GPS data reveal that the Brahmaputra Valley has broken from the Indian Plate and rotates clockwise relative to India about a point a few hundred kilometers west of the Shillong Plateau. The GPS ...velocity vectors define two distinct blocks separated by the Kopili fault upon which 2–3 mm/yr of dextral slip is observed: the Shillong block between longitudes 89 and 93°E rotating clockwise at 1.15°/Myr and the Assam block from 93.5°E to 97°E rotating at ≈1.13°/Myr. These two blocks are more than 120 km wide in a north‐south sense, but they extend locally a similar distance beneath the Himalaya and Tibet. A result of these rotations is that convergence across the Himalaya east of Sikkim decreases in velocity eastward from 18 to ≈12 mm/yr and convergence between the Shillong Plateau and Bangladesh across the Dauki fault increases from 3 mm/yr in the west to >8 mm/yr in the east. This fast convergence rate is inconsistent with inferred geological uplift rates on the plateau (if a 45°N dip is assumed for the Dauki fault) unless clockwise rotation of the Shillong block has increased substantially in the past 4–8 Myr. Such acceleration is consistent with the reported recent slowing in the convergence rate across the Bhutan Himalaya. The current slip potential near Bhutan, based on present‐day convergence rates and assuming no great earthquake since 1713 A.D., is now ~5.4 m, similar to the slip reported from alluvial terraces that offsets across the Main Himalayan Thrust and sufficient to sustain a Mw ≥ 8.0 earthquake in this area.
Key Points
New GPS velocity field in eastern HimalayaShillong Plateau is independent from IndiaStrain accumulated since the last earthquake is sufficient for a M > 8 earthquake
A number of geophysical observations reveal that the underthrusting Indian continental plate beneath the southern Tibetan Plateau is laterally torn and segmented, with contrasting subduction angles ...along strike. However, the mechanism of such kind of slab tearing remains unclear. Here, a series of 3‐D high‐resolution numerical models are conducted, which suggest that the lateral variation (either gradually or abruptly changing) of boundary convergence rate plays a critical role in the tearing of underthrusting continental slab. The abruptly changing convergence with a large lateral contrast could lead to slab tearing all by itself, whereas the gradually changing convergence should be combined with pre‐existing weakness in the underthrusting block. The structures and properties of overriding block play secondary roles. Finally, we propose that Indian slab tearing could be attributed to the time‐dependent and lateral variation of convergence rate during the Indian‐Asian collision and the pre‐existing Indian lithospheric weakness.
Plain Language Summary
Horizontal slab tearing associated with the final slab break‐off occurs in many places of the global subduction‐collision system, the mechanism of which is easier for understanding and is mainly driven by the negative buoyancy of the subducted slab. However, for the underthrusting Indian continental plate beneath the southern Tibetan Plateau, the slab tearing and fragmentation occurs along several vertical planes perpendicular to the collision zone, which is identified by a number of geophysical explorations. It is rather difficult for a coherent subducting slab to tear along these sub‐vertical planes due to the lack of major forces in the along‐strike direction. Thus, the mechanism of Indian slab tearing is widely debated and remains unclear. In this study, many speculations have been tested by conducting a series of 3‐D high‐resolution numerical models. The model results indicate that the Indian slab tearing may be resulted from the time‐dependent and lateral variation of convergence rate between the Indian‐Asian collision and the resulting rotation of Indian continent, as well as the pre‐existing weakness within the Indian plate. Alternatively, the structures and properties of overriding Tibetan plate only play secondary roles.
Key Points
Systematic 3‐D numerical models are conducted to study the mechanism of Indian slab tearing along vertical planes beneath Tibetan Plateau
Lateral variation of boundary convergence rate (either abruptly or gradually changing) is generally required for such slab tearing
Indian slab tearing may be attributed to the gradually changing collision rate along strike and the pre‐existing lithospheric weakness
A strong 50–35 Ma decrease in India‐Asia convergence is generally ascribed to continent‐continent collision. However, a convergence rate increase of similar magnitude occurred between ∼65–50 Ma. An ...earlier increase occurred at ∼90 Ma. Both episodes of accelerated convergence followed upon arrival of a mantle plume below and emplacement of a large igneous province (LIP) on the Indian plate. We here first confirm these convergence rate trends, reassessing the Indo‐Atlantic plate circuits. Then, using two different numerical models, we assess whether plume head arrival and its lateral asthenospheric flow may explain the plate velocity increases and whether decreased plume flux and increasing continent‐plume distance may explain deceleration, even without continental collision. The results show that plume head arrival can indeed lead to absolute Indian plate motion accelerations on the order of several cm/yr, followed by decelerations on timescales similar to the reconstructed fluctuations. The 90 Ma increase could potentially be explained as response to the Morondova mantle plume alone. The 65–50 Ma convergence rate increase, however, is larger than can be explained by plume head spreading alone. We concur with previous hypotheses that plume‐induced weakening of the Indian continental lithosphere‐asthenosphere coupling and an increased slab pull and ridge push efficiency are the most likely explanations for the large convergence rate increase. The post‐50 Ma decrease is best explained by orogeny‐related increased trench resistivity, decreased slab pull due to continental subduction, and possibly restrengthening of lithosphere‐asthenosphere coupling upon plume demise.
