Russell N. Pysklywec

Mantle lithosphere delamination driving plateau uplift and synconvergent extension in eastern Anatolia

Eastern Anatolia is the site of lithospheric thinning, plateau uplift, heating, and synconvergent extension. Using numerical geodynamic experiments, we test the hypothesis that these tectonic anomalies are all related and the consequence of delamination of the mantle lithosphere. Our findings indicate that delamination during plate convergence results in ~2-km-high plateau uplift. The removal of mantle lithosphere induces distinct regions of contraction and thickening, as well as extension and thinning of the crust. The latter occurs even within a regime of plate shortening, although it is muted with increasing plate convergence. Detachment of the delaminating slab results in minor surface topographic perturbation, but only above the delamination hinge. The plateau uplift and pattern of surface contraction and/or extension are consistent with a topographic profile at 42°E and geologically interpreted zone of synconvergent extension at eastern Anatolia.

Near‐surface diagnostics of dripping or delaminating lithosphere

[1] In various geological regions, it has been postulated that the mantle lithosphere has been thinned or completely removed. Two of the primary removal mechanisms that have been put forward include: (1) delamination, a wholesale peeling away of a coherent block of the mantle lithosphere, and (2) lithospheric ‘‘dripping,’’ viscous Rayleigh-Taylor instability of the mantle lithosphere. Using computational models, we investigate several near-surface observables to determine if these may be diagnostic of either (often ambiguous) removal mechanism. Surface topography associated with delamination has a broad region of uplift above the lithospheric gap and a localized and mobile zone of subsidence at the delaminating hinge. With dripping lithosphere, the topographic expression is symmetric and fixed above the downwelling. Delamination of mantle lithosphere is more efficient than dripping for thermal heating of the crust; the onset is more rapid and the elevated temperatures persist longer. The resultant crustal P-T-t paths show modest pressure variations and high temperature increases with large-scale delamination or dripping. Delamination also causes contraction directly above the (migrating) hinge and distal extension. Dripping lithosphere induces superimposed contraction and extension above and symmetric about the viscous instability. For all the observables, if only a portion of the mantle lithosphere is removed by viscous instability (delamination inherently removes all of the mantle lithosphere), the differences between the two removal mechanisms are even more pronounced. With only partial removal of the mantle lithosphere, uppermost mantle lithosphere remains well coupled to the crust, leading to lower surface temperature variations and broad zones of crustal deformation/thickening.

Mantle flow, dynamic topography, and rift-flank uplift of Arabia

Rift-flank uplift adjacent to the Red Sea is asymmetric, i.e., a broad tilt of the entire Arabian plate along an axis parallel to the rift and more localized uplift on the African shoulder. A suite of models has been proposed to explain this pattern, but no model has considered the dynamic effects of large-scale mantle flow. Recent high-resolution images from seismic tomography show a massive, anomalously slow shear velocity structure that emerges from the core-mantle boundary beneath South Africa and that reaches close to the surface at the Red Sea. This buoyant megaplume has been identified as the driving mechanism for anomalously high topography in southern Africa and rifting in East Africa; in this paper we investigate its role in present-day African-Arabian topography. In particular, we present predictions of dynamic topography based on viscous-flow simulations initiated using seismically inferred mantle heterogeneity. These predictions demonstrate that viscous stresses associated with mantle flow are responsible for the long-wavelength signal in African-Arabian flank uplift. Our results do not preclude localized topographic contributions from other processes, particularly within the near field of the Red Sea.

The Role of Subduction‐Induced Subsidence in the Evolution of the Karoo Basin

The stratigraphic record indicates that during the Late Carboniferous to Early Triassic, South Africa experienced an episode of large‐scale subsidence that resulted in the development of the Karoo Basin. The southern margin of the region was contemporaneously dominated by an episode of compressional tectonics, and there is evidence for subduction of the paleo‐Pacific plate beneath the Karoo Basin. We propose that the long‐wavelength component of the basin subsidence resulted from the deflection of the lithosphere due to mantle flow coupled to the adjacent subduction. This was followed by a recovery phase during which the entire region was uplifted on cessation of subduction (and subsequent loss of dynamic support for subsidence), contributing to the present‐day high‐elevation topography of South Africa. Numerical models of mantle convection are presented and show that a shallow‐dipping slab is able to reconcile the observed long‐wavelength subsidence of the basin. A significant component of the near‐field sedimentation is also explained by the model; however, matching the total stratigraphic record in this region requires the tandem effects of subduction‐induced deflection and flexural thrust loading of the lithosphere.

