Simon Hanmer

Asymmetrical pull-aparts and foliation fish as kinematic indicators

Abstract From field observations, asymmetrical boudins, pinch-and-swell structures and ellipsoidal volumes of oblique foliation segments in anisotropic rocks lacking competent layers are potentially useful kinematic indicators in shear zones. They are derived by the modification of initially symmetrical pull-aparts and/or layering by superposed layer-parallel shearing. The response of boudins to such shear is a synthetic flow of those corner volumes lying within the extensional quadrants of the kinematic framework of the deformation into the inter-boudin gaps (Type 1 asymmetrical pull-aparts). The response of pinch-and-swell structure is a back-rotation of the ‘swells’ to form Type 2 asymmetrical pull-aparts. Discrete extensional shears in the ‘pinches’ may be either absent (Type 2A) or present (Type 2B). Analogues of Types 2A and 2B asymmetrical pull-aparts occur in anisotropic materials with no compositionally distinct competent members where ellipsoidal volumes contain back-rotated foliation segments (foliation fish). The field-based kinematic interpretation was successfully tested by laboratory experiment wherein initially symmetrical trains of pre-formed competent, internally anisotropic boudins and pinch-and-swell structures were deformed in bulk simple shear. A mechanical model is presented in terms of two material properties of the inclusions (competence/viscosity and internal rheological anisotropy) in an attempt to account for the observed flow in the experiments and the deduced flow and kinematic significance of the natural examples. In the presence of angular corners and associated geometrical stress concentrations, competence/viscosity controls the response to shear in the boudins. In the absence of angular corners, internal anisotropy controls the response to shear in the pinch-and-swell structures and in the foliation fish.

Anorthosites and related granitoids in the Grenville orogen: A product of convective thinning of the lithosphere?

In the central and southwestern Grenville province in Canada and the northeastern United States, anorthosites and related granitoids were emplaced during two distinct pulses, at ca. 1.16–1.13 Ga and 1.09–1.05 Ga, each following major crustal thickening events. U-Pb age constraints on movement along major ductile shear zones indicate that they were emplaced within extending crust in an overall convergent orogen. We suggest that convective thinning of the lithosphere played a significant role in their emplacement and in the tectonic evolution of the Grenvillian orogeny.

Great Slave Lake Shear Zone, Canadian Shield: reconstructed vertical profile of a crustal-scale fault zone

Abstract The Great Slave Lake Shear Zone (GSLSZ) of the northwestern Canadian Shield, a 25 km wide, early Proterozoic, transcurrent, dextral, mylonite zone, is a type example of a crustal-scale fault zone profile. With time: 1. (1) the metamorphic grade decreased from granulite to greenschist facies; 2. (2) the locus of high strain narrowed and jumped laterally, abandoning relatively older mylonites. Progressively younger mylonites represent both cooler temperatures and lower pressures; thus the sequence of narrowing mylonite belts represents a series of progressively shallower seated sections through the GSLSZ; 3. (3) deformation evolved through the brittle-ductile transition, initially producing non-dilational penetrative breccias, followed first by discrete non-dilational faulting and subsequently by dilational, quartz “stockworks”. Developing crustal-scale shear zones grow to an optimum width dictated by the requirement of minimum geologically reasonable strain rates for given conditions of wall rock rheology and displacement rate. Independent of variations in relative plate motion, mature shear zones change width and migrate laterally as a function of inferred thermally activated rheological changes within the wall rocks.

