Geological Association of Canada [GA]

 CC:701B  Tuesday  1030h

From Microstructures to Tectonics: Multiple Scales of Deformation in the Lithosphere I

Presiding:  S Lin, University of Waterloo; B Lafrance, Laurentian University; D Jiang, University of Western Ontario; Y D Kuiper, Boston College; J W Waldron, University of Alberta; P Simony, University of Calgary


Scale Invariance and Fractals in Structural Geology

Hobbs, B (, CSIRO, PO Box 1130, Bentley, WA 6102, Australia
* Ord, A ( AB: It is commonly observed that structures in deformed metamorphic rocks are scale invariant. This means that structures such as shear zones, folds, boudinage and fractures of similar shape and with similar relationships to each other occur at all scales from micro to regional. It is of interest to understand the processes that lead to such scale invariance. Does scale invariance represent a true fractal geometry that arises from the operation of one or more processes that are themselves independent of scale? Or is the scale invariance only apparent and arises from mixing of results from different mechanisms, each with their own length scales, operating and overlapping at different length scales to produce an apparent scale independent data set? We show that many structures in deformed rocks arise from coupling between processes such as deformation, fluid flow, mineral reactions, chemical diffusion and thermal transport. This coupling leads to scale invariance but this is not a fractal geometry across the scales. The feedback relations between these processes is expressed as a dominance of some processes at specific scales yet all of the coupled processes produce similar structures at the scale at which they dominate. Coupled deformation-thermal transport-mineral reaction processes dominate at regional scales, deformation-mineral reaction processes dominate at outcrop scales and deformation-mineral reaction-chemical diffusion processes dominate at micro-scales. This coupling leads to strain-rate softening as well as strain softening or hardening. Strain-rate softening processes produce shear zones, folding, boudinage and fracturing in materials that would be structure-less in uncoupled systems. The precise nature of the coupling between these processes depends on whether the processes are endothermic (dehydration mineral reactions, melting, grain size reduction, preferred orientation development, fracturing) or exothermic (inelastic deformation, retrograde mineral reactions, melt crystallisation) and also on the scale of the process. For slow (non-seismic) strain-rates endothermic mineral reactions destabilise deformation at the scale of an outcrop but stabilise deformation at the regional scale. Exothermic mineral reactions destabilise deformation at all scales and hence are commonly associated with shear zones.


Structural Hetherogeneities in the pre-Alpine Gabbros of the Dent Blanche Nappe, Western Alps

Spalla, M (, CNR-IDPA, Sezione di Milano, Via Mangiagalli, 34, Milano, I I-20133, Italy
Spalla, M (, Dipartimento di Scienze della Terra, Università di Milano, Via Mangiagalli, 34, Milano, I I- 20133, Italy
* Gosso, G (, CNR-IDPA, Sezione di Milano, Via Mangiagalli, 34, Milano, I I-20133, Italy
* Gosso, G (, Dipartimento di Scienze della Terra, Università di Milano, Via Mangiagalli, 34, Milano, I I- 20133, Italy
Baletti, L

