fold anatomy

Folding buckles strata away from the linear, planar, and horizontal.

Folds may be symmetrical or asymmetrical. In a symmetrical fold (image at left), the axial plane is vertical and the limbs (sides) dip symmetrically from the axis. In an asymmetrical fold, the axial plane is tilted from the vertical with one limb dipping more steeply than the other.

▪ the axial plane of a fold is an imaginary plane surface that divides a fold as symmetrically as possible.

▪ the fold axis is a line drawn along the points of maximum curvature of a layer of a fold (parallel to the hinge in anticlines and synclines).

▪ the plunge of a fold is the angle between the horizontal plane and the fold axis (when this is not not horizontal).

▪ the plunge direction is the geographic quadrant towards which the plunge is directed

By convention, plunge direction is expressed as a three-digit number, and the plunge by a two-digit number. Thus, a line plunging 35º toward the azimuth 35º will be noted: 45º->035

Monoclinal folding drapes strata as though over a ledge, whereas hinge folding wraps the limbs of a fold as though hinged around the fold axis.

Because the eroded surface of a plain could fail to indicate the sub-surface folding relationships of ridges of resistant strata, folding is most easily elucidated by examining structures eroded through folds (side view of diagram at left).





However, the apparent shape of a fold may be distorted by the angle of erosional exposure relative to the fold axis (image at left).


(Try this with a stick of celery.)




├ .. On geological maps, the stike and dip of bedding, rather than axes and folds, are indicated.

▪ faulting ▪ fault attributes – net-slip, dip-slip, dip vector, strike-slip, vertical-throw, horizontal-throw, horizontal vector, heave, rake

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geological maps

A geological map provides a graphic representation of selected geological features within a desired surface topographic or subsurface area. The size and relative position of each feature on the map corresponds to its correct geographic situation according to an established scale and projection. Mapped features included in the map key include geologic units, stratigraphic contour lines, fault lines, strike and dip lines, and other symbols. Information can be extrapolated from surface mapping in order to postulate the distribution of geological features in the subsurface. Resulting hypothetical structural models can provide the basis for exploring the landmass in search of its resources.

Commonly, geological features cannot be measured continuously over large areas, so other forms of evidence are employed to infer the delineation of large or hidden features. Surface geological features may be traceable in bedrock outcroppings (ground surveys), from air photographs (photogeological reconnaissance) and/or from satellite images. Subsurface geological features may be traceable in boreholes using cores, cuttings and/or geophysical logs. Geophysical surveys (measuring the magnetic, gravitational, or seismic properties) provide information that helps delineate geological features in the subsurface.

A Geographic Information System (GIS) is an efficient way to manage, analyse and display spatial data. Data from a variety of different sources can be rapidly computer-overlaid for viewing and analysis.

[links: images: LANDSAT image gallery, Geology - Oman; Seafloor Mapping, Bahia, Brazil; websites: Geological Map of Canada : Structural Geology (animations) : Geologic Maps from USGS National Park Service : Geological Maps, UWisc : USGS National Geologic Map Database : USGS Geology in the Parks : Animations : Plate Tectonics : Rocks & Minerals : Geologic Time : Glossary : Geomorphic Provinces : Sand Dunes : Caves : Glaciers : Coasts : NPS Park Geology Tour home]

mapping

A geological map provides a graphic representation of selected geological features within a desired surface topographic or subsurface area. The size and relative position of each feature on the map corresponds to its correct geographic situation according to an established scale and projection. When mapping out a region, standardized terms are employed to describe the spatial orientation (attitude) of planar and linear elements of folds, faults, and other geological structures (diagram):

├ . The strike of a plane is the compass direction of the line formed by the intersection of the horizontal plane with the inclined plane under consideration. So, strike marks the geographical direction perpendicular to dip. ├ Strike and dip are depicted on geological maps by a long line (strike) with a short perpendicular line (dip) and a number indicating the angle of dip (degrees).

In North Amercia, strike is often expressed as the angle E or W of true North (0º-90º). In the European system, compass directions are expressed as azimuths. The azimuth is measured clockwise along an horizontal plane from the true North direction to the strike line (0º-359º). N=0º, E=90º, S=180º, W=270º.

A strike tending 26º east of true north would be expressed as N26ºE in Canada and the US, and as 026º in the European system. Similarly 74º west of true north would be expressed as N74ºW in the North American system, and as (360-74=) 286º in the European system.

├ .. The dip of a plane is analogous to plunge, and is the angle in degrees measured from an horizontal plane down to the inclined plane under consideration. That is, dip is the angle between the inclined plane and the horizontal plane, and is measured along a vertical plane perpendicular to the strike line of the plane.

Strike and dip directions of a fold are always mutually perpendicular, though two planes could have the same numerical strike (direction) and dip (angle). That is, a plane inclined at 45º to the horizontal (dip) that is facing SSE (135º) could have a strike (direction) of 45º East (o45º).

Strike and dip are differentiated in North America by expressing the direction according to the geographic quadrant faced by the planes. By European convention, strike is expressed as a three digit azimuthal number, and dip as a two digit angular number. Thus, a plane striking 25º and dipping 45º toward the southwest would be noted: 025-45SW.

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