Key Points
India‐Asia plate convergence underwent strong acceleration and deceleration
These cycles relate to arrival of mantle plumes below India
The strong deceleration around 50 Ma probably relates to India‐Asia collision
Even if still little known, the most significant nonlinear effect during nonlinear vibrations of continuous systems is the increase of damping with the vibration amplitude. The literature on ...nonlinear vibrations of beams, shells and plates is huge, but almost entirely dedicated to model the nonlinear stiffness and completely neglecting any damping nonlinearity. Experiments presented in this study show a damping increase of six times with the vibration amplitude. Based on this evidence, the nonlinear damanaping of rectangular plates is derived assuming the material to be viscoelastic, and the constitutive relationship to be governed by the standard linear solid model. The material model is then introduced into a geometrically nonlinear plate theory, carefully considering that the retardation time is a function of the vibration mode shape, exactly as its natural frequency. Then, the equations of motion describing the nonlinear vibrations of rectangular plates are derived by Lagrange equations. Numerical results, obtained by continuation and collocation method, are very successfully compared to experimental results on nonlinear vibrations of a rectangular stainless steel plate, validating the proposed approach. Geometric imperfections, in-plane inertia and multi-harmonic vibration response are included in the plate model.
The Sunda Plate is a minor tectonic plate bounded by tectonically active convergent boundaries, below which are subducting: the Philippine Sea Plate to the east and the Indo‐Australian Plate to the ...south and west. It is thus an ideal tectonic setting for investigating the interaction between subduction and asthenospheric flow. To better understand mantle interactions within the two nearly perpendicular subduction zones, we characterize seismic anisotropy by conducting a source‐side sS splitting analysis, which allows us to improve spatial resolution of anisotropic fabrics, in particular underneath the backarc regions, which are poorly constrained by previous studies. In the backarc of the Java‐Banda subduction zone, a gradual fast‐axis rotation from trench normal in the west to trench parallel in the east is clearly observed. We attribute this rotation to the interactions between the 2‐D corner flow in the Java wedge and a squeezed asthenospheric flow by the highly arcuate Banda slab. In the backarc of the Philippine subduction zone, the fast‐axis direction transitions from trench normal in the central south to trench oblique in the north; the trench normal is attributed to the mantle wedge corner flow, whereas the trench oblique is likely deflected by the eastward subduction of the South China Sea Plate. Hence, the mantle flow system beneath the Sunda Plate is composed of various types of flow developed in the mantle wedges. Their interactions play an important role in influencing greatly the regional geodynamics in the upper mantle above the 670‐km discontinuity.
Plain Language Summary
In this study, we constrained the seismic anisotropy patterns (i.e., the variation of seismic velocities with propagating directions) in the upper mantle beneath the backarc regions of Java‐Banda and Philippines located in Southeast Asia by analyzing the fast polarization directions of sS wave. It is generally believed that the mantle seismic anisotropy is mainly caused by the crystallographic preferred orientation of mantle minerals such as olivine as the consequence of ductile deformation induced by mantle flow. Therefore, measurements of seismic anisotropy are probably the best tool available to directly probe the mantle flow patterns in the currently tectonic active regions, especially with the importance in understanding dynamic processes such as transport of melt and volatile in the mantle wedge above the subduction zone. Our results depicted that 2‐D corner flow, which stands for the mass circulation in the wedge‐shaped mantle and is mechanically dragged by the viscously coupled descending slab beneath, is the dominant flow pattern in three studied subduction settings. More importantly, we noted that the 2‐D corner flow interacts with the lateral flow, which is orthogonal to the corner flow direction and induced by the highly arcuate Banda slab, and that the flow in north Philippines appears to be deflected by the eastward subducting South China Sea Plate. Our observations reveal the complex dynamic interactions of various types of flow in these tectonic regions.
