How to Describe a Fold Using a Steronet

1 each point may be counted. Determination of hinge line positions axial surface orientation and interlimb angles from field data using Rick Allmendingers stereonet software.

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Slide point Y 35 degrees along E-W axis so that pole to bedding is restored to the center of the net.

. General description how to determine hinge line orientations axial surfaces and interlimb angles using the stereonet. Folding alters the distribution of the original poles. From North read the fold axis trend and plunge.

Stereonet plotting by hand is usually done using a piece of tracing paper over a stereonet from a book. The stereonet is a representation of the lower half of a sphere. A fold is a bend in a layered rock caused by compressive stress buckling or passive draping of layers over a deeper structure including normal faults or around a resistant object.

Rotate the overlay counter-clockwise until β lies on the N-S axis. It is the relative ages of the rock strata that separate anticlines from antiforms. C Plot both limbs of the fold from part b on a stereographic projection.

At each grid node count the number of points in the circle and plot no. FOLD ORIENTATION Two pieces of information required to define fold attitude. Fold axis trend and plunge Fold axial plane strike and dip FOLD CLASSIFICATION Younging direction aka up-direction.

Non-cylindrical folds with curved hinge lines do not have fold axes and for the purpose of detailed structural analysis for example stereographic representation it is necessary to subdivide them into several cylindrical folds each with a. A fold axis is a geometric imaginary straight line which when moved parallel to itself through space generates the shape of the fold. 1Requires some component of ductile deformation.

Strike is measured from a horizontal line that lies in the plane it is the direction in degrees from north like azimuth which we covered of. 2Imparts permanent strain on rocks. Using the intersection and the angle between the planes estimate the plunge and trend of the fold axis and the interlimb angle of the fold.

Original top or upper side of a sedimentary or volcanic bed Can be determined using some kinds of primary sedimentary. 3Requires high P and T and therefore significant burial. Prior to folding the poles to a bedding plane plot as a single point or a concentration of points on a stereonet.

If the plane is folded about a straight fold axis cylindrical folding then the orientation of the poles will range from those on one limb to those on the other and all the. An Antiform can be used to describe any fold that is convex up. This analysis involves analyzing structures shown on beautiful rich detailed and well-prepared outcrop bedrock geology of a real region.

This point is β the fold axis. The hinge is also the highest point on a stratum along the top of the fold. Folds display a wide range of shapes and result from a wide range of.

1 Starting with the overlay and stereonet N marks coinciding mark the strike azimuth on the primitive circle. For strike values in. Use the points where the contours cross the fold hinge to determine the average trend and plunge of the fold hinge.

However because the stereonet is in 2D and a sphere is 3D all the 3D data needs to be projected into the 2D plane. The hinge of an anticline refers to the location where the curvature is greatest also called the crest. Strike and dip are two measurements used to describe the orientation of a plane such as the limb of a fold.

With the great circle pinned to North count 90 o from the circle along the east-west axis the count must pass through the centre of the stereonet. A stereonet is a tool used by geologists to analyze and interpret orientation data such as planes and lines. Rotate net so that point Y is located on East-West axis of net.

Parts of a Fold 3. Students construct a number of stereonets step-by-step and the map-scale simple fabrics come to life. Folding爀刀攀焀甀椀爀攀猀 猀漀洀攀 挀漀洀瀀漀渀攀渀琀 漀昀 搀甀挀琀椀氀攀 搀攀昀漀爀洀愀琀椀漀渀屲Imparts permanent strain on rocks爀刀攀焀甀椀爀攀猀 栀椀最栀 倀 愀渀搀 吀Ⰰ 愀渀搀 琀栀攀爀攀晜漀爀攀 猀椀.

Students also prepare an accurate cross-section using data from the stereonet analysis. Definition of Fold 2. D Lastly describe the type of fold you mapped your description should use almost all of the words covered in class Part II.

Attitude of Folds in the Cascades. A stereonet allows the stereographical projection of three-dimensional information onto a two-dimensional plane usually as a piece of paper or computer image and is used as a tool applied to a range of geological problems including the removal of structural tilt. This may sound a bit contrived actually it is but it provides us with a sensible approach to fold analysis particularly using stereographic projections.

Lets remove the effects of Y tilting using the stereonet Technique a. Use a 1 circle 1 of area of stereonet and move it in grid fashion across the Pi diagram c. Lay grid on stereonet equal area net b.

In the simplest case a dip and strike measurement on each limb of a cylindrical fold will plot on a stereonet as two great circles the intersection of which is a point that represents the trend and plunge of the hinge. Plot poles to bedding for formation O and Y b. 2 For strike values in the northern part of the stereonet for strikes ranging from 270 clockwise through 000 to 090 rotate the labeled tick mark to N.

Stereonets involve the projection of lines or planes onto a sphere. Such a fold may range from microscopic crinkle to great arches and troughs even. It is frequently seen that the strata forming the earths crust have been not only tilted out of the horizontal but also bent and buckled into folds.

A piece of tracing paper for every stereonet problem A blank stereonet attached to this lab A thumbtack A thumbtack protector Place the thumbtack through the center of the blank stereonet so that the sharp end is. Just a few weeks ago the class went the Cascades near French Cabin Creek and recorded a number of strikes and dips of folded meta-sedimentary rocks as well.

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