Stratigraphic control of slip localization within the Cambro-Ordovician Sedimentary Sequence during the late stages of emplacement of the Moine Thrust Zone, Scotland

Abstract

Geodynamic models of orogenic wedges usually involve significant weakening occurring along basal detachments to allow large-scale horizontal displacement. The mechanical weakening that allows these structures to accommodate major horizontal shortening is commonly attributed to the presence of mineralogically weak fault rocks, the development of overpressured pore fluids or other various strain-induced softening mechanisms related to grain-size reduction.
Here, we explore the role of lithological control over weakening mechanisms - and the potential role played by dynamic weakening during co-seismic slip - that lead to the localization of slip along specific units in the tectonic context of large-scale thrust faults, employing an approach that combines field observations, petrography and microstructural documentation and frictional experiments to investigate the sedimentary rocks of the Cambro-Ordovician sequence of the Moine Thrust Zone foreland.
The Cambro-Ordovician shallow-marine sedimentary succession was involved in an intricate system of imbricates and duplexes within the structurally lower thrust sheets of the Moine Thrust Zone, during the late stages of emplacement of this Caledonian thrust belt. The fine-grained clastic units and carbonate-dominated formations of this sedimentary succession have been appointed as the lithologies in which slip tends to localise within the imbricates and the detachments. The slip localization is, generally, attributed to frictional weakness of these sedimentary units.
Field work corroborated the observations previously made by other authors, and expanded on that, appointing other stratigraphic unis potentially localizing slip. At a grain-scale, the rocks involved in the lower structural sheets of the Moine Thrust Zone deform primarily in a brittle manner, with a significant additional contribution from fluid-assisted diffusion mechanisms. Pressure solution, evidenced by changes in the shape of minerals along cleavage surfaces and the presence of dissolution seams and caps, is widespread throughout the studied rock sequences. The profuse occurrence of this grain-scale mechanism makes it very likely a syn-deformational fluid-influx occurred. This is substantiated by the anomalously high content of authigenic K-feldspar found particularly to affect the fault rocks derived from the fine-grained clastic units of the Cambro-Ordovician sequence. The compositional heterogeneities within the sedimentary succession are enhanced by the K-enriched fluid alteration affecting the lithologies differently, and have markedly influenced the way that these rocks accommodate deformation.
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Cambro-Ordovician powdered rocks were tested in a rotary shear apparatus, to investigate the frictional strength and stability of the gouges. The mechanical data derived from these experiments show that, different from what was previously proposed, the rheological contrast produced by the frictional weakness of the Cambro-Ordovician protoliths is not enough plausibly to explain the deformation localization observed in the field. Our experiments demonstrated that the fine-grained clastic and carbonate-dominated lithologies – previously appointed as weak layers - exhibit a substantial shift from original frictional and stability behaviours, following an imposed episode of co-seismic slip, particularly in the presence of a hydrous fluid. Initially, all the investigated lithologies exhibit similar frictional strength and velocity-strengthening behaviour (prone to stable sliding). After a simulated earthquake is imposed, the fine-grained clastic units change drastically their behaviour to a consistent velocity-weakening response – this is a shift to an unstable behaviour, that allows the material to host and, furthermore, nucleate other earthquakes. The carbonate-dominated gouges retain their stable behaviour but show a significant drop in frictional strength. The units that show a significant change in frictional response are those largely observed to concentrate slip within the MTZ lower sheets. Based on these results, we propose an alternative model to account for those observations and to help explain the observed strain partitioning in the field.
Our work on the Cambro-Ordovician synthetic gouges suggest that beyond lubricating faults during co-seismic slip, the occurrence of a seismic slip episode can trigger lasting changes in frictional strength and stability of fault gouges. These findings have important repercussions for earthquake propagation at shallower crustal depths and provide fresh perspective on the role played by the frictional properties controlling deformation localization within the foreland sedimentary rocks of Moine Thrust Zone and potentially other foreland fold and thrust belts.