Journal of Geodynamics最新文献

Amphibole peridotites and neighboring hornblendites are often found in subduction zones. To understand the effect of amphibole-rich rocks on seismic anisotropy in subduction zones, we studied the lattice-preferred orientation (LPO) of olivine and amphibole in amphibole peridotites and neighboring hornblendites in Gapyeong, South Korea. The major minerals of amphibole peridotites were olivine (31–51% in volume), amphibole (28–47%), and orthopyroxene (7–16%). Amphibole in amphibole peridotites showed relatively high SiO₂ and low TiO₂ and Na₂O contents (S-type amphibole), indicating that it was formed under supra-subduction conditions. Amphibole in amphibole peridotites showed the type-I, type-II, and type-IV LPO, whereas amphibole in neighboring hornblendites showed the type-III and type-IV LPO. In the case of olivine, most samples showed a mixture of A- and B-type LPO, and one sample showed a mixture of B- and C-type LPO. Many serpentine inclusions were found in olivines. Fourier-transform infrared (FTIR) analysis of the samples showed that the olivines contained a large amount of water (∼29000–45000 ppm H/Si). We also found many dislocations in olivines. These observations indicate that samples showing a mixture of A- and B-type LPO and a mixture of B- and C-type LPO of olivine were deformed under water-rich conditions by dislocation creep. In amphibole peridotites, the P-wave anisotropy of olivine was relatively low (0.9–4.8%), whereas the P-wave anisotropy of amphibole was high (6.5–17.7%). The maximum S-wave anisotropy of olivine was also relatively low (0.87–2.89%), whereas the maximum S-wave anisotropy of amphibole was high (3.81–15.19%). In hornblendites, the P-wave anisotropy and maximum S-wave anisotropy of amphibole were high (6.9–13.6% and 4.27–10.61%, respectively). The P-wave anisotropy and maximum S-wave anisotropy of the amphibole peridotites were in the range of 2.6–8.4% and 1.73–7.30%, respectively. The seismic velocity and anisotropy pattern of amphibole peridotites were more similar to those of amphibole than those of olivine, indicating that the seismic properties of amphibole peridotites were more strongly affected by amphibole than by olivine. Furthermore, the seismic anisotropy of the mixture of amphibole peridotite and hornblendite in subduction zones was also found to be significantly affected by the amphibole LPO in hornblendite.

The mechanisms of magma ascent, transport and emplacement of the volcanic pile from LIPs are key issues regarding the understanding of the complex construction of volcanic systems and magma transport through the crust. The integrated approach of morphology of the volcanic and subvolcanic bodies, whole rock geochemistry and ASM data provides a robust tool to unravel the flow dynamics of the volcanic bodies and the sequence of the magmatic events. Such approach allows building up a model of construction of the lava pile considering the role played by the plumbing system. This paper investigates the emplacement mechanism of sill and lava flows of the Serra Geral Group (SGG), in the southern part of Mesozoic Paraná-Etendeka Magmatic Province (PEMP) in Brazil. Geologic and geochemical analysis were integrated with anisotropy of magnetic susceptibility (AMS), anisotropy of anhysteretic remanent magnetization (AARM) and rock magnetism experiments on 14 sampling sites in the Morro da Igreja region, top of the lava pile in Santa Catarina. The area is composed of low-Ti basaltic (with predominance of rubbly pahoehoe) and silicic lava flows, both intruded by a tabular sill that reaches a few hundred meters wide. The mafic lava flows are composed of Gramado, Urubici and Esmeralda magma-type basalts and andesite basalts. The silicic lava flows are massive or foliated, classified as Palmas type dacites. The sill has regular columnar joints and is classified as Esmeralda type. Magnetic mineralogy data suggests magnetite or Ti-poor magnetite as the main magnetic mineral for all the igneous rocks, and AARM results show anomalous fabric for the lava flows and normal or intermediate fabric for the sill. Besides the anomalous fabric sites, AMS data provide reliable directional data to infer flow direction for the sill, with initial propagation towards NE, followed by a preferential SE direction of magma flow. For the silicic rocks, the presence of vertical and inclined magnetic foliation suggest a lava dome geometry. The dynamics of the magmatic flow of the sill and lava flows associated with the compositional characteristics of the rocks allows stablishing the stratigraphy of the magmatic events in the area as well as the proposition of an emplacement model for the sequence.

