Reconstruction/Interpretation of Macroscopic Structures from Discontinuous Mesoscopic Observations: a Major Task of Field/Structural Geologists

Geologists face a difficult task when mapping metasedimentary rocks over large areas where outcrops are scarce: reconstruction/interpretation of the macroscopic structure from information gathered at isolated outcrops. Situations can range from very simple to very complex, but most have a unique solution. The basic technique derives from Shackleton's rule: to compare the stratigraphic (polarity) and geometric (angular relation between bedding and cleavage) criteria at each outcrop, to evaluate accordance - if they are accordant, the structure is very simple because antiforms (geometry) are anticlines (polarity) and synforms are synclines (upward facing folds); otherwise antiforms and synforms are, respectively, synclines and anticlines, and the macro-structure is more complex (downward facing folds). When studying, for instance, polyphase deformed metasedimentary rocks of accretionary wedges, the geologist often finds a second deformation phase with a concomitant cleavage, and he must be much more careful...