STRUCTURAL AND STRAIN ANALYSIS OF CRYOGENIAN STRATA, GHUBRAH BOWL, NORTHERN OMAN

Basement rocks in the Jebel Akhdar massif of northern Oman preserve glaciogenic deposits associated with Cyrogenian glaciations. In this study, we examine these low-grade metasedimentary rocks exposed in the Ghubrah Bowl, an erosional window in the Jebel Akhdar massif. Our purpose is to (1) characterize the structural geometry in the Ghubrah Bowl, (2) quantify the finite strain in these deformed rocks, and (3) determine the timing of deformation with ⁴⁰Ar/³⁹Ar geochronology of syn-tectonic sericite.

In the Jebel Akhdar massif, the Ghubrah Formation consists primarily of diamictite interbedded with sandstone, siltstone, and conglomerate. Diamictites contain clasts of sandstone, siltstone, granite, and metavolcanic rock in a fine-grained matrix. Clasts range from granules to boulders. Harder clasts tend to be subangular and poorly aligned with low aspect ratios (<1.5:1), whereas fine-grained rock clasts are well-aligned with large aspect ratios (>3:1). Primary layering in the Ghubrah Formation generally dips to the NW, whereas the penetrative foliation strikes E-W and dips to the S. A prominent elongation lineation/pencil structure plunges gently to moderately to the south. Rf/phi strain analysis at the hand sample and thin section scale reveal moderate constrictional strain (L>S tectonites). We interpret the strain data to indicate that these rocks experienced two episodes of deformation. Outsized clasts of granite and sandstone are mantled by distinctive wing-like structures, herein defined as double-duckbill structures, that are developed in the pressure shadow. These double-duckbill structures include more recrystallized minerals (sericite and calcite) than elsewhere in the foliated diamictite. Overall, the Ghubrah Formation experienced N-S elongation that is incompatible with N-S shortening of the Jebel Akhdar massif during late Cretaceous to Paleogene contraction.