Joseph Martinod

Mantle dynamics, uplift of the Tibetan Plateau, and the Indian Monsoon

Convective removal of lower lithosphere beneath the Tibetan Plateau can account for a rapid increase in the mean elevation of the Tibetan Plateau of 1000 m or more in a few million years. Such uplift seems to be required by abrupt tectonic and environmental changes in Asia and the Indian Ocean in late Cenozoic time. The composition of basaltic volcanism in northern Tibet, which apparently began at about 13 Ma, implies melting of lithosphere, not asthenosphere. The most plausible mechanism for rapid heat transfer to the midlithosphere is by convective removal of deeper lithosphere and its replacement by hotter asthenosphere. The initiation of normal faulting in Tibet at about 8 (± 3) Ma suggests that the plateau underwent an appreciable increase in elevation at that time. An increase due solely to the isostatic response to crustal thickening caused by Indias penetration into Eurasia should have been slow and could not have triggered normal faulting. Another process, such as removal of relatively cold, dense lower lithosphere, must have caused a supplemental uplift of the surface. Folding and faulting of the Indo-Australian plate south of India, the most prominent oceanic intraplate deformation on Earth, began between about 7.5 and 8 Ma and indicates an increased north-south compressional stress within the Indo-Australian plate. A Tibetan uplift of only 1000 m, if the result of removal of lower lithosphere, should have increased the compressional stress that the plateau applies to India and that resists Indias northward movement, from an amount too small to fold oceanic lithosphere, to one sufficient to do so. The climate of the equatorial Indian Ocean and southern Asia changed at about 6–9 Ma: monsoonal winds apparently strengthened, northern Pakistan became more arid, but weathering of rock in the eastern Himalaya apparently increased. Because of its high altitude and lateral extent, the Tibetan Plateau provides a heat source at midlatitudes that should oppose classical (symmetric) Hadley circulation between the equator and temperate latitudes and that should help to drive an essentially opposite circulation characteristic of summer monsoons. For the simple case of axisymmetric heating (no dependence on longitude) of an atmosphere without dissipation, theoretical analyses by Hou, Lindzen, and Plumb show that an axisymmetric heat source displaced from the equator can drive a much stronger meridianal (monsoonlike) circulation than such a source centered on the equator, but only if heating exceeds a threshold whose level increases with the latitude of the heat source. Because heating of the atmosphere over Tibet should increase monotonically with elevation of the plateau, a modest uplift (1000–2500 m) of Tibet, already of substantial extent and height, might have been sufficient to exceed a threshold necessary for a strong monsoon. The virtual simultaneity of these phenomena suggests that uplift was rapid: approximately 1000 m to 2500 m in a few million years. Moreover, nearly simultaneously with the late Miocene strengthening of the monsoon, the calcite compensation depth in the oceans dropped, plants using the relatively efficient C4 pathway for photosynthesis evolved rapidly, and atmospheric CO2 seems to have decreased, suggesting causal relationships and positive feedbacks among these phenomena. Both a supplemental uplift of the Himalaya, the southern edge of Tibet, and a strengthened monsoon may have accelerated erosion and weathering of silicate rock in the Himalaya that, in turn, enhanced extraction of CO2 from the atmosphere. Thus these correlations offer some support for links between plateau uplift, a downdrawing of CO2 from the atmosphere, and global climate change, as proposed by Raymo, Ruddiman, and Froehlich. Mantle dynamics beneath mountain belts not only may profoundly affect tectonic processes near and far from the belts, but might also play an important role in altering regional and global climates.

Flat subduction dynamics and deformation of the South American plate: Insights from analog modeling

Received 14 June 2007; revised 13 January 2008; accepted 12 March 2008; published 21 June 2008. [1] We present lithospheric-scale analog models, investigating how the absolute plates’ motion and subduction of buoyant oceanic plateaus can affect both the kinematics and the geometry of subduction, possibly resulting in the appearance of flat slab segments, and how it changes the overriding plate tectonic regime. Experiments suggest that flat subductions only occur if a large amount of a buoyant slab segment is forced into subduction by kinematic boundary conditions, part of the buoyant plateau being incorporated in the steep part of the slab to balance the negative buoyancy of the dense oceanic slab. Slab flattening is a long-term process (� 10 Ma), which requires the subduction of hundreds of kilometers of buoyant plateau. The overriding plate shortening rate increases if the oceanic plateau is large enough to decrease the slab pull effect. Slab flattening increases the interplate friction force and results in migration of the shortening zone within the interior of the overriding plate. The increase of the overriding plate topography close to the trench results from (1) the buoyancy of the plate subducting at trench and (2) the overriding plate shortening. Experiments are compared to the South American active margin, where two major horizontal slab segments had formed since the Pliocene. Along the South American subduction zone, flat slab segments below Peru and central Chile/NW Argentina appeared at � 7 Ma following the beginning of buoyant slab segments’ subduction. In northern Ecuador and northern Chile, the process of slab flattening resulting from the Carnegie and Iquique ridges’ subductions, respectively, seems to be active but not completed. The formation of flat slab segments below South America from the Pliocene may explain the deceleration of the Nazca plate trenchward velocity. Citation: Espurt, N., F. Funiciello, J. Martinod, B. Guillaume, V. Regard, C. Faccenna, and S. Brusset (2008), Flat subduction dynamics and deformation of the South American plate: Insights

