Thermal maturity of the Hawasina units and origin of the Batinah Mélange (Oman Mountains). Insights from clay minerals

The Oman Mountains developed during Cretaceous to Cenozoic time by obduction of the Semail Ophiolite on top of the Arabian rifted margin. The tectonic pile of the orogenic system is composed of three major domains, which from bottom to top are: (i) the proximal domain of the Arabian rifted margin; (ii) the Hawasina Nappe, including rocks pertaining to the distal portions of the Arabian rifted margin; (iii) the Upper Cretaceous Semail Ophiolite. However, in NE Oman (Batinah Coastal Plain), exposures of Hawasina rocks are resting above the ophiolite. What is the origin of the uncommon structural position of Hawasina rocks in the Batinah Coastal Plain? To address this question, an extensive dataset of X-ray diffraction analyses from rocks of the Hawasina units has been used to reconstruct their thermal and burial history since the Late Cretaceous. Temperature-dependent clay minerals indicate that the Hawasina units experienced different levels of thermal maturity from early diagenetic to anchizone conditions depending on their structural position during orogenic build-up. Rocks from the Hawasina units resting above the ophiolite contain random-ordered mixed layer illite-smectite (I–S) with an illite content between 30 and 45% displaying low levels of thermal maturity reflecting early diagenetic conditions that were acquired because of limited sedimentary burial (600 m) and minor heating during the early Pliocene. Deep-water rocks of the Hawasina units below the ophiolite are characterized by long-range ordered mixed layer I–S with an illite content between 80 and 90% and by a mineralogical assemblage of rectorite and pyrophyllite displaying more evolved levels of thermal maturity in deep diagenetic/anchizone conditions that were acquired during the obduction of the 4100 to 5500 m thick Semail Ophiolite. We finally propose a new evolutionary scheme for the genesis of the Batinah Mélange, that was not buried by the ophiolite but was transported by gravity-driven mass transport on top of the ophiolite. Our results may further improve the assessment of thermal maturity of Arabian passive margin deposits for hydrocarbon exploration purposes.