Zhong-Hai Li

Subduction-induced mantle flow, finite strain, and seismic anisotropy: Numerical modeling

Surface measurements of shear wave splitting patterns are widely used to infer the mantle circulation around subducting slabs; however, the relation between mantle flow and seismic anisotropy is still elusive. Finite strain is a direct measurement of time-dependent deformation and has been proposed as a proxy for the crystal-preferred orientation (CPO) of mantle minerals. We have conducted a series of numerical models to systematically investigate the mantle flow, finite strain, olivine CPO, and SKS wave splitting in oceanic subduction zones with variable slab width. They demonstrate that the preferred orientations of olivine a axes generally agree with the long (extensional) axes of the finite strain ellipsoid (FSE), even in these very complex mantle flow fields; however, neither the a axis nor the FSE axes necessarily aligns with the instantaneous mantle velocity vector. We identify two domains with distinct deformation mechanisms in the central subplate mantle, where simple shear induced by plate advance dominates at shallow depths and produces trench-normal fast splitting, while pure shear induced by slab rollback dominates the deeper mantle and results in trench-parallel fast splitting. The SKS splitting patterns are thus dependent on the competing effects of these two mechanisms and also on the subduction partition ratio γ = Xp/Xt: trench parallel when γ 1. In addition, different mantle deformation mechanisms and SKS splitting patterns are observed in the mantle wedge and around the slab edges, which may aid in the general interpretation of seismic anisotropy observations in natural subduction zones.

A review on the numerical geodynamic modeling of continental subduction, collision and exhumation

Continental subduction and collision normally follows oceanic subduction, with the remarkable event of formation and exhumation of high-to ultra-high-pressure (HP-UHP) metamorphic rocks. Based on the summary of numerical geodynamic models, six modes of continental convergence have been identified: pure shear thickening, folding and buckling, one-sided steep subduction, flat subduction, two-sided subduction, and subducting slab break-off. In addition, the exhumation of HP-UHP rocks can be formulated into eight modes: thrust fault exhumation, buckling exhumation, material circulation, overpressure model, exhumation of a coherent crustal slice, episodic ductile extrusion, slab break-off induced eduction, and exhumation through fractured overriding lithosphere. During the transition from subduction to exhumation, the weakening and detachment of subducted continental crust are prerequisites. However, the dominant weakening mechanisms and their roles in the subduction channel are poorly constrained. To a first degree approximation, the mechanism of continental subduction and exhumation can be treated as a subduction channel flow model, which incorporates the competing effects of downward Couette (subduction) flow and upward Poiseuille (exhumation) flow in the subduction channel. However, the (de-)hydration effect plays significant roles in the deformation of subduction channel and overriding lithosphere, which thereby result in very different modes from the simple subduction channel flow. Three-dimensionality is another important issue with highlighting the along-strike differential modes of continental subduction, collision and exhumation in the same continental convergence belt.

Material transportation and fluid-melt activity in the subduction channel: Numerical modeling

The subduction channel is defined as a planar to wedge-like area of variable size, internal structure and composition, which forms between the upper and lower plates during slab subduction into the mantle. The materials in the channel may experience complex pressure, temperature, stress and strain evolution, as well as strong fluid and melt activity. A certain amount of these materials may subduct to and later exhume from >100 km depth, forming high to ultra-high pressure rocks on the surface as widely discovered in nature. Rock deformation in the channel is strongly assisted by metamorphic fluids activities, which change composition and mechanical properties of rocks and thus affect their subduction and exhumation histories. In this study, we investigate the detailed structure and dynamics of both oceanic and continental subduction channels, by conducting high-resolution petrological-thermomechanical numerical simulations taking into account fluid and melt activities. The numerical results demonstrate that subduction channels are composed of a tectonic rock melange formed by crustal rocks detached from the subducting slab and the hydrated mantle rocks scratched from the overriding plate. These rocks may either extrude sub-vertically upward through the mantle wedge to the crust of the upper plate, or exhume along the subduction channel to the surface near the suture zone. Based on our numerical results, we first analyze similarities and differences between oceanic and continental subduction channels. We further compare numerical models with and without fluid and melt activity and demonstrate that this activity results in strong weakening and deformation of overriding lithosphere. Finally, we show that fast convergence of orogens subjected to fluid and melt activity leads to strong deformation of the overriding lithosphere and the topography builds up mainly on the overriding plate. In contrast, slow convergence of such orogens leads to very limited deformation of the overriding lithosphere and the mountain building mainly occurs on the subducting plate.

