Absolute ages of multiple generations of brittle structures by U-Pb dating of calcite

Reuben J. Hansman1, Richard Albert2, Axel Gerdes2, and Uwe Ring1 1Department of Geological Sciences, Stockholm University, SE-106 91 Stockholm, Sweden 2Department of Geosciences, Goethe University Frankfurt, 60438 Frankfurt am Main, Germany

ABSTRACT al., 2013). Hematite (U-Th)/He dating from fault zones is another tech- Direct dating of brittle structures is challenging, especially abso- nique used to date thermal anomalies generated by fault slip (Ault et al., lute dating of diagenesis followed by a series of superimposed brittle 2015), although the high temperatures needed to reset the hematite He deformation events. We report 22 calcite U-Pb ages from tectonites system during faulting may not commonly occur at shallow crustal depths. and carbonate host rocks that date 3 diagenetic and 6 brittle deforma- U-Pb dating of calcite fibers has only recently been used for dating tion events. Results show that U-Pb dating of calcite fibers from these brittle faulting (Roberts and Walker, 2016; Ring and Gerdes, 2016; Nuriel structures is compatible with overprinting relationships. Ages indicate et al., 2017). This method is suitable for carbonates and other rocks that that diagenesis occurred between 147 ± 6 Ma and 103 ± 34 Ma, and contain calcite veins and fibers and is not fraught with closure temperature was followed by top-to-the-south, layer-parallel shearing due to ophio- issues, but is technically challenging because of low U concentrations lite obduction at 84 ± 5 Ma (2σ errors). Sheared top-to-the-northeast, (<10 ppm). The few applications of U-Pb dating of calcite fibers have layer-parallel veins were dated as 64 ± 4 Ma and are interpreted to only focused on the direct dating of a single deformation phase and not have developed during postobduction exhumation. After this event, multiple tectonic events. We apply this technique for dating a series of a series of strike-slip structures, which crosscut and reactivated older superimposed brittle deformation events. We study the Al Hajar Moun- faults due to northwest-southeast horizontal shortening, were dated tains in Oman that have undergone various shallow-level deformation as 55 ± 22 Ma and 43 ± 6 Ma. Eight ages from strike-slip faults and events since the Late Cretaceous that ultimately resulted in the poorly thrusts resulting from northeast-southwest shortening range from 40.6 understood development of high topography. ± 0.5 Ma to 16.1 ± 0.2 Ma. The youngest ages are from minor overprint- ing fibers ranging in age between 7.5 ± 0.9 Ma and 1.6 ± 0.6 Ma. Our BRITTLE STRUCTURES IN THE AL HAJAR MOUNTAINS results show that U-Pb dating of calcite fibers can be successfully used Carbonate rocks at the base of the Al Hajar Mountains in Oman (Fig. 1) to constrain a complicated succession of brittle deformation structures record several generations of overprinting brittle structures (Fig. 2) con- that encompasses two orogenies and an intervening extension period. taining calcite fibers (Gomez-Rivas et al., 2014), and provide a unique opportunity for dating calcite that crystallized from the Cretaceous to INTRODUCTION Quaternary. The Al Hajar Mountains, situated on the northeast margin of Brittle structures such as fractures, veins, and faults characterize the the Arabian continental plate, consist of a pre-Permian basement that is kinematics and tectonic history in the shallow crust (Marrett and All- unconformably covered by a middle Permian to Late Cretaceous carbon- mendinger, 1990). In applied geology, the absolute timing of brittle struc- ate platform (Glennie et al., 1974). In the Late Cretaceous, continental tures is important to understand the evolution of ore deposits (Blundell, slope-rise sedimentary rocks and the Semail Ophiolite