Periodic instabilities during compression or extension of the lithosphere 1. Deformation modes from an analytical perturbation method

We argue that the plastic rheology of the lithosphere, rather than its “recoverable” elastic properties, is responsible for the systematic development of periodic instabilities during compression or extension. We use a linear perturbation model with analytical solutions to calculate the instability modes for various rheologies. The growth of such periodic instabilities is enhanced by the highly nonlinear stress-strain rheologies encountered in the brittle layers of the lithosphere. On the contrary, ductile layers, deforming according to high temperature creep flow laws, tend to inhibit these instabilities. For oceanic domains, we assume that the only brittle layer is the upper part of the lithosphere. In compression, the only valid instability is a buckling whose wavelength is around 4 times the thickness of the brittle layer. Calculated wavelengths and growth rates are consistent with observations available for the Indian Ocean. For continental domains, a reasonable assumption is the existence of two plastic layers, one in the upper crust, the other in the upper mantle, for Moho temperatures between 450°C and 600°C. In compression and extension, two instabilities develop a long wavelength instability involving the whole lithosphere (coupling mode) and a short wavelength instability involving the crust and controlled by the upper brittle layer (intrinsic crustal mode). In compression, the coupling mode is a whole-lithosphere buckling, with a wavelength about 4 times the thickness of the active lithosphere (the two plastic layers plus the intermediate ductile layer). In extension, the coupling mode is a boudinage of opposite phase in the two plastic layers and a folding of the intermediate ductile layer. The intrinsic crustal mode is a crustal buckling in compression, crustal boudinage in extension. Neither deflects the Moho. The intrinsic crustal mode is favored by an increase in thermal gradient and by a decrease in strength of the ductile lower crust.

Horizontal subduction zones, convergence velocity and the building of the Andes.

We discuss the relationships between Andean shortening, plate velocities at the trench, and slab geometry beneath South America. Although some correlation exists between the convergence velocity and the westward motion of South America on the one hand, and the shortening of the continental plate on the other hand, plate kinematics neither gives a satisfactory explanation to the Andean segmentation in general, nor explains the development of the Bolivian orocline in Paleogene times. We discuss the Cenozoic history of horizontal slab segments below South America, arguing that they result from the subduction of oceanic plateaus whose effect is to switch the buoyancy of the young subducting plate to positive. We argue that the existence of horizontal slab segments, below the Central Andes during Eocene-Oligocene times, and below Peru and North-Central Chile since Pliocene, resulted (1) in the shortening of the continental plate interiors at a large distance from the trench, (2) in stronger interplate coupling and ultimately, (3) in a decrease of the trenchward velocity of the oceanic plate. Present-day horizontal slab segments may thus explain the diminution of the convergence velocity between the Nazca and South American plates since Late Miocene.

Dynamical effects of subducting ridges: insights from 3-D laboratory models

SUMMARY We model using analogue experiments the subduction of buoyant ridges and plateaus to study their effect on slab dynamics. Experiments show that simple local (1-D) isostatic considerations are not appropriate to predict slab behaviour during the subduction of a buoyant ridge perpendicular to the trench, because the rigidity of the plate forces the ridge to subduct with the dense oceanic lithosphere. Oceanic ridges parallel to the trench have a stronger effect on the process of subduction because they simultaneously affect a longer trench segment. Large buoyant slab segments sink more slowly into the asthenosphere, and their subduction result in a diminution of the velocity of subduction of the plate. We observe a steeping of the slab below those buoyant anomalies, resulting in smaller radius of curvature of the slab that augments the energy dissipated in folding the plate and further diminishes the velocity of subduction. When the 3-D geometry of a buoyant plateau is modelled, the dip of the slab above the plateau decreases, as a result of the larger velocity of subduction of the dense ‘normal’ oceanic plate on both sides of the plateau. Such a perturbation of the dip of the slab maintains long time after the plateau has been entirely incorporated into the subduction zone. We compare experiments with the present-day subduction zone below South America. Experiments suggest that a modest ridge perpendicular to the trench such as the present-day Juan Fernandez ridge is not buoyant enough to modify the slab geometry. Already subducted buoyant anomalies within the oceanic plate, in contrast, may be responsible for some aspects of the present-day geometry of the Nazca slab at depth.

