Tectonophysics最新文献

The links between tectonics, surface processes and magmatism govern the evolution of rifted and transform margins. Quantifying the control of surface and deep Earth processes, lithosphere rheology and plate kinematics is challenging because of their non-linear interactions. We designed and conducted systematic 3D magmatic-thermo-mechanical numerical experiments coupled with surface processes modelling to better understand the formation of rifted and transform continental margins. Oceanic transform faults are formed by either the opposite polarity of oceanic detachment faults or their formation is linked to the gradual interaction between two propagating rift and spreading centers.

Lower divergence velocities, faster crustal and slower mantle thinning, lower surface processes (i.e. erosion and sedimentation) rates, and lower mantle potential temperature lead to the formation of magma-starved continental margins, mantle exhumation and eventually the formation of a stable transform fault zone with a magma-starved, deep transform valley. Suppressed melting and small-scale mantle instabilities govern the along-ridge variation of magmatic and non-magmatic segments, often leading to V-shaped zero-offset oceanic fracture zones. In contrast, faster divergence, lithospheric mantle inherited weak zones, enhanced erosion and sedimentation, result in enhanced mantle melting, and rift magmatism and the formation of a spreading center in the transform zone. Models simulating the temporal increase of divergence velocities show the evolution from an initial magma-poor to a final magma-rich oceanic basin.

In models without simulating mantle melting, enhanced surface processes lead to delayed break-up linked to a longer continental hyper-extended stage. However, enhanced surface processes and a more localized and accelerated lithospheric mantle thinning can promote earlier mantle melting and the formation of magma-chambers beneath the crust.

The tempo of subduction-related magmatic activity over geological time is episodic. Despite intense study and its importance to crustal growth, the fundamental drivers of this episodicity remains unclear. We demonstrate quantitatively a first order relationship between arc flare-up events and high subduction flux. The volume of oceanic lithosphere entering the mantle is the key parameter that regulates the proportion and rate of H₂O entering the sub-arc. New estimates of subduction zone H₂O flux over the last 150 million-years indicate a three- to five-fold increase in the proportion of H₂O entering the sub-arc during the most recent global pulse of magmatism. Step changes in H₂O flux enable proportionally greater partial melting in the sub-arc mantle leading to a flare-up episode. Similar magmatic flare-ups in the ancient Earth could be related to variability in slab flux associated with supercontinent cycles.

We identify possible sources of seismic anisotropy beneath India by synthesizing 2064 well-constrained shear-wave splitting parameters determined from a consistent analysis of waveforms recorded at 357 broadband seismic stations. Our effort includes compilation of previous results, reanalysis of old data, analysis of new data from previous networks and new stations. Our results reveal that the average delay time for entire India and its constituent tectonic provinces is $\sim$0.83 s suggesting moderate strength of anisotropy. Although the fast polarization azimuths (FPAs) are scattered, a NE trend appears dominant. Due to significant correlation of FPAs with the APM direction and lack of correlation between i) splitting parameters and backazimuths and ii) average delay times and lithospheric thickness, we conclude that the major contribution to anisotropy is from shearing in the upper part of the asthenosphere or a transitional layer from the base of the lithosphere to the upper part of the asthenosphere. Further, we postulate that a weakly anisotropic lithosphere in northern, central and south-eastern India is due to frozen anisotropy from past tectonic events. Northern and central India, Arunachal Himalaya and southern part of Burmese arc have simple anisotropy. Application of the spatial coherency technique reveals a source depth of 290 km for northern India. However, for south-eastern India and northern part of the Burmese arc, a two-layer model, with frozen-in and present-day anisotropy in the upper layer, and shearing and mantle flow in the lower layer, respectively, fits the anisotropy. In southern India, a large deviation of the FPAs from APM suggests imprints of deformation related to past tectonic events. A two-layer model, with frozen-in anisotropy in the upper and lower layers, is plausible. Variation in FPAs in the central part of the Indian shield is attributed to deflection in mantle flow at the northern edge of the lithospheric keel.

