The role of inheritance in forming rifts and rifted margins and building collisional orogens: a Biscay-Pyrenean perspective

The aim of this paper is to provide a conceptual framework that integrates the role of inheritance in the study of rifts, rifted margins and collisional orogens based on the work done in the OROGEN project, which focuses on the Biscay-Pyrenean system. The Biscay-Pyrenean rift system resulted from a complex multistage rift evolution that developed over a complex lithosphere pre-structured by the Variscan orogenic cycle. There is a general agreement that the Pyrenean-Cantabrian orogen resulted from the reactivation of an increasingly mature rift system along-strike, ranging from a mature rifted margin in the west to an immature and segmented hyperextended rift in the east. However, different models have been proposed to explain the preceding syn-rift evolution and its influence on the subsequent reactivation. Results from the OROGEN project show a sequential reactivation of rift inherited decoupling horizons and identify the specific role of exhumed mantle, hyperextended and necking domains during reactivation. They also highlight the contrasting fate of segment centres vs. segment boundaries during convergence, explaining the non-cylindricity of internal parts of collisional orogens. Results from the OROGEN project also suggest that the role of inheritance is more important during the initial stages of subduction and collision, which may explain the complexity of internal parts of orogenic systems. In contrast, once tectonic systems get more mature, orogenic evolution becomes mostly controlled by first-order physical processes as described in the Coulomb Wedge theory for instance. This may account for the simpler and more continuous architecture of external parts of collisional orogens. It may also explain why most numerical models can reproduce mature orogenic and rift architectures with better accuracy compared to the initial stages of such systems. Thus, while inheritance may not explain steady-state processes, it is a prerequisite for comprehending the initial stages of tectonic systems. The new concepts developed from the OROGEN research are now ready to be tested at other orogenic systems that result from the reactivation of rifted margins, such as the Alps, the Colombian cordilleras and the Caribbean, Taiwan, Oman, Zagros or Timor.

Rheological inheritance controls the formation of segmented rifted margins in cratonic lithosphere

Observations from rifted margins reveal that significant structural and crustal variability develops through the process of continental extension and breakup. While a clear link exists between distinct margin structural domains and specific phases of rifting, the origin of strong segmentation along the length of margins remains relatively ambiguous and may reflect multiple competing factors. Given that rifting frequently initiates on heterogenous basements with a complex tectonic history, the role of structural inheritance and shear zone reactivation is frequently examined. However, the link between large-scale variations in lithospheric structure and rheology and 3-D rifted margin geometries remains relatively unconstrained. Here, we use 3-D thermo-mechanical simulations of continental rifting, constrained by observations from the Labrador Sea, to unravel the effects of inherited variable lithospheric properties on margin segmentation. The modelling results demonstrate that variations in the initial crustal and lithospheric thickness, composition, and rheology produce sharp gradients in rifted margin width, the timing of breakup and its magmatic budget, leading to strong margin segmentation.

Decoding low-temperature thermochronology signals in Alpine-type orogens: modelling the role of rift thermal imprint into continental collision

Resolving the timing of initiation and propagation of continental accretion associated with increasing topography and exhumation is a genuinely challenging task using low-temperature thermochronology. We present an integrated thermo-mechanical and low-temperature thermochronology modelling study of tectonically-inverted hyper-extended rift systems. Model low-temperature thermochronology data sets for four widely used thermochronological systems are generated from fourteen locations across a model doubly-vergent orogen. Our approach allows prediction of specific, distinct low-temperature thermochronological signatures for each domain of the two rifted margins that, in turn, enable deciphering which parts of the margins are involved in orogenic wedge development. Our results show that a combination of zircon (U-Th)/He and apatite fission-track data allows diagnostic investigation of model orogen tectonics and offers the most valuable source of thermochronological information for the reconstruction of the crustal architecture of the model inverted rifted margins. Comparison of model data for inverted rifted margins with model data for non-inverted, purely thermally-relaxed rifted margins enables assessing the actual contribution of tectonic inversion with respect to thermal relaxation to cooling during convergence. Similarities between our modelling results and published low-temperature thermochronology data from the Pyrenees provide new insights into the evolution of Alpine-type, double-wedged orogenic systems from rifting to collision. In particular, they suggest that the Pyrenean Axial Zone mainly consists of the inverted lower plate necking and hyper-extended domains while the North Pyrenean Zone represents the inverted upper plate distal rifted margin. This is in good agreement with previous, independent reconstructions from literature, showing the power that our integrated study offers in identifying processes involved in orogenesis, especially early inversion, as well as reconstruction of pre-orogenic crustal architecture of hyper-extended rifted margins.

Basement-decoupled hyperextension rifting: The tectono-stratigraphic record of the salt-rich Pyrenean necking zone (Arzacq Basin, SW France)

Half grabens and supra-detachment basins correspond to end-member basin types of magma-poor rift settings, each of them showing a characteristic stratigraphic architecture. The occurrence of a basement-cover décollement has been shown to drastically change the stratigraphic architecture of half graben basins, however, the effect of such basement-cover décollement remains to be documented in supra-detachment basins formed during hyper-extension. We investigate the tectono-stratigraphic record of the Arzacq Basin (SW France) recording the formation of a salt-rich Cretaceous hyperextended rift system. Combining 2-D and 3-D seismic reflection calibrated from well data, we show that this basin is an asymmetric syn-rift extensional syncline growing above a pre-kinematic salt layer. By mapping the sub-salt basement, we show that the formation of this syncline is controlled by the South-Arzacq Fault (SAF), soling in the sub-salt basement. Based on crosscutting relationships and the observed southward migration of syn-rift depocenters, this N110°-striking, 20°-dipping structure accommodates >10 km of thick-skinned extension. The overlying supra-salt cover coherently glided, following the basement geometry. The 3-D segmentation of the SAF and the sub-salt stratigraphic architecture of the Arzacq Basin suggest a roughly dip-slip kinematic. A post-kinematic kilometer-scale uplift is documented on the southern side of the Arzacq Basin. It may result from the increasing lithospheric thinning and thermal support at the end of asymmetric hyperextension. As salt commonly occurs in extensional settings, we believe that our description of the tectono-stratigraphic record of a basement-decoupled supra-detachment basin has global applicability to unleash the tectono-stratigraphic evolution of worldwide hyper-extended rifted margins.