Petrofabric, P-wave anisotropy and seismic reflectivity of high-grade tectonites

Abstract Ji, S., Salisbury, M.H. and Hanmer, S., 1993. Petrofabric, P-wave anisotropy and seismic reflectivity of high-grade tectonites. Tectonophysics, 222: 195–226. In order to understand seismic anisotropy and its influence on the reflectivity of lower crustal fault zones, we have undertaken experimental measurements of P-wave velocities up to 600 MPa for 20 granulite and upper amphibolite facies mylonites from the Snowbird tectonic zone (Canada). The rocks, whose composition ranges from ultramafic to felsic, have experienced extremely large ductile strain. It is found that the amphibolitic mylonites exhibit significant Vp anisotropy (6–13% at 600 MPa), whereas the pyroxene-bearing granulite-facies mylonites and the quartz or feldspathic (tonalitic, granitic and diatexitic) mylonites are quasi-isotropic. In order to constrain interpretation of the measured Vp properties, microstructural analyses have been systematically performed in the samples studied. It is shown that the interaction between feldspar and quartz or pyroxene and the absence of amphibole cause anisotropy in the granulite facies mylonites to be low. The amphibolitic mylonites are strongly anisotropic because of a large volume fraction of hornblende with strong LPO ([001]∥lineation, and (100)∥foliation). The anisotropy pattern in the amphibolitic mylonites is consistently orthorhombic: Vp(X) >Vp(Y) >Vp(Z), where X is parallel to stretching lineation, Y is normal to the lineation in the foliation plane and Z is normal to the foliation plane. But the anisotropy pattern in the mica-rich mylonites is transversely isotropic: Vp(X) = Vp(Y) >Vp(Z). If sufficiently large volumes of the crust display such patterns, this difference may be an important indicator for kinematic analysis in the middle to lower crust using modern seismic techniques. The study demonstrates that while seismic reflectivity is strongly lithology-controlled, fabric-induced anisotropy can both enhance and decrease seismic reflectivity. Hence, the discontinuity of seismic reflections along ductile shear zones in the lower crust is not necessarily indicative of the discontinuity of the shear zones, but may indicate changes in composition or metamorphic grade.

Magnetotelluric and teleseismic study across the Snowbird Tectonic Zone, Canadian Shield: A Neoarchean mantle suture?

[1]xa0The Snowbird tectonic zone (STZ) is a fundamental boundary within Canadas Western Churchill Province, one of the worlds largest yet poorly-known fragments of Archean crust. Geophysical data from a collocated magnetotelluric and teleseismic transect across the northeastern segment of the STZ provide an image of its subsurface geometry and indicate that it may have been previously mislocated. The model suggests that (1) the STZ has played a major role in the Neoarchean assembly and Paleoproterozoic reworking of the western Canadian Shield, (2) it was reactivated in a manner comparable to other crustal-scale features such as the Kapuskasing zone of the Superior Province, Canada, and the Redbank thrust of the Arunta block, central Australia, and (3) it juxtaposes mantle blocks with contrasting geophysical properties, revealing a lithosphere-scale overlap of the leading edges of the Rae and Hearne domains. The STZ thus records plate interactions in the Neoarchean comparable in scale with that of modern orogenic belts.

Back folds in the core of the Himalayan orogen: An alternative interpretation

The hanging wall of the South Tibetan detachment system in the central Nepal Himalaya is characterized by regional-scale, northeast-verging folds, classically interpreted as gravity-induced structures developed during down-to-the-north extensional shearing along the detachment system. New structural observations and balanced cross sections of the Tethyan sedimentary sequence in the Kali Gandaki area and new U-Pb geochronology support an alternative interpretation. The northeast-verging folds developed before ductile extensional shearing along the detachment system, thereby recording some of the earliest contraction of this part of the orogen. We propose a new model in which the northeast-verging folds of the Kali Gandaki area represent the northern part of a late Eocene to Oligocene contractional fan structure.

Great Slave Lake shear zone, NW Canada : mylonitic record of Early Proterozoic continental convergence, collision and indentation

Abstract Great Slave Lake shear zone, in the NW Canadian Shield, is an excellent example of the kinematic, magmatic and thermal evolution of a crustal-scale shear zone associated with a collisional continental boundary. It is a 25 km wide corridor of granulite to lower greenschist facies mylonites and cataclastic fault rocks developed in the deep-seated parts of an Early Proterozoic (2.0–1.9 Ga) magmatic arc, constructed on the upper (Rae) plate at the contact between the Archean Slave and Rae continents. The rocks of Great Slave Lake shear zone, the Thelon magmatic arc and the Taltson magmatic zone are all components of the same magmatic arc, but their geological histories reflect different aspects of the continental interaction.