We employed shear-wave splitting analysis on both teleseismic SKS and S waves, and S waves from deep (150–250 km) local earthquakes collected from a dense array with 43 temporary broadband seismic stations and nine long-term seismic stations centered at Mount Taranaki to characterize the upper-mantle dynamics in the southwestern North Island of New Zealand, in areas previously unexamined for shear-wave splitting. We observed predominantly trench-parallel fast polarizations and strikingly large delay times over 3 s from teleseismic analysis. In contrast, local S analysis yielded a sharp transition of fast-polarization from trench-parallel in the northeast to trench-normal in the southwest. Trench-parallel fast-polarization from teleseismic analysis may be attributed to sub-slab trench-parallel flow or to trench-parallel fractures in the subducting slab. More importantly, we attribute large delay times to deep upper-mantle (200–400 km depth) deformation, possibly associated with the dynamic interaction between the downgoing slab and the 410-km discontinuity or with the lithosphere delamination near the Taranaki-Ruapehu line. In contrast, the trench-parallel anisotropy from the local S waves in the northeast could be caused by fluid-bearing cracks in the crust of the Taupō Volcanic Zone and/or by trench-parallel fractures in the subducting slab resulting from outer rise bending. The abrupt change to trench-normal may be related to stress variations in the downgoing slab at different depths.

We characterize and quantify the microstructure, hydrogen concentrations, and seismic properties of a tectonically exhumed sliver of oceanic lithospheric mantle outcropping in the Moa Island (Leti archipelago, Timor-Tanimbar outer-arc). The 18 spinel peridotites (lherzolites and harzburgites) have coarse-porphyroclastic microstructures and olivine crystal-preferred orientations (CPO) with axial-[010] (also known as AG-type) or [100](010) (A-type) patterns, similar to those observed in peridotitic xenoliths from oceanic mantle lithosphere. These coarse-porphyroclastic microstructures are variably overprinted by growth of strain-free olivine neoblasts and crystallization of secondary pyroxenes. Recrystallized fractions vary from 6.9 up to 31.3%. The interstitial (cuspate) shapes and CPOs of clinopyroxene, uncorrelated with the olivine CPOs, indicate that refertilization by a reactive melt percolation post-dated deformation. Seismic properties are calculated based on the modal compositions and CPOs of all samples. Increase in the recrystallized olivine fraction decreased the seismic anisotropy, since static recrystallization produced some dispersion of the CPO, but did not change drastically the texture acquired during deformation. Mean seismic velocities (mean Vp = 7.9 km.s⁻¹; mean Vs = 4.5 km.s⁻¹) and anisotropy (mean maximum S wave polarization anisotropy = 4.5%), estimated by considering coherent orientation of the foliation and lineation of all samples, are within the range of typical values for the uppermost mantle. The nominally anhydrous minerals contain small amounts of hydrogen (olivine: 13–18 ppm H₂O by weight; orthopyroxene: 58–175 wt ppm H₂O and clinopyroxenes: 244–288 wt ppm H₂O). A bulk water content of 50 wt ppm H₂O is estimated based on nomminally anhydrous minerals for the Moa peridotites, in agreement with previous estimates for the oceanic mantle lithosphere based on peridotitic xenoliths. This is the first direct measurement of hydrogen concentrations in peridotites from an oceanic mantle lithosphere which experienced melt extraction.

We studied fault core geometry and mechanical properties of exhumed basement rocks at two localities (Storskora and Lislaskora) in Sotra Island, western Norway. We combine outcrop studies with in-situ measurements of the rock stiffness (Young's modulus) to characterize the faults. Faults were investigated both along and across strike using multiple 1D scanlines on the outcrop. Our results show that both fault core thickness and stiffness values vary along the faults. Thicker fault cores (up to 0.8 m and 1.9 m in Storskora and Lislaskora loaclities, respectively) show higher values of stiffness (Young's modulus) up to 70 GPa. The stiffness values of the fault core are generally higher than those measured on the damage zone of the faults in this area. The presence of epidote and compacted fault gouge in the fault core can cause the increase in estimated fault core stiffness. In contrast, fractures are dominant in the damage zones causing local reductions in the stiffness. A map of recent seismic events in this area shows potential seismic activities along some of the major exposed faults in the Sotra Island (e.g. Rustefjorden Fault). Based on the evidence from outcrop, inferred displacements, and interpretation of an available reflection seismic section, we found that the exposed faults could be secondary and part of the damage zone of the Øygarden Fault Complex in the east margin of the rift system in the North Sea. The results of this study could be utilized to predict the architecture and changes in rock stiffness of basement-involved faults in the subsurface.

