Lab 8: Folds and their map patterns

Transcription

1 Lab 8: Fols an their map patterns Fall Fols are one of the most common tectonic structures evelope in eforme rocks. They form in rocks containing planar features such as seimentary being, lithologic layering in metamorphic schists an gneisses, or planar features such as cleavage or schistosity prouce uring an earlier eformational event. They occur at all scales from the thin section to mountain ranges. At larger scales, complex fols may be ifficult to unerstan while mapping in the fiel. Instea, we rely on the pattern that evelops on the map. In aition to looking at the overall fol pattern of the units, we can gain information about the 3-D shape of the fols by analyzing other ata such as strike an ips of the units, shapes an orientations of small-scale fols that are observe in the outcrop, an the orientation of fabrics (e.g. cleavages an lineations) that evelope uring foling. 1.1 Terminology an jargon Hinge line: lines joining the points of maximum curvature on a fol. A physical property, so it has a location as well as an orientation. Hinge plane/surface: the plane or surface containing the hinge lines of successive layers Crest (or trough) line: line of highest (or lowest) elevation on a fol. Also a physical property. Crest (or trough) plane/surface: the plane or surface containing the crest/trough lines. Limbs: zones of minimum curvature, the close to planar part of the fol between the hinges. Fol axis: a theoretical line which, if move through space, woul trace out a closely fitting surface to a fol (in particular, the fol axis generates a cylinrical fol, an real fols are only approximately cylinrical for small regions). Axial plane/surface: plane or surface which connects the hinge lines on ajacent layers. Often approximate as bisecting the fol limbs. Axial surface trace: intersection of axial plane an groun surface. Enveloping surface: a plane or surface containing hinge lines of ajacent fols. Meian surface: a plane or surface containing inflection lines of ajacent fols. Fol train: a series of fols. Symmetric fols: fols in which (1) the meian surface is planar, (2) the axial plane is perpenicular to the meian surface an (3) the fols are bilaterally symmetrical about their axial planes. Asymmetric fols: fols that are not symmetric. Limbs of asymmetric fols are of unequal length. Asymmetric fols are often classifie as "Z-fols" or "S-fols" epening on the sense of overturn. Vergence: irection of acute angle mae by intersection of hinge surface an enveloping surface. Often use to infer a sense of shear, a irection of tectonic transport or the closure of large-scale fols. Cylinrical fol: a fol whose hinge is a straight line; this is the kin of fol generate by moving a line (fol axis) through space parallel to itself. 1

2 Non-cylinrical fol: a fol whose hinge line is curve. Essentially all fols are non-cylinrical, but a noncylinrical fol can usually be broken own into sub-regions which are cylinrical. Anticline: a fol with oler rocks in its core. Syncline: a fol with younger rocks in its core. Antiform: a fol whose limbs ip away from the hinge: that is, it opens ownwar (upsie own U-shape). Synform: a fol whose limbs ip towars the hinge: i.e., it opens upwar. Vertical fol: a fol whose hinge line is vertical or nearly so. Recline fol: a fol whose axial surface is incline an whose hinge plunges own the ip of the axial surface. Recumbent fol: a fol whose axial surface is horizontal or nearly so. Overturne fol: a fol where one limb is "upsie-own". That is, oler bes are foun above younger bes. Dip Isogon: theoretical line connecting points of equal inclination between two fole surfaces (fig. 6). One common scheme for classifying fols is base on whether isogons are 1) convergent isogons; 2) parallel; or 3) ivergent an variations in orthogonal thickeness an axial trace thickness. Parallel fol: a fol in which the thickness measure orthogonally across the layer bounaries is constant throughout the fol. Concentric fol: a subclass of parallel fol whose inner an outer surfaces are efine by circular arcs having a common center. Similar fol: a fol characterize by (1) parallel ip isogons; (2) a ecrease in orthogonal thickness from hinge to limb; (3) constant axial trace thickness. Interlimb angle: the angle between limbs of a fol (fig. 7). This is also the basis for a classification scheme: gentle 120 to 180 open 70 to 120 close 30 to 70 tight 2 to 30 isoclinal 0 to 2 Harmonic fols In a fole multilayer, fols eventually ie out parallel to the axial surface (unless they hit a free surface first). The epth of foling is the length own the axial surface that fols persist. Harmonic fols persist for a greater epth than many multiples of the fol half-wavelength, isharmonic fols ie out within a istance on the orer of the fol s wavelength. Parasitic folsfoling often evelops simultaneously at many scales such that smaller fols are foun on the limbs of larger fols. These are terme parasitic fols, an it is common to istinguish between "first orer fols" (map scale); "secon-orer fols" (tens or hunres of meters) an thir an higher orer fols (meters to millimeters). Preictable changes in the asymmetry or vergence of parasitic fols are useful for etermining the geometry of lower-orer fols. Polyphase foling: a term use when there are multiple episoes of foling. 1.2 Stereonets an foling Prior to foling, the poles to a being plane plot as a single point or a concentration of points on a stereonet. Foling alters the istribution of the original poles. If the plane is fole about a straight fol axis (cylinrical foling), then the orientation of the poles will range from those on one limb to those on the other, an all the orientations will lie in an imaginary plane perpenicular to the fol axis. Information from fole being is usually plotte in one of two ways. A Pi plot is a stereonet plot of poles to being. If the being was cylinrically fole, the poles will all plot roughly on a great circle perpenicular 2