Intraplate tectonics: feedback between radioactive thermal weakening and crustal deformation driven by mantle lithosphere instabilities

Abstract Two-dimensional viscous–plastic numerical experiments show that weak radioactive crust may be susceptible to significant tectonic activity by the effect of Rayleigh–Taylor-type instability of the sub-crustal lithosphere below a continental interior. In particular, crust having a localized or regional enrichment of radioactive elements can respond to an underlying mantle lithosphere downwelling by initial subsidence then thickening and uplift owing to the positive feedback among thickening, heating, and weakening. Such models may later undergo orogenic deflation as spontaneous crustal extension/thinning and surface subsidence occur while mantle downwelling continues. Strong homogeneous crust subsides dynamically above mantle downwellings and undergoes essentially no internal deformation. A uniform distribution of radioactive elements through the crust, or a concentration at depth, is much more effective in causing thermal weakening of the crust and subsequent intraplate deformation than with radioactive material concentrated nearer the surface. For wide regions of high heat production in the crust the length scale of the intracrustal tectonic zone is controlled by the width of the mantle downwelling in contrast with narrow heat production regions which control their own length scale. In general, the numerical experiments demonstrate that the presence of radioactive elements may make the crust vulnerable to intraplate tectonic deformation and represent a primary control on the way the strength of the crust evolves during sub-crustal forcing. The results may explain first-order features of intraplate tectonics including the subsidence mechanism for intracontinental basins, orogenic crustal contraction, thickening and surface uplift, and subsequent extension and crustal deflation. The processes have particular implications for regions of high radioactive heat production and juvenile crust which would more likely have a higher concentration of radioactive elements in the lower crust.

Surface topography and internal strain variation in wide hot orogens from three-dimensional analogue and two-dimensional numerical vice models

Abstract The post-accretionary deformation of wide, hot orogens is characterized by pure-shear or transpressional shortening of relatively weak lithosphere (the orogen) between converging stronger blocks (the vice). We report on a series of analogue vice models and compare the resulting three-dimensional strain fields and surface topographies to equivalent two-dimensional numerical experiments. In the analogue models a rheologically stratified (frictional/viscous) weak orogenic lithosphere overlying a viscous asthenosphere is squeezed between converging strong lithospheric blocks. Ductile lower crust and mantle in the weak lithosphere is free to flow laterally, parallel to the orogen. The Argand number describes the model dynamics and strongly controls both the orogenic relief and the degree of lower crustal orogen parallel stretching in the analogue models. Cross sections of numerical and analogue experiments display consistent geometries in which upper crustal deformation is characterized by upright folding compared to apparently decoupled horizontal strains in the lower crust. The relative buoyancy and degree of orogen parallel flow in the lower crust of the analogue models has a dramatic influence on three-dimensional strain fields and the kinematics of upper crustal curvilinear shear zones. The analogue and numerical results demonstrate the importance of three-dimensional effects in determining the structure of natural orogens and compare favourably to field and geophysical observations of large hot orogens in the geological record.

Mantle flow mechanisms for the large-scale subsidence of continental interiors

Evidence in the geological record shows that continental interiors periodically undergo enigmatic episodes of large-scale subsidence. We propose that mantle flow associated with the descent of cold plumes and slabs, and their interaction with the endothermic phase change at 660 km depth, may provide a plausible mechanism for these epeirogenic events. Simulations of mantle convection that incorporate the thermodynamic effects of the phase change and both depth- and temperature-dependent viscosity are used to model the descent of plumes and slabs through the 660 km boundary. We find that the plume-slab flow scenarios are capable of supporting topographic deflections of amplitudes of ∼1 km and horizontal wavelengths of ∼1000 km that persist over time scales of 100–150 m.y.

Surface erosion control on the evolution of the deep lithosphere

Russell N. Pysklywec [ Geology, v. 34, No. 4, pp. 225–228 (April 2006)][1] Figure 1 has been corrected and replaced by the author. The corrected Figure 1 is below. ![Figure][2] [1]: /lookup/doi/10.1130/G21963.1 [2]: pending:yes

Mantle avalanche as a driving force for tectonic reorganization in the southwest Pacific

Abstract The mechanism responsible for the recent, dramatic reorganization of the tectonic plate boundary in the New Hebrides region of the southwest Pacific has remained elusive. We propose that an ongoing avalanche of cold, dense slab material into the lower mantle, imaged by high-resolution seismic tomographic methods, provides the necessary driving force for this enigmatic evolution. Numerical experiments demonstrate that the avalanche model reconciles a broad suite of observational constraints, including the change in polarity of plate subduction, the rapid migration of the New Hebrides arc and opening of the North Fiji Basin, and the present-day geometry of slabs associated with both active and extinct subduction zones.

The surface tectonics of mantle lithosphere delamination following ocean lithosphere subduction: Insights from physical‐scaled analogue experiments

Many postulated lithospheric removal events occur in regions with an earlier history of subduction, but the relationship between the two processes has not been explored. In this work, we use physical-scaled analogue experiments to investigate the evolution from ocean lithosphere subduction to collision and possible delamination of the mantle lithosphere from the crust. We test how varying the magnitude of plate convergence alters the behavior of the subduction-delamination model. Our experiments show that a retreating ocean proplate can evolve to continental mantle lithosphere delamination. Negative surface topography is supported at the delamination hinge, and this migrates back with the peeling lithosphere. With high plate convergence, delamination is suppressed. Rather, the crust and mantle lithosphere split at the collision zone in a form of flake tectonics as oncoming procrust is accreted on top of the retroplate and the promantle lithosphere subducts below. Localized high topography develops at this zone of crustal accretion and thickening. The results suggest that delamination may be a continental continuation of plate retreat and that lithospheric removal is triggered by the transition from one process to another.