| 1. |
- Ring, Uwe, et al.
(författare)
-
The Samail subduction zone dilemma : Geochronology of high-pressure rocks from the Saih Hatat window, Oman, reveals juxtaposition of two subduction zones with contrasting thermal histories
- 2024
-
Ingår i: Earth-Science Reviews. - 0012-8252 .- 1872-6828. ; 250
-
Forskningsöversikt (refereegranskat)abstract
- The Samail Ophiolite in the Oman Mountains formed at a Cretaceous subduction zone that was part of a wider Neo-Tethys plate-boundary system. The original configuration and evolution of this plate-boundary system is hidden in a structurally and metamorphically complex nappe stack below the Samail Ophiolite. Previous work provided evidence for high-temperature metamorphism high in the nappe pile (in the metamorphic sole of the Samail Ophiolite), and high-pressure metamorphism in the deepest part of the nappe pile (Saih Hatat window), possibly reflecting a downward younging, progressive accretion history at the Samail subduction zone. However, there is evidence that the two subduction-related metamorphic events are disparate, but temporally overlapping during the mid-Cretaceous. We present the first geochronologic dataset across the entire high-pressure nappe stack below the Samail Ophiolite, and the shear zones between the high-pressure nappes. Our 22 new Rb-Sr multimineral isochron ages from the Saih Hatat window, along with independent new field mapping and kinematic reconstructions, constrain the timing and geometry of tectonometamorphic events. Our work indicates the existence of a highpressure metamorphic event in the nappes below the ophiolite that was synchronous with the hightemperature conditions in the metamorphic sole. We argue that the thermal conditions of these synchronous metamorphic events can only be explained through the existence of two Cretaceous subduction zones/segments that underwent distinctly different thermal histories during subduction infancy. We infer that these two subduction zones initially formed at two perpendicular subduction segments at the Arabian margin and subsequently rotated relative to each other and, as a consequence, their records became juxtaposed: (1) The hightemperature metamorphic sole and the Samail Ophiolite both formed above the structurally higher, outboard, 'hot' and rotating Samail subduction zone and, (2) the high-pressure nappes developed within the structurally lower, inboard, 'cold' Ruwi subduction zone. We conclude that the formation and evolution of both subduction zones were likely controlled by the density structure of the mafic-rock-rich Arabian rifted margin and outermost Arabian Platform, and the subsequent arrival of the buoyant, largely mafic-rock-free, full-thickness Arabian lithosphere, which eventually halted subduction at the southern margin of Neo-Tethys.
|
|
| 2. |
- Hansman, Reuben J., et al.
(författare)
-
Absolute ages of multiple generations of brittle structures by U-Pb dating of calcite
- 2018
-
Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 46:3, s. 207-210
-
Tidskriftsartikel (refereegranskat)abstract
- Direct dating of brittle structures is challenging, especially absolute dating of diagenesis followed by a series of superimposed brittle deformation events. We report 22 calcite U-Pb ages from tectonites and carbonate host rocks that date 3 diagenetic and 6 brittle deformation events. Results show that U-Pb dating of calcite fibers from these structures is compatible with overprinting relationships. Ages indicate that diagenesis occurred between 147 +/- 6 Ma and 103 +/- 34 Ma, and was followed by top-to-the-south, layer-parallel shearing due to ophiolite obduction at 84 +/- 5 Ma (2 sigma errors). Sheared top-to-the- northeast, layer-parallel veins were dated as 64 +/- 4 Ma and are interpreted to have developed during postobduction exhumation. After this event, a series of strike-slip structures, which crosscut and reactivated older faults due to northwest-southeast horizontal shortening, were dated as 55 +/- 22 Ma and 43 +/- 6 Ma. Eight ages from strike-slip faults and thrusts resulting from northeast-southwest shortening range from 40.6 +/- 0.5 Ma to 16.1 +/- 0.2 Ma. The youngest ages are from minor overprinting fibers ranging in age between 7.5 +/- 0.9 Ma and 1.6 +/- 0.6 Ma. Our results show that U-Pb dating of calcite fibers can be successfully used to constrain a complicated succession of brittle deformation structures that encompasses two orogenies and an intervening extension period.
|
|
| 3. |
- Hansman, Reuben J., et al.
