II Basic Techniques in Structural Geology Field measurements

II. Basic Techniques in Structural Geology • • • Field measurements and mapping Terminology on folds and folds Stereographic projections From maps to cross-sections Seismic Imaging

Davis and Reynolds, ‘Structural Geology’

• A geologic map represents the geometric relationship between the various rock units faults at the surface. • Any interpretation of the subsurface must be consistent with the geologic control, and style of deformation revealed by the map; as well as with the information available from the subsurface (wells, seismic profiles).

Amanz Gressly 1859… Desor & Gressly 1859 Gressly’s prediction before the tunnel…

Gressly’s subsurface prediction was an extrapolation of surface geology… Desor & Gressly 1859

After the tunnel… Desor & Gressly 1859 Gressly’s prediction before the tunnel…

Construction of a cross-section • Define the plane section (in general vertical but not necessarily). • Determine topographic profile (without any vert. exaggeration) • Plot geologic data (measurements of strike and dip; intersections of stratigraphic contacts; faults). • Extrapolate and interpolate data.

Apparent dip angles can be determined from vector algebra, stereonets or graphs

Davis and Reynolds, ‘Structural Geology’

Projecting the map, using the fold axis…

Davis and Reynolds, ‘Structural Geology’

Orthographic projections

Structure Contour Lines

Extrapolation and interpolation - Some assumptions are needed to extrapolate. - Is the proposed section a possible geometry?

Principles of ‘balanced’ cross sections • The true section is retrodeformable. • Any retrodeformable section is a possible model. Not unique however in general. • The ‘space’ of retrodeformable section is large. We need assumptions to limit possibilities: – Conservation of mass is often assumed to convert to conservation of volume (not correct in case of compaction or metamorphism). – if plane strain is assumed or if the structure is cylindrical conservation of volume converts to conservation of area NB: Balanced = retrodeformable

Parallel folding Parallel folds commonly form by a deformation mechanism called flexural slip, where folding is accommodated by motions on minor faults that occur along some mechanical layering -- usually bedding. Flexural-slip surfaces, which can be observed in core or outcrop, may vary in spacing from a few millimeters to several tens of meters in spacing. In that case of flexural slip folding there is conservation of bed length and of bed thickness

Two popular methods can then be used to construct balanced cross sections (applies to profile sections across cylindrical structures) -The Busk Method (Busk, 1929) - The Kink Method

The Busk’ method… parallel and concentric folds

The Busk’ method…

The Kink method for a parallel fold… The kink method is based on the assumption of flexural slip folding in the limit where dip angles varie only across axial surfaces (it’s equivalent to the Busk method with infinite curvature within dip domains and zero curvature within axial surface). g -If bed thickness is constant, the axial surface bisects the angle between the fold limbs Axial angle : g g

The Kink method for a parallel fold… - Dip angles are constant within dip domains separated by axial surfaces. -the axial surface bisects the angle between the fold limbs

The Kink method for a parallel fold… - where two axial surfaces intersect, a new axial surface is formed. Its dip angle bisects the dip angles of the adjacent dip domains

The Kink method for a parallel fold… Holland, 1914

The kink method is more general than the Busk method (any curve can be divided in straight segments).

Constructing a balanced crosssection from the kink method • Assemble data (surface and subsurface observations) • Define dip domains, positions and dip angles of axial surface. • Extrapolate at depth by trials and errors. (you will need an eraser, experience will help). • Test that the section is indeed retrodeformable.

Two possible interpretations of structural measurements at the surface and along an exploration well. Which one is most plausible?

The kink method is more general than the Busk method (any curve can be divided in straight segments).

This section is retrodeformable and is thus a plausible model (but not a unique solution).

Minimum Shortening: 17. 5 km A balanced cross-section generally yields a lower bound on the amount of shortening (because of erosion)

Fault-bend folding (Rich, 1934)… Passive Axial surface Active Axial surface The non planar fault has a flat-ramp-flat geometry. Translation of the thrust sheet along that fault requires axial surfaces. Note the difference between the active and passive axial surfaces

Fault-bend folding … Courtesy of Frederic Perrier

Constructing a balanced crosssection from the kink method • Assemble data (surface and subsurface observations) • Define dip domains, positions and dip angles of axial surface. • Extrapolate at depth by trials and errors. (you will need an eraser, experience will help). • Test that the section is indeed retrodeformable.

A balanced cross section might be checked and eventually retrodeformed based on the principle of conservation of area and conservation of bed length

Curvimetric Shortening & planimetric shortening Curvimetric Shortening: Sc= Lc-l Planimetric shortening: Sa=Asr/h Structural relief: Asr Undeformed depth of decollement below bed: h Area of Shortening: A Conservation of area implies: A=Asr= Sc*h. NB: Both quantities refer to a particular bed.

Curvimetric Shortening & planimetric shortening Curvimetric Shortening: Sc= Lc-l Planimetric shortening: Sa=Asr/h Conservation of area and bed length implies : - curvimetric shortening = planimetric shortening - Asr should increase linearly with elevation above decollement (h).

Sc= 0, 95 km Asr/h=2. 6 km Sc= 0, 95 km Asr/h=0. 95 km If area and bed length is preserved during folding then b is the most plausible solution …or a is correct but some diapirism is involved

Structural relief: Asr Equality of curvilinear and planimetric shortening can be used to either check the section or predict the depth to the decollement. NB: Be careful with the possibility of diapirism

Structural elevation in balanced cross sections: the problem of filling space

Balanced cross-section Across the Pine Mountain (Southerm Appalachians) (Mitra, 1988)

(Mitra, 1988) (Suppe, 1983)

Fault-bend-folding +Imprication/Duplex…

Structural elevation in balanced cross sections: the problem of filling space