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# Lec.05 - 5. STRUCTURE CONTOURS AND MAP PATTERNS I ...

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Unformatted text preview: 8/26/11 5. STRUCTURE CONTOURS AND MAP PATTERNS I Main Topics A Structure contours B Strike of beds on a geologic map with a topographic base C Appearance of planar beds on a geologic map D Appearance of folded beds on a geologic map with a topographic base 8/26/11 GG303 1 5. STRUCTURE CONTOURS AND MAP PATTERNS II Structure contours A A line or curve (contour) that marks the intersecQon of a horizontal plane with some geologic surface; this surface need not be planar. Strike lines are tangent to structure contours (see Fig. 5.1). 8/26/11 GG303 2 1 8/26/11 5. STRUCTURE CONTOURS AND MAP PATTERNS II Structure contours (cont.) B A geologic map can be thought of as the collecQon of points marking the intersecQons between structure contours (red) and the corresponding black topographic contours. C See the “html help desk” for Matlab funcQons surfc and contour3. 8/26/11 GG303 3 5. STRUCTURE CONTOURS AND MAP PATTERNS III Strike of beds on a geologic map with a topographic base A Lines of strike are horizontal (i.e., a series of points of equal elevaQon). For a surface (or layer) of constant strike, a line of strike (i.e., a traverse at equal elevaQon) lies along the surface (or layer) rather than cu[ng across the surface (or layer); (see Fig. 5.3). B Lines of strike can be determined by locaQng where a contact intersects a given contour line in more than one point; these points of intersecQon lie along strike. This is easiest where a contact is steep. 8/26/11 GG303 4 2 8/26/11 5. STRUCTURE CONTOURS AND MAP PATTERNS III Strike of beds on a geologic map with a topographic base A Lines of strike are horizontal (i.e., a series of points of equal elevaQon). For a surface (or layer) of constant strike, a line of strike (i.e., a traverse at equal elevaQon) lies along the surface (or layer) rather than cu[ng across the surface (or layer); (see Fig. 5.3). 8/26/11 GG303 5 5. STRUCTURE CONTOURS AND MAP PATTERNS III Strike of beds on a geologic map with a topographic base (cont.) B Lines of strike can be determined by locaQng where a contact intersects a given contour line in more than one point; these points of intersecQon lie along strike. This is easiest where a contact is steep. 8/26/11 GG303 6 3 8/26/11 5. STRUCTURE CONTOURS AND MAP PATTERNS IV Appearance of planar beds on a geologic map A Planar beds have a constant strike and a constant dip B Strike lines along structure contours are parallel and straight C Strike lines along structure contours are evenly spaced D Dip direcQon is constant 8/26/11 GG303 7 5. STRUCTURE CONTOURS AND MAP PATTERNS V Appearance of folded beds on a geologic map A The strike and/or dip of a folded bed varies with posi7on B Strike lines along structure contours might or might not be parallel; the strike of folded layers does not necessarily change. 1 If strike lines are parallel, then the strike is constant (±180°) and the axis of the fold is horizontal 2 If strike lines are not parallel, then the strike is not constant and the axis of the fold plunges (e.g., fold with a verQcal fold 8/26/11 Surface curls over fold axis; Fold axis points into page; Structure contours in red Surface curls over fold axis; Fold axis points into page; Structure contours in red GG303 8 4 8/26/11 5. STRUCTURE CONTOURS AND MAP PATTERNS V Appearance of folded beds on a geologic map (cont.) C If a folded layer changes dip, then strike lines along structure contours with a uniform contour interval will not be evenly spaced. D Dip direcQon and magnitude may or may not be constant (e.g., fold with a horizontal fold axis). E Cross secQons and maps together are powerful 3 ­D visualizaQon tools, whether on paper or on a computer. 8/26/11 GG303 9 5 ...
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