Magnetobiochronology of lower Pliocene marine sediments from the lower Guadalquivir 1 Basin : insights into the tectonic evolution of the Strait of Gibraltar area 2 3

32 The Gibraltar Arc is a complex tectonic region, and several competing models have been 33 proposed to explain its evolution. We study the sedimentary fill of the Guadalquivir Basin to 34 identify tectonic processes occurring when the re-opening of the Strait of Gibraltar led to the 35 re-establishment of Mediterranean outflow. We present a chronostratigraphic framework 36 for the lower Pliocene sediments from the lower Guadalquivir Basin (SW Spain). The 37 updated chronology is based on magnetobiostratigraphic data from several boreholes of this 38 basin. Our results show that the studied interval in the La Matilla core is early Pliocene, and 39 further provide better constraints on the sedimentary evolution of the basin during this 40 period. Migrating depositional facies led to younger onset of sandy deposition basinward. In 41 the northwestern passive margin, a 0.7-Ma period of sedimentary bypass related to a sharp 42 decrease in sedimentation rates and lower sea levels resulted from the tectonic uplift of the 43 forebulge. In contrast, high sedimentation rates with continuous deep marine sedimentation 44 are recorded at the basin center due to continuous tectonic subsidence and west45 southwestward progradation of axial depositional systems. The marginal forebulge uplift, 46


INTRODUCTION
The lower Guadalquivir Basin, located in SW Spain, is the westernmost sector of the 54 Guadalquivir foreland basin of the Betic Cordillera (Fig. 1). This area was part of the marine  In this study, we provide the first detailed chronostratigraphic framework for the LM core based 108 on magnetostratigraphic and biostratigraphic methods, along with information from other cores in 109 the area to assess the tectono-sedimentary evolution of the basin and the neighboring Gibraltar Arc  The study area is located in the western sector of the Guadalquivir Basin (SW Spain) (Fig. 1). 114 This basin is an ENE-WSW-elongated foreland basin bounded to the N by the passive Iberian  (Flores, 1987;Sierro et al., 1993), is the Gibraleón Formation (Civis et al., 1987). This marine unit 131 consists of greenish-bluish clays including glauconitic silts at the base. The contact between the 132 Niebla and Gibraleón formations is interpreted as a condensation level (Sierro et al., 1996;Abad, 133 2010), associated with a maximum flooding event. In the Seville province, the Gibraleón  Units that consist of allochthonous sediments; and 3) the Internal Zones of the Betic Cordillera. 163 The Guadalquivir Basin was formed during the NW emplacement of the Betic front, which 164 occurred progressively from the E to the W of the basin due to the oblique convergence of the 165 African and Eurasian plates (Serpelloni et al., 2007). The emplacement of the outermost unit of

187
The LM core is a 276-m-long continuous core originating from the western lower Guadalquivir        shallower. Therefore, the ChRMs likely provide a reliable record of the polarity reversals of the 318 geomagnetic field. For the sake of quality and because the azimuth of the borehole is unknown, 319 only type 1 and 2 ChRM inclinations have been used to identify polarity intervals in the studied 320 core (Fig. 7a). Each polarity interval has been determined using at least two consecutive type 1 or inclinations and a significant increase in the sand fraction in the studied sediments (Fig. 7a, e).   N1 to R2 according to the PF events described above (Fig. 8). The first of the PF events is R2 correlate with chrons C3n.1r, C3n.1n and C2Ar, respectively (Fig. 8). Taking into account the 394 presence of G. puncticulata in the lowermost sample, which provides a maximum age of 4.52 Ma 395 for the base of the record, we conclude that the interval N1 represents a genuine magnetozone that 396 can be straightforwardly correlated to chron C3n.2n (Figs. 2 and 8). infer the onset of the continental sedimentation at an age not older than 3.95 Ma in the LM core 415 (Fig. 8). The lower Pliocene age (i.e., within chron C2Ar) of the interval of uncertain polarity in 416 the upper part of the core confirms the poor recording fidelity of the sandy sediments from this 417 interval. We attribute this poor fidelity to postdepositional realignment of detrital magnetite during 418 later polarity periods (Fig. 8). 419 We propose an age model for the LM core with three tie points provided by the tops of chrons 420 C3n.2n, C3n.1r, and C3n.1n. Using these tie points, sedimentation rates can be calculated for the 421 different intervals (Table 2). R1 is calculated to have a sedimentation rate of 42.9 cm/kyr.

422
Extrapolating this sedimentation rate downward to the bottom of N1, we infer an age of 4.5 Ma 423 for the bottom of the LM record (Fig. 8). The sedimentation rate for N2 is 49.9 cm/kyr. For the 424 upper part of the record (R2 and uncertain polarity interval), we extrapolate the sedimentation rates 425 of N2 (49.9 cm/kyr) (Fig. 8). In summary, this chronology demonstrates that the LM core 426 represents a continuous 550-kyr early Pliocene marine record (4.5-3.95 Ma) (Fig. 8), which may   The main PF events identified in the MT core are the PF events 2, 3, 4, and 6 of Sierro et al.

507
To estimate the vertical tectonic motions around the M/P boundary at the Villamanrique-1 (VM) 508 borehole, we apply a simplified backstripping analysis in an interval from 6.50 to 5.20 Ma (920 to 509 650 m core depth) encompassing the Miocene-Pliocene boundary (5.33 Ma, 700 m core depth).

510
The analysis accounts for sediment compaction, isostatic subsidence due to the sediment weight 511 and water column, observed paleodepth change and eustatic sea-level fluctuations (Watts, 1988;512 Allen and Allen, 1990). For the sediment compaction correction, we used surface porosity of 0.63,   to integrate the information on sedimentation rates, lithologies, and formation boundaries obtained 574 from the LM core with that from other boreholes along a NW-SE transect from the passive margin 575 towards the active margin of the basin (Fig. 9). In the northwestern part of the basin, the continuous