(författare)
-
Jabal Hafit anticline (UAE and Oman) formed by decollement folding followed by trishear fault-propagation folding
- 2018
-
Ingår i: Journal of Structural Geology. - : Elsevier BV. - 0191-8141 .- 1873-1201. ; 117, s. 168-185
-
Tidskriftsartikel (refereegranskat)abstract
- Creating three-dimensional (3D) models to replicate geological structures is crucial in discerning the geometry and kinematics of an orogen. The Jabal Hafit anticline, in the foreland of the Al Hajar Mountains, extends through Oman and the United Arab Emirates and is a relatively simple and well exposed structure. Surprisingly, previous studies of this anticline have presented conflicting interpretations regarding the strain field and the timing of deformation. In this study a structural 3D geological model of the Jabal Hafit anticline is constructed and demonstrates that the anticlines geometry can be reproduced by a two-phase model: (1) flexural slip folding, followed by (2) trishear fault-propagation folding. The trishear mode occurred due to a west-dipping backthrust that propagated from the decollement, to accommodate WSW-directed shortening. The different geometries of the various biostratigraphic layers indicate that anticline growth occurred during the late Oligocene to early -middle Miocene. This timing supports the recently established age for uplift of the Al Hajar Mountains.
|
|
| 4. |
- Hansman, Reuben J., et al.
(författare)
-
Late Eocene Uplift of the Al Hajar Mountains, Oman, Supported by Stratigraphy and Low-Temperature Thermochronology
- 2017
-
Ingår i: Tectonics. - 0278-7407 .- 1944-9194. ; 36:12, s. 3081-3109
-
Tidskriftsartikel (refereegranskat)abstract
- Uplift of the Al Hajar Mountains in Oman has been related to either Late Cretaceous ophiolite obduction or the Neogene Zagros collision. To test these hypotheses, the cooling of the central Al Hajar Mountains is constrained by 10 apatite (U-Th)/He (AHe), 15 fission track (AFT), and four zircon (U-Th)/He (ZHe) sample ages. These data show differential cooling between the two major structural culminations of the mountains. In the 3km high Jabal Akhdar culmination AHe single-grain ages range between 392 Ma and 101 Ma (2 sigma errors), AFT ages range from 518 Ma to 324 Ma, and ZHe single-grain ages range from 62 +/- 3Ma to 39 +/- 2 Ma. In the 2 km high Saih Hatat culmination AHe ages range from 26 +/- 4 to 12 +/- 4 Ma, AFT ages from 73 +/- 19Ma to 57 +/- 8 Ma, and ZHe single-grain ages from 81 +/- 4 Ma to 58 +/- 3 Ma. Thermal modeling demonstrates that cooling associated with uplift and erosion initiated at 40 Ma, indicating that uplift occurred 30 Myr after ophiolite obduction and at least 10 Myr before the Zagros collision. Therefore, this uplift cannot be related to either event. We propose that crustal thickening supporting the topography of the Al Hajar Mountains was caused by a slowdown of Makran subduction and that north Oman took up the residual fraction of N-S convergence between Arabia and Eurasia.
|
|
| 5. |
- Hansman, Reuben J., et al.
(författare)
-
Oligocene–Miocene trishear fault-propagation folding of the Jabal Hafit Anticline, supported by a three-dimensional geological model; and assessing structure-from-motion (SfM) photogrammetry of unmanned aerial vehicle (UAV) photographs for mapping
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Creating three-dimensional (3D) models to replicate geological structures is crucial in discerning the geometry and kinematics of a field area. Adopting new technologies such as unmanned aerial vehicle (UAV) acquired photographs processed by structure-from-motion (SfM) software to create terrain models will enhance structural mapping. The Jabal Hafit anticline which extends through Oman and the United Arab Emirates is a relatively simple and well exposed structure, located in the foreland of the Al Hajar Mountains. Surprisingly, previous studies of this anticline have presented conflicting interpretations regarding the strain field and the timing of deformation. In this study a structural 3D geological model of the Jabal Hafit anticline is constructed and shows that its geometry can be reproduced by a trishear fault-propagation fold. This fold formed above a west-dipping thrust that accommodated WSW–directed shortening. The different geometries of the various biostratigraphic layers indicate that anticline growth occurred during the late Oligocene through to the early–middle Miocene. This timing supports the recently established age for uplift of the Al Hajar Mountains. The anticline was also used as a test site for carrying out UAV-SfM mapping and shows that this is an invaluable tool for the field geologist.
|
|
| 6. |
- Hansman, Reuben J., et al.
