II Basic Techniques in Structural Geology Field measurements

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

Two types of fault-related fold (drawn here assuming flexural slip folding) Fault-bend fold Fault-propagation fold

Fault-Bend Fold Fault Bend Folds (FBF) =ramp anticlines - throughgoing faults - slip is conserved. - Fold amplitude is determined by the thickness above the lower hanging wall flat. - Once the fold has achieved maximum amplitude, it only grows in width.

Duplex system

Thrust fault hangingwall and footwall ramps and flats…

Fault-Propagation Fold Fault Propagation Folds (FPF) - fault slip is not conserved. Fault displacement is taken up by folding - Fault tip is marked by a ductile ‘process zone’. - Ideally, FPF’s evolve into FBF’s

Fault-tip Folds Fault-propagation fold (non-parallel fold) Detachment fold (folding is always nonparallel) Fault-propagation Fold (parallel fold)

‘Drag folds’ Footwall rocks are commonly deformed into synclines Could also relate to fault propagation

Footwall syncline in the Canadian Cordillera

Let’s practice…

• Primary strucure due to cross-bedding stratification. Not related to deformation

Describe: from: http: //earth. leeds. ac. uk/learnstructure/index. htm from: http: //www. stmarys. ca/academic/science/geology/st

similar fold parallel fold from: http: //earth. leeds. ac. uk/learnstructure/index. htm from: http: //www. stmarys. ca/academic/science/geology/struc Recumbent tight similar syncline and anticline Upright open parallel syncline

try to sketch: shallowly plunging, upright, tight, similar syncline Assume fold axis plunges toward N 45 E

Example from the Argentinian Cordillera

What is it?

Fault related Folds all from: http: //earth. leeds. ac. uk/learnstructure/index. htm

Fault related Folds Fault-bend Fold Fault-tip Fold http: //earth. leeds. ac. uk/learnstructure/index. htm

A case study of a fold and thrust belt: The North Tian Shan Piemont A B (Charreau et al, 2008)

Growth strata record of Kink Band migration (Huerguosi Anticline) A B (Charreau et al, 2008) (Suppe et al. , 1997; Hubert-Ferrari et al, 2007)

Modeling 12 equations ex: Kinematic model (Charreau et al, 2008)

(Charreau et al, 2008)

Modeling dip angles Bedding measured in surface Bedding expected from the pure Fault-bend fold model Bedding expected from the fault-bend fold model with growth strata (Charreau et al, 2008)

Long Term History of Shortening (Charreau et al, 2008)

Sedimentation History k te a r n Ac s– ac b gy Pig tio a l u m u c phy Ac tesa r n o i at cumul igra t a r t s to magne (Charreau et al, 2008)