Early Cenozoic Listwaenite Formation at a Major Extensional Fault Zone of the Oman Mountains (Fanja Area) – Insights from Structural Analysis and U-Pb Carbonate Dating
Listwaenites are carbonatized and/or silicified serpentinite rocks. These rocks are common in the Fanja area (~50 km to the SW of Muscat) and typically associated with or near the structural base of the Semail Ophiolite at major extensional faults. The listwaenite forms largely unfolded, tabular bodies with thicknesses of several meters to tens of meters and lengths of a few meters to a few kilometers. Ophiolite obduction onto the Arabian margin and platform ensued to the SW during the Late Cretaceous in the course of the Arabia-Eurasia convergence. During obduction, the Arabian margin descended into a subduction zone and was eventually rapidly exhumed, forming the initial Saih Hatat Dome. Obduction was followed by major top-to-the-NE extension until the Paleocene/Eocene transition (Stage I; Grobe et al., 2019, their Fig. 12). During this event the initial Jabal Akhdar Dome formed. The northern margins of the Jabal Akhdar and Saih Hatat domes are flanked by a major fault zone with a throw of few to several kilometers (Frontal Range Fault, Mattern and Scharf, 2018). The activity of this fault zone ensued immediately after obduction of the ophiolite (Stage I) and again during the Late Eocene to Miocene (Stage II; Hansman et al., 2017; Mattern and Scharf, 2018). The origin of the listwaenite is debated. It either formed within the subduction zone during Late Cretaceous obduction (Falk and Kelemen, 2015) or after obduction along extensional faults in the cause of orogenic collapse (e.g., Wilde et al., 2002; Mattern and Scharf, 2018; Bailey et al., 2019). Our structural investigations revealed that listwaenite remained unfolded. This is in contrast to what is expected within a subduction zone. At one outcrop, a 15 m long and 5 m wide listwaenite body is brittlely to ductilely deformed. Furthermore, listwaenite may form injections into the postobductional latest Cretaceous Al-Khod Formation near Fanja along the Frontal Range Fault. Thus, the listwaenite is postobductional in age. However, it is unclear if the listwaenite formed during Stage I or II along the Frontal Range Fault. For verification we performed U-Pb dating using the LA-SF-ICPMS (e.g., Woodhead and Petrus, 2019) on carbonate veins within the listwaenite. Veins with a thickness of ≤0.5 cm are common within the listwaenite. These veins form a complex network. We interpret the veins to have formed during the final stage of the listwaenite formation because these veins appear undeformed. Two U-Pb dated carbonate veins within the listwaenite near Fanja yielded an age of 55±4 Ma. Thus, the listwaenite formed during Stage I of the Frontal Range Fault activity. Furthermore, the direction of Sigma 3 at this site was NNE-SSW which is in agreement with the postobductional extensional stress field at other parts of northeastern Oman (e.g., Grobe et al., 2019).
AAPG Datapages/Search and Discovery Article #90362 ©2019 AAPG Middle East Region, Geosciences Technology Workshop, 2nd Edition Structural Styles of the Middle East, Muscat, Oman, December 9-11, 2019