkaline basalts formed along the Kunlun strike-slip fault with the aim of understanding their petrogenesis and the composition of mantle sources beneath the northeastern Tibetan Plateau. High Nb/U and Ce/Pb ratios and relatively depleted bulk-rock Sr-Nd-Pb isotope compositions corroborate the mantle origin of these alkaline basalts. Homogeneous and low 87Sr/86Sr of clinopyroxene indicates negligible crustal contamination during magmatic evolution. Low δ26Mg in the alkaline basalts and positive correlations with Hf/Sm and Ti/Ti* indicate that the basalts were derived from mantle that was metasomatized by melts derived from sedimentary carbonates during the Paleo-Tethyan seafloor subduction. Based on 40Ar/39Ar dating results, it appears that the alkaline basaltic magmatism in the northeastern Tibetan Plateau occurred simultaneously with Kunlun strike-slip faulting. These observations suggest that the India-Asia convergence must have reactivated ancient subduction plate boundaries and resulted in strike-slip faulting along these suture zones within and around the Tibetan Plateau. The eruption of low-volume and deeply rooted alkaline basalts may have been controlled by fractures associated with the strike-slip fault systems.
The slip behavior of crustal faults is known to be controlled by the composition of the fault gouge, but the exact mechanisms, especially considering the role of water-rock interactions, are still under investigation. Here, we use a geochemical approach measuring the cation exchange capacity (CEC) of several phyllosilicate minerals and non-clays, using CEC as a proxy for the ability to bind water to the mineral surfaces and/or develop electrostatic forces between particles. Laboratory shearing experiments show that low CEC materials (<3 mEq/100 g) tend to exhibit high friction, low cohesion, and velocity-weakening frictional behavior. Phyllosilicate minerals exhibit CEC values up to 78 mEq/100 g and correspondingly lower friction coefficients, higher cohesion, and a tendency for velocity-strengthening friction. Zeolite behavior is atypical, exhibiting a high CEC value typical of phyllosilicates but the strength and frictional characteristics of a non-clay with low CEC. This suggests that the structure of the mineral is important for non-phyllosilicates. For phyllosilicates, our results can be explained by water bound to mineral surfaces, creating bridges of hydrogen or van der Waals bonds between particles. The enhanced particle bonding for high CEC materials is consistent with high cohesion under zero effective stress conditions, and lowered friction by trapping bound water between the mineral surfaces under normal load. Bound water may explain the tendency for velocity-strengthening friction in high CEC materials by hindering a Dieterich-type time-dependent frictional strengthening mechanism.
Dating young lava flows is essential for understanding volcano's eruption frequency, yet challenging due to methodological limitations of commonly used dating techniques. Ruapehu (Aotearoa New Zealand) produced many lava flows during the Holocene, but constraints on the timing of these eruptions are scarce. Here, we use paleomagnetic dating to deliver new eruption ages of 18 lava flows with uncertainties ranging between 500 and 2,700 years (at the 95% confidence level). Comparison between lava flows' paleomagnetic directions and a local paleosecular variation record indicates that the large lava flow field located on the Whakapapa area was emplaced during at least three distinct eruptive episodes between 10600 and 7400 BP. Two of these episodes closely followed a large collapse event that affected Ruapehu's northern area and generated large volumes of lava between 10600 and 8800 BP, with the third episode producing less voluminous lava flows between 8100 and 7400 BP. Following a smaller collapse of the southeastern sector of the edifice at ca. 5300 BP, several low-volume lava flows were emplaced during at least two distinct eruptive episodes prior to ca. 1000 BP, which supplied the Whangaehu valley with lava. The youngest age inferred from our data represents the youngest eruption age provided for a lava flow outside Ruapehu's summit region. This research provides greater detail to the Holocene effusive chronology at Ruapehu, shedding light on partial cone reconstructions after edifice collapses during the Holocene, and the time relationships between trends observed in its effusive and explosive activity.
Bamboo corals are promising archives of paleoceanographic conditions. Existing calibrations for element-to-calcium ratio (El/Ca) proxies of bamboo corals, however, are not necessarily calibrated to contemporaneous environmental parameters, thus weakening the reliability of the proxies. Here, we aim at calibrating the proxies by comparing El/Ca in the outermost surface of the calcareous skeletons of live-collected bamboo corals from the South China Sea (SCS) with modern environmental records. Statistical analysis suggests that Mg/Ca and Ba/Ca can be expressed as a function of in situ seawater temperature and silicate concentration, respectively, that is, Mg/Ca (mmol/mol) = 2.17 ± 0.51 * T (°C) + 74.90 ± 2.66 and Ba/Ca (μmol/mol) = 0.070 ± 0.020 * Silicate (μmol/kg) + 7.27 ± 2.42. The slope of the Mg/Ca-T equation from this study is slightly different from that in a previous study on bamboo corals, likely due to taxonomic and/or geographic differences of the corals and/or differences in sampling strategy and pre-treatment method. Intra- and inter-coral variations have small effects on Mg/Ca, yielding an uncertainty of 2.04 mmol/mol in Mg/Ca (95% confidence interval), equivalent to 0.94°C in estimated temperature. The slope of the Ba/Ca-silicate equation is the same as that in a previous study, suggesting little effect of geographic difference on Ba/Ca. Intra- and inter-coral variations in Ba/Ca are larger than those in Mg/Ca, possibly reflecting incorporation of multiple Ba-rich particulate phases and/or highly variable nutrient concentrations in the micro-environment near corals. These new calibrations allow reconstructions of paleo-temperature and nutrient concentration in the SCS on decadal and longer timescales.
