Distinguishing coeval patterns of contraction and collapse around flow lobes in mass transport deposits

Abstract

Gravity-driven mass transport deposits (MTDs) form by the downslope-directed movement of sediment associated with slope failure. Simple models suggest that extension forms at the upslope (head) area, contraction is focussed in the downslope toe of the slump, while differential shear associated with strike-slip is restricted to the lateral margins of the slump. Although the head and toe are considered to be dominated by layer-parallel shear (LPS), differential layer-normal shear (LNS) may be generated around the lateral margins of slumps and potentially also within MTDs where flow has been separated into different ‘lobes’. Despite this realisation that LNS must form, there has been little work into the geometries and spatial relationships of resulting structures. Using the late Pleistocene Lisan Formation exposed around the Dead Sea Basin as our case study, we examine detailed (<10 m) relationships of folds and thrusts created during LNS and LPS, as well as investigating the role of broadly coeval extension that may reactivate these structures. We also undertake analysis of anisotropy of magnetic susceptibility (AMS) fabrics to determine flow and shear relationships around folds and detachments created during LNS and LPS. Our study shows that LPS results in gently-curvilinear fold hinges that arc around the transport direction while LNS results in cylindrical fold hinges developed oblique or sub-parallel to transport. Such folds may be recumbent or upright, and associated with lateral ramps marking areas of differential LNS within the MTD. These structures are interpreted to accommodate variations in the amount and direction of downslope-directed movement resulting in LNS around the margins of individual flow ‘lobes’ that are developed over tens of metres. These ‘lobes’ display broadly down-slope transport with locally radial flow that results in along-strike shortening between lobes. Our analysis of AMS fabrics shows that they are controlled by slump folds, but magnetic fabrics do not differentiate how these folds were created in zones of LPS or differential LNS. AMS taken from gouge formed along detachments marked by differential LNS provide a first-order indicator for the transport direction. In addition, AMS fabrics in gouge or fluidised layers directly beneath thrust ramps, reveals prolate fabrics marking a component of strike-parallel flow along the branching intersections of thrust ramps and flats. Extensional faults directly reactivate existing thrusts, or create new extensional faults that are sub-parallel to thrusts or cut across them at steeper angles. Extension is part of the same MTD event as a sedimentary cap that is deposited out of suspension following slope failure, overlies and locally thickens into the hangingwall of extensional faults to create ‘growth’ sequences. Extensional reactivation and ‘collapse’ of original thrusts may help explain why contraction is apparently ‘missing’ from many seismic sections across MTDs.