3 to the fol axis. The hanout shows a variety of fol geometries an the corresponing stereonet plots. Look through these carefully until they make sense to you. Note the assumption in each of the plots that the collection of ata is equally istribute along the length of the fol, an thus the ensity of points correspons to the length of limb. The tangent planes to cylinrically fole being intersect at the fol axis. A Beta plot can mae by plotting strike an ip measurements as great circles (rather than as poles to being) an fining the point where all the great circles intersect. The Beta an Pi methos o essentially the same thing, but the Pi plot is easier to work with. The orientation of the axial plane to a cylinrical fol can be foun with the orientations of two lines that are containe by the axial plane. One available line is the fol axis. By efinition, the fol axis must lie in the axial plane. A secon line is the trace of the axial plane across the groun. Draw trace on the map by connecting the points of maximum curvature for several fole bes an measure its general tren irectly from the map. Fin the axial surface by fitting a great circle through the points representing the fol axis an the surface trace of the axial plane. Another option is to raw a plane through the fol axis that bisects the fol limbs. This is an not strictly correct, but it is a reasonable an often mae approximation for rawing fols in cross section. 1.3 Down plunge projections Up to this point, all of the cross sections we have looke at or constructe have been oriente vertically. In regions of fole rocks, it is often esirable to raw what is calle a own plunge projection of the fol structure, a cross section rawn perpenicular to the plunge of a fol. Starting with a map outcrop of a fol, contacts are projecte from their location on the map into the plane of the cross section. Depening on if you are oing a vertical cross section or a own plunge projection, the location of the contact on the plane of the cross section can be foun at an elevation above the base line by multiplying the perpenicular map istance to the contact by the sine or tangent of the plunge. Vertical cross-section Down-plunge cross-section plunge (p) tan(p) plunge (p) sin(p) Plane of section Plane of section To raw a own plunge projection (or for that matter, any section of a plunging structure), start by rawing a square gri on the map parallel an perpenicular to the bearing of the plunge (which shoul be parallel an perpenicular to the line of the section as well). Conense the gri on the cross section, keeping the spacing constant in the irection perpenicular to plunge (parallel to line of section), but conensing the vertical scale by (gri spacing) * (sine (plunge)). Then project points to appropriate elevations above the reference line of cross section using the gris as guies. Obviously, the closer the gri spacing, the more accuracy that you will have on your own plunge projection, but the longer it will take to raw your section. Balance these two factors accor- 3

4 ingly. sin(p) When topography is involve, rawing the own plunge projection becomes a little more complicate. However, it is still possible to raw a section fairly quickly. The elevation becomes a factor in where on the cross section a particular feature plots. sin(p) hcos(p) plunge p p h 2 Exercises 1. Pick a fol on the table. Sketch the fol an ientify, on your sketch: the hinge line, axial plane, the limbs. Ientify the rock type. Are there linear fabric elements (eg. mineral lineations, stretching lineations, or intersection lineations?) present? Classify the fol accoring to the following schemes: (1) cylinricity; (2) symmetry; (3) tightness; (4) Ramsay s classification (using orthogonal thickness, axial trace thickness an the convergence of isogons). 2. Down plunge projection. The fols in figure 1 plunge 30 ue west. a. A fol hinge traces an a few strike an ip symbols (on t worry about the ip, just the irection) to the map. b. Construct a own plunge projection. c. Write a sentence or two on why its important to raw a fol perpenicular to the plunge rather than vertically. 3. Down plunge projection with topography. The fol in figure 2 plunges 25 ue north. Construct an E- W profile of the fol perpenicular to the plunge with 340 m as the zero atum line. Use the formula given in the hanout, but remember to convert all your measurements from the map an the elevations using the given scale. Interpolate elevations between contour lines. 4. Polyphase foling. Schematic map vies of several polyphase fols are shown in figure 3. For each of these 4 fol interference patterns, raw the traces of the F1 (first foling) an F2 (secon foling) hinge surfaces. Also raw schematic cross sections along lines AA an BB. What is the approximate angular relation between the F1 an F2 fol axes? 4

5 5. Stereonet analysis. With the attitue ata given in figure 4, construct a Beta iagram an a Pi iagram. What is the tren an plunge of the fol axis? (feel free to use a stereonet program for this, no nee to o it by han). 6. Stereonet analysis. With the attitue ata given in figure 5, construct a contoure Pi iagram. Determine the following: a. tren an plunge of the fol axis b. approximate attitue of the axial plane c. approximate interlimb angle. approximate fol style. Illustrate with a sketch. 7. Mapping fols an stereonet analysis. As seen in the map in figure 6, a series of sanstone an shales are unconformably overlain by a conglomerate unit. There are no faults in the area. Map symbols are: Strike an ip of bes Strike an ip at joints (arrow points ownip) Vertical joint (strike is line without arrow) Strike an ip of cleavage Tren an plunge of minor fols Tren an plunge of lineations a. Make a "soli" geologic map from the given outcrop area, using extrapolative license. Remember the simplest interpretation is usually the right one. Pay attention to minor fol asymmetries as you complete the map. Lightly color the map an use a re pencil to raw in appropriate hinge surface traces, inicating if its an anticline, syncline, overturne anticline or overturne syncline. There is more that one sanstone an shale unit. b. Analyze the fol structures by plotting on a single stereonet (using ifferent symbols) i) poles to being ii) poles to slaty cleavage iii) minor fol axes Base on the plots, what is the style of foling? What is the orientation of the major fol axial surface in the center of the map? Calculate the angle between the axial surface an each limb of the major fol. Roughly sketch an characterize the fols? c. On a separate stereonet plot, using ifferent symbols, plot i) poles to joints in the sanstones an shales ii) poles to joints in the conglomerate Do the joints appear to be genetically relate to the fols in the shales/sanstones? Are the joints in the conglomerates genetically relate to the joints in the shales/sanstones? If so, why? 5