Development of fault and vein networks in a carbonate sequence near Hayl al-Shaz, Oman Mountains

Abstract

We present a high-resolution structural study on the dip slope of the southern flank of Jabal Shams in the central Oman Mountains. The objectives of the study were: (1) to test existing satellite-based interpretations of structural elements in the area; (2) prepare an accurate geological map; and (3) collect an extensive structural dataset of fault and bedding planes, fault throws, veins and joints. These data are compared with existing models of tectonic evolution in the Oman Mountains and the subsurface, and used to assess the applicability of these structures as analogs for fault and fracture systems in subsurface carbonate reservoirs in Oman. The complete exposure of clean rock incised by deep wadis allowed detailed mapping of the complex fault, vein and joint system hosted by Member 3 of the Cretaceous Kahmah Group. The member was divided into eight units for mapping purposes, in about 100 m of vertical stratigraphy. The map was almost exclusively based on direct field observations. It includes measurement of fault throw in many locations and the construction of profiles, which are accurate to within a few meters. Ground-truthing of existing satellite-based interpretations of structural elements showed that faults can be mapped with high confidence using remote-sensing data. The faults range into the subseismic scale with throws as little as a few decimeters. However, the existing interpretation of lineaments as cemented fractures was shown to be incorrect: the majority of these are open fractures formed along reactivated veins. The most prominent structure in the study area is a conjugate set of ESE-striking faults with throws resolvable from several centimeters to hundreds of meters. These faults contain bundles of coarse-grained calcite veins, which may be brecciated during reactivation. We interpret these faults to be a conjugate normal- to oblique fault set, which was rotated together with bedding during the folding of the Al Jabal al-Akhdar anticline. There are many generations of calcite veins with minor offset and at high-angle-to-bedding, sometimes in en-echelon sets. Analysis of clear overprinting relationships between veins at high-angle-to-bedding is consistent with the interpretations of Holland et al. (2009a); however we interpret the anticlockwise rotation of vein strike orientation to start before and end after the normal faulting. The normal faults post-date the bedding-parallel shear veins in the study area. Thus these faults formed after the emplacement of the Semail and Hawasina Nappes. They were previously interpreted to be of the same age as the regional normal- to oblique-slip faults in the subsurface of northern Oman and the United Arab Emirates, which evolved during the early deposition of the Campanian Fiqa Formation as proposed by Filbrandt et al. (2006). We interpret them also to be coeval with the Phase I extension of Fournier et al. (2006). The reactivation of these faults and the evolution of new veins was followed by folding of the Al Jabal al-Akhdar anticline and final uplift and jointing by reactivation of pre-existing microveins. Thus the faults in the study area are of comparable kinematics and age as those in the subsurface. However they formed at much greater depth and fluid pressures, so that direct use of these structures as analogs for fault and fracture systems in subsurface reservoirs in Oman should be undertaken with care.

Brittle Deformation of Carbonated Peridotite—Insights From Listvenites of the Samail Ophiolite (Oman Drilling Project Hole BT1B)

“…This was followed by NE‐SW shortening in the Miocene to Pliocene during large‐scale folding of the Jebel Akhdar and Saih Hatat domes, which tilted the earlier structures (Gomez‐Rivas et al, 2014; Grobe et al, 2018). Widespread strike‐slip faults postdate the large‐scale folding (Gomez‐Rivas et al, 2014; Grobe et al, 2018; Virgo et al, 2013). At the northern flank of Jebel Akhdar, final exhumation of the core of the anticlinorium occurred along detachments (Grobe et al, 2018).…”

Structure and age relationship of joint sets on the Lilstock Benches, UK, based on mapping a full resolution UAV-based image

“…The presence of such low angle X-intersections is intriguing, because if joints are uncemented fractures, even with very high anisotropy of the horizontal stress, crosscutting is not possible at such a low angle (Renshaw and Pollard, 1995): instead, the younger joints will abut on the older one without crossing over into the adjacent block. However, joints can cross older joints if cementation of the older joint partly restores the shear strength (Virgo et al, 2013(Virgo et al, , 2014(Virgo et al, , 2016. If joints are completely invisible to the stress field because they are "glued" together with a vein of exactly the same strength and elastic modulus, joints can cross without any deflection.…”