Jocelyn Barbarand

Improved modeling of fission-track annealing in apatite

Abstract Two abiding issues impact the reliability of apatite fission-track analysis and thermal history inversion in particular: reproducibility of track-length measurements and variability of annealing kinetics. In the companion to this paper, we addressed the first by demonstrating that using c-axis projection to normalize track lengths for crystallographic angle improves reproducibility among measurements acquired by Barbarand et al. (2003a, 2003b). We continue here by looking at the effect of c-axis projection on extrapolation of laboratory measurements to geological time scales. We find that c-axis projection improves agreement between predictions of empirical annealing models fit to measurements by Barbarand et al. (2003b) analysts 1 and 3, further corroborating its effectiveness in ameliorating observer bias. Furthermore, these annealing models closely match predictions from the Carlson et al. (1999) annealing data set, which was obtained by a different analyst using a different etching protocol. Normalizing for angle thus allows us to create a single annealing model that encompasses both data sets and etching techniques. By combining the different compositional varieties of apatite in the two data sets, the resulting model provides an improved basis for estimating apatite kinetic properties in unknowns using proxies such as composition, etch-figure dimension, and unit-cell parameters. Predictions from a fanning curvilinear model form accord with conventional expectations of annealing at high and low temperatures, and its use for thermal history inversion will not introduce spurious late cooling events.

Improved measurement of fission-track annealing in apatite using c-axis projection

Abstract Apatite fission-track length data are used extensively for thermal history inversion. However, several studies have documented instances of poor reproducibility of length data. We address this problem by using c-axis projection to normalize track lengths for crystallographic angle in the extensive laboratory annealing data set acquired by Barbarand et al. (2003a, 2003b). A new simplification reduces the c-axis projection model from six to four fitted parameters. Normalizing for track angle using c-axis projection improves every aspect of length measurement reproducibility examined. It accelerates convergence of mean length in single analyses; increases consistency among replicate measurements by a single analyst; enhances consistency of measurements of the same mounts by different analysts; and improves the match between analyses conducted with and without Cf-irradiation. C-axis projection is also shown to enhance the thermal sensitivity of length data. Based on these results, we assert that c-axis projection is a good means of compensating for observer bias, although it does not overcome differences caused by experimental error.

Polyphased uplift and erosion of the Cévennes (southern France): An example of slow morphogenesis

The Cevennes are bordering the French Massif Central and the Gulf of Lion margin. The morphogenesis of this area results from an interaction between deep-seated and superficial processes, whose origin and timing is still discussed. We attempt a reconstruction of the surrection and erosion history of the area through a multidisciplinary approach including geology, geomorphology, thermochronology and geochronology. Thermochronology shows that the Cevennes basement underwent some 2 km denudation in mid-Cretaceous time. Analyses of the sediments preserved on uplifted surfaces and in peripheral sedimentary basins indicate a differential surface uplift of the Cevennes, of the surrounding calcareous plateaus, and of the coastal plain, that occurred in several stages during the Tertiary. Early Miocene rifting of the Gulf of Lion margin and opening of the NW Mediterranean drastically modified the drainage network. Geomorphology analyses of the incised rivers and karst network suggest that most of the incision results from uplift that occurred sometime in the Serravalian-Tortonian interval. U/Th dating of calcite concretions in karsts allows to chronologically bracket the formation of some fluvial terraces, and to find very low incision rates during the Pleistocene. Most of the morphogenesis predates the Quaternary. This ongoing study shows an example of polyphased and very slow morphogenesis, with present-day landscape including elements as old as Cretaceous.

Constraints on palaeodrainage evolution induced by uplift and exhumation on the southern flank of the Zagros–Iranian Plateau

Foreland sedimentary rocks from the northern Fars region of Iran contain a record of deformation associated with the Cenozoic collision between Arabia and Eurasia that resulted in formation of the Zagros orogen. The timing of the deformation associated with this event is poorly known. To address this we conducted a study of Miocene foreland sedimentary rocks (19.7–14.8 Ma) of the Chahar–Makan syncline using clast composition, clay mineralogy and low-temperature fission-track dating. The results showed that most of the sedimentary rocks were sourced from ophiolitic rocks. Detrital apatite fission-track (AFT) age signatures of Miocene sedimentary rocks record exhumation in the hanging wall of the Main Zagros Thrust and confirm that the change from underthrusting of the stretched Arabian margin to widespread crustal thickening and deformation in the Zagros region is no younger than 19.7 Ma. A transition from Late Oligocene to Mesozoic–Eocene AFT detrital age signatures between 19.7–16.6 Ma and 16.6–13.8 Ma is interpreted to reflect a possible rearrangement of palaeodrainage distribution that resulted from folding and expansion–uplift of the Zagros–Iranian Plateau region. Supplementary material: Details of the succession of the study area, a complete presentation of sample preparation and fission-track age measurements of apatite grains, and examples of X-ray diffractograms are available at http://www.geolsoc.org.uk/SUP18502.

