THE CONTROL OF MECHANICAL STRATIGRAPHY ON THE FORMATION OF TRIANGLE ZONES

Triangle zones form in fold-and-thrust belts, in places where foreland -vergence is lost and both fore- and backthrusts form. We have reviewe d the stratigraphy and geometry of triangle zones in fold-and-thrust b elts worldwide. Our review indicates that there are two basic types of triangle zones: Type I involves only a single decollement horizon and is generally associated with cores of detachment folds; Type ii invol ves two or more decollement horizons and is found at the frontal porti on of passive-roof duplexes. We propose that mechanical stratigraphy i s a controlling factor in the formation and development of triangle zo nes. Type I triangle zones form over very weak decollements, such as e vaporites. Type II triangle zones have three mechanical units: a duple x in relatively strong units, overlain by a cover sequence in weaker r ocks separated by a roof decollement. The duplex and decollements invo lved in Type II triangle zones are not much different in strength than those found in thrust belts without triangle zones. However, the cove r sequence is consistently weaker and shalier in thrust belts with tri angle zones than in those without, suggesting that the strength of the cover stratigraphy determines where triangle zones do and do not form . Reconstructions of the frontal stratigraphy of the Wyoming thrust be lt suggest that triangle zones may not have formed in this region unti l late stages of thrust belt evolution, when significant synorogenic d eposits accumulated at the deformation front. The synorogenic deposits often provide the weak shale-rich rocks for the cover sequence to the triangle zone, which is thrust backwards and bent up over a duplex. O ften, the backthrust may be located within the synorogenic deposits, i nstead of at their base. The location of the backthrust in the synorog enic deposits is tentatively proposed to coincide with a mechanical bo undary related to the smectite to illite transition.