Anne Replumaz

Dynamic ups and downs of the Himalaya

Fast uplift and exhumation of the Himalaya and Tibet and fast subsidence in the foreland basin portray the primary Neogene evolution of the Indian-Eurasian collision zone. We relate these events to the relative northward drift of India over its own slab. Our mantle-flow model derived from seismic tomography shows that dynamic topography over the southward-folded Indian slab explains the modern location of the foreland depocenter. Back in time, our model suggests that the stretched Indian slab detached from the Indian plate during the indentation of the Eurasian plate, and remained stationary underneath the northward-drifting Indian continent. We model the associated southward migration of the dynamic deflection of the topography and show that subsidence has amounted to ∼6000 m in the foreland basin since 15 Ma, while the dynamic surface uplift of the Himalaya amounted to ∼1000 m during the early Miocene. While competing with other processes, transient dynamic topography may thus explain, to a large extent, both the uplift history of the Himalaya and subsidence of its foreland basin, and should not be ignored.

Reconciling subduction dynamics during Tethys closure with large-scale Asian tectonics: Insights from numerical modeling

We use three-dimensional numerical models to investigate the relation between subduction dynamics and large-scale tectonics of continent interiors. The models show how the balance between forces at the plate margins such as subduction, ridge push, and far-field forces, controls the coupled plate margins and interiors evolution. Removal of part of the slab by lithospheric break-off during subduction destabilizes the convergent margin, forcing migration of the subduction zone, whereas in the upper plate large-scale lateral extrusion, rotations, and back-arc stretching ensue. When external forces are modeled, such as ridge push and far-field forces, indentation increases, with large collisional margin advance and thickening in the upper plate. The balance between margin and external forces leads to similar convergent margin evolutions, whereas major differences occur in the upper plate interiors. Here, three strain regimes are found: large-scale extrusion, extrusion and thickening along the collisional margin, and thickening only, when negligible far-field forces, ridge push, and larger far-field forces, respectively, add to the subduction dynamics. The extrusion tectonics develops a strong asymmetry toward the oceanic margin driven by large-scale subduction, with no need of preexisting heterogeneities in the upper plate. Because the slab break-off perturbation is transient, the ensuing plate tectonics is time-dependent. The modeled deformation and its evolution are remarkably similar to the Cenozoic Asian tectonics, explaining large-scale lithospheric faulting and thickening, and coupling of indentation, extrusion and extension along the Asian convergent margin as a result of large-scale subduction process.

Timing and rate of exhumation along the Litang fault system, implication for fault reorganization in Southeast Tibet

The Litang fault system that crosses the Litang Plateau, a low relief surface at high elevation (~4200–4800u2009m above sea level) that is not affected by regional incision, provides the opportunity to study exhumation related to tectonics in the SE Tibetan Plateau independently of regional erosion. Combining apatite and zircon fission track with apatite (U-Th)/He thermochronologic data, we constrain the cooling history of the Litang fault system footwall along two transects. Apatite fission track ages range from 4 to 16u2009Ma, AHe ages from 2 to 6u2009Ma, and one zircon fission track age is ~99u2009Ma. These data imply a tectonic quiet period sustained since at least 100u2009Ma with a slow denudation rate of ~0.03u2009km/Ma, interrupted at 7 to 5u2009Ma by exhumation at a rate between 0.59 and 0.99u2009km/Ma. We relate that faster exhumation to the onset of motion along the left-lateral/normal Litang fault system. That onset is linked to a Lower Miocene important kinematic reorganization between the Xianshuihe and the Red River faults, with the eastward propagation of the Xianshuihe fault along the Xiaojiang fault system and the formation of the Zhongdian fault. Such strike-slip faults allow the sliding to the east of a wide continental block, with the Litang fault system accommodating differential motion between rigid blocks. The regional evolution appears to be guided by the strike-slip faults, with different phases of deformation, which appears more in agreement with an “hidden plate-tectonic” model rather than with a “lower channel flow” model.

Asian collisional subduction: A key process driving formation of the Tibetan Plateau

Using silicone slabs as a model analogue for lithospheric plates subducting into a box of glucose syrup, as an analogue of the mantle, we explore the subduction of continental lithosphere in a context of intercontinental collision. The continental indenter pushed by a piston, reproducing the collision, attached to a dense oceanic plate, subducts to two-thirds of the depth of the mantle box. We show that, surprisingly, the continental plate attached to the back wall of the box subducts, even if not attached to a dense oceanic slab. The engine of this subduction is not the weight of the slab, because the slab is lighter than the mantle, but the motion of the piston, which generates horizontal tectonic forces. These are transmitted to the back wall plate through the indenter and the upper plate at the surface, and by the advancing indenter slab through the mantle at shallow depth. We define this process as collisional subduction occurring in a compressional context. The collisional subduction absorbs between 14% and 20% of the convergence, and represents an unexplored component of collisional mass balance. The transmission of tectonic forces far from the collision front favors the formation of a wide plateau. Our experiments reproduce adequately the amount and geometry of the Asian lithosphere subduction episodes inferred during the collision, leading us to conclude that it reproduces adequately the physics of such process.

Numerical simulation of a class of models that combine several mechanisms of dissipation

We study a class of time evolution models that contain dissipation mechanisms exhibited by geophysical materials during deformation: plasticity, viscous dissipation and fracture. We formally prove that they satisfy a Clausius-Duhem type inequality. We describe a semi-discrete time evolution associated with these models, and report numerical 1D and 2D traction experiments, that illustrate that several dissipation regimes can indeed take place during the deformation. Finally, we report 2D numerical simulation of an experiment by Peltzer and Tapponnier, who studied the indentation of a layer of plasticine as an analogue model for geological materials.

Wet tropical climate in SE Tibet during the Late Eocene

Cenozoic climate cooling at the advent of the Eocene-Oligocene transition (EOT), ~33.7u2009Ma ago, was stamped in the ocean by a series of climatic events albeit the impact of this global climatic transition on terrestrial environments is still fragmentary. Yet archival constraints on Late Eocene atmospheric circulation are scarce in (tropical) monsoonal Asia, and the paucity of terrestrial records hampers a meaningful comparison of the long-term climatic trends between oceanic and continental realms. Here we report new sedimentological data from the Jianchuan basin (SE Tibet) arguing for wetter climatic conditions in monsoonal Asia at ~35.5u2009Ma almost coevally to the aridification recognized northwards in the Xining basin. We show that the occurrence of flash-flood events in semi-arid to sub-humid palustrine-sublacustrine settings preceded the development of coal-bearing deposits in swampy-like environments, thus paving the way to a more humid climate in SE Tibet ahead from the EOT. We suggest that this moisture redistribution possibly reflects more northern and intensified ITCZ-induced tropical rainfall in monsoonal Asia around 35.5u2009Ma, in accordance with recent sea-surface temperature reconstructions from equatorial oceanic records. Our findings thus highlight an important period of climatic upheaval in terrestrial Asian environments ~2–4 millions years prior to the EOT.