were thrust on top 2002), oil migration (Fink et al., 2016), fault control of geothermal reser- of the carbonate platform (Searle and Cox, 1999). Subsequently, the Al voirs (Jolie et al., 2015), and earthquake recurrence intervals (Cowan, 1999). For tectonics research, the dating of brittle structures is vital for compre- hending shallow-level deformation and rates of fluid flow in the brittle crust. Areas that have undergone polyphase deformation exhibit sets of struc- tures that crosscut each other, providing a relative time sequence of the deformation events. A powerful tool for interpreting deformation of the shallow crust is absolute dating of brittle structures, but this is inherently difficult. Advances in dating brittle structures have focused on Ar/Ar and Rb-Sr dating of illite from fault gouge (van der Pluijm et al., 2001). Gouge dating is often not practical, as temperatures in the shallow crust are low and may prevent complete syntectonic crystallization of illite, which occurs at temperatures >100 °C. Furthermore, the closure temperature of illite is also debated and depends on the cooling rate and clay mineral- ogy (Ring et al., 2017). Another approach is to date fibers and vein fillings. If crustal fluid pressure exceeds the tensile strength of a rock, the fluid pressure causes an opening where minerals can crystallize (Ramsay, 1980). Sm-Nd dating of calcite veins has been carried out (Uysal et al., 2007), but it is only suc- cessful for samples with a large range in Sm-Nd ratios. Another method Figure 1. A: Simplified geological map of the central Al Hajar Mountains, Oman (adapted from Le Métour et al., 1993). B: Schematic northeast- is U-Th dating of calcite slickenfibers (Nuriel et al., 2012), but that can southwest cross section of the Jabal Akhdar culmination (not to scale). only be applied to time scales of 1–600 k.y. (Pickering, 2017). In addition, C: Present tectonic configuration of the Arabian plate (modified from Th-Pb dating of monazite formed within fractures can be used (Berger et Stern and Johnson, 2010).

GEOLOGY, March 2018; v. 46; no. 3; p. 207–210 | GSA Data Repository item 2018050 | https://doi.org/10.1130/G39822.1 | Published online 4 January 2018 ©GEOLOGY 2018 Geological | Volume Society 46 | ofNumber America. 3 For| www.gsapubs.org permission to copy, contact [email protected]. 207 and Zeh, 2009), and plotted using Isoplot add-in (Ludwig, 2012). Figure Figure 2. A: Sketch of DR1 shows thick section images and LA-ICP-MS spot locations (see crosscutting brittle struc- the GSA Data Repository for more details on the analytical procedure). tures related to nine tectonic events at Jabal Akhdar, Oman (including RESULTS diagenesis 1) (adapted Ages (n = 22) from 11 thick sections were obtained (Table 1); the ages from Gomez-Rivas et are from 1 fossil calcite cement, 4 limestone matrices, and 17 calcite al., 2014; see text for a fibers that formed within the various brittle structures. These structures detailed explanation and GSA Data Repository; see are associated with 6 tectonic events, and range in age from 84 ± 5 Ma footnote 1). B: Top-to-the- to 1.6 ± 0.6 Ma (2σ errors). Matrix and fossil calcite ages range from northeast layer-parallel 147 ± 6 Ma to 103 ± 34 Ma and were sampled from the upper Sahtan veins (event 4) offsetting Group, which, based on foraminifera, has a Kimmeridgian to Portland- a normal fault (event 3) at Wadi Nakhr (hammer ian (157–145 Ma) depositional age (Beurrier et al., 1986). The analytical length is 33 cm). C: results (Table DR2) were plotted in Tera-Wasserburg concordia diagrams Strike-slip vein (event 5) (Fig. DR2) and ages were calculated as lower isochron intercepts using crosscutting layer-parallel Isoplot 3.75 (Ludwig, 2012). veins (event 4) at Wadi Four ages (CV-1, CV-4_V2, CV-5_M1, and SF-1_V1b) have