Lithosphere delamination in continental collisional orogens: A systematic numerical study

Lithosphere delamination is believed to have played a major role in mountain building; however, the mechanism and dynamics of delamination remain poorly understood. Using a 2-D high-resolution thermomechanical model, we systematically investigated the conditions for the initiation of lithosphere delamination during orogenesis of continental collision and explored the key factors that control the various modes of delamination. Our results indicate that the negative buoyancy from lithosphere thickening during orogenesis could cause delamination, when the reference density of the lithospheric mantle is not lower than that of the asthenosphere. In these cases, compositional rejuvenation of depleted continental lithosphere by magmatic/metasomatic plume- and/or subduction-induced processes may play crucial roles for subsequent lithosphere delamination. If the reference density of the lithospheric mantle is less than that of the asthenosphere, additional promoting factors, such as lower crust eclogitization, are required for delamination. Our numerical simulations predict three basic modes of lithosphere delamination: pro-plate delamination, retro-plate delamination, and a transitional double-plates (both the pro-plate and retro-plate) delamination. Pro-plate delamination is favored by low convergence rates, high lithospheric density, and relatively strong retro-plate, whereas retro-plate delamination requires a weak retro-plate. The Northern Apennines and Central Northern Tibetan Plateau are possible geological analogs for the pro-plate and retro-plate delamination modes, respectively. Our model also shows significant impact of delamination on the topographic evolution of orogens. Large-scale lithosphere delamination in continental collision zones would lead to wide and flat plateaus, whereas relatively narrow and steep mountain belts are predicted in orogens without major delamination.

Crustal structure beneath Namche Barwa, eastern Himalayan syntaxis: New insights from three-dimensional magnetotelluric imaging

The eastern terminations of the Himalayan orogeny, named Namche Barwa, are considered a vital natural laboratory in the Tibetan plateau for geodynamics due to its distinctive geological and geomorphological characteristics. Magnetotelluric (MT) data measured at 83 sites around the Namche Barwa are imaged by three-dimensional (3D) inversion to better reveal the crustal structure of the eastern Himalaya. The results show a complex and heterogeneous electrical structure beneath the Namche Barwa. The electrical conductors distributed in the middle and lower crust around the Namche Barwa provide additional evidence for the “crustal flow” model if they are considered as some parts of the flow in a relatively large-scale region. The near-surface resistivity model beneath the inner part of Namche Barwa conforms with the locations of hot spring and fluid inclusions, the brittle–ductile transition and the 300 °C–400 °C isotherm from previous hydrothermal studies. Relatively resistive upper crust (>800 Ωm) is underlain by a more conductive middle to lower crust (<80 Ωm). The electrical characteristics of the thermal structure at shallow depth indicate an accumulation of hydrous melting, a localized conductive steep dipping zone for decompression melting consistent with the “tectonic aneurysm” model for explaining the exhumation mechanism of metamorphic rocks at Namche Barwa. The results also imply that both surface processes and local tectonic responses play a vital role in the evolution of Namche Barwa. An alternative hypothesis that the primary sustained heat source accounts for the local thermal–rheological structure beneath Namche Barwa is also discussed.

Effects of crustal eclogitization on plate subduction/collision dynamics: Implications for India-Asia collision

Abstract2D thermo-mechanical models are constructed to investigate the effects of oceanic and continental crustal eclogitization on plate dynamics at three successive stages of oceanic subduction, slab breakoff, and continental subduction. Crustal eclogitization directly increases the average slab density and accordingly the slab pull force, which makes the slab subduct deeply and steeply. Numerical results demonstrate that the duration time from initial continental collision to slab breakoff largely depends on the slab pull force. Specifically, eclogitization of subducted crust can greatly decrease the duration time, but increase the breakoff depth. The detachment of oceanic slab from the pro-continental lithosphere is accompanied with obvious exhumation of the subducted continental crust and a sharp uplift of the collision zone in response to the disappearance of downward drag force and the induced asthenospheric upwelling, especially under the condition of no or incomplete crustal eclogitization. During continental subduction, the slab dip angle is strongly correlated with eclogitization of subducted continental lower crust, which regulates the slab buoyancy nature. Our model results can provide several important implications for the Himalayan-Tibetan collision zone. For example, it is possible that the lateral variations in the degree of eclogitization of the subducted Indian crust might to some extent contribute to the lateral variations of subduction angle along the Himalayan orogenic belt. Moreover, the accumulation of highly radiogenic sediments and upper continental crustal materials at the active margin in combination with the strong shear heating due to continuous continental subduction together cause rising of isotherms in the accretionary wedge, which facilitate the development of crustal partial melting and metamorphism.