Vaste gabbro intrusion in the continental lithosphere is a marker of lithospheric thinning: association of mantle peridotites, gabbros and continental crust is an important key for the understanding of passive margins. Gabbros included in continental units of orogens are therefore basic tools to infer duration and mechanisms active during ocean opening. Many gabbro bodies within the continental pre-Alpine crust of the Alps display a Permian-Triassic age, are concentrated in the Austroalpine and Southalpine domains and are often intruded into metamorphic rocks deformed under high temperature and intermediate- to low pressure conditions. They are interpreted, with the contemporaneous metamorphism often developed in their country rocks, as signals of a positive thermal anomaly associated to lithospheric thinning, indicated by some authors as precursor of the continental rift that led to opening of the Ligurian-Piemontese ocean (Dal Piaz et alii, 1977; Lardeaux & Spalla, 1991; Schuster et alii, 2001; Rebay & Spalla, 2001, Spalla & Gosso, 2003; Marotta & Spalla, 2007; Spalla & Marotta, 2007). The Dent Blanche nappe is the part of the Austroalpine domain of the Western Alps containing the highest density of Permian gabbro bodies, mainly hosted in metagranitoids (Arolla gneisses); outstanding igneous bodies are in the Matterhorn, in Mont Collon - Dent de Bertol (Dal Piaz et alii, 1977; Monjoie et alii, 2005) and at Cima della Sassa (Diehl et alii, 1952, Pennacchioni & Guermani, 1993). The contact between the gabbro bodies and the Arolla metagranitoid gneisses is defined by a thick and persistent mylonitic horizon. The polyphase Alpine deformation of these gabbros is highly hetherogeneous; our detailed structural investigation reconstructed the Alpine and pre-Alpine evolutions of the gabbroic complex of Cima della Sassa. In the context of the Permian gabbros of the Alps, the Sassa body maintains at best the igneous characters over huge weakly deformed volumes, preserved during the structural evolution of the Alpine convergence; these low-strain volumes, are affected by incomplete metamorphic re-equilibrations, leaving space to well preserved pre-Alpine mineral assemblages. This situation offers details of igneous and metamorphic pre- Alpine structures and textures, that suggest interpretations of the emplacement mechanisms of this mafic complex and of the geodynamic setting. In addition some evolutionary steps of the Alpine structural and metamorphic history of the Dent Blanche nappe have been added. REFERENCES DAL PIAZ G.V., DE VECCHI G. and HUNZIKER J.C. (1977) - Schweiz. Mineral. Petrogr. Mitt., 57: 59-88. DIEHL E.A., MASSON R. and STUTZ A.H. (1952) - Mem. Ist. Geol. Min. Univ. Padova, 17: 5-52. LARDEAUX J.M. and SPALLA M.I. (1991) - Journ. metam. Geol., 9: 35-59. MAROTTA A.M. and SPALLA M.I. (2007) - Tectonics, 26: TC4016, doi:10.1029/2006TC002047, 1-27. MONJOIE P., BUSSY F., LAPIERRE H. and PFEIFER H.R. (2005) - Lithos, 83: 317-346. PENNACCHIONI G. and GUERMANI A. (1993) - Mem. Sci. Geol., 45: 37-55. REBAY G. and SPALLA M.I. (2001) - Lithos, 58: 85-104. SCHUSTER R., SCHARBERT S., ABART R. and FRANK W. (2001) - Mitt. Ges. Geol. Bergbaustud. Oesterr., 45: 111-141. SPALLA M.I. and GOSSO G. (2003) - Mem. Sci. Geol., 54: 105 - 108. SPALLA M.I. and MAROTTA A.M. (2007) - Per. Mineral., 76: 211-252.


Vertical Variation of Deformation in Continental Strike-Slip Regimes

* Jiang, D (, University of Western Ontario, Department of Earth Sciences, London, ON N6A 5B7, Canada