mean Muaydin. D: En échelon veins (event 9) overprinting squared weighted deviates (MSWD) > 2.0. This indicates some scatter a strike-slip vein (event 7) in the data that may be attributed to an underestimated analytical error, at Wadi Nakhr. variation of ages within a calcite vein or veins, a slight open U-Pb system behavior in some domains, or no complete initial equilibration of the Pb isotopes (Rasbury and Cole, 2009). Therefore, these results should be interpreted cautiously. Samples CV-2a, CV-2b_V3, and CV-12 have a Hajar Mountains were affected by postorogenic­ extension, followed by limited 238U/206Pb spread, which produces a low-precision result. late Eocene to Miocene shortening (Warrak, 1996), creating large-scale anticlines forming the Jabal Akhdar and Saih Hatat culminations (Fig. 1) DISCUSSION and the current high topography (Hansman et al., 2017). This study is the first of its kind to carry out LA-ICP-MS U-Pb dating Gomez-Rivas et al. (2014) demonstrated that nine brittle tectonic of calcite to directly date multiple diagenetic and tectonic events. The events occurred from the Late Cretaceous to the Neogene in the Jabal ability to precisely constrain the absolute timing of six brittle events, Akhdar culmination. Structures associated with these nine events are: which span the Late Cretaceous through to the Quaternary, is validated. (1) bedding-confined veins and diagenetic stylolites; (2) top-to-the-south U-Pb dating of diagenetic events together with the fracture history have layer-parallel shearing during the Late Cretaceous orogeny; (3) normal implications for crustal fluid flow, oil migration, and the ability to date and oblique faults; and (4) top-to-the-northeast, layer-parallel veins (both carbonate rocks that have no biostratigraphic controls, such as micrites. 3 and 4 formed during postorogenic extension and footwall unroofing); All U-Pb calcite ages are consistent with the relative sequence of (5) conjugate strike-slip faults during northwest-southeast shortening; events as outlined by Gomez-Rivas et al. (2014). The ages obtained from (6) conjugate strike-slip faults resulting from east-west shortening; the matrix are 147 ± 6 Ma, 130 ± 3 Ma, and 103 ± 34 Ma, and that from (7) conjugate strike-slip veins due to north-south to northeast-southwest a calcite cement in a fossil is 113.3 ± 1.6 Ma. Sample CV-5_M1 (137 ± shortening; (8) doming and thrusting of the Jabal Akhdar anticline; and 8 Ma, MSDW = 17) is a mixed age and cannot be used. These ages are (9) joints resulting from the present-day stress field, northwest-southeast interpreted to date three separate diagenetic episodes related to event 1 compression (Fig. 2). (Fig. 3); these diagenetic episodes are 0–15 m.y. and 12–30 m.y. younger This study utilizes U-Pb geochronology on calcite with laser ablation– than the biostratigraphic age of deposition (157–145 Ma). The calcite inductively coupled plasma–mass spectrometry (LA-ICP-MS) to directly cement in the fossil is even younger (30–45 m.y. after deposition), and is date the brittle structures in the central Al Hajar Mountains. These results interpreted to be a late diagenetic overprint. If any earlier diagenetic events provide the first absolute ages of diagenetic events 1 and tectonic events occurred, they have been overprinted by later episodes. This disparity 2, 4, 5, 7, 8, and 9 as defined by Gomez-Rivas et al. (2014). between depositional and diagenetic ages has previously been observed in calcite cements in ammonites from Africa (Li et al., 2014). Li et al. METHODS (2014) showed that U-Pb ages of calcite cements were overprinted by a Structural field work and sampling was carried out in Wadi Nakhr diagenetic event 10–15 m.y. after deposition, similar to our results. and Wadi Muaydin as well as near the towns of Dank and Tiwi (Fig. 