Key Points
Systematic investigations of seismic anisotropy in Java‐Banda, and Philippines were conducted by source‐side sS splitting analyses
Two‐dimensional corner flow, squeezed lateral flow by highly arcuate slab, and deflected flow by edge effect are observed
Dynamic interactions within two adjacent subduction zones are suggested
The belt boundary thrust within the Cretaceous–Neogene accretionary complex of the Shimanto Belt, southwestern Japan, extends for more than ~ 1 000 km along the Japanese islands. A common ...understanding of the origin of the thrust is that it is an out of sequence thrust as a result of continuous accretion since the late Cretaceous and there is a kinematic reason for its maintaining a critically tapered wedge. The timing of the accretion gap and thrusting, however, coincides with the collision of the Paleocene–early Eocene Izanagi–Pacific spreading ridges with the trench along the western Pacific margin, which has been recently re‐hypothesized as younger than the previous assumption with respect to the Kula‐Pacific ridge subduction during the late Cretaceous. The ridge subduction hypothesis provides a consistent explanation for the cessation of magmatic activity along the continental margin and the presence of an unconformity in the forearc basin. This is not only the case in southwestern Japan, but also along the more northern Asian margin in Hokkaido, Sakhalin, and Sikhote‐Alin. This Paleocene–early Eocene ridge subduction hypothesis is also consistent with recently acquired tomographic images beneath the Asian continent. The timing of the Izanagi–Pacific ridge subduction along the western Pacific margin allows for a revision of the classic hypothesis of a great reorganization of the Pacific Plate motion between ~ 47 Ma and 42 Ma, illustrated by the bend in the Hawaii–Emperor chain, because of the change in subduction torque balance and the Oligocene–Miocene back arc spreading after the ridge subduction in the western Pacific margin.
白亜系−第三系四万十帯を二分する境界スラストは総延長1000キロメートルを超える。この断層は，後期白亜紀以来継続的に成長した付加体の臨界尖形維持という力学的理由によって形成された順序外スラストであると一般的には考えられている。しかし，付加の中断とスラスト形成のタイミングは西太平洋縁辺での暁新世−前期始新世のイザナギ−太平洋海嶺の海溝との衝突時期に一致する。イザナギ−太平洋海嶺沈み込みは，西南日本弧のみならず，北海道，サハリン，シホテアリンなどのアジア大陸縁辺での火成活動の中断や前弧海盆の不整合，アジア大陸下のトモグラフィーをよく説明できる。イザナギ−太平洋海嶺沈み込みによるトルクバランスの変化はハワイ−天皇海山列の屈曲に現れるような47~42Maの太平洋プレートの広域的な運動の変化に現れ、その後の漸新世−中新世の背弧拡大へ繋がったと言えそうである。
The ridge subduction hypothesis is consistent with the cessation of magmatic activity along the continental margin and the presence of an unconformity in the forearc basin in the western Pacific margin. A Izanagi–Pacific ridge subduction model revises the classic hypothesis of a great reorganization of the Eocene Pacific Plate motion and the Oligocene–Miocene back arc spreading after the ridge subduction in the western Pacific margin.
Abstract
It is widely believed that groups of hot spots in different regions of the world are in relative motion at rates of 10 to 30 mm a
−1
or more. Here we present a new method for analyzing ...geologically current motion between groups of hot spots beneath different plates. In an inversion of 56 globally distributed, equally weighted trends of hot spot tracks, the dispersion is dominated by differences in trend between different plates rather than differences within plates. Nonetheless the rate of hot spot motion perpendicular to the direction of absolute plate motion,
v
perp
, differs significantly from zero for only 3 of 10 plates and then by merely 0.3 to 1.4 mm a
−1
. The global mean upper bound on |
v
perp
| is 3.2 ± 2.7 mm a
−1
. Therefore, hot spots move slowly and can be used to define a global reference frame for plate motions.
Key Points
We propose a new method for estimating the motion of groups of hot spots using relative plate velocities and the trends of hot spot tracks
Motion differs from zero for only 3 of 10 plates; nominal speeds: 1.3–3.9 mm a
−1
, lower bounds: 0.3–1.4 mm a
−1
, and upper bounds 2.3–6.3 mm a
−1
Rates of motion between groups of hot spots thus are much lower than 10 mm a
−1
to 30 mm a
−1
or more as currently assumed by many researchers
PDF