Active and ancient continental strike-slip regimes like the Tanlu fault system in east China and the San Andreas fault system in the west United States are diffused deformation zones of hundreds to over a thousand kilometers wide. The long term deformation of the upper (∼15km) crust in these regions is commonly believed to have been largely achieved by slip on strands of crustal faults subparallel to the system boundary and by internal inelastic deformation of the fault-bounded blocks. How the ductile part of the lithosphere deforms, and to what extent the lithosphere is coupled, remain poorly understood. Most investigations of the flow of the ductile part of the lithosphere have assumed a Newtonian rheology with a constant viscosity, ignoring the strong exponential dependence of rock viscosity on temperature. In this contribution, I examine the consequence of the temperature-dependence of rock viscosity on the long term deformation of the ductile part of the continental lithosphere in an ideal transform plate boundary environment. I investigate the effects of various possible boundary conditions on the flow of the ductile lithosphere and on the vertical coupling of the continental lithosphere to identify the relative significance of various boundary constraints. It is shown that when subjected to a prescribed top boundary condition, appropriate for the long term motion of the upper brittle crust, and side boundary conditions due to relative plate motion, the ductile part of the lithosphere deforms primarily in response to the side boundary conditions. Although the flow in the ductile layer asymptotically approaches a simple shear as depth increases, the upper crust discrete, fault-block motion extends down as localized shear zones into the ductile lithosphere much farther than the constant viscosity model would predict. However, this down-cutting localization cannot penetrate a major rheological discontinuity. Strain localizations in the lithospheric mantle which are observed in many orogen-scale transcurrent and transpressional zones must be due to the effect of strong side boundary conditions. When the side boundary conditions are progressively relaxed, the transform motion (transcurrent shearing on vertical planes) cannot be maintained over a great depth range. If the system is subjected to a prescribed motion from the bottom (Moho), the motion dies out quickly upwards. This suggests that deformation in the continental lithosphere can only be transferred down the geothermal gradient not against it. Therefore tectonic deformation cannot be bottom driven. To maintain a lithosphere scale transform deforming regime, the region must be subjected to strong side boundary conditions. Therefore the long term tectonic deformation must be largely side driven. In a side-driven scenario, the motion of all rheological layers (the upper crust, the lower crust, and the mantle) share a common set of side boundary conditions. As a natural consequence of this, the motion in all rheological layers, on the large scale, are similar and approximate a transcurrent simple shear. This provides an explanation for many recent observations that the elastic strain field on the earth surface as determined from geodetic measurements generally agrees with long-term geological deformation. These observations have been used by some to argue for a basal-driven continental deformation model where the upper crustal faults are weak and fault-bounded blocks follow the motion of the ductile layer beneath them. Such a proposal is implausible once temperature-dependence viscosity of the lithosphere is taken into consideration, because as this investigation demonstrates a motion cannot be relayed upward, against the geothermal gradient.


Problems With Determining the Kinematic Vorticity Number From the Shape Fabric of Rigid Porphyroclasts

* Li, C (, University of Western Ontario, Department of Earth Sciences, London, ON N6A 5B7, Canada
Liu, R (, University of Western Ontario, Department of Earth Sciences, London, ON N6A 5B7, Canada

Shape preferred orientations (SPOs) of rigid porphyroclasts in mylonites have been used by many authors to estimate the kinematic vorticity number (Wk) of the hosting shear zone. There are many assumptions in this practice. First, it is assumed that the porphyroclasts behave according to Jeffery's theory. Second, the current method applies only to plane-strain deformation situations. Third, the method also requires that either the porphyroclasts be spheroidal, or if triaxially shaped, they must have one principal axis parallel to the vorticity vector of the flow. Finally, it is also tacitly assumed that the strain in the shear zone is sufficiently high that porphyroclasts with aspect ratios more than a critical value (Rc=[(1+Wk)/(1-Wk)]1/2)have reached their stable positions. These assumptions are too restrictive for the method to be applicable to natural shear zones. Many recent studies suggest that rigid porphyroclasts in mylonites may not behave according to Jeffery's theory. Besides, plane-strain deformations are perhaps rare and it is very unlikely that porphyroclasts will have one principal axis parallel to the vorticity vector of the flow. In this contribution we first assume that Jeffery's theory applies, and investigate, by numerical modeling, the consequences of relaxing other assumptions. Second, we use porphyroclast dada from some mylonites to evaluate the applicability of Jeffery's theory. We show that, in practice, Wk cannot be estimated accurately from porphyroclast SPO by the current method. This is because to determine Wk accurately the finite strain must be enormously high (λ11/231/2>108), many orders of magnitude higher than the finite strain any natural shear zone can possibly reach. For finite strain values typical of natural shear zones, the current method will overestimate Wk when it is lower than 0.4, and underestimate it when it is higher than 0.7. These conclusions suggest that accurate Wk estimate is practically not possible even if Jeffery's theory applies to all rigid porphyroclasts in shear zones. Our comparison of rigid porphyroclast data from mylonites with our numerical modeling results suggests that Jeffery's theory is not applicable to most rigid porphyroclasts in natural shear zones, and further indicate that Wk cannot be estimated with the current method.


Is the Larder Lake - Cadillac Deformation Zone a Transpression Zone?

