Cécile Gautheron

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.

Oligocene–Miocene burial and exhumation of the Southern Pyrenean foreland quantified by low-temperature thermochronology

The central Pyrenees experienced an episode of rapid exhumation in Late Eocene–Early Oligocene times. Erosional products shed from the range during this time were deposited in large palaeovalleys of the southern flank of the Axial Zone, leading to significant sediment accumulation. A recent numerical modelling study of the post-orogenic exhumation and relief history of the central Axial Zone allowed us to constrain this valley-filling episode in terms of timing and thickness of conglomeratic deposits. This paper aims to test these results for the southern fold-and-thrust belt using apatite fission-track and (U–Th)/He analysis on detrital samples from the Tremp–Graus and Ager basins. Inverse thermal-history modelling of the low-temperature thermochronology data indicates that the fold-and-thrust belt was covered during the Late Eocene to Miocene by 0.7–1.6 km of sediments and confirms the timing of re-excavation of the valleys during the Miocene. A detailed analysis of the apatite (U–Th)/He results shows that the significant scatter in grain ages can be explained by the influence of alpha-recoil damage with varying effective uranium content together with distinct pre-depositional thermal histories; the age scatter is consistent with initial exhumation of the sediment sources during the Triassic and Early Cretaceous. Supplementary materials: Modelling results obtained using the Gautheron et al. (2009) diffusion algorithm are available at www.geolsoc.org.uk/SUP18560.

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.

Rift-to-collision transition recorded by tectono-thermal evolution of the northern Pyrenees

The impact of rift-related processes on tectonic and thermal evolution of collisional orogens is poorly documented. Here, we study the northern Pyrenees, a region that has preserved a geological record of the transition from rifting to collision. Using modeling of new low-temperature thermochronological data, including fission track and (U-Th)/He on apatite and zircon, we propose a temporal reconstruction of the inversion of the European rifted margin. Our data confirm that rifting and related cooling started in the Late Paleozoic-Triassic. Throughout the Jurassic and Early Cretaceous the European margin recorded slow heating during postrift subdsidence. Modeling of thermochronological data allows distinguishing subsidence and denudation controlled by south dipping normal faults in granitic massifs that reflect a second episode of crustal thinning at 130–110 Ma. Following onset of convergence at 83 Ma, shortening accumulated into the weak and hot Albian-Cenomanian rift basins floored by both hyperextended continental crust and exhumed subcontinental mantle. The lack of cooling during this initial stage of convergence is explained by the persistence of a high geothermal gradient. The onset of exhumation-related cooling is recognized in the whole Pyrenean region at 50–35 Ma. This timing reveals that the main phase of mountain building started when hyperextended rift basins closed and collision between proximal domains of the rifted margin occurred.

Timing and rate of exhumation along the Litang fault system, implication for fault reorganization in Southeast Tibet

The Litang fault system that crosses the Litang Plateau, a low relief surface at high elevation (~4200–4800 m above sea level) that is not affected by regional incision, provides the opportunity to study exhumation related to tectonics in the SE Tibetan Plateau independently of regional erosion. Combining apatite and zircon fission track with apatite (U-Th)/He thermochronologic data, we constrain the cooling history of the Litang fault system footwall along two transects. Apatite fission track ages range from 4 to 16 Ma, AHe ages from 2 to 6 Ma, and one zircon fission track age is ~99 Ma. These data imply a tectonic quiet period sustained since at least 100 Ma with a slow denudation rate of ~0.03 km/Ma, interrupted at 7 to 5 Ma by exhumation at a rate between 0.59 and 0.99 km/Ma. We relate that faster exhumation to the onset of motion along the left-lateral/normal Litang fault system. That onset is linked to a Lower Miocene important kinematic reorganization between the Xianshuihe and the Red River faults, with the eastward propagation of the Xianshuihe fault along the Xiaojiang fault system and the formation of the Zhongdian fault. Such strike-slip faults allow the sliding to the east of a wide continental block, with the Litang fault system accommodating differential motion between rigid blocks. The regional evolution appears to be guided by the strike-slip faults, with different phases of deformation, which appears more in agreement with an “hidden plate-tectonic” model rather than with a “lower channel flow” model.