Upper mantle deformation is mainly controlled by the mechanical behavior of olivine. Crystallographic preferred orientations (CPOs) develop in olivine due to crystal-plastic deformation during mantle flow, where the a-axes of olivine polycrystalline aggregates are aligned with the flow direction. Therefore, the observed CPO in olivine-rich rocks is used as an indicator of the mantle flow direction. Experimental data show that olivine rheology is strongly controlled by the microstructure. While the influence of plastic deformation is in general well characterized, the role of dynamic recrystallization during deformation is not totally understood, limiting our ability to interpret the deformation history of naturally deformed rocks. This contribution presents microdynamic numerical simulations of olivine polycrystalline aggregates with different iron content (i.e., fayalite content) with the aim of exploring the CPO and grain size response to dynamic recrystallization. We use a full-field approach with an explicit simulation of viscoplastic deformation and dynamic recrystallization processes under simple shear boundary conditions up to high strain. The simulations show that the CPOs are similar and practically reach the same maximum regardless of the iron content. CPOs are characterized by a single cluster of a-axis and two-clusters of b-axis, reveling a joint activity of the easy glide [100](010) and the moderate strength [100](010) slip systems. High-strain domains of our models are consistent with experimental results, showing an A-type fabric with double maxima, and where the CPO is aligned with the shear direction. The model provides a deeper understanding of the dynamic recrystallization influence on olivine CPOs resulting from plastic deformation.
A large mantle helium anomaly and separate domains of high heat flow are the predominant manifestations of bimodal magmatic activity in the Milford valley. The mantle helium anomaly (1.9–2.6 R/Ra) covers 270 km2 and is subdivided into two separated domains: a cold shallow groundwater regime and high temperature hydrothermal activity. The zone of anomalous heat flow covers >100 km2 and is also subdivided into two adjacent domains, comprising hydrothermal activity at Roosevelt Hot Springs (RHS) (3–7 W/m2) and conductive heat flow (100–180 mW/m2). While the transfer of heat and mantle helium at RHS are coupled, heat and helium transfer are decoupled in the adjacent cold groundwater regime to the west. Both the mantle helium and geothermal anomalies are attributed to recent mafic-felsic magmatic intrusions of >400 km3, however, the absence of volcanic eruptions <500,000 years indicates magmas stall before rising to shallow crustal level <10 km depth. Deep level magmatism produces a felsic composition melt, which is inferred to be responsible for the widespread and near uniform range of diluted mantle helium values. A thick and impermeable mass of crystalline granitic basement rock at the mid-crustal level divides the ascent of mantle helium into separate flow paths. It may also impede the rise of buoyant magma trapping thermal energy that facilitates partial melting, slow cooling, and development of a thick thermal aureole. Partitioning of convective and conductive thermal regimes and independent flow paths supplying deeply derived helium characterize the development of a large long-lived magma-related geothermal system.
The Caribbean-South America subduction zone is a flat subduction zone, with Laramide-style thick-skinned uplifts occurring in the Merida Andes, Sierra de Perija Range, and Santa Marta Massif. Geodetic measurements and historical seismicity show this region is storing strain energy and is capable of a mega-thrust earthquake (M ≥ 8.0). Previous seismic investigations of the lithosphere and upper mantle in this area are either very large scale, very local, or only peripheral to this area; therefore, details of the Caribbean plate subduction geometry beneath the Maracaibo block remain unclear. In this study, we used a new data set acquired by the Caribbean-Merida Andes seismic experiment (CARMA), which comprised 65 temporary broadband stations and 44 permanent stations from the Colombian and Venezuelan national seismic networks. We jointly inverted ambient noise Rayleigh wave Z/H ratios, phase velocities in the 8–30 s band and ballistic Rayleigh wave phase velocities in 30–80 s band to construct a 3-D S-wave velocity model in the area between 75°–65°W and 5°–12°N. The 3-D model reveals a general increase in crust thickness from the trench to the southeast. An anomalous area is the Lake Maracaibo, which is underlaid by the thinnest crystalline crust in the region. This observation may indicate that the Maracaibo block is experiencing a contortion deformation within the crust. We also identified a high velocity anomaly above the subducting Caribbean slab, likely representing a detached piece of eclogitized Caribbean large igneous province from the base of the Maracaibo block. Additionally, our Vs model clearly indicates a slab tear within the subducted Caribbean slab, approximately beneath the Oca-Ancon Fault.
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