Syn-tectonic sedimentation and thrust-and-fold kinematics at the intra-mountain Montalbán Basin (northern Iberian Chain, Spain)

Abstract

The Montalbán Basin (30 × 15 km wide) is located within the Iberian Chain (N Spain), and filled with syn-tectonic Tertiary (Eocene to Lower Miocene) deposits, with a cumulated thickness of about 2 000 m. The depth to the Palaeozoic basement is maximum at the western part and gradually shallows toward the east, as inferred from gravimetric data. The Montalbán Basin is surrounded by an E–W, N-verging, shallow-dipping thrust to the south and a NW–SE-trending anticline, involving the Palaeozoic, to the north. The southern thrust shows a minimum displacement of 8 km and results from the inversion of Mesozoic normal faults. The northern anticline is asymmetric, its southern limb being vertical in most of its length. Mesozoic extensional faults and Variscan thrusts have acted as controls on the location of the frontal limb of this anticline. The sedimentary, syn-tectonic filling of the Montalbán Basin, can be divided into five (M1 to M5) tectono-sedimentary (unconformity-bounded) units, each with a fining upwards-coarsening upwards vertical evolution. Their sedimentary environment corresponds to alluvial fans, with conglomerates at the basin margins grading to evaporites and limestones at the basin centre. The geometry and kinematics of the structures limiting the basin strongly controlled its sedimentary evolution: the Montalbán Basin was wider during M1 and M2, with a northward provenance of alluvial fans, and closed to its present-day dimensions from M3 to M5, coinciding with the largest displacement and uplift of the surrounding structures, which became the main source areas.