thrust wedges with décollement levels and syntectonic erosion

7/29/2019 Thrust Wedges with Dcollement Levels and Syntectonic Erosion

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Thrust Wedges with DcollementLevels and Syntectonic Erosion: AView from Analog Models

Aycan Yildirim


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STRUCTURAL CONCEPTSThin-skinned deformation: Shortening that onlyinvolves the sedimentary covers.Thick-skinned deformation: Shortening thatinvolves basement rocks

M. Mouyen, et al. / Journal of Geodynamics, vol 48, (2009), p 284-291

Thin-skinned fold andthrust belts are wedgeshaped (Chappel, 1978).


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The relative strength of the dcollements and surrounding rocksaffects the development of active- or passive-roof duplexes (trianglezones).

dcollementMore intense deformation

Weaker deformation

DECOLLEMENT


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Backthrusting - UnderthrustingThese features can be explained Mechanical Stratigraphy concept.

The shale layers weaker than the better consolidated limestone layers. Thetheory states that as shales are weaker than limestone, the angle (1 between

1 and the fractures in shales must be smaller than the angle (2) between 1

and the fractures in limestones. The fractures in the shales are more

subhorizontal than the fractures in the limestones.


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No exumation onthe accreted basalmaterial

Critical Taperangle is measuredfrom this model.


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Effect of shortening rate on structural evolution ?In slowly shortened models >> a gentle topographic taper of about

12 during the early stages of deformation and was maintainedthroughout the deformation history.

In contrast, a high frontal taper of 10 or more formed in modelssubjected to rapid shortening.

In addition

The amount of friction along the dcollement affects the shape ofthe wedge.

A low-angle slope reflects a low-friction dcollementA higher-angle slope reflects a higher-friction dcollement


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Erosion

Exumation

Backthrusting


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The moreextensivebackthrustsdevelopment

at the rear ofthe wedgeMW3 is likelyfavored bylesser rate oferosion that

provided moretime to thewedge toreach criticaltaper betweeneach step oferosion.


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Erosion

Duplexing

Antiformal stack

Exumation

Underthrusting


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Pop-up structures

Antiformal Stack

Exumation


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Small-scale duplexes

No basal material

exumation

No major backthrust


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a) The extent of erosionalremoval increases with

shortening for all erodedthrust wedges, and thisparameter is higher in thrustwedges without dcollements.

RESULTS

b) The ratio of basalunderthrusting is the highestin the eroded thrust wedgeswithout dcollements.

c) When more material isremoved by erosion from thesurface, then more material isaccreted at the base of modelwedges.


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Natural Examples

MW5

Southern Foothills of theCanadian Rocky Mountains


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MW4


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MW6

thrust wedges with décollement levels and syntectonic erosion

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