* Lafrance, B (, Department of Earth Sciences, Laurentian University, Ramsey Lake Road, Sudbury, ON P3E 2C6, Canada

The Larder Lake - Cadillac deformation zone (LLCDZ) in the southern Abitibi subprovince in eastern Ontario is a 100 to 500 metre wide high strain zone that hosts several past-producing gold mines, including the giant Kerr-Addison Mine. The LLCDZ generally straddles a contact zone where older mafic volcanic and ultramafic rocks of the ca. 2706 Ma Larder Lake Group are interleaved with younger ca. 2675 Ma Timiskaming assemblage rocks. The LLCDZ is a D2 structure that postdates early D1 imbrication of the Larder Lake Group and Timiskaming assemblage. It is characterized by a strong moderately- to steeply-plunging L2 stretching lineation and by the intensification of a regional S2 foliation which is axial planar to east-striking regional F2 folds. The LLCDZ has been previously interpreted as a transpression zone that formed during D2 north-south shortening. Variations in the strike of the LLCDZ from northeast-, to east-, to southeast-striking are thought to predate the development of the deformation zone. East-striking segments experienced bulk coaxial strains during D2 north-south shortening, whereas northeast- and southeast-striking segments underwent, respectively, sinistral and dextral transpressive deformations. This interpretation is based on the orientation of S2 with respect to the strike of lithological contacts along the LLCDZ, clockwise along northeast-striking segments and anticlockwise along southeast-striking segments, and on the asymmetry of kinematic indicators on horizontal outcrop surfaces that are oriented at high angles to the stretching lineation. Mapping along the LLCDZ indicates that (1) all segments underwent bulk coaxial strain regardless of their strike, (2) S2 is a strong flattening fabric characterized by symmetrical strain shadows around clasts and detrital grains, (3) asymmetrical kinematic indicators are rare and where present are observed on surfaces parallel to the stretching lineation and perpendicular to foliation, (4) variations in the strike of the LLCDZ is due to late superposed folding, and (5) S2 can retain the same structural facing but vary in orientation from clockwise to anticlockwise to lithological contacts due to the formation of doubly-plunging F2 folds. These observations collectively suggest that deformation along the LLCDZ is not transpressive. The LLCDZ is interpreted as a zone of strong transposition and bulk coaxial strains where variable non-vertical extension and extrusion resulted in the formation of localized shear zones with slips roughly parallel to the stretching lineation.


Microstructural Controls on Shear Zone localization During Formation of the Tuscan Nappe, Isola Palmaria, Northern Apennine, Italy

* Molli, G (, Dipartimento di Scienze della Terra, Università di Pisa, via Santa Maria, 53, Pisa, 56126, Italy
White, J C (, Department of Geology, University of New Brunswick, 2 Bailey Dr., Fredericton, NB E3B 5A3, Canada
White, J C (, Dipartimento di Scienze della Terra, Università di Pisa, via Santa Maria, 53, Pisa, 56126, Italy
Kennedy, L (, Department of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, BC V6T 1Z4, Canada
Taini, V, Dipartimento di Scienze della Terra, Università di Pisa, via Santa Maria, 53, Pisa, 56126, Italy

