Through the Plate Tectonics theory, a given present-day orogen results from the succession and stacking of different “orogenic stages” through time within a convergent plate boundary. Each of this stage is characterized by a distinctive geological record that is the combination of a deformation style (i.e. structure and spatial evolution), reliefs, basins and sedimentary dynamics, PT-t metamorphic tracks, fluids/magmatic record and, importantly, by which pre-orogenic crustal domain (oceanic/rift domains) this stage is shortening/consuming. Each of these phases is separated from the other by a sharper transitional stage that is referred to a “critical events”.
The succession of these orogenic stages is not random and follow a logical order what defines a “lifecyle”. From this point of view, any orogenic system is getting mature through time going across “maturity” stages and critical events. Present-day orogenic systems can be representative of different maturity stages as far orogenesis can stop prematurely if plate convergence stops. This can explain part of the observed worldwide diversity of mountain belts. The same “maturity” concept can also apply to the pre-orogenic divergence evolution that precedes the formation of an orogen by convergence. Different maturity stages can also be defined from early continental rifts to mature oceanic basins. In the OROGEN research program, we propose that the combination of the divergence and convergence maturity of an orogenic system defines the “OROGENIC ID” of a mountain belt in a new “genetic” orogenic nomenclature – Oro-genetics -. We further think that the first order geological characteristics of an orogenic system can be predicted from its determined “Orogenic ID”.

First of all, this issue is not only relevant for exploration of hydrocarbons but is also fundamental for any geo-resources from which their formation and preservation depend on the geological evolution of a given area (e.g. H2, He, Minerals etc…). As mentioned in my talk, we should also make clear what depends on the local geological parameters (preceding both Alpine divergence and convergence) from what is genetic in a given orogen. Genetic parameters be easily predicted. There are two different aspects that should be considered:

1) How can we determine the orogenic track of an orogenic system depending on its divergence and convergence maturities (what we refer as the “Oro-genic ID”).
2) How does the orogenic track impact the development / preservation / distribution of a petroleum system (i.e. which oro-genic ID applies).

To solve question 1, surface/sub-surface geological data is required to determine at what divergent and convergent maturities the plate boundary stopped during the last “Wilson Cycle”. The key observations that enable to determine the Orogenic IDs will be provided within the OROGEN “head paper” to be released before the end of the year. An algorithm to tackle this is in development at M&U.

Question 2 cannot be solved before obtaining the answer to question 1. Then, each parameter of a petroleum system needs to be linked with the genesis of the orogenic system. The maturity stage at which it develops, the tectonic domain at which it deposits and what tectonic domains were destroyed by orogenesis until present-day. This approach provides keys to predict where a layer of rock was deposited and what overprints may have suffered after its deposition during the different tectonic domains of the orogen.
Thus, this approach is focused on determining the consequences that the different tectonic stages have for the development, preservation and potential destruction of a petroleum system.

For the Pyrenees, in-situ and thermochronological studies show that there is a limited cooling path of sourcing basement areas (reliefs) during this intermediate “proto-collisional” phase (e.g. Bellahsen et al., 2020, Waldner et al., PhD, 2021, Bernard et al., 2021). Detrital thermochronology however suggests that sourcing reliefs may exist out of the present-day high range (Axial Zone). Indeed, rift ages were obtained from L-T detrital thermochonology within Paleocene sediments of peripheral orogenic basins (Ternois et al., 2019, PhD). It strongly argues that sourcing reliefs may have been located further north. (within the North Pyrenean Zone) and is inherited from the former early orogenic phase made of inverted rift domains (e.g. Mouthereau et al., 2014; Tugend et al., 2015; Grool et al., 2018; Ternois et al., 2019; Gomez-Romeu et al., 2019). What was shown is that orogenic strain is actually accommodated in a diffuse way in a relatively wide area at that time, corresponding to (at least) the future axial zone as suggested by Waldner et al. (2021 & PhD). Structurally speaking, strain distribution is not expecting to generate high and located reliefs but rather limited/smoother reliefs by comparison with former and latter (strain localized) orogenic phases. If sourcing, the axial zone would therefore be smooth “dome” as suggested within Waldner’s PhD. The fact that the Paleozoic detrital component is continuously decreasing during the tectonic quiescence (Ternois et al., 2019 & PhD) further supports the limited exhumation and erosion of basement rocks (and therefore formation of reliefs) during this phase. It may also be acknowledged that convergence rates deduced from plate kinematic models (themselves based on the restoration of the Atlantic, e.g. Macchivelli et al., 2017 and references therein) agree on a significantly smaller convergence rates accommodated within the Pyrenees at that time (some are even suggesting extension that is not supported by field evidences so far). These kinematic boundary conditions would anyway result in limiting the formation of reliefs (what is then questionable is the reason of this kinematic change…).

An alternative way to get an image of the pre-collisional topographies is to study fossilized example of early orogenic systems that failed to reach a mature collision. In the Pyrenean system, the best example may be the present-day Basque belt as suggested by Ducoux et al. (2019) and Mirò et al (2020, PhD). These authors suggest that this part of the Pyrenean system never reached a mature collisional stage and stopped while inverting the rift (until the necking zone). Present-day (and Cenozoic) topographies are not related to crustal overthickening (collision) but rather due to the thin-skinned shortening of Meso-Cenozoic rocks above a near top-basement salt décollement. This would correspond to expectations as reliefs for this transitional orogenic phase.