Richard Horne

An evaluation of flexural-slip folding in the Meguma Group was conducted in the hinge area of the Lawrencetown Anticline (Halifax area) and the Ovens Anticline (Ovens area). Flexural slip in these areas was accommodated on mainly bedding-parallel movement horizons consisting of simple slip surfaces and flexural-slip duplexes. Lateral and frontal ramp movement horizons are locally associated with bedding-parallel movement horizons, forming a linked system accommodating flexural-slip strain. Thrusting locally accompanied flexural-slip. Discordant shear fractures in the Ovens Anticline are kinematically related to flexural slip. Movement horizons are typically coated with slickenfibres and quartz veins commonly occupy flexural-slip structures in the Ovens Anticline, and fluid pressure was likely important during flexural slip. Movement lineations are roughly perpendicular to the fold hinge or systematically form an acute angle with the hinge in the direction of the fold plunge, suggesting flexural slip during non-cylindrical fold growth. Shear sense indicators include slickenfibres, flexural-slip duplex geometry, displaced conjugate veins and thrust geometry, and invariably show a reverse sense which changes systematically across the fold.

The average spacing of bedding-parallel movement horizons ranges from approximately 1-4 metres and spacing is inversely related to limb dip. The flexural-slip amount determined for a section of the Ovens Anticline from displaced discordant veins indicates that flexural-slip accounts for 4o-8o of limb dip. Slip amount is variable, with the largest slip occurring on flexural-slip duplexes and flexural-slip shear strain is uniform across the limb. Spacing and slip amount data suggest that increasing flexural-slip strain during incremental fold growth is accommodated by increased movement on early formed movement horizons and continuous formation of new movement horizons.

Bedding-cleavage relations, the distribution of minor folds and cleavage-parallel strain defined by pressure shadows on arsenopyrite suggest a synfolding origin for cleavage and indicate flexural slip was accompanied by significant layer-parallel shortening in fold hinges, including the flat tops of box folds. Flexural slip is a brittle, post metamorphic deformation which deforms the contact aureole of the ca. 370 Ma South Mountain Batholith. A post metamorphic age is supported by 40Ar/39Ar age data which constrain regional metamorphism to ca. 410-385 Ma but synfolding cleavage-parallel strain and the emplacement of (flexural-slip) quartz veins at ca. 375 Ma. A model of folding has been proposed which includes (i) initial box-fold development, (ii) synfolding cleavage development and layer-parallel shortening in the flat segments of box folds with flexural- flow on the limbs under metamorphic conditions and (iii) post metamorphic flexural-slip accompanied by additional cleavage-parallel strain, including layer-parallel shortening in fold hinges.

A synfolding, flexural-slip model is proposed for the auriferous quartz vein array at the Ovens, including synchronous emplacement of bedding-parallel and discordant veins. Most Meguma Gold Districts share general features with the Ovens which are consistent with a flexural- slip model, suggesting a flexural-slip model may be an appropriate and testable model for Meguma gold deposits.

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Pages: 265
Supervisor: Nicholas Culshaw