QUANTIFYING STRAIN AND CONSTRAINING THE DEFORMATION TIMING IN NEOPROTEROZOIC GLACIOGENIC STRATA FROM JEBEL AKHDAR, NORTHERN OMAN

Neoproterozoic rocks exposed in northern Oman record glaciogenic sedimentation during Cryogenian glaciations. Although the depositional history of these rocks is well studied, the significance of post-depositional deformation is largely unconstrained. We examine low-grade metasedimentary rocks exposed in the Ghubrah Bowl, an erosional window in the Jebel Akhdar massif, in order to quantify 3D finite strain, understand deformation kinematics and timing. The older Ghubrah and younger Fiq formations comprise a >1 km thick sequence dominated by diamictite with clasts of siltstone and sandstone, with lesser amounts of granite and metavolcanic rock in a fine-grained quartz + sericite ± chlorite matrix. Clasts range from granules to boulders. Clasts of harder lithologies are subangular and poorly aligned with low aspect ratios, whereas clasts of fine-grained rocks are well-aligned with large aspect ratios. Bedding generally dips to the NW but is gently folded in accord with the overall regional structure. A penetrative foliation strikes E-W and dips to the S. Locally, a prominent elongation lineation/pencil structure occurs and plunges gently to moderately to the S.

Rf/phi strain analysis in the diamictites reveals a range of 3D strain geometries (flattening to constriction) with strain ratios up to 3:1 in XZ sections. Strain is strongly partitioned, as clasts of igneous rock have low aspect ratios and are only weakly aligned. Penetrative strain in clast-supported sandstones is negligible. Outsized clasts of granite and sandstone are mantled by distinctive symmetric pressure shadows (double-duckbill structures) that include more recrystallized minerals than elsewhere in the diamictite. Deformation occurred under sub-greenschist facies conditions and syn-tectonic sericite grew in pressure shadows at temperatures below closure for Ar diffusion in muscovite. ⁴⁰Ar/³⁹Ar geochronology of sericite in pressure shadows yields ages as young as 90 Ma, which are interpreted as mixed ages containing an older detrital and/or diagenetic component and a dominant younger fraction that formed during penetrative deformation. Deformation is associated with emplacement and loading by the Semail Ophiolite & Hawasina Group which was emplaced over the autochthonous Neoproterozoic sequence in the late Cretaceous.