1). The oldest calcite fibers dated are from event 2 structures, which record Samples from Wadi Nakhr and Muaydin are from Jurassic to Early Cre- ophiolite obduction that occurred from 97 to 74 Ma (Searle and Cox, 1999). taceous carbonates. Rocks collected from Dank and Tiwi are from reverse The U-Pb calcite age of 84 ± 5 Ma has an elevated scatter (MSDW = 6.6), faults within middle Eocene limestones, and were not part of the Gomez- which indicates a mixed age due to multiple calcite crystallization episodes Rivas et al. (2014) study. (For details of the rock samples refer to Table during obduction (Fig. 3). Events 3 and 4 are the result of upper crustal DR1 in the GSA Data Repository1.) postobduction extensional deformation (Mann et al., 1990), prior to shorten- From the samples, 13 thick sections were screened for U-Pb dating, of ing events 5–9. The U-Pb calcite age of 64 ± 4 Ma from an event 4 structure which 11 could be dated, using a 193 nm ArF excimer laser coupled to an indicates a late episode of footwall unroofing by normal faulting. Event 5 Element2 SF (sector field technology) ICP-MS. Raw data were corrected shortening structures have calcite ages of 55 ± 22 Ma and 43 ± 6 Ma; these offline using an in-house Microsoft Excel spreadsheet program (Gerdes ages reveal that event 5 occurred during the middle Eocene, and is the earli- est age of shortening that records the onset of the Al Hajar Mountains uplift. For event 7 we obtained an age of 22 ± 4 Ma, sampled from a major 1 GSA Data Repository item 2018050, methods and detailed results, is avail- able online at http://www.geosociety.org/datarepository/2018/, or on request from set of conjugate strike-slip faults with well-developed slickenfibers. Two [email protected] or Documents Secretary, GSA, P.O. Box 9140, Boulder, samples from event 8 structures resulted in four well-defined ages from CO 80301, USA. reverse faults due to northeast-southwest shortening. These ages are

208 www.gsapubs.org | Volume 46 | Number 3 | GEOLOGY TABLE 1. SAMPLE DETAILS AND U-Pb CALCITE AGES Sample Structure Kinematics Event number Age (Ma) ± 2σ MSWD Number of spots

Wadi Nakhr: Lat 23.204344°N, Long 57.212668°E Sahtan Group Carbonates CV-1 Top-to-the-S layer-parallel vein N-S shortening 284* 56.6 40 CV-2a Matrix Diagenesis 1103 34 0.77 CV-2b_M1 Matrix Diagenesis 1130 31.9 28 CV-2b_V1 Top-to-the-NE layer-parallel vein NE-SW extension 46441. 220 CV-2b_V2 Overprinting fiber (minor domain) NW-SE shortening 91.6 0.61.1 6 CV-2b_V3† Normal reactivated as reverseNW-SE shortening 97.5 0.91.3 15 CV-4_V1 Normal reactivated as strike slip NW-SE shortening 54361. 630 CV-4_V2 Overprinting fiber (minor domain) NW-SE shortening 96*43.18 CV-4_V3 Overprinting fiber (minor domain) NW-SE shortening 93.1 1. 91.2 6 CV-5_M1 Matrix Diagenesis 1137*8 17 30 CV-5_V1 WSW dip strike-slip slickenfiber NW-SE shortening 96.9 0.91.2 28 CV-6_M1 Matrix Diagenesis 1147 60.5 5 CV-6_F1 Calcite cement in fossil Diagenesis 1113.3 1. 61.2 19 Dank: Lat 23.550180°N, Long 56.358890°E Hadhramaut Group Limestone CV-7_V1 Thrust (km scale) NNE-SSW shortening 840.0* 0.52.4 38 CV-7_V2 Thrust (km scale) NNE-SSW shortening 816.10.2 1. 319 Wadi Muaydin: Lat 22.993440°N, Long 57.672030°E Kahmah Group Carbonates CV-8 Conjugate strike-slip slickenfiber NNE-SSW shortening 72240.534 CV-12E dip sinistral strike-slip vein NW-SE shortening 555220.8 21 Tiwi: Lat 22.825500°N, Long 59.084220°E Hadhramaut Group Limestone SF-1_V1a Reverse fault NE-SW shortening 840.60.5 1. 617 SF-1_V1b Reverse fault NE-SW shortening 839.0* 1. 42.1 15 SF-1_V2a Reverse fault NE-SW shortening 833.30.5 0.721 SF-1_V2b Reverse fault NE-SW shortening 833.10.2 0.933 SF-1_V3 Reverse fault NE-SW shortening 821. 50.3 1. 034 Note: MSWD is mean square of weighted deviates. *Mixed ages (MSWD > 2.0). †Different thick section.

