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Ductile structures course

Modules containing structural tools and skills specific to moderately deformed ductile terranes in low to high grade metamorphic rocks.

Note that these topics assume knowledge of the content of the Basic course. [The Advanced Structural Analysis course covers complex multiply deformed terranes and assumes as background the material that is in this module]

Instructor: Rod Holcombe

Terranes dominated by ductile deformation (at low to moderate metamorphic grades) are dominated by folds, cleavage, and shear zones, but brittle and semi-brittle elements can contribute significantly to mineralisation. Although multiple deformation structures are present in most such terranes, the modules in this course are aimed at terranes in which a single generation of structures dominates. In contrast to thin-skinned terranes, folds commonly have significant plunges and disharmonic styles and thus require quite different cross-section techniques to those appropriate in most thin-skinned terranes. The presence of a cleavage provides additional, but commonly under-utilised, mapping tools.

Key concepts are: strain and fold/fabric development; fold style; disharmonic folding, enveloping surfaces and transposition; shear zones and flow kinematics Key tools are: trend surface (form line mapping); bedding and younging data (particularly in monotonous lithologies); fabric generations and overprinting; vergence domains; stereographic projection orientation analysis; projection techniques for cross-sections; shear zone analysis

Many problem areas (particularly in slate belts) are only problems because the rock types are monotonous and definition of stratigraphy is poor. Such areas are generally quite tractable but require particular care to concentrate on gathering sufficient bedding, younging, and vergence data in order to replace stratigraphic mapping by form line maps or pseudo-structure contour maps.

The following modules assume a number of the Basic modules as required background.

Ductile structures topics

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