Multiphase Structural Evolution of a Continental Margin During Obduction Orogeny: Insights From the Jebel Akhdar Dome, Oman Mountains

Abstract

The structural evolution of the carbonate platform in the footwall of the Semail ophiolite emplaced onto the passive continental margin of Arabia helps to better understand the early stages of obduction‐related orogens. These early stages are rarely observable in other orogens as they are mostly overprinted by later mountain building phases. We present an extensive structural analysis of the Jebel Akhdar anticline, the largest tectonic window of the Oman Mountains, and integrate it on different scales. Outcrop observations can be linked to plate motion data, providing an absolute timeframe for structural generations consistent with radiometric dating of veins. Top‐to‐S overthrusting of the Semail ophiolite and Hawasina nappes onto the carbonate platform during high plate convergence rates between Arabia and Eurasia caused rapid burial and overpressure, generation and migration of hydrocarbons, and bedding‐confined veins, but no major deformation in the carbonate platform. At reduced convergence rates, subsequent tectonic thinning of the ophiolite took place above a top‐to‐NNE, crustal‐scale ductile shear zone, deforming existing veins and forming a cleavage in clay‐rich layers in early Campanian times. Ongoing extension occurred along normal‐ to oblique‐slip faults, forming horst‐graben structures and a precursor of the Jebel Akhdar dome (Campanian to Maastrichtian). This was followed by NE‐SW oriented ductile shortening and the formation of the Jebel Akhdar dome, deforming the earlier structures. Thereafter, exhumation was associated with low‐angle normal faults on the northern flank of the anticline. We correlate the top‐to‐NNE crustal‐scale shear zone with a similar structure in the Saih Hatat window to develop a unified model of the tectonic evolution of the Oman Mountains.

The Effect of Obliquity of Slip in Normal Faults on Distribution of Open Fractures

“…This depth is not only controlled by mechanical stratigraphy resulting from interlayering of lava flows and ash and scoria layers; lava flows themselves have different strengths depending on their degree of fracturing and jointing. Furthermore, the occurrence of open fractures at depth depends on fluid pressure (Townend and Zoback, 2000); in cohesive carbonates dilatant open fractures have been observed at depths of several kilometers, associated with high fluid pressures (Hilgers et al, 2006;Grobe et al, 2018). We are not aware of fluid pressure data from Iceland measured at depths >1 km and consequently as of now the depth below which open fractures can be maintained remains speculative.…”

Tectonic Evolution of the Northern Oman Mountains, Part of the Strait of Hormuz Syntaxis: New Structural and Paleothermal Analyses and U‐Pb Dating of Synkinematic Calcite

“…The highlighted differences in the tectonic evolution of Central Oman Mountains and Musandam are likely indicative of noncylindrical structures for the Oman Mountains. Another evidence of noncylindrical structures is provided by minor differences in the tectonic evolution of the three tectonic windows of the Central and Southeastern Oman Mountains (Hawasina, Jabal Akhdar, and Saih Hatat), highlighted by Grobe et al (2018).…”

“…Owing to a lack of geochronological data, an earlier beginning of Cenozoic compression in the Musandam area cannot be excluded, as tentatively indicated by the dashed thrust in Figure 10c. This tectonic event may be interpreted as a result of the onset of collision between the Arabian and Eurasian plates (Figure 10c; e.g., Glennie et al, 1990; Grobe et al, 2018; Loosveld et al, 1996; Searle, 1988b; Searle et al, 2014). Underthrusting of Arabia beneath Eurasia began at least 35 Ma ago (Figure 10c; Bigi et al, 2018; Mouthereau et al, 2012).…”

Gondwana accretion tectonics and implications for the geodynamic evolution of eastern Arabia: first structural evidence of the existence of the Cadomian Orogen in Oman (Jabal Akhdar Dome, Central Oman Mountains)

“…The final doming of the JAD occurred from Eocene to Miocene (Hansman et al, 2017(Hansman et al, , 2018Grobe et al, 2018Grobe et al, , 2019. This process was accompanied by extensional shearing at the northern and eastern margin of the dome (Mattern and Scharf, 2018;Scharf et al, 2019), and tilting of the Arabian rocks along a WNW-ENE trending rotation axis at the northern and southern margin of the dome by ∼30° and ∼20°, respectively (compare Beurrier et al, 1986;Rabu et al, 1986).…”

“…Obduction was followed by extension (e.g., Fournier et al, 2006;Searle, 2007;Mattern and Scharf, 2018;Grobe et al, 2018Grobe et al, , 2019 as indicated by extensional faults, which displaced the ophiolite Searle, 2007). It is worth mentioning that both SHD and JAD display large-scale listric normal faults bounding their flanks (Searle, 2007).…”