Subduction initiation dynamics along a transform fault control trench curvature and ophiolite ages

Understanding how new subduction zones form is essential for complete articulation of plate tectonic theory. Formation of new subduction zones by collapse of oceanic transform faults or fracture zones is suggested on the basis of empirical evidence. This process has heretofore been investigated with two-dimensional (2-D) numerical models, which thus ignore its intrinsic three-dimensional (3-D) geometry, lateral propagation, and dynamics. Here, we investigate a 3-D thermomechanical model, in which old and thick oceanic lithosphere (plate) is separated by a transform fault from a thinner and younger oceanic plate containing a transform-orthogonal spreading ridge. The results suggest that the older plate starts to sink spontaneously at the ridge–transform fault junction, and then subduction initiation laterally propagates along the transform away from the ridge. Two key factors control the 3-D subduction initiation (SI) dynamics in nature: (1) the age of the sinking plate, which controls its negative buoyancy; and (2) the thermal structure of the overriding plate, which reflects its spreading history. Our numerical models not only shed new light on the SI dynamics of Cenozoic subduction zones (e.g., the Izu-Bonin-Mariana zone in the Pacific Ocean), but also have implications for fossil convergent plate margins (e.g., the Bitlis-Zagros suture zone, west of the Strait of Hormuz). In the latter case, systematic variations in ages of supra–subduction zone ophiolites may reflect diachronous SI and its lateral propagation.

Subduction Mode Selection During Slab and Mantle Transition Zone Interaction: Numerical Modeling

Global seismic tomography of the subduction zones shows that the subducting slabs could either stagnate around the 660-km discontinuity, or penetrate into the lower mantle. The stagnating slabs also have various morphologies. These are directly related to the interaction between the subducting slabs and the mantle transition zone (MTZ), the dynamics of which are still debated. Using a 2-D thermo-mechanical model, we systematically investigated the modes of subduction in the mantle transition zone and explored the key constraints of various subduction styles. Four basic subduction modes are obtained in the numerical experiments, including one with slab penetrating through the 660-km discontinuity and three other modes with slab stagnating in the MTZ (i.e. folding, lying and rolling-back). The numerical models indicate that the age of subducting oceanic plate, the thickness of overriding continental lithosphere and the convergence velocity play crucial roles in the dynamics of subducting slab and MTZ interaction. In general, the young subducting slab favors the penetration or folding mode, whereas the old subducting slab tends to result in lying or rolling-back mode, although other parameters can also affect. Our models also show a strong correlation between the subduction mode selection and dip angle of the slab tip when reaching the 660-km phase boundary.

Multi-terrane structure controls the contrasting lithospheric evolution beneath the western and central–eastern Tibetan plateau

The Tibetan plateau is manifested by contrasting along-strike lithospheric structures, but its formation mechanism and the relationship with the heterogeneous multi-terrane configuration is a challenging problem. Here we conduct systematic numerical modeling to explore the roles of width, density, and rheological properties of the multiple terranes in the lithospheric evolution of the Tibetan plateau, which reveals two distinct collision modes. In Mode-I, the lithospheric mantles of both the strong and weak terranes in the Tibetan plate are completely detached, followed by the underthrusting of Indian lithosphere beneath the whole plateau. Alternatively, Mode-II is characterized by full detachment of the weak terranes, but (partial) residue of the strong terranes during collision. These two contrasting modes, broadly consistent with the lithospheric structures of western and central–eastern Tibetan plateau, respectively, are strongly dependent on the along-strike variation of the width of the strong Lhasa–Qiangtang terranes.The Tibetan plateau is manifested by contrasting along-strike lithospheric structures, but the correlation with multi-terrane configuration remains challenging. Here, the authors show the crucial roles of the original geometric shape of accreted terranes in regulating the lithospheric evolution of Tibetan plateau.