Isochores and 3-D visualization of rising and falling slat diapirs

Diapir fall, which was predicted by physical models, has been identified in salt provinces, such as the South Atlantic margins, the North Sea, and the Paradox Basin (Colorado-Utah). However the 3-D geometry of falling diapirs and their country rock is still poorly understood. 3-D visualization and i sochore patterns from a physical model help elucidate this geometry. The model initially comprised a unit of viscous silicone overlain by a prek inematic sand unit. Sand units representing brittle sediments were deposite d episodically during gravity gliding and spreading. Regional extension tri ggered and eventually widened salt walls, causing them to sag. The 3-D visu alization shows that regional hydrocarbon migration, which tends to be seaw ard during diapir rise and landward during diapir fall, can potentially be orthogonal to local migration along grabens at soft-linked zones of relay r amps. Furthermore, anticlinal culminations may form (1) in horsts that bend along strike and (2) adjoining the fork of Y-shaped salt walls. Sequential isochore maps of the overburden show how patterns of sedimentati on, deformation, and underlying salt thickness changed through time. Isocho res of prekinematic units record only strain: thinned belts record early ex tension. In contrast, isochores of synkinematic units record mostly thickne ss variations due to deposition on actively deforming topography. Isochores above sagging diapirs identify the thickest part of crestal depocenters, w here the most rapid sagging occurred in regions of maximum extension near t he unbuttressed downdip part of the gravity-spreading system. Additionally, asymmetric isochore patterns may reveal underlying half-grabens or tilted symmetric grabens. In relay systems, overlying isochores may indicate which part of a salt wall rose to compensate for sagging elsewhere in the relay. (C) 1999 Elsevier Science Ltd. All rights reserved.