Abstract
Abstract Whether thrusts are ramp-dominated and form imbricate fans or run out onto the syn-orogenic surface, forming ‘thrust-allochthons’, is governed by the activity of secondary ‘upper’ detachments along the syn-orogenic surface, activations of which are inhibited by syn-kinematic sedimentation at the thrust front. In the northern Apennines, where thrust systems are ramp-dominated and form an emergent imbricate fan, syn-kinematic sedimentation was abundant and accumulated ahead and above each thrust. In the southern Apennines, the far-travelled Lagronegro allochthon achieved its high displacements (>65 km) while the foredeep basin received little sediment. The imbricate fan at the front of the main Himalayan arc developed within a foredeep that experienced high rates of syn-kinematic sedimentation. In contrast, further west, the Salt Range Thrust emerged into a distal, weakly developed foredeep with significantly reduced rates of sediment accumulation. Displacements were strongly localized onto this thrust (c. 25 km displacement) which activated an upper detachment along the syn-orogenic surface. It is an arrested thrust-allochthon. Lateral variations into the adjacent, ramp-dominated but still salt-detached, Jhelum fold-belt are marked by increases in syn-kinematic sedimentation. As sedimentation styles can vary in space and time, individual thrusts and thrust systems can evolve from being allochthon prone to imbricate dominated.
Highlights
Kinematic explanations of fold–thrust structures are commonly illustrated graphically as developed in stratigraphic templates that are laterally unvarying
The aim of this paper is to explore these influences, the role of sedimentation at the toe of a thrust sheet, on the gross structure of thrust systems
Imbricate fan tectonic contraction (a) no active upper detachment basal detachment = floor thrust (b) active upper detachment = thrust flat ramp single thrust surface single thrust (b) multiple array of thrust ramps tectonic contraction future thrust-allochthon basal detachment = lower thrust flat imbricate fan (a) cumulative displacement (c) distance on restored section are ramp-dominated so displacements are significantly less than the thickness of strata involved in the structure
Summary
Kinematic explanations of fold–thrust structures are commonly illustrated graphically as developed in stratigraphic templates that are laterally unvarying. Emergent imbricate fans and duplexes both rely on regionally extensive detachment horizons, such as over-pressured shales or evaporites, to preferentially form floor-thrusts It is these geometries that, since the work of Cadell (1889), have been widely reproduced in analogue models (reviewed by Graveleau et al 2012), interpreted from seismic sections through accretionary prisms at subduction zones In the settings described here, the structures are formed in, and at the margins of, foredeeps: they are foreland fold– thrust belts It is the surface processes of deposition rather than erosion that are likely to be more important. Analogue models show that the spacing and geometry of imbricate thrusts, together with their relative timing and activity, change depending on syn-kinematic sedimentation (Storti & McClay 1999; Bonnet et al 2008; Barrier et al 2013). Elsewhere it is argued that the relative partitioning of sedimentation ahead of the thrust wedge strongly influences the geometry of the thrust belt (Butler et al 2019)
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