Middle Pleistocene seismically induced clay diapirism in an intraplate zone, western Brittany, France

Abstract

The Brittany region of France is located in a low seismicity intraplate zone. Most of the instrumented earthquakes are limited to a shallow crustal depth without surface rupture. A paleoseismological analysis was performed on deposits on the Crozon Peninsula and in the Elorn estuary. We highlight hydroplastic deformations induced by liquefaction leading to clay diapirism, which were likely triggered by past earthquakes. This diapirism seems to be frequent in continental nonconsolidated sediments and to develop on the inherited tectonic structures, when a shallow water table and confining layers exist. Timing of deformation is dated using paleoenvironmental data, and electron spin resonance and infrared-stimulated luminescence dating methods. Two seismic periods were identified in western Europe during early Marine Oxygen Isotope Stage (MIS) 10 (~380 ka) and early MIS 8 (~280–265 ka). The lack of similar deformations affecting the Holocene tidal deposits in the Bay of Brest suggests that the magnitude of the triggering paleoearthquakes is probably higher (Mw~6) than the recent events (Mw5.4). These unusual intraplate major paleoearthquakes need specific factors affecting the far-field crustal stress loading to be triggered, such as a brief acceleration of the Africa-Eurasia lithospheric plate convergence, glacio-isostatic stress perturbations associated with the onset of major glaciations in northern Europe, or other processes induced by orbital forcing.

Is the Machecoul fault the source of the ∼M6 1799 Vendée earthquake (France)?

“…Several of the fault segments were also considered as reactivated syn-to post-Pliocene or during the Quaternary (see Jomard et al 2017, and references therein). This is not an unique case in Brittany: indeed, Quaternary activity was also described in Crozon Peninsula where basement faults were reactivated during Pleistocene palaeo-earthquakes (Van Vliet-Lanoë et al 2018). Reactivated faults during the Quaternary have been debated close to the north of the Loire estuary, that is to the north of our studied area (Fig.…”

Le gisement paléolithique inférieur de Pen Hat (Crozon, Bretagne) et son contexte stratigraphique régional

“…De nouvelles datations, cette fois par luminescence stimulée par infrarouge (IRSL sur feldspaths potassiques ; tab. 2) ont été réalisées sur la coupe (Van Vliet-Lanoë et al, 2019a), permettant de dater plusieurs unités supérieures, et notamment l'unité PH7 du MIS 6. Ces nouveaux éléments de datation radiométrique impliquent de facto un encadrement de l'âge du niveau archéologique entre un âge maximal de 473 à 333 ka (MIS 11 à 9) et un âge minimal de 180-154 ka (MIS 6).…”

“…Le sable marin fin pédogénéisé (PH3-PH4 ; figs. 2 & 4) est remanié lors de la seconde crise séismique observée régionalement vers 280 ka (Van Vliet-Lanoë et al, 2019a).…”

“…Sur la base d'enregistrements sédimentaires et pédo-sédimentaires et, de datations radiométriques et physiques, les formations littorales pléistocènes de la côte occidentale de l'Europe ont été découpées en trois membres (Van Vliet-Lanoë et al, 2000, 2019a, dont les unités stratigraphiques ont été corrélées aux stades isotopiques marins (MIS) définis par Railsback et al (2015), occupant la plateforme jusqu'à environ +5 m NGF au moins huit fois depuis le MIS 15 :…”

“…Nous traiterons de la nature de l'industrie lithique observée à la base du stratotype de Pen Hat en Crozon et de ce que le gisement apporte à la connaissance des premiers peuplements bretons et plus largement du Paléolithique inférieur d'Europe occidentale. La reprise des datations de 2018 (Van Vliet-Lanoë et al, 2019a) a permis de lever certaines ambiguïtés stratigraphiques (tab. 2).…”