High-pressure-low-temperature (HP-LT) rocks such as eclogites and blueschists preserve valuable information about the history and geodynamics of subduction zones. HP-LT rocks exposed in Iran formed during subduction of Paleo-Tethys and Neo-Tethys oceanic lithosphere beneath the Iran microplate and were subsequently exhumed from the Permian to the Early Eocene. The Shanderman and Anarak complexes are associated with Paleo-Tethys, and the Shahrekord, Hajiabad-Esfandaghe, Sistan, Makran, and Sabzevar complexes are related to Neo-Tethys. The assemblage garnet + omphacite + amphibole (Na-, Na-Ca) + white mica + albite ± zoisite formed at the peak metamorphism stage of the eclogites, and amphibole (Na-, Na-Ca) + albite ± white mica ± zoisite ± epidote ± lawsonite formed in blueschists. Retrograde metamorphism replaced peak assemblages with epidote-amphibolite and greenschist facies mineral assemblage (Ca-amphibole + Na-Ca plagioclase + epidote + chlorite). For the various complexes, geothermobarometry calculations and phase diagram modeling show peak P-T conditions of 23 kbar - 600 °C (Shanderman), 14 kbar - 560 °C (Anarak), 25 kbar - 670 °C (Shahrekord), 17 kbar - 530 °C (Hajiabad-Esfandaghe), 24 kbar - 650 °C (Sistan), 15 kbar - 560 °C (Makran), and 17 kbar - 570 °C (Sabzevar). Their retrograde P-T conditions are ∼6 kbar - 470 °C, 7 kbar - 500 °C, 6 kbar - 530 °C, 10 kbar - 450 °C, 8 kbar - 500 °C, 7 kbar − 500 °C and 6 kbar - 490 °C, respectively. The obtained P-T conditions represent a geothermal gradient of 12–16 °C/km for the Paleo-Tethys HP-LT complexes and 7–11 °C/km for the Neo-Tethys. Field and petrographic studies reveal that subduction metamorphism occurred mostly along clockwise P-T paths, except for the Hajiabad-Esfandaghe, Makarn and a subunit of the Anarak complexes that endured counter-clockwise paths. The differences may show a greater subduction angle of Neo-Tethys oceanic lithosphere than Paleo-Tethys, resulting in a cold geothermal gradient and development of continental back-arc basins during the Mesozoic.

We investigate the presence of the quasi-Love wave (qL) at 51 seismic stations of a temporary seismic network across the western Arabia-Eurasia collision zone. We quantify the intensity of the qL observations from the April 12, 2014 Solomon Islands earthquake by calculating the peak-to-peak amplitude ratios of the qL and Love waves, and compare them with predicted qL intensities from previous shear-wave splitting results. We determine the polarity, timing, and period-dependence of the qL observations within the period range of 50–100 s. Our analysis reveals that the qL observations at stations in the Zagros and Alborz mountain belts exhibit opposite characteristics. In contrast to the Alborz stations, the intensity of qL observations at the Zagros stations exhibits relatively negligible dependence on the period, while their receiver-scatterer distances are considerably period-dependent. We approximately locate the anisotropic gradients that generate the qL waves. Our results suggest that a lithospheric gap is responsible for the shallow and abrupt variation in the belt-parallel trend of fast-axis orientations in the westernmost part of the Zagros. Additionally, the period/depth dependence of the anisotropic gradients along the boundary between the central Zagros and central Iran provides insight into the variation in the downward dip of the Arabian lithosphere. The anisotropic gradient located to the north of the Doruneh fault in eastern Iran indicates its role as a major shear zone and lithospheric boundary. Finally, we observe that the spatial distribution of the anisotropic gradient in northeastern Iran matches the higher strain-rate areas in the Kopet Dagh Mountains, suggesting coupling between the lithospheric mantle and crust in that region.

We investigate the complex flow field around the Vrancea slab in Romania, a steeply sinking seismogenic lithospheric block that experienced lateral tear-off and possible rotation. The Vrancea slab is located beneath the South-East Carpathians and generates frequent seismicity despite its remote location from active collisional boundaries. We analyse core-refracted shear wave (SKS) splitting recorded by permanent broadband seismic stations from the Romanian Seismic Network for periods up to 10 years, and compare our results with seismic tomography of the upper mantle. We identify several stations with large backazimuthal variations of SKS fast axis polarization and delay times both in the slab hinge zone, the back-arc and the circum-slab region, indicating complex mantle deformation patterns. To investigate the effect of a two-level rotated slab we invert SKS waveforms using cross-convolutional misfit combined with a neighbourhood search algorithm to model two layers of anisotropy. In the shallow mantle, anisotropy aligns with the upper slab strike and reorients along the strike of the lower slab at depths below the hinge zone. In the back-arc trench-perpendicular anisotropy switches to trench-parallel, due to the recent retreat and roll-back of the slab. Our results have important implications for understanding SKS interference from subducted slab fragments and provide evidence of the recent retreat, break-off and rotation of two Vrancea slab levels sinking into the upper mantle.

