Upslope-verging back thrusts developed during downslope-directed slumping of mass transport deposits

G.I. Alsop, S. Marco, R. Weinberger, T. Levi

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Abstract

While much research has recently been focussed on downslope-verging systems of gravity-driven fold and thrust belts within mass transport deposits (MTDs), rather less attention has been paid to back thrusts, which are defined as displaying the opposite vergence to the main transport direction in thrust systems. A fundamental question arises over whether back thrusts in downslope-verging MTDs record actual movement back upslope. In order to address this issue, we have examined exceptional outcrops of Pleistocene fold and thrust systems developed in MTDs around the Dead Sea Basin. Back thrusts can be interpreted in terms of a ‘downslope-directed underthrust model’, where material moves down slope and is driven into the footwall of the back thrust, resulting in the ‘jacking up’ of the largely passive hangingwall. Our data support this underthrust model and include the observation that stratigraphic units may be markedly thickened (up to 250%) in the footwall of back thrusts. This thickening is a consequence of pure shear lateral compaction as the ‘wedge’ of sediment is driven into the footwall to create an underthrust. In addition, back thrusts may be rotated as new back thrusts form in their footwalls, ultimately resulting in overturned thrusts. The observation that steeper back thrusts typically accommodate less displacement than gently-dipping back thrusts suggests that steepening occurred during back thrusting, and is therefore a consequence of ‘footwall wedging’. Contrary to some recent interpretations, we demonstrate that back thrusts can develop in gravity-driven systems and cannot therefore be used to distinguish different emplacement mechanisms for MTDs.
Original languageEnglish
Pages (from-to)45-61
Number of pages17
JournalJournal of Structural Geology
Volume100
Early online date15 May 2017
DOIs
Publication statusPublished - Jul 2017

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slumping
mass transport
thrust
footwall
gravity
fold and thrust belt
compaction
emplacement
outcrop

Keywords

  • back thrust
  • MTD
  • slump
  • Dead Sea Basin

Cite this

Upslope-verging back thrusts developed during downslope-directed slumping of mass transport deposits. / Alsop, G.I.; Marco, S.; Weinberger, R.; Levi, T.

In: Journal of Structural Geology, Vol. 100, 07.2017, p. 45-61.

Research output: Contribution to journalArticle

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abstract = "While much research has recently been focussed on downslope-verging systems of gravity-driven fold and thrust belts within mass transport deposits (MTDs), rather less attention has been paid to back thrusts, which are defined as displaying the opposite vergence to the main transport direction in thrust systems. A fundamental question arises over whether back thrusts in downslope-verging MTDs record actual movement back upslope. In order to address this issue, we have examined exceptional outcrops of Pleistocene fold and thrust systems developed in MTDs around the Dead Sea Basin. Back thrusts can be interpreted in terms of a ‘downslope-directed underthrust model’, where material moves down slope and is driven into the footwall of the back thrust, resulting in the ‘jacking up’ of the largely passive hangingwall. Our data support this underthrust model and include the observation that stratigraphic units may be markedly thickened (up to 250{\%}) in the footwall of back thrusts. This thickening is a consequence of pure shear lateral compaction as the ‘wedge’ of sediment is driven into the footwall to create an underthrust. In addition, back thrusts may be rotated as new back thrusts form in their footwalls, ultimately resulting in overturned thrusts. The observation that steeper back thrusts typically accommodate less displacement than gently-dipping back thrusts suggests that steepening occurred during back thrusting, and is therefore a consequence of ‘footwall wedging’. Contrary to some recent interpretations, we demonstrate that back thrusts can develop in gravity-driven systems and cannot therefore be used to distinguish different emplacement mechanisms for MTDs.",
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N1 - Acknowledgements SM acknowledges the Israel Science Foundation (ISF grant No. 1436/14) and the Ministry of National Infrastructures, Energy and Water Resources (grant #214-17-027). RW was supported by the Israel Science Foundation (ISF grant No. 1245/11).

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