OPHIOLITE OBDUCTION AND THRUST SYSTEMS: SIMPLE THRUST STACKING OR GREATER COMPLEXITY AS SHOWN BY THE OMAN MOUNTAINS?

Ophiolite obduction scenarios require emplacement of successively lower "thrust slices" from an external oceanic domain over a "passive" continental margin. The Samail Ophiolite of Oman is one of the best examples with a Late Cretaceous closure of the Tethys Ocean by "piggy-back" thrusting of oceanic lithosphere, seamount volcanic complexes (Haybi), basinal facies (Hawasina Group) and slope facies (Sumeini Group) over the Arabian passive margin (Hajar Supergroup). This geometry suggests craton-directed thrusting with domal culminations related to deep-seated, footwall and lateral ramps implying passive footwall (margin) behavior. However, in both the platform and slope-facies carbonate rocks exposed in the Saih Hatat and Hawasina windows, the dominant structures are regional-scale folds verging away from the craton; these folds have important consequences for craton-directed overthrusting models. The original passive Arabian margin is involved with "thrusting" and over-folding of the margin to the NE in the Late Cretaceous (~76 to 70Ma), as the ophiolite is emplaced over the top to the SW or SSW. For Saih Hatat, the development of these structures also postdates an earlier blueschist to eclogite facies metamorphic event that affects the continental margin. Permissible structural scenarios that incorporate these folds and shear zones include: 1) lateral escape from a rising buoyant crustal slice along the former subduction interface, 2) back-folding (retrocharriage) associated with major oceanwards-directed underthrusting, or 3) simple underthrusting of the margin by the oceanic realm. The domal culminations previously assoicated with Cretaceous thrust systems appear to post date the structures associated with ophiolite emplacement. Fission track dating suggests that uplift and present day morphology of the Oman Mountains is due to Tertiary, post-early Miocene overthrusting and folding.