(1) 40.6 ± 0.5 Ma, 40.0 ± 0.5 Ma, 39.0 ± 1.4 Ma; (2) 33.3 ± 0.5 Ma, 33.1 ± 0.2 Ma; (3) 21.5 ± 0.3 Ma; and (4) 16.1 ± 0.2 Ma. The ages indicate that doming and reverse faulting were episodic and started in the late Eocene at the latest and continued until the early Miocene. Gomez-Rivas et al. (2014) separated strike-slip event 7 (NNE–SSW shortening) from the doming and thrusting event 8 (northeast-southwest shortening), but allowed for the possibility that they could have been contemporaneous. Both events provided overlapping U-Pb calcite ages and have comparable kinemat- ics; this supports a coeval interpretation. The large-scale Jabal Akhdar dome forms the highest topography in Oman and therefore these ages constrain the timing of the Al Hajar Mountains uplift, which is supported by fission-track and (U-Th)/He ages of 40–15 Ma (Hansman et al., 2017). Event 9 structures are described as hairline veins by Gomez-Rivas et al. (2014) and were generated by the present-day northwest-southeast compressive stress field. An age of 6.9 ± 0.9 Ma was obtained from a thin en échelon calcite vein crosscutting event 7 structures (Fig. 2D) and is consistent with northwest-southeast shortening. In addition, 3 other thick sections contained minor calcite fibers that overprint older fibers and are dated as 7.5 ± 0.9 Ma, 3.1 ± 1.9 Ma, and 1.6 ± 0.6 Ma. Sample CV-4_V2 (MSDW = 3.1) is a mixed age (6 ± 4 Ma) and cannot be used to date a single event. We speculate that the present-day stress regime was established by the late Miocene. The timing of uplift that produced the present topography in the Al Hajar Mountains is contestable. Some suggest that uplift is the result of Miocene Zagros collision (Al-Lazki et al., 2002), which occurred more than 400 km north-northwest of Jabal Akhdar. Our calcite U-Pb ages provide absolute constraints on the timing of horizontal shortening events 5–8, which resulted in crustal thickening and the formation of the Al Hajar Mountains. These ages support a late Eocene onset of uplift that ceased by the early Miocene, as suggested in Hansman et al. (2017), where it was proposed that deformation in the Al Hajar Mountains was Figure 3. Summary of 7 dated events (from 1 to 9) in the Al Hajar Moun- related to a slowing of Makran subduction, causing north Oman to take tains, Oman, with the corresponding calcite U-Pb ages (±2σ errors). up the residual fraction of north-south convergence between Arabia and Events 3 and 6 were not dated in this study.

GEOLOGY | Volume 46 | Number 3 | www.gsapubs.org 209 Eurasia, leading to crustal thickening and high topography. In addition, low-temperature thermochronology: Tectonics, v. 36, https://doi​ .org​ /10.1002​ ​ the ages presented in this study provide significant insight into hydro- /2017TC004672. Heap, M.J., Baud, P., Reuschlé, T., and Meredith, P.G., 2014, Stylolites in lime- carbon migration in Oman. Not only do diagenetic processes, such as stones : Barriers to fluid flow?: Geology, v. 42, p. 51–54, https://doi​ ​.org​ stylolite generation, greatly increase porosity in carbonates (Heap et al., /10.1130​/G34900​.1. 2014), but brittle structures can act as conduits that result in increased Jolie, E., Moeck, I., and Faulds, J.E., 2015, Quantitative structural-geological fluid flow. Oil is currently extracted from the same carbonate platform exploration of fault-controlled geothermal systems—A case study from the rocks in the foreland basin of Oman, 100 km southwest of Wadi Nakhr. Basin-and-Range Province, Nevada (USA): Geothermics, v. 54, p. 54–67, https://​doi​.org​/10​.1016​/j​.geothermics​.2014​.10​.003. While no oil traps have been discovered in the mountains, at least two Le Métour, J., Platel, J.P., Béchennec, F., Berthiaux, A., Chevrel, S., Dubreuilh, petroleum migration events have been documented based on solid bitu- J., Roger, J., and Wyns, R., 1993, Geological map of Oman: Muscat, Oman, men found in calcite veins in Wadi Nakhr (Fink et al., 2016). The first Directorate General of Minerals, Oman Ministry of Petroleum and Minerals, oil migration exploited preexisting fractures related to event 1, prior to scale 1:1,000,000. Li, Q., Parrish, R.R., Horstwood, M.S.A., and McArthur, J.M., 2014, U-Pb dat- obduction. The second migration is correlated with events 7 and 8, when ing of cements in Mesozoic ammonites: Chemical Geology, v. 376, p. 76–83, northeast-southwest shortening generated strike-slip faults and uplift of https://​doi​.org​/10​.1016​/j​.chemgeo​.2014​.03​.020. the Al Hajar Mountains. Understanding the absolute ages of structures Ludwig, K.R., 2012, User’s Manual for Isoplot 3.75. A Geochonological Toolkit and diagenetic events controlling oil migration will assist significantly for Microsoft Excel: Berkeley Geochronology Center Special Publication, in structurally complex reservoir dynamics. v. 5, p. 1–75. 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