Eocene exhumation of the Tuareg Shield (Sahara Desert, Africa)

The arch-and-basin geometry that characterizes North Africa was achieved at the end of Paleozoic times. It has been subsequently reactivated during the Mesozoic-Cenozoic with, in particular, the development of large topographic anomalies. Among these, the Tuareg Shield forms a topographic high in which the Pan-African basement reaches 2400 m above sea level (Hoggar core). While Cretaceous sedimentary remnants suggest a possible stage of subsidence during the Mesozoic, currently the area forms a swell, emphasized by Cenozoic volcanic episodes since 35 Ma. In this context, we present the first apatite (U-Th)/He thermochronological data acquired across this swell, with mean ages ranging from 78 ± 22 Ma to 13 ± 3 Ma. These results demonstrate the existence of a widespread Eocene exhumation of the shield before volcanic activity began, which reflects large-scale vertical processes. In the northeastern part of the swell, Cretaceous continental sedimentary remnants unconformably lying on the basement close to our samples evidence that they were near the surface at that time. This study shows that basement rocks have undergone subsequent heating at ∼60–80 °C, suggesting a burial of more than 1 km after the Early Cretaceous. This conclusion can be possibly extended over the whole Tuareg Shield.

A scenario for late Neogene Andean shortening transfer in the Camisea Subandean zone (Peru, 12°S): Implications for growth of the northern Andean Plateau

Precise knowledge of the timing of deformation in the Subandean zone of the Andean Plateau is a prerequisite for deciphering the late Neogene growth of the Andean Plateau. In this paper, we report new apatite fission-track (AFT) and vitrinite reflectance (Ro) data for a regional balanced cross section of the Camisea Basin in the central Peruvian Subandean zone, adjacent to the northern Andean Plateau. The balanced cross section shows that the structure of this basin is characterized by a broad internal passive roof duplex and external thrust- related anticlines. The balanced cross-section restoration shows 53 km (39%) of total horizontal shortening. We sampled Paleozoic to Cenozoic sedimentary strata for AFT and Ro analyses along the ∼4-km-thick vertical profile of the Mainique back thrust (passive roof thrust), the innermost preserved Subandean structure. Young components of AFT ages are spread between ca. 6 Ma and ca. 24 Ma. A break in the slope in the AFT ages determines the geometry of the Miocene partial annealing zone and the exhumation of the Mainique back thrust at ca. 6 Ma. Sequential restoration calibrated by AFT and Ro data indicates that the last ∼23 km horizontal shortening were accommodated by the Camisea thrust system over the past ∼6 m.y., giving a mean shortening rate of 3.8 mm/yr. Using this shortening rate for the first ∼30 km horizontal shortening, we calculate that the Andean shortening transfer into the Peruvian Subandean zone initially started at ca. 14 Ma. This result suggests that the transfer of shortening from the northern Andean Plateau to the Subandean zone occurred prior to the removal of dense lithosphere previously reported to have occurred between ca. 10 Ma and ca. 7 Ma. We rather propose that the late Neogene growth of the northern Andean Plateau mostly resulted from a continuous crustal shortening combined with lower-crustal flow.

Postrift history of the eastern central Atlantic passive margin: Insights from the Saharan region of South Morocco

The passive margin of South Morocco is a low-elevated passive margin. It constitutes one of the oldest margins of the Atlantic Ocean, with an Early Jurassic breakup, and little geological data are available concerning its postrift reactivation so far. We investigated the postrift thermal history of the onshore part of the margin with low-temperature thermochronology on apatite crystals. Fission track and (U-Th-Sm)/He ages we obtained are significantly younger than the breakup (~190 Ma). Fission track ages range from 107 ± 8 to 175 ± 16 Ma, with mean track lengths from 10.7 ± 0.3 to 12.5 ± 0.2 µm. (U-Th-Sm)/He ages range from 14 ± 1 to 185 ± 15 Ma. Using inverse modeling of low-temperature thermochronological data, we demonstrate that the South Moroccan continental margin underwent a complex postrift history with at least two burial and exhumation phases. The first exhumation event occurred during Late Jurassic/Early Cretaceous, and we attribute this to mantle dynamics rather than to intrinsic rifting-related processes such as flexural rebound. The second event, from Late Cretaceous to early Paleogene, might record the onset of Africa/Europe convergence. We show a remarkably common behavior of the whole Moroccan passive margin during its early postrift evolution. The present-day differences result from a segmentation of the margin domains due to the Africa/Europe convergence. Finally we propose that varying retained strengths during rifting and also the specific crustal/lithospheric geometry of stretching explain the difference between the topographical expressions on the continental African margin compared to its American counterpart.