The Pelotas Batholith corresponds to the eastern margin of the Dom Feliciano Belt in southernmost Brazil. It comprises multiple intrusions formed by successive tectonic-magmatic processes during the Brasiliano / Pan-African Cycle. One of these intrusive igneous bodies is the Piquiri Syenite Massif (PSM), recently described as a multi-intrusive body formed by three successive pulses, dated by LA-MC-ICP-MS (U-Pb in zircon), with the oldest pulse located at the margins and the youngest in the centre of the intrusion. Pulse 1 (609.3 ± 1.5 Ma) comprises fine- to medium-grained equigranular syenite to quartz syenite and alkali-feldspar quartz syenite, with colour index M′15–30. Pulse 2 (603.4 ± 3.9 Ma) comprises medium- to coarse-grained equigranular alkali-feldspar syenite and alkali-feldspar quartz syenite, with colour index M′ 5–15 and fragments of Pulse 1 varieties. Pulse 3 (588.8 ± 3.1 to 583.2 ± 1.8)comprises medium- to coarse-grained inequigranular quartz syenites, with colour index M′ 2–10, containing fragments of pulses 1 and 2 varieties. As defined by the AMS data, the PSM magnetic fabric is concordant with the magmatic fabric, parallel to the outer edges of the body, dipping towards the centre. However, the construction and emplacement of the body have been the subject of different interpretations. The main objective of this work is to relate the general morphology of the massif and the behaviour of the pulses within the massif with the processes of formation and ascent of the magmas that have built it. For this, terrestrial geophysical surveys were carried out to obtain gravimetric data, with which the Bouguer Anomaly map was generated. Modelling by gravimetric inversion was carried out in nine profiles using tools from the Oasis Montaj software, which was the basis for constructing a three-dimensional geological model using the Leapfrog Geo software. Associating the geological model with the magmatic and magnetic foliations, including the magnetic lineation, it was possible to determine a change in behaviour between the youngest and the two oldest pulses related to the ascension process. In addition, the body’s general shape, with the greatest depths located in the eastern region, allowed us to relate the rise of magma to a structure in the region.

Global Positioning System (GPS) stations in Lombok, Indonesia showed postseismic displacement following the 2018 Lombok earthquake sequence which occurred on 28 July 2018 (Mw 6.4), 5 August 2018 (Mw 6.9), 9 August 2018 (Mw 5.9), and the 19 August 2018 (Mw 6.3 and Mw 6.9). This research has investigated the contribution of afterslip and viscoelastic relaxation during the one-month period after the mainshock on 19 August 2018. This study utilizes continuous GPS data available from 20 August 2018–19 September 2018. The analysis reveals that the afterslip was distributed at the shallower portion of the fault, complementary to the deeper coseismic slip of the mainshock. Viscoelastic relaxation, involving a gravitational Maxwell viscoelastic response with viscosity of 9.0 × 10¹⁷ Pa s, was estimated. This research also found that the depth of elastic upper crust in this region is 50 km. Observations made during the one-month period after the mainshock revealed that afterslip was responsible for most of the ongoing postseismic deformation. Although the viscoelastic relaxation contributed only insignificantly during the one-month period, this research shows that viscoelastic relaxation is expected to increase with time.

The Northern Calcareous Alps (NCA) (Austria) consist of detached Austroalpine covers, comprising mainly Permian evaporites and Triassic-Jurassic carbonates, topped by unconformable clastic sediments (Gosau Basin, Late Cretaceous-Eocene). The NCA experienced a polyphase deformation during the Eoalpine phase (Jurassic? mid-Cretaceous) followed by post-Eocene transpressive collisional tectonics. In the study area, the oldest regional structure is the Eoalpine NW-ward thrusting of the Dachstein Unit over the Hallstatt Unit. During the Eocene-Oligocene the dextral WNWENE Wolfgangsee fault developed between Osterhorn and Höllengebirge Units. The top-to-N phase followed (Oligocene?-Early/Mid. Miocene), related to the final emplacement of the Tirolic Units on the Bajuvaric and Rheno-Danubian Flysch, which in the Höllengebirge Unit resulted into intense thrusting and folding related to the Schafberg thrust. In the Middle Miocene the development of the Königsee-Lammertal-Traunsee (KLT) sinistral fault led to the major structures in the area. Two stepped strike-slip sectors (SSW-NNE) and (SW-NE/N-S) delimited a restraining bend, where the WNW-ESE Nussensee thrust and the Weissenbach anticline were formed. This deformation history disagrees with some of the available geodynamic models for the area, providing instead fundamental constrains for the Adria paleogeography and the onset of thrusting in the NCA.