Inferred thrust imbrication, deformation gradients and the Lachlan Transverse Zone in the Eastern Belt of the Lachlan Orogen, New South Wales

R. A. Glen*, D. Wyborn

*Corresponding author for this work

Research output: Contribution to journalArticle

30 Citations (Scopus)

Abstract

Re-examination of the Ordovician geology between Mandurama and Bigga in the Lachlan Orogen of central western New South Wales has produced new interpretations of the stratigraphy and structural geology. The Abercrombie beds have been previously inferred to comprise an Ordovician turbidite package with interbedded black shale bands. Although hampered by a paucity of fossil ages, new data suggest that the Ordovician geology of this region instead represents an imbricate stack of Lower Ordovician turbidites (Adaminaby Group) and Upper Ordovician black shales (Warbisco Shale). Structural data from the north of this region suggest that duplication occurred in a D} event (with formation of broadly east-west to west-northwest-trending thrust slices or fold limbs) and was accompanied by formation of cleavage and isoclinal folds. Thrusting of the Adaminaby Group and Warbisco Shale over or under the Lower Ordovician Coombing Formation (southern part of the Molong volcanic belt) also occurred at this time. East-vergent imbrication and thrusting and formation of a regional near-meridional steeply west-dipping cleavage occurred in the D2 event, when D1 thrusts or folds were folded around overturned (east-vergent) D2 folds. These new data also suggest that there is a north-to-south gradient in the intensity of the D2 deformation, with D2 effects decreasing from south to north approaching the Lachlan Transverse Zone. Such a gradient mirrors similar but more subtle local changes from the north. Together, they imply that the Lachlan Transverse Zone was a major zone of weakness during north-south shortening that resulted in the formation of D1 structures but was relatively rigid in local areas during the regional D2 deformation that resulted from east-west shortening when it formed a major tear/accommodation zone. This D2 rigidity may be caused by strength imparted by the earlier emplacement of large (variably mineralised) intrusive/volcanic complexes along the transverse zone.

Original languageEnglish
Pages (from-to)49-68
Number of pages20
JournalAustralian Journal of Earth Sciences
Volume44
Issue number1
Publication statusPublished - 1997
Externally publishedYes

Keywords

  • Adaminaby Group
  • Deformation
  • Lachlan Orogen
  • Lachlan Transverse Zone
  • Ordovician
  • Thrust faults
  • Warbisco Shale

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