Vertical movements along the northern border of the West African Craton: the Reguibat Shield and adjacent basins

The absence of a sedimentary record on large cratonic areas often prevents the reconstruction of the history of their vertical movements. The Reguibat Rise, belonging to the West African Craton, is typically a large basement high, whose Meso-Cenozoic history is poorly known because of the extreme reduction of this sedimentary record. In this paper we present the first thermochronological data from the centre of the Reguibat Rise, and combine them with the geometry of the sedimentary formations in the adjoining Tindouf and Taoudeni basins situated north and south, respectively. Fission track analysis on apatite yields ages from 256±21 Ma to 139±8 Ma, and 120±10 Ma by (U–Th)/He dating. Two competing scenarios are tested based on these data with thermal history modelling. We favour the scenario that includes a Late Jurassic/Early Cretaceous cooling of the samples, based on stratigraphical constraints and the thermochronological results. We then propose a new model for the evolution of this region and reveal the occurrence of a previously unknown major exhumation event at the Jurassic/Cretaceous transition accounting for the main present-day structures.

Detailed mineralogy and petrology of manganese oxyhydroxide deposits of the Imini district (Morocco)

Manganese ore in the Cenomanian-Turonian dolostone of the Imini district (south of the High Atlas, Morocco) displays a high Mn content due to the occurrence of pyrolusite, cryptomelane, hollandite sensu stricto, coronadite, romanechite, and lithiophorite. The orebodies occur mainly as three stratabound layers along the ~25-km-long ore belt following a WSW-ENE direction. Using scanning electron microscopy (SEM-EDS) coupled with X-ray diffraction (XRD), we here refine the mineralogy and petrology of Mn oxides and oxyhydroxides in order to constrain the paragenetic sequence and define the main processes of ore formation. Deposition and concentration of Mn oxyhydroxides follow a two-step sequence after dolomitization: (1) replacement of dolomite fabric (dolomite micrite and dolomite rhombs) by hollandite group minerals leading to a textural conservation and then (2) neoformation of collomorphous aggregates in an opened and brecciated system. These observations are consistent with a multistage evolution including multiple reworking and brecciation. Pyrolusite is the main Mn oxide observed in the Imini district during both early and late stages. The superficial environment provides supergene conditions suitable for the deposition of only Mn oxyhydroxides. This supergene environment and the occurrence of multiple bands of hollandite group minerals indicate an external migration (allochthonous supply) of Mn and associated elements, contemporaneous to an in situ chemical and mechanical dissolution of the host dolostone. The lithological heterogeneities of dolostone and its chemical environment compared to less permeable surrounding rocks may have provided the conditions to concentrate Mn oxides and oxyhydroxides.

Mesozoic evolution of NW Africa: implications for the Central Atlantic Ocean dynamics

The Central Atlantic Ocean opened during the Early Jurassic and represents the oldest portion of the Atlantic Ocean. Although the American margin has been well studied, the onshore evolution of its African counterpart is poorly understood. We investigated the evolution of a c. 1300 km transect across the Reguibat Shield (Morocco, Mauritania, Algeria) in the northern West African Craton using low-temperature thermochronology. Fourteen samples were dated using apatite fission-track analysis. Nine of these samples were also dated using (U–Th–Sm)/He analysis. Fission-track ages range from 118 ± 10 to 497 ± 61 Ma, with mean track lengths between 11.2 ± 0.4 and 12.5 ± 0.2 µm. (U–Th–Sm)/He single-grain ages range from 32 ± 3 to 396 ± 32 Ma. Through forward and inverse thermal modelling, we demonstrate that the craton underwent kilometre-scale exhumation between the Early–Middle Jurassic and the Late Cretaceous. Based on our new results, published data on NW Africa and data from the conjugate eastern North American passive margin, we show that this post-rift Early–Middle Jurassic to Early Cretaceous exhumation affected both margins to a similar areal extent and simultaneously. Transient mantle dynamic support is suggested to account for the major erosional phase recorded on both margins. Supplementary material: The method for thermal modelling, the thermal modelling for all samples and the relationship between apatite chemistry and (U–Th)/He ages are available at https://doi.org/10.6084/m9.figshare.c.3770918