Philippe Fullsack

Mechanical Model for the Tectonics of Doubly Vergent Compressional Orogens

A mechanical model of crustal shortening and deformation driven by the relative convergence of rigid, underlying mantle plates explains many features of convergent orogens. Results based on numerical models and supported by sandbox models show that a Coulomb crustal layer subject to basal velocity boundary conditions corresponding to asymmetric detachment and subduction of the underlying mantle passes through three stages of orogenic growth: (1) block uplift bounded by step-up shear zones; (2) development of a low-taper wedge over the underthrusting mantle plate; and (3) development of a low-taperwedge overlying the overthrusting mantle plate and verging in the opposite direction. When modified by isostasy, basal viscous flow, surface erosion and denudation, and sedimentation, the resultant model orogens exhibit a variety of styles with characteristics in common with small, rapidly denuded orogens, large orogens with plateaus and extensional characteristics, and active subduction margins with doubly vergent accretionary wedges and deformed fore-arc basins.

Erosional control of active compressional orogens

Denudation has long been acknowledged as a process that contributes to the unroofing of compressional orogens. It has, however, mainly been considered as a passive process, not one that can dictate or control the tectonic evolution. This view prevails despite the knowledge that the style of deformation is controlled by the interplay of gravitational and tectonic stresses: an interplay that is sensitive to the mass removed by denudation.

Factors controlling the Alpine evolution of the central Pyrenees inferred from a comparison of observations and geodynamical models

Geodynamical numerical modeling has been combined with crustal structural restoration of the central Pyrenees in order to gain insight into fundamental processes that control the evolution of collisional orogens. Models are based on deformation of the crust by stresses transmitted upward from kinematic basal boundary conditions corresponding to the subduction of part of the lithosphere. The influence of inherited crustal heterogeneities, denudation, subcrustal loads, and crustal mechanical properties, consistent with well-constrained crustal partial restored cross sections of the central Pyrenees, is investigated by progressively incorporating them into model experiments. The primary result inferred from the modeling is that the asymmetry of the central Pyrenees double-wedge, seen as strain partitioning and in the morphological evolution, is a consequence of the asymmetric distribution of inherited crustal heterogeneity. The tectonic style of the central Pyrenees is the result of the inversion of the Early Cretaceous extensional fault system, during the early stages of the collision, and the reactivation of Hercynian heterogeneities during the late stages. Most of the upper crustal mass that entered the orogen during the calculated 165 km of convergence was accommodated by an increase of upper crustal cross sectional area or lost by denudation. To explain the upper crustal mass partitioning, as well as the geometry of the foreland basins and the preservation of synorogenic deposits in piggyback basins, a subduction load has to be applied to the models. Lower crust and mantle lithosphere were consumed by the mantle.

The continental collision zone, South Island, New Zealand: Comparison of geodynamical models and observations

The South Island zone of oblique continent-continent convergence occurs along a 400 km-long section of the modern Australia-Pacific plate boundary zone, across which about 50 km of shortening has been accommodated since about 10 Ma. The orogen comprises a central mountain range (Southern Alps) flanked on both sides by what are interpreted to be foreland basins. Two essential features that characterize the orogen are (1) the degree of denudation that accompanied deformation, and (2) a fundamental structural asymmetry. The architectural asymmetry of the orogen can be explained by plane strain, finite element models of continental convergence incorporating mantle subduction. Comparison of model and orogen polarity implies that Pacific plate mantle subducts. The models predict two crustal-scale dipping shear zones that form above the point where the Pacific mantle subducts. The localized one more distant from the incoming plate (retro-step-up shear zone) corresponds to the Alpine fault, whereas its conjugate (pro-step-up shear zone) corresponds to the distributed strain and thrusting along the eastern margin of the mountain belt. Parameters that modify the model boundary conditions (top surface, degree of denudation; basal zone, subduction load, crust-mantle velocity discontinuity, subduction of lower crust, mantle retreat, and distributed decrease in mantle velocity) and the internal strength of the crust (two-layer crust with moderate coupling, temperature distribution, strain weakening) are varied in a series of numerical model calculations that establish the combination of material properties and boundary conditions that lead to different cross sectional architectures of the modeled collision zone. In turn, these are compared with observations about the South Island orogen. The calculations show how the style and extent of deformation across the whole orogen depend on the rheological properties of the crustal layer and on the balance between its internal strength and the combined effects of the boundary and gravitational stresses. North to south along-strike differences in the width and two-dimensional architecture of the orogen, simulated in the experiments by varying the model parameters, can be explained by a combination of southward increases in preconvergent crustal thickness, geothermal gradient, convergence, and potentially subduction retreat, with the added possibility of a southward decrease in the component of lower crustal subduction.

