Fold and thrust systems in Mass Transport Deposits

Abstract

Improvements in seismic reflection data from gravity-driven fold and thrust systems developed in offshore Mass Transport Deposits (MTDs) reveal a number of significant features relating to displacement along thrusts. However, the data are still limited by the resolution of the seismic method, and are unable to provide detail of local fold and thrust processes. Investigation of exceptional gravity-driven contractional structures forming part of MTDs in lacustrine deposits of the Dead Sea Basin, enables us to present the first detailed outcrop analysis of fold and thrust systems cutting unlithified ‘soft’ sediments. We employ a range of established geometric techniques to our case study, including dip isogons, fault-propagation fold charts and displacement-distance diagrams previously developed for investigation of thrusts and folds in lithified rocks. Fault-propagation folds in unlithified sediments display tighter interlimb angles compared to models developed for lithified sequences. Values of stretch, which compares the relative thickness of equivalent hangingwall and footwall sequences measured along the fault plane, may be as low as only 0.3, which is significantly less than the minimum 0.5 values reported from thrusts cutting lithified rocks, and reflects the extreme variation in stratigraphic thickness around thrust-related folds. We suggest that the simple shear component of deformation in unlithified sediments may modify the forelimb thickness and interlimb angles to a greater extent than in lithified rocks. The average spacing of thrust ramps and the thickness of the thrust sequence display an approximate 5:1 ratio across a range of scales in MTDs. In general, thicker hangingwall and footwall sequences occur with larger thrust displacements, although displacement patterns on thrusts cutting unlithified (yet cohesive) sediments are more variable than those in lithified rocks. Line-length restoration of thrust systems in MTDs reveals 42% shortening, which reduces to 35% in overlying beds. A 23% reduction in shortening by folding and thrusting along individual thrusts suggests that heterogeneous lateral compaction may increase by ∼10% towards the sediment surface. Thrust systems cutting unlithified sediments display distinct steps in cumulative displacement-distance plots representing increased rates of slip along the floor thrust, while displacement-distance plots along individual thrusts also reveal ‘horizontal steps’ relating to lithological variation. Competent units cut by thrust ramps may display the greatest displacement, which then progressively reduces both upward and sometimes downward along the ramp. This relationship demonstrates that ramps do not necessarily propagate upwards from the underlying flat as in some traditional models, but rather initiate by offset of competent horizons in the hangingwall of the detachment. Critical taper angles in MTDs may be an order of magnitude less than in accretionary complexes or lithified rocks. Overall, thrusts cutting unlithified sediments in MTDs display more variable displacement, and more pronounced displacement gradients toward fault tips, compared to thrusts cutting lithified sequences.