Fold and Thrust Systems in Mass‐Transport Deposits Around the Dead Sea Basin

Abstract

The late Pleistocene Lisan Formation outcropping around the Dead Sea preserves exceptional 3D exposures of fold and fault systems generated during soft-sediment slumping associated with mass-transport deposits (MTDs). Vergence of slump folds outcropping to the west of the Dead Sea is generally (>90%) toward the east and consistent with depocenter-directed movement of MTDs. On a regional scale, the direction of slumping inferred from the fold and thrust geometries systematically varies along the entire ~100 km length of the western Dead Sea Basin. The fold and thrust systems are interpreted to form part of a large-scale radial system of MTDs directed toward the basin's depocenter and to be triggered by earthquakes generated along the Dead Sea Fault. The progressive evolution of MTDs is broadly categorized into initiation, translation, cessation, relaxation, and compaction phases. Deformation may become most evolved where slope failure initiates and MTD movement commences. Consequently, many MTDs display the greatest deformation in their central area, and this diminishes toward the leading edge where MTD displacement was still initiating. Thrust packages may define piggyback sequences during MTD translation, while in other cases, overstep thrust sequences are formed, leading to a range of crosscutting and overprinting relationships. The recognition that MTDs may be reworked by younger seismically triggered events suggests that in some cases, the seismic recurrence interval may be shorter than anticipated. Our observations of heterogeneous lateral compaction, together with upright folding that represents an early phase of layer-parallel shortening, may help explain why some contraction is apparently missing from seismic sections across large-scale MTDs. In addition, conjugate normal faults that strike parallel to the dip of the paleoslope indicate a component of non-plane strain deformation associated with “out-of-plane” movement. MTDs in the Dead Sea Basin are best described as having open-ended toes (a variant of frontally confined toes) in which further movement is impeded by downslope strata that form a “soft” buttress. Observations that contraction and extension may overprint one another at any position within an MTD suggest that second-order flow cells may locally interact with one another and are linked to “pockets” of “surging” or “slackening” flow during overall downslope translation of the MTD. The spacing of thrust ramps within MTDs are linearly related to the thickness of the section to define an approximate 5 : 1 ratio across a range of scales. In summary, the MTDs in the Lisan Formation exposed around the Dead Sea Basin are triggered by seismic events, may be facilitated by mechanical heterogeneity linked to detrital-rich horizons, and are ultimately controlled by the paleoslope orientation.