During the Aptian and Albian thick terrigenous and carbonate successions of sediments up to 5000 m thick, including shallow water rudist limestones (Urgonian facies), were deposited in the Basque– Cantabrian Basin of northern Spain as a result of an intense rift‐related subsidence. Based on criteria from surface and subsurface data, syn‐sedimentary faults, folds, angular unconformities, diapirs and sub‐basins are distinguished within the Urgonian successions. Faults are grouped into N– S, E– W, NW– SE and NE– SW families and most are normal and strike‐slip. Folds are gentle anticlines and synclines related to major faults. The angular unconformities have small hiatuses, poor lateral continuity and they are associated with either folds or tilted blocks. Diapirs are related to the intersection of major basement faults and in at least one instance the diapir was fossilized by Late Albian times. Strong differential subsidence controlled by basement faults determined the division of the basin into many subbasins of different sizes, which acted as depocentres (e.g. Bilbao). Despite the tectonic inversion which affected the basin during the Tertiary and created thrusts in their margins and centre, the present position of the syn‐sedimentary tectonic structures gives approximate clues about the broad structural style and this reveals the original model of basin extension. Features characteristic of strike‐slip identified in different parts of the basin are displaced geological lines, wrench corridors, drag effects, thickness shifts, paired uplifts and basins, vaulting of ‘slabs’, decreasing displacements, horsetail and fault splays, ‘chessboard’, oroflexural bending, pull‐apart geometries, in‐line horst slices, and restraining/releasing bends. Sinistral strike‐slip movements along major NW– SE faults are supposedly responsible for transtension, which characterized the basin particularly during the Albian. In this scenario, the main wrench movements would have concentrated along the Oiz domain (Biscay Tertiary Synclinorium) and is a situation that has more in common with the strike‐slip model proposed by some workers for the western Pyrenees, than with the simple extension models proposed for the northern margin of the Bay of Biscay.

However salt has a viscous rheology, overburden rocks adjacent to salt diapirs have a brittle rheology. Evidence of deformation within the overburden has been described from diapirs worldwide. Gravitydriven deposits are also present along the flanks of several diapirs. The well-known example from the La Popa Basin in northern Mexico shows that such deposits may be organized into halokinetic sequences. This leads to several questions: (i) How does diapir growth contribute to overburden deformation? (ii) Are halokinetic sequence models valid for other areas beyond the La Popa Basin. The Bakio diapir and its well-exposed overburden in Basque Country, Spain provides key elements to address these questions. The Bakio diapir consists of Triassic red clays and gypsum and is flanked by synkinematic middle to upper Albian units that thin towards the diapir. The elongate diapir parallels the Gaztelugatxe normal fault to the NE: both strike NE-SW and probably formed together during the middle Albian, as synkinematic units onlap the fault scarp. The diapir is interpreted as a reactive diapir in response to middle Albian motion on the Gaztelugatxe fault. The rate of salt rise is estimated to be about 500 m Myr À1 during this passive stage. During Late Albian, the diapir evolved passively as the Gaztelugatxe fault became inactive. Synkinematic units thinning towards the diapir, major unconformities, slumps and other gravity-driven deposits demonstrate that most deformation related to diapir growth occurred at the sea floor. Halokinetic sequences composed of alternating breccias and fine-grained turbidites recorded cyclic episodes of diapir flank destabilization. This work provides insights into drape fold and halokinetic sequence models and offers a new simple method for estimating rates of diapir growth. This method may be useful for outcrop studies where biostratigraphical data are available and for other passive diapirs worldwide.

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In the mid-Cretaceous Lasarte sub-basin (LSB) [northeastern Basque-Cantabrian Basin (BCB)] contemporaneous and syn-depositional thin-and thick-skinned extensional tectonics occur due to the presence of a ductile detachment layer that decoupled the extension. Despite the interest in extension modes of rift basins bearing intra-stratal detachment layers, complex cases remain poorly understood. In the LSB, field results based on mapping, stratigraphic, sedimentological and structural data show the relationship between growth strata and tectonic structures. Syn-depositional extensional listric faults and associated folds and faults have been identified in the supra-detachment thinskinned system. But stratigraphic data also indicate the activation of sub-detachment thick-skinned extensional faults coeval with the development of the thin-skinned system. The tectono-sedimentary evolution of the LSB, since the Late Aptian until the earliest Late Albian, has been interpreted based on thin-and thick-skinned extensional growth structures, which are fossilized by post-extensional strata. The development of the thin-skinned system is attributed to the presence of a ductile detachment layer (Upper Triassic Keuper facies) which decoupled the extension from deeper sub-detachment basement-involved faulting under a regional extensional/transtensional regime.

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