Deformation of the crust is believed to occur dominanlly by cataclasis at low temperatures and/or effective confining pressure, by pressure solution at intermediate temperatures, and by dislocation creep at high temperatures. Each flow mechanism gives rise to distinctive microscopic and small scale structures.
Brittle deformation with grain fracture leading to a reduction of particle diameter is characteristic of cataclastic flow.
Pressure solution produces grain shape fabrics by intercrystalline diffusion assisted by the presence of water. Grains may change shape at constant mass, or decrease in mass (and therefore in size) by long range diffusion: mass is then not locally conserved. Reduction of grain diameter leads to increased rates of deformation (strain softening).
Distinctive spaced cleavage zones form by pressure solution in which mineral species are redistributed due to different rates of deformation: the displacement field is discontinuous and deformation non-isochemical. Tectonic veins associated with pressure solution structures probably form by local mass transport; thus brittle and ductile mechanical behaviour coexist.
Dislocation creep produces grain shape fabrics by intracrystalline deformation. and may cause grain size reduction by subgrain formation and recrystallization. Preferred crystallogra-phic orientations can arisefromdislocation glide. Mass is conserved and deformation is believed to be essentially isochemical. Small scale structures formed by dislocation creep are ductile, with a continuous displacement field.
The geologic record of the Archean in the Superior Province of Canda is divisible into two major portions; an older, high grade basement (>3.5 Ga), and younger (3.0 Ga to 2.6 Ga) rocks that are arranged in easterly-trending zones (Ga, Gigayear, is used to mean 109 years). The younger rocks comprise alternating volcanic (low grade greenstone superbelts) and sedimentary (paragneiss superbelts) terrains. The contrasting lithologie and metamorphic nature of the superbelts is tentatively explained in terms of a mantle convection cell system. Closely analogous Archean assemblages of southern Africa developed about 0.5 Ga earlier, attesting to the heterogeneous nature of the Archean crust and its diachronous evolution in different parts of the world.
This photo-essay presents a few pictures showing some of the features of meta-ultramafic komatiitic submarine flows of Archean age from the spectacular La Motte - Vassan ultramafic belt, situated northwest of Val d'Or in northwest Quebec. The writer has spent the past five field seasons ( 1972-1977) in this region and the volcano-sedimentary foreland in Abitibi-est, Temiscaningue and Rouyn-Noranda counties. The material shown here is part of a larger publication (Imreh. 1978) produced by the Québec Ministère des Richesses naturelles.
Spinifex textures are shown in Figures 1 and 2: microspinifex in Figure 4 and (at a larger scale) Figure 5. Not all flows show spinifex: Figure 3 shows the contact between two flows without spinifex. Micro-spinifex in pockets within massive flows, as shown in Figures 4 and 5. is always limited to the upper parts of flows without spinifex zones Details of spinifex textures, as well as mega-pillow structure (Figure 6) can be used to indicate the tops of the flows
Les six photos choisies montrent quelques-uns des traits morpho-faciologiques caractéristiques des coulées méta-ultramafiques komatiitiques archéens tels que vus sur les affleurements. Les coulées avec et sans zone à spinifex sont passées en revue ainsi que la morphologie de la surface figée, indicateur irréfutable de la nature de la mise en place et de la polarité des laves ultramafiques sous-marines. Les photos ont été prises dans le sillon majeure de la Motte-Vassan (Abitibi-Est) où ces coulées sont particulièrement bien développées.
Ces photos sont publié avec l'autorisation du Ministère des Richesses naturelles du Québec.