Claudio L. Rosenberg

Deformation and recrystallization of plagioclase along a temperature gradient: an example from the Bergell tonalite

Abstract Syn- to post-mylonitic tilting of the Bergell tonalite allows investigation of the deformation and recrystallization of plagioclase grains along a temperature gradient from the lower to the upper amphibolite facies. At the lowest temperatures recrystallization occurs by nucleation and growth of new grains having a different composition from the old grains. In contrast, at the highest temperatures recrystallization occurs by subgrain rotation associated with grain boundary migration. The temperature increase is inferred to induce this transition in two ways. First, at higher temperatures the anorthite content of the new grains is higher and thus the comositional difference between old and new grains is lower. Hence, the chemical driving potential is small compared with recrystallization at lower temperatures. Second, higher temperatures facilitate climb of dislocations and thus subgrain formation as well as grain boundary mobility. The temperature increase is also associated with a transition in the dominant deformation mechanism. Intracrystalline plasticity dominates deformation at lower temperatures, whereas at higher temperatures deformation occurs primarily by diffusion-accommodated grain boundary sliding, as indicated by the weakening of the indicatrix preferred orientation and the formation of a mixed plagioclase–biotite matrix with increasing deformation. Therefore, the strength of the mid to lower crust may be overestimated by dislocation creep flow laws for plagioclase. Lower crust rheology for polyphase rocks is better approximated by constitutive relationships involving diffusion accommodated grain boundary sliding.

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.

Observations from the floor of a granitoid pluton: Inferences on the driving force of final emplacement

An east-west profile across the tilted Bergell pluton exposes a 10-km-thick interval in terms of crustal depth. Consequently, the floor as well as the root and “side” of the main intrusive body of the pluton crop out at the surface and a tentative three-dimensional geometry is constructed. At the highest crustal level, the geometry and deformation features at the margin of the pluton indicate ballooning, whereas the folded floor of the main intrusive body indicates synmagmatic shortening related to regional deformation. These contrasting features are best explained by shortening of the base of the pluton which caused an expansion at a higher crustal level. Final emplacement of the pluton into higher crustal levels was, therefore, not driven primarily by buoyancy, but rather by regional deformation within deeper levels of the crust.

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.

The role of fault zones and melts as agents of weakening, hardening and differentiation of the continental crust: a synthesis

Abstract The rheology of crustal fault zones containing melts is governed primarily by two strain-dependent mechanical discontinuities: (1) a strength minimum parallel to mylonitic foliation just below the active brittle-viscous (b-v) transition; (2) the anatectic front, which marks the upper depth limit of anatectic flow. The mode of syntectonic melt segregation in fault zones is determined by the scale of strain localization and melt-space connectivity, to an extent dependent on strain, strain rate and melt fraction in the rock. Melt drains from the mylonitic wall rock into dilatant shear surfaces, which propagate sporadically as veins. Anatectic flow at natural strain rates therefore involves melt-assisted creep punctuated by melt-induced veining. On the crustal scale, dilatant shear surfaces and vein networks serve as conduits for the rapid, buoyancy-driven ascent of transiently overpressured melt from melt-source rocks at or just below the anatectic front to sinks higher in the crust. Strength estimates for natural rocks that experienced anatectic flow indicate that melts weaken the continental crust, particularly in depth intervals where they spread laterally beneath low-permeability layers or along active shear zones with a pronounced mylonitic foliation. However, acute weakening associated wit h strength drops of more than an order of magnitude occurs only during short periods (103–105 a) of crustal-scale veining. Cooling and crystallization at the end of these veining episodes is fast and hardens the crust to strengths at least as great as, and in some cases greater than, its pre-melting strength. Repeated melt-induced weakening then hardening of fault zones may be linked to other orogenic processes that occur episodically (shifting centres of clastic sedimentation and volcanism) and has implications for stress transmission across orogenic wedges and magmatic arcs.

Indentation model of the Eastern Alps and the origin of the Tauern Window

A lithospheric model scaled for density and viscosity was shortened obliquely in front of an indenter whose shape and orientation match those of the South Alpine indenter. Deformation was initially localized into subparallel sets of conjugate transpressive thrusts. These structures evolved into folds, which were strongly amplified and converged closer together during ongoing shortening, finally forming a series of tight antiforms. At the same time, this tightly folded structure locally accommodated significant orogen-parallel extension, attaining 100% in front of the leading edge of the indenter, but only 15% on the scale of the entire model. The internal architecture and fault pattern of the model in map view are very similar to those of the Tauern Window in the Eastern Alps, emphasizing the importance of localization of folding and shortening rather than orogen-parallel extension for its formation.

Mechanisms and orientation of melt segregation paths during pure shearing of a partially molten rock analog (norcamphor–benzamide)

Abstract In-situ deformation experiments were performed on partially molten analog materials (norcamphor in the presence of a benzamide–norcamphor melt) undergoing pure shearing at a constant melt fraction of 0.13. Melt in the samples induces a strain-dependent transition from purely dislocation creep to dislocation creep associated with minor intergranular fracturing and grain boundary sliding (GBS). Intergranular fractures drain the melt from initially isotropic melt pockets to grain boundaries. Along such boundaries, grain-boundary migration recrystallization is inhibited, while GBS occurs. Intergranular melt pockets occur along grain boundaries oriented subparallel to the shortening direction, but melt must have migrated parallel to the elongation direction of the samples, as indicated by melt accumulations at both extruding ends of the sample. Intergranular melt pockets parallel to the elongation direction were only rarely observed, because melt was rapidly expelled from these sites. Nevertheless, these grain boundaries are the pathways of melt segregation in the samples.

