Processes: Triangle zones form at the foreland margins of fold and thrust belts in response to sub-horizontal tectonic or gravitational stress causing foreland-verging thrust faults rooted in a basal detachment to flatten upward into an upper detachment zone. Injection of thrust wedges along this upper zone uplifts the overlying para-autochtonous section. The upper detachment zone normally follows an incompetent shaley or evaporitic unit which also forms the upper seal to the envelope of overpressured section and any included or migrating hydrocarbons. Contrary to classical theory relating thrusting to overpressuring, a pre-existing basal overpressured shale is not a prerequisite for thrusting. Instead, the topographic uplift resulting from the wedging process itself generates overpressure at the tip of the wedge. Foreland-progressing repetition of the thrust wedging process forms a thrust belt with a fold belt in the overlying para-autochtonous section.

Problems: The foreland-facing dip slope of a triangle zone is an ideal locus for out-of- sequence gravity slides which, if not recognized, may be drilled in mistake for older and more prospective tectonic structures, and can cause major difficulties in constructing balanced cross-sections. Gravity slides, which occur at both present and former foreland margins during stages of major structural and topographic relief, may themselves have second-order triangle zones at their frontal edges. Errors in the dating of orogenies have resulted where upper detachment surfaces have been misinterpreted as post-tectonic unconformities, especially where long - distance thrusting has inserted an exotic allochtonous section into the foreland sequence.

Petroleum prospects: There are three principal types of structural traps associated with triangle zones; 1) thrust-repeated reservoir units within a triangle zone, examples of which are ubiquitous; 2) fault-propagation and fault-bend folds associated with backthrusts in the para-autochtonous section overlying the triangle zone occur along the eastern margin of the Andes; 3) lenses of fractured and contorted materials within Upper Cretacous detachment zones may have provided the fractured shale reservoirs in oilfields flanking uplifts in the western U.S. A.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah