CONTROLS ON THRUST TECTONICS ALONG BASEMENT-COVER DETACHMENT

Decollement at the base of fold-and-thrust belts follows frequently th e basement-cover contact. Pre-existing local conditions near this cont act have a strong influence on thrust topography. Underneath the Orobi c thrust in the Southern Alps of Italy, a whole series of different ad aptations to local conditions can be examined. Far away from asperitie s the decollement layer itself is attenuated, and the lost material is piled up in duplex stacks more externally. Obstacles like hinterland dipping normal faults are overcome by larger duplexes requiring decoll ement along minor, less efficient detachment horizons. An upward rampi ng thrust encountering a graben with incompetent fill splits into many branches transposing the graben to a stack of imbrications. If the de tachment has reached already the regional decollement horizon and a gr aben starts within the basement below however, the detachment may step down to the base of the incompetent fill, rotating the graben deposit s antithetically and creating ''extensional duplexes''. Basement highs within the graben get detached and subsequently appear as imbrication s between the cumulated basin deposits. Another possibility for a thru st to step down to a lower detachment in faulted area includes synthet ical, domino-like rotation of the slabs between the faults. A thrust t hus has numerous possibilities to react to pre-existing, not perfect l ayercake conditions.