Summary

The lithosphere is central to our plate tectonic view of the Earth and it is also a key component in the evolution of the Earth System. Our objective is an improved understanding of the behaviour and geological role of the lithosphere, recognizing that continental plates are not rigid and that the mechanics of deformation is fundamental to an improved understanding. Plates deform where they interact at their boundaries and in their interiors. They interact with the atmosphere, hydrosphere and biosphere at their surface and with the thermally convecting mantle below. These interactions are responsible for the mesoscale tectonic components (e.g. rifted and transcurrent margins, convergent margins, orogens, plateaus and their associated sedimentary basins) that store the geological record of Earth evolution.

Our approach is to develop geodynamical models as analogues to the natural system. This approach has been rewarding in our recent research on convergent margins and orogeny. The subduction model of orogenesis (Fig. 1) has been tested and compared with small orogens (Alps, Southern Alps New Zealand, Pyrenees, orogens studied by Lithoprobe, Coast Ranges California) and the potential for the climate control of orogens outlined. The novelty and significance of our interdisciplinary collaborative research (Canadian Institute for Advanced Research - Earth Systems Evolution Program, Lithoprobe) is that it addresses geological, geophysical, geomorphic and climate processes within ancient and modern orogens and other zones of deformation both in terms of the underlying mantle, lithospheric and surface components, and as an integrated system. The objectives of the next phase of the program are: development of a unified model of orogenesis; incorporation of mantle deformation into the models; a dynamical understanding of fold and thrust belts; improved surface process models, and; the coupling of the surface process models to tectonic models that correctly predict localized internal and surface deformation.
 

Relationship to University Strategic Plan

Specific research proposed here is a component of the broader objectives of Dalhousie and the Canadian Institute for Advanced Research in Earth Systems/Ocean/Petroleum Research (Fig. 2). Currently, Dalhousie and CIAR fund Geodynamics research by supporting Chris Beaumont, and Structural Geology/Tectonics research by supporting Dr. Djordje Grujic. His interests complement geodynamics in two ways- research on the evolution of orogens (mountain belts), e.g. Himalayan-Tibetan orogen, and in laboratory analogue experiments that parallel our numerical experiments.

Dalhousie plans to add a faculty member specialising in earth surface processes, probably with emphasis on the measurement of rates of surface erosion etc. using innovative radionuclide techniques. This observational emphasis complements our more theoretical/modelling approach to surface processes.

CIAR has committed financial support for a faculty appointment in the Geochemistry of Modern and Ancient Oceans if this application is successful. This position both strengthens Ocean Studies research and Earth System research at Dalhousie. In the latter case, oceans are a reservoir for material produced by tectonics and then eroded and transported to the sea. Direct input of river, ice and wind born sediment is one monitor of these processes and the isotopic geochemistry of precipitated sediments or contained in sedimentary organic matter is another. For example, because the oceans are well mixed on relatively short timescales, strontium and neodymium isotopes in suitable sediments of known age from worldwide locations act to tape record the pulse of plate tectonics and lithospheric deformation. Other radiometric and stable isotope systems provide powerful constraints on climate variations and biological activity, in addition to tectonic and volcanic processes, throughout Earth history.

Dalhousie also agrees to a fifth faculty position in Petroleum Geology, also a priority area, if this proposal is successful. Salt Tectonics is a potential research field, further complementing structural, tectonic and geodynamic research and adding an analogue experimental laboratory. Deep water salt tectonics of continental margins is a frontier research problem regarded as especially important by the petroleum industry. This position will have direct connections with the Atlantic Canada Petroleum Institute and may be partly supported by it. These linkages are illustrated in Figure 2.
 

Introduction and Motivation

The focus of our research is lithospheric deformation during orogenesis (mountain building) and the associated interactions of the solid earth, hydrosphere and atmosphere acting as a system (Fig. 1). This field of research is important because orogeny is the principal process that creates continents. Most of our research is collaborative and significant progress has been achieved by numerical model experiments combined with comparisons with observations. The motivation is to gain an improved understanding of the dynamics of the model systems and to determine whether the same interactions control natural systems. Space for this proposal is limited, therefore I have focussed on current and proposed research. The way in which this integrates with published results may be viewed at http://adder.ocean.dal.ca/research/crc2000.html which contains an expanded version of the proposal.
 

Progress Report Principal Results of Projects - Mantle Subduction (MS) Models of Orogenesis This research addresses a class of problems in which crustal deformation is forced by subduction of the underlying mantle lithosphere (Figs. 1 and 3). The proposition is that continental and oceanic mantle lithospheres behave in a similar way during convergence - they subduct. What are the styles of deformation of the overlying crust and to what degree are compressional orogens mechanical analogues of accretionary wedges? Below, I list the results of specific projects designed to address this problem. Table 1 relates developments to the basic doubly-vergent model. Model types are: Class 1: Crustal deformation driven by mantle subduction (MS) - Plane strain vertical section (MS-PS), Thermal Mechanical (MS-TM) - Thin crustal sheet (MS-TS) - Fully 3D (MS-3D) Class 2: Whole lithosphere deformation by lateral indentors (WL) - Single thin sheet (WL-TS) 1) Basic Model (MS-PS). Willett, Beaumont and Fullsack (Geology 21, p.371, 1993) demonstrates underlying concepts (Fig. 1), the generality of the basic mantle subduction model and relates the model results to small orogens (Fig. 3a style), large orogens and active margins.  2) Thin Sheet Models Forced by Basal Boundary Conditions (MS-TS). Ellis, Fullsack and Beaumont (Geophys. J.I., 120, p.24, 1995). The mantle subduction model can also be investigated in planform. It is shown that there is a non-dimensional parameter, the Ampferer number (ratio of basal coupling to crustal strengths) that characterizes the problem in addition to n(the creep power law exponent) and the Argand number (ratio of gravitational force to crustal strength).  3) Arbitrary-Lagrangian - Eulerian Finite Element Method. Fullsack (Geophys, J.I., 120, p.1, 1995) describes the mathematics, numerical techniques, and provides examples with accuracy analysis, of the plane-strain, large deformation, visco-plastic, Stokes flow, finite element methods used for our PS and TS models. 4) Strain Partitioning in Obliquely-Convergent Mantle Subduction Models. Braun and Beaumont (J. Geophys. Res., 100, p.18059, 1995) use a fully MS- 3D model (development by Braun) to demonstrate that, in weakly transpressional orogen models with a Griffith crust, strain is partitioned into thrust-sense pro- and retro-shears (as in basic PS-MS model, Fig. 3a) co-existing with and bounding strike-slip ‘flower’ type structures. Under stronger transpression, the strike-slip ‘flowers’ merge with the step-up shears to create non-partitioned oblique-slip zones. The partitioned mode corresponds closely with the Coast Ranges California, whereas the non-partitioned mode is like the continental collision zone, South Island, New Zealand. 5) Styles of Small Compressional Orogens (MS-PS). Beaumont, Fullsack and Hamilton (Tectonophysics, 232, p.119, 1994). Shows results of a range of numerical experiments for small convergence in relation to initial crustal thickness. Specifically, the effects of plasticity and power law creep, thermal structure of the crust, degree of crustal subduction and denudation are investigated. Predictions are made for basic seismic reflectivity patterns in orogens. Oppositely vergent primary structures in orogens do not require a change in polarity of subduction. 5a) Ellis et al. (Can. J. Earth Sci., 35, p1323, 1998) demonstrate the dramatic change in deformation style when the crust of the orogeny is weak and becomes squeezed between the stronger vise jaws of the external crust. 6) Large Orogens (MS-PS), Himalaya-Tibet (H-T). Willett and Beaumont (Nature, 369, p.642, 1994) use large orogen MS-PS models. The work combines these models with ‘rollback’ (ie. retreat of the subducting mantle lithosphere) to propose a model of the H-T system. Comparison of the numerical experiments with the first order asymmetry of the H-T orogen and position of the Indus-Tsangpo suture indicates these observations are best satisfied by subduction of Asian lithospheric mantle. An alternative interpretation of the results, which has the same effective basal boundary condition, is the northward advance of the subduction zone (S, Fig. 1) as the collision progresses (advancing or ablative subduction). 7) Subduction, the Subduction-Collision Transition, and Collision. Evolution from subduction to collision is a common and fundamental process in orogenesis. Beaumont, Ellis et al. (Geology, 24, p.675, 1996) show that the doubly-vergent wedge model of orogens (the basic model) can be extended to the subduction/transition precursor phases and that the buoyancy of the subducted lithosphere is a primary control in the transition (ie. slab pull, delamination and slab-breakoff may control the transition to double-vergence). The mechanism seems to apply to the Neoalpine collision in the Swiss Alps and explains the onset of ‘retrocharriage’. 8) Effect of Subduction Zone Retreat on PS-MS Models. Waschbusch and Beaumont (J. Geophys. Res., 101, p.28133, 1996) investigate the consequences of the motion of the subducting slab away from the overriding (retro-) plate during convergence. Retreat of the subduction zone (ie. hinge point of the subducting plate) leads to tectonic underplating in the space created between the pro- and retro- lithospheres. The thickness and amount of deformation of the tectonic underplate are simple functions of the subduction zone retreat rate and fraction of the crustal layer that subducts. Model predictions agree with the low elevation, limited exhumation and single-vergence style of many orogens formed at retreating plate boundaries. 9) Subduction-Accretion Modes at Convergent Margins. Beaumont et al. (J. Geophys. Res., 104, p. 15169, 1999) propose conceptual modes of deformation in systems that also allow sediment subduction. Model experiments confirm their dynamical feasibility. 9a) Ellis et al. (J. Geophys. Res., 104, p. 17573, 1999) extend the convergent margin analysis to include episodic accretion that, among other results, shows how large-scale fold nappes may form. 10) Thermal-Mechanical MS-TM Coupled Models. Coupling between mechanics and heat is a first-order process in orogens. Application of MS-TM models to Barrovian metamorphism, specifically heating by tectonic accretion of radioactive material (TARM), is discussed in Jamieson, Beaumont et al. (Geol. Soc. Lond. Spec. Publ. 138, p. 23, 1998). Unpublished results (http://adder.ocean.dal.ca/ (Jamieson et al. presentations)) illustrate the corresponding results for Large Hot Orogens (LHO) with plateaus.