Marble mylonites of the Bancroft shear zone: Evidence for extension in the Canadian Grenville

Detailed field mapping in marbles from the Central Metasedimentary belt (CMB) near Bancroft, Ontario, shows that isolated exposures of marble mylonite are part of a regionally extensive shear zone, the Bancroft shear zone. Shear-sense indicators in this southeast-dipping zone give movement of the hanging wall toward the southeast, indicating extension. Local crosscutting relationships indicate that the extension is younger than the regional thrusting event that produced the predominant structural fabric. On a regional scale, however, the extensional and compressional fabrics are essentially parallel. The marbles preserve a variety of textures ranging from light-colored, coarse samples to light-colored, weakly foliated samples with remnant coarse calcite grains to banded fine-grained samples to dark, extremely fine-grained samples. These various textures represent progressive stages in the development of marble mylonite, from protolith to ultramylonite. We conclude that the Bancroft shear zone marks the Bancroft-Elzevir subdomain boundary of the CMB and that late extension is responsible for the observed differences in lithology and metamorphic grade of these Grenville subdomains.

Modest movements, spectacular fabrics in an intracontinental deep-crustal strike-slip fault: Striding-Athabasca mylonite zone, NW Canadian Shield

Geometry and strain partitioning within lower-crustal intraplate strike-slip shear zones can be extremely complex, compared with analogous structural levels of interplate strike-slip shear zones sited at plate margins. Striding-Athabasca mylonite zone, Canadian Shield, is a spectacular ca. 500 km long granulite facies continental intraplate shear zone. The shear zone is composed of Middle Archean granulite facies annealed mylonites (ca. 3.13 Ga) and Late Archean (ca. 2.62-2.60 Ga) granulite facies ribbon mylonite belts, which thread a sinuous course along a chain of crustal-scale ‘lozenges’ cored by relatively stiff rocks of mafic to intermediate composition. To the northeast, the mylonites form a N-S-trending, 5–10 km thick, dextral strike-slip belt. To the southwest, this bifurcates into a pair of conjugate strike-slip shear zones, overlain by a contemporaneous dip-slip shear zone. n nStriding-Athabasca mylonite zone was kinematically inefficient as a strike-slip fault and cannot have accommodated large wallrock displacements. Nevertheless, spectacular granulite facies ribbon mylonites were formed throughout the shear zone, reflecting the very high temperatures (ca. 850–1000 °C), high recrystallization rate/strain rate ratios, and the transpressive nature of the deformation (Wk < 1), possibly accommodated by significant volume loss by magma migration.

Significance of crustal-scale shear zones and synkinematic mafic dykes in the Nagssugtoqidian orogen, SW Greenland: a re-examination

The published accounts of the structural geology and tectonic evolution of the Early Proterozoic Nagssugtoqidian orogen, SW Greenland, require modification in several fundamental respects. The geometry of the orogen has been attributed to important displacements on major conjugate shear zones and thrust zones. From new field observations, we are unable to confirm that crustal-scale shear zones have played an important role in Nagssugtoqidian tectonics. The ‘dextral’ component of ‘conjugate’ shear zones is demonstrably sinistral and transpressive, strike-slip shear zones are incipient, and a major thrust zone within the orogen cannot be confirmed at its projected inland location. The sinistral strike-slip Nordre Stromfjord shear zone is an order of magnitude smaller than previously thought. It comprises an array of non-linked segments of annealed mylonite and cannot have accommodated large displacements. The dextral ‘Itivdleq shear zone’ is characterised by heterogeneous, sinistral, noncoaxial flow, but compared with deformation elsewhere in the orogen, it is not a large-scale zone of strain localisation. ‘Ikertoq thrust zone’ is not an orogen-scale shear zone, but appears to be part of a large-scale, rheological boundary. Application of an en relais fracture array model to what is classically identified as the Kangaˆmiut mafic dyke swarm suggests that the sinistral vorticity determined throughout much of the Nagssugtoqidian orogen is detectable in the Archean foreland up to 150 km south of the orogenic front.