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Syntectonic deposits and punctuated limb rotation in an Albian submarine transpressional fold (Mutriku village, Basque-Cantabrian basin, northern Spain)

¹ Estratigrafia eta Paleontologia Saila, Euskal Herriko Unibertsitatea, 644 postakutxa, 48080 Bilbao, Spain
² Natural History Museum, Cromwell Road, London SW7 5BD, UK
³ Estratigrafia eta Paleontologia Saila, Euskal Herriko Unibertsitatea, 644 postakutxa, 48080 Bilbao, Spain

Deep-water syntectonic deposits and angular unconformities record the denudation and deformation history of the middle Cretaceous Aitzeta structure, interpreted here as a monoclinal syncline associated with the high-angle reverse Mutriku fault (Basque- Cantabrian basin, northern Spain). Sedimentological and structural analyses, combined with a precise chronostratigraphy based on ammonites, permit us to document in detail the history of this syncline during 0.53 m.y. (Late Albian, Callihoplites auritus Subzone). Tectonism occurred during two major and two minor short- lived pulses. The sedimentary signatures of these pulses include unconformities and resedimented deposits derived from the uplifted rocks, showing inverted clast stratigraphy. In contrast, intervening phases of tectonic calm were characterized by gentle hemipelagic sedimentation. The major tectonic pulses were characterized by strong rotation and uplift of the syncline limb. The two major pulses coincided with the lower and upper limits of the auritus Subzone, respectively, and resulted in the following limb-rotation and uplift values: 42° and 250 m in the first pulse and 66° and 100–140 m in the second pulse. A maximum duration of 0.12 m.y. for each major pulse has been calculated, from which limb-rotation and uplift rates of 0.35°/k.y. and 2.08 m/k.y. (first pulse) and 0.55°/k.y. and 0.83–1.16 m/ k.y. (second pulse) are estimated. Growth strata and angular unconformity geometries, temporal variation of deformation rates, and bedding-parallel faulting indicate folding by progressive limb rotation and associated flexural slip. Local northwest- southeast compression is deduced in formation of the Aitzeta syncline. The presence of bedding-parallel oblique faulting and minor drag folds suggests a component of right-oblique movement along the Mutriku fault.

Key Words: Albian • Basque • folding • rates • sedimentation • transpression