Interpreting fold and fault geometries from within the lateral to oblique parts of slumps: A case study from the Anglo-Brabant Deformation Belt (Belgium)

Using a rigorous comparison of as many slump fold-based and slump-related fault-based methods as possible, also small complex data-sets can be used to deduce a reliable slump transport direction and sense. This is exemplified using late Early Ordovician to Middle Ordovician slump features of the Anglo-Brabant Deformation Belt. The suggested procedure involves (A) an analysis of fold data by means of (1) the mean axis method, (2) the separation arc method, (3) the axial–planar intersection method, (4) the fold hinge azimuth and interlimb angle method, (5) the fold axial surface strike and interlimb angle method and (6) the axial surface dip and dip direction method, (B) an analysis of slump-related fault data by means of (1) the mean fault orientation method, (2) the fault separation method, (3) the fault intersection method and (4) the best-fit girdle to fault poles method, and finally (C) a rigorous comparison of the results of the different methods. l possible diagnostic criteria are put forward for distinguishing slump deformation features trending at low angles to transport direction (oblique to lateral parts of slump sheets or internal lobes or tongues) and slump deformation features trending at high angles to transport direction (frontal to central parts of slump sheets). These criteria include the relationship between fold interlimb angle and axial surface dip, the plunge of the fanning axis of the reverse slump-related faults and the plunge of the intersection of the reverse and normal slump-related faults. Such criteria greatly facilitate the interpretation of the often confusing results of many classical methods. Moreover, the plunge azimuth of the intersection of the normal and reverse slump-related faults and the mismatch between slump transport direction inferred from fold-based and fault-based methods, may both be diagnostic for dextrally deformed or sinistrally deformed slump sheet parts.