Rift flank uplift at the Gulf of California: No requirement for asthenospheric upwelling

Continental rifts are commonly flanked by zones of high elevation, but the cause of uplift remains controversial. Proposed uplift mechanisms include active and induced asthenospheric upwelling, and isostatically driven lithospheric flexure. Discrimination between these hypotheses requires close constraint of the timing of rift flank uplift and crustal extension. Here, we focus on the well-preserved Neogene Gulf of California rift. The western rift margin is characterized by a prominent east-facing kilometer-scale escarpment, which bounds a west-tilted, topographically asymmetric rift flank. We exploit west-draining canyons incised into the rift flank to constrain the timing of uplift to between ca. 5.6 and 3.2 Ma using 40Ar/39Ar dating of lavas, which show cut-and-fill relationships to the canyons. Rift flank uplift closely followed the onset of slip on the principal fault of the Loreto rift segment at ca. 8–6 Ma, the age of which we obtain from apatite (U-Th)/He and fission-track thermochronologic analysis of rift escarpment exhumation. Uplift was therefore coeval with lithospheric rupture and the onset of oceanic spreading between ca. 6 and 3 Ma, but post-dates a proposed asthenospheric upwelling event by ∼8–10 Ma. The timing of uplift is inconsistent with either active or induced upwelling as uplift mechanisms, and we conclude that rift flank uplift was driven by the flexural response to lithospheric unloading.

(Un)Coupled thrust belt-foreland deformation in the northern Patagonian Andes: new insights from the Esquel-Gastre sector (41°30’–43° S)

The Patagonian Andes represent a unique natural laboratory to study surface deformation in relation to deep slab dynamics. In the sector comprised between latitudes 41°30’ and 43°S, new apatite (U-Th)/He ages indicate a markedly different unroofing pattern between the ‘broken foreland’ area (characterized by Late Cretaceous to Paleogene exhumation) and the adjacent Andean sector to the west, which is dominated by Miocene-Pliocene exhumation. These unroofing stages can be confidently ascribed to inversion tectonics involving reverse fault-related uplift and concomitant erosion. Late Cretaceous-Paleogene shortening and exhumation are well known to have affected also the thrust belt sector of the study area during a prolonged stage of flat-slab subduction. Therefore, the different ages of near-surface unroofing documented in this study suggest coupling of the deformation between the thrust belt and its foreland during periods of flat-slab subduction (e.g. during Late Cretaceous-Paleogene times), and dominant uncoupling during periods of steep-slab subduction and rollback, even when these are associated with high convergence rates (i.e. > 4 cm/year), as those documented in Miocene times for the Patagonian Andes.

Slab flattening, magmatism, and surface uplift in the Cordillera Occidental (northern Peru)

The impact of subduction processes on surface uplift and relief building in the Andes is not well understood. In northern Peru, we have access to a modern flat subduction zone (3°–15°S) where both the geometry and timing of the flattening of the slab are well constrained. Some of the highest Andean peaks, the Cordillera Blanca (6768 m) and the Cordillera Negra (5187 m), are located just above the Peruvian flat slab. This is a perfect target to explore the impact of slab flattening and associated magmatism on Andean topography and uplift. We present new apatite (U-Th)/He and fission-track data from three vertical profiles in the Cordillera Blanca and the Cordillera Negra. Time-temperature inverse modeling of the thermochronologi-cal data suggests that regional exhumation in the Cordillera Occidental started at ca. 15 Ma, synchronous with the onset of subduction of the Nazca Ridge and eastward movement of regional magmatism. We propose that ridge subduction at 15 Ma and onset of slab flattening drove regional surface uplift, with an important contribution of mag-matism to relief building in the Cordillera Occidental.

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

Foreland exhumation controlled by crustal thickening in the Western Alps

In alpine-type collision belts, deformation of the foreland may occur as a result of forward propagation of thrusting and is generally associated with thin-skinned deformation mobilizing the sedimentary cover in fold-and-thrust belts. Locally, foreland deformation can involve crustal-scale thrusting and produce large-scale exhumation of crystalline basement resulting in significant relief generation. In this study, we investigate the burial and exhumation history of Tertiary flexural basins located in the Western Alpine foreland, at the front of the Digne thrust sheet (southeast France), using low-temperature apatite fission-track and (U-Th)/He thermochronology. Based on the occurrence of partially to totally reset ages, we document 3.3–4.0 km of burial of these basin remnants between ca. 12 Ma and 6 Ma, related to thin-skinned thrust-sheet emplacement without major relief generation. The onset of exhumation is dated at ca. 6 Ma and is linked to erosion associated with significant relief development. This evolution does not appear to have been controlled by major climate changes (Messinian crisis) or by European slab breakoff. Rather, we propose that the erosional history of the Digne thrust sheet corresponds to basement involvement in foreland deformation, leading to crustal thickening. Our study highlights the control of deep-crustal tectonic processes on foreland relief development and its erosional response at mountain fronts.