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

The Montalban Basin (30 x 15 km wide) is located within the Iberian Chain ( N Spain), and filled with syn-tectonic Tertiary (Eocene to Lower Miocene) d eposits, with a cumulated thickness of about 2 000 m. The depth to the Pala eozoic basement is maximum at the western part and gradually shallows towar d the east, as inferred from gravimetric data. The Montalban Basin is surro unded 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 t hrust shows a minimum displacement of 8 km and results from the inversion o f Mesozoic normal faults. The northern anticline is asymmetric, its souther n limb being vertical in most of its length. Mesozoic extensional faults an d Variscan thrusts have acted as controls on the location of the frontal li mb of this anticline. The sedimentary, syn-tectonic filling of the Montalba n Basin, can be divided into five (M1 to M5) tectono-sedimentary (unconform ity-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 t he basin strongly controlled its sedimentary evolution: the Montalban 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 becam e the main source areas. (C) 2000 Editions scientifiques et medicales Elsev ier SAS.