Triangle zones - Geometry, kinematics, mechanics, and the need for appreciation of uncertainties

Triangle zones are important structures found in foreland fold-and-thrust belts all over the world and are commonly associated with tectonic wedging. However, tectonic wedging and consequently the formation of passive hinterland verging roof thrusts requires particular mechanic conditions such as pre-fractured rocks, syntectonic sedimentation, or a specific stratigraphic layering with variable mechanically weak and rigid formations. In this contribution models of triangle zones and hypotheses regarding their formation are reviewed. Our results show that the term triangle zone is often used in a contradictory sense and interpretations are affected by large uncertainties. Passive roof thrusts are not necessarily required in all cases and suggested models of triangle zones, even if they are geometrically and kinematically viable, are hampered by their mechanical implications. With respect to a large number of published triangle zones and associated kinematic as well as mechanic models we present a new definition and classification scheme, which is discussed and applied to natural examples worldwide. We show that natural examples of triangle zones can be separated into two types: (1) detachment dominated and (2) ramp dominated triangle zones. Both types imply particular mechanic conditions of involved detachments and regional dynamics. Regarding the large uncertainties associated with triangle zone interpretation we suggest to only use the term triangle zone in a very stringent manner and only if irrefutable evidences for stratigraphic repetitions within a duplex are given. The results should then be validated with additional kinematic and mechanic considerations. Geometric and kinematic uncertainties as well as implications of the proposed model explaining observed triangular structures should be reflected in the nomenclature used.