Mark R. Handy

Flow laws for rocks containing two non-linear viscous phases: A phenomenological approach

Abstract Mylonitic rocks and rock-analogue materials reveal two basic types of structure: (1) a load-bearing framework (LBF) of strong phase contains isolated pockets of weak phase; (2) interconnected layers of weak phase (IWL) separate boudins and clasts of strong phase. Aggregates with the LBF microstructure are characterized by nearly uniform strain rate. Stress is concentrated in the load-bearing framework. In aggregates with an IWL microstructure, strain rate and sometimes also stress are higher in the interconnected weak phase than in the boudins and clasts of strong phase. The degree of stress and strain partitioning depends strongly on the viscous strength contrast and on the relative amounts of the constituent mineral phases. Based on these observations, the rheology of two-phase rock is modelled with separate functions for LBF and IWL microstructures. A new flow law is derived for rock with IWL structure in which two phases undergo dislocation creep. The flow law expresses composite creep strength in terms of temperature, bulk strain rate and the volume proportions and creep parameters of the minerals in the rock. Strain rate and stress are averaged in the constituent phases and slip along phase boundaries maintains strain compatibility within the aggregate. Composite strengths predicted with the IWL flow law fall well within the uniform stress and uniform strain rate bounds and are generally consistent with the viscous strengths of experimentally deformed bimineralic aggregates. A hypothesis of viscous strain energy minimization is used to determine the relative stability of the LBF and IWL microstructures. During steady-state creep, the IWL microstructure is predicted to be stable over a broad range of two-phase compositions and mineral strength contrasts, whereas the LBF microstructure is stable only in rocks with low volume proportions of weak phase and low to moderate mineral strength contrasts. The IWL flow law indicates that rheological stratification in the lithosphere depends strongly on rock composition, especially in rocks with low volume proportions of a weak phase and high mineral strength contrasts.

The kinematics of movements along the Insubric Line and the emplacement of the Ivrea Zone

Schmid, SM., Zingg, A. and Handy, M., 1987. The kinematics of movements along the Insubric Line and the emplacement of the Ivrea Zone. In: H.J. Zwart, M. Martens, I. van der Molen, C.W. Passchier, C. Spiers and R.L.M. Vissers (Editors), Tectonic and Structural Processes on a Macro-, Meso- and Micro-Scale. Tecta~a~hys~~, 135: 47-66. The Insubric Line west of Locamo is characterized by a 1 km thick greenschist facies mylonite belt. East of Locamo, these mylonites are overprinted by a discrete brittle fault. The mylonites are derived from basement units of the Central Alps (Sesia Zone) and the Southern Alps (Ivrea Zone) as well as from the Permo-Mesozoic cover of the Southern Alps (Canavese). Sense of shear criteria indicate that the mylonites accomm~ated backt~s~g followed by dextral strike-slip motion. Mylonitization during backtbrusting was synchronous with backfolding of the Central Alpine nappes under higher metamorphic conditions. A horizontal temperature gradient resulted from the rapid juxtaposition of the warm Central Alpine block against the cold Southern Alpine block. Mylonites formed during the later dextral strike-slip event are related to large transcurrent displacements in the Central Alps deduced from regional kinematic considerations. Thus, both mylo~t~ation events are contempor~eous with deformation to the north and south of the Insubric Line (Insubric phase) extensively modifying the pre-Insubric crustal configuration of the Alps. The Insubric phase post-dates the Bergell intrusion (30 m.y.). The emplacement of the geophysical Ivrea body is a combined effect of vertical uplift due to E-W directed crustal thinning during the Early Jurassic and underplating by continental crust associated with Late Cretaceous compression. A deep crustal normal fault (Pogallo Line), subsequently rotated during Tertiary Alpine orogenesis, separates deeper parts of the Southern Alpine crust (Ivrea Zone) from intermediate crustal levels (Strona-Ceneri Zone). The rigid Ivrea body localir.ed large strains within the Insubric mylonite belt and is responsible for the present curvature of the Insubric Line. In~~uetion

Deformation regimes and the rheological evolution of fault zones in the lithosphere: the effects of pressure, temperature, grainsize and time

