Noncoaxial macroscopically plastic deformation of weakly cemented rudaceous siliciclastic sediments – a remarkable occurrence from western Anatolia
Nicholas J. Verge
2, Chestnut Close, Middle Assendon, Henley-on-Thames, Oxfordshire, RG9 6AY
nickverge@earthmachine.freeserve.co.uk
At shallow crustal levels, where a fault develops within weakly cemented sediments of low strength, sediment deformation to facilitate relative motion of the fault walls, may be distributed through a greater volume than had the sediments been cemented to a greater degree and consequently, stronger. Where this occurs, because of the low strength of such sediments, it is possible that they may undergo distributed non-coaxial ductile deformation as a result of being able to flow in a macroscopically plastic manner. Described is a remarkable occurrence of what is thought to be such deformation in extremely rudaceous siliciclastic sediments that were then, and still are, weakly cemented.
Within the western Anatolian segment of the modern Afro-Eurasian Plate Convergence Zone, mid-Cenozoic extreme horizontal extension-vertical thinning of tectonically thickened continental lithosphere was facilitated by a system of extensional detachments, which resulted in the exhumation of deep-crustal rocks of the Menderes Metamorphic Core-complex, as their footwall. In contrast, the hangingwall of the active detachment system was the site of the accumulation of mostly footwall derived & commonly boulder-grade, siliciclastic fluvial sediments, deposited across alluvial fans, within a supa-detachment extensional basin. Within these sediments, an isolated zone has been found, of very limited extent, wherein they possess a marked LS-shape fabric. This is manifest by a preferred alignment of the strongest lithoclasts & matrix grains; strings of clast-/grain-fragments resulting from the brittle breakage and pull-apart of weaker ones along tensional and shear fractures; and by highly elongated aggregates of fine-grained rock fragments derived from the pervasive cataclasis of the weakest clasts/grains. Also present are: a dominant set of metre- to millimetre-scale ductile fabric-relative extensional matrix shear zones; asymmetric matrix strain shadows around clasts that stair-step in an identical sense, together with clasts & arrays of clast fragments that have undergone a consistent senses of rotation &/or extensional relative displacement to produce imbricate & V-pull-apart structures. On this basis of the presence of these structures it is inferred these sediments experienced a high degree of noncoaxial deformation during which they & especially their finer matrix, were able to deform by macroscopic plastic flow.
No crystal-plastic deformation of the common rock-forming minerals or metamorphic mineral growth occurred within these sediments during their deformation. Instead, it is concluded that their macroscopically plastic flow is inferred to have been by grain boundary sliding and clast/matrix grain brittle deformation. However, despite this, these sediments now possess macro-(& perhaps micro-)structures resembling those typical of porphyroclastic mylonites produced by non-coaxial deformation at much higher temperatures. Consequently, the term soft-sediment "pseudomylonite" is suggested for sediments exhibiting such structures as a result of having been highly deformed, in the above described manner, at very low temperatures, whilst having been only weakly cemented.