The Kosi River flows from the eastern Nepal Himalaya into the state of Bihar (India) and has experienced frequent avulsions, causing extensive flood-related damage. Because of this avulsive behavior, the Kosi is called the “Sorrow of Bihar.” The avulsion of 2008 was the most catastrophic avulsion event recorded for the Kosi and has been attributed primarily to hydrological and sedimentological processes that formed a super-elevated river channel and caused avulsion. Detailed topographic analysis of the region near the Kosi exit from the Himalaya, using mean-corrected and resampled 1-arc, Shuttle Radar Topography Mission (SRTM) and Real Time Kinematic Global Positioning System (RTK-GPS) datasets, reveals that the Kosi channel is super-elevated only relative to its eastern floodplain. The western floodplain elevation is similar to or higher than the Kosi channel in the region between the Kosi River exit from the main eastern Nepal Himalaya and the Kosi barrage at the Indo-Nepal border. Structurally, the Kosi exits the Himalaya in the transition zone between the closed Trijuga dun to the west and the Dharan salient to the east. The Trijuga dun is closed by the Main Frontal thrust (MFT)-related frontal topography or the Outer Churia Hills. The eastern slopes of these hills induce west-to-east topographic slope in the channel, such that topographic avulsion indices are highest only in the Outer Churia Hills affected parts of the Kosi Channel and the 2008 avulsion region. Therefore, our preferred model for the primary control on the channel's asymmetric, metastable, super-elevation is the influence of the tectonically controlled MFT-related Outer Churia Hills on the Kosi River channel. Geomorphological processes have operated in the Kosi channel in this backdrop. This study emphasizes that detailed structural and topographic analysis of river exits from mountain belts like the Himalaya can provide better insights into river channel metastability and avulsion worldwide.

Simple mechanical arguments suggest that slip along interlocked, rough faults, damages surrounding rocks. The same arguments require that the scale of secondary damage is proportional to the size of geometric irregularities along the main fault. This relationship could apply at all scales, but has, so far, been difficult to observe at the 10s to 100 s of km scales of large, natural faults, often because large-scale deformation is distributed across wide, complex plate-boundary fault systems, like the San Andreas Fault. The geometry and geology of another large-scale plate-boundary strike slip fault—the Queen Charlotte Fault (QCF)—is, in contrast, especially simple. Here, we show that observations of secondary deformation are well-aligned with predictions of stress variations caused by geometric irregularities along the QCF, suggesting a geometric relationship between primary fault geometry and secondary deformation. The analytic stress solution reveals that the highest stresses and highest likelihood of failure are confined to a zone of influence (ZOI) with a width quantified by $\mathrm{ZOI}=\lambda /2\pi$, where λ is the wavelength of geometric variations along the main fault. This simple model is consistent with ∼100-km-scale observations along the QCF and can theoretically be used to predict the width of secondary deformation at all scales.

The most abundant serpentine mineral in subduction settings, antigorite has one of the highest water storage capacities and is involved in seismicity. Seismic wave velocities of antigorite are important for detecting and quantifying serpentinization within the mantle wedge and the subducting oceanic plate. At present, the elastic properties of antigorite at high pressures and temperatures are unclear. In this study, we have investigated pressure-volume-temperature (P-V-T) data and thermodynamic properties of antigorite using first-principles molecular dynamics (FPMD) simulations. Using these simulations results, we computed the relevant thermoelastic parameters and estimated compressional and shear wave velocities ($v_P$ and $v_S$) of antigorite in subduction conditions. A simplified velocity model of antigorite with its coexisting mantle anhydrous phases was introduced to help us understand the potential effect of serpentinization on the seismic velocity of mantle rocks. Combined with seismic observations, we re-evaluated some velocity anomalies within forearc mantle wedges and established reliable serpentinization budgets. These results can provide preliminary evaluations and reliable constraints on serpentinization and water content in mantle rocks, which has important implications for understanding global plate dynamics and the deep water cycle.