Abstract A new series of two- and three-dimensional deformation regime maps facilitating the simultaneous display of petrological, microstructural and rock-mechanical information illustrates the pressure, temperature and grainsize dependencies of brittle, crystal-plastic and granular creep regimes in the lithosphere. The predictions made using the extrapolated laboratory data are in qualitative agreement with inferences drawn from microstructural studies in naturally deformed monomineralic rocks. The deformation regime maps are used to show that the rheological evolution of a fault zone is related both to its location within a compositionally heterogeneous lithospheric section and to the pressure-temperature-time history of that section. Strain localization into ductile shear zones is accelerated if grainsize reduction leads to a change from grainsize-insensitive to grainsize-sensitive creep. In rocks whose rheology is approximated by the flow of one weak mineral, this process is favored at high stress-low temperature and/or retrograde thermal conditions. For polymineralic rocks in either prograde or retrograde thermal settings, reaction-induced or -enhanced switches to grainsize-sensitive creep can occur at depths corresponding to the intersection of the ambient geothermal gradient or pressure-temperature-time path with the limits of mineral stabilities for a specified rock composition. The consideration of the effects of pressure, temperature, grainsize and syntectonic reactions on fault rheology results in an evolutionary rheological model of an extending lithosphere which is more complex than existing models.

The tectonic and rheological evolution of an attenuated cross section of the continental crust: Ivrea crustal section, southern Alps, northwestern Italy and southern Switzerland

The tectonic and rheological evolution of the southern Alpine continental crust is reconstructed from structural, petrological, and radiometric studies in the Ivrea and Strona-Ceneri basement units. The deep crust of the southern Alps acquired its present compositional and metamorphic zonation during Paleozoic magmatism and amphibolite-to granulite-facies regional metamorphism. Inferred strength contrasts between lower crustal and upper mantle rocks in the Ivrea zone are low at the high temperatures of regional metamorphism. Late Paleozoic transtension and basic to intermediate magmatism in all crustal levels preceded extensional faulting associated with the formation of a passive continental margin during early Mesozoic time. Extensional uplift and cooling of the basement section is coupled with crustal-scale trends of increasing rheological stratification, grain size reduction, and strain localization. Mylonitic shear zones in the lower crust and a broad zone of noncoaxial shear at the base of the intermediate crust in the Ivrea zone are inferred to have controlled the high-strain rheology of the attenuating southern Alpine crust. The deep crust in the thinned Ivrea crustal cross section transferred noncoaxial strain from the upper mantle to the intermediate and upper crust. Final uplift and exposure of the deep southern Alpine crust occurred during Late Cretaceous to Tertiary Alpine thrusting.

Tectonometamorphic history of the Ivrea Zone and its relationship to the crustal evolution of the Southern Alps

Abstract The Ivrea Zone represents a cross-section through the lower continental crust of the Southern Alpine basement. A first long episode with regional metamorphism and associated polyphase deformation ended in the Variscan. Subsequent Late Paleozoic magmatic activity may have occurred during an early stage of crustal attenuation. Late Paleozoic and Early Mesozoic crustal thinning is accommodated by conjugate high-temperature shear zones within the granulite facies Ivrea Zone and by low-angle normal faulting within the Pogallo Ductile Fault Zone, at the base of the intermediate crust. The age and kinematics of the Pogallo Ductile Fault Zone are consistent with the occurrence of Early Mesozoic extensional basins in the Southern Alpine sediments. Exhumation and final steepening of the Ivrea Zone during the Alpine Orogeny did not substantially alter its internal structure exept in the vicinity of the Insubric Line. Thus, the Ivrea Zone and the adjacent Strona-Ceneri Zone represent a good example of highly attenuated lower and intermediate continental crust.

