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Effects of Basement Structure, Sedimentation and Erosion on Thrust Wedge Geometry: An Example from the Quebec Appalachians and Analogue Models
The Taconian fold and thrust belt of the Quebec Appalachians displays typical structures such as inverted normal faults, ramp and flat structures, sub-horizontal detachments, triangle zones and backthrusts. The development of these structures is not, however, consistent along the belt and seems to be spatially related to variations in palaeotopography and stratigraphic architecture of the Middle–Late Ordovician foreland basin, which developed in front of the Taconian tectonic wedge. A triangle zone is bounded by backthrusts in the southeastern limb of the Chambly-Fortierville syncline and by imbricate faults of the thrust wedge parautochthonous units. The triangle zone dies out to the northeast as the basement shallows. Reinterpretation of reflection seismic lines shows that there is a link between the structural geometry of the thrust belt and depth to basement and the presence of pre-existing basement structures. The deflection of frontal thrusts relative to the general SW–NE strike of the orogen occurs over an oblique, deep-seated basement escarpment.
In order to better understand factors controlling the formation of structures in the study area, analogue sandbox modelling was used to study thrust systems developed above irregular basement structures and affected by syntectonic sedimentation and erosion. Sand layers were constructed with a decollement level; step-like escarpments were introduced in the model’s basement, oriented either parallel or oblique to the shortening direction. Basement depth variations in models controlled the geometry of thrust wedges. Deflection and rotation of frontal detachments was influenced by the presence of an oblique, steep basement escarpment in front of the growing wedge, by gradual changes of sand layer thickness across the ramp and by differential slip along the basal detachment. Deflection developed due to differential forward motion of the hanging wall along the strike of the fault and counter-clockwise rotation about a vertical axis. Syntectonic sedimentation induced forward propagation of flat detachments, resulting in the formation of a piggyback basin. Syntectonic erosion and sedimentation affected the thrust wedge kinematics; a series of backthrusts and/or triangle zones developed at the rear of the wedge. These structures developed above the deeper-seated basement and died out towards basement highs. Our modelling results and geological data support the hypothesis that pre-existing basement structure, syntectonic sedimentation, and erosion could be responsible for the development of triangle zones and frontal thrust deflection in the Quebec Appalachians.
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