Examples of Proposed Research Themes - Orogen Tectonics, Lithosphere-Mantle Interactions, and Surface Processes

An assessment of our stand alone and collaborative research indicates several themes that warrant further attention and development. Some of these themes are outlined below and linked to specific projects, noting collaborations and research training. The fold and thrust belt theme and our research on the development of numerical techniques are discussed in the CFI module. We provide an attractive environment for interdisciplinary research training in a range of fields: applied mathematics / numerical methods / computer science (modern parallel CFD techniques) / geodynamics / geology / geomorpology / Earth system evolution. We have successfully trained undergrad., grad., p.d.f., and technical research personnel (see c.v. section 4). Two have been appointed to university faculty positions in the last year. In addition, we will have 4 visiting researchers during the next year and we have spawned research groups in Canada and internationally. There is every reason to expect to continue and expand this research training.

Toward a Unified Model of Orogenesis

A general question concerns the development of a unified model of orogenesis (Fig. 1). Which processes are key to orogenesis? One conceptual classification of progressive orogenesis (Fig. 3) suggests that change in lithospheric strength, probably through temperature increase and thermal weakening, is a crucial process that explains plateau development and normal faulting/extension. It can be argued that while the lithosphere is cold and strong the associated Types 1 and 2 crustal orogenesis (Fig.3) are consistent with the mantle subduction model. Once an orogen becomes large, hot, or both, it is less likely that the motion of the mantle lithosphere in contact with the base of the crust will be like subduction. It is therefore essential to develop models to investigate the circumstances under which continental mantle lithosphere subducts in a plate-like manner analogous to oceanic lithosphere. What are the other styles of deformation of the continental lithosphere? How do these styles evolve during orogenesis? What is the role of temperature, and other factors that change material properties, during this evolution?
 

Lithosphere-Mantle Interactions during Orogenesis (with R. Pysklywec NSERC PDF, R. Huismans PDF, P. Fullsack numerical analyst, B. Lee Res. Tech.).

In recent research we have developed modelling techniques based on a new F.E. environment -SOPALE, developed by Philippe Fullsack, a member of our group. This more dynamical approach expands the model domain to include all of the lithosphere and the underlying more fluid viscous upper mantle, thereby relaxing the kinematic subduction boundary conditions at the base of the crust. Lithospheric convergence is specified as a far-field boundary condition and deformation evolves as a density-driven flow determined by the model rheology, convergence velocity and density field. The formulation is a large deformation Arbitrary-Lagrangian-Eulerian (ALE) implicit finite element coupled thermal-mechanical model for creeping flows with viscous/plastic/frictional material properties. For pressure sensitive, (e.g. frictional) materials the calculation must include the total gravitational forces and a true deformable upper free surface. SOPALE meets these requirements, has passed a range of analytical and other tests, and can efficiently solve 100 by 400 finite element meshes, allowing crustal and lithospheric properties to be resolved. Although these are only medium resolution models, a more refined parallel code has been developed and is currently being tested.

Large Convergence Lithospheric Scale Models

Pysklywec et al.(Geology, 28, p. 655, 2000) have focussed on prototype model experiments designed to address the fate of mantle lithosphere during orogenesis. In these SOPALE experiments the lithosphere is a viscous/plastic multilayer with density stratification (Von Mises crust and upper mantle lithosphere with strain softening; linear uniform viscosity lower mantle lithosphere; and lower density, lower uniform viscosity fluid upper mantle). The lithosphere is inserted into the model domain (2400 by 600 km) at constant velocity at the model boundary, the upper surface is stress free, and other boundaries are free to slip. Although these models are not realistic Earth equivalents (e.g. they have simplified material properties), they exhibit a suite of deformation styles: subduction/underthrusting (including asymmetric and symmetric ‘double’ subduction), slab breakoff, the well-studied Rayleigh-Taylor (RT) viscous density-driven instability (‘dripping’), and combinations of these modes. Unpublished experiments also exhibit reversals in subduction polarity and delamination events.