Fracture-driven intrusion and upwelling of a mid-crustal pluton fed from a transpressive shear zone—The Rieserferner Pluton (Eastern Alps)

The Rieserferner Pluton was emplaced at a depth of 12–15 km into steeply dipping, greenschist-facies mylonitic rocks of the Austroalpine basement, just south of the Tauern Window (Eastern Alps). Intrusion occurred during north-south–directed shortening and east-west horizontal extension in front of the rigid Southern Alpine Indenter. The regional strain field is transpressive, with a strong coaxial component, and is characterized by east-west stretching. Tonalitic melt ascended through a feeder channel preserved in the steep southern part of the Rieserferner Pluton, within and adjacent to the steep mylonitic foliation of a major shear zone, the Defereggen-Antholz-Vals (DAV) Line. Melt ascent was perpendicular to the north-south shortening direction in the country rocks. Magma emplacement involved melt-induced hydro-fracturing to form a subhorizontal, tabular pluton that protruded northward from the DAV Line into the previously folded country rocks. During the late stage of emplacement, buoyant upwelling of the partly recrystallized magma induced doming of the pluton roof as well as vertical ductile shortening of the directly overlying country rocks. Magma pressure therefore locally exceeded the lithostatic pressure. Thermal modeling constrains the maximum time for doming and solidification of the pluton to have been 32,000 yr. The wavelength of the domes in the pluton roof indicates the viscosity contrast between country rock and partly crystallized tonalite in the pluton to have been at least 100:1.

Syntectonic melt pathways in granitic gneisses, and melt-induced transitions in deformation mechanisms

Abstract Partial melting of granodioritic gneisses in the contact aureole of the Bergell Pluton (Central Alps) occurred during regional deformation. Melting occurred in the presence of water, which was released from the pluton. The presence of melt is evidenced by local segregations of granite in shear zones, veins and dykes, and by grain-scale interstitial films of K-feldspar and quartz that do not occur in the unmelted protolith. These films are oriented parallel, as well as perpendicular to the foliation plane. In contrast, on the outcrop scale, cm-wide leucosome veins are oriented almost exclusively parallel to the foliation plane, indicating foliation-parallel flow. The partially molten granitic rocks contain dm-long clasts of restitic, well-foliated gneiss, showing a higher competence than their granitic matrix. K-feldspar is lacking in these clasts, the microstructure of which is characterized by elongate aggregates of quartz and feldspar, both dynamically recrystallized. In contrast, the granitic matrix is characterised by a random distribution of minerals, whith a shape preferred orientation defining a weak foliation. These microstructures are indicative of granular flow, whereas the microstructures of the clasts indicate dislocation creep involving dynamic recrystallization. The presence of K-feldspar controls the onset of melting and thus the transition from dislocation creep to granular flow. The weakening resulting from this transition is indicated by the formation of strong clasts in a weaker matrix.

The western termination of the SEMP Fault (eastern Alps) and its bearing on the exhumation of the Tauern Window

Abstract The SEMP (Salzach–Ennstal–Mariazell–Puchberg) Fault strikes along more than 300 km from the southern margin of the Vienna Basin to the northern Tauern Window accommodating a sinistral displacement of 60 km during Tertiary time. We present new structural data, showing that the SEMP Fault continues into the Tauern Window within a 50 km long mylonitic belt of approximately 2 km width, which we term the Ahorn shear zone. This sinistral shear zone, which marks the northern boundary of the Zentral Gneiss, strikes E to ENE, dips subvertically, and is characterized by gently W-dipping to subhorizontal stretching lineations. S-side-up kinematic indicators in the Y–Z fabric plane and a pronounced southward increase in the inferred temperature of sinistral shearing are observed within the shear zone. Microstructural observations indicate that deformation of quartz at the northernmost boundary of the Zentral Gneiss occurred by dislocation glide with only incipient dynamic recrystallization, suggesting a temperature of approximately 300 °C. Further south, temperatures greater than 300 °C are inferred because all samples are affected by dynamic recrystallization of quartz, and dynamic recrystallization of feldspars also occurred in the southernmost part of the shear zone. These findings point to transpressive deformation accommodating a significant component of south-side-up displacement in addition to sinistral shearing. The sinistral mylonitic foliation forms the axial-plane foliation of the large-scale, ENE-striking upright folds of the western Tauern Window. From east to west, deformation becomes increasingly distributed, passing from an area of interconnected shear zones in the east to a homogeneously deformed mylonitic belt in the west, which terminates into a belt of WNW-striking, upright folds. From the above, we suggest the following: (1) the SEMP Fault extended beyond the brittle-ductile transition to a depth where temperatures exceeded 500 °C (>20 km depth?). These mylonites should be included in the seismic interpretation profiles as a major crustal discontinuity; (2) the large-amplitude, upright folds of the Tauern Window formed at the same time as the sinistral mylonites, and hence during south-side-up differential displacement; and (3) part of the 60 km lateral displacement of the SEMP fault is transferred into a vertical displacement at the western end of the Ahorn shear zone and into a fold belt accommodating NNE-oriented shortening, west of the Ahorn shear zone.