(författare)
-
Structural architecture and Late Cretaceous exhumation history of the Saih Hatat Dome (Oman), a review based on existing data and semi-restorable cross-sections
- 2021
-
Ingår i: Earth-Science Reviews. - : Elsevier BV. - 0012-8252 .- 1872-6828. ; 217
-
Forskningsöversikt (refereegranskat)abstract
- The Saih Hatat Dome in the Al Hajar Mountains provides an outstanding opportunity to study subduction/exhumation processes coeval with obduction of the Semail Ophiolite. The exceptionally good outcrop conditions offer a unique opportunity to constrain the geometry of this subduction/obduction complex. In this review, the metamorphic, structural, and tectonic evolution of the Oman high-pressure complex in the Saih Hatat Dome is discussed. New structural cross-sections are developed and are used to interpret a geometrically feasible tectonic model for the Saih Hatat Dome. Our review highlights the importance of two major tectonic boundaries: (1) The As Sheik Shear Zone which separates the high pressure rocks of the As Sifah Unit (1.7–2.3 GPa and 510–550 °C) from the overlying Hulw Unit (1.0–1.2 GPa and 250–300 °C), and was active at ~79–76 Ma; and (2) the Upper Plate–Lower Plate Discontinuity, which forms a major surface in the landscape and developed by ~76–74 Ma, cutting through structures of the HP rocks in the lower plate (footwall). This discontinuity is associated with a pronounced strain gradient, notably in its upper plate (hanging wall), and separates rocks that have markedly different deformation geometry. The Upper Plate–Lower Plate Discontinuity initiated with a modest dip angle, making it a neutral structure in terms of crustal shortening vs extension. As a result, there is no discernable break in P-T conditions across it. The upper plate is dominated by the Saih Hatat Fold Nappe, forming between ~76 and 70 Ma. Subsequently, the upper plate has been dissected by a number of NNE-dipping thrusts at ~70–66 Ma, followed by normal faults at <~66 Ma. Our review and tectonic model indicate that the Oman high-pressure rocks were exhumed in a contractional tectonic setting that was possibly driven by forced return flow assisted by buoyancy forces. During this exhumation, when the rocks reached the greenschist-facies middle crust the Upper Plate–Lower Plate Discontinuity formed, as a shallow, south-dipping backthrust. Final exhumation of the high-P rocks was achieved by late normal faults after ~66 Ma.
|
|
| 7. |
- Hansman, Reuben J., et al.
(författare)
-
Workflow : From photo-based 3-D reconstruction of remotely piloted aircraft images to a 3-D geological model
- 2019
-
Ingår i: Geosphere. - 1553-040X. ; 15:4, s. 1393-1408
-
Tidskriftsartikel (refereegranskat)abstract
- Geological field mapping is a vital first step in understanding geological processes. During the 20th century, mapping was revolutionized through advances in remote sensing technology. With the recent availability of low-cost remotely piloted aircraft (RPA), field geologists now routinely carry out aerial imaging without the need to use satellite, helicopter, or airplane systems. RPA photographs are processed by photo-based three-dimensional (3-D) reconstruction software, which uses structure-from-motion and multi-view stereo algorithms to create an ultra-high-resolution, 3-D point cloud of a region or target outcrop. These point clouds are analyzed to extract the orientation of geological structures and strata, and are also used to create digital elevation models and photorealistic 3-D models. However, this technique has only recently been used for structural mapping. Here, we outline a workflow starting with RPA data acquisition, followed by photo-based 3-D reconstruction, and ending with a 3-D geological model. The Jabal Hafit anticline in the United Arab Emirates was selected to demonstrate this workflow. At this anticline, outcrop exposure is excellent and the terrain is challenging to navigate due to areas of high relief. This makes for an ideal RPA mapping site and provides a good indication of how practical this method may be for the field geologist. Results confirm that RPA photo-based 3-D reconstruction mapping is an accurate and cost-efficient remote sensing method for geological mapping.
|
|
| 8. |
- Peillod, Alexandre, et al.
(författare)
-
The role of ductile flow of the lower crust in controlling heat advection in the footwall of the Naxos extensional fault system (Aegean Sea, Greece)
-
Tidskriftsartikel (refereegranskat)abstract
- Geothermobarometric and structural work indicates that metamorphism in the footwall of the migmatitic Naxos core complex is strongly coupled to heat advection in the lower crust. Using the average pressure-temperature (P-T) method of THERMOCALC for geothermobarometric calculations coupled with detailed textural work, we constructed P-T paths for nine samples that have different spatial relationships to the Naxos extensional fault system and a migmatite dome in the center of the Naxos core complex. All nine samples show early near isothermal decompression. The late, cooling segment of the P-T paths shows systematic spatial differences in temperature gradients relative to the distance of the samples below the Naxos extensional detachment (representing the top of the Naxos extensional fault system). The differences in late thermal gradients correlate with finite strain, a proxy for the intensity of ductile flow. High finite strain correlates with high thermal gradients that do not change with distance below the Naxos detachment, whereas low finite strain correlates with downward decreasing thermal gradients. The difference in late thermal gradients and finite strain define a thermal asymmetry controlled by ductile flow, which we interpret to show that ductile flow controlled advection of heat and thus high-grade metamorphism on Naxos. We conclude that advective heating is an important driver of metamorphism in extensional settings.
|
|