The behaviour is controlled by: 1) rheological stratification -plastic properties give the models a greater range of styles than viscous properties alone; 2) mechanisms of strain localization -focussed as opposed to distributed deformation; 3) density stratification - determines growth rates for RT instabilities and critical forces for breakoff instabilities. The importance of the results is that the rich range of behaviours achieved with the simplified models indicates that there is probably an equally interesting range of natural behaviours yet to be properly characterized. A combination of refined seismic tomography and model experiments holds the promise to observe and then describe the physics of these modes. We propose to investigate the factors that lead to each of the modes and their sensitivity to material properties.

Small Convergence Lithospheric Scale Models (with R. Pysklywec NSERC PDF, B. Lee Res. Tech.)

Current research (e.g. T. Stern et al., in press, J. Geophys. Res.; E. R. Klosko et al., Geophys. Res. Lett., 26, p. 1497,1999; M. Kohler, J. Geophys. Res., 104, p. 15025, 1999; P. Molnar et al., Science, 286, p. 516, 1999) has given a new observational impetus to the debate concerning subduction versus convective removal (RT instability) of lithospheric mantle even below orogens where convergence is small (~100km) e.g. Southern Alps, New Zealand, Transverse Ranges, California. Seismic evidence indicates symmetric distributed thickening of mantle lithosphere, yet the deformation style of the crust of the Southern Alps, for example, coincides with the asymmetric mantle subduction model (Beaumont et al., J. Geophys. Res., 101, p. 3333,1996).

Small convergence SOPALE model experiments (in progress) indicate one way to resolve this dichotomy; strong, plastic upper mantle lithosphere may asymmetrically underthrust/subduct while viscous lower mantle lithosphere tectonically thickens and develops an RT instability (Fig. 4). This figure only represents one early result and we propose a complete range of sensitivity tests for these models in which the rheology, density, and localization mechanisms will be systematically varied. In addition, the sensitivity to the b.c's and denudation of the upper surface (coupling to surface processes) will be investigated. A second series of fully thermally-mechanically coupled models is also proposed to determine the effect of crustal radioactive heating, strain related heating, and the diffusive redistribution of heat. The equivalent higher resolution parallel numerical models will be used to measure the numerical accuracy of the results, particularly in regard to strain softening/hardening and associated localization.



Coupling of Erosion and Tectonics in Large Hot Orogens: Application to Himalayan Tectonics (with R. Jamieson collab. faculty, M. Nguyen Res. Tech., F. Bigler Phd student, D. Grujic collab. faculty).

The Himalayan - Tibetan system, the largest active orogen, is a focus of major international research programs. A pervasive feature along the southern flank of the Himalaya is the Higher Himalaya Crystalline Belt (HHCB), a zone of high metamorphic grade rocks progressively exhumed from a depth of approximately 30km during approximately the last 20Ma. The belt is bounded by an upper normal sense shear (STD) and a lower thrust sense shear (MCT). Although several mechanical explanations have been proposed for the origin of this structure (e.g. L. Royden and Burchfiel, B.C., Geology, p. 13679, 1985; L. Royden, J. Geophys. Res., 101, p. 17679,1996; D. Grujic et al., Tectonophys., 260, p. 21, 1996) the overall evolution is not fully understood. In particular, the potential links with inferred zones of crustal partial melt beneath the southern Tibetan Plateau requires investigation. Insights may be derived from coupled thermal-mechanical models (Fig. 5) that demonstrate outward mid-crustal channel/extrusion type flows as the viscosity of this region decreases with 'incipient melting' (zones outlined in red). These flows remain buried in the crust under conditions of low surface erosion rates. However, when influenced by a focussed erosion front the extruded region becomes tectonically coupled to the denudation (velocity field Figs. 5a,b) and is exhumed and exposed (Fig.5b). This configuration has much in common with the HHCB geometry, the connection to the 'melt' zone beneath the adjacent plateau, and the fact that the Himalaya are an erosional front where monsoonal/orographic precipitation maximizes glacial/river incision. This is an example of an Earth System problem which has implications for orogen tectonics, the properties of sediments derived from the orogen, and the strontium/neodymium isotope signature of seawater. We propose to investigate this type of coupling between surface processes and extrusion flows in large hot orogens to determine its general character and whether it is a viable model for the HHCB.