The western promontory of La Spezia, Italy comprises extensive outcroppings of carbonates that form part of the Tuscan nappe, a continent-derived unit of the northern Apennine nappe stack. Formation of the Tuscan nappe during deformation of the northern Apennitic accretionary wedge began in early Miocene and it is now exposed as part of the kilometer-scale west-facing La Spezia fold that deformed the nappe stack in late Miocene. This study examines fine-grained carbonates (∼ 5μm) deformed within bedding-parallel shear zones at shallow structural levels with temperature less than 250°C. The shear zones of interest occur within the Upper Triassic/early Liassic limestones and dolomites ('Portoro' beds) of Isola Palmaria, an island at the southern termination of the promontory that exposes the inverted limb of the La Spezia fold. These early, bedding- parallel shear zones collectively accommodate large amounts of deformation associated with the crustal thickening. Two distinctive types of shear zones are observed in the study area. Each has distinctive microstructural and micromechanical characteristics that can be tracked during progressive deformation of pre-existing features and fabrics. Type A zones develop within the host micritic limestone and exhibit development of an optical shape-preferred orientation (SPO) that can be linked by transmission electron microscopy (TEM) to dislocation glide on r{10- 14} and f {-1012} planes. Pre-existing bedding-perpendicular veins are progressively develop dislocation glide, dislocation network formation and rotation recrystallization, at which point they cannot be differentiated from primary micrite. Electron backscattered electron diffraction (EBSD) indicates that neither deformed vein nor micrite develop crystallographic preferred orientations (CPO) despite significant intracrystalline deformation. Grain boundaries throughout this shear zone are heavily decorated with voids consistent with the extensive fluid flow in these zones noted in previous studies. Type B shear zones localize within initially coarse- grained bedding-parallel calcite veins. The sequential optical microstructures comprise twinning, with twin bulging and migration, followed by shear zone parallel bands of dynamically recrystallized grains until an equigranular calc-mylonite is produced at high strain. In contrast to Type A vein calcite, grain boundaries are generally undecorated, and strong CPOs of r, f and c(0001) planes develop consistent with the mesoscopic displacement sense of the shear zones. An outstanding paradox is that no definitive basal plane dislocations have been identified despite their inference from EBSD data. Our observations demonstrate that dislocation-mediated accommodation processes and dynamic recrystallization play an important role during deformation at shallow structural levels. Type A shear zones are an example of dislocation glide coexisting with fluid-mediated grain boundary diffusion and relative displacement (grain boundary sliding) such that no CPO is formed. Type B zones demonstrate the ease with which coarse-grained calcite accommodates intracrystalline deformation, acting as a primary mechanism for localizing deformation. The lower diffusivity of the tighter' grain boundaries inhibits relative grain displacement, even at small grain size, and allows a quasi-uniform deformation that enables generation of the strong CPO.


Deformation partitioning in transpressional shear zones: an example from the Superior Boundary Zone, Manitoba, Canada

* Kuiper, Y D (, Boston College, 140 Comm Ave, Chestnut Hill, MA 02467, United States
Lin, S (, University of Waterloo, 200 Univ Ave W, Waterloo, ON N2L 3G1, Canada
Jiang, D (, University of Western Ontario, 1151 Richmond St, London, ON N6A 5B7, Canada

In progressive deformation, planar and linear fabrics, such as foliations, lineations and fold hinge lines, follow complex rotation patterns, especially in triclinic shear zones. Because triclinic shear zones are common, rotation patterns are expected to be complex in most shear zones. However, deformation across a shear zone may be partitioned into various domains. We describe shear zones along the Superior Boundary Zone (SBZ) in which the overall triclinic flow is partitioned into approximately monoclinic and orthorhombic domains. The SBZ is a transitional zone of deformation between the Archean Superior Province and the Paleoproterozoic Trans-Hudson Orogen. Mylonitic shear zones within it show evidence for thinning across the zones. The Aiken River shear zone (ARSZ) is a 1-1.5 km wide ~E-W trending, dextral, north-side-up, mylonite zone. SE-plunging folds in the Split Lake Block, north of the ARSZ, consistently become tighter, and turn to horizontal, towards the ARSZ, indicating a shear zone-parallel horizontal stretching component. Lineations and fold hinge lines within the mylonite zone generally plunge moderately to the west, indicating the simple shear direction. The shear zone thus has an overall triclinic symmetry. If simple shear had played a significant role adjacent to the mylonite zone, the fold hinge lines of the Split Lake Block would not have rotated directly towards horizontal orientations, without rotating towards a moderately westerly plunge first. Flow was therefore approximately orthorhombic (pure shear). If pure shear had played a significant role within the mylonite zone, lineations and fold hinge lines would have rotated towards horizontal. Flow within the mylonite zone was therefore approximately monoclinic (simple shear). We conclude that shear zones with triclinic symmetry may be partitioned into approximately monoclinic and orthorhombic domains. A transitional domain with triclinic symmetry is likely to exist, but may be narrow. Furthermore, as exemplified by the transpressive shear zones of the SBZ, simple shear may be concentrated in relatively narrow zones with relatively sharp boundaries, while pure shear is distributed over broad zones with diffuse boundaries.