Shortening of analogue models of the continental lithosphere: New hypothesis for the formation of the Tibetan plateau

Initial stages of compression produce periodic buckling of models analogous to a four-layer continental lithosphere. With further shortening, amplification of the buckles occurs by thrust faulting at inflection points in both brittle layers of the upper crust and upper mantle. The rescaled results suggest that a continental lithosphere under compression will buckle with a first-order wavelength of about 200 km and a second-order wavelength of 20–30 km. Continued shortening results in the amplification of undulations. Troughs sink and become compressional basins while deformation keeps going on major thrusts within the upper mantle, thus giving rise to an irregular topography of the Moho. Fifty percent shortening results in a 60-km-thick continental crust. We argue that this model history may explain the elevation and deep crustal structure of the Tibetan plateau.

Periodic instabilities during compression of the lithosphere: 2. Analogue experiments

We have modeled the behavior of the continental and oceanic lithospheres under compression, using materials with analogous properties in laboratory experiments, to study the development of lithospheric buckling. Periodic instabilities, which are a major deformation process during the compression of the lithosphere, have already been described by several authors using an analytical perturbation method. At small strains, analogue experiments corroborate most of the results obtained by the perturbation method: (1) the deformation modes (geometrical relationships of interfaces and related wavelengths) are mainly dependent on the spatial distribution of the brittle layer(s), and (2) the amplitude of buckling is an exponential function of the horizontal strain. Some departure from the perturbation method occurs when there are two instabilities growing concurrently. The breakdown of the exponential growth occurs for strains of about 5%, and is concomitant with the appearance of thrust faults. In experiments including one brittle layer, which model the compression of the oceanic lithosphere, faults are regularly located at the inflection points of the folds. In experiments including two brittle layers, which model continental lithosphere, faults form more complicated patterns with an asymmetrical deep thrust overlain by a fan-shaped symmetrical thrust system in the upper brittle layer. Such fault geometries give some new highlights on typical compressive geological structures such as those encountered in Central Asia.

A geomorphological approach to determining the Neogene to Recent tectonic deformation in the Coastal Cordillera of northern Chile (Atacama)

The large (≈10000 km2) and local-scale ( 300 m) sedimentary succession was deposited east of the AFS. The succession fills previously deep paleovalleys. And it consists of gravel, so-called “Atacama Gravels”, which passes laterally into fine-grained playa related deposits near the AFS. We interpret the deposition of this succession as a result of a blocking closure of the valley flowing from the Precordillera due to the activity on AFS. A pedimentation episode followed sediment deposition and is locally strongly re-incised by the main modern-day river valleys draining the Precordillera. Incision may result from either regional uplift of the forearc, and/or from more localized activity on the AFS. Furthermore, Recent (Quaternary?) tectonic activity on the AFS has been observed which is consistent with a localized relative uplift of the crustal block west of the AFS.

An analog experiment for the Aegean to describe the contribution of gravitational potential energy

The southern Aegean seafloor exhibits clear evidence of internal deformation (stretching) as shown by tectonics, seismology and space geodesy. We use an analog three-layer laboratory experiment of sand, silicone putty and honey to investigate the deformation of the southern Aegean lithosphere. The model is installed in a box and confined by a vertical wall. We open a gate in the wall and observe the deformation of the two upper layers due to buoyancy forces. The general pattern of the deformation of the southern Aegean is found in the analog model. We observe the formation of an arc spreading outward with time, the extension is radial in the inner part, but parallel to the arc in the external part and of comparable importance. At both ends of the gate we observe strike-slip motion (dextral in the western part, sinistral in the eastern part). Rotation (clockwise in the western part, counterclockwise in the eastern part) of up to 40° is seen on both sides of the gate but is also present, with a smaller amplitude, far in the internal region, partially due to distributed shear. The spreading is associated with the thinning of the two upper layers and affects a region of dimensions comparable to the length of the free boundary. This spreading does not propagate inward with time. Some pieces of material located near the active boundary remain undeformed during the experiment.

Slope and climate variability control of erosion in the Andes of central Chile

Climate and topography control millennial-scale mountain erosion, but their relative impacts remain matters of debate. Confl icting results may be explained by the infl uence of the erosion threshold and daily variability of runoff on long-term erosion. However, there is a lack of data documenting these erosion factors. Here we report suspended-load measurements, derived decennial erosion rates, and 10 Be-derived millennial erosion rates along an exceptional climatic gradient in the Andes of central Chile. Both erosion rates (decennial and millenial) follow the same latitudinal trend, and peak where the climate is temperate (mean runoff ~500 mm yr ‐1 ). Both decennial and millennial erosion rates increase nonlinearly with slope toward a threshold of ~0.55 m/m. The comparison of these erosion rates shows that the contribution of rare and strong erosive events to millennial erosion increases from 0% in the humid zone to more than 90% in the arid zone. Our data confi rm the primary role of slope as erosion control even under contrasting climates and support the view that the infl uence of runoff variability on millennial erosion rates increases with aridity.