Styles of crustal deformation in compressional orogens caused by subduction of the underlying lithosphere

Abstract Crustal-scale deformation is calculated for models in which the driving mechanism corresponds to the asymmetric detachment and underthrusting of the underlying mantle lithosphere. The plane-strain finite-element model results provide indications of the styles of deformation to be expected in small compressional orogens. In particular these styles occur where shortening of the mantle lithosphere is achieved by the nearly rigid convergence between lithospheric mantles and the subduction of one mantle beneath the other. The crust is modelled using Coulomb plastic (frictional) and thermally-activated power-law viscous rheologies and the effects of compositional layering and variable geothermal gradients are included. Results are presented for a range of models in which the strength of the coupling between the model crust and its basal boundary, surface denudation, partial and total subduction of the crust, and compositional layering are examined. The results show the development of inclined step-up shear zones, which are a consequence of conjugate thrusting in regions of Coulomb-controlled rheology. These zones link to sub-horizontal shear zones which occur where composition and temperature render the crust viscously weak. The model strain fields are interpreted in terms of deformation on discrete planes and the seismic reflectivity fabric that may be associated with this discrete deformation. Finally, we ask whether similar seismic reflectivity fabric can be recognized in observations from small compressional orogens.

Mechanical model for subduction-collision tectonics of Alpine-type compressional orogens

Alpine-type orogens are characterized by three distinct convergent tectonic phases: subduction with deformation that has primarily single vergence, a transition from subduction to collision, and continental collision with double vergence. Although the Cenozoic history of the European Alps has additional complexities, a mechanical explanation for these three phases would provide the necessary crustal-scale framework in which to develop an understanding of the smaller-scale processes. We present results from a simple numerical model, which explain the mechanics of these three phases as a consequence of the changing buoyancy of the lithosphere subducted beneath the orogen. The development and exhumation of a subduction complex, suture zone, and basement nappe stack (Piemont suture, Penninic Nappes); the presence of a crustal-scale back fold and thrust (Insubric Line); and uplift of basement on the pro- (European) side of the orogen (external basement massifs) may be explained as a simple consequence of changing dynamics during the transition from subduction to collision.

Barrovian regional metamorphism: where’s the heat?

Abstract Coupled thermal-mechanical models of convergent orogens offer a novel way to investigate the interactions between heat and tectonics that lead to regional metamorphism. In this study, the effects of different distributions of heat-producing material in the crust and upper mantle on crustal thermal histories and deformation fields are investigated. The models involve subduction-driven collision with moderate convergence and erosion rates. For models involving standard continental crust, where heat production is initially concentrated in the upper crust, P-T-t paths do not intersect the field of typical Barrovian P-T conditions. However, heat-producing material can be tectonically redistributed, for example, by subduction of crustal rocks to upper mantle depths, or by formation of thick accretionary wedges or continental margin sequences during convergence. Models that include a wedge of heat-producing material in the upper mantle generate high temperatures in the lower crust and upper mantle that lead to a change in orogenic style; radioactive heating of partially subducted crustal material on time scales of 10–30 Ma yields temperatures high enough for partial melting. However, crustal P-T-t paths are unlikely to intersect the Barrovian field unless erosion or convergence rates change. Models that include a crustal-scale region with moderate, uniform heat production, simulating a large accretionary wedge or tectonically thickened continental margin sequence, generate P-T-t paths that intersect the Barrovian field. However, as convergence proceeds, the heat-producing region is deformed, eroded, and reduced in volume, so that the model orogen begins to cool down after about 20 Ma. The model results provide an explanation for many first-order tectonic and metamorphic features of small orogens, including metamorphic styles ranging from blueschists to the Barrovian series to granulites, late-orogenic granitoid magmatism, and the crustal-scale tectonic features associated with regional metamorphic belts. We conclude that the thermal state of an orogen is controlled by the evolving competition between cooling by subduction and radioactive heating within the deforming orogen.

Modeling the behavior of the continental mantle lithosphere during plate convergence

In studying orogenic processes, the mechanism of viscous Rayleigh-Taylor–type removal of gravitationally unstable lithosphere is often invoked to explain the behavior of the mantle lithosphere. Using numerical models, we consider this mechanism and explore alternate styles of deep lithospheric deformation during tectonic convergence. The numerical experiments incorporate a mix of viscous and plastic rheologies to model the mechanical evolution of the lithosphere-asthenosphere system. Our results suggest that there are a number of deformational modes of the model mantle lithosphere: (1) a dripping or Rayleigh-Taylor–type instability; (2) an asymmetric underthrusting or subduction; (3) symmetric, ablative plate consumption; (4) slab breakoff, the failure and detachment of the strong lithosphere; and (5) mixed modes with combinations of these processes. The development of the modes is controlled by the rate of convergence associated with the background tectonic regime, the density field, and the rheology of the mantle lithosphere. It is important to determine whether these modes occur in the Earth beneath collisional orogens.

Tectonic assembly of inverted metamorphic sequences

Inverted metamorphic sequences are characterized by peak metamorphic temperatures that increase structurally upward, with isograds that are typically parallel to associated thrust faults. A fully coupled thermal-mechanical model for convergent orogens shows how an inverted sequence can be tectonically assembled in a crustal-scale ductile shear zone at moderate to high rates of synorogenic erosion. Model inverted sequences form by tectonic juxtaposition of points with widely differing initial positions that reach peak temperatures at different times and in different places within the model orogen, and thus do not represent metamorphic or thermal gradients. Inverted crustal isotherms are not required to produce model inverted isograds. Model results agree well with metamorphic pressure-temperature data from the Main Central Thrust zone of the central Nepal Himalayas.