Syntectonic melt pathways during simple shearing of a partially molten rock analogue (Norcamphor-Benzamide)

Norcamphor-benzamide aggregates were used as analogues for partially molten quartzofeldspathic rock. Drained, constant displacement rate, simple shearing of partially molten norcamphor-benzamide aggregates produces strain localization within melt-bearing, extensional shear fractures. These fractures interconnect to form shear surfaces whose orientation with respect to the shear zone boundary is like that of synthetic shear bands (C′ surfaces) in naturally deformed, mylonitic rocks. The shear bands channel overpressured melt from dilatant grain boundaries in the norcamphor-benzamide aggregate to undeformed, low-pressure areas adjacent to the deforming sample. The rapid expulsion of melt in the aggregate along shear bands hinders the attainment of a rheological critical melt percentage (20%) within the shear zone as a whole. However, this melt percentage is achieved within the melt-bearing shear bands. Deformation within the shear bands involves a strain-dependent switch from intergranular fracturing and dislocation creep to diffusion-accommodated, grain-boundary sliding, whereas the matrix adjacent to the shear bands continues to deform by dislocation creep. At a shear strain of approximately γ = 0.7 the melt-bearing shear bands coalesce to form interconnected weak layers subparallel to the shear zone boundaries. The strength of the aggregate could not be measured but is inferred to decrease markedly once the melt-bearing layers interconnect. When the melt completely crystallizes and deformation ceases, the former presence of melt within the shear bands is only betrayed by the alignment of benzamide grain boundaries parallel to the shear bands.

The evolution of high-temperature mylonitic microfabrics: evidence from simple shearing of a quartz analogue (norcamphor)

Plane strain simple shearing of norcamphor (C7H10O) in a see-through deformation rig to a shear strain of γ = 10.5 at a homologous temperature of Th = 0.81 yields a microfabric similar to that of quartz in amphibolite facies mylonite. Synkinematic analysis of the norcamphor microfabric reveals that the development of a steady-state texture is linked to changes in the relative activities of several grain-scale mechanisms. Three stages of textural and microstructural evolution are distinguished: (1) rotation and shearing of the intracrystalline glide planes are accommodated by localized deformation along three sets of anastomozing microshears. A symmetrical c-axis girdle reflects localized pure shear extension along the main microshear set (Sa) oblique to the bulk shear zone boundary (abbreviated as SZB); (2) progressive rotation of the microshears into parallelism with the SZB increases the component of simple shear on the Sa microshears. Grain-boundary migration recrystallization favours the survival of grains with slip systems oriented for easy glide. This is associated with a textural transition towards two stable c-axis point maxima whose skeletal outline is oblique with respect to the Sa microshears and the SZB; and (3) at high shear strains (γ > 8), the microstructure, texture and mechanism assemblage are strain invariant, but strain continues to partition into rotating sets of microshears. Steady state is therefore a dynamic, heterogeneous condition involving the cyclic nucleation, growth and consumption of grains.

Reconstructing the Alps–Carpathians–Dinarides as a key to understanding switches in subduction polarity, slab gaps and surface motion

Palinspastic map reconstructions and plate motion studies reveal that switches in subduction polarity and the opening of slab gaps beneath the Alps and Dinarides were triggered by slab tearing and involved widespread intracrustal and crust–mantle decoupling during Adria–Europe collision. In particular, the switch from south-directed European subduction to north-directed “wrong-way” Adriatic subduction beneath the Eastern Alps was preconditioned by two slab-tearing events that were continuous in Cenozoic time: (1) late Eocene to early Oligocene rupturing of the oppositely dipping European and Adriatic slabs; these ruptures nucleated along a trench–trench transfer fault connecting the Alps and Dinarides; (2) Oligocene to Miocene steepening and tearing of the remaining European slab under the Eastern Alps and western Carpathians, while subduction of European lithosphere continued beneath the Western and Central Alps. Following the first event, post-late Eocene NW motion of the Adriatic Plate with respect to Europe opened a gap along the Alps–Dinarides transfer fault which was filled with upwelling asthenosphere. The resulting thermal erosion of the lithosphere led to the present slab gap beneath the northern Dinarides. This upwelling also weakened the upper plate of the easternmost part of the Alpine orogen and induced widespread crust–mantle decoupling, thus facilitating Pannonian extension and roll-back subduction of the Carpathian oceanic embayment. The second slab-tearing event triggered uplift and peneplainization in the Eastern Alps while opening a second slab gap, still present between the Eastern and Central Alps, that was partly filled by northward counterclockwise subduction of previously unsubducted Adriatic continental lithosphere. In Miocene time, Adriatic subduction thus jumped westward from the Dinarides into the heart of the Alpine orogen, where northward indentation and wedging of Adriatic crust led to rapid exhumation and orogen-parallel escape of decoupled Eastern Alpine crust toward the Pannonian Basin. The plate reconstructions presented here suggest that Miocene subduction and indentation of Adriatic lithosphere in the Eastern Alps were driven primarily by the northward push of the African Plate and possibly enhanced by neutral buoyancy of the slab itself, which included dense lower crust of the Adriatic continental margin.

The origin of shape preferred orientations in mylonite: inferences from in-situ experiments on polycrystalline norcamphor

Abstract Polycrystalline norcamphor (C 7 H 10 O) undergoing plane strain simple shear in a see-through deformation rig develops intergranular microshears whose activity and orientation is closely related to shape preferred orientation (SPO) of dynamically recrystallized grains and grain aggregates. Intergranular microshears nucleate at 80–90° to the sample’s shear zone boundary (SZB) and rotate synthetically toward this boundary. They accommodate increasing amounts of incremental shear strain at angles ranging from 40° to 60° ( S b orientation) with respect to the SZB. With progressive simple shear, both the rotation rate and the amount of strain accommodated by these microshears decrease, but strain accommodation increases as the microshears attain a lower angle (10–30°, S a orientation) to the SZB. The microshears gradually deactivate as they acquire inclination angles to the SZB of less than 10°. The deactivation of such microshears is accompanied by the nucleation of fresh, high angle microshears. This heterogeneous deformation is associated with two shape preferred orientations in the norcamphor mylonite: A steep, oblique grain SPO comprising the long axes of dynamically recrystallized grains (40–60°) subparallel to the S b -oriented microshears and gently inclined domainal SPO (20°) subparallel to the S a -oriented microshears. The domain SPO is defined by the length axes of grain aggregates with a uniform crystallographic preferred orientation. The angle between domainal and grain SPOs is a potential measure of the bulk vorticity during mylonitization.

Decoupling and its relation to strain partitioning in continental lithosphere: insight from the Periadriatic fault system (European Alps)

Abstract The Periadriatic fault system (PFS) is an array of late orogenic faults (35-15 Ma) in the retro-wedge of the Alpine orogen that accommodated dextral transpression during oblique indentation by the southern Alpine crust. Decoupling along the leading edges of the southern Alpine indenter occurred where inherited lithological and rheological contrasts were accentuated by lateral thermal gradients during emplacement of the warm orogenic retro-wedge next to the cold indenter. In contrast, decoupling within the core and retro-wedge of the orogen occurred in a network of folds and mylonitic faults. In the Eastern Alps, this network comprises conjugate sets of upright, constrictional folds, strike-slip faults and low-angle normal faults that accommodated nearly coaxial NNE-SSW shortening and E-W extensional exhumation of the Tauern thermal dome. The dextral shear component of oblique convergence was taken up by a discrete, brittle fault parallel to the indenter surface. In the Central and Western Alps, a steep mylonitic backthrust, upright folds, and low-angle normal faults effected transpressional exhumation of the Lepontine thermal dome. Mylonitic thrusting and dextral strike-slip shearing along the steep indenter surface are transitional along strike to low-angle normal faults that accommodated extension at the western termination of the PFS. The areal distribution of poles to mylonitic foliation and stretching lineation of these networked structures is related to the local shape and orientation of the southern Alpine indenter surface, supporting the interpretation of this surface as the macroscopic shearing plane for all mylonitic segments of the PFS. We propose that mylonitic faults nucleate as viscous instabilities induced by cooling, or more often, by folding and progressive rotation of pre-existing foliations into orientations that are optimal for simple shearing parallel to the eigenvectors of flow. The mechanical anisotropy of the viscous continental crust makes it a preferred site of decoupling and weakening. Networking of folds and mylonitic fault zones allow the viscous crust to maintain strain compatibility between the stronger brittle crust and upper mantle, while transmitting plate forces through the lithosphere. Decoupling within the continental lithosphere is therefore governed by the symmetry and kinematics of